Dynamic coordination of cations and catalytic selectivity on zinc-chromium oxide alloys during syngas conversion

Metal oxide alloys (for example A(x)B(y)O(z)) exhibit dramatically different catalytic properties in response to small changes in composition (the A:B ratio). Here, we show that for the ternary zinc-chromium oxide (ZnCrO) catalysts the activity and selectivity during syngas (CO/H-2) conversion strongly depend on the Zn:Cr ratio. By using a global neural network potential, stochastic surface walking global optimization and first principles validation, we constructed a thermodynamics phase diagram for Zn-Cr-O that reveals the presence of a small stable composition island, that is, Zn:Cr:O = 6:6:16 to 3:8:16, where the oxide alloy crystallizes into a spinel phase. By changing the Zn:Cr ratio from 1:2 to 1:1, the ability to form oxygen vacancies increases appreciably and extends from the surface to the subsurface, in agreement with previous experiments. This leads to the critical presence of a four-coordinated planar Cr2+ cation that markedly affects the syngas conversion activity and selectivity to methanol, as further proved by microkinetics simulations.