Effect of hydrogen sulfide and methanethiol adsorption on acidic properties of metal oxides: An infrared study

Article abstract of DOI:10.1021/jp0126762

The adsorption of H₂S and CH₃SH on SiO₂, TiO₂, Al₂O₃, and ZrO₂ and the resulting changes of surface acidity were investigated by IR spectroscopy using CO and 2,6-dimethylpyridine (DMP) as probe molecules. H₂S and CH₃SH formed H-bond with surface OH groups of all the adsorbents at least, at low temperatures. These molecules were rather strong bases, and shifted the silanol OH band by 210-240 and 330-400/cm, respectively. For silica, H-bond was the only mode of adsorption but was not restricted to the formation of 1:1 complexes. At high coverage, additional frequency shifts of the SH and OH bands revealed the interaction of two molecules with one silanol group or the formation of polymeric chains at the surface. Two kinds of induced Bronsted acidity were established. In the presence of an excess of adsorbed sulfur-containing molecules, a reversible increase of OH groups acid strengths was observed for all the four oxides. It was induced by molecular H₂S H-bonded to the oxygen atom of surface OH groups. For silica, this interaction resulted in an increase of the v(OH) frequency shift with increasing coverage of H₂S or CH₃SH. For alumina, titania, and zirconia, the enhanced OH acidity led to additional frequency shifts of v(CO) and perturbed v(OH) bands observed on CO adsorption at 77 K. For Al₂O₃, ZrO₂, and TiO₂, dissociative adsorption of H₂S and CH₃SH led to the appearance of novel OH groups accounting for the irreversible, at least at 300 K, increase of Bronsted acidity revealed by the higher number of H-bonded Co and protonated DMP.

Full text of DOI:10.1021/jp0126762

1350
J. Phys. Chem. B 2002, 106, 1350-1362
Effect of Hydrogen Sulfide and Methanethiol Adsorption on Acidic Properties of Metal
Oxides: An Infrared Study
†
†,‡
‡
,†
†
A. Travert, O. V. Manoilova, A. A. Tsyganenko, F. Maug e´ ,* and J. C. Lavalley
Laboratoire Catalyse et Spectrochimie, UMR CNRS 6506, ISMRA, UniVersit e´ de Caen, 14050 Caen,
Cedex, France, and Institute of Physics, St. Petersburg UniVersity, St. Petersburg, 198504, Russia
ReceiVed: July 11, 2001
Adsorption of H
were studied by means of IR spectroscopy using CO (at 77 K) and 2,6-dimethylpyridine (DMP) (at 300 K)
as probe molecules. Both H S and CH SH form H-bond with surface OH groups of all the adsorbents. For
2 3 2 2 3 2 2
S and CH SH on SiO , Al O , TiO , and ZrO and the resulting changes of surface acidity
2
3
silica it is the only way of adsorption, while coordination as well as dissociative adsorption leading to formation
of OH groups and molecular water occur on the three other oxides. Two types of induced Brønsted acidity
were established, according to the mechanism of adsorption of the sulfur-containing compound. In the presence
of large amount of sulfur-containing molecules, a reversible increase of the OH group acidity occurs, as
revealed by the frequency shifts of the bands of H-bonded CO molecules and of perturbed OH groups on
increasing H
is explained by the interaction of molecular H
ZrO , dissociative adsorption of H S and CH SH results in the appearance of new OH groups that account
2
S coverage, or by the increased intensity of protonated DMP in the presence of H
2
S. This effect
2
S with oxygen atom of the OH group. For Al
O
2 3
, TiO , and
2
2
2
3
for an irreversible, at least at 300 K, increase of Brønsted acidity revealed by higher intensity of corresponding
bands of H-bonded CO and of protonated 2,6-dimethylpyridine. These new OH groups correspond to the
most acidic of those that normally exist at the surface of the considered metal oxides. In no case the S-H
groups of adsorbed molecules or surface SH groups formed by dissociation could account for the observed
Brønsted acidity increase.
Introduction
the bending vibration has even lower intensity and occurs at
-
1
about 1183 cm , where some of the oxides are not transparent
to the IR beam. The appearance of bands due to new OH groups
or to molecular water is the most direct evidence for the
dissociative adsorption of H2S. An alternative way to obtain
information about interaction of sulfur-containing molecules
with surface sites of oxides is to study the adsorption of thiols
such as methanethiol. Indeed, this compound has chemical
properties close to those of H2S and is much more favorable
for infrared study. It possesses a methyl group which gives rise
to strong infrared bands and its SH stretching vibration has a
much higher molar extinction coefficient than that of H2S. On
alumina, CH3SH dissociates at low coverage, whereas molecular
adsorption occurs at higher coverage, as in the case of H2S
Hydrogen sulfide is a compound that is present as a reactant
or product in important catalytic processes, such as hydrotreat-
ment or Claus reaction. For hydrotreatment, it has been reported
that H2S can promote or inhibit the activity depending on the
1
H2S concentration and on the reaction temperature. For the
supported hydrotreatment catalysts, H2S was shown to modify
surface properties not only of the sulfide phase but also of the
alumina support.2 Moreover, surface properties of metal oxides,
which can be used as Claus catalysts, can be greatly influenced
by treatment in H2S.4
,3
,5
Several papers deal with the mechanism of H2S interaction
with alumina.6 For this oxide, two modes of H2S adsorption
-11
6
-8
were reported,
molecular adsorption or dissociation. The
6
adsorption.
former occurs either on coordinately unsaturated cations with
the formation of a coordinate bond or via hydrogen bond to
surface OH groups or to previously adsorbed H2S molecules.
Dissociative adsorption takes place on the strong Lewis acid-
base pair sites and leads to appearance of acidic OH group and
Much less is known about H2S adsorption on other oxides.
On silica, H2S and CH3SH only reversibly interact with surface
11-16
silanol groups.
However, CH3SH adsorption is stronger than
H2S adsorption. On titania, coordination to surface cations is
17,18
-
6,7
2-
predominant
while on zirconia, as on alumina, the first doses
SH species or to S ions and either two OH groups or one
water molecule4
,8,11-14
of H2S and CH3SH dissociate, then molecular adsorption takes
per every dissociated molecule of H2S.
6
,19,20
place.
To obtain additional information about the adsorption sites
At low H2S coverage on alumina, dissociative adsorption is
generally predominant while non dissociative adsorption mainly
occurs at high H2S coverage.
1
1-13
of H2S, Desyatov et al.
have monitored the occupation of
It should be noted that weak bands observed in the region of
stretching S-H vibrations do not enable one to distinguish
between molecular and dissociative adsorption of H2S, while
surface sites using CO, CDF3 and pyridine as probe molecules
for Lewis acid sites, basic sites and proton-donating centers,
1
1
respectively. In most cases, such as Al2O3, a large amount of
Lewis acid and basic sites were blocked by H2S. This was
considered as evidence for dissociative adsorption. For the phos-
phated silica surface, after H2S dissociation the authors have
detected an increased number of protonic sites that were tenta-
tively associated with the formation of new POH or PSH groups.
*
To whom correspondence should be addressed. Phone: +33-
(
0)2.31.45.28.24. Fax: +33(0)2.31.45.28.22. E-mail: Francoise.Mauge@
ismra.fr.
†
‡
Laboratoire Catalyse et Spectrochimie.
Institute of Physics.
1
0.1021/jp0126762 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/22/2002

