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Hydrogen generation by acetic acid steam reforming over Ni-based catalysts derived from La1−xCexNiO3 perovskite
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Hydrogen generation by acetic acid steam reforming over Ni-based catalysts derived from La1¡xCexNiO3 perovskite Lin Li, Bo Jiang, Dawei Tang*, Qian Zhang, Zhouwei Zheng Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116024, China
article info
abstract
Article history:
In this work, La1xCexNiO3 perovskite-type oxide derived catalysts were synthesized for
Received 10 January 2018
hydrogen generation from acetic acid steam reforming process (AcSR), and the effect of Ce
Received in revised form
substitution was investigated. Various techniques were applied to characterize the prop-
14 February 2018
erties of synthesized catalysts including X-ray diffraction (XRD), H2 temperature-
Accepted 19 February 2018
programmed reduction (TPR), H2 chemisorption, transmission electron microscopy
Available online 13 March 2018
(TEM), N2 adsorption-desorption, and thermogravimetric analysis (TGA). The characterization results demonstrated that the perovskite-type oxide derived catalysts possessed
Keywords:
uniform Ni dispersion and small Ni particle size. The adding of Ce remarkably increased
Hydrogen
the metal support interaction, improved the coke resistance and enhanced the water gas
Steam reforming
shift reaction. The reactivity and stability tests were conducted in a fixed-bed reactor. The
Acetic acid
La0.9Ce0.1NiO3 exhibited the best performance for AcSR with an average H2 yield of 90%, an
Perovskite
average acetic acid conversion of 95%, and an accumulated coke deposition of 11.8 wt% in a
La1xCexNiO3
30-h stability test. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Increasing concerns about the exhaustion of fossil fuels and the environmental pollution caused by the energy demand render hydrogen energy an appealing alternative [1]. At present, more than 50% hydrogen is generated by steam reforming of hydrocarbons which results in remarkable carbon emission [2,3]. Thus, if the H2 generation is derived from biomass source, significant environmental benefits towards the hydrogen economy could be achieved due to the theoretically closed carbon loop [4,5]. Fast pyrolysis is one of the most widely used technologies to convert biomass into bio-oil. Due
to the complicated composition of bio-oil, which depends on the conditions of pyrolysis and the source of biomass, bio-oil should be further updated. Steam reforming (SR) of bio-oil is a prospective way to convert the energy stored in bio-oil into easily usable hydrogen. Acetic acid is one of the major compositions in bio-oil, occupying a percentage up to 32 wt%. Therefore, acetic acid steam reforming (AcSR) has been widely used as a model to understand the conditions on the design efficient catalysts for bio-oil steam reforming process [6e8]. AcSR occurs together with water-gas-shift reaction (WGS) simultaneously. The overall reaction of AcSR is present as Eq. (1), and the major side reactions in AcSR are listed in Eqs. (3)e(5), including the acetic acid thermal decomposition
* Corresponding author. E-mail address: [email protected] (D. Tang). https://doi.org/10.1016/j.ijhydene.2018.02.128 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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reactions and ketonization of acetic acid, during which the formed acetone is considered as a coke precursor [9]. The main culprits of catalysts deactivation in AcSR are the coke deposition on the active sites and metal sintering due to the harsh reaction temperatures ranging from 500 C to 800 C. CH3 COOH þ 2H2 O/2CO þ 4H2
(1)
CH3 COOH#2CO þ 2H2
(2)
CH3 COOH#CH4 þ CO2
(3)
CH3 COOH/C2 H4 ; C2 H6 ; C3 H4 ; coke
(4)
2CH3 COOH/ðCH3 Þ2 CO þ H2 O þ CO2
(5)
Supported noble metal catalysts, such as Ru, Rh, Pt and Ir, show high catalytic activities and excellent stabilities. Seshan and co-workers have studied acetic acid steam reforming over Pt/ZrO2 catalysts to elucidate its reaction mechanism [10]. It suggests that the severe coke deposition of Pt/ZrO2 was due to the form acetone, a primary coke precursor, derived from the ketonization reaction. On the contrary, Pt/CeO2 shows less coke formation in AcSR due to the enhanced acetone reforming reaction caused by facile redox properties of CeO2 [11]. Basagiannis et al. found Ru and Rh based catalysts showed high activity in AcSR process [12,13]. Lemonidou et al. [14] demonstrated that the La2O3 modification on Rh based CexZr1xO2 catalysts not only efficiently suppresses the sintering by improving the metal support interaction (MSI) but also inhibits the coke deposition by facilitating the exchange of lattice oxygen. Although noble metal based catalysts show high performance in AcSR, their application at large scales is limited by high costs. Non-noble metal catalysts, especially the Nibased catalysts, have been extensively investigated in reforming process due to its low-cost and outstanding capability for C-C and C-H bond rupture [15]. Recently, Nibased catalysts derived from some particular structure such as hydrotalcite-like structure and phyllosilicate structure has attracted much attention due to the improved Ni dispersion and the enhanced MSI [16,17]. Perovskite type-like oxide catalyst precursor also shows the same properties in the steam reforming process [18,19]. Perovskites are mixed oxides with a general formula of ABO3, in which A is an alkaline earth or rare earth metal and B is generally a transition metal element. The partial substitution of cations in A and B position would provoke significant changes to its properties such as improving the thermal stability and increasing the mobility of oxygen ion vacancies [20,21]. Wu et al. [22] have investigated the Ca substitution of La in a La1xCaxNiO3 catalyst in glycerol steam reforming, and they have concluded that the perovskite structure could improve the distribution of containing elements and therefore enhance the MSI. M. de Lima et al. [20] have investigated the effect of La substitution by the Ce oxide on the performance of a La1xCexNiO3 perovskite oxide precursor in steam reforming of ethanol, and the results showed that the Ce doping could effectively decrease the Ni crystal size and increase the amount of oxygen vacancies.
Among these substitution elements, CeO2 has been recognized as an active promoter to improve the reforming activity of acetone, which is a coke precursor in AcSR. In this work, La1xCexNiO3 perovskite-type oxide derived catalysts were synthesized for hydrogen generation from AcSR process. The effect of Ce doping on the performance of La1xCexNiO3 was investigated, and various techniques were applied to characterize the properties of the synthesized catalysts, including Xray diffraction (XRD), H2 temperature-programmed reduction (TPR), H2 chemisorption, transmission electron microscopy (TEM), N2 adsorption-desorption, and thermogravimetric analysis (TGA). The activity and stability tests were carried out in a fixed-bed reactor.
Experimental Preparation of catalysts A series of La1xCexNiO3 (x ¼ 0, 0.05, 0.1, 0.2, and 0.3) were synthesized by citrate decomposition method. An aqueous solution with an appropriate molar ratio of La(NO3)3$6H2O (Aladdin, 99.99%), Ce(NO3)3$6H2O (Aladdin, 99.95%) and Ni(NO3)2$6H2O (Aladdin, 98%) was prepared. Then, a concentrated solution of citric acid (Aladdin, 99.8%) was added into the prepared solution, in such a way that the ratio of equivalent grams of metal to equivalent grams of citric acid would be unity, to form the metallic amorphous citrates. The obtained solution was heated to 60 C and kept for 9 h, followed by evaporating for 30 min using a vacuum rotary evaporator. The viscous solution was further dried for 12 h at 120 C. The spongy material was first grinded and then calcined in two stages: first at 550 C for 3 h and then at 900 C for 5 h.
Characterization XRD was performed with a 2q ranges from 10 to 80 by the graphite filtered Cu Ka radiation (l ¼ 1.5406 A). The N2 adsorption-desorption was used to characterize the physical texture by a Micrometrics Tristar 3000 analyzer at 77 K. The morphology of fresh catalysts was obtained from a HRTEM (Tecnai F30) at 300 kV. H2-TPR was carried out to detect the metal support interaction (MSI) by a Micrometrics Autochem Ⅱ device, and a flow rate of 30 mL min1 of 10% H2/N2 was introduced for reduction. TGA was carried out using a thermal system (STA449F3, NETZSCH Corp.) under a flow of air (50 mL min1) with a heating rate of 10 K min1. The amount of carbon deposition was calculated via the mass profiles in TGA results.
