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

Ce2(MoO4)3 synthesized with oleylamine and oleic acid as additives for photocatalysis: effect of preparation method

Ce₂(MoO₄)₃ was prepared using dielectric barrier discharge (DBD) plasma method, co-precipitation method and hydrothermal method, respectively, with water/ethanol (W/O) as solvent, oleylamine (OAm) and oleic acid (OAc) as additives. Preparation method showed significant influence on the morphological and structural properties, as well as photocatalytic performance. Ce₂(MoO₄)₃ synthesized with DBD plasma (MO-P) was mainly flowerlike nanosheets, which were beneficial to promoting electron transfer and providing more space for catalytic activity. Also, MO-P samples exhibited more oxygen vacancies, which were conducive to the photocatalytic performance. What's more, MO-P showed lower PL intensity and narrow energy gap, which implied a slow photoelectron-hole pair recombination rate and an increased electron transfer rate. The degradation rate of methyl orange (50 mg/L) could achieve 98% within 12 min with 0.5 g/L MO-P. Hydroxyl radicals (·OH) and superoxide radicals (·O₂^-) played a major effect. Plasma synthesis method exhibited potential application prospect in photocatalysts preparation.

TiO2-modified CeVO4 catalyst for the selective catalytic reduction of NOx with NH3

A series of TiO2-modified CeVO4 catalysts were prepared by the homogeneous precipitation method, among which the CeVTi5 catalyst showed the best low-temperature NH3-selective catalytic reduction (NH3-SCR) activity under high GHSV....

Insight into the properties of MnO2-Co3O4-CeO2 catalyst series for the selective catalytic reduction of NOx by C3H6 and NH3

A series of MnO₂-Co₃O₄-CeO₂ catalysts with different ceria loading (0.75, 1.26 and 1.88 Ce/Mn molar ratio) were synthesized by a co-precipitation technique and the catalytic activity was tested for selective catalytic reduction of NO_x by C₃H₆ or NH₃. The catalysts were characterized by various physicochemical techniques to examine the effect of ceria loading on the properties of catalysts, such as crystallinity of metal species, surface area, porosity, and acidity using physical adsorption analysis, SEM-EDX, H₂-TPR, XRD, NH₃-TPD and in-situ FTIR spectroscopy. Ceria loading had a significant effect on the reduction of NO_x, with the catalyst having low amount of ceria loading (Ce/Mn = 0.75) showing excellent performance at low-temperature conditions, but the activity declined at higher temperature. The high ceria loading (Ce/Mn = 1.88) catalyst showed poor activity compared to the counterparts owing to the lower number of acid sites and the resulting lower adsorption capacity.

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.

Adjustment of operation temperature window of Mn-Ce oxide catalyst for the selective catalytic reduction of NOx with NH3

In order to enhance the catalytic activity of Mn-Ce oxide catalyst for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR), W was introduced as a promoter. With the doping of W, the NO_x conversion over Mn₃CeO_x catalyst above 150 °C was increased, and the N₂O production was significantly decreased. Even in the present of water vapour, Mn₃CeW_0.3O_x still showed a good SCR activity. H₂-TPR and XPS results suggested that the doping of tungsten could inhibit the charge imbalance and reducibility, which would inhibit NO oxidation to NO₂ over Mn₃CeO_x. As a result, the NO_x conversion below 150 °C over Mn₃CeW_0.3O_x was slightly lower than that over Mn₃CeO_x. Since the NO_x production and the NH₃ conversion during the NH₃ oxidation of Mn₃CeO_x were inhibited after the doping of W, the NO_x conversion above 150 °C over Mn₃CeW_0.3O_x was higher than that over Mn₃CeO_x. The transient reaction demonstrated that the doped W species on Mn₃CeW_0.3O_x could inhibit the N₂O produced by the Langmuir-Hinshelwood mechanism. Kinetic study proved that ν_SCR over Mn₃CeW_0.3O_x was obviously higher that over Mn₃CeO_x, ν_NSCR and ν_C-O over Mn₃CeO_x were much higher than those of Mn₃CeW_0.3O_x, which were consistent with the SCR activity.

