Theoretical study of structure sensitivity on ceria-supported single platinum atoms and its influence on carbon monoxide adsorption

Density functional theory (DFT) calculations explore the stability of a single platinum atom on various flat, stepped, and defective ceria surfaces, in the context of single-atom catalysts (SACs) for the water-gas shift (WGS) reaction. The adsorption properties and diffusion kinetics of the metal strongly depend on the support termination with large stability on metastable and stepped CeO2(100) and (210) surfaces where the diffusion of the platinum atom is hindered. At the opposite, the more stable CeO2(111) and (110) terminations weakly bind the platinum atom and can promote the growth of metallic clusters thanks to fast diffusion kinetics. The adsorption of carbon monoxide on the single platinum atom supported on the various ceria terminations is also sensitive to the surface structure. Carbon monoxide weakly binds to the single platinum atom supported on reduced CeO2(111) and (211) terminations. The desorption of the CO2 formed during the WGS reaction is thus facilitated on the latter terminations. A vibrational analysis underlines the significant changes in the calculated scaled anharmonic CO stretching frequency on these catalysts.

Platinum-Loaded Cerium Oxide Capable of Repairing Neuronal Homeostasis for Cerebral Ischemia-Reperfusion Injury Therapy

Effective neuroprotective agents are required to prevent neurological damage caused by reactive oxygen species (ROS) generated by cerebral ischemia-reperfusion injury (CIRI) following an acute ischemic stroke. Herein, it is aimed to develop the neuroprotective agents of cerium oxide loaded with platinum clusters engineered modifications (Ptn-CeO2). The density functional theory calculations show that Ptn-CeO2 could effectively scavenge ROS, including hydroxyl radicals (·OH) and superoxide anions (·O2 -). In addition, Ptn-CeO2 exhibits the superoxide dismutase- and catalase-like enzyme activities, which is capable of scavenging hydrogen peroxide (H2O2). The in vitro studies show that Ptn-CeO2 could adjust the restoration of the mitochondrial metabolism to ROS homeostasis, rebalance cytokines, and feature high biocompatibility. The studies in mice CIRI demonstrate that Ptn-CeO2 could also restore cytokine levels, reduce cysteine aspartate-specific protease (cleaved Caspase 3) levels, and induce the polarization of microglia to M2-type macrophages, thus inhibiting the inflammatory responses. As a result, Ptn-CeO2 inhibits the reperfusion-induced neuronal apoptosis, relieves the infarct volume, reduces the neurological severity score, and improves cognitive function. Overall, these findings suggest that the prominent neuroprotective effect of the engineered Ptn-CeO2 has a significant neuroprotective effect and provides a potential therapeutic alternative for CIRI.

Understanding the Reducibility of CeO2 Surfaces by Proton-Electron Transfer from CpCr(CO)3H

CeO2 is a popular material in heterogeneous catalysis, molecular sensors, and electronics and owes many of its special properties to the redox activity of Ce, present as both Ce3+ and Ce4+. However, the reduction of CeO2 with H2 (thought to occur through proton-electron transfer (PET) giving Ce3+ and new OH bonds) is poorly understood due to the high reduction temperatures necessary and the ill-defined nature of the hydrogen atom sources typically used. We have previously shown that transition-metal hydrides with weak M-H bonds react with reducible metal oxides at room temperature by PET. Here, we show that CpCr(CO)3H (1) transfers protons and electrons to CeO2 due to its weak Cr-H bond. We can titrate CeO2 with 1 and measure not only the number of surface Ce3+ sites formed (in agreement with X-ray absorption spectroscopy) but also the lower limit of the hydrogen atom adsorption free energy (HAFE). The results match the extent of reduction achieved from H2 treatment and hydrogen spillover on CeO2 in a wide range of applications.

