Novel MoO3/CeO2–ZrO2 catalyst for the selective catalytic reduction of NOx by NH3

An overview of the deactivation mechanism and modification methods of the SCR catalysts for denitration from marine engine exhaust

Selective catalytic reduction (SCR) technology is currently the most effective deNO_x technology and has broad application prospects. Moreover, there is a large NO_x content in marine engine exhaust. However, the marine engine SCR catalyst will be affected by heavy metals, SO₂, H₂O_(g), hydrocarbons (HC) and particulate matter (PM) in the exhaust, which will hinder the removal of NO_x via SCR. Furthermore, due to the high loading operation of the marine engine and the regeneration of the diesel particulate filter (DPF), the exhaust temperature of the engine may exceed 600 °C, which leads to sintering of the SCR catalysts. Therefore, the development of new catalysts with good tolerances to the above emissions and process parameters is of great significance for further reducing NO_x from marine engines. In this work, we first elaborate on the mechanism of the SCR catalyst poisoning caused by marine engine emissions, as well as the working mechanism of SCR catalysts affected by the engine exhaust temperature. Second, we also summarize the current technologies for improving the properties of SCR catalysts with the aim of enhancing the resistance and stability under complex working conditions. Finally, the challenges and perspectives associated with the performance optimization and technology popularization of marine SCR systems are discussed and proposed further. Consequently, this review may provide a valuable reference and inspiration for the development of catalysts and improvement in the denitration ability of SCR systems matched with marine engines.

Synthesis of W-modified CeO2/ZrO2 catalysts for selective catalytic reduction of NO with NH3

In this paper, a series of tungsten–zirconium mixed binary oxides (denoted as W_mZrO_x) were synthesized via co-precipitation as supports to prepare Ce_0.4/W_mZrO_x catalysts through an impregnation method. The promoting effect of W doping in ZrO₂ on selective catalytic reduction (SCR) performance of Ce_0.4/ZrO₂ catalysts was investigated. The results demonstrated that addition of W in ZrO₂ could remarkably enhance the catalytic performance of Ce_0.4/ZrO₂ catalysts in a broad temperature range. Especially when the W/Zr molar ratio was 0.1, the Ce_0.4/W_0.1ZrO_x catalyst exhibited the widest active temperature window of 226–446 °C (NO_x conversion rate > 80%) and its N₂ selectivity was almost 100% in the temperature of 150–450 °C. Moreover, the Ce_0.4/W_0.1ZrO_x catalyst also exhibited good SO₂ tolerance, which could maintain more than 94% of NO_x conversion efficiency after being exposed to a 100 ppm SO₂ atmosphere for 18 h. Various characterization results manifested that a proper amount of W doping in ZrO₂ was not only beneficial to enlarge the specific surface area of the catalyst, but also inhibited the growth of fluorite structure CeO₂, which were in favor of CeO₂ dispersion on the support. The presence of W was conducive to the growth of a stable tetragonal phase crystal of ZrO₂ support, and a part of W and Zr combined to form W–Zr–O_x solid super acid. Both of them resulted in abundant Lewis acid sites and Brønsted acid sites, enhancing the total surface acidity, thus significantly improving NH₃ species adsorption on the surface of the Ce_0.4/W_0.1ZrO_x catalyst. Furthermore, the promoting effect of adding W on SCR performance was also related to the improved redox capability, higher Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio and abundant surface chemisorbed oxygen species. The in situ DRIFTS results indicated that nitrate species adsorbed on the surface of the Ce_0.4/W_0.1ZrO_x catalyst could react with NH₃ due to the activation of W. Therefore, the reaction pathway over the Ce_0.4/W_0.1ZrO_x catalyst followed both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms at 250 °C.

Interaction of W with Zr improved NH₃-SCR performance via enhancing redox and surface acidity.

The promoting effect of support pretreatment with sulfate acid on the Ca resistance of a CeO2/ZrO2 catalyst for NH3-SCR of NOx with NH3

CaSO4 was formed through the reaction between S and Ca to relieve the effect of Ca-poisoning on the catalyst.

Revealing the effect of paired redox-acid sites on metal oxide catalysts for efficient NOx removal by NH3-SCR

