The hydrogenation of butene (C4H8) to butane (C4H10) on certain oxide catalysts is believed to proceed according to the sequence of elementary reactions. C4H8(g)+1k1k1C4H81(fast)H2(g)+2k2k2H22(fast)C4H81+H22k1C4H101+2(slow)C4H101k1C4H10(g)+1(fast) In the above mechanism, 1 and 2 are two distinctly different types of surface sites. Only H2 adsorbs on 2, while both C4H8 and C4H10 adsorb on 1 but H2 does not