Role of surface science in catalysis

Around the time of World War I, Langmuir advanced a simple theory of chemisorption and showed how it could be used to formulate rate laws for reactions occurring on surfaces. From that time on, surface science has played an important role in heterogeneous catalysis. Between the two world wars, simple studies of extents of adsorption by catalyst surfaces led to the concept of activated adsorption and to a universally used method for determining the high surface areas associated with the pore structures of catalytic materials. After World War II, the application of various spectroscopic and structural probes made it possible to investigate catalyst surfaces at a more microscopic level. Studies with idealized surfaces such as the faces of single crystals in ultra-high vacuum apparatus also made their appearance. By the end of the twentieth century, direct information was being obtained on the rates of elementary reactions of well-defined surface species. The results of such work are beginning to put “finishing touches” on the great insight of early pioneers in surface science and heterogeneous catalysis. Much has been accomplished, but exciting opportunities still remain.     

Tight-binding studies of surface effects on electronic structure of CdSe nanocrystals: the role of organic ligands, surface reconstruction, and inorganic capping shells

We utilize a tight-binding model to study the effects of surface structure on electronic properties of CdSe clusters. The model takes into account experimental information about structure and shape of the nanocrystals, as well as the nature and distribution of capping ligands. The effects of both organic capping ligands and inorganic capping shells on the densities of states (DOS) and on the single-particle absorption spectra of the clusters are calculated for various cluster shapes and sizes, and are compared to results for clusters with truncated surfaces. For organic capping ligands, the effect of ligand hybridization is investigated and a simple model of surface reconstruction is developed. Both ligand hybridization and surface reconstruction are seen to have a major influence on the band edge electronic and optical properties. Inorganic capping shells give rise to differential localization of valence and conduction band edge states, with the hole primarily confined to the core region and the electron more evenly distributed over both core and shell. Received 21 September 1998 / Received in final form: 15 December 1998     

The role of surface science experiments in understanding heterogeneous catalysis

Heterogeneous catalysis is of enormous industrial importance for the large scale production of chemicals. A continuous research effort is devoted to improving the activity and selectivity of catalysts. Research is also expected to replace gradually empiricism by a more fundamental understanding of the governing factors of catalysis. Surface science in particular is capable of providing this fundamental information because of the advent of a multitude of novel surface characterization techniques. These new surface analytical tools operative under vacuum conditions are used to characterize catalytic surfaces with respect to composition, crystallographic and electronic structure. At the same time catalytic reactions are studied on these well characterized surfaces. A comparison of results from these studies with those obtained by more traditional catalysis research, e.g. at high pressure and on supported catalysts, turns out to be most interesting and informative.     

Infrared photoacoustic spectroscopy in catalysis and surface science

The analysis of solid samples can often be a difficult problem for the researchers dealing with infrared (IR) spectroscopy. In conventional absorption spectroscopy the measurement of absorption is transferred to that of the radiation transmitted through the sample. Three methods stand out as being more suitable for studying solid materials. These methods are: diffuse reflectance (DR), photoacoustic spectroscopy (PAS), and Fourier transform (FT) Raman. All three methods require little or no sample preparation, and therefore are ideal for the samples that may change during the preparation as mineral oil mulls or KBr disks. In the case of PAS, the adsorbed radiation is determined directly via its heat and hence the sound produced in the sample. Fourier transform infrared PAS (FT-IR/PAS) is one of the main IR techniques which can be successfully applied in catalysis and surface science research. Recent examples of this spectroscopic technique application will be presented.     

有机超微粒的制备及其尺寸效应的研究

有机超微粒是国际上刚刚起步的研究领域,是纳米科技领域和有机光电子领域的重要前沿课题.文章从有机功能小分子出发,在制备粒径和形状可控的、高度单分散的纳米超微粒的基础上,首次系统地研究了有机超微粒的电子态随尺寸大小的变化过程.发现有机超微粒和无机超微粒一样具有显著的尺寸效应,而且更具多样性.该项研究工作为探索和比较无机和有机材料介观尺寸效应的异同点这一科学问题奠定了坚实的基础,对于理解有机分子晶体这类传统材料中的基本过程和现象以及开发新型光电材料和器件也极具意义.     

