Structure-Directing Role of Support on Hg⁰ Oxidation over V₂O₅/TiO₂ Catalyst Revealed for NO_x and Hg⁰ Simultaneous Control in an SCR Reactor

Abstract

The crystal structure of TiO2 strongly influences the physiochemical properties of supported active sites and thus the catalytic performance of the as-synthesized catalyst. Herein, we synthesized TiO2 with different crystal forms (R = rutile, A = anatase, and B = brookite), which were used as supports to prepare vanadium-based catalysts for Hg0 oxidation. The Hg0 oxidation efficiency over V2O5/TiO2-B was the best, followed by V2O5/TiO2-A and V2O5/TiO2-R. Further experimental and theoretical results indicate that gaseous Hg0 reacts with surface-active chlorine species produced by the adsorbed HCl and the reaction orders of Hg0 oxidation over V2O5/TiO2 catalyst with respect to HCl and Hg0 concentration were approximately 0 and 1, respectively. The excellent Hg0 oxidation efficiency over V2O5/TiO2-B can be attributed to lower redox temperature, larger HCl adsorption capacity, and more oxygen vacancies. This work suggests that to achieve the best simultaneous removal of NO x and Hg0 on state-of-the-art V2O5/TiO2 catalyst, a combination of anatase and brookite TiO2-supported vanadyl tandem catalysts is supposed to be employed in the SCR reactor, and the brookite-type catalyst should be on the downstream of the anatase-based catalyst due to the inhibition of NH3 on Hg0 oxidation.

Unveiling the role of microwave induction on V2O5@AC catalysts with enhanced activity for low temperature NH3-SCR reaction: an experimental and DFT study

“…5a, the V2p 3/2 spectra could be split into two peaks by conducting peak fitting deconvolutions in the binding energy range, and the peak located at 516.2 eV could be ascribed to V 4+ species of vanadium oxides and the peak located at 517.5 eV could be attributed to V 5+ species of vanadium oxides. 40 Note that vanadium species existed on the 3V 2 O 5 @AC and 3V 2 O 5 @AC-300 catalysts in both V 5+ and V 4+ valence states, with V 5+ being the dominant species. It can be obviously observed from Fig.…”

“…The peak centered at 530.6 eV was marked as O γ , which corresponded to the lattice oxygen anions (O 2− ) in vanadium oxides, the peak centered at 531.7 eV was ascribed to surface adsorbed oxygen, such as O 2 − and O − (oxygen-deficient), denoted as O β , and the peak centered at 533.3 eV was attributed to the oxygen species in hydroxyl-like groups (denoted as O α ). 40,41 As shown in Fig. 5d, after microwave treatment, the ratio of O β /(O α + O β + O γ ) on 3V 2 O 5 @AC-300 (22.2%) was remarkably improved compared to that on 3V 2 O 5 @AC (6.2%), illustrating that microwaves bring more chemisorbed oxygen in the 3V 2 O 5 @AC-300 catalyst, accompanied with more oxygen vacancies.…”

Change in the Electronic Environment of the VOx Active Center via Support Modification to Enhance Hg Oxidation Activity

“…To further investigate the effect of reaction conditions, the Gibbs free energy changes (Δ G ) of the reaction steps in the Hg oxidation were calculated using a typical reaction temperature of 623 K (Figure S10). Even after we considered the reaction condition by calculating the Δ G , the trend observed in the relative energy profiles was not changed. Particularly, the HCl dissociation step still remained as the rate-determining step (RDS) for the V 2 O 5 /P-aTiO 2 model and the HgCl 2 desorption step as the RDS for the other three models.…”

“…In many previous studies, VO x has been known to exist in various forms, such as monomers, dimers, and monolayers on the TiO 2 support. , Particularly, VO x was reported to be deposited in the form of dimeric V 2 O 5 on the TiO 2 support when loaded with low contents of 1 wt % in several SCR and Hg oxidation experiments. , The stoichiometry of V 2 O 5 can lead to various configurations on the TiO 2 surface depending on the crystalline phase. Thus, the GA was adopted to determine the adsorption configuration of V 2 O 5 on the TiO 2 support (Figure ).…”

“…Mercury (Hg) emission during fossil fuel consumption has posed a threat to the global environment due to its high toxicity and bioaccumulation ability. − To date, Hg removal methods, such as coal washing, thermal coal treatment, adsorption, and catalytic oxidation, have been widely studied. − Among them, the catalytic oxidation has been considered to be an effective approach to the Hg elimination. , Generally, Hg in flue gas can exist as both elemental Hg (Hg 0 ) and oxidized Hg (Hg 2+ ) in compounds, and the oxidation state of Hg is known to critically affect the efficiency of Hg removal in the flue gas treatment facility; Hg 0 is highly volatile and insoluble in water (∼50 μg/L), not being effectively eliminated, while Hg 2+ is soluble in water and can be easily removed by wet flue gas desulfurization systems. , Among many, the selective catalytic reduction (SCR) catalysts, such as TiO 2 -supported vanadia (VO x /TiO 2 ), have attracted extensive attention owing to their catalytic and economic efficiency; the use of V 2 O 5 /TiO 2 catalysts has been shown to be effective in promoting Hg oxidation under specific reaction conditions . These conditions typically include a high temperature of 623 K and the presence of oxidizing agents, such as oxygen or chlorine . Not only is the SCR catalyst active to both the Hg oxidation and NO x reduction, but also it can utilize the hydrochloric acid (HCl) species as a specific oxidant, which is produced by NO x reduction with ammonium chloride (NH 4 Cl) .…”