Catalytic Oxidation of CO over Nanocrystalline La1–xCexNiO3 Perovskite‐Type Oxides

Nanocrystalline La_1–xCe_xNiO₃ (x = 0.1, 0.3, 0.5, 0.7, 0.9) perovskite‐type oxide catalysts prepared by the Pechini method were employed in catalytic CO oxidation and the effect of substitution of La by Ce on CO conversion was evaluated. The results indicated the remarkable effect of La substitution with Ce on the catalytic performance at low temperatures. The reaction temperature had a significant influence on the stability of the catalysts. The La_0.1Ce_0.9NiO₃ sample exhibited the highest activity among the prepared catalysts in CO oxidation reaction. In addition, the influence of different parameters including pretreatment condition, feed ratio, and gas hourly space velocity (GHSV) on the catalytic performance was examined. The optimum catalyst proved high stability under severe reaction conditions in the presence of water vapor and CO₂ in the feed stream.     

CO在铂催化剂上选择氧化的反应机理和本征动力学研究(续)

用气相色谱脉冲技术研究了在负载型铂催化剂上CO选择氧化过程的反应机理,获知了CO氧化和H2氧化两个平行竞争反应各自的反应历程和CO氧化具有高选择性的原因,并推导出动力学方程。采用最小容积准则的序贯实验设计法安排了动力学实验点。应用Gauss-Newton法对加权非线性最小二乘形式的目标函数进行了参数优化。对模型的残差分析证实了机理研究得到的动力学方程是合适的。     

CO_2 methanation over TiO_2–Al_2O_3 binary oxides supported Ru catalysts

TiO_2 modified Al_2O_3 binary oxide was prepared by a wet-impregnation method and used as the support for ruthenium catalyst. The catalytic performance of Ru/TiO_2–Al_2O_3catalyst in CO_2 methanation reaction was investigated. Compared with Ru/Al_2O_3 catalyst, the Ru/TiO_2–Al_2O_3catalytic system exhibited a much higher activity in CO_2 methanation reaction. The reaction rate over Ru/TiO_2–Al_2O_3 was 0.59 mol CO_2·(g Ru)1·h-1, 3.1 times higher than that on Ru/Al_2O_3[0.19 mol CO_2·(gRu)-1·h-1]. The effect of TiO_2 content and TiO_2–Al_2O_3calcination temperature on catalytic performance was addressed. The corresponding structures of each catalyst were characterized by means of H_2-TPR, XRD, and TEM. Results indicated that the averaged particle size of the Ru on TiO_2–Al_2O_3support is 2.8 nm, smaller than that on Al_2O_3 support of 4.3 nm. Therefore, we conclude that the improved activity over Ru/TiO_2–Al_2O_3catalyst is originated from the smaller particle size of ruthenium resulting from a strong interaction between Ru and the rutile-TiO_2 support, which hindered the aggregation of Ru nanoparticles.     

二氧化铈载金催化剂催化CO氧化的研究进展

将金纳米颗粒负载在二氧化铈上,形成负载型催化剂,能应用于多种反应体系,表现出良好的催化活性,有着广阔的发展前景。综述了二氧化铈载金催化剂在一氧化碳催化氧化中的应用研究进展,介绍了二氧化铈载金催化剂中金的结构、价态和纳米尺寸,载体二氧化铈的形貌、结构、尺寸和氧空位以及金和载体二氧化铈两者之间的相互作用对一氧化碳催化氧化性能的影响。     

负载型La2-xSrxCoO4类钙钛矿复合氧化物CO催化氧化性能

采用柠檬酸络合-浸渍法制备了以镁铝尖晶石MgAl2O4为载体、复合氧化物La2-xSrxCoO4(X=0.2、0.4、0.6和0.8)为活性组分的催化剂,通过XRD、H2-TPR和BET等分析方法对催化剂进行表征,利用固定床微型反应器测试催化剂对CO氧化的催化性能,并考察水蒸汽和SO2对其活性的影响.结果表明,X=0....     

