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without PVP for H2 selectivity enhancement over CO2 and N2 gases
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Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases Rajesh Kumar a, Kamakshi b, Shivani Shisodia a, Manoj Kumar a, Kamlendra Awasthi a,* a b
Department of Physics, Malaviya National Institute of Technology, Jaipur, Rajasthan, 302017, India Department of Physics, Banasthali University, Banasthali, Rajasthan, 304022, India
article info
abstract
Article history:
The hydrogen-based economy is one of the possible approaches toward to eliminate the
Received 31 January 2018
problem of global warming, which are increases because of the gathering of greenhouse gases.
Received in revised form
Palladium (Pd) is well-known material having a strong affinity to the hydrogen absorbing
6 June 2018
property and thus appropriate material to embed in the membrane for the improvement of
Accepted 14 June 2018
selective permeation of hydrogen gas. In present work, we have functionalized polycarbonate
Available online xxx
(PC) membranes with the help of UV irradiation to embed the Pd nanoparticles in pores as well as on the surface of the PC membrane. Use of Pd Nanoparticles is helpful to enhance the H2
Keywords:
selectivity over other gases (CO2, N2, etc.). Also, the UV based modification of membrane in-
Hydrogen selectivity
creases the attachment of Pd Nanoparticles. Further to enhance the Pd nanoparticles attach-
Functionalization
ment, we used PVP binder with Pd nanoparticles solution. Gas permeability measurements of
Pd nanoparticles
functionalized PC membranes have been carried out, and better selectivity of hydrogen has
PC membrane
been found in the functionalized and Pd nanoparticle binded membrane. PC membrane with
Gas permeability
48 h UV irradiated and Pd NPs with PVP have been found to have maximum selectivity and permeability for H2 gas. All the samples being characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and UVeVis spectroscopy for their morphological and structural investigation. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The supreme dominating energy sources are the fossil fuels, but they are suffering from the petroleum crises as well as environmental scarcity. To overcome this subject, it is necessary to explore new sustainable clean energy sources. Hydrogen energy will be a suitable option for the future. The main advantage of using hydrogen as an energy source is a “green fuel source” which does not damage the ecological system [1e3]. Hydrogen energy is not only the clean energy
source but also the offshoot in the form of pure water. So to utilise hydrogen as an energy source and prevent the environment from the global warming, it is required to purify hydrogen gas from the other gases and impurities [4,5]. The purity of the hydrogen is a leading parameter to decide the efficiency of the system which is based on the hydrogen energy such as hydrogen fuel cell. New materials and methods are being developed for the hydrogen purification/separation but the membrane-based separation process is the advanced, reliable and low-cost with better results [6,7].
* Corresponding author. E-mail address: [email protected] (K. Awasthi). https://doi.org/10.1016/j.ijhydene.2018.06.094 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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The membranes have a multiplicity of applications such as separation processes for liquid and gaseous mixtures, catalysis, biomedical applications, etc. Particularly for the gas separation application, membrane has an advantage like quick mass transfer rate and selectivity, efficiency, low cost, environmentfriendly, easy cleaning process [6,8,9]. In the field of membrane separation, many methodologies have been reported in the literature such as blend polymer [10e12], CNT composites [13e15], zeolite composites [16,17], graphene composites [18e20], nanoparticles composites [21e24], track-etched membranes [25e30], block-copolymers [31,32], functionalized membranes [26,33] etc. Despite several advantages, these membranes suffer from a problem in the from trade-off relationship between selectivity and permeability. Functionalization of the membrane is a technique to alter the polymeric membranes surface and molecular structure by providing the direct or indirect energy source. Functionalization of the membrane and use of gas sensitive nanoparticles is a noble strategy in the sense of synthesis process, reproducibility, permeability and higher selectivity for a particular gas. There are various functionalization methods for polymeric surface, such as chemical [25], plasma [23,34], UV irradiation [35,36], ion irradiation [37] etc. Out of these functionalization approaches, UV irradiation is one of the best approach due to its simplicity and controlled surface modification. For nanoparticle deposition in functionalized nanochannels, we selected Palladium (Pd) nanoparticles [38,39] over other nanoparticles like Platinum [40], Nickel [41], Titanium [42] And Metal Alloys [43e45] etc. due to maximum absorption property of Pd for H2 Gas and it can be reversed by the pressure/temperature gradient. H2/He selectivity of 2.2 was reported by Masakoto Kanezashi et al. [46] with PdeSiO2 layer of thickness 300 nm at a higher temperature. Satisfactory improvement in selectivity of H2/N2 was also observed by Xiaojuan Hu et al. [47] by using Pd/Pencil/ Al2O3 membrane with a Pd thickness of 5 mm. S. Simon et al. [48] used Polyetherimide/Palladium composite membranes for the H2/ CO2 selectivity and found nearly 3. So, the outstanding interaction of the Pd with H2 molecule give as a direction to use Pd Nanoparticles for the H2 separation by using polymeric membranes. In present manuscript, functionalization of track-etched PC membranes have been carried out via UV-irradiation at different time, and its effect on gas permeability and selectivity of H2 over CO2 and N2 gases has been reported. Further, to enhance H2 selectivity, Pd nanoparticles have deposited into the pores as well as on the surface. We have also used PVP as a binder for the Pd nanoparticles to improve the H2 selectivity. These membranes show improved permeability and selectivity and can be used for H2 separation. This method gives an opportunity to use these membranes with easy fabrication process and low cost for commercial use. The process followed for this study is briefly represented by the schematic diagram as shown in Fig. 1.
