Electron paramagnetic resonance spectroscopy of catalytic surfaces

Electron paramagnetic resonance spectroscopy of catalytic surfaces

Colloidsand SurfacesA: Physicochemicaland EngineeringAspects, 72 (1993) 353-363 353 Elsevier Science Publishers B.V., Amsterdam Electron paramagnet...

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Colloidsand SurfacesA: Physicochemicaland EngineeringAspects, 72 (1993) 353-363

353

Elsevier Science Publishers B.V., Amsterdam

Electron paramagnetic resonance spectroscopy of catalytic surfaces Russell F. Howe

Department of Physical Chemistry, University of New South Wales, Box 1, Kensington, N.S.W. 2033, Australia (Received 24 August 1992; accepted 5 J a n u a r y 1993)

Abstract A review is presented of the application of electron paramagnetic resonance (EPR) spectroscopy to study surfaces of catalytic interest. Spectra of Mo(V) in supported molybdena catalysts illustrate how EPR can assist in the characterization of such materials. The spectra of an adsorbed stable radical, the superoxide ion, are used to show how surface sites on catalysts can be probed by EPR. Unstable radicals which are conceivable intermediates in catalytic reaction mechanisms can be identified via spin trapping experiments, either chemical or matrix isolation. The mobility of adsorbed radicals on surfaces can be studied by EPR. Finally, recent progress in observing EPR spectra from welldefined single-crystal surfaces is discussed.

Keywords."Catalytic surfaces; electron paramagnetic resonance; molybdenum(V).

Introduction The possibilities of using electron pararnagnetic resonance (EPR) spectroscopy to investigate catalytic surfaces were first discussed by O'Reilly [1] in a 1960 review article at a time when very few experiments had actually been done. Since then, the sensitivity of the E P R technique and its ability to provide unique information a b o u t the microscopic environment on a catalytic surface have been exploited in literally thousands of studies and n u m e r o u s reviews have appeared [2 6]. This paper does not intend to survey the literature in a comprehensive manner. Rather, an attempt is made to illustrate the wide scope of E P R spectroscopy as a surface technique by reference to selected examples only. The selection reflects the author's own particular interests and

Correspondenceto. R.F. Howe, Dept. of Physical Chemistry, University of New South Wales, Box 1, Kensington, N.S.W. 2033, Australia. 0927-7757/93/$06.00

© 1993

preferences. The objectives are to convince the E P R spectroscopist that catalytic surfaces offer m a n y interesting spectroscopic problems, and to convince the catalytic chemist that E P R spectroscopy has a great deal to offer as a technique for characterizing surfaces and observing adsorbed species,

Catalyst characterization The E P R spectrum of a paramagnetic transition metal ion is extremely sensitive to the coordination environment in which the metal ion is placed. This fact has been exploited in m a n y E P R studies of supported transition metal oxide catalysts. These materials consist of a transition metal oxide component dispersed to a greater or lesser extent on a high surface area insulating or semiconductor oxide such as silica, alumina or titania. The catalytic chemist wishes to k n o w what oxidation states of the transition metal are present

ElsevierScience Publishers B.V. All rights reserved.

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R.F. Howe~Colloids Surfaces A: Physicochem. Eng. Aspects 72 (1993) 353-363

under various different conditions of activation or catalytic reaction, what ligands are in the coordination sphere, and how well dispersed the metal ions are. EPR spectroscopy can in principle answer all three questions. Consider, for example, the case of supported MoO3 catalysts. These find many applications in hydrogenation, polymerization, metathesis, partial oxidation and hydrodesulphurization processes [7]. Thermal reduction in hydrogen of aluminasupported MOO3, for instance, produces a broad asymmetric EPR signal due to Mo(V), which has been widely studied. Most authors have assigned the signal to a distorted octahedral (square pyramidal) coordination [8], although tetragonally distorted tetrahedral symmetry has also been suggested [9]. Double integration of the Mo(V) signal gives spin concentrations typically corresponding to only a few per cent of the amount of Mo(V) determined to be present by reduction stoichiometry measurements [10] or X-ray photoelectron spectroscopy [11]. The EPR invisible fraction of Mo(V) might be considered to occupy high symmetry sites and not therefore be observed because of rapid spin-lattice relaxation. This possibility was dismissed by Abdo et al. [12], who found no new signals appearing when spectra were measured at 4.2 K. They proposed that the "missing" Mo(V) must be strongly coupled magnetically. Coupling between Mo(V) in close proximity on the alumina support is clearly related to the important question of how well the transition metal component is dispersed when the catalyst is prepared by conventional impregnation methods. Direct EPR evidence for coupling between Mo(V) in conventionally prepared supported molybdena catalysts comes from reaction of the reduced catalysts with gaseous hydrogen halides [13]. Ligand substitution reactions of the type Mo(V)O + 2HX --, M o ( V ) X 2 + H 2 0 and Mo(V)O Mo(V)+ 2HX ~ 2Mo(V)X + H20

