Probing the Role of a Non-Thermal Plasma (NTP) in the Hybrid NTP Catalytic Oxidation of Methane

Article abstract of DOI:10.1002/anie.201703550

Three recurring hypotheses are often used to explain the effect of non-thermal plasmas (NTPs) on NTP catalytic hybrid reactions; namely, modification or heating of the catalyst or creation of new reaction pathways by plasma-produced species. NTP-assisted methane (CH₄) oxidation over Pd/Al₂O₃ was investigated by direct monitoring of the X-ray absorption fine structure of the catalyst, coupled with end-of-pipe mass spectrometry. This in situ study revealed that the catalyst did not undergo any significant structural changes under NTP conditions. However, the NTP did lead to an increase in the temperature of the Pd nanoparticles; although this temperature rise was insufficient to activate the thermal CH₄ oxidation reaction. The contribution of a lower activation barrier alternative reaction pathway involving the formation of CH₃(g) from electron impact reactions is proposed.

Full text of DOI:10.1002/anie.201703550

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Probing the role of a non-thermal plasma (NTP) in the hybrid
NTP-catalytic oxidation of CH4
Authors: E Gibson, C Stere, B Curran-McAteer, W Jones, G Cibin, D
Gianolio, A Goguet, P Wells, R Catlow, P Collier, P Hinde,
and Christopher Hardacre
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703550
Angew. Chem. 10.1002/ange.201703550
Link to VoR: http://dx.doi.org/10.1002/anie.201703550
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COMMUNICATION
Probing the role of a non-thermal plasma (NTP) in the hybrid NTP-
catalytic oxidation of CH4
[
a,b]
[c]
[d]
[a,e]
Emma K Gibson,*
Cristina E Stere, Bronagh Curran-McAteer, Wilm Jones,
Giannantonio
[
f]
[f]
[d]
[a,f,g]
[a,b]
Cibin, Diego Gianolio, Alexandre Goguet,* Peter P. Wells,
C. Richard A. Catlow,
Paul
[
h]
[h]
[c]
Collier, Peter Hinde, Christopher Hardacre*
0
[1]
Abstract: Three recurring hypothesis are often used to explain the
effect of non-thermal plasma (NTP) on NTP-catalytic hybrid
reactions, namely, modification or heating of the catalyst or the
ability of plasma produced species to open up new reaction
pathways. By directly monitoring the catalyst by X-ray absorption
fine structure (XAFS) coupled with end of pipe mass spectrometry
PdO-like phase to Pd . Unfortunately, of all the catalysts
currently reported, none are sufficiently active under cold start
conditions, with most catalysts requiring light-off temperatures of
[
2]
around 400°C. Such high temperatures are required due to the
high activation barriers for the dehydrogenation of CH to form
surface adsorbed CH * and H* which is thought to be the rate
determining step. For example, Jørgensen et al. predicted that
the extraction of the first H from CH , had an activation barriers
of 0.99 and 0.79 eV over Pd(111) and Pd(100), respectively.
One known method of inducing catalytic activity for kinetically
restricted reactions at low temperatures is the coupling of non-
thermal plasmas (NTPs) with catalysis. Recent examples are the
4
3
4 2 3
the NTP assisted CH oxidation over Pd/Al O was investigated. This
in situ study has shown that the catalyst did not undergo any
significant structural changes under NTP conditions. However, the
NTP did lead to the Pd nanoparticle temperature increasing but this
4
[
3]
temperature rise was insufficient to activate the thermal CH
4
oxidation reaction. The contribution of a lower activation barrier
alternative reaction pathway involving the formation of CH3(g) via
electron impact reactions is proposed.
selective catalytic reduction of NO
shift without the need of an external heating source. Similarly,
the NTP assisted CH oxidation has been reported at both low
x
, VOC removal and water gas
4
temperature, where no additional heat source is applied, and at
elevated temperatures, where the catalyst is also heated to
temperatures up to 300 °C.
