Dongsen Mao

Effect of TiO2 crystal structure on the catalytic performance of Co3O4/TiO2 catalyst for low-temperature CO oxidation

Co3O4 catalysts supported on TiO2 with different crystalline structures (anatase (A), rutile (R) and P25 (Degussa)) were prepared by a deposition–precipitation method, and characterized by nitrogen adsorption/desorption, XRD, HR-TEM, EPR, Raman spectroscopy, XPS and H2-TPR techniques. The results show that Co3O4/TiO2 (A) exhibited the highest activity among the three Co3O4/TiO2 catalysts: CO can be completely oxidized to CO2 at −43 °C. When rutile TiO2 or P25 were used as the support, its catalytic activity was decreased obviously, because the TiO2 crystal structure has an influence on the physicochemical and catalytic properties of the Co3O4 catalysts. The results show that the Co3O4/TiO2 (A) catalyst contains Ti3+ species, which is in an unstable state and can affect the properties of Co3O4 by the interaction between the deposited Co3O4 and anatase TiO2 support. The Co3O4/TiO2 (A) catalyst exhibits highly defective structure and good oxygen adsorption ability. The reducibility of Co3O4 is improved by the anatase TiO2 support, resulting in Co3O4/TiO2 (A) possessing the better redox property than the other Co3O4/TiO2 catalysts, which is an important factor for its high catalytic activity.

Low-temperature CO oxidation on CuO/CeO2 catalysts: the significant effect of copper precursor and calcination temperature

CuO/CeO2 catalysts for CO oxidation were prepared by impregnation using different Cu precursors (acetate, nitrate, chloride, and sulfate) and calcined at 500 or 800 °C. Their physicochemical properties were characterized by TG-DSC, XRD, TEM, N2 adsorption, Raman, XPS, H2-TPR, CO-TPD, and DRIFTS. The results show that CuO is the dominant Cu species in all cases except for copper sulfate calcined at 500 °C, under which temperature some Cu2(OH)3Cl are also obtained from copper chloride. CuO/CeO2 prepared from copper acetate and calcined at 500 °C shows the best activity for CO oxidation, due to the presence of more finely dispersed CuO and stronger synergistic effects. The synergistic effects can induce the formation of Cu+ (the better CO adsorption sites) and activate the lattice oxygen, thus exerting a crucial role in the catalytic process. However, the residual Cl− and SO42− have a negative effect on the synergistic effects, resulting in low activity in CO oxidation.

Synthesis of cubic ordered mesoporous YPO4:Ln3+ and their photoluminescence properties

Cubic ordered mesoporous yttrium phosphate (YPO4) and lanthanide-doped yttrium phosphates (YPO4:Ln3+, Ln3+ = Eu3+, Tb3+ and Ce3+) have been synthesized successfully through the nanocasting route by employing the Y(NO3)3:Ln(NO3)3/H3PO4/HNO3 system as a guest unit and KIT-6 silica as a hard template host, and their textural and structure properties were characterized by X-ray diffraction, transmission electronic microscope and nitrogen adsorption at low temperature. The results show that the cubic ordered mesoporous YPO4 and YPO4:Ln3+ materials exhibit high surface area, large pore volume and uniform pore size distribution. Photoluminescence (PL) measurements exhibit their optical properties such as red, green and blue emission, and their PL intensities can be altered by the dopant Ln3+ concentration, without the need for doping or grafting external fluorophore species.

Biodiesel synthesis over the CaO–ZrO2 solid base catalyst prepared by a urea–nitrate combustion method

CaO–ZrO2 solid base catalysts with Ca/Zr ratios varying from 4/6 to 9/1 were prepared via a urea–nitrate combustion method and used in the transesterification of soybean oil with methanol to produce biodiesel. The catalysts were characterized using N2 adsorption, XRD, SEM and CO2-TPD techniques, and tested for biodiesel synthesis. The results show that a new phase of CaZrO3 has been formed for the investigated CaO–ZrO2 catalysts. With the increase in Ca/Zr molar ratio, the total basic sites over the catalyst increase and a maximum is obtained over the CaO–ZrO2 catalyst with a Ca/Zr ratio of 8/2. A similar variation trend of biodiesel yield is observed, suggesting that the catalytic activity correlates well with the total basic sites on the catalyst surface. Furthermore, the turnover frequency (TOF) has been calculated for various CaO–ZrO2 catalysts and the result revealed that the catalytic activity also depends on the strength of basic sites. The urea–nitrate combustion method was demonstrated to be a simple, fast and effective method for the preparation of CaO–ZrO2 solid base catalysts, which could be effectively applied for biodiesel synthesis.

