Low O2 dissociation barrier on Pt(111) due to adsorbate–adsorbate interactions

Repulsive Lateral Interaction of Water Molecules at the Initial Stages of Adsorption in Microporous AlPO4-11 According to 27Al NMR and DFT

Lateral (adsorbate-adsorbate) interactions between adsorbed molecules affect various physical and chemical properties of microporous adsorbents and catalysts, influencing their functional properties. In this work, we studied the hydration of microporous AlPO4-11 aluminophosphate, which has an unusually ordered structure upon adsorption of water vapor, and according to 27Al NMR data, only tetrahedrally or octahedrally coordinated Al sites are present in the AlPO4-11. These 27Al NMR data are consistent with the results of density functional theory (DFT) calculations of hydrated AlPO4-11, which revealed the presence of a strong repulsive lateral interaction at the initial stage of adsorption, suppressing the adsorption of water on neighboring (separated by one -O-P-O- bridge) Al crystallographic sites. As a result, of all the different aluminum sites, only half of the Al1 sites adsorb two water molecules and acquire octahedral coordination.

Understanding the mechanisms and design principles for oxygen evolution and oxygen reduction activity on perovskite catalysts for alkaline zinc–air batteries

The high cost and limited availability of the precious metal catalysts required for catalysing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in metal–air batteries restrict the marketing of these clean energy technologies.

Catalytic Oxidation of CO on a Curved Pt(111) Surface: Simultaneous Ignition at All Facets through a Transient CO-O Complex*

The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.

Augmented Pairwise Additive Interaction Model for Lateral Adsorbate Interactions: The NO-CO Reaction System on Rh(100) and Rh(111)

Lateral adsorbate interactions have been acknowledged to play an important role in heterogeneous catalytic kinetics. To quantify such energies efficiently and accurately, a lattice-gas-based augmented pairwise additive interaction model was proposed. The model Hamiltonian is defined as the summation over all isolated binding energies and all pairwise interactions, with the pairwise interaction used for each adsorbate pair consisting of the corresponding isolated pairwise interaction and some needed modification terms if the surroundings meet specific conditions. The parameters used in this augmented pairwise additive interaction model for the NO-CO reaction system on Rh(100) and Rh(111) were collected based on density functional theory (DFT) calculations. The reliability of the model was examined by the reproduction of some DFT-based predictions and experimental observations. Results suggest that the lateral interactions on Rh(100) are relatively short range and greatly susceptible to the lateral displacements of adsorbates, whereas on Rh(111), the lateral interactions are relatively long range and the lateral displacements of adsorbates are more limited. In addition, the Brønsted-Evans-Polanyi relations for adsorbate diffusion were constructed and a modification strategy was proposed for certain diffusion processes. This work opens up the possibility of accurate microkinetic modeling for this reaction system with a faithful account of lateral interactions and motivates the extensions to other complex surface systems.

Self-Assembled Dendritic Pt Nanostructure with High-Index Facets as Highly Active and Durable Electrocatalyst for Oxygen Reduction

The durability issues of Pt catalyst should be resolved for the commercialization of proton exchange membrane fuel cells. Nanocrystal structures with high-index facets have been recently explored to solve the critical durability problem of fuel cell catalysts as Pt catalysts with high-index facets can preserve the ordered surfaces without change of the original structures. However, it is very difficult to develop effective and practical synthetic methods for Pt-based nanostructures with high-index facets. The current study describes a simple one-pot synthesis of self-assembled dendritic Pt nanostructures with electrochemically active and stable high-index facets. Pt nanodendrites exhibited 2 times higher ORR activity and superior durability (only 3.0 % activity loss after 10 000 potential cycles) than a commercial Pt/C. The enhanced catalytic performance was elucidated by the formation of well-organized dendritic structures with plenty of reactive interfaces among 5 nm-sized Pt particles and the coexistence of low- and high-index facets on the particles.

Thermodynamic assessment of the oxygen reduction activity in aqueous solutions

The hydrogen bonding of hydrophilic oxygen reduction intermediates to water has large effects on scaling relations and volcano plots.

ORR viability of alumina-supported platinum nanocluster: exploring oxidation behaviour by DFT

Despite abundant use of alumina-supported platinum nanoclusters as catalyst for various chemical reactions, their potential as an ORR catalyst is yet to be explored. Therefore, the present study aimed to assess the viability of alumina supported platinum clusters as ORR catalysts.

Cooperation between adsorbates accounts for the activation of atomic layer deposition reactions

Atomic layer deposition (ALD) is a technique for producing conformal layers of nanometre-scale thickness, used commercially in non-planar electronics and increasingly in other high-tech industries. ALD depends on self-limiting surface chemistry but the mechanistic reasons for this are not understood in detail. Here we demonstrate, by first-principle calculations of growth of HfO2 from Hf(N(CH3)2)4-H2O and HfCl4-H2O and growth of Al2O3 from Al(CH3)3-H2O, that, for all these precursors, co-adsorption plays an important role in ALD. By this we mean that previously-inert adsorbed fragments can become reactive once sufficient numbers of molecules adsorb in their neighbourhood during either precursor pulse. Through the calculated activation energies, this 'cooperative' mechanism is shown to have a profound influence on proton transfer and ligand desorption, which are crucial steps in the ALD cycle. Depletion of reactive species and increasing coordination cause these reactions to self-limit during one precursor pulse, but to be re-activated via the cooperative effect in the next pulse. This explains the self-limiting nature of ALD.

A DFT study of oxygen dissociation on platinum based nanoparticles

Density functional theory calculations are performed on 38 and 79 metal atom truncated octahedron clusters to study oxygen dissociation as a model for the initial stage of the oxygen reduction reaction. Pure platinum and alloyed platinum-titanium core-shell systems are investigated. It is found that barrierless oxygen dissociation occurs on the (111) facet of the pure platinum clusters. A barrier of ~0.3 eV is observed on the (100) facet. For the alloyed cluster, dissociation barriers are found on both facets, typically ~0.6 eV. The differences between the two systems are attributed to the ability of oxygen to distort the (111) surface of the pure platinum clusters. We show that flexibility of the platinum shell is crucial in promotion of fast oxygen dissociation. However, the titanium core stabilises the platinum shell upon alloying, resulting in a less easily distortable surface. Therefore, whilst an alloyed platinum-titanium electrocatalyst has certain advantages over the pure platinum electrocatalyst, we suggest alloying with a more weakly interacting metal will be beneficial for facilitating oxygen dissociation.

Development of a ReaxFF potential for Pt–O systems describing the energetics and dynamics of Pt-oxide formation

A ReaxFF force field description of Pt–O systems has been developed, validated and applied to oxygen diffusion on Pt(111).

Oxidation of Pt(111) under near-ambient conditions

The oxidation of Pt(111) at near-ambient O2 pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O K edge reveal significant temperature- and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin α-PtO2 trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.