HIERARCHICAL REDUCED MODELS FOR CATALYTIC COMBUSTION - H-2 AIR MIXTURES NEAR PLATINUM SURFACES/

The catalytic ignition and gas-phase ignition of H-2 in air over plati num are studied in a stagnation-point flow geometry using various gas- phase reaction mechanisms, surface reaction mechanisms, and transport models. Systematically reduced models are developed using a new method ology based on reaction path analysis at turning points along two-para meter bifurcation diagrams. The different reduced models, including al gebraic ignition criteria, quantitatively reproduce the catalytic and gas-phase ignition temperatures of the full, spatially distributed mod el governed by a few thousand equations. Caution is needed in applying reduced mechanisms developed for homogeneous combustion to catalytic combustion. For example, it is found that a reduced gas-phase reaction mechanism, derived for homogeneous combustion alone, must be augmente d by H2O2 chemistry for the homogeneous-heterogeneous problem. It is a lso shown that the rate-limiting surface-reaction step changes with co nditions, and common assumptions used in catalysis of fast adsorption- desorption steps of reactants (partial equilibrium) break down under m any conditions. Various lumped one-step surface reaction rates are der ived to model catalytic combustion of H-2/air mixtures over platinum s urfaces. Finally, our analysis shows that under sufficiently fuel-lean conditions, platinum behaves as a source of adsorbed OH species promo ting gas-phase ignition. Desorption of adsorbed OH needs then to be in cluded for accurate model predictions.