CARBON NANOFIBRE GROWTH FROM LOW TEMPERATURE METHANE DECOMPOSITION
OVER SKELETAL TRANSITION METAL CATALYSTSJames Highfield1, Yook Si Loo1, Ziyi Zhong1, Ruijiang Li1 & Benjamin Grushko2
1 Applied Catalysis Technology, Institute of Chemical & Engineering Sciences, 1 Pesek Road, Jurong Island, SINGAPORE S627833. 2 Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 GERMANY
CH4 C + 2 H2 H298 K = + 74.5 kJ mol-1
A. Direct eco-friendly route to “CO-free” H2 and speciality carbons.
B. Single-metal & multinary (alloy ?) skeletal catalysts from quasicrystals (QC)
Route: Al65-75(TM/Cu)35-25 arc melt/anneal XRD ideally single-phase QC
P
N2
T
Dual-set pointTemperature
Controller
Dual Lock Gas valve
To vacuum
Gas cylinder with pressure gauge
T
Heating Block
Gas Diffuser and Filter
Top lid
Pre-calibrated volume (100 ml) for calculation of gas consumption
Purge line
Gas cylinder with pressure gauge
P
N2O
/He
5M NaOH Chemical Liquid Feed pump
Knock-out pot
Drain line
P
< 5 barg
[Selective leach of Al 5 M NaOH under N2]
In-situ washed, dried, “passivated” catalyst
custom leaching rig
characterization
TEMXRD
XRFBET
catalytic testing [TG-FTIR/MS]
TEM micrograph of fresh skeletal Co (ex Al13Co4) Typical TG curve for CH4 decompositon (skeletal Co)
1. abrupt onset of weight gain (blue curve) above 350 C;
2. rapid establishment of fixed rate (10% per h @ 400 C)
250 C
300 C
350 C
400 C
TEM micrographs of carbon nanofibres on skeletal cobalt deposited at 400 C (up to 50 wt. % as carbon)
TG%
0.0
1.0
2.0
3.0
Time/h 1.0 2.0 3.0 4.0
on carburization Co “needles” broken into fine “teardrops”
metal dusting corrosion?
more proof of irreversible change
new activity below 300 C !
0.00160 0.00165 0.00170 0.00175 0.00180 0.00185 0.00190 0.00195-4
-2
0
2 Ln rate Linear Fit of Data1_E
ln r
ate
1/T K
Methane Coking kinetics: 250-330 C Co new2: 11/04/05
Eapp
= 129 +/- 6 kJ.mol-1330 C
300 C
280 C
250 C
Oven Temp.
(C)
Rate of wt. gain (%) per hour in CH4/H2 flow‡
Ni ex Al2Ni
GF
Ni9Cu
ex
Al22Ni9Cu
Ni/SA(65% Ni)
Aldrich
Fe21Cu5
ex
Al74Fe21Cu5
Feex
Al5Fe2
Co20Cu14
ex
Al67Co20Cu14
Coex
Al13Co4
Ru22Cu7
ex
Al71Ru22Cu7
Ruex
Al76Ru24
250 0.01 0.01 0.03 0.08 -- 0.003 0.05 0.005 0.001
280 0.28
300 0.06 0.017 0.080 0.35 0.004 0.02 0.82
330 1.4 2.50
350 0.15 0.030 0.065 zero 0.35 0.07 0.02 1.7 3.30 0.02
360 2.6 -
380 5.3 -
400 8.80 2.20 0.001 1.5 4.4 0.57 8.1 10.6 11.25 0.80 0.07
TG analysis of CH4 decomposition: in-situ pre-reduced samples & controls
‡ CH4 + 2 % H2 [12 ml/min; 1:1 N2]
Red: start at 400 C, then T [new low-T activity]
Blue: as for Red, then switch to CH4/N2 at 250 C & T [dramatic inhibition by H2!]
Green: mean of increasing rate (Fe-containing samples) [long induction phase?]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.00.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
dTG
(m
g/m
in)
H2 v
ol.
%
Time (h)
450 C hold
300 C
StoichiometryH2 : C = 2.15 : 1
1.6 ml min-1 H2 71 mol. min-1
0.40 mg min-1 C or 33 g at. min-1
Eapp = 129 +/- 6 kJ mol-1
Surface area = 30–160 m2 g-1
Al = 5-10 wt.%Na < 1 wt.%
Amorphous (except Ni)
Proof of unimolecular decompn: CH4 C + 2 H2
Rate of C deposition vs. H2 level [T = 450 C; cat. Fe19Ni9; CH4: 80 ml min-1]
Summary
1. Skeletal metals made from quasicrystalline precursors are “triggered” into CH4 conversion at T > 350 C, yielding nanofibrous carbons & H2 in the ratio C:H2 = 1:2;
2. Pre-carburization leads to irreversible metal decrepitation, akin to “metal dusting corrosion”, creating particles in the range 20-50 nm well suited for filament growth;
3. 1st-row TMs Co, Ni, Fe, & their combinations most active, while Cu moderates activity;
4. Despite remarkable low-T activity, CH4 conversion is still quite low (< 2% at 400 C);
5. Process operation would need high recycle ratios and rapid (in-situ?) removal of product H2, a powerful inhibitor.
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