Weizheng Weng

Oxidative dehydrogenation of propane over a series of low-temperature rare earth orthovanadate catalysts prepared by the nitrate method

A series of high‐purity rare earth orthovanadates were prepared by the nitrate method and found to be effective low‐temperature catalysts for the oxydehydrogenation of propane at 320°C, at which no reactions occurred over the catalysts reported in the literature, and, thus, may be of practical significance. The catalytic performances of LnVO4 (Ln = Y, Ce–Yb) at 500°C were much better than those of rare earth orthovanadate catalysts and also slightly exceeded that of magnesium orthovanadate Mg3(VO4)2 reported in the literature. LnVO4 (Ln = Y, Ce–Yb) materials were tetragonal active phases which could stabilize the existence of active sites for the oxydehydrogenation of propane. Some catalysts with a certain amount of LnVO3 reduced from LnVO4 (Ln = Ho–Yb) under reaction atmosphere exhibited better redox properties and catalytic performances possibly due to the existence of biphasic catalytic synergy. LaVO4 was a monoclinic unstable active phase, although its bulk structure did not change after reaction. The remarkable deactivation of the LaVO4 catalyst was probably due to that LaVO4 could not stabilize the existence of surface active sites.

Enhanced photodegradation activity of methyl orange over Z-scheme type MoO3–g-C3N4 composite under visible light irradiation

Novel Z-scheme type MoO3–g-C3N4 composites photocatalysts were prepared with a simple mixing–calcination method, and evaluated for their photodegradation activities of methyl orange (MO). The optimized MoO3–g-C3N4 photocatalyst shows a good activity with a kinetic constant of 0.0177 min−1, 10.4 times higher than that of g-C3N4. Controlling various factors (MoO3–g-C3N4 amount, initial MO concentration, and pH value of MO solution) can lead to the enhancement of the photocatalytic activity of the composite. Only MoO3 and g-C3N4 are detected with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) spectra. N2 adsorption and UV-vis diffuse reflectance spectroscopy (DRS) results suggest that the addition of MoO3 slightly affects the specific surface area and the photoabsorption performance. The transmission electron microscopy (TEM) image of MoO3–g-C3N4 indicates a close contact between MoO3 and g-C3N4, which is beneficial to interparticle electron transfer. The high photocatalytic activity of MoO3–g-C3N4 is mainly attributed to the synergetic effect of MoO3 and g-C3N4 in electron–hole pair separation via the charge migration between the two semiconductors. The charge transfer follows direct Z-scheme mechanism, which is proven by the reactive species trapping experiment and the ˙OH-trapping photoluminescence spectra.

Mechanistic study of partial oxidation of methane to synthesis gas over supported rhodium and ruthenium catalysts using in situ time-resolved FTIR spectroscopy.

Abstract In situ time-resolved FTIR spectroscopy was used to study the reaction mechanism of partial oxidation of methane to synthesis gas and the interaction of CH 4 /O 2 /He (2/1/45) gas mixture with adsorbed CO species over SiO 2 and γ-Al 2 O 3 supported Rh and Ru catalysts at 500–600°C. It was found that CO is the primary product for the reaction of CH 4 /O 2 /He (2/1/45) gas mixture over H 2 reduced and working state Rh/SiO 2 catalyst. Direct oxidation of methane is the main pathway of synthesis gas formation over Rh/SiO 2 catalyst. CO 2 is the primary product for the reaction of CH 4 /O 2 /He (2/1/45) gas mixture over Ru/γ-Al 2 O 3 and Ru/SiO 2 catalysts. The dominant reaction pathway of CO formation over Ru/γ-Al 2 O 3 and Ru/SiO 2 catalysts is via the reforming reactions of CH 4 with CO 2 and H 2 O. The effect of space velocity on the partial oxidation of methane over SiO 2 and γ-Al 2 O 3 supported Rh and Ru catalysts is consistent with the above mechanisms. It is also found that consecutive oxidation of surface CO species is an important pathway of CO 2 formation during the partial oxidation of methane to synthesis gas over Rh/SiO 2 and Ru/γ-Al 2 O 3 catalysts.

Catalytic performance, structure, surface properties and active oxygen species of the fluoride-containing rare earth (alkaline earth)-based catalysts for the oxidative coupling of methane and oxidative dehydrogenation of light alkanes

Abstract The fluoride-containing catalysts, mostly of the rare earth (alkaline earth)-based system, demonstrate good catalytic performances for the oxidative coupling of methane and oxidative dehydrogenation of ethane, propane as well as iso-butane. The results of structural analysis of the catalysts for oxidative coupling of methane show that the promoting effects of the fluoride in the catalysts may be principally related to the phase–phase interaction between fluoride and oxide. Compared to the corresponding alkaline earth oxide promoted rare earth oxide catalyst system, an alkaline earth fluoride-promoted rare earth oxide catalyst system is less basic and will therefore be favorable to reduce CO2 inhibition in the reaction of methane oxidative coupling. However, there is no simple correlation between the acidity/basicity of a methane oxidative coupling catalyst and its catalytic performance. In the experiments of in situ spectroscopic characterizations carried out at the temperature from 650°C to 800°C, O−2 species was detected over five fluoride-containing rare earth (alkaline earth)-based catalysts for methane oxidative coupling reaction, and the reactions between O−2 species and CH4 to form C2H4 and the corresponding side-products were observed using in situ IR over four catalysts, which suggest that O−2 is probably the active oxygen species for the methane oxidative coupling reaction over the corresponding catalysts.