Activity and stability tests The activity and stability tests were performed in a packedbed reactor. The experimental procedure, relevant calculations and reactor set-up were shown in our previous studies [2]. The catalyst of 500 mg was placed in the middle of the reactor. Before the reaction, the catalysts were reduced by a pure H2 (100 mL min1) at 700 C for 3 h. The mixture of acetic acid and water with a steam to carbon ratio (S/C) of 3 was injected into the evaporator at a flow rate of 4 mL h1. The N2
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as a carrier gas was also introduced into the reactor at a flow rate of 150 mL min1. The reaction temperatures were set at 650 C, 700 C and 750 C. The gas products were analyzed by two gas chromatographs. One is equipped with a flame ionization detector (FID) and a Porapak-Q column with N2 as the carrier gas to analyze the organic species such as acetone. The other one is integrated with a thermal conductivity detector (TCD) and a TDX-01 column using He as the carrier gas to monitor hydrogen, carbon dioxide, carbon monoxide, and methane. The conversion, H2 yield and selectivity were calculated as follows: YH2 ¼
moles of hydeogen produced 100% 4 moles of acetic acid feed
X¼
acetic acid feed unreacted acetic acid 100% acetic acid feed
S¼
moles of carbon of each carbonaceous product 100% 2 moles of reacted acetic acid feed
Results and discussion Characterization of catalysts XRD patterns of as-prepared La1xCexNiO3 perovskite type oxides are shown in Fig. 1. The diffraction peaks at 23.2 , 47.6 and 58.8 was indexed to the (1 0 1), (2 0 2) and (3 0 0) planes of LaNiO3, indicating that the perovskite is successfully synthesized by the citric acid decomposition method. With the gradual increase of Ce doping, the peaks centered at 33.1 , 37.2 and 43.3 became significant, which could be assigned to the CeO2 (2 0 0), NiO (1 1 1) and NiO (2 0 0) planes, respectively. These three peaks were significant for La0.7Ce0.3NiO3 and La0.8Ce0.2NiO3 samples. Thus, the saturation point of Ce doping was reached at x ¼ 0.2. For all the catalysts precursors,
Fig. 1 e XRD profiles of different perovskite precursors. a) La0.7Ce0.3NiO3, b) La0.8Ce0.2NiO3, c) La0.9Ce0.1NiO3, d) La0.95Ni0.05NiO3 and e) LaNiO3.
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there was no peak shift of LaNiO3 characteristic peak, which could be attributed to the similar cation radius between La3þ (1.17 A) and Ce3þ (1.15 A). When x was less than 0.1, there were not any appreciable NiO and CeO2 peaks, suggesting the Ce cations was incorporated into the perovskite structure and the substituted perovskite was formed [20,23]. It has been demonstrated by XPS that high fraction substitution of Ce3þ ions in La1xCexNiO3 would result in the segregation of CeO2 and most of the Ce ions essentially existed as Ce4þ oxidation state [24]. Ghasdi et al. [25] have reported a substitution limit for Ce segregation in La1xCexCoO3, which is between 5% and 10%. The partial substitution of Ce3þ for La3þ leads to the conversion of Co3þ to Co2þ so as to keep the charge neutrality. The ionic radius increase due to the Co ions conversion would result in a decrease of the tolerance factor and therefore lead to the segregation of Ce. Besides, the NiO segregation is accompanied with the CeO2 segregation. This phenomenon is associated with the decrease of the La/Ni atomic ratio as well as with the fact that extra CeO2 cannot react with the NiO to form a perovskite structure. TEM was carried out to investigate the morphologies of as prepared perovskite structure precursors, as shown in Fig. 2. It is obvious that with the increase of Ce doping the metal oxides segregation occurs for La0.7Ce0.3NiO3 and La0.8Ce0.2NiO3, and the prepared precursors were successfully with low agglomeration. In order to investigate the reduction behavior of asprepared perovskite type precursor, TPR treatment was performed. As shown in Fig. 3A, there were three peaks at 590 K, 570 K and 770 K for all samples, respectively. Generally, the H2 consumption peaks in the case of perovskites result from the reduction of B site metal cation in the ABO3 structure. The first peak centered around 590 K could be derived from the reduction of Ni3þ in the perovskite structure to an intermediate valence Ni cation, leading to the formation of La4Ni3O10. The second peak at 670 K would be attributed to the reduction of La4Ni3O10 to Ni2þ, resulting in the formation of La2NiO4. The third peak at 770 K corresponded to the complete reduction of Ni2þ in La2NiO4 to produce La2O3, CeO2 and Ni0 metallic nanoparticles. This result implies that the reduction treatment at 973 K in the fixed-bed reactor could completely destroy the perovskite structure and Ni0 nanoparticles could deposit on the lanthanum oxides. Sierra Gallego et al. [26] have demonstrated that the reduction of LaNiO3-d proceeds in three steps as follows by an in-situ XRD technique. 4LaNiO3 þ 2H2 /La4 Ni3 O10 þ Ni0 þ 2H2 O
(6)
La4 Ni3 O10 þ 3H2 /La2 NiO4 þ 2Ni0 þ La2 O3 þ 3H2 O
(7)
La2 NiO4 þ H2 /Ni0 þ La2 O3 þ H2 O
(8)
Assuming that LaNiO3 is the only phase existed in these samples, the H2 consumption intensity ratio between the second peak and the first peak should be two. However, for all the samples, the ratio was obviously lower than two. It suggested that the concomitant formation of Ni0 occurred as listed in Eq. (6) and Eq. (7). The TPP profiles of La0.8Ni0.2O3 and La0.7Ni0.3O3 were slightly different from the others. The first and third peaks were inconspicuous, and a new peak ranging from 900 K to 1100 K appeared. The incorporation of the first
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Fig. 2 e TEM profiles of different catalysts. a) LaNiO3; b) La0.95Ni0.05NiO3; c) La0.9Ce0.1NiO3; d), f) La0.8Ce0.2NiO3; and e) La0.7Ce0.3NiO3.
two peaks is due to the decomposition of the perovskite structure, as evidenced in the XRD results. Therefore, the first peaks for La0.8Ni0.2O3 and La0.7Ni0.3O3 are mainly derived from the reduction of Ni2þ to Ni0, while the reduction in two steps of Ni3þ to Ni0 from the perovskite occurs in a limited extend [20]. The broad reduction peaks from 900 K to 1100 K were detected, which are likely due to the reduction of bulk CeO2 at high Ce substitution fraction. According to the H2-TPR, the synthesized catalysts were reduced at 973 K for 1 h. XRD patterns of the reduced catalysts are shown in Fig. 3B. As expected, the diffraction patterns of
all the samples were significantly different from those of the previous as-prepared perovskite-type precursors. It is apparent that all the samples lost their perovskite characteristic peaks after the reduction at 973 K. Meanwhile, new peaks indexed to La(OH)3, Ni, and La2O3 were revealed. In contrast with the other three samples, the reduced La0.8Ni0.2O3 and La0.7Ni0.3O3 exhibited basically the same characteristic peaks except for the peaks at 28.5 , 33.1 , 47.5 and 56.3 , which could be assigned to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of CeO2. In addition, the diffraction peaks of CeO2 were not observed until the Ce doping reached 0.2, revealing CeO2 is
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distinguished for all the samples, and therefore the Ni crystal size (listed in Table 1) was calculated by the Scherrer equation from this plane. It is obvious that the Ni crystal size gradually increases when the Ce substitution was more than 0.1. This was in line with the H2 chemisorption results listed in Table 1. Liu et al. [27] have reported that the aggregation of excessive Ce doping could blockade the mesopores or cover the accessible Ni active sites, thus exerting a negative effect on Ni dispersion. Consequently, the Ni catalyst derived from La0.9Ni0.1O3 shows the highest Ni dispersion and smallest Ni crystal size. The physical properties of these reduced catalysts were listed in Table 1. All the samples possessed BET surface areas of lower than 5.0 m2 g1, which is the inherent characteristic of these materials prepared after calcination at high temperature.
Activity and stability tests
Fig. 3 e (A) TPR profiles of different catalysts. a) La0.7Ce0.3NiO3, b) La0.8Ce0.2NiO3, c) La0.9Ce0.1NiO3, d) La0.95Ni0.05NiO3 and e) LaNiO3. (B) XRD profiles of reduced metal supported catalysts derived from different perovskite precursors. a) La0.7Ce0.3NiO3, b) La0.8Ce0.2NiO3, c) La0.9Ce0.1NiO3, d) La0.95Ni0.05NiO3 and e) LaNiO3.
