Search Results (26)

Nitrogen oxides (NO_x) and chlorobenzene (CB) released during waste incineration and iron ore sintering pose significant threats to both the atmosphere and human health, necessitating effective control measures. Vanadium-based catalysts are commonly employed for the simultaneous control of NO_x and CB; however, their catalytic performance requires further enhancement. In this study, the NH₃-SCR activity and CB catalytic oxidation (CBCO) activity were significantly enhanced by doping the V10W/Ti catalyst with Ce. During the multi-pollutant control (MPC) reaction, the optimized 15CeV10W/Ti catalyst demonstrated NO_x conversion approaching 100% and N₂ selectivity exceeding 95% at temperatures between 210 and 450 °C. Additionally, it achieved CB conversion nearing 100% and CO₂ selectivity above 80% at temperatures above 350 °C. These results were markedly superior to those of the conventional commercial 1%V₂O₅–10%WO₃/TiO₂ catalyst. Characterization studies indicated that the 15CeV10W/Ti catalyst possessed improved redox performance and more acidic sites. In the MPC reaction, the declined CBCO activity, compared to the CB separate oxidation, can be attributed primarily to the competitive adsorption of NH₃ with CB. Conversely, the observed decrease in NO_x conversion at lower temperatures was primarily due to the suppression of the oxidation of NO to NO₂ by CB. Full article

V₂O₅-WO₃(MoO₃)/TiO₂ catalysts are widely used in industrial denitrification (deNO_x) processes based on the selective catalytic reduction (SCR) technique. To lower their cost and shorten the production cycle, V₂O₅-MoO₃/TiO₂ catalysts with and without CeO₂ modification were prepared using the ball milling method. This study demonstrates that the CeO₂-modified catalyst has high NO conversion and a broader temperature window due to the decreased amount of easily reducible vanadium species and the enhanced property of oxygen species activation in CeO₂. Meanwhile, the SO₂ resistance of the catalyst is restrained due to the strong adsorption and oxidation of SO₂ over CeO₂ in the catalyst. Full article

Nickel-based catalysts are regarded as the most excellent urea oxidation reaction (UOR) catalysts in alkaline media. Whatever kind of nickel-based catalysts is utilized to catalyze UOR, it is widely believed that the in situ-formed Ni³⁺ moieties are the true active sites and the as-utilized nickel-based catalysts just serve as pre-catalysts. Digging the pre-catalyst effect on the activity of Ni³⁺ moieties helps to better design nickel-based catalysts. Herein, five different anions of OH⁻, CO₃²⁻, SiO₃²⁻, MoO₄²⁻, and WO₄²⁻ were used to bond with Ni²⁺ to fabricate the pre-catalysts β-Ni(OH)₂, Ni-CO₃, Ni-SiO₃, Ni-MoO₄, and Ni-WO₄. It is found that the true active sites of the five as-fabricated catalysts are the same in situ-formed Ni³⁺ moieties and the five as-fabricated catalysts demonstrate different UOR activity. Although the as-synthesized five catalysts just serve as the pre-catalysts, they determine the quantity of active sites and activity per active site, thus determining the catalytic activity of the catalysts. Among the five catalysts, the amorphous nickel tungstate exhibits the most superior activity per active site and can catalyze UOR to reach 158.10 mA·cm^–2 at 1.6 V, exceeding the majority of catalysts. This work makes for a deeper understanding of the pre-catalyst effect on UOR activity and helps to better design nickel-based UOR catalysts. Full article