Acidic Properties of Metal Oxides
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1351
Few attempts were undertaken to establish correlation be-
tween catalytic activity induced by H2S or CH3SH and their
mechanism of adsorption. Ziolek et al. studied the effect of
used as a specific probe for creation and poisoning of Brønsted
and Lewis acid sites.
4
sulfidation on a series of various metal oxides, such as MgO,
TiO2, ZrO2, CeO2, and Al2O3. The authors have shown that
sulfidation by treatment in H2-H2S mixture at elevated tem-
peratures modify their activity and selectivity in some acid-
catalyzed reactions and have related catalytic properties with
the nature of oxides and the ways of H2S adsorption. Okamoto
Experimental Section
2
-1
1
. Catalysts. SiO2 (Degussa, 200 m g ), γ-Al2O3 (GFSC,
2
-1
2
23 m g ) from Rhone-Poulenc, and TiO2 (Degussa P25, 58
2
-1
m g ), which contains 0.2% SiO2, 0.3% Al2O3 , and 0.3%
HCl, were used. ZrO2 was synthesized from zirconium n-
propylate. It was dried at 383 K and calcined in air at 823 K,
7
et al. observed that isomerization of butene-1 on alumina is
2
-1
and had a specific surface area of 84 m g .
poisoned by dissociative adsorption of H2S.
2
. Infrared Spectroscopy. Two IR cells were used: (1) a
5
Hosotsubo et al. studied the influence of H2S on the catalytic
glass cell for room temperature (RT) measurements and (2) a
low-temperature stainless steel cell described elsewhere.29 The
latter cell is equipped with crystalline ZnSe inner windows that,
in combination with the outer KBr windows, enabled us to
register spectra at 77-300 K in the region comprised between
activity of a wide range of metal ion exchanged Y zeolites, metal
ions supported on silica-alumina or on pure silica and alumina,
for several acid-catalyzed reactions, such as cumene cracking,
toluene disproportionation, and o-xylene isomerization or dis-
proportionation. From infrared data, it was concluded that the
increase in the catalytic activity observed for zeolites after H2S
treatment is a result of the formation of new OH groups, acting
as Brønsted acid sites. For metal ions supported on silica or
alumina, the increased activity was associated with the presence
of irreversibly adsorbed H2S. However, for silica-alumina the
effect of promoting action of H2S was associated with H2S
molecules, apparently, coordinated to metal ions. The effect was
different, depending on the cations, for all the supports it was
-
1
4
000 and 500 cm . All the low-temperature spectra were
recorded in the presence of 0.5-1 Torr of He in the inner
volume of the cell for better thermal contact of sample with
the cooled environment. After removal of liquid nitrogen, the
temperature, measured by a thermocouple inserted in the volume
for coolant, could be changed by gradual heating of the cell
from 77 K up to room temperature.
Samples (about 5 mg cm ) were pressed into 2 cm disks
and first heated under oxygen (10-15 Torr), at 670-1070 K
for 30 min to remove organic contaminations and then evacuated
at the same temperature for 1 h.
H2S (or CH3SH) was adsorbed either at room temperature or
at about 120 K, to increase the amount of adsorbed molecules
and to prevent their dissociation that could be activated at 300
K. In the latter case, a small amount of the studied gas was
introduced into the cooled cell, which already contained helium.
The cell temperature was then increased from 77 K up to about
-
2
2
2+
2+
2+
the most pronounced for Cd , Ag , and Cu ions and almost
negligible for pure oxides used as a support.
However, although H2S was shown to promote or inhibit the
catalytic activity of metal oxides in acid-catalyzed reactions,
no clear relationship between the activity and the nature,
strength, and concentration of surface acidic sites present after
H2S adsorption has been reported.
The aim of this study was to established correlation between
the ways of H2S or CH3SH interaction with the surface of oxides
and the resulting changes of surface acidity in order to
understand the origin of their influence on the catalytic
properties.
1
20 K in order to evaporate the sulfur-containing molecules,
which had condensed on the cooled walls of the cell. When the
increase of pressure and the changes in the IR spectrum indicated
H2S (or CH3SH) adsorption on the sample, the cell was cooled
again up to 77 K before CO admitting. In some cases, two
Adsorption of H2S and CH3SH was studied by means of
infrared spectroscopy on four oxides that exhibit quite different
acid-base properties: SiO2, Al2O3, TiO2, and ZrO2. To detect
the modifications of surface acidic properties caused by adsorp-
tion, two probe molecules were used, carbon monoxide and 2,6-
dimethylpyridine. At 77 K CO interacts both with Lewis and
-
1
sharp bands at 2544 and 2526 cm due to the film of solid
H S on the cold cell windows could be distinguished, then these
2
were subtracted to obtain the spectra of pure adsorbed H S. As
a next step, the cell was evacuated at about 120 K to decrease
the amount of adsorbed H S (or CH SH), and again the surface
2
2
3
21-23
Brønsted acid sites.
Its interaction with free hydroxy groups
was characterized by CO adsorption at 77 K. Then the same
procedure was repeated after raising the temperature and
evacuation at 300 K.
To characterize the samples by CO adsorption before or after
contact with sulfur-containing molecules, about 8 Torr of CO
at equilibrium pressure were admitted into the cell. Then,
evacuations during increasing times were performed to study
the spectrum dependence on the CO coverage.
Experiments on the catalyst acidity monitored with DMP as
a probe molecule were carried out at room temperature. In this
case, to study the effect of H2S or CH3SH adsorption, two
different procedures were used. In the first case, small calibrated
H2S doses were successively introduced up to saturation. Then,
after evacuation at room temperature, 1 Torr of DMP was let
results both in a downward shift of OH stretching frequency
and in the increase of ν(CO) whose value can be used as a
measure of OH group acidity. Nevertheless, for various metal
oxides the spectra of adsorbed CO could be rather complex and
do not always enable one to detect easily the creation of
Brønsted acid sites. Therefore, 2,6-dimethylpyridine (DMP)
adsorption was also studied. Due to its higher basicity (proton
24
affinity (PA) ) 963.0 kJ/mol ) as compared with pyridine (PA
2
4
)
930.0 kJ/mol ), DMP is more sensitive to Brønsted acidity
2
5
than pyridine. Application of DMP for the surface acidity
26-28
studies and band assignment has been considered elsewhere.
The 1660-1580 cm-1 region is particularly informative, since
there occur the bands of ν8a and ν8b vibrations of DMP, the
most sensitive to adsorption. Bands between 1655 and 1625
-5
into the cell that was again evacuated up to about 2 × 10
-
1
+
cm correspond to protonated DMP (DMPH ), whereas those
between 1618 and 1580 cm are due to coordinated DMP or
weakly adsorbed DMP.
Torr. In the second case, DMP was first adsorbed until a pressure
-
1
-5
of 2 × 10 Torr was reached, then doses of hydrogen sulfide
were introduced as above, and spectra were recorded in the
presence of H2S gas and after its removal. The latter procedure
was used for the quantitative measurements. Typically, the
Thus, in this work CO was used to get information about the
nature and strength of acid sites of oxide surfaces before or
after contact with H2S and CH3SH, while DMP adsorption was
+
intensity of DMPH bands increased rapidly with the first doses

1352 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Travert et al.
Figure 1. IR spectra of H
2 3
S (A) and CH SH (B) adsorbed on silica pretreated at 970 K. 1: Before adsorption. 2: After adsorbate addition at 120
K. 3: After pumping off the excess of adsorbate at the same temperatures. All the spectra are recorded at 77 K.
-
1
of introduced H2S (up to about 300 µmol g ), and then
saturation took place, when the pressure of H2S was several
Torr.
spectrum 2). As in the case of H2S, upon evacuation at 120 K,
it decreases in intensity and shifts toward higher wavenumbers,
up to 3410 cm . Most of the bands of adsorbed CH3SH appear
-1
The concentration of Brønsted sites (site number per nm2)
induced by H2S or CH3SH adsorption on the studied metal
oxides was estimated from the spectra of adsorbed DMP, taking
into account the surface area of the oxide disk and assuming
that the integrated absorption coefficient for the complex band
as doublets. In particular, the band of ν(SH) vibration presents
-
1
two components at 2585 and 2540 cm . Evacuation at 120 K
mainly diminishes the intensities of the lower wavenumber
components of the C-H and S-H bands (Figure 1B, spectrum
3). This points to the existence of at least two different forms
of molecular adsorption.
+
of two ν8 DMPH vibrations of protonated form does not
depend on the adsorbent. Its value was determined in a special
experiment with adsorption of small calibrated doses of DMP
onto the surface of HY zeolite. On this sample, in the conditions
of experiment, all the introduced DMP was adsorbed and
protonated, and the integrated absorption was related to the
quantity of adsorbate admitted into the cell. The measured value
1.2. CO Adsorption. In accordance with the earlier stud-
ies,2
1,23,32
2
CO adsorption at 77 K on SiO shifts the band of
-
1
free silanol groups to 3670 cm and results in the appearance
of bands of H-bonded and physisorbed CO species at 2157 and
-
1
2140 cm , respectively (Table 1).
After H2S or CH3SH preadsorption at 120 K, the ν(CO) band
+
of ꢀ for ν8a and ν8b vibrations of DMPH was found to be 17
-1
at 2157 cm presents a lower intensity, as could be expected
-
1 30
(
3 cm mmol .
from the smaller amount of remaining free silanols capable of
hydrogen bonding. The magnitude of the ν(OH) shift, however,
is the same as without preadsorbed species.
Commercial oxygen, helium, and carbon monoxide were
purified by passing through a trap cooled by liquid nitrogen.
Helium and DMP were also purified by contacting with
preliminary outgassed zeolite at 77 K or room temperature,
respectively.
1
.3. DMP Adsorption. Admission of 1 Torr of DMP at 300
K results in a complete disappearance of the band of free silanol
groups, while a broad band due to perturbed OH groups appears
Spectra were recorded on a Nicolet FT-IR 710 spectrometer
-1
at ca. 2900 cm . The changes are essentially reversible since
-
1
with 4 cm resolution.
most part of adsorbed DMP could be removed by prolonged
evacuation at 300 K. The ν8 bands of adsorbed DMP occur at
Results
-1
1
602 and 1580 cm and do not shift with surface coverage.
These data show that H-bonding with silanol groups is the only
adsorption mode of DMP.
1
. Silica. 1.1. H2S and CH3SH Adsorption. After activation
at 970 K, silica spectrum exhibits a single hydroxy band at 3750
cm due to isolated silanol groups (Figure 1A.B, spectra 1).31
When excess H2S is added at about 120 K, the intensity of this
band decreases and a broad band of perturbed OH groups
-1
Introduction of H2S or CH3SH at 300 K before or after DMP
adsorption does not affect the spectrum of the latter. When most
part of DMP is removed, one could observe weak manifestations
of H2S or CH3SH adsorption on silanol groups and a very slight
-
1
appears at 3510 cm (Figure 1A, spectrum 2). In the ν(SH)
vibration region, an intense band at 2576 cm develops with
shoulders at 2608 and 2597 cm . When excess H2S is removed
-
1
2
decrease of the remaining DMP bands. Subsequent H2S
-1
addition at 120 K to the sample, with preadsorbed DMP, leads
to a slight shift of the band of perturbed OH groups toward
lower frequencies.
-
1
by evacuation at 120 K, the intensity of the band at 2576 cm
decreases, whereas those bands at 2608 and 2597 cm- remain
1
almost unaffected (Figure 1A, spectrum 3). The intensity of the
2. Al O . 2.1. H S and CH SH Adsorption. After activation
2
3
2
3
-
1
band of perturbed OH groups at 3510 cm
also diminishes,
at 770 K, the spectrum of Al O presents ν(OH) bands at 3795,
2
3
-
1
whereas its maximum shifts toward higher values, up to about
3775, 3730, and 3680 cm (Figure 2A, spectrum 1). H2S
addition at 120 K (Figure 2A, spectrum 2) leads to the decrease
or complete disappearance of the free OH bands. In parallel,
appearance of a broad band due to perturbed OH groups at about
-
1
3
540 cm .
Adsorption of CH3SH on silica in excess leads to the
-
1
appearance of perturbed ν(OH) band at 3360 cm (Figure 1B,

