The Effect of Surface Composition and Structure on Elementary Chemical Reactions on Metallic Substrates

Studies of chemical reactions on well-defined surfaces have considerable potential for providing fundamental knowledge of surface reactivity and guidelines for the understanding of catalytic materials. with the use of low energy election deffraction (LEED)[1,2] surface structures can be diferentiated and the effect of these structures on surface reactivity determined. In addition, the relative case of obtaining surfaces of known composition utilizing Auger electron spectroscopy (AES)[3-5] makes it possible to study metal surfaces with less than 1% impurity and, moreover, to prepare surfaces with known coverages of adatoms such as carbon, oxygen, and sulfur in order to study the effects of these spacies on the activity and selectivity of the surface for given reactants. In some cases, as illustrated of the surface adatoms.     

The Role of Redox,Acid-Base and Collective Properties and of Cristalline State of Heterogeneous Catalysts in the Selective Oxidation of Hydrocarbons

The main aspects of heterogeneous selective oxidation on metal oxides are considered, in particular the importance of acid--base and redox properties. Particular emphasis is placed on the dynamics of the surface under catalytic conditions which involves a redox process, dehydroxylation of the surface, reversible formation of point defects and shear plane transformations, changes in collective properties due to the adsorption of reactants/products and reduction of the solid (electrical conductivity variations), amorphous state of a few surface top layers, etc. All these aspects affect the reaction mechanism and, subsequently, the activity and selectivity of the reactions and result in a very complex system, particularly difficult to characterise. However, despite this great complexity some general conclusions can be drawn.     

Reactions of late transition metal complexes with molecular oxygen

Limited natural resources, high energy consumption, economic considerations, and environmental concerns demand that we develop new technologies for the sustainable production of chemicals and fuels. New methods that combine the selective activation of C-H bonds of hydrocarbons with oxidation by a green oxidant such as molecular oxygen would represent huge advances toward this goal. The spectacular selectivity of transition metals in cleaving C-H bonds offers the potential for the direct use of hydrocarbons in the production of value-added organics such as alcohols. However, the use of oxygen, which is abundant, environmentally benign, and inexpensive (particularly from air), has proven challenging, and more expensive and less green oxidants are often employed in transition-metal-catalyzed reactions. Advances in the use of oxygen as an oxidant in transition-metal-catalyzed transformations of hydrocarbons will require a better understanding of how oxygen reacts with transition metal alkyl and hydride complexes. For alkane oxidations, researchers will need to comprehend and predict how metals that have shown particularly high activity and selectivity in C-H bond activation (e.g. Pt, Pd, Rh, Ir) will react with oxygen. In this Account, we present our studies of reactions of late metal alkyls and hydrides with molecular oxygen, emphasizing the mechanistic insights that have emerged from this work. Our studies have unraveled some of the general mechanistic features of how molecular oxygen inserts into late metal hydride and alkyl bonds along with a nascent understanding of the scope and limitations of these reactions. We present examples of the formation of metal hydroperoxide species M-OOH by insertion of dioxygen into Pt(IV)-H and Pd(II)-H bonds and show evidence that these reactions proceed by radical chain and hydrogen abstraction pathways, respectively. Comparisons with recent reports of insertion of oxygen into other Pd(II)-H complexes, and also into Ir(III)-H and Rh(III)-H complexes, point to potentially general mechanisms for this type of reaction. Additionally, we observed oxygen-promoted C-H and H-H reductive elimination reactions from five-coordinate Ir(III) alkyl hydride and dihydride complexes, respectively. Further, when Pd(II)Me(2) and Pt(II)Me(2) complexes were exposed to oxygen, insertion processes generated M-OOMe complexes. Mechanistic studies for these reactions are consistent with radical chain homolytic substitution pathways involving five-coordinate M(III) intermediates. Due to the remarkable ability of Pt(II) and Pd(II) to activate the C-H bonds of hydrocarbons (RH) and form M-R species, this reactivity is especially exciting for the development of partial alkane-oxidation processes that utilize molecular oxygen. Our understanding of how late transition metal alkyls and hydrides react with molecular oxygen is growing rapidly and will soon approach our knowledge of how other small molecules such as olefins and carbon monoxide react with these species. Just as advances in understanding olefin and CO insertion reactions have shaped important industrial processes, key insight into oxygen insertion should lead to significant gains in sustainable commercial selective oxidation catalysis.     

