The impact of surface science on the commercialization of chemical processes

Surface science developed instruments for atomic- and molecular-scale studies of catalyst surfaces, their composition and structure, both in a vacuum and at high pressures, under reaction conditions (bridging the pressure gap). Surfaces ranging from single crystals, nanoparticles and thin films to porous high surface area catalytic materials have been studied. Classes of surface structure sensitive and insensitive reactions have been identified by surface science studies, including ammonia synthesis, hydrodesulfurization, reforming, combustion and hydrogenation. Rates of reactions often vary by orders of magnitude between using the right and the wrong surface structures. The roles of many promoters that modify the catalyst surface structures and bonding of adsorbates have been verified. Surface reaction intermediates could be identified and the mobility of adsorbates and the adsorbate induced reconstruction of the catalysts attest to the dynamic nature of the catalytic systems during the reaction turnover. The important active sites for catalysis include the low coordination surface step, kink, oxygen and chloride ion vacancies sites and sites at oxide-metal interfaces. Uncovering the molecular ingredients of heterogeneous catalysts will have a major impact on the understanding of reaction selectivity to help the evolution of green chemistry and selective reaction of many types.     

The Surface Science of Catalytic Selectivity

Catalysis science research employing model systems (single crystal surfaces) focused on understanding and enhancing catalytic activity (turnover rate). The catalyst surface structure and the mobility of adsorbed species are key ingredients that control activity. Catalytic selectivity is the focus of research in the foreseeable future to develop environmentally benign chemical processes that approach 100% selectivity. The catalyst surface structure, selective site blocking, bifunctional catalysis, and oxide–metal interfaces have been recognized as some of the features of reaction selectivity. New two-dimensional model catalyst systems are being fabricated by electron beam and photolithographies for molecular studies of selectivity. New methods are employed to develop three-dimensional high surface area catalysts with precise control of metal particle size, surface structure, and location in the mezopores.     

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.     

The 13th International Symposium on Relations Between Homogeneous and Heterogeneous Catalysis—An Introduction

Over 40 years, there have been major efforts to aim at understanding the properties of surfaces, structure, composition, dynamics on the molecular level and at developing the surface science of heterogeneous and homogeneous catalysis. Since most catalysts (heterogeneous, enzyme and homogeneous) are nanoparticles, colloid synthesis methods were developed to produce monodispersed metal nanoparticles in the 1–10 nm range and controlled shapes to use them as new model catalyst systems in two-dimensional thin film form or deposited in mezoporous three-dimensional oxides. Studies of reaction selectivity in multipath reactions (hydrogenation of benzene, cyclohexene and crotonaldehyde) showed that reaction selectivity depends on both nanoparticle size and shape. The oxide-metal nanoparticle interface was found to be an important catalytic site because of the hot electron flow induced by exothermic reactions like carbon monoxide oxidation.     

Selective Nanocatalysis of Organic Transformation by Metals: Concepts, Model Systems, and Instruments

Monodispersed transition metal (Pt, Rh, Pd) nanoparticles (NP) in the 0.8–15 nm range have been synthesized and are being used to probe catalytic selectivity in multipath organic transformation reactions. For NP systems, the turnover rates and product distributions depend on their size, shape, oxidation states, and their composition in case of bimetallic NP systems. Dendrimer-supported platinum and rhodium NPs of less than 2 nm diameter usually have high oxidation states and can be utilized for catalytic cyclization and hydroformylation reactions which previously were produced only by homogeneous catalysis. Transition metal nanoparticles in metal core (Pt, Co)––inorganic shell (SiO₂) structure exhibit exceptional thermal stability and are well-suited to perform catalytic reactions at high temperatures (>400 °C). Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules. The facile rearrangement of NP catalysts required for catalytic turnover makes nanoparticle systems (heterogeneous, homogeneous and enzyme) excellent catalysts and provides opportunities to develop hybrid heterogeneous-homogeneous, heterogeneous-enzyme and homogeneous-enzyme catalyst systems.     

