Ni(100)

Surface Science 183 (1987) L279-L284 North-Holland, A m s t e r d a m

L279

SURFACE SCIENCE LETTERS AN IR-STUDY O F T H E ~ - C O C O - A D S O R P T I O N S T A T E IN T H E S Y S T E M CO/H/Ni(IO0) B.E. H A Y D E N *, R. K L A U S E R and A.M. BRADSHAW Fritz-Haber-lnstitut der Max-Planck-Gesellsehaft, Faradayweg 4-6, D-IO00 Berlin 33, Germany Received 28 November 1986; accepted for publication 30 December 1986

The weakly b o u n d E-CO co-adsorption state in the system C O / H / N i ( 1 0 0 ) has been investigated by IR reflection-absorption spectroscopy (IRAS). Its C - O stretching frequency changes from 2095 to 2115 cm -1 as a function of increasing coverage. Experiments with 12CO and 13CO mixtures show that this shift is composed of an increase in frequency of 48 cm -1 due to dipole-dipole coupling and a decrease of 24 cm -1 due to "static" or chemical effects. The negative chemical shift can be correlated with the appearance of satellites in the photoelectron spectrum as in other weak chemisorption systems.

Exposure of CO to a Ni(100) surface subsequent to saturation exposure with hydrogen yields a sharp peak at 200 K in both masses 2 and 28 in thermal desorption spectroscopy (TDS) [1]. At higher temperatures the characteristic desorption peaks for CO and adsorbed hydrogen are also observed. The system C O / H / N i ( 1 0 0 ) is thus unusual in that a chemisorption state is formed which is not present when either of the species is adsorbed on the surface alone. This genuine co-adsorption state - designated E-CO by Goodman et al. [1] - has also been extensively investigated by White and co-workers using HREELS, UPS and XPS [2-4]. To gain a deeper understanding of the nature of the interaction between CO and hydrogen in the E-state it is necessary, however, to obtain more detailed information as to the influence of hydrogen on the electronic and vibrational properties of the co-adsorbed CO molecule. On account of its intrinsically high resolution, infrared reflection-absorption spectroscopy (IRAS) is very suitable for studying the physical processes associated with the position and shape of vibrational bands. In particular, it has been possible in recent years to analyse coverage-dependent frequency shifts of vibrational modes in adsorbed diatomic molecules. Such shifts contain two contributions, the first being due to coupling between the oscillat* Permanent address: School of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.

0039-6028/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

L280

B.E. Hayden et al. / 1R-study of N-CO co-adsorption state in CO / H / Ni(I O0)

ing dipoles and the second to a change in the electronic distribution between the metal and the adlayer. These effects can be separated by the measurement of the vibrational frequencies of isotopic mixtures (e.g. 12CO and aBCO) as a function of relative concentration [5]. Such experiments have been performed for CO adsorption on Pt(111) [6], Pt(100) [7], Pd(100) [8], Cu(100) [9], C u ( l l l ) [10] and Cu(ll0) [11] as well as for N 2 on Ni(ll0) [12] and NO on P t ( l l l ) [13]. Whereas the dipole coupling shift is always positive, the "static", or "chemical" shift is positive in the case of strong chemisorption (e.g. C O / P d ) but negative in the case of weak chemisorption (e.g. CO/Cu). The system NO/Pt(111) is a possible exception in this classification scheme: "on-top" NO shows a small negative shift but is relatively strongly bound. In this Letter we apply the method of isotopic mixtures to 27-CO in the system C O / H / N i ( 1 0 0 ) and discuss the results in the context of recent angle-resolved photoemission data for the same system [14]. The experiments were performed in an infrared reflection system which has been described in a recent review article [15] and used in several previous studies (e.g. refs. [8,11,13]). The Ni(100) crystal was mechanically and electrochemically polished, cleaned in situ by argon ion bombardment at 470 and 750 K and annealed at about 1000 K. To remove the residual carbon the surface was treated with oxygen followed by flashing in hydrogen at high temperature (see ref. [16]). Fig. 1 shows the coverage dependence of the C - O stretching frequency in IRAS for CO/H(sat)/Ni(100) at 100 K. The absorption band shifts from 2095 to 2115 cm -1 with a halfwidth of 12 cm -1. Comparison with the C - O stretching band for the c(2 • 2)-CO-overlayer on a clean Ni(100) surface [16] reveals that the frequency of Z-CO is about 40 cm -1 higher. No other bands were observed below 200 K, indicating that only the 2;-CO state is present on the surface at low temperature. On heating the crystal to 220 K - a temperature just above the 27-CO desorption peak - and cooling down again to 100 K the peak at 2115 cm -1 disappears and a new band at 1975 cm -1 is observed (fig. 2b). This is a frequency in the spectral region corresponding to CO adsorbed on bridging sites. Clearly part of the N-CO has been desorbed and part has migrated onto other sites. If the remaining hydrogen is desorbed by heating to about 350 K the C - O stretch shifts back to a frequency of about 2075 cm -1 (fig. 2c), corresponding to the formation of "on-top" CO in the c(2 • 2) overlayer. The low temperature desorption peak in TDS and the high stretching frequency of CO in the N-state is strongly reminiscent of the C O / C u system. To further investigate this similarity various mixtures of 12CO and 13CO were adsorbed on the H(sat)/Ni(100) surface. The technique utillses the fact that only molecules with similar frequency exhibit strong dipole-dipole coupling; the frequency difference between 13CO and 12CO is sufficiently large (47 cm-1) for two such species to be essentially decoupled dynamically. The shift

