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Insights into Heterogeneous Catalysis through Surface Science Techniques
C H A P T E R
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Insights into Heterogeneous Catalysis through Surface Science Techniques C.P. Vinod Catalysis Division and Center of Excellence on Surface Science, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
12.1 INTRODUCTION Surface science and heterogeneous catalysis belong to two traditions with increasing overlap over the years. Heterogeneous catalysis is believed to be a surface phenomenon where several technologically and industrially relevant reactions and processes occur either at the interface or at the actual surface itself [1]. A detailed understanding on the nature of bond breaking and making, the intermediates and the energetic involved needs characterization tools which can probe the events in the time scale and with sufficient spatial resolution. Over the years surface science techniques have been instrumental in throwing light onto many of the fundamental problems of heterogeneous catalysis. The important recognition of this being the Nobel Prize in Chemistry to Professor Gerahard Ertl at the Fritz Haber Institute, Berlin on the studies on the molecular level understanding of many fundamental chemical processes including the ammonia synthesis catalyst [2]. The area of surface science is making rapid strides thanks to the improvements in the instrumentation thereby pushing the limits of data acquisition and thereby the quality of the data generated. The advancement of experimental surface science techniques including the development of new operando tools has had a major impact in our atomic and molecular level understanding of materials. This means a greater knowledge of structure, dynamic, composition, and thermodynamic properties of the surface which are important factors for predicting materials for desired chemical properties. The rapid stride made by surface science over the last decade meant that the new tools could be used in ultrahigh vacuum or at near ambient (realistic)
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conditions [3]. The outlook of surface science took a major phase shift with the development of probe techniques to the likes of STM and AFM. These techniques which are developing at an unimaginable scale: the major improvement in terms of surface catalysis being the development of reactor STM which can probe surfaces at near ambient conditions [4]. The fact that these microscopes can give atomically resolved images under high pressures and reacting conditions, make them powerful tools in deducing hidden atomic scale information of several exciting catalytic problems. The developments in the probe techniques will not be covered and this chapter is outlined such that the advancement made by two techniques: X-ray Photoelectron Spectroscopy and Vibrational Spectroscopy in catalyst characterization and its application in understanding the genesis of catalysis will be described with relevant examples from the literature. Vibrational spectroscopy has been a major characterization tool for many fundamental problems in chemistry [5]. There are several variations of the surface science counterpart of vibrational spectroscopy. Among them Sum Frequency Generation spectroscopy (SFG) and PM-IRRAS (polarization modulation infrared reflection absorption spectroscopy) are routinely used for identifying the vibrational features of surface adsorbates. Surface science research groups have immensely benefited from using these techniques for model catalysts.
12.2 X-RAY PHOTOELECTRON SPECTROSCOPY UNDER NEAR AMBIENT CONDITIONS (APXPS) X-ray photoelectron spectroscopy has been a major surface science tool in identifying active species in many catalytic systems. XPS is based on the simple principle of the photoelectric effect where a material when illuminated by soft X-rays (1.5 keV) results in the generation of photoelectrons which can be energy analyzed using an electron analyzer. By knowing the kinetic energies of the photoelectron, the binding energy of the electron and hence the chemical environment of the elements in the sample specimen can be determined. The photoemission process can be explained by the equation
KE = hν − BE − ϕ,
(12.1)
where KE is the kinetic energy of the emitted photoelectron, hν (h—Planck constant (6.62 × 10−34 Js), ν—frequency (Hz) of the radiation) is the energy of the exciting X-ray source, BE is the binding energy of the emitted electron, and ϕ is the work function of the material. Since the relative areas of the photoelectron spectrum give the information of the elemental composition from 5 to 10 nm, XPS becomes a surface sensitive technique which gives quantitative and qualitative information. The conventional photoelectron spectrometers are operated under UHV conditions where the experiments are carried out at pressures better than 10−6 mbar. Such high to ultrahigh vacuum is required as the emitted photoelectrons can be scattered by gas phase before they reach the electron analyzer resulting in weak or no electron counts. The application of pressures exceeding 10−5 mbar is not recommended in typical XPS instruments owing to the sensitivity of the electron detector toward higher pressures in the spectrometer, and is limited by attenuation of the signal due to inelastic scattering of electrons in the gas phase.
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The use of ultrahigh vacuum photoelectron spectroscopy has given phenomenal fundamental results associated with surface science of materials, but poses severe limitations in studying catalytic surface science. The weakly adsorbed species tend to desorb from the surfaces by mere evacuation making it impossible to track the nature and in turn identification of possible active species in a catalytic cycle. Similarly, stability of active phases of materials for instance changes depending on the operating pressure. So for a realistic understanding of catalytic phenomena it is ideal to perform XPS under near ambient conditions. Since the discovery of photoelectron spectroscopy there has been a constant impetus in pushing the pressure limit and getting closer to ambient pressure conditions. Recently, the history of the development of this field has been covered by an excellent review by Knop-Grieke and co-workers [6]. One of the earlier high pressure instruments were the ones with the sample located inside a high pressure gas cell [7] or a high pressure cell with a nozzle doser pushed over the sample [8]. The calculation of the attenuation of the electron beam passing through the gas phase was taken into consideration during the development of the first generation of ambient pressure X-ray photoelectron spectrometers by Roberts and co-workers [7]. The attenuation of the electron beam (Iattenuated) passing through 1 mm thick gas layer is given by Eq. (12.2)
Iattenuated = Ivac exp(−P/λ),
(12.2)
where Ivac is the intensity of the photoelectron in vacuum, P is the pressure in torr, and λ is the inelastic mean free path of the electron in a gas environment of 1 torr pressure. At low kinetic energies of the electrons λP can be calculated by Eq. (12.3) [9]
λP = 4(2σP )1/2 .
(12.3)
Calculations show that only 14% of the photoelectrons pass through to the analyzer at 1 torr of gas pressure. Some of the critical areas that need attention during the instrumentation of an ambient pressure XPS in order to get a good signal-to-noise ratio is worth mentioning. The intensity of the X-rays impinging on the sample, the X-ray window which separates the high pressure sample region from the source from which X-rays are generated, the absorption of X-rays by the gas phase, scattering of photoelectrons by the gas phase, and the collection of photoelectrons by the analyzer. The thickness and area of the window will determine the mechanical stability and the transmission rate of the X-rays across the window. Aluminum or Si3N4 are conventional X-ray windows used in ambient pressure XPS instruments. A 100 nm thick Si3N4 window with area 2.5 × 2.5 m2 can hold a pressure of 10 mbar. There is an exponential decay of the photoelectron intensity due to inelastic collisions with the gas phase molecules and the distance between the sample and the aperture of the analyzer is a critical parameter in the design of the ambient pressure XPS spectrometer. For a 0.5 mm aperture, a 1 mm distance between the sample and the entrance of the analyzer is proven to be optimum. In the current generation of ambient pressure X-ray photoelectron spectrometers, the sample is placed close to a differentially pumped analyzer so that gas molecules can escape increasing the mean free path length of the electrons. This coupled with an electrostatic lens can significantly improve the photoelectron count reaching the analyzer. A schematic of the principle of a typical ambient pressure XPS is shown in Figure 12.1 along with the analyzer cone used for accepting the electrons toward the hemispherical analyzer.
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FIGURE 12.1 Schematic of an ambient pressure X-ray photoelectron spectrometer.
The X-rays which pass through the window reach the sample kept in a high pressure environment relevant to the catalytic reaction. The aperture size of the differentially pumped analyzer cone is also a trade-off between the count of electrons reaching the analyzer to the pressure developed near the first differential pumping stage. The X-ray source can be either conventional Al Kα/Mg Kα or from a synchrotron source. The advantage of high brilliance and exceptionally good resolution makes most of the APXPS studies being carried out using synchrotron radiations. However, there is a major interest in developing instruments based on conventional X-ray sources [10–12] which sometimes can be helpful in avoiding problems associated with radiation damages of samples and molecules routinely encountered while using high intensity X-ray (synchrotron) sources [13].
12.3 VIBRATIONAL SPECTROSCOPY AT HIGH PRESSURES Over the years infrared spectroscopy has become a powerful technique for studying adsorbed molecules and catalyst support [14–16]. The in situ studies involving powder catalyst in the presence of reactant molecules and at temperatures relevant to catalysis have given valuable information in elucidating catalysis phenomena and reaction mechanisms. Fast scanning Fourier transform IR instruments in conjunction with in situ cells can deliver data with considerable ease and time. Traditionally surface scientists have relied upon High Resolution Electron Energy Loss Spectroscopy (HREELS) and Reflection Absorption Infrared Spectroscopy (RAIRS) to carry out fundamental surface science studies. Electrons (HREELS) and photons (RAIRS) are allowed to specularly reflect from the flat surface and the resulting vibrations are detected using a suitable analyzer. These studies done on flat metal surfaces like a single crystal surface or model catalyst surface where metal nanoparticle deposited on thin oxide layer are carried out under ultrahigh vacuum conditions. Again UHV is required
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while doing HREELS to circumvent the problem of inelastic scattering of low energy electrons that are used to excite the adsorbed molecules on surface. Conventional RAIRS spectrometers also work at high vacuum conditions as high pressure experiments will have strong gas phase signals which will bury the vibrational features from the surface. So to study model surface reactions from flat surfaces at catalytically relevant pressures and temperatures, two techniques were developed which have been successfully used over last several years. These are polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and Sum Frequency Generation IR Spectroscopy (SFG). The basic concept behind the two techniques is explained in the next section.
