908
M. J. D. Low
AND
KUN-ICHIMATSUSHITA
Infrared Spectra of the Surface Species Produced by Reactions of Ammonia with Germania Gel by M. J. D. Low and Kun-ichi Matsushita Department of Chemistry, N e w York Univresity, New Y o r k , Neu) York
I0453
(Received August 8 9 , 1 9 6 8 )
Infrared spectra were recorded from 4000 to 500 om-' of ammonia sorbed on dehydroxylated germania gel. Extensive dissociation of ammonia occurred, with the formation of Ge-OH groups. The spectra showed that beside physisorbed ammonia, which gave rise to absorptions at 3600-2500, 3420, 3312, and 1610 cm-I as well as a previously unreported one near 1260 cxndl, at least four other surface species were formed. Bands at 3363, 3267, 1600, 1210, and 700 cm-l were attributed to ammonia weakly coordinated to surface germanium atoms. Bands at 3467, 3393, and 1535 cm-I were attributed to a more strongly bonded primary amine which might be the precursor of a secondary amine giving rise to a band at 3397 em-'. -4minor band near 2195 cm-I was attributed to Ge-H structure.
Introduction -4lthough germanium-containing devices are widely used and "real" germanium surfaces have been studied extensively, relatively little is known about the nature of the surface species formed when gases become adsorbed on germanium or germanium oxide surfaces. However, infrared spectroscopic techniques are useful for the study of such reactions and, using germania gel as analog of real germanium, the formation of some surface groupings has recently been described. The formation of hydroxyls' and Ge-H groups2 was observed, and some surface structures resulting from the reactions of germania with acetic and propionic acids3 as well as with methanol, formaldehyde, and formic acid4 have been described. Those studies have now been extended to the reaction of ammonia with germania gel surfaces.
Experimental Section The preparation of the germania gel and many other experimental procedures have been described elsewhere.* Prior to exposure to ammonia, a fresh pellet was degassed at 500" for 5 hr, heated for 5 hr in 300 Torr of oxygen at 500°, and then degassed for 5 hr a t 550". Ammonia of 99.99% purity was purchased from Matheson Co. and was vacuum distilled and subjected to freeze-pump-thaw cycles. Spectra were recorded with a Perkin-Elmer Model 621 spectrophotometer. As the transmittances of the samples were below 20%, screens were extensively used in the reference beam and ordinate scale expansions were used as required.
Results and Discussion Trace A of Figure 1 is a typical "background" spectrum of well-degassed germania gel and shows a weak band near 3670 em-' (hereafter termed the Ge-OH band) attributed'J to isolated surface Ge-OH groups; there are also very strong absorptions of the germania below 1600 cm-l with "windows" near 1300-1100 and 700-500 cm-l. When such a specimen was exposed to The Journal of Physical Chemistry
ammonia at 25" at low pressure (spectrum €3, Figure 1j the Ge-OH band increased in intensity, a broad band attributed1t2 to hydrogen-bonded hydroxyls formed near 3570 cm-I (e.g., spectrum C, Figure 1), and other absorptions were observed over the entire spectral range which was examined. The intensity of the Ge-OH band declined with increasing ammonia pressure (spectra A, B, Figure 2 ) but increased when sorbed ammonia was removed. On degassing, the Ge-OH and 3570-cm-' bands exhibited behavior very much like that observed2 when hydroxylated germania was degassed. Many of the absorptions, especially those in the K-H region, broadened and were obscured when the amount of sorbed ammonia was increased. A very broad absorption formed at higher ammonia pressures in the 3600-2500-cm-' range (spectrum B, Figure 2) and, as in the case of ammonia sorption on porous glass5 or silica,6 is attributed to physically adsorbed ammonia. Although the spectra were complex, it was possible to use observations on changes in band intensities and position, band shapes, and the like, to separate the numerous bands into several groups. Three major groups are given in Table I. Other, minor bands will be considered later. All bands of groups A, B, and C could be observed at low ammonia pressure. With increasing pressure, group A bands grew more than group B bands; group C bands did not seem to be affected, but it was difficult to be certain of this because the group A bands obscured the others. Although all of these bands could be diminished by pumping a t low temperature, the group C bands were diminished more slowly than others and (1) M. J. D. Low and P. Ramamurthy, Chem. Commun., 733 (1966). (2) M. J. D. Low, N. Madison, and P . Ramamurthy, Surface Sci., in press. (3) J. C. McManus and M. J. D. Low, J. P h y s . Chem., 7 2 , 2378 (1968). ( 4 ) J. C . RlcManus, K-I. Matsushita, and 31. J . D . Low, C a n . J.