Materials and methods Materials Track-etched polycarbonate (PC) membranes having regular pore diameter of 0.1 mm purchased from Whatman
International Ltd. USA. For the Pd nanoparticles synthesis, we used different chemicals such as Palladium chloride (PdCl2) Trisodium citrate (Na3C6H5O7), Sodium borohydride (NaBH4), Sulfuric acid (H2SO4) from Sigma-Aldrich, India and Potassium permanganate (KMnO4), Hydrochloric acid (HCl), Ethanol (C2H6O) from Merck Millipore, India. Other chemical Polyvinylpyrrolidone (PVP) from Loba Chemicals, India and PVDF syringe filter of 0.2 mm from GE Healthcare, USA. All the chemicals and products are used without any alteration.
Synthesis of Pd nanoparticles Chemical method was followed for the preparation of Pd nanoparticles. We have followed the similar method for preparation of Pd nanoparticles as reported earlier [13,33].
UV irradiation and Pd nanoparticles deposition For the time-based UV irradiation process, we used G8T5 UV lamp of 25-W power. The wavelength of the UV lamp is approximately 253 nm, and this wavelength belongs from the C region of the UV spectra. For the Pd nanoparticles deposition, UV irradiated membranes kept in the Pd solution for fixed time and volume. And during this time, it was also cross-checked that the deposition of particles was equal from both sides of the sample in the solution. For using PVP as binder for Pd nanoparticles, we used 0.001 gm/ml solution of PVP in Pd nanoparticles solution. The samples are listed in Table 1 based on the UV irradiation, Pd nanoparticles deposition time and with or without PVP.
Permeability measurements For the gas permeability measurements, we used the gas permeability setup, and details of set up have been reported earlier [13]. Gas permeability for all the samples has calculated by equation (1), which is based on the Fick's law. To minimize the error, the gas permeability data has been taken 6 times and an average of gas permeability data has been given for every sample. The pressure range during the experiment was lies between the 5e10 psi. P¼
K*d*s Dp*t
(1)
All the symbols in the equation have their common significance, K is cell constant of the setup, thickness of the sample membrane is defined by d and S used for displacement of mercury slug with in U-tube of gas permeability setup, Dp denote pressure difference across the sample and t is the time taken by Hg slug for the specific distance of the Hg slug.
Membrane characterization Surface morphology of functionalized membranes and Pd nanoparticles deposition within pore as well as on surface were studied by the Field Emission scanning electron microscopy (FESEM) (Nova Nano FE-SEM 450) at the accelerating voltage of 15 kV. To analyze, the characteristic bonds of PC and the structural changes due to functionalization and Pd nanoparticle deposition, Fourier Transform Infrared
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 1 e Schematic diagram of process Gas permeation through (a) the pristine PC membrane (b) UV irradiated PC membrane and (c) Pd Nanoparticles deposited in UV irradiated PC membrane.
Table 1 e Sample representation (S0eS6) based on the UV irradiation time and Pd nanoparticles deposition time. Membrane (pore size) PC PC PC PC PC PC PC
(0.1 (0.1 (0.1 (0.1 (0.1 (0.1 (0.1
mm) mm) mm) mm) mm) mm) mm)
UV irradiation time 0 36 h 36 h 36 h 48 h 48 h 48 h
Pd nanoparticles deposition time (with or without PVP) 0 0 24 24 0 24 24
h h (with PVP) h h (with PVP)
spectroscopy (FTIR) was used in the range of 400 cm1 4000 cm1, with the help of a Spectrum 2 Perkin Elmer setup. Raman scattering spectra were recorded on a AIRIX STR 500 Raman spectroscopy having an excitation source laser of 785 nm and power intensity of 50 mW. Carbonate bond cleavage confirmed by the UVeVis spectra by using the Agilent Technologies carry 60 UVeVis setup in the range of 300e900 nm.
Results and discussion Electronic energy of the polymer molecule changes when the polymer irradiated in the UV region [49]. These electronic changes outcome in the form of physical and chemical changes. UV region has sufficient energy that it can split the molecules bonds of the polymer and be able to create free radicals. In our case, when PC is irradiated by UV radiation, photo-degradation (bond splitting) phenomena arises. Photodegradation can be understood by photo-fries rearrangement mechanism, well reported in the literature [35,36]. Basically, UV Irradiation affects the CeO bond of the carbonate group in the PC molecular structure. Carbonate group selectively absorbed the energy in the UV region and starts the chain scission process and in parallel, the process of the crosslinking continued by the small molecules. The completion of the process is the formation of the phenyl free radicals. These free radicals are helpful in the improvement of hydrogen selectivity over other gases.
