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 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.     

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.     

Directions of theoretical and experimental investigations into the mechanisms of heterogeneous catalysis

The roles of the atomic structure and the electronic structure of the active surface sites in bonding of reactants and causing bond breaking or bond formation have been the focus of theoretical studies. In addition to calculations on static systems, usually clusters, modelling of the transition states and the dynamics of elementary reaction steps (adsorption, dissociation, surface diffusion, desorption) have been performed. Variations of electronic structure of elements across the periodic table have been shown to be responsible for the unique importance of transition metals in catalysis.Experimental studies utilize catalysts with well-characterized structure (zeolites, crystal surfaces) and information about surface structure, composition and chemical bonding of adsorbates becomes available on the molecular level. Deliberate alteration of catalyst structure, surface composition by alloying and electronic structure by addition of electron donor and electron acceptor promoters have been utilized to modify reaction rates and selectivity. This way many of the molecular ingredients of heterogeneous catalytic reactions have been identified.In recent years evidence has been accumulating that indicates periodic and long term restructuring of the catalyst surface as necessary for chemical change and reaction turnover. These findings point to the need of time resolved studies and in-situ investigations of both the substrate and the adsorbate sides of the surface chemical bonds simultaneously on a time scale shorter than the reaction turnover frequency.Close collaboration between theorists and experimentalists is essential if we are to succeed in designing heterogeneous catalysts.     

Molecular surface science of C–H bond activation and polymerization catalysis

Surface science studies of heterogeneous catalysis use model systems ranging from single crystals to monodispersed nanoparticles in the 1–10 nm range. Molecular studies reveal that bond activation (C–H, H–H, C–C, C≡O) occurs at 300 K or below as the active metal sites simultaneously restructure. The strongly adsorbed molecules must be mobile to free up these sites for continued turnover of reaction. Oxide–metal interfaces are also active for catalytic turnover. Examples using C–H and C = O activation are described to demonstrate these properties. Polymerization catalysis demonstrates a strong dependence upon catalyst surface structure, which allows for the selectivity to be tuned by the choice of Ziegler-Natta surface preparation. Novel preparation methods of model catalyst arrays in two and three dimensions are opening the door to a complete understanding of catalytic reaction selectivity.     

Electron Spin Echo Modulation Studies of Dipolar Hyperfine Interactions of Catalytic Reaction Intermediates Involving Transition Metal Ions on Oxide Surfaces

The application of electron spin echo modulation (ESEM) pulsed electron spin resonance methods to detect dipolar hyperfine interactions of catalytic reaction intermediates involving transition metal ions on oxide surfaces is described. This provides a method to determine critical aspects of the geometrical structure of such intermediates. One example involves catalysis of ethylene dimerization by paramagnetic Ni⁺ on silica. ESEM results show direct coordination of two and three ethylene molecules to Ni⁺ as reaction intermediates in ethylene dimerization, depending on the activation temperature of the catalyst pretreatment. A second example involves catalysis of ethylene dimerization by paramagnetic Pd⁺ in X—zeolite. ESEM results show direct coordination of successively one and two molecules of ethylene to Pd⁺ prior to dimerization. Thus ESEM methods provide a powerful tool to develop a molecular picture of the course of catalytic reactions on surfaces.     

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 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.     

Cooperative Catalysis: A New Development in Heterogeneous Catalysis

Whereas cooperative effect in catalysis, in which multiple chemical interactions participate cooperatively to achieve significant enhancement in catalytic activity and/or selectivity, is common in enzymatic reactions, it has been sparingly employed in heterogeneous catalytic systems. Here, some recent literature examples of abiotic catalysis, with emphasis on heterogeneous systems, that employ cooperation between acid and base and two metal centers are briefly described to demonstrate the principles involved. Since effective cooperation places strict demand on the positions of the different functional groups, new synthetic methods and strategies are needed to design and construct structures useful for cooperative catalysis. Recent progress in our laboratory in synthesizing new nanocage structures that possess molecular-size cavities, atomic layer thick, porous shells with internal functional groups is described. These recent developments suggest possibilities of new catalytic transformations that have not been attempted before. This is illustrated with two speculative examples utilizing cooperative catalysis: oxidative hydrolytic desulfurization and terminal carbon activation of hydrocarbon molecules.     