Methane is a major contributor to climate change, with a global
warming potential at least 21 times higher than that of CO
2
,
[4]
consequently, its release into the atmosphere must be
stringently controlled. In addition to controlling the release from
landfill, biomass burning and from leakage from natural gas
storage and distribution, emission abatement due to methane
slip in automotive vehicles must also be addressed.
Three recurring hypotheses are often proposed to explain
the assistance which NTP’s give to catalytic reactions. Firstly,
that the plasma modifies the catalyst, secondly, that the plasma
heats the catalyst, and thirdly, that the assistance of the plasma
permits new reaction pathways to occur. The NTP has been
shown, in some cases, to alter the catalysts itself by, changing
One solution is to use catalytic total oxidation to produce
2
CO and water. Palladium is a known efficient catalyst for
[
5]
[6,7]
methane oxidation, and has been studied extensively with
varying hypotheses of the active phase being reported, from a
the oxidation state or metal surface area
of the components.
Several attempts have been made to determine the temperature
of a catalyst during NTP reactions, using thermocouples placed
near or in the catalyst bed, via infrared cameras or optical
[
8]
[
a]
Dr. E. K. Gibson, Dr P. P. Wells
emission spectroscopy. However, no study has yet directly
measured the temperature of the metal nanoparticles within the
catalyst and compared this with the overall bed temperature
during a NTP assisted catalytic reaction. The interaction of
radicals, electrons or photons produced by the NTP with the
catalyst and the adsorbed molecules may open up new reaction
pathways, for instance the direct reaction of gas phase radicals
with adsorbed species, i.e. a direct Eley-Rideal mechanism
UK Catalysis Hub, Research Complex at Harwell, Rutherford
Appleton Laboratory, Harwell Oxon, Didcot, OX11 0FA, UK
E-mail: emma.gibson@rc-harwell.ac.uk, a.goguet@qub.ac.uk,
c.hardacre@manchester.ac.uk
[
[
b]
c]
Dr. E. K. Gibson
Department of Chemistry, University College London
20 Gordon Street, London, WC1 0AJ, UK
Dr. C. E Stere, Prof. C. Hardacre
School of Chemical Engineering & Analytical Science
University of Manchester, The Mill (C56), Sackville Street,
Manchester, M13 9PL, UK
[
9]
could occur. Desorption from the catalyst surface may also be
[
10]
aided by electron impact.
To investigate which of these
[
d]
Miss B. Curran-McAteer, Prof. A. Goguet
School of Chemistry and Chemical Engineering
Queen’s University Belfast
hypotheses are operating under NTP assisted catalysis, in situ
investigations are crucial. Whilst very few in situ combined
[
11]
Belfast BT9 5AG, N. Ireland, U.K
plasma catalysis studies have been performed,
one recent
[
[
[
[
e]
f]
Mr W. Jones
example is the investigation of the hydrocarbon selective
Cardiff Catalysis Institute, School of Chemistry, Cardiff University,
Cardiff, CF10 3AT
Dr. G. Cibin, Dr. D. Gianolio, Dr P. P. Wells
Diamond Light Source, Harwell Science and Innovation Campus,
Chilton, Didcot, OX11 0DE, UK
[12]
catalytic reduction (HC-SCR) of NO
x
over Ag/Al
2 3
O .
This
investigation used a modified DRIFTS set up to follow the
adsorbates during the thermal and the NTP enhanced reactions;
providing invaluable information on adsorbed species and the
mechanism of the reaction. However, to the best of our
knowledge, no in situ structural studies have been undertaken to
characterize the catalyst during NTP assisted reactions. In the
g]
h]
Dr P. P. Wells
School of Chemistry, University of Southampton, Southampton,
SO17 1BJ, UK
Dr. P. Collier, Dr. P. Hinde
Johnson Matthey Technology Centre, Reading, UK
Supporting information for this article is given via a link at the end of
the document. Open access data is available via the University of
Manchester Pure Research Portal
study reported herein, we have probed the NTP assisted CH
oxidation, in the absence of any applied external heating, using
in situ XAFS spectroscopy. The results provided significant new
4
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insights into the role of plasma induced heating effects in the
NTP assisted process.
