Comparative Study on Steam Reforming of Single-and

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Comparative study on steam reforming of single- and multicomponent model compounds of biomass fermentation for producing biohydrogen over mesoporous Ni/MgO catalyst Yajing Wang, Tingting Ji, Xiaoxuan Yang, and Yuhe Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02130 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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Comparative study on steam reforming of single- and multi-component model compounds of biomass fermentation for producing biohydrogen over mesoporous Ni/MgO catalyst Yajing Wang, Tingting Ji, Xiaoxuan Yang, Yuhe Wang* Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education; College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China

Corresponding author Tel: +86-451-88060570 E-mail: [email protected]

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ABSTRACT Steam reforming (SR) of single model molecules (acetone, butanol and ethanol) and a specific mixture (butanol/acetone/ethanol = 6/3/1, mass ratio) assumed as a model of bio-butanol raw mixture has been researched over mesoporous Nix/MgO catalysts (x represented nickel mole percent in Ni–Mg) at 673–873 K. The result showed reactant conversion at lower temperatures allowed ranking the three single-molecule compounds as the function of SR activity as follows: ethanol > acetone > butanol. The catalytic performance of mesoporous Ni0.12/MgO catalyst with lower Ni component containing enough active sites outperformed that of mesoporous Ni0.16/MgO catalyst which was more inclined to sinter in all SR tests of single-component compounds except at 673 K. On the contrary, the catalytic behavior of Ni0.16/MgO (richer in nickel) was superior to that of Ni0.12/MgO catalyst in SR of multi-component compounds. Moreover, in acetone-butanol-ethanol steam reforming (ABESR), SR of ethanol suffered competition from acetone and butanol, suppressing ethanol molecules access to the limited active sites of the catalyst. As a consequence, the conversion of ethanol in SR of multi-component compounds decreased compared to that in SR of pure ethanol.

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1. INTRODUCTION Hydrogen is regarded as a clean alternative with high energy density. Steam reforming (SR) of bio-oil has always got more attention as a versatile catalytic route from both potential industrial interest and environmentally-friendly perspective for hydrogen preparation.1 Bio-oil is a complex mixture ranging from low to high molecule weight oxygenated compounds like acid, ketones, aldehydes, alcohols, phenols, esters, sugars and derivatives.2 To simplify SR of bio-oil, the SR of single model compounds of bio-oil for hydrogen production has been the subject of many attempts.3‒9 However, it is limited for screening a suitable catalyst, which was only determined by their behavior in SR of pure molecules, applying to SR of bio-oil. If so, it would neglect diverse molecule structure and the interplay between oxygenated compounds which could have much of an effect on the product distribution and carbon deposition in SR process. As a consequence, more thorough and detailed research is needed for the SR of bio-oil. Against this background, Iriondo et al. designed the experiment, SR of glycerine, ethanol and glycerine-ethanol mixture tests were performed over Ni/Al2O3–La2O3 catalyst. The result showed that glycerine and possibly intermediate products competed with ethanol for the metal active sites, leading to a low accessibility towards ethanol. As a consequence, the reduction of ethanol conversion was pronounced in SR of glycerine-ethanol mixture with respect to higher by-products selectivity compared to SR of pure ethanol molecule.10 Garbarino et al. investigated the SR of ethanol-phenol mixture over 5% Ni–Al2O3 catalyst to evaluate the possibility of their mutual reaction between different molecules co-exist in the syngas. They found that the catalyst catalyzed the alkylation of phenol through ethanol at low temperature (720 K).11 Chitsazan et al. studied the possible interaction

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between different tar molecules over Ni/Al2O3 catalysts. The result indicated the presence of phenol suppressed the SR of ethanol at 873 K and below, making for competitive reactions such as dehydration and dehydrogenation. The co-presence of phenol and naphthalene appeared to lead to a slight mutual restrain of their SR processes.12 Bio-butanol produced by biomass fermentation, a potential renewable source for hydrogen production, has a unique charm due to its availability such as high energy density, low vapor pressure, weak corrosion and safety.13 In light of biomass feedstock and the fermentation conditions employed, the composition of bio-butanol raw mixture varied but it mainly was composed of butanol, acetone and ethanol with approximately mass ratio of 6:3:1.14 It is well known that acetone and ethanol can be applied to SR for hydrogen preparation,9,15 so SR of bio-butanol raw mixture will give the opportunity to save separation cost and availability of different compositions co-exist for decentralized hydrogen production. There are only a few literatures about SR of bio-butanol raw mixture, which almost pay attention to the potential significance that the influence of different catalysts or operating conditions on acetone-butanol-ethanol steam reforming (ABESR).16‒20 However, the comparative study concerning the influence of same catalysts on three pure molecules (acetone, butanol and ethanol) and bio-butanol raw mixture during SR has not been reported at all to our knowledge. Thus, a systematic understanding concerning the behavior of SR of a series of bio-butanol raw mixture model components including acetone, butanol, ethanol and mixture of them needs clarification. The optional SR catalysts are categorized into precious and transition metal catalysts. Despite the former (Rh, Ru, Pd and Pt) showed superior catalytic activity and anti-carbon property in severe

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tests,21 nickel was invariably selected for the wide availability due to higher quality and lower cost.22,23 The reason of mesoporous MgO served as support has been reported in our previous work, because the catalytic activity of Ni-based mesoporous MgO catalyst can be markedly enhanced in comparison with conventional MgO as support.24 Ni supported on mesoporous MgO, as representative reforming catalyst, exhibited satisfactory catalytic performance in SR reaction.9,25 In this work, the operating parameters were established under these conditions previously identified as suitable. In order to obtain a high yield of H2 and sustain the catalyst stability, all the tests were carried out under high S/C of 20, in large stoichiometric excess. This paper will describe and discuss a comparative study on SR of single- and multi-component model compounds of biomass fermentation over mesoporous Ni/MgO catalyst. This work mainly considered the effects of reaction temperature, and of Ni content, in order to study the difference of catalytic behavior of mesoporous Ni/MgO catalyst in various SR reaction systems and research the interplay between oxygenated compounds with properties. This will not only indicate the feasible path to prepare desirable catalysts for SR reaction, but also further approach the real component of bio-oil and provide the possibility for the industrialization of SR of bio-oil. 2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The catalyst was prepared via the incipient wetness method using mesoporous MgO as support. The procedure of the support synthesis was the same as reported in a previous work.26 For comparisons, various contents of nickel nitrate precursor (the nickel mole percent in Ni–Mg was 0.12 and 0.16, respectively) were impregnated to MgO support. All the aqueous mixtures were followed by drying and calcination at 923 K for 5 h. The prepared two