Acidic Properties of Metal Oxides
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1353
TABLE 1: Band Positions (in cm-1) of H-Bonded CO and of the Corresponding Bands of Perturbed OH Groups (in
Parentheses) for the Studied Oxides at Different Surface Coverages by CO Adsorbed at 77 K, before and after Treatment
by H S
2
activation
temperature/K
H
2
S adsorbed
at 120 K
H
2
S and evacuation
at 120 K
H
2
S and evacuation
at 300 K
oxide
SiO
after activation
2
970
2157
2157
(3670)
(3670)
a
Al
2
2
O
3
3
770
2155-2160
2158-2163
(3540-3510)
2160-2165
(3515-3500)
2156-2160
(3545-3530)
2155-2162
(3560- 3540)
a
(3610-3560)
Al
O
1070
770
∼2155-2160
3600-3560)
2156-2158, 2149-2151
3575-3555)
2152-2156
3610-3600)
2157-2163
(3520-3505)
2155-2158
(3550-3530)
2153-2159
(3570-3550)
2152-2158
(3580-3555)
(
TiO
2
(
ZrO
2
770
2152-2156
(3560-3550)
(
a
The two values represent the band shift when the CO coverage decreases from saturation (in bold) down to the lowest intensities that enable
band position measurements to be made.
740 and 3700 cm- which become more intense than before
1
3
H2S adsorption.
CH3SH addition at 120 K to the sample pretreated at 770 K
leads to a decrease of the bands of free OH groups, mainly of
-
1
that at 3775 cm and to the appearance of broad bands of
-
1
OH and SH vibrations at about 3150 and 2450 cm . The
C-H bands of adsorbed CH3SH appear at 2990, 2932, 2855,
-
1
1
1
442, and 1321 cm for both the samples treated at 770 and
070 K.
2
.2. CO Adsorption. CO adsorption at 77 K on alumina
pretreated at 770 K results in the decrease of free OH bands
and appearance of the band of perturbed OH groups at 3610
-
1
cm (Figure 3, spectrum 1). Bands of adsorbed CO arise at
-
1
2
235, 2180, 2155, and 2145 (sh) cm and are assigned to
coordination on strong and weak Lewis sites, H-bonding, and
Figure 2. A. IR spectra of H
K (A) or 1070 K (B). 1: Before adsorption. 2: After H
2
S adsorbed on alumina pretreated at 770
S addition at
21
physisorption, respectively. With decreasing amount of ad-
2
-1
sorbed CO, the band at 2145 cm is the first to disappear, while
120 K. 3: After pumping off the excess of H
2
S at 120 K. All the spectra
-
1
that at 2155 cm strongly diminishes and shifts up to 2160
are recorded at 77 K.
-
1
cm (Table 1). Finally, after removal of the most part of
adsorbed CO, only the bands first detected at 2235 and 2180
-
1
-1
3
470 cm , a ν(SH) band at about 2550 cm with a tailing on
-
1
cm remain in the spectrum with their maxima shifted up to
-
1
the low-frequency side and a weak δ(HSH) band at 1177 cm
are observed. Removal of the excess of H2S at 120 K (Figure
-1
2
245 and 2200 cm , respectively. On evacuation, the maximum
of the band of perturbed OH groups shifts to lower wavenum-
2
A, spectrum 3) restores the bands of free OH groups at 3730
-
1
bers, to ca. 3560 cm (Table 1) and ultimately disappears
-
1
and 3680 cm , whereas the high frequency ν(OH) bands at
795 and 3775 cm-1 are still perturbed. Evacuation also results
in the decrease and broadening of the bands at 3470 and 2550
-
1
together with the CO band at 2160 cm assigned to H-bonded
CO molecules.
3
Raising the pretreatment temperature up to 1070 K (Figure
-
1
-1
cm , whose maxima are shifted to 3240 and 2520 cm ,
respectively. The band of the δ(HSH) mode can no longer be
detected.
4
, spectrum 1) results in several changes in the spectrum of
adsorbed CO. The bands of H-bonded CO and of perturbed OH
groups occur at almost the same positions as those for the sample
activated at 770 K but present a much weaker intensity, as
expected from the reduced number of surface OH groups. On
the contrary, the bands of coordinated CO species at about 2235
Heating the sample up to 300 K in the presence of H2S
followed by cooling to 77 K lead to an increase in the intensities
-
1
of the SH band at 2520 cm and of the broad perturbed OH
-
1
-1
band at about 3400 cm . A δ(HOH) band at 1620 cm ,
characteristic of molecular water, also arises. It could not be
removed by evacuation at 300 K. The bands of free OH groups
completely disappear and could not be restored by pumping at
-1
and 2180 cm are more intense, and are observed in the
presence of gas phase even at 300 K.
H2S preadsorption at 120 K on the sample pretreated at 770
K (Figure 3, spectrum 2) results in noticeable changes in the
spectrum of adsorbed CO. The intensity of the band at 2180
1
20 K.
After raising the pretreatment temperature up to 1070 K
-1
cm decreases significantly, indicating the occupation of
(Figure 2B, spectrum 1) three bands of isolated OH groups
surface cations by H S. The band of H-bonded CO species
2
-1
-1
remain at 3800, 3740, and 3700 cm . Subsequent H2S addition
at 120 K (Figure 2B, spectrum 2) leads to the appearance of a
appears at 2158 cm accompanied by the band of perturbed
-
1
OH groups at 3540 cm . Lowering CO coverage leads to a
-
1
-1
broad band at 3200 cm of perturbed OH groups and ν(SH)
shift of the CO/OH band up to 2163 cm while the perturbed
-1
-1
band at 2508 cm . Removal of the excess of H2S by evacuation
at 120 K decreases their intensities (Figure 2B, spectrum 3).
Heating at 300 K followed by evacuating at the same temper-
ature lead to the disappearance of these bands. This is ac-
companied by an increase in the intensity of the OH bands at
OH band shifts further up to 3510 cm (Table 1). Comparison
of the spectra recorded at low CO coverage in the absence and
in the presence of adsorbed H S clearly indicates a stronger
2
acidity of surface OH groups in the latter situation since the
-
1
ν(CO) increases by 3 cm , while the maximum of the