occur on exposure of the catalyst to HC1 or HBr at room temperature, causing an approximately threefold increase in integrated Mo(V) spin concentration. Figure 1 (curve a) shows for example the EPR

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~

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1"9162 !

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Fig. I. Mo(V) signals from 95Mo-enriched molybdena-alumina catalyst treated with HCI (reproduced with permission from Ref. 1_13]): curve a, observed composite spectrum; curve b, simulated composite spectrum generated by combining the simulated signals of coupled curve c and isolated curve d oxychloromolybdenum(V) species.

R.F. Howe~Colloids Surjaces A: Physicochem. Eng. Aspects 72 (1993) 353 363

spectrum obtained after reaction of a 9 5 M o enriched catalyst with HC1. This spectrum can be satisfactorily simulated by superposition of the signals in Fig. 1 (curves b and c): one, showing well-resolved 95Mo hyperfine splitting, due to magnetically isolated oxychloro Mo(V) species, and the second broad signal attributed to the same species in close proximity and therefore dipolar broadened. The enhancement of Mo(V) spin concentration on reaction with HX is attributed to partial removal of antiferromagnetic coupling between adjacent Mo(V) ions via bridging oxide ligands (this phenomenon is reversible; on outgassing, adsorbed water reacts with the oxychloro species to liberate HC1 and restore the original EPR signal). Similar experiments have been undertaken with silicasupported molybdenum catalysts prepared by conventional impregnation [14], and with molybdenum-loaded pillared clay catalysts prepared from an organometallic precursor [15]. In all these cases the reduced catalysts evidently contain extended regions of strongly coupled molybdenum; the reaction with hydrogen halides appears to be a useful way of testing for such interactions. Alternative catalyst preparation methods can produce much higher dispersions of the transition metal component than conventional impregnation. Che et al. [ 16] have, for example, made an extensive study of grafted silica-supported molybdenum catalysts prepared by reaction of MoC15 with the hydroxyl groups on the silica support. A consequence of the higher dispersion of molybdenum in these grafted catalysts is a much improved resolution. Figure 2 shows the spectrum of a reduced grafted catalyst. Three different Mo(V) signals were resolved (note the use of third derivative presentation to enhance the resolution). The signals have been assigned to 4-, 5- and 6-coordinate Mo(V) species respectively, based on a study of their reactions with adsorbed probe molecules H20 and CO [173. Well-resolved Mo(V) signals can also be obtained for conventionally prepared silicasupported molybdenum catalysts if they are photoreduced at low temperatures. Figure 3 shows

355

spectra (observed and simulated) obtained by photoreduction of an impregnated molybdena silica catalyst photoreduced in hydrogen at 20 K [18]. The spectrum was analysed in terms of three component signals, two with axial g tensors and one with orthorhombic g tensors. All three signals show 1H superhyperfine splitting (confirmed by reducing in deuterium). The high resolution of the photoreduced catalysts can be attributed to the fact that the catalyst is "frozen"; no coordination vacancies are formed, and the species observed are the initial products of attack of hydrogen on the various oxomolybdenum components present. In thermal reduction, however, substantial rearrangement of the coordination sphere can occur, and a range of coordination environments is produced, which results in a poorly resolved EPR signal. On warming the photoreduced catalysts to room temperature or slightly above, the 1H splittings were lost, and the resulting EPR signal became similar to that formed on thermal reduction. Howe and Seyedmonir attributed these changes to transfer of protons from the molybdena phase to the silica support, generating new SiOH groups [18]. An alternative approach to obtaining better structural definition and hence spectroscopic resolution is to examine transition metal ions cationexchanged in zeolites. The zeolite as a threedimensional crystalline surface provides an excellent opportunity to study the coordination of transition metal ions and model the reactivity of transition metal ion sites on supported oxide catalysts. Stable radicals on surfaces