Chem., in press. ( 5 ) &I. J. D. Low. N. Ramasabramanian, and V. V. Subba Rao, J. P h y s . Chem., 71, 1726 (1967). (6) N. W. Cant and L. €1. Little, Can. J. Chem., 4 3 , 1252 (1965).
909
REACTIONS OF AMMONIA WITH GERMANIA GEL Table I: Major Bands of Adsorbed Ammonia Grow A cm-1
Group B cm-1 H
Physisorbed ammonia 3420 3312 1610
H
\I/
.. N
H
Group C cm-1 H
\
/H N.
I
Ge
Ge
3363 3267 1600
Figure 1. Ammonia sorption-low pressure. A well-degassed germania specimen (spectrum A) was exposed to 0.5 Torr of NHs for 1 hr at 25" and degassed for 30 min at 25" (spectrum B). The sample was then degassed at 25" for 10 hr (spectrum C ) and for 30 min at 100" (D), 215" (E), 335" (F), and 410" (G). The ordinates are displaced to avoid overlapping. The segments of spectra C to G near 3000 aiid near 2200 cm-1, as well as those of the insert showing the 900-2OO-cm-' range, were recorded at 5 times the ordinate expansion. 1
L
I
*
I
3500
I
,
'
I
I
I
3 m
,
I
,:
E I
I
2 m
I
I
I
I
I
1500
I
I
I
&)
I
'
Figure 2. Ammonia sorption-high pressure. A well-degassed germania specimen (spectrum A) was exposed to 26 Torr of ammonia at 25" for 12 hr (spectrum B) and was then degassed for 5 min at 25" (C), 16 hr at 25" (D), and 30 min at 110" (E). The ordinates are displaced.
were removed by pumping above 100". Group A bands declined somewhat more rapidly than group B bands and were almost completely removed by pumping at 25" for 0.5 day. Group B bands were retained after pumping a t 25" for several days. However, differences in rates were difficult t o discern in the N-H region. Also, a band increased and shifted from 1603 to 1610 cm-l when the ammonia sorption increased, declined, and shifted to 1603 em-' when much of the weakly held ammonia was removed, and then declined further and shifted to 1600 cm-I on further degassing near 100". The decline and shift from 1610 em-l
1210 700
3467 3393
va
1535
6 ("2) 6. ("a) PF ( "a)
vs
(N-W (N-H)
ad ("a) N
-1260
Assignment
paralleled the decline of the very broad band of physically adsorbed ammonia and of an absorption near 1260 cm-' (the latter appears as a broad shoulder in spectrum B, Figure 2). Such effects suggested that the same weakly held species responsible for the 36002500-, 3420-, 3312-, and 126O-cm-' absorptions caused much of the absorptions near 1610 em-', while a second weakly but somewhat more tightly held species brought about the absorption at 1600 cm--l and other group B bands. A third species was responsible for group C. The position and behavior of the 3420-, 3312-, and 1610-~m-~bands and the broad 3600-2500-cm-' absorption are very similar to those found when ammonia was physically adsorbed on a variety of solids. The effects were reviewed by Little.' There is consequently little doubt that the group A bands were caused by physically adsorbed ammonia. The band assignments are given in Table I and are like those reported prev i o u ~ l ywith , ~ the exception of that a t 1260 em-'. Such an absorption has not been observed before because adsorbents such as silicas or aluminas used in earlier studies absorb very strongly in the region where the low-frequency ammonia deformation band would be expected. The 1260-crn-' absorption exhibits a fairly large shift from the positions of the deformation bands of gaseous (968, 932 em-') and crystalline* (1060 ern-') ammonia, but the shift is in the direction expected for a strongly hydrogen-bonded s y ~ t e m . ~Unfortunately, ,~ the occurrence of the absorption as a broad shoulder causes uncertainty about its position, so that these considerations must remain qualitative. The five bands termed group B show good correspondence with those of typical amine complexes. These have been reviewed by Nakamoto.lo Also, the (7) L. H. Little, "Infrared Spectra of Adsorbed Species," Academic Press, New York, N. Y., 1966. ( 8 ) M. E. Jacox and D. E. Milligan, Spectrochim. Acta, 19, 1173
(1963). (9) V. N. Abramov. A. V. Kiselev, and V. I. Lygin, Russ. J. Phus" Chem., 38, 1020 (1964). (10) K. Nakamoto, "Infrared Spectra of Inorganic and Coordination Compounds," John Wiley and Sons, Inc., New York, N. Y., 1963, p 143 ff. Volume 76,Number 4 April 1869
910
M. J. D. Low
