Solid-solid palladium-catalysed water reduction with zinc: mechanisms of hydrogen generation and direct hydrogen transfer reactions

Facile generation of hydrogen gas from water takes place under moderate con ditions in the presence of zinc powder and catalytic palladium on carbon; 8 2% conversion of zinc is obtained. An unusually large kinetic isotope effec t is observed using D2O (k(H)/k(D) = 14), which may reflect the cleavage of both O-H bonds in the rate-determining step. Experiments using D2O-H2O mix tures evidence that water molecules adsorbed on the catalyst surface underg o H-D exchange reactions (with molecules from the solvent bulk) that are ap proximately 100 times faster than the hydrogen generation reaction. The pri mary factors in this system appear to be palladium-hydrogen and zinc-oxygen interactions. Conversely, in the presence of an organic hydrogen acceptor, such as benzaldehyde, a different course is realised, consisting of direct hydrogen transfer from "zinc-activated" water to the substrate, without th e participation of Pd-H intermediates. Quantitative hydrogenation of benzal dehyde to benzyl alcohol, and of aromatic nitro compounds to the correspond ing amines, is obtained. Another application of the above system is the spe cific deutero-dehalogenation of aromatic halides. Possible mechanisms and t he implications of a chemical reaction involving two macroscopic solid part icles are discussed.