Use of molecular beams for kinetic measurements of chemical reactions on solid surfaces

In this review we survey the contributions that molecular beam experiments have provided to our understanding of the dynamics and kinetics of chemical interactions of gas molecules with solid surfaces. First, we describe the experimental details of the different instrumental setups and approaches available for the study of these systems under the ultrahigh vacuum conditions and with the model planar surfaces often used in modern surface-science experiments. Next, a discussion is provided of the most important fundamental aspects of the dynamics of chemical adsorption that have been elucidated with the help of molecular beam experiments, which include the development of potential energy surfaces, the determination of the different channels for energy exchange between the incoming molecules and the surface, the identification of adsorption precursor states, the understanding of dissociative chemisorption, the determination of the contributions of corrugation, steps, and other structural details of the surface to the adsorption process, the effect to molecular steering, the identification of avenues for assisting adsorption, and the molecular details associated with the kinetics of the uptake of adsorbates as a function of coverage. We follow with a summary of the work directed at the determination of kinetic parameters and mechanistic details of surface reactions associated with catalysis, mostly those promoted by late transition metals. This discussion we initiate with an overview of what has been learned about simple bimolecular reactions such as the oxidation of CO and H₂ with O₂ and the reaction of CO with NO, and continue with the review of the studies of more complex systems such as the oxidation of alcohols, the conversion of organic acids, the hydrogenation and isomerization of olefins, and the oxidative activation of alkanes under conditions of short contact times. 6 Reactions on supported nanoparticles: Materials gap, 7 Low-probability reactions: Pressure gap of this review deal with the advances made in the use of molecular beams with more realistic models for catalysis, using surfaces comprised of metal nanoparticles dispersed on the oxide surfaces used as catalyst support and high-flux beams to approach the pressures used in catalysis. The next section deals with the study of systems associated with fields other than catalysis, mainly with the etching and oxidation of semiconductor surfaces and with the chemistry used to grow thin solid films by chemical means (chemical vapor deposition, CVD, or atomic layer deposition, ALD). We end with a personal assessment of the past accomplishments, present state, and future promise of the use of molecular beams for the study of the kinetics of surface reactions relevant to practical applications.     

交换关联泛函近似方法对Ru催化剂上甲烷和乙烷形成的影响

密度泛函理论作为多相催化研究中的一个强有力工具,常被用于获得催化过程中关键的热力学及动力学参数,如吸附能、反应焓、活化能垒和速率常数等. 理解密度泛函交换关联近似方法对于揭示催化剂的催化性能及机理至关重要. 本文报道了六种不同的交换关联泛函近似方法,包括PBE、RPBE、BEEF+vdW、optB86b+vdW、SCAN和SCAN+rVV10,对金属Ru(0001)和Ru(1011)表面上甲烷和乙烷形成过程中涉及到的中间体的吸附能、反应能和活化能垒的影响. 当基元反应中反应物和产物与表面的配位数不同时,理论计算的反应能大小强烈依赖于交换关联密度泛函的选择. 对于涉及多个基元步骤的总反应,反应能的计算偏差会逐渐累积,从而导致不同的交换关联泛函近似方法之间的巨大差异. 由于不同泛函对反应涉及到的中间体吸附能之间存在差异,交换关联泛函近似方法的选择显著地影响Ru(0001)表面上甲烷、乙烯和乙烷的选择性. 然而,不同泛函近似方法对于Ru(0001) 和Ru(1011)表面上基元反应的能垒以及结构敏感性影响不大. 本工作不仅揭示了交换关联密度泛函近似方法在理论计算研究催化领域的局限性,也强调了选择合适的交换关联泛函方法对于正确评估催化剂活性和选择性的重要性.     

Dynamic Investigation of Mechanism of Catalytic Reactions as Revealed by Spectroscopic Techniques

The study of chemisorption of gases on catalyst surfaces is of primary importance in elucidating the mechanism of heterogeneous catalysis. Accordingly, the adsorption of each of the gases which participate in the catalysis has been studied separately in many cases. However, chemisorption on the catalyst surface during the progress of reaction, generally speaking, cannot be estimated from the adsorption equilibrium of reactants and products measured separately for each species, but depends upon the mechanism of the reaction, the interaction among the adsorbed species, the formation of reaction intermediates or surface complexes, and the area of active region on the surface.     

