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Sodium borohydride hydrolysis by using ceramic foam supported bimetallic and trimetallic catalysts
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Sodium borohydride hydrolysis by using ceramic foam supported bimetallic and trimetallic catalysts Esra Balkanli*, Halit Eren Figen Yildiz Technical University, Chemical Engineering Department, Davutpasa Campus, Topkapi 34210, Istanbul, Turkey
article info
abstract
Article history:
In this study, bimetallic and trimetallic catalysts with different contents on ceramic foam
Received 24 September 2018
support were prepared in an integrated continuous system and performances of catalysts
Received in revised form
were investigated using a fixed bed reactor. Bimetallic copper-cobalt, lithium-cobalt, and
30 November 2018
platinum and palladium added bimetallic (trimetallic) catalysts were prepared and char-
Accepted 4 December 2018
acterized by SEM for morphological structure analysis, BET for surface area measurements,
Available online 24 December 2018
and XRD and XPS for crystal structure analysis. In the hydrogen production tests carried out at different flow rates and temperatures, Pd included trimetallic catalysts performed
Keywords:
slightly better than Pt added bimetallic catalysts. Although Pd catalysts have low activity
Hydrogen generation
than Pt catalysts according to literature, Pd catalyst prepared on ceramic foam had higher
Sodium borohydride
activity. In this work, PdeLiCo and PdeCuCo catalysts demonstrated highest hydrogen
Hydrolysis
production rates (respectively 4.76 ml/min and 4.69 ml/min) as well as highest specific
Catalyst
surface area (7.301 m2/g for PdeLiCo, 11.821 m2/g for PdCuCo).
Ceramic foam
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction An increase of 2.1% in global energy demand compared to 2017 record has increased the use of fossil resources, along with the decline in energy efficiency, increasing global carbon dioxide emissions and the environmental impact [1]. The main challenges are to find the alternative and clean energy to replace the most used energy source in the world, to reverse the impact of the greenhouse effect and to decarbonize or minimize the carbon fuel usage [2]. Hydrogen is the noncarbon based energy system that can overcome these disadvantages [3]. Today, technologies and innovations offer the opportunity to produce clean hydrogen without pollutants and emissions. Hydrogen in water and hydrocarbons in nature can be obtained by several methods using renewable or non-renewable sources. Although hydrogen is the most
promising energy carrier, its storage properties are still challenging and need improvement [2,4e6]. There are several approaches for hydrogen storage and transportation including those used today [2,7,8]. Among them, hydrides are also a promising technique for onboard hydrogen storage, in terms of mild hydrolysis properties [9]. Boron-containing hydrides have also importance in liquid forms [8,10], and sodium borohydride (NaBH4) has many advantages among chemical hydrides as hydrogen carriers [11]. First studied by Schlesinger in 1952, this material has become prominent regarding its ease of use, non-combustibility, long shelf life of the solutions, and optimum control of hydrolysis via catalytic processes [12]. By thermolysis and hydrolysis reactions, stored hydrogen in the NaBH4 can be released [13]. The hydrolysis reaction between NaBH4 and water is exothermic and highly important since half of the hydrogen
* Corresponding author. E-mail address: [email protected] (E. Balkanli). https://doi.org/10.1016/j.ijhydene.2018.12.010 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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comes from water, and it increases the gravimetric hydrogen amount from 7.4% to 10.8 (w) %. To achieve 10.8% hydrogen output, NaBH4 requires 2 moles of water in the reaction. However, during the reaction, due to hydration of sodium metaborate (NaBO2) to sodium tetra hydroxyborate (NaB(OH)4), the solubility of NaBH4 in solution decreases and hydrogen storage capacity is reduced under 7.3% [11,13e15]. Spontaneously occurring self-hydrolysis reactions can be stabilized with pH change to achieve higher hydrolysis efficiency with homogenous and heterogeneous catalysts [12,13,16,17]. Despite the high hydrogen yield, homogeneous catalytic reactions are difficult to control and to recycle of the catalyst and separation of the by-product is challenging [16]. Sodium borohydride solutions which have been stabilized by alkalizing (mostly NaOH) can be activated by heterogeneous metallic catalysis to observe the hydrogen generation [11]. The main advantage of the alkali-stabilized reaction is that it can create an on-off system which is controlled by heterogeneous catalysts [12,13]. Many catalysts have been studied over the years since Schlesinger's experiments (1953) [17] with heterogeneous catalysts. Throughout research, cobalt [17e20] and nickel [19,20] catalysts were mostly studied because of their activities in hydrolysis were better than other non-noble metals such as iron, copper, manganese etc. [17,18,21]. Later, behavior of noble metal catalysts in hydrolysis was investigated. Platinum [18,22] and ruthenium [12,18,20] were observed to be most active catalysts in the noble metals [16,18,23]. Transition metal and noble metal combinations (such as NiCo [24], CoCu [25], CoMo [26], RuCo [27], PtRu [27], etc.) were investigated alongside with/without supported materials to approach the highest hydrogen yield and activity. The surface areas can be increased by using supports such as carbon nanotubes [28], silisium oxide [29], lithium cobalt oxide [29], alumina, titanium oxide [29], resins [12] and copper sheets [30]. Although blocking by sodium metaborate faster than the powder forms, supported form catalysts prevent dragging in the reactor [13,14]. Although majority of the studies were carried out in batch reactors, the behavior of sodium borohydride in continuous systems was also investigated. The first continuous system was run by the Millennium Cell with temperature-integrated reactors [31,32], involving operating the car with a continuous hydrogen production system proving the storage capacity of sodium borohydride as hydrogen fuel [31]. Subsequently, continuous systems were integrated with different types of reactor configurations, and small-scale power generation systems were established [33e37]. While working on a continuous system, adaptation of the reactor and scaling-up features for the catalysts come into prominence. Such features can be obtained by using foam-like supported catalysts for achieving high activities, for example, nickel foams [38] and bimetallic foams [39]. In this research, we investigated SiC foam due to its feasible features, physical durability and large geometric surface area [40]. Also, ceramic foams facilitate hydrogen output due to wide pores and large fluid contact area. Eugenio et al. (2016) [39] found that copper materials were less sensitive towards boron-induced deactivation, copper was chosen as a bimetallic catalyst besides highly efficient LiCo precursor catalyst [41]. As results that shown below, CuCo bimetalic catalyst performs better than
LiCo catalysts in the continuous hydrogen generation system. Also, trimetallic Pd and Pt include catalysts’ that were coated on LiCo and CuCo bimetallic catalysts, hydrogen performances were investigated. Contrary to the low hydrogen generation profile of palladium catalyst compared to that of platinum catalyst in the literature [18], the present study found significant improvement for palladium catalyst in terms of its trimetallic forms with CuCo and LiCo.
Experimental study Materials Cobalt nitrate hexahydrate (CoN2O6,99%) from ABCR GmbH&Co.KG, lithium nitrate anhydrous (LiNO3,99%) and dihydrogen hexachloride platinate (IV) hexahydrate (H2PtCl6$6H2O, 99,9%) from Alfa Aesar, copper (II) nitrate (Cu(NO3)2$3H2O) from Carlo Erba and palladium (II) nitrate hydrate (Pd(NO3)2.xH2O) from Sigma Aldrich were used for the synthesis of catalyst. Ceramic foam material which is used as catalyst support has been provided by Yamer Industrial Products Company. Gray-black colored silicon carbide-based ceramic foam has pore density in the range of 8e60 ppi and 80e90% porosity. Before catalyst preparation, support material has been calcinated at 1273 K for 5 h and then shaped cylindrically with the dimension of 7 20 mm for the reactor setup. Prepared support materials were wash coated in the integrated system for 30 min at room conditions with 1:1 M ratio of LiNO3 and Co(NO3)2 or Cu(NO3)2$3H2O and Co(NO3)2 solutions. After that, wash coated supports were dried at 293 K temperature and were calcinated at 973 K for 2 h. The amount of catalyst loaded was determined according to Eq. (1a), and the process was repeated until the loading percentage was within the range of 8e10. At the end of the loading, LiCo or CuCo doped catalysts were prepared. For trimetallic catalyst preparation, the same process was applied with 0.05 M H2PtCl6$6H2O or 0.05 M Pd(NO3)2.xH2O solutions for one cycle on prepared LiCo and CuCo doped catalysts. catalyst% ¼
mcatalyst msupport 100 msupport
(1a)
Activations of the loaded catalysts were carried out by NaBH4 reduction. Reduction of each loaded catalyst was performed separately in the batch system with 0.075 M NaBH4 solution.
