Local Electron Environment Regulation of Spinel CoMn2O4 Induced Effective Reactant Adsorption and Transformation of Lattice Oxygen for Toluene Oxidation.

In contrast to numerous studies on oxygen species, the interaction of volatile organic compounds (VOCs) with oxides is also critical to the catalytic reaction but has hardly been considered. Herein, we develop a highly efficient Pt atom doped spinel CoMn2O4 (Pt-CoMn) for oxidation of toluene at low temperature, and the toluene conversion rate increased by 18.3 times (129.7 versus 7.1 × 10-11 mol/(m2·s)) at 160 °C compared to that of CoMn2O4. Detailed characterizations and density functional theory calculations reveal that the local electron environment of the Co sites is changed after Pt doping, and the formed electron-deficient Co sites in turn strengthen the interaction with toluene. Adsorbed toluene will react with lattice oxygen in Pt-CoMn and CoMn catalysts and convert into benzoate intermediates, and the consumption rate of benzoate is closely related to the activation of gaseous oxygen. Significantly, the abundant bulk defects of Pt-CoMn help to open the reaction channel in the CoMn spinel, which acts as an oxygen pump to promote the transformation of bulk lattice oxygen into surface lattice oxygen at lower temperatures, thus accelerating the conversion rate of benzoate intermediates into CO2 and enhancing low-temperature combustion of toluene. Pt-CoMn developed here emphasizes the regulation of VOCs adsorption strength and lattice oxygen transformation processes on CoMn2O4 by adjusting the local electron environment, which will provide new guidance for the design of efficient oxide catalysts for catalytic oxidation.

Disentangling Local Interfacial Confinement and Remote Spillover Effects in Oxide-Oxide Interactions.

Supported oxides are widely used in many important catalytic reactions, in which the interaction between the oxide catalyst and oxide support is critical but still remains elusive. Here, we construct a chemically bonded oxide-oxide interface by chemical deposition of Co3O4 onto ZnO powder (Co3O4/ZnO), in which complete reduction of Co3O4 to Co0 has been strongly impeded. It was revealed that the local interfacial confinement effect between Co oxide and the ZnO support helps to maintain a metastable CoOx state in CO2 hydrogenation reaction, producing 93% CO. In contrast, a physically contacted oxide-oxide interface was formed by mechanically mixing Co3O4 and ZnO powders (Co3O4-ZnO), in which reduction of Co3O4 to Co0 was significantly promoted, demonstrating a quick increase of CO2 conversion to 45% and a high selectivity toward CH4 (92%) in the CO2 hydrogenation reaction. This interface effect is ascribed to unusual remote spillover of dissociated hydrogen species from ZnO nanoparticles to the neighboring Co oxide nanoparticles. This work clearly illustrates the equally important but opposite local and remote effects at the oxide-oxide interfaces. The distinct oxide-oxide interactions contribute to many diverse interface phenomena in oxide-oxide catalytic systems.

Enhancement of toluene oxidation performance over La1-xCoO3-δ perovskite by lanthanum non-stoichiometry.

La1-xCoO3-δ catalysts with different non-stoichiometry of lanthanum ions were synthesized by using the sol-gel method, and their catalytic performance in toluene combustion was investigated. The results showed that the catalytic activity and stability of A-site nonstoichiometric La1-xCoO3-δ were improved to a certain extent compared with pure LaCoO3 perovskite. Among them, the La0.9CoO3-δ catalyst gave the best catalytic performance for toluene oxidation. It achieved 90% toluene conversion at 205°C under the conditions of a WHSV (weight hourly space velocity) of 22,500 mL/(g·hr) and a 500 ppmV-toluene concentration. Various characterization techniques were used to investigate the relationship between the structure of these catalysts and their catalytic performance. It was found that the non-stoichiometric modification of the lanthanum ion at position A in LaCoO3 changed the surface element state of the catalyst and increased the oxygen vacancy content, thus, combined with improved reducibility, improving toluene degradation on the catalyst.

