Utilizing Quantitative in Situ FTIR Spectroscopy To Identify Well-Coordinated Pt Atoms as the Active Site for CO Oxidation on Al2O3-Supported Pt Catalysts

Visualizing Dynamic Single Atom Catalysis

Many industrial chemical processes, including for producing fuels, foods, pharmaceuticals, chemicals and environmental controls, employ heterogeneous solid state catalysts at elevated temperatures in gas or liquid environments. Dynamic reactions at the atomic level play a critical role in catalyst stability and functionality. In situ visualization and analysis of atomic-scale processes in real time under controlled reaction environments can provide important insights into practical frameworks to improve catalytic processes and materials. This review focuses on innovative real time in situ electron microscopy (EM) methods, including recent progress in analytical in situ environmental (scanning) transmission EM (E(STEM), incorporating environmental scanning TEM (ESTEM) and environmental transmission EM (ETEM), with single atom resolution for visualizing and analysing dynamic single atom catalysis under controlled flowing gas reaction environments. ESTEM studies of single atom dynamics of reactions, and of sintering deactivation, contribute to a better-informed understanding of the yield and stability of catalyst operations. Advances in in situ technologies, including gas and liquid sample holders, nanotomography, and higher voltages, as well as challenges and opportunities in tracking reacting atoms, are highlighted. The findings show that the understanding and application of fundamental processes in catalysis can be improved, with valuable economic, environmental, and societal benefits.

Facilely Fabricated Single-Site Ptδ+-O(OH)x- Species Associated with Alkali on Zirconia Exhibiting Superior Catalytic Oxidation Reactivity

Fabrication of robust isolated atom catalysts has been a research hotspot in the environment catalysis field for the removal of various contaminants, but there are still challenges in improving the reactivity and stability. Herein, through facile doping alkali metals in Pt catalyst on zirconia (Pt-Na/ZrO2), the atomically dispersed Ptδ+-O(OH)x- associated with alkali metal via oxygen bridge was successfully fabricated. This novel catalyst presented remarkably higher CO and hydrocarbon (HCs: C3H8, C7H8, C3H6, and CH4) oxidation activity than its counterpart (Pt/ZrO2). Systematically direct and solid evidence from experiments and density functional theory calculations demonstrated that the fabricated electron-rich Ptδ+-O(OH)x- related to Na species rather than the original Ptδ+-O(OH)x-, serving as the catalytically active species, can readily react with CO adsorbed on Ptδ+ to produce CO2 with significantly decreasing energy barrier in the rate-determining step from 1.97 to 0.93 eV. Additionally, owing to the strongly adsorbed and activated water by Na species, those fabricated single-site Ptδ+-O(OH)x- linked by Na species could be easily regenerated during the oxidation reaction, thus considerably boosting its oxidation reactivity and durability. Such facile construction of the alkali ion-linked active hydroxyl group was also realized by Li and K modification which could guide to the design of efficient catalysts for the removal of CO and HCs from industrial exhaust.

Enhanced Catalytic Oxidation Reactivity over Atomically Dispersed Pt/CeO2 Catalysts by CO Activation

The elevation of the low-temperature oxidation activity for Pt/CeO2 catalysts is challenging to meet the increasingly stringent requirements for effectively eliminating carbon monoxide (CO) from automobile exhaust. Although reducing activation is a facile strategy for boosting reactivity, past research has mainly concentrated on applying H2 as the reductant, ignoring the reduction capabilities of CO itself, a prevalent component of automobile exhaust. Herein, atomically dispersed Pt/CeO2 was fabricated and activated by CO, which could lower the 90% conversion temperature (T90) by 256 °C and achieve a 20-fold higher CO consumption rate at 200 °C. The activated Pt/CeO2 catalysts showed exceptional catalytic oxidation activity and robust hydrothermal stability under the simulated working conditions for gasoline or diesel exhausts. Characterization results illustrated that the CO activation triggered the formation of a large portion of Pt0 terrace sites, acting as inherent active sites for CO oxidation. Besides, CO activation weakened the Pt-O-Ce bond strength to generate a surface oxygen vacancy (Vo). It served as the oxygen reservoir to store the dissociated oxygen and convert it into active dioxygen intermediates. Conversely, H2 activation failed to stimulate Vo, but triggered a deactivating transformation of the Pt nanocluster into inactive PtxOy in the presence of oxygen. The present work offers coherent insight into the upsurging effect of CO activation on Pt/CeO2, aiming to set up a valuable avenue in elevating the efficiency of eliminating CO, C3H6, and NH3 from automobile exhaust.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Unravelling the origin of reaction-driven aggregation and fragmentation of atomically dispersed Pt catalyst on ceria support

