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ZnO-supported palladium-based catalysts have been shown in recent years to be both active and selective towards the steam reforming of methanol, although they are still considered to be less active than traditional copper-based catalysts. The activity of PdZn catalysts can be significantly improved by supporting them on alumina. Here we show that the Pd/ZnO/Al 2 O 3 catalysts have better long-term stability when compared with commercial Cu/ZnO/Al 2 O 3 catalysts, and that they are also stable under redox cycling. The Pd/ZnO/Al 2 O 3 catalysts can be easily regenerated by oxidation in air at 420 ºC followed by re-exposure to reaction conditions at 250 ºC, while the Cu/ZnO based catalysts do not recover their activity after oxidation. Reduction at high temperatures (>420 ºC) leads to Zn loss from the alloy nanoparticle surface resulting in a reduced catalyst activity. However, even after such extreme treatment, the catalyst activity is regained with time on stream under reaction conditions alone, leading to highly stable catalysts. These findings illustrate that the nanoparticle surface is dynamic and changes drastically depending on the environment, and that elevated reduction temperatures are not necessary to achieve high CO 2 selectivity.

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The selectivity towards CO2 during steam reforming of methanol on Pd increases in the order Al2O3 < ZrO2 < ZnO. However, conventional catalyst preparation can damage the ZnO surface, even causing complete dissolution. The faceted, prismatic ZnO crystals in the support (Aldrich) get easily destroyed during catalyst preparation. We show in this work that, by using organic precursors, the faceted ZnO particles can be preserved. The role of ZnO morphology on reactivity for methanol steam reforming (MSR) is explored. Since the MSR reactivity and selectivity is also a function of the particle size of the nanoparticles as well as the presence of the PdZn ordered alloy phase, we have controlled for both these parameters to derive the true influence of the support. We find that the catalyst prepared from an organic precursor is more active than one prepared from acidic precursors, despite having similar particle size and extent of bulk PdZn ordered alloy formation. The results suggest that preserving certain ZnO surfaces is beneficial, and the ZnO support may play an important role in the overall reaction of methanol steam reforming.

No abstract