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Microcalorimetric and FT-IR spectroscopic study of the adsorption of methanol on TiO2 (anatase)
Colloids Elsevier
and Surfaces, 16 (1985) Science Publishers B.V.,
95-102 Amsterdam
95 -
Printed
in The Netherlands
MICROCALORIMETRIC AND FT-IR SPECTROSCOPIC STUDY OF THE ADSORPTION OF METHANOL ON TiO, (ANATASE)
PIER
FRANCESCO
’ Istituto
di Chimica, P. le Kennedy, 16129
’ ICFAM, (Received
CNR,
ROSSI’*
Genova
11 February
and GUIDO
Facoltii Genova
di Ingegneria, (Italy)
BUSCAl Universitti
di Genova,
Fiera
de1 Mare,
(Italy) 1985;
accepted
in final form
26 April
1985)
ABSTRACT The adsorption of methanol on TiO, (anatase) activated under vacuum at different temperatures has been studied by adsorption microcalorimetry and FT-IR spectroscopy. Both dissociative and undissociative chemisorption of methanol molecules have been found corresponding to the evolution of very high adsorption heats (qdiff> 150 kJ mall’). At higher coverages, hydrogen bonded and dimeric adsorbed species are formed corresponding to differential adsorption heats of 50-120 kJ mall’.
INTRODUCTION
The interaction of alcohols with oxide surfaces is commercially important, being involved in a number of industrial processes, e.g. in the fields of heterogeneous catalysis and paint technology. Moreover, alcohols may also be regarded as suitable probe molecules in surface chemistry. Titanium dioxide, in the form of anatase, is a useful support of both metal catalysts for Fischer-Tropsch [l] and other syntheses [Z] and of vanadium-oxide catalysts for hydrocarbon selective oxidations [3 3, as well as being an important white pigment. Its surface has been studied in detail and satisfactory surface models have been developed [ 41. A FT-IR, TPD and pulse catalytic study of the interaction of methanol with anatase has recently been published [5] ; the experimental results, which are in rather good agreement with previous work of Carrizosa et al. [6], have led to a suggested identification of a number of surface species, as well as their temperature and pressure existence ranges. As a further development of such work, we report here a heat-flow microcalorimetric study in order to evaluate energetically the individual stages of adsorption. Previous studies concerning similar systems [7,8] have indicated that this technique may allow identification of adsorbed species, obtained by spectroscopic methods, to be improved.
0166-6622/85/$03.30
o
1985
Elsevier
Science
Publishers
B.V.
96 EXPERIMENTAL
Titanium dioxide samples were Degussa P 25 (anatase 90%, rutile 10% by XRD; BET surface area 50 m2 g-’ by N, adsorption). Before adsorption they were heated in air at 673 K in order to remove hydrocarbon and chlorine impurities [4]. Evacuation at ,373 K (Tloo sample) or at 673 K (To”0 sample) for 1 h was then carried out (P = 10m5 Torr). Methanol was RPE-ACS product from Carlo Erba (Milan, Italy). The calorimetric measurements were carried out using a Tian-Calvet heat-flow microcalorimeter at room temperature equipped with a Setaram NV 724 amplifier nanovoltmeter and a Servotrace Sefram recorder. The cells and ramps were similar to those reported by Della Gatta et al. [9]. The IR spectra were recorded with a Nicolet MXI Fourier Transform spectrometer. The self supporting pressed discs underwent similar pretreatments as for microcalorimetric measurements into the IR cell. RESULTS
AND DISCUSSION
The microcalorimetric on anatase are reported
and volumetric adsorption isotherms of methanol in Fig. 1. They are of the Langmuir type. For
OE
N
06
E 2 c
0
OL
0.2
C
I
,
I
I
1
2
4
6
8
10
P Fig. 1. Calorimetric of methanol on TiO,
I
I
12
14
torr
(n, (qint versus I’) and volumetric evacuated at 400°C (‘I’,,,) and 100°C
versus (T,,,).
