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Oxygen adsorption on the Pt(110)(1 × 2) surface studied with EELS
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Surface Science 284 (1993) 121-128 North-Holland
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Oxygen adsorption J. Schmidt,
Ch. Stuhlmann
Institut fiir Grenzfldenforschung
on the Pt( llO)( 1 x 2) surface studied with EELS and H. Ibach und Vakuumphysik, Forschungszentrum
Jiilich, Postfach 1913, D-51 70 Jiilich, Germany
Received 10 September 1992; accepted for publication 3 November 1992
The adsorption of oxygen on the Pt(llOX1 X 2) surface was investigated by electron energy loss spectroscopy (EELS). At 30 K and low exposures oxygen is adsorbed as two different peroxolike molecular species in on-top and twofold coordinated sites with O-O stretching frequencies of 860 and 930 cm-‘, respectively. With increasing exposure oxygen is additionally adsorbed in a superoxo form with ~o_o = 1250 cm-‘. Heating the surface leads to partial dissociation and partial desorption of the oxygen molecules. At about 100 K the superoxo species desorbs completely; desorption and dissociation of the peroxo species begins at 125 K and is completed at 200 K. At 300 K oxygen is adsorbed in atomic form, two Pt-0 vibrations with frequencies of 480 and 330 cm-r are observed.
1. Introduction
Oxygen adsorbs in molecular form on Pt surfaces at low temperatures, whereas at room temperature dissociative adsorption takes place. In particular, the system oxygen on Pt(ll1) was investigated with a broad variety of methods. Vibrational studies with EELS at about 100 K have been reported by Steininger et al. [l], Gland et al. [21 and Fisher and coworkers [3]. According to Steininger et al. at 100 K oxygen is molecularly adsorbed as a bridge-bonded species revealing an O-O stretching frequency of 700 cm-’ and an on-top species with vo_o = 875 cm-‘, both of the peroxo type (Oz-). After adsorption at 300 K they observed a single loss at 480 cm-’ which was attributed to the perpendicular Pt-0 vibration of oxygen atoms adsorbed in threefold hollow sites. The results of Gland et al. differ merely in the interpretation of the 700 cm-’ loss. They ascribe this loss to molecular oxygen adsorbed at defect sites. In contrast, Outka and coworkers [4] proposed in a NEXAFS study that oxygen molecules are bound in a superoxo (0;) state on Pt(ll1) at about 100 K. Only few investigations dealt with oxygen adsorption on the Pt(llOX1 x 2) surface. The com-
monly accepted model of this surface is of the missing row type as confirmed by several studies [5-81. The surface is formed of close-packed rows of Pt atoms in [liO] direction, but every second [liO]-row is missing. A rather open surface is thus formed with broad troughs between the closepacked rows. The ridges and valleys of the troughs are composed of the [l-i01 atomic rows, the walls forming (111) microfacets. In recent work [lo-131 oxygen adsorption on Pt(ll0) at temperatures around 100 K has been studied with TDS, XPS, photoemission of adsorbed xenon (PAX) and work function measurements. The results indicate that the observed species are similar to those on Pt(ll1). Adsorption takes place in two steps: first oxygen molecules are adsorbed as a bridgebonded species on the [liO] rows in the valleys of the reconstructed surface, in a second step at higher coverages adsorption on the (111) microfacets occurs. TDS spectra show two low temperature desorption peaks of molecular oxygen with desorption temperatures of about 180 and 200 K [9,121. After adsorption at 300 K only atomic oxygen is found. Primarily dissociative adsorption on the [ITO] atomic rows takes place, followed by adsorption on the (111) microfacets [10,14]. TDS measurements show a single desorption peak due
0039~6028/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
to atomicadsorbed o~ge~ at about $00 IS. Moiec~ar and atomic adso~tio~ ~mpete for the same adsu~tion sites [ISJ. In this paper we will report our results of EELS measurements on this system at temperatures between 30 K and room temperature.
The e~e~~euts were carried out in a two level stainless steel UHV chamber. The lower level housed a high resolution electron energy loss spectrometer, while the upper level contained a CMA for Auger electron spectroscopy (AES), a three-grid LEED optics, a quadrupoie mass spectrometer for residual gas analysis and a gas inlet system. The ~~~rn~~was pied by a turborno~~~~ pump, an ion getter pump and a liquid nitrogen cooled titanium subhmation pump. After baking out a base pressure of about 5 X 1O-11 mbar was achieved, the residual gas being mostly hyd~ogen~ The sample could be coofed down to 30 K with liquid He, Heating was decoyed by electron bombardment from the backside of the crystal and temperature was measured with a Nick-Ni therm~ouple attached to the sample holder. The crystal was annealed ex situ for 9 h at 2200 K in an 0, ambient. In situ the crystal was further cleaned by s~utte~n~ with 3 keV Ne ions at room tempera~re, f~~owed by a~~~~~ at 900 K for several minutes, The sputte~g-heating cycles were repeated several times until nu ~nt~iu~tio~s co&d be detected with AES and EELS. The clean surface revealed the typioar (1 x 2) LEED pattern, the diffraction spots were slightly streaky in the [l%l] direction. EELS measurements were perfo~ed in the specular direction with an angle of incidence Sj of 59” and primary beam energy -E, of 3.3 eV in ah spectra. The energy re~lu~on was set to 35 -r as shown by the full width at half maximum F&j of the dastic peak. For oxygen ddsing the UHV chamber was aged with 0, and the 0, pressure was measured with an ion~a~o~ gauge. ‘The readings were corrected for the ~~s~ndi~~ se~i~~~ of the
gauge, the exposure pre~ure was kept in the 3.0W9mbar range.
