Coadsorption of carbon monoxide and hydrogen on iridium single crystals

65

Surface Science 217 (1989) 65-77 North-Holland, Amsterdam

COADSORFTION OF CARBON MONOXIDE ON IRIDIUM SINGLE CRYSTALS Ts.S. MARINOVA

AND HYDROGEN

and D.V. CHAKAROV

Institute of General and Inorganic Chemistry, Bulgarian Academy

of Sciences, Sofia 1040, Bulgaria

Received 5 December 1988; accepted for publication 20 February 1989

The coadsorption of CO and Ha on the (111) and (110) faces of iridium has been investigated at temperatures of 170-400 K. It is established by HREELS that the CO molecule is terminally bonded to both surfaces within the whole range of coverages. The non-reactive character of coadsorption of the two gases is confirmed. The adsorption of CO on a surface with pre-adsorbed hydrogen leads to displacement of the hydrogen, the surface being completely poisoned with respect to hydrogen adsorption when the CO coverage reaches saturation. The mechanisms of hydrogen displacement by CO are discussed with a view to the adsorption sites occupied by the adsorbates on the two surfaces under consideration.

1. Introduction Investigation of the adsorption and coadsorption of CO and H, on the surface of iridium is of interest for at least two reasons: (i) these systems are appropriate models for advancing theoretical concepts on chemisorption and (ii) in view of the high activity of iridium in reactions with the participation of CO and hydrogen, the systems are of interest for catalysis. The coadsorption of the two gases is widely investigated on group VIII transition metals [l-7]. The general conclusion is that carbon monoxide displaces preadsorbed hydrogen from the metal surface and blocks further hydrogen adsorption, i.e. a passivation and poisoning of the surface with respect to hydrogen adsorption is observed. In the case of iridium these processes have been studied on the (110) face alone [8]. We are of the opinion that it is of interest to use the effects of coadsorption of these gases for elucidating the adsorption behaviour of hydrogen. The results presented in this paper supplement earlier studies on the coadsorption of CO and H, on other transition metals. 2. Experimental The experiments were carried out with an ultrahigh vacuum ESCALAB II (VG) apparatus at a residual gas pressure below 1 X lo-* Pa using the 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

66

Ts. S. Marinova, D. V. Chakarov / Coaclsorption of CO and H2 on Ir

methods of thermal desorption mass-spectrometry (TDS), high resolution electron energy loss spectroscopy (HREELS) and ultraviolet photoelectron spectroscopy (UPS). The Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) methods were applied to prepare the single crystal surfaces for adsorption and for the determination of the CO coverages. TDS measurements were carried out with a SQ-300 quadrupole mass-spectrometer whose computerized registration system allowed simultaneous study of the changes of several atomic masses. The HREELS measurements were performed using a primary electron beam with an energy of 3.0 eV. The resolution was 9-10 meV (72-80 cm-‘) at a counting rate of the registration system of about 1 x lo5 counts/s for a clean Ir surface. All spectra were recorded in the specular direction towards the incident electron beam ( tii = 0, = 45 o ). He1 radiation (hv = 21.2 eV normal emission) was used for the UPS experiments. The resolution was about 0.15 eV as measured from the Fermi edge of the Ag valence band. The iridium single crystals were prepared by a standard crystallographic technology. Both surfaces of the crystals were polished mechanically. The deviation from the preset crystallographic direction was smaller than 0.5 o for Ir(ll0) and f1° for Ir(ll1). The crystals were heated resistively by tungsten wires which were spot-welded to the samples. Cooling with liquid nitrogen at 160-170 K was achieved through the holder on which the samples were mounted. The temperature was controlled by a thermocouple (tungsten/ tungsten-rhenium) spot-welded on the lateral crystal surface. The cleaning procedure of the surfaces included heating cycles in oxygen (Po, = 1 X 10eh Pa) at 1200 K for several minutes, annealing in vacuum at 1900 K for about 30 s, (the heating up to 1900 K was achieved by electron impact, at 15-17 mA/cm*, 2 kV) and ion bombardment (Ar+, 20pAA/cm*, 2 kV).