Metal-support interaction plays a crucial role in governing the stability and activity of atomically dispersed platinum catalysts on ceria support. The migration and aggregation of platinum atoms during the catalytic reaction leads to the redistribution of active sites. In this study, by utilizing a multimodal characterization scheme, we observed the aggregation of platinum atoms at high temperatures under reverse water gas shift reaction conditions and the subsequent fragmentation of platinum clusters, forming "single atoms" upon cooling. Theoretical simulations of both effects uncovered the roles of carbon monoxide binding on perimeter Pt sites in the clusters and hydrogen coverage in the aggregation and fragmentation mechanisms. This study highlights the complex effects of adsorbate and supports interactions with metal sites in Pt/ceria catalysts that govern their structural transformations under in situ conditions.

Effect of Dynamic and Preferential Decoration of Pt Catalyst Surfaces by WOx on Hydrodeoxygenation Reactions

Catalysts containing Pt nanoparticles and reducible transition-metal oxides (WOx, NbOx, TiOx) exhibit remarkable selectivity to aromatic products in hydrodeoxygenation (HDO) reactions for biomass valorization, contrasting the undesired aromatic hydrogenation typically observed for metal catalysts. However, the active site(s) responsible for the high selectivity remains elusive. Here, theoretical and experimental analyses are combined to explain the observed HDO reactivity by interrogating the organization of reduced WOx domains on Pt surfaces at sub-monolayer coverage. The SurfGraph algorithm is used to develop model structures that capture the configurational space (∼1000 configurations) for density functional theory (DFT) calculations of a W3O7 trimer on stepped Pt surfaces. Machine-learning models trained on the DFT calculations identify the preferential occupation of well-coordinated Pt sites (≥8 Pt coordination number) by WOx and structural features governing WOx-Pt stability. WOx/Pt/SiO2 catalysts are synthesized with varying W loadings to test the theoretical predictions and relate them to HDO reactivity. Spectroscopy- and microscopy-based catalyst characterizations identify the dynamic and preferential decoration of well-coordinated sites on Pt nanoparticles by reduced WOx species, consistent with theoretical predictions. The catalytic consequences of this preferential decoration on the HDO of a lignin model compound, dihydroeugenol, are clarified. The effect of WOx decoration on Pt nanoparticles for HDO involves WOx inhibition of aromatic ring hydrogenation by preferentially blocking well-coordinated Pt sites. The identification of preferential decoration on specific sites of late-transition-metal surfaces by reducible metal oxides provides a new perspective for understanding and controlling metal-support interactions in heterogeneous catalysis.

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd

The use of visible photon fluxes to influence catalytic reactions on metal nanoparticle surfaces has attracted attention based on observations of reaction mechanisms and selectivity not observed under equilibrium heating. These observations suggest that photon fluxes can selectively impact the rates of certain elementary steps, creating nonequilibrium energy distributions among various reaction pathways. However, quantitative studies validating these hypotheses on metal nanoparticle surfaces are lacking. We examine the influence of continuous wave visible photon fluxes on the CO desorption rates from 1 to 2 nm diameter Pt and Pd nanoparticle surfaces supported on γ-Al2O3. Temperature-programmed desorption measurements quantified via diffuse reflectance infrared Fourier transform spectroscopy demonstrate that visible photon fluxes significantly enhanced the rate of CO desorption from Pt nanoparticles in a wavelength-dependent manner. 440 nm photons most efficiently promoted CO desorption from Pt nanoparticle surfaces, aligning with the excitation energy for the interfacial electronic transition within the Pt-CO bond. Conversely, visible photon fluxes had no measurable influence on CO desorption rates from Pd nanoparticle surfaces after accounting for photon-induced heating. Density functional theory calculations demonstrate that the Pt-CO bond exhibits a narrower LUMO resonance, stronger coupling between the photoexcitation and forces induced on the metal-C bond, and vibrational energy dissipation that more effectively couples to desorption as compared to Pd-CO. These results demonstrate the specificity photons provide in facilitating chemical reactions on metal nanoparticle surfaces and substantiate the idea that photon fluxes can steer processes and outcomes of catalytic reactions in ways not achievable by equilibrium heating.

