The kinetic effect of H2O pressure on CO hydrogenation over different Rh cluster sizes

The effect of H₂O pressure (P_H2O) on CH₄ formation turnover rate (TOF_CH4) was evaluated as a function of Rh cluster size. In-situ DRIFTS and kinetic measurements were used to study the CO hydrogenation at methanation conditions on two Rh/Al₂O₃ catalysts with different cluster sizes (Rh-1nm and Rh-3nm). It was observed a significant effect of P_H2O on TOF_CH4 over Rh-1nm catalysts, while the rate on Rh-3nm resulted insensible to the H₂O pressure. The mean Rh cluster sizes were estimated by H₂ chemisorption, TEM and XPS analyses. It is confirmed the structural sensitivity of the CO methanation reaction on supported Rh catalysts as the TOF_CH4 resulted significantly higher in the larger clusters. Nonetheless, lower apparent activation energy was measured in smaller Rh clusters, which is successfully explained by the energies involved into the parameter of the proposed L-H kinetic model. CO adsorption isotherms were obtained from in-situ DRIFTS experiments at 280–340 °C. The enthalpy and entropy values for CO adsorption indicate that the CO binds more strongly on the low-coordinated surface of Rh atoms. In-situ DRIFTS results demonstrate that the CO coverage is unaffected by P_H2 and P_H2O, regardless of Rh dispersion, thus excluding H* and H₂O* as most abundant surface intermediates of the Kinetics. The infrared and kinetic measurements on both catalysts are consistent with a mechanism of CO bond dissociation assisted by H*. Data are well represented by a single-site-L-H model that successfully captures the effect of P_H2, P_CO and P_H2O on TOF_CH4. This model contains a parameter (k’) that represents the ratio between the rates of C* removal by O* and H* at Rh surface and explains the stronger kinetic effect of P_H2O on TOF_CH4 observed over smaller Rh clusters.