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NixO/MgO solid solutions were denoted as Ni0.12O/MgO and Ni0.16O/MgO, respectively. Before the reaction, the catalyst precursors were reduced to obtain catalysts, marked as Ni0.12/MgO and Ni0.16/MgO, with well-dispersed Ni particles. 2.2. Characterization. N2 adsorption–desorption test was conducted in a Micromeritics ASAP 2020 nitrogen adsorption apparatus at 77 K, in order to determine textural properties of the fresh calcined samples. Prior to the analysis, each sample was pre-treated under vacuum at 473 K for 3 h. The specific surface area was calculated using the BET method, the total pore volume and average pore size were estimated from desorption branch of the isotherm based on BJH analysis. The temperature-programmed reduction (TPR) measurement was recorded using a PCA-1200 apparatus equipped with a TCD detector to examine reducibility of the catalysts. About 50 mg of the samples was firstly flushed with 45 mL/min of Ar at 393 K for 0.5 h to eliminate impurities and subsequent cooled to room temperature. The analysis was performed until the baseline of TCD became stabilized. The temperature was increased up to 923 K kept for 1 h under 10% v/v H2/Ar with a flow rate of 45 mL/min. The amount of hydrogen consumed was monitored by a TCD detector and determined by the integration of its profile, using CuO as a calibration. The X-ray photoelectron spectroscopy (XPS) was carried out on the reduced catalysts with Al Ka radiation, to determine surface composition and metal particle size. The position of the C 1s peak (284.8 eV) was regarded as internal calibrant to correct the binding energies for both reduced catalysts due to electrostatic charging. The Ni particle size was calculated based on the XPS intensity ratios of the Ni 2p3/2 and Mg 1s peaks (INi/IMg) using a model according to the article reported by Kuipers et al..27

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Transmission electron microscopy (TEM) characterization was used to observe the morphology and measure the metal particle sizes of the reduced catalysts. About 20 mg powder sample was dispersed in 10 mL ethanol by ultrasonic pretreatment for 1 h to avoid agglomeration. A drop of the suspension was deposited on a Cu grid and then left to dry. The Ni particle size was estimated by counting at least 300 particles to obtain statistically reliable information. Main physical and chemical properties of the fresh catalysts were listed in Table 1. 2.3. Catalytic Activity Test. SR of model compounds of bio-butanol raw mixture obtained by bio-fermentation process was conducted at atmospheric pressure in a fixed-bed reactor. The soluble feedstocks were the solution of acetone, butanol, ethanol and ABE mixture. In all cases, 0.3 g catalyst precursors (40–60 mesh sieved) were loaded in the quartz tube and then reduced under H2/Ar flow with 10% (v/v) of H2 at 923 K for 1 h to activate the mesoporous MgO supported Ni catalysts. Before supplying the feedstock, the catalyst bed was swept out using carrier gas N2 with a flow rate of 45 mL/min until H2 could not be detected on the on-line gas chromatographs equipped with TCD detector. All aqueous solution of reactants with a molar ratio of S/C = 20 mol/mol was pumped into a vaporizing chamber (473 K) by an injection pump, corresponding to WHSV = 16 h-1, when the reaction temperature was turned to specified one. Reactor furnace temperature was varied from 673 to 873 K. The flow rate of effluent gas was determined through a bubble flow meter. The on-line gas analyzer determined the concentrations of H2, CO, CH4, CO2 and C2H4 based on the TCD detector for every 0.5 h to show transient behavior. The exhaust from the reactor passed through condensing collector operated at 273 K and then the liquid products including unconverted reactant, H2O and by-products were analyzed using FID detector over 1 h of reaction. The final data was

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determined at steady state (6 h on stream) in all tests. Reactants conversion was defined as follows:

Xreactant(%) =

Freactant, in − Freactant, out Freactant, in

× 100%

(1)

where Xreactant is the reactant conversion, Freactant,in and Freactant,out represent the molar flow rates of reactant at the inlet and outlet, respectively. The yield of H2 was defined by the Eqs. (3)–(5) as a percentage of the maximum allowed by stoichiometry, considering single-component reactants or the contribution of multi-component reactants. YH2(%) =

FH2, out × 100% ∑ vi × Freactant, in

(2)

where YH2 is the hydrogen yield, FH2,out represents the molar flow rate of hydrogen at the outlet, vi is the ratio of the stoichiometric reaction coefficients. i.e. 8 hydrogen molecules for each acetone molecule fed, 12 for butanol, 6 for ethanol during SR reactions as follows. CH3COCH3 + 5H2O → 3CO2 + 8H2

(3)

C4H9OH + 7H2O → 4CO2 + 12H2

(4)

C2H5OH + 3H2O → 2CO2 + 6H2

(5)

The selectivity of gaseous products was defined by Eq. (6) Sp(%) =

molp ∑ molsp

(6)

× 100%

where the Sp is the selectivity for hydrogen and other gases, molp represents the mole number of each gaseous product, molsp represents the total mole number of gaseous products. 3. RESULTS 3.1. Catalyst Characterization. N2 adsorption–desorption isotherms for the fresh calcined