either amorphous or well dispersed in the catalysts. The reflection peaks of Ni0 are inconspicuous with the increase of Ce doping amount, indicating the Ni particles are smaller and highly dispersed in the presence of CeO2. This could be either ascribed to the formation of Ni-Ce mixed oxide due to the strong interaction between Ni and CeO2 or attributed to the dilution effect caused by CeO2 addition to separate Ni nanoparticles [15]. No NiCe peak in XRD profiles is due to the small crystallite sizes of the NiCeO solution. The reflection peaks at 44.4 corresponding to the (0 1 1) plane of metallic Ni could be
The acetic acid conversions at different temperatures are shown in Fig. 4A. For all the samples, the acetic acid conversion increased with the reaction temperature, which is due to the endothermic nature of AcSR. The acetic acid conversion of the five catalysts decreased as follows: La0.9Ce0.1NiO3 > La0.95Ce0.05NiO3 > La0.8Ce0.2NiO3 > LaNiO3 > La0.7Ce0.3NiO3, corresponding with the Ni dispersion listed in Table 1. Since Ni is recognized as the active site for C-C and C-H bond rupture, the catalyst with higher active surface area is conducive to promote the conversion of acetic acid and other C-containing intermediates. Pu et al. [6] have reported that decreasing the nickel particle size could enhance the acetic acid conversion and increase the H2 yield. Lemonidou et al. have reported the temperature effect on the hydrogen production over Ni and Ru catalysts, and they also discovered the same trend of the acetic acid conversion with temperature [28]. It has been demonstrated that increasing the reaction temperature could not only remarkably promote the acetic acid steam reforming but also the by-products such as CH4, acetone and ketene [29]. The products distribution (700 C) of these catalysts are present in Fig. 4B. However, the distribution of the products is not similar, implying different extent of the reaction pathways. Steam reforming of acetic acid is a complicated system, involving different routines for acetic acid conversion and several secondary reactions between the reforming byproducts. Acetic acid is not a thermal stable reactant, which could be converted to other organics before reaching or on the catalyst bed via homogeneous/heterogeneous reactions such as decomposition reaction (Eq. (2)), decarboxylation (Eq. (3)) and ketonization (Eq. (5)). Consequently, the undesirable byproducts generated. These side reactions would occur together with the steam reforming, thereby reducing the hydrogen yield. It is noting that steam reforming is not the only reaction contributes to the hydrogen generation, and the acetone and methane steam reforming also play an important role in distribution of products, especially at high temperatures. In the present test, the CH4 selectivity was low for all the samples, indicating the methanation was inhibited under the reaction conditions due to its extremely exothermic nature. The small amount methane would be mainly derived from the decarboxylation reaction (Eq. (3)) and the cracking of the organics. As shown in Fig. 4B, the CH4 selectivity variation of different
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Table 1 e Properties of reduced catalysts. Samples LaNiO3 La0.95Ni0.05NiO3 La0.9Ce0.1NiO3 La0.8Ce0.2NiO3 La0.7Ce0.3NiO3 a b c
Surface area m2/g
Average pore diameter nm
Pore volume cm3/g
2.6 3.4 4.1 2.7 2.2
56.1 40.3 38.5 36.1 37.2
0.01 0.02 0.04 0.01 0.01
Crystal size of Ni contentb Ce contentb Metal dispersionc Nia nm wt% wt% m2/gcat 23.6/28.2 20.5 18.8/21.5 21.7 26.2
31.2 30.5 29.7 28.7 29.1
0.0 12.5 22.1 39.1 44.7
0.15 0.24 0.36 0.16 0.11
Determined by Scherrer equation from XRD profiles (reduced/spent). Determined by ICP-OES. Determined by H2 chemisorption.
Fig. 4 e (A) Conversion of acetic acid at various temperatures. (B) H2 yield and C-containing species selectivity.
catalysts corresponds with the Ni dispersion. Ni is active not only for methane steam reforming but also for the acetic acid cracking due to its high capacity for C-C rupture. Consequently, the samples such as La0.8Ce0.2NiO3, LaNiO3 and La0.7Ce0.3NiO3 with low Ni dispersion probably do not possess enough nickel sites for the occurrence of both acetic acid steam reforming and methane steam reforming, thus leading to the high selectivity to CH4. The lack of enough active Ni sites also could contribute to the high CO selectivity [30]. In contrast with LaNiO3 derived catalysts, the other Ce substitution perovskite derived samples showed high H2 yield accompanied with higher CO2 selectivity and lower CO selectivity. As the CO2/CO ratio has been identified as an indicator for WGS reactivity, the improved H2 yields is associated with the Ce substitution. It is well known that the CeO2-containing catalysts show higher selectivity to H2 and CO2 in steam reforming process due to the excellent redox property and high oxygen storage capacity of CeO2. Specifically, the metal Ni could adsorb the acetic acid and break its C-C bond, while oxygen vacancies in CeO2 would activate the water to generate the OH groups, which in turn react with CxH and CxOyH intermediate species to produce CO2 and H2. The WGS reactivity is improved by Ce promotion; therefore, the more Ce doping, the higher WGS reactivity. However, excessive Ce doping would bury the Ni particles, which would decrease the active surface area, leading to the loss of acetic acid conversion. The loss of the capability of C-C rupture, in turn, has a negative effect on WGS reactivity. Therefore, an optimum Ce/Ni ratio is important to obtain a high performance in AcSR. The formation of acetone suggests that the ketonization reactions also proceed with much lower rates. In general, steam reforming is recognized as a severe reaction under harsh conditions and always suffers from catalyst deactivation. To better understand the substitution effect of La by Ce over perovskite-type catalyst in AcSR, LaNiO3 and La0.9CeO0.1NiO3 derived catalysts were screened out with a 30h stability test. According to the results of activity tests, the stability tests were performed at 750 C. The acetic acid conversion with time of the two catalysts are shown in Fig. 5. The deactivation degree for the two catalysts is significantly distinguishable. The conversion of La0.9CeO0.1NiO3 derived catalysts was steady at the first 5 h (around 100%), then it decreased to 93% after 10 h and leveled off. In contrast, the acetic acid conversion of LaNiO3 derived catalyst plunged to 55% in the first 17 h and then kept stable at the last 13 h. Coke deposition is the main culprit for catalyst deactivation in AcSR. In fact, acetone has been regarded as a major coke
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Fig. 5 e Stability tests results of LaNiO3 and La0.9Ce0.1NiO3. precursor in AcSR. A plausible explanation is that the surface defect sites are necessary for the initial deprotonation of carboxylic acids, while the presence of adjacent Lewis acid and Brønstred basic sites is crucial for ketonization. Subsequently the aldol-condensation of acetone is facile, producing mesityl oxide, which further undergoes oligomerization, leading to coke deposition and thus rapidly deactivating the catalyst by blocking active catalytic sites [9]. As shown in Fig. 4B, the acetone selectivity of LaNiO3 derived catalyst is significantly higher than that of La0.9CeO0.1NiO3 derived catalyst. Therefore, the high coke deposition could be expected on LaNiO3 derived catalysts. It has been reported that the facile redox properties of CeO2 could promote the activation of steam and therefore reform acetone more efficiently, leading to a low coke deposition on La0.9CeO0.1NiO3 derived catalyst. The Ni crystal sizes after the stability tests are also listed in Table 1. The enhanced sintering resistance of La0.9CeO0.1NiO3 derived catalyst contributes to the higher stability. Recent studies have demonstrated that CeO2 exhibits the capability to stabilize metal particles against thermal sintering due to the improved MSI [31].
Characterization of spent catalysts The XRD profiles of spent catalysts are shown in Fig. 6A. The peaks at 25.1 , 25.8 , 27.6 , 30.3 , 47.4 , 50.2 and 56.9 could be indexed to (1 0 0), (1 0 1), (1 0 2), (1 0 3), (1 0 4), (1 1 4) and (1 1 6) planes of La2O2CO3. Cerritos et al. [32] have reported that the La2O3 modified Ni particles could react with CO2 to form La2O2CO3 (Eq. (9)), which would further eliminate the coke deposition around Ni particles (Eq. (10)). La2 O3 þ CO2 4La2 O2 CO3
(9)
La2 O2 CO3 þ C/La2 O3 þ 2CO
(10)
Moreover, the reflection peaks of graphite at 26.6 were detected for both catalysts. The graphite peak of Ce substitution catalyst is less intense than that of LaNiO3 derived catalyst, confirming the coke deposition is inconspicuous on La0.9CeO0.1NiO3. In addition, the sharp diffraction peak of Ni at
Fig. 6 e (A) XRD profiles and (B) TGA-DTG profiles of two spent catalysts. a) La0.9Ce0.1NiO3 and b) LaNiO3.
41.5 and 44.5 suggests that the LaNiO3 derived catalyst suffers from sintering. And the Ni particle size increased to 28.2 nm after the stability test. The agglomerated Ni particles would result in the decrease of the active surface, further leading to the loss of the capability towards C-C bond rupture. The amount of deposited coke was determined by TGA, and the DTG profiles of the two catalysts were also present in Fig. 6B. It is worth noting that the spent catalysts were first pretreated at 800 C for 0.5 h in N2 flow before the TGA tests so as to eliminate the influence of La2O2CO3 decomposition. The initial increase of the mass for both samples occurred, which is due to the oxidation of metallic Ni particles. It is obvious that the amount of coke deposition on La0.9Ce0.1NiO3 sample (11.8%) is less than that on LaNiO3 sample (17.3%), indicating the coke deposition is effectively suppressed. It has been reported that the small crystals possessed a high saturation concentration of carbon and therefore a low driving force for carbon diffusion through the nickel crystals [33]. The lattice oxygen released from the CeO2 also could react with the carbonaceous species as soon as it forms and therefore keep the metal surface free of coke deposition [34]. Both the small particle size and the CeO2 substitution contribute to the less
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coke deposition on La0.9Ce0.1NiO3 sample. Additionally, from the TGA profiles, two peaks appeared on LaNiO3 sample while only one peak occurred on La0.9Ce0.1NiO3 sample. The peaks at different temperatures should be assigned to different carbonaceous species on the spent catalyst surface. The existence of two distinguishable peaks has been reported previously. It is assumed that the first peak at lower temperature is related to the combustion of amorphous carbon species, while the second coke deposition on higher temperature could be attributed to the oxidation of graphitic carbon [35]. Furthermore, it is also reported that the coke near the Ni particles could be oxidized more easily than that far from the Ni particles [35]. It is apparent that the main peaks on La0.9Ce0.1NiO3 sample are at the lower temperature range, indicating more coke deposition is close to Ni particles. The coke deposition, which is far from the Ni particles, could be effectively eliminated by the CeO2.