Understanding the nature of active sites on metal oxide catalysts in the selective catalytic reduction (SCR) of NO by NH₃ (NH₃-SCR) is a crucial prerequisite for the development of novel efficient NH₃-SCR catalysts. In this work, two CeO₂-based SCR catalyst systems with diverse acidic metal oxides-CeO₂ interfaces, i.e., Nb₂O₅-CeO₂ (Nb₂O₅/CeO₂ and CeO₂/Nb₂O₅) and WO₃-CeO₂ (WO₃/CeO₂ and CeO₂/WO₃), were prepared and used to reveal the relationship between NH₃-SCR activity and surface acidity/redox properties. In combination with the results of the NH₃-SCR activity test and various characterizations, it was found that the NH₃-SCR performance of Nb₂O₅-CeO₂ and WO₃-CeO₂ catalysts was highly dependent on the strong interactions between the redox component (CeO₂) and acidic component (Nb₂O₅ or WO₃), as well as the amount of paired redox-acid sites. From a quantitative perspective, an activity-surface acidity/redox property relationship was proposed. For both Nb₂O₅-CeO₂ and WO₃-CeO₂ catalysts systems operated at the more concerned low-temperature range (200 °C), the NH₃-SCR activity in low NO_x conversion region (< 40%) was mainly dominated by the surface acidity of catalysts, while the NH₃-SCR activity in high NO_x conversion region (> 40%) was more determined by redox properties.

Study on the mechanism of synthetic (Ce,La)CO3F sulfuric acid acidification and NH3-SCR loaded with Mn and Fe

A hydrothermal method was used to synthesise (Ce,La)CO3F grain simulated minerals, in accordance with the Ce-La ratio of bastnaesite in the mineralogy of the Bayan Ebo process. The NH3-SCR catalytic activity of the synthesised (Ce,La)CO3F was improved by loading transition metals Mn and Fe and sulphuric acid acidification treatments. The activity test results showed that the catalysts which were simultaneously acidified with sulphuric acid and loaded with transition metals Mn and Fe had a NO x conversion of 92% at 250 °C. XRD, SEM, XPS and in situ Fourier transform infrared spectroscopy (FTIR) were used to investigate the physical phase structure, surface morphology, reaction performance and mechanism of the catalysts, to provide theoretical guidance for the specific reaction path of cerium fluorocarbon ore in the NH3-SCR reaction. The results showed that the introduction of transition metals and sulphuric acid greatly increases the proportion of adsorbed oxygen (Oα) and facilitates the adsorption of NH3 and NO. The catalyst surface metal sulphate and metal oxide species act as the main active components on the catalyst surface to promoted the reaction, and cracks and pores appear on the surface to facilitate the adsorption of reactive gases. The reaction mechanism of the SO4 2--Mn-Fe/(Ce,La)CO3F catalyst, and characterisation of the adsorption and conversion behaviour of the reactive species on the catalyst surface, were investigated by Fourier transform infrared spectroscopy (FTIR). The results showed that the catalyst follows the E-R and L-H mechanisms throughout the reaction, with the E-R mechanism being the main reaction. The reaction species were NH4 +/NH3 species in the adsorbed state and NO. The NH3(ad) species on the Lewis acidic site is the main NH3(g) adsorbed species for the reaction, bonded to Ce4+ in the carrier (Ce,La)CO3F to participate in the acid cycle reaction, and undergo a redox reaction on the catalyst surface to produce N2 and H2O. The SO4 2- present on the catalyst surface can also act as an acidic site for the adsorption of NH3. The above results indicated the excellent performance of the SO4 2--Mn-Fe/(Ce,La)CO3F catalyst, which provided a theoretical basis for the high value utilization of bastnaesite.

Enhanced activity and alkali metal resistance in vanadium SCR catalyst via co-modification with Mo and Sb

Enhanced surface acidity contributed significantly to the catalytic activity and alkali metal resistance of the optimum catalysts.

Poisoning-Resistant NOx Reduction in the Presence of Alkaline and Heavy Metals over H-SAPO-34-Supported Ce-Promoted Cu-Based Catalysts

Selective catalytic reduction (SCR) of NO_x using NH₃ in the presence of alkaline and heavy metals is still an issue in the application of a stationary source. Reported here is the rational design of a novel H-SAPO-34-supported ceria-promoted copper-based catalyst (CuCe/H-SAPO-34) that demonstrates exceptional resistance against alkali (K), alkaline earth (Ca), and heavy metal (Pb) poisoning during SCR of NO_x. The H-SAPO-34 support contained numerous acid sites that allowed Cu-based catalysts to maintain their catalytic activity while also resisting poisoning by K and Ca. Decorating the catalyst with CeO₂ promoted the low-temperature deNO_x activity by accelerating the redox cycle with Cu species and assisted the H-SAPO-34 in capturing Ca and Pb. H-SAPO-34-supported ceria-promoted copper oxide catalysts prevented the irreversible combination of K, Ca, or Pb with the active centers, providing the catalyst with excellent poisoning resistance. This work provides a strategy for the development of high-performance, poisoning-resistant catalysts for NH₃-SCR of NO_x in the presence of alkaline and heavy metals.

NO oxidative activity of mesoporous LaMnO3 and LaCoO3 perovskite nanoparticles by facile molten-salt synthesis

LaMnO₃ and LaCoO₃ perovskite nanoparticles have sprung up as the PGM-free catalysts for NO oxidation.