The electronic structure effect in heterogeneous catalysis

Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture is given of the chemisorption bond. It is suggested that the adsorption bond strength and hence the activity of transition metal surfaces as catalysts for chemical reactions can be related to certain characteristics of the surface electronic structure.     

Interfacial Reactions in Glass–Ceramic-to-Metal Seals

During the sealing of glass–ceramics to metals, undesirable interfacial reactions may occur between constituents of the glass and diffusing metal species, and any reaction products formed may have serious consequences on the lifetime behavior of seal components. This paper reviews the factors influencing the lifetime behavior of glass–ceramic-to-metal systems, with particular emphasis given to the effect that such interfacial reactions can have on the resultant and longer term properties. Steps that can be taken to enhance desirable interfacial reactions and minimize those that are undesirable are highlighted. In addition, in order to aid in the understanding of complex alloy systems, it is of interest to study both the interfacial reactions and diffusion characteristics of simplified systems without the complicating issue of multiple element diffusion. Thus, a preliminary investigation is highlighted in which a lithium zinc silicate glass nucleated with P₂O₅ has been sealed to high-purity Fe, Ni, and Cr metals and the resultant diffusion into the glass of each metal monitored as a function of sealing temperature and time. The data generated have been compared with information from multicomponent alloy systems and an attempt has been made to explain the differences noted.     

Multifunctional graphene‐based nanostructures for efficient electrocatalytic reduction of oxygen

Graphene derivatives have been used extensively as a functional support for nanoparticle catalysts in diverse applications, in particular, oxygen reduction reactions (ORR) at fuel cell cathodes. This review summarizes recent progress in this area of research, where the catalytic performance is evaluated within the context of stabilization of metal nanoparticles against sintering/aggregation and metal–substrate interactions that manipulate the electronic properties of metal nanoparticles and hence the bonding interactions with reaction intermediates. Also discussed are the latest breakthroughs of heteroatom‐doped graphene derivatives as effective metal‐free catalysts for oxygen reduction. In addition, the review includes a perspective on the development of effective ORR catalysts with a focus on a further understanding of the ORR mechanism as well as on other two‐dimensional layered nanostructures such as MoS₂ that have been observed to exhibit electrocatalytic activity for oxygen reduction. Leading mechanistic models are discussed to account for the electrocatalytic activity. © 2015 Society of Chemical Industry     

负载型金属催化剂的热稳定机制

负载型金属催化剂是一类重要的催化材料,在石油炼制、环境保护以及材料合成等领域起着重要的作用。然而,由于活性金属在反应环境下容易烧结团聚,以致活性降低乃至失活,因此,如何提高其热稳定性成为负载型金属催化剂研究的一个关键问题。概述了催化剂的金属团聚成因及其稳定机制。简要介绍了Ostwald效应以及颗粒合并长大两种团聚模型,从热力学角度解释了导致催化剂烧结团聚的原因。总结了现阶段几种提高负载型金属催化剂热稳定性能的方法,具体包括以包覆封装隔离为原理的物理方法,以及以形成化学键为基础的化学方法,可为进一步开发高热稳定性的负载型金属催化剂提供借鉴。     

RECENT DEVELOPMENTS IN FREE RADICAL POLYMERIZATION AT HIGH CONVERSION—DIFFUSION CONTROLLED TERMINATION AND PROPAGATION

A deeper understanding of the dynamics of macromolecules in polymer melts (reptation theory—de Genne and others refer to PVC-Cleveland on long term aging of glassy polymers) has sparked a number of investigations on the modelling of the termination and propagation reactions in free radical homopolymerization. These recent contributions will be reviewed and some statements will be made about limitations of the present models and recommendations will be made for future study.     

