Al2O3 catalyst

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JIEC 3158 1–13 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

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Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/Al2O3 catalyst Kang Min Kima , Byeong Sub Kwaka , No-Kuk Parkb , Tae Jin Leeb , Sang Tae Leec , Misook Kanga,* a b c

Department of Chemistry, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea School of Chemical Engineering, College of Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea Wooshin Com., Gyeongsan, Gyeongbuk 38470, Republic of Korea

A R T I C L E I N F O

Article history: Received 3 September 2016 Received in revised form 18 October 2016 Accepted 29 October 2016 Available online xxx Keywords: Propane steam reforming Hydrogen production Fe-promoter 30NixFeyO/70Al2O3 Particle agglomeration

A B S T R A C T

In this study, we investigated the role of Fe oxide as a promoter to improve the redox properties of Ni in PSR (propane steam reforming), thereby extending its lifetime and enabling it to be more easily oxidized by the CO generated to produce CO2. Bimetallic NiFe supported on g-Al2O3 (30NixFeyO/70Al2O3) samples are prepared as catalysts and characterized by XRD, TEM, H2-TPR, TPO, and XPS. Moreover, a mechanism for propane steam reforming over the 30NixFeyO/70Al2O3 catalyst is proposed on the basis of the GC and mass spectroscopy results and CO-, C3H8-, and H2O-temperature programmed desorption analysis. Consequently, the Fe components in the 30NixFeyO/70Al2O3 catalysts suppressed the agglomeration between the Ni and Al particles. The catalytic performances on the 30NixFeyO/70Al2O3 catalysts are improved by the reduced deposition of carbon compared to that on the 30NiO/70Al2O3 one: at 700
C, the hydrogen selectivity amounted to 86% in the 30Ni0.8Fe0.2O/70Al2O3 catalyst compared to 79% in the 30NiO/70Al2O3 one. ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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Introduction

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Hydrogen has attracted interest as an efficient and environmentally friendly energy carrier. The methods of hydrogen production include water electrolysis [1], the partial oxidation or gasification of heavy oil or coal [2], and steam reforming of hydrocarbons [3]. It should be noted however that from an economic viewpoint, the steam reforming of hydrocarbons has been highlighted as one of the most reasonable methods of producing hydrogen. Currently, the hydrocarbons used as the hydrogen sources in the steam reforming reaction can be separated into two kinds: firstly, light paraffins such as methane [4], ethane [5], propane [6] and butane [7], and secondly, oxidized compounds such as methanol [8], dimethylether [9], ethanol [10] and acetic acid [11]. In particular, propane has many advantages as a hydrogen source candidate; it is a gas at standard temperature and pressure and a by-product of natural gas processing and petroleum refining, is commonly used as a fuel for engines, oxygas torches, portable stoves and residential central heating, but is

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* Corresponding author. Fax: +82 53 815 5412. E-mail address: [email protected] (M. Kang).

compressible to a transportable liquid. Thus, propane is a potential candidate as a hydrogen carrier, because of its common storage (LP gases) and existing widely spread infrastructure. Additionally, propane steam reforming (PSR) is the most economical pathway in terms of the hydrogen yield, since hydrogen is produced from steam as well as propane, as shown in the following equation for propane overall steam reforming:

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C3H8 + 6H2O ! 3CO2 + 10H2 DH
298 = 499 kJ/mol

(1)

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Since industrial operations always use excess steam to minimize catalyst deactivation, the maximum yield of hydrogen per mole of propane fed can reach 10. In practice, PSR is performed at high temperatures over Nibased catalysts. However, the high temperature required may favor several routes to the formation of carbon deposits and the decomposition of the hydrocarbons. In addition, Ni catalysts tend to agglomerate and then lose their active surface area under PSR conditions, resulting in short catalyst lifetimes. Consequently, novel preparation and promotion techniques that can resist rapid catalyst deactivation by coking and sintering are essential. Much of this research effort has focused on developing Ni catalysts with improved resistance to coke formation by adding promoters. Alkali

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http://dx.doi.org/10.1016/j.jiec.2016.10.046 1226-086X/ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: K.M. Kim, et al., Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/ Al2O3 catalyst, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.046

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metals, such as K2O [12] and MgO [13], have been shown to improve the coking resistance by enhancing the carbon gasification, but at the cost of reduced catalytic activity. The effects of novel metals, such as Pt, Rh and Ru, have also been investigated [14–16], and their introduction into Ni-based catalysts to increase their hydrogen selectivity and decrease their coke deposition showed dramatic results. More recently, bi or tri metallic catalytic species have been investigated. Laosiripojana et al. [17] reported that adding CeO2 to Ni/Al2O3 catalysts enhanced the nickel dispersion and reactivity of the carbon deposits, leading to improved catalytic activity and stability in the steam reforming of propane. Althenayan et al. [18] researched the use of bimetallic Co–Ni/Al2O3 catalysts for propane dry reforming, in order to determine the intrinsic reaction rate simultaneously with the carbon-induced deactivation coefficient from the transient rate data over an extended period of time (up to 72 h), for propane dry reforming over a Co–Ni catalyst at 823–973 K. Malaibari et al. [19] also investigated the effect of Mo as a promoter in aluminasupported Ni catalysts for PSR at 450
C. Mo promotion showed a beneficial effect by both decreasing the rate of carbon deposition and increasing the catalytic activity. In order to develop more durable and cheaper catalysts for use in PSR, this study broke new ground. Fe oxide was introduced as a promoter to improve the redox properties of Ni, thereby extending its lifetime and enabling it to be more easily oxidized by the CO generated to produce CO2. Iron oxide can be thermally reduced by CO during PSR as follows: Fe2O3 + 3CO ! 2Fe + 3CO2. Additionally, partial reduction with hydrogen at about 400
C gives magnetite, a black magnetic material that contains both Fe(III) and Fe(II) through the reaction: 3Fe2O3 + H2 $ 2Fe3O4 + H2O and this reaction can occur reversely also. Thus, in this study, we attempted to apply bimetallic NiFe catalysts supported on g-Al2O3 to PSR. The prepared catalysts were characterized by XRD, TEM, H2O-, C3H8and CO-TPD, H2-TPR, TPO, and XPS. In addition, the nature of the promoter action afforded by Fe oxide was examined. The results were applied to the design of practical catalysts for PSR.