Fig. 1 shows a series of loss spectra recorded at 30 K after different exposures in the range of 0.345 L 0,. In ah spectra the dominate fess appears at N 800 em-‘, shifting to g60 cm-l with ~~r~i~g exposure. At 1 L 0, (fig. lb> a loss at 920 cm-’ evolves, which with ~~~asiug exposures gains utensil ~rn~ar~ to the former one and shifts to 930 cm-i. It is knowt~ that nearly ah metal dioxygen complexes can be divided into peroxo and super0x0 complexes according to the characte~sties of the dioxygen ligand. The double bond in gaseous 0, is reduced to a bond with bond order 1 in peroxo species (Oz-) and bond order 1.5 in superoxo (0;) species as one or two metal electrons are donated to the pariahs vacant antibonding rr; orbitdts of the mobcular oxygen during formation d the metal dio~gen complexes. Roth types occur in two different configurations as shown in fig. 2 [16,17].Infrared data of metal dio~ge~ compiexes summarized in the review articles of Vaska iI61 and Jones et al. j17J indicate an Q-O stretching frequency of ~-93~ cm-l for the peroxo complexes (Ia) and 790430 em-” for (Ib). The (Ia) complex of a specific metal atom gene&y reveals a higher 0-O stretching frequency than the ~~es~nd~g (Ib) complex. For the superoxo ~mpiexes (IIa) and @lb) ~~_.o= 110%1195 cm-’ and yo_o = 1075P%? cm-l were found, res~etiveiy. Referring to the data collected by Vaska and Jones et al,, we assign the low frequency loss at 860 cm-l to the Q-Q stretching vibration of peroxo-like species (Ibj in budge-ended con&urations and the high frequency Ioss at 930 em- ’ to the O-O s~tching ~bratio~ of peroxo (Ia) species adsorbed in on-top positions. This assign ment is also in accordance with the inte~ret~tio~ of ~te~iuger et al. [I]. The observed peroxo O-Q stretch~g frequencies in our e~er~ents, as well
J. Schmidt et al. / Oxygen adsorption on the Pt(llO)(l
peroxo
I
x250
123
X 2) surface studied with EELS
Co m (Ia) voo = 800-930 cm-’
0.3L
1Ib) VoO= 790 - 880 cm-r
O-O bond order: 1 60 superoxo
P m
(110) voo =llOO-1195cm-' O-O
,O%oHm m Mb) voo=1075-1122cm-l
bond order: I.5
Fig. 2. Different dioxygen metal complexes. The O-O bond order 1 in peroxo-like complexes is symbolically represented by full lines, whereas O-O bonds in superoxo complexes with bond order 1.5 are drawn as full together with broken lines. The peroxo species (Ial is assigned to the 930 cm-’ loss, the 860 cm-’ loss is due to the peroxo species (Ib). The loss at about 1250 cn-’ in figs. Id and le is attributed to a superoxo species.
1270
931'155
e)
25 L
e;b
32. 'T9
x50
x750
: 0
1000
2000
3000
Energy Loss km-') Fig. 1. EELS spectra of molecularly adsorbed oxygen after different exposures at 30 K. 0, exposure: (a) 0.3 L, (b) 1 L, (cl 3 L, (d) 10 L and (e) 25 L. In (d), (e) the channeltron voltage had to be reduced during the recording of the elastically reflected beam, whereas for the rest of both spectra the same channeltron voltage as in (al-(c) was used. Therefore the magnification factors given in (d) and (e) are only rough estimates.
as those observed by Steininger et al. [l], who found frequencies of vo_o = 700 cm-’ for the peroxo (Ib) species and 860 cm-’ for the (Ia) species, are modified compared to the gas phase values due to the influence of the specific adsorption geometries. Nevertheless, the occurrence of two losses in the expected frequency ranges and the relative values of the O-O stretching frequencies of the two different peroxo species given by Vaska and Jones et al. allow a clear assignment. The adsorption process of the peroxo species clearly takes place in two steps. At low exposures only (Ib) species are detected, with increasing exposures 2 1 L 0, an increasing amount of type (Ia) is found. This could be explained by the larger coordination number of type (Ib), i.e., the reduction of the number of possible adsorption sites for this species at high coverages. As already noted, a two step process with primary adsorption as a bridge-bonded species was also postulated by Freyer et al. [lo] and Fusy and Ducros 111,121for adsorption temperatures around 100 K. Measurements of the work function change after different 0, exposures indicate sequential adsorption of oxygen molecules in two different adsorption sites
and PAX ~vestigatio~s show that molecular oxygen is adsorbed firstly in the valleys of the reconstructed surface, followed by adsorption on the microfacets, The transition between adsorption on the atomic rows and adsorption on the microfacets occurs at about 1 L O2 exposure, which compares favourably with our observations. Whether oxygen molecules are firstly adsorbed on the close-packed atomic rows of the reconstructed surface and then on the (111) microfacets can not be decided on the basis of our results. Further evidence for the assig~ent of the 860 and 930 cm-r losses to peroxo (Ib) and (Ia) species will be given later (cf. section 3.4). Fig, lc shows an additional loss at 1250 cm-*. In accordance with the above classification scheme we ascribe this frequency to the O-O stretching vibration of a superoxo species. This species was not observed in the investigations cited above because its desorption temperature lies below 100 K (cf. section 3.3). During the formation of a superox~l~e oxygen molecule only one electron is transferred from the metal, so that this species should occur at an adso~tio~ site in which a small orbital overlap favours a small charge transfer compared to adsorption in the vslleys or on the mic~fa~ts~ where a large orbital overlap results in a large charge transfer and formation of a peroxo-like species. An adsorption site with the necessary small orbital overlap is found on the ridges of the reconstructed surface. At even higher exposures in figs, ld and le a loss at 1550 cm-i due to ph~isorbed 0, appears rpsaSphase frequency of 0,: 15% cm-r>. The sim~taueous presence of all species allows a clarification of the discrepancy between Stei~i~ger et al, fl’j and Outka and coworkers [4] regarding the assignment of the molecular losses of oxygen adsorbed on pt(lll) to peroxo or superoxo species. The similarity of the oxygen species on PtCllO) and Pt(ll1) leads to the conclusion that the species observed by Steininger et al, IlJ and Gland and coworkers [2] are indeed of the peroxo type and not of the superoxo type as proposed by Qutka et al. I41, The appearence of physisor~d 0, in fig. ld is accompanied by a drastic increase of the reflectiv-
