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Surface Studies By IR Spectroscopy
2320 SURFACE STUDIES BY IR SPECTROSCOPY
Surface Studies By IR Spectroscopy Norman Sheppard, University of East Anglia, Norwich, UK Copyright © 1999 Academic Press
The investigation of surfaces, and of molecular layers adsorbed on surfaces, by electromagnetic radiation has been carried out principally by infrared spectroscopy. This is because of the high sensitivity of present-day Fourier transform infrared (FT-IR) spectrometers; the capability for IR spectroscopy to obtain data from mixed-phase samples with gas/solid, liquid/solid, or gas/liquid interfaces; and because of the availability of very large databases relating the positions and relative strengths of infrared absorptions to structural features of organic and inorganic molecules. As described below, the sampling techniques used differ substantially whether the systems under investigation involve finely divided samples (powders or porous solids) or whether the surface involved is flat. In the early history of the subject, spectral features relating to adsorbed layers and other surface phenomena could only be detected if very high area finely divided samples were used so that the radiation beam could pass through many interfaces. However, since the advent of FT-IR spectrometers, infrared sensitivity has so much improved that nowadays a measurable spectrum can be produced from even a single monolayer on a flat surface. After reviewing the experimental techniques involved, we survey the principal applications of the infrared method under the headings surface characterization, physical adsorption, and chemisorption and catalysis.
Experimental techniques High-area, finely divided, surfaces
Surfaces, because of their unsaturated surface fields, normally require to be cleaned from contamination derived from the atmospheric environment before systematic research can be carried out on them. For finely divided samples of high area it is adequate to mount them in a high-vacuum enclosure (~106 mbar) provided with infrared-transparent windows and the means for treating the sample in oxygen or hydrogen at elevated temperatures. The samples themselves are most often studied in transmission, usually in the form of pressed discs derived from powders. These are prepared using a hydraulic press
VIBRATIONAL, ROTATIONAL & RAMAN SPECTROSCOPIES Applications in a manner similar to that used for the standard potassium bromide pressed-disc sampling procedure for IR spectroscopy. The pressure required for coherent disc formation is greater for the commonly studied oxide layers than for the softer potassium bromide, but the discs so prepared remain porous for adsorption studies. Alternatively, a powdered sample can often be made to cohere on an infraredtransparent disc, or on a fine metal grid, through sublimation or by deposition from a solvent. Finely divided metal samples require that the opaque particles are separated from each other for transmission purposes. Usually this is done by distributing (supporting) them on the surface of an oxide which is transparent over relevant regions of the spectrum. Such samples are prepared by depositing metal salts from solution on the oxide particles followed by evaporation of the solvent; a disc is then pressed from the mixed powder and inserted in the IR vacuum cell; finally, the salt is reduced in hydrogen at appropriate temperatures so as to form metal particles distributed over the surface of the oxide support. Very high area powders of silica and alumina, of areas between 200 and 300 m2 g1, are commercially available and are frequently used as metal supports. They have the advantages that they are largely infrared-transparent down to ~1300 or ~1100 cm1 respectively, and hence permit the study of many group-characteristic absorptions from organic adsorbates. Silica is a more catalytically inert support than alumina. Samples prepared as described above can be good models for working catalysts of either the oxide or metal types and many infrared studies of surface phenomena are undertaken in conjunction with catalytic investigations. Loose powders can alternatively be studied by diffuse reflection, with the advantage for kinetic studies that surface reactions, rather than diffusion processes, are more likely to be rate determining than is the case with the fine-pored pressed discs. Low-area flat surfaces
Adsorbents in the form of flat surfaces are of very low area and normally ultra-high vacuum (UHV) conditions (~1010 mbar) are required in order to
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preserve them from contamination. Where the substrate is transparent, infrared spectra of the surface layers can be obtained either by transmission or by reflection; when the substrate is non-transmitting, as in the case of metals, then reflection is normally used. Experiments involving flat surfaces allow the application of polarized radiation for the purpose of obtaining information about the orientation of the adsorbed molecules with respect to the surface. For transparent substrates the used of radiation polarized in or perpendicular to the plane of incidence, in combination with the measured angle of incidence, can determine the direction of the dipole change with respect to the surface associated with each group-characteristic vibration. The orientations of even flexible molecules with respect to the surface can be deduced from such measurements. In the case of metals the effect of the free response of the conduction-electrons to a charge above the surface can be modelled in terms of an image of opposite sign at the same distance below the surface as is shown in Figure 1. In the infrared context it is the dipole moment change associated with the vibration that interacts with the radiation. Figure 1 shows that a component of such a dipole change that is parallel to the surface is cancelled out by its image, whereas a component perpendicular to the surface is doubled in magnitude. Hence only modes with perpendicular dipole components are IR allowed; in general these are those modes of vibration that are symmetrical with respect to all the symmetry elements associated with the surface complex. For example, a CO molecule adsorbed perpendicular with respect to the surface will give absorption bands from the QCO or QCM (M = metal) bond-stretching modes but not from the OCM bending modes. Such considerations constitute the metal surface selection rule (MSSR), which is widely used for the determination of molecular orientation or, if this is known, as an aid in the assignment of vibrational modes. For work with metals, reflectionabsorption infrared spectroscopy(RAIRS) uses near-grazing incidence in order to maximize the strength of the electric vector of the incident infrared radiation that is perpendicular to the surface. UHV is normally required when studying low-area flat surfaces (exceptionally this would not be a requirement if the adsorbate, such as a surfactant, is capable of displacing surface impurities) and this requires sophisticated equipment. Also, the high sensitivity needed for the measurement of spectra from single monolayers requires the use of FT-IR spectrometers with selective photoconductive infrared detectors; the mercury/cadmium telluride detector which covers the major range of the spectrum down
Figure 1 Charges and their images near metal surfaces; the origin of the metal surface selection rule (MSSR).
to ~700 cm1 is widely used. Figure 2 illustrates a typical experimental arrangement for RAIRS on a metal surface under UHV conditions. Spectroscopic work carried out on single crystals with known types of adsorption sites, such as are readily available for metals, are of great use in interpreting the more complex spectroscopic phenomena obtained from finely divided samples. Individual particles of the latter can exhibit facets with a variety of atomic arrangements and adsorption sites which can be studied one-at-atime on single crystals. Figure 3 shows the different atomic arrangements, and hence adsorption sites, of the (111), (100) and (110) faces of a face-centred cubic metallic lattice. UHV facilities also permit complementary spectroscopic methods involving particles such as electrons (as in high-resolution electron energy loss spectroscopy) or diffraction methods (as in low-energy electron diffraction) to be employed in order to characterize the same system further. Adsorption on metal electrodes, which can be cleaned in solution by electrode reactions, is also studied by RAIRS. There is added interest in the effects of the variable electrode potential on the spectra and structures of the adsorbed species. The surfaces of infrared-transparent materials that are available in the form of shaped and polished crystals, such as silicon or germanium, can be studied with good sensitivity by using attenuated total internal reflection (ATR) in conjunction with multiple reflection procedures. Sum frequency generation (SFG) is a recent spectroscopic development in which two laser beams, one in the visible region and the other of variable frequency in the infrared region, generate infraredmodulated signals in the visible region at the sum of the two frequencies. As the signals come only from the interface and not from the bulk, this technique is being exploited in high-pressure catalyst work and for surfactant research.
