Blomfield1972NH4Si

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Chemisorption of Ammonia on Silica
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Departmetzt of Physical atzd Itlorganic Chemistry, The University of Western Australin,
Nedlnnds, Western Australia, 6009
Received August 31, 1972
The interaction of ammonia with silicas prepared by a variety of methods was studied to resolve conflicting reports of the mode of ammonia adsorption and to determine the role of chlorine impurities (present
in the silica) in the adsorption process. Results for the adsorption of water on silica assisted in making band
assignments and competition between ammonia and water for silica adsorption sites was observed. It was
concluded that dehyd;oxylated silicas contain sites which dissociateammonia to form Si-NH2 groups having
- infrared bands at 3540.3450. and 1550cm-' (the surface amine -HrouDs
. are not dis~lacedbv added water).
'The presence of chlorine in the silica is not a prerequisite for chemisorption of ammonia.
L'interaction de I'ammoniac avec des silices prCparCes selon plusieurs mtthodes a ttC ttudite afin
d'Clucider les problemes issus du mode d'adsorption de l'ammoniac et afin de dtterminer le rBle des
impuretts du type chlore (presentes dans la silice) dans le processus d'adsorption. Les r6sultats obtenus
lors de l'adsorption de l'eau sur la silice ont contribui 2 l'attribution des bandes et la compCtition entre
I'adsorption de l'eau et celle de I'ammoniac sur les sites de la silice a CtC CtudiCe. I1 a CtC conclu que les
silices dthydroxylees contiennent des sites susceptibles de dissocier I'ammoniac ce qui entrainent laformation de groupes Si-NHz possidant eninfra-rouge des bandes 3540,3450, et 1550cm-' (les groupes amines
en surface ne sont pas dCplacCs lorsque I'on additionne de l'eau). La prtsence du chlore dans la silice n'est
[Traduit par le journal]
pas une condition nCcessaire a la chCmisorption de l'ammoniac.
Can I . Chrm.. 51, 1771 (1973)
Mapes and E~schens (1) reported that
ammonia was only physically adsorbed to pure
silica and that no chemisorption occurred. Later
reports for silicas evacuated at temperatures
of 200-800 "C confirmed these results (2-6).
Bands which appear at 3400, 3320, and 1625
cm-l in the infrared spectrum have been
assigned to ammonia hydrogen bonded to
hydroxyl groups on the silica surface (2, 4, 6).
More recently, extra bands at approximately
3525, 3440, and 1555 cm-' have been reported
in the spectrum of ammonia adsorbed on silica
and these have been assigned to vibrations of a
Si-NH,
group formed by chemisorption of
ammonia to the silica (8-10). However, other
workers using similar samples and similar pretreatment temperatures still found no evidence
for chemisorption (6, 11, 12).
In one case for which chemisorption was
reported (8), the silica was prepared by a method
involving the use of hydrochloric acid solution
and it was found that the band intensities of
chemisorbed groups were enhanced by pretreatment of the silica with chlorine or carbon
tetrachloride vapors. The other reports of
chemisorption considered adsorption onto Cab0-Sil (10) or Aerosil (9) silicas and we have
found that these powders contain traces of
chlorine or chloride impurities. Folman (13)
had previously reported the formation of
Si-NH, groups when ammonia was adsorbed
on chlorinated porous silica glass. It is possible
that the presence of chlorine in silica samples is
necessary for chemisorption of ammonia. The
formation of Si-NH, groups has been reported
for ammonia adsorbed on unchlorinated porous
silica glass (14) but the existence of such groups
was questioned (15) in view of the many reports
which suggest that ammonia is not chemisorbed
to silica.
In order to resolve the question of chemisorption of ammonia on silica and the role of chlorine
and to distinguish between primary amine and
secondary amine groups for ammonia adsorbed
on porous silica glass (14, 16), the adsorption
of ammonia onto silicas prepared by a variety
of methods was investigated using infrared
spectroscopy. It was concluded that dehydroxylated silicas contain sites which dissociate
ammonia to form Si-NH, groups. The presence
of chlorine in the silicas is not a prerequisite for
chemisorption of ammonia.
Experimental
A variety of silica powders was studied. Cab-0-Sil
HS-5 and Cab-0-Sil M-5 silicas were supplied by
Godfrey L. Cabot Corporation, Boston, Massachusetts.
Aerosil silica is manufactured by Degussa, Frankfurt a m
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1772
CAN. J. CHEM. VOL. 5 1 . 1973
Main, Germany. These silicas are prepared by the flame
hydrolysis of silicon tetrachloride. A silica powder
prepared by precipitation from sodium silicate was
obtained (17).
Ludox SM silica was supplied by Dupont, Wilmington,
Delaware, as an aqueous sol. Freeze-drying this suspension (initially diluted ten times with distilled, de-ionized
water) produced a fluffy white powder. A modified silica
was prepared by adding dilute hydrochloric acid to the
sol before freeze-drying. The resulting flocculation led to
a decrease in the surface area of the chlorinated powder
so obtained.
A silica powder was prepared by the hydrolysis of
ethyl ortho-silicate. Approximately 10 g of ethyl orthosilicate was added to 250 ml of distilled, de-ionized water
in a polythene beaker and the solution was stirred continuously for 4 days at 20 "C. The resulting silica suspension was freeze-dried.