Adsorption and valence electronic states of nitric oxide on metal surfaces

Among fundamental diatomic molecules, the adsorption of carbon monoxide (CO) and nitric oxide (NO) on metal surfaces has been a subject of intensive research in the surface science community, partly owing to its relevance to heterogeneous catalysis used for environmental control. Compared to the rather well-defined adsorption mechanism of CO, that of NO is less understood because the adsorption results in much more complex reactions. The complexity is ascribed to the open-shell structure of valence electrons, making the molecule readily interact with the metal surface itself as well as with co-adsorbed molecules. Furthermore, the interaction crucially depends on the local structure of the surface. Therefore, to elucidate the interaction at the molecular scale, it is essential to study the valence state as well as the bonding geometry for individual NO molecules placed in a well-defined environment on the surface. Scanning tunneling microscopy (STM) is suitable for this purpose. In this review, we summarize the knowledge about the interaction of NO with metal surfaces, mainly focused on the valence electronic states, followed by recent studies using STM and atomic force microscopy (AFM) at the level of individual molecules.     

In situ surface dynamics in heterogeneous catalysis

During the former half of the last century the mechanism of heterogeneous catalysis had been studied, keeping the catalyst in a black box, and on the basis of the information outside of the black box, it was discussed just from mere conjectures. The author initiated a method to study directly the behavior of the working catalyst surface, looking into the inside of the black box by measuring adsorption on the working catalyst surface. In the same period many varieties of recent physical tools to study the solid surfaces have been developed and were applied to study the in situ dynamics of working catalyst surface. However, even if some chemisorbed species were observed on the working catalyst surface, it does not follow that they are reaction intermediates. A new dynamic approach to identify the dynamic behavior of each of the chemisorbed species under the reaction conditions, had been proposed by the author by use of “isotope jump method”, in which labeled species are replaced during the course of reaction to study the behavior of each of the adsorbed species under the reaction conditions. By using such new techniques we could not only identify the reaction path and the rate-determining step, but also new phenomena which are called “adsorption assisted processes” were discovered, It is, accordingly, hoped that by means of new nanotechnologies recently developed to study the behavior of single molecules on solid surfaces the nature of heterogeneous catalysis should make a remarkable advances on the basis of this in situ dynamic methods. In this review article emphasis has been put in the fundamental methods of dynamic approaches.     

Surface studies of supported model catalysts

Metal particles grown by vapour deposition on clean and well-defined oxide surfaces are used as model catalysts. These new model catalysts allow, unlike metal single crystals, a study of size and support effects in heterogeneous catalysis. The structure, the electronic properties and the reactivity of these supported model catalysts have been studied, in situ, by a large number of surface science techniques. In order to get relevant information from those studies it is necessary to control the nucleation and growth in order to get uniform collections of metal particles. The preparation conditions and the characterisation methods will be reviewed. Particles with well-defined shapes are obtained by epitaxial growth at high temperature on clean ordered surfaces. The electronic properties of the small metal particles depend not only on their size but also on their shape. The chemisorption properties are strongly related to the surface structure of the particles. The interplay between the surface structure, the local electronic properties and the adsorption energy will be discussed for CO chemisorption. The presence of the support plays an important role in the control of the particle morphology. Furthermore, it can increase the adsorption rate. The intrinsic heterogeneity of the supported model catalysts has to be taken into account to understand in detail the catalytic reactions. The reaction rate cannot be considered as an average on the different crystalline facets present on the particle. Finally, we will discuss the possibility to study in situ and at the atomic level simple chemical reactions on supported catalysts.     

Impact of surface science on the understanding of kinetics of heterogeneous catalytic reactions

The kinetics of chemical reactions in gas and liquid phases are usually described by employing the conventional mass-action law equations. The laws governing the kinetics of heterogeneous catalytic reactions are as a rule much more complex due to adsorbate–adsorbate lateral interactions, surface heterogeneity, spontaneous and adsorbate-induced changes in a surface, and/or limited mobility of reactants. The importance of these factors was recognized by the heterogeneous catalysis community far before the surface-science era. Only with the development of surface science, however, has it become possible to study in detail the non-ideality of rate processes on solid surfaces. In the present paper, we survey the main conceptual results currently available in this field and illustrate the impact of surface science on its development. Specifically, we outline the approaches used to describe elementary reaction steps and the whole reaction kinetics near and far from equilibrium, including such topics as kinetic phase transitions, pattern formation, kinetic oscillations and chaos, and pressure- and structure-gap problems. All these phenomena and problems are demonstrated to provide promising opportunities for further experimental and theoretical studies.     

