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Study of the ammonia chemisorption on silica-gel by wide-line nuclear magnetic resonance
Volume 22, number
1
1.5 September
CHEXlICAL PHYSICS LETTERS
I973
STUDY OF THE AMMONIA CHEMISORPTION ON SILICA-GEL BY WIDE-LINE NUCLEAR 1MACNETIC RESONANCE J.L. BONARDET and J.P. FRAISSARLI f ahoratoirc dc Chimie Geh’rale (Ckirnie des S~rrJiices),Utrirvrsi~~ParfsVI, 75 230 Paris Cedes 05, Frarzce
Received 2 April 1973 Rcviscd manuscript received 25 May 1973
This study dciines the nnture of ammonia adsorbed on silica-gel at 25°C. NH3 molcculcs zre partly adsorbed on the OH groups, When the contact time between gas and solid is sufficient a dissociative reaction of NH3 occurs on the silica-gel, with the formation of NH2 chemisorbed groups.
1. Introduction
wide-line
and absorption
‘IIX z-nod&&n
The kinetic studies of ammonia adsorption on silicagel [ 1 ] , made with a MacEkin balance, show two different stages at ambient temperature: first, a rapid one, corresponding to physical adsorption and, secondly, a slower one due to NH, chemical adsorption on the hydrosyl groups located on the adsorbent surface [2] _ If the contact-time between gas and solid is not too long, this quantity of am~nonia thus adsorbed may be entirely eliminated at 50°C under lo-” torr. But, if this time js about 60 hours at 40°C or 5 hours at 15O”C, a chemically rtdsorbed ammonia quantity re= mains on the sample even after treatment at 150°C under vacuum; this quantity depends on the initial temperature treatment of the solid.
superimposed
frequency on
mode,
at 60 ME-fz frrquency.
of the sinusdidni
the directing
field Iim
Ho field for the signal
was 20 Hz, and its amplitude was aIways much lower than the experimental line width. We
detection
have allowed for the Hm contribution tion of the second moment.
3. Theoretical
calculations
to the cakuia-
of the second moment
The van We& formula for the second moment. the case of a powder sampfe is [5!
in
2. Experimental The ammonia adsorptions were made at 35°C under 11 torr, on a silica-gel prepared according to Planck’s method [3], and heated at 4OO*C under
The first term is the contribution of theN nuclei, having spin I and magnetic moment gp,,I, which are at resonance; the second term is due to the non-
10-4 torr. The surface area was 390 m2/g of SKI2 and the water concentration was 26 mg/g. This water is only in the form of OH superficial groups. NXR experiments were carried out at liquid nitrogen tem-
resonating nuclei having spin 11, magnetic moment gfpo If and interacting with the I spins. ~i,:i,k and ‘j,,p
perature,
using a Varian
DP 6Xl spectrometer,
on a
are lengths of vectors joining a resonating nucfeus j and either another one k or a non-resonating nucleus f. In the case of protons interacting among themselves 75
Volume 22, number 1
CHEMICAL PHYSICS LETTERS Tabte 1 Calculated second moments
:
15 September
in a rigid lattice
Bond angle
Length
LEIlgth
M2 (H) only due to protonic interactions iG’>
hr, (GZi
H2O
104”s
NH3 NH4
106”8 109O.5 iO5’9 .---
I.520 1.628 I.685 1.620
1.015 1.032 1.015
29.20 38.70 47.20 13.40
29.20 40.75 49.03 20.45
Chemid compound
I
NH2
Fig. 1. First derivative signal, registrcd at -196°C after pumping at 40°C under IO4 torr,
of (a) bulk water; (c) ~~lrnoni~ adsorbed at 25°C under 11 ton; (b) s~ccl~rn
and with one 14N nitrogen nucleus, the formula is
Calculated second moments for some compounds are given in table 1.