ity of the surface observed as an enhanced intensity of the elastically reflected beam lelast,though no long range order of the oxygen overlayer could be detected. Because of the high reflective of the surface after 0, exposures 2 10 L (figs. Id and le) the ~~a~eltron voltage had to be reduced during the recording of the elastically reflected beam in these spectra, so that the magnification factors given in figs. Id and le are only estimates. In fig. le the ~tensities of the peroxo losses are unch~g~d compared to fig. Id, whereas the intensities of the losses due to the U-O stretching vibrations of molecules in a superoxo state at 1250 cm-’ and physisorbed oxygen molecules at 1550 cm-i increased, The origin of the band observed at 383 cm”” in fig. la and 410 cm‘-’ in fig. lb is not clear. Steininger et al. [lf and Gland and coworkers [Z] observed on Pt(lll) at low temperatures a loss at .-.+ 380 cm-” and ascribed it to the Pt-0 ~bration of a peroxo species. The frequency of this vibration in matrix isolated PtCO,) lies in the 375-415 cm-i range [IS]_ Assigning the 400 cm-’ loss to the Pt-0 vibration of the (Ib) species, one would expect an increasing intensity of this band as the loss due to the O-O stretching vibration of the (Ib) species at = 800 cm-’ grows with increasing exposure. The ~o~~~ed loss intensi~ l/l&,t of the 400 cm-r vibration is, however, essentiahy constant for exposures between 0.05 and 1 L 02, a range in which the intensity of the 800 cm-i loss increases si~i~~~~ (the ~o~es~~ding spectra are not shown in fig. 1X A dissociative adso~tion of oxygen on defect sites at low temperatures would result in a loss near the observed frequency (cf. section 3.2) and could explain the saturation of the loss intensity at low coverages. The reconstructed Pt{llO) surface is quite open, so that a high defect density is expected. The streaky LEEID pattern also indicates a certain degree of disorder of the clean surface. Furthermore, a small amount of atomic oxygen on Pt(llO> after adsorption at 120 K was also found by Freyer et al. [lo], Another ~ter#ti~ feature of fig. 1 is the observed correlation of the 400 cm‘-’ band due to atomic oxygen with the superoxo loss: the 400 cm-” loss vanishes at w 3 L 0,
J. Schmidt et al. / Oxygen adsorption on the Pt(llO)~l X 2) surface studied with EELS
125
exposure, when the superoxo loss at 1250 cm-’ appears (figs. lb and 1~). We ascribe this behaviour to the influence of the molecular super0x0 species on the bonding of atomic oxygen to the metal surface. It might be possible, that the charge transfer from the metal to the superoxo species reduces the charge transfer to the atomically bound oxygen. The reduced charge transfer to the oxygen atoms results in a smaller dynamic dipole moment of the Pt-0 vibration and a decrease of the intensity of the corresponding loss below the limit of detectability.
Figs. 3a-3d show loss spectra recorded after 0.1, 0.3, 1 and 6 L 0, exposure at 300 K, respectively. In fig. 3a a small loss at 450 cm-’ is seen. With increasing exposures the 450 cm-’ loss shifts to higher frequencies (* 480 cm-’ after 6 L O2 exposure in fig. 3d). In fig. 3d a second loss at about 330 cm-’ appears, which is detected after approximately 3 L 0, exposure. Off specular measurements show that all losses are due to dipole scattering. In accordance with the results of Steininger et al, [l] and Gland et al. [21 for oxygen adsorption on the Pt(ll1) surface at room temperature, the 480 cm-l loss is assigned to the perpendicular Pt-0 vibration of chemisorbed oxygen atoms. We ascribe the 330 cm-i loss, which was not ob-
a)
x200 I y'
0
6L
1000
I zoo0
Energy Loss
3000
km-‘)
Fig. 3. EELS spectra of atomically adsorbed oxygen. Oxygen adsorption at 300 K: (a) 0.1, (b} 0.3, (c) 1 and Cd)6 L 0,.
served by Steininger and Gland on Pt(lll), to the hindered translations of oxygen atoms adsorbed on the (111) microfacets of the reconstructed
b)
low exposures
high exposures
Fig. 4. Surface structure of the Pt(llOX1 X 2) surface showing the adsorption sites of oxygen atoms after (a) low exposures and (b) high exposures at room temperature. Open circles represent second and third layer Pt atoms, shaded circles represent first layer Pt atoms and the dark circles represent oxygen atoms.
Pt(ll0) surface. The hindered translations should become dipole active on the reconstructed Pt(ll0) surface, because the Pt-0 bonds for atoms chemisorbed on the microfacets are inclined against the surface normal (see fig. 4), so that the dynamic dipole moment connected with these modes posseses a component perpendicular to the surface. In contrast, the dynamical dipole moment connected with the hindered translations of oxygen atoms chemisorbed on Pt(ll1) lies parallel to the surface and therefore is totally screened by the image dipole induced in the metal. Considering the morphology of the reconstructed Pt(ll0) surface and the exposure dependence of the atomic losses the following picture schematically depicted in fig. 4 seems to be reasonable: For low exposures oxygen is dissociatively adsorbed in a fourfold coordinated site on the [liO] atomic rows in the valleys of the reconstructed surface (see fig. 4a). In this site only the perpendicul~ Pt-0 vibration at 480 cm-’ is detectable. The hindered translations are screened, because their dipole moment lies parallel to the surface. At exposures 2 3 L 0, adsorption preferentially takes place in threefold coordinated adsorption sites on the (111) microfacets and the loss due to the hindered translations at 330 cm-’ becomes visible. Adsorption on the ridges of the reconstructed surface is unlikely to occur, because atomic oxygen should be chemisorbed in at least threefold coordinated sites. Adsorption on the rows of the reconstructed Pt(ll0) surface for exposures I 1 L 0, at 300 K followed by adsorption on the (111) microfacets for exposures 2 1 L 0, was also observed by Freyer et al. [lo] and Ducros and Merrill 1141. 3.3. Desorption and dissociationof molecular oxysen Heating the layers of molecularly adsorbed oxygen leads to partial dissociation and partial desorption of the oxygen molecules as shown in figs. 5a-5d. The sample was heated to the temperature indicated in the figure, then the temperature was held fixed and the loss spectra were recorded. The physisorbed 0, molecules and the
0
too0 Energy
Loss
2000 km-’
3ooo
)
Fig. 5. Desorption and dissociation of molecularly adsorbed oxygen. The surface was exposed to 25 L 0, at 30 K and spectra were recorded at (a) 60 K, (b) 125 K, (c) 175 K and (d) 200 K.
