| AVS 62nd International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS+AS+EN-TuA |
| Session: | Mechanistic Insight of Surface Reactions: Catalysis, ALD, etc. - II |
| Presenter: | Satoshi Ohno, The University of Tokyo, Japan |
| Authors: | S. Ohno, The University of Tokyo, Japan M. Wilde, The University of Tokyo, Japan K. Fukutani, The University of Tokyo, Japan |
| Correspondent: | Click to Email |
Pd-dissolved hydrogen is an essential ingredient in the highly selective hydrogenation of olefinic C=C double bonds catalyzed by Pd, yet the particular role played by H below the surface has long been debated controversially. Some proposed that absorbed H atoms become directly involved in hydrogenation reactions after they emerge from the metal interior onto the catalyst surface in an energetic state. Others considered that sizeable populations of subsurface sites by absorbed hydrogen indirectly activate surface-adsorbed hydrogen by altering the electronic structure of the catalyst.
To resolve this dispute we have studied the hydrogenation reaction of cis-2-butene to butane on a Pd(110) model catalyst surface with temperature-programmed desorption (TPD) and 1H(15N, ag)12C nuclear reaction analysis (NRA) that reveals the hydrogen distribution on and beneath the surface. TPD demonstrates that the catalytic hydrogenation reaction proceeds efficiently between 160 and 250 K. NRA under the hydrogenation reaction condition, on the other hand, shows that the H concentration in the Pd subsurface region is as small as 0.5 at. %. Thus, the scenario of indirect surface-hydrogen activation through large quantities of H in the subsurface sites appears rather unrealistic for our experimental conditions. We furthermore elucidate that the butane reaction yield scales linearly with the number of Pd-dissolved H atoms that reach the surface after diffusion from the Pd bulk. This observation clarifies that the Pd-catalyzed olefin hydrogenation is triggered by the emergence of bulk-dissolved hydrogen onto the Pd surface. Our NRA H profiles also demonstrate that the catalytic reaction proceeds on the Pd surface fully saturated with chemisorbed hydrogen. This surface hydrogen is considered important, as it possibly prevents deactivation of reactive surface hydrogen species in vacant chemisorption sites.
Finally, the TPD spectrum of butene shows four peaks at 140, 165, 190, and 225 K, suggesting multiple butene-adsorption modes onto Pd(110) surfaces. Reactive TPD experiments in presence of absorbed hydrogen exhibit a significant decrease in the 165 K peak, identifying this feature as the reactive butene species in the catalytic hydrogenation reaction.