Role of the Acid Site for Selective Catalytic Oxidation of NH₃ over Au/Nb₂O₅

Abstract

Selective catalytic oxidation (SCO) of NH3 to harmless N2 and H2O is an ideal technology for its removal. To develop air purification systems for a living environment, catalysts that can work at room temperature with high selectivities to N2 are required. However, it has been a technical challenge because the reported catalysts either needed high operating temperatures or showed low selectivities to N2. In this study, we first demonstrated that acidic metal-oxide-supported gold catalysts showed good N2 selectivities compared with that of other metal-oxide-supported gold catalysts. A gold catalyst with niobium oxide synthesized by the hydrothermal method as a support showed high catalytic activity and high selectivity to N2 at low temperatures (18% NH3 conversion with 100% N2 selectivity at 25 °C) and at high temperatures (100% NH3 conversion with 95% N2 selectivity at 245 °C). Important roles of Brønsted acid sites and formation of active oxygen sites in improving N2 selectivity were revealed in this study. To the best of our knowledge, this is the first report of efficient catalysts that presented high NH3 conversion with high N2 selectivity at 25 °C which will offer great scopes for commercial applications related to control of odors. In addition, this breakthrough finding that acid sites would greatly affect N2 selectivity and catalytic activity will provide a new trend in designing efficient catalysts not only for SCO of NH3 but also for the other selective catalytic oxidation.

Praseodymia–titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides

“…Then N 2 gas was introduced at −180 °C with a flow rate of 50 mL min −1 for 30 min to fully exclude the gas phase and weakly adsorbed CO species. 36–38…”

Heterogeneous Gold Catalysis: From Discovery to Applications

“…As discussed in the previous section, the catalytic activity of supported AuNPs catalysts is extremely dependent on the size of AuNPs. The preparation of supported AuNPs catalysts is certainly considered as a strategic aspect strongly affecting the size of AuNPs [32][33][34]. Among the various methods for the preparation of Au catalysts, the impregnation approach finds particularly frequent applications.…”

Progress on Noble Metal-Based Catalysts Dedicated to the Selective Catalytic Ammonia Oxidation into Nitrogen and Water Vapor (NH3-SCO)

“…Thus, Lippits et al [77] observed the enhanced NH 3 conversion (full NH 3 conversion at 338 • C with N 2 selectivity below 50%) over Au/Al 2 O 3 after its doping with CeO x (able to provide and store oxygen) and Li 2 O (responsible for decreasing the catalyst acidity and thus improved oxygen adsorption). Recently, Lin et al [12] reported the acidic metal-oxide-supported gold catalyst (Au/Nb 2 O 5 ) with improved N 2 selectivity compared to other metal-oxide-supported gold catalysts (e.g., Au/SiO 2 , Au/Al 2 O 3 , Au/Fe 2 O 3 , etc., Figure 6a). Specifically, Au/Nb 2 O 5 contains both Brønsted and Lewis acid sites that allowed NH 3 oxidation according to hydrazine mechanism (N 2 H 4 as the intermediate) and imide mechanism (HNO as the intermediate), respectively (Figure 6b).…”

“…Various kinds of catalysts were studied for NH 3 -SCO, including noble metals (e.g., Ru [11]), supported noble metals (e.g., Au/Nb 2 O 5 , Au/ZrO 2 [12]), (mixed) metal oxides, supported metal oxides, modified zeolites (e.g., Pt-CuMgAlO x [13], Ag-USY [13]), etc. These groups of catalysts investigated in NH 3 -SCO were summarized by Jabło ńska et al [7,14], Gao et al [15] and Lan et al [16].…”