Characterization Characterization of the catalysts were performed with wellknown instrumental techniques: X-Ray Diffractometry (XRD, Philips Panalytical X'Pert-Pro diffractometer, CuKa radiation) for crystal structure identification; Phase identification were made by reference to International Center for Crystal Database (ICDD) 2018; X-Ray Photoelectron Spectrometry (XPS, Thermo K-Alpha X-Ray Photoelectron Spectrometer), calibration of binding energies were made with C 1s peak at 284.5 eV as a reference, Aluminum anode (Al Ka ¼ 1468.3 eV) for electron structure determination; Brunauer-Emmet-Teller
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Fig. 1 e Integrated continuous system for performance tests.
(BET, Quantachrome, Autosorb Instrument, under N2 adsorptive gas and He carrier gas at 77 K after outgassing at 0.6 Pa and 473 K) for specific surface area calculation; Inductively Coupled Plasma- Mass Spectrometry (ICP-MS, Agilent 7700) for quantification of metal content. Before the ICP-MS readings, the sample was then treated in a microwave digester; Scanning Electron Microscopy (SEM, Zeiss Evo LS 10) for surface morphology and microstructure.
separation section and a simple liquid-gas exchange system for gas measurement. NaBH4/NaOH solution with 1:1 were fed to the packed reactor from under with various flow (1, 1.5, 2 ml/min) rates by peristaltic pump. Evolved gas was monitored after separation from liquid in the separation zone. Separated reactant and byproduct were collected. Reactions were carried out at room temperature, 40 C and 60 C.
Hydrogen production system development
Results and discussion The performance tests of the catalysts were carried out on an integrated continuous system (Fig. 1). The system consists of a peristaltic pump (Cole Parmer MasterFlex L/S 7518-40, ±0.1), silicon piping, a packed reactor, a custom designed liquid-gas
In the integrated continues system, six different bimetallic and trimetallic catalysts were prepared by wash coating method. Characteristic results of XRD, XPS, BET and elemental
Table 1 e Characterization results of catalysts. Catalyst Silicon Carbide (Support Material) LieCo PdeLieCo
PteLieCo
CueCo PdeCueCo
PteCueCo
Crystal Phases
Phase Structure
Metal Content (Weight, %)
Specific Surface Area (m2/g ±5%)
SiC Li2O Co3O4 PdO Li2O Co3O4 PtO2 Li2O Co3O4 CuO Co3O4 PdO CuO Co3O4 PtO2 CuO Co3O4
Hexagonal Cubic Cubic Tetragonal Cubic Cubic Tetragonal Cubic Cubic Monoclinic Cubic Tetragonal Monoclinic Cubic Tetragonal Monoclinic Cubic
0.19e6.24 (Li e Co) 0.05e0.11 e 4.62 (Pd e Li e Co)
4.041 5.172 7.301
0.07e 0.14 e 7.84 (Pd e Li e Co)
5.026
5.95e6.49 (Cu e Co) 0.06e2.95 e 3.18 (Pd e Cu e Co)
4.316
0.05e0.11 e 4.62 (Pt e Cu e Co)
11.821
5.615
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Table 2 e XPS spectra of the prepared catalysts. Catalysts
Peak
Binding Energy
Assignment
LieCo PdeLieCo
Co2p3 Co2p3 Pd3d5 Co2p3 Pt4d5 Co2p3 Cu2p3 Co2p3 Cu2p3 Pd3d5 Co2p3 Cu2p3 Pt4d5
779.85 779.85 336.6 779.8 317.38 779.85 933.47 779.77 933.47 336.97 779.79 933.5 316.79
Co₃O₄ Co₃O₄ PdO Co₃O₄ PtO2 Co₃O₄ CuO Co₃O₄ CuO PdO Co₃O₄ CuO PtO2
PteLieCo CueCo PdeCueCo
PteCueCo
Fig. 2 e XRD Patterns of catalysts; a. SiC Support, b. LiCo, c. PdeLiCo, d. PteLiCo, e. CuCo, f. PdeCuCo, g. PteCuCo.
analyses of all catalysts are presented in Table 1. The specific surface area of the support material was found as 4.041 m2/g and an increase in BET content was observed following each catalyst loading. According to the given data, PdeCuCo had the highest surface area among the catalysts with an area of 11.821 m2/g which indicates that adding palladium to catalysts increases the surface area.
Crystal phases and structures of catalysts are shown in Table 1. Fig. 2 shows that desired phases for catalyst are formed. SiC based support catalysts were formed with given structures respectively; Li2O and Co3O4 are cubic, PdO and PtO2 are tetragonal, and CuO is monoclinic in structure. Due to the support material, it is difficult to determine the oxide conditions in XRD peaks. Therefore XPS analyses were performed to clarify the chemical composition and oxide forms of catalysts which results were shown in Fig. 3. The presence of lithium element can be difficult and misleading to clarify in XPS spectra due to low binding energy with XPS hence the lithium graph is not given [42].
Fig. 3 e XPS spectra of catalysts, a. Co2p, b. Cu2p, c. Pd3d and d. Pt4d.
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Fig. 4 e SEM micrographs of catalysts; a. SiC Support, b. CueCo, c. PteCueCo, d. PdeCueCo, e. LieCo, f. PteLieCo, g. PdeLieCo.
As can be seen from Table 2, the average binding energy of each cobalt containing catalyst is 779.7 eV which corresponds to Co3O2 oxide structure [43,44]. Moreover, prepared catalysts’ average binding energy can be matched respectively; with Cu2p3 spectrum, 993.5 eV binding energy representing the Cu(II)O [45]; with Pd3d5 spectrum, 336.7 eV binding energy representing the
PdO [46] and with Pt4d5, 317 ± 3 eV binding energy representing the PtO2 oxide form [47]. Hence, analysis results show that the applied catalyst preparation method for catalyst loading on selected support material was successfully carried out. Surface morphological results of prepared catalysts and blank support material are shown in Fig. 4.
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production trends for catalysts with LiCo, PteLiCo, PdeLiCo, CuCo, PteCuCo, and PdeCuCo can be seen in Fig. 5. NaBH4(aq)þ2H2O(l)/NaBO2(aq)þ4H2(g) (10,8 (w.)%, 293e296 K)
Fig. 5 e Hydrogen Generation Results at room Temperature.
The support material was rough and had a porous structure. Also, it has been observed that all loaded catalyst accumulated in common regions on the surface area.
Performance Testing of Prepared Catalysts Hydrolysis reaction of Sodium Borohydride was investigated in a continuous flow system (Equation (1b)) The hydrogen
(1b)
When the system is operated in a continuous flow, it discharges the active regions of the catalysts together with the flow, which allows them to interact more with the fed reactants, as the by-product of the reaction (sodium metaborate) is removed from the reactor with the flow. Thus, the same conversion rate occurs at unit time intervals (the same amount of hydrogen). As a nature of a continuous flow system, catalytic activity does not change. As a result, each catalyst keeps its linear trend and produces hydrogen at a steady state. As shown in Fig. 5, noble metals perform better than nonnoble metals. However, as there was not much difference in performance catalysts, hydrogen output was constantly observed in similar trends. In general, constant hydrogen output is observed for each catalyst in the continuous system. A comparison between lithium and copper shows that the CuCo catalyst is more active than LiCo catalyst. Otherwise, CuCo had the same effect in similar experiments with 2:1 and
Fig. 6 e Hydrogen generation rate at different temperature for a. PdeLiCo, b. PdeCuCo.
