Дисертації з теми "Nitrogen reduction reaction (NRR)"

L'azote joue un rôle indispensable pour toute vie sur terre et pour le développement des êtres humains. À l'heure actuelle, la seule technologie de synthèse de l'ammoniac à l'échelle industrielle est le procédé mis au point par Haber et Bosch au début du XXe siècle, qui utilise les phases gazeuses N2 et H2. Cependant, le procédé Haber-Bosch nécessite des conditions difficiles, des équipements complexes et une consommation d'énergie élevée, et fonctionne avec de faibles taux de conversion, ce qui est incompatible avec les exigences d’un développement durable. Par rapport à la méthode Haber-Bosch, l'électrocatalyse est l'une des voies prometteuses qui permet d'intégrer l'électricité produite à partir de technologies d'énergies renouvelables pour la production d'ammoniac à température ambiante et à pression ambiante. Un défi spécifique est lié au développement de nouveaux électrocatalyseurs/électrodes dans le but de parvenir à une production d'ammoniac à faible coût, à grande échelle et délocalisée. Compte tenu ces défis scientifiques , ce travail de doctorat se concentre sur trois aspects principaux de la réaction électrocatalytique de réduction de l'azote (NRR) : i) ingénierie et conception de l'électrocatalyseur, ii) conception de l'électrode et de la cellule du dispositif électrochimique et iii) amélioration et optimisation des conditions de réaction, afin d'améliorer les performances de la synthèse de l'ammoniac. La plupart des activités de recherche de ce travail de doctorat sur la synthèse et la caractérisation des matériaux électrocatalytiques et l'assemblage/le test des électrodes dans des dispositifs électrochimiques non conventionnels ont été menées au laboratoire CASPE de l'université de Messine. En outre, une période de 12 mois a été passée en cotutelle avec l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), où des voies de synthèse avancées ont été explorées pour la préparation d'électrocatalyseurs à base de composés organométalliques qui ont été utilisés comme électrodes plus actives dans la RRN. Cette thèse de doctorat est organisée en cinq grands chapitres. Le chapitre 1 se concentre sur les questions de fixation de l'azote et sur la description du processus industriel de Haber-Bosch, avec un aperçu des implications générales liées à ses besoins élevés en énergie. Le chapitre 2 fait référence aux matériaux électrocatalytiques développés pour la préparation des électrodes : 1) les matériaux hybrides organiques-inorganiques de type MOF, une classe de matériaux poreux très prometteurs pour leurs caractéristiques particulières de surface spécifique élevée et leurs propriétés ajustables ainsi que pour la possibilité de créer des sites catalytiques actifs spécifiques grâce aux groupes fonctionnels et aux centres d'ions métalliques ; 2) les MXènes, une classe de matériaux en carbure ou nitrure de métal à structure bidimensionnelle (2D), qui ont récemment suscité un grand intérêt pour un large éventail d'applications, notamment la catalyse et la fixation de N2, pour leurs propriétés uniques de conductivité métallique et de nature hydrophile des surfaces terminées par un hydroxyle ou un oxygène. Les chapitres 3 à 5 présentent et analysent les résultats expérimentaux. Le chapitre 3 concerne la préparation d'une série d'électrodes à base de Fe-MOF (Fe@Zn/SIM-1) et leur test dans la réaction NRR en utilisant un réacteur triphasé de pointe, fonctionnant en phase gazeuse. Dans le chapitre 4, une série de matériaux améliorés à base de Fe-MOF (incluant un dopage additionel par un métal alcalin du MOF UiO-66-(COOH)2), synthétisés par une technique de réaction d'échange de cations pour remplacer le proton de l'acide carboxylique par un cation de fer, sont présentés. Enfin, le chapitre 5 fait référence à l'exploration des matériaux avancés à base de MXène (Ti3C2 MXène) et à la tentative de synthèse d'une nanoarchitecture 3D à partir de catalyseurs à base de MXène en 2D
Nitrogen plays an indispensable role for all life on earth and for the development of human beings. Industrially, nitrogen gas is converted to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed spontaneously by nature. At present, the only industrial-scale ammonia synthesis technology is the process developed by Haber and Bosch in the early 20th century using gas phase N2 and H2 as the feeding gases. However, the Haber-Bosch process requires harsh conditions, complex equipment and high energy consumption, and operates with low conversion rates, which are inconsistent with economic and social growing development requirements. Compared to the Haber-Bosch method, electrocatalysis is one of the promising routes that can integrate electricity produced from renewable energy technologies for the production of ammonia at room temperature and ambient pressure. A specific challenge is related to the development of novel electrocatalysts/electrodes with the aim to achieve a low-cost, large-scale and delocalized production of ammonia. In view of the above key scientific issues, this PhD work focuses on three main aspects of the electrocatalytic nitrogen reduction reaction (NRR): i) engineering and design of the electrocatalyst, ii) electrode and cell design of the electrochemical device and iii) improvement and optimization of the reaction conditions, to enhance the performances of ammonia synthesis. Most of the research activities of this PhD work about synthesis and characterization of the electrocatalytic materials and assembling/testing of the electrodes in unconventional electrochemical devices were carried out at the laboratory CASPE (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. Moreover, during the three years, a period of 12 months was spent in cotutelle with the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where advanced synthesis routes were explored for the preparation of organometallic-based electrocatalysts to be used as more active electrodes in NRR. The PhD thesis is organized in five main chapters. Chapter 1 focuses on N2 fixation issues and on describing the industrial Haber-Bosch process, with an overview of the general implications related to its high energy requirements. Chapter 2, instead, refers to the electrocatalytic materials developed in this PhD work for the preparation of the electrodes: 1) the Metal-organic Frameworks (MOFs), a class of porous materials very promising for their peculiar characteristics of high surface area, tunable properties, organic functionality and porosity, as well as for the possibility of creating specific catalytic active sites thanks to both the functional groups and the metal ion centres; 2) the MXenes, a class of metal carbide or nitride materials with a two-dimensional (2D) structure, which have recently attracted a large interest for a broad range of applications, including catalysis and N2 fixation, for their unique properties of metallic conductivity and hydrophilic nature of the hydroxyl or oxygen terminated surfaces. In Chapters 3-5, the experimental results are presented and discussed. Chapter 3 concerns the preparation of a series of Fe-MOF-based (Fe@Zn/SIM-1) electrodes and their testing in NRR by using an advanced engineered three-phase reactor, working in gas-phase. In Chapter 4, a series of improved Fe-MOF-based materials (Fe-based and Fe-alkali metal-based MOF UiO-66-(COOH)2), synthesized by cation exchange reaction technique to replace the proton of carboxylic acid with an iron cation, are presented. Finally, Chapter 5 refers to the exploration of advanced MXene materials (Ti3C2 MXene) and to the attempt of synthesizing a 3D nanoarchitecture starting from 2D-dimensional MXene-based catalysts

This dissertation relates to the application of density functional theory to the design of novel nanoelectrocatalysts for various electrochemical reduction reactions such as carbon dioxide reduction reactions, carbon monoxide reduction reactions and nitrogen reduction reactions. Many electrocatalysts with high activity, excellent selectivity and stability were designed and engineered using first principle calculations. These findings could potentially guide the experimentalists for creating clean and sustainable energy resources.

This thesis reports a computational discovery and design of highly efficient electrocatalysts for various of electrochemical reactions. The method is based on the Density Functional Theory (DFT) by using Vienna ab initio simulation package (VASP). This project is a step forward in developing the low-cost, high activity, selectivity, stability and scalability for the electrochemical reactions, which could make a contribution to the global-scale green energy system for a clean and sustainable energy future.

Electrocatalysis is key to both sensitive electrochemical sensing and efficient electrochemical energy conversion. Despite high catalytic activity, traditional metal catalysts have poor stability, low selectivity, and high cost. Metal-free, carbon-based materials are emerging as alternatives to metal-based catalysts because of their attractive features including natural abundance, environmental friendliness, high electrical conductivity, and large surface area. Altering surface functionalities and heteroatom doping are effective ways to promote catalytic performance of carbon-based catalysts. The first chapter of this dissertation focuses on developing electrode modification methods for electrochemical sensing of biomolecules. After electrochemical pretreatment, glassy carbon demonstrates impressive figures-of-merit in detecting small, redox-active biomolecules such as DNA bases and neurotransmitters. The results highlight a simplified surface modification procedure for producing efficient and highly selective electrocatalysts. The next four chapters focus on evaluating nitrogen-doped carbon nano-onions (𝑛-CNOs) as electrocatalysts for oxygen reduction and CO2 reduction. 𝑛-CNOs exhibit excellent electrocatalytic performance toward O2 to H2O reduction, which is a pivotal process in fuel cells. 𝑛-CNOs demonstrate excellent resistance against CO poisoning and long-term stability compared to state-of-the-art Pt/C catalysts. In CO2 electrochemical conversion, 𝑛-CNOs demonstrate significant improvement in catalytic performance toward reduction of CO2 to CO with a low overpotential and high selectivity. The outstanding catalytic performance of 𝑛-CNOs originates from the asymmetric charge distribution and creation of catalytic sites during incorporation of nitrogen atoms. High contents of pyridinic and graphitic N are critical for high catalytic performance. This work suggests that carbon-based materials can be outstanding alternatives to traditional metal-based electrocatalysts when their microstructures and surface chemistries are properly tailored.

This thesis presents a comprehensive review of practical strategies to enhance the catalytic activity of M-N-C materials. The practical strategies can be extended to engineer external factors to break the linear scaling relationships and to further enhance the catalytic performances. In order to design the next-generation higher-performance catalysts, this project was a step forward in developing strain and heterostructure method to achieve a superior HER performance and a ORR performance beyond the limit.

