Heterogeneous catalysis plays a significant role in the chemical industry and the global economy. Most heterogeneous catalysts in the chemical industry and laboratory consist of supported metal nanoparticles, clusters and isolated (single) atoms. Understanding structure sensitivity and identifying the active site or sites are crucially essential for designing efficient catalysts. To determine the active sites of a catalyst for a particular chemical reaction, in-situ/operando spectroscopy, such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption fine structure (XAFS) spectroscopy, is usually implemented as characterization tools. However, understanding the limitation of the characterization tools is crucial to eliminate misleading conclusions. Therefore, the main object of this work is not only to characterize the catalyst before and after the reaction but to investigate the reliability of the characterization tools as well as the stability of the metal clusters and single atoms during CO oxidation. There are four main findings that will be present in this work. First, a high-flux X-ray beam can induce structural change that leads to a reduction of the metal and agglomeration of metal clusters. This finding is very important since X-ray beam damage is uncommon for heterogeneous catalysis as for homogeneous catalysts and biological samples. In the study, the effect of high-flux X-ray on the Rh clusters and nanoparticles was highlighted along with providing mitigation strategies in order to reduce the damage caused by the high-flux X-ray beam. The second important finding is about the characterization of Rh clusters and nanoparticles during CO reduction treatment using DRIFTS. In this study, the integration of low-temperature CO oxidation kinetics as a characterization tool with DRIFTS, XAFS and scanning/transmission electron microscopy (STEM) was found to be necessary to improve the characterization of Rh single atoms. Implementing CO oxidation measurements at low temperatures can provide a rough estimation of the percentage of Rh single atoms. The third finding is related to the stability of Rh clusters upon exposure to CO, oxygen and CO oxidation at different temperatures. The study shows an unexpected dynamic structural change that the Rh cluster undergoes during exposure to oxygen even at room temperature in which the Rh clusters disperse to form Rh single atoms. This dispersion phenomenon was found to be size, gas environment and temperature dependent. For example, small clusters tend to disperse while large nanoparticles resist dispersion. additionally, increasing the temperature to ∼ 160 with CO and oxygen lead to an increase in the percentage of Rh single atoms. More importantly, the dispersed catalyst (Rh single atoms) exhibits higher CO oxidation activity than Rh nanoparticles by 350x. This finding can also be used for Rh single atoms synthesis for different oxide supports such as MgAl2O4, TiO2, and CeO2. Finally, the fourth finding is about investigating the CO oxidation kinetics and mechanism. The kinetics of Rh single atoms differ from Rh nanoparticles. Implementing in-situ spectroscopy helps to identify the resting state of the Rh complex during CO oxidation which is Rh(CO)2. By combining CO oxidation kinetics and in-situ spectroscopy, the plausible mechanism was suggested to be Eley-Rideal/Mars Van Krevelen mechanism. / Doctor of Philosophy / Heterogeneous catalysts are solid materials that scientists and chemical engineers use to convert undesirable raw reactants (liquid or gas) to other products (liquid or gas). One example of a heterogeneous catalyst is a catalytic converter used in most cars around the world. One goal of the catalytic converter is to convert CO (toxic gas) to CO2 (less toxic). The catalyst in a catalytic converter contains precious metals as nanoparticles such as Platinum (Pt), Palladium (Pd) and Rhodium (Rh) deposits on oxide supports (inert materials) such as Al2O3. These Pt, Pd and Rh nanoparticles help to accelerate the chemical reaction (e.g.CO oxidation) in which converting the toxic gas CO to CO2 at a relatively low temperature compared to if the reaction proceeds without those metal nanoparticles. In order to improve the performance of the catalyst, scientists and engineers implement characterization techniques to identify the active site based on the shape and size of the nanoparticles. One method to improve the catalyst performance is to decrease the particle size below 2 nm or even to reach isolated atoms. Unfortunately, synthesizing isolated (single) atoms supported on oxide support is very challenging. One main discovery presented in this work is that Rh single atoms can be synthesized using a simple but effective method. More importantly, Rh single atoms show higher performance than Rh nanoparticles by 350 times which helps to convert CO the toxic gas to CO2 at room temperature. This finding is important in which that the synthesis presented here can be used for different chemical reactions such as methane oxidation and methanol carbonylation.

