Single-Atom Alloys for the Electrochemical Oxygen Reduction Reaction.

Single-atom alloys (SAAs) consisting of isolated transition-metal atoms doped in the surface of coinage metal hosts exhibit unique catalytic properties, harnessing the high activity of the dopant metals with the selectivity of the coinage metal hosts. Here we use density functional theory (DFT) to study SAAs comprised of Ni, Pd, Pt, Co and Rh doped into Ag and Au hosts, as candidate electrocatalysts for the oxygen reduction reaction (ORR) in proton-exchange membrane (PEM) fuel-cells. Our calculations reveal that the PdAu SAA exhibits a slightly lower theoretical overpotential, enhanced selectivity for 4-e- ORR, and tolerance to CO-poisoning compared to Pt(111). While the number of active sites of PdAu SAA is lower than that of Pt(111), the aforementioned desirable properties could bring the overall catalytic performance thereof close to that of Pt/C, indicating that the PdAu SAA could be a viable material for electrocatalytic ORR in PEM fuel-cells.

Adlayer structure and lattice size effects on catalytic rates predicted from KMC simulations: NO oxidation on Pt(111).

Repulsive and/or attractive interactions between surface adsorbates have an important effect on the structure of the adsorbate layer and consequently on the rate of heterogeneous catalytic reactions. Thus, developing reaction models that take into account adsorbate-adsorbate interactions is crucial for making accurate predictions of the catalytic rate and surface coverage during reaction. In the present work, we employ kinetic Monte Carlo simulation to model the catalytic NO oxidation on Pt (111), adopting a cluster expansion (CE) Hamiltonian approach for treating the aforementioned interactions. We investigate CEs of increasing complexity, ranging from pairwise 1st nearest neighbor to long-range and many-body terms. We show that energetic models incorporating solely short-range interactions result in ordered adlayer structures, which are disrupted by anti-phase boundaries and defective regions when the size of the periodic lattice is non-commensurate to the structure of the stable adlayer. We find that O2 dissociates on sites located in these defective regions, which are predominantly responsible for the activity, and the predicted catalytic rate is strongly depended on the lattice size. Such effects are absent when employing non-periodic lattices, whereon the catalytic activity appears more intense on edges/corner sites. Finally, inclusion of long-range interactions in the model Hamiltonian induces relative disorder in the adsorbate layer, which is ascribed to the "softening" of the repulsive interactions between adspecies. Under these circumstances, the distribution of activation energies for O2 dissociation is broader as compared to short-range interaction models and on this basis we explain the disparate catalytic rate predictions when using different CEs.

Lonely Atoms with Special Gifts: Breaking Linear Scaling Relationships in Heterogeneous Catalysis with Single-Atom Alloys.

We discuss a simple yet effective strategy for escaping traditional linear scaling relations in heterogeneous catalysis with highly dilute bimetallic alloys known as single-atom alloys (SAAs). These systems, in which a reactive metal is atomically dispersed in a less reactive host, were first demonstrated with the techniques of surface science to be active and selective for hydrogenation reactions. Informed by these early results, PdCu and PtCu SAA nanoparticle hydrogenation catalysts were shown to work under industrially relevant conditions. To efficiently survey the many potential metal combinations and reactions, simulation is crucial for making predictions about reactivity and guiding experimental focus on the most promising candidate materials. This recent work reveals that the high surface chemical heterogeneity of SAAs can result in significant deviations from Brønsted-Evans-Polanyi scaling relationships for many key reaction steps. These recent insights into SAAs and their ability to break linear scaling relations motivate discovery of novel alloy catalysts.

Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C-H activation.

