Identification of the Active Sites for Low Temperature CO Oxidation over Nanocrystalline Co3O4 Catalysts

The microstructural properties of dry‐grinding derived Co₃O₄ catalysts pretreated under different atmospheres, in relation to the activities on CO oxidation were investigated. The Co₃O₄ synthesized by soft reactive grinding and pretreated with O₂ resulted in the best activity, with 100% conversion of CO at ?52 °C, superior to that of Co₃O₄ pretreated with He. To find out the active sites on Co₃O₄ for low temperature CO oxidation, the characterizations of the cobalt oxides had been investigated by means of N₂ physisorption, XRD, TEM, H₂‐TPR, CO‐titration, XPS and O₂‐TPD technologies. XPS of Co2p results show that it is difficult to ascribe the difference in catalytic performance to the surface concentration of active Co³⁺ sites. A correlation between the activity and the CO‐titration and O₂‐TPD results for Co₃O₄ reveals that a high abundance of readily accessible superficial electrophilic oxygen (O^?) species is important for achieving a high activity. Therefore, CO oxidation takes place on the surface active oxygen sites in Co₃O₄ crystallites via the suprafacial mechanism.     

高效SnO2纳米片催化剂用于CO氧化

SnO2是一种具有丰富表面缺位氧的n型半导体,其晶格氧亦可还原.另外其熔点高达1630oC,具有较高的热稳定性能.在过去的几十年中, SnO2主要用作气敏材料.而其作为催化材料的性能,特别是用于大气污染治理则鲜见报道.在过去的几年中,本课题组系统研究了SnO2的催化化学,发现利用传统共沉淀法制备的SnO2纳米粉末,在焙烧温度高于500 oC时,其比表面积通常低于20 m2/g,因而限制了其氧化活性.在SnO2晶格中掺杂Fe、Cr、Mn、Ce和Ta等形成固溶体可有效提高其比表面积并产生更多的活性氧物种,因而其对CO和CH4的氧化活性及稳定性大幅度提高.本课题组近期研究结果表明,采用熔盐法制备的高纯SnO2纳米棒单晶比SnO2纳米颗粒和纳米微球等具有更优异的CO氧化活性,260 oC即可完全氧化CO.且在240–260 oC狭窄温度区间发生转化率突跃,表现出类似贵金属的催化行为.值得指出的是, SnO2纳米棒的比表面积(1 m2/g)远低于其他几种形貌的材料,且无活泼氧存在.但研究表明SnO2纳米棒具有优先暴露的(110)活泼晶面,是导致其活性优良的主要原因.另外,我们采用简单共沉淀法成功制备了高比表面介孔Cu-Sn复合氧化物纳米片(196 m2/g),其在140 oC即可将CO完全氧化,且具有优良的抗水失活性能.因此, SnO2的形貌是影响其催化活性的主要因素之一,但迄今未见较系统深入的研究.在上述工作基础上,本文通过水热法,不添加任何有机模板剂,成功制备了厚度约10 nm的介孔SnO2纳米片和纳米棒及纳米颗粒混合物样品;采用常规共沉淀法制备了SnO2纳米颗粒.并将以上三种不同形貌的SnO2纳米材料用于CO氧化.利用SEM、XRD、N2吸附-脱附、H2-TPR和XPS探讨了不同形貌SnO2催化剂的体相结构和表面性质及其对催化性能的影响.与SnO2纳米颗粒相比,介孔SnO2纳米片具有高的比表面积、孔体积及更活泼的氧中心,因此后者CO氧化活性远高于前者.在空速18000 mL/(g·h)时, SnO2纳米片在260 oC即可完全氧化CO.而SnO2纳米颗粒的CO完全氧化温度高于360 oC. SnO2纳米棒和纳米颗粒的混合样品虽然其比表面积和孔体积及表面活性氧的活性仅略高于SnO2纳米颗粒,但XRD定量结果表明,其具有更多的暴露(110)活泼晶面,因而活性也高于SnO2纳米颗粒. SnO2纳米片催化剂的寿命及抗水性能测试结果表明,该催化剂具有良好的稳定性,且水蒸气仅对其活性产生可恢复的影响.进一步优化其性能, SnO2纳米片有可能用于实际汽车尾气状况下的CO催化清除.     