Les acides carboxyliques aliphatiques sont utilisés dans de nombreux secteurs industriels et leur importance économique augmente. Ils sont actuellement produits en grande quantité, grâce à des procédés utilisant le C0 qui est principalement non- renouvelable. L'anhydride carbonique est une molécule potentiellement écologique, renouvelable et abondante. Cette thèse décrit l'étude et l'optimisation d'un système catalytique homogène de Rh, utilisé pour produire des acides carboxyliques aliphatiques à partir de substrats oxygénés, C02 et H2. Le système consiste en un précurseur de Rh, un additif à base d'iodure et un ligand PPh3, fonctionnant dans un réacteur discontinu sous une pression de C02 et de H2. Les conditions de réaction ont été optimisées pour chaque classe de substrats étudiés: alcools primaires et secondaires, cétones, aldéhydes et époxydes. 30 molécules différentes ont été converties en acides carboxyliques, conduisant à des rendements jusqu'à 80%. En plus, le système a été étudié avec une approche de « Design of Experiment », ce qui a permis d'obtenir des informations supplémentaires concernant les paramètres étudiés. Le mécanisme de réaction et les espèces catalytiques actives ont été étudiés par différentes manipulations comme des réactions compétitives, des expériences de RMN et l'utilisation de molécules marquées. La réaction est composée de transformations non catalytiques et de deux étapes catalytiques. La réaction se déroule à travers une réaction de reverse Water Gas Shift (rWGSR) transformant le C02 et l'H2 en C0 et H20, qui sont consommés dans l'hydrocarboxylation suivante de l'alcène formé in situ pour livrer l'acide carboxylique. Le système catalytique est similaire aux catalyseurs traditionnels à base du Rh pour les réactions de carbonylation et de Water Gas Shift. Le PPh3 est nécessaire pour fournir des ligands supplémentaires, permettant au catalyseur de fonctionner avec une quantité minimale de ligand toxique de C0. En plus, un système catalytique hétérogène a été étudié pour la même réaction. « Single Atom Catalysts » (SACs) reçoit beaucoup plus d'attention que les solutions catalytiques, car il présente à la fois les avantages des catalyseurs homogènes (sélectivité, haute activité) et des catalyseurs hétérogènes (séparation et recyclage faciles). Des atomes de rhodium simples dispersés sur du graphène dopé avec l'N ont été synthétisés et caractérisés, obtenant des informations concernant la structure chimique et physique du matériau. Finalement, ils ont été testés ainsi que les catalyseurs pour l'activation du C02, la production d'acides carboxyliques, les réactions d'hydrogénation et d'hydrogénolyse / Aliphatic carboxylic acids are used in many industrial sectors and their importance from an economical point of view is increasing. They are currently produced in large quantities, through processes exploiting the mostly non-renewable C0 as C1 synthon. Carbon dioxide is a potential environmentally friendly, renewable and abundant C1 building block. The aim of this work is to provide a catalytic protocol converting C02, H2 and oxygenated substrates to obtain useful chemicals, like carboxylic acids.To this end a homogeneous catalytic Rh system, used to produce aliphatic carboxylic acids starting from oxygenated substrates, C02 and H2 was investigated and optimized. The system consists of a Rh precursor, iodide additive and PPh3 ligand working in a batch reactor under C02 and H2 pressure. The reaction conditions were optimized for each class of investigated substrates: primary alcohols, secondary alcohols, ketones, aldehydes and epoxides. The reaction scope was investigated and 30 different molecules were converted into carboxylic acids, leading to yields of up to 80%. ln addition, the system was studied using a Design of Experiment approach, obtaining additional information regarding the studied parameters.The reaction mechanism and the catalytically active species were studied, by different experiments like competitive reactions, NMR and labelling experiments. This investigation resulted in a deeper knowledge of the reaction pathway, composed of some non-catalytic transformations and two catalytic steps. The reaction proceeds through a reverse Water Gas Shift Reaction (rWGSR) transforming C02 and H2 into C0 and H20, which are consumed in the following hydrocarboxylation of the in-situ formed alkene to give the final carboxylic acid product. The catalytic system is similar to traditional Rh carbonylation and Water Gas Shift catalysts. The PPh3 is needed to supply additional ligands allowing the catalyst to work in reaction conditions with a minimal amount of toxic C0 ligand. ln addition, a heterogeneous catalytic system was investigated for the same reaction. Single atom catalysts (SACs) are receiving much attention as catalytic