superoxo species desorb ahnost completely already at 60 K (see fig. 5a). The peroxo losses at 60 K are essentially unchanged compared to 30 K. Furthe~ore the intensi~ of the elastically reflected beam is strongly reduced at all investigated temperatures between 60 and 200 K in figs. 5a-5d, so that the whole spectra are recorded with full channeltron voltage. At about 125 K (fig. 5b) first evidence for atomic oxygen is found as shown by the appearence of the Pt-0 vibration at about 460 cm-‘. The intensities of the peroxo losses are strongly reduced compared to fig. 5a. With rising temperature (fig. 5c at 175 K) the intensities of the molecular losses decrease further and the Pt-0 loss gains intensity. At 200 K (fig. 5d) only atomic oxygen is found. In fig. 5d, the intensity of the atomic loss at 470 cm-’ normalized with respect to the elastic peak equals
J. Schmidt
et al. ,’ Oxygen
aako&on
on the Ptfll#)(l
the normalized loss intensity found after = 0.5 L 0, exposure at 300 K. Referring to Freyer et al. [lOI 0.5 L 0, exposure at room temperature corresponds to a coverage 8 = 0.1. Freyer et al. estimated the saturation coverage at low temperatures 020 I0 to @,,,= 1.35 and the maximum coverage at 300 K to t$,, = 0.35, so that 8 < @,,( T = 300 K) < f&( T = 120 K). Noting that we started with a saturated layer of oxygen molecules at low temperatures this relation shows, that at high temperatures saturation coverage is not achieved. We conclude that during annealing of the saturated Iayer partial desorption of the oxygen molecules takes piace. Our
r
x 2) surface
studied
with EELS
127
results are in general accordance with the desorption temperatures of the peroxo species measured with TDS by Ohno et al. and Fusy and coworkers [9,111. 3.4. Coadsorption of molecular and atomic oxygen Fig. 6 shows the influence of coadsorbed atomic oxygen on the adsorption properties of molecular oxygen. In all cases the surface was preexposed to 6 L 0, at room temperature followed by the exposure indicated in the figure at 30 K Fig. 6a corresponding to an exposure of 0.3 L 0, at 30 K shows a peroxo loss at m 890 cm-’ and the two atomic losses at about 480 and 330 cm-‘. Increasing the exposure leads to an increase of the intensity of the 890 cm-’ loss compared to the atomic losses (see fig. 6b). At 3 L O2 exposure (fig. 6c) a new strong loss at 950 cm-’ due to the second peroxo species appears, and the 890 cm-’ peak is only slightly higher than in fig. 6b. After 25 L 0, exposure the former 890 cm-i loss is only detected as a small as~met~ of the 950 cm-i peak. These results are a further evidence for the assignment of the low frequency peroxo species to the bridge-bonded type (Ib), whereas the high frequency species shouId be of type (Ia), which is adsorbed in an on-top position: preadsorption of atomic oxygen diminishes preferentially the number of possible adsorption sites for the species with the higher coordination number.
4. Summary
0
1000
2000
3000
Energy Loss km-‘) Fig. 6. Spectra of coadsorbed atomic and molecular oxygen. The surface was exposed to 6 L 0, at room temperature, followed by O2 exposure oE (a) 0.3 L, (b) I L, (cl 3 L and fd) 25 L at 30 K. Channeltron voltage reduced for the elastically reflected beam in fe) (see caption to fii. 1).
The adsorption of oxygen molecules on the reconstructed Pt(llOX1 x 2) surface was studied by EELS. After oxygen adsorption at 30 K in a first step at low coverages oxygen is adsorbed in molecular form as a bridge-bonded peroxo species with ~o_o = 860 cm -I. When the oxygen exposure is increased, adsorption takes place as a peroxo species in an on-top site with yo_o = 930 cm-‘. With further increasing coverages oxygen molecules are additionally adsorbed as a super0x0 species with an O-O stretching frequency of about 1250 cm-i. We believe that these molecules
128
J. Scan
et al, / Oxygen adscnption on the Pt(f 10)fl x 2) surface s&died with EELS
are chemisorbed on the ridges of the reconstructed surface. High exposures lead to physisorption of 0, molecules with vo_o very near the gas phase frequency. Upon heating the molecular 0, is gradually converted into atomic oxygen and partially desorbs. After adsorption at room temperature at low coverages oxygen is atomically adsorbed mostly on the atomic rows of the reconstructed surface with a perpendicular Pt-0 stretching frequency of N 480 em-r. Additionally the hindered trandations parallel to the microfacets are observed with Y~_~ = 330 cm-‘.
References [I] H. Steininger, S. Lehwatd and H. Jbach, Surf. Sci. 123 (1982) 1. [2] J.L. Gland, B.A. Sexton and G.B. Fisher, J. Vat. Sci. Technol. 17 (1980) 144.
[3] G.B. Fisher, B.k Sexton and J.L. Gland, Surf. Sci. 95 (1980) 587. [4] D.A. Outka, J. Stiihr, W. Jark, P. Stevens, J. Solomon and R.J. Madi Pbys. Rev. 3 35 (1987) 4119. [S] A.M. Lahee, R.J. Blake and W. Abison, Surf. Sci. 151 (1985) L153. 161 G.L. Kellogg, Phys. Rev. Lett. 35 (1985) 2168. [7] MS, Daw, Surf. Sci. 166 (1986) L166. [8] S. Foiles, Surf. Sci. 191 (1987) L779. [9] Y. Ohno and T. Matsushima, Surf. Sci. 241 (1991) 47. [lo] N. Freyer, M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci. 166 (1986) 206. [ll] J. Fusy and R. Ducros, Surf. Sci. 214 (1989) 337. [12] R. Ducros and J. Fusy, Appl. Surf. Sci. 44 (1990) 59. [13] KC. Prince, K. Diickers, K Horn and V. Chab, Surf. Sci. 200 (1988) L451. [14] R. Ducros and R.P. Merrill, Surf. Sci. 5.5 (1976) 227. 1151 M. WiIf and P.T. Dawson, Surf. Sci. 65 (1977) 399. 1161 L. Vaska, Act. Chem. Res. 9 (1976) 175. [17] R.D. Jones, D.A. Summervilie and F. Basolo, Chem. Rev. 79 (1979) 139. [18] H. Huber, W. Klotzbiicher, G.A. Ozin, A. van der Voet, Can. J. Chem. 51 (1973) 2722.
