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 metal–support interaction. Global optimization algorithms together with Hubbard-corrected density functional theory calculations (DFT+U) are used to identify the stable and metastable morphologies of Pt₃–Pt₆ clusters supported on pristine and defective CeO₂(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 Pt₆ 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 H₂ 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 Ru­(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 H₂ and the UV–vis driven evolution of O₂. 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 Car–Parrinello 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 aromatization–dearomatization 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 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

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

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

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 metal–oxide interfaces, as demonstrated by fundamental studies on inverse model catalysts. We investigate the effects of the metal–oxide interface on the surface reactivity of ceria (CeO₂) 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 ≈9 Å. 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 Å (corresponding to two O–Ce–O 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/CeO₂ 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 Co–O network and phosphate groups. The stable crystallites consist of bis-oxo-bridged Co centers that assemble into layered structures (edge-sharing CoO₆ 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 ∼1 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 motifswhich are believed to be essential for the catalytic activity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 O₂ 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