3. Experimental results 3.1. CO adsorption on Ir(I 11) and Ir(1 IO) The results from thermal desorption studies can be summarized as follows: at low coverages the thermal desorption spectra have a maximum at - 600 k for Ir(ll0) and - 570 K for Ir(ll1). With increasing coverages the maxima are shifted to lower temperatures and the spectra change, additional components being observed in the desorption peaks: two for Ir(ll0) and one for Ir(ll1). The desorption energies were found to be Ed = 35 + 2 kcal/mol and Ed = 33 f 2 kcal/mol for Ir(ll0) and Ir(llO), respectively. The kinetic curves of adsorption plotted on the basis of data from TDS and XPS measurements (the

Ts. S. Marinovw,D. V, Chakarov / Coadwrptionof CO and H, on Ir

Ir(lll)

67

-i-CO

Hel

hv=21.2

eV

T=170K

16

14

(1

$0

ENERGY

L

‘

BELOW

1

1

0

E, [eV]

Fig. 1. He1 (hv = 21.2 eV) UPS difference spectra AN(E) for various CO exposures on clean Ir(ll1) surface at 170 K: (a) -0.2 L, (b) -0.5 L, (c) - 5.1 L.

intensity of the O(ls) peak was measured due to overlapping of C(ls) with iridium peaks) showed that at an adsorption temperature T,= 170 K and a CO pressure of 1 x low6 Pa on the (111) face, saturation was attained at about 6 L whereas on the (110) face this was achieved at about 4 L. Under the above conditions the initial sticking coefficient was close to one (0.8 for (111) and 0.9 for (110)). The saturation coverage was about (9.5 + 0.4) X 1014 molecules/cm2 for both surfaces at 170 K. Figs. 1 and 2 show the ultraviolet He1 difference spectra (exposed minus clean surface) depending on the exposure to CO at 170 K. The peaks observed in these spectra are denoted as 5u/la and 4a according to the electron energies of the corresponding molecular orbitals of CO (5~ = 8.5 eV, la = 11.5 eV, 4a = 14.0 eV for CO in the gas phase [9]). With increasing coverage, the quickest increase is observed for the 5a/llr peak at - 8 eV below E,. The

68

TX S. Marinoua, D. V. Chakarov

Ir(l10)

/ Coadsorption

of CO and H2 on Ir

+ CO

He I.

hvz21.2

eV

T=170K

4

:

!

:

:

:

:

:

:

lb

14

11

IO

8

6

4

I

0

ENERGY

BELOW

E, [eV]

Fig. 2. He1 UPS difference spectra AN(E) for various CO exposures on clean Ir(ll0) 170 K: (a) -0.2 L, (b) -0.45 L, (c) -4.2 L.

surface at

shift of the energies with increasing coverages and temperatures is weak and affects the 4a peaks only. The above experimental results are in good agreement with those obtained earlier [lo-121 on the same systems. They are given again here for the sake of comparison with results obtained during coadsorption of CO and hydrogen. Figs. 3 and 4 show the vibrational spectra of CO and the two surfaces under investigation at low temperatures (160 K for Ir(ll1) and 170 K for Ir(ll0)) and different exposures. The adsorption of CO causes a strong decrease in reflectance of the surfaces (about ten times with saturated coverages). The vibrations observed are assigned as V-CM in the region about 490 -’ and V-CO at about 2000 cm-‘. Comparison (table 1) with the results of zzer authors concerning CO adsorption on transition metals shows [13-181 that under the above conditions metal adsorption is nondissociative and the bonding of the CO molecule with the metal surface is terminal over the whole

TX S. Marinova, D. V. Chakarov

lrtlll

/ Coadsorption of CO and Hz on Ir

69

1 + CO

i 2000 1000 ENERGY

3000 LOSS.cm-1

Fig. 3. Vibration spectra of CO adsorbed on Ir(ll1) at 170 K as function of exposure: (a) clean surface, (b) 0.1 L, (c) 0.22 L, (d) 0.45 L,(e) 0.7 L, (f) 1.6 L, (g) 3.6 L, (h) 4.2 L, (i) 5.1 L,(j) 7.9 L.