Les oxydes d'azote (no et no#2), utilises lors du retraitement des combustibles nucleaires, peuvent etre engendres par voie electrochimique a partir d'acide nitrique. Ce procede ne produit pas d'effluents genants et, de ce fait, constitue une alternative seduisante au procede actuel. Des etudes de voltamperometrie menee a l'aide d'une electrode de platine en milieu nitrique concentre ont montre l'existence de phenomenes de reduction entre 0,05 v/enh et 0,3 v/enh et entre 0,5 v/enh et 1 v/enh. La determination des mecanismes reactionnels mis en jeu dans le domaine de potentiel le plus eleve a ete effectuee: (1) par des methodes classiques de microelectrolyse, (2) par des methodes de macroelectrolyse, (3) par l'utilisation du couplage spectroscopie - electrochimie. Il a ainsi ete montre que la reduction de l'acide nitrique est initiee par l'acide nitreux, reduit electrochimiquement en monoxyde d'azote qui reduit ensuite chimiquement l'acide nitrique. Cette reaction chimique engendre a nouveau l'acide nitreux ce qui met ainsi en evidence le caractere autocatalytique du processus de reduction. L'apparition de dioxyde d'azote dans les produits gazeux montre qu'une ou plusieurs autres reactions chimiques couplees a la reaction electrochimique de reduction de l'acide nitreux interviennent. Tant que la valeur du potentiel de l'electrode de platine est superieure a 0,8 v/enh, les seuls produits de la reduction indirecte de l'acide nitrique sont l'acide nitreux, le monoxyde d'azote et le dioxyde d'azote. Pour un potentiel d'electrode inferieur, le monoxyde d'azote est reduit en protoxyde d'azote n#2o. Le parametre potentiel joue donc un role important vis a vis de la selectivite du procede de production des oxydes d'azote no et no#2. Cependant, des electrolyses en mode intentiostatique ont demontre que, grace au caractere autocatalytique du processus reactionnel, la contrainte de potentiel peut etre aisement maitrisee lors d'une mise en uvre de type industriel

Metal nanoparticles have been a primary focus in areas of catalysis and electrocatalysis applications as a result of their large surface area-to-volume ratios. While there is an increased interest in understanding the properties and behaviors of metal nanoparticles, they can become expensive over time. Recent research has incorporated the idea of using heteroatom-doped materials as a cheaper catalytic alternative to metal nanoparticles. In this study nitrogen-doping and phosphorous-doping techniques were applied to chemical vapor-deposited carbon ultramicroelectrodes in order to study the electrocatalytic properties toward the oxygen reduction reaction and the enhanced affinity for the deposition of gold nanoparticles onto the electrodes.

La reduction electrochimique de (pyradinyl-2)-5- ou (pyridyl-2)-5 methyl-4 dithiole-1,2 thiones-3 est etudiee; elle conduit a des pyrrolo (1,2-a) pyrazines, ou des indolizines respectivement. Des hypotheses sont proposees pour expliquer l'activite anthelminthique des composes du titre

Les oxazaborolidines sont devenues un puissant outil pour la reduction enantioselective des cetones avec des exces enantiomeres eleves. Cependant la separation catalyseur / produit est difficile a realiser, le milieu etant homogene. Le defit etait de fixer une oxazaborolidine a la surface d'un metal catalyseur d'hydrogenation. Ainsi bh#3 serait remplace par h#2 et l'inducteur chiral ne serait pas detruit lors de la separation de l'alcool, realisee par simple decantation. Dans un premier temps nous avons prepare des nano-particules de borure de nickel de composition massique nib#2. Ce catalyseur a ete caracterise par plusieurs methodes physico-chimiques (analyse centesimale, surface bet, surface metallique, infra-rouge, microscopie electronique, auger). Nous avons ensuite greffe une oxazaborolidine sur la surface de nib#2 par addition d'un amino alcool. Plusieurs amino alcools ont ete etudies, differents substrats ont ete hydrogenes (isophorone, 4-methyl-2-pentanone), les exces enantiomeres obtenus sont faibles (respectivement 4% et 3,6%). Cependant il est important de signaler qu'un catalyseur utilise plusieurs fois conserve la meme enantioselectivite et que si l'amino alcool s (+) conduit a l'alcool r (-), l'emploi de l'isomere r (-) de l'amino alcool fournit l'alcool s (+) avec le meme exces enantiomere. Les faibles exces enantiomeres obtenus lors de ces hydrogenations contrastent avec les excellents resultats (ee = 95%) observes lors de la reduction par bh#3. Le complet accord observe entre les proprietes enantioselectives des deux methodes de reductions par bh#3 (homogene et heterogene) met en evidence la formation d'oxazaborolidine. Cette molecule chirale est tres solidement fixee a la surface des particules de catalyseur.

Cette thèse concerne l’étude de la réaction photoélectrochimique de réduction du CO2 (PEC CO2RR) sur le semi-conducteur de type p CuCo2O4 en abordant le rôle cocatalytique des RTIL à base d'imidazolium par microscopie photoélectrochimique à balayage (SPECM). Le CuCo2O4 a été étudié dans différents électrolytes supports, notamment une solution aqueuse, une solution de mélange binaire (25 vol.% [C2mim][BF4]/H2O et 25 vol.% [C4mim][BF4]/H2O) et des liquides ioniques pur pour explorer par SPECM le rôle des RTIL dans les performances des PEC. Un courant de photoréduction significativement amélioré sous l'éclairage UV-vis et visible est obtenu dans une solution à 25 vol.% [C2mim][BF4]/H2O. Seul le CO généré par la PEC CO2RR a été détecté sur une fibre optique à double sonde - ultra-microélectrode (OF-UME) développée au laboratoire et sur une électrolyse en volume sous illumination. La formation de CO à des potentiels plus positifs que la valeur thermodynamique est rapportée ici et il est clairement indiqué que la réduction directe du CO2 à la surface de l'électrode n'est pas le mécanisme. Un schéma de réaction possible pour la PEC CO2RR par l'intermédiaire de [C2mim]+ est proposé. Ainsi, nos résultats ont démontré pour la première fois le rôle cocatalytique de [C2mim]+ pour le PEC CO2RR. En outre, la CO2RR électrochimique a également été étudiée sur divers catalyseurs de métaux de transition, d'azote et de carbone (M–N–Cs). 25%Fe25%Co–N–C a montré la meilleure performance parmi les M–N–Cs étudiés. La présence de sites Co a fourni un effet synergique pour la génération de microcubes distribués riches en Fe, qui agissent comme des sites actifs dans la CO2RR électrochimique
This thesis studies photoelectrochemical CO2 reduction reaction (PEC CO2RR) on p-type semiconductor CuCo2O4 addressing the cocatalytic role of imidazolium based RTILs by scanning photoelectrochemical microscopy (SPECM). CuCo2O4 was studied in different solvent supporting electrolyte systems including: aqueous solution (0.1 M KHCO3 and 0.1 M Na2SO4), binary mixture solution (25 vol.% [C2mim][BF4]/H2O and 25 vol.% [C4mim][BF4]/H2O) and pure RTILs ([C2mim][BF4], [C4mim][BF4]) to explore by SPECM the role of RTILs in CuCo2O4 semiconductor PEC performance. Significantly enhanced photoreduction current under both UV-vis and visible light illumination is reported in 25 vol.% [C2mim][BF4]/H2O solution. Only CO generated from PEC CO2RR was detected using an in-situ detection method based on a home-made dual tip optical fiber-ultramicroelectrode (OF-UME) and from bulk electrolysis under illumination. The formation of CO at potentials more positive than the thermodynamic value clearly points out that direct CO2 reduction on the electrode surface is not the mechanism. A possible reaction scheme for the PEC CO2RR mediated by [C2mim]+ is proposed. Thus, our results have demonstrated for the first time the cocatalytic role of [C2mim]+ for the PEC CO2RR. In addition, electrochemical CO2RR has also been studied on various synthesized transition metal–nitrogen–carbon catalysts (M–N–Cs) by rotating disk electrode. 25%Fe25%Co–N–C exhibited the best performance among the studied M–N–Cs in this thesis. The presence of Co sites in that catalyst provided synergic effect for the generation of distributed Fe-rich microcubes, which act as active sites in electrochemical CO2RR

L'objectif de ce travail était d'évaluer les paramètres importants responsables des excès énantiomériques obtenus par des modèles biométiques chiraux du NADH. Cette stéréodifférenciation serait le fruit d'une synergie entre l'orientation de la fonction carbonyle et l'encombrement stérique du à l'auxiliaire chiral. Cette étude nous a, de plus, permis d'élaborer deux des modèles chiraux du NADH les plus performants en synthèse asymétrique. Dans une seconde partie, nous avons envisagé la synthèse de modèles plus stables et plus réactifs en série benzo[b]naphthyridine et indolo[2,3-b]pyridine

Dans une premiere partie on determine les zones monophasiques des diagrammes des phases pseudoternaires de plusieurs microemulsions. Dans l'une d'elles l'isophorone remplace l'huile. Ensuite les chapitres ii et iii sont consacres aux reactions de photocycloaddition (a+a, a+b) d'alpha -enones (isophorone, coumarine, dimethylthymine). On montre que les phenomenes observes sont lies aux differentes localisations possibles des substrats, aux mecanismes de reaction et aux structures des milieux. En conclusion, la localisation des reactifs a l'interface permet la combinaison des effets de concentration, d'organisation et de proximite et conduit a des rendements, des regio et des stereoselectivite eleves