Conclusions The La1xCexNiO3 perovskite type oxides were prepared and tested in AcSR. The nickel-based catalysts derived from the perovskite-type oxides possessed high Ni dispersion after the reduction treatment. The Ce substitution of La showed significant effect on the properties of perovskite-type oxide, and La0.9Ce0.1NiO3 sample exhibited the best performance due to its small Ni particles and high Ni surface areas. Meanwhile, Ce doping could remarkably promote the acetone reforming, which is generated from the ketonization reaction in AcSR, and water gas shift reaction. All the above properties contribute to the high catalytic activity in AcSR. Additionally, the deactivation resulted from coke deposition and Ni sintering of Ni-based catalysts in AcSR was effectively suppressed. This is due to improved MSI and coke deposition resistance from Ce substitution.
Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51706030) and the China Postdoctoral Science Foundation (No. 2017M611219).
references
[1] Cortright RD, Davda RR, Dumesic JA. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002;418:964e7. [2] Jiang B, Zhang C, Wang K, Dou B, Song Y, Chen H, et al. Highly dispersed Ni/montmorillonite catalyst for glycerol steam reforming: effect of Ni loading and calcination temperature. Appl Therm Eng 2016;109(Part A):99e108. [3] Dou B, Zhang H, Cui G, Wang Z, Jiang B, Wang K, et al. Hydrogen production by sorption-enhanced chemical looping steam reforming of ethanol in an alternating fixedbed reactor: sorbent to catalyst ratio dependencies. Energy Convers Manag 2018;155:243e52.
[4] Jiang B, Dou B, Wang K, Zhang C, Li M, Chen H, et al. Sorption enhanced steam reforming of biodiesel by-product glycerol on Ni-CaO-MMT multifunctional catalysts. Chem Eng J 2017;313:207e16. [5] Dou B, Wang K, Jiang B, Song Y, Zhang C, Chen H, et al. Fluidized-bed gasification combined continuous sorptionenhanced steam reforming system to continuous hydrogen production from waste plastic. Int J Hydrogen Energy 2016;41:3803e10. [6] Pu J, Nishikado K, Wang N, Nguyen TT, Maki T, Qian EW. Core-shell nickel catalysts for the steam reforming of acetic acid. Appl Catal B Environ 2018;224:69e79. [7] Wang S, Li X, Guo L, Luo Z. Experimental research on acetic acid steam reforming over CoeFe catalysts and subsequent density functional theory studies. Int J Hydrogen Energy 2012;37:11122e31. [8] Iwasa N, Yamane T, Takei M, Ozaki J-I, Arai M. Hydrogen production by steam reforming of acetic acid: comparison of conventional supported metal catalysts and metalincorporated mesoporous smectite-like catalysts. Int J Hydrogen Energy 2010;35:110e7. [9] Li D, Li X, Gong J. Catalytic reforming of oxygenates: state of the art and future prospects. Chem Rev 2016;116(19). [10] Takanabe K, Aika K-I, Inazu K, Baba T, Seshan K, Lefferts L. Steam reforming of acetic acid as a biomass derived oxygenate: bifunctional pathway for hydrogen formation over Pt/ZrO2 catalysts. J Catal 2006;243:263e9. [11] Matas Gu¨ell B, Babich I, Nichols KP, Gardeniers JGE, Lefferts L, Seshan K. Design of a stable steam reforming catalystda promising route to sustainable hydrogen from biomass oxygenates. Appl Catal B Environ 2009;90:38e44. [12] Basagiannis A, Verykios X. Catalytic steam reforming of acetic acid for hydrogen production. Int J Hydrogen Energy 2007;32:3343e55. [13] Basagiannis AC, Verykios XE. Influence of the Carrier on steam reforming of acetic acid over Ru-based catalysts. Appl Catal B Environ 2008;82:77e88. [14] Lemonidou AA, Vagia EC, Lercher JA. Acetic acid reforming over Rh supported on La2O3/CeO2eZrO2: catalytic performance and reaction pathway analysis. ACS Catal 2013;3:1919e28. [15] Li D, Zeng L, Li X, Wang X, Ma H, Assabumrungrat S, et al. Ceria-promoted Ni/SBA-15 catalysts for ethanol steam reforming with enhanced activity and resistance to deactivation. Appl Catal B Environ 2015;176e177:532e41. [16] Wu GW, Zhang CX, Li SR, Huang ZQ, Yan SL, Wang SP, et al. Sorption enhanced steam reforming of ethanol on NiCaO-Al2O3 multifunctional catalysts derived from hydrotalcite-like compounds. Energy Environ Sci 2012;5: 8942e9. [17] Bian Z, Suryawinata IY, Kawi S. Highly carbon resistant multicore-shell catalyst derived from Ni-Mg phyllosilicate nanotubes@silica for dry reforming of methane. Appl Catal B Environ 2016;195:1e8. [18] Zhang Q, Li L, Jiang B, Wang K, Tang D, Dou B. An intelligent oxygen Carrier of La2 xSrxNiO4l for hydrogen production by chemical looping reforming of ethanol. Int J Hydrogen Energy 2017;42:17102e11. [19] Zhang Q, Li L, Jiang B, Tang D, Dou B. Hydrogen by chemical looping reforming of ethanol: the effect of promoters on La2xMxNiO4-l (M¼ Ca, Sr and Ce) oxygen carriers. Chem Eng Sci 2017;174:259e67. [20] de Lima SM, da Silva AM, da Costa LOO, Assaf JM, Mattos LV, Sarkari R, et al. Hydrogen production through oxidative steam reforming of ethanol over Ni-based catalysts derived from La1xCexNiO3 perovskite-type oxides. Appl Catal B Environ 2012;121e122:1e9.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 6 7 9 5 e6 8 0 3
[21] Li L, Song Y, Jiang B, Wang K, Zhang Q. A novel oxygen Carrier for chemical looping reforming: LaNiO3 perovskite supported on montmorillonite. Energy 2017;131:58e66. [22] Wu G, Li S, Zhang C, Wang T, Gong J. Glycerol steam reforming over perovskite-derived nickel-based catalysts. Appl Catal B Environ 2014;144:277e85. [23] Franchini CA, Aranzaez W, de Farias AMD, Pecchi G, Fraga MA. Ce-substituted LaNiO3 mixed oxides as catalyst precursors for glycerol steam reforming. Appl Catal B Environ 2014;147:193e202. [24] Lima S, Assaf J, Pena M, Fierro J. Structural features of La1 x CexNiO3 mixed oxides and performance for the dry reforming of methane. Appl Catal Gen 2006;311:94e104. [25] Ghasdi M, Alamdari H, Royer S, Adnot A. Electrical and CO gas sensing properties of nanostructured La1 xCexCoO3 perovskite prepared by activated reactive synthesis. Sensor Actuator B Chem 2011;156:147e55. n F, Barrault J, Tatiboue €t J-M, Batiot[26] Gallego GS, Mondrago Dupeyrat C. CO2 reforming of CH4 over LaeNi based perovskite precursors. Appl Catal Gen 2006;311:164e71. [27] Liu D, Quek XY, Cheo WNE, Lau R, Borgna A, Yang Y. MCM-41 supported nickel-based bimetallic catalysts with superior stability during carbon dioxide reforming of methane: effect of strong metalesupport interaction. J Catal 2009;266:380e90. [28] Vagia EC, Lemonidou AA. Investigations on the properties of ceriaezirconia-supported Ni and Rh catalysts and their performance in acetic acid steam reforming. J Catal 2010;269:388e96.
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[29] Hu X, Lu G. Comparative study of alumina-supported transition metal catalysts for hydrogen generation by steam reforming of acetic acid. Appl Catal B Environ 2010;99:289e97. [30] Li Z, Hu X, Zhang L, Liu S, Lu G. Steam reforming of acetic acid over Ni/ZrO2 catalysts: effects of nickel loading and particle size on product distribution and coke formation. Appl Catal Gen 2012;417e418:281e9. [31] Senanayake SD, Rodriguez JA, Stacchiola D. Electronic metalesupport interactions and the production of hydrogen through the water-gas shift reaction and ethanol steam reforming: fundamental studies with well-defined model catalysts. Top Catal 2013;56:1488e98. [32] Carrera Cerritos Rl, Fuentes Ramirez R, Aguilera Alvarado AF, Martinez Rosales JM, Viveros Garcia TS, Galindo Esquivel IR. Steam reforming of ethanol over Ni/Al2O3 La2O3 catalysts synthesized by Sol Gel. Ind Eng Chem Res 2010;50:2576e84. ndez E, Yu Z, Tøtdal B, [33] Chen D, Christensen KO, Ochoa-Ferna Latorre N, et al. Synthesis of carbon nanofibers: effects of Ni crystal size during methane decomposition. J Catal 2005;229:82e96. [34] Zhang C, Li S, Li M, Wang S, Ma X, Gong J. Enhanced oxygen mobility and reactivity for ethanol steam reforming. AIChE J 2012;58:516e25. [35] Ma H, Zeng L, Tian H, Li D, Wang X, Li X, et al. Efficient hydrogen production from ethanol steam reforming over Lamodified ordered mesoporous Ni-based catalysts. Appl Catal B Environ 2016;181:321e31.
