纳米金催化剂在CO低温氧化和选择性氧化中的研究进展

介绍了纳米金催化剂在CO低温氧化和丙烯直接环氧化反应中的研究进展。在CO低温氧化反应中,催化剂的活性相和载体都具有明显的尺寸效应,纳米金颗粒和载体之间的相互作用主要表现载体不仅可以改变纳米金颗粒的大小和形状,而且也影响了氧的活化,从而提高反应活性;在丙烯直接环氧化反应中,由H2和O2在金颗粒表面反应生成的过氧化物种是反应中间体;在选择性氧化和选择性加氢反应中,金催化剂表现出优良的活性和稳定性。     

Selective CO oxidation in the presence of hydrogen over supported Pt catalysts promoted with transition metals

Selective CO oxidation in the presence of excess hydrogen was studied over supported Pt catalysts promoted with various transition metal compounds such as Cr, Mn, Fe, Co, Ni, Cu, Zn, and Zr. CO chemisorption, XRD, TPR, and TPO were conducted to characterize active catalysts. Among them, Pt-Ni/γ-Al₂O₃ showed high CO conversions over wide reaction temperatures. For supported Pt-Ni catalysts, Alumina was superior to TiO₂ and ZrO₂ as a support. The catalytic activity at low temperatures increased with increasing the molar ratio of Ni/Pt. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Ni and Pt was determined to be 5. This Pt-Ni/γ A1₂O₃ showed no decrease in CO conversion and CO₂ selectivity for the selective CO oxidation in the presence of 2 vol% H₂O and 20 vol% CO₂. The bimetallic phase of Pt-Ni seems to give rise to stable activity with high CO₂ selectivity in selective oxidation of CO in H₂-rich stream.     

负载银催化剂的氧性质和CO氧化活性

运用ＸＲＤ、ＴＰＤ、ＴＰＲ技术研究了催化剂Ａｇ／Ａｌ２Ｏ３、Ａｇ／ＣｅＯ２、Ａｇ／ＴｉＯ２的氧性质及ＣＯ氧化活性。Ａｇ／Ａｌ２Ｏ３催化剂的ＣＯ氧化活性最高。催化剂的ＣＯ氧化活性顺序与还原易难顺序相一致，但与催化剂氧脱出顺序没有对应关系。     

Chaos and synchronisation in heterogeneous catalytic systems: CO oxidation over Pd zeolite catalysts

The influence of experimental parameters on the structure of global reaction rate oscillations and the coupling of local oscillators on a catalyst bed in a continuous stirred tank reactor is studied for the oxidation of CO on zeolite supported palladium catalysts. Global coupling can be achieved via mass transfer through the gas phase or via heat transfer in the case of a support of high heat conductivity. Characteristic differences in the activity of catalysts as well as in the period and the amplitude of the oscillations are related to the size of the palladium clusters and can be simulated by adding the state of the oxidation of the metal surface as a parameter to a common kinetic model. The analysis of observed chaotic behaviour leads to the conclusion that diffusional chaos characteristic of a distributed system is observed on the level of the zeolite crystallite that supports the palladium clusters.     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

CO oxidation and oxygen-assisted CO adsorption/desorption on Ag/MnOx catalysts

A series of Ag-doped manganese oxide catalyst were synthesized by the reflux method in an acid medium. The surface structure of the catalysts was characterized by N₂ adsorption, XRD and TEM experiments. The catalysts showed excellent catalytic activity for CO oxidation. The adsorption and oxidation of CO on a 1.0% Ag/MnO_x catalyst between 393 and 493 K were studied by means of single pulse experiments in a TAP reactor. The adsorption of CO was reversible at these temperatures and CO₂ was formed in an oxidation reaction of CO and lattice oxygen. Curve fitting to the experimental TAP response curves of the reactant and product was used to determine the kinetic parameters for the elementary steps. The activation energies were 83 kJ/mol for CO desorption, 31 kJ/mol for CO₂ desorption, and 116 kJ/mol for the surface CO oxidation by lattice oxygen. In addition, the effect of coadsorbed O₂ on CO adsorption was studied by the TAP technique. Below 353 K, there was a sharp increase, by about one order of magnitude, in the rate constant of CO adsorption promoted by the presence of coadsorbed O₂.     

