The absorption and locking-in of hydrogen in copper

The mechanism of methanol synthesis from a CO/CO2/H2 feed over a Cu/ZnO/Al2O3 catalyst is reviewed. It is shown that it is the CO2 component of the gas stream which forms the methanol and that Cu metal is the active component of the catalyst. The CO2 is shown to adsorb dissociatively on the Cu forming a surface oxide which on further CO2 adsorption produces a surface carbonate. Hydrogenation of the surface carbonate produces an adsorbed formate, hydrogenation of which is the rate determining step in methanol synthesis. It is a consequence of methanol being produced from CO2 that, at steady state, the surface of the Cu will be in a partially oxidised state. Oxidation of the Cu(110) surface produces a (2×1) O overlayer. Hydrogen desorption studies show that H atoms are both adsorbed on the surface of the Cu and absorbed into the bulk of the Cu. The (2×1) O overlayer forms a barrier to the desorption of H atoms from the subsurface regions of the Cu. Removal of 40% of the O(a) in the (2×1) O overlayer results in a sudden reconstruction of the overlayer to the Cu (211) structure, the process of which allows the desorption of the subsurface H atoms.