Low temperature toluene oxidation over Pt nanoparticles supported on yttria stabilized-zirconia

Article abstract of DOI:10.1007/s10562-013-1071-x

Toluene oxidation was measured over Pt nanoparticles synthesized using a modified polyol reduction method and deposited on ionically conductive yttria-stabilized zirconia (Pt/YSZ) for three different loadings (1.1, 0.8, 0.4 %), and non-ionically conductive γ-alumina (Pt/γ-Al ₂O₃) as a comparison (metal loading 0.7 %). It was found that nanoparticles supported on YSZ, tested as a support for the first time for toluene oxidation, have greater catalytic activity compared to a conventional γ-Al₂O₃ support in spite of a lower specific area and Pt dispersion. This could be explained by the stronger metal-support interactions between Pt and YSZ due to the ionic conductivity of YSZ and presence of oxygen vacancies.

Full text of DOI:10.1007/s10562-013-1071-x

Catal Lett
DOI 10.1007/s10562-013-1071-x
Low Temperature Toluene Oxidation Over Pt Nanoparticles
Supported on Yttria Stabilized-Zirconia
•
Holly A. E. Dole Rima J. Isaifan Foteini M. Sapountzi
•
•
•
Leonardo Lizarraga Daniel Aubert Agnes Princivalle
•
•
•
Philippe Vernoux Elena A. Baranova
Received: 16 May 2013 / Accepted: 7 July 2013
Ó Springer Science+Business Media New York 2013
Abstract Toluene oxidation was measured over Pt
nanoparticles synthesized using a modified polyol reduc-
tion method and deposited on ionically conductive yttria-
stabilized zirconia (Pt/YSZ) for three different loadings
(1.1, 0.8, 0.4 %), and non-ionically conductive c-alumina
(Pt/c-Al2O3) as a comparison (metal loading 0.7 %). It was
found that nanoparticles supported on YSZ, tested as a
support for the first time for toluene oxidation, have greater
catalytic activity compared to a conventional c-Al2O3
support in spite of a lower specific area and Pt dispersion.
This could be explained by the stronger metal-support
interactions between Pt and YSZ due to the ionic con-
ductivity of YSZ and presence of oxygen vacancies.
1 Introduction
A growing global concern about air pollution has sparked
the interests of many research groups around the world.
Volatile organic compounds (VOCs) are carbon-based
chemicals that are among the pollutants that can cause
environmental and health problems. There are several
methods that have been demonstrated to control VOC
emissions; however, the use of catalytic oxidation is found
to be efficient and cost-effective. Many different catalysts
and supports have been studied for this purpose, including
metal oxides [1], and noble metals supported by metal
oxides or carbon [2] or a mixture of metal oxides [3].
Yttria-stabilized zirconia (YSZ), a metal oxide support,
is known to be an O2- conductor due to the presence of
oxygen vacancies inside its crystallographic structure. This
oxide has been extensively studied as a material for oxygen
sensors and fuel cells [4]. In the last 25 years, YSZ has
received attention as a catalytic support due to its ionic
conductivity properties especially in studies of electro-
chemical promotion of catalysis (EPOC); also called the
non faradaic electrochemical modification of the catalytic
activity (NEMCA) effect [5]. Vayenas and collaborators
[5] were the first to show that the migration of ionic species
from a solid electrolyte to the gas exposed catalyst surface
induced by electrical polarizations can improve catalytic
performances. Vayenas et al. [6] have described EPOC as
an electrically controlled strong metal support interaction
(SMSI). In addition, it was found that ionic oxygen species
can thermally migrate without any electrical polarization
on nanoparticles of metallic catalysts supported on ioni-
cally conducting ceramics, such as YSZ [7]. Recently,
Masui et al. [8] have shown that the mobility of oxygen in
the near surface region at the Pt nanoparticles dispersed on
Ce0.64Zr0.15Bi0.21O0.1895 can promote the oxidation activity
Keywords Toluene oxidation ꢀ Ionic conductivity ꢀ
Yttria-stabilized zirconia ꢀ Oxygen vacancies ꢀ
Platinum
H. A. E. Dole ꢀ R. J. Isaifan ꢀ E. A. Baranova (&)
Department of Chemical and Biological Engineering and Center
for Catalysis Research and Innovation, University of Ottawa,
161 Louis-Pasteur St., Ottawa, ON K1N 6N5, Canada
e-mail: elena.baranova@uottawa.ca
F. M. Sapountzi ꢀ L. Lizarraga ꢀ P. Vernoux
Institut de Recherches sur la catalyse et l’environnement de
´
Lyon (IRCELYON), UMR 5256, CNRS, Universite Lyon 1,
2 Avenue Albert Einstein, 69626 Villeurbanne, France
D. Aubert ꢀ A. Princivalle
`
´
Laboratoire de Synthese et Fonctionnalisation des Ceramiques,
UMR3080, CNRS/Saint-Gobain - 550, Av. Alphonse Jauffret,
84306 Cavaillon Cedex, France
123