Bifunctional zeolites-silver catalyst enabled tandem oxidation of formaldehyde at low temperatures.

Bifunctional catalysts with tandem processes have achieved great success in a wide range of important catalytic processes, however, this concept has hardly been applied in the elimination of volatile organic compounds. Herein, we designed a tandem bifunctional Zeolites-Silver catalyst that enormously boosted formaldehyde oxidation at low temperatures, and formaldehyde conversion increased by 50 times (100% versus 2%) at 70 °C compared to that of monofunctional supported silver catalyst. This is enabled by designing a bifunctional catalyst composed of acidic ZSM-5 zeolite and silver component, which provides two types of active sites with complementary functions. Detached acidic ZSM-5 activates formaldehyde to generate gaseous intermediates of methyl formate, which is more easily oxidized by subsequent silver component. We anticipate that the findings here will open up a new avenue for the development of formaldehyde oxidation technologies, and also provide guidance for designing efficient catalysts in a series of oxidation reactions.

Synergistic effects in Mn-Co mixed oxide supported on cordierite honeycomb for catalytic deep oxidation of VOCs.

A series of Co-Mn mixed oxide catalyst supported on a cordierite monolith was facilely synthesized by ultrasonic impregnation. Its catalytic performance was evaluated in the combustion of toluene, ethyl acetate and its mixture. It was observed that with incorporating Mn into Co3O4, the formation of solid solution with spinel structure could significantly improve the catalytic activity of pure phase Co3O4. And the monolithic Co0.67Mn0.33Ox catalyst showed the best catalytic performance in the catalytic oxidation of toluene and ethyl acetate which could be completely oxidized at 220 and 180°C respectively under the reaction velocity (WHSV) about 45,000 mL/(g•hr) and pollutant concentration of 500 ppmV. The total conversion temperature of the VOCs mixture was at 230°C (500 ppmV toluene and 500 ppmV ethyl acetate) and determined by the temperature at which the most difficult molecule was oxidized. The excellent catalytic performance of monolithic Co0.67Mn0.33Ox was attributed to the higher content of Mn3+, Co3+, surface adsorbed oxygen and better redox ability. The prepared catalyst showed the good mechanical stability, reaction stability, and good adaptability to different reaction conditions.

Effect of alkaline earth metal promoter on catalytic activity of MnO2 for the complete oxidation of toluene.

Herein, Na+ and Ca2+ are introduced to MnO2 through cation-exchange method. The presence of Na+ and Ca2+ significantly enhance the catalytic activity of MnO2 in toluene oxidation. Among them, the Ca-MnO2 catalyst exhibits the best catalytic activity (T50 = 194°C, T90 = 215°C, Ea = 57.2 kJ/mol, reaction rate 8.40 × 10-10 mol/(sec⋅m2) at 210°C. T50 and T90: the temperature of 50% and 90% toluene conversion; Ea: apparent activation energy) and possess high tolerance against 2.0 vol.% water vapor. Results reveal that the increased acidic sites of the MnO2 sample can enhance the adsorption of gaseous toluene, and the mobility of oxygen species and the content of reactive oxygen species in the catalyst are significantly improved due to the formed oxygen vacancy. Thus these two factors result in excellent catalytic performance for toluene oxidation combining with the weak CO2 adsorption ability.

Promotion of NH3-SCR activity by sulfate-modification over mesoporous Fe doped CeO2 catalyst: Structure and mechanism.