solution, since they have both the advantages of homogeneous (selectivity, high activity) and heterogeneous (easy separation and recycling) catalysts. Single Rh atoms dispersed on N-doped graphene were synthesized and characterized, obtaining information regarding the chemical and physical structure of the material. Eventually, they were tested as catalysts for C02 activation, carboxylic acid production, hydrogenation and hydrogenolysis reactions / Aliphatische Carbonsauren werden in vielen industriellen Bereichen verwendet und ihre wirtschaftliche Bedeutung nimmt zu. Sie werden derzeit in gror.en Mengen hergestellt, indem das meistens nicht erneuerbare Kohlenmonoxid als C1-Synthon genutzt wird. Kohlendioxid ist ein potenziell umweltfreundlicher, erneuerbarer und abundanter C1-Baustein. Das Ziel dieser Arbeit ist die Entwicklung eines Protokolls zur katalytischen Umwandlung von C02, H2 und sauerstoffhaltigen Substraten, um nützliche Chemikalien, wie Carbonsauren zu erhalten. Zu diesem Zweck wird ein homogenes Rh-Katalysatorsystems zur Herstellung aliphatischer Carbonsauren aus sauerstoffhaltigen Substraten, C02 und H2 untersucht und optimiert. Das System besteht aus Rh-Prakursor, lodid-Additiv und PPh3 als Ligand, die in einem Batchreaktor unter C02 und H2 eingesetzt werden. Die Reaktionsbedingungen wurden für folgende Substratklassen optimiert: primare Alkohole, sekundare Alkohole, Ketone, Aldehyde und Epoxide. Es wurden insgesamt 30 verschiedene Substrate mit Ausbeuten bis zu 80% zu Carbonsauren umgesetzt. Darüber hinaus wurde das System mit einem ,,Statistische Versuchsplanung"-Ansatz untersucht, um zusatzliche lnformationen zu den untersuchten Parametern zu erhalten. Mechanismus und katalytisch aktive Spezies wurden durch verschiedene Experimente wie Konkurrenzreaktionen, NMR- und Markierungsexperimenten untersucht. Dies erschloss den Reaktionsweg, der aus mehreren nicht-katalytischen Transformationen und zwei katalytischen Schritten besteht. Die Reaktion verlauft durch eine ,,reverse Wassergas-Shift-Reaktion" (rWGSR), die C02 und H2 in C0 und H20 umwandelt. Diese werden wiederum bei der nachfolgenden Hydrocarboxylierung des in-situ gebildeten Alkens unter Bildung der Carbonsaure verbraucht. Das katalytische System ahnelt herkômmlichen Rh-Carbonylierungs- und WGSR-Katalysatoren. PPh3 fungiert als zusatzlicher Ligand, der es dem Katalysator ermôglicht unter den gleichen Reaktionsbedingungen mit minimaler Menge toxischen C0 als Liganden zu arbeiten. Zusatzlich wurde ein heterogenes katalytisches System für die gleiche Reaktion untersucht. ,,Single atom catalysts" (SACs) erhalten gror.e Aufmerksamkeit als neue Katalysatorklasse. Sie kombinieren die Selektivitat und hohe Aktivitat homogener und die einfache Abtrennung und Recycling heterogener Katalysatoren Verschiedene Katalysatoren aus auf N-dotiertem Graphen dispergierten Rh-Atomen, wurden synthetisiert und charakterisiert. Dadurch wurden lnformationen über die chemische und physikalische Struktur des Materials gewonnen und als Katalysatoren für C02-Aktivierung, Carbonsauresythese, Hydrierung und Hydrogenolyse getestet

Copper-based materials are one of the most promising catalysts for performing transformations of important organic compounds in both academic and industrial operations. However, it is challenging to consistently synthesize highly active and stable copper species as heterogeneous catalysts due to their relatively high surface energy. As a result, agglomeration usually occurs, which limits the catalytic activities of the copper species. The work presented in this dissertation shows different synthetic strategies for obtaining active and stable copper-based materials by modifying chemical/physical properties of copper nanoparticles (NPs). Emphasis is placed on discussing specific catalytic systems, including carbon-supported catalysts (monometallic and bimetallic copper-based heterogeneous catalysts) and titania-supported catalysts, and their advantages in terms of catalytic performance. In recent years, there has been increasing interest in using metal-organic frameworks (MOFs) as a sacrificial template to obtain carbon-supported NPs via a thermolysis process. The advantages of using MOFs to prepare carbon supported nanomaterials are a fine distribution of active particles on carbon matrix without post-synthesis treatments and corresponding increased catalytic activity and stability in many reaction conditions. To better understand the potential of this synthetic approach, MOF pyrolyzed products have been characterized. Then, they were applied as heterogeneous catalysts for several chemical reactions. In particular, the high energy copper-based MOF, CuNbO-1, was decomposed to obtain an amorphous copper species supported on carbon (a-Cu@C). This catalyst was found to be highly active for