1354 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Travert et al.
Figure 3. IR spectra of CO (8 Torr) adsorbed at 77 K on alumina pretreated at 770 K. 1: On pure alumina. 2: On alumina with preadsorbed H
after removing the excess of H S by pumping at 120 K. Dotted line: spectra before CO adsorption.
2
S
2
2
Figure 4. IR spectra of CO (8 Torr) adsorbed at 77 K on alumina pretreated at 1070 K. 1: on pure alumina. 2: After preadsorption of H S at 120
K. 3: After subsequent evacuation at 120 K. 4: After evacuation at 300 K. Dotted line: spectra before CO adsorption.
corresponding band of perturbed OH groups is shifted by 50
cm toward lower frequencies.
because the above-mentioned growth of the OH bands after
-
1
contact with H2S at RT reveals the creation of new OH groups
as a result of dissociative adsorption. However, the band position
CH3SH preadsorption also leads to a decrease in the intensity
of the bands of CO adsorbed on Lewis acid sites and hydroxy
groups of the sample pretreated at 770 K, although no frequency
shifts that could point to acidity changes were observed.
-
1
of CO adsorbed on such hydroxyls (2155 cm ) is not as high
as after H2S removal at 120 K (Table 1).
Preadsorption of CH3SH affects the spectrum of adsorbed
CO in a similar way. Bands of coordinated CO disappear.
Removing the excess of CH3SH at RT from the sample
pretreated at 1070 K leads to the appearance of a new OH band
Exposing the sample pretreated at 1070 K to a large dose of
H2S followed by subsequent adsorption of CO leads to the
appearance of two ν(CO) bands at 2157 and 2144 cm- (Figure
1
-
1
with a maximum at 3700 cm . CO adsorption at 77 K results
4
, spectrum 2). Their intensities are lower than in the absence
-
1
in a band of perturbed OH groups at 3540 cm and a band of
of preadsorbed H2S. The perturbed OH band appears in this
-
1
-
1
adsorbed CO at 2159 cm that slightly shifts to higher
wavenumbers with decreasing coverage.
case at 3520 cm . Removing the excess of H2S at 120 K
Figure 4, spectrum 3) leads to an increase of the intensity of
(
-
1
-1
the band at 2157 cm , while its maximum shifts to 2160 cm
and even to 2165 cm-1 with decreasing the CO coverage (Table
). Simultaneously, the band of perturbed OH groups shifts
It should be noticed that in no case CO adsorption gives rise
to any perturbation of stretching SH bands or displaces adsorbed
H2S or CH3SH.
1
-
1
down to ca. 3500 cm , the lowest observed position for
alumina. The intensity of this band, as well as that of the
corresponding band of H-bonded CO, become noticeably higher
than before H2S adsorption, if the latter is removed at 300 K
2.3. DMP Adsorption. When DMP is adsorbed on alumina
activated at 670 K, the hydroxy groups are perturbed, leading
-
1
to the appearance of a strong band centered at 2960 cm . A
large number of bands assigned to adsorbed DMP also arise.
The most sensitive to adsorption are those that correspond to
(Figure 4, spectrum 4). Same features occur when H2S is
-
1
adsorbed at 300 K with subsequent evacuation at the same
temperature. The increased amount of H-bonded CO is expected,
the ν8a and ν8b modes observed in the 1650-1550 cm region
-
1
(Figure 5, spectra 1 and 2). The intense band at 1594 cm

Acidic Properties of Metal Oxides
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1355
spectrum 3), H2S addition leads to a decrease in the intensity
-1
of the band of coordinated species (1615 cm ) and an increase
-1
of those corresponding to the protonated (1652 and 1628 cm )
-
1
and H-bonded (1603 cm ) species. A shoulder also appears at
-
1
1
643 cm and could be removed on evacuation at 300 K
(spectrum 4). From the comparison with the spectral features
of the δ(HOH) band which characterizes H2O formation when
H2S is added, one can exclude that this shoulder is due to water.
+
It evidently characterizes specific DMPH species.
When DMP is added after H2S preadsorption, the increase
in the intensity of the bands of protonated form at the expense
of those due to coordination also occurs. However, in this case
the band intensity of protonated molecules is lower than when
H2S is added to preadsorbed DMP. It should be noted that, in
the presence of H2S in the gas phase, the bands of protonated
species present a higher intensity than after evacuation (spectra
Figure 5. Effect of H
pretreated at 670 K in the ν
DMP (1 Torr). 2: After evacuation at 300 K. 3: After subsequent
addition of H S (1 Torr). 4: After evacuation at 300 K.
2
S on the spectrum of DMP adsorbed on alumina
8
vibration region. 1: After addition of
3
and 4 in Figure 5). Quantitative estimation of the number of
2
Brønsted acid sites (see Table 2 below) shows that it increases
-
2
-2
-2
-2
-2
TABLE 2: Effect of H S on Surface Concentration of
2
from 0.4 × 10 nm on pure alumina to 5.8 × 10 nm
2
-2
Brønsted Acid Sites (Number of Sites per 100 nm )
Determined from IR Spectra of Adsorbed DMP
when H2S is added. This amount decreases to 5.3 × 10 nm
when weakly adsorbed H2S is removed on evacuation.
before H
addition
2
S
in the presence of
after evacuation
at 300 K
Adsorption of CH3SH also affects the spectrum of adsorbed
DMP. It leads to the complete elimination of the band of
coordinated DMP species and to an increase in the intensity of
DMPH . However the amount of protonated DMP is lower than
that observed after H2S addition.
oxide
Al
TiO
ZrO
H
2
2 3
S (5 Torr) Al O
O
2 3
0.4
5.9
0.2
5.8
29.8
12.6
5.3
21.9
7.1
+
2
2
3
. TiO2. 3.1. H2S and CH3SH Adsorption. After treatment at
disappears following a short evacuation at 300 K, indicating
7
70 K, the spectrum of titania sample displays a sharp ν(OH)
band at 3742 cm and a broader one at about 3675 cm
Figure 6A, spectrum 1). H2S adsorption at 120 K perturbs all
the OH groups giving rise to a broad band near 3300 cm
that it is associated with the physisorbed DMP molecules. The
-
1
-1
intensity of the bands at 1603 and 1580 cm-1 decreases
(
simultaneously with that of perturbed OH groups. This enables
one to assign these two bands to the ν8a and ν8b modes of DMP
molecules that form H-bond with surface hydroxy groups, in
-
1
(
Figure 6A, spectrum 2) and to the intense ν(SH) band with a
-
1
2
complex contour and a maximum at 2544 cm . A weak band
of the bending mode of H S could also be distinguished at 1174
agreement with previous studies. A pair of weak bands at 1615
-
1
and 1580 cm that resist to evacuation at 300 K are assigned
to ν8a and ν8b vibration modes of DMP coordinated to surface
cations. These bands could be removed by evacuation at 370 K
2
-
1
cm . Evacuation at 120 K results in a decrease of the intensity
of the S-H band, especially from the high-frequency side,
allowing one to distinguish maxima at 2544, 2535, and 2526
-
1
together with two others at 1652 and 1628 cm (weak) that
-
1
+
2
cm . The band of perturbed OH groups also diminishes and
correspond to ν8a and ν8b modes of protonated DMPH .
-1
shifts to 3270 cm (Figure 6A, spectrum 3), while the OH band
at 3742 cm recovers its intensity almost completely and the
band at 3675 cm becomes much more intense than before
H2S addition, evidently, as a result of H2S dissociation.
Subsequent introduction of H2S after preadsorption of DMP
at 300 K and removal of weakly bound molecules by evacuation
at the same temperature, results in the disappearance of bands
due to free OH groups and to the concomitant growth of the
bands of perturbed hydroxyls. As in the case of H2S adsorbed
alone at 300 K, the band of ν(SH) vibrations could be seen at
-
1
-
1
H2S adsorption at 300 K, leads to the appearance of a ν(SH)
band at 2530 cm and perturbation of free OH groups. A strong
broad band near 3400 cm also appears and is accompanied
-1
-
1
-1
2
585 cm . In the spectrum of adsorbed DMP (Figure 5,
Figure 6. IR spectra of H
2
S (A) and CH
3
SH (B) adsorbed on titania pretreated at 770 K. 1: Before adsorption. 2: After addition of adsorbate at
S bands.]
1
20 K. 3: After evacuation at 120 K. All the spectra are recorded at 77 K. [*: Solid H
2

