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

Nitric acid production represents the largest source of N2O in the chemical industry, with a global annual emission of 400 kt N2O. The high impact of N2O on the environment as greenhouse gas and stratospheric ozone depletor, and the ongoing agreements and prospective regulations calls for the development of efficient and economical systems for N2O mitigation, but no mature commercial technology is yet available. In this review, the current state-of-the-art for N2O control in the nitric acid manufacture is presented. The formation of N2O and the process are analyzed and several options for reducing its emissions are discussed, depending on the position in the process. Primary abatement options deals with modifications in the ammonia oxidation catalyst, secondary abatement with options between the ammonia converter and the absorber, tertiary abatement with options in the tail-gas upstream of the expander, and quaternary abatement with options in the tail-gas downstream of the expander. The abatement technologies are evaluated based on the technical advantages and disadvantages, and cost efficiency.

Formation and control of N2O in nitric acid production: Where do we stand today?

A method is presented for estimating an amount of ammonia stored in a urea-based SCR catalyst based on a dynamic model of the catalyst. The model takes into account chemical and physical properties of the catalyst, such as catalyst volume, the number of available ammonia storage cites, adsorption and desorption dynamics, as well as poisoning, thermal aging, and different catalyst operating temperatures, and generates the estimate based on a measured or estimated amount of NOx in an exhaust gas mixture upstream of the catalyst, an amount of reductant injected into the catalyst to facilitate NOx reduction, and on a measured value of NOx in an exhaust gas mixture downstream of the catalyst. The estimated ammonia storage amount is then used to control the amount of reductant injected into the catalyst to maintain desired ammonia storage amount such that maximum NOx conversion efficiency coupled with minimum ammonia slip are achieved.

Exhaust gas aftertreatment systems

Effects of hydrothermal aging on NH3-SCR reaction over Cu/zeolites

Selective Catalytic Reduction of Nitric Oxide by Ammonia over Cu-FAU Catalysts in Oxygen-Rich Atmosphere

A series of Fe−zeolite-beta (Fe−BEA and 57Fe−BEA) has been prepared either by exchanging BEA (Si/Al = 12.5) with Fe(NO3)3 or by impregnation. The identification of Fe species in Fe−BEA has been car...

Identification of Iron Species in Fe-BEA: Influence of the Exchange Level

An Fe-Zeolite-beta (Fe-BEA) catalyst, composed only of Fe cations or oxocations in charge compensation sites of the zeolite, is active in the simultaneous removal of NO (1500 ppm) and N2O (1000 ppm) by NH3 (2500 ppm) in the presence of O2 (3 vol.%). In temperature-programmed surface reaction (TPSR) experiments (ramp: 10 K min−1 from 423 to 823 K, space velocity: 200,000 h−1), the light-off temperature, at 50% conversion, is shifted to lower values viz. by 20 K for NO (≈590 K) and 40 K for N2O (≈670 K), compared to those found when processing NO and N2O alone. It is proposed that the removal of surface oxygen O*, coming from the interaction of N2O with iron sites, is faster with NO than with NH3. NO2 which is then formed reacts in turn very fast with NO and NH3 through the classical selective catalytic reduction (SCR) of NOx by NH3 in the presence of O2.

The simultaneous catalytic reduction of NO and N2O by NH3 using an Fe-zeolite-beta catalyst

The kinetics of the selective catalytic reduction (SCR) of NO by NH3 in the presence of O2 has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

The influence of ammonia on the reduction of N2O in presence of oxygen over Fe‐zeolite has been studied. It is found that BEA zeolite is the most efficient host structure for iron ions to catalyse the reduction of N2O with NH3.

Stéphane Kieger

Papers

Selective Catalytic Reduction of Nitric Oxide by Ammonia over Cu-FAU Catalysts in Oxygen-Rich Atmosphere

Identification of Iron Species in Fe-BEA: Influence of the Exchange Level

The simultaneous catalytic reduction of NO and N2O by NH3 using an Fe-zeolite-beta catalyst

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

Catalytic reduction of N2O by NH3 in presence of oxygen using Fe‐exchanged zeolites