Electron spectroscopy applied to the study of reactivity at metal surfaces — A review

Surface Science 0 North-Holland

63 (1977) 291-314 Publishing Company

ELECTRON SPECTROSCOPY APPLIED TO THE STUDY OF REACTIVITY METAL SURFACES - A REVIEW

AT

Richard W. JOYNER School of Chemistry, University of Bradford, Bradford ED 7 1 DP, England

Photoelectron spectroscopy (PES) has become an important tool in the study of surfaces and the solid state. This review discusses information obtained on the interaction of gases with metal surfaces. The fundamentals of electron spectroscopy are briefly reviewed and experimental methods outlined. Approaches to the interpretation of both X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectra (UPS) are discussed. The chemisorption of carbon monoxide is examined in detail, PES distinguishes two situations, where CO retains its molecular identity and where dissociation occurs. Bonding of the molecule is considered, as are factors affecting dissociation. Some XPS investigations of metal oxidation are examined and the significance of the several oxygen 1s peaks observed is discussed. The adsorption and decomposition of organic molecules has been studied primarily by UPS. Several investigations are summarized, and the decomposition of formic acid considered in detail. The application of electron excited Auger electron spectroscopy is briefly discussed.

1. Introduction Photoelectron spectroscopy (PES) has, over the last five years been applied to the study of a substantial number of chemisorption systems and gas/metal interactions. The purpose of this review is to consider the extent and degree to which PES has advanced our knowledge of the adsorbed state and of surface reactivity. In the photoelectron experiment a sample, which may be gaseous [ 11, liquid [2] or solid [3], is exposed to a beam of monochromatic electromagnetic radiation. When the photon energy of the radiation exceeds the work function of a solid sample (ionization potential for a gas) electron emission occurs [4] and the photoelectron spectrum is a plot of number of electrons versus electron energy. A typical photoelectron spectrum thus consists of discrete peaks superimposed on a background of electrons which have suffered inelastic scattering, and secondaries. The peaks correspond to occupied valence and core levels in the solid, and the kinetic energy may be approximated by KE=hv-En,

(la)

where hu is the phtoton energy and E, is the electron binding energy. For a core electron level the binding energy (referred to some suitable reference zero) is a reflection of the electronic environment of the atom which is related to 291

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/ Electron

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its involvement in chemical bonding. For valence levels the photoelectron spectrum would, in the absence of complicating factors reflect the occupied density of states of the solid. Both core and valence level spectra can be changed by chemisorption or surface reaction. The information available from PES is thus, potentially of the greatest possible interest and has resulted in a rapid growth in its application to many areas of chemistry and physics as well as surface science. Eq. (la) is, unfortunately, only approximately true and we must now consider some of the complicating factors involved. In most electron s?ectrorneters for the study of solids and surfaces the reference zero for both kinetic energies and binding energies is taken to be the Fermi level of the sample, which is held in electrical contact with the spectrometer. Measured kinetic energy and binding energy are then related by KE=hv-Es-&,

(lb)

where &p is the work function of the spectrometer. Determination of this quantity is tedious and electron spectrometers are usually calibrated by studying samples of known binding energy. Thus, referred to the Fermi level, the binding energy of the 4f,,, level of clean gold is accepted to be 83.8 f 0.1 eV, while that of copper 2p3j2 is 932.8 eV [.5]. The remaining complications are due to the physics of the photoemission process, and mention must be made of relaxation, spin-orbit splitting, spin-spin or multiplet splitting, shake up processes and final state (direct transition) effects.

2. Relaxation During the photoemission process the remaining passive electrons in the atom, molecule or solid are polarized towards the electron hole. This increases the kinetic energy of photoelectron, resulting of course in a decrease in the measured binding observable energy. The relaxation energy E, is, in principle an experimentally quantity in the gas phase, through the operation of the “Lever Rule” [6] relating the core electron binding energy to the photoelectron spectrum. In the solid state the existence of extrinsic electron loss features is likely to vitiate the application of the Lever Rule and ER cannot be calculated from experiment. Relaxation energies have been considered by Shirley [7] who identifies two components: ER = EA •tEEA.

E, allows for relaxation of electrons in the photoemitting atom, and is called the atomic term and EEA, the extra-atomic term, involves relaxation of the surrounding atoms in a solid. Gadzuk [8] has treated relaxation in the adsorbed phase using the classical theory of the image charge and suggests that the extra-atomic

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293

relaxation due to adsorption may be l-3 eV, and also notes that shifts within the same principal quantum number should differ by <20%. In surface studies the absolute magnitude of the relaxation shift is of little interest, of greater importance, however is the quantity A.!?n the change in relaxation between the molecule in the gas phase, and the molecule in the adsorbed layer. We return to this point below.

3. Spin-orbit

and multiplet

splittings

Removal of an electron from a completely filled p shell gives rise to two possible ionic states, with J = l/2 or 3/2. These states can have quite different energies, and a doublet, intensity ratio of components 2 : 1, arises in the photoelectron spectrum. Similar splittings result for d and f orbitals. Photoionization from a paramagnetic species can result in two states of different energies, if the ion has 2 unpaired electrons. This is referred to as spin-spin or multiplet splitting.

4. Electron shake up In the photoemission process a valence electron may sometimes be excited to a “higher” energy level (lower binding energy) and the energy of this excitation is lost to the photoelectron. This process, referred to as shake-up results in a peak in the photoelectron spectrum removed by a few electron volts from the main photoelectron peak, and at higher binding energies. Fig. 8 shows the shake up phenomenon in the 2~3,~ spectrum of copper oxide (CuO). The intensity of the shake up process can be large in the solid state and selection rules for shake up processes are a matter of much interest [9]. The occurrence of shake-up can have diagnostic value, the shake-up features prominent in copper(I1) compounds, for example (fig. 8) are not observed in copper(I) compounds or copper metal [lo].

5. Final state effects When photoemission occurs within a crystalline solid the spectrum represents a coupling between the initial state (band structure), the photon field and the final state. For excited states energies near the Fermi level the intensity of photoemission is thus strongly modulated by variation in the density of allowed final states and the need to conserve the wave vector k of the photoelectron. The consequence of this is a strong dependence of valence band photoemission on photon energy, as shown in fig. 1, This dependence becomes less severe at higher energies.

