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Al2O3 catalysts as hydrogen storage intermediate
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Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate Dexiang Zhang*, Jing Zhao, Yuanyuan Zhang, Xilan Lu Department of Chemical Engineering for Energy Resources, Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
article info
abstract
Article history:
Hydrogen storage as an expanding research topic was investigated to discover and study
Received 18 August 2015
new materials. However, phenanthrene (PHE) for the deeply hydrogenation reaction and
Received in revised form
hydrogen storage capacity can be used as a promising liquid organic hydrogen carrier. The
27 November 2015
hydrogenation of phenanthrene over the NiMo/Al2O3 catalysts was carried out in the 30 mL
Accepted 30 November 2015
tubing-bomb micro-reactor. The catalysts were prepared by coprecipitation and charac-
Available online xxx
terized by X-ray diffraction(XRD), Scanning electronic microscopy coupled with Energy
Keywords:
troscopy(XPS), temperature programmed reduction (TPR) and temperature-programmed
dispersive spectrometer (SEM-EDS), N2 physical adsorption, X-ray photoelectron specHydrogen storage
desorption (TPD) of H2. The results show that the interaction between the NiMo and the
Organic intermediate
support increase with the calcination temperature and that Ni and Mo were present as Ni2þ
Phenanthrene
and Mo4þ on the support. The hydrogenated products were analyzed by Gas
Catalytic hydrogenation
Chromatography-Mass Spectrometer (GC-MS). The results show that the catalyst calci-
NiMo/Al2O3 catalyst
nated at 600 C, exhibits excellent high activity performance and the hydrogenation conversion of phenanthrene over the catalyst can reach above 70% at 400 C. The selectivity of octahydrophenanthrene (OHP), octahydroanthracene (OHA), tetrahydrophenanthrene (THP) and dihydrophenanthrene (DHP) are 9.07%, 34.84%, 18.36% and 32.85%. The reaction pathway of phenanthrene hydrogenation is proposed. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to meet the ever increasing clean energy demand, zero carbon emission fuel such as hydrogen is required [1,2]. Hydrogen is a light element that has low volumetric density (84 g/m3), and high gravimetric energy density of 28.68 kcal/g and is three times more than that of gasoline [3]. Hydrogen
can be used as an alternative energy carrier. An efficient method for hydrogen storage, transportation and delivery to point of usage is a prerequisite for any hydrogen fuel energy system [2]. Among wide variety of hydrogen storage technologies liquid organic hydrides provide several advantages such as relatively higher hydrogen capacity on both the weight and volume basis [1,2]. Moreover, liquid organic hydride carriers are simple, safe and feasible handling for hydrogen storage.
* Corresponding author. Tel.: þ86 21 64252367; fax: þ86 21 64252737. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2015.11.173 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Hydrogen addition via liquid organic hydride carriers is based on the reversible catalytic hydrogenationedehydrogenation reactions and covalently bound to a liquid carrier substance via hydrogenation [4e6]. Hydrogen is released via dehydrogenation in place of energy demand. The hydrogen carrying liquid itself is not consumed but can be reloaded and used in further cycles [7]. Dehydrogenation process substances have been discussed as potential liquid organic hydrides candidates, e.g. methylcyclohexane-toluene [4], cycloalkanes [8], ammonia-borane-based systems [6,9] and heterocyclic aromatic hydrocarbons like N-ethylcarbazole [10], 9ethylcarbazole [11], while hydrogenation of unsaturated organic compounds as liquid organic hydrogen carriers for hydrogen storage is investigated little. Generally, alkenes, alkynes and unsaturated aromatic can be used as hydrogen storage materials. It is reported that benzene, toluene, and naphthalene [12] as the useful model compounds are hydrogenated to upgrade of coal liquids and diesel fuels. Considering the sources and toxicity, phenanthrene is the more aromatic compounds in the coal tar and the lower toxicity than others. However, one of the main difficulties in the hydrogenation is the acquisition of a hydrogenation catalyst that is capable of optimizing the hydrogenation activity, the selectivity toward hydrogenated aromatic hydrocarbons under extreme operating conditions (high pressure, high temperature). Previous research has indicated that Pt based catalysts are reported as the most efficient catalysts for selective hydrogenation of phenanthrene [13]. However, Pt is high cost and less abundance so that researchers use various non-noble monometallic and bimetallic catalysts to reduce the Pt contents [1,14e17]. Many experimentals [18e20] provide many insights into the nature of the active phase on hydrotreating catalysts. These catalysts consist of well-dispersed MoS2 nanocrystallites supported on g-alumina and promoted by cobalt or nickel atoms. In the present work, the catalysts of the alumina (Al2O3) as a support for nickel (Ni) and molybdenum (Mo) have been investigated for the hydrogenation of phenanthrene as hydrogen storage materials. In order to determine various physico-chemical and morphological, the NieMo catalyst was characterized by the Scanning Electronic Microscopy coupled to Energy Dispersive Spectrometer (SEM-EDS), N2-physical adsorption, temperature programmed reduction(TPR), temperature programmed desorption(TPD) and X-ray photoelectron spectroscopy(XPS) analysis. Moreover, we carried out hydrogenation experimentals using model compounds: PHE to elucidate the properties of catalyst and proposed the reaction pathways.
