Sol-gel synthesis of Pt/Al2O3 catalysts: Effect of Pt precursor and calcination procedure on Pt dispersion

Article abstract of DOI:10.1016/j.molcata.2006.06.018

Pt/Al₂O₃ catalysts are used in a wide variety of reactions. Tailoring the catalyst structure is important in order to efficiently and effectively utilize the noble metal. In this work, the effects of Pt precursor (Pt(NH₃)₄Cl₂, Pt(C₅H₅N)₄Cl₂, Pt(CH₃NH₂)₄Cl₂, or Pt(C₄H₉NH₂)₄Cl₂) and calcination procedure (heating rates of 2 °C/min or 10 °C/min in different atmospheres) have been investigated for 1.5 wt% Pt/Al₂O₃ catalysts prepared by sol-gel synthesis. After drying, calcination, and reduction, the Pt dispersion was measured by H₂ chemisorption. The catalyst structures were characterized using X-ray diffraction, N₂ adsorption, and transmission electron microscopy. The Pt precursors as well as the calcination procedures influenced the Pt dispersion. Higher dispersions were obtained using a lower heating rate (2 °C/min), an ammonia precursor, and a flowing gas stream (as opposed to static). Monitoring the emissions during calcination with time resolved mass spectrometry indicated that decomposition of the precursors could be achieved in helium and that subsequent treatment in oxygen was not required. Differential thermal analysis indicated that larger heat flows resulted at the higher heating rate (10 °C/min) and with the pyridine precursor, compared to the ammonia precursor. The larger heat flows may have caused more sintering and, thus, a lower dispersion. Toluene hydrogenation was used as a model reaction to demonstrate that the catalysts with higher dispersions had higher activities and better catalyst stability.

Full text of DOI:10.1016/j.molcata.2006.06.018

Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
Sol–gel synthesis of Pt/Al2O3 catalysts: Effect of Pt precursor
and calcination procedure on Pt dispersion
Linjie Hu, Kenneth A. Boateng, Josephine M. Hill∗
Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr NW Calgary, Alberta T2N 1N4, Canada
Received 26 January 2006; received in revised form 31 May 2006; accepted 2 June 2006
Available online 18 July 2006
Abstract
Pt/Al2O3 catalysts are used in a wide variety of reactions. Tailoring the catalyst structure is important in order to efficiently and effectively utilize
the noble metal. In this work, the effects of Pt precursor (Pt(NH3)4Cl2, Pt(C5H5N)4Cl2, Pt(CH3NH2)4Cl2, or Pt(C4H9NH2)4Cl2) and calcination
procedure (heating rates of 2 ◦C/min or 10 ◦C/min in different atmospheres) have been investigated for 1.5 wt% Pt/Al2O3 catalysts prepared by
sol–gel synthesis. After drying, calcination, and reduction, the Pt dispersion was measured by H2 chemisorption. The catalyst structures were
characterized using X-ray diffraction, N2 adsorption, and transmission electron microscopy. The Pt precursors as well as the calcination procedures
influenced the Pt dispersion. Higher dispersions were obtained using a lower heating rate (2 ◦C/min), an ammonia precursor, and a flowing gas
stream (as opposed to static). Monitoring the emissions during calcination with time resolved mass spectrometry indicated that decomposition of
the precursors could be achieved in helium and that subsequent treatment in oxygen was not required. Differential thermal analysis indicated that
larger heat flows resulted at the higher heating rate (10 ◦C/min) and with the pyridine precursor, compared to the ammonia precursor. The larger
heat flows may have caused more sintering and, thus, a lower dispersion. Toluene hydrogenation was used as a model reaction to demonstrate that
the catalysts with higher dispersions had higher activities and better catalyst stability.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Platinum catalyst; Alumina; Sol–gel method; Calcination; Dispersion
1. Introduction
cesses are influenced by several parameters which control the
homogeneity and the microstructure of the derived material
[2].
Supported noble metal catalysts are widely used in different
catalytic processes, such as petroleum refining and automotive
exhaust emission control. Currently there is much interest in
tailored catalyst design due to the need to produce new cata-
lysts to meet the growing world energy demand and to develop
new technologies to make more active, selective, and stable cat-
alysts. Sol–gel synthesis is an attractive catalyst preparation
method for supported noble metal catalysts [1]. This method
can be used to produce catalysts with uniform metal distribu-
tion, tuneable particle size, high surface area, and stable dis-
persion. The technique is based on the hydrolysis of molecular
precursors, mostly metal or semi-metal alkoxides. Hydrolysis
and polycondensation reactions lead to the formation of oxo
polymers or metal oxides. These fundamental chemical pro-
Supported catalysts prepared via the sol–gel method gen-
erally exhibit better thermal stability compared with car-
rier systems generated by impregnation and co-impregnation
because of strong metal–support interactions [3–5]. For exam-
ple, metal/ceramic composites have been prepared within a SiO2
matrix that are nano-sized, non-agglomerated, and homoge-
neously distributed with narrow particle size distributions and
adjustable metal loadings [6–8]. Zou et al. [9] studied the ther-
mal stability of silica supported Pd catalysts prepared by the
sol–gel method, and found that the Pd particles were stable at
temperatures of up to 650 ◦C for 22 h in an O2 atmosphere. Cho
et al. [10] studied sol–gel processed Pt/Al2O3 with XAFS, and
found the Pt particles are partially buried in the alumina. This
burial of particles was the cause of a strong metal support inter-
action and resulted in high sintering resistance of metal particles
at 600 ◦C in air.
∗
Corresponding author. Tel.: +1 403 210 9488; fax: +1 403 284 4852.