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Hydrogen generation by acetic acid steam reforming over Ni-based catalysts derived from La1¡xCexNiO3 perovskite Lin Li, Bo Jiang, Dawei Tang*, Qian Zhang, Zhouwei Zheng Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116024, China
article info
abstract
Article history:
In this work, La1xCexNiO3 perovskite-type oxide derived catalysts were synthesized for
Received 10 January 2018
hydrogen generation from acetic acid steam reforming process (AcSR), and the effect of Ce
Received in revised form
substitution was investigated. Various techniques were applied to characterize the prop-
14 February 2018
erties of synthesized catalysts including X-ray diffraction (XRD), H2 temperature-
Accepted 19 February 2018
programmed reduction (TPR), H2 chemisorption, transmission electron microscopy
Available online 13 March 2018
(TEM), N2 adsorption-desorption, and thermogravimetric analysis (TGA). The characterization results demonstrated that the perovskite-type oxide derived catalysts possessed
Keywords:
uniform Ni dispersion and small Ni particle size. The adding of Ce remarkably increased
Hydrogen
the metal support interaction, improved the coke resistance and enhanced the water gas
Steam reforming
shift reaction. The reactivity and stability tests were conducted in a fixed-bed reactor. The
Acetic acid
La0.9Ce0.1NiO3 exhibited the best performance for AcSR with an average H2 yield of 90%, an
Perovskite
average acetic acid conversion of 95%, and an accumulated coke deposition of 11.8 wt% in a
La1xCexNiO3
30-h stability test. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Increasing concerns about the exhaustion of fossil fuels and the environmental pollution caused by the energy demand render hydrogen energy an appealing alternative [1]. At present, more than 50% hydrogen is generated by steam reforming of hydrocarbons which results in remarkable carbon emission [2,3]. Thus, if the H2 generation is derived from biomass source, significant environmental benefits towards the hydrogen economy could be achieved due to the theoretically closed carbon loop [4,5]. Fast pyrolysis is one of the most widely used technologies to convert biomass into bio-oil. Due
to the complicated composition of bio-oil, which depends on the conditions of pyrolysis and the source of biomass, bio-oil should be further updated. Steam reforming (SR) of bio-oil is a prospective way to convert the energy stored in bio-oil into easily usable hydrogen. Acetic acid is one of the major compositions in bio-oil, occupying a percentage up to 32 wt%. Therefore, acetic acid steam reforming (AcSR) has been widely used as a model to understand the conditions on the design efficient catalysts for bio-oil steam reforming process [6e8]. AcSR occurs together with water-gas-shift reaction (WGS) simultaneously. The overall reaction of AcSR is present as Eq. (1), and the major side reactions in AcSR are listed in Eqs. (3)e(5), including the acetic acid thermal decomposition
* Corresponding author. E-mail address: [email protected] (D. Tang). https://doi.org/10.1016/j.ijhydene.2018.02.128 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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reactions and ketonization of acetic acid, during which the formed acetone is considered as a coke precursor [9]. The main culprits of catalysts deactivation in AcSR are the coke deposition on the active sites and metal sintering due to the harsh reaction temperatures ranging from 500 C to 800 C. CH3 COOH þ 2H2 O/2CO þ 4H2
(1)
CH3 COOH#2CO þ 2H2
(2)
CH3 COOH#CH4 þ CO2
(3)
CH3 COOH/C2 H4 ; C2 H6 ; C3 H4 ; coke
(4)
2CH3 COOH/ðCH3 Þ2 CO þ H2 O þ CO2
(5)
Supported noble metal catalysts, such as Ru, Rh, Pt and Ir, show high catalytic activities and excellent stabilities. Seshan and co-workers have studied acetic acid steam reforming over Pt/ZrO2 catalysts to elucidate its reaction mechanism [10]. It suggests that the severe coke deposition of Pt/ZrO2 was due to the form acetone, a primary coke precursor, derived from the ketonization reaction. On the contrary, Pt/CeO2 shows less coke formation in AcSR due to the enhanced acetone reforming reaction caused by facile redox properties of CeO2 [11]. Basagiannis et al. found Ru and Rh based catalysts showed high activity in AcSR process [12,13]. Lemonidou et al. [14] demonstrated that the La2O3 modification on Rh based CexZr1xO2 catalysts not only efficiently suppresses the sintering by improving the metal support interaction (MSI) but also inhibits the coke deposition by facilitating the exchange of lattice oxygen. Although noble metal based catalysts show high performance in AcSR, their application at large scales is limited by high costs. Non-noble metal catalysts, especially the Nibased catalysts, have been extensively investigated in reforming process due to its low-cost and outstanding capability for C-C and C-H bond rupture [15]. Recently, Nibased catalysts derived from some particular structure such as hydrotalcite-like structure and phyllosilicate structure has attracted much attention due to the improved Ni dispersion and the enhanced MSI [16,17]. Perovskite type-like oxide catalyst precursor also shows the same properties in the steam reforming process [18,19]. Perovskites are mixed oxides with a general formula of ABO3, in which A is an alkaline earth or rare earth metal and B is generally a transition metal element. The partial substitution of cations in A and B position would provoke significant changes to its properties such as improving the thermal stability and increasing the mobility of oxygen ion vacancies [20,21]. Wu et al. [22] have investigated the Ca substitution of La in a La1xCaxNiO3 catalyst in glycerol steam reforming, and they have concluded that the perovskite structure could improve the distribution of containing elements and therefore enhance the MSI. M. de Lima et al. [20] have investigated the effect of La substitution by the Ce oxide on the performance of a La1xCexNiO3 perovskite oxide precursor in steam reforming of ethanol, and the results showed that the Ce doping could effectively decrease the Ni crystal size and increase the amount of oxygen vacancies.
Among these substitution elements, CeO2 has been recognized as an active promoter to improve the reforming activity of acetone, which is a coke precursor in AcSR. In this work, La1xCexNiO3 perovskite-type oxide derived catalysts were synthesized for hydrogen generation from AcSR process. The effect of Ce doping on the performance of La1xCexNiO3 was investigated, and various techniques were applied to characterize the properties of the synthesized catalysts, including Xray diffraction (XRD), H2 temperature-programmed reduction (TPR), H2 chemisorption, transmission electron microscopy (TEM), N2 adsorption-desorption, and thermogravimetric analysis (TGA). The activity and stability tests were carried out in a fixed-bed reactor.
Experimental Preparation of catalysts A series of La1xCexNiO3 (x ¼ 0, 0.05, 0.1, 0.2, and 0.3) were synthesized by citrate decomposition method. An aqueous solution with an appropriate molar ratio of La(NO3)3$6H2O (Aladdin, 99.99%), Ce(NO3)3$6H2O (Aladdin, 99.95%) and Ni(NO3)2$6H2O (Aladdin, 98%) was prepared. Then, a concentrated solution of citric acid (Aladdin, 99.8%) was added into the prepared solution, in such a way that the ratio of equivalent grams of metal to equivalent grams of citric acid would be unity, to form the metallic amorphous citrates. The obtained solution was heated to 60 C and kept for 9 h, followed by evaporating for 30 min using a vacuum rotary evaporator. The viscous solution was further dried for 12 h at 120 C. The spongy material was first grinded and then calcined in two stages: first at 550 C for 3 h and then at 900 C for 5 h.
Characterization XRD was performed with a 2q ranges from 10 to 80 by the graphite filtered Cu Ka radiation (l ¼ 1.5406 A). The N2 adsorption-desorption was used to characterize the physical texture by a Micrometrics Tristar 3000 analyzer at 77 K. The morphology of fresh catalysts was obtained from a HRTEM (Tecnai F30) at 300 kV. H2-TPR was carried out to detect the metal support interaction (MSI) by a Micrometrics Autochem Ⅱ device, and a flow rate of 30 mL min1 of 10% H2/N2 was introduced for reduction. TGA was carried out using a thermal system (STA449F3, NETZSCH Corp.) under a flow of air (50 mL min1) with a heating rate of 10 K min1. The amount of carbon deposition was calculated via the mass profiles in TGA results.