Reactivity Switch of Platinum with Gallium: From Reverse Water Gas Shift to Methanol Synthesis

The development of efficient catalysts for the hydrogenation of CO2 to methanol using "green" H2 is foreseen to be a key step to close the carbon cycle. In this study, we show that small and narrowly distributed alloyed PtGa nanoparticles supported on silica, prepared via a surface organometallic chemistry (SOMC) approach, display notable activity for the hydrogenation of CO2 to methanol, reaching a 7.2 molCH3OH h-1 molPt-1 methanol formation rate with a 54% intrinsic CH3OH selectivity. This reactivity sharply contrasts with what is expected for Pt, which favors the reverse water gas shift reaction, albeit with poor activity (2.6 molCO2 h-1 molPt-1). In situ XAS studies indicate that ca. 50% of Ga is reduced to Ga0 yielding alloyed PtGa nanoparticles, while the remaining 50% persist as isolated GaIII sites. The PtGa catalyst slightly dealloys under CO2 hydrogenation conditions and displays redox dynamics with PtGa-GaOx interfaces responsible for promoting both the CO2 hydrogenation activity and methanol selectivity. Further tailoring the catalyst interface by using a carbon support in place of silica enables to improve the methanol formation rate by a factor of ∼5.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Promoting Catalytic Performance Involving Hydrogen Spillover by Ion Exchange of Pt@A Catalysts to Regulate Reactant Adsorption

Zeolite-encapsulated metal nanoparticle systems have exhibited interesting catalytic performances via the hydrogen spillover process, yet how to further utilize the function of zeolite supports to promote catalytic properties in such a process is still challenging and has rarely been investigated. Herein, to address this issue, the strategy to strengthen the adsorption energy of reactant onto the zeolite surface via a simple ion exchange method has been implemented. Ion-exchanged linde type A (LTA) zeolite-encapsulated platinum nanoclusters (Pt@NaA, Pt@HA, Pt@KA, and Pt@CaA) were prepared to study the influence of ion exchange on the catalytic performance in the model reaction of hydrogenation of acetophenone to 1-phenylethanol. The reaction results showed that the Pt@CaA catalyst exhibited the best catalytic activity in the series of encapsulated catalysts, and the selectivity of 1-phenylethanol approached 100%. As revealed by density functional theory (DFT) calculations and acetophenone temperature-programmed desorption (acetophenone-TPD) experiments, in comparison with introduced cations of Na+, H+, and K+, ion-exchanged Ca2+ on the zeolite maximumly enhanced the adsorption of carbonyl groups in acetophenone, playing a critical role in achieving the highest activity and excellent catalytic selectivity among the Pt@A catalysts.

Interrogating site dependent kinetics over SiO2-supported Pt nanoparticles

A detailed knowledge of reaction kinetics is key to the development of new more efficient heterogeneous catalytic processes. However, the ability to resolve site dependent kinetics has been largely limited to surface science experiments on model systems. Herein, we can bypass the pressure, materials, and temperature gaps, resolving and quantifying two distinct pathways for CO oxidation over SiO2-supported 2 nm Pt nanoparticles using transient pressure pulse experiments. We find that the pathway distribution directly correlates with the distribution of well-coordinated (e.g., terrace) and under-coordinated (e.g., edge, vertex) CO adsorption sites on the 2 nm Pt nanoparticles as measured by in situ DRIFTS. We conclude that well-coordinated sites follow classic Langmuir-Hinshelwood kinetics, but under-coordinated sites follow non-standard kinetics with CO oxidation being barrierless but conversely also slow. This fundamental method of kinetic site deconvolution is broadly applicable to other catalytic systems, affording bridging of the complexity gap in heterogeneous catalysis.

Modulating the Electronic States of Pt Nanoparticles on Reducible Metal-Organic Frameworks for Boosting the Oxidation of Volatile Organic Compounds

The adsorption and activation of pollutant molecules and oxygen play a critical role in the oxidation reaction of volatile organic compounds (VOCs). In this study, superior adsorption and activation ability was achieved by modulating the interaction between Pt nanoparticles (NPs) and UiO-66 (U6) through the spatial position effect. Pt@U6 exhibits excellent activity in toluene, acetone, propane, and aldehyde oxidation reactions. Spectroscopic studies, 16O2/18O2 kinetic isotopic experiments, and density functional theory (DFT) results jointly reveal that the encapsulated Pt NPs of Pt@U6 possess higher electron density and d-band center, which is conducive for the adsorption and dissociation of oxygen. The toluene oxidation reaction and DFT results indicate that Pt@U6 is more favorable to activate the C-H of toluene and the C═C of maleic anhydride, while Pt/U6 with lower electron density and d-band center exhibits a higher oxygen dissociation temperature and higher reactant activation energy barriers. This study provides a deep insight into the architecture-performance relation of Pt-based catalysts for the catalytic oxidation of VOCs.