Promotional role of ceria on cobaltosic oxide catalyst for low-temperature CO oxidation

Ceria modified Co3O4 catalysts for low temperature CO oxidation were prepared by a precipitation–oxidation method, and characterized by low-temperature N2 adsorption/desorption, TPR, O2–TPD, CO–TPD and transient–response reaction. The roles of ceria in CeO2–Co3O4 catalyst and the effect of pretreatment on the performance of CeO2–Co3O4 for CO oxidation were investigated in detail. The results show that the presence of CeO2 can increase its surface area, reduce the crystal size of Co3O4, and improve obviously the catalytic activity and stability of Co3O4 for CO oxidation, such as its T100 is only −60 °C. It was also found that the addition of CeO2 can not only promote the adsorption of O2 and the reaction of adsorbed CO with surface oxygen species to form CO2, but also increase the CO2 desorption speed. The pretreatment method can affect the catalytic activity of CeO2–Co3O4, the catalyst treated in N2 exhibits higher catalytic activity for low-temperature CO oxidation due to formation of oxygen vacancy. The catalyst reduced in H2 shows lower activity for CO oxidation although it has more surface oxygen vacancies, because of the difficult desorption of CO2 on the reduced CeO2–Co3O4 catalyst.

The role of iron oxide in the highly effective Fe-modified Co3O4 catalyst for low-temperature CO oxidation

A series of iron modified cobalt oxide catalysts (FeaCobOx (b:a = MCo:MFe, 10 < x < 15)) were prepared by a co-precipitation method, characterized by nitrogen adsorption–desorption, XRD, Raman spectroscopy, XPS, H2-temperature programmed reduction, CO-temperature-programmed desorption, O2-temperature-programmed desorption and time-resolved CO titration, and their catalytic activities for CO oxidation were evaluated. When Co:Fe is 8:2 (mol), the Fe2Co8Ox catalyst exhibits a very high catalytic activity, in which CO can be completely converted to CO2 at −80 °C. The results show that the addition of Fe to Co3O4 can increase its surface area and inhibit the agglomeration of iron oxide, improve the reduction behaviour of Co3O4, optimize the ratio of Co3+:Co2+ on the catalyst surface, and promote CO adsorption and CO2 desorption on the catalyst surface. The oxygen species on Fe2Co8Ox are more active than those on Co3O4, and when the feed gas is lacking in oxygen the lattice oxygen of Fe2Co8Ox can easily overflow to the surface to participate in the oxidation of CO.

Enhanced CO oxidation activity of CuO/CeO2 catalyst prepared by surfactant-assisted impregnation method

A modified CuO/CeO2 catalyst was prepared by surfactant-assisted impregnation method and showed better catalytic activity for low temperature CO oxidation than that from conventional impregnation method. The physicochemical properties of different CuO/CeO2 catalysts were characterized by thermogravimetric and differential scanning calorimetric measurements (TG-DSC), X-ray diffraction (XRD), N2 adsorption-desorption, Raman spectroscopy, H2 temperature-programmed reduction (H2-TPR), temperature-programmed desorption of O2 (O2-TPD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results suggested that the addition of hexadecyl trimethyl ammonium bromide (CTAB) into the impregnation solution could improve the dispersion of CuO species, which could facilitate Cu2+ incorporating into CeO2 lattice and strengthened the synergistic effects between CuO and CeO2, making the lattice oxygen more active, and eventually resulting in enhanced activity for CO oxidation.