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surface science ,... :.:& ~:r?.:..: .‘.‘.y ...,,., ..........i.:.: .,.,.... ““Q’.A .........‘.:.~,..:.:.:.:.:.;...::~:.:::::::~~~~
Surface Science 284 (1993) 121-128 North-Holland
...........:,..I.: ,,,: ;,;,. ““‘-‘*A “..‘....A’......... :,., w:~:::~:$$.:$:.* ...........~.~.~...... ‘.‘S .A :.~~~~~~:.:.~.~~~.~...~.: :,:.: ”( _
Oxygen adsorption J. Schmidt,
Ch. Stuhlmann
Institut fiir Grenzfldenforschung
on the Pt( llO)( 1 x 2) surface studied with EELS and H. Ibach und Vakuumphysik, Forschungszentrum
Jiilich, Postfach 1913, D-51 70 Jiilich, Germany
Received 10 September 1992; accepted for publication 3 November 1992
The adsorption of oxygen on the Pt(llOX1 X 2) surface was investigated by electron energy loss spectroscopy (EELS). At 30 K and low exposures oxygen is adsorbed as two different peroxolike molecular species in on-top and twofold coordinated sites with O-O stretching frequencies of 860 and 930 cm-‘, respectively. With increasing exposure oxygen is additionally adsorbed in a superoxo form with ~o_o = 1250 cm-‘. Heating the surface leads to partial dissociation and partial desorption of the oxygen molecules. At about 100 K the superoxo species desorbs completely; desorption and dissociation of the peroxo species begins at 125 K and is completed at 200 K. At 300 K oxygen is adsorbed in atomic form, two Pt-0 vibrations with frequencies of 480 and 330 cm-r are observed.
1. Introduction
Oxygen adsorbs in molecular form on Pt surfaces at low temperatures, whereas at room temperature dissociative adsorption takes place. In particular, the system oxygen on Pt(ll1) was investigated with a broad variety of methods. Vibrational studies with EELS at about 100 K have been reported by Steininger et al. [l], Gland et al. [21 and Fisher and coworkers [3]. According to Steininger et al. at 100 K oxygen is molecularly adsorbed as a bridge-bonded species revealing an O-O stretching frequency of 700 cm-’ and an on-top species with vo_o = 875 cm-‘, both of the peroxo type (Oz-). After adsorption at 300 K they observed a single loss at 480 cm-’ which was attributed to the perpendicular Pt-0 vibration of oxygen atoms adsorbed in threefold hollow sites. The results of Gland et al. differ merely in the interpretation of the 700 cm-’ loss. They ascribe this loss to molecular oxygen adsorbed at defect sites. In contrast, Outka and coworkers [4] proposed in a NEXAFS study that oxygen molecules are bound in a superoxo (0;) state on Pt(ll1) at about 100 K. Only few investigations dealt with oxygen adsorption on the Pt(llOX1 x 2) surface. The com-
monly accepted model of this surface is of the missing row type as confirmed by several studies [5-81. The surface is formed of close-packed rows of Pt atoms in [liO] direction, but every second [liO]-row is missing. A rather open surface is thus formed with broad troughs between the closepacked rows. The ridges and valleys of the troughs are composed of the [l-i01 atomic rows, the walls forming (111) microfacets. In recent work [lo-131 oxygen adsorption on Pt(ll0) at temperatures around 100 K has been studied with TDS, XPS, photoemission of adsorbed xenon (PAX) and work function measurements. The results indicate that the observed species are similar to those on Pt(ll1). Adsorption takes place in two steps: first oxygen molecules are adsorbed as a bridgebonded species on the [liO] rows in the valleys of the reconstructed surface, in a second step at higher coverages adsorption on the (111) microfacets occurs. TDS spectra show two low temperature desorption peaks of molecular oxygen with desorption temperatures of about 180 and 200 K [9,121. After adsorption at 300 K only atomic oxygen is found. Primarily dissociative adsorption on the [ITO] atomic rows takes place, followed by adsorption on the (111) microfacets [10,14]. TDS measurements show a single desorption peak due
0039~6028/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
to atomicadsorbed o~ge~ at about $00 IS. Moiec~ar and atomic adso~tio~ ~mpete for the same adsu~tion sites [ISJ. In this paper we will report our results of EELS measurements on this system at temperatures between 30 K and room temperature.
The e~e~~euts were carried out in a two level stainless steel UHV chamber. The lower level housed a high resolution electron energy loss spectrometer, while the upper level contained a CMA for Auger electron spectroscopy (AES), a three-grid LEED optics, a quadrupoie mass spectrometer for residual gas analysis and a gas inlet system. The ~~~rn~~was pied by a turborno~~~~ pump, an ion getter pump and a liquid nitrogen cooled titanium subhmation pump. After baking out a base pressure of about 5 X 1O-11 mbar was achieved, the residual gas being mostly hyd~ogen~ The sample could be coofed down to 30 K with liquid He, Heating was decoyed by electron bombardment from the backside of the crystal and temperature was measured with a Nick-Ni therm~ouple attached to the sample holder. The crystal was annealed ex situ for 9 h at 2200 K in an 0, ambient. In situ the crystal was further cleaned by s~utte~n~ with 3 keV Ne ions at room tempera~re, f~~owed by a~~~~~ at 900 K for several minutes, The sputte~g-heating cycles were repeated several times until nu ~nt~iu~tio~s co&d be detected with AES and EELS. The clean surface revealed the typioar (1 x 2) LEED pattern, the diffraction spots were slightly streaky in the [l%l] direction. EELS measurements were perfo~ed in the specular direction with an angle of incidence Sj of 59” and primary beam energy -E, of 3.3 eV in ah spectra. The energy re~lu~on was set to 35 -r as shown by the full width at half maximum F&j of the dastic peak. For oxygen ddsing the UHV chamber was aged with 0, and the 0, pressure was measured with an ion~a~o~ gauge. ‘The readings were corrected for the ~~s~ndi~~ se~i~~~ of the
gauge, the exposure pre~ure was kept in the 3.0W9mbar range.
