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Effect of the amine group content on catalytic activity and stability of mesoporous silica supported Pd catalysts for additive-free formic acid dehydrogenation at room temperature
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Effect of the amine group content on catalytic activity and stability of mesoporous silica supported Pd catalysts for additive-free formic acid dehydrogenation at room temperature Min-Ho Jin a,b,1, Ju-Hyoung Park b,c,1, Duckkyu Oh a, Jong-Soo Park a, Kwan-Young Lee b,**, Dong-Wook Lee a,* a Advanced Materials and Devices Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeongro, Yuseong, Daejeon 305-343, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, Sungbuk-gu, Seoul 136-701, Republic of Korea c Clean Fuel Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeongro, Yuseong, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
A strong metal-support interaction (SMSI) between amine-functionalized silica supports and
Received 9 October 2018
Pd nanoparticles is one of important factors to determine the catalytic activity of additive-
Received in revised form
free formic acid dehydrogenation at room temperature over Pd/NH2-silica catalysts. How-
27 December 2018
ever, there are few reports on the effect of the content of amine functional groups on the
Accepted 29 December 2018
SMSI and catalytic performance for formic acid dehydrogenation. In this study, we tried to
Available online 24 January 2019
maximize the content of amino-propyl groups on the surface of mesoporous silica supports (KIE-6) via hydroxylation of KIE-6 surface before amine functionalization and investigated the effect of the content of amine functional groups on the catalytic activity and stability for formic acid dehydrogenation. As a result, Pd/NH2-hydroxylated KIE-6 (Pd/NH2-OH-KIE-6) catalysts with more amine functional groups provided higher initial catalytic activity (595 mol H2 mol catalyst1h1) than Pd/NH2-KIE-6 catalysts. However, Pd/NH2-KIE-6 catalysts showed higher catalytic stability in comparison with Pd/NH2-OH-KIE-6 catalysts. After various characterizations of catalysts, it was demonstrated that the enhanced initial catalytic activity of Pd/NH2-OH-KIE-6 catalysts is attributed to the higher ratio of Pd/PdO derived from the increased content of amine groups of NH2-OH-KIE-6 supports. In contrast, the low surface area of NH2-OH-KIE-6 promoted the aggregation of Pd nanoparticles on Pd/NH2-OHKIE-6 catalysts, which resulted in the lower catalytic stability of Pd/NH2-OH-KIE-6 catalysts than Pd/NH2-KIE-6 catalysts. Thus it was concluded that confinement of Pd nanoparticles to the pores of supports is a more dominant factor to achieve higher catalytic stability, while the initial catalytic activity is affected by the electronic state of Pd nanoparticle determined by the content of amine functional groups on the surface of supports. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.-Y. Lee), [email protected] (D.-W. Lee). 1 Min-Ho Jin and Ju-Hyoung Park contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2018.12.208 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Hydrogen as clean energy carrier has received increasing attention [1e8], however safe and efficient storage system of hydrogen are still a difficult challenge [1e5]. Formic acid, which is nontoxic, noncorrosive, and inflammable, is one of promising candidates for hydrogen storage materials because of its high volumetric (53 g/L) and gravimetric hydrogen density (4.4 wt%). Formic acid can be decomposed either by dehydrogenation (HCOOH 4 H2 þ CO2 G ¼ 48.4 kJ mol1) or dehydration (HCOOH 4 H2O þ CO G ¼ 28.5 kJ mol1) pathways [9,10]. For the combination of formic acid as a liquid organic hydrogen carrier (LOHC) and polymer electrolyte membrane fuel cell (PEMFC), dehydrogenation of formic acid is highly desired [11e13]. However, the dehydration reaction should be avoided because generated CO gas deactivates fuel cell catalysts and catalysts of formic acid dehydrogenation [14]. Recently, homogeneous catalysts have been studied and they showed excellent catalytic activity [15e21]. Nevertheless, the disadvantages of homogeneous catalysts, such as the difficulty of separation and reusability, triggered the development of heterogeneous catalysts for formic acid dehydrogenation [22,23], and there have been a number of publications reporting excellent catalytic activity [24e49]. To date, the optimal catalyst for additive-free formic acid dehydrogenation at room temperature is considered to be the Pd supported on amine functionalized silica because O-H bonding of formic acid is broken by basic eNH2 groups [25]. In the catalyst system, a strong metal-support interaction (SMSI) between amine-functionalized silica supports and active Pd nanoparticles is one of important factors to determine the catalytic activity of additive-free formic acid dehydrogenation at room temperature [50,51]. In this context, it is necessary to investigate the effect of amine functional groups on catalytic activity. However, there are few reports on the effect of the content of amine functional groups on the catalytic performance of the Pd catalysts supported on amine functionalized silica. In this study, we tried to maximize the content of aminopropyl groups on the surface of mesoporous silica supports (KIE-6) via hydroxylation of KIE-6 surface before amine functionalization and investigated the effect of the content of amine functional groups on the catalytic activity and stability for formic acid dehydrogenation by comparing the Pd catalysts supported on hydroxylated and amine functionalized KIE-6 with the Pd catalysts supported on amine functionalized KIE-6.
Experimental methods Preparation of KIE-6 A silica sol having a diameter of about 5 nm was synthesized through a base-catalyzed hydrolysis and condensation reaction of tetraethyl orthosilicate (TEOS: Aldrich 98%). A mixture solution of NH3 and H2O was added to a mixture solution of TEOS and EtOH under vigorous stirring at 50 C. The molar ratio of TEOS: NH3: H2O: EtOH was 1:0.084:53.6:40.7. After the
final mixture was refluxed for 3 h at 50 C, colloidal silica sol with particle size of about 5 nm was successfully synthesized. In order to obtain mesoporous silica (KIE-6), 4 g of glycerol and sulfuric acid (10 wt% of glycerol) were added to 30 mL of the as-prepared colloidal silica sol. The mixture solution was dried at 150 C for 24 h in air for solvent removal and precarbonization of glycerol, resulting in the nanocomposite of silica nanoparticles and precarbonized glycerol. After the nanocomposite was calcined at 550 C for 2 h in air, the mesoporous silica KIE-6 was obtained.
Hydroxylation of KIE-6 surface To maximize the amine functionalization of KIE-6 surface, we conducted hydroxylation of KIE-6 surface prior to amine functionalization of KIE-6 surface. In a typical synthesis, 1 g of mesoporous silica (KIE-6) was added into 67 mL of 17.7 wt% hydrochloric acid aqueous solution and the mixture solution was stirred for 12 h. After washing three times with distilled water and drying at room temperature, we obtained hydroxylated KIE-6 (OH-KIE-6).