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Fig. 7 e Hydrogen evaluation for PdeLiCo catalyst for various flow rate a. at 20 C, b. at 40 C, c. at 60 C.
0.5:1 NaBH4/NaOH ratios. As the residence time in the reactor depends on the flow rate (5 min for 0.5 ml/min, 2.5 min for 1 ml/min and 1.25 min for 2 ml/min), 480 ml hydrogen output was observed at a flow rate of 2 ml/min and 2:1 NaBH4/NaOH
ratio, although 700 ml hydrogen output was observed with CuCo catalyst at the same conditions. The activity and the regeneration state of the used catalysts were observed through repeated runs. Also, the catalyst regeneration was
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Fig. 8 e Hydrogen evaluation for PdeCuCo catalyst for various flow rate a. at 20 C, b. at 40 C, c. at 60 C.
carried out by washing the catalysts with hot water before proceeding to another cycle. Although LiCo exhibited high performance in the first experiment cycle, it showed a mean reduction of 14.7% after regeneration and reintroduction into
the system. However, such a sharp decrease was not observed in CuCo catalyst at the same period. This proves that copper is more resistant to deactivation that may occur with metaborate [39].
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On the other hand, hydrogen production rate for CuCo catalyst has increased 22% with 0.05%Pt addition and 17% with 0.06% Pd addition. For the catalyst LiCo; hydrogen production rate has considerably increased to 53% with 0.07% Pt addition and to 59% with 0.05% Pd addition onto the catalyst surface. It was found that the efficiency values of the prepared catalyst at room temperature followed the order of PdeLiCo (4.76 ml/min); Pde CuCo (4.69 ml/min); PteCuCo (4.61 ml/min); PteLiCo (4.49 ml/min); CuCo (3.82 ml/min) and LiCo (3.01 ml/ min). Since the conversions with, Pd added catalysts indicated slightly better activity than Pt added catalysts at room temperatures. Pd added catalysts were investigated in detail at different conditions in terms of flow rates and temperatures. For PdeLiCo and PdeCuCo catalysts, a comparison of catalytic performance at different temperatures and at 1 ml/min flow rate is shown in Fig. 6. According to this figure, these catalysts have the similar tendency in hydrogen generation rate with rising temperature. For PdeCuCo catalyst, the reaction was performed at three different temperatures: at 20 C, 40 C, and 60 C resulted in 25% increase in hydrogen generation rate (at 40 C compared to 20 C) and 5% (at 60 C compared to 40 C). For PdeLiCo catalyst corresponding increase rates were 19% and 8%. Fig. 7 and Fig. 8 represent the hydrogen generation rate (mmol/s) and total volume of hydrogen generation for Pde LiCo and PdeCuCo catalysts at different conditions. As Figs. 7a and 8a, hydrogen generation rates for each catalyst are almost similar at different flow rates at 20 C. For PdeCuCo catalyst, hydrogen generation rate increase nearly 2.5 times when the temperature is raised from 20 C to 60 C. For PdeLiCo catalyst, this rate increases nearly three times at the same condition (Fig. 7b, c, 8b, 8c). On the other hand, according to hydrogen generation versus time graph in Figs. 7 and 8, PdeCuCo catalyst's total conversion is higher than that of PdeLiCo at 40 C and at 1, 1.5 and 2 ml/min flow rates. Total conversions are 86%,73%, 65% for PdeLiCo and 89%, 80%, 76% for PdeCuCo at the same temperature (40 C) and flow rates. The catalysts' total hydrogen conversion rates tended towards each other at all flow rates as the temperature increased from 40 C to 60 C and, PdeCuCo catalyst was slightly more efficient than Pde LiCo.
Conclusion In the present work, LiCo, PteLiCo, PdeLiCo, CuCo, PteCuCo and PdeCuCo catalysts on ceramic foam support prepared by integrated wash coating system and catalysts performances of hydrogen production in terms of noble metal loading were carried on with the purpose of sodium borohydride hydrolysis in a continuous reactor system. The results obtained in this study are as follows; Although SiC support material is quite tough and not easy to shape, it provides a superior support for catalysts, due to its porous structure and durability. It has not been damaged in the continuous flow reactor and during
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calcination steps. The surface area of 4.041 m2/g has been increased as catalyst loading. Due to the crystalline structure of copper (monoclinic), CuCo catalyst rendered hydrogen production at a higher yield than LiCo catalyst [39]. Although LieCo catalyst has the least hydrogen production rate, its Pd added version (PdeLiCo) has the highest conversion rate. Only 0.05% addition of Pd increased the catalytic performance to 59%. Efficiency values of the prepared catalysts at room temperature followed the order of PdeLiCo (4.76 ml/min); Pde CuCo (4.69 ml/min); PteCuCo (4.61 ml/min); PteLiCo (4.49 ml/min); CuCo (3.82 ml/min) and LiCo (3.01 ml/min). Although Pd catalysts have low activity for NaBH4 hydrolysis, the trimetallic oxide forms (PdeCueCo,PdeLieCo) have slightly better catalytic activity than Pt included trimetallic catalysts (PteLieCo, PteCueCo). Hydrogen production rate increased as temperature increased. When the reaction temperature for catalyst Pde CuCo was increased from 20 C to 40 C and from 40 C to 60 C, hydrogen production rates increased by 25% and 5%, respectively. For PdeLiCo catalyst these values are 19% and 8%. While no significant difference was observed between the hydrogen production rates of catalysts PdeCuCo and Pde LiCo when flow rates changed at 20 C, a remarkable difference occurred when the reaction temperature increased from 20 C to 60 C for 2 ml/min flow rate, thus a 2.5 times increase was observed at the hydrogen production rates of catalysts PdeLiCo and PdeCuCo. For other flow rates this increase was 1.3 times for 1 ml/min and approximately 2 times for 1.5 ml/min.
As a final conclusion, Pd included CuCo and LiCo catalysts can be recommended for use in a continuous system to provide maximum hydrogen output with the optimum condition (at 40 C 1.5 ml/min or at 60 C 2 ml/min).
Acknowledgement This work has been financially supported by Research Fund of the Yildiz Technical University (YTU, Project No: 2013-07-01GEP03) and authors are gratefully acknowledged for Yamer Industrial Products Company, Izmir-Turkey, for their helpful contribution to supporting the ceramic foam material.
references
[1] IEA IEA. Global energy & CO2 status report 2017. 2018. [2] Ball M, Weeda M. The hydrogen economy - vision or reality? Int J Hydrogen Energy 2015;40(25):7903e19. [3] Dutta S. A review on production, storage of hydrogen and its utilization as an energy resource. J Ind Eng Chem 2014;20(4):1148e56. [4] Dincer I, Acar C. Review and evaluation of hydrogen production methods for better sustainability. Int J Hydrogen Energy 2015;40(34):11094e111.
