Single-atom catalysts reveal the dinuclear characteristic of active sites in NO selective reduction with NH3

Abstract

High-performance catalysts are extremely required for controlling NO emission via selective catalytic reduction (SCR), and to acquire a common structural feature of catalytic sites is one key prerequisite for developing such catalysts. We design a single-atom catalyst system and achieve a generic characteristic of highly active SCR catalytic sites. A single-atom Mo1/Fe2O3 catalyst is developed by anchoring single acidic Mo ions on (001) surfaces of reducible α-Fe2O3, and the individual Mo ion and one neighboring Fe ion are thus constructed as one dinuclear site. As the number of the dinuclear sites increases, SCR rates increase linearly but the apparent activation energy remains almost unchanged, evidencing the identity of the dinuclear active sites. We further design W1/Fe2O3 and Fe1/WO3 and find that tuning acid or/and redox properties of dinuclear sites can alter SCR rates. Therefore, this work provides a design strategy for developing improved SCR catalysts via optimizing acid-redox properties of dinuclear sites.

Single-atom site catalysts for environmental catalysis

“…However, the development of high-performance NH3-SCR catalysts operated under various hash conditions is extremely urgent as regulation related to pollution emission become more and more stringent. Regarding this, Tang and coworkers developed Mo1/Fe2O3 SAC with isolated Mo ions and one neighboring Fe ions to better understand the active sites and design high-performance NH3-SCR catalysts at the atomic level [65]. Several other Mo1/Fe2O3 SACs with different Mo loading have also been developed to clarify the real active sites over Mo1/Fe2O3 for NH3-SCR reaction.…”

“…However, since various species, such as CO, NO, C3H6 and C3H8, are involved in fueled engine exhaust systems, both TWCs and DOCs should simultaneously eliminate all these harmful components. Similar to independent catalysis of CO oxidation and NO reduction, both CO and NO could easily be eliminated from gasoline-fueled engine exhaust systems by SACs [58,65,66]. However, the usefulness of SACs and small clusters with ensemble sites catalysts have been the debate for HC (CH4, C3H6 and C3H8) oxidation [56,58,66,128].…”

Single‐atom Automobile Exhaust Catalysts

“…[144] However, it is still debated whether the SCR reaction proceeds via the mononuclear V or the dinuclear V 2 sites. [145] This disagreement is mainly ascribed to the difficulty of precisely designing uniform mononuclear or dinuclear sites. Other types of metal oxides, including single MnO x , [146] mixed metal (Mn, Fe, Ce, Cu, W, Ti, Sn, Nb) oxides, [146][147] and the TiMO x (Cu, Fe, Co, Mn, Cr), CeWO x , and CeZrO x -based solid solution catalysts, [139f,148] all require acid and redox properties.…”

“…Recently, Tang and Chen's group designed a single-site Mo 1 /Fe 2 O 3 catalyst where a uniform MoÀ Fe dinuclear site was assembled with one discrete acidic Mo ion and its neighboring redox Fe ion. [145] The characterization and atomic-scale structure of the Mo 1 /Fe 2 O 3 catalyst can be seen in Figure 14. The calculated TOF (NO conversion amount per second per MoÀ Fe dinuclear site) on the Mo 1 /Fe 2 O 3 catalyst is~1.7 × 10 À 3 s À 1 at 270°C, which is comparable to the TOFs of 1.34 × 10 À 3 s À 1 and 4.80 × 10 À 3 s À 1 (NO conversion amount per second per V site at 277°C) over commercial 0.8 wt% V 2 O 5 /TiO 2 and 1.5 wt% V 2 O 5 /TiO 2 , respectively.…”

“…Furthermore, the SCR rate can be adjusted by selectively altering the acid or redox sites, which provides a more comprehensive identification of the SCR active site. The discrepancy in activity that results from modifying acid-redox properties (MoÀ Fe, WÀ Fe, and FeÀ W) in SACs [145] seems to explain some previously observed experimental phenomena, including (1) some dimeric V sites have a higher SCR rate than monomeric V sites; [150] (2) the promotional effect of WO 3 or MoO 3 in the classical V 2 O 5 /TiO 2 catalyst is due to the possible partial formation of dinuclear WÀ V or MoÀ V sites; [151] and (3) the SCR reaction preferentially proceeds at acid-redox oxide interfaces (MoÀ Fe, WÀ Fe, WÀ Ce oxides, etc.). [152] The application of SACs in the SCR reaction is still uncommon, but the further study of this uniform dinuclear structure by selectively tailoring acid-redox sites would help researchers understand the SCR reaction mechanism, cast light on the nature of the SCR active site, and propose reasonable protocols to design more attractive SACs.…”