Based on the ability of the reduced samples to dissociate CO molecule, we are firmly expected that the reduced copper-free perovskite is active for the synthesis of hydrocarbons while the[r]
Trang 1AUTHOR QUERY FORM Journal: JSAMD
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Trang 2Original Article
perovskites for CO hydrogenation
Q2 Nguyen Tien Thao*, Le Thanh Son
Faculty of Chemistry, Vietnam National University Hanoi, 19 Le Thanh Tong ST, Hanoi, 10999, Viet Nam
a r t i c l e i n f o
Article history:
Received 15 June 2016
Accepted 28 July 2016
Available online xxx
Keywords:
CO hydrogenation
Metal dispersion
CueCo
Perovskite
a b s t r a c t
synthesis from syngas The experimental results indicated that the activities in CO dissociation and
different from those in the extra-perovskite lattice The overall catalytic activity in syngas conversion is correlated with the CoeCu metal surface, but the alcohol productivity e productivity of alcohols
© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi
1 Introduction
Perovskites, mixed oxides of the general formula ABO3, have
extensively been applied in many fields due to their particular
compositional structure [1] In principle, the ideal perovskite
structure is cubic with the space-group Pm3m-Oh in which the A
cation occupies at the body center, the cation (B) is at the cube
corner, and the oxygen stays at the midpoint of the cube edges[1,2]
By this way, the perovskite derivatives may be synthesized by the
replacement of another element in A and/or B position[1,3,4] In
the present work, we have partially substituted Co3þin
lanthanum-cobaltate by Cu2þ to obtain La(Co,Cu)O3 perovskite catalyst
pre-cursors The partial reduction of La(Co,Cu)O3 perovskites may
further produce metallic copper-cobalt metals those originate
intentionally from the perovskite lattice As a result, a finely
dispersed metal catalyst from perovskite precursors would be
ex-pected to use for several applications[2,4] In experimental, Crespin
and Hall[5]had produced Co0/La2O3from the reduction of LaCoO3
under hydrogen atmosphere Fierro et al.[6]received the Ni/La2O3
after the complete reduction of LaNiO3at 705 K Bedel et al.[4]only
carried out the partial reduction of La(Co,Fe)O3 orthorhombic
perovskites at 723 K, producing a small amount of metallic cobalt while the perovskite lattice still preserved Thus, the perovskite product has exhibited a high catalytic activity in many applications such as CO oxidation[7]hydrogenation of ethylene[8], reforming
of CO2 [9], and conversion of syngas (H2/CO) into many useful chemicals and liquid fuels[10,11] The latter conversion is a very important process since a mixture of alcohols is a crucial gasoline additive or green vehicle fuel today[12e14]
In our previous work, we have reported some novel character-istics of lanthanum-cobaltate nanoperovskites prepared by mechano-synthesis method[10,11,14] The reduction of such ma-terials leads to the formation of a well-homogenized supported bimetallic alloy Furthermore, the co-existence of two transition metal ions in the solid lattice results in the formation of dual sites which are active for many oxidation-reduction applications [4,15e19] This article is to present a way for the preparation of metals supported catalysts for the conversion of carbon monoxide into oxygenated compounds
2 Experimental 2.1 Catalyst preparation
A series of CoeCu bases perovskites were synthesized by mechano-synthesis method The stoichiometric proportions of commercial lanthanum, copper, and cobalt oxides (99%, Aldrich)
* Corresponding author Fax: þ84 (04) 3824 1140
E-mail address: ntthao@vnu.edu.vn (N Tien Thao).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
http://dx.doi.org/10.1016/j.jsamd.2016.07.011
2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
Journal of Science: Advanced Materials and Devices xxx (2016) 1e6
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Trang 3were mixed together with three hardened steel balls
(diameter¼ 11 mm) in a hardened steel crucible (50 mL) A SPEX
high energy ball mill working at 1000 rpm was used for
mechano-synthesis Milling was carried out for 8 h prior to a second milling
step with an alkali additive Then, the resulting powder was mixed
to 50% sodium chloride (99.9%) and further milled under the same
conditions for 12 h before washing the additives with distilled
water A sample was added into a beaker containing 1200 mL water
and stirred by magnetic stirring for 90 min prior to being
sedi-mented for 3e5 h After the clean water is removed, the slurry was
dried in oven at 60e80C before calcination at 250C for 150 min.
