Cu/ZnO/Al2O3 catalysts for oxidative steam reforming of methanol: the role of Cu and the dispersing oxide matrix

Cu/ZnO/Al2O3 catalysts (5–45 Cu at.%) derived from layered double hydroxide (LDH) precursors were studied for oxidative steam reforming of methanol (OSRM). The precursors were prepared by homogeneous precipitation with urea. The catalysts were obtained by thermal decomposition of the precursors and subsequent reduction in H2 stream. XRD, SEM, N2 adsorption, TPR and NH3 TPD techniques were employed for characterization. Catalytic activity tests were carried out in a fixed bed flow reactor at T = 200–400°C, H2O/CH3OH/O2 molar ratios = 1.1/1/0.12 (CH3OH concentration = 17.8%), GHSV = 60000 h-1. Tests of simple steam reforming (SRM), partial oxidation (POM) and CH3OH decomposition (DEC) were also carried out. TPR measurements showed that redox properties depended on the composition of the samples and on the nature of the phases present in the precursors. The area of metallic Cu, measured by N2O passivation method, was correlated to Cu content. The size of Cu particles was smaller than 10 nm for Cu content up to 18 at.%. NH3 TPD measurements showed acid sites with a wide strength distribution, the strongest ones being mainly related to Al2O3 or Zn aluminate. Catalytic activity was influenced by the chemical composition: kinetic constants for OSRM varied unevenly with Cu surface area, while those for SRM increased with Cu surface area. A reaction mechanism agreeing with OSRM, SRM and DEC data was hypothesized. The mechanism involved an oxidation-reduction cycle of Cu and also the participation of the oxide matrix.