Catalytic behavior of YBa2Cu3O7-x in the partial oxidation of methanol to formaldehyde

The partial oxidation of methanol to formaldehyde was studied over YBa₂Cu₃O_7-x catalyst in a flow reactor. The structural change of YBa₂Cu₃O_7-x before and after the reaction was measured by means of XRD and iodometric titration method. The catalytic characteristics of YBa₂Cu₃O_7-x for the partial oxidation of methanol to formaldehyde was due to copper ions. It was found that Cu⁺² was responsible for the higher selectivity for formaldehyde.     

Characteristics of Y1Ba2Cu3O7-x high-T c superconductor prepared by partial melting process

The superconducting Y₁Ba₂Cu₃O_7-x (123) and the Y₂Ba₁Cu₁O₅(211) powders in this study were prepared by pyrophoric synthesis method with Y₂O₃ (99.9 %), BaCO₃ (99.9 %), and CuO (99.9 %) powders. Samples of 123 and 211 pellets were first prepared and then piled to have a 123/211/123 arrangement before a partial melting process was applied for phase change of the center piece (211) to 123 phase through a peritectic reaction. The process parameters were a melting temperature of 1,040–1,070 °C and the mass of the 123 piece ranging from 0.4 to 1.2 g. The superconductivity, such as critical temperature (T_c), and mass susceptibility(χ) of bulk 123 and 211 samples were measured by AC four point probe method and AC susceptometer, respectively. The experimental results can be summarized as follows: the best preparation condition in the range examined was a melting temperature of 1,060 °C and mass ratio [123(A)/211(B)] of 2:1 with melting time of 30 minutes to yield the T_c of 88.5 K.     

Partial oxidation of methane catalyzed by YBa2Cu3Ox in a solid state electrochemical reactor

The catalytic oxidation of methane was studied between 840 and 1110 K on a YBa₂Cu₃O* mixed-conducting electrode deposited on an yttria stabilized zirconia solid electrolyte. The lattice oxygen supplied electrochermically to the catalyst surface actively participated in the oxidation reaction, with CO as the major product, and CO₂, CH₃OH and HCHO as the minor products. The reaction rate was found to be of first order with respect to methane. The rate constant depended strongly on the electrochemically imposed bias. Under DC bias condition, the reaction rate was enhanced by as much as two orders of magnitude over that observed under open-circuit conditions. An Eley-Rideal mechanism for the interaction of CH₄ with surface oxygen is proposed as the rate-determining step.     

Ambient temperature oxidation of carbon monoxide using a Cu2Ag2O3 catalyst

The mixed copper–silver oxide, Cu₂Ag₂O₃, has been prepared by co-precipitation and tested for ambient temperature carbon monoxide oxidation. The catalyst demonstrated appreciable low temperature oxidation activity and the catalyst aged for 4 h was the most active. Carbon monoxide conversion increased with time-on-stream, reaching steady state after ca. 1000 min. Acomparison of the catalytic activity has been made with a representative sample of a high activity hopcalite, mixed copper/manganese oxide catalyst. On the basis of CO oxidation rate data corrected for the effect of catalyst surface area the Cu₂Ag₂O₃, aged for 4 h was at least as active as the hopcalite.     

Effects of MgO promoter on properties of Ni/Al2O3 catalysts for partial oxidation of methane to syngas

The effects of MgO promoter on the physicochemical properties and catalytic performance of Ni/Al₂O₃ catalysts for the partial oxidation of methane to syngas were studied by means of BET, XRD, H₂-TPR, TEM and performance evaluation. It was found that the MgO promoter benefited from the uniformity of nickel species in the catalysts, inhibited the formation of NiAl₂O₄ spinel and improved the interaction between nickel species and support. These results were related to the formation of NiO-MgO solid solution and MgAl₂O₄ spinel. Moreover, for the catalysts with a proper amount of MgO promoter, the nickel dispersiveness was enhanced, therefore making their catalytic performance in methane partial oxidation improved. However, the excessive MgO promoter exerted a negative effect on the catalytic performance. Meanwhile, the basicity of MgO promoted the reversed water-gas shift reaction, which led to an increase in CO selectivity and a decrease in H₂ selectivity. The suitable content of MgO promoter in Ni/Al₂O₃ catalyst was ∼7 wt-%. Translated from Journal of Fuel Chemistry and Technology, 2006, 34(4): 450–455 [译自: 燃料化学学报]     