E-mail address: jhill@ucalgary.ca (J.M. Hill).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.06.018

52
L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
The difficulty with using sol–gel synthesis is that the cata-
precursor using the BBI5 probe with the sample dissolved in
dimethyl sulfoxide (DMSO) or deuterated chloroform (CDCl3).
Pt-containing dry alumina gels were prepared using a sol–gel
method similar to the procedure by Cho et al. [10]. Deionized
water was mixed with aluminium tri-sec-butoxide (ATB) in an
H2O/ATB molar ratio of 100, and then stirred for 30 min at room
temperature. Next, a 0.1 g/ml HNO3 solution was added drop-
wise to the mixture, and stirred for 10 min. During the stirring
the ATB decomposed resulting in a phase containing sec-butanol
forming on top of a phase containing the sol. After separating
sec-butanol from the mixture, additional HNO3 solution was
added to the sol until the HNO3/Al ratio reached 0.5. Finally,
the Pt precursor was added to the alumina sol, which was stirred
at room temperature for 1 h, and then sonicated for 30 min. The
sol was then placed in the fume hood for 48 h to allow the gel to
form and the solvent water to evaporate. The dry gel was further
dried at 110 ◦C for 12 h, and then at 200 ◦C for 2 h. The final
material consisted of yellow cubic particles.
lyst properties are very sensitive to changes in the processing
conditions [1,2,11–14]. During the sol–gel process, a gel forms
because of the condensation of partially hydrolyzed species
into a three-dimensional polymeric network. Ward and Ho [14]
suggest that any factors that affect either or both of these reac-
tions (hydrolysis and condensation) are likely to impact the final
properties of the gel, and in turn, the properties of the final cat-
alyst. These factors, as stated, include type of precursor, type of
solvent, water content, acid or base content, precursor concen-
tration, and temperature [14]. These sol–gel parameters have
been extensively studied over SiO2-based catalytic materials
[1,2,11–14].
Alumina supported platinum catalysts also have been pre-
pared using a sol–gel method [1,4,5,10,15–19]. Romero-Pascual
et al. [4] studied the influence of platinum content, metal precur-
sor, and water/alkoxide ratio on platinum particle size and ther-
mal stability. An optimum water/alkoxide ratio was found by the
researchers. The platinum particles derived from Pt(AcAc)2 are
larger than those from H2PtCl6. Khelifi and Ghorbel [11] studied
the influence of hydrolysis and gelation process on thermal sta-
bility of Pt particles. Various gelation processes were performed
either by the help of water, acetic acid, or by a slow condensa-
tion without a hydrolysis source. The catalyst prepared by acetic
acid exhibited better sintering resistance. Few publications have
been found, however, about the influence of calcination or pre-
treatment on the platinum dispersion for Pt/Al2O3. In addition,
there are few studies on the effects of platinum precursors other
than H2PtCl6 and Pt(AcAc)2.
In this paper, 1.5 wt% Pt/Al2O3 catalysts were prepared
using a single-step sol–gel method with various Pt precur-
sors and calcination procedures. The platinum precursors
include Pt(NH3)4Cl2, Pt(CH3NH2)4Cl2, Pt(C5H5N)4Cl2, and
Pt(C4H9NH2)4Cl2 which all have nitrogen-containing ligands
and are soluble in water. The effects of ligand in the Pt pre-
cursor, as well as the calcination procedure and atmosphere,
have been investigated by a combination of catalyst characteri-
zation methods and calcination exhaust stream analysis by mass
spectrometry. Specifically, the catalysts have been characterized
using nitrogen physisorption, hydrogen chemisorption, X-ray
diffraction, and transmission electron microscopy.
2.2. Catalyst calcination
For comparison, several methods of calcination were used.
A portion of the dry gel was calcined by a one-step process.
This dry gel was calcined at 550 ◦C in one of three ways: (1) in
flowing oxygen for 2 h in a U-tube flow reactor heated on the
outside by an electric furnace; (2) in flowing air for 2 h in the
same U-tube flow reactor or (3) in static air in a muffle furnace
for 2 h. In order to investigate the influence of heating rate, two
ramping rates were used—2 or 10 ◦C/min. The other portion of
the dry gel was calcined by a two-step process. The gel was
first calcined at 550 ◦C (2 or 10 ◦C/min heating rate) in flowing
helium for 0.5 h. After cooling to 50 ◦C, the flow was switched
to pure oxygen. The temperature was then ramped to 550 ◦C at
2 or 10 ◦C/min and held for 2 h.
For some of the calcinations, the exhaust gas composition
was monitored during the temperature ramp using a Cirrus 200
Quadrupole Mass Spectrometer system (MKS) to determine
which products were being produced during the calcination.
The catalysts have been named according to the Pt precursor
and the calcination treatment. For instance, Al-1 refers to a cat-
alyst containing only alumina and calcined in oxygen at 550 ◦C,
while Py-2 refers to a catalyst prepared with a Pt–pyridine pre-
cursor and calcined in two steps with helium first and then
oxygen (see Tables 1 and 2).
2. Experimental
2.1. Catalyst preparation
2.3. Catalyst characterization
The platinum precursors were prepared by dissolving PtCl2
(Sigma–Aldrich, +99% purity) in an aqueous solution of NH3,
CH3NH2, n-butylamine or pyridine. The solvent and excess
ligands were removed by open dish drying in a fume hood.