H. A. E. Dole et al.
of toluene. This phenomenon adds to the appeal of using
YSZ as an alternative support in the catalytic removal of
harmful pollutants such as VOCs.
dispersion ð%Þ ¼ ðMWPt ꢀ 600Þ=ðq ꢀ d ꢀ aPt ꢀ NAÞ
ð1Þ
where MWPt is the molecular weight of Pt (195.08 gꢀmol-1),
q is the density of Pt (21.09 gꢀcm-3), d is the average particle
diameter (from TEM images), aPt is the surface area of one Pt
atom (8.06 9 10-20 m2ꢀatom-1), and NA is Avogadro’s
number. Table 1 summarizes the results of the character-
ization of each catalyst.
Among the large variety of VOCs, toluene can be con-
sidered a representative of aromatic hydrocarbons and its
oxidation at low temperatures has a significant practical
interest. In this work, platinum nanoparticles with an
average colloidal size of 2.5 0.5 nm and narrow size
distribution were synthesized using a modified polyol
method. As-prepared colloids were deposited on YSZ and
c-Al2O3 (conventional support used in heterogeneous
catalysis [9]) and tested for toluene oxidation. The effect of
metal loading and support nature on the catalytic activity
was investigated.
2.2 Catalytic Activity Measurements
The catalytic activity measurements of the Pt nanoparticles
deposited on YSZ and c-Al2O3 were carried out at atmo-
spheric pressure in a continuous flow U-shaped quartz
reactor. Approximately 50 mg of catalyst was placed into
the reactor. The reactive mixture was composed of toluene
vapour (Alfa Aesar, 99.8 %), O2 (Air Liquid, 99.995 %
purity) and He (Air Liquid, 99.995 % purity) as a carrier
gas. In order to adjust the VOC concentration, two He
streams were employed: the first one was introduced into a
regulated saturator to maintain a constant flow rate of tol-
uene vapour and the other one balanced the overall con-
centration. Further details of this setup are described by
Benard et al. [2]. The reactant and product gases were
analyzed by an on-line gas chromatograph (Perkin Elmer
Clarus 500) equipped with a thermal conductivity detector
(TCD) and several columns (Shin Carbon ST, Q PLOT, 2
OV101 and molecular sieve) to quantify toluene, O2, CO2
and CO. The detection limit was 2 ppmv for carbon dioxide
and no other product was detected. From the outlet con-
centrations of toluene ([C7H8]OUT) and CO2 ([CO2]OUT),
and the inlet concentration of toluene ([C2H7]IN), the tol-
uene conversion, catalytic rate (r) and the turnover fre-
quency (TOF) was calculated with the respective
equations:
2 Experimental
2.1 Preparation and Characterization of Pt
Nanoparticles
Platinum nanoparticles deposited on YSZ and c-Al2O3
were synthesized using a modified polyol reduction method
[10, 11]. First, Pt nanoparticles in the form of colloids were
synthesized in ethylene glycol (anhydrous 99.8 %, Sigma
Aldrich) at 160 °C, starting from PtCl4 (Alfa Aesar,
99.9 % metals basis) precursor salt using a sodium
hydroxide (EM Science, ACS grade) concentration of
0.08 M as described previously [12]. The resulting dark
brown colloidal solution was deposited on 8 mol % Y2O3-
stabilized ZrO2 (YSZ) (Tosoh, specific surface area
*13 m2ꢀg-1), and c-Al2O3 (Alfa-Aesar, specific surface
area 120 m2ꢀg-1) supports. The supported nanoparticles
were extensively washed with deionized water
(18 MXꢀcm) and separated by centrifugation (YSZ), then
dried in air at 60 °C.
C7H8 Conversion ð%Þ ¼ ½CO2ꢁOUT
=
À
Á
ð2Þ
X-ray diffraction (XRD, Rigaku Ultima IV diffractometer
7 ꢀ ½C7H8ꢁOUT þ½CO2ꢁOUT ꢂ 100%
using Cu Ka source) in the focused beam geometry with a
ꢀ
ꢁ
1mol 273K
ꢂ
divergence slit of 2/3 degree,
a scan speed of
r ¼ Total Gas Flow ðL=sÞ ꢂ
0.17 degreeꢀmin-1 and a scan step of 0.06 degrees between
30 and 50° 2h, was also used to determine the crystallite size
of the colloidal Pt nanoparticles for comparison with TEM
results. The supported Pt nanoparticles were characterized
by TEM (JEOL 2010 LaB6) using a replica technique [13].
The use of Image J software allowed for the determination of
Pt particles size distribution through measuring at least 200
nanoparticles from the TEM images. Preparation of these
TEM samples is described in details previously [12].
Dispersion of platinum was determined for the sup-
ported Pt nanoparticles using the H2-pulse chemisorption
technique [14]. The resulting values were compared with
dispersion calculations from the TEM images using the
following equation [15],
22:4L 298K
½C7H8ꢁIN
C7H8 Conversion ð%Þ
ꢂ
ꢂ
ð3Þ
ð4Þ
106ppm
100%
TOF ¼ r=NG
where NG is the moles of active metal sites (mol) (calcu-
lated from the dispersion values determined from Eq. 1).
A PC with a dedicated program controlled the valve
injection system, data acquisition and integration of chro-
matographic peaks. Blank experiments have shown that the
quartz reactor was catalytically inert at least up to 600 °C.
The gas compositions were controlled by mass flow con-
trollers (Brooks, 5850 TR Series) and were the following:
300 ppm C7H8 with approximately 20 % O2 and He
123

