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1.
Au/TiO2 catalysts prepared by a deposition–precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature‐independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion–temperature characteristics to the well‐known S‐shape, with activation energies slightly below 30 kJ mol?1. Sample characterization by XAFS and electron microscopy and a low‐temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas‐phase O2 at 90 K already over the freeze‐dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII‐containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0.  相似文献   

2.
Au/Mg(OH)2 catalysts have been reported to be far more active in the catalytic low‐temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO2 catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)2 with CO2 formed during the low‐temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO2 also catalyzes CO2 formation, but does not allow for CO2 desorption, which results in self‐poisoning. At higher temperatures (above 0 °C), however, CO2 formation is rate‐limiting, which results in a much higher activity for Au/TiO2 under these reaction conditions.  相似文献   

3.
The development of oxidation catalysts that are resistant to sulfur poisoning is crucial for extending the lifespan of catalysts in real-working conditions. Herein, we describe the design and synthesis of oxide-metal interaction (OMI) catalyst under oxidative atmospheres. By using organic coated TiO2, an oxide/metal inverse catalyst with non-classical oxygen-saturated TiO2 overlayers were obtained at relatively low temperature. These catalysts were found to incorporate ultra-small Pd metal and support particles with exceptional reactivity and stability for CO oxidation (under 21 vol % O2 and 10 vol % H2O). In particular, the core (Pd)-shell (TiO2) structured OMI catalyst exhibited excellent resistance to SO2 poisoning, yielding robust CO oxidation performance at 120 °C for 240 h (at 100 ppm SO2 and 10 vol % H2O). The stability of this new OMI catalyst was explained through density functional theory (DFT) calculations that interfacial oxygen atoms at Pd−O−Ti sites (of oxygen-saturated overlayers) serve as non-metal active sites for low-temperature CO oxidation, and change the SO2 adsorption from metal(d)-to-SO2(π*) back-bonding to much weaker σ(Ti−S) bonding.  相似文献   

4.
The oxidation of CO in the presence of hydrogen (PROX process) was investigated on bimetallic Au-Rh catalysts at 300–373 K by Fourier transform infrared spectroscopy and mass spectroscopy. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied. The IR studies revealed the formation of bi- and monodentate carbonates, bicarbonates and hydrocarbonates on the catalysts surfaces; these surface species proved to be not involved in the surface reactions. The formation of adsorbed formaldehyde was observed on all surfaces, except 1% (0.25Au+0.75Rh)/TiO2. Adsorbed CO2 (as the surface product of CO oxidation) was not detected on any surface. The presence of both O2 and H2 reduced the surface concentration of CO adsorbed on the metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO2 was formed in the highest amount in the CO+O2 mixture, the presence of H2 suppressed the amount of CO2 produced. This negative effect of H2 was the lowest on the 1% Rh/TiO2 and 1% (0.25Au+0.75Rh)/TiO2 catalysts.  相似文献   

5.
Platinum group metal (PGM) catalysts are the current standard for control of pollutants in automotive exhaust streams. Aside from their high cost, PGM catalysts struggle with CO oxidation at low temperatures (<200 °C) due to inhibition by hydrocarbons in exhaust streams. Here we present a ternary mixed oxide catalyst composed of copper oxide, cobalt oxide, and ceria (dubbed CCC) that outperforms synthesized and commercial PGM catalysts for CO oxidation in simulated exhaust streams while showing no signs of inhibition by propene. Diffuse reflectance IR (DRIFTS) and light‐off data both indicate low interaction between propene and the CO oxidation active site on this catalyst, and a separation of adsorption sites is proposed as the cause of this inhibition resistance. This catalyst shows great potential as a low‐cost component for low temperature exhaust streams that are expected to be a characteristic of future automotive systems.  相似文献   