Preparation of catalyst supports via aminefunctionalization of KIE-6 and OH-KIE-6 surface In a typical synthesis, 1.5 g of KIE-6 or OH-KIE-6 was added into 150 mL of toluene solution, followed by the addition of 3.75 mL of 3-aminopropyl trimethoxysilane (APTMS). The final mixture solution was refluxed at 110 C for 3 h without stirring and then filtered and washed repeatedly with toluene for removal of unreacted APTMS. After drying the sample at room temperature for 24 h, the amine-functionalized KIE-6 (NH2KIE-6) or amine-functionalized OH-KIE-6 (NH2-OH-KIE-6) was prepared.
Preparation of Pd catalysts for formic acid dehydrogenation NH2-KIE-6 and NH2-OH-KIE-6 were used as a catalyst support for preparation of Pd catalysts for formic acid dehydrogenation. In a typical synthesis, 0.095 g of 10 wt% palladium (II) nitrate aqueous solution (PM RESEARCH) was added into 10 mL of distilled water and then 0.18 g NH2-KIE-6 or NH2-OHKIE-6 was added. Afterward, 2 mL of 0.85 M NaBH4 was added into the solution and was stirred for 1 h. After the solution was centrifuged, washed, and was dried, Pd/NH2-KIE-6 and Pd/ NH2-OH-KIE-6 catalysts were prepared.
Additive-free formic acid dehydrogenation at roomtemperature We used the Teflon-lined stainless steel batch reactor connected with the gas burette system filled with water. 0.055 g of catalyst was located in the batch reactor at room temperature and then purged with nitrogen for 30 min. Afterward, a mixture of 0.19 mL of formic acid (95%, Aldrich) and 10 mL of distilled water was injected through a rubber septum to initiate the reaction. After the first test of additive-free formic acid dehydrogenation at room temperature, the catalyst was separated from reaction solution by centrifugation, and
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washed, then dried at room temperature. The dried catalyst used again in the additive-free formic acid dehydrogenation at room temperature.
Characterization For characterization of the KIE-6, OH-KIE-6, NH2-KIE-6, and NH2-OH-KIE-6 supports, nitrogen sorption isotherms, fourier transform infrared (FTIR), thermogravimetric analysis (TGA), elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were conducted. Nitrogen sorption isotherms of materials were obtained using a Micromeritics ASAP 2420 instrument, after the degassing of samples was carried out at 200 C for 5 h. FTIR spectra were obtained with a Thermo Nicolet 5700 instrument. Thermogravimetric analysis (TGA) and Elemental analysis (EA) were conducted on a TGA Q500 and a Thermo Scientific FLASH EA2000 instrument, respectively. XPS and HAADF-STEM analyses were performed using a Kratos 165XP and a FEI/TECNAI G5 instrument. The composition of product gas was confirmed by gas chromatography (Agilent 6890) with a carbon 1010 PLOT fused silica capillary column (30 m 0.53 mm, SUPELCO) and thermal conductivity detector (TCD).
Results and discussion Amine functionalization of mesoporous silica (KIE-6) and hydroxylated mesoporous silica (OH-KIE-6) supports In the previous study, we reported the control in the pore structure of mesoporous silica (KIE-6) supports using a glycerol template and the effect of the pore structure on catalytic activity for additive-free formic acid dehydrogenation at room temperature [47]. As a result, we demonstrated that the improved catalytic activity was achieved by catalyst supports having three dimensionally interconnected pore structures with large pore volume, because such pore structure provided easy diffusion of reactant molecules to the active site. In other previous publication, we reported that stirring time and types of Pd precursors influenced the particle size and dispersion of Pd nanoparticles, and high dispersion of Pd nanoparticles on the amine functionalized KIE-6 supports gave improved catalytic activity [48]. In summary, the pore structure of supports and dispersion of Pd nanoparticles are the most significant factors to determine catalytic activity of Pd/NH2-KIE-6 catalysts. In this paper, we will report the effect of the content of amine groups of amine functionalized KIE-6 (NH2-KIE-6) supports on the pore properties and dispersion of Pd nanoparticles for Pd/NH2-KIE-6 catalysts. To investigate the effect of the content of amine groups of NH2-KIE-6 supports, we prepared two different NH2-KIE-6 supports. The first one (NH2-KIE-6) is prepared by conducting amine functionalization of KIE-6 surface, and the second one (NH2-OH-KIE-6) is prepared by conducting the amine functionalization after hydroxylation of KIE-6 surface. Fig. 1 and Table 1 show nitrogen sorption results of mesoporous silica (KIE-6), hydroxylated KIE-6 (OH-KIE-6), amine-functionalized and hydroxylated and amineKIE-6 (NH2-KIE-6),
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functionalized KIE-6 (NH2-OH-KIE-6). Comparing OH-KIE-6 with KIE-6, a significant change in pore properties of KIE-6 was not observed after hydroxylation of KIE-6. However, comparing NH2-KIE-6 with KIE-6 and NH2-OH-KIE-6 with OHKIE-6, the pore properties of KIE-6 and OH-KIE-6 decreased after amine functionalization. Especially, in the case of OHKIE-6 supports, amine functionalization led to a considerable decrease in pore properties. Fig. 2 shows fourier transform infrared (FTIR) spectra for NH2-KIE-6 and NH2-OH-KIE-6. The peaks at 786e793 and 1023e1038 cm1 for NH2-KIE-6 and NH2-OH-KIE-6 correspond to symmetric and asymmetric stretching vibration of Si-O-Si. Furthermore, the bands at 1559 and 2936 cm1 for NH2-KIE-6, and 1560 and 2932 cm1 for NH2-OH-KIE-6 were assigned to NH bending and C-H stretching vibration. On the basis of Fig. 2, it was confirmed that amino propyl groups were successfully functionalized on surface of KIE-6 and OH-KIE-6 supports. In addition, the intensity of the peaks at 1560 and 2932 cm1 for NH2-OH-KIE-6 is much higher than that for NH2-KIE-6, indicating that NH2-OH-KIE-6 has more amino propyl groups on the surface than NH2-KIE-6. To estimate the amount of amino propyl groups attached to KIE-6 and OH-KIE-6 supports, we also conducted thermal gravimetric analysis (TGA) of NH2-KIE-6 and NH2-OH-KIE-6. Fig. 3 shows thermal gravimetric analysis (TGA) and differential thermogravimetric (DTG) curves of NH2-KIE-6 and NH2OH-KIE-6. The main two peaks were observed below 110 C and in the range of 300e650 C. The first peak below 110 C is attributed to desorption of the physically adsorbed water, and the second peak is assigned to decomposition of amino propyl groups. NH2-OH-KIE-6 showed 17.1% of weight loss, while NH2-KIE-6 showed 7.4% of weight loss. It is because amine functionalization after hydroxylation of KIE-6 surface resulted in a significant increase in amino propyl groups on NH2-OHKIE-6. As amino propyl groups are functionalized on the KIE-6 surface by condensation between ethoxy groups of APTMS and hydroxyl groups of KIE-6 surface, more hydroxyl groups on KIE-6 surface lead to more amino propyl groups functionalized on KIE-6 surface. To confirm the increased amount of amino propyl groups on NH2-OH-KIE-6, elemental analysis (EA) was conducted. Table 2 shows elemental analysis data for KIE-6, NH2-KIE-6, OH-KIE-6 and NH2-OH-KIE-6. Comparing OH-KIE-6 with KIE-6, the content of hydrogen for OH-KIE-6 increased because of hydroxylation on KIE-6 surface. In the case of NH2-KIE-6 and NH2-OH-KIE-6, carbon and nitrogen were detected because of functionalization of amino propyl groups on KIE-6 and OH-KIE-6 surface. However, the content of carbon and nitrogen for NH2-OH-KIE-6 is about 1.7 times higher than that for NH2-KIE-6. On the basis of Figs. 2 and 3 and Table 2, it was demonstrated that the content of amino propyl groups functionalized on KIE-6 surface significantly increased by hydroxylation of KIE-6 surface before amino propyl functionalization.
