Temperature-programmed desorption/decomposition with simultaneous DRIFTS analysis: Adsorbed pyridine on sulfated ZrO2 and Pt-promoted sulfated ZrO2

Article abstract of DOI:10.1016/S0040-6031(03)00305-8

The thermal stability and reactivity of adsorbed pyridine on sulfated zirconia and Pt/sulfated zirconia were studied by temperature-programmed desorption (TPD) coupled with infrared spectroscopic and mass spectrometric analyses. Adsorption of pyridine on sulfated zirconia produced two types of adsorbed species: a pyridinium ion on the Br?nsted acid site (Pyr-B) giving rise to infrared bands at 1638, 1611, 1486, and 1540 cm^-1 and a covalently bound species on Lewis acid sites (Pyr-L), giving characteristic bands at 1486 and 1445 cm^-1 at the same rate, accompanied by the desorption of sulfates to produce gaseous SO₃. TPD studies showed that Pyr-L nearly completely decomposed to CO₂ at 773-823 K, whereas more than 60% Pyr-B remained on the surface in the same temperature range, indicating that Pyr-B possesses higher thermal stability than Pyr-L. The addition of Pt to sulfated zirconia lowers the decomposition temperature of sulfate as SO₂. The study shows that that temperature-programmed desorption/decomposition with simultaneous infrared spectroscopic and mass spectrometric analyses is an effective approach for identifying the structure of the adsorbed species leading to decomposed products. TPD of adsorbed pyridine is not a reliable approach for measuring acid strength of the acid/ base catalysts.

Full text of DOI:10.1016/S0040-6031(03)00305-8

Thermochimica Acta 407 (2003) 61–71
Temperature-programmed desorption/decomposition with
simultaneous DRIFTS analysis: adsorbed pyridine on
sulfated ZrO2 and Pt-promoted sulfated ZrO2
Robert W. Stevens Jr.a, Steven S.C. Chuanga,∗, Burtron H. Davisb
a
Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906, USA
Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA
b
Received 18 January 2003; received in revised form 7 May 2003; accepted 8 May 2003
Abstract
The thermal stability and reactivity of adsorbed pyridine on sulfated zirconia and Pt/sulfated zirconia were studied by
temperature-programmed desorption (TPD) coupled with infrared spectroscopic and mass spectrometric analyses. Adsorption
of pyridine on sulfated zirconia produced two types of adsorbed species: a pyridinium ion on the Brønsted acid site (Pyr-B)
giving rise to infrared bands at 1638, 1611, 1486, and 1540 cm−1 and a covalently bound species on Lewis acid sites (Pyr-L),
giving characteristic bands at 1486 and 1445 cm−1 at the same rate, accompanied by the desorption of sulfates to produce
gaseous SO3. TPD studies showed that Pyr-L nearly completely decomposed to CO2 at 773–823 K, whereas more than 60%
Pyr-B remained on the surface in the same temperature range, indicating that Pyr-B possesses higher thermal stability than
Pyr-L. The addition of Pt to sulfated zirconia lowers the decomposition temperature of sulfate as SO2. The study shows that
that temperature-programmed desorption/decomposition with simultaneous infrared spectroscopic and mass spectrometric
analyses is an effective approach for identifying the structure of the adsorbed species leading to decomposed products. TPD
of adsorbed pyridine is not a reliable approach for measuring acid strength of the acid/base catalysts.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Sulfated zironia; Pt-promoted sulfated zirconia; Temperature-programmed desorption; Temperature-programmed decomposition;
TPD; Adsorption; Dynamics; In situ; Infrared spectroscopy; IR; Diffuse reflectance Fourier transform infrared spectroscopy; DRIFTS; Mass
spectrometry
1. Introduction
change in the property of the compound by moni-
toring weight changes and evolution of the gaseous
species [1–3].
Temperature-programmed desorption (TPD) and
decomposition is an approach for determining the
thermal stability and properties of a compound. The
approach typically involves heating of the sample
at a controlled rate and continuous measurement of
In heterogeneous catalysis research, temperature-
programmed desorption/decomposition technique has
been widely used to characterize the nature of ad-
sorbed species on the catalysts, reaction pathways,
and the acidity/basicity of the oxides [4,5]. Devel-
opment of a low cost residual gas analyzer or mass
spectrometer (MS) has allowed determination of des-
orption/decomposition profiles of multiple species by
∗
Corresponding author. Tel.: +1-330-972-6993;
fax: +1-330-972-5856.