12.3.1 Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) Polarization modulation infrared reflection absorption spectroscopy is based on the selective absorption of s- and p-polarized light by the surface and gas phase species. The absorption of s-polarized light on the metal surface is zero and hence all the signal amounts to the gas phase. In contrast, the p-polarized light excites both gas phase and surface vibrations. Thus by collecting the spectra using s- and p-polarized light and then carefully subtracting the one from the other gives specific vibrations of molecules adsorbed on the surface. The polarization of the light falling on the sample is modulated between p-polarized light (polarization parallel to the plane of incidence) and s-polarized light (polarization perpendicular to the plane of incidence) at a frequency using a photoelastic modulator (PEM). By this a differential reflectance spectrum
Ip − Is R = R Ip + Is can be obtained where Ip is the spectrum obtained with the p-polarized light Is with that of s-polarized light. The polarization of light is carried out using a photoelastic modulator where the optical element is compressed (stretched), the horizontal polarization component (parallel to the modulator axis) travels slightly faster (slower) than the vertical component [17,18]. The phase difference between the two components is called the retardation, R. When the peak retardation is set to one half of the light wavelength (half-wave retardation mode), the PEM rotates the light polarization by 90°. The polarization of the light impinging on the sample can thus be switched with a frequency of 37 kHz between p and s polarizations. Because the two polarization states occur twice within each PEM oscillating cycle, the sampling frequency is 74 kHz. Consequently, the p (surface and gas phase) and s (gas phase) spectra are acquired nearly simultaneously. Since the effective absorption of the surface species by s-polarized light is zero (metal surface selection rule) [19] after demodulation of the signal, (p-s) spectra are obtained which characterize the vibrational signature of the surface species, whereas the s spectra characterize the corresponding gas phase absorption (see Figure 12.2). The major advantage of PM-IRRAS technique lies in the simultaneous detection of gas phase and surface species which can give tremendous insight into the reaction mechanism [21,22]. The conventional IRRAS spectrometer which gives maximum sensitivity for the light
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FIGURE 12.2 The generation of p-and s-polarized light used in PM-IRRAS technique. (Reproduced with permission from [20] IOP Copyright © Journal of Physics-Condensed Matter 2008.)
beam reflected from the surface makes the flatness of the sample important criteria in PM-IRRAS experiments. However, recently Baiker and co-workers have demonstrated that if dispersed properly on a substrate even powder samples can also be employed for PM-IRRAS studies [23]. Apart from the ability to do high pressure experiments, data acquisition of a PM-IRRAS spectrum is extremely fast and a spectral range of 800–4000 cm−1 can be acquired making this method a very special technique for the surface science community.
12.3.2 Sum Frequency Generation Spectroscopy Vibrational Sum Frequency Generation is another useful technique that can be performed in the mbar to bar pressure regime for studying the interface and interfacial reactions. In this technique two short picosecond laser pulses are spatially and temporally overlapped on the sample. One of the beams is in the visible range with a fixed frequency (ωvis) and the other is a tunable beam in the mid-IR range (ωIR). In the case of a vibrational resonance of a molecule adsorbed at the surface-gas interface, the two light waves falling on the surface interact and generate a wave at the sum of their frequencies (ωSFG = ωIR + ωvis), resulting in a signal in the visible (e.g., blue1) region. In a simplified picture one can envision this as a vibrational transition from the ground state to an excited state, followed by excitation to a higher-energy virtual state and relaxation through an anti-Stokes Raman process (Figure 12.3a). By tuning the IR wavelength (wavenumber) and monitoring the SFG intensity, an adsorbate vibrational spectrum is obtained. According to the applicable selection rules, a vibrational mode must be simultaneously IR and Raman active to be SFG active. Therefore, SFG is not allowed in media with inversion symmetry, as in the centrosymmetric bulk of a noble metal or in the isotropic gas phase, but has a finite value at the catalyst-gas interface, where the inversion symmetry is broken. Because this nonlinear process produces only a small signal, high incident light intensities, i.e., pulsed lasers, are required. As can be seen from Figure 12.3, there can be different ways in which SFG spectra can be generated. The scanning mode is the one where the IR range is tuned over the vibrational region of interest and typically takes several minutes for acquisition. In contrast the broadband 1
For interpretation of color in Figure 12.3, the reader is referred to the web version of this book.
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FIGURE 12.3 The various modes of collection of spectrum in Sum Frequency Generation Spectroscopy. (Adapted from Ref. [24].)
SFG is acquired by using ultrashort pulses of 150 fs broad IR pulse of width 150 cm−1 overlapping with a narrow band visible pulse (Figure 12.3b). Thus only that part of the IR transition that is in resonance with vibrational transition will result in the sum frequency signal (ωSFG = ωIR + ωvis). The broadband method allows generating the vibrational spectrum with few laser pulses without scanning the IR region and allows a comparatively faster data acquisition (Figure 12.3c). Another advantage of SFG spectroscopy lies in using this method for doing time-resolved pump-probe experiments. This is done by exciting the surface with an intense picosecond or femtosecond near IR laser pulse followed by a time delayed weak IR + Vis light to probe the vibrational dynamics of the adsorbed molecules or intermediates on the surface which has short lifetimes (Figure 12.3d). The advantage of using a laser allows one to use a polarized light source for excitation of the adsorbed molecules. The conventional method is using p-polarized UV light and p-polarized IR light, generating p-polarized SFG signal (Figure 12.3e). Since parallel polarized light is the one that is most sensitive to adsorbed species, this mode is useful in characterizing metal-adsorbate interfaces.
12.4 SURFACE SCIENCE STUDIES USING HIGH PRESSURE TECHNIQUES One of the first studies carried out using ambient pressure XPS on the interaction of CO on Pd(1 1 1) surface clearly demonstrated the inevitable need of high pressure techniques in understanding the catalysis phenomena [25] (see Figure 12.4).
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FIGURE 12.4 The C1s spectrum collected at 10−6 and 10−1 mbar of CO on a Pd(111) surface. (Adapted from Ref. [25].)
The C1s spectrum recorded at high vacuum after CO exposure and at 10−1 mbar of CO clearly demonstrates the presence of CO adsorbed on the threefold hollow site (285.6 eV) and CO on the top site (286.3 eV). The population of CO on the top site is clearly higher in the high pressure regime which otherwise is not well resolved in the UHV experiments. The binding energy assignments of CO are also supported by high resolution synchrotron experiments [26]. The vibrational spectroscopy employing SFG carried out in the same study showed that the SFG spectrum measured at 10−6 mbar and 400 K gave evidence of CO adsorbed on the threefold hollow sites (1910 cm−1), whereas, at higher CO coverages (10−6 mbar at 300 K), mixtures of CO on the threefold hollow sites and bridging CO (1935 cm−1), along with a small amount of on-top CO, were produced on the Pd(1 1 1) surface. Increasing the CO pressure to 1 mbar shifted the frequency of the hollow/bridge species to 1948 cm−1 and increased the intensity of the on-top species (2083 cm−1) clearly showing one-to-one correspondence and the evidence of weakly bound species which otherwise are not detected under UHV conditions. Similarly, in situ characterization of the various phases formed during the reaction can have a huge impact in the understanding of oxidation catalysis. Ketteler and co-workers identified various oxide phases formed on the Pd(1 1 1) surface by dosing oxygen at 1 mbar pressure [27]. The presence of a metastable (transforming to surface oxide) subsurface oxide layer in the pressure range of 10−2 and 40 Pa and at temperatures <650–850 K was detected during the high pressure XPS experiments. The identification of such a phase has given insight into the general trend in which metals transform into bulk oxides. A metastable subsurface oxide lies intermediate between a bulk oxide and surface oxide structure. A direct transformation of a surface oxide to bulk oxide is separated by a high activation barrier. Thus the following mechanism is proposed for the transformation of a metal, in particular Pd to bulk metal oxide as illustrated by cartoon in Figure 12.5. During the reduction of the bulk oxide to the metal, the system bypasses the metastable phase because the thermal energy can overcome the activation barrier resulting in a direct transformation.
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FIGURE 12.5 Cartoon illustrating the oxidation reduction changes occurring on Pd(111) surface as revealed by ambient pressure XPS studies. The dark spheres represent Palladium atoms and lighter spheres oxygen atoms.
The CO oxidation reaction on Pt surfaces has been an area of great interest for the surface science community because of the variety of surface structures that can form on the surface [28,29], role of surface structure on the oxidation [29,30], the oscillations and spatiotemporal patterns generated on the surface [31–34]. In situ oxidation study of Pt(1 1 0) and the reactivity of the different oxides formed on the surface with CO was probed recently using variety of techniques including ambient pressure XPS [35]. The O1s spectra obtained during 0.5 mbar O2 showed two distinct features that can be related to chemisorbed oxygen and surface oxide at binding energy values 529.7 and 530.8 eV (Figure 12.6). The oxygen-treated surface at 0.5 mbar was further titrated with CO and the time-resolved spectra (collected every 12 s) shown in Figure 12.6a. The time-resolved spectrum clearly shows the buildup of on-top CO (532.6 eV) and the depletion of chemisorbed oxygen and surface oxygen. The spectra collected at 36 and 276 s are shown in Figure 12.6b and c and a careful analysis of the time-resolved spectra showed a faster rate of disappearance of chemisorbed oxygen compared to the surface oxide. The surface oxide formed on Pt(1 1 0) under high oxygen pressure has α-PtO2 phase which has features similar to those of the oxide formed on a platinum nanoparticle. Methanol decomposition on metal surfaces is a hot area of research because of the potential application of this reaction in H2 generation for onboard fuel cell application [36]. The dehydrogenation pathway of this reaction on palladium surface has been studied using model surfaces to gain further knowledge on the deactivation of the catalyst [37]. The possible species for the deactivation like formation of carbon or carbonaceous species (CHx; x = 0−3) by cleavage of the methanolic CO bond (i.e., the CO bond within a CHxO molecule; x = 1−4). The dissociation of the reaction product CO is also less likely but cannot be excluded as the source of carbon [24]. There is a consensus regarding the first step of the decomposition of methanol say the formation of methoxy species [38–40] but further scission of CO bond in the methoxy species is debated in the literature [41–44]. The in situ XPS and SFG studies carried out by Morkel et al. [45] have given more insights into this reaction on Pd surface. Figure 12.7 shows the XPS and SFG spectrum obtained at different pressures (5 × 10−7 to 0.1 mbar) and temperature (300 and 400 K) of methanol exposure. The C1s binding energy values (284 eV) after exposing methanol vapor on Pd(1 1 1) surface at 300 and 400 K demonstrates the build up of CHx species on the surface. The concentration of CHx on the palladium surface is evidently higher at 400 K compared to the CO (285.6 eV) species. The corresponding SFG spectrum obtained at 300 K for both 5 × 10−7 and 0.1 mbar showed weak feature at 1920 cm−1 due to the bridged or
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FIGURE 12.6 (a) Time-resolved O1s XPS spectra obtained during the exposure of CO on an oxygen covered Pt(1 1 0) surface generated at 0.5 mbar O2. (b) O1s spectrum at 36 s and (c) 276 s. (Adapted and reprinted with permission from [35] Copyright © 2011, American Chemical Society.)
