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Manganese-based mullite-type and perovskite-type mixed oxides were prepared by sol–gel method, and then palladium was loaded onto the mixed oxides by wet impregnation method. The catalytic performance for methane combustion, the influence of the supports, and the interaction between noble metal and the mixed oxides were investigated via experiments and theoretical calculations. The Pd/SmMn2O5 catalyst exhibited superior low-temperature catalytic activity for methane combustion (T50 = 328 °C), which was superior to the Pd/perovskites catalysts and even better than Pd/Al2O3 catalyst. It was ascribed to the abundant labile surface lattice oxygen species with high mobility and the uniformly dispersed Pd species on SmMn2O5, as revealed by TEM, CO-chemisorption, H2-TPR and XPS. DFT calculations verified the significant metal-support interaction via electron transfer between Pd and SmMn2O5, which further activated the lattice oxygen and improved the catalytic performance. It demonstrates the prospect of utilizing SmMn2O5 mullite to optimize the typical noble metal catalysts. [ABSTRACT FROM AUTHOR]

Pr-modified MnOx catalyst was synthesized through a facile co-precipitation process, and the results showed that MnPrOx catalyst exhibited much better selective catalytic reduction (SCR) activity and SO2 resistance performance than pristine MnOx catalyst. The addition of Pr in MnOx catalyst led to a complete NO conversion efficiency in 120–220 °C. Moreover, Pr-modified MnOx catalyst exhibited a superior resistance to H2O and SO2 compared with MnOx catalyst. After exposing to SO2 and H2O for 4 h, the NO conversion efficiency of MnPrOx catalyst could remain to 87.6%. The characterization techniques of XRD, BET, hydrogen-temperature programmed reduction (H2-TPR), ammonia-temperature programmed desorption (NH3-TPD), XPS, TG and in situ diffuse reflectance infrared spectroscopy (DRIFTS) were adopted to further explore the promoting effect of Pr doping in MnOx catalyst on SO2 resistance performance. The results showed that MnPrOx catalyst had larger specific surface area, stronger reducibility, and more L acid sites compared with MnOx catalyst. The relative percentage of Mn4+/Mnn+ on the MnPrOx-S catalyst surface was also much higher than those of MnOx catalyst. Importantly, when SO2 exists in feed gas, PrOx species in MnPrOx catalyst would preferentially react with SO2, thus protecting the Mn active sites. In addition, the introduction of Pr might promote the reaction between SO2 and NH3 rather than between SO2 and Mn active sites, which was also conductive to protect the Mn active sites to a great extent. Since the presence of SO2 in feed gas had little effect on NH3 adsorption on the MnPrOx catalyst surface, and the inhibiting effect of SO2 on NO adsorption was alleviated, SCR reactions could still proceed in a near-normal way through the Eley-Rideal (E-R) mechanism on Pr-modified MnOx catalyst, while SCR reactions through the Langmuir-Hinshelwood (L-H) mechanism were suppressed slightly. [ABSTRACT FROM AUTHOR]

The three bipyridine‐proline chiral ligands (Z) with different structural units as highly active species were successfully introduced on Zn‐modified bimodal mesoporous silicas (BMMs). The representative heterogeneous catalysts (ZZnBMMs‐n) containing different molar ratios of Z/Zn (5, 20, 50, 100, 200%) were prepared, and their catalytic performances were evaluated in the asymmetric aldol reactions between p‐nitrobenzaldehyde and cyclohexanone, showing the satisfactory activities (yields) and good stereoselectivity (dr and ee). Meanwhile, their microstructural features and physicochemical properties were characterized by various methods. Particularly, the fractal evolution, the pair distance distribution function, and the Porod plots of the ZZnBMMs catalysts were demonstrated by SAXS analysis. The results revealed that the interfacial layer thickness and their mass fractal (Dm) value gradually increased with the enhanced molar ratios of Z/Zn, along with the decrease of pore size and densification of volume, suggesting the successful Z‐grafting into the mesoporous channels of Zn‐modified matrixes. The essential relationships between the fractal features and their catalytic performance (yield, dr and ee) were explored, showing the larger the Dm values, the better catalytic performance. Besides, the uniform distributions of Zn and S elements in ZZnBMMs catalysts and the formation of enamine‐imine intermediates during the asymmetric aldol reactions were confirmed. [ABSTRACT FROM AUTHOR]

