Structure–reactivity relationship for bimetallic electrodes: Pt overlayers and PtAu surface alloys on Au(111)

Thermally Methanol Oxidation via the Mn1@Co3O4(111) Facet: Non-CO Reaction Pathway

Co3O4, as the support of single-atom catalysts, is effective in electron-structure modulation to get distinct methanol adsorption behaviors and adjustable reaction pathways for the methanol oxidation reaction. Herein, we considered the facets that constitute a Co vacancy of the Co3O4(111) facet and a foreign metal atom M (M = Fe, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, Mn) leading to single-atom catalysts. The Mn1@Co3O4(111) facet is the facet considered the most favorable among all of the possible terminations. Oxygen adsorption, decomposition, and its co-adsorption with methanol are the vital steps of methanol oxidation at the exposed Mn1@Co3O4(111) facet, giving rise to the stable configuration: two O* and one CH3OH* adsorbates. Then, the Mn1@Co3O4(111) facet activates the O-H and C-H bonds within CH3OH*, advances CH3O* → H2CO* → HCOO* → COO*, and releases the products H2, H2O, and CO2 consecutively.

Ag@Pd bimetallic structures for enhanced electrocatalytic CO2 conversion to CO: an interplay between the strain effect and ligand effect

Electrochemical CO₂ reduction reactions provide a promising path to effectively convert CO₂ into valuable chemicals and fuels for industries. Among the many CO₂ conversion catalysts, Pd stands out as a promising catalyst for effective CO₂ to CO conversion. Here, using the misfit strain strategy, Ag@Pd bimetallic nanoparticles with different Pd overlayer contents were prepared as CO₂ reduction catalysts. By varying the Pd overlayer content, all the Ag@Pd bimetallic nanoparticles exhibited superior CO₂ conversion performance over their Pd and Ag nanoparticle counterparts. An optimal Pd-to-Ag ratio of 1.5 : 1 yielded the highest CO faradaic efficiency of 94.3% at -0.65 V vs. RHE with a high CO specific current density of 3.9 mA cm^-2. It was found that the Pd content can substantially affect the interplay between the strain effect and ligand effect, resulting in optimized binding properties of the reaction intermediates on the catalyst surface, thereby enhancing the CO₂ reduction performance.

Engineering gold-platinum core-shell nanoparticles by self-limitation in solution

Core-shell particles with thin noble metal shells represent an attractive material class with potential for various applications ranging from catalysis to biomedical and pharmaceutical applications to optical crystals. The synthesis of well-defined core-shell architectures remains, however, highly challenging. Here, we demonstrate that atomically-thin and homogeneous platinum shells can be grown via a colloidal synthesis method on a variety of gold nanostructures ranging from spherical nanoparticles to nanorods and nanocubes. The synthesis is based on the exchange of low binding citrate ligands on gold, the reduction of platinum and the subsequent kinetically hindered growth by carbon monoxide as strong binding ligand. The prerequisites for homogeneous growth are low core-binding ligands with moderate fast ligand exchange in solution, a mild reducing agent to mitigate homonucleation and a strong affinity of a second ligand system that can bind to the shell's surface. The simplicity of the described synthetic route can potentially be adapted to various other material libraries to obtain atomically smooth core-shell systems.

Vacancy assisted diffusion on single-atom surface alloys

Bimetallic surfaces can exhibit an improved catalytic activity through tailoring the concentration and/or the arrangement of the two metallic components. However, in order to be catalytically active, the active bimetallic surface structure has to be stable under operating conditions. Typically, structural changes in metals occur via vacancy diffusion. Based on the first-principles determination of formation energies and diffusion barriers we have performed kinetic Monte-Carlo (kMC) simulations to analyse the (meta-)stability of PtRu/Ru(0001), AgPd/Pd(111), PtAu/Au(111) and InCu/Cu(100) surface alloys. In a first step, here we consider single-atom alloys together with one vacancy per simulation cell. We will present results of the time evolution of these structures and analyse them in terms of the interaction between the constituents of the bimetallic surface.

Bimetallic PtAu electrocatalysts for the oxygen reduction reaction: challenges and opportunities

Highly active, durable oxygen reduction reaction (ORR) electrocatalysts have an essential role in promoting the continuous operation of advanced energy technologies such as fuel cells and metal-air batteries. Considering the scarce reserve of Pt and its unsatisfactory overall performance, there is an urgent demand for the development of new generation ORR electrocatalysts that are substantially better than the state-of-the-art supported Pt-based nanocatalysts, such as Pt/C. Among various nanostructures, bimetallic PtAu represents one unique alloy system where highly contradictory performance has been reported. While it is generally accepted that Au may contribute to stabilizing Pt, its role in modulating the intrinsic activity of Pt remains unclear. This perspective will discuss critical structural issues that affect the intrinsic ORR activities of bimetallic PtAu, with an eye on elucidating the origin of seemingly inconsistent experimental results from the literature. As a relatively new class of electrodes, we will also highlight the performance of dealloyed nanoporous gold (NPG) based electrocatalysts, which allow a unique combination of structural properties highly desired for this important reaction. Finally, we will put forward the challenges and opportunities for the incorporation of these advanced electrocatalysts into membrane electrode assemblies (MEA) for actual fuel cells.