Device relevant organic films and interfaces: A surface science approach

Here the morphology, molecular orientation and electronic structure of in situ prepared para-sexiphenyl (6P) and α-sexithiophene (6T) films studied with atomic force microscopy, near edge X-ray absorption fine structure spectroscopy and valence band photoemission are presented. Attention is given to the differences between different organic crystallite orientations and the pitfalls in the interpretation of area averaging surface sensitive techniques that can arise from inhomogeneities in the films, which commonly occur even on single crystal inorganic substrates. The growth of organic-organic heterostructures is then considered for sexithiophene films grown on homogeneous upright (6P(0 0 1)) and lying (6P(2 0 3)) crystalline films. In both cases, the orientation of the substrate molecules is imposed on the molecules of the second species and thick films of upright-on-upright or lying-on-lying could be produced. The organic substrates are thus shown to be excellent templates for further organic film growth that do not require the stringent UHV conditions of inorganic templates.     

X-ray studies of the structure and electronic behavior of alkanethiolate-capped gold nanoparticles: the interplay of size and surface effects

We report a study of the structure and electronic properties of a series of thiol-capped Au nanoparticles (NP) of nominal sizes of 1.6, 2.4, and 4.0 nm. Transmission electron microscopy, x-ray powder diffraction, x-ray absorption fine structure, and x-ray photoemission spectroscopy have been used to investigate the size-dependent systematics of lattice contraction and charge redistribution of these NPs. It is found that the lattice contracts and the d charge at the Au atom site depletes relative to bulk Au as the size of the NP decreases. The implication of these observations is discussed in terms of the interplay of quantum-size and surface effect.     

Fractal analysis of size effects and surface morphology effects in catalysis and electrocatalysis

Fractal geometry tools are used in order to analyze several related problems in surface science, catalysis, and electrocatalysis. The effects of complex morphologies of adsorbents, catalysts, and electrodes on various molecular processes with these materials are determined both theoretically and experimentally. It is shown that fractal geometry provides a convenient and natural tool for the elucidation of geometry-performance relations in heterogeneous chemistry. Issues covered are particle size effects in physisorption and chemisorption; morphology effects on a variety of catalytic processes with unsupported catalysts (including coal liquefaction, alkene polymerizations, oxidations, dehydrogenations, and esterifications); surface accessibility effects on molecular interactions in an Eley-Rideal mechanism; surface patterning effects on concentration profiles near the surface; and electrode-morphology effects on a variety of electrochemical and electrocatalytic processes. The domains of applicability of the fractal approach to these problems is discussed.     

The surface chemistry of catalysis: new challenges ahead

Surface scientists have over the last few decades greatly advanced the atomic level understanding of the surface chemical reactions associated with heterogeneous catalysis. Nonetheless, many fundamental questions still remain unanswered. In this review, a critical analysis of the state of the art of this field is provided, and a number of future research directions are suggested.     

The diamond surface: atomic and electronic structure

Experimental studies of the diamond surface with primary emphasis on the (111) surface are presented. Aspects of the diamond surface which are addressed include (1) the electronic structure, (2) the atomic structure, and (3) the effect of termination of the lattice by foreign atoms. Limited studies of graphite are discussed for comparison with the diamond results. Experimental results from valence band and core level photoemission spectroscopy (PES), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) are used to study and characterize both the clean and hydrogenated surface. In addition, the interaction of hydrogen with the diamond surface is examined using results from vibrational high resolution low energy electron loss spectroscopy and photon stimulated ion desorption (PSID) yield at photon energies just above the carbon k-edge. Both EELS and PSID verify that the mechanically polished 1 × 1 surface is hydrogen terminated and also that the reconstructed 2×2/2×1 surface is hydrogen free. We apply this basic knowledge of the clean diamond surface and of diamond-hydrogen systematics to understanding of the fundamental growth characteristics of diamond films.     