Ce travail est consacre a la synthese, la caracterisation physico-chimique et l'application de films de polymeres conducteurs electroniques (pce) incluant des heteropolyanions (hpa). Ces entites minerales, connues pour leurs remarquables proprietes redox, sont immobilisees en tant qu'anion dopant lors de la synthese electrochimique des pce. Dans un premier temps, nous montrons qu'une electrode modifiee par un film de poly(3-methyl thiophene) dope par un hpa de structure de keggin xm#1#2o#4#0#n#- (x = p, si ; m = w, mo et n = 3, 4) presente l'electroactivite des deux partenaires. La reponse electrochimique associee a l'hpa n'est pas modifiee apres son immobilisation, pour peu que les conditions de l'electrosynthese du film soient optimisees (rapport monomere/hpa et valeur du potentiel impose). La geometrie particuliere de l'hpa n'induit pas une structure organisee dans le poly(3-methyl thiophene). Afin de remonter a une explication structurale du materiau, nous entreprenons, dans un second temps, l'etude d'un compose modele base sur un oligomere du thiophene (6t)#4#p#+/(pmo#1#2o#4#0)#p#- (6t represente l'hexamere du thiophene et 3 < p < 4). Malheureusement, la tres faible solubilite de ce sel ne permet pas des cristallisations adequates pour des etudes cristallographiques. Les films de pce/hpa peuvent realiser des electrocatalyses, a condition de choisir la bonne association. Dans ce sens, nous mettons en evidence qu'une electrode modifiee par un film de poly(n-methyl pyrrole) incluant un hpa mixte fepw#1#1o#3#9(h#2o)#4#- presente une remarquable stabilite electrochimique et activite electrocatalytique vis-a-vis de la reduction de no#2#-. Le mecanisme de reduction en phase immobilisee passe par un complexe fer-nitrosyl fepw#1#1o#3#9(no)#5#- qui assure la selectivite de cette catalyse chimique electroassistee. Cette electrode modifiee permet de la meme maniere la reduction catalytique du no dissous dans l'eau. Cette molecule, actuellement reconnue comme mediateur intercellulaire essentiel, peut etre detectee in vivo a partir d'une fibre de carbone modifiee par le depot catalytique precedent. Cette etude, realisee en collaboration avec r. Cespuglio (inserm, lyon) constitue un des rares exemples de detection fonctionnelle et en temps reel de cette molecule

Synthèse d'hydroxy- ou dihydroxy amino-3 tetrahydro-1,2,3,4 quinoléines par réduction par NABH4 ou par hydrogénations sur palladium d'amino-3 quinoléines ; études de relation structure activité ; calculs MNDO et CNDO

Dans une première partie, synthèse d'alkyl-4 dihydro-1,4 benzyl-1 et -phényl-1p éthyl-1 (diméthyl-4,4 oxazoline-2yl-2)-3 pyridines et étude de leur activité réductrice vis-a-vis de p-nitrobenzaldéhyde et de benzenéglyoxylate de méthyle en présence de mg**(2+) ; dans la seconde partie, étude de modèles plus énantiosélectifs : méthyl-1 dihydro-1,4 n-(hydroxyméthyl-1 propyl) nicotinamide (=méthyl-1 a), o-, m- et p- xylylene-1, 1' bis-a, dihydro-1,4 methyl-1 nicotinate de (dihydro-1,4 methyl-1 nicotinoylamino)-2 butyle et le cyclophane correspondant à ce dernier composé (pont xylylene entre les azotes des pyridines) ; interaction entre les noyaux pyridines

The development of efficient energy conversion technologies, such as electrocatalysis process converting electricity derived from renewable energy to various forms of chemical energy, provides a highly desired pathway to produce transportable fuels and value-added chemicals from low-cost feedstocks like water and air, simultaneously to ease the fossil fuel reliance as well as the greenhouse gases emissions. Ammonia (NH3) and hydrogen peroxide (H2O2) are globally important chemicals as basic building blocks in industry and promising carbon-free hydrogen carriers. Production of NH3 from N2 (NRR) or H2O2 from O2 (2e– ORR) through the sustainable and energy-saving electrocatalysis process are therefore highly meaningful. A crucial step in conducting these processes is to develop efficient electrocatalysts for effective activation of the reactants and selective formation of the desired products. Therefore, this Thesis aims to design and synthesize novel nanostructured materials as efficient electrocatalysts for nitrogen and oxygen reduction reactions. Besides the electrocatalyst engineering, energy devices combining various reactions/techniques are also elaborately designed as demonstration for future practical applications. In this Thesis, a systematic review on the recent research progress for the application of main group elements on NRR is firstly provided by investigating their interaction with N2 and the strategies for suppression of the undesired hydrogen evolution reaction (HER) (Chapter 2). This chapter provides a concise but comprehensive understanding on various reaction pathways for NRR and strategies towards N2 activation and HER suppression. The first part of this Thesis (Chapter 3 and 4) focus on a comprehensive optimization and accurate evaluation of NRR performance by investigating aspects such as electrocatalyst and electrolyte. Firstly, semiconducting bismuth nanosheet was for the first time reported to be promising candidate for ambient NRR. The high NRR electrocatalytic activity of the Bi NS originates from the sufficient exposure of edge sites coupled with effective p-orbital electron delocalization. Secondly, trace amount of nitrate and nitrite were found to exist in some lithium salts such as Li2SO4 and LiClO4, which are usually used as electrolytes. Reduction of those nitrogen oxyanions (NOx –) causes false positive results for NRR. To avoid these false positive results and to make the best practice of NRR research, simple but versatile spectrophotometric methods were employed to quantitatively determine NOx – contaminations, followed by effective high-temperature annealing strategy to eliminate them. The second part of this Thesis (Chapter 5) focus on exploration of novel strategy for efficient nitrogen fixation other than the present one-step NRR process. It is proposed that fixation of N2-to-NH3 can be decoupled to a two-step process with one problem effectively solved in each step, in which facile activation of N2 to NOx – is realized by a non-thermal plasma technique and highly selective conversion of NOx – to NH3 by electrocatalytic reduction. In the third part of this Thesis (Chapter 6), the electrochemical reduction of O2 via a twoelectron reaction pathway for sustainable and decentralized H2O2 production was investigated on a nitrogen-rich few-layered graphene (N-FLG). A positive correlation between the content of pyrrolic-N and the H2O2 selectivity is experimentally observed. The critical role of pyrrolic- N is elucidated by the variable intermediate adsorption profiles as well as the dependent negative shifts of the pyrrolic-N peak on X-ray absorption near edge structure spectra. A practical device coupling electrochemical H2O2 production with furfural oxidation was then assembled to achieve high-value products on both anode and cathode with optimized energy efficiency.
Thesis (Ph.D.) -- School of Chemical Engineering and Advanced Materials, 2021

