Pollution from nitrogen oxides, NOx, has an adverse effect on the environment and public health. The abatement from the exhaust of stationary sources has been solved by selective catalytic reduction (SCR) of NOx to dinitrogen by reaction with ammonia over a heterogeneous catalyst. Vanadia on titania promoted with tungsta is the most widely used industrial catalyst for this purpose. It decreases the NOx emission down below legislation limits for both automotive and stationary applications. The suggested catalytic cycles all contain both vanadium(V) and (IV), but the degree of reduction under SCR conditions is questioned. Here we show the results of an EPR study under operando SCR conditions as well as in the oxidized and reduced resting states in a fixed bed plug flow reactor operating at high space velocities. We find that vanadium maintains the oxo group even in the reduced phase as vanadyl(IV), VO2+, which is the dominant reduced species, comprising up to 50 % of the total vanadium content. During the SCR reaction the vanadium(IV) concentration is highly dependent on the space velocity imposed on the catalytic reactor. It is present as a significant fraction during operando measurements of a stressed catalyst bed. By comparison, under typical industrial conditions the space velocity is much lower and the temperature is higher, which leaves the majority of the catalyst in the oxidized, vanadium(V), resting state. The results are put into context of a new chemically consistent proposal for the catalytic cycle for the SCR reaction.[1] [1] T. V. W. Janssens et. al. ACS Catal. 2015, 5, 2832.

Quantification of vanadium(IV) on SCR catalysts by operando electron paramagnetic resonance (EPR) spectroscopy

251st National Meeting (2016)

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