A chloride-free (as determined by X-ray fluorescence
spectroscopy) silica powder was prepared by refluxing
approximately 3 g of Cab-0-Sil HS-5 silica for 6 h in
concentrated nitric acid. This silica was then washed
repeatedly in distilled, de-ionized water, dried at 100 "C
and crushed to a fine powder.
Each of these powders was compressed under a pressure of 10 tons/sq in. to form rectangular pellets of 10-35
mg/cmz. Samples were heated to 400 "C in oxygen before
use to remove hydrocarbon impurities.
Pellets of Aerosil silica were suspended in refluxing
thionyl chloride for 7 days to induce the exchange of
hydroxyl groups on the silica surface by chloride groups.
A similar method has been used to chlorinate porous
silica glass (1 3).
The possible effect of iron impurities introduced onto
the silica through compression in the steel pellet presses
was investigated. Samples of silica - ferric oxide were
prepared by soaking pellets of Cab-0-Sil HS-5 silica in a
solution of 0.1 M ferric chloride in 2 M hydrochloric acid.
These were then heated in flowing oxygen at 400 "C for
10-12 h to decompose the ferric chloride.
F o r adsorption onto all samples, anhydrous ammonia
obtained from Matheson Company Inc., was used.
Surface areas were determined by the B.E.T. method
using nitrogen as the adsorbate. The relative chlorine
concentrations in each type of silica was determined by
X-ray fluorescence spectroscopy using a "Geigerflex X-ray
Spectrometer" model K G - 315 manufactured by RigakuDenki Company Ltd., Tokyo, Japan.
Infrared spectra were recorded on a Perkin-Elmer 521
spectrophotometer in the region 4000 to 1300 cm-'. The
spectral slit width was approximately 4 at 3800 cm-' and
approximately 2 cm-' in the 1600 cm-' region. A modified windlass cell (16) with a Kanthal wound, silica
furnace section permitted evacuation of samples at
temperatures up to 900 "C. All spectra were recorded at
room temperature and usually a second silica pellet was
placed in the reference beam of the spectrometer to
compensate for absorption bands due to Si--0 vibrations
in spectra below 2000 cm-'. Weak bands in the region
3800-3600 cm-' and 1800-1500 cm-' are due to uncompensated water vapor.
Results and Discussion
Spectra of a typical sample of Cab-0-Sil HS-5
silica recorded before and after the addition of
ammonia are shown in Fig. 1. Approximately
5 Torr of ammonia was admitted to a sample
which had been evacuated at 450 "C (Fig. la)
and bands appeared in the spectrum at 3400,
3320,3250,2980, 1625, and 1550 cm-' (Fig. 1b).
The bands at 3400, 3320, and 1625 cm-' have
been assigned to the N-H vibrations of ammonia hydrogen bonded to surface hydroxyl groups
(2, 4), and the band at 2980 cm-' is due to the
0-H
stretching vibration of these hydrogen
bonded groups (4). A broad weak band at 3250
cm-' (Fig. lb) is probably due to the first overtone of the N-H deformation fundamental at
1625 cm-'.
Condensing the gas into a liquid air cooled
side-arm on the cell removes all the bands due
to this hydrogen bonded ammonia from the
spectrum and only bands at 3450 and 1550 cm-'
remain (Fig. lc). If a larger pressure of gas is
admitted to the sample and then condensed the
intensities of these bands increase and an additional band appears at 3520 cm-' (Fig. Id). The
thickness of samples used in this study was
greater than for previous studies in which these
bands were undetected (4). These three bands
remain in the spectrum of the sample after
evacuation at 450 "C and must be produced by
one or more species which are chemisorbed to
the silica. Folman (13) assigned bands at 3520
and 3540 cm-' to Si-NH, groups when ammonia was adsorbed on chlorinated porous silica
glass and others have found similar bands for
ammonia on dehydroxylated silicas (8-10). All
previous reports of ammonia adsorbed on pure
silica evacuated at temperatures below 600 "C
have concluded that only physical adsorption
occurs but the results aboveindicate that chemisorbed groups are formed as well. It is likely that
the smaller gas pressures, the thinner samples,
and shorter times of standing used in these
former studies produced only low concentrations
of chemisorbed species and these were not
detected.
The spectrum of Cab-0-Sil HS-5 silica after
evacuation at 850 "C (Fig. le) contains a very
sharp peak at 3747 cm-' due to the isolated
hydroxyl groups on the silica surface. Admission
of approximately 5 Torr of ammonia to this
sample produces the spectrum shown in Fig. If.
