二氧化碳在Pd/SiO2和La-Pd/SiO2上的催化氢化反应

对CO₂在附载型Pd催化剂上的氢化反应的研究迄今报道不多^[1,2],且都在加压条件下进行,对常压下助催剂如何改善Pd的催化行为未见报道。本文探讨了Pd/SiO₂催化剂,未加与掺加助催剂La(NO₃)₃后,其Pd粒大小、CO₂的吸附性能及CO₂/H₂催化活性等方面都有显著差异。     

Nucleation Phenomenon in Silica Xerogels and Pd/SiO2, Ag/SiO2, Cu/SiO2 Cogelled Catalysts

Pd/SiO₂, Ag/SiO₂ and Cu/SiO₂ xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Pd, Ag and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). It appears that, in cogelled samples, the metal complex acts as a nucleation agent in the formation of silica particles. The resulting catalysts are then composed of completely accessible metallic crystallites with a diameter of about 3 nm located inside silica porous particles with a monodisperse microporous distribution. Xerogels without metal synthesized with EDAS and TEOS (C. Alié, R. Pirard, A.J. Lecloux, and J.-P. Pirard, J. Non-Cryst. Solids 289, 88 (2001)) verify this hypothesis of nucleation by EDAS.     

Structure of Pd/SiO2 sol-gel and impregnated catalysts

Palladium catalysts were prepared by sol-gel and impregnation techniques both in acid and basic media. The sol-gel catalysts resulted in larger PdO crystallites than the impregnated catalysts. In both preparation methods, an acid medium promoted the formation of small metal crystallites which provided catalytic activity. A stronger metal-support interaction was found in sol-gel catalysts synthesized from a square planar palladium complex. Deactivation results in phenylacetylene hydrogenation have been attributed to palladium hydrate formation or to deposition of carbonaceous compounds. The preparation mode determined the contribution of each deactivating reaction.     

Studies on TiO2/SiO2 and Pd/TiO2/SiO2 Catalysts in Photoreduction of CO2 with H2O to Methanol

The development of industry induced a massive increase in the emission of carbon dioxide into the atmosphere. A large amount of CO₂ and its general availability causes that it could be a cheap reactant in a reaction that runs in a way similar to photosynthesis in plants. Pure TiO₂ and metal doped TiO₂ are the most studied semiconductor catalysts for photoreduction of CO₂. The TiO₂/SiO₂ and Pd/TiO₂/SiO₂ catalysts were prepared and studied by temperature-programmed desorption, X-ray diffraction analysis, SEM-EDS, temperature-programmed reduction and then used for the methanol synthesis. The photoactivity of Pd/TiO₂/SiO₂ catalysts in the reduction of CO₂ with H₂O was tested at room temperature using photoreactor equipped with 16 lamps. The wavelength was characteristic of near ultraviolet. Post-reaction products were identified with gas chromatograph equipped with the flame ionization detector. Pd doping made the catalysts photoactive and the photoactivity of catalysts was changing as follows: 1%Pd/5%TiO₂/SiO₂ > 1% Pd/10% TiO₂/SiO₂ > 1% Pd/15% TiO₂/SiO₂. Optimum ultraviolet radiation time in the photoreduction of CO₂ to methanol was 7 h. An addition of Pd does not change the surface of the carrier.     

Thallium as an Additive Modifying the Selectivity of Pd/SiO2 Catalysts

Catalytic properties of palladium and bimetallic palladium–thallium catalysts supported on SiO₂ in the reaction of glucose oxidation to gluconic acid were studied. Catalysts modified with thallium showed better selectivity and activity than palladium catalysts. XRD studies proved the presence of intermetallic interactions, which probably increase the selectivity of Pd–Tl/SiO₂ catalysts. Particular attention was paid to the losses of thallium and palladium from the catalysts during the catalytic reaction.     

