Identification of surface states on finely divided supported palladium catalysts by means of inelastic incoherent neutron scattering

The purpose of the present investigation was to utilize the inelastic incoherent neutron scattering (INS) technique to reveal changes at the surface of technical catalysts under the influence of hydrogen in gas/solid interactions and during chemical reactions in a liquid-phase process. The formation and the properties of supported palladium hydride and changes of the hydrogen-related surface chemistry of the corresponding activated carbon supports in 20% Pd/C catalysts after short-term and long-term hydrogen cycling at different hydrogen pressures and temperatures were studied. The spectra indicate that hydrogenation of the activated carbon support by hydrogen spillover occurs to, partly, give a material that strongly resembles a-C:H (amorphous hydrogenated carbon). Indications for different relaxation phenomena and long-range phase coherence inside of supported particles of palladium hydride compared to hydrogenated palladium black were obtained. A 5% Pd/C catalyst after use in C-C coupling reactions, the Heck reaction of bromobenzene and styrene to stilbenes, was also studied after subsequent solvent extraction. Evidence for a preferential adsorption and accumulation of cis-stilbene at the catalyst surface was obtained. INS allows identification of a certain isomer from a complex reaction mixture preferentially adsorbed at the surface of a finely divided industrial heterogeneous catalyst.