Significance of Epitaxial Growth of PtO2 on Rutile TiO2 for Pt/TiO2 Catalysts

TiO2-supported Pt species have been widely applied in numerous critical reactions involving photo-, thermo-, and electrochemical-catalysis for decades. Manipulation of the state of the Pt species in Pt/TiO2 catalysts is crucial for fine-tuning their catalytic performance. Here, we report an interesting discovery showing the epitaxial growth of PtO2 atomic layers on rutile TiO2, potentially allowing control of the states of active Pt species in Pt/TiO2 catalysts. The presence of PtO2 atomic layers could modulate the geometric configuration and electronic state of the Pt species under reduction conditions, resulting in a spread of the particle shape and obtaining a Pt/PtO2/TiO2 structure with more positive valence of Pt species. As a result, such a catalyst exhibits exceptional electrocatalytic activity and stability toward hydrogen evolution reaction, while also promoting the thermocatalytic CO oxidation, surpassing the performance of the Pt/TiO2 catalyst with no epitaxial structure. This novel epitaxial growth of the PtO2 structure on rutile TiO2 in Pt/TiO2 catalysts shows its potential in the rational design of highly active and economical catalysts toward diverse catalytic reactions.

Migration and aggregation of Pt atoms on metal oxide-supported ceria nanodomes control reverse water gas shift reaction activity

Single-atom catalysts (SACs) are particularly sensitive to external conditions, complicating the identification of catalytically active species and active sites under in situ or operando conditions. We developed a methodology for tracing the structural evolution of SACs to nanoparticles, identifying the active species and their link to the catalytic activity for the reverse water gas shift (RWGS) reaction. The new method is illustrated by studying structure-activity relationships in two materials containing Pt SACs on ceria nanodomes, supported on either ceria or titania. These materials exhibited distinctly different activities for CO production. Multimodal operando characterization attributed the enhanced activity of the titania-supported catalysts at temperatures below 320 ˚C to the formation of unique Pt sites at the ceria-titania interface capable of forming Pt nanoparticles, the active species for the RWGS reaction. Migration of Pt nanoparticles to titania support was found to be responsible for the deactivation of titania-supported catalysts at elevated temperatures. Tracking the migration of Pt atoms provides a new opportunity to investigate the activation and deactivation of Pt SACs for the RWGS reaction.

High or Low Coordination: Insight into the Active Site of Pt Nanoparticles toward CO Oxidation

The catalytic activity of metal nanoparticles (NPs) is highly dependent on the coordination environment of the surface sites. Understanding the role of different sites in reactions is essential for gaining insights into catalytic activity and the precise design of catalysts. Herein, we used first-principles calculation-based kinetic Monte Carlo simulations to investigate correlations between different sites on Pt NPs in CO oxidation reactions. Low-coordinated (LC) sites favor the CO adsorption and reaction, whereas the oxygen mainly adsorbs on high-coordinated (HC) sites and diffuses to LC sites for reaction at low temperatures. Compared with step-dominated and terrace-dominated structures, the step-terrace structures exhibit higher activities. This reveals that the catalytic performance is not simply determined by the sites where the reaction occurs but is dramatically affected by the kinetic synergies between different sites. A proper way to optimize the activity of Pt catalysts is to balance the LC and HC sites.

The Role of In Situ/Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis

Heterogeneous catalysts undergo thermal- and/or adsorbate-induced dynamic changes under reaction conditions, which consequently modify their catalytic behavior. Hence, it is increasingly crucial to characterize the properties of a catalyst under reaction conditions through the so-called "operando" approach. Operando IR spectroscopy is probably one of the most ubiquitous and versatile characterization methods in the field of heterogeneous catalysis, but its potential in identifying adsorbate- and thermal-induced phenomena is often overlooked in favor of other less accessible methods, such as XAS spectroscopy and high-resolution microscopy. Without detracting from these techniques, and while aware of the enormous value of a multitechnique approach, the purpose of this Review is to show that IR spectroscopy alone can provide relevant information in this field. This is done by discussing a few selected case studies from our own research experience, which belong to the categories of both "single-site"- and nanoparticle-based catalysts.

