WO3–V2O5 Active Oxides for NOx SCR by NH3: Preparation Methods, Catalysts’ Composition, and Deactivation Mechanism—A Review

A comprehensive review of deactivation and modification of selective catalytic reaction catalysts installed in cement kilns

After the ultralow emission transformation of coal-fired power plants, cement production became China's leading industrial emission source of nitrogen oxides. Flue gas dust contents at the outlet of cement kiln preheaters were as high as 80-100 g/m3, and the calcium oxide content in the dust exceeded 60%. Commercial V2O5(-WO3)/TiO2 catalysts suitable for coal-fired flue gas suffer from alkaline earth metal Ca poisoning of cement kiln flue gas. Recent studies have also identified the poisoning of cement kiln selective catalytic reaction (SCR) catalysts by the heavy metals lead and thallium. Investigation of the poisoning process is the primary basis for analyzing the catalytic lifetime. This review summarizes and analyzes the SCR catalytic mechanism and chronicles the research progress concerning this poisoning mechanism. Based on the catalytic and toxification mechanisms, it can be inferred that improving the anti-poisoning performance of a catalyst enhances its acidity, surface redox performance-active catalytic sites, and shell layer protection. The data provide support in guiding engineering practice and reducing operating costs of SCR plants. Finally, future research directions for SCR denitrification catalysts in the cement industry are discussed. This study provides critical support for the development and optimization of poisoning-resistant SCR denitrification catalysts.

Disposal of spent V2O5-WO3/TiO2 catalysts: A regeneration principle based on structure-activity relationships from carrier transformations

Characterization and evaluation of hazardous spent V2O5-WO3/TiO2 catalysts are critical to determining their treatment or final disposal. This study employs a thermal approach to simulate the preparation of spent catalysts derived from commercial V2O5-WO3/TiO2 catalysts and investigate the structure-activity relationship of the carrier changes during the deactivation process. The results indicate that the catalyst carrier undergoes two processes: an increase in grain size and a transformation in crystal structure. Both structural and catalytic investigations demonstrate that the grain size for catalyst deactivation is 24.62 nm, and the formation of CaWO4 occurs before the crystalline transformation. The specific surface area is susceptible to an increase in grain size. The reactions of selective catalytic reduction involve the participation of both Brønsted acid and Lewis acid sites. The deactivation process of the carrier initially affects Brønsted acid sites, followed by a reduction in Lewis acid sites, resulting in a decline in NH3 adsorption capacity and oxidation. Correlation analysis reveals that changes in the physicochemical properties of the catalyst reduce the NO conversion, with the order being The grain size > Total acid amount > The surface area. It is recommended to recycle the spent catalyst if the carrier grain size is less than 25 nm. The findings of this investigation contribute to expanding the database for evaluating and understanding the physicochemical properties of spent catalysts for disposal.

Innovations in WO3 gas sensors: Nanostructure engineering, functionalization, and future perspectives

This review critically examines the progress and challenges in the field of nanostructured tungsten oxide (WO3) gas sensors. It delves into the significant advancements achieved through nanostructuring and composite formation of WO3, which have markedly improved sensor sensitivity for gases like NO2, NH3, and VOCs, achieving detection limits in the ppb range. The review systematically explores various innovative approaches, such as doping WO3 with transition metals, creating heterojunctions with materials like CuO and graphene, and employing machine learning models to optimize sensor configurations. The challenges facing WO3 sensors are also thoroughly examined. Key issues include cross-sensitivity to different gases, particularly at higher temperatures, and long-term stability affected by factors like grain growth and volatility of dopants. The review assesses potential solutions to these challenges, including statistical analysis of sensor arrays, surface functionalization, and the use of novel nanostructures for enhanced performance and selectivity. In addition, the review discusses the impact of ambient humidity on sensor performance and the current strategies to mitigate it, such as composite materials with humidity shielding effects and surface functionalization with hydrophobic groups. The need for high operating temperatures, leading to higher power consumption, is also addressed, along with possible solutions like the use of advanced materials and new transduction principles to lower temperature requirements. The review concludes by highlighting the necessity for a multidisciplinary approach in future research. This approach should combine materials synthesis, device engineering, and data science to develop the next generation of WO3 sensors with enhanced sensitivity, ultrafast response rates, and improved portability. The integration of machine learning and IoT connectivity is posited as a key driver for new applications in areas like personal exposure monitoring, wearable diagnostics, and smart city networks, underlining WO3's potential as a robust gas sensing material in future technological advancements.