Sample name
Sample Representation
PC_0.1 PC_0.1_UV_36 PC_0.1_UV_36_Pd_24 PC_0.1_UV_36_Pd_PVP_24 PC_0.1_UV_48 PC_0.1_UV_48_Pd_24 PC_0.1_UV_48_Pd_PVP_24
S0 S1 S2 S3 S4 S5 S6
UVeVis spectroscopy The UVeVis absorption spectroscopy measurements were done in the range of 200e900 nm for all the samples (S0eS6). For the samples (S1, S2, S3), which were irradiated for 36 h by UV having same spectra and similarly for the samples (S4, S5, S6) getting same spectra having the irradiation time of 48 h. So we are comparing the fitted data of UVeVis spectra for the unirradiated S0 and irradiated samples S1 & S4 as shown in Fig. 2. Two shoulder bands nearly 365 nm and 320 nm can be clearly seen in Fig. 2. The presence of the both bands can be interpret by the photo-fries mechanism of the PC. These two bands most probably due to the p e p* (~320 nm) and n e p* (~365 nm) transmission of carbonyl group respectively. The intensity of both the bands increases with the UV-irradiation time. This intensity increment of the bands is a clear sign of higher bond-cleavage of carbonate group with UV-irradiation time.
Raman spectroscopy Fig. 3 shows the Raman spectra of all the samples in the range of 500e2000 cm1 and all fundamental peaks of polycarbonate are present and can be seen in Fig. 3. But there was small shift in the Raman frequencies of pristine PC membrane (S0) and UV-irradiated PC membrane (S1eS6). These shifts are due to change in the structure of surfaces for UV-irradiated membrane as compared to pristine. All the observed peak frequencies and corresponding bonds
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Table 2 e Raman Peaks and corresponding bonds. Wavenumber (cm1) 574, 634 703,731 826 886 918, 1109, 1146, 1177, 1234 1005 1387,1441, 1462 1601 1773
Fig. 2 e UVeVis spectra for the sample S0, S1 and S4 in the range of 300e700 nm, those are resembled to the Pristine PC, UV irradiated PC for 36 h and UV irradiated PC for 48 h respectively.
are presented in Table 2. These shift or changes in frequency also show that the UV irradiation has small effect on Raman frequencies in the range of 2000e4000 cm1. If we enlarge the region 2750-3150 cm1 as shown in Fig. 4, we found that peaks at 2937 cm1 and 3071 cm1 shows intensity growth after the UV irradiation samples (S1eS6) as a result of the photo-fries process on the PC surface.
Fourier Transform Infrared spectroscopy (FTIR) Fig. 5 shows the FTIR spectrum of all samples (S0eS6) in the range of 400e4000 cm1 and have all the peaks corresponding to elementary bonds of polycarbonate [50,51]. For the analysis samples (S1eS6) are compared with the base sample of polycarbonate S0. There was no remarkable shift in the spectra (400-4000 cm1), but peaks related to carbonated group shows
Type of Vibration Phenyl ring vibration CeH out-of-plane bending Phenyl ring vibration OeC(O)eO stretching CeOeC stretching CeH bending in-plane CH3 deformation Phenyl ring vibration C¼O stretching
intensity variation. Peak 2965 cm1 is corresponding to CeH3 stretching vibration, 1768 cm1 corresponding to free carbonyl stretching motion and 1168 cm1 represents CeO stretching (carbonyl group). When the UV irradiation time of samples is increased, a slight increment has been observed for both the peaks at 1768 cm1 and 1168 cm1. These peaks are corresponding to carbonyl group and the increment is due to enhancement in stretching of the CeO bond. Also, absorption peak around 2965 cm1 associated with CeH stretching arises due to UV irradiation. It means that when the samples have been exposed to UV light, then bond formation takes place due to the photo-fries mechanism. UV light generate the changes in the PC surface which is responsible for the variation in carbonate (1780 cm1) to phenylene (1520 cm1) intensity ratio.
Scanning electron microscopy (SEM) The morphological study of the samples was done with the help of scanning electron microscopy (SEM) and observed SEM images are shown in Fig. 6. From the SEM images, it's clear that the average pore size of all the sample is ~0.1 mm. These membranes were dipped in Pd nanoparticle solution and there is very less possibility to cover the pore by the Pd nanoparticles because the Pd nanoparticle size in the range of 6 ± 1 nm [13] and pore size is 0.1 mm. In the SEM images, bright
Fig. 3 e Raman spectra of all the samples (S0eS6) in the range of 500e200 cm¡1. Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 4 e Raman spectra of all the samples (S0eS6) in the range of 2750e3150 cm¡1.
Fig. 5 e Fourier-transform infrared spectroscopy (FTIR) spectra of the all the samples (S0eS6) in the range of 400e3100 cm¡1.
points (yellowish) indicates the number of Pd nanoparticles on the surface as well as in pores. In Fig. 6(C), it can be seen that the Pd nanoparticles are in the pores as well as on the surface too. But the clustering of Pd nanoparticles on surface of membranes as well as in pores can be controlled by using the PVP as binder. From Fig. 6(d) it can observe that when PVP is used with Pd nanoparticles, there was minimum clustering on the surface and as a result the surface as well as the pores boundary have enhanced brightness in sample S6 with uniform distribution of nanoparticles. Energy-dispersive X-ray spectroscopy (EDS) spectra of sample S2 also confirms the existence of Palladium (supporting data). Using PVP binder with Pd nanoparticles is an effective way to have maximum attachment of nanoparticles on polymer surface as well as in pores.
