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FT-IR STUDY OF THE OH SURFACE GROUPS ON CORDIERITE AEROGEL
M.-I. Baraton, T. Merle-Mejean, P. Quintard, V. Lorenzelli
To cite this version:
M.-I. Baraton, T. Merle-Mejean, P. Quintard, V. Lorenzelli. FT-IR STUDY OF THE OH SURFACE
GROUPS ON CORDIERITE AEROGEL. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-239-
C4-243. �10.1051/jphyscol:1989439�. �jpa-00229523�
R E W E DE PHYSIQUE
APPLIQUBE
Colloque C4, Supplement au n04, Tome 24, avril 1989
FT-IR STUDY OF THE OH SURFACE GROUPS ON CORDIERITE AEROGEL
M.-I. BARATON, T. MERLE-MEJEAN, P. QUINTARD and V. LORENZELLI*
Laboratoire d e Spectrometrie Vibratiomelle, CNRS UA-320, 123 Avenue Albert Thomas, F-87060 Limoges. France
"Istituto di Chimica, Facolta d i Ingegneria, Ple K e ~ e d y , I-16129 Genova, Italie
~ i s u r n k - Nous avons dtudi6 une c o r d i d r i t e h h a u t e s u r f a c e spdcifique par spectroscopie inffarouge e n transmission. Apr& a c t i v a t i o n d e s &chantillons, nous a v o n s m i s e n Cvidence l a p r e s e n c e d e s groupes hydroxyles. L ' a c t i o n du mCthano1 et 116change isotopique a v e c l e deut6rium o n t prouv6 q u e t o u s ces groupes sont lids a u x a t o m e s d e silicium d e surface.
Abstract - A high s p e c i f i c s u r f a c e a r e a c o r d i e r i t e has. b e e n investigated by i n f r a r e d transmission spectroscopy. A f t e r a c t i v a t i o n of t h e samples, w e have evidenced t h e presence of hydroxyl groups. T r e a t m e n t s with m e t h a n o l and isotopic e x c h a n g e with deuterium have proved t h a t all of them a r e bound t o t h e s u r f a c e silicium atoms.
1 - INTRODUCTION
C o r d i e r i t e i s a c e r a m i c m a t e r i a l promising i n t h e e l e c t r o n i c and c a t a l y t i c fields. The study of t h e s e c a t a l y t i c p r o p e r t i e s g o e s through t h e investigation of t h e surface. From t h i s viewpoint, t h e Fourier t r a n s f o r m i n f r a r e d (FT-IR) s p e c t r o s c o p y i s a very useful tool a s long a s t h e specific s u r f a c e a r e a of t h e compound i s high enough. The sol-gel p r e p a r a t i o n process provides u s with such a s u r f a c e a r e a which allows t o c h a r a c t e r i z e t h e O H groups o n c o r d i e r i t e as a f i r s t s t e p of a s u r f a c e study.
2 - EXPERIMENTAL
The preparation of c o r d i e r i t e aerogel has b e e n r e l a t e d e l s e w h e r e /1,2/. According t o t h i s process, t h e a e r o g e l is o b t a i n e d with a high specific s u r f a c e a r e a (400 m2.g- I ), a low density (0.3) and hydrophylic c h a r a c t e r . T h e p r o p e r t i e s of t h e bulk have previously been studied / 3 / .
T h e s p e c t r a a r e recorded using a FT-IR s p e c t r o m e t e r (Nicolet 5DX) with a 4 cm-I resolution.
C o r d i e r i t e i s pressed i n grid-supported pellets. The samples a r e introduced inside a vacuum h e a t a b l e cell allowing in-situ experiments. We have a c t i v a t e d them by heating up t o 723 K a n d t h e n cooling down t o room t e m p e r a t u r e , both under evacuation. Prior t o a n y i n t r o d u c t i o n of a d s o r b a t e s t h e pressure i s a b o u t mB.
Methanol ( R P Normapur from Prolabo) i s dried o n m o l e c u l a r sieves b e f o r e adding. D2 (from Alphagaz-Air liquidel i s used a s provided.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989439
3 - RESULTS AND DISCUSSION
The spectrum of a n activated sample presents a sharp absorption band a t 3747 cm-l (figure la). As in silica /4,5/, i t is assigned to the v(OH) stretching vibration of t h e surface silanol groups, f r e e from any hydrogen bond. On the cordierite surface, we c a n expect t o find OH groups bound t o A1 and Mg a t o m s whose vibration frequencies a r e a t 3700, 3745, 3760 and 3785 cm-I (AI-OH on Y - alumina) and a t 3752 and 3610 cm-I (Mg-OH o n magnesia) /6/. However, we have been unable to evidence these absorptions. This lack of t h e expected bands has already been noticed o n silica- alumina /4/ and Mg A1204 171.
Fig. 1 - Evolution o f the cordierite spectrum a f t e r addition o f D : Spectrum o f the activated c o r d i e r i t e (a). Addition o f 7 rnB D2 a t 673K f o r l 0 m n
(k),
30mn ( c ) and 60mn ( d ) .Fig. 2 - Isotopic exchange - Integmted surface (S) versus time : 1st dose P(D2) = 6 mB
-
y 2nd dose P(D2) = 355 mB.