4. Ex~er~en~aI
-_
1973
resuits and discussion
Absorption signal of bulk water of the silica-gel sample heated at 400°C under vacuum shows only a one-line spectrum (a in fig. I); its shape is between gaussian and lorentzian forms. The data are given .in table 2. We can deduce that the half width at halfintensity is much lower than the quadratic mean width. Considering thtit the experiment is performed at -f96”C, we would suggest that the protonic exchange
c
between tiie different sites is a sIow process and that the signal width is due to inhomogeneous distribution of hydroxyl groups on the solid surface. Their concentration is low enough so that each proton is in a local weak field; thus the signal is a narrow line with a small width. Nevertheless, few strongly interacting nuclei (:! geminal OH for example) are sufficient to bring an imbortant contribution to the second moment and therefore make its square root larger than the reaf width [13]_
After the ammonia sdsorption is made at 2S°C under 11 torr, the recorded signal (spectrum c, in fig. 1, tabfa 2) may be due to the superposition of three seemingly individual lines, with minima and m*ma at about 20.382 G (Iine I), k2.04 G (line 2) and +4.22 G (line 3). The expe&nental second moment-is 7.30 G2.
~~~~_
7.80
10-9 torr
0.765
at 153°C under
2
ammonia desorption
10-Otorr
2
7.80
8.44 0.765
11 lorr
ammonia dcsorption at 40°C under
0.765 4.08
3
at 35OC under
ammonia adsorption
2.:
3.2
-
* 0.1
A 0.1
7.30 * 0.05
0.90 i 0.05
0.720
I
~__----
nr,
bulk water
AH peak (G’)
line
Espcrimcntal speclrum
Table 2
.__--_
--
16.465
17.350 18.235
17.350
15.755 2.655 2.655 1.770
-
1.770
2.130
3.190
-
IS.235
4.485
4.485
37.200
17.350 18.945
17.350
17.350
17.350
__._.__.___._ ___..--
20 20 20 20
3.540
_
I5 6.18 2.13 2.62 9.44
rotating NH3 rigid NH; rigid NH3 rotating NH3 rigid NH2 rigid NH:
--
3.71 3.36
rigid Nil? 22.135
rigid NH? (2.5 mg)
9.54 2.91
28.10 7.25
rigid NH3 rotating NH3
rigid NH3 rotating NH3
(G2)
tllC0KtiCill
adsorbed phase
rif*
Connected
Assumed state of the
Of thC dsOrbcd
22.135 22.135
_
6.380
_
54.55
Number p 10”’ of protons assumed as Total _~ ___~~_ amounl NHl Nfl; NIla Of1 p 1o-?O
~.__.-_ ~_.___~_ ~~______~
of adsorbed ammonia. Tbcorclical second moments as a function of tbc assumed .hlC
number lo peak (G) .____~___~
Sample
phase
Charactcrislics of the registrcd spectra with Ihcquantity
Volume 52. number 1
CHEMICAL PHYSICS LETTERS
Considering that this value is the mean of the second moments of surface water and adsorbant and that the interactions between these two types of protons are weak enough, the second moment relative to the lone adsorbed phase is 10.2 G2. This value is much lower than the second moment of the various forms in rigid lattice, mentioned in table 1, especially for NH3 (40.75 Gz). Let us notice that, in the latter case, the second moment of the whole recorded signal must be 28.10 G2 (to be compared to 7.30 G2). We have to allow that even at liquid nitrogen temperature, there is a narrowing of the line ascribed to movement of protons. Andrew and Bersohn [6] showed that spectrum 01, which is characteristic of a group of three nuclei of spin % at the corners of an equilateral triangle, consists of a central line and three pairs of lines symmetrically disposed about the centre. Due to their broadening by the local field of all neighbouring groups these lines are in general not resolved and, in fact, the spectrum is composed of a centra1 Iine and a symmetric doublet. In the case of triangular groups reorienting about the three-fold axis, the spectrum fl also only includes a central line and a symmetric doublet having roughly half the distance of the doublet in the rigid lattice. The related second moment is one fourth the value observed in a rigid lattice. This is, for example, the case for soiid ammonia at 90°K (II!, = 9.7 G2) [7] Then, according to these theoretical considerations, it xcms that both lines 2 