range of coverages, contrary to the case of platinum [14], i.e. no transition from on-top to bridge bonding is observed on iridium with the populating of the adsorption layer, irrespective of the fact that data obtained by the other methods (LEED, TDS [lO,ll]) indicate an analogy (on evaporated iridium films [19], bands at 2050 and 1900 cm-’ are assigned as on-top bridge bonded CO). The differences in the CO frequencies observed on the two surfaces can be ascribed to different electron densities, differences in charge transfer and hence, different degrees of weakening of the C-O bond. Comparison with the different energies of CO desorption from the two surfaces can also be made. Fig. 5 shows the changes, with increasing coverage, in the V-CM and V-CO frequencies and in the intensity of the V-CO peak normalized with respect to the elastic peak. The data presented concern low-temperature adsorption. The change in frequencies with coverage is more pronounced in the case of the Ir(ll0) surface (from 2012 to 2070 cm-‘). For the Ir(ll1) face, a smaller change is observed (2025-2050 cm-‘). A similar shift is noted in ref. [14] for CO adsorbed on platinum and is attributed to interaction with the adlayer. The frequency change of the V-CM peak is the same (within the error limits)

TX S. Marinova, D. V. Chakarov / CoacLForption of CO and Hz on Ir

70

a 0

1000

2000 EN

,

3000

E RGY,cm-1

Fig. 4. Vibration spectra of CO adsorbed on Ir(ll0) at 170 K as function of exposure: (a) clean surface(b) 0.1 L, (c) 0.25 L, (d) 0.6 L, (e) 1.3 L, (f) 2.8 L, (g) 4.2 L, (h) 5.0 L, (i) 6.3 L.

for the two surfaces (of the order of lo-15 cm-‘, i.e. 475-490 cm-‘). At room and higher temperatures (measurements at 300, 350 and 400 K), a shift to higher frequencies occurs. It is evident from fig. 5 that the changes in intensity and frequency of V-CO vibrations are different for the two surfaces under consideration. This is probably due to the different densities of the adsorption layers and hence, to the formation of different structures. In ref. [20] the maximum in relative Table 1 or adsorbed CO in on-top and bridge sites (the Comparison of v-CO frequencies (in cm-‘) frequency shifts with coverage are given in brackets) Site

On-top Bridge

Ir(ll1) [this work]

Pt(ll1)

Ni(ll1)

Ni(ll0)

Pd(ll0)

1141

WI

Ir(ll0) [this work]

Pt(ll0)

P31

v51

P71

Ml

2010

2025

2094

2040

2012

2100

1960

2115

(16) 1840

(17) 1846

(50) 1840

(62) 1900

(-)

(80)

(80)

(12)

Ir films

(83) -

(25) _ _

(58) _ -

(-) _ -

Ts. S. Marinova, D. V. Chakarov / Coadrorption of CO and H2 on Ir

Ir(lll)+CO Ir(1

:

IO)

+CO

-

-

-

0

71

; t _: 15.. 2 \ cl 11.. ” L 11., a. t-

0

0 0

cl

0

0

0

C 9.. 0

“7 z Y I-0 c -L - 5...

’ .

I

I

a

’

I

,7 0

I

I

.

0

1

0

b I

I

. 1

1

4

I

6

I

EXPOSVRE.

Fig. 5. Dependence of V-CM (a) and v-CO @) frequencies and v-CO peak intensity (c) with CO exposure. T = 170 K.

intensity of the V-CO peak is attributed to the appearance of a new surface structure on platinum. However, in our case this transition is not associated with the alteration of type of the adsorption site. 3.2. Coahorption

of hydrogen and carbon monoxide

The experiments were carried out at low (160-170 K) and room (350 and 400 K) temperatures. A hydrogen coverage was formed on the clean surface and then exposed to CO. This succession of procedures was chosen due to the lower sticking coefficient of hydrogen and in order to obtain a hydrogen coverage with a definite value. The control measurements performed in the opposite order during adsorption led to qualitatively the same results.TDS

12

Ts. S. Marinova, D. V. Chakarov / Coadrorption of CO and H2 on Ir

(lr(llO)+EOL

H2)+CO

TEMPERATURE.K

Fig. 6. Thermal desorption spectra for H, from hydrogen saturated iridium surfaces after various CO exposures: (a) 0 L, (b) 0.4 L CO, (c) 1.0 L CO, (d) 1.8 L CO. Heating rate 9.5 K/s for Ir(ll1) and 7.5 K/s for Ir(ll0).