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Experimental

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Preparations of the 30NixFeyO/70Al2O3 catalysts

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The bimetallic NiFe catalysts supported on g-Al2O3 (30NixFeyO/ 70Al2O3) were prepared using impregnation methods [20] with a Q3 Ni-based catalyst content of 30 wt.% and Fe/Ni atomic ratios of 0, 0.1, 0.25, and 0.43. The 70 wt.% g-Al2O3 (Junsei Co., Japan) was mixed in ethanol solvent at room temperature and the suspension was stirred for 1 h. Then, NiCl2 (99.99%, Junsei Co., Japan) and FeCl2 (99.99%, Junsei Co., Japan) as the Ni and Fe sources, respectively, were added to the suspension containing 70 wt.% g-Al2O3 powder, and the mixture was stirred at 40
C. At this time, the catalysts were labeled NixFey, where x and y indicate the atomic %. The NixFey main catalytic species were loaded onto the external surfaces of alumina and the total loaded weights in all of the samples were the same, viz. 30 wt.%. After aging for 3 h, the final solutions were evaporated at 70
C for 6 h and dried at 50
C for 24 h in an oven. The final samples were treated thermally at 800
C for 3 h in air, and reduced by H2/argon (1:10 ratio) at 700
C for 2 h to generate the NiFe oxide composites. The following four experimental bimetallic NiFe catalysts supported on Al2O3 were prepared for comparison: 30NiO/70Al2O3, 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3.

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Physical properties of the 30NixFeyO/70Al2O3 catalysts

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The 30NiO/70Al2O3 and 30NixFeyO/70Al2O3 catalysts were examined by powder XRD (model MPD from PANalytical) using

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nickel-filtered CuKa radiation (30.0 kV, 30.0 mA) over the 2u range of 10–100
. The core and shell shapes of the catalyst particles were determined by high-resolution TEM (H-7600, Hitachi, Japan) operated at 120 kV. The BET (Brunauer, Emmett and Teller) surface areas of the catalysts were measured using a Belsorp II instrument. The BET surface measurement was performed by nitrogen gas adsorption using a continuous flow method with a mixture of nitrogen and helium as the carrier gas. A tube was filled with 0.2 g of the sample under an N2 atmosphere and then out-gassed for 1 h at 200
C before the measurements. After pre-treatment, the samples were cooled down to room temperature and then exposed to liquid nitrogen for 2 h. In the a-plot method, the adsorption volume, Vads (p/p0), which was normalized to Vads for the reference material, is used as a new x-axis to plot the adsorption isotherms for the samples of interest. The discrete adsorption data for the reference material was interpolated numerically to generate a continuous x-axis. The XPS (AXIS-NOVA Kratos Inc.) spectroscopy measurements of Ni2p, Fe2p Al2p, O1s, and C1s using a nonmonochromatic AlKa (1486.6 eV) X-ray source were performed at the Yeungnam University instrumental center, Korea. The powders were pelletized at 1.2  104 kPa for 1 min and the 1.0-mm pellets were stored overnight in a vacuum (1.0  107 Pa) to remove the water molecules from the surface before the measurements. The base pressure in the system was less than 1 109 Pa. The experiments were performed using a 200-W source power and an angular acceptance of 5
. The analyzer axis formed a 90
angle with the specimen surface. The Shirley function was used to subtract the background for the XPS data analysis. The signals were fitted using the mixed Lorentzian–Gaussian curves.

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Gas adsorption abilities of the 30NixFeyO/70Al2O3 catalysts