The bands at 3420, 3337, 3320, 3050, and 1625
cm- which disappear when the gas is condensed
(Fig. lg) are similar to those for ammonia
physically adsorbed to Cab-0-Sil evacuated at
450 "C (Fig. lb) except that the band corre-
'
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4000
3600
3200
2800
17CO
1500
Frequency (crn-' )
FIG. I . The infrared spectrum of a sample of Cab-0-Sil HS-5 silica: a,after evacuation for 3 h at 450 "C; b, after
the addition of approximately 5 Torr of ammonia; c, after condensing the gas; d, after adding 150Torr of ammonia
and condensing the gas; e, after evacuation for 5 h at 850 " C ; ] ,after adding approximately 5 Torr of ammonia; g, after
condensing the gas.
sponding to the asymmetric N-H
stretching
vibration of these hydrogen bonded ammonia
molecules has shifted from 3400 to 3420cm-' and
the half-width has decreased. This band is now
very sharp suggesting that, unlike the case for
Cab-0-Sil evacuatec! at 450 "C (Fig. lb), the am-
monia molecules, hydrogen bonded to single isolated hydroxyl groups are also isolated from one
another and from nearby hydroxyl groups and
there is no interaction between adjacent groups.
Similar sharpening of bands of co-ordinated
ammonia were reported on high temperature
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1774
C A N . J . CHEM. VOL. 51, 1973
evacuated porous glass (16). The 0-H
stretching vibration of these OH...NH, groups
has shifted from 2980 cm-' (Fig. lb) to 3050
cm-' (Fig. If) indicating that the hydrogen
bonds between the ammonia and the hydroxyl
groups are weaker for ammonia adsorbed on the
more dehydrated sample. The band at 3337 cm-'
is due to the symmetric N-H stretching vibration of gaseous ammonia.
The bands at 3520,3450, and 1550 cm-' which
remain in the spectrum after the gas has been
condensed (Fig. lg) are similar to those found
by other workers (8-10). The bands at 3450 and
1550 cm-' arise from the symmetric stretching
and the asymmetric deformation modes respectively of the species Si-NH,.
It will be shown
later that the asymmetric stretching mode of this
group has a band at 3540 cm-' and that the
three bands at 3540, 3450, and 1550 cm-' arise
species which remains
from a single Si-NH,
on the sample after evacuation at 400°C or
above. These assignments are confirmed by
deuteration studies with ND, adsorbed on
deuterated silica. The corresponding Si-ND,
stretching bands occur at 2630 and 2526 cm-'.
It is possible that a hydroxyl species formed
during the reaction of ammonia with the silica,
has a band at 3520 cm-' which overlaps this
3540 cm-' band for spectra of samples after the
gas is condensed (Fig. lg).
In addition, a weak shoulder at 3400 c h - '
which was not reported in previous studies
(8-10) was present in the spectrum of amnlonia
chemisorbed to dehydrated silica (Fig. Ig), and
this band disappears after evacuation of the
sample at 100-150 "C. The assignment of this
band to strongly physically adsorbed ammonia
will be considered in a later section.
Similar results were obtained for ammonia
chemisorbed to Cab-0-Sil M-5 and Aerosil
silicas, although the concentration of chemisorbed species was less than for Cab-0-Sil HS-5
which has a larger surface area. Samples were
dehydrated at either 450, 650, or 800 "C before
admission of ammonia and for each type of
silica, the concentration of the chemisorbed
species was greatest for samples which had been
evacuated at 800 "C. The intensities of these
amine bands also increased if larger ammonia
gas pressures were admitted, if the sample was
left to stand in the gas for longer periods (12 h)
or if the sample was heated in the gas. No further
increase was observed when the evacuated
sample was immersed in liquid ammonia.
Pellets prepared from the Ludox silica did not
transmit infrared radiation as readily as the other
silicas and their spectra have a steeply sloping
background due to scattering losses. When
100 Torr of ammonia was admitted to a sample
which has been evacuated at 450 "C, bands
appeared in the spectrum at 3450, 3400, 3320,
and 1550 cm-' after the gas was condensed.
Heating the sample at 450 "C in ammonia gas
increased the concentration of the chemisorbed
species and a band appeared at 3520 cm-'. The
bands at 3400 and 3320 cm-' were not affected
by the heat treatment and remain after evacuation at 20 "C but disappear after evacuation at
150 "C.
The concentration of the chemisorbed species
formed on the Ludox sample evacuated at
600 "C after treatment with ammonia is greater
than after the 450 "C evacuation. However a
similar concentration of Si-NH, groups results
from heating the sample in the gas. The bands
at 3400 and 3320 cm-' are present and disappear
after evacuation at 150 "C.Gross sintering of the
Ludox silica occurs after evacuation at 800 "C
and no chemisorption of ammonia was detected
then.
Samples of the silica prepared from sodium
silicate (17) were more transparent to infrared
radiation than the Ludox silica. Similar spectra
were obtained after admission and evacuation
of ammonia to pellets dehydrated at 450 or
800 "C. As well as bands due to Si-NH, groups,
the infrared spectrum contained bands at 3400
and 3320 cm-' which could be removed by
evacuating the sample at 150 "C.
For the Cab-0-Sil and Aerosil silicas, admission of ammonia after evacuation at 600-800 "C
produced a band at 3400 cm-' in the spectrum
and this remained after evacuation of the sample
at 20 "C (Fig. lg) but disappeared after evacuation at 15&200 "C. This band was not present
in the spectra of samples which had been dehydrated at only 450 "C before ammonia was
added (Fig. Id). It is probable that a second
band at 3320 cm-' accompanies this 3400 cm-'
band but it is very weak and its presence is often
difficult to detect. No band at 1625 cm-' was
found after evacuation of the ammonia at 20 "C
for any of the silicas studied.
