Pt on Fecralloy catalyses methane partial oxidation to syngas at high pressure

Article abstract of DOI:10.1016/j.cattod.2015.11.018

μ-Gas-to-Liquids technology (μ-GTL) can potentially reduce natural gas that is flared throughout the world. Integrating syngas production (at high pressure) together with Fischer-Tropsch in the same vessel addresses both investment costs and operating costs challenges related to μ-GTL. Pt on a Fecralloy woven metal support effectively oxidizes methane in the presence of O₂/Ar mixtures. Both CO and H₂ selectivity are higher over the Fecralloy compared to either a commercial Pt-gauze or a Pt/Rh gauze at 900 °C and 20 bar. The catalyst activity is stable over 1 h but the 80-300 nm Pt particles agglomerate and form popcorn shape particles that are as large as 1150 nm.

Full text of DOI:10.1016/j.cattod.2015.11.018

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Pt on Fecralloy catalyses methane partial oxidation to syngas at
high pressure
Cristian Neagoea,b, Daria C. Boffito , Zhenni Ma , Cristian Trevisanut ,
Gregory S. Patience
a
a
a
a
a,∗
Department of Chemical Engineering, Polytechnique Montréal, C.P. 6079, Succ. CV, Montréal, H3C 3A7 Québec, Canada
ME Resource Corp., Suite 900 – 555 Burrard Street Vancouver, V7X-1M8 BC, Canada
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2015
Received in revised form 3 September 2015
Accepted 14 November 2015
Available online xxx
ꢀ-Gas-to-Liquids technology (ꢀ-GTL) can potentially reduce natural gas that is flared throughout the
world. Integrating syngas production (at high pressure) together with Fischer–Tropsch in the same ves-
sel addresses both investment costs and operating costs challenges related to ꢀ-GTL. Pt on a Fecralloy
woven metal support effectively oxidizes methane in the presence of O2/Ar mixtures. Both CO and H2
selectivity are higher over the Fecralloy compared to either a commercial Pt-gauze or a Pt/Rh gauze at
◦
9
00 C and 20 bar. The catalyst activity is stable over 1 h but the 80–300 nm Pt particles agglomerate and
Keywords:
form popcorn shape particles that are as large as 1150 nm.
Catalytic POX
Methane partial oxidation
High pressure
Syngas
©
2015 Elsevier B.V. All rights reserved.
Fecralloy
Catalytic gauze
Coking
Regeneration
1
. Introduction
produce syngas are at ambient pressure and high temperature
◦
(
<900 C) whereas the thermodynamics of the Fischer–Tropsch step
◦
The World Bank launched the Global Gas Flaring Reduction
favour high pressure (<20 bar) and low temperature (>320 C). From
a thermodynamic perspective, the CPOX reaction is more efficient
at low pressure and it is a challenge to convert CH4 to CO and
H2 selectively at high pressure. Newitt and Haffner [1] pioneered
high pressure methane partial oxidation in 1932. Much later, Lott
and Sliepcevich [2] designed a bench process to study hydrocarbon
partial oxidation at 200,000 psi.
partnership in 2002 as part of the World Summit on Sustainable
Development. They estimated that petroleum companies flare as
much as $35 billion per year of natural gas (NG). Building pipeline
infrastructure is uneconomic for remote reservoirs or for reservoirs
with a short life expectancy. The economics of converting low vol-
umes of gas to Fischer–Tropsch fuels, methanol, DME (gasoline),
ammonia, or ethylene might be attractive if the investment costs
can be maintained at the standard for large scale units. Investment
in gas-to-liquids processes (Fischer–Tropsch fuels, for example) is
In CPOX, molecular O2 partially oxidizes CH4 to CO and H . It
2
requires 1/2 mole of O per mole of CH (Eq. (1)), whereas complete
2
4
combustion of CH4 requires 2 moles of O2 per mole of methane
(Eq. (2)). Complete combustion is 20 times more exothermic than
partial oxidation
1
00,000 $ per bbl of oil produced.
Most NG conversion technologies rely on producing syngas (H2
and CO) in the first step via steam methane reforming (SMR), auto-
thermal reforming (ATR) or partial oxidation POX). Our final goal
is to combine CPOX with the Fischer–Tropsch process in one sin-
gle vessel in order to reduce both investment and operating costs.
