Reaction Kinetics and Mechanism: Model Studies on Metal Single Crystals [P.H. Emmett Award Address, 1983]

In the past decade remarkable progress has been made in the surface science of catalysis. Studies on both small single crystal metal particles on supporting materials and on large (bulk) single crystals point toward a more refined understanding of the fundamental processes underlying catalytic chemistry and, hopefully, toward a rational design of catalytic materials. It seems clear that concepts gleaned from studies on single crystal surfaces will strongly influence the research with supported metal catalysts and ultimately affect the nature of the materials used as practical catalysts.     

多相光催化分解H_2S中阴离子表面活性剂对半导体催化剂CdS表面的影响

研究了在H_2S碱性溶液中,CdS粉末催化剂存在时,光催化分解H_2S释氢和生成硫反应。考察了阴离子表面活性剂——十二烷基硫酸钠(SDS)对催化剂的表面性质和催化活性的影响。通过模拟该反应体系,用电化学方法测定了单晶CdS电极在上述反应体系中加入SDS(浓度低于临界胶团浓度CMC值)后的平带电位的变化。结果表明:单晶CdS电极的平带电位,由于该体系加入SDS而正移,与n型多晶半导体CdS在加入SDS的H_2S碱性溶液中,光催化分解H_2S的释氢量减少相一致。并探讨了在该体系中,由于表面活性剂的阴离子与S~(2-)在单晶CdS电极表面上的竞争吸附,而引起单晶CdS电极的平带电位正移。     

The structure sensitivity and insensitivity of catalytic reactions in light of the adsorbate induced dynamic restructuring of surfaces

The classification of catalytic reactions to structure sensitive and structure insensitive types has stood up well to the tests of experiments over the past two decades. Model studies using metal single crystal surfaces explored the relationship between the reaction kinetics and the atomic surface structure. These investigations uncovered the adsorbate induced dynamic restructuring of metal surfaces on the timescales of chemisorption, catalytic reactions and on longer times controlled by mass transport. The causes of reaction structure sensitivity and insensitivity are reinterpreted in view of the dynamic restructuring of the metal catalysts.     

金属有机配合物的合成及晶体培养

金属有机配合物因具有独特的催化活性、光学特性、磁学性质和生物学性质等而受到人们的广泛关注。它们在磁性材料、发光材料、非线性光学材料、微孔材料的研制方面扮演着重要的角色。本文详细介绍了金属有机配合物的6种合成方法及它们各自的优缺点,总结出了晶体生长的几种关键因素并对各种因素进行了详细的分析。     

Nanosensors: towards morphological control of gas sensing activity. SnO2, In2O3, ZnO and WO3 case studies

Anisotropy is a basic property of single crystals. Dissimilar facets/surfaces have different geometric and electronic structure that results in dissimilar functional properties. Several case studies unambiguously demonstrated that the gas sensing activity of metal oxides is determined by the nature of surfaces exposed to ambient gas. Accordingly, a control over crystal morphology, i.e. over the angular relationships, size and shape of faces in a crystal, is required for the development of better sensors with increased selectivity and sensitivity in the chemical determination of gases. The first step toward this nanomorphological control of the gas sensing properties is the design and synthesis of well-defined nanocrystals which are uniform in size, shape and surface structure. These materials possess the planes of the symmetrical set {hkl} and must therefore behave identically in chemical reactions and adsorption processes. Because of these characteristics, the form-controlled nanocrystals are ideal candidates for fundamental studies of mechanisms of gas sensing which should involve (i) gas sensing measurements on specific surfaces, (ii) their atomistic/quantum chemical modelling and (ii) spectroscopic information obtained on same surfaces under operation conditions of sensors.     

Surface Science Studies on the Zirconia-Based Model Catalysts

Zirconia possesses ideal chemical and mechanical stability properties. It has been widely used in many technical applications such as gas sensors, protective coatings and heterogeneous catalysis. In particular, in heterogeneous catalysis, zirconia has been used in many catalytic reactions not only as the metal catalysts’ support but also as the pure catalyst; it can be also used as an additive to improve the catalytic performances of the catalysts. To gain fundamental understanding of the roles that zirconia plays in catalysis, significant surface science studies based on zirconia model catalysts have been performed. In this paper, we will present a short review of recent surface science studies on the zirconia-based model catalysts. These model catalysts include single crystalline yttria-stabilized zirconia surfaces, zirconia thin films which were grown on metal single crystal surfaces and zirconia-supported metal catalysts. Besides the focuses on the surface chemistry of model zirconia surfaces, the surface structures and adsorption/reaction properties of the zirconia-supported metal catalysts will be also addressed.     