Role of the surface state of Ni/Al2O3 in partial oxidation of CH4

The effect of gas phase O₂ and reversibly adsorbed oxygen on the decomposition of CH₄ and the surface state of a Ni/Al₂O₃ catalyst during partial oxidation of CH₄ were studied using the transient response technique at atmospheric pressure and 700°C. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H₂ can be produced from non‐selective oxidation of CH₄ by reversibly adsorbed oxygen which is more active in oxidizing CH₄ completely than NiO via the Rideal–Eley mechanism and both the conversions of CH₄ and O₂ and the selectivities to CO and H₂ are very low. Therefore, keeping the catalyst surface in the reduced state is the precondition of high conversion of CH₄ and high selectivities to CO and H₂. The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH₄. During partial oxidation of CH₄, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduced state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH₄, and the indirect oxidation mechanism is not able to gain dominance in the reaction under the experimental conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Synergistic Effect of Adsorption-Photocatalysis for Removal of Organic Pollutants on Mesoporous Cu2V2O7/Cu3V2O8/g-C3N4 Heterojunction

Cu₂V₂O₇/Cu₃V₂O₈/g-C₃N₄ heterojunctions (CVCs) were prepared successfully by the reheating synthesis method. The thermal etching process increased the specific surface area. The formation of heterojunctions enhanced the visible light absorption and improved the separation efficiency of photoinduced charge carriers. Therefore, CVCs exhibited superior adsorption capacity and photocatalytic performance in comparison with pristine g-C₃N₄ (CN). CVC-2 (containing 2 wt% of Cu₂V₂O₇/Cu₃V₂O₈) possessed the best synergistic removal efficiency for removal of dyes and antibiotics, in which 96.2% of methylene blue (MB), 97.3% of rhodamine B (RhB), 83.0% of ciprofloxacin (CIP), 86.0% of tetracycline (TC) and 80.5% of oxytetracycline (OTC) were eliminated by the adsorption and photocatalysis synergistic effect under visible light irradiation. The pseudo first order rate constants of MB and RhB photocatalytic degradation on CVC-2 were 3 times and 10 times that of pristine CN. For photocatalytic degradation of CIP, TC and OTC, it was 3.6, 1.8 and 6.1 times that of CN. DRS, XPS VB and ESR results suggested that CVCs had the characteristics of a Z-scheme photocatalytic system. This study provides a reliable reference for the treatment of real wastewater by the adsorption and photocatalysis synergistic process.     

The catalytic oxidation of CO on superconductor Ba2YCu3O7–8

The measurements of MS-TPD, TPRS, the electrical conductivity and kinetics on Ba₂YCu₃O_7-8 show that the catalytic activity of CO oxidation is closely related to properties such as the amounts and sites of oxygen, and electrical conductivity. Based on the experimental results a reaction mechanism has been suggested.     

Catalytic hydrogenation of CO over the doped perovskite oxide YBa2Cu3O7-x catalysts

