Promotion effect of Ce and Ta co-doping on the NH3-SCR performance over V2O5/TiO2 catalyst.

NH3-SCR (SCR: Selective catalytic reduction) is an effective technology for the de-NOx process from both mobile and stationary pollution sources, and the most commonly used catalysts are the vanadia-based catalysts. An innovative V2O5-CeO2/TaTiOx catalyst for NOx removal was prepared in this study. The influences of Ce and Ta in the V2O5-CeO2/TaTiOx catalyst on the SCR performance and physicochemical properties were investigated. The V2O5-CeO2/TaTiOx catalyst not only exhibited excellent SCR activity in a wide temperature window, but also presented strong resistance to H2O and SO2 at 275 ℃. A series of characterization methods was used to study the catalysts, including H2-temperature programmed reduction, X-ray photoelectron spectroscopy, NH3-temperature programmed desorption, etc. It was discovered that a synergistic effect existed between Ce and Ta species. The introduction of Ce and Ta enlarged the specific surface area, increased the amount of acid sites and the ratio of Ce3+, (V3++V4+) and Oα, and strengthened the redox capability which were related to synergistic effect between Ce and Ta species, significantly improving the NH3-SCR activity.

Inhibiting the Formation of Toxic HCN Induced by C3H6 Coexistence and Enhancing C3H6 Resistance of Cu-Zeolite in the NH3-SCR Reaction.

The presence of light hydrocarbons (HCs) in diesel exhaust, specifically C3H6, significantly affects the performance of the state-of-the-art Cu-SSZ-13 zeolite NH3-SCR catalysts. It also leads to the formation of highly toxic HCN, posing risks to the environment and human health. In this work, the highly toxic HCN formation is inhibited, and the C3H6 resistance of Cu-SSZ-13 is improved by secondary metal modification via doping with rare earth/transition metal elements. Upon introduction of C3H6, the activity of Cu-SSZ-13 significantly decreases at medium-high temperatures. This is primarily due to the competitive reaction between C3H6 and NH3, which compete for the NH3 reductant required in the NH3-SCR reaction, resulting in the production of HCN. The unfavorable effect is alleviated on the modified catalysts due to their enhanced oxidation capabilities toward C3H6 and the HCHO intermediate, facilitating the complete oxidation of C3H6 to COx. This inhibits the undesirable partial oxidation reaction between C3H6 and NH3, thereby improving the activity of Cu-SSZ-13 at medium to high temperatures and significantly reducing the formation of highly toxic HCN.

Enhanced NOx reduction on CePO4 catalysts: Cu-loading, phosphotungstic acid, and insights from In-situ DRIFTs and DFT.

This study investigates the effects of varying Cu/Ce doping ratios on the NH3-SCR denitrification efficiency using Cu-HPW/CePO4 catalysts, where CePO4 serves as the support and copper-doped phosphotungstic acid (HPW) acts as the active phase. The NH3-SCR reaction mechanism was studied by In-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (In-situ DRIFTs) and Density Functional Theory (DFT). In-situ DRIFTs were employed to delve into the intricacies of adsorption and transformation dynamics at the surface sites of catalysts. This approach furnished a robust theoretical foundation aimed at augmenting the efficacy of low-temperature denitrification catalysts. DFT calculations were used to systematically investigate the reaction pathways, intermediates, transition states, and energy barriers over the HPW structure model to complete the NH3-SCR reaction. Empirical evidence suggests that modifying the catalysts with copper substantially enhances their denitrification efficacy and extends their operational temperature spectrum. A notable initial increase in denitrification efficiency was observed with increasing levels of copper modification, followed by a decline. Within the HPW-O15H site, the NH3-SCR reaction advances through both the E-R and L-H mechanisms, encompassing processes such as NH3 adsorption, intermediate formation and transformation, and product release.

Electron transferring with oxygen defects on Ni-promoted Pd/Al2O3 catalysts for low-temperature lean methane combustion.

Methane (CH4) is the second most consequential greenhouse gas after CO2, with a substantial global warming potential. The CH4 catalytic combustion offers an efficient method for the elimination of CH4. However, improving the catalytic performance of Pd-based materials for low-temperature CH4 combustion remains a big challenge. In this study, we synthesized an enhanced Pd/5NiAlOx catalyst that demonstrated superior catalytic activity and improved water resistance compared to the Pd/Al2O3 catalyst. Specifically, the T90 was decreased by over 100 °C under both dry and wet conditions. Introducing Ni resulted in an enormously enhanced number of oxygen defects on the obtained 5NiAlOx support. This defect-rich support facilitates the anchoring of PdO through increased electron transfer, thereby inhibiting the production of high-valence Pd(2+δ)+ and stimulating the generation of unsaturated Pd sites. Pd0 can effectively activate surface oxygen and PdO plays a significant role in activating CH4, resulting in high activity for Pd/5NiAlOx. On the other hand, the increased water resistance of Pd/5NiAlOx was mainly due to the generation of *OOH species and the lower accumulation of surface -OH species during the reaction process.