Selective methanation of CO over supported Ru catalysts

The catalytic performance of supported ruthenium catalysts for the selective methanation of CO in the presence of excess CO₂ has been investigated with respect to the loading (0.5–5.0 wt.%) and mean crystallite size (1.3–13.6 nm) of the metallic phase as well as with respect to the nature of the support (Al₂O₃, TiO₂, YSZ, CeO₂ and SiO₂). Experiments were conducted in the temperature range of 170–470 °C using a feed composition consisting of 1%CO, 50% H₂ 15% CO₂ and 0–30% H₂O (balance He). It has been found that, for all catalysts investigated, conversion of CO₂ is completely suppressed until conversion of CO reaches its maximum value. Selectivity toward methane, which is typically higher than 70%, increases with increasing temperature and becomes 100% when the CO₂ methanation reaction is initiated. Increasing metal loading results in a significant shift of the CO conversion curve toward lower temperatures, where the undesired reverse water–gas shift reaction becomes less significant. Results of kinetic measurements show that CO/CO₂ hydrogenation reactions over Ru catalysts are structure sensitive, i.e., the reaction rate per surface metal atom (turnover frequency, TOF) depends on metal crystallite size. In particular, for Ru/TiO₂ catalysts, TOFs of both CO (at 215 °C) and CO₂ (at 330 °C) increase by a factor of 40 and 25, respectively, with increasing mean crystallite size of Ru from 2.1 to 4.5 nm, which is accompanied by an increase of selectivity to methane. Qualitatively similar results were obtained from Ru catalysts supported on Al₂O₃. Experiments conducted with the use of Ru catalyst of the same metal loading (5 wt.%) and comparable crystallite size show that the nature of the metal oxide support affects significantly catalytic performance. In particular, the turnover frequency of CO is 1–2 orders of magnitude higher when Ru is supported on TiO₂, compared to YSZ or SiO₂, whereas CeO₂- and Al₂O₃-supported catalysts exhibit intermediate performance. Optimal results were obtained over the 5%Ru/TiO₂ catalyst, which is able to completely and selectively convert CO at temperatures around 230 °C. Addition of water vapor in the feed does not affect CO hydrogenation but shifts the CO₂ conversion curve toward higher temperatures, thereby further improving the performance of this catalyst for the title reaction. In addition, long-term stability tests conducted under realistic reaction conditions show that the 5%Ru/TiO₂ catalyst is very stable and, therefore, is a promising candidate for use in the selective methanation of CO for fuel cell applications.     

Studies of the activation process over Pd perovskite-type oxides used for catalytic oxidation of toluene

Calcined and reduced catalysts Pd/LaBO₃ (B = Co, Fe, Mn, Ni) were used for the total oxidation of toluene. Easiness of toluene destruction was found to follow the sequence based on the T₅₀ values (temperature at which 50% of toluene is converted): Pd/LaFeO₃ > Pd/LaMnO_3+δ > Pd/LaCoO₃ > Pd/LaNiO₃. In order to investigate the activation process (calcination and reduction) in detail, the reducibility of the samples was evaluated by H₂-TPR on the calcined catalysts. Additionally, characterization of the Pd/LaBO₃ (B = Co, Fe) surface was carried out by X-ray photoelectron spectroscopy (XPS) at each stage of the global process, namely after calcination, reduction and under catalytic reaction at either 150 or 200 °C for Pd/LaFeO₃ and either 200 or 250 °C for LaCoO₃. The different results showed that palladium oxidized entities were totally reduced after pre-reduction at 200 °C for 2 h (2 L/h, 1 °C/min). As LaFeO₃ was unaffected by such a treatment, for the other perovskites, the cations B are partially reduced as B³⁺ (B = Mn) or B²⁺ even to B⁰ (B = Co, Ni). In the reactive stream (0.1% toluene in air), Pd⁰ reoxidized partially, more rapidly over Co than Fe based catalysts, to give a Pd²⁺/Pd⁴⁺ and Pd⁰/Pd²⁺/Pd⁴⁺ surface redox states, respectively. Noticeably, reduced cobalt species are progressively oxidized on stream into Co³⁺ in a distorted environment. By contrast, only the lines characteristic of the initial perovskite lattice were detected by XRD studies on the used catalysts. The higher activity performance of Pd/LaFeO₃ for the total oxidation of toluene was attributed here to a low temperature of calcination and to a remarkable high stability of the perovskite lattice whatever the nature of the stream which allowed to keep a same palladium dispersion at the different stages of the process and to resist to the oxidizing experimental conditions. On the contrary, phase transformations for the other perovskite lattices along the process were believed to increase the palladium particle size responsible of a lower activity.     