Systematic Characterization of Electronic Metal-Support Interactions in Ceria-Supported Pt Particles

Electronic metal-support interactions affect the chemical and catalytic properties of metal particles supported on reducible metal oxides, but their characterization is challenging due to the complexity of the electronic structure of these systems. These interactions often involve different states with varying numbers and positions of strongly correlated d or f electrons and the corresponding polarons. In this work, we present an approach to characterize electronic metal-support interactions by means of computationally efficient density functional calculations within the projector augmented wave method. We describe Ce3+ cations with potentials that include a Ce4f electron in the frozen core, overcoming prevalent convergence and 4f electron localization issues. We systematically explore the stability and chemical properties of different electronic states for a Pt8/CeO2(111) model system, revealing the predominant effect of electronic metal-support interactions on Pt atoms located directly at the metal-oxide interface. Adsorption energies and the reactivity of these interface Pt atoms vary significantly upon donation of electrons to the oxide support, pointing to a strategy to selectively activate interfacial sites of metal particles supported on reducible metal oxides.

Isotope Exchange Raman Spectroscopy (IERS): A Novel Technique to Probe Physicochemical Processes In Situ

A novel in situ methodology for the direct study of mass-transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges, is developed. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, which is not accessible by conventional methods, enabling complementary insights for the study of ion-transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) is demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary-ion mass spectrometry (ToF-SIMS) characterization and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness, and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate the understanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research, and beyond.

Metal-Support Interaction and Charge Distribution in Ceria-Supported Au Particles Exposed to CO

Understanding how reaction conditions affect metal-support interactions in catalytic materials is one of the most challenging tasks in heterogeneous catalysis research. Metal nanoparticles and their supports often undergo changes in structure and oxidation state when exposed to reactants, hindering a straightforward understanding of the structure-activity relations using only ex situ or ultrahigh vacuum techniques. Overcoming these limitations, we explored the metal-support interaction between gold nanoparticles and ceria supports in ultrahigh vacuum and after exposure to CO. A combination of in situ methods (on powder and model Au/CeO₂ samples) and theoretical calculations was applied to investigate the gold/ceria interface and its reactivity toward CO exposure. X-ray photoelectron spectroscopy measurements rationalized by first-principles calculations reveal a distinctly inhomogeneous charge distribution, with Au⁺ atoms in contact with the ceria substrate and neutral Au⁰ atoms at the surface of the Au nanoparticles. Exposure to CO partially reduces the ceria substrate, leading to electron transfer to the supported Au nanoparticles. Transferred electrons can delocalize among the neutral Au atoms of the particle or contribute to forming inert Au^δ- atoms near oxygen vacancies at the ceria surface. This charge redistribution is consistent with the evolution of the vibrational frequencies of CO adsorbed on Au particles obtained using diffuse reflectance infrared Fourier transform spectroscopy.

Dispersion and stability mechanism of Pt nanoparticles on transition-metal oxides

The heterogeneous catalysts of Pt/transition-metal oxides are typically synthesized through calcination at 500 °C, and Pt nanoparticles are uniformly and highly dispersed when hydrogen peroxide (H₂O₂) is applied before calcination. The influence of H₂O₂ on the dispersion and the stability of Pt nanoparticles on titania-incorporated fumed silica (Pt/Ti-FS) supports was examined using X-ray absorption fine structure (XAFS) measurements at the Pt L₃ and Ti K edges as well as density functional theory (DFT) calculations. The local structural and chemical properties around Pt and Ti atoms of Pt/Ti-FS with and without H₂O₂ treatment were monitored using in-situ XAFS during heating from room temperature to 500 °C. XAFS revealed that the Pt nanoparticles of H₂O₂-Pt/Ti-FS are highly stable and that the Ti atoms of H₂O₂-Pt/Ti-FS support form into a distorted-anatase TiO₂. DFT calculations showed that Pt atoms bond more stably to oxidized-TiO₂ surfaces than they do to bare- and reduced-TiO₂ surfaces. XAFS measurements and DFT calculations clarified that the presence of extra oxygen atoms due to the H₂O₂ treatment plays a critical role in the strong bonding of Pt atoms to TiO₂ surfaces.