The surface science of enzymes

One of the largest challenges to science in the coming years is to find the relation between enzyme structure and function. Can we predict which reactions an enzyme catalyzes from knowledge of its structure—or from its amino acid sequence? Can we use that knowledge to modify enzyme function? To solve these problems we must understand in some detail how enzymes interact with reactants from its surroundings. These interactions take place at the surface of the enzyme and the question of enzyme function can be viewed as the surface science of enzymes. In this article we discuss how to describe catalysis by enzymes, and in particular the analogies between enzyme catalyzed reactions and surface catalyzed reactions. We do this by discussing two concrete examples of reactions catalyzed both in nature (by enzymes) and in industrial reactors (by inorganic materials), and show that although analogies exist and the two kinds of catalyst can be described by similar tools, nature and human effort have come up with different solutions. This on the other hand implies that new and improved catalysts may be made by learning from nature.     

The surface science of xerography

Over the past four decades xerography, the dry ink marking process developed by the photocopy industry, has grown from nothing into a $170 billion industry worldwide. This amazing commercial success is due to the fact that during this period, xerographic technology experienced constant and often-dramatic improvement created by sustained industrywide research and development. Indeed, the development of the xerographic copying and printing industry is one of the great applied surface science successes of all time. In this article we outline the story of the advances in xerographic technology during the past four decades, describe the profound dependence on these advances of the control of surface and interface properties of increasingly sophisticated multi-component materials systems, and indicate the potential impact on the industry of the continuing development of the surface and interface science of the multi-component materials packages used in xerographic technology.     

Radial vibration of ultra-small nanoparticles with surface effects

An elastic model to predict radial vibration of ultra-small nanoparticles is proposed and the main reason of frequency shifts (comparing with classical elastic model) in ultra-small nanoparticles is interpreted. Taking the curvature-dependent surface theory into account, the effects of surface on the radial vibrations of nanoparticles are investigated with our new model. Both the atomic and the present models are calculated and their results agree well. It argues that the surface effects are remarkable on the radial vibrations of ultra-small nanoparticles and surface elasticity plays the main role rather than surface stress which is the previous understanding. The curvature-dependence of surface effects cannot be ignored when the particle is small enough. For the low-order radial vibration, the surface effects are more noteworthy.     

A prospective: Surface science and catalysis at the nanoscale

Surface science has progressed from its beginnings, which focused on simple materials and adsorbates, to the study of much more complex materials. In this article I focus on the developments in the field of nanoparticle surface science, especially those relating to advances in our understanding of heterogeneous catalysis. Methods to make such materials and to characterise oxidic supports have advanced enormously in the last few years, but efforts in the field are still rather limited and patchy. Such work will expand significantly in the next few years and many new, exciting discoveries await us. Important areas for development include the fabrication of ordered arrays of monodisperse nanoparticles, imaging small metal particles at atomic resolution and carrying such investigations out under high pressure/temperature conditions in order to identify active sites on nanoparticles under realistic conditions.     

The effects of vacuum annealing on the structure and surface chemistry of iron:nickel alloy nanoparticles

