Effect of Regeneration Period on the Selectivity of Synthesis Gas of Ba-Hexaaluminates in Chemical Looping Partial Oxidation of Methane

Article abstract of DOI:10.1021/acscatal.8b03855

Chemical looping partial oxidation of CH₄ is a promising method for producing syngas with a suitable H₂/CO ratio and avoiding the risk of explosion and use of an expensive air separation plant. However, regeneration of reduced oxygen

Full text of DOI:10.1021/acscatal.8b03855

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Article
The effect of regeneration period on the selectivity of synthesis gas of
Ba-hexaaluminates in chemical looping partial oxidation of methane
Qianqian Shen, Fei Huang, Ming Tian, Yanyan Zhu, Lin Li, Junhu Wang, and Xiaodong Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03855 • Publication Date (Web): 19 Nov 2018
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The effect of regeneration period on the selectivity of synthesis gas
of Ba-hexaaluminates in chemical looping partial oxidation of
methane
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Qianqian Shen,a,b Fei Huang,c Ming Tian,*,a Yanyan Zhu,*,b Lin Li,a Junhu Wang,a
Xiaodong Wang*,a
aDalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan
Road, Dalian 116023 (P. R. China).
bCollege of Chemical Engineering, 229 Taibai Bei Road, Northwest University, Xi’an
710069 (P. R. China).
cFujian Normal University, 8 Shangsan Road, Fujian 350007 (P. R. China).
*Corresponding author: E-mail: tm1982@dicp.ac.cn; yanyanzhu2006@126.com;
xdwang@dicp.ac.cn
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Abstract
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Chemical looping partial oxidation of CH4 is a promising method to produce
syngas with suitable H2/CO ratio, and avoiding risk of explosion and expensive air
separation plant. However, regeneration of reduced OCs by O2 usually leads to the
enrichment of surface electrophilic O2- or O- species thus the CO2 formation and the
decrease in total CO selectivity. In this work, it was found that CO selectivity of
BaFe3Al9O19 hexaaluminate (BF3A) OC was greatly improved from 58% to 83% by
tuning regeneration period with CH4 conversion of 86%, H2/CO ratio of 2, syngas
yield of 4.2 mmol/g without significant carbon deposition. This resulted from the
decrease of regeneration time (from 15 min to 4.2 min) leading to the decline in Fe3+
in Al(1), Al(2) and Al(3) sites (Fe1, Fe2 and Fe3) active for combustion of CH4, and
the increase in reduced Fe2+ to Fe0 responsible for the partial oxidation of CH4.
Consequently, the amount of CO2 decreased while that of CO increased significantly.
In addition, un-oxidized surface Fe0 resulted from shorter regeneration period (5 min
and 4.2 min) acted as catalysts for dry reforming of CH4 and CO2 at the beginning of
reaction, which also contributed to the increase of CO formation for these two
samples.
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Key words: CH4 conversion; Chemical looping partial oxidation; The improvement of
syngas; Regeneration period; Ba-hexaaluminate
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1. Introduction
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The conversion of methane as the main component of natural gas and shale gas to
high value-added chemicals is getting more economically attractive since crude oils
have become depleted and natural gas has been more available due to the discovery of
shale gas reservoirs.1, 2 Currently, methane is industrially transformed to chemicals
and fuels through the indirect route, e.g. it is converted to syngas (CO+H2) and then
the syngas is used as feedstock for the production of methanol, and liquid fuels by
Fischer-Tropsch synthesis.3, 4 Three representative pathways to reform methane to
form syngas are steam reforming (SR, equation 1),5 dry reforming (DR, equation 2)6
and partial oxidation (PO, equation 3).7
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CH4 + H2O → CO + 3H2, Ho298K= 206 kJ mol-1
CH4 + CO2→ 2CO + 2H2, Ho298K= 247 kJ mol-1
CH4 + 1/2O2→ CO + 2H2, Ho298K= -36 kJ mol-1
(1)
(2)
(3)
The H2/CO ratio for SR and DR is 3 and 1, respectively, which is not suitable for the
downstream process such as methanol production and Fischer-Tropsch synthesis.8
Moreover, these two reactions are endothermic and energy intensive. In comparison,
PO is mildly exothermic and its H2/CO ratio is favourable for the downstream process
for the production of chemicals.9 Unfortunately, this reaction has to require a costly
air separation to supply pure oxygen and has risk of explosion because of mixing
methane and oxygen, which hamper the industrial implementation of this route.10
Thus, a novel technique for the conversion of methane to syngas with suitable H2/CO
ratio, and avoiding risk of explosion and expensive air separation plant is highly
desired.