CO oxidation over alumina supported platinum catalyst

The oxidation of CO over Pt/Al₂O₃ has been studied using combined FTIR andin situ reaction cell. During reaction the stretching frequency of the adsorbed carbonyl species remained constant over a temperature range during which a change in the CO conversion occurred. The range of conversion during which this invariance was observed was considerably greater for used catalyst than for fresh Pt/Al²O³. The formation of islands of CO and the role of these in the overall reaction mechanism is discussed.     

Oxidation Reactions over Multi-Component Catalysts: Low-Temperature CO Oxidation and the Total Oxidation of CH4

A series of NM/MO _x /Al₂O₃ (NM = Pd, Ag, Pt, and Au) catalysts were prepared and tested in the oxidation of CO and CH₄. The catalysts were characterized with X-ray diffraction and transmission electron microscopy. Where addition of MO _x generally does not seem to affect the catalyst activity in CH₄ oxidation, a large enhancement in CO oxidation was observed. Fourier transform infrared spectroscopy has been used to identify the role of MO _x as a promoter for low-temperature CO oxidation. The results were found to support a Mars and van Krevelen type model.     

富氢气体中CO选择性氧化的研究进展

详细概述了富氢气体中CO选择性氧化反应的贵金属和非贵金属催化剂,简要介绍了反应条件对催化剂活性的影响,指出控制催化剂的尺寸是金催化剂研究的重点,铂和其它非贵金属的抗CO2性和抗水性还有待进一步改善;通过便宜的添加剂、合适的制备方法和处理条件等手段来提高催化剂活性和选择性是有效的途径。     

A Mechanistic Study of Low Temperature CO Oxidation over Cobalt Oxide

The CO oxidation over Co₃O₄ and Co₃O₄/Al₂O₃ has been investigated using flow reactor and in situ FTIR studies. Cobalt oxide shows very high activity even at room temperature. However, a gradual deactivation takes place during reaction. The deactivated catalyst shows the presence of two different carbonate species and one graphite-like species. A possible mechanism for the deactivation is discussed.     

The Ligand Effect: CO Desorption from Pt/Ru Catalysts

Density Functional Theory (DFT) calculations of the adsorption energy of CO, for a platinum overlayer on Ru(0001), have been performed. For all coverages a significant reduction in the binding energy of up to 0.5 eV has been observed compared to that obtained on Pt(111). In addition, a Steady‐State Isotopic Transient Kinetic Analysis (SSITKA) study has been performed to determine the desorption rate dependence on the partial pressure of CO over commercial Pt/Ru electrocatalysts. As expected, no significant difference in the rate of exchange of CO at any given pressure is observed on going from Pt to Pt/Ru electrocatalysts when the diluted gas used was argon since the CO states will be filled to the same desorption energy for the two catalysts. However on changing the diluent gas to hydrogen, a reduction in the exchange rate for CO is observed clearly reflecting the lower CO binding energy and the increased competition for sites at the surface of the catalyst. The reduction efficiency of the Pt/Ru electrocatalyst was also studied and found to be highly dependent on whether CO or hydrogen was used. These results will be discussed with reference to the anode catalysis of the Polymer Electrolyte Membrane Fuel Cell (PEMFC).     

Potential dependence of segregation and surface alloy formation of a Ru modified carbon supported Pt catalyst

Ruthenium modified carbon supported platinum catalysts have been shown to have a similar activity towards carbon monoxide oxidation as conventionally prepared bimetallic PtRu alloy catalysts. In this study the effect of the applied electrode potential and potential cycles on the location and oxidation state of the Ru species in such Ru modified Pt/C catalysts was investigated using in situ EXAFS collected at both the Ru K and Pt L₃ absorption edges. The as prepared catalyst was found to consist of a Pt core with a Ru oxy/hydroxide shell. The potential dependent data indicated alloying to form a PtRu phase at 0.05 V versus RHE and subsequent dealloying to return to the Ru oxy/hydroxide decorated Pt surface at potentials greater than 0.7 V. The Ru-O distances obtained indicate that both Ru³⁺ and Ru⁴⁺ species are present on the surface of the Pt particles at oxidising potentials; the former is characteristic of the as prepared Ru modified Pt/C catalyst and following extensive periods at potentials above 0.7 V and the latter of the Ru oxide species on the PtRu alloy.     

