Effects of gas-phase conditions and particle size on the properties of Cu(111)-supported ZnyOx particles revealed by global optimization and ab initio thermodynamics

The characterization of the interaction between nano or sub-nanoparticles with a support nowadays increasingly relies on computational modeling by means of the density functional theory calculations. These provide valuable atomic-detail understanding of the structure and energetics of supported clusters, but it is still challenging to find (or design) structural models that are representative of real systems in terms of size, structure, and composition. In this study, we have applied an extensive and systematic approach combining global optimization based on an evolutionary algorithm with atomistic ab initio thermodynamics for finding stable structures of a relevant material for catalytic methanol synthesis: Cu(111)-supported ZnyOx clusters. We identify the ZnO3 motif as the elementary building block of such clusters, on which we recently have investigated the full catalytic process for methanol synthesis [Reichenbach, T.; Mondal, K.; Jäger, M.; Vent-Schmidt, T.; Himmel, D.; Dybbert, V.; Bruix, A.; Krossing, I.; Walter, M.; Moseler, M. J. Catal., 2018,360, 168-174]. With the collection of global minima of Cu(111)-supported ZnyOx clusters resulting from this large-scale global optimization effort, we assess the effect of size, gas-phase conditions, and support interactions on the phase diagrams, reactivity, and structural properties of the ZnyOx particles. We find moderate size-effects that are mostly related to the differences in stable Zn/O ratios of the identified global minima and to the formation of different sites in larger clusters. In contrast, large differences in the oxidation state of the clusters as defined by the gas-phase conditions significantly affect the geometry, electronic structure, and reactivity of the ZnyOx particles. This highlights the importance of thoroughly sampling structures with different stoichiometry and appropriately assessing their stability using a detailed thermodynamics analysis.