Pd-C surface phase as an essential parameter of selective alkyne hydrogenation

Palladium is one of the most widely applied metals in catalytic processes both in homogeneous and heterogeneous systems. In either case selectivity is a major concern to make industrial processes economically feasible. Palladium particles are able to add one hydrogen molecule to both alkenes and alkynes, hence the question may arise how the catalyst prohibits total hydrogenation of alkynes and multiple unsaturated hydrocarbon. Most of the explanations offered for this question consider the presence of carbonaceous overlayer on the palladium surface. We studied the hydrogenation of 1-pentyne over various palladium catalysts under different conditions. In line with the literature data on alkyne hydrogenation, 1-pentyne hydrogenation on palladium catalysts is characterized by two significantly different regimes: at high pressures (and) with high hydrogen excess pentane is by far the main product. At lower pressures or/and with lower H2/C5 ratios, hydrogenation is much slower, but almost totally selective to 1-pentene. Pulse hydrogenation, in-situ TEOM, in-situ XPS and HRTEM reveals that this turn of selectivity is related to an especial carbon retention. It is unequivocally established that carbon dissolves into the palladium lattice (mainly in the near-surface region) and a palladium-carbon surface phase (PdC) builds up in the early stage of the reaction. This, and not the clean palladium surface, is the active phase in the regime of selective hydrogenation of alkynes on a typical catalyst. The formation of Pd-C is strongly suppressed at high p(H2), at which condition hydrogenation is non-selective. We propose that the role of dissolved carbon and the Pd-C surface phase is to exclude bulk dissolved hydrogen participating to the reaction. The genesis of the active surface includes the total fragmentation of significant amount of reactant molecules. Further experiments with C2/C3/C5 alkynes and alkenes indicate that Pd-C formation is a general process during selective triple bond hydrogenation, but it does not build up from the corresponding alkenes, making the hydrogenation sites different from alkynes and alkenes