Perovskite oxide structured YBa₂Cu₃O₇-x(YBCO) has been first prepared by carbonate precipitation and then modified with palladium or ruthenium by impregnation on the perovskite oxide, while cobalt was co-precipitated simultaneously in the same pH range with perovskite oxide. After characterization the catalysts were used in the temperature range 300–450°C, in the pressure range 1–9 atmospheres and for H₂/CO ratios in the range 1–4 in a differential plug flow reactor for the hydrogenation of carbon monoxide to give hydrocarbons. The perovskite oxide (YBCO) 20% (w/w) and doped 2% (w/w) cobalt oxide catalyst were prepared by the wet chemical method from their nitrate solutions and oxidized at 950°C. Perovskite oxide (Dursun, G. & Winterbottom, J. M., J. Chem. Technol Biotechnol. 63 (1995) 113–16) was also doped with palladium and ruthenium metal by impregnation followed by oxidation at 250°C. The catalysts prepared were characterized by using TemperatureProgrammed Reduction (TPR) to observe the reduction temperature and also to measure total and metal surface area. The modified perovskite oxide on alumina, ruthenium- and cobalt-doped catalysts, has been shown to give a better conversion and also selectivity towards saturated hydrocarbons compared with palladium-doped catalyst. The temperature effect of these catalysts is more consistent, giving a steady increase of conversion with increasing temperature. Although increase of pressure increases the conversion, it causes very little change in product distribution. The activation energy of palladium- and ruthenium-doped, and cobalt co-precipitated catalysts for the reaction has been measured to be 55 kJ mol⁻¹, 75 kJ mol⁻¹ and 50 kJ mol⁻¹ respectively. A general rate equation of the form r=k[H₂]^m[CO]ⁿ has been observed and found to be applicable at the pressures and temperatures used for the catalytic system studied and found to be m≌1·0 for palladium-doped, m≌1·2 for ruthenium-doped and m≌0·95 for cobalt co-precipitated catalysts as n becomes zero or negligibly less than zero. The mechanism of reaction to produce hydrocarbons from syngas has been deduced from the results. It appeared that the carbon monoxide insertion mechanism has been more evident for palladium-doped catalysts whereas the carbide mechanism plays the main role for the ruthenium-doped and cobalt co-precipitated catalysts. © 1998 Society of Chemical Industry     

ESR, FT-Raman spectroscopic and ethanol partial oxidation studies on MoO3/SnO2 catalysts made by metal oxide vapor synthesis

A series of SnO₂-supported MoO₃ catalysts were prepared by the metal oxide vapor synthesis (MOVS) technique. ESR studies indicated the presence of highly dispersed Mo⁵⁺ species in both octahedral and tetrahedral coordination environments at all the loadings studied. At the highest MoO₃ loading of 12 wt%, the formation of MoO₃ microcrystallites was indicated from the lower intensity of the ESR signal. Raman studies also showed the presence of well dispersed surface molybdate species up to 4.4 wt% MoO₃ loading, and the peaks corresponding to microcrystallites of molybdena were observed at 12 wt% MoO₃ loading. The ethanol partial oxidation activities of the catalysts increased with increase in MoO₃ loading and the catalyst with 4.4 wt% molybdena content showed the highest activity; all the MOVS catalysts showed 100% selectivity to acetaldehyde at low conversions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure Characteristics and NO Selective Catalytic Reduction Property of Sn x Zr1-x O2 Solid Solution Catalysts

Novel catalysts, Sn_xZr_1-xO₂ solid solutions, for NO selective catalytic reduction:NO SCR) are reported. They have much higher activity and selectivity than SnO₂ and ZrO₂. Sn⁴⁺ is the main reductive sites as proved by TPR. The dilution of Sn sites by the coexisting Zr causes a suppression of propene combustion and consequently promoted the selective reduction of NO. The rutile structure might be beneficial to NO SCR.     

Partial oxidation of ethane by in situ generated H2O2

Selective partial oxidation of ethane to ethanol and acetaldehyde by in situ generated H₂O₂ has been achieved under cathodic current passing through a carbon supported Nafion-H catalytic membrane. A correlation between H₂O₂ generation rate and reaction rate has been found.     

