Towards understanding the improved stability of palladium supported on TS-1 for catalytic combustion

Article abstract of DOI:10.1039/c6cp00319b

A novel Pd supported on TS-1 combustion catalyst was synthesized and tested in methane combustion under very lean and under highly humid conditions (<1%). A notable increase in hydrothermal stability was observed over 1900 h time-on-stream experiments, where an almost constant, steady state activity obtaining 90% methane conversion was achieved below 500 °C. Surface oxygen mobility and coverage plays a major role in the activity and stability of the lean methane combustion in the presence of large excess of water vapour. We identified water adsorption and in turn the hydrophobicity of the catalyst support as the major factor influencing the long term stability of combustion catalysts. While Pd/Al₂O₃ catalyst shows a higher turn-over frequency than that of Pd/TS-1 catalyst, the situation reversed after ca. 1900 h on stream. Two linear regions, with different activation energies in the Arrhenius plot for the equilibrium Pd/TS-1 catalyst, were observed. The conclusions were supported by catalyst characterization using H₂-chemisorption, TPD, XPS analyses as well as N₂-adsorption-desorption, XRD, SEM, TEM. The hydrophobicity and competitive adsorption of water with oxygen is suggested to influence oxygen surface coverage and in turn the apparent activation energy for the oxidation reaction.

Full text of DOI:10.1039/c6cp00319b

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Towards understanding the improved stability of
palladium supported on TS-1 for catalytic
combustion†
Cite this:DOI: 10.1039/c6cp00319b
Adi Setiawan,ab Jarrod Friggieri,a Hadi Hosseiniamoli,a Eric M. Kennedy,a
Bogdan Z. Dlugogorski,c Adesoji A. Adesinad and Michael Stockenhuber*a
A novel Pd supported on TS-1 combustion catalyst was synthesized and tested in methane combustion
under very lean and under highly humid conditions (o1%). A notable increase in hydrothermal stability
was observed over 1900 h time-on-stream experiments, where an almost constant, steady state activity
obtaining 90% methane conversion was achieved below 500 1C. Surface oxygen mobility and coverage
plays a major role in the activity and stability of the lean methane combustion in the presence of large
excess of water vapour. We identified water adsorption and in turn the hydrophobicity of the catalyst
support as the major factor influencing the long term stability of combustion catalysts. While Pd/Al2O3
catalyst shows a higher turn-over frequency than that of Pd/TS-1 catalyst, the situation reversed after
ca. 1900 h on stream. Two linear regions, with different activation energies in the Arrhenius plot for the
equilibrium Pd/TS-1 catalyst, were observed. The conclusions were supported by catalyst characterization
using H2-chemisorption, TPD, XPS analyses as well as N2-adsorption–desorption, XRD, SEM, TEM. The
hydrophobicity and competitive adsorption of water with oxygen is suggested to influence oxygen
surface coverage and in turn the apparent activation energy for the oxidation reaction.
Received 15th January 2016,
Accepted 10th March 2016
DOI: 10.1039/c6cp00319b
www.rsc.org/pccp
an irreversible deactivation can be induced by the presence of
water where the active site (palladium oxide) transforms into
1. Introduction
Catalytic combustion of methane is a potential solution to mitigate a less active site (palladium hydroxide).3,4,7 Ciuparu et al.,
methane emissions as it can operate outside the flammability suggested that the hydroxyl groups produced by the methane
range and production of NOx is essentially absent due to its combustion reaction are bound strongly on the surface8 and
relatively low reaction temperatures. Supported Pd catalysts are accumulate on the support.9 Therefore, when external water
considered to be excellent catalysts for this reaction.1 Poisoning is introduced the surface becomes saturated and the rate of
by water and other contaminants can have long-term deleterious desorption is decreased.8
effects, and significantly reduce the potential for their use in
catalytic combustion of fugitive methane emission from coal nature and redox activity, the TS-1 zeotype is known as a more
mines ventilation air.2 hydrophobic material compared to other zeolites.10 Due to their
In addition to its well defined, crystalline microporous
Earlier studies reported that water significantly inhibits microporous and well defined pore structure, zeolites often are
3,4
the activity of Pd on Al2O3 and Pd on zeolites5,6 at lower used to support redox functionalities because of their ability to
temperatures, which is due to competition between water and stabilise small clusters. The titanium silicalite-1 (TS-1) materials
methane for adsorption on catalyst active sites.3,4,7 Furthermore, initially prepared by Taramasso et al. were effectively used as
a catalyst for the selective oxidation of alkenes and alcohol
substrates using hydrogen peroxide.11–13 Using photoassisted
deposition (PAD) method, nanosize Pd particles were successfully
a Priority Research Centre for Energy (PRCfE), Discipline of Chemical Engineering,
School of Engineering, The University of Newcastle, Callaghan, NSW 2308,
deposited on TS-1 and tested for direct synthesis of H2O2 from H2
and O2.14 Very recently, the preparation of Au–Pd nano-particles
supported on hierarchical TS-1 catalysts was evaluated for selec-
tive oxidation of benzyl alcohol using in situ generated hydrogen
peroxide.15 It was found that the use of TS-1 as catalyst support
improved metal dispersion.15 It is generally agreed that the
Australia. E-mail: Michael.Stockenhuber@newcastle.edu.au;
Fax: +61 2 4921 6893; Tel: +61 2 4985 4433
b Mechanical Engineering Department, Faculty of Engineering,
Universitas Malikussaleh, Bukit Indah, Lhokseumawe, 24352, Indonesia
c School of Engineering and Information Technology, Murdoch University, Murdoch,
WA 6150, Australia
d ATODATECH LLC, Brentwood, CA 94513, USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp00319b dispersion of the noble metal is one of the key factors for
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improving the activity of palladium catalysts for methane the active particle size and metal dispersion by performing
oxidation.1 These properties are thought to improve the long hydrogen chemisorption at 35 1C, in the pressure range
term stability of a combustion catalyst and thus we investigated between 30 and 90 mbar (vacuum measurement by Pfeiffer
TS-1 as a support material for palladium catalysts under long- CMR 362-TPG 261, data logging by LabVIEW version 7.1.1).