Stability of oxidized states of free-standing and ceria-supported PtOx particles

Nanostructured materials based on CeO2 and Pt play a fundamental role in catalyst design. However, their characterization is often challenging due to their structural complexity and the tendency of the materials to change under reaction conditions. In this work, we combine calculations based on the density functional theory, a machine-learning assisted global optimization method (GOFEE), and ab initio thermodynamics to characterize stable oxidation states of ceria-supported PtyOx particles in different environments. The collection of global minima structures for different stoichiometries resulting from the global optimisation effort is used to assess the effect of temperature, oxygen pressure, and support interactions on the phase diagrams, oxidation states, and geometries of the PtyOx particles. We thus identify favoured structural motifs and O:Pt ratios, revealing that oxidized states of free-standing and ceria-supported platinum particles are more stable than reduced ones under a wide range of conditions. These results indicate that studies rationalizing activity of ceria-supported Pt clusters must consider oxidized states, and that previous understanding of such materials obtained only with fully reduced Pt clusters may be incomplete.

A theoretical study of the electrochemical reduction of CO2 on cerium dioxide supported palladium single atoms and nanoparticles

Pd/CeO₂ catalysts show superior catalytic performance owing to their optimal cycling activity and stability. In this study, single-atom Pd and eight-atom Pd nanoparticle clusters were supported on the surface of CeO₂(110) to investigate the effect of loaded-metal size on the catalytic performance of the Pd-CeO₂ system for CO₂ reduction. We investigated the CO₂ reduction reaction (CRR) that produces C₁ products (CO, HCOOH, CH₃OH, and CH₄) on Pd₈/CeO₂ and Pd/CeO₂ by density functional theory. The structures, CO₂ adsorption configurations, and CO₂ reduction mechanisms of these two electrocatalysts were systematically studied. Subsequently, different reduction pathways on Pd₈/CeO₂ and Pd/CeO₂ were investigated to identify the optimal reaction pathway for further assessment. The results showed that both of these catalysts are more selective towards the production of CH₃OH than CH₄. Moreover, compared to Pd/CeO₂ and Pd₄/CeO₂ (from a previously reported study) the production of CH₃OH via the CRR on Pd₈/CeO₂ exhibited the lowest limiting potential. These results demonstrate the superiority of Pd₈/CeO₂ as an electrocatalyst for the electrochemical reduction of CO₂ to CH₃OH.

Adsorption and Oxidation of CO on Ceria Nanoparticles Exposing Single-Atom Pd and Ag: A DFT Modelling

Various COx species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCOx moieties are found to be stabilised in the order MCO < MCO2 < MCO3, similar to the trend for COx species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO2 transformation are found for O4Pd site of the Pd/Ce21O42 model featuring a square-planar coordination of the Pd2+ cation. The removal of one O atom and formation of the O3Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO2-species are strongly bound in palladium carboxylate complexes, whereas the CO2 molecule easily desorbs from oxide-supported AgCO2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO3 nor AgCO3 carbonate intermediates before CO2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface COx moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO2 and CO32- species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction.

Recent advances in catalytic systems in the prism of physicochemical properties to remediate toxic CO pollutants: A state-of-the-art review

Carbon monoxide (CO) is the most harmful pollutant in the air, causing environmental issues and adversely affecting humans and the vegetation and then raises global warming indirectly. CO oxidation is one of the most effective methods of reducing CO by converting it into carbon dioxide (CO₂) using a suitable catalytic system, due to its simplicity and great value for pollution control. The CO oxidation reaction has been widely studied in various applications, including proton-exchange membrane fuel cell technology and catalytic converters. CO oxidation has also been of great academic interest over the last few decades as a model reaction. Many review studies have been produced on catalysts development for CO oxidation, emphasizing noble metal catalysts, the configuration of catalysts, process parameter influence, and the deactivation of catalysts. Nevertheless, there is still some gap in a state of the art knowledge devoted exclusively to synergistic interactions between catalytic activity and physicochemical properties. In an effort to fill this gap, this analysis updates and clarifies innovations for various latest developed catalytic CO oxidation systems with contemporary evaluation and the synergistic relationship between oxygen vacancies, strong metal-support interaction, particle size, metal dispersion, chemical composition acidity/basicity, reducibility, porosity, and surface area. This review study is useful for environmentalists, scientists, and experts working on mitigating the harmful effects of CO on both academic and commercial levels in the research and development sectors.

