Methane partial oxidation by the lattice oxygen of the LaNiO3- perovskite. A pulse

study

Germán Sierra Gallego, Catherine Batiot-Dupeyrat2, Joël Barrault 2 and Fanor Mondragón1

1 Institute of Chemistry, University of Antioquia, A.A. 1226, Medellin, Colombia.

2Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, Université de Poitiers,

Ecole Supérieure d’Ingénieurs de Poitiers, 40, avenue du Recteur Pineau, 86022 Poitiers

Cedex, France.

* Corresponding author: G. Sierra, e-mail: geasierraga@unal.edu.co, Universidad Nacional de Colombia – Escuela de Ingeniería de Materiales, Minas - Sede Medellín. Tel: 57-4-4255286. Address: Carrera 80 # 65-223, Bloque M3-020

Abstract

LaNiO3 perovskite was prepared by the self-combustion method and tested as catalyst for

CH4 activation using the oxygen lattice at 700 °C and 800 °C. Based on the

nonstoichiometry the perovskite formula is written as La+3Ni3+0.368Ni2+

0.632O2.684. When

unreduced LaNiO3- perovskite is used, only -type oxygen species were responsible for the

partial CH4 oxidation.

Over non reduced perovskite high CH4 conversions to H2 and CO were obtained. CH4

conversion and H2, CO and CO2, selectivities under quasi-stationary conditions were 100%,

80%, 98% and 2 % respectively at 800°C. At the beginning of the reaction, a complete

oxidation of methane to CO2 and H2O took place, whereas a partial oxidation to CO and H2

was observed after that period. In such conditions the H2/CO molar ratio obtained was 1.7,

indicating a contribution of the parallel reverse water gas shift reaction.

Keywords: Methane partial oxidation; lattice oxygen; LaNiO3- perovskite; oxygen storage

materials

2

1. Introduction

Syngas is an important feedstock for Fischer–Tropsch and methanol synthesis [1-2], as well

as a precursor of hydrogen production. Most of the syngas is currently obtained from

natural gas in which the main component is methane.

To date, a large-scale industrial route for syngas production comprises methane steam

reforming (MSR), but an alternative technology is based on the catalytic partial oxidation

of methane (POM):

H°298 = -36 kJ/mol

This process has some comparatively important differences with the well established steam

reforming of methane. POM is mildly exothermic and does not require a high operating

pressure and hence is more energy efficient [3-9]. The major drawback is related to the high

costs of oxygen plants, and the use of dense ceramic membranes with mixed oxygen-ionic

and electronic conductivity [10-11].

Many catalysts have been studied for the POM reaction. Most of them include non-noble

(Ni, Co, Fe) and noble (Ru, Pt, Ir, Os, Rh, Pd) transition metals, usually deposited as fine

particles over porous supports [12-13]. The production of synthesis gas by partial

oxidation of CH4 is also possible in using the SOFCs (solid oxide fuel cells).

The oxidation technologies using SOFC-type reactors integrate oxygen separation and

partial oxidation into a single step for methane conversion. Materials such as RuOx–ZrO2

(8% mol Y2O3) composites [14-15], pyrochlore materials [16-17], ceria based materials

3

[18] and perovskites [19-20] have been used as anodes or cathodes in solid oxide fuel cells

(SOFCs).

A particularly attractive option for POM [21-23] and SOFCs 24-28 consists in the use of

mixed metal oxide precursors with a perovskite structure, since these materials have also an

exceptional high thermal stability. The general formula of these oxides is ABO3, in which

the cation A of a larger size is responsible for the thermal resistance of the catalyst whereas

the cation B of smaller size accounts for the catalytic performance. Moreover, these oxides,

due to their redox properties, are particularly attractive for the partial oxidation of methane

[29].

Previous studies have shown that the particular behavior of the ABO3 perovskites as

oxidation catalysts is mainly due to the specific mobility of the lattice oxygen [30-31].

In this work, in the presence of LaNiO3 perovskite the partial oxidation of methane was

studied using only lattice oxygen species instead of gaseous oxygen. It was evidenced that

oxygen from the lattice selectively oxidized methane with production of syngas.