term reaction conditions. To our knowledge, for catalytic A H/Pd = 1/1 stoichiometry was assumed for H2 chemisorption
combustion of lean methane mixtures, palladium supported on metallic palladium.18 Mean particle diameter was estimated
on TS-1 has not been previously reported.
based on dispersion values using the following formula:19
Attempts to improve the tolerance against water poisoning
were undertaken by supporting palladium on composite materials
6 ꢁ Ca ꢁ M ꢁ 109
dp ¼
r ꢁ D ꢁ Na
16
such as LaMnO3–ZrO2 and TiO2–SiO2.17 As reported in the
literature,17 mixing titanium and silica oxides avoids or delays
the reaction between Pd and H2O. A hydrothermal stability test
was reported recently by Liu et al.4 using Ni-modified alumina
as support at a reaction temperature of 600 1C. Our recent work
on a long term stability test (1150 h) over Pd/Al2O3 catalyst
disclosed that the level of 90% methane conversion is achievable
at temperatures r500 1C in the presence of water in the feed.2
For the current work, we prepared a new palladium catalyst
using titanium silicalite (TS-1) zeolite as support for methane
combustion to gain a better understanding of the underlying
principles that govern catalyst stability. The resistance of this
catalyst was evaluated during catalytic combustion of lean
methane in air over an extended period of time. The long term
stability test was performed under conditions where the level of
methane conversion was maintained at or above 90% at a gas
hourly space velocity of 100 000 hꢀ1. The physical and chemical
properties of Pd/TS-1 catalyst were investigated in order to
understand the advantages of this catalyst in comparison with
conventional palladium on alumina catalysts.
where Ca is concentration of surface metal atom (equal to
1.27 ꢁ 1019 atoms mꢀ2),19 M is Pd atomic mass, r is Pd
volumetric mass (equal to 12.02 ꢁ 106 g mꢀ3),19 D is metal
dispersion and Na is Avogadro number. Temperature-programmed
desorption (TPD) analysis was undertaken using a purpose built
TPD apparatus with a Pfeiffer Prisma quadrupole mass analyser for
detection. Zeiss Sigma VP FESEM served to capture the scanning
electron microscopy (SEM) images of the sample using a secondary
electron (SE) detector and backscattered electron (BE) detector.
A JEOL 2200 FS transmission electronic microscope (TEM) with
EDS and a scanning transmission electron microscope (STEM)
system was used for imaging nano-sized Pd particles and support
as well as measuring the particle size distribution. Palladium
loading was quantified using a Varian 715-ES inductively coupled
plasma optical emission spectrometer (ICP-OES). Powder X-ray
diffraction patterns of catalysts were examined using Cu Ka
radiation with a Philips X’Pert diffractometer. Diffractograms
were collected in the 2y angle range from 21 to 901 with 0.0081
2y step resolution.
For surface analysis, X-ray photoelectron spectroscopy (XPS)
was conducted using monochromated Al K alpha (energy
1486.68 eV) radiation and the emitted photoelectrons were
analyzed using an ESCALAB250Xi manufactured by Thermo
Scientific, UK. To minimize the possibility of sample contami-
nation, spent samples were immediately transferred into sealed
containers after stopping the reaction by cooling in helium.
The sample was mounted on the stub using indium tape.
The energy scale of the XPS spectra was referenced to the
adventitious carbon adsorption at 284.6 eV. The spectro-
2. Experimental
2.1 Catalyst preparation
Catalyst containing 1.4 wt% Pd/TS-1 was prepared by wet
impregnation of a commercial titanium silicalite (TS-1) support
(ENI-Polimeri Europa SpA) with an aliquot of Pd(II) nitrate
solution (10 wt% in 10 wt% nitric acid, Sigma-Aldrich) mixed
using a mortar and pestle. Water was added drop-wise while
mixing, until a paste was formed. The resulting paste was dried
in an oven at 110 1C for approximately 20 h. The dried catalyst
was then ground, pressed and sieved to 250–425 mm. A tubular
fixed-bed reactor was employed for the calcination of the dried
solid catalyst in air at 500 1C for 2 h followed by purging in
helium while slowly cooling the catalyst to desired reaction
temperature. For comparison, a commercially available 1.0 wt%
Pd/Al2O3 (Sigma-Aldrich) was used and activated in air at 500 1C
for 4 h followed by purging in H2 for 2 h.2 In contrast to our
previous investigation Pd/TS-1 catalyst was activated in air only.
Thus, it is expected Pd2+ is the only species observed over the
used sample.
meter was calibrated against binding energies of Au 4f7/2
83.96 eV, Ag 3d5/2 = 368.21 eV and Cu 2p3/2 = 932.62 eV.
=
2.3 Catalytic activity assessment
The activity of the catalyst was measured in a tubular, stainless
steel micro reactor. The inlet mixture was 7000 ppm CH4,
3.2 vol% H2O balanced with air at a gas hourly space velocity
(GHSV) of 100 000 hꢀ1. The inlet and outlet gas mixtures were
analyzed using an online gas chromatograph equipped with a
thermal conductivity detector (TCD) and concentric-packed
single column (Alltech CTR-I). The reactant mixture was passed
through a saturator and a humidity probe was installed at the
outlet. The reaction temperature was measured with a K-type
thermocouple placed into the catalyst bed.
2.2 Catalyst characterization
The surface areas of the catalysts were measured by nitrogen
For catalytic stability studies, a second, separate reactor
adsorption at 77 K using a Gemini 11 2370 surface area set-up was used and operated continuously under varying reaction
analyzer. A volumetric glass apparatus was employed to assess conditions (temperature was increased in order to achieve a 90%
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level of conversion for methane). Throughout the study, the feed
concentration of methane and carbon dioxide were kept constant
at 7000 ppm and 10 000 ppm, respectively. The saturator system
was operated at ambient temperature (25 1C ꢂ 3 1C) resulting in a
relative humidity of ca. 85% (corresponding to a H2O(v) pressure of
between 30 000–40 000 ppm). This humidity is well within the
range of a typical coal mine as reported in the literature.20 The
targeted 90% methane conversion was maintained by increasing
the catalyst bed temperature as the catalyst deactivated during
time on stream up to 1900 h.