The formation of stable radical anions or cations has long been used as a probe of one-electron donor or acceptor sites on catalytic surfaces [19]. The superoxide ion O2 is one such species whose EPR spectrum contains much useful information about the surface sites on which it is formed [20]. The g tensor components (particularly the low field g= component) are found to depend on the formal

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356

R.F. Howe~ColloidsSurfaces A: Physicochem. Eng. Aspects 72 (1993) 353-363

50G

I

® 5*

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Fig. 2. Spectrumof a reducedgraftedMo-SiO2 catalystat 77 K: curvea, First derivative;curve b, third derivative.(Reproducedwith permission from Ref.[17].) oxidation state of the metal ion on which the 0 2 is generated; this can be understood in terms of the lifting of the degeneracy of the 0 2 H orbitals by the electrostatic field associated with the surface site. Surface sites having a nuclear spin (e.g. 27A13+, 95Mo6+) produce hyperfine splitting in the 0 2 spectrum. The use of lVO-enriched oxygen to generate O2 gives further information about surface site geometry as revealed by the extent of inequivalence of the two oxygen atoms. Formation of 0 2 on a catalytic surface will occur spontaneously if the surface contains transition metal ions able to act as one-electron donors (e.g. MoS+). Alternatively, UV- or 7-irradiation may be needed to generate electrons and initiate electron transfer to adsorbed oxygen. As an example of the use of O2 to probe surface sites, consider the interaction of oxygen with Mo(VI)-exchanged zeolite ZSM-5 [21]. Figure 4 shows spectra measured following adsorption of oxygen onto the Mo-ZSM-5 after various different pretreatments. The 0 2 signal generated following hydrogen reduction of the zeolite (Fig. 4 (curve a)) has g tensor components identical to those of 0 2 on reduced molybdena-silica catalysts formed through the electron transfer reaction [20] 02 + Mo s+ --* M06+02

This signal also showed the same 95Mo and 170 hyperfine splitting patterns as 0 2 on molybdena-silica when the corresponding isotope enrichment experiments were undertaken. The signal was destroyed if the sample was exposed to 2,6 -dimethylpyridine vapour. The 2,6 -dimethylpyridine molecule is too large to enter the zeolite channels. Thus the 0 2 species responsible for the signal in Fig. 4 (curve a) must be formed on Mo(V) sites located on the external surface of the zeolite or at channel openings, and not within the zeolite channels. These sites appear to resemble in every respect those found on a conventional molybdena-silica catalyst. Adsorption of oxygen following reduction of the Mo-ZSM-5 in ammonia rather than hydrogen produced a second 0 2 species, whose contribution to the spectrum could be enhanced by repeated reduction (Fig. 4, curves b and c). The second signal has a larger g= value than the first (2.021 compared with 2.016), but shows similar 95Mo hyperfine splitting when 95Mo-enriched molybdenum is used. The 170 hyperfine splitting pattern of the second signal indicates that the two oxygen atoms are less inequivalent than those in the first 0 2 species (AAxx=8 G instead of 13 G), and the second signal was unaffected by exposure of the sample to 2,6 -dimethylpyridine.

R.F. [towe/Colloids Surfaces A: Physicochem. Eng. Aspects 72 (1993) 353-363

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c Fig. 3. Spectrum of Mo SiO 2 catalyst photoreduced at 20 K: curve a, observed; curve b, calculated. (Reproduced with permission from Ref. [l 8].)

The second 0 2 species was thus identified as being formed on Mo 5+ sites within the zeolite channels. These sites were generated by the ammonia reduction treatment; the shifted gz= value of the 0 2 suggests that the bonding of O2 to the internal M o 6 + sites is not identical to that on the external sites, and may involve a degree of covalency made possible by the low coordination environment of M o 6 + within the zeolite channels.