reduction, oxidation, and N-arylation reactions without further tuning or optimization. Higher catalyst turnover numbers for each of these transformations were obtained when comparing a-Cu@C activity to that of similar Cu-based materials. To improve catalyst performance, a secondary metal can be introduced to create synergistic effects with the parent copper species. In order to gain insights into the role of the second metal, a well-known Cu-MOF, HKUST-1, was doped with nickel, cobalt, and silver solutions, followed by a decomposition process with 2,4,6-trinitrotoluene (TNT) as additive. This additive was used to enhance the rapid thermolysis of the bimetallic MOFs. In these bimetallic systems, the addition of a second metal was found to help in dispersing both metals over the carbon composite support and in influencing the particle size and oxidation state of the metals. Catalytic performance showed that even <1% of a secondary metal increased the rate for nitrophenol reduction. Optimal catalytic performance was achieved using a Ni-CuO@C bimetallic catalyst. Another synthetic strategy for Cu-catalyst preparation involves using the deposition-precipitation method, in which a copper catalyst anchored on a titania support was synthesized at low weight % in order to obtain a single atom catalyst (1-Cu/TiO2). The higher copper loading catalyst, 5-Cu/TiO2, was synthesized as a benchmark catalyst for comparison. The copper structure in the synthesized catalysts was investigated by powder X-ray diffraction (PXRD), Raman, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX), X-ray photoelectron spectroscopy (XPS), N2 physisorption and inductively coupled plasma mass spectrometry (ICP-MS) in order to characterize physical and chemical properties. STEM-EDX observations showed single atom copper species less than 0.75 nm in size, as well as nanoparticles with an average diameter of ~1.31 nm. This catalyst was highly active in the reduction of nitro-aromatic compounds with NaBH4 at room temperature. The small to atomic level sizes of the Cu species and multiple oxidation states of Ti species were found to play a crucial role in the catalytic activity.

Graphene and its functionalized derivatives, such as nitrogen-doped graphene, have recently become a popular substrate material for the proton exchange membrane fuel cell (PEMFC) due to its enhanced electrical conductivity, electrochemical stability, and increased surface area when compared to the conventional, carbon black. In order to further develop the alternative fuel industry, the Pt catalyst within the PEMFC must also be considered. Single Pt atoms have a higher surface area to volume ratio when compared to nanoparticles, thus offering the potential to create a more affordable and efficient PEMFC. In this thesis, electrode materials comprising single Pt atoms and clusters, produced using atomic layer deposition (ALD) on various C derivatives, including graphene, N-doped graphene, carbon nanotubes (CNTs), and N-doped CNTs (NCNTs) are investigated through the utilization of aberration corrected transmission electron microscopy. Structural and chemical analysis was performed on thermally exfoliated N-doped graphene and CVD-produced graphene that was exposed to N+ ion sources. It was determined that the thermally exfoliated N-doped graphene maintained the short-range order of the graphene lattice; however, local inhomogeneities existed for the total N concentration, and the specific N-dopants within and between graphene sheets. More importantly, Pt atoms and clusters were observed and determined to be primarily stabilized at the edge of the N-doped graphene sheets. The stabilization of the Pt atoms and clusters resulted in a significantly higher mass and specific activity for the hydrogen evolution reaction, when compared to the use of a graphene substrate and Pt nanoparticles on C black. The N+ ion implantation in the CVD graphene showed the incorporation of N-dopants; however, electron energy loss spectroscopy revealed structural damage to thin sheets. NCNTs were also characterized in this thesis as possible gas containers, and as a substrate material to examine the effects of varying ALD conditions. It was determined that the NCNTs were an effective N2 gas conduit, wherein a decreasing pressure was observed with an increase to the inner diameter of the nanotubes. Using similar NCNTs, the effect of dosing time, temperature, and substrate on the Pt size were analyzed using ALD. While no singular condition resulted in the sole production of single Pt atoms, modifying both the substrate and dosing time were shown to provide the greatest potential for producing individual Pt atom catalysts. / Thesis / Doctor of Philosophy (PhD)