2. Experimental

2.1. Catalyst preparation.

The LaNiO3 perovskite was prepared by the self combustion method [32]. Glycine

(H2NCH2CO2H) used as ignition promoter was added to an aqueous solution of metal

nitrates with the appropriate stoichiometry, in order to get a NO3-/NH2 = 1 ratio. The

resulting solution was slowly evaporated until a vitreous green gel was obtained. The gel

was heated up to 250 °C at which the ignition reaction occured yielding a powdered

precursor which still contained carbon residues. After calcination at 700°C for 8 hours all

of the remaining carbon species were eliminated leading to the formation of the perovskite

structure.

4

2.2. Characterization

The catalysts were characterized before and after the catalytic test by powder X-ray

diffraction (XRD) using a Siemens D-5000 diffractometer with CuK1 = 1.5406 and CuKα2

=1.5439 Ǻ, operated at 40 kV and 40 mA. The diffraction patterns were recorded in the 2

range 10-90° with a step of 0.016° and 4s per step.

Transmission and scanning electron microscopy were carried out on a Philips CM120

instrument, with a LaB6 filament and equipped with an energy dispersive X-ray analyzer

(EDX). The catalyst powder was first dispersed in iso-propanol after grinding in an agate

mortar and thereafter put on Cu grids for TEM observation.

Temperature programmed reduction (TPR) experiments were carried out with a

Micromeritics Autochem 2910 in a flow of 5% hydrogen in argon (50 mL/min), with a

heating rate of 5 °C/min, from room temperature to 900 °C.

The specific surface area was obtained from the adsorption – desorption isotherm of N2

using 30 % N2/Ar as the adsorbate in a Micromeritics Flowsorb II 2300 apparatus at –196

°C. All samples were degassed for 30 minutes at 623 K before measurement

Nonstoichiometric oxygen was experimentally determined by a thermogravimetric method

[33-34], using a 2950 TA instrument microbalance.

In the transient TGA experiments, 15 mg of the perovskite was put on the sample pan. Then

the sample was heated up from room temperature to the desired temperature at a rate of 20

°C/min under a flow of pure O2 at 1 atm. After the sample weighted reaches a steady-state

value, the gas in the furnace was switched to helium.

Pulse experiments using a CH4 flow were carried out in a quartz reactor (i.d. = 6 mm) at

700 °C and 800 °C under atmospheric pressure. Prior to each reaction, 50 mg of the powder

catalyst was loaded into the reactor and preheated at 500 °C under 30 ml/min flow of He/O2

5

(0.1% of oxygen) for 30 min in order to remove residual gases. Then, the reactor was

cooled down to room temperature and the catalyst exposed to CH4 pulses while the

temperature of the reactor was increased from room temperature to 700 °C or 800 °C. The

amount of gas in each pulse was 7.7 mol, and the time interval between pulses varied

from 1 min to 15 min. Between every CH4 pulse, a flow of He/O2 (0.1% of oxygen) used as

carrier gas passed through the catalyst bed. The products of CH4 conversion were analyzed

on-line with a mass spectrometer. The same type experiment was performed using a

reduced perovskite catalyst.

The CH4 and CO2 conversions and the CO and H2 selectivities were defined as follows:

Carbon balance:

The subscripts i and Det respectively mean inlet and outlet respectively.

3. Results and discussion

3.1. Sample characterization

XRD patterns of the perovskite LaNiO3 after calcination and after reduction are shown in

Figure 1.

After calcination at 700 °C only LaNiO3 perovskite structure (Figure 1b) was observed. The

diffraction lines are representative of the LaNiO3 rhombohedral phase (JCPDF Card No.

6

33-0711- Figure 1a). After reduction treatment under hydrogen at 700°C, the LaNiO3

perovskite structure was completely transformed into Ni° and La2O3 (Figure1c), which are

the only phases observed.