Low temperature toluene oxidation
balance. The overall flow rate was held constant at
4 Lꢀh-1, resulting in a space velocity of approximately
16,000 h-1. A K-type thermocouple was attached to the
reactor at the catalyst bed in order to monitor the tem-
perature during the heat-up period. The entire reactor was
placed in a cylindrical furnace, which was attached to a
temperature controller (Eurotherm). The sample was
heated from 25 to 250 °C at a rate of 2 °Cꢀmin-1. The
temperature was held constant at 250 °C for 5 min and
then decreased to room temperature. Three heating cycles
were performed for each catalyst to observe the stability of
the catalyst.
3 Results and Discussion
TEM characterization was carried out for the supported Pt
nanoparticles. Figure 1 shows representative TEM micro-
graphs and corresponding size distributions for the Pt
nanoparticles supported on YSZ (0.8 % metal loading) and
c-Al2O3 (0.7 % metal loading). The TEM images show
that the Pt nanoparticles are homogeneously dispersed.
From these images, an average Pt particle size was found
to be 2.96 and 2.34 nm, for Pt/YSZ and Pt/c-Al2O3,
respectively. This is comparable to the average particle
size determined from XRD for the corresponding colloid,
which suggests that the modified polyol reduction method
is effective in achieving finely dispersed metallic particles.
The Pt dispersion value of the Pt/c-Al2O3 catalyst, mea-
sured by H2-pulse chemisorption, is similar to that esti-
mated from TEM images (Eq. 1 and Table 1). This
confirms the good homogeneity of the sample. However,
on YSZ, the Pt dispersion measured by H2-pulse chemi-
sorption is much smaller than that estimated from TEM.
This statistic determination of the mean diameter is not
accurate in that case, probably due to the low specific
surface area of the sample. A few large Pt particles are
most likely present on the sample; therefore, suggesting the
TEM images are not representative of the entire Pt/YSZ
sample.
Catalytic oxidation of toluene was used as a model
reaction, representing aromatic hydrocarbons, in order to
evaluate the performance of the Pt nano-catalysts. Figure 2
shows the toluene conversion as a function of the tem-
perature achieved from the Pt/YSZ2 catalyst for three
cycles. Toluene conversion starts from around 100 °C and
reaches 100 % at 180 °C. The performance of each cata-
lyst is relatively stable after the first cycle as the results
during the second and third cycle are similar. This slight
deactivation of the catalyst between the first and second
cycle can be attributed to sintering of the Pt nanoparticles
since they are heated on-stream for the first time to
250 °C.
123