NH3-NO SCR Catalysts for Engine Exhaust Gases Abatement: Replacement of Toxic V2O5 with MnOx to Improve the Environmental Sustainability

Mn-WO3/TiO2 catalysts were investigated for Selective Catalytic Reduction (SCR) of NO with NH3. The catalysts were synthesized by wetness impregnation method with different Mn loadings (1.5-3-12 wt%) on 8wt%WO3/TiO2. All three catalysts were compared with 8wt%WO3/TiO2 and bare MnOx oxide, used as references. The 1.5wt%Mn-8wt%WO3/TiO2 exhibited the highest performance in NO conversion and N2 selectivity. A commercial catalyst, based on titania supported vanadia and tungsta, (V2O5-WO3/TiO2), widely used for its high efficiency, was also investigated in the present work. The morphological, structural, redox and electronic properties of the catalysts and their thermal stability were studied by several techniques (N2 adsorption/desorption, X-ray diffraction, H2 temperature-programmed reduction, NH3 temperature programmed desorption, X-ray photoelectron spectroscopy).

The aim of this paper is to study the effect of different Mn loadings on 8wt%WO3/TiO2 with the ambition to obtain highly active and selective catalysts in a large window of temperature. The replacement of toxic vanadium used in the classic V2O5-WO3/TiO2 catalyst with MnOx in the best performing catalyst, 1.5wt%Mn-8wt%WO3/TiO2, represents an important achievement to improve the environmental sustainability.

Insights of Selective Catalytic Reduction Technology for Nitrogen Oxides Control in Marine Engine Applications

The international shipping industry is facing increasingly stringent limitations on nitrogen oxide (NOx) emissions. New solutions for reducing NOx 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 NOx removal for the selected engine operating conditions was evaluated in a wide range of temperature and NOx emissions values and for different quantities of the reduction agent (AdBlue or ammonia) added to exhaust gases. The investigated technological solution resulted in efficient NOx emission control from a marine engine.

High-Dispersed V2O5-CuOX Nanoparticles on h-BN in NH3-SCR and NH3-SCO Performance

Typically, to meet emission regulations, the selective catalytic reduction of NO_X with NH₃ (NH₃-SCR) technology cause NH₃ emissions owing to high NH₃/NO_X ratios to meet emission regulations. In this study, V-Cu/BN-Ti was used to remove residual NO_X and NH₃. Catalysts were evaluated for selective catalytic oxidation of NH₃ (NH₃-SCO) in the NH₃-SCR reaction at 200–300 °C. The addition of vanadium and copper increased the number of Brønsted and Lewis acid sites available for the reaction by increasing the ratio of V⁵⁺ and forming Cu⁺ species, respectively. Furthermore, h-BN was dispersed in the catalyst to improve the content of vanadium and copper species on the surface. NH₃ and NO_X conversion were 98% and 91% at 260 °C, respectively. Consequently, slipped NH₃ (NH₃-Slip) emitted only 2% of the injected ammonia. Under SO₂ conditions, based on the NH₃ oxidation reaction, catalytic deactivation was improved by addition of h-BN. This study suggests that h-BN is a potential catalyst that can help remove residual NO_X and meet NH₃ emission regulations when placed at the bottom of the SCR catalyst layer in coal-fired power plants.

Ce1−xFexVO4 with Improved Activity for Catalytic Reduction of NO with NH3

A series of Ce1−xFexVO4 (x = 0, 0.25, 0.50, 0.75, 1) catalysts prepared by modified hydrothermal synthesis were used for selective catalytic reduction (SCR) of NOx with NH3. Among them, Ce0.5Fe0.5VO4 showed the highest catalytic activity. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction using H2 (H2-TPR), and temperature-programmed desorption of NH3 (NH3-TPD). The results indicated the formation of Ce-Fe-V-O solid solutions. The average oxidation states (AOS) of Ce, Fe, V, and O atoms changed obviously with the incorporation of Fe3+ into CeVO4, and the acidity of Ce0.5Fe0.5VO4 differs from that of CeVO4 and FeVO4. The presence of more acid sites and a sharp increase in active oxygen species in Ce0.5Fe0.5VO4 effectively improved the selective catalytic reduction (SCR) activity.

Hydrometallurgical process development to recycle valuable metals from spent SCR deNOX catalyst

Spent catalyst, containing vanadium and tungsten oxide in a TiO₂ glass fiber matrix, pose a risk of environmental contamination due to the high toxicity of its metal oxides if leached into the soil when disposed in landfills. Due to the increasing demand of metals and the continuous depletion of primary resources there is an growing necessity for recycling and reprocessing of spent catalysts and other secondary metal sources for environmental and economical reasons. Study of spent SCR catalyst soda roasting process with dissolved NaOH compared with the usual NaOH dry roasting and its influence in the subsequent water leaching. After optimization, the ideal parameters are roasting using a 0.4 ratio of NaOH/spent SCR catalyst in solution for 2 h at 973 K and de-ionized water leaching for 30 min, at 298 K with a pulp density of 30%. The research results show an important reduction of the roasting temperature and leaching time during the processing of spent SCR catalyst obtaining a 95.4% W and 80.2% V leaching efficiency liquor. Silicon compounds are one of the main impurities leached alongside the valuable metals and in this work, the silicon compounds leached are reduced significantly with the aim of avoiding the de-silication post-processing of the leach liquor. The main advantage of the proposed process is the increase of the leaching efficiency of vanadium and tungsten with a minimization of silicon impurities in a shorter time regardless of the leaching temperature.