The resulting Pt precursors were Pt(NH3)4Cl2, Pt(C5H5N)4Cl2,
Pt(CH3NH2)4Cl2, and Pt(C4H9NH2)4Cl2. The platinum content
in the Pt precursors was determined using ICP-MS (Galbraith
Labs Inc.). Elemental analysis for C, H, and N content in the
Pt precursors were performed using a Perkin-Elmer 2400 CHN
Analyzer. Proton and carbon NMR (Bruker AMX300) were also
performed to confirm the identities of the groups present in the
The N2 adsorption–desorption isotherms for the catalysts
were measured on an AUTOSORB-1C (Quantachrome) instru-
ment. All samples were evacuated at 120 ◦C until the outgas
rate was below 15 ␮mHg/min (or 2 Pa/min) prior to analysis.
The specific surface area was calculated using the BET method.
The total pore volume was determined at a relative pressure
P/Po = 0.99. Pore size distributions were calculated from the
desorption isotherms using the Barrett, Joyner, and Halenda
(BJH)method. Thedesorptionlegoftheisothermispreferredfor
pore analysis because it is thermodynamically more stable than

L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
53
Table 1
Properties of sol–gel alumina and Pt-containing samples before and after calcination
Sample
Pt precursor
Treatment
Surface area (m2/g)
Pore volume (ml/g)
Al-200
Al-1
Al-2
N-200
N-1
N-2
Py-200
Py-1
Al2O3 only
Al2O3 only
Al2O3 only
Pt(NH3)4Cl2
Pt(NH3)4Cl2
Pt(NH3)4Cl2
Pt(C5H5N)4Cl2
Pt(C5H5N)4Cl2
Pt(C5H5N)4Cl2
–
Drying in air at 200 ◦C (2 h)
O2 550 ◦C (2 h)
9.4
281
254
0
269
262
0
272
254
208
0.012
0.35
0.30
–
0.34
0.33
–
0.32
0.32
0.31
He 550 ◦C (0.5 h), O2 550 ◦C (2 h)
Drying in air at 200 ◦C (2 h)
O2 550 ◦C (2 h)
He 550 ◦C (0.5 h), O2 550 ◦C (2 h)
Drying in air at 200 ◦C (2 h)
O2 550 ◦C (2 h)
He 550 ◦C (0.5 h), O2 550 ◦C (2 h)
–
Py-2
Commercial ␥-Al2O3
Note: Error is 5% in the surface area and the pore volume measurements.
the adsorption leg due to the lower Gibb’s free energy change
[20].
database) for identification. If possible, the average crystallite
diameter of metallic Pt was calculated using Scherer’s method,
DPt = Kλ/βcos θ, where the constant K was taken as 0.9 and β
was the full width at half maximum (FWHM) of the Pt(3 1 1)
peak at 2θ = 81.3◦.
Transmission electron microscopy (TEM) images were
recorded on an H-7000 transmission electron microscope
(Hitachi) at 75 kV. The samples were ground to a fine pow-
der, and mixed with acetone to make a suspension. A drop of
the suspension was placed on a lacey carbon nickel grid, which
was subsequently dried at room temperature before the measure-
ment.
H2 chemisorption measurements were carried out on the
same AUTOSORB-1C instrument. For these measurements,
approximately 1.0 g of catalyst was placed in a quartz U-tube
(i.d. = 10 mm), and reduced in a H2 flow of 15 ml/min at 300 ◦C
for 2 h. After reduction, the sample cell was evacuated at 300 ◦C
for 2 h, and then cooled to 40 ◦C for the H2 chemisorption
measurement. The H2 monolayer uptake of the catalysts was
calculated by extrapolating the H2 adsorption isotherm to zero
pressure. The Pt particle diameter (dPt) was calculated using the
formula, dPt = 6V/S, where V is the volume of total metallic Pt,
and S is the active Pt surface area, assuming the Pt2+ ions were
reduced completely and the Pt particles were spherical in shape.
An adsorption stoichiometry of one hydrogen atom adsorbed
per surface Pt atom (H/Pts = 1) was assumed. The percent Pt
dispersion was calculated by dividing the number of exposed
surface Pt atoms (as determined by H2 chemisorption) by the
total amount of Pt in the catalyst.
2.4. Differential thermal and thermogravimetric analyses
Differential thermal and thermogravimetric analyses
(DTA/TGA) were performed on three samples (A1-200, N-200,
and Py-200) to examine the thermal and gravimetric changes
that occur in those samples during calcination. A DSC/TGA
Q600 instrument (TA Instruments) was used for this analysis.