Chemical looping partial oxidation (CLPOx) was proposed to overcome these
drawbacks above by a two-step method for the production of syngas using lattice
oxygen of oxygen carriers (OC).11-16 In this process, CH4 is reacted with lattice
oxygen to form CO and H2 and then the reduced OC is re-oxidized by air. Lattice
oxygen is used to react with CH4 instead of gaseous oxygen, which allows
considerable cost savings without air separation unit and avoids risk of explosion.
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Excellent performance of OC with high CH4 conversion, syngas selectivity, good
redox stability and carbon-resistance is of great importance to the realization of this
process. Various studies have been contributed to improving syngas selectivity based
on the development of novel OC materials.17-23 Shafiefarhood et al.24 designed a
Fe2O3@La0.8Sr0.2FeO3-δ core-shell redox catalysts for CLPOx of methane in order to
avoid the direct contact of CH4 and Fe2O3, which significantly increased the
selectivity of CO. Mishra et al.25 found that BaMnO3 showed higher syngas selectivity
than CaMnO3 did, which was because CaMnO3 contains significantly higher amounts
of loosely bound oxygen species that were highly active for the formation of CO2.
Chen et al.26 reported that syngas yield of Ni-promoted WO3/Al2O3 could be
substantially increased by about 2.7-fold in comparison with its unmodified
counterpart, which was attributed to that surface-grafted nickel species could
effectively activate methane molecules and catalyze the partial oxidation reaction.
The studies above focus on the effect of the modification of OC materials on the
syngas selectivity.
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Besides the variation of OCs, oxidation atmosphere in the re-oxidation process also
influences the syngas selectivity in the reduction step greatly.27 More et al.28 found
that compared with air oxidation, CO2 as oxidants could keep Ni in its metallic state
which catalyzed the gasification of carbon deposited on Ni thus leading to the
increase of syngas selectivity. Zhu et al.29 also discovered that the syngas selectivity
of LaFe3Al9O19 hexaaluminate OC was much higher when using CO2 as oxidants than
that using O2. This resulted from that CO2 could selectively regenerate Fe3+-O5(Tr)
and Fe3+-O4(Th) sites which were highly active for the CH4 partial oxidation to
syngas. H2O as a soft oxidant used to recover OCs was also reported to be able to
increase the syngas selectivity in the reduction step. Neal et al.30 used a low
concentration of steam (2%) to regenerate iron species to FeO, which avoided the
initially complete oxidation reaction of CH4 resulting in the increase of CO selectivity.
Compared with these weak oxidants, using air/oxygen to replenish OCs can usually
completely regenerate their structure due to the strong oxidization. Moreover, this
process is mildly exothermic, which is highly energy efficient. However, regeneration
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of OCs by O2 as strong oxidants usually results in the enrichment of surface
electrophilic O2- and O- species which was highly active for the CO2 formation,
thereby the total CO selectivity decreases. Thus, it is highly desired to enhance the
selectivity of syngas when using O2 as regeneration agent for CLPOx of CH4.
In our previous work, BaFe3Al9O19 hexaaluminate OC exhibited high activity for
complete oxidation of CH4 due to significant CO2 formation in the initial of reduction
reaction leading to low syngas selectivity.31, 32 In this paper, its CO selectivity could
be greatly enhanced from 58% to 83% by tuning regeneration period with CH4
conversion of 86%, H2/CO of 2 without significant increase in amount of carbon
deposited. The effect of regeneration period on the micro-structure of OCs was
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studied in detail by a series of characterization techniques such as HRTEM and Fe
Mössbauer spectroscopy. Then the underlying reason for the improvement of CO
selectivity was revealed.
