Computational Study of the NH₃ de-NOₓ Selective Catalytic Reduction on the Mono-Cu Active Site of Cu-SSZ-13

The release of greenhouse gases such as NOX and the resulting climate change is a major global issue. Diesel engines are a prominent emitter and to mitigate against this, NOX is catalysed to H2O and N2 in the exhaust gas flue by the zeolite Cu-SSZ-13 via the SCR reaction. The exact reaction mechanism this proceeds by is currently unknown despite significant progress in recent years. A precise understanding of the reaction will allow for rational design of new catalysts allowing for higher conversion of NOX into H2O and N2. This study provides new insights into the speciation of the active copper complex and further elucidates the reaction pathway and the elementary reaction steps, using a computational methodology. It is found that the form of the active Cu complex is dynamic and shifting with multiple potential accessible minimum energy states under reaction conditions, and they can have significantly different geometries. The effect that different geometries have upon the activation barriers for steps in the reaction cycle is something overlooked by literature studies and is shown here to have an important effect; the lowest energy starting structure does not always give the lowest energy reaction pathway. Different reaction routes are proposed in literature, here we compare the routes on the same energy scale which has not previously been studied. It is found that for the reduction half cycle formation of NH4NO2 via HONO is the lowest energy route, but others may become active at higher temperatures. The form of the active Cu complex during the oxidation half cycle is debated in the SCR literature with the general consensus being that it is dimeric at low temperatures when Cu is mobile. It is shown here that the oxidation half-cycle can proceed on a monomer via a peroxynitrite intermediate previously unknown to the SCR literature.