With the rapid development of industrialization, the emission of nitrogen oxides (NO_x) has become a global environmental issue. Uranium is the primary fuel used in nuclear power generation. However, the production of uranium, typically based on the uranyl nitrate method, usually generates large amounts of nitrogen oxides, particularly NO₂, with concentrations in the exhaust gas exceeding 10,000 ppm. High concentrations of nitrogen dioxide are also produced during silver electrolysis processing and the treatment of waste electrolyte solutions. Traditional V-W/TiO₂ NH₃-SCR catalysts typically exhibit high catalytic activity at temperatures ranging from 300 to 400 °C, under conditions of low NO_x concentrations and high gas hourly space velocity. However, their performance is not satisfying when reducing high concentrations of NO₂. This study aims to optimize the traditional V-W/TiO₂ catalysts to enhance their catalytic activity under conditions of high NO₂ concentrations (10,000 ppm) and a wide temperature range (200–400 °C). On the basis of 3 wt% Mo/TiO₂, various loadings of V₂O₅ were selected, and their catalytic activities were tested. Subsequently, the optimal ratios of active component vanadium and additive molybdenum were explored. Simultaneously, doping with WO₃ for modification was selected in the V-Mo/TiO₂ catalyst, followed by activity testing under the same conditions. The results show that: the NO_x conversion rates of all five catalysts increase with temperature at range of 200–400 °C. Excessive loading of MoO₃ decreased the catalytic performance, with 5 wt% being the optimal loading. The addition of WO₃ significantly enhanced the low-temperature activity of the catalysts. When the loadings of WO₃ and MoO₃ were both 3 wt%, the catalyst exhibited the best denitrification performance, achieving a NO_x conversion rate of 98.8% at 250 °C. This catalyst demonstrates excellent catalytic activity in reducing very high concentration (10,000 ppm) NO₂, at a wider temperature range, expanding the temperature range by 50% compared to conventional SCR catalysts. Characterization techniques including BET, XRD, XPS, H₂-TPR, and NH₃-TPD were employed to further study the evolution of the catalyst, and the promotional mechanisms are explored. The results revealed that the proportion of chemisorbed oxygen (O_α) increased in the WO₃-modified catalyst, exhibiting lower V reduction temperatures, which are favorable for low-temperature denitrification activity. NH₃-TPD experiments showed that compared to MoO_x species, surface WO_x species could provide more acidic sites, resulting in stronger surface acidity of the catalyst. Full article

To address the environmental pollution caused by nitrogen oxides, V₂O₅-WO₃/TiO₂ is widely used as a catalyst based on selective catalytic reduction (SCR) technology. However, spent SCR catalysts pose a potential hazard to the environment due to the presence of heavy metals. This problem continues to plague countries with predominantly thermal power generation, and landfills as the dominant disposal method wastes significant metal resources. Previous research into the recovery of these metal resources has received considerable attention. Here, we summarise the methods of recovery and find that research trends are beginning to move towards improving the added value of recovered products. One very promising application is photocatalysts; however, the atomic efficiency of current methods is not satisfactory. Therefore, this review first focuses on the regeneration of spent SCR catalysts and the processes used for elemental extraction to clarify what forms of V, W and Ti can be obtained from existing processes. This is followed by providing directions for the conversion of spent SCR catalysts into photocatalysts with improvements based on such processes. From a different perspective, this also provides a new resource for photocatalysts and is expected to significantly reduce the cost of photocatalyst production. Full article

In this study, a V₂O₅-Sb₂O₃/TiO₂ monolithic catalyst was modified by introducing WO₃. The WO₃-modified catalyst exhibited enhanced catalytic activity in the measuring temperature range of 175–320 °C. The changes in dispersion of vanadia species were investigated by ultraviolet-visible (UV-Vis) spectroscopy and H₂ temperature-programmed reduction (H₂-TPR). A durability test was conducted in a wet SO₂-containing atmosphere at 220 °C for 25 h. The sulfate deposition was estimated by temperature-programmed decomposition (TPDC) of sulfates, thermo-gravimetric (TG) analysis, and temperature-programmed desorption (TPD) of NH₃. Isothermal SO₂ oxidation and temperature-programmed surface reaction (TPSR) of NH₄HSO₄ with NO were performed. Based on these characterizations, effects of WO₃ modification on the sulfate tolerance of the catalyst were explored. Full article

