Density Functional Theory Study Of Gas Adsorption And Dissociation On Rh And RhMn Surfaces

Catalytic conversion of syngas (carbon monoxide and hydrogen) from reforming of natural gas and coal to hydrocarbon (Fishcher-Tropsch synthesis) and oxygenates attracts extensive efforts due to the limited resources of petroleum available on the planet. Among them, rhodium (Rh) is a unique catalyst for its higher selectivity on the production of C2 oxygen-containing compounds, such as acetic acid, ethanol and acetaldehyde. Moreover, the catalytic activity and/or selectivity can be improved further by introducing manganese (Mn) in Rh catalysts. Theoretical study on the atomistic level can help understand the mechanism of syngas conversion. In the present paper, density functional theory calculations have been employed to study the effects of alloy on energetics and preferential adsorption sites of atomic (H, C, N, O, S), molecular (N2, NO, CO) and radical (CH3, OH) adsorption on RhMn (111) alloy surface, the effect of Mn on CO dissociation on flat and step RhMn surface are also investigated.The calculation results show that independent on adsorbates (not including O and OH), the interactions between these species and Rh atoms are preferential, and enhanced in general due to the ligand effects induced by Mn nearby. In contrast, oxygen-containing species (atomic oxygen and hydroxyl) prefer to coordinate with Mn atom due to the significant hybridization between oxygen and Mn, a manifestation of the ensemble effects. The order of the binding energies on RhMn alloy surface from the least to the most strongly bound is N2 < CH3 < CO < NO < H < OH < O < N < S < C, which is also found on Rh (111) surface, due to the distinct reactivity of these species overwhelming the ligand/ensemble effects present in surface alloy. The calculation of CO dissociation on close-packed Rh (111), RhMn (111), stepped Rh (553) and RhMn (553) surfaces shows that, the dissociation of CO is endothermic with respect to chemisorbed CO on these surfaces, and the activation barrier is sensitive to the surface structure. More open and stepped surfaces which provide strong binding sites for C and O adatoms are favorable for CO dissociation. The addition of Mn to the Rh surface can lower the dissociation barrier by increasing the activity of Rh and strong bonding with O, and this effect is not structure sensitive. The CO dissociation barrier follows following sequence, Rh (111) > RhMn (111) > Rh (553) > RhMn (553).