Abstract
Graphene-based materials modified with transition metals, and their potential utilization as hydrogen storage devices, are extensively studied in the last decades. Despite this widespread interest, a comprehensive understanding of the intricate interplay between graphene-based transition metal systems and H2 molecules remains incomplete. Beyond fundamental H2 adsorption, the activation of H2 molecule, crucial for catalytic reactions and hydrogenation processes, may occur on the transition metal center. In this study, binding modes of H2 molecules on the circumcoronene (CC) decorated with Cr or Fe atoms are investigated using the DFT methods. Side-on (η2-dihydrogen bond), end-on and dissociation modes of H2 binding are explored for high (HS) and low (LS) spin states. Spin state energetics, reaction energies, QTAIM and DOS analysis are considered. Our findings revealed that CC decorated with Cr (CC-Cr) emerges as a promising material for H2 storage, with the capacity to store up to three H2 molecules on a single Cr atom. End-on interaction in HS is preferred for the first two H2 molecules bound to CC-Cr, while the side-on LS is favored for three H2 molecules. In contrast, CC decorated with Fe (CC-Fe) demonstrates the capability to activate H2 through H–H bond cleavage, a process unaffected by the presence of other H2 molecules in the vicinity of the Fe atom, exclusively favoring the HS state. In summary, our study sheds light on the intriguing binding and activation properties of H2 molecules on graphene-based transition metal systems, offering valuable insights into their potential applications in hydrogen storage and catalysis.
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1 Introduction
The coordination and activation of the hydrogen molecule (H2) by transition metal centers have attracted increased scientific interest since Kubas’s groundbreaking discovery of a new type of σ-bonding [1]. In this η2-dihydrogen bond or “side-on” interaction, often referred to as the Kubas interaction, the H–H bond undergoes elongation from its typical length of 0.74 Å, corresponding to a free H2 molecule, up to 0.80–0.90 Å [2, 3]. Such interaction between the transition metal (TM) center and the H2 molecule or molecules holds significant potential for hydrogen storage technologies [4, 5], a crucial element in the pursuit of hydrogen as a future renewable and high-capacity energy source [6, 7]. Consequently, numerous studies have explored TM-containing materials and their potential application as hydrogen storage devices [8,9,10,11,12,13,14,15,16,17].
Furthermore, catalytic hydrogenation (or hydrogen catalysis) stands as one of the most important chemical processes in industrial chemistry [18, 19]. Within the hydrogenation mechanism, the activation of H2 molecules is a prerequisite, followed by the controlled dissociation of the H–H bond [4]. Naturally, such H2 molecule activation and bond dissociation primarily occurs on TM centers. For example, the adsorption, activation and dissociation of the hydrogen molecule on Pd-decorated graphene surfaces has been reported recently [20, 21]. It was found that the interaction of H2 molecule with graphene surface decorated with one Pd atom leads to an elongation of the H–H bond to 0.86 Å, while the adsorption of a larger number of H2 molecules on Pd4 cluster deposited on graphene surface leads also to the dissociation of some H2 molecules [20]. In our previous works, we have investigated the H2 adsorption on circumcoronenes (CCs) doped with various first row TM atoms [13, 14, 22], but the dissociation of H2 molecule was found only in the case of a negatively charged Mn-3N-doped CC [14].
In the presented study, the interaction between the H2 molecules and the CCs decorated with Cr and Fe atoms is investigated using DFT methods and quantum theory of atoms in molecules (QTAIM) [23]. The choice of Cr and Fe is inspired by our previous work, indicating that Cr-, and Fe-doped CCs are promising adsorbents of H2 molecules [13]. TM-decorated graphene surfaces are potentially capable of a larger number of interactions with H2 molecules when compared to their TM-doped analogs, primarily due to their lower steric hindrance [22, 24]. Decoration of graphene surfaces with transition metals (CC-TM) can be experimentally achieved for example via an electrochemical deposition [25,26,27] or by the direct current magnetron sputtering [28, 29]. Pasti et al. [30] conducted a noteworthy computational study summarizing the calculated adsorption energies of elements from H to Rn on the pristine graphene surface. According to the results presented therein [30], the adsorption of Fe on the pristine graphene surface should be energetically more favorable when compared to Cr [30].
