Production of hydrogen from methanol over binary Cu/ZnO catalysts: Part II. Catalytic activity and reaction pathways

Abstract

The activity for conversion of methanol into hydrogen was investigated over binary Cu/ZnO catalysts derived from precursors prepared by two different techniques, viz. oxalates formed in microemulsion and hydroxycarbonates formed in aqueous solution. Some distinct differences in the reaction pathways were observed. During partial oxidation of methanol under a sub-stoichiometric oxygen/methanol ratio, the microemulsion materials exhibited considerably higher combustion activity in the low-temperature region than a catalyst prepared in aqueous solution. Over the former, oxygen was quickly converted by methanol combustion, after which steam reforming was initiated, producing hydrogen at the expense of water and gradually decreasing the net heat of reaction. Hence, a reaction sequence for the partial oxidation reaction over microemulsion catalysts is proposed, consisting of consecutive methanol combustion and steam reforming, followed by decomposition when all oxygen has been consumed. Over the hydroxycarbonate catalyst, the reaction ignited at a higher temperature, directly producing hydrogen by partial oxidation of methanol. When the two types of catalysts were evaluated in the steam reforming reaction, all catalysts displayed the typical S-shaped dependence of methanol conversion on temperature. However, there was a downward shift in the temperature at which methanol reached complete conversion, favouring the hydroxycarbonate material. Hydrogen was produced selectively over all catalysts, but carbon monoxide formation was more pronounced over the microemulsion materials. The differences in catalytic behaviour are discussed in terms of catalyst morphology and the valence state of Cu in the working catalyst.