Comparison of conventional versus microwave heating of the platinum catalysed oxidation of carbon monoxide over EUROPT-1 in a novel infrared microreactor cell

Article abstract of DOI:10.1016/j.molcata.2006.12.035

The oxidation of carbon monoxide over a 6.3% Pt/SiO₂ catalyst (EUROPT-1) is studied under conventional and microwave heating and found to display similar activity. The capability of the cell to observe the adsorbed gases spectroscopically is also demonstrated.

Full text of DOI:10.1016/j.molcata.2006.12.035

Journal of Molecular Catalysis A: Chemical 269 (2007) 1–4
Comparison of conventional versus microwave heating of the platinum
catalysed oxidation of carbon monoxide over EUROPT-1
in a novel infrared microreactor cell
Ian Silverwood∗, Gordon McDougall∗∗, Gavin Whittaker
School of Chemistry, University of Edinburgh, Joseph Black Building, West Mains Road, Edinburgh, Scotland, United Kingdom
Received 18 December 2006; accepted 20 December 2006
Available online 4 January 2007
Abstract
The oxidation of carbon monoxide over a 6.3% Pt/SiO2 catalyst (EUROPT-1) is studied under conventional and microwave heating and found
to display similar activity. The capability of the cell to observe the adsorbed gases spectroscopically is also demonstrated.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Microwave; In situ infrared spectroscopy; Carbon monoxide; Platinum
1. Introduction
Thecatalyticoxidationofcarbonmonoxideisofconsiderable
importance in a number of fields. In addition to the elimination
of CO emission in both vehicle exhausts and other combus-
tion sources [13], interest exists in areas such as purification of
gas streams [14] and regeneration of closed-cycle CO2 lasers
[15]. Conventional heating of the reaction is well understood
[16], although reports of the application of microwave heating
are limited to the work by Perry et al. [17–19]. Their research
concerns carbon monoxide oxidation over ␥-alumina supported
Pt and Pd catalysts under conventional and microwave heating.
In contrast to reports relating to other heterogeneous catalytic
reactions [10,20], no rate enhancement was observed, and it
was suggested that the difficulties of thermometry in microwave
fields may result in inaccurate measurements of activity.
Use of a silica supported platinum catalyst for CO oxidation
under microwave heating has not previously been reported and it
is possible that the dielectric properties of the support may cause
dissimilar results. This work was thus undertaken to investigate
the effects of microwave irradiation upon such a catalytic system
using EUROPT-1, a well characterised 6.3% Pt/SiO2 catalyst
[21].
The use of microwaves to heat chemical reactions began
in the 1980s [1] and has attracted interest ever since. Today,
laboratory-scale microwave ovens are increasingly common in
the realm of liquid phase organic synthesis with considerable
enhancements in rate [2] often arising from the utilisation of
this method. This effect is generally agreed to result from the
superheating of solvents which may occur in a microwave
system [3] and a number of reviews on liquid-phase organic
reactions have been presented [4,5].
Microwave ovens may also allow greater efficiency due to the
fundamental differences in the mechanism of heating of materi-
als under microwave irradiation from conventional heating [6]. It
is possible to heat only the target rather than maintaining an oven
or vessel at an elevated temperature and this has led to its use in
the sintering of ceramics [7,8] as well as its more familiar use
in food processing [9]. Heating solid phase and heterogeneous
reactions with microwave radiation has also shown differences
from conventional heating [10–12], although agreement on the
source of these effects has not yet been reached.
∗
Corresponding author (pre-publication). Tel.: +44 131 650 4733;
2. Experimental
f∗a∗x: +44 131 650 6453.
Corresponding author (post-publication). Tel.: +44 131 650 7284;
fax: +44 131 650 6453.
The microreactor cell used for the experiments has been
described in detail previously [22] and consists of a parallel plate
applicator which surrounds a ceramic sample holder. The sample
E-mail addresses: Ian.Silverwood@gmail.com (I. Silverwood),
G.McDougall@ed.ac.uk (G. McDougall).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.12.035

