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.

Reduction/oxidation half-cycles of the selective catalytic reduction of NO with NH 3 (NH 3 -SCR) at 200 °C were investigated using in situ and operando spectroscopies to propose a general mechanism for four different catalysts (TiO 2 -supported and bulk vanadium oxides and Cu-AFX and Cu-CHA zeolites). The reduction half-cycle is initiated by the reaction of NH 3 on Lewis acid sites [V(V) or Cu(II); L-NH 3 ] and NO, followed by the gradual reaction of NH 3 on Brønsted acid sites (B-NH 3 ) and NO; this results in the formation of V(IV) or Cu(I) and protons (H + ) on the surface, along with N 2 and H 2 O. The UV−vis measurements for the reduction half-cycle indicate that N 2 is continuously generated until the surface V(V) or Cu(II) species is depleted. The subsequent reoxidation of the reduced catalysts under O 2 leads to the regeneration of V(V) or Cu(II) and the reaction of surface H + , yielding H 2 O (oxidation half-cycle). The higher consumption rates of B-NH 3 and L-NH 3 under NO + O 2 than those under NO, which has been previously reported in the literature, were explained based on the continuous reduction/oxidation of V(V)/ V(IV) or Cu(II)/Cu(I) where the regenerated V(V) or Cu(II) site is reused in the subsequent (second) reduction half-cycle. Namely, upon the recovery of V(V) or Cu(II) via reoxidation, the leftover B-NH 3 species react with the supplied NO to yield N 2 ; this suggests that B-NH 3 is not a spectator but a reservoir of NH 3 to participate in the second reduction half-cycle possibly via the migration of NH 3 or HONO species. These results provide comprehensive evidence of the general mechanism of NH 3 -SCR, which was found to be applicable to both V and Cu catalysts.

Single-atom catalysts show outstanding catalytic activity owing to their different electronic and structural properties from those of nanoparticulate catalysts. For Au single-atom catalysts, however, metal oxides that can stabilize a single atomic state are still limited and the stabilizing effect is not well known. Here, we report Au single-atom catalysts supported on NiO (Au1/NiO), their structural features, and their catalytic properties. Atomic Au was dispersed on the surface of NiO with Ni vacancy sites. The formation of Au single-atom sites was more favorable on an NiO surface with Ni vacancy sites than on a clean NiO surface. The Au single atoms were cationic, and their charge states depended on the crystal facets [Au3+-like on NiO(100); Au+-like on NiO(110)]. The turnover frequency (TOF) of 0.011 wt % Au1/NiO for CO oxidation (0.41 s–1) was higher than the TOFs of nanoparticulate Au catalysts supported on NiO (0.033 s–1) at room temperature. Au1/NiO showed a high catalytic stability (120 °C, 120 h) to CO oxidation reaction.