Spin trapping of unstable free radicals Radical species have long been proposed as important intermediates in catalytic reaction mech-

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~=

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= ~'OO3

Fig. 4. EPR spectra of O 2 in M o - Z S M - 5 (ion exchanged): curve a, after H 2 reduction; curve b, after single N H 3 reduction; curve c, after repeated NH 3 reduction. (Reproduced with permission from Ref. [21].)

anisms, either adsorbed on the catalyst surface or desorbed into the gas phase. Direct observation of such radical intermediates under the conditions of a high temperature catalytic reaction remains a difficult problem. Spin trapping methods can, however, be used to advantage. A spin trapping experiment of major significance in the currently topical area of methane activation

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catalysis was the detection by Driscoll et al. [22] of methyl radicals desorbed from lithium-doped MgO catalysts used for the oxidative coupling of methane: 2CH4 + 2-1-O2~ C 2 H 6 + H 2 0 These authors used a matrix isolation technique to trap and identify gas phase radicals downstream from the catalyst. The yield of methyl radicals was found to correlate directly with the concentration of paramagnetic Li+O - centres in the catalyst itself, leading to the conclusion that abstraction of hydrogen from adsorbed C H 4 by O centres forms methyl radicals which then desorb into the gas phase: C H 4 + O - -~ CH; + O H -

The subsequent chemistry undergone by the gas phase methyl radicals is not simple; more than 100 mechanistic steps have been proposed to account for all the observed effects of experimental variables on oxidative coupling conversions and product distributions [23]. Nevertheless, the convincing demonstration by matrix isolation spin trapping that gas phase methyl radicals are a key intermediate in the oxidative coupling reaction has dominated all subsequent research aimed at developing a commercially viable oxidative coupling process. This is a rare example of catalyst development being driven by spectroscopic identification of a reaction intermediate. Spin trapping is more conventionally associated with reaction of an unstable radical with a diamagnetic spin trap reagent to form a stable radical: R" + T ~ R T " Analysis of the spectrum of the stable adduct may in favourable cases allow the radical R" to be identified. Spin trapping has been applied to detect radicals desorbed from a ZSM-5 zeolite catalyst during conversion of dimethyl ether to hydrocarbons [24]. The mechanism of formation of carbon carbon bonds from methanol or dimethyl ether over the ZSM-5 catalyst has been the subject of intensive

research [25]; the suggestion of a radical pathway certainly adds a new dimension to the debate. Closer examination of the data in Ref. [24] indicates, however, that the link between the radicals trapped and the mechanism of carbon-carbon bond formation is by no means clearly established. Clarke et al. [24] do not identify the adduct responsible for the EPR spectrum they observe. Comparison of the 14N (about 15 G) and 1H (about 2.5 G) splittings with those of known adducts of the e-phenyl-N-tert-butylnitrone (PBN) spin trap used suggests that the radical trapped is probably "OH rather than the alkyl or alkoxy radical implied in Ref. [24]. The question of whether radicals generated deep within the zeolite channel system can escape into the gas phase at the temperatures of the dimethyl ether conversion to hydrocarbons (22-350°C) also remains open. There is no doubt, however, that radicals are generated in some manner over the zeolite catalyst; the suggestion by Clarke et al. that ~r-bond homolysis leading to radical formation may be more prevalent in catalysis than previously recognized is a valid proposition which must be further investigated by EPR spectroscopy. An illustration of the spin trapping technique applied to an adsorbed species rather than a desorbed radical is the work of Matsuzaki et al. on hydrogen adsorbed on zinc oxide [26]. The rapid and reversible chemisorption of hydrogen on zinc oxide has been shown by IR spectroscopy to produce ZnH and OH species. When HD is adsorbed, an equilibrium isotope effect results in a preponderance of ZnH, OD over ZnD, OH [27]. Figure 5 shows EPR spectra obtained when a ZnO surface containing adsorbed H 2 (Fig. 5, curve a) and adsorbed HD (Fig. 5, curve b) respectively was washed with a solution of the spin trap PBN under oxygen free conditions. The seven-line spectrum (curve a) in Fig. 5 is due to the hydrogen atom adduct of PBN. The spectrum in curve b is dominated by the same seven-line signal, but superimposed is a weaker 18-line signal due to the corresponding deuterium atom adduct, in an intensity ratio approximately equal to the equilibrium

