Alkene chemistry on the palladium surface: nanoparticles vs single crystals

The adsorption of trans-2-pentene, cis-2-pentene, and 1-pentene on Pd(111) and Pd/Al2O3 model catalysts has been studied using temperature-programmed desorption (TPD). Each molecule reacts in an identical manner on the Pd(111) surface. Three distinct molecular adsorption states are observed, which have been assigned to multilayer, pi-bonded pentene and interchanging di-sigma-bonded pentene/pentyl groups. The latter species undergo coverage-dependent stepwise dehydrogenation. For trans-2-pentene on D-2 preadsorbed Pd(111), a H-D exchange reaction occurs, resulting in D-substituted pentene, which molecularly desorbs or dehydrogenates on heating similar to nonexchanged pentene. Pentane is not formed as a hydrogenation product on Pd(111). On Pd nanoparticles, dehydrogenation proceeds more readily than on Pd(111). In addition, the extent of the H-D exchange reaction is considerably greater on particles. In contrast to Pd(111), the hydrogenation reaction occurs on the Pd particles. Data show that di-sigma-bonded pentene is the precursor for both the H-D exchange reaction and the pentane formation, with each occurring via a pentyl group. This pentyl group reacts either by beta-H elimination to form pentene or by reductive elimination to form pentane. The results for pentenes are compared with those for ethene. We have found that, under the low-pressure conditions studied, alkene hydrogenation only occurs in the presence of subsurface hydrogen. The accessibility of the subsurface hydrogen atoms is enhanced on the particles, due to the nanoscale dimensions, relative to that on crystals. The results are rationalized on the basis of a model of overlapping desorption states, which may predict both the feasibility of alkene hydrogenation on Pd catalysts and the active species involved in the reaction. (C) 2004 Elsevier Inc. All rights reserved.