Rh single atoms on TiO2 dynamically respond to reaction conditions by adapting their site

A review on transition metal oxides in catalysis

Transition Metal Oxides (TMOs) have drawn significant attention due to their diverse range of properties and applications. The partially filled d orbitals of the transition metal ions, with highly electronegative oxygen atoms, give rise to unique electronic structures that lead to multiple applications due to their magnetic, optical, and structural properties. These properties have a direct influence on chemical reactions that enable tailoring materials for specific applications in catalysis, such as electrocatalysis and photocatalysis. While the potential of TMOs is promising, their development for enhanced functional properties poses numerous challenges. Among these challenges, identifying the appropriate synthesis processes and employing optimal characterization techniques are crucial. In this comprehensive review, an overview of recent trends and challenges in the synthesis and characterization of highly functional TMOs as well as ceramics will be covered with emphasis on catalytic applications. Mesoporous materials play a key role in augmenting their functionality for various applications and will be covered. Ab-initio modeling aspects for the design and development of novel TMO will be also discussed.

Dynamic structural evolution of MgO-supported palladium catalysts: from metal to metal oxide nanoparticles to surface then subsurface atomically dispersed cations

Supported noble metal catalysts, ubiquitous in chemical technology, often undergo dynamic transformations between reduced and oxidized states-which influence the metal nuclearities, oxidation states, and catalytic properties. In this investigation, we report the results of in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, and other physical characterization techniques, bolstered by density functional theory, to elucidate the structural transformations of a set of MgO-supported palladium catalysts under oxidative treatment conditions. As the calcination temperature increased, the as-synthesized supported metallic palladium nanoparticles underwent oxidation to form palladium oxides (at approximately 400 °C), which, at approximately 500 °C, were oxidatively fragmented to form mixtures of atomically dispersed palladium cations. The data indicate two distinct types of atomically dispersed species: palladium cations located at MgO steps and those embedded in the first subsurface layer of MgO. The former exhibit significantly higher (>500 times) catalytic activity for ethylene hydrogenation than the latter. The results pave the way for designing highly active and stable supported palladium hydrogenation catalysts with optimized metal utilization.

Understanding the Dynamic Aggregation in Single-Atom Catalysis

The dynamic response of single-atom catalysts to a reactive environment is an increasingly significant topic for understanding the reaction mechanism at the molecular level. In particular, single atoms may experience dynamic aggregation into clusters or nanoparticles driven by thermodynamic or kinetic factors. Herein, the inherent mechanistic nuances that determine the dynamic profile during the reaction will be uncovered, including the intrinsic stability and site-migration barrier of single atoms, external stimuli (temperature, voltage, and adsorbates), and the influence of catalyst support. Such dynamic aggregation can be beneficial or deleterious on the catalytic performance depending on the optimal initial state. Those examples will be highlighted where in situ formed clusters, rather than single atoms, serve as catalytically active sites for improved catalytic performance. This is followed by the introduction of operando techniques to understand the structural evolution. Finally, the emerging strategies via confinement and defect-engineering to regulate dynamic aggregation will be briefly discussed.

Generating active metal/oxide reverse interfaces through coordinated migration of single atoms

Identification of active sites in catalytic materials is important and helps establish approaches to the precise design of catalysts for achieving high reactivity. Generally, active sites of conventional heterogeneous catalysts can be single atom, nanoparticle or a metal/oxide interface. Herein, we report that metal/oxide reverse interfaces can also be active sites which are created from the coordinated migration of metal and oxide atoms. As an example, a Pd1/CeO2 single-atom catalyst prepared via atom trapping, which is otherwise inactive at 30 °C, is able to completely oxidize formaldehyde after steam treatment. The enhanced reactivity is due to the formation of a Ce2O3-Pd nanoparticle domain interface, which is generated by the migration of both Ce and Pd atoms on the atom-trapped Pd1/CeO2 catalyst during steam treatment. We show that the generation of metal oxide-metal interfaces can be achieved in other heterogeneous catalysts due to the coordinated mobility of metal and oxide atoms, demonstrating the formation of a new active interface when using metal single-atom material as catalyst precursor.

