Partial Hydrogenation of 1,3-Butadiene on Hydrogen-Precovered Pd(110) in the Balance of π-Bonded C₄ Hydrocarbon Reactions

The hydrogenation and dehydrogenation of C₄ hydrocarbon molecules (1,3-butadiene, 1-butene, <i>trans</i>-2-butene, <i>cis</i>-2-butene, and <i>n</i>-butane) on the hydrogen-precovered Pd(110) surface have been investigated by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). 1,3-Butadiene was found to be adsorbed molecularly on the surface below 350 K. Further heating of the surface resulted in decomposition, forming hydrocarbons at 350 K and finally the graphite layer at 550 K. The butene isomers and <i>n</i>-butane adsorbed on the surface were, however, relatively unstable compared with 1,3-butadiene when heated. Some of the adsorbed butenes were desorbed, and the species that remained on the surface were dehydrogenated to 1,3-butadiene between 150 and 250 K. <i>n</i>-Butane on the surface showed similar reaction behavior except for the lower dehydrogenation and desorption temperature. Our findings indicate that the dehydrogenations of π-bonded C₄ hydrocarbons on the Pd surface show significantly different pathways compared with those of the σ-bonded C₄ hydrocarbon on Pt and Ru surfaces. Here, we discuss the selective partial hydrogenation of 1,3-butadiene on hydrogen-precovered Pd(110) in terms of the reactivity of the butenes and butanes.