E-mail address: schuang@uakron.edu (S.S.C. Chuang).
0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0040-6031(03)00305-8

62
R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
monitoring the composition of the evolving gases from
either the decomposition or desorption of the sample.
This information obtained from MS measurement has
been used (i) to determine adsorption energetics of
small molecules such as H2, (ii) to elucidate the nature
of adsorbed species and catalyst surface, and (iii) to
postulate the chemical identity of adsorbed species and
their reaction pathways occurring during the TPD pro-
cess. Although postulations derived from TPD study
can assist in the understanding of catalysis as well as
catalyst development, they could lead to confusion and
controversy.
Direct measurement of the surface species dur-
ing the TPD will provide the chemical identity of
adsorbed species leading the desorption and decom-
position profiles—direct evidence for elucidation of
reaction pathways and catalytic properties. Infrared
(IR) spectroscopy is one of the few techniques that
allow in situ observation of adsorbed species under
reaction conditions. Although TPD and IR have been
extensively used in catalyst characterization, simulta-
neous use of these two techniques are rarely reported
[5,6].
We developed a technique involving in situ IR com-
bined with MS to study changes in adsorbate and
variation in gas-phase composition during the catalytic
reaction [7]. This technique has been applied to moni-
toring the change in material properties and the evolv-
ing gas during the transient condition in which the
step and pulse changes in the reactant concentration
were applied to the catalytic reactor system [8–10].
We have recently employed this technique to investi-
gate temperature-programmed desorption of adsorbed
pyridine on sulfated ZrO2 (SZ) [11]. Our interest in
the SZ catalysts stems from (i) its unique activity for
isomerization of alkenes at low temperature [12–14]
and (ii) the oxidative properties of SZ exhibited dur-
ing TPD of benzene and pyridine [15–18]. The latter
is reflected in the observation of CO2 accompanied by
the release of SO2.
pyridinium ions and covalently bound pyridine during
temperature-programmed desorption/decomposition.
In this paper, we studied the interaction between pyri-
dine and SZ and Pt-SZ by TPD approach coupled
with MS and IR. IR allows us to observe the surface
interaction between adsorbed pyridine and sulfate
during TPD. Combining IR with MS not only allow
in situ observation of pyridine adsorption dynamics
but also greatly enhance the capability of the TPD
technique for determining reaction pathway, the na-
ture of adsorbed species, and catalyst surface states;
this improved approach allows us to draw an unam-
biguous conclusion on the role of Pyr-B and Pyr-L in
the formation of CO2 and SO2. This study reveals ad-
vantages of the TPD coupled with MS and IR and its
potential for application in thermoanalytical studies.
2. Experimental
2.1. Catalyst preparation
Preparation of ZrO2 involves: (i) precipitation of
zirconium hydroxide by mixing a 0.3 M aqueous so-
lution of ZrCl4 with an excess amount of ammonium
hydroxide at a pH of 10.5 and (ii) drying the precip-
itate at 383 K for 50 h. The dried ZrO2 was sulfated
by immersing it into 0.5 M H2SO4 (15 cm3/g of ZrO2)
and stirred for 2 h. PtSO42−/ZrO2 was prepared by
impregnating the SO42−/ZrO2 solid with a solution of
H2PtCl6·6H2O in deionized water. The resulting sam-
ples (both SZ and Pt-SZ) were calcined in air at 873 K
for 2 h and stored.
Total sulfur content was determined to be 3.18 and
3.43%, respectively, for SZ and 4% Pt/SZ by a Leco
SC432 instrument. BET surface area of the two sam-
ples, SZ and 4% Pt/SZ was determined to be 139 and
123 m2/g, respectively.