The analysis conditions were selected so as to mimic the calci-
nation procedure. Three different types of tests were performed
as follows: (1) all samples (A1-200, N-200, and Py-200) were
heated under air flow from room temperature to 550 ◦C at
Powder X-ray diffraction (XRD) spectra were recorded on a
Multiflex X-ray diffractometer (Rigaku) using Cu K␣1 radiation
˚
(λ = 1.54056 A) at 40 kV tube voltage and 40 mA tube current
with a scanning speed of either 0.2 or 2◦/min. The XRD pat-
terns were referenced to the powder diffraction files (ICDD-FDP
Table 2
Pt dispersions of Pt/Al2O3 samples after various calcination procedures
Sample Pt precursor
Calcination procedure
Heating rate
(◦C/min)
Pt metal
content (%)
Active metal surface
area (m2/g)
Dispersion
(%)
Particle
diameter (nm)
N-1
N-2
N-3
N-4
N-5
N-6
Py-1
Py-2
Py-3
Py-4
Py-5
Py-6
MA-1
BA-1
Pt(NH3)4Cl2
Pt(NH3)4Cl2
Pt(NH3)4Cl2
Pt(NH3)4Cl2
Pt(NH3)4Cl2
Pt(NH3)4Cl2
Pt(C5H5N)4Cl2
Pt(C5H5N)4Cl2
Pt(C5H5N)4Cl2
Pt(C5H5N)4Cl2
Pt(C5H5N)4Cl2
Pt(C5H5N)4Cl2
Pt(CH3NH2)4Cl2
O2 550 ◦C (2 h)
10
10
2
2
2
1.58
1.58
1.58
1.58
1.58
1.58
1.43
1.43
1.43
1.43
1.43
1.43
1.54
1.19
1.4
2.3
4.1
4.1
3.4
4.1
2.2
2.0
3.1
2.7
2.0
3.1
2.2
0.34
35
59
105
104
87
106
63
56
88
77
58
89
3.2
1.9
1.1
1.1
1.3
1.1
1.8
2.0
1.3
1.3
2.0
1.3
1.9
9.9
He 550 ◦C (0.5 h), O2 550 ◦C (2 h)
O2 550 ◦C (2 h)
He 550 ◦C (0.5 h), O2 550 ◦C (2 h)
Static air in muffle furnace, 550 ◦C (2 h)
Flowing air, 550 ◦C (2 h)
2
O2 550 ◦C (2 h)
He 550 ◦C (0.5 h), O2 550 ◦C (2 h)
O2 550 ◦C (2 h)
10
10
2
2
2
2
2
2
He 550 ◦C (0.5 h), O2 550 ◦C (2 h)
Static air in muffle furnace, 550 ◦C (2 h)
Flowing air, 550 ◦C (2 h)
Static air in muffle furnace, 550 ◦C (2 h)
59
11
Pt(C4H9NH2)4Cl2 Static air in muffle furnace, 550 ◦C (2 h)
Note: (1) H2 chemisorption was performed after reducing the catalyst on line at 300 ◦C for 2 h in flowing H2. (2) Error is 5% in the dispersion measurements.

54
L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
2 ◦C/min and held at 550 ◦C for 30 min; (2) sample N-200
was heated under air flow from room temperature to 550 ◦C at
10 ◦C/min and held at 550 ◦C for 30 min and (3) sample N-200
was heated under He flow from room temperature to 550 ◦C
at 10 ◦C/min and held at 550 ◦C for 30 min. Heat flow, mass
loss, and differential temperatures were recorded during the
analyses.
2.5. Reactivity testing
Four catalyst samples, N-4, Py-4, MA-1, and BA-1 (see
Table 2) were tested for reactivity in a fixed bed reactor using
hydrogenation of toluene at atmospheric pressure as a model
reaction. The reactor was a quartz tube with inner diameter of
7 mm. Approximately 400 mg of catalyst was used for each
run. All the catalysts particles were sieved to the same size
range, 90–250 ␮m. Reactions were conducted at temperatures
between 60 and 270 ◦C at 30 ◦C intervals. A liquid hourly space
velocity (LHSV) of 1 h−1 was used, with a H2 to toluene volu-
metric ratio of 1250. The catalysts were reduced in flowing H2
for 2 h at 300 ◦C and then the temperature was reduced to the
reaction temperature. The reactor effluent was analyzed using a
gas chromatograph (Agilent 6890) equipped with a GS-GasPro
PLOT column and a flame ionization detector (FID). The reac-
tor came to steady state after approximately 30 min on stream.
The steady-state compositions were used to calculate activities.
After 120 min on stream at one temperature, the temperature
was increased by 30 ◦C. Once at 270 ◦C, the reactor was cooled
to 60 ◦C and the testing and temperature cycle repeated. One
complete temperature cycle between 60 and 270 ◦C constituted
one run. Three runs were done for each of the four catalysts.
Fig. 1. Pore size distributions and adsorption isotherms of samples Al-1 and
commercial ␥-Al2O3 (Alfa Aesar). Sample Al-1 is representative of all in-house
prepared samples with or without platinum.
Based on the elemental analysis performed by ICP-MS (Gal-
braith Labs), the platinum content of the catalysts are as follows:
1.58%forN-batchcatalysts, 1.43%forPy-batchsamples, 1.54%
for MA-1, and 1.19% for BA-1. These Pt metal contents are
shown in Table 2, with the Pt dispersions as determined by H2
chemisorption. The dispersions range between 11 and 106%.
The metal surface areas used to calculate the dispersions are also
shown in Table 2. Dispersions above 100% may be attributed
to errors in the Pt metal content, errors in the adsorbed H2
determination, or hydrogen spillover from the Pt. The Autosorb-
1C was calibrated with the Quantachrome standard reference
(1%Pt/Al2O3), and had an error of 0.5%.
3. Results and discussion
3.1. Physical properties of catalysts
In general, a lower heating rate (2 ◦C/min versus 10 ◦C/min)
results in an increased dispersion. In addition, the precursor may
also significantly influence the dispersion, as will be discussed
in more detail in the following sections. In this work, all com-
parisons are made between the same batches of catalyst. Despite
batch to batch variability, the overall trends are consistent.
The diameter of the Pt particles can be estimated from the
hydrogen uptake and is given in Table 2. For all catalysts, except
BA-1, the average particle sizes of Pt are less than 3.2 nm. X-ray
diffraction can also be used to estimate the particle size provided
that the particles are larger than approximately 4–5 nm. Fig. 2
shows the XRD spectra of the prepared catalysts and a commer-
cial ␥-Al2O3. The XRD pattern of aluminium oxide can be quite
different depending on the preparation method and crystalline
phaseaccordingtotheICDD-FDPdatabase. ThespectrainFig. 2
indicate that the commercial ␥-Al2O3 sample and the in-house
prepared samples have similar structures although the latter alu-
mina has smaller crystalline size as indicated by the broader
peaks at 2θ = 67.3◦. The XRD patterns of the samples are all
quite similar despite different platinum particle sizes, calcina-
tion procedures, and platinum precursors. This result indicates
that the alumina crystalline structure is not sensitive to these
Table 1 contains the surface areas and pore volumes for cata-
lyst samples prepared using different Pt precursors and treatment
procedures. The samples Al-200, N-200, and Py-200 exhibit
very low surface areas (<10 m2/g), which indicates that alu-
minium hydroxide and nitrate were not decomposed to refrac-
tory oxide Al2O3 after being dried in air at 200 ◦C for 2 h. The
remainingcatalystshadsurfaceareasbetween254and281 m2/g,
and pore volumes between 0.30 and 0.35 ml/g. The pore size
distributions were similar for all of the calcined catalysts, with
mean pore diameters of 3.8 nm. These results indicate that the
precursor and calcination procedure had little effect on the bulk
physical structure of the catalysts.