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The H2-TPR experiments for the as-synthesized 30NixFeyO/ 70Al2O3 samples were conducted using the same equipment as that used for the thermo-gravimetric analysis (TGA) experiment (Shinco com., Korea). Approximately 0.05 g of the catalyst was pre-treated under flowing argon gas (30 mL min1) at 300
C for 1 h and then cooled to 50
C. The analysis was carried out by increasing the catalyst temperature from room temperature to 900
C at a rate of 5
C min1 under H2 (5 vol.%)/Ar with a flow rate of 50 mL min1. The adsorption abilities of the catalysts for the CO and C3H8 gases were measured from the CO- and C3H8-TPD experiments performed using a BELCAT (Bel Japan Inc., Japan). Each catalyst (0.05 g) was placed in the quartz reactor of the TPD apparatus. The catalysts were pretreated at 300
C for 1 h under a He flow (30 mL min1) to remove the physically absorbed water and impurities. CO (5 vol.% CO/He) and C3H8 (25 vol.% C3H8/He) gases were injected into the reactor for 1 h at a rate of 50 mL min1 at 50
C. The physically absorbed CO and C3H8 gases were removed by evacuating the catalyst samples at 50
C for 30 min. The furnace temperature was increased from 50 to 900
C at a rate of 10
C min1 under He flow. The desorbed CO and C3H8 gases were detected using a TCD detector. The activation energy for water desorption in the catalysts was determined by a TGA apparatus equipped with a micro thermodifferential and gravimetric analyzer (Shinco com., Korea). The samples were analyzed after coming into contact with saturated (NH4)2SO4 for 24 h to maintain identical water vapor conditions. Blaine and Kissinger [21] presented a useful equation to calculate the activation energies of various thermal reactions based on the shifts of the maximum deflection temperature of the DTA thermograms upon the changing of the heat rates, as follows. The Kissinger equation was selected and is given as ln(C/Tm2) = Ea/RTm + a constant, where C is the DTA heating rate (
C/min), Tm is the crystallization peak temperature, Ea is the activation

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energy required for crystallization and R is the gas constant (1.987 cal/mol). Propane steam reforming (PSR) reaction on the 30NixFeyO/70Al2O3 catalysts The reactor for PSR in this study was designed as a fixed bedtype [22], and the devices that were employed are shown in Fig. 1: it consists of a feed gas (C3H8) supply, a water supply, a quartz reactor, and analysis equipment (GC). The catalytic activities were measured at 700
C for a reaction time of 10 h at a steam-topropane ratio of 1:6 (mol.%) with a GHSV (gas hourly space velocity) of 6000 h1. The catalyst (0.4 g) was pelletized to a 20–24 mesh and then packed with a small amount of quartz wool to prevent the catalyst from moving in the fixed-bed quartz reactor, which was then mounted vertically inside the furnace. All of the catalysts were reduced in situ under hydrogen (10 mL min1) for 2 h at 700
C before each run. In this study, the amount of steam was adjusted by regulating the temperature according to the partial pressure law [23]. The flow rate was kept constant at 10.0 mL/min for propane gas (25.0 vol.%). Argon gas was used to carry the vaporized mixture into the reactor. The samples were pre-reduced by H2 (5 vol.%)/Ar gases at 700
C for 2 h before the reaction. The reaction products during PSR were measured by an on-line gas chromatograph (Donam DS6200, Donam Company, Korea) equipped with a thermal conductivity detector (TCD) and flame ionizing detector (FID). H2, CO, CO2, CH3CHO, CH3COCH3, and CH3COOH were detected using the TCD, whereas the FID was used to detect CH4, C2H4, C2H6, C3H8, and the other products. Additionally, mass spectroscopy (BelMass, Bel Japan Inc., Japan) was used to confirm the intermediates evolved during PSR. The C3H8 conversion and CH4, CO2, CO, and H2 selectivities were defined as: C3H8 conversion (%) = ([C3H8]in  [C3H8]out)/[C3H8]in  100,

(2)

H2 (CO, CO2, or CH4) selectivity (%) = [H2 (CO, CO2, or CH4)]out/ ([CO2 + CO + CH4 + H2]out  100. (3)

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Results and discussion

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Characteristics of the 30NixFeyO/70Al2O3 catalysts

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Based on the mechanism of the hydrocarbon reforming reaction, it was confirmed that its rate increases in the presence of metallic gradients [24]. Therefore, many studies have examined the reforming reaction after reducing the catalysts. The catalytic reduction temperature is also very important, and the activity of the catalyst is dependent on the reduction temperature. Therefore, to determine the proper pre-reduction temperature for the samples before PSR, the changes corresponding to the reduction of the Ni and Fe components were observed in the H2-TPR profiles of all of the fresh samples, as shown in Fig. 2. Generally, in H2-TPR, the peak area corresponds to the hydrogen uptake and the peak location depends on how easily the catalyst species are reduced [25]. The peak locations for the three 30NixFeyO/70Al2O3 catalysts were almost the same, but the amounts of Ni reduced in them decreased in proportion to the amount of Fe added. The NiO isolated from the 30NiO/70Al2O3 and 30NixFeyO/70Al2O3 samples was considered to be reduced to metallic Ni [26] at approximately 380–550
C, and the peaks were separated into two curves, corresponding to slightly different oxidation states. Another sharper and larger Ni-reduction curve was exhibited at higher temperatures in the range of 700–900
C in both the 30Ni/70Al2O3 and 30NixFeyO/70Al2O3 samples, which corresponds to the reduction of NiO to Ni in the spinel structured NiAl2O4, and the peak location was shifted to a slightly higher temperature in 30NixFeyO/70Al2O3 compared to that in 30NiO/70Al2O3. This appears to be due to the influence of the added iron. Furthermore, there are three types of reduction curves at around 350, 500, and 750
C, respectively, on 30FeO/70Al2O3, which correspond to the reductions of the Fe oxide in its various oxidation states and, in particular, the isolated Fe components were reduced at higher temperature compared to the Ni components. The temperature to reduce the exposed NiO and FeO ingredients were kept at 700
C before PSR. It is expected that some of the oxidized Ni or Fe components might remain after PSR, which would affect their catalytic activity.

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Fig. 1. Equipment configuration of batch bed type reactor for propane steam reforming.

