Introduction

The energy crisis and greenhouse effect caused by the emission of carbon dioxide (CO2) are the two serious global problems at the present day and even remain in the next 50 years1, which stimulated the current research interest in efficient conversion of CO2 into valuable fuels and chemicals2,3,4. However, due to the negative adiabatic electron affinity (EA) and large ionization potential (IP), the CO2 molecule is thermodynamically stable and kinetically inert, thus making the conversion difficult under normal conditions5. To overcome these challenges, we need to understand the basic chemical processes of the conversion and seek for highly efficient, cost-effective, and environmentally sound catalysts. Since formic acid (FA) has been widely used as a medium for hydrogen storage and an industrial chemical, catalytic hydrogenation of CO2 to FA becomes one of the most common and promising way to utilize CO2.

Recently, systems containing frustrated Lewis pairs (FLPs) have been found as effective catalysts for H2 activation6,7,8, CO2 reduction9,10,11 and hydrogenation12,13,14 for the production of C1 fuels. As we know that a FLP contains both Lewis acid and base centers, and the most common active Lewis pairs are B/N, B/P, Al/N and Al/P. Furthermore, a cationic Lewis acid component has also been extended to silicon15,16, carbon17, in ref. 9 and even the transition-metal (Zr18,19 and Ti20) complexes, while the Lewis base component has been extended to O9, carbenes21, ethers22, ketones23, and sulfides24. The reduction of CO2 via FLPs usually consists of two major steps: hydrogen activation and hydrogen transfer to CO2, where a hydrogen molecule is first split into a proton (H+) and a hydride (H−), and then CO2 is reduced via a concerted or sequential transfer of H+ and H− to CO2. By theoretical calculations, Liu et al. found a relationship between these two steps, i.e. a stronger FLP results in a lower energy barrier for H2 activation, but in a higher energy barrier for H transfer12.

Inspired by the mechanism of CO2 hydrogenation by FLPs, here we raise a question: whether the clusters consisting of the active element for FLPs such as B/N, B/P, Al/N and Al/P etc. could act as catalysts for H2 splitting and CO2 further hydrogenation?

In the past few years, experiment and theoretical research efforts have been devoted to (XY) n (M = B, Al, Be, Mg; X = N, P, O) nanostructures such as nanocages, nanohorns, nanotubes, and nanowires25,26,27,28,29. Theoretical studies found that the fullerene-like cages (XY)12 with Th symmetry were the most stable geometry30,31. Moreover, B12N12 has been synthesized and detected by laser desorption time-of-flight mass spectrometry32. Al-, Ga- doped33 and Li-, Na- decorated34 stable B12N12 clusters have been also theoretically studied. In addition, previous studies indicated that BN35, AlN36,37 and BeO38,39 clusters can absorb H2 molecularly due to the polar bond between B and N, Al and N, Be and O with different electron affinities. Moreover BN clusters can also capture CO240,41,42. In fact, the most special point for cluster catalysis is that the addition or removal of a single atom can have a substantial influence on the activity and selectivity of reaction, which provides us the basis for converting CO2 to different products with different efficiencies by introducing different atoms to the cluster surface or inside the cluster with more flexibilities and diversities. Based on these points, in the present work we systematically study H2 dissociation (2H*) and CO2 hydrogenation using M12x12 (M = B, Al, Be, Mg; X = N, P, O) cage clusters, and explore the involved mechanisms.

Results and Discussions

Geometry structures

For M12x12 type cluster with Th symmetry as B12N12, Al12N12, B12P12, Al12P12, Be12O12 and Mg12O12, they all consist of eight 6-membered rings (6-MR) and six 4-membered rings (4-MR), as was shown on Fig. 1(a). To improve the binding with CO2 and H2, Al, Ga [Fig. 1(b)] and Li, Na [Fig. 1(c)] are introduced to B12N12 cage, the geometry parameters can be found in Table S1 of Supporting Information, where there are two types of M-X bonds: the bond shared by two 6-MRs (labelled MX-66), and the other one shared between a 4-MR and 6-MR (labelled MX-64), the former is longer than the latter for all studied nano-cages.

Figure 1: Geometry sketches.
figure 1

(a) MX clusters (M1 = B, Al, Be, Mg; X = N, P or O); (b) AlB11N12 and GaB11N12 (M2 = Al, Ga); (c) LiB12N12 and NaB12N12 (M3 = Li, Na).