It appears that some residual ammonia
physically adsorbed to the silica remains after
evacuation at 20 "C to produce the bands at
3400 and 3320 cm-'. The Ludox silica and the
silica precipitated from sodium silicate (both
1775
BLOMFIELD AND LITTLE: CHEMISORPTION OF AMMONIA ON SILICA
TABLE
1. Relative chlorine contents and surface areas of various silicas
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Relative chlorine
content (X-ray
fluoresence analysis)
Silica
C.P.S.
Cab-0-Sil HS-5
Cab-0-Sil M-5
Aerosil
Silica precipitated from sodium silicate (17)
Dried Ludox
Dried Ludox -t HCI solution
Hydrolyzed ethyl-ortho silicate
Cab-0-Sil nitric acid leached
Aerosil treated with thionyl chloride
Standard silicate rock (U.S. Geological
Survey reference G-2 (20)) containing
108 p.p.m. chlorine
28
0
52
68
70
152
66
0
470
(
Surface area after
evacuation at 800 "C
m2/g
278
165
157
224
240
187
260
152
140
170
227
200
-
-
30
prepared from solution) will probably consist
of particles containing fine pores in which hydrogen bonded ammonia molecules might be more
strongly held. For the Cab-0-Sil and Aerosil
silicas (prepared at over 1000 "C) it is only after
evacuation at temperatures above 600°C with
some loss of surface area, that sufficient ammonia remains after evacuation at 20 "C to produce
weak bands at 3400 and 3320 cm-' in the spectrum. The fact that the band due to the asymmetric stretching vibration of these ammonia
molecules appears at 3400 cm-' and not 3420
cm-' (Fig. 1f, g ) suggests that these molecules
exist in a more perturbing environment.
Griffiths et al. (18) reported bands at 3350 and
3280 cm-' when ammonia was adsorbed on
silica-supported platinum or on pure silica Samples. They assign these bands to ammonia
chemisorbed to platinum sites (Pt-NH, groups)
in the first instance and ammonia bound to
siloxane groups NH,-OISi /Si)
Surface area after
evacuation at 450 "C
m2/g
in the second.
They did not evacuate the gaseous ammonia from
the samples to verify that these chemisorbed ammonia groups were in fact chemically bound to
the adsorbent.
In the present investigation, much higher
resolution of bands could be obtained and no
bands at these frequencies were observed for any
of the ammonia-silica systems we have studied.
Bands at 3360 and 3260 cm-' were observed for
ammonia on silica supported oxides in the
presence of gaseous ammonia (19). These bands
were ascribed to physically adsorbed ammonia
molecules. I n view of the poor resolution for the
spectra in the study of Griffiths et al. (18) and
the lack of corroborative evidence to support
their assignments (1-6, 8-16) it seems unlikely
that chemically bound NH, groups are adsorbed
onto silica adsorbents. However, the assignment
of bands to Pt-NH,
groups may still apply.
Pellets of silica prepared from ethyl orthosilicate were very opaque and infrared transmission was less than 2% at 4000 cm-', even for
very thin pellets. However, after attenuation of
the reference beam, infrared spectra could be
recorded. Bands at 3450 and 1550cm-' were
present in the spectra of samples dehydrated
at 450 and 800 "C after adsorption and subsequent evacuation of ammonia. It appears that
this silica chemisorbs ammonia to form Si-NH,
groups.
Analysis of a pellet of Cab-0-Sil HS-5 by
X-ray fluorescence spectroscopy revealed that
for the elemental range sodium to tungsten,
small concentrations of iron and chlorine atoms
were present as well as the silicon atoms. The
iron impurity possibly resulted from the steel
press used to prepare the pellets, while the chlorine impurity was possibly derived from the
silicon tetrachloride used in the manufacture of
the Cab-0-Sil, although pellets of Cab-0-Sil
M-5 contained no detectable chlorine. Each
type of silica was analyzed for chlorine and the
results are shown in Table 1. The result for a
standard silicate rock (U.S. Geological Survey,
ref. G-2 (20)) containing 108 p.p.m. chlorine,
is included for comparison.
Cab-0-Sil M-5 silica containing no detectable
chlorine will form Si-NH,
groups after reac[I]
\
7Si-C'
+ NH3
-
\
H-C1
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1776
CAN. J. CHEM. VOL. 51. 1973
tion with ammonia. It appears therefore that the
chemisorption of ammonia on silica does not
depend upon the presence of chlorine in the
silica. The possibility still exists that chlorine
impurities will react according to the following
equation to produce more Si-NH,
groups
as postulated by Folman (13).
To test this possibility, a sample of Cab-0-Sil
HS-5 powder was treated with nitric acid to
remove chlorine (Table 1). This chlorine-free
silica still reacted with ammonia and in spite of
a slight reduction in surface area (Table l), the
intensities of the Si-NH, bands in the spectrum
of the sample were similar to those for ammonia
adsorbed on untreated Cab-0-Sil HS-5. It
appears that surface chlorine does not play a
prominent role in the production of surface
amine groups for the silicas studied.
