Vibrational spectra of ammonia adsorbed on Fe(110)

Surface Science 119 (1982) L357-L362 North-Holland Publishing C o m p a n y

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SURFACE SCIENCE LETTERS VIBRATIONAL SPECTRA OF AMMONIA ADSORBED ON Fe(ll0) W. ERLEY and H. IBACH lnstitut fllr Grenzfliichenforschung und Vakuurnphysik, Kernforschungsanlage Jiilich, D-5170 Jiilich, Fed. Rep. of Germany Received 6 April 1982

EELS spectra of a m m o n i a adsorbed on a F e ( l l 0 ) surface at 120 K reveal three different adsorption states of molecular ammonia. Thermal processing of the a m m o n i a covered Fe(ll0) surface to 315 K indicates fragmentation of the N H 3 molecules into atomic hydrogen and nitrogen. Formation of an N H 2 intermediate is not observed whereas the existence of NHad species cannot be excluded at present.

In view of the wide use of iron catalysts for the ammonia synthesis reaction, the adsorption of NH 3 on iron surfaces and the possible formation of intermediate surface compounds are of particular interest for elucidating the elementary reaction steps. Detailed investigations of the NHa/Fe(110) system have been performed by Weiss et al. [1] using LEED, UPS, AES, thermal desorption spectroscopy and w0rkfunction measurements, and by Drechsler et al. [2] using SIMS. Both studies report the molecular adsorption of ammonia at low temperatures (130 K), and the existence of an intermediate surface species, most probably NHad, between 300 and 400 K. High resolution electron energy loss spectroscopy (EELS) is a particularly appropriate technique to study adsorption and decomposition of ammonia on metal surfaces. Vibrational spectra of adsorbed ammonia reported by Sexton and Mitchell for Pt(111) [4] and by Gland et al. for Ag(110) [3] indicate three distinct adsorption states for molecular ammonia. On both surfaces no fragmentation of the adsorbed ammonia was observed upon thermal processing. The present experiments were performed in an UHV system which will be described elsewhere [5]. The EELS spectrometer is a single pass type. The sample could be cooled down to 120K and heated from the rear by the radiation from a tungsten filament. Details of the sample cleaning procedure have been described in a previous paper [6]. Ammonia of 99.8% purity and deuterated ammonia of 99% isotopic purity was introduced via an adjustable leak valve. All spectra were taken in the specular reflected direction using a fixed angle of incidence of 70 ° and a primary electron beam of 2.4 eV. A set of vibrational spectra taken after exposing a clean Fe(110) surface at 120 K to increasing doses of ammonia are shown in fig. 1. At low exposures 0039-6028/82/0000-0000/$02.75 © 1982 North-Holland

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W. Erley, H. lbach / Vibrational spectra of N H 3 on Fe(llO)

NH3onFe(110) {=120K

1105

i

~~1~

~1290

0,3L

20011

ZL ENERGY LOSS (em -I1

Fig. l. Vibrational spectra of NH 3 adsorbed on Fe(ll0) at 120 K taken after three different exposures.

(0.05 L) only three losses are observed: a strong loss at 1170 c m i a n d two m u c h w e a k e r ones at 350 a n d 3310 c m - ~ . A f t e r an e x p o s u r e of 0.3 L the losses at 1170 a n d 3310 c m -~ shift to lower frequencies a n d new losses a p p e a r at 1450, 1640 a n d 3370 c m - l . T h e loss at 360 cm - l strongly increases in intensity. A s the e x p o s u r e is increased to 2 L, two a d d i t i o n a l losses at 160 a n d 1190 cm i are o b s e r v e d a n d the loss at 360 c m - t b e c o m e s the most p r o m i n e n t one. It should be p o i n t e d out that the 1190 c m ~ loss does not result from a f r e q u e n c y shift o f the 1105 c m - t loss (observed at 0.3 L) as the 1105 c m z loss r e m a i n s clearly visible in the s p e c t r u m a a s h o u l d e r on the left side of the 1190 c m - l loss. F u r t h e r e x p o s u r e to a m m o n i a does not result in a significant c h a n g e o f the spectrum. The spectra are similar to those o b t a i n e d for a m m o n i a a d s o r p t i o n on the P t ( l l 1) a n d Ag(110) surfaces [3,4]. T h e v i b r a t i o n a l s p e c t r u m o b t a i n e d at low coverge (0.05 L) m a y be a t t r i b u t e d either to m o l e c u l a r a m m o n i a or to the p a r t i a l l y d i s s o c i a t e d species, N H 2 a n d N H . F o r an N H 2 species with C2v s y m m e t r y three d i p o l e excited v i b r a t i o n a l m o d e s s h o u l d b e observed. H o w e v e r we reject this p o s s i b i l i t y as no H N H scissor m o d e n e a r 1600 c m - ~ is observed. F o r an N H species with C2v or C s