1356 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Travert et al.
Figure 7. IR spectra of CO adsorbed at 77 K on titania pretreated at 770 K. 1: On pure titania. 2: After H
2
S adsorption at 120 K. 3: After
evacuation of H S at 120 K. 4: After evacuation of H S at 300 K. Dotted line: spectra taken before CO adsorption.
2
2
-
1
by a δ(HOH) band at 1614 cm , characteristic of molecular
When H2S is adsorbed at 300 K and pumped off at the same
temperature, the above-mentioned formation of new OH groups
and water leads to the increased intensity of the band of
H-bonded CO (Figure 7, spectrum 4). However, these new OH
groups do not exhibit stronger acidity than the initial OH groups
of pure oxide, even at small CO coverage. As seen from the
Table 1, the differences in the band positions of perturbed OH
groups and corresponding ν(CO) bands are within the limits of
experimental error.
water, that could not be completely removed by evacuation at
3
00 K.
CH3SH adsorption at 120 K gives rise to a stronger perturba-
tion of OH groups than that observed following H2S adsorption
under the same conditions (Figure 6B, spectrum 2). The
-
1
appearance of a ν(SH) band at 2520 cm with a shoulder at
-
1
2
495 cm indicates the presence of nondissociated CH3SH.
.2. CO Adsorption. On TiO2, CO adsorption at 77 K results
in the decrease of the intensity of the free OH band at 3742
3
As observed in the case of alumina, the bands in the ν(SH)
region are never affected by CO addition.
-
1
cm and in the appearance of perturbed hydroxy band at 3575
-
1
cm (Figure 7, spectrum 1; Table 1). In the region of CO
stretching vibrations, an intense band appears at 2179 cm-
accompanied by weaker bands at 2207, 2165, 2156 (sh), 2149,
and 2141 cm . Evacuation at increasing temperatures indicates
that the thermal stability of CO surface species increases with
increasing ν(CO) frequency. The band of physisorbed CO at
141 cm disappears upon evacuation. The bands at 2156 and
149 cm could be removed by short evacuation at 77 K
together with the OH perturbation, and could be assigned to
H-bonded CO species. The weak band at 2165 cm disappears
after prolonged evacuation and is assigned to CO adsorption
on weak Lewis acid sites.21 The 2207 and 2179 cm bands,
shifted to higher wavenumbers, can be observed in the presence
of CO gas even at 300 K and are due to CO coordinated to two
types of Lewis acid sites.33
The effect of H2S preadsorption on the spectra of adsorbed
CO depends on the amount of adsorbed H2S. After exposure to
H2S at 120 K (Figure 7, spectrum 2), CO does not induce any
changes, either in ν(OH) or in ν(SH) region. It only gives rise
3
.3. DMP Adsorption. DMP adsorption at 300 K on TiO2
1
pretreated at 670 K leads to the perturbation of free OH groups
with the appearance of a broad band with two maxima at about
-1
-1
2
800 and 2000 cm and to the numerous bands assigned to
adsorbed DMP. After evacuation at 300 K, the decrease in
intensity of the perturbed ν(OH) band is accompanied by the
-
1
2
2
-1
decrease in intensity of intense band at 1600 cm that is due
-
1
to the elimination of H-bonded DMP species. The bands
characteristic of protonated species remain at 1645 and 1625
-1
-1
-1
cm . A shoulder at 1607 cm could also be distinguished,
which is due to DMP molecules coordinated to Lewis acid sites.
-1
When H2S is introduced after DMP evacuation at 300 K, the
intensity of the bands of protonated DMP at 1645 and 1625
-
1
+
cm increases while a new DMPH band appears at 1634
-
1
cm . Simultaneously, a decrease in the intensity of the band
-
1
assigned to DMP coordinated to Lewis acid sites (1607 cm )
is observed. Quantitative estimations (see Table 2) show that
H2S addition leads to ca. a 5-fold increase of the number of
detected Brønsted acid sites, their total concentration being ca.
-
1
to weak band of physisorbed CO at 2142 cm . This indicates
a complete coverage of titania surface by H2S which hinders
CO adsorption both on the cationic sites and on the acidic OH
groups.
-2
0
.3 nm . About one-third of these newly formed sites disappear
after removal of gaseous H2S. Adsorbing H2S before DMP also
+
results in the appearance of supplementary DMPH species,
the amount of which is increased at the expense of the
coordinated species.
Removal of excess H2S at 120 K followed by CO adsorption
(
Figure 7, spectrum 3) leads to the appearance of the band of
-
1
Introduction of CH SH after DMP adsorption leads to similar
perturbed hydroxyls at 3545 cm , while the corresponding ν-
3
-
1
features. But, as observed in the case of alumina, the increase
(
CO) band appears at 2156 cm . These bands are more intense
+
in intensity of the bands of DMPH species to the detriment of
than on pure titania, evidently due to the creation of new acidic
OH groups. On lowering the CO coverage, the latter band moves
to 2160 cm and the band of perturbed OH groups reaches ca.
those of coordinated DMP is less pronounced as compared to
-
1
the case of H S adsorption.
2
-
1
3
530 cm (Table 1). When CO adsorption is performed on
4. ZrO2. 4.1. H2S and CH3SH Adsorption. After activation
at 770 K, ZrO2 presents two ν(OH) bands at about 3780 and
CH3SH precovered sample, the ν(CO) band and ν(OH) band
perturbed by CO appears at wavenumbers similar to those
obtained for pure titania.
-
1
3670 cm . As seen from the spectrum 2 in Figure 8A, H2S
adsorption at 120 K leads to disappearance of the high-frequency

Acidic Properties of Metal Oxides
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1357
670 cm- and the appearance of a band of perturbed hydroxyls
1
3
-1
at 3610 cm (Figure 9, spectrum 1; Table 1). CO bands appear
-
1
at 2183 (sh), 2171, 2152, and 2141 cm . The last two bands,
assigned to H-bonded and physisorbed CO, disappear after
removal of gaseous CO at 77 K, whereas the initial spectrum
in the OH region is restored. With the decreasing CO coverage,
the two other ν(CO) bands corresponding to coordinated CO
-
1
shift to higher wavenumbers (up to 2207 cm ), in agreement
with earlier studies.
The IR spectrum obtained for the sample with preadsorbed
H2S indicates that CO adsorption shifts the OH band from 3665
-
1
to 3560 cm (Figure 9, spectrum 2). The intensity of the
-
1
ν(CO) band at 2152 cm becomes higher than in the initial
spectrum, indicating hydroxy groups creation. On the other hand,
-
1
-1
the ν(CO) band at 2171 cm and the shoulder at 2183 cm
Figure 8. IR spectra of H
2
S (A) and CH
3
SH (B) adsorbed on zirconia
strongly decrease, indicating the poisoning of Lewis acid sites.
On CO desorption, the band of perturbed OH groups diminishes
pretreated at 770 K. 1: Before adsorbate adsorption. 2: After adsorbate
addition at 120 K. 3: After adsorbate evacuation at 120 K. All the
spectra are recorded at 77 K.
-1
-1
and moves to 3550 cm , while the band at 2152 cm shifts
-
1
to 2156 cm (Table 1). After evacuation at 120 K, the ν(OH)
band of the hydroxy groups perturbed by CO is slightly shifted
toward low wavenumbers whereas the frequency of the corre-
-
1
band, while a broad ν(OH) band centered at c.a. 3300 cm
arises, accompanied by a δ(HOH) band at 1590 cm , a broad
-1
-
1
sponding ν(CO) increases up to 2162 cm (Table 1).
-
1
band ν(SH) at 2540 cm , and a weak δ(HSH) band at 1173
When molecular H2S is removed by evacuation at 300 K
-1
cm . Subsequent evacuation diminishes the intensity of the last
(Figure 9, spectrum 3), the bands of coordinatively bonded CO
-
1
two bands, whereas the band at 3670 cm increases consider-
ably in intensity and shifts to 3655 cm (Figure 8A, spec-
trum 3).
species disappear almost completely. On the other hand, the
band of H-bonded CO is much more intense than in the case of
CO adsorption on pure zirconia. Spectra recorded at low CO
coverage show that band positions of the perturbed OH groups
and H-bonded CO are only slightly different from those
observed on the pure oxide, indicating a small increase of OH
acidity with respect to initial zirconia sample (Table 1). When
CH3SH is adsorbed at 120 K, no increase of OH acidity is
detected by CO adsorption after removal the excess of adsorbate,
neither at 120 K nor at 300 K.
-
1
H2S adsorption at RT or at 120 K followed by an increase of
the temperature up to 300 K in the presence of H2S leads to
further increase in the intensity of bands at 3655, 3300 and 1590
-
1
cm corresponding respectively to the ν(OH) modes of the
surface hydroxyls and to the stretching and bending mode of
molecularly adsorbed water. In the ν(SH) region, a weak and
-
1
broad band with two maxima at 2570 and 2540 cm are
observed together with a weak δ(HSH) band at 1163 cm . After
evacuation at 300 K, the ν(SH) and δ(HSH) bands disappear
-1
As in the case of other oxides, CO adsorption caused no
perturbation in the SH stretching region.
-
1
together with the bands of water at 3300 and 1590 cm .
4
.3. DMP Adsorption. DMP adsorption on zirconia leads
CH3SH adsorption at low temperature (Figure 8B, spectrum
to the complete disappearance of the high-frequency band at
2) also perturbs mainly the high-frequency OH groups and gives
-1
3
3
780 cm and a decrease in of the intensity of the band at
-
1
rise to a broad ν(OH) band at ca. 3300 cm , a very weak SH
-1
670 cm whereas a broad band of perturbed hydroxy groups
-1
stretching band at 2500 cm , and CH stretching bands at 2984,
-1
appears at ca. 3150 cm . Strong bands of adsorbed DMP arise
at 1610 and 1580 cm with a shoulder at 1603 cm . Weak
bands at 1652 and 1627 cm corresponding to protonated
species are also observed.
-
1
2
926, and 2847 cm .
.2. CO Adsorption. CO adsorption at 77 K on zirconia results
in the decrease in intensity of both free OH bands at 3780 and
-1
-1
-
1
4
Figure 9. IR spectra of CO adsorbed at 77 K on zirconia pretreated at 770 K. 1: On pure sample. 2: After H
heating in H S up to 300 K. Dotted line: spectra taken before CO adsorption.
2
S adsorption at 120 K. 3: After
2