and 3 correspond to NH3 rotating around its three-fold axis. Indeed, this has the shape of the previous theoretical fl spectrum with dipolar broadening; besides, the distance between shoulders of the real signal is such that we can calculate for a rotating triangle having a side 1.6 A; moreover, the second moment (10.2 G2) is exactly equal to the theoretical one for such a rotator. Accordingly, the recorded spectrum is the result of the merger of two lines: the first one, central and narrow, is due to the OH groups (line 1); the second one, wider, with two symmetric shoulders, is due to adsorbed NH3 rotating around its axis, These results are in ageement with those of Kvlitidze et al. [s] Under the ammonia equilibrium pressure of 11 torr, line 1 of spectrum c is more intense than the one of the hydroxyls of the solid (spectrum a). At first sight this seems to be due to the addition of a part of line 2. But, after desorption at 40°C under vacuum (spectrum
78
15 September 1973
b), this central line 1 increases slightly; in fact when line 2 disappears, line 1 should decrease. In our opinion this development proves that, before the desorption, some hydroxyl groups at the solid surface are in strong interaction with adsorbed NH3 molecules; then their representative line is low, broad and hidden by the large component above (lines 2 and 3). This result verifies that NH, molecules are partly adsorbed on +he OH groups. In addition high-resolution NMR points out that, at ambient temperature, there is a protonic exchange between the solid surface and some adsorbed molecules, the jump frequency of this motion being about lo3 Hz [2]. 4.3. Cherl fisorptioa After desorption at 40°C under vacuum of the ammonia which had being adsorbed for three days, spectrum b of the sample cooled down to -196’C is composed of rhe narrow line 1 and a doublet with a very low intensity. The second moment is 3.2 G’. ‘A%en the desoqtion is carried out at 15O’C under vacuum, the chemisorbed ammonia content is 2.5 mg/g of solid; the NMR spectrum resembles the previous one; but the intensity of the wide line is slightly weaker. The value (2.20 G2) of the second moment is lower than the previous one, but much higher than that due only to the OH groups (0.90 G2). In table 2, one can find the theoretical second moments of the sample after ammonia descrption at 40 and 150°C under vacuum, assuming that the configuration of the chemisorbed phase is rigid NH:, rigid NH, or NH3 rotating around its three-fold axis, and rigid NH2. In every one of these two samples, the rigid NH: and NH, forms do not agree with the experimental second moment. The calculated second moments of the sample desorbed at 150°C are 2.13 or 2.60 G” when we assume the configuration of the adsorbed phase to be either NH3 rotating around its axis or rigid NH,. These values are close to the experimental one. HoweYeT, the shape of the wide component of tl~e recorded spectrum seems to be tyFical of a protonic pairing interaction but not of a triangular interaction. In addition the narrow line 1 is higher than for the solid alone (spectrum s); this proves that NH3 adsorption produces OH groups at the solid surface. When the partial ammonia desorption is carried out
CHEMICAL PHYSICS LETTERS
Volume 22, number 1
at only 40°C under vacuum, the calculated second moment of this sample agrees exactly with the experimental value (3.2 C2) ifwe assume that the remaining adsorbed phase is in two forms: a rigid NH7 form whose quantity is equal to ;he adsorbed phase of the sample at 150°C and an NH, form, rotating around its three-fold axis; in this case the calculated M, = 3.30 GZ. Then, when the contact.time between gas and solid is sufficiently long, there is an NH, chemisorbed phase; that is to say, a slow reaction occurs between NH, and silica-gel as shown in the equation:
Of” Pi OF Si Si Si
/I\ /l\/l\
Si i- i%$
/I\
*rOrH *FOF
-+ Si
Si
Si
Si
/I\
/I\
/I\
/i\
.
