Article pubs.acs.org/JPCA
Chemical Reaction CO+OH• → CO2+H• Autocatalyzed by Carbon Dioxide: Quantum Chemical Study of the Potential Energy Surfaces Artem ̈ E. Masunov,*,†,‡,§,∥,⊥ Elizabeth Wait,†,‡ and Subith S. Vasu# †
NanoScienece Technology Center, ‡Department of Chemistry, §Department of Physics, and ∥Florida Solar Energy Center, University of Central Florida, 12424 Research Parkway, Ste 400, Orlando, Florida 32826, United States ⊥ National Research Nuclear University MEPhI, Kashirskoye shosse 31, Moscow, 115409, Russia # Center for Advanced Turbomachinery and Energy Research (CATER), Mechanical and Aerospace Engineering University of Central Florida, Orlando, Florida 32816, United States ABSTRACT: The supercritical carbon dioxide medium, used to increase efficiency in oxy combustion fossil energy technology, may drastically alter both rates and mechanisms of chemical reactions. Here we investigate potential energy surface of the second most important combustion reaction with quantum chemistry methods. Two types of effects are reported: formation of the covalent intermediates and formation of van der Waals complexes by spectator CO2 molecule. While spectator molecule alter the activation barrier only slightly, the covalent bonding opens a new reaction pathway. The mechanism includes sequential covalent binding of CO2 to OH radical and CO molecule, hydrogen transfer from oxygen to carbon atoms, and CH bond dissociation. This reduces the activation barrier by 11 kcal/mol at the rate-determining step and is expected to accelerate the reaction rate. The finding of predicted catalytic effect is expected to play an important role not only in combustion but also in a broad array of chemical processes taking place in supercritical CO2 medium. It may open a new venue for controlling reaction rates for chemical manufacturing.
1. INTRODUCTION Oxy−fuel combustion in supercritical carbon dioxide (sCO2) offers several advantages in fossil energy technology.1 It can significantly increase power plant efficiency2 and drastically reduce pollutants.3 In oxy−fuel combustion, the fuel is burned in pure oxygen (O2) and CO2 diluent instead of nitrogen from the air.4 This yields combustion products consisting mainly of CO2 and H2O, avoids pollution by nitrogen oxides, and opens the possibility for easy carbon capture and sequestration.5 To reduce wasting fuel or oxidant, oxy−fuel systems may operate close to stoichiometry, so that the combustion temperature is controlled by adjusting the amount of diluent.6 The increase in efficiency is determined by the near-liquid density of sCO2 working fluid before entering the turbine.7,8 However, the effects of supercritical CO2 on the combustion kinetics are not well understood. Computational studies can greatly assist in this understanding,9−12 considering instrumental limitations of kinetic experiments at high pressure.13 Here we focus on OH• + CO → H• + CO2 reaction, the second most essential reaction in combustion, which represents the main heat release step.14 Because of its importance in combustion, it had been studied extensively. Rate constants for OH + CO reaction were found to be nearly independent of temperature between 80 and 500 K, while increasing with temperature above 500 K.15 This strong non-Arrhenius © 2016 American Chemical Society