The effect of the content of amino propyl groups on catalytic activity and stability for additive-free formic acid dehydrogenation at room-temperature We investigated the effect of the content of amino propyl groups on catalytic activity and stability by using NH2-KIE-6
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Fig. 1 e Nitrogen sorption results of mesoporous silica (KIE-6), hydroxylated KIE-6 (OH-KIE-6), amine-functionalized KIE-6 (NH2-KIE-6), and hydroxylated and amine-functionalized KIE-6 (NH2-OH-KIE-6). (a) isotherms (b) BJH desorption pore size distributions.
Table 1 e Pore properties for KIE-6, OH-KIE-6, NH2-KIE-6 and NH2-OH-KIE-6 obtained from nitrogen sorption tests. Materials KIE-6 OH-KIE-6 NH2-KIE-6 NH2-OH-KIE-6 a b c
d
SABET [m2/g]a
SAmicro [m2/g]b
Vtot [cm3/g]c
D [nm]d
405 432 162 15
10.8 22.0 7.5 3.2
0.83 0.86 0.37 0.05
7.2 7.4 7.0 8.79
BET surface area. Micropore surface area calculated from a t-plot. Total pore volume taken from the volume of nitrogen adsorbed at P/Po ¼ 0.995. BJH desorption average pore diameter.
and NH2-OH-KIE-6 as a catalyst support for additive-free formic acid dehydrogenation at room temperature. Fig. 4 shows reusability test results for formic acid dehydrogenation over Pd (5 wt%)/NH2-KIE-6 and Pd (5 wt%)/NH2-OH-KIE-6. The total volume of product gas (H2 and CO2) at 70 min and turnover frequency (TOF) at initial 10 min and 25 C for Pd (5 wt%)/NH2OH-KIE-6 in 1st run were 200 mL and 595 mol H2 mol catalyst1 h1, which were almost 1.4 times higher than that for Pd (5 wt %)/NH2-KIE-6 catalyst (170 mL and 419 mol H2 mol catalyst1
h1). However, The total volume of product gas (H2 and CO2) at 70 min and turnover frequency (TOF) at initial 10 min and 25 C for Pd (5 wt%)/NH2-KIE-6 in 4th run were 154 mL and 310 mol H2 mol catalyst1h1, which were similar that Pd (5 wt %)/NH2-OH-KIE-6 catalyst (152 mL and 300 mol H2 mol catalyst1h1). After 5 th run, the catalytic activity of Pd (5 wt%)/NH2-KIE-6 was higher than Pd (5 wt%)/NH2-OH-KIE-6. Fig. 5 shows variation of catalytic activity with an increase in the number of reusing for catalysts. Initially, the conversion and TOF of Pd (5 wt%)/NH2-OH-KIE-6 catalysts were much higher than those of Pd (5 wt%)/NH2-KIE-6 catalysts. However, the decrease in the conversion and TOF for Pd (5 wt%)/NH2OH-KIE-6 was much larger than that for Pd (5 wt%)/NH2-KIE-6, as number of reusability increased. On the basis of Figs. 4 and 5, it was confirmed that the increase in the content of amino propyl groups on catalyst supports can enhance the initial catalytic activity, however it significantly decreases the catalytic stability.
Correlation between catalytic activity and the content of amino propyl groups on catalyst supports To investigate the correlation between the catalytic stability and amino propyl groups on catalyst supports, we carried out
Fig. 2 e FT-IR spectra of (a) amine-functionalized KIE-6 (b) amine-functionalized OH-KIE-6.
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Fig. 3 e TGA and DTG curves of (a) amine-functionalized KIE-6 (b) amine-functionalized OH-KIE-6.
Table 2 e Elemental analysis (EA) data. Materials KIE-6 NH2-KIE-6 OH-KIE-6 NH2-OH-KIE-6
Carbon [%]
Hydrogen [%]
Nitrogen [%]
e 9.2 e 15.2
0.6 2.4 0.7 3.7
e 3.2 e 5.4
X-ray photoelectron spectroscopy (XPS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analyses for the catalysts. Fig. 6 presents XPS (Pd 3d5/2 and Pd 3d3/2) spectra for Pd (5 wt%)/NH2KIE-6 and Pd (5 wt%)/NH2-OH-KIE-6 catalysts. The peaks at 335.4e335.5 eV and 340.5e340.9 eV correspond to 3d5/2 and 3d3/2 components for zero-valence metallic state of Pd nanoparticles and the peaks at 337.4e337.6 eV and 342.4e342.8 eV correspond to 3d5/2 and 3d3/2 components for PdO. The intensity ratio of Pd peaks and PdO peaks for Pd (5 wt%)/NH2OH-KIE-6 catalysts was higher than that for Pd (5 wt%)/NH2KIE-6 catalysts, indicating that Pd (5 wt%)/NH2-OH-KIE-6 catalysts have more Pd nanoparticles than Pd (5 wt%)/NH2-KIE-6 catalysts before the reaction. The increased content of amine groups of NH2-OH-KIE-6 supports promoted the reduction of Pd precursors. This is the reason why initial catalytic activity of Pd (5 wt%)/NH2-OH-KIE-6 catalysts is much higher than that of Pd (5 wt%)/NH2-KIE-6 catalysts.
