wall surface and particle surface reactions
 

Hi all,I can't understand the definition,meaning and difference between wall surface and particle surface reactions in the fluent,for example a reaction of a gas on the surface of a solid in a porous medium of this solid is wall or particle surface reaction or both of them?

I have also doubt with wall surface reaction and particle surface reaction. we already solve chemical reaction by volumetric reaction type then why we need to solve wall surface reaction and particle surface reaction.
Please explain with examples.
Thanks for advance.