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Figure 2 The optical arrangement of an FT-IR spectrometer for reflection–absorption (RAIRS) work in ultrahigh vacuum (UHV). A, detector; B, KBr lens: C, KBr window; D, UHV chamber; E, sample; F, Michelson interferometer; G, Globar source; P, grid polarizer. Reprinted from Chesters MA (1986) Journal of Electron Spectroscopy and Related Phenomena 38:123 Copyright (1986), with permission from Elsevier Science.
Infrared contributions to our knowledge of surfaces are mostly short-range in type and involve the identification of different types of site through the adsorption of probe molecules chosen for this purpose. CO is a well known probe of surface sites on metal surfaces. As discussed in the Chemisorption section below, its QCO bond-stretching vibration has distinct wavenumber ranges for adsorption on linear (on-top), twofold and threefold bridging sites. Although linear and twofold sites can occur on each of the surfaces shown in Figure 3, the threefold one is specifically characteristic of (111) surfaces and can be used to identify such facets on metal particles. Distinctions can sometimes be made between twofold CO sites on different facets. The wavenumbers of CO absorptions can also be used to characterize surface cation sites of different charge (different formal oxidation states) on transition metal oxides as shown in Figure 4 for a partially reduced Ni oxide sample. For hydrocarbon adsorption the characteristic spectrum of ethylidyne (CH3C) also plays a useful role in identifying (111) facets on finely divided metals. One of the earliest discoveries of surface infrared spectroscopy was that oxide surfaces, such as those of SiO2 or Al2O3, retain chemisorbed OH groups after the removal of water molecules adsorbed from the atmosphere. These can only be removed by hightemperature treatment and are presumably generated by the reaction of ambient water molecules with otherwise free valencies on the surface of the oxide lattice, according to the reaction O2 + H2O → 2OH or its covalent equivalent. In the case of alumina, for example, individual absorptions amongst a multiplicity of OH bond-stretching absorptions can be identified with linear, two- and threefold adsorption sites, for each of two types of surface aluminium atoms which in the bulk lattice have four- or six-fold coordinations, i.e. are in formal IV or VI oxidation states. Silica has only four silicon coordination and correspondingly simpler QOH spectrum consisting
Surface characterization The long-range patterns of surface atomic arrangements are principally monitored by low-energy electron diffraction (LEED). Whereas in principle the top layers of a lattice have different frequencies and hence wavenumbers from those of the bulk lattice, the associated absorptions often fall within a spectral region dominated by the latter and are hence difficult to identify. Transition metal oxides are exceptional in that the variable valency associated with the metallic element can lead to the generation of surface M = O groups (M = metal) that give absorptions of notably higher wavenumber than those of the lattice modes.
Figure 3 The arrangements of atoms, and the resulting adsorption sites, on the (100), (111) and (110) surfaces of a facecentred-cubic metal.
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num, metal surfaces adsorb oxygen from the atmosphere leading, in the cases of aluminium and iron, to the production of multilayer passivating oxide films or (with the participation of absorbed water) thick films of rust, respectively. Infrared spectroscopy can monitor these surface corrosion processes.
Physical adsorption
Figure 4 The IR spectrum of CO adsorbed on partially reduced NiY zeolite showing the oxidation-state dependence of QCO. Reproduced with permission of John Wiley & Sons Ltd. from Davydov AA (1984) Infrared Spectroscopy of Adsorbed Species on the Surfaces of Transition Metal Oxides, Copyright John Wiley & Sons Ltd.
mainly of absorptions of pairs of free and hydrogenbonded OH groups on adjacent sites. An acidic oxide such as alumina exhibits relatively inert OH groups, strongly acidic OH groups that are capable of proton donation (Brønsted acidity), plus aluminium ions that act as electron-deficient sites (Lewis acidity). The relative proportions of these on such oxide surfaces are analysed using the infrared spectrum of adsorbed pyridine. The spectra are measured in the 17001500 cm1 region. Pyridine hydrogen-bonded to the weaker surface OHs gives a weakly perturbed spectrum; that interacting with the strongly acidic OHs forms the pyridinium ion by proton transfer which has a characteristic additional absorption at 1540 cm1; that interacting with metalcoordination sites shows a change in wavenumber of a skeletal absorption near 1600 cm1. Basic oxides frequently adsorb carbon dioxide from the atmosphere to give surface carbonates according to the reaction O2 + CO2 → CO32, a process readily monitored by infrared spectroscopy. With the well known exceptions of gold and plati-
In this section we consider the spectroscopic study of the association of molecules with surfaces by intermolecular forces ranging from van der Waals to strong hydrogen bonding. Figure 5 shows the absorption bands in the QCH bond-stretching region from methane adsorbed on porous silica glass. In the gas phase the triply degenerate QCH mode at 3019 cm 1 is infrared active and is to be identified with the strong band from the adsorbed species at ~3006 cm 1. On the surface an additional feature has appeared in the spectrum at 2899 cm 1 which is readily identified as the gas-phase forbidden QCH breathing mode, known from gasphase Raman spectroscopy to occur at 2917 cm 1. The one-sided surface forces have distorted the original tetrahedral shape of the methane molecule so as to cause this mode to become active. The considerable breath of the ~3006 cm 1 absorption of the surface species, notably less than that of the gas-phase vibrationrotation band, was interpreted in terms of quasi-free rotation of the molecule about a single axis perpendicular to the silica surface. The spectrum of Figure 6 shows the interaction of the very acidic surface OH group on zeolite HY with adsorbed ethene. The low wavenumber, broad profile, and intensification of the shifted QOH absorption upon ethene adsorption indicate a hydrogen bond of considerable strength, comparable to that between water molecules, even although the bonding is only to the S-electrons of the adsorbed ethene. This complex
Figure 5 The IR spectrum of CH4 adsorbed on high-area porous silica glass in the QCH bond-stretching region showing the presence of a gas-phase forbidden absorption. Reproduced with permission of the Royal Society from Sheppard N and Yates DJC (1956) Proceedings of the Royal Society of London, Series A 238: 69.
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Figure 7 The IR spectrum of cyclohexane adsorbed on a Pt(111) surface. The broad absorption near 2600 cm–1 is from a form of hydrogen bonding between axial CH bonds and surface Pt atoms. Reprinted from Chesters MA and Gardener P (1990) Spectrochimica Acta, Part A 46: 1011, Copyright (1990), with permission from Elsevier Science.