• Pr-modified MnO x catalysts were prepared via co-precipitation method. • Pr modification promoted low-temperature catalytic performance and SO 2 resistance. • A highly dispersion of metal oxides could be achieved by Pr modification on MnO x catalysts. • Lewis acid sites and chemisorbed oxygen were enriched on the surface of MnPrO x catalysts. Manganese oxides (MnO x) are a promising catalyst in selective catalytic reduction of NO with NH 3 due to their excellent low-temperature catalytic activities. However, MnO x still suffer from poor N 2 selectivity and SO 2 resistance. In this work, rare earth element Pr was used to modify MnO x catalysts via co-precipitation method, and the effects of Pr modification on SCR performance of MnO x catalysts had been investigated. The temperature window of MnPrO x -0.1 catalyst with nearly 100% NO conversion efficiency ranged from 120 to 220 °C, and Pr modification on MnO x catalyst had improved the resistance to SO 2 obviously. The characterization results indicated that Pr modification on MnO x catalyst could increase the specific surface area, dispersity, reducibility, relative percentages of surface Mn4+ and chemisorbed oxygen species (O α). NH 3 -TPD and in-situ DRIFTS results revealed that Pr modification increased the adsorption of NH 3 on catalyst surface, resulting in more amount of Lewis acid sites on the surface of MnPrO x -0.1 catalyst. All of these factors led to an excellent catalytic performance of Pr-modified MnO x catalysts. In-situ DRIFTS results implied that SCR reactions over Pr-modified MnO x catalyst followed both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. [Display omitted] [ABSTRACT FROM AUTHOR]

Axially‐unfixed 2,2′‐bipyridine‐based chiral catalysts were synthesized using enantiopure amino acids as chiral sources, which were successfully used in asymmetric aldol reactions of p‐nitrobenzaldehyde with cyclohexanone, while the bulkier aldehyde derivatives (2‐naphthaldehyde, 9‐anthracenecarboxaldehyde, and 1‐pyrenecarboxaldehyde) were selected to further elucidate the catalytic properties. Particularly, the influences of the bipyridine‐proline chiral structures and the polarities of used solvents (petroleum ether, toluene, CH2Cl2, ethanol, DMF, DMSO, and water) on the catalytic performance were investigated. The results indicated that the structure of bipyridine catalysts, the molecular volume of aldehydes, and the polarity of solvents have significant effects on the catalytic activities, in which, the smaller steric effects were conducive to the improvement of the yields and stereoselectivities for asymmetric aldol reaction, along with the increased polarity of used solvents and the decreased molecular volume of aldehydes. Meanwhile, the chemical identity of all compounds was confirmed by 1H‐NMR, 13C‐NMR, HRMS, and HPLC analysis. [ABSTRACT FROM AUTHOR]

Cu-SAPO-34 catalysts with varied Cu loadings were synthesized through ion exchange to study the influence of water on the NH3-SCR reaction. The catalytic activities were evaluated by selective catalytic reduction of NO under a reactant feed in the presence/absence of water. Transient experiments were designed to study the response of NO conversion to the presence of water. H2-TPR and DFT calculations were performed to study the reducibility of Cu species. NH3-TPD and XPS were conducted to reveal the migration of Cu species. The results show that water could remarkably improve NO reduction activities and the promoting effect is more significant on the catalyst with low Cu loading. Both transient and permanent influences were found in this promoting phenomenon. For the transient influence, water has been proved to accelerate the re-oxidation half-cycle. Moreover, water can enhance the promoting effect of the SCR feed on the migration of Cu species. These unanchored Cu ions migrate to defect sites to form active sites, which lead to a permanent influence of water. [ABSTRACT FROM AUTHOR]

