Hydrogen Production by Methanol Steam Reforming: Catalytic Performance of Supported-Pd on Zinc-Cerium Oxides´ Nanocomposites

Abstract

Two series of supported palladium catalysts (2 wt%) were prepared on ZnO–CeO2nanocomposites (Zn-to-Ce atomic ratio between 0.5 and 2) obtained by oxalate or carbonate coprecipitation (OC and CC series,respectively). Methanol steam reforming (MSR) reaction was tested in a wide range of temperature(398–623 K) for CH3OH/H2O = 1/1 gas mixtures. Pd on pure CeO2was only able to decompose methanolto CO, under 523 K, but the reverse water gas shift reaction took place at higher temperatures. The MSRreaction only occurred in the presence of zinc oxide and the selectivity to CO2was higher for the CCseries, due to the better dispersion of the ZnO phase over these carbonate-derived nanocomposites.Although the CO2selectivity seems to be modulated by the reverse water gas shift reaction, the palladiumsupported on the mixed oxides was more stable than Pd/ZnO, which continuously deactivated. A detailedcharacterization by high resolution atomic microscopy, X-ray photoelectron spectroscopy and a novelcarbon monoxide step chemisorption technique, proved the formation of bulk and surface PdZn alloyingin the ternary catalyst, Pd supported on the nanosized ceria and zinc oxide supports. It is concluded thatalthough a better catalytic stability was observed on the ZnO–CeO2 nanocomposites, the employment oftemperatures higher than 450 K would impose an insurmountable limitation in terms of CO2 selectivity.