Strength Of Basic Sites Research Articles

The role of ceria in promoting Ni catalysts supported on phosphate‐modified zirconia for the partial oxidation of methane

AbstractThe catalytic partial oxidation of methane (POM) is aimed at the mitigation of CH4 (a highly potent greenhouse gas) from the environment and the synthesis of syngas with a high H2/CO ratio. Herein, to enhance the POM reaction, Ni‐supported phosphate‐modified‐zirconia were synthesized with promotor “Ce” to achieve high H2/CO ratio (2.4–3.2). The catalysts were characterized by surface area and porosity, X‐ray diffraction, RAMAN, temperature‐programmed experiments (TPR, CO2‐TPD, and TPO), and TEM. Increasing the ceria addition over 10Ni/PO4 + ZrO2 resulted in lower crystallinity, higher dispersion of active sites, and enhanced the surface area of catalyst. The unique and prominent reducibility and basicity of NiO‐species and surface oxide ions, respectively, are particularly notable at 4 wt.% ceria loading. At a reaction temperature of 600°C, the highest concentration of active sites and a unique concentration of moderate strength basic sites can be achieved with 4 wt.% ceria loading over 10Ni/PO4 + ZrO2. This leads to 44% conversion of CH4, 36% yield of H2, 35% yield of CO2, and H2/CO ratio of 3.16 for the POM reaction. The cyclic H2TPR‐O2TPO‐H2TPR experiment confirms the reorganization of the active site towards high temperature under oxidizing gas O2 and reducing gas H2 gas stream during the POM reaction.

Self-heating photothermal synergistic catalytic decomposition of carcinogenic formaldehyde by α-MnO2 at ambient temperature: Functional roles of surface acidic-basic sites

Formaldehyde is a major pollutant in indoor atmosphere, and the realization of complete mineralization of formaldehyde at room temperature is of great significance and practical demand for improving indoor air quality. Surface acidic-basic sites, as one of the important surface properties that have significant influence on catalytic oxidation of formaldehyde, revealing their functions is beneficial for the development of efficient formaldehyde decomposition materials. Herein, the α-MnO2 acidic-basic sites were regulated and its effects on self-heating photothermal catalytic oxidation of formaldehyde were further contrastive investigated. The acidic-basic sites showed distinct functions: the basic sites exhibited a significant promotion on formaldehyde oxidation; While the acidic sites greatly inhibited the reactivity. Particularly, the basicity enhanced α-MnO2 could reach the 100% conversion of formaldehyde at ambient temperature. Furthermore, the self-heating photothermal mechanism was consonant with the thermal-assisted photocatalysis mechanism, rather than traditional photo-drive thermal catalysis mechanism. The strength of basic sites significantly increased the content of surface hydroxyl groups, and the electron-rich basic sites were favorable for the activation of oxygen, thereby generating more reactive oxygen species for the decomposition of formaldehyde. Moreover, in-situ DRIFTS reveals that the enhancement of basic sites can accelerate the rate oxidation of HCHO to generate formates and carbonates. This work clarified the influence of surface acid-base properties on formaldehyde photothermal decomposition.

Effect of Acid‐Base Properties of Oxide Catalysts on Glycerol Transformation into Diglycerides

AbstractGlycerol oligomerization in a liquid phase was studied over the acid‐base bifunctional catalysts for the following oxides: TiO2, ZrO2, CeO2, binary CeO2−ZrO2 systems, and purely basic oxide MgO. The phase composition and surface area have been estimated by XRD and BET. The acid and basic properties of catalysts have been characterized using the quasi‐equilibrium thermal desorption (QE‐TD) of NH3 (acidity) and CO2 (basicity). Attenuated total reflectance FTIR (ATR‐FTIR) spectroscopy provided qualitative insights into the interactions in the oxide‐glycerol systems. A comparison of catalytic and acid‐base properties of the oxide systems showed that the reaction requires the presence of both acid and base sites in the catalyst. Moreover, the strength of base sites should exceed the strength of acid sites.

