Catalytically Active Rh Sub‐Nanoclusters on TiO2 for CO Oxidation at Cryogenic Temperatures

The discovery that gold catalysts could be active for CO oxidation at cryogenic temperatures has ignited much excitement in nanocatalysis. Whether the alternative Pt group metal (PGM) catalysts can exhibit such high performance is an interesting research issue. So far, no PGM catalyst shows activity for CO oxidation at cryogenic temperatures. In this work, we report a sub‐nano Rh/TiO₂ catalyst that can completely convert CO at 223 K. This catalyst exhibits at least three orders of magnitude higher turnover frequency (TOF) than the best Rh‐based catalysts and comparable to the well‐known Au/TiO₂ for CO oxidation. The specific size range of 0.4–0.8 nm Rh clusters is critical to the facile activation of O₂ over the Rh–TiO₂ interface in a form of Rh?O?O?Ti (superoxide). This superoxide is ready to react with the CO adsorbed on TiO₂ sites at cryogenic temperatures.     

Controlling carbon monoxide emissions from automobile vehicle exhaust using copper oxide catalysts in a catalytic converter

Carbon monoxide (CO) is a very poisonous gas present in the atmosphere. It has significant effects on human beings, animals, plants and the climate. Automobile vehicle exhaust contributes 64% of the CO pollution in urban areas. To control this exhaust pollution, various types of catalysts in catalytic converters have been investigated. Increasing costs of noble metals as a catalyst in automobile vehicles motivates the investigation of material that can be substituted for noble metals. Among the non-noble metals, copper (Cu) is found to be the most capable and highly active catalyst for CO oxidation, compared to precious metal catalysts. Lower cost, easy availability and advance preparation conditions with stabilizers, promoters and so on, make Cu a good choice as an auto exhaust purification catalyst. The oxidation of CO proceeds very quickly over Cu°, followed by Cu⁺ and Cu²⁺. The Cu₂O catalyst is more active in an O₂-rich atmosphere than in O₂-lean conditions. The reduced species of copper (Cu⁰, Cu⁺) are essential for better CO oxidation but smaller Cu particles could be less active than the higher ones. There is a great deal of research available on the Cu catalyst for CO oxidation, but there is a gap in the literature for a review article individually applied to the Cu catalyst for CO oxidation. To fill this gap, the present review updates information on Cu catalysts in the purification of exhaust gases.     

铂族金属催化剂低温 CO氧化研究近期进展

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.     

金单原子催化剂上一氧化碳低温氧化

CO低温氧化对于基础研究和实际应用均具有重要意义.自上世纪八十年代日本的 Haruta教授发现氧化物负载金催化剂对 CO氧化的超高活性以来,负载金催化剂受到了广泛关注与深入研究,被认为是目前活性最高的 CO氧化催化剂.在诸多影响 CO氧化活性的因素中,纳米金的粒子尺寸是最重要因素之一.目前主流观点认为对于 CO氧化,纳米金有一个最优尺寸范围,在0.5–5 nm,而 Au原子/离子(Au3+, Au+)的活性则低一到两个数量级.因此,一般认为负载金单原子催
　　化剂对于 CO氧化反应的活性要比金纳米粒子和团簇低很多.然而,最近几年的理论与实验研究均表明,金单原子负载于合适的载体上可以显示出与金纳米粒子和团簇相当的活性.本文对这些新进展进行综述,阐述金单原子催化剂对 CO氧化的独特反应性能. Gates教授研究组进行了大量关于正价金对 CO氧化影响的研究,其中包括孤立的金原子(Au+).他们的研究发现, CO氧化活性随价态降低而降低,表明正价金对 CO氧化至关重要.此外,他们的研究也表明,孤立金原子对 CO氧化的活性(TOF)比金纳米粒子低一到两个数量级.然而,在他们的研究中,有几个因素可能导致催化剂的低活性.首先,他们一般采用非或弱还原性的载体.而载体的还原性对金催化剂上 CO氧化活性影响非常巨大.另外,他们所用的金原子前驱体为配合物,在催化剂制备与反应过程中配体并没有去除,可能也是导致催化剂活性低的原因之一.与此相反,张涛课题组近期采用氯金酸为前驱体,通过简单的吸附浸渍法制备了一系列负载金单原子催化剂.同时用相同的载体制备了负载金纳米粒子催化剂进行对比,可以排除载体等其它影响因素.对比结果显示,单原子催化剂均显示出与纳米粒子相当的 TOF(单位表面 Au原子)和更高的反应速率(单位重量金).首先制备了氧化铁负载金单原子催化剂,该催化剂在室温即展现出可观活性, TOF值与2–3 nm金粒子 TOF值相当(~0.5 s–1).更有趣也更重要的是,该催化剂在高温(200oC以上)展现出非常高的反应稳定性,在200oC反应100 h无失活.在300和400oC反应50 h也无失活发生,为开发高温稳定的金催化剂提供了新途径.其次制备了氧化钴负载金单原子催化剂,该催化剂以0.05%金负载量即可实现室温全转化,其 TOF值高达1.4 s–1.然而该催化剂在达到高活性之前必须首先在反应气氛中进行高温处理,这限制了其实用性.此外,催化剂需经反应气氛活化的原因尚待进一步研究.随之又制备了氧化铈负载金单原子催化剂,对富氢条件下 CO选择氧化不仅具有高活性,而且具有极高的 CO选择性.进一步研究结合理论计算表明,高选择性来自氧化铈负载的金单原子不能解离活化氢,对于氢气氧化活性极低,从而导致 CO氧化的高选择性.理论研究方面也有进展. Camellone等计算发现金原子可以取代 CeO2(111)面上的 Ce原子形成 Au+并促进 CO氧化.然而该金原子会扩散至氧空位形成带负电荷的 Auδ-,阻止 CO和 O2吸附,因而使催化剂失活.李隽课题组利用从头算分子动力学模拟首次发现氧化铈和氧化钛负载的 Au纳米粒子在 CO氧化过程中可以形成单原子的现象,并将之称为动态单原子催化剂. Yang等则计算了二维材料 BN负载 Au单原子催化 CO氧化并发现反应优先通过三原子 E-R机理进行.     

