Hydrogen production by partial oxidation of methanol over gold catalysts supported on TiO 2-MO x (M = Fe, Co, Zn) composite oxides

Abstract

Hydrogen production by partial oxidation of methanol (POM) has been investigated over Au/TiO 2-MO x (M = Fe, Co and Zn) catalysts in the temperature range of 423–548 K. The catalysts were characterized by ICP, BET, XRD, TEM and XPS analyses. The XRD analysis confirms the desired structure and phase purity of Fe 2O 3, Co 3O 4, ZnO and TiO 2 samples and the presence of gold in these materials. TEM observations show that the gold particles are stabilized against sintering during calcination and after catalytic tests, in the presence of MO x in Au/TiO 2 catalysts. The XPS analysis detects the existence of metallic gold (Au 0), non-metallic gold (Au δ+ ) and Au 2O 3 species in the uncalcined catalyst samples both before and after reaction, and the existence of metallic gold (Au 0) and Au 2O 3 species in the calcined catalyst samples. The catalytic activity of Au/TiO 2 for the POM reaction to produce hydrogen is improved by using additional support (MO x ), probably due to a combination of factors, such as increasing the mobility of the lattice oxygen, maintaining the adequate oxidation state of the active gold particles and controlling the sintering of gold particles. Therefore, MO x can act as a structural promoter and/or as a cocatalyst. The most active catalyst is Au/TiO 2-Fe 2O 3. Although Fe 2O 3 in Au/TiO 2 catalysts increases the catalytic activity, a surfeit of Fe 2O 3 lowers the activity for hydrogen formation. Calcination of the catalyst samples results in a decrease of the catalytic activity. The sample dried at 373 K in air exhibits the highest activity for POM reaction. Both methanol conversion and hydrogen selectivity are increased with increase in reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation, steam reforming and decomposition. CO produced by methanol decomposition and/or by reverse water gas shift is subsequently transformed into CO 2 and H 2 by the water gas shift and/or CO oxidation.