Cu-SAPO-34 SCR catalyst has drawn much attention because of its high NH 3 -SCR activity and N 2 selectivity. Different copper species as active sites for SCR reaction in Cu-SAPO-34 catalyst were studied by DFT calculations, primarily focusing on copper sites in six-membered rings (6 MR) and eight-membered rings (8 MR). The relative energies of various Cu species were calculated and organized as phase diagrams to investigate the stabilities of Cu species. The diagrams show that Cu 2+ can exist in 6 MR, and [Cu-O-Cu] 2+ tends to form in 8 MR.The DFT results show that the adsorption energies for NH 3 on Cu + and Cu 2+ in 8 MR are the highest and but NH 3 can hardly dissociate in 8 MR, which indicate isolated Cu ions located in 8 MR will hinder the NH 3 -SCR reaction. However, the additional Cu ion and O atom in 8 MR effectively reduces the dissociation energy barrier. These results indicate that isolated Cu 2+ is the active site in 6 MR, and [Cu-O-Cu] 2+ is the active site in 8 MR. Besides, the NH 3 dissociation on Cu 2+ and [Cu-O-Cu] 2+ are the key steps for the reactions in 6 MR and 8 MR. [ABSTRACT FROM AUTHOR]

• The partial substitution of Mn by Ce significantly enhanced N 2 selectivity. • It resulted in a mild redox property and an enhanced surface acidity. • It led to the formation of NH 4 NO 2 and the consumption of -NH 2 on catalyst surface. • It inhibited the oxidation reaction of NH 3 species via L -H mechanism. Owing to excellent low-temperature activity and environmentally friendliness, Mn-based catalyst is commonly considered as one of the promising alternatives to traditional V-based catalysts for NH 3 -SCR of NO. However, Mn-based catalysts still suffer from some drawbacks, such as poor N 2 selectivity and narrow operating temperature window. Herein, MnTiO x catalyst was synthesized through a facile coprecipitation method. Ce was adopted to partially substitute Mn in MnTiO x catalyst to promote N 2 selectivity and high-temperature activity. The partial substitution of Mn by Ce in MnTiO x catalyst could significantly improve N 2 selectivity and widen operating temperature window. The higher substitution rate of Mn by Ce was, the better N 2 selectivity of Ce MnTiO x catalyst was. Partial substitution of Mn by Ce led to mild redox property, enhanced surface acidity, lower surface Mn4+ and chemisorbed oxygen species proportion on catalyst surface. Besides, partial substitution of Mn by Ce was in favor of the generation NH 4 NO 2 species in NH 3 oxidation reaction, and boosted the consumption of -NH 2 species during NH 3 -SCR reaction, which was conducive to N 2 formation. The reactions between adsorbed NH 3 species and oxygen species through Langmuir-Hinshelwood mechanism might be inhibited by partial substitution of Mn by Ce, thus effectively restraining N 2 O formation. [Display omitted] [ABSTRACT FROM AUTHOR]

Red mud, as industrial solid waste, causes severe environmental problems such as soil alkalization and groundwater pollution. In this work, we researched and developed the red mud as a selective catalytic reduction catalyst for NO x removal with NH 3 (NH 3 -SCR). After selective dissolution and specific heat treatment, different Ce precursors were used to modifying its physical and chemical properties. The results showed that Ce(NO 3) 3 and Ce(NH 4) 2 (NO 3) 6 modified red mud (RM cn and RM can) had excellent SCR performance below 300 °C. Ce(SO 4) 2 modified red mud (RM cs) showed relatively low NO x conversions at 200–300 °C. The redox property was improved with the Ce(NO 3) 3 and Ce(NH 4) 2 (NO 3) 6 , while depressed with the Ce(SO 4) 2. Agglomerates generated on the RM cs and blocked the accumulated pores due to the formation of Ce 2 (SO 4) 3. The surface acidity of RM cs enhanced with increased adsorption for ammonia. However, these new adsorbed ammonia species, highly related to the sulfate from the Ce 2 (SO 4) 3 , were inert and did not react with the adsorbed or gaseous NO species at 200–300 °C. The abundant surface lattice oxygen from CeO 2 microcrystals improved the catalytic oxidation capacity of the RM cn and RM can. • Red mud, as industrial waste, was efficiently recovered and used as SCR catalysts. • Cerium modified red mud catalysts exhibited NO x conversion above 90% at 200–300 °C. • The surface lattice oxygen and Fe–O–Ce bridges enhanced the reducibility. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two carbonates of Na 2 CO 3 and (NH 4) 2 CO 3 are used for SrO-based catalyst synthesis. • SrO-M/Na exhibits a better catalytic activity in comparison to SrO-M/NH 4. • SrO-M/NH 4 shows a better reusability by contrast to SrO-M/Na. • The existence of Na enhances the amount of lattice oxygen for SrO-M/Na. In present study, sodium carbonate (Na 2 CO 3) and ammonium carbonate ((NH 4) 2 CO 3) are separately selected as precipitant to synthesize the strontium oxide (SrO) based solid catalyst, namely, SrO–M/Na and SrO–M/NH 4 for biodiesel production through transesterification. The influence of precipitant on catalyst physicochemical property, catalytic activity as well as reusability are investigated and compared. To begin with, transesterification parameters of catalyst amount, molar ratio of methanol to oil, reaction time, and reaction temperature are concerned to comprehensively evaluate catalytic performance of the two catalysts. Moreover, catalyst reusability is evaluated under the optimal conditions. With catalyst amount of 4 wt.%, molar ratio of methanol to oil of 9 at 65 ℃ within 30 min, SrO–M/Na exhibits a better catalytic activity in comparison to SrO–M/NH 4 , whereas, the reusability is in reverse sequence. In order to reveal the reason behind, catalysts are characterized by thermogravimetric/differential thermogravimetric/scanning calorimetry (TG/DTG/DSC), X–ray diffraction (XRD), X–ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), N 2 –adsorption/desorption, basic strength, basicity and vibrating sample magnetometer (VSM). [ABSTRACT FROM AUTHOR]

