Revealing the structure-activity relationship of Pt1/CeO2 with 17O solid-state NMR spectroscopy and DFT calculations

Single-atom catalysts (SACs) have attracted significant interest due to their exceptional and tunable performance, enabled by diverse coordination environments achieved through innovative synthetic strategies. However, various local structures of active sites pose significant challenges for precise characterization, a prerequisite for developing structure-activity relationships. Here, we combine 17O solid-state NMR spectroscopy and DFT calculations to elucidate the detailed structural information of Pt/CeO2 SACs and their catalytic behaviors. The NMR data reveal that single Pt atoms, dispersed from clusters with water vapor, exhibit a square planar geometry embedded in CeO2 (111) surface, distinct from the original clusters and other conventionally generated Pt single atoms. The square planar Pt/CeO2 SAC demonstrates improved CO oxidation performance compared to Pt/CeO2 SAC with octahedral coordination, due to moderately strong CO adsorption and low energy barriers. This approach can be extended to other oxide-supported SACs, enabling spatially resolved characterization and offering comprehensive insights into their structure-activity relationships.

Does H2 Temperature-Programmed Reduction Always Probe Solid-State Redox Chemistry? The Case of Pt/CeO2

Redox reactions on the surface of transition metal oxides are of broad interest in thermo, photo, and electrocatalysis. H2 temperature-programmed reduction (H2-TPR) is commonly used to probe oxide reducibility by measuring the rate of H2 consumption during temperature ramps, assuming that this rate is controlled by oxide reduction. However, oxide reduction involves several elementary steps, such as H2 dissociation and H-spillover, before surface reduction and H2O formation occur. In this study, we evaluated the kinetics of H2 consumption over CeO2 and Pt/CeO2 with varying Pt loadings and structures to identify the elementary steps probed by H2-TPR. Literature often attributes changes in H2-TPR characteristics with Pt addition to increased CeO2 reducibility. However, our analysis revealed that the H2 consumption rate is measurement of the rate of H-spillover at Pt-CeO2 interfaces and is determined by the concentration of Pt species on Pt nanoclusters that dissociate H2. Therefore, lower temperature H2 consumption observed with Pt addition does not indicate higher CeO2 reducibility. Measurements on samples with mixtures of Pt single-atoms and nanoclusters demonstrated that H2-TPR can effectively quantify dilute Pt nanocluster concentrations, suggesting caution in directly linking H2-TPR characteristics to oxide reducibility while highlighting alternative material insights that can be gleaned.

Digging Its Own Site: Linear Coordination Stabilizes a Pt1/Fe2O3 Single-Atom Catalyst

Determining the local coordination of the active site is a prerequisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems and much emphasis is placed on density functional theory computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (11̅02) facet of α-Fe2O3; a common support material in SACs. Using a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudolinear coordination to surface oxygen atoms. Despite breaking three surface Fe-O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O-Pt-O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase. Moreover, we conclude that extensive structural searches are necessary to determine realistic active site geometries in single-atom catalysis.

Insight into the Dynamic Nature of the Pt-CeO2 Interface in Dry Reforming of Methane

Single-atom catalysts (SACs) are attractive in one-carbon (C1) chemistry because of their high atom efficiency. However, it is a great challenge for understanding the dynamic roles of SACs under operating conditions. Here, isolated Pt atoms trapped on defective CeO2 surface are investigated by experiments, especially operando techniques, which offers basic understanding of the nature and dynamic evolution of the Pt-CeO2 interface in dry reforming of methane (DRM). The Pt-Olattice configuration is highly active for CH4 dissociation at the expense of the Olattice atoms, which in turn promotes the H-assisted dissociation of CO2. The transformation of Pt atoms between positive and metallic states is driven by the DRM reaction, which is essential for rendering highly efficient catalysis. The dynamic evolution of Pt atoms favors to eliminate the reactive intermediates, such as carbonates and formates. The dynamic nature of the Pt-CeO2 interface in the DRM reaction shows a similar picture to the Yin and Yang transformation in ancient Chinese Tai Ji wisdom.