Behavior of nickel supported on calcium-enriched hydroxyapatite samples for CCU-methanation and ICCU-methanation processes

The viability of Ni supported on Ca-enriched hydroxyapatite (HAP) samples has been investigated for the intensification of CCU-methanation and ICCU-methanation decarbonization processes. Even working with a surface density of added Ca three times larger than that required for a deposition of a theoretical monolayer, there is no apparent segregation of its resulting phases. Moreover, the progressive addition of Ca dramatically increases both the number and the strength of basic sites. Likewise, this markedly increases the Ni dispersion and the density of Ni lattice defects. The addition of 1–3 wt% Ca induces a dramatic improvement of the CCU-methanation activity and selectivity, when compared with the Ni/HAP sample. The optimal catalyst shows a good resistance under a long-term stability test at 375 °C for 90 h TOS. A compromise between the surface basicity and the distribution of Ni active sites proves to be the key factor influencing the efficiency of the investigated materials. In the ICCU-methanation process the Ni/Ca(3)/HAP DFM with added Ca loading close to the theoretical monolayer exhibits the best performance. For instance, it proves to be the most efficient to convert the adsorbed CO2 to CH4, during the hydrogenation period, reaching a CH4 density close to 155 μmolCH4 g−1 at 400 °C. It is supposed that the methanation activity involves CO2 stored only on surface CaO species and it is most likely improved over DFMs exhibiting short Ni-CaO distances. Interestingly, the performance of our Ni/Ca(3)/HAP DFM clearly outperforms that reported over 15%Ni/15%CaO/Al2O3 and 9.3%Ni/CaO reference DFMs. We conclude that a dispersion on HAP support of Ca loadings close to the theoretical monolayer becomes a promising alternative to basic DFMs.

Synthesis of dimethyl carbonate from methanol and propylene carbonate over the Ca-Zr catalyst modified by transition metals

A series of Ca-Zr catalysts modified by different transition metals were prepared by the sol-gel method and their catalytic performance in the synthesis of dimethyl carbonate (DMC) from methanol and propylene carbonate (PC) by transesterification at low temperature was investigated. The results indicate that the selectivity to DMC of various transition metal-modified Ca-Zr catalysts follows the order of Co-Ca-Zr > Cu-Ca-Zr > Ca-Zr > Fe-Ca-Zr > Ni-Ca-Zr > Zn-Ca-Zr. For the transesterification over the Co-Ca-Zr catalyst, in particular, the conversion of PC reaches 84.3% with a selectivity of 94.5% to DMC after reaction for 2 h under 35 °C, a methanol/PC molar ratio of 15, and catalyst amount of 4%. Combining with the XRD, FT-IR, XPS and CO2-TPD results, it is revealed that increasing the strength of basic sites can raise the conversion of PC, whereas increasing the density of basic sites leads to a decrease of the selectivity to DMC. As a result, the Co-modified Ca-Zr (Co-Ca-Zr) catalyst, with the lowest density of surface basic sites but the highest fraction of strong basic sites, exhibits a high conversion of PC and a high selectivity to DMC for the transesterification of PC with methanol at a low temperature.

Guanylation of NH2-MIL-125 under green condition: An approach to strengthen basic sites of NH2-functionalized materials for enhanced base catalysis

The NH2-MIL-125, an amine functionalized metal-organic framework, possesses good amount and strength of basic sites due to –NH2 groups, and exhibits significant activity for base catalyzed Claisen-Schmidt condensation of acetophenone and benzaldehyde to synthesize chalcone under solvent free condition. In order to further enhance the basicity and catalytic activity of NH2-MIL-125, the material was modified by the guanylation reaction. The guanylation of NH2-MIL-125 using an aqueous solution of cyanamide at room temperature (30 °C) converted some of –NH2 groups into guanidyl functionalities (16% modification) without affecting the framework structure. The guanidyl groups in the resulted material act as strong basic sites producing a stronger solid base catalyst than NH2-MIL-125. The modified NH2-MIL-125 exhibited enhanced catalytic activity for solvent free Claisen-Schmidt condensation reaction. The study shows that NH2-MIL-125 and its modified form (having guanidyl groups) are potential solid base catalysts. The developed protocol for guanylation could also be useful for the modification of amine functionalized materials under green condition to strengthen their surface basic sites for enhanced catalytic performance.