In-Depth Understanding of the Oxidative Compatibility of Volatile Organic Compounds with Mn2O3 and Pt-Loaded Catalysts.

Designing suitable catalysts for efficiently degrading volatile organic compounds (VOCs) is a great challenge due to the distinct variety and nature of VOCs. Herein, the suitability of different typical VOCs (toluene and acetone) over Pt-based catalysts and Mn2O3 was investigated carefully. The activity of Mn2O3 was inferior to Pt-loaded catalysts in toluene oxidation but showed superior ability for destroying acetone, while Pt loading could boost the catalytic activity of Mn2O3 for both acetone and toluene. This suitability could be determined by the physicochemical properties of the catalysts and the structure of the VOC since toluene destruction activity is highly reliant on Pt0 in the metallic state and linearly correlated with the amount of surface reactive oxygen species (Oads), while the crucial factor that affects acetone oxidation is the mobility of lattice oxygen (Olat). The Pt/Mn2O3 catalyst shows highly active Pt-O-Mn interfacial sites, favoring the generation of Oads and promoting Mn-Olat mobility, leading to its excellent performance. Therefore, the design of abundant active sites is an effective means of developing highly adaptive catalysts for the oxidation of different VOCs.

Identification of Direct Anchoring Sites for Monoatomic Dispersion of Precious Metals (Pt, Pd, Ag) on CeO2 Support.

Monoatomic dispersion of precious metals on the surface of CeO2 nanocrystals is a highly practical approach for dramatically reducing the usage of precious metals while exploiting the unique properties of single-atom catalysts. However, the specific atomic sites for anchoring precious metal atoms on the CeO2 support and underlying chemical mechanism remain partially unknown. Herein, we show that the terminal hydroxyls on the (100) surface are the most stable sites for anchoring Ag atoms on CeO2 , indicating that CeO2 nanocubes are the most efficient substrates to achieve monoatomic dispersion of Ag. Importantly, the newly identified chemical mechanism for single-metal-atom dispersion on CeO2 nanocubes appears to be generic and can thus be extended to other precious metals (Pt and Pd). In fact, our experiments also show that atomically dispersed Pt/Pd species exhibit morphology- and temperature-dependent CO selectivity in the catalytic CO2 hydrogenation reaction.

Effects of impregnation sequence on the NH3-SCR activity and hydrothermal stability of a Ce-Nb/SnO2 catalyst.

Hydrothermal stability is crucial for the practical application of deNOx catalyst on diesel vehicles, for the selective catalytic reduction of NOx with NH3 (NH3-SCR). SnO2-based materials possess superior hydrothermal stability, which is attractive for the development of NH3-SCR catalyst. In this work, a series of Ce-Nb/SnO2 catalysts, with Ce and Nb loading on SnO2 support, were prepared by impregnation method. It was found that, the NH3-SCR activities and hydrothermal stabilities of the Ce-Nb/SnO2 catalysts significantly varied with the impregnation sequences, and the Ce-Nb(f)/SnO2 catalyst that firstly impregnated Nb and then impregnated Ce exhibited the best performance. The characterization results revealed that Ce-Nb(f)/SnO2 possessed appropriate acidity and redox capability. Furthermore, the strong synergistic effect between Nb and Sn species stabilized the structure and maintained the dispersion of acid sites. This study may provide a new understanding for the effect of impregnation sequence on activity and hydrothermal stability and a new environmental-friendly NH3-SCR catalyst with potential applications for NOx removal from diesel and hydrogen-fueled engines.

Insight into the mechanisms of activity promotion and SO2 resistance over Fe-doped Ce-W oxide catalyst for NOx reduction.