2.2. Experimental apparatus
It has been well established that pyridine adsorption
onto the SZ and Pt/SZ catalysts produces two types
of adsorbed species: a pyridinium ion on the Brønsted
acid site (Pyr-B) giving rise to infrared bands at 1638,
1611, 1486, and 1540 cm−1 and a covalently bound
species on Lewis acid sites (Pyr-L), giving character-
istic bands at 1486 and 1445 cm−1 [19–23]. A key
question that remains to be addressed is the role of
The experimental apparatus, shown in Fig. 1, con-
sists of three sections: (i) a gas metering section, (ii) a
reactor section, and (iii) an effluent gas analysis sec-
tion. The gas metering section consists of mass flow
controllers (Brooks 5850; not shown), which are used
to deliver controlled gas flows to the reactor system,
as well as a pyridine saturator. He flow was directed

R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
63
He (30 cm3/min)
H2 (30 cm3/min)
4-port Valve
Vent
Gas inlet system
SZ Sample
Vent
IR
Beam
Pyridine
saturator
IR
Bench
Mass
DRIFTS
Spectrometer
Fig. 1. Schematic of experimental apparatus.
into the saturator through a pair of interconnected
three-way valves, delivering a gaseous flow of pyri-
dine (room temperature, partial pressure ≈28 mmHg)
to the reactor. The reactor system is made up of an
in situ diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) reactor (Spectra-Tech Inc.)
that resides inside an IR bench (Nicolet Magna 550).
The effluent section was analyzed via a quadrapole
mass spectrometer (Balzers QMG 112). The dynam-
ics of the catalyst surface, its adsorbates, and product
formation during the course of the reaction study can
be studied with this arrangement.
Seventy five milligrams of sample was used in each
experiment and placed into the DRIFTS reactor. Each
experiment was analyzed continuously with both IR
and MS. The collected IR spectra had a resolution
of 4 cm−1 and were a result of 32 coadded scans to
improve the signal/noise ratio. The catalyst was pre-
treated in situ by heating the reactor from room tem-
perature to 873 K at a rate of 10 K/min and held for
2 h in a flowing He environment. The reactor was then
cooled back to room temperature during which IR
spectra of the clean catalyst surface were collected as
a function of temperature, hereafter referred to as pre-
treated backgrounds. The IR bench is a single-beam
instrument and therefore it is necessary to collect back-
ground spectra (reference spectra) of the catalyst prior
to the experiments.
IR data are presented in two formats in this paper:
(i) single beam and (ii) absorbance units. Single beam
is essentially a plot consisting of background spectra.
Single beam is akin to transmittance in the sense that
absorbing species are shown in a negative direction;
however, it differs from transmittance in that the base-
line is curved due to the shape of the IR source spec-
trum itself (Fig. 2(a)). The second format, absorbance
units, uses the background spectra collected following
the sample pretreatment; absorbance spectra are calcu-
lated from background (prior to experimentation) and
sample (during experimentation) spectra of matching
temperature. The formula used to calculate absorbance
is as follows: A(ν) = −log{I(ν)/I0(ν)}, where I(ν)
and I0(ν) refer to IR intensities of the sample and
background spectra, respectively, as a function of fre-
quency (wavenumber). Typical spectra of this type are
shown in Fig. 3(a). Absorbance spectra relative to the
pretreated background gave a clean baseline due to
the matched temperatures of the sample/background

64
R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
0.4
Temp (K)
313
373
473
573
673
773
873
873 (2 h)
4000
(a)
3600
3200
2800
Wavenumber (cm-1)
2400
2000
1600
1200
5e-10
Pyr (m/e = 79)
H2 (m/e = 2)
CO2 (m/e = 44)
H2O (m/e = 18)
SO (m/e = 48)
SO2 (m/e = 64)
SO3 (m/e = 80)
373 473 573 673 773
Temp (K)
873 (35 min)
(b)
Fig. 2. (a) IR and (b) MS analysis during the pretreatment of SZ from 40 to 600 ◦C. Background infrared spectra are displayed in (a).
The shaded bar on the temperature axis of (b) indicates constant temperature.
spectra and are further rationalized by the fact that the
point of reference is a clean surface.
were maintained at 30 cm3/min throughout all of the
experiments.