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Fig. 2. H2-TPR profiles of the three fresh samples, 30NiO/70Al2O3, 30FeO/7070Al2O3 and 30Ni0.8Fe0.2O/70Al2O3. 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263

The crystallinities of the synthesized samples before (A) and after (B) hydrogen pre-treatment were examined by XRD, as shown in Fig. 3. In the hydrogen pre-treated samples (Fig. 3A), the main XRD peaks for the 30NiO/70Al2O3 samples were observed at 2u = 37.25
(111), 43.28
(200), 63.88
(220), 75.42
(311), 79.41
(222), and 95.58
(400), which correspond to cubic crystalline NiO [27]. Special peaks for spinal structured NiAl2O4 [JSPDS no. 01-0781601, Fd-3m, Cubic] and FeAl2O4 [JSPDS no. 00-003-0894, Fd-3m, Cubic] were observed in all of the 30NixFeyO/70Al2O3 samples, however, there were no peaks for NiO or FeO, which means that the Ni and Fe components are very well dispersed over the surface of Al2O3, causing them to have a perfect spinel structure. Generally, the crystallite size decreases with increasing line-broadening of the peaks. The crystallite size was estimated using Scherrer’s equation [28], t = 0.9l/bcos u, where l is the wavelength of the incident X-rays, b is the full width at half maximum height in radians, and u is the diffraction angle in radians. The estimated crystallite sizes based on the (111) and (311) planes of the NiO and NiAl2O4 formed in the 30NiO/70Al2O3, 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3 samples were 9.04, 7.29, 9.21, and 17.4 nm, respectively. Consequently, the crystallites sizes decreased with increasing amount of Fe added.

On the other hand, Fig. 3B shows the XRD patterns after the hydrogen pre-treatment. The main XRD peaks for metallic cubic crystalline Ni [JSPDS no. 01-1258, Fm-3m] after reduction in all of the samples were observed at 2u = 44.37
(111), 51.60
(200), 76.08
(220), and 92.09
(311). Small peaks, which were assigned to spinel structured NiAl2O4, were also observed in all of the reduced samples, which means that the Ni and Fe components are well combined with the Al2O3 support ingredients. The estimated crystallite sizes based on the (111) plane of the formed Ni were compared, and the values were 20.12, 23.76, 24.92, and 25.82 nm, respectively, in the 30NiO/70Al2O3, 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70 Al2O3, and 30Ni0.7Fe0.3O/70Al2O3 samples. In particular, the sizes were increased 2.23 fold compared to those in the 30Ni/70Al2O3 sample before H2-reduction. Fig. 4 shows the TEM image and TEM-elemental mapping on the reduced 30Ni0.8Fe0.2O/70Al2O3 as a representative sample. The scale bars of the images are 100 nm. Rectangular-shaped Ni, NiFe alloy, and NiAl2O4 particles, approximately 30, 50, and 100 nm in size, respectively, were observed in the reduced 30Ni0.8Fe0.2O/ 70Al2O3 sample, and there was no significant aggregation between the metal oxides. The particle sizes decreased with increasing amount of Fe added, and there was little aggregation between the

Fig. 3. XRD patterns of 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 before and after pre-reduction.

Please cite this article in press as: K.M. Kim, et al., Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/ Al2O3 catalyst, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.046

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Fig. 4. TEM and elements-mapping images of 30Ni0.8Fe0.2O/70Al2O3 after pre-reduction. 286 287 288 289 290 291 292 293

metal oxides. This suggests that the Fe components existed between the Ni particles, preventing any strong agglomeration between them. The elemental mapping showed that all of the components, Ni, Fe, Al, and O, were evenly distributed and their concentrations were reliable as expected. Fig. 5 displays the survey spectra derived from the quantitative XPS analysis of the Ni2p, Fe2p, Al2p, and O1s peaks for all of the reduced samples, viz. 30NiO/70Al2O3, 30Ni0.9Fe0.1O/70Al2O3,

30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3. The difference between the 2p3/2 and 2p1/2 peaks was 17.49 eV in each case. The 2p3/2 spin-orbital photoelectron of Ni before PSR was located at a binding energy of 853.8–860.0 eV, which was assigned to NiO in NiAl2O4 for all of the samples [29]. On the other hand, the curves were slightly changed to a lower binding energy in the three samples, 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3, corresponding to lower oxidation states of

Fig. 5. Survey spectra from quantitative XPS of the Ni2p, Fe2p, Al2p, and O1s peaks for the 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts after pre-reduction.

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the Ni components, even though the area of the peaks was smaller. From this result, it was deduced that the reduction potentials of Ni2 + ! Ni0 and Fe2+ ! Fe0 were 0.25 and 0.44 eV, respectively, and, thus, that the reduction of Ni is performed more rapidly than that of Fe in the 30NixFeyO/70Al2O3 samples, so the added Fe ingredients should facilitate the reduction of Ni oxide. With increasing concentration of Fe added, the peak intensities according to Fe oxide increased. During PSR, Fe oxides can strongly attract propane molecules, and Ni oxides can act as an oxygen donor to the thermally cracked CH2CH4 or CH4 molecules from propane in the formation of CH3CHO and CO as intermediates. In contrast, the spin-orbital spectra of Fe2p3/2 and Fe2p1/2 for FeO in FeAl2O4 over the 30NixFeyO/70Al2O3 samples revealed broad peaks at 712.0 and 725.6 eV, respectively [30], and the difference between the two orbitals, HOMO and LUMO, was 13.6 eV. The peak locations were not changed, but the intensities were more sharply dependent on the amount of Fe added. The Al2p and O1s spin-orbital photoelectrons were located at binding energies of 76.4 and 533.0 eV, respectively, which were assigned to Al and O in Al2O3 or NiAl2O4 for all of the samples. The curves were shifted to a very slightly lower binding energy in the 30NixFeyO/70Al2O3 samples compared to 30NiO/70Al2O3. Additionally, the atomic compositions for the O, Al, Ni, and Fe components obtained by XPS analysis (%) after reduction are also compared in the right-hand table. The atomic ratios for Ni/Fe were 3.26, 2.63, and 1.59 in 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/ 70Al2O3, respectively, which differ from the initial amounts used in the synthesis step. The amounts of Ni and Fe present on the surface were not affected greatly by the amount inserted during synthesis, which also indicates their high dispersion. In addition, the accurate quantitative analysis of a surface by XPS analysis alone is difficult. However, the atomic amount of Fe increased proportionally in the order of 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3, thus these values seem to be somewhat consistent.