Mechanism-Guided Design of Photocatalysts for CO2 Reduction toward Multicarbon Products

Photocatalytic reduction of CO2 to high value-added multicarbon (C2+) products is an important way to achieve sustainable production of green energy but limited by the low efficiency of catalysts. One fundamental issue lies in the high complexity of catalyst structure and reaction process, making the rational catalyst design and targeted performance optimization a grand challenge. Herein, we performed a mechanism-guided design of photocatalysts for CO2 reduction by using the experimentally reported Cu doped TiO2 (Cu-TiO2) with high C3H8 selectivity and well-defined structure as the prototype. Our mechanistic study highlights three key factors for C3H8 formation, i.e., formation of double O vacancies (Vdi-O) for selectivity, C-C coupling for activity, and Vdi-O recovery for durability. More importantly, Vdi-O formation/recovery and C-C coupling are negatively correlated, indicating that ideal candidates should achieve a balance between oxygen vacancy (VO) formation and C-C coupling. On this basis, TiO2 with the doping of two adjacent Cu atoms (Cu-Cu-TiO2) was designed with enhanced performance for CO2 photoreduction toward C3H8. Furthermore, a simple descriptor (Nμ, "effective d electron number") based on inherent atomic properties was constructed to uncover the underlying causes of the performance variation of different systems. These results provide new insights into the "structure-performance" relation of metal oxide-based photocatalysts, thus offering useful strategies for the rational design of excellent catalysts for CO2 photoreduction.

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.

Catalytic Structure Design by AI Generating with Spectroscopic Descriptors

Generative artificial intelligence has depicted a beautiful blueprint for on-demand design in chemical research. However, the few successful chemical generations have only been able to implement a few special property values because most chemical descriptors are mathematically discrete or discontinuously adjustable. Herein, we use spectroscopic descriptors with machine learning to establish a quantitative spectral structure-property relationship for adsorbed molecules on metal monatomic catalysts. Besides catalytic properties such as adsorption energy and charge transfer, the complete spatial relative coordinates of the adsorbed molecule were successfully inverted. The spectroscopic descriptors and prediction models are generalized, allowing them to be transferred to several different systems. Due to the continuous tunability of the spectroscopic descriptors, the design of catalytic structures with continuous adsorption states generated by AI in the catalytic process has been achieved. This work paves the way for using spectroscopy to enable real-time monitoring of the catalytic process and continuous customization of catalytic performance, which will lead to profound changes in catalytic research.

High-speed mapping of surface charge dynamics using sparse scanning Kelvin probe force microscopy

Unraveling local dynamic charge processes is vital for progress in diverse fields, from microelectronics to energy storage. This relies on the ability to map charge carrier motion across multiple length- and timescales and understanding how these processes interact with the inherent material heterogeneities. Towards addressing this challenge, we introduce high-speed sparse scanning Kelvin probe force microscopy, which combines sparse scanning and image reconstruction. This approach is shown to enable sub-second imaging (>3 frames per second) of nanoscale charge dynamics, representing several orders of magnitude improvement over traditional Kelvin probe force microscopy imaging rates. Bridging this improved spatiotemporal resolution with macroscale device measurements, we successfully visualize electrochemically mediated diffusion of mobile surface ions on a LaAlO3/SrTiO3 planar device. Such processes are known to impact band-alignment and charge-transfer dynamics at these heterointerfaces. Furthermore, we monitor the diffusion of oxygen vacancies at the single grain level in polycrystalline TiO2. Through temperature-dependent measurements, we identify a charge diffusion activation energy of 0.18 eV, in good agreement with previously reported values and confirmed by DFT calculations. Together, these findings highlight the effectiveness and versatility of our method in understanding ionic charge carrier motion in microelectronics or nanoscale material systems.

Heck Migratory Insertion Catalyzed by a Single Pt Atom Site

Single-atom catalysts (SACs) have generated excitement for their potential to downsize metal particles to the atomic limit with engineerable local environments and improved catalytic reactivities and selectivities. However, successes have been limited to small-molecule transformations with little progress toward targeting complex-building reactions, such as metal-catalyzed cross-coupling. Using a supercritical carbon-dioxide-assisted protocol, we report a heterogeneous single-atom Pt-catalyzed Heck reaction, which provides the first C-C bond-forming migratory insertion on SACs. Our quantum mechanical computations establish the reaction mechanism to involve a novel C-rich coordination site (i.e., PtC4) that demonstrates an unexpected base effect. Notably, the base was found to transiently modulate the coordination environment to allow migratory insertion into an M-C species, a process with a high steric impediment with no previous example on SACs. The studies showcase how SACs can introduce coordination structures that have remained underexplored in catalyst design. These findings offer immense potential for transferring the vast and highly versatile reaction manifold of migratory-insertion-based bond-forming protocols to heterogeneous SACs.

Modeling Single-Atom Catalysis

Electronic structure calculations represent an essential complement of experiments to characterize single-atom catalysts (SACs), consisting of isolated metal atoms stabilized on a support, but also to predict new catalysts. However, simulating SACs with quantum chemistry approaches is not as simple as often assumed. In this work, the essential factors that characterize a reliable simulation of SACs activity are examined. The Perspective focuses on the importance of precise atomistic characterization of the active site, since even small changes in the metal atom's surroundings can result in large changes in reactivity. The dynamical behavior and stability of SACs under working conditions, as well as the importance of adopting appropriate methods to solve the Schrödinger equation for a quantitative evaluation of reaction energies are addressed. The Perspective also focuses on the relevance of the model adopted. For electrocatalysis this must include the effects of the solvent, the presence of electrolytes, the pH, and the external potential. Finally, it is discussed how the similarities between SACs and coordination compounds may result in reaction intermediates that usually are not observed on metal electrodes. When these aspects are not adequately considered, the predictive power of electronic structure calculations is quite limited.