Figure 6 The IR spectrum of ethene adsorbed on the acid OH groups of HY zeolite. Solid line, ethene adsorbed; dashed line, background. Reproduced with permission of the Royal Society of Chemistry from Liengme BV and Hall WK (1966) Transactions of the Faraday Society 62: 3229.
is clearly an intermediate in the higher temperature formation of the carbenium ion C2H . Hydrogen bonds are normally considered to form between acidic XH groups and electron-rich bases. However, surface infrared spectroscopy, in conjunction with HREELS, has shown that such bonds can also occur between electron-rich CH bonds of paraffins and electron-deficient sites on metal surfaces. Figure 7 shows the spectrum of cyclohexane adsorbed on the (111) surface of platinum. The very broad band centred at ~2620 cm 1 is from a proportion of CH bonds of the adsorbed cyclohexane in a hydrogen-bonded type of environment. As the separation between the three parallel axial CH bonds on one side of the cyclohexane molecule is almost exactly the separation between adjacent Pt atoms on a threefold site of the (111) surface, it is clear that the hydrogen bond is of the type CHPt. Figure 8 is a spectrum taken at 33 K in the QCH region of CD3H adsorbed on the (100) face of the face-centred-cubic lattice of NaCl. It is seen that there are two well resolved absorption bands, one
Figure 8 The IR spectrum from CHD3 adsorbed at 33 K on NaCl(100). Ep and Es refer to radiation polarized in and perpendicular to the plane of incidence, respectively. Reprinted with permission from Davis KA and Ewing GE (1997) Journal of Chemical Physics 107: 8073. Copyright 1997, American Institute of Physics.
sharp and the other broad, resulting from CH bonds that are oriented differently with respect to the surface. One of these occurs with the incident light polarized in the plane of incidence but is eliminated when the light is polarized perpendicular to this; the other is present in both spectra. The former band hence has its QCH vibrational dipole change perpendicular to the surface, whereas the direction of the
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latter has both parallel and perpendicular components. Considerations of relative intensities, taking into account the angle of incidence, show that the broader low-wavenumber band is from CH bonds that are at ~70° with respect to the surface. It is hence concluded that the parent CH4 molecules are adsorbed with three of their four CH bonds on the surface. Even in the absence of the special effects of hydrogen bonding, bandwidths of 10 cm1 or more are common from adsorbed species on polycrystalline substrates owing to interactions with sites that differ in their detailed environments. Absorptions obtained from adsorbed species on single-crystal planes with uniform and well defined sites can, in contrast, be very sharp with bandwidths of less than 1 cm1. In the case of methane itself, and of a number of other molecules such as CO and CO2, the resolution of the spectra on alkali metal halide single-crystal surfaces are such that even the fine-structure splitting caused by the vibrational couplings of more than one molecule in the surface unit cell can readily be resolved.
Chemisorption and catalysis The quantitative and energetic aspects of the chemisorption of molecules on surfaces have long been investigated but, until in the 1950s it became possible to obtain infrared spectra, the actual structures of the surface species could only be a matter for speculation. The spectra show that in fact finely divided adsorbents give absorptions from several different surface species, and that the nature of the latter can vary as a function of coverage. Simpler spectra are obtained on single-crystal surfaces of known atomic arrangements. However, even so, the deductions of the structures of the chemisorbed species can be difficult because of uncertainties related to the effects of surface bonding on the spectra of the attached groups. The usual group-characteristic wavenumber ranges can no longer be assumed to be reliable because of the electron-donating or -withdrawing properties of the surface atoms and also, when there is multiple bonding to the surface, because of strains associated with cyclic bonding features. The procedure adopted is to use the spectra to suggest possible alternative structures for the adsorption complexes, and then to look for molecular analogues of known structures whose spectra can be obtained for comparative pattern-recognition purposes. This approach is well exemplified by the results obtained for chemisorption on metal surfaces, an area much studied because of the ready availability of single crystals of metals which can be cut so as to display particular surface planes.
Figure 9 The IR spectra of CO adsorbed on the silica-supported metals Cu, Pt, Ni and Pd. Absorptions above 2000 cm–1 are from linear (on top) CO bonded to one metal atom; those below this value are from CO bridge-bonded to two or three metal atoms. Reprinted with permission from Eischens RP, Pliskin WA and Francis S A (1954) Journal of Chemical Physics, 22: 1786. Copyright 1954 American Institute of Physics.
Figure 9 shows high-coverage spectra obtained from CO chemisorbed on the silica-supported metals Cu, Pt, Ni and Pd. The several metals show different proportions of absorption bands above and below 2000 cm 1 which are characteristic of adsorption on linear (on-top) and bridged sites, respectively. These structural assignments were deduced by comparison with the spectra of metal carbonyls. The spectral ranges attributable to such surface species are as follows: linear, 21202000 cm 1; twofold bridge, 2000 ~1870 cm 1; and threefold bridge ~1900 1800 cm1. These ranges apply whichever crystal face is involved. Within each range the characteristic absorptions increase in wavenumbers with increasing coverage. This is caused by strong vibrational coupling within the array of parallel molecules on the surface, mostly of a dipolar nature related to the exceptional strength of the QCO absorptions. The mixture of linear and bridged CO species found from the spectra from the finelydivided samples is caused by adsorption on different sites, usually different facets, on the metal particles. Figure 10 shows QCO spectra at full coverage from chemisorption on an Rh(111) single-crystal electrode. These are plotted as a function of the
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Figure 10 The IR spectra of CO adsorbed at full coverage on an Rh(111) electrode in 0.1M NaClO4 at various electrode potentials. Reprinted from Chang SC and Weaver MJ (1990) Surface Science 238: 142, Copyright (1990), with permission from Elsevier Science.
electrode potential (with respect to a standard Ag/ AgCl electrode) in 0.1M NaClO4 and show the interest of this additional variable in electrode work. It is seen that at the lowest electrode potential the spectrum is dominated by the absorption at 1886 cm1 from a bridged species but at higher potentials, before desorption sets in, a linear species becomes dominant, absorbing at 2029 cm1. More generally, work on electrodes shows that, at a given coverage, negative potentials favour bridged species over linear species and that the wavenumbers of QCO absorptions from linear species increase in value with increasingly positive electrode potentials a milder version of the dependence of QCO on metal oxidation state reported above. Hydrocarbons on metal surfaces provide greater challenges in spectral interpretation and we choose the example of ethene chemisorbed on different metal surfaces. Here the relevant model compounds are inorganic binuclear or trinuclear metal clusters with the hydrocarbon ligand of interest and additional
CO ligands occupying the positions of the metal atoms of the surface complex. One of the unexpected aspects of the adsorption of ethene is that (111) faces of many metals are covered by the dissociative ethylidyne species CH3CM3 (M = metal) near room temperature. Its spectrum was attributed to this structure by comparison with the spectrum of the model compound (CH3C)Co3(CO)9, considered as a possibility because electron diffraction had shown that the CC bond of the adsorbed species is perpendicular to the surface. This example shows the importance of the metalsurface selection rule (MSSR). For this species, as a ligand or as a surface complex, the modes of vibration are fully separable into those with dipole changes either perpendicular or parallel to the surface (parallel or perpendicular to the CC bond, respectively). Only the former modes are active under the MSSR but both sets are active in the infrared spectrum of the model compound. Figure 11 compares the infrared spectrum of the ethylidyne species on the Pt(111) surface with that of the model compound. The bands marked with asterisks in the spectra of the model compound (in order of decreasing wavenumber, QCH3 symmetrical stretch, G&+3 symmetrical bend and QCC stretch) are those which give dipole changes perpendicular to the surface; the other doubly degenerate modes give dipole changes parallel to the surface (QCH3 asymmetric stretch, GCH3 asymmetric bend and CH3 rock). The positions of the missing modes of the surface species, indicated by arrows, have been identified in the HREEL spectrum of the same system where the selection rules are more relaxed. It has been shown by spectroscopy that at low temperatures ethene adsorbs on Pt(111) as the
Figure 11 A comparison of the IR spectrum from ethene adsorbed on Pt(111) at room temperature with that of the model compound (CH3C)Co3(CO)9. Asterisks indicate absorptions of the model compound allowed in the spectrum of the adsorbed species by the metal-surface selection rule; arrows indicate other bands observed by HREELS. Courtesy Chesters MA.