9968
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 9 9 5 9 e9 9 6 9
[5] Momirlan M, Veziroglu TN. Current status of hydrogen energy. Renew Sustain Energy Rev 2002;6(1e2):141e79. [6] Sherif SA, Barbir F, Veziroglu TN. Principles of hydrogen energy production, storage and utilization. J Sci Ind Res India 2003;62(1e2):46e63. [7] Fakioglu E, Yurum Y, Veziroglu TN. A review of hydrogen storage systems based on boron and its compounds. Int J Hydrogen Energy 2004;29(13):1371e6. [8] Eberle U, Felderhoff M, Schuth F. Chemical and physical solutions for hydrogen storage. Angew Chem Int Ed 2009;48(36):6608e30. [9] Moussa G, Moury R, Demirci UB, Sener T, Miele P. Boronbased hydrides for chemical hydrogen storage. Int J Energy Res 2013;37(8):825e42. [10] Marrero-Alfonso EY, Beaird AM, Davis TA, Matthews MA. Hydrogen generation from chemical hydrides. Ind Eng Chem Res 2009;48(8):3703e12. [11] Mohring RM, Wu Y. Hydrogen generation via sodium borohydride. Hydrogen in Materials and Vacuum Systems 2003;671:90e100. [12] Amendola SC, Sharp-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, Binder M. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25(10):969e75. [13] Brack P, Dann SE, Wijayantha KGU. Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH4) solutions. Energy Sci Eng 2015;3(3):174e88. [14] Demirci UB. The hydrogen cycle with the hydrolysis of sodium borohydride: a statistical approach for highlighting the scientific/technical issues to prioritize in the field. Int J Hydrogen Energy 2015;40(6):2673e91. [15] Demirci UB, Miele P. Chemical hydrogen storage: 'material' gravimetric capacity versus 'system' gravimetric capacity. Energy Environ Sci 2011;4(9):3334e41. [16] Demirci UB, Akdim O, Andrieux J, Hannauer J, Chamoun R, Miele P. Sodium borohydride hydrolysis as hydrogen generator: issues, state of the art and applicability upstream from a fuel cell. Fuel Cell 2010;10(3):335e50. [17] Schlesinger HI, Brown HC, Finholt AE, Gilbreath JR, Hoekstra HR, Hyde EK. Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen. J Am Chem Soc 1953;75(1):215e9. [18] Herbert CAB, Brown C. New, highly active metal catalysts for the hydrolysis of borohydride. J Am Chem Soc 1962;84:1493e4. [19] Liu BH, Li ZP, Suda S. Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. J Alloy Comp 2006;415(1e2):288e93. [20] Walter JC, Zurawski A, Montgomery D, Thornburg M, Revankar S. Sodium borohydride hydrolysis kinetics comparison for nickel, cobalt, and ruthenium boride catalysts. J Power Sources 2008;179(1):335e9. [21] Suda S, Sun YM, Liu BH, Zhou Y, Morimitsu S, Arai K, Tsukamoto N, Uchida M, Candra Y, Li ZP. Catalytic generation of hydrogen by applying fluorinated-metal hydrides as catalysts. Appl Phys a-Mater 2001;72(2):209e12. [22] Bai Y, Wu CA, Wu F, Yi BL. Carbon-supported platinum catalysts for on-site hydrogen generation from NaBH4 solution. Mater Lett 2006;60(17e18):2236e9. [23] Netskina OV, Kochubey DI, Prosvirin IP, Kellerman DG, Simagina VI, Komova OV. Role of the electronic state of rhodium in sodium borohydride hydrolysis. J Mol Catal Chem 2014;390:125e32. [24] Ingersoll JC, Mani N, Thenmozhiyal JC, Muthaiah A. Catalytic hydrolysis of sodium borohydride by a novel nickel-cobaltboride catalyst. J Power Sources 2007;173(1):450e7.
[25] Ding XL, Yuan XX, Jia C, Ma ZF. Hydrogen generation from catalytic hydrolysis of sodium borohydride solution using Cobalt-Copper-Boride (Co-Cu-B) catalysts. Int J Hydrogen Energy 2010;35(20):11077e84. [26] Zhuang DW, Kang Q, Muir SS, Yao XD, Dai HB, Ma GL, Wang P. Evaluation of a cobalt-molybdenum-boron catalyst for hydrogen generation of alkaline sodium borohydride solution-aluminum powder system. J Power Sources 2013;224:304e11. [27] Demirci UB, Garin F. Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate. J Alloy Comp 2008;463(1e2):107e11. [28] Pena-Alonso R, Sicurelli A, Callone E, Carturan G, Raj R. A picoscale catalyst for hydrogen generation from NaBH4 for fuel cells. J Power Sources 2007;165(1):315e23. [29] Kojima Y, Suzuki K, Fukumoto K, Sasaki M, Yamamoto T, Kawai Y, Hayashi H. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int J Hydrogen Energy 2002;27(10):1029e34. [30] Cho KW, Kwon HS. Effects of electrodeposited Co and Co-P catalysts on the hydrogen generation properties from hydrolysis of alkaline sodium borohydride solution. Catal Today 2007;120(3e4):298e304. [31] Mohring RM, Luzader RE. A sodium borohydride on-board hydrogen generator for powering fuel Cell and internal combustion engine vehicles. California: Future Transportation Technology Conference; 2001. [32] Zhang Q, Smith G, Wu Y, Mohring R. Catalytic hydrolysis of sodium borohydride in an auto-thermal fixed-bed reactor. Int J Hydrogen Energy 2006;31(7):961e5. [33] Kim T. NaBH4 (sodium borohydride) hydrogen generator with a volume-exchange fuel tank for small unmanned aerial vehicles powered by a PEM (proton exchange membrane) fuel cell. Energy 2014;69:721e7. [34] Zhang JS, Zheng Y, Gore JP, Fisher TS. 1 kW(e) sodium borohydride hydrogen generation system Part I: experimental study. J Power Sources 2007;165(2):844e53. [35] Sousa T, Rangel CM. A dynamic two phase flow model for a pilot scale sodium borohydride hydrogen generation reactor. Int J Hydrogen Energy 2014;39(10):5291e300. [36] Lim MH, Chung J. The development and evaluation of a flow-type reactor utilizing the thermal siphon effect to generate hydrogen. Int J Hydrogen Energy 2014;39(25):13079e86. [37] Wang FC, Fang WH. The development of a PEMFC hybrid power electric vehicle with automatic sodium borohydride hydrogen generation. Int J Hydrogen Energy 2017;42(15):10376e89. [38] Dai HB, Liang Y, Wang P, Yao XD, Rufford T, Lu M, Cheng HM. High-performance cobalt-tungsten-boron catalyst supported on Ni foam for hydrogen generation from alkaline sodium borohydride solution. Int J Hydrogen Energy 2008;33(16):4405e12. [39] Eugenio S, Demirci UB, Silva TM, Carmezim MJ, Montemor MF. Copper-cobalt foams as active and stable catalysts for hydrogen release by hydrolysis of sodium borohydride. Int J Hydrogen Energy 2016;41(20):8438e48. [40] Twigg MV, Richardson JT. Fundamentals and applications of structured ceramic foam catalysts. Ind Eng Chem Res 2007;46(12):4166e77. [41] Kojima Y, Suzuki K, Fukumoto K, Kawai Y, Kimbara M, Nakanishi H, Matsumoto S. Development of 10 kW-scale hydrogen generator using chemical hydride. J Power Sources 2004;125(1):22e6. [42] ThermoScientificXPS, https://xpssimplified.com/elements/ lithium.php, (July 10, 2018). [43] ThermoScientificXPS, https://xpssimplified.com/elements/ cobalt.php, (July 10, 2018).
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[44] Du ZJ, Xiong DH, Verma SK, Liu BS, Zhao XJ, Liu LF, Li H. A low temperature hydrothermal synthesis of delafossite CuCoO2 as an efficient electrocatalyst for the oxygen evolution reaction in alkaline solutions. Inorg Chem Front 2018;5(1):183e8. [45] ThermoScientificXPS, https://xpssimplified.com/elements/ copper.php, (July 10, 2018).
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[46] ThermoScientificXPS, https://xpssimplified.com/elements/ palladium.php, (July 10, 2018). [47] Shyu JZ, Otto K. Identification of platinum phases on gamma-alumina by xps. Appl Surf Sci 1988;32(1e2):246e52.