The primary objective of this computational study is to gain deeper insights into the nature of H2 binding on the Cr-, and Fe-decorated CCs (denoted CC-Cr and CC-Fe, respectively), see Fig. 1. Spin state energetics and optimized geometries of CC-TMs are reported first. The interaction of CC-TMs with H2 is explored in three distinct binding modes: i) side-on H2 binding to CC-TM, typically resulting in the formation of the η2-dihydrogen bond [2, 3]; (ii) dissociation of the H2 molecule on the TM atom; iii) end-on H2 binding to the TM atom, which leads to the formation of only one direct TM-H interaction [13] (such interaction is rather of van der Waals nature; see e.g. Figure 4, including the corresponding PDOS plots for Cr). The interaction of CC-TMs is studied with up to four H2 molecules, considering a high (HS) and low (LS) spin state preference, see e.g. Figure 5. Additionally, QTAIM and PDOS analyses are employed to evaluate the bonding characteristics in the electron density and the molecular orbital representation, respectively.
Initial structures of a circumcoronene (CC) and b circumcoronene decorated with iron (CC-Fe). Atomic colors: grey–carbon, pink–hydrogen, orange–iron
2 Computational details
Circumcoronene (CC) is a polyaromatic hydrocarbon, consisting of 54 carbons arranged in 19 hexagonal rings, see Fig. 1a. CC is a hexagonal fragment of graphene surface terminated with hydrogen atoms [31]. Due to its sufficient size, it is frequently used in computational studies as a model graphene quantum dot [32,33,34], for example as a scavenger of CO2 molecules [35]. The optimized structure of CC was taken from the work [13] and subsequently decorated with one transition metal (TM), in particular iron (Fe) or chromium (Cr). The TM atom was placed above the hollow site of the hexagonal ring in the middle of the CC structure in the initial geometries, see Fig. 1b.
The B3LYP-GD3 [36,37,38,39,40]/6-311G** [41,42,43] geometry optimizations of the CC-Cr and CC-Fe were performed in the Gaussian16 software package [44]. A vibrational analysis was conducted to ensure that the optimized geometries represent minima on the potential energy surface. The geometry optimizations with modified redundant internal coordinate of CC-Cr and CC-Fe, in their energetically preferred spin states, were carried out to evaluate the potential energy curves of the interaction between the Cr and Fe atoms and the CC surface. In these optimizations, the distance between the Cr or Fe atom and the CC surface was forcibly modified with a linear step of 0.1 Å. Subsequently, one to four interacting H2 molecules were placed into the vicinity of the Cr (or Fe) atom in the optimized CC-Cr (or CC-Fe) structures. These systems were then re-optimized using the same level of theory as mentioned above. Following the re-optimization, including the vibrational analysis, QTAIM analysis [23] was performed to estimate atomic charges, spin populations, as well as to assess the nature of interactions involving CC-Cr, Cr–H, and Fe-H bonds. This evaluation was based on bond critical point (BCP) characteristics, including electron density ρBCP, Laplacian of electron density ∆ρBCP, and delocalization indexes (DIs). The QTAIM analysis was carried out with the AIMAll package [45] utilizing Gaussian16 formatted check-point files. Partial density of states (PDOS) analysis was performed using GaussSum [46] to obtain the PDOS plots of valence orbital overlaps. Reaction energies were computed for the H2 B3LYP-GD3/6-311G** optimized geometry, yielding EDFT(H2) = – 1.179571 a.u. (hartree). Finally, the Vesta software was used for visualization of the studied systems [47].
3 Results
3.1 Decoration of the CC surface with Cr and Fe atoms
The first task of this study was to find the energetically preferred spin states of the studied systems. Calculated relative energies of the CC-Cr and CC-Fe in different spin states are presented in Table 1, including the calculated distances between the CC backbone and the TM atom. Selected B3LYP-GD3/6-311G** optimized structures of CC-Cr and CC-Fe are shown in Fig. 2.
B3LYP-GD3/6-311G** optimized structures of CC-Cr and CC-Fe (in different spin states). Atomic colors: grey–carbon, pink–hydrogen, blue–chromium, orange–iron
As can be seen in Table 1, the heptet spin state (from hereafter abbreviated as high-spin, HS) is energetically favored in the case of CC-Cr. According to the QTAIM analysis, approximately five and a half of the unpaired electrons (5.5 e) are localized on the Cr atom, and the remaining 0.5 e is delocalized over the CC backbone. Notably, there is a charge transfer of 0.3 e from Cr to CC. The preferred position of Cr in the HS CC-Cr system is at the top site, i.e., directly above a C atom (see Fig. 2a). The distance between the Cr atom and the CC backbone is 2.03 Å for the singlet state and 2.50 Å for the HS state, as listed in Table 1. The singlet spin state (from hereafter abbreviated as low-spin, LS) is disfavored by 365 kJ mol−1 relative to HS, see Table 1.