behavior has been attributed to the intricate minimum energy paths (MEP), which have near-isoenergetic barriers in the entrance (OH• + CO) and exit (H• + CO2) channels15,16 and long-lived intermediate species. Several ab initio potential energy surface (PES) studies17−19 report existence of two possible intermediates, HOCO and HCO2, which form in the OH• + CO reaction.17 The minimum energy pathway proceeds via formation of cis-HOCO intermediate (Figure 1). Another intermediate, HCO2, has a lower activation energy for hydrogen dissociation but a relatively high activation barrier for its formation from trans-HOCO via intramolecular isomerization. The previous computational studies were focused on chemical transformations in vacuum or inert diluent.15,16,18,19 The effects of pressure, temperature, and quantum-mechanical tunneling on the kinetics of the reaction were studied.17 To the best of our knowledge, the effects of supercritical CO2 on the rates of product formation had not been analyzed yet. However, it is widely known that the presence of the small molecules (such as water) may alter the reaction pathways20 or molecular conformations,21 and either accelerate22 or slow down23,24 the reactions both in the gas phase and in the condensed state Received: March 30, 2016 Revised: June 1, 2016 Published: June 28, 2016 6023
DOI: 10.1021/acs.jpca.6b03242 J. Phys. Chem. A 2016, 120, 6023−6028
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The Journal of Physical Chemistry A
(including main-group atomization energies, barrier heights, and noncovalent interaction energies). However, long-range dispersion is still missing in the M11 functional.32 For this reason, we used Grimme’s three-body dispersion correction (GD3)33 with S8 and SR6 parameters chosen to be S8 = 0.0 and SR6 = 1.619 (values, optimized for a similar M06-2X functional). The 6-311G** basis set34 was selected for compatibility with CBS-QB3 model chemistry.35 Since CBSQB3 method employs optimization at uncorrected DFT level, it is not expected to describe van der Waals (vdW) complexes well. Therefore, we replace B3LYP functional with the dispersion-corrected M11+GD3 method to obtain geometries and frequencies. The modified compound method is dubbed CBS-QM11 and is benchmarked below. The pathway search was made using the following protocol: (1) optimization of the product/reactant van der Waals complex; (2) relaxed potential energy scan along selected bond formation/breaking; (3) transition state (TS) optimization starting form the highest point on that scan; (4) frequency analysis for identification of that TS as first-order critical point; (5) an intrinsic reaction coordinate36 (IRC) search for two (forward and backward) minimum energy pathways (MEP) starting from the transition state; (6) optimization into the local minima to continue both MEPs found in step 5 and then a repeat of step 2 above.
Figure 1. Relative energies (kcal/mol) of the stationary points along the reaction pathways for the HO+CO → H+CO2 reaction. The lowest energy pathway is in green, the intermediate one is in blue, and the higher one is in red (adapted from ref 19).
environment. In this work, we investigate how PES for the OH + CO → H+CO2 reaction is altered in the presence of one CO2 molecule. We hypothesize that trans-HOCO to HCO 2 isomerization may proceed via an intermolecular mechanism with a lower activation barrier. We also investigate the possibility of a spectator CO2 molecule influencing the activation barriers by forming a weak complex with the HOCO intermediate.
3. RESULTS AND DISCUSSION We begin with validation of the M11 exchange-correlation functional30 and the CBS-QM11 model chemistry for the reactive system in question. The energies of the stationary points, predicted in this work at M11+GD3/6-311G** and CBS-QM11 theory level, are reported in Table 1. The highlevel ab initio values from the literature are also shown for comparison. One can see that our M11+GD3 results deviate from the most recent CCSD(T)-F12 ab initio data by less than 3 kcal/mol (less than 2 kcal/mol for CBS-QM11 results). The maximal deviation between two advanced ab initio theory level (CCSD(T) and MRCI//CAS) is somewhat larger (3.8 kcal/ mol). Therefore, the M11+GD3 theory level has sufficient accuracy for this reactive system, and it is used to summarize our results in Table 2, Figures 2 and 3 and in the following discussion.