Activity and stability tests The activity and stability tests were performed in a packedbed reactor. The experimental procedure, relevant calculations and reactor set-up were shown in our previous studies [2]. The catalyst of 500 mg was placed in the middle of the reactor. Before the reaction, the catalysts were reduced by a pure H2 (100 mL min1) at 700 C for 3 h. The mixture of acetic acid and water with a steam to carbon ratio (S/C) of 3 was injected into the evaporator at a flow rate of 4 mL h1. The N2
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as a carrier gas was also introduced into the reactor at a flow rate of 150 mL min1. The reaction temperatures were set at 650 C, 700 C and 750 C. The gas products were analyzed by two gas chromatographs. One is equipped with a flame ionization detector (FID) and a Porapak-Q column with N2 as the carrier gas to analyze the organic species such as acetone. The other one is integrated with a thermal conductivity detector (TCD) and a TDX-01 column using He as the carrier gas to monitor hydrogen, carbon dioxide, carbon monoxide, and methane. The conversion, H2 yield and selectivity were calculated as follows: YH2 ¼
moles of hydeogen produced 100% 4 moles of acetic acid feed
X¼
acetic acid feed unreacted acetic acid 100% acetic acid feed
S¼
moles of carbon of each carbonaceous product 100% 2 moles of reacted acetic acid feed
Results and discussion Characterization of catalysts XRD patterns of as-prepared La1xCexNiO3 perovskite type oxides are shown in Fig. 1. The diffraction peaks at 23.2 , 47.6 and 58.8 was indexed to the (1 0 1), (2 0 2) and (3 0 0) planes of LaNiO3, indicating that the perovskite is successfully synthesized by the citric acid decomposition method. With the gradual increase of Ce doping, the peaks centered at 33.1 , 37.2 and 43.3 became significant, which could be assigned to the CeO2 (2 0 0), NiO (1 1 1) and NiO (2 0 0) planes, respectively. These three peaks were significant for La0.7Ce0.3NiO3 and La0.8Ce0.2NiO3 samples. Thus, the saturation point of Ce doping was reached at x ¼ 0.2. For all the catalysts precursors,
Fig. 1 e XRD profiles of different perovskite precursors. a) La0.7Ce0.3NiO3, b) La0.8Ce0.2NiO3, c) La0.9Ce0.1NiO3, d) La0.95Ni0.05NiO3 and e) LaNiO3.
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there was no peak shift of LaNiO3 characteristic peak, which could be attributed to the similar cation radius between La3þ (1.17 A) and Ce3þ (1.15 A). When x was less than 0.1, there were not any appreciable NiO and CeO2 peaks, suggesting the Ce cations was incorporated into the perovskite structure and the substituted perovskite was formed [20,23]. It has been demonstrated by XPS that high fraction substitution of Ce3þ ions in La1xCexNiO3 would result in the segregation of CeO2 and most of the Ce ions essentially existed as Ce4þ oxidation state [24]. Ghasdi et al. [25] have reported a substitution limit for Ce segregation in La1xCexCoO3, which is between 5% and 10%. The partial substitution of Ce3þ for La3þ leads to the conversion of Co3þ to Co2þ so as to keep the charge neutrality. The ionic radius increase due to the Co ions conversion would result in a decrease of the tolerance factor and therefore lead to the segregation of Ce. Besides, the NiO segregation is accompanied with the CeO2 segregation. This phenomenon is associated with the decrease of the La/Ni atomic ratio as well as with the fact that extra CeO2 cannot react with the NiO to form a perovskite structure. TEM was carried out to investigate the morphologies of as prepared perovskite structure precursors, as shown in Fig. 2. It is obvious that with the increase of Ce doping the metal oxides segregation occurs for La0.7Ce0.3NiO3 and La0.8Ce0.2NiO3, and the prepared precursors were successfully with low agglomeration. In order to investigate the reduction behavior of asprepared perovskite type precursor, TPR treatment was performed. As shown in Fig. 3A, there were three peaks at 590 K, 570 K and 770 K for all samples, respectively. Generally, the H2 consumption peaks in the case of perovskites result from the reduction of B site metal cation in the ABO3 structure. The first peak centered around 590 K could be derived from the reduction of Ni3þ in the perovskite structure to an intermediate valence Ni cation, leading to the formation of La4Ni3O10. The second peak at 670 K would be attributed to the reduction of La4Ni3O10 to Ni2þ, resulting in the formation of La2NiO4. The third peak at 770 K corresponded to the complete reduction of Ni2þ in La2NiO4 to produce La2O3, CeO2 and Ni0 metallic nanoparticles. This result implies that the reduction treatment at 973 K in the fixed-bed reactor could completely destroy the perovskite structure and Ni0 nanoparticles could deposit on the lanthanum oxides. Sierra Gallego et al. [26] have demonstrated that the reduction of LaNiO3-d proceeds in three steps as follows by an in-situ XRD technique. 4LaNiO3 þ 2H2 /La4 Ni3 O10 þ Ni0 þ 2H2 O
(6)
La4 Ni3 O10 þ 3H2 /La2 NiO4 þ 2Ni0 þ La2 O3 þ 3H2 O
(7)
La2 NiO4 þ H2 /Ni0 þ La2 O3 þ H2 O
(8)
Assuming that LaNiO3 is the only phase existed in these samples, the H2 consumption intensity ratio between the second peak and the first peak should be two. However, for all the samples, the ratio was obviously lower than two. It suggested that the concomitant formation of Ni0 occurred as listed in Eq. (6) and Eq. (7). The TPP profiles of La0.8Ni0.2O3 and La0.7Ni0.3O3 were slightly different from the others. The first and third peaks were inconspicuous, and a new peak ranging from 900 K to 1100 K appeared. The incorporation of the first
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Fig. 2 e TEM profiles of different catalysts. a) LaNiO3; b) La0.95Ni0.05NiO3; c) La0.9Ce0.1NiO3; d), f) La0.8Ce0.2NiO3; and e) La0.7Ce0.3NiO3.
two peaks is due to the decomposition of the perovskite structure, as evidenced in the XRD results. Therefore, the first peaks for La0.8Ni0.2O3 and La0.7Ni0.3O3 are mainly derived from the reduction of Ni2þ to Ni0, while the reduction in two steps of Ni3þ to Ni0 from the perovskite occurs in a limited extend [20]. The broad reduction peaks from 900 K to 1100 K were detected, which are likely due to the reduction of bulk CeO2 at high Ce substitution fraction. According to the H2-TPR, the synthesized catalysts were reduced at 973 K for 1 h. XRD patterns of the reduced catalysts are shown in Fig. 3B. As expected, the diffraction patterns of
all the samples were significantly different from those of the previous as-prepared perovskite-type precursors. It is apparent that all the samples lost their perovskite characteristic peaks after the reduction at 973 K. Meanwhile, new peaks indexed to La(OH)3, Ni, and La2O3 were revealed. In contrast with the other three samples, the reduced La0.8Ni0.2O3 and La0.7Ni0.3O3 exhibited basically the same characteristic peaks except for the peaks at 28.5 , 33.1 , 47.5 and 56.3 , which could be assigned to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of CeO2. In addition, the diffraction peaks of CeO2 were not observed until the Ce doping reached 0.2, revealing CeO2 is
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distinguished for all the samples, and therefore the Ni crystal size (listed in Table 1) was calculated by the Scherrer equation from this plane. It is obvious that the Ni crystal size gradually increases when the Ce substitution was more than 0.1. This was in line with the H2 chemisorption results listed in Table 1. Liu et al. [27] have reported that the aggregation of excessive Ce doping could blockade the mesopores or cover the accessible Ni active sites, thus exerting a negative effect on Ni dispersion. Consequently, the Ni catalyst derived from La0.9Ni0.1O3 shows the highest Ni dispersion and smallest Ni crystal size. The physical properties of these reduced catalysts were listed in Table 1. All the samples possessed BET surface areas of lower than 5.0 m2 g1, which is the inherent characteristic of these materials prepared after calcination at high temperature.
Activity and stability tests
Fig. 3 e (A) TPR profiles of different catalysts. a) La0.7Ce0.3NiO3, b) La0.8Ce0.2NiO3, c) La0.9Ce0.1NiO3, d) La0.95Ni0.05NiO3 and e) LaNiO3. (B) XRD profiles of reduced metal supported catalysts derived from different perovskite precursors. a) La0.7Ce0.3NiO3, b) La0.8Ce0.2NiO3, c) La0.9Ce0.1NiO3, d) La0.95Ni0.05NiO3 and e) LaNiO3.