Hydrogen Spillover Enhanced Hydroxyl Formation and Catalytic Activity Toward CO Oxidation at the Metal/Oxide Interface

H₂‐promoted catalytic activity of oxide‐supported metal catalysts in low‐temperature CO oxidation is of great interest but its origin remains unknown. Employing an FeO(111)/Pt(111) inverse model catalyst, we herewith report direct experimental evidence for the spillover of H(a) adatoms on the Pt surface formed by H₂ dissociation to the Pt?FeO interface to form hydroxyl groups that facilely oxidize CO(a) on the neighboring Pt surface to produce CO₂. Hydroxyl groups and coadsorbed water play a crucial role in the occurrence of hydrogen spillover. These results unambiguously identify the occurrence of hydrogen spillover from the metal surface to the noble metal/metal oxide interface and the resultant enhanced catalytic activity of the metal/oxide interface in low‐temperature CO oxidation, which provides a molecular‐level understanding of both H₂‐promoted catalytic activity of metal/oxide ensembles in low‐temperature CO oxidation and hydrogen spillover.     

A Study of Structure and Catalytic Activity of Ag-Fe Mixed Oxide Catalysts for CO oxidation

Much effort is made to reduce CO to environment pollution^[1]. The precious metal (such as Pt, Pd, Rh) are well known to apply in the total oxidation to treat with the exhaust gas emission and CO, which have good situation in high activity and stability^[2]. Due to the high cost and limited availability of precious metal, people become more and more interested in searching base metals to replace precious metal, especially,transition metal oxide catalysts. In our study, the results indicate that the Ag/Fe (1:1) catalyst exhibits highest active to CO oxidation. It is detected that the highly dispersed state of σ-AgFeO₂ is the active site for CO oxidation.     

一氧化碳低温催化氧化

一氧化碳 (CO) 催化氧化反应因在实际生活中应用广泛而受到人们普遍关注,如激光器中微量CO的消除、封闭体系中CO的消除、汽车尾气净化以及质子交换膜燃料电池中少量CO的消除等。本文总结了近年来CO低温催化氧化研究进展,包括催化剂及其制备方法、CO氧化反应机理以及不同环境气氛对催化剂CO低温氧化性能的影响。催化剂的制备方法主要包括传统浸渍法、共沉淀法、沉积-沉淀法、溶胶-凝胶法、离子交换法、化学气相沉积法、溶剂化金属原子浸渍法等。催化剂可分为贵金属催化剂、非贵金属催化剂、以分子筛为载体的催化剂和合金催化剂等。CO氧化反应机理方面的相关报道较多,人们针对不同催化剂体系提出了各种假设。不同环境气氛对催化剂CO低温氧化性能的影响主要分为H₂O、CO₂、H₂和其它气氛等4部分进行描述。最后对该领域的发展前景进行了展望。     

Monolithically Integrated Spinel MxCo3−xO4 (M=Co,Ni, Zn) Nanoarray Catalysts: Scalable Synthesis and Cation Manipulation for Tunable Low‐Temperature CH4 and CO Oxidation

A series of large scale M_xCo_3−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.     

