Role of MgO over γ-Al2O3-supported Pd catalysts for carbon dioxide reforming of methane

Abstract

A series of Pd/Al2O3 catalysts with varying Mg loading (1, 3, 7 and 10wt%) were investigated for carbon dioxide reforming of methane. It was observed that the initial catalytic activities and long-term stabilities in terms of both CO2 and CH4 conversions increased with increasing Mg content when its loading was below 7wt%. When Mg content was up to 10wt%, the initial activity and stability decreased. Moreover, Pd7Mg/Al2O3 displayed the highest H2 and CO yields. Characterization results conducted on catalysts before and after reaction test demonstrated that MgO mainly presented in amorphous form for catalysts with additive content below 7wt%; while a fraction of MgO transformed into crystalline form when more additive was introduced to the catalyst. The amorphous MgO significantly enhanced surface metal dispersion, decreased average Pd crystallite size and thus improved resistances against both metal sintering and carbon deposition, which contributed to the enhanced initial activities and long-term stabilities. On the contrary, the crystalline MgO decreased exposed Pd active sites, caused metal sintering and thus led to increased amount of carbon deposition owing to both its negative decoration effect on surface metal and deterioration effect on textural characteristics of support. These factors were responsible for the worse catalytic performance of Pd10Mg/Al2O3 as compared to that of Pd7Mg/Al2O3.