6.
Summary. The thermal and photoassisted catalytic oxidation of CO at metal oxide supported RuO2·xH2O was studied at room temperature. Contrary to neat RuO2·xH2O the supported catalysts suffer from fast deactivation attributed to strong adsorption of the reaction product carbon dioxide. The latter can be efficiently removed from the catalyst surface at elevated temperatures. In some cases, i.e. for catalysts supported with selected n-type semiconductors (TiO2, SnO2, WO3), efficient CO2 desorption and good, constant catalytic activity was observed upon visible light irradiation. Under such conditions the CO to CO2 conversion observed for RuO2·xH2O/TiO2 was nearly as good and stable as for the unsupported catalyst. It is suggested that light absorption promotes carbon dioxide desorption through positive charging of the catalyst surface.  相似文献   

7.
Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs4[α‐SiW12O40]?n H2O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was ?67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Auδ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.  相似文献   

8.
林坚  王晓东  张涛 《催化学报》2016,(11):1805-1813
CO氧化可能是多相催化领域最常见的反应,它不仅能作为探针反应研究催化剂结构、反应活性位等,而且在诸多实际过程如空气净化、汽车尾气污染物控制、燃料电池所用氢源净化等扮演重要角色.最早的 CO氧化催化剂为霍加拉特剂,其组分主要为 CuO与 MnO2混合氧化物,然而在实际应用过程中存在低温活性低、吸湿易失活等缺点.1987年, Haruta等发现湿化学法制备的氧化物负载 Au催化剂表现出非常高的低温 CO氧化活性及耐水稳定性,其 Au粒子以纳米尺度分散,进而引发了催化研究领域的“淘金热”及纳米催化研究热潮.而 CO氧化通常作为考察 Au催化剂结构性质的探针反应,也成为考核其它金属催化剂是否具有高活性的判据之一. Pt族金属上 CO氧化反应从 Langmuir等研究开始至今已有100多年,然而低温下该金属催化剂活性与 Au催化剂相比要低一个数量级.本质原因为 Pt族金属上 CO吸附较强, O2吸附与活化受到抑制,而该步骤被认为是 CO氧化的速控步,因而表现出较低的催化活性.通常 Pt族金属催化剂需要100oC以上 CO才能脱附, O2进而得以吸附.目前研究人员采取多种策略,其基本原则为削弱 Pt族金属上 CO吸附强度或者提供其它活性位供 O2吸附与活化.本综述将概括近十年来Pt族金属催化剂 CO氧化研究进展,主要总结室温甚至超低温条件下的研究成果.高活性 CO氧化催化剂主要是通过采用可还原氧化物为载体或助剂,或者改变催化剂表面性质如使表面富 OH基物种来形成. Au催化剂的研究发现,改变金属粒子尺寸极有可能获得不同寻常的催化性能,而常规的 Pt族金属催化剂研究主要是在纳米尺度.近期人们发现逐渐减小 Pt族金属粒子尺寸,从纳米到亚纳米甚至单原子时,其电荷状态逐渐呈正价形式,这有利于削弱其 CO吸附强度.此外,可通过增强金属载体间的相互作用,改变金属载体接触方式,如从核壳到交叉结联结构,构筑出更多的金属载体界面,使得 O2更容易吸附与活化或稳定更多的 OH基物种进而在此界面与吸附的 CO反应.伴随着表征技术的发展, CO氧化机理的认识也更加深入,这给催化剂的设计带来更多新的思路.(1)改变 CO吸附活化位,将 CO吸附活化位从金属转移到载体上,从而大大降低 CO吸附强度,活化的 CO物种在反应过程中容易溢流到金属载体界面处,这甚至有利于超低温度下(–100oC左右) CO氧化.(2)改变 O2活化形式. O2通常在 Pt族金属上容易以解离氧原子形式存在,通过改变载体、金属载体界面性质使得 O2以分子氧形式活化,如形成超氧或过氧物种,这有利于降低 CO氧化的活化能垒,进而提高其低温甚至超低温下 CO氧化活性.今后,设计并合成出在超低温度下能够氧化 CO的 Pt族金属催化剂将成为 CO氧化催化剂研究的重要方向之一.  相似文献   

