How can I add a decomposed reaction on the wall surface coated by the catalyst? Thanks in advance.

Define the wall surface as a boundary separately from other walls. Depending on what the reaction at the wall surface, you may need to write UDF file and input at that wall surface.

Thanks a lot for xiaofish! My problem is a multiphase reaction ,and how can I use the UDF? Can you explain it in details? Thanks in advance!