L'azoto gioca un ruolo indispensabile per la vita sulla terra e per lo sviluppo degli esseri umani. Industrialmente, è necessario convertire l'azoto gassoso in ammoniaca (NH3) per la produzione di fertilizzanti, in modo da integrare la quantità di azoto fissata spontaneamente in natura. Attualmente, l'unica tecnologia di sintesi dell'ammoniaca su scala industriale è il processo sviluppato da Haber e Bosch all'inizio del XX secolo che utilizza N2 e H2 come gas di alimentazione. Tuttavia, il processo Haber-Bosch richiede condizioni molto drastiche, apparecchiature complesse e porta ad un elevato consumo energetico, operando inoltre a bassi tassi di conversione che non sono coerenti con le esigenze sempre crescenti di sviluppo economico e sociale. In alternativa al metodo Haber-Bosch, l'elettrocatalisi rappresenta una delle vie più promettenti che possono integrare l'elettricità prodotta da tecnologie di energia rinnovabile con la produzione di ammoniaca a temperatura ambiente e a pressione atmosferica. Una sfida specifica è legata allo sviluppo di nuovi elettrocatalizzatori/elettrodi con l'obiettivo di ottenere una produzione di ammoniaca a basso costo, su larga scala e delocalizzata sul territorio. Alla luce delle suddette questioni scientifiche fondamentali, questo lavoro di dottorato si concentra su tre aspetti principali legati alla reazione elettrocatalitica di riduzione dell'azoto (NRR): i) l’ingegneria e la progettazione dell'elettrocatalizzatore, ii) la progettazione dell'elettrodo e del dispositivo elettrochimico e iii) il miglioramento e l’ottimizzazione delle condizioni di reazione, per migliorarne le prestazioni nella sintesi dell'ammoniaca. La maggior parte delle attività di ricerca di questo dottorato, dalla sintesi e caratterizzazione dei materiali elettrocatalitici all'assemblaggio/collaudo degli elettrodi in dispositivi elettrochimici non convenzionali, sono state svolte presso il laboratorio CASPE (Laboratorio di Catalisi per la Produzione e l'Energia Sostenibile) dell'Università di Messina. Durante i tre anni, un periodo di 12 mesi è stato inoltre trascorso in cotutela con l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), dove sono state studiate tecniche di sintesi avanzate per la preparazione di elettrocatalizzatori a base organometallica da utilizzare come elettrodi cataliticamente attivi nella NRR. La tesi di dottorato è organizzata in cinque capitoli principali. Il capitolo 1 si concentra sulle questioni di fissazione dell’azoto e sulla descrizione del processo industriale Haber-Bosch, con una panoramica sulle implicazioni generali relative al suo elevato fabbisogno energetico. Vengono poi presentati i metodi alternativi per la fissazione elettrochimica dell'azoto, con un'ampia descrizione dei vantaggi, legati alle condizioni più favorevoli (cioè temperatura ambiente e pressione atmosferica) e degli svantaggi, e discutendo gli elementi da sviluppare per una futura implementazione di questa tecnologia, includendo anche una descrizione del possibile meccanismo di reazione, ancora non del tutto chiaro in letteratura. Il capitolo 2, invece, si riferisce alla descrizione dei materiali elettrocatalitici sviluppati in questo lavoro di dottorato per la preparazione degli elettrodi: 1) i “Metal-Organic Frameworks” (MOF), una classe di materiali porosi molto promettenti per le loro caratteristiche peculiari di elevata superficie, proprietà adattabili, funzionalità organica e porosità, oltre che per la possibilità di creare specifici siti attivi catalitici grazie sia ai gruppi funzionali che ai centri ionici metallici; 2) i MXeni, una classe di materiali a base di carburi o nitruri metallici con struttura bidimensionale (2D), che hanno recentemente attirato un grande interesse per una vasta gamma di applicazioni, tra cui la catalisi e la fissazione di N2, per le loro proprietà uniche di conducibilità metallica e la natura idrofila delle superfici con terminali idrossilici o di ossigeno. Nei capitoli 3-5 vengono presentati e discussi i risultati sperimentali. Il capitolo 3 riguarda la preparazione di una serie di elettrodi di MOF a base di Fe (Fe@Zn/SIM-1) e il loro test nella NRR utilizzando un reattore trifasico avanzato, che lavora in fase gassosa. Questo nuovo dispositivo funziona a temperatura ambiente e a pressione atmosferica, e possiede il controelettrodo e l’elettrodo di riferimento immersi in una semicella anodica (dove avviene l'ossidazione di H2O a O2) contenente un elettrolita liquido (l'anolita), mentre la semicella catodica per la NRR opera in fase gassosa senza elettrolita liquido. Questo tipo di reattore elettrocatalitico è quindi molto diverso dai reattori elettrocatalitici convenzionali che operano in fase liquida, con il grande vantaggio di evitare problematiche legate alla bassa solubilità e al trasporto di N2 nell'elettrolita, e di permettere inoltre un più facile recupero dell'ammoniaca prodotta. I risultati ottenuti da questi test elettrocatalitici in fase gassosa sono stati molto utili per migliorare la progettazione degli elettrodi a base di MOF, evidenziando i limiti di questo tipo di materiali in termini di contenuto di N, stabilità e possibilità di preparare elettrocatalizzatori più avanzati mediante carbonizzazione. Un'ampia parte di questo capitolo è stata dedicata allo sviluppo di nuove strategie sperimentali per evitare i falsi positivi nella rilevazione dell'ammoniaca, che è uno degli argomenti più investigati negli ultimi due anni dai ricercatori che lavorano sulla NRR. Mentre in letteratura sono stati recentemente proposti protocolli molto accurati che utilizzano tecniche analitiche avanzate (basati sull’azoto marcato 15N), in questo lavoro viene invece suggerita una metodologia più semplice basata sull'analisi spettrofotometrica UV-visibile (accoppiata a test in bianco con gas inerti in luogo dell’azoto) che hanno permesso con successo di evitare contaminazioni da ammoniaca e identificare i falsi positivi, anche se tecniche analitiche più sofisticate sono sicuramente necessarie per confermare definitivamente la vera fonte di ammoniaca. Nel capitolo 4 viene presentata una serie di materiali MOF migliorati (MOF UiO-66-(COOH)2 a base di Fe o Fe e metalli alcalini), sintetizzati con la tecnica di reazione a scambio cationico per sostituire il protone dell'acido carbossilico con un catione di ferro. Rispetto ai materiali Fe@Zn/SIM-1, questa nuova classe di MOF è più stabile in acqua e non contiene atomi di azoto nella sua struttura. I risultati hanno dimostrato che il Fe@UiO-66-(COOH)2 ottenuto mediante l'80% di scambio cationico (con un contenuto effettivo di Fe di circa 8% in peso) è stato il miglior elettrocatalizzatore testato tra i vari materiali MOF a base di Fe sintetizzati. Le prestazioni nella NRR dipendono fortemente dal design della cella e dell'elettrodo. Più in dettaglio, è stato ottenuto un rendimento di ammoniaca di 1.19 μg•h-1•mgcat-2 con una configurazione di strati assemblati ed ordinati nel modo seguente: i) Nafion (la membrana), ii) MOF a base di Fe (l'elettrocatalizzatore), iii) il GDL (lo strato di diffusione gassosa a base di carbonio) e iv) un ulteriore strato di Fe-MOF. È stato anche esplorato l'effetto del voltaggio applicato, con un potenziale ottimale di -0.5 V vs RHE per massimizzare l'attività nella NRR e limitare la reazione collaterale di evoluzione dell'idrogeno. Inoltre, come attualmente utilizzato nei catalizzatori industriali per il processo Haber-Bosh, è stata studiata anche l'introduzione del potassio negli elettrocatalizzatori, al fine di facilitare il trasferimento di carica dagli ioni K- verso la superficie del catalizzatore a base di ferro, bilanciando il chemisorbimento dissociativo tra H2 e N2, e sopprimendo le reazioni collaterali, migliorandone così sia l'attività che la stabilità. I risultati ottenuti sono molto promettenti, anche se sono necessari ulteriori studi per migliorare le loro prestazioni nella NRR, per superare le limitazioni legate ai materiali MOF stessi, soprattutto a causa della loro bassa conducibilità e stabilità. Infine, il capitolo 5 si riferisce all'esplorazione di materiali avanzati, i MXeni (Ti3C2 MXeni), e al tentativo di sintetizzare una nanoarchitettura 3D partendo dalla loro forma bidimensionale. Per comprendere il ruolo della nanostruttura dei materiali MXeni nella NRR, “nanoribbons” (nano-nastri) di Ti3C2 sono stati trattati con KOH per ottenere una forma finale di strutture porose tridimensionali (3D). In particolare, l'obiettivo di questa parte di lavoro è stato quello di indagare come la conversione dei “nanoribbons” di Ti3C2 in strutture tridimensionali influenzi la reattività nella NRR condotta nel dispositivo elettrochimico in fase gassosa. È stata anche effettuata una caratterizzazione completa dei “nanoribbons” di MXeni (SEM, TEM, HRTEM, XRD, XPS e EDX). I risultati hanno mostrato che la nanostruttura tridimensionale porta ad un significativo miglioramento dell'attività di fissazione di N2 a causa della formazione di siti esposti di Ti-OH. È stata anche osservata una relazione lineare tra il tasso di formazione di ammoniaca e la quantità di ossigeno sulla superficie dei Ti3C2 MXeni.
Nitrogen plays an indispensable role for all life on earth and for the development of human beings. Industrially, nitrogen gas is converted to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed spontaneously by nature. At present, the only industrial-scale ammonia synthesis technology is the process developed by Haber and Bosch in the early 20th century using gas phase N2 and H2 as the feeding gases. However, the Haber-Bosch process requires harsh conditions, complex equipment and high energy consumption, and operates with low conversion rates, which are inconsistent with economic and social growing development requirements. Compared to the Haber-Bosch method, electrocatalysis is one of the promising routes that can integrate electricity produced from renewable energy technologies for the production of ammonia at room temperature and ambient pressure. A specific challenge is related to the development of novel electrocatalysts/electrodes with the aim to achieve a low-cost, large-scale and delocalized production of ammonia. In view of the above key scientific issues, this PhD work focuses on three main aspects of the electrocatalytic nitrogen reduction reaction (NRR): i) engineering and design of the electrocatalyst, ii) electrode and cell design of the electrochemical device and iii) improvement and optimization of the reaction conditions, to enhance the performances of ammonia synthesis. Most of the research activities of this PhD work about synthesis and characterization of the electrocatalytic materials and assembling/testing of the electrodes in unconventional electrochemical devices were carried out at the laboratory CASPE (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. Moreover, during the three years, a period of 12 months was spent in cotutelle with the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where advanced synthesis routes were explored for the preparation of organometallic-based electrocatalysts to be used as more active electrodes in NRR. The PhD thesis is organized in five main chapters. Chapter 1 focuses on N2 fixation issues and on describing the industrial Haber-Bosch process, with an overview of the general implications related to its high energy requirements. The alternative methods based on the electrochemical nitrogen fixation are then presented, with a wide description of pros and cons related to the milder conditions (i.e., room temperature and atmospheric pressure) and by discussing the elements to be developed for a future implementation of this technology, including a description of the possible reaction mechanism, which is still unclear in literature. Chapter 2, instead, refers to the electrocatalytic materials developed in this PhD work for the preparation of the electrodes: 1) the Metal-organic Frameworks (MOFs), a class of porous materials very promising for their peculiar characteristics of high surface area, tunable properties, organic functionality and porosity, as well as for the possibility of creating specific catalytic active sites thanks to both the functional groups and the metal ion centres; 2) the MXenes, a class of metal carbide or nitride materials with a two-dimensional (2D) structure, which have recently attracted a large interest for a broad range of applications, including catalysis and N2 fixation, for their unique properties of metallic conductivity and hydrophilic nature of the hydroxyl or oxygen terminated surfaces. In Chapters 3-5, the experimental results are presented and discussed. Chapter 3 concerns the preparation of a series of Fe-MOF-based (Fe@Zn/SIM-1) electrodes and their testing in NRR by using an advanced engineered three-phase reactor, working in gas-phase. This novel device operates at room temperature and atmospheric pressure, with counter and reference electrodes immersed into an anode half-cell (where the oxidation of H2O to O2 occurs) containing a liquid electrolyte (the anolyte), while the cathode half-cell for NRR operates in gas phase without a liquid electrolyte (electrolyte-less conditions). This type of electrocatalytic reactor is thus quite different from the conventional electrocatalytic reactors operating in liquid phase, with the main advantages of avoiding issues related to the low N2 solubility and transport in the electrolyte, and allowing an easier recovery of ammonia. The results obtained from these electrocatalytic tests in gas-phase were very useful to improve the design of the MOFs-based electrodes, evidencing the limits of these kinds of materials in terms of N content, stability and possibility to prepare more advanced electrocatalysts by carbonization. A wide part of this chapter was dedicated to the development of new experimental strategies for avoiding false positive in the detection of ammonia, which is one of the topics most studied from scientists working in NRR in the last two years. As accurate protocols were recently suggested in literature, also using advanced analytical techniques (i.e. using 15N labelled nitrogen), an easier methodology based on UV-visible spectrophotometric analysis (coupled with blank tests with inert gases) was suggested in this work to avoid ammonia contaminations and false positives, although more sophisticated analytical techniques may definitely confirm the real source of ammonia. In Chapter 4, a series of improved Fe-MOF-based materials (Fe-based and Fe-alkali metal-based MOF UiO-66-(COOH)2), synthesized by cation exchange reaction technique to replace the proton of carboxylic acid with an iron cation, are presented. With respect to Fe@Zn/SIM-1, this new class of MOFs are more stable in water and do not contain nitrogen atoms in their structure. Results evidenced that 80% cation exchange Fe@UiO-66-(COOH)2 (with an effective Fe content of around 8 wt.%) was the best electrocatalyst among the tested Fe-based MOF synthesized materials. The performances in NRR highly depended on cell and electrode design. More in detail, an ammonia yield of 1.19 μg•h-1•mgcat-2 was obtained with an assembling configuration of layers ordered as i) Nafion (the membrane), ii) Fe-based MOF (the electrocatalyst), iii) GDL (the carbon gas diffusion layer) and iv) a further layer of Fe-MOF. The effect of applied voltage was also explored, indicating an optimal voltage of -0.5 V vs. RHE to maximize activity in NRR and limiting the side hydrogen evolution reaction. Moreover, as currently used in the industrial catalysts for Haber-Bosh process, the introduction of potassium in the electrocatalysts was also investigated, in order to facilitate charge transfer from K- ions to the iron-based catalyst surface, balancing the dissociative chemisorption between H2 and N2, and suppressing side reactions, thus improving both activity and stability. These results were very promising, although a further experimentation is needed to improve their performances in NRR, to overcome limitations related to MOF materials themselves, majorly due to their low conductivity and stability. Finally, Chapter 5 refers to the exploration of advanced MXene materials (Ti3C2 MXene) and to the attempt of synthesizing a 3D nanoarchitecture starting from 2D-dimensional MXene-based catalysts. To understand the role of the nanostructure of MXene materials in NRR, Ti3C2 nanosheets were treated with KOH to obtain a final shape of three-dimensional (3D) porous frameworks nanoribbons. Specifically, the objective of this research was to investigate how the conversion of Ti3C2 nanosheets to 3D-like nanoribbons influence the NRR reactivity in the gas-phase electrochemical device. A full characterization of MXenes nanoribbons (SEM, TEM, HRTEM, XRD, XPS and EDX) was also presented. Results showed that the 3D-type nanostructure (nanoribbons) leads to a significant enhancement of the N2 fixation activity due to the formation of exposed Ti-OH sites. A linear relationship was observed between ammonia formation rate and amount of oxygen on the surface of Ti3C2 MXene.
L'azote joue un rôle indispensable pour toute vie sur terre et pour le développement des êtres humains. Industriellement, l'azote gazeux est converti en ammoniac (NH3) et en engrais riches en azote pour compléter la quantité d'azote fixée spontanément par la nature. À l'heure actuelle, la seule technologie de synthèse de l'ammoniac à l'échelle industrielle est le procédé mis au point par Haber et Bosch au début du XXe siècle, qui utilise les phases gazeuses N2 et H2. Cependant, le procédé Haber-Bosch nécessite des conditions difficiles, des équipements complexes et une consommation d'énergie élevée, et fonctionne avec de faibles taux de conversion, ce qui est incompatible avec les exigences d’un développement durable. Par rapport à la méthode Haber-Bosch, l'électrocatalyse est l'une des voies prometteuses qui permet d'intégrer l'électricité produite à partir de technologies d'énergies renouvelables pour la production d'ammoniac à température ambiante et à pression ambiante. Un défi spécifique est lié au développement de nouveaux électrocatalyseurs/électrodes dans le but de parvenir à une production d'ammoniac à faible coût, à grande échelle et délocalisée. Compte tenu ces défis scientifiques, ce travail de doctorat se concentre sur trois aspects principaux de la réaction électrocatalytique de réduction de l'azote (NRR) : i) ingénierie et conception de l'électrocatalyseur, ii) conception de l'électrode et de la cellule du dispositif électrochimique et iii) amélioration et optimisation des conditions de réaction, afin d'améliorer les performances de la synthèse de l'ammoniac. La plupart des activités de recherche de ce travail de doctorat sur la synthèse et la caractérisation des matériaux électrocatalytiques et l'assemblage/le test des électrodes dans des dispositifs électrochimiques non conventionnels ont été menées au laboratoire CASPE (Laboratory of Catalysis for Sustainable Production and Energy) de l'université de Messine. En outre, une période de 12 mois a été passée en cotutelle avec l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), où des voies de synthèse avancées ont été explorées pour la préparation d'électrocatalyseurs à base de composés organométalliques qui ont été utilisés comme électrodes plus actives dans la RRN. Cette thèse de doctorat est organisée en cinq grands chapitres. Le chapitre 1 se concentre sur les questions de fixation de l'azote et sur la description du processus industriel de Haber-Bosch, avec un aperçu des implications générales liées à ses besoins élevés en énergie. Les méthodes alternatives basées sur la fixation électrochimique de l'azote sont ensuite présentées, avec une large description des avantages et des inconvénients liés aux conditions plus douces (c'est-à-dire la température ambiante et la pression atmosphérique) et en discutant des éléments à développer pour une future mise en œuvre de cette technologie, y compris une description du mécanisme de réaction possible, encore débattu dans la littérature. Le chapitre 2 fait référence aux matériaux électrocatalytiques développés pour la préparation des électrodes : 1) les matériaux hybrides organiques-inorganiques de type MOF, une classe de matériaux poreux très prometteurs pour leurs caractéristiques particulières de surface spécifique élevée et leurs propriétés ajustables ainsi que pour la possibilité de créer des sites catalytiques actifs spécifiques grâce aux groupes fonctionnels et aux centres d'ions métalliques ; 2) les MXènes, une classe de matériaux en carbure ou nitrure de métal à structure bidimensionnelle (2D), qui ont récemment suscité un grand intérêt pour un large éventail d'applications, notamment la catalyse et la fixation de N2, pour leurs propriétés uniques de conductivité métallique et de nature hydrophile des surfaces terminées par un hydroxyle ou un oxygène. Les chapitres 3 à 5 présentent et analysent les résultats expérimentaux. Le chapitre 3 concerne la préparation d'une série d'électrodes à base de Fe-MOF (Fe@Zn/SIM-1) et leur test dans la réaction NRR en utilisant un réacteur triphasé de pointe, fonctionnant en phase gazeuse. Ce nouveau dispositif fonctionne à température ambiante et à la pression atmosphérique, avec des électrodes de comptage et de référence immergées dans une demi-cellule anodique (où se produit l'oxydation de H2O en O2) contenant un électrolyte liquide (l'anolyte), tandis que la demi-cellule cathodique pour le NRR fonctionne en phase gazeuse sans électrolyte liquide. Ce type de réacteur électrocatalytique est donc très différent des réacteurs électrocatalytiques classiques fonctionnant en phase liquide, avec les principaux avantages d'éviter les problèmes liés à la faible solubilité et au transport de N2 dans l'électrolyte, et de permettre une récupération plus facile de l'ammoniac. Les résultats obtenus lors de ces essais électrocatalytiques en phase gazeuse ont été très utiles pour améliorer la conception des électrodes à base de MOFs, mettant en évidence les limites de ce type de matériaux en termes de teneur en N, de stabilité et de possibilité de préparer des électrocatalyseurs plus avancés par carbonisation. Une grande partie du chapitre 3 a été consacrée au développement de nouvelles stratégies expérimentales pour éviter les faux positifs dans la détection de l'ammoniac, qui est l'un des sujets les plus étudiés par les scientifiques travaillant dans la NRR ces deux dernières années. Comme des protocoles précis ont été récemment suggérés dans la littérature, utilisant également des techniques analytiques avancées (c'est-à-dire utilisant de l'azote marqué à 15N), une méthodologie plus facile basée sur l'analyse spectrophotométrique UV-visible (couplée à des essais à blanc avec des gaz inertes) a été suggérée dans ce travail pour éviter les contaminations par l'ammoniac et les faux positifs, bien que des techniques analytiques plus sophistiquées puissent définitivement confirmer la source réelle d'ammoniac. Dans le chapitre 4, une série de matériaux améliorés à base de Fe-MOF (incluant un dopage additionel par un métal alcalin du MOF UiO-66-(COOH)2), synthétisés par une technique de réaction d'échange de cations pour remplacer le proton de l'acide carboxylique par un cation de fer, sont présentés. En ce qui concerne le Fe@Zn/SIM-1, cette nouvelle classe de MOF est plus stable dans l'eau et ne contient pas d'atomes d'azote dans sa structure. Les résultats ont montré que l'échange cationique à 80 % Fe@UiO-66-(COOH)2 (avec une teneur effective en Fe d'environ 8 % en poids) était le meilleur électrocatalyseur parmi les matériaux synthétisés de MOF à base de Fe testés. Les performances du NRR dépendaient fortement de la conception de la cellule et de l'électrode. Plus en détail, un rendement en ammoniac de 1.19 μg•h-1•mgcat-2 a été obtenu avec une configuration d'assemblage de couches ordonnées comme i) Nafion (la membrane), ii) MOF à base de Fe (l'électrocatalyseur), iii) GDL (la couche de diffusion de gaz carbonique) et iv) une autre couche de Fe-MOF. L'effet de la tension appliquée a également été exploré, indiquant une tension optimale de -0,5 V par rapport à la RHE pour maximiser l'activité dans le NRR et limiter la réaction latérale d'évolution de l'hydrogène. En outre, comme c'est le cas actuellement dans les catalyseurs industriels pour le procédé Haber-Bosh, l'introduction de potassium dans les électrocatalyseurs a également été étudiée, afin de faciliter le transfert de charge des ions K- à la surface du catalyseur à base de fer, en équilibrant la chimisorption dissociative entre H2 et N2, et en supprimant les réactions secondaires, ce qui améliore à la fois l'activité et la stabilité. Ces résultats étaient très prometteurs, bien qu'une nouvelle expérimentation soit nécessaire pour améliorer leurs performances dans les NRR, afin de surmonter les limitations liées aux matériaux MOF eux-mêmes, principalement en raison de leur faible conductivité et de leur stabilité. Enfin, le chapitre 5 fait référence à l'exploration des matériaux avancés à base de MXène (Ti3C2 MXène) et à la tentative de synthèse d'une nanoarchitecture 3D à partir de catalyseurs à base de MXène en 2D. Pour comprendre le rôle de la nanostructure des matériaux à base de MXène dans la NRR, des nanofeuilles de Ti3C2 ont été traitées au KOH pour obtenir une forme finale de nanorubans à armature poreuse tridimensionnelle (3D). Plus précisément, l'objectif de cette recherche était d'étudier comment la conversion des nanofeuilles de Ti3C2 en nanorubans tridimensionnels influençait la réactivité du NRR dans le dispositif électrochimique en phase gazeuse. Une caractérisation complète des nanorubans MXenes (SEM, TEM, HRTEM, XRD, XPS et EDX) a également été présentée. Les résultats ont montré que la nanostructure de type 3D (nanorubans) conduit à une amélioration significative de l'activité de fixation du N2 en raison de la formation de sites Ti-OH exposés. Une relation linéaire a été observée entre le taux de formation d'ammoniac et la quantité d'oxygène à la surface du Ti3C2 MXene.