多功能胶囊催化剂在多相催化中的应用研究进展

Capsule catalysts composed of pre-shaped core catalysts and layer zeolites have been widely used in the tandem reactions where multiple continuous reactions are combined into one process. They show excellent catalytic performance in heterogeneous catalysis, including the direct synthesis of middle isoparaffins or dimethyl ether from syngas, as compared to the conventional hybrid catalysts. The present review highlights the recent development in the design of capsule catalysts and their catalytic applications in heterogeneous catalysis. The capsule catalyst preparation methods are introduced in detail, such as hydrothermal synthesis method, dual-layer method, physically adhesive method and single crystal crystallization method. Furthermore, several new applications of capsule catalysts in heterogeneous catalytic processes are presented such as in the direct synthesis of liquefied petroleum gas from syngas, the direct synthesis of para-xylene from syngas and methane dehydroaromatization. In addition, the development in the design of multifunctional capsule catalysts is discussed, which makes the capsule catalyst not just a simple combination of two different catalysts, but has some special functions such as changing the surface hydrophobic or acid properties of the core catalysts. Finally, the future perspectives of the design and applications of capsule catalysts in heterogeneous catalysis are provided.     

From fundamental studies of reactivity on single crystals to the design of catalysts

One of the prominent arguments for performing surface science studies have for many years been to improve and design new and better catalysts. Although surface science has provided the fundamental framework and tools for understanding heterogeneous catalysis until now there have been extremely few examples of actually designing new catalysts based solely on surface science studies. In this review, we shall demonstrate how a close collaboration between different fundamental disciplines like structural-, theoretical-and reactivity-studies of surfaces as well as a strong interaction with industry can have strong synergetic effects and how this was used to develop a new catalyst. As so often before the studies reviewed here were not initiated with the objective to solve a specific problem, but realizing that a new class of very stable two-dimensional alloys could be synthesized from otherwise immiscible metals made it possible to present a new solution to a specific problem in the industrial catalysis relating to methane activation in the steam reforming process. Methane is the main constituent of natural gas and it is an extremely important raw material for many large scale chemical processes such as production of hydrogen, ammonia, and methanol. In the steam reforming process methane and water are converted into a mixture of mainly hydrogen and carbon monoxide, the so-called synthesis gas. Industrially the steam reforming process usually takes place over a catalyst containing small nickel crystallites highly dispersed on a porous support material like aluminum/magnesium oxides in order to achieve a high active metal area. There is a general consensus that the rate limiting step of this process is the dissociative sticking of methane on the nickel surface. Driven by the desire to understand this step and hopefully be able to manipulate the reactivity, a large number of investigations of the methane/nickel interaction have been performed using nickel single crystals as model catalysts. The process has been investigated, both under thermal conditions and by using supersonic molecular beams elucidating the dynamical aspects of the interaction. The results obtained will be reviewed both with respect to the clean and modified nickel surfaces. Especially the two-dimensional gold–nickel alloy system will be considered since the fundamental results here have lead to the invention of a new nickel based catalyst, which is much more resistant to carbon formation than the conventional nickel catalysts. This may be one of the first examples of how fundamental research can lead to the invention of new catalysts. Other overlayer/alloy combinations, their stability, and reactivity are briefly discussed with respect to manipulation of the surface reactivity towards methane.     