P) adsorption
isotherms
97
the powder pretreated by evacuation at 673 K (Z’4,,0), they tend to asymptotic values of 9.8 I.tmol m-* (adsorbedamount n,) and -0.97 J me2 (integral adsorption heat qtit). However, such curves also indicate that a significant amount of methanol (6 ymol m-2 ) is adsorbed when the equilibrium pressure is still undetectable under our experimental conditions, Such a first strong adsorption step is followed by a weaker one that corresponds to methanol pressures of l-11 Torr (condensation vapour pressure of methanol 100 Torr at 21.2%). The curve of the differential adsorption heats (4diff, Fig. 2) shows that on T400 in the first adsorption region (where equilibrium pressures are still undetectable), very high heats are evolved. Such a very high qdiff value (> 200 kJ mol-‘) at very low coverages, decrease to values of -110 kJ mol-’ when -2.5 pmol mm2 are adsorbed. The qdiff curve is then stabilized near such a value until 6 pmol me2 are adsorbed, when the equilibrium pressure rises to a detectable value. Finally, Qdiff decreases again slowly towards the value of the methanol condensation heat (39.2 kJ mol-‘), while equilibrium pressures increase up to 11 Torr. It is remarkable that the n, value (6 pmol m-‘) that corresponds to the end of the strongly exothermic adsorption step (q&f > 110 kJ mol-‘) corresponds to one adsorbed molecule each 27.6 AZ, which is slightly lower
0
2
4
6 no gmol
Fig. 2. Differential evacuated at 400°C
and integral and 100°C.
(in the
m
inset)
-2
adsorption
0
heats
of methanol
on TiO,
98
than the estimated area of the methanol molecule (29.7 .A2 [ lo]). We must conclude that, at the end of this step, the monolayer is already complete; in the following step, when qdiff values are still above the condensation heat, dimeric or oligomeric adsorbed species are then likely to be formed. It is noteworthy that these species have not been observed on alumina
[Ill *
Such a “multilayer” exothermic adsorption does not take place on Tloo where a strong adsorption step also occurs (qdiff > 100 kJ mol-‘), but involves a very limited amount of sites (< 1 pmol m-‘). Successively, a plateau is observed near qdiff = 70 kJ mol-‘, while exothermic adsorption (adsorption heat higher than condensation heat) ends only slightly after the theoretical monolayer coverage is reached. Fourier transform infrared spectra indicate that, at very low coverages, two distinct adsorbed forms of methanol are formed simultaneously, whose concentration ratio (as deduced by intensities of their absorption bands) does not vary appreciably with the increasing coverage. The spectra under these conditions are reported in Figs 3 and 4, and the assignments of the observed bands are reported in Table 1. Such adsorbed species are identified respectively as I, an undissociated form of methanol, not involved in Hbonding; and II, methoxy groups formed by methanol dissociation,
\‘... \ ‘I
8
5
.’
\‘...
: ./...,
\“.
5
‘.,,,
.-
E
\
2 ,”
_:’ ‘..
\\
\
k
,,
,,
‘.__/’
1
3600
I
3400
:
jt
:I
1’
\
3800
:/
::
,,
\
/
I’
‘.
I
3200
3000
2800
I
Wavenumbers
Fig. 3. FT-IR spectra of TiO, pressed disc evacuated at 400°C (dashed line), after admission of methanol (undetectable equilibrium pressure, full and point lines), in equilibrium with 5 Torr methanol vapour (broken line).
99
Wavenumbers
Fig. 4. FT-IR spectra of the adsorbed species formed on TiO, after admission of methanol (undetectable equilibrium pressure, full line) and in contact with 5 Torr methanol vapour. The background spectrum of activated TiO, has been subtracted, in order to allow the observation of the bands near the black-out due to bulk TiO, absorptions (below 1000 cm-‘).