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294

spectroscopy

hY I eV) i 90

N(e)

a

L

OeV

Energy

Fig. 1. The valence band photoelectron Freeouf, Erbudak and Eastman [ 791.)

spectrum

of gold at different

photon

energies.

(After

6. PES as a surface technique Photoelectron spectroscopy, Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) all owe their surface sensitivity to the very strong nature of the interactions between electrons and matter. Thus an electron with 50
7. Experimental

aspects

The experimental aspects of PES will be considered only very briefly with reference particularly to photon sources, types of analyzer and the influence of experimental geometry. Most experiments are carried out in commercially manufactured equipment [ 121 and the importance of using ultra-high vacuum techniques @ < 1 x 1o-8 P a ) m conjunction with PES for the study of surfaces is now reasonably well recognized. An important trend, which should continue, is the application of PES in conjunction with other surface techniques such as LEED, flash desorption and work function measurements.

8. Photon sources The information obtained in the PES experiment depends critically on the photon energy employed. Two regimes may be identified, ultraviolet photoelectron

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295

1JleV

;:; 1L85

IL87

‘--&

Fig. 2. The intrinsic line width of the aluminium I(, X-ray source. (After Siegbahn et al. [ 31.)

spectroscopy (UPS) referring to energies below -50 eV and X-ray photoelectron spectroscopy (XPS) to photon energies in excess of 1 keV. Gas discharge lamps provide UPS photons, the helium lamp with lines at 21.28 and 40.8 eV being most popular. The line width is narrow (
9. Electron analyzers Photoelectron spectroscopy requires an analyzer of high resolution and adequate transmission and two designs currently enjoy popularity. In Europe the concentric

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BE Fig. 3. Angular resolved photoemission from direction. (After Lloyd, Quinn and Richardson

a (100) (181.)

copper

crystal

along

the

i?% symmetry

henlispheri~~ analyzer (CHA) predo~nates [16] while in the tJSA the double pass cylindrical mirror (DPCMA) is favoured tl7J. No detailed comparisons are available, but is seems likely that the performance of the two designs is similar.

10. Experimental geometry Physical considerations require that ultraviolet photoemission from a metal single crystal, with or without an adsorbed layer, should be a very sensitive function of the polar and azimuthal angle of electron emission. Such angular studies are now being carried out by many groups and do show the expected angular sensitivity. Fig. -3 gives angular resolved photoe~n~ssion from a clean (100) copper surface along the I’X symmetry direction [ 181. Such studies of angular resolved UPS are likely to become very important in the future, they are currently, however, few in number and we shall mainly here concern ourselves with angular integrated studies. Angular dependent XPS studies [19] are tractable at a much less sophisticated level. No dependence on azimuthal angle is assumed, and intensity is considered to depend on the path length of the photoelectron through the solid. Thus, bulk sensitive peaks are severely attenuated on approaching grazing exit angle from the normal, whereas peaks from an adsorbed monolayer suffer much less attenuation. Such angular studies are thus of use in separating surface species from incorporated layers.

11. Interpretation of photoelectron spectra We can now examine in detail some PES studies of chemisorption systems; it may initially be of value though, to look at the various approaches which are currently in use to the discussion of photoelectron spectra for surface studies.

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291

11.1. UPS Three distinct approaches can be discussed in this area, all being subject to the requirement of consistency with information from other techniques. The simplest method is merely to use the UPS spectrum as s fingerprint, conclusions being reached by comparison of spectra generated in comparable situations. This approach has a number of pitfalls, thus surface oxygen and carbon each tend to produce a broad resonance at 5-8 eV binding energy. More disturbing is the case of oxygen on polyc~stalline platinum where no change is observed in the He1 or He11 spectrum [203, although XPS indicates substantial oxygen adsorption 12 l] . Probably the most common approach in the discussion of UPS of adsorption systems involves comparison of chemisorption levels with the gas phase spectrum of the relevant molecule. Gas phase IIeI spectra are available for most simple molecules, many being included in the compilation of Turner et al. [lb]. Gas phase spectra, however, use as a reference the potential energy of an electron at rest in vacuum at infinity, and not the Fermi level of the spectrometer, and this difference in energy reference levels has the form of a work function (r$). Much has been written [22] about the correct choice of this term and no consensus has yet emerged. It can probably be agreed, however, that, at least for a given adsorbent on a given singIe crystalline adsorbate, Qthas a constant value. It therefore differs from the instantaneous work function during adsorption which is, of course, coverage dependent. The difference between gas phase and adsorbed phase binding energies can therefore be written, following Demuth and Eastman [23] as ABE = BE(s) - BEt,d,t = # + UK

+ AEn,

where AER is the change in relaxation energy between the two states, and which will be due in large part to differences in extra-atomic relaxation. A!7n is the change in binding energy due to changes in chemical bonding and is the quantity of ultimate interest. The practise of compa~ng adsorbate levels with gas phase spectra has been criticized by Quinn [24] but in the case of physisorption and condensation at least seems to be well justified. Thus Yu et al. [25] have examined a number of condensed molecules on an MoSz substrate. In the condensed phase it is assumed that aEn = 0 and therefore changes are due to the ($ t A!?n) term. Yu et al. showed that these changes were constant for all orbitals within any one adsorbent. The conclusion, although lacking in theoretical justi~cation can be extended even to core levels. Table 1 shows values of A(BE) for core and valence levels for a number of condensed and physisorbed species and it can be seen that differences are small, showing no systematic trend. In dealing with chemisorbed species the assumption of AEn = 0 can sometimes be applied to certain of the orbitals which may, on chemical grounds be expected not

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298

Table 1 Binding energy differences for gas phase and physisorbed phases for core and valence molecules condensed on nickel or copper films at -80 K (data from Brundle [26]). Molecule

levels;

A(BE) (eV) Core levels 6.6

H20

Valence -6.0

6.2

6.5

(32

c

1s

6.6

6.5

Physisorbed

0 1s

6.9

co2

c

1s

5.1

Multilayers

0 1s

5.8

332

s

2P

6.9

0 1s

6.8

H2S

levels

5.3

7.0

participate in the bonding [23]. The justification of this procedure is pragmatic only, and it must be applied with care. The remaning approach to UPS spectra concerns comparisons with calculations, which may be of varying degrees of sophistication. The ideal calculation deals with the band structure of the metal, the adsorbed layer, includes both initial and final state, and will be suitable for comparison only with the angle resolved experiment. Such calculations have not yet appeared although work is in progress among theoreticians. to