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Figure 12 The structures of the principal adsorbed species from the adsorption of ethene on metal surfaces; [1] S-complex; [2] the di-V species; [3] ethylidyne (CH3C).
MCH2CH2M (di-σ adsorbed) species with a cyclic C2M2 skeleton and that this transforms into ethylidyne on warming to near room temperature. The (111) faces of other metals, notably Pd and Cu, show low-temperature spectra from another less strongly perturbed H2CCH2 adsorbed species in which there is bonding from a single metal atom to the S-electron distribution of the C=C double bond. Its spectrum is closer to that of ethene itself but with those modes which involve CC stretching occurring at lower wavenumbers. The structures of these three species are shown in Figure 12. Figure 13 shows two spectra from ethene adsorbed on a silica-supported Pt sample, of the type used in catalysis, at 180K and at room temperature. On these are indicated absorptions from the above three species with the MSSRallowed modes still dominant. It is seen that the spectra from the catalyst sample are comprehensively accounted for in terms of the species that had been identified one-at-time on single-crystal surfaces.
For the purpose of catalysis, the structure of the surface-adsorbed reactant should be sufficiently perturbed in order to promote reactivity, but not so strongly adsorbed that it cannot be removed by reaction. Below the temperature for the onset of catalysis, controlled by the energy of activation, the reactive species will be one or more of the chemisorbed species. The spectra of such species will weaken or disappear when catalysis commences while less reactive species are retained. In the case of ethene hydrogenation over metal catalysts, the order of reactivity in the presence of hydrogen is [1] > [2] > [3], with [3], the ethylidyne species, being very slow to be removed. By room temperature over Pt/ SiO2, when the di-V species has all been converted to ethylidyne, it is clear that the S-species, [1], is the catalytically active one. On Pt single crystals this mainly occurs on non-close-packed planes, and it may be inferred that catalytic reduction occurs on rougher, non-(111), surfaces of the metal particles. In a similar manner, it has been shown by single-crystal spectroscopy that the reactive species in the reduction of nitrogen to ammonia over the Fe catalyst (the Haber process) is a di-σ species involving the NN molecule chemisorbed to two Fe atoms which dissociates to adsorbed N atoms during catalysis. The transition metal oxides form the other principal class of catalysts. These differ from the metals in that they have both acid and base sites in the same surface (the metal and oxygen atoms/ions, respectively) and react differently according to which of these properties is dominant. Figure 14 shows the infrared spectrum from the heterolytic dissociation of hydrogen on polycrystalline ZnO to given surface
Figure 13 The IR spectra of ethene adsorbed on silica-supported Pt (A) at 180 K and (B) at room temperature, labelled according to the structural assignments of the absorption bands. Reprinted with permission from Mohsin SB, Trenary M and Robota H Journal of Physical Chemistry, (1988) 92: 5229 and (1991) 95: 6657. Copyright 1988,1991, American Chemical Society.
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High-resolution electron energy loss spectroscopy (HREELS), with its higher sensitivity but lower resolution, has played a strongly complementary role to IR in the study of molecules adsorbed on singlecrystal metal surfaces. Inelastic neutron scattering (INS) and inelastic electron tunnelling spectroscopy (IETS) have found more limited applications to the study of the adsorption of molecules on high-area surfaces.
Figure 14 The IR spectra of hydrogen adsorbed on ZnO. Reproduced with permission of the Royal Society of Chemistry from Hussain G and Sheppard N (1990) Journal of the Chemical Society, Faraday Transactions 86: 1615.
HZn+ and OH groups. Oxides have mostly been studied in high-area form to date, including the zeolites whose acidic activity occurs on well defined sites within the pores of the crystalline material. Flat single crystals of oxides are difficult to clean in UHV because of their insulating properties. Single-crystal spectroscopic work on oxides is increasing being carried out on thin films grown epitaxially on metal surfaces.
Other vibrational spectroscopic techniques for surfaces Raman spectroscopy provides valuable complementary vibrational information to IR spectroscopy but its applications to adsorbed molecules has been principally limited to the study of finely divided samples for reasons of reduced sensitivity. Exceptionally, flat monolayers of long-chain surfactant have given Raman spectra using multireflection techniques. Surface-enhanced Raman spectroscopy (SERS) gives greatly enhanced sensitivity but only for molecules adsorbed on the roughened surfaces of the coinage metals, particularly silver.
See also: ATR and Reflectance IR Spectroscopy, Applications; High Resolution Electron Energy Loss Spectroscopy, Applications; Inelastic Neutron Scattering, Applications; Inelastic Neutron Scattering, Instrumentation; IR Spectroscopy, Theory; Raman and IR Microspectroscopy; Surface-Enhanced Raman Scattering (SERS), Applications.
Further reading Bell AT and Hair ML (1980) Vibrational Spectroscopies for Adsorbed Species, ACS Symposium Series 137. Washington, DC: American Chemical Society. Clark RJH and Hester RE (eds) (1988) Spectroscopy of Surfaces, Advances in Spectroscopy, Vol 16. New York: Wiley. Davydov AA (1984) Infrared Spectroscopy of Adsorbed Species on the Surfaces of Transition Metal Oxides. New York: Wiley. Sheppard N and De La Cruz C (1996, 1998) Vibrational spectra of hydrocarbons adsorbed in metals. Advances in catalysis, Part I, 41: 1112; Part II, 42: 181313. Sheppard N and Nguyen TT (1978) The vibrational spectra of CO chemisorbed on the surfaces of metal catalysts. In: Clark RJH and Hester RE (eds) Advances in Infrared and Raman Spectroscopy, Vol. 5. London: Heyden. Suëtaka W (1995) Surface Infrared and Raman Spectroscopy Methods and Applications. New York: Plenum Press. Willis RF (ed.) (1980) Vibrational Spectra of Adsorbates, Springer Series in Chemical Physics 15. Berlin: SpringerVerlag. Yates JT Jr and Madey TE (1987) Vibrational Spectroscopy of Molecules on Surfaces. New York: Plenum Press.