Ce travail de thèse a consisté en l’étude de catalyseurs ultradispersés, composés de clusters nanométriques ou d’atomes isolés (single-atom catalysts, SACs) de métal, une nouvelle classe de catalyseurs faisant actuellement l’objet d’un engouement mondial. Les systèmes Pt/?-Al2O3 et Pt/TiO2 ont été préparés, caractérisés et testés en oxydation de CO et photogénération d’hydrogène, respectivement. Plusieurs méthodes d’imprégnation et de traitement thermique ont été comparées, notamment grâce à l’analyse de la dispersion du platine par microscopie électronique en transmission à balayage (STEM). Pour la préparation de SACs, notre choix s’est finalement porté sur l’imprégnation à humidité naissante d’une faible charge de précurseur Pt(NH3)4(NO3)2, suivie d’une calcination à l’air. L’étude des performances catalytiques et de l’évolution structurale des catalyseurs au cours des réactions a permis de montrer que les atomes isolés (cations) de platine étaient moins actifs que les clusters (réduits) pour les deux systèmes catalytiques étudiés. Dans le cas de Pt/?-Al2O3, des suivis par spectroscopie d’absorption X (XAS) operando en rayonnement synchrotron, spectroscopie infrarouge par réflexion diffuse (DRIFTS) operando et microscopie environnementale (E-STEM) ont montré la formation et la déstabilisation des SACs, cette dernière étant toutefois moindre en conditions oxydantes. En effet, l’oxygène stabilise le platine isolé via la formation de plusieurs liaisons Pt-O-Al comme montré par modélisation DFT, alors que la présence d’un composé réducteur (CO, H2) conduit à la formation de clusters, mobiles sur leur support. Ce travail met en évidence les limites possibles concernant la stabilisation et la mise en œuvre des SACs dans des réactions catalytiques impliquant des conditions réductrices / This PhD work is focused on the study of ultradispersed catalysts, composed of nanometer-sized clusters or isolated atoms (single-atom catalysts, SACs) of metal, a new class of catalysts which are currently the object of worldwide interest. The Pt/?-Al2O3 and Pt/TiO2 systems were prepared, characterized and evaluated for CO oxidation and hydrogen photogeneration, respectively.Several methods of impregnation and thermal treatment were compared, in particular through platinum dispersion analysis using scanning electron microscopy (STEM). For the preparation of SACs, our choice finally turned to the incipient wetness impregnation of Pt(NH3)4(NO3)2 precursor at low loading, followed by calcination in air.For the two catalytic systems of interest, the study of the performances and the structural evolution of the catalysts during the reactions shows that isolated Pt atoms (cations) are less active than their (reduced) cluster counterparts.In the case of Pt/?-Al2O3, operando X-ray absorption spectroscopy (XAS) using synchrotron radiation, operando diffuse reflectance infrared spectroscopy (DRIFTS), and environmental microscopy (E-STEM) allowed us to monitor the SAC formation and destabilization, the latter being however limited in oxidizing conditions. Indeed, the presence of oxygen stabilizes single Pt atoms via the formation of several Pt-O-Al bonds as shown by DFT modeling, whereas the presence of a reducing compound (CO, H2) leads to the formation of Pt clusters, mobile on their support.This work highlights the possible limitations in the stabilization and implementation of SACs for catalytic reactions involving reducing conditions

Excessive CO2 emissions can negatively impact society and our planet. Reduction of CO2 is one potential avenue for its abatement. One of the most significant challenges to reducing CO2 is its extremely stable linear form. Experimentally, Cu/TiO2 has shown promise for CO2 photocatalytic reduction. Dispersed atomic catalysts can achieve high catalytic efficiency on a per atom basis. Active sites also typically having lower coordination number, and therefore may be more reactive. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (Ov) and photoexcited electrons on supported single atom catalysts and CO2 reduction. Cu atoms with Ov have shown to aid in the process of bent, anionic CO2 formation. In the first step involving CO2 dissociation (CO2* --> CO* + O*), a single Cu atom in Ov lowered the activation barrier to 0.10 - 0.19 eV, which could enable fast reduction of CO2 even at room temperature, in agreement with experimental findings. A photoexcited electron model was shown to readily promote Cu binding to the surface vacancy, and CO2 adsorption and direct dissociation. Finally, we briefly compare our results to calculations of supported single Pt atoms to determine how metals besides Cu may behave as photocatalysts for CO2 reduction, and we found a single Pt with Ov can promote CO2 dissociation. Our results show that tailoring TiO2 surfaces with defects in conjunction with atomic catalysts may lead to useful catalysts in the photoreduction of CO2.