11.2. XPS The main effort in discussing XPS spectra concerns core electron binding energies of both adsorbate and adsorbent. Considering the adsorbate first, simple molecules such as 02, CO, C02, H20, H,S, nitrogen containing compounds and organic species have been most popular. The core levels involved have therefore been oxygen Is, carbon Is, sulphur 2p and nitrogen Is. Table 2 shows the range of binding energies (referred to the Fermi level) which have been reported, in each case a spread of 7-9 eV is observed, quite similar to the range of binding energies noted for free molecules. In the molecular case change in binding energy has been quite successfully correlated with the changing charge on the atom involved, [la], increasing negative charge resulting in a decrease in the core electron binding energy. In the condensed phase no quantitative attempt has been made to relate charge and binding energy, although interrelationship is often assumed [30,3 I]. Another empirical correlation, which seems to be of quite general validity relates decreasing binding energy with increasing heat of adsorption. Thus the high binding energy species in table 2 are all desorbed

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299

Table 2 The range of core electron binding energies observed in surface studies Level

Range (eV)

System

Oxygen 1s

531

r-CO on molybdenum [27] Oxygen on lead [ 281 Condensed CO2 on Ni 1261 “Carbide” carbon [29] Condensed H2S [26] Sulphided lead (281 Condensed NO (261 NO and NO2 chemisorbed on Ni [26]

-528.5 Carbon 1s Sulphur 2p Nitrogen 1s

290 -283 163.9 - -160 -405 -391.8

below 295 K while the low binding energy relate to stable, often refractory species. The generality of this observation suggests that trends in BE can be associated predominantly with changes in EB although of course trends may also occur in the ER term of eq. (3). The relationship between BE and heat of adsorption is not linear, marked changes in BE often reflecting only small changes in AHads when the absolute value of this quantity is small. When the heat of adsorption is large (>150 kJ mol-‘) the binding energy often seems insensitive to changes in the heat of adsorption [32]. Attention is also paid to changes in the binding energy of the substrate during adsorption, as will be seen these are of particular interst in the study of metal oxidation. 12. Interaction

of carbon monoxide

with metals

The UPS spectra of carbon monoxide adsorbed on metals fall into two distinct types, as shown in fig. 5. In the first, of which nickel has been most thoroughly studied [29,33] there is a similarity between the adsorbate resonances and the gas phase spectrum of CO, which has been often remarked. Similar spectra are noted on platinum [20], copper [29,34], iron [35], ruthenium [36], the virgin state on molybdenum [27] and the weakly held (Yand y states on tungsten [37] and molybdenum [27]. In the second class, which comprises the P states on W [37,40] and MO [28] as well as iron [35], nickel [29,38] and titanium [39] none of the peaks which are similar to those of gas phase carbon monoxide are present, instead there is a broad peak 6-8 eV below the Fermi level. The two classes are also differentiated by their core level binding energies, in the first case the oxygen 1s binding energies fall within the broad range 531.7-537 eV, and the carbon 1s values are -286-290 eV. In the latter instance the oxygen 1s values cluster at -530.0 eV and the carbon 1s values at -283 eV. From these results the identification of the surface species is comparatively simple and certain. In the first case the similarity to the gas phase spectra supports the

300

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classical interpretation of carbon monoxide as molecular (associative) with bonding to the metal surface through the carbon atom. In the second class, which is confined to examples ofhigh heat of adsorption (AHads > 200 kJ mall’) it is clear that the molecular identity of the adsorbate has been lost. We have noted that both carbon and oxygen at the surface generate broad UPS peaks in the 6-8 eV binding energy range. The XPS peak positions confirm that the surface can be regarded as M + C + 0 rather than M-CO. The oxygen Is value, at 530 eV is in the same position as chemisorbed oxygen [41]. More significantly the carbon 1 s binding energies, at -283 eV are in similar positions to those of the metal carbides [42]. This assertion of dissociative adsorption based on PES results was rather novel since, until 1972 it had been generally considered that CO adsorption was invariable molecular. The evidence for this has been discussed by Comer [43] and King [44] who had tentatively proposed that P-CO on tungsten might be dissociative. Both UPS and XPS results suggest that any residual bonding between carbon and oxygen at the surface is weak and this is consistent with observed isotopic mixing [45]. Recently Fuggle and Menzel [46] have shown that the oxygen KLzL2 Auger shape, which is expected to be very sensitive to chemical environment, in the &CO state on tungsten is identical with that for chemisorbed oxygen and quite different to that from molecular carbon monoxide.

13. Molecularly

adsorbed CO

The analysis of the UPS spectrum of molecularly adsorbed CO is the subject of discussion. Originally Eastman and Cashion [33] assigned the two prominent peaks to the 50 and Irr orbitals of the free molecule, based on comparison with the gas phase spectrum; this assignment was followed in subsequent studies [29,33]. In 1974 several workers questioned this assignment, based on the failure to observe the 4u orbital, even using helium II (40.8 eV) excitation [47]. A new assignment was suggested by Plummer, and another by Lloyd, based on comparison with UPS of metal carbonyls. There have been numerous calcuations [48], both empirical and ab-initio of the energy levels in Ni-CO clusters and a detailed account of the discussion is given in the excellent review by Bradshaw et al. 1491. It is generally accepted that the original assignment is incorrect and it seems likely that angular resolved UPS may generate a definitive answer to the problem. From a reactivity viewpoint the question is of limited interest, although it is clear that the 5a level - the lone pair of electrons projecting away from the carbon atom ~ is strongly perturbed in chemisorption. The analysis of the UPS spectra has concentrated attention on the u contribution to the M-CO bond as reflected by changes in the 50 orbital. In metal carbonyl from metal to vacant rr* chemistry importance is also given to “back-bonding” ligand orbitals. Thus the zero valent metal state is more effectively stabilized by ligands such as CO and (Y--G dipyridyl which have suitable acceptor orbitals, com-