Titania-Supported Cu-Single-Atom Catalyst for Electrochemical Reduction of Acetylene to Ethylene at Low-Concentrations with Suppressed Hydrogen Evolution

Electrochemical acetylene reduction (EAR) is a promising strategy for removing acetylene from ethylene-rich gas streams. However, suppressing the undesirable hydrogen evolution is vital for practical applications in acetylene-insufficient conditions. Herein, Cu single atoms are immobilized on anatase TiO2 nanoplates (Cu-SA/TiO2 ) for electrochemical acetylene reduction, achieving an ethylene selectivity of ≈97% with a 5 vol% acetylene gas feed (Ar balance). At the optimal Cu-single-atom loading, Cu-SA/TiO2 is able to effectively suppress HER and ethylene over-hydrogenation even when using dilute acetylene (0.5 vol%) or ethylene-rich gas feeds, delivering a 99.8% acetylene conversion, providing a turnover frequency of 8.9 × 10-2  s-1 , which is superior to other EAR catalysts reported to date. Theoretical calculations show that the Cu single atoms and the TiO2 support acted cooperatively to promote charge transfer to adsorbed acetylene molecules, whilst also inhibiting hydrogen generation in alkali environments, thus allowing selective ethylene production with negligible hydrogen evolution at low acetylene concentrations.

Structure Sensitivity of CO2 Hydrogenation on Ni Revisited

Despite the large number of studies on the catalytic hydrogenation of CO2 to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO2 hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2-12 nm), we found that the active sites responsible for the conversion of CO2 to CO are different from those for the subsequent hydrogenation of CO to CH4. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO2 hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO2 hydrogenation and demonstrated that direct dissociation of CO2 to CO is followed by the conversion of strongly bonded carbonyls to CH4. These findings provide essential insights into the much debated structure sensitivity of CO2 hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts.

Selective Reduction of NO into N2 Catalyzed by Rh1-Doped Cluster Anions RhCe2O3-5. J Am Chem Soc 2023;145:18658-18667. [PMID: 37572057 DOI: 10.1021/jacs.3c06565] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Catalytic conversion of toxic nitrogen oxide (NO) and carbon monoxide (CO) into nitrogen (N2) and carbon dioxide (CO2) is imperative under the weight of the increasingly stringent emission regulations, while a fundamental understanding of the nature of the active site to selectively drive N2 generation is elusive. Herein, in combination with state-of-the-art mass-spectrometric experiments and quantum-chemical calculations, we demonstrated that the rhodium-cerium oxide clusters RhCe2O3-5- can catalytically drive NO reduction by CO and give rise to N2 and CO2. This finding represents a sharp improvement in cluster science where N2O is commonly produced in the rarely established examples of catalytic NO reduction mediated with gas-phase clusters. We demonstrated the importance of the unique chemical environment in the RhCe2O3- cluster to guide the substantially improved N2 selectivity: a triatomic Lewis "acid-base-acid" Ceδ+-Rhδ--Ceδ+ site is proposed to strongly adsorb two NO molecules as well as the N2O intermediate that is attached on the Rh atom and can facilely dissociate to form N2 assisted by both Ce atoms.

Thermally Methanol Oxidation via the Mn1@Co3O4(111) Facet: Non-CO Reaction Pathway

Co3O4, as the support of single-atom catalysts, is effective in electron-structure modulation to get distinct methanol adsorption behaviors and adjustable reaction pathways for the methanol oxidation reaction. Herein, we considered the facets that constitute a Co vacancy of the Co3O4(111) facet and a foreign metal atom M (M = Fe, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, Mn) leading to single-atom catalysts. The Mn1@Co3O4(111) facet is the facet considered the most favorable among all of the possible terminations. Oxygen adsorption, decomposition, and its co-adsorption with methanol are the vital steps of methanol oxidation at the exposed Mn1@Co3O4(111) facet, giving rise to the stable configuration: two O* and one CH3OH* adsorbates. Then, the Mn1@Co3O4(111) facet activates the O-H and C-H bonds within CH3OH*, advances CH3O* → H2CO* → HCOO* → COO*, and releases the products H2, H2O, and CO2 consecutively.