A CeO2/ZrO2-TiO2 Catalyst for the Selective Catalytic Reduction of NOx with NH3

In this study, CeZr0.5TiaOx (with a = 0, 1, 2, 5, 10) catalysts were prepared by a stepwise precipitation approach for the selective catalytic reduction of NOx with NH3. When Ti was added, all of the Ce-Zr-Ti oxide catalysts showed much better catalytic performances than the CeZr0.5Ox. Particularly, the CeZr0.5Ti2Ox catalyst showed excellent activity for broad temperature range under high space velocity condition. Through the control of pH value and precipitation time during preparation, the function of the CeZr0.5Ti2Ox catalyst could be controlled and the structure with highly dispersed CeO2 (with redox functions) on the surface of ZrO2-TiO2 (with acidic functions) could be obtained. Characterizations revealed that the superior catalytic performance of the catalyst is associated with its outstanding redox properties and adsorption/activation functions for the reactants.

Deactivation by HCl of CeO2-MoO3/TiO2 catalyst for selective catalytic reduction of NO with NH3

The effect of HCl on a CeO₂–MoO₃/TiO₂ catalyst for the selective catalytic reduction of NO with NH₃ was investigated with BET, XRD, NH₃-TPD, H₂-TPR, XPS and catalytic activity measurements. The results showed that HCl had an inhibiting effect on the activity of the CeO₂–MoO₃/TiO₂ catalyst. The deactivation by HCl of the CeO₂–MoO₃/TiO₂ catalyst could be attributed to pore blockage, weakened interaction among ceria, molybdenum and titania, reduction in surface acidity and degradation of redox ability. The Ce³⁺/Ce⁴⁺ redox cycle was damaged because unreactive Ce³⁺ in the form of CeCl₃ lost the ability to be converted to active Ce⁴⁺ in the SCR reaction. In addition, a decrease in the amount of chemisorbed oxygen and the concentrations of surface Ce and Mo was also responsible for the deactivation by HCl of the CeO₂–MoO₃/TiO₂ catalyst.

The effect of HCl on a CeO₂–MoO₃/TiO₂ catalyst for the selective catalytic reduction of NO with NH₃.

Promotional Effects of Ti on a CeO2-MoO3 Catalyst for the Selective Catalytic Reduction of NOx with NH3

In this study, Ti was doped to CeO₂-MoO₃ to promote the catalytic performance for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). The preparation method for CeMo_0.5Ti_aO_x (a = 0, 1, 2, 5, 10) catalysts was a stepwise precipitation method. When Ti was doped, all of the Ce-Mo-Ti catalysts exhibited remarkably improved NO_x conversion and N₂ selectivity than the CeMo_0.5O_x without Ti. The CeMo_0.5Ti₅O_x with excellent activity in a broad temperature range was selected as an optimal catalyst to investigate the effects of Ti addition. The formation process analysis of the CeMo_0.5Ti₅O_x showed that, the Mo and Ti species first precipitated together from the mixed solution with the increase of pH, and then Ce species precipitated onto the Mo-Ti precipitates. The obtained catalyst exhibited remarkably facilitated NO_x and NH₃ adsorption, enhanced charge imbalance, promoted redox property, and improved surface acidity, which are all important reasons for the excellent catalytic performance of an NH₃-SCR catalyst.

Simultaneous removal of elemental mercury and NO from simulated flue gas using a CeO2 modified V2O5–WO3/TiO2 catalyst

The redox cycle (V⁴⁺ + Ce⁴⁺ ↔ V⁵⁺ + Ce³⁺) over V₂O₅–WO₃/TiO₂–CeO₂ plays a key role in Hg⁰ oxidation and NO conversion.

Novel Fe–Ce–Ti catalyst with remarkable performance for the selective catalytic reduction of NOx by NH3

The redox cycle (Ce⁴⁺ + Fe²⁺ ↔ Ce³⁺ + Fe³⁺) over the Fe–Ce–Ti catalyst contributes to the activation of NO_x and NH₃ and thus the formation of reaction intermediates, leading to the high catalytic performance for the NH₃-SCR of NO_x.

Fabrication and formation mechanism of Ce2O3–CeO2–CuO–MnO2/CNTs catalysts and application in low-temperature NO reduction with NH3

Ce₂O₃–CeO₂–CuO–MnO₂/CNTs catalysts were synthesized via a redox strategy, and presented 58–85% NO conversion at 80–180 °C.

Ceria–zirconia modified MnOx catalysts for gaseous elemental mercury oxidation and adsorption

The developed ceria–zirconia modified MnO_x catalysts were found to exhibit enhanced Hg⁰ oxidation and removal performance.

A novel Ce–Sb binary oxide catalyst for the selective catalytic reduction of NOx with NH3

The synergetic effect between Sb and Ce not only increases the surface acidity of the catalyst but also enhances the redox property, both of which contribute to improving NH₃-SCR activity.