Catalytic Properties of Cr2O3 Doped with MgO Supported on MgF2 and Al2O3

Catalytic properties of Cr₂O₃ supported on MgF₂ or Al₂O₃ have been modified by magnesium oxide. The catalysts have been obtained by the co-impregnation method and characterised by: BET, XRD and TPR. As follows from the results, the oxides supported on magnesium fluorine react with each other already at 400 °C, leading to formation of an amorphous spinel-like phase. On the Al₂O₃ support such an MgCr₂O₄ spinel has appeared at much higher temperatures. The addition of magnesium oxide has a significant effect on the activity and selectivity of the catalysts studied in the CO oxidation reaction at room temperature and in the reaction of cyclohexane dehydrogenation. The magnesium–chromium catalysts supported on MgF₂ have been found to show much higher activity and selectivity than the analogous systems supported on Al₂O₃.     

Effect of the metal oxide loading on the activity of silica supported MoO3 and V2O5 catalysts in the selective partial oxidation of methane

Precipitated silica catalysts loaded with either MoO₃ (0.2–4.0 wt%) or V₂O₅ (0.2–5.3 wt%) have been studied in the selective partial oxidation of methane to formaldehyde with molecular oxygen at 520 °C. The functionality of the SiO₂ surface towards the formation of HCHO is significantly promoted by V₂O₅, while it is depressed by the MoO₃.     

Catalytic partial oxidation of methane to syngas in a fixed-bed reactor with an O2-distributor: The axial temperature profile and species profile study

Catalytic partial oxidation of methane (CPOM) to syngas has been investigated in a fixed-bed reactor with an O₂-distributor (FR-OD). The axial temperature profile and species profile along the Ni or Rh-based catalyst bed have been measured at different conditions. As the O₂ was distributed radially into the catalyst bed through several rows of holes arranged at the special zone of the OD, a microenvironment maintaining a low O₂/CH₄ ratio (0.10–0.22) was provided in the catalyst bed. The hotspot phenomena appeared at the entrance of the catalyst bed have been effectively controlled. A more uniform temperature profile along the catalyst bed has been given, which is beneficial to the stability of catalyst and the safety of reactor operation.     

The catalytic activity of the ceramic superconductor YBa2Cu3O7 − δ

The superconductivity of the ceramic solid YBa₂Cu₃O_{7 ? δ}, prepared by heating Y₂O₃, BaCO₃ and CuO at 920°C for 22 h, was checked by the Meissner effect. Chemical analysis established the formula of the compound prepared. A thermostated aqueous suspension of the superconductor, treated with a solution of H₂O₂, produced oxygen, whose volume was measured at intervals in a gas burette. From the initial rates of the first-order reaction at different temperatures, the activation enthalpy and entropy were 16 kJ mol^?1 and ?210 J K^?1 mol^?1 respectively. The rates of gas evolution were similar to those obtained when a MnO₂ sample was used as a catalyst. Neither the superconductor nor its semiconductor phase photocatalysed the decomposition of KMnO₄ solution. Evidence of the catalysed decomposition of N₂H⁺₅ by the superconductor is presented.     

Semiconductive and Redox Properties of Ti and Zr Pyrophosphate Catalysts (TiP2O7 and ZrP2O7). Consequences for the Oxidative Dehydrogenation of n-Butane

The electrical conductivity of titanium and zirconium pyrophosphates used as catalysts in n-butane oxidative dehydrogenation has been measured under oxygen and n-butane at 400 and 500 °C and under subsequent exposures to both gases at the catalytic reaction temperature. The two compounds appeared to be p-type semiconductors under air with positive holes as the main charge carriers but became n-type when contacted with n-butane. If their conductivities are comparable as p-type semiconductors (within one order of magnitude), by contrast, they differ by 3 orders of magnitude when being n-type semiconductors. These results explain the difference in catalytic reaction mechanism encountered on the two solids. The alkane activation was proposed to be related in both cases to the p-type semiconducting properties of the solids, likely through hydrogen abstraction by a surface O^- species, forming a C₄H₉ radical which will similarly undergo a second hydrogen abstraction to form butenes. The changes in activation energy and in selectivity on TiP₂O₇ at higher temperatures (>450 C) are indicative of a change in mechanism, possibly with the transient formation of an alkoxide species.     

