Activation of Au/TiO2 catalyst for CO oxidation

Changes in a Au/TiO(2) catalyst during the activation process from an as-prepared state, consisting of supported AuO(x)(OH)(4-2x)(-) species, were monitored with X-ray absorption spectroscopy and FTIR spectroscopy, complemented with XPS, microcalorimetry, and TEM characterization. When the catalyst was activated with H(2) pulses at 298 K, there was an induction period when little changes were detected. This was followed by a period of increasing rate of reduction of Au(3+) to Au(0), before the reduction rate decreased until the sample was fully reduced. A similar trend in the activation process was observed if CO pulses at 273 K or a steady flow of CO at about 240 K was used to activate the sample. With both activation procedures, the CO oxidation activity of the catalyst at 195 K increased with the degree of reduction up to 70% reduction, and decreased slightly beyond 80% reduction. The results were consistent with metallic Au being necessary for catalytic activity.     

A density functional theory study of hydrogen dissociation and diffusion at the perimeter sites of Au/TiO2

Reactivity of supported gold catalysts is a hot topic in catalysis for many years. This communication reports an investigation on the dissociation of molecular hydrogen at the perimeter sites of Au/TiO(2) and the spillover of hydrogen atoms from the gold to the support using density functional theory calculations. It is found that the heterolytic dissociation is favoured in comparison with homolytic dissociation of molecular hydrogen at the perimeter sites. However, the surface oxygen of the rutile TiO(2)(110) surface at these sites can be readily passivated by the formed OH, suggesting that further dissociation of molecular hydrogen may occur at pure gold sites.     

Reactivity of molecularly chemisorbed oxygen on a Au/TiO2 model catalyst

We present results of an investigation into the reactivity of molecularly chemisorbed oxygen with CO on a Au/TiO2 model catalyst at 77 K. We previously discovered that exposing the model catalyst sample to a radio-frequency-generated plasma jet of oxygen results in co-population of both atomically and molecularly chemisorbed oxygen species on the sample. We tested the reactivity of the molecularly chemisorbed oxygen by comparing the CO2 produced from a sample populated with both species to the CO2 produced from a sample that has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. Samples that are populated with both species consistently result in greater CO2 produced than samples with only atomic oxygen. We interpret this result to indicate that molecularly chemisorbed oxygen on the sample can directly participate in the CO oxidation reaction. The reactivity of molecularly chemisorbed oxygen has been investigated for five different gold coverages (0.5, 0.75, 1, 1.25, and 2 ML), and we observe that there is a greater fractional difference in the CO2 produced (difference between sample populated with both molecularly and atomically adsorbed oxygen and sample populated solely with atomically adsorbed oxygen) for the 1 ML Au coverage than for the other coverages for equivalent oxygen plasma-jet exposures. However, it is not possible to unambiguously conclude that this observation is directly related to a particle size effect on the chemistry since the absolute O(2,a) and O(a) content on the various surfaces is different for all the coverages studied because of the plasma-jet technique that we employed for populating the surfaces with oxygen. Unfortunately, this precludes a direct comparison of the reactivity of molecular oxygen in the carbon monoxide oxidation reaction as a function of gold coverage and hence particle size.     

KOH改性对TiO2结构及其负载金催化剂CO氧化反应活性的影响

以不同浓度KOH处理钛酸丁酯水解产物得到KOH改性TiO2载体.使用N2吸附、热重-差热分析-示差扫描量热、X射线粉末衍射、紫外可见漫反射光谱、透射电镜及X射线光电子能谱研究了KOH对TiO2结构、形貌的影响.结果表明,较低浓度的KOH对TiO2有较显著的改性效果,改变了TiO2的电子结构.对KOH-TiO2负载的金催...     