Kinetic measurements were carried out over the fresh and
long-term used catalysts using a wet feed containing 7000 ppm
CH4, 3.2% H2O balanced with air. Carbon dioxide was not
added to the feed since the reaction rate is independent of the
partial pressure of CO2.21 The turn-over frequency (TOF),
defined as the moles of methane converted per second and
per mole of active site was calculated under conditions where
the methane conversion level was below 15%. The number of
active sites on the catalyst was estimated based on palladium
dispersion measurements. Pd dispersion was also calculated
based on the average Pd particle size measured by TEM analysis
(for fresh and used catalysts) using the formula mentioned above.
Fig. 1 TPD spectra of CO2 (m/z = 44) desorbed from Pd/TS-1 catalysts.
CO2 was produced under vacuum on the surface of catalysts subsequent
to CH4 in situ adsorption at 110 1C in presence of water. Heating ramp =
5 1C minꢀ1
.
active sites measured by TEM analysis. Fig. 2 shows the Arrhenius
plots for Pd/TS-1 catalyst. As a comparison, the activity of Pd/Al2O3
catalyst was tested under similar conditions and also plotted in
Fig. 2. Table 1 summarises the kinetic data of both catalysts.
Comparison of the Arrhenius plots shows that over palladium
supported on TS-1 catalysts a lower rate per active site compared
to Pd/Al2O3 catalyst was observed. Note that a TS-1 sample
(without Pd loading) was also tested under similar condition as
described previously and conversion of methane was not
detected at temperatures as high as 510 1C confirming that
TS-1 zeolite itself is not an active catalyst for methane oxidation
under our reaction conditions. The deposited palladium particles
on TS-1 are responsible for converting methane to carbon dioxide.
In the presence of water in the feed, the turn-over frequencies
of fresh Pd/TS-1 and Pd/Al2O3 catalysts at reaction temperature of
312 1C were 0.007 sꢀ1 and 0.015 sꢀ1, respectively. Estimation
of the apparent activation energy resulted in 247 kJ molꢀ1
for Pd/TS-1 compared to 159 kJ molꢀ1 for Pd/Al2O3 catalyst.
3. Results and discussion
3.1 Catalyst activity and stability under wet feed conditions
In addition to catalytic activity measurements in the micro-
reactor, the surface reactions involved in methane combustion
over the Pd/TS-1 were studied using temperature programmed
desorption (TPD). The sample was heated under vacuum at
500 1C for 1 h to remove any pre-adsorbed compounds. At
110 1C, methane (99.99%) was adsorbed. Subsequently, the
products were desorbed and the gas phase was analysed.
During the desorption experiments, a significant quantity of
CO2 was produced, suggesting methane is being oxidised by
lattice oxygen of the catalyst. This result suggests that the
surface oxygen participates in the methane oxidation reaction,
following a Mars–van Krevelen mechanism under these condi-
tions.22,23 Fig. 1 shows TPD profiles of CO2 (m/z = 44) desorbed
from Pd/TS-1 catalyst. TPD spectra indicate that a significant
rate of desorption of CO2 was detected at temperatures ranging
from 270 1C to 400 1C with a maximum desorption rate at
395 1C. It is expected that in the plug-flow reactor study,
methane oxidation can utilize lattice oxygen from the catalyst
over this temperature range.
The catalytic performance of Pd/TS-1 catalyst was measured
for total oxidation of methane in the presence of water vapour
in the feed. For initial activation, the catalyst was calcined in air
at 500 1C for 2 h then cooled to 200 1C in a flow of helium.
A feed of 7000 ppm methane and 3.2 vol% H2O in air was
passed through the catalyst bed with a GHSV of 100 000 hꢀ1
.
The experiment was started by gradually increasing the furnace
temperature. The performance of the catalyst was evaluated in
terms of turn-over frequencies in the range of 2–11% methane
conversion level, and it has been normalized by the number of
Fig. 2 The Arrhenius plots of Pd/TS-1 and Pd/Al2O3 catalysts under wet
feed conditions. The reactions were run under feed conditions consisting
of 7000 ppm CH4, 3.2 vol% H2O in air at GHSV = 100 000 hꢀ1
.
= Pd/TS-1;
= Pd/Al2O3.
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Table 1 Kinetic data for Pd/TS-1 and Pd/Al2O3 catalysts
partially offsetting the effect of the increased availability of
oxygen. A change in activation energy was attributed to the
stability of PdO under varying conditions which goes hand in
hand with a change in the reaction order of water in the kinetic
expression.23 This behaviour is consistent with our observations
of the importance of oxygen coverage on the activity.
Samples
Ea (kJ molꢀ1
)
TOF (sꢀ1
)
Pd/TS-1, fresh
Pd/Al2O3, fresh
Pd/TS-1, used
Pd/Al2O3, used
247
159
0.007a
0.015a
0.066b
0.120b
269 (E1) and 48 (E2)
128
a
b
To assess the usefulness of the catalytic system for a practical
application, evaluation of catalyst stability of the Pd/TS-1 was
performed using a separate reactor configuration which was
operated continuously under varying reaction temperatures in
TOF was calculated at temperature of 312 1C. TOF was calculated at
temperature of 365 1C.
The activation energy of the conversion over the fresh Pd/Al2O3 order to maintain a 90% level of methane conversion. The feed
catalyst is consistent with the data reported in the literature21,23 composition for these long term stability tests was adjusted,
where under humid conditions (feed containing 2–2.5% H2O) the mimicking the ventilation air composition typically emitted
apparent activation energies were in the range of 150–166 kJ molꢀ1
.
from a coal mine (7000 ppm CH4; 10 000 ppm CO2; 30 000 to
The apparent activation energies reported are similar for alumina 40 000 ppm H2O balance air). Fig. 3 shows the catalyst bed
and zirconia supported catalysts and in turn little influence of reactor temperature of Pd/TS-1 over 1900 h.
metal support interactions are expected for our alumina supported
catalyst as well.