(11
This reaction which was first detected by Peri using IR spectroscopy, is similar to the dissociative chemisorption of water [lo] with the formation of two OH groups, as shown in steps (a) and (b) of the reaction:
15 September 1973
of siloxane bonds capable of being broken; so it must be with the step of the initial dehydration of the solid. In fact reaction (1) does not occur when the surface is almost covered with OH groups, because the number of the siloxane bonds at the surface is very low; it rises to a maximum when the outgassing temperature under vacuum of the solid is about 4OCi”C, and these dissociations become negligible as soon as this temperature is higher than 6OO”C, that is when the su$erf;ciaI OH number is very Iow. We have seen, that the Hz0 or NH, molecular adsorption partly occurs by means of the superficial silanol groups. In our opinion this adsorption increases the polar~~t~o~ of these groups (with protonic exchange} thus the neigh~uuring Si-0 bonds are poiarized. These bonds being more ionic can be broken more easily and give the reactions (I) and (2). Kvlividze et al. f8] and Kiselev l12] consider that the HZ0 or NH, adsarption involves the establisllment of donor-acceptor bonds, using the electron doublet of oxygen or nitrogen and the free orbitafs of the Si atom of a sihmol groups. This mechanism which of course involves a large polarization of the Si-0 bonds agrees as 5vel.I with ouz results.
5. Conclusion 426 -
Si Si Si Si . /l\/l\ /l\/l\ fd P\
A
01
This dissociative reaction of water is also slow at room temperature; in particular, it is much slower than the molecule adsorption. Besides, this chemisorption makes further dehydration of the solid easier IlO]. So, the two OH arising from the water dissociation are not necessarily recombined when further dehydration occurs. This result is easily explained through eq. (2): the probability of eliminating the OH(,) and 0Ht2) groups in order to obtain the dehydrated surface(c) is much lower with the partly
dehydrated
Ac~awled~ement Discussions work.
with B. fmehk were very helpful
in this
SLIT-
face (4 than with the surface(b) after’waterchemi. sorption. Accordingly, the HZ0 molecule is dissociated in the vicinity of the hydroxyl grol’ps remaining at the solid surface after the first outgassing. In the case of NH, chemisorption, we obtain similar results. If the OH groups do not play a role in Hz0 and NH, dissociations, these reactions
This study defines the nature of ammor&a!rdsorbed on silica-gel, and particularly corroborates the dissociative reaction of NH, on this solid. Besides it shows the interest of the wide-line XMR in the siudy of ihe structure of a chemisorbed complex.
must increase with the number
References 111J.L. Bonardet and J.P. Fraissard, un~ubli~cd. 121J.L. Bonardet and J.P. Fraissard, hlobilitd et CataIgse,
Journ& de la Saci~t~ch~ique Be&, LOLWGI, Szptembre 1972, IndustrieChimiqueBeIge 38 (1973) 370. 131 Plank, J. Colloid Sci. 2 (1947) 413. [41 E.R. Andrew, Phys Rev. 91 (1953) 425.
79
Volume
22,
number
1
CHEMICAL PHYSICS LETTERS
[S] J.H. van Vieck, Phys. Rev. 74 (1948) 1168. [6f E.R. Andrew and R. Bersohn, J. Chem. Phys. 18 (1950) 159. [7] H-S. Gutowsky and G.E. Pake. J. Chem. Phys. 18 (19501 162. [8] V.I. Kvlividze, R.A. Bran=, V.F. Kiss&v and G.M. Bliznadov, J. Catalysis 13 (1969) 2%. [9] J.B. Peri, J. Phys. Chem. 70 (1966) 2937.
15 September
1973
[ 101 J. Demarquay, J.P. Frtissard and 13. lmelik, Compt. Rend. Acad. Sii. (Paris) 273C (1971) 1405.
[ 111 /t.v. Kissctev, V.I. Lygin and T.I. Titova, Russian 1. Phys. C’hen. 38 (1964) 1487. 1121 V.F. Kisselzv, Dokf. Akad. NaL’auk SSSR 176 (1967) 124. [ 13 I J. Demarquay, J. Fraissard and B. Imelik, to be pubIished.