The Balzers QMG 112 mass spectrometer could
measure 8 m/e signals simultaneously; m/e values
of 79, 80, 64, 48, 44, 2, 4, and 18 were monitored
corresponding to pyridine, SO3, SO2, SO, CO2,
H2, He, and H2O, respectively. Total gas flow rates
2.3. Pyridine adsorption (acidity characterization)
Prior to the adsorption of pyridine, the reactor was
heated to 423 K. This temperature was selected for the

R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
65
(b)
0.4
0.2
0
(a)
0.3
-0.2
Time (min)
-0.4
S=O (1407 cm-1)
Pyr-L (1444 cm-1)
S-OH (3633 cm-1)
H3O+ (1673 cm-1)
After He purge
(45.5 min)
-0.6
Pyr-B (1540 cm-1)
-0.8
13.5
3.96
0
0.5
1
1.5
2
2.5
3
Time (min)
1.25
0.52
5e-10
(c)
H2 (m/e = 2)
0.33
0.21
0.0
CO2 (m/e = 44)
SO (m/e = 48)
3800 3650 3500 1800 1700 1600 1500 1400 1300
SO3 (m/e = 80)
H2O (m/e = 18)
SO2 (m/e = 64)
Pyr (m/e = 79)
-1
Wavenumber (cm )
0
10
20
30
40
50
60
Time (min)
Fig. 3. (a) IR analysis relative to a pretreated background, (b) Peak intensities of selected species, and (c) MS analysis during pyridine
adsorption over SZ. Times indicated are relative to the time of pyridine introduction.
ease of comparison with literature data and to insure
that pyridine (bp = 389 K) did not condense on the
catalyst surface. Gaseous pyridine was sent to the reac-
tor by redirecting the He carrier gas through the pyri-
dine saturator. The SZ sample was exposed to pyridine
for about 15 min, during which changes in concentra-
tion of adsorbate and gaseous species were recorded
via IR and MS. The pyridine/He flow was then re-
placed by only He in order to remove the weakly bound
(physisorbed) pyridine from the surface, leaving only
the stronger chemisorbed species, while maintaining
the reactor at 423 K until no further changes were ob-
served via IR or MS. Finally, the reactor was cooled
to 313 K and a final IR measurement was collected to
characterize the pyridine-saturated acid surface.
2.4. Temperature-programmed desorption (TPD)
Following the pyridine adsorption, the reactor was
heated at a rate of 10 K/min under He flow from 313 to
873 K. Transient changes to the sample were recorded
via IR and MS during the heating profile.
2.5. Hydrogen treatment
The objective of this experiment is to alter the
concentration of Brønsted acid sites on the surface,
thereby altering the acidity of the surface [12]. This
presumes that H2 addition to the surface of SZ will
produce an elevated level of Brønsted acid sites.
Furthermore, the addition of Pt as a promoter to the

66
R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
catalyst should result in an even higher concentration
of Brønsted acid sites when combined with the H2
treatment. The stability of the pyridine complexes to
the samples with differing relative levels of Brønsted
and Lewis acid sites can be characterized through
TPD study.
Separate sets of experiments were conducted with
fresh catalysts to examine the effect of H2 on the
Brønsted acidity of the SZ catalysts. After pretreat-
ment and before pyridine adsorption, He flow was re-
placed by H2 (30 cm3/min) with a four-port valve for a
period of 15 min at 423 K. The H2 was then switched
back to He and the system was allowed to return to
a steady state condition. The remaining stages of the
experiment, beginning with pyridine adsorption, were
conducted as previously described.
3.2. Pyridine adsorption
Fig. 3(a) shows absorbance spectra that are relative
to a pretreated background during pyridine adsorp-
tion at 150 ◦C. Exposure of the SZ sample to pyridine
led to a temporary formation of the 1676 cm−1 band
which has been assigned to hydronium, H3O+ [25].
This species is a precursor for pyridine adsorption. Its
disappearance was accompanied by the formation of
the pyridinium ion (Brønsted acid site complex; here-
after referred to as “Pyr-B”) at 1638, 1613, 1489, and
1542 cm−1, as well as that of covalently bound pyri-
dine (Lewis acid site complex; hereafter referred to as
“Pyr-L”) at 1489, and 1445 cm−1 concurrently with a
−1
=
decrease in S O at 1413 cm
.