Formation of Surface and Quantum-Well States in Ultra Thin Pt Films on the Au(111) Surface

The electronic structure of the Pt/Au(111) heterostructures with a number of Pt monolayers n ranging from one to three is studied in the density-functional-theory framework. The calculations demonstrate that the deposition of the Pt atomic thin films on gold substrate results in strong modifications of the electronic structure at the surface. In particular, the Au(111) s-p-type Shockley surface state becomes completely unoccupied at deposition of any number of Pt monolayers. The Pt adlayer generates numerous quantum-well states in various energy gaps of Au(111) with strong spatial confinement at the surface. As a result, strong enhancement in the local density of state at the surface Pt atomic layer in comparison with clean Pt surface is obtained. The excess in the density of states has maximal magnitude in the case of one monolayer Pt adlayer and gradually reduces with increasing number of Pt atomic layers. The spin-orbit coupling produces strong modification of the energy dispersion of the electronic states generated by the Pt adlayer and gives rise to certain quantum states with a characteristic Dirac-cone shape.

Water adsorption on bimetallic PtRu/Pt(111) surface alloys

The adsorption of water on bimetallic PtRu/Pt(111) surface alloys has been studied based on periodic density functional theory calculations including dispersion corrections. The Ru atoms of the PtRu surface alloy interact more strongly with water than Pt atoms, as far as both single water molecules and ice-like hexagonal structures are concerned. Within the surface alloy layer, the lateral ligand effect reducing the local reactivity of the surface atoms with increasing Ru content is more dominant than the opposing geometric effect due to the tensile strain. The structural preference for the Ru atoms also prevails at room temperature, as ab initio molecular dynamics simulations show.

The role of the CO adsorption on Pt monolayers supported on flat and stepped Au surfaces: a density functional investigation

Ultrathin metal films supported on transition-metal surfaces have been considered as promising catalysts as their chemical activity can be controlled by substrate strain, composition, and ligand effects, however, our atomistic understanding of the atomic structure of those systems is far from satisfactory.

Electronic and structural study of Pt-modified Au vicinal surfaces: a model system for Pt–Au catalysts

Morphology and electronic properties of Pt nanostructures are influenced by the underlying Au surface orientation and surface alloy formation, respectively.

A kinetic Monte Carlo study of Pt on Au(111) with applications to bimetallic catalysis

Pt-decorated Au nanostructures and bimetallic PtAu nanoparticles have been shown to act as catalysts. Consequently we investigate the formation of extended Pt decorations of an Au island edge on Au(111) as possible catalysts. The investigation is by simulation using the kinetic Monte Carlo method. The effects of varying the rate of deposition of Pt atoms and the simulation temperature on the morphology of the resulting Pt nanostructures were investigated. The thickness and roughness of the nanostructures are readily influenced, with temperature being the more important factor. A combination of both high temperature and low deposition rate was the most effective at reducing the roughness. PtAu alloying in the Au island edge was identified. This work is (to the best of our knowledge) the first kinetic Monte Carlo simulation study of the formation of Pt nanostructures on Au. We demonstrate how the morphology of the Pt nanostructures can be controlled. The nanostructures studied here, comprising an adjustable mix of Pt overlayers and novel 1D PtAu surface alloys, are expected to be of considerable interest as potential bimetallic nano-catalysts.

Tailoring the reactivity of bimetallic overlayer and surface alloy systems

Changing the composition and structure of a bimetallic surface system modifies its electronic properties and thus its catalytic activity. On the basis of density functional theory calculations, the electronic factors underlying the modified properties of bimetallic surfaces such as overlayer systems and, in particular, surface alloys will be discussed. It will be demonstrated that by mixing two metals a new metallic compound can result whose properties are not intermediate but beyond those of both constituents, so that for example by adding a relatively inert metal a more reactive surface can result. Besides the reactivity, the stability of the bimetallic systems will also be briefly discussed.

Energetics driving the short-range order in CuxPd1-x/Ru(0001) monolayer surface alloys

The energetics determining the distinct short-range order in two-dimensional (2D) monolayer Cu(x)Pd(1-x) surface alloys on a Ru(0001) substrate were investigated by Monte Carlo simulations and density functional theory calculations. Using a 2D lattice gas Hamiltonian based on effective pair interaction (EPI) parameters, the EPIs were derived for different Cu concentrations with Monte Carlo (MC) simulations by comparing with the atomic distributions obtained from atomic resolution STM images and the related Warren-Cowley short-range order parameters (Hoster et al., Phys. Rev. B, 2006, 73 165413). The ground state structures and mixing energies at 0 K derived from these EPIs agree well with mixing energies determined from DFT calculations of different ordered surface alloys. Additional MC simulations yield rather low transition temperatures which explain the absence of ordered 2D phases in the experiments. The consequences of our findings for the use of alloy surfaces and surface alloys as model systems for adsorption and catalytic reaction studies are discussed.

CO adsorption on Cu–Pd alloy surfaces: ligand versus ensemble effects

The CO adsorption on ordered Cu-Pd alloy surfaces and surface alloys has been studied using density functional theory (DFT) within the framework of the generalized gradient approximation (GGA). On the surface alloys, the CO adsorption energy at the top sites decreases with increasing concentration of the more reactive metal Pd. This surprising ligand effect is caused by the effective compressive strain induced by the larger size of the Pd atoms. On the other hand, at the most favorable adsorption sites the CO binding becomes stronger with increasing Pd concentration which is caused by an ensemble effect related to the availability of higher coordinated adsorption sites. At the surfaces of the bulk alloys, the trends in the adsorption energy as a function of the Pd concentration are less clear because of the strong Pd-Cu interaction and the absence of effective strain effects.