Experimental Catalysts preparation The NiMo/Al2O3 catalysts were synthesized by coprecipitation. Briefly, the sample was obtained from wellmixed aluminum nitrate, nickel nitrate and ammonium molybdate solutions by dropping the solution of ammonium carbonate and keeping the pH about 9. The precipitate was filtered, thoroughly washed with deionized water,
aged for 12 h and then dried overnight at 110 C, and calcined at 500 C for 5 h to prepare a mixed oxide sample. Additionally, the calcination temperature of other catalysts was 600 C, 700 C and 800 C, respectively. The catalysts are denoted as Catx, where x is the calcination temperature of catalysts.
Catalyst characterization The specific surface areas of the support and oxide catalysts were evaluated from the nitrogen adsorptionedesorption isotherms at 196 C in a Micromeritics ASAP 2020 apparatus, after being degassed at 200 C and 1.3 102 Pa for 24 h. The XRD analysis was obtained with a Siemens D5000 diffractometer. The XRD analysis was carried out in the scanning angle (2q) range of 10e80 C. The SEM tests were used the Field Emission Scanning Electron Microscope (S-4800, Hitachi Ltd.) to observe the morphology and surface distribution of Nickel and Molybdenum at 15.0 kV. All samples have been placed on the conductive Tapes and sputtered with gold under vacuum using K550 unit and then observed with back-scattered electrons detectors, equipped with an energy dispersive spectrometer (EDS analysis). H2-TPR experiments were carried out using a AMI300 autoabsorber with a TCD detector. The TPR analysis of the oxidic NiMo/g-Al2O3 catalyst (150 mg) was performed in a quartz reactor. The catalyst sample and reactor were placed in a furnace and heated up. Firstly for the samples pretreatment, the temperature was increased from room temperature to 350 C, at a rate of 10 C/min and held for 1 h. Then, the temperature was again decreased up to 50 C at a rate of 20 C/ min and then held for 30 min.The temperature was increased from 50 C to 900 C, at a rate of 10 C/min and held for 1 h. H2-TPD experiments were carried out using a AMI300 autoabsorber with a TCD detector. About 150 mg of catalyst sample was placed in the center of a quartz tube reactor with quartz wool plugs. Prior to reduction, the operation condition of reduction was similar to those used in the TPR analysis and then the samples were cooled in 30 mL/min Ar flow to a final temperature of 50 C at 10 C/min. The Ar was replaced by H2(10%)/Ar gas mixture with a flow-rate of 30 mL/min and held for 60 min. Then the H2(10%)/Ar was replaced Ar gas with a flow rate of 30 mL/min and the temperature of the reactor was increased linearly to 600 C at 10 C/min and then retained isothermally for 30 min. X-ray photoelectron spectra (XPS) were analyzed by a Thermo Scientific ESCALAB 250Xi spectrometer with a monochromatic Al K Alpha radiation (300 W, 15 kV, 1486.6 eV) and with a multi-channel detector. Samples were transferred to the analysis chamber under inert atmosphere. Spectra of powder samples were recorded in the constant pass energy mode at 29.35 eV, using a 720 mm diameter analysis area. Charge referencing was measured against adventitious carbon (C1s 284.8 eV).
Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Catalytic reaction: effect on hydrogenation depth and the amount of hydrogen storage by using different catalysts The NiMo/Al2O3 catalysts were presulfided in the autoclave. Initially, 10 g catalyst and 100 mL CS2 (5% in the n-heptane) solution was joined in the autoclave and then ventilated with nitrogen leak detection. Then the temperature was increased to 230 C and held isothermally for 4 h at this temperature with a hydrogen initial pressure of 7 MPa and the stirring rate of 200 r/min. The hydroprocessing experiments of phenanthrene (95%, CP) used in the present study without further purification were performed in the 30 mL tubing-bomb micro-reactor. Initially, 0.3 g phenanthrene, 0.025 g catalysts and 2 g ndodecane solution were added into micro oscillation reactor. Experiments were performed under 9 MPa (H2 initial pressure) and contact time 30 min and carried out at 400 C. Identification of the hydrogeneration products of phenanthrene was analyzed by GC/MS using an Agilent 7890A gas chromatograph coupled with Agilent 5975C mass detector equipped with a HP-5 capillary column, a flame ionization detector (FID) and helium as carrier gas with a flow rate of 0.7 mL/min. The detector and injector temperatures were 310 C and the column temperature was started at 60 C for 8 min, then heated to 300 C at a heating rate of 5 C/min and holding 5 min at 300 C. And the mass spectrum condition is as follows: solvent delay, 6 min; electron impact ion source temperature, 230 C; quadruple spectrometer temperature, 150 C; and scan from 50 to 300.
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temperature rising, which is shown by the high intensity and narrower diffraction peak. After the loading of Ni and Mo, no high intensity peak which can be attributed to NiO and MoO3 was observed. From Fig. 2, the surface structure of catalysts presents only a morphological habit and the surface distribution of Nickel and Molybdenum is uniform. Therefore, this indicates active component well disperse on the surface of support. But some small peaks at 2q ¼ 19 , 31 , 57 characteristic of NiAl2O4 (PDF 10-339), are found in the XRD patterns of Cat3 and Cat4. It is suggested that the interaction between the nickel and Al2O3 gradually increases when the calcination temperature is above 600 C.
