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

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?

Reduction of N2O with CO over FeMFI zeolites: influence of the preparation method on the iron species and catalytic behavior

https://iopscience.iop.org/article/10.1088/1468-6996/16/3/036002/pdf

Perovskite-type catalytic materials for environmental applications

The decomposition of N2O, and the catalytic reduction by NH3 of N2O and N2O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of FeII-zeolite with N2O (α-oxygen), are formed in largest amounts with this material. The decomposition of N2O is promoted by addition of NH3, and even more with NH3 + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N2O originated from the fast surface reaction between α-oxygen O∗ and NO∗ to yield NO2∗, which in turn reacts immediately with NH3.

Catalytic decomposition of N2O and catalytic reduction of N2O and N2O + NO by NH3 in the presence of O2 over Fe-zeolite

Three options for removal of N 2 O using a promoted Fe-ZSM-5 catalyst have been explored under conditions representative for the off gases of a nitric acid plant: catalytic decomposition and selective catalytic reduction (SCR) using propane or methane as a reductant. Catalytic decomposition of N 2 O takes place at temperatures above 400°C at a space velocity of 10,000 h −1 . The addition of propane lowers the temperature for N 2 O conversion with about 100°C. In propane-assisted SCR of N 2 O, the emissions of unreacted hydrocarbons and of CO are low and also NO x reduction takes place. Methane is more difficult to be activated by the catalyst, which causes lower N 2 O destruction efficiency and the output of unreacted methane. In all cases, N 2 O conversion is higher at elevated pressure.

D.F Bakker

Papers

Catalytic removal of N2O in model flue gases of a nitric acid plant using a promoted Fe zeolite