Fig. 1 shows a series of loss spectra recorded at 30 K after different exposures in the range of 0.345 L 0,. In ah spectra the dominate fess appears at N 800 em-‘, shifting to g60 cm-l with ~~r~i~g exposure. At 1 L 0, (fig. lb> a loss at 920 cm-’ evolves, which with ~~~asiug exposures gains utensil ~rn~ar~ to the former one and shifts to 930 cm-i. It is knowt~ that nearly ah metal dioxygen complexes can be divided into peroxo and super0x0 complexes according to the characte~sties of the dioxygen ligand. The double bond in gaseous 0, is reduced to a bond with bond order 1 in peroxo species (Oz-) and bond order 1.5 in superoxo (0;) species as one or two metal electrons are donated to the pariahs vacant antibonding rr; orbitdts of the mobcular oxygen during formation d the metal dio~gen complexes. Roth types occur in two different configurations as shown in fig. 2 [16,17].Infrared data of metal dio~ge~ compiexes summarized in the review articles of Vaska iI61 and Jones et al. j17J indicate an Q-O stretching frequency of ~-93~ cm-l for the peroxo complexes (Ia) and 790430 em-” for (Ib). The (Ia) complex of a specific metal atom gene&y reveals a higher 0-O stretching frequency than the ~~es~nd~g (Ib) complex. For the superoxo ~mpiexes (IIa) and @lb) ~~_.o= 110%1195 cm-’ and yo_o = 1075P%? cm-l were found, res~etiveiy. Referring to the data collected by Vaska and Jones et al,, we assign the low frequency loss at 860 cm-l to the Q-Q stretching vibration of peroxo-like species (Ibj in budge-ended con&urations and the high frequency Ioss at 930 em- ’ to the O-O s~tching ~bratio~ of peroxo (Ia) species adsorbed in on-top positions. This assign ment is also in accordance with the inte~ret~tio~ of ~te~iuger et al. [I]. The observed peroxo O-Q stretch~g frequencies in our e~er~ents, as well
J. Schmidt et al. / Oxygen adsorption on the Pt(llO)(l
peroxo
I
x250
123
X 2) surface studied with EELS
Co m (Ia) voo = 800-930 cm-’
0.3L
1Ib) VoO= 790 - 880 cm-r
O-O bond order: 1 60 superoxo
P m
(110) voo =llOO-1195cm-' O-O
,O%oHm m Mb) voo=1075-1122cm-l
bond order: I.5
Fig. 2. Different dioxygen metal complexes. The O-O bond order 1 in peroxo-like complexes is symbolically represented by full lines, whereas O-O bonds in superoxo complexes with bond order 1.5 are drawn as full together with broken lines. The peroxo species (Ial is assigned to the 930 cm-’ loss, the 860 cm-’ loss is due to the peroxo species (Ib). The loss at about 1250 cn-’ in figs. Id and le is attributed to a superoxo species.
1270
931'155
e)
25 L
e;b
32. 'T9
x50
x750
: 0
1000
2000
3000
Energy Loss km-') Fig. 1. EELS spectra of molecularly adsorbed oxygen after different exposures at 30 K. 0, exposure: (a) 0.3 L, (b) 1 L, (cl 3 L, (d) 10 L and (e) 25 L. In (d), (e) the channeltron voltage had to be reduced during the recording of the elastically reflected beam, whereas for the rest of both spectra the same channeltron voltage as in (al-(c) was used. Therefore the magnification factors given in (d) and (e) are only rough estimates.
as those observed by Steininger et al. [l], who found frequencies of vo_o = 700 cm-’ for the peroxo (Ib) species and 860 cm-’ for the (Ia) species, are modified compared to the gas phase values due to the influence of the specific adsorption geometries. Nevertheless, the occurrence of two losses in the expected frequency ranges and the relative values of the O-O stretching frequencies of the two different peroxo species given by Vaska and Jones et al. allow a clear assignment. The adsorption process of the peroxo species clearly takes place in two steps. At low exposures only (Ib) species are detected, with increasing exposures 2 1 L 0, an increasing amount of type (Ia) is found. This could be explained by the larger coordination number of type (Ib), i.e., the reduction of the number of possible adsorption sites for this species at high coverages. As already noted, a two step process with primary adsorption as a bridge-bonded species was also postulated by Freyer et al. [lo] and Fusy and Ducros 111,121for adsorption temperatures around 100 K. Measurements of the work function change after different 0, exposures indicate sequential adsorption of oxygen molecules in two different adsorption sites
and PAX ~vestigatio~s show that molecular oxygen is adsorbed firstly in the valleys of the reconstructed surface, followed by adsorption on the microfacets, The transition between adsorption on the atomic rows and adsorption on the microfacets occurs at about 1 L O2 exposure, which compares favourably with our observations. Whether oxygen molecules are firstly adsorbed on the close-packed atomic rows of the reconstructed surface and then on the (111) microfacets can not be decided on the basis of our results. Further evidence for the assig~ent of the 860 and 930 cm-r losses to peroxo (Ib) and (Ia) species will be given later (cf. section 3.4). Fig, lc shows an additional loss at 1250 cm-*. In accordance with the above classification scheme we ascribe this frequency to the O-O stretching vibration of a superoxo species. This species was not observed in the investigations cited above because its desorption temperature lies below 100 K (cf. section 3.3). During the formation of a superox~l~e oxygen molecule only one electron is transferred from the metal, so that this species should occur at an adso~tio~ site in which a small orbital overlap favours a small charge transfer compared to adsorption in the vslleys or on the mic~fa~ts~ where a large orbital overlap results in a large charge transfer and formation of a peroxo-like species. An adsorption site with the necessary small orbital overlap is found on the ridges of the reconstructed surface. At even higher exposures in figs, ld and le a loss at 1550 cm-i due to ph~isorbed 0, appears rpsaSphase frequency of 0,: 15% cm-r>. The sim~taueous presence of all species allows a clarification of the discrepancy between Stei~i~ger et al, fl’j and Outka and coworkers [4] regarding the assignment of the molecular losses of oxygen adsorbed on pt(lll) to peroxo or superoxo species. The similarity of the oxygen species on PtCllO) and Pt(ll1) leads to the conclusion that the species observed by Steininger et al, IlJ and Gland and coworkers [2] are indeed of the peroxo type and not of the superoxo type as proposed by Qutka et al. I41, The appearence of physisor~d 0, in fig. ld is accompanied by a drastic increase of the reflectiv-
