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Low Pressure RAIRS Studies of Model Catalytic Systems Emma L. Wilson and Wendy A. Brown* Department of Chemistry, UniVersity College London, 20 Gordon Street, London, WC1H 0AJ United Kingdom ReceiVed: December 22, 2009; ReVised Manuscript ReceiVed: January 29, 2010
In this review we examine the use of reflection absorption infrared spectroscopy (RAIRS) as a tool for studying model catalyst surfaces. Model catalysts allow us to study the structural and physical properties of their surfaces at an atomic level in a controlled environment in order to gain a fundamental understanding of how “real-life” catalysts work and how they can be improved. RAIRS is a particularly good technique for examining model catalyst surfaces as we can gain information about the morphology of the surface and about any intermolecular interactions that may be present. RAIRS can also be used to study surfaces in high pressure environments, as well as at ultrahigh vacuum, unlike many other standard surface science techniques. Here we review a number of examples of where RAIRS has been used to further the knowledge of model catalyst composition and to study the interactions of model catalyst surfaces with small gas molecules. A range of studies have been carried out examining the temperature, particle size and gas pressure dependencies of the adsorption of small gas molecules on a wide range of model catalyst surfaces. 1. Introduction The recent increase in public awareness of environmental issues has led to a much higher demand for research in the area of environmental heterogeneous catalysis and in particular for catalysts involved in the removal of polluting gases from the atmosphere. In order to gain a more fundamental understanding of how such catalysts work and how they can be improved, it is necessary to study the structural and physical properties of surfaces at an atomic level. Real life catalysts, such as automobile catalytic converters, are often very chemically complicated and consist of a large number of individual components which together contribute to the catalytic activity of the overall system. By studying selected components of such systems in controlled environments, it is hoped that a picture of how the catalyst works as a whole can be built up. The simplest model catalyst systems consist of small molecules adsorbed on single crystal metal surfaces in ultrahigh vacuum (UHV). The use of UHV provides an easily characterized, reproducible surface on which adsorbate-substrate interactions and reactions can be studied. Consequently a large number of studies have been carried out on such systems (for examples see various reviews and references therein1-3). However, there are obvious limitations to studying such welldefined single crystal surfaces as catalyst analogues, as they do not adequately represent the complex, defect-rich, and multicomponent surfaces of real catalysts. One way of introducing defects into the surface of a crystal is to use stepped and kinked surfaces. These surfaces allow deviations away from a perfect flat crystal, while still maintaining a well-defined surface. As such, a large number of studies of small gas molecules adsorbed on metal stepped and kinked surfaces have been carried out. For more details of studies of adsorption on stepped and kinked surfaces, see a review by Vattuone et al.4 The majority of more recent model catalyst studies have progressed further in complexity to look at systems which consist of metal nanoparticles deposited on metal oxide substrates. Although, to a certain extent, these studies still rely on * Corresponding author. E-mail:
[email protected].
the use of single crystal surfaces, they allow us to build up an ever more complex picture of how catalysts function, as demonstrated in Figure 1. The insulating properties of most bulk oxides limit the surface science techniques that can be used to study this type of system, and hence, the majority of studies involve the use of oxide thin films grown on metal single crystal substrates. A number of studies have also been carried out to observe the interactions between small gas molecules and the oxide surfaces themselves (for examples see refs 5-8). Model catalyst surfaces have been studied using a wide variety of surface science techniques, including reflection absorption infrared spectroscopy (RAIRS, also known as IRAS/ IRRAS, infrared reflection-absorption spectroscopy), and hence, a number of review articles have been written on the subject.9-13 However, no previous review articles have focused specifically on the role of RAIRS in studying model catalysts. As well as RAIRS, there are a number of other techniques which can give information about the vibrational modes of adsorbed species on a surface, including electron energy loss spectroscopy (EELS),14 sum frequency generation (SFG) spectroscopy,15,16 and photoelectron diffraction (PED).17 Although PED is not a vibrational technique, it has proved key in understanding the relationship between the vibrational frequency of a probe molecule and its adsorption site.18,19 For more details of how these techniques work and examples of how they are used, see references given and references therein. All of these techniques can give information about the vibrational modes of surface adsorbates, which in turn can be used to determine, for example, the morphology of the surface and to give information about intermolecular interactions. RAIRS has the disadvantage of only being able to see modes which have a component of the vibration perpendicular to the surface, whereas EELS is also able to see species parallel to the surface when operated in off-specular mode. PED and SFG are also able to give information about the orientation of adsorbed gas molecules. SFG and PED also have the advantage of being able to give electronic structure information, with PED having particularly good chemical sensitivity, allowing identification of the chemical state of each species present by looking
10.1021/jp912080t 2010 American Chemical Society Published on Web 03/18/2010
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Emma Wilson is currently a post-doctoral research associate at the Department of Chemistry at UCL, working with Dr. Wendy Brown and Prof. Helen Fielding on a new project to design and build an apparatus to study ultra-fast surface science. Previous to this, she studied for her Ph.D., also at UCL, under the supervision of Prof. Geoff Thornton and Dr. Wendy Brown, on the subject of model catalyst surfaces. Her interests involve studying adsorption and reactions on metal and model catalyst surfaces.