Single atom catalysts are prospective class of materials, which holds promises to reach the ultimate limit of improvement in catalyst performance, selectivity, lifetime and cost reduction. The ability to efficiently capture the adsorbates at the active sites is the key prerequisite for catalytic transformation to the products. In this respect, our experimental study aims to describe the interaction of gas molecules (H2O, CO, O2 and NO) with single metal atoms (Rh, Ir, Cu) on the magnetite surface employing scanning tunneling microscopy.

Single-atom and cluster catalysts have been receiving significant interest due to not only their capability to approach the limit of atom efficiency but also to explore fundamentally unique properties. Supported Pt-group single atoms and clusters catalysts in the subnanometer size regime maximize the metal utilization and were reported to have extraordinary activities and/or selectivities compared with nanoparticles for various reactions including hydrogenation reactions. However, the relationship between metal nuclearity, electronic and their unique catalytic properties are still unclear. Thus, it is crucial to establish their relations for better future catalyst design. Ethylene hydrogenation and acetylene hydrogenation are two important probe reactions with the simplest alkene and alkyne, and they have been broadly studied as the benchmark reactions on the various catalyst systems. However, the catalytic properties and reaction mechanism of those hydrogenation reactions for metal nuclearitiy in the subnanometer regime is still not well understood. In this study, we applied different characterization techniques including x-ray absorption fine structure (XAFS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy(XPS), diffuse reflectance infrared spectroscopy (DRIFTS), calorimetry and high-resolution scanning transmission electron microscopy (STEM) to investigate the structure of Pt/TiO2 and Pd/COF single-atom catalysts and tested their catalytic properties for hydrogenation reactions. In order to develop such relations, we varied the nuclearity of Pt supported on TiO2 from single atoms to subnanometer clusters to larger nanoparticles. For acetylene hydrogenation, Pt in the subnanometer size regime exhibits remarkably high selectivity to ethylene compared to its nanoparticle counterparts. The high selectivity is resulted from the decreased electron density on Pt and destabilization of C2H4, which were rationalized by X-ray photoelectron spectroscopy and calorimetry results. On the other hand, the activity of H2 activation and acetylene hydrogenation decreased as Pt nuclearity decreased. Therefore, our results show there's a trade-off between activity and selectivity for acetylene hydrogenation. Additionally, the kinetics measurements of ethylene hydrogenation and acetylene hydrogenation were performed on Pt/TiO2 catalysts, and they found to be structure sensitive for both reactions, which the reaction orders and activation energy changes as particles size change. The activity of ethylene hydrogenation decreases, and activation energy increase from 43 to 86 kJ/mol, as Pt nuclearity decreased from an average size of 2.1 nm to 0.7 nm and single atoms. The reaction orders in hydrocarbons (ethylene and acetylene) were less negative on subnanometer clusters and single atoms in contract to nanoparticles. The results imply that hydrocarbons, ethylene and acetylene species, do not poison the catalyst on Pt in the subnanometer size regime, and hydrogen activation turn to competitive adsorption path with surface hydrocarbons species. Moreover, single atom Pd supported on imine-linked covalent organic framework was synthesized, characterized by a various of techniques including X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, and evaluated its catalytic properties for ethylene hydrogenation. The XAS results show that Pd atoms are isolated and stabilized by two covalent Pd–N and Pd-Cl bonds. DRIFTS of CO adsorption shows a sharp symmetrical peak at 2130 cm−1. The Pd single atoms are active for hydrogenation of ethylene to ethane at room temperature. The reaction orders in C2H4 and H2 were 0.0 and 0.5 suggesting that ethylene adsorption is not limiting while hydrogen forms on Pd through dissociative adsorption. / Doctor of Philosophy / More than 90% of chemicals come from petroleum and natural gas, and most of these chemicals are composed of alkene and alkyne, hydrocarbons containing at least one double bonds or triple bonds, such as ethylene, propylene, butenes, butadiene. These small hydrocarbon molecules with carbon-carbon bonds (double or triple) are in great interest of fundamental study and serve as probe units for understanding more complex reactions. Catalysts are materials that can be added to a chemical reaction to accelerate the specific rate of reactions. Most