Outstanding mechanistic questions in heterogeneous catalysis

This Feature Article summarizes key results from our studies on the kinetics of chemical reactions on solid surfaces as they relate to heterogeneous catalysis. Emphasis is placed on the identification of specific issues of relevance to the design of catalytic processes, particularly the need for highly selective catalytic reactions. From the kinetic point of view, issues of selectivity translate into controlling relative activation energies within a small fraction of their absolute value. In the case of catalytic reforming of hydrocarbons, the nature of the final products is determined by the regioselectivity of an early dehydrogenation from surface alkyl intermediates (from the alpha, beta. or gamma position), a step greatly affected by the electronic structure of the metal used as catalyst. In the case of partial oxidation reactions, both the geometry of the local ensemble of surface atoms in the catalytic site and the presence of surface species such as hydroxide groups play determining roles in defining selectivity. A third factor is adsorption geometry, which often varies with changing reaction conditions: variations in concentration of the modifier in chiral catalysis, for example. may lead to dramatic changes in the enantioselectivity of hydrogenation processes. High pressures can induce new types of bonding between adsorbates and surfaces as well, opening new mechanistic routes for catalytic reactions: the hydrogenation of olefins, where pi bonding occurs on top of strongly bonded carbonaceous deposits, falls into this category. Finally, the inhomogeneous distribution of adsorbates within the surface can be manifested in the formation of local islands. and those can promote specific catalytic reactions; witness, for instance, the effect of islanding on reactivity in the catalytic oxidation of carbon monoxide and during the catalytic reduction of nitrogen oxide.