碩士
國立臺灣科技大學
化學工程系
99
Proton exchange membrane fuel cell (PEMFC), a high energy efficient and environmentally friendly system, is considered to be a future power supplier. The major barriers for PEMFC commercialization are high cost and insufficient cycle life, which mainly arise from the platinum-based catalysts of limited supply. N-doped carbon and non-noble metal chalcogenides are potentially substitutes for platinum catalysts, although their activities are less than platinum. In this investigation, we attempt to promote the activity of cobalt selenide with N-doped carbon. We coated cobalt and iron doped ethylenediamine–formaldehyde chelate complexes on Vulcan support. The N-doped carbon catalyst was subsequently synthesized by heat treatment and wet ball-mill to break down aggregations.The optimal catalyst of N-doped carbon demonstrates an onset potential 0.831 V (vs. NHE), half-wave potential (E1/2) 0.667 V (vs. NHE) when reducing saturated oxygen in 0.5 M H2SO4, measured with the rotating disk voltammetry. It has excellent stability, showing only 0.031V E1/2 decay after 1000 cycles. It also generated less than 3.7% H2O2, accompanying oxygen reduction. This N-doped carbon catalyst contains 5.09 wt% nitrogen, measured with elemental analysis. XPS analysis reveals four types of nitrogen sites. Among them, the pyridinic-N site occupies 40.28%, the pyrrolic-N site takes up 24.80%, and the quarternary-N and pyridinic oxide make up the rest. Raman results indicate a moderate distortion of the graphitized edge, with ID/IG 1.05. TEM shows the morphology of the catalyst. The catalyst activity is improved when cobalt selenium (CoSe2) is integrated with N-doped carbon properly, even though the activity improvement and the stability of this compound catalyst are much less than the catalyst of RuSe2 and N-doped carbon that our group synthesized previously. One salient feature of CoSe2 and N-doped carbon is its relatively low material cost.The best compound catalyst exhibits an onset potential 0.792 V (vs. NHE), half-wave potential (E1/2) 0.711 V (vs. NHE) on oxygen reduction. However, CoSe2 seems unable to withstand the 1000 CV cycles of stability test, the compound catalyst shows a 0.067 V E1/2 decay after 1000 cycles. It also shows a higher H2O2 yield 6.4%. Two crystalline phases are found in CoSe2 catalyst, including the 21 nm particles of orthorhombic phase and the 28-29 nm particles of cubic phase.

