Paper SS1-MoA8
H/D Exchange on Pd Nanoparticles: Effects of cis-2-butene Co-adsorption, and Implications for Isomerization and Hydrogenation of Alkenes over Pd

Monday, October 18, 2010, 4:20 pm, Room Picuris

On Pd(111), HD production from H₂ and D₂ is believed to occur by a Langmuir-Hinshelwood mechanism: H + D --> HD. However, there has been discussion about the possibility that subsurface hydrogen plays a role in the associative desorption of hydrogen, and consequently the H/D exchange reaction.

The ability of Pd nanoparticles to accommodate weakly bound subsurface hydrogen differs from that of Pd(111) single crystals. As such, we have studied H/D exchange over well-defined Pd nanoparticles and Pd(111) for comparison, in the temperature range between 200 K and 350 K.

Over the Pd nanoparticles, the HD production occurs at a slightly higher rate than over Pd(111), but with a similar temperature dependence. From both the pressure and temperature dependence of the steady state reaction rate, it is not possible to tell if there is an influence from subsurface hydrogen on the HD production, over either the single crystals or the nanoparticles.

Unexpectedly, co-adsorbed cis-2-butene nearly deactivates the H/D exchange reaction, while the catalyzed isomerization of and hydrogenation of cis-2-butene occur with persistent activity under these reaction conditions – indicating that hydrogen/deuterium is still available on the catalyst despite deactivation of the H/D exchange reaction.

We interpret these results to be indicative of a “portal” model for dissociative hydrogen adsorption in the presence of butene. In a “portal” model, adsorption occurs at dispersed sites on the surface, and from there the adsorbates diffuse to the rest of the surface prior to reaction. Under this interpretation, hydrocarbons block most of the surface sites for hydrogen, thus inhibiting both dissociative adsorption of hydrogen molecules and recombination of adsorbed hydrogen atoms. Consequently, hydrogen molecules dissociate on the minority of open spaces remaining, and the formed hydrogen atoms diffuse between the organic adsorbates. These hydrogen atoms then react with organic adsorbates, while only a small percentage of hydrogen atoms find an open space at the same time as a second hydrogen/deuterium atom to desorb with: thereby preventing HD formation under these conditions, while persistent isomerization and hydrogenation occurs. These findings may have important kinetic and mechanistic implications for alkene hydrogenation and isomerization over Pd catalysts, and potentially other transition metal catalysts.