Effect of MoO3 on vanadium based catalysts for the selective catalytic reduction of NOx with NH3 at low temperature

Interface sites on vanadia-based catalysts are highly active for NOx removal under realistic conditions

TiO2-supported V2O5 catalysts are commonly used in NOx reduction with ammonia due to their robust catalytic performance. Over these catalysts, it is generally considered that the active species are mainly derived from the vanadia species rather than the intrinsic structure of V-O-Ti entities, namely the interface sites. To reveal the role of V-O-Ti entities in NH3-SCR, herein, we prepared TiO2/V2O5 catalysts and demonstrated that V-O-Ti entities were more active for NOx reduction under wet conditions than the V sites (V=O) working alone. On the V-O-Ti entities, kinetic measurements and first principles calculations revealed that NH3 activation exhibited a much lower energy barrier than that on V=O sites. Under wet conditions, the V-O-Ti interface significantly inhibited the transformation of V=O to V-OH sites thus benefiting NH3 activation. Under wet conditions, meanwhile, the migration of NH4+ from Ti site neighboring the V-O-Ti interface to Ti site of the V-O-Ti interface was exothermic; thus, V-O-Ti entities together with neighboring Ti sites could serve as channels linking NH3 pool and active centers for activation of NH4+. This finding reveals that the V-O-Ti interface sites on V-based catalysts play a crucial role in NOx removal under realistic conditions, providing a new perspective on NH3-SCR mechanism.

Mercury removal using various modified V/Ti-based SCR catalysts: A review

Growing levels of mercury pollution has made countries urgently need a suitable mercury treatment technology. Among various technologies, heterogeneous oxidative mercury removal via different modified V/Ti-based SCR catalysts is considered as a promising approach due to excellent economic value and removal efficiency. Although various related modification experiments have been worked in recent years, the research on the performance, including activity and resistance, and mechanism of catalysts still needs to be improved, so it is necessary to summarize these experiments to guide further work. This article will review many modifications start from the V/Ti catalyst. Not only the performance of these catalysts, but also a lot of speculation about the mercury removal mechanism are include in our research. In addition, the characteristics of some modified catalysts have been linked with their oxidation mechanism and structural changes by comparing many studies, and finally attributed to some special properties of the corresponding modifiers. We expect this study will clarify the research progress of modified V/Ti-based SCR catalysts in mercury removal, and guide future modification so that some properties of the catalyst can be improved in a targeted manner.

Molybdenum-decorated V2O5–WO3/TiO2: surface engineering toward boosting the acid cycle and redox cycle of NH3-SCR

Submonolayer Mo-decorated V₂O₅–WO₃/TiO₂ provides abundant vanadia species and unsaturated V⁴⁺ species, accelerating the acid and redox cycling of low-temperature NH₃-SCR.

Recent Progress on Improving Low-Temperature Activity of Vanadia-Based Catalysts for the Selective Catalytic Reduction of NOx with Ammonia

Selective catalytic reduction of NOx with NH3 (NH3-SCR) has been successfully applied to abate NOx 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.

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

The mode of operation of titania-supported vanadia (VOx) catalysts for NOx abatement using ammonia selective catalytic reduction (NH3-SCR) is still vigorously debated. We introduce a new high surface area VOx/TiO2/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VOx species on amorphous TiO2 domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NOx conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH2 and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NOx conversion is attributed to the presence of highly dispersed VOx species and the moderate acidity of VOx supported on TiO2/SBA-15.

The Deactivation of Industrial SCR Catalysts—A Short Review

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.

NOx Emissions and Nitrogen Fate at High Temperatures in Staged Combustion

Staged combustion is an effective technology to control NOx emissions for coal-fired boilers. In this paper, the characteristics of NOx emissions under a high temperature and strong reducing atmosphere conditions in staged air and O2/CO2 combustion were investigated by CHEMKIN. A methane flame doped with ammonia and hydrogen cyanide in a tandem-type tube furnace was simulated to detect the effects of combustion temperature and stoichiometric ratio on NOx emissions. Mechanism analysis was performed to identify the elementary steps for NOx formation and reduction at high temperatures. The results indicate that in both air and O2/CO2 staged combustion, the conversion ratios of fuel-N to NOx at the main combustion zone exit increase as the stoichiometric ratio rises, and they are slightly affected by the combustion temperature. The conversion ratios at the burnout zone exit decrease with the increasing stoichiometric ratio at low temperatures, and they are much higher than those at the main combustion zone exit. A lot of nitrogen compounds remain in the exhaust of the main combustion zone and are oxidized to NOx after the injection of a secondary gas. Staged combustion can lower NOx emissions remarkably, especially under a high temperature (≥1600 °C) and strong reducing atmosphere (SR ≤ 0.8) conditions. Increasing the combustion temperature under strong reducing atmosphere conditions can raise the H atom concentration and change the radical pool composition and size, which facilitate the reduction of NO to N2. Ultimately, the increased OH/H ratio in staged O2/CO2 combustion offsets part of the reducibility, resulting in the final NOx emissions being higher than those in air combustion under the same conditions.

