Detailed Characterization of MoO_x-Modified Rh Metal Particles by Ambient-Pressure XPS and DFT Calculations

Characterization of Mo species on Rh metal particles in Rh-MoOx/C catalyst for C-O hydrogenolysis reactions was carried out with in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations. The as-prepared Rh-MoOx/C sample with high oxidation states of Rh3+, Rh4+ and Mo6+ was reduced with 0.2 Torr of H2. All the Rh species and most Mo species were reduced to Rh0 and Mo4+ during the heating to 570 K. The thickness of the Mo layers after the reduction was calculated by a numerical analysis based on the XPS intensity ratio and a uniform adlayer stacking model. The calculated averaged thickness was about 0.3 nm, suggesting the formation of Mo species with a monolayer order on the metallic Rh particle surface. The O 1s XPS showed the presence of Mo-OH and Mo-O oxygen species with a ratio of 1.4 in reduced Rh-MoOx/C. Considering the presence of residual Mo6+ species with a large number of Mo6+-O bonds, the ratio of Mo4+-OH to Mo4+-O is estimated to be 2, suggesting the formation of MoO(OH)2. The DFT calculations of various monomeric MoOxHy species on the Rh(111) surface were carried out, and the stability was examined by the ab initio thermodynamic approach. The relative stability is changed when the surface coverage of hydrogen atoms is taken into account. The MoO(OH)2 species is calculated to be the most stable species in the presence of surface hydrogens over a wide range of conditions, including those for the catalytic reaction (ca. 400 K, 100-1 MPa H2).