B.E. Hayden et al. / IR-study of ~-CO co-adsorption state in CO / H/Ni(IO0)

CO/H(sat)

- Ni(lO0)

L281

2115 cm -I

I "5 ~176

exposure L

~

E

~

~

/

0

r

\"----~1o

\

~ .o~ ~ . - - _ _ - - o~

3 0

~

2

01

-_.-----~ 00

'6

2020

J

I

20 0 2100 Wovenumber (cm -1)

2140

Fig. 1. Coverage dependence of the C - O stretching frequency of ~-CO at 100 K.

i

CO/H / Ni(100)

2115 cm-1

CO(sat) I H(sat)- Ni (I00) .

a

I

>,

~c c

heated to 220 K: desorption of [ species

~

~ .

1975cm-1 k

~,., 2075 cm -1

heated to 340K: desorption of remaining H

. . 1900 . . 1920 . 1880

.1940

C

1960

1980 20(X) 20120 2( ~ 0 WavenMmber (cm-I )

~ ' ~

20I60

20~}0

I 2100

I 2120

' 2140

Fig. 2. Changes in the C - O stretching region for CO(sat)/H(sat)/Ni(100) (a), after desorbing the 2~-species (b) and the remaining hydrogen (c).

L282

B.E. Hayden et al. / IR-study of .~-CO coIadsorption state in CO / H / Ni(I O0)

due the static effect remains, however. For a particular coverage the relative concentration of the two isotopes on the surface is varied to establish the so-called dilution limit [6], e.g. a single 12CO molecule surrounded only by 13CO. Under these conditions the "static" shift for the particular coverage can be determined; from the total shift for that coverage the dipole-dipole contribution is then established. The results for N-CO are shown in fig. 3 where the dipole-dipole and static contributions are plotted together with the total shift for the pure isotope. At saturation CO coverage the net shift of 24 c m - l (fig. 1) is seen to be composed of a frequency increase of 48 cm-1 due to the dipole-dipole effect which is partially cancelled by a decrease due to the "static" effect of 24 cm -1. Such negative shifts have also been found for C O / C u ( l l 0 ) (~0s= - 4 4 cm -1 [111), CO/Cu(100) (~0s= - 3 0 cm -1 [9]) and C O / C u ( l l l ) (~0s = - 2 7 cm -1 [10]) as well as for NO/Pt(111) (~0s = - 1 3 cm -1, "on-top" species [13]) and N2/Ni(110 ) [12]. (No full data set is available for the latter case. It was, however, established that the "static" and dipole-dipole shifts are in opposite direction.) The last twenty years have seen a substantial effort in the interpretation of coverage-dependent frequency shifts in vibrational spectroscopy [5,17-20]. In particular, models to describe dipole-dipole coupling have included the interaction of the dynamic dipole moment of the adsorbed molecule with both the

CO/H/Ni(IO0): frequency

s h i f t of

E - CO b a n d &wo(O)

4030=E 20._= 10& 0-

6"

O - - ~ w (0)

(exposure,L) 9

',

14

.

16

-f

t

-10I

-20

o-

&w s

181

-30 Fig. 3. Coverage dependence of the components of the frequency shift for C - O stretch in ,~-CO. (zx): dipolar coupling shift, &~oo; ([3): "static" or chemical shift, A~s; (O): net shift, Ao: = Aw D + A~ s.

B.E. Hayden et al. / IR-study of ~,-CO co-adsorption state in CO / H / Ni(I O0)