2. Experimental
2.1 OCs preparation
BaFe3Al9O19 (denoted as BF3A) hexaaluminate OC was synthesized by the
two-step method proposed by Huang et al.33, 34 The first step was to prepare BF3A
precursor, 0.52 g of Ba(NO3)2, 6.75 g of Al(NO3)3·9H2O and 2.42 g of
Fe(NO3)3·9H2O were dissolved in 52 ml of N,N-dimethylacetamide (DMAC) under
stirring overnight at room temperature, and then poured into 15.98 g of NH3·H2O and
58.5 ml of DMAC mixed solution under stirring for 0.5 h to form the precursor
precipitate. The precipitate was filtered and washed with ethanol for three times. The
second step was the resin coating process wherein the precipitate was dispersed in 280
ml of ethanol under ultrasonic treatment for 0.5h and then were added into 17.62 g of
resorcinol, 19.46 g of formaldehyde and 8.57g of NH3·H2O step by step. After that,
the mixture was further treated by ultrasonic for 1 h. Subsequently, the precipitate was
o
filtered, washed with ethanol one time and then dried at 120 C overnight. Finally, the
o
sample was calcined in argon atmosphere until the temperature reached 1200 C and
then the gas switched to air in high-temperature tube furnace for 6 h.
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For nitric acid washing experiment, a certain amount of OCs was added to nitric
acid with the concentration of 1 mol/L and vigorous stirred at room temperature for 3
h. Then, the mixed solution was filtered, washed with ultrapure water for 3 times and
dried in an oven at 120 oC overnight.
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2.2 OCs characterization
The X-ray diffraction (XRD) patterns were obtained to analyze the crystal phase
composition before and after cycling using a PANalytical X’Pert-Pro powder X-ray
diffractometer (U=40 KV, I=40 mA) with Cu Kα monochromatized radiation
(λ=0.15418nm), and the diffraction angle (2θ) was scanned from 10o to 80o.
The morphology of the hexaaluminates was studied by scanning electron
microscopy (SEM) on a JEOL JSM-7800F electron microscope.
Scanning transmission electron microscopy (STEM) and elemental mapping were
performed using a field-emission transmission electron microscope (JEOL
JEM-2100F). The samples were dispersed in ethanol and then dropped onto a
carbon-coated 300-mesh copper grid and dried in an oven.
The 57Fe Mössbauer spectra were recorded at room temperature with a
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spectrometer working in constant acceleration mode with the Co γ-quantum source
in Rh matrix. The powder samples were used approximately 10mg/cm2 of natural iron.
During preparation of BF3A OC, 0.02 g 57Fe/g Fe was added to improve the
signal-to-noise of samples. All spectra were fitted by the MossWinn package and the
isomer shifts (IS) were given relative to the centroid of α-Fe at room
temperature.
2.3 OCs reactivity evaluation
The performance of as-prepared BF3A and that after regeneration for different time
for CLPOx of CH4 was investigated in a U-shape quartz fixed bed reactor under
atmospheric pressure. OC (0.2g) with a particle size of 20-40 mesh was placed in a
quartz tube and a K-type thermocouple was used to monitor the reaction temperature.
Prior to each measurement, pure He (50 ml/min) was flowed to the reactor from room
temperature to 900 oC to remove the possible impurities on the OC. When the reaction
temperature reached 900 oC, the reactions between 5% CH4 and 5% O2 were switched
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and the total gas flow was kept to 15ml/min. Helium (He) was used as the carrier gas
to prevent mixing between methane and gas-phase oxygen. The reduction time was 8
min and the oxidation time was 15min, 6min, 5min and 4.2min. The product
components were analyzed by a quadruple mass spectrometer system (InProcess
Instruments GAM 200). The mass spectrometer signals were calibrated
accurately before the experiment using the calibration gas with known
composition. Blank experiment using inert silica with the same void volume at
identical conditions suggested low contribution of gas-phase reactions (<2%
methane conversion of gas-phase reaction).