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Sodium borohydride hydrolysis by using ceramic foam supported bimetallic and trimetallic catalysts Esra Balkanli*, Halit Eren Figen Yildiz Technical University, Chemical Engineering Department, Davutpasa Campus, Topkapi 34210, Istanbul, Turkey
article info
abstract
Article history:
In this study, bimetallic and trimetallic catalysts with different contents on ceramic foam
Received 24 September 2018
support were prepared in an integrated continuous system and performances of catalysts
Received in revised form
were investigated using a fixed bed reactor. Bimetallic copper-cobalt, lithium-cobalt, and
30 November 2018
platinum and palladium added bimetallic (trimetallic) catalysts were prepared and char-
Accepted 4 December 2018
acterized by SEM for morphological structure analysis, BET for surface area measurements,
Available online 24 December 2018
and XRD and XPS for crystal structure analysis. In the hydrogen production tests carried out at different flow rates and temperatures, Pd included trimetallic catalysts performed
Keywords:
slightly better than Pt added bimetallic catalysts. Although Pd catalysts have low activity
Hydrogen generation
than Pt catalysts according to literature, Pd catalyst prepared on ceramic foam had higher
Sodium borohydride
activity. In this work, PdeLiCo and PdeCuCo catalysts demonstrated highest hydrogen
Hydrolysis
production rates (respectively 4.76 ml/min and 4.69 ml/min) as well as highest specific
Catalyst
surface area (7.301 m2/g for PdeLiCo, 11.821 m2/g for PdCuCo).
Ceramic foam
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction An increase of 2.1% in global energy demand compared to 2017 record has increased the use of fossil resources, along with the decline in energy efficiency, increasing global carbon dioxide emissions and the environmental impact [1]. The main challenges are to find the alternative and clean energy to replace the most used energy source in the world, to reverse the impact of the greenhouse effect and to decarbonize or minimize the carbon fuel usage [2]. Hydrogen is the noncarbon based energy system that can overcome these disadvantages [3]. Today, technologies and innovations offer the opportunity to produce clean hydrogen without pollutants and emissions. Hydrogen in water and hydrocarbons in nature can be obtained by several methods using renewable or non-renewable sources. Although hydrogen is the most
promising energy carrier, its storage properties are still challenging and need improvement [2,4e6]. There are several approaches for hydrogen storage and transportation including those used today [2,7,8]. Among them, hydrides are also a promising technique for onboard hydrogen storage, in terms of mild hydrolysis properties [9]. Boron-containing hydrides have also importance in liquid forms [8,10], and sodium borohydride (NaBH4) has many advantages among chemical hydrides as hydrogen carriers [11]. First studied by Schlesinger in 1952, this material has become prominent regarding its ease of use, non-combustibility, long shelf life of the solutions, and optimum control of hydrolysis via catalytic processes [12]. By thermolysis and hydrolysis reactions, stored hydrogen in the NaBH4 can be released [13]. The hydrolysis reaction between NaBH4 and water is exothermic and highly important since half of the hydrogen
* Corresponding author. E-mail address: [email protected] (E. Balkanli). https://doi.org/10.1016/j.ijhydene.2018.12.010 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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comes from water, and it increases the gravimetric hydrogen amount from 7.4% to 10.8 (w) %. To achieve 10.8% hydrogen output, NaBH4 requires 2 moles of water in the reaction. However, during the reaction, due to hydration of sodium metaborate (NaBO2) to sodium tetra hydroxyborate (NaB(OH)4), the solubility of NaBH4 in solution decreases and hydrogen storage capacity is reduced under 7.3% [11,13e15]. Spontaneously occurring self-hydrolysis reactions can be stabilized with pH change to achieve higher hydrolysis efficiency with homogenous and heterogeneous catalysts [12,13,16,17]. Despite the high hydrogen yield, homogeneous catalytic reactions are difficult to control and to recycle of the catalyst and separation of the by-product is challenging [16]. Sodium borohydride solutions which have been stabilized by alkalizing (mostly NaOH) can be activated by heterogeneous metallic catalysis to observe the hydrogen generation [11]. The main advantage of the alkali-stabilized reaction is that it can create an on-off system which is controlled by heterogeneous catalysts [12,13]. Many catalysts have been studied over the years since Schlesinger's experiments (1953) [17] with heterogeneous catalysts. Throughout research, cobalt [17e20] and nickel [19,20] catalysts were mostly studied because of their activities in hydrolysis were better than other non-noble metals such as iron, copper, manganese etc. [17,18,21]. Later, behavior of noble metal catalysts in hydrolysis was investigated. Platinum [18,22] and ruthenium [12,18,20] were observed to be most active catalysts in the noble metals [16,18,23]. Transition metal and noble metal combinations (such as NiCo [24], CoCu [25], CoMo [26], RuCo [27], PtRu [27], etc.) were investigated alongside with/without supported materials to approach the highest hydrogen yield and activity. The surface areas can be increased by using supports such as carbon nanotubes [28], silisium oxide [29], lithium cobalt oxide [29], alumina, titanium oxide [29], resins [12] and copper sheets [30]. Although blocking by sodium metaborate faster than the powder forms, supported form catalysts prevent dragging in the reactor [13,14]. Although majority of the studies were carried out in batch reactors, the behavior of sodium borohydride in continuous systems was also investigated. The first continuous system was run by the Millennium Cell with temperature-integrated reactors [31,32], involving operating the car with a continuous hydrogen production system proving the storage capacity of sodium borohydride as hydrogen fuel [31]. Subsequently, continuous systems were integrated with different types of reactor configurations, and small-scale power generation systems were established [33e37]. While working on a continuous system, adaptation of the reactor and scaling-up features for the catalysts come into prominence. Such features can be obtained by using foam-like supported catalysts for achieving high activities, for example, nickel foams [38] and bimetallic foams [39]. In this research, we investigated SiC foam due to its feasible features, physical durability and large geometric surface area [40]. Also, ceramic foams facilitate hydrogen output due to wide pores and large fluid contact area. Eugenio et al. (2016) [39] found that copper materials were less sensitive towards boron-induced deactivation, copper was chosen as a bimetallic catalyst besides highly efficient LiCo precursor catalyst [41]. As results that shown below, CuCo bimetalic catalyst performs better than
LiCo catalysts in the continuous hydrogen generation system. Also, trimetallic Pd and Pt include catalysts’ that were coated on LiCo and CuCo bimetallic catalysts, hydrogen performances were investigated. Contrary to the low hydrogen generation profile of palladium catalyst compared to that of platinum catalyst in the literature [18], the present study found significant improvement for palladium catalyst in terms of its trimetallic forms with CuCo and LiCo.
Experimental study Materials Cobalt nitrate hexahydrate (CoN2O6,99%) from ABCR GmbH&Co.KG, lithium nitrate anhydrous (LiNO3,99%) and dihydrogen hexachloride platinate (IV) hexahydrate (H2PtCl6$6H2O, 99,9%) from Alfa Aesar, copper (II) nitrate (Cu(NO3)2$3H2O) from Carlo Erba and palladium (II) nitrate hydrate (Pd(NO3)2.xH2O) from Sigma Aldrich were used for the synthesis of catalyst. Ceramic foam material which is used as catalyst support has been provided by Yamer Industrial Products Company. Gray-black colored silicon carbide-based ceramic foam has pore density in the range of 8e60 ppi and 80e90% porosity. Before catalyst preparation, support material has been calcinated at 1273 K for 5 h and then shaped cylindrically with the dimension of 7 20 mm for the reactor setup. Prepared support materials were wash coated in the integrated system for 30 min at room conditions with 1:1 M ratio of LiNO3 and Co(NO3)2 or Cu(NO3)2$3H2O and Co(NO3)2 solutions. After that, wash coated supports were dried at 293 K temperature and were calcinated at 973 K for 2 h. The amount of catalyst loaded was determined according to Eq. (1a), and the process was repeated until the loading percentage was within the range of 8e10. At the end of the loading, LiCo or CuCo doped catalysts were prepared. For trimetallic catalyst preparation, the same process was applied with 0.05 M H2PtCl6$6H2O or 0.05 M Pd(NO3)2.xH2O solutions for one cycle on prepared LiCo and CuCo doped catalysts. catalyst% ¼
mcatalyst msupport 100 msupport
(1a)
Activations of the loaded catalysts were carried out by NaBH4 reduction. Reduction of each loaded catalyst was performed separately in the batch system with 0.075 M NaBH4 solution.