Several pellets of Aerosil silica were treated
with thionyl chloride to exchange the surface
hydroxyl groups by chlorine. Only partial
chlorination of these samples was achieved since
their spectra still contained a strong band due
to SiOH groups. Folman (13) reported the
exchange of 90% of the hydroxyls on porous
silica glass for samples which had been treated
with thionyl chloride for 14 days. Boehm (21)
has found it was possible to react only 50% of
the hydroxyls on Aerosil with thionyl chloride.
After evacuation of these chlorinated samples
at 450 or 800 "C, ammonia was admitted to the
cell. In each case the intensities of Si-NH,
bands ~ r o d u c e dbv reaction with the ammonia
were no greater than for those formed on untreated Aerosil. Weak bands at 3150, 3050, and
2805 cm- ' due to NH4CI (13) were formed but
there seemed to be no relationship between the
concentration of NH4Cl produced and the
intensity of the Si-NH, bands. A side reaction
1 between ammonia and adsorbed chlorine
atoms may have occurred but this was not the
major reaction for the formation of Si-NH,
groups. Analysis of one of these chlorinated
pellets indicated a marked increase in chlorine
content (Table 1).
Presumably these chlorine groups, replacing
hydroxyls on the Aerosil pellets, were covalently
bound to silicon atoms. A sample of Ludox
silica which had been treated with hydrochloric
acid solution before drying also contained an
increased concentration of chlorine (Table l),
probably in an ionic form on the silica surface.
In this case, results for reaction with ammonia
were identical with those results obtained using
the untreated Ludox silica. Thus it can be seen
that the ability for silica to chemisorb ammonia
is independent of the presence of chlorine or
chloride and is a property of the silica itself.
It is possible that traces of iron introduced
into the silica samples during compression in the
steel pellet press and detected by X-ray fluorescence analysis, may have affected the adsorption of ammonia onto the silica. Any iron in the
silica samples would most likely be present as
ferric oxide since the pellets were heated in
oxygen before they were placed in the spectral
cell. Consequently the adsorption of ammonia
onto samples of ferric oxide supported in Cab0-Sil was investigated and these results have
been presented elsewhere (22). After admission
of ammonia, bands at 3450 and 1550 cm-' were
present in the spectra of the silica-ferric oxide
samples and more intense bands (assigned to
Fe-NH
roups) appeared at 3380, 3290, and
1605 crn'lg If the bands at 3520, 3450, and
1550 cm-' observed in the spectra of ammonia
chemisorbed to silica (Fig. I) were due to ammonia chemisorbed to iron impurities, additional
(and more intense) bands due to Fe-NH,
groups should also have been detected. This was
not the case and it appears that the possible iron
impurities in the silica samples were not responsible for the chemisorption of ammonia to produce bands in the spectrum at 3520, 3450, and
1550 cm-' (Fig. 1).
It has been proposed that the Si-NH, groups
on the silica surface are formed by the reaction
of ammonia with "highly strained" or ionic
siloxane bridges (8).
[21
/O\
Si
Si
+ NH3 ->
Si-NH2
+ SiOH
When ammonia is adsorbed on almost completely dehydroxylated porous silica glass, bands
at 3747 and 3704 cm-' due to isolated SiOH
and BOH groups respectively, appear at the same
time as bands due to amine groups (16). Ammonia reacts with the B-0-B
sites on the porous
glass more readily than with the siloxane sites.
Attempts to completely dehydroxylate a silica
pellet were not successful. A pellet of Cab-0-Sil
which has been evacuated for 5 d at 800 "C
retained 75% of the free hydroxyl groups that
were present after only 8 h evacuation at 800 OC
(measured by comparison of the optical densities
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BLOMFIELD AND LITTLE: CHEMIS(IRPTION O F AMMONIA ON SILICA
1777
It has been shown (19) that one of the perof the band at 3747 cm-I). When ammonia was
turbed hydroxyl groups which exist on mildly
added to this sample, bands due to Si-NH,
groups appeared in the spectrum but no apparent evacuated silica and rehydrated silica has a band
increase in the intensity of the hydroxyl band at 3520 cm-' which disappears from the spectrum of the sample after evacuation at 300 "C.
was observed.
If the strained siloxane sites on dehydrated It is possible that the SiOH groups formed by
silica can abstract a hydrogen from ammonia the reaction of ammonia with the silica surface
by the mechanism above, it is possible that an (reaction 2) will be in a strongly perturbed
stretching
environment such that their 0-H
adjacent site might react with the Si-NH,
groups formed and a secondary amine group frequency occurs at 3520 cm- '. These groups
would result. By comparison with spectra of are removed by evacuation at 300 "C. The
various silazanes and disilazanes, such groups rigorously dehydrated porous silica glass sample
should have an N-H stretching vibration in the investigated previously (16) contained no surface
region 3350-3370 cm-' (23) although it is groups before the admission of ammonia and
possible that this vibration may occur at a higher the majority of the hydroxyl groups formed by
frequency and overlap one of the bands due to reaction with ammonia would exist in an unperthe Si-NH,
groups. Low et al. (14) reported turbed environment and given rise to the 3747
that a band at 3455 cm-I (which has been cm-I band rather than the 3520cm-' band
assigned to a secondary B-NH-B
group on discussed above. For the band to occur at 3747
porous glass (14, 15)) was obscured by the cm-', there must be no hydrogen bonding
Si-NH, band at 3459 cm-'. In order to investi- between the OH and NH, groups shown in eq. 2.