3.2. Characterization by electron microscopy

SEM analysis was carried out after the perovskite calcination at 700°C and TEM

characterization was done after the reduction treatment with H2. The micrographs presented

in the Figure 2a shows that the auto combustion method leads to the formation of a porous

LaNiO3 with a sponge appearance. TEM micrographs obtained after reduction of LaNiO3

under hydrogen at 700 °C clearly shows the presence of spherical particles of nickel

(Figure 2b). Based on the TEM micrographs, the average size of metallic particles was

calculated using the following equation:

where ni is the particle number and di is the characteristic diameter of particles [35]. The

calculation gives an average particle size around 15 nm.

3.3. Reductive behavior and perovskite structural formula

The TPR profile of LaNiO3 shown in the Figure 3 indicates that the reduction proceeds in

three steps; the successive changes of the perovskite structure being determined by in-situ

XRD measurements [36].

4LaNiO3 + 2H2 La4Ni3O10 + Ni° + 2H2O

La4Ni3O10 + 3H2 La2NiO4 + 2Ni° + La2O3 + 3H2O

La2NiO4 + H2 Ni° + La2O3 + H2O

7

From these experiments the amount of hydrogen consumed at each step was determined

and compared to the expected value obtained from the stoichiometric formula LaNiO3. The

experimental value was found to be lower than the theoretical one and this difference can

result of a deviation from the oxygen stoichiometry, in agreement to the work of

Wachowski et al. [37]. Thus, using the amount of consumed hydrogen, the formula of the

perovskite structure was recalculated as LaNiO2.68. The TPR also indicates that the

reduction of LaNiO2.68 into La2O3 and metallic Ni° is completed at around 610°C.

In an other set of experiments, a TPR of the perovskite LaNiO2.68 was first performed

leading to Ni°/La2O3, then the solid was re-oxidized in air at 800°C and a second TPR was

carried out. The two TPR profiles were very similar, indicating that the oxidation-reduction

process was reversible. To confirm that suggestion, XRD spectra of re-oxidized samples in

air at 800°C and after the second H2 TPR was recorded (Figure 4b and 4 d). The results

clearly confirmed that the perovskite phase was restored upon the reoxidizing treatment

with an apparently lower symmetry compared to that of the starting material.

3.4. Characterization of oxygen non stoichiometric by thermo-gravimetric analysis

Figure 5 shows the weight changes after modification of the carrier gas (cycles N2 O2) at

800°C as follows: under nitrogen flow (curves a-b and c-d) and under oxygen flow (curves

b-c and d-e). The initial mass loss under the nitrogen flow corresponds to desorption of

water and carbon dioxide. When the carrier gas is switched from oxygen to nitrogen at 800

°C the weight of the LaNiO2.68 sample decreases with time on stream as shown in Figure 5.

Weight of the sample is restored if nitrogen is replaced by oxygen. The rate of the weight

change is slower in the oxygen desorption period (curve c-d), than in the absorption period

(curve d-e).

8

These results mean that a part a lattice oxygen can be removed from the simple at 800°C.

The overall oxygen content and the average Ni oxidation state were determined using the

thermo-gravimetric analysis data. Thus, in the formula LaNiO3− value is estimated to be

0.32 (in our experimental conditions), so that the perovskite formula can be rewritten as

La+3Ni3+0.37Ni2+

0.63O2.68.

This result is quite important because it shows that in the LaNiO3 perovskite about 60 % of

the nickel in the structure correspond to Ni2+ species with the formation of oxygen

vacancies, while about 37 % of the Ni remains as Ni3+.

In several previous publications, it was also reported that the decomposition of LaNiO3 to

produce the structures La2NiO4 or La4Ni3O10 and NiO occurred at elevated temperatures

between 800 and 1100 °C in both air and inert gases [38-39]. To verify if the perovskite

structure was maintained during the nitrogen→oxygen→nitrogen→oxygen cycles at

900°C, an in-situ XRD analysis was performed. It can be observed (Figure 6) that during

the oxygen adsorption and desorption steps the perovskite structure remains unchanged

showing a high stability in these conditions or perhaps a rather low phase transformation

(with a slow kinetics to be observed), which could explain the reversibility of the process as

shown in Figure 5.