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MgO and NixO/MgO (the precursor of Nix/MgO) samples are plotted in Figure 1A. In all cases, the samples possessed a typical capillary condensation step of mesoporous structures, and the adsorption–desorption isotherms could be classified as type IV following the IUPAC classification. The parallelism of the branches forming the hysteresis loops, which located at high relative pressures range of 0.8–1.0, revealing that the existence of narrow slit-shaped pores was ascribed to the stack of the nanoplates and there always were some macropores.28,29 In addition, it was found that the specific surface areas and pore volumes decreased with increasing nickel loadings in terms of the total nitrogen adsorbed amount (see Table 1), indicating that nickel species impregnated into the pores of mesoporous MgO support. The pore size distribution of fresh calcined MgO and NixO/MgO could be observed in Figure 1B, all samples presented a distinguishable double family of pores. One centered at 3 nm, the other was situated in scope of 7−70 nm, while average pore size was about 25 nm, indicating that the mesopores played a primary role, together with a small amount of macropores. The characteristics of MgO and NixO/MgO samples are summarized in Table 1. The H2-TPR experiments for NixO/MgO samples were performed to simulate the reductive pre-treatment applied prior to catalysis (923 K, 1 h). About four irregular reduction peaks of NiO were observed in Figure 2. According to the position of reduction peaks, the profiles for NixO/MgO samples could be divided into two major regions. The first region associated with reduction of free NiO was defined from 473 to 673 K, the second region attributed to sublayer Ni2+ reduction was defined from 673 to 973 K.30 It was well known that the interaction between MgO support and NiO could suppress the reduction of NiO and restrain the growth of Ni particles, due to form NiO–MgO solid solution.31 Through comparison of catalysts with various loadings, it was found that the

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location of peaks shifted towards left and broadened with the increasing loadings. This suggested that the interaction between nickel species with MgO support in Ni0.12O/MgO was stronger than that of Ni0.16O/MgO, as shown in Table 1, the Ni reducibility of Ni0.12/MgO (17.77%) was lower than that of Ni0.16/MgO (21.37%), and Ni particle size determined by XPS of Ni0.12/MgO (5.76 nm) was lower than that of Ni0.16/MgO (6.91 nm). The surface morphology and metal dispersion of the reduced Ni0.12/MgO and Ni0.16/MgO catalysts were investigated by TEM. As observed in Figure 3, some Ni particles (dark spherical) were formed and uniformly distributed on the external surface of the support. A representative HRTEM image of dark spherical is given in Figure 3a, Ni particles were identified with lattice fringe diffraction with lattice spacing of 0.203 nm for the Ni0 (111) plane (JCPDS Card No. 65-2865). In Table 1, the average Ni particle sizes determined by TEM for both catalysts distributed in the range of 5.5–7.0 nm, which were almost identical with the results measured by XRS analysis. 3.2. Catalytic Activity Studies. A series of SR tests were conducted over the mesoporous Ni0.12/MgO and Ni0.16/MgO catalysts to evaluate the influence of the different Ni loadings and reaction temperature on the catalytic performance. The distribution of gaseous products of model compounds was relatively stable under various reaction temperatures. The catalytic performance of a given catalyst was assessed in terms of reactant conversion, hydrogen yield and selectivity towards gaseous products. 3.2.1. Acetone Steam Reforming (ASR). The conversion of acetone and the outlet gas composition achieved for the ASR over Ni0.12/MgO catalysts are listed in Table 2, upper part. The acetone was mainly converted to CO, CO2 and a small amount of CH4 in the narrow temperature

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range of 723–873 K. The catalytic activity of Ni0.12/MgO catalyst rose with increased temperature and the trends in selectivity to H2 and CO were alike. The acetone was almost fully converted above 773 K. These could be explained by the fact that SR of acetone and the reverse water gas shift (RWGS) reaction were endothermic reactions, which were thermodynamically favored at higher temperature. Although under the high temperature (873 K), the production of CH4 by thermal decomposition of acetone would occur, SR of CH4 still hold the advantage, as evidenced by the traces of methane in Table 2, upper part. The same experiments were done over Ni0.16/MgO catalyst (see Table 2, lower part), whose results were very parallel to that of Ni0.12/MgO catalysts in the reaction temperature range. The selectivity trend to CO was contrary to that of CH4 which could be effectively removed by steam at higher temperature. The best performance was obtained at 873 K where H2 yield approached 85.12% with the minimum of methane selectivity (1.05%), revealing that the ASR mainly took place SR reaction. Over both catalysts, the higher selectivity of CH4 appeared at 673 K, the methanation reaction which influenced the product distribution was deduced as primary reason for the sharp reduction of H2. In addition, the Ni0.16/MgO catalyst provided higher H2 yield compared to Ni0.12/MgO catalyst in ASR at 673 K. It was worthy that the CO/CO2 product ratio and the amount of CH4 for Ni0.12/MgO were always lower than counterpart for Ni0.16/MgO catalysts at 723–873 K, demonstrating that the reforming ability of Ni0.12/MgO catalyst outperformed that of Ni0.16/MgO catalyst at this temperature region. 3.2.2. Butanol Steam Reforming (BSR). The experimental data obtained for Ni0.12/MgO catalysts in BSR reaction are summarized in Table 3, upper part. At 673 K, the conversion of butanol

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was just 68.37% with H2 yield of 12.36%. But the conversion reached 99.92% with increasing reaction temperature to 723 K, leading to the sharp increase of H2 yield. The butanol was totally converted to COx + H2 above 823 K, essentially with no methane formation, indicating that SR of butanol became the main reaction. At 873 K, H2 yield reached the value of 84.58% with 5.01% selectivity to CO and 16.26% to CO2, reflecting the high activity of Ni0.12/MgO catalyst for BSR. The same experiments were performed over Ni0.16/MgO catalyst. From the Table 3, lower part, it could be seen that the H2 yield increased gradually with rising temperature in 673–823 K. As the temperature reached 823 K, the H2 yield went through a maximum of 81.75% and the methane production was zero, and the selectivity of H2 and CO2 rose at the expense of CO. However, the temperature continued to rise above 823 K, H2 yield slowly decreased along with the increase of CO selectivity, because RWGS was thermodynamically feasible. For Ni0.12/MgO and Ni0.16/MgO catalysts, the selectivity of H2 was lower (54.31 and 56.32%), while CO selectivity (12.05 and 11.39%) and CH4 selectivity (14.39 and 13.46%) were higher at 673 K (see Table 3), respectively. This demonstrated that decomposition reaction of butanol may take place according to the following reaction (Eq. (7)).20 It appeared to be definitely less active for the two catalysts in ASR where the small quantities of non-reformed intermediate products including ethene and butaldehyde were detected due to poor ability of SR at this stage. C4H9OH → CO + 2CH4 + H2 + C

