Search Results (8)

In recent years, increased attention to air pollutants such as NO_x has led the scientific community to focus meaningfully on developing strategies for NO_x reduction. Selective catalytic reduction of NO_x by ammonia (NO SCR by NH₃) is currently the main method to remove NO_x from diesel engine exhaust emissions. The catalysts with typical V₂O₅-WO₃/TiO₂ (VWTi) composition are widely used in NH₃-SCR for their high NO_x conversion activity, low cost, and robustness, especially concerning sulfur poisoning. However, in real diesel engine working conditions, the thermal and hydrothermal aging of catalysts can occur after several hours of operation at high temperature, affecting the catalytic performance. In this study, the stability of a commercial VWTi monolith, self-supported and containing glass fibers and bentonite in its matrix, was investigated as a case study. In laboratory conditions, NO SCR tests were performed for 50 h in the range of 150 to 350 °C. Subsequently, the VWTi monolith was thermally and hydrothermally aged at 600 °C for 6 h. The thermal aging increased the NO_x conversion, especially at low temperature (<250 °C), while the hydrothermal aging did not affect the SCR. The differences in NO_x conversion before and after aging were associated with the change in vanadium and tungsten oxide surface coverage and with the reduction in the surface area of catalysts. In order to correlate the change in SCR activity with the modifications occurring after aging processes, the monolithic samples were characterized by several techniques, namely XRD, SSA and pore analysis, TPR, XPS, Raman, TGA and SEM/EDX. Full article

The international shipping industry is facing increasingly stringent limitations on nitrogen oxide (NO_x) emissions. New solutions for reducing NO_x emitted by marine engines need to be investigated to find the best technology. Selective Catalytic Reduction (SCR) is an advanced active emissions control technology successfully used in automotive diesel engines; it could be applied to marine engines with ad-hoc solutions to integrate it in the exhaust of large engines. In this study, a commercial SCR was tested at the exhaust of a diesel engine in inlet gas conditions typical of a marine engine. The SCR system consisted of a custom monolith (provided by Hug-Engineering AG) that enabled seamless integration for a broad range of engine sizes; the active phases were V₂O₅ (3 wt%)-WO₃ (7 wt%)-TiO₂ (75 wt%). The monolith was studied at the laboratory scale for its in-depth chemical/physical characterization and by means of an intermediate-scale engine, reproducing the exhaust gas conditions of a full-scale marine engine. The system’s effectiveness in terms of NO_x removal for the selected engine operating conditions was evaluated in a wide range of temperature and NO_x emissions values and for different quantities of the reduction agent (AdBlue or ammonia) added to exhaust gases. The investigated technological solution resulted in efficient NO_x emission control from a marine engine. 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

A hydrodynamic cavitation method was used to maximize the effect of destructuration of a honeycomb monolithic support of a spent Selective Catalyst Reduction (SCR) catalyst—V₂O₅-WO₃/TiO₂-type—for extracting crystalline titanium and tungsten oxides from the cordierite surface. A high relative inlet pressure of 40 MPa was applied to a divergent nozzle for obtaining high shear stresses of the submerged cavitating jets and intensive micro- and nano-jets and shock waves acting on the particle surface of the milled catalyst. Scanning Electron Microscopy (SEM) analysis indicated the compact morphology of the thin metal oxide layer at the surface of the cordierite support and the high content of Ti and W elements in the sample. Energy dispersive spectroscopy (EDAX) performed along with TEM investigations on different nano-zones from the sample established the elemental composition of WO₃-TiO₂ agglomerates separated after hydrodynamic cavitation processing and identified as independent nanocrystalline structures through Bright Field Transmission Electron Microscopy (BF-TEM) and High Resolution Transmission Electron Microscopy (HR-TEM) measurements. The tetragonal anatase phase of TiO₂ and cubic phase of WO₃ were established by both interplanar d spacing measurements and X-ray diffraction analysis. The photoelectrochemical results showed the possible second life application of automotive catalysts. Full article

