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Solid-State Polymerization of CO from Catalytic Photoexcitation: An Ab Initio Molecular Dynamic Study Xue Yong, John S Tse, and Choong-Shik Yoo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10157 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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The Journal of Physical Chemistry

Solid-State Polymerization of CO2 from Catalytic Photoexcitation: An Ab Initio Molecular Dynamic Study Xue Yong,1 John S. Tse1* and Choong Shik Yoo2 1

Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada Department of Chemistry, Washington State University, Pullman, WA 99164-2816, U.S.A. Molecular dynamics, polymerization, CO2

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ABSTRACT: Extended form of CO2 (X-CO2) with fully sp3 hybridized C atoms bonded with O atoms is a novel type of energetic material. However, the synthesis requires formidably high temperature and high pressure for commercial production. We propose an alternate approach to produce X-CO2 by photo-excitation of molecular CO2 and through ab initio molecular dynamic simulations, demonstrated the formation of a similar polymeric solids at considerably lower pressure and temperature. This polymeric XCO2 phase can be recovered under ambient pressure with an estimated energy density of 8.63 kJ/g.

INTRODUCTION Solid carbon dioxide (CO2) has several high-pressure polymorphs ranging from molecular to fully extended covalent solids (X-CO2)1. The fully 4-coordinated polymorphs (CO2-V) has a silica-like structure and was discovered by heating molecular solids of CO2 (CO2-III) at 40 GPa to 1800 K2. The reaction is promoted by the large compression energy (P∆V) that helped to overcome the energy barrier in breaking the very strong double C=O bonds. The X-CO2 formed is a new material and expected to possess novel properties, such as optical nonlinearity, low compressibility and high energy density. However, the synthesis conditions2-4 are too severe for practical industrial production. For the practical use of X-CO2 it is imperative to find an effective path to lower the transition pressure and recover it under ambient condition. It is well-known that upon photolysis at 4.89 eV5 the closed shell linear molecular CO2 is excited to a bent triplet6-8 diradical. Di-radicals are chemically active and, in the solid state, while the relative molecular positions are restricted, the radicals can be effective precursors for polymerization reactions. Furthermore, due to the high reactivity of the radical, it is expected that only a small number of photo-excited radicals are needed to initiate the process. To explore the feasibility and the conditions for the formation of extended X-CO2 using photo-excitation, ab initio Molecular dynamics (AIMD) calculations in the in the isobaric-isothermal (NPT) ensemble were performed on CO2 Cmca molecular solid phase (CO2-III)9, the experimentally observed stable phase at 300 K and pressure higher than 10 GPa. The outline of the paper is as follows. First, the polymerization of solid CO2 upon single molecule excitation at different thermodynamic conditions is investigated. The effect of the parameters used in the variable pressure MD dynamics is presented in this section. Then the effect of multiple excitations on the polymerization process were studied. Finally, the decomposition and energy density of the quenched recovered X-CO2 obtained are evaluated. This paper ends with a discussion on how the proposed method to exper-

imental verification and the relevance to industrial production of extended CO2 polymers. COMPUTATIONAL METHODOLOGY AIMD simulations in the NPT10-11 ensemble with Langevin thermostats were performed on CO2 Cmca phase9. A model consisted of 96 atoms (32 CO2) was used. Excited triplet CO2 in the solid was generated by promoting an electron from the ground to the conduction band of a randomly selected molecule. Spin polarized MD calculations were performed at a variety temperature and pressure conditions, viz. 1200 K and 2000 K at 12 GPa, 500 K, 800 K and 1200 K at 15 GPa, 300 K, 350 K, 400 K, 500 K, 800 K and 22 GPa, 800K at 25 GPa, and 800 K at 50 GPa. An integration time-step of 1.0 fs was used. Each MD trajectory was run for at least 10 ps and, in several cases, up to 150 ps. In most calculations, the fictitious mass for the crystal lattice was set to 10. To examine the effect of the box dynamics, additional calculations were performed with a heavier fictitious mass of 20. To investigate the effect of multiply excited CO2, two randomly chosen CO2 molecules was excited and calculations were performed at 12 GPa, 15 GPa and 22 GPa. To calculate the energy content upon thermal decomposition of the recovered product, an interfacevacuum region was created by doubling the longest axis of the model system. The decomposition energy was estimated from total energy difference calculated from micro-canonical NVT calculations of the recovered product at 80 K and after decomposition at 350K under ambient pressure. The Vienna ab initio simulation package, VASP12-15, a plane wave basis electronic structure code employing projected augmented wave potentials for the atoms was used for all calculations. The Perdew, Burke, and Ernzerhof (PBE)16 exchange-correlation functional was employed. A plane wave basis set with an energy cutoff 400 eV was used. In view of the large supercell and that the system is a large gap insulator, only one k-point (Γ) was used to sample the Brillouin Zone. RESULTS AND DISCUSSIONS