Fig. 6 shows the adsorption–desorption isotherm curves of N2 at 77 K for the reduced samples, 30NiO/70Al2O3, 30Ni0.9Fe0.1O/ 70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3. Using Kelvin’s equation [31], the radius of the pores, in which capillary condensation occurs actively, can be determined as a function of the relative pressure (P/P0). The mean pore diameter, Dp, was calculated from Dp = 4VT/S, where VT is the total volume of the pores and S is the BET surface area. According to the IUPAC classification, all of the isotherms belonged to type IV [32]. The hysteresis slopes were observed at higher relative pressures in all of the samples, indicating the presence of bulk mesopores formed between the particles. The BET specific surface areas were significantly higher in the 30NixFeyO/70Al2O3 samples (94.5– 100.9 m2 g1) than in the 30NiO/70Al2O3 one (56.67 m2 g1). In general, the specific surface areas in regular particles are strongly related to the particle sizes: the lower the particle size, the higher the surface area. These results suggest that the particle sizes were smaller in the 30NixFeyO/70Al2O3 samples due to the addition of Fe. The total pore volumes increased with increasing amount of Fe added, showing the same trend in all of the samples. The pore size distribution (PDS) is an important characteristic for porous materials. Among these methods, the BJH (Barrett–Joyner– Halenda) plot provides a suitable method for the measurement of micro- or meso-pores [33]. Through the BJH plot, the pore size distributions and average pore diameters in the 30NixFeyO/ 70Al2O3 samples were found to be slightly sharper and smaller, depending on the amount of Fe added, than those in 30NiO/ 70Al2O3.

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Gas adsorption abilities on the 30NixFeyO/70Al2O3 catalysts

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The reactivity of the catalyst is strongly associated with its adsorption capacity for the reactant or intermediate gases. In general, a catalyst having a strong adsorption capacity for these gases exhibits superior catalytic activity. Chemisorption is a kind of

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Fig. 6. N2-adsorption/desorption isotherm curves (A) and pore size distributions determined from the BJH (Barrett–Joyner–Halenda) plots (B) of the 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts after pre-reduction.

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adsorption which involves a chemical reaction between the surface and the adsorbent: new chemical bonds are generated at the adsorbent surface. An important aspect of chemisorption is in heterogeneous catalysis, which involves molecules reacting with each other via the formation of chemisorbed intermediates [34]. After the chemisorbed species combine, the product desorbs from the surface. This study attempted the thermal programmed chemisorption for propane gas and steam as the reactant feeds and CO which is generated in the final stage. In the first step, tests of C3H8-chemisorption were performed and the desorption curves for chemisorbed C3H8 are exhibited in Fig. 7A. Additionally, the evolution of the mass spectrometer signals (m/z) recorded during propane desorption with O2 valence carriers over 30NixFeyO/ 70Al2O3 is shown in Fig. 7B. Propane chemisorption experiments were conducted in order to investigate the interaction of the molecular species with the surface oxygen in the catalytic surfaces derived from the 30NixFey/70Al2O3 samples. The C3H8 desorption profiles were characterized over one broad temperature range, 400–600
C, corresponding to C3H8 desorption on the metal species. The temperature locations were almost the same for all of the samples, but the areas of the curves were significantly larger in the case of 30NixFeyO/70Al2O3. In particular, the desorbed area was the largest in the case of 30Ni0.8Fe0.2O/70Al2O3, where it was 2.8 times larger than that for 30Ni/70Al2O3, which means that considerably more C3H8 molecules were adsorbed on the surface of the bimetallic NiFeO surfaces than on the Ni surface. Otherwise, the desorption curves were separated into three curves, and on the basis of mass spectroscopy, assigned to H2, CH4, CH3CHO, CO, and CO2 desorptions [35]. Fig. 7B presents the mass spectra for the desorbed molecules in O2 valence for the three desorption temperatures, 500, 750, and 850
C. Signals at m/z = 2 (H2), 16 (CH4), 28 (CO), 43 (CH3CHO), and 44 (CO2) were exhibited over the 30Ni0.8Fe0.2O/70Al2O3 sample. The mass spectroscopy signals did not vary with the desorption temperature in any of the segmental compounds, except for CO2. These segmental compounds are

7

Fig. 8. CO-TPD profiles of the pre-reduced 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts.