2. COMPUTATIONAL DETAILS All calculations were performed with the Gaussian 2009 suite of programs.25 Geometry optimizations and normal-mode analysis were performed by using density functional theory (DFT). DFT was shown to be an invaluable tool to predict linear26 and nonlinear spectra,27,28 as well as hyperpolarizability.29 For this work, we selected the M11 exchange-correlation functional.30 This functional belongs to the class of meta-GGA hybrids, where a larger fraction of Hartree−Fock exchange31 ensures the accurate description of the activated complexes, while dependence on the kinetic energy density recovers an accurate description of the stable molecular configurations. Unlike older meta-GGA hybrids, M11 includes range separation, so that fraction of Hartree−Fock exchange varies from 42.8% at short interelectron distances to 100% at the asymptotic limit. M11 was optimized for across-the-board superior performance
Table 1. Relative Energies (kcal/mol) for the Critical Points on PES for the CO + OH• → CO2 + H• Reaction, Predicted at Different Theory Levelsa
a
stationary point
MRCI//CASb
CCSD(T)c
CCSD(T)-F12d
M11/6-311G**+GD3
CBS-QM11
trans-HOCO cis-HOCO OH···CO OH···OC tor-TS cis-TS2 trans-TS4 cis-TS1 trans-TS1 HCO2 H·CO2-TS3 H + CO2 OH + CO
0 1.92 26.98 28.39 9.36 30.67 38.42 29.85 25.38
0 1.86 27.91 28.98 9.33 31.54 37.96 32.72 29.14
0 1.77 27.33 28.37 9.30 32.00 38.37 32.71 29.01 15.95 20.88 6.97 29.59
0.00 1.22 29.49 30.06 9.58 32.08 38.28 34.84 30.63 18.75 20.15 7.13 31.57
0.00 2.25 28.08 28.87 9.41 31.60 37.74 33.85 29.86 17.87 19.86 6.19 30.27
See text for details. bReference 17. cReference 18 dReference 19. 6024
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The Journal of Physical Chemistry A Table 2. Predicted Relative Energies (kcal/mol) of Stationary Points on Reaction Pathwaysa stationary points, Figure 1
OH···OC
cis-TS1
cis-HOCO
cis-TS2
H + CO2
ΔE
M11/6-311G**+GD3 M11/6-311G**+GD3+ZPE stationary points, Scheme 1
30.06 26.04 RC11
34.84 31.33 TS12
1.22 1.05 IN13
32.08 26.89 TS14
7.13 1.18 IN15
30.86 25.84 ΔE
M11/6-311G**+GD3 M11/6-311G**+GD3+ZPE stationary points, Scheme 1 (contd)
25.16 21.95
44.67 42.61 TS16
24.47 24.11 IN17
36.49 36.27 TS18
17.44 19.20 PC19
19.51 20.66 ΔE
14.27 13.31 IN23
16.82 12.23 TS24
5.07 0.15 PC25
17.03 13.17 ΔE
IN15
M11/6-311G**+GD3 M11/6-311G**+GD3+ZPE stationary points, Scheme 2
17.44 19.20 RC21
34.47 32.37 TS22
M11/6-311G**+GD3 M11/6-311G**+GD3+ZPE stationary points, Scheme 3
25.16 22.47 RC31
25.87 22.77 TS32
−4.98 −4.11 IN33
29.69 25.19 TS34
4.87 −0.20 PC35
34.67 29.30 ΔE
M11/6-311G**+GD3 M11/6-311G**+GD3+ZPE stationary points, Scheme 4
25.52 22.47 RC41
26.79 24.38 TS42
−2.95 −1.95 IN43
47.96 43.45 TS44
6.07 −3.41 PC45
50.91 45.40 ΔE
M11/6-311G**+GD3 M11/6-311G**+GD3+ZPE
28.50 25.04
29.44 26.81
−2.52 −2.08
47.75 43.35
4.86 0.03
50.27 45.43
a For the first two lines, the trans-HOCO intermediate is chosen as the reference point; for all other lines, the trans-HOCO + CO2 system is chosen as the reference point. Both potential energies and enthalpies at 0 K, including the zero-point vibrational energy (ZPE), are reported. Activation barrier heights (ΔE) for the rate-limiting step are shown in the last column.
Scheme 1. Structures of the Complexes, Intermediates, and Transition States, along the Pathway witha Covalently Bound CO2 Molecule
Figure 2. Relative energies (kcal/mol) of the reaction pathway shown on Scheme 1, with one covalently bound CO2 molecule (trans-HOCO + CO2 system is chosen as the reference point).
Figure 3. Relative energies (kcal/mol) of two of the reaction pathways shown shown on Scheme 2 (in black) and Scheme 3 (in red) with spectator CO2 molecule (the trans-HOCO + CO2 system is chosen as the reference point).
