Selective partial hydrogenation of 1,3-butadiene to butene on Pd(110): Specification of reactant adsorption states and product stability

TPD, scanning tunneling microscopy, and HREELS were used to study the selective partial hydrogenation of C₄H₆ to C₄H₈ on hydrogen-precovered Pd(110). The effective partial hydrogenation of C₄H₆ on the Pd(110) surface was realized due to the high hydrogenation activity of weakly π-bonded C₄H₆ combined with the instability of C₄H₈ produced at the reaction temperature. C₄H₆ hydrogenation did not take place in the sparse domains of < 0.25 ML, in which C₄H₆ was in a strongly π-bounded state. C₄H₆ formed dense domains and turned into a weakly π-bonded state in these dense domains on increasing C₄H₆ coverage. Only the weakly π-bonded C₄H₆ was hydrogenated to C₄H₈ at about 200 K. In contrast, the produced C₄H₈ was not stabilized on the surface above 200 K because C₄H₈ preferred to desorb at the temperature it was produced or was dehydrogenated to C₄H₆ when any part of the C₄H₈ remained on the Pd surface.