the absence of plasma, under CH
4
oxidation conditions. Spectra
collected at two positions, 2.5 mm and 7.5 mm from the start of
the catalyst bed are shown in Figure 3 and the fitting parameters
are shown in Table 1. In both cases the oscillations are
dampened when the plasma is on compared to when it is off. No
other differences are observed in the spectra, for example no
shift in phase or additional features are observed which would
indicate changes in distance to nearest neighbours or changes
in the co-ordination around the absorber atom. Furthermore, on
turning off the plasma, no change was found compared with the
fresh catalyst (Figure S4) demonstrating the reversibility of the
changes and indicating that no significant permanent changes to
the nanoparticle structure had occurred. It should be noted that
XAFS is a bulk averaging technique, therefore, we cannot
exclude that some minor non-reversible changes to the Pd
nanoparticles may occur, which are below the detection level of
the technique. However, the EXAFS results are consistent with
transmission electron micrographs of the catalyst before and
after the plasma treatment which showed similar particle sizes
The NTP activated oxidation of CH
sample 1, Table S1) was examined at applied voltages of 6 and
kV at 22.5 kHz. In the absence of an externally applied heating
source, 55% (6 kV) and 67% (7 kV) CH conversions were
observed. In the presence of the catalyst high selectivities were
found (CO:CO , 1:10.8 (6 kV) and 1:11.5 (7 kV)) and negligible
amounts of were formed. In addition, temperature
4 2 3
over a 2% Pd/Al O
(
7
4
2
H
2
a
programmed oxidation measurement of the catalyst following the
o
methane oxidation showed that little carbon oxidation (>600 C)
had occurred (Figure S1). It should be noted that, in the absence
of the catalyst, although the conversion was similar (68% at 7
kV), significantly more CO was formed (CO:CO , 1:1.1). This
2
may also be compared with the reaction over the support alone
in the presence of the plasma which had a reduced conversion
2
of 59% and a CO:CO ratio of 1:2.1. These demonstrate the
importance of the catalyst on determining the selectivity of the
reaction. The specific energy input (SEI) calculated for the 6 and
2
.1 ± 1.0 and 2.9 ± 1.4 nm and no change in the shape of the
-
1
7
2
kV plasma in the presence of the catalyst was 2.133 kJ L and
.637 kJ L , respectively.
nanoparticles observed (Figure S5).
-
1
Similar conversions/selectivities were also obtained during
the XAFS spectroscopy investigation monitored by end of pipe
mass spectrometry analysis (sample 1, 52% conversion at 6 kV,
Figure S2). The setup, shown in Figures 1 and S3, allowed
XAFS measurements along the length of the packed catalyst
bed to be monitored. XAFS measurements were performed at
the Pd K-edge on the B18 beamline at the Diamond Light
Source, UK.
Figure 2. Normalised XANES spectra of the Pd foil reference, PdO reference,
the fresh catalyst (sample 1) under helium, the catalyst during the thermal CH
4
oxidation reaction, during the plasma activated CH
only reaction under plasma conditions.
4
oxidation and during CH
4
Figure 1. Setup for in situ measurements using XAFS spectroscopy coupled
with MS for methane oxidation using plasma.
The X-ray absorption near edge structure (XANES) spectra
of the fresh catalyst (sample 1), under thermal CH
4
oxidation
reaction (352 °C) and under CH oxidation using the plasma are
4
very similar as shown in Figure 2. On first inspection, there is
little impact of the plasma on the catalyst, with the spectra
closely resembling that of PdO. When oxygen is removed from
4
the system under plasma, leaving just 5000 ppm CH and He
balance, the catalyst is reduced, as shown by a shift in the edge
position of 2 eV and the spectra are very similar to that of the Pd
foil reference. Analysis of the extended X-ray absorption fine
structure (EXAFS) region, reveals only very subtle differences in
the spectra when comparing the plasma activated case and in
2
Figure 3. k (k) data of CH
(
4
oxidation at the start (position 2.5) and middle
position 7.5) of the catalyst (sample 1) bed, under plasma on and plasma off
conditions and the CH only experiment under plasma on and off conditions.
