Dissociative Adsorption of Molecular Hydrogen on BN-Doped

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Dissociative Adsorption of Molecular Hydrogen on BN-Doped Graphene-Supported Aluminum Clusters Deepak Kumar,† Sailaja Krishnamurty,*,‡ and Sourav Pal*,§,∥ †

Physical Chemistry Division, National Chemical Laboratory, Homi Bhabha Road, Pune 411 008, India Functional Materials Division, Central Electrochemical Research Institute, Karaikudi 600 006, India § Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India ‡

S Supporting Information *

ABSTRACT: The present work demonstrates dissociative adsorption of molecular hydrogen on supported and unsupported aluminum clusters (Aln, n = 4−8, 13) using density functional theory based calculations. The studies reveal that the presence of a BN-doped graphene surface support reduces the dissociative adsorption barrier of the bond in molecular hydrogen on even atom clusters. In particular, supported Al6 demonstrates a barrier-less dissociative adsorption toward the H2 molecule. These results demonstrate the excellent potential of supported Al nanoparticles for hydrogen storage and also the potential of doped graphene systems are catalyzing supports.

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INTRODUCTION The major contribution to the worldwide utilized energy comes from the fossil fuel sources. Harmful, toxic emissions and greenhouse gases such as CO2 produced from the fossil-fuels result in an adverse effect on the environment. The above-mentioned environmental concerns resulting from the fossil-fuel-based energy sources have both thrown a major challenge as well as necessitated the development of alternative sustainable energy sources. Hydrogen storage may be a possible and alternative source of clean and sustainable energy.1 The stumbling block for this energy route is that the hydrogen gas storage requires high pressure (700 bar pressure used for hydrogen tank systems in vehicles).2 On the other hand, storage in the liquid phase requires low temperatures (20.28 K at which hydrogen is in liquid state at atmospheric pressure)3 and is further associated with high volume, energy loss, and safety risks. In both of the states of (gas as well as liquid) storage, hydrogen can be either in a physisorbed form or a chemisorbed form. A significant amount of research has been and is still being devoted to enhancing the storage of hydrogen on porous materials such as clathrate hydrates,4 metal organic frameworks,5 and carbon nanotubes.6 Most of these materials have high storage densities only at cryogenic temperatures.

Hydrogen storage by physisorption has been widely studied on various porous materials like carbon or metal−organic frameworks (MOFs).The physisorption of hydrogen on the porous support material emanating from the van der Waals forces is typically observed at 77 K. These low temperatures are disadvantageous for automobile applications and, just as importantly, hydrogen storage for vehicular applications mandates critical conditions in terms of volume, weight, operation temperature, hydrogen sorption rates, and safety. Chemisorptions of molecular hydrogen on light materials may be a promising alternative for hydrogen storage. Promising candidates in that respect are lightweight complex metal hydrides.7 In recent years, Li, Na, Mg, Ca, and K salts of AlH4−, NH2−, and BH4− (alanats, amides, and boranes) have received most attention.8−10 During the past decade, sodium alanate (NaAlH4) has received the most attention as a potential candidate for hydrogen storage. However, all types of metal hydrides have kinetic barriers which restrict the hydrogenation and dehydrogenation rates.11 In short, a single material that satisfies all the storage criteria has yet to be envisaged. According to recent studies, transition metal and alkali metal clusters exhibit a potential to serve as alternative hydrogen storage materials over coming to the above-mentioned disadvantages. In this area, Pd and Pt clusters have been extensively explored for hydrogen dissociation. Owing to the stability issues associated with these clusters, stabilization of metal clusters on graphene and other carbon materials is being explored in the recent years.12 In this context, Cabraia et.al13 demonstrated the hydrogen dissociation on graphene supported Pd clusters.

Table 1. Ground State Conformations and Total Cohesive Energies (in eV) of Pristine Aln Clusters, n = 4−8, 13 n

cluster (ground state conformation)

Ec = nE(Al) − E(Aln)

4 5 6 7 8 13

Al4 Al5 Al6 Al7 Al8 Al13

6.75 9.48 12.70 16.28 18.57 34.85 © XXXX American Chemical Society

Received: August 5, 2017 Revised: October 31, 2017 Published: November 1, 2017 A

DOI: 10.1021/acs.jpcc.7b07789 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Different modes of adsorption of molecular hydrogen on the pristine Al4 cluster.

