Acetaldehyde adsorption and catalytic decomposition on Pd(1 1 0) and the dissolution of carbon

The reaction of acetaldehyde with the Pd(1 1 0) surface has been studied using a molecular beam reactor, TPD and LEED. Below 270 K acetaldehyde sticks to the surface with a high initial probability (∼0.8), but no gas phase products evolve. When the reaction is run at >270 K, hydrogen evolves into the gas phase early in the reaction together with methane in a non-steady-state fashion, but above 300 K there is a very efficient steady-state catalytic reaction at the surface; this reaction is the decarbonylation of acetaldehyde to produce methane and carbon monoxide in the gas phase. This behaviour continues up to about 400 K. However, when acetaldehyde is dosed at 423 K, the reaction rate slowly evolves through a maximum to a very low catalytic rate. Upon carrying out reactor experiments at 473 K and above, the reaction mechanism changes to total dehydrogenation, and CO and H2 are produced at high steady-state rate, not withstanding the fact that carbon is continually being deposited onto the surface. This carbon does not appear to affect the reaction, which takes place on a surface with a c(2 × 2)-C layer present, since the extra carbon is lost from the reaction zone by diffusion into the bulk of the crystal.