Monolithically Integrated Spinel MxCo3−xO4 (M=Co,Ni, Zn) Nanoarray Catalysts: Scalable Synthesis and Cation Manipulation for Tunable Low‐Temperature CH4 and CO Oxidation

A series of large scale M_xCo_3?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.     

高活性Co3O4负载单原子Au催化剂室温催化CO氧化

CO低温氧化是多相催化领域研究最多的反应之一.作为简单、典型的探针反应,其不仅具有重要的基础研究价值,而且在环境污染消除等方面也有着非常重要的实际应用价值.金属氧化物如铜锰(Hopcalite)、铜铬复合氧化物以及氧化钴等都具有优异的低温CO氧化活性.然而氧化物催化剂热稳定性低、反复启动性能差、以及对硫化物、水等物质敏感,严重制约了其实际应用.相对而言,负载型贵金属催化剂因具有较高的CO氧化活性、反应稳定性以及热稳定性而受到关注.但是贵金属价格昂贵、资源稀少,使其持续应用面临严峻挑战.为了提高贵金属利用效率、降低贵金属使用量,在负载型贵金属催化剂中,贵金属多以纳米尺度分散于高比表面载体上.由于多相催化一般在纳米粒子表面发生,只有表面金属原子能够接触到反应物,因而贵金属原子利用率仍然有待提高.最近本课题组成功开发以原子级分散的单原子催化剂并提出“单原子催化”的概念.后续研究以及其他研究人员相继证明氧化物负载贵金属单原子具有高活性和/或不同于纳米粒子的反应性能,表明开发单原子催化剂是最大化贵金属利用效率、降低贵金属用量的可行途径.对于CO氧化而言,目前普遍认为负载Au催化剂具有最高活性.然而负载Au单原子催化剂是否具有活性仍存争议：理论计算表明氧化物负载Au单原子催化剂具有很好的活性,但是缺少实验证据;目前已有一些氧化物负载Au正价离子催化剂的报道,结果也都表明Au单原子活性远低于纳米粒子或纳米团簇.最近本课题组发现氧化铁负载Au单原子不仅具有与Au纳米粒子相当的单位活性位(TOF)活性而且具有更高的单位金属重量(反应速率)活性以及非常高的反应稳定性.本文将载体拓展到氧化钴,开发了具有更高活性的氧化钴负载Au单原子催化剂, Au负载量仅为0.05 wt%即可在室温条件下实现CO完全转化. Co3O4载体用Co(NO3)3与Na2CO3通过共沉淀法制备,400 oC焙烧.然后通过简单的沉淀吸附法制备Co3O4负载Au单原子催化剂(Au1/Co3O4),确保Au单原子能够分散于载体的表面.具有原子分辨率的球差校正高分辨电镜照片显示Au原子确实以单原子形式分散于载体上.催化剂在第一个循环中活性并不非常高,但是在第二个循环中活性提高非常明显,可以在室温条件下实现CO全转化.为了弄清楚活性提高的原因,我们用惰性气体(He)、氧化性气体(5%O2/He)以及还原性气体(5%CO/He)对催化剂进行了热处理,但是活性提高并不明显.由此推断催化剂是在第一个循环反应过程中发生了某些变化,导致活性显著提高.空白载体实验表明Co3O4载体本身虽然具有反应活性,但是远不如负载少量Au原子活性高,表明Au原子或Au原子与载体一起起到高活性的作用.稳定性研究表明该催化剂在室温条件下容易失活,但经惰性气体或氧化气体处理后活性可恢复,表明不是结构性失活而是可逆失活,说明单原子非常稳定.     

Rhenium- and ruthenium-containing catalysts for neutralization of automobile exhaust

The rhenium- and ruthenium-containing catalysts are active in the oxidation of CH _x and CO and in the reduction of NO _x . Comparative testing of catalyst samples under laboratory conditions and on an engine stand demonstrated that these catalysts outperform the known commercial catalysts in the neutralization of exhaust gas from automotive gasoline engines. It is, therefore, possible to completely replace expensive Rh and partially replace Pt and Pd with cheaper components—Re and Rh—in the manufacturing of catalytic converters.     

