Distinctive Roles of Chemisorbed Atomic Oxygen and Dioxygen in Methane Catalytic Oxidation on Pt{110}

The isothermal oxidation of C a formed by CH 4 dissociation has been studied on Pt{110} using molecular beam techniques at a surface temperature of 650 K. The reaction products CO and CO 2 are produced in the ratio of 9:1, respectively, for all carbon coverages studied (0.03−0.8 ML). When the O 2 beam is turned on, there is a sharp rise in the rate of CO production (less than 0.1 s) which is attributed for the first time to the surface reaction of C a with short-lived chemisorbed molecular dioxygen, O 2c . This is followed by a slow rise to a maximum rate that is due to the surface reaction between C a and adsorbed atomic oxygen, O a, which accumulates more slowly on the surface. The initial rate of CO formation from O 2c decreases as the C a precoverage is increased, while the time taken to reach the maximum in the rate from the C a + O a reaction increases. The CO rate maximum always occurs earlier than that of CO 2, for all carbon coverages. The rate constant for the C a + O 2c reaction decreases by a factor of 6 up to a C a precoverage of 0.2 ML and then stays almost constant up to saturation.