Specific surface areas and pore size of catalysts The BET surface areas and pore volumes determined by nitrogen physical adsorption and surface active component ratio of Ni/Mo amount of all the catalysts are presented in Table 1 and the N2 adsorption isotherms were shown in Fig. 3. The nitrogen adsorptionedesorption isotherms are of type IV in the IUPAC classification and with a similar shape to that observed in the case of the support. All the pore of catalysts are mesopores, the specific surface areas decrease and mean pore size increases with increasing the calcination temperature, in spite of that pore volume has no obvious change. Meanwhile, the ratio of Ni/Mo amount on the surface of catalysts firstly increases and then decreases through EDS test. The variation can be attributed not only to the presence of large particles that the higher calcination temperature, the more crystallinity, but also to the increase interaction between active component and support.
Results and discussion
Reducibility of catalysts
Catalysts phase analysis and surface structure
TPR is complimentary technique that helps identify the oxidation states of a reducible metal oxide and estimate the interaction of the metal oxide with the support. The H2-TPR profiles of all the samples are presented in Fig. 4. All the samples exhibit one main reduction peak at relatively high temperature and shifts to high temperature region with increasing the calcination temperature. It may be attributed to the reduction of Ni(II) species being in a strong interaction with the alumina support, that is, amorphous Ni aluminate species, which are visible in the XRD patterns (Fig. 1). While, the lower reduction peak at around 370 C, can be assigned to the reduction of Mo species particles with no interaction with the support and correspond to the reduction of Mo (VI) to Mo (IV) of NiMoO4 or MoO3. It demonstrates the difference in the interactions between Ni2þ and the support with varying calcination temperature and the reducibility of the Mo4þ species increased with increasing the calcination temperature. This leads to a shift in the peaks corresponding to the reduction of Ni2þ species interacting with the support from around 700 C to around 780 C.
The X-ray powder diffraction patterns of all the samples are shown in Fig. 1 and the SEM-EDS photographs of the catalysts are given in Fig. 2. The XRD patterns in Fig. 1 show three broad peaks at 37 , 46 and 67 , respectively, which are corresponding to the characteristic value of 2 theta for the g-Al2O3 phase as it closely matched with JCPDS card No. 48-367. Moreover, crystallinity increases with the calcination
Dispersion of the active metal component
Fig. 1 e XRD patterns of catalysts.
The dispersion of the active metal component of the sample is estimated by temperature programmed desorption of H2. The strength of H2 desorption can be determined by the
Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Fig. 2 e The SEM-EDS photographs of Catalysts.
temperature at which the adsorbed H2 desorbs. Based on the desorption temperature, there are mainly two kinds of attractions leading to absorption of hydrogen molecules onto solid surface, physical adsorption and chemical adsorption. All of the samples exhibit that a very small peak at approximately 120 C is attributed to physisorption on the catalyst surface and that the peak at the high temperature region represents chemisorption in the catalyst. However, dispersion values calculated the peak area in all samples are very small. These low dispersion values are largely due to the presence of a low surface concentration of Ni2þ and Mo4þ in the samples, which form the amorphous or poorly crystalline Ni silicate species, and which are visible in the XRD patterns (Fig. 1), SEM-EDS photographs (Fig. 2) and TPR profiles (Fig. 4). From the Fig. 5, Cat1 and Cat4 are similar to the monocomponent NiO and Al2O3 sample, It indicates that active component Mo species are finely dispersed on the support surface. Comparison with Cat3, the active components of Cat2 are well dispersed on the surfaces of the carriers.
Table 1 e The characteristic parameters and surface active component ratio of Ni/Mo amount of NiMo catalysts. Sample
ABET (m2/g)
y (mL/g)
Mean pore d (nm)
Ratio of Ni/Mo amount (wt%)
Cat1 Cat2 Cat3 Cat4
306.85 215.82 245.23 117.78
0.55 0.46 0.57 0.40
6.92 8.06 9.48 14.22
1.80 1.71 2.21 2.15
Fig. 3 e The N2 adsorption isotherms of different catalysts.
X-ray photoelectron spectroscopy The chemical species present on the surface of the sulfided samples were evaluated by XPS. Fig. 6 shows the Ni2p, Mo3d, S2p and C1s XPS spectra of the sulfide catalysts as being representative of this series of samples. In the catalysts, the Ni 2p photoemission line was deconvoluted into two components at ca.853.6 eV and 856.5 eV, which can be ascribed to sulfide nickel and nickel remaining as NiMoO4, respectively. Considering that the B.E. for Ni2þ ions in NiS is close to 852.9 eV [21], it indicates that Ni2þ ions are embedded in the structure of MoS2, probably forming the NieMoeS phase, i.e. after sulfidation at 400 C [22]. In the XPS spectrum, The Mo 3d photoemission line region of XPS spectra centered at 226.9 eV exhibit the typical Mo 3d5/2 photoemission, which is characteristic of MoS2 species. The Mo 3d5/2 value is typical of Mo(VI) [23] and is very close to those observed for MoO3 (232.6 eV) and
Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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NiMoO4 (232.5 eV) due to partially sulfided OeMoeS [24]. The sulfided catalyst presents a lower binding energy peak at 161.9 eV in the S 2p energy region, which is characteristic of sulfide (S2) species.