2.2 Characterization
The elemental chemical analysis of copper and cobalt in the
perovskites was performed by atomic absorption spectroscopy
using a PerkineElmer 1100B spectrometer Phase analysis and
crystal domain size determination were performed by powder
X-ray diffraction (XRD) using a SIEMENS D5000 diffractometer with
CuKaradiation (l¼ 1.54059 Å) Bragg's angles between 15 and 75
were collected at a rate of 1/min
To measure the real surface area of the reduced perovskites, two
other BET experiments were performed using aflow system
(RXM-100, Advanced Scientific Designs, Inc., ASDI) First, 70e100 mg of
catalyst was calcined at 773 K (ramp of 5 K/min) under 20 mL/min
of O2/He (20 vol %) for 90 min and then evacuated at 723 K for
90 min (P 8.5 108mmHg) Nitrogen adsorption was carried
out at 77 K Each point of the adsorption isotherm was established
by introducing a given amount of nitrogen from the reaction
manifold into the reactor Temperature Programmed Reduction
(TPR) experiments were carried out after evacuating N2adsorption
(BET measurement) TPR of the catalysts was then carried out by
ramping under 4.65 vol.% of H2/Ar (20 mL/min) from room
tem-perature to 773 K (5 K/min) for 90 min The second BET
measure-ment of the sample after reduction was also done in situ
Chemisorption performance with H2at 373 K and CO at room
temperature was carried out after the second BET measurement
The H2-chemisorption performance was similar to steps for BET
measurement of nitrogen After the first isotherm that contains
both physical adsorption and chemisorption was collected, the
sample was evacuated at adsorption temperature for 5e10 min in
order to remove all physically adsorbed species prior to do the
second adsorption The difference between thefirst and the second
isotherm gives the chemisorption isotherm H2-and CO-uptake
were determined by extrapolating the straight-line portion of the
adsorption isotherms to zero pressure as represented inFig 1
CO dissociation tests on the reduced samples were carried out
using a RXM-100 system Prior to pretreatment, 40e50 mg of
catalyst was ramped at 10 K/min up to the calcination temperature
(773 K) under 20 vol.% O2/He (20 mL/min) for 90 min and then
cooled down to room temperature under aflow of 20 mL/min He
for 60 min in order to remove the physically adsorbed gas The
pretreatment of the catalysts was then carried out from room
temperature up to 798 K (5 K/min) for 90 min under 4.65 vol % of
H2/Ar (20 mL/min) and then cooled down to reaction temperature
under aflowing 20 mL/min of He A number of CO/He (0.586 vol %)
pulses (0.25 mL) were then injected and passed through the reactor
prior to on line analysis using mass spectrometer (UTI-100) The m/
z signals 2, 18, 28, 44 were collected
2.3 Catalytic activity
The catalytic tests were carried out in a stainless-steel
contin-uous fixed-bed flow micro-reactor (BTRSeJr PC, Autoclave
Engi-neers) The reaction pressure was controlled using a back-pressure
regulator The syngas mixture (H2/CO¼ 2/1) was diluted in helium (20 vol %) A mixture of reactants and inert gas was supplied from a pressurized manifold via individual mass flow controllers The catalyst pellet size was 40 mesh Catalysts were pretreated in situ underflowing 5 vol.% of H2/Ar (20 mL/min) prior to each reaction test The temperature was kept at 523 K (3 h), and 773 K (2h30) with a ramp of 2 K/min Then, the reactor was cooled down to the reaction temperature while pressure was increased to 69 bars by feeding a reaction mixture of gases The products were analyzed using a gas chromatograph equipped with two capillary columns and an automated online gas sampling valve maintained at 443 K
The temperature of transfer line between the reactor and the valves was kept at 493 K in order to avoid any product condensation