Activity of Nickel Supported Over Nanorod-Shaped Strontium Hydroxyapatite Catalysts in Selective Methanation of CO & CO2

Nickel modified strontium hydroxyapatite (Ni/Sr-HAP) supported catalysts are studied in selective methanation of CO/CO2. In this work, a new type of nano rod-shaped strontium hydroxyapatite-based catalysts with two different sizes and aspect ratios were prepared by sol–gel method and in next step, Ni precursor was wet impregnated i.e., denoted as Ni/Sr-HAP and Ni/Sr-HAP(F127). The catalytic tests were performed in CO and CO2 methanation reactions and evaluated the light-off temperatures curves (225–450 °C) under ambient pressure in a fixed-bed flow reactor. The physicochemical properties of the prepared catalysts were characterized by XRD, N2 physisorption, TEM, SEM, TPR, CO2/H2-TPD and H2-chemisorption techniques. From XRD analysis, both Ni/Sr-HAP and Ni/Sr-HAP(F127) were identified as the hydroxyapatite type structure with high crystallinity and very low intensity peaks corresponds to strontium phosphates as the main phase and structure. The TEM and SEM micrographs of Ni/Sr-HAP catalysts displayed a nano- rod shaped morphology with different dimensions and exhibited average Ni particle size of ~ 9.2 nm. While Ni/Sr-HAP(F127) shown the rod size in the length in the range of 100–250 nm and width in the range of 20–40 nm with average Ni particle size 5.7 nm. The F127 mediated support Sr-HAP synthesis i.e., Ni/Sr-HAP(F127) mesoporous catalyst possessed higher metal surface with smaller Ni particles size and possessed higher CO2 adsorption capacity. The medium strength basic sites of Ni/Sr-HAP catalyst played an important role in methanation reactions. Based on the characterization and the catalysts activity results, small sized nanorods of Ni/Sr-HAP(F127) is the most active and selective catalyst due to the higher Ni dispersion, higher amounts of medium basic sites, and reducibility character than the bigger nanorods based Ni/Sr-HAP catalyst.

Activated Carbon Produced from the Hydrothermal Treatment of Glucose with KOH Activation for Catalytic Absorption of CO2 in a BEA-AMP Bi-Solvent Blend.

The amine-based postcombustion CO2 capture (PCC) process involves absorption of CO2 into a solvent and then regenerating the solvent to produce CO2. In this study, the effect of an activated carbon (AC) catalyst, synthesized through hydrothermal treatment and furnace activation on CO2 absorption in a 4M BEA/AMP amine blend, was evaluated and compared with that of a KMgO/CNTs (1:4) catalyst. The physical and chemical properties of AC were investigated with a scanning electron microscope (SEM), CO2 temperature-programmed desorption (CO2-TPD), Brunauer-Emmett-Teller (BET), powder X-ray diffraction (XRD), and thermogravimetric analyzer (TGA) and compared with the KMgO/CNTs (1:4) catalyst. The results showed that when compared against noncatalytic CO2 absorption, AC enhanced the linear rate of CO2 absorption by 33.3%, while for KMgO/CNTs, it was reported as 25.5%. The relatively higher surface area, combined with the higher number and strength of basic sites of AC relative to the KMgO/CNTs (1:4) catalyst, provided effective basic reaction sites for CO2 absorption, thereby enhancing the rate of CO2 absorption into the amine. AC was also relatively easier to synthesize which would provide a good replacement for the CNT-based catalyst which has carcinogenic tendencies.