Variation in physical and mechanical properties with coating thickness in epoxy–diamine–aluminum system

The mechanical properties and glass‐transition temperature within different thickness organic coatings made of diglycidyl ether of bisphenol A epoxy resin and 3‐aminomethyl‐3,5,5‐trimethylcyclohexylamine hardener are determined. The coatings are deposited on aluminum alloy (1050) substrates after degreasing. Dynamic mechanical thermal analysis and differential scanning calorimetry experiments are carried out on debonded coatings before and after the material from the opposed surface to the polymer/metal interface is removed by polishing. The results clearly show that the values of the physical and mechanical properties in those coatings depend on their thickness, but there is no gradient of properties within such coatings. Therefore, at a given thickness, those properties are homogeneous within the coating. To gain a better fundamental understanding of this behavior, a qualitative model involving the chemical reactions that take place at the epoxy/metal interface and the related diffusion phenomena is given. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 891–895, 2005     

Catalysis on Supported Metals

It is often commented that the practice of catalysis has preceded its understanding. The comment is nowhere more valid than in the area of catalytic action on transition metal surfaces which is the primary basis of the present review. We shall attempt here to survey some developments which have brought about a renewal of interest in work with metals supported on “inert” carriers as a means of furthering understanding of catalytic action. Summaries of work with supported catalysts from a number of aspects have been given from time to time [1-7].     

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.     

Bimetallic Cobalt Based Catalysts

Highly dispersed, oxide- or zeolite-supported bimetallic catalysts are widely used in the catalytic industry, such as in catalytic reforming, nitrogen industry and gas-to-liquid technology. The paper highlights the nano-sized Co-based bimetallic system in terms of correlation between structure and reactivity/selectivity promoted by the second metal. Opposite to the bulk alloys nano-sized bimetallic catalysts are extremely sensitive to the structure, morphology, valence state of the supporting oxide material in which the nano-particles are embedded. In this case, one of the less reducible components, which strongly interact with the supports, may stabilize the second, more noble metals, and thus the latter can be stabilized in highly dispersed state. Conversely, addition of noble metal to the hardly reducible component may facilitate reduction, which causes the retardation of the deactivation process of some hydrocarbon reaction. The future trend is the application of bimetallic nano-particles although careful consideration and experimentation should be taken to elucidate the structure of such type of catalyst. The various effects of Co-based bimetallic particles, such as particle size, metal/support interface, morphology and electronic effects, on the activity/selectivity in given catalytic reactions will be discussed.     

Complementary structure sensitive and insensitive catalytic relationships

The burgeoning field of nanoscience has stimulated an intense interest in properties that depend on particle size. For transition metal particles, one important property that depends on size is catalytic reactivity, in which bonds are broken or formed on the surface of the particles. Decreased particle size may increase, decrease, or have no effect on the reaction rates of a given catalytic system. This Account formulates a molecular theory of the structure sensitivity of catalytic reactions based on the computed activation energies of corresponding elementary reaction steps on transition metal surfaces. Recent progress in computational catalysis, surface science, and nanochemistry has significantly improved our theoretical understanding of particle-dependent reactivity changes in heterogeneous catalytic systems. Reactions that involve the cleavage or formation of molecular pi-bonds, as in CO or N(2), must be distinguished from reactions that involve the activation of sigma-bonds, such as CH bonds in methane. The activation of molecular pi-bonds requires a reaction center with a unique configuration of several metal atoms and step-edge sites, which can physically not be present on transition metal particles less than 2 nm. This is called class I surface sensitivity, and the rate of reaction will sharply decrease when particle size decreases below a critical size. The activation of sigma chemical bonds, in which the activation proceeds at a single metal atom, displays a markedly different size relationship. In this case, the dependence of reaction rate on coordinative unsaturation of reactive surface atoms is large in the forward direction of the reaction, but the activation energy of the reverse recombination reaction will not change. Dissociative adsorption with cleavage of a CH bond is strongly affected by the presence of surface atoms at the particle edges. This is class II surface sensitivity, and the rate will increase with decreasing particle size. Reverse reactions such as hydrogenation typically show particle-size-independent behavior. The rate-limiting step for these class III reactions is the recombination of an adsorbed hydrogen atom with the surface alkyl intermediate and the formation of a sigma-type bond. Herein is our molecular theory explaining the three classes of structure sensitivity. We describe how reactions with rates that are independent of particle size and reactions with a positive correlation between size and rate are in fact complementary phenomena. The elucidation of a complete theory explaining the size dependence of transition metal catalysts will assist in the rational design of new catalytic systems and accelerate the evolution of the field of nanotechnology.     