Carbon monoxide and carbon dioxide were separated using a capillary column (Carboxen™ 1006 PLOT, 30 m 0.53 mm) con-nected to the TCD Quantitative analysis of all organic products was carried out using the second capillary column (Wcot fused silica,
60 m 0.53 mm, Coating Cp-Sil 5CB, DF ¼ 5.00mm) connected to a FID detector (Varian CPe 3800) and mass spectrometer (Varian Saturn 2200 GC/MS/MS)
3 Results and discussion 3.1 Catalyst characteristics The physical properties of all fresh catalyst samples are measured and shown inTable 1 X-ray diffraction spectra of all samples were collected (but not shown) and the crystal phase is presented inTable 1 [10] Although Table 1 shows each sample contains at least two components, but it is noted that the main phase is perovskite a long with a very small amount of starting metal oxide material(s) as impurities [10] Both LaCoO3 and La(Co,CuO)O3 perovskites are presented as the well-structured
0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40
0 10 20 30 40 50
Pressure (torr)
H 2
-Total adsorption Chemisorption Physisorption
Fig 1 H 2 e Chemisorption at 373 K over LaCo 0.4 Cu 0.6 O 3 reduced at 773 K.
Table 1 Properties of the synthesized perovskites.
area (m 2 /g) b
Chemical composition
a XRD spectra were compared to JCPDS files: P: Perovskite (JCPDS No 48e0123);
Co 3 O 4 (JCPDS No 42e1467); CuO(JCPDS No 45e0937).
b BET surface area before reduction.
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Please cite this article in press as: N Tien Thao, L.T Son, Production of cobalt-copper from partial reduction of La(Co,Cu)O perovskites for CO
Trang 4rhombohedral perovskite as shown inFig 1 [3e5,10] The crystal
domain, determined from the FWHM of the (102) diffraction peak
using Scherrer's equation after Warren's correction of instrumental
broadening, is in the range from 7.9 to 10.5 nm The third column in
Table 1indicates that all ground perovskites samples have medium
surface area, ranging from 20 to 60 m2/g.Fig 2
The reducibility of the ground perovskites is interpreted
through the H2-TPR analysis as represented inFig 3 It is clearly
observed that the copper efree perovskite sample shows two
distinct visible peaks at 670 and 960 K The low temperature peak is
firmly ascribed to the reduction of Co3 þto Co2 þand the other broad
peak is attributed to the complete reaction of cobalt divalent to
metallic phase, in good harmony with the results reported by
several groups [1,4e6,9,10] A similar H2-TPR feature is also
observed for LaCo0.9Cu0.1O3, the profile slightly shifts to the lower
temperature (Fig 3) Thus, the first shaped-peak is observed at
650 K while the second is at 840 K It is worthily noted that the H2
-TPR baseline was completely recovered at 950 K showing that the
reduction is essentially terminated at much lower temperature as
compared with the case of LaCoO3 [10,12] The calculation of H2
consumed amount balance indicates that the reduction of Co3þand
Cu2þto Co2þand Cu0below 671 K, whereas that of Co2þto Co0at
840 K [4,11,15] An increased amount of intra-perovskite lattice
copper leads to a significant affect on the perovskite reducibility
[15] For LaCo0.7Cu0.3O3sample, the lower peak is visible, but the
other is very broadening from 643 to 943 K When a larger amount
of intra-lattice cobalt is replaced by copper ions, the two distinct
peaks in H2-TRP trace of the perovskite sample seems to coalesce
into a single peak at 687 K while that of the physical mixture of
Cu2O/LaCoO3still preserves two visible peaks at 670 and 1018 K
This is interpreted by the assumption of mutual interaction
be-tween cobalt and copper ions in the reduced form; the reduced
copper metal is to promote the reducibility of cobalt ions in the
framework when copper was essentially extracted from the
perovskite lattice at lower reduction temperature as demonstrated
by H2-TPR results[1,10,15,19] Moreover, hydrogen is well known to