In the case of CC-Fe, the pentet spin state (high spin, HS) is the energetically favored spin state, see Table 1. Once again, the singlet spin state (low-spin, LS) is the most energetically disfavored, with a difference of approximately 282 kJ mol−1 relative to HS, see Table 1. Contrary to the position of Cr in CC-Cr, the Fe atom in CC-Fe is located at the hollow site (see Fig. 2b, c) in the distance of 1.71 (LS) and 3.28 Å (HS) from the CC backbone. For comparison, the hollow sites of CC have been also identified as the energetically favored adsorption sites for alkali metal ions [48]. According to the QTAIM analysis, all four unpaired electrons are localized on the Fe atom in the HS TM-Fe, with only a tiny charge transfer from Fe to CC (0.06 e), see Table 1.
As can be observed, the distance (TM charge) between the Cr or Fe atom and the CC backbone increases (decreases) with a growing number of unpaired electrons (spin multiplicity) in both CC-Cr and CC-Fe systems. This increase in distance is more pronounced in the case of CC-Fe, see Table 1, and it could potentially lead to a destabilization of the studied system. However, the presence of H2 molecules in the vicinity of the TM atom leads to a shortening of the TM-CC distances. This aspect will be explored and discussed in the forthcoming sections, where the focus will be on the LS and HS states for both CC-Cr and CC-Fe systems interacting with up to four hydrogen molecules.
To obtain a better insight into the interaction between the CC backbone and Cr or Fe atom, the potential energy curves of Cr–CC and Fe–CC systems in their energetically preferred HS states were evaluated, see Fig. 3. Optimized Cr-CC and Fe-CC distances of 2.50 and 3.27 Å, respectively, are confirmed by the minima on the potential energy curves for both studied systems. The potential energy curve of Cr–CC interaction contains, besides the global minimum found at 2.50 Å, also a local minimum at approximately 3.0 Å, see Fig. 3a. The energy difference between these two minima is approximately 3.32 kJ mol–1, which corresponds to the Boltzmann ratio of ca. 80:20. A similar double-well on the potential energy curve was also reported earlier for Cr-Cr interaction in dichromium complexes [49, 50], with energy difference between the two minima of 6.75 kJ mol–1 [49]. Interestingly, the double-well reaction energetic profile was recently reported for CO2 reacting with differently charged graphene quantum dots, where the energetic preference of outer or inner minima changes with the increasing negative charge [35]. On the other hand, the potential energy curve of Fe – CC interaction has only one well-defined minimum, see Fig. 3b.
Potential energy curves of Cr–CC (blue) and Fe–CC (orange) systems calculated with the B3LYP-GD3/6-311G** method
3.2 Single hydrogen binding on CC-Cr
In the case of a single hydrogen molecule (H2) binding to the CC-Cr system all three abovementioned modes of hydrogen interaction are illustrated in Fig. 4, along with the corresponding PDOS plots.