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Single CO2 Excitation A series of MD simulations at different temperatures and pressures were performed. At 12 GPa, with a single excited CO2 molecule, no reaction was observed at 1200 K and 2000 K after 10 ps. Similarly, no reaction was observed at 22 GPa and 300 K even though the CO2 molecules were brought closer to each other by higher pressure (Figure 1a). However, as will be explained in more detail later (vide supra), the delay in the reaction was simply a kinetic effect and not an indication that the reaction of CO2 di-radical with neighbouring CO2 molecules is not feasible at low pressure. At 22 GPa and 350 K, the formation of a dimer between the photo-excited CO2 with a neighbour was observed (Figure 1b). Upon further heating to 500 K, CO2 molecules started to react with each other and eventually formed polymeric chains (Figure 1c). It is obvious that high temperature facilitated the movements of CO2 molecules and promoted reactions between the excited triplet CO2 with its neighbours. Thus, to accelerate the reaction further, the model system was heated to 800 K. At this temperature, chemical reactions proceeded almost immediately after a CO2 molecule was excited by rapidly propagating to other CO2 molecules through the formation of new chemical bonds. The X-CO2 formed (Figure 2) was a mixture of sp2 and sp3 carbon. In this instance, out of the 32 CO2 molecules in the model, 13 carbon became four-coordinated (sp3), and 19 became three-coordinated (sp2) to the oxygen atoms. When the temperature was raised to 2000 K, the number of fourcoordinated C increased to 16, which accounted for half of the total number. If the pressure was increased to 25 GPa at 800 K, the number of four-coordinated C further increased to 19 with 13 three-coordinated C. The number of sp2 and sp3 C centres in the product was significantly affected both by the temperature and external pressure as the proportion of fourcoordinated C continued to increase at 50 GPa. The mobility of CO2 molecules in the solid increased with the temperature while the intermolecular distances decreased as the pressure rose. Consequently, the probability for contacts of the photoexcited CO2 with its neighbours was increased, and this accelerated the reaction and increased the concentration of sp3 carbons.

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The theoretical results show the combination of photoexcitation and high temperature is an effective way of promoting the polymerization process in the synthesis of X-CO2. To confirm the kinetic nature of the reaction, we repeated the MD calculations using a heavier fictitious mass and a much longer simulation time. The heavier mass of the simulation box slowed down the fluctuations of the model box allowing more time for the CO2 molecules to migrate and increasing the chance for the radical to interact with its nearest neighbours. Indeed, the same reaction was observed even at 350 K and 15 GPa. This is strong evidence suggesting that, in the experimental time scale, the synthesis of X-CO2 could be achieved at even much lower temperature and pressure.

Figure 2 Projected final snapshots of the solid CO2 model in (a) ab-plane and (b) ac-plane phase from the MD trajectory, showing the structural transformation upon single-molecule photoexcitation at 22 GPa and 800 K. (C in grey and O in red).

