Partial oxidation of methanol to produce hydrogen over Cu-Zn-based catalysts

In this work Cu/ZnO and Cu/ZnO/Al2O3 catalysts have been studied for the partial oxidation of methanol with oxygen to produce hydrogen. These Cu-Zn based catalysts showed high activity for the partial oxidation of methanol and it was found that the catalytic activity is directly related to the copper metal surface area. In the series Cu-Zn with copper relative content of 20-70 wt%, the catalyst Cu40Zn60 (Cu 40 wt% and Zn 60 wt%) which showed the highest copper area gave the best results for the partial oxidation of methanol. The activation energies and TOF (turnover frequencies) varied with the Cu-Zn catalyst composition. For catalysts with low copper loading very high E-a and TOF were obtained (for Cu30Zn70Ea = 482 kJ/mol and TOF ca. 200 min(-1) at 497-499 K) whereas for higher copper contents the E-a and TOF decreased tending to constant values (for Cu70Zn30Ea = 71 kJ/mol and TOF = 160 min(-1) at 497-499 K). These results are discussed in terms of a possible effect of the Cu-ZnO interaction which depends on the catalyst composition. Catalytic experiments with Cu40Zn55Al5 (Cu 40 wt%, Zn 55 wt% and Al 5 wt%) showed that the presence of aluminium has an inhibiting effect producing slightly lower methanol conversion. On the other hand, higher selectivities for H-2 and CO2 were obtained with only traces of the undesirable carbon monoxide. Moreover, the Al is very important for catalyst stability and life-time experiments showed that Cu40Zn55Al5 is stable during the partial oxidation of methanol with no significant change in activity and selectivity even after 110 h operation at 503 K. The catalyst Cu40Zn60 with no Al, deactivates rapidly after 20 h reaction at 503 K. Experiments using N2O as oxidant showed higher activity to convert methanol but producing large amounts of H2O and CO. The impregnation of catalyst with Na produced similar effect increasing the selectivity for H2O and CO. The results presented seem to indicate that the copper metal is active for partial oxidation of methanol to H-2 and CO2 whereas Cu+1 favour the formation of H2O and CO. CU+2 as CuO shows very low activity for methanol conversion producing only CO2 and H2O. (C) 1997 Elsevier Science B.V.