Oxidative Methanol Reforming Reactions on CuZnAl Catalysts Derived from Hydrotalcite-like Precursors

Abstract

Copper–zinc–aluminum (CZA) catalysts prepared by the homogeneous precipitation of chloride salts in the presence of urea to produce hydrogen selectively by partial oxidation and oxyreforming reactions of methanol were used. The crystalline phases of the precursors and of the air-calcined CZA samples were identified by X-ray diffraction. Temperature-programmed reduction revealed that both CuO and CuAl2O4 phases were present in the calcined catalysts. Prereduced catalysts were tested in both partial oxidation (O2/CH3OH=0.3–0.4) and oxyreforming reactions (O2/CH3OH/H2O=0.3 : 1 : 1.1) of methanol exhibiting high activity for CH3OH conversion and very high selectivity for H2 production. The product distributions obtained in the partial oxidation of methanol (POM) reaction as a function of CH3OH and O2 conversion indicate clearly that both conversion and selectivity become strongly dependent on the O2 concentration in the gas phase and much less on the catalyst composition investigated. At low methanol conversion, when O2 is not completely consumed, combustion and decomposition of methanol are the dominant reactions, however when O2 is near or completely consumed, the methanol conversion goes beyond the expected stoichiometric POM values (60 and 80% for O2/CH3OH molar ratios of 0.3 and 0.4, respectively). For the oxyreforming reaction, the simultaneous increase in H2 selectivity at the expense of water, with a less marked increase of CO selectivity, points to the contribution of steam reforming of methanol. The Auger parameter of copper of the catalysts used in the partial oxidation (1849.1 eV) and in the oxyreforming (1849.3 eV) reactions indicates that Cu2O species are present on the active catalyst surface. These Cu2O structures, developed on the catalyst surface during on-stream operation, appear to be sufficiently large to be also observe d by X-ray diffraction of the catalysts used in both reactions.