Co3O4 nanosheets: synthesis and catalytic application for CO oxidation at room temperature

Hexagonal β-Co(OH)2 nanosheets with edge length of 50 nm and thickness of 10 nm were hydrothermally synthesized with the aid of triethylamine.Upon calcination at 350°C in air,the β-Co(OH)2 nanosheets was converted into Co3O4 nanosheets with a similar dimension.Structural analyses during the calcination process identified that the β-Co(OH)2 precursor was initially dehydrated to HCoO2 and subsequently transferred into Co3O4.When being applied to catalyze CO oxidation at room temperature,the Co3O4 nanosheets exhibited a higher activity than the conventional spherical nanoparticles.This was perhaps related to the partial exposure of the{112}planes over the Co3O4 nanosheets.The porous structure generated during the calcination process also provided significant amounts of surface defects,which might contribute to the enhanced catalytic activity as well.     

Investigation of CO and formaldehyde oxidation over mesoporous Ag/Co3O4 catalysts

CO and formaldehyde (HCHO) oxidation reactions were investigated over mesoporous Ag/Co₃O₄ catalysts prepared by one-pot (OP) and impregnation (IM) methods. It was found that the one-pot method was superior to the impregnation method for synthesizing Ag/Co₃O₄ catalysts with high activity for both reactions. It was also found that the catalytic behavior of mesoporous Co₃O₄ and Ag/Co₃O₄ catalysts for the both reactions was different. And the addition of silver on mesoporous Co₃O₄ did not always enhance the catalytic activity of final catalyst for CO oxidation at room temperature (20 °C), but could significantly improve the catalytic activity of final catalyst for HCHO oxidation at low temperature (90 °C). The high surface area, uniform pore structure and the pretty good dispersion degree of the silver particle should be responsible for the excellent low-temperature CO oxidation activity. However, for HCHO oxidation, the addition of silver played an important role in the activity enhancement. And the silver particle size and the reducibility of Co₃O₄ should be indispensable for the high activity of HCHO oxidation at low temperature.     

Pd-Cu-Clx/Al2O3催化剂对CO氧化的稳定性

CO催化氧化广泛应用于空气净化、机动车尾气治理和CO气体传感器中.在CO氧化催化剂设计与制备过程中,催化剂与使用环境密切相关.例如工业和机动车尾气净化需要在高温(200–600°C)下进行,而对于半密闭空间(隧道或者地下停车场)空气净化需要在室温和高相对湿度下进行.频繁冷启动导致半密闭空间CO浓度累积而超过排放控制标准,因此制备室温、高相对湿度下CO氧化催化剂是面临的重要问题之一.负载型Wacker催化剂对于CO低温催化氧化的研究一直受到广泛关注.环境中少量水的存在会促进负载型Wacker催化剂对CO的低温氧化性能,但随着水沉积量的增加,活性位点将被覆盖,并且Pd和Cu活性组分之间的紧密结构被破坏,从而导致催化剂的失活,即催化剂的稳定性变差.因此,为了提高催化剂在高相对湿度下的稳定性,利用二乙氧基二甲基硅烷对Al2O3载体进行硅烷化处理,以增加载体的疏水性,考察载体疏水改性对CO低温氧化过程中催化剂稳定性的影响.催化剂的稳定性测试结果表明,在0°C,100%相对湿度条件下,未改性催化剂在约20 h内CO转化率由81%下降到50%;载体硅烷化后制备的催化剂在反应进行150 h后,CO转化率仍保持在78%,即反应活性未见降低.由此表明催化剂载体经有机硅烷改性后,可显著增强催化剂在低温、高相对湿度下的稳定性.N2吸附/脱附和水吸附实验结果表明,载体硅烷化改性并未对催化剂的比表面积产生影响,但显著降低了催化剂上水沉积速度和沉积量,未改性催化剂的初始吸水速度是改性后催化剂的4倍,但改性后催化剂的饱和吸水率仅占未改性催化剂的1/3.X射线衍射结果表明,载体预处理后活性物种Cu2(OH)3Cl晶粒尺寸有所增加.氢气程序升温还原、X射线光电子能谱结果表明,载体硅烷化预处理改善了催化剂中Cu和Pd物种的化学分布及接触状态,增加了与Pd物种紧密接触的Cu物种的量,从而促进了Cu物种的还原.与此同时,载体硅烷化显著降低了催化剂表面Cl离子的浓度,从而影响到对CO吸附.为了进一步研究水与催化剂稳定性之间的关系,采用原位红外漫反射(In situ DRIFT)对催化剂进行表征.负载型Wacker催化剂对CO氧化反应机理为:Pd是CO氧化反应的活性中心,通过Pd和Cu物种之间的氧化还原循环来实现CO氧化,且Pd+比Pd2+具有更高的CO氧化性能.反应气氛中水的存在,有利于CO在Pd+上氧化、以及金属态Pd被Cu2+物种再氧化的过程,同时水也显著促进了催化剂表面碳酸盐的生成以及抑制了活性物种Pd+生成.与表面碳酸盐累积相比,水对于活性物种Pd+生成的抑制作用是导致催化剂活性降低的主要原因.     

