Reply to “Comment on ‘Nanohole-Structured and Palladium-Embedded 3D Porous Graphene for Ultrahigh Hydrogen Storage and CO Oxidation Multifunctionalities’” Won G. Hong,† Rajesh Kumar,‡ Jeong Young Park,§,⊥ Hae Jin Kim,*,† and Il-Kwon Oh*,∥ †
Nano-Bio Electron Microscopy Research Group, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea ‡ Center for Semiconductor Components, State University of Campinas (UNICAMP), 13083-870 Campinas, Sao Paulo, Brazil § Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), ⊥Graduate School of EEWS, and ∥Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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our samples were redrawn in Figure 1a. The activated carbon follows the Langmuir-type absorption isotherm shape, while 3D-Pd-E-PG shows an almost linear isotherm response, as can be seen in Figure 1a. Because the linear isotherm shape in 3DPd-E-PG can be strongly related to the slow adsorption and desorption kinetics of hydrogen storage, we calculated the mass transfer coefficient (Ksap) at each absorption step using the linear driving force (LDF) model.5 The LDF model has been widely accepted to assess the mass transfer coefficient for adsorbate uptake under a constant adsorption temperature. This model is also useful in catalytic reactions requiring manipulation of transport rates in gas adsorption kinetic studies.5 The time of uptake and the adsorbed mass are renormalized at each adsorption step, so that the mass transfer coefficient (Ksap) is obtained at each specific adsorption step.6 Based on the LDF model, the adsorption kinetics can be analyzed using curve fitting of the pressure decay process for equilibrium pressure. The Ksap value obtained using the LDF model approximation indicates the overall kinetic rate constant, covering surface diffusion as well as pore diffusion. The Ksap value at each adsorption step and the dependencies of Ksap on the adsorbed amount are shown in Figure 1b,c. From the fitted results, we were able to observe that the Ksap values of MEGO, Pd-D-G, and 3D Pd-E-PG decrease with increasing amount of adsorption. Furthermore, the Ksap values of MEGO, Pd-D-G, and 3D Pd-E-PG are much higher than that of activated carbon, implying that slow adsorption kinetics occurs in MEGO, Pd-DG, and 3D Pd-E-PG. From these results, we speculate that our samples have an adsorption mechanism different from that of activated carbon: this is the so-called spillover effect resulting from the existence of the Pd nanoparticles and the slow adsorption kinetics. Additionally, we investigated the separation factor (adsorbate in gas phase/adsorbate in solid phase, α) of our samples for each adsorption point using recorded adsorption rate measure-
irst of all, we appreciate that Klechikov and Talyzin read our article carefully and gave helpful comments. Hereafter, we refer to Klechikov and Talyzin as “authors”. The authors claim that the hydrogen dissociation required for the spillover mechanism is not expected at 77 K. If the authors would read the article carefully, they would find our comment on this spillover effect: “The increase of H2 uptake in Pd−D−G can be tentatively attributed to the hydrogen spillover effect” in the first sentence on page 7349 of the ACS Nano article. We only tentatively suggested our speculation on the increased hydrogen storage in the palladium−graphene hybrid structures in our article. Also, we would like to argue that hydrogen spillover at 77 K is a controversial issue. In fact, a recently published article clearly reported: “The inelastic neutron scattering (INS) method is uniquely capable of revealing the state of hydrogen, in either atomic or molecular form. The INS results indicate a direct quantitative evaluation of the volume of hydrogen adsorbed on activated carbon in atomic form via spillover. Atomic hydrogen spillover was observed from a Pd catalyst, to activated carbon fibers loaded at 77 K with 2.5 wt % H2” on page 103.2 In addition, Jiménez et al.3 and Contescu et al.4 reported the enhanced hydrogen storage capacity of transition metal−carbon structures at 77 K, based on the spillover mechanism. The spillover mechanism might be extended to explain the mechanism for the increased hydrogen storage of palladium− carbon hybrid structures at 77 K as well as at room temperature. However, we think that further studies of the spillover mechanism at 77 K should be performed using both experimental and theoretical investigations to resolve this controversial issue. As reported in the Supporting Information of the ACS Nano article,1 the high-pressure volumetric apparatus was calibrated with LaNi5 (1.46 wt %) at 313 K, and the absorption isotherm of activated carbon (Maxsorb, surface area ∼3000 m2/g) at 77 K was measured using computer-controlled commercial PCT (Belsorp-HP) and 99.9999% hydrogen gas. See Figures S3 and S4 in the Supporting Information of the ACS Nano article.1 The hydrogen isotherm responses for the activated carbon and for © 2016 American Chemical Society
Received: July 1, 2016 Published: October 25, 2016 9057
DOI: 10.1021/acsnano.6b04370 ACS Nano 2016, 10, 9057−9060
Letter to the Editor
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Figure 1. Hydrogen adsorption responses of MEGO, Pd-D-G, 3D-Pd-E-PG, and activated carbon at 77 K. (a) Adsorption isotherm shapes, (b) Ksap value at each adsorption step, (c) Ksap value according to storage amount, and (d) separation values at individual adsorption steps.