4
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Table 1. EXAFS Fitting Parameters for Pd K-Edge spectra taken under CH
4
oxidation and CH
4
only conditions for sample 1.
[
c]
2
2
Conditions
Absorber-
Scatterer
E
0
(eV)
CN
R
eff (Å)


Temperature
(plasma on (°C))
(plasma on data)
0.0043(5)
0.0099(7)
0.0095(9)
0.0038(4)
0.0088(6)
0.0092(8)
0.0097(7)
0.0158(9)
Pd_O
4.2(6)
4.2(6)
4.9(8)
3.9(2)
4.3(6)
4.7(8)
7.5(5)
2(2)
2.024(6)
3.058(6)
3.445(6)
2.026(5)
3.059(6)
3.449(6)
2.76(9)
0.0025(6)
0.0059(8)
0.0054(9)
0.0022(5)
0.0059(7)
0.0053(8)
0.0057(2)
0.0081(1)
[
a]
CH
4
oxidation, plasma off, front 2.5
Pd_Pd1
Pd_Pd2
Pd_O
-0.7(8)
3.9(2)
203±32
mm)
[
a]
CH
7.5 mm)
4
oxidation, plasma off, middle
Pd_Pd1
Pd_Pd2
Pd_Pd1
Pd_Pd2
152±28
207±32
(
[
b]
4
CH cracking, plasma off, middle
5(1)
(
7.5 mm)
3.8(6)
2
[
[
0
a] Fitting parameters: S = 0.85 as determined by the use of a Pd foil standard; Fit range 3.0 < k < 16.0, 1. < R < 3.5; number of independent points = 20.5.
b] Fitting parameters: 1.2 < R <4, 3.4 < k < 10.6, number of independent points = 12.6
We propose that this subtle dampening of the oscillations
is due to an increase in the temperature of the palladium under
the application of the plasma, with the mean squared thermal
provided a reactor temperature of ~210 °C; which is consistent
with the exothermicity of the CH oxidation.
4
2
These estimated temperatures of the catalyst bed are
significantly higher than those measured using an IR camera,
disorder parameter ( ) increasing and consequent decreasing
the amplitude of the EXAFS oscillations. Similarly weak
oscillations were also observed for a range of catalyst loadings
and particle sizes (samples 1-4, Table S1) under plasma
conditions. As no other measurable changes occur to the
o
Figure 4 (60-90 C), which is a technique commonly used to
[
8]
determine plasma reaction temperatures. This is not surprising
as the IR camera predominantly measures the outside wall of
the reactor and will significantly underestimate the temperature
within the packed bed.
2
catalyst, the change in  can be used to estimate the
temperature of the Pd nanoparticles on the catalyst. A data set
was obtained on cooling the catalyst from 500 °C to room
temperature under air. This showed no structural changes on
cooling and was used to determine a calibration curve of the
2
variation of  with temperature.
The calibration curve and fitting parameters are shown in
the supplementary information, Figures S6-S9 and Tables S2
and S3. This data was then used to determine the temperature
of the catalysts when activated by the plasma. To fit the data
upon cooling, the EXAFS fit was performed allowing the
2
coordination numbers (CN),  values and distances to refine.
The determined CN’s were then fixed when fitting the plasma
2
activated spectrum, allowing only  and distances to refine.
This value was then used to estimate the temperature of the
catalyst when activated by the plasma (Table S4). For all the
Figure 4. Temperature profile along the catalyst bed (x direction) using an
infrared camera, where x are arbitrary units, dependant on the camera focus.
catalysts studied the estimated temperatures ranged from 138 to
o
1
79 C in the middle of the bed. In addition, measurements were
made for sample 1 comparing the front and middle of the bed
and the temperatures were determined as 203 ± 32°C and 152 ±
To determine if the observed heating of the catalyst was
due to the exothermicity of the CH
by the plasma, the reaction was performed in the absence of O
Under these conditions the reaction of methane is endothermic
4
oxidation reaction or induced
2
8 °C, respectively. The fitting data (Table 1) and the data and fit
are shown in Figure S7. The higher temperature at the front of
the bed is expected as the CH oxidation is exothermic and the
2
.