(7)

3.2.3. Ethanol Steam Reforming (ESR). The reaction results are showed in Table 4 as a function of the reaction temperature during ESR over the different catalysts. The ethanol was close to complete conversion to H2, CO, CH4 and CO2 as the detected products over two catalysts in the

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temperature range of 723–873 K. At 673 K, the conversion of ethanol was 81.83% for Ni0.12/MgO catalyst (see upper part), producing organic liquid byproducts such as aldehyde, ethene and acetone. As in fact happened, this indicated that parallel reactions situated in different sites occurred,32 concerning ethanol dehydration reaction according to Eq. (8), ethanol dehydrogenation to acetaldehyde according to Eq. (9) and decarbonylation of acetaldehyde according to Eq. (10), respectively. CH3CH2OH → CH3CHO + H2

(8)

CH3CH2OH → C2H4 + H2O

(9)

2CH3CHO → CH3COCH3 + CO + H2

(10)

Furthermore, the CH4/CO ratio at 673 K was almost equal to 1 on Ni0.12/MgO catalyst (see Table 4, upper part), suggesting that the prevalent reaction was ethanol decomposition according to (Eq. (11)) at low temperature region. CH3CH2OH → CH4 + CO + H2

(11)

For Ni0.12/MgO, the catalytic performance increased with the increase of the reaction temperature and maximized value of H2 yield (78.17%) at 873 K, the selectivity of CO and CO2 accounted for 9.72 and 15.35%, respectively, and CH4 vanished. In the case of the Ni0.16/MgO catalyst (see Table 4, lower part), the optimum temperature was at 823 K, the yield towards H2 as the main product was 64.83% while the trace of CH4 was detected at the exit of the reactor. The slight decrease in H2 yield arose from the RWGS reaction at 873 K, as evidenced by the increase in the selectivity of CO. At 673 K, the CH4/ (CO + CO2) ratio was well below unit on Ni0.16/MgO catalyst, demonstrating that SR and water gas shift (WGS) acted as the

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main reaction, possibly followed by ethanol decomposition reaction. In contrast to what happened over Ni0.12/MgO catalyst, the ethanol decomposition reaction was less favored over Ni0.16/MgO catalyst. 3.2.4. Acetone-Butanol-Ethanol (Bio-Butanol Raw Mixture) Steam Reforming (ABESR). The SR results are listed in Table 5 (upper part) for the feed containing acetone, butanol and ethanol with steam on the fresh Ni0.12/MgO catalyst. The comparison of the data obtained in SR of multi-component compounds could be regarded as indicative of the interplay between different molecules co-exist in the feedstock for H2 yield. Butaldehyde, acetaldehyde, ethene and acetone were also detected besides the common gaseous products of the ABESR reaction below 773 K. Unfortunately, the acetone conversion for ABESR could not be established in all cases, because acetone could be formed by aldol condensation of ethanal produced through dehydrogenation reaction of ethanol.33 Although the conversions of butanol and ethanol were above 50% at 673 K, the yield towards H2 was lower, implying that intermediates could not be effectively reformed in the ABESR process. The mixture was completely steam reformed at 873 K with H2 yield of 78.16% and methane was very low. The Ni0.12/MgO catalysts gave a little indication in behavior where the H2 yield and the selectivity to C1 products were only small differences from 773 to 873 K. Ni0.16/MgO catalysts were active for the SR of bio-butanol raw mixture with very high H2 yield (see Table 5, lower part). The behavior of Ni0.16/MgO picked up with raising temperature and passed through its maximum of 82.20% at 823 K, the outlet dry gas was consisted of 72.61% H2, 7.86% CO, 19.5% CO2 and 0.03% CH4. In contrast to what happened with Ni0.12/MgO catalyst, SR of bio-butanol raw mixture gave rise to significant changes to the catalytic performance of Ni0.16/MgO

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catalyst which was even more sensitive to varied temperature region from 773 to 873 K. The CO/CO2 ratio obtained over Ni0.16/MgO was lower than that over Ni0.12/MgO, evidencing that the equilibrium of SR versus other reactions such as Eqs. (7)–(11) was moved to SR the more the higher the Ni loading was. Obviously, the catalytic performance of Ni0.12/MgO was definitely lower than that of Ni0.16/MgO. It seemed that butanol was more easily steam reformed (full conversion at 823 K) than ethanol (full conversion at 873 K) over Ni0.16/MgO catalysts, while on the Ni0.12/MgO catalyst butanol and ethanol were completely steam reformed at 873 K. This indicated that SR of bio-butanol raw mixture involved extended Ni metal particles that were more abundant on Ni0.16/MgO catalysts. The lack of nickel sites attenuated the catalytic performance of ABESR reaction even under higher temperature over Ni0.12/MgO catalyst. 3.2.5. Catalytic Stability of the Nix/MgO Catalyst in ABESR. The catalytic stability of Nix/MgO catalysts for ABESR at 823 K for 20 h is shown in Figure 4. The Ni0.12/MgO catalyst (see Figure 4A) experienced the change of butanol conversion with a reduction of 2.6% after 20 h. Meanwhile, the initial yield of hydrogen was 78.7% which diminished to 68.6% within 20 h TOS. The maximization of hydrogen selectivity was ca. 69.4% in the first 1 h TOS. In contrast, for Ni0.16/MgO catalyst (see Figure 4B), the initial conversion of the butanol and the selectivity of hydrogen progressively declined from 100 to 98.75% and from 74.1 to 67.9% within 20 h TOS, respectively. The values for yield of H2 were within the scope of 83.67−76.60% during the runtime. In addition, the content of undesired by-products over Ni0.12/MgO was significantly higher than that over Ni0.16/MgO. However, as in the case of Ni0.12/MgO catalyst, the decline rate of ethanol conversion was slightly faster than that of butanol. Based on experiment results, it could be concluded that the catalytic activity and