Ceria-based catalysts for the selective catalytic reduction of NOx with NH3 (NH3-SCR) are always subject to deactivation by sulfur poisoning. In this study, Fe-doped Ce-W mixed oxides, which were synthesized by the co-precipitation method, improved the SCR activity and SO2 durability at low temperatures of undoped Ce-W oxides. The improved low-temperature activity was mainly due to the enhancement of redox properties at low temperatures and more active oxygen species, together with the adsorption and activation of more abundant NOx species, facilitating the "fast SCR" reaction. In the presence of SO2, doping with Fe species effectively prevented sulfate deposition on the CeW catalyst, due to the interaction between Fe, Ce, and W species inducing electron transfer among different metal sites and altering the electron distribution. The competitive adsorption behavior between NO and SO2 was changed by Fe doping, in which the adsorption and oxidation of SO2 were restrained. Besides, the elevated NO oxidation accelerated the decomposition of ammonium bisulfate, causing the SCR reaction to not be greatly suppressed. Hence, Fe-doped Ce-W oxides catalysts showed excellent sulfur resistance. This study provides an in-depth understanding of efficient Ce-based catalysts for SO2-tolerance strategies.

Advances in emission control of diesel vehicles in China.

Diesel vehicles have caused serious environmental problems in China. Hence, the Chinese government has launched serious actions against air pollution and imposed more stringent regulations on diesel vehicle emissions in the latest China VI standard. To fulfill this stringent legislation, two major technical routes, including the exhaust gas recirculation (EGR) and high-efficiency selective catalytic reduction (SCR) routes, have been developed for diesel engines. Moreover, complicated aftertreatment technologies have also been developed, including use of a diesel oxidation catalyst (DOC) for controlling carbon monoxide (CO) and hydrocarbon (HC) emissions, diesel particulate filter (DPF) for particle mass (PM) emission control, SCR for the control of NOx emission, and an ammonia slip catalyst (ASC) for the control of unreacted NH3. Due to the stringent requirements of the China VI standard, the aftertreatment system needs to be more deeply integrated with the engine system. In the future, aftertreatment technologies will need further upgrades to fulfill the requirements of the near-zero emission target for diesel vehicles.

Hydrothermal Aging Treatment Activates V2O5/TiO2 Catalysts for NOx Abatement.

Thermal stability is crucial for the practical application of deNOx catalysts. Vanadia-based catalysts are widely applied for the selective catalytic reduction of NOx with NH3 (NH3-SCR). Generally, hydrothermal aging at high temperatures induces the deactivation of deNOx catalysts. However, in this work, a remarkable increase in low- and medium-temperature NH3-SCR activity was observed for a V2O5/TiO2 catalyst after hydrothermal aging treatment, especially at 750 °C for 16 h. After the vanadia-based catalyst was hydrothermally treated at 750 °C, the specific surface area decreased and the surface VOx density and surface V ratio increased significantly. Therefore, the aged catalyst presented more abundant polymeric vanadyl species than the fresh one. Furthermore, the redox capability was improved markedly after hydrothermal treatment due to the strong interaction of vanadia and titania, contributing to the NH3-SCR reaction. 750 °C is the optimal temperature to activate the V2O5/TiO2 catalyst, improving the SCR performance significantly. This study provides an in-depth understanding of vanadia-based catalysts for practical applications.

Developing a thermally stable Co/Ce-Sn catalyst via adding Sn for soot and CO oxidation.

The thermal stability of the catalysts is of particular importance but still a big challenge for working under harsh conditions at high temperature. In this study, we report a strategy to improve the thermal stability of the ceria-based catalyst via introducing Sn. XRD, Rietveld refinement, and other characterizations results indicated that the formation of Sn-Co solid solution plays a key role in the thermal stability of the catalyst. The developed ternary 3%Co/Ce0.5Sn0.5O2 catalyst not only exhibits outstanding thermal stability and resistance to SO2 and H2O for soot oxidation from diesel vehicle exhaust but also remains extraordinary thermal stability for CO oxidation. Remarkably, even after thermal aging at 1000°C, it still possessed high catalytic activity similar to that of the fresh catalyst.

Influence of NOx on the activity of Pd/θ-Al2O3 catalyst for methane oxidation: Alleviation of transient deactivation.

Alumina supported Pd catalyst (Pd/Al2O3) is active for complete oxidation of methane, while often suffers transient deactivation during the cold down process. Herein, heating and cooling cycle tests between 200 and 900°C and isothermal experiments at 650°C were conducted to investigate the influence of NOx on transient deactivation of Pd/θ-Al2O3 catalyst during the methane oxidation. It was found that the co-fed of NO alleviated transient deactivation in the cooling ramp from 800 to 500°C, which was resulted from the in situ formation of NO2 during the process of methane oxidation. Over the Pd/θ-Al2O3, thermogravimetric analysis and O2 temperature programmed oxidation measurements confirmed that transient deactivation was due to the decomposition of PdO particles and the hysteresis of Pd reoxidation, while the metal Pd entities were less active for methane oxidation than the PdO ones. CO pulse chemisorption and scanning transmission electron microscopy characterizations rule out the NO2 effect on Pd size change. Powder X-ray diffraction and X-ray photoelectron spectroscopy characterizations were used to obtain palladium status of Pd/θ-Al2O3 before and after reactions, indicating that in lean conditions at 650°C, the presence of NO2 increases the content of active PdO on the catalyst surface, thus benefits methane oxidation. Homogeneous reaction between CH4, O2, and NOx may be partially responsible for the alleviation above 650°C. The interesting research of alleviation in transient deactivation by NOx, the components co-existing in exhausts, are of great significance for the application.

