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The oxygen activity in the solid determines the phase composition which should be stable in the case of oxygen transfer reactions at oxidic catalyst. Its value results from the rates of the oxygen uptake by the solid and the oxygen transfer to an acceptor in the gas phase, e.g. a hydrocarbon or ammonia. This oxygen activity in the catalyst can be determined under reaction conditions with the aid of a potentiometric method based on the application of solid ion conductors. In that case the catalyst is simultaneously one electrode of an electrochemical cell, the oxygen activity and the reaction rate being measured simultaneously. The method has been applied during the investigation of the reduction of nitrogen monoxide by ammonia on copper oxide and a typical SCR catalyst on the basis of vanadia‐titania. The reaction was studied in the domain 300 <ϑ/°C < 390 at reactant concentrations up to 4000 ppm, the oxygen content in the gas phase being varied between 0 and 10 Vol.%. The results demonstrate unambiguously a correlation between oxidation state of the catalyst and the reaction kinetics. The rate increasing influence of oxygen is accompanied by the transition of V4+ to V5+ in the case of the SCR catalyst. In the case of copper oxide catalyst the reduction rate of NO is decreased at increasing oxygen concentration at the transition to Cu2+. In addition the increased N2O formation could be observed. In spite of the high oxygen excess a quasiequilibrium of oxygen chemisorption can be excluded in any case.

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