Encapsulation of palladium nanoparticles by multiwall carbon nanotubes‐graft‐poly(citric acid) hybrid materials

Citric acid was polymerized onto the surface of functionalized multiwall carbon nanotubes (MWCNT‐COOH) and MWCNT‐graft‐poly(citric acid) (MWCNT‐g‐PCA) hybrid materials were obtained. Due to the grafted poly(citric acid) branches, MWCNT‐g‐PCA hybrid materials not only were soluble in water but also were able to trap water soluble metal ions. Reduction of trapped metal ions in the polymeric shell of MWCNT‐g‐PCA hybrid materials by reducing agents such as sodium borohydride led to encapsulated metal nanoparticles on the surface of MWCNT. Herein palladium nanoparticles were encapsulated and transported by MWCNT‐g‐PCA hybrid materials (MWCNT‐g‐PCA‐EPN) and their application as nanocatalyst toward Heck reaction in different conditions was investigated. The catalytic activity of palladium ions supported by MWCNT‐g‐PCA hybrid materials (MWCNT‐g‐PCA‐PdCl₂) toward Heck reactions is much more than for MWCNT‐g‐PCA‐EPN. Structure, characteristics and catalytic activity of synthesized systems was investigated using spectroscopy and microscopy methods. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

非贵金属催化剂催化氧化甲醛的研究进展

室内装修装饰材料释放出的甲醛是一级致癌物质,严重危害人类的身体健康。催化氧化法在较为温和的条件下可以将甲醛转化为无毒的CO₂和H₂O,是一种非常有效的消除甲醛技术。其中该技术采用的非贵金属催化剂因资源丰富,价格低廉,目前研究的最为广泛。本文综述了近年来非贵金属催化剂的研究进展,主要介绍了单一金属氧化物和复合金属氧化物的研发情况;对这些催化剂的合成、结构及甲醛催化氧化性能进行了分析,并总结了一部分甲醛催化氧化的反应机理。另外还对非贵金属催化剂未来的研究发展方向进行了展望：通过各种制备方法调控金属氧化物的结构、形貌、比表面积,进而暴露大面积活性面和丰富优化活性位来提高催化性能;同时借助一些先进的表征技术和理论计算方法探索非贵金属催化剂结构和催化性能之间的关系,深入探讨甲醛催化氧化反应机理等基础问题。     

The Nanoscience Revolution: Merging of Colloid Science, Catalysis and Nanoelectronics

The incorporation of nanosciences into catalysis studies has become the most powerful approach to understanding reaction mechanisms of industrial catalysts and designing new-generation catalysts with high selectivity. Nanoparticle catalysts were synthesized via controlled colloid chemistry routes. Nanostructured catalysts such as nanodots and nanowires were fabricated with nanolithography techniques. Catalytic selectivity is dominated by several complex factors including the interface between active catalyst phase and oxide support, particle size and surface structure, and selective blocking of surface sites, etc. The advantage of incorporating nanosciences into the studies of catalytic selectivity is the capability of separating these complex factors and studying them one by one in different catalyst systems. The role of oxide–metal interfaces in catalytic reactions was investigated by detection of continuous hot electron flow in catalytic nanodiodes fabricated with shadow mask deposition technique. We found that the generation mechanism of hot electrons detected in Pt/TiO₂ nanodiode is closely correlated with the turnover rate under CO oxidation. The correlation suggests the possibility of promoting catalytic selectivity by precisely controlling hot electron flow at the oxide–metal interface. Catalytic activity of 1.7–7.2 nm monodispersed Pt nanoparticles exhibits particle size dependence, demonstrating the enhancement of catalytic selectivity via controlling the size of catalyst. Pt–Au alloys with different Au coverage grown on Pt(111) single crystal surface have different catalytic selectivity for four conversion channels of n-hexane, showing that selective blocking of catalytic sites is an approach to tuning catalytic selectivity. In addition, presence and absence of excess hydrogen lead to different catalytic selectivity for isomerization and dehydrocyclization of n-hexane on Pt(111) single crystal surface, suggesting that modification of reactive intermediates by the presence of coadsorbed hydrogen is one approach to shaping catalytic selectivity. Several challenges such as imaging the mobility of adsorbed molecules during catalytic reactions by high pressure STM and removing polymeric capping agents from metal nanoparticles remain.     