Effect of hydroxyl and Mo doping on activity and carbon deposition resistance of hydroxyapatite supported NixMoy catalyst for syngas production via DRM reaction

The doping of the second metal Mo is expected to further enhance the carbon deposition resistance of Ni-based catalysts for syngas production via dry reforming of methane (DRM). In this study, the hydroxyapatite (HAP) was used as the support and a small Mo dosage was doped in the Ni-based catalysts to investigate the effect of intrinsic hydroxyl and Mo doping on the catalytic activity and carbon deposition resistance in DRM reaction. The catalyst characterization results show that both Ni and Mo are doped into the HAP structure with relatively uniform dispersion. The basic site strength of Ni4Mo0.2-HAP catalyst containing Mo is significantly higher than that of without Mo. The Mo dopant significantly improves the initial catalytic activity, but has minimal effect on the stability enhancement. Whether the catalyst is pre-reduced or not is crucial to the initial activity of the DRM reaction, the non-pre-reduced catalysts will go through a “self-activation” stage at the beginning of the reaction, where the “hydroxyl group” are proved to play as an “oxygen supply” for the partial oxidation of CH4 or the oxidation of the carbon deposition in the initial stage. Only trace amount of carbon deposition is found after 100 h of DRM reaction on Ni3Mo0.2-HAP catalyst. The NiMo-HAP catalysts exhibit excellent initial activity and resistance to carbon deposition due to the synergistic effect of Ni–Mo alloy and hydroxyl groups in the hydroxyapatite support.

Stable Engineered Trimetallic Oxide Scaffold as a Catalyst for Enhanced Solvent-Free Conversion of CO2 into Value-Added Products

This work demonstrates the development of mesoporous trimetallic oxide scaffolds (TOSs) as heterogeneous catalysts for the solvent-free transformation of CO2 into various value-added products. The TOS catalyst was prepared using a solution-combustion protocol as a time- and energy-saving method using cobalt, magnesium, and cerium metal nitrate salts as precursors and ethylene glycol as the fuel system and compared with their monometallic counterparts. Characterization suggested strong metal–metal and metal–oxygen interactions in the 3D-interconnected hierarchical porous network, which resulted in substantial alteration in the electronic, structural, and physicochemical properties. This resulted in an appreciable surface area, acid–base cooperative sites, and a larger pore volume in the catalyst. Thereafter, the CoMgCe-TOS catalyst was first used for the solvent-free cyclization of o-phenylenediamines and CO2 to produce benzimidazoles. In the presence of dimethylamine borane as a reductant, benzimidazoles were obtained in 94% yield at 100 °C under pressurized conditions, along with good recyclability for 12 cycles. It was established that selectivity toward benzimidazole improved upon the incorporation of basic metals (Mg and Ce), with Co aiding in the formation of cooperative Lewis acid–base sites. A plausible mechanism was also predicted, wherein the overall conversion of reactants for the N-formylation step was influenced by the number and strength of basic sites, and the cyclization step for selectively obtaining benzimidazole was affected by the acidic site strength. Besides the production of benzimidazole, the catalyst was also highly active for the synthesis of cyclic carbonates using epoxides and CO2 under optimized reaction conditions. This work therefore provides a new and greener route for the synthesis of benzimidazoles and organic carbonates, which can be easily adapted for scale-up applications.

CO2 methanation over Ni-Al LDH-derived catalyst with variable Ni/Al ratio

CO2 methanation is a promising technology to recycle CO2 into useful chemicals, fuels, and energy, avoiding its emissions in the atmosphere, as well as for the purification of H2 streams containing CO2. In this work, Ni-Al LDH-derived catalysts with Ni/Al ratio between 0.5 and 4 were prepared by co-precipitation and evaluated in CO2 methanation. The samples were characterized by N2 physisorption, X-ray diffraction, temperature-programed reduction, temperature-programmed desorption (CO2-TPD, H2-TPD) and oxidation. Catalytic tests were carried out in a fixed-bed reactor at atmospheric pressure, inlet mixture of H2:CO2:N2 = 4:1:15 and GHSV = 60000 mL (gcat h)−1, in stepwise mode (200–400 °C) and stability at 300 °C. The catalysts presented high activity and selectivity, reaching 92.3 % of CO2 conversion at 300 °C, along with 100 % CH4 selectivity for the catalyst with NiAl = 2 due to its high number of weak-to-medium strength basic sites. The amount of H2-chemisorbed was higher for NiAl = 1, whereas the highest number of basic sites was for NiAl = 2. These results indicate that LDH-derived Ni-Al catalysts with a Ni/Al ratio between 1 and 2 would be suitable for CO2 methanation.