Surface Studies By IR Spectroscopy Norman Sheppard, University of East Anglia, Norwich, UK Copyright © 1999 Academic Press
The investigation of surfaces, and of molecular layers adsorbed on surfaces, by electromagnetic radiation has been carried out principally by infrared spectroscopy. This is because of the high sensitivity of present-day Fourier transform infrared (FT-IR) spectrometers; the capability for IR spectroscopy to obtain data from mixed-phase samples with gas/solid, liquid/solid, or gas/liquid interfaces; and because of the availability of very large databases relating the positions and relative strengths of infrared absorptions to structural features of organic and inorganic molecules. As described below, the sampling techniques used differ substantially whether the systems under investigation involve finely divided samples (powders or porous solids) or whether the surface involved is flat. In the early history of the subject, spectral features relating to adsorbed layers and other surface phenomena could only be detected if very high area finely divided samples were used so that the radiation beam could pass through many interfaces. However, since the advent of FT-IR spectrometers, infrared sensitivity has so much improved that nowadays a measurable spectrum can be produced from even a single monolayer on a flat surface. After reviewing the experimental techniques involved, we survey the principal applications of the infrared method under the headings surface characterization, physical adsorption, and chemisorption and catalysis.
Experimental techniques High-area, finely divided, surfaces
Surfaces, because of their unsaturated surface fields, normally require to be cleaned from contamination derived from the atmospheric environment before systematic research can be carried out on them. For finely divided samples of high area it is adequate to mount them in a high-vacuum enclosure (~106 mbar) provided with infrared-transparent windows and the means for treating the sample in oxygen or hydrogen at elevated temperatures. The samples themselves are most often studied in transmission, usually in the form of pressed discs derived from powders. These are prepared using a hydraulic press
VIBRATIONAL, ROTATIONAL & RAMAN SPECTROSCOPIES Applications in a manner similar to that used for the standard potassium bromide pressed-disc sampling procedure for IR spectroscopy. The pressure required for coherent disc formation is greater for the commonly studied oxide layers than for the softer potassium bromide, but the discs so prepared remain porous for adsorption studies. Alternatively, a powdered sample can often be made to cohere on an infraredtransparent disc, or on a fine metal grid, through sublimation or by deposition from a solvent. Finely divided metal samples require that the opaque particles are separated from each other for transmission purposes. Usually this is done by distributing (supporting) them on the surface of an oxide which is transparent over relevant regions of the spectrum. Such samples are prepared by depositing metal salts from solution on the oxide particles followed by evaporation of the solvent; a disc is then pressed from the mixed powder and inserted in the IR vacuum cell; finally, the salt is reduced in hydrogen at appropriate temperatures so as to form metal particles distributed over the surface of the oxide support. Very high area powders of silica and alumina, of areas between 200 and 300 m2 g1, are commercially available and are frequently used as metal supports. They have the advantages that they are largely infrared-transparent down to ~1300 or ~1100 cm1 respectively, and hence permit the study of many group-characteristic absorptions from organic adsorbates. Silica is a more catalytically inert support than alumina. Samples prepared as described above can be good models for working catalysts of either the oxide or metal types and many infrared studies of surface phenomena are undertaken in conjunction with catalytic investigations. Loose powders can alternatively be studied by diffuse reflection, with the advantage for kinetic studies that surface reactions, rather than diffusion processes, are more likely to be rate determining than is the case with the fine-pored pressed discs. Low-area flat surfaces
Adsorbents in the form of flat surfaces are of very low area and normally ultra-high vacuum (UHV) conditions (~1010 mbar) are required in order to
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preserve them from contamination. Where the substrate is transparent, infrared spectra of the surface layers can be obtained either by transmission or by reflection; when the substrate is non-transmitting, as in the case of metals, then reflection is normally used. Experiments involving flat surfaces allow the application of polarized radiation for the purpose of obtaining information about the orientation of the adsorbed molecules with respect to the surface. For transparent substrates the used of radiation polarized in or perpendicular to the plane of incidence, in combination with the measured angle of incidence, can determine the direction of the dipole change with respect to the surface associated with each group-characteristic vibration. The orientations of even flexible molecules with respect to the surface can be deduced from such measurements. In the case of metals the effect of the free response of the conduction-electrons to a charge above the surface can be modelled in terms of an image of opposite sign at the same distance below the surface as is shown in Figure 1. In the infrared context it is the dipole moment change associated with the vibration that interacts with the radiation. Figure 1 shows that a component of such a dipole change that is parallel to the surface is cancelled out by its image, whereas a component perpendicular to the surface is doubled in magnitude. Hence only modes with perpendicular dipole components are IR allowed; in general these are those modes of vibration that are symmetrical with respect to all the symmetry elements associated with the surface complex. For example, a CO molecule adsorbed perpendicular with respect to the surface will give absorption bands from the QCO or QCM (M = metal) bond-stretching modes but not from the OCM bending modes. Such considerations constitute the metal surface selection rule (MSSR), which is widely used for the determination of molecular orientation or, if this is known, as an aid in the assignment of vibrational modes. For work with metals, reflectionabsorption infrared spectroscopy(RAIRS) uses near-grazing incidence in order to maximize the strength of the electric vector of the incident infrared radiation that is perpendicular to the surface. UHV is normally required when studying low-area flat surfaces (exceptionally this would not be a requirement if the adsorbate, such as a surfactant, is capable of displacing surface impurities) and this requires sophisticated equipment. Also, the high sensitivity needed for the measurement of spectra from single monolayers requires the use of FT-IR spectrometers with selective photoconductive infrared detectors; the mercury/cadmium telluride detector which covers the major range of the spectrum down
Figure 1 Charges and their images near metal surfaces; the origin of the metal surface selection rule (MSSR).
to ~700 cm1 is widely used. Figure 2 illustrates a typical experimental arrangement for RAIRS on a metal surface under UHV conditions. Spectroscopic work carried out on single crystals with known types of adsorption sites, such as are readily available for metals, are of great use in interpreting the more complex spectroscopic phenomena obtained from finely divided samples. Individual particles of the latter can exhibit facets with a variety of atomic arrangements and adsorption sites which can be studied one-at-atime on single crystals. Figure 3 shows the different atomic arrangements, and hence adsorption sites, of the (111), (100) and (110) faces of a face-centred cubic metallic lattice. UHV facilities also permit complementary spectroscopic methods involving particles such as electrons (as in high-resolution electron energy loss spectroscopy) or diffraction methods (as in low-energy electron diffraction) to be employed in order to characterize the same system further. Adsorption on metal electrodes, which can be cleaned in solution by electrode reactions, is also studied by RAIRS. There is added interest in the effects of the variable electrode potential on the spectra and structures of the adsorbed species. The surfaces of infrared-transparent materials that are available in the form of shaped and polished crystals, such as silicon or germanium, can be studied with good sensitivity by using attenuated total internal reflection (ATR) in conjunction with multiple reflection procedures. Sum frequency generation (SFG) is a recent spectroscopic development in which two laser beams, one in the visible region and the other of variable frequency in the infrared region, generate infraredmodulated signals in the visible region at the sum of the two frequencies. As the signals come only from the interface and not from the bulk, this technique is being exploited in high-pressure catalyst work and for surfactant research.