Permeability and selectivity For the gas separation application, we used H2, N2 and CO2 gases to examine the gas permeability and selectivity for all the samples. The obtained data of the gas permeability are shown in Figs. 7 and 8. As evident from the gas permeability measurements, H2 shows the maximum permeability not only due to smallest molecular size in comparison to other two gases (N2 & CO2) but also other factors like gas molecule interaction with surface, action energy and diffusion rate in the polymeric medium and all these factors are favourable for the higher permeation of H2 gas. The spherical shape of H2 molecules also helps in the higher permeability rate then N2 and CO2. In case of CO2 and N2, CO2 has smaller molecular (3.30 A) size than N2 (3.64 A). But because of the linear shape
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 6 e Scanning electron microscopy (SEM) of the sample (a) Pristine PC (S0) (b) UV irradiated for 48 h (S4) (c) UV irradiated for 48 h with deposition of Pd Nanoparticles for 24 h (S5) (d) UV irradiated for 48 h with deposition of Pd nanoparticles.
Fig. 7 e Graph of the gas permeability data for the samples (S0eS3). S0 is for the without UV irradiation pristine PC and S1e S3 samples having the UV irradiation time of 36 h.
gas molecule of CO2, it requires more activation energy for diffusion through porous membrane than N2 gas molecules, which leads CO2 to lower permeation in comparision to N2. UV-irradiation process of membrane increases the number of free radicals/group on the surface as well as on pore inner walls. These free group are ready to accept the hydrogen molecules according to the functionalize mechanism. In the presence of pressure gradient of hydrogen, group donates or transport hydrogen to the next group or molecule of polymer in the low concentration zone of H2. Such action of these free
group increases the permeability of hydrogen. But for N2 and CO2 these group works as obstacle. So, there was not effective increment or decrement found in the CO2 and N2 permeability in all the samples (Figs. 7 and 8). In case of UV irradiated sample S1 and S4, the selectivity H2/N2 and H2/CO2 is higher (Fig. 9) in the S4 because of the high UV-irradiation time (48 h). In the second step, we deposited Pd nanoparticles in the UVirradiated samples (S2, S5) without PVP and (S3, S6) with PVP. Pd nanoparticles have supreme absorption property of hydrogen due to its lattice structure. In the presence of H2 gas
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 8 e Graph of the gas permeability data for the samples (S0, S4eS6). S0 is for the without UV irradiation pristine PC and S4eS6 samples having the UV irradiation time of 48 h.
Fig. 9 e Graph of selectivity data of H2 over CO2 and N2 for all the samples (S0eS6). medium, Pd shows two types of interaction; a and b crystalline phases. With low concentration of hydrogen, Pd shows a phase and b phases. In both phases, lattice have the face centered cubic (FCC) structure with different lattice unit length. The octahedral void of the FCC lattice is perfect place to grip the H2 because of its smaller molecular size. The absorption phenomena also reversible, means diffusion of H2 through lattice will be higher if there was temperature/pressure gradient is available. So, in our case we use pressure gradient, that's why Pd deposited samples shows the higher permeability and selectivity of hydrogen. The UV irradiation of the samples improves the attachment of Pd nanoparticles with membrane in comparison to pristine membrane. More attachment of Pd nanoparticles within PC membrane is responsible for higher permeability and selectivity of H2. PVP helps in increasing the numbers of Pd nanoparticles on surface as well as in pores of PC membranes. Membranes having 48 h UV irradiation time and higher number of Pd nanoparticles (sample S6) show the highest permeability of H2 and also the maximum selectivity for H2/N2 and H2/CO2, 3.43 and 3.92 respectively.
Conclusions The permeability and selectivity of the gases through the polymeric membrane can be controlled by the selective deposition of nanoparticles and the UV irradiation process. It was concluded from above results that with increment in UV irradiation time the attachment of Pd nanoparticles have enhanced. For better attachment of the nanoparticles, PVP binder has an important role for making highly permeable and selective membrane for hydrogen. Maximum selectivity was found 3.43 and 3.92 for H2/N2 and H2/CO2 respectively. So, we have faith in the fact that such kind of the membrane has potential use in gas separation and purification.
Acknowledgements The authors acknowledge the financial support received from SERB, New Delhi for ECR Award (ECR/2016/001780) and MRC, MNIT Jaipur for characterization facilities.