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The conversion of CH4 (X), the product selectivity (S), the ratio of H2/CO (R)
and the amount of CO2 (nCO2 ) and CO formation (nCO ) in mmol/g during the
reduction process were defined as followed:
t
 yCOdt VC
F   y
red
CO2
0
X 
100
F  yCH 4,in t
red
(1)
t
dt
F   y
red
CO2
0
SCO2

100
t
 yCO )dt VC
F  (y
red
CO2
0
(2)
(3)
t
F   y dt
red
CO
0
SCO
SH 2
R 


100
t
 yCO )dt VC
F  (y
red
CO2
0
t
F  0.5 y dt
red
H 2
0
100
t
 yCO )dt VC
F  (y
red
CO2
0
(4)
(5)
t
F   y dt
red
H2
0
t
F   y dt
red
co
0
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F   y dt
red
CO 2
0
nCO2

24.4W
OC
(6)
(7)
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t
F   y dt
red
CO
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0
nCO

24.4W
OC
Where Fred is the total volume flow rate of the effluent gas in ml/s, yCH yH yCO and
4,in
2
2
,
,
yCO are the volume fraction of CH4 in the inlet gas, and that of H2, CO2 and CO in the
effluent gas in the reduction step, respectively, WOC is the initial weight of OC in the
bed in g, and
VC is the volume of carbon produced in ml which is calculated as:
t
V  F   y
CO2,ox  yCO,oxdt
C
ox
0
(8)
Where Fox is the total volume flow rate of the effluent in ml/s, yCO2,ox and
yCO,ox are the volume fraction of CO2 and CO in the effluent gas in the
re-oxidation step.
The yield of synthesis gas
n(2H
2
 CO)
was calculated as:
t
F  (2 y  y )dt
red
H
CO
2
0
n(2H2  CO) 
24.4W
OC
(9)
3. Results and discussion
3.1 Reactivity test and structure characterization of BF3A-x OCs
3.1.1 The effect of regeneration period on the performance of BF3A OC
Fig. 1 presents the kinetic curves of BF3A OC in the reduction process after
re-oxidation for different time (denoted as BF3A-x, x indicated oxidation time). It
could be seen that CO2 formation significantly declined with the reduction of
regeneration period from 15 min to 4.2 min (Fig. S1(a)). In addition, CO formed
earlier after regeneration for shorter period (Fig. S1(b)). For example, it was about
112 s when CO began to form for BF3A-15 while it decreased to 88 s for BF3A-6. For
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BF3A-5 and BF3A-4.2, CO formed at the beginning of reduction reaction, indicating
that CO formation increased with the shortening regeneration period. The results
above presented that CO2 formation significantly decreased while CO increased after
regeneration for shorter time.
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The amount of CO2 and CO formed for BF3A-x is given in Table 1. It could be seen
that the former reached as high as 0.38 mmol/g for BF3A-15 and it decreased with the
regeneration time. For BF3A-4.2, it was only 0.12 mmol/g, about decreased by 3 times.
On the other hand, the amount of CO increased from 0.62 mmol/g for BF3A-15 to
0.83 mmol/g for BF3A-4.2 with the decrease of regeneration time. The decrease of
amount of CO2 and increase of that of CO formation resulted in the CO selectivity
markedly increasing from 58% to 83% with the decrease of regeneration time (Fig.
2a). Despite the regeneration time decreased by almost 4 times, CH4 conversion,
H2/CO ratio and the amount of carbon deposited changed a little (Fig. 2(b, c, d)),
indicating that active species in BF3A OC for partial oxidation of CH4 was still
recovered. The stability of BF3A-x during multiple redox cycles was shown in Fig. 3.
Their performance was almost constant during 20 redox cycles even though the
regeneration time decreased to 4.2 min with CH4 conversion of 86%, CO selectivity
of 87%, H2/CO ratio of 2 and syngas productivity of 4.2 mmol/g, indicating that good
redox stability of BF3A OC could be maintained even if re-oxidation time decreased
by 4 times. It was generally accepted that the enrichment of surface electrophilic O2-
or O- species by regeneration of O2 led to the CO2 formation in the initial period of
reaction thus the decrease of total CO selectivity.35, 36 In the present work, syngas
selectivity could be significantly improved due to the decrease of CO2 and increase of
CO formation by tuning regeneration period of O2.
3.1.2 X-ray diffraction patterns of BF3A-x OCs
Fig. 4 shows X-ray diffraction (XRD) patterns of BF3A OC before reaction and
after different regeneration time. For fresh BF3A OC, the diffraction peaks attributed
to β-Al2O3 phase and MP-type hexaaluminate (JCPDS No. 1-84-1788) were observed.
