Probing crystalline phase effect of rare earth metal oxides (REO) on Cu/REO (RE=Gd, Eu, Sm) catalysts for methanol steam reforming (MSR) to produce H2

Abstract

In this work, to study the phase structure effect, three groups of Cu/REO catalysts were prepared with cubic and monoclinic Gd2O3, Eu2O3 and Sm2O3 supports for MSR reaction to produce H2. Based on CH3OH conversion and H2 yield, the reaction performance of the catalysts ranks as Cu/Sm2O3-M > Cu/Sm2O3–C > Cu/Gd2O3-M > Cu/Gd2O3–C > Cu/Eu2O3-M > Cu/Eu2O3–C. For the same kind of REO, Cu supported on the monoclinic support shows better performance than on the cubic one. Despite the phase structure difference, Sm2O3 is the best support among all the three kinds of REOs. Compared with Cu/REO catalysts prepared with cubic supports, the corresponding catalysts prepared with monoclinic supports generally possess more surface oxygen vacancies, which can generate more surface active oxygen (O2–) and moderate basic sites. Moreover, the contents of Cu + on the catalysts follow the same sequence. The reaction performance is positively related to the amount of these three types of surface sites. But metallic Cu0 species is necessary to maintain the Cu+ ↔ Cu0 redox cycle. Furthermore, on a catalyst with good performance, those vital surface reaction intermediates can be stabilized during the reaction. Cu/Sm2O3-M possesses the largest quantities of these surface sites, and has the appropriate amount of Cu+ and Cu0 after reduction, thereby displaying the optimal performance in all the catalysts. In conclusion, evident support crystal structure effect is observed for Cu/REO catalysts, and a monoclinic phase REO is a better support than the respective cubic phase one.