Microstructured Au/TiO2 model catalyst systems

The preparation of microstructured Au/TiO2 model catalysts as a first step toward micrometer-scale parallel studies on model catalysts and toward studies of mesoscopic effects in catalytic reactions was investigated by atomic force microscopy and X-ray photoelectron spectroscopy. The model systems, which consist of micrometer-size active areas covered with Au nanoparticles that are separated by similarly sized inactive areas free of Au particles, are fabricated by combining optical lithography methods for microstructuring and ultrahigh vacuum evaporation for Au nanoparticle deposition and by applying suitable cleaning steps. It is demonstrated that practically perfect microstructures with Au nanoparticles of catalytically relevant sizes (2-3-nm diameter) on a clean TiO2 substrate can be produced this way and that the processing steps do not affect the deposited Au nanoparticles, neither in size nor in lateral distribution.     

Au/γ-MnO2催化剂催化无溶剂条件下甲苯氧化反应

采用氧化还原法制备了α, δ, γ-MnO₂载体, 采用原位还原法制备了Au负载量为0.5%-3.0%的Au/γ-MnO₂催化剂, 并采用X射线衍射、扫描电镜、透射电镜和N₂物理吸附等手段对其进行了表征. 透射电镜照片表明Au/γ-MnO₂催化剂中Au颗粒的大小约为10 nm. 采用无溶剂存在下的甲苯氧化反应测试所制备样品的催化活性. 结果表明, 甲苯转化率随着Au负载量的增加而增大. 这是由于Au颗粒数量增多, 尺寸减小的缘故. 同时, 负载Au颗粒对苯甲醛具有较高的选择性. Au/γ-MnO₂催化剂具有良好的重复使用性.     

Unexpected promotion of Au/TiO2 by nitrate for CO oxidation

The catalytic activity for Au/TiO2 for CO oxidation can be significantly enhanced by the addition of nitrates and this may relate to the variable catalyst performance observed in many studies.     

单晶Au表面和Au/TiO2（110）模型催化剂表面CO氧化反应机理和活性位

在分子尺度上介绍了Au/TiO2(110)模型催化剂表面和单晶Au表面CO氧化反应机理和活性位、以及H2O的作用.在低温(<320 K), H2O起着促进CO氧化的作用, CO氧化的活性位位于金纳米颗粒与TiO2载体界面(Auδ+–Oδ––Ti)的周边. O2和H2O在金纳米颗粒与TiO2载体界面边缘处反应形成OOH,而形成的OOH使O–O键活化,随后OOH与CO反应生成CO2.300 K时CO2的形成速率受限于O2压力与该反应机理相印证.相反,在高温(>320 K)下,因暴露于CO中而导致催化剂表面重组,在表面形成低配位金原子.低配位的金原子吸附O2,随后O2解离,并在金属金表面氧化CO.     

Au/TiO2催化剂上非催化CO氧化反应中活性氧物种的生成和消除

在定量的瞬时产物分析(TAP)反应器中,于80 oC下采用CO脉冲和O2脉冲补充等方法,研究了高温(400 oC)焙烧的Au/TiO2催化剂上活性氧物种的移除反应活性,特别是活性氧物种的性质。以往的研究大多关注的是CO催化氧化反应中活性氧物种及其性质,在典型的反应条件下该物种的形成和消除是可逆的;而本研究表明,催化剂直接焙烧后就存在额外的氧物种;该物种对CO氧化反应也具有活性,但其在典型的反应条件下不生成或生成很少。基于此,讨论了Au/TiO2催化剂上CO氧化反应的机理,特别是不同活性氧物种的作用。     

Electrocatalytic oxidation of nitric oxide at TiO2–Au nanocomposite film electrodes