3.5. Reactivity of lattice oxygen in the oxidation of methane.

The partial oxidation of methane by pulsed reaction over the LaNiO3- catalysts was studied

at 700 °C and 800 °C. First, a blank experiment was carried out without catalyst under

usual experimental conditions in order to evidence the thermal decomposition of methane.

In all cases the CH4 conversion was lower than 1%. Then, methane pulses were introduced

9

each 15 minutes over the non reduced LaNiO3- perovskite, in order to examine the

reactivity of lattice oxygen (between pulses, carrier gas He/O2 (0.1% O2) was continuously

flowed through the catalyst bed).

At 700°C there was no catalytic activity and therefore the CH4 conversion remained equal

to that observed in the blank test. At 800 °C the results are displayed in the Figure 7. The

only products detected were H2, CO, and CO2, which indicates that methane is effectively

oxidized by the lattice oxygen of the perovskite oxide (the amount of water produced in the

reaction was not quantified). Figure 7 also shows that CO and H2 productions begin almost

at the same time when the CO2 production goes through a maximum before declining

(pulse 18). The decline in the CO2 production suggests a decrease of the concentration of

surface active oxygen species.

After 25 pulses the amount of H2 and CO indicates that the partial oxidation (POM) of

methane is taking place at a significant extent with a very high selectivity. At this point it is

possible that the hydrogen reduces the catalyst so that Ni° particles are available as active

sites for the methane activation. Then it is suggested that carbon species can be formed over

the metal active sites from the dissociation of methane and further completely oxidized to

synthesis gas by the oxygen lattice. Under steady conditions the CH4 conversion is 100%

and the CO2, CO and H2 selectivities are 2, 98 and 80 % respectively without formation of

carbon deposits at the catalyst surface.

It is expected that if the reaction followed the stoichiometry of the POM, the CO and H2

selectivities should be similar, with a H2/CO molar ratio equal to 2. In our case, the

selectivity to H2 is lower giving a H2/CO molar ratio of 1.7. This result suggest that there is

a contribution of the reverse water gas shift reaction.

10

From the comparison of the results of pulsed experiments at 700 ºC and 800 °C, it is

evident that -type oxygen species (atomic oxygen originating from the bulk which desorbs

at high temperatures) are the only oxygen species responsible for the partial oxidation

reaction. Thus, it is clear that the bulk lattice oxygen of LaNiO3- perovskite should

participate in the partial oxidation of CH4 to H2 and CO. The TGA experiments (Figure 5)

show that there are non stoichiometric oxygen and oxygen vacancies involved in the

oxygen transport through the lattice 40. In a recent publication it was found that the

selectivity towards the total and partial oxidation of methane using La0.3Sr0.7Fe0.8M0.2O3−δ

(M=Ga, Al) perovskites, was related to the amount of nonstoichiometric oxygen 41. The

relationship between the rate of oxygen migration and the concentration of oxygen

vacancies was also confirmed by Rossetti et al 42.

A complementary experiment using the pulse technique was conducted at 800°C in order to

study the behavior of the lattice oxygen during a quasi-continuous operating reaction. The

results are displayed in the Figure 8. The experiments can be described as follows:

In the first part (segment (a) of Figure 8), the pulse sequence is the same as the one

described above. Pulses of methane were injected each 15 min, while the temperature was

increased from room temperature to 800 °C. After that, a higher oxygen concentration was

used (He/O2 - 5% of oxygen) during 30 min, with the aim of re-oxidizing the sample and

refill the maximum number of the oxygen vacancies, (segment (b) in the Figure 8). Then 30

pulses of methane were injected again at intervals of 15 min using He/O2 (0.1% of oxygen)

as carrier gas (segment (c) of the Figure 8. In order to approach a continuous operation and

to determine how fast the catalyst can recover the oxidation capability, pulses of CH 4 were

injected at intervals of 1 min, (segment (d) Figure 8) Finally the pulses frequency was

reduced to every 15 min (segment (e) Figure 8). The carbon debalance of all these

11

experiments is reported in the Figure 9. The selectivities and conversion profiles shown in