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H2 and CO2 Activation

In order to determine the configuration with the lowest energy for H2 and CO2 adsorption on the surface of the cluster, a number of different initial structures have been used for optimization. The results of stable H2/M12x12, CO2/M12x12 complexes as well as their corresponding transition state (TS) structures are shown in Fig. 2. For the convenience of discussions, we distinguish the physisorption (P) from the chemical functionalization (C) for the molecules on the cages. As seen from Fig. 2, in the process of physisorption, H2 and CO2 molecules are weakly adsorbed on the clusters with minor changes in geometry. While in the chemical adsorption, H2 is dissociated forming 2H* and CO2 is chemically activated forming CO2*. The corresponding geometry parameters as well as their TSs are shown in Tables S2 and S3, and the interaction energies of H2 and CO2 physisorption and chemical absorption are given in Table S4 of Supporting Information.

Figure 2
figure 2

Geometry sketches for H2 and CO2 physiortioon (P), chemisorption (C) and the transition states (TS) in MX-64 and MX-66 configurations.

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The activation energies of H2 on MX-64 and MX-66 are labeled as and , respectively, while the activation energies of CO2 on MX-64 and MX-66 are labeled as and respectively. The energy barriers for H2 activation () are calculated as the Gibbs energy difference between the (the TS for H2 activation) and the initial state of H2 adsorption:

Similarly, the energy barriers for the CO2 activation () are calculated as the Gibbs energy difference between the (the TS for CO2 activation) and the initial state of CO2 adsorption:

The calculated results are listed in Table 1, and typical structures with H2 and CO2 either in physisorption, or chemisorption as well as their transition states are given in the Fig. 2. All the Gibbs energy barrier for H2 activation on MX-64 are all lower than which of the MX-66. When the activation barrier is overcome, H2 can be dissociated generating hydridic (Ha) and protic (Hb) hydrogens.

Table 1 Gibbs energy barrier for H2 (ΔGH), and CO2 activation (ΔGC), All values are in eV.
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Instead for CO2 activation, the Gibbs energy barrier on MX-64 are all higher than that of the MX-66 except for Al12P12 and Be12O12 clusters. The activation barriers of CO2 are lower than that of H2 for the studied systems except for B12P12. The Al, Ga doped and Li, Na decorated B12N12 cages have lower activation energy barriers for H2 and CO2 than those of the pristine B12N12. This illustrates that the doping with Al and Ga as well as decoration with Li and Na can increase the activity of B12N12 cluster.

To clarify the effect of H2 and CO2 adsorptions on the electronic structures of nano-cages, natural bond orbital (NBO) analyses are performed and the results are listed in Table S5, from which one can see that upon the adsorption, charges on the cages are redistributed due to the geometry change and charge transfer. For example, in all the cases CO2 received electrons from cages resulting in the activation. The charges on M sites in all clusters are decreased upon the H2 adsorption, while increased upon the CO2 adsorption.

The different behaviors in H2 activation on MX-64 and MX-66 are due to the different activities between them. As seen from Table S5, more charges are on M and X sites in MX-64 than those in MX-66, which makes the former more active with a lower H2 activation barrier as compared with the latter one.

2H* transfer mechanism

Two reaction pathways for CO2 hydrogenation on Lewis pair moiety have been identified, one involves the physisorped CO2 reacting with the chemisorbed 2H*, and the other one involves the physisorped H2 reacting with CO2*. For the latter, the reaction barrier for hydrogenation of the activated CO2* is usually very high as found by Ye12 (2.65 eV in UiO-66-P-BF2 catalyst) and by us (2.84 eV for MX-64 and 2.97 eV for MX-66 of B12N12), and this pathway leads to the formation of chemisorbed HCO and OH ([HCO + OH]*) instead of HCOOH as shown in Supporting Information (Fig. S1). Consequently, in the following discussions, we only focused on the first path way for HCOOH formation.

