Recent advances in automotive catalysis for NO_x emission control by small-pore microporous materials

Abstract

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

Understanding Catalytic Reactions over Zeolites: A Density Functional Theory Study of Selective Catalytic Reduction of NOx by NH3 over Cu-SAPO-34

“…Some researchers 16,17 argued that a mixture of Cu(I)-Cu(II) oxidation states will exist during standard SCR process, but it is not generally accepted yet. 4 Another interesting phenomenon is that fast SCR (equation 2) reacts more rapid than standard SCR (equation 1), of which the intrinsic reason is still under debate. Some studies 18,19 suggested that in fast SCR, NO and NO 2 would bind in gas phase to form N 2 O 3 , followed by a hydrolysis to nitrous acid (N 2 O 3 + H 2 O −−→ 2HONO).…”

“…Among zeolites, a majority of studies in past decade were conducted on copper exchanged small-pore CHAtype zeolites (Cu-SAPO-34 and Cu-SSZ-13) because of their high NO x conversion as well as N 2 selectivity, non-toxicity, wider operation temperature range, and excellent hydrothermal stability in diesel engines. [2][3][4]9 Owing to the uniqueness of zeolite structure, the reaction mechanism of NH 3 -SCR differs a lot between zeolites and traditional catalysts. For example, there are two kinds of possible active sites in Cu exchanged CHA-type zeolites: the Brønsted acid (H in the zeolite framework) and Lewis acid (introduced Cu ion) site.…”

“…[1][2][3][4] Among possible solutions, selective catalytic reduction (SCR) of NO x by ammonia (NH 3 -SCR) over metal-exchanged CHA-type zeolites has attracted great attention in recent years. As the main N-containing compounds from diesel engines is NO (>90%), the main reaction of NH 3 -SCR can be expressed as: 5…”

Coke Formation in a Zeolite Crystal During the Methanol‐to‐Hydrocarbons Reaction as Studied with Atom Probe Tomography

“…[3,4] Commercially,one of the most important zeolites is ZSM-5 with MFI framework topology,w hich has become ubiquitous in petroleum refining and chemical manufacturing.…”

“…[3,4] Commercially,one of the most important zeolites is ZSM-5 with MFI framework topology,w hich has become ubiquitous in petroleum refining and chemical manufacturing. [2] Thee normous quantities at which this material is utilized at the global scale continue to drive research targeting improved performance.…”