According to the thermodynamic analysis, the best conditions to
−
1
CH + 1/2O2 = CO + 2H ꢁH = −36, 000 kJ kmol
(1)
(2)
4
2
−
1
CH + 2O CO + 2H OꢁH = −800, 000 kJ kmol
4
2
2
2
The most common industrial reforming catalysts are based on
Ni that cost less than noble metals. However, methane cokes on
Ni more rapidly than noble metals and according to XPS analyses
surface carbon species, such as carbide species and carbon fila-
ments that form on the surface, may alter the catalyst structure
[3]. Together with Ni other CPOX catalysts include Co supported on
∗ Corresponding author.
E-mail address: gregory-s.patience@polymtl.ca (G.S. Patience).
http://dx.doi.org/10.1016/j.cattod.2015.11.018
0
920-5861/© 2015 Elsevier B.V. All rights reserved.
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ZSM-5 [4], Ni/Ru system with addition of steam [5], Ni/Co/Ru [6],
Ni-Pt/La0.2Zr0.4Ce0.4Ox [7]. Noble metal catalysts remain the best
candidates for CPOX because they are most stable and CO and H2
yields are higher [8] and because carbon does not dissolve in those
elements [9].
Methane pyrolyses on reduced metal sites forming hydrogen
and carbon adspecies. Carbon reacts with oxygen to form CO [10].
Both Pt and Rh are active noble catalysts but Rh selectivity is higher
[
11,12]. Basini et al. [13] studied the thermal decomposition on the
surface of Rh based catalyst and reported differences between gas
and solid temperatures. Methane combusted or oxidized partially
in the front end of the catalytic bed while near the end of the bed,
less energetic reactions – steam reforming, CO2 reforming and the
water gas shift – predominated. To take advantage of the spatial
differences across the catalyst bed, Smith et al. [14] segment the
reactor by placing catalysts optimized for each reaction in differ-
ent parts of the bed: at the reactor inlet, they placed combustion
catalysts and after they place a reforming catalyst
Fig. 1. Optical microscope image of FA tissue provided by Eratec.
Thermodynamics and reaction kinetics depend on pressure.
Reaction rates increase with pressure while maintaining the GHSV
constant over Pt while the rate is less prominent for Rh [15]. Cerium
sides. An IR camera monitored the surface temperature of the FA.
−
1
The solution for the alumina-ceria supports contained 0.07 mol L
oxides (CeO , Ce O ) improve noble metal stability and the oxygen
2
2
3
−
1
cerium(IV) ammonium nitrate and 0.44 mol L aluminium nitrate
adsorption rates [16].
◦
mixture. The catalyst calcined for 4 h at 600 C with the same
Partial oxidation reactors are smaller than steam reformers
but operate less efficiently [15]. Their main advantage is that
they can react higher hydrocarbon feedstocks and NOx and SOx
emissions are lower. Many reactor types have been adapted to par-
tially oxidize methane: fixed beds [17,18], fluidized beds [19,20],
membranes [21,22], monoliths [23,24], and gauzes [25], chemical
looping technology [26,27] and cyclic fixed beds [28].
temperature ramp as the initial FA oxidation step in the oven.
In the final two steps, platinum salt solutions were sprayed on
◦
◦
the FA+support and they calcined at 1000 C for 4 h (with a 5 C
−
1
min ramp). We sprayed 2.5 mL of a chloroplatinic acid hexahy-
drate (PtH Cl ×6H O) solution on each side of the coated FA. The
2
6
2
−
1
−1
and 0.13 mol L
concentration of the Pt salt was 0.051 mol L
for the 1%Pt/FA and the 5%Pt/FA, respectively. The final weight
ratio of Al2O3:CeO2 was 79:21. All the reagents were certified high
purity chemicals by Sigma– Aldrich and were used without further
purification.
New reactor classes for CPOX include microliths and coated
®
metal plate reactors. We develop woven Fecralloy metal fibres
(
FA) as a catalsyst support. Fecralloy resists temperatures and have
excellent thermal properties [29]. Catalyst can be added to FA by
slurry deposition of impregnated alumina [30], by spraying an
atomized solution of precursor [31], electro-synthesis [32] or by
2.2. Catalyst characterization
growing the support on the existing Al O3 on the surface of the FA
2
[
33].
