Reactivity of Methanol Steam Reforming on ZnPd Intermetallic Catalyst: Understanding from Microcalorimetric and FT-IR Studies

Methanol steam reforming (MSR), catalyzed by Pd/ZnO, is a promising process to produce onboard hydrogen for fuel cell. The reactivity of Pd/ZnO, especially selectivity to CO2 and H2, changes with the formation of ZnPd intermetallic compound and ZnPd–ZnO interface. In this work, we measured the adsorption energies and natures of adsorbed species on ZnO, Pd/ZnO, and ZnPd/ZnO catalysts by combining adsorption microcalorimetry and infrared spectroscopy with the reactants (methanol and water) and intermediate (formaldehyde) of MSR as probe molecules and correlated the adsorption energies to the reactivities of the samples. ZnO exhibits weak molecular adsorption for methanol but strong interaction with water. In contrast, the adsorption energy gap between methanol and water decreases on Pd/ZnO and disappears on ZnPd/ZnO. This might be responsible for the highest activity of MSR in ZnPd/ZnO, since methanol could competitively adsorb and react with water. Although the introduction of Pd into ZnO lowers the thermodynamic stability of adsorbed formaldehyde, the formed ZnPd intermetallic compound strengthens the bonding of adsorbed formaldehyde, allowing the further reaction with water that results in the progress of the reaction pathway to CO2 and H2. This might be an important factor for the high selectivity to CO2 and H2 on ZnPd/ZnO.