Intrinsic and external active sites of single-atom catalysts

Active components with suitable supports are the common paradigm for industrial catalysis, and the catalytic activity usually increases with minimizing the active component size, generating a new frontier in catalysis, single-atom catalysts (SACs). However, further improvement of SACs activity is limited by the relatively low loading of single atoms (SAs, which are heteroatoms for most SACs, i.e., external active sites) because of the highly favorable aggregation of single heteroatoms during preparation. Research interest should be shifted to investigate SACs with intrinsic SAs, which could circumvent the aggregation of external SAs and consequently increase the SAs loading while maintaining them individual to further improve the activity. In this review, SACs with external or intrinsic SAs are discussed and, at last, the perspectives and challenges for obtaining high-loading SACs with intrinsic SAs are outlined.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ∼1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ∼5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H₂. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

Dynamic evolution of the active center driven by hemilabile coordination in Cu/CeO2 single-atom catalyst

Hemilability is an important concept in homogeneous catalysis where both the reactant activation and the product formation can occur simultaneously through a reversible opening and closing of the metal-ligand coordination sphere. However, this effect has rarely been discussed in heterogeneous catalysis. Here, by employing a theoretical study on CO oxidation over substituted Cu₁/CeO₂ single atom catalysts, we show that dynamic evolution of metal-support coordination can significantly change the electronic structure of the active center. The evolution of the active center is shown to either strengthen or weaken the metal-adsorbate bonding as the reaction proceeds from reactants, through intermediates, to products. As a result, the activity of the catalyst can be increased. We explain our observations by extending hemilability effects to single atom heterogenous catalysts and anticipate that introducing this concept can offer a new insight into the important role active site dynamics have in catalysis toward the rational design of more sophisticated single atom catalyst materials.

Activity Origin of the Nickel Cluster on TiC Support for Nonoxidative Methane Conversion

Designing an active and selective catalyst for nonoxidative conversion of methane under mild conditions is critical for natural gas utilization as a chemical feedstock. Here, we demonstrate that the origin of the selective nonoxidative conversion of methane by the titanium carbide supported nickel cluster arises from the formation of a nickel carbide site under the reaction conditions, which could stabilize the CH_x intermediate to facilitate the C-C coupling, but further coking is rather limited. The reaction mechanism reveals that the C₂ products can be formed via a key -CH_x-CH₃ intermediate. In addition, we demonstrate that boration of the nickel cluster site can improve the methane conversion toward C₂ products. That higher activity and selectivity from the moderate rise in d orbital energy levels can therefore be considered as a descriptor of the catalyst effectiveness. These findings provide an understanding of the dynamic behavior of the single nickel cluster toward methane conversion to C₂ products and guidance for their future rational design.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Genesis of Active Pt/CeO2 Catalyst for Dry Reforming of Methane by Reduction and Aggregation of Isolated Platinum Atoms into Clusters

Atomically dispersed metal catalysts offer the advantages of efficient metal utilization and high selectivities for reactions of technological importance. Such catalysts have been suggested to be strong candidates for dry reforming of methane (DRM), offering prospects of high selectivity for synthesis gas without coke formation, which requires ensembles of metal sites and is a challenge to overcome in DRM catalysis. However, investigations of the structures of isolated metal sites on metal oxide supports under DRM conditions are lacking, and the catalytically active sites remain undetermined. Data characterizing the DRM reaction-driven structural evolution of a cerium oxide-supported catalyst, initially incorporating atomically dispersed platinum, and the corresponding changes in catalyst performance are reported. X-ray absorption and infrared spectra show that the reduction and agglomeration of isolated cationic platinum atoms to form small platinum clusters/nanoparticles are necessary for DRM activity. Density functional theory calculations of the energy barriers for methane dissociation on atomically dispersed platinum and on platinum clusters support these observations. The results emphasize the need for in-operando experiments to assess the active sites in such catalysts. The inferences about the catalytically active species are suggested to pertain to a broad class of catalytic conversions involving the rate-limiting dissociation of light alkanes.

Elucidation of the Active Site for the Oxygen Evolution Reaction on a Single Pt Atom Supported on Indium Tin Oxide

Single-atom catalysts (SACs) have attracted attention for their high catalytic activity and selectivity, but the nature of their active sites under realistic reaction conditions, involving various ligands, is not well-understood. In this study, we use density functional theory calculations and grand canonical basin hopping to theoretically investigate the active site for the oxygen evolution reaction (OER) on a single Pt atom supported on indium tin oxide, including the influence of the electrochemical potential. We show that the ligands on the Pt atom change from Pt-OH in the absence of electrochemical potential to PtO(OH)₄ in electrochemical conditions. This change of the chemical state of Pt is associated with a decrease of 0.3 V for the OER overpotential. This highlights the importance of accurately identifying the nature of the active site under reaction conditions and the impact of adsorbates on the electrocatalytic activity. This theoretical investigation enhances our understanding of SACs for the OER.