Decomposition of methanol-d4 on Rh nanoclusters supported by thin-film Al2O3/NiAl(100) under near-ambient-pressure conditions

The decomposition of methanol-d4 (CD3OD) on Rh nanoclusters grown by the deposition of Rh vapors onto an ordered thin film of Al2O3/NiAl(100) was studied, with various surface-probe techniques and largely under near-ambient-pressure (NAP) conditions. The results showed a superior reactivity of small Rh clusters (diameter < 1.5 nm) exposed to CD3OD at 5 × 10-3-0.1 mbar at 400 K; the gaseous production of CO and D2 from decomposed methanol-d4 per Rh surface site on the small Rh clusters with diameters of ∼1.1 nm was nearly 8 times that on large ones with diameters of ∼3.5 nm. The promotion of reactivity with decreased cluster size under NAP conditions was evidently greater than that under ultrahigh vacuum conditions. Moreover, the concentration of atomic carbon (C*; where * denotes adsorbate)-a key catalyst poisoner-yielded from the dissociation of CO* from dehydrogenated methanol-d4 was significantly smaller on small clusters (diameter < 1.5 nm). The NAP size effect on methanol-d4 decomposition involved the surface hydroxyl (OH*) from the little co-adsorbed water (H2O*) that was dissociated at a probability dependent on the cluster size. H2O* was more likely dissociated into OH* on small Rh clusters, by virtue of their more reactive d-band structure, and the OH* then effectively promoted the O-D cleavage of methanol-d4, as the rate-determining step, and thus the reaction probability; on the other hand, the OH* limited CO* dissociation on small Rh clusters via both adsorbate and lateral effects. These results suggest that the superior properties of small Rh clusters in both reactivity and anti-poisoning would persist and be highly applicable under "real-world" catalysis conditions.

How Pt Influences H2 Reactions on High Surface-Area Pt/CeO2 Powder Catalyst Surfaces

The addition of platinum-group metals (PGMs, e.g., Pt) to CeO2 is used in heterogeneous catalysis to promote the rate of redox surface reactions. Well-defined model system studies have shown that PGMs facilitate H2 dissociation, H-spillover onto CeO2 surfaces, and CeO2 surface reduction. However, it remains unclear how the heterogeneous structures and interfaces that exist on powder catalysts influence the mechanistic picture of PGM-promoted H2 reactions on CeO2 surfaces developed from model system studies. Here, controlled catalyst synthesis, temperature-programmed reduction (TPR), in situ infrared spectroscopy (IR), and in situ electron energy loss spectroscopy (EELS) were used to interrogate the mechanisms of how Pt nanoclusters and single atoms influence H2 reactions on high-surface area Pt/CeO2 powder catalysts. TPR showed that Pt promotes H2 consumption rates on Pt/CeO2 even when Pt exists on a small fraction of CeO2 particles, suggesting that H-spillover proceeds far from Pt-CeO2 interfaces and across CeO2-CeO2 particle interfaces. IR and EELS measurements provided evidence that Pt changes the mechanism of H2 activation and the rate limiting step for Ce3+, oxygen vacancy, and water formation as compared to pure CeO2. As a result, higher-saturation surface hydroxyl coverages can be achieved on Pt/CeO2 compared to pure CeO2. Further, Ce3+ formed by spillover-H from Pt is heterogeneously distributed and localized at and around interparticle CeO2-CeO2 boundaries, while activated H2 on pure CeO2 results in homogeneously distributed Ce3+. Ce3+ localization at and around CeO2-CeO2 boundaries for Pt/CeO2 is accompanied by surface reconstruction that enables faster rates of H2 consumption. This study reconciles the materials gap between model structures and powder catalysts for H2 reactions with Pt/CeO2 and highlights how the spatial heterogeneity of powder catalysts dictates the influence of Pt on H2 reactions at CeO2 surfaces.

Decomposition of methanol-d4 on a thin film of Al2O3/NiAl(100) under near-ambient-pressure conditions

We have studied the decomposition of methanol-d4 on thin film Al2O3/NiAl(100) under near-ambient-pressure conditions, with varied surface-probe techniques and calculations based on density-functional theory. Methanol-d4 neither adsorbed nor reacted on Al2O3/NiAl(100) at 400 K under ultrahigh vacuum conditions, whereas they dehydrogenated, largely to methoxy-d3 (CD3O*, * denoting adsorbates) and formaldehyde-d2 (CD2O*), on the surface when the methanol-d4 partial pressure was increased to 10-3 mbar and above. The dehydrogenation was facilitated by hydroxyl (OH* or OD*) from the dissociation of little co-adsorbed water; a small fraction of CD2O* interacted further with OH* (OD*) to form, via intermediate CD2OOH* (CD2OOD*), formic acid (DCOOH* or DCOOD*). A few surface carbonates were also yielded, likely on the defect sites of Al2O3/NiAl(100). The results suggest that alumina not only supports metal clusters but also participates in reactions under realistic catalytic conditions. One may consider accordingly the multiple functions of alumina while designing ideal catalysts.

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.

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.