W. Erley, H. Ibach / Vibrational spectra of NHj on Fe(l lO)

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symmetry (respectively), two or four vibrational modes should be detectable, in contrast to the observation of three vibrational modes. Hence we give preference to an N H 3 molecule with C3v symmetry and the losses are assigned as the F e - N stretching (350 c m - I ) , the symmetric N H 3 deformation (1170 c m - l ) . Further evidence for the presence of molecular ammonia in this low coverage state will be given below in connection with the deuteration experiments. The C3v symmetry for the adsorbed ammonia molecule implies that the two-fold symmetry of the Fe(110) surface does not contribute to the over-all symmetry of the iron-ammonia adsorption complex. This indicates weak interaction of the ammonia molecules with the iron surface, a view which is supported by the small shift of the N H stretching vibration of the adsorbed molecule (3310 cm - I ) with respect to a free N H 3 molecule (3337 cm-~). The frequency of the deformation mode (1170 cm - I ) is close to the corresponding mode (1156 cm-1) in the hexamine salt Fe(NH3)6CI 2. By analogy with the results obtained for the NH3/Ag(110) system [4] the spectrum shown in fig. 1 after an exposure of 0.3 L may be attributed to a second state of chemisorbed ammonia. The losses at 3370, 3290 and 1640 c m - 1 are interpreted as the degenerate and symmetric N H 3 stretching and degenerate deformation modes, respectively. In excellent agreement with the present results, frequencies of 3380 , 3290 and 1610 cm -1 have been reported in an infrared study of ammonia adsorption on iron dispersed on silica [9]. Similar to the Ag(110) and Pt(l 11) results, the loss at 1450 c m - i is a combination of the losses at 350 and 1105 c m - i. The appearance of the degenerate N H 3 stretching and deformation modes is consistent with an adsorption complex of C s symmetry. The lowering of symmetry may be a consequence of a stronger interaction between the adsorbate molecules and the substrate. This view is supported by the down-shift of both the symmetric N H 3 stretching and deformation modes. Hydrogen bonding of the ammonia adsorbed in the second state has been considered as a possibility [4], and may explain the increased interaction with the substrate. The two intense losses at 160 and 350 cm -I (140 and 280 cm - I for ND3) observed after an ammonia exposure of 2 L are interpreted as a hindered translation and a hindered rotation (libration), of condensed solid ammonia. The small shoulder at 1100 cm - t indicates that the chemisorption state of ammonia is still present on the Fe(110) surface. Unfortunately, measurements carried out with N D 3 are complicated by exchange reactions with hydrogen from the wails of the system. Mass spectrometer observations during the inlet of N D 3 revealed considerable amounts of mass 17, 18 and 19, which are due to N H 3, N H 2 D and N H D 2. As this problem could be only partially overcome by flushing the system with D 2 prior to the N D 3 inlet, no adsorption spectrum of pure deuterated ammonia can be presented here. Dosing the Fe(110) surface at 120 K with 0.05 L N D 3 leads to the spectrum shown in fig. 2, where instead of a single loss at 910 c m - I due to the symmetric

W. Erley, H. lbach / Vibrational spectra of NH~

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33

I

on

Fe(llO)

Fe (110) T = 120 K 0,05L ND3

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178(

0

"~0

'

2600

~

-

ENERGY LOSS (cm -1)

Fig. 2. Vibrational spectrum taken after exposing the Fe(110) surface at 120 K to 0.05 L ND 3.

deformation mode of adsorbed N D 3, three additional modes at 1000, 1100 and at 1180 cm -I are observed. The latter is clearly due to the symmetric deformation mode of N H 3 (fig. 1), and the remaining two may be attributed to the symmetric deformation modes of N H 2 D and N D z H , respectively. Nevertheless the spectrum shown in fig. 2 is a very useful result as it immediately proves the existence of molecular ammonia in the low coverage state (0.05 L). Fig. 3 shows a sequential heating of the Fe(110) sample after an exposure to 2 L ammonia at 120 K. All spectra were recorded after the sample had re-cooled to 120 K. Heating to 195 K causes the desorption of the condensed solid ammonia