The variation in the intensity of all the major IR
bands with time in Fig. 3(b) was plotted along that of
MS intensity in Fig. 3(c) to illustrate the dynamics of
pyridine adsorption over SZ. The combined IR and MS
observation revealed a number of key features on pyri-
dine adsorption on SZ which was not previously re-
ported. Further pyridine exposure led to (i) increasing
intensities of all bands for Pyr-B and Pyr-L, (ii) losses
of Zr–OH and S–OH bands 3730 and 3639 cm−1, re-
3. Results and discussion
3.1. Pretreatment
Fig. 2 shows the IR spectra of SZ and the MS anal-
ysis of the reactor effluent as a function of tempera-
ture during the He pretreatment. Upon heating from
313 to 473 K, H2O and H2 were evolved (Fig. 2(b)).
The difference in the temperatures which yielded a
maximum evolution of H2 and H2O (456 K versus
408 K) indicates that the H2 signal is not simply a
fragment of H2O. Furthermore, increasing temper-
ature to 473 K led to the growth of both the H2O
−1
=
spectively, (iii) decrease in the S O band at 1405 cm
and growth of a broad band in the 1339 cm−1 region,
and (iv) evolution of H2O, H2, and SOx (Fig. 3(c)).
Comparison of the MS fragments of SO3, SO2, and SO
suggests that the major sulfur species evolved is SO3.
Pyr-B and Pyr-L changed at the same rate, as shown
in Fig. 3(b), suggesting that adsorption of pyridine on
Brønsted and Lewis sites occurred at the same rate. It
is interesting to note that the intensities of the adsorbed
species became constant after 2 min of pyridine expo-
sure, whereas gaseous SOx, H2O, and H2 continued to
increase with pyridine as shown in Fig. 3(c). Contin-
uous evolution of SOx (Fig. 3(c)) without variation of
IR profiles (Fig. 3(b)) can be attributed to the fact that
the upper portion of the sample, which is analyzed by
the DRIFTS, has been saturated with adsorbed pyri-
dine while the remaining sample continues to uptake
pyridine as well as desorb SOx. Upon switching from
pyridine to He, the SOx, H2O, and H2 immediately
decreased.
at 1646 cm−1 and S O at 1370–1400 cm as well
as formation terminal Zr–OH band at 3739 cm−1
[24]. Further increase in temperature led to growth
−1
=
=
of the S O bond, which shifted to higher frequency
as its intensity increases and is ultimately centered at
1394 cm−1 as well as a decrease in the broad H2O
band in the 3300–3800 cm−1 region. After holding
the reactor at a final temperature of 873 K for 2 h,
H2O was completely removed, whereas both termi−na1l
=
OH and S O became stable at 3739 and 1394 cm
,
respectively. It is important to note that no other
species desorbed during the pretreatment of the sam-
ple (Fig. 2(b)), indicating that the sulfate groups on the
surface of the sample were stable up to at least 873 K.
All other samples gave very similar behavior to that
depicted in Fig. 2; thus their individual results are not
shown.
=
The decrease in the S O stretch can be attributed to
a loss of sulfate species (i.e. displaced pyridine adsorp-
tion, as evidenced by the formation of SO3 (Fig. 3(c))
and/or to an interaction between sulfate and pyridine

R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
67
resulting in a change to the double bond character
mation of bridged OH on Zr (3650–3700 cm−1),
of S O [26]. The losses of Zr–OH and S–OH cou-
adsorbed H2O (1600–1620 cm−1), and a loss of S O
=
=
pled with the formation of the Pyr-B complex band
at 1540 cm−1 shows that both of these species may
serve as Brønsted acid sites according to the following
scheme:
(1370–1430 cm−1) was observed. No SOx species
were observed to desorb via MS (not shown), thus
=
the decrease in S O intensity is not due to displace-
ment of SOx from the SZ surface. It also should
be noted that the absence of both terminal OH and
S–OH, which would be observed at approximately
3730 and 3630 cm−1, respectively, does not indi-
cate that these species were not present. It simply
means that the concentration of these species did not
change. The formation of the terminal OH is clearly
seen during the pretreatment in Fig. 2. The 4% Pt/SZ
–OH + Pyr → –O− + +H-Pyr
The loss intensity of S–OH is greater than that
of Zr–OH, suggesting that S–OH is a more signifi-
cant Brønsted acid site. Pyridine adsorption over the
Pt-promoted SZ sample (Pt/SZ) follows the same
trends as those reported above. Hence, individual re-
sults of pyridine adsorption over Pt/SZ are not shown.