Investigations of Structure Sensitivity in Heterogeneous Catalysis: From Single Crystals to Monodisperse Nanoparticles

Metal surface structure is often a crucial component in determining the activity and selectivity of heterogeneous catalytic reactions. Many important industrial reactions, such as ammonia synthesis, catalytic combustion, Fischer–Tropsch synthesis, and hydrocarbon reforming have been labeled as structure-sensitive. Metal single crystal studies utilizing ultra high vacuum techniques have repeatedly shown the importance of surface structure in reaction kinetics. Recent advances in the field of colloidal synthesis allow for fine control of the size and shape of metal nanoparticles, which permits catalytic studies of structure sensitivity to be performed on nanometer sized catalysts. It is clear that in order to optimize the performance of a catalyst, a complete molecular level understanding of the role of surface structure in the reaction of interest is essential. This article aims to review the importance of surface structure in heterogeneous catalysts, ranging from single crystals to size and shape controlled nanocatalysts.     

Decomposition of N2O on Perovskite-Related Oxides

Mixed metal oxides crystalizing in a perovskite-related structure have long been of interest to solid state chemists and physicists because of their technologically important physical properties. The ready availability of a family of isomorphic solids with controllable physical properties makes these oxides suitable for basic research in catalysis. These mixed metal oxides are more advantageous and are better catalytic materials than simple oxides because: (i) the crystal structure can accomodate various metal ions and can stabilize unusual and mixed valence states of active metal ion; (ii) appropriate formulation of these oxides leads to easy tailoring of many desirable properties such as valence state of transition metal ion, distance between active sites, binding energy, diffusion of oxygen in the lattice, magnetic and conducting properties of the solid; (iii) the catalytic activity can be correlated to solid state properties since many of their solid state properties are thoroughly understood; (iv) the surface of these oxides can be regenerated by suitable activation procedure.     

Decomposition of N2O on Perovskite-Related Oxides

Mixed metal oxides crystalizing in a perovskite-related structure have long been of interest to solid state chemists and physicists because of their technologically important physical properties. The ready availability of a family of isomorphic solids with controllable physical properties makes these oxides suitable for basic research in catalysis. These mixed metal oxides are more advantageous and are better catalytic materials than simple oxides because: (i) the crystal structure can accomodate various metal ions and can stabilize unusual and mixed valence states of active metal ion; (ii) appropriate formulation of these oxides leads to easy tailoring of many desirable properties such as valence state of transition metal ion, distance between active sites, binding energy, diffusion of oxygen in the lattice, magnetic and conducting properties of the solid; (iii) the catalytic activity can be correlated to solid state properties since many of their solid state properties are thoroughly understood; (iv) the surface of these oxides can be regenerated by suitable activation procedure.     

Catalytic effect of metal cations on the formation of carbon nanotubes inside the channels of AlPO4-5 crystal

Single-walled 0.4 nm carbon nanotubes (SWNTs) were fabricated using various metal cation substituted AlPO₄-5 (MeAPO-5) molecular sieves as the template. The catalytic behavior varies with different metal incorporation. The incorporation of metal cations (Mn, Mg, Co) and Si gives rise to the formation of negatively charged frameworks and Brønsted acid sites. These frameworks thus play an important catalytic role in the pyrolysis and conversion of the organic molecules to SWNTs within the crystal channels. It is shown that the MeAPO-5 single crystals have a higher density of SWNTs than those crystals without the metal cations. The experimental results agree with the predictions of first-principles calculations, which show the metal-incorporated frameworks to be favorable for the SWNTs formation.     

Ordered macroporous bimetallic nanostructures: design, characterization, and applications

Ordered porous metal nanomaterials have current and future potential applications, for example, as catalysts, as photonic crystals, as sensors, as porous electrodes, as substrates for surface-enhanced Raman scattering (SERS), in separation technology, and in other emerging nanotechnologies. Methods for creating such materials are commonly characterized as "templating", a technique that involves first the creation of a sacrificial template with a specific porous structure, followed by the filling of these pores with desired metal materials and finally the removal of the starting template, leaving behind a metal replica of the original template. From the viewpoint of practical applications, ordered metal nanostructures with hierarchical porosity, namely, macropores in combination with micropores or mesopores, are of particular interest because macropores allow large guest molecules to access and an efficient mass transport through the porous structures is enabled while the micropores or mesopores enhance the selectivity and the surface area of the metal nanostructures. For this objective, colloidal crystals (or artificial opals) consisting of three-dimensional (3D) long-range ordered arrays of silica or polymer microspheres are ideal starting templates. However, with respect to the colloidal crystal templating strategies for production of ordered porous metal nanostructures, there are two challenging questions for materials scientists: (1) how to uniformly and controllably fill the interstitial space of the colloidal crystal templates and (2) how to generate ordered composite metal nanostructures with hierarchical porosity. This Account reports on recent work in the development and applications of ordered macroporous bimetallic nanostructures in our laboratories. A series of strategies have been explored to address the challenges in colloidal crystal template techniques. By rationally tailoring experimental parameters, we could readily and selectively design different types of ordered bimetallic nanostructures with hierarchical porosity by using a general template technique. The applications of the resulting nanostructures in catalysis and as substrates for SERS are described. Taking the ordered porous Au/Pt nanostructures as examples for applications as catalysts, the experimental results show that both the ordered hollow Au/Pt nanostructure and the ordered macroporous Au/Pt nanostructure exhibit high catalytic ability due to their special structural characteristics, and their catalytic activity is component-dependent. As for SERS applications, primary experimental results show that these ordered macroporous Au/Ag nanostructured films are highly desirable for detection of DNA bases by the SERS technique in terms of a high Raman intensity enhancement, good stability, and reproducibility, suggesting that these nanostructures may find applications in the rapid detection of DNA and DNA fragments.     

