surface reaction

Jason

I'm modeling 2A(g)+2B(s)--> 2C(g) + D(g). A, B, C, D are defined as fluid in mixture-template, while B is surface species. In the wall surface reaction, I defined the above reaction, and the rate component for B is 0. However, the simulation can not achieve mass balance. The outlet of B mole fraction is even more than the reduce of A mole fraction. Do you know any reason. Meanwhile there is not formation of D at surface. I'm using 2D Fluent 6.0. Thanks.