hollow bonded CO. The 440 K SFG spectrum showed no resonance for CO species indicating that the surface is saturated with CHx (x = 0–3) species which on exposure to O2 at 450 K cleans the surface. Similar results were also reported by Borasio and co-workers using PM-IRRAS [22]. Ambient pressure XPS was recently used for studying oxidation and reduction of Pd(1 0 0) and Pd nanoparticles supported on silica [46]. The study was motivated by the fact that much less is known about the oxidation of the Pd particles and the role of Pd oxide formed during the catalytic oxidation reactions. The initial stages of the oxidation of the metal or metal nanoparticle can possibly give us a better understanding and solve some of the controversies existing in determining the active phase of the catalyst under the reaction conditions [30,47–50]. The ambient pressure XPS results prove that the oxide phase formation starts at a lower temperature on nanoparticle compared to the single crystal and switches to metallic form in the reduction cycle. The CO was found to adsorb on the PdO surface indicating that under real catalyst the CO oxidation to CO2 proceeds via Mars-van Krevelen mechanism. CO is also found
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400 K
300 K —6
a
0.1 mbar
0.1 mbar
10 mbar
a
a
FIGURE 12.7 C1s spectra collected at 300 and 400 K after exposing the Pd(1 1 1) surface to methanol vapor at 300 and 400 K. The corresponding SFG spectrum is shown at the bottom. (Adapted from Ref. [45].)
to dissociate on the nanoparticle surface forming palladium carbide which could be the source of deactivation of the catalyst. Based on this study the following mechanism for the CO oxidation reaction has been proposed on palladium nanoparticle as shown in Figure 12.8. There are several other reports on the application of high pressure surface science techniques on model catalyst surface where experiments are carried out on nanoparticles supported on well-defined oxide surfaces. A major literature in the last two decades has been focused in addressing the genesis of the catalysis by gold [51–61]. The mode of activation of O2 on supported gold catalyst has been debated in the literature where the activation has been proposed to happen both experimentally and theoretically on the bilayer gold, perimeter interface between metal nanoparticle and oxide support and even on under co-ordinated sites
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FIGURE 12.8 The initial carbide covered Pd nanoparticle subjected to O2 (1 bar at 150 °C) removes carbon and produces surface oxide. Increasing the temperature to 200 °C in O2 created bulk oxide. Switching to CO (0.5 bar) cycle at 120 °C reduces the surface back to metallic Pd by oxide decomposition along with the CO2 production. The surface is converted to carbide phase once the oxide layer is completely removed and the cycle continues. (Adapted and reprinted from Ref. [46] Copyright © 2011, American Physical Society.)
[62–67]. Recent ambient pressure XPS studies on the interaction of 1 torr of O2 for several hours on Au supported on TiO2 showed no evidence for their interaction with Au nanoparticles [13]. However, in the presence of X-rays the intensity of the metallic Au peak (84 eV) reduced in time whereas an additional peak at higher binding energy (85.3 eV) showed the presence of X-ray induced oxidation of the Au clusters. The results demonstrate that in the CO oxidation reaction the activation of O2 is unlikely to happen on the Au clusters and extreme care to be taken while interpreting the data obtained using high intensity X-ray sources like synchrotron radiation. NO and O2 adsorption was also a subject matter for APXPS studies by Salmeron and co-workers on TiO2 and SiO2 [68]. They could identify species like N (399.3 eV), NO3 (405.1 eV) and (NO)2 (401.9 eV) at 240 mtorr of NO on Au/TiO2 surface and not on Au/SiO2 and was revealed by further investigations that NO adsorbs only on TiO2. Bimetallic nanoparticles have received a lot of interest in recent years because of the surprising trends in reactivity compared to their monometallic counterparts [69]. Also tuning the surface structure and atomic arrangement of the catalyst can result in engineering nextgeneration catalyst [70–73]. The surprising catalytic activity of Au–Pd for oxidation of primary alcohols [72], the superior activity of Au–Ni for steam reforming [71], high hydrogen storage capacity of Pd–Cd [73] system all demonstrate that future generation of catalysts requires better understanding of the surface properties. Since XPS is an ideal tool for studying the chemical composition of the surface and the oxidation state, the bimetallic nanostructures are extensively probed using this technique. Combining ambient pressure XPS, using a tunable X-ray source from synchrotron radiation one can very well do a depth profiling of the nanostructures under the reactive gaseous environment [74–76]. Bimetallic nanoparticles with core-shell morphology made of Rh–Pd, Rh–Pt, and Pt–Pd of varying composition when subjected to reductive and oxidative gaseous environment were shown to exhibit interesting surface segregation properties [76]. The ambient pressure XPS studies carried out under
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FIGURE 12.9 Pd (3d) and Rh (3d) spectra collected in the presence of 100 mtorr of CO and NO from a Rh0.5Pd0.5 bimetallic cluster. The cartoon at the bottom shows the dynamic behavior of the core-shell particle during the CO-NO switching cycle. (Adapted and reprinted with permission from [76] Copyright © 2010, American Chemical Society.)
oxidizing and reducing environment showed that RhxPd1−x and RhxPt1−x undergo reversible segregation of the metal under oxidizing and reducing conditions, no such effects were found in the Pt–Pd system (see Figure 12.9). APXPS spectrum collected at 100 mtorr of CO and NO shown in figure demonstrates the segregation of one metal onto the surface evidenced by a substantial increase in the photoelectron intensity from one metal to the other while switching the gases from oxidizing to reducing conditions. Thus, switching from 100 mTorr CO to 100 mTorr NO at 300 °C there is a noticeable decrease in the Pd (3d) intensity with a corresponding increase in the Rh (3d) intensity. Metal-support interaction [77] which plays a very important role in catalysis has been studied on Ni/CeO2 system using ambient pressure XPS [78,79]. The photoemission carried out at 1 torr of hydrogen produced dynamic changes on the Ni/Ceria system which is depicted in Figure 12.10. The Ce 4d spectrum collected before the reduction in H2 at 500 °C shows features characteristic of fully oxidized Ce(IV) phase. But in the presence of 1 torr H2 and at temperature 500 °C the Ce 4d spectrum shows partially reduced CeO2−x phase as evidenced by a reduction in the 110 eV peak, which is similar to the one reported in the literature [80]. This is accompanied by
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FIGURE 12.10 (a) Dynamic changes associated with Ce(4d) spectrum collected at different temperatures in H2 at 1 torr pressure. (b) The cartoon depicts the formation of partially reduced ceria. (Adapted from Ref. [79].)
a reduction in the Ni 2p XPS signal caused due to the migration of the ceria to the nickel nanoparticle surface which is related to the SMSI effect. This process is reversible through simple evacuation. The schematic in Figure 12.10b depicts the dynamic events happening on the surface during the in situ reduction process.
12.5 CONCLUSION AND OUTLOOK The understanding of catalysis phenomena has improved tremendously over the last few years mainly due to the developments in the field of surface science. The new generation of in situ tools wherein the model reactions can be probed under realistic conditions has given invaluable data wherein molecular level insights on how the real-world catalyst responding to reactive conditions can be deduced. As we push further the limits of instrumentation, design and development of smarter and robust functional materials can be engineered from the surface science inputs where catalyst will be able to perform with superior selectivity and activity.
Acknowledgments The author thanks Dr. Sourav Pal, Director CSIR-NCL, and Dr. C.S. Gopinath for providing support and encouragement. The help of A.B. Vysakh is acknowledged for recreating some of the images.
References [1] A.L. Robinson, Science 185 (1974) 772. [2] G. Ertl, Angew. Chem. Int. Ed. 47 (2008) 3524. [3] C.O. Arean, B.M. Weckhuysen, A. Zecchina, Phys. Chem. Chem. Phys. 14 (2012) 2125.
