A series of manganese-cerium oxide catalysts were prepared by co-precipitation method and used for low temperature selective catalytic reduction (SCR) of NO x with ammonia in the presence of excess O 2 . These catalysts were characterized by X-ray diffraction (XRD), surface area measurement and FTIR. The experimental results showed that the best Mn-Ce mixed-oxide catalyst yielded 95% NO conversion at 150 °C at a space velocity of 42,000 h −1 . As the manganese content was increased from 0 to 40% (i.e. the molar ratio of Mn/(Mn+Ce)), NO conversion increased significantly, but decreased at higher manganese contents. The most active catalyst was obtained with a molar Mn/(Mn+Ce) ratio of 0.4. Only N 2 rather than N 2 O was found in the product when the temperature was below 150 °C. At higher temperatures, trace amounts of N 2 O were detected. A mechanistic pathway for this reaction was proposed based on earlier findings and FTIR results obtained in this work. The initial step was the adsorption of NH 3 on Lewis acid sites of catalyst, followed by reaction with nitrite species to produce N 2 and H 2 O. Possible intermediates are proposed and all the intermediates could transform into NH 2 NO, which could further react to produce N 2 and H 2 O.

MnOx-CeO2 mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH3 at low temperatures

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

Reduction of NOx with Ammonia over Fe/MFI: Reaction Mechanism Based on Isotopic Labeling

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+.

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

The interaction of O2 with Fe supported on zeolite MFI has been investigated by the in situ UV Raman technique. Previously, the formation of adsorbed superoxide ions, O2-, on the Fe/MFI prepared by sublimation of FeCl3 vapor onto HMFI, was identified by ESR at 77 K. In situ Raman data indicate that adsorbed peroxide ions, O22-, are formed on the same catalysts with Fe/Al = 1 even at 300 K. A band at 730 cm-1, which was found to be sensitive to the partial pressure of O2, is tentatively assigned to the stretching vibration of peroxide ions. With the 18O2 isotope, a red shift of 32 cm-1 was observed for the 730 cm-1 band. Raman features of other catalyst samples show that the binuclear iron sites in the Fe/MFI are essential for the formation of peroxide ions. As proposed before, the same bridging sites between two Fe ions, that are occupied by O2- in the calcined catalyst, are also able to adsorb di-oxygen complexes.

Qi Sun

Papers

Reduction of NOx with Ammonia over Fe/MFI: Reaction Mechanism Based on Isotopic Labeling

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

UV-Raman characterization of iron peroxo adsorbates on Fe/MFI catalyst with high activity for NOx reduction