R.F. Howe~Colloids SurJdces A." Physicochem. Eng. Aspects 72 (1993) 353 363

359

decomposition of water supports the frequently cited mechanistic role of these radicals in the reaction mechanism. Direct detection of unstable radicals

8

+ Fig. 5. Spectrum of the reaction product of PBN and H z adsorbed on Z n O (curve a), and spectrum of adsorbed H D

(curve b).

isotope ratio reported from the IR experiments. The spin trapping experiment thus appears to be detecting exclusively hydrogen atoms attached to Zn / + sites. Spin trapping has also been applied to the detection of radicals generated during photocatalysis. Jaeger and Bard [28], for example, added spin trap reagents to aqueous solutions in contact with anatase or platinized anatase powders. On irradiation with light of energy greater than that of the anatase band gap, complex EPR spectra containing overlapping signals from up to four different spin adducts appeared. Adducts of the radicals "OH and HO~ were identified by comparison with signals of individual authentic examples. The remaining species remained unidentified, but are probably due to decomposition products of the spin traps, something which remains an occupational hazard of the spin trapping experiment. Nevertheless, the detection of "OH and HO~ during photoassisted

The low temperatures at which photocatalysis is conducted, and the important role of free radicals in photocatalytic reaction mechanisms, make photocatalysts ideal subjects for low temperature in situ EPR studies. Irradiation of a semiconductor photocatalyst with photons of energy exceeding the band gap will produce holes and electrons; the subsequent reactions of these oxidizing and reducing species respectively with molecules adsorbed on the catalyst surface (as opposed to unproductive recombination) are the key to photocatalysis. We have attempted to identify the initial products of reaction of holes and electrons produced in an anatase photocatalyst by observing in situ EPR spectra of the catalyst irradiated with UV light at 4.2 K [-29]. Figure 6 shows spectra obtained on irradiation of hydrated and deuterated anatase samples respectively in vacuo. Two signals appeared when the light source was turned on. The relative intensities of these signals varied from one experiment to another; their absolute intensities reached limiting values after short irradiation times and remained unchanged thereafter, as long as the light source remained on. When irradiation was stopped, the signals rapidly decayed at 4.2 K, although they could be restored on further irradiation. The higher field signal in Fig. 6 is attributed to interstitial Ti 3+ cations, formed by electron trapping at interstitial Ti 4 + defect sites. The lower field signal was unchanged when the experiment was repeated with a deuterated anatase. The absence of 1H (or 2H) hyperfine splitting means that this signal cannot be due to hydroxyl radicals, which would be the expected product of hole trapping at surface hydroxide ions. The g tensor components are, however, consistent with an O- species, presumably formed by hole trapping at oxide ions. In vacuo at 4.2 K, the EPR experiment thus detects

R.F. Howe~Colloids SurJaces A." Physicochem. Eng. Aspects 72 (1993) 353 363

360 2.016

..j B

4.2 K), the sample is warmed to room temperature. The absence of 170 hyperfine splitting in the spectrum when lVO-enriched oxygen is employed indicates, however, that it is generated from hydroxyl groups or adsorbed water and not from

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01;

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A

02. 1.960

50 gauss

Fig. 6. EPR spectra of hydrated anatase (curve a) and deuterated anatase (curve b) irradiated in vacuo at 4 K. (Reproduced with permission from Re['. [29].)

small steady state concentrations of trapped holes and electrons when the catalyst is irradiated; when irradiation is stopped, these decay to zero with a half-life of several minutes at 4.2 K, presumably due to recombination. When irradiation is carried out in the presence of oxygen, the trapped electron (Ti 3+) signal is not seen, and the trapped hole signal (O-) is stable indefinitely at 77 K. Oxygen is known to be an effective scavenger of electrons; removal of trapped electrons through reaction with oxygen stabilizes the trapped holes. On dehydrated anatase surfaces, the product of electron transfer to oxygen is the O2 radical anion [30]. On the hydrated surface, however, O2 is not generated from gaseous oxygen, so that the electron scavenging process must involve two-electron transfer to form peroxide. An O~- species is generated on the hydrated surface when, following irradiation in oxygen at 77 K (or