REFERENCES
303
[4] P.B. Rasmussen, B.L.M. Hendriksen, H. Zeijlemaker, H.G. Ficke, J.W.M. Frenken, Rev. Sci. Instrum. 69 (1998) 3879. [5] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Complexes, Part B, third ed., Wiley, New York, 1980. [6] A. Knop-Gericke, E. Kleimenov, M. Havecker, R. Blume, D. Teschner, S. Zafeiratos, R. Schlogl, V.I. Bukhtiyarov, V.V. Kaichev, I.P. Prosvirin, A.I. Nizovskii, H. Bluhm, A. Barinov, P. Dudin, M. Kiskinova, Advances in Catalysis, vol. 52, Elsevier Academic Press Inc, San Diego, 2009 p. 213. [7] R.W. Joyner, M.W. Roberts, K. Yates, Surf. Sci. 87 (1979) 501. [8] D.M. Littrell, B.J. Tatarchuk, J. Vac. Sci. Technol. A Vac. Surf. Films 4 (1986) 1608. [9] S. Dushman, J.M. Lafferty, John Wiley & Sons, New York, 1962 p. 806. [10] F. Tao, Chem. Commun. 48 (2012) 3812. [11] A. Jürgensen, N. Esser, R. Hergenröder, Surf. Interface Anal. 144 (2012) 1100. [12] K. Roy, C.P. Vinod, C.S. Gopinath, J. Phys. Chem. C 117 (2013) 4717. [13] P. Jiang, S. Porsgaard, F. Borondics, M. Kober, A. Caballero, H. Bluhm, F. Besenbacher, M. Salmeron, J. Am. Chem. Soc. 132 (2010) 2858. [14] R.G. Greenler, J. Chem. Phys. 44 (1966) 310. [15] J. Ryczkowski, Catal. Today 68 (2001) 263. [16] C. Lamberti, A. Zecchina, E. Groppo, S. Bordiga, Chem. Soc. Rev. 39 (2010) 4951. [17] K.W. Hipps, G.A. Crosby, J. Phys Chem. 83 (1979) 555. [18] B.J. Barner, M.J. Green, E.I. Saez, R.M. Corn, Anal. Chem. 63 (1991) 55. [19] F.M. Hoffmann, Surf. Sci. Rep. 3 (1983) 107. [20] G. Rupprechter, C. Weilach, J. Phys. Condens. Matter 20 (2008). [21] A. Urakawa, T. Burgi, H.P. Schlapfer, A. Baiker, J. Chem. Phys. 124 (2006). [22] M. Borasio, O.R. de la Fuente, G. Rupprechter, H.-J. Freund, J. Phys. Chem. B 109 (2005) 17791. [23] E. Roedel, A. Urakawa, A. Baiker, Catal. Today 155 (2010) 172. [24] G. Rupprechter, MRS Bull. 32 (2007) 1031. [25] V.V. Kaichev, I.P. Prosvirin, V.I. Bukhtiyarov, H. Unterhalt, G. Rupprechter, H.-J. Freund, J. Phys. Chem. B 107 (2003) 3522. [26] S. Surnev, M. Sock, M.G. Ramsey, F.P. Netzer, M. Wiklund, M. Borg, J.N. Andersen, Surf. Sci. 470 (2000) 171. [27] G. Ketteler, D.F. Ogletree, H. Bluhm, H. Liu, E.L.D. Hebenstreit, M. Salmeron, J. Am. Chem. Soc. 127 (2005) 18269. [28] W.X. Li, L. Osterlund, E.K. Vestergaard, R.T. Vang, J. Matthiesen, T.M. Pedersen, E. Lægsgaard, B. Hammer, F. Besenbacher, Phys. Rev. Lett. 93 (2004) 146104. [29] B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett. 89 (2002) 046101. [30] F. Gao, Y. Wang, Y. Cai, D.W. Goodman, J. Phys. Chem. C 113 (2008) 174. [31] R. Imbihl, G. Ertl, Chem. Rev. 95 (1995) 697. [32] A.v. Oertzen, A.S. Mikhailov, H.H. Rotermund, G. Ertl, J. Phys. Chem. B 102 (1998) 4966. [33] A.v. Oertzen, H.H. Rotermund, A.S. Mikhailov, G. Ertl, J. Phys. Chem. B 104 (2000) 3155. [34] A. von Oertzen, A. Mikhailov, H.H. Rotermund, G. Ertl, Surf. Sci. 350 (1996) 259. [35] D.R. Butcher, M.E. Grass, Z. Zeng, F. Aksoy, H. Bluhm, W.-X. Li, B.S. Mun, G.A. Somorjai, Z. Liu, J. Am. Chem. Soc. 133 (2011) 20319. [36] W.-H. Cheng, Acc. Chem. Res. 32 (1999) 685. [37] R. Shiozaki, T. Hayakawa, Y.-y. Liu, T. Ishii, M. Kumagai, S. Hamakawa, K. Suzuki, T. Itoh, T. Shishido, K. Takehira, Catal. Lett. 58 (1999) 131. [38] S.M. Francis, J. Corneille, D.W. Goodman, M. Bowker, Surf. Sci. 364 (1996) 30. [39] A.K. Bhattacharya, M.A. Chesters, M.E. Pemble, N. Sheppard, Surf. Sci. 206 (1988) L845. [40] M. Mavrikakis, M.A. Barteau, J. Mol. Catal. A: Chem. 131 (1998) 135. [41] K. Christmann, J.E. Demuth, J. Chem. Phys. 76 (1982) 6308. [42] J.L. Davis, M.A. Barteau, Surf. Sci. 187 (1987) 387. [43] J.L. Davis, M.A. Barteau, Surf. Sci. 235 (1990) 235. [44] J.A. Gates, L.L. Kesmodel, J. Catal. 83 (1983) 437. [45] M. Morkel, V.V. Kaichev, G. Rupprechter, H.J. Freund, I.P. Prosvirin, V.I. Bukhtiyarov, J. Phys. Chem. B 108 (2004) 12955. [46] R. Westerstrom, M.E. Messing, S. Blomberg, A. Hellman, H. Gronbeck, J. Gustafson, N.M. Martin, O. Balmes, R. van Rijn, J.N. Andersen, K. Deppert, H. Bluhm, Z. Liu, M.E. Grass, M. Havecker, E. Lundgren, Phys. Rev. B 83 (2011) 115440.
304
12. Insights into Heterogeneous Catalysis through Surface Science Techniques
[47] F. Gao, S.M. McClure, Y. Cai, K.K. Gath, Y. Wang, M.S. Chen, Q.L. Guo, D.W. Goodman, Surf. Sci. 603 (2009) 65. [48] B.L.M. Hendriksen, S.C. Bobaru, J.W.M. Frenken, Surf. Sci. 552 (2004) 229. [49] R. van Rijn, O. Balmes, R. Felici, J. Gustafson, D. Wermeille, R. Westerström, E. Lundgren, J.W.M. Frenken, J. Phys. Chem. C 114 (2010) 6875. [50] F. Gao, Y. Wang, D.W. Goodman, J. Phys. Chem. C 114 (2010) 6874. [51] A.S.K. Hashmi, G.J. Hutchings, Angew. Chem. Int. Ed. 45 (2006) 7896. [52] J.K. Norskov, T. Bligaard, B. Hvolbaek, F. Abild-Pedersen, I. Chorkendorff, C.H. Christensen, Chem. Soc. Rev. 37 (2008) 2163. [53] M. Haruta, Cattech 6 (2002) 102. [54] G.J. Hutchings, M. Brust, H. Schmidbaur, Chem. Soc. Rev. 37 (2008) 1759. [55] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [56] T. Kiyonaga, Q. Jin, H. Kobayashi, H. Tada, Chemphyschem 10 (2009) 2935. [57] J.F. Jia, K. Haraki, J.N. Kondo, K. Domen, K. Tamaru, J. Phys. Chem. B 104 (2000) 11153. [58] C. Bianchi, F. Porta, L. Prati, M. Rossi, Top. Catal. 13 (2000) 231. [59] T. Janssens, B. Clausen, B. Hvolbæk, H. Falsig, C. Christensen, T. Bligaard, J. Nørskov, Top. Catal. 44 (2007) 15. [60] N. Lopez, T.V.W. Janssens, B.S. Clausen, Y. Xu, M. Mavrikakis, T. Bligaard, J.K. Norskov, J. Catal. 223 (2004) 232. [61] R. Meyer, C. Lemire, S.K. Shaikhutdinov, H. Freund, Gold Bull. 37 (2004) 72. [62] M.S. Chen, D.W. Goodman, Science 306 (2004) 252. [63] N. Lopez, J.K. Nørskov, J. Am. Chem. Soc. 124 (2002) 11262. [64] Z.-P. Liu, X.-Q. Gong, J. Kohanoff, C.n. Sanchez, P. Hu, Phys. Rev. Lett. 91 (2003) 266102. [65] J.A. van Bokhoven, C. Louis, J.T. Miller, M. Tromp, O.V. Safonova, P. Glatzel, Angew. Chem. Int. Ed. 45 (2006) 4651. [66] N. Weiher, A.M. Beesley, N. Tsapatsaris, L. Delannoy, C. Louis, J.A. van Bokhoven, S.L.M. Schroeder, J. Am. Chem. Soc. 129 (2007) 2240. [67] M. Kotobuki, R. Leppelt, D.A. Hansgen, D. Widmann, R.J. Behm, J. Catal. 264 (2009) 67. [68] T. Herranz, X.Y. Deng, A. Cabot, P. Alivisatos, Z. Liu, G. Soler-Illia, M. Salmeron, Catal. Today 143 (2009) 158. [69] D. Wang, Y. Li, Adv. Mater. 23 (2011) 1044. [70] J.K. Norskov, T. Bligaard, J. Rossmeisl, C.H. Christensen, Nat. Chem. 1 (2009) 37. [71] F. Besenbacher, I. Chorkendorff, B.S. Clausen, B. Hammer, A.M. Molenbroek, J.K. Norskov, I. Stensgaard, Science 279 (1998) 1913. [72] D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362. [73] B.D. Adams, G. Wu, S. Nigro, A. Chen, J. Am. Chem. Soc. 131 (2009) 6930. [74] S. Alayoglu, F. Tao, V. Altoe, C. Specht, Z.W. Zhu, F. Aksoy, D.R. Butcher, R.J. Renzas, Z. Liu, G.A. Somorjai, Catal. Lett. 141 (2011) 633. [75] F. Tao, M.E. Grass, Y.W. Zhang, D.R. Butcher, J.R. Renzas, Z. Liu, J.Y. Chung, B.S. Mun, M. Salmeron, G.A. Somorjai, Science 322 (2008) 932. [76] F. Tao, M.E. Grass, Y.W. Zhang, D.R. Butcher, F. Aksoy, S. Aloni, V. Altoe, S. Alayoglu, J.R. Renzas, C.K. Tsung, Z.W. Zhu, Z. Liu, M. Salmeron, G.A. Somorjai, J. Am. Chem. Soc. 132 (2010) 8697. [77] R.W. Joyner, J.B. Pendry, D.K. Saldin, S.R. Tennison, Surf. Sci. 138 (1984) 84. [78] S.J. Tauster, Acc. Chem. Res. 20 (1987) 389. [79] A. Caballero, J.P. Holgado, V.M. Gonzalez-delaCruz, S.E. Habas, T. Herranz, M. Salmeron, Chem. Commun. 46 (2010) 1097. [80] L. Kundakovic, D.R. Mullins, S.H. Overbury, Surf. Sci. 457 (2000) 51.