Size-Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at Low Temperature and the Consequences for CO Oxidation Activity

Understanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al2 O3 using in situ/operando spectroscopy and ex situ aberration-corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperature and the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Scalable Synthesis of Flexible Single-Atom Monolithic Catalysts for High-Efficiency, Durable CO Oxidation at Low Temperature

The creation of single-atom catalysts in a large-size, high-yield, and stable form represents an important direction for high-efficiency industrial catalysis in the future. Herein, we report a strategy to synthesize flexible single-atom monolithic catalysts (SAMCs) based on the hierarchical 3D assembly of single-atom-loaded oxide ceramic nanofibers. The nanofibers, which can be produced in a continuous and scalable manner, serve as an ideal support for single atoms spontaneously and almost completely exposed at the surface through the Kirkendall effect-enabled in situ ion migration during the spinning process, resulting in both high yield and large loading quantity. Moreover, the hierarchical 3D assembly of these nanofibers into a porous, flexible structure endows the SAMCs with the advantages of sufficient infiltration and oscillation tolerance when faced with high-throughput gaseous media, leading to both high catalytic efficiency and excellent durability. As a proof-of-concept demonstration, a Pt SAMC is synthesized, which exhibits 100% CO oxidation at low temperature (∼170 °C), excellent invariance toward high-frequency (10 Hz) oscillation, and high structural stability from 25 to 300 °C. This work is beneficial for the large-scale production of SAMCs in broad industrial applications.

Preformed Pt Nanoparticles Supported on Nanoshaped CeO2 for Total Propane Oxidation

Pt-based catalysts have been widely used for the removal of short-chain volatile organic compounds (VOCs), such as propane. In this study, we synthesized Pt nanoparticles with a size of ca. 2.4 nm and loaded them on various fine-shaped CeO2 with different facets to investigate the effect of CeO2 morphology on the complete oxidation of propane. The Pt/CeO2-o catalyst with {111} facets exhibited superior catalytic activity compared to the Pt/CeO2-r catalyst with {110} and {100} facets. Specifically, the turnover frequency (TOF) value of Pt/CeO2-o was 1.8 times higher than that of Pt/CeO2-r. Moreover, Pt/CeO2-o showed outstanding long-term stability during 50 h. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that the excellent performance of Pt/CeO2-o is due to the prevalence of metallic Pt species, which promotes C-C bond cleavage and facilitates the rapid removal of surface formate species. In contrast, a stronger metal-support interaction in Pt/CeO2-r leads to easier oxidation of Pt species and the accumulation of intermediates, which is detrimental to the catalytic activity. Our work provides insight into the oxidation of propane on different nanoshaped Pt/CeO2 catalysts.

Facet-Controlled Cu2O Support Enhances Catalytic Activity of Pt Nanoparticles for CO Oxidation

The metal-support interaction plays a crucial role in determining the catalytic activity of supported metal catalysts. Changing the facet of the support is a promising strategy for catalytic control via constructing a well-defined metal-support nanostructure. Herein, we developed cubic and octahedral Cu₂O supports with (100) and (111) facets terminated, respectively, and Pt nanoparticles (NPs) were introduced. The in situ characterizations revealed the facet-dependent encapsulation of the Pt NPs by a CuO layer due to the oxidation of the Cu₂O support during the CO oxidation reaction. The CuO layer on Pt at cubic Cu₂O (Pt/c-Cu₂O) significantly enhanced catalytic performance, while the thicker CuO layer on Pt at octahedral Cu₂O suppressed CO conversion. The formation of a thin CuO layer is attributed to the dominant Pt-O-Cu bond at the Pt/c-Cu₂O interface, which suppresses the adsorption of oxygen molecules. This investigation provides insight into designing high-performance catalysts via engineering the interface interaction.

Fabrication of supported Pt/CeO2 nanocatalysts doped with different elements for CO oxidation: theoretical and experimental studies

Supported Pt/CeO₂ catalysts have been widely used in carbon monoxide (CO) oxidation; however, the high oxygen vacancy formation energy (E_vac) in the process leads to the poor performance of these catalysts. Herein, we explored different element (Pr, Cu, or N) doped CeO₂ supports using Ce-based metal-organic frameworks (MOFs) as precursors via calcination treatment. The obtained CeO₂ supports were used to load Pt nanoparticles. These catalysts were systematically characterized by various techniques, and they showed superior catalytic activity for CO oxidation compared to undoped catalysts which could be attributed to the formation of Ce³⁺, and high amounts of O_ads/(O_ads + O_lat) and Pt^δ+/Pt_total. Moreover, density functional theory calculations with on-site Coulomb interaction correction (DFT+U) were performed to provide atomic-scale insights into the reaction process by the Mars-van Krevelen (M-vK) mechanism, which revealed that the element-doped catalysts could simultaneously reduce the adsorption energies of CO and lower reaction energy barriers in the *OOCO associative pathway.

Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation

Catalysts can undergo structural changes during the reaction, affecting the number and/or the shape of active sites. For example, Rh can undergo interconversion between nanoparticles and single atoms when CO is present in the reaction mixture. Therefore, calculating a turnover frequency in such cases can be challenging as the number of active sites can change depending on the reaction conditions. Here, we use CO oxidation kinetics to track Rh structural changes occurring during the reaction. The apparent activation energy, considering the nanoparticles as the active sites, was constant in different temperature regimes. However, in a stoichiometric excess of O₂, there were observed changes in the pre-exponential factor, which we link to changes in the number of active Rh sites. An excess of O₂ enhanced CO-induced Rh nanoparticle disintegration into single atoms, affecting catalyst activity. The temperature at which these structural changes occur depend on Rh particle size, with small particle sizes disintegrating at higher temperature, relative to the temperature required to break apart bigger particles. Rh structural changes were also observed during in situ infrared spectroscopic studies. Combining CO oxidation kinetics and spectroscopic studies allowed us to calculate the turnover frequency before and after nanoparticle redispersion into single atoms.

Identifying the morphology of Pt nanoparticles for the optimal catalytic activity towards CO oxidation

The morphology of nanoparticles (NPs) is crucial for determining their catalytic performance. The dramatic changes in the morphology of metal NPs during reactions observed in many in situ experiments pose great challenges for the identification of the geometry for optimal catalytic activities, which arouses the controversial understanding of the reaction mechanism. In this work, taking CO oxidation as a model reaction, we coupled a multiscale structure reconstruction model with kinetic Monte Carlo simulations to study the catalytic performance of the Pt NPs with changing morphology and reaction conditions. Through the quantitative analysis of contour plots for turnover frequencies, we show that the NPs with more well-coordinated sites exhibit optimal activity under CO-rich conditions at higher temperatures, while the reactivity of NPs with more low-coordination sites is optimal under O₂-rich conditions at lower temperatures. Further analysis indicates that the competitive adsorption of CO and O₂ plays the key role, in which the structure with optimal activity has a closer CO and O coverage. This work not only reconciles the controversy of the active geometry in the experiments, but offers an efficient method to guide the rational design of high-performance catalysts.

Effects of thermal aging on the electronic and structural properties of Pt-Pd and toluene oxidation activity

Catalytic oxidation is a feasible method for remediating volatile organic compounds (VOCs), due to its lower energy consumption and mineralization of VOCs into H₂O and CO₂. Noble metal-based catalysts are preferred for the catalytic oxidation of VOCs because of their superior activity, but they are usually deactivated by thermal aging which sinters the metal particles. Here, we report that Pt-Pd/Al₂O₃ thermally aged at 700-900 °C in air showed enhanced catalytic activity for toluene oxidation in humid conditions. There were electronic and structural changes in the thermally aged Pt-Pd/Al₂O₃, as confirmed by numerous analyses. Both Pt and Pd existed in a metallic rather than oxidized state without additional reduction steps. The noble metal particles were assembled to form Pt-Pd alloy, in the form of isolated Pd atoms surrounded by Pt atoms. This specific alloy structure was found to be crucial to the observed enhancement in catalytic toluene oxidation at low temperature.

Effect of Hydroxyl Groups on Metal Anchoring and Formaldehyde Oxidation Performance of Pt/Al2O3

Pt/Al₂O₃ catalysts showing excellent activity and stability have been used in various reactions, including HCHO oxidation. Herein, we prepared Pt-Na/Al₂O₃ catalysts with a Pt content of 0.05 wt % to reveal the key factors determining the anchoring of Pt as well as the catalytic activity and mechanism of HCHO oxidation. Pt-Na/nano-Al₂O₃ (denoted as Pt-Na/nAl₂O₃) catalysts with 0.05 wt % Pt content could completely oxidize HCHO to CO₂ at room temperature, which is the lowest Pt content used in HCHO catalytic oxidation to our knowledge. After Na addition, terminal hydroxyl groups (denoted as HO-μ_ter) on nano-Al₂O₃ were transformed to doubly bridging hydroxyl groups between Na and Al (denoted as HO-μ_bri(Na-Al)), which atomically dispersed Pt species. Pt anchoring further promoted the regeneration of HO-μ_bri(Na-Al) by activating O₂ and H₂O, oxidizing HCHO to CO₂ directly by the fast reaction step ([HCOO^-] + [OH]_a → CO₂ + H₂O). Our study revealed that the HO-μ_bri(Na-Al) synergistically generated by HO-μ_ter and Na species provided anchoring sites for Pt species.