For comparison, a commercial ␥-Al2O3 (Alfa Aesar) was
characterized. The surface area of the commercial alumina is
slightly lower (208 m2/g) than the surface areas of the prepared
catalysts, while the pore volume is similar (Table 1). Fig. 1 com-
pares the pore size distributions and adsorption isotherms of the
commercial alumina and sample Al-1, which is representative
of all the calcined catalysts. The in-house prepared alumina is
similar to the commercial alumina in terms of its pore structure.

L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
55
Fig. 3. XRD patterns of reduced Pt/Al2O3 catalysts obtained with slow scans
(0.2◦/min).
Fig. 2. XRD patterns of alumina samples and Pt/Al2O3 catalysts. All samples
were calcined. Catalysts were reduced at 300 ◦C for 2 h with flowing H2.
parameters, which is consistent with the BET measurements
with respect to the constant pore structure and surface area.
The peaks for Pt overlap, at least partially, with alumina
at most diffraction angles, except for the Pt(3 1 1) peak at
2θ = 81.3◦. In order to obtain accurate Pt particle size infor-
mation, slow scans (0.2◦/min) were performed in the range of
78–90◦ 2θ. TheresultsareshowninFig. 3andareconsistentwith
the average particle size of Pt calculated from the chemisorp-
tion results. Pt is undetectable for catalysts N-5 and N-6, while
Py-5 and MA-1 both have a peak at 2θ = 81.3◦ that is too small
for estimation of particle size. The calculated average particle
size of Pt in catalyst BA-1 is 23 nm, which is larger than that
estimated by hydrogen chemisorption (10 nm).
CHNanalysis, theseresultsconfirmedthepresenceofa(C5H5N)
group in the precursor.
The selection of platinum precursor was based on the hypoth-
esis that the larger the precursor the better the resulting Pt
dispersion. During calcination, platinum (oxide) atoms may
agglomerate into clusters. Assuming that the agglomeration
occurs as a series of binary interactions, the rate of agglom-
eration is related to the distance between any two platinum
atoms. Thus, the ligand of the Pt(II) precursor would act as
space holder for the platinum atoms. As shown in Table 2,
the precursors are all nitrogen-containing molecules and sol-
uble in water. The molecular diameters of the precursors were
estimated by bond lengths and are approximately 5, 7, 12, and
˚
13 A for Pt(NH3)4Cl2, Pt(CH3NH2)4Cl2, Pt(C5H5N)4Cl2, and
3.2. Effect of platinum precursor
Pt(C4H9NH2)4Cl2, respectively.
Fig. 4 illustrates that the results were exactly the opposite of
what was expected. That is, the smaller the precursor, the better
the dispersion. The results shown in Fig. 4 are those taken from
Table 2 for samples N-5, Py-5, MA-1, and BA-1. The Pt disper-
sion is strongly dependent on the precursor and increases from
11% for BA-1 (largest precursor) to 87% for N-5 (smallest pre-
cursor). These samples were calcined in a muffle furnace in static
air. Although, as discussed below, calcining in static air results
in lower dispersions than flowing air, the trend of increasing
dispersion with decreasing precursor size did not change with a
change in the calcination procedure.
The platinum precursors – Pt(NH3)4Cl2, Pt(CH3NH2)4Cl2,
Pt(C5H5N)4Cl2, and Pt(C4H9NH2)4Cl2 – were analyzed with
CHN analysis to obtain the C, H, and N ratios contained in the
catalysts. Based on the CHN analysis results, it was confirmed
that the desired precursors were obtained. Carbon NMR of the
Pt(C4H9NH2)4Cl2 precursor showed a CH3 chemical shift at
13.7 ppm and CH2 at 19.4, 32.7, and 46.1 ppm. Proton NMR
of the same Pt(C4H9NH2)4Cl2 confirmed the presence of the
CH3 and CH2 peaks in addition to the NH2 peak at ca. 5.6 ppm
chemical shift. These results in addition to the CHN analysis
confirmed the presence of a (C4H9NH2) group in the precur-
sor. Proton NMR of the Pt(C5H5N)4Cl2 precursor revealed an
aromatic structure with the first CH at 8.9 ppm, the second at
7.5 ppm, and the third at 7.9 ppm. Combined with results of
The extremely low dispersion of BA-1, obtained using
Pt(C4H9NH2)4Cl2 as precursor, may be caused partly by the
relatively poor solubility of the precursor in water. During the
precursor preparation, the water solution (15 ml water, contain-

56
L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
On all other catalysts, no Pt particles were visible, consistent
with the hydrogen chemisorption and XRD results.
The chemical nature of the Pt precursors and their inter-
action with the support precursor (i.e., silane as precursor of
silica) may also be a factor in the resulting metal dispersion.