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301

pared to ligands like ethylenediamine where no acceptor orbitals occur [50]. In chemisorption also, the metal-CO bond is strengthened (increasing AHa+_) where the Fermi level crosses the d band compared to copper, silver and gold, where the d band is full. The calculations of Doyen and Ertl [48] suggest that u bonding accounts for only -50 kJ mol-’ of the heat of adsorption of CO on palladium or nickel. Joyner and Roberts [30] have invoked r-r bonding to explain the relationship between oxygen 1s binding energy and heat of CO adsorption, which is shown in fig. 4. Back bonding is expected to increase the negative charge on the oxygen atom (which is likely to be relatively unaffected or reduced by (Tbonding) and thus reduce the binding energy, as is observed. Back bonding is likely to cause changes in the UPS near the Fermi level, which are notoriously difficults to interpret, it should however be clearly observable in XPS core level binding energies. Following Brundle [26], we apply eq. (3) in an attempt to extract and compare values of A.!?n. We take the lower peak in the He11 spectrum to be the 40 orbital, substantially unperturbed in the chemisorption bond, therefore with aEn = 0. Further it is assumed that, as was demonstrated for the condensed phase, AEn (and of course 4) is the same for both core and valence levels, and we can therefore calculate AEn for the oxygen 1s level. For the cases of strong molecular chemisorption, nickel, platinum, palladium, ruthenium and iron, with similar values of heat of adsorption, A./?‘uvalues are all about +2 eV. For the weakest chemisorption studied, y-CO on molybdenum, AEn by contrast is -3.2 eV and for the cases of intermediate heat of adsorption (o states on tungsten, the virgin state on molybdenum, and copper), the Eu values are in the range -0.4 to t1.0 eV [Sl]. We therefore believe that the decrease in oxygen 1s binding energies reflects increasing metal to ligand

BE

eV 537

535

0

Pt.

0

NI.

Cl

41-w

A

*2-W

0

Gu

v

X-M0

q 533

0

#J,

‘.

531

100

200

, ‘.

\

300

‘%ds kJ mol

Fig. 4. Relation between oxygen (After Joyner and Roberts [ 301.)

1s binding

energy

and heat of carbon

-1

monoxide

adsorption.

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R. W. Joyner /Electron

spectroscopy

donation and this is supported by IR stretching frequencies for the chemisorbed molecules [52]. The positive values of AEn noted for nickel, etc. are consistent with charge transfer to the adsorbed species. In contrast to the systematic trend observed for AE, no regular variation occurs in the @+ AE, term, and this accounts for the failure of Yu et al. [53] to correlate UPS spectra with heat of adsorption.

14. Factors affecting carbon monoxide dissociation Following Joyner and Roberts [30] we have noted that dissociation of the molecule is associated with a threshold heat of adsorption of about 260 kJ mol-’ at 295 K. We can now consider thermodynamic and kinetic factors which affect the occurrence of dissosiation. From a thermodynamic standpoint it appears that the metal must be capable of forming a stable carbide. Thus carbon monoxide can be dissociated by nickel [29,38] but not by platinum where desorption occurs exclusively [S4]. When the thermodynamic condition is fulfilled there remains kinetic control, for each metal there is a threshold temperature below which dissociation is very slow. For tungsten and molybdenum this threshold is below room temperature and for nickel it is between 370 and 420 K [29]. The case of iron, as discussed by Kishi and Roberts 13.51 is particularly fascinating, and is shown in fig. 5, where dissociation commences, slowly at 295 K. The dissociation threshold temperature shows an inverse relation to heat of adsorption which can be understood by reference to fig. 6. The activation energy for dissociation can be calculated from the threshold temperature, assuming first order kinetics and a standard pre-exponential factor (1013 s-l). The activation energy E,, and the time for half reaction, tl12, at temperature T are related by (4) For carbon monoxide on iron eq. (4) suggests an activation energy for dissociation of about 90 kJ mol-r, compared with a value of about 120 kJ mol-’ for nickel. In the case of molybdenum the threshold temperature [27] is below 170 K, indicating an activation energy of <50 kJ mol-‘, and a similar value is expected for tungsten. Kishi and Roberts [35] have shown that the activation energy for dissociation can be increased by the presence of impurity at the surface. Thus preadsorption of sulphur by iron prevents the dissociation of molecularly chemisorbed carbon monoxide, at 298 K. The dissociation of carbon monoxide at metal surfaces is of catalytic significance, particularly in the Fischer-Tropsch process: CO + H2 + C2-C2e

hydrocarbons

and alcohols.

(5)

R. W.Joyner /Electron spectroscopy

303

BE

Fig, 5. Helium II spectra for carbon monoxide adsorbed on iron. Curve 1: clean iron; curve 2: iron + CO at 80 K (molecular adsorption); curve 3: iron + CO at 295 K after 1000 s (molecular and dissociated CO present); curve 4: iron + CO at 350 K (wholly dissociative adsorption). (After Kiski and Roberts [ 351.) Fig. 6. The activation barrier for carbon monoxide dissociation.

Effective catalysts are iron, nickel cobalt and ruthenium and adsorptiun is expected to be dissociative at the temperatures (420-570 K) and high pressures used.

15. Interaction with oxygen Fig. 7 shows the change in the oxygen Is signal from the surface of a copper (100) single crystal during exposure to oxygen [55]. There is no change in the peak shape, other than a small increase in binding energy, as the oxygen coverage increases. The copper core levels are unchanged in peak position, decreasing in peak height by -10% during oxygen uptake. The low energy electron diffraction pattern, by contrast shows several distinct changes, from the (I X 1) clean surface mesh to an “oblique” structure, to (t/‘2 X v/2)R45” and finally, above 2000 L exposure to (42 X 2t/2)R45”. We h ave argued that the first two adsorbed structures reflect chemisorption of oxygen only, without movement of surface metal atoms, and that the final (42 X 242)R45” mesh is due to reconstruction. The XPS spectra appear to be insensitive to this important change in surface structure. The distinction between the chemisorbed layer and the reconstructed, NiO layer which is well established from LEED studies, has also been studied by XPS. Norton and Tapping [56] observe only small changes in the shape or positions of the oxygen 1s peak during exposure of a nickel film to oxygen, but note that changes commence in the Ni 2p,,* spectrum at the exposure (-10 L) where oxide nucleation starts. Between 12 and 18 L exposure the Ni 2p3j2 peak is broadened and attenu-

304

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5f6 Fig. 7. Oxygen 1s spectra (After Braithwaite, Joyner

570

observed during adsorption and Roberts (551.)

spectroscopy

y

BE

eV

of oxygen

on copper

(100)

at 295 K.