After re-oxidation, the MP-type hexaaluminate for all the samples was transformed
completely to β-Al2O3 phase, which was in line with the results of our previous
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work .31, 32 In addition, all the diffractogram after re-oxidation shifted to a lower 2θ
values compared with that of fresh BF3A and the shifting extent became larger with
the decrease of regeneration time (Fig. 4(b)). Meanwhile, the cell parameters of BF3A
after oxidation for shorter time were larger (Table 2), suggesting that the decrease of
regeneration time caused that more Fe ions in lower oxidation state could not be
oxidized. It was worthwhile to note that diffraction peaks associated with Fe0 (JCPDS
No. 1-1262) were observed besides β-Al2O3 phase for BF3A-5 and BF3A-4.2 and its
intensity for BF3A-5 was lower than that for BF3A-4.2. This could be also confirmed
by HRTEM results wherein some sphere particles with size of about 50-100 nm only
composed of Fe without Ba, Al and O element were clearly observed on the surface of
plate-like hexaaluminates for these two samples (Fig. 5 (a and b)). This should result
from shorter regeneration period leading to that Fe0 formed in the reduction step was
not oxidized in the re-oxidization step. Combined with our results of reactivity test
that syngas formed at the beginning of reduction reaction for BF3A-5 and BF3A-4.2
(Fig. 1 and Fig. S1(b)), this might be associated with these Fe0 species resulted from
shorter regeneration time.
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3.1.3 SEM characterization of BF3A-x OCs
SEM images of BF3A-x OCs are presented in Fig. 6. The morphology of BF3A
after regeneration remained unchanged compared with fresh sample still showing
typically plate-like hexaaluminate crystallites even though the re-oxidization time
decreased by about 4 times, indicating that hexaaluminate structure could be also
recovered after regeneration for shorter time.
3.1.4 57Fe Mössbauer spectroscopy of BF3A-x OCs
Fig. 7 shows 57Fe Mössbauer spectroscopy of BF3A-x and the corresponding
parameters are listed in Table 3. For BF3A-15, two magnetic sextets were observed.
One with IS of 0.27 mm/s, QS of 0 mm/s and hyperfine field of 486 KOe was
attributed to Fe3+ in Al(3) sites (Fe3) in β-Al2O3 phase while the other one with IS of
0.47 mm/s, QS of 0.55 mm/s and hyperfine field of 385 KOe was associated with Fe3+
in Al(1) sites (Fe1).24 Their relative area (A) was 16% and 25%, respectively. In
addition, two doublets with IS of 0.25 mm/s and 0.7 mm/s were attributed to Fe3+ in
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Al(2) sites (Fe2) (48%) and Fe2+ (5%), respectively.37-40 With the decrease of
regeneration time to 6 min, the amount of Fe3 and Fe2 did not change but that of Fe1
decreased to 12%, which resulted in the amount of Fe2+ increasing to 18%. Further
decreasing regeneration time to 5 min, the amount of Fe3 remained unchanged but Fe1
completely disappeared and that of Fe2 decreased to 44% causing that of Fe2+ further
increased to 32%. Moreover, a new sextet with IS of -0.05 mm/s, QS of -0.08 mm/s
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and magnetic field of 331 KOe was seen which was associated with Fe0,41,
suggesting that the decrease in regeneration time resulted in that not only Fe2+ but also
Fe0 could not be re-oxidized to Fe3+. For BF3A-4.2, the amount of Fe3 also decreased
besides that of Fe2 further decreasing to 25% as well as that of Fe2+ and Fe0 increasing
to 47% and 18%, respectively. 57Fe Mössbauer spectroscopy results presented that the
shortening of regeneration period caused that Fe1, Fe2 and Fe3 could not be recovered,
which led to the decrease of their amount and the increase of that of Fe2+ and Fe0.
Considering the results of reactivity test, it could be inferred that the decline of CO2
formation might originate from the decrease of Fe1, Fe2 and Fe3 while increase of CO
amount might be due to the enhancement of Fe2+ and Fe0.
3.2 Discussion
Our reactivity test results demonstrated that CO selectivity of BF3A OC in the
reduction step increased with the shortening of regeneration period, which resulted
from the decrease in amount of CO2 and the increase in that of CO formation. In the
following section, the underlying reason why CO selectivity increased would be
uncovered.