Controlled synthesis and luminescent properties of assembled spherical YPxV1−xO4:Ln3+ (Ln = Eu, Sm, Dy or Tm) phosphors with high quantum efficiency

YPxV1−xO4 (x = 0.3–0.9) assembled spheres with tetragonal phase were prepared hydrothermally under a simple and mild method with the assistance of EDTA. The structures and shapes of the prepared samples were significantly affected by the reaction conditions (hydrothermal treatment time, organic additive, pH value and the amount of EDTA in the synthesis solution). The characteristic emission of the spherical YPxV1−xO4:Ln3+ (Ln = Eu, Sm, Dy or Tm) phosphors were investigated in detail. The results showed that the light color of the YP0.3V0.7O4 microspheres can be easily adjusted by doping different lanthanide activators; Y0.93Sm0.07P0.3V0.7O4 and Y0.95Eu0.05P0.3V0.7O4 exhibited strong orange-red and red emission, respectively, and Y0.93Sm0.07P0.3V0.7O4 has a higher quantum efficiency of 76.8%. For the Y0.97Dy0.03PxV1−xO4 (x = 0.3–0.9) samples, both emission intensities of Dy3+ and VO43− increase with increasing the P amount, and the quantum efficiency of Y0.97Dy0.03P0.9V0.1O4 can reach 92%. It can also be found that the light color of the Y0.97Dy0.03PxV1−xO4 samples can be tuned by changing the ratio of P/V, and Y0.97Dy0.03P0.5V0.5O4 can emit white light under UV excitation.

Effects of alkaline-earth oxides on the performance of a CuO–ZrO2 catalyst for methanol synthesis via CO2 hydrogenation

CuO–ZrO2 catalysts doped with alkaline-earth oxides were prepared by a urea-nitrate combustion method. The catalysts were characterized with N2 adsorption, N2O titration, XRD, H2-TPR, XPS and CO2-TPD techniques and tested for methanol synthesis from CO2 hydrogenation. With the incorporation of alkaline-earth oxides, the copper surface area increases remarkably, whereas the reducibility of CuO in the catalyst decreases. The doping of alkaline-earth oxides leads to an increase in the strength and contribution of the strong basic site on the catalyst surface. The results of catalytic tests indicate that the conversion of CO2 depends not only on the copper surface area but also on the reducibility of CuO in the catalyst, and the latter is a predominant factor for CaO-, SrO- and BaO-doped CuO–ZrO2 catalysts. The selectivity to methanol is related to the basicity of the catalyst. Moreover, the influence of the doping amount of MgO on the properties of CuO–ZrO2 was investigated, and the optimum catalytic activity is obtained as the amount of MgO doping is 5 mol%.

Synthesis of cubic and hexagonal ordered mesoporous YVO4:Eu3+ and their photoluminescence properties

Cubic and hexagonal ordered mesoporous Eu3+-doped yttrium vanadate (YVO4:Eu3+) have been synthesized successfully through the nanocasting route with a Y(NO3)3/Eu(NO3)3/NH4VO3/HNO3/ethanol system as a guest unit and KIT-6 or SBA-15 silica as the hard template host, and were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM) and low-temperature nitrogen adsorption. The prepared YVO4:Eu3+ samples have the characteristic cubic (Ia3d) (or hexagonal (p6mm)) ordered mesostructure based on different hard templates, and possess high surface area, large pore volume and uniform pore size distribution. Photoluminescence (PL) measurement shows that the main red emission peaks of two mesoporous YVO4:Eu3+ samples appear at 618 nm, and different mesostructures lead to different optimum concentrations of Eu3+ dopant and different PL intensities. For the hexagonal mesoporous YVO4:Eu3+, when Eu3+ dopant is 5(mol)% the highest PL intensity can be reached; for the cubic one, optimized Eu3+ amount is 8 mol%. The interaction between Eu3+ ions in hexagonal (p6mm) mesoporous YVO4:Eu3+ is more active than that in cubic (Ia3d) mesoporous YVO4:Eu3+, thus the absorbed energy is dissipated by nonradiation; while the cubic mesoporous YVO4:Eu3+ has more Eu3+ luminescent sites, compared with the hexagonal mesostructure one.