Adsorbate-Induced Structural Evolution of Pd Catalyst for Selective Hydrogenation of Acetylene

This work describes the transformation of Pd species under reaction conditions to control the reactivity of heterogeneous catalysts, in which the real active sites for hydrogenation differ from simple Pd sites on a carbon nanofiber. Specifically, in the presence of C2H2/C2H4/H-2 reaction mixtures, a permeable amorphous hydrocarbon overlayer is formed with simultaneous insertion of carbon atoms into the Pd lattice that drives the formation of low-coordinated Pd-carbide, thus providing more reactive Pd-C-sub@C-layer sites (C-sub: subsurface carbon; C-layer: carbon layer on the surface). The combination of these surface and subsurface effects hinders Pd-hydride, weakens ethylene adsorption confirmed by density functional theory (DFT) calculation, and thus improves catalytic behavior for selective hydrogenation of acetylene (93% ethylene selectivity at 100% conversion) with long-term stability. In the absence of hydrogen, a denser more crystalline overlayer is formed with severely restricted permeability, resulting in significantly lower activity. Moreover, by X-ray absorption spectroscopy (XAS) and in situ X-ray diffraction (XRD), it is demonstrated that different active sites dominate this catalytic reaction depending on the choice of adsorbate and exposure temperature. This work represents an alternative and arguably a simpler manner to design more reactive catalytic sites with the characteristics of long-term stability and facile preparation, which would enable promising industrial applications.