Dramatic Effects of Gallium Promotion on Methanol Steam Reforming Cu–ZnO Catalyst for Hydrogen Production: Formation of 5 Å Copper Clusters from Cu–ZnGaOx

A new class of copper, zinc, and gallium mixed oxides (CuZnGaOx) 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.