Plausible reaction schemes can be written to account for the behaviour of the observed signals [29] involving for example reaction of holes with hydroxide ions or adsorbed water at temperatures above 77 K to form hydroxyl radicals which dimerize to form peroxide. Many aspects of this chemistry remain speculative, although it may be noted that the involvement of peroxide species in water photolysis over anatase photocatalysts has been indicated from other experiments [31]. It is clear, however, that the in situ EPR technique has considerable potential for tracing photocatalytic reaction mechanisms, and many further applications can be anticipated.

Mobility of adsorbed species The rates at which adsorbed species migrate across catalyst surfaces to find active sites or other adsorbed species with which to react can influence strongly the overall reaction rates in catalytic processes. There are, however, a very limited number of experimental techniques which can provide quantitative information about rates and activation barriers for surface migration or diffusion. EPR spectroscopy is one such technique. Table 1 summarizes activation energies for surface diffusion determined by EPR for a number of adsorbates and adsorbents. With one exception, these data were derived from analysis of temperaturedependent line shapes in conventional continuous wave (CW) EPR experiments. The line shape variations from a rigid limit powder pattern to an isotropic (or, in the case of two-dimensional averaging, an axial line shape) spectrum, and the corresponding changes in apparent hyperfine coupling constants, span a range of correlation times from about 10 ~ to 10 -l° s. Clarkson and Kooser [32] applied the saturation transfer technique to extend

R.F. Howe/Colloids Surfaces A" Physicochem. Eng. Aspects 72 (1993) 353-363

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TABLE I

Single-crystal surfaces

Activation energies for surface diffusion

Major advances in the understanding of reaction mechanisms in catalysis by metals have come from the application of surface science techniques to well defined single-crystal metal surfaces. The structural definition available at a single-crystal surface has allowed detailed models of active sites and the configurations of adsorbed reactants to be constructed. Such structural definition is not present in the polycrystalline powder samples used in most EPR studies of catalyst surfaces. Zomack and Baberschke [34] have recently reported a successful attempt to observe EPR spectra from submonolayer quantities of a radical adsorbed on a single-crystal metal surface prepared and characterized by surface analytical methods in ultrahigh vacuum (UHV). These authors constructed a UHV chamber in which an Ag(ll0) crystal could be cleaned and characterized, dosed with adsorbate at a known concentration, then lowered into a quartz section mounted in an EPR cavity for measurement of EPR spectra. Adsorption of small amounts of NO2 onto the clean Ag(ll0) surface gave no detectable EPR signals. This can be understood in terms of coupling of the magnetic moment of the unpaired electron on NO2 with conduction electrons of the metal. Similar problems were encountered in earlier attempts to observe radicals on clean metal films [35]. If, however, the Ag(ll0) surface was first dosed with several monolayers of krypton, EPR spectra could be clearly seen from NO2 molecules subsequently adsorbed. Figure 7 shows spectra of N O 2 o n thin krypton layers on Ag(110) at 20 K. The crystal surface was oriented parallel to the static field direction. At low coverage, the spectrum (Fig. 7, curve a) is consistent with NO2 molecules being oriented in the plane of the surface. As the coverage is increased (Fig. 7, curve b) additional features appear, indicating the onset of random three-dimensional orientation; the spectrum approaches that of NO 2 matrix isolated in Kr (Fig. 7, curve d). Figure 7 (curve c) is the spectrum obtained from a high (multilayer) coverage of NO2

Adsorbed species

Surface

E (kJ mol ~)

Ref.