12
Insights into Heterogeneous Catalysis through Surface Science Techniques C.P. Vinod Catalysis Division and Center of Excellence on Surface Science, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
12.1 INTRODUCTION Surface science and heterogeneous catalysis belong to two traditions with increasing overlap over the years. Heterogeneous catalysis is believed to be a surface phenomenon where several technologically and industrially relevant reactions and processes occur either at the interface or at the actual surface itself [1]. A detailed understanding on the nature of bond breaking and making, the intermediates and the energetic involved needs characterization tools which can probe the events in the time scale and with sufficient spatial resolution. Over the years surface science techniques have been instrumental in throwing light onto many of the fundamental problems of heterogeneous catalysis. The important recognition of this being the Nobel Prize in Chemistry to Professor Gerahard Ertl at the Fritz Haber Institute, Berlin on the studies on the molecular level understanding of many fundamental chemical processes including the ammonia synthesis catalyst [2]. The area of surface science is making rapid strides thanks to the improvements in the instrumentation thereby pushing the limits of data acquisition and thereby the quality of the data generated. The advancement of experimental surface science techniques including the development of new operando tools has had a major impact in our atomic and molecular level understanding of materials. This means a greater knowledge of structure, dynamic, composition, and thermodynamic properties of the surface which are important factors for predicting materials for desired chemical properties. The rapid stride made by surface science over the last decade meant that the new tools could be used in ultrahigh vacuum or at near ambient (realistic)
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conditions [3]. The outlook of surface science took a major phase shift with the development of probe techniques to the likes of STM and AFM. These techniques which are developing at an unimaginable scale: the major improvement in terms of surface catalysis being the development of reactor STM which can probe surfaces at near ambient conditions [4]. The fact that these microscopes can give atomically resolved images under high pressures and reacting conditions, make them powerful tools in deducing hidden atomic scale information of several exciting catalytic problems. The developments in the probe techniques will not be covered and this chapter is outlined such that the advancement made by two techniques: X-ray Photoelectron Spectroscopy and Vibrational Spectroscopy in catalyst characterization and its application in understanding the genesis of catalysis will be described with relevant examples from the literature. Vibrational spectroscopy has been a major characterization tool for many fundamental problems in chemistry [5]. There are several variations of the surface science counterpart of vibrational spectroscopy. Among them Sum Frequency Generation spectroscopy (SFG) and PM-IRRAS (polarization modulation infrared reflection absorption spectroscopy) are routinely used for identifying the vibrational features of surface adsorbates. Surface science research groups have immensely benefited from using these techniques for model catalysts.
12.2 X-RAY PHOTOELECTRON SPECTROSCOPY UNDER NEAR AMBIENT CONDITIONS (APXPS) X-ray photoelectron spectroscopy has been a major surface science tool in identifying active species in many catalytic systems. XPS is based on the simple principle of the photoelectric effect where a material when illuminated by soft X-rays (1.5 keV) results in the generation of photoelectrons which can be energy analyzed using an electron analyzer. By knowing the kinetic energies of the photoelectron, the binding energy of the electron and hence the chemical environment of the elements in the sample specimen can be determined. The photoemission process can be explained by the equation
KE = hν − BE − ϕ,
(12.1)
where KE is the kinetic energy of the emitted photoelectron, hν (h—Planck constant (6.62 × 10−34 Js), ν—frequency (Hz) of the radiation) is the energy of the exciting X-ray source, BE is the binding energy of the emitted electron, and ϕ is the work function of the material. Since the relative areas of the photoelectron spectrum give the information of the elemental composition from 5 to 10 nm, XPS becomes a surface sensitive technique which gives quantitative and qualitative information. The conventional photoelectron spectrometers are operated under UHV conditions where the experiments are carried out at pressures better than 10−6 mbar. Such high to ultrahigh vacuum is required as the emitted photoelectrons can be scattered by gas phase before they reach the electron analyzer resulting in weak or no electron counts. The application of pressures exceeding 10−5 mbar is not recommended in typical XPS instruments owing to the sensitivity of the electron detector toward higher pressures in the spectrometer, and is limited by attenuation of the signal due to inelastic scattering of electrons in the gas phase.
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The use of ultrahigh vacuum photoelectron spectroscopy has given phenomenal fundamental results associated with surface science of materials, but poses severe limitations in studying catalytic surface science. The weakly adsorbed species tend to desorb from the surfaces by mere evacuation making it impossible to track the nature and in turn identification of possible active species in a catalytic cycle. Similarly, stability of active phases of materials for instance changes depending on the operating pressure. So for a realistic understanding of catalytic phenomena it is ideal to perform XPS under near ambient conditions. Since the discovery of photoelectron spectroscopy there has been a constant impetus in pushing the pressure limit and getting closer to ambient pressure conditions. Recently, the history of the development of this field has been covered by an excellent review by Knop-Grieke and co-workers [6]. One of the earlier high pressure instruments were the ones with the sample located inside a high pressure gas cell [7] or a high pressure cell with a nozzle doser pushed over the sample [8]. The calculation of the attenuation of the electron beam passing through the gas phase was taken into consideration during the development of the first generation of ambient pressure X-ray photoelectron spectrometers by Roberts and co-workers [7]. The attenuation of the electron beam (Iattenuated) passing through 1 mm thick gas layer is given by Eq. (12.2)
Iattenuated = Ivac exp(−P/λ),
(12.2)
where Ivac is the intensity of the photoelectron in vacuum, P is the pressure in torr, and λ is the inelastic mean free path of the electron in a gas environment of 1 torr pressure. At low kinetic energies of the electrons λP can be calculated by Eq. (12.3) [9]
λP = 4(2σP )1/2 .
(12.3)
Calculations show that only 14% of the photoelectrons pass through to the analyzer at 1 torr of gas pressure. Some of the critical areas that need attention during the instrumentation of an ambient pressure XPS in order to get a good signal-to-noise ratio is worth mentioning. The intensity of the X-rays impinging on the sample, the X-ray window which separates the high pressure sample region from the source from which X-rays are generated, the absorption of X-rays by the gas phase, scattering of photoelectrons by the gas phase, and the collection of photoelectrons by the analyzer. The thickness and area of the window will determine the mechanical stability and the transmission rate of the X-rays across the window. Aluminum or Si3N4 are conventional X-ray windows used in ambient pressure XPS instruments. A 100 nm thick Si3N4 window with area 2.5 × 2.5 m2 can hold a pressure of 10 mbar. There is an exponential decay of the photoelectron intensity due to inelastic collisions with the gas phase molecules and the distance between the sample and the aperture of the analyzer is a critical parameter in the design of the ambient pressure XPS spectrometer. For a 0.5 mm aperture, a 1 mm distance between the sample and the entrance of the analyzer is proven to be optimum. In the current generation of ambient pressure X-ray photoelectron spectrometers, the sample is placed close to a differentially pumped analyzer so that gas molecules can escape increasing the mean free path length of the electrons. This coupled with an electrostatic lens can significantly improve the photoelectron count reaching the analyzer. A schematic of the principle of a typical ambient pressure XPS is shown in Figure 12.1 along with the analyzer cone used for accepting the electrons toward the hemispherical analyzer.
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FIGURE 12.1 Schematic of an ambient pressure X-ray photoelectron spectrometer.
The X-rays which pass through the window reach the sample kept in a high pressure environment relevant to the catalytic reaction. The aperture size of the differentially pumped analyzer cone is also a trade-off between the count of electrons reaching the analyzer to the pressure developed near the first differential pumping stage. The X-ray source can be either conventional Al Kα/Mg Kα or from a synchrotron source. The advantage of high brilliance and exceptionally good resolution makes most of the APXPS studies being carried out using synchrotron radiations. However, there is a major interest in developing instruments based on conventional X-ray sources [10–12] which sometimes can be helpful in avoiding problems associated with radiation damages of samples and molecules routinely encountered while using high intensity X-ray (synchrotron) sources [13].
12.3 VIBRATIONAL SPECTROSCOPY AT HIGH PRESSURES Over the years infrared spectroscopy has become a powerful technique for studying adsorbed molecules and catalyst support [14–16]. The in situ studies involving powder catalyst in the presence of reactant molecules and at temperatures relevant to catalysis have given valuable information in elucidating catalysis phenomena and reaction mechanisms. Fast scanning Fourier transform IR instruments in conjunction with in situ cells can deliver data with considerable ease and time. Traditionally surface scientists have relied upon High Resolution Electron Energy Loss Spectroscopy (HREELS) and Reflection Absorption Infrared Spectroscopy (RAIRS) to carry out fundamental surface science studies. Electrons (HREELS) and photons (RAIRS) are allowed to specularly reflect from the flat surface and the resulting vibrations are detected using a suitable analyzer. These studies done on flat metal surfaces like a single crystal surface or model catalyst surface where metal nanoparticle deposited on thin oxide layer are carried out under ultrahigh vacuum conditions. Again UHV is required
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while doing HREELS to circumvent the problem of inelastic scattering of low energy electrons that are used to excite the adsorbed molecules on surface. Conventional RAIRS spectrometers also work at high vacuum conditions as high pressure experiments will have strong gas phase signals which will bury the vibrational features from the surface. So to study model surface reactions from flat surfaces at catalytically relevant pressures and temperatures, two techniques were developed which have been successfully used over last several years. These are polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and Sum Frequency Generation IR Spectroscopy (SFG). The basic concept behind the two techniques is explained in the next section.