Table 2. Interatomic Distances (in Å) and Adsorption Energies Ead (in eV) of Pristine Al4H2 Complexes in Various Adsorption Modesa cluster H2@Al4 H2@Al4 H2@Al4 H2@Al4 H2@Al5 H2@Al6 H2@Al7 H2@Al8 H2@Al13 a

dH−H

orientation perpendicular bridge mode parallel mode parallel mode parallel mode parallel mode parallel mode parallel mode parallel mode

mode on on on on on on on

atom bond bond bond bond bond bond

dAl−H

0.75 0.75 2.534 2.539 2.698 2.268 2.396 4.484 3.442

3.50 3.593, 1.606, 1.605, 1.597, 1.198, 1.731, 1.599. 1.593,

3.901 1.751 1.750 1.722 1.948 1.732 1.599 1.594

dAl−Al

Ead

2.57 2.562 2.609 2.610 2.559 2.715 2.774 2.807 3.101

0.03 0.03 −1.05 −1.06 −0.74 −1.25 −0.42 −1.0 −1.06

Also given are the interatomic distances (in Å) and adsorption energies Ead (in eV) of pristine AlnH2 complexes (n = 5, 6, 8, and 13).

Figure 2. Final orientations of H2 chemisorbed pristine Aln (n = 4−8, 13) clusters.

Figure 3. Hydrogen dissociation on supported aluminum clusters.

Pd clusters being expensive, Al clusters which have already demonstrated an excellent catalytic potential may be explored as an alternative means. In addition, doping is a potential and promising way to increase the catalytic potential of carbonbased supports such as graphene. The sp2 hybridization of carbon atoms in graphene affects by the doping of various atoms (B, N, BN). The electron deficient and rich nature of B and N atoms helps to alter significantly the electronic properties of the graphene. The reason for selecting the N and B atoms to replace C atoms is the atomic masses of these dopants are closer to a carbon atom.

It was reported that sodium alanates show the better kinetics and low-temperature hydrogen storage to the nanoscale level.14 Hence, in this work we study the dissociation of molecular hydrogen using sodium alanate materials (used aluminum clusters as a proxy for sodium alanate) on BN-doped graphene supports. Our periodic DFT studies provide insight into the role of aluminum and carbon support in promising storage capacity. The methodology is given in Computational Methods. Results and Discussion also contains results on dissociated state of molecular H2 on pristine Aln clusters and BN-doped B


Article

The Journal of Physical Chemistry C Table 3. Adsorption Energy (eV) for H2 Adsorption on Aln Clusters Supported by BN-Doped Graphene system

dH−H

dAl−B

dAl−C

H2@Al4@BN_graphene H2@Al5@BN_graphene H2@Al6@BN_graphene H2@Al7@BN_graphene H2@Al8@BN_graphene H2@Al13@BN_graphene

4.90 2.576 2.814 2.686 2.764 4.150

2.499 2.515 2.565 2.710 2.859 2.862

2.023 2.006 2.057 1.990 2.109 2.081

dH−Al 1.597, 1.609, 1.588, 1.599, 1.604, 1.590,

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H2@Aln

H2@Aln@BN_graphene

4 5 6 7 8 13

0.14 0.50 0.46 0.86 0.40 0.41

0.25 0.63 0.004 1.07 0.25 0.30

dAl−Al

Ead

Ead (DFT + D3)