We deposited Pt over a FA microlith to partially oxidize CH4 to
An X-ray Philipps X’pert diffractometer (XRD) recorded the
diffraction patterns of the FA, FA+support and FA catalysts with
syngas at high pressure in air. We evaluated the activity of spray-
coating the Fecralloy woven construction at 1 bar and 20 bar at
1
positions together with two commercial gauzes (Pt and Pt/Rh). At
high pressure, the thermodynamic equilibrium selectivity to coke
is about 5% but experimentally the Fecralloy produced as much as
Cu K˛ (1.5406 A˚ ) radiation at 50 kV and 40 mA at room tempera-
◦
◦
ture. The diffraction angle varied from 0 and 90 , with an incidence
of 1 .
173 K. We compared conversion and selectivity of three FA com-
◦
2
.3. Catalytic partial oxidation – CPOX
4
0% coke at ambient pressure and 20 bar.
The high-pressure CPOX reactor was a 8 mm quartz tube that
was placed in a stainless steel tube (Fig. 2). The FA catalyst bed
consisted of 10 disks x 8 mm ID stacked on top of each other with a
total bed height of 25 mm. We loaded a single 8 mm diameter disk
for tests with the pure Pt metallic gauze and the Pt/10%Rh gauze.
2
. Experimental
2.1. Catalysts synthesis
The gas manifold included CH , 30%O2 in Ar and Ar. Bronkhorst
mass flow controllers (MFC) dosed the gas flows to the reactor
We synthesized several catalysts and tested two commercial
4
gauzes. The FA has a mass fraction of 20% chromium, 5% aluminum,
yttrium >0.1%, 0.3% silicon, 0.08% manganese, 0.03% copper, 0.03%
carbon and the balance iron. This metallic support resembles a
woven fabric (Fig. 1) with a wire diameter of 10 ␮m. The first com-
mercial sample was a 52 mesh 99.9% Pt gauze (wire diameter – dw
and maintained the O at 21%. A spring-loaded back-pressure valve
2
maintained the reactor pressure at 20 bar.
A type-K thermocouple monitored the temperature at top of
the catalyst bed (effluent side). Horn et al. [34] and de Smet et al.
[
35] tested alternative strategies to measure bed temperatures with
–
0.1 mm from Sigma–Aldrich) and the second was an 80 mesh 90%
thermocouples but an IR camera is better to identify radial and axial
non-uniformities. The gas entered the quartz tube cold and trav-
eled 150 mm before reaching the FA. The time it takes for the gas
to reach the FA is insufficient to heat it to the reaction tempera-
ture. Preheating the gas increases the reaction rates in the bed and
increases the conversion (and selectivity). Under some conditions,
the temperature exceeded 1173 K.
Pt on 10% Rh gauze (dw=0.076 mm from Alfa Aesar). The catalysts
we prepared are based on a Fecralloy woven (FA) support coated
with alumina or alumina-ceria solutions. In the first step, air oxi-
◦
dized 2.5 g of FA at 1000 C in a furnace for 3 h. The furnace ramped
◦
−1
the temperature from ambient to the set point at 5 C min . In the
◦
second step, we placed the oxidized FA on a hot plate at 300 C and
sprayed 7.5 mL of a 0.44 mol L Al(NO )×9H O solution on both
−
1
3
2
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VENT
TT
1
PI
2
MFC-4F
Electric heater
Catalyst
4WV-2
MFC-3F
GC
MFC-2F
PI
1
Quadrupole
MS
MFC-1F
VENT
4WV-1
Ar
Fig. 2. 8 mm ID quartz reactor with a multi-gas manifold and an electric furnace.
2.4. Analytical
as large as 3000 nm (Fig. 5a). The Pt particles remained uniformly
dispersed across the surface even after several hours under reac-
◦
A quadrupole MS (Hiden QIC-200) monitored effluent gas con-
tion conditions – 1 bar, 900 C and a volume fraction of 30% CH .
4
centration on-line at a frequency of 3 Hz. Because of the overlap
at 28 amu we created a synthetic air bottle with oxygen and argon
rather than nitrogen. We corrected the CO signal by subtracting the
We fed the oxygen/argon mixture over the catalyst after every
contribution of CO . We calibrated the mass spectrometer before
2
and after each experiment with a mix of Ar, CO, CO , CH4 and H2 at
2
two levels.
The 4WV-1 directed the feed gas to either the reactor or the sec-
ond four way valve (4WV-2). The second valve directed the effluent
gas to the vent or to the analytical instruments. When we calibrated
the MS with the feed gases, we purged the reactor with Ar through
the needle valve V-1.
