Mass Transfer Effects in Stagnation Flows on a Porous Catalyst: Water-Gas-Shift Reaction Over Rh/Al2O3

Water-gas-shift (WGS) and reverse water-gas-shift (RWGS) reactions are numerically investigated in a stagnation-flow on a porous Rh/Al2O3 catalyst. External and internal mass transfer effects are studied using three different models for the mass transport and chemical conversion inside the porous catalyst: the dusty-gas model, a set of reaction-diffusion equations, and the effectiveness factor approach. All three models are coupled with the boundary layer equations to describe the potential flow on the stagnation disc, and a multi-step surface reaction mechanism is implemented. The numerically predicted species profiles in the external boundary layer are compared with recently measured profiles. Internal mass transfer limitations are more significant than external ones in case of the 100 μm thick catalyst layer. The effects of catalyst structure (thickness, mean pore diameter, porosity, tortuosity) as well as flow rate and pressure on chemical conversion are discussed.

Two-dimensional spatial resolution of concentration profiles in catalytic reactors by planar laser-induced fluorescence: NO reduction over diesel oxidation catalysts

Planar laser-induced fluorescence (PLIF) enables noninvasive in situ investigations of catalytic flow reactors. The method is based on the selective detection of two-dimensional absolute concentration maps of conversion-relevant species in the surrounding gas phase inside a catalytic channel. Exemplarily, the catalytic reduction of NO with hydrogen (2 NO+5 H2 →2 H2 O+2 NH3 ) is investigated over a Pt/Al2 O3 coated diesel oxidation catalyst by NO PLIF inside an optically accessible channel reactor. Quenching-corrected 2D concentration maps of the NO fluorescence above the catalytic surface are obtained under both, nonreactive and reactive conditions. The impact of varying feed concentration, temperature, and flow velocities on NO concentration profiles are investigated in steady state. The technique presented has a high potential for a better understanding of interactions of mass transfer and surface kinetics in heterogeneously catalyzed gas-phase reactions.