On the Impact of Solvation on a Au/TiO₂ Nanocatalyst in Contact with Water

Water, the ubiquitous solvent, is also prominent in forming liquid–solid interfaces with catalytically active surfaces, in particular, with promoted oxides. We study the complex interface of a gold nanocatalyst, pinned by an F-center on titania support, and water. The ab initio simulations uncover the microscopic details of solvent-induced charge rearrangements at the metal particle. Water is found to stabilize charge states differently from the gas phase as a result of structure-specific charge transfer from/to the solvent, thus altering surface reactivity. The metal cluster is shown to feature both “cationic” and “anionic” solvation, depending on fluctuation and polarization effects in the liquid, which creates novel active sites. These observations open up an avenue toward “solvent engineering” in liquid-phase heterogeneous catalysis

Fluxionality of Au Clusters at Ceria Surfaces during CO Oxidation: Relationships among Reactivity, Size, Cohesion, and Surface Defects from DFT Simulations

Density functional theory (DFT) calculations are used to identify correlations among reactivity, structural stability, cohesion, size, and morphology of small Au clusters supported on stoichiometric and defective CeO₂(111) surfaces. Molecular adsorption significantly affects the cluster morphology and in some cases induces cluster dissociation into smaller particles and deactivation. We present a thermodynamic rationalization of these effects and identify Au₃ as the smallest stable nanoparticle that can sustain catalytic cycles for CO oxidation without incurring structural/morphological changes that jeopardize its reactivity. The proposed Mars van Krevelen reaction pathway displays a low activation energy, which we explain in terms of the cluster fluxionality and of labile CO₂ intermediates at the Au/ceria interface. These findings shed light on the importance of cluster dynamics during reaction and provide key guidelines for engineering more efficient metal–oxide interfaces in catalysis

Probing the Reactivity of Pt/Ceria Nanocatalysts toward Methanol Oxidation: From Ionic Single-Atom Sites to Metallic Nanoparticles

Single-atom catalysts represent the ultimate extreme in heterogeneous catalysis for the maximum dispersion of mononuclear catalytic metal particles on supporting surfaces. Ultralow Pt loading has been achieved on nanostructured ceria surfaces that allow for stabilizing metallic and ionic Pt sites that are anchored at surface defects. Here, we assess the chemical reactivity of these different Pt species, which are experimentally known to coexist on Pt–ceria nanocatalysts, by taking methanol oxidation as a chemical probe. Our density functional theory calculations demonstrate that Pt²⁺ and Pt⁴⁺ single-ion species do not promote methanol oxidation by themselves. Instead, metallic sites of supported sub-nanometer Pt particles are always required to promote the oxidation reaction. Our finding generalizes the conclusions of recent photoemission experiments in the context of H₂ oxidation by ceria/Pt nanocatalysts. Moreover, the simulations predict that surface hydroxide groups may act as cocatalyst for the direct methanol oxidation to formaldehyde, thus proposing a viable strategy for catalyst design

Effects of Thermal Fluctuations on the Hydroxylation and Reduction of Ceria Surfaces by Molecular H₂

The hydroxylation of oxide surfaces driven by molecular H₂ dissociation plays a central role in a wide range of catalytic redox reactions. The high reducibility and oxygen storage capacity of ceria (CeO₂) surfaces account for its extensive use as active catalyst support in these redox reactions. By means of ab initio molecular dynamics simulations, we investigate the hydroxylation and reduction of ceria surfaces and demonstrate the so-far unrecognized effects of atomic thermal fluctuations into the mechanism and kinetics of H₂ dissociation. The reaction free-energy hypersurface is sampled and mapped at finite temperature by combining Hubbard-<i>U</i> density functional theory (DFT+<i>U</i>), ab initio molecular dynamics, metadynamics, and umbrella sampling methods. Our molecular dynamics simulations show that the explicit inclusion of thermal fluctuations into the reaction thermodynamics alters the mechanism of H₂ dissociation, changes the nature of the rate-limiting transition state, and decreases the activation temperatures by more than 25%. The results are discussed in the context of kinetic measurements and provide novel insight into the hydroxylation and reduction steps that control the catalytic activity and selectivity of ceria surfaces

Catalytic Proton Dynamics at the Water/Solid Interface of Ceria-Supported Pt Clusters

Wet conditions in heterogeneous catalysis can substantially improve the rate of surface reactions by assisting the diffusion of reaction intermediates between surface reaction sites. The atomistic mechanisms underpinning this accelerated mass transfer are, however, concealed by the complexity of the dynamic water/solid interface. Here we employ ab initio molecular dynamics simulations to disclose the fast diffusion of protons and hydroxide species along the interface between water and ceria, a catalytically important, highly reducible oxide. Up to 20% of the interfacial water molecules are shown to dissociate at room temperature via proton transfer to surface O atoms, leading to partial surface hydroxylation and to a local increase of hydroxide species in the surface solvation layer. A water-mediated Grotthus-like mechanism is shown to activate the fast and long-range proton diffusion at the water/oxide interface. We demonstrate the catalytic importance of this dynamic process for water dissociation at ceria-supported Pt nanoparticles, where the solvent accelerates the spillover of ad-species between oxide and metal sites

Bulk Hydroxylation and Effective Water Splitting by Highly Reduced Cerium Oxide: The Role of O Vacancy Coordination

Reactions of reduced cerium oxide CeO_<i>x</i> with water are fundamental processes omnipresent in ceria-based catalysis. Using thin epitaxial films of ordered CeO_<i>x</i>, we investigate the influence of oxygen vacancy concentration and coordination on the oxidation of CeO_<i>x</i> by water. Upon changing the CeO_<i>x</i> stoichiometry from CeO₂ to Ce₂O₃, we observe a transition from a slow surface reaction to a productive H₂-evolving CeO_<i>x</i> oxidation with reaction yields exceeding the surface capacity and indicating the participation of bulk OH species. Both the experiments and the ab initio calculations associate the effective oxidation of highly reduced CeO_<i>x</i> by water to the next-nearest-neighbor oxygen vacancies present in the bixbyite c-Ce₂O₃ phase. Next-nearest-neighbor oxygen vacancies allow for the effective incorporation of water in the bulk via formation of OH^– groups. Our study illustrates that the coordination of oxygen vacancies in CeO_<i>x</i> represents an important parameter to be considered in understanding and improving the reactivity of ceria-based catalysts