A GC (Agilent 7890B) with two TCD detectors sampled the efflu-
ent gas every 7 min to verify the MS signal. The columns in the
GC were a capillary POLT Q column, (30 m × 0.53 mm × 40 ␮m) and
a Molsieve column (30 m × 0.53 mm × 30 ␮m). The oven tempera-
◦
ture was constant at 50 C.
3
. Results and discussion
3.1. Catalyst characterization
The X-ray diffraction pattern of the uncoated FA has three peaks
(
Fig. 3a) that represent the main components of Fecralloy - FeCr
◦
[
36]. After depositing the Al O3 and calcining at 600 C, together
2
with the original Fecralloy peaks, several other peaks appear that
correspond to ˛-alumina (Fig. 3b). Fe and Cr were absent in the
EDX spectra, which confirms that either spraying the surface with
the aluminium nitrate solution coated it entirely or that alumina
comes to the surface during the calcination step [33]. Even though
the percentage of Pt is low, the diffractogram clearly shows Pt peaks
(Fig. 3c) on the FA after spraying the chloroplatinic acid hexahy-
drate solution and calcining.
Calcining alumina increases the loading capacity of the catalyst.
The intensity of Pt peaks after calcining the coated FA support is
much higher than when we deposited Pt on the support without
calcining (Fig. 4).
Spraying the Pt-solution over the coated FA disperses the Pt
uniformly over the surface (Fig. 5b and c). Most of the particles
vary from 80 nm to 300 nm (Fig. 5d). However, a few particles are
Fig. 3. (a) Diffractogram of Fecralloy (FA) with the three peaks corresponding to FeCr
(
JCPDS 34-0396). (b) Diffractogram of the FA coated with alumina: ˛-Al2O3 (JCPDS
1
3
0-0173). (c) Diffractogram of the 1%Pt/FA. The label “X” corresponds to FeCr (JCPDS
4-0396) while “O” corresponds to the metallic Pt (JCPDS 04-0802). Unlabeled peaks
belong to ␣-Al2O3 (JCPDS 10-0173).
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Fig. 6. SEM micrographs of the 1%Pt/FA coated gauze after reaction at 20 bar.
Fig. 4. Diffractogram of 1%Pt/FA (a) without calcining between alumina and Pt coat-
ing; (b) standard synthesis.
experiment with methane to remove any carbon that accumulated
on the catalyst. SEM images and EDX analysis confirmed that no
coke remained on the catalyst after the 2 min oxygen treatment,
Fig. 6.
XRD analysis (Fig. 7) confirms that the catalytic gauze phase
composition remained unchanged after the activity tests at 20 bar.
We ascribe the difference in the peaks at the same 2-Theta to a lim-
itation in the XRD analytical protocol and not to a structural change
in the catalyst composition. X’pert diffractometers are more suit-
able for powders than for coated surfaces. The analysis of the fresh
Fig. 7. Diffractogram of 1%Pt/FA after reaction at 20 bar (with “popcorn effect”).
Fig. 5. The white dots in the SEM micrographs represent Pt particles on the surface of the of the 1%Pt/FA catalyst. At low magnification (–1000×) two particles are as large as
3
000 nm. At the higher resolution (b – 5000× and c – 10,000×) the Pt particles appear uniformly dispersed across the surface. Most particles lie between 80 nm and 300 nm
(
d – 20,000×).
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Table 1
Methane conversion and product selectivity measured at 1 bar and 20 bar for commercial Pt gauzes and Fecralloy catalysts operating 1173 K.
WHSV, h 1
−
XCH4
%
SCO%
SH2
%
SCO2 %
SC%
H2 /CO
Catalys type
P, bar
Pt Gauze
Pt Gauze
Pt/Rh gauze
Pt/Rh gauze
1
20
1
20
1
20
1
20
1
59
59
59
59
11
11
11
11
11
11
37
60
26
74
70
85
88
87
67
81
60
38
24
32
54
46
53
38
37
40
5
24
7
44
25
42
48
34
13
37
13
24
51
11
16
16
9
13
21
25
27
37
25
57
30
38
38
48
43
35
0.2
1.2
0.6
2.8
0.9
1.8
1.5
1.8
0.7
1.9
1
1
5
5
1
1
%Pt/FA
%Pt/FA
%Pt/FA
%Pt/FA
%Pt/Ce/FA
%Pt/Ce/FA
20
catalyst was easier than for the used one because the gauze was flat.