1
1.5 September
CHEXlICAL PHYSICS LETTERS
I973
STUDY OF THE AMMONIA CHEMISORPTION ON SILICA-GEL BY WIDE-LINE NUCLEAR 1MACNETIC RESONANCE J.L. BONARDET and J.P. FRAISSARLI f ahoratoirc dc Chimie Geh’rale (Ckirnie des S~rrJiices),Utrirvrsi~~ParfsVI, 75 230 Paris Cedes 05, Frarzce
Received 2 April 1973 Rcviscd manuscript received 25 May 1973
This study dciines the nnture of ammonia adsorbed on silica-gel at 25°C. NH3 molcculcs zre partly adsorbed on the OH groups, When the contact time between gas and solid is sufficient a dissociative reaction of NH3 occurs on the silica-gel, with the formation of NH2 chemisorbed groups.
1. Introduction
wide-line
and absorption
‘IIX z-nod&&n
The kinetic studies of ammonia adsorption on silicagel [ 1 ] , made with a MacEkin balance, show two different stages at ambient temperature: first, a rapid one, corresponding to physical adsorption and, secondly, a slower one due to NH, chemical adsorption on the hydrosyl groups located on the adsorbent surface [2] _ If the contact-time between gas and solid is not too long, this quantity of am~nonia thus adsorbed may be entirely eliminated at 50°C under lo-” torr. But, if this time js about 60 hours at 40°C or 5 hours at 15O”C, a chemically rtdsorbed ammonia quantity re= mains on the sample even after treatment at 150°C under vacuum; this quantity depends on the initial temperature treatment of the solid.
superimposed
frequency on
mode,
at 60 ME-fz frrquency.
of the sinusdidni
the directing
field Iim
Ho field for the signal
was 20 Hz, and its amplitude was aIways much lower than the experimental line width. We
detection
have allowed for the Hm contribution tion of the second moment.
3. Theoretical
calculations
to the cakuia-
of the second moment
The van We& formula for the second moment. the case of a powder sampfe is [5!
in
2. Experimental The ammonia adsorptions were made at 35°C under 11 torr, on a silica-gel prepared according to Planck’s method [3], and heated at 4OO*C under
The first term is the contribution of theN nuclei, having spin I and magnetic moment gp,,I, which are at resonance; the second term is due to the non-
10-4 torr. The surface area was 390 m2/g of SKI2 and the water concentration was 26 mg/g. This water is only in the form of OH superficial groups. NXR experiments were carried out at liquid nitrogen tem-
resonating nuclei having spin 11, magnetic moment gfpo If and interacting with the I spins. ~i,:i,k and ‘j,,p
perature,
using a Varian
DP 6Xl spectrometer,
on a
are lengths of vectors joining a resonating nucfeus j and either another one k or a non-resonating nucleus f. In the case of protons interacting among themselves 75
Volume 22, number 1
CHEMICAL PHYSICS LETTERS Tabte 1 Calculated second moments
:
15 September
in a rigid lattice
Bond angle
Length
LEIlgth
M2 (H) only due to protonic interactions iG’>
hr, (GZi
H2O
104”s
NH3 NH4
106”8 109O.5 iO5’9 .---
I.520 1.628 I.685 1.620
1.015 1.032 1.015
29.20 38.70 47.20 13.40
29.20 40.75 49.03 20.45
Chemid compound
I
NH2
Fig. 1. First derivative signal, registrcd at -196°C after pumping at 40°C under IO4 torr,
of (a) bulk water; (c) ~~lrnoni~ adsorbed at 25°C under 11 ton; (b) s~ccl~rn
and with one 14N nitrogen nucleus, the formula is
Calculated second moments for some compounds are given in table 1.
