I am simulating slow flow through a channel, where diffusion is dominant over convection. In this instance, O2 is reacting with a site surface catalyst to form N2. I would like to see a steady buildup of N2 and a steady loss of O2, but instead the N2 and O2 are rapidly diffusing through the reaction wall. I suspect this is because there is no specification of zero-flux through the wall. 
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