The concentration of oxygen species over SiO2-supported Rh and Ru catalysts and its relationship with the mechanism of partial oxidation of methane to synthesis gas

The partial oxidation of methane (POM) to synthesis gas over SiO2-supported Rh and Ru catalysts was studied by in situ microprobe Raman and in situ time-resolved FTIR (TR-FTIR) spectroscopies. The results of in situ microprobe Raman spectroscopic characterization indicated that no Raman band of Rh2O3 was detected at 500 °C over the Rh/SiO2 catalyst under a flow of a CH4/O2/Ar (2/1/45, molar ratio) mixture, while the Raman bands of RuO2 can even be detected at 600°C over the Ru/SiO2 catalyst under the same atmosphere. The experiments of in situ TR-FTIR spectroscopic characterizations on the reactions of CH4 over O2 pre-treated Rh/SiO2 and Ru/SiO2 catalysts indicated that the products of CH4 oxidation over Rh/SiO2 and Ru/SiO2 greatly depend on the concentration of O2− species over the catalysts. On the catalysts with high concentration of O2−, CH4 will be completely oxidized to CO2. However, if the concentration of O2− species over the catalysts is low enough, CH4 can be selectively converted to CO without the formation of CO2. The parallel experiments using in situ TR-FTIR spectroscopy to monitor the reaction of the CH4/O2/Ar (2/1/45, molar ratio) mixture over Rh/SiO2 and Ru/SiO2 catalysts show that the mechanisms of synthesis gas formation over the two catalysts are quite different. On the Rh/SiO2 catalyst, synthesis gas is mainly formed by the direct oxidation of CH4, while on the Ru/SiO2 catalyst, the dominant pathway of synthesis gas formation is via the sequence of total oxidation of CH4 followed by reforming of unconverted CH4 with CO2 and H2O. The significant difference in the mechanisms of partial oxidation of CH4 to synthesis gas over Rh/SiO2 and Ru/SiO2 catalysts can be well related to the difference in the concentration of O2− species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the two metals.

Photodegradation of RhB over YVO4/g-C3N4 composites under visible light irradiation

A series of novel YVO4/g-C3N4 photocatalysts were prepared by a facile mixing and calcination method. The obtained composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, ultraviolet visible diffuse reflection spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and a photocurrent–time experiment. The rhodamine B dye was selected as a model pollutant to evaluate the photocatalytic activity of the as-prepared YVO4/g-C3N4 composite. It shows that the photocatalytic activity of g-C3N4 can be largely improved by the doping of YVO4. The optimal YVO4 content is determined to be 25.8 wt%; and the corresponding degradation rate is 2.34 h−1, about 2.75 folds that of pure g-C3N4. A possible mechanism of YVO4 on the enhancement of visible light performance is proposed. It suggests that YVO4 plays a key role, which may lead to efficiently suppressing the recombination of photogenerated charge carriers, consequently, improving the visible light photoactivity.

Comparative study on the mechanisms of partial oxidation of methane to syngas over rhodium supported on SiO2 and γ-Al2O3

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO2- and γ-Al2O3-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the in situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH4 to syngas is the main pathway of the POM reaction over Rh/SiO2 catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/γ-Al2O3 catalysts. The results of TPR characterization indicate that Rh supported on γ-Al2O3 is more difficult to reduce than Rh supported on SiO2. The IR experiments of CO adsorption over Rh/SiO2 and Rh/γ-Al2O3 after the POM reaction reveal that the surface of the Rh/γ-Al2O3 catalyst contains more partially oxidized rhodium (Rh+) species as compared to the Rh/SiO2 catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO2 and Rh/γ-Al2O3 catalysts can be related to the difference in the surface concentration of O2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.

Oscillations during partial oxidation of methane to synthesis gas over Ru/Al2O3 catalyst

Ministry of Science and Technology of China [2005CB221401]; National Natural Science Foundation of China [20873111]; Key Science & Technology Specific Projects of Fujian Province [2009HZ10102]

Sinter-Resistant Pd/SiO2 Nanocatalyst Prepared by Impregnation Method

National Basic Research Program of China [2010CB732303]; National Natural Science Foundation of China [20873111, 20923004, 21033006]; Key Scientific Project of Fujian Province, China [2009HZ0002-1]

Active-oxygen species on non-reducible rare-earth-oxide-based catalysts in oxidative coupling of methane

From supplementary in situ Raman spectroscopic studies of active-oxygen species on non-reducible rare-earth-oxide-based catalysts in the oxidative coupling of methane (OCM) and structural adaptability considerations, further support has been obtained for our proposal that there may be an active and elusive precursor (of O 2− and O 22− adspecies), most probably O 32− formed from reversible redox coupling of an O2 adspecies at an anionic vacancy with a neighboring O2− in the surface lattice. This active precursor may initiate H abstraction from CH4 and be itself converted to OH−+O 2−, or it may abstract an electron from the oxide lattice and be converted to O 22−+O−. The prospect of developing this type of OCM catalysts is discussed.