Ab Initio Periodic Density Functional Studies Of The Adsorptions Of Atoms And Molecules On Rh(111)

Ab initio total energy calculations have been carried out for the adsorption of atoms and molecules on the Rh(lll) surface. Rhodium has been shown to be one of the most effective catalysts in heterogeneous catalysis, which is of great technological and commercial importance, since this metal can catalyze the hydrogenation of CO to produce hydrocarbons, and can also catalyze efficiently the oxidation of CO, NO and some unburned hydrocarbons in controlling automobile exhaust. A detailed knowledge of the chemisorption of atoms and molecules on the rhodium surface would be very useful to gain some insight into the nature of the interaction between adsorbates and substrates and to understand the corresponding catalytic processes. In this work, ab initio total energy calculations using density functional theory with the generalized gradient approximation have been performed for the adsorption of CO, Na, K, Cs and CH3 on the Rh(lll) surface. The geometric, energetic, electronic and vibrational properties of these adsorption systems all have been provided. The preferred adsorption site has been identified and the nature of the interaction between adsorbates and substrate has been discussed.For CO adsorption on the Rh(lll) surface, our results show that atlow coverage CO prefers to adsorb at top site and at high coverage one molecule occupies top site while the other two molecules occupy hep and fcc hollow sites respectively. This is not consistent with the earlier experiment but in good agreement with the recent experimental results. DOS analysis showed that the 3a molecular orbital is not involved in bonding with the surface, but there are mixing between metal d orbital and 4a-, 5a- and l7i-derived states. It was also indicated that the most significant contribution to CO-Rh bonding comes from 5a orbital. To gain insight into the nature of the adsorption state, the work function change as a function of coverage has been examined. We found that there is nearly no charge redistribution during the coverage increasing from 0.25 and 0.33ML, while more charge transfer from substrate to adsorbate as the coverage increases from 0.33ML to 0.75ML. The fundamental vibrational properties of CO adsorption on the Rh(lll) surface have been calculated, especially for the unobserved low frequency vibrational modes. The results show that as the coverage increases the top C-0 stretching frequency increases. The frequency shift of 15cm'1 in the (V3x-73)R30?structure and 30cm"1 in the (2><2)-3CO structure have been attributed to the dipole-dipole coupling and chemical effect respectively. The site assignments, optimized geometries and calculated vibrational frequencies are found to be in good agreement with the experimental results.For the adsorption of Na and K adsorbed in (V3xV3)R30?and (2x2) patterns on the Rh(lll) surfaces, the most stable adsorption sites were found to be hep hollow sites in these cases. The differences in binding energies between the on-top and hep-hollow sites were found to become smaller with increasing size of AM, implying that top site adsorption becomes more possible for larger alkali metal adsorption. This finding isin excellent agreement with experiments. For the adsorption of Cs adsorbed in (V3xV3)R30 and (2x2) patterns on the Rh(lll) surfaces, the most stable adsorption sites were found to be top and hep hollow sites respectively. The adsorbate-substrate interaction is relatively weaker and the differences in binding energies between hep and top sites is smaller, as compared with the results of Na and K adsorption. Therefore, for Cs adsorption, only small displacement of Cs-coordinated Rh atoms is needed to overcompensate this smaller binding energy difference. Our results have suggested that Cs experience smaller substrate electron-density corrugation as a consequence of larger atomic size. This effect may explain the preference for the on-top position for big Cs atom in the (2><2) phase. Our calculations also indicated that partial charge has transferred from AM adsorbate to Rh substrate, which is sli...