Wilson and Brown to the poor frequency resolution available with EELS and PED. The use of SFG to study model catalyst surfaces has been reviewed by Rupprechter.16 EELS and PED have only been used in a handful of studies, for example Sakamoto et al. used EELS to study NOx and SOx on a Pt/Ba model catalyst28 and Yoshihara et al.29 used PED to study Cu/ZnO surfaces. This review focuses exclusively on the use of RAIRS to investigate adsorption and reactions on model catalysts under low pressure conditions. Due to the large number of publications concerning studies of model catalyst surfaces, and the number of reviews on the subject,9-13 only an overview of the most recent research, using RAIRS as the primary investigative technique is presented here. In particular, the main focus here is on studies of metal nanoparticles supported on oxide surfaces, because of its application to environmental catalysis. CO is the most common adsorbate used for these studies, in part due to the current interest in converting CO into less environmentally polluting products. CO is also an excellent probe molecule for RAIRS studies and has been exhaustively studied on single crystal metal surfaces. We will also give examples of model catalyst systems where methanol and NO have been used as the adsorbate, although there are relatively few examples where this is the case. Model catalyst systems using bimetallic nanoparticles will also be discussed. 2. Model Catalyst Growth Methods
Wendy Brown is currently a Reader in Physical Chemistry at the Department of Chemistry at University College London (UCL), where she has been since 1998. Prior to joining UCL, she held a research fellowship at Peterhouse, University of Cambridge working with the group of Professor Sir David King, in whose group she also studied for her Ph.D. Her research interests involve studying adsorption and reactions on a range of surfaces ranging from metals and oxides to model interstellar dust grains.
at chemical shifts. PED has also been used for site identification, where previous site assignments from combined RAIRS and low energy electron diffraction (LEED) studies had been found to be incorrect.18-21 Unlike the other spectroscopic techniques, RAIRS has a very high spectral resolution and therefore can separately resolve closely positioned peaks, which may be observed as a single species using other techniques. It is also able to identify adsorption on different metal planes which may exist together on a surface or nanoparticle. For example, larger Pd nanoparticles often consist of multiple facets which are easily identifiable by recording RAIR spectra for adsorbates that show vibrations sensitive to different crystal planes, such as CO. RAIRS is also particularly suitable for studying such surfaces in high pressure environments (for example see refs 15 and 22-26), as in real catalysts in industrial processes. While SFG can be used to give time-resolved information, more recently time-resolved RAIRS has also been developed.27 For studying model catalyst surfaces RAIRS and SFG are the most commonly used vibrational spectroscopy methods, due
All of the model catalyst systems discussed in this review consist of an oxide thin film on which supported metal nanoparticles are adsorbed, as illustrated in Figure 2. This is a good model system for many industrial heterogeneous catalysts. Thin oxide films, grown on a metal single crystal, are often chosen because the majority of surface science techniques require the use of a conducting sample. The use of RAIRS to study adsorption on insulating surfaces, including most bulk metal oxides, is limited due to poor reflectivity, hence resulting in a 2- to 3-fold drop in sensitivity compared to an experiment on a highly reflective metal surface.30 Insulating surfaces can therefore be more effectively studied when grown as thin films on top of a metallic substrate, using a method known as the buried metal layer approach.30-32 If the film thickness is considerably smaller than the wavelength of the infrared radiation being used, the reflectivity is that of the metallic substrate, increasing the sensitivity. Despite the fact that the infrared light is reflecting from the metal substrate, the chemistry observed is that of the oxide thin film. The model catalyst surface is usually grown in UHV conditions to prevent the inclusion of any contaminants. The exact method of production of the oxide thin film varies depending on the substrate and the chosen metal oxide. The most common method for oxide thin film growth involves vapor depositing the metal onto the substrate in an O2 atmosphere. In order to achieve an ordered oxide thin film the deposited oxide is usually annealed. For example, for Al2O3 thin films grown on Ta(110), aluminum is vapor deposited in a 1 × 10-6 Torr atmosphere of O2 at 900 K, before being annealed to 1200 K in O2 to create an ordered Al2O3 film with a (1 × 1) LEED pattern.33 However, when Al2O3 thin films are grown on NiAl, no deposition of Al is required. In this case 1200 L of O2 is dosed at elevated temperatures and the sample is then annealed to 1150-1200 K for 5 min.34 A multistage growth procedure is also sometimes required for oxide systems, for example to grow CeO2-x(111) on Pt(111). Wilson et al.35 vapor deposited cerium onto a clean Pt(111) surface at 300 K and then annealed it to 1020 K in UHV, giving
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Figure 1. Pictorial representation of how model catalyst surfaces are becoming progressively more complex. The figure shows STM images of model systems. Reproduced from ref 154. Reproduced by permission of the PCCP owner societies.