catalysts are supported noble metals thus increase the utilization of metal atoms are important. Decreasing the particle size to increase the metal dispersion is the simple approach to maximize the atom efficiency. However, it is not well understood how do the electronic property and catalytic performance change as particle size decrease. In this work, we focus on the structure sensitivity on catalysts in sub-nanometer region. Supported Pt and Pd catalysts, known to be highly active for hydrogenation reactions, are studied on hydrogenation reactions of acetylene and ethylene, the simplest alkene and alkyne. The Pd and Pt catalysts with particle sizes ranging from single atoms, sub-nanometer clusters and nanoparticles were prepared, characterized and tested for hydrogenation reactions mentioned above. The results show that significantly change in electronic property, catalytic performance (activity and/or selectivity) and reaction kinetics of the catalysts as the particle size changing from nanometer to sub-nanometer region. The fundamental understanding of structure sensitivity on catalysts and their relations between surface structure, electronic property and catalytic performance presented in this work can help the researchers design better catalysts for future work.

Supported noble metal catalysts make the best utilization of noble metal atoms. Recent advances in nanotechnology have brought many attentions into the rational design of catalysts in the nanometer and subnanometer region. Recent studies showed that catalysts in the subnanometer regime could have extraordinary activity and selectivity. However, the structural performance relationships behind their unique catalytic performances are still unclear. To understand the effect of particle size and shape of noble metals, it is essential to understand the fundamental reaction mechanism. Single atoms catalysts and subnanometer clusters provide a unique opportunity for designing heterogeneous catalysts because of their unique geometric and electronic properties. CO oxidation is one of the important probe reactions. However, the reaction mechanism of noble single atoms is still unclear. Additionally, there is no agreement on whether the activity of supported single atoms is higher or lower than supported nanoparticles. In this study, we applied different operando techniques including x-ray absorption fine structure (XAFS), diffuse reflectance infrared spectroscopy (DRIFTS), with other characterization techniques including calorimetry and high-resolution scanning transmission electron microscopy (STEM) to investigate the active and stable structure of Ir/MgAl2O4 and Pt/CeO2 single-atom catalysts during CO oxidation. With all these characterization techniques, we also performed a kinetic study and first principle calculations to understand the reaction mechanism of single atoms for CO oxidation. For Ir single atoms catalysts, our results indicate that instead of poisoning by CO on Ir nanoparticles, Ir single atoms could adsorb more than one ligand, and the Ir(CO)(O) structure was identified as the most stable structure under reaction condition. Though one CO was strongly adsorbed during the entire reaction cycle, another CO could react with the surface adsorbed O* through an Eley-Rideal reaction mechanism. Ir single atoms also provide an interfacial site for the facile O2 activation between Ir and Al with a low barrier, and therefore O2 activation step is feasible even at room temperature. For Pt single-atom catalysts, our results showed that Pt(O)3(CO) structure is stable in O2 and N2 at 150 °C. However, when dosing CO at 150 °C, one surface O* in Pt(O)3(CO) could react with CO to form CO2, and the reacted O* can be refilled when flowing O2 again at 150 °C. This suggests that an adsorbed CO is present in the entire reaction cycle as a ligand, and another gas phase CO could react with surface O* to form CO2 during low-temperature CO oxidation. Supported single atoms synthesized with conventional methods usually consist of a mixture of single atoms and nanoparticles. It is important to quantify the surface site fraction of single atoms and nanoparticles when studying catalytic performances. Because of the unique reaction mechanism of Ir single atoms and Ir nanoparticles, we showed that kinetic measurements could be applied as a simple and direct method of quantifying surface site fractions. Our kinetic methods could also potentially be applied to quantifying other surface species when their kinetic behaviors are significantly different. We also benchmarked other in-situ and ex-situ methods of quantifying surface site fraction of single atoms and nanoparticles. To bridge the gap between single atoms and nanoparticles and have a better understanding of the effect of nuclearity on CO oxidation, we also studied supported Ir subnanometer clusters