ated, and there is marked attenuation of the associated shake up feature. Norton and Tapping argue that these changes are a consequence of the formation of a semiconducting layer. Studies of nickel oxidation are complicated by the observation of a second oxy gen 1s peak, with binding energy of -53 1.4 eV. This peak has also been observed in the oxidation of polycrystalline zinc [57] and lead samples [Sg]. There exists, as yet, no consensus regarding its origin, possibilities include chemisorption on the oxide [59], “true” chemisorption [60], incorporation [57] and formation of Ni203 [61]. The following pieces of evidence may be relevant:(1) Norton reports [56] that, although it may constitute 30-50% of the adsorbed oxygen on unannealed nickel films it represents not more than 10% of adsorbed oxygen on well annealed polycrystalline foils. Similarly it cannot be detected, at 300 K on an annealed single crystal of nickel. Surface heterogeneity thus appears to have an important role. (2) Ion bombardment of oxidized nickel initially reduces the size of the higher binding energy peak, and not that of the peak at 529.7 eV. This might support chemisorption on the oxide, but is not in keeping with Briggs study of XF’S angular

R. W. foyner

9?9

L

/Electron

911

9?3

305

spectroscopy

I

945

919

ev

Binding Energy

a

(ii)

-6

920

916

,

1

912

t

*

908 ev

Kinetic Energy b Fig. 8. (a) Copper 2p3/2 peak; curve (i): clean copper; curve (ii): copper (100) after exposure to 5 Torr of oxygen at 295 K; curve (iii): copper (II) oxide. (b) The copper L23 W Auger peak generated by X-ray bombardment; as (a). (After Braithwaite et al. [55).)

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R. W. Joyner /Electron

spectroscopy

dependence at zinc [57], where the 531.5 eV peak showed a more “brllk-like” dependence than the 530 eV peak. (3) On nickel the 531.5 eV peak is more prominent at 400 K than at 300 K, this probably excludes chemisorption on the metal [60], which is unactivated but might support chemisorption on the oxide which could well be activated. (4) On nickel the higher binding energy peak is more prominent during adsorption at 80 K than at 29.5 K [56,60]. Oxygen is also observed at higher binding energies on copper at 80 K [SS]. (5) On a lead surface after prolonged oxidation the 532 eV peak is much larger than that at 529.5 eV [58], in which case oxygen chemisorption on either metal or oxide would seem to be excluded. Evans et al. have, however, discussed the possibility that the higher binding energy peak may reflect surface contamination. (6) Although the higher binding energy peak on nickel could be due to NizOs no equivalent oxide can be postulated for zinc. For lead, although higher oxides exist it seems probable that their binding energies are not very different from PbO [62]. This diversity of evidence precludes a single unifying conclusion. It seems probable that more than one type of adsorption, or adsorbed species can give rise to an oxygen 1s peak at 531.5-532.0 eV binding energy. In particular low temperture adsorption (<298 K) and high temperature adsorption can be distinguished. It also seems likely that combined LEED/PES studies could be of value. Oxidation of copper beyond the monolayer stage raises the need to distinguish between copper (I) oxide (CuaO) and copper (II) oxide (CuO). As we have noted above the appearance of shake-up satepites close to the copper core levels is characteristic of the presence of Cu2+ and such satellites do emerge after the oxidation of Cu(100) in 5 Torr of oxygen at 295 [55] (fig. 8). The Auger spectrum (excited by X-ray bombardment) however shows a peak at 915 eV, kinetic energy, compared with 918 eV which would be characteristic of pure copper (II) oxide. This difference in kinetic energy suggests that copper (I) oxide is the major oxide present, and the copper (II) oxide component of the surface can be decomposed into CuO (+OJ by heating to 520 K in ultrahigh vacuum. In the above discussion we have concentrated on XPS studies of metal oxidation and oxygen chemisorption. There have also been UPS studies of the oxidation of nickel [56], copper [63], silver [63], gold [63], tungsten [40], lead [58], zinc [57], cesium and strontium [64]. The UPS spectra provide a sensitive fingerprint of the surface, but are often difficult to discuss at a more sophisticated level.

16. Interaction of organic molecules with metal surfaces The study of the interaction of organic molecules with metal surfaces is attractive since there are often several paths open to the reactant molecule, interest also being stimulated by the possibility of relating the results to heterogeneous catalysis. The first published work [23] concerned the condensation and adsorption of

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301

benzene, ethylene and acetylene on a (111) nickel surface. Eq. (3) was applied to the calculation of bonding shifts, assuming that AEn was the same for all valence orbitals and heats of chemisorption were calculated from Grimley’s theory [65]. The values deduced, ethylene = 13.5 kJ mol-‘, benzene = 340 kJ mol-’ and acetylene = 490 kJ mol-’ are in the expected order, and in agreement with the observed dehydrogenation of ethylene to acetylene at the nickel surface. The dehydrogenation of ethylene had previously been noted in exchange studies and by LEED [65]. Egelhoff, Linnett and Perry have studied the interaction of methanol and formaldehyde with the (100) phase of tungsten [64] at 310 K. At exposures below 1 L. the two molcules generate similar UPS spectra, which were interpreted as decomposition into “/3-CO” and hydrogen. As noted above, the fl state of carbon monoxide on tungsten is itself dissociated. At higher coverages dissociation ceases, adsorption of both methanol and formaldehyde becoming molecular as indicated by comparison with gas phase spectra. The angular dependence of the methanol spectra were also shown to be consistent with molecular adsorption. Mason et al. have examined the interaction of some fluoro-alkenes and alkynes with platinum (100) and (111) [67] using both XPS and UPS. In some cases adsorption was dissociative, with the halogen being desorbed from the surface, and the extent of dissociation was monitored by measuring, e.g., the fluorine Is/carbon 1s areas ratio. Adsorption of CHsC-CFs, CHa=CH-CF, and CF,=CFCl was non dissociative on both crystal planes. By comparison, vinyl fluoride (H,C=CHF) was 40% dissociated at Pt(lOO) and nearly 80% dissociated on the (111) surface. Vinyl chloride also, is extensively dissociated by the (100) plane. As was the case for carbon monoxide on iron [35] dissociation could be prevented by impurity adsorption, Mason et al. choosing to use carbon monoxide. It was suggested that dissociation occurred by intermolecular H-halogen elimination reaction and that only certain metal sites on the surface were active.

