Atomic Scale Catalysis: Structure and Reactions of Chromium Oxide Species on Transition Aluminas

Chromium-supported transition aluminas are widely used for de-hydrogenation of alkanes, yet an understanding of the structure and relevant chemical reactions on atomic scale is lacking. With first-principles calculations and Z-contrast scanning transmission electron microscopy observations, we show that dispersed CrO$_x$ species supported on transition aluminas are critical for catalytic activities. The atomic-scale species induce dissociation of alkanes whereas crystalline chromium oxide inhibits such dissociation. Contrary to prior models, chromium atoms do not participate in the catalytic reactions, but act as binding centers to which active oxygen atoms are bound. We also show that $\eta$-alumina is more efficient to support the CrO$_x$ species while nucleation of crystalline Cr$_2$O$_3$ is facilitated on $\gamma$-alumina. The difference is attributed to the different vacancy distributions in $\eta$- and $\gamma$-aluminas.