High efficiency removal of NO using waste calcium carbide slag by facile KOH modification

Waste calcium carbide slags (CS), which are widely applied to desulfurisation, are not typically used in denitration. Herein, to well achieve waste control by waste, a facile and high-efficiency denitration strategy is developed using KOH to modify the calcium carbide slags (KCS). Various KCS samples were investigated using a series of physical and chemical characterisations. The performance test results showed that the KOH concentration and reaction temperature are the main factors affecting the denitration efficiency of KCS, and CS modified with 1.5 mol/L KOH (KCS-1.5) can achieve 100% denitration efficiency at 300°C. Such excellent removal efficiency is due to the catalytic oxidation of the oxygen-containing functional groups derived from the KCS. Further studies showed that KOH treatment significantly increased the concentration of oxygen vacancies, nitro compounds, and basic sites of CS. This study provides a novel strategy for the resource utilisation of waste CS in the future.

To be support or promoter: the mode of introducing ceria into commercial V2O5/TiO2 catalyst for enhanced Hg0 oxidation

Ce-modified commercial vanadium-based catalysts are still in a rapid development stage in terms of optimizing Hg⁰ oxidation performance. Due to the universal property of ceria, it can act as either support or promoter to supported vanadium-based catalysts. However, the introduction mode of Ce on the Hg⁰ oxidation is still unclarified. Herein, introducing Ce to vanadium-based catalysts as a promoter (VCe/Ti) plays a more effective role in the Hg⁰ oxidation than only doping Ce into TiO₂ support (V/CeTi). It is revealed that the strong interaction between V and Ce increases the orbital hybridization, and reduces the lowest unoccupied molecular orbital (LUMO) of V, which is conducive to adsorbing and activating HCl. The excellent performance of the VCe/Ti catalyst can be ascribed to its superior redox ability, stronger HCl adsorption capacity, abundant surface oxygen vacancies, and the redox equilibrium (Ce³⁺ + V⁵⁺ ↔ Ce⁴⁺ + V⁴⁺), which improves electron transfer, and thus the catalytic activity. This work provides the potential application of Ce-modified V-based catalysts for the simultaneous control of NO_x and Hg⁰ in stationary sources.

Elimination of PCDD/Fs over Commercial Honeycomb-Like Catalyst of V2O5-MoO3/TiO2 at Low Temperature: From Laboratory Experiments to Field Study

With the need for ultra-low emissions and the strict regulation of PCDD/Fs from MSWI plants, traditional SCR catalysts have been applied to remove PCDD/Fs. In this study, we compared one typical commercial V2O5-MoO3/TiO2 catalyst’s performance in removing PCDD/Fs under laboratory and industrial conditions. Various characterization methods like XRF, XPS, BET, and H2-TPR were applied to analyze the catalyst’s properties. The laboratory results showed that the adsorption could significantly affect the removal at low temperatures. The RE on PCDD/Fs was 59.4% (55.0% for toxicity RE), 88.5% (90.3%), and 78.0% (76.0%) at 160 °C, 180 °C, and 200 °C, respectively, showing that 180 °C is the most suitable operation temperature for this V2O5-MoO3/TiO2 catalyst. The field study was conducted at 180 °C, and the results revealed that the competition between water vapor and the interaction of SO2 could lower the RE. However, comparisons between laboratory and field conditions showed that this V2O5-MoO3/TiO2 catalyst still showed good stability, with only a 6.8% drop.

Protection Effect of Ammonia on CeNbTi NH3-SCR Catalyst from SO2 Poisoning

CeNbTi catalyst was poisoned in different sulfur poisoning atmospheres at 300 °C for 6 h and then was evaluated for selective catalytic reduction (SCR) of NOx with NH3. The catalyst deactivation upon SO2 exposure was effectively inhibited in the presence of NH3. Temperature-programmed decomposition (TPD) analyses were applied to identify deposit species on the poisoned catalysts by comparison with several groups of reference samples. Diffuses reflectance infrared Fourier transform spectroscopy (DRIFTS) over CeNbTi catalysts with different poisoning pretreatments and gas purging sequences were designed to investigate the roles of NH3 in the removal of surface sulfites and sulfates. More ammonium sulfates including ammonium bisulfate and ammonium cerium sulfate were generated instead of inert cerium sulfate in these conditions. The mechanisms about the formation and transformation of surface deposits upon sulfur poisoning w/wo NH3 were explored, which provided a basis for developing Ce-based mixed oxides as SCR catalysts for stationary sources.

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

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.

Like Cures like: Detoxification Effect between Alkali Metals and Sulfur over the V2O5/TiO2 deNOx Catalyst

The V₂O₅/TiO₂ (VTi) catalyst has been widely employed for the NH₃ selective catalytic reduction (NH₃-SCR) reaction, and sulfur (S) and alkali metals (K) were usually considered as poisons during this reaction. In this work, the synergistic effect of S and K over the VTi catalyst for the NH₃-SCR reaction was analyzed and discussed. It is surprisingly observed that the synergistic effects of S and K exhibited a detoxification effect on the NH₃-SCR reaction. That is, although the VTi catalyst exhibited moderate resistance to S poisoning and unsatisfactory resistance to K deactivation, the SCR activity was restored to close to fresh VTi when K and S coexisted. This detoxification effect also could occur between other alkali metals (e.g., Ca and Na) and sulfur. X-ray photoelectron spectroscopy and charge density difference studies both indicate that the introduction of K could significantly affect the electronic structure of V, but this toxic effect was recovered by the further addition of S because of the strong interaction between S and K. Therefore, this detoxification effect can occur in the practical reaction atmosphere, which alleviates the alkali metal poisoning of commercial catalysts.