Highly dispersed Rh single atoms over graphitic carbon nitride as a robust catalyst for the hydroformylation reaction

Rhodium-catalysed hydroformylation, effective tool in bulk and fine-chemical synthesis, predominantly uses soluble metal complexes. For that reason, the metal leaching and the catalyst recycling are still the major drawbacks of this process. Single-atom catalysts have emerged as a powerful tool to combine the advantages of both homogeneous and heterogeneous catalysts. Since using an appropriate support material is key to create stable, finely dispersed, single-atom catalysts, here we show that Rh atoms anchored on graphitic carbon nitride are robust catalysts for the hydroformylation reaction of styrene.

Challenges and Opportunities in Engineering the Electronic Structure of Single-Atom Catalysts

Controlling the electronic structure of transition-metal single-atom heterogeneous catalysts (SACs) is crucial to unlocking their full potential. The ability to do this with increasing precision offers a rational strategy to optimize processes associated with the adsorption and activation of reactive intermediates, charge transfer dynamics, and light absorption. While several methods have been proposed to alter the electronic characteristics of SACs, such as the oxidation state, band structure, orbital occupancy, and associated spin, the lack of a systematic approach to their application makes it difficult to control their effects. In this Perspective, we examine how the electronic configuration of SACs can be engineered for thermochemical, electrochemical, and photochemical applications, exploring the relationship with their activity, selectivity, and stability. We discuss synthetic and analytical challenges in controlling and discriminating the electronic structure of SACs and possible directions toward closing the gap between computational and experimental efforts. By bringing this topic to the center, we hope to stimulate research to understand, control, and exploit electronic effects in SACs and ultimately spur technological developments.

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.

Synergistic promotions between CO2 capture and in-situ conversion on Ni-CaO composite catalyst

The integrated CO₂ capture and conversion (iCCC) technology has been booming as a promising cost-effective approach for Carbon Neutrality. However, the lack of the long-sought molecular consensus about the synergistic effect between the adsorption and in-situ catalytic reaction hinders its development. Herein, we illustrate the synergistic promotions between CO₂ capture and in-situ conversion through constructing the consecutive high-temperature Calcium-looping and dry reforming of methane processes. With systematic experimental measurements and density functional theory calculations, we reveal that the pathways of the reduction of carbonate and the dehydrogenation of CH₄ can be interactively facilitated by the participation of the intermediates produced in each process on the supported Ni-CaO composite catalyst. Specifically, the adsorptive/catalytic interface, which is controlled by balancing the loading density and size of Ni nanoparticles on porous CaO, plays an essential role in the ultra-high CO₂ and CH₄ conversions of 96.5% and 96.0% at 650 °C, respectively.

Computational and Experimental Characterization of the Ligand Environment of a Ni-Oxo Catalyst Supported in the Metal-Organic Framework NU-1000

Heterogeneous catalysts exhibit significant changes in composition due to the influence of operating conditions, and these compositional changes can have dramatic effects on catalytic performance. For traditional bulk metal heterogeneous catalysts, relationships between composition and catalytic operating conditions are well documented. However, the influence of operating conditions on the compositions of single-site heterogeneous catalysts remains largely unresolved. To address this, we report a combined computational and experimental characterization of a Ni oxo catalyst under catalytic hydrogenation conditions. Specifically, pair distribution function (PDF) analysis is combined with ab initio thermodynamic modeling to investigate ligand environments present on a Ni oxo cluster supported in the metal-organic framework NU-1000. Comparisons of the experimentally observed and simulated Ni-O coordination numbers and Ni-O, Ni···Ni, and Ni···Zr distances provide insight into the Ni ligand environment under H₂ (g). These comparisons suggest significant OH and H₂O content and, further, that different Ni ions within the cluster and/or NU-1000 structure may comprise subtly different numbers of these ligands. Further, the observation of significant H₂O content under H₂ (g) suggests that the NU-1000 support supplies H₂O to the cluster. Examples of ligand environments that could lead to the observed PDFs are provided. The combination of simulations and experiments provides new insights into the ligand environment for Ni-NU-1000 catalysts that will be useful for understanding the ligand environments of other single-site Ni catalysts as well.

Ethene Hydroformylation Catalyzed by Rhodium Dispersed with Zinc or Cobalt in Silanol Nests of Dealuminated Zeolite Beta

