Surface kinetics and energetics from single crystal adsorption calorimetry lineshape analysis: Methyl from methyl iodide on Pt(1 1 1)

The first use of single crystal adsorption calorimetry (SCAC) to probe the kinetics of surface chemical reactions is presented. It is applied to study a common situation encountered in catalytic mechanisms, wherein a gas quickly populates a molecularly adsorbed state that then converts to more stable products on a slower timescale (10-1000 ms). We show that for such a two-step process, detailed analysis of the heat-detector signal's time-response lineshape to a single pulse of gas provides the rate constant for the second step and the heats of reaction for both of the elementary reactions. We apply this analysis to the dissociative adsorption of methyl iodide (CH₃I) on Pt(1 1 1) at 270 K, to measure the heats of reaction for the elementary steps involved and the rate constant for the slow step, all as a detailed function of coverage. At low coverage, the reaction is CH₃I_g → CH ₃I_ad → CH_3,ad + I_ad, followed at high coverage by CH_3,ad → CH_ad + 2H_ad and CH_3,ad + H_ad → CH_4,g (above 0.08 ML total CH_3,ad plus I_ad). These results provide the rate constant for the dissociation of CH₃I_ad and the heats of formation of both CH_3,ad and CH_ad. These two heats agree with values determined at 320 K where the rates are so fast that lineshape analysis is not needed, proving the validity of the lineshape analysis method introduced here for analyzing SCAC data.