According to the first pathway, CO2 is firstly physiosorbed on MX-2H*(P), and then forms HCOOH (C) via the transition state (TS) (Fig. 3). For B12N12 nano cage, the reaction barrier for H2 activation on MX-64 (1.19 eV) is lower than that on MX-66 (1.27 eV), while the 2H* transfer barrier on MX-64 (1.28 eV) is higher than that on MX-66 (1.19 eV). The IRC calculations show that the hydride and protons are transferred to CO2 simultaneously. In other words, the H transfer step is essentially the donation of a hydride and a proton from the ion-pair products to CO2 via a concerted mechanism. Tables S6 and S7 list the corresponding geometry parameters, transition states, and interaction energies for CO2 and HCOOH.

Figure 3
figure 3

Geometry sketches for 2H* transfer from MX cluster to CO2 in physisorption (P), chemisorption (C) and the transition states (TS) on MX-64 (a) and MX-66 (b) of the MX-2H* clusters.

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When attempting to bind CO2 with Mg12O12−2H* in a way shown in Fig. 3(P), we cannot find any stable structures from our calculations. While CO2 can directly bonds at Mg and O sites (as shown in Fig. 4a) with a stronger binding energy of −0.5 eV (Table S4). Similarly, when introducing HCOOH to Mg12O12 shown in Fig. 3(C), one H atom of HCOOH was taken away by the O atom in Mg12O12 nano-cage (Fig. 4b). This suggests that Mg12O12 is not competent to be the catalyst for CO2 hydrogenation to HCOOH.

Figure 4
figure 4

(a) Physisorption of CO2 on Mg12O12−2H*; (b) HCOOH on Mg12O12−2H*. (The jade-blue sphere representing Mg atom, red sphere representing O atom, gray sphere representing C atom and white sphere representing H, respectively).

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The activation energies of H2 transfer on MX-64 and MX-66 are labeled as and and respectively. The energy barriers for H2 activation () are calculated as the Gibbs energy difference between the TSHT (the TS for H2 transfer) and the initial state of CO2 the physisorbed on MX-2H*:

The corresponding activation energies for H2 transfer are listed in Table 2, where one can see that the activation energies of H transfer are all higher than 1.0 eV, except for Be12O12 cage. Just the opposite to the H2 activation process in Table 1, all the Gibbs energy barriers for 2H* transfer on MX-64 are all higher than that of MX-66.

Table 2 Gibbs energy barrier for H2 transfer (ΔGHT) to form HCOOH, all values are in eV.
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Based on the overall consideration of H2 activation and 2 H* transfer barriers as listed in Tables 1 and 2, one can find that introducing Al, Ga, Li, and Na to B12N12 cage has definitely decreased the H2 activation energy barrier but increased the 2H transfer energy as compared to the pristine B12N12.

In order to be more intuitive, a potential energy surfaces of the reaction pathway are shown in Fig. 5 showing a balance between H2 activation and H transfer. Thus, the interaction between the cluster and H2 is extremely important, the stronger catalyst with more strength to activate the hydrogen molecule would promote a faster hydrogen activation process. On the other hand, a stronger catalyst has more strength to keep the hydrogen, thus would slow down the hydrogen transfer process. This is in accordance with the general Sabatier principle43.

Figure 5
figure 5

Relative potential energy surfaces for H2 activation and 2H* transfer pathway in Al, Ga doped, Li, Na decorated B12N12 and pristine B12N12 with Mx-66 bond as example.

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To understand the trend of protonation activation barriers for the studied nanocages, we analyze the charges on C site of CO2. In the free standing state, C carries 1.069 e. When CO2 is adsorbed with MX-66 configuration on B12N12-2H*, NaB12N12-2H* and LiB12N12-2H*, the charges increase to 1.086, 1.084, and 1.087 e, respectively. The increased charges on C site make it more active to easily bind H with smaller barriers. The similar mechanism can also be applied to other cages.

2H* transfer one by one with stepwise mechanism

When checking the overall the activation energy barrier of H2 activation and transfer (seen from Tables 1 and 2), one can find that many clusters such as Al12N12, NaB12N12 and AlB11N12 etc. have lower H2 activation but higher 2 H* transfer barriers. To search for a lower barrier of H transfer, the other mechanism needs to be investigated further. We find that a new stepwise mechanism exists for CO2 hydrogenation only on Be12O12 cage as shown in Fig. 6.