B3LYP-GD3/6-311G** optimized structures of CC-Cr with one H2 molecule (top) and PDOS plots (bottom): a side-on H2 binding, b H2 dissociation, c end-on H2 binding. Atomic colors: grey–carbon, pink–hydrogen, blue–chromium
The calculated absolute and relative energies of the LS and HS CC-Cr systems in the presence of interacting H2 molecules are compiled in Table 2, while the calculated Cr–H, Cr-CC and selected H–H distances are shown in Table 3. In the case of CC-Cr + H2 system, the HS end-on H2 binding is energetically the most favored over either the HS side-on H2 binding (with a relative energy of 22 kJ mol−1) or HS H2 dissociation (with a relative energy of 118 kJ mol−1). The reaction energy of the end-on H2 binding (H2 + HSCC-Cr → HSCC-Cr + H2) is –9.6 kJ mol−1. The LS optimized geometries are energetically even less favored. For comparison, in the case of the LS state, the H2 side-on geometry is 134 kJ mol−1 over the preferred HS H2 end-on geometry. Distance between the hydrogens of H2 in the end-on geometry is 0.75 Å, which corresponds to the H–H bond-length in a free H2 molecule. The shorter Cr–H distance is 2.93 Å and the distance between Cr and the CC backbone is 2.54 Å, as indicated in Table 3. Albeit the HS (LS) side-on geometry is energetically disfavored, it is interesting to point out that the distance between the hydrogens in the side-on geometry is 0.78 (0.86) Å and both hydrogens are ca 1.90 (1.71) Å away from Cr. Additionally, the distance between Cr and CC is 2.17 (1.55) Å, see Table 3. For comparison, previously reported B3LYP-GD3/6-311G** Cr–H distances of Cr-doped CC with side-on H2 molecule are approximately 2.1 (1.8) Å for HS (LS), respectively [13], see reference compounds in Table 3. In addition, Xiang et al. reported the Cr–H distance of 1.756 Å for Cr-doped CC plus one H2 using PBE functional with Grimme dispersion correction (spin state not shown) [51]. For the sake of completeness, in the case of H2 dissociation over the HS (LS) CC-Cr, two Cr–H bonds, having lengths of 1.66 (1.60) Å, are formed and the distance between the Cr and the CC backbone is 2.28 (2.04) Å.
A deeper understanding of the nature of the Cr–H interaction can be obtained from the QTAIM analysis. Selected BCP characteristics and DI values of the Cr–H interactions are compiled in Table S1. In the case of the energetically favored HS CC-Cr system with the end-on H2 binding mode, only a weak interaction between Cr and H2 is observed (DI value is 0.11). This interaction falls under the category of van der Waals stabilization, characterized as pure physisorption. This observation is further supported by the absence of orbital overlap in the corresponding PDOS plot (Fig. 4c). Due to the long range (van der Waals) nature of interaction in the HS CC-Cr + H2 end-on system, there are minimal changes in QTAIM charge and spin population compared to CC-Cr, see Table S2. In contrast, the obtained DI(Cr–H) values of 0.28 corresponding to the energetically disfavored HS CC-Cr with side-on H2 binding, point on the formation of η2-dihydrogen bonds [2, 3]. In this case (HS side-on H2 binding), the charge on TM is close to one, indicating oxidation of Cr and the CC backbone is negatively charged, see Table S2. Last but not least, the energetically even less favored H2 dissociation in HS CC-Cr leads to a formation of two equivalent chemical bonds between Cr and H. Formation of Cr–H bonds is supported by visible orbital overlap in the region of around –6 eV in the PDOS plot (Fig. 4b) as well as by the calculated QTAIM parameters (ρBCP of around 0.1 bohr−3 and DI of around 0.8, see Table S1). It is worth noting that a DI value of 1.0 formally corresponds to a single covalent bond [52]. Once again, Cr undergoes oxidation, and both H2 atoms carry a negative charge of – 0.5 e, see Table S2.
3.3 Binding of multiple hydrogens on CC-Cr
The calculated absolute and relative energies of the CC-Cr systems with multiple H2 molecules, in different spin states, are compiled in Table 2. The calculated Cr–H and Cr-CC distances of the CC-Cr systems with up to four H2 molecules are presented in Table 3. It is found that in the case of two H2 molecules bound to the CC-Cr system, the HS state with two end-on H2 molecules is still energetically favored, see Table 2. The reaction energy of the end-on H2 binding (2H2 + HSCC-Cr → HSCC-Cr + 2H2) is –19.8 kJ mol−1. The CC-Cr + 2H2 system with one or both H2 molecules bound via side-on interaction is energetically disfavored by approximately 20 kJ mol−1. On the other hand, the side-on binding of two hydrogens to the LS CC-Cr is disfavored by around 50 kJ mol−1. (Note that in the case of single H2 molecule it was 130 kJ mol−1 and for three H2 molecules LS side-on mode is energetically preferred, as discussed below). In the case of the energetically preferred HS (end-on mode) CC-Cr + 2H2 system, the calculated Cr–H and CC-Cr distances agree with the ones of CC-Cr + H2, being approximately 2.94 Å and 2.52 Å, respectively. In this system with two end-on H2 molecules, the low DI(Cr–H) values (below 0.10, not shown here) are consistent with large Cr–H distances mentioned earlier. Conversely, in the non–preferred LS system, the DI(Cr–H) values of about 0.35 indicate the formation of coordination bond.