The mesoporous Fe doped CeO2 catalyst after modifying organic sulfate functional groups show an excellent activity with above 80% NOx conversion in a temperature range of 250-450 °C. These organic-like sulfate groups bound to the Fe-O-Ce species leads to the strong electron interaction between Fe3+-O-Ce4+ species and sulfate groups, which modifies the acidity and redox properties of catalyst. The strong ability of SË­O/S-O in sulfate groups to accommodate electrons from a basic molecule is a driving force in the generation of acidic properties, and thus promotes to produce new Brønsted acid sites. The bondage of Fe-O-Ce species obviously inhibits the creation of thermostable bidentate NO3- species. Besides, the redox cycles between Fe3+ and Ce4+ are disrupted, thus inhibiting NH3 oxidation at medium-high temperatures and resulting in the increase of NOx conversion. Furthermore, the in situ DRIFTS results show that for the fresh samples, the coordinate NH3 reacts not only with NO3- through L-H mechanism, but also with oxygen species to form NOx. Differently for sulfated sample, the coordinate NH3 might react with achieved NO2 instead of the oxygen species through E-R mechanism, meanwhile the NH4+ could react with the NO3- species through L-H mechanism.

Simultaneous cobalt(III)-histidine reduction and aerobic denitrification by Paracoccus versutus LYM.

Cobalt(II)-histidine [Co(II)His] is potentially a better alternative to ferrous complexes in the chemical absorption-biological reduction (CABR) flue gas denitrification process in view of its higher oxygenation reversibility. Though with excellent O2-resistant ability, Co(II)His was still gradually oxidized into Co(III)His, losing NO binding capacity. Thus, Co(III)His biological reduction is an indispensable step in CABR process. Co(III)His reduction by Paracoccus versutus LYM under aerobic condition in the presence of nitrate or nitrite was investigated. Results indicated that simultaneous Co(III)His reduction and aerobic denitrification were achieved by strain LYM. Co(III)His reduction was significantly promoted by denitrification process, but dramatically inhibited by 5-15 mM sulfite. Co(II)His absorbent regeneration could be facilitated by adjusting O2 supply properly or adding nitrogen and carbon source regularly. These findings provide a basis for the application of Co(II)His as the absorbent in the CABR process and qualify P. versutus LYM as an applicable and competitive strain for this process.

Tuning oxygen vacancy concentration of MnO2 through metal doping for improved toluene oxidation.

Oxygen vacancy acts an important role in adjusting the chemical properties of MnO2. In this paper, two-dimensional MnO2 catalysts with different oxygen vacancy concentration are obtained by doping Cu2+. It is researched that the K+ species in the interlayer of birnessite-type MnO2 can be substituted during the Cu2+ doping process. Meanwhile, this process will generate the oxygen vacancy. Interestingly, the formation of an appropriate numbers of oxygen vacancy in MnO2 distinctly enhances the low-temperature reducibility and oxygen species activity, which improves the catalytic activity for the toluene oxidation (T100 = 220 °C, Ea=43.6 kJ/mol). However, an excessive concentration of oxygen vacancy in MnO2 sample performs against the activity improvement for toluene oxidation. In situ DRIFTS are applied to elucidate the main intermediates and conversion pathway on MnO2-OV3 with moderate concentration of oxygen vacancy. The results demonstrate that the adsorbed toluene can interact with oxygen species of catalyst to form physisorbed benzaldehyde, aldehydic adsorbate and benzoate species. In addition, it is found that the oxygen vacancy concentration plays an important effect on the oxidation of benzoate species owing to the acceleration effect of oxygen vacancy in the activation of gaseous oxygen.

Distribution of organophosphate esters between the gas phase and PM2.5 in urban Dalian, China.