Catalysts for hydroformylation of ethene were prepared by grafting Rh into nests of ≡SiOZn-OH or ≡SiOCo-OH species prepared in dealuminated BEA zeolite. X-ray absorption spectra and infrared spectra of adsorbed CO were used to characterize the dispersion of Rh. The Rh dispersion was found to increase markedly with increasing M/Rh (M = Zn or Co) ratio; further increases in Rh dispersion occurred upon use for ethene hydroformylation catalysis. The turnover frequency for ethene hydroformylation measured for a fixed set of reaction conditions increased with the fraction of atomically dispersed Rh. The ethene hydroformylation activity is 15.5-fold higher for M = Co than for M = Zn, whereas the propanal selectivity is slightly greater for the latter catalyst. The activity of the Co-containing catalyst exceeds that of all previously reported Rh-containing bimetallic catalysts. The rates of ethene hydroformylation and ethene hydrogenation exhibit positive reaction orders in ethene and hydrogen but negative orders in carbon monoxide. In situ IR spectroscopy and the kinetics of the catalytic reactions suggest that ethene hydroformylation is mainly catalyzed by atomically dispersed Rh that is influenced by Rh-M interactions, whereas ethene hydrogenation is mainly catalyzed by Rh nanoclusters. In situ IR spectroscopy also indicates that the ethene hydroformylation is rate limited by formation of propionyl groups and by their hydrogenation, a conclusion supported by the measured H/D kinetic isotope effect. This study presents a novel method for creating highly active Rh-containing bimetallic sites for ethene hydroformylation and provides new insights into the mechanism and kinetics of this process.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Development of Power-to-X Catalytic Processes for CO2 Valorisation: From the Molecular Level to the Reactor Architecture

Nowadays, global climate change is likely the most compelling problem mankind is facing. In this scenario, decarbonisation of the chemical industry is one of the global challenges that the scientific community needs to address in the immediate future. Catalysis and catalytic processes are called to play a decisive role in the transition to a more sustainable and low-carbon future. This critical review analyses the unique advantages of structured reactors (isothermicity, a wide range of residence times availability, complex geometries) with the multifunctional design of efficient catalysts to synthesise chemicals using CO2 and renewable H2 in a Power-to-X (PTX) strategy. Fine-chemistry synthetic methods and advanced in situ/operando techniques are essential to elucidate the changes of the catalysts during the studied reaction, thus gathering fundamental information about the active species and reaction mechanisms. Such information becomes crucial to refine the catalyst’s formulation and boost the reaction’s performance. On the other hand, reactors architecture allows flow pattern and temperature control, the management of strong thermal effects and the incorporation of specifically designed materials as catalytically active phases are expected to significantly contribute to the advance in the valorisation of CO2 in the form of high added-value products. From a general perspective, this paper aims to update the state of the art in Carbon Capture and Utilisation (CCU) and PTX concepts with emphasis on processes involving the transformation of CO2 into targeted fuels and platform chemicals, combining innovation from the point of view of both structured reactor design and multifunctional catalysts development.

Oxidative decomposition of dimethyl methylphosphonate on rutile TiO2(110): the role of oxygen vacancies

The decomposition of dimethyl methylphosphonate (DMMP, (CH₃O)₂P(O)(CH₃)), a simulant to the toxic nerve agent Sarin, on the rutile TiO₂(110) surface has been studied with temperature programmed desorption (TPD) and density functional theory (DFT) calculations. The reactivity of the TiO₂(110) surface for DMMP decomposition is shown to be low, with mainly molecular desorption and only a small fraction of methanol and formaldehyde decomposition products seen from TPD at around 650 K. In addition, this amount of products is similar to the number of O vacancies on the surface. DFT calculations show that O vacancies are key for P-OCH₃ bond cleavage of DMMP, lowering the barrier by 0.7 eV and enabling the reactive process to occur at around 600 K. This is explained by the closer position of DMMP with respect to the surface in the presence of O vacancies. Calculations show that the produced methoxy groups can transform into gas phase formaldehyde and methanol at the considered temperature (600 K), in agreement with experiments. O-C bond cleavage of DMMP is also a viable pathway at such a high temperature (600 K) for DMMP decomposition on r-TiO₂, even in the absence of O vacancies, but the formation of a gas phase product is energetically unfavorable. O vacancies hence are the active sites for decomposition of DMMP into gas phase products on r-TiO₂(110).

Single-Atom Catalysis: Insights from Model Systems

The field of single-atom catalysis (SAC) has expanded greatly in recent years. While there has been much success developing new synthesis methods, a fundamental disconnect exists between most experiments and the theoretical computations used to model them. The real catalysts are based on powder supports, which inevitably contain a multitude of different facets, different surface sites, defects, hydroxyl groups, and other contaminants due to the environment. This makes it extremely difficult to determine the structure of the active SAC site using current techniques. To be tractable, computations aimed at modeling SAC utilize periodic boundary conditions and low-index facets of an idealized support. Thus, the reaction barriers and mechanisms determined computationally represent, at best, a plausibility argument, and there is a strong chance that some critical aspect is omitted. One way to better understand what is plausible is by experimental modeling, i.e., comparing the results of computations to experiments based on precisely defined single-crystalline supports prepared in an ultrahigh-vacuum (UHV) environment. In this review, we report the status of the surface-science literature as it pertains to SAC. We focus on experimental work on supports where the site of the metal atom are unambiguously determined from experiment, in particular, the surfaces of rutile and anatase TiO₂, the iron oxides Fe₂O₃ and Fe₃O₄, as well as CeO₂ and MgO. Much of this work is based on scanning probe microscopy in conjunction with spectroscopy, and we highlight the remarkably few studies in which metal atoms are stable on low-index surfaces of typical supports. In the Perspective section, we discuss the possibility for expanding such studies into other relevant supports.