Fig. 5 e Variation of catalytic activity with an increase in the reusability number of catalysts.
Fig. 7 shows STEM results of Pd/NH2-KIE-6 and Pd/NH2-OHKIE-6 catalysts. Pd nanoparticle size after reusability test of Pd/NH2-KIE-6 and Pd/NH2-OH-KIE-6 catalysts increased from 2.30 to 2.47 nm and from 2.41 to 3.48 nm, respectively. On the basis of Figs. 4, 5 and 7, it was found that decrease of the catalytic activity for reusability test shown in Figs. 4 and 5 was attributed to the increase of Pd nanoparticle size. Moreover, as the increase in Pd nanoparticle size for Pd (5 wt%)/NH2-OHKIE-6 catalysts was larger than that for Pd (5 wt%)/NH2-KIE-6
Fig. 4 e Reusability test results for formic acid dehydrogenation over (a) Pd (5 wt%)/NH2-KIE-6 and (b) Pd (5 wt%)/NH2-OH-KIE6.
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Fig. 6 e XPS spectra of catalysts. (a) Pd (5 wt%)/NH2-KIE-6 (b) Pd (5 wt%)/NH2-OH-KIE-6.
Fig. 7 e STEM images of catalysts. (a) Pd (5 wt%)/NH2-KIE-6 catalyst before reusability (b) Pd (5 wt%)/NH2-KIE-6 catalyst after 10 times reusability (c) Pd (5 wt%)/NH2-OH-KIE-6 catalyst before reusability (d) Pd (5 wt%)/NH2-OH-KIE-6 catalyst after 10 times reusability.
catalysts (Fig. 7), the decrease in the catalytic activity for Pd (5 wt%)/NH2-OH-KIE-6 was much larger than that for Pd (5 wt %)/NH2-KIE-6 (Figs. 4 and 5). Meanwhile, as shown in Fig. 1 and Table 1, NH2-OH-KIE-6 showed the considerable decrease in surface area and pore volume after amine functionalization in comparison with NH2-KIE-6. Such low surface area of NH2-
OH-KIE-6 led to deposition of Pd nanoparticles on external surface of supports rather than inside pores of supports. The Pd nanoparticles on external surface of NH2-OH-KIE-6 supports promoted the increase in Pd particle size through aggregation, which resulted in lower catalytic stability of Pd (5 wt %)/NH2-OH-KIE-6 catalysts than Pd (5 wt%)/NH2-KIE-6
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catalysts. Thus it was concluded that the increased content of amine groups of supports can promote the reduction of Pd precursors and enhance initial catalytic activity, whereas the increased content of amine groups of supports leads to the low surface area of supports and decreases catalytic stability through easy aggregation of Pd nanoparticles. From the viewpoint of catalytic stability, the use of KIE-6 supports without hydroxylation is suitable for additive-free formic acid dehydrogenation over Pd/NH2-KIE-6 catalysts.
Conclusions To investigate correlation between an increase in the content of amine groups and catalytic activity and stability in additivefree formic acid dehydrogenation at room-temperature, we prepared two different supports of NH2-KIE-6 and NH2-OHKIE-6. The NH2-KIE-6 is prepared by conducting amine functionalization of KIE-6 surface, and the NH2-OH-KIE-6 is prepared by conducting the amine functionalization after hydroxylation of KIE-6 surface. As a result, the content of amino propyl groups functionalized on OH-KIE-6 surface significantly increased by hydroxylation of KIE-6 surface before amino propyl functionalization. However, the NH2-OHKIE-6 showed the considerable decrease in surface area and pore volume after amine functionalization in comparison with NH2-KIE-6. The increased content of amine groups of NH2-OH-KIE-6 supports provided higher ratio of Pd/PdO, so that initial catalytic activity of Pd/NH2-OH-KIE-6 catalysts was much higher than that of Pd/NH2-KIE-6 catalysts. In contrast, the low surface area of NH2-OH-KIE-6 promoted the aggregation of Pd nanoparticles on Pd/NH2-OH-KIE-6 catalysts, which resulted in lower catalytic stability of Pd/NH2-OH-KIE-6 catalysts than Pd/NH2-KIE-6 catalysts. Thus it was confirmed that the increased content of amine groups of supports can enhance the initial catalytic activity by the higher ratio of Pd/ PdO, whereas it can decrease the catalytic stability because of aggregation of Pd nanoparticles derived from the low surface area of supports.
Acknowledgements This work was supported by a research program (B7-2461-03) of the Korea Institute of Energy Research (KIER).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.208.