We investigated the concentrations and seasonal variations of organophosphate esters (OPEs) in the gas phase and PM2.5 (particulate matter with an aerodynamic diameter <2.5 µm) in an urban area of Dalian, China, as well as their gas-particle partitioning. The total concentrations of OPEs in the gas phase were in the range of 0.056-6.38 ng/m3 with the mean concentration of 0.83 ± 1.24 ng/m3, while the concentrations of OPEs in the PM2.5 were in the range of 0.32-3.46 ng/m3 with the mean concentration of 1.21 ± 0.67 ng/m3. Tris-(1-chloro-2-propyl) phosphate (TCIPP) was the dominant congener in the gaseous phase, followed by tris-(2-chloroethyl) phosphate (TCEP) and tri-n-butylphosphate (TNBP), whereas TCEP was the dominant species in the PM2.5, followed by TCIPP and triphenyl phosphate (TPHP). Seasonality was discovered for OPEs in both gas phase and PM2.5 with their concentrations higher in hot seasons, which may due to the temperature-driven emission or gas-particle partitioning. The PM2.5-bound fractions of OPEs varied significantly between seasons. Tricresyl phosphate (TMPP), tri(2-ethylhexyl) phosphate (TEHP), 2-ethylhexyl diphenyl phosphate (EHDPP), and TPHP were mostly adsorbed onto fine particles, while TNBP, TCEP, TCIPP, and tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) distributed in both gas and fine particle phases. The predicted PM2.5-bound fractions by Koa-based model were closer to the measurements for TCIPP, TDCIPP, and TPHP, whereas the predictions by Junge-Pankow model were closer to the measurements for TMPP and tris (2-butoxyethyl) phosphate (TBOEP). However, the predictions of both models cannot accurately match the measured gas-particle partitioning of TNBP and TCEP.

Effects of cobalt-histidine absorbent on aerobic denitrification by Paracoccus versutus LYM.

To overcome the problem that ferrous complexes are easily oxidized by O2 and then lose NO binding ability in the chemical absorption-biological reduction (CABR) process, cobalt(II)-histidine [Co(II)His] was proposed as an alternative. To evaluate the applicability of Co(II)His, the effects of CoHis absorbent on the aerobic denitrification by Paracoccus versutus LYM were investigated. Results indicated that His significantly promoted nitrite reduction. The inhibition effects of CoHis absorbent could be substantially alleviated by increasing the initial His/Co2+ to 4 or higher. CoHis with concentrations of 4, 8, 12, 16 and 20 mM presented no distinct effect on nitrite reduction, but slightly inhibited the reduction of nitrate, resulting in longer lag of nitrate reduction, and obviously promoted the growth of strain LYM. In the presence of 5, 10, 15 and 20 mM CoHis absorbent, the main denitrification product was N2 (not less than 95.0%). This study is of significance in verifying the applicability of Co(II)His in the CABR process, and provides a referable CoHis absorbent concentration as 20 mM with an initial His/Co2+ of 4 for the future experiments.

Highly catalytic activity of Mn/SBA-15 catalysts for toluene combustion improved by adjusting the morphology of supports.

Rod-like, hexagonal and fiber-like SBA-15 mesoporous silicas were synthesized to support MnOx for toluene oxidation. This study showed that the morphology of the supports greatly influenced the catalytic activity in toluene oxidation. MnOx supported on rod-like SBA-15 (R-SBA-15) displayed the best catalytic activity and the conversion at 230°C reached more than 90%, which was higher than the other two catalysts. MnOx species consisted of coexisting MnO2 and Mn2O3 on the three kinds of SBA-15 samples. Large amounts of Mn2O3 species were formed on the surface and high oxygen mobility was obtained on MnOx supported on R-SBA-15, according to the H2 temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) results. The Mn/R-SBA-15 catalyst with greater amounts of Mn2O3 species possessed a large amount of surface lattice oxygen, which accelerated the catalytic reaction rate. Therefore, the surface lattice oxygen and high oxygen mobility were critical factors on the catalytic activity of the Mn/R-SBA-15 catalyst.

Mechanistic Investigation into the Effect of Sulfuration on the FeW Catalysts for the Selective Catalytic Reduction of NOx with NH3.