Insight into the hydrogen oxidation electrocatalytic performance enhancement on Ni via oxophilic regulation of MoO_2

Exploring platinum-group-metal(PGM)free electrocatalysts for hydrogen oxidation reaction(HOR)in alkaline media is essential to the progress of anion exchange membrane fuel cells(AEMFCs).In this work,a Ni/MoO₂ heterostructure catalyst with comparable HOR activity in alkaline electrolyte with PGM catalyst was prepared by a simple hydrothermal-reduction method.Remarkably,the Ni/MoO₂ presents a mass kinetic current density of 38.5 mA mgNi^-1 at the overpotential of 50 mV,which is higher than that of the best PGM free HOR catalyst reported by far.Moreover,the HOR performance of Ni/MoO₂ under 100 ppm CO shows negligible fading,together with the superior durability,render it significant potential for application in AEMFCs.A particular mechanistic study indicates that the excellent HOR performance is ascribed to the accelerated Volmer step by the incorporation of MoO₂.The function of MoO₂ was further confirmed by CO striping experiment on Pt/C-MoO₂ that MoO₂ can facilitated OH adsorption thus accelerate the HOR process.On account of the high performance and low cost,the Ni/MoO₂ electrocatalyst encourages the establishment of high performance PGM free catalyst and shows significant potential for application in AEMFCs.     

High throughput screening of low temperature CO oxidation catalysts using IR thermography

The catalytic oxidation of carbon monoxide to carbon dioxide is an important process used in several areas such as respiratory protection, industrial air purification, automotive emissions control, CO clean-up of flue gases and fuel cells. Research in this area has mainly focused on the improvement of catalytic activity at low temperatures. Numerous catalyst systems have been proposed, including those based on Pt, Pd, Rh, Ru, Au, Ag, and Cu, supported on refractory or reducible carriers or dispersed in perovskites. Well known commercial catalyst formulations for room temperature CO oxidation are based on CuMn2O4 (hopcalite) and CuCoAgMnOx mixed oxides. We have applied high-throughput and combinatorial methodologies to the discovery of more efficient catalysts for low temperature CO oxidation. The screening approach was based on a hierarchy of qualitative and semi-quantitative primary screens for the discovery of hits, and quantitative secondary screens for hit confirmation, lead optimization and scale-up. Parallel IR thermography was the primary screen, allowing one wafer-formatted library of 256 catalysts to be screened in approximately 1 hour. Multi-channel fixed bed reactors equipped with imaging reflection FTIR spectroscopy or GC were used for secondary screening. Novel RuCoCe compositions were discovered and optimized for CO oxidation and the effect of doping was investigated for supported and bulk mixed oxide catalysts. Another family of active hits that compare favorably with the Pt/Al2O3 benchmark is based on RuSn, where Sn can be used as a dopant (e.g. RuSn/SiO2) and/or as a high surface area carrier (e.g., SnO2 or Sn containing mixed metal oxides). Also, RuCu binary compositions were found to be active after a reduction pretreatment with hydrogen.     

Property and structure of various platinum catalysts for low-temperature carbon monoxide oxidations

The oxidation of carbon monoxide (CO) has received more attention in the last two to three decades owing to its importance in different fields. To control this CO pollution, catalytic converters have been investigated. Different types of catalysts have been used in a catalytic converter for CO emission control purposes. Platinum (Pt)-based noble metal catalysts show great potential for CO oxidation in catalytic converters with high thermal stability and tailoring flexibility. Pt metal catalysts modified with promoters such as alkali metals and reducible metal oxides have received great attention for their superior catalytic activities in CO oxidation. Temperature, close environment of the catalyst, and chemical composition in the surface layer of the catalyst have a huge effect on the active phase dispersion and O₂ adsorption capacity of the Pt metal catalysts. The main difference in activities of Pt metal catalyst for CO oxidation in O₂ or H₂ atmosphere has found. The addition of supports in Pt metal catalysts has improved their performances and reduced their cost. These improvement strongly depends on the surface structure, morphology, number of active sites, and various Pt-O interactions. Many research articles have already been published in CO oxidation over Pt metal catalysts, but no review article dedicated to CO oxidation is available in the literature.     