A f t e r adding 7 m B of D 2 a t 6 7 3 K t o t h e a c t i v a t e d sample, w e observe a sharp band occuring a t 2760 c m - l (figure 1) a t t r i b u t e d t o t h e v ( 0 D ) s t r e t c h i n g mode, while simultaneously t h e i n t e n s i t y of t h e v ( 0 H ) band decreases. The isotopic e x c h a n g e does not p r o c e e d t o c o m p l e t i o n under a low pressure, e v e n by adding subsequent doses. The intensity r a t i o s of t h e v ( 0 D ) and v(OH) bands have b e e n c a l c u l a t e d a s functions of t h e c o n t a c t times, t h e t e m p e r a t u r e s of t h e sample a n d t h e doses.
S o m e r e s u l t s a r e given i n f i g u r e 2 : w h a t e v e r t h e dose and t h e pressure a r e t h e c u r v e s v ( 0 H ) a n d v (OD) versus t h e t i m e follow t h e s a m e evolution, w i t h a n a s y m p t o t i c behavior corresponding t o t h e e x c h a n g e equilibrium.
A p r e s s u r e higher t h a n 200 m B i s needed t o r e a c h t h e c o m p l e t e e x c h a n g e e v e n though i t i s not spontaneous but a r a t h e r slow process. I t i s t o t a l l y reversible by introducing H2.
S o w e c a n conclude t h a t all t h e OH groups a r e l o c a t e d o n t h e surface.
Adsorbed m e t h a n o l o n t h e c o r d i e r i t e s u r f a c e gives rise t o m a n y bands over t w o m a i n regions : 3700 -2800 c m - I and 1500 - 1300 c m - l
.
We w e r e particularly i n t e r e s t e d i n t h e f i r s t range c o r r e s p o n d ~ n g t o t h e absorption of t h e v(OH) s t r e t c h i n g v i b r a t i o n s (figure 3). Adding 0.07 m B of m e t h a n o l a t room t e m p e r a t u r e l e a d s t o t h e d i s a p p e a r a n c e of t h e v ( 0 H ) at 3747 c m - l a n d t h e a p p e a r a n c e of a broad band c e n t e r e d at 3375 c m - l a n d of t w o sharp bands (2966 a n d 2853 c m - 1 ) with shoulders (2997 a n d2921 cm-I).
Fig. 3
-
Difference spectrum featuring the modification o f the cordierite spectrum a f t e r adding 0.07 mB of methanol a t room temperature.T h e f i r s t t w o f e a t u r e s result from t h e f o r m a t i o n of a hydrogen bond b e t w e e n t h e s u r f a c e silanols a n d t h e m e t h a n o l OH group. The v ( C H ) s t r e t c h i n g f r e q u e n c i e s of H-bonded m e t h a n o l should b e slightly p e r t u r b e d c o m p a r e d with t h e f r e e molecule.
Under e v a c u a t i o n a t room t e m p e r a t u r e , a very f e w silanol groups a r e r e s t o r e d
,
b u t a t 373 K w e o b s e r v e t h e decreasing o f t h e 3350 cm-' band and of both 2966 a n d 2841 c m - l bands, included t h e i r shoulders (flgure 4).
We t h e r e f o r e assign them t o t h e v(CH) s t r e t c h i n g vibrations of t h e H-bonded methanol.REVUE DE PHYSIQUE A P P L I Q U ~ E
Fig. 4 - Difference spectrum : the evolution o f the spectrum of m_ythanol adsorbed on cordierite by evacuation at 373 K. The modification o f the band at 3747 cm ( v (OH)) is partly due to a temperature effect /10,1 l / .
However, a f t e r pumping off f o r o n e hour a t 6 7 3 K t h e spectrum of c o r d i e r i t e still p r e s e n t s t w o w e a k bands a t 2964 and 2856 c m - I ( f i g u r e 5). A t this t e m p e r a t u r e t h e H-bond no longer exists.
C o m p a r i n g w i t h s i l i c a 181, w e a t t r i b u t e t h e m t o m e t h o x y l g r o u p o n s i l i c i u m (Si-OCH3).
N o t all t h e sample i s m e t h y l a t e d a t room t e m p e r a t u r e and under t h a t pressure. N e i t h e r w a s t h e silica s u r f a c e e v e n a t higher t e m p e r a t u r e a n d under s a t u r a t e d m e t h a n o l vapour, b e c a u s e of t h e s t e r i c hindrance preventing a c o m p l e t e r e a c t i o n 191.
Fig. 5 - Difference spectrum : adsorbed methanol on the cordierite surface a f t e r evacuation at 6 73K.
4 - CONCLUSION
By FT-IR spectroscopy we have pointed out t h a t all the OH groups of cordierite a r e located on t h e surface. Their stretching vibration frequency and their behavior regarding deuteration and methyl- lation look very much t h e same a s in silica.
So we c a n conclude t h a t all t h e evidenced OH groups belong t o t h e silanol species. No absorption f r e q u e n c y c o r r e s p o n d i n g t o AI-OH or Mg-OH s u r f a c e g r o u p s h a s b e e n o b s e r v e d so f a r . Experiments a r e still running for characterizing t h e other c a t a l y t i c sites on t h e cordierite surface and their acidic or basic activities.
The authors a r e indebted to Dr H; VESTEGHEM (E.N.S.C.I., Lirnoges, France) for providing them with cordierite aerogels.
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