Iron tungsten (FeW) catalyst is a potential candidate for the selective catalytic reduction (SCR) of NOx with ammonia because of its excellent performance in a wide operating window. Sulfur poisoning effects in SCR catalysts have long been recognized as a challenge in development of efficient catalysts for applications. In this paper, the impact of sulfuration on catalyst structure, NH3-SCR reaction performance and mechanism was systematically investigated through spectroscopic and temperature-programmed approaches. The sulfuration inhibited the SCR activity at low temperatures (<300 °C), while no evident effect was observed at high temperatures (≥300 °C). After sulfuration for FeW oxides catalyst, the organic-like with covalent S═O bonds sulfate species were mainly formed over the FeW catalysts. Combining TPD with in situ DRIFTS results, it was found that the Lewis and the Brønsted acidity were enhanced by the interaction between metal species and sulfate species due to the strong electron withdrawing effect of the S═O double bonds. The in situ DRIFTS study showed that the formation of NO2 was hindered, leading to the "fast-SCR" pathway was partly cut off by the sulfuration process and thereby the loss of SCR activity at low temperatures. However, the Langmuir-Hinshelwood reaction pathway between adsorbed NH3/NH4+ species and nitrate species was facilitated and dominated at high temperatures, making the as-synthesized FeW catalysts resistant to SO2 poisoning.

Superior Performance of Fe1-xWxOδ for the Selective Catalytic Reduction of NOx with NH3: Interaction between Fe and W.

Novel iron-tungsten catalysts were first developed for the selective catalytic reduction of NOx by NH3 in diesel exhaust, achieving an excellent performance with a wide operating temperature window above 90% NOx conversion from 225 or 250 to 450 °C (GHSVs of 30 000 or 50 000 h-1). It also exhibited a pronounced stability and relatively high NOx conversion in the presence of H2O, SO2 and CO2. The introduction of W resulted in the formation of α-Fe2O3 and FeWO4 species obtained by HRTEM directly. The synergic effect of two species contributed to the high SCR activity, because of the increased surface acidity and electronic property. The FeWO4 with octahedral [FeO6]/[WO6] structure acted as the Brønsted acid sites to form highly active NH4+ species. Combining DFT calculations with XPS and UV-vis results, it was found that the fine electron interaction between α-Fe2O3 and FeWO4 made the electron more easily transfer from W6+ sites to Fe3+ sites, which promoted the formation of NO2. Judging by the kinetics and SCR activity studies, the Fe0.75W0.25Oδ with an appropriate W amount showed the strongest interaction, and thereby the lowest activation energy of 39 kJ•mol-1 and optimal catalytic activity. These findings would be conducive to the reasonable design of NH3-SCR catalysts by adjusting the fabrication.

Highly dispersed Fe2O3 on carbon nanotubes for low-temperature selective catalytic reduction of NO with NH3.

Highly dispersed Fe2O3 nanoparticles supported on carbon nanotubes, prepared by a simple ethanol-assisted impregnation method, showed above 90% NO conversion and selectivity at low temperatures (200-325 °C). Moreover excellent durability and stability towards SO2/H2O was obtained.

Clickable periodic mesoporous organosilicas: synthesis, click reactions, and adsorption of antibiotics.

Pharmaceutical antibiotics are not easily removed from water by conventional water-treatment technologies and have been recognized as new emerging pollutants. Herein, we report the synthesis of clickable azido periodic mesoporous organosilicas (PMOs) and their use as adsorbents for the adsorption of antibiotics. Ethane-bridged PMOs, functionalized with azido groups at different densities, were synthesized by the co-condensation of 1,2-bis(trimethoxysilyl)ethane (BTME) and 3-azidopropyltrimethoxysilane (AzPTMS), in the presence of nonionic-surfactant triblock-copolymer P123, in an acidic medium. Four different alkynes were conjugated to azide-terminated PMOs by means of an efficient click reaction. The clicked PMOs showed improved adsorption capacity (241 µg g(-1)) for antibiotics (ciprofloxacin hydrochloride) compared with azido-functionalized PMOs because of the enhanced π-π stacking interactions. These results indicate that click reactions can introduce multifunctional groups onto PMOs, thus demonstrating the great potential of PMOs for environmental applications.

Clickable SBA-15 to screen functional groups for adsorption of antibiotics.

Pharmaceutical antibiotics, as emerging contaminants, are usually composed of several functional groups that endow them with the ability to interact with adsorbents through different interactions. This makes the preparation of adsorbents tedious and time-consuming to screen appropriate functionalized materials. Herein, we describe the synthesis of clickable SBA-15 and demonstrate its feasibility as a screening material for the adsorption of antibiotics based on similar adsorption trends on materials with similar functional groups obtained by a click reaction and cocondensation/grafting methods.

