Selective catalytic reduction of NOx by NH3 over V2O5-WO3 supported by titanium isopropoxide (TTIP)-treated TiO2
Graphical abstract
Introduction
The combustion of fossil fuels produces nitrogen oxides (NO, NO2, and N2O, collectively denoted as NOx) which are responsible for several forms of environmental pollution such as smog, particulate matter 2.5, and acid rain and are therefore strictly regulated [1], [2], [3], [4]. Consequently, much attention has been drawn to the reduction of NOx emissions from stationary sources which exploit the energy of fossil fuel combustion, including power plants, industrial boilers, cement kilns, and turbines. Typical NOx removal techniques include selective non-catalytic reduction (SNCR), selective catalytic reduction (SCR), and storage-reduction. Among these methods, SCR is viewed as the most chemically efficient [4], [5], [6], as it requires lower temperatures than SNCR (300–400 °C vs. 850–1100 °C, respectively) and complies with stricter NOx regulations. SCR can be performed with ammonia as a reducing agent (NH3-SCR), in which NOx is converted into N2 and H2O [7], [8].
NH3-SCR can be categorized into two types: a high dust system located immediately after the exhaust gas and a low dust system at the tail end of scrubbers and dust collectors. In most cases, high dust systems located just behind the exhaust gas use a conventional V2O5-WO3 (V-W) catalyst due to its excellent NOx removal efficiency and reasonable sulfur resistance [9]. However, these catalysts require temperatures above 300 °C and cannot be used to treat the relatively cold (<250 °C) exhaust fumes of steel, cement, and glass factories [10], [11], [12], [13], [14]. Therefore, research has been focused on the development of catalysts promoting NH3-SCR at low temperatures, as exemplified by Cu- or Mn-based catalysts [8], [13], [15], [16], [17], [18], [19] and zeolite-based catalysts prepared using ion exchange [20], [21], [22], [23]. It is important to note that operation at low temperatures may result in active site blockage and destruction due to the deposition of sulfates such as NH4HSO4 and (NH4)2SO4, but ammonium sulfate (AS) can be removed by heating at >350 °C to regenerate the exhaust system [24], [25], [26], [27], [28]. Therefore, a practical SCR catalyst that resists sulfur poisoning and exhibits stable performance over a wide temperature range (240–450 °C) is needed for use in both systems. To achieve this, synthesis of highly dispersed catalysts is essential to increase their long-term durability.
Although some V-W catalysts have been commercialized because of their high sulfur resistance and stable performance, their activity at low temperatures still needs to be enhanced such as by increasing the number of active sites and dispersion uniformity [29], [30], [31]. Surface modification has been widely used to increase the number of active sites in SCR catalysts. A recent study has correlated the SCR activity of vanadium with its coordination environment, interaction with the catalyst support, VOx dispersion and composition, and the number of dinuclear sites [32], [33], [34]. Moreover, depending on whether and how their active sites are connected by weak van der Waals forces, catalysts may exhibit different redox properties, bond strengths, and other parameters affecting the coordination environment [35], [36]. WOx is commonly used as a promoter of supported VOx catalysts, enhancing the activity of vanadium sites and increasing the NH3 adsorption capacity of the catalyst surface [13], [37], [38], [39], [40], [41]. TiO2 is a popular support for heterogeneous catalysts, exhibiting unique optoelectronic properties that strongly depend on the synthetic route. For example, local non-stoichiometric or geometric defects on the support surface directly affect its electronic structure, influence its interactions with the active materials [42], [43], and provide other advantages [44]. The surface modification of TiO2 was shown to affect the atomic arrangements of V and W and thus influenced the ability of the supported species to promote surface reactions, even when the contents of V2O5 and WO3 remained unchanged [45]. Titanium isopropoxide (TTIP) is commonly used as a TiO2 precursor, affording TiO2 particles with rod-like or spherical shapes (depending on the solvent). The use of ethanol as a solvent allows for pore size control [46], [47]. Herein, we used different loadings of TTIP to induce the formation of surface oxygen defects on TiO2 and investigated how this treatment affected the NH3-SCR activity of the supported V-W catalysts. We also investigated the previously unexplored distribution of VOx and WOx sites on the catalyst surface, the chemical states of the elements on this surface, and the reactions occurring thereon.
Section snippets
Catalyst synthesis
V2O5-WO3/TiO2 catalysts were prepared using wet impregnation methods. Anatase TiO2 (CristalACTiV™ DT-51, Tronox) treated with TTIP (97%, Alfa Aesar) was prepared using a co-impregnation method. Surface-treatment was performed with TiO2 using several TTIP: bulk TiO2 mass ratios (1, 3, 5, 7, 100). The TTIP was dissolved in anhydrous ethanol and the mixture was stirred with the bulk TiO2 for 1 hour to mix well. Then, distilled water (H2O:TTIP = 110:1, mol/mol) was added dropwise while stirring.
SCR performance
Fig. 1 shows the effects of temperature on the SCR performances of 2V10WTi_Ref, 2V10WTi_3TTIP, and 2V10WTi_100TTIP, (where 3 and 100 refer to the TTIP wt%) revealing that NOx removal efficiency decreased in the order of 2V10WTi_3TTIP > 2V10WTi_Ref > 2V10WTi_100TTIP at a given temperature and peaked at 350 °C for a given catalyst. Within the investigated temperature range, the N2 and N2O selectivities of 2V10WTi_3TTIP were close to those of 2V10WTi_Ref, whereas 2V10WTi_100TTIP showed an
Conclusion
The NH3-SCR activity of TiO2-supported V-W catalysts was correlated with the distribution of VOx and WOx sites on their surface, chemical reactivity, and chemical states. The catalyst prepared with a TTIP loading of 100 wt% was less active than that supported by commercially available TiO2 due to encapsulation of V and W species in the former case. Hence, we focused on catalysts prepared using low TTIP loadings of 1–7 wt%, and the best performance was observed for a loading of 5 wt%.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was financially supported by the Ministry of Trade, Industry and Energy (MOTIE) [grant number 20005721], the Ministry of Environment [grant number 2020003060014], and the Ministry of Science and ICT (MSIT) [grant number JA200009]. We would like to express our sincere thanks to them for their support.
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