Wall surface reaction problem
 

I have a problem(all in gas phase) of surface-catalyzed reaction. In my case, a kind of species in atomic form will recombinate to become its molecular form once it contact with wall. But the molecular form proportionally exist at the very beginning. I want to use the wall surface reaction module to solve. But which kind of speices should be set as the site species? Can anybody give me some advice? Thanks a lot!