To examine the pathway leading to the reaction product, we analyzed the MD trajectory at 22 GPa and 800 K from a single excited CO2 since the simulation resulted in a fully extended polymeric structure consisting of three and four-fold coordinated C atoms. A snapshot of the final structure from the MD trajectory is shown in Figure 2. The reaction product can be described as an extended 3D-network constructed from linked planar CO3 and tetrahedral CO4 units. Among the four coordinated C, three of the C-O bond lengths are about 1.35 Å while the remaining one is usually longer, about 1.44 Å. The four oxygen atoms form a tetrahedron that links with other species via corner sharing. The three-coordinated CO3 form a planar carbonate structure with one short C-O bond of 1.22 Å and two longer C-O bond lengths ranging from 1.35 Å to 1.40 Å. The hybridization of C changed from sp in CO2 to the sp2 in CO3 and then to the sp3 in the four-coordinated environment. The presence of CO4 and CO3 in the product are revealed in the calculated vibrational density of states shown in Figure 3. To clarify the assignment, we extracted the atom positions of the CO4 and CO3 species from the MD trajectory and calculated the individual vibrational density of states. The results are presented in Figure S1. We found the CO3 and CO4 vibrations are coupled as both species are linked through a common oxygen atom. Nevertheless, the peak at 1829 cm-1 can be assigned to the carbonyl stretch. The peaks at 985 cm-1, 1185 cm-1, and 1386 cm-1 are characteristic of CO417-18 with some contribution from the –O-(C=O)=O- vibrational modes of CO3 carbonates19-20, while the peak at 640 cm-1 is unique to the C-O-C stretching/bending mode in CO4 tetrahedral.

Figure 1 Final snapshots of the solid CO2 model upon the singlemolecule photo-excitation at 22 GPa and (a) 300 K (b), 350 K, and (c) 500 K. (C in grey and O in red).


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(a)

(b) Figure 3 Vibrational density of states (in arbitrary unit, a.u.) calculated from the Fourier transform of the atomic autocorrelation functions of the X-CO2 obtained at 800 K and 22 GPa.

To examine the kinetic effect, we extended the NPT simulation at 22 GPa and 800 K to 134 ps. Salient features of the structural changes in the model with time are shown in Figure 4. In the first 120 ps after initial compression, apart from the shortening of c and b cell lengths, little change in the a axes or the lattice angles was observed. In this initial stage, the external pressure led to compression of the (010) planes of the Cmca structure. After 120 ps, the crystal lattice was found to have sheared, and the length of the a axes started to contract. Figure 5 shows several snapshots taken from the MD trajectory. The reaction was initiated by the photo-excited bent CO2 perpendicular (Figure 5a) to the (010) plane. This formed a reaction centre that helped to bring neighbouring CO2 in the adjacent layer along the a direction closer together. The O atoms of the bent CO2 then reacted with another molecule in the (010) plane (Figure 5b) producing CO3. This resulted in the shortening of the b and c cell axes. When more CO2 molecules were moved to the reactive CO3, O-C-O- centres then chains and cages started to form. Upon further compression, the planar CO3 were twisted, and the π bonds were broken. Subsequently, when another CO2 situated perpendicularly to the planar CO3 was brought closer, CO4 (Figure 5d) was formed. This process led to a sudden decrease in the c axes at 58.1 ps (Figure 4). A closer examination found the structure to be composed of a mixture of CO3 and CO4 (Figure 6a−b) with the CO3 located mostly in the (010) plane. In addition, the number of CO4 had increased to 5 within 0.1 ps. The distortion of the lattice from orthorhombic symmetry (Figure 4) occurred at around 120 ps, due to the large length change of the a axes as more CO3 reacted and increased the number of CO4 units from 10 to 13. This analysis demonstrates the importance of kinetics as a longer MD run permitted the CO2 to propagate, and the relaxation of the crystal lattice led to the formation of more CO4 species. Scheme 1 depicts the schematic of the reactions. Since triplet CO2 is a di-radical, the primary process was the insertion of the unpaired electron on the radical O atom into the C atom (σ*) of a neighbouring CO2. This initiated the subsequent polymerization reactions forming -O-C-O- chains.

(c)

Figure 4 Evolution of (a) lattice constants (in Å), (b) lattice angles (in degree, °), and (c) lattice volume (in Å3) of the CO2 structural model with time in the NPT AIMD simulations at 22 GPa and 800 K and 50 GPa and 800 K upon single-molecule photo-excitation.



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occurred when the C atoms of the two CO3 groups situated on top of each other. This configuration, however, was less energetically favourable due to the repulsion interaction between the CO3 units. Consequently, at 50 GPa and 800 K, most of the sp3 C centres were CO4.