0 2 Oz O2 O2 NO2 DTBN a CO 2 C6H ~-

SiO 2 Ti/Vycor Ag/Vycor Co/AI20 3 Vycor SiO2 NaX HY

2.4 2.0 24 2.0 2.0 7.6 16.5 9.2

[33] [38] [-32] [39] [40] [413 [42] [43]

aDTBN = di-tert-butyl nitroxide.

this time scale down to the millisecond regime, and showed that 0 2 adsorbed on Ag + sites on a Vycor-supported silver catalyst undergoes a much slower reorientational diffusion than 0 2 on silica gel [333. A similar conclusion was reached by Howe [6] from saturation transfer studies of O ] on ZnO. The activation energies in Table 1 are broadly speaking consistent with qualitative expectations. On surfaces where there is no strong specific interaction with an anchoring site, the adsorbed radicals undergo relatively free diffusion. In zeolites, mobility is restricted by the zeolite channel structure; on supported metal oxide catalysts the metal ion sites anchor adsorbed species much more strongly, and mobility may be detected only on the saturation transfer time scale or (as in the case of O f on silica-supported molybdena E63) not at all. Quantitative determination of correlation times and hence activation energies from either conventional CW EPR or saturation transfer EPR spectra requires assumption of a motional model and computer simulation of observed line shapes. The growing availability of pulsed EPR techniques means that direct measurements of surface diffusion can now be contemplated, eliminating the problems associated with interpreting CW EPR line shapes. There is a need for reliable quantitative data on mobilities of species adsorbed on catalyst surfaces.

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<4/> 3040

3140

HIG

3240

Fig. 7. EPR spectra of NO2 adsorbed on Kr on Ag(ll0) at 20 K: curve a, 0.01 L NO2 on 10 L Kr; curve b, 1 L NO2 on I00 L Kr; curve c, 100 L NO2 on clean Ag(ll0); curve d, 0.1 L NO2 in matrix of 104 L Kr (1 L = 1 Langmuir). (Reproduced with permission from Ref. [34].) on Ag(1 10) without preadsorbed Kr. Under these conditions, a signal is detected, but the broad lines suggest a randomly oriented film. The importance of the Zomack and Baberschke experiment is that it demonstrates the feasibility of studying orientations of adsorbed radicals on welldefined single-crystal surfaces, and of combining EPR spectroscopy with established surface science techniques. The problem of coupling to the conduction electrons means the technique will have limited interest as far as metal surfaces are concerned. The potential, however, for studying radicals adsorbed on well-defined oxide or semiconductor surfaces in this manner offers many new possibilities in the area of catalysis. Another class of single-crystal surfaces amenable to study by EPR is represented by zeolites. Many EPR studies have been reported of radicals and transition metal ions in zeolites, but these have used polycrystalline samples in which the structural definition and orientation of the zeolite lattice is largely lost; the resulting spectra show the characteristic powder patterns of polycrystalline samples

from which principal components of g and hyperfine tensors can usually be extracted, but the orientations of the corresponding principal axes are unknown. Chao and Lunsford [36] have reported singlecrystal EPR spectra of Cu 2 ÷ ions in the zeolite chabazite. Chabazite occurs naturally as large single crystals several millimetres in size. Ion exchange of Cu 2+ into such crystals gave wellresolved EPR spectra measured as a function of crystal orientation; three magnetically inequivalent sites were determined and the location of these sites inferred. Chabazite has a relatively small channel size and is thus of little current interest from a catalytic viewpoint. Larger pore zeolites (ZSM-5; Y) can be grown synthetically to crystal sizes of at least several hundred microns [37]. The sensitivity of current EPR spectrometers (less than 1012 spins per Gauss) means that radicals or transition metal ions in such synthetic zeolite single crystals should be observable. Manipulating and orienting submillimetre crystals in the magnetic field will not be trivial; this is nevertheless an experiment which needs to be done, and which will open up many new applications of EPR spectroscopy to zeolite catalysis.

Concluding remarks This paper has attempted to convey the broad scope of EPR spectroscopy as a technique for studying catalytic surfaces. All the examples presented have utilized conventional CW EPR techniques. EPR spectroscopy is now undergoing a renaissance as pulsed Fourier transform methods have become commercially available. There are also exciting new developments in areas such as high field EPR spectroscopy. The application of these new methods to catalytic surfaces offer many new and different opportunities to understand the origins of catalytic activity.

References 1 P.E. O'Reilly, Adv. Catal., 12 (1960) 31. 2 M. Che, NATO Advanced Study Institute Ser. C, 61 (1980) 79.

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