Catalytic hydrogenation of phenanthrene for hydrogen storage intermediate
Fig. 4 e H2-TPR profiles of catalysts.
Fig. 5 e H2-TPD profiles of catalysts.
Recently, hydrogenation of aromatic molecules in general has received a lot of attention for the production of high performance diesel fuels [25]. In this study, phenanthrene as the liquid organic hydride is chosen to hydrogenate carrier because it is one of the main components of polycyclic aromatic compounds in low temperature coal tar that the output has reached thousands of tons every day and their nature is very aromatic that results in low-quality fuel products. However, there is no in-depth study concerning the mechanism of catalytic hydrogenation of phenanthrene for hydrogen storage purpose. The catalytic hydrogenation (HYD) of PHE over the NiMo/ Al2O3 sulfide catalysts at 400 C and 9 MPa (hydrogen initial pressure) was investigated and the results are presented in Fig. 7. During the HYD reaction, several products (see Table 2) were detected and confirmed by the combination of GCeMS. DHP, THP and OHP are hydrogenated and main hydrogenation products. DP and 6-BTN are the products of the hydrocracking pathway, while the contents are less. Besides products from the HYD and hydrocracking pathways, three other products, PHA, OHA and THA, were detected as well. These three products might be due to subsequent isomerization (ISO) reactions of the HDA products. Fig. 7 shows that at 400 C, the
Fig. 6 e XPS of Ni 2p, Mo 3d, S 2p and C1s for the Cat2. Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Fig. 7 e Catalytic results for HDA of phenanthrene over NiMo catalysts.
Table 2 e Compound and its abbreviation. Compound
hydrogenation reaction pathways of phenanthrene under the experimental condition were proposed in the Fig. 8. In the process, the main products consist of OHP, OHA, THP and DHP, and their selectivity rate are 9.07%, 34.84%, 18.36% and 32.85%, respectively. Moreover, the effect on the reaction temperature is remarkable that OHA and OHP are easier to dehydrogenation than THP at high reaction temperature. It indicates that the double ring compounds are prior mono-ring compounds to dehydrogenation reaction. Hydrogenation reactions are divided into three types: hydrogenation, isomerism and hydrocracking. During in the process, one hydrogen molecule is added to the unsaturated double CeC bond and finally makes it saturated. Hydrogenation of phenanthrene potentially results into 7 mol of H2 per each mole of phenanthrene. This results in higher hydrogen production contents of hydrogenated phenanthrene for hydrogen storage, and bimetallic NiMo/Al2O3 is non-noble metal based potential candidate catalyst for hydrogenation of liquid organic intermediate and worth in the industrial application.
Abbreviation
Phenanthrene 9,10-dihydrophenanthrene 1,2,3,4-tetrahydrophenanthrene 1,2,3,4,4a,5,6,10b-octahydrophenanthrene(cis) 1,2,3,4,4a,5,6,10b-octahydrophenanthrene(trans) 1,2,3,4-tetrahydroanthracene 1,2,3,4,5,6,7,8-octahydroanthracene Perhydroanthracene(cis) Perhydroanthracene(trans)
PHE DHP THP Cis-OHP Trans-OHP THA OHA Cis-PHA Trans-PHA
PHE conversion rate decreased in the order: Cat2 > Cat3 > Cat4 > Cat1, and that the Cat2 (calcination temperature 600 C) exhibited higher HDA activity than the other catalysts. It is important to identify the structures of the reaction products which would lead to a better understanding of the mechanism of this reversible reaction. Based on the generation and concentration variation of the intermediate products,
Conclusions The main goal of this study was to prepare the NiMo catalysts and evaluate their hydrogenation activity of phenanthrene for hydrogen storage. The NiMo catalysts are prepared by coprecipitation. The characteristics of the catalysts show that the interaction between active component and support increases with the calcination temperature, especially the formation of NiAl2O4 by NiO and Al2O3 can affect the surface active component ratio of Ni/Mo amount and decrease the catalytic activity when the calcination temperature exceeds 700 C. Moreover, Ni and Mo are presented as Ni2þ and Mo4þ on the support. The Cat2 at calcination temperature of 600 C exhibits excellent catalytic activity for the hydrogenation of phenanthrene. The conversion yield of phenanthrene exceeds 70% based on Cat2 and is higher than that of other catalysts. Moreover, the selectivity of OHP, OHA, THP and DHP are 9.07%, 34.84%, 18.36% and 32.85% in the hydrotreating process with
Fig. 8 e Reaction pathways for PHE hydrogenation. Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Cat2. Further, the reaction pathway of phenanthrene hydrogenation was speculated and the double ring hydrogenated phenanthrene is easier to dehydrogenation than the mono ring compounds. These results provide a new perspective on the preparation of highly active metal oxide catalysts supported on alumina and phenanthrene might be a potential hydrogen storage material.