Design strategies for the molecular level synthesis of supported catalysts

Supported catalysts, metal or oxide catalytic centers constructed on an underlying solid phase, are making an increasingly important contribution to heterogeneous catalysis. For example, in industry, supported catalysts are employed in selective oxidation, selective reduction, and polymerization reactions. Supported structures increase the thermal stability, dispersion, and surface area of the catalyst relative to the neat catalytic material. However, structural and mechanistic characterization of these catalysts presents a formidable challenge because traditional preparations typically afford complex mixtures of structures whose individual components cannot be isolated. As a result, the characterization of supported catalysts requires a combination of advanced spectroscopies for their characterization, unlike homogeneous catalysts, which have relatively uniform structures and can often be characterized using standard methods. Moreover, these advanced spectroscopic techniques only provide ensemble averages and therefore do not isolate the catalytic function of individual components within the mixture. New synthetic approaches are required to more controllably tailor supported catalyst structures. In this Account, we review advances in supported catalyst synthesis and characterization developed in our laboratories at Northwestern University. We first present an overview of traditional synthetic methods with a focus on supported vanadium oxide catalysts. We next describe approaches for the design and synthesis of supported polymerization and hydrogenation catalysts, using anchoring techniques which provide molecular catalyst structures with exceptional activity and high percentages of catalytically significant sites. We then highlight similar approaches for preparing supported metal oxide catalysts using atomic layer deposition and organometallic grafting. Throughout this Account, we describe the use of incisive spectroscopic techniques, including high-resolution solid state NMR, UV-visible diffuse reflectance (DRS), UV-Raman, and X-ray absorption spectroscopies to characterize supported catalysts. We demonstrate that it is possible to tailor and isolate defined surface species using a molecularly oriented approach. We anticipate that advances in catalyst design and synthesis will lead to a better understanding of catalyst structure and function and, thus, to advances in existing catalytic processes and the development of new technologies.     

Monodisperse Metal Nanoparticle Catalysts: Synthesis, Characterizations, and Molecular Studies Under Reaction Conditions

We aim to develop novel catalysts that exhibit high activity, selectivity and stability under real catalytic conditions. In the recent decades, the fast development of nanoscience and nanotechnology has allowed synthesis of nanoparticles with well-defined size, shape and composition using colloidal methods. Utilization of mesoporous oxide supports effectively prevents the nanoparticles from aggregating at high temperatures and high pressures. Nanoparticles of less than 2?nm sizes were found to show unique activity and selectivity during reactions, which was due to the special surface electronic structure and atomic arrangements that are present at small particle surfaces. While oxide support materials are employed to stabilize metal nanoparticles under working conditions, the supports are also known to strongly interact with the metals through encapsulation, adsorbate spillover, and charge transfer. These factors change the catalytic performance of the metal catalysts as well as the conductivity of oxides. The employment of new in situ techniques, mainly high-pressure scanning tunneling microscopy (HPSTM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) allows the determination of the surface structure and chemical states under reaction conditions. HPSTM has identified the importance of both adsorbate mobility to catalytic turnovers and the metal substrate reconstruction driven by gaseous reactants such as CO and O₂. APXPS is able to monitor both reacting species at catalyst surfaces and the oxidation state of the catalyst while it is being exposed to gases. The surface composition of bimetallic nanoparticles depends on whether the catalysts are under oxidizing or reducing conditions, which is further correlated with the catalysis by the bimetallic catalytic systems. The product selectivity in multipath reactions correlates with the size and shape of monodisperse metal nanoparticle catalysts in structure sensitive reactions.     

The Mechanism of HDS Catalysis

The mechanism of heterogeneous catalytic reactions is much more difficult to elucidate than that of homogeneous systems. Despite the facilities provided by physical methods for investigating the surface of solids, obtaining detailed information on the structure of the active component in real heterogeneous catalysts presents difficulties due to the nonuniform chemical composition of the surface species. Some of these surface species are totally inactive in catalysis, and others can catalyze the given chemical reaction by different pathways and according to different mechanisms. This results in a change of selectivity to the desired product and the appearance of intermediates and reaction by-products. Furthermore, the effect of the reaction medium on the catalyst gains importance during a catalytic process when, at high temperature and pressure, one type of surface species is transformed into another, thus changing the mechanism and direction of the catalyzed reaction.     

The Mechanism of HDS Catalysis

The mechanism of heterogeneous catalytic reactions is much more difficult to elucidate than that of homogeneous systems. Despite the facilities provided by physical methods for investigating the surface of solids, obtaining detailed information on the structure of the active component in real heterogeneous catalysts presents difficulties due to the nonuniform chemical composition of the surface species. Some of these surface species are totally inactive in catalysis, and others can catalyze the given chemical reaction by different pathways and according to different mechanisms. This results in a change of selectivity to the desired product and the appearance of intermediates and reaction by-products. Furthermore, the effect of the reaction medium on the catalyst gains importance during a catalytic process when, at high temperature and pressure, one type of surface species is transformed into another, thus changing the mechanism and direction of the catalyzed reaction.