In order to increase the longevity of contaminant retention on the particle surface, a method is sought to improve the corrosion resistance of bimetallic iron nickel nanoparticles (INNP) used for the remediation of contaminated water, and thereby extend their industrial lifetime. A multi-disciplinary approach was used to investigate changes induced by vacuum annealing (<5 × 10^?8 mbar) at 500 °C on the bulk and surface chemistry of INNP. The particle size was determined to increase significantly as a result of annealing and the thickness of the surface oxide increased by 50%. BET analysis recorded a decrease in INP surface area from 44.88 to 8.08 m² g^?1, consistent with observations from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which indicated the diffusion bonding of previously discrete particles at points of contact. X-ray diffraction (XRD) confirmed that recrystallisation of the metallic cores had occurred, converting a significant fraction of initially amorphous iron nickel alloy into crystalline FeNi alloy. X-ray photoelectron spectroscopy (XPS) indicated a reduction in the proportion of surface iron oxide and a change in its stoichiometry related to annealing-induced disproportionation. This was also evidenced by an increased proportion of Fe(0) and Ni(0) to Fe- and Ni-oxides, respectively. The data also indicated the concurrent development of boron oxide at the metal surfaces, which accounts for the overall increase measured in surface oxide thickness. The improved core crystallinity and the presence of passivating impurity phases at the INNP surfaces may act to improve the corrosion resistance and reactive lifespan of the vacuum annealed INNP for environmental applications.     

Sonochemical fabrication of inorganic nanoparticles for applications in catalysis

Catalysis covers almost all the chemical reactions or processes aiming for many applications. Sonochemistry has emerged in designing and developing the synthesis of nano-structured materials, and the latest progress mainly focuses on the synthetic strategies, product properties as well as catalytic applications. This current review simply presents the sonochemical effects under ultrasound irradiation, roughly describes the ultrasound-synthesized inorganic nano-materials, and highlights the sonochemistry applications in the inorganics-based catalysis processes including reduction, oxidation, degradation, polymerization, etc. Or all in all, the review hopes to provide an integrated understanding of sonochemistry, emphasize the great significance of ultrasound-assisted synthesis in structured materials as a unique strategy, and broaden the updated applications of ultrasound irradiation in the catalysis fields.     

The effects of vacuum annealing on the structure and surface chemistry of iron nanoparticles

In order to increase the longevity of contaminant retention, a method is sought to improve the corrosion resistance of iron nanoparticles (INP) used for remediation of contaminated water and thereby extend their industrial lifetime. A multi-disciplinary approach was used to investigate changes induced by vacuum annealing (<5 × 10^?8 mbar) at 500 °C on the bulk and surface chemistry of INP. The particle size did not change significantly as a result of annealing but the surface oxide thickness decreased from an average of 3–4 nm to 2 nm. BET analysis recorded a decrease in INP surface area from 19.0 to 4.8 m² g^?1, consistent with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations which indicated the diffusion bonding of previously discrete particles at points of contact. X-ray diffraction (XRD) confirmed that recrystallisation of the metallic cores had occurred, converting a significant fraction of poorly crystalline iron to bcc α-Fe and Fe₂B phases. X-ray photoelectron spectroscopy (XPS) indicated a change in the surface oxide stoichiometry from magnetite (Fe₃O₄) towards wüstite (FeO) and the migration of boron and carbon to the particle surfaces. The improved core crystallinity and the presence of passivating impurity phases at the INP surfaces may act to improve the corrosion resistance and reactive lifespan of the vacuum annealed INP for environmental applications.     

高压对有机化合物晶体结构和电子结构的作用

压力作为与温度同等重要的一个环境物理参量，在众多研究领域中越来越受到重视。经过几十年的发展，现在已经可以实现对超高压的可控调节。将高压作为一个重要的环境物理参量引入到科学研究的各个领域，将会大大扩展研究的范围．文章介绍了近年发展起来的一种简单安全的高压产生技术，以及利用这种高压产生技术在超高压条件下开展的一些有机化合物光物理性质的研究工作。     

高压对有机化合物晶体结构和电子结构的作用

压力作为与温度同等重要的一个环境物理参量,在众多研究领域中越来越受到重视.经过几十年的发展,现在已经可以实现对超高压的可控调节.将高压作为一个重要的环境物理参量引入到科学研究的各个领域,将会大大扩展研究的范围.文章介绍了近年发展起来的一种简单安全的高压产生技术,以及利用这种高压产生技术在超高压条件下开展的一些有机化合物光物理性质的研究工作.