V₂O₅-WO₃/TiO₂ as a commercial selective catalytic reduction (SCR) catalyst usually used at middle-high temperatures was modified by loading of MnO_x for the purpose of enhancing its performance at lower temperatures. Manganese oxides were loaded onto V-W/Ti monolith by the methods of impregnation (I), precipitation (P), and in-situ growth (S), respectively. SCR activity of each modified catalyst was investigated at temperatures in the range of 100–340 °C. Catalysts were characterized by specific surface area and pore size determination (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR), etc. Results show that the loading of MnO_x remarkably enhanced the SCR activity at a temperature lower than 280 °C. The catalyst prepared by the in-situ growth method was found to be most active for SCR. Full article

One of the most harmful compounds are nitrogen oxides. Currently, the common industrial method of nitrogen oxides emission control is selective catalytic reduction with ammonia (NH₃-SCR). Among all of the recognized measures, NH₃-SCR is the most effective and reaches even up to 90% of NOx conversion. The presence of the catalyst provides the surface for the reaction to proceed and lowers the activation energy. The optimum temperature of the process is in the range of 150–450 °C and the majority of the commercial installations utilize vanadium oxide (V₂O₅) supported on titanium oxide (TiO₂) in a form of anatase, wash coated on a honeycomb monolith or deposited on a plate-like structures. In order to improve the mechanical stability and chemical resistance, the system is usually promoted with tungsten oxide (WO₃) or molybdenum oxide (MoO3). The efficiency of the commercial V₂O₅-WO₃-TiO₂ catalyst of NH₃-SCR, can be gradually decreased with time of its utilization. Apart from the physical deactivation, such as high temperature sintering, attrition and loss of the active elements by volatilization, the system can suffer from chemical poisoning. All of the presented deactivating agents pass for the most severe poisons of V₂O₅-WO₃-TiO₂. In order to minimize the harmful influence of H₂O, SO₂, alkali metals, heavy metals and halogens, a number of methods has been developed. Some of them improve the resistance to poisons and some are focused on recovery of the catalytic system. Nevertheless, since the amount of highly contaminated fuels combusted in power plants and industry gradually increases, more effective poisoning-preventing and regeneration measures are still in high demand. Full article

In this work, we set out to investigate the deactivation of a commercial V₂O₅-WO₃/TiO₂ monolith catalyst that operated for a total of 18,000 h in a selective catalytic reduction unit treating the exhaust gases of a municipal waste incinerator in a tail end configuration. Extensive physical and chemical characterization analyses were performed comparing results for fresh and aged catalyst samples. The nature of poisoning species was determined with regards to their impact on the DeNOx catalytic activity which was experimentally evaluated through catalytic tests in the temperature range 90–500 °C at a gas hourly space velocity of 100,000 h⁻¹ (NO = NH₃ = 400 ppmv, 6% O₂). Two simple regeneration strategies were also investigated: thermal treatment under static air at 400–450 °C and water washing at room temperature. The effectiveness of each treatment was determined on the basis of its ability to remove specific poisoning compounds and to restore the original performance of the virgin catalyst. Full article

Firing of biomass can lead to rapid deactivation of the vanadia-based NH₃-SCR catalyst, which reduces NO_x to harmless N₂. The deactivation is mostly due to the high potassium content in biomasses, which results in submicron aerosols containing mostly KCl and K₂SO₄. The main mode of deactivation is neutralization of the catalyst’s acid sites. Four ways of dealing with high potassium contents were identified: (1) potassium removal by adsorption, (2) tail-end placement of the SCR unit, (3) coating SCR monoliths with a protective layer, and (4) intrinsically potassium tolerant catalysts. Addition of alumino silicates, often in the form of coal fly ash, is an industrially proven method of removing K aerosols from flue gases. Tail-end placement of the SCR unit was also reported to result in acceptable catalyst stability; however, flue-gas reheating after the flue gas desulfurization is, at present, unavoidable due to the lack of sulfur and water tolerant low temperature catalysts. Coating the shaped catalysts with thin layers of, e.g., MgO or sepiolite reduces the K uptake by hindering the diffusion of K⁺ into the catalyst pore system. Intrinsically potassium tolerant catalysts typically contain a high number of acid sites. This can be achieved by, e.g., using zeolites as support, replacing WO₃ with heteropoly acids, and by preparing highly loaded, high surface area, very active V₂O₅/TiO₂ catalyst using a special sol-gel method. Full article