=
sample exhibited a slightly larger loss of S O as com-
pared to unpromoted SZ. Although the intensity of
these various species depends on the Pt loading, the
wavenumber of these species, which reflects their na-
ture, remained unchanged. These results suggest that
the presence of Pt does not alter the surface chemistry
upon H2 exposure.
3.3. Hydrogen treatment
Fig. 4 illustrates IR spectra following the H2
treatment of both SZ and 4% Pt/SZ, relative to a
pretreated background, at 423 K. In both cases, for-
0.1
4% Pt/SZ
SZ
4000
3500
3000
2500
2000
1500
Wavenumber (cm-1)
Fig. 4. Comparison of IR spectra, relative to a pretreated sample background, of H2-treated samples following He flushing.

68
R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
0.4
1e-10
Temp (K)
H2O (m/e = 18)
SO3 (m/e = 80)
873
823
773
673
SO2 (m/e = 64)
SO (m/e = 48)
573
473
373
Pyr (m/e = 79)
CO2 (m/e = 44)
313
3900 3700 3500 1700 1600 1500 1400 1300
373
473
573
673
773
873 (15 min)
-1
(a)
(b)
Wavenumber (cm )
Temp (K)
Fig. 5. (a) IR relative to a pretreated sample background and (b) MS analysis during the pyridine TPD from 40 to 600 ◦C over SZ. The
rate of heating used was 10 ◦C/min.
3.4. Pyridine TPD
pletely removed at 823 K; at 873 K it appears that
Pyr-L has returned. It is believed that this returning
peak at 1444 cm−1 is not Pyr-L but rather a result of
the surface rearrangement due to sulfate decomposi-
Fig. 5 show the IR and MS analyses as a function
of temperature during the TPD from 40 to 873 K over
SZ catalyst. Increasing temperature from 313 to 573 K
−1
=
tion. The return of the S O band at 1411 cm (be-
comes less negative) is quite noteworthy. Recall that
the exposure of the SZ catalyst to pyridine resulted in
resulted in a slight depletion of Zr–OH at 3729 cm−1
,
S–OH at 3638 cm−1 [24], and Pyr-L at both 1575 and
1445 cm−1 (Fig. 5(a)), and was accompanied by evo-
lution of H2O in the reactor effluent (Fig. 5(b)). In-
creasing the temperature further from 573 to 773 K
resulted in a decreasing of all IR intensities and evo-
lution of CO2. No pyridine desorbed throughout the
experiment as indicated by the flat m/e = 79 pro-
file of Fig. 5(b). Part of adsorbed pyridine is oxidized
to CO2, indicating that pyridine TPD is not a valid
method to gauge acid strength. Further heating from
773 to 873 K resulted in decomposition of the sulfate
group as indicated by the SO2 and SO profiles, evo-
lution of a second, larger CO2 peak at 813 K and de-
=
the loss of S O (Fig. 3) and this was accompanied by
=
evolution of SO3. The return of the S O while pyridine
decomposes suggests that its initial loss was not solely
due to the decomposition of surface sulfate but rather,
at least in part, due to an interaction of the adsorbed
pyridine with the sulfate group resulting in a change
to its chemical bonding (i.e. double bond to single
bond).
Fig. 6 depicts similar IR and MS analyses during the
TPD over H2-treated 4% Pt-SZ from 313 to 873 K in
He flow. Increasing temperature resulted in the same
trends to the adsorbed Pyr-B and Pyr-L as those in
Fig. 5, however, there are notable differences: (i) the
=
creases in the intensities of S O, Zr–OH, and S–OH.