Enhanced Formaldehyde Oxidation Performance of the Mesoporous TiO2(B)-Supported Pt Catalyst: The Role of Hydroxyls

As one of the crystal phases of titania, TiO₂(B) was first utilized as a catalyst carrier for the oxidation of formaldehyde (HCHO). The mesoporous TiO₂(B) loaded with Pt nanoparticles enhanced the HCHO oxidation reaction whose reaction rate was 4.5-8.4 times those of other crystalline TiO₂-supported Pt catalysts. Simultaneously, Pt/TiO₂(B) exhibited long-term stable HCHO oxidation performance. The structural characterization results showed that in comparison with Pt/anatase, Pt/TiO₂(B) had more abundant hydroxyls, facilitating increasing the content of oxygen species. Studies on the role of hydroxyls in HCHO oxidation of Pt/TiO₂(B) illustrated that synergistic involvement of terminally bound hydroxyls and bridging hydroxyls in HCHO oxidation accelerated the transformation from HCHO to formate via dioxymethylene. Moreover, hydroxyls could avoid the accumulation of excessive formate on Pt/TiO₂(B) and promote the rapid oxidation of CO. Accordingly, the hydroxyl groups could accelerate each substep of formaldehyde oxidation, which enabled Pt/TiO₂(B) to exhibit excellent formaldehyde oxidation performance.

NaCl-Templated Ultrathin 2D-Yttria Nanosheets Supported Pt Nanoparticles for Enhancing CO Oxidation Reaction

Morphology of support is of fundamental significance to the fabrication of highly efficient catalysts for CO oxidation reaction. Many methods for the construction of supports with specific morphology and structures greatly rely on controlling general physical and chemical synthesis conditions such as temperature or pH. In this paper, we report a facile route to prepare yttria nanosheet using NaCl as template to support platinum nanoparticles exhibiting higher CO oxidation activity than that of the normally prepared Pt/Y2O3. With the help of TEM and SEM, we found that Pt NPs evenly distributed on the surface of NaCl modified 2D-nanosheets with smaller size. The combination of XAFS and TEM characterizations demonstrated that the nano-size Pt species with PtxOy structure played an essential role in the conversion of CO and kept steady during the CO oxidation process. Moreover, the Pt nanoparticles supported on the NaCl templated Y2O3 nanosheets could be more easily reduced and thus exposed more Pt sites to adsorb CO molecules for CO oxidation according to XPS and DRIFTS results. This work offers a unique and general method for the preparation of potential non-cerium oxide rare earth element oxide supported nanocatalysts.

Identification of the Stable Pt Single Sites in the Environment of Ions: From Mechanism to Design Principle

For single-atom (SA)-based catalysis, it is urgent to understand the nature and dynamic evolution of SA active sites during the reactions. In this work, an example of Pt SA-Zn_0.5 Cd_0.5 S (Pt SA-ZCS) is found to display interesting phenomena when facing the Brownian collision of ions in aqueous photocatalysis. Via synchrotron radiation, surface techniques, microscopy, and theory calculations, the results show that two kinds of Pt sites exist: Pt_Zn-sub -S₃ (Pt substituting the Zn site) and Pt_ads -S₂ (Pt adsorbing on the surface). In Na₂ S, the S^2- can coordinate with Pt atoms and peel them from the Pt_ads -S₂ sites, but leaves more stable Pt_Zn-sub -S₃ sites, bringing a low but stable catalytic activity (19.40 mmol g^-1 h^-1 ). Meanwhile, in ascorbic acid, the ascorbic acid ions show less complex ability with Pt atoms, but can decrease the migration barrier of Pt_ads -S₂ sites (67.18 down to 35.96 mmol g^-1 h^-1 , 52.03% drop after 6 h). Therefore, the Pt_ads -S₂ sites gradually aggregate into nanoclusters, bringing a high but decayed catalytic activity. Moreover, a Pt SA-ZCS-Sulfur composite is designed mainly covered by Pt_Zn-sub -S₃ sites accordingly (max: 79.09 mmol g^-1 h^-1 , 5% drop after 6 h and QE: 14.0% at 420 nm), showing a beneficial strategy "from mechanism to design principle."

