Ag Size/Structure-Dependent Effect on Low-Temperature Selective Catalytic Oxidation of NH₃ over Ag/MnO₂

Abstract

Selective catalytic oxidation of ammonia (NH3-SCO) into N2 and H2O is considered to be a promising technique to eliminate NH3 pollution. Various Ag/MnO2 catalysts were prepared and applied to low-temperature (especially <100 °C) NH3-SCO. The Ag/MnO2-X (X indicating the calcination temperature) catalyst had a variable Ag size and structure depending on the calcination temperature from 200 to 500 °C. The Ag/MnO2-400 catalyst showed 10% NH3 conversion at 35 °C, and the temperature was 90 °C for the complete removal of 50 ppm NH3. More importantly, the N2 selectivities of NH3-SCO over Ag/MnO2-400 at the temperatures ranging from 30 to 120 °C were higher than 96%. Turnover frequencies of NH3 oxidation, N2 selectivity, and byproducts depended on the Ag size/structure. Ag/MnO2-400 with Ag particles of 2.4 nm diameter had the highest density of Ag particles over rod-shaped MnO2 during the reaction, which would be favorable for the formation of adsorbed NO and NH2NO intermediates to form N2 and H2O.

Understanding the Distinct Effects of Ag Nanoparticles and Highly Dispersed Ag Species on N2 Selectivity in NH3–SCO Reaction

“…We observed an Ag particle size/structure-dependent effect on the NH 3 –SCO catalytic performance. The Ag/MnO 2 -400 catalyst showed 10% NH 3 conversion at 35 °C and 100% NH 3 conversion with 99% N 2 selectivity at 100 °C . We propose that the rearrangement of Ag atoms on Al 2 O 3 can effectively change the properties of the Ag structure, which, therefore, adjusts the N 2 selectivity of the catalyst.…”

“…The Ag/ MnO 2 -400 catalyst showed 10% NH 3 conversion at 35 100% NH 3 conversion with 99% N 2 selectivity at 100 °C. 31 We propose that the rearrangement of Ag atoms on Al 2 O 3 can effectively change the properties of the Ag structure, which, therefore, adjusts the N 2 selectivity of the catalyst. On the other hand, NH 3 −SCO is a complicated multistep reaction with many corresponding intermediates.…”

Theoretical design principles of metal catalysts for selective ammonia oxidation from high throughput computation

“…It is in accord with the experimental phenomenon that the turnover frequency of supported Ag nanoparticles decreases with the increasing particle size. 1 In general, the volcano relation indicates that a moderate E(N*) (À5.00 $ À4.00 eV) could achieve the most desirable NH 3 oxidation activity, which gives a specic theoretical principle for the catalyst screening for NH 3 oxidation. Then, the relation of N 2 formation rates of (100) and ( 111) against binding energies of N and O atoms (E(N*) and E(O*)) is displayed in Fig.…”

“…Ammonia (NH 3 ) is a colorless atmospheric pollutant with a pungent smell and is harmful to human health, and even an ultra-low concentration of ammonia could hurt the eyes, skin and respiratory tract of humans. 1,2 Ammonia emission originates from diverse sources involving agricultural production, industrial processes, and transportation, which pose a great challenge to the control and elimination of ammonia emission. [3][4][5][6] Selective catalytic oxidation of ammonia (NH 3 -SCO) to innocuous N 2 and H 2 O is considered the key enabling technology for ammonia removal due to its good safety and low cost.…”

Ammonia Abatement via Selective Oxidation over Electron-Deficient Copper Catalysts

“…A variety of supported catalysts, including noble metals, transition metal oxides, and bimetallic (containing both noble and non-noble metals) catalysts, have been explored for NH 3 -SCO applications. Supported noble metals, such as Pt, − Ag, − Au, and Ru, are highly active at low temperatures, but their application is hindered by the poor selectivity to N 2 , the easy formation of NO x byproducts, and the high cost. Transition metal oxides, despite their relatively lower activities compared with those of noble metals at low temperatures, offer opportunities for highly efficient slip NH 3 abatement thanks to their high N 2 selectivity and low cost. , Among others, Cu-based catalysts are particularly promising because they often demonstrate relatively high activity and excellent N 2 selectivity. − Nevertheless, the temperature achieving complete NH 3 conversion, which was found to be above 240 °C for most of the reported Cu-based catalysts, ,− is higher than that of typical flue gases after NH 3 -SCR treatments and requires additional energy input to achieve in practical applications.…”