Rational design of hierarchically structured porous catalysts for autothermal reforming of methane

It is shown that the performance of a commercial, meso-macroporous catalyst for the autothermal reforming of methane on Ni/Al2O3 could be improved significantly by optimizing the macroporosity and the size of the macropores. The commercial catalyst, taken as a base case, contains macropores with an average diameter of 2 μm and mesopores with an average diameter of 20 nm. The kinetics of Xu and Froment (1989a) for steam reforming and the water gas shift reaction were employed, in combination with kinetics for the total oxidation of methane. Multicomponent molecular diffusion, Knudsen diffusion and viscous flow were accounted for in the modeling of transport in the macropores. At typical reaction conditions, Knudsen diffusion dominates transport in the mesopores; the effect of pore surface roughness on Knudsen diffusion was included in the simulations. Both the macropore size and the macroporosity influence the overall conversion; increases of up to 40–300% with respect to a commercial catalyst are possible. A larger macroporosity typically favors a lower CO/H2 ratio, that is, a higher selectivity toward hydrogen, when the reverse reaction of the water gas shift reaction dominates, and vice versa. Temperature gradients in the catalyst increase with macroporosity, as a result of the lower thermal conductivity of the solid porous material, but the maximum temperature in the catalyst was around 10 K above that at the outer surface at the investigated operating conditions.