Biomedical surface science: Foundations to frontiers

Surfaces play a vial role in biology and medicine with most biological reactions occurring at surfaces and interfaces. The foundations, evolution, and impact of biomedical surface science are discussed. In the 19th century, the first observations were made that surfaces control biological reactions. The advancements in surface science instrumentation that have occurred in the past quarter of a century have significantly increased our ability to characterize the surface composition and molecular structure of biomaterials. Similar advancements have occurred in material science and molecular biology. The combination of these advances have allowed the development of the biological model for surface science, where the ultimate goal is to gain a detailed understanding of how the surface properties of a material control the biological reactivity of a cell interacting with that surface. Numerous examples show that the surface properties of a material are directly related to in vitro biological performance such as protein adsorption and cell growth. The challenge is to fully develop the biological model for surface science in the highly complex and interactive in vivo biological environment. Examples of state-of-the-art biomedical surface science studies on surface chemical state imaging, molecular recognition surfaces, adsorbed protein films, and hydrated surfaces are presented. Future directions and opportunities for surface scientists working in biomedical research include exploiting biological knowledge, biomimetics, precision immobilization, self-assembly, nanofabrication, smart surfaces, and control of non-specific reactions.     

Adsorption on model fractal surfaces

A simple model for the adsorption of gas molecules on fractal surfaces is formulated. For surface reactions where adsorption is the rate determining step, considerations based on the probability of adsorption are sufficient to describe the kinetics. Different values of fractal dimension correspond to different values of rate constant for adsorption. A linear relationship between the dynamics and the geometric properties of a solid surface as well as a statistical mechanical relationship between internal partition functions of the gas molecule and the adsorption complex are obtained.     

基于机器学习的合金催化剂表面吸附能预测

近年来,机器学习方法逐渐成为多相催化中的一种关键研究手段. 二元合金材料作为重要的催化剂之一,在双功能催化剂的筛选中受到了广泛的关注. 本文提出了一个将机器学习方法应用在预测催化性质上的整体框架,从而快速预测原子、分子在金属和二元合金表面的吸附能. 通过测试不同的机器学习方法来评估它们对于该问题的适用性,并将树集成的方法与压缩感知方法相结合,利用约6×10⁴个吸附能数据构建了预测模型. 相对于线性比例关系,该方法可以更准确地预测大量合金上的吸附能(预测的均方根误差降低一半),并且更通用地预测各种吸附物的能量,为发现新的双金属催化剂铺平了道路.     

Infrared Photothermal Deflection Spectroscopy of Carbon-Supported Metal Catalysts

The infrared (IR) study of surface species and reactions occurring on solids has contributed markedly to our understanding of surface chemistry and heterogeneous catalysis, and a wide variety of solids has been examined. However, an entire class of catalysts consisting of metal dispersed on carbon has never been studied because the carbon catalyst support absorbs IR radiation so strongly in even the thinnest practical layers that conventional IR techniques fail. As IR data would be helpful in understanding how such catalysts function, we have explored the feasibility of examining carbon-supported catalysts with photothermal beam deflection spectroscopy (PDS) and describe the first IR spectra ot surface species on a Ni-on-carbon catalyst.     

Atomic-scale surface science phenomena studied by scanning tunneling microscopy

Following the development of the scanning tunneling microscope (STM), the technique has become a very powerful and important tool for the field of surface science, since it provides direct real-space imaging of single atoms, molecules and adsorbate structures on surfaces. From a fundamental perspective, the STM has changed many basic conceptions about surfaces, and paved the way for a markedly better understanding of atomic-scale phenomena on surfaces, in particular in elucidating the importance of local bonding geometries, defects and resolving non-periodic structures and complex co-existing phases. The so-called “surface science approach”, where a complex system is reduced to its basic components and studied under well-controlled conditions, has been used successfully in combination with STM to study various fundamental phenomena relevant to the properties of surfaces in technological applications such as heterogeneous catalysis, tribology, sensors or medical implants. In this tribute edition to Gerhard Ertl, we highlight a few examples from the STM group at the University of Aarhus, where STM studies have revealed the unique role of surface defects for the stability and dispersion of Au nanoclusters on TiO₂, the nature of the catalytically active edge sites on MoS₂ nanoclusters and the catalytic properties of Au/Ni or Ag/Ni surfaces. Finally, we briefly review how reaction between complex organic molecules can be used to device new methods for self-organisation of molecular surface structures joined by comparatively strong covalent bonds.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

Chemical reactions on metal oxide surfaces investigated by vibrational spectroscopy