12.3.1 Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) Polarization modulation infrared reflection absorption spectroscopy is based on the selective absorption of s- and p-polarized light by the surface and gas phase species. The absorption of s-polarized light on the metal surface is zero and hence all the signal amounts to the gas phase. In contrast, the p-polarized light excites both gas phase and surface vibrations. Thus by collecting the spectra using s- and p-polarized light and then carefully subtracting the one from the other gives specific vibrations of molecules adsorbed on the surface. The polarization of the light falling on the sample is modulated between p-polarized light (polarization parallel to the plane of incidence) and s-polarized light (polarization perpendicular to the plane of incidence) at a frequency using a photoelastic modulator (PEM). By this a differential reflectance spectrum
Ip − Is R = R Ip + Is can be obtained where Ip is the spectrum obtained with the p-polarized light Is with that of s-polarized light. The polarization of light is carried out using a photoelastic modulator where the optical element is compressed (stretched), the horizontal polarization component (parallel to the modulator axis) travels slightly faster (slower) than the vertical component [17,18]. The phase difference between the two components is called the retardation, R. When the peak retardation is set to one half of the light wavelength (half-wave retardation mode), the PEM rotates the light polarization by 90°. The polarization of the light impinging on the sample can thus be switched with a frequency of 37 kHz between p and s polarizations. Because the two polarization states occur twice within each PEM oscillating cycle, the sampling frequency is 74 kHz. Consequently, the p (surface and gas phase) and s (gas phase) spectra are acquired nearly simultaneously. Since the effective absorption of the surface species by s-polarized light is zero (metal surface selection rule) [19] after demodulation of the signal, (p-s) spectra are obtained which characterize the vibrational signature of the surface species, whereas the s spectra characterize the corresponding gas phase absorption (see Figure 12.2). The major advantage of PM-IRRAS technique lies in the simultaneous detection of gas phase and surface species which can give tremendous insight into the reaction mechanism [21,22]. The conventional IRRAS spectrometer which gives maximum sensitivity for the light
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FIGURE 12.2 The generation of p-and s-polarized light used in PM-IRRAS technique. (Reproduced with permission from [20] IOP Copyright © Journal of Physics-Condensed Matter 2008.)
beam reflected from the surface makes the flatness of the sample important criteria in PM-IRRAS experiments. However, recently Baiker and co-workers have demonstrated that if dispersed properly on a substrate even powder samples can also be employed for PM-IRRAS studies [23]. Apart from the ability to do high pressure experiments, data acquisition of a PM-IRRAS spectrum is extremely fast and a spectral range of 800–4000 cm−1 can be acquired making this method a very special technique for the surface science community.
12.3.2 Sum Frequency Generation Spectroscopy Vibrational Sum Frequency Generation is another useful technique that can be performed in the mbar to bar pressure regime for studying the interface and interfacial reactions. In this technique two short picosecond laser pulses are spatially and temporally overlapped on the sample. One of the beams is in the visible range with a fixed frequency (ωvis) and the other is a tunable beam in the mid-IR range (ωIR). In the case of a vibrational resonance of a molecule adsorbed at the surface-gas interface, the two light waves falling on the surface interact and generate a wave at the sum of their frequencies (ωSFG = ωIR + ωvis), resulting in a signal in the visible (e.g., blue1) region. In a simplified picture one can envision this as a vibrational transition from the ground state to an excited state, followed by excitation to a higher-energy virtual state and relaxation through an anti-Stokes Raman process (Figure 12.3a). By tuning the IR wavelength (wavenumber) and monitoring the SFG intensity, an adsorbate vibrational spectrum is obtained. According to the applicable selection rules, a vibrational mode must be simultaneously IR and Raman active to be SFG active. Therefore, SFG is not allowed in media with inversion symmetry, as in the centrosymmetric bulk of a noble metal or in the isotropic gas phase, but has a finite value at the catalyst-gas interface, where the inversion symmetry is broken. Because this nonlinear process produces only a small signal, high incident light intensities, i.e., pulsed lasers, are required. As can be seen from Figure 12.3, there can be different ways in which SFG spectra can be generated. The scanning mode is the one where the IR range is tuned over the vibrational region of interest and typically takes several minutes for acquisition. In contrast the broadband 1
For interpretation of color in Figure 12.3, the reader is referred to the web version of this book.
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FIGURE 12.3 The various modes of collection of spectrum in Sum Frequency Generation Spectroscopy. (Adapted from Ref. [24].)
SFG is acquired by using ultrashort pulses of 150 fs broad IR pulse of width 150 cm−1 overlapping with a narrow band visible pulse (Figure 12.3b). Thus only that part of the IR transition that is in resonance with vibrational transition will result in the sum frequency signal (ωSFG = ωIR + ωvis). The broadband method allows generating the vibrational spectrum with few laser pulses without scanning the IR region and allows a comparatively faster data acquisition (Figure 12.3c). Another advantage of SFG spectroscopy lies in using this method for doing time-resolved pump-probe experiments. This is done by exciting the surface with an intense picosecond or femtosecond near IR laser pulse followed by a time delayed weak IR + Vis light to probe the vibrational dynamics of the adsorbed molecules or intermediates on the surface which has short lifetimes (Figure 12.3d). The advantage of using a laser allows one to use a polarized light source for excitation of the adsorbed molecules. The conventional method is using p-polarized UV light and p-polarized IR light, generating p-polarized SFG signal (Figure 12.3e). Since parallel polarized light is the one that is most sensitive to adsorbed species, this mode is useful in characterizing metal-adsorbate interfaces.
12.4 SURFACE SCIENCE STUDIES USING HIGH PRESSURE TECHNIQUES One of the first studies carried out using ambient pressure XPS on the interaction of CO on Pd(1 1 1) surface clearly demonstrated the inevitable need of high pressure techniques in understanding the catalysis phenomena [25] (see Figure 12.4).
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FIGURE 12.4 The C1s spectrum collected at 10−6 and 10−1 mbar of CO on a Pd(111) surface. (Adapted from Ref. [25].)
The C1s spectrum recorded at high vacuum after CO exposure and at 10−1 mbar of CO clearly demonstrates the presence of CO adsorbed on the threefold hollow site (285.6 eV) and CO on the top site (286.3 eV). The population of CO on the top site is clearly higher in the high pressure regime which otherwise is not well resolved in the UHV experiments. The binding energy assignments of CO are also supported by high resolution synchrotron experiments [26]. The vibrational spectroscopy employing SFG carried out in the same study showed that the SFG spectrum measured at 10−6 mbar and 400 K gave evidence of CO adsorbed on the threefold hollow sites (1910 cm−1), whereas, at higher CO coverages (10−6 mbar at 300 K), mixtures of CO on the threefold hollow sites and bridging CO (1935 cm−1), along with a small amount of on-top CO, were produced on the Pd(1 1 1) surface. Increasing the CO pressure to 1 mbar shifted the frequency of the hollow/bridge species to 1948 cm−1 and increased the intensity of the on-top species (2083 cm−1) clearly showing one-to-one correspondence and the evidence of weakly bound species which otherwise are not detected under UHV conditions. Similarly, in situ characterization of the various phases formed during the reaction can have a huge impact in the understanding of oxidation catalysis. Ketteler and co-workers identified various oxide phases formed on the Pd(1 1 1) surface by dosing oxygen at 1 mbar pressure [27]. The presence of a metastable (transforming to surface oxide) subsurface oxide layer in the pressure range of 10−2 and 40 Pa and at temperatures <650–850 K was detected during the high pressure XPS experiments. The identification of such a phase has given insight into the general trend in which metals transform into bulk oxides. A metastable subsurface oxide lies intermediate between a bulk oxide and surface oxide structure. A direct transformation of a surface oxide to bulk oxide is separated by a high activation barrier. Thus the following mechanism is proposed for the transformation of a metal, in particular Pd to bulk metal oxide as illustrated by cartoon in Figure 12.5. During the reduction of the bulk oxide to the metal, the system bypasses the metastable phase because the thermal energy can overcome the activation barrier resulting in a direct transformation.
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FIGURE 12.5 Cartoon illustrating the oxidation reduction changes occurring on Pd(111) surface as revealed by ambient pressure XPS studies. The dark spheres represent Palladium atoms and lighter spheres oxygen atoms.
The CO oxidation reaction on Pt surfaces has been an area of great interest for the surface science community because of the variety of surface structures that can form on the surface [28,29], role of surface structure on the oxidation [29,30], the oscillations and spatiotemporal patterns generated on the surface [31–34]. In situ oxidation study of Pt(1 1 0) and the reactivity of the different oxides formed on the surface with CO was probed recently using variety of techniques including ambient pressure XPS [35]. The O1s spectra obtained during 0.5 mbar O2 showed two distinct features that can be related to chemisorbed oxygen and surface oxide at binding energy values 529.7 and 530.8 eV (Figure 12.6). The oxygen-treated surface at 0.5 mbar was further titrated with CO and the time-resolved spectra (collected every 12 s) shown in Figure 12.6a. The time-resolved spectrum clearly shows the buildup of on-top CO (532.6 eV) and the depletion of chemisorbed oxygen and surface oxygen. The spectra collected at 36 and 276 s are shown in Figure 12.6b and c and a careful analysis of the time-resolved spectra showed a faster rate of disappearance of chemisorbed oxygen compared to the surface oxide. The surface oxide formed on Pt(1 1 0) under high oxygen pressure has α-PtO2 phase which has features similar to those of the oxide formed on a platinum nanoparticle. Methanol decomposition on metal surfaces is a hot area of research because of the potential application of this reaction in H2 generation for onboard fuel cell application [36]. The dehydrogenation pathway of this reaction on palladium surface has been studied using model surfaces to gain further knowledge on the deactivation of the catalyst [37]. The possible species for the deactivation like formation of carbon or carbonaceous species (CHx; x = 0−3) by cleavage of the methanolic CO bond (i.e., the CO bond within a CHxO molecule; x = 1−4). The dissociation of the reaction product CO is also less likely but cannot be excluded as the source of carbon [24]. There is a consensus regarding the first step of the decomposition of methanol say the formation of methoxy species [38–40] but further scission of CO bond in the methoxy species is debated in the literature [41–44]. The in situ XPS and SFG studies carried out by Morkel et al. [45] have given more insights into this reaction on Pd surface. Figure 12.7 shows the XPS and SFG spectrum obtained at different pressures (5 × 10−7 to 0.1 mbar) and temperature (300 and 400 K) of methanol exposure. The C1s binding energy values (284 eV) after exposing methanol vapor on Pd(1 1 1) surface at 300 and 400 K demonstrates the build up of CHx species on the surface. The concentration of CHx on the palladium surface is evidently higher at 400 K compared to the CO (285.6 eV) species. The corresponding SFG spectrum obtained at 300 K for both 5 × 10−7 and 0.1 mbar showed weak feature at 1920 cm−1 due to the bridged or
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FIGURE 12.6 (a) Time-resolved O1s XPS spectra obtained during the exposure of CO on an oxygen covered Pt(1 1 0) surface generated at 0.5 mbar O2. (b) O1s spectrum at 36 s and (c) 276 s. (Adapted and reprinted with permission from [35] Copyright © 2011, American Chemical Society.)