4. Ex~er~en~aI
-_
1973
resuits and discussion
Absorption signal of bulk water of the silica-gel sample heated at 400°C under vacuum shows only a one-line spectrum (a in fig. I); its shape is between gaussian and lorentzian forms. The data are given .in table 2. We can deduce that the half width at halfintensity is much lower than the quadratic mean width. Considering thtit the experiment is performed at -f96”C, we would suggest that the protonic exchange
c
between tiie different sites is a sIow process and that the signal width is due to inhomogeneous distribution of hydroxyl groups on the solid surface. Their concentration is low enough so that each proton is in a local weak field; thus the signal is a narrow line with a small width. Nevertheless, few strongly interacting nuclei (:! geminal OH for example) are sufficient to bring an imbortant contribution to the second moment and therefore make its square root larger than the reaf width [13]_
After the ammonia sdsorption is made at 2S°C under 11 torr, the recorded signal (spectrum c, in fig. 1, tabfa 2) may be due to the superposition of three seemingly individual lines, with minima and m*ma at about 20.382 G (Iine I), k2.04 G (line 2) and +4.22 G (line 3). The expe&nental second moment-is 7.30 G2.
~~~~_
7.80
10-9 torr
0.765
at 153°C under
2
ammonia desorption
10-Otorr
2
7.80
8.44 0.765
11 lorr
ammonia dcsorption at 40°C under
0.765 4.08
3
at 35OC under
ammonia adsorption
2.:
3.2
-
* 0.1
A 0.1
7.30 * 0.05
0.90 i 0.05
0.720
I
~__----
nr,
bulk water
AH peak (G’)
line
Espcrimcntal speclrum
Table 2
.__--_
--
16.465
17.350 18.235
17.350
15.755 2.655 2.655 1.770
-
1.770
2.130
3.190
-
IS.235
4.485
4.485
37.200
17.350 18.945
17.350
17.350
17.350
__._.__.___._ ___..--
20 20 20 20
3.540
_
I5 6.18 2.13 2.62 9.44
rotating NH3 rigid NH; rigid NH3 rotating NH3 rigid NH2 rigid NH:
--
3.71 3.36
rigid Nil? 22.135
rigid NH? (2.5 mg)
9.54 2.91
28.10 7.25
rigid NH3 rotating NH3
rigid NH3 rotating NH3
(G2)
tllC0KtiCill
adsorbed phase
rif*
Connected
Assumed state of the
Of thC dsOrbcd
22.135 22.135
_
6.380
_
54.55
Number p 10”’ of protons assumed as Total _~ ___~~_ amounl NHl Nfl; NIla Of1 p 1o-?O
~.__.-_ ~_.___~_ ~~______~
of adsorbed ammonia. Tbcorclical second moments as a function of tbc assumed .hlC
number lo peak (G) .____~___~
Sample
phase
Charactcrislics of the registrcd spectra with Ihcquantity
Volume 52. number 1
CHEMICAL PHYSICS LETTERS
Considering that this value is the mean of the second moments of surface water and adsorbant and that the interactions between these two types of protons are weak enough, the second moment relative to the lone adsorbed phase is 10.2 G2. This value is much lower than the second moment of the various forms in rigid lattice, mentioned in table 1, especially for NH3 (40.75 Gz). Let us notice that, in the latter case, the second moment of the whole recorded signal must be 28.10 G2 (to be compared to 7.30 G2). We have to allow that even at liquid nitrogen temperature, there is a narrowing of the line ascribed to movement of protons. Andrew and Bersohn [6] showed that spectrum 01, which is characteristic of a group of three nuclei of spin % at the corners of an equilateral triangle, consists of a central line and three pairs of lines symmetrically disposed about the centre. Due to their broadening by the local field of all neighbouring groups these lines are in general not resolved and, in fact, the spectrum is composed of a centra1 Iine and a symmetric doublet. In the case of triangular groups reorienting about the three-fold axis, the spectrum fl also only includes a central line and a symmetric doublet having roughly half the distance of the doublet in the rigid lattice. The related second moment is one fourth the value observed in a rigid lattice. This is, for example, the case for soiid ammonia at 90°K (II!, = 9.7 G2) [7] Then, according to these theoretical considerations, it xcms that both lines 2 