As already noted, the LS side-on H2 binding (three η2-dihydrogen bonds) is found to be energetically preferred for CC-Cr + 3H2 and CC-Cr + 4H2 (Cr–H distances are around 1.7 Å). The reaction energy of the side-on H2 binding (3H2 + HSCC-Cr → LSCC-Cr + 3H2) is – 38.9 kJ mol−1 and for four hydrogens –48.7 kJ mol−1, which corresponds to an average binding energy of approximately – 13 kJ mol−1 per one H2 molecule. Note that an ideal energy for reversible H2 molecule adsorption should lie within the range from –14 to –58 kJ mol−1 [53]. In the case of CC-Cr + 3H2 system, the LS state is lower in energy by 9 kJ mol−1 when compared to the HS end-on one, see Table 2. The change of spin state preference can be assigned to the H2 strong ligand field character which favors the existence of TM atoms in the LS states [4]. This preference is achieved after the third H2 molecule becomes coordinated to CC-Cr. In CC-Cr + 4H2, the fourth H2 molecule is found in the distance of 3.37 Å from Cr atom, see Table 3. This finding corresponds with the earlier reports on the H2 binding capacity of the first row TM atoms on graphene-like surfaces [8, 13, 22, 51]. Note that for the LS single H2 molecule bound to CC-Cr, the Cr atom is oxidized (charge close to one) and most (ca. 80%) of negative charge is found on the CC moiety with the rest (ca. 20%) on H2 molecule, see Table S2. The same is confirmed for the side-on LSCC-Cr + 3H2 system, with Cr charge of 0.87 e and CC charge of – 0.51 e (average charge of hydrogens is – 0.06 e). In the case of CC-Cr + 4H2 system, the HS state with four H2 molecules in an end-on mode (Cr–H distances are around 3.0 Å) is disfavored by only less than 1 kJ mol−1 when compared to the preferred LS, see Table 2 (a 42:58 Boltzmann ratio for HS:LS). This unusually low difference in energy suggests the possibility of both scenarios coexisting and should be considered in future studies.
Moreover, the delocalization indexes of Cr–H bonds in the LS CC-Cr + 3H2 system are approximately 0.40, which points on the formation of coordination bonds between the H2 molecules and Cr atom. For comparison, the DI(Cr–H) values between the fourth H2 molecule and the Cr atom are significantly lower (0.01 and 0.02), with no bond critical points detected. Again, such finding corresponds with the calculated Cr–H distances presented in Table 3.
Another interesting parameter which should be monitored is the distance between the Cr atom and the CC backbone. In the case of LS CC-Cr systems, the Cr-CC bond distance increases with the growing number of side-on bound H2 molecules, 1.55 Å (one H2), 1.58 Å (two H2), 1.69 Å (three H2) and 1.67 Å (four H2), see Table 3. In the case of HS CC-Cr, the distance of Cr-CC has the following trend with the increasing number of H2 molecules: 2.17 Å (one H2), 2.52 Å (two H2), 2.30 Å (three H2) and 2.11 Å (four H2). When considering the energetically preferred LS CC-Cr + 3H2 (or + 4H2) system, one can assume that the Cr atom with bound H2 molecules is stabilized over the CC backbone (Cr-CC distances lower than 1.70 Å) due to charge transfer from Cr to CC. For completeness, the optimized structures of the energetically preferred CC-Cr plus two, three and four H2 molecules are shown in Fig. 5.