Dual-template synthesis of defect-rich mesoporous Co3O4 for low temperature CO oxidation

CO oxidation is a benchmark in heterogeneous catalysis for evaluation of redox catalysts due to its practical relevance in many applications and the fundamental problems associated with its very high activity at low temperatures. Among which, Co₃O₄ is one of the most active non-precious metal catalysts. Exposed crystal planes and cobalt sites are considered to be important for its high catalytic activity. Herein, we demonstrate an enhanced CO oxidation activity by a defect-rich mesoporous Co₃O₄ that prepared by a designed dual-template method. Two different kinds of silicas are used as hard-templates at the same time, resulting in a defect-rich mesoporous Co₃O₄ with a surface area as high as 169 m²/g. This catalyst exhibited a very high catalytic activity for low temperature CO oxidation with a light-off temperature at −73 ^oC under the space velocity of 80,000 mL h^-1 g_cat^-1. Further studies reveal that the high surface area promotes the lattice oxygen mobility, surface rich of Co²⁺ species and active oxygen species are crucial for the high catalytic activity. Moreover, the dual-template approach paves a way towards the design and construction of high-surface-area mesoporous metal oxides for various applications.     

Co3O4-decorated carbon nanotubes as a novel efficient catalyst in the selective oxidation of benzoins

An efficient and facile air oxidation of benzoins in the presence of the heterogeneous catalyst, Co₃O₄–CNTs nanocomposite, has been developed. It thus constitutes a very simple, clean, economical and selective method for the aerobic preparation of benzils from the corresponding benzoins. The catalyst can be re-used a number of times without losing its catalytic activity to a greater extent.     

Ordered mesoporous Co3O4 as highly active catalyst for low temperature CO-oxidation

Cubic ordered mesoporous Co(3)O(4), prepared via the nanocasting pathway using KIT-6 as hard template, was found to be an excellent catalyst for low temperature CO oxidation, with the activity clearly depending on surface area and pore systems of the catalysts.     

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.     

介孔Co3O4-CeO2复合氧化物的制备及在CO选择性氧化中的应用

以硝酸钴和硝酸铈为前驱物，SBA-15为硬模板，利用双溶剂法制备了Co3O4-CeO2介孔复合氧化物，通过X-射线衍射、N2吸脱附测试、程序升温还原和透射电子显微镜等技术对活性组分及载体进行了表征，并且与浸渍法和共沉淀法所制备的催化剂进行了对比分析。结果表明，相比于浸渍法和共沉淀法，采用双溶剂法制备的介孔Co3O4-CeO2复合氧化物具有均匀的介孔结构、较小的颗粒尺寸、较大的比表面积和较高的活性组分分散度。此外，CO氧化脱除评价显示与常规的共沉淀法和浸渍法所制备的催化剂相比该介孔复合氧化物具有较高的反应活性和选择性，其高活性主要归因于较高的比表面积和活性组分的高分散度。     

Highly selective oxidation of amines to imines by Mn2O3 catalyst under eco-friendly conditions

Enhancing the selectivity of imines for the oxidative self-coupling of primary amines was found to be challenging in the heterogeneous catalysis. Three different manganese oxides (M-3, M-4, M-5) were synthesized by controlling the calcination temperature using a simple template-free oxalate route. The prepared manganese oxides were systematically characterized using XRD, N₂ sorption, SEM, TEM, XPS, H₂-TPR techniques. M-4 gave 96.2% selectivity of imine at 100% conversion of benzylamine, which was far more superior than other existing protocols. Mn³⁺/Mn⁴⁺ ratio was found to affect the selectivity of the imines. The probable reaction pathway for amines oxidation catalyzed by manganese oxides was proposed for the first time.     