Figure 2. SEM and TEM images of nanohole-structured and palladium-embedded 3D porous graphene (3D-E-PG) sample. (a) SEM image of large-scale 3D-E-PG sample, (b) TEM image of 3D-E-PG, and (c,d) high-resolution TEM images showing lattice spacing of Pd.
separation factor can be directly related to the linear shape of the adsorption isotherm and the overall mechanism of H2 uptake in our samples. However, in the case of activated carbon, the uptake of H2 in solid phase starts very rapidly from the initial step, resulting in a Langmuir-type response. However, the increased separation value could be obtained
ment data. As can be seen in Figure 1d, the separation factors of our samples decrease after the third adsorption point. This means that most of the H2 uptake in the solid phase occurs after the third adsorption. The decreased values of the separation factors with increasing adsorption steps reveal the increase of H2 uptake in the solid phase. This response of the 9058
DOI: 10.1021/acsnano.6b04370 ACS Nano 2016, 10, 9057−9060
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to direct contact between the Pd particles and the edge sites of the r-GO, as shown in Figure 2c. In the right inset of Figure 1 produced by Klechikov and Talyzin, r-GO decorated with Pd nanoparticles is shown, but the contrast of the Pd particles is rather vague, indicating that the Pd nanoparticles are not clearly metallic but could be encapsulated by oxide or carbonates. These encapsulated or contaminated Pd particles can give rise to lower hydrogen storage performance, which could also be the origin of the nonenhancement of hydrogen uptake observed by Klechikov and Talyzin on r-GO decorated with Pd particles. Because of this issue, it is required to show the chemical or oxidation states of the surface of the Pd nanoparticles using X-ray photoelectron spectroscopy, for example, in order to explain the major discrepancy between the authors’ results and Kumar et al.’s results. Regarding the hydrogen storage data provided by the authors, there are many studies on the decrease of H2 storage capacity after the introduction of a transition metal catalyst system; this is because of a decrease in the surface area and nanopore volume due to pore blockage. However, enhanced H2 storage capacity can be achieved on an adsorbent with a nanoporous structure or on transition metal catalysts, such as Pd, Pt, and Ni. However, the authors’ data simply decrease compared with pure graphene. Here, we want to highlight that the nanopores with sub-nanometer scale are not nanoholes that are produced by Pd nanoparticles with diameters in the range of 10−100 nm. With respect to using a Pd catalyst to improve the hydrogen storage capacity, Contescu et al. 4 have demonstrated that nanopores may be a suitable adsorption site for H2 molecules in the presence of a Pd catalyst. In addition, several researchers have reported that these adsorption sites are size-dependent,8,9 and our samples have a suitable nanopore structure with a 0.75 nm pore width. These studies provide a possible explanation as to how molecular H2 was stored in the nanopores. Ismail et al. have reported that Pd nanoparticles work as good catalysts at 80 K and improve the hydrogen storage capacity of r-GO nanosheets.10 By using physically and chemically modulated nanoporous 3D graphenebased materials with Ni species, Kim et al. have also suggested that Ni species and the microporous structure play prominent roles in extremely high H2 storage capacity.11 Based on the reasons we address above, we conclude that Klechikov and Talyzin failed to produce a nanohole-structured and palladium-embedded porous graphene structure. Considering the authors’ results shown in the SEM images and their hydrogen storage data, the authors did not successfully produce the samples, and as such, they cannot insist that their hydrogen storage data are correct. The palladium particles produced by the authors may be contaminated with oxygen or carbon or may not be fully treated. We think that it is not meaningful to mention hydrogen storage data without reporting the XPS data and pore size distribution for all samples and without synthesizing the same structure as 3D-Pd-E-PG. We would like to emphasize that all characterization data in the ACS Nano article,1 including morphological structures, chemical analyses, and hydrogen storage, were obtained with careful consideration and faithful investigation.