Selective catalytic reduction of NO x with NH3: opportunities and challenges of Cu-based small-pore zeolites.

Zeolites, as efficient and stable catalysts, are widely used in the environmental catalysis field. Typically, Cu-SSZ-13 with small-pore structure shows excellent catalytic activity for selective catalytic reduction of NO x with ammonia (NH3-SCR) as well as high hydrothermal stability. This review summarizes major advances in Cu-SSZ-13 applied to the NH3-SCR reaction, including the state of copper species, standard and fast SCR reaction mechanism, hydrothermal deactivation mechanism, poisoning resistance and synthetic methodology. The review gives a valuable summary of new insights into the matching between SCR catalyst design principles and the characteristics of Cu2+-exchanged zeolitic catalysts, highlighting the significant opportunity presented by zeolite-based catalysts. Principles for designing zeolites with excellent NH3-SCR performance and hydrothermal stability are proposed. On the basis of these principles, more hydrothermally stable Cu-AEI and Cu-LTA zeolites are elaborated as well as other alternative zeolites applied to NH3-SCR. Finally, we call attention to the challenges facing Cu-based small-pore zeolites that still need to be addressed.

Reaction Pathways of Standard and Fast Selective Catalytic Reduction over Cu-SSZ-39.

Cu-SSZ-39 exhibits excellent hydrothermal stability and is expected to be used for NOx purification in diesel vehicles. In this work, the selective catalytic reduction (SCR) activities in the presence or absence of NO2 were tested over Cu-SSZ-39 catalysts with different Cu contents. The results showed that the NOx conversion of Cu-SSZ-39 was improved by NO2 when NO2/NOx = 0.5, especially for the catalysts with low Cu loadings. The kinetic studies showed two kinetic regimes for fast SCR from 150 to 220 °C due to a change in the rate-controlling mechanism. The activity test and diffuse reflectance infrared Fourier transform spectra demonstrated that the reduction of NO mainly occurred on the Cu species in the absence of feed NO2, and when NO2/NO = 1, NO could react with NH4NO3 on the Brønsted acid sites in addition to undergoing reduction on Cu species. Thus, NO2 can promote the SCR reaction over Cu-SSZ-39 by facilitating the formation of surface nitrate species.

Adsorption-Induced Active Vanadium Species Facilitate Excellent Performance in Low-Temperature Catalytic NOx Abatement.

Vanadia-based catalysts have been widely used for catalyzing various reactions, including their long-standing application in the deNOx process. It has been commonly considered that various vanadium species dispersed on supports with a large surface area act as the catalytically active sites. However, the role of crystalline V2O5 in selective catalytic reduction of NOx with NH3 (NH3-SCR) remains unclear. In this study, a catalyst with low vanadia loading was synthesized, in which crystalline V2O5 was deposited on a TiO2 support that had been pretreated at a high temperature. Surprisingly, the catalyst, which had a large amount of crystalline V2O5, showed excellent low-temperature NH3-SCR activity. For the first time, crystalline V2O5 on low-vanadium-loading catalysts was found to be transformed to polymeric vanadyl species by the adsorption of NH3. The generated active polymeric vanadyl species played a crucial role in NH3-SCR, leading to remarkably enhanced catalytic performance at low temperatures. This new finding provides a fundamental understanding of the metal oxide-catalyzed chemical reaction and has important implications for the development and commercial applications of NH3-SCR catalysts.

Investigation of suitable precursors for manganese oxide catalysts in ethyl acetate oxidation.