experiments (fig. 6) show that even small CO amounts (0.1-0.2 L) cause a change in population of the adsorption states of hydrogen. On both surfaces this effect is more pronounced with respect to the strongly bound (&) state of hydrogen. When the CO exposure increases, the hydrogen amount in the & state decreases and its desorption maximum is shifted to lower temperatures. With flc, > 0.4 for the (111) face and @co > 0.5 for the (110) face this state completely disappears. A similar phenomenon is observed during CO + hydrogen coadsorption and described in ref. [8] for the Ir(ll0) surface. With a further increase of CO exposures, the amount of weakly bound hydrogen (pi) also decreases. Surfaces with saturated CO coverages are completely poisoned for hydrogen adsorption. Experiments with hydrogen coverages corresponding to the filling of the & state only (5-10 L) show that after titration with CO displaced strongly bound hydrogen occupies the & state. This transition is demonstrated in fig. 7 which shows the UPS difference spectra of the two surfaces covered with hydrogen and then exposed to CO. The experiments were carried out as follows: at 170 K the surface was exposed to 5 L hydrogen (according to TDS data under these conditions only the & state is occupied). After that, the spectra shown in figs. 7a and 7d were recorded. They exhibit only weak changes associated with a change in work function and a certain decrease in intensity of the d-band of the metal immediately below E,. The spectra taken after the addition of 0.5 L CO (figs. 7b and 7e) already contain not only the peaks typical of CO but also a peak at about 6 eV below the Fermi level. This peak can be ascribed to H(ls) electrons, as already shown in refs. [21,22]. With increasing CO exposure (the spectra in figs. 7c and 7f), the hydrogen peak overlaps with the peaks for CO. When the same experiment is performed at room temperature (coverage 15 L hydrogen, exposed to 0.5 L CO), the peak at - 6 eV does not appear. It can be concluded that at this temperature the displaced & hydrogen has left the surface due to the low bond energy in the & state.

Ts. S. Marinova, I). V. Chtrkurov / Coadsmption of CO and H; on Ir

73

6eV H/lr(?lO)+CC

H/ir(lll)+CC

I

c

b

[* I.

16 14 12 10 ENERGY

8

6 4 BELOW

2

0 E,[eV]

Fig. 7. He1 UPS difference spectra AN(E) for various CO exposures on & hydrogen precovered iridium surfaces: (a) 0 L CO, (b> 0.5 L CO on Ir(fll), (c) 1.4 L CO on Ir(lll), (d) 0 L CO, (e) 0.5 L CO on Ir(llO), (f) 1.5 L CO on Ir(l10). T = 170 K.

ENERGY

LOSS

.cm-1

Fig. 8. Vibrational spectra of hydrogen and CO coadsorbed on (a) Ir(lll) and (b) Ir(lI0). The sawed hydrogen coverage is exposed with 0.1 L CO at 170 K.

14

Ts. S. Marinova, D. V. Chakarov / CoaAorption

of CO and H2 on Ir

The low intensity of the hydrogen-induced vibrations [23] makes their observation simultaneously with those of CO at comensurable coverages impossible. Fig. 8 shows the vibrational spectra of the two surfaces with hydrogen and carbon monoxide adsorbed on them. The & hydrogen-induced shifts of the V-CO frequencies are at - 25 cm-’ for Ir(ll0) and about 15 cm-’ for Ir(ll1). A broadening of the peaks is also observed and is attributed to CO. In the case of V-CM this broadening can be attributed to spectral overlapping with Y-HM whereas the broadening of V-CO is probably due to the presence of molecules with different surroundings. The broadening of V-CO is more pronounced on the Ir(ll0) surface. Experiments at room and higher temperatures (up to the temperature of hydrogen desorption) confirmed the nonreactive character of coadsorption of the two gases.

