Process rate is diffusion controlled in bulk gas phase, i.e. the reaction rate of surface reaction is depending on the mass diffused into the surface. Currently I just use a very large surface reaction rate (set preexponent constant to a huge value). Is this the right way, or do you have other method? Thanks

That sounds OK to me. Basically you are saying that there are no kinetic limitations and that its based on the mass flux due to diffusion through the boundary layer.

I often use such an approach for the oxidation reaction to trigger ignition in steady-state simulations.

Greg