Abstract
A series of in-situ experiments were carried out on mechanisms and microkinetic modeling analysis was conducted of a CuMnZn (ca. 28.0 wt.% Cu, 23.3 wt.% Mn, and 48.7 wt.% Zn) catalyst for the partial oxidation of methanol reaction. In comparison with CuZn (ca. 29.2 wt.% Cu, and 70.8 wt.% Zn), CuMnZn catalyst with the structure of copper-manganese spinel CuMn2O4 performed with higher methanol conversion and hydrogen selectivity. During POM reaction, copper and manganese might reduce to more active species, such as Cu0, Cu+, and Mn2+, and enhance the adsorption at lower temperature. Furthermore, ethoxy and monodentate formate must be the dominators below ignition temperature (≦180 ℃). In contrast, when reaching ignition temperature (>180 ℃), the consumption of formate species were, correspondingly, to generate hydrogen. For more in-depth understanding, kinetic modeling analysis of these data was conducted with rate equations which have rendered it possible to derive six steps comprising methanol adsorption, oxygen adsorption, surface reaction, hydrogen desorption, water desorption, and carbon dioxide desorption. The apparent activation energy of 16.5 kcal/mole of CuMnZn catalyst, in contrast, is much lower than a general CuZn-based catalyst and can be initiated at lower temperature. Thus, loading manganese with copper seems to be a synergistic phenomenon which would lead to effective catalytic activity in the partial oxidation of methanol. A simple and very inexpensive method was created to modulate oxygen vacancies on the non-precious metallic CuZn-based catalyst surface. The identification and quantification of these oxygen vacancies on vZ (ZnO containing oxygen vacancies) and the catalytic activities of CvZ (Cu on ZnO containing oxygen vacancies, ca. 30 wt.% Cu and 70 wt.% Zn) during partial oxidation of methanol (POM) reaction are discussed. The vZ was calcined in a nitrogen atmosphere at various temperatures (450 ℃, 500 ℃ and 550 ℃), and catalytic activities of CvZ catalysts prepared in deposition precipitation (DP) and co-precipitation (CP) and CZr (Cu on ZrO2, ca. 30 wt.% Cu and 70 wt.% Zr) catalysts were performed. Both CP-CvZ-450 and DP-CvZ-450 catalysts present excellent catalytic performance with 100% of CMeOH and 95% of SH2 at 250℃. They also maintained 70% of CMeOH and 75% SH2 at 150℃; especially, Sco was kept at 0~4% at T < 250 ℃, with outstanding stability for DP-CvZ-450 catalyst, as well. Moreover, a CO-free situation could be achieved for both DP-CvZ-500 and DP-CvZ-550 which contain more oxygen vacancies. These oxygen vacancies on the surface: enhanced an affinity for adsorbing reactant oxygen atoms; induced decomposition of intermediates species, and; can even catalyze CO oxidation at a lower temperature.