Figure 5 Snapshots of the solid CO2 model from the NPT simulation at 22 GPa and 800 K at different stages: (a) initial (b) 2.0 ps, (c) 15.0 ps, and (d) 15.5 ps upon single-molecule photoexcitation. Blue circles highlight the structural changes described in the text. (C in grey and O in red).

(a)

(b)

Figure 6 Snapshots of the solid CO2 model from the NPT simulation at 22 GPa and 800 K at (a) 58.1 ps and (b) 58.2 ps upon single-molecule photo-excitation (C in grey and O in red).

Scheme 1 The primary reactions of the polymerization of solid CO2 upon single-molecule photo-excitation

To explore how pressure can affect the carbon coordinations, we examined the trajectory of the MD calculations performed at 50 GPa and 800 K, which was a continuation from the compression at 22 GPa and 800 K. We found that the CO2 model system had sheared significantly with an obvious shortening of the a axes (Figure 4a). The Higher pressure helped to increase the number of distorted planar CO3 species. We found two different reactions involving “twisted” CO3 on the formation of sp3 carbon atoms which are depicted in Figure 7 and Figure S1. The first one was via the formation of a C-O bond with a close-by CO3 (Figure 7a−b and Figure S1a−b). The interaction was facilitated by one of the O atoms of the second CO3 located on top of the twisted CO3. A second way was to react with yet another CO3 through the formation of a C-C bond (Figure 7c−d and Figure S1c−d). This reaction only

Figure 7 Snapshots of the solid CO2 model from the NPT simulation at 50 GPa and 800 K at step (a) 90, (b) 100 (c) 394, and (d) 400 upon single-molecule photo-excitation. Blue circles highlight the structural changes described in the text. (C in grey and O in red).

The Effect of Multiple Excitation Encouraged by the results from a single photo-excited CO2, we expected that multiple excited CO2 would further enhance the reaction kinetics and reduce the pressure and temperature required. To investigate this effect, two randomly selected CO2 molecules were excited. At 12 GPa at 300 K and 500 K, two isolated dimers were formed immediately, but no further reaction was observed in the next 50 ps. When the temperature was increased to 1200 K, C-O chains with sp2 C and sp3 C centres (Figure 8a) were formed after 40 ps. Once again, the long molecular separation at low compression apparently limited the rate of the polymerization reaction. When the pressure was slightly increased to 15 GPa at 500 K, two CO2 dimers were formed readily (Figure 8b). When raised to 800 K (Figure 8c), two tetramers were formed. At 1200 K, the migration of CO2 increased rapidly and more CO2 moved to the nascent CO2 dimer, forming a trimer and the polymerization reaction proceeded to form extended CO2 with mixed sp2 and sp3 C-O coordinations (Figure 8d). As above, we also examined the atomic trajectory from the simulation with two excited CO2 at 15 GPa and 1200 K. Figure 9 shows selected snapshots from the NPT calculation. Similar to the case of the single excited CO2 presented above (vide supra), the reaction started with a bent excited CO2. In this case, both excited bent CO2 reacted with their respective neighbours immediately, creating the dimer precursors for the subsequent polymerization reaction. Since there were two reaction centres, the polymerization reaction rate had accelerated, transforming solid CO2 rapidly into connected CO3 and CO4 in just under 1 ps. The results showed the initial formation of the dimer and the subsequent polymerization process was similar to that of the single excited CO2, but the reaction obviously had proceeded much faster. In experiments,


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since multiple CO2 will be excited simultaneously, we expect the formation of X-CO2 will occur readily. Therefore, multiple photo-excited CO2 lower the energy barrier while high temperature increases the diffusion rate and fully extended X-CO2 can be synthesized at lower temperature and pressure than static compression to 40 GPa and 1800 K.

pressure and low temperature and has a favourable heat of decomposition making it a promising high-energy material.

Figure 10 (a) Quench-recovered structure of the high pressure and high temperature (22 GPa and 800 K) form of X-CO2 to ambient pressure. (b) The decomposed product of X-CO2 obtained. (C in grey and O in red).