Analysing the Performance of Ammonia Powertrains in the Marine Environment

This study develops system-level models of ammonia-fuelled powertrains that reflect the characteristics of four oceangoing vessels to evaluate the efficacy of ammonia as an alternative fuel in the marine environment. Relying on thermodynamics, heat transfer, and chemical engineering, the models adequately capture the behaviour of internal combustion engines, gas turbines, fuel processing equipment, and exhaust aftertreatment components. The performance of each vessel is evaluated by comparing its maximum range and cargo capacity to a conventional vessel. Results indicate that per unit output power, ammonia-fuelled internal combustion engines are more efficient, require less catalytic material, and have lower auxiliary power requirements than ammonia gas turbines. Most merchant vessels are strong candidates for ammonia fuelling if the operators can overcome capacity losses between 4% and 9%, assuming that the updated vessels retain the same range as a conventional vessel. The study also establishes that naval vessels are less likely to adopt ammonia powertrains without significant redesigns. Ammonia as an alternative fuel in the marine sector is a compelling option if the detailed component design continues to show that the concept is practically feasible. The present data and models can help in such feasibility studies for a range of vessels and propulsion technologies.

Cu/ZSM5-Geopolymer 3D-Printed Monoliths for the NH3-SCR of NOx

Geopolymer-based monoliths manufactured by direct ink writing, containing up to 60% by weight of presynthesized ZSM5 with low Si/Al ratio, were investigated as structured catalysts for the NH3-SCR of NOx. Copper was introduced as the active metal by ion exchange after a preliminary acid treatment of the monoliths. Monolithic catalysts were characterized by morphological (XRD and SEM), textural (BET and pore size distribution), mechanical (compressive strength), chemical (ICP–MS), redox (H2-TPR) and surface (NH3-TPD) analyses, showing the preservation of Cu-exchanged zeolite features in the composite monoliths. NH3-SCR tests, carried out on both monolithic and powdered samples in the temperature range 70–550 °C, confirmed that composite monoliths provide a very good activity and a high selectivity to N2 over the whole range of temperatures explored due to the hierarchical structure of the materials, in addition to a good mechanical resistance—mostly related to the geopolymer matrix.

Improved Sulfur Resistance of COMMERCIAl V2O5-WO3/TiO2 SCR Catalyst Modified by Ce and Cu

The accumulation of NH4HSO4 leads to the deactivation of commercial V2O5-WO3/TiO2 catalyst (VWTi) in practical application. The commercial catalyst is modified with 0.3 wt. % Ce and 0.05 wt. % Cu (donated as VWCeCuTi), and its sulfur resistance is noticeably improved. After loading 20 wt. % NH4HSO4, the NOx conversion of VWCeCuTi-S remains 40% at 250 °C, higher than that of VWTi-S (25%). Through a series of characterization analyses, it was found that the damaged surface areas and acid sites are the key factors for the deactivation of S-poisoned samples. However, surface-active oxygen and NO adsorption are increased by NH4HSO4 deposition, and the L–H mechanism is promoted over S-poisoned samples. Due to the interaction between V, Ce and Cu, the surface-active oxygen over VWCeCuTi-S is increased, and then NO adsorption is promoted. In addition, VWCeCuTi-S obtains a higher V5+ ratio and a better redox property than VWTi-S, which in turn accelerates the NH3-SCR reaction. More NO adsorption and encouraged reaction contribute to the better sulfur resistance of VWCeCuTi.

Aftertreatment DeNOx Systems for Future Light Duty Lean-Burned Emission Regulations

Future light duty vehicles in Europe and the United States are required to be certified, owing to progressively more and more stringent regulations [...]

Poisoning of SCR Catalysts by Alkali and Alkaline Earth Metals

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.

The Structure Effect on the Activity and Strength of an Industrial Honeycomb Catalyst Derived from Different Ti Sources

A new honeycomb production process was proposed with both filter cake (from hydrothermal treatment of metatitanic acid) and industrial titanium dioxide as Ti sources. The strength of the obtained pilot product was comparable with the current industrial honeycomb product from only filter cake, but its denitration (DeNOx) efficiency was elevated up to 15 percentage points. Multiple characterizations revealed the filter cake and industrial titanium dioxide to be composed of primary particles and secondary particles, respectively, and the introduced secondary particles from industrial titanium dioxide resulted in increased specific surface area and pore size/volume, facilitating the exposure of more active sites with improved activity. Moreover, a positive correlation property was found between the honeycomb strength and crystallinity for the samples from different titanium sources. The filter cake with rich hydroxyl groups and weak crystallinity could be fused more easily among these primary particles to have a higher strength than industrial titanium dioxide, and the primary particle of the filter cake could fill the pile pore of industrial titanium dioxide and act as a solid phase binder to acquire good strength for the honeycomb using both the filter cake and titanium dioxide as Ti sources. The improved honeycomb product with good activity and strength may have more widespread application for the purification of low temperature flue gas in industry.