H. A. E. Dole et al.
Fig. 1 TEM images (left) and
corresponding histograms
(right) of Pt nanoparticles
prepared with 0.08 M NaOH:
a supported on YSZ,
(a)
30
25
20
15
10
5
b supported on c-Al2O3
0
Particle Size (nm)
(b)
25
20
15
10
5
0
Particle Size (nm)
100
80
60
40
20
0
1st Cycle
2nd Cycle
3nd Cycle
1.1%
0.8%
0.4%
100
80
60
40
20
0
50
75
100
125
150
175
200
50
75
100
125
150
175
200
225
Temperature (oC)
Temperature (oC)
Fig. 2 Toluene conversion as a function of temperature for Pt/YSZ2
(metal loading: 0.8 wt%) for three consecutive cycles
Fig. 3 Effect of metal loading on catalytic activity of Pt/YSZ
catalysts (second cycle is shown)
Figure 3 evaluates the effect of metal loading of Pt
nanoparticles deposited on YSZ. It is evident that metal
loading has a slight effect on catalytic performance with,
overall, a higher loading corresponding to higher catalytic
performance at low temperatures (i.e., lower than 140 °C).
A similar trend was shown in a previous study where 1.5,
0.9 and 0.56 % Pt was deposited on Al2O3 was evaluated
for toluene and propene oxidation [16]. This trend was
explained that with higher metal loading, lower dispersion
is apparent, meaning that the particle size is relatively lar-
ger. With larger particles, the Pt–O bond strength decreases,
causing more reactive oxygen species adsorbed on the Pt
sites to react with the adsorbed toluene species. However, in
this study, different metal loadings on YSZ did not show a
significant difference in dispersion or average particle size
which could explain the slight effect observed.
123