be easily dissociated to hydrogen atoms on metallic copper sites
Consequently, the reduction of cobalt ions (Co3þ and Co2þ) by
atomic hydrogen is presumably taken place at lower temperatures
[1,10,15] Thus, the reduction of La(Co,Cu)O3is to provide afinely
dispersed CoeCu atoms on the catalyst surface and the formation of
bimetallic alloy is not ruled out[19]
To determine the dispersion of metals formed, we have
measured both H2and CO chemisorptions for the reduced
perov-skite forms Unfortunately, the determination of each individual
component dispersion level is a very difficult task due to a synergic
interaction between two metals after the partial reduction of
perovskites[10,11,15,19] In this case, we have recorded total H2and
CO chemisorbed volume (mL/g) of each sample (Fig 1) The volume
of H2and CO uptake of all reduced samples is in turn presented in Table 2
The reduced samples were investigated the ability of CO dissoci-ation to C* intermediate which further hydrogenates to carbon skel-eton through performing the dissociation of CO versus temperature programmed from room temperature to 798 K The relationship be-tween CO dissociation conversion and temperature is displayed in Fig 4 It seems that the CO decomposition level is related with the chemical composition and dissociation temperatures[15,17,20] The presence of copper component gives rise to slight decreased CO dissociation conversion in the temperature range of 500e798 K The
CO dissociation conversion decreases with the order of LaCoO3> CuO/ LaCoO3> LaCo0.7Cu0.3O3> LaCo0.4Cu0.6O3 LaCo0.9Cu0.1O3(Fig 4) It
is well known that cobalt metal has shown a very good activity in the dissociation of CO while copper is inactive for the CO splitting [17,21,22] In this case, copper plays an important role in the synthesis
of alcohols through the protection of the OH functional groups during the hydrogenation conditions[11,20,22] Thus the higher CO con-versions over the cobalt-rich samples are certainly comprehensive [4,21] Based on the ability of the reduced samples to dissociate CO molecule, we are firmly expected that the reduced copper-free perovskite is active for the synthesis of hydrocarbons while the perovskite containing copper may act as promising catalysts for the hydrogenation of CO to linear primary alcohols
3.2 Catalytic activity in hydrogenation of carbon monoxide All the prepared samples are pretreated under hydrogen flow-rate at 798 K prior to test for the hydrogenation of carbon monoxide
at 548 K, 68.9 bars and space velocity VVH¼ 5000 h1(H2/CO/
He¼ 8/4/3) The products contain a mixture of linear primary al-cohols and n-alkane in addition to small amounts of secondary alcohols and isoparafins and the formation of products is believed
to be associated with the catalyst metal surface[22] Thus, we presented the correlation between CO conversion and product selectivity versus the CO-chemisorbed volume uptake With the exception of mixture Cu2O/LaCoO3sample,Fig 5shows
an increased CO conversion with the CO chemisorbed-volume in
LaCoO3> LaCoO3> LaCo0.4Cu0.6O3> LaCo0.9Cu0.1O3 The selectivity
to alcohols obtained over these catalyst samples are presented in Fig 6 Although product selectivity seems to be well correlated with the CO-chemisorbed volume, it should be less meaningful as compared the product selectivities at different conversion values (Fig 6) Thus, a comparison between the productivities may give more insight into the catalytic behavior[10,22] Undoubtedly,Fig 6
0 300 600 900 1200 1500 1800
LaCo Cu O
LaCo Cu O
LaCo Cu O
Cu O/LaCoO
LaCoO 2θ (degree)
Fig 2 XRD patterns of all catalyst precursor samples.
0 5 10 15 20 25
Temperature (K)
LaCoO 3
LaCo0.4Cu0.6O3
Cu 2 O/LaCoO 3
LaCo0.9Cu0.1O3
Fig 3 H 2 -TPR profiles for the catalyst samples from room temperature to 1050 K in
20 mL/min of 4.65 vol.% of H 2 /Ar flowrate.