and 3 correspond to NH3 rotating around its three-fold axis. Indeed, this has the shape of the previous theoretical fl spectrum with dipolar broadening; besides, the distance between shoulders of the real signal is such that we can calculate for a rotating triangle having a side 1.6 A; moreover, the second moment (10.2 G2) is exactly equal to the theoretical one for such a rotator. Accordingly, the recorded spectrum is the result of the merger of two lines: the first one, central and narrow, is due to the OH groups (line 1); the second one, wider, with two symmetric shoulders, is due to adsorbed NH3 rotating around its axis, These results are in ageement with those of Kvlitidze et al. [s] Under the ammonia equilibrium pressure of 11 torr, line 1 of spectrum c is more intense than the one of the hydroxyls of the solid (spectrum a). At first sight this seems to be due to the addition of a part of line 2. But, after desorption at 40°C under vacuum (spectrum
78
15 September 1973
b), this central line 1 increases slightly; in fact when line 2 disappears, line 1 should decrease. In our opinion this development proves that, before the desorption, some hydroxyl groups at the solid surface are in strong interaction with adsorbed NH3 molecules; then their representative line is low, broad and hidden by the large component above (lines 2 and 3). This result verifies that NH, molecules are partly adsorbed on +he OH groups. In addition high-resolution NMR points out that, at ambient temperature, there is a protonic exchange between the solid surface and some adsorbed molecules, the jump frequency of this motion being about lo3 Hz [2]. 4.3. Cherl fisorptioa After desorption at 40°C under vacuum of the ammonia which had being adsorbed for three days, spectrum b of the sample cooled down to -196’C is composed of rhe narrow line 1 and a doublet with a very low intensity. The second moment is 3.2 G’. ‘A%en the desoqtion is carried out at 15O’C under vacuum, the chemisorbed ammonia content is 2.5 mg/g of solid; the NMR spectrum resembles the previous one; but the intensity of the wide line is slightly weaker. The value (2.20 G2) of the second moment is lower than the previous one, but much higher than that due only to the OH groups (0.90 G2). In table 2, one can find the theoretical second moments of the sample after ammonia descrption at 40 and 150°C under vacuum, assuming that the configuration of the chemisorbed phase is rigid NH:, rigid NH, or NH3 rotating around its three-fold axis, and rigid NH2. In every one of these two samples, the rigid NH: and NH, forms do not agree with the experimental second moment. The calculated second moments of the sample desorbed at 150°C are 2.13 or 2.60 G” when we assume the configuration of the adsorbed phase to be either NH3 rotating around its axis or rigid NH,. These values are close to the experimental one. HoweYeT, the shape of the wide component of tl~e recorded spectrum seems to be tyFical of a protonic pairing interaction but not of a triangular interaction. In addition the narrow line 1 is higher than for the solid alone (spectrum s); this proves that NH3 adsorption produces OH groups at the solid surface. When the partial ammonia desorption is carried out
CHEMICAL PHYSICS LETTERS
Volume 22, number 1
at only 40°C under vacuum, the calculated second moment of this sample agrees exactly with the experimental value (3.2 C2) ifwe assume that the remaining adsorbed phase is in two forms: a rigid NH7 form whose quantity is equal to ;he adsorbed phase of the sample at 150°C and an NH, form, rotating around its three-fold axis; in this case the calculated M, = 3.30 GZ. Then, when the contact.time between gas and solid is sufficiently long, there is an NH, chemisorbed phase; that is to say, a slow reaction occurs between NH, and silica-gel as shown in the equation:
Of” Pi OF Si Si Si
/I\ /l\/l\
Si i- i%$
/I\
*rOrH *FOF
-+ Si
Si
Si
Si
/I\
/I\
/I\
/i\
.