either amorphous or well dispersed in the catalysts. The reflection peaks of Ni0 are inconspicuous with the increase of Ce doping amount, indicating the Ni particles are smaller and highly dispersed in the presence of CeO2. This could be either ascribed to the formation of Ni-Ce mixed oxide due to the strong interaction between Ni and CeO2 or attributed to the dilution effect caused by CeO2 addition to separate Ni nanoparticles [15]. No NiCe peak in XRD profiles is due to the small crystallite sizes of the NiCeO solution. The reflection peaks at 44.4 corresponding to the (0 1 1) plane of metallic Ni could be
The acetic acid conversions at different temperatures are shown in Fig. 4A. For all the samples, the acetic acid conversion increased with the reaction temperature, which is due to the endothermic nature of AcSR. The acetic acid conversion of the five catalysts decreased as follows: La0.9Ce0.1NiO3 > La0.95Ce0.05NiO3 > La0.8Ce0.2NiO3 > LaNiO3 > La0.7Ce0.3NiO3, corresponding with the Ni dispersion listed in Table 1. Since Ni is recognized as the active site for C-C and C-H bond rupture, the catalyst with higher active surface area is conducive to promote the conversion of acetic acid and other C-containing intermediates. Pu et al. [6] have reported that decreasing the nickel particle size could enhance the acetic acid conversion and increase the H2 yield. Lemonidou et al. have reported the temperature effect on the hydrogen production over Ni and Ru catalysts, and they also discovered the same trend of the acetic acid conversion with temperature [28]. It has been demonstrated that increasing the reaction temperature could not only remarkably promote the acetic acid steam reforming but also the by-products such as CH4, acetone and ketene [29]. The products distribution (700 C) of these catalysts are present in Fig. 4B. However, the distribution of the products is not similar, implying different extent of the reaction pathways. Steam reforming of acetic acid is a complicated system, involving different routines for acetic acid conversion and several secondary reactions between the reforming byproducts. Acetic acid is not a thermal stable reactant, which could be converted to other organics before reaching or on the catalyst bed via homogeneous/heterogeneous reactions such as decomposition reaction (Eq. (2)), decarboxylation (Eq. (3)) and ketonization (Eq. (5)). Consequently, the undesirable byproducts generated. These side reactions would occur together with the steam reforming, thereby reducing the hydrogen yield. It is noting that steam reforming is not the only reaction contributes to the hydrogen generation, and the acetone and methane steam reforming also play an important role in distribution of products, especially at high temperatures. In the present test, the CH4 selectivity was low for all the samples, indicating the methanation was inhibited under the reaction conditions due to its extremely exothermic nature. The small amount methane would be mainly derived from the decarboxylation reaction (Eq. (3)) and the cracking of the organics. As shown in Fig. 4B, the CH4 selectivity variation of different
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Table 1 e Properties of reduced catalysts. Samples LaNiO3 La0.95Ni0.05NiO3 La0.9Ce0.1NiO3 La0.8Ce0.2NiO3 La0.7Ce0.3NiO3 a b c
Surface area m2/g
Average pore diameter nm
Pore volume cm3/g
2.6 3.4 4.1 2.7 2.2
56.1 40.3 38.5 36.1 37.2
0.01 0.02 0.04 0.01 0.01
Crystal size of Ni contentb Ce contentb Metal dispersionc Nia nm wt% wt% m2/gcat 23.6/28.2 20.5 18.8/21.5 21.7 26.2
31.2 30.5 29.7 28.7 29.1
0.0 12.5 22.1 39.1 44.7
0.15 0.24 0.36 0.16 0.11
Determined by Scherrer equation from XRD profiles (reduced/spent). Determined by ICP-OES. Determined by H2 chemisorption.
Fig. 4 e (A) Conversion of acetic acid at various temperatures. (B) H2 yield and C-containing species selectivity.
catalysts corresponds with the Ni dispersion. Ni is active not only for methane steam reforming but also for the acetic acid cracking due to its high capacity for C-C rupture. Consequently, the samples such as La0.8Ce0.2NiO3, LaNiO3 and La0.7Ce0.3NiO3 with low Ni dispersion probably do not possess enough nickel sites for the occurrence of both acetic acid steam reforming and methane steam reforming, thus leading to the high selectivity to CH4. The lack of enough active Ni sites also could contribute to the high CO selectivity [30]. In contrast with LaNiO3 derived catalysts, the other Ce substitution perovskite derived samples showed high H2 yield accompanied with higher CO2 selectivity and lower CO selectivity. As the CO2/CO ratio has been identified as an indicator for WGS reactivity, the improved H2 yields is associated with the Ce substitution. It is well known that the CeO2-containing catalysts show higher selectivity to H2 and CO2 in steam reforming process due to the excellent redox property and high oxygen storage capacity of CeO2. Specifically, the metal Ni could adsorb the acetic acid and break its C-C bond, while oxygen vacancies in CeO2 would activate the water to generate the OH groups, which in turn react with CxH and CxOyH intermediate species to produce CO2 and H2. The WGS reactivity is improved by Ce promotion; therefore, the more Ce doping, the higher WGS reactivity. However, excessive Ce doping would bury the Ni particles, which would decrease the active surface area, leading to the loss of acetic acid conversion. The loss of the capability of C-C rupture, in turn, has a negative effect on WGS reactivity. Therefore, an optimum Ce/Ni ratio is important to obtain a high performance in AcSR. The formation of acetone suggests that the ketonization reactions also proceed with much lower rates. In general, steam reforming is recognized as a severe reaction under harsh conditions and always suffers from catalyst deactivation. To better understand the substitution effect of La by Ce over perovskite-type catalyst in AcSR, LaNiO3 and La0.9CeO0.1NiO3 derived catalysts were screened out with a 30h stability test. According to the results of activity tests, the stability tests were performed at 750 C. The acetic acid conversion with time of the two catalysts are shown in Fig. 5. The deactivation degree for the two catalysts is significantly distinguishable. The conversion of La0.9CeO0.1NiO3 derived catalysts was steady at the first 5 h (around 100%), then it decreased to 93% after 10 h and leveled off. In contrast, the acetic acid conversion of LaNiO3 derived catalyst plunged to 55% in the first 17 h and then kept stable at the last 13 h. Coke deposition is the main culprit for catalyst deactivation in AcSR. In fact, acetone has been regarded as a major coke
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Fig. 5 e Stability tests results of LaNiO3 and La0.9Ce0.1NiO3. precursor in AcSR. A plausible explanation is that the surface defect sites are necessary for the initial deprotonation of carboxylic acids, while the presence of adjacent Lewis acid and Brønstred basic sites is crucial for ketonization. Subsequently the aldol-condensation of acetone is facile, producing mesityl oxide, which further undergoes oligomerization, leading to coke deposition and thus rapidly deactivating the catalyst by blocking active catalytic sites [9]. As shown in Fig. 4B, the acetone selectivity of LaNiO3 derived catalyst is significantly higher than that of La0.9CeO0.1NiO3 derived catalyst. Therefore, the high coke deposition could be expected on LaNiO3 derived catalysts. It has been reported that the facile redox properties of CeO2 could promote the activation of steam and therefore reform acetone more efficiently, leading to a low coke deposition on La0.9CeO0.1NiO3 derived catalyst. The Ni crystal sizes after the stability tests are also listed in Table 1. The enhanced sintering resistance of La0.9CeO0.1NiO3 derived catalyst contributes to the higher stability. Recent studies have demonstrated that CeO2 exhibits the capability to stabilize metal particles against thermal sintering due to the improved MSI [31].
Characterization of spent catalysts The XRD profiles of spent catalysts are shown in Fig. 6A. The peaks at 25.1 , 25.8 , 27.6 , 30.3 , 47.4 , 50.2 and 56.9 could be indexed to (1 0 0), (1 0 1), (1 0 2), (1 0 3), (1 0 4), (1 1 4) and (1 1 6) planes of La2O2CO3. Cerritos et al. [32] have reported that the La2O3 modified Ni particles could react with CO2 to form La2O2CO3 (Eq. (9)), which would further eliminate the coke deposition around Ni particles (Eq. (10)). La2 O3 þ CO2 4La2 O2 CO3
(9)
La2 O2 CO3 þ C/La2 O3 þ 2CO
(10)
Moreover, the reflection peaks of graphite at 26.6 were detected for both catalysts. The graphite peak of Ce substitution catalyst is less intense than that of LaNiO3 derived catalyst, confirming the coke deposition is inconspicuous on La0.9CeO0.1NiO3. In addition, the sharp diffraction peak of Ni at
Fig. 6 e (A) XRD profiles and (B) TGA-DTG profiles of two spent catalysts. a) La0.9Ce0.1NiO3 and b) LaNiO3.
41.5 and 44.5 suggests that the LaNiO3 derived catalyst suffers from sintering. And the Ni particle size increased to 28.2 nm after the stability test. The agglomerated Ni particles would result in the decrease of the active surface, further leading to the loss of the capability towards C-C bond rupture. The amount of deposited coke was determined by TGA, and the DTG profiles of the two catalysts were also present in Fig. 6B. It is worth noting that the spent catalysts were first pretreated at 800 C for 0.5 h in N2 flow before the TGA tests so as to eliminate the influence of La2O2CO3 decomposition. The initial increase of the mass for both samples occurred, which is due to the oxidation of metallic Ni particles. It is obvious that the amount of coke deposition on La0.9Ce0.1NiO3 sample (11.8%) is less than that on LaNiO3 sample (17.3%), indicating the coke deposition is effectively suppressed. It has been reported that the small crystals possessed a high saturation concentration of carbon and therefore a low driving force for carbon diffusion through the nickel crystals [33]. The lattice oxygen released from the CeO2 also could react with the carbonaceous species as soon as it forms and therefore keep the metal surface free of coke deposition [34]. Both the small particle size and the CeO2 substitution contribute to the less
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coke deposition on La0.9Ce0.1NiO3 sample. Additionally, from the TGA profiles, two peaks appeared on LaNiO3 sample while only one peak occurred on La0.9Ce0.1NiO3 sample. The peaks at different temperatures should be assigned to different carbonaceous species on the spent catalyst surface. The existence of two distinguishable peaks has been reported previously. It is assumed that the first peak at lower temperature is related to the combustion of amorphous carbon species, while the second coke deposition on higher temperature could be attributed to the oxidation of graphitic carbon [35]. Furthermore, it is also reported that the coke near the Ni particles could be oxidized more easily than that far from the Ni particles [35]. It is apparent that the main peaks on La0.9Ce0.1NiO3 sample are at the lower temperature range, indicating more coke deposition is close to Ni particles. The coke deposition, which is far from the Ni particles, could be effectively eliminated by the CeO2.