17. Decomposition

of formic acid

The following are the main features of a study of formic acid decomposition at copper, nickel and gold surfaces, which Professor Roberts and I have recently completed [68], using both XPS and UPS. The helium I spectrum of formic acid condensed on copper at 80 K is shown, together with the gas phase spectrum of Formic acid in fig. 9. A very good comparison exists except for the single broad peak at BE = 6.4 eV in the condensed phase, the gas phase equivalent having two well resolved peaks. The oxygen 1s peak for the condensed phase is also a broad singlet, compared to a doublet peak noted for gas phase acetic acid. We believe that both of these changes are a result of hydrogen bonding known to occur in the crystal structure of solid formic acid. On warning to 295 K marked changes occur in the UPS spectrum (fig. 9a) and in

308

He fl)

BEQfeV

a Hell)

0

5.0

10,O eV BEcFj

b Fig. 9. Helium I spectra. (a) Curve (i): clean copper; curve (ii): copper after exposure to formic acid at 1O-6 Torr and 295 K. (b) Curve (i): copper with formic acid condensed, at 80 K; curve (ii): gas phase formic acid. (After Joyner and Roberts (681.)

R. W. Joyner /Electron

spectroscopy

309

the oxygen 1s spectrum, which shifts over 2 eV to lower binding energy (531.7 eV) and decreases in width from FWHM = 3.4 eV at 80 K to 1.5 eV at 295 K. The changes in the XPS are helpful in diagnosing the fate of the adsorbed acid; the relative area of carbon 1s to oxygen 1s is unchanged on warming to 295 K, showing that the carbon/oxygen stoichiometry is still CO*. The marked decrease in the oxygen 1s peak width, however suggests that the two oxygen atoms become chemically very similar. Only two surface species are consistent with these requirements: adsorbed carbon dioxide and the adsorbed formate ion, HCOO-. We know that carbon dioxide does not chemisorb on copper at 295 K and it seems certain, therefore that the spectra are due to the adsorbed formate ion. Infrared studies [69], have also suggested the existence of the formate ion at metal surfaces, although at much higher gas phase pressures. The UPS binding energies observed at 295 K are compared with those calculated [70] for the formate ion, in fig. 10. There is a good correspondence between calculated and experimental values, particularly since we may assume that the three calculated orbitals at -4.65, -5.1 and -5.2 eV would appear as a single peak in the experimental spectrum. The existence of the formate ion is thus established at the copper and nickel surfaces (where the spectra are identical to those noted on copper). There is also evidence that the formate ion exists on gold, although only between 160 and 190 K. The UPS spectra and other evidence [71] suggest that decomposition into the ion BE e” (F’ o.o-

1.0 -

-

BP-4

EXPERIMENT Fig. 10. Comparison anion. (After Joyner

of experimental binding and Roberts [68].)

CALCULATED energies

with

those

calculated

for the formate

310

R. W. Joyner /Electron

spectroscopy

Table 3 Activation energy calculated for formate ion decomposition at the metal surface (kJ mol-‘) ___ .__.,.. _ ~_ ~. _ _ 115-130 90 SO- 60

Metai

__.__~ Copper Nickel Gold

Data from ref. [68] and references

Activation energy observed for catalytic decomposition over the metal surface [FICOOH -+ HZ + CO?] (kJ mol-‘)

.~ ~~ .-. .. . 100-I 8555.-

-. ~- - .---I

_~ ~~~-__

10 95 60

therein.

involves three steps: (1) Breaking of hydrogen bonds in the condensed layer. (2) Reorientation of the molecular skeleton from parallel to the metal to perpendicular to the metal surface. (3) Dissociation, releasing a proton. On each of the three metals studied increasing the surface temperature eventually results in the decomposition of the formate ion, with rapid desorption of the decomposition products and regeneration of a clean metal surface. The temperature at which the decottlposition rate becomes rapid on each metal allows the activation energy for decomposition to be calculated from eq. (4) (table 3). The calculated activation energies are in excellent agreement with those observed for the gas phase catalytic reaction: IICOOH --f H2 + CO:! over the three metals, showing quite conchtsively that deconlposition of the formate ion at the surface is rate determining. The decomposition of formic acid provides an opportunity to test the operation of an “electronic factor” which is often referred to in the earlier catalytic literature. The idea has been revived by Boudart (721, who suggests that the similar catalytic activity of platinum and tungsten carbide can be explained by their similar valence band photoelectron spectra. Applying this conception to formic acid decomposi-

Table 4 Activity

for HCOOH

Increase

1

decomposition

MetaI

Density

Gold Iron Silver Nickel Copper PIatinum

Low High Low High Low High

of states at Fermi level

R. W. Joyner / Electron

spectroscopy

311

tion, table 4 shows a number of metals in order of increasing activity, together with the density of the metal d band at the Fermi level, Clearly no obvious correlation exists, nor can one be found by considering the full density of states curves. Although electronic factors are of paramount importance in catalysis, as in all chemistry, their isolation may be, in general, a rather complex process.

18. Surface reactivity studied by Auger electron spectroscopy Core holes generated in a solid in the energy range of interest decay predominantly by Auger electron emission [73]. As we have seen the Auger spectrum can be very useful in PES studies. Core holes, and thence Auger emission can, however, be generated by electron bombardment, and electron-excited Auger electron spectroscopy (EE-AES) predates XPS in its application to the study of surface phenomena. Its use is confined mainly to elemental detection and analysis at the surface, being less powerful than PES for two main reasons: (a) The electron beam is usually damaging to the surface layer and may cause structural re-arrangement, desorption, adsorption or decomposition of the adsorbed species. (b) Chemical effects in AES are often more difficult to interpret than changes in an UPS or XPS spectrum. Changes in peak shape are as common as clearly observable shifts. The change in carbon KLL peak shape has diagnostic usefulness [74]. Electron excited AES has as its strengths, in relation to photoelectron spectroscopy, the ability to achieve reasonable signal strength either very rapidly, or from a very small area. The latter application is of particular interest in materials science, while the former is valuable in the study of the kinetics of surface reaction. A number of investigations in this area have been reviewed recently [73d], including the adsorption and reactivity of sulphur at platinum surfaces 1751, the oxidation of carbon at tungsten surfaces [76] and the desorption of nitrogen by tungsten [77]. To these should be added the determination of oxygen adsorption isotherms on tungsten 1781.