B3LYP-GD3/6-311G** optimized structures of CC-Cr plus two H2 molecules (a), three H2 molecules (b), and four H2 molecules (c). Atomic colors: grey–carbon, pink–hydrogen, blue – chromium
3.4 Single hydrogen binding on CC-Fe
Similar as in the case of CC-Cr, all three different modes of H2 binding on CC-Fe are considered (side-on H2 binding, H2 dissociation and end-on H2 binding), see Fig. 6. Calculated absolute as well as relative energies of the CC-Fe systems in LS and HS states, with and without H2 molecules are compiled in Table 4. Calculated Fe-H, Fe-CC and selected H–H distances are presented in Table 5. As can be seen from Table 4, the H2 dissociation on CC-Fe in the HS state is energetically preferred over the side-on (83 kJ mol−1) and end-on (36 kJ mol−1) H2 binding modes. The H2 dissociation reaction energy (H2 + HSCC-Fe → HSCC-Fe + H2) is – 45.7 kJ mol−1. Again, in this process, Fe is oxidized and each hydrogen is negatively charged by –0.5 e, see Table S3. The LS CC-Fe has a less negative total energy, regardless the mode of H2 binding, when compared to the preferred HS one by approximately 190 kJ mol−1. The distance between the hydrogens of the dissociated H2 molecule is 3.05 Å, both hydrogen atoms are in the distance of 1.63 Å from Fe and the distance between Fe and CC backbone is 2.66 Å (by 0.6 Å shorter than for the HS CC-Fe system), see Table 5. In this HS system two equivalent chemical bonds are formed between Fe and H atoms, because the DI value is 0.78 in both Fe-H bonds (note that the DI value of single covalent bond is formally equal to 1.0 [52]). Formation of the chemical bond between the Fe atom and both hydrogens can be also seen upon the overlap in the region from –10 to –6 eV in the corresponding PDOS plot (see Fig. 6b). For comparison, the calculated B3LYP-GD3/6-311G** Fe-H distances in a single FeH2 molecule are 1.53 Å and 1.62 Å for LS and HS, respectively, see reference compounds in Table 5. Note that, the HS FeH2 is energetically preferred over the LS FeH2 by approximately 231 kJ mol−1. On the other hand, the experimentally measured Fe-H bond in the FeH2 radical in the gas phase is 1.665 Å [54]. Further comparison can be made to Fe-doped CC, where the side-on H2 binding mode is energetically favored, with the calculated Fe-H distances of 1.78 Å [13]. The Fe atom in Fe-doped CC is bound to the CC backbone more tightly when compared to the one in CC-Fe, hence its interaction with H2 molecule is limited to the formation of η2-dihydrogen bond [13]. Even longer Fe-H binding distances (1.936 Å) were reported for Fe-doped graphene surface composed of 42 C atoms [8]. Such result can be attributed to the use of basis set of lower quality (LanL2DZ) and the missing long-range dispersion correction.
B3LYP-GD3/6-311G** optimized structures of CC-Fe plus H2 molecule: a side-on H2 binding, b H2 dissociation, c end-on H2 binding. Atomic colors: grey–carbon, pink–hydrogen, orange – iron
It is interesting to point out that in the end-on H2 binding mode, the H–H distance is 0.75 Å, while in the side-on binding mode, it elongates to 0.85 Å. This elongation is the usual consequence of the formation of the η2-dihydrogen bond [2, 3] between the metal atom and the H2 molecule (Fe-H distance is 1.66 Å). In the case of the end-on H2 binding on HS CC-Fe, the distance between the proximal hydrogen and the Fe atom is 2.95 Å, see Table 5. Even though, the LS CC-Fe is not favored, in the case of side-on H2 binding mode, the H–H distance is 0.88 Å, and the Fe-H distances are 1.58 and 1.63 Å, see Table 5. In the LS CC-Fe with dissociated H2 molecule, the distance between the hydrogens is 2.19 Å and the Fe-H distances are around 1.50 Å.
3.5 Binding of multiple hydrogens on CC-Fe
In this section, the interaction of CC-Fe with two and three H2 molecules is discussed. In the presence of two hydrogens bound to the CC-Fe system, the HS state with the dissociated H2 molecule is energetically preferred again, see Table 5. The reaction energy for two hydrogens (2H2 + HSCC-Fe → HSCC-Fe + 2H2) is –55.9 kJ mol−1 whereas for three hydrogens is – 67.2 kJ mol−1. The LS (or HS) CC-Fe with two H2 molecules bound in side-on mode is energetically disfavored by 24 (or 39) kJ mol−1 when compared to the H2 dissociated HS CC-Fe one. Distance of the Fe atom from the CC backbone in the HS state with the dissociated H2 molecule is 2.54 Å, see Table 5. The dissociation of one H2 molecule is found to be energetically preferred also in the case of the HS CC-Fe system with three interacting H2 molecules. In the case of the HS CC-Fe + 3H2 with one dissociated H2 molecule, the remaining two H2 molecules are more than 2.7 Å away from the Fe atom. Naturally, the Fe atom becomes occupied upon the formation of two Fe-H bonds, which hinders its interaction with additional H2 molecules. This dissociated HS state is preferred by 45 kJ mol−1 over the HS CC-Fe with three H2 molecules bound in the side-on mode (Fe-H distances are around 1.8 Å), see Table 4. For completeness, the optimized structures of the energetically preferred CC-Fe plus two and three H2 molecules are shown in Fig. 7.