[11]
[12]
[13]
Acknowledgments [14]
This work was supported by the China 973 Program (2011CB201304). [15]
references
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Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
Available online at www.sciencedirect.com
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Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate Dexiang Zhang*, Jing Zhao, Yuanyuan Zhang, Xilan Lu Department of Chemical Engineering for Energy Resources, Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
article info
abstract
Article history:
Hydrogen storage as an expanding research topic was investigated to discover and study
Received 18 August 2015
new materials. However, phenanthrene (PHE) for the deeply hydrogenation reaction and
Received in revised form
hydrogen storage capacity can be used as a promising liquid organic hydrogen carrier. The
27 November 2015
hydrogenation of phenanthrene over the NiMo/Al2O3 catalysts was carried out in the 30 mL
Accepted 30 November 2015
tubing-bomb micro-reactor. The catalysts were prepared by coprecipitation and charac-
Available online xxx
terized by X-ray diffraction(XRD), Scanning electronic microscopy coupled with Energy
Keywords:
troscopy(XPS), temperature programmed reduction (TPR) and temperature-programmed
dispersive spectrometer (SEM-EDS), N2 physical adsorption, X-ray photoelectron specHydrogen storage
desorption (TPD) of H2. The results show that the interaction between the NiMo and the
Organic intermediate
support increase with the calcination temperature and that Ni and Mo were present as Ni2þ
Phenanthrene
and Mo4þ on the support. The hydrogenated products were analyzed by Gas
Catalytic hydrogenation
Chromatography-Mass Spectrometer (GC-MS). The results show that the catalyst calci-
NiMo/Al2O3 catalyst
nated at 600 C, exhibits excellent high activity performance and the hydrogenation conversion of phenanthrene over the catalyst can reach above 70% at 400 C. The selectivity of octahydrophenanthrene (OHP), octahydroanthracene (OHA), tetrahydrophenanthrene (THP) and dihydrophenanthrene (DHP) are 9.07%, 34.84%, 18.36% and 32.85%. The reaction pathway of phenanthrene hydrogenation is proposed. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to meet the ever increasing clean energy demand, zero carbon emission fuel such as hydrogen is required [1,2]. Hydrogen is a light element that has low volumetric density (84 g/m3), and high gravimetric energy density of 28.68 kcal/g and is three times more than that of gasoline [3]. Hydrogen
can be used as an alternative energy carrier. An efficient method for hydrogen storage, transportation and delivery to point of usage is a prerequisite for any hydrogen fuel energy system [2]. Among wide variety of hydrogen storage technologies liquid organic hydrides provide several advantages such as relatively higher hydrogen capacity on both the weight and volume basis [1,2]. Moreover, liquid organic hydride carriers are simple, safe and feasible handling for hydrogen storage.
* Corresponding author. Tel.: þ86 21 64252367; fax: þ86 21 64252737. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2015.11.173 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Hydrogen addition via liquid organic hydride carriers is based on the reversible catalytic hydrogenationedehydrogenation reactions and covalently bound to a liquid carrier substance via hydrogenation [4e6]. Hydrogen is released via dehydrogenation in place of energy demand. The hydrogen carrying liquid itself is not consumed but can be reloaded and used in further cycles [7]. Dehydrogenation process substances have been discussed as potential liquid organic hydrides candidates, e.g. methylcyclohexane-toluene [4], cycloalkanes [8], ammonia-borane-based systems [6,9] and heterocyclic aromatic hydrocarbons like N-ethylcarbazole [10], 9ethylcarbazole [11], while hydrogenation of unsaturated organic compounds as liquid organic hydrogen carriers for hydrogen storage is investigated little. Generally, alkenes, alkynes and unsaturated aromatic can be used as hydrogen storage materials. It is reported that benzene, toluene, and naphthalene [12] as the useful model compounds are hydrogenated to upgrade of coal liquids and diesel fuels. Considering the sources and toxicity, phenanthrene is the more aromatic compounds in the coal tar and the lower toxicity than others. However, one of the main difficulties in the hydrogenation is the acquisition of a hydrogenation catalyst that is capable of optimizing the hydrogenation activity, the selectivity toward hydrogenated aromatic hydrocarbons under extreme operating conditions (high pressure, high temperature). Previous research has indicated that Pt based catalysts are reported as the most efficient catalysts for selective hydrogenation of phenanthrene [13]. However, Pt is high cost and less abundance so that researchers use various non-noble monometallic and bimetallic catalysts to reduce the Pt contents [1,14e17]. Many experimentals [18e20] provide many insights into the nature of the active phase on hydrotreating catalysts. These catalysts consist of well-dispersed MoS2 nanocrystallites supported on g-alumina and promoted by cobalt or nickel atoms. In the present work, the catalysts of the alumina (Al2O3) as a support for nickel (Ni) and molybdenum (Mo) have been investigated for the hydrogenation of phenanthrene as hydrogen storage materials. In order to determine various physico-chemical and morphological, the NieMo catalyst was characterized by the Scanning Electronic Microscopy coupled to Energy Dispersive Spectrometer (SEM-EDS), N2-physical adsorption, temperature programmed reduction(TPR), temperature programmed desorption(TPD) and X-ray photoelectron spectroscopy(XPS) analysis. Moreover, we carried out hydrogenation experimentals using model compounds: PHE to elucidate the properties of catalyst and proposed the reaction pathways.