Structured films of TiO₂ (anatase) nanoparticles (ca. 6 nm diameter) and gold nanoparticles (nominal 20 nm diameter) are formed via a layer-by-layer deposition procedure. TiO₂ nanoparticles are deposited with a Nafion polyelectrolyte binder followed by calcination to give a mesoporous thin film electrode. Gold nanoparticles are incorporated into this film employing a poly(diallyldimethylammonium chloride) polyelectrolyte binder followed by calcination to give a stable mesoporous TiO₂–gold nanocomposite. This methodology allows well-defined and structured films to be formed which are re-usable after a 500 °C heat treatment in air.Electrochemical experiments are performed in aqueous KCl and buffer solutions and for the oxidation of nitric oxide, NO, and nitrite in phosphate buffer solution. It is shown that the NO oxidation occurs as a highly effective electrocatalytically amplified process at the surface of the gold nanocomposite probably with co-evolution of oxygen, O₂. In contrast, the oxidation of nitrite to nitrate occurs at the same potential but without oxygen evolution. A mechanistic scheme for the amplified NO detection process is proposed.     

Dynamic electronic structure of a Au/TiO2 catalyst under reaction conditions

The electronic structure of a highly active Au/TiO2 powder catalyst was probed in situ by synchrotron X-ray photoelectron spectroscopy (XPS) in the 10-1 mbar range. The electronic structure of the Au component was found to respond sensitively to changes in temperature and indicated the absence of bulklike metallic Au under the conditions of highest catalytic activity. Concurrent modification of interfacial sites adjacent to Au on the TiO2 support was not evident from the Ti photoemission, but may have been below the detection limit of XPS.     

Adsorption of O2 and oxidation of CO at Au nanoparticles supported by TiO2(110)

Density functional theory calculations are performed for the adsorption of O2, coadsorption of CO, and the CO+O2 reaction at the interfacial perimeter of nanoparticles supported by rutile TiO2(110). Both stoichiometric and reduced TiO2 surfaces are considered, with various relative arrangements of the supported Au particles with respect to the substrate vacancies. Rather stable binding configurations are found for the O2 adsorbed either at the trough Ti atoms or leaning against the Au particles. The presence of a supported Au particle strongly stabilizes the adsorption of O2. A sizable electronic charge transfer from the Au to the O2 is found together with a concomitant electronic polarization of the support meaning that the substrate is mediating the charge transfer. The O2 attains two different charge states, with either one or two surplus electrons depending on the precise O2 adsorption site at or in front of the Au particle. From the least charged state, the O2 can react with CO adsorbed at the edge sites of the Au particles leading to the formation of CO2 with very low (approximately 0.15 eV) energy barriers.     

Oxygen-assisted reduction of Au species on Au/SiO2 catalyst in room temperature CO oxidation

An unexpected oxygen-assisted reduction of cationic Au species by CO was found on a Au/SiO(2) catalyst at room temperature and the produced metallic Au species plays an essential role in CO oxidation on Au/SiO(2).     

A key to the storage stability of Au/TiO(2) catalyst

The effect of Au(3+) percentage in Au/TiO(2) on its storage stability at room temperature was studied by varying the drying temperature and storage duration of a deposition-precipitation prepared Au/TiO(2) sample. Carefully-designed room temperature storage in a desiccator, in the dark to exclude any interference of light irradiation, was referenced to the freezing storage (255 K) in a refrigerator. The samples were characterized by well-calibrated H(2)-TPR, TEM and TG measurements. Reduction of Au(3+) ions and agglomeration of metallic Au particles were shown to be the main reasons for the deterioration of Au/TiO(2) during desiccator-storage. Correlating the percentage of Au(3+) ions, determined by H(2)-TPR, with the storage stability of Au/TiO(2) for CO oxidation at 273 K revealed that Au/TiO(2) samples with higher Au(3+) percentages (>90%) were much more stable during the desiccator-storage than those with higher percentages of metallic Au. Residual water in fresh Au/TiO(2) before storage showed a promotional effect on gold reduction and agglomeration during storage. By maximizing the percentage of Au(3+) ions and minimizing the residual water in the fresh sample, the deterioration of the Au/TiO(2) catalyst was successfully avoided during desiccator-storage of up to 150 days in dark. A possible mechanism of Au/TiO(2) deterioration during the desiccator-storage was also discussed.