Boosting the Alkali/Heavy Metal Poisoning Resistance for NO Removal by Using Iron-Titanium Pillared Montmorillonite Catalysts

It is still a big challge to improve the alkali and heavy metal resistance of deNO_x catalysts for selective catalytic reduction (SCR) of NO_x with NH₃. In this study, a novel catalyst developed by pillaring montmorillonite with iron and titanium (Fe-Ti-MMT) was proposed. It is quite interesting that high resistance to alkaline and heavy metals has been demonstrated by using Fe-Ti-MMT catalysts. It has been demonstrated that the specific pillaring synthesis procedure and further addition of the Ti pillared sites greatly contributed to the wide active temperature window and enhanced the resistance to alkali and heavy metal. The higher ratio of active Fe²⁺ species, more active acid sites, and enhanced ammonia adsorption indicated the remarkable activity as well as K and Pb resistance. Moreover, the K and Pb poisons would promote the generation of active adsorbed NO_x species on the Fe-Ti-MMT but induce the formation of stable inactive ones on that of Fe-MMT, which greatly tuned the reaction pathways and improved the reaction rate for Ti modified Fe pillared MMT catalysts. The strategy of incorporating Ti into the Fe pillared MMT catalysts strongly provides a novel inspiration for keeping excellent NH₃-SCR performance in the presence of alkali/heavy metal for NO_x removal.

VxMn(4-x)Mo3Ce3/Ti catalysts for selective catalytic reduction of NO by NH3

A series of vanadium based catalysts (VxMn(4-x)Mo3Ce3/Ti) with different vanadium (x wt.%) and manganese ((4-x) wt.%) contents have been prepared by the wet impregnation method and investigated for selective catalytic reduction (SCR) of NO_x by NH₃ in the presence of 8 vol.% H₂O and 500 ppmV SO₂. The physicochemical characteristics of the catalysts were thoroughly characterized. The SCR of NO_x by NH₃ (NH₃-SCR) activity, especially the low-temperature activity, significantly increased with increasing V₂O₅ content in the catalyst until the V₂O₅ content reached 1.5 wt.%, which corresponds well with the redox properties of the catalyst. All of the metal oxides were well dispersed and strongly interacted with each other on the catalyst surface. V mainly exists in the V⁵⁺ state in the catalysts. The strong synergistic effect between the vanadium and cerium species led to formation of more Ce³⁺ species, and that between the vanadium and manganese species contributed to formation of more manganese species with low valences. All of the catalysts exhibited strong acidity, while the redox properties determined the NH₃-SCR activity, especially the low-temperature activity. H₂O and SO₂ had severe inhibiting effects on the activity of V1.5Mn2.5Mo3Ce3/Ti. However, good H₂O and SO₂ resistance and high NO_x conversion by V1.5Mn2.5Mo3Ce3/Ti could be achieved in the presence of SO₂ and almost no decline was observed in a long-term test at 275°C for 168 hr in the presence of SO₂ and H₂O, which can be attributed to the sulfate species formed on the catalyst surface.

CeO2 grafted with different heteropoly acids for selective catalytic reduction of NOx with NH3

The CeO₂ catalysts grafted with heteropoly acid (i.e., HPA) could enhance their catalytic performance for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). In comparison to HSiW/CeO₂, HPMo/CeO₂, and commercial V₂O₅-WO₃/TiO_x catalysts, HPW/CeO₂ catalysts showed the best SCR performance. XPS and DRIFTS demonstrated that the amount of HPA on HPW/CeO₂ was more than those on HSiW/CeO₂ and HPMo/CeO₂. H₂-TPR results indicated that reducibility of HPMo/CeO₂ was stronger than those of HSiW/CeO₂ and HPW/CeO₂, resulting in the high-temperature performance loss. According to kinetic results, below 250 °C, k_SCR-ER and k_SCR-LH of HPW/CeO₂ were higher than those of HSiW/CeO₂, meanwhile k_side of both HSiW/CeO₂ and HPW/CeO₂ were low. Therefore, HPW/CeO₂ had the better SCR performance than HSiW/CeO₂. As NH₃ was completely consumed, SCR activity depended on the ratio of SCR reaction in the consumption of NH₃. The selectivity of SCR reaction, NSCR reaction, and C-O reaction of HSiW/CeO₂ were almost the same as those of HPW/CeO₂ above 250 °C, resulting in the NO_x conversion of HPW/CeO₂ was basically the same as that of HSiW/CeO₂ above 250 °C. Due to the lowest k_SCR-ER and k_SCR-LH, and highest k_side, NO_x conversion of HPMo/CeO₂ was the worst compared to HSiW/CeO₂ and HPW/CeO₂ catalysts.