ity of the surface observed as an enhanced intensity of the elastically reflected beam lelast,though no long range order of the oxygen overlayer could be detected. Because of the high reflective of the surface after 0, exposures 2 10 L (figs. Id and le) the ~~a~eltron voltage had to be reduced during the recording of the elastically reflected beam in these spectra, so that the magnification factors given in figs. Id and le are only estimates. In fig. le the ~tensities of the peroxo losses are unch~g~d compared to fig. Id, whereas the intensities of the losses due to the U-O stretching vibrations of molecules in a superoxo state at 1250 cm-’ and physisorbed oxygen molecules at 1550 cm-i increased, The origin of the band observed at 383 cm”” in fig. la and 410 cm‘-’ in fig. lb is not clear. Steininger et al. [lf and Gland and coworkers [Z] observed on Pt(lll) at low temperatures a loss at .-.+ 380 cm-” and ascribed it to the Pt-0 ~bration of a peroxo species. The frequency of this vibration in matrix isolated PtCO,) lies in the 375-415 cm-i range [IS]_ Assigning the 400 cm-’ loss to the Pt-0 vibration of the (Ib) species, one would expect an increasing intensity of this band as the loss due to the O-O stretching vibration of the (Ib) species at = 800 cm-’ grows with increasing exposure. The ~o~~~ed loss intensi~ l/l&,t of the 400 cm-r vibration is, however, essentiahy constant for exposures between 0.05 and 1 L 02, a range in which the intensity of the 800 cm-i loss increases si~i~~~~ (the ~o~es~~ding spectra are not shown in fig. 1X A dissociative adso~tion of oxygen on defect sites at low temperatures would result in a loss near the observed frequency (cf. section 3.2) and could explain the saturation of the loss intensity at low coverages. The reconstructed Pt{llO) surface is quite open, so that a high defect density is expected. The streaky LEEID pattern also indicates a certain degree of disorder of the clean surface. Furthermore, a small amount of atomic oxygen on Pt(llO> after adsorption at 120 K was also found by Freyer et al. [lo], Another ~ter#ti~ feature of fig. 1 is the observed correlation of the 400 cm‘-’ band due to atomic oxygen with the superoxo loss: the 400 cm-” loss vanishes at w 3 L 0,
J. Schmidt et al. / Oxygen adsorption on the Pt(llO)~l X 2) surface studied with EELS
125
exposure, when the superoxo loss at 1250 cm-’ appears (figs. lb and 1~). We ascribe this behaviour to the influence of the molecular super0x0 species on the bonding of atomic oxygen to the metal surface. It might be possible, that the charge transfer from the metal to the superoxo species reduces the charge transfer to the atomically bound oxygen. The reduced charge transfer to the oxygen atoms results in a smaller dynamic dipole moment of the Pt-0 vibration and a decrease of the intensity of the corresponding loss below the limit of detectability.
Figs. 3a-3d show loss spectra recorded after 0.1, 0.3, 1 and 6 L 0, exposure at 300 K, respectively. In fig. 3a a small loss at 450 cm-’ is seen. With increasing exposures the 450 cm-’ loss shifts to higher frequencies (* 480 cm-’ after 6 L O2 exposure in fig. 3d). In fig. 3d a second loss at about 330 cm-’ appears, which is detected after approximately 3 L 0, exposure. Off specular measurements show that all losses are due to dipole scattering. In accordance with the results of Steininger et al, [l] and Gland et al. [21 for oxygen adsorption on the Pt(ll1) surface at room temperature, the 480 cm-l loss is assigned to the perpendicular Pt-0 vibration of chemisorbed oxygen atoms. We ascribe the 330 cm-i loss, which was not ob-
a)
x200 I y'
0
6L
1000
I zoo0
Energy Loss
3000
km-‘)
Fig. 3. EELS spectra of atomically adsorbed oxygen. Oxygen adsorption at 300 K: (a) 0.1, (b} 0.3, (c) 1 and Cd)6 L 0,.
served by Steininger and Gland on Pt(lll), to the hindered translations of oxygen atoms adsorbed on the (111) microfacets of the reconstructed
b)
low exposures
high exposures
Fig. 4. Surface structure of the Pt(llOX1 X 2) surface showing the adsorption sites of oxygen atoms after (a) low exposures and (b) high exposures at room temperature. Open circles represent second and third layer Pt atoms, shaded circles represent first layer Pt atoms and the dark circles represent oxygen atoms.
Pt(ll0) surface. The hindered translations should become dipole active on the reconstructed Pt(ll0) surface, because the Pt-0 bonds for atoms chemisorbed on the microfacets are inclined against the surface normal (see fig. 4), so that the dynamic dipole moment connected with these modes posseses a component perpendicular to the surface. In contrast, the dynamical dipole moment connected with the hindered translations of oxygen atoms chemisorbed on Pt(ll1) lies parallel to the surface and therefore is totally screened by the image dipole induced in the metal. Considering the morphology of the reconstructed Pt(ll0) surface and the exposure dependence of the atomic losses the following picture schematically depicted in fig. 4 seems to be reasonable: For low exposures oxygen is dissociatively adsorbed in a fourfold coordinated site on the [liO] atomic rows in the valleys of the reconstructed surface (see fig. 4a). In this site only the perpendicul~ Pt-0 vibration at 480 cm-’ is detectable. The hindered translations are screened, because their dipole moment lies parallel to the surface. At exposures 2 3 L 0, adsorption preferentially takes place in threefold coordinated adsorption sites on the (111) microfacets and the loss due to the hindered translations at 330 cm-’ becomes visible. Adsorption on the ridges of the reconstructed surface is unlikely to occur, because atomic oxygen should be chemisorbed in at least threefold coordinated sites. Adsorption on the rows of the reconstructed Pt(ll0) surface for exposures I 1 L 0, at 300 K followed by adsorption on the (111) microfacets for exposures 2 1 L 0, was also observed by Freyer et al. [lo] and Ducros and Merrill 1141. 3.3. Desorption and dissociationof molecular oxysen Heating the layers of molecularly adsorbed oxygen leads to partial dissociation and partial desorption of the oxygen molecules as shown in figs. 5a-5d. The sample was heated to the temperature indicated in the figure, then the temperature was held fixed and the loss spectra were recorded. The physisorbed 0, molecules and the
0
too0 Energy
Loss
2000 km-’
3ooo
)
Fig. 5. Desorption and dissociation of molecularly adsorbed oxygen. The surface was exposed to 25 L 0, at 30 K and spectra were recorded at (a) 60 K, (b) 125 K, (c) 175 K and (d) 200 K.