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stability of Ni0.16/MgO catalyst were superior to those of Ni0.12/MgO catalyst. 4. DISCUSSION 4.1. Steam Reforming of Single-Component Compound. The reactant molecules and steam competed for the metallic active sites on catalyst surface during SR process.34 Bearing in mind that the onset of the SR activity was caused by the activation of water at higher temperature,35 so water could not be effectively activated at the low temperature (673 K). Thus, the decomposition of reactant dominated and catalytic activity of SR decreased. These were responsible for abundant by-product generation in SR of acetone, butanol and ethanol, respectively. As a consequence, as soon as the temperature headed towards the high regions, the reactants and intermediates were steam reformed dramatically improved. Furthermore, the minimum of reaction temperature for coming to completion relied on the size and type of reactant molecule. The larger the molecule, the higher the temperature required, because SR process concerned the breakage of more chemical bonds.12 This could be confirmed by the experiment data stemmed from SR of acetone, butanol and ethanol at lower temperatures, respectively. The SR of ethanol could effectively proceed at the temperature as low as 673 K, and the followed were acetone and butanol. Surprisingly, the conversion of three reactants to COx + H2 decreased in the orders: butanol > acetone > ethanol in the temperature range 723–873 K. As it was evident, ESR process involved a more complicated reaction network, which included many competing reactions in ESR such as mainly Eqs. (8)–(11). For ASR reaction, the acetone molecule acted as both the reactant molecule and the carbon precursor. As in fact happened, the ASR process could run a risk of carbon formation which would cover Ni active sites to diminish the SR activity. On the contrary, BSR reaction was more selective, CH4 being the only by-product

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besides COx. The analysis of the experiment data discussed above manifested that both Ni-based mesoporous MgO catalyst gave rise, at same condition, to almost complete SR of butanol and ethanol (irrespective of acetone) above 773 K. The behaviors of Ni0.12/MgO for SR of single-component compounds were quite similar at tested temperature region. Despite the relatively low Ni loadings, the Ni0.12/MgO catalyst exhibited better catalytic performance compared to Ni0.16/MgO catalyst in 723–873 K, suggesting that the Ni0.12/MgO catalyst had enough Ni active sites to effectively catalyze SR of pure compound. Nickel aggregation will usually happen when the reaction temperature (873 K) is higher than Tamman temperature of metallic nickel (863 K).36 Thus, the catalyst with sufficiently charged nickel would be inclined to agglomerate at higher temperature, resulting in improving the chance of carbon deposition and deactivating. This could be understood based on the SR experiments that showed the H2 yield over Ni0.16/MgO at 823 K was better than one at 873 K during SR of butanol and ethanol, respectively (see Tables 3 and 4, upper part). The catalytic behavior at low temperature, affirmed by the SR of acetone, butanol and ethanol experiments, clearly exhibited that at 673 K the conversion of reactants on the Ni0.12/MgO was always lower than that on the Ni0.16/MgO. To conclude, the Ni0.16/MgO was more active due to lower temperature application. Interestingly, the optimum temperature for Ni0.12/MgO was higher than that for Ni0.16/MgO catalyst, it seemed likely that the optimum temperature for SR reaction was determined by the property of the resulting Ni particles.32 4.2. Steam Reforming of Multi-Component Compounds. Over both catalysts, the butanol conversion in ABESR was similar to when butanol was single fed. Nevertheless, the presence of

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acetone and butanol seemed to affect the ethanol conversion which deviated from the obtained one during ESR, even though this effect was diminished on Ni0.16/MgO catalyst. And the deviation trend decreased as the temperature rose. In order to further confirm whether the decrease of ethanol conversion was ascribed to the presence of acetone or butanol, SR mixtures of ethanol/acetone and ethanol/butanol tests were carried out at same condition (see Figure 5). As observed, the ethanol conversion decreased with the addition of acetone or butanol, respectively. This observation indicated that acetone and butanol competed with ethanol for Ni active sites which were used to facilitate cleavage of chemical bonds, hindering ethanol access to the active sites of the catalyst. In fact, ethanol and butanol had an identical functional group (the hydroxyl) which was involved in the first adsorption step.32 However, the volatility of butanol was less than that of ethanol. Therefore, butanol preferentially adsorbed on a limited number of Ni active sites on the catalyst surface (see Figure 6, step 1). On the other hand, acetone, known as carbon precursor, was easier converted to carbon covering active sites, leading to the reduction of ethanol conversion (see Figure 6, steps 2 and 3). Focusing on the characterization results of catalyst precursors (see Table 1), the BET surface areas of both catalyst precursors are approximate (44.57 and 40.28 m2/g, respectively). However, the composition and reducibility of Nix/MgO catalysts revealed diversity of surface Ni0 content. As XPS data indicated, the Ni/Mg ratio of Ni0.16/MgO catalyst (16.77/83.23) was higher than that of Ni0.12/MgO catalyst (12.92/87.08). This demonstrated that the former had higher exposure of nickel species, which was responsible for the better reforming behavior observed in ABESR process. These results lead to the conclusion that Ni0.16/MgO catalyst could be regard as candidate for SR of multi-component compounds.