similar to the intermediates and products produced during the reforming reaction for propane. CO-TPD experiments were performed for all of the samples, as shown in Fig. 8. The majority of the adsorbed CO desorbs as CO2 at high temperatures, but some molecular desorption of CO occurs at temperatures as low as 50–250
C and, indeed, CO desorption begins immediately upon the initiation of the temperature ramp [36]. In this study, the CO desorption profiles were characterized over two broad temperature ranges in 30NiO/70Al2O3 centered at 350 and 600
C, corresponding to CO and CO2 desorption on the metal species, respectively. The temperatures decreased and the curve intensities increased significantly in the 30NixFeyO/70Al2O3 samples, which mean that considerably more CO molecules were adsorbed on the surface of the metallic NiFe than on the mono Ni component. Additionally the adsorbed area was the largest in the

Fig. 7. C3H8-TPD profiles (A) and mass spectra (B) on the pre-reduced 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts.

Please cite this article in press as: K.M. Kim, et al., Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/ Al2O3 catalyst, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.046

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Fig. 9. Correlation graphs of ln(C/Tm2) versus Tm deduced by Kissinger equation for the pre-reduced 30NiO/70Al2O3 and 30Ni0.8Fe0.2O/70Al2O3 catalysts. 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443

30Ni0.8Fe0.2O/70Al2O3 sample. In general, a rapid catalytic reaction occurs when many reactants are well-adsorbed over the catalyst. On the other hand, water (H2O) molecules constitute the main gas in hydrocarbon steam reforming. Water molecules are involved in the water gas shift reaction to convert CO to CO2. The H2Odesorption experiments for two representative samples, 30NiO/ 70Al2O3 and 30Ni0.8Fe0.2/70Al2O3, were performed using a TG instrument, and their activation energies were calculated from the slopes, which were obtained from the plots [37] of ln(C/Tm2) versus 1/Tm, as shown in Fig. 9. The activation energy was larger in 30Ni0.8Fe0.2O/70Al2O3 than in 30NiO/70Al2O3: the difference in the activation energies reached 2.78 fold. Additionally, the water desorption rate was higher in the 30Ni0.8Fe0.2O/70Al2O3 sample. This means that the water molecules were absorbed better on the surface of the catalyst having the larger desorption activation energy. It is expected that the steam added to the propane feed gas will adsorb on the surface of the Fe component rather than that of Ni during the propane steam reforming reaction. PSR performances over the 30NixFeyO/70Al2O3 catalysts Fig. 10 shows the propane conversion and hydrogen selectivity during propane steam reforming on the two representative samples of 30NiO/70Al2O3 and 30Ni0.8Fe0.2O/70Al2O3, according to the reaction temperature in the range from 500 to 800
C with an interval of 50
C for 1 h at a GHSV 6000 h1. Depending on the

reaction temperature, both the propane conversion and hydrogen selectivity increased in the two samples, and the propane conversion was 100% above 650
C in both samples. However, the rate of increase of the hydrogen selectivity was greater in the 30Ni0.8Fe0.2O/70Al2O3 catalyst than that in 30NiO/70Al2O3 and the increase was particularly marked at 700
C in both samples. At 700
C, the hydrogen selectivity amounted to 86% in the 30Ni0.8Fe0.2O/70Al2O3 catalyst, whereas it was only 79% in 30NiO/70Al2O3 at the same temperature. Thus, on the basis of this result, the optimum temperature was fixed at 700
C in this study. Fig. 11 exhibits the product distributions and the absolute values for all of the samples evolved during propane steam reforming for 1 h at 700
C for a GHSV of 6000 h1. Here, the propane conversions were close to 100% in all of the samples, however the absolute amounts of hydrogen generated differed depending on the catalyst (Fig. 11A); the absolute value exceeded 80 mL in the 30Ni0.8Fe0.2O/70Al2O3 catalyst, which is more than the value of 20 mL emitted in the 30NiO/70Al2O3 one. Regarding the product distributions during PSR, there are only three molecules in the 30NiO/70Al2O3 catalyst, viz. H2, CH4, and CO, as the main products, whereas more CO2 gas was emitted in the case of the 30NixFeyO/70Al2O3 catalysts. This is evidence that the water gas shift reaction proceeds readily during propane steam reforming over the 30NixFeyO/70Al2O3 catalysts. However, no intermediates, except for acetaldehyde and ethylene molecules, were observed in

Fig. 10. The propane conversion and hydrogen selectivity on 30NiO/70Al2O3 and 30Ni0.8Fe0.2O/70Al2O3 catalysts according to the reaction temperatures.

Please cite this article in press as: K.M. Kim, et al., Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/ Al2O3 catalyst, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.046

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Fig. 11. Product distributions (A) for all of the catalysts, and gas chromatography (B) and mass spectroscopy (C) results for the 30Ni0.8Fe0.2O/70Al2O3 catalyst during PSR.