It has been confirmed previously that the water vapour
present in the feed would be the primary factor for catalyst
The reaction rate measurement reported in the literature was deactivation in addition to the water vapour produced from
also not affected by external diffusion and the close comparison reaction.2 As plotted in Fig. 3, the initial temperature to achieve
with our activation energies suggests the absence of diffusion as 90% conversion (T90) was 430 1C and the bed temperature was
well. Indeed, our own experimental diffusion test did not show then slowly increased to maintain the methane conversion
a significant influence of flow rates on conversion. Changing level. The bed temperature appears to plateau starting from
the flow rate and keeping space velocity constant resulted in a 160 h until 1900 h time-on-stream. Hydroxyl group accumula-
variation of the conversion from 6.4% to 6.9% (within the tion on the active site, carbon deposits and the increase in
experimental error) when the flow rate was changed from reaction temperature resulted in a slowly alteration of the
80 ml minꢀ1 to 120 ml minꢀ1 under a steady state conversion oxygen surface coverage. This argument is supported by our
at a GHSV of 100 000 hꢀ1. Internal diffusion for the TS-1 catalyst TPD data as discussed below. To evaluate the performance of
is possible, but the Thiele modulus was significantly smaller the catalyst under conditions relevant to the combustion of
than 1 (0.11) clearly suggesting (together with the lack of ventilation air methane under practically relevant conditions,
external diffusion) the absence of Knudsen diffusion.24 Pore data over Pd/TS-1 catalyst is compared with data that was
diffusion was checked by altering the pellet size of 250–425ꢀm1m and reported earlier over Pd supported on alumina catalysts2 and
425–500 mm which resulted in a rate of 4.6 ꢁ 10ꢀ3 mol s gcat
plotted in Fig. 3. It can be observed that the practical benchmark
ꢀ1
ꢀ1
(308 1C) and 4.9 ꢁ 10ꢀ3 mol sꢀ1 gcat (310 1C), respectively. of 90% conversion can be maintained at lower temperatures over
This indicates no observable effect of pore diffusion on the TS-1 supported compared to alumina supported Pd catalysts.
activity of the catalyst.
The Arrhenius plots of Pd/TS-1 and Pd/Al2O3 measured
The activation barrier over the Pd/TS-1 and Pd/Al2O3 catalysts following long-term time on stream experiments are plotted
is significantly high under conditions when the combustion gas in Fig. 4. Our estimation of the rate of reaction over used Pd/TS-1
was saturated with water vapour. A possible explanation for this and Pd/Al2O3 catalysts at 365 1C results in the TOF of 0.066 sꢀ1
behaviour can be related to the availability of labile oxygen for the
combustion. As indicated above, the catalytic combustion can
utilise stored surface oxygen and thus a Mars–van Krevelen type
mechanism will be relevant in the temperature window studied.
Clear evidence can be found in Fig. 1, where the reaction with
methane and the absence of any oxidant results in the formation
of CO2, suggesting the reaction of methane occurs with stored
surface oxygen. Evidence for the existence of mobile, stored
oxygen can also be observed in oxygen desorption shown in
Fig. 5. The ‘‘effective’’ surface coverage of oxygen is expected to
be higher than at lower temperatures. Competitive adsorption
between O2 and H2O is expected to have an effect, which would
result in lower oxygen surface coverage over the Pd/Al2O3
catalyst, in turn resulting in a smaller rate constant. However,
increasing the reaction temperature will result in a reduced
relative coverage of water/O2 (heat of adsorption of water on
supported Pd catalysts is lower than that of oxygen25,26) only
Fig. 3 Catalyst bed temperature required for 90% CH4 conversion over
Pd/TS-1 and Pd/Al2O3 catalysts. Feed: 7000 ppm CH4, 10 000 ppm CO2,
30 000–40 000 ppm H2O balance air, GHSV 100 000 hꢀ1
.
= TOS
experiment over Pd/Al2O3;
= TOS experiment over Pd/TS-1.
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and 0.120 sꢀ1, respectively. It is observed that over the used
Pd/TS-1 catalyst the slope of the line (and in turn the apparent
activation energy) changes at ca. 360 1C, while over the used
Pd/Al2O3 catalyst no change in the apparent activation energy was
observed. At reaction temperatures below 360 1C, the Arrhenius
plot of Pd/TS-1 suggests the activation energy is 369 kJ molꢀ1
.
Above this reaction temperature, the activation energy decreases
to 48 kJ molꢀ1. In principle, this behaviour can be due to
(i) diffusion effects, (ii) change in the mechanism and (iii) due
to coverage dependency of the apparent activation enthalpy.27 As
mentioned above, external diffusion can be excluded since no
variation of the conversion was observed upon changing the
flow rate. It is also suggested that internal diffusion for the TS-1
catalyst is negligible. Furthermore, the reduction in apparent
activation energy is significantly larger than 50%, also suggesting
the absence of internal diffusion.28
Fig. 5 TPD spectra of oxygen (m/z = 32) desorbed from Pd/Al2O3 and
Pd/TS-1 catalysts. Oxygen was co-adsorbed with H2O at 300 1C. Heating
ramp = 5 1C minꢀ1
.
= Pd/Al2O3;
= Pd/TS-1.
The changes in activation energy in methane combustion
over supported noble metal catalysts have been reported in the
literature.24,28,29 It was found that at lower temperatures,
higher activation energies were observed which was suggested
to be related to the heat of adsorption of oxygen on precious metal.
The apparent higher activation energy at lower temperatures may
be expressed as the sum of activation energies for oxidation of
palladium and for methane oxidation by the resulting palladium
oxide.28 As shown in Fig. 5, oxygen TPD from Pd on TS-1 (after
co-adsorption with water at 300 1C) discloses a desorption peak of
O2 at ca. 480 1C whereas this peak was not observed over Pd on
alumina subject to the same conditions. This suggests that at a
reaction temperature of 365 1C, most of the labile oxygen will have
desorbed reducing the surface coverage of oxygen. This in turn (as
suggested in ref. 27) results in a decreasing apparent activation
energy, as was observed in Fig. 4. The Arrhenius plot of the used
Pd/TS-1 catalyst also suggests that the water molecules on the
surface of Pd/TS-1 catalyst start to desorb at temperature of 360 1C
resulting in an increase in rate of reaction. No accumulation of
hydroxyl is expected at this temperature which is supported by
TPD spectra as plotted in Fig. 6 and 7. On the other hand, TPD
spectra of water desorbed from Pd/Al2O3 catalyst at temperatures
o500 1C shows a very high intensity of water adsorbed on the
surface of catalyst which persistently inhibits the reaction.
