First-Principles Study about the Effect of Coverage on H2 Adsorption and Dissociation over a Rh(100) Surface

The adsorption and dissociation of H-2 with different coverages over the Rh(100) surface have been systematically investigated to probe into the effect of coverage on H-2 adsorption and dissociation. Here, the results are obtained using the density functional theory (DFT) method together with the periodic slab model. Both the parallel and vertical modes of H-2 adsorption on the Rh(100) surface have been identified, and the detailed studies corresponding to H-2 adsorption and dissociation at different coverages are presented. Our results show that the parallel mode of a single H-2 adsorbed on the Rh(100) surface is more favorable than the vertical mode, in which the top site is the most stable adsorption site. However, the parallel modes of single H-2 adsorbed at the bridge and 4F hollow sites, as well as the vertical mode of single H-2 adsorbed at the 4F hollow site are all the dissociative adsorption. On the other hand, with the increasing of H-2 coverage from low to high, the most stable adsorption configurations of H-2 is the parallel adsorption mode at the top site, and the adsorption energies of these adsorbed H-2 molecules will decrease gradually until the saturated adsorption with H-2 coverage of 6/12 ML, further, the dissociation of these adsorbed H-2 molecules is more favorable both kinetically and thermodynamically than their desorption, suggesting that the dissociation of the adsorbed H-2 molecule is more favored than their desorption. Finally, considering the dissociative adsorption of the single H-2 molecule with the parallel modes at the bridge and 4F hollow sites, as well as the vertical mode at the 4F hollow site, our results still show that the adsorptions of H-2 with different coverages at these sites are still the dissociative adsorption with the dissociative H atoms adsorbed on the Rh surface. Therefore, H-2 dominantly exists in the form of H atoms on Rh catalyst under realistic conditions.