Direct IR observation of vibrational properties of carbonyl species formed on Pd nano-particles supported on amorphous carbon: comparison with Pd/SiO2-Al2O3

By diluting optically opaque carbon-supported Pd particles in silica Aerosil we succeeded in observing the IR bands of adsorbed carbonyls and extracting information on the particle dispersion. Comparison with literature single crystal data and with silica-alumina supported Pd allowed us to make an assignment in terms of linear and 2-fold bridged carbonyls formed on Pd(111) and Pd(100) faces. Two Pd/C samples have been investigated. The relative intensities of the two carbonyl families observed on the two samples are consistent with the Pd dispersion independently measured with CO chemisorption, TEM and EXAFS analysis.     

Impact of palladium silicide formation on the catalytic properties of Pd/SiO2 catalysts in liquid-phase semihydrogenation of phenylacetylene

In this study, palladium silicide was formed on the sol–gel derived SiO₂ supported Pd catalysts when they were prepared by ion-exchange method using Pd(NH₃)₄Cl₂ as a palladium precursor. No other palladium phases (PdO or Pd⁰) were evident after calcinations at 450 °C for 3 h. The Pd/SiO₂ catalysts with Pd silicide formation were found to exhibit superior performance than commercial SiO₂ supported ones in liquid-phase semihydrogenation of phenylacetylene. From XPS results, the binding energy of Pd 3d of palladium silicide on the Pd/SiO₂ catalyst shifted toward larger binging energy, indicating that Pd is electron deficient. This could probably result in an inhibition of a product styrene on the Pd surface and hence high styrene selectivities were obtained at high phenylacetylene conversions. The formation of Pd silicide, however, did not have much impact on specific activity of the Pd catalysts since the TOFs were quite similar among the various catalysts with or without palladium silicides if their average particle sizes were large enough. The TOFs decreased by an order of magnitude when palladium dispersion was very high and their average particle sizes were smaller than 3–5 nm.     

State of palladium in deposited Pd/SiO2 catalysts for carbonylation of ethylene

Conclusions In deposited Pd/SiO₂ catalyst for the carbonylation of ethylene with CO the catalytically active form is Pd(O).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2627–2629, November, 1978.The authors are indebted to V. E. Shubin and V. A. Shvets for taking the EPR spectra.     

Pd/CexZr1-xO2/SiO2催化剂的制备及其甲烷催化燃烧性能的研究

采用浸渍法制备了Pd质量分数为0.1%~1.0%的Pd/SiO2和Pd/5%CexZr1-xO2/SiO2（x0.0~1.0）系列催化剂,在微型固定床反应器上对催化剂的甲烷催化燃烧性能进行了评价,用XRD、H2-TPR等分析测试技术对催化剂的结构进行了表征。结果表明,Pd/SiO2和Pd/CexZr1-xO2/SiO2催化剂都具有较好的甲烷催化燃烧活性,CexZr1-xO2可以明显提高催化剂的催化活性,并且Ce和Zr的比值对催化剂催化活性也有显著的影响。XRD和TPR显示,Pd/CexZr1-xO2/SiO2催化剂中的CexZr1-xO2对PdO的分散性和还原性有较好的促进作用。     

Hydrogen adsorption kinetics on Pd/Ce0.8Zr0.2O2

Hydrogen adsorption on Pd/Ce(0.8)Zr(0.2)O(2) was studied by temperature-programmed reduction, volumetric measurements and IR spectroscopy. Hydrogen uptake and reduction rate at 353 K are strongly dependent on the hydrogen pressure. At relatively high hydrogen partial pressure, reduction involves PdO, the surface and a significant fraction of the bulk of the ceria based oxide. Formation of oxygen vacancies even at low temperature (<373 K) is observed. The hydrogen adsorption process is mainly irreversible, as is shown by an increase in the (2)F(5/2)-->(2)F(7/2) electronic transition of Ce(3+) with hydrogen pressure and surface dehydroxylation. This "severe" reduction has a negative effect on the subsequent hydrogen adsorption capability. The decrease of hydrogen uptake capacity and rate during adsorption can be associated with the partial loss of superficial OH and the presence of Ce(3+), which deactivates Pd electronically.     

Thermal decomposition mechanism and kinetics of Pd/SiO2 nanocomposites in air atmosphere

Journal of Thermal Analysis and Calorimetry - SiO2 and Pd/SiO2 nanocomposites were prepared using sol–gel method and characterized by X-ray diffraction, Fourier transform infrared spectra,...     