1358 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Travert et al.
Evacuation at 370 K restores the OH bands at 3780 and 3670
the ν(OH) wavenumbers at increased coverage is expected for
such a structure. The increase in the ν(OH) frequency shift of
silanol groups upon H-bond is caused by an additional H2S
interaction with their oxygen atoms. This leads to an increase
of the OH group acidity induced by its interaction with H2S
acting as an acid. The same process is proposed for CH3SH
adsorption. This phenomenon was well demonstrated in a recent
-
1
cm , although the former one presents a lower intensity than
before adsorption. Evacuation also dramatically decreases the
intensity of the band assigned to perturbed OH groups. In the
-1
ν8 vibrations region, a shoulder at 1603 cm , which corresponds
to H-bonded DMP, disappears. The pair of bands at 1610 and
-1
1
580 cm that is assigned to coordinately bound DMP species
3
7
is still present and resists to an evacuation at 423 K.
Addition of H2S at 300 K to the sample with preadsorbed
DMP results in a further decrease of the intensity of the band
study which has shown that coadsorption of acidic and basic
molecules on silica could even lead to the protonation of the
3
7
bases by the silanol group.
-1
at 3670 cm and the appearance of a broad absorption centered
This explanation is also in agreement with the lowering of
-
1
at 3000 cm . In the ν8 region, the band of coordinated DMP
ν(SH) frequency observed at high CH3SH or H2S coverages.
-
1
+
at 1610 cm disappears, and bands of DMPH species increase
6
Indeed, Saur et al. have reported that CH3SH molecule linked
-
1
in intensity at 1652 (weak), 1636, and 1627 cm . Evacuation
at 300 K leads to a decrease of the intensity of the latter bands,
as well as of those due to H-bonded DMP and of absorption
around 3000 cm , whereas a partial restoration of the band of
free OH groups and coordinately bound species is observed.
The concentration of Brønsted acid sites detected in the presence
of H2S, about 0.12 nm is twice greater than that for alumina,
but still much lower than that for titania (Table 2). Almost one-
half of these sites are due to reversible adsorption of H2S and
disappear after evacuation.
via its sulfur atom to the surface has higher CH and SH
stretching vibration frequencies as compared to CH3SH molecule
bound via its hydrogen atom. Accordingly, at low coverage,
CH3SH is H-bonded through its sulfur atom and presents a high
-
1
-1
ν(SH) maximum at 2585 cm and high-frequency components
of the CH bands. At higher coverages, supplementary CH3SH
molecules interact by H-bond via their protons with oxygen
atoms of silanol groups, which would account for the ν(SH)
-
2
-1
band at 2545 cm and low-frequency CH components.
2
. H2S and CH3SH Adsorption on Alumina, Titania, and
When the sample is first contacted with H2S at 300 K,
subsequent DMP addition does not reveal any remaining Lewis
acid site whereas the bands of protonated DMP species appear
with a lower intensity than that observed when H2S is adsorbed
after DMP.
Adsorption of CH3SH after DMP introduction also leads to
the decrease of the bands of coordinated DMP and to the
appearance of very weak bands of protonated species.
Zirconia. On alumina, titania, and zirconia, the presence of
Lewis acid sites and of OH groups with various acidities gives
rise to several adsorption modes of H2S and CH3SH. Perturba-
tion of surface OH groups by weakly adsorbed H2S could also
be observed for these metal oxides. Such reversible adsorption
leads to perturbed ν(OH) bands, which, contrary to the case of
silica, shift to lower wavenumbers when H2S coverage de-
creases.
Pumping at about 120 K removes at least a part of H-bonded
H2S. However, the spectra of initial OH groups of metal oxides
are only partially recovered. The low-frequency ν(OH) bands
are totally restored or become more intense than before H2S
adsorption, whereas the high-frequency OH bands are either
not observed or have lower intensity. The latter are shifted and
present reduced intensities when H2S is adsorbed at 300 K.
Discussion
1
. H2S and CH3SH Adsorption on Silica. Our results on
H2S interaction with silica are in agreement with those of Mayer
and Bastick,1
5,16
who show that the only adsorption mode of
H2S is hydrogen bonding. No evidence for H2S dissociation or
for creation of new acid sites was found in the present study.
-
1
Two bands at 2608 and 2597 cm , not reported in ref 16, that
arise simultaneously with the OH perturbation by small doses
of H2S, can be assigned, respectively, to the symmetric and
antisymmetric S-H stretching vibrations of hydrogen bonded
molecules. Because H2S is a rather strong base, it shifts the
Both H2S and CH3SH molecules could be adsorbed by
coordination via their sulfur atom to the coordinately unsaturated
surface cations. This form of adsorption should remain at the
surface after removal of physisorbed and H-bonded molecules.
In fact, after pumping at about 120-140 K, weak ν(SH) bands
could still be observed at wavenumbers rather different from
those of weakly bound molecules. The spectra of CO adsorbed
on the samples with preadsorbed H2S show that most of the
Lewis acid sites are still inaccessible for CO. However, it is
difficult to determine whether these sites are occupied by
molecular H2S or by the products of its dissociation. Observation
of the δ(HSH) bending vibration mode of molecular H2S at
-
1
silanol ν(OH) band by 210-240 cm , twice more than CO
-
1 21
-1 34,35
(80 cm
) or ozone (about 100 cm
) and more than
-
1 36
ethylene (152 cm
). As for CH3SH, it shifts the silanol OH
-
1
band by 330-400 cm in agreement with its stronger basicity:
the proton affinity (PA) of CH3SH is 773.4 kJ/mol as compared
with 705.0 kJ/mol for H2S.24
It is noticeable that after H2S and CH3SH adsorption, the
ν(OH) frequency shift increases with the amount of adsorbate.
The same effect was earlier observed for ozone adsorbed on
silica.34 The stronger perturbation of OH groups at increased
coverage can be explained by interaction of a second molecule
with the oxygen atom of the silanol group:3
-
1 38
about 1183 cm
could be helpful for identification of
molecular adsorption. However, its extinction coefficient is very
low, and the corresponding band was observed at 1175-1163
-
1
cm only at highest coverages, i.e., in the presence of weakly
bound H2S. The adsorption mode of CH3SH is easier to
determine. Simultaneous observation of the bands of SH and
CH vibrations in the spectra of CH3SH adsorbed on titania or
alumina at low temperatures enables one to conclude that in
these conditions some molecules are not dissociated. It is
assumed that, under the same conditions, coordination of H2S
also occurs at low temperature on the three metal oxides. Then,
the higher intensity of ν(OH) bands and the absence of S-H
stretching bands that was observed after heating to 300 K point
to H2S dissociation at higher temperatures.
2
This process should finally lead to polymeric chains or two-
dimensional film of H2S and silanol groups. Thus, lowering of