B3LYP-GD3/6-311G** optimized structures of CC-Fe plus two H2 molecules (a) and three H2 molecules (b). Atomic colors: grey–carbon, pink–hydrogen, orange–iron
To sum up, the findings from the previous sections suggest that CC-Fe has the capability to cleave H2 molecules, resulting in the formation of an “iron dihydride” residue that coats the CC backbone. Even though iron dihydride is known to be an unstable molecule, it has already found its role in catalysis or hydrogen storage applications [55, 56]. Based on these results, it can be inferred that Fe-decorated CCs may have potential applications in facilitating the formation of FeH2 intermediates within catalytic cycles, particularly in hydrogenation reactions or related processes. This insight highlights the potential of CC-Fe systems in hydrogen-related catalysis and underscores their relevance in hydrogen storage technologies.
4 Conclusions
In summary, the DFT-level investigation of the interaction between Cr- and Fe-decorated circumcoronenes with H2 molecules revealed distinct preferences for H2 binding modes and spin states in these systems. CC-Cr demonstrates a tunable spin state preference and H2 interaction that changes depending on the number of interacting H2 molecules. It prefers a weak end-on interaction with one and/or two H2 molecules in the HS state. However, with three interacting H2 molecules on CC-Cr, the LS side-on interaction becomes favored. When four H2 molecules are present, both LS side-on and HS end-on modes are close in energy. The LS side-on binding mode on CC-Cr involves the formation of coordination bonds between the Cr atom and three H2 molecules, as evidenced by calculated Cr–H distances of approximately 1.70 Å and corresponding DI values of about 0.4. In contrast, the HS end-on interaction in CC-Cr is of van der Waals character.
On the contrary, CC-Fe consistently prefers the high spin state with one dissociated H2 molecule, regardless of the presence of additional H2 molecules. Upon the interaction with CC-Fe, the H–H bond is cleaved and the FeH2 moiety is formed over the CC backbone. The calculated Fe-H distances of 1.63 Å and DI values of approximately 0.8 indicate the formation of chemical bonds with characteristics of single covalent Fe-H bonds.
In conclusion, CC-Cr appears to be a material suitable for H2 storage due to its tunable spin state preference and ability to interact with multiple H2 molecules. Conversely, CC-Fe demonstrates a consistent capability for H2 activation and H–H bond breaking, making it potentially valuable in catalytic processes that involve hydrogen activation. These findings shed light on the diverse roles that transition metal-decorated CCs can play in hydrogen-related applications.
Data availability
Data available on request from the authors.
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Acknowledgements
This work received financial support from Slovak Grant Agencies APVV (contracts No. APVV-19-0087 and APVV-20-0213) and VEGA (contracts No. 1/0324/24 and 1/0175/23). This work received also financial support from Fundação para a Ciência e a Tecnologia (FCT/MCTES) through national funds (LAQV-REQUIMTE, grant UID/QUI/50006/2020). MM and LB would like to thank the Operational Program Integrated Infrastructure for the project: "Support of research activities of Excellence laboratories STU in Bratislava", Project no. 313021BXZ1, co-financed by the European Regional Development Fund. We are also grateful to the HPC center at the Slovak University of Technology in Bratislava, which is a part of the Slovak Infrastructure of High Performance Computing (SIVVP project, ITMS code 26230120002, funded by the European Region Development Fund) for the computational time.
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Open access funding provided by The Ministry of Education, Science, Research and Sport of the Slovak Republic in cooperation with Centre for Scientific and Technical Information of the Slovak Republic.
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Simona Müllerová: Investigation, Writing – original draft. Michal Malček: Conceptualization, Investigation, Writing – original draft. Lukas Bucinsky: Conceptualization, Writing – review & editing, Supervision. Maria Natália Dias Soeiro Cordeiro: Writing – review & editing, Supervision.
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Müllerová, S., Malček, M., Bucinsky, L. et al. Exploring hydrogen binding and activation on transition metal-modified circumcoronene. Carbon Lett. 34, 1495–1506 (2024). https://doi.org/10.1007/s42823-024-00709-1
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DOI: https://doi.org/10.1007/s42823-024-00709-1