The adsorption and decomposition of methanol on Fe(110)

Surface Science 133 (1983) L469-L474 North-Holland Publishing Company

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SURFACE SCIENCE LETTERS T H E A D S O R P T I O N AND D E C O M P O S I T I O N OF M E T H A N O L ON Fe(110) P.H. McBREEN, W. ERLEY and H. IBACH lnstitut fl~r Grenzfl~ichenforschung und Vakuumpkvsik, Kernfor~ehungsanlage Ji~/wk, Po,~(/~t<'k 1 Ol 3, D 51 70 Jiilich, Fed. Rep. of Germany Received 10 May 1983: accepted for publication 26 July 1983

The EEL spectrum of methanol adsorbed on Fe(110) at 120 K varies markedly as a function of coverage and temperature. At high coverage the spectrum is that of molecularly adsorbed methanol. On heating to 300 K, a surface methoxy species is formed which remains stable to 435 K. Low coverage at 120 K results in a spectrum displaying the samc losses, but different relative intensities, as the surface methoxy group. A loss at 170 cm I in the low coverage regime is possibly due to an intcrmolecular vibration in hydrogen bond stabilized methanol clusters.

We present EEL spectra for the adsorption and decomposition of methanol on the Fe(ll0) surface. Demuth and lbach [1] found that f o r . N i ( l l l ) at 140 K methanol bonds to the nickel surface via the O atom and that the O - H bond remains intact. At higher temperature the O - H bond breaks and a surface methoxy species is formed. Further heating results in complete decomposition to CO and H 2. The chemistry of methanol on Fe and Ni is quite different. The methoxy species is stable on iron to 435 K [2,3] and on nickel to - 3 0 0 K [10]. As pointed out by several workers [4,3] this difference in stability may have some bearing on the fact that Fe Fischer-Tropsch catalysts produce methanol whereas their Ni counterparts do not. It is of interest to see if the surface vibrational spectrum, as measured by EELS, can give some clues as to the origin of the difference. EELS and IR spectroscopy have been applied to the adsorption of methanol on Cu(100) [6], Pt(111) [7], Pd(100) [8] and N i ( l l l ) [1], and Cu(100) [9], Ni(100) [10] and Pt(111) [11], respectively. Experiments were performed in an UHV system which will be described in detail elsewhere [12]. Spectra were taken at a fixed angle of reflection of 70 ° from the sample normal and the primary beam energy was 2.4 eV. Details of the sample preparation and surface cleaning procedure have been described in a recent paper [13]. CH3OH of 99.95% purity was introduced via a variable leak valve. 0039-6028/83/0000-0000/$03.00 © 1983 North-Holland

P. tL McBreen et aL / Methanol on F e ( l l O f

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The EEL spectra for methanol adsorbed on Fe(110) at 120 K are shown in fig. 1. In fig. 2 the changes in the potential applied to the Fe(ll0) sample to compensate for adsorbate induced work function changes are given. The latter results indicate a total work function change of approximately - 1.8 eV, which is typical of that observed for methanol adsorption on a variety of metals [14]. Comparing the EELS and Ag, results one can see that the work function change levels off at 2 L exposure whereas the EEL spectrum changes significantly in the 2 - 4 L exposure region. It is thus necessary before proceeding to establish the coverage versus exposure relationship. Exposure was effected via the ambient at 5 × 10 ~s Torr (uncorrected ion gauge reading). Assuming a sticking coefficient for methanol of approximately unity [3] this dosing configuration is likely to yield submonolayer coverage for a 1 L exposure. We first discuss the results for high coverage methanol. At exposures above 2 L losses associated with the methanol O - H group, at - 750 and 3300 cm become prominent. The presence of methanol on the surface at high coverage can be deduced by comparing the observed losses with those for liquid or solid methanol (table 1). Fig. 3 shows that on heating the methanol saturated Fe(ll0) surface to 300 K the spectrum transforms into that characteristic of a surface methoxy. Recently, the EEL spectra of methoxy species on a number of transition and noble metals have been reported [1,6-8,19]. Also, Blyholder and Neff [2] have

CH30Hon Fe(110) "333~°

~i

*l°°l/

.333,

T=120K

2s50

>.F-Z LIJ I---

z

"33/

0

.100\ 1~70

1000

3300

2(?00

ENERGY LOSS

3000 (cm-ll

Fig. l . EEL spectra for C H 3 O H on F e ( l l 0 ) at 120 K as a [unction o[ exposure:

(c) 3 L.

(a) I

L;

(b) 2 L;

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P. If. McBreen et al. / Methanol on Fe(llO)

-2.