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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[39] Rahimpour MR, Samimi F, Babapoor A, Tohidian T, Mohebi S. Palladium membranes applications in reaction systems for hydrogen separation and purification: a review. Chem Eng Process: Process Intensification 2017;121:24e49. [40] Sankir M, Semiz L, Serin RB, Sankir ND. Hydrogen generation from nanoflower platinum films. Int J Hydrogen Energy 2015;40:8522e9. [41] Wang M, Song J, Wu X, Tan X, Meng B, Liu S. Metallic nickel hollow fiber membranes for hydrogen separation at high temperatures. J Membr Sci 2016;509:156e63. [42] Acharya NK, Kulshrestha V, Awasthi K, Kumar R, Jain AK, Singh M, et al. Gas permeation study of Ti-coated, tracketched polymeric membranes. Vacuum 2006;81:389e93. [43] Lee SM, Xu N, Kim SS, Li A, Grace JR, Lim CJ, et al. Palladium/ ruthenium composite membrane for hydrogen separation from the off-gas of solar cell production via chemical vapor deposition. J Membr Sci 2017;541:1e8. [44] Jo YS, Lee CH, Kong SY, Lee KY, Yoon CW, Nam SW, et al. Characterization of a Pd/Ta composite membrane and its application to a large scale high-purity hydrogen separation from mixed gas. Separ Purif Technol 2017;200:221e9. [45] Kumar Vijay Y, Kumar Jain A, Acharya N, Awasthi K, Kulshrestha V. Temperature dependence study of gas permeability in metal doped composite polymeric
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Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases Rajesh Kumar a, Kamakshi b, Shivani Shisodia a, Manoj Kumar a, Kamlendra Awasthi a,* a b
Department of Physics, Malaviya National Institute of Technology, Jaipur, Rajasthan, 302017, India Department of Physics, Banasthali University, Banasthali, Rajasthan, 304022, India
article info
abstract
Article history:
The hydrogen-based economy is one of the possible approaches toward to eliminate the
Received 31 January 2018
problem of global warming, which are increases because of the gathering of greenhouse gases.
Received in revised form
Palladium (Pd) is well-known material having a strong affinity to the hydrogen absorbing
6 June 2018
property and thus appropriate material to embed in the membrane for the improvement of
Accepted 14 June 2018
selective permeation of hydrogen gas. In present work, we have functionalized polycarbonate
Available online xxx
(PC) membranes with the help of UV irradiation to embed the Pd nanoparticles in pores as well as on the surface of the PC membrane. Use of Pd Nanoparticles is helpful to enhance the H2
Keywords:
selectivity over other gases (CO2, N2, etc.). Also, the UV based modification of membrane in-
Hydrogen selectivity
creases the attachment of Pd Nanoparticles. Further to enhance the Pd nanoparticles attach-
Functionalization
ment, we used PVP binder with Pd nanoparticles solution. Gas permeability measurements of
Pd nanoparticles
functionalized PC membranes have been carried out, and better selectivity of hydrogen has
PC membrane
been found in the functionalized and Pd nanoparticle binded membrane. PC membrane with
Gas permeability
48 h UV irradiated and Pd NPs with PVP have been found to have maximum selectivity and permeability for H2 gas. All the samples being characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and UVeVis spectroscopy for their morphological and structural investigation. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The supreme dominating energy sources are the fossil fuels, but they are suffering from the petroleum crises as well as environmental scarcity. To overcome this subject, it is necessary to explore new sustainable clean energy sources. Hydrogen energy will be a suitable option for the future. The main advantage of using hydrogen as an energy source is a “green fuel source” which does not damage the ecological system [1e3]. Hydrogen energy is not only the clean energy
source but also the offshoot in the form of pure water. So to utilise hydrogen as an energy source and prevent the environment from the global warming, it is required to purify hydrogen gas from the other gases and impurities [4,5]. The purity of the hydrogen is a leading parameter to decide the efficiency of the system which is based on the hydrogen energy such as hydrogen fuel cell. New materials and methods are being developed for the hydrogen purification/separation but the membrane-based separation process is the advanced, reliable and low-cost with better results [6,7].
* Corresponding author. E-mail address: [email protected] (K. Awasthi). https://doi.org/10.1016/j.ijhydene.2018.06.094 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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The membranes have a multiplicity of applications such as separation processes for liquid and gaseous mixtures, catalysis, biomedical applications, etc. Particularly for the gas separation application, membrane has an advantage like quick mass transfer rate and selectivity, efficiency, low cost, environmentfriendly, easy cleaning process [6,8,9]. In the field of membrane separation, many methodologies have been reported in the literature such as blend polymer [10e12], CNT composites [13e15], zeolite composites [16,17], graphene composites [18e20], nanoparticles composites [21e24], track-etched membranes [25e30], block-copolymers [31,32], functionalized membranes [26,33] etc. Despite several advantages, these membranes suffer from a problem in the from trade-off relationship between selectivity and permeability. Functionalization of the membrane is a technique to alter the polymeric membranes surface and molecular structure by providing the direct or indirect energy source. Functionalization of the membrane and use of gas sensitive nanoparticles is a noble strategy in the sense of synthesis process, reproducibility, permeability and higher selectivity for a particular gas. There are various functionalization methods for polymeric surface, such as chemical [25], plasma [23,34], UV irradiation [35,36], ion irradiation [37] etc. Out of these functionalization approaches, UV irradiation is one of the best approach due to its simplicity and controlled surface modification. For nanoparticle deposition in functionalized nanochannels, we selected Palladium (Pd) nanoparticles [38,39] over other nanoparticles like Platinum [40], Nickel [41], Titanium [42] And Metal Alloys [43e45] etc. due to maximum absorption property of Pd for H2 Gas and it can be reversed by the pressure/temperature gradient. H2/He selectivity of 2.2 was reported by Masakoto Kanezashi et al. [46] with PdeSiO2 layer of thickness 300 nm at a higher temperature. Satisfactory improvement in selectivity of H2/N2 was also observed by Xiaojuan Hu et al. [47] by using Pd/Pencil/ Al2O3 membrane with a Pd thickness of 5 mm. S. Simon et al. [48] used Polyetherimide/Palladium composite membranes for the H2/ CO2 selectivity and found nearly 3. So, the outstanding interaction of the Pd with H2 molecule give as a direction to use Pd Nanoparticles for the H2 separation by using polymeric membranes. In present manuscript, functionalization of track-etched PC membranes have been carried out via UV-irradiation at different time, and its effect on gas permeability and selectivity of H2 over CO2 and N2 gases has been reported. Further, to enhance H2 selectivity, Pd nanoparticles have deposited into the pores as well as on the surface. We have also used PVP as a binder for the Pd nanoparticles to improve the H2 selectivity. These membranes show improved permeability and selectivity and can be used for H2 separation. This method gives an opportunity to use these membranes with easy fabrication process and low cost for commercial use. The process followed for this study is briefly represented by the schematic diagram as shown in Fig. 1.
