Modulating the Formation and Evolution of Surface Hydrogen Species on ZnO through Cr Addition

Formation and evolution of active hydrogen species from H-2 on a metal oxide is of great importance in many hydrogenation reactions, while understanding the nature of surface hydrogen species remains a great challenge. Herein, high-vacuum-based transmission infrared spectroscopy coupled with temperature-programmed desorption has been applied to study surface hydrogen species over ZnO and ZnCrOx catalysts. We show that D-2 dissociates readily to form Zn-D and O-D on ZnO at 153 K, and simultaneous Zn-D and O-D desorption in the form of D-2 occurs at 223 K. This corroborates reversible dissociation and recombination of D-2 on ZnO. ( Cr-)Zn-D and (Cr-)O-D species appear on ZnCr2O4 in D-2 at 273 K, indicating a higher barrier for D-2 activation than that on ZnO. (Cr-)Zn-D can diffuse to form (Cr-)O-D at 573 K, and then desorption of surface O-D groups requires a higher temperature of 943 K. Theoretical studies demonstrate that surface geometry plays a crucial role in the evolution of the surface hydrogen species. The polar ZnCr2O4(111) surface can stabilize metal-H species due to the high barrier for its surface diffusion and the thermodynamic obstruction for its association with O-H, in contrast with the nonpolar ZnO(10 (1) over bar0) surface. These results provide insights into the tuning of the stability of surface metal-H species on metal oxide catalysts, which will contribute to tailoring the hydrogenation processes in the H-involved catalytic reactions.