gate the possible formation of secondary amine
An alternative explanation for the high rate
groups when ammonia is added to silica, a of decrease of the 3520-3540 cm-' band with
careful study was made of the desorption of evacuation at temperatures up to 400 "C may be
chemisorbed ammonia. Samples of Cab-0-Sil provided by the observation that often species
HS-5 silica evacuated at 800 "C were exposed to at very low coverages show an abnormally low
250 Torr of ammonia gas and evacuated for intensity for the band due to the asymmetric
2 h periods at 100 OC intervals. After each evacu- stretching vibration compared to that for the
ation, a spectrum of the sample plus adsorbed symmetric stretch. Only a t higher coverages does
species was recorded. The intensity changes of the asymmetric stretching band become apprecithe N-H
vibrations were similar for all the ably intense. This phenomenon has been obsamples studied and a typical sequence of spectra served for many systems: e.g. ethylene polymers
is shown in Fig. 2. After evacuation at 100 OC on Ziegler type catalysts where the band due to
(Fig. 2a) the band due to the asymmetric N-H
the asymmetric CH, stretching vibration is weak
stretching vibration is broad and is centered at compared to the symmetric stretch at low
3520-3540 cm-'. After evacuation at 300 "C coverage (24); also for the asymmetric stretching
(Fig. 2b), this band has become quite sharp and is vibration of co-ordinated ammonia on silica now centered at 3540 cm-'. For evacuation at boric oxide and porous silica glass (16). No
temperatures up to 400 " C , the optical density explanation can be given for these unusual
of this band decreases more rapidly than the intensity changes, although it must be apprecioptical densities of the bands at 3450 and 1550 ated that the band due to the asymmetric
cm-' but above this temperature all three bands stretching vibration of gaseous ammonia is
decrease at similar rates.
weak and only becomes appreciable when interThese observations suggest that as well as molecular interactions occur in condensed
Si-NH,
groups, a species having a band at states.
3520 cm-' is formed by the chemisorption of
For desorption of the chemisorbed species
ammonia onto silica and this is removed after from silica at temperatures above 400 " C , the
evacuation at 300 "C. The bands due to the NH2 three bands at 3540, 3450, and 1550cm-'
groups are relatively stable at this temperature decrease at approximately equal rates suggesting
and it would be expected that a secondary amine that a single species produces all three bands.
group should be equally stable. Therefore it is Consequently these bands are assigned to the
unlikely that the species producing the 3520 asymmetric stretching, the symmetric stretching,
cm-' band is an amine group.
and the asymmetric deformation modes, respec-
1778
CAN. J. CHEM. VOL. 51, 1973
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I
L
4000
I
I
I
I
3600
3200
'
2800
Frequency (cm- )
FIG.2. A sample of Cab-0-Sil HS-5 was evacuated for 10 h at 800 OC and after cooling, 250 Torr of ammonia was
admitted. Spectra were recorded: a, after subsequent evacuation for 2 h at 100 "C; b, after evacuation for 2 hat 300°C;
c, after evacuation for 2 h at 500 "C; d, after evacuation for 2 h at 700 OC.
tively, of the N-H
vibrations of Si-NH,
groups. These stretching frequencies are approximately 50 cm-' higher than the corresponding
frequencies for aminosilanes (23) but the
deformation frequency at 1550 cm- agrees
very closely.
It appears that no secondary amine groups
are formed by the chemisorption of ammonia
on silica. It has been proposed (14, 15) that the
band at 3450-3455 cm-' which appears in the
spectrum of ammonia chemisorbed to porous
'
silica glass is due to a secondary 'B-NH-
/
species and this is overlapped by the bands due
groups (14). For the Si-NH,
to the Si-NH,
groups discussed above, the intensity of the
band at 3540 cm-' is less than one-fifth of the
intensity of the band at 3450 cm-'. Therefore it
is probable that the band a t 3450-3455 cm-'
reported in the porous silica glass studies (14, 15)
is in fact the symmetric stretching vibration of
Si-NH,
groups. The accompanying band at
1779
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BLOMFlELD AND LITTLE: CHEMlSO1RPTION O F AMMONIA ON SILICA
3540 cm-' is very weak and cannot be distinguished from the 3568cm-' band due to
B-NH, groups. Thus it seems doubtful whether
secondary amine groups exist on either ammonia - porous glass or ammonia-silica systems.
For each type of silica examined, no chemisorption of ammonia was found until the sample
had been evacuated a t a temperature of 400 "C
or higher. The concentration of the Si-NH,
groups produced by reaction with ammonia
increased as the temperature of dehydration was
increased above this temperature. A sample of
Cab-0-Sil M-5 silica was evacuated a t successively higher temperatures and after each evacuation, 70Torr of ammonia was admitted.
Spectra for this sample after evacuation a t 200,
400, 600, and 750 "C are shown in Fig. 3. Fine
structure due to gaseous ammonia was evident
in the spectra when ammonia was standing over
the sample.
After evacuation at 200 "C (Fig. 3a) the sample
retained most of the weakly hydrogen bonded
hydroxyls (having an infrared band a t 3650 cm-'
(7)) and possibly some of the strongly hydrogen
bonded groups having a band a t 3520 cm-' (7).