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[22] Zhu Q-L, Xu Q. Liquid organic and inorganic chemical hydrides for high-capacity hydrogen storage. Energy Environ Sci 2015;8:478e512. [23] Cho J, Lee S, Han J, Yoon SP, Nam SW, Choi SH, et al. Importance of ligand effect in selective hydrogen formation via formic acid decomposition on the bimetallic Pd/Ag catalyst from first-principles. J Phys Chem C 2014;118:22553e60. [24] Bi Q-Y, Lin J-D, Liu Y-M, He H-Y, Huang F-Q, Cao Y. Dehydrogenation of formic acid at room temperature: boosting palladium nanoparticle efficiency by coupling with pyridinic-nitrogen-doped carbon. Angew Chem Int Ed 2016;55:11849e53. [25] Karatas Y, Bulut A, Yurderi M, Ertas IE, Alal O, Gulcan M, et al. PdAu-MnOx nanoparticles supported on aminefunctionalized SiO2 for the room temperature dehydrogenation of formic acid in the absence of additives. Appl Catal B Environ 2016;180:586e95. [26] Mori K, Dojo M, Yamashita H. Pd and Pd Ag Nanoparticles within a Macroreticular basic resin : an efficient catalyst for hydrogen production from formic acid decomposition. ACS Catal 2013;3:1114e9. [27] Yan J-M, Wang Z-L, Gu L, Li S-J, Wang H-L, Zheng W-T, et al. AuPd-MnOx/MOF-graphene: an efficient catalyst for hydrogen production from formic acid at room temperature. Adv Energy Mater 2015;5:1500107. [28] Wang ZL, Ping Y, Yan JM, Wang HL, Jiang Q. Hydrogen generation from formic acid decomposition at room temperature using a NiAuPd alloy nanocatalyst. Int J Hydrogen Energy 2014;39:4850e6. [29] Tedsree K, Li T, Jones S, Chan CWA, Yu KMK, Bagot PAJ, et al. Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst. Nat Nanotechnol 2011;6:302e7. [30] Zhu Q-L, Tsumori N, Xu Q. Sodium hydroxide-assisted growth of uniform Pd nanoparticles on nanoporous carbon MSC-30 for efficient and complete dehydrogenation of formic acid under ambient conditions. Chem Sci 2014;5:195e9. [31] Lee DW, Jin MH, Park JC, Lee CB, Oh D, Lee SW, et al. Wasteglycerol-directed synthesis of mesoporous silica and carbon with superior performance in room-temperature hydrogen production from formic acid. Sci Rep 2015;5:1e11. [32] Caner N, Bulut A, Yurderi M, Ertas IE, Kilal H, Kaya M, Zahmakiran M. Atomic layer deposition-SiO2 layers protected PdCoNi nanoparticles supported on TiO2 nanopowders: exceptionally stable nanocatalyst for the dehydrogenation of. Appl Catal B Environ 2017;210:470e83. [33] Hattori M, Einaga H, Daio T, Tsuji M. Efficient hydrogen production from formic acid using TiO2 -supported AgPd@Pd nanocatalysts. J Mater Chem 2015;3:4453e61. [34] Lv Q, Feng L, Hu C, Liu C, Xing W. High-quality hydrogen generated from formic acid triggered by in situ prepared Pd/C catalyst for fuel cells. Catal Sci Technol 2015;5:2581e4. [35] Li SJ, Ping Y, Yan J-M, Wang H-L, Wu M, Jiang Q. Facile synthesis of AgAuPd/graphene with high performance for hydrogen generation from formic acid. J Mater Chem 2015;3:14535e8. [36] Jin F, Zeng X, Liu J, Jin Y, Wang L, Zhong H, et al. Highly efficient and autocatalytic H2O dissociation for CO2 reduction into formic acid with zinc. Sci Rep.DOI: 10.1038/step04503.
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Effect of the amine group content on catalytic activity and stability of mesoporous silica supported Pd catalysts for additive-free formic acid dehydrogenation at room temperature Min-Ho Jin a,b,1, Ju-Hyoung Park b,c,1, Duckkyu Oh a, Jong-Soo Park a, Kwan-Young Lee b,**, Dong-Wook Lee a,* a Advanced Materials and Devices Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeongro, Yuseong, Daejeon 305-343, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, Sungbuk-gu, Seoul 136-701, Republic of Korea c Clean Fuel Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeongro, Yuseong, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
A strong metal-support interaction (SMSI) between amine-functionalized silica supports and
Received 9 October 2018
Pd nanoparticles is one of important factors to determine the catalytic activity of additive-
Received in revised form
free formic acid dehydrogenation at room temperature over Pd/NH2-silica catalysts. How-
27 December 2018
ever, there are few reports on the effect of the content of amine functional groups on the
Accepted 29 December 2018
SMSI and catalytic performance for formic acid dehydrogenation. In this study, we tried to
Available online 24 January 2019
maximize the content of amino-propyl groups on the surface of mesoporous silica supports (KIE-6) via hydroxylation of KIE-6 surface before amine functionalization and investigated the effect of the content of amine functional groups on the catalytic activity and stability for formic acid dehydrogenation. As a result, Pd/NH2-hydroxylated KIE-6 (Pd/NH2-OH-KIE-6) catalysts with more amine functional groups provided higher initial catalytic activity (595 mol H2 mol catalyst1h1) than Pd/NH2-KIE-6 catalysts. However, Pd/NH2-KIE-6 catalysts showed higher catalytic stability in comparison with Pd/NH2-OH-KIE-6 catalysts. After various characterizations of catalysts, it was demonstrated that the enhanced initial catalytic activity of Pd/NH2-OH-KIE-6 catalysts is attributed to the higher ratio of Pd/PdO derived from the increased content of amine groups of NH2-OH-KIE-6 supports. In contrast, the low surface area of NH2-OH-KIE-6 promoted the aggregation of Pd nanoparticles on Pd/NH2-OHKIE-6 catalysts, which resulted in the lower catalytic stability of Pd/NH2-OH-KIE-6 catalysts than Pd/NH2-KIE-6 catalysts. Thus it was concluded that confinement of Pd nanoparticles to the pores of supports is a more dominant factor to achieve higher catalytic stability, while the initial catalytic activity is affected by the electronic state of Pd nanoparticle determined by the content of amine functional groups on the surface of supports. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.-Y. Lee), [email protected] (D.-W. Lee). 1 Min-Ho Jin and Ju-Hyoung Park contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2018.12.208 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Hydrogen as clean energy carrier has received increasing attention [1e8], however safe and efficient storage system of hydrogen are still a difficult challenge [1e5]. Formic acid, which is nontoxic, noncorrosive, and inflammable, is one of promising candidates for hydrogen storage materials because of its high volumetric (53 g/L) and gravimetric hydrogen density (4.4 wt%). Formic acid can be decomposed either by dehydrogenation (HCOOH 4 H2 þ CO2 G ¼ 48.4 kJ mol1) or dehydration (HCOOH 4 H2O þ CO G ¼ 28.5 kJ mol1) pathways [9,10]. For the combination of formic acid as a liquid organic hydrogen carrier (LOHC) and polymer electrolyte membrane fuel cell (PEMFC), dehydrogenation of formic acid is highly desired [11e13]. However, the dehydration reaction should be avoided because generated CO gas deactivates fuel cell catalysts and catalysts of formic acid dehydrogenation [14]. Recently, homogeneous catalysts have been studied and they showed excellent catalytic activity [15e21]. Nevertheless, the disadvantages of homogeneous catalysts, such as the difficulty of separation and reusability, triggered the development of heterogeneous catalysts for formic acid dehydrogenation [22,23], and there have been a number of publications reporting excellent catalytic activity [24e49]. To date, the optimal catalyst for additive-free formic acid dehydrogenation at room temperature is considered to be the Pd supported on amine functionalized silica because O-H bonding of formic acid is broken by basic eNH2 groups [25]. In the catalyst system, a strong metal-support interaction (SMSI) between amine-functionalized silica supports and active Pd nanoparticles is one of important factors to determine the catalytic activity of additive-free formic acid dehydrogenation at room temperature [50,51]. In this context, it is necessary to investigate the effect of amine functional groups on catalytic activity. However, there are few reports on the effect of the content of amine functional groups on the catalytic performance of the Pd catalysts supported on amine functionalized silica. In this study, we tried to maximize the content of aminopropyl groups on the surface of mesoporous silica supports (KIE-6) via hydroxylation of KIE-6 surface before amine functionalization and investigated the effect of the content of amine functional groups on the catalytic activity and stability for formic acid dehydrogenation by comparing the Pd catalysts supported on hydroxylated and amine functionalized KIE-6 with the Pd catalysts supported on amine functionalized KIE-6.
