The Production, Purification and Characterization of Carbon Nanotubes for Hydrogen Storage
By
Jorge Ivan Salazar Gomez
Main Supervisor:
Prof. Peter J. Hall
Second Supervisor:
Dr. Len Belouis
INTRODUCTION
• The discovery of the fullerenes [1] and the carbon nanotubes [2] opened a new area of research in both the theoretical and the experimental field. Since the discovery of the single wall carbon nanotubes SWNTs, many chemical and physical properties have been predicted.
• Different techniques have been applied to produce carbon nanotubes: electric arc discharge [2,3,4], laser ablation [5,6], chemical vapor deposition (CVD) [7,8,9], being the last one the method that shows large scale production, but with the disadvantage of the formation of amorphous carbon and other impurities, which must be removed by physical or chemical methods. Therefore, the purification process [10,11] is a key step in the production of carbon nanotubes addressed toward hydrogen storage [12] or other applications.
• The characterization [13] becomes an important part of the process because it gives information about the catalyst and the carbon samples before and after purification, so giving information about the actual nature of the samples produced and how to control and improve them.
AIM
• The aim of this project focuses on achieving the best conditions for the production of carbon nanotubes by the CVD method, directed towards hydrogen storage. The storage of hydrogen by physical adsorption or by electrochemical methods is one of the most promising applications of carbon nanotubes due to their possible use in fuel cells, especially for the transportation sector, implying a clean process and therefore reducing the global contamination from CO2.
OBJECTIVES
1) To evaluate the effect of:
• Catalyst composition
• Temperature
• Time
• Flow rate of feed gas
• Particle size
2) To elucidate the impact of the processes of washing, graphitization and activation on the properties of the nanotubes.
3) To characterize the catalysts and the carbons using different techniques that permit to understand the structure and properties of these materials.
4) To evaluate the physical and electrochemical hydrogen storage.
EXPERIMENTAL
Catalyst preparation sol-gel method
Calcination Cu:Ni Calcination Cu:Ni:Mg
CVD (DEON) CVD (SP, CAT)
C2H4, 600oC C2H4, CH4,
600oC, 800oC
CATALYST REMOVAL
Washing with acids
Adding acid and stirring For minimum 4 h and filtering
Passing acid through the samples (filtering)
To rinse with deionized Water until pH 7
To dry in vacuum
GRAPHITIZATION
To heat at 10oC/min until 1500oC in inert atmosphere (Ar 100 ml/min)
To leave the samples 2h
To cool until room temperature
To put an amount of sample in an horizontal furnace and purge 30 min.
ACTIVATION (CO2)
To put an amount of the samples in an horizontal furnace
To purge 30 min. with Ar
To heat at 10oC/min from room temperature until 850oC
To change the gas by CO2 and to leave the sample for 1-4 h.
To cool until room temperature with Ar
CVD REACTION APPARATUS
RESULTS
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
Yield (g/g)
Yie
ld (
g/g
)
Weight % Ni
10
20
30
40
50
60
70
0 20 40 60 80 100
Yie
ld o
f C
arb
on
(g
/g)
Reaction Time (min)
DEON: Cu:Ni = 40:60
SP: Cu:Ni:Mg = 10:20:70
CAT: Cu:Ni:Mg = 0.3:0.7:3.0
No termination time observed
Catalyst still active
Constant Reaction rate
No diffusional effects
0
10
20
30
40
50
60
70
450 500 550 600
Yie
ld o
f C
arb
on
(g
/g)
Reaction Temperature (oC)
0
0.5
1
1.5
2
2.5
3
3.5
4
0.0011 0.00115 0.0012 0.00125 0.0013 0.00135 0.0014
y = 35.33 - 25490x R2= 0.99892
ln(k
)
1/T (1/K)
y = 14.36 - 9287.1x R2= 0.99964
SAMPLES PARTICLE SIZE (µm) YIELD (g/g)
DEON 011(i)a x < 150 40.36
DEON 011(ii)a 212 > x > 150 31.01
DEON 011(iii)a x > 212 33.89
DEON 011(i)b x < 150 41.14
Below 500oC Ea = 211.924 kJ.mol-1
Above 500oC Ea = 77.213 kJ.mol-1
BET Surface Area
0
50
100
150
200
250
300
350
0 0.2 0.4 0.6 0.8 1
raw
washed
Graphitised
Ad
sorp
tio
n a
mo
un
t (c
c/g
)
Relative pressure
0
20
40
60
80
100
0 0.2 0.4 0.6 0.8 1
010a(600oC-80ml/min)010a(Activated)
010e(600oC-80ml/min)010e(Washed)
010f(600oC-80ml/min)010f(Graphitized)
Ad
sorp
tio
n A
mo
un
t (c
c/g
)
Relative Pressure
0
50
100
150
200
250
0 0.2 0.4 0.6 0.8 1
09e(500oC-10ml/min)
09d(600oC-80ml/min)
09c(600oC-57ml/min)
09c(Graphitized)
09b(600oC-33ml/min)
09a(600oC-10ml/min)
Relative pressure
Ad
sorp
tio
n a
mo
un
t (c
c/g
)
a) b) c)
1
00
1
00
0 11
11
1n
nn
m
PPCP
PC
PPnP
Pn
PP
C
W
W
X-Ray Diffraction
0
200
400
600
800
1000
1200
20 30 40 50 60 70 80 90
DEON 01a (Raw)
DEON 01a (W)
DEON01a (G)
Co
un
ts
2 THETA
0
500
1000
1500
2000
2500
3000
3500
4000
10 20 30 40 50 60 70 80 90
Cat. SP1Cat. SP2SP1aSP2aSP2b
Inte
ns
ity/
a.u
.
