Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust

Increasingly stringent emission legislations, such as US 2010 and Euro VI, for NOx in mobile applications will require the use of intensification of NOx reduction aftertreatment technologies, such as the selective catalytic reduction (SCR). Due to the required higher deNOx efficiency, a lot of efforts have recently been concentrated on the optimization of the SCR systems for broadening the active deNOx temperature window as widely as possible, especially at low temperatures, enhancing the catalysts durability, and reducing the cost of the deNOx system. This paper provides a comprehensive overview of the state-of-the-art SCR technologies, including the alternative ammonia generation from the solid reductants, Vanadium-based, Cu-zeolite (CuZ) and Fe-zeolite (FeZ) based, and the novel chabazite zeolite with small pore size SCR catalysts. Furthermore, the progresses of the highly optimized hybrid approaches, involving combined CuZ and FeZ SCR, passive SCR, integration of DOC + (DPF, SCR), as well as SCR catalyst coated on DPF (referred as SCRF hereinafter) systems are well discussed. Even though SCR technology is considered as the leading NOx aftertreatment technology, attentions have been paid to the adverse by-products, such as NH3 and N2O. Relevant regulations have been established to address the issues.