4
[13]
and results in coupling products.
During the endothermic non-
rate will be highest towards the start of the bed of catalyst. Using
Aspen software (Aspen Technology), a simulation of the reaction
using the same reactants concentrations and assuming
thermodynamic equilibrium (i.e. full methane conversion)
oxidative CH coupling, the EXAFS data, shown in Figure 2,
4
when the plasma is both on and off resembles that of the Pd foil,
therefore, the PdO catalyst has been, unsurprisingly, reduced on
2
removal of oxygen from the feed gas. Using the value of 
calculated from this data, the estimated temperature during the
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plasma was 207 ± 32 °C (Table 1). From these results we
conclude that the plasma is responsible for the observed heating
effect.
transition point where methane activation becomes rate limiting,
it is likely that the plasma activation of methane in the gas phase
then leads to a reduced activation barrier for the surface process
and thus the ability of the NTP-process to occur at much
reduced temperatures. It cannot be discounted that the Pd
nanoparticles may become more defective in the presence of
The fact that there is no significant nanoparticle size or
catalyst loading dependent changes on the temperatures
calculated from the XAFS may suggest that the surface of the
whole catalyst (nanoparticle and oxide) is being heated. The
XAFS data only probes the Pd and this, however, does not
preclude the alumina surface from also being heated. In this
case the support and nanoparticle would be in thermal
equilibrium, therefore, leading to similar changes in temperature
for all the catalysts studied.
the plasma and more open faces have been reported to offer a
[3]
lower activation barrier for CH
4
dehydrogenation.
However,
this effect is likely to be small compared with the pre-activation
of the CH in the gas phase under plasma conditions.
4
Acknowledgements
Taking account of all the data acquired, the estimated
temperature of the nanoparticles during NTP activated CH
oxidation is 162 ± 24 °C, within the error of the calculated
temperature from the exothermicity of the methane oxidation
4
The authors acknowledge the Diamond Light Source for
provision of beamtime (SP12986-1 and SP10306-9). The UK
Catalysis Hub is kindly thanked for resources and support
provided via our membership of the UK Catalysis Hub
Consortium and funded by EPSRC (Portfolio Grants
EP/K014706/2, EP/K014668/1, EP/K014854/1, EP/K014714/1
and EP/I019693/1). Open access data can be found via the
University of Manchester research portal. We also thank Eleanor
Dann and Rahmin Gholami for their help.
reaction. Therefore,
a clear difference in temperature is
observed between the EXAFS estimation and that measured via
the infrared camera. Almost a two-fold increase in temperature
of the nanoparticles is measured compared to the overall
temperature of the catalyst bed. However, this temperature
(
162 °C) is not high enough to activate the thermal reaction, as
observed from the light-off curve of the thermal reaction for
sample 1, shown in Figure 5.
Keywords: non-thermal plasma • EXAFS spectroscopy •
100
oxidation• heterogeneous catalysis • methane
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
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Figure 5. Light-off profile 5000 ppm CH , 5% O , 5% Ar and He balance.
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1
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Entry for the Table of Contents
COMMUNICATION
Author(s), Corresponding Author(s)*
Emma K Gibson,* Cristina E Stere,
Bronagh Curran-McAteer, Wilm Jones,
Giannantonio Cibin, Diego Gianolio,
Alexandre Goguet,* Peter P. Wells, C.
Richard A. Catlow, Paul Collier, Peter
Hinde, Christopher Hardacre*
Text for Table of Contents
CH
4
combustion has been undertaken using a hybrid non-thermal plasma-Pd based
Title
catalyst system without heating the catalyst bed. Using in-situ X-ray absorption
spectroscopy no significant structural changes were found in the catalyst. Although,
the plasma was found to heat the Pd nanoparticles, this was not sufficient energy to
activate the reaction leading to a proposed alternative reaction pathway.
Probing the role of a non-thermal
plasma (NTP) in the hybrid NTP-
catalytic oxidation of CH
4
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