CO Oxidation Behavior of Copper and Copper Oxides

Carbon monoxide oxidation activities over Cu, Cu₂O, and CuO were studied to seek insight into the role of the copper species in the oxidation reaction. The activity of copper oxide species can be elucidated in terms of species transformation and change in the number of surface lattice oxygen ions. The propensity of Cu₂O toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enables Cu₂O to exhibit higher activities than the other two copper species. The non-stoichiometric metastable copper oxide species formed during reduction are very active in the course of CO oxidation because of its excellent ability to transport surface lattice oxygen. Consequently, the metastable cluster of CuO is more active than CuO, and the activity will be significantly enhanced when non-stoichiometric copper oxides are formed. In addition, the light-off behaviors were observed over both Cu and Cu₂O powders. CO oxidation over metallic Cu powders was lighted-off because of a synergistic effect of temperature rises due to heat generation from Cu oxidation as well as CO oxidation over the partially oxidized copper species.     

超临界二氧化碳介质中的烃类氧化反应

介绍了有关烷烃、烯烃、芳烃化合物在超临界CO2介质中进行的氧化反应。以超临界CO2介质代替有机溶剂的环境友好方式,并采用环境友好的氧化剂氧气或过氧化氢,研究烃类化合物的氧化反应,对于化工中间体的开发研究以及绿色石油化工的发展具有重要意义和应用价值。研究结果表明,以超临界CO2作为反应介质的氧化反应,具有广阔的发展前景。     

石墨烯负载铜在富氢条件下催化CO氧化的研究

石墨烯具有许多优良性质,在燃料电池和催化领域应用前景广阔。以石墨烯为载体负载不同含量的Cu在富氢条件下催化CO氧化的影响。     

Structure sensitivity of CO oxidation over model Au/TiO2 2 catalysts

Model catalysts of Au clusters supported on TiO₂ thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO₂ catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)^-1 × s^-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO₂(001)/Mo(100) is structure sensitive. This revised version was published online in July 2006 with corrections to the Cover Date.     

Partial oxidation of methane to synthesis gas over some titanates based perovskite oxides

Ca_1–x - _x Sr _x TiO₃-based mixed oxide catalysts containing chromium, iron, cobalt or nickel were prepared and used in the oxidation of methane. The catalyst containing cobalt or nickel showed high activity for the synthesis gas production from methane. In the case of nickel containing catalyst, nickel oxide originally separated from the perovskite structure was easily reduced to nickel metal, which showed synthesis gas production activity. In the case of the cobalt containing catalyst, pretreatment with methane was required for high activity. Reduced metallic cobalt was formed from the perovskite structure, which revealed relatively high selectivity for the oxidative coupling of methane, and afforded synthesis gas production. Both the catalysts also catalyzed carbon dioxide reforming of methane and especially both high activity and selectivity were observed over the nickel containing catalyst.     

CO oxidation over promoted Pt catalysts

A study of CO oxidation by O₂ over Pt catalysts, promoted by MnO_x and CoO_x, is described. The activities of Pt/SiO₂, Pt/MnO_x/SiO₂ and Pt/CoO_x/SiO₂ are compared with commercial Pt/Al₂O₃, Pt/Rh/Al₂O₃ and Pt/CeO_x/Al₂O₃ catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found: Pt/CoO_x/SiO₂ > Pt/MnO_x/SiO₂, Pt/CeO_x/Al₂O₃ > Pt/Al₂O₃, Pt/Rh/Al₂O₃, Pt/SiO₂. Over Pt/CoO_x/SiO₂ CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoO_x/SiO₂ in the CO/O₂ reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoO_x/SiO₂, Pt/MnO_x/SiO₂ or Pt/CeO_x/Al₂O₃ catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoO_x/SiO₂ catalyst in CO oxidation.     

Pd/Al2O3催化剂上低浓度CO氧化的研究

采用共沉淀法和浸渍法制备了2%Pd-Al2O3催化剂,在固定反应器中评价不同制备方法(共沉淀和浸渍)、助剂(1%K和1%Mg)、预处理方法(快速氧化和缓慢还原)、空速(726h-1、1638h-1和5274h-1)情况下2%Pd-Al2O3催化剂对CO氧化的催化活性。结果表明,共沉淀法制备的2%Pd50%Zr-Al2O3催化剂有较高的反应活性,该催化剂经缓慢还原预处理活化,在温度为70℃时就可以使低浓度的CO完全氧化。