4. Discussion The effects observed during coadsorption of the two gases should be explained with the peculiarities of their adsorption on Ir(ll1) and Ir(ll0) surfaces. As is well known [22], low-temperature adsorption of hydrogen on these surfaces leads to the appearance of two thermal desorption states, & and &, with desorption maxima at about 200 and 370 K. HREELS studies showed hydrogen in the & state to be atomically adsorbed in threefold hollow sites on the two surfaces. In the case of Ir(llO), atoms from the second atomic layer of the metal also participate in the bond [23]. Two explanations of the origin of the two thermal desorption states are possible: (i) the states are a result of strong lateral interactions during population of the adsorption layer [24] and (ii) two types of adsorption sites on the surface are occupied. In the case of Ir(ll1) there are two types of threefold sites on the surface, and they are distinguished by the presence (hcp hollow) of metal atoms directly below (in the second metal layer) or its absence (fee hollow). On Ir(ll0) adsorption proceeds at (111) facets of the reconstructed (llO)-(1 X 2) surface, and a bond may be formed by H with an atom from the second surface layer in the first case, and with two atoms in the second case. The possibility of hydrogen penetration under the surface should not be overlooked. The above experimental results show that on both surfaces the effect of CO is concentrated on strongly bound hydrogen. If we accept the first model for the origin of the & state, the suppression of this state should be due to screening of the above interactions between the hydrogen atoms which have in fact produced the state. With this assumption, the question arises why hydrogen is invisible in the HREEL and UP spectra obtained at coverages corresponding to the population of this state alone.

TX S. Marinova, D. V. Chakarov / Coadrorption of CO and H2 on Ir

75

Another objection against this explanation is the fact that the lateral interactions on the two surface, Ir(ll1) and Ir(llO), which have different reliefs, lead to the same result. The difference in lateral interactions on the two surfaces can be seen in fig. 5 where these differences for CO can be attributed to the ordering of different structures in the adsorbed layer. The assumptions concerning hydrogen (unfortunately LEED data are not available for Ir) which are based on symmetry considerations also indicate the population of different structures. The results obtained on hydrogen displacement from the surface by CO also reject the hypothesis about lateral interactions. As was shown earlier for Ir(ll0) [8], it is a case not only of hydrogen displacement from a state but of simultaneous appearance of conditions favouring hydrogen transitions to another state. The degree of this shift depends on the number of free & adsorption sites. In experiments of Weinberg et al. [8], where only one state is occupied, this shift is complete, whereas in the case where free sites are absent, the displaced hydrogen is desorbed. Experiments on isotope exchange [25] showed hydrogen to be rapidly passing from one adsorption form to the other, tunneling the relatively low potential barrier between the two forms [26]. On the other hand, at 160-170 K carbon monoxide is sufficiently mobile on the surface. Therefore, a modification of the surface (change in electron density around the adsorption site occupied by CO) should occur in such a way as to hinder hydrogen penetration below the surface layer. It was already shown that on both surfaces the CO molecule forms linear bonds, while the hydrogen atoms occupy threefold hollow sites, i.e. no direct displacement from the same adsorption site takes place. TDS experiments on the displacement of strongly bound hydrogen and UPS data on the transition of & into & allow distinction between hydrogen dissolution in the metal bulk with rising temperature and hydrogen penetration below the first atomic layer. Hydrogen in the & state dissolved in the metal bulk should be insensitive to CO adsorption, contrary to the case where it is in the subsurface layer. Thus, the results obtained on the coadsorption of the two gases permit considering the & hydrogen as an atom dissolved in the metal and dissociated to a proton and an electron in the conductance band, but localized immediately below the surface (at about 0.5 A according to ref. [27]). This assumption explains why adsorption at room temperature or low-temperature adsorption corresponding to the filling of the & state only leads to the formation of chernisorbed hydrogen which is not present in the photoelectron spectrum and in the energy loss spectra. As was already pointed out in section 3, experiments at 170-400 K confirmed the nonreactive character of coadsorption of the two gases. This result is a direct consequence of the nondissociative character of CO adsorption on the iridium surface under consideration [28]. The effect of adsorbed & hydrogen consists in a decrease of the CO bond energy, which is reflected by an increase in the V-CO frequency. The above interaction is a result of the

16

Ts. S. Muinoua,

D. V. Chakarov

/ Coaabption

of CO and H, on Ir

collision between hydrogen corresponding to the M+-H- state and CO, since the two species have weak bonds with the surface and compete with each other in the attraction of electrons from the metal. This interpretation explains why with the increase of CO coverage, the amount of hydrogen adsorbed on the metal surface decreases gradually till the complete poisoning of the surface for hydrogen adsorption.