Furthermore, Pyr-L at 1444 cm−1 is essentially com-
loss of S O is not completely restored, (ii) Zr–OH
=

R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
69
0.5
1e-10
Temp (˚C)
H2O (m/e = 18)
600
500
400
SO3 (m/e = 80)
SO2 (m/e = 64)
SO (m/e = 48)
300
200
100
40
Pyr (m/e = 79)
CO2 (m/e = 44)
373
473
573
673
773
873 (15 min)
3900 3700 3500 1700
1600
1500
1400
1300
-1
Temp (K)
(a)
Wavenumber (cm )
(b)
Fig. 6. (a) IR relative to a pretreated sample background and (b) MS analyses during the pyridine TPD from 40 to 600 ◦C over H2-treated
4% Pt/SZ. The rate of heating used was 10 ◦C/min.
and S–OH exhibited no changes over the course of the
temperature range studied, and (iii) the CO2 desorp-
tion peak (Fig. 6(b)) is one single peak with a maxi-
mum at about 823 K rather the than two peaks shown
in Fig. 5. These differences can be due to either the
H2 treatment, Pt promotion, or a combination of both.
The individual effects of H2 and Pt can be seen in
Fig. 7.
Fig. 7 depicts a summary of both MS and IR anal-
yses of pyridine TPD over (a) SZ, (b) H2-treated SZ,
(c) 4% Pt/SZ, and (d) H2-treated 4% Pt/SZ. IR intensi-
ties have been normalized in order to clearly illustrate
trend information as well as to ease comparison of
the data from the separate experiments. This summary
highlights a number of key features: (i) the intensity
of Pyr-B decreased only slightly at temperatures be-
low 873 K throughout all of the runs, suggesting that
it is a relatively stable species, (ii) Pyr-L decreased
steadily with the increase in temperature from 313 to
773 K; at 773 K, Pyr-L appeared to decompose to yield
pyridine adsorption cannot be attributed solely to sul-
fate displacement, which led to the formation of SO3.
=
The recovery of S O appears to be a result of the de-
composition of Pyr-L.
The similarity between the IR/MS profiles of
Fig. 7(a) and (b) as well as between Fig 7(c) and
(d) suggest that H2 treatment has no effect on the
chemistry of the pyridine–SZ interactions. In contrast,
comparing the TPD profile of Pt/SZ in Fig. 7(c) and
(d) to that of SZ in Fig. 7(a) and (b) clearly demon-
strates that Pt led to instability to the sulfate groups of
the surface as shown by the lower evolution temper-
ature of SO2 (673 K versus 793 K). This is comple-
mented by a continuous evolution of CO2, as opposed
to two distinct decomposition peaks. Prior to 773 K,
the evolution of CO2 appears to correspond to the
decrease in Pyr-L and it is thus believed that the CO2
initially comes from oxidation of the pyridine-Lewis
complex. Following 773 K, the sulfate group decom-
posed and it becomes difficult to deduce the surface
chemistry.
=
CO2, and (iii) S O character of the surface increased
steadily with temperature while Pyr-L and Pyr-B de-
Decomposition of adsorbed pyridine and sulfates
makes pyridine TPD an unreliable tool for gauging the
=
creased, indicating that the initial loss of S O during

70
R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
1
1
0.5
0.5
0
Pyr-B
Pyr-L
S=O
Pyr-B
Pyr-L
S=O
0
-0.5
-1
-0.5
-1
SO2
Pyr
SO2
Pyr
CO2
CO2
373
473 573 673 773
Temp (K)
873 (Cont)
373 473 573 673 773
Temp (K)
873 (Cont)
(b)
(a)
1
1
0.5
0
0.5
0
Pyr-B
Pyr-L
S=O
Pyr-B
Pyr-L
S=O
-0.5
-1
-0.5
-1
SO2
SO2
Pyr
Pyr
CO2
CO2
373 473 573 673 773
Temp (K)
873 (Cont)
373 473 573 673 773
Temp (K)
873 (Cont)
(d)
(c)
Fig. 7. Pyridine TPD summary. Mass spectral and infrared analyses of selected species showing compositional changes in surface adsorbates
and the gaseous effluent as a function of temperature over (a) SZ, (b) H2-treated SZ, (c) 4% Pt/SZ, and (d) H2-treated 4% Pt/SZ.
−1
=
acid strength of the acid/base catalysts. The difference
in the sulfate concentration before and after pyridine
adsorption makes the correlation between the catalytic
reaction rate data and the Lewis to Brønsted acid sites
ratio dubious. We recommend that desorption behavior
of probe molecules be closely monitored with multi-
ple techniques such as mass and infrared spectroscopy
before being applied to probing the properties of a
material.
led to suppression of S O band at 1413 cm and
desorption of sulfate as SO3.