碩士
國立臺灣大學
化學研究所
102
To promote the commercial of fuel cell, designing a high activity, high stability and low cost catalyst was a critical issue. Recently, the catalyst which iron nitride nanoparticle deposited on the zero dimensional and three dimensional carbon support was synthesized under the ammonia atmosphere and demonstrated a well performance in activity, electron transfer and yield of hydrogen peroxide in oxygen reduction reaction. However, its activity was still lower than commercial Platinum catalyst. The electronic structure was an important factor to enhance the activity and inhabit the side reaction of catalyst. In present study, we established method to improve the activity by importing an electron donor, platinum for example, into the iron nitride catalyst enhance the back donation of active site. In the present study, the characteristics of catalyst were identified by following technology. Crystal structure identified X-ray powder diffraction. X-ray Absorption Near Edge Structure (XANES) study by using synchrotron radiation was applied for the d-band vacancy of iron and platinum. The oxygen reduction performance was identified by cyclic voltammery. Confirm the introduction of platinum into iron nitride catalyst would enhance the catalytic activity.

碩士
國立中央大學
化學工程與材料工程學系
106
Fuel cells can directly convert chemical energy from a fuel into electricity with high power density, efficiency and in a more environmentally friendly fashion. The oxygen reduction reaction (ORR) is the main reaction on the cathode of fuel cells, and this reaction is limited by its kinetically slow reaction, which in turn decides the overall performance of fuel cells. Traditionally, metallic materials such as platinum and its alloys are used at the cathode. Recently, non-metallic materials such as carbon nanotubes and nitrogen-doped graphene (NG) have seen increased research in the field. Graphene and its derivatives are helpful for electrocatalytical application in fuel cells because of their electronic properties. There has been report that NG and carbon defects facilitate the oxygen reduction reaction (ORR) on the cathode in fuel cells. Raman spectroscopy is used for quick, robust and precise molecular identification. However, the quite small cross-section of common molecules and rather weak signal. Surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman signal of molecules. The SERS effects come from two major mechanisms: electromagnetic mechanism (EM) and chemical enhancement mechanism (CM). Graphene-enhanced Raman scattering (GERS), used graphene as substrate for Raman enhancement, is developing up a new way to study CM and reinforce the practical application of the SERS. In addition, NG on SERS effects has been investigated recently on both experimental and theoretical study which show better SERS effects than pristine graphene. In this study, for the ORR section, we investigate the ORR reactivity of NG by using density functional theory (DFT), a computational quantum mechanical technique. Four doped sites and five models are comprehensively studied: quaternary nitrogen (NQ), pyrrolic nitrogen (N5), pyridinic nitrogen (N6, N6nH) and three-pyridinic nitrogen (3N6). Models for possible sites during each step of the oxygen reduction reaction were set up and visualized to provide a platform to calculate the free energy of ORR reaction pathway to determine the suitability of each doping scenario for ORR reaction. All models except N5 react in associative mechanisms and N5 react in dissociative mechanisms. The calculated free energy pathway demonstrated that the ranking of the reactivity of ORR reaction of different nitrogen configurations from high to low is N6, NQ, N6nH, 3N6, N5. Spin density and charge density aid in describing levels of reactivity. For the GERS section, we investigate the Raman spectra and electronic properties of periodic and cluster pristine and nitrogen-doped graphene models, and the dye molecule R6G. We describe the interaction between R6G and a systematic series of nitrogen-doped graphene: quaternary (NQ), pyrrolic (N5), pyridinic (N6, N6nH) and three-pyridinic (3N6). Density of state (DOS) and work function are calculated to quantify the GERS mechanism. We compared the simulated Raman spectrum of both R6G and R6G on NG, and the result shows enhancement factor (EF) of 3-68 times. Results of density of state (DOS) has shown that R6G on NQ has the energy gap of LUMO-EF which indicate that NQ can have highest potential on GERS effects. Our calculated results of Raman spectra also demonstrated that NQ is the best candidate to the GERS effects.

Metal air battery has captured the spotlight recently as a promising class of sustainable energy storage for the future energy systems. Metal air batteries offer many attractive features such as high energy density, environmental benignity, as well as ease of fuel storage and handling. In addition, wide range of selection towards different metals exists where different energy capacity can be achieved via careful selection of different metals. The most energy dense systems of metal-air battery include lithium-air, aluminum-air and zinc-air. Despite the choice of metal electrode, oxygen reduction (ORR) occurs on the air electrode and oxidation occurs on the metal electrode. The oxidation of metal electrode is a relatively facile reaction compared to the ORR on the air electrode, making latter the limiting factor of the battery system. The sluggish ORR kinetics greatly affects the power output, efficiency, and lifetime of the metal air battery. One solution to this problem is the use of active, affordable and stable catalyst to promote the rate of ORR. Currently, platinum nanoparticles supported on conductive carbon (Pt/C) are the best catalyst for ORR. However, the prohibitively high cost and scarcity of platinum raise critical issues regarding the economic feasibility and sustainability of platinum-based catalysts. Cost reduction via the use of novel technologies can be achieved by two approaches. The first approach is to reduce platinum loading in the catalyst formulation. Alternatively platinum can be completely eliminated from the catalyst composition. The aim of this work is to identify and synthesize alternative catalysts for ORR toward metal air battery applications without the use of platinum re other precious metals (i.e., palladium, silver and gold). Non-precious metal catalysts (NPMC) have received immense international attentions owing to the enormous efforts in pursuit of novel battery and fuel cell technologies. Different types of NPMC such as transition metal alloys, transition metal or mixed metal oxides, chalcogenides have been investigated as potential contenders to precious metal catalysts. However, the performance and stability of these catalysts are still inferior in comparison. Nitrogen-doped carbon materials (NCM) are an emerging class of catalyst exhibiting great potential towards ORR catalysis. In comparison to the metal oxides, MCM show improved electrical conductivity. Furthermore, NCM exhibit higher activity compared to chalcogenides and transition metal alloys. Additional benefits of NCM include the abundance of carbon source and environmental benignity. Typical NCM catalyst is composed of pyrolyzed transition metal macrocycles supported by high surface area carbon. These materials have demonstrated excellent activity and stability. However, the degradation of these catalysts often involves the destruction of active sites containing the transition metal centre. To further improve the durability and mass transport of NCM catalyst, a novel class of ORR catalyst based on nitrogen-doped carbon nanotubes (NCNT) is investigated in a series of studies. The initial investigation focuses on the synthesis of highly active NCNT using different carbon-nitrogen precursors. This study investigated the effect of using cyclic hydrocarbon (pyridine) and aliphatic hydrocarbon (ethylenediamine) towards the formation and activity of NCNT. The innate structure of the cyclic hydrocarbon promotes the formation of NCNT to provide higher product yield; however, the aliphatic hydrocarbon promotes the formation of surface defects where the nitrogen atoms can be incorporated to form active sites for ORR. As a result, a significant increase in the ORR activity of 180 mV in half-wave potential is achieved when EDA was used as carbon-nitrogen precursor. In addition, three times higher limiting current density was observed for the NCNT synthesized from ethylenediamine. Based on the conclusion where highly active NCNT was produced from aliphatic hydrocarbon, similar carbon-nitrogen precursors with varying carbon to nitrogen ratio in the molecular structure (ethylenediamine, 1, 3-diaminopropane, 1, 4-diaminobutane) were adapted for the synthesis of NCNT. The investigation led to the conclusion that higher nitrogen to carbon ratio in the molecular structure of the precursors benefits the formation of active NCNT for ORR catalysis. The origin of such phenomena can be correlated with the higher relative nitrogen content of the resultant NCNT synthesized from aliphatic carbon precursor that provided greater nitrogen to carbon ratio. As the final nitrogen content increased in the molecular structure, the half-wave potential of the resultant NCNT towards ORR catalysis was increased by 120 mV. The significant improvement hints the critical role of nitrogen content towards ORR catalysis. To further confirm the correlation between the nitrogen content and ORR activity, another approach was used to control the final nitrogen content in the resultant NCNT. In the third investigation, a carbon-nitrogen precursor (pyridine) was mixed with a carbon precursor (ethanol) to form an admixture. The relative proportion of the two components of the admixture was varied to produce NCNT with different nitrogen content. By adopting this methodology, potential effect of different carbon-nitrogen precursors on the formation of NCNT can be eliminated since the same precursors were used for NCNT synthesis. Based on the electrochemical evaluations, the nitrogen content can be positively correlated to ORR activity. Among the NCNT samples, 41% higher limiting current density was achieved for 0.7 at. % increase in overall nitrogen content. Furthermore, the selectivity of the NCNT catalyst with higher nitrogen content favours the production of water molecule—the favourable product in metal-air battery by 43%. ORR catalyst is an outer-sphere electron transfer reaction whereby the reactants interact with the surface of catalysts. Consequently, the surface structure can be a determining factor towards the ORR activity of the NCNT in addition to the nitrogen content. In the forth investigation, the surface structure of NCNT was tailored to differentiate the ORR activity of smooth and rugged surface while controlling the overall nitrogen content to be similar. NCNT having different surface structures but similar nitrogen content (approximately 2.7 to 2.9 at. %) were successfully synthesized using different synthesis catalysts. Comparison of the two NCNT catalysts showing different surface structure resulted in a 130 mV increased in half-wave potential favouring the NCNT with more rugged surface structure. This study provided insights to the potential effects of synthesis catalyst towards directing the surface structure and the ORR activity of NCNT. Through a series of studies, the important parameters affecting the ORR performance of NCNT were elucidated and the most active NCNT catalyst synthesized was used for testing in a prototype zinc-air battery. The fifth study evaluated the performance of NCNT catalyst in different concentrations of alkaline electrolyte and at different battery voltage. An increase in the electrolyte’s alkaline strength improved the battery performance to a certain degree until the increasing viscosity impeded the performance of the battery system. The zinc-air battery employing NCNT as ORR catalyst produced a maximum battery power density of 69.5 mWcm-2 in 6M potassium hydroxide. The fifth study illustrated the great potential of NCNT towards the ORR catalysis for metal-air batteries. In combination, the series of investigations presented in this document provide a comprehensive study of a novel material and its application towards ORR catalysis in metal air batteries. Specifically, this report provides insights into the fundamentals of NCNT synthesis; the origins of ORR activity and the optimal operating conditions of NCNT in a prototype zinc-air battery. The excellent performance of NCNT warrants further studies of this material in greater details, and the information presented in this document will create a basis for future investigations towards ORR catalysis.