Figure 6: Relative potential energy surfaces for H2 activation and H* stepwise transfer pathway on Mx-64 (redline) and MX-66 (blue line) bond for Be12O12.
figure 6

(The yellow sphere representing Be atom, red sphere representing O atom, gray sphere representing C atom and white sphere representing H, respectively).

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By following this reaction mechanism, the first H transfer is a rate-limiting step-with an activation energy of 0.22 eV on MX-64 bond (red line on Fig. 6) and 0.36 eV on the MX-66 bond (blue line on Fig. 6), In contrast, the 2H* transfer activation on MX-64 and MX-66 bonds is 0.45 eV and 0.24 eV, respectively. Then, we can conclude that 2H* simultaneously transfer to CO2 on MX-66 bond has lower activation energy than that of the one H transfer stepwise, but on MX-64 bond the situation is opposite.

For practical applications of an efficient catalyst, both the H2 activation and transfer barriers should be comparable or lower than 1 eV44. Among all systems studied here, Be12O12 is the most promising catalyst, where the H2 activation barrier is close to 1 eV (1.04 eV) on MX-64 bond, and the following H* transfer barriers are all lower than 1 eV, (0.45 eV by the 2H* simultaneously transfer mechanism, while 0.22 eV by the H* stepwise transfer mechanism). Therefore, the reaction pathway on Be12O12 MX-64 bond has the lowest barrier based on the overall consideration of the H2 activation (1.04 eV) and H* stepwise transfer (0.22 eV).

Conclusions

In summary, based on the DFT and MP2 calculations, a series M12X12 nano-cages have been studied for activating H2 and CO2 to form HCOOH. The hydrogenation process mainly consists of H2 activation to 2H*, and then 2H* further transfer to CO2 forming a HCOOH molecule. Two kinds of H* transfer mechanisms are found: one involves 2H* simultaneous transfer, and the other is a stepwise H* transfer to CO2. The two mechanisms result in the same product HCOOH. Moreover, Al, Ga doped and Li, Na decorated B12N12 cages have lower H2 activation energy barriers, but higher 2H* transfer activation barriers than that of the pristine B12N12. For practical applications, in order to have an efficient catalyst to reduce CO2, we should search for a catalyst that has a balance between the energy barriers for H2 activation and the H transferring. Among all the systems studied here, Be12O12 is found to be the most promising catalyst, its reaction pathway on MX-64 bond has the lowest barriers (1.04 eV for H2 activation and 0.22 eV for H* transferring). This conclusion would motivate experimental work in the future.

Methods

Since many theoretical calculations have demonstrated that different DFT functions (e.g. B97D, ω-B97X-D, and M06-2X) and basis sets (e.g. 6–31 G*, 6–31 + G**) led to very similar results for the systems only containing main group elements for H2 activations7,8,11. In this work, all the geometry optimizations are performed at the M06-2X/6–31 + G** level as implemented in Gaussian 09 package45. Solvent effects are taken into account by using the polarizable continuum model (PCM) with toluene as a solvent. The highly parameterized, empirical exchange correlation functional, M06-2X, developed by Zhao and Truhlar, was shown to better describe the main-group thermochemistry and kinetics than other density functionals such as B3LYP46. Moreover, this hybrid density M06-2X functional has been previously proved to have a good reliability in computing molecular binding energies of H2 and CO2 on FLPs47. Frequency calculations are carried out at the same level to characterize the nature of the stationary points along the reaction coordinates. No imaginary frequencies were found for the local minima, and one and only one imaginary frequency was found for the transition state. The Natural Bond Orbital (NBO 3.1) program48, was used to calculate the natural charges at the M06-2X/6–31 + G** level of theory. The thermal contributions at room temperature (298.15 K) including the specific free energies were obtained from a harmonic analysis, and accurate electronic energies were obtained from frequency calculations using Møller-Plesset second-order perturbation theory (MP2)49,50 with the cc-pVTZ triple-ζ quality basis51,52. Using the optimized geometries and starting from the TS, intrinsic reaction coordinate (IRC) calculations are performed to verify the true connection of the reactants, the transition states and the products for both H2, CO2 activation and H transfer processes.

Additional Information

How to cite this article: Zhu, H. et al. Be12O12 Nano-cage as a Promising Catalyst for CO2 Hydrogenation. Sci. Rep. 7, 40562; doi: 10.1038/srep40562 (2017).

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