Wenxiang Tang

MOF Thin Film-Coated Metal Oxide Nanowire Array: Significantly Improved Chemiresistor Sensor Performance.

A strategy for combining metal oxides and metal-organic frameworks is proposed to design new materials for sensing volatile organic compounds, for the first time. The prepared ZnO@ZIF-CoZn core-sheath nanowire arrays show greatly enhanced performance not only on its selectivity but also on its response, recovery behavior, and working temperature.

Oxalate route for promoting activity of manganese oxide catalysts in total VOCs’ oxidation: effect of calcination temperature and preparation method

A novel template-free oxalate route was applied to synthesize mesoporous manganese oxides with high surface area (355 m2 g−1) and well-defined mesopores which can be obtained in large quantities. The physicochemical properties of the materials were characterized by means of TG, XRD, SEM, TEM, H2-TPR and XPS techniques. All catalysts were tested on catalytic deep oxidation of benzene, and the effects of calcination temperature on the features of catalyst structure and catalytic activity were investigated. Manganese oxides prepared by oxalate route exhibited better catalytic activities for complete oxidation of benzene, toluene and o-xylene as compared with related manganese oxides prepared by other different methods (NaOH route, NH4HCO3 route and nanocasting strategy), and especially the temperature for benzene conversion of 90% on the oxalate-derived manganese oxide catalysts was 209 °C, which is 132 °C lower than required for the catalyst prepared by NaOH route. The catalytic performance of manganese oxide is correlated with surface area, pore size, low-temperature reducibility and distribution of surface species. The mole ratio of Mn4+/Mn2+ on the samples which performed with better catalytic activity was close to 1.0. This is good for the redox process of Mn4+ ↔ Mn3+ ↔ Mn2+ which is the key factor in determining the activity on MnOx, further indicating that the oxalate route is good for keeping the distribution of manganese oxidation states at an appropriate degree. A possible process of VOCs’ complete oxidation on manganese oxide catalysts is discussed. In addition, the best catalyst was highly stable with prolonged time on stream and was resistant to water vapor.

Preparation of hierarchical layer-stacking Mn-Ce composite oxide for catalytic total oxidation of VOCs

Hierarchical layer-stacking Mn-Ce composite oxide with mesoporous structure was firstly prepared by a simple precipitation/decomposition procedure with oxalate precursor and the complete catalytic oxidation of VOCs (benzene, toluene and ethyl acetate) were examined. The Mn-Ce oxalate precursor was obtained from metal salt and oxalic acid without any additives. The resulting materials were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), hydrogen temperature programmed reduction (H-2-TPR) and X-ray photoelectron spectroscopy (XPS). Compared with Mn-Ce composite oxide synthesized through a traditional method (Na2CO3 route), the hierarchical layer-stacking Mn-Ce composite oxide exhibited higher catalytic activity in the complete oxidation of volatile organic compounds (VOCs). By means of testing, the data revealed that the hierarchical layer-stacking Mn-Ce composite oxide possessed superior physiochemical properties such as good low-temperature reducibility, high manganese oxidation state and rich adsorbed surface oxygen species which resulted in the enhancement of catalytic abilities.

Hierarchical hollow ZnO cubes constructed using self-sacrificial ZIF-8 frameworks and their enhanced benzene gas-sensing properties

Novel hierarchical ZnO hollow cubes are constructed using the interpenetrated 0D nanoparticles through directly decomposing the Zn-based metal–organic frameworks (Zn-MOF, ZIF-8). After decomposition at 450 °C for 1 h, the as-prepared ZnO well maintains the original ZIF-8 shape with relatively high surface area (45 m2 g−1), thereby realizing fast surface reaction kinetics of benzene molecules. In contrast to singular 0D ZnO nanoscale counterparts, the unique ZnO nanostructure assembly renders the well exposed surfaces and defect states, enhancing significantly chemical sensitivity towards gaseous benzene. The present work provides a facile and versatile approach for designing the high-performance chemical sensing materials.

Importance of porous structure and synergistic effect on the catalytic oxidation activities over hierarchical Mn–Ni composite oxides

Hierarchically porous manganese–nickel composite oxides (MNCOs) were successfully synthesized by an oxalate route and further applied for catalytic removal of benzene. Among these catalysts, the best one was Mn2Ni1 mixed oxides which exhibited uniform hierarchical lithops-like topography, a rich porous structure and a high surface area of 201.1 m2 g−1. The temperature required for a benzene conversion of 90% over this catalyst was ca. 232 °C under the conditions of a benzene concentration of 1000 ppm in air and a high space velocity of 120000 mL g−1 h−1, which was 54 °C lower than that over the non-porous MNCO particles prepared by a traditional approach. The reaction kinetic study showed that the apparent activation energy (45.2 kJ mol−1) for the total oxidation of benzene over the Mn2Ni1 oxide catalyst was much lower than those (72.4–97.2 kJ mol−1) over other catalysts. With XPS and H2-TPR analyses, the porous MNCOs have a higher content of surface-adsorbed oxygen species and better low-temperature reducibility which can be ascribed to a possible synergetic effect between Mn and Ni ions in the spinel mixed oxides.