superoxo species desorb ahnost completely already at 60 K (see fig. 5a). The peroxo losses at 60 K are essentially unchanged compared to 30 K. Furthe~ore the intensi~ of the elastically reflected beam is strongly reduced at all investigated temperatures between 60 and 200 K in figs. 5a-5d, so that the whole spectra are recorded with full channeltron voltage. At about 125 K (fig. 5b) first evidence for atomic oxygen is found as shown by the appearence of the Pt-0 vibration at about 460 cm-‘. The intensities of the peroxo losses are strongly reduced compared to fig. 5a. With rising temperature (fig. 5c at 175 K) the intensities of the molecular losses decrease further and the Pt-0 loss gains intensity. At 200 K (fig. 5d) only atomic oxygen is found. In fig. 5d, the intensity of the atomic loss at 470 cm-’ normalized with respect to the elastic peak equals
J. Schmidt
et al. ,’ Oxygen
aako&on
on the Ptfll#)(l
the normalized loss intensity found after = 0.5 L 0, exposure at 300 K. Referring to Freyer et al. [lOI 0.5 L 0, exposure at room temperature corresponds to a coverage 8 = 0.1. Freyer et al. estimated the saturation coverage at low temperatures 020 I0 to @,,,= 1.35 and the maximum coverage at 300 K to t$,, = 0.35, so that 8 < @,,( T = 300 K) < f&( T = 120 K). Noting that we started with a saturated layer of oxygen molecules at low temperatures this relation shows, that at high temperatures saturation coverage is not achieved. We conclude that during annealing of the saturated Iayer partial desorption of the oxygen molecules takes piace. Our
r
x 2) surface
studied
with EELS
127
results are in general accordance with the desorption temperatures of the peroxo species measured with TDS by Ohno et al. and Fusy and coworkers [9,111. 3.4. Coadsorption of molecular and atomic oxygen Fig. 6 shows the influence of coadsorbed atomic oxygen on the adsorption properties of molecular oxygen. In all cases the surface was preexposed to 6 L 0, at room temperature followed by the exposure indicated in the figure at 30 K Fig. 6a corresponding to an exposure of 0.3 L 0, at 30 K shows a peroxo loss at m 890 cm-’ and the two atomic losses at about 480 and 330 cm-‘. Increasing the exposure leads to an increase of the intensity of the 890 cm-’ loss compared to the atomic losses (see fig. 6b). At 3 L O2 exposure (fig. 6c) a new strong loss at 950 cm-’ due to the second peroxo species appears, and the 890 cm-’ peak is only slightly higher than in fig. 6b. After 25 L 0, exposure the former 890 cm-i loss is only detected as a small as~met~ of the 950 cm-i peak. These results are a further evidence for the assignment of the low frequency peroxo species to the bridge-bonded type (Ib), whereas the high frequency species shouId be of type (Ia), which is adsorbed in an on-top position: preadsorption of atomic oxygen diminishes preferentially the number of possible adsorption sites for the species with the higher coordination number.
4. Summary
0
1000
2000
3000
Energy Loss km-‘) Fig. 6. Spectra of coadsorbed atomic and molecular oxygen. The surface was exposed to 6 L 0, at room temperature, followed by O2 exposure oE (a) 0.3 L, (b) I L, (cl 3 L and fd) 25 L at 30 K. Channeltron voltage reduced for the elastically reflected beam in fe) (see caption to fii. 1).
The adsorption of oxygen molecules on the reconstructed Pt(llOX1 x 2) surface was studied by EELS. After oxygen adsorption at 30 K in a first step at low coverages oxygen is adsorbed in molecular form as a bridge-bonded peroxo species with ~o_o = 860 cm -I. When the oxygen exposure is increased, adsorption takes place as a peroxo species in an on-top site with yo_o = 930 cm-‘. With further increasing coverages oxygen molecules are additionally adsorbed as a super0x0 species with an O-O stretching frequency of about 1250 cm-i. We believe that these molecules
128
J. Scan
et al, / Oxygen adscnption on the Pt(f 10)fl x 2) surface s&died with EELS
are chemisorbed on the ridges of the reconstructed surface. High exposures lead to physisorption of 0, molecules with vo_o very near the gas phase frequency. Upon heating the molecular 0, is gradually converted into atomic oxygen and partially desorbs. After adsorption at room temperature at low coverages oxygen is atomically adsorbed mostly on the atomic rows of the reconstructed surface with a perpendicular Pt-0 stretching frequency of N 480 em-r. Additionally the hindered trandations parallel to the microfacets are observed with Y~_~ = 330 cm-‘.
References [I] H. Steininger, S. Lehwatd and H. Jbach, Surf. Sci. 123 (1982) 1. [2] J.L. Gland, B.A. Sexton and G.B. Fisher, J. Vat. Sci. Technol. 17 (1980) 144.
[3] G.B. Fisher, B.k Sexton and J.L. Gland, Surf. Sci. 95 (1980) 587. [4] D.A. Outka, J. Stiihr, W. Jark, P. Stevens, J. Solomon and R.J. Madi Pbys. Rev. 3 35 (1987) 4119. [S] A.M. Lahee, R.J. Blake and W. Abison, Surf. Sci. 151 (1985) L153. 161 G.L. Kellogg, Phys. Rev. Lett. 35 (1985) 2168. [7] MS, Daw, Surf. Sci. 166 (1986) L166. [8] S. Foiles, Surf. Sci. 191 (1987) L779. [9] Y. Ohno and T. Matsushima, Surf. Sci. 241 (1991) 47. [lo] N. Freyer, M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci. 166 (1986) 206. [ll] J. Fusy and R. Ducros, Surf. Sci. 214 (1989) 337. [12] R. Ducros and J. Fusy, Appl. Surf. Sci. 44 (1990) 59. [13] KC. Prince, K. Diickers, K Horn and V. Chab, Surf. Sci. 200 (1988) L451. [14] R. Ducros and R.P. Merrill, Surf. Sci. 5.5 (1976) 227. 1151 M. WiIf and P.T. Dawson, Surf. Sci. 65 (1977) 399. 1161 L. Vaska, Act. Chem. Res. 9 (1976) 175. [17] R.D. Jones, D.A. Summervilie and F. Basolo, Chem. Rev. 79 (1979) 139. [18] H. Huber, W. Klotzbiicher, G.A. Ozin, A. van der Voet, Can. J. Chem. 51 (1973) 2722.