Ceria–tungsten–tin oxide catalysts with superior regeneration capacity after sulfur poisoning for NH3-SCR process

A combined study on the anti-sintering ability, SO2-poisoning mechanism and thermal regeneration property of CeWSnOx catalysts for NH3-SCR reaction.

Comparative study of HSOA-/SOA2- versus H3-BPO4B- functionalities anchored on TiO2-supported antimony oxide-vanadium oxide-cerium oxide composites for low-temperature NOX activation

TiO₂-supported antimony oxide-vanadium oxide-cerium oxide (SVC) imparts Lewis acidic (L)/Brönsted acidic (B) sites, labile (O_α)/mobile oxygens (O_M), and oxygen vacancies (O_V) for selective catalytic NO_X reduction (SCR). However, these species are harmonious occasionally, readily poisoned by H₂O/sulfur/phosphorus/carbon, thus limiting SCR performance of SVC. Herein, a synthetic means is reported for immobilizing HSO_A^-/SO_A^2- (A= 3-4) or H_3-BPO₄^B- (B= 1-3) on the L sites of SVC to form SVC-S and SVC-P. HSO_A^-/SO_A^2-/H_3-BPO₄^B- acted as additional B sites with distinct characteristics, altered the properties of O_α/O_M/O_V species, thereby affecting the SCR activities and performance of SVC-S and SVC-P. SVC-P activated Langmuir-Hinshelwood-typed SCR better than SVC-S, as demonstrated by a greater O_α-directed pre-factor and smaller binding energy between O_α and NO. Meanwhile, SVC-S provided a larger B-directed pre-factor, thereby outperforming SVC-P in activating Eley-Rideal-typed SCR that dictated the overall SCR activities. Compared with SVC-S, SVC-P contained fewer O_V species, yet, had higher O_M mobility, thus enhancing the overall redox cycling feature, while providing greater Brönsted acidity. Consequently, the resistance of SVC-P to H₂O or soot were greater than or similar to that of SVC-S. Conversely, SVC-S revealed greater tolerance to hydro-thermal aging and SO₂ than SVC-P. This study highlights the pros and cons of HSO_A^-/SO_A^2-/H_3-BPO₄^B- functionalities in tailoring the properties of metal oxides in use as SCR catalysts.

Simultaneous removal of NOx and SO2 from simulated marine ship flue gas in a novel wet scrubbing system based on divided diaphragm seawater electrolysis technology: efficiency optimization and economic assessment

This work constructed a divided diaphragm seawater electrolysis system with two tandem packed towers for the synergistic removal of NO_x and SO₂. The first tower was mainly used to oxidize NO and SO₂ by AC (active chlorine), and the second tower was used to further absorb NO_x. The factors affecting on NO removal, including ACC (active chlorine concentration), pH value, initial NO concentration and temperature in the oxidation tower were investigated. Moreover, the effect of different inlet gas concentrations and current values were explored. The results showed that with the increase of ACC, the NO and NO_x removal efficiency increased rapidly, but when the ACC was higher than 500 mg/L [Cl₂], the removal efficiency did not increase further in the oxidation tower. Low pH values in the oxidation tower were favorable for NO removal. NO removal efficiency reached a maximum at 40 °C. Higher NO and SO₂ concentrations were favorable for NO removal. The decline of pH in the anode cell was not conducive to the storage of AC in the continuous electrolysis removal process. NO_x and SO₂ were almost completely removed after being scrubbed in the oxidation and absorption towers. The relationship between current and removal efficiency of NO and SO₂ in the oxidation tower was also analyzed. Finally, the removal mechanism and the application prospects were discussed.

Research on NO and SO2 removal using TiO2-supported iron catalyst with vaporized H2O2 in a catalytic oxidation combined with absorption process

Simultaneous removal of NOx and SO₂ is carried out by an oxidation-absorption process, which NO oxidized by active hydroxyl radicals (·OH) derived from catalytic decomposition of vaporized H₂O₂ over Fe₃O₄/TiO₂ and then adsorbed by NaOH solution along with SO₂. Fe₃O₄/TiO₂ synthesized by wet impregnation method with an additional reduction under H₂ atmosphere was characterized by XRD, FTIR, BET, XPS, and VSM analysis. Effects of H₂O₂ concentration, H₂O₂ injection rate, reaction temperature, gas flow rate, and flue gas component on simultaneous removal were investigated. The experimental results show that NO can be effectively oxidized by highly reactive ·OH radicals generated from H₂O₂ decomposition over Fe₃O₄/TiO₂ catalyst, and removal efficiencies of 93.31% for NO, 85.90% for NOx, and 100% for SO₂ were obtained. The surface zero-valent iron (Fe⁰) and divalent iron (Fe²⁺) are the key factors of the catalytic oxidation with hydroxyl radical. H₂O₂ adsorption and dissociation mechanism on catalyst surface was studied using DFT calculation. The calculation results demonstrate that H₂O₂ prefers to dissociate on iron containing surface, and ·OH radicals generation follow by Haber-Weiss (H-W) mechanism. The stable oxidative product of HNO₂ and HNO₃ were generated through NO/NO₂ and H₂O₂ co-adsorption on the FeO/TiO₂ (0 0 1) surface.

The role of vanadium species during SO2 removal over a V2O5/AC catalyst

Mechanism of SO₂ removal over a V₂O₅/AC catalyst.