La掺杂TiO2负载Pt催化剂制备及其室温氧化低浓度甲醛

甲醛是室内装修污染的重要组分,已严重危害到人们身体健康,在室温条件下消除甲醛引起了人们的广泛兴趣.目前室温清除甲醛主要有物理吸附法、光催化法、等离子体技术及催化氧化技术.物理吸附法主要采用活性炭等作为吸附剂,其初期吸附效果较好,但当吸附饱和之后会重新释放甲醛造成二次污染;光催化法和等离子体技术需要特殊装置,不适合室内室温环境脱除甲醛;而催化氧化技术则可直接将甲醛转化为无毒无害的水和CO2,因而备受关注.Pt/TiO2被认为是目前消除甲醛最有效的催化剂.为进一步降低贵金属Pt的用量及增强其稳定性(Pt被氧化后其活性会降低),本文首次采用稀土La掺杂锐钛矿型TiO2,负载少量Pt后用于室内低浓度(0.5 ppm)甲醛的催化氧化.活性测试结果表明,纯TiO2催化剂上甲醛转化率在5%以下,有可能是物理吸附或可见光催化所致.负载0.5%Pt后,Pt/TiO2和Pt/La-TiO2甲醛转化率均高于80%,尤其是La掺杂活性高达96%以上,且在连续反应8 h甚至延长至40 h后其活性均未见下降趋势.电镜结果表明,La掺杂Pt/La-TiO2催化剂中Pt粒径从未掺杂的2.2 nm降至1.7 nm;CO程序升温脱附测试表明,Pt/La-TiO2/Pt的分散度达66%,而未掺杂样品仅为51%;X射线光电子能谱测试表明,Pt/La-TiO2的表面氧物种高于Pt/TiO2催化剂,说明La掺杂增强了Pt和载体间的相互作用.为探讨Pt/La-TiO2商业化应用前景,将粉体Pt/La-TiO2涂覆在堇青石蜂窝陶瓷上制备成整体催化剂.该整体催化剂在容积为2 m3的密室测试中5 min内即可将浓度为0.5 ppm的甲醛将至0.02 ppm以下.该催化剂在存放3个月后活性略有下降,但在10 min内仍可将甲醛浓度降至0.08 ppm,达到室内甲醛排放标准.综上,本文成功制备了La掺杂Pt/La-TiO2用于室内低浓度甲醛催化氧化,该催化剂表现出优异的催化性能.通过多种表征手段表明,La修饰后贵金属Pt纳米粒子尺寸减小、分散度提高及Pt与载体间相互作用增强是其活性优异的主要原因.以Pt/La-TiO2粉体制备的整体催化剂同样表现出了高的催化性能,具有工业应用前景.     

Zirconia supported La, Co oxides and LaCoO3 perovskite: structural characterization and catalytic CO oxidation

ZrO₂-supported La, Co oxide catalysts with different La, Co loading (2, 6, 8, 12 and 16 wt.% as LaCoO₃) were prepared by impregnation of tetragonal ZrO₂ with equimolar amounts of La and Co citrate precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. Catalytic CO oxidation was performed at 298–800 K. XRD revealed the presence of tetragonal zirconia with traces of the monoclinic phase. LaCoO₃ perovskite was also detected for loading higher than 6%. XAS experiments suggested that at high loading LaCoO₃ and Co₃O₄ were formed, while at low loading, La, Co oxide species interacting with support, and hard to be structurally defined, prevailed. The catalysis study evidenced that the catalytic activity was due to segregated and highly dispersed cobalt oxide species.     