T=120 K ~ ] ~ _

T=~SK

~

ENER6Y LOSS (cm-I1

Fig. 3. Vibrational spectra of N H 3 on Fe(110) adsorbed at 120 K and after warming to 195 and 315 K, respectively:

W. Erley, H. lbach / Vibrational spectra of NH j on Fe(l 1O)

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layer as indicated by the nearly complete disappearance of the vibrational mode at 350 cm-~. Simultaneously, the mode at 1170 c m - l is shifted to 1105 c m - l , the latter being characteristic for molecular ammonia adsorbed in the second layer. This is in agreement with the findings of Weiss et al. [1] who reported an N H 3 desorption maximum at 165 K. The observed shifts in some of the peaks and the appearance of an additional feature at 1580 c m - l may possibly indicate the formation of an intermediate surface species. Heating to 315 K leads to the observance of losses at 500, 880, 1020, 1170, and 3310 cm-~. The losses at 1170 and 3310 c m - l are identified as chemisorbed N H 3 in the first layer (fig. 1) as both losses increase in intensity during the sample cools to 120K. These losses are therefore assumed tO be caused by re-adsorption from the residual gas. The losses at 880 and 1020 c m - l may be identified as the asymmetric and symmetric stretching vibrations of atomically adsorbed hydrogen, in accordance to a previous investigation of H 2 on Fe(110)

[l]. The loss at 500 cm-~ is due to the symmetric stretching vibration of atomically adsorbed nitrogen. Fig. 4 shows a vibrational spectrum obtained after an exposure of 30 L N H 3 to the Fe(110) surface held at 450 K. No other species than nitrogen was present on the surface as verified by AES. A similar result for the F e - N stretching vibration (500 cm -1) has been reported in an infra-red study on ammonia adsorption at evaporated iron films

[i11. Hence, apart from the small amount of re-adsorbed ammonia, the spectrum in the lower part of fig. 3 is due to atomic hydrogen and nitrogen which indicates fragmentation of N H 3 molecules after the sample is heated above room temperature. The existence of an adsorbed N H 2 species can be excluded as no scissor vibration near 1600 cm-~ is observed. However, at present, the existence of an intermediate species like NHad cannot definitely be excluded as the corresponding N H stretching and bending modes may be obscured by the re-adsorption of ammonia.

I ~ i I.~ I~

I~on Fe(110)

' '

ENERGY LOSS (crn-s) Fig. 4. Vibrational spectrum taken after exposingthe Fe(l ]0) surface at 450 K to 30 L NH 3.

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IV. Erley, tt. Ibach / Vibrational spectra of N H 3 on Fe(llO)

B e c a u s e o f the c o m p l i c a t i o n s w h i c h arise d u r i n g t h e d e u t e r a t i o n a n d therm a l p r o c e s s i n g e x p e r i m e n t s ( i s o t o p i c e x c h a n g e r e a c t i o n s a n d r e - a d s o r p t i o n ) , an e x t e n s i o n o f this w o r k u s i n g a d o s e r is c u r r e n t l y u n d e r w a y .

References [1] M. Weiss, G. Ertl and F. Nitschk6, Appl. Surface Sci. 2 (1979) 61,4. [2] M. Drechsler, H. Hoinkes, H. Kaarmann, H. Wilsch, G. Ertl and M. Weiss, Appl. Surface Sci. 3 (1979) 217. [3] J.L. Gland, B.A. Sexton and G.E. Mitchell, Surface Sci. 115 (1982) 623. [4] B.A. Sexton and G.E. Mitchell, Surface Sci. 99 (1980) 523. [5] W. Erley, to be published. [6] W. Erley, J. Vacuum Sci. Technol. 18 (1981) 472. [7] T. Shimanouchi, Tables of Molecular Vibrational Frequencies, Consolidated Vol. I, NSRDSNBS 39. [8] K.H. Schmidt and A. Miiller, Coord. Chem. Rev. 19 (1976) 41. [9] T. Nakata and S. Matsushita, J. Phys. Chem. 72 (1968) 458. [10] A.M. Bar6 and W. Erley, Surface Sci. 112 (1981) L759. [11] T. Okawa, T. Onishi and K. Tamaru, Chemistry Letters (1977) 1077.