The extensive amount of platinum required in order to facilitate the oxygen reduction reaction (ORR) occuring at the cathode of low temperature fuel cells provides cost limitations to the sustainable commercialization of this technology. The development of electrocatalyst materials with either reduced or eliminated platinum dependency is an urgent necessity. The present work investigates the application of nitrogen doped carbon nanotubes (N-CNTs) and their composites as electrocatalyst materials for the ORR. First, N-CNTs are investigated as platinum support materials for proton exchange membrane fuel cells. They were found to result in improved ORR activity in comparison with undoped CNT supported platinum, due to the enhanced catalyst-support interactions and electronic properties induced by nitrogen heteroatoms incorporated into the graphitic structure of CNTs. Second, N-CNTs synthesized from a variety of different precursor materials were investigated as ORR electrocatalysts in alkaline conditions. The influence of the precursor materials was illustrated with improved ORR activity and nitrogen concentration observed for N-CNTs synthesized with precursor materials containing higher nitrogen to carbon contents. Highly active N-CNTs based on ethylenediamine were fabricated into thin, free standing films for use as a stand-alone cathode catalyst layer in an alkaline anion exchange membrane fuel cell. Finally, metal-free N-CNTs were developed and demonstrated to provide promising ORR in the absence of any metal interactions.

Although Cu-SSZ-13 zeolites are used commercially in diesel engine exhaust after-treatment for abatement of toxic NO_x pollutants via selective catalytic reduction (SCR) with NH₃, molecular details of its active centers and mechanistic details of the redox reactions they catalyze, specifically of the Cu(I) to Cu(II) oxidation half-reaction, are not well understood. A detailed understanding of the SCR reaction mechanism and nature of the Cu active site would provide insight into their catalytic performance and guidance on synthesizing materials with improved low temperature (< 473 K) reactivity and stability against deactivation (e.g. hydrothermal, sulfur oxides). We use computational, titration, spectroscopic, and kinetic techniques to elucidate (1) the presence of two types of Cu²⁺ ions in Cu-SSZ-13 materials, (2) molecular details on how these Cu cations, facilitated by NH₃ solvation, undergo a reduction-oxidation catalytic cycle, and (3) that sulfur oxides poison the two different types of Cu²⁺ ions to different extents at via different mechanisms.

Copper was exchanged onto H-SSZ-13 samples with different Si:Al ratios (4.5, 15, and 25) via liquid-phase ion exchange using Cu(NO₃)₂ as the precursor. The speciation of copper started from the most stable Cu²⁺ coordinated to two anionic sites on the zeolite framework to [CuOH]⁺ coordinated to only one anionic site on the zeolite framework with increasing Cu:Al ratios. The number of Cu²⁺ and [CuOH]⁺ sites was quantified by selective NH₃ titration of the number of residual Brønsted acid sites after Cu exchange, and by quantification of Brønsted acidic Si(OH)Al and CuOH stretching vibrations from IR spectra. Cu-SSZ-13 with similar Cu densities and anionic framework site densities exhibit similar standard SCR rates, apparent activation energies, and orders regardless of the fraction of Z₂Cu and ZCuOH sites, indicating that both sites are equally active within measurable error for SCR.

The standard SCR reaction uses O₂ as the oxidant (4NH₃ + 4NO + O₂ -> 6H₂O + 4N₂) and involves a Cu(I)/Cu(II) redox cycle, with Cu(II) reduction mediated by NO and NH₃, and Cu(I) oxidation mediated by NO and O₂. In contrast, the fast SCR reaction (4NH₃ + 2NO + 2NO₂ -> 6H₂O + 4N₂) uses NO₂ as the oxidant. Low temperature (437 K) standard SCR reaction kinetics over Cu-SSZ-13 zeolites depend on the spatial density and distribution of Cu ions, varied by changing the Cu:Al and Si:Al ratio. Facilitated by NH₃ solvation, mobile Cu(I) complexes can dimerize with other Cu(I) complexes within diffusion distances to activate O₂, as demonstrated through X-ray absorption spectroscopy and density functional theory calculations. Monte Carlo simulations are used to define average Cu-Cu distances. In contrast with O₂-assisted oxidation reactions, NO₂ oxidizes single Cu(I) complexes with similar kinetics among samples of varying Cu spatial density. These findings demonstrate that low temperature standard SCR is dependent on Cu spatial density and requires NH₃ solvation to mobilize Cu(I) sites to activate O₂, while in contrast fast SCR uses NO₂ to oxidize single Cu(I) sites.

We also studied the effect of sulfur oxides, a common poison in diesel exhaust, on Cu-SSZ-13 zeolites. Model Cu-SSZ-13 samples exposed to dry SO₂ and O₂ streams at 473 and 673 K. These Cu-SSZ-13 zeolites were synthesized and characterized to contain distinct Cu active site types, predominantly either divalent Cu²⁺ ions exchanged at proximal framework Al sites (Z₂Cu), or monovalent CuOH+ complexes exchanged at isolated framework Al sites (ZCuOH). On the model Z₂Cu sample, SCR turnover rates (473 K, per Cu) catalyst decreased linearly with increasing S content to undetectable values at equimolar S:Cu molar ratios, while apparent activation energies remained constant at ~65 kJ mol^-1, consistent with poisoning of each Z₂Cu site with one SO₂-derived intermediate. On the model ZCuOH sample, SCR turnover rates also decreased linearly with increasing S content, yet apparent activation energies decreased monotonically from ~50 to ~10 kJ mol^-1, suggesting that multiple phenomena are responsible for the observed poisoning behavior and consistent with findings that SO₂ exposure led to additional storage of SO₂-derived intermediates on non-Cu surface sites. Changes to Cu²⁺ charge transfer features in UV-Visible spectra were more pronounced for SO₂-poisoned ZCuOH than Z₂Cu sites, while X-ray diffraction and micropore volume measurements show evidence of partial occlusion of microporous voids by SO₂-derived deposits, suggesting that deactivation may not only reflect Cu site poisoning. Density functional theory calculations are used to identify the structures and binding energies of different SO₂-derived intermediates at Z₂Cu and ZCuOH sites. It is found that bisulfates are particularly low in energy, and residual Brønsted protons are liberated as these bisulfates are formed. These findings indicate that Z₂Cu sites are more resistant to SO₂ poisoning than ZCuOH sites, and are easier to regenerate once poisoned.

Selective catalytic reduction (SCR) of NO_x using NH₃ as a reductant (4NH₃+ 4NO + O₂ 6H₂O + 4N₂) over Cu-SSZ-13 zeolites is a commercial technology used to meet emissions targets in lean-burn and diesel engine exhaust. Optimization of catalyst design parameters to improve catalyst reactivity and stability against deactivation (hydrothermal and sulfur poisoning) necessitates detailed molecular level understanding of structurally different active Cu sites and the reaction mechanism. With the help of synthetic, titrimetric, spectroscopic, kinetic and computational techniques, we established new molecular level details regarding 1) active Cu site speciation in monomeric and dimeric complexes in Cu-SSZ-13, 2) elementary steps in the catalytic reaction mechanism, 3) and deactivation mechanisms upon hydrothermal treatment and sulfur poisoning.

