An overview is given of the selective catalytic reduction of NOx by ammonia (NH3‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH3‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3‐SCR reaction over Fe‐ZSM‐5, with isolated species being the ...

https://www.tandfonline.com/doi/pdf/10.1080/01614940802480122

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

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

Catalysis for NOx abatement

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Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

The catalytic removal of nitrogen oxide (NOx) under lean‐burn conditions is one of the most important targets in catalysis research. Some lean‐NOx control technologies such as the direct decomposition of NOx, NOx storage‐reduction (NSR), and selective catalytic reduction (SCR) using different reducing agents (diesel soot, NH3, or hydrocarbon) are described. The reaction mechanism of NSR, which is the most promising technology, together with some novel NSR catalysts is discussed. Some mechanisms of SCR of NOx by hydrocarbon (HC‐SCR) were classified into two categories: one is the adsorption/dissociation mechanism, and the other is the oxidation‐reduction mechanism. Based on the discussion of the reaction mechanism, the influence of some factors (catalyst support, metal loading, calcination temperature, catalyst preparation method, oxygen, reducing agents, water, and sulfur) on the activity of HC‐SCR catalyst is discussed. It seems that Ag/Al2O3 catalyst offers the most promising for SCR of NOx by hydrocarb...

Recent Advances in Catalytic DeNOX Science and Technology

The reactions of acetaldehyde, acetic acid, and nitromethane with NO2 on a BaNa−Y zeolite are monitored with FTIR spectroscopy, typically at 473 K. Isotopic labeling was used to help elucidate reaction pathways and intermediates. The mechanism that has been deduced for the reaction of NO2 with acetaldehyde starts with the formation of acetate ions and subsequently the aci-anion of nitromethane. NO2 can react with the aci-anion of nitromethane, which is stabilized in the ionic environment of the zeolite to form a proposed O2NCH2NO2- intermediate. At 473 K, the O2NCH2NO2- intermediate is expected to swiftly lose NO2 and H2O yielding the formonitrile oxide anion, CNO-, which can isomerize to NCO- and react with a proton to form HNCO. The formation of an O2NCH2NO2- intermediate is consistent with the observation of both isotopomers of HNCO when 15NO2 is a reactant. Also, as expected for an O2NCH2NO2- intermediate, all four isotopomers of N2O are observed. In the next step, NH3 and CO2 are formed from HNCO + H...

NOx Reduction from Diesel Emissions over a Nontransition Metal Zeolite Catalyst: A Mechanistic Study Using FTIR Spectroscopy

The mechanism of selective catalytic reduction (SCR) of NOx with NH3 over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH3 then to flowing NO + O2 or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH4NO2. In situ FTIR results in flowing NO + NH3 + O2 at different temperatures show that NH3 is strongly adsorbed and reacts with impinging NOx. The intensity of the NH4NO2 bands initially increases with temperature, but passes through a maximum at 120 °C because the nitrite decomposes to N2 + H2O. The mechanistic model rationalizes that the consumption ratio of NO and NH3 is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H_2O enhances the rate because it is needed to form NH4NO2. At low temperature, when adsorbed H2O is abundant it lowers the rate because it competes with NOx for adsorption sites.

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

Reduction over zeolite-based catalysts of nitrogen oxides in emissions containing excess oxygen: Unraveling the reaction mechanism

Reduction in an H2 flow at 600 °C of Fe/MFI prepared by chemical vapor deposition, followed by its exposure to N2O at 250 °C, produces a highly active state characterized by an unusual TPR spike at 200 °C. In situ X-ray absorption near-edge structure, X-ray absorption fine structure data and literature data on DFT calculations suggest that in this state some Fe will be present in the oxidation state of Fe4+.

Bin Wen

Papers

NOx Reduction from Diesel Emissions over a Nontransition Metal Zeolite Catalyst: A Mechanistic Study Using FTIR Spectroscopy

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

Reduction over zeolite-based catalysts of nitrogen oxides in emissions containing excess oxygen: Unraveling the reaction mechanism

Identification of highly active iron sites in N2O-activated Fe/MFI

Identification by Isotopic Exchange of Oxygen Deposited on Fe/MFI by Decomposing N2O