The Influence of Surface Structure and Adlayer Composition on the Kinetics and Mechanisms of Gas-Surface Reactions

Citation

Engstrom, James Robert (1987) The Influence of Surface Structure and Adlayer Composition on the Kinetics and Mechanisms of Gas-Surface Reactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6b69-ch24. https://resolver.caltech.edu/CaltechETD:etd-03122008-080659

Abstract

The theoretical formulation of a frequency response technique, based on surface temperature modulation, for the study of the kinetics of elementary gas-surface reactions is described in detail. The formalism is developed for analyzing adsorption, desorption, and both unimolecular and bimolecular surface reactions. The technique possesses a distinct advantage over competitive techniques, such as modulated molecular beam reactive scattering, since the evaluation of activation barriers for elementary surface reactions is straightforward even in the presence of strong nonlinearities and requires no knowledge of the composition and/or configuration of the adlayer. The technique has been employed successfully to investigate the adsorption, desorption and oxidation of CO on the Pt(110)-(1x2) surface. The oxidation of CO has been studied with emphasis on examining the kinetics of the elementary bimolecular surface reaction CO(a) + O(a) → CO₂(g). The activation energy is found to correlate well with the oxygen adatom concentration, the variation being quite nonlinear. This nonlinear variation is interpreted as a manifestation of the dominant role that the local configuration of the reactants play in determining the reaction dynamics. It is suggested that the power of the technique can be increased significantly by either coupling it with time-resolved, surface-sensitive spectroscopies or employing localized (e.g., laser) heating techniques to provide the modulation in surface temperature. The chemisorption of hydrogen on both the Ir(111) and Pt(110)-(1x2) surfaces has been examined by employing primarily thermal desorption mass spectrometry. Comparisons of the results obtained on the (111) and (110)-(1x2) surfaces of both iridium and platinum suggest strongly that local surface structure has a profound influence on the kinetics of adsorption of hydrogen on these surfaces. Surface structure also influences greatly the desorption kinetics of hydrogen via the mediation of "through-metal" adatom-adatom interactions. In particular, both attractive and repulsive interactions are clearly manifest within the higher binding energy β₂-adstates on the (110)-(1x2) surfaces, these β₂ states associated with adsorption into one-dimensional channels which are composed of high-coordination "trough" sites. However, only repulsive interactions were apparent on the (111) surfaces or for the lower binding energy β₁-adstates on the (110)-(1x2) surfaces, these β₁ states associated with adsorption on the (111) microfacets. The hydrogenolysis of several alkanes and cycloalkanes has been examined on both the (111) and (110)-(1x2) single crystalline surfaces of iridium for hydrocarbon partial pressures near 1 Torr, hydrogen partial pressures near 100 Torr, and surface temperatures between 400 and 700 K. The choice of these two surfaces has proven to be decisive for the unambiguous assessment of the effects of surface geometry on both catalytic activity and selectivity. Concerning the hydrogenolysis of alkanes, excepting the special case of n-butane on Ir(110)-(1x2), a mechanism involving a rate-limiting unimolecular C-C bond cleavage in an adsorbed, dehydrogenated hydrocarbon fragment describes well the variations in both the specific activity and selectivity of hydrogenolysis with variations in the reactant partial pressures. In cases where similar activities and selectivities are observed over the two surfaces, essentially identical reaction intermediates (mechanisms) are implicated. On the other hand, in cases where different activities and/or selectivities are observed over the two surfaces, distinct reaction intermediates (mechanisms) are implicated. In the particular case of the reaction of n-butane, the selectivity for ethane was identified explicitly with the participation (concentration) of low-coordination-number metal surface atoms. It is suggested that the intermediate that leads to the high selectivity for ethane is a mononuclear metallacycle pentane; the formation of this intermediate is sterically forbidden on (111) surfaces. Concerning the hydrogenolysis and hydrogenation of cycloalkanes, the Ir(110)-(1x2) surface possesses a greater specific activity (per metal surface atom basis) with respect to the Ir (111) surface for both the hydrogenation (to propane) and the hydrogenolysis (to methane and ethane) of cyclopropane. This result is interpreted by invoking a greater intrinsic activity (for these reactions) for the low-coordination-number metal surface atoms that are present in higher concentrations on Ir(110)-(1x2).

Unlike the reactions of alkanes, for cyclopropane under "severe" reaction conditions (i.e., large hydrocarbon-to-hydrogen partial pressure ratios and/or high temperatures), a transformation in the composition of the adlayer occurs, with the coverage of the carbonaceous residue increasing from one-half of a monolayer to nearly a full monolayer. Concomitant with the adlayer transformation, the selectivity of the reaction of cyclopropane and hydrogen shifts dramatically from methane (hydrogenolysis) to propylene (isomerization). The dominance of the (relatively facile) isomerization channel on the "carburized" surface is interpreted as a manifestation of a weakened interaction between the metal surface and the relevant adsorbed reaction intermediates.

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