Tailoring the Local Environment of Platinum in Single-Atom Pt1 /CeO2 Catalysts for Robust Low-Temperature CO Oxidation

A single-atom Pt₁ /CeO₂ catalyst formed by atom trapping (AT, 800 °C in air) shows excellent thermal stability but is inactive for CO oxidation at low temperatures owing to over-stabilization of Pt²⁺ in a highly symmetric square-planar Pt₁ O₄ coordination environment. Reductive activation to form Pt nanoparticles (NPs) results in enhanced activity; however, the NPs are easily oxidized, leading to drastic activity loss. Herein we show that tailoring the local environment of isolated Pt²⁺ by thermal-shock (TS) synthesis leads to a highly active and thermally stable Pt₁ /CeO₂ catalyst. Ultrafast shockwaves (>1200 °C) in an inert atmosphere induced surface reconstruction of CeO₂ to generate Pt single atoms in an asymmetric Pt₁ O₄ configuration. Owing to this unique coordination, Pt₁ ^δ+ in a partially reduced state dynamically evolves during CO oxidation, resulting in exceptional low-temperature performance. CO oxidation reactivity on the Pt₁ /CeO₂ _TS catalyst was retained under oxidizing conditions.

Unraveling CO adsorption on model single-atom catalysts

Understanding how the local environment of a "single-atom" catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper₁, silver₁, gold₁, nickel₁, palladium₁, platinum₁, rhodium₁, and iridium₁ species on Fe₃O₄(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag₁-CO) or weaken it (as for Ni₁-CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.

Atomically Dispersed Pt-group Catalysts: Reactivity, Uniformity, Structural Evolution, and Paths to Increased Functionality

The development of experimental and computational tools that give accurate and visual active site descriptions has renewed research interest in atomically dispersed metal catalysts. In this perspective, we describe our approach to synthesizing and understanding atomically dispersed Pt-group metals on oxide supports. Using site-specific characterization, we show that these metal species have distinct reactivity from metal clusters. We argue that producing materials where all metal sites have identical local coordination is key to both accurately assessing catalytic properties and achieving single-site behavior. Methods for assessing site uniformity are considered. We show that producing uniform metal species allows us to describe their structure at the atomic scale and understand how this structure evolves under different conditions. Finally, we suggest pathways to increased functionality for atomically dispersed catalysts, through control of their local coordination and steric environment and through cooperativity with different sites.

Surface Reactivity of Ag-Modified Ceria to Hydrogen: A Combined Experimental and Theoretical Investigation

We investigate the mechanism of H₂ activation on Ag-modified cerium oxide surfaces, of interest for different catalytic applications. The study is performed on thin epitaxial cerium oxide films, investigated by X-ray photoemission spectroscopy to assess the changes of both the Ag oxidation state and the concentration of Ce³⁺ ions, O vacancies, and hydroxyl groups on the surface during thermal reduction cycles in vacuum and under hydrogen exposure. The results are interpreted using density functional theory calculations to model pristine and Ag-modified ceria surfaces. Although the reactivity of ceria toward H₂ oxidation improves when a fraction of Ce cations is substituted with Ag, the concentration of reduced Ce³⁺ ions in Ag-modified ceria is found to be lower than in pure ceria under the same conditions. This behavior is observed even though the number of surface oxygen vacancies caused by the thermal treatment under hydrogen exposure is larger for the Ag-modified surface. These results are explained in terms of a change of the oxidation state of the surface Ag, which is able to acquire some of the extra surface electrons created by the oxygen vacancies and the adsorbed hydrogen atoms. Our findings provide new insights into the reactivity of Ag-modified ceria, which has been proposed as a promising alternative to platinum electrodes in electrochemical devices.

Theoretical Studies on the Stability and Reactivity of the Metal-Doped CeO2(100) Surface: Toward H2 Dissociation and Oxygen Vacancy Formation

Surface doping is a common method to improve the performance of nanostructured materials. Different dopants will affect the structure and catalytic reactivity of the support. For a comprehensive understanding of the doping effects of metals doped into CeO₂, we conducted density functional theory (DFT) studies on the stabilities and geometry structures of transition-metal atoms (M = Fe, Co, Ni, Cu; Ru, Rh, Pd, Ag; Os, Ir, Pt, Au) doped into CeO₂(111), (110), and (100) surfaces. Moreover, the reactivity for H₂ dissociation and oxygen vacancy formation are systematically investigated on M-doped CeO₂(100) surfaces. The greater the binding energies of doped M atoms on the CeO₂ surface, the more difficult the formation of oxygen vacancies. The doped Co and Ir atoms do not directly participate in H₂ activation but serve as a promoter to make the H-H bond to break easily. The Cu, Ru, Pd, Ag, Pt, and Au atoms could act as the catalytically active center for H₂ dissociation and greatly reduce the activation energy barrier. Besides, it is easier to generate H₂O (W^M) and a surface oxygen vacancy from the intermediate H2^M/H4^M than from H3^M/H5^M, which is related to the acid-base interaction between H_Ce/M* and H_O* in H2^M/H4^M. This work could provide theoretical insights into the atomic structure characteristics of the transition-metal-doped CeO₂(100) surface and give ideas for the design of hydrogenation catalysts.