The effect of Pt precursor on the Pt dispersion and particle
size has been studied over sol–gel processed Pt/SiO2 by Lem-
bacher [2]. The studied precursors included Pt(AcAc)2, PtCl2,
H2PtCl6, Na2PtCl6, Pt(CN)2, and Pt(NH3)4(NO3)2. PtCl2 led
to the highest Pt dispersion when a nitrogen-containing silane,
H2NCH2CH2NH(CH2)3Si(OEt)3 (AEAPTS), was used as sil-
ica precursor. The researchers ascribed this result to the strong
interaction of Pt(II) and the silica precursor, while Pt(IV) cannot
form a complex with nitrogen-containing molecules with a sil-
ica precursor. In the work done herein, the Pt precursors will not
react with the aluminium precursors and so anchoring of some
of the Pt precursors cannot explain the different dispersions that
were obtained.
3.3. The effect of calcination procedure on Pt dispersion
Fig. 4. Influence of platinum precursors on dispersion in Pt/Al2O3 catalysts.
The dry gels were calcined in a muffle furnace at 550 ◦C for 2 h with a heating
rate of 2 ◦C/min, and reduced at 300 ◦C for 2 h in flowing H2.
A heating rate of 2 ◦C/min under all calcination conditions
resulted in higher Pt dispersions than using a heating rate of
10 ◦C/min. Fig. 6 presents the results for the catalysts derived
from Pt(NH3)4Cl2. Samples N-1 (35% dispersion) and N-3
(105% dispersion) were both calcined in O2 at 550 ◦C for 2 h,
while samples N-2 (59% dispersion) and N-4 (104% disper-
sion) were calcined in a two-step process involving helium and
then oxygen. Clearly a slower heating rate results in higher dis-
persions. The same trend is observed for the catalysts prepared
with Pt(C5H5N)4Cl2 as the precursor (Py-catalysts). For exam-
ple, the Pt dispersion for Py-3, heated at 2 ◦C/min, was 88%,
compared to a Pt dispersion of 63% for Py-1, which was heated
at 10 ◦C/min. We have not found any literature demonstrating
that a slower heating rate is better for the preparation of Pt/Al2O3
catalysts, although this result is expected based on the results for
silica-supported catalysts.
ing 0.1 g of Pt) had to be heated to 60 ◦C in order to dissolve
this precursor. The precursor did not precipitate in the sol or
wet gel because of the presence of sufficient water (ca. 50 ml).
The precursor, Pt(C4H9NH2)4Cl2, probably precipitated, how-
ever, during water removal. If the precipitate separated from the
alumina gel during subsequent heat treatments, large Pt parti-
cles may be formed. TEM analysis of the reduced BA-1 catalyst
(Fig. 5) indicated that large Pt particles have formed. The darker
particles in Fig. 5(a) are on the order of 200 nm in size and
are likely Pt particles or agglomerates. The lighter particles are
alumina. In Fig. 5(b), Pt particles are visible at the edges of the
alumina particles with sizes of 10–50 nm. A homogenous Pt dis-
tribution was not obtained from this Pt precursor as the majority
of the alumina particles did not contain any visible Pt particles.
Fig. 5. (a and b) TEM images of catalyst BA-1.

L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
57
Fig. 7. Influence of gas flow during calcination on Pt dispersion of Pt/Al2O3
catalysts. All the catalysts were calcined either in flowing air or in static air
(muffle furnace) at a heating rate of 2 ◦C/min.
Fig. 6. Influence of heating rate during calcination on Pt dispersion of Pt/Al2O3
catalysts. The dry gel was calcined in flowing gas at a heating rate of 2 or
10 ◦C/min.
3.4. Calcination studies performed by mass spectrometry
The atmosphere during calcination also affected the final Pt
dispersion, but the magnitude of the effect depended on the
heating rate and Pt precursor. For example, samples N-1 and
N-2, were both prepared from the same precursor and heated
at the same rate (10 ◦C/min) during calcination, but had signifi-
cantly different Pt dispersions (35% versus 59%). In this case, a
higher dispersion was obtained by heating in He before heating
in O2. Conversely, for samples N-3 and N-4 (Table 2), heated
at 2 ◦C/min, the Pt dispersion was the same regardless of the
calcination procedure. When Pt(C5H5N)4Cl2 was used as the Pt
precursor, a comparison of Py-1 with Py-2, and Py-3 with Py-4
(Table 2), indicated that He pretreatment before calcination in
O2 resulted in a lower Pt dispersion compared to no He pre-
treatment. Lembacher has shown that pretreatment in an inert
atmosphere is beneficial for Pt dispersion on silica supports [2].
For Pt/SiO2 the average particle size of Pt decreased from 22 nm
to less than 3 nm by pretreating the catalyst with argon before
exposure to air at 400 ◦C [2]. The author suggested that, during
the one-step calcination, the Pt particles sintered due to local
overheating caused by rapid oxidation of organic groups in the
silane precursor. It is unclear, however, how the organic groups
in the metal precursors affect the dispersion during calcination.
Heating in a muffle furnace is a much simpler method than
heating a catalyst in a flow apparatus. In these experiments,
however, treatment in static air (i.e., the muffle furnace) resulted
in much lower Pt dispersions than treatment in a U-tube on a
flow apparatus. For example, as shown in Fig. 7 and Table 2,
sample N-5 was calcined in the muffle furnace and had a Pt
dispersion of 87% compared to a dispersion of 106% for sample
N-6, calcined in flowing air. Similar results were obtained for
samples Py-5 and Py-6. Likely the water produced during the
calcination was not removed quickly enough without flowing
gases. The presence of water may have promoted the sintering
of the Pt particles [15,21,22].
