CHx (x=1-3) species, central in many catalytic reactions occurring on transition metal surfaces, have been extensively studied from both of experimental and theoretical points of view. Among them, rhodium is a unique catalyst due to its high selectivity and activity for the synthesis of higher oxygenated compounds, especially in methane partial oxidation to syngas and subsequent C2-oxygenates. Although significant efforts have been pursued on the experimental characterization of small hydrocarbons on Rh surfaces, there are still many remain. For example, in CO hydrogenation over promoted Rh catalysts it is widely accepted that during the formation of C2-oxygenates, CO is inserted into the surface-alkyl group and generates CHxCO species. There is, however, no agreement on which CHx species will be inserted by CO. [1-2] To unravel the microscopic mechanisms, it is mandatory to study the elementary steps for insertion which begins with co-adsorption of CHx and CO on these surfaces, and studied by density functional theory calculations.[3] It is found that CHx (x=1-3) radicals prefer three-fold hollow sites on Rh(111) surfaces, and the bond strength between CHx and Rh(111) follows the order of CH3 < CH2 < CH. When the coverage is larger than 1/4 ML, considerable repulsion is built up between adsorbates. We find that surface work function decreases after CHx adsorption due to charge transfer from the radicals to the substrate. For adsorbed CH3, charge back donation from the substrate to C-H bond is identified and results in C-H stretch mode-softening. The mode-softening is however prevented by co-adsorbed CO via charge competition. Conversely, C-O stretch is softened by enhanced backfilling of anti-bonding states of five sigma orbital from the CH3. Finally, the C 1s surface core level shifts (SCLS) for CHx with and without the presence of CO are calculated, and compared with experiments.
Reference: [1] H. Treviño, G. D. Lei, W. M. H. Sachtler, J.Catal. 154, 245 (1995). [2] B. S. Bunnik, G. J. Kramer, J. Catal. 242 (2), 309 (2006). [3] B. Hammer, L. B. Hansen, J. K. Nørskov, Phys. Rev. B 59, 7413 (1999).
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