Based on the air quality in Europe - 2019 report of European Environment Agency, nitrogen oxides (NOx) are included to the most harmful air pollutants in terms of damage to ecosystems. Moreover, NO2 is one of the most Europe’s dangerous pollutants to human health. Anthropogenic emissions of NOx are mainly generated by combustion of fossil fuels. Nitrogen oxides being emitted into the atmosphere cause such environmental problems as acid rains, acidification of soil, lakes and rivers, eutrophication and photochemical smog.
The most effective and widely used technology achieved for today to purify flue gases from NOx is selective catalytic reduction using ammonia (NH3-SCR de-NOx). Nowadays, one of the most significant directions in the field of NH3-SCR de-NOx is applying of new nonconventional methods and preparation of new catalysts possessing high specific surface area, uniformity, dispersion of active sites, high activity and selectivity. Atomic layer deposition (ALD) is an attractive technique for deposition of uniformly distributed active catalytic layers or nanoparticles on highly porous substrates being characterized by a complex structure and highly developed surface, where the application of conventional methods (e.g. impregnation or deposition precipitation) can be problematic. The significant advantage of ALD application for preparation of supported catalysts is that the preparation process can be controlled on the atomic scale providing the required thickness of an active layer synthesized with accuracy of up to one atom. Moreover, ALD ensures the formation of catalysts in a gas phase instead of a liquid phase, which enhance the possibility of active species to be deposited inside pores which are very small in size.
In this study ALD was applied for preparation of VxOy-based NH3-SCR de-NOx catalysts. Highly porous silica gel powder (63-100μm) with a specific surface area of up to 450 m2·g-1 was used as a substrate for active layer deposition. VxOy/SiO2 with different metal loadings (wt.%) and (VxOy+TiO2)/SiO2 catalysts were prepared by applying the following metal precursors: vanadium (V) tri-i-propoxy oxide (VTIP) and titanium tetrachloride (TiCl4), while deionised water was used as the co-reactant. ICP-OES results revealed that vanadium loadings in VxOy/SiO2 catalysts were 0.3, 0.7, 1.1 and 1.6 wt.%, while the metals loading in the TiO2-promoted VxOy/SiO2 catalyst was 1.04 wt.% and 0.15 wt.% for V and Ti, respectively. According to XPS spectra V2O3 and V2O5 species were identified in the prepared VxOy/SiO2 catalyst with the predominance of vanadium (V) compounds (V2O5/V2O3 ratio was 1.6 and 6.3 for as-synthesized and calcined samples, respectively). V2O5 is found as an active vanadium form for NH3-SCR de-NOx. Additionally, TEM, XRD and BET analyses were conducted to provide a comprehensive characterization of the catalysts prepared.