The absorption and locking-in of hydrogen in copper

Abstract The mechanism of methanol synthesis from a CO/CO 2 /H 2 feed over a Cu/ZnO/Al 2 O 3 catalyst is reviewed. It is shown that it is the CO 2 component of the gas stream which forms the methanol and that Cu metal is the active component of the catalyst. The CO 2 is shown to adsorb dissociatively on the Cu forming a surface oxide which on further CO 2 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 CO 2 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.