Materials and methods Materials Track-etched polycarbonate (PC) membranes having regular pore diameter of 0.1 mm purchased from Whatman
International Ltd. USA. For the Pd nanoparticles synthesis, we used different chemicals such as Palladium chloride (PdCl2) Trisodium citrate (Na3C6H5O7), Sodium borohydride (NaBH4), Sulfuric acid (H2SO4) from Sigma-Aldrich, India and Potassium permanganate (KMnO4), Hydrochloric acid (HCl), Ethanol (C2H6O) from Merck Millipore, India. Other chemical Polyvinylpyrrolidone (PVP) from Loba Chemicals, India and PVDF syringe filter of 0.2 mm from GE Healthcare, USA. All the chemicals and products are used without any alteration.
Synthesis of Pd nanoparticles Chemical method was followed for the preparation of Pd nanoparticles. We have followed the similar method for preparation of Pd nanoparticles as reported earlier [13,33].
UV irradiation and Pd nanoparticles deposition For the time-based UV irradiation process, we used G8T5 UV lamp of 25-W power. The wavelength of the UV lamp is approximately 253 nm, and this wavelength belongs from the C region of the UV spectra. For the Pd nanoparticles deposition, UV irradiated membranes kept in the Pd solution for fixed time and volume. And during this time, it was also cross-checked that the deposition of particles was equal from both sides of the sample in the solution. For using PVP as binder for Pd nanoparticles, we used 0.001 gm/ml solution of PVP in Pd nanoparticles solution. The samples are listed in Table 1 based on the UV irradiation, Pd nanoparticles deposition time and with or without PVP.
Permeability measurements For the gas permeability measurements, we used the gas permeability setup, and details of set up have been reported earlier [13]. Gas permeability for all the samples has calculated by equation (1), which is based on the Fick's law. To minimize the error, the gas permeability data has been taken 6 times and an average of gas permeability data has been given for every sample. The pressure range during the experiment was lies between the 5e10 psi. P¼
K*d*s Dp*t
(1)
All the symbols in the equation have their common significance, K is cell constant of the setup, thickness of the sample membrane is defined by d and S used for displacement of mercury slug with in U-tube of gas permeability setup, Dp denote pressure difference across the sample and t is the time taken by Hg slug for the specific distance of the Hg slug.
Membrane characterization Surface morphology of functionalized membranes and Pd nanoparticles deposition within pore as well as on surface were studied by the Field Emission scanning electron microscopy (FESEM) (Nova Nano FE-SEM 450) at the accelerating voltage of 15 kV. To analyze, the characteristic bonds of PC and the structural changes due to functionalization and Pd nanoparticle deposition, Fourier Transform Infrared
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 1 e Schematic diagram of process Gas permeation through (a) the pristine PC membrane (b) UV irradiated PC membrane and (c) Pd Nanoparticles deposited in UV irradiated PC membrane.
Table 1 e Sample representation (S0eS6) based on the UV irradiation time and Pd nanoparticles deposition time. Membrane (pore size) PC PC PC PC PC PC PC
(0.1 (0.1 (0.1 (0.1 (0.1 (0.1 (0.1
mm) mm) mm) mm) mm) mm) mm)
UV irradiation time 0 36 h 36 h 36 h 48 h 48 h 48 h
Pd nanoparticles deposition time (with or without PVP) 0 0 24 24 0 24 24
h h (with PVP) h h (with PVP)
spectroscopy (FTIR) was used in the range of 400 cm1 4000 cm1, with the help of a Spectrum 2 Perkin Elmer setup. Raman scattering spectra were recorded on a AIRIX STR 500 Raman spectroscopy having an excitation source laser of 785 nm and power intensity of 50 mW. Carbonate bond cleavage confirmed by the UVeVis spectra by using the Agilent Technologies carry 60 UVeVis setup in the range of 300e900 nm.