Light-Induced Structural Dynamic Evolution of Pt Single Atoms for Highly Efficient Photocatalytic CO2 Reduction

Revealing the structural evolution of the real active site during photocatalysis is very important for understanding the catalytic mechanism, but it remains a great challenge. By employing single atoms (SAs) as the mechanism research platform, we investigated the variation of the SA structure under light and the corresponding reaction pathway controlment mechanism. In particular, taking the defect anchoring strategy, Pt SAs are anchored on the metal ion vacancy-rich ZnNiTi layered double hydroxide-etched (ZnNiTi-LDHs-E) support. It is proved by CO-Fourier transform infrared and X-ray absorption fine structure characterization methods that the Pt SAs could gain photoelectrons to form cationic Pt(IV), electron-rich Pt(II), and near-neutral Ptδ+ species at different light intensities. By in situ inducing the above different Pt SAs in photocatalytic CO2 reduction, a dramatic product distribution is observed: (1) under weak light, Pt(IV) SAs cannot activate CO, so CO cannot be further transformed into hydrocarbons; (2) under the moderate light, electron-rich Pt(II) SAs could cooperate with adjacent LDH surface sites (Ni2+/Ti4+) to open up the C-C coupling route for C2H6 generation; and (3) Pt SAs in the state of near-neutral Ptδ+ could directly hydrogenate CO into CH4. This work reveals the structural evolution of Pt SAs in photocatalysis and the corresponding effect on catalytic performance, which provides a new idea for the construction of highly efficient photocatalysts.

The lanthanide doping effect on toluene catalytic oxidation over Pt/CeO2 catalyst

The present work was undertaken to know the lanthanide doping effect on the physicochemical properties of Pt/CeO₂ catalysts and their catalytic activity for toluene oxidation. A series of lanthanide ions (La, Pr, Nd, Sm and Gd) were incorporated into ceria lattice by hydrothermal method, and the Pt nanoparticles with equal quality were successfully loaded on various ceria-based supports. Their catalytic performance toward toluene oxidation shows a remarkable lanthanide-doping effect, and the activity is much dependent on the ion radius and valence state of dopants. Owing to smaller ion radius and low valence state, the dopant of Gd would form more Gd-Ce complex and less GdO₈-type complex, generating more oxygen vacancies and then promoting oxygen replenishment. Furthermore, the high concentration of oxygen vacancy would drive electrons to transfer from support to metal, and thus electron-rich and under-coordinated Pt particles that are favorable for toluene adsorption and dissociation are obtained. Attributing to above positive factors, the doping of Gd would effectively enhance the catalytic oxidation of toluene over Pt/CeO₂ catalyst. In addition, the Pt/CeGdO₂ sample exhibits an excellent reaction stability and resistance of concentration impact.

The concept of active site in heterogeneous catalysis

Catalysis is at the core of chemistry and has been essential to make all the goods surrounding us, including fuels, coatings, plastics and other functional materials. In the near future, catalysis will also be an essential tool in making the shift from a fossil-fuel-based to a more renewable and circular society. To make this reality, we have to better understand the fundamental concept of the active site in catalysis. Here, we discuss the physical meaning - and deduce the validity and, therefore, usefulness - of some common approaches in heterogeneous catalysis, such as linking catalyst activity to a 'turnover frequency' and explaining catalytic performance in terms of 'structure sensitivity' or 'structure insensitivity'. Catalytic concepts from the fields of enzymatic and homogeneous catalysis are compared, ultimately realizing that the struggle that one encounters in defining the active site in most solid catalysts is likely the one we must overcome to reach our end goal: tailoring the precise functioning of the active sites with respect to many different parameters to satisfy our ever-growing needs. This article ends with an outlook of what may become feasible within the not-too-distant future with modern experimental and theoretical tools at hand.

A simple method to regulate surface hydroxy groups on Al2O3 for improving catalytic oxidation performance for HCHO on Pt/Al2O3

For supported noble metal catalysts, the properties of the support had great influence on the state of noble metals. Herein, we chose Al2O3 as model to find a simple method...

Infrared spectroscopic monitoring of solid-state processes

We put a spotlight on IR spectroscopic investigations in materials science by providing a critical insight into the state of the art, covering both fundamental aspects, examples of its utilisation, and current challenges and perspectives focusing on the solid state.

Self-adaptive structure and catalytic performance of the Pt–Sn/Al2O3 propane dehydrogenation catalyst regenerated by dichloroethane oxychlorination

Oxychlorination regeneration is an important and effective regeneration method for Pt based catalysts for propane dehydrogenation.