Experimental methods Preparation of KIE-6 A silica sol having a diameter of about 5 nm was synthesized through a base-catalyzed hydrolysis and condensation reaction of tetraethyl orthosilicate (TEOS: Aldrich 98%). A mixture solution of NH3 and H2O was added to a mixture solution of TEOS and EtOH under vigorous stirring at 50 C. The molar ratio of TEOS: NH3: H2O: EtOH was 1:0.084:53.6:40.7. After the
final mixture was refluxed for 3 h at 50 C, colloidal silica sol with particle size of about 5 nm was successfully synthesized. In order to obtain mesoporous silica (KIE-6), 4 g of glycerol and sulfuric acid (10 wt% of glycerol) were added to 30 mL of the as-prepared colloidal silica sol. The mixture solution was dried at 150 C for 24 h in air for solvent removal and precarbonization of glycerol, resulting in the nanocomposite of silica nanoparticles and precarbonized glycerol. After the nanocomposite was calcined at 550 C for 2 h in air, the mesoporous silica KIE-6 was obtained.
Hydroxylation of KIE-6 surface To maximize the amine functionalization of KIE-6 surface, we conducted hydroxylation of KIE-6 surface prior to amine functionalization of KIE-6 surface. In a typical synthesis, 1 g of mesoporous silica (KIE-6) was added into 67 mL of 17.7 wt% hydrochloric acid aqueous solution and the mixture solution was stirred for 12 h. After washing three times with distilled water and drying at room temperature, we obtained hydroxylated KIE-6 (OH-KIE-6).
Preparation of catalyst supports via aminefunctionalization of KIE-6 and OH-KIE-6 surface In a typical synthesis, 1.5 g of KIE-6 or OH-KIE-6 was added into 150 mL of toluene solution, followed by the addition of 3.75 mL of 3-aminopropyl trimethoxysilane (APTMS). The final mixture solution was refluxed at 110 C for 3 h without stirring and then filtered and washed repeatedly with toluene for removal of unreacted APTMS. After drying the sample at room temperature for 24 h, the amine-functionalized KIE-6 (NH2KIE-6) or amine-functionalized OH-KIE-6 (NH2-OH-KIE-6) was prepared.
Preparation of Pd catalysts for formic acid dehydrogenation NH2-KIE-6 and NH2-OH-KIE-6 were used as a catalyst support for preparation of Pd catalysts for formic acid dehydrogenation. In a typical synthesis, 0.095 g of 10 wt% palladium (II) nitrate aqueous solution (PM RESEARCH) was added into 10 mL of distilled water and then 0.18 g NH2-KIE-6 or NH2-OHKIE-6 was added. Afterward, 2 mL of 0.85 M NaBH4 was added into the solution and was stirred for 1 h. After the solution was centrifuged, washed, and was dried, Pd/NH2-KIE-6 and Pd/ NH2-OH-KIE-6 catalysts were prepared.
Additive-free formic acid dehydrogenation at roomtemperature We used the Teflon-lined stainless steel batch reactor connected with the gas burette system filled with water. 0.055 g of catalyst was located in the batch reactor at room temperature and then purged with nitrogen for 30 min. Afterward, a mixture of 0.19 mL of formic acid (95%, Aldrich) and 10 mL of distilled water was injected through a rubber septum to initiate the reaction. After the first test of additive-free formic acid dehydrogenation at room temperature, the catalyst was separated from reaction solution by centrifugation, and
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washed, then dried at room temperature. The dried catalyst used again in the additive-free formic acid dehydrogenation at room temperature.
Characterization For characterization of the KIE-6, OH-KIE-6, NH2-KIE-6, and NH2-OH-KIE-6 supports, nitrogen sorption isotherms, fourier transform infrared (FTIR), thermogravimetric analysis (TGA), elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were conducted. Nitrogen sorption isotherms of materials were obtained using a Micromeritics ASAP 2420 instrument, after the degassing of samples was carried out at 200 C for 5 h. FTIR spectra were obtained with a Thermo Nicolet 5700 instrument. Thermogravimetric analysis (TGA) and Elemental analysis (EA) were conducted on a TGA Q500 and a Thermo Scientific FLASH EA2000 instrument, respectively. XPS and HAADF-STEM analyses were performed using a Kratos 165XP and a FEI/TECNAI G5 instrument. The composition of product gas was confirmed by gas chromatography (Agilent 6890) with a carbon 1010 PLOT fused silica capillary column (30 m 0.53 mm, SUPELCO) and thermal conductivity detector (TCD).
