KINETIC COUPLING AND HYDROGEN SURFACE FUGACITIES IN HETEROGENEOUS CATALYSIS .1. ALKANE REACTIONS ON TE/NAX, H-ZSM5, AND GA/H-ZSM5

Hydrogen removal occurs by recombinative desorption and by hydrogen transfer during dehydrogenation steps required for alkane and cycloalkane conversion on Te/NaX, H-ZSM5, and Ga/H-ZSM5 catalysts. Recombinative desorption limits the rate of n-heptane and methylcyclohexane aromatization on Te/NaX and prevents equilibration between gas-phase H2 and H-adatoms formed in intermediate dehydrogenation steps. The resulting high surface hydrogen fugacities lead to low steady-state concentrations of required unsaturated intermediates. Te ions catalyze rate-limiting hydrogen desorption steps during alkane reactions on Te/NaX. On H-ZSM5, hydrogen removal limits the rate of propane conversion to aromatics. Hydrogen adatoms are removed predominantly by reactions with coadsorbed hydrocarbon fragments, leading to high cracking selectivity. Ga ions introduce a recombinative desorption function that partially relieves the resulting high hydrogen surface fugacities and allows dehydrogenation steps to occur without concurrent cracking. Thus, Ga ions increase aromatics selectivity by providing a "porthole" for the removal of hydrogen adatoms as dihydrogen. We propose that rate-limiting hydrogen desorption steps, and the high surface hydrogen fugacities that result, control the rate and selectivity of dehydrogenation and related reactions on many nonmetal surfaces. © 1992.