Enhanced CO sensing properties of Pd modified ZnO porous nanosheets

Noble metal is usually used to improve the gas sensing performance of metal oxide semiconductor (MOS) due to its better catalytic properties. In this work, we reported a synthesis of Pd/ZnO nanocomposite by an in situ reduction with ascorbic acid (AA). It was found that Pd/ZnO sensor has excellent selectivity to CO and the response of the Pd/ZnO sensor towards 100 ppm CO was as high as 15 (R_a/R_g), obviously higher than that of the pristine ZnO sensor (1.4) when the working temperature is 220 °C. Moreover, the pure ZnO sensor almost has no selectivity to CO, but the Pd/ZnO sensor has excellent selectivity to CO, which may be ascribed to the electronic sensitization of Pd. Our present results demonstrate that the Pd can significantly improve the gas-sensing performance of metal oxide semiconductor and the obtained sensor has great potential in monitoring coal mine gas.     

Eosin Y/Pt/SiO2催化剂光催化还原水制氢

构建了具有较高可见光还原水制氢性能的Eosin Y/Pt/SiO₂催化体系, 详细考察了二氧化硅性质、曙红Y与二氧化硅的混合方式以及光照强度等因素对光敏化催化剂制氢性能的影响. 实验结果表明: 二氧化硅的比表面积增大, 析氢速率随之提高; 光照强度过高或过低都不利于提高光量子效率; 与曙红Y浸渍法吸附在二氧化硅表面制备的催化剂相比, 原位物理混合制备的催化剂光敏化析氢速率和稳定性均有显著提高.     

Cu/SiO2模型催化剂上甲醇部分氧化制氢反应研究

采用浸渍法，制备了不同组成的Cu/SiO2、Cu/Zn/SiO2催化剂，考察了Cu负载量及Cu/Zn比对甲醇部分氧化抽制氢（CH3OH|1/2O2→2H2 CO2）反应的影响，结果显示，当Cu的负载量为10％、Cu/Zn比为7：3时，催化剂活性最好。H2-TPR、XRD、XPS等表征结果表明，催化剂的制氢活性与Cu^0有关，而大量Cu^ 与Cu^2 的存在则不利于催化剂活性的提高。Zn助剂的引入，有利于分散Cu^0物种，提高催化剂的活性；但由于同时稳定了Cu^ 物种，导致Cu2O物种的大量生成，从而提高了催化剂的还原温度，抑制了Cu^0的氧化还原过程（Cu^2 →Cu^ →Cu^0或Cu^ →Cu^2 ），降低了催化剂的活性。因此，对于Cu/Zn/M催化剂，存在一个最佳的Cu/Zn比。     

Co-Ni／SiO2催化剂催化乙酸重整制氢反应研究

用浸渍法制备了一系列Co-Ni/SiO2催化剂,利用固定床反应器对催化剂在乙酸重整制氢反应中的催化性能进行了测试,详细研究了Co-Ni配比、活性组分负载量,反应温度,空速及水碳比对催化剂活性的影响.实验结果表明,该催化剂能高效催化乙酸重整反应.当Co和Ni摩尔比例为0.5∶1,活性组分质量百分含量为15%时催化剂呈现最好的催化活性,在水碳摩尔比为7.5∶1,温度大于500℃时可使乙酸完全转化,并且氢和二氧化碳选择性均达95%以上.     

Characterization of Hydrogen Adsorption on Palladium and Pd/c Using a Radiotracer

Hydrogen adsorption on palladium black and palladium supported on activated carbon was characterized by the Temperature Programmed Desorption technique (TPD) using the radiotracer tritium. It is shown that there are five energetic desorbed hydrogens with peak maxima at —50, 35, 115, 350, and 580°C. An additional peak at — 120°C hydrogen desorbed from activated carbon was observed on Pd/C. An isotopic hydrogen separation experiment was designed to demonstrate that hydrogen only dissociatively adsorbed on palladium surfaces whereas it was associatively adsorbed on activated carbon.     

