Selective catalytic oxidation of NH3 over noble metal-based catalysts: state of the art and future prospects

Abstract

Selective catalytic oxidation of ammonia to nitrogen and water vapor (NH₃-SCO) is considered to be an efficient technique to eliminate hazardous and pungent gaseous NH₃ is mainly emitted from selective catalytic reduction of NO_x with NH₃ units using appropriate catalysts. Due to the satisfactory low-temperature ammonia oxidation activity, noble metal-based catalysts are the primary groups applied in the NH₃-SCO reaction on account of their unique intrinsic properties. Meanwhile, it is essential that selectivity towards byproducts, including NO, NO₂ and N₂O, is as low as possible so that the high N₂ selectivity meets the stringent environmental policies in practical scenarios. To provide an overview of the NH₃-SCO reaction, this review starts with a general understanding of reaction mechanisms and catalyst promotion strategies. After that, the noble metal-based catalysts, namely Pt, Ag, Ru, Ir, Pd, Au and Rh-based catalysts, are discussed from the aspects of the regulation of active species, the adsorption and activation of reactants and intermediates, the formation of N-containing products and the coupling effects with NH₃-SCR catalysts. Finally, a condensed summary of previous work as well as reasonable suggestions and perspectives on future trends in this field are presented. To the best of our knowledge, this is the first comprehensive review on the application of noble metal-based catalysts in the NH₃-SCO reaction and we hope that this article will shed light on the rational catalyst design and in understanding the structure–property relationships of noble metal-based NH₃-SCO catalysts.