Catalytic activity of natural clinoptilolite modified with iron by various methods in selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR)

Abstract

Nitrogen oxides (NOₓ) are one of the most harmful air pollutants emitted by stationary sources. The mixture of NOₓ has a great contribution to acid rain and photochemical smog formation and ozone layer depletion. In order to abate the amount of nitrogen oxides, governments around the world implemented a number of emission control legislations. Meeting more severe restrictions can be achieved only by using effective methods of NOₓ reduction. One of the widespread and efficient technologies used on the industrial scale is selective catalytic reduction with ammonia (NH₃-SCR). The reaction between nitrogen and the redUCing agent (NH₃) takes place on the surface of the catalyst, consequently, nitrogen and water vapor are yielded. The commercial catalyst of the process is V₂O₅-WO₃(MoO₃)- TiO₂. Despite satisfactory activity above 300°C, it causes some operational problems. The most important are its narrow temperature window and secondary contamination by toxic vanadia and oxidation of SO₂ to SO₃. The mentioned problems can be solved by the replacement of the commercial system by substitutive catalyst, free of these drawbacks. One of the most promising precursors of new NH₃-SCR catalysts are natural zeolites, such as clinoptilolite. It is mainly due to its acidic character, micro- and mesopore texture and high thermal stability. Vanadium active phase can be replaced by iron species that have already found wide application in NH₃-SCR. Therefore, in the presented work, natural clinoptilolite was modified with iron by co-precipitation and adsorption from solution and tested as a catalyst of NH₃-SCR. It was found that even raw zeolite exhibits 56% of NO conversion at 450°C. Catalytic activity of the material was much higher when co-precipitation was applied as the modification procedure. Furthermore, when the active phase was introduced by adsorption from solution, the amount of emitted N₂O was higher in the entire temperature range of the reaction. XRD analysis performed in the temperature range of 100-500 °C indicated that the structure of both the raw zeolite and the catalysts remains stable within the temperature range of the reaction.