(11
This reaction which was first detected by Peri using IR spectroscopy, is similar to the dissociative chemisorption of water [lo] with the formation of two OH groups, as shown in steps (a) and (b) of the reaction:
15 September 1973
of siloxane bonds capable of being broken; so it must be with the step of the initial dehydration of the solid. In fact reaction (1) does not occur when the surface is almost covered with OH groups, because the number of the siloxane bonds at the surface is very low; it rises to a maximum when the outgassing temperature under vacuum of the solid is about 4OCi”C, and these dissociations become negligible as soon as this temperature is higher than 6OO”C, that is when the su$erf;ciaI OH number is very Iow. We have seen, that the Hz0 or NH, molecular adsorption partly occurs by means of the superficial silanol groups. In our opinion this adsorption increases the polar~~t~o~ of these groups (with protonic exchange} thus the neigh~uuring Si-0 bonds are poiarized. These bonds being more ionic can be broken more easily and give the reactions (I) and (2). Kvlividze et al. f8] and Kiselev l12] consider that the HZ0 or NH, adsarption involves the establisllment of donor-acceptor bonds, using the electron doublet of oxygen or nitrogen and the free orbitafs of the Si atom of a sihmol groups. This mechanism which of course involves a large polarization of the Si-0 bonds agrees as 5vel.I with ouz results.
5. Conclusion 426 -
Si Si Si Si . /l\/l\ /l\/l\ fd P\
A
01
This dissociative reaction of water is also slow at room temperature; in particular, it is much slower than the molecule adsorption. Besides, this chemisorption makes further dehydration of the solid easier IlO]. So, the two OH arising from the water dissociation are not necessarily recombined when further dehydration occurs. This result is easily explained through eq. (2): the probability of eliminating the OH(,) and 0Ht2) groups in order to obtain the dehydrated surface(c) is much lower with the partly
dehydrated
Ac~awled~ement Discussions work.
with B. fmehk were very helpful
in this
SLIT-
face (4 than with the surface(b) after’waterchemi. sorption. Accordingly, the HZ0 molecule is dissociated in the vicinity of the hydroxyl grol’ps remaining at the solid surface after the first outgassing. In the case of NH, chemisorption, we obtain similar results. If the OH groups do not play a role in Hz0 and NH, dissociations, these reactions
This study defines the nature of ammor&a!rdsorbed on silica-gel, and particularly corroborates the dissociative reaction of NH, on this solid. Besides it shows the interest of the wide-line XMR in the siudy of ihe structure of a chemisorbed complex.
must increase with the number
References 111J.L. Bonardet and J.P. Fraissard, un~ubli~cd. 121J.L. Bonardet and J.P. Fraissard, hlobilitd et CataIgse,
Journ& de la Saci~t~ch~ique Be&, LOLWGI, Szptembre 1972, IndustrieChimiqueBeIge 38 (1973) 370. 131 Plank, J. Colloid Sci. 2 (1947) 413. [41 E.R. Andrew, Phys Rev. 91 (1953) 425.
79
Volume
22,
number
1
CHEMICAL PHYSICS LETTERS
[S] J.H. van Vieck, Phys. Rev. 74 (1948) 1168. [6f E.R. Andrew and R. Bersohn, J. Chem. Phys. 18 (1950) 159. [7] H-S. Gutowsky and G.E. Pake. J. Chem. Phys. 18 (19501 162. [8] V.I. Kvlividze, R.A. Bran=, V.F. Kiss&v and G.M. Bliznadov, J. Catalysis 13 (1969) 2%. [9] J.B. Peri, J. Phys. Chem. 70 (1966) 2937.
15 September
1973
[ 101 J. Demarquay, J.P. Frtissard and 13. lmelik, Compt. Rend. Acad. Sii. (Paris) 273C (1971) 1405.
[ 111 /t.v. Kissctev, V.I. Lygin and T.I. Titova, Russian 1. Phys. C’hen. 38 (1964) 1487. 1121 V.F. Kisselzv, Dokf. Akad. NaL’auk SSSR 176 (1967) 124. [ 13 I J. Demarquay, J. Fraissard and B. Imelik, to be pubIished.