TABLE
1
IR bands (cm-‘) on TiO,-anatase
and
assignments
of
the
low-coverage-adsorbed
Species I, undissociated
Species II, dissociated
CH,OH
3470 2953 2849
_a 2930 2830
3681 2956 2844
1470
1462
1365
-
1474 1337
&s(CH,) 6 (OH)
1162 1016
1151 1125
1145 1033
P(CH,)
species
of
methanol
gas
[I41 u(OH) 1
Q(CH,)~
1478 1465
sa&CH,)
y(CO)
aA new OH absorption is formed by dissociation, at 3650 cm-‘. bSuch bands are due to Fermi resonance with the first overtone formation of methyl groups, so their description is approximate.
of the symmetric
de-
100
Such assignments agree with calorimetric data. The first exothermic step we have observed is assigned to the simultaneous formation of such species. The qdiff values we have observed are very similar to those we have measured for strong methanol adsorption on alumina [ll] and to that reported for methanol dissociative adsorption on ZnO [12] ; the slight difference may perhaps be explained by taking into account the contribution of the undissociated adsorbed form, probably chemisorbed coordinatively on Lewis acid sites. Such a species has not been observed on ZnO [12]. In fact, in the case of water adsorption, it has been shown that the qdiff value due to undissociative chemisorption is higher than that due to dissociation [ 131, even if the entropic contribution is reversed, so justifying the similar conditions of formation. Calorimetric data then improve substantially our previous assignments [ 51. When saturation of the strong adsorption sites is reached, the IR spectra suddenly change. The bands due to the surface OH groups on the adsorbant (3730-3640 cm-‘, Fig. 3) disappear, indicating that such groups begin to act as adsorption sites. A broad OH stretching band is formed, showing at least two components, one near 3420 cm-’ and one at 3150 cm-’ (very broad). Under such conditions the COH deformation band of undissociatively chemisorbed methanol is not perturbed, indicating that such a species does not interact with methanol “overlayers”. The CH stretching bands are now observed at 2945, 2925 and 2825 cm-‘. The conditions of existence of such species correspond to those in which the plateau at 110 kJ mol-’ is observed. The more reasonable model for the surface species predominant in this situation involves a rather strong, possibly multiple interaction by H-bonding between methanol and surface OH. We have already emphasized that such a species at temperatures intermediate between 100 and 200°C gives rise to methoxy groups, surface OH being no more restored. This indicates that at least two different mechanisms, active in different conditions, may produce methoxy groups on TiO, : (i) dissociation on acid-base pair sites and (ii) condensation with surface OH. The latter has been observed only to a very small extent on alumina [ll], where the first is largely predominant. At even higher coverages, the reversible perturbation of the OH deformation band of undissociatively chemisorbed methanol is observed (Fig. 3), which indicates that it acts as an adsorption site of a second methanol molecule. The formation of adsorbed dimers, whose high qdiff heat of formation (100-50 kJ mol-‘) is justified by the enhanced acidity of methanol induced by chemisorption, is then evident both from volumetric isotherms and IR spectra. Such a species has not been observed either on T,,, or on alumina [ll] , where non-hydrogen-bonded undissociatively chemisorbed species do not form. The prominent OH stretching band at 3150 cm-’ (very broad) seems to indicate that H-bonded species where the hydroxylic hydrogen of meth-
101
an01 interacts with basic sites of the surface, with 5 Torr of methanol vapour.
are also present,
in contact
CONCLUSIONS
This work has shown that heat-flow microcalorimetry is a good technique to improve, by energetic characterization, the identification of adsorbed species. Our study concerning the interaction of methanol on anatase indicates: (a) the existence of dissociative chemisorption sites, as already shown on alumina [ll, 151 and iron oxide [16], (b) the existence of sites that allow undissociative chemisorption, (c) the high reactivity towards methanol of the surface OH groups that form strong hydrogen bonds with methanol, and are able to form methoxy groups by condensation at temperatures intermediate between 25 and 2OO”C, (d) the ability of undissociatively chemisorbed methanol to act as adsorption sites for a second molecule to produce adsorbed dimers and (e) the existence of weakly bonded methanol interacting, via H-bonding, with surface basic sites at high coverages.
