6 research outputs found
On the Impact of Solvation on a Au/TiO<sub>2</sub> Nanocatalyst in Contact with Water
Water, the ubiquitous solvent, is also prominent in forming
liquidâsolid
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 âcationicâ
and âanionicâ solvation, depending on fluctuation and
polarization effects in the liquid, which creates novel active sites.
These observations open up an avenue toward âsolvent engineeringâ
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 CeO<sub>2</sub>(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 Au<sub>3</sub> 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 CO<sub>2</sub> 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 metalâoxide 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 Ptâceria nanocatalysts,
by taking methanol oxidation as a chemical probe. Our density functional
theory calculations demonstrate that Pt<sup>2+</sup> and Pt<sup>4+</sup> 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 H<sub>2</sub> 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 H<sub>2</sub>
The hydroxylation of oxide surfaces
driven by molecular H<sub>2</sub> dissociation plays a central role
in a wide range of catalytic redox
reactions. The high reducibility and oxygen storage capacity of ceria
(CeO<sub>2</sub>) 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
H<sub>2</sub> 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 H<sub>2</sub> 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<sub><i>x</i></sub> with water
are fundamental processes omnipresent in ceria-based
catalysis. Using thin epitaxial films of ordered CeO<sub><i>x</i></sub>, we investigate the influence of oxygen vacancy concentration
and coordination on the oxidation of CeO<sub><i>x</i></sub> by water. Upon changing the CeO<sub><i>x</i></sub> stoichiometry
from CeO<sub>2</sub> to Ce<sub>2</sub>O<sub>3</sub>, we observe a
transition from a slow surface reaction to a productive H<sub>2</sub>-evolving CeO<sub><i>x</i></sub> 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<sub><i>x</i></sub> by water to the next-nearest-neighbor oxygen vacancies
present in the bixbyite c-Ce<sub>2</sub>O<sub>3</sub> phase. Next-nearest-neighbor
oxygen vacancies allow for the effective incorporation of water in
the bulk via formation of OH<sup>â</sup> groups. Our study
illustrates that the coordination of oxygen vacancies in CeO<sub><i>x</i></sub> represents an important parameter to be considered
in understanding and improving the reactivity of ceria-based catalysts