We cut the catalyst into disks and fixed it into the reactor which
deformed the surface and might have changed the peak intensity.
The lower signal to noise ratio in the XRD pattern of the used cat-
alysts, as well as the similar width as the XRD pattern of the fresh
confirm that the differences are mainly due to an instrument issue.
The Pt-particle shape was no longer spherical after reacting at
We report the activity data for all the experiments at 60 min,
except for 5%Pt/FA, for which the activity tests were only 30 min
long due to the difficult temperature control.
For all catalysts, the H /CO ratio increases with increasing
2
pressure. The optimum ratio is slightly greater than two for
Fischer–Tropsch reactors (FT). The ratio at low pressure varied from
as 0.2 (for the Pt Gauze) to 1.5 (for the 5% Pt/FA catalyst), all of which
are unacceptably low for FT synthesis. The ratio varied from 0.9 to
2.8 at high pressure. The ratio hovered around 1.9 for the FA cata-
lysts, which would be acceptable for FT synthesis, particularly for
Fe based FT catalysts. These catalysts have water gas shift proper-
ties and can produce hydrogen in situ (Eq. (6)) thereby increasing
2
0 bar (Fig. 6). Not only do the particles adopt a popcorn from, they
agglomerate and reach 1150 nm.
During the experiment the dispersion of Pt on the support
decreased by 75%. Herman reported a high degree of sintering
already in 1961 [37]. The dispersion of Pt decreases with time-
on-stream and with increasing temperature. The high reaction
temperature (1173 K) is responsible for the sintering. The pres-
ence of oxygen in the feed also promotes particle aggregation [38].
Spraying the Pt solution on the surface of the heated FA promotes
faster crystallization than other techniques. In fact, the particle size
of the fresh catalyst was 300 nm, which is 10 to 15 times larger than
Pt particles on Al O supports. Larger Pt clusters are less stable to
the H /CO ratio to an optimal value
2
CO + H2O ꢀ CO2 + H2
ꢁH 2o = −41 kJ mol−1
(6)
98
Conversion was higher for the FA catalyst compared to the
gauzes, which we expected since the WHSV was almost six times
higher for the tests with the gauzes. Methane conversion is higher
at 20 bar compared to 1 bar for all catalysts. We maintained the
same mass flow rate and expected that the conversion would be
significantly greater at the higher pressure because the residence
time is 20 times longer (Fig. 8). However, the equilibrium conver-
2
3
sintering than smaller ones due to the lower strength of anchor-
ing on the acidic sites of the alumina support [39]. We envisage a
decrease of the catalyst activity for protracted periods of time.
◦
◦
sion at 1 bar and 900 C is 98% while it is 85% at 20 bar and 900 C.
The FA catalyst have essentially achieved equilibrium conversion
at 20 bar and are further from equilibrium at 1 bar. The gauze cat-
alytic activity is excellent although lower than the FA catalysts. We
loaded 1.0 g of FA catalyst of which 0.01 g Pt was distributed uni-
formly across the surface. Although there was six times as much Pt
in the gauze reactor since it is a bulk catalyst only a fraction of the
Pt contacts the methane.
3.2. Catalytic partial oxidation
We tested the performance of two commercial catalysts – Pt
Gauze and Pt/Rh Gauze – and three Fecralloy supported Pt catalysts
1% Pt/FA, 5% Pt /FA and 1% Pt/Ce/FA at 1173 K and 1 bar and 20 bar
Table 1). Methane conversion is the ratio of the difference in molar
–
(
flow rate in (Fin,CH4 ) and out (Fout,CH4 ) and the molar feed rate going
to the reactor (Eq. (3))
The CO selectivity was generally higher at lower pressure and
reached 60% for the Pt gauze (Fig. 9). Only the Pt/10%Rh gauze had
a marginally higher selectivity at higher pressure.
F
− Fout,CH4
in,CH4
in,CH4
X
CH4
=
· 100
(3)
F
The selectivity to each carbon species (CO, CO2 and C) is the ratio
of the flow rate at the exit of each of these species, (Fout,i) and the
methane that reacts (Eq. (4))
Fout,i
Si = F
· 100, i = CO , CO, C
(4)
2
− Fout,CH4
in,CH4
The exit molar flow rate relies on the entrance molar flow rate and
the MS signal of Ar, yout,Ar (Eq. (5))
Fin,Ar
FT =
(5)
yout,Ar
−
1
We define the weight hourly space velocity as the ratio of g h
CH4 to the gram of catalyst. We maintained the same WHSV at
bar as at 20 bar for each catalyst (Table 1). The reactor operated
at a WHSV 59 h for the commercial gauzes and 11 h for the FA-
catalysts.