3.2 Characterization of catalysts
The surface properties of Pd/TS-1 in comparison with Pd/Al2O3
catalysts were investigated utilizing TPD of oxygen. Prior to
analysis, the sample was pre-treated under vacuum at 500 1C for
1 h at a heating ramp of 5 1C minꢀ1. Oxygen was co-adsorbed
with H2O at 300 1C and desorbed at 550 1C. Fig. 5 shows the
oxygen TPD spectra (m/z = 32) of Pd/TS-1. In the presence of
water, Fig. 5 shows that maximum rate of O2 desorption over
Pd/TS-1 was observed at 480 1C whereas in contrast over
Pd/Al2O3 no peak was observed at this temperature range. These
observations can be interpreted that the oxygen adsorption
capacity is higher on the Pd/TS-1 catalyst. Similar phenomena
were reported in the literature where high dispersion of the
palladium on the support resulted in a higher oxygen mobility.30
As observed from TEM and chemisorption results, the Pd
dispersion is larger on Pd/TS-1 compared to Pd/Al2O3 catalyst.
The support has also been suggested to influence on the
ability of palladium to adsorb oxygen and there was a strong
Fig. 4 The Arrhenius plots of Pd/TS-1 and Pd/Al2O3 catalysts measure
after long-term stability tests (used catalysts). The reactions were run
under wet feed conditions consisting of 7000 ppm CH4, 3.2 vol% H2O in
Fig. 6 TPD curves of water desorption from Pd/TS-1 and Pd/Al2O3
catalysts. H2O was adsorbed at 110 1C. Heating ramp = 5 1C minꢀ1
.
air at GHSV = 100 000 hꢀ1
.
= used Pd/TS-1 and
= used Pd/Al2O3.
= Pd/TS-1;
= Pd/Al2O3.
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water at 110 1C and heating to 350 1C for 1 h, then cooling
to room temperature. TPD analysis started from room tem-
perature increasing to a temperature of 850 1C. Prior to these
experiments, both catalysts were heated under vacuum at
500 1C for 1 h. The spectra of H2O desorbed from both catalysts
are plotted in Fig. 7. This plot discloses a significant desorption
peak at 450 1C over Pd/Al2O3 catalyst highlighting a stronger
adsorption state of water on the surface. The opposite behaviour
is observed from Pd/TS-1 catalyst, where no significant peak was
detected at temperature below 500 1C.
The complexity of water adsorption–desorption for MFI-zeolitic
adsorbents has been reported in the literature, especially for TS-1,
ZSM-5 and silicalite-1 (pure silica).10 It was found that among
these zeolites, TS-1 is the most demonstrably hydrophobic
material. It was reported that for hydrocarbons adsorption,
Fig. 7 TPD curves of water desorption from Pd/TS-1 and Pd/Al2O3
catalysts after heating at 350 1C for 1 h. H2O adsorption at 110 1C. Heating
ramp = 5 1C minꢀ1
.
= Pd/TS-1;
= Pd/Al2O3.
correlation between the oxygen adsorption capacity of precious TS-1 exhibited behaviour similar to that of ZSM-5 zeolite,
metal and its catalytic activity.28,31
however for water adsorption the intensity of TS-1 was as low
The effect of water vapour on palladium supported on TS-1 as the rate of water desorbed from pure silica.10 This argument
catalyst was investigated using TPD analysis. Water was selected is in line with what has been observed in TPD curves plotted in
as the adsorbate with the purpose of investigating the inter- Fig. 6 and 7. TS-1 has very low concentration of adsorbed water
actions between water, active sites and the support material. The and in turn helps to maintain the stability of Pd active sites
intensity of water (m/z = 18) desorbed from the support material during reaction. The hydrophobicity of TS-1 was suggested due
(pure TS-1) and Pd/TS-1 catalyst are provided in Fig. S1 of ESI.† to the absence of surface acidity (strong acid and hydrogen
Prior to adsorption, both samples were pre-heated for 1 h at bonding), instead of an increased dispersion interaction.35
500 1C with a heating rate of 5 1C minꢀ1 to remove any pre- More recent investigation also reported that the hydrophobic
adsorbed compounds. Water was adsorbed at 110 1C to avoid properties of TS-1 is due to the titanium atoms in the tetrahedral
any weakly bound adsorbate retained on the sample. Fig. S5 of position of the MFI structure which create the hydrophobic
ESI† suggests that the TPD spectra of Pd/TS-1 are identical to environment.10
what was observed in water desorption from the unmodified
The adsorption and desorption of nitrogen was performed
TS-1 support material. There are high and low temperature on pure TS-1 support, Pd/TS-1 and Pd/Al2O3 catalysts in order to
desorption peaks observed in both samples spectra. A low measure the surface area and the pore sizes. The adsorption–
intensity peak detected at 240 1C indicates that there is a small desorption isotherm of Pd/TS-1 and Pd/Al2O3 catalysts are
number of water molecules adsorbed on the catalyst surface. The plotted in Fig. S2 of the ESI.† The textural properties of all
higher temperature peaks appearing at 510 1C and 660 1C are samples tested are provided in Table 2.