• Two different N 2 O formation mechanisms over V-based SCR catalyst are proposed. • The decomposition of NH 4 NO 3 is the main way of N 2 O formation at the temperatures below 300 °C. • At high temperatures, NH 3 is oxidized to form NH by V active site and then reacts with NO to produce N 2 O. • The deep oxidation of the adsorbed NH 3 is the rate-determined step for N 2 O formation at high temperatures. Formation of N 2 O during NH 3 -SCR reaction limits the temperature window of SCR catalyst. In the present study, a series of kinetics experiments were carried out over V-based catalyst to study the N 2 O formation characteristics at different temperatures. Transient and temperature-programmed reactions show that the thermal decomposition of solid NH 4 NO 3 formed by the reaction of NH 3 with NO 2 is the main way of N 2 O formation at low temperatures (below 300 °C). At the temperature above 300 °C, N 2 selectivity of NO reduction over V-based catalysts decreases with the increase of the gaseous NH 3 concentration and the V content of the catalysts. DFT calculations were applied to understand the formation mechanism of N 2 O at high temperature. The results show that the NH is found to be formed by NH 3 oxidative dehydrogenation over V Lewis acid site and subsequently reacts with NO to produce N 2 O. In this process, the deep oxidation of the adsorbed NH 3 is the rate-determined step, which can be aggravated by the high temperature and high V content. [ABSTRACT FROM AUTHOR]

Formation of NH 4 HSO 4 and its deposition on catalyst surface are the main obstacle for the application of NH 3 -SCR technology at low temperature. In this work, the deposited NH 4 HSO 4 was found to be highly active towards NO x in the NO/NO 2 contained atmosphere on the V 2 O 5 -WO 3 /TiO 2 catalyst. The reaction profiles indicate that the NO/NO 2 /O 2 mixture can rapidly react with NH 4 + of NH 4 HSO 4 above 100 °C, while NO/O 2 mixture can only strongly react with NH 4 HSO 4 above 300 °C. Regenerations of NH 4 HSO 4 -deactivated catalyst were performed in both NO/O 2 and NO/NO 2 /O 2 atmospheres and the performances were compared. Results from FTIR, XPS and EDS characterizations indicate the S-containing groups of deposited NH 4 HSO 4 will be stabilized as metal sulfate after NH 4 + is consumed by NO x . The BET and BJH analysis shows that the NO/NO 2 /O 2 mixture can recover the pore structure of NH 4 HSO 4 -deposited catalyst more effectively than NO/O 2 mixture does. The SCR activity tests reveal that the regeneration by NO/NO 2 /O 2 mixture can remarkably restore the activity of NH 4 HSO 4 -deactivated catalyst. [ABSTRACT FROM AUTHOR]