Hydrogen peroxide sensing at MnO2/carbonized nanostructured polyaniline electrode

Manganese dioxide modified carbonized nanostructured polyaniline (MnO₂/Carb-nanoPANI) was prepared via a novel hydrothermal procedure. The synthesized material was characterized using XRD, SEM and TG-DTA analysis. Furthermore, MnO₂/Carb-nanoPANI was examined as electrode material for potential application in the field of electroanalysis. It showed a high electrocatalytic activity for the sensing of hydrogen peroxide in an aqueous media.     

A study of the redox properties of MoOx/SiO2

A sample of MoOx/SiO2, in which all of the Mo cations are present as isolated mono-oxo molybdate moieties, was prepared and investigated to understand the redox chemistry of such molybdate species and their ability to exchange oxygen with O2 and H2O. Raman spectroscopy was used to monitor the exchange of 18O for 16O in the Mo=O bond of isolated molybdate species, whereas mass spectrometry was used to follow the isotopic composition of the gaseous species, i.e., O2 and H2O. Reduction in H2 at 920 K results in the loss of one O atom per Mo atom, and consistent with this, all of the Mo(VI) cations are reduced to Mo(IV) cations. Raman spectroscopy shows that virtually all Mo=O bonds of the original molybdate species are lost upon reduction. While reoxidation of Mo(IV) cations by O2 is quantitative, studies using 18O2 reveal that only a small part of the newly formed Mo=O bonds are 18O labeled, and that the balance are 16O labeled, indicating that O-atom exchange between the support, SiO2, and the supported MoOx species occurs during reoxidation. Rapid exchange of O atoms was observed upon exposure of both bare SiO2 and MoOx/SiO2 to H2(18)O at 920 K, and the presence of MoOx species was found to enhance the rate of exchange. By contrast, very slow exchange of O atoms was observed when the oxidized catalyst was exposed to 18O2 at 920 K. In situ observations of the catalyst during exposure to a mixture of H2 and 18O2 at 920 K showed that all of the Mo cations remained in the VI oxidation state and that O atom exchange occurred at a rate comparable to that observed upon exposure to H2(18)O. The results of this investigation suggest that reoxidation of Mo(IV) cations following H2 reduction involves the formation of a Mo-peroxide species and subsequent O atom migration from such a species to the SiO2 support. It is proposed that the steady-state oxidation of H2 also involves the formation of Mo-peroxide species by interaction of O2 with a small number of Mo(IV) centers. The Mo-peroxide species are then rapidly reduced by H2 to form H2O and a Mo=O bond. The rapid exchange of O atoms between the gas phase and the catalyst observed during steady-state oxidation of H2 is attributed to interactions of the product H2O with the catalyst, rather than to O atom migration originating from the Mo-peroxide species formed on the catalyst surface.     

Mechanistic Study of Partial Oxidation of Methane to Syngas over c Pd/SiO2 Catalyst

Partial oxidation of methane (POM) to make syngas has been largely studied in recent years because of its potential to reduce the cost of syngas. Two reaction schemes have been proposed for the reaction:one is the sequence of combustion of CH₄ followed by reforming of unconverted CH₄ with CO₂ and H₂O,and the other is the direct partial oxidation of CH₄ to CO and H₂ without the experience of CO₂ and H₂O as reaction intermediates. In the industrial process, if the combustion-reforming mechanism predominantly contributes to the conversion of methane to syngas, severe heat management problems have to be taken into account. Therefore, the elucidation of the reaction pathway is of vital importance. In this paper, in situ time-resolved FTIR (in situ TR-FTIR) spectroscopy was used to study the POM reaction over lwt%Pd/SiO₂. The results of catalytic performance evaluation on the POM reaction over lwt%Pd/SiO₂ under different space velocity are also presented. It is expected that the additional proof can be presented to interpret POM mechanism.     

Mesoporous TiO2/SiO2 composite nanofibers with selective photocatalytic properties

Mesoporous TiO2/SiO2 composite nanofibers with a diameter of 100-200 nm and silica shell thickness of 5-50 nm have been fabricated by a sol-gel combined two-capillary co-electrospinning method; the composite nanofibers exhibited selective photocatalytic activity based on the decomposition of Methylene Blue, Active Yellow and Disperse Red.