Characterization Characterization of the catalysts were performed with wellknown instrumental techniques: X-Ray Diffractometry (XRD, Philips Panalytical X'Pert-Pro diffractometer, CuKa radiation) for crystal structure identification; Phase identification were made by reference to International Center for Crystal Database (ICDD) 2018; X-Ray Photoelectron Spectrometry (XPS, Thermo K-Alpha X-Ray Photoelectron Spectrometer), calibration of binding energies were made with C 1s peak at 284.5 eV as a reference, Aluminum anode (Al Ka ¼ 1468.3 eV) for electron structure determination; Brunauer-Emmet-Teller
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Fig. 1 e Integrated continuous system for performance tests.
(BET, Quantachrome, Autosorb Instrument, under N2 adsorptive gas and He carrier gas at 77 K after outgassing at 0.6 Pa and 473 K) for specific surface area calculation; Inductively Coupled Plasma- Mass Spectrometry (ICP-MS, Agilent 7700) for quantification of metal content. Before the ICP-MS readings, the sample was then treated in a microwave digester; Scanning Electron Microscopy (SEM, Zeiss Evo LS 10) for surface morphology and microstructure.
separation section and a simple liquid-gas exchange system for gas measurement. NaBH4/NaOH solution with 1:1 were fed to the packed reactor from under with various flow (1, 1.5, 2 ml/min) rates by peristaltic pump. Evolved gas was monitored after separation from liquid in the separation zone. Separated reactant and byproduct were collected. Reactions were carried out at room temperature, 40 C and 60 C.
Hydrogen production system development
Results and discussion The performance tests of the catalysts were carried out on an integrated continuous system (Fig. 1). The system consists of a peristaltic pump (Cole Parmer MasterFlex L/S 7518-40, ±0.1), silicon piping, a packed reactor, a custom designed liquid-gas
In the integrated continues system, six different bimetallic and trimetallic catalysts were prepared by wash coating method. Characteristic results of XRD, XPS, BET and elemental
Table 1 e Characterization results of catalysts. Catalyst Silicon Carbide (Support Material) LieCo PdeLieCo
PteLieCo
CueCo PdeCueCo
PteCueCo
Crystal Phases
Phase Structure
Metal Content (Weight, %)
Specific Surface Area (m2/g ±5%)
SiC Li2O Co3O4 PdO Li2O Co3O4 PtO2 Li2O Co3O4 CuO Co3O4 PdO CuO Co3O4 PtO2 CuO Co3O4
Hexagonal Cubic Cubic Tetragonal Cubic Cubic Tetragonal Cubic Cubic Monoclinic Cubic Tetragonal Monoclinic Cubic Tetragonal Monoclinic Cubic
0.19e6.24 (Li e Co) 0.05e0.11 e 4.62 (Pd e Li e Co)
4.041 5.172 7.301
0.07e 0.14 e 7.84 (Pd e Li e Co)
5.026
5.95e6.49 (Cu e Co) 0.06e2.95 e 3.18 (Pd e Cu e Co)
4.316
0.05e0.11 e 4.62 (Pt e Cu e Co)
11.821
5.615
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Table 2 e XPS spectra of the prepared catalysts. Catalysts
Peak
Binding Energy
Assignment
LieCo PdeLieCo
Co2p3 Co2p3 Pd3d5 Co2p3 Pt4d5 Co2p3 Cu2p3 Co2p3 Cu2p3 Pd3d5 Co2p3 Cu2p3 Pt4d5
779.85 779.85 336.6 779.8 317.38 779.85 933.47 779.77 933.47 336.97 779.79 933.5 316.79
Co₃O₄ Co₃O₄ PdO Co₃O₄ PtO2 Co₃O₄ CuO Co₃O₄ CuO PdO Co₃O₄ CuO PtO2
PteLieCo CueCo PdeCueCo
PteCueCo
Fig. 2 e XRD Patterns of catalysts; a. SiC Support, b. LiCo, c. PdeLiCo, d. PteLiCo, e. CuCo, f. PdeCuCo, g. PteCuCo.
analyses of all catalysts are presented in Table 1. The specific surface area of the support material was found as 4.041 m2/g and an increase in BET content was observed following each catalyst loading. According to the given data, PdeCuCo had the highest surface area among the catalysts with an area of 11.821 m2/g which indicates that adding palladium to catalysts increases the surface area.
Crystal phases and structures of catalysts are shown in Table 1. Fig. 2 shows that desired phases for catalyst are formed. SiC based support catalysts were formed with given structures respectively; Li2O and Co3O4 are cubic, PdO and PtO2 are tetragonal, and CuO is monoclinic in structure. Due to the support material, it is difficult to determine the oxide conditions in XRD peaks. Therefore XPS analyses were performed to clarify the chemical composition and oxide forms of catalysts which results were shown in Fig. 3. The presence of lithium element can be difficult and misleading to clarify in XPS spectra due to low binding energy with XPS hence the lithium graph is not given [42].
Fig. 3 e XPS spectra of catalysts, a. Co2p, b. Cu2p, c. Pd3d and d. Pt4d.
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Fig. 4 e SEM micrographs of catalysts; a. SiC Support, b. CueCo, c. PteCueCo, d. PdeCueCo, e. LieCo, f. PteLieCo, g. PdeLieCo.
As can be seen from Table 2, the average binding energy of each cobalt containing catalyst is 779.7 eV which corresponds to Co3O2 oxide structure [43,44]. Moreover, prepared catalysts’ average binding energy can be matched respectively; with Cu2p3 spectrum, 993.5 eV binding energy representing the Cu(II)O [45]; with Pd3d5 spectrum, 336.7 eV binding energy representing the
PdO [46] and with Pt4d5, 317 ± 3 eV binding energy representing the PtO2 oxide form [47]. Hence, analysis results show that the applied catalyst preparation method for catalyst loading on selected support material was successfully carried out. Surface morphological results of prepared catalysts and blank support material are shown in Fig. 4.
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production trends for catalysts with LiCo, PteLiCo, PdeLiCo, CuCo, PteCuCo, and PdeCuCo can be seen in Fig. 5. NaBH4(aq)þ2H2O(l)/NaBO2(aq)þ4H2(g) (10,8 (w.)%, 293e296 K)
Fig. 5 e Hydrogen Generation Results at room Temperature.
The support material was rough and had a porous structure. Also, it has been observed that all loaded catalyst accumulated in common regions on the surface area.
Performance Testing of Prepared Catalysts Hydrolysis reaction of Sodium Borohydride was investigated in a continuous flow system (Equation (1b)) The hydrogen
(1b)
When the system is operated in a continuous flow, it discharges the active regions of the catalysts together with the flow, which allows them to interact more with the fed reactants, as the by-product of the reaction (sodium metaborate) is removed from the reactor with the flow. Thus, the same conversion rate occurs at unit time intervals (the same amount of hydrogen). As a nature of a continuous flow system, catalytic activity does not change. As a result, each catalyst keeps its linear trend and produces hydrogen at a steady state. As shown in Fig. 5, noble metals perform better than nonnoble metals. However, as there was not much difference in performance catalysts, hydrogen output was constantly observed in similar trends. In general, constant hydrogen output is observed for each catalyst in the continuous system. A comparison between lithium and copper shows that the CuCo catalyst is more active than LiCo catalyst. Otherwise, CuCo had the same effect in similar experiments with 2:1 and
Fig. 6 e Hydrogen generation rate at different temperature for a. PdeLiCo, b. PdeCuCo.