We have demonstrated that Cu in Cu-SSZ-13 speciates as two distinct isolated sites, nominally divalent Cu^II and monovalent [Cu^II(OH)]⁺ complexes exchanged at paired Al and isolated Al sites, respectively. This Cu site model accurately described a wide range of zeolite chemical composition, as evidenced by spectroscopic (Infrared and X-ray absorption) and titrimetric characterization of Cu sites under ex situ conditions and in situ and operando SCR reaction conditions. Monovalent [Cu^II(OH)]⁺ complexes have been further found to condense to form multinuclear Cu-oxo complexes upon high temperature oxidative treatment, which have been characterized using UV-visible spectroscopy, CO-temperature programmed reduction and dry NO oxidation as a probe reaction. Structurally different isolated Cu sites have different susceptibilities to H₂ and He reductions, but are similarly susceptible to NO+NH₃ reduction and have been found to catalyze NO_x SCR reaction at similar turnover rates (per Cu^II; 473 K) via a Cu^II/Cu^I redox cycle, as their structurally different identities are masked by NH₃ solvation during reaction.

Molecular level insights on the low temperature Cu^II/Cu^I redox mechanism have been obtained using experiments performed in situand in operando coupled withtheory. Evidence has been provided to show that the Cu^II to Cu^I reduction half-cycle involves single-site Cu reduction of isolated Cu^II sites with NO+NH₃, which is independent of Cu spatial density. In contrast, the Cu^I to Cu^II oxidation half-cycle involves dual-site Cu oxidation with O₂ to form dimeric Cu-oxo complexes, which is dependent on Cu spatial density. Such dual-site oxidation during the SCR Cu^II/Cu^I redox cycle requires two Cu^I(NH₃)₂sites, which is enabled by NH₃solvation that confers mobility to isolated Cu^I sites and allows reactions between two Cu^I(NH₃)₂ species and O₂. As a result, standard SCR rates depend on Cu proximity in Cu-SSZ-13 zeolites when Cu^I oxidation steps are kinetically relevant. Additional unresolved pieces of mechanism have been investigated, such as the reactivity of Cu dimers, the types of reaction intermediates involved, and the debated role of Brønsted acid sites in the SCR cycle, to postulate a detailed reaction mechanism. A strategy has been discussed to operate either in oxidation or reduction-limited kinetic regimes, to extract oxidation and reduction rate constants, and better interpret the kinetic differences among Cu-SSZ-13 catalysts.

The stability of active Cu sites upon sulfur oxide poisoning has been assessed by exposing model Cu-zeolite samples to dry SO₂ and O₂ streams at 473 and 673 K, and then analyzing the surface intermediates formed via spectroscopic and kinetic assessments. Model Cu-SSZ-13 zeolites were synthesized to contain distinct Cu active site types, predominantly either divalent Cu^II ions exchanged at proximal framework Al (Z₂Cu), or monovalent [Cu^IIOH]⁺ complexes exchanged at isolated framework Al (ZCuOH). SCR turnover rates (473 K, per Cu) decreased linearly with increasing S content to undetectable values at equimolar S:Cu ratios, consistent with poisoning of each Cu site with one SO₂-derived intermediate. Cu and S K-edge X-ray absorption spectroscopy and density functional theory calculations were used to identify the structures and binding energies of different SO₂-derived intermediates at Z₂Cu and ZCuOH sites, revealing that bisulfates are particularly low in energy, and residual Brønsted protons are liberated at Z₂Cu sites as bisulfates are formed. Molecular dynamics simulations also show that Cu sites bound to one HSO₄^- are immobile, but become liberated from the framework and more mobile when bound to two HSO₄^-. These findings indicate that Z₂Cu sites are more resistant to SO₂poisoning than ZCuOH sites, and are easier to regenerate once poisoned.

The stability of active Cu sites on various small-pore Cu-zeolites during hydrothermal deactivation (high temperature steaming conditions) has also been assessed by probing the structural and kinetic changes to active Cu sites. Three small-pore, eight-membered ring (8-MR) zeolites of different cage-based topology (CHA, AEI, RTH) have been investigated. With the help of UV-visible spectroscopy to probe the Cu structure, in conjunction with measuring differential reaction kinetics before and after subsequent treatments, it has been suggested that the RTH framework imposes internal transport restrictions, effectively functioning as a 1-D framework during SCR catalysis. Hydrothermal aging of Cu-RTH results in complete deactivation and undetectable SCR rates, despite no changes in long-range structure or micropore volume after hydrothermal aging treatments and subsequent SCR exposure, highlighting beneficial properties conferred by double six-membered ring (D6R) composite building units. Exposure aging conditions and SCR reactants resulted in deleterious structural changes to Cu sites, likely reflecting the formation of inactive copper-aluminate domains. Therefore, the viability of Cu-zeolites for practical low temperature NO_x SCR catalysis cannot be inferred solely from assessments of framework structural integrity after aging treatments, but also require Cu active site and kinetic characterization after aged zeolites are exposed to low temperature SCR conditions.

In the past decade, rechargeable batteries attracted the attention from the researchers in search for renewable and sustainable energy sources. Up to date, lithium-ion battery is the most commercialized and has been supplying power to electronic devices and hybrid and electric vehicles. Lithium-ion battery, however, does not satisfy the expectations of ever-increasing energy and power density, which of their limits owes to its intercalation chemistry and the safety.1-2 Therefore, metal-air battery drew much attention as an alternative for its high energy density and a simple cell configuration.1 There are several different types of metal-air batteries that convey different viable reaction mechanisms depending on the anode metals; such as Li, Al, Ca, Cd, and Zn. Redox reactions take place in a metal-air cell regardless of the anode metal; oxidation reaction at the anode and reduction reaction at the air electrode. Between the two reaction, the oxygen reduction reaction (ORR) at the air electrode is the relatively the limiting factor within the overall cell reactions. The sluggish ORR kinetics greatly affects the performance of the battery system in terms of power output, efficiency, and durability. Therefore, researchers have put tremendous efforts in developing highly efficient metal air batteries and fuel cells, especially for high capacity applications such as electric vehicles. Currently, the catalyst with platinum nanoparticles supported on carbon material (Pt-C) is considered to exhibit the best ORR activities. Despite of the admirable electrocatalytic performance, Pt-C suffers from its lack of practicality in commercialization due to their prohibitively high cost and scarcity as of being a precious metal. Thus, there is increasing demand for replacing Pt with more abundant metals due economic feasibility and sustainability of this noble metal.3-5 Two different attitudes are taken for solution. The first approach is by optimizing the platinum loading in the formulation, or the alternatively the platinum can be replaced with non-precious materials. The purpose of this work is to discover and synthesize alternative catalysts for metal-air battery applications through optimized method without addition of precious metals. Different non-precious metals are investigated as the replacement of the precious metal including transition metal alloys, transition metal or mixed metal oxides, and chalcogenides. These types of metals, alone, still exhibits unsatisfying, yet worse, kinetics in comparison to the precious metals. Nitrogen-doped carbon material is a recently well studied carbon based material that exhibits great potential towards the cathodic reaction.6 Nitrogen-doped carbon materials are found to exhibit higher catalytic activity compared to the mentioned types of metals for its improved conductivity. Benefits of the carbon based materials are in its abundance and minimal environmental footprints. However, the degradation of these materials has demonstrated loss of catalytic activity through destruction of active sites containing the transition metal centre, ultimately causing infeasible stability. To compensate for these drawbacks and other limits of the nitrogen-doped carbon based catalysts, nitrogen-doped carbon nanotubes (NCNT) are also investigated in the series of study. The first investigation focuses on a development of a simple method to thermally synthesize a non-precious metal based nitrogen-doped graphene (NG) electrocatalyst using exfoliated graphene (Ex-G) and urea with varying amounts of iron (Fe) precursor. The morphology and structural features of the synthesized electrocatalyst (Fe-NG) were characterized by SEM and TEM, revealing the existence of graphitic nanoshells that potentially contribute to the ORR activity by providing a higher degree of edge plane exposure. The surface elemental composition of the catalyst was analyzed through XPS, which showed high content of a total N species (~8 at.%) indicative of the effective N-doping, present mostly in the form of pyridinic nitrogen groups. The oxygen reduction reaction (ORR) performance of the catalyst was evaluated by rotating disk electrode voltammetry in alkaline electrolyte and in a zinc-air battery cell. Fe-NG demonstrated high onset and half-wave potentials of -0.023 V (vs. SCE) and -0.110 V (vs. SCE), respectively. This excellent ORR activity is translated into practical zinc-air battery performance capabilities approaching that of commercial platinum based catalyst. Another approach was made in the carbon materials to further improve the cost of the electrode. Popular carbon allotropes, CNT and graphene, are combined as a composite (GC) and heteroatoms, nitrogen and sulfur, are introduced in order to improve the charge distribution of the graphitic network. Dopants were doped through two step processes; nitrogen dopant was introduced into the graphitic framework followed by the sulfur dopant. The coexistence of the two heteroatoms as dopants demonstrated outstanding ORR performance to those of reported as metal free catalysts. Furthermore, effects of temperature were investigated through comparing ORR performances of the catalysts synthesized in two different temperatures (500 ??? and 900 ???) during the N-doping process (consistent temperature was used for S-doping). Through XPS analysis of the surface chemistry of catalysts produced with high temperature during the N-doping step showed absence of N-species after the subsequent S-doping process (GC-NHS). Thus, the synergetic effects of the two heteroatoms were not revealed during the half-cell testing. Meanwhile, the two heteroatoms were verified in the catalyst synthesized though using low temperature during the N-doping process followed by the S-doping step (GC-NLS). Consequently, ORR activity of the resulting material demonstrated promising onset and half-wave potentials of -0.117 V (vs. SCE) and -0.193 V (vs. SCE). In combination of these investigations, this document introduces thorough study of novel materials and their performance in its application as ORR catalyst in metal air batteries. Moreover, this report provides detailed fundamental insights of carbon allotropes, and their properties as potential elecrocatalysts and essential concepts in electrochemistry that lies behind zinc-air batteries. The outstanding performances of carbon based electrocatalyst are reviewed and used as the guides for further direction in the development of metal-air batteries as a promising sustainable energy resource in the future.