Removal of As2O3 in coal-fired flue gas by metal oxides: Effects of adsorption temperature and flue gas components

Arsenic emitted from coal-fired power plants has attracted widespread attention. Metal oxides are promising adsorbents for arsenic removal, but the influence laws and mechanisms of temperature and flue gas components on their arsenic removal performance are unclear. In this work, systematic research on the effects of adsorption temperature and flue gas components on As2O3 removal by Fe2O3, CaO, and γ-Al2O3 was conducted. The results show that As2O3 removal ability satisfies Fe2O3 > CaO > γ-Al2O3 at 300–900 °C, which is mainly related to their basic site strength. At 500 °C, Fe2O3 has the largest As2O3 removal capacity (adsorption capacity: 158.4 μg/g; removal efficiency: 84.68%). Increasing adsorption temperature is beneficial to the transformation of arsenic to high-valent arsenic (As(V)) on the surface of Fe2O3, CaO, and γ-Al2O3. Low-concentration SO2 will compete with As2O3 for adsorption on the three metal oxides surface, which thereby inhibits their As2O3 removal ability. The presence of NO has an obvious inhibitory effect on the As2O3 removal of Fe2O3, CaO, and γ-Al2O3, which is little affected by the change of NO concentration. Fe2O3 is a metal oxide adsorbent with strong arsenic removal potential. Compared with basic simulated flue gas (BSFG) components, the presence of SO2, NO, and SO2+NO in the simulated flue gas all inhibits the adsorption of As2O3 by Fe2O3. Finally, future work on arsenic removal from coal-fired flue gas adsorbents is presented. This work has important engineering applications and theoretical research guiding significance for the development of high-performance arsenic removal adsorbents.

The Study of MgO and/or ZrO2 Modified Al2O3 for CO2 Adsorption

Alumina (Al2O3) is widely used as a sorbent in CO2 adsorption and as a catalyst support in CO2 utilization. For this application, the CO2 adsorption ability is the most important property to be improved. Thus, this work presents the idea to modify the surface of a commercial alumina using an Mg-Zr mixed oxide. With this material, sites for CO2 adsorption can be created by generating oxygen mobility property simultaneously with the basicity. To prove this hypothesis, alumina modified with MgO, ZrO2, or Mg-Zr mixed oxide was prepared by an incipient wetness impregnation. The physicochemical properties of the bare alumina and the alumina modified with MgO, ZrO2, or Mg-Zr mixed oxide were investigated using N2 adsorption-desorption and X-ray diffraction. The achievement of oxygen mobility creation was quantitatively evaluated by O2-temperature programmed desorption measurement. The CO2 adsorption performance with the distribution of basic site strength was determined by CO2-temperature programmed desorption measurements. According to the results, the highest oxygen mobility was found in the alumina modified with Mg-Zr mixed oxide, which is approximately 1.7 times that of the bare alumina. The alumina modified with Mg-Zr mixed oxide showed the highest CO2 capacity by approximately 1.4 times compared to the alumina. It is because the cubic ZrO2 stabilized with MgO provided the number of oxygen vacancy sites that can be filled by oxygen atoms in adsorbed CO2 corresponding to the moderate and strong basic sites.

Aceh Local Dolomite Modified by Alkali Metals as a Low-Cost Solid Inorganic Catalyst for Biodiesel Synthesis

Solid inorganic-based catalyst which mainly contains calcium oxide and magnesium oxide has been successfully prepared from local Aceh dolomite minerals. The calcination process of dolomite particles (150 μm) in the air environment for 4 hours changed those calcium and magnesium carbonates compounds into their oxide form, which are subsequently referred to as inorganic oxides. Furthermore, the calcined local dolomite has been chemically modified using alkaline metal precursors (lithium, sodium and cesium precursors). Physicochemical investigation on the modified dolomite indicated that introduction of lithium metal on modified dolomite surface has a positive impact on increasing the number and strength of basic sites on the catalyst. The modified catalyst had proved its catalytic activity for biodiesel synthesis with the yield values of 82;78;51 respectively.

