CO-Induced Aggregation and Segregation of Highly Dilute Alloys: A Density Functional Theory Study

Abstract

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.

Engineering the Surface Architecture of Highly Dilute Alloys: An ab Initio Monte Carlo Approach

“…11 Interestingly, the binding of adsorbates on SAA isolated dopant atoms is often weaker as compared to a pure dopant catalyst, which allows for the facile desorption of products and tolerance to common poisons. [12][13][14][15][16][17] Though their application has been met with resounding success in the catalysis of a number of chemical processes, the use of SAAs is not ubiquitous. In some instances, dispersed dopant atoms are not capable of activating chemical bonds, with this task requiring contiguous active dopant metal sites.…”

“…Indeed, it is well-known that the alloy structure and composition strongly affect the adsorption energy of surface intermediates [23][24][25][26] and the catalytic performance during a chemical process. 27 Accordingly, a number of experimental and theoretical studies investigating the structure of alloy systems under vacuum [28][29][30][31][32] versus reactive conditions 13,14,20,22,[33][34][35][36][37][38][39][40][41] (i.e. in the presence of adsorbates) have appeared in the literature.…”

On the behaviour of structure-sensitive reactions on single atom and dilute alloy surfaces

“…In our previous studies, we have explored the adsorption of CO on the top dopant site of a number of Cu-, Ag-, and Aubased SAA surfaces. 18,39 A highlight of our work was that the binding strength of CO on SAA(111) and (100) surfaces is almost identical. 18 Therefore, our results suggested that isolated dopant atoms on flat facets show very similar reactivity, which appears to be independent of the geometry of the catalytic surface.…”

“…18,39 A highlight of our work was that the binding strength of CO on SAA(111) and (100) surfaces is almost identical. 18 Therefore, our results suggested that isolated dopant atoms on flat facets show very similar reactivity, which appears to be independent of the geometry of the catalytic surface. To some extent, this phenomenon can be attributed to the limited electronic and spatial overlap between the dopant atom and its surrounding host metal atoms.…”

“…16,17 This electronic characteristic is attributed to the occasionally limited electronic interactions between the dopant atom and its coordination environment, and brings about the high reactivity of SAAs. 16 Furthermore, calculations performed by our group suggested that CO exhibits very similar binding strengths on the top dopant site of two lowindex SAA surfaces, (100) and (111), 18 which are the most commonly exposed surfaces on face-centred cubic metals. 19 This finding implies that adsorbate binding is not affected by the geometry of the host (at least for CO), and gives rise to questions of far-reaching significance for future surface science and theoretical investigations: are isolated dopant atoms on flat (111) and (100) surfaces equivalent in terms of reactivity?…”