ACKNOWLEDGEMENTS
The authors are indebted to Professor V. Lorenzelli for helpful discussions and to F. Guzzo for technical collaboration. This work has been supported by Minister0 della Pubblica Istruzione (Progetto Nazionale “Struttura e Reattivita delle Superfici”).
REFERENCES
5 6 7 8 9 10 11
M.A. Vannice, J. Catal., 74 (1982) 199. S. Matsuda and A. Kato, Appl. Catal., 8 (1983) 149. M.S. Wainwright and N.R. Forster, Catal. Rev., 19 (1979) 211. G. Busca, H. Saussey, 0. Saur, J.C. Lavalley and V. Lorenzelli, Appl. Catal., 14 (1985) 245. G. Busca, P. Forzatti, J.C. Lavalley and E. Tronconi, in B. Imelik (Ed.), Catalysis by acids and bases, Elsevier, Amsterdam 1985, p. 15. I. Carrizosa, G. Munuera and S. Castanar, J. Catal., 49 (1977) 265. B. Fubini, Rev. Gen. Therm., 209 (1979) 297. P.F. Rossi and G. Busca, J. Therm. Anal., 29 (1984) 745. G. Della Gatta, B. Fubini and G. Venturello, J. Chim. Phys., 70 (1973) 64. V.M. Young and A.D. Crowell, Physical adsorption of gases, Butterworth, London, 1962. G. Busca, P.F. Rossi, Phys. Chem., in press.
V. Lorenzelli,
M. Benaissa,
J. Travert
and J.C.
Lavalley,
J.
102 12 13 14 15 16
T. Morimoto, B. Fubini, G. A. Senallach, R.G. Greenler, G. Busca and
M. Kiriki and M. Nagao, J. Phys. Chem., 84 (1980) 2058. Della Gatta and G. Venturello, J. Colloid Interface Sci., 64 (1978) R. Meyer and H.H. Gunthard, J. Mol. Spectros., 52 (1974) 94. J. Chem. Phys., 37 (1962) 2094. V. Lorenzelli, J. Catal., 66 (1980) 155.
470.
and Surfaces, 16 (1985) Science Publishers B.V.,
95-102 Amsterdam
95 -
Printed
in The Netherlands
MICROCALORIMETRIC AND FT-IR SPECTROSCOPIC STUDY OF THE ADSORPTION OF METHANOL ON TiO, (ANATASE)
PIER
FRANCESCO
’ Istituto
di Chimica, P. le Kennedy, 16129
’ ICFAM, (Received
CNR,
ROSSI’*
Genova
11 February
and GUIDO
Facoltii Genova
di Ingegneria, (Italy)
BUSCAl Universitti
di Genova,
Fiera
de1 Mare,
(Italy) 1985;
accepted
in final form
26 April
1985)
ABSTRACT The adsorption of methanol on TiO, (anatase) activated under vacuum at different temperatures has been studied by adsorption microcalorimetry and FT-IR spectroscopy. Both dissociative and undissociative chemisorption of methanol molecules have been found corresponding to the evolution of very high adsorption heats (qdiff> 150 kJ mall’). At higher coverages, hydrogen bonded and dimeric adsorbed species are formed corresponding to differential adsorption heats of 50-120 kJ mall’.
INTRODUCTION
The interaction of alcohols with oxide surfaces is commercially important, being involved in a number of industrial processes, e.g. in the fields of heterogeneous catalysis and paint technology. Moreover, alcohols may also be regarded as suitable probe molecules in surface chemistry. Titanium dioxide, in the form of anatase, is a useful support of both metal catalysts for Fischer-Tropsch [l] and other syntheses [Z] and of vanadium-oxide catalysts for hydrocarbon selective oxidations [3 3, as well as being an important white pigment. Its surface has been studied in detail and satisfactory surface models have been developed [ 41. A FT-IR, TPD and pulse catalytic study of the interaction of methanol with anatase has recently been published [5] ; the experimental results, which are in rather good agreement with previous work of Carrizosa et al. [6], have led to a suggested identification of a number of surface species, as well as their temperature and pressure existence ranges. As a further development of such work, we report here a heat-flow microcalorimetric study in order to evaluate energetically the individual stages of adsorption. Previous studies concerning similar systems [7,8] have indicated that this technique may allow identification of adsorbed species, obtained by spectroscopic methods, to be improved.
