Gas A is diffusing from the bulk gas phase to the catalyst surface, where it reacts instantaneously and irreversibly in a heterogeneous reaction as follows:

A$->0\mathrm{.}5$B

Gas B then diffuses back out gas film to the main stream composed of A

and B$.$ Assume the appropriate concentration of A and B across the film thickness $($d$).$

Obtain the expression of molar flux of A through the film.