NH3-SCR (selective catalytic reduction) is important process for removal of NOx. However, water vapor included in exhaust gases critically inhibits the reaction in a low temperature range. Here, we report bulk W-substituted vanadium oxide catalysts for NH3-SCR at a low temperature (100–150 °C) and in the presence of water (~20 vol%). The 3.5 mol% W-substituted vanadium oxide shows >99% (dry) and ~93% (wet, 5–20 vol% water) NO conversion at 150 °C (250 ppm NO, 250 ppm NH3, 4% O2, SV = 40000 mL h−1 gcat−1). Lewis acid sites of W-substituted vanadium oxide are converted to Brønsted acid sites under a wet condition while the distribution of Brønsted and Lewis acid sites does not change without tungsten. NH4+ species adsorbed on Brønsted acid sites react with NO accompanied by the reduction of V5+ sites at 150 °C. The high redox ability and reactivity of Brønsted acid sites are observed for bulk W-substituted vanadium oxide at a low temperature in the presence of water, and thus the catalytic cycle is less affected by water vapor.
Practical catalysts
that work at a low temperature for selective
catalytic reduction of NO
x
using NH3 (NH3–SCR) have been required to treat NO
x
at the outlet temperature in boiler systems
(100–150 °C). In this paper, we report bulk vanadium oxide
catalysts that show NH3–SCR activity at a low temperature
below 150 °C. Defective bulk vanadium oxide (V(V)+V(IV)) catalysts
were synthesized by the calcination of vanadium(IV)-oxalate at 270
°C (1–4 h). The reaction rate per mol of surface vanadium
atom of the catalyst calcined at 270 °C for 2 h (V 270-2, 6.4
× 10–2 molNO molV
–1 s–1) was 10–14 times faster
than those of conventional 1–9 wt % V2O5/TiO2 (4.5 × 10–3–6.1 ×
10–3 molNO molV
–1 s–1), indicating that bulk vanadium oxide is more
favorable for NH3–SCR and V(IV) species enhance
the activity. The NH3–SCR of V 270-2 is driven by
the Lewis acid mechanism, which proceeds faster than the Brønsted
acid mechanism.
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