attributed to desorption of surface hydroxyl groups resulting in
In general, the isotherm adsorption plots of our samples
partial destruction of the lattice suggested in the literature.32–34 fall into type-IV as classified by IUPAC which is typically the
Fig. 6 shows the comparison of TPD spectra of water desorbed behaviour of mesoporous materials.36 Interestingly, the desorption
from Pd/Al2O3 and Pd/TS-1 catalysts. The TPD spectra of Pd/Al2O3 curves of Pd/TS-1 catalyst (Fig. S2a of ESI†) is shifted slightly to a
suggest the presence of multiple desorption peaks at temperature higher volume over the pressure range while Pd/Al2O3 catalyst
below 500 1C and a higher desorption state at 610 1C. The area desorption plot (Fig. S2b of ESI†) is shifted only at a relative
of the low temperature water desorption peak from Pd/Al2O3 is pressure higher than 0.7. As reported in the literature, these shifts
much bigger compared to the higher temperature peak (ratio = indicate enhanced adsorption over the range of relative pressures,
0.73/0.27). Our data suggests that Pd/Al2O3 catalyst adsorbs more which suggests the coexistence of micro and mesopores within
water at lower temperature, while Pd/TS-1 catalyst shows signifi- these catalysts.37 As shown in Table 2, the average mesopore
cantly reduced adsorption capacity for water at lower temperatures, diameter calculated using Barrett–Joyner–Halenda (BJH) method
which are relevant for the catalytic methane combustion.
results in 5.4 nm and 16.8 nm for Pd/TS-1 and Pd/Al2O3 catalysts,
Further investigation of the H2O-TPD phenomena were respectively. Not surprisingly the surface area of Pd/TS-1 catalyst
carried-out over Pd/TS-1 and Pd/Al2O3 catalysts by adsorbing is much higher compared to Pd supported on alumina catalyst,
Table 2 N2-physisorption calculation results
Sample
Parameter
TS-1
Pd/TS-1
Used Pd/TS-1
Pd/Al2O3
Used Pd/Al2O3
Langmuir surface area (m2 gꢀ1
BJH average pore diameter (Å)
)
641.9
30.7
0.07
492.8
54.1
0.11
476
23.6
0.06
212.9
167.6
0.66
146.5
158.7
0.4
BJH adsorption pore volume (cm3 gꢀ1
)
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due to the microporous nature of TS-1. The surface area estimated
by Langmuir method discloses a decrease from 641.9 m2 gꢀ1 to
492.8 m2 gꢀ1 in the presence the Pd nano-particles on TS-1 which
suggests the deposition of some Pd in the pores of the TS-1
support.
Nitrogen physisorption analysis over the used Pd/TS-1 catalyst
suggests that the surface area decreased only ca. 3% after 1900 h
time-on-stream experiment. In contrast, a significant loss in
surface area was observed over the used Pd/Al2O3 catalyst where
the surface area decreased from 212.9 m2 gꢀ1 to 146.5 m2 gꢀ1
(ca. 31% drop) after 1100 h TOS experiment.
The crystallinity of palladium supported on TS-1 was inves-
tigated using powder X-ray diffraction (XRD) analysis. Fig. S3 of
ESI† shows XRD patterns for Pd/TS-1 catalyst in comparison
to XRD pattern of pure TS-1 support material. Diffraction
reflections of both samples confirm the presence of MFI-type
zeolite structure and high crystallinity of these samples.10,15
A very small, broad reflection at 34.81 2y observed in the XRD
pattern of calcined Pd/TS-1 catalyst suggesting the existence of
palladium oxide (0 1 2) reflection on the TS-1 support. This is
consistent with a low net metal loading (from ICP-OES result)
Fig. 8 SEM images of (a) TS-1 zeolite; (b) Pd/TS-1; and (c) Pd/Al2O3
catalyst.
and indicates the well-dispersed nature of palladium particles This minor change is most-likely related to the transformation
on TS-1 zeolite (from H2-chemisorption result). The low intensity of palladium particle. This argument is further substantiated
(high FWHM) of PdO reflections does not enable an estimation of by our finding from TEM analysis. It is suggested that the
the palladium particle size. An estimation of the active particle formation and removal of hydroxide compounds from the
size calculated from the results of H2-chemisorption at 35 1C surface of catalyst can potentially accelerate the sintering
shows that average particle diameter is 7.6 nm resulting in 14.8% process,38 either sintering of the catalyst or support material.39
dispersion. A comparable result was obtained from hydrogen The surface diffusion of the single phase solid is significantly
chemisorption of Pd/Al2O3 catalyst which is suggesting a palladium increased at temperatures near to the Hu¨ttig temperature
dispersion of 14.2% with an average particle size of 7.9 nm.
(0.2–0.3 ꢁ melting point).38 Assuming the melting point of
Exploring the possibility of a morphological transformation palladium is 1552 1C, the Hu¨ttig temperature is then in the
during catalyst preparation and methane oxidation was carried range of 310 1C to 466 1C. In Fig. 3 we can see that after 150 h,
out using scanning electron microscopy (SEM). Fig. 8 shows the bed temperatures were increased from 410 1C to 430 1C
SEM images of TS-1 zeolite and Pd/TS-1 catalysts at the same which is inside the Hu¨ttig temperature range, and thus, a surface
magnifications. The topography of fresh TS-1 zeolite was captured diffusion of palladium nano-particle would be expected.
by a secondary electron (SE) detector and shown in Fig. S4 of ESI.†
Palladium particle size and distribution of Pd/TS-1 catalyst
Using a backscattered electron (BE) detector, the images of TS-1 was observed under transmission electron microscope (TEM)
support material and Pd/TS-1 catalyst were captured as shown in before and after long-term stability test. Fig. 9a shows the
Fig. 8a and b. Comparing both images, SEM micrograph of fresh representative TEM micrograph of fresh catalyst, including
Pd/TS-1 catalyst reveals very small, brighter particles distributed particle size distribution plot and the representative image of
over the whole area of TS-1 surface which are interpreted as signal the used catalyst is provided in Fig. 9b. According to our
from palladium particles. In this image, the palladium particle is statistical analysis, the size of palladium particle distributed
recognized as the brighter particle due to its higher atomic weight on the surface of the fresh catalyst ranged from 1 nm to 11 nm
(see the arrows). This has been confirmed by energy dispersive with the average size of 4 nm. An increase in particle size was
spectroscopy (EDS) which was performed during analysis. A less observed over the used catalyst, where the average particle size
bright particle distribution is observed from Pd/Al2O3 SEM image of palladium was 8 nm. This increase is most likely related to
(see Fig. 8c). These images suggest that the nature of TS-1 support the deactivation observed over 1900 h time-on-stream experiment.
helps in establishing a better palladium particle distribution. This A similar phenomena was reported over Pd/Al2O3 catalyst, where
is in agreement to what has been reported in the literature that the particle size of palladium increased after long-term stability
the morphology and microstructure of nano-sized Pd particle is test under wet feed conditions.2 In Fig. 9c and d, the representative
affected by the nature of its support.4
TEM images and particle distribution of fresh and used Pd/Al2O3
The SEM images of fresh and used Pd/TS-1 catalysts are catalysts are shown. The average Pd particle size of Pd/Al2O3
compared in Fig. S5 of ESI.† It can be seen that no significant catalyst increased from 2.6 nm to 8.2 nm after high humidity,
measurable change in morphology was observed under SEM. long time-on-stream catalyst evaluation.