hollow bonded CO. The 440 K SFG spectrum showed no resonance for CO species indicating that the surface is saturated with CHx (x = 0–3) species which on exposure to O2 at 450 K cleans the surface. Similar results were also reported by Borasio and co-workers using PM-IRRAS [22]. Ambient pressure XPS was recently used for studying oxidation and reduction of Pd(1 0 0) and Pd nanoparticles supported on silica [46]. The study was motivated by the fact that much less is known about the oxidation of the Pd particles and the role of Pd oxide formed during the catalytic oxidation reactions. The initial stages of the oxidation of the metal or metal nanoparticle can possibly give us a better understanding and solve some of the controversies existing in determining the active phase of the catalyst under the reaction conditions [30,47–50]. The ambient pressure XPS results prove that the oxide phase formation starts at a lower temperature on nanoparticle compared to the single crystal and switches to metallic form in the reduction cycle. The CO was found to adsorb on the PdO surface indicating that under real catalyst the CO oxidation to CO2 proceeds via Mars-van Krevelen mechanism. CO is also found
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400 K
300 K —6
a
0.1 mbar
0.1 mbar
10 mbar
a
a
FIGURE 12.7 C1s spectra collected at 300 and 400 K after exposing the Pd(1 1 1) surface to methanol vapor at 300 and 400 K. The corresponding SFG spectrum is shown at the bottom. (Adapted from Ref. [45].)
to dissociate on the nanoparticle surface forming palladium carbide which could be the source of deactivation of the catalyst. Based on this study the following mechanism for the CO oxidation reaction has been proposed on palladium nanoparticle as shown in Figure 12.8. There are several other reports on the application of high pressure surface science techniques on model catalyst surface where experiments are carried out on nanoparticles supported on well-defined oxide surfaces. A major literature in the last two decades has been focused in addressing the genesis of the catalysis by gold [51–61]. The mode of activation of O2 on supported gold catalyst has been debated in the literature where the activation has been proposed to happen both experimentally and theoretically on the bilayer gold, perimeter interface between metal nanoparticle and oxide support and even on under co-ordinated sites
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FIGURE 12.8 The initial carbide covered Pd nanoparticle subjected to O2 (1 bar at 150 °C) removes carbon and produces surface oxide. Increasing the temperature to 200 °C in O2 created bulk oxide. Switching to CO (0.5 bar) cycle at 120 °C reduces the surface back to metallic Pd by oxide decomposition along with the CO2 production. The surface is converted to carbide phase once the oxide layer is completely removed and the cycle continues. (Adapted and reprinted from Ref. [46] Copyright © 2011, American Physical Society.)
[62–67]. Recent ambient pressure XPS studies on the interaction of 1 torr of O2 for several hours on Au supported on TiO2 showed no evidence for their interaction with Au nanoparticles [13]. However, in the presence of X-rays the intensity of the metallic Au peak (84 eV) reduced in time whereas an additional peak at higher binding energy (85.3 eV) showed the presence of X-ray induced oxidation of the Au clusters. The results demonstrate that in the CO oxidation reaction the activation of O2 is unlikely to happen on the Au clusters and extreme care to be taken while interpreting the data obtained using high intensity X-ray sources like synchrotron radiation. NO and O2 adsorption was also a subject matter for APXPS studies by Salmeron and co-workers on TiO2 and SiO2 [68]. They could identify species like N (399.3 eV), NO3 (405.1 eV) and (NO)2 (401.9 eV) at 240 mtorr of NO on Au/TiO2 surface and not on Au/SiO2 and was revealed by further investigations that NO adsorbs only on TiO2. Bimetallic nanoparticles have received a lot of interest in recent years because of the surprising trends in reactivity compared to their monometallic counterparts [69]. Also tuning the surface structure and atomic arrangement of the catalyst can result in engineering nextgeneration catalyst [70–73]. The surprising catalytic activity of Au–Pd for oxidation of primary alcohols [72], the superior activity of Au–Ni for steam reforming [71], high hydrogen storage capacity of Pd–Cd [73] system all demonstrate that future generation of catalysts requires better understanding of the surface properties. Since XPS is an ideal tool for studying the chemical composition of the surface and the oxidation state, the bimetallic nanostructures are extensively probed using this technique. Combining ambient pressure XPS, using a tunable X-ray source from synchrotron radiation one can very well do a depth profiling of the nanostructures under the reactive gaseous environment [74–76]. Bimetallic nanoparticles with core-shell morphology made of Rh–Pd, Rh–Pt, and Pt–Pd of varying composition when subjected to reductive and oxidative gaseous environment were shown to exhibit interesting surface segregation properties [76]. The ambient pressure XPS studies carried out under
12.4 Surface Science Studies Using High Pressure Techniques
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FIGURE 12.9 Pd (3d) and Rh (3d) spectra collected in the presence of 100 mtorr of CO and NO from a Rh0.5Pd0.5 bimetallic cluster. The cartoon at the bottom shows the dynamic behavior of the core-shell particle during the CO-NO switching cycle. (Adapted and reprinted with permission from [76] Copyright © 2010, American Chemical Society.)
oxidizing and reducing environment showed that RhxPd1−x and RhxPt1−x undergo reversible segregation of the metal under oxidizing and reducing conditions, no such effects were found in the Pt–Pd system (see Figure 12.9). APXPS spectrum collected at 100 mtorr of CO and NO shown in figure demonstrates the segregation of one metal onto the surface evidenced by a substantial increase in the photoelectron intensity from one metal to the other while switching the gases from oxidizing to reducing conditions. Thus, switching from 100 mTorr CO to 100 mTorr NO at 300 °C there is a noticeable decrease in the Pd (3d) intensity with a corresponding increase in the Rh (3d) intensity. Metal-support interaction [77] which plays a very important role in catalysis has been studied on Ni/CeO2 system using ambient pressure XPS [78,79]. The photoemission carried out at 1 torr of hydrogen produced dynamic changes on the Ni/Ceria system which is depicted in Figure 12.10. The Ce 4d spectrum collected before the reduction in H2 at 500 °C shows features characteristic of fully oxidized Ce(IV) phase. But in the presence of 1 torr H2 and at temperature 500 °C the Ce 4d spectrum shows partially reduced CeO2−x phase as evidenced by a reduction in the 110 eV peak, which is similar to the one reported in the literature [80]. This is accompanied by
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FIGURE 12.10 (a) Dynamic changes associated with Ce(4d) spectrum collected at different temperatures in H2 at 1 torr pressure. (b) The cartoon depicts the formation of partially reduced ceria. (Adapted from Ref. [79].)
a reduction in the Ni 2p XPS signal caused due to the migration of the ceria to the nickel nanoparticle surface which is related to the SMSI effect. This process is reversible through simple evacuation. The schematic in Figure 12.10b depicts the dynamic events happening on the surface during the in situ reduction process.
12.5 CONCLUSION AND OUTLOOK The understanding of catalysis phenomena has improved tremendously over the last few years mainly due to the developments in the field of surface science. The new generation of in situ tools wherein the model reactions can be probed under realistic conditions has given invaluable data wherein molecular level insights on how the real-world catalyst responding to reactive conditions can be deduced. As we push further the limits of instrumentation, design and development of smarter and robust functional materials can be engineered from the surface science inputs where catalyst will be able to perform with superior selectivity and activity.
Acknowledgments The author thanks Dr. Sourav Pal, Director CSIR-NCL, and Dr. C.S. Gopinath for providing support and encouragement. The help of A.B. Vysakh is acknowledged for recreating some of the images.
References [1] A.L. Robinson, Science 185 (1974) 772. [2] G. Ertl, Angew. Chem. Int. Ed. 47 (2008) 3524. [3] C.O. Arean, B.M. Weckhuysen, A. Zecchina, Phys. Chem. Chem. Phys. 14 (2012) 2125.