This sample did not chemically react with
ammonia since no N-H bands remained after
evacuation of gaseous ammonia (Fig. 3c). After
evacuation a t 400 "C, only groups having a band
a t 3747 cm-' with a shoulder a t approximately
3650 cm-' remained (Fig. 3d). A weak band now
appeared a t 3450 cm-' in the spectrum of the
sample after treatment with ammonia (Fig. 3f ) .
Evacuation a t 600°C removed most of the
hydrogen bonded hydroxyls from the silica and
the band a t 3650 cm-' had almost disappeared
(Fig. 3g). Bands due to Si-NH, groups formed
by reaction with ammonia were now quite strong
(Fig. 3i). The concentration of these chemisorbed
groups was greater still (Fig. 31) on a sample
which had been evacuated a t 750 "C before
treatment with ammonia (Fig. 3j).
It appears that the ammonia chemisorption
sites are formed when those hydroxyl groups
which produce a band a t 3650 cm-' in the
infrared spectrum are removed by evacuation at
400 "C and above. The greater the dehydroxylation of the silica surface, the greater the number
of chemisorption sites that are formed.
It is considered that ammonia is physically
adsorbed to the free hydroxyl groups on the
silica surface in preference t o hydroxyl groups
which are hydrogen bonded t o one another (4).
I
I
4000
3600
3200
2800
Frequency ( c m - ' )
FIG.3. The infrared spectrum of Cab-0-Sil M-5
silica: a, evacuated at 200°C; b, after the addition of
70Torr of ammonia; c, after evacuation at 20°C; d,
sample evacuated at 400 "C; e, after addition of 70 Torr
of ammonia; f, after evacuation at 20 "C; g, sample
evacuated a t 600°C; h, after addition of 70Torr of
ammonia; i, after evacuation at 20 "C; j, sample evacuated at 750 "C; k, after addition of 70 Torr of ammonia;
I, after evacuation at 20 "C.
For temperatures of evacuation up to 600 "C,
the intensity of the band a t 3747 cm-' in the
spectrum of a silica sample is almost constant
(Fig. 3a, ci, g) suggesting that the number of
sites for physical adsorption of ammonia mole-
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1780
C A N . J . C H E M . VOL.. 55, 1973
cules should be approximately the same after
evacuation at any temperature in this range.
However, when 70 Torr of ammonia was
admitted to a sample of silica which had been
dried at 200, 400, or 600 "C (Fig. 3b, e, h), the
concentration of physically adsorbed ammonia
decreased as the evacuation pre-treatment temperature increased. On a sample evacuated at
200 "C (Fig. 36) bands at 3400 and 3320 cm-'
due to N-H vibrations and at approximately
2980cm-' due to 0-H
vibrations of the
0-H...NH3 groups are very intense. The corresponding decrease in the intensity of the isolated
hydroxyl band is also large. The low frequency
of the 0-H
stretching band at 2980 cm-'
suggests that the average strength of hydrogen
bonds which are formed is large.
There was less adsoiption when 70 Torr of
ammonia was admitted to the same sample after
evacuation at 400 "C and the hydrogen bonded
0-H
stretching frequency increased to 3000
cm-'. After evacuation at 600 "C, the sample
physically adsorbed even less ammonia (Fig. 3h)
and these adsorbed molecules were less strongly
held (the 0-H stretching frequency shifted to
3080 cm-'). The band at 3747 cm-', due to free
hydroxyls not interacting with ammonia, was
greater than after ammonia adsorption on the
sample evacuated at lower temperatures. Many
of the hydroxyl groups which adsorbed ammonia
molecules in these previous cases will no longer
act as physical adsorption sites. The adsorption
of ammonia on the sample after evacuation at
750 "C (Fig. 3k) was even less but the initial concentration of free hydroxyl groups had also
decreased (Fig. 3j).
These results suggest that the preferred site
for the physical adsorption of ammonia is the
hydroxyl group which is illustrated in 1. A
similar conclusion has been reached previously
(24).
This is a group which is free to bond to ammonia
molecules but is also involved in bonding to
adjacent groups. The 0-H bond will vibrate
freely so will have a stretching frequency at
approximately 3747 cm-' but the oxygen of the
group is bound to an adjacent hydroxyl group
(which would have an 0-H
stretching frequency at 3650 cm-'). The free hydrogen on this
group may be expected to be somewhat more
electropositive than that on a completely isolated
hydroxyl group. The adjacent hydrogen bond
(I) would induce the removal of electron density
from the free hydrogen, although possibly not
sufficiently to greatly alter the 0-H stretching
frequency (McDonald reported that a weakly
perturbed hydroxyl having a stretching frequency at 3740 cm-' existed on the surface of
evacuated Cab-0-Sil). Ammonia molecules adsorbed to such a hydrogen would be somewhat
more strongly held than molecules on completely
isolated hydroxyls.
The concentration of these electropositive
groups is reduced as the temperature of evacuation of the silica is increased and very few of
these groups exist after evacuation at 600 "C
(the shoulder at 3650 cm-' is very weak for the
spectrum of silica evacuated at this temperature
(Fig. 3g)). The only hydroxyl adsorption site
which remains is one which is completely isolated
from its neighbors and not subject to perturbation by adjacent hydroxyl groups. Ammonia
adsorbed to such a group will be less strongly
bound and the 0-H stretching frequency for the
0-H...NH3 group shifts to 3080 cm- '.