0166-6622/85/$03.30
o
1985
Elsevier
Science
Publishers
B.V.
96 EXPERIMENTAL
Titanium dioxide samples were Degussa P 25 (anatase 90%, rutile 10% by XRD; BET surface area 50 m2 g-’ by N, adsorption). Before adsorption they were heated in air at 673 K in order to remove hydrocarbon and chlorine impurities [4]. Evacuation at ,373 K (Tloo sample) or at 673 K (To”0 sample) for 1 h was then carried out (P = 10m5 Torr). Methanol was RPE-ACS product from Carlo Erba (Milan, Italy). The calorimetric measurements were carried out using a Tian-Calvet heat-flow microcalorimeter at room temperature equipped with a Setaram NV 724 amplifier nanovoltmeter and a Servotrace Sefram recorder. The cells and ramps were similar to those reported by Della Gatta et al. [9]. The IR spectra were recorded with a Nicolet MXI Fourier Transform spectrometer. The self supporting pressed discs underwent similar pretreatments as for microcalorimetric measurements into the IR cell. RESULTS
AND DISCUSSION
The microcalorimetric on anatase are reported
and volumetric adsorption isotherms of methanol in Fig. 1. They are of the Langmuir type. For
OE
N
06
E 2 c
0
OL
0.2
C
I
,
I
I
1
2
4
6
8
10
P Fig. 1. Calorimetric of methanol on TiO,
I
I
12
14
torr
(n, (qint versus I’) and volumetric evacuated at 400°C (‘I’,,,) and 100°C
versus (T,,,).
P) adsorption
isotherms
97
the powder pretreated by evacuation at 673 K (Z’4,,0), they tend to asymptotic values of 9.8 I.tmol m-* (adsorbedamount n,) and -0.97 J me2 (integral adsorption heat qtit). However, such curves also indicate that a significant amount of methanol (6 ymol m-2 ) is adsorbed when the equilibrium pressure is still undetectable under our experimental conditions, Such a first strong adsorption step is followed by a weaker one that corresponds to methanol pressures of l-11 Torr (condensation vapour pressure of methanol 100 Torr at 21.2%). The curve of the differential adsorption heats (4diff, Fig. 2) shows that on T400 in the first adsorption region (where equilibrium pressures are still undetectable), very high heats are evolved. Such a very high qdiff value (> 200 kJ mol-‘) at very low coverages, decrease to values of -110 kJ mol-’ when -2.5 pmol mm2 are adsorbed. The qdiff curve is then stabilized near such a value until 6 pmol me2 are adsorbed, when the equilibrium pressure rises to a detectable value. Finally, Qdiff decreases again slowly towards the value of the methanol condensation heat (39.2 kJ mol-‘), while equilibrium pressures increase up to 11 Torr. It is remarkable that the n, value (6 pmol m-‘) that corresponds to the end of the strongly exothermic adsorption step (q&f > 110 kJ mol-‘) corresponds to one adsorbed molecule each 27.6 AZ, which is slightly lower
0
2
4
6 no gmol
Fig. 2. Differential evacuated at 400°C
and integral and 100°C.