In modern dual-pressure nitric acid plants, the tail gas temperature usually exceeds 300 °C. The NH₃-SCR catalyst used in this temperature range must be resistant to thermal deactivation, so commercial vanadium-based systems, such as V₂O₅-WO₃ (MoO₃)-TiO₂, are most commonly used. However, selectivity of this material significantly decreases above 350 °C due to the increase in the rate of side reactions, such as oxidation of ammonia to NO and formation of N₂O. Moreover, vanadium compounds are toxic for the environment. Thus, management of the used catalyst is complicated. One of the alternatives to commercial V₂O₅-TiO₂ catalysts are natural zeolites. These materials are abundant in the environment and are thus relatively cheap and easily accessible. Therefore, the aim of the study was to design a novel iron-modified zeolite catalyst for the reduction of NO_x emission from dual-pressure nitric acid plants via NH₃-SCR. The aim of the study was to determine the influence of iron loading in the natural zeolite-supported catalyst on its catalytic performance in NO_x conversion. The investigated support was firstly formed into pellets and then impregnated with various contents of Fe precursor. Physicochemical characteristics of the catalyst were determined by XRF, XRD, low-temperature N₂ sorption, FT-IR, and UV–Vis. The catalytic performance of the catalyst formed into pellets was tested on a laboratory scale within the range of 250–450 °C using tail gases from a pilot nitric acid plant. The results of this study indicated that the presence of various iron species, including natural isolated Fe³⁺ and the introduced FexOy oligomers, contributed to efficient NO_x reduction, especially in the high-temperature range, where the NO_x conversion rate exceeded 90%. Full article

In recent years, low-temperature SCR (Selective Catalytic Reduction) denitrification technology has been popularized in non-power industries and has played an important role in the control of industrial flue gas NO_x emissions in China. Currently, the most commonly used catalysts in industry are V₂O₅-WO₃(MoO₃)/TiO₂, MnO₂-based catalysts, CeO₂-based catalysts, MnO₂-CeO₂ catalysts and zeolite SCR catalysts. The flue gas emitted during industrial combustion usually contains SO₂, moisture and alkali metals, which can affect the service life of SCR catalysts. This paper summarizes the mechanism of catalyst poisoning and aims to reduce the negative effect of NH₄HSO₄ on the activity of the SCR catalyst at low temperatures in industrial applications. It also presents the outstanding achievements of domestic companies in denitrification in the non-power industry in recent years. Much progress has been made in the research and application of low-temperature NH₃-SCR, and with the renewed demand for deeper NO_x treatments, new technologies with lower energy consumption and more functions need to be developed. Full article

The application of secondary NO_x control methods in medium to low-capacity furnaces is a relatively new topic on the energy market and thus requires further research. In this paper, the results of full-scale research of SNCR and hybrid SNCR + SCR methods applied into a 29 MW_th solid fuel fired stoker boiler is presented. The tests were performed for a full range of boiler loads, from 33% (12 MW_th) to 103% (30 MW_th) of nominal load. A novel SNCR + SCR hybrid process was demonstrated based on an enhanced in-furnace SNCR installation coupled with TiO₂-WO₃-V₂O₅ catalyst, which provides extra NO_x reduction and works as an excess NH₃ “catcher” as well. The performance of a brand-new catalyst was evaluated in comparison to a recovered one. The emission of NO_x was reduced below 180 mg NO_x/Nm³ at 6% O₂, with ammonia slip in flue gas below 10 mg/Nm³. Special attention was paid to the analysis of ammonia slip in combustion products: flue gas and fly ash. An innovative and cost-effective method of ammonia removal from fly ash was presented and tested. The main idea of this method is fly ash recirculation onto the grate. As a result, ammonia content in fly ash was reduced to a level below 6.1 mg/kg. Full article

Selective catalytic reduction (SCR) is the most efficient NO_X removal technology, and the vanadium-based catalyst is mainly used in SCR technology. The vanadium-based catalyst showed higher NO_X removal performance in the high-temperature range but catalytic efficiency decreased at lower temperatures, following exposure to SO_X because of the generation of ammonium sulfate on the catalyst surface. To overcome these limitations, we coated an NH₄⁺ layer on a vanadium-based catalyst. After silane coating the V₂O₅-WO₃/TiO₂ catalyst by vapor evaporation, the silanized catalyst was heat treated under NH₃ gas. By decomposing the silane on the surface, an NH₄⁺ layer was formed on the catalyst surface through a substitution reaction. We observed high NO_X removal efficiency over a wide temperature range by coating an NH₄⁺ layer on a vanadium-based catalyst. This layer shows high proton conductivity, which leads to the reduction of vanadium oxides and tungsten oxide; additionally, the NO_X removal performance was improved over a wide temperature range. These findings provide a new mothed to develop SCR catalyst with high efficiency at a wide temperature range. Full article

