Redox behaviour of vanadium during hydrogen–oxygen exposure of the V2O5-WO3/TiO2 SCR catalyst at 250 °C

A series of experiments was conducted to reveal the impact on the catalyst and NH3-SCR activity of an industrial deNOx catalyst from Haldor Topsøe A/S (V2O5-WO3/TiO2) by exposure to H2 and H2/O2. Treatment of the SCR catalyst was done by exposure for 60 min with three different concentrations of H2 (0–2%) in an 8% O2/N2 mixture, where the SCR activity was measured before and after the hydrogen treatment. The results show that the activity of the SCR catalyst is only negligibly affected during exposure to the H2/O2 gas and in all cases it returned reversibly to the initial NOx conversion rate after reexposure to the standard NH3-SCR gas. In situ electron paramagnetic resonance (EPR) and UV–vis spectroscopy at 250 °C suggest that a significant part of V4+ present during SCR conditions was oxidized to V5+ during exposure to 2% H2 + 8% O2. However, the distribution of vanadium in oxidation states V4+ and V5+ returned to the initial redox equilibrium in less than an hour after the hydrogen–oxygen treatment, emphasizing the reversibility of the V4+-V5+ system.

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► Reversibility of the V2O5-WO3/TiO2 SCR catalyst in hydrogen-oxygen mixtures at 250 °C is studied.
► In-situ UV–vis spectroscopy suggests the occurrence of a mild reduction of V5+ in diluted H2/N2.
► The isosbestic point observed at 250 °C affirms that only V4+ and V5+-species is present.
► Increased VO2+ level (EPR) correlates with activity in the transient after H2/O2 treatment.
► The vanadium redox system is completely reversible upon return to normal SCR gas after reduction.