(in the
m
inset)
-2
adsorption
0
heats
of methanol
on TiO,
98
than the estimated area of the methanol molecule (29.7 .A2 [ lo]). We must conclude that, at the end of this step, the monolayer is already complete; in the following step, when qdiff values are still above the condensation heat, dimeric or oligomeric adsorbed species are then likely to be formed. It is noteworthy that these species have not been observed on alumina
[Ill *
Such a “multilayer” exothermic adsorption does not take place on Tloo where a strong adsorption step also occurs (qdiff > 100 kJ mol-‘), but involves a very limited amount of sites (< 1 pmol m-‘). Successively, a plateau is observed near qdiff = 70 kJ mol-‘, while exothermic adsorption (adsorption heat higher than condensation heat) ends only slightly after the theoretical monolayer coverage is reached. Fourier transform infrared spectra indicate that, at very low coverages, two distinct adsorbed forms of methanol are formed simultaneously, whose concentration ratio (as deduced by intensities of their absorption bands) does not vary appreciably with the increasing coverage. The spectra under these conditions are reported in Figs 3 and 4, and the assignments of the observed bands are reported in Table 1. Such adsorbed species are identified respectively as I, an undissociated form of methanol, not involved in Hbonding; and II, methoxy groups formed by methanol dissociation,
\‘... \ ‘I
8
5
.’
\‘...
: ./...,
\“.
5
‘.,,,
.-
E
\
2 ,”
_:’ ‘..
\\
\
k
,,
,,
‘.__/’
1
3600
I
3400
:
jt
:I
1’
\
3800
:/
::
,,
\
/
I’
‘.
I
3200
3000
2800
I
Wavenumbers
Fig. 3. FT-IR spectra of TiO, pressed disc evacuated at 400°C (dashed line), after admission of methanol (undetectable equilibrium pressure, full and point lines), in equilibrium with 5 Torr methanol vapour (broken line).
99
Wavenumbers
Fig. 4. FT-IR spectra of the adsorbed species formed on TiO, after admission of methanol (undetectable equilibrium pressure, full line) and in contact with 5 Torr methanol vapour. The background spectrum of activated TiO, has been subtracted, in order to allow the observation of the bands near the black-out due to bulk TiO, absorptions (below 1000 cm-‘).
TABLE
1
IR bands (cm-‘) on TiO,-anatase
and
assignments
of
the
low-coverage-adsorbed
Species I, undissociated
Species II, dissociated
CH,OH
3470 2953 2849
_a 2930 2830
3681 2956 2844
1470
1462
1365
-
1474 1337
&s(CH,) 6 (OH)
1162 1016
1151 1125
1145 1033
P(CH,)
species
of
methanol
gas
[I41 u(OH) 1
Q(CH,)~
1478 1465
sa&CH,)
y(CO)
aA new OH absorption is formed by dissociation, at 3650 cm-‘. bSuch bands are due to Fermi resonance with the first overtone formation of methyl groups, so their description is approximate.
of the symmetric
de-
100
Such assignments agree with calorimetric data. The first exothermic step we have observed is assigned to the simultaneous formation of such species. The qdiff values we have observed are very similar to those we have measured for strong methanol adsorption on alumina [ll] and to that reported for methanol dissociative adsorption on ZnO [12] ; the slight difference may perhaps be explained by taking into account the contribution of the undissociated adsorbed form, probably chemisorbed coordinatively on Lewis acid sites. Such a species has not been observed on ZnO [12]. In fact, in the case of water adsorption, it has been shown that the qdiff value due to undissociative chemisorption is higher than that due to dissociation [ 131, even if the entropic contribution is reversed, so justifying the similar conditions of formation. Calorimetric data then improve substantially our previous assignments [ 51. When saturation of the strong adsorption sites is reached, the IR spectra suddenly change. The bands due to the surface OH groups on the adsorbant (3730-3640 cm-‘, Fig. 3) disappear, indicating that such groups begin to act as adsorption sites. A broad OH stretching band is formed, showing at least two components, one near 3420 cm-’ and one at 3150 cm-’ (very broad). Under such conditions the COH deformation band of undissociatively chemisorbed methanol is not perturbed, indicating that such a species does not interact with methanol “overlayers”. The CH stretching bands are now observed at 2945, 2925 and 2825 cm-‘. The conditions of existence of such species correspond to those in which the plateau at 110 kJ mol-’ is observed. The more reasonable model for the surface species predominant in this situation involves a rather strong, possibly multiple interaction by H-bonding between methanol and surface OH. We have already emphasized that such a species at temperatures intermediate between 100 and 200°C gives rise to methoxy groups, surface OH being no more restored. This indicates that at least two different mechanisms, active in different conditions, may produce methoxy groups on TiO, : (i) dissociation on acid-base pair sites and (ii) condensation with surface OH. The latter has been observed only to a very small extent on alumina [ll], where the first is largely predominant. At even higher coverages, the reversible perturbation of the OH deformation band of undissociatively chemisorbed methanol is observed (Fig. 3), which indicates that it acts as an adsorption site of a second methanol molecule. The formation of adsorbed dimers, whose high qdiff heat of formation (100-50 kJ mol-‘) is justified by the enhanced acidity of methanol induced by chemisorption, is then evident both from volumetric isotherms and IR spectra. Such a species has not been observed either on T,,, or on alumina [ll] , where non-hydrogen-bonded undissociatively chemisorbed species do not form. The prominent OH stretching band at 3150 cm-’ (very broad) seems to indicate that H-bonded species where the hydroxylic hydrogen of meth-
101
an01 interacts with basic sites of the surface, with 5 Torr of methanol vapour.