It is a pleasure to acknowledge guidance and support from, and many stimulating discussions with Professor M.W. Roberts. Financial support from I.C.I. Ltd. (Petrochemicals Division) is also acknowledged.

References [l] (a) K. Siegbahn et al. ESCA Applied 1969). (b) D.W. Turner

et al., Molecular

to

Photoelectron

Free Molecules Spectroscopy

(North-Holland, (Wiley, London.

Amsterdam, 1970).

312

R. W. Joyner / Electron spectroscopy

[2j K. Siegbahn,

J. Electron Spectr. 5 (1974) 3, 1059. [3] K. Siegbahn et al., Electron Spectroscopy for Chemical Analysis (Almqvist and Wiksells, Uppsala, 1967). [4] Discovered by Einstein in 1905. [S] M. Johansson, J. Hedman, A. Bendtsson, M. Klasson and K. Nillson, J. Electron Spectr. 2 (1973) 295. [6] R. Manne and T. Aberg, Chem. Phys. Letters 7 (1970) 282. [7] D.A. Shirley, J. Electron Spectr. 5 (1974) 135. [S] J.W. Gadzuk, J. Vacuum Sci. Technol. 12 (1975) 289. [9] M.A. Brisk and A.D. Baker, J. Electron Spectr. 7 (1975) 197; T. Robert and G. Offergeld, Chem. Phys. Letters 29 (1975) 606. [lo] DC. Frost, A. lshitani and CA. McDowell, Mol. Phys. 24 (1972) 861. [ll] See e.g. 1. Lindau and W.E. Spicer, J. Electron Spectr. 3 (1974) 409; or C.J. Powell, Surface Sci. 44 (1974) 29. [12] C.R. Brundle, M.W. Koberts, D. Latham and K. Yates, J. Electron Spectr. 3 (1974) 241. [ 13 ] T.A. Carlsson, Faraday Disc. Chem. Sot. 60 (1975) 30. [ 141 H. Ebel and N. Gurker, J. Electron Spectr. 5 (1974) 799; A.F. Carley and R.W. Joyner, unpublished results. [15] J.C. Fuggle, T.E. Madey, M. Steinkilberg and D. Menzel, Surface Sci. 52 (1975) 521. [ 16 J Manufactured by V.C. Scientific Ltd., and A.E.I. Ltd. [ 171 Manufactured by Physical Electronics Industries, Inc. [ 181 D.R. Lloyd, C.M. Quinn and N.V. Richardson, J. Phys. C. (Solid State Phys.) 8 (1973) L 371. [19] C.S. Fadley, J. Electron Spectr. 5 (1975) 725; Faraday Disc. Chem. Sot. 60 (1975) 18. [20] P. Biloen and A.A. Holscher, Faraday Disc. Chem. Sot. 58 (1974) 106. [21] R.W. Joyner, Faraday Disc. Chem. Sot. 58 (1974) 138. [22] For example see: (a) A.F. Carley, R.W. Joyner and M.W. Roberts, Chem. Phys. Letters 27 (1974) 580. (b) H.D. Hagstrum, Surface Sci. 54 (1976) 197. (c) Faraday Disc. Chem. Sot. 58 (1974) 90, R.W. Joyner, D. Menzel and others. 123) J. Demuth and D.E. Eastman, Phys. Rev. Letters 32 (1974) 1123. [24] C.M. Quinn, Faraday Disc. Chem. Sot. 60 (1975) 139. [25] K.Y. Yu, J.C. McMenamin and W.E. Spicer, J. Vacuum Sci. Technol. 12 (1975) 286. [26] C.R. Brundle and A.F. Carley, I:araday Disc. Chem. Sot. 60 (1975) 51. 1271 S.J. Atkinson, C.R. Brundle and M.W. Roberts, F’araday Disc. Chem. Sot. 58 (1974) 62. [28] K. Kishi and M.W. Roberts, J.C.S. 1:araday I, 71 (1975) 1721. [29] R.W. Joyner and M.W. Roberts, J.C.S. Faraday I, 70 (1974) 1819. (301 R.W. Joyner and M.W. Roberts, Chem. Phys. Letters 29 (1974) 447. (311 C.R. Brundle, J. Vacuum Sci. Technol. 13 (1976) 301. 132 1 K.W. Joyner and M.W. Roberts, Chem. Phys. Letters 28 (1974) 246. 1331 DE. Eastman and J.K. Cashion, Phys. Rev. Letters 27 (1971) 1520. 1341 H. Conrad, G. Ertl, J. Kiippers and E.E. Latta, Solid State Commun. 17 (1975) 613; C.K. Brundle, Faraday Disc. Chem. Sot. 60 (1975) 138. [35] K. Kishi and M.W. Roberts, J.C.S. Faraday I, 71 (1975) 1715. [36] J.C. Fuggle and D. Menzel, Surface Sci., 53 (1975) 21. [37] A.M. Bradshaw, D. Menzel and M. Steinkilberg, Japan. J. Appl. Phys. Suppl. 2 (1974) 841; Chem. Phys. Letters 28 (1974) 516. [38] D.E. Eastman, J.E. Demuth and J.M. Baker, J. Vacuum Sci. Technol. 11 (1974) 273. (391 D.E. Eastman and J.E. Demuth, Japan. J. Appl. Phys. Suppl. 2 (1974) 827. [40] W.F. Egelhoff, D.L. Perry and J.W. Linnett, Faraday Disc. Chem. Sot. 58 (1975) 35. [41] J.T. Yates, T.E. Madey and N.E. Erickson, Surface Sci. 43 (1974) 257.