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5. CONCLUSION To summarize, the catalytic behavior of mesoporous Nix/MgO catalysts was directly related to the type of reactant molecules in SR process. The increase of nickel loadings promoted the growth of Ni particles, meanwhile, increased the number of Ni0 on the catalyst surface. For single-component compounds, Ni0.12/MgO catalyst possessed enough active sites to activate chemical bonds, while higher metal loadings were more inclined to sinter. On the contrary, the activity in SR of multi-component compounds behaved the better when the surface Ni0 content was higher of the catalyst. These findings pointed to an alternative way for employing adequate catalyst for SR reaction. ACKNOWLEDGMENTS This work was supported by the Overseas Scholars Program of the Department of Education, Heilongjiang Province (No. 1155h019). REFERENCES (1) Zhang, Y.; Brown, T. R.; Hu, G.; Brown, R. C., Comparative techno-economic analysis of biohydrogen production via bio-oil gasification and bio-oil reforming. Biomass. Bioenerg. 2013, 51, 99-108. (2) Fogassy, G.; Thegarid, N.; Toussaint, G.; van Veen, A. C.; Schuurman, Y.; Mirodatos, C., Biomass derived feedstock co-processing with vacuum gas oil for second-generation fuel production in FCC units. Appl. Catal. B: Environ. 2010, 96, 476-485. (3) Bossola, F.; Evangelisti, C.; Allieta, M.; Psaro, R.; Recchia, S.; Dal Santo, V., Well-formed, size-controlled ruthenium nanoparticles active and stable for acetic acid steam reforming. Appl. Catal. B:

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Environ. 2016, 181, 599-611. (4) Hu, X.; Zhang, L.; Lu, G., Pruning of the surface species on Ni/Al2O3 catalyst to selective production of hydrogen via acetone and acetic acid steam reforming. Appl. Catal. A: Gen. 2012, 427-428, 49-57. (5) Li, J.; Yu, H.; Yang, G.; Peng, F.; Xie, D.; Wang, H.; Yang, J., Steam reforming of oxygenate fuels for hydrogen production: A thermodynamic study. Energ. Fuel. 2011, 25 (6), 2643-2650. (6) Koike, M.; Li, D.; Watanabe, H.; Nakagawa, Y.; Tomishige, K., Comparative study on steam reforming of model aromatic compounds of biomass tar over Ni and Ni–Fe alloy nanoparticles. Appl. Catal. A: Gen. 2015, 506, 151-162. (7) Park, J. E.; Yim, S. D.; Kim, C. S.; Park, E. D., Steam reforming of methanol over Cu/ZnO/ZrO2/Al2O3 catalyst. Int. J. Hydrogen Energ. 2014, 39, 11517-11527. (8) Dhanala, V.; Maity, S. K.; Shee, D., Roles of supports (γ-Al2O3, SiO2, ZrO2) and performance of metals (Ni, Co, Mo) in steam reforming of isobutanol. RSC Adv. 2015, 5 (65), 52522-52532. (9) Wurzler, G. T.; Rabelo-Neto, R. C.; Mattos, L. V.; Fraga, M. A.; Noronha, F. B., Steam reforming of ethanol for hydrogen production over MgO-supported Ni-based catalysts. Appl. Catal. A: Gen. 2016, 518, 115-128. (10) Iriondo, A.; Barrio, V. L.; El Doukkali, M.; Cambra, J. F.; Güemez, M. B.; Requies, J.; Arias, P. L.; Sánchez-Sánchez, M. C.; Navarro, R.; Fierro, J. L. G., Biohydrogen production by gas phase reforming of glycerine and ethanol mixtures. Int. J. Hydrogen Energ. 2012, 37 (2), 2028-2036. (11) Garbarino, G.; Lagazzo, A.; Riani, P.; Busca, G., Steam reforming of ethanol-phenol mixture on Ni/Al2O3: Effect of Ni loading and sulphur deactivation. Appl. Catal. B: Environ. 2013, 129, 460-472. (12) Chitsazan, S.; Sepehri, S.; Garbarino, G.; Carnasciali, M. M.; Busca, G., Steam reforming of

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biomass-derived organics: Interactions of different mixture components on Ni/Al2O3 based catalysts. Appl. Catal. B: Environ. 2016, 187, 386-398. (13) Lee, S. Y.; Park, J. H.; Jang, S. H.; Nielsen, L. K.; Kim, J.; Jung, K. S., Fermentative butanol production by clostridia. Biotechnol. Bioeng. 2008, 101 (2), 209-28. (14) Jin, C.; Yao, M.; Liu, H.; Lee, C.-f. F.; Ji, J., Progress in the production and application of n-butanol as a biofuel. Renew. Sust. Energ. Rev. 2011, 15 (8), 4080-4106. (15) Guil-Lopez, R.; Navarro, R. M.; Ismail, A. A.; Al-Sayari, S. A.; Fierro, J. L. G., Influence of Ni environment on the reactivity of Ni–Mg–Al catalysts for the acetone steam reforming reaction. Int. J. Hydrogen Energ. 2015, 40 (15), 5289-5296. (16) Cai, W.; de la Piscina, P. R.; Homs, N., Oxidative steam reforming of bio-butanol for hydrogen production: effects of noble metals on bimetallic CoM/ZnO catalysts (M=Ru, Rh, Ir, Pd). Appl. Catal. B: Environ. 2014, 145, 56-62. (17) Cai, W.; Homs, N.; de la Piscina, P. R., Renewable hydrogen production from oxidative steam reforming of bio-butanol over CoIr/CeZrO2 catalysts: Relationship between catalytic behaviour and catalyst structure. Appl. Catal. B: Environ. 2014, 150-151, 47-56. (18) Cai, W.; de la Piscina, P. R.; Homs, N., Hydrogen production from the steam reforming of bio-butanol over novel supported Co-based bimetallic catalysts. Bioresour. Technol. 2012, 107, 482-6. (19) Cai, W.; de la Piscina, P. R.; Gabrowska, K.; Homs, N., Hydrogen production from oxidative steam reforming of bio-butanol over CoIr-based catalysts: effect of the support. Bioresour. Technol. 2013, 128, 467-71. (20) Cai, W.; Homs, N.; de la Piscina, P. R., Efficient hydrogen production from bio-butanol oxidative