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Fig. 7 e Hydrogen evaluation for PdeLiCo catalyst for various flow rate a. at 20 C, b. at 40 C, c. at 60 C.
0.5:1 NaBH4/NaOH ratios. As the residence time in the reactor depends on the flow rate (5 min for 0.5 ml/min, 2.5 min for 1 ml/min and 1.25 min for 2 ml/min), 480 ml hydrogen output was observed at a flow rate of 2 ml/min and 2:1 NaBH4/NaOH
ratio, although 700 ml hydrogen output was observed with CuCo catalyst at the same conditions. The activity and the regeneration state of the used catalysts were observed through repeated runs. Also, the catalyst regeneration was
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Fig. 8 e Hydrogen evaluation for PdeCuCo catalyst for various flow rate a. at 20 C, b. at 40 C, c. at 60 C.
carried out by washing the catalysts with hot water before proceeding to another cycle. Although LiCo exhibited high performance in the first experiment cycle, it showed a mean reduction of 14.7% after regeneration and reintroduction into
the system. However, such a sharp decrease was not observed in CuCo catalyst at the same period. This proves that copper is more resistant to deactivation that may occur with metaborate [39].
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On the other hand, hydrogen production rate for CuCo catalyst has increased 22% with 0.05%Pt addition and 17% with 0.06% Pd addition. For the catalyst LiCo; hydrogen production rate has considerably increased to 53% with 0.07% Pt addition and to 59% with 0.05% Pd addition onto the catalyst surface. It was found that the efficiency values of the prepared catalyst at room temperature followed the order of PdeLiCo (4.76 ml/min); Pde CuCo (4.69 ml/min); PteCuCo (4.61 ml/min); PteLiCo (4.49 ml/min); CuCo (3.82 ml/min) and LiCo (3.01 ml/ min). Since the conversions with, Pd added catalysts indicated slightly better activity than Pt added catalysts at room temperatures. Pd added catalysts were investigated in detail at different conditions in terms of flow rates and temperatures. For PdeLiCo and PdeCuCo catalysts, a comparison of catalytic performance at different temperatures and at 1 ml/min flow rate is shown in Fig. 6. According to this figure, these catalysts have the similar tendency in hydrogen generation rate with rising temperature. For PdeCuCo catalyst, the reaction was performed at three different temperatures: at 20 C, 40 C, and 60 C resulted in 25% increase in hydrogen generation rate (at 40 C compared to 20 C) and 5% (at 60 C compared to 40 C). For PdeLiCo catalyst corresponding increase rates were 19% and 8%. Fig. 7 and Fig. 8 represent the hydrogen generation rate (mmol/s) and total volume of hydrogen generation for Pde LiCo and PdeCuCo catalysts at different conditions. As Figs. 7a and 8a, hydrogen generation rates for each catalyst are almost similar at different flow rates at 20 C. For PdeCuCo catalyst, hydrogen generation rate increase nearly 2.5 times when the temperature is raised from 20 C to 60 C. For PdeLiCo catalyst, this rate increases nearly three times at the same condition (Fig. 7b, c, 8b, 8c). On the other hand, according to hydrogen generation versus time graph in Figs. 7 and 8, PdeCuCo catalyst's total conversion is higher than that of PdeLiCo at 40 C and at 1, 1.5 and 2 ml/min flow rates. Total conversions are 86%,73%, 65% for PdeLiCo and 89%, 80%, 76% for PdeCuCo at the same temperature (40 C) and flow rates. The catalysts' total hydrogen conversion rates tended towards each other at all flow rates as the temperature increased from 40 C to 60 C and, PdeCuCo catalyst was slightly more efficient than Pde LiCo.
Conclusion In the present work, LiCo, PteLiCo, PdeLiCo, CuCo, PteCuCo and PdeCuCo catalysts on ceramic foam support prepared by integrated wash coating system and catalysts performances of hydrogen production in terms of noble metal loading were carried on with the purpose of sodium borohydride hydrolysis in a continuous reactor system. The results obtained in this study are as follows; Although SiC support material is quite tough and not easy to shape, it provides a superior support for catalysts, due to its porous structure and durability. It has not been damaged in the continuous flow reactor and during
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calcination steps. The surface area of 4.041 m2/g has been increased as catalyst loading. Due to the crystalline structure of copper (monoclinic), CuCo catalyst rendered hydrogen production at a higher yield than LiCo catalyst [39]. Although LieCo catalyst has the least hydrogen production rate, its Pd added version (PdeLiCo) has the highest conversion rate. Only 0.05% addition of Pd increased the catalytic performance to 59%. Efficiency values of the prepared catalysts at room temperature followed the order of PdeLiCo (4.76 ml/min); Pde CuCo (4.69 ml/min); PteCuCo (4.61 ml/min); PteLiCo (4.49 ml/min); CuCo (3.82 ml/min) and LiCo (3.01 ml/min). Although Pd catalysts have low activity for NaBH4 hydrolysis, the trimetallic oxide forms (PdeCueCo,PdeLieCo) have slightly better catalytic activity than Pt included trimetallic catalysts (PteLieCo, PteCueCo). Hydrogen production rate increased as temperature increased. When the reaction temperature for catalyst Pde CuCo was increased from 20 C to 40 C and from 40 C to 60 C, hydrogen production rates increased by 25% and 5%, respectively. For PdeLiCo catalyst these values are 19% and 8%. While no significant difference was observed between the hydrogen production rates of catalysts PdeCuCo and Pde LiCo when flow rates changed at 20 C, a remarkable difference occurred when the reaction temperature increased from 20 C to 60 C for 2 ml/min flow rate, thus a 2.5 times increase was observed at the hydrogen production rates of catalysts PdeLiCo and PdeCuCo. For other flow rates this increase was 1.3 times for 1 ml/min and approximately 2 times for 1.5 ml/min.
As a final conclusion, Pd included CuCo and LiCo catalysts can be recommended for use in a continuous system to provide maximum hydrogen output with the optimum condition (at 40 C 1.5 ml/min or at 60 C 2 ml/min).
Acknowledgement This work has been financially supported by Research Fund of the Yildiz Technical University (YTU, Project No: 2013-07-01GEP03) and authors are gratefully acknowledged for Yamer Industrial Products Company, Izmir-Turkey, for their helpful contribution to supporting the ceramic foam material.
references
[1] IEA IEA. Global energy & CO2 status report 2017. 2018. [2] Ball M, Weeda M. The hydrogen economy - vision or reality? Int J Hydrogen Energy 2015;40(25):7903e19. [3] Dutta S. A review on production, storage of hydrogen and its utilization as an energy resource. J Ind Eng Chem 2014;20(4):1148e56. [4] Dincer I, Acar C. Review and evaluation of hydrogen production methods for better sustainability. Int J Hydrogen Energy 2015;40(34):11094e111.