L283

dipole and image dipole fields of neighbouring molecules and have also considered the dipole-self-image effects as well as the screening out of these interactions by the electronic polarisability of the adlayer. The situation with regard to the "static" or chemical shift is less satisfactory and so far only qualitative explanations have been offered. The positive shift obtained in strong chemisorption systems can be rationalised in terms of the "back-bonding" model used in coordination chemistry [21]. An increase in coverage gives rise to (substrate-mediated) intermolecular repulsion and a concomitant decrease in adsorption energy. This manifests itself as a reduction in the extent of the interaction between occupied metal states and the empty 2~r orbital of CO. The C - O stretching frequency then shifts back in the direction of its gas phase value. If, on the other hand, the chemisorption bond is weak and the contribution of ~r bonding less important, the coverage-dependent behaviour of the o donation may dominate. A reduction in the amount of 5 o donation as coverage increases results in a reduction in frequency of the C - O stretch since this orbital is slightly anti-bonding [22]. A comparison of the frequencies of free CO (2143 cm -1) and CO + (2184 cm -1) makes this clear. In a previous paper we have proposed an alternative explanation for the "negative" shift [11], in which the occupancy of the broadened, but largely empty ~b state changes as a function of coverage. This picture is, however, not consistent with more recent photoemission and inverse photoemission data. Pritchard's model seems to provide the most convincing explanation. Heskett et al. [23] have recently made the observation that those diatomic adsorption systems which give a negative chemical frequency shift in vibrational spectroscopy also show satellite lines due to multielectron excitations in their photoelectron spectra. This may be understood as follows. In weak chemisorption systems the 2~r orbital is not strongly involved in bonding, as mentioned above. The screening of the hole in the photoionisation process consists of the 2~r orbital being pulled down in energy and becoming more strongly occupied in the final state, i.e. the occupied, "bonding" % state gains substantial 2~r character. Since this process for weak chemisorption is neither complete (as in strong chemisorption) nor negligible (as in physisorption) two final ionic states result and may be described as "well-screened" (main peak) and "poorly screened" (satellite). For this reason we have investigated the valence level photoelectron spectrum of 1J-CO using synchrotron radiation [14]. As predicted by Heskett et al., there is indeed a satellite on the 40 line at about 1.5 eV higher binding energy. Again there is a very strong resemblance to the C O / C u system where a similar satellite at almost exactly the same energy relative to the main line is observed. In summary, we emphasize that the formation of the ~-CO co-adsorption state in the system C O / H / N i ( 1 0 0 ) gives rise to a single C - O stretch in the vibrational spectrum. It can only be partially desorbed, the remaining species migrating onto other, probably bridging sites. The net coverage-dependent

L284

B.E. Hayden et al. / IR-study of Z-CO co-adsorption state in CO / H / Ni(I O0)

f r e q u e n c y shift c o n t a i n s a c o n t r i b u t i o n d u e to c h a n g e s i n the b o n d i n g ( c h e m ical shift) of 24 c m - 1 to lower f r e q u e n c i e s w h i c h is also o b s e r v e d for o t h e r w e a k d i a t o m i c c h e m i s o r p t i o n systems. T h i s m a y b e c o r r e l a t e d w i t h the a p p e a r a n c e o f a satellite o n the 4 0 l i n e i n the U V p h o t o e l e c t r o n s p e c t r u m . T h i s w o r k h a s b e e n s u p p o r t e d i n p a r t b y the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t t h r o u g h S o n d e r f o r s c h u n g s b e r e i c h 6 a n d the F o n d s der C h e r n i s c h e n Industrie.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

D.W. Goodman, J.T. Yates, Jr. and T.E. Madey, Surface Sci. 93 (1980) L135. G.E. Mitchell, J.L. Gland and J.M. White, Surface Sci. 131 (1983) 167. B.E. Koel, D.E. Peebles and J.M. White, Surface Sci. 125 (1983) 709. B.E. Koel, D.E. Peebles and J.M. White, Surface Sci. 125 (1983) 739. See the section on coupling effects in adlayers in P. Holiins and J. Pritchard, Progr. Surface Sci. 19 (1985) 275. A. Crossley and D.A. King, Surface Sci. 68 (1977) 528. A. Crossley and D.A. King, Surface Sci. 95 (1980) 131. A. Ortega, F.M. Hoffmann and A.M. Bradshaw, Surface Sci. 119 (1982) 79. R. Ryberg, Surface Sci. 114 (1982) 627. P. Hollins and J. Pritchard, Surface Sci. 89 (1979) 486. D.P. Woodruff, B.E. Hayden, K.C. Prince and A.M. Bradshaw, Surface Sci. 123 (1982) 397. G.N. Burland and J. Pritchard, unpublished results. B. Hayden, Surface Sci. 131 (1983) 419. R. Klauser, M. Surman, Th. Lindner and A.M. Bradshaw, to be published. B.E. Hayden, in: Methods of Surface Characterization, Vol. 4, Eds. J.T. Yates, Jr. and T.E. Madey (Plenum, New York, in press). R. Klauser, W. Spiess, A.M. Bradshaw and B.E. Hayden, J. Electron Spectrosc. Related Phenomena 38 (1986) 187. R.M. Hammaker, S.A. Francis and R.P. Eischens, Spectrochim. Acta 21 (1965) 1295. G.D. Mahan and A.A.-Lucas, J. Chem. Phys. 68 (1978) 1344. M. Scheffler, Surface Sci. 81 (1979) 562. B.N.J. Persson and A. Liebsch, Surface Sci. 110 (1981) 356. G. Blyholder, J. Phys. Chem. 68 (1964) 2771. P. Hollins and J. Pritchard, in: Vibrational Spectroscopies for Adsorbed Species, ACS Syrup. No. 137 (Am. Chem. Soc., Washington, DC, 1980). D. Heskett, E.W. Plummer and R.P. Messmer, Surface Sci. 139 (1984) 558.