Hydrogen production by methanol steam reforming: Catalytic performance of supported-Pd on zinc–cerium oxides’ nanocomposites

Abstract

Two series of supported palladium catalysts (2wt%) were prepared on ZnO–CeO2 nanocomposites (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 (398–623K) for CH3OH/H2O=1/1 gas mixtures. Pd on pure CeO2 was only able to decompose methanol to CO, under 523K, but the reverse water gas shift reaction took place at higher temperatures. The MSR reaction only occurred in the presence of zinc oxide and the selectivity to CO2 was higher for the CC series, due to the better dispersion of the ZnO phase over these carbonate-derived nanocomposites. Although the CO2 selectivity seems to be modulated by the reverse water gas shift reaction, the palladium supported on the mixed oxides was more stable than Pd/ZnO, which continuously deactivated. A detailed characterization by high resolution atomic microscopy, X-ray photoelectron spectroscopy and a novel carbon monoxide step chemisorption technique, proved the formation of bulk and surface PdZn alloying in the ternary catalyst, Pd supported on the nanosized ceria and zinc oxide supports. It is concluded that although a better catalytic stability was observed on the ZnO–CeO2 nanocomposites, the employment of temperatures higher than 450K would impose an insurmountable limitation in terms of CO2 selectivity.