1
Fig. 8. Methane conversion to syngas at 1173 K and 1 bar and 20 bar. The WHSV
for the Pt gauzes (a – pure Pt gauze, b – 10%Rh/Pt) are 6 times lower than for the
FA-catalysts (c – 1%Pt/FA, d – 5%Pt/FA, e – 1%Pt/Ce/FA).
−1
−1
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Fig. 9. The selectivity for CO at 1173 K for 1 bar and 20 bar. (a) Pure Pt gauze, (b)
0%Rh/Pt, (c) 1%Pt/FA, (d) 5%Pt/FA, (e) 1%Pt/Ce/FA.
1
Fig. 12. The evolution in time of the conversion of CH4, selectivity of H2 and CO for
%Pt/FA at 1 bar and 1173 K
1
CeO2 disperses Pt better that should reduce coking and increase
both CO and H yield [41]. The water gas shift reaction became more
2
important for bimetallic catalysts prepared by classic impregnation
and precipitation methods [42]. On the other hand, ceria promoted
Pt catalyst on Al O3 prepared by incipient wetness impregnation
2
selectively oxidizes CO from hydrogen rich streams [43]. In the case
of 1%Pt/Ce/FA, the CeO increases CH conversions to total combus-
2
4
tion products CO2 and H O. The high selectivity for CO and H2 at
2
2
high pressure correlate with lower levels of coke compared with
low pressure case (Fig. 15). This implies that the water gas shift
reaction is more prominent at high pressure.
Fig. 10. The selectivity for H2 at 1173 K for 1 bar and 20 bar. (a) pure Pt gauze, (b)
1
0%Rh/Pt, (c) 1%Pt/FA, (d) 5%Pt/FA, (e) 1%Pt/Ce/FA.
The activity of the 1%Pt/FA decreases in the first 20 min then sta-
bilizes at 70% CH conversion at atmospheric pressure (Figs. 12–14).
The CO selectivity slowly decreases initially to stabilize after 15 min
at 54%. The selectivity of H2 follows the same trend as for methane
conversion. The symmetry between the selectivity to water and H2
indicates that hydrogen reacts with gas phase oxygen, which is con-
The challenge for this reaction is to produce hydrogen. At these
4
high temperatures, oxygen reacts readily with hydrogen to form
water. Gas phase oxygen may oxidize the H , the CO and the CH .
2
4
Furthermore, hot spots in the catalytic bed decrease the yield of
hydrogen [40]. The highest selectivity to hydrogen was 48% over the
% Pt/FA (Fig. 10) while the equilibrium hydrogen selectivity is 99%.
sistent with previous work [44]. Besides methane combustion and
reforming reactions, oxygen reacts with H2 and CO. The activity of
5
Both the Pt/Rh gauze and the 1% Pt/FA exceeded 40% selectivity. The
equilibrium selectivity at 20 bar is 93%, which is more than double
the best selectivity achieved by the catalysts we tested. de Smet
et al. [35] also reported low hydrogen selectivity for gauze reactors.
The Pt/10%Rh gauze produced the most CO2 (Fig. 11).
In the first 20% of the catalytic bed oxygen may combust
methane, which is highly exothermic [15]. Further along the bed
steam may reform unreacted methane and hydrogen evolves due to
the water gas shift. This would explain the low selectivity of hydro-
gen and the presence of water at the exit of the reactor. Adding
1
6
%Pt/FA at 20 bar and of 5%Pt/FA at 1 bar were constant during the
0 min test.
At 20 bar, 5%Pt/FA converts more methane during the first 5 min
and then remains stable thereafter. Both CO and H selectivity were
2
constant during the entire experiment.
Fig. 11. The selectivity for CO2 at 1173 K for 1 bar and 20 bar. (a) Pure Pt gauze, (b)
Fig. 13. CH4 conversion and selectivity to H2 and CO for 5%Pt /FA at 20 bar and
1
0%Rh/Pt, (c) 1%Pt/FA, (d) 5%Pt/FA, (e) 1%Pt/Ce/FA.