The role of Mo species in Ni-Mo catalysts for dry reforming of methane

The Ni-Mo catalyst has attracted significant attention due to its excellent coke-resistance in dry reforming of methane (DRM) reaction, but its detailed mechanism is still vague. Herein, Mo-doped Ni (Ni-Mo_x) and MoO_x adsorbed Ni surfaces (MoO_x@Ni) are employed to explore the DRM reaction mechanism and the effect of coke-resistance. Due to the electron donor effect of Mo, the antibonding states below the Fermi level between Ni and C increase and the adsorption of C decrease, thereby inhibiting the carbonization of Ni. On account of the strong Mo and O interaction, more O atoms gather around Mo, which inhibits the oxidation of Ni and may promote the formation of MoO_x species on the Ni-Mo catalyst. The presence of Mo-O species promotes the carbon oxidation, forming a unique redox cycle (MoO_x ↔ MoO_x-1) similar to the Mars-van Krevelen (MvK) mechanism, explaining the excellent anti-carbon deposition effect on the Ni-Mo catalyst.

Advanced Strategies for Stabilizing Single-Atom Catalysts for Energy Storage and Conversion

Well-defined atomically dispersed metal catalysts (or single-atom catalysts) have been widely studied to fundamentally understand their catalytic mechanisms, improve the catalytic efficiency, increase the abundance of active components, enhance the catalyst utilization, and develop cost-effective catalysts to effectively reduce the usage of noble metals. Such single-atom catalysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment. However, freestanding single atoms are thermodynamically unstable, such that during synthesis and catalytic reactions, they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas. Therefore, developing innovative strategies to stabilize single-atom catalysts, including mass-separated soft landing, one-pot pyrolysis, co-precipitation, impregnation, atomic layer deposition, and organometallic complexation, is critically needed. Many types of supporting materials, including polymers, have been commonly used to stabilize single atoms in these fabrication techniques. Herein, we review the stabilization strategies of single-atom catalyst, including different synthesis methods, specific metals and carriers, specific catalytic reactions, and their advantages and disadvantages. In particular, this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts, including their functions as carriers for metal single atoms, synthetic templates, encapsulation agents, and protection agents during the fabrication process. The technical challenges that are currently faced by single-atom catalysts are summarized, and perspectives related to future research directions including catalytic mechanisms, enhancement of the catalyst loading content, and large-scale implementation are proposed to realize their practical applications.

Graphical Abstract

Single-atom catalysts are characterized by high metal dispersibility, weak coordination environments, high catalytic activity and selectivity, and the highest atom utilization. However, due to the free energy of the large surface area, individual atoms are usually unstable and are prone to agglomeration during synthesis and catalytic reactions. Therefore, researchers have developed innovative strategies, such as soft sedimentation, one-pot pyrolysis, coprecipitation, impregnation, step reduction, atomic layer precipitation, and organometallic complexation, to stabilize single-atom catalysts in practical applications. This article summarizes the stabilization strategies for single-atom catalysts from the aspects of their synthesis methods, metal and support types, catalytic reaction types, and its advantages and disadvantages. The focus is on the application of polymers in the preparation and stabilization of single-atom catalysts, including metal single-atom carriers, synthetic templates, encapsulation agents, and the role of polymers as protection agents in the manufacturing process. The main feature of polymers and polymer-derived materials is that they usually contain abundant heteroatoms, such as N, that possess lone-pair electrons. These lone-pair electrons can anchor the single metal atom through strong coordination interactions. The coordination environment of the lone-pair electrons can facilitate the formation of single-atom catalysts because they can enlarge the average distance of a single precursor adsorbed on the polymer matrix. Polymers with nitrogen groups are favorable candidates for dispersing active single atoms by weakening the tendency of metal aggregation and redistributing the charge densities around single atoms to enhance the catalytic performance. This review provides a summary and analysis of the current technical challenges faced by single-atom catalysts and future research directions, such as the catalytic mechanism of single-atom catalysts, sufficiently high loading, and large-scale implementation.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Single-Atom-Based Catalysts for Photocatalytic Water Splitting on TiO2 Nanostructures

H2 generation from photocatalytic water splitting is one of the most promising approaches to producing cost-effective and sustainable fuel. Nanostructured TiO2 is a highly stable and efficient semiconductor photocatalyst for this purpose. The main drawback of TiO2 as a photocatalyst is the sluggish charge transfer on the surface of TiO2 that can be tackled to a great extent by the use of platinum group materials (PGM) as co-catalysts. However, the scarcity and high cost of the PGMs is one of the issues that prevent the widespread use of TiO2/PGM systems for photocatalytic H2 generation. Single-atom catalysts which are currently the frontline in the catalysis field can be a favorable path to overcome the scarcity and further advance the use of noble metals. More importantly, single-atom (SA) catalysts simultaneously have the advantage of homogenous and heterogeneous catalysts. This mini-review specifically focuses on the single atom decoration of TiO2 nanostructures for photocatalytic water splitting. The latest progress in fabrication, characterization, and application of single-atoms in photocatalytic H2 generation on TiO2 is reviewed.

