Probe the formation and evolution of palladium carbide during selective alkyne hydrogenation

Evolution of active sites is a common fact, which originates from entropy gains or enthalpy losses that effectively regulates the selectivities and rates during catalysis, however, is very difficult to probe. In this work, a PdCu organic precursor was synthesized and dispersed well on SiO 2 support by impregnation method (c.a. Pd 1 Cu 1 /SiO 2 ). Within the initial stage of phenylacetylene hydrogenation, Pd 1 Cu 1 /SiO 2 showed severe overhydrogenation. Consequently, Pd species were carburized to PdCuC x ensembles and facilitated the styrene formation significantly (>99% conv., >98% selec.). Interestingly, the activation process was highly dependent on the type and concentration of the reactant gas, in which the increase of alkyne concentration promoted the evolution of active sites (i.e. carbonization of Pd) and improved styrene formation, whereas the promotion of hydrogen stabilized palladium hydride and hindered carbonization process, which led to the overhydrogenation. These findings were well evidenced by catalytic performance evaluation and relative characterizations, i.e. quasi-in situ XPS and HRTEM etc. Clearly, the dynamics of PdCuC x ensemble generation were significantly correlated to the concentrations of the alkyne and hydrogen, which could modify the selective hydrogenation behaviour greatly.