R. W. Joyner / Electron spectroscopy

313

1421 L. Ramqvist, J. Appl. Phys. 42 (1971) 2113. [43] R. Gomer, Royal Society of London Discussion of Physics and Chemistry of Surfaces, London, 1972. (441 D.A. King, C.G. Goymour and J.T. Yates, Proc. Roy. Sot. (London) A331 (1972) 361. [45] T.E. Madey, J.T. Yates and R.C. Stern, J. Chem. Phys. 42 (1965) 1372. [46] J.C. Fuggle and D. Menzel, personal communication, and to be published. [47] Faraday Disc. Chem. Sot. 58 (1975) 134, discussion contributions by E.W. Plummer, D.L. Lloyd and others. [48] G. Doyen and G. Ertl, Surface Sci. 43 (1974) 197; G. Blyholder, J. Vacuum Sci. Technol. 11 (1974) 865; Surface Sci. 42 (1974) 865; I.P. Batra and 0. Robaux, J. Vacuum Sci. Technol. 12 (1975) 242; LP. Batra and P.S. Bagus, Solid State Commun. 16 (1975) 1097; L.S. Cederbaum, W. Domcke, W. von Niessen and W. Brenig, 2. Physik B21 (1975) 381. [49] A.M. Bradshaw, LS. Cederbaum and W. Domcke, in: Structure and Bonding, Vol. 24, Eds. J.D. Dunitz et al. (Springer, Berlin, 1975). [SO] L.F. Orgei, Transition Metal Chemistry (Methuen, London, 1965). [s l] Calcuiation of bEB for copper is difficult since ass~nment of the spectrum is in dispute; that of Conrad et al. [ 34 ] has been used. [52] A.M. Bradshaw and J. Pritchard, Proc. Roy. Sot. (London) A316 (1970) 169. [.53] K.Y. Yu, W.E. Spicer, I. Lindau, P. Pianetta and S.F. Lin, J. Vacuum Sci. Technol. 13 (1976) 277. (541 R.W. Joyner and M.W. Roberts, unpublished results. [55] M.J. Braithwaite, R.W. Joyner and M.W. Roberts, Faraday Disc. Chem. Sot. 60 (1975) 89. (561 P.R. Norton and R.L. Tapping, Faraday Disc. Chem. Sot. 60 (1975) 71, and discussion remarks therein. ]57] D. Briggs, Faraday Disc. Chem. Sot. 60 (1975) 81. [58] S. Evans and J.M. Thomas, J.C.S. Faraday Trans. II, 71 (1975) 320. [59] P.J. Page, D.L. Trimm and P.M. Williams, J.C.S. Faraday I, 69 (1974) 1769. [60] CR. Brundle, Faraday Disc. Chem. Sot. 60 (1975) 51, and discussion remarks therein. [61] K.S. Kim and N. Winograd, Surface Sci. 43 (1974) 625. [62] J.M. Thomas and M.J. Tricker, J.C.S. Faraday 1, 71 (1975) 336. [63] S. Evans, E.L. Evans, D.E. Parry, M.J. Tricker, M.J. Walters and J.M. Thomas, Faraday Disc. Chem. Sot. 58 (1974) 97; S. Evans, D.E. Parry and J.M. Thomas, Faraday Disc. Chem. Sot. 60 (1974) 102. [64] CR. Helms and W.E. Spicer,Phys. Rev. Letters 28 (1972) 565; 31 (1973) 1307; 32 (1974) 228. [65] G. Maire, J.R. Anderson and B.B. Johnson, Proc. Roy. Sot. (London) A 320 (1970) 227. [66] W.F. Egelhoff, D.L. Perry and J.W. Linnett, J. Electron Spectr. 5 (1974) 339; Faraday Disc. Chem. Sot. 60 (1975) 127. (671 T.A. Clarke, I.D. Gay and R.L. Mason, Chem. Phys. Letters 27 (1974) 172; 27 (1974) 562; Faraday Disc. Chem. Sot. 60 (1975) 119. [68] R.W. Joyner and M.W. Roberts, Proc. Roy. Sot. (London) A350 (1976) 107. [69] J. Fahrenfort, L.L. van Reyen and W.M.H. Sachtler, in: Proc. Symp. Mechanism of Heterogeneous Catalysis, Ed. J.H. de Boer (Elsevier, Amsterdam, 1960). [70] M. Barber, personal communication quoted in ref. 1681. (?l] M. Ito and W. Suetako, Chem. Letters (1973) pp. 757. [72] R.B. Levy and M. Boudart, Science 181 (1973) 547. [73] See e.g.: (a) C.C. Chang, Surface Sci. 25 (1971) 53; (b) J.C. Tracy, NATO Summer School, Ghent, 1972; (c) C.J. Todd, Vacuum 23 (1973) 195;

R. W. Joyner / Electron spectroscopy

314

1741 [75] [76] [77] [78] [79]

(d) R.W. Joyner and M.W. Roberts, in: Surface and Defect Properties of Solids, Vol. 4, Eds. M.W. Roberts and J.M. Thomas. (Chem. Sot., London, 1975) p. 68. T.W. Haas, J.T. Grant and G.J. Dooley, in: Proc. 2nd Intern. Symp. on Adsorption/ Desorption Phenomena, Ed. F. Ricca (Academic Press, London, 1973). H.P. Bonzel and R. Ku, J. Gem. Phys. 58 (1973) 4617;59 (1973) 1641; Surface Sci. 40 (1973) 85. Y. Vishwanath and L.D. Schmidt, J. Chem. Phys. 59 (1973) 4184. R.W. Joyner, J. Rickman and M.W. Roberts, J.C.S. Faraday I, 70 (1974) 1825. U. Banninger and E.B. Bas, Surface Sci. 50 (1975) 279. .I. Freeouf, M. Erbudak and DE. Eastman, Solid State Commun. 13 (1973) 771.

Discussion is closely correlated with defects. R. Haul (Hannover): In solid state chemistry “reactivity” With respect to surfaces, e.g. for metal-oxygen systems we know as yet very little about defect structures. There exists the principal difficulty that in using UHV electron spectroscopy techniques the surface is not in a thermodynamic equilibrium state which would e.g. for binary systems only be defined by a constant temperature and partial oxygen pressure. How would you judge the potentialities and limits of the surface analysis techniques in this context? R. W. Joyner: The influence of defects at surfaces has been studied in several ways. It has been shown that imperfections such as out of phase domains in the adsorbed layer can be examined through, for example, the occurrence of streaking in the LEED pattern *. Defects such as steps and kinks can be introduced into the surface, and studied directly by LEED t. The requirement of uhv or very low pressures does impose limitaions, particularly in relating results to situations of catalytic interest, which will be overcome only gradually.

* C.S. McKee, D.L. Perry and M.W. Roberts, Surface Sci. 39 (1973) 176; C.S. McKee, Roberts and M.L. Williams, Advan. Colloid Interface Sci., to be published. t See, e.g., G.A. Somorjai and D.W. Blakely, Nature 258 (1975) 580.

M.W.