are also present,
in contact
CONCLUSIONS
This work has shown that heat-flow microcalorimetry is a good technique to improve, by energetic characterization, the identification of adsorbed species. Our study concerning the interaction of methanol on anatase indicates: (a) the existence of dissociative chemisorption sites, as already shown on alumina [ll, 151 and iron oxide [16], (b) the existence of sites that allow undissociative chemisorption, (c) the high reactivity towards methanol of the surface OH groups that form strong hydrogen bonds with methanol, and are able to form methoxy groups by condensation at temperatures intermediate between 25 and 2OO”C, (d) the ability of undissociatively chemisorbed methanol to act as adsorption sites for a second molecule to produce adsorbed dimers and (e) the existence of weakly bonded methanol interacting, via H-bonding, with surface basic sites at high coverages.
ACKNOWLEDGEMENTS
The authors are indebted to Professor V. Lorenzelli for helpful discussions and to F. Guzzo for technical collaboration. This work has been supported by Minister0 della Pubblica Istruzione (Progetto Nazionale “Struttura e Reattivita delle Superfici”).
REFERENCES
5 6 7 8 9 10 11
M.A. Vannice, J. Catal., 74 (1982) 199. S. Matsuda and A. Kato, Appl. Catal., 8 (1983) 149. M.S. Wainwright and N.R. Forster, Catal. Rev., 19 (1979) 211. G. Busca, H. Saussey, 0. Saur, J.C. Lavalley and V. Lorenzelli, Appl. Catal., 14 (1985) 245. G. Busca, P. Forzatti, J.C. Lavalley and E. Tronconi, in B. Imelik (Ed.), Catalysis by acids and bases, Elsevier, Amsterdam 1985, p. 15. I. Carrizosa, G. Munuera and S. Castanar, J. Catal., 49 (1977) 265. B. Fubini, Rev. Gen. Therm., 209 (1979) 297. P.F. Rossi and G. Busca, J. Therm. Anal., 29 (1984) 745. G. Della Gatta, B. Fubini and G. Venturello, J. Chim. Phys., 70 (1973) 64. V.M. Young and A.D. Crowell, Physical adsorption of gases, Butterworth, London, 1962. G. Busca, P.F. Rossi, Phys. Chem., in press.
V. Lorenzelli,
M. Benaissa,
J. Travert
and J.C.
Lavalley,
J.
102 12 13 14 15 16
T. Morimoto, B. Fubini, G. A. Senallach, R.G. Greenler, G. Busca and
M. Kiriki and M. Nagao, J. Phys. Chem., 84 (1980) 2058. Della Gatta and G. Venturello, J. Colloid Interface Sci., 64 (1978) R. Meyer and H.H. Gunthard, J. Mol. Spectros., 52 (1974) 94. J. Chem. Phys., 37 (1962) 2094. V. Lorenzelli, J. Catal., 66 (1980) 155.
470.