Photocatalytic Activity of Cellulose Acetate Nanoceria/Pt Hybrid Mats Driven by Visible Light Irradiation

A photocatalytic system for the degradation of aqueous organic pollutants under visible light irradiation is obtained by an innovative approach based on ceria/platinum (Pt) hybrid nanoclusters on cellulose acetate fibrous membranes. The catalytic materials are fabricated by supersonic beam deposition of Pt nanoclusters directly on the surface of electrospun cellulose acetate fibrous mats, pre-loaded with a cerium salt precursor that is transformed into ceria nanoparticles directly in the solid mats by a simple thermal treatment. The presence of Pt enhances the oxygen vacancies on the surface of the formed ceria nanoparticles and reduces their band gap, resulting in a significant improvement of the photocatalytic performance of the composite mats under visible light irradiation. Upon the appropriate pretreatment and visible light irradiation, we prove that the most efficient mats, with both ceria nanoparticles and Pt nanoclusters, present a degradation efficiency of methylene blue of 70% and a photodegradation rate improved by about five times compared to the ceria loaded samples, without Pt. The present results bring a significant improvement of the photocatalytic performance of polymeric nanocomposite fibrous systems under visible light irradiation, for efficient wastewater treatment applications.

A Hydrothermally Stable Irreducible Oxide-Modified Pd/MgAl2 O4 Catalyst for Methane Combustion

Catalytic combustion is promising in removing trace amounts of CH₄ to address serious environmental concerns. Supported Pd-based catalysts are most effective but often suffer from low stability in applications owing to the water-vapor-induced sintering. Herein, we develop a universal strategy to prepare irreducible-oxide-modified Pd/MgAl₂ O₄ catalysts which show high activity and excellent stability against both hydrothemal aging at elevated temperatures and deactivation in long-term reaction under wet conditions. The addition of irreducible oxides inhibited the deep oxidation of Pd in the oxygen-rich conditions, which preserved not only the epitaxial structure but also a suitable active phase of Pd-PdO_x on MgAl₂ O₄ , thus promoting both activity and stability. This work provides new insights into the effect of metal-oxide interaction on CH₄ combustion and offers an avenue to design hydrothermally stable and active combustion catalysts for industrial applications.

CeO2(111) electronic reducibility tuned by ultra-small supported bimetallic Pt-Cu clusters

Controlling Ce4+ to Ce3+ electronic reducibility in a rare-earth binary oxide such as CeO2 has enormous applications in heterogeneous catalysis, where a profound understanding of reactivity and selectivity at the atomic level is yet to be reached. Thus, in this work we report an extensive DFT-based Basin Hopping global optimization study to find the most stable bimetallic Pt-Cu clusters supported on the CeO2(111) oxide surface, involving up to 5 atoms in size for all compositions. Our PBE+U global optimization calculations indicate a preference for Pt-Cu clusters to adopt 2D planar geometries parallel to the oxide surface, due to the formation of strong metal bonds to oxygen surface sites and charge transfer effects. The calculated adsorption energy values (Eads) for both mono- and bimetallic systems are of the order of 1.79 up to 4.07 eV, implying a strong metal cluster interaction with the oxide surface. Our calculations indicate that at such sub-nanometer sizes, the number of Ce4+ surface atoms reduced to Ce3+ cations is mediated by the amount of Cu atoms within the cluster, reaching a maximum of three Ce3+ for a supported Cu5 cluster. Our computational results have critical implications on the continuous understanding of the strong metal-support interactions over reducible oxides such as CeO2, as well as the advancement of frontier research areas such as heterogeneous single-atom catalysts (SAC) and single-cluster catalysts (SCC).

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.

