manuscript-methane partial oxidation by the lattice oxygen of the lanio3 perovskite. a pulse study
TRANSCRIPT
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: [email protected], 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
0 200 400 600 800
Temperature (°C)
Inte
nsity
(a.u
)
23
Figure 4
10 20 30 40 50 60 70 80 90
(a)
2
(b)
(c)
(d)
24
°°
Figure 5
97
98
99
100
0 50 100 150 200 250Time (min)
Wei
ght
loss
(%
)
25
N2 O2 N2 O2
2.06 %
(a)
(b)
(c)
(d)
(e)
Figure 6
20 30 40 50 60 70 80 90
26
(a)
(b)
(c)
(d)
Figure 7
0
20
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60
80
100
0 5 10 15 20 25 30 35 40 45 50
Pulse number
Con
vers
ion
(%)
0
20
40
60
80
100
Sel
ecti
vity
(%
)
27
Figure 8
28
(a) (d) (e)(b) (c)
0 10 20 40 60 80 100 1200
20
40
60
80
100
0
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Selectivity (%)
Conversion (%)
Pulse number
0 10 20 40 60 80 100 120
0
20
40
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100
0
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80
100
H2/C
O
Conver
sion (
%)
Pulse number
Figure 9
-100
-80
-60
-40
-20
0
20
40
60
80
100
0 10 20 30 40 50 60 70 80 90 100
Pulse number
CB
(%
)
29
(a) (b) (d) (e)(c)
30