Infrared spectroscopy characterization of the chemistry of C4 hydrocarbons on Pt(111) single-crystal surfaces

The uptake and thermal chemistry of a number of C-4 hydrocarbons on Pt(111) single-crystal surfaces were characterized by reflection-absorption infrared spectroscopy (RAIRS). The compounds studied include butane, 1-butene, cis-2-butene, trans-2-butene, and 1,3-butadiene as well as 1-iodobutane, 2-iodobutane, 1-bromo-2-butene, and 1-bromo-3-butene, precursors for the formation of corresponding alkyl and alkenyl intermediates. Much of the chemistry reported here agrees with previous work on similar systems, and conforms with the general idea of hydrogenation, dehydrogenation, and H-D exchange reactions occurring by single hydrogen incorporation and elimination steps according to the so-called Horiuti Polanyi mechanism. In addition, though, a couple of surprising observations are presented of particular relevance to the mechanism of carbon-carbon double bond isomerizations. In particular, it was seen that although the isomerization of trans- to cis-2-butene is more favorable and occurs at lower temperatures than the reverse cis-to-trans conversion, beta-hydride elimination from the common 2-alkyl intermediate produces trans-2-butene preferentially. This indicates that it is the relative adsorption energy of the alkene what determines selectivity in these reactions.