The intensity changes of the band due to the
free hydroxyl groups which remain at 3747 cm-'
after adsorption of ammonia (compare Figs.
3b, e, and h ) suggests that less of the free hydroxyls on the silica act as adsorption sites when
the sample has been evacuated at 600 "C than
after evacuation at 400 "C and 200 "C when
many of these groups would have been weakly
perturbed, existing as hydroxyl pairs (1).
To a sample of silica evacuated at 800 "C,
20 Torr of water vapor was added and then
evacuated at 20 "C. When ammonia was
admitted to this sample, the gas was physically
adsorbed to the surface hydroxyl groups but
could be all removed by evacuation at 20 "C and
the spectrum returned to its original contour.
The rehydroxylated silica surface no longer contained sites which could chemisorb ammonia.
However, when water vapor was added to a
sample of silica having Si-NH, groups on its
surface (Fig. Ig), the water was chemisorbed
to form the hydroxyl groups with bands at
3720 and 3520 cm-' and the adsorbed water did
not displace any of the Si-NH, groups. Evacuation at 300 "C removed most of the ~;ewly
BLOMFIELD AND LITTLE: CHEMISORPTION OF AMMONIA ON SILICA
6. I. D. CHAPMAN
and M. L. HAIR.Trans. Faraday Soc.
61, 1507 (1965).
7. R. S. MCDONALD.
J. Phys. Chem. 62, 1168 (1958).
8. J. B. PERI.J. Phys. Chem. 70,2937 (1966).
Si-NH,
g r o u p s a n d isolated h y d r o x y l g r o u p s .
9. B. CAMARA,
H. D U N K E Nand
, P. FINK.Z. Chem. 8 ,
155 (1968).
T h u s t h e adsorwtion o f w a t e r c a n still t a k e wlace
10. L. ABRAMS
and J. W. SUTHERLAND.
J. Phys. Chem.
i n the presence of amine groups on the silica
73, 3160 (1969).
surface whereas
of ammonia as 1 I. P. PICHAT,
M.-V. MATHIEU,
and B. IMELIK.
J . Chim.
Si-NH2
g r o u p s is blocked by p r e - a d s o r p t i o n
phys, 66, 845 (1969).
of w a t e r vaDor. I t is ~ r o b a b l et h a t b o t h w a t e r 12. M. L. HAIRand W . HERTL.J. Phvs. Chem. 73. 4269
(1969).
a n d a m m o n i a r e a c t w i t h t h e s a m e t y p e o f sites
13. M. FOLMAN.
Trans. Faraday Sot. 57, 2000 (1961).
of dehydroxylated silica b u t w a t e r c a n also react
14. M. J. D. Low, N . RAMASUBRAMANIAN,
and V . V.
w i t h sites which a r e n o t involved i n c h e m i s o r p SUBBA
RAO.J . Phys. Chem. 71, 1726 (1967).
t i o n of a m m o n i a .
15. N. W . CANTand L. H. LITTLE.J. Catal. 12. 134
(1968).
and L. H. LITTLE.J. Catal. 21, 149
16. G. A. BLOMFIELD
Thanks are due to Mr. P. Bannister of the Department
(1971).
of Geology for the X-ray fluorescence analyses. Financial
and I. E. PUDDINGTON.
Can. J . Chem.
17. A. F. SIRIANNI
assistance from the Australian Research Grants Commit33, 391 (1955).
tee is gratefully acknowledged.
18. D. W. L. GRIFFITHS,H. E. HALLAMand W. J .
THOMAS.
Trans. Faraday Soc. 64, 3361 (1968).
and L. H. LITTLE.TObe published.
1. J. E. MAPESand R. P. EISCHENS.
J. Phys. Chem. 58, 19. G. A. BLOMFIELD
Geochim. Cosmochim. Acta, 31,289
20. F. J. FLANAGAN.
1059 (1954).
(1967).
2. A. V. KISELEV,
V. I. LYGIN,and T. I. TITOVA.
RUSS.
21. H.-P. BOEHM.Angew Chem. Int. Ed. Engl. 5, 533
J. Phys. Chem. 38, 1487 (1964).
(1966).
3. T. W. BOYLE,W. J. GAW,and R. A. Ross. J. Chern.
22. G. A. BLOMFIELD
and L. H. LITTLE.J. Catal. In
SOC.240 (1965).
press.
Can.J. Chem. 43,1252
4. N.W. CANTand L. H. LITTLE.
23. A. MARCHAND,
M.-T. FOREL,F. METKAS,and J .
(1965).
VALADE.J. Chim. Phys. 61, 343 (1964).
and F. GESMUNDO.
Ann. Chim.
5. 1'. LORENZELLI
24. N. W. CANTand L. H. LITTLE.Unpublished results.
(Rome), 55, 628 (1965).
f o r m e d hydroxyl g r o u p s , leaving i n t h e s p e c t r u m
a band due to the
hydroxyls at 3650 cm-' as
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1781
hydrogen bonded
as bands due to
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