REFERENCES
303
[4] P.B. Rasmussen, B.L.M. Hendriksen, H. Zeijlemaker, H.G. Ficke, J.W.M. Frenken, Rev. Sci. Instrum. 69 (1998) 3879. [5] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Complexes, Part B, third ed., Wiley, New York, 1980. [6] A. Knop-Gericke, E. Kleimenov, M. Havecker, R. Blume, D. Teschner, S. Zafeiratos, R. Schlogl, V.I. Bukhtiyarov, V.V. Kaichev, I.P. Prosvirin, A.I. Nizovskii, H. Bluhm, A. Barinov, P. Dudin, M. Kiskinova, Advances in Catalysis, vol. 52, Elsevier Academic Press Inc, San Diego, 2009 p. 213. [7] R.W. Joyner, M.W. Roberts, K. Yates, Surf. Sci. 87 (1979) 501. [8] D.M. Littrell, B.J. Tatarchuk, J. Vac. Sci. Technol. A Vac. Surf. Films 4 (1986) 1608. [9] S. Dushman, J.M. Lafferty, John Wiley & Sons, New York, 1962 p. 806. [10] F. Tao, Chem. Commun. 48 (2012) 3812. [11] A. Jürgensen, N. Esser, R. Hergenröder, Surf. Interface Anal. 144 (2012) 1100. [12] K. Roy, C.P. Vinod, C.S. Gopinath, J. Phys. Chem. C 117 (2013) 4717. [13] P. Jiang, S. Porsgaard, F. Borondics, M. Kober, A. Caballero, H. Bluhm, F. Besenbacher, M. Salmeron, J. Am. Chem. Soc. 132 (2010) 2858. [14] R.G. Greenler, J. Chem. Phys. 44 (1966) 310. [15] J. Ryczkowski, Catal. Today 68 (2001) 263. [16] C. Lamberti, A. Zecchina, E. Groppo, S. Bordiga, Chem. Soc. Rev. 39 (2010) 4951. [17] K.W. Hipps, G.A. Crosby, J. Phys Chem. 83 (1979) 555. [18] B.J. Barner, M.J. Green, E.I. Saez, R.M. Corn, Anal. Chem. 63 (1991) 55. [19] F.M. Hoffmann, Surf. Sci. Rep. 3 (1983) 107. [20] G. Rupprechter, C. Weilach, J. Phys. Condens. Matter 20 (2008). [21] A. Urakawa, T. Burgi, H.P. Schlapfer, A. Baiker, J. Chem. Phys. 124 (2006). [22] M. Borasio, O.R. de la Fuente, G. Rupprechter, H.-J. Freund, J. Phys. Chem. B 109 (2005) 17791. [23] E. Roedel, A. Urakawa, A. Baiker, Catal. Today 155 (2010) 172. [24] G. Rupprechter, MRS Bull. 32 (2007) 1031. [25] V.V. Kaichev, I.P. Prosvirin, V.I. Bukhtiyarov, H. Unterhalt, G. Rupprechter, H.-J. Freund, J. Phys. Chem. B 107 (2003) 3522. [26] S. Surnev, M. Sock, M.G. Ramsey, F.P. Netzer, M. Wiklund, M. Borg, J.N. Andersen, Surf. Sci. 470 (2000) 171. [27] G. Ketteler, D.F. Ogletree, H. Bluhm, H. Liu, E.L.D. Hebenstreit, M. Salmeron, J. Am. Chem. Soc. 127 (2005) 18269. [28] W.X. Li, L. Osterlund, E.K. Vestergaard, R.T. Vang, J. Matthiesen, T.M. Pedersen, E. Lægsgaard, B. Hammer, F. Besenbacher, Phys. Rev. Lett. 93 (2004) 146104. [29] B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett. 89 (2002) 046101. [30] F. Gao, Y. Wang, Y. Cai, D.W. Goodman, J. Phys. Chem. C 113 (2008) 174. [31] R. Imbihl, G. Ertl, Chem. Rev. 95 (1995) 697. [32] A.v. Oertzen, A.S. Mikhailov, H.H. Rotermund, G. Ertl, J. Phys. Chem. B 102 (1998) 4966. [33] A.v. Oertzen, H.H. Rotermund, A.S. Mikhailov, G. Ertl, J. Phys. Chem. B 104 (2000) 3155. [34] A. von Oertzen, A. Mikhailov, H.H. Rotermund, G. Ertl, Surf. Sci. 350 (1996) 259. [35] D.R. Butcher, M.E. Grass, Z. Zeng, F. Aksoy, H. Bluhm, W.-X. Li, B.S. Mun, G.A. Somorjai, Z. Liu, J. Am. Chem. Soc. 133 (2011) 20319. [36] W.-H. Cheng, Acc. Chem. Res. 32 (1999) 685. [37] R. Shiozaki, T. Hayakawa, Y.-y. Liu, T. Ishii, M. Kumagai, S. Hamakawa, K. Suzuki, T. Itoh, T. Shishido, K. Takehira, Catal. Lett. 58 (1999) 131. [38] S.M. Francis, J. Corneille, D.W. Goodman, M. Bowker, Surf. Sci. 364 (1996) 30. [39] A.K. Bhattacharya, M.A. Chesters, M.E. Pemble, N. Sheppard, Surf. Sci. 206 (1988) L845. [40] M. Mavrikakis, M.A. Barteau, J. Mol. Catal. A: Chem. 131 (1998) 135. [41] K. Christmann, J.E. Demuth, J. Chem. Phys. 76 (1982) 6308. [42] J.L. Davis, M.A. Barteau, Surf. Sci. 187 (1987) 387. [43] J.L. Davis, M.A. Barteau, Surf. Sci. 235 (1990) 235. [44] J.A. Gates, L.L. Kesmodel, J. Catal. 83 (1983) 437. [45] M. Morkel, V.V. Kaichev, G. Rupprechter, H.J. Freund, I.P. Prosvirin, V.I. Bukhtiyarov, J. Phys. Chem. B 108 (2004) 12955. [46] R. Westerstrom, M.E. Messing, S. Blomberg, A. Hellman, H. Gronbeck, J. Gustafson, N.M. Martin, O. Balmes, R. van Rijn, J.N. Andersen, K. Deppert, H. Bluhm, Z. Liu, M.E. Grass, M. Havecker, E. Lundgren, Phys. Rev. B 83 (2011) 115440.
304
12. Insights into Heterogeneous Catalysis through Surface Science Techniques
[47] F. Gao, S.M. McClure, Y. Cai, K.K. Gath, Y. Wang, M.S. Chen, Q.L. Guo, D.W. Goodman, Surf. Sci. 603 (2009) 65. [48] B.L.M. Hendriksen, S.C. Bobaru, J.W.M. Frenken, Surf. Sci. 552 (2004) 229. [49] R. van Rijn, O. Balmes, R. Felici, J. Gustafson, D. Wermeille, R. Westerström, E. Lundgren, J.W.M. Frenken, J. Phys. Chem. C 114 (2010) 6875. [50] F. Gao, Y. Wang, D.W. Goodman, J. Phys. Chem. C 114 (2010) 6874. [51] A.S.K. Hashmi, G.J. Hutchings, Angew. Chem. Int. Ed. 45 (2006) 7896. [52] J.K. Norskov, T. Bligaard, B. Hvolbaek, F. Abild-Pedersen, I. Chorkendorff, C.H. Christensen, Chem. Soc. Rev. 37 (2008) 2163. [53] M. Haruta, Cattech 6 (2002) 102. [54] G.J. Hutchings, M. Brust, H. Schmidbaur, Chem. Soc. Rev. 37 (2008) 1759. [55] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [56] T. Kiyonaga, Q. Jin, H. Kobayashi, H. Tada, Chemphyschem 10 (2009) 2935. [57] J.F. Jia, K. Haraki, J.N. Kondo, K. Domen, K. Tamaru, J. Phys. Chem. B 104 (2000) 11153. [58] C. Bianchi, F. Porta, L. Prati, M. Rossi, Top. Catal. 13 (2000) 231. [59] T. Janssens, B. Clausen, B. Hvolbæk, H. Falsig, C. Christensen, T. Bligaard, J. Nørskov, Top. Catal. 44 (2007) 15. [60] N. Lopez, T.V.W. Janssens, B.S. Clausen, Y. Xu, M. Mavrikakis, T. Bligaard, J.K. Norskov, J. Catal. 223 (2004) 232. [61] R. Meyer, C. Lemire, S.K. Shaikhutdinov, H. Freund, Gold Bull. 37 (2004) 72. [62] M.S. Chen, D.W. Goodman, Science 306 (2004) 252. [63] N. Lopez, J.K. Nørskov, J. Am. Chem. Soc. 124 (2002) 11262. [64] Z.-P. Liu, X.-Q. Gong, J. Kohanoff, C.n. Sanchez, P. Hu, Phys. Rev. Lett. 91 (2003) 266102. [65] J.A. van Bokhoven, C. Louis, J.T. Miller, M. Tromp, O.V. Safonova, P. Glatzel, Angew. Chem. Int. Ed. 45 (2006) 4651. [66] N. Weiher, A.M. Beesley, N. Tsapatsaris, L. Delannoy, C. Louis, J.A. van Bokhoven, S.L.M. Schroeder, J. Am. Chem. Soc. 129 (2007) 2240. [67] M. Kotobuki, R. Leppelt, D.A. Hansgen, D. Widmann, R.J. Behm, J. Catal. 264 (2009) 67. [68] T. Herranz, X.Y. Deng, A. Cabot, P. Alivisatos, Z. Liu, G. Soler-Illia, M. Salmeron, Catal. Today 143 (2009) 158. [69] D. Wang, Y. Li, Adv. Mater. 23 (2011) 1044. [70] J.K. Norskov, T. Bligaard, J. Rossmeisl, C.H. Christensen, Nat. Chem. 1 (2009) 37. [71] F. Besenbacher, I. Chorkendorff, B.S. Clausen, B. Hammer, A.M. Molenbroek, J.K. Norskov, I. Stensgaard, Science 279 (1998) 1913. [72] D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362. [73] B.D. Adams, G. Wu, S. Nigro, A. Chen, J. Am. Chem. Soc. 131 (2009) 6930. [74] S. Alayoglu, F. Tao, V. Altoe, C. Specht, Z.W. Zhu, F. Aksoy, D.R. Butcher, R.J. Renzas, Z. Liu, G.A. Somorjai, Catal. Lett. 141 (2011) 633. [75] F. Tao, M.E. Grass, Y.W. Zhang, D.R. Butcher, J.R. Renzas, Z. Liu, J.Y. Chung, B.S. Mun, M. Salmeron, G.A. Somorjai, Science 322 (2008) 932. [76] F. Tao, M.E. Grass, Y.W. Zhang, D.R. Butcher, F. Aksoy, S. Aloni, V. Altoe, S. Alayoglu, J.R. Renzas, C.K. Tsung, Z.W. Zhu, Z. Liu, M. Salmeron, G.A. Somorjai, J. Am. Chem. Soc. 132 (2010) 8697. [77] R.W. Joyner, J.B. Pendry, D.K. Saldin, S.R. Tennison, Surf. Sci. 138 (1984) 84. [78] S.J. Tauster, Acc. Chem. Res. 20 (1987) 389. [79] A. Caballero, J.P. Holgado, V.M. Gonzalez-delaCruz, S.E. Habas, T. Herranz, M. Salmeron, Chem. Commun. 46 (2010) 1097. [80] L. Kundakovic, D.R. Mullins, S.H. Overbury, Surf. Sci. 457 (2000) 51.