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the GC signal during PSR at 700
C, possibly because the reaction temperature was too high. Thus, the reaction temperature was reduced to 450
C in an attempt to identify any intermediates. As a result, a small amount of ethylene (C2H4) molecules was seen in the GC signal during PSR at 450
C, as shown in Fig. 11B. In order to observe all of the intermediates evolved during PSR, mass spectroscopy was used and the resultant signals are shown in Fig. 11C: the signals at m/z = 14, 26, 28, and 29 were assigned to CH2, C2H2, C2H4, and CH3CHO, respectively. The mechanism of CH4 steam reforming has been extensively studied [38], however, less research has focused on the propane and higher hydrocarbon steam reforming mechanisms [39]. In the present work, the solid– gas mechanism involves the reactions between hydrocarbons (C3H8, CH4, and C2H4) and/or an intermediate surface hydrocarbon species with lattice oxygen at the NiFeO surface. Based on this result, the mechanism in PSR was expected to be as follows: The main reaction is accompanied by cracking to produce methane and ethylene on the surface of metallic Ni in the catalysts (*) with the possible hydrogenation of the latter. The cracking reaction is C3H8 + * ! C2H4 + CH4. Here, C2H6 produced by the hydrogenation of C2H4 was not detected in this study. The production of CH4 as an intermediate was favored under our conditions and various reactions proceeded on the surface of the catalysts: the C2H4 formed was converted to CH3CHO on the surface of the oxidized metal in the catalyst and then converted to CH4 and CO. After that, the CH4 was converted to H2 and CO. Finally, the steam (H2O) enables the reduced metal species in the catalysts to recover by delivering oxygen with the production of H2 (water gas shift reaction).

498

C2H4 + *Ox ! CH3CHO + *Ox1,

(4)

499

CH3CHO + * ! CH4 + CO*,

(5)

500

CH4 + *Ox ! 2H2 + C*Ox,

(6)

501

C*Ox ! CO + *Ox1,

(7)

502

CO* ! CO + *,

(8)

503

CO + *Ox ! CO2 + *Ox1,

(9)

(10)

504

Fig. 12 exhibits the C3H8 conversion and hydrogen selectivity during propane steam reforming for all of the catalysts, 30NiO/ 70Al2O3, 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3, at a time on stream of 10 h at 700
C for a GHSV of 6000 h1. The propane was perfectly converted to the products with 100% conversion and these perfect conversions were maintained for 10 h in all of the samples. The hydrogen production was the highest over the 30Ni0.8Fe0.2O/70Al2O3 sample, reaching 89% after 10 h, whereas it was 67% in the case of 30NiO/ 70Al2O3. Generally, the presence of CO degrades the active catalyst, due to catalyst poisoning by CO molecules [40]. The CO evolution was the smallest in the 30Ni0.8Fe0.2O/70Al2O3 sample. On the other hand, the hydrogen selectivity decreased significantly in the 30Ni0.7Fe0.3O/70Al2O3 sample. This is possibly because the increase in the amount of Fe led to the acceleration of the Fisher– Tropsch reaction (CO + H2 ! CH4), leading to the reverse methane reforming reaction. Therefore, there is an optimal amount of Fe of 0.2 mol. Based on the product distribution, the introduction of an appropriate amount of Fe into the 30Ni/70Al2O3 catalyst during PSR has a favorable effect on the catalytic performance, due to the enhancement of the water gas shift reaction, resulting in less catalytic deactivation. Therefore, these results highlight the synergic effect of Fe and Ni on the catalytic performance in PSR.

505

Characteristics of the catalysts after PSR

528

The XRD patterns of the used 30NixFeyO/70Al2O3 catalysts were analyzed, in order to determine the structural changes in the metal species of the catalysts after PSR, as shown in Fig. 13. The diffraction lines for all of the used catalysts were similar to those of the fresh ones (the pre-reduced catalysts). Metallic Ni species after PSR were seen in all of the catalysts, and the intensities were the strongest in the 30NiO/70Al2O3 sample, however the peaks were smaller and the (111) planes were shifted to lower angles in the 30NixFeyO/70Al2O3 samples. Using Scherrer’s equation, the crystallite sizes estimated based on the (111) plane of the Ni formed in the 30NiO/70Al2O3, 30Ni0.9Fe0.1O/70Al2O3, 30Ni0.8Fe0.2O/70Al2O3, and 30Ni0.7Fe0.3O/70Al2O3 samples were 33.70, 27.86, 26.14, and

529

H2O + *Ox1 ! H2 + *Ox

Please cite this article in press as: K.M. Kim, et al., Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/ Al2O3 catalyst, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.046

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Fig. 12. Propane conversions and hydrogen productions versus reaction times for the 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts. 541 542 543 544 545 546 547 548 549 550 551 552 553 554

26.21 nm, respectively, which provide proof that the Ni particles after the reaction were strongly aggregated in the 30NiO/70Al2O3 sample. On the other hand, the peaks which were assigned to NiAl2O4 became somewhat bigger in the case of 30NixFeyO/ 70Al2O3, which means that the NiO components remained in the 30NixFeyO/70Al2O3 samples. Fig. 14 shows the survey spectra derived from the quantitative XPS spectra of the Ni2p, Fe2p, Al2p, and O1s peaks for the used catalyst after PSR. The peak locations for the 2p3/2 spin-orbital photoelectron, which was assigned to Ni2+ in all of the catalysts, did not change compared to those in the fresh catalysts, but the intensities were greatly decreased in the 30NiO/70Al2O3 sample. This trend was also observed for the Fe2p3/2 spin-orbitals, and the peak decreased significantly when there were no Fe ions in the

Fig. 13. XRD patterns of the used 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts after PSR.

