Based on the air quality in Europe - 2019 report of European Environment Agency, nitrogen oxides (NO_x) 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 NO_x 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 NO_x is selective catalytic reduction using ammonia (NH₃-SCR de-NO_x). Nowadays, one of the most significant directions in the field of NH₃-SCR de-NO_x 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 V_xO_y-based NH₃-SCR de-NO_x 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. V_xO_y/SiO₂ with different metal loadings (wt.%) and (V_xO_y+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 V_xO_y/SiO₂ catalysts were 0.3, 0.7, 1.1 and 1.6 wt.%, while the metals loading in the TiO₂-promoted V_xO_y/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 V_xO_y/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-NO_x. Additionally, TEM, XRD and BET analyses were conducted to provide a comprehensive characterization of the catalysts prepared.