5. Conclusion The present investigation confirms the nonreactive character of coadsorption of carbon monoxide and hydrogen on the surface of iridium at 170-400 K. The adsorbates cause no structural changes in the reconstructed Ir(llO)(1 X 2) surface. It is established that CO is adsorbed nondissociatively on clean and hydrogen-covered Ir(ll1) and Ir(ll0) surfaces forming a terminal bond with the metal atoms. No transition to bridge bonding of CO on Ir(ll1) is found with increasing coverage. The presence of hydrogen produces no significant changes in the UP and HREEL spectra of adsorbed carbon monoxide. The adsorption of CO on a hydrogen-covered surface leads to displacement of strongly bound (&) hydrogen, and at &, > 0.4 or 0.5 (for the Ir(ll1) and Ir(ll0) surfaces, respectively), also to a displacement of & hydrogen. UPS measurements have shown that at low temperatures the strongly bound hydrogen displaced by CO becomes weakly bound. On the basis of the adsorption sites occupied by CO and hydrogen on the surfaces under investigation, the mechanism of direct displacement of & hydrogen is rejected. The displacement of & hydrogen occurs at high CO coverages due to a strong H-CO repulsive interaction.

References [l] [2] [3] [4] [5] [6] [7] (81 [9] [lo] [ll]

V.H. Baldwin, Jr. and J.B. Hudson, J. Vacuum Sci. Technol. 8 (1971) 49. E.D. Williams, P.A. Thiel, W.H. Weinberg and J.T. Yates, Jr., J. Chem. Phys. 72 (1980) 3496. H. Conrad, G. Ertl and E.E. Latta, J. Catalysis 35 (1974) 363. L.J. Richter, T.A. Germer and W. Ho, Surface Sci. 195 (1988) L 182. J. Banhofer, M. Hock and J. Kiippers, J. Electron Spectrosc. Related Phenomena 44 (1987) 55. L. Westerlund, L. Jonsson and S. Anderson, Surface Sci. 199 (1988) 109. R.W. McCabe and L.D. Schmidt, Surface Sci. 65 (1977) 189. D.E. Ibotson, T.S. Wittrig and W.H. Weinberg, Surface Sci. 97 (1980) 297. D.W. Turner, C. Beker, C.R. Bnmdle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, New York, 1970) p. 49. J.L. Taylor, D.E. Ibotson, W.H. Weinberg, J. Chem. Phys. 69 (1978) 2298. C.M. Comrie and W.H. Weinberg, J. Chem. Phys. 64 (1976) 250.

Ts. S. Marinova, D. V. Chakarov / Coadsorption of CO and H_, on Ir

II

[12] P.A. Zhdan, G.K. Boreskov, AL Boronin, A.P. Schepelin, W.F. Egelhoff and W.H. Weinberg, Surface Sci. 71 (1978) 267. [13] D. Reinalda and V. Ponec, Surface Sci. 91 (1979) 113. [14] B.E. Hayden and A.M. Bradshaw, Surface Sci. 125 (1983) 787. [15] V.A. Sobyanin, Poverkhnost 10 (1986) 65. [16] M.A. Chesters, G.S. McDougall, M.E. Pemble and N. Sheppard, Surface Sci. 164 (1985) 425. [17] J. Banhofer, M. Hock and J. Kippers, Surface Sci. 191 (1987) 395. [18] M. Trenary, K.J. Uram and J.T. Yates, Jr., Surface Sci. 157 (1985) 512. [19] J.F. Herrod, R.W. Roberts and E.F. Rissman, J. Phys. Chem. 71 (1967) 343. [20] H. Froitzheim, M. Schulze, J. Electron Spectrosc. Related Phenomena 45 (1987) 19. [21] P. Hofmamr and D. Menzel, Surface Sci. 152/153 (1985) 382. [22] D.E. Ibotson, T.S. Wittrig and W.H. Weinberg, J. Chem. Phys. 72 (1980) 4885. [23] D.V. Chakarov and Ts. Marinova, Surface Sci. 204 (1988) 147. [24] K. Christmann, G. Ertl and T. Pignet, Surface Sci. 54 (1976) 365. [25] V.J. Mimeault and R.S. Hansen, J. Chem. Phys. 45 (1966) 2240. [26] J.P. Muscat, Surface Sci. 148 (1984) 237. [27] J. Horiuti and T. Toya, in: Solid State Surface Science, Vol. 1, Ed. M. Green (Dekker, New York, 1969) p. 35. [28] K.W. Frese, Jr., Surface Sci. 202 (1988) 277.