catalysts. In particular, the peak intensity remains the strongest in the 30Ni0.8Fe0.2O/70Al2O3 sample. Eventually, the reduced Ni was re-oxidized by the Fe oxide during PSR, resulting in the formation of CH3CHO and CO as intermediates. The observation of peak separation in the Al2p spectra in the 30NiO/70Al2O3 sample is quite unusual: the amount of aluminum oxide with reduced states at around 75.0 eV was lower, but a new peak was observed at 78 eV with a more highly oxidized state. This is considered to be due to the conversion of the isolated aluminum from the Ni components to Al(OH)3 during PSR [41]. This trend is the same in the O1s spectra. It likely corresponds to more highly oxidized oxygen; probably the O of the OH in Al(OH)3. To determine the amounts and shapes of the carbon deposited on each catalyst, the amounts of carbon deposited were measured from the C1s-XPS analysis, as shown in Fig. 15. There is a peak at 286 eV which is assigned to carbon deposited on the surface of the catalysts after PSR. It can be assumed to correspond to three main kinds of carbon species, viz. C–O, C–C or C–H, as well as a small amount of CNTs [42,43]. The peak locations were almost the same, except for the 30Ni0.8Fe0.2O/70Al2O3 sample, where they were shifted to a higher binding energy, corresponding to a carbon component in a more highly oxidized state. It is estimated that the carbon deposited on the catalysts was in the form of lumps of carbon, and a very small amount of CNTs was produced. The peak area, which corresponds to the amount of carbon deposited, was the largest in the used 30NiO/70Al2O3 sample and the smallest in the 30Ni0.8Fe0.2O/70Al2O3 sample, indicating that the level of catalytic degradation caused by coke formation was reduced over the latter. This shows that the carbon did not grow over the Fe sites, suggesting that water could adsorb easily over them, indicating that Fe plays a role in changing CO to CO2, promoting the production of hydrogen with less catalytic deterioration. On the basis of the PSR performances and the physicochemical analysis on the catalysts, in particular the mass spectroscopy results, Scheme 1 presents the proposed PSR mechanism over the

Please cite this article in press as: K.M. Kim, et al., Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/ Al2O3 catalyst, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.046

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Fig. 14. Survey spectra from quantitative XPS of the Ni2p, Fe2p, Al2p, and O1s peaks for the 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts after PSR.

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30NixFeyO/70Al2O3 catalysts. During propane reforming, the metallic Ni component plays an important role in the dehydrogenation and thermal cracking resulting from the propane gases. These gases are transformed into C2H4 and CH4 as intermediates and acetaldehyde (CH3CHO) is generated by ethylene oxidation over NiFeO. Then, acetaldehyde is thermally cracked catalytically to produce CH4 and CO. The CH4 and CO obtained participate in other reactions, such as the CH4-stem reforming reaction and the CO–water gas shift reaction over Ni and FeO, respectively. Their gas adsorption abilities were compared through H2O-TPD experiments (Fig. 9). On the basis of the CO- and H2O-TPD experiments, it can be inferred that water molecules are well adsorbed on the Fe components and, thus, the CO-water gas shift reaction takes

place more predominantly over 30NixFeyO/70Al2O3 than over 30NiO/70Al2O3. Finally, the CO molecules obtained by CH4-SR should be converted to CO2 and H2 via a secondary CO-WGS reaction. On the other hand, the amount of CO2 molecules evolved over the 30NiO/70Al2O3 sample was too small and, consequently, CO molecules are deposited as carbon lumps on the 30NiO/ 70Al2O3 catalyst surface, resulting in catalytic deactivation. Contrary to our expectations, however, no deterioration of the catalyst occurred. In this experiment, the addition of Fe oxide contributed significantly to preventing sintering between the inter-Ni particles. Therefore, Fe oxide as a promoter helps produce a high hydrogen yield in the PSR reaction while inducing the emission of CO2.

Fig. 15. Survey spectra from quantitative XPS of the C1s peaks for the 30NiO/70Al2O3 and 30Ni0xFeyO/70Al2O3 catalysts after PSR.

Please cite this article in press as: K.M. Kim, et al., Effective hydrogen production from propane steam reforming over bimetallic co-doped NiFe/ Al2O3 catalyst, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.046

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Scheme 1. The expected model for propane steam reforming over the 30Ni0xFeyO/70Al2O3 catalyst. 616 617

Conclusions

633

The goal of this study was to improve the catalytic stability of the 30NiO/70Al2O3 catalyst through the introduction of Fe as a subcatalytic species. Three types of 30NixFeyO/70Al2O3 catalysts were prepared and there existed an optimum Ni/Fe ratio, which was Ni0.8:Fe0.2. The hydrogen yield in PSR was the highest on the 30Ni0.8Fe0.2O/70Al2O3 catalyst, reaching 83% with 100% propane conversion. In addition, more C3H8 and CO gases adsorbed over the 30Ni0.8Fe0.2O/70Al2O3 catalyst, resulting in the formation of a greater amount of CO2 gas. During PSR, C2H4, CH4, and CH3CHO molecules were identified as intermediates. A trace amount of carbon lumps was deposited over the 30Ni0.8Fe0.2/70Al2O3 catalyst after the PSR reaction. Overall, this study proved that the introduction of Fe along with Ni in PSR has a favorable effect on the stable production of hydrogen gas, with significantly less catalytic deactivation due to catalytic poisoning by CO molecules, since these are transformed to CO2 through the water gas shift reaction.

634

Acknowledgments

635

642

This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2015H1C1A1035639), and by the Basic Research Science and Technology Projects through the National Research Foundation of Korea Grant funded by the Ministry of Science, ICT & Future Planning (No. 2015R1A1A3A04001268), for which the authors are very grateful.

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