The formation mechanism of Ag/SBA-15 nanocomposites prepared via in-situ pH-adjusting method.

In this paper, the mesostructures and formation mechanism of Ag/SBA-15 nanocomposites by in-situ "pH-adjusting" method were studied. Characterization results, such as X-ray powder diffraction, N2 adsorption-desorption isotherms and Transmission electron microscope, showed that the resultant materials exhibited highly ordered hexagonal mesoporous structures, which were strongly correlated to the pH values in the synthesis solution. The pH value also strongly affected the incorporation of silver nanoparticles into the channels. When the pH value of the solution was < or = 7.5, the highly ordered Ag/SBA-15 nanocomposites were synthesized successfully; moreover, more and more Ag(NH3)+ reacts with Si-OH groups in the channels, which resulted in the increase of silver nanoparticles in the channels with the increase of the pH value. Further increasing the pH value, overmuch OH- reacts with Ag+, leading to the decrease of the coated Ag species and the structural order of Ag/SBA-15 nanocomposites. It was found that silver ions could be automatically reduced to silver atoms during the crystallization procedure.

Comparison of dynamic adsorption/desorption characteristics of toluene on different porous materials.

Four different types of adsorbents, SBA-15, MCM-41, NaY and SiO2, were used to study the dynamic adsorption/desorption of toluene. To further investigate the influence of pore structure on its adsorption performance, two SBA-15 samples with different microspores were also selected. It is shown that microporous material NaY has the largest adsorption capacity of 0.2873 mL/g, and the amorphous SiO2 exhibits the least capacity of 0.1003 mL/g. MCM-41 also shows a lower break through capacity in spite of the relatively small pore diameter, because it can not provide the necessary small geometric confinement for the tiny adsorbates. However, the mesoporous SBA-15 silica with certain micropore volume shows relatively higher adsorption capacity than that of MCM-41 silica. The presence of micropores directly leads to an increase in the dynamic adsorption capacity of toluene. Although NaY has the highest adsorption capacity for toluene, its complete desorption temperature for toluene is high (> 350 degrees C), which limits its wide application. On the contrary, mesoporous silica materials exhibits a good desorption performance for volatile organic compounds at lower temperatures. Among these materials mesoporous SBA-15 samples, with a larger amount micropores and a lower desorption temperature, are a potentially interesting adsorbent for the removal of volatile organic compounds. This behavior should been related with the best synergetic effect of mesopores and micropores.

Photocatalytic degradation of gaseous toluene over Ag-doping TiO2 nanotube powder prepared by anodization coupled with impregnation method.

In this work, Ag-doping TiO(2) nanotubes were prepared and employed as the photocatalyst for the degradation of toluene. The TiO(2) nanotube powder was produced by the rapid-breakdown potentiostatic anodization of Ti foil in chloride-containing electrolytes, and then doped with Ag through an incipient wetness impregnation method. The samples were characterized by scanning electron microscope, high-resolution transmission electron microscopy, X-ray diffraction, surface photovoltage measurements, X-ray photoelectron spectroscopy and N(2) adsorption. The nanotubular TiO(2) photocatalysts showed an outer diameter of approximately 40nm, fine mesoporous structure and high specific surface area. The photocatalytic activity of Ag-doping TiO(2) nanotube powder was evaluated through photooxidation of gaseous toluene. The results indicated that the degradation efficiency of toluene could get 98% after 4h reaction using the Ag-doping TiO(2) nanotubes as the photocatalyst under UV light illumination, which was higher than that of the pure TiO(2) nanotubes, Ag-doping P25 or P25. Benzaldehyde species could be observed during the photocatalytic oxidation monitored by in situ FTIR, and the formed benzaldehyde intermediate during reaction would be partially oxidized into CO(2) and H(2)O.