Supported hybrid early and late transition metal catalysts for the synthesis of polyethylene with tailored molecular weight and chemical composition distributions

Supported hybrid catalysts using metallocenes and a nickel diimine catalyst were synthesized and used for ethylene slurry polymerization and ethylene/1-hexene copolymerization. Two types of metallocenes, together with a nickel diimine catalyst were supported onto SiO₂ through chemical bonding, and a borate compound was physisorbed for the activation of the catalysts. These supported hybrid catalysts had high catalyst activities and made free-flowing polymer particles. The chemical composition distribution, molecular weight averages and distributions of resultant polymers were controlled by catalyst structure and polymerization conditions such as reaction temperature and the use of α-olefin. According to GPC-IR, ¹³C NMR and CEF characterization results of some polymers, more 1-hexene was incorporated in the high molecular weight region, short chain branches were generated by the chain walking mechanism in low molecular weight region. The morphologies of the resulting particles were investigated by SEM.     

The Surface Structure of and Catalysis by Platinum Single Crystal Surfaces

The importance of the atomic structure of solid surfaces and of adsorbed molecules in heterogeneous catalysis has been advocated by many scientists throughout the years. Early studies by Balandin [l], for example, have suggested the presence of close-packed structures of adsorbed molecules that are intermediates in catalytic reactions. In his view both the atomic structure of the substrate metal surface and the structure of the adsorbed molecules were of primary importance in carrying out certain types of catalytic reactions. In the past, however, the difficulties of determining the atomic structure of catalyst surfaces proved to be almost insurmountable, mostly because of the configuration of the catalyst systems. In the case of metal catalysts most commercial catalyst systems consist of finely dispersed metal particles that are deposited on a high surface area support, most frequently silica or alumina. Even at present the atomic structure of such polydispersed systems cannot be ascertained unambiguously, although the application of several techniques, for example x-ray diffraction, small angle x-ray scattering, electron microscopy, and the electron microprobe have helped to define many of its structural and chemical characteristics. The difficulties of unraveling the role of surface structures in surface reactions were compounded by the variable active surface area of catalyst systems that also markedly influences the rates of catalytic reactions. The effects of surface area, particle size distribution, and structure had to be separated before the role of atomic surface structure in heterogeneous catalysis could be explored by definitive studies.     

The Surface Structure of and Catalysis by Platinum Single Crystal Surfaces

The importance of the atomic structure of solid surfaces and of adsorbed molecules in heterogeneous catalysis has been advocated by many scientists throughout the years. Early studies by Balandin [l], for example, have suggested the presence of close-packed structures of adsorbed molecules that are intermediates in catalytic reactions. In his view both the atomic structure of the substrate metal surface and the structure of the adsorbed molecules were of primary importance in carrying out certain types of catalytic reactions. In the past, however, the difficulties of determining the atomic structure of catalyst surfaces proved to be almost insurmountable, mostly because of the configuration of the catalyst systems. In the case of metal catalysts most commercial catalyst systems consist of finely dispersed metal particles that are deposited on a high surface area support, most frequently silica or alumina. Even at present the atomic structure of such polydispersed systems cannot be ascertained unambiguously, although the application of several techniques, for example x-ray diffraction, small angle x-ray scattering, electron microscopy, and the electron microprobe have helped to define many of its structural and chemical characteristics. The difficulties of unraveling the role of surface structures in surface reactions were compounded by the variable active surface area of catalyst systems that also markedly influences the rates of catalytic reactions. The effects of surface area, particle size distribution, and structure had to be separated before the role of atomic surface structure in heterogeneous catalysis could be explored by definitive studies.     

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.     