A new V2O5–MoO3–TiO2–SO42−nanostructured aerogel catalyst for diesel DeNOxtechnology

A new V₂O₅–MoO₃–TiO₂–SO₄²⁻nanostructured aerogel catalyst exhibits superior SCR activity compared to the V₂O₅–WO₃/TiO₂commercial catalyst (EUROCAT) at high temperatures (375–500 °C).

Effect of preparation methods on the performance of CuFe-SSZ-13 catalysts for selective catalytic reduction of NOx with NH3

CuFe-SSZ-13 catalyst showed excellent performance in the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) for diesel engine exhaust purification. To investigate the effect of preparation methods on NH₃-SCR performance, Fe was loaded into one-pot synthesized Cu-SSZ-13 catalysts through solid-state ion-exchange (SSIE), homogeneous deposition precipitation (HDP) and liquid ion-exchange (IE), respectively. Three CuFe-SSZ-13 catalysts showed similar SO₂ resistance, which was better than that of Cu-SSZ-13. The improvement was attributed to the protection of Fe species. Hydrothermal stability of three CuFe-SSZ-13 catalysts was significantly different, which was attributed to the state of active species caused by different preparation methods. Compared with the other two catalysts, more active species existed inside the zeolite pores of CuFe-SSZ-13_SSIE. During hydrothermal aging, the aggregation of these active species in the pores caused the collapse of catalyst structure, ultimately leading to the deactivation of CuFe-SSZ-13_SSIE. In contrast, Fe species was dispersed better on the surface over CuFe-SSZ-13_IE, enhancing the hydrothermal stability of catalysts. Consequently, Fe loading effectively improved the resistance of SO₂ and H₂O over Cu-SSZ-13. For CuFe-SSZ-13, large amounts of active species located inside the zeolite pores are not beneficial for the hydrothermal stability.

Alkali resistance promotion of Ce-doped vanadium-titanic-based NH3-SCR catalysts

The effect of K deactivation on V₂O₅/WO₃-TiO₂ and Ce-doped V₂O₅/WO₃-TiO₂ catalysts in the selective catalytic reduction (SCR) of NO_x by NH₃ was studied. Ce-doped V₂O₅/WO₃-TiO₂ showed significantly higher resistance to K deactivation than V₂O₅/WO₃-TiO₂. Ce-doped V₂O₅/WO₃-TiO₂ with K/V=4 (molar ratio) showed 90% NO_x conversion at 350°C, whereas in this case V₂O₅/WO₃-TiO₂ showed no activity. The fresh and K-poisoned V₂O₅/WO₃-TiO₂ and Ce-doped V₂O₅/WO₃-TiO₂ catalysts were investigated by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), NH₃-temperature progress decomposition (NH₃-TPD), X-ray photoelectron spectroscopy (XPS) and H₂-temperature program reduction (H₂-TPR). The effect of Ce doping on the improving resistance to K of V₂O₅/WO₃-TiO₂were discussed.

NH3-SCR performance and the resistance to SO2 for Nb doped vanadium based catalyst at low temperatures

Niobium oxide as the promoter was doped in the V/WTi catalyst for the selective catalytic reduction (SCR) of NO. The results showed that the addition of Nb₂O₅ could improve the SCR activity at low temperatures and the 6wt.% additive was an appropriate dosage. The enhanced reaction activity of adsorbed ammonia species and the improved dispersion of vanadium oxide might be the reasons for the elevation of SCR activity at low temperatures. The resistances to SO₂ of 3V6Nb/WTi catalyst at different temperatures were investigated. FTIR spectrum and TG-FTIR result indicated that the deposition of ammonium sulfate species was the main deactivation reason at low temperatures, which still exhibited the reactivity with NO above 200°C on the catalyst surface. There was a synergistic effect among NH₃, H₂O and SO₂ that NH₃ and H₂O both accelerated the catalyst deactivation in the presence of SO₂ at 175°C. The thermal treatment at 400°C could regenerate the deactivated catalyst and get SCR activity recovered. The particle and monolith catalysts both kept stable NO_x conversion at 225°C with high concentration of H₂O and SO₂ during the long time tests.

Iron oxide-based catalysts for low-temperature selective catalytic reduction of NO x with NH3

Selective catalytic reduction (SCR) is now an established NO x removal technology for industrial flue gas as well as for diesel engine exhaust gas. However, it is still a big challenge to develop a novel low-temperature catalyst for NH3-SCR of NO x , especially at a temperature below 200°C. In the past few years, many studies have demonstrated the potential of iron (Fe)-based catalysts as low-temperature catalysts for NH3-SCR of NO x . Herein, we summarize the recent progress and performance of Fe-based catalysts for low-temperature NH3-SCR of NO x . Catalysts are divided into three categories: single Fe x O y , Fe-based multimetal oxide, and Fe-based multimetal oxide with support catalysts. The catalytic activity and selectivity of Fe-based catalysts are systematically analyzed and summarized in light of some key factors such as activation energy, specific surface area, morphology, crystallinity, preparation method and precursor, acid sites, calcination temperature, other metal dopant/substitute, and redox property of catalysts. In addition, H2O/SO2 tolerance and the NH3-SCR reaction mechanism over Fe-based catalysts, including Eley-Rideal and Langmuir-Hinshelwood mechanism, are emphasized. Lastly, the perspectives and future research directions of low-temperature NH3-SCR of NO x are also proposed.