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Trang 5shows the productivity of alcohols decreases monotonically with
CO uptake in the order of LaCo0.7Cu0.3O3> LaCo0.4Cu0.6O3> Cu2O/
LaCoO3 > LaCo0.9Cu0.1O3 > LaCoO3 This observation is not with
respect to the order of CO chemisorbed volume, but in good
agreement with the ratio of H2volume uptake/BET surface area of
the sample after reduction (Fig 7) This phenomenon is explained
by the composition of the catalyst surface and the available
abun-dance of bimetallic cobalt-copper sites on the catalyst surface after
reduction[11,16,22e24]
Certainly, the presence of intra-lattice copper LaCo0.7Cu0.3O3)
has a promotional effect on the formation of alcohols as compared
with the extra-lattice copper (Cu2O/LaCoO3) or the copper-free
perovskite sample [11,23] The formation of La(Co,Cu)O3
perov-skite precursors would provide intimidate dual copper-cobalt sites
which are prerequisite for the formation of alcohols from CO and H2
[17,19,25] This issue is further supported by the examination of the
catalytic activity at different pretreatment conditions.Table 2
dis-plays the alcohol selectivity/productivity versus the reduction
temperatures obtained on LaCo0.7Cu0.3O3 It is noted that the
alcohol productivity gradually increases and reaches a maximal value at 773 K and then sharply decreased at higher reduction temperatures This observable trend is explained by the fact that the surface composition is strongly associated with pretreatment temperature because of the reduction of CoeCu based perovskites happening in a multiple-step process at different temperatures [1,2,4,8,10] The surface concentration of cobalt and copper metals
is very sensitive to the reduction temperatures[4,5,9,11,15,21,25]
As an increased in H2-reduction temperature, the (Cu0eCo0)surface/ (CueCo)totalmolar surface ratio is varied and probably approached
a highest value around 773 K as elucidated by hydrogen chemi-sorption data (Table 2)[23]
A higher reduction temperature gives rise to a sintering of atomic copper metals and as consequence the active sites for the formation of OH alcohol functional group gradually decreases
Indeed, it was widely reported that the reduction of perovskites can
be described either by the contrasting-sphere model or by the
Table 2
Effect of hydrogen pretreatment temperature on alcohol productivity over sample LaCo 0.7 Cu 0.3 O 3 in CO hydrogenation at 548 K (VVH ¼ 5000 h 1 , 69 bar, H 2 /CO/He ¼ 8/4/3).
20 30 40 50 60 70 80 90 100
Temperature (K)
LaCoO3 CuO/LaCoO3 LaCo0.7Cu0.3O3
LaCo0.9Cu0.1O3
LaCoO 3
LaCo 0.7 Cu 0.3 O 3
Fig 4 CO dissociation ability at different temperatures on the reduced samples after
pre-treatment at 798 K in H 2 /Ar (0.586 vol % CO/He pulses (0.25 mL) were then
injected through the catalyst).
5 15 25 35 45 55 65 75
CO Conversion (%) and Alcohol productivity (mg/g cat /h) 0.012
0.014 0.016 0.018
0.028
Conversion
Alcohols Cu2O/LaCoO3
LaCo 0.7 Cu 0.3 O 3
LaCo 0.9 Cu 0.1
LaCoO 3
Fig 5 Correlation between the volume of CO chemisorbed uptake and CO
hydroge-nation activity at 548 K (VVH ¼ 5000 h 1 , 69 bar, H 2 /CO/He ¼ 8/4/3).
0 10 20 30 40 50 60 70 80 90 100 Alcohol and hydrocarbon selectivity (wt.%) 0.012
0.014 0.016 0.018
0.028
Hydrocarbons Alcohols
Cu 2 O/LaCoO 3
LaCo0.4Cu0.6O3
LaCo0.9Cu0.1O3 LaCoO 3
Fig 6 Correlation between the volume of CO chemisorbed uptake and CO hydroge-nation activity at 548 K (VVH ¼ 5000 h 1 , 69 bar, H 2 /CO/He ¼ 8/4/3).