钒系、钛系Ziegler—Natta催化体系电子作用过程描述

对Ziegler-Natta催化剂的形成和发展进行了介绍,并对其催化体系电子作用过程进行了描述。重点阐述了烷基铝化合物中铝原子与烷基的电子层分布情况、过渡金属元素的电子结构特点,描述以烷基铝和过渡金属卤化物为代表的类阴离子型催化剂和路易斯酸等阳离子型催化剂反应的具体过程以及钒催化剂的催化机理等。     

The surface science concepts of heterogeneous catalysis. The building of complex catalysts systems on single crystal surfaces

Surface science studies have aided the molecular formulation and the elucidation of the concepts of heterogeneous catalysis. These include the thermal activation of bond breaking, the unique, chemical activity of rough surfaces, the dynamic chemisorption - induced restructuring of surfaces and the coadsorption bond. The high catalytic activities of bimetallic catalysts and oxide- metal interfaces can be rationalized using these concepts. Complex model catalyst systems can be built by vapour deposition of the various ingredients on metal crystal surfaces.     

Environmental high resolution electron microscopy of gas‐catalyst reactions

Environmental electron microscopy has become an important scientific method for fundamental studies of dynamic chemical reaction processes in heterogeneous catalysis and of catalytic growth of carbon nanotubes. Outstanding contributions are resulting from the ability to observe gas‐catalyst surface reactions in situ, on the atomic scale. A great deal of structural and chemical information including lattice modification of working catalysts is possible. This is key to understanding novel reaction processes, including release mechanism of structural oxygen in oxide catalysts in selective oxidation of hydrocarbons and to designing improved catalysts. This brief survey of the recent spectacular developments in environmental high resolution electron microscopy shows that new opportunities are being opened up in catalysis. This revised version was published online in August 2006 with corrections to the Cover Date.     

镁基固体催化剂催化酯交换反应制备生物柴油研究进展

生物柴油是一种重要的可再生能源,非均相催化甘油酯酯交换反应制备生物柴油是当前的研究热点,而高活性的非均相催化剂则是非均相催化工艺的核心。镁基催化剂由于其原料来源广泛、价格低廉,受到了广泛关注。综述近年来镁基催化剂催化酯交换反应制备生物柴油的研究进展,重点介绍了镁基催化剂制备方法、组成、结构、反应条件等对催化酯交换反应活性的影响,并探讨了镁基催化剂目前存在的不足以及今后的发展方向。     

Building and characterization of catalysts on single crystal surfaces

The components of complex catalyst systems include transition metals, oxides, alkali metal or halogen additives and strongly chemisorbed sulfur or carbonaceous species. Modern surface science techniques identified the chemical and structural roles of these ingredients when used in combination with catalytic reaction rate studies. Using metal single crystal substrates, catalyst components have been deposited from the vapor phase and the complex catalyst system could be built this way. The Pt-Re-S, Rh-TiO₂, and Mo-Co-S systems are reviewed and the requirements for the proper surface structure and composition to obtain high reaction rates and selectivities are discussed.     

Untersuchung von Heterogen-Katalysatoren mit modernen Oberflächen-Analysenmethoden

Study of heterogeneous catalysts by modern surface analysis methods . The past decade has been one of tempestuous development in methods of surface analysis as a result of significant improvements of vacuum techniques and electronic components. The principal methods available for studying the chemical composition of catalysts are ESCA, Auger spectroscopy, ISS (ion scattering spectroscopy), and SIMS (secondary ion mass spectrometry). The “classical scanning microscope” and various “micro-range” analyzers, e. g. the scanning Auger microscope, permit elucidation of catalyst surface structures. Advantages and drawbacks of the various methods are presented with the aid of selected examples and the limits of the analysis of industrial catalysts are discussed.     