Reducing the Cost and Preserving the Reactivity in Noble‐Metal‐Based Catalysts: Oxidation of CO by Pt and Al–Pt Alloy Clusters Supported on Graphene

The oxidation mechanisms of CO to CO₂ on graphene‐supported Pt and Pt‐Al alloy clusters are elucidated by reactive dynamical simulations. The general mechanism evidenced is a Langmuir–Hinshelwood (LH) pathway in which O₂ is adsorbed on the cluster prior to the CO oxidation. The adsorbed O₂ dissociates into two atomic oxygen atoms thus promoting the CO oxidation. Auxiliary simulations on alloy clusters in which other metals (Al, Co, Cr, Cu, Fe, Ni) replace a Pt atom have pointed to the aluminum doped cluster as a special case. In the nanoalloy, the reaction mechanism for CO oxidation is still a LH pathway with an activation barrier sufficiently low to be overcome at room temperature, thus preserving the catalyst efficiency. This provides a generalizable strategy for the design of efficient, yet sustainable, Pt‐based catalysts at reduced cost.     

Investigation of performances of commercial diesel oxidation catalysts for CO,C3H6, and NO oxidation

Four commercial monolithic diesel oxidation catalysts (DOCs) with two different platinum group metal (PGM) loadings and Pt:Pd ratios of 1:0, 2:1, 3:1 (w/w) were investigated systematically for CO, C₃H₆, and NO oxidation, CO-C₃H₆ co-oxidation, and CO-C₃H₆-NO oxidation reactions via transient activity measurements in a simulated diesel engine exhaust environment. As PGM loading increased, light-off curves shifted to lower temperatures for individual and co-oxidation reactions of CO and C₃H₆. CO and C₃H₆ were observed to inhibit theoxidation of themselves and each other. Addition of Pd to Pt was found to enhance CO and C₃H₆ oxidation performance of the catalysts while the presence and amount of Pd was found to increase the extent of self-inhibition of NO oxidation. NO inhibited CO and C₃H₆ oxidation reactions while NO oxidation performance was enhanced in the presence of CO and C₃H₆ probably due to the occurrence of reduced Pt and Pd sites during CO and C₃H₆ oxidations. The optimum Pt:Pd ratio for individual and co-oxidations of CO, C₃H₆, and NO was found to be Pt:Pd = 3:1 (w/w) in the range of experimental conditions investigated in this study.     

Catalytically Active and Spectator Ce3+ in Ceria‐Supported Metal Catalysts

Identification of active species and the rate‐determining reaction steps are crucial for optimizing the performance of oxygen‐storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce³⁺ species in a ceria‐supported platinum catalyst during CO oxidation are short‐lived and therefore cannot be observed under steady‐state conditions. Using time‐resolved resonant X‐ray emission spectroscopy, we quantitatively correlated the initial rate of Ce³⁺ formation under transient conditions to the overall rate of CO oxidation under steady‐state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce³⁺ was present as spectators. This approach can be applied to various catalytic processes involving oxygen‐storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.     

钙钛矿型氧化物负载纳米金属催化剂结构的动态调变及其催化氧化CO反应性能

研究了钛酸钡和钛酸钙担载的Ag和Pt纳米催化剂的表面结构随氧化-还原处理过程的动态变化及其对CO完全氧化反应性能的影响.发现氧化物担载的Ag催化剂在氧化处理后其催化活性较还原处理的高; X射线衍射(XRD)和X射线光电子能谱(XPS)表征结果表明,氧化处理能够提高载体表面Ag颗粒的分散度,而还原处理导致Ag颗粒的聚集,从而降低了催化氧化CO反应的活性.氧化-还原处理改变了担载Ag纳米粒子的尺寸并影响其CO氧化反应活性.与此相反,氧化物担载的Pt催化剂在还原处理后所表现出的CO氧化反应活性较氧化处理的高; 对比研究发现,氧化和还原处理后Pt纳米粒子的尺寸基本相同,但是氧化处理的样品中Pt表面物种以氧化态为主,而还原处理后Pt表面物种主要为金属态.Pt纳米粒子表面化学状态随氧化-还原处理的调变是导致表面催化活性差异的主要原因.     

Steady-state photocatalytic epoxidation of propene by O2 over V2O5SiO2 photocatalysts

A silica-supported, lowly loaded vanadium oxide (V2O5/SiO2) photocatalyst promotes the photocatalytic epoxidation of propene with O2 at steady state in a flow reactor system. Very little deep oxidation of propene into CO2 takes place over V2O5/SiO2, in contrast to the results obtained over a TiO2 photocatalyst in which total oxidation is the main path. With each loading, the sums of the selectivities into propene oxide (PO) and propanal (PA) at steady state were almost the same. The monomeric VO4 tetrahedral species dispersed on SiO2 yield PO under UV irradiation. The less dispersed vanadium oxide species on SiO2 promote the isomerization of PO into PA. We utilized a flow reactor system in which the short contact time reduced the isomerization and resultant decomposition of PO over the catalyst surface.