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I. Silverwood et al. / Journal of Molecular Catalysis A: Chemical 269 (2007) 1–4
holderhasagastightsealtotheapplicatorplatesandconnections
for admission of the reactant gases outwith the microwave field.
This allows the atmosphere surrounding the catalyst to be con-
trolled and the cell operated as a flow reactor. Resistive cartridge
heaters are held in the applicator plates to allow conventional
heating.
Self-supporting catalyst wafers were formed by pressing
circa 30 mg of the finely powdered EUROPT-1 catalyst under
10 tonnes in a 13 mm die for 10 s. A 1 mm diameter shielded
thermocouple, passed through the ground applicator plate, was
placed in contact with the sample.
Gas flow to the cell was controlled using four MKS mass flow
controllers; carbon monoxide (99.5%) was supplied by Argo; He
(99.996%), H2 (99.99%) and compressed air were supplied by
BOC. EUROPT-1 was provided by Johnson Matthey.
Mass spectrometry of the exhaust gases was conducted with a
Macrovision Plus quadrapole analyser, with associated vacuum
and sample admission systems, from Spectra. The gas inlet was
via a fused silica capillary inserted into the cell’s outlet gas
connection so that the sampling point was within 5 cm of the
catalyst sample. This arrangement should ensure that the effluent
gases reach the sampling point in less than 0.5 s.
The microwave source used was an ASTeX AX-2100
microwave generator, a variable power 2.45 GHz source allow-
ing remote control and generating up to 1 kW. The source was
connected to a Thurlby Thandar TG215 function generator for
power control and the power was varied sinusoidally between 0
and 200 W with 0.1 Hz frequency.
Catalyst wafers were pretreated by oxidation in 15 sccm air at
circa 423 K for 1 h, followed by reduction under 15 sccm hydro-
gen at the same temperature. The samples were allowed to cool
under hydrogen, then purged under 20 sccm He for 5 min before
the reaction mixture of 5 sccm CO and 15 sccm air, correspond-
ing to a 1:1.3 ratio of CO:O, was passed over the catalyst disc.
The cell and catalyst were then raised to the desired tempera-
ture by conventional heating and the reaction system allowed to
equilibrate.
Under conventional heating alone, IR data was averaged
over a period of 30 s to give improved signal to noise. Where
microwaves were applied in addition to conventional heating,
data was collected with a time resolution of approximately 1 s.
Recording was initiated and after a short period of circa 10–20 s
at the set temperature, the oscillating microwave power was
enabled for 120 s. The infrared spectrometer used for all data
collection was a Bio-Rad FTS-6000 instrument fitted with a
narrow band MCT detector.
Fig. 1. CO2 production (a) and recorded temperature (b) over EUROPT-1 under
microwave heating as a function of time.
A significant jump was seen on enabling the microwave
power, followed by smaller 0.1 Hz oscillations. This larger ini-
tial temperature step is believed to result from overshoot in the
output from the microwave power supply when first switched
on. The production of CO2 increases with time as the microwave
energy heats the sample, with oscillations clearly seen at 0.1 Hz,
corresponding to the frequency at which the microwave energy is
modulated. The CO2 production appears to slightly lag behind
the recorded temperature, possibly due to the self-sustaining
nature of the reaction. Each microwave cycle raises the temper-
ature by approximately 4.5 K and the CO2 conversion by 0.05%.
Over the 120 s irradiation the temperature rises in total by 18 K
and the conversion by 0.2%.
The production of CO2 over the catalyst between 300 and
440 K is presented in Fig. 2. This shows the variation in the par-
tial pressure of mass 44 recorded under both heating regimes.
The data obtained from the conventionally heated experiment is
plotted as a dashed line, and shows the CO oxidation increas-
ing with temperature. Hysteresis is seen on cooling due to
the reaction exotherm sustaining the reaction. The microwave
heated data is shown as a series of solid traces, with the
CO2 level as a function of temperature recorded from the
3. Results
During an irradiation cycle, oscillations in the CO2 produc-
tion, sample temperature and spectral features caused by the
modulation of the microwave power were clearly evident at
almostalltemperatures. TheCO2 pressuretraceforsuchaperiod
is reproduced in Fig. 1 along with the corresponding temperature
recorded. The data corresponds to the introduction of microwave
heating to the reacting system initially at 367 K. At this point,
the CO conversion was approximately 0.7%.
Fig. 2. CO2 production over EUROPT-1 under microwave (a, solid line) and
conventional heating (b, dashed line).