Catalytic reduction of NO_x by CO on hydro-talcites-derived mixed oxides Co AIM and MgAlM (M = Cr, Mn, Fe, Co, Ni, Cu)

Two series of mixed oxides, CoAlM and MgAlM (M= Cr, Mn, Fe, Co, Ni, Cu) , were prepared by calcining their corresponding hydrotalcite-like compounds (HTLc) . The ratio of Mg: Al: M (or Co: Al: M) was 3:1:1. The catalytic activity of all samples for the reaction of NO CO was investigated. The results showed that the activity of CoAlM was much higher than that of MgAlM. The structure and the property of redox were characterized by XRD and H2-TPR. The results indicated that only MgO phase was observed after calcining MgAlM hydrotalcites, and the transition metals became more stable. The spinel-like phase appeared in all of CoAlM samples after the calcination, and the transition metals were changed to be more active, and easily reduced . The activities of three series of mixed oxides CoAlCu obtained from different preparation methods, different ratio of Co:Al:Cu and at different calcination temperatures, were studied in detail for proposing the mechanism of reaction. The ability of adsorption of NO and CO     

介孔Fe-Cu复合金属氧化物纳米粉催化剂催化低温CO氧化

以环氧丙烷为凝胶剂,采用简便低廉的无表面活性剂的溶胶-凝胶法制备了一系列不同Cu/Fe摩尔比的高比表面积介孔Fe-Cu复合氧化物纳米粉末。运用微反应器-色谱体系考察了它们在低温CO氧化反应中的催化性能。采用X射线衍射、N2吸附-脱附、热重-差热分析、程序升温还原、傅里叶变换红外光谱和透射电镜对所制样品进行了表征。结果表明,这些介孔Fe-Cu复合氧化物催化剂具有纳米晶结构、窄的孔径分布和高的比表面积,在低温CO氧化反应中表现出高的活性和稳定性。 CuO的添加影响了Fe2O3的结构和催化性能。当CuO含量为15 mol%时,催化剂具有最高的比表面积和催化活性,在低温CO氧化反应中表现出较高的催化稳定性。     

Kinetics of low temperature CO oxidation over Pt/SnO2 catalyst

In a CO−O₂ stoichiometric mixture, the kinetic parameters, reaction order, rate constant and activation energy of CO oxidation over a Pt/SnO₂ catalyst have been measured using a fixed bed flow reactor near 0°C. The results show that it is a first-order reaction. The activation energy of CO oxidation over Pt/SnO₂ prepared with SnO₂ calcined at 300°C was approximately 21 kJ/mol. The activation energy of CO oxidation over Pt/SnO₂ changed slowly with SnO₂ calcination temperature above 400°C, and reached approximately 45 kJ/mol.     

Pt-表层限域的表面FeOx结构用于低温CO氧化反应

通过简单的浸渍-还原并随后在酸性溶液中处理制备了Pt-Cu双金属催化剂. 利用电感耦合等离子体发射光谱、X射线衍射和X射线光电子能谱对不同处理条件下的Pt-Cu纳米粒子的结构和组成进行了表征. 研究发现, Pt-Cu催化剂在高温H₂中还原形成了PtCu₃合金结构, 酸洗处理后形成了包含Pt-骨架(Pt-skeleton)的表面结构和PtCu₃合金核的纳米粒子. 而Pt-骨架结构又可通过在H₂中退火转变为规整的Pt-表层(Pt-skin)结构. Pt-表层表面修饰Fe氧化物后在CO选择氧化反应中表现出较好的催化性能. 通过此方法制备的三金属Pt-Cu-Fe催化剂可达到与Pt-Fe相近的高活性, 并且催化剂中Pt的用量大大降低.     

Catalytic reduction of NOx by CO on hydrotalcites-derived mixed oxides CoAlM and MgAlM (M=Cr,Mn, Fe,Co, Ni,Cu)

Two series of mixed oxides, GIAlM and MgAlM (M= G, Mn, Fe, Co, Ni, Cu), were prepared by calcining their corresponding hydrotalcite-like compounds (HTLc). The ratio of Mg: Al: M (or Co: Al: M) was 3:1:1. The catalytic activity of all samples for the reaction of NO + CO was investigated. The results showed that the activity of CoAlM was much higher than that of MgAlM. The structure and the property of redox were characterized by XRD and H₂-TPR. The results indicated that only MgO phase was observed after calcining MgAlM hydrotalcites, and the transition metals became more stable. The spinel-like phase appeared in all of CoAlM samples after the calcination, and the transition metals were changed to be more active, and easily reduced. The activities of three series of mixed oxides CoAlCu obtained from different preparation methods, Different ratio of Co: Al: Cu and at different calcination temperatures. were studied in detail for proposing the mechanism of reaction. The ability of adsorption of NO and CO were investigated respectively for supporting the mechanism.     

