No abstract

A new class of copper, zinc, and gallium mixed oxides (CuZnGaO x ) with different chemical compositions obtained by a coprecipitation technique is identified as a highly active catalyst for the low-temperature, direct steam reforming of methanol to supply hydrogen gas to portable fuel cell devices. Their catalytic activity and selectivity are found to be critically dependent on the copper surface area, catalyst structure, and metal–support interaction, etc. As a result, temperature-programmed reduction has been used to investigate the copper ion reducibility and resulting copper speciation; N2O chemisorption and advanced microscopies to determine specific copper surface area, dispersion, and particle size; XRD to investigate the catalyst structure; EPR spectroscopy to probe the environment of Cu2+ species; and AC impedance spectroscopy to probe the mobility of trapped ions in solids. It is proposed that Ga incorporation into Cu–Zn oxide leads to the formation of a nonstoichiometric cubic spinel phase containing interstitial Cu+ ions, which can produce in situ a high population of extremely small 5 Å copper clusters at high dispersion on a defective ZnGa2O4 surface for effective catalysis.

Utilization of hydrogen gas (and carbon dioxide) from methanol steam reforming reaction directly without CO post-treatment to supply proton exchange membrane fuel cells for mobile applications is an attractive option. CuZnGaOx based mixed oxides prepared by co-precipitation are found to be active as catalysts for the reforming reaction. It is also found that the use of lower temperature and a faster substrate flow rate with a shorter contact time with the catalyst bed can significantly reduce the CO level in the product gas stream. At 150 °C this class of oxides gives a decent methanol conversion but can also totally suppress the CO production at a short contact time, which is in a sharp contrast with conventional CuZnOx based catalysts that give a significant degree of CO formation. Characterization using Diffuse Reflectance Infrared Fourier Transform (DRIFT) analysis presented in this work clearly suggests the importance of the interface between copper metal-defective oxides for the catalysis. Mechanistic aspects of this reaction are therefore discussed in this paper.