Novel Sn–Ce/Al2O3 Catalyst for the Selective Catalytic Reduction of NOx Under Lean Conditions

Effect of additives, Ce and Mn, on the catalytic performance of Sn/Al₂O₃ catalyst prepared by sol–gel method for the selective reduction of NO_x with propene under lean conditions was studied. Sn–Ce/Al₂O₃ catalysts exhibited higher activity than Sn/Al₂O₃ catalyst and the optimum Ce loading is 0.5–1%. The promoting effect of Ce is to enhance the oxidation of NO to NO₂ and facilitate the activation of propene, both of which are important steps for the NO_x reduction. The presence of oxygen contributes to the oxidation of NO and shows a promoting effect.     

The Partial Oxidation of CH3OH to CO2 and H2 Over a Cu/ZnO/Al2O3 Catalyst

The partial oxidation of CH₃OH to CO₂ and H₂ over a Cu/ZnO/Al₂O₃ catalyst has been studied by temperature-programmed oxidation (TPO) using N₂O and O₂ as the oxidant. Post-reaction analysis of the adsorbate composition of the surface of the catalyst was determined by temperature-programmed desorption (TPD). The temperature dependence of the composition of the mixture of products formed by TPO was shown to depend critically on the partial pressure of the oxidant, with the highest partial pressure of oxygen used (10% O₂ in He, 101 kPa—the CH₃OH partial pressure was 17% throughout), producing marked non-Arrhenius fluctuations on temperature programming. Unsurprisingly, therefore, the adsorbate composition of the catalyst revealed by post-reaction TPD was also found to be determined by the partial pressure of the oxidant. Using high partial pressures of oxidant (5% and 10% O₂ in He, 101 kPa), the only adsorbate detected was the bidentate formate species adsorbed on Cu. Lowering the oxygen partial pressure to 2% in He (101 kPa) revealed a catalyst surface on which the bidentate formate on Cu was the dominant intermediate with the formate on Al₂O₃ also being present. A further lowering of the partial pressure of the oxidant, obtained by using N₂O as the oxidant (2% N₂O in He, 101 kPa), resulted in a surface on which the formate adsorbed on ZnO was the dominant adsorbate with only a small coverage of the Cu by the bidentate formate.     

Changes in Ba Phases in BaO/Al2O3 upon Thermal Aging and H2O Treatment

The effects of thermal aging and H₂O treatment on the physicochemical properties of BaO/Al₂O₃ (the NO_x storage component in the lean NO_x trap systems) were investigated by means of X-ray diffraction (XRD), BET, TEM/EDX and NO₂ TPD. Thermal aging at 1000 °C for 10 h converted dispersed BaO/BaCO₃ on Al₂O₃ into low surface area crystalline BaAl₂O₄. TEM/EDX and XRD analysis showed that H₂O treatment at room temperature facilitated a dissolution/reprecipitation process, resulting in the formation of a highly crystalline BaCO₃ phase segregated from the Al₂O₃ support. Crystalline BaCO₃ was formed from conversion of both BaAl₂O₄ and a dispersed BaO/BaCO₃ phase, initially present on the Al₂O₃ support material after calcinations at 1000 and 500 °C, respectively. Such a phase change proceeded rapidly for dispersed BaO/BaCO₃/Al₂O₃ samples calcined at relatively low temperatures with large BaCO₃ crystallites observed in XRD within 10 min after contacting the sample with water. Significantly, we also find that the change in barium phase occurs even at room temperature in an ambient atmosphere by contact of the sample with moisture in the air, although the rate is relatively slow. These phenomena imply that special care to prevent the water contact must be taken during catalyst synthesis/storage, and during realistic operation of Pt/BaO/Al₂O₃ NO_x trap catalysts since both processes involve potential exposure of the material to CO₂ and liquid and/or vapor H₂O. Based on the results, a model that describes the behavior of Ba-containing species upon thermal aging and H₂O treatment is proposed.