with the average size less than 0.7 nm (< 13 atoms) prepared by both inorganic precursor and organometallic complex Ir4(CO)12. Low-temperature CO adsorption indicates that CO and O2/O could co-adsorb on Ir subnanometer clusters, however on larger nanoparticle the particle surface is covered by CO only. Additional co-adsorption of CO and O2 was studied by CO and O2 calorimetry at room temperature. CO oxidation results showed that Ir subnanometer clusters are more active than Ir single atoms and Ir nanoparticles at all conditions, and this could be explained by the competitive adsorption of CO and O2 on subnanometer clusters. / Doctor of Philosophy / CO oxidation is one of the important reactions in catalytic converters. Three-way catalysts, typically supported noble metals, are very efficient at high temperature but could be poisoned by CO at cold start. Better designed catalysts are required to improve the performance of the catalytic converter to lower the emissions of gasoline engines. To reach this goal, more efficient use of the noble metal is required. Single-atom catalysts consist of isolated noble metal atoms supported on different supports, which provide the best utilization of noble metal atoms and provides a new opportunity for a better design of heterogeneous catalysts. The unique electronic and geometric properties of metal single atoms catalysts could lead to a better activity and selectivity. Subnanometer clusters have also been shown to have unique electronic properties. With a better understanding of the structure of supported single atoms and subnanometer clusters, their catalytic performance can be optimized for better catalysts in the catalytic converter and other applications. In this work, we applied in-situ and operando characterization, kinetic studies and first principle calculations aiming to understand the active and stable structure of noble metal single atoms and vi subnanometer clusters under reaction condition, and their reaction mechanisms during CO oxidations. For MgAl₂O₄ supported Ir single atoms, our results suggest that CO could be co-adsorbed with O₂/O under reaction conditions. These multiple ligands adsorption leads to a unique reaction mechanism during CO oxidation. Though one CO was adsorbed during the whole reaction cycle, another gas phase CO could react with the O* species co-adsorbed with CO through an Eley-Rideal mechanism. This suggests that Ir single atoms are no longer poisoned by CO, and on the other hand the O₂ can be activated on an interfacial site with a low reaction barrier. Ir subnanometer clusters showed higher activities than Ir single atoms and nanoparticles. In-situ IR and high energy resolution fluorescence detected – X-ray absorption near edge spectroscopy (HERFD-XANES) showed that CO could co-adsorb with O₂ at room temperature, and this competitive adsorption could explain the high activity during CO oxidation. Supported Ir single atoms and subnanometer clusters are not poisoned by CO and O₂ could be co-adsorbed, this could be potentially applied to solve the poisoning of catalyst in the catalytic converter at cold start temperature. We also performed kinetic study on CeO₂ supported Pt single atoms. Similar behavior was observed, and we showed that the CO and O co-adsorbed complex is stable in O₂ and N₂, but could react in CO. With the understanding of the active structure of noble metal single atoms and the origin of activities, better-designed catalysts can be synthesized to improve the activity and selectivity of low-temperature oxidation reactions.

Decentralized electrochemical production of hydrogen peroxide (H₂O₂) is an attractive alternative to the industrial anthraquinone process, the application of which is hindered by the lack of high-performance electrocatalysts in acidic media. Herein, a novel catalyst design strategy is reported to optimize the Pd sites in pure metallic aerogels by tuning their geometric environments and electronic structures. By increasing the Hg content in the Pd–Hg aerogels, the Pd–Pd coordination is gradually diminished, resulting in isolated, single-atom-like Pd motifs in the Pd₂Hg₅ aerogel. Further heterometal doping leads to a series of M–Pd₂Hg₅ aerogels with an unalterable geometric environment, allowing for sole investigation of the electronic effects. Combining theoretical and experimental analyses, a volcano relationship is obtained for the M–Pd₂Hg₅ aerogels, demonstrating an effective tunability of the electronic structure of the Pd active sites. The optimized Au–Pd₂Hg₅ aerogel exhibits an outstanding H₂O₂ selectivity of 92.8% as well as transferred electron numbers of ≈2.1 in the potential range of 0.0–0.4 V/RHE. This work opens a door for designing metallic aerogel electrocatalysts for H₂O₂ production and highlights the importance of electronic effects in tuning electrocatalytic performances.