2322 SURFACE STUDIES BY IR SPECTROSCOPY
Figure 2 The optical arrangement of an FT-IR spectrometer for reflection–absorption (RAIRS) work in ultrahigh vacuum (UHV). A, detector; B, KBr lens: C, KBr window; D, UHV chamber; E, sample; F, Michelson interferometer; G, Globar source; P, grid polarizer. Reprinted from Chesters MA (1986) Journal of Electron Spectroscopy and Related Phenomena 38:123 Copyright (1986), with permission from Elsevier Science.
Infrared contributions to our knowledge of surfaces are mostly short-range in type and involve the identification of different types of site through the adsorption of probe molecules chosen for this purpose. CO is a well known probe of surface sites on metal surfaces. As discussed in the Chemisorption section below, its QCO bond-stretching vibration has distinct wavenumber ranges for adsorption on linear (on-top), twofold and threefold bridging sites. Although linear and twofold sites can occur on each of the surfaces shown in Figure 3, the threefold one is specifically characteristic of (111) surfaces and can be used to identify such facets on metal particles. Distinctions can sometimes be made between twofold CO sites on different facets. The wavenumbers of CO absorptions can also be used to characterize surface cation sites of different charge (different formal oxidation states) on transition metal oxides as shown in Figure 4 for a partially reduced Ni oxide sample. For hydrocarbon adsorption the characteristic spectrum of ethylidyne (CH3C) also plays a useful role in identifying (111) facets on finely divided metals. One of the earliest discoveries of surface infrared spectroscopy was that oxide surfaces, such as those of SiO2 or Al2O3, retain chemisorbed OH groups after the removal of water molecules adsorbed from the atmosphere. These can only be removed by hightemperature treatment and are presumably generated by the reaction of ambient water molecules with otherwise free valencies on the surface of the oxide lattice, according to the reaction O2 + H2O → 2OH or its covalent equivalent. In the case of alumina, for example, individual absorptions amongst a multiplicity of OH bond-stretching absorptions can be identified with linear, two- and threefold adsorption sites, for each of two types of surface aluminium atoms which in the bulk lattice have four- or six-fold coordinations, i.e. are in formal IV or VI oxidation states. Silica has only four silicon coordination and correspondingly simpler QOH spectrum consisting
Surface characterization The long-range patterns of surface atomic arrangements are principally monitored by low-energy electron diffraction (LEED). Whereas in principle the top layers of a lattice have different frequencies and hence wavenumbers from those of the bulk lattice, the associated absorptions often fall within a spectral region dominated by the latter and are hence difficult to identify. Transition metal oxides are exceptional in that the variable valency associated with the metallic element can lead to the generation of surface M = O groups (M = metal) that give absorptions of notably higher wavenumber than those of the lattice modes.
Figure 3 The arrangements of atoms, and the resulting adsorption sites, on the (100), (111) and (110) surfaces of a facecentred-cubic metal.
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num, metal surfaces adsorb oxygen from the atmosphere leading, in the cases of aluminium and iron, to the production of multilayer passivating oxide films or (with the participation of absorbed water) thick films of rust, respectively. Infrared spectroscopy can monitor these surface corrosion processes.
Physical adsorption
Figure 4 The IR spectrum of CO adsorbed on partially reduced NiY zeolite showing the oxidation-state dependence of QCO. Reproduced with permission of John Wiley & Sons Ltd. from Davydov AA (1984) Infrared Spectroscopy of Adsorbed Species on the Surfaces of Transition Metal Oxides, Copyright John Wiley & Sons Ltd.
mainly of absorptions of pairs of free and hydrogenbonded OH groups on adjacent sites. An acidic oxide such as alumina exhibits relatively inert OH groups, strongly acidic OH groups that are capable of proton donation (Brønsted acidity), plus aluminium ions that act as electron-deficient sites (Lewis acidity). The relative proportions of these on such oxide surfaces are analysed using the infrared spectrum of adsorbed pyridine. The spectra are measured in the 17001500 cm1 region. Pyridine hydrogen-bonded to the weaker surface OHs gives a weakly perturbed spectrum; that interacting with the strongly acidic OHs forms the pyridinium ion by proton transfer which has a characteristic additional absorption at 1540 cm1; that interacting with metalcoordination sites shows a change in wavenumber of a skeletal absorption near 1600 cm1. Basic oxides frequently adsorb carbon dioxide from the atmosphere to give surface carbonates according to the reaction O2 + CO2 → CO32, a process readily monitored by infrared spectroscopy. With the well known exceptions of gold and plati-
In this section we consider the spectroscopic study of the association of molecules with surfaces by intermolecular forces ranging from van der Waals to strong hydrogen bonding. Figure 5 shows the absorption bands in the QCH bond-stretching region from methane adsorbed on porous silica glass. In the gas phase the triply degenerate QCH mode at 3019 cm 1 is infrared active and is to be identified with the strong band from the adsorbed species at ~3006 cm 1. On the surface an additional feature has appeared in the spectrum at 2899 cm 1 which is readily identified as the gas-phase forbidden QCH breathing mode, known from gasphase Raman spectroscopy to occur at 2917 cm 1. The one-sided surface forces have distorted the original tetrahedral shape of the methane molecule so as to cause this mode to become active. The considerable breath of the ~3006 cm 1 absorption of the surface species, notably less than that of the gas-phase vibrationrotation band, was interpreted in terms of quasi-free rotation of the molecule about a single axis perpendicular to the silica surface. The spectrum of Figure 6 shows the interaction of the very acidic surface OH group on zeolite HY with adsorbed ethene. The low wavenumber, broad profile, and intensification of the shifted QOH absorption upon ethene adsorption indicate a hydrogen bond of considerable strength, comparable to that between water molecules, even although the bonding is only to the S-electrons of the adsorbed ethene. This complex
Figure 5 The IR spectrum of CH4 adsorbed on high-area porous silica glass in the QCH bond-stretching region showing the presence of a gas-phase forbidden absorption. Reproduced with permission of the Royal Society from Sheppard N and Yates DJC (1956) Proceedings of the Royal Society of London, Series A 238: 69.
2324 SURFACE STUDIES BY IR SPECTROSCOPY
Figure 7 The IR spectrum of cyclohexane adsorbed on a Pt(111) surface. The broad absorption near 2600 cm–1 is from a form of hydrogen bonding between axial CH bonds and surface Pt atoms. Reprinted from Chesters MA and Gardener P (1990) Spectrochimica Acta, Part A 46: 1011, Copyright (1990), with permission from Elsevier Science.