光热协同催化净化挥发性有机物的研究进展及展望

挥发性有机物（VOCs）是一类重要的空气污染物。催化氧化技术可以将VOCs转化为无毒的CO₂和H₂O,是有效的治理方式之一。针对传统的热催化氧化技术的高能耗和光催化净化VOCs技术的低效率问题,光催化耦合强化热催化的光热协同催化净化VOCs技术近些年来受到广泛关注,并表现出比传统热催化或光催化净化技术更优异的净化性能。总结了近年国内外研究者在光热催化净化VOCs领域所取得的主要研究进展,重点讨论了光热协同作用机制的认知和光热协同催化材料的设计理念,包括贵金属型和金属氧化物型光热协同催化材料,并对光热协同催化净化技术的未来发展方向进行了展望。     

过渡金属磷化物催化剂综述

过渡金属磷化物催化剂具有特殊的晶体结构,在催化加氢脱硫、加氢脱氮及电化学制氢反应中具有优异的催化活性。主要简述过渡金属磷化物催化剂的结构、制备方法和应用。多金属、复合多功能型过渡金属磷化物催化剂将在催化制氢反应中受到更多的关注。     

Oxidation of carbon nanofibers and materials obtained on their basis

The results of the oxidation of carbon nanofibers and materials obtained on their basis are presented; these results demonstrate that the nanofibers were formed by carbon with different degrees of crystal structure ordering. The experimental data supported previous hypotheses that amorphous carbon results from the decomposition of metal carbides. The subsequent formation of spatial structures and the appearance of crystalline carbon species resulted from catalytic graphitization. It was demonstrated that sorbents can be prepared based on carbon nanofibers after pyrolytic consolidation followed by activation, and these sorbents are more effective than well-known sorbents.     

Abnormally large deuterium uptake on small transition metal clusters

Deuterium uptake experiments on gas phase transition metal cluster cations of Ni, Pt and Rh show that the small (< 10 Å dia.) clusters can bind many (up to 8) deuterium atoms per metal atom in the cluster, in contast to (H(D)/M)_max ratios near unity typically reported for single crystal metal surfaces and in previous uptake experiments on nickel and iron clusters [11]. Abnormally large (H(D)/M)_max ratios appear to be the rule rather than the exception for small transition metal clusters, an effect which has strong implications in chemical and catalytic processes involving hydrogen chemisorption.     

Synthesis of Supported Catalysts by Atomic Layer Deposition

A promising new method of catalyst synthesis is atomic layer deposition (ALD). ALD is a variation on chemical vapor deposition wherein metals, oxides and other materials are deposited on surfaces via a sequence (usually binary) of self-limiting reactions. The self-limiting character of the reactions makes it possible to achieve uniform deposits on high-surface-area porous solids and, hence, produce practical catalytic materials. The ability to deposit uniform layers in a sequence makes it possible to fabricate the support and then construct the catalytic metal and/or metal oxide species and add modifier layers in any desired order. This article will provide a short introduction to the technique of ALD and its application to the synthesis of supported catalytic metal nanoparticles and oxide monolayers.     

三核铜配合物的合成、表征及其催化性能

采用溶剂热法合成了以三核碘化亚铜（CuI）四面体结构为活性中心的硅氢加成反应催化剂,探讨了物料比对产物收率的影响。结果说明了当配体与碘化亚铜的摩尔比为1：6时,产物收率最高。通过元素分析、傅里叶红外光谱分析、X射线光电子能谱分析、X射线单晶体衍射分析、紫外可见光光谱分析、热失重分析对配合物的化学组成、空间结构及性能进行表征,并进一步通过甲基苯基乙烯基树脂和甲基苯基含氢硅油的硅氢加成反应进行催化固化效果验证。结果说明了在催化剂填加量为0.04%、固化温度为150℃的优化条件下反应24h,共混体系固化效果最佳。该配合物对硅氢加成反应具有很好的催化性能,并且原料成本低、制备方法简单、晶体颗粒方便储存,有望解决硅氢加成反应中贵金属催化剂的高成本问题。