Role of Polarons in Single-Atom Catalysts: Case Study of Me1 [Au1, Pt1, and Rh1] on TiO2(110)

The local environment of metal-oxide supported single-atom catalysts plays a decisive role in the surface reactivity and related catalytic properties. The study of such systems is complicated by the presence of point defects on the surface, which are often associated with the localization of excess charge in the form of polarons. This can affect the stability, the electronic configuration, and the local geometry of the adsorbed adatoms. In this work, through the use of density functional theory and surface-sensitive experiments, we study the adsorption of Rh1, Pt1, and Au1 metals on the reduced TiO2(110) surface, a prototypical polaronic material. A systematic analysis of the adsorption configurations and oxidation states of the adsorbed metals reveals different types of couplings between adsorbates and polarons. As confirmed by scanning tunneling microscopy measurements, the favored Pt1 and Au1 adsorption at oxygen vacancy sites is associated with a strong electronic charge transfer from polaronic states to adatom orbitals, which results in a reduction of the adsorbed metal. In contrast, the Rh1 adatoms interact weakly with the excess charge, which leaves the polarons largely unaffected. Our results show that an accurate understanding of the properties of single-atom catalysts on oxide surfaces requires a careful account of the interplay between adatoms, vacancy sites, and polarons.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11244-022-01651-0.

Promotional Effect of H2 Pretreatment on the CO PROX Performance of Pt1/Co3O4: A First-Principles-Based Microkinetic Analysis

Atomic Pt studded on cobalt oxide is a promising catalyst for CO preferential oxidation (PROX) dependent on its surface treatment. In this work, the CO PROX reaction mechanism on Co₃O₄ supported single Pt atom is investigated by a comprehensive first-principles based microkinetic analysis. It is found that as synthesized Pt₁/Co₃O₄ interface is poisoned by CO in a wide low temperature window, leading to its low reactivity. The CO poisoning effect can be effectively mitigated by a H₂ prereduction treatment, that exposes Co ∼ Co dimer sites for a noncompetitive Langmuir-Hinshelhood mechanism. In addition, surface H atoms assist O₂ dissociation via "twisting" mechanism, avoiding the high barriers associated with direct O₂ dissociation path. Microkinetic analysis reveals that the promotion of H-assisted pathway on H₂ treated sample helps improve the activity and selectivity at low temperatures.

Reverse water-gas shift reaction catalyzed by diatomic rhodium anions

The reverse water-gas shift (RWGS, CO₂ + H₂ → CO + H₂O, ΔH₂₉₈ = +0.44 eV) reaction mediated by the diatomic anion Rh₂^- was successfully constructed. The generation of a gas-phase H₂O molecule and ion product [Rh₂(CO)_ads]^- was identified unambiguously at room temperature and the only elementary step that requires extra energy to complete the catalysis is the desorption of CO from [Rh₂(CO)_ads]^-. This experimentally identified Rh₂^- anion represents the first gas-phase species that can drive the RWGS reaction because it is challenging to design effective routes to yield H₂O from CO₂ and H₂. The reactions were performed by using our newly developed double ion trap reactors and characterized by mass spectrometry, photoelectron spectroscopy, and high-level quantum-chemical calculations. We found that the order that the reactants (CO₂ or D₂) were fed into the reactor did not have a pronounced impact on the reactivity and the final product distribution (D₂O and Rh₂CO^-). The atomically precise insights into the key steps to guide the reaction toward the RWGS direction were provided.

Single-Atom Catalysis: From Simple Reactions to the Synthesis of Complex Molecules

To date, the scope of single-atom catalysts (SAC) in liquid-phase transformations is rather limited owing to stability issues and the inability to activate complex substances. This calls for a better design of the catalyst support that can provide a dynamic coordination environment needed for catalytic action, and yet retain robustness against leaching or aggregation. In addition, the chemical orthogonality of SAC is useful for designing tandem or multicomponent reactions, in which side reactions common to metal nanoparticles are suppressed. In this review, the intrinsic mechanism will be highlighted that controls reaction efficiency and selectivity in SAC-catalyzed pathways, as well as the structural dynamism of SAC under complex liquid-phase conditions. These mechanistic insights are helpful for the development of next-generation SAC systems for the synthesis of high-value pharmaceuticals through late-stage functionalization, sequential and multicomponent strategies.