焙烧温度对Au/Fe2O3选择性氧化富氢气体中CO催化性能的影响

甲醇重整在线制氢作为质子交换膜燃料电池的燃料成为当前研究的热点。受重整反应动力学及热力学的限制，使得甲醇重整气(富氢气体)中除含有大量的氢气外还含有少量的CO，CO极易吸附在燃料电池阳极催化剂表面，使电池性能下降，因而必须去除重整气中的CO，选择性氧化脱除富氢气     

Co3O4纳米晶表面晶格氧催化氧化CO的理论研究

近年来,纳米科学技术的迅速发展给催化领域,特别是多相催化带来了新的机遇和挑战.科学家们开始着眼于在纳米尺度上对催化剂结构和催化性能进行表征、控制和设计.Co3O4作为一种重要的半导体金属氧化物材料,由于其优异的氧化还原性质,在锂电池、气体传感器以及多相催化领域得到了十分广泛的应用.最近,研究者发现Co3O4纳米晶在催化CO低温氧化和CH4活化等一系列重要反应中表现出显著的反应活性和晶面效应,表明有效设计和合成特定的高活性、高选择性的纳米晶面,对催化领域的发展将具有十分重要的意义.因而,从原子层面对纳米晶所表现出的这种高活性和晶面效应进行深入解释,将为高效催化剂设计提供重要指导.低温CO氧化作为一种重要的催化反应在燃料电池、空气净化与汽车尾气处理中具有重要的应用价值.本文采用密度泛函理论对Co3O4纳米晶催化CO氧化反应的机理、晶面效应以及结构敏感性进行了理论研究.首先,研究了CO在Co3O4(001)和(011)表面Co,CoOo和Co-Ot三种不同位点的吸附扩散行为,发现CO在Co位点表现出较强的吸附行为,但这种吸附构型需要克服很高的能垒(～1 eV)才能转变到Co-O离子对位点,在低温下这种转变将不可能发生,因此我们推断CO在Co位点的吸附对Co3O4催化CO氧化的晶面效应没有显著影响.接着,对CO在Co-O离子对位点抽提晶格氧生成CO2的反应机理进行了研究.我们发现,(011)表面Co-Ot位点可以较强地吸附CO(吸附能-1.15 eV),并十分容易夺取晶格氧离子(能垒0.26 eV),具有很低的势能面,因而其CO氧化活性明显大于(001)面.为了更清楚地理解这种晶面效应和结构敏感效应的本质,我们提出将CO2形成步的过渡态在反应路径上的能级作为反应活性指标.这种活性指标兼顾考虑了CO在Co-O氧位点的吸附覆盖度和CO2形成步的反应能垒,可以近似理解为反应的表观活化能.据此我们得出,Co3O4不同表面不同品格位点催化CO氧化的反应活性顺序为:(011)-Co-Ot＞＞(001)-Co-Oo＞(011)-Co-Oo＞(001)-Co-Ot.由于CO吸附和CO2形成步都涉及到表面被还原的过程,我们因此发现CO催化氧化活性的高低与表面晶格氧位点的可还原性具有正相关性.这种表面不同位点的还原性可以直接通过对空穴形成能的计算获得,降低表面氧空穴的生成能将有利于提高CO氧化的活性.催化设计的终极目标是在对催化活性位点的本质及反应机理深入认识的基础上在原子层面上对催化剂进行可控设计,从而实现催化剂材料的高效、经济的利用.本文研究表明离子对活性位点是Co3O4纳米晶催化CO氧化反应的活性位点,其中阳离子负责对CO的吸附,阴离子则负责CO2的形成过程,这种协同作用实现了Co3O4纳米晶的高反应活性.我们相信,寻找有效的方法在催化剂表面增加离子对位点活性中心的数目是一种实现高性能催化剂设计的途径.