Mass spectrometry (MS) was used to better understand what
species are being formed during the calcination. This analysis
was used in conjunction with XRD analysis. The alumina sup-
port was analyzed first, and then several samples containing Pt
were analyzed. The alumina gel is soluble in water after drying
at 200 ◦C for 2 h, indicating the dry gel has the same or similar
composition with the sol or wet gel except for solvent (H2O)
content. In order to investigate the structure of the dry gel (Al-
200 in Table 2), the XRD patterns were recorded before and
after calcinations (spectra not shown). The XRD pattern of the
dry gel was different from that of the sol–gel Al2O3 (Al-1 and
Al-2). Referencing the ICDD database, the dry gel had a similar
structure to Al(OH)3, indicating the dry gel did not decompose
after drying for 2 h at 200 ◦C in air. The composition of the dry
gel can be represented by Al(NO3)δ(OH)3−δ, or more precisely
Al(NO3)δOx(OH)3−δ−2x because of condensation [23]. Here,
δ ≈ 0.5 since the molar ratio of HNO3/Al is 0.5 during peptiza-
tion. After calcination, the mass loss of the dry gel is 45 4%.
From the mass loss, the value of x can be calculated, which will
be discussed in Section 3.5. The calcined alumina samples (Al-
1 and Al-2) had structures similar to a commercial ␥-Al2O3 as
described previously.
Fig. 8 shows the evolution of CO2 (mass 44) and NO2 (mass
46) during the calcination of the dry gel in oxygen or in helium.
Masses 18 (H2O) and 30 were also monitored during the exper-
iment. During calcination water was produced from the decom-
position of Al(OH)3. The water, however, condensed before the
inlet to the mass spectrometer and, thus, the water signal is not
reliable and not shown. The signals from masses 46 and 30 were
identical in shape and, thus, mass 30 is attributed to the crack-
ing pattern of NO2 [24]. The plots in Fig. 8 are nearly identical
whether the atmosphere is helium or oxygen. In both cases, CO2
is detected at temperatures between 240 and 470 ◦C, while NO2

58
L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
helium or oxygen atmosphere. The quantity of CO2 evolved,
however, depended on the presence of the precursor. Emissions
of CO2 for the gel derived from Pt(NH3)4Cl2 (N-200) and for a
blank alumina gel (Al-200) were similar. In comparison, the gel
derived from Pt(C5H5N)4Cl2 (Py-200) produced approximately
20 times more CO2 than either N-200 or Al-200, consistent with
the carbon content of the precursor. The additional emitted CO2
came from the oxidation of the pyridine contained in the dry gel
for Py-200.
3.5. Chemical reactions during calcination
As mentioned above, the composition of the alu-
mina dry gel before calcination can be represented by
Al(NO3)δOx(OH)3−δ−2x, δ ≈ 0.5. The value of x can then be
calculated based on mass loss during calcination. The final
solid product after calcination is Al2O3, and the mass losses
are 45 4% during calcination. Thus, the reaction during calci-
nation can be described as follows:
Fig. 8. CO2 and NO2 evolution monitored by mass spectrometry during calci-
nation of dry gel alumina (Al-200) in flowing O2 or He (20 ml/min, 10 ◦C/min).
is detected at temperatures above 250 ◦C. The CO2 originated
from the oxidation of an organic compound in the gel produced
during the hydrolysis of ATB. The organic compound is likely
2-butanol; however, 2-butanol was not detected in the emissions.
The organics were oxidized by oxygen or the produced NOx in
the absence of oxygen (i.e., helium atmosphere). Following the
treatment in flowing He, the same sample was monitored dur-
ing treatment in flowing O2 and no NO2 or CO2 was detected.
Theseresultssuggestthatcompletedecompositionoccurreddur-
ing the pre-treatment in helium and that treatment with oxygen
was unnecessary.
2Al(NO3)0.5Ox(OH)2.5−2x
= Al2O3 + NO2 + (2.5 − 2x)H2O + 0.25O2
(1)
The calculated value of x is between 0 and ∼0.8. On average,
if the mass loss is 45%, x ≈ 0.4. That is, 100 g of the dry gel
would produce 25 g of NO2, 4.3 g of O2, and 16 g of H2O. Water
condensation in the lines after the calcination vessel was evident.
The condensed water had a yellow color and was strongly acidic
(pH < 1), indicating that HNO3 likely has been formed from the
following reactions in the exit stream:
The NO2, NO, and H2O emissions during the calcination
of Pt-containing dry gels (N-200 and Py-200) were similar to
that for blank alumina gel (Al-200) in either oxygen or helium.
The CO2 evolution during the calcination of Al-200, N-200,
and Py-200 is shown in Fig. 9. The plots are similar for a
4NO2 + O2 + 2H2O = 4HNO3
3NO2 + H2O = NO + 2HNO3
(2)
(3)
Water vapor has been reported to enhance Pt sintering in
Pt/Al2O3 during calcination and reduction [15,21,22]. The pres-
ence of excess water vapor may have been a factor in the lower
dispersions obtained when the calcination was performed in
static air (muffle furnace) rather than flowing air, in which the
water would have been removed and not accumulated on the sur-
face of the catalyst. Similarly, the lower heating rate (2 ◦C/min)
may be beneficial because of the slower decomposition, and
hence water evolution, of the alumina gel. The gas flow rate and
amount of catalyst was kept constant throughout the calcination
tests.
