Copper based catalysts for the selective ammonia oxidation into nitrogen and water vapour—Recent trends and open challenges

Abstract

More restrictive standards of Euro VI concerning nitrogen oxide (NOx=NO, NO2) emissions necessitate an enhanced urea injection, while generating a higher ammonia slip, which is also precisely limited. For this reason, ammonia slip catalysts (ASC) are an essential part of efficient aftertreatment systems. Currently, supported noble metal catalysts are applied but possess limited selectivity to nitrogen. Copper based catalysts present a promising alternative for the selective catalytic ammonia oxidation into nitrogen and water vapour (NH3-SCO). This review article focusses on NH3-SCO as appropriate solution to abate unreacted ammonia particularly after the selective catalytic reduction of NOx with ammonia (NH3-SCR, DeNOx). A brief overview of potential catalyst systems is provided, followed by a comprehensive discussion of copper based catalysts. Potential material classes including oxides, exchanged zeolites, modified clays or mixed forms are described systematically. The review focusses on structure-performance correlations covering copper loading, redox properties of CuO species and available acid sites of the catalysts. Another emphasis concerns the influence of the feed composition on the catalytic performance including the content of oxygen and water vapour or sulphur oxide in the feed. Finally, the proposed i-SCR mechanism over copper based catalysts, including bimetallic systems, is described and critically reviewed followed by general conclusions together with a discussion of promising research directions.