Low temperature toluene oxidation
The effect of the support on the catalytic activity in
terms of toluene conversion, turnover frequency, and
intrinsic activity of the Pt nanoparticles deposited on YSZ
(metal loading of 0.8 and 0.4 %) and c-Al2O3 (metal
loading of 0.7 %) are shown in Fig. 4. It is evident that for
similar metal loadings, the catalytic activity of the Pt
nanoparticles supported on YSZ is slightly higher or
comparable to those supported on c-Al2O3. This trend is
more obvious when the catalytic rate is normalized for the
active surface area (i.e., turnover frequency); whereby, Pt/
YSZ had a lower active surface area compared to Pt/c-
Al2O3 resulting in higher TOF values with increasing
temperature. More specifically, the TOF values at both 120
and 140 °C are shown in Table 1. Overall, for both tem-
peratures, the TOF value is higher for Pt/YSZ compared to
Pt/c-Al2O3. Additionally, the same trend was observed
when the catalytic rate was normalized for mass of Pt
where the metal loading and amount of catalyst employed
for each test was accounted for.
(a)
(b)
(c)
100
80
60
40
20
0
Table 1 summarizes the performances of the Pt/YSZ
and Pt/c-Al2O3 catalysts along with other similar catalysts
evaluated in the literature for toluene oxidation [2, 8, 17–
19]. Each catalyst from literature, containing similar Pt
loadings and reaction conditions, was compared to those
evaluated in the current study in terms of T20 and T50 (i.e.,
temperature at which 20 and 50 % conversion occur,
respectively). In general, the Pt/YSZ catalyst appears to
have conversion at a lower temperature compared to the
other findings. However, Masui et al. [8] found for a much
higher Pt loading (5 wt%) that toluene conversion was
achieved at slightly lower temperatures. Similarly, Santos
et al. [18] found conversion at lower temperatures for a
similar Pt loading but using a different synthesis method
and TiO2 as a support, as well as a high concentration of
toluene, i.e. 4 g carbon/m3 instead of 1.09 g carbon/m3 in
our study.
90
120
150
180
210
Temperature (oC)
35
30
25
20
15
10
5
(a)
(b)
(c)
It is apparent that the catalytic performance of the Pt/
YSZ catalyst, for a similar loading, is similar in terms of
light-off temperature, to that of the Pt/c-Al2O3 catalyst.
This trend is also shown through the activation energy for
toluene conversions of less than 20 % for each catalyst that
was determined using the following Arrhenius relationship.
0
90
120
150
180
210
Temperature (oC)
(a)
(b)
(c)
ln r ¼ ꢃEa=ðRTÞ þ ln A
ð5Þ
70
60
50
40
30
20
10
0
where r is the toluene oxidation rate (molꢀs-1), Ea is the
apparent activation energy (Jꢀmol-1), R is the universal gas
constant (Jꢀmol-1ꢀK-1), T is the reactor temperature (K),
and A is the pre-exponential factor. Plots of ln r versus 1/T
were constructed, and the activation energies were calcu-
lated for each catalyst from the slope of a linear fit. Table 1
shows that Pt/YSZ, for a similar loading, has a similar
activation energy to that of Pt/c-Al2O3. Radic et al. [20]
have interpreted the toluene oxidation on Pt using Mars van
Krevelen (MVK) or Eley–Rideal (E–R) mechanisms. The
MVK model has been widely reported for hydrocarbons
oxidation [21, 22]. In both cases (MVK and E–R), the
reaction rate is controlled by two steps: the oxygen
chemisorption on Pt and the surface reaction between
oxidized hydrocarbons intermediate and adsorbed oxygen.
90
120
150
180
210
Temperature (oC)
Fig. 4 Comparison of catalytic performance of Pt supported on
a YSZ2 (0.8 wt%); b YSZ3 (0.4 wt%); and c c-Al2O3 (0.7 wt %)
(second cycle is shown)
123

H. A. E. Dole et al.
Radic et al. [20] have calculated that the activation energy
value for the surface reaction is around 73 kJꢀmol-1 and is
not affected by the Pt particle size. On the other hand, the
activation energy value for oxygen chemisorptions
decreases from 122 kJꢀmol-1 to 108 kJꢀmol-1 when the Pt
particle size increases from 1 to 15 nm. These latter values
are in good agreement with the apparent activation values
found in the current study, i.e., 107–137 kJꢀmol-1, con-
sidering Pt particles of 2.5 0.5 nm. Therefore, it is
suggested that the rate-determining step of the toluene
oxidation, in the current experimental conditions, is the
oxygen chemisorption on Pt. This indicates that more
reactive oxygen species may exist on Pt/YSZ compared to
that on Pt/c-Al2O3 which could explain the slightly higher
catalytic activity of toluene oxidation.
on the YSZ support compared to Pt on c-Al2O3 (2.34 nm)
starting from the same precursor colloids. Despite the
lower dispersion, Pt/YSZ showed a slightly higher reaction
rate per, both, active surface area and mass of Pt for toluene
oxidation. This is attributed to stronger MSI between Pt
and YSZ due to the ionic conductivity of YSZ and presence
of oxygen vacancies suggesting that YSZ is a promising
alternative support for toluene oxidation.
Acknowledgments The authors acknowledge the financial support
from Natural Science and Engineering Research Council (NSERC).
Dr. N. De Silva at the University of Ottawa for ICP measurements
and the microscopy service of IRCELYON.
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