Coke deposition over Ni-based catalysts for dry reforming of methane: effects of MgO-Al2O3 support and ceria, lanthana promoters

The coke deposition, a major challenge in the dry reforming of methane process (DRM), was investigated over the different Ni-based catalysts varied, Ni loading 5–12.5 wt% over γ-Al2O3, MgO content 30–70 wt% in MgO.Al2O3 and Ce, La promoters, prepared by incipient wetness impregnation method. The prepared catalysts were studied for the DRM activity and stability under the severe reaction conditions of 800 °C, 1 atm, CH4/CO2 = 1 and GHSV 1700 h−1 in a bench-scale reactor. The higher catalytic activity and coking resistance were observed at 5% Ni over γ-Al2O3 and 30% MgO in MgO.Al2O3 unpromoted catalysts attributed to the corresponding strong metal-support interaction and the highly stable MgAl2O4 spinel with a large number of medium strength basic sites. While the stability tests over the highly active promoted catalysts up to 76 h exhibited the pressure drop build-up of 0.40 bar in order of NiCeLa/MgO.Al2O3 < NiCe/γ-Al2O3 < NiLa/γ-Al2O3, similar to total carbon and the graphitic carbon deposition. The comparatively higher activity and stability over the catalyst NiCeLa/30MgO.70Al2O3 was assigned to the mainly amorphous coke formation. Also, the regeneration of a spent catalyst NiCe/γ-Al2O3 in the graphitic carbon regime further increased the pressure drop to 0.55 bar, revealing the difficulty of graphitic carbon regeneration. In addition, the regenerated catalyst exhibited decreased catalytic activity and the rapid increase of pressure drop 1.03 bar due to the catalyst deactivation.

Synthetic gas production by dry reforming of methane over Ni/Al2O3–ZrO2 catalysts: High H2/CO ratio

Alumina prepared by the sol-gel method, was impregnated with zirconia (5, 15 and 30 wt.%). Subsequently, the resulting Al2O3–ZrO2 supports were impregnated with 15% Ni to obtain the Ni/Al2O3–ZrO2 catalysts. The obtained catalysts were characterized by BET, SEM, XRD, H2-TPR and TPD- CO2. The catalytic activity was studied by means of dry reforming of methane (DRM) for syngas production. The catalysts displayed different physicochemical properties and trends of their catalytic activity as a function of the ZrO2 content in the mixed oxide supports. For instance, ZrO2 (5 wt %) in the catalyst, led to enhanced concentration of the medium strength basic sites and increased specific surface area, yielding thus the best performance in the DRM, with low carbon deposition after 36 h of reaction, compared with the other catalysts. This indicates that during the DRM reaction, this catalyst can provide more surface oxygen to prevent carbon deposits that could deactivate the catalyst.

Factors affecting primary and secondary pathways in CO2 hydrogenation to methanol over CuZnIn/MZrOx (La, Ti or Y)

CO2 hydrogenation to methanol was investigated over CuZnIn/MZrOx (M = La, Y or Ti) catalysts at 20 bar and at 225 °C with the purpose to elucidate the effects of the metal oxide promoter for ZrO2 and InOx for CuZnOx on reaction scheme of product formation, catalyst activity and product selectivity. Although the additives do not affect the overall scheme of product formation, they are relevant for the rates of direct CO2 conversion into methanol or CO and of methanol decomposition to CO and H2. This is due to their influence on dispersion of CuOx and the strength of basic sites for CO2 adsorption. Highly dispersed CuOx species are essential for CO2 conversion but not for methanol selectivity. The selectivity is improved over polymerized CuOx with strong basicity. This catalyst property can be controlled through promoting of (i) ZrO2 with La2O3/Y2O3 or (ii) CuZnOx with InOx. The latter promoter is crucial for hindering the undesired methanol decomposition to carbon monoxide.