9.
An 48‐step sur face reaction mechanism with thermodynamic consistent kinetic data is presented for the catalytic conversion of the gaseous chemical system H2/O2/H2O/CO/CO2/CH4 over Rh/Al2O3 catalysts. Total and partial oxidation as well as steam reforming and dry reforming of methane over Rh catalysts is studied experimentally and numerically at varying temperature and composition. The results are used to extend the kinetic schemes we developed for H2 oxidation, CO oxidation kinetics, and the water‐gas‐shift reactions in former studies. Aside from the experiments in a stagnation‐flow reactor presented here, we modeled a number of experiments from the literature to test the newly established kinetic scheme.  相似文献   

10.
Mn–TiO2 catalysts were utilized as an ozonation catalyst for the first time to study the simultaneous catalytic ozonation of Hg0 and NO at low flue gas temperatures. BET, SEM–EDS, XRD, XPS, H2-TPR, NO x -TPD and Hg0-TPD were used to characterize the catalysts. The Mn–TiO2 catalyst, in which the molar content of metal Mn was 60%, exhibited the best catalytic activities of Hg0 and NO oxidation, compared with other Mn–TiO2 catalysts. It was found that within the range of experiment, the catalytic ozonation efficiency of Hg0 and NO was higher than that of ozonation or catalytic oxidation. The results also showed that the presence of NO gas inhibited the catalytic ozonation of elemental mercury, and the inhibition was enhanced with the NO inlet concentration, while few elemental mercury molecules did promote the catalytic ozonation of NO. The addition of H2O vapor promoted the catalytic ozonation of Hg0 and NO. In addition, 0.6Mn–TiO2 catalyst demonstrated a good TOS and cyclic stability. The catalytic ozonation of NO and Hg0 on Mn–TiO2 catalyst likely followed the Langmuir–Hinshelwood mechanism, where the hydroxyl radicals reacted with adjacently adsorbed NO molecules and elemental mercury on catalyst surface.  相似文献   

11.
Spatially and temporally resolved energy-dispersive EXAFS (EDE) has been utilised in situ to study supported Rh nanoparticles during CO oxidation by O2 under plug-flow conditions. Three distinct phases of Rh supported upon Al2O3 were identified by using EDE at the Rh K-edge during CO oxidation. Their presence and interconversion are related to the efficiency of the catalysts in oxidising CO to CO2. A metallic phase is only found at higher temperatures (>450 K) and CO fractions (CO/O2 > 1); an oxidic phase resembling Rh2O3 dominates the active catalyst under oxygen-rich conditions. Below about 573 K, and in CO-rich environments, high proportions of isolated Rh(I)(CO)2 species are found to co-exist with metallic Rh nanoparticles. Alongside these discrete situations a large proportion of the active phase space comprises small Rh cores surrounded by layers of active oxide. Confinement of Rh to nanoscale domains induces structural lability that influences catalytic behaviour. For CO oxidation over Rh/Al2O3 there are two redox phase equilibria alongside the chemistry of CO and O adsorbed upon extended Rh surfaces.  相似文献   

12.
利用沉积沉淀法制备了Pt/TiO2催化剂, 将其在不同温度下焙烧, 以得到不同颗粒尺寸的Pt. 并将这些样品用于CO催化氧化反应以及反应动力学研究. 结果表明: 焙烧温度对催化剂有明显影响, Pt 颗粒尺寸随着焙烧温度的升高而增加; 与此同时, CO催化活性随焙烧温度的升高呈先增加后降低的趋势, 其中, 400℃焙烧的样品表现出最高的催化活性. 反应动力学结果表明, 催化剂上CO氧化反应表观速率方程为r=5.4×10-7pCO0.17pO20.36,说明在该催化剂上CO氧化遵循Langmuir-Hinshelwood机理. 同时, 对催化剂进行了CO化学吸附红外光谱和O2化学吸附表征. 结果表明, 随着焙烧温度的升高, 催化剂上CO和O2吸附量均呈现先升高后降低的趋势, 这与反应结果和反应动力学方程一致, 说明反应受到催化剂表面上CO和O2吸附浓度的影响. 而在400℃焙烧的催化剂上, CO和O2吸附量均最高, 因此其反应活性也最好. 这可能是焙烧过程影响了Pt 和TiO2之间的相互作用引起的.  相似文献   