The control of ethyl acetate emissions from fermentation and extraction processes in the pharmaceutical industry is of great importance to the environment. We have developed three Mn2O3 catalysts by using different Mn precursors (MnCl2, Mn(CH3COO)2, MnSO4), named as Mn2O3-Cl, -Ac, -SO4. The tested catalytic activity results showed a sequence with Mn precursors as: Mn2O3-Cl > Mn2O3-Ac > Mn2O3-SO4. The Mn2O3-Cl catalyst reached a complete ethyl acetate conversion at 212℃ (75℃ lower than that of Mn2O3-SO4), and this high activity 100% could be maintained high at 212℃ for at least 100 hr. The characterization data about the physical properties of catalysts did not show an obvious correlation between the structure and morphology of Mn2O3 catalysts and catalytic performance, neither was the surface area the determining factor for catalytic activity in the ethyl acetate oxidation. Here we firstly found there is a close linear relationship between the catalytic activity and the amount of lattice oxygen species in the ethyl acetate oxidation, indicating that lattice oxygen species were essential for excellent catalytic activity. Through H2 temperature-programmed reduction (H2-TPR) results, we found that the lowest initial reduction temperature over the Mn2O3-Cl had stronger oxygen mobility, thus more oxygen species participated in the oxidation reaction, resulting in the highest catalytic performance. With convenient preparation, high efficiency, and stability, Mn2O3 prepared with MnCl2 will be a promising catalyst for removing ethyl acetate in practical application.

Superior Oxidative Dehydrogenation Performance toward NH3 Determines the Excellent Low-Temperature NH3-SCR Activity of Mn-Based Catalysts.

Mn-based oxides exhibit outstanding low-temperature activity for the selective catalytic reduction of NOx with NH3 (NH3-SCR) compared with other catalysts. However, the underlying principle responsible for the excellent low-temperature activity is not yet clear. Here, the atomic-level mechanism and activity-limiting factor in the NH3-SCR process over Mn-, Fe-, and Ce-based oxide catalysts are elucidated by a combination of first-principles calculations and experimental measurements. We found that the superior oxidative dehydrogenation performance toward NH3 of Mn-based catalysts reduces the energy barriers for the activation of NH3 and the formation of the key intermediate NH2NO, which is the rate-determining step in NH3-SCR over these oxide catalysts. The findings of this study advance the understanding of the working principle of Mn-based SCR catalysts and provide a fundamental basis for the development of future generation SCR catalysts with excellent low-temperature activity.

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 NOx with NH3 (NH3-SCR), W was introduced as a promoter. With the doping of W, the NOx conversion over Mn3CeOx catalyst above 150 °C was increased, and the N2O production was significantly decreased. Even in the present of water vapour, Mn3CeW0.3Ox 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 Mn3CeOx. As a result, the NOx conversion below 150 °C over Mn3CeW0.3Ox was slightly lower than that over Mn3CeOx. Since the NOx production and the NH3 conversion during the NH3 oxidation of Mn3CeOx were inhibited after the doping of W, the NOx conversion above 150 °C over Mn3CeW0.3Ox was higher than that over Mn3CeOx. The transient reaction demonstrated that the doped W species on Mn3CeW0.3Ox could inhibit the N2O produced by the Langmuir-Hinshelwood mechanism. Kinetic study proved that νSCR over Mn3CeW0.3Ox was obviously higher that over Mn3CeOx, νNSCR and νC-O over Mn3CeOx were much higher than those of Mn3CeW0.3Ox, which were consistent with the SCR activity.

Synergistic Effects of Multicomponents Produce Outstanding Soot Oxidation Activity in a Cs/Co/MnOx Catalyst.

The control of soot emission from diesel vehicles is of extraordinary importance to the environment, and catalytic removal of soot is a highly effective and clean method. Here, we report a novel, non-noble metal catalyst for application in the catalytic combustion of soot with superb activity and resistance to H2O and SO2. MnOx oxide was prepared via a hydrothermal method, and then, Cs and Co were loaded on MnOx by impregnation. The 5%Cs/1%Co/MnOx catalyst displayed excellent catalytic activity with values of T10 (332 °C), T50 (371 °C), and T90 (415 °C) under loose contact. The as-prepared catalysts were investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), H2 temperature-programmed reduction (TPR), O2 temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS). The results suggest that, after the introduction of Cs and Co into the MnOx oxide, more NO2 molecules take part in soot oxidation, exhibiting higher NO2 utilization efficiency; this is due to the synergistic effects of multiple components (Cs, Co, and Mn) promoting the generation of more surface-active oxygen and then accelerating the reaction between NO2 and soot. This study provides significant insights into the development of high-efficiency catalysts for soot oxidation, and the developed 5%Cs/1%Co/MnOx catalyst is a promising candidate for application in diesel particulate filters.

Significant promotion effect of the rutile phase on V2O5/TiO2 catalysts for NH3-SCR.

A large amount of polymeric vanadyl species owing to higher interaction energy between vanadia and anatase than rutile and the synergistic effect of vanadium oxides, anatase and rutile TiO2 contributes to an excellent NH3-SCR activity of the vanadia-based catalysts with high rutile content and low specific surface area.