Metal-doped ceria nanoparticles: stability and redox processes

Doping oxide materials by inserting atoms of a different element in their lattices is a common procedure for modifying properties of the host oxide. Using catalytically active, yet expensive noble metals as dopants allows synthesizing materials with atomically dispersed metal atoms, which can become cost-efficient catalysts. The stability and chemical properties of the resulting materials depend on the structure of the host oxide and on the position of the dopant atoms in it. In the present work we analyze by means of density functional calculations the relative stability and redox properties of cerium dioxide (ceria) nanoparticles doped with atoms of four technologically relevant transition metals - Pt, Pd, Ni and Cu. Our calculations indicate that the dopants are most stable at surface positions of ceria nanoparticles, highlighting the role of under-coordinated sites in the preparation and characterization of doped nanostructured oxides. The energies of two catalytically important reduction reactions - the formation of oxygen vacancies and homolytic dissociative adsorption of H₂ - are found to strongly depend on the position of the doping atoms in nanoparticulate ceria.

Decomposition behavior of platinum clusters supported on ceria and γ-alumina in the presence of carbon monoxide

Carbon monoxide facilitates decomposition of platinum clusters supported on ceria, while carbonylated platinum clusters are stable on gamma alumina.

2D–3D structural transition in sub-nanometer PtN clusters supported on CeO2(111)

Transition metal particles dispersed on oxide supports are used as heterogeneous catalysts in numerous applications.

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.

SOx Tolerant Pt/TiO2 Catalysts for CO Oxidation and the Effect of TiO2 Supports on Catalytic Activity

We developed a new technique for mitigating catalyst deactivation caused by SO2 in exhaust gases. A series of 0.1 wt %-Pt/TiO2 catalysts with different surface, crystal, and pore structures were prepared and tested for CO oxidation activity in the presence of SO2 and H2O. The order of the CO oxidation activity under the influence of SO2 was much different from that in the absence of SO2. Catalysts with a high ratio of larger pores exhibited higher catalytic activity under the influence of SO2 and H2O in the temperature range of 250-300 °C, whereas other parameters, such as BET surface area and crystal structure of the TiO2 support, had minor effects on the CO oxidation activity. The oxidation state of Pt differed significantly depending on the kind of TiO2 support. Some catalysts were less active without H2 reduction pretreatment due to the presence of oxidized Pt species.

Towards stable single-atom catalysts: strong binding of atomically dispersed transition metals on the surface of nanostructured ceria

Surface oxygen sites on CeO₂ nanostructures are able to bind atoms of various transition metals strong enough to prevent their sintering. This finding opens a knowledge-driven way to prepare stable single-atom catalysts with maximum metal efficiency.

O vacancies on steps on the CeO2(111) surface

Cerium dioxide is a compound important for heterogeneous catalysis, energy technologies, biomedical applications, etc. One of its most remarkable properties is low O vacancy (Ovac) formation energy Ef. Nanostructuring of ceria was shown to decrease Ef and to make the oxide material more active in oxidative reactions. Here we investigate computationally formation of Ovac on CeO2(111) surfaces nanostructured by steps with experimentally observed structures. To facilitate the search for Ovac + 2Ce(3+) configurations that yield the lowest Ef values we proposed and employed an efficient computational scheme where DFT + U calculations were preceded by a pre-screening procedure based on the results of plain DFT calculations. Ef values on the steps were calculated to be up to 0.7 eV lower than on a regular CeO2(111) surface. Some energetically stable Ovac + 2Ce(3+) configurations were found to include subsurface Ce(3+) ions. The present results quantify to what extent the roughness of the CeO2(111) surface affects its reducibility.

The role of charge transfer in the oxidation state change of Ce atoms in the TM13–CeO2(111) systems (TM = Pd, Ag, Pt, Au): a DFT + U investigation

Despite extensive studies of transition-metal (TM) clusters supported on ceria, the role of TM atoms in the oxidation state change of Ce atoms is still unclear.

The structure and stability of reduced and oxidized mononuclear platinum species on nanostructured ceria from density functional modeling

The most stable neutral and ionic mononuclear platinum species and their positions on a ceria nanoparticle under different conditions are identified.

A DFT + U computational study on stoichiometric and oxygen deficient M–CeO2 systems (M = Pd1, Rh1, Rh10, Pd10 and Rh4Pd6)

Molecular and dissociative adsorption processes of ethanol on stoichiometric and O-defected CeO₂(111) surfaces alone as well as in the presence of one metal atom (Pd or Rh) are studied using spin-polarized density functional theory (DFT) with the GGA + U method (U_eff = 5.0 eV).