Acidic Properties of Metal Oxides
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1359
Appearance of ν(OH) bands with enhanced intensities could
be regarded as the most direct evidence for H2S dissociation. It
was established for Al2O3 pretreated at 1070 K and for ZrO2
and TiO2 pretreated at 770 K, when adsorption of H2S at low
temperature was followed by an evacuation at 120 K. For TiO2,
this was also observed after CH3SH adsorption at 300 K. For
the three metal oxides, the ν(OH) bands that are due to
dissociation present the same spectral features than the lowest
frequency ν(OH) bands that are observed in the spectra of pure
H-bonding. This H-bond gives rise to a large displacement of
-1
the OH frequency, up to 2900 cm , in agreement with the high
2
4,28
basicity of DMP (PA ) 963.0 kJ/mol.
).
Alumina, titania, and zirconia exhibit a much wider variety
of surface sites than silica. H-bond with surface OH groups
results in perturbed ν(OH) bands and ν(CO) bands which
positions depend on the coverage of adsorbed molecules. Thus,
for alumina the perturbed ν(OH) and ν(CO) bands move on
-
1
-1
desorption from 3610 and 2155 cm to 3560 and 2160 cm ,
respectively (Table 1). This phenomenon can be accounted for
the presence of several types of OH groups with different acid
strengths. On average, the Brønsted acidity characterized at high
39-41
oxides. According to current models of oxide surfaces,
these
bands correspond to the hydroxy groups bound to the highest
possible number of metal atoms. Hence, one can consider that
the supplementary OH groups formed upon dissociation result
from a proton transfer from the adsorbed molecules to coordi-
nately unsaturated surface oxygen atoms.
-
1
CO coverage by the band at 2155 cm is weaker than that for
silica. However, the hydroxy groups that are probed at low CO
-
1
coverage are considerably more acidic (ν(CO) at 2160 cm ,
When H2S is adsorbed at 300 K, a band appeared at about
Table 1). For titania, the presence of two well resolved bands
-
1
-1
-1
-1
1
620 cm for Al2O3, 1614 cm for TiO2, and 1590 cm for
at 2156 and 2149 cm in the spectrum of CO could be assigned
ZrO2. It is always accompanied by a strong broad absorption
band of OH stretching vibrations and is assignable to the bending
mode of adsorbed H2O. For zirconia, these changes already
occur at 120 K indicating that at this temperature proton mobility
is sufficiently high for OH groups recombination. Until water
is formed, H2S adsorption at 120 K is essentially reversible since
one can readily recover most of the initial spectrum of surface
OH groups on evacuation. However, once water is formed, the
restoration of the high-frequency OH bands is not possible upon
subsequent evacuation. This could be explained taking into
account the basic character of these OH groups that are bound
to one metal atom.41 Presumably, proton transfer from H2S to
these OH groups leads to the formation of a surface SH group
and a water molecule:
to adsorbed CO on hydroxy groups presenting different acid
strengths. After evacuation at 120 K, the perturbed OH band
-
1
shifts from 3575 to 3555 cm while the two ν(CO) bands are
-1
observed at 2158 and 2151 cm . For zirconia, the spectra show
-
1
ν(CO) band at 2152-2156 cm , depending on CO coverage,
and thus, do not reveal Brønsted acidity stronger than silica.The
spectra of DMP reveal the presence of Brønsted sites on the
surface of alumina, titania, and zirconia, thus showing that some
of their hydroxy groups are acidic enough for protonation.
Coordinated DMP species reveal stronger Lewis acidity of
-
1
Al2O3 (ν8a at ca. 1615 cm ) as compared with TiO2 and ZrO2
-1
(ca. 1610 cm ), in agreement with the results reported with
2
5
pyridine as a probe molecule. CO adsorption provides more
detailed information about Lewis acidity. On alumina, two types
of Lewis sites could be distinguished, leading to ν(CO) bands
-
1
M + MOH +H S f MSH + M + H O
at 2230-2220 and 2200-2180 cm , depending on CO
coverage, and are associated with CUS (coordinately unsaturated
sites) corresponding to tetrahedrally and octahedrally coordi-
2
2
3
. Acidic Properties of Pure Oxides. Spectra of adsorbed
2
1,22
CO and DMP provide detailed information about the nature and
concentration of acid sites of oxides. CO adsorption on OH
group results in frequency shift of the OH stretching vibration
toward lower wavenumbers and in the increase of the adsorbed
CO frequency. Both shifts could be used as a measure of proton
acidity of surface hydroxy groups. Coordination to Lewis acid
sites results in more important ν(CO) shifts to higher wave-
numbers. The extinction coefficient of CO adsorbed on oxides
nated cations, respectively.
The dependence of the position
of both ν(CO) bands on the coverage is caused by lateral
45
interactions between adsorbed CO molecules. On titania, three
types of Lewis acid sites are observed, leading to ν(CO) bands
-1 21
at 2213-2207, 2195-2179, and 2165 cm . For zirconia, the
band of CO molecules bound to Lewis acid sites occurs at
-
1 43,44
2207-2171 cm .
4. Modification of Acidic Properties by H2S and CH3SH
Adsorption. Adsorption of H2S and CH3SH leads to strong
modifications of surface acidic properties, revealed in the spectra
of adsorbed CO and DMP. Few changes are observed for silica.
For Al2O3, TiO2, and ZrO2, a decrease in Lewis acid sites
concentration is observed, which is accounted for their occupa-
tion by adsorbed sulfur-containing molecules or by the products
of their dissociation. Simultaneously, an increase of the number
of Brønsted acid sites as well as modification of their strength
take place. This accounts for the increased intensities of the
bands of protonated DMP, of H-bonded CO, and of the
corresponding bands of perturbed OH groups, as well as of the
shifts of the band positions of CO and perturbed OH groups.
One could anticipate that hydrogen bonding of an SH group
with an adsorbed molecule, such as CO, should result in a
downward shift of the ν(SH) band and in the appearance of a
corresponding ν(CO) band. However, such a phenomenon was
never observed. We can thus conclude that the acidity of SH
group is too low to be the source of proton acidity. Hence, the
increased number of Brønsted acid sites is not associated with
the presence of SH groups themselves.
4
2
does not differ too much from that of a free molecule. Thus
the intensities of the bands of chemisorbed CO at saturating
conditions could be used for a rough comparison of the
concentrations of various adsorption sites.
The position of the ν8a band of adsorbed DMP could also
characterize the strength of hydrogen bond or coordination.
However the observed shifts values are rather small, and the
bands are not always resolved. Because of steric hindrance, the
molecule cannot interact with all the Lewis acid sites, but it is
a good probe for Brønsted acidity, since the spectrum of
+
DMPH ion presents characteristic ν8a and ν8b vibration bands
-
1
25-28
in the 1670-1620 cm region.
Our spectra of CO and DMP adsorbed on metal oxides prior
H2S or CH3SH addition are in agreement with the earlier
data.2
1,23,25,44
For silica, H-bond is the only adsorption mode of
carbon monoxide. It shifts the frequencies of silanol groups and
-
1
CO molecules to 3670 and 2157 cm , respectively. These
values are not dependent on coverage, thus indicating a high
degree of homogeneity of the acid strength of OH groups. DMP
-
1
adsorption on silica leads to only one ν8a band at 1602 cm .
This indicates that silanol acidity is not sufficient for the proton
transfer, and the only adsorption mode of DMP on silica is also
One can distinguish at least two different effects of sulfur
compounds on the Brønsted acidity, depending on the adsorption