I. Silverwood et al. / Journal of Molecular Catalysis A: Chemical 269 (2007) 1–4
3
Fig. 4. IR spectrum for CO oxidation over EUROPT-1 after 90 s of microwave
heating (a) and under conventional heating over a similar temperature range (b).
Fig. 3. Time resolved IR spectra of CO oxidation of EUROPT-1 under
microwave heating.
4. Discussion
thermocouple in the cell during each 2 min irradiation cycle
for a series of seven experiments over the temperature range
studied.
Even at low conversion, the CO2 production is a sensitive
measure of the effective catalyst temperature as the conver-
sion increases significantly with rising temperature. In studies
of microwave heating of heterogeneous systems, accurate and
meaningful measurement of temperature is always an issue [17].
Here, the similarity of the gradient of each individual segment
representing CO2 production under microwave heating, shown
in Fig. 2, to the corresponding section of the conventionally
heated conversion curve suggest that the thermocouple accu-
rately represents the effective temperature of the bulk catalyst.
Furthermore, the increased CO2 production in all the individual
microwave experiments appears merely that to be expected from
theequivalenttemperaturerampfromconventionalheating. This
suggests that for the EUROPT-1 catalyst, microwave heating is
homogeneousor, ifheterogeneouswiththemetalparticleexperi-
encing disproportionate heating compared with the support, then
the rate of dissipation of heat to the support is sufficiently rapid
that the bulk temperature measured is an accurate representation
of the temperature of the active site for the catalytic reaction.
In the infrared spectra shown in Figs. 3 and 4, the behaviour
of the adsorbed CO features will relate the temperature of the
catalyst metal particles while the band from the desorption of
water will be principally associated with the SiO2 support mak-
ing up the bulk of the catalyst. The extract at 90 s shown in
Fig. 4a shows the consequence of the 15 K rise in temperature
from the initial temperature of 367 K under microwave heat-
ing. Fig. 4b is the corresponding spectrum produced by the
ratio of spectra recorded at 367 and 384 K under purely conven-
tional heating. Like the effect of conventional and microwave
heating on conversion shown in Fig. 2, there is again close cor-
respondence between the changes apparent as a consequence of
the microwave heating and those observed from the equivalent
temperature rise produced by conventional heating. Only in the
region around 2100 cm−1 do the curves differ significantly and
this may be attributed to the low photometric accuracy of the
experiment at this point due to the near extinction of the linear
CO feature.
Infrared spectra from the reacting system are a potentially
more sensitive probe of the state of the catalyst under microwave
heating than the conversion and temperature data. Fig. 3 shows
the infrared spectra corresponding to the data presented in Fig. 1.
The spectra are presented as a greyscale, time-resolved plot with
higher absorbance values represented by the increasingly lighter
shades. The background spectrum for the production of the
absorbance spectra was recorded at the set temperature immedi-
ately prior to the acquisition of the data set and so the spectra are
essentially featureless until the introduction of the microwave
power at 20 s indicating the stability of the reaction system.
The only significant feature evident during the initial 20 s is
∼2100 cm−1. The overlapping absorbance of the gas phase and
linearly adsorbed CO lead to near zero transmission at this point
and so to intense miscancellation features at this wavelength in
the ratioed data. A number of spectral features clearly develop
with time under irradiation and oscillate in intensity in time with
the applied microwave power and the changes in temperature
and conversion noted in Fig. 1. These features are perhaps more
evident in an individual absorbance spectrum extracted from
the dataset and shown in Fig. 4a. This shows a spectrum col-
lected after 90 s under microwave irradiation. The most intense
feature in the region around 2079 cm−1 is due to the linearly
adsorbed CO and shows saturation. The gas phase CO2 peaks
are visible around 2340 cm−1 and, in Fig. 3, increase in inten-
sity with time due to the rising levels of CO oxidation as the
temperature rises. A corresponding decrease in the gas phase
CO can also be seen in the R-branch at 2167 cm−1, although
the large linear adsorbed CO peak obscures the P-branch. The
adjacent positive feature at 2229 cm−1 is caused by the heat-
ing of the gas phase resulting in an increased population of
higher rotational states [22]. The features between 1950 and
1710 cm−1 are due to changes in the bridged CO band [23], and
a strong negative peak due to the desorption of water is seen at
1630 cm−1
.

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I. Silverwood et al. / Journal of Molecular Catalysis A: Chemical 269 (2007) 1–4
These results appear to agree well with the earlier studies
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5. Conclusions
The rate of CO2 production over the EUROPT-1 catalyst and
the nature of the species absorbed on the catalyst surface in the
temperature range 300–400 K appears unaffected by the method
of heating used in as much as the increase in rate and change in
surface composition noted when microwave heating is applied
at a given temperature can be replicated by the equivalent tem-
perature rise induced by conventional means. While reports of
increased activity under microwave heating for other catalytic
systems continue to require further explanation, there appears to
be no enhancement for the CO oxidation reaction over platinum.
Acknowledgements
The authors would like to thank Mr. David Paden for
manufacture of the microreactor cell used, and the University
of Edinburgh for funding Ian Silverwood through a Charles
Kemball Memorial studentship.