2.496 2.630 2.635 2.596 2.838 2.782

−1.18 −0.50 −1.30 −0.35 −0.31 −0.61

−1.62 −0.52 −1.28 −0.38 −0.37 −0.66

RESULTS AND DISCUSSION a. Chemisorption of H2 with Pristine Aluminum Clusters. As mentioned earlier, the choice of considering Al clusters with 4−13 atoms comes from the fact that the clusters with this size range have been found to be catalytically active toward molecules such as CI, N2, and H2O, and so forth.22−24 We note that the adsorption of molecular hydrogen on aluminum clusters has been studied earlier by various groups. Hemert et. al25 reported the hydrogen dissociation on the small size (n = 2−6) aluminum clusters. The effect of charge and doping on hydrogen dissociation on aluminum clusters are also studied by Yarovsky et. al.26 These all previous studies were based on atom-centered basis set. In present work, our approach is based on using the periodic DFT for hydrogen dissociation on pristine and BN-doped graphene supported aluminum clusters. Earlier reported ground state geometries for Aln (n = 4−8, 13) clusters are taken and optimized. The ground state geometries along with their cohesive energies are given in Table 1. H2 molecule is adsorbed on these gas phase Al clusters so as to have reference values in order to quantify the role of the support. Different modes of H2 adsorption are possible and are demonstrated on Al4 cluster in Figure 1. Interatomic distances for various H2−Al4 complexes are given in Table 2. From Table 2 and Figure 1, we note that H2 molecule does not chemisorb in vertical and perpendicular modes. It chemisorbs and gets activated when chemisorbed in parallel and single site mode as demonstrated in the figure. This is the case for Al clusters of all sizes. For the case of Al4 cluster, H2 adsorption in parallel mode is nearly −1.05 eV with a HH bond dissociation. The final HH bond length on Al4 cluster is 2.53 Å as compared to 0.75 Å in the free gas state. Adsorption studies of H2 on other pristine Aln (n = 5−8, 13) clusters also reveals that a dissociative exothermic adsorption in parallel mode is favored. The structural parameters of the most favorable final conformation AlnH2 (n = 5, 6, 7, 8, 13) complexes are given in Table 2. The final orientations of the AlnH2 (n = 4−8, 13) clusters are given in Figure 2. It is noted that even clusters and Al13 have higher energies for dissociative adsorption of molecular hydrogen. b. Aluminum Clusters Supported on BN-Doped Graphene. On the basis of our observations presented above as well as earlier observations that Al clusters favor a dissociative adsorption for H2 molecule, it is interesting to identify a catalyzing support for the same. Our earlier work has shown that the Aln clusters are not stable on the pristine graphene surface. However, Al clusters adsorb favorably on BN doped graphene sheets. It has been earlier noted that the stable orientations of Aln clusters to the graphene plane are Al atom atop on the carbon atom or CB bond of the BN-doped surface. The optimized structures of Al clusters on BN doped graphene are given in the Supporting Information Table S1. c. Interaction of H2 with Aln Clusters Supported on BN-Doped Graphene. As in the case of pristine Al clusters,

Table 4. Activation Energy Barrier (in eV) Calculated by Nudged Elastic Band Method (NEB) for H2 Dissociation on Supported Aln Clusters size of clusters

1.608 1.839 1.902 1.704 1.856 1.590

graphene supported Aln clusters. Finally, conclusions of the work are presented in last section (Conclusions).

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COMPUTATIONAL METHODS We use Vienna ab initio simulation package (VASP)15 to perform all the density functional theory (DFT) based firstprinciple calculations. The projected augmented wave (PAW)16 method is employed using an energy cut off 520 eV to describe plane wave basis set. We use the PBE17 functional for electronic structure calculations. The two-dimensional graphene sheet structures are simulated using periodic boundary conditions and to avoid the interactions between the different nearest neighboring layers a vacuum space of 20 Å was created along the Z-direction. The 5 × 5 super cell with 50 atoms is used as graphene surface model and the optimized C−C bond length in graphene sheet is 1.42 Å. The structural optimization of all geometries is carried out using the conjugate gradient method.18 The Brillouin zone is sampled by a (2 × 2 × 1) K point grid using the Monkhorst−Pack scheme.19 For DOS calculations, the Monkhorst−Pack generated (9 × 9 × 1) set of K points. The adsorption energy (Ead) of Aln clusters on graphene of BN-doped graphene is calculated using the following equation Ead = E(Al n@graphene/BN_graphene) − E(Al n) − E(graphene/BN_graphene)

Here E(Aln@graphene/BN_graphene) represents the energy of the optimized structure of Aln clusters supported by graphene or BN-doped graphene surfaces. The E(Aln) and E(graphene/ BN_graphene) represent the energy of pure Aln clusters and graphene/BN_graphene, respectively. We have included semiempirical DFT−D3 type dispersion correction20 for the longrange interaction between adsorption of molecular hydrogen and supported aluminum clusters. To calculate the activation energy barriers we have performed nudged elastic band (NEB)21 method-based transition energy search as incorporated in VASP. Seven intermediate images are used in each NEB pathway. C


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Figure 4. Total density of states (TDOS) and projected density of states (PDOS) of Aln clusters supported by BN-doped graphene.