1360 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Travert et al.
mode of the sulfur-containing molecule. The first effect that
we shall discuss below is the acidity enhancement caused by
molecular H2S or CH3SH. The second effect could be detected
after removal of the adsorbed sulfur-containing molecules and
is caused by the products of their dissociation. It will be
discussed later (section 4.2)
SCHEME 1
4
.1. Acidity Induced by Molecular Adsorption of H2S and
CH3SH. On silica, the additional shift to lower frequencies of
perturbed ν(OH) bands on increasing coverage in H2S or
CH3SH is attributed to the interaction of a second molecule with
the oxygen atom of the silanol group. This phenomenon can be
interpreted as an increase of the OH group acidity induced by
concentration is always much greater than that measured for
initial samples. The above spectral data on CO adsorption enable
one to associate a part of the Brønsted acidity which is
eliminated by pumping at 300 K to the acidity induced by
molecular adsorption of H2S, while the dissociative adsorption,
evidently, accounts for irreversible generation of Brønsted sites,
at least at 300 K.
For alumina, CO adsorption show that, after elimination of
molecular H2S by pumping at 300 K, Brønsted acid sites present
the same strength than those initially present (Table 1). On
zirconia, after evacuation at 300 K, some Brønsted acid sites
still present an increased acid strength, but weaker than after
H2S addition at low temperature (Table 1). On titania, complex-
ity of CO spectra does not allow any conclusion to be drawn
with respect to the variation of OH acid strength after H2S
removal at 300 K.
It is believed that the mechanism of acidity enhancement in
the presence of molecular H2S slightly differs from that observed
for silanol groups. Indeed on silica, H2S (or CH3SH) interaction
with oxygen atom of the silanol group is only possible if a first
H2S molecule already interacts as a base with hydrogen of the
OH groups, whereas on alumina, titania, and zirconia the OH
groups presenting an enhanced acidity still remain accessible
for H-bond, since CO is able to detect the acidity increase. This
means that molecular H2S that account for acidity enhancement
is more strongly bonded on these oxides than on silica. On metal
oxides, the H2S molecules that are coordinated to surface cations
can simultaneously interact with oxygen of an adjacent OH
group, leading to an increase in its acidity. At 300 K, a proton
transfer should occur since water formation is observed for all
three oxides. Scheme 1 is therefore proposed. Such a scheme
can also explain the large width and low frequency position
generally observed for the SH band as well as the comparatively
high energy of H2S adsorption. This interaction increases the
acidity of the OH group, which is reflected in the large ν(OH)
frequency lowering on CO adsorption and high position of the
ν(CO) band.
3
7
the adsorption of the second molecule acting as an acid.
However, this effect cannot be detected by coadsorption of CO,
because sulfur-containing molecules which are more basic than
CO preferentially form H-bond with silanol groups. Hence, CO
can only adsorb on the remaining free OH groups.
Coadsorption of H2S or CH3SH with DMP at 300 K also
indicates competitive adsorption on silanol groups. At 120 K
addition of H2S to silica with preadsorbed DMP does not lead
to protonation, but to a slight increase in the ν(ÃΗ) frequency
shift upon H-bonding with DMP. This reveals an increase of
OH groups acidity due to H2S. Addition of a more acidic
compound like SO2 under the same conditions results in an
almost complete transformation of H-bonded DMP to the
3
7
protonated form. Clearly, the acidity of H2S molecules is not
strong enough to enhance that of silanol groups to afford DMP
protonation.
For the other metal oxides, the effect of molecular adsorption
is observed following exposure to a large amount of H2S and
outgassing at 120 K. On alumina, titania and zirconia, coad-
sorption of H2S and CO at 120 K leads to lower intensity of
H-bonded CO, indicating a lower accessibility of hydroxy
groups to CO. This is to be expected at high coverage. However,
spectra recorded for small CO coverage clearly reveal the
enhancement of OH group acidity, as compared to that of pure
oxides. In fact, under these conditions, the lowest positions of
perturbed ν(OH) bands and the highest frequencies of H-bonded
CO were detected (Table 1). For example, in the case of
alumina, after contact with H2S at 120 K and for low CO
coverage, the band of perturbed OH groups appears at 3500
-1
cm while the corresponding ν(CO) band increases up to 2165
-
1
cm , whereas on pure alumina, these bands occur at 3560 and
-
1
2
160 cm , respectively. After removal of molecular H2S by
-
1
pumping at 300 K, these bands appear at 3530 and 2158 cm ,
i.e., at values close to those for pure Al2O3 surface.
Conversely, no clear enhancement of acidic strength can be
detected when coadsorption of CH3SH and CO are performed
at low temperature. This can be related to the weaker acidity of
CH3SH compared to H2S, leading to a weaker interaction with
the oxygen atom of OH group.
This source of Brønsted acidity cannot be attributed to
adsorbed H2S, since the ν(SH) bands are not perturbed by CO.
Furthermore, this acidity is inhibited in the presence of excess
H2S. Similarly, one cannot relate this Brønsted acidity to H2O
molecules since for Al2O3 and TiO2 an increase in acidity is
observed at 120 K, whereas water formation does not occur at
this temperature. Thus, acidity enhancement is more likely
induced by the presence of reversibly adsorbed H2S molecules,
which modifies OH group acidity.
When the sample is heated in the presence of H2S up to 300
K, proton transfer from coordinated H2S to the OH group could
be activated, leading to the formation of coordinated water
molecules and surface SH group. The water formed, which
resists to evacuation at 300 K, could remain coordinated to the
cation. Water, like H2S in the previous scheme, could also
interact with OH groups, increasing their acidity. Compared to
H2S, however, the enhancement of the OH acidity by H2O
should be limited by the partial consumption of basic OH groups
and by the weaker acidity of water compared to that of H2S.
Water formation may also explain the irreversible perturbation
Another evidence for Brønsted acidity induced by molecular
+
H2S is in the reversible increase of the intensity of DMPH in
+
the presence of the excess of H2S. Data obtained from DMPH
intensity measurements after exposure to H2S are summarized
in the Table 2. For all the three oxides, the concentration of the
Brønsted acid sites increases after H2S addition. Subsequent
evacuation at 300 K shows that these changes are partially
reversible. The amount of protonated DMP decreases after
removal of gaseous H2S, but the final Brønsted acid site
-
1
of high-frequency OH band at ca. 3795 cm observed on
alumina even after evacuation of adsorbed H2S at 300 K.
4
1
According to current models of oxide surfaces, these OH

Acidic Properties of Metal Oxides
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1361
groups are bound to one aluminum cation (type I OH groups)
and present a higher basicity than other types of OH groups.
Hence, type I OH groups could play the role of proton-acceptors
in H-bond. These groups are less acidic than type III groups
that absorb at ca. 3700 cm-1 and which can form stronger
H-bond, but only as proton donors. The fact that after removal
of H-bonded H2S, the type III OH groups are recovered while
the type I OH groups are still perturbed is not surprising if one
assumes that these groups interact with coordinated H2S
molecules via their oxygen atoms at low temperature and react
with H2S at higher temperature to form water. On alumina
surface, this happens at about 300 K, while on zirconia water
formation occurs already at 120 K, evidently, because of the
more basic character of the type I OH groups bound to
zirconium.
of ν8a and ν8b bands of adsorbed 2,6-dimethylpyridine were used
to distinguish molecules protonated on Brønsted sites, coordi-
nately bound to Lewis acid sites, and H-bonded to surface
hydroxyls.
Both H S and CH SH form H-bond with surface OH groups
2
3
of all the adsorbents at least, at low temperatures. These
molecules are rather strong bases, and shift the silanol OH band
-
1
by 210-240 and 330-400 cm , respectively. For silica,
H-bond is the only mode of adsorption but it is not restricted
to the formation of 1:1 complexes. At high coverage, additional
frequency shifts of the OH and SH bands reveal the interaction
of two molecules with one silanol group or the formation of
polymeric chains at the surface.
For the three other metal oxides, spectra of adsorbed H S or
2
CH SH with coadsorbed CO or DMP provide evidence for the
3
On silica, the silanol groups are sufficiently acidic for the
proposed mechanism of strong Brønsted acidity enhancement
by molecular H2S, but unlike alumina, there are no other sites
to fix H2S molecules close to the hydroxy groups. It should be
noted that acidity increases caused by adsorbed molecular H2S
occupation of surface Lewis acid sites by coordinatively bound
molecules if adsorbed at low temperatures. At 300 K or higher
temperatures, the increased intensity of ν(OH) and δ(ΗOH) is
accounted for the dissociative adsorption of the sulfur-containing
molecule, leading to the formation of surface OH groups and
5
-
2-
was reported by Hosotsubo for silica, alumina, and silica-
molecular water. The formation of SH or S ions is not
detected by IR, but it could account for the occupation of Lewis
acid sites when molecular H S or CH SH are no more detectable
alumina modified by transition metal cations. Such cations were
shown to be the sites of H2S adsorption, then interaction of
adsorbed molecules with the adjacent silanol groups or type I
hydroxyls of alumina could account for the acidity increase
observed under conditions allowing molecular adsorption of H2S
on the cations to take place.
2
3
in the spectra. Water molecules are formed as a result of the
protonation of basic type I OH groups by H S or CH SH. This
2
3
takes place below 120 K on ZrO and 300 K on Al O and
2
2
3
TiO2.
4
.2. Modification of Acidic Properties after Dissociation of
Two types of induced Brønsted acidity were established. In
the presence of an excess of adsorbed sulfur-containing mol-
ecules, a reversible increase of OH groups acid strengths was
observed for all the four oxides. It was shown to be induced by
H2S and CH3SH. Another mechanism of Brønsted acidity
generation was evidenced after H2S adsorption and subsequent
evacuation at 300 K on Al2O3, TiO2, and ZrO2. The increased
intensity of OH bands is indicative for H2S dissociative
adsorption. The absence of any bands in the S-H stretching
region could be due to their very low extinction coefficient.
Spectra of adsorbed CO reveal an increased amount of H-bonded
CO molecules and a decreased number of Lewis acid sites that,
evidently, are occupied by the products of the reaction of H2S
molecular H S H-bonded to the oxygen atom of surface OH
2
groups. For silica, this interaction leads to an increase of the
ν(OH) frequency shift with increasing coverage of H S or
2
Ch SH. In the case of alumina, titania, and zirconia, the
3
enhanced OH acidity results in additional frequency shifts of
ν(CO) and perturbed ν(OH) bands observed on CO adsorption
at 77 K.
2-
with the surface (SH, OH, S , and H2O).
On alumina, titania, and zirconia, DMP adsorption after H2S
interaction at 300 K reveals a significant increase of the number
of Brønsted acid sites. Protonated species are also formed, even
in a lower aboundance than after H2S addition to the sample
with preadsorbed DMP. In both cases, most part of the
protonated form cannot be removed by evacuation and, hence,
should be associated with the formation of acidic OH groups
produced by H2S dissociation.
For Al2O3, TiO2, and ZrO2, the spectral features of the new
hydroxy groups and their behavior toward CO are similar to
those of the most acidic hydroxy groups present on the pure
oxide. Indeed, the newly created OH groups are formed,
according to recent models of oxide surfaces,39,40,41 by proton
attachment to coordinatively unsaturated surface oxide ions that
are bound to the maximum number of metal ions possible for
the crystal lattice: three for Al2O3 and ZrO2 and two for TiO2.
The positions of the bands assigned to these hydroxyls are
comparable to those observed following H2S adsorption.
For Al O , TiO , and ZrO , dissociative adsorption of H S
2
3
2
2
2
and CH SH results in the appearance of new OH groups that
3
account for the irreversible, at least at 300 K, increase of
Brønsted acidity revealed by the higher number of H-bonded
CO and of protonated DMP. These new OH groups are the most
acidic among those that normally exist at the surface of the
considered metal oxides. In no case the S-H groups of adsorbed
molecules or surface SH groups reveal any proton-donating
ability that could account for the increase in Brønsted acidity.
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