In-Situ Vibrational Spectroscopic Studies on Model Catalyst Surfaces at Elevated Pressures

Elucidation of complex heterogeneous catalytic mechanisms at the molecular level is a challenging task due to the complex electronic structure and the topology of catalyst surfaces. Heterogeneous catalyst surfaces are often quite dynamic and readily undergo significant alterations under working conditions. Thus, monitoring the surface chemistry of heterogeneous catalysts under industrially relevant conditions such as elevated temperatures and pressures requires dedicated in situ spectroscopy methods. Due to their photons-in, photons-out nature, vibrational spectroscopic techniques offer a very powerful and a versatile experimental tool box, allowing real-time investigation of working catalyst surfaces at elevated pressures. Infrared reflection absorption spectroscopy (IRAS or IRRAS), polarization modulation-IRAS and sum frequency generation techniques reveal valuable surface chemical information at the molecular level, particularly when they are applied to atomically well-defined planar model catalyst surfaces such as single crystals or ultrathin films. In this review article, recent state of the art applications of in situ surface vibrational spectroscopy will be presented with a particular focus on elevated pressure adsorption of probe molecules (e.g. CO, NO, O₂, H₂, CH₃OH) on monometallic and bimetallic transition metal surfaces (e.g. Pt, Pd, Rh, Ru, Au, Co, PdZn, AuPd, CuPt, etc.). Furthermore, case studies involving elevated pressure carbon monoxide oxidation, CO hydrogenation, Fischer–Tropsch, methanol decomposition/partial oxidation and methanol steam reforming reactions on single crystal platinum group metal surfaces will be provided. These examples will be exploited in order to demonstrate the capabilities, opportunities and the existing challenges associated with the in situ vibrational spectroscopic analysis of heterogeneous catalytic reactions on model catalyst surfaces at elevated pressures.     

适用于丙烯二聚反应非均相催化剂的研究进展

丙烯二聚反应是生产4-甲基-1-戊烯、1-己烯等端碳烯烃的重要手段,其产物可作为特种高分子材料单体、汽油添加剂和化工有机中间体,关键技术在于高效催化剂的开发。非均相催化剂相比于均相催化剂因易回收、对环境污染小等优点受到了学者们的广泛关注。本文综述了丙烯二聚非均相催化剂的研究进展,依据固体碱催化剂与固体酸催化剂在丙烯二聚反应中不同的反应机理,比较了两者各自的优缺点。回顾了固体碱催化剂的发展历程及工业化应用,并以碱金属钾为例总结了反应机理。详细介绍了固体酸催化剂中的固体磷酸催化剂、分子筛催化剂和负载型过渡金属催化剂。针对固体碱催化剂制备条件苛刻等问题,提出了改善催化装置的新思路;针对固体酸催化剂选择性不足的劣势,指出了应该进一步完善机理,并合理设计酸性载体与过渡金属相结合的催化剂。     

Heterogeneous Catalytic Oxidation of Acrolein to Acrylic Acid: Mechanism and Catalysts

Partial oxidation of acrolein is a commercially important reaction, its product—acrylic acid—being widely used industrially for producing resins, dyes, glues, nonwoven fabrics, etc.

Partial oxidation of acrolein is also a convenient model reaction because: (1) the number of reaction products is moderate (CO, CO₂, acrylic acid) and (2) their difference in acid-base properties from the starting material makes it possible to select desirable catalysts by applying directly and efficiently Boreskov's concept of intermediate chemical interaction of a catalyst with reaction mixture components. According to this concept [1], the transformation of surface intermediates (SI) formed in the interaction of reactants with a catalyst's surface is determined by the structure and bond energy of these SI.

The study of the reaction mechanism includes determination of structures and energy characteristics of the surface intermediates and the elucidation of their connection with catalyst chemical composition and reaction routes to particular products. This reliable information helps us to understand the nature of catalyst action and to elaborate the theory of catalyst selection. We have used this method to approach the problem of the systematic selection of catalysts for the oxidation of acrolein to acrylic acid. The review summarizes the research done in the lnstitute of Catalysis of the Siberian Branch of the Russian Academy of Sciences during recent years.