Palladium-zinc bimetallic catalysts for the steam reforming of methanol

In this work, we have prepared Pd/ZnO/Al2O3 catalysts that exhibit remarkable stability to in-situ oxidation reduction cycles and show higher activity than Cu/ZnO-based catalysts. These catalysts were characterized by XRD, TEM, EDS, and FTIR in order to elucidate their bulk and surface characteristics. We find that the nanoparticle surface is dynamic and changes drastically depending on the environment. The ability of the catalyst to recover its performance, even after high temperature oxidation, is elucidated through the TEM and XRD investigations.;Although most early studies on the steam reforming of methanol were focused on copper-based catalysts, due to their high activity and selectivity, the current focus has shifted away from copper due to its fast deactivation, poor thermal stability, and pyrophoric nature. Group VIII metals, such as Pd, favor methanol decomposition to CO and H2 but Pd/ZnO was found to become selective towards CO2 formation when reduced at temperatures above 300 °C. This was attributed to formation of the PdZn alloy due to spill-over of atomic hydrogen from the Pd metal to the ZnO. While the extent of alloy formation was increased at higher reduction temperatures, our recent research has shown that the selectivity towards CO2 does not increase monotonically with the extent of alloy formation. Particle size, as well as catalyst preparation conditions (aqueous vs. non aqueous) can play a major role. We have found that many of the problems in working with ZnO support can be avoided by co-impregnation of Pd and Zn on alumina. The Pd/Zn/Al2O3 catalyst system shows activity better than the commercial Cu/ZnO/Al203 catalysts along with remarkable stability. The catalytic sites can be reformed under reaction conditions, even after oxidation at elevated temperatures.;We prepared a number of Pd/ZnO/Al2O3 catalysts via impregnation of palladium and zinc nitrate precursors which were exposed to a number of pretreatments. The crystallite size and composition were studied using STEM and EDS. The bulk composition was determined by XRD, while the average surface composition was analyzed by using FTIR. Measurements of the steam reforming of methanol were performed in tubular packed-bed reactors of 1.75 mm ID with the effluent of the reactor being analyzed using gas chromatography.;While the Cu based catalyst shows steady deactivation with time, the PdZn based catalyst remains stable, after the initial drop in activity. Furthermore, we found that the deactivated PdZn catalysts can be regenerated by a short treatment in air followed by exposure to reaction conditions, while the Cu based catalyst could not be regenerated after similar treatment. Upon oxidation, the alloy is broken up to PdO and ZnO, but these species are then reduced back to recreate small nanoparticles with PdZn surfaces under reaction conditions at 250 °C resulting in re-dispersion of the catalyst. Analysis of individual metal particles in the microscope allowed us to determine bulk and surface composition. We found that the particles were uniform in composition with H2 reduction temperatures around 300 °C, while higher reduction temperatures lead to Zn depletion from the catalyst surface resulting in lowered performance. However, we found that the Zn-depleted catalyst surface could be regenerated simply by exposure to reaction conditions. We hypothesize that the surface of the catalyst is in a meta-stable state which reorders and reaches equilibrium under reaction conditions.;The inherent benefit for Pd/ZnO-based based catalysts, over Cu/ZnO-based ones, is that the active site for the steam reforming of methanol is an alloyed PdZn surface on the nanoparticle. This alloy surface is dynamic and changes depending on the gas environment. We find that the reaction environment helps to create the optimal surface composition, leading to catalysts that can be easily regenerated and/or re-dispersed, a potential benefit for steam reforming catalysts. PdZn alloy methanol steam reforming catalysts are highly stable, in contrast to Cu/ZnO-based catalysts.