Abstract
Transition metal catalysts are used in a wide variety of industrial processes, including steam reforming of methane, which is the main source of synthesis gas used for producing ammonia, large hydrocarbons, and olefins. Dissociative chemisorption by initial C-H bond cleavage is considered to be the rate limiting step in steam reforming. At the high surface temperatures used in industrial reactors (ca. 800 K) molecular desorption is highly competitive to dissociative chemisorption. Alkanes can be molecularly physisorbed to the surface through van der Waals interactions at low surface temperatures (Ts < 300 K), but rapidly desorb at high temperatures. It is critical to understand both the kinetics of molecular desorption and dissociative chemisorption, as well as how they relate to each other, in order to better model industrial catalysts and catalytic processes. This study reports on the thermal rate constants for molecular desorption and dissociative chemisorption, and the role van der Waals interactions play in stabilizing the dissociative chemisorption for alkanes ranging from methane to n-nonane.
The molecular desorption rate constants were determined by analysis of temperature programmed desorption (TPD) spectra, to extract pre-exponential factors (νd) and desorption energies (ED). The pre-exponential factors and desorption energies are reported for the branched alkanes of isobutane, neopentane, and 2,2,3,3-tetramethylbutane (TMB). The latter two molecules were found to have weaker desorption energies than their n-alkane isomer counterparts due to their branched geometry and different van der Waals attraction to the surface that falls off as 1/z^3.
The rate constants for dissociative chemisorption of alkanes from methane to n-nonane were calculated from a fit of alkane thermal dissociative sticking coefficients (DSC) to a precursor-mediated thermal trapping (PMTT) model, which assumes gas molecules undergo full energy exchange with the surface and thermalize to the surface temperature prior to undergoing either desorption or reaction, with thermal rate constants. Even for molecules which are known to have poor gas-surface energy exchange (e.g. methane), the PMTT model was able to adequately model thermal equilibrium DSCs, to yield reaction activation energies (Ea,r) and pre-exponential factors (νr). For the majority of alkanes, νd > νr with both values increasing with increasing alkane size. TMB was found to have special reactivity at temperatures above 560 K, where it underwent trapping-mediated C-C bond pyrolysis with a νr,C-C and Ea,C-C significantly larger than that of νd and ED.
Additional studies were done into the formation of graphene on Pt(111), a material which has been a topic of great interest in the recent decade because of its unique electrical properties as a semi-conductor, by TMB chemical vapor deposition. Although larger alkanes were shown by this study to be more reactive on Pt(111) than smaller alkanes, TMB formed smaller coverages (1.35 ML) of graphene than ethylene (2.57 ML). This was determined to be due to many scattered graphene islands, between which were regions inaccessible to the large TMB molecules, which require an ensemble of binding sites to adsorb and react.
The transition state and products of dissociative chemisorption were found to be stabilized by van der Waals interactions, which the ED of the reagent gas molecules well approximates. Ea,r decreased proportional to the increase in ED as molecular size increased, yielding an Evans-Polanyi correlation with a slope dEa,r/dED = -0.50 ± 0.04. In general, larger alkanes were found to be more reactive on surfaces than smaller alkanes. Because van der Waals interactions cause dissociative chemisorption to become more exothermic, it was possible to predict heats of reaction and Pt-C bond energies for a wide range of alkanes, from ethane to n-nonane, using data from methane and isobutane Pt(111) single crystal calorimetry experiments. The Evans-Polanyi correlation was reproduced between the apparent activation energies and heats of reaction yielding a slope dEa,r/dED = -0.46 ± 0.09. This study demonstrates a method of predicting heats of reaction and activation energies for surface reactions using data from a small set of very difficult single crystal calorimetry experiments.