Results and discussion Amine functionalization of mesoporous silica (KIE-6) and hydroxylated mesoporous silica (OH-KIE-6) supports In the previous study, we reported the control in the pore structure of mesoporous silica (KIE-6) supports using a glycerol template and the effect of the pore structure on catalytic activity for additive-free formic acid dehydrogenation at room temperature [47]. As a result, we demonstrated that the improved catalytic activity was achieved by catalyst supports having three dimensionally interconnected pore structures with large pore volume, because such pore structure provided easy diffusion of reactant molecules to the active site. In other previous publication, we reported that stirring time and types of Pd precursors influenced the particle size and dispersion of Pd nanoparticles, and high dispersion of Pd nanoparticles on the amine functionalized KIE-6 supports gave improved catalytic activity [48]. In summary, the pore structure of supports and dispersion of Pd nanoparticles are the most significant factors to determine catalytic activity of Pd/NH2-KIE-6 catalysts. In this paper, we will report the effect of the content of amine groups of amine functionalized KIE-6 (NH2-KIE-6) supports on the pore properties and dispersion of Pd nanoparticles for Pd/NH2-KIE-6 catalysts. To investigate the effect of the content of amine groups of NH2-KIE-6 supports, we prepared two different NH2-KIE-6 supports. The first one (NH2-KIE-6) is prepared by conducting amine functionalization of KIE-6 surface, and the second one (NH2-OH-KIE-6) is prepared by conducting the amine functionalization after hydroxylation of KIE-6 surface. Fig. 1 and Table 1 show nitrogen sorption results of mesoporous silica (KIE-6), hydroxylated KIE-6 (OH-KIE-6), amine-functionalized and hydroxylated and amineKIE-6 (NH2-KIE-6),
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functionalized KIE-6 (NH2-OH-KIE-6). Comparing OH-KIE-6 with KIE-6, a significant change in pore properties of KIE-6 was not observed after hydroxylation of KIE-6. However, comparing NH2-KIE-6 with KIE-6 and NH2-OH-KIE-6 with OHKIE-6, the pore properties of KIE-6 and OH-KIE-6 decreased after amine functionalization. Especially, in the case of OHKIE-6 supports, amine functionalization led to a considerable decrease in pore properties. Fig. 2 shows fourier transform infrared (FTIR) spectra for NH2-KIE-6 and NH2-OH-KIE-6. The peaks at 786e793 and 1023e1038 cm1 for NH2-KIE-6 and NH2-OH-KIE-6 correspond to symmetric and asymmetric stretching vibration of Si-O-Si. Furthermore, the bands at 1559 and 2936 cm1 for NH2-KIE-6, and 1560 and 2932 cm1 for NH2-OH-KIE-6 were assigned to NH bending and C-H stretching vibration. On the basis of Fig. 2, it was confirmed that amino propyl groups were successfully functionalized on surface of KIE-6 and OH-KIE-6 supports. In addition, the intensity of the peaks at 1560 and 2932 cm1 for NH2-OH-KIE-6 is much higher than that for NH2-KIE-6, indicating that NH2-OH-KIE-6 has more amino propyl groups on the surface than NH2-KIE-6. To estimate the amount of amino propyl groups attached to KIE-6 and OH-KIE-6 supports, we also conducted thermal gravimetric analysis (TGA) of NH2-KIE-6 and NH2-OH-KIE-6. Fig. 3 shows thermal gravimetric analysis (TGA) and differential thermogravimetric (DTG) curves of NH2-KIE-6 and NH2OH-KIE-6. The main two peaks were observed below 110 C and in the range of 300e650 C. The first peak below 110 C is attributed to desorption of the physically adsorbed water, and the second peak is assigned to decomposition of amino propyl groups. NH2-OH-KIE-6 showed 17.1% of weight loss, while NH2-KIE-6 showed 7.4% of weight loss. It is because amine functionalization after hydroxylation of KIE-6 surface resulted in a significant increase in amino propyl groups on NH2-OHKIE-6. As amino propyl groups are functionalized on the KIE-6 surface by condensation between ethoxy groups of APTMS and hydroxyl groups of KIE-6 surface, more hydroxyl groups on KIE-6 surface lead to more amino propyl groups functionalized on KIE-6 surface. To confirm the increased amount of amino propyl groups on NH2-OH-KIE-6, elemental analysis (EA) was conducted. Table 2 shows elemental analysis data for KIE-6, NH2-KIE-6, OH-KIE-6 and NH2-OH-KIE-6. Comparing OH-KIE-6 with KIE-6, the content of hydrogen for OH-KIE-6 increased because of hydroxylation on KIE-6 surface. In the case of NH2-KIE-6 and NH2-OH-KIE-6, carbon and nitrogen were detected because of functionalization of amino propyl groups on KIE-6 and OH-KIE-6 surface. However, the content of carbon and nitrogen for NH2-OH-KIE-6 is about 1.7 times higher than that for NH2-KIE-6. On the basis of Figs. 2 and 3 and Table 2, it was demonstrated that the content of amino propyl groups functionalized on KIE-6 surface significantly increased by hydroxylation of KIE-6 surface before amino propyl functionalization.
The effect of the content of amino propyl groups on catalytic activity and stability for additive-free formic acid dehydrogenation at room-temperature We investigated the effect of the content of amino propyl groups on catalytic activity and stability by using NH2-KIE-6
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Fig. 1 e Nitrogen sorption results of mesoporous silica (KIE-6), hydroxylated KIE-6 (OH-KIE-6), amine-functionalized KIE-6 (NH2-KIE-6), and hydroxylated and amine-functionalized KIE-6 (NH2-OH-KIE-6). (a) isotherms (b) BJH desorption pore size distributions.
Table 1 e Pore properties for KIE-6, OH-KIE-6, NH2-KIE-6 and NH2-OH-KIE-6 obtained from nitrogen sorption tests. Materials KIE-6 OH-KIE-6 NH2-KIE-6 NH2-OH-KIE-6 a b c
d
SABET [m2/g]a
SAmicro [m2/g]b
Vtot [cm3/g]c
D [nm]d
405 432 162 15
10.8 22.0 7.5 3.2
0.83 0.86 0.37 0.05
7.2 7.4 7.0 8.79
BET surface area. Micropore surface area calculated from a t-plot. Total pore volume taken from the volume of nitrogen adsorbed at P/Po ¼ 0.995. BJH desorption average pore diameter.
and NH2-OH-KIE-6 as a catalyst support for additive-free formic acid dehydrogenation at room temperature. Fig. 4 shows reusability test results for formic acid dehydrogenation over Pd (5 wt%)/NH2-KIE-6 and Pd (5 wt%)/NH2-OH-KIE-6. The total volume of product gas (H2 and CO2) at 70 min and turnover frequency (TOF) at initial 10 min and 25 C for Pd (5 wt%)/NH2OH-KIE-6 in 1st run were 200 mL and 595 mol H2 mol catalyst1 h1, which were almost 1.4 times higher than that for Pd (5 wt %)/NH2-KIE-6 catalyst (170 mL and 419 mol H2 mol catalyst1
h1). However, The total volume of product gas (H2 and CO2) at 70 min and turnover frequency (TOF) at initial 10 min and 25 C for Pd (5 wt%)/NH2-KIE-6 in 4th run were 154 mL and 310 mol H2 mol catalyst1h1, which were similar that Pd (5 wt %)/NH2-OH-KIE-6 catalyst (152 mL and 300 mol H2 mol catalyst1h1). After 5 th run, the catalytic activity of Pd (5 wt%)/NH2-KIE-6 was higher than Pd (5 wt%)/NH2-OH-KIE-6. Fig. 5 shows variation of catalytic activity with an increase in the number of reusing for catalysts. Initially, the conversion and TOF of Pd (5 wt%)/NH2-OH-KIE-6 catalysts were much higher than those of Pd (5 wt%)/NH2-KIE-6 catalysts. However, the decrease in the conversion and TOF for Pd (5 wt%)/NH2OH-KIE-6 was much larger than that for Pd (5 wt%)/NH2-KIE-6, as number of reusability increased. On the basis of Figs. 4 and 5, it was confirmed that the increase in the content of amino propyl groups on catalyst supports can enhance the initial catalytic activity, however it significantly decreases the catalytic stability.