Significantly enhanced Pb resistance of a Co-modified Mn–Ce–Ox/TiO2 catalyst for low-temperature NH3-SCR of NOx

Co modification can significantly improve the performance for low-temperature NH₃-SCR of NO_x and the Pb resistance of the Mn–Ce–O_x/TiO₂ catalyst.

Nanostructured MoO3 for Efficient Energy and Environmental Catalysis

This paper mainly focuses on the application of nanostructured MoO₃ materials in both energy and environmental catalysis fields. MoO₃ has wide tunability in bandgap, a unique semiconducting structure, and multiple valence states. Due to the natural advantage, it can be used as a high-activity metal oxide catalyst, can serve as an excellent support material, and provide opportunities to replace noble metal catalysts, thus having broad application prospects in catalysis. Herein, we comprehensively summarize the crystal structure and properties of nanostructured MoO₃ and highlight the recent significant research advancements in energy and environmental catalysis. Several current challenges and perspective research directions based on nanostructured MoO₃ are also discussed.

Enhancing Water Resistance of a Mn-Based Catalyst for Low Temperature Selective Catalytic Reduction Reaction by Modifying Super Hydrophobic Layers

OMS-2 catalysts exhibit excellent selective catalytic reduction (SCR) activity at low temperature but weak H₂O resistance restricts its industrial application. To remarkably improve the water resistance of Mn-based catalysts is a key technical problem. In this work, the H₂O endurance and self-cleaning properties of OMS-2 catalysts are remarkably improved by the facile process, construction of hydrophobic coating. The performance of the hydrophobic layer on the bulk OMS-2 catalyst surface could be effectively controlled by adjusting the polydimethylsiloxane (PDMS) vapor deposition temperature. It is discovered that the 200 °C catalyst obtained super hydrophobic properties and formed with a contact angle of 160.3°, which not only exhibited satisfactory NH₃-SCR activity at low temperatures (140-300 °C) but also dramatically improved H₂O endurance and self-cleaning performance. Moreover, the mechanism of improving H₂O resistance and stability of the 200 °C catalyst was investigated in detail through a series of characterizations. Although the SCR activity of the 200 °C catalyst decreased slightly because of the combination of some active species (O_α and Mn³⁺) with PDMS, the H₂O passivation of the active species was eliminated. The advantage of self-cleaning was confirmed by the analysis of surface species and simulation experiments, which could avoid the accumulation of intermediates on the surface and promote the stability of the OMS-2 catalyst for NH₃-SCR at low temperature. This method of constructing special coating might be a huge step to remarkably improve the H₂O endurance properties of the catalyst and provided a new concept for future industrial application.

Fabrication of Highly Dispersed Cu-Based Oxides as Desirable NH3-SCR Catalysts via Employing CNTs To Decorate the CuAl-Layered Double Hydroxides

In this study, three kinds of CuAl-LDO/CNT (LDO, layered double oxide) catalysts were prepared by the assembly of CNTs and CuAl-LDH (LDH, layered double hydroxides) as well as subsequently structural topological transformation. The effects of the assembly method on the surface structure property and the DeNO_x performance of the prepared samples were systematically investigated. It was found that three CuAl-LDO/CNT catalysts showed preferable NH₃-SCR catalytic performance compared with CuAl-LDO where the catalyst CuAl-LDO/CNTs(I) exhibited optimum NO_x conversion (>80%) and N₂ selectivity (>90%) within 180-300 °C. Such fine catalytic performance can be attributed to the proper surface acidity and redox ability of the catalyst, which might be correlated with the high dispersion of Cu-based active centers caused by the induced nucleation and effective separation action of LDH by carbon nanotubes. In addition, the outstanding H₂O and SO₂ resistance of the CuAl-LDO/CNTs(I) catalyst was also obtained because of the synergistic effect between CuAl-LDO and CNTs, which could greatly promote the activation and decomposition of ammonium sulfate at lower temperatures.