2 Theta
a) b)
nd sinSAMPLE
TEMPERATURE(oC)
C2H4 FLOW RATE(ml/min)
d-SPACING(nm)
DEON 09a 600 10 0.3437DEON 09b 600 33 0.3405DEON 09c 600 57 0.3418DEON 09d 600 80 0.3405DEON 09e 500 10 0.3445DEON 09f 700 10 0.3406, 0.3427SP1a 600 80 3.441SP2a 600 80 3.426SP2b 700 80 3.422
TGA
Graphitized more stable
Raw carbon is more reactive
Washed carbons have medium reactivity
0
20
40
60
80
100
0
200
400
600
800
1000
0 10 20 30 40 50 60
SP1a (600oC-90 min)
SP2a (600oC-90 min)
SP2d (700oC-90 min)
SP2e (700oC-90 min)
SP2f (600oC-30 min)
SP2g (700oC-30 min)
Temperature (oC)
Temperature (oC)
Temperature (oC)
Temperature (oC)
Temperature (oC)
Temperature (oC)
Wei
gh
t lo
ss (
%) T
emp
eratu
re (
oC)
Time (min)
SAMPLEVOLATILES
(%)PURE CARBON
(%)ASHES (%)
SP1a-2 5.01 93.06 1.95SP2a-1 6.48 93.01 0.58SP2d-1 6.90 92.17 0.82SP2e-1 5.06 94.52 0.39SP2f-1 17.36 79.10 3.57SP2g-1 9.63 89.76 0.56
ELECTROCHEMICAL STORAGE
CYCLEDENSITY
DELOADING(mA/g)
CAPACITY(mAh/g)
H:Cratio
WEIGHT(%)
DEON 09d-1 100 34.94 1:64 0.130
DEON 09d-2 50 10.27 1:124 0.067
DEON 09d-3 100 15.07 1:104 0.080
DEON 09d-4 50 14.09 1:158 0.053
DEON 09d-5 5 8.82 1:160 0.052
DEON 09d-6 20 13.64 1:172 0.048
DEON 09d-7 20 13.99 1:171 0.049
DEON 09d-8 50 19.60 1:114 0.073
DEON 09d-9 23 13.93 1:163 0.051
DEON 09d-10 100 20.53 1:109 0.076
DEON 09d-11 50 38.39 1:125 0.066
DEON 09d-12 23 8.24 1:129 0.065
CA-01 100 57.33 1:39 0.210
CA-02 200 69.05 1:32 0.260
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
100 110 120 130 140
DEON 09d
Cycle 10
Cycle1
PO
TE
NT
IAL
(V
)
CAPACITY (mAh/g)
Discharging process after 1 and 10 cycles for the sample DEON 09d previously loaded electrochemically with hydrogen at a current density of 1000 mA/g for 1h in a 6M KOH solution.
MS-TPD
0
5 10-10
1 10-9
1.5 10-9
2 10-9
2.5 10-9
3 10-9
3.5 10-9
4 10-9
50 100 150 200 250 300 350 400 450
DEON 09d He (2nd cycle)
Mass 1Mass 2Mass 16Mass 17
Partia
l P
res
su
re (
To
rr)
Temperature (oC)
Adsorption at 10 bar for 24h at room temperature.
Desorption at 20oC/min in He as carrier.
RAMAN SPECTROSCOPY
0
500
1000
1500
2000
2500
800 1200 1600 2000 2400 2800 3200
Inte
nsi
ty/a
.u.
Raman Shift/cm-1
406 nm
785 nm
457 nm
011(ii)a-1011(ii)a-3011(ii)a-3
0
1000
2000
3000
4000
5000
6000
400 800 1200 1600 2000 2400 2800
406 nm517nm632 nm
Inte
nsi
ty/a
.u.
Raman Shift/cm-1
CAT 3B
G-Band at 1575 cm-1
D-Band at 1312 cm-1
G/D = 3.87
Mainly Semiconducting
G-Band at 1589 cm-1 (shoulder at 1546 cm-1)D-Band at 1351 cm-1
G/D = 1.19Metallic and Semiconducting
Small-Angle Neutron Scattering (SANS)
0.01
0.1
1
10
100
1000
0.001 0.01 0.1 1
PJH1414-1434m-SP2a-Dry
SP2a DrySP2a CM
d
d
Q)(
cm-1
)
Q (Å-1)
0.1
1
10
100
1000
0.001 0.01 0.1 1
PJH1416-1436m-09d-Dry
DEON 09d DryDEON 09d CM
d
d
Q)(
cm-1
)
Q (Å-1)
Raw data for time-of-flight technique with corrections for instrument background and transmission.
TEM
250nm
5nm
100nm 5nm
CONCLUSIONS
•The best catalyst composition for the synthesis of the carbon nanofibers is Cu:Ni=40:60 and the optimum temperature is 600oC, at higher temperature catalyst deactivation appears.
• The washing process with nitric acid was effective in the removal of catalyst particles and induced some ordering. It had little effect on the surface area.
• The graphitization process enhances the chemical stability of the nanofibres and induces more order (formation of bundles and reduction of aggregated pores) and enhances crystallinity, but it decreases the capacity of adsorption.
• The activation process with CO2 opens some of the tubes, but it does not
apparently remove the amorphous carbon. The selectivity though appears to be better than oxygen.
•The Raman results indicate that the nanotubes formed are mainly semiconducting, but a high proportion of nanofibers and impurities are present.
• The capacity of adsorption for hydrogen is very low for the raw samples, but higher uptakes are expected for purified samples.
REFERENCES
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