Proton Transfer Processes in Water Clusters

Elucidation of some of the basic mechanisms and principles of proton transfer in clusters is achieved by systematic evaluation of experimental and theoretical results that were obtained throughout the last decade from the authors' studies on ionic water clusters and, more recently, on protonated rare gas clusters. Among the processes studied are: Blackbody radiation-induced, stepwise desolvation of clusters when stored in an ion trap; ionic dissociation and recombination of HCl in water clusters; metal oxidation in aqueous clusters; formation of nascent hydrogen in reactions of solvated metal cations with acids; and the behavior of ionic water clusters as acidic or basic micro-solutions. Infrared spectroscopic studies of size-selected water clusters are expected to advance our knowledge of structure and dynamics of water clusters and of excess protons therein.     

Innovative technologies in the control of lipid oxidation

Transition metal promoted oxidative degradation reactions impact quality, shelf‐life, and nutritional content of many packaged foods. When trace metals are present in packaged foods, they can initiate degradation of nutritional compounds such as unsaturated fatty acids, carotenoids, antioxidants, phytosterols, and many vitamins. These reactions occur at metal concentrations naturally occurring in foods (often in the low parts per billion range) therefore complete metal removal is not a practical solution to inhibit these reactions. Chelators such as EDTA can be added to inhibit metal‐promoted oxidation, however there is significant consumer and industry demand to eliminate EDTA from product formulations. Other natural chelators such as citric acid are ineffective in many foods and in some cases can actually increase oxidation rates by increasing metal solubility. An alternate approached to control transition metal reactivity is to utilize active packaging technologies. However, current approaches to antioxidant active packaging can impact packaging film mechanical properties and often exhibit low antioxidant activity. This review article surveys and critically reviews advances in the control of lipid oxidation, paying particular attention to novel advances in both food product formulation (e. g. additives, food microstructure) as well as food packaging. We introduce a new concept of active packaging in which metal ion chelation by non‐migratory active packaging materials may enable removal of synthetic food additives from product formulations while maintaining product quality and shelf‐life.     

Polystyrene-supported thiosemicarbazone–transition metal complexes: synthesis and application as heterogeneous catalysts

Functionalized polymers are found to be highly efficient in immobilizing transition metal ions. Crosslinked polystyrene supported Schiff's base complexes of metal ions such as Fe(III), Co(II), Ni(II) and Cu(II) are very effective as heterogeneous catalysts. The catalytic activity of these metal complexes has been studied in the decomposition of H₂O₂ and in the epoxidation of cyclohexene and styrene. The reactions show a first order dependence on the concentration of both the substrates and the catalyst. The influence of the degree of crosslinking of the polymer support on the rate of reactions has been studied. The metal complexes show low catalytic activity at low crosslink density (2% and 5%) but 10% crosslinked resins show higher activity. A possible mechanism for the reactions is suggested. © 1999 Society of Chemical Industry     

Catalysis on Supported Metals

It is often commented that the practice of catalysis has preceded its understanding. The comment is nowhere more valid than in the area of catalytic action on transition metal surfaces which is the primary basis of the present review. We shall attempt here to survey some developments which have brought about a renewal of interest in work with metals supported on “inert” carriers as a means of furthering understanding of catalytic action. Summaries of work with supported catalysts from a number of aspects have been given from time to time [1-7].     

Life of the Tafel equation: Current understanding and prospects for the second century

The life of Tafel equation is considered briefly as evolution in the understanding of Tafel's empiric parameters in the framework of various phenomenological and theoretical approaches. Modern theories of the interfacial charge transfer reactions are employed to explain the behavior of transfer coefficient versus electrode overvoltage and deviations of this quantity from 0.5 at low overvoltage. The effects of intramolecular reorganization, orbital overlap, reactant quantum modes and solvent dynamics are addressed and illustrated by model calculations. An attempt is made to propose new explanations of some data reported in the literature.