Figure 6 The IR spectrum of ethene adsorbed on the acid OH groups of HY zeolite. Solid line, ethene adsorbed; dashed line, background. Reproduced with permission of the Royal Society of Chemistry from Liengme BV and Hall WK (1966) Transactions of the Faraday Society 62: 3229.
is clearly an intermediate in the higher temperature formation of the carbenium ion C2H . Hydrogen bonds are normally considered to form between acidic XH groups and electron-rich bases. However, surface infrared spectroscopy, in conjunction with HREELS, has shown that such bonds can also occur between electron-rich CH bonds of paraffins and electron-deficient sites on metal surfaces. Figure 7 shows the spectrum of cyclohexane adsorbed on the (111) surface of platinum. The very broad band centred at ~2620 cm 1 is from a proportion of CH bonds of the adsorbed cyclohexane in a hydrogen-bonded type of environment. As the separation between the three parallel axial CH bonds on one side of the cyclohexane molecule is almost exactly the separation between adjacent Pt atoms on a threefold site of the (111) surface, it is clear that the hydrogen bond is of the type CHPt. Figure 8 is a spectrum taken at 33 K in the QCH region of CD3H adsorbed on the (100) face of the face-centred-cubic lattice of NaCl. It is seen that there are two well resolved absorption bands, one
Figure 8 The IR spectrum from CHD3 adsorbed at 33 K on NaCl(100). Ep and Es refer to radiation polarized in and perpendicular to the plane of incidence, respectively. Reprinted with permission from Davis KA and Ewing GE (1997) Journal of Chemical Physics 107: 8073. Copyright 1997, American Institute of Physics.
sharp and the other broad, resulting from CH bonds that are oriented differently with respect to the surface. One of these occurs with the incident light polarized in the plane of incidence but is eliminated when the light is polarized perpendicular to this; the other is present in both spectra. The former band hence has its QCH vibrational dipole change perpendicular to the surface, whereas the direction of the
SURFACE STUDIES BY IR SPECTROSCOPY 2325
latter has both parallel and perpendicular components. Considerations of relative intensities, taking into account the angle of incidence, show that the broader low-wavenumber band is from CH bonds that are at ~70° with respect to the surface. It is hence concluded that the parent CH4 molecules are adsorbed with three of their four CH bonds on the surface. Even in the absence of the special effects of hydrogen bonding, bandwidths of 10 cm1 or more are common from adsorbed species on polycrystalline substrates owing to interactions with sites that differ in their detailed environments. Absorptions obtained from adsorbed species on single-crystal planes with uniform and well defined sites can, in contrast, be very sharp with bandwidths of less than 1 cm1. In the case of methane itself, and of a number of other molecules such as CO and CO2, the resolution of the spectra on alkali metal halide single-crystal surfaces are such that even the fine-structure splitting caused by the vibrational couplings of more than one molecule in the surface unit cell can readily be resolved.
Chemisorption and catalysis The quantitative and energetic aspects of the chemisorption of molecules on surfaces have long been investigated but, until in the 1950s it became possible to obtain infrared spectra, the actual structures of the surface species could only be a matter for speculation. The spectra show that in fact finely divided adsorbents give absorptions from several different surface species, and that the nature of the latter can vary as a function of coverage. Simpler spectra are obtained on single-crystal surfaces of known atomic arrangements. However, even so, the deductions of the structures of the chemisorbed species can be difficult because of uncertainties related to the effects of surface bonding on the spectra of the attached groups. The usual group-characteristic wavenumber ranges can no longer be assumed to be reliable because of the electron-donating or -withdrawing properties of the surface atoms and also, when there is multiple bonding to the surface, because of strains associated with cyclic bonding features. The procedure adopted is to use the spectra to suggest possible alternative structures for the adsorption complexes, and then to look for molecular analogues of known structures whose spectra can be obtained for comparative pattern-recognition purposes. This approach is well exemplified by the results obtained for chemisorption on metal surfaces, an area much studied because of the ready availability of single crystals of metals which can be cut so as to display particular surface planes.
Figure 9 The IR spectra of CO adsorbed on the silica-supported metals Cu, Pt, Ni and Pd. Absorptions above 2000 cm–1 are from linear (on top) CO bonded to one metal atom; those below this value are from CO bridge-bonded to two or three metal atoms. Reprinted with permission from Eischens RP, Pliskin WA and Francis S A (1954) Journal of Chemical Physics, 22: 1786. Copyright 1954 American Institute of Physics.
Figure 9 shows high-coverage spectra obtained from CO chemisorbed on the silica-supported metals Cu, Pt, Ni and Pd. The several metals show different proportions of absorption bands above and below 2000 cm 1 which are characteristic of adsorption on linear (on-top) and bridged sites, respectively. These structural assignments were deduced by comparison with the spectra of metal carbonyls. The spectral ranges attributable to such surface species are as follows: linear, 21202000 cm 1; twofold bridge, 2000 ~1870 cm 1; and threefold bridge ~1900 1800 cm1. These ranges apply whichever crystal face is involved. Within each range the characteristic absorptions increase in wavenumbers with increasing coverage. This is caused by strong vibrational coupling within the array of parallel molecules on the surface, mostly of a dipolar nature related to the exceptional strength of the QCO absorptions. The mixture of linear and bridged CO species found from the spectra from the finelydivided samples is caused by adsorption on different sites, usually different facets, on the metal particles. Figure 10 shows QCO spectra at full coverage from chemisorption on an Rh(111) single-crystal electrode. These are plotted as a function of the
2326 SURFACE STUDIES BY IR SPECTROSCOPY
Figure 10 The IR spectra of CO adsorbed at full coverage on an Rh(111) electrode in 0.1M NaClO4 at various electrode potentials. Reprinted from Chang SC and Weaver MJ (1990) Surface Science 238: 142, Copyright (1990), with permission from Elsevier Science.
electrode potential (with respect to a standard Ag/ AgCl electrode) in 0.1M NaClO4 and show the interest of this additional variable in electrode work. It is seen that at the lowest electrode potential the spectrum is dominated by the absorption at 1886 cm1 from a bridged species but at higher potentials, before desorption sets in, a linear species becomes dominant, absorbing at 2029 cm1. More generally, work on electrodes shows that, at a given coverage, negative potentials favour bridged species over linear species and that the wavenumbers of QCO absorptions from linear species increase in value with increasingly positive electrode potentials a milder version of the dependence of QCO on metal oxidation state reported above. Hydrocarbons on metal surfaces provide greater challenges in spectral interpretation and we choose the example of ethene chemisorbed on different metal surfaces. Here the relevant model compounds are inorganic binuclear or trinuclear metal clusters with the hydrocarbon ligand of interest and additional
CO ligands occupying the positions of the metal atoms of the surface complex. One of the unexpected aspects of the adsorption of ethene is that (111) faces of many metals are covered by the dissociative ethylidyne species CH3CM3 (M = metal) near room temperature. Its spectrum was attributed to this structure by comparison with the spectrum of the model compound (CH3C)Co3(CO)9, considered as a possibility because electron diffraction had shown that the CC bond of the adsorbed species is perpendicular to the surface. This example shows the importance of the metalsurface selection rule (MSSR). For this species, as a ligand or as a surface complex, the modes of vibration are fully separable into those with dipole changes either perpendicular or parallel to the surface (parallel or perpendicular to the CC bond, respectively). Only the former modes are active under the MSSR but both sets are active in the infrared spectrum of the model compound. Figure 11 compares the infrared spectrum of the ethylidyne species on the Pt(111) surface with that of the model compound. The bands marked with asterisks in the spectra of the model compound (in order of decreasing wavenumber, QCH3 symmetrical stretch, G&+3 symmetrical bend and QCC stretch) are those which give dipole changes perpendicular to the surface; the other doubly degenerate modes give dipole changes parallel to the surface (QCH3 asymmetric stretch, GCH3 asymmetric bend and CH3 rock). The positions of the missing modes of the surface species, indicated by arrows, have been identified in the HREEL spectrum of the same system where the selection rules are more relaxed. It has been shown by spectroscopy that at low temperatures ethene adsorbs on Pt(111) as the
Figure 11 A comparison of the IR spectrum from ethene adsorbed on Pt(111) at room temperature with that of the model compound (CH3C)Co3(CO)9. Asterisks indicate absorptions of the model compound allowed in the spectrum of the adsorbed species by the metal-surface selection rule; arrows indicate other bands observed by HREELS. Courtesy Chesters MA.