This is consistent with N2-physisorption results where only
XPS spectra of fresh and used Pd/TS-1 catalyst are displayed
3% of surface area decreased during 1900 h TOS experiment. in Fig. 10. The binding energies (BE) and surface composition
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Fig. 9 TEM images and particle size distribution of fresh and used catalysts: (a) fresh Pd/TS-1; (b) used Pd/TS-1; (c) fresh Pd/Al2O3 and (d) used Pd/Al2O3.
are provided in Table 3. Prior to analysis, the fresh sample was the long term (1900 h) stability tests as plotted in Fig. 3. The
calcined in air at 500 1C for 2 h, while the used sample was from spectra of the fresh catalyst sample (Fig. 10a) reveals a single
distinct peak representing the Pd 3d5/2 core level transitions,
similar to what can be observed from the used sample (Fig. 10b).
No metallic Pd was detected from the surface of the used sample.
The presence of PdO2 at binding energies of 337.2–337.4 eV
has been assigned by comparing with shifts reported in the
literature40–42 with the oxide most likely formed during calcina-
tion of Pd/TS-1 catalyst. The unique formation of Pd4+ on
zeolite supported Pd catalyst has been confirmed by Bi and
Lu who studied Pd/NaZSM-5 calcined at 600 1C where a binding
energy was observed at 337.7 eV.43 On the other hand, they also
observed that upon calcination of Pd/Al2O3, the binding energy
of Pd 3d5/2 was 336.6 eV (which can be identified as Pd2+),
relatively similar to what was found from our Pd/Al2O3 catalysts
(Fig. 10c). XAS investigations are currently underway to unequivo-
cally identify the state of the Pd functionality. This suggests the
formation of Pd4+ on Pd/Al2O3 catalysts is unlikely. Meanwhile,
palladium supported on zeolite can stabilize the presence of PdO2
through the special space structure of the zeotype.43
The binding energy of a peak at Pd 3d5/2 core level of the
time-on-stream used catalyst (Fig. 10b) is shifted 0.2 eV towards
a higher BE compared to the fresh catalyst. However, the shape
of deconvoluted peak at Pd 3d5/2 core level of the used catalyst
Fig. 10 XPS spectra of Pd 3d core level of (a) fresh Pd/TS-1, (b) used
Pd/TS-1 and (c) used Pd/Al2O3 catalysts.
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Table 3 Surface composition and binding energies at Pd 3d core level
References
Surface composition (%)
Pd 3d5/2 peak
1 P. Gelin and M. Primet, Appl. Catal., B, 2002, 39, 1–37.
2 A. Setiawan, J. Friggieri, E. M. Kennedy, B. Z. Dlugogorski and
M. Stockenhuber, Catal. Sci. Technol., 2014, 4, 1793–1802.
3 R. Burch, F. J. Urbano and P. K. Loader, Appl. Catal., A, 1995,
123, 173–184.
Sample
position (eV) Pd
O
Ti Si
Al
C
(a)
(b)
(c)
Pd/TS-1, fresh 337.2
Pd/TS-1, used 337.4
Pd/Al2O3, used 336.3
0.5 64.1 0.3 33.1
0.3 63.6 0.2 32.7
—
—
37.1
2.12
3.12
0.2 59.2
—
—
4 Y. Liu, S. Wang, T. Sun, D. Gao, C. Zhang and S. Wang,
Appl. Catal., B, 2012, 119–120, 321–328.
5 D. Gao, S. Wang, C. Zhang, Z. Yuan and S. Wang,
Chin. J. Catal., 2008, 29, 1221–1225.
6 C. Shi, L. Yang and J. Cai, Fuel, 2007, 86, 106–112.
7 F. H. Ribeiro, M. Chow and R. A. Dallabetta, J. Catal., 1994,
146, 537–544.
8 D. Ciuparu, N. Katsikis and L. Pfefferle, Appl. Catal., A, 2001,
216, 209–215.
9 W. R. Schwartz, D. Ciuparu and L. D. Pfefferle, J. Phys. Chem.
C, 2012, 116, 8587–8593.
10 D. P. Serrano, G. Calleja, J. A. Botas and F. J. Gutierrez,
Sep. Purif. Technol., 2007, 54, 1–9.
11 M. Taramasso, G. Perego and B. Notari, US Pat., US 4410501,
United States, 1983.
is virtually identical to the fresh catalyst peak pattern. As shown
in Table 3, the ratios of Pd to Ti or Pd to Si are similar for used
and fresh catalyst. In addition, comparison of a plot of Ti 2p
core-level of fresh and used Pd/TS-1 spectra (see Fig. S6 and
Table S1 of ESI†) indicates no shift in binding energy. These
suggest no significant changes in chemical/oxidation state of
Pd/TS-1 catalyst following the long term operation under surrogate
VAM gas, and that the Pd phase in Pd/TS-1 catalyst is stable and
less sensitive to water poisoning as no other species was able to be
detected from this spectra. In contrast, Pd on alumina shows
significant variation of the Pd signature2 which can be attributed
to the formation of palladium hydroxide. The argument is inline
with TPD plots (Fig. 6 and 7) as well as the comparison bed furnace
temperatures plotted in Fig. 3.
