14 research outputs found
Mechanisms for Oxidative Unzipping and Cutting of Graphene
We identify mechanisms and surface precursors for the
nucleation
and growth of extended defects on oxidized graphene. Density functional
theory calculations show that the formation of surface structures
capable to initiate the unzipping and cracking of the oxidized C network
is strongly influenced by the constraint of the graphitic lattice
on the surface functional groups. Accounting for this effect on the
preferential spatial patterning of O adsorbates allows us to revise
and extend the current models of graphene oxidative unzipping and
cutting. We find that these processes are rate limited by O diffusion
and driven by the local strain induced by the O adspecies. Adsorbate
mobility is ultimately recognized as a key factor to control and to
prevent the C-network breakdown during thermal processing of oxidized
graphene
Role of Cluster Morphology in the Dynamics and Reactivity of Subnanometer Pt Clusters Supported on Ceria Surfaces
We study subnanometer (sub-nm) Pt
clusters supported by highly reducible oxide surfaces and establish
the role of cluster morphology in the thermodynamics and kinetics
of surface processes relevant for reactivity, namely cluster mobility,
reverse oxygen spillover, and oxygen vacancy formation. The relationships
between cluster morphology and reactivity are rarely considered in
computational studies because of the large domain and complexity of
the potential energy surface, particularly in the presence of strong
metalâsupport interaction. Global optimization algorithms together
with Hubbard-corrected density functional theory calculations (DFT+U)
are used to identify the stable and metastable morphologies of Pt<sub>3</sub>âPt<sub>6</sub> clusters supported on pristine and
defective CeO<sub>2</sub>(111) surfaces. Our systematic exploration
for these sub-nm Pt particles shows that the charge of the supported
cluster, its bonding to the substrate, and the degree of ceria reduction
depend on the metal/oxide interface area and on the cluster morphology.
Concerning reaction thermodynamics and kinetics, the use of global
optimization methods leads to very different results as compared to
usual minimization procedures. By allowing for morphology changes
during reaction, the energetics of reverse O spillover changes from
highly endothermic to exothermic and leads to new minimum-energy reaction
and diffusion mechanisms. The diffusion kinetics predicts clusters
as small as Pt<sub>6</sub> to be resistant to sintering on ceria surfaces.
The relevance of these findings for larger metal clusters and for
supporting oxide nanoparticles is discussed as well as their connection
with the recent literature
Reaction Mechanisms of Water Splitting and H<sub>2</sub> Evolution by a Ru(II)-Pincer Complex Identified with Ab Initio Metadynamics Simulations
Water splitting is at the basis of artificial photosynthesis
for
solar energy conversion into chemical fuels. While the oxidation of
water to molecular oxygen and the reduction of protons to molecular
hydrogen are typically promoted by different catalysts, the RuÂ(II)-pincer
complex recently synthesized by Kohl et al. [<i>Science</i> <b>2009</b>, <i>324</i>, 74] has been shown to promote
both the thermal driven formation of H<sub>2</sub> and the UVâvis
driven evolution of O<sub>2</sub>. Here, we investigate, through density
functional theory calculations, a portion of the catalytic cycle,
focusing on the formation of hydrogen. We adopt an explicit description
of the solvent and employ metadynamics coupled with the CarâParrinello
method to study the reaction mechanism and determine the activation
free energies. Our simulations predict a novel catalytic cycle, which
has considerably lower activation energies than earlier proposals
and which does not involve the sequential aromatizationâdearomatization
of the PNN ligand of the complex. This work clearly demonstrates the
general importance of an explicit description of the solvent for a
predictive modeling of chemical reactions that involve the active
participation of the solvent
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
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
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
Water Adsorption and Dissociation at Metal-Supported Ceria Thin Films: Thickness and Interface-Proximity Effects Studied with DFT+U Calculations
The chemistry of several catalytic processes can be controlled
by tuning metalâoxide interfaces, as demonstrated by fundamental
studies on inverse model catalysts. We investigate the effects of
the metalâoxide interface on the surface reactivity of ceria
(CeO<sub>2</sub>) thin films supported by a copper metal surface.
Our density functional theory (DFT+U) calculations reveal that the
interface has impact on the surface water adsorption and dissociation
when the thickness of the ceria film is below â9 Ă
. On
thinner films, the energetics of adsorption and dissociation display
a significant variation, which arises from a combination of thickness
and interface-proximity effects, and which we rationalize in terms
of charge-density response at the adsorbate-oxide and oxide-metal
interfaces. The adsorption energy is maximized for film thicknesses
of 5.5 Ă
(corresponding to two OâCeâO trilayers),
while thinner films affect primarily the relative stability between
molecular, semidissociated, and dissociated water adsorption. These
results provide useful insights into the effect of low-dimensional
ceria species in Cu/CeO<sub>2</sub> catalysts
Atomistic Structure of Cobalt-Phosphate Nanoparticles for Catalytic Water Oxidation
Solar-driven water splitting is a key photochemical reaction that underpins the feasible and sustainable production of solar fuels. An amorphous cobalt-phosphate catalyst (Co-Pi) based on earth-abundant elements has been recently reported to efficiently promote water oxidation to protons and dioxygen, a main bottleneck for the overall process. The structure of this material remains largely unknown. We here exploit <i>ab initio</i> and classical atomistic simulations combined with metadynamics to build a realistic and statistically meaningful model of Co-Pi nanoparticles. We demonstrate the emergence and stability of molecular-size ordered crystallites in nanoparticles initially formed by a disordered CoâO network and phosphate groups. The stable crystallites consist of bis-oxo-bridged Co centers that assemble into layered structures (edge-sharing CoO<sub>6</sub> octahedra) as well as in corner- and face-sharing cubane units. These layered and cubane motifs coexist in the crystallites, which always incorporate disordered phosphate groups at the edges. Our computational nanoparticles, although limited in size to âź1 nm, can contain more than one crystallite and incorporate up to 18 Co centers in the cubane/layered structures. The crystallites are structurally stable up to high temperatures. We simulate the extended X-ray absorption fine structure (EXAFS) of our nanoparticles. Those containing several complete and incomplete cubane motifsî¸which are believed to be essential for the catalytic activityî¸display a very good agreement with the experimental EXAFS spectra of Co-Pi grains. We propose that the crystallites in our nanoparticles are reliable structural models of the Co-Pi catalyst surface. They will be useful to reveal the origin of the catalytic efficiency of these novel water-oxidation catalysts
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
Dual Path Mechanism in the Thermal Reduction of Graphene Oxide
Graphene is easily produced by thermally reducing graphene oxide. However, defect formation in the C network during deoxygenation compromises the charge carrier mobility in the reduced material. Understanding the mechanisms of the thermal reactions is essential for defining alternative routes able to limit the density of defects generated by carbon evolution. Here, we identify a dual path mechanism in the thermal reduction of graphene oxide driven by the oxygen coverage: at low surface density, the O atoms adsorbed as epoxy groups evolve as O<sub>2</sub> leaving the C network unmodified. At higher coverage, the formation of other O-containing species opens competing reaction channels, which consume the C backbone. We combined spectroscopic tools and ab initio calculations to probe the species residing on the surface and those released in the gas phase during heating and to identify reaction pathways and rate-limiting steps. Our results illuminate the current puzzling scenario of the low temperature gasification of graphene oxide