3.2.1 The reason for decrease of CO2 formation for BF3A-x
57Fe Mössbauer spectroscopy of BF3A-15 and BF3A-6 after reduction time of 105s
and 75s (denoted as BF3A-15-R and BF3A-6-R) when no CO formation was observed
(Fig. 1(a, b)) was carried out and the results are shown in Fig. S2 and corresponding
parameters are listed in Table S1. For BF3A-15-R and BF3A-6-R, both Fe3 and Fe1
completely disappeared and that of Fe2 decreased from 54% to 44% compared with
BF3A-15 and BF3A-6 (Table 4). This result indicated that all of Fe3, Fe1 and Fe2 were
responsible for CO2 formation. The correlation between the total amount of these
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three crystallographic sites and that of CO2 formation during reduction reaction is
shown in Fig. 8. It could be clearly seen that the amount of CO2 formation decreased
with the total that of Fe3, Fe1 and Fe2, which demonstrated that shortening of
regeneration time led to the significant decline in amount of high activity for CH4
combustion of Fe3, Fe1 and Fe2, thereby the CO2 formation decreased during
reduction reaction.
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3.2.2 Active species for syngas formation for BF3A-x
Our reactivity test results showed that curves of CO concentration differed greatly
after regeneration for different time (Fig. S1(b)). For BF3A-15 and BF3A-6, CO
concentration increased significantly after a fixed reaction time, which was denoted as
reaction i (Fig. S3). For BF3A-5 and BF3A-4.2, small amount of CO could be detected
at the beginning of reaction besides reaction i, which was denoted as reaction ii (Fig.
S3). The amount of CO formed in these two reactions was given in Table S2. In the
following section, we would identify the active sites for these two reactions
separately.
In order to determine active species for reaction i, 57Fe Mössbauer spectroscopy of
deeper reduction (480 s) for these four samples (denoted as BF3A-x-Re) was
performed (Fig. S4 and Table S3). For BF3A-15-Re, the amount of Fe2 decreased
from 44% to 18% compared with BF3A-15-R (Table 4), which was responsible for
CO2 formation during deeper reduction (after reaction time of 105 s) since Fe2 was
active for combustion of CH4 as discussed in the last section. In addition, Fe0
appeared and its amount reached 18%, which was due to the reduction of Fe2+,
indicating that the reduction of Fe2+ to Fe0 contributed to CO formation in reaction i
(
CH4  Fe2  CO  2H2  Fe0 ). Further decreasing regeneration time to 6 min, the
amount of Fe0 increased to 19% (Table 4 and S3) resulted from enhanced reduction of
Fe2+, which led to the increase in the amount of CO from 0.62 mmol/g to 0.69 mmol/g
(Table S2). For BF3A-5-Re and BF3A-4.2-Re, the amount of Fe0 further increased by
20% (28%-8%) and 22% (40%-18%) compared with BF3A-5 and BF3A-4.2 (Table 4
and S3), resulting in the increase of CO formation to 0.72 mmol/g and 0.78 mmol/g,
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respectively. In order to correlate amount of reduced Fe2+ and reactivity, the amount
of CO formed in reaction i was given in Fig. 9. It could be clearly observed that CO
increased with the reduction of Fe2+ to Fe0, indicating that the decline of regeneration
time led to the enhancement of reduction of Fe2+ to Fe0, which was responsible for
increased CO formation in reaction i.
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For BF3A-5 and BF3A-4.2, small amount of CO formed at the beginning of reaction
(reaction ii). TEM results indicated that Fe0 nanoparticles were clearly observed on
the surface on these two OCs. These Fe0 particles might catalyze the dry reforming of
CH4 and CO2 formed.43-45 In order to confirm this, we used nitric acid (1 mol/L) to
remove these Fe0 particles on the surface at room temperature and then examined if
the syngas would form at the beginning of reaction. The X-ray diffraction patterns of
BF3A-5 and BF3A-4.2 after nitric acid washing showed that the diffraction peaks of
57
Fe0 could hardly be detected (Fig. S5). In addition, Fe Mössbauer spectroscopy of
these two samples presented that no subspectra attributed to Fe0 could be observed
(Fig. S6 and Table S4), indicating that Fe0 particles were successfully removed on the
surface of these two samples. Moreover, syngas indeed could not be detected for the
samples after nitric acid washing (Fig. 10), which gave solid evidence that these Fe0
particles resulted from shorter regeneration period acted as catalysts for the dry
reforming of CH4 and CO2 leading to syngas formation (reaction ii:
Fe0
CH4  CO2  2CO  2H2 ) in the initial of reaction for BF3A-5 and BF3A-4.2.