CO2 Methanation Using Multimodal Ni/SiO2 Catalysts: Effect of Support Modification by MgO, CeO2, and La2O3

Ni/oxide-SiO2 (oxide: MgO, CeO2, La2O3, 10 wt.% target concentration) catalyst samples were prepared by successive impregnation of silica matrix, first with supplementary oxide, and then with Ni (10 wt.% target concentration). The silica matrix with multimodal pore structure was prepared by solvothermal method. The catalyst samples were structurally characterized by N2 adsorption-desorption, XRD, SEM/TEM, and functionally evaluated by temperature programmed reduction (TPR), and temperature programmed desorption of hydrogen (H2-TPD), or carbon dioxide (CO2-TPD). The addition of MgO and La2O3 leads to a better dispersion of Ni on the catalytic surface. Ni/LaSi and Ni/CeSi present a higher proportion of moderate strength basic sites for CO2 activation compared to Ni/Si, while Ni/MgSi lower. CO2 methanation was performed in the temperature range of 150–350 °C and at atmospheric pressure, all silica supported Ni catalysts showing good CO2 conversion and CH4 selectivity. The best catalytic activity was obtained for Ni/LaSi: CO2 conversion of 83% and methane selectivity of 98%, at temperatures as low as 250 °C. The used catalysts preserved the multimodal pore structure with approximately the same pore size for the low and medium mesopores. Except for Ni/CeSi, no particle sintering occurs, and no carbon deposition was observed for any of the tested catalysts.

Effect of crystallographic structure of MnO2 on degradation of 2-CEES

In this study, four MnO2 samples with different crystallographic structures (α-, β-, δ- and γ-MnO2) were synthesized by the hydrothermal method. The relationship between the characteristics of the four crystalline samples and their degradation ability against 2-chloroethyl ethyl sulfide (2-CEES), a simulant of the chemical warfare agent sulfur mustard, and the relevant reaction mechanism were investigated. Characterization data indicated that the various crystal types displayed different specific surface areas, frequency of lattice defects, amounts of adsorbed/inserted water, and numbers and strength of basic sites. The combined effects of these differences caused different degradation activity in 2-CEES degradation to be observed. The reaction followed pseudo-first-order kinetics, whereby γ-MnO2 exhibited the highest degradation activity. The mechanism of 2-CEES degradation was studied by gas chromatography and infrared spectroscopy methods, and data indicated the key role played by substrate hydrolysis and oxidation in the degradation process.

Promoting the surface active sites of defect BaSnO3 perovskite with BaBr2 for the oxidative coupling of methane

In this work, to develop better OCM catalysts, especially with improved ethylene selectivity, the promotional effects of BaBr2 on defect BaSnO3 has been investigated by changing the BaBr2/BaSnO3 molar ratio. XRD and Raman have substantiated that BaBr2 and BaSnO3 co-exists in all the BaBr2-modified catalysts. Compared with pure defect BaSnO3, all the modified catalysts exhibit significantly improved OCM performance, particularly with extremely high ethylene selectivity. Electrical conductivity measurement have proven that defect BaSnO3 is a p-type semi-conductor favouring OCM reaction, whose surface possesses abundant vacancies. O2-TPD, CO2-TPD and electrical conductivity dependence on O2 partial pressure results have indicated that BaBr2 addition can enhance the surface oxygen vacancy concentration of BaSnO3, which facilitates the conversion of surface lattice O2− into OCM reactive oxygen sites, such as O22− and O2- anions. At the same time, a larger amount of moderate strength basic sites have been formed. EPR, in situ Raman and XPS results have demonstrated that the quantities of these active oxygen sites can be evidently improved by BaBr2 addition. More importantly, a type of new O2δ- (0 <δ < 1) sites beneficial to OCM reaction have also been formed. Notably, XPS results have revealed that the amount of O22− sites, which can directly convert CH4 into ethylene through carbene intermediates, has been remarkably increased due to the interaction between BaBr2 and BaSnO3. As a consequence, the OCM reaction performance, especially ethylene selectivity and yield on the BaBr2-modified BaSnO3, has been significantly improved.