Alkyne Hydrogenation over Pd Catalysts: A New Paradigm

The hydrogenation of 1-pentyne over various palladium catalysts was studied under different conditions. In the regime of selective hydrogenation, as observed by in-situ X-Ray Photoelectron Spectroscopy (XPS), significant amount of subsurface carbon and a Pd-C surface phase builds up in the early stage of the reaction. These species inhibit the emergence of bulk-dissolved hydrogen to the surface, which is reactive but unselective. Carbon laydown was also observed by Tapered Element Oscillating Microbalance (TEOM) and by catalytic pulse experiments with greater laydown occurring in the selective regime. The effect of carbon dissolution in the crystal lattice near the surface was evidenced by High-Resolution Transmission Electron Microscopy (HRTEM). In alkyne hydrogenation the active phase of palladium catalysts is a Pd-C surface phase in the regime of selective hydrogenation. As self-hydrogenation (hydrogen from dissociated pentyne) was shown to be unselective as well, only surface hydrogen from the gas phase is available to generate the alkene. The issue of structure-sensitivity of alkyne hydrogenation over palladium catalysts is discussed in terms of structure-sensitive carbon deposition and carbon dissolution into the metal lattice