Simultaneous measurement of adsorbed species and
gaseous species evolved from SZ and Pt/SZ surfaces
during TPD shows Pyr-L nearly completely decom-
posed to CO2 at 773–823 K; more than 60% Pyr-B re-
mained on the surface in the same temperature range.
Pyr-B exhibited higher thermal stability than Pyr-L.
No major difference in the behavior of Pyr-B and
Pyr-L was observed on SZ and Pt/SZ. The major effect
of Pt is to promote the desorption of sulfate as SO2.
The study demonstrates that temperature-programmed
desorption/decomposition with simultaneous analysis
is an effective approach in identifying the structure of
the adsorbed species leading to decomposed products.
4. Conclusion
Combined IR with MS analysis shows that pyridine
adsorption produced Pyr-B and Pyr-L at the same rate,

R.W. Stevens Jr. et al. / Thermochimica Acta 407 (2003) 61–71
71
Acknowledgements
[11] R.W. Stevens Jr., S.S.C. Chuang, Appl. Catal. A, in press.
[12] X. Song, A. Sayari, Catal. Rev.-Sci. Eng. 38 (1996) 329.
[13] B.Q. Xu, W.M.H. Sachtler, J. Catal. 165 (1997) 231.
[14] S. Hammache, J.G. Goodwin Jr, J. Catal. 211 (2002)
316.
This work was partially financed by the Ohio Board
of Regents Grant R4552-OBR.
[15] K.T. Wan, C.B. Khouw, M.E. Davis, J. Catal. 158 (1996) 311.
[16] E.C. Sikabwe, M.A. Coelho, D.E. Resasco, R.L. White, Catal.
Lett. 34 (1995) 23.
References
[17] A. Jatia, C. Chang, J.D. MacLeod, T. Okubo, M.E. Davis,
Catal. Lett. 25 (1994) 21.
[18] W.M. Hua, C.X. Miao, J.M. Chen, Z. Gao, Mater. Chem.
Phys. 45 (1996) 220.
[19] D.J. Zalewski, S. Alerasool, P.K. Doolin, Catal. Today 53
(1999) 419.
[1] J.L. Falconer, J.A. Schwarz, Catal. Rev.-Sci. Eng. 25 (1983)
141.
[2] R.J. Cvetanovic, Y. Amenomiya, Catal. Rev. 6 (1972) 21.
[3] S.A. Hedrick, S.S.C. Chuang, Thermochim. Acta 315 (1998)
159.
[20] T. Yamaguchi, T. Jin, K. Tanabe, J. Phys. Chem. 90 (1986)
3148.
[4] R.J. Gorte, Catal. Today 28 (1996) 405.
[5] R. Krishnamurthy, S.S.C. Chuang, Thermochim. Acta 262
(1995) 215.
[6] Y. Chi, S.S.C. Chuang, J. Phys. Chem. B 104 (2000) 4673.
[7] S.S.C. Chuang, M.A. Brundage, M.W. Balakos, G. Srinivas,
Appl. Spectrosc. 49 (1995) 1151.
[8] S.S.C. Chuang, M.A. Brundage, M.W. Balakos, Appl. Catal.
A 151 (1997) 333.
[9] Y. Chi, S.S.C. Chuang, Catal. Today 62 (2000) 303.
[10] M.V. Konduru, S.S.C. Chuang, X. Kang, J. Phys. Chem. B
105 (2001) 10918.
[21] B.H. Davis, R.A. Keogh, S. Alerasool, D.J. Zalewski, D.E.
Day, P.K. Doolin, J. Catal. 183 (1999) 45.
[22] C. Zhang, R. Miranda, B.H. Davis, Catal. Lett. 29 (1994) 349.
[23] B. Li, R.D. Gonzalez, Catal. Today 46 (1998) 55.
[24] E.E. Platero, M.P. Mentruit, Catal. Lett. 30 (1995) 31.
[25] F. Babou, G. Coudurier, J.C. Vedrine, J. Catal. 152 (1995)
341.
[26] T. Jin, T. Yamaguchi, K. Tanabe, J. Phys. Chem. 90 (1986)
4794.