This work describes the transformation of Pd species under reaction condition 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/H2 reaction mixtures, a permeable amorphous hydrocarbon overlayer is formed with simultaneous insertion of carbon atoms into Pd lattice that drives the formation of low coordinated Pd-carbide, thus providing more reactive Pd-Csub@Clayer sites (Csub: subsurface carbon; Clayer: 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 improve catalytic behaviour for selective hydrogenation of acetylene (93% ethylene selectivity @ 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 2 spectroscopy (XAS) and in-situ X-ray diffraction (XRD), it is demonstrated that different active sites dominate this catalytic reaction depending on choice of adsorbate and exposure temperature. This work represents an alternative and arguably simpler manner to design more reactive catalytic sites with the characteristics of long-term stability and facile preparation which would enable promising industrial application.

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