The recent availability of shale gas has led to a renewed interest in C-H bond activation as the first step towards the synthesis of fuels and fine chemicals. Heterogeneous catalysts based on Ni and Pt can perform this chemistry, but deactivate easily due to coke formation. Cu-based catalysts are not practical due to high C-H activation barriers, but their weaker binding to adsorbates offers resilience to coking. Using Pt/Cu single-atom alloys (SAAs), we examine C-H activation in a number of systems including methyl groups, methane and butane using a combination of simulations, surface science and catalysis studies. We find that Pt/Cu SAAs activate C-H bonds more efficiently than Cu, are stable for days under realistic operating conditions, and avoid the problem of coking typically encountered with Pt. Pt/Cu SAAs therefore offer a new approach to coke-resistant C-H activation chemistry, with the added economic benefit that the precious metal is diluted at the atomic limit.

Carbon Monoxide Poisoning Resistance and Structural Stability of Single Atom Alloys.

Platinum group metals (PGMs) serve as highly active catalysts in a variety of heterogeneous chemical processes. Unfortunately, their high activity is accompanied by a high affinity for CO and thus, PGMs are susceptible to poisoning. Alloying PGMs with metals exhibiting lower affinity to CO could be an effective strategy toward preventing such poisoning. In this work, we use density functional theory to demonstrate this strategy, focusing on highly dilute alloys of PGMs (Pd, Pt, Rh, Ir and Ni) with poison resistant coinage metal hosts (Cu, Ag, Au), such that individual PGM atoms are dispersed at the atomic limit forming single atom alloys (SAAs). We show that compared to the pure metals, CO exhibits lower binding strength on the majority of SAAs studied, and we use kinetic Monte Carlo simulation to obtain relevant temperature programed desorption spectra, which are found to be in good agreement with experiments. Additionally, we consider the effects of CO adsorption on the structure of SAAs. We calculate segregation energies which are indicative of the stability of dopant atoms in the bulk compared to the surface layer, as well as aggregation energies to determine the stability of isolated surface dopant atoms compared to dimer and trimer configurations. Our calculations reveal that CO adsorption induces dopant atom segregation into the surface layer for all SAAs considered here, whereas aggregation and island formation may be promoted or inhibited depending on alloy constitution and CO coverage. This observation suggests the possibility of controlling ensemble effects in novel catalyst architectures through CO-induced aggregation and kinetic trapping.

MoS2 monolayer catalyst doped with isolated Co atoms for the hydrodeoxygenation reaction.

The conversion of oxygen-rich biomass into hydrocarbon fuels requires efficient hydrodeoxygenation catalysts during the upgrading process. However, traditionally prepared CoMoS2 catalysts, although efficient for hydrodesulfurization, are not appropriate due to their poor activity, sulfur loss and rapid deactivation at elevated temperature. Here, we report the synthesis of MoS2 monolayer sheets decorated with isolated Co atoms that bond covalently to sulfur vacancies on the basal planes that, when compared with conventionally prepared samples, exhibit superior activity, selectivity and stability for the hydrodeoxygenation of 4-methylphenol to toluene. This higher activity allows the reaction temperature to be reduced from the typically used 300 °C to 180 °C and thus allows the catalysis to proceed without sulfur loss and deactivation. Experimental analysis and density functional theory calculations reveal a large number of sites at the interface between the Co and Mo atoms on the MoS2 basal surface and we ascribe the higher activity to the presence of sulfur vacancies that are created local to the observed Co-S-Mo interfacial sites.

Controlling Hydrogen Activation, Spillover, and Desorption with Pd-Au Single-Atom Alloys.

Key descriptors in hydrogenation catalysis are the nature of the active sites for H2 activation and the adsorption strength of H atoms to the surface. Using atomically resolved model systems of dilute Pd-Au surface alloys and density functional theory calculations, we determine key aspects of H2 activation, diffusion, and desorption. Pd monomers in a Au(111) surface catalyze the dissociative adsorption of H2 at temperatures as low as 85 K, a process previously expected to require contiguous Pd sites. H atoms preside at the Pd sites and desorb at temperatures significantly lower than those from pure Pd (175 versus 310 K). This facile H2 activation and weak adsorption of H atom intermediates are key requirements for active and selective hydrogenations. We also demonstrate weak adsorption of CO, a common catalyst poison, which is sufficient to force H atoms to spill over from Pd to Au sites, as evidenced by low-temperature H2 desorption.