Higher Oxidation State Responsible for Ozone Decomposition at Room Temperature over Manganese and Cobalt Oxides: Effect of Calcination Temperature

The heterogeneous catalytic decomposition of ozone was investigated over unsupported manganese and cobalt oxide at room temperature. All catalysts were characterized by X-ray diffraction (XRD), N2 adsorption–desorption (Brunauer–Emmet–Teller method), H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The catalytic activity test indicated that these oxides had a good activity on ozone conversion meanwhile the catalysts remained highly active over time under reaction conditions. The treated temperature of the catalyst had a significant impact on the performance of ozone abatement and the samples treated at lower temperature showed higher activity. The surface area decreased obviously when developing the calcination temperature and H2-TPR results demonstrated that much higher oxidation state of metal ions and active oxygen species were maintained on the surface under low treated temperature. XPS analysis showed that there were higher oxidation states of metal ions (Mn4+ and Co3+) and adsorbed oxygen species on the surface of catalysts treated at lower temperature, both of which play a significant role in ozone decomposition. However, the activity of manganese oxide was higher than that of cobalt oxide and the possible reason for this phenomenon was discussed.

Enthalpies of formation of rare earth-3d metal alloys and intermetallic compounds

The enthalpies of formation of rare earth-3d metal alloys and intermetallic compounds have been calculated by Miedemas semiempirical method. The calculations agree well with experimental enthalpy data and with phase diagram information. A brief comment on Miedemas model is given and the possibility of predicting the enthalpies of formation of multicomponent alloys and intermetallic compounds is discussed.

Design and synthesis of porous non-noble metal oxides for catalytic removal of VOCs

The design and synthesis of highly active non-noble metal oxide catalysts, such as transition- and rare-earth-metal oxides, have attracted significant attention because of their high efficiency and low cost and the resultant potential applications for the degradation of volatile organic compounds (VOCs). The structure-activity relationships have been well-studied and used to facilitate design of the structure and composition of highly active catalysts. Recently, non-noble metal oxides with porous structures have been used as catalysts for deep oxidation of VOCs, such as aromatic hydrocarbons, aliphatic compounds, aldehydes, and alcohols, with comparable activities to their noble metal counterparts. This review summarizes the growing literature regarding the use of porous metal oxides for the catalytic removal of VOCs, with emphasis on design of the composition and structure and typical synthetic technologies.

Crystal structure and magnetism of LaCo13−x−yFexSiy compounds

The crystal structure and magnetic properties of LaCo13−x−yFexSiy compounds were investigated by means of x‐ray powder diffraction and magnetization measurements. The substitution of Si for Co induces an order–disorder transition from the cubic NaZn13‐type to its tetragonal derivative structure, while the substitution of Fe for Co does not induce such a phase transition. After annealing treatment, the homogeneous range of the cubic phase is narrowed and that of the tetragonal phase is extended. From crystallographic and thermodynamic points of view, the sta‐ bility of the cubic and the tetragonal phases is discussed. The measured magnetic moment of LaCo13−x−yFexSiy coincides well with the theoretical prediction based on the magnetic valence model. Within the framework of this model, LaCo13−x−yFexSiy compounds can be considered as strong ferromagnets and their magnetic moment can be theoretically predicted. The composition dependence of Curie temperature is discussed within the mean field approximation. At...

Understanding low temperature oxidation activity of nanoarray-based monolithic catalysts: from performance observation to structural and chemical insights

Monolithic catalysts have been widely used in automotive, chemical, and energy relevant industries. Nanoarray-based monolithic catalysts have been developed, demonstrating high catalyst utilization efficiency and good thermal/mechanical robustness. Compared with the conventional washcoat-based monolithic catalysts, they have shown advances in precise and optimum microstructure control and feasibility in correlating materials structure with properties. Recently, the nanoarray-based monolithic catalysts have been studied for low temperature oxidation of automotive engine exhaust and exhibited interesting and promising catalytic activities. This review focuses on discussing the key structural parameters of nanoarray catalyst that affect the catalytic performance from the following aspects: (1) geometric shape and crystal planes, (2) guest atom doping and defects, (3) array size and size-assisted active species loading, and (4) the synergy effect of metal oxide in composite nanoarrays. Prior to the discussion, an overview of the current status of synthesis and development of the nanoarray-based monolithic catalysts is introduced. The performance of these materials in low temperature simulated engine exhaust oxidation is also demonstrated. We hope this review will elucidate the science and chemistry behind the good oxidation performance of the nanoarray-based monolithic catalysts and serve as a timely and useful research guide for rational design and further improvement of the nanoarray-based monolithic catalysts for automobile emission control.