Abstract
The hydrogen production by the catalytic partial oxidation of methanol (POM) over rare earth oxide (RE = La, Dy, Gd, and Ce)-modified ZnO-supported silver catalysts, as well as silver supported on Ce(1-x)Gd(x)O(y-)modified ZnO catalysts, was investigated. The effect of the temperature on the activity was studied in the range between 150 and 400 degrees C, and the catalyst stability was monitored with time-on-stream (TOS). The addition of rare earth metal oxide promoters resulted in a significant improvement in the catalytic performance. The optimal performance in methanol oxidation was achieved using AgCe20Zn, which exhibited a hydrogen selectivity of 90.8% with 95.2% methanol conversion at 350 degrees C; however, the catalyst suffered from marked deactivation with TOS. The good stability of the Ag/ZnO catalyst was verified using a gadolinium promoter. Doping of the AgCe20Zn catalyst by Gd greatly enhanced its life span over at least 24 h on-stream and markedly reduced the CO content (down to <1%). X-ray diffraction analysis indicated the formation of a CeGd solid solution; hence, more oxygen vacancies were generated by the substitution of Ce4+ cations with Gd3+. It is proposed that the oxygen vacancies in the AgCeGdZn catalysts provide a method to increase the Ag-support interaction and inhibit metal sintering. H-2-temperature-programmed reduction and transmission electron microscopy results confirmed that the reducibility and dispersion of the Ag particles on the AgCe20Zn catalyst was greatly enhanced by Gd doping, which could contribute to the good long-term activity observed for the POM reaction over AgCeGdZn catalysts.