Conclusions The La1xCexNiO3 perovskite type oxides were prepared and tested in AcSR. The nickel-based catalysts derived from the perovskite-type oxides possessed high Ni dispersion after the reduction treatment. The Ce substitution of La showed significant effect on the properties of perovskite-type oxide, and La0.9Ce0.1NiO3 sample exhibited the best performance due to its small Ni particles and high Ni surface areas. Meanwhile, Ce doping could remarkably promote the acetone reforming, which is generated from the ketonization reaction in AcSR, and water gas shift reaction. All the above properties contribute to the high catalytic activity in AcSR. Additionally, the deactivation resulted from coke deposition and Ni sintering of Ni-based catalysts in AcSR was effectively suppressed. This is due to improved MSI and coke deposition resistance from Ce substitution.
Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51706030) and the China Postdoctoral Science Foundation (No. 2017M611219).
references
[1] Cortright RD, Davda RR, Dumesic JA. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002;418:964e7. [2] Jiang B, Zhang C, Wang K, Dou B, Song Y, Chen H, et al. Highly dispersed Ni/montmorillonite catalyst for glycerol steam reforming: effect of Ni loading and calcination temperature. Appl Therm Eng 2016;109(Part A):99e108. [3] Dou B, Zhang H, Cui G, Wang Z, Jiang B, Wang K, et al. Hydrogen production by sorption-enhanced chemical looping steam reforming of ethanol in an alternating fixedbed reactor: sorbent to catalyst ratio dependencies. Energy Convers Manag 2018;155:243e52.
[4] Jiang B, Dou B, Wang K, Zhang C, Li M, Chen H, et al. Sorption enhanced steam reforming of biodiesel by-product glycerol on Ni-CaO-MMT multifunctional catalysts. Chem Eng J 2017;313:207e16. [5] Dou B, Wang K, Jiang B, Song Y, Zhang C, Chen H, et al. Fluidized-bed gasification combined continuous sorptionenhanced steam reforming system to continuous hydrogen production from waste plastic. Int J Hydrogen Energy 2016;41:3803e10. [6] Pu J, Nishikado K, Wang N, Nguyen TT, Maki T, Qian EW. Core-shell nickel catalysts for the steam reforming of acetic acid. Appl Catal B Environ 2018;224:69e79. [7] Wang S, Li X, Guo L, Luo Z. Experimental research on acetic acid steam reforming over CoeFe catalysts and subsequent density functional theory studies. Int J Hydrogen Energy 2012;37:11122e31. [8] Iwasa N, Yamane T, Takei M, Ozaki J-I, Arai M. Hydrogen production by steam reforming of acetic acid: comparison of conventional supported metal catalysts and metalincorporated mesoporous smectite-like catalysts. Int J Hydrogen Energy 2010;35:110e7. [9] Li D, Li X, Gong J. Catalytic reforming of oxygenates: state of the art and future prospects. Chem Rev 2016;116(19). [10] Takanabe K, Aika K-I, Inazu K, Baba T, Seshan K, Lefferts L. Steam reforming of acetic acid as a biomass derived oxygenate: bifunctional pathway for hydrogen formation over Pt/ZrO2 catalysts. J Catal 2006;243:263e9. [11] Matas Gu¨ell B, Babich I, Nichols KP, Gardeniers JGE, Lefferts L, Seshan K. Design of a stable steam reforming catalystda promising route to sustainable hydrogen from biomass oxygenates. Appl Catal B Environ 2009;90:38e44. [12] Basagiannis A, Verykios X. Catalytic steam reforming of acetic acid for hydrogen production. Int J Hydrogen Energy 2007;32:3343e55. [13] Basagiannis AC, Verykios XE. Influence of the Carrier on steam reforming of acetic acid over Ru-based catalysts. Appl Catal B Environ 2008;82:77e88. [14] Lemonidou AA, Vagia EC, Lercher JA. Acetic acid reforming over Rh supported on La2O3/CeO2eZrO2: catalytic performance and reaction pathway analysis. ACS Catal 2013;3:1919e28. [15] Li D, Zeng L, Li X, Wang X, Ma H, Assabumrungrat S, et al. Ceria-promoted Ni/SBA-15 catalysts for ethanol steam reforming with enhanced activity and resistance to deactivation. Appl Catal B Environ 2015;176e177:532e41. [16] Wu GW, Zhang CX, Li SR, Huang ZQ, Yan SL, Wang SP, et al. Sorption enhanced steam reforming of ethanol on NiCaO-Al2O3 multifunctional catalysts derived from hydrotalcite-like compounds. Energy Environ Sci 2012;5: 8942e9. [17] Bian Z, Suryawinata IY, Kawi S. Highly carbon resistant multicore-shell catalyst derived from Ni-Mg phyllosilicate nanotubes@silica for dry reforming of methane. Appl Catal B Environ 2016;195:1e8. [18] Zhang Q, Li L, Jiang B, Wang K, Tang D, Dou B. An intelligent oxygen Carrier of La2 xSrxNiO4l for hydrogen production by chemical looping reforming of ethanol. Int J Hydrogen Energy 2017;42:17102e11. [19] Zhang Q, Li L, Jiang B, Tang D, Dou B. Hydrogen by chemical looping reforming of ethanol: the effect of promoters on La2xMxNiO4-l (M¼ Ca, Sr and Ce) oxygen carriers. Chem Eng Sci 2017;174:259e67. [20] de Lima SM, da Silva AM, da Costa LOO, Assaf JM, Mattos LV, Sarkari R, et al. Hydrogen production through oxidative steam reforming of ethanol over Ni-based catalysts derived from La1xCexNiO3 perovskite-type oxides. Appl Catal B Environ 2012;121e122:1e9.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 6 7 9 5 e6 8 0 3
[21] Li L, Song Y, Jiang B, Wang K, Zhang Q. A novel oxygen Carrier for chemical looping reforming: LaNiO3 perovskite supported on montmorillonite. Energy 2017;131:58e66. [22] Wu G, Li S, Zhang C, Wang T, Gong J. Glycerol steam reforming over perovskite-derived nickel-based catalysts. Appl Catal B Environ 2014;144:277e85. [23] Franchini CA, Aranzaez W, de Farias AMD, Pecchi G, Fraga MA. Ce-substituted LaNiO3 mixed oxides as catalyst precursors for glycerol steam reforming. Appl Catal B Environ 2014;147:193e202. [24] Lima S, Assaf J, Pena M, Fierro J. Structural features of La1 x CexNiO3 mixed oxides and performance for the dry reforming of methane. Appl Catal Gen 2006;311:94e104. [25] Ghasdi M, Alamdari H, Royer S, Adnot A. Electrical and CO gas sensing properties of nanostructured La1 xCexCoO3 perovskite prepared by activated reactive synthesis. Sensor Actuator B Chem 2011;156:147e55. n F, Barrault J, Tatiboue €t J-M, Batiot[26] Gallego GS, Mondrago Dupeyrat C. CO2 reforming of CH4 over LaeNi based perovskite precursors. Appl Catal Gen 2006;311:164e71. [27] Liu D, Quek XY, Cheo WNE, Lau R, Borgna A, Yang Y. MCM-41 supported nickel-based bimetallic catalysts with superior stability during carbon dioxide reforming of methane: effect of strong metalesupport interaction. J Catal 2009;266:380e90. [28] Vagia EC, Lemonidou AA. Investigations on the properties of ceriaezirconia-supported Ni and Rh catalysts and their performance in acetic acid steam reforming. J Catal 2010;269:388e96.
6803
[29] Hu X, Lu G. Comparative study of alumina-supported transition metal catalysts for hydrogen generation by steam reforming of acetic acid. Appl Catal B Environ 2010;99:289e97. [30] Li Z, Hu X, Zhang L, Liu S, Lu G. Steam reforming of acetic acid over Ni/ZrO2 catalysts: effects of nickel loading and particle size on product distribution and coke formation. Appl Catal Gen 2012;417e418:281e9. [31] Senanayake SD, Rodriguez JA, Stacchiola D. Electronic metalesupport interactions and the production of hydrogen through the water-gas shift reaction and ethanol steam reforming: fundamental studies with well-defined model catalysts. Top Catal 2013;56:1488e98. [32] Carrera Cerritos Rl, Fuentes Ramirez R, Aguilera Alvarado AF, Martinez Rosales JM, Viveros Garcia TS, Galindo Esquivel IR. Steam reforming of ethanol over Ni/Al2O3 La2O3 catalysts synthesized by Sol Gel. Ind Eng Chem Res 2010;50:2576e84. ndez E, Yu Z, Tøtdal B, [33] Chen D, Christensen KO, Ochoa-Ferna Latorre N, et al. Synthesis of carbon nanofibers: effects of Ni crystal size during methane decomposition. J Catal 2005;229:82e96. [34] Zhang C, Li S, Li M, Wang S, Ma X, Gong J. Enhanced oxygen mobility and reactivity for ethanol steam reforming. AIChE J 2012;58:516e25. [35] Ma H, Zeng L, Tian H, Li D, Wang X, Li X, et al. Efficient hydrogen production from ethanol steam reforming over Lamodified ordered mesoporous Ni-based catalysts. Appl Catal B Environ 2016;181:321e31.