Less than 10% of the plastics generated globally are recycled, while the rest are incinerated, accumulated in landfills, or leach into the environment. New technologies are emerging to chemically recycle...

The atomic scale structure of the active sites in heterogeneous catalysts is central to their reactivity and selectivity. Therefore, understanding active site stability and evolution under different reaction conditions is key to the design of efficient and robust catalysts. Herein we describe theoretical calculations which predict that carbon monoxide can be used to stabilize different active site geometries in bimetallic alloys and then demonstrate experimentally that the same PdAu bimetallic catalyst can be transitioned between a single-atom alloy and a Pd cluster phase. Each state of the catalyst exhibits distinct selectivity for the dehydrogenation of ethanol reaction with the single-atom alloy phase exhibiting high selectivity to acetaldehyde and hydrogen versus a range of products from Pd clusters. First-principles based Monte Carlo calculations explain the origin of this active site ensemble size tuning effect, and this work serves as a demonstration of what should be a general phenomenon that enables in situ control over catalyst selectivity.

Highly dilute binary alloys composed of an active platinum group metal (PGM) and a more inert coinage metal are important in the field of catalysis, as they function as active and selective catalysts. Their catalytic properties depend on the surface "ensemble" of PGM atoms, whose size may be altered under reactive conditions. We use density functional theory and investigate the interaction of CO, a molecule common in numerous industrially important chemistries, with alloys that are composed of a PGM (Pt, Pd, Rh, Ir, Ni) doped in coinage metal hosts (Cu, Au, Ag). We study the adsorption of CO on the (211) step and (100) facet and compare our results to those previously obtained on the (111) facet. We determine strong correlations between the adsorption energies of CO across the facets and highlight the corresponding thermochemical scaling relations. Finally, we study the stability of isolated surface dopant atoms with respect to aggregation into clusters and segregation into the bulk, both in the presence and absence of CO. We find that strong COdopant interactions significantly influence the morphology of the catalyst surface, suggesting that it may be possible to establish control over the ensemble size of the dopant by tuning PCO.

Adopting low-index single-crystal surfaces as models for metal nanoparticle catalysts has been questioned by the experimental findings of adsorbate-induced formation of subnanometer clusters on several single-crystal surfaces. We used density functional theory calculations to elucidate the conditions that lead to cluster formation and show how adatom formation energies enable efficient screening of the conditions required for adsorbate-induced cluster formation. We studied a combination of eight face-centered cubic transition metals and 18 common surface intermediates and identified systems relevant to catalytic reactions, such as carbon monoxide (CO) oxidation and ammonia (NH 3 ) oxidation. We used kinetic Monte Carlo simulations to elucidate the CO-induced cluster formation process on a copper surface. Scanning tunneling microscopy of CO on a nickel (111) surface that contains steps and dislocations points to the structure sensitivity of this phenomenon. Metal-metal bond breaking that leads to the evolution of catalyst structures under realistic reaction conditions occurs much more broadly than previously thought.

Highly dilute alloys of platinum group metals (PGMs)-(Pt, Rh, Ir, Pd, and Ni) with coinage metals (Cu, Au and Ag) serve as highly selective and coke-resistant catalysts in a number of applications. The catalytic behaviour of these materials is governed by the size and shape of the surface "ensembles" of PGM atoms. Therefore, establishing a means of control over the topological architecture of highly dilute alloy surfaces is crucial to optimising their catalytic performance. In the present work, we use on-lattice Monte Carlo (MC) simulations that are parameterised by density functional theory (DFT) derived energetics, in order to investigate the surface aggregation of PGM atoms under vacuum conditions and in the presence of CO. We study several highly dilute alloy surfaces at various PGM loadings, including Pd/Au(111), Pd/Ag(111), Pt/Cu(111), Rh/Cu(111), Ir/Ag(111) and Ni/Cu(111). Under vacuum conditions, we observe a thermodynamic preference for dispersion of PGM as single atoms in the surface of the coinage metal host, on all examined alloy surfaces except Ir/Ag(111), where Ir atom aggregation and island formation is preferred. By evaluating the alloy surface structure in the presence of CO, we determine that the size and shape of PGM ensembles can be manipulated by tuning the partial pressure of CO (PCO) on the Pd/Au(111), Pd/Ag(111), Ir/Ag(111) and Ni/Cu(111) surfaces. In contrast, we determine that Pt/Cu(111) and Rh/Cu(111) highly dilute alloys are unresponsive to changes in PCO with Rh and Pt dispersing as isolated single atoms within the host matrix, irrespective of gaseous composition. Our findings suggest that it may be possible to fine-tune the surface architecture of highly dilute binary alloys for optimised catalytic performance.

The development of platinum group metal-free (PGM-free) catalysts, which can efficiently reduce pollution-causing emissions, is an important task for overcoming major environmental challenges. In particular, nitrogen oxides (NOx) are major...

Metal alloys are ubiquitous in many branches of heterogeneous catalysis, and it is now fairly well established that the local atomic structure of an alloy can have a profound influence on its chemical reactivity. While these effects can be difficult to probe in nanoparticle catalysts, model studies using well defined single crystal surfaces alloyed with dopants enable these structure-function correlations to be drawn. The first step in this approach involves understanding the alloying mechanism and the type of ensembles formed. In this study, we examined the atomic structure of RhCu single-atom alloys formed on Cu(111), Cu(100), and Cu(110) surfaces. Our results show a striking difference between Rh atoms alloying in Cu(111) vs the more open Cu(100) and Cu(110) surface facets. Unlike Cu(111) on which Rh atoms preferentially place-exchange with Cu atoms in the local regions above step edges leaving the majority of the Cu surface free of Rh, highly dispersed, homogeneous alloys are formed on the Cu(100) and ( 110) surfaces. These dramatically different alloying mechanisms are understood by quantifying the energetic barriers for atomic hopping, exchange, swapping, and vacancy filling events for Rh atoms on different Cu surfaces through theoretical calculations. Density functional theory results indicate that the observed differences in the alloying mechanism can be attributed to a faster hopping rate, relatively high atomic exchange barriers, and stronger binding of Rh atoms in the vicinity of step edges on Cu(111) compared to Cu(110) and Cu(100). These model systems will serve as useful platforms for examining structure sensitive chemistry on single-atom alloys.

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 O 2 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 O 2 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.