The effect of SMSI (strong metal-support interaction) behavior on CO adsorption and hydrogenation on Pd catalysts: II. Kinetic behavior in the methanation reaction

Abstract

The specific activity of Pd in the methanation reaction can show a 200-fold variation depending upon the support used. In contrast, the CH 4 turnover frequency is independent of Pd crystallite size, at least over the range of 3–30 nm particles. The most active catalyst is TiO 2-supported Pd in which an SMSI state has been induced, and silica-supported Pd is the least active. Turnover numbers increase in the following order: Pd SiO 2 ⪡ Pd SiO 2-Al 2O 3 , Pd Al 2O 3 − Pd TiO 2 (448 K) < Pd TiO 2 (SMSI). All CO partial pressure dependencies were near zero, while H 2 partial pressure dependencies shifted from unity to near one-half for the TiO 2-supported catalysts. A reaction model for Pd is proposed which is consistent with this behavior, with ir spectra of CO adsorbed on these Pd surfaces, and with recent results in the literature on CO hydrogenation. This model involves no assumptions regarding CO coverage, and assumes CH 4 forms from surface carbon produced by a hydrogen-assisted CO dissociation step. The higher N values for Pd TiO 2 in the SMSI state are consistent with this model and appear to be a consequence of much higher surface concentrations of hydrogen compensating for markedly reduced CO coverages, thereby resulting in a rate enhancement.