1173 K with respect to time
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Fig. 16. Thermodynamic calculations for the selectivity for graphite at 1 bar and
0 bar at 1173 K.
Fig. 14. CH4 conversion and selectivity to H2 and CO for 1%Pt /Ce/FA at 20 bar and
2
1
173 K with respect to time.
follow this tendency except the 1%Pt/Ce/FA. At high pressure, the
added cerium decreased the amount of coke.
At 1 bar the CeO2 promoted catalyst activity was constant
Not only does coke reduce the yield of CO, it must be removed
periodically to ensure the steady operation of a plant. Removing the
coke with air complicates the operation of a FT reactor downstream
since these catalysts require a reducing environment. Any oxygen
breaking through while the CPOX catalyst is regenrated will affect
the reactivity of the FT catalyst. Furthermore, oxygen that would
otherwise react with CH4 to form CO might react with hydrogen to
form water.
for 60 min. Meanwhile, at 20 bar, after 30 min, the conversion of
methane and both CO and H2 selectivities dropped by 15%. Carbon
deposition on cerium and sintering (weakened mechanical stabil-
ity) deactivated the catalyst [42].
3
.3. Coke
The major challenge facing commercializing CPOX technology is
Several mechanisms could account for the large difference for
the higher than expected coke selectivity we measured compared
to the equilibrium calculations. The theoretical calculations pre-
sume that methane reacts with oxygen stoichiometrically to from
syngas. However, if oxygen combusts a fraction of the methane
at the entrance of the bed, the stoichiometric ratio along the bed
would be lower. It is unlikely that iron is a source of coke for the
Fecralloy catalysts. If it were a source of coke, we would expect to
see more coke on the Fecralloy catalysts compared to the Pt gauzes
(Fig. 16).
to reduce the rate at which coke forms on the catalyst. Early stud-
ies demonstrated that “whiskers” as well as “encapsulated” forms
of carbon form on noble metal supported catalysts [45]. The type
of coke and the yield depends strongly on the support [46]. The
two main sources of carbon are due to methane pyrolysis and the
Boudouard reaction.
We simulated coke formation with the FactSage 6(R) thermody-
namic package. The calculations minimize Gibbs free energy and we
assumed that coke formed graphite. For all catalysts the solid car-
bon produced during the reaction at 1173 K was much greater than
thermodynamics (Fig. 15). The thermodynamic calculations predict
that more coke forms at higher pressure at 1173 K and some exper-
imental evidence supports this trend [47]. All the tested catalysts
4. Conclusions
Gas-to-liquids technology is uneconomic at a scale of 10 bbl per
day with conventional technology. Integrating the partial oxida-
tion step with Fischer–Tropsch synthesis at high pressure reduces
investment costs, processes equipment and operating complexity.
Several challenges remain to develop a dual reactor vessel that
◦
operates at a temperature difference of 600 C and 20 bar. While the
thermodynamic equilibriums favour both high selectivity towards
H2 and CO, selectivity towards coke is unacceptably high.
Woven Fecralloy are promising supports to partially oxidize
methane because they resist high temperatures, are highly con-
ductive, and can be molded into shapes. We successfully dispersed
Pt over the surface of FA microlith that we coated with ˛-Al O and
2
3
˛
-Al O /CeO . The selectivity to syngas of these FA microlith was
2 3 2
◦
better than commercial Pt gauzes at 900 C and 1 bar and 20 bar.
The methane conversion reached the thermodynamic equilibrium
at 20 bar.
Except for the 90%Pt/10%Rh gauze and 1%Pt/Ce/FA, CO selectiv-
ity was greater at 1 bar than at 20 bar whereas the selectivity for
the coated catalyst were higher at 20 bar. We attributed the low H2
yield to combustion of hydrogen with unreacted oxygen in the gas
phase. The 1% Pt/FA and 5% Pt/FA catalysts were stable at 20 bar but
Fig. 15. Selectivity to carbon deposition compared with the thermodynamic calcu-
lation for graphite produced as a function of pressure. Open symbols 1 bar, filled
symbols 20 bar. at different pressures.
5
% Pt/Ce/FA lost about 10% of his activity in 1 h. We are now testing
long term stability.
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Please cite this article in press as: C. Neagoe, et al., Pt on Fecralloy catalyses methane partial oxidation to syngas at high pressure, Catal.
Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.11.018