Figure 8 are similar to those presented in the Figure 7. Comparison of segments (a) and (c)

shows that re-oxidation process of the catalyst is reversible, and that the oxygen vacancies

can be easily regenerated. Under steady conditions CH4 conversion and CO2, CO and H2

selectivities are the same as those obtained at 700°C (Figure 7). Methane conversions as

well as CO and H2 selectivities were constant during 25 pulses, segment (c). At the same

time, carbon debalance (Figure 9) revealed that there was not carbon deposition on the

catalyst surface during the pulsed experiments. This result suggests that partial oxidation is

the main reaction taking place and that the catalytic methane pyrolysis, if any, is a minor

reaction.

The results show that the oxygen required in the partial oxidation is i) extracted from the

LaNiO3- lattice during the CH4 pulses and ii) regenerated by direct activation of molecular

oxygen contained of the carrier gas.

Indeed if the time between pulses was reduced from 15 min to 1min, segment (d), CH4

dissociation was observed and carbon deposition took place on the LaNiO3- perovskite

surface due to a lower surface concentration of oxygen species close to the carbon species

deposited on the active sites which could also indicate a limited oxygen mobility. During

these 20 consecutive CH4 pulses in segment (d), only H2 was formed and CO selectivity

decreased towards zero. Here, carbon deposits were detected, indicating that pyrolysis of

CH4 takes place (Figure 9).

After re-increasing the pulse intervals to 15 min (segment (e)), CO was produced again and

a large amount of CO2 was also detected. The carbon balance is considerably lowered due

to the oxidation of the carbonaceous deposits over the catalyst surface, which originated

from the previous methane dissociation (segment (e) in the Figure 9).

12

3.6. Methane partial oxidation mechanism over the LaNiO3- perovskite

Taking into account the above observations and the results published in the literature, the

following mechanism is suggested.

First, it is necessary to consider the oxygen incorporation into the lattice. This process has

been studied using isotopic oxygen exchange with different oxides. It is generally accepted

that O2 incorporation into the lattice occurred as follows, [43 - 45].

Several species can appear as intermediates in the reduction of molecular oxygen, such as

O2−, O2

2−, and O−. In this work we did not attempt to distinguish between these three surface

oxygen species, and therefore they are denoted as O*. The Kröger-Vink notation is used for

lattice defects: V··o denotes an oxygen vacancy, O×

O a regular lattice oxygen ion, and e’ an

electron. The subscripts s and b refer to “surface” and “bulk,” respectively. This distinction

is necessary because only the surface lattice oxygen will be accessible for the catalytic

reaction. The same argument also holds for oxygen vacancies.

In the first part of the reaction, when there is a high concentration of reactive lattice

oxygen, CH4 is completely oxidized to form CO2 and H2O

A low concentration of extractible lattice oxygen favors selective oxidation of methane to

CO and H2, and the reaction mechanism can be expressed as:

13

The methane molecule can be also dissociated by the oxygen species according to the

following equation 46:

In this step, the adsorbed H atom may combine to form hydrogen which easily desorbs. The

chemisorbed carbon species are either oxidized to CO by lattice oxygen or oxidized to CO2.

According to the above scheme, the product distribution of the POM reaction over

LaNiO3- catalyst was determined by the concentration of surface oxygen species, which is

affected by the migration rate of oxygen from the bulk toward the surface.

Finally, the re-oxidation process can be written as function of an incorporation and

migration process:

According to our results, these vacancies can be re-filled with oxygen either by direct

activation of oxygen from the gas phase or by lattice oxygen diffusion. Lattice oxygen

diffusion may provide an alternative pathway for replenishment of the surface lattice

oxygen consumed by methane. This may explain the high methane conversion over the

LaNiO3 catalyst.

14

4. Conclusions

We have shown that the perovskite LaNiO3 prepared by the auto combustion method has a

nonstoichiometric oxygen and that its formula can be rewritten as La+3Ni3+0.368Ni2+

0.632O2.684.

Perovskite oxygen lattice has catalytic activity towards CH4 partial oxidation at 800 °C

while at a lower temperature the conversion is negligible.