VOC deep oxidation over Pt catalysts using hydrophobic supports

The active hydrophobic-supported Pt catalysts were synthesized for VOC deep oxidation at low temperature (less than 200°C). The destruction of VOC could cost less at lower temperature due to less energy consumption. The advantage using hydrophobic support was that moistures from atmosphere and oxidation would not be adsorbed on the surface. Thus the active sites would not be cloaked and catalyst activity could be maintained, especially at low temperature. The hydrophobicity of supports was characterized by wetting angles. Porous SDB (styrene divinylbenzene copolymer) was found near 113°, indicating high hydrophobicity. Three Pt catalysts were prepared on SDB and activated carbons by incipient wetness method. Specific surface areas were measured by nitrogen adsorption. The thermal stability of SDB catalyst was examined by TGA, and found no degradation below 200°C in air. The surface compositions of catalysts were analyzed by EDS. XRD showed that Pt was well dispersed on supports after hydrogen reduction at 160°C. The chemical states of Pt were investigated by XPS, and suggested that the oxidized Pt^IV might be the active sites in the reaction. The deep oxidation of toluene/air mixture was carried out to test the activity of catalysts. Pt/SDB showed the highest activity among the catalysts and could completely oxidize 90 ppm toluene/air at VHSV=21 000 h⁻¹, 150°C. Redox mechanism was proposed to reveal the enhanced kinetic rates. The results suggested that the rate of toluene oxidation might be enhanced due to the fact that water, one of the products, was expelled from the hydrophobic surface.     

Preferential CO oxidation over supported Pt catalysts

Preferential CO oxidation reaction has been carried out at a gas hourly space velocity of 46,129 h^?1 over supported Pt catalysts prepared by an incipient wetness impregnation method. Al2O3, MgO-Al₂O₃ (MgO=30 wt% and 70 wt%) and MgO were employed as supports for the target reaction. 1 wt% Pt/Al₂O₃ catalyst exhibited very high performance (X _CO >90% at 175 ^°C for 100 h) in the reformate gases containing CO₂ under severe conditions. This result is mainly due to the highest Pt dispersion, easier reducibility of PtO _x , and easier electron transfer of metallic Pt. In addition, 1 wt% Pt/Al₂O₃ catalyst was also tested in the reformate gases with both CO₂ and H₂O to evaluate under realistic condition.     

Application of alumina supported gold-based catalysts in total oxidation of CO and light hydrocarbons mixture

Gold nanoparticles supported on alumina have been produced using the anionic exchange method and ammonia washing procedure. The catalysts are tested in the reaction of total oxidation of a mixture of light hydrocarbons and carbon monoxide in order to study the possibility of application in the reduction of cold start emissions. The obtained results are promising according to the temperature range observed for the oxidation of unsaturated hydrocarbons. The results obtained for acetylene confirms the difference of oxidation of this hydrocarbon over gold catalysts. An ageing procedure has been employed. This procedure does not affect the comportment of the catalysts versus hydrocarbon oxidation.     

Mesoporous Co-Al oxide nanosheets as highly efficient catalysts for CO oxidation

We report mesoporous Co-Al oxide nanosheets (Co_xAl-Ns, where x denotes the Co/Al ratio in the samples) prepared by calcination of CoAl-hydrotalcite and subsequent alkaline treatment. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy measurements show that the prepared Co-Al oxide nanosheets (Co_xAl-Ns) are very thin (10–15 nm) and exhibit high mesoporosity (3–5 nm). Catalytic CO oxidation tests reveal that the Co_xAl-Ns exhibit excellent catalytic performances at relatively low temperatures: for example, the Co_2.5Al-Ns catalyst could achieve 99% CO conversion at −98°C. Kinetic studies and experimental investigations indicate that the high activity of the Co_2.5Al-Ns sample is strongly related to the abundance of active sites associated with the large Brunauer–Emmett–Teller surface area. The Co_2.5Al-Ns catalyst also achieves full conversion of CO in tests performed with a gas mixture simulating automobile exhaust gas at 200°C. After loading the Co_2.5Al-Ns on a porous ceramic substrate, the obtained Co_2.5Al-Ns/PC shows high activity and stability in CO oxidation process. These features are potentially important for future industrial applications of these catalysts.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

Preferential CO oxidation over Pt/3A zeolite catalysts in H2-rich gas for fuel cell application

Even traces of CO in the hydrogen-rich gas fed to proton exchange membrane fuel cells (PEMFC) poison the platinum anode electrode and dramatically decrease the cell power output. In this work, several 1%Pt catalysts, all on 3A-type zeolite support, were prepared starting from different precursors and employing different methods: cation exchange (CE), wet impregnation (WI) and incipient wetness impregnation (IWI). The catalysts were characterised in terms of platinum dispersion and tested under realistic conditions in the quest of a catalyst for the removal of CO via the CO preferential oxidation (CO-PROX) reaction. The best catalytic performance was shown by the 1%Pt–3A catalyst prepared by IWI starting from Pt(NH₃)₄Cl₂ as a precursor, thanks to its highest platinum dispersion.     