The effect of the Pt precursor on dispersion may be related
to the composition of the precursor. In this work, the Pt precur-
sors with organic ligands yielded lower Pt dispersions than the
precursor containing ammonia. The dry gel containing a pyri-
dine Pt(II) ligand (Py-200) produces approximately 20 times the
amount of CO2 during calcination than the dry gel containing
an ammonia Pt(II) ligand (N-200). The localized heating from
the oxidation of the organic ligands may have been sufficient to
result in sintering of the Pt particles. Lembacher [2] suggested
that localized heating caused by rapid oxidation of the support
precursor, silane, resulted in sintering in Pt/SiO2 catalysts. In
contrast to the work on Pt/SiO2 [2], with Pt/Al2O3 He treat-
Fig. 9. CO2 evolution monitored by mass spectrometry during calcination of
Al-200, N-200, and Py-200 in flowing O2 or He (20 ml/min, 10 ◦C/min).

L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
59
Fig. 11. Toluene hydrogenation activity of N-4, Py-4, MA-1, and BA-1 catalysts
at 240 ◦C for three different runs: (᭹) run 1, (ꢀ) run 2, and (ꢀ) run 3. The
numbers given in brackets on the x-axis are the Pt dispersions of each catalyst.
largest change in heat flow occurred on sample N-200 heated
in air at 10 ◦C/min, which is consistent with the lower disper-
sions obtained for samples heated at 10 ◦C/min than 2 ◦C/min
(Table 2). These results support the theory that localized heating
during calcination may contribute to sintering of the Pt resulting
in lower dispersions.
3.7. Reactivity tests—toluene hydrogenation
Fig. 11 shows a comparison of the production of methylcy-
clohexane (MCH) over catalysts produced from the different
precursors (N-4, Py-4, MA-1, and BA-1) at 240 ◦C and atmo-
spheric pressure. Three runs were performed for each catalyst
and in each case, the only product was methylcyclohexane. Cat-
alyst BA-1 had the lowest Pt dispersion (11%) and the lowest
production of MCH. In contrast, catalyst N-4 had the highest
Pt dispersion (104%) and the highest production rate. Catalysts
Py-4 and MA-1 had intermediate Pt dispersions (77 and 59%)
and intermediate activities. The activity of N-4 stayed within
2.5% of the mean activity of this catalyst. The other catalysts,
however, had larger decreases in activity over time.
Fig. 10. Thermogravimetric (a) and differential thermal analysis (b) of A1-200,
N-200, and Py-200 heated at 2 ◦C/min in air. The differential thermal analysis
is also shown for N-200 heated at 2 ◦C/min in He, and at 10 ◦C/min in air.
ments before calcination in oxygen only improved the dispersion
for the ammonia precursor with a heating rate of 10 ◦C/min. The
mass spectrometer results indicated that the pyridine and ammo-
nia precursors were completely oxidized in He and a secon−d
treatment in O2 was not required. The decomposition of NO3
groups produced significant amounts of O2 and NOx which can
act as oxidants. Likely these species were sufficient to com-
pletely decompose the precursors.
4. Conclusions
3.6. Results of differential thermal and thermogravimetric
analyses
Pt dispersions ranging between 11 and 106% were obtained
for 1.5 wt% Pt/Al2O3 catalysts prepared by sol–gel syn-
thesis. The Pt dispersion of the catalyst was found to be
strongly dependent on the platinum precursor, and con-
trary to our expectation, a larger precursor molecule did
not result in better dispersion. Specifically, in terms of
highest Pt dispersion, Pt(NH3)4Cl2 was the best precursor
followed by Pt(CH3NH2)4Cl2, Pt(C5H5N)4Cl2, and finally
Pt(C4H9NH2)4Cl2. The latter precursor resulted in a very poor
dispersion, likely because of its poor solubility. The dispersion
also depended on the calcination procedure. The use of flowing
gas instead of static air, and a lower heating rate (2 ◦C/min rather
than 10 ◦C/min) resulted in higher Pt dispersions. The presence
The thermogravimetric and differential thermal analysis of
samples Al-200, N-200, and Py-200 are shown in Fig. 10. In
agreement with the mass spectrometry results, the mass loss in
an air atmosphere corresponded to decomposition within the
temperature range of 200–450 ◦C (Fig. 10(a)). The differential
thermalanalysisisshownfortwoheatingrates(2and10 ◦C/min)
as well as two atmospheres (helium and air) for sample N-200.
More heat was evolved during the calcination of Py-200 (compa-
rable to Py-3 with a dispersion of 88%) than for the calcination
of N-200 (comparable to N-3 with a dispersion of 105%). The

60
L. Hu et al. / Journal of Molecular Catalysis A: Chemical 259 (2006) 51–60
of accumulated water (from treatment in static air or from too
high a heating rate) and/or localized heating effects (dependent
onthedecompositionoftheprecursor)resultedinsinteringofthe
Pt and lower dispersions. Pretreatment in helium before oxygen
did not improve the dispersion. Toluene hydrogenation exper-
iments indicated that higher activities and selectivities can be
achieved with catalysts containing more highly dispersed Pt.
Thus, it is desirable to optimize catalyst preparation to achieve
high dispersion.
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We would like to acknowledge Suncor Energy Inc., Alberta
Energy Research Institute, and the Natural Sciences and Engi-
neering Research Council (NSERC) for funding this project.
We thank Mr. R. Humphrey and Mr. W. Dong at the Microscopy
and Imaging Facility, University of Calgary, for assistance with
electron microscopy, Ms. L. Klatzel-Mudry at the Department of
Geology and Geophysics, University of Calgary, for assistance
with XRD analysis, Ms. Q. Wu of the Chemical Instrumenta-
tion Lab at the Department of Chemistry, University of Calgary,
for assistance with NMR and elemental analysis, Dr. A. Has-
san for his assistance with preparing platinum precursors, and
Drs. J. Perez-Zurita and A. Mahmoudkhani for performing DTA
analysis.
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