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steam reforming over bimetallic Co–Ir/ZnO catalysts. Green Chem. 2012, 14 (4), 1035. (21) Liguras, D. K.; Kondarides, D. I.; Verykios, X. E., Production of hydrogen for fuel cells by steam reforming of ethanol over supported noble metal catalysts. Appl. Catal. B: Environ. 2003, 43 (4), 345-354. (22) Wang, S.; Cai, Q.; Zhang, F.; Li, X.; Zhang, L.; Luo, Z., Hydrogen production via catalytic reforming of the bio-oil model compounds: Acetic acid, phenol and hydroxyacetone. Int. J. Hydrogen Energ. 2014, 39 (32), 18675-18687. (23) Bizkarra, K.; Barrio, V. L.; Yartu, A.; Requies, J.; Arias, P. L.; Cambra, J. F., Hydrogen production from n-butanol over alumina and modified alumina nickel catalysts. Int. J. Hydrogen Energ. 2015, 40 (15), 5272-5280. (24) Yang, X.; Wang, Y.; Li, M.; Sun, B.; Li, Y.; Wang, Y., Enhanced hydrogen production by steam reforming of acetic acid over a Ni catalyst supported on mesoporous MgO. Energ. Fuel. 2016, 30 (3), 2198-2203. (25) Yu, M.; Zhu, K.; Liu, Z.; Xiao, H.; Deng, W.; Zhou, X., Carbon dioxide reforming of methane over promoted NixMg1−xO (111) platelet catalyst derived from solvothermal synthesis. Appl. Catal. B: Environ. 2014, 148-149, 177-190. (26) Yang, X.; Wang, Y.; Wang, Y., Significantly improved catalytic performance of Ni-based MgO catalyst in steam reforming of phenol by inducing mesostructure. Catalysts 2015, 5, 1721-1736. (27) Kuipers, H. P. C. E.; van Leuven, H. C. E.; Visser, W. M., The characterization of heterogeneous catalysts by XPS based on geometrical probability 1: Monometallic catalysts. Surf. Interface Anal. 1986, 8, 235-242.

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(28) Veldurthi, S.; Shin, C.-H.; Joo, O.-S.; Jung, K.-D., Synthesis of mesoporous MgO single crystals without templates. Micropor. Mesopor. Mat. 2012, 152, 31-36. (29) Ling, Z.; Zheng, M.; Du, Q.; Wang, Y.; Song, J.; Dai, W.; Zhang, L,; Ji, G.; Cao, J., Synthesis of mesoporous MgO nanoplate by an easy solvothermal-annealing method. Solid State Sci. 2011, 13, 2073-2079. (30) Parmaliana, A.; Arena, F.; Frusteri, F.; Giordano, N., Temperature-programmed reduction study of NiO–MgO interactions in magnesia-supported Ni catalysts and NiO–MgO physical mixture. J. Chem. Soc., Faraday Trans. 1990, 86, 2663-2669. (31) Bradford, M. C. J.; Vannice, M. A., Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity. Appl. Catal. A Gen. 1996, 142 (1), 73-96. (32)

Garbarino,

G.;

Wang, C.; Valsamakis, I.;

Chitsazan,

S.;

Riani,

P.;

Finocchio,

E.;

Flytzani-Stephanopoulos, M.; Busca, G., A study of Ni/Al2O3 and Ni–La/Al2O3 catalysts for the steam reforming of ethanol and phenol. Appl. Catal. B: Environ. 2015, 174-175, 21-34. (33) Nishiguchi, T.; Matsumoto, T.; Kanai, H.; Utani, K.; Matsumura, Y.; Shen, W. J.; Imamura, S., Catalytic steam reforming of ethanol to produce hydrogen and acetone. Appl. Catal. A: Gen. 2005, 279 (1-2), 273-277. (34) Marquevich, M.; Medina, F.; Montané, D., Hydrogen production via steam reforming of sunflower oil over NiAl catalysts from hydrotalcite materials. Catal. Commun. 2001, 2, 119-124. (35) Garbarino, G.; Chitsazan, S.; Phung, T. K.; Riani, P.; Busca, G., Preparation of supported catalysts: A study of the effect of small amounts of silica on Ni/Al2O3 catalysts. Appl. Catal. A: Gen. 2015, 505, 86-97.

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(36) Trane, R.; Dahl, S.; Skjøth-Rasmussen, M. S.; Jensen, A. D., Catalytic steam reforming of bio-oil. Int. J. Hydrogen Energ. 2012, 37 (8), 6447-6472.

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Table 1. Physical and Chemical Properties of Mesoporous MgO Supported Ni Catalysts pore Ni loadings

average

surface

BET surface

Ni size (nm) b

samples (wt. %)

Ni

volume

pore size

(cm3/g)

b

c

reducibility

composition

(nm)

(%)

(Ni/Mg)d

XPSd

TEMe

areaa (m2/g)

MgO

－

98.42

0.80

25.72

－

－

－

－

Ni0.12/MgO

15.96

44.57

0.31

25.41

17.77

12.92/87.08

5.76

5.62

Ni0.16/MgO

20.62

40.28

0.28

24.20

21.37

16.77/83.23

6.91

6.84

a

Surface areas of NixO/MgO solid solutions were calculated through BET method.

b

Total pore volume and average pore size of NixO/MgO solid solutions were obtained by BJH

desorption branch. c

Ni reducibility was calculated by TPR quantification.

d

Surface composition (atomic ratio) and Ni particle size of catalysts were determined by XPS

measurement. e

Ni particle size was calculated using weighted average according to the TEM results.

