Coordination dependence of carbon deposition resistance in partial oxidation of methane on Rh catalysts

Abstract

In this paper, we used the density functional theory (DFT) calculations for analyzing the property of carbon deposition resistance of size-dependence of Rh-clusters supported on anatase for partial oxidation of methane (POM). In addition, we used three-different coordination number of Rh-based models that were Rh1, Rh4 and Rh13. We considered two major routes for the carbon deposition processes which were CH4* dissociation and CHx* oxidation. The apparent energy barrier of CH4* dissociation on Rh1/Ana-OV (2.95 eV) was much higher than that on Rh4/Ana-OV (1.84 eV) and Rh13/Ana-OV (1.85 eV), whereas, the energy barrier of the reaction (CH2* + O* → CH2O*) enlarged with the growing coordination number: Rh1/Ana-OV (0.58 eV) < Rh4/Ana-OV (0.71 eV) < Rh13/Ana-OV (2.23 eV). Thus, the Rh-catalysts with low coordination number, Rh1/Ana-OV, could be the one with the strong resistance for the carbon deposition and high catalytic activity. In fact, the results of the energetic span model analysis indicated that the catalytic activity of Rh1/Ana-OV (3.32 × 106 s−1) was better than that of Rh4/Ana-OV (3.18 × 104 s−1) and Rh13/Ana-OV (1.65 × 10-2 s−1) at the temperature of 973 K. The possible reasons why the single Rh catalyst (Rh1/Ana-OV) has the strong carbon deposition resistance property can be explained as the dissociation of CHx* often requires more active sites because its products such as CH* or C*, which are much more active, and are usually adsorbed in threefold hollow site or more. However CHx* oxidation reaction needs relatively less active sites because the products like CH2O* is a more saturation species and often adsorbed on top or bridge sites. Moreover, the low coordination number sites can meet such kind of requirement. Thus, single-atom catalysts are the candidates to reduce the carbon deposition for POM processes: its low coordination number contributes to the CHx* oxidation process and hinder the CH4* dissociation except the first C–H bond scission step.