Exploiting the dynamic properties of Pt on ceria for low-temperature CO oxidation

Low-temperature redispersion leads to an intermediate Pt monolayer arrangement, very active for CO oxidation.

Effects of Ni loading on the physicochemical properties of NiOx/CeO2 catalysts and catalytic activity for NO reduction by CO

The NO reduction by CO reaction was investigated using NiO_x/CeO₂ catalysts with different Ni loadings. Surface NiO_x controls the catalytic activity which was related to the molecular structure and reducibility of the catalysts.

Dynamic charge and oxidation state of Pt/CeO2 single-atom catalysts

The catalytic activity of metals supported on oxides depends on their charge and oxidation state. Yet, the determination of the degree of charge transfer at the interface remains elusive. Here, by combining density functional theory and first-principles molecular dynamics on Pt single atoms deposited on the CeO₂ (100) surface, we show that the common representation of a static metal charge is oversimplified. Instead, we identify several well-defined charge states that are dynamically interconnected and thus coexist. The origin of this new class of strong metal-support interactions is the relative position of the Ce(4f) levels with respect to those of the noble metal, allowing electron injection to (or recovery from) the support. This process is phonon-assisted, as the Ce(4f) levels adjust by surface atom displacement, and appears for other metals (Ni) and supports (TiO₂). Our dynamic model explains the unique reactivity found for activated single Pt atoms on ceria able to perform CO oxidation, meeting the Department of Energy 150 °C challenge for emissions.

Quantitative Analysis of the Oxidation State of Cobalt Oxides by Resonant Photoemission Spectroscopy

Quantitative assessment of the charge transfer phenomena in cobalt oxides and cobalt complexes is essential for the design of advanced catalytic materials. We propose a method for the evaluation of the oxidation state of cobalt oxides with mixed valence states using resonant photoemission spectroscopy. The method is based on the calculation of the resonant enhancement ratio (RER) from the heights of the resonant features associated with the Co³⁺ and Co²⁺ states. The nature of the corresponding states was corroborated by means of density functional calculations. We employed a well-ordered Co₃O₄(111) film to calibrate the RER with respect to the atomic Co³⁺/Co²⁺ ratio. The method was applied to monitor the reduction of a well-ordered Co₃O₄(111) film to CoO(111) upon annealing under exposure to isopropanol. We demonstrate that this method yields the stoichiometry of cobalt oxides at a level of accuracy that cannot be achieved when fitting the Co 2p core level spectra.

Highly Reducible Nanostructured CeO2 for CO Oxidation

Ceria in nanoscale with different morphologies, rod, tube and cube, were prepared through a hydrothermal process. The structure, morphology and textural properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and isothermal N2 adsorption-desorption. Ceria with different morphologies were evaluated as catalysts for CO oxidation. CeO2 nanorods showed superior activity to the others. When space velocity was 12,000 mL·gcat−1·h−1, the reaction temperature for 90% CO conversion (T90) was 228 °C. The main reason for the high activity was the existence of large amounts of easily reducible oxygen species, with a reduction temperature of 217 °C on the surface of CeO2 nanorods. Another cause was their relatively large surface area.

Can the state of platinum species be unambiguously determined by the stretching frequency of an adsorbed CO probe molecule?

The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functional) results of pertinent systems - platinum surfaces, nanoparticles and clusters as well as reduced or oxidized platinum moieties on a ceria support. Our results for CO adsorbed on-top on metallic Pt(0), with C-O vibrational frequencies in the region 2018-2077 cm(-1), suggest that a decrease of the coordination number of the platinum atom, to which CO is bound, by one lowers the CO frequency by about 7 cm(-1). This trend corroborates the Kappers-van der Maas correlation derived from the analysis of the experimental stretching frequency of CO adsorbed on platinum-containing samples on different supports. We also analyzed the effect of the charge of platinum species on the CO frequency. Based on the calculated vibrational frequencies of CO in various model systems, we concluded that the actual state of the platinum species may be mistaken based only on the measured value of the C-O vibrational frequency due to overlapping regions of frequencies corresponding to different types of species. In order to identify the actual state of platinum species one has to combine this powerful technique with other approaches.

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

Nanomaterials coated with atomically dispersed platinum on ceria are structurally dynamic and show high potential for applications in fuel cells.