0.01 0.02 0.03 0.04 0.05 0.06 0.07
Perovskite catalysts
2 )
LaCoO 3 LaCo 0.9 Cu 0.1 O 3 LaCo 0.7 Cu 0.3 O 3 LaCo 0.4 Cu 0.6 O 3 Cu 2 O/LaCoO 3
Fig 7 The correlation between H 2 -volume uptake (mL/g)/S BET ratio and the samples reduced at 773 K prior to chemisorption of hydrogen at 373 K.
N Tien Thao, L.T Son / Journal of Science: Advanced Materials and Devices xxx (2016) 1e6 4
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Please cite this article in press as: N Tien Thao, L.T Son, Production of cobalt-copper from partial reduction of La(Co,Cu)O perovskites for CO
Trang 6nucleation mechanism[1,2,5,8,9] Thus, the total metal surface area
is strongly dependant on the pretreatment conditions[8,10,18,20]
In the present study, the LaCo0.7Cu0.3O3is reduced at 773 K gives
the most effective catalyst for the formation of higher alcohols from
CO hydrogenation reaction
The alcohol product distribution is presented inFig 8which
contains a mixture of linear primary alcohols from methanol to
heptanol [10,22] The product distribution is recalculated as
Anderson-Schulz-Flory (ASF) rule and the plot between ln(wt.%/n)
versus carbon number (n) is drawn in Fig 9 [20,22] Since the
carbon chain growth factor of alcohols (designated asa1) is not on
par with that of hydrocarbons (a3), we have recalculated the second
one (a2) excluding methanol because methanol may be
indepen-dently produced by different pathways[11,19,26e30] In the case,
the alcohol chain growth factor (a2) of C2OHe C7OH stays at middle
value between a1 and a3 This indicates that the formation of
skeletal carbons of primary alcohols occurs parallel to that of
hy-drocarbons on cobalt catalyst surface[20,22,24e27] A close
dis-tance between cobalt and copper sites on catalyst surface has
steered the formation of hydrocarbons into primary alcohols by
insertion of undissociated CO molecule absorbed on copper sites
[11,19,20,22,28e30]
4 Conclusions
A set of La(Co,Cu)O3perovskite samples prepared by grounding
method was pretreated in H2prior to test for the CO hydrogenation
reaction The presence of copper ions in the perovskite lattice
results in a significant effect on the perovskite reducibility Under the same pretreatment conditions the LaeCoeCu based perovskites
is easily reduced, yielding metallic cobalt and copper sites dispersed over a La2O3matrix The CO dissociation ability of cobalt
is remarkably affected by the presence of neighboring copper atoms The overall activity of the catalysts in syngas conversion strongly depends on pretreatment temperature and the metal surface area The intra-framework copper increases the formation
of higher alcohols Alcohol and hydrocarbon productivity are strongly dependant on reducing conditions The highest alcohol productivity was about 0.07 galcohol/gcat/h on the LaCo0.7Cu0.3O3 perovskite precursor
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0 5 10 15 20 25 30 35 40 45 50
623 723 773 823
Temperature (K)
Fig 8 Effect of hydrogen pretreatment temperature on alcohol distribution over
sample LaCo 0.7 Cu 0.3 O 3 in CO hydrogenation at 548 K (VVH ¼ 5000 h 1 , 69 bar, H 2 /CO/
He ¼ 8/4/3).
-4 -3 -2 -1 0 1 2 3 4 5 6
Carbon number
α 3 = 0.43 α2 = 0.42
α1 = 0.38
Fig 9 ASF distribution of products obtained at pretreatment temperature of 773 K
over LaCo 0.7 Cu 0.3 O 3 (548 K, VVH ¼ 5000 h 1 , 69 bar, H 2 /CO/He ¼ 8/4/3
(a1 ¼ C 1 OHeC 7 OH,a2 ¼ C 2 OHeC 7 OH,a3 ¼ C1eC10 hydrocarbons).
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N Tien Thao, L.T Son / Journal of Science: Advanced Materials and Devices xxx (2016) 1e6 6
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Please cite this article in press as: N Tien Thao, L.T Son, Production of cobalt-copper from partial reduction of La(Co,Cu)O perovskites for CO