Identification of Surface States on Finely Divided Supported Palladium Catalysts by Means of Inelastic Incoherent Neutron Scattering

Abstract

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.

Neutron Scattering Investigations of Hydride Species in Heterogeneous Catalysis

“…The INS study performed by Albers et al on Pd nanoparticles supported on AC showedt hat hydrogenation/dehydrogenation cycles could eliminate subsurface hydrides due to the restructuring of dislocations and lattice defects. [50] It was also shown that high temperature and pressure favored hydrogen spillover,l eading to the formation of surfacea morphous hydrogenated carbon. Analysis of as pent 5% Pd/C catalyst, which was used in the reaction of bromobenzene and styrene to form trans and cis-stilbene, revealed surface deposition of cis-stilbene, despite most of the product being trans-stilbene.…”

“…The INS study performed by Albers et al. on Pd nanoparticles supported on AC showed that hydrogenation/dehydrogenation cycles could eliminate subsurface hydrides due to the restructuring of dislocations and lattice defects . It was also shown that high temperature and pressure favored hydrogen spillover, leading to the formation of surface amorphous hydrogenated carbon.…”

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

“…The detailed identification of the specific properties of the surface atomic layers of carboneous supports used for the preparation of Pd/C catalysts has been studied recently by means of inelastic incoherent neutron scattering. This technique allows the behavior of activated carbons and carbon blacks in the presence of adsorbed hydrogen to be studied and the vibrational states of protons on and inside carbon‐supported PdNPs and PtNPs to be identified 44a. Well‐known and typical applications of such materials are the hydrogenation reactions that are important for the synthesis of fine‐chemicals intermediates, vitamins, and pharmaceuticals 2.…”

Nanopartikel als regenerierbare Katalysatoren: an der Nahtstelle zwischen homogener und heterogener Katalyse

“…Neuere Untersuchungen mit inelastischer inkohärenter Neutronenstreuung befassten sich insbesondere mit den spezifischen Eigenschaften der atomaren Oberflächenschichten von Kohlenstoffträgern für Pd/C‐Katalysatoren. Mit dieser Technik wurde das Verhalten von Aktivkohlematerial und Industrierußen (carbon blacks) in Gegenwart von Wasserstoffadsorbat sowie die Schwingungszustände von Protonen auf der Oberfläche und im Innern der auf dem Kohlenstoffträger fixierten Pd‐NPs und Pt‐NPs untersucht 44a. Häufige und charakteristische Anwendungen solcher Materialien sind Hydrierungsreaktionen zur Herstellung von Zwischenstufen in der Feinchemikaliensynthese sowie von Vitaminen und Pharmazeutika 2.…”