In all of the NEB calculations, the initial configuration is a physisorbed state in which the molecular H2 is placed ∼3.0 Å from the Aln clusters and final state is the dissociation of molecular H2 on Aln clusters. The activation barriers and the corresponding minimum energy reaction paths for H2 on supported Aln clusters are given in Table 4 and demonstrated in Figure 3. Our calculations show that the BN-doped graphene surface lowers the activation barriers of Al4, Al6, Al8, and Al13 clusters. In case of odd clusters (Al5 and Al7) activation barriers are very high in pristine Aln clusters as well as supported Aln clusters. So, Al5 and Al7 clusters are not suitable for H2 dissociation. The effect of BN-doped graphene surface is very promising as a support of Aln clusters. The dissociation of H2 on supported Al6 clusters is a barrier-less process. The activation barriers for other clusters (Al4, Al8, and Al13) are also very low. This confirms

various modes of H2 adsorption are studied on supported Aln. The molecular dissociation states of H2 on supported Aln clusters are shown in Figure 3. The geometrical parameters and the adsorption energies of the dissociated states are given in Table 3. Supported Al4 and Al6 retain high dissociative adsorption energy for H2 molecule. Supported Al13 also shows reasonable adsorption energy though in lesser strength as compared to its pristine counterpart. d. Barriers and Underlying Electronic Factors for Dissociative Adsorption of H2 on Supported versus Unsupported Aln Clusters. To explore the effect of BN-doped graphene surface on the dissociation of H2 on Aln clusters, the activation barriers are also calculated for dissociation of H2 on pristine Aln clusters and BN-doped graphene-supported Aln clusters. The activation barriers are calculated by NEB method. D


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the role of BN-doped graphene to increase the catalytic reactivity of Aln clusters. To understand the electronic structure of supported Aln clusters and trend of activation barriers, we also performed the density of states (DOS) calculations for Aln clusters supported by BN-doped graphene. The DOS calculations are given in Figure 4. The potential increase in the catalytic reactivity of Aln clusters (Al4, Al6, Al8, and Al13) due to their p-orbitals closer to the Fermi level. The p-orbitals in the case of Al5 and Al7 clusters are lying very far or in conduction band which is not available for the interaction of H2. However, in other Aln clusters the p-orbitals are very near to the Fermi level, so easily available for binding with H2.

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CONCLUSIONS In this paper, we have demonstrated the adsorption of hydrogen molecule on model Al nanoclusters supported on BN-doped graphene surface. BN-doped graphene plays an important role in decreasing the kinetic activation barriers for dissociation of molecular hydrogen on aluminum clusters. The aluminum clusters of various sizes (Al4, Al6, Al8, and Al13) are seen to be suitable candidates for hydrogen dissociation. Significantly, the dissociation of molecular hydrogen on BN-doped graphene supported Al6 cluster is a barrier-less process. These results could pave the way for designing a new genre of technologically important catalytically active materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07789. Adsorption energies of Aln clusters supported by BNdoped graphene and Cartesian coordinate of the optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. ORCID

Sourav Pal: 0000-0002-4836-639X Present Address ∥

(S.P.) Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.K acknowledges UGC for funding of the SRF (Senior Research Fellowship) and M. Dixit for valuable discussion. S.K. acknowledges the High-Performance Computing facility at CSIR−CECRI, Karaikudi, and CSC-0129 (MSM project) for funding. S.P. acknowledges the grant from the J. C. Bose Fellowship of DST toward fulfillment of this work.

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REFERENCES

(1) Turner, J. A. Sustainable hydrogen production. Science 2004, 305 (5686), 972−974. (2) Eberle, U.; Muller, B.; von Helmolt, R. Fuel cell electric vehicles and hydrogen infrastructure: status. Energy Environ. Sci. 2012, 5 (10), 8780−8798. E


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The Journal of Physical Chemistry C (25) Pino, I.; Kroes, G. J.; Van Hemert, M. C. Hydrogen dissociation on small aluminum clusters. J. Chem. Phys. 2010, 133, 184304− 184316. (26) Henry, J. D.; Yarovsky, I. Dissociative adsorption of hydrogen molecule on aluminum clusters: Effect of charge and doping. J. Phys. Chem. A 2009, 113, 2565−2571.

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Dissociative Adsorption of Molecular Hydrogen on BN-Doped

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