The most successful method to unravel the microscopic mechanisms governing reactions in heterogeneous catalysis is the “surface science” approach which is based on well-controlled studies on model catalysts (usually single crystal surfaces) under ultrahigh vacuum (UHV) conditions [G. Ertl, Angew. Chem. 47 (2008) 3524]. In this review our recent vibrational spectroscopic studies on selected model reactions at various single-crystalline metal oxide surfaces are summarized. Two vibrational spectroscopic methods, high resolution electron energy loss spectroscopy (HREELS) and Fourier-transform infrared spectroscopy (FTIRS), were applied to characterize the adsorbed species and to elucidate the elementary processes of chemical reactions at oxide surfaces ranging from well-defined single crystals to modified surfaces with deliberately introduced defects. The combination of both methods allows us to extend the vibrational spectroscopic studies from ideal to complex systems.     

Chirality of molecular nanostructures on surfaces via molecular assembly and reaction: manifestation and control

The formation of chiral nanostructures via molecular assembly and reaction on solid surfaces is a ubiquitous surface process due to the symmetry-breaking at 2D surface. Studying chirality during the adsorption, assembly, and reaction of molecules on 2D solid surfaces at molecular level not only sheds deep insights into the enantioselective heterogeneous catalysis, chiral recognition, origin and evolution of chirality, and many important physical chemistry processes but also provides an important strategy to create chiral nanostructures. Here, we give a survey of recent advances in chiral expression and control in molecular assemblies and reactions on surfaces. We firstly give a brief introduction to the general concepts of chiral molecular nanostructures on surfaces. And then we focus on the induction and control of chirality expressed in molecular assemblies. The recent developments in the control strategies such as chiral co-adsorber, chiral auxiliary, chiral solvent, chiral templated surfaces, as well as the underlying mechanism to achieve the chiral induction and amplification, are reviewed. After that, we review the studies of chirality expressed in on-surface synthesis which has been proved to be a promising strategy to fabricate covalently bonded low-dimensional nanostructures and materials. In this respect, we introduce the chiral expression in the intramolecular and intermolecular coupling reactions on surfaces. In addition, we survey the methods to steer the stereoselectivity of on-surface reactions including the design of precursor structure, steric hindrance effect, substrate, kinetic parameters et al. Finally, the future outlook in this field is discussed.     

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.     

Relation between adsorption and catalysis in the case of NiO and Co3O4

Reversed flow-inverse gas chromatography is a quick, precise and effective methodology to characterize physicochemical properties of adsorbents. This is extended to the experimental measurement of the adsorption energy distribution function as well as of the differential energy of adsorption due to lateral interactions of molecules adsorbed on two catalysts, namely Co₃O₄ and NiO. Thus, the nature and the strength of the adsorbate-adsorbent and adsorbate-adsorbate interactions are extracted in order to give detailed answers to the questions: (a) where are the molecules on the heterogeneous surface and (b) which is the nature of the surface chemical bonds? Thus, adsorption of 1-butene was found to take place immediately and irreversibly. It holds a deep relation between adsorption and catalysis of 1-butene over these catalysts. As a consequence, the adsorption of 1-butene in the presence of hydrogen leads to isobutane and/or n-butane, depending on the temperature. It can be seen from the adsorption/desorption kinetic constants that the adsorption of 1-butene on Co₃O₄ is one order higher than over NiO. This fact in connection with the bigger activation energy and the lower kinetic coefficients concerning hydrogenation reaction over NiO shows that Co₃O₄ is a better catalyst for this kind of catalysis.     

Tracking hydroxyl adsorption on TiO2 (1 1 0) through secondary emission changes

In surface science, rutile TiO₂ continues to be one of the most studied surfaces and in the catalysis field numerous groups study how adsorbates interact with this surface. All groups face the difficult problem of reproducibility due to surface preparation unknowns like defect concentration and the continuous aging of the crystals. Recent studies, using STM imaging, showed that hydroxyl adsorption takes place even in very good vacuum conditions. Upon adsorption, the surface electric field is reduced and the work function decreases. We found that this change may be readily detected in the onset energy of the secondary electrons. By following the onset region of secondary electron emission it is possible to track hydroxyl adsorption in quantities well below the detection level of XPS and LEIS. With this knowledge, we show that the time elapsed after surface preparation and water partial pressure should be accounted in the study of TiO₂ surfaces.