Results and discussion Electronic energy of the polymer molecule changes when the polymer irradiated in the UV region [49]. These electronic changes outcome in the form of physical and chemical changes. UV region has sufficient energy that it can split the molecules bonds of the polymer and be able to create free radicals. In our case, when PC is irradiated by UV radiation, photo-degradation (bond splitting) phenomena arises. Photodegradation can be understood by photo-fries rearrangement mechanism, well reported in the literature [35,36]. Basically, UV Irradiation affects the CeO bond of the carbonate group in the PC molecular structure. Carbonate group selectively absorbed the energy in the UV region and starts the chain scission process and in parallel, the process of the crosslinking continued by the small molecules. The completion of the process is the formation of the phenyl free radicals. These free radicals are helpful in the improvement of hydrogen selectivity over other gases.
Sample name
Sample Representation
PC_0.1 PC_0.1_UV_36 PC_0.1_UV_36_Pd_24 PC_0.1_UV_36_Pd_PVP_24 PC_0.1_UV_48 PC_0.1_UV_48_Pd_24 PC_0.1_UV_48_Pd_PVP_24
S0 S1 S2 S3 S4 S5 S6
UVeVis spectroscopy The UVeVis absorption spectroscopy measurements were done in the range of 200e900 nm for all the samples (S0eS6). For the samples (S1, S2, S3), which were irradiated for 36 h by UV having same spectra and similarly for the samples (S4, S5, S6) getting same spectra having the irradiation time of 48 h. So we are comparing the fitted data of UVeVis spectra for the unirradiated S0 and irradiated samples S1 & S4 as shown in Fig. 2. Two shoulder bands nearly 365 nm and 320 nm can be clearly seen in Fig. 2. The presence of the both bands can be interpret by the photo-fries mechanism of the PC. These two bands most probably due to the p e p* (~320 nm) and n e p* (~365 nm) transmission of carbonyl group respectively. The intensity of both the bands increases with the UV-irradiation time. This intensity increment of the bands is a clear sign of higher bond-cleavage of carbonate group with UV-irradiation time.
Raman spectroscopy Fig. 3 shows the Raman spectra of all the samples in the range of 500e2000 cm1 and all fundamental peaks of polycarbonate are present and can be seen in Fig. 3. But there was small shift in the Raman frequencies of pristine PC membrane (S0) and UV-irradiated PC membrane (S1eS6). These shifts are due to change in the structure of surfaces for UV-irradiated membrane as compared to pristine. All the observed peak frequencies and corresponding bonds
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Table 2 e Raman Peaks and corresponding bonds. Wavenumber (cm1) 574, 634 703,731 826 886 918, 1109, 1146, 1177, 1234 1005 1387,1441, 1462 1601 1773
Fig. 2 e UVeVis spectra for the sample S0, S1 and S4 in the range of 300e700 nm, those are resembled to the Pristine PC, UV irradiated PC for 36 h and UV irradiated PC for 48 h respectively.
are presented in Table 2. These shift or changes in frequency also show that the UV irradiation has small effect on Raman frequencies in the range of 2000e4000 cm1. If we enlarge the region 2750-3150 cm1 as shown in Fig. 4, we found that peaks at 2937 cm1 and 3071 cm1 shows intensity growth after the UV irradiation samples (S1eS6) as a result of the photo-fries process on the PC surface.
Fourier Transform Infrared spectroscopy (FTIR) Fig. 5 shows the FTIR spectrum of all samples (S0eS6) in the range of 400e4000 cm1 and have all the peaks corresponding to elementary bonds of polycarbonate [50,51]. For the analysis samples (S1eS6) are compared with the base sample of polycarbonate S0. There was no remarkable shift in the spectra (400-4000 cm1), but peaks related to carbonated group shows
Type of Vibration Phenyl ring vibration CeH out-of-plane bending Phenyl ring vibration OeC(O)eO stretching CeOeC stretching CeH bending in-plane CH3 deformation Phenyl ring vibration C¼O stretching
intensity variation. Peak 2965 cm1 is corresponding to CeH3 stretching vibration, 1768 cm1 corresponding to free carbonyl stretching motion and 1168 cm1 represents CeO stretching (carbonyl group). When the UV irradiation time of samples is increased, a slight increment has been observed for both the peaks at 1768 cm1 and 1168 cm1. These peaks are corresponding to carbonyl group and the increment is due to enhancement in stretching of the CeO bond. Also, absorption peak around 2965 cm1 associated with CeH stretching arises due to UV irradiation. It means that when the samples have been exposed to UV light, then bond formation takes place due to the photo-fries mechanism. UV light generate the changes in the PC surface which is responsible for the variation in carbonate (1780 cm1) to phenylene (1520 cm1) intensity ratio.
Scanning electron microscopy (SEM) The morphological study of the samples was done with the help of scanning electron microscopy (SEM) and observed SEM images are shown in Fig. 6. From the SEM images, it's clear that the average pore size of all the sample is ~0.1 mm. These membranes were dipped in Pd nanoparticle solution and there is very less possibility to cover the pore by the Pd nanoparticles because the Pd nanoparticle size in the range of 6 ± 1 nm [13] and pore size is 0.1 mm. In the SEM images, bright
Fig. 3 e Raman spectra of all the samples (S0eS6) in the range of 500e200 cm¡1. Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 4 e Raman spectra of all the samples (S0eS6) in the range of 2750e3150 cm¡1.
