Catalysts are materials that alter the rate of chemical reactions and are broadly used to lower costs, improve yields and to reduce unwanted pollutants. To properly understand the catalytic mechanism it is necessary to study the system under reaction conditions, due to the dynamic nature of the catalysts composition. Measuring in this manner is increasing being adopted, although it necessitates more complex experimental arrangements. Recently, equipment has been commissioned at ISIS that allows QENS measurement under such flow conditions. We aim to exploit this equipment with a reacting system for the the first time to demonstrate our new abilities. The reaction proposed is the selective catalytic reduction of ammonia, used in diesel engine exhaust remediation. We will measure the diffusion of ammonia in an industrial catalyst under different conditions, including during reaction.