/

o

of

/

~

O

f

0

0 0

1

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s

EXPOSURE [ L) Fig. 2. Contact potential adjustment versus coverage data for CH3OH on Fe(ll0) at 120 K.

shown that methanol decomposes on Fe catalysts to yield methoxy species stable to approximately 450 K. The intense loss at 1050 c m - : , in fig. 3, is due to the C - O stretch in the methoxy group. For the methoxy saturated surface, no change in the relative intensity of the loss at 1880 cm -~ due to adsorbed CO was observed on dosing with 9 L CO at 300 K. However, when the surface methoxy was left overnight at 300 K in U H V a sixfold decrease in the ratio of the 1050 c m - 1 loss to the 1880 cm-1 loss was observed. The combined results indicate that the latter change arose from methoxy decomposition rather than from contamination due to residual CO. The weak losses due to hydrogen adsorbed on F e ( l l 0 ) [15] are presumably obscured by the intense methoxy peaks. At 410 K the methoxy losses were greatly attenuated and were fully

Table 1 A comparison of vibrational losses (cm :) observed for methanol on Fe(ll0) at 120 K and IR data [51 for liquid methanol; the corresponding values for deuterated species are given in brackets Assignment of observed losses

Experimentally observed frequencies (cm - i) Adsorbed methanol

Liquid methanol

2L

3L

F e - O stretch Torsion O H bend CO stretch

380 530 710 1035

750 (560) 1050 (1010)

680/790 1030

CH 3 deformation

1450 1480 2850 2950 3250

1470 (1125)

1450 1480 2834 2980 3328

C H 3 s-stretch CH 3 d-stretch OH stretch

2850 (2110) 2950 (2270) 3300 (2470)

[.472

P. tt. McBreen et al. / Methanol on Fe(llO)

,020

CH]OH on Fe (110)

~100

T = 300 K

370 i

i

/2 I-z

333 1430

~82o ~,

2050 i

290

2840 i i

a

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.3

190000c

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6oo ENERGY LOSS

26oo

:A)o

(cm-1;

Fig. 3. Methoxy species formed on warming methanol saturated sample to 300 K. The second spectrum was obtained by recooling lhe sample to 120 K

removed at 445 K. Losses at 220, 480 and 500 cm- ~ remained and may be attributed to adsorbed oxygen [16]. In table 2 the losses associated with the methoxy group on Fe(ll0) are listed alongside those for methoxy on Ni(111) as measured by Demuth and Ibach [1] and Ibach and Mills [19]. The spectra are almost identical despite the fact that methoxy on Ni(111) is not stable above room temperature. On the surface of both metals the decomposition process presumably involves the abstraction of hydrogen by the metal surface and the breaking of the metal-oxygen bond. Thus of the losses clearly visible in the EEL spectra one might expect that the methoxy o x y g e n - m e t a l stretching frequency and the separation between the symmetric and degenerate C H 3 stretching vibration losses might be diagnostic of the decomposition process. However, within experimental error both spectra are the same and thus the EEL spectra do not indicate the origin of the difference in stability of the methoxy groups on Ni and Fe. The variation in stability of surface methoxy over a range of metals has been correlated with the

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P. II. McBreen et al. / Methanol on Fe(110)

Table 2 A comparison of ,.ibrational losses (cm 1) for a methoxy group on Fe(110) with those for methox'., on N i ( l l l ) {17] and for, low coverage methanol on Fe(ll0): corresponding frequencies for deuterated species are given in brackets Assignment of observed losses

Experimentally observed frequencies (cm 1) Methoxy group on Fe(ll0)

Methanol intermolecular stretch Surface phonon M e t a l - O stretch Torsion (or oxygen contamination Torsion (or H ,O contamination) CO stretch

1050 (1025)

CH~ deformation CO stretch, overtone CH ~ s-stretch C H ) d-stretch

1435 2085 2860 (2080) 2930 (2220)

Ni(lll)

Low coverage CH~OH (1 L) Ice(1 It)) 170

250 380 (375) 530 670

220 375 520

380 550

730 875 1035 1170 1440

690 1020

2825 2920

2850 2950

1450

heats of adsorption of the decomposition products [7]. However, in the case of Fe and Ni the difference may, perhaps, be accounted for by reference to thermal desorption data obtained for CH3OH on Ni(100) [20,10], Fe(100) [3] and for CO and H 2 on Ni(100) [20,10] and Fe(110) [15]. On Fe(ll0), H 2 desorbs at - 440 K [15] which is approximately the temperature at which the methoxy group decomposes. Similarly, hydrogen desorbs from nickel in the temperature region of methoxy decomposition [10]. In the decomposition reaction the removal of hydrogen from the surface to yield active sites, with regard to further CH bond breaking, may well be the rate determining step on a m e t h o x y / h y d r o g e n saturated surface. It is interesting to note that on metals where the methoxy species decomposes well below 300 K (Pt(111) [7] and Ru(001) [21]) the methoxy CO stretch is at least 20 c m - t lower than that for methoxy on metals where it is stable to 300 K or higher. Thus perhaps a lower CO stretching frequency is indicative of a less stable methoxy configuration. We now turn to the adsorption of methanol at low coverage (fig. la). The losses at 370 and 690 cm -1 indicate that both methoxy and methanol are present on the surface. In the case of CH3OH on Pd(100) Christmann and Demuth found from TDS, UPS and EELS data that at 77 K the first 20% of methanol adsorbing on the surface decomposes to methoxy. Although the spectrum obtained for methanol on Fe(110) at 120 K following a 1 L exposure (fig. la) is similar to that for the methoxy group observed at high temperature