Comparison of support synthesis methods for TiO2and the effects of surface sulfates on its activity toward NH3-SCR

Addition of SO₄²⁻inhibits the transformation of TiO₂from anatase to rutile and generates sulfate salts to increase the surface acidity.

Novel Homogeneous and Mesoporous MnOx-Doped Ceria Nanosheets as Catalysts for Low-Temperature Selective Catalytic Reduction

The development of a catalyst for the selective catalytic reduction (SCR) of NOx is essential for purifying air and the denitration of coal-burning exhaust. Herein, we prepare novel MnOx-CeO2 nanosheets with porous structures by a homogeneous coordination precipitation (HCP) method which exhibit a high NO removal efficiency above 90% in the SCR reaction at low temperature (150–240°C). The MnOx-CeO2(HCP) catalysts have a higher Brunauer–Emmett–Teller (BET) surface area and more homogeneous distribution of Mnx+ in the CeO2 lattice than those prepared by co-precipitation and precursor mixture combustion methods according to BET, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. Together with a higher ratio of Mn4+, Ce3+, and Oα, the above properties are responsible for the high catalytic performances of MnOx-CeO2(HCP) in the SCR of NOx.

Comprehensive study of the promotional mechanism of F on Ce–Mo/TiO2 catalysts for wide temperature NH3-SCR performance: the activation of surface Ti–F bonds

The CeFMoTiO_x catalyst was prepared via a solvothermal method. It confirmed that O_α was not only generated during the transformation between Ce⁴⁺ and Ce³⁺ but was also released from F⁻ species of the surface Ti–F bonds.

Doping effect of Sm on the TiO2/CeSmOx catalyst in the NH3-SCR reaction: structure–activity relationship, reaction mechanism and SO2 tolerance

A good balance between the redox properties and surface acidity induces the high activity of the Sm doped TiO₂/CeO₂ catalyst.

Rotation-Assisted Hydrothermal Synthesis of Thermally Stable Multiwalled Titanate Nanotubes and Their Application to Selective Catalytic Reduction of NO with NH3

Titanate nanotubes are widely applied in various fields, including photocatalysts and electronic devices, but their weak thermal stability limits their application for catalyst support. Here, we found that titanate nanotubes with a thick multiwalled structure of 15 layers or more can be prepared by using rotation-assisted hydrothermal synthesis. The porous structure of conventional nanotubes synthesized without rotation collapsed easily after thermal treatment, whereas the nanotubes having a thick multiwalled structure retained their pore structure and the specific surface area (∼300 m²/g) even after calcination at 400 °C in air. Systematic variation of rotation speed suggested that rotation in the synthesis process accelerated the stacking of layered titanate nanosheets, which are known to be intermediates of nanotubes. Thus, the rapid assembly of titanate nanosheets facilitated by rotation led to the formation of nanotubes with a multiwalled structure. Overly fast rotation, however, caused excessive stacking and created a thicker structure that cannot be easily wrapped into nanotubes. Therefore, it is essential to maintain the optimum rotation speed to obtain both the nanotube morphology and the thick multiwalled structure. Vanadium-tungsten-oxide catalyst supported on the multiwalled titanate nanotubes was used in NH₃-selective catalytic reduction, which showed stable NO _x reduction performance with high selectivity to N₂, which may originate from the suppressed sintering of VO _x on multiwalled nanotubes. This study demonstrates that the morphology of nanotubes can be tuned by controlling the degree of interaction supplied by external forces.