Experimental Catalysts preparation The NiMo/Al2O3 catalysts were synthesized by coprecipitation. Briefly, the sample was obtained from wellmixed aluminum nitrate, nickel nitrate and ammonium molybdate solutions by dropping the solution of ammonium carbonate and keeping the pH about 9. The precipitate was filtered, thoroughly washed with deionized water,
aged for 12 h and then dried overnight at 110 C, and calcined at 500 C for 5 h to prepare a mixed oxide sample. Additionally, the calcination temperature of other catalysts was 600 C, 700 C and 800 C, respectively. The catalysts are denoted as Catx, where x is the calcination temperature of catalysts.
Catalyst characterization The specific surface areas of the support and oxide catalysts were evaluated from the nitrogen adsorptionedesorption isotherms at 196 C in a Micromeritics ASAP 2020 apparatus, after being degassed at 200 C and 1.3 102 Pa for 24 h. The XRD analysis was obtained with a Siemens D5000 diffractometer. The XRD analysis was carried out in the scanning angle (2q) range of 10e80 C. The SEM tests were used the Field Emission Scanning Electron Microscope (S-4800, Hitachi Ltd.) to observe the morphology and surface distribution of Nickel and Molybdenum at 15.0 kV. All samples have been placed on the conductive Tapes and sputtered with gold under vacuum using K550 unit and then observed with back-scattered electrons detectors, equipped with an energy dispersive spectrometer (EDS analysis). H2-TPR experiments were carried out using a AMI300 autoabsorber with a TCD detector. The TPR analysis of the oxidic NiMo/g-Al2O3 catalyst (150 mg) was performed in a quartz reactor. The catalyst sample and reactor were placed in a furnace and heated up. Firstly for the samples pretreatment, the temperature was increased from room temperature to 350 C, at a rate of 10 C/min and held for 1 h. Then, the temperature was again decreased up to 50 C at a rate of 20 C/ min and then held for 30 min.The temperature was increased from 50 C to 900 C, at a rate of 10 C/min and held for 1 h. H2-TPD experiments were carried out using a AMI300 autoabsorber with a TCD detector. About 150 mg of catalyst sample was placed in the center of a quartz tube reactor with quartz wool plugs. Prior to reduction, the operation condition of reduction was similar to those used in the TPR analysis and then the samples were cooled in 30 mL/min Ar flow to a final temperature of 50 C at 10 C/min. The Ar was replaced by H2(10%)/Ar gas mixture with a flow-rate of 30 mL/min and held for 60 min. Then the H2(10%)/Ar was replaced Ar gas with a flow rate of 30 mL/min and the temperature of the reactor was increased linearly to 600 C at 10 C/min and then retained isothermally for 30 min. X-ray photoelectron spectra (XPS) were analyzed by a Thermo Scientific ESCALAB 250Xi spectrometer with a monochromatic Al K Alpha radiation (300 W, 15 kV, 1486.6 eV) and with a multi-channel detector. Samples were transferred to the analysis chamber under inert atmosphere. Spectra of powder samples were recorded in the constant pass energy mode at 29.35 eV, using a 720 mm diameter analysis area. Charge referencing was measured against adventitious carbon (C1s 284.8 eV).
Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Catalytic reaction: effect on hydrogenation depth and the amount of hydrogen storage by using different catalysts The NiMo/Al2O3 catalysts were presulfided in the autoclave. Initially, 10 g catalyst and 100 mL CS2 (5% in the n-heptane) solution was joined in the autoclave and then ventilated with nitrogen leak detection. Then the temperature was increased to 230 C and held isothermally for 4 h at this temperature with a hydrogen initial pressure of 7 MPa and the stirring rate of 200 r/min. The hydroprocessing experiments of phenanthrene (95%, CP) used in the present study without further purification were performed in the 30 mL tubing-bomb micro-reactor. Initially, 0.3 g phenanthrene, 0.025 g catalysts and 2 g ndodecane solution were added into micro oscillation reactor. Experiments were performed under 9 MPa (H2 initial pressure) and contact time 30 min and carried out at 400 C. Identification of the hydrogeneration products of phenanthrene was analyzed by GC/MS using an Agilent 7890A gas chromatograph coupled with Agilent 5975C mass detector equipped with a HP-5 capillary column, a flame ionization detector (FID) and helium as carrier gas with a flow rate of 0.7 mL/min. The detector and injector temperatures were 310 C and the column temperature was started at 60 C for 8 min, then heated to 300 C at a heating rate of 5 C/min and holding 5 min at 300 C. And the mass spectrum condition is as follows: solvent delay, 6 min; electron impact ion source temperature, 230 C; quadruple spectrometer temperature, 150 C; and scan from 50 to 300.
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temperature rising, which is shown by the high intensity and narrower diffraction peak. After the loading of Ni and Mo, no high intensity peak which can be attributed to NiO and MoO3 was observed. From Fig. 2, the surface structure of catalysts presents only a morphological habit and the surface distribution of Nickel and Molybdenum is uniform. Therefore, this indicates active component well disperse on the surface of support. But some small peaks at 2q ¼ 19 , 31 , 57 characteristic of NiAl2O4 (PDF 10-339), are found in the XRD patterns of Cat3 and Cat4. It is suggested that the interaction between the nickel and Al2O3 gradually increases when the calcination temperature is above 600 C.