13.
The effect of the microstructure of titanium dioxide on the structure, thermal stability, and catalytic properties of supported CuO/TiO2 and CuO/(CeO2-TiO2) catalysts in CO oxidation was studied. The formation of a nanocrystalline structure was found in the CuO/TiO2 catalysts calcined at 500°C. This nanocrystalline structure consisted of aggregated fine anatase particles about 10 nm in size and interblock boundaries between them, in which Cu2+ ions were stabilized. Heat treatment of this catalyst at 700°C led to a change in its microstructure with the formation of fine CuO particles 2.5–3 nm in size, which were strongly bound to the surface of TiO2 (anatase) with a regular well-ordered crystal structure. In the CuO/(CeO2-TiO2) catalysts, the nanocrystalline structure of anatase was thermally more stable than in the CuO/TiO2 catalyst, and it persisted up to 700°C. The study of the catalytic properties of the resulting catalysts showed that the CuO/(CeO2-TiO2) catalysts with the nanocrystalline structure of anatase were characterized by the high-est activity in CO oxidation to CO2.  相似文献   

14.
A series of large scale MxCo3?xO4 (M=Co, Ni, Zn) nanoarray catalysts have been cost‐effectively integrated onto large commercial cordierite monolithic substrates to greatly enhance the catalyst utilization efficiency. The monolithically integrated spinel nanoarrays exhibit tunable catalytic performance (as revealed by spectroscopy characterization and parallel first‐principles calculations) toward low‐temperature CO and CH4 oxidation by selective cation occupancy and concentration, which lead to controlled adsorption–desorption behavior and surface defect population. This provides a feasible approach for scalable fabrication and rational manipulation of metal oxide nanoarray catalysts applicable at low temperatures for various catalytic reactions.  相似文献   

15.
Supported Au catalysts for low-temperature CO oxidation were prepared by solvated metal atom impregnation (SMAI) and conventional impregnation (CI). X-ray photoelectron spectroscopy investigations indicated that gold in all the samples was in the metallic state. TEM and XRD measurements showed that the mean diameter of Au particles prepared by SMAI was smaller than that of those prepared by CI with the same gold content. Catalytic tests showed that SMAI catalysts had higher CO oxidation activity than CI catalysts with the same compositions. Both SMAI and CI Au/TiO2catalysts exhibited high activities in low temperature CO oxidation. Full CO conversion was obtained at 323 K for 3.1 wt.% Au/TiO2 (SMAI) catalyst, which displayed higher activity than the 3.1 wt.% Au/D-72(SMAI) and 3.1 wt.% Au/TiO2(CI). Although the sizes of gold particles prepared by the same method and supported on both TiO2 and resin were comparable, the Au/TiO2 catalysts showed significantly higher activities than the Au/resin catalysts with the same Au contents under the same reaction conditions. These results prove that not only the gold particle size, but also the support plays a key role in CO oxidation. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

16.
Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs4[α‐SiW12O40]⋅n H2O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was −67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Auδ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.  相似文献   