• Co 0.5 Mn 1 Fe 0.25 Al 0.75 O x -LDO showed remarkable N 2 -selectivity enhancement. • Use of Co reduced significantly NH 3 oxidation to N 2 O. • Electron transfer between Co3+/Co2+ and Mn4+/Mn3+ suppressed N 2 O formation. • Transient NH 3 -SCR clarified the role of Co in improving N 2 -selectivity. • Transient NH 3 -SCR strongly suggested L-H mechanism on CoMnFeAlO x -LDO at 250 °C. A Co 0.5 Mn 1 Fe 0.25 Al 0.75 O x -LDO catalyst was developed which showed excellent performance for the low-temperature NH 3 -SCR. NO x conversions ∼100% were achieved in the whole 100-250 °C range, while after 10-h operation at 150 °C with 100 ppm SO 2 /5 vol% H 2 O in the feed, the NO x conversion was maintained at 80%. This catalyst provided a much better N 2 -selectivity than the Mn 1 Fe 0.25 Al 0.75 O x -LDO and Mn 1 Al 1 O x -LDO, especially at 150-300 °C. It was found that Co 0.5 Mn 1 Fe 0.25 Al 0.75 O x possessed higher surface acidity and reducibility, while XPS analyses indicated an electron transfer between Co3+/Co2+ and Mn4+/Mn3+ redox cycles, leading to a much lower N 2 O formation, supported by Density Functional Theory (DFT) calculations. Detailed analysis of gas responses obtained upon various step-gas switches was performed, which allowed to measure the surface concentration and reactivity of preadsorbed NO x -s and NH x -s leading to N 2 and N 2 O. Transient kinetic and DFT studies strongly suggested likely mechanisms of NH 3 -SCR and the critical role of Co for N 2 -selectivity enhancement. [ABSTRACT FROM AUTHOR]

• NH 4 HSO 4 can easily form in the gas phase and then deposit on the catalyst surface to deactivate the SCR catalyst during the low-temperature SCR process. • A molecular-level reaction mechanism for the reaction of NH 4 HSO 4 over V/Ti catalyst is proposed. • NO 2 can remarkably enhance the reaction of NH 4 HSO 4 at low-temperature since NO 2 is an efficient oxidant for V4+ reoxidation. • Commercial VWTi catalyst exhibits excellent sulfur tolerance at low temperatures when NO 2 was contained in the reaction atmosphere. The deactivation of NH 3 -selective catalytic reduction (SCR) catalysts due to NH 4 HSO 4 deposition at low temperatures (< 300 °C) is still a significant challenge. In this work, we present a comprehensive mechanism describing the formation, deposition, and reaction of NH 4 HSO 4 on a V 2 O 5 /TiO 2 catalyst using a combination of theoretical and experimental methods. The results show that NH 4 HSO 4 is mainly formed in the gas phase through the nucleation of SO 3 , H 2 O, and NH 3 and then deposits onto the catalyst surface. The decomposition of NH 4 HSO 4 on the surface of the V 2 O 5 /TiO 2 catalyst consists of two steps: NO is reduced by the NH 4 + of NH 4 HSO 4 forming N 2 and H 2 O by transferring an electron to the adjacent vanadium site, followed by a reoxidation of the reduced vanadium site by either O 2 or NO 2. At low temperatures, due to the weak reoxidizing ability of O 2 , the reaction of NH 4 HSO 4 with NO in the NO/O 2 mixture is rather slow. Adding NO 2 can remarkably enhance the decomposition of NH 4 HSO 4 on the catalyst surface. Our results reveal that the rate-determining step of the reaction between NH 4 HSO 4 and NO/O 2 is the reoxidation of the reduced vanadium site and that NO 2 is a better reoxidizing agent than O 2 , which has been confirmed by X-ray photoelectron spectroscopy analysis and the designed transient response method experiments. Finally, the catalyst sulfur tolerance test has proven that the commercial V 2 O 5 -WO 3 /TiO 2 catalyst can successfully maintain its long-term activity for NO x reduction in SO 2 -contained flue gas at 250 °C due to the rapid decomposition of deposited NH 4 HSO 4 on the catalyst surface by the NO/NO 2 mixture. [ABSTRACT FROM AUTHOR]