Specific surface areas and pore size of catalysts The BET surface areas and pore volumes determined by nitrogen physical adsorption and surface active component ratio of Ni/Mo amount of all the catalysts are presented in Table 1 and the N2 adsorption isotherms were shown in Fig. 3. The nitrogen adsorptionedesorption isotherms are of type IV in the IUPAC classification and with a similar shape to that observed in the case of the support. All the pore of catalysts are mesopores, the specific surface areas decrease and mean pore size increases with increasing the calcination temperature, in spite of that pore volume has no obvious change. Meanwhile, the ratio of Ni/Mo amount on the surface of catalysts firstly increases and then decreases through EDS test. The variation can be attributed not only to the presence of large particles that the higher calcination temperature, the more crystallinity, but also to the increase interaction between active component and support.
Results and discussion
Reducibility of catalysts
Catalysts phase analysis and surface structure
TPR is complimentary technique that helps identify the oxidation states of a reducible metal oxide and estimate the interaction of the metal oxide with the support. The H2-TPR profiles of all the samples are presented in Fig. 4. All the samples exhibit one main reduction peak at relatively high temperature and shifts to high temperature region with increasing the calcination temperature. It may be attributed to the reduction of Ni(II) species being in a strong interaction with the alumina support, that is, amorphous Ni aluminate species, which are visible in the XRD patterns (Fig. 1). While, the lower reduction peak at around 370 C, can be assigned to the reduction of Mo species particles with no interaction with the support and correspond to the reduction of Mo (VI) to Mo (IV) of NiMoO4 or MoO3. It demonstrates the difference in the interactions between Ni2þ and the support with varying calcination temperature and the reducibility of the Mo4þ species increased with increasing the calcination temperature. This leads to a shift in the peaks corresponding to the reduction of Ni2þ species interacting with the support from around 700 C to around 780 C.
The X-ray powder diffraction patterns of all the samples are shown in Fig. 1 and the SEM-EDS photographs of the catalysts are given in Fig. 2. The XRD patterns in Fig. 1 show three broad peaks at 37 , 46 and 67 , respectively, which are corresponding to the characteristic value of 2 theta for the g-Al2O3 phase as it closely matched with JCPDS card No. 48-367. Moreover, crystallinity increases with the calcination
Dispersion of the active metal component
Fig. 1 e XRD patterns of catalysts.
The dispersion of the active metal component of the sample is estimated by temperature programmed desorption of H2. The strength of H2 desorption can be determined by the
Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Fig. 2 e The SEM-EDS photographs of Catalysts.
temperature at which the adsorbed H2 desorbs. Based on the desorption temperature, there are mainly two kinds of attractions leading to absorption of hydrogen molecules onto solid surface, physical adsorption and chemical adsorption. All of the samples exhibit that a very small peak at approximately 120 C is attributed to physisorption on the catalyst surface and that the peak at the high temperature region represents chemisorption in the catalyst. However, dispersion values calculated the peak area in all samples are very small. These low dispersion values are largely due to the presence of a low surface concentration of Ni2þ and Mo4þ in the samples, which form the amorphous or poorly crystalline Ni silicate species, and which are visible in the XRD patterns (Fig. 1), SEM-EDS photographs (Fig. 2) and TPR profiles (Fig. 4). From the Fig. 5, Cat1 and Cat4 are similar to the monocomponent NiO and Al2O3 sample, It indicates that active component Mo species are finely dispersed on the support surface. Comparison with Cat3, the active components of Cat2 are well dispersed on the surfaces of the carriers.
Table 1 e The characteristic parameters and surface active component ratio of Ni/Mo amount of NiMo catalysts. Sample
ABET (m2/g)
y (mL/g)
Mean pore d (nm)
Ratio of Ni/Mo amount (wt%)
Cat1 Cat2 Cat3 Cat4
306.85 215.82 245.23 117.78
0.55 0.46 0.57 0.40
6.92 8.06 9.48 14.22
1.80 1.71 2.21 2.15
Fig. 3 e The N2 adsorption isotherms of different catalysts.
X-ray photoelectron spectroscopy The chemical species present on the surface of the sulfided samples were evaluated by XPS. Fig. 6 shows the Ni2p, Mo3d, S2p and C1s XPS spectra of the sulfide catalysts as being representative of this series of samples. In the catalysts, the Ni 2p photoemission line was deconvoluted into two components at ca.853.6 eV and 856.5 eV, which can be ascribed to sulfide nickel and nickel remaining as NiMoO4, respectively. Considering that the B.E. for Ni2þ ions in NiS is close to 852.9 eV [21], it indicates that Ni2þ ions are embedded in the structure of MoS2, probably forming the NieMoeS phase, i.e. after sulfidation at 400 C [22]. In the XPS spectrum, The Mo 3d photoemission line region of XPS spectra centered at 226.9 eV exhibit the typical Mo 3d5/2 photoemission, which is characteristic of MoS2 species. The Mo 3d5/2 value is typical of Mo(VI) [23] and is very close to those observed for MoO3 (232.6 eV) and
Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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NiMoO4 (232.5 eV) due to partially sulfided OeMoeS [24]. The sulfided catalyst presents a lower binding energy peak at 161.9 eV in the S 2p energy region, which is characteristic of sulfide (S2) species.