Fig. 5 e Fourier-transform infrared spectroscopy (FTIR) spectra of the all the samples (S0eS6) in the range of 400e3100 cm¡1.
points (yellowish) indicates the number of Pd nanoparticles on the surface as well as in pores. In Fig. 6(C), it can be seen that the Pd nanoparticles are in the pores as well as on the surface too. But the clustering of Pd nanoparticles on surface of membranes as well as in pores can be controlled by using the PVP as binder. From Fig. 6(d) it can observe that when PVP is used with Pd nanoparticles, there was minimum clustering on the surface and as a result the surface as well as the pores boundary have enhanced brightness in sample S6 with uniform distribution of nanoparticles. Energy-dispersive X-ray spectroscopy (EDS) spectra of sample S2 also confirms the existence of Palladium (supporting data). Using PVP binder with Pd nanoparticles is an effective way to have maximum attachment of nanoparticles on polymer surface as well as in pores.
Permeability and selectivity For the gas separation application, we used H2, N2 and CO2 gases to examine the gas permeability and selectivity for all the samples. The obtained data of the gas permeability are shown in Figs. 7 and 8. As evident from the gas permeability measurements, H2 shows the maximum permeability not only due to smallest molecular size in comparison to other two gases (N2 & CO2) but also other factors like gas molecule interaction with surface, action energy and diffusion rate in the polymeric medium and all these factors are favourable for the higher permeation of H2 gas. The spherical shape of H2 molecules also helps in the higher permeability rate then N2 and CO2. In case of CO2 and N2, CO2 has smaller molecular (3.30 A) size than N2 (3.64 A). But because of the linear shape
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 6 e Scanning electron microscopy (SEM) of the sample (a) Pristine PC (S0) (b) UV irradiated for 48 h (S4) (c) UV irradiated for 48 h with deposition of Pd Nanoparticles for 24 h (S5) (d) UV irradiated for 48 h with deposition of Pd nanoparticles.
Fig. 7 e Graph of the gas permeability data for the samples (S0eS3). S0 is for the without UV irradiation pristine PC and S1e S3 samples having the UV irradiation time of 36 h.
gas molecule of CO2, it requires more activation energy for diffusion through porous membrane than N2 gas molecules, which leads CO2 to lower permeation in comparision to N2. UV-irradiation process of membrane increases the number of free radicals/group on the surface as well as on pore inner walls. These free group are ready to accept the hydrogen molecules according to the functionalize mechanism. In the presence of pressure gradient of hydrogen, group donates or transport hydrogen to the next group or molecule of polymer in the low concentration zone of H2. Such action of these free
group increases the permeability of hydrogen. But for N2 and CO2 these group works as obstacle. So, there was not effective increment or decrement found in the CO2 and N2 permeability in all the samples (Figs. 7 and 8). In case of UV irradiated sample S1 and S4, the selectivity H2/N2 and H2/CO2 is higher (Fig. 9) in the S4 because of the high UV-irradiation time (48 h). In the second step, we deposited Pd nanoparticles in the UVirradiated samples (S2, S5) without PVP and (S3, S6) with PVP. Pd nanoparticles have supreme absorption property of hydrogen due to its lattice structure. In the presence of H2 gas
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Fig. 8 e Graph of the gas permeability data for the samples (S0, S4eS6). S0 is for the without UV irradiation pristine PC and S4eS6 samples having the UV irradiation time of 48 h.
Fig. 9 e Graph of selectivity data of H2 over CO2 and N2 for all the samples (S0eS6). medium, Pd shows two types of interaction; a and b crystalline phases. With low concentration of hydrogen, Pd shows a phase and b phases. In both phases, lattice have the face centered cubic (FCC) structure with different lattice unit length. The octahedral void of the FCC lattice is perfect place to grip the H2 because of its smaller molecular size. The absorption phenomena also reversible, means diffusion of H2 through lattice will be higher if there was temperature/pressure gradient is available. So, in our case we use pressure gradient, that's why Pd deposited samples shows the higher permeability and selectivity of hydrogen. The UV irradiation of the samples improves the attachment of Pd nanoparticles with membrane in comparison to pristine membrane. More attachment of Pd nanoparticles within PC membrane is responsible for higher permeability and selectivity of H2. PVP helps in increasing the numbers of Pd nanoparticles on surface as well as in pores of PC membranes. Membranes having 48 h UV irradiation time and higher number of Pd nanoparticles (sample S6) show the highest permeability of H2 and also the maximum selectivity for H2/N2 and H2/CO2, 3.43 and 3.92 respectively.
Conclusions The permeability and selectivity of the gases through the polymeric membrane can be controlled by the selective deposition of nanoparticles and the UV irradiation process. It was concluded from above results that with increment in UV irradiation time the attachment of Pd nanoparticles have enhanced. For better attachment of the nanoparticles, PVP binder has an important role for making highly permeable and selective membrane for hydrogen. Maximum selectivity was found 3.43 and 3.92 for H2/N2 and H2/CO2 respectively. So, we have faith in the fact that such kind of the membrane has potential use in gas separation and purification.
Acknowledgements The authors acknowledge the financial support received from SERB, New Delhi for ECR Award (ECR/2016/001780) and MRC, MNIT Jaipur for characterization facilities.
Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094
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Please cite this article in press as: Kumar R, et al., Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.094