12 F. Qiu, X. Wang, X. Zhang, H. Liu, S. Liu and K. L. Yeung,
Chem. Eng. J., 2009, 147, 316–322.
4. Conclusion
13 B. Notari, Catal. Today, 1993, 18, 163–172.
14 K. Mori, Y. Miura, S. Shironita and H. Yamashita, Langmuir,
2009, 25, 11180–11187.
15 I. Moreno, N. F. Dummer, J. K. Edwards, M. Alhumaimess,
M. Sankar, R. Sanz, P. Pizarro, D. P. Serrano and G. J.
Hutchings, Catal. Sci. Technol., 2013, 3, 2425–2434.
16 S. Specchia, P. Palmisano, E. Finocchio, M. A. L. Vargas and
G. Busca, Appl. Catal., B, 2009, 92, 285–293.
17 G. D. Carlo, G. Melaet, N. Kruse and L. F. Liotta, Chem.
Commun., 2010, 46, 6317–6319.
18 P. Castellazzi, G. Groppi and P. Forzatti, Appl. Catal., B,
2010, 95, 303–311.
19 J. R. Anderson and K. C. Pratt, Introduction to characterization
and testing of catalysts, Academic Press, Sydney, Orlando, 1985.
20 S. Su, H. Chen, P. Teakle and S. Xue, J. Environ. Manage.,
2008, 86, 44–62.
The catalytic activity and stability of palladium supported on
TS-1 catalyst was investigated in catalytic combustion of a lean
methane/air mixture and compared with Pd/Al2O3 catalyst.
A notable increase in hydrothermal stability was observed over
1900 h time-on-stream experiments, where a constant steady
state activity to obtain 90% conversion was achieved below
500 1C. The Arrhenius plot of Pd-TS-1 catalyst obtained after
long-term stability test indicates two linear regions with different
apparent activation energies and consequently a change in the
oxygen surface coverage dependent on the reaction temperature is
proposed. Catalyst characterization suggests that water adsorption
and in turn the hydrophobicity of the catalyst support plays
important role in enhancing the stability of the catalyst. The data
obtained disclose that the activity of Pd/TS-1 catalyst was hindered
at lower temperature by water present in the feed, however at
temperatures higher than 350 1C water is not present on the
surface resulting in a lower activation energy compared to
Pd/Al2O3. The hydrophobic properties of titanium silicalite
support helps in preventing the accumulation of hydroxyl on
the support and also increase the amount of available oxygen.
21 J. C. van Giezen, F. R. van den Berg, J. L. Kleinen, A. J. van
Dillen and J. W. Geus, Catal. Today, 1999, 47, 287–293.
22 K.-I. Fujimoto, F. H. Ribeiro, M. Avalos-Borja and E. Iglesia,
J. Catal., 1998, 179, 431–442.
23 D. Ciuparu and L. Pfefferle, Appl. Catal., A, 2001, 209,
415–428.
24 D. L. Trimm and C.-W. Lam, Chem. Eng. Sci., 1980, 35,
1405–1413.
25 G. Li, P. Xiao and P. Webley, Langmuir, 2009, 25,
10666–10675.
26 G. D. Zakumbaeva and S. V. Artamonov, React. Kinet. Catal.
Lett., 1979, 10, 183–186.
27 H. Lynggaard, A. Andreasen, C. Stegelmann and P. Stoltze,
Prog. Surf. Sci., 2004, 77, 71–137.
Acknowledgements
We duly acknowledge the financial support from ACARP. A. S.
thanks the Aceh Province Government, Indonesia for their
sponsorship. We thank Jane Hamson for assistance with ICP
analysis and Gizelle Sanchez Combita for her help in TPD
calibration. We are grateful to University of Newcastle for
XRD, SEM and TEM analyses at EM/X-ray unit, and the Australian 28 C. F. Cullis and B. M. Willatt, J. Catal., 1983, 83, 267–285.
Synchrotron for the use of their facilities.
29 T. R. Baldwin and R. Burch, Appl. Catal., 1990, 66, 359–381.
This journal is ©the Owner Societies 2016
Phys. Chem. Chem. Phys.

View Article Online
Paper
PCCP
30 B. Stasinska, A. Machocki, K. Antoniak, M. Rotko, J. L. 37 D. P. Serrano, R. Sanz, P. Pizarro, I. Moreno and S. Medina,
Figueiredo and F. Gonalves, Catal. Today, 2008, 137, 329–334. Appl. Catal., B, 2014, 146, 35–42.
31 D. V. Ivanov, L. G. Pinaeva, L. A. Isupova, E. M. Sadovskaya, 38 D. L. Trimm, in Studies in Surface Science and Catalysis,
I. P. Prosvirin, E. Y. Gerasimov and I. S. Yakovleva,
Appl. Catal., A, 2013, 457, 42–51.
32 J. B. Peri, J. Phys. Chem., 1966, 69, 220–230.
ed. H. B. Calvin and B. B. John, Elsevier, 1991, pp. 29–51.
39 J. H. Lee and D. L. Trimm, Fuel Process. Technol., 1995, 42,
339–359.
33 M. Zamora and A. Cordoba, J. Phys. Chem., 1978, 82, 40 D. Gao, C. Zhang, S. Wang, Z. Yuan and S. Wang, Catal.
584–588. Commun., 2008, 9, 2583–2587.
34 M. Egashira, M. Nakashima, S. Kawasumi and T. Seiyama, 41 A. M. Venezia, G. Di Carlo, G. Pantaleo, L. F. Liotta,
J. Phys. Chem., 1981, 85, 4125–4130. G. Melaet and N. Kruse, Appl. Catal., B, 2009, 88, 430–437.
35 R. S. Drago, S. C. Dias, J. M. McGilvray and A. L. M. L. 42 S. P. Chenakin, G. Melaet, R. Szukiewicz and N. Kruse,
Mateus, J. Phys. Chem. B, 1998, 102, 1508–1514.
J. Catal., 2014, 312, 1–11.
36 K. S. W. Sing, Pure Appl. Chem., 1982, 54, 2201–2218.
43 Y. Bi and G. Lu, Appl. Catal., B, 2003, 41, 279–286.
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