Complete oxidation of methane to CO2 and H2O occurs during the first step of the reaction

(initial pulses), whereas partial oxidation to CO and H2 is found to be the main reaction

during the subsequent pulses. A high concentration of available lattice oxygen promotes

total oxidation of CH4 to CO2 and H2O, while a low concentration of lattice oxygen favors

selective oxidation of methane to CO and H2. High methane conversions and large amounts

of H2 and CO were produced using non reduced perovskite, with a CH4 conversion and H2,

CO and CO2, selectivities under steady conditions of 100%, 80%, 98% and 2 %

respectively at 800°C. The H2/CO ratio obtained was 1.7, indicating a contribution of the

reverse water gas shift reaction.

Acknowledgements

The authors are grateful to the PICS program: "Valorization of natural gas and Fischer-

Tropsch synthesis" for the financial support given. F. Mondragon and G. Sierra

acknowledge to the University of Antioquia for the financial support of the Sostenibilidad

Program and to Colciencias for the support of the project 1115-06-17639. G. Sierra thanks

COLCIENCIAS and the University of Antioquia for the PhD scholarship.

15

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Figures caption:

Figure 1. XRD patterns of the LaNiO3 (a) JCPDF Card No. 330711, (b) after calcination at 700°C (c) and after reduction treatment. (°) Ni°, () La2O3.

Figure 2. (a) SEM micrographs of LaNiO3- after calcination and (b) TEM micrograph obtained after reduction of LaNiO3- under hydrogen at 700°C.

Figure 3 : TPR curve of the perovskites LaNiO3, catalyst weight : 160mg, gas flow rate : 5% H2/Ar, 50mL/min

Figure 4. Powder X-ray diffraction patterns for (a) La2O3 JCPDF Card No.05602, (b) LaNiO3 after TPR experiment up to 800°C (Ni°/La2O3), (c) LaNiO3 JCPDF Card No. 330711 and (d) LaNiO3 reduced after re-oxidation in air at 800°C for 1h. (°) Ni°.

Figure 5. Weight change of LaNiO3 during nitrogen→oxygen→nitrogen→oxygen cycles at 800°C.

Figure 6. DRX of LaNiO3 during nitrogen→oxygen→nitrogen→oxygen cycles. (a) at room temperature, (b) at 900 °C over oxygen, (c) at 900 °C over nitrogen and (d) at 900 °C over oxygen. (■) sample holder.

Figure 7. Methane pulsed oxidation reaction (7.7 mol/pulse) over non reduced LaNiO3-

perovskite at 800 °C. A pulse was injected every 15 min. (▲) CH4, () CO2, () CO and (■) H2. The perpendicular dotted line represents the pulse number when the temperature of the reactor reached 800 °C.

Figure 8. Methane pulsed oxidation reaction (7.7 mol/pulse) over non reduced LaNiO3-

perovskite at 800 °C. Pulse composition: (a) CH4 (every 15 min), (b) O2/He - 5% of oxygen (every 15min), (c) CH4 (every 15 min), (d) CH4 (every 1min) and (e) CH4 (every 15 min).

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(▲) CH4, () CO2, () CO, (■) H2. The perpendicular dotted line represents the pulse number when the temperature of the reactor reached 800 °C.

Figure 9. Carbon debalance for the methane pulses series of the Figure 8.

Figure 1

20 40 60 80

(a)

2

(b)

(c)

21

°

°

Figure 2

22

100 nm 1µm

(a) (b)

Figure 3

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Temperature (°C)

Inte

nsity

(a.u

)

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Figure 4

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(a)

2

(b)

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(d)

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°°

Figure 5

97

98

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100

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Wei

ght

loss

(%

)

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N2 O2 N2 O2

2.06 %

(a)

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(c)

(d)

(e)

Figure 6

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(a)

(b)

(c)

(d)

Figure 7

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ion

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Figure 8

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(a) (d) (e)(b) (c)

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H2/C

O

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sion (

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Figure 9

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Pulse number

CB

(%

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(a) (b) (d) (e)(c)

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