Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts—A review

Excellent activity and selectivity of Cu-SSZ-13 in the selective catalytic reduction of NOx with NH3

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.

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

Selective catalytic reduction (SCR) of NO with NH3 over manganese substituted iron titanate catalysts was fully studied. The low temperature SCR activity was greatly enhanced when partial Fe was substituted by Mn, although the N-2 selectivity showed some decrease to a certain extent. The Mn substitution amounts showed obvious influence on the catalyst structure, redox behavior and NH3/NOx adsorption ability of the catalysts. Among FeaMn1-aTiOx (a = 1, 0.75, 0.5, 0.2, 0) serial catalysts, Fe0.5Mn0.5TiOx with the molar ratio of Fe:Mn = 1: 1 showed the highest SCR activity, because the interaction of iron, manganese and titanium species in this catalyst led to the largest surface area and the highest porosity, the severest structural distortion and most appropriate structural disorder, the enhanced oxidative ability of manganese species, the highest mobility of lattice oxygen, the proper ratio of Bronsted acid sites and Lewis acid sites together with the enhanced NOx adsorption capacity. (C) 2009 Elsevier B.V. All rights reserved.

Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Lowenstein’s rule. Further, exchanged CuII ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as CuIIOH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordinatio...

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites

The chemistry of the NO/NO2–NH3 “fast” SCR reaction over Fe-ZSM5 investigated by transient reaction analysis

Study of a Fe–zeolite-based system as NH3-SCR catalyst for diesel exhaust aftertreatment

Ammonia blocking of the “Fast SCR” reactivity over a commercial Fe-zeolite catalyst for Diesel exhaust aftertreatment

The activity and the mechanism of the main reactions in the NO/NO2–NH3 SCR reacting system were comparatively investigated over a Fe- and a Cu-promoted commercial zeolite catalyst for the aftertreatment of Diesel exhausts. A dynamic micro-kinetic model in close agreement with all the details of the SCR catalytic chemistry was also developed.

A. Grossale

Papers

The chemistry of the NO/NO2–NH3 “fast” SCR reaction over Fe-ZSM5 investigated by transient reaction analysis

Study of a Fe–zeolite-based system as NH3-SCR catalyst for diesel exhaust aftertreatment

Ammonia blocking of the “Fast SCR” reactivity over a commercial Fe-zeolite catalyst for Diesel exhaust aftertreatment

NH3–NO/NO2 SCR for Diesel Exhausts Aftertreatment: Reactivity, Mechanism and Kinetic Modelling of Commercial Fe- and Cu-Promoted Zeolite Catalysts

FTIR in situ mechanistic study of the NH3NO/NO2 “Fast SCR” reaction over a commercial Fe-ZSM-5 catalyst