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P. 11..tl~ Breen e: aL / Ah'thanol (,n Fe(l 1O)

(fig. 3) there are c o n s i d e r a b l e d i f f e r e n c e s b e t w e e n the two spectra. F o r i n s t a n c e the elastic p e a k a n d the C O s t r e t c h i n g loss are m u c h m o r e intense in fig. 3. At 1 L e x p o s u r e the elastic i n t e n s i t y d e c r e a s e d by a f a c t o r of six r e l a t i v e 1o that for the c l e a n surface, a n d no f u r t h e r c h a n g e w,as o b s e r v e d on i n c r e a s i n g the e x p o s u r e to 4 L. C h r i s t l n a n n a n d D e m u t h [8] r e p o r t no o b s e r v a b l e L E E D p a t t e r n for C H 3 O H on Pd(100) a n d the a b o v e results for F e ( l l 0 ) m a y be seen in t e r m s of a d i s o r d e r e d a d s o r b a t e layer. D i s o r d e r f o l l o w i n g C H ~ O H a d s o r p t i o n c o u l d p o s s i b l y arise f r o m h y d r o g e n b o n d i n d u c e d clustering. I n d e e d the loss at 170 c m -~ in fig. l a p r o v i d e s s o m e e v i d e n c e for such clustering. B o t h P a s s c h i e r et al. [17] a n d W o n g a n d W h a l l e y [181 assign a b a n d at 160 cm ~ in the s p e c t r u m of solid m e t h a n o l to an i n t e r m o l e c u l a r s t r e t c h i n g f r e q u e n c y . F o r a C H 3 O H s u r f a c e cluster o f the f o l l o w i n g t y p e CH~ O-H Fe

CH 3 ... O-H Fe

the n o n - o b s e r v a t i o n of O H losses at - 3300 c m for by s u r f a c e s e l e c t i o n rule c o n s i d e r a t i o n s .

~ c o u l d p o s s i b l y be a c c o u n t e d

T h e a u t h o r s wish to a c k n o w l e d g e the skilful t e c h n i c a l assistance of Mrs. C. Damerow.

References [11 [21 [31 [4] ]5] [61 [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

J.E. Demuth and H. lbach, Chem. Phys. Letters 60 (1979) 395, G. Blyholder and L.D, Neff, J. Chem. Phys. 70 (1966) 893. J.B. Benziger and R.J. Madix. J. Catalysis 65 (1980) 36. G. Blyholder and W.V. Wyatt, J. Phys, Chem. 70 (1966) 1745. M. Falk and E. Whalley, J. Chem. Phys. 34 (1961) 1554. B.A. Sexton, Surface Sci. 88 (1979) 299. B.A, Sexton, Surface Sci. 102 (1981) 271. K. Christmann and J.E. Demuth, J. Chem. Phys. 76 (1982) 6308, 5639. R. Rydberg, Chem. Phys. Letters 83 (1981) 423. F.L Baudais. A,J, Borschke. J.D. Fedyk and M.J. Dignam, Surface ScL 100 11980) 210. H.J. Krebs and H. Ltith, in: Vibrations in Adsorbed Layers, Proc. Jtilich Conf., 1978. W. Erley, P.H, McBreen and H. lbach, J. Catalysis, submitted. W. Erley and H. Ibach, J. Electron Spectrosc. and Related Phenomena, submitted. R.J. Madix, Advan. Catalysis 29 (1980) 1. A.M. Bard and W. Erley, Surface Sci. 112 (1981) L759. W. Erley and H. Ibach, Solid State Commun. 20 (1976) 229. W.F. Passchier, E.R. Klompmaker and M, MandeL Chem. Phys. Letters 4 (1970) 485. P.T.T. Wong and E. Whalley, J. Chem. Phys. 55 (1971) 1830. H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic Press, New York, 1982) p. 320. [20] D.W. Goodman, J.T. Yates, Jr. and T.E. Madey, Surface Sci. 93 (1980) L143. [21] J. Hrbek, R.A. dePaola and F.M. Hoffmann, J. Vacuum Sci. Technol. AI (1983) 1222.