Figure 8 Final snapshots of the solid CO2 model after two CO2 molecules were photo-excited from the AIMD simulations at (a) 12 GPa and 2000 K, 15 GPa and (b) 500 K, (c) 800 K, and (d) 1200 K. (C in grey and O in red).

Figure 9 Structural evolution of the solid CO2 model after two CO2 molecules were photo-excited from NPT calculations at 15 GPa and 1200 K at step (a) 100 and (b) 105. (C in grey and O in red).

Decomposition Processes and Energy Density Estimation To investigate whether the predicted high pressure (22 GPa and 800 K) product can be quench-recovered, an NPT simulation at 0 GPa and 300 K was performed for 10 ps. MD results confirm the X-CO2 product is metastable, and the structure can be maintained at 300 K and 0 GPa (Figure 10a), suggesting it could be recovered under ambient pressure and temperature. We studied thermal decomposition of the product and the accompanying heat evolution. For this purpose, the longest axes of the model was doubled to create a solid-vacuum interface. After a 10 ps NVT calculation at 0 GPa and 300 K, the X-CO2 decomposed into CO2, cyclic carbonate rings and a few short C-O chain molecules (Figure 10b). From the total energy difference between the initial X-CO2 and the decomposed product, we found the reaction was exothermic with an energy release of about 8.63 kJ/g. It is noteworthy that the density of the recovered X-CO2 produced by photo-excitation is 2.65 g/cm3, which is about two-thirds of the recovered CO2-V21. A lower density is because not all the C were converted to fully sp3 bonds. Yet the theoretical results show the X-CO2 generated by photo-excitation of solid Cmca CO2 is stable at ambient

CONCLUSIONS A new and efficient synthetic route for X-CO2 through catalytic photo-excitation of molecular solid CO2-III has been proposed. Results from AIMD simulations showed that a 3D extended X-CO2 network with four- and three- coordinated C atoms connected by oxygen can be formed even at pressure as low as 15 GPa and 1200 K. These conditions are much lower than 40 GPa and 1800 K, which are required for static compression. Under a realistic experimental time-frame, the pressure and temperature required are expected to be significantly lower than the theoretical prediction. The mechanisms for the polymerization reaction are identical for single or multiple excited CO2 molecules. Furthermore, the reaction is controlled by the freedom of CO2 migration and therefore should proceed readily on a realistic experimental time scale at substantially lower pressure and temperature than the predicted conditions. The CO2 singlet → triplet excitation energy of 4.89 eV is accessible with two-photon absorption. The energy is lower than that of materials usually used as anvils for high pressure experiments, e.g., the indirect band gap energy of diamond of 5.5 eV22 and cubic boron nitride (c-BN) of 6.36 eV23 under ambient pressure. It is noteworthy that the band gap of diamond increases with pressure up to 370 GPa, as shown recently from X-ray energy loss spectroscopy24. Therefore, the laser photon will not be blocked, and the experimental condition for the proposed photo-excited reaction is achievable in a diamond anvil cell experiment25. It is noteworthy that there has been a report that the formation of CO2-V can be catalyzed with prolong irradiation with near infrared-radiation26 where a visible laser has been used for Raman measurements in situ during the irradiation with the heating NIR laser. It is possible two-photon UV excitations may help to initialize the polymerization reaction. Furthermore, electronic transition from the ground singlet to excited triplet state is spin forbidden and will require spin-orbit coupling and, therefore, it is not an efficient process. However, as shown here, only a few excited CO2 are needed to prompt the reaction. The relatively mild pressuretemperature synthetic conditions by photoexcitation may be amendable to the current large-press technology for industrialscale production.

ASSOCIATED CONTENT Supporting Information Vibrational density of states plots and selected snapshots from MD trajectories. The Supporting Information is available free of charge on the ACS Publications website.



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AUTHOR INFORMATION Corresponding Author *Tel.: + 1-306-966-6410. E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

NOTES The authors declare no competing financial interest

ACKNOWLEDGMENT XY acknowledges the financial support from the China Scholarship Council. JST ad CSY would like to thank Dr, Judah Goldwasser and John Paschkewitz for the DRAPR (W31P4Q-121-0009) support.

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