9968
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 9 9 5 9 e9 9 6 9
[5] Momirlan M, Veziroglu TN. Current status of hydrogen energy. Renew Sustain Energy Rev 2002;6(1e2):141e79. [6] Sherif SA, Barbir F, Veziroglu TN. Principles of hydrogen energy production, storage and utilization. J Sci Ind Res India 2003;62(1e2):46e63. [7] Fakioglu E, Yurum Y, Veziroglu TN. A review of hydrogen storage systems based on boron and its compounds. Int J Hydrogen Energy 2004;29(13):1371e6. [8] Eberle U, Felderhoff M, Schuth F. Chemical and physical solutions for hydrogen storage. Angew Chem Int Ed 2009;48(36):6608e30. [9] Moussa G, Moury R, Demirci UB, Sener T, Miele P. Boronbased hydrides for chemical hydrogen storage. Int J Energy Res 2013;37(8):825e42. [10] Marrero-Alfonso EY, Beaird AM, Davis TA, Matthews MA. Hydrogen generation from chemical hydrides. Ind Eng Chem Res 2009;48(8):3703e12. [11] Mohring RM, Wu Y. Hydrogen generation via sodium borohydride. Hydrogen in Materials and Vacuum Systems 2003;671:90e100. [12] Amendola SC, Sharp-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, Binder M. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25(10):969e75. [13] Brack P, Dann SE, Wijayantha KGU. Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH4) solutions. Energy Sci Eng 2015;3(3):174e88. [14] Demirci UB. The hydrogen cycle with the hydrolysis of sodium borohydride: a statistical approach for highlighting the scientific/technical issues to prioritize in the field. Int J Hydrogen Energy 2015;40(6):2673e91. [15] Demirci UB, Miele P. Chemical hydrogen storage: 'material' gravimetric capacity versus 'system' gravimetric capacity. Energy Environ Sci 2011;4(9):3334e41. [16] Demirci UB, Akdim O, Andrieux J, Hannauer J, Chamoun R, Miele P. Sodium borohydride hydrolysis as hydrogen generator: issues, state of the art and applicability upstream from a fuel cell. Fuel Cell 2010;10(3):335e50. [17] Schlesinger HI, Brown HC, Finholt AE, Gilbreath JR, Hoekstra HR, Hyde EK. Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen. J Am Chem Soc 1953;75(1):215e9. [18] Herbert CAB, Brown C. New, highly active metal catalysts for the hydrolysis of borohydride. J Am Chem Soc 1962;84:1493e4. [19] Liu BH, Li ZP, Suda S. Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. J Alloy Comp 2006;415(1e2):288e93. [20] Walter JC, Zurawski A, Montgomery D, Thornburg M, Revankar S. Sodium borohydride hydrolysis kinetics comparison for nickel, cobalt, and ruthenium boride catalysts. J Power Sources 2008;179(1):335e9. [21] Suda S, Sun YM, Liu BH, Zhou Y, Morimitsu S, Arai K, Tsukamoto N, Uchida M, Candra Y, Li ZP. Catalytic generation of hydrogen by applying fluorinated-metal hydrides as catalysts. Appl Phys a-Mater 2001;72(2):209e12. [22] Bai Y, Wu CA, Wu F, Yi BL. Carbon-supported platinum catalysts for on-site hydrogen generation from NaBH4 solution. Mater Lett 2006;60(17e18):2236e9. [23] Netskina OV, Kochubey DI, Prosvirin IP, Kellerman DG, Simagina VI, Komova OV. Role of the electronic state of rhodium in sodium borohydride hydrolysis. J Mol Catal Chem 2014;390:125e32. [24] Ingersoll JC, Mani N, Thenmozhiyal JC, Muthaiah A. Catalytic hydrolysis of sodium borohydride by a novel nickel-cobaltboride catalyst. J Power Sources 2007;173(1):450e7.
[25] Ding XL, Yuan XX, Jia C, Ma ZF. Hydrogen generation from catalytic hydrolysis of sodium borohydride solution using Cobalt-Copper-Boride (Co-Cu-B) catalysts. Int J Hydrogen Energy 2010;35(20):11077e84. [26] Zhuang DW, Kang Q, Muir SS, Yao XD, Dai HB, Ma GL, Wang P. Evaluation of a cobalt-molybdenum-boron catalyst for hydrogen generation of alkaline sodium borohydride solution-aluminum powder system. J Power Sources 2013;224:304e11. [27] Demirci UB, Garin F. Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate. J Alloy Comp 2008;463(1e2):107e11. [28] Pena-Alonso R, Sicurelli A, Callone E, Carturan G, Raj R. A picoscale catalyst for hydrogen generation from NaBH4 for fuel cells. J Power Sources 2007;165(1):315e23. [29] Kojima Y, Suzuki K, Fukumoto K, Sasaki M, Yamamoto T, Kawai Y, Hayashi H. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int J Hydrogen Energy 2002;27(10):1029e34. [30] Cho KW, Kwon HS. Effects of electrodeposited Co and Co-P catalysts on the hydrogen generation properties from hydrolysis of alkaline sodium borohydride solution. Catal Today 2007;120(3e4):298e304. [31] Mohring RM, Luzader RE. A sodium borohydride on-board hydrogen generator for powering fuel Cell and internal combustion engine vehicles. California: Future Transportation Technology Conference; 2001. [32] Zhang Q, Smith G, Wu Y, Mohring R. Catalytic hydrolysis of sodium borohydride in an auto-thermal fixed-bed reactor. Int J Hydrogen Energy 2006;31(7):961e5. [33] Kim T. NaBH4 (sodium borohydride) hydrogen generator with a volume-exchange fuel tank for small unmanned aerial vehicles powered by a PEM (proton exchange membrane) fuel cell. Energy 2014;69:721e7. [34] Zhang JS, Zheng Y, Gore JP, Fisher TS. 1 kW(e) sodium borohydride hydrogen generation system Part I: experimental study. J Power Sources 2007;165(2):844e53. [35] Sousa T, Rangel CM. A dynamic two phase flow model for a pilot scale sodium borohydride hydrogen generation reactor. Int J Hydrogen Energy 2014;39(10):5291e300. [36] Lim MH, Chung J. The development and evaluation of a flow-type reactor utilizing the thermal siphon effect to generate hydrogen. Int J Hydrogen Energy 2014;39(25):13079e86. [37] Wang FC, Fang WH. The development of a PEMFC hybrid power electric vehicle with automatic sodium borohydride hydrogen generation. Int J Hydrogen Energy 2017;42(15):10376e89. [38] Dai HB, Liang Y, Wang P, Yao XD, Rufford T, Lu M, Cheng HM. High-performance cobalt-tungsten-boron catalyst supported on Ni foam for hydrogen generation from alkaline sodium borohydride solution. Int J Hydrogen Energy 2008;33(16):4405e12. [39] Eugenio S, Demirci UB, Silva TM, Carmezim MJ, Montemor MF. Copper-cobalt foams as active and stable catalysts for hydrogen release by hydrolysis of sodium borohydride. Int J Hydrogen Energy 2016;41(20):8438e48. [40] Twigg MV, Richardson JT. Fundamentals and applications of structured ceramic foam catalysts. Ind Eng Chem Res 2007;46(12):4166e77. [41] Kojima Y, Suzuki K, Fukumoto K, Kawai Y, Kimbara M, Nakanishi H, Matsumoto S. Development of 10 kW-scale hydrogen generator using chemical hydride. J Power Sources 2004;125(1):22e6. [42] ThermoScientificXPS, https://xpssimplified.com/elements/ lithium.php, (July 10, 2018). [43] ThermoScientificXPS, https://xpssimplified.com/elements/ cobalt.php, (July 10, 2018).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 9 9 5 9 e9 9 6 9
[44] Du ZJ, Xiong DH, Verma SK, Liu BS, Zhao XJ, Liu LF, Li H. A low temperature hydrothermal synthesis of delafossite CuCoO2 as an efficient electrocatalyst for the oxygen evolution reaction in alkaline solutions. Inorg Chem Front 2018;5(1):183e8. [45] ThermoScientificXPS, https://xpssimplified.com/elements/ copper.php, (July 10, 2018).
9969
[46] ThermoScientificXPS, https://xpssimplified.com/elements/ palladium.php, (July 10, 2018). [47] Shyu JZ, Otto K. Identification of platinum phases on gamma-alumina by xps. Appl Surf Sci 1988;32(1e2):246e52.