two bands in the N-H stretching region are similar to bands attributed to ammonia coordinated to surface boron atoms on porous glass and boria-impregnated silica16and the stretching bands and the two bending bands are similar to bands attributed to ammonia coordinated to surface aluminum atoms. The various bands observed on chemisorption of ammonia on aluminas and silica-aluminas were reviewed by Little.7 I n view of these similarities, the group B bands are attributed to a weakIy bound surface complex consisting of ammonia coordinated to a germanium atom, The assignments given in Table I are based on earlier ~ n e s . ~ , ~ The J O 700-cm-' rocking band has not been previously reported, because it falls in a region in which previously used adsorbents absorb very strongly. A Ge-N stretching band, which would be expected in the 300-40O-~m-~rangello was not observed because the germania was opaque below about 500 cm-'. The two bands at 3467 and 3393 cm-l of group C fall in the N-H stretching region, closely fit Bellamy and Williams' equation relating the frequencies of asymmetric and symmetric stretching fundamentals of primary amines" and also are similar to those of -XH2 species formed on porous g l a ~ s silicas, , ~ ~ ~ and aluminas.' The 2535-cm-l band was superimposed on an intense band of the germania, and consequently was weak and indistinct. However, spectra of amido complexes suggest that an -NH2 deformation bandlo would occur near 1500-1600 cm-', and bands assigned to such a mode of dissociatedly adsorbed ammonia were observed a t 1555 cm-l with silica12 and a t 1560 and 1510 cm-1 with a1~mina.l~It is consequently not unreasonable to assign the group C bands to a primary surface amine formed by a reaction
H 0
\ / N
H
OH
which will also lead to the formation of the bands of isolated and perturbed hydroxyl groups. One stretching band of the primary amine was observed near 3467 cm-" a t relatively high stages of coverage, declined, and shifted to near 3470 cm-' on degassing a t lOO", and along with the 1535-cm-' band, disappeared above 100". The second stretching band was originally observed near 3390 cm-', declined, and shifted to near 3393 cm-' on mild pumping, but then shifted to 3397 cm-1 and declined only s~owlywith increasing severity of degassing conditions. The 3397cm-I band was removed at 500". The effects suggest that some o€ the weakly bound material was converted to a relatively strongly bound species. The interpretation is difficult and somewhat uncertain, because only one band is available. However, the single 3397-cm-' band €ails in the region of N-H stretching fundamentals The Journal of Physical Chemistry
AND
KUN-ICHIMATSUSHITA
where a single band due to a secondary amine would be expected. In view of this and the manner of formation of the band, the 3397-cm-' band is assigned to a surface secondary amine formed by a reaction such as
H
\ / N I Ge
H
H
I
Ge
+ HzO
N
OH --+
/ \
Ge
(2)
Ge
The water formed could remain adsorbed or react with the germania to produce hydroxyls, either of which would contribute to the broad 3570-cm-' band, or be desorbed at the higher temperatures. .4 small band was formed near 2966 cm-', increased slightly when weakly bound material was desorbed (spectra B-D, Figure 2), but declined only slightly a t higher temperatures. Two smaller bands could be observed near 2920 and 2870 cm-' by means of scale expansion (Figure 1). The three bands appeared to be largely independent of the change of other bands, and persisted even after degassing at 500". The 2966-cm-' band (and also the two other less intense bands) falls in the range expected for a stretching fundamental of KHlf, on the basis of earlier work on surface NH4+ reviewed by Little.' Bands attributable to an NH4+ deformation were not observed, but these, if present, would be superimposed on the intense absorption of the germania in the 1300-1600-~m-~region, so that detection would be difficult. The small bands might be caused by NHc+ formed by a reaction of ammonia with hydroxyls, but the assignment is specdative. An alternate explanation is that the bands were caused by a carbonaceous contaminant. A very small band whose behavior was independent of that of other bands was detected near 2195 cm-l. It began to decline on pumping near 200" and exhibited behavior much like that of bands formed by the reaction of water or hydrogen with germania.2 The 2195-cm-' band is consequently attributed to surface Ge-H groups. These could be formed directly through the dissociation of ammonia or from the water2 formed in reaction 2. The mechanism is uncertain. However, the Ge-H band was not significantly changed when the amount of sorbed ammonia was increased, suggesting that the reaction leading to the formation of the Ge-H species could not occur extensively because one of the reactants-possibly a portion of the surface which consisted of Ge or GeO-was present to a limited extent.
Acknowledgment. Support by means of a grant from the National Center for Air Pollution Control is gratefully acknowledged. (11) L. J. Bellamy and R . L. Williams, Spectrochbm. Acta, 9, 341 (1967). (12) J , B. Peri, J , PhUs, Chem., ,o, 2 9 3 , (1966). ( 1 3 ) J. B. Peri. ( b i d . , 69, 231 (1965).