微波法制备的Pt/C催化剂对甲醇和CO的电催化氧化

用微波间断升温法制备了3种Pt/C催化剂,运用循环伏安和线行扫描方法测试甲醇和吸附态CO在不同方法制备的Pt/C催化剂上的电催化氧化情况。发现在酸性溶液中,对于相同Pt载量的Pt(2)和Pt(3)催化剂,Pt(3)具有较小的Pt平均粒径及较高的电催化活性;对于具有较高Pt载量的Pt(1)催化剂,具有最小的平均粒径和最高的电催化活性。     

过渡金属对Pt/SiC催化剂上CO氧化性能的影响

以SiC为载体、Pt为活性组分、过渡金属Fe、Co和Ni为助剂,采用浸渍法制备CO氧化催化剂。考察浸渍方法、助剂及其负载量、空速和催化剂焙烧温度等对Pt/SiC催化剂性能的影响。结果表明,助剂的加入提高了活性组分Pt在载体表面的分散度,并产生一定的相互作用,从而提高了催化剂活性,其中,铁助剂的助催化效果较好。共浸渍法制备的催化剂的催化活性优于分步浸渍法,Pt-Fe/SiC催化剂制备中焙烧温度500 ℃时,催化剂活性较佳,适量Fe助剂的添加能够显著提高Pt/SiC催化剂的活性。     

Influence of an Fe promoter on silica-supported Pt/SnO x catalysts used for low-temperature CO oxidation

Silica-supported Pt/SnO_x catalysts used for low-temperature CO oxidation have been prepared without and with an Fe promoter. Reaction studies demonstrate that the addition of the Fe promoter results in higher catalytic activity in the presence of 8 at% CO₂ and a lower decay rate. Ion scattering spectroscopy (ISS) has been used to examine the outermost atomic layers of the promoted and nonpromoted catalysts before and after activation by a reductive pretreatment. The nonpromoted catalyst exhibits agglomeration of the platinized tin oxide film exposing the catalytically inactive silica support. This agglomeration does not occur when Fe is present, and a large catalytically active surface area is maintained during the reduction.     

Cu-Zr-Ce-O复合氧化物催化剂上CO选择性氧化性能

将共沉淀法制备的Cu-Zr-Ce-O复合氧化物催化剂应用于富氢气体中CO的选择性氧化反应,研究了ZrO₂掺杂量及预处理方法对催化剂性能的影响,并通过H₂-TPR、CO-TPR等手段对催化剂进行了表征.结果表明,掺杂ZrO₂的Cu₁Zr₁Ce₉O_δ催化剂在160～200℃之间具有99％以上的CO转化率和相对较高的选择性.掺杂适量的ZrO₂能够提高催化剂的热稳定性和储氧能力,促进催化剂表面吸附氧向晶格氧的转化.经氧气预处理的Cu₁Zr₁Ce₉O_δ催化剂活性最高.催化剂上Cu⁺/Cu²⁺氧化还原离子对和表面的晶格氧含量均影响催化剂的活性,但在富氢气氛下,表面的晶格氧对催化剂的性能影响较大.     

CO氧化催化剂研究进展

CO氧化反应是控制汽车尾气污染和理论研究的重要课题之一。贵金属催化剂活性好，寿命长，但由于资源缺乏，价格上涨，人们一直研究用非贵金属或少量贵金属来替代贵金属催化剂。稀土在CO氧化催化剂中的作用越来越引起人们的重视。介绍了贵金属、非贵金属和稀土催化剂。     

Negative activation energies in CO oxidation over an icosahedral Au/Mg(OH)2 catalyst

In the course of our studies on CO oxidation over Au/Mg(OH)₂ we have discovered a catalyst which exhibits an apparent negative activation energy when studied under ultra‐dry conditions (80 ppb moisture content). A review of current literature suggests that the oxidation of CO may occur by a reaction between CO and OH radicals and not by oxygen as previously thought. Substantial differences in catalytic behaviour between low and high temperature suggest that the reaction is complex and that more than one reaction pathway is present. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO selective oxidation in hydrogen-rich gas over platinum catalysts

The kinetics of CO oxidation in hydrogen-rich gas on Pt/mordenite (Pt/MD) or Pt/Al₂O₃ were investigated over a wide range of CO (0.4–1.8%) and O₂ concentrations (0.26–1.14%). The integral flow measurements showed that both the catalysts that could remove CO from 1% to ppm-level Pt/MD had a wider operation temperature range than Pt/Al₂O₃, especially towards lower temperatures.     

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.