PNNL

Abstract

Atomic resolution aberration-corrected HAADF-STEM imaging has been used to directly image the basal (001) plane of the catalytically relevant M1 phase in MoVTeNb complex oxides. Facets {010}, {120} and {210} are identified as the most stable lateral terminations of the crystals. For each of these surface facets the average concentration of active sites is determined directly from the images. The most stable {010} facet does not expose active sites and is therefore regarded to be inactive. As a consequence, large flattened M1 particles are concluded to have a lower activity than other morphologies. This hypothesis is validated by a statistical analysis of two samples with rod vs. plate morphology. The concentration of sites and the catalytic activity in ethane oxidative dehydrogenation correlate perfectly, demonstrating quantitatively that the differences in reactivity between MoVTeNbOx catalysts of equal crystallinity and chemical composition depend primarily on the terminating facets that constitute the elongated catalyst particles.