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Figure 12 The structures of the principal adsorbed species from the adsorption of ethene on metal surfaces; [1] S-complex; [2] the di-V species; [3] ethylidyne (CH3C).
MCH2CH2M (di-σ adsorbed) species with a cyclic C2M2 skeleton and that this transforms into ethylidyne on warming to near room temperature. The (111) faces of other metals, notably Pd and Cu, show low-temperature spectra from another less strongly perturbed H2CCH2 adsorbed species in which there is bonding from a single metal atom to the S-electron distribution of the C=C double bond. Its spectrum is closer to that of ethene itself but with those modes which involve CC stretching occurring at lower wavenumbers. The structures of these three species are shown in Figure 12. Figure 13 shows two spectra from ethene adsorbed on a silica-supported Pt sample, of the type used in catalysis, at 180K and at room temperature. On these are indicated absorptions from the above three species with the MSSRallowed modes still dominant. It is seen that the spectra from the catalyst sample are comprehensively accounted for in terms of the species that had been identified one-at-time on single-crystal surfaces.
For the purpose of catalysis, the structure of the surface-adsorbed reactant should be sufficiently perturbed in order to promote reactivity, but not so strongly adsorbed that it cannot be removed by reaction. Below the temperature for the onset of catalysis, controlled by the energy of activation, the reactive species will be one or more of the chemisorbed species. The spectra of such species will weaken or disappear when catalysis commences while less reactive species are retained. In the case of ethene hydrogenation over metal catalysts, the order of reactivity in the presence of hydrogen is [1] > [2] > [3], with [3], the ethylidyne species, being very slow to be removed. By room temperature over Pt/ SiO2, when the di-V species has all been converted to ethylidyne, it is clear that the S-species, [1], is the catalytically active one. On Pt single crystals this mainly occurs on non-close-packed planes, and it may be inferred that catalytic reduction occurs on rougher, non-(111), surfaces of the metal particles. In a similar manner, it has been shown by single-crystal spectroscopy that the reactive species in the reduction of nitrogen to ammonia over the Fe catalyst (the Haber process) is a di-σ species involving the NN molecule chemisorbed to two Fe atoms which dissociates to adsorbed N atoms during catalysis. The transition metal oxides form the other principal class of catalysts. These differ from the metals in that they have both acid and base sites in the same surface (the metal and oxygen atoms/ions, respectively) and react differently according to which of these properties is dominant. Figure 14 shows the infrared spectrum from the heterolytic dissociation of hydrogen on polycrystalline ZnO to given surface
Figure 13 The IR spectra of ethene adsorbed on silica-supported Pt (A) at 180 K and (B) at room temperature, labelled according to the structural assignments of the absorption bands. Reprinted with permission from Mohsin SB, Trenary M and Robota H Journal of Physical Chemistry, (1988) 92: 5229 and (1991) 95: 6657. Copyright 1988,1991, American Chemical Society.
2328 SURFACE STUDIES BY IR SPECTROSCOPY
High-resolution electron energy loss spectroscopy (HREELS), with its higher sensitivity but lower resolution, has played a strongly complementary role to IR in the study of molecules adsorbed on singlecrystal metal surfaces. Inelastic neutron scattering (INS) and inelastic electron tunnelling spectroscopy (IETS) have found more limited applications to the study of the adsorption of molecules on high-area surfaces.
Figure 14 The IR spectra of hydrogen adsorbed on ZnO. Reproduced with permission of the Royal Society of Chemistry from Hussain G and Sheppard N (1990) Journal of the Chemical Society, Faraday Transactions 86: 1615.
HZn+ and OH groups. Oxides have mostly been studied in high-area form to date, including the zeolites whose acidic activity occurs on well defined sites within the pores of the crystalline material. Flat single crystals of oxides are difficult to clean in UHV because of their insulating properties. Single-crystal spectroscopic work on oxides is increasing being carried out on thin films grown epitaxially on metal surfaces.
Other vibrational spectroscopic techniques for surfaces Raman spectroscopy provides valuable complementary vibrational information to IR spectroscopy but its applications to adsorbed molecules has been principally limited to the study of finely divided samples for reasons of reduced sensitivity. Exceptionally, flat monolayers of long-chain surfactant have given Raman spectra using multireflection techniques. Surface-enhanced Raman spectroscopy (SERS) gives greatly enhanced sensitivity but only for molecules adsorbed on the roughened surfaces of the coinage metals, particularly silver.
See also: ATR and Reflectance IR Spectroscopy, Applications; High Resolution Electron Energy Loss Spectroscopy, Applications; Inelastic Neutron Scattering, Applications; Inelastic Neutron Scattering, Instrumentation; IR Spectroscopy, Theory; Raman and IR Microspectroscopy; Surface-Enhanced Raman Scattering (SERS), Applications.
Further reading Bell AT and Hair ML (1980) Vibrational Spectroscopies for Adsorbed Species, ACS Symposium Series 137. Washington, DC: American Chemical Society. Clark RJH and Hester RE (eds) (1988) Spectroscopy of Surfaces, Advances in Spectroscopy, Vol 16. New York: Wiley. Davydov AA (1984) Infrared Spectroscopy of Adsorbed Species on the Surfaces of Transition Metal Oxides. New York: Wiley. Sheppard N and De La Cruz C (1996, 1998) Vibrational spectra of hydrocarbons adsorbed in metals. Advances in catalysis, Part I, 41: 1112; Part II, 42: 181313. Sheppard N and Nguyen TT (1978) The vibrational spectra of CO chemisorbed on the surfaces of metal catalysts. In: Clark RJH and Hester RE (eds) Advances in Infrared and Raman Spectroscopy, Vol. 5. London: Heyden. Suëtaka W (1995) Surface Infrared and Raman Spectroscopy Methods and Applications. New York: Plenum Press. Willis RF (ed.) (1980) Vibrational Spectra of Adsorbates, Springer Series in Chemical Physics 15. Berlin: SpringerVerlag. Yates JT Jr and Madey TE (1987) Vibrational Spectroscopy of Molecules on Surfaces. New York: Plenum Press.