According to the discussion above, manipulating regeneration time led to the
remarkable drop of Fe1, Fe2 and Fe3, highly active for CH4 combustion, and increase
in the amount of reduced Fe2+ to Fe0, responsible for the partial oxidation of CH4. As
a result, CO2 decreased and CO formation increased significantly. Specially, for
BF3A-5 and BF3A-4.2, appearance of Fe0 catalyzed the dry reforming of CH4 and
CO2 at the beginning of reaction, which also contributed to the increase of CO
formation. This provided a possible strategy to decrease CO2 formation in the initial
of reaction due to the surface electrophilic oxygen species when OCs were
regenerated by O2 thus increasing CO selectivity.
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4. Conclusions
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In this work, the effect of regeneration period on the syngas selectivity of BF3A
OC was investigated. It was for the first time found that tuning regeneration period
could remarkably increase its CO selectivity from 58% to 83% with CH4 conversion
of 86%, H2/CO ratio of 2, syngas yield of 4.2 mmol/g without evident carbon
deposition, which was due to significant decline of CO2 and increase of CO formation.
The decrease of regeneration time resulted in decline of Fe1, Fe2 and Fe3 which
exhibited high activity for combustion of CH4 thus CO2 formation, and the
enhancement of reduced Fe2+ to Fe0 which was responsible for the partial oxidation of
CH4 thus the increase of CO formation. Specially, for BF3A-5 and BF3A-4.2,
un-oxidized Fe0 formed in the reduction step due to shorter regeneration period
catalyzed dry reforming of CH4 and CO2 leading to small amount of CO formation in
the initial of reaction, which provided the possibility to decrease the amount of CO2 at
the beginning of reaction resulted from surface electrophilic O2- or O- species if
reduced OCs were regenerated by O2 thus the increase of CO selectivity. The present
work proposes a novel method to enhance the syngas selectivity based on the
manipulation of regeneration period.
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Supporting Information. Additional experimental results of the performance and
characterization of catalysts including Figures S1− S6 and Tables S1−S4
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Acknowledgment
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We are grateful for the financial support provided by the National Science
Foundation of China (NSFC) grants (21303137, 21406225, 21573232, 21676269,
21706254 and 21808033) and Postdoctoral Science Foundation of China
(2014M561261).
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44. Kim, S. M.; Abdala, P. M.; Margossian, T.; Hosseini, D.; Foppa, L.;
Armutlulu, A.;
van Beek, W.;
Comas-Vives, A.;
Coperet, C.; Muller, C.,
Cooperativity and Dynamics Increase the Performance of Nife Dry Reforming
Catalysts. J. Am. Chem. Soc. 2017, 139, 1937-1949.
45. Zhang, F. S.; Song, Z. L.; Zhu, J. Z.; Sun, J.; Zhao, X. Q.; Mao, Y. P.;
Liu, L.; Wang, W. L., Factors Influencing CH4-CO2 Reforming Reaction over Fe
Catalyst Supported on Foam Ceramics under Microwave Irradiation. Int. J. Hydrogen
Energy 2018, 43, 9495-9502.
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Figure 1 The kinetic curves of reduction process for BF3A-x hexaaluminate OC (a)
BF3A-15, (b) BF3A-6, (c) BF3A-5 and (d) BF3A-4.2
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BF3A-4.2
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Samples
Samples
Figure 2 CO and CO2 selectivity (a), CH4 conversion (b), H2/CO ratio (c) and C yield
(d) of BF3A-x
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Number of cycle
Figure 3 Performances of BF3A-x during multiple redox cycles, CH4 conversion (a),
CO2 selectivity (b), CO selectivity (c), H2 selectivity (d), H2/CO ratio (e) and
synthesis gas yield (f)
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β
 Fe  Al2O3  MP
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BF3A-5
BF3A-4.2
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2 /degree
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Figure 4 X-ray diffraction patterns of fresh BF3A and BF3A-x, (a) 10-80o and (b)
30-40o
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Figure 5 STEM images of (a) BF3A-5, (b) BF3A-4.2, EDS elemental mapping and
line scans results of BF3A-5 and BF3A-4.2
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Figure 6 SEM images of BF3A-15 (a), BF3A-6 (b), BF3A-5 (c) and BF3A-4.2 (d)
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Figure 7 Room temperature 57Fe Mössbauer spectra of BF3A-x
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