Catalytic hydrogenation of phenanthrene for hydrogen storage intermediate
Fig. 4 e H2-TPR profiles of catalysts.
Fig. 5 e H2-TPD profiles of catalysts.
Recently, hydrogenation of aromatic molecules in general has received a lot of attention for the production of high performance diesel fuels [25]. In this study, phenanthrene as the liquid organic hydride is chosen to hydrogenate carrier because it is one of the main components of polycyclic aromatic compounds in low temperature coal tar that the output has reached thousands of tons every day and their nature is very aromatic that results in low-quality fuel products. However, there is no in-depth study concerning the mechanism of catalytic hydrogenation of phenanthrene for hydrogen storage purpose. The catalytic hydrogenation (HYD) of PHE over the NiMo/ Al2O3 sulfide catalysts at 400 C and 9 MPa (hydrogen initial pressure) was investigated and the results are presented in Fig. 7. During the HYD reaction, several products (see Table 2) were detected and confirmed by the combination of GCeMS. DHP, THP and OHP are hydrogenated and main hydrogenation products. DP and 6-BTN are the products of the hydrocracking pathway, while the contents are less. Besides products from the HYD and hydrocracking pathways, three other products, PHA, OHA and THA, were detected as well. These three products might be due to subsequent isomerization (ISO) reactions of the HDA products. Fig. 7 shows that at 400 C, the
Fig. 6 e XPS of Ni 2p, Mo 3d, S 2p and C1s for the Cat2. Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Fig. 7 e Catalytic results for HDA of phenanthrene over NiMo catalysts.
Table 2 e Compound and its abbreviation. Compound
hydrogenation reaction pathways of phenanthrene under the experimental condition were proposed in the Fig. 8. In the process, the main products consist of OHP, OHA, THP and DHP, and their selectivity rate are 9.07%, 34.84%, 18.36% and 32.85%, respectively. Moreover, the effect on the reaction temperature is remarkable that OHA and OHP are easier to dehydrogenation than THP at high reaction temperature. It indicates that the double ring compounds are prior mono-ring compounds to dehydrogenation reaction. Hydrogenation reactions are divided into three types: hydrogenation, isomerism and hydrocracking. During in the process, one hydrogen molecule is added to the unsaturated double CeC bond and finally makes it saturated. Hydrogenation of phenanthrene potentially results into 7 mol of H2 per each mole of phenanthrene. This results in higher hydrogen production contents of hydrogenated phenanthrene for hydrogen storage, and bimetallic NiMo/Al2O3 is non-noble metal based potential candidate catalyst for hydrogenation of liquid organic intermediate and worth in the industrial application.
Abbreviation
Phenanthrene 9,10-dihydrophenanthrene 1,2,3,4-tetrahydrophenanthrene 1,2,3,4,4a,5,6,10b-octahydrophenanthrene(cis) 1,2,3,4,4a,5,6,10b-octahydrophenanthrene(trans) 1,2,3,4-tetrahydroanthracene 1,2,3,4,5,6,7,8-octahydroanthracene Perhydroanthracene(cis) Perhydroanthracene(trans)
PHE DHP THP Cis-OHP Trans-OHP THA OHA Cis-PHA Trans-PHA
PHE conversion rate decreased in the order: Cat2 > Cat3 > Cat4 > Cat1, and that the Cat2 (calcination temperature 600 C) exhibited higher HDA activity than the other catalysts. It is important to identify the structures of the reaction products which would lead to a better understanding of the mechanism of this reversible reaction. Based on the generation and concentration variation of the intermediate products,
Conclusions The main goal of this study was to prepare the NiMo catalysts and evaluate their hydrogenation activity of phenanthrene for hydrogen storage. The NiMo catalysts are prepared by coprecipitation. The characteristics of the catalysts show that the interaction between active component and support increases with the calcination temperature, especially the formation of NiAl2O4 by NiO and Al2O3 can affect the surface active component ratio of Ni/Mo amount and decrease the catalytic activity when the calcination temperature exceeds 700 C. Moreover, Ni and Mo are presented as Ni2þ and Mo4þ on the support. The Cat2 at calcination temperature of 600 C exhibits excellent catalytic activity for the hydrogenation of phenanthrene. The conversion yield of phenanthrene exceeds 70% based on Cat2 and is higher than that of other catalysts. Moreover, the selectivity of OHP, OHA, THP and DHP are 9.07%, 34.84%, 18.36% and 32.85% in the hydrotreating process with
Fig. 8 e Reaction pathways for PHE hydrogenation. Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173
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Cat2. Further, the reaction pathway of phenanthrene hydrogenation was speculated and the double ring hydrogenated phenanthrene is easier to dehydrogenation than the mono ring compounds. These results provide a new perspective on the preparation of highly active metal oxide catalysts supported on alumina and phenanthrene might be a potential hydrogen storage material.
[11]
[12]
[13]
Acknowledgments [14]
This work was supported by the China 973 Program (2011CB201304). [15]
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Please cite this article in press as: Zhang D, et al., Catalytic hydrogenation of phenanthrene over NiMo/Al2O3 catalysts as hydrogen storage intermediate, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.11.173