Correlation between catalytic activity and the content of amino propyl groups on catalyst supports To investigate the correlation between the catalytic stability and amino propyl groups on catalyst supports, we carried out
Fig. 2 e FT-IR spectra of (a) amine-functionalized KIE-6 (b) amine-functionalized OH-KIE-6.
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Fig. 3 e TGA and DTG curves of (a) amine-functionalized KIE-6 (b) amine-functionalized OH-KIE-6.
Table 2 e Elemental analysis (EA) data. Materials KIE-6 NH2-KIE-6 OH-KIE-6 NH2-OH-KIE-6
Carbon [%]
Hydrogen [%]
Nitrogen [%]
e 9.2 e 15.2
0.6 2.4 0.7 3.7
e 3.2 e 5.4
X-ray photoelectron spectroscopy (XPS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analyses for the catalysts. Fig. 6 presents XPS (Pd 3d5/2 and Pd 3d3/2) spectra for Pd (5 wt%)/NH2KIE-6 and Pd (5 wt%)/NH2-OH-KIE-6 catalysts. The peaks at 335.4e335.5 eV and 340.5e340.9 eV correspond to 3d5/2 and 3d3/2 components for zero-valence metallic state of Pd nanoparticles and the peaks at 337.4e337.6 eV and 342.4e342.8 eV correspond to 3d5/2 and 3d3/2 components for PdO. The intensity ratio of Pd peaks and PdO peaks for Pd (5 wt%)/NH2OH-KIE-6 catalysts was higher than that for Pd (5 wt%)/NH2KIE-6 catalysts, indicating that Pd (5 wt%)/NH2-OH-KIE-6 catalysts have more Pd nanoparticles than Pd (5 wt%)/NH2-KIE-6 catalysts before the reaction. The increased content of amine groups of NH2-OH-KIE-6 supports promoted the reduction of Pd precursors. This is the reason why initial catalytic activity of Pd (5 wt%)/NH2-OH-KIE-6 catalysts is much higher than that of Pd (5 wt%)/NH2-KIE-6 catalysts.
Fig. 5 e Variation of catalytic activity with an increase in the reusability number of catalysts.
Fig. 7 shows STEM results of Pd/NH2-KIE-6 and Pd/NH2-OHKIE-6 catalysts. Pd nanoparticle size after reusability test of Pd/NH2-KIE-6 and Pd/NH2-OH-KIE-6 catalysts increased from 2.30 to 2.47 nm and from 2.41 to 3.48 nm, respectively. On the basis of Figs. 4, 5 and 7, it was found that decrease of the catalytic activity for reusability test shown in Figs. 4 and 5 was attributed to the increase of Pd nanoparticle size. Moreover, as the increase in Pd nanoparticle size for Pd (5 wt%)/NH2-OHKIE-6 catalysts was larger than that for Pd (5 wt%)/NH2-KIE-6
Fig. 4 e Reusability test results for formic acid dehydrogenation over (a) Pd (5 wt%)/NH2-KIE-6 and (b) Pd (5 wt%)/NH2-OH-KIE6.
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Fig. 6 e XPS spectra of catalysts. (a) Pd (5 wt%)/NH2-KIE-6 (b) Pd (5 wt%)/NH2-OH-KIE-6.
Fig. 7 e STEM images of catalysts. (a) Pd (5 wt%)/NH2-KIE-6 catalyst before reusability (b) Pd (5 wt%)/NH2-KIE-6 catalyst after 10 times reusability (c) Pd (5 wt%)/NH2-OH-KIE-6 catalyst before reusability (d) Pd (5 wt%)/NH2-OH-KIE-6 catalyst after 10 times reusability.
catalysts (Fig. 7), the decrease in the catalytic activity for Pd (5 wt%)/NH2-OH-KIE-6 was much larger than that for Pd (5 wt %)/NH2-KIE-6 (Figs. 4 and 5). Meanwhile, as shown in Fig. 1 and Table 1, NH2-OH-KIE-6 showed the considerable decrease in surface area and pore volume after amine functionalization in comparison with NH2-KIE-6. Such low surface area of NH2-
OH-KIE-6 led to deposition of Pd nanoparticles on external surface of supports rather than inside pores of supports. The Pd nanoparticles on external surface of NH2-OH-KIE-6 supports promoted the increase in Pd particle size through aggregation, which resulted in lower catalytic stability of Pd (5 wt %)/NH2-OH-KIE-6 catalysts than Pd (5 wt%)/NH2-KIE-6
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catalysts. Thus it was concluded that the increased content of amine groups of supports can promote the reduction of Pd precursors and enhance initial catalytic activity, whereas the increased content of amine groups of supports leads to the low surface area of supports and decreases catalytic stability through easy aggregation of Pd nanoparticles. From the viewpoint of catalytic stability, the use of KIE-6 supports without hydroxylation is suitable for additive-free formic acid dehydrogenation over Pd/NH2-KIE-6 catalysts.
Conclusions To investigate correlation between an increase in the content of amine groups and catalytic activity and stability in additivefree formic acid dehydrogenation at room-temperature, we prepared two different supports of NH2-KIE-6 and NH2-OHKIE-6. The NH2-KIE-6 is prepared by conducting amine functionalization of KIE-6 surface, and the NH2-OH-KIE-6 is prepared by conducting the amine functionalization after hydroxylation of KIE-6 surface. As a result, the content of amino propyl groups functionalized on OH-KIE-6 surface significantly increased by hydroxylation of KIE-6 surface before amino propyl functionalization. However, the NH2-OHKIE-6 showed the considerable decrease in surface area and pore volume after amine functionalization in comparison with NH2-KIE-6. The increased content of amine groups of NH2-OH-KIE-6 supports provided higher ratio of Pd/PdO, so that initial catalytic activity of Pd/NH2-OH-KIE-6 catalysts was much higher than that of Pd/NH2-KIE-6 catalysts. In contrast, the low surface area of NH2-OH-KIE-6 promoted the aggregation of Pd nanoparticles on Pd/NH2-OH-KIE-6 catalysts, which resulted in lower catalytic stability of Pd/NH2-OH-KIE-6 catalysts than Pd/NH2-KIE-6 catalysts. Thus it was confirmed that the increased content of amine groups of supports can enhance the initial catalytic activity by the higher ratio of Pd/ PdO, whereas it can decrease the catalytic stability because of aggregation of Pd nanoparticles derived from the low surface area of supports.
Acknowledgements This work was supported by a research program (B7-2461-03) of the Korea Institute of Energy Research (KIER).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.12.208.
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