Figure 2. Schematic representation of a typical oxide supported metal nanoparticle, with the most common adsorption sites indicated.
rise to a (2 × 2) LEED pattern. This LEED pattern was thought to be due to the formation of a Pt-Ce alloy. Subsequent annealing to 1020 K in an O2 pressure of 1 × 10-5 mbar then resulted in a CeO2-x(111) thin film which gave rise to a (1.4 × 1.4) LEED pattern with respect to the pattern for the clean Pt(111) surface. Film thicknesses are often calculated by studying the attenuation of peaks in Auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS), as used for SiO225 and CeO235 thin films. Alternatively deposition rates can also be measured with a quartz crystal microbalance, as used by Wolter et al. to measure the Pd deposition rate.36 Metal nanoparticles are usually deposited onto the thin film oxide by vapor deposition. Depending on the oxide substrate
and the size and structure of the nanoparticles required, they are sometimes stabilized by annealing and/or O2 treatment. This method was used by Schalow et al.,37 to grow a Pd/Fe3O4 model catalyst. In these experiments Pd was vapor deposited onto the Fe3O4 film before being annealed at 600 K and then stabilized by five cycles of O2 and CO exposure at 500 K. Scanning tunnelling microscope (STM) images showed that this procedure produced ∼7 nm wide well ordered Pd nanoparticles with (111) and (100) facets. On different substrates, for example CeO2, it has been shown that similar well-ordered Pd nanoparticles can be produced without the use of the additional annealing and stabilization steps.35,38
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3. Results A wide variety of model catalyst surfaces have been studied, consisting of a range of metal oxide surfaces onto which metal nanoparticles are deposited. One of the most commonly studied species of metal nanoparticle is Pd, due to its use in automobile catalytic converters. Model-catalyst investigations into Pd clusters supported on various metal oxides, such as TiO2,39,40 Al2O3,41-46 SiO2,46,47 MgO,48 and ZnO,49 have taken place. Investigations into oxide supported Pt and Rh nanoparticles (for examples see ref 13) have also been carried out due to the use of these transition metals in catalytic converters; however, these systems have been less extensively investigated using RAIRS. RAIRS has, however, been used to study other metal/ oxide systems, such as Au supported on Al2O3,50,51 SiO2,52,53 FeO(111),54,55 CeO2,56 and TiO257-59 thin films. These systems are of interest due to Au’s ability to catalyze the CO oxidation reaction. RAIRS has also been used to study the phonon modes of the oxide surfaces themselves and to investigate how these are affected by the presence of metal nanoparticles.60,61 3.1. Oxide Supported Monometallic Nanoparticles. A large variety of experimental and theoretical techniques have been used to study oxide supported metal nanoparticles. The most common experimental studies use powdered and mixed samples to carry out rate studies to determine the efficiency of a particular mix of components for catalyzing specific reactions, for example see ref 62. These methods show that the mixture of materials used works to catalyze specific reactions, but the actual mechanism of the catalytic reaction on the atomic scale cannot be deduced. In order to determine the mechanisms occurring at metal-oxide interfaces at an atomic level, surface science studies are required. UHV conditions are used to eliminate any contaminants and to ensure that only the reactants required are present. In this way the interaction of a small number of components can be studied individually in order to gain a better understanding of the system as a whole. 3.1.1. CO Adsorption. CO adsorbed on metal surfaces is one of the most highly studied surface science systems. This has arisen due to the huge number of industrial applications, particularly within environmental heterogeneous catalysis, which involve these components. Also, CO bonds easily to many surfaces, and CO-metal bonding has been well characterized by the Blyholder model,63,64 making it an ideal system for fundamental studies. CO acts as an excellent probe molecule for RAIRS studies of metal surfaces, and the observed CO stretching frequencies can provide various details, including information about the surface morphology, adsorption site, and CO-surface interactions. Assignments of vibrational bands observed for CO adsorbed on metal nanoparticles are most often made by comparison with previous studies