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Table 2. Acetone Steam Reforming (ASR) Experiment Results over Nix/MgO Catalyst with respect to Acetone Conversion (Xi), Hydrogen Yield (YH2) and Selectivity (Si) to Gaseous Products on Different Temperature

ASR temperature experiment over Ni0.12/MgO catalyst

Temperature (K)

Xacetone

YH2

SH2

SCO

SCH4

SCO2

673

75.24

24.25

37.85

20.45

21.98

19.72

723

98.11

71.85

65.83

5.24

1.98

26.95

773

99.80

75.87

66.11

5.74

1.32

26.83

823

99.95

83.23

66.91

7.09

0.30

25.70

873

99.96

88.36

69.77

9.11

0.02

21.10

ASR temperature experiment over Ni0.16/MgO catalyst

Temperature (K)

Xacetone

YH2

SH2

SCO

SCH4

SCO2

673

88.16

54.25

55.75

13.53

14.75

15.97

723

99.92

62.25

64.20

6.85

1.84

27.11

773

99.94

67.25

65.25

9.13

1.52

24.10

823

99.95

81.26

67.95

10.76

1.14

20.15

873

99.96

85.12

69.41

12.19

1.05

17.35

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Table 3. Butanol Steam Reforming (BSR) Experiment Results over Nix/MgO Catalyst with respect to Butanol Conversion (Xi), Hydrogen Yield (YH2) and Selectivity (Si) to Gaseous Products on Different Temperature

BSR temperature experiment over Ni0.12/MgO catalyst

Temperature (K)

Xbutanol

YH2

SH2

SCO

SCH4

SCO2

SC2H4

673

68.37

12.36

54.31

12.05

16.97

13.46

3.21

723

97.97

62.53

74.10

2.82

0.21

22.87

－

773

99.93

72.50

73.81

3.34

0.04

22.81

－

823

100

82.66

75.56

4.91

－

19.53

－

873

100

84.58

78.73

5.01

－

16.26

－

BSR temperature experiment over Ni0.16/MgO catalyst

Temperature (K)

Xbutanol

YH2

SH2

SCO

SCH4

SCO2

SC2H4

673

69.56

17.85

56.32

11.39

15.17

14.39

2.73

723

99.29

58.79

71.65

6.94

3.72

17.69

－

773

99.94

69.33

73.72

9.38

0.05

16.85

－

823

99.96

81.75

74.93

6.56

－

18.51

－

873

100

73.08

69.51

12.76

－

17.73

－

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Table 4. Ethanol Steam Reforming (ESR) Experiment Results over Nix/MgO Catalyst with respect to Ethanol Conversion (Xi), Hydrogen Yield (YH2) and Selectivity (Si) to Gaseous Products on Different Temperature

ESR temperature experiment over Ni0.12/MgO catalyst

Temperature (K)

Xethanol

YH2

SH2

SCO

SCH4

SCO2

SC2H4

673

81.83

33.17

46.15

18.95

14.88

15.01

5.01

723

99.86

64.33

62.22

6.76

1.49

29.53

－

773

99.97

73.67

69.61

8.25

0.03

22.06

－

823

99.98

75.50

72.32

8.34

0.01

19.33

－

873

100

78.17

74.93

9.72

－

15.35

－

ESR temperature experiment over Ni0.16/MgO catalyst

Temperature (K)

Xethanol

YH2

SH2

SCO

SCH4

SCO2

SC2H4

673

95.91

48.50

51.97

19.09

6.06

18.79

4.09

723

99.91

57.17

61.14

10.14

1.08

26.68

0.96

773

99.96

59.17

62.42

11.78

0.04

25.76

－

823

99.97

64.83

65.17

9.20

0.03

25.60

－

873

99.94

51.53

61.15

14.37

0.05

24.43

－

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Energy & Fuels

Table 5. Acetone-Butanol-Ethanol Steam Reforming (ABESR) Experiment Results over Nix/MgO Catalyst with respect to Reactant Conversion (Xi), Hydrogen Yield (YH2) and Selectivity (Si) to Gaseous Products on Different Temperature

ABESR temperature experiment over Ni0.12/MgO catalyst

Temperature (K)

Xbutanol

Xethanol

YH2

SH2

SCO

SCH4

SCO2

SC2H4

673

67.94

74.73

22.75

44.52

19.82

15.51

11.55

8.60

723

97.35

99.35

52.92

55.76

9.20

7.81

21.83

5.40

773

99.82

99.58

74.49

65.82

9.85

4.32

19.80

0.21

823

99.93

99.91

76.21

68.47

10.17

2.96

18.40

－

873

100

100

78.16

71.24

8.17

1.05

19.54

－

ABESR temperature experiment over Ni0.16/MgO catalyst

Temperature (K)

Xbutanol

Xethanol

YH2

SH2

SCO

SCH4

SCO2

SC2H4

673

71.01

84.00

26.15

52.62

16.56

10.02

14.29

6.51

723

99.12

99.82

67.73

60.86

5.08

4.17

28.22

1.67

773

99.92

99.65

75.50

66.81

7.37

1.49

24.33

－

823

100

99.96

82.20

72.61

7.86

0.03

19.50

－

873

100

100

79.23

69.59

10.12

0.11

20.18

－

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Figure 1. Nitrogen adsorption–desorption isotherms (A) and BJH desorption pore size distributions (B) for fresh calcined samples (a) mesoporous MgO, (b) Ni0.12O/MgO and (c) Ni0.16O/MgO.

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Energy & Fuels

Figure 2. TPR patterns for fresh calcined samples (a) Ni0.12O/MgO and (b) Ni0.16O/MgO.

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Figure 3. TEM images for reduced catalysts (a) Ni0.12/MgO and (b) Ni0.16/MgO.

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Energy & Fuels

Figure 4. Reactant conversion (Xi), Hydrogen yield (YH2) and selectivity (Si) to gaseous products vs. reaction time over mesoporous Ni0.12/MgO (A) and Ni0.16/MgO (B) catalysts. Reaction conditions: 0.3 g catalyst, 823 K, N2 flow rate = 45 mL/min, WHSV = 16 h-1, S/C = 20 mol/mol.

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Figure 5. Effects of feedstocks on the conversions of ethanol and H2 yield over Ni0.12/MgO catalysts at 673 K.

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Energy & Fuels

Figure 6. The assumed mechanism involved in the ABESR at the Ni/MgO catalyst surface.

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Comparative Study on Steam Reforming of Single-and

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