after saturated adsorption, indicating that the amount of H2 uptake in the gas phase is relatively larger than that in the solid phase. Regarding the isotherm shape, we agree that the general isotherm of carbonaceous materials at 77 K can follow a Langmuir-type isotherm shape. However, it has become clear that a different shape of hydrogen adsorption−desorption isotherm could be shown by modified and nanostructured carbonaceous materials. The authors can find examples in the literature3,7 that do not show the typical isotherm shape in the case of several graphene derivatives, such as chemically reduced graphene oxide and thermally exfoliated graphene oxide. Hudson et al.7 reported that the hydrogen uptake capacity increases linearly at 77 K with respect to the applied H2 pressure; the isotherms of the three different r-GO samples show that the shape of the adsorption−desorption isotherm may be different depending on the characteristics of the r-GO samples. Hudson et al.7 clearly reported that CR-GO (chemically reduced graphene oxide) follows an almost linear increase in hydrogen storage at 77 K, as can be seen in Figure 6, while TR-GO (thermally reduced graphene oxide) shows the typical isotherm shape shown in Figure 5.7 Hudson et al.7 wrote this in their conclusion: “We have also observed that the hydrogen uptake capacity of the graphene sample depends not only on its surface area but also its defects concentration. More defective graphene surface exhibits higher hydrogen uptake behavior.” In another study, a clear change in the shape of the hydrogen absorption−desorption isotherm was observed with Ni-modified carbon nanofiber at 77 K.3 Our samples are produced by using microwave-exfoliated methods. The isotherm shapes of the graphene derivatives are highly dependent on various parameters, such as the surface-tovolume ratio, oxygen functional groups, layer number, interlayer spacing, and defect sites. Many papers reporting hydrogen storage by graphene derivatives show quite different isotherm shapes with various storage wt %. We believe that the derivatives of reduced graphene oxide can show different isotherm shapes regardless of the typical shape of the activated carbon. Here, we want to clearly mention that Figure 5a,c in our ACS Nano paper1 simply shows a schematic diagram to explain the mechanism of hydrogen adsorption with our current understanding. This is not an accurate model, nor were the figures drawn to real scale. We clearly described the sizes of the nanoholes and Pd particles, as shown in Figures 1 (SEM images) and 2 (TEM images) of the ACS Nano paper.1 Additionally, more SEM and TEM images of 3D-Pd-E-PG cleary showing nanohole-structured and palladium-embedded 3D porous graphene are provided in Figure 2. Most interestingly, large palladium particles (10−100 nm) show destructive behavior that produces nanoholes in the graphene surfaces, while smaller-sized palladium (∼10 nm) particles are supported on the graphene surfaces. The authors claim that the holes in the r-GO sheets around the Pd particles were formed after on-air microwave or simple heat treatments. However, in the insets in Figure 1 produced by Klechikov and Talyzin, the formation of the holes near the Pd particles is not clear. The SEM image on the left shows holes that are separated from the Pd particles, which is quite different from the morphology of 3D Pd-E-PG reported by Kumar et al. (see Figure 1e in ref 1). In the results reported by Kumar et al., Pd particles are formed inside the holes of the r-GO, giving rise
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DOI: 10.1021/acsnano.6b04370 ACS Nano 2016, 10, 9057−9060
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Creative Research Initiative program, funded by the National Research Foundation of Korea (NRF) (No. 2015R1A3A2028975). REFERENCES (1) Kumar, R.; Oh, J. H.; Kim, H. J.; Jung, J. H.; Jung, C. H.; Hong, W. G.; Kim, H. J.; Park, J. Y.; Oh, I. K. Nanohole-structured and Palladium-embedded 3D Porous Graphene for Ultrahigh Hydrogen Storage and Co Oxidation Multifunctionalities. ACS Nano 2015, 9, 7343−7351. (2) Konda, S. K.; Chen, A. Palladium Based Nanomaterials for Enhanced Hydrogen Spillover and Storage. Mater. Today 2016, 19, 100−108. (3) Jiménez, V.; Ramírez-Lucas, A.; Sánchez, P.; Valverde, J. L.; Romero, A. Improving Hydrogen Storage in Modified Carbon Materials. Int. J. Hydrogen Energy 2012, 37, 4144−4160. (4) Contescu, C. I.; Brown, C. M.; Liu, Y.; Bhat, V. V.; Gallego, N. C. Detection of Hydrogen Spillover in Palladium-Modified Activated Carbon Fibers during Hydrogen Adsorption. J. Phys. Chem. C 2009, 113, 5886−5890. (5) Glueckauf, E.; Coates, J. E. Theory of Chromatography IV. J. Chem. Soc. 1947, 1315−1321. (6) Chihara, K.; Koide, S.; Nomoto, M.; Amari, Y. Experiment and Simulation Study of Multi-component Gas Adsorption on MSC3A by Volumetric Method. 2013 AIChE Annual Meeting, November 3−8, 2013; Vol. 333. (7) Hudson, M. S. L.; Raghubanshi, H.; Awasthi, S.; Sadhasivam, T.; Bhatnager, A.; Simizu, S.; Sankar, S. G.; Srivastava, O. N. Hydrogen Uptake of Reduced Graphene Oxide and Graphene Sheets Decorated with Fe Nanoclusters. Int. J. Hydrogen Energy 2014, 39, 8311−8320. (8) Ihm, Y.; Cooper, V. R.; Peng, L.; Morris, J. R. The Influence of Dispersion Interactions on the Hydrogen Adsorption Properties of Expanded Graphite. J. Phys.: Condens. Matter 2012, 24, 424205. (9) Xia, Y.; Yang, Z.; Zhu, Y. Porous Carbon-materials for Hydrogen Storage: Advancement and Challenges. J. Mater. Chem. A 2013, 1, 9365−9381. (10) Ismail, N.; Madian, M.; El-Shall, M. S. Reduced Graphene Oxide Doped with Ni/Pd Nanoparticles for Hydrogen Storage Application. J. Ind. Eng. Chem. 2015, 30, 328−335. (11) Kim, T. K.; Cheon, J. Y.; Yoo, K.; Kim, J. W.; Hyun, S.; Shin, H. S.; Joo, S. H.; Moon, H. R. Three-dimensional Pillared Metallomacrocycle−graphene Frameworks with Tunable Micro- and Mesoporosity. J. Mater. Chem. A 2013, 1, 8432−8437.
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