17.
The direct methanol fuel cell (DMFC) is considered as a promising power source, because of its abundant fuel source, high energy density and environmental friendliness. Among DMFC anode materials, Pt and Pt group metals are considered to be the best electrocatalysts. The combination of Pt with some specific transition metal can reduce the cost and improve the tolerance toward CO poisoning of pure Pt catalysts. In this paper, the geometric stabilities of PtFe/PdFe atoms anchored in graphene sheet and catalytic CO oxidation properties were investigated using the density functional theory method. The results show that the Pt (Pd) and Fe atoms can replace C atoms in graphene sheet. The CO oxidation reaction by molecular O2 on PtFe–graphene and PdFe–graphene was studied. The results show that the Eley–Rideal (ER) mechanism is expected over the Langmuir–Hinshelwood mechanism for CO oxidation on both PtFe–graphene and PdFe–graphene. Further, complete CO oxidation on PtFe–graphene and PdFe–graphene proceeds via a two‐step ER reaction: CO(gas) + O2(ads) → CO2(ads) + O(ads) and CO(gas) + O(ads) → CO2(ads). Our results reveal that PtFe/PdFe commonly embedded in graphene can be used as a catalyst for CO oxidation. The microscopic mechanism of the CO oxidation reaction on the atomic catalysts was explored.  相似文献   

18.
The effects of ceria and zirconia on the structure–function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ‐Al2O3) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ‐Al2O3 from [Ce(acac)3] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ‐Al2O3. The structure–function correlations of ceria and/or zirconia‐doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier‐transform spectroscopy/energy‐dispersive extended X‐ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time‐resolved, in situ conditions. CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ‐Al2O3 samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.  相似文献   

19.
An approach for significantly suppressing N2O formation in reduction of NO by NH3 over V2O5–WO3/TiO2 (VWT) catalyst has been studied by coating different amounts of a Fe-exchanged zeolite (FeZ) onto the catalyst. FeZ-promoted VWT samples were characterized using N2 sorption, X-ray diffraction (XRD) analysis, and NH3 adsorption/desorption techniques to understand the primary role of FeZ in lowering N2O production levels. At high temperatures (≥450 °C), VWT gave N2O production with high concentrations, while N2O formation was noticeably reduced when using FeZ-promoted catalysts, which also showed somewhat lower NO removal activities (<5 %) at all temperatures. N2 sorption and XRD measurements revealed no perceptible physical or chemical alterations of each constituent, even in VWT catalysts after FeZ coating following high-temperature calcination. Adsorption of NH3 on unpromoted and FeZ-promoted catalysts and subsequent desorption yielded very complicated spectra for N2O that might primarily come from NH3 oxidation, and the interaction between V–NO species at temperatures >580 °C. NO on neighboring sites seems to be produced via decomposition of N2O generated at lower temperatures. The FeZ in the promoted VWT catalysts could be responsible for N2O decomposition and N2O reduction with unreacted NH3 at temperatures >400 °C, thereby significantly lowering N2O emission levels. This promotional effect bodes well for use in many industrial deNO x applications.  相似文献   

20.
The effect of the state of hydrated surface of the bulk oxide photocatalysts, TiO2, CeO2, and ZnO on the rate of UV-induced oxidation of CO with atmospheric oxygen was studied. The activity of dehydroxylated catalyst samples evacuated at temperatures of >350 °C toward CO photooxidation decreases in the series CeO2 > ZnO ≈ TiO2, while that of partially hydrated samples after pretreatment at 20 °C changes in the order TiO2 > ZnO ≥ CeO2 ≈ 0. According to the results, the difference in the photocatalytic activity toward CO oxidation on the dehydrated ZnO, TiO2, and CeO2 catalysts is attributable to different concentrations of oxygen vacancies, which are formed more readily after high-temperature treatment on ZnO and CeO2 and thus promote higher rate of CO photooxidation. Using a new technique for recording transmittance IR spectra, it was found that photoirradiation in the presence of adsorbed water and O2 gives peroxides and hydroperoxides, with their concentrations decreasing in the series TiO2 >> ZnO >> CeO2. Most likely, these species are active intermediates of CO photooxidation with oxygen in the presence of adsorbed water. The hydrophobization effect was detected upon TiO2 modification with zinc, resulting in removal of surface acid sites capable of adsorbing water. The TiO2 modification with zinc increases the activity of CO photooxidation with respect to the oxidation catalyzed by samples pretreated at low temperatures (20—60 °C).  相似文献   

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