Modern catalysts for internal combustion engine applications are traditionally constituted by honeycomb substrates on which a coating of the catalytically active phase is applied. Due to the laminar flow of the gases passing through their straight channels, these structures present low heat and mass transfer, thus leading to relatively large catalyst sizes to compensate for the low catalytic activity per unit of volume. Better conversion efficiency can be achieved if three-dimensional periodic structures are employed, because of the resulting gases’ tortuous paths. Furthermore, the increased catalytic activity implies a reduction in the overall catalyst volume, which can translate to a decreased usage of precious metals as active phase. By exploiting the ceramic Stereolithography technique (i.e., SLA) it is nowadays possible to accurately 3D print complex alumina-based lattices to be used as ceramic substrates for catalysis. In this work, closed-walls lattices consisting of a rotated cubic cell of 2 mm dimensions were designed, 3D printed via SLA and finally washcoated with V₂O₅-WO₃-TiO₂. The samples were tested for the selective catalytic reduction of NO by NH₃ in a heated quartz glass reactor and the performance of the innovative 3D-printed substrate was compared with the catalytic efficiency of the conventional cordierite honeycombs. Full article

This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO₂) and precious metals. WO_x particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO₃-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO₃ catalysts for DeNO_x applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis). Full article

Ca poisoning behavior is inevitable for high-calcium content flue gas, so V₂O₅-WO₃/TiO₂ (VWT) and V₂O₅-WO₃-CeO₂/TiO₂ (VWCeT) catalysts with different vanadium content have been prepared and the Ca-doped catalysts are compared in this manuscript. The result shows Ce addition can both promote the NO conversion and the alkali resistance. Lower Ca addition for 0.1VWCeT catalyst promotes its oxidability and Ce modification is more suitable for low vanadium catalysts. The total acidity and the reducibility of catalysts decline after Ca doping, and the reducibility of the active species on catalysts has been strengthened by Ce addition. CeO₂ based catalysts with lower Ca loading struggle to resist sulfur poisoning, while higher Ca loading favors SO₂ adsorption and also physically reduces the cerium acidification process. In the presence of SO₂, additional Brønsted acid sites are formed in Ca rich catalyst. The dynamic NH₃ adsorption has been investigated, shows that Ca doping content on catalyst is critical for SCR reaction, and the catalyst is more susceptible to SO₂ initially in alkali flue gas during the actual application, but the sulfur resistance may increase with the alkali-poisoning effect aggravated by Ca doping. Full article

SCR still represents the most widely applied technique to remove nitrogen oxides from flue gas from both stationary and mobile sources. The catalyst lifetime is greatly affected by the presence of poisoning compounds in the exhaust gas that deactivate the catalysts over time on stream. The progressive and widespread transition towards bio-derived fuels is pushing research efforts to deeply understand and contrast the deactivating effects of some specific poisons among those commonly found in the emissions from combustion processes. In particular, exhaust gases from the combustion of bio-fuels, as well as from municipal waste incineration plants and marine engines, contain large amounts of alkali and alkaline earth metals that can severely affect the acid, redox, and physical properties of the SCR catalysts. This review analyzes recent studies on the effects of alkali and alkaline earth metals on different types of SCR catalysts divided into three main categories (conventional V₂O₅-WO₃/TiO₂, supported non-vanadium catalysts and zeolite-based catalysts) specifically focusing on the impact of poisons on the reaction mechanism while highlighting the different type of deactivation affecting each group of catalysts. An overview of the different regeneration techniques aimed at recovering as much as possible the original performance of the catalysts, highlighting the pros and cons, is given. Finally, current research trends aiming to improve the tolerance towards alkali-poisoning of SCR catalysts are reported. Full article

Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) has been successfully applied to abate NO_x from diesel engines and coal-fired industries on a large scale. Although V₂O₅-WO₃(MoO₃)/TiO₂ catalysts have been utilized in commercial applications, novel vanadia-based catalysts have been recently developed to meet the increasing requirements for low-temperature catalytic activity. In this article, recent progress on the improvement of the low-temperature activity of vanadia-based catalysts is reviewed, including modification with metal oxides and nonmetal elements and the use of novel supports, different synthesis methods, metal vanadates and specific structures. Investigation of the NH₃-SCR reaction mechanism, especially at low temperatures, is also emphasized. Finally, for low-temperature NH₃-SCR, some suggestions are given regarding the opportunities and challenges of vanadia-based catalysts in future research. Full article