of CO adsorbed on metal single crystals and other model catalyst systems. However, caution must be exercised, as the oxide support can alter the catalytic reactivity, and hence vibrational frequencies, of CO on the metal nanoparticles. For example ceria has been shown to alter the catalytic reactivity of dispersed Au.65,66 Also note that vibrational frequencies can only be used as a guide to site assignments, as previous studies18-21 have shown that assignments based on vibrational frequencies alone are not always correct. In particular, this has been shown to be the case for the CO/Pd(111) system,21 where the 1936 cm-1 band initially observed by Bradshaw et al. and assigned as a bridge band67 must now be assigned as a CO-Pd(111) 3-fold band.20,21 In order to avoid confusion in this review, all peak assignments refer to the
Wilson and Brown assignments made in the papers referenced, which may not always be correct. Where possible this has been noted in the text. 3.1.1.1. Pd Nanoparticles. There have been a large number of studies of CO adsorbed on Pd surfaces, including single crystal surfaces, stepped surfaces, and oxide supported Pd nanoparticles. This is largely due to the interest generated by the use of Pd in three-way automobile catalysts. The extensive studies of CO on single crystal surfaces, using a variety of surface science techniques, have also provided a wealth of data for comparison with more complex systems such as oxide supported nanoparticles. A summary of the results obtained from CO RAIRS studies of oxide supported Pd systems is given in Table 1. 3.1.1.1.1. Pd/Al2O3. Al2O3 supported Pd nanoparticle surfaces are one of the most widely studied model catalyst systems. This is mainly due to the introduction of Pd-only three way catalysts in the 1990s, which consisted of Pd nanoparticles supported on an amorphous Al2O3 structure, whereas Rh and Pt had been largely used previously.68 Extensive research of CO adsorption on Pd clusters grown on Al2O3 thin films has been carried out.33,36,43,44,46,69-79 Within the large number of studies that have employed vibrational spectroscopic techniques to observe the CO interaction with the Pd/Al2O3 surface, a wide variety of methods have been employed to grow the Pd clusters. This enables the effects of different temperature conditions on the structure of Pd clusters to be observed, and CO has been used as a probe in order to learn about the surface structure.36,72 Studies have shown that the size of the Pd clusters has a significant effect on the types and intensities of vibrational modes observed.33,36,46,72,73 The effects of elevated temperatures and high pressure CO exposure on the Pd/Al2O3 surface have also been studied,33,46,71,75 as this mimics the operating conditions of a catalytic converter more closely than low temperature and pressure experiments. An example of RAIR spectra of CO adsorbed on a Pd/Al2O3 model catalyst, as a function of sample temperature and Pd cluster size, is shown in Figure 3. Wolter et al.36,72 used CO RAIRS to compare Pd nanoparticles deposited on an Al2O3/NiAl(110) substrate at different temperatures. Pd was deposited at 90 and 300 K and RAIR spectra were taken after CO saturation of the surface. Three peaks were identified for both sets of spectra. A high frequency peak at approximately 2100 cm-1 was assigned to a CO-Pd atop stretch. The other two peaks were assigned to CO-Pd bridge species, on the terraces of the Pd nanoparticles (1930-1970 cm-1) and on the edges of the aggregates (1970-2000 cm-1). Previous work on Pd/Al2O3 systems found similar results.33,50 However, the previous work had assigned the 1970-2000 cm-1 band as bridge bonded CO on Pd(100) facets, due to the cuboctahedral shape of larger Pd particles found by STM69 which consists of both (111) and (100) facets. The (100) assignment was ruled out by Wolter et al.36 due to the low ratio of (100) sites compared to (111) sites. However, little can be learnt from the comparative intensities of the two bridge bound peaks using RAIRS, as it is postulated that intensity transfer occurs to the higher frequency species.36,72 A further article by Yudanov79 combined RAIRS, SFG, and density functional theory (DFT) studies. The ∼1970 cm-1 band was assigned to bridge bonded CO on particle edges as the DFT calculations indicated strong bonding of CO to particle edge sites. Pd nanoparticles deposited at 90 K showed a much higher intensity peak associated with CO-Pd bridge bonded species at edges, compared to Pd particles grown at 300 K, due to the