     

Fundamental Investigations of Enantioselective Heterogeneous Catalysis

Enantioselective heterogeneous catalysis is an important and rapidly expanding research area. The two most heavily researched examples of this type of catalysis are the enantioselective hydrogenation of α-ketoesters over Pt-based catalysts and the enantioselective hydrogenation of β-ketoesters over Ni-based catalysts. These systems share one extremely important common feature—the enantioselective surface reaction is controlled by the presence of adsorbed chiral molecules (modifiers) on the surface of the metal component of the catalyst. In each system, a number of models have been proposed to explain the enantioselective behavior in the light of catalytic experiments. In recent years, surface science has begun to address the issues relevant to this branch of catalysis. This article reviews to what extent surface science has enabled the verification of the proposed models and, in addition, what new light surface science has shed on the possible mechanisms of enantioselective heterogeneous catalysis.     

Preparation and characterization of catalyst thin films

Many devices used in catalysis are based on high surface area materials in which catalytic reactions are carried out inside pores, channels and other confined cavities for which the deposition of catalyst thin films is required. This paper provides an overview of the methods in use for the preparation and characterization of catalyst thin films, and focuses more specifically on thin films involved in the electropromotion of catalysis (EPOC). In fact, EPOC or NEMCA (Non-Faradic Electrochemical Modification of Catalytic Activity) have shown the importance of being able to combine electrical contacts between catalytic metals and ion conducting oxide layers as well as to develop in the same system catalytic materials with large specific surface area. The different aspects of thin film preparation and characterization are described in relation to catalyst thin films deposition. Multimodal and hierarchic porous structures can be obtained from the assembly of catalyst thin films with various carrier materials, anticipating more efficient catalytic systems. Chemical and physical coating techniques are compared with a special attention on those useful for the preparation of thin films with controlled porous structure and morphology. With regard to EPOC systems, electrode and electrolyte materials of interest for electrochemical catalytic devices are listed and typical examples of systems based on electrocatalyst thin films are given.     

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.     

Role of support in lean DeNO x catalysis

This article reviews the accumulated theoretical results, in particular density functional theory calculations, on two catalytic processes, CO oxidation and NO reduction on metal surfaces. Owing to their importance in automotive emission control, these two reactions have generated a lot of interest in the last 20 years. Here the pathways and energetics of the involved elementary reactions under different catalytic conditions are described in detail and the understanding of the reactions is generalized. It is concluded that density functional theory calculations can be applied to catalysis to elucidate mechanisms of complex surface reactions and to understand the electronic structure of chemical processes in general. The achieved molecular knowledge of chemical reactions is certainly beneficial to new catalyst design.     

Model catalysis by metals and alloys: From single-crystal surfaces to well-defined nano-particles

During the last decades surface science has played an important role for modelling surface chemistry on metallic surfaces and understanding catalytic processes for simple reactions. Progresses are now made possible by the recent development of specific tools for characterising at the nanoscale level model materials and following the kinetics of reactions on these materials of small area in dedicated reactors.It needs the preparation, and characterisation at the atomic level, of well-defined materials. They can be either single-crystal surfaces having a well-defined orientation or in shape of model supported nano-particles. Most has been done in the framework of monometallic materials, but more difficult is the preparation of well-defined alloy surfaces and surface alloys, and the elaboration of alloy nano-particles well-defined in size and composition.Kinetic studies in dedicated reactors show how the catalytic behaviour of model samples may depend on the specific sites present at surface. For example, surface sites having a low coordination number (like steps, kinks, edges and corners) are very efficient for bond-breaking. In bi-metallics, ad-layers of a given metal on a foreign substrate may show new and original structures having very specific catalytic properties. Thus, works on catalysis at the atomic scale proposes new active/selective sites, and it is now a challenge to design new industrial catalysts on the basis of these fundamental works.In order to move near the conditions for real catalysis one has now to bridge the “pressure gap”, i.e. to make in situ (during reaction under pressure of reactants) characterisation of both the surface itself and ad-species. This needs the development of specifics tools able to work in such conditions. This is a challenge for today and future works in the field of model catalysis.     

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.     

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.