search from nonbonded complex IN17 and located two transition states: one for intermolecular transfer of H atom to O atom (TS16), and another one for C−H bond dissociation (TS18). While IRC search from TS18 resulted in products vdW complex PC19, the MEP starting from TS16 lead to the polymeric intermediate IN15. Polymerization of carbon dioxide was studied both experimentally and theoretically under high pressure condition.37,38 It is conceivable that IN15 or similar species may play a role in this polymerization. Next, we investigated several bond dissociation pathways for IN15 and
Next we investigate a possibility that CO2 molecules from the environment will bind covalently and alter the reaction mechanism. Specifically, the H atom transfer from O to C atom in trans-HOCO intermediate may proceed via intermolecular mechanism, thus relieving the steric constraints of 3membered ring in transition state of 1,2-hydrogen shift (transTS4 in Figure 1). Indeed, we discovered the intermolecular mechanism, and it is reported in Scheme 1. We started our 6025
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The Journal of Physical Chemistry A found CO dissociation via TS14 has the lowest energy. Resulting vdW complex IN13 finally dissociates into the reactants vdW complex RC11 via transition state TS12. The energies of the respective species are reported in Figure 2, and Table 2 (which also lists activation barrier heights). As one can see, the activation barrier for the mechanism shown on Scheme 1 is lower than the one for the mechanism with no additional CO2 molecules (by 11 kcal/mol before and by 5 kcal/mol after zero-point vibrational energy correction). Here we assume collisional thermalization for all the intermediates. It is a reasonable assumption at high pressure limit typical for supercritical fluid. We also investigate the structures of the nonbonded complexes between HOCO intermediate and spectator CO2 molecule. The spectator CO2 molecule forms relatively strong (3−6 kcal/mol) van der Waals complexes with both reactants and intermediates. The geometry of these complexes is determined not only by dispersion interaction, but also by stronger donor−acceptor contacts.39−41 While oxygen atoms may play the role of lone pair donors, both hydrogen and carbon atoms are electron deficient and are able to play the role of acceptors. Therefore, cis-HOCO may form two contacts of this nature (IN23, Scheme 2), while trans-HOCO may form
Scheme 3. Structures of the Complexes, Intermediates, and Transition States along the trans-HOCO Bond Dissociation Pathway with a Spectator CO2 Molecule
Scheme 4. Structures of the Complexes, Intermediates, and Transition States along the trans-HOCO Bond Dissociation Pathway with a Spectator CO2 Molecule
Scheme 2. Structures of the Complexes, Intermediates, and Transition States along the cis-HOCO Bond Dissociation Pathway with a Spectator CO2 Molecule
complexes where trans-HOCO donates electron pair (IN33 and IN43), the OH bond becomes stronger and activation barrier is nearly doubled (50.91 and 50.27 kcal/mol). Since MEP data in Scheme 3 and 4 are quantitatively similar, only Schemes 2 and 3 are illustrated in Figure 3. Finally, in complex IN55, the trans-HOCO intermediate forms a hydrogen bond to an additional CO2 molecule. Breaking the OH covalent bond in this intermediate is accompanied by formation of another covalent bond instead of dissociation. Since this mechanism does not result in formation of products, we do not report associated potential energies. However, the transition complex in the hydrogen transfer process includes the formation of a partially covalent C··C interaction, and this may play role in diffusion of hydrogen atom through the dense CO2 medium.
only one donor−acceptor contact. In the latter case, the hydroxyl oxygen atom (IN33, Scheme 3), or the terminal oxygen atom (IN43, Scheme 4) donates its lone pair. One contact also forms when the hydrogen of trans-HOCO acts as the lone pair acceptor (IN55, Scheme 5). We optimized the three complexes described (IN23, IN33, and IN43) and located the transition states, corresponding to the O−H and OC−OH bond breaking in each of them. The relative energies of the stationary points are collected in Table 2. The pathway without CO2 is also shown for comparison. One can see that in all four mechanisms the rate limiting step is dissociation of OH bond. The activation barrier for this step for the mechanism described on Scheme 2 (34.67 kcal/mol) is comparable with the activation barrier in the absence of additional CO2 molecule (33.30 kcal/mol). In case of
4. CONCLUSIONS We investigated the potential surfaces for the CO + OH• → CO2 + H• reaction in the presence of additional CO2 molecule. Depending on the complex structure, the activation barrier for covalent OH bond dissociation is either comparable or strongly (17−20 kcal/mol) increased. We also found a new reaction 6026
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Scheme 5. Structures of the Complexes, Intermediates, and Transition States along the trans-HOCO Hydrogen Exchange Pathway with Another CO2 Molecule
mechanism, where the additional CO2 molecule covalently binds the OH radical and then to the CO molecule, forming a new intermediate. This intermediate undergoes hydrogen transfer from oxygen to carbon atoms with subsequent CH bond dissociation. All activation barriers along the reaction pathway are less than 20 kcal/mol, considerably lower than activation barrier in the absence of additional CO2 molecule (31 kcal/mol). Hence, the product of CO + OH• → CO2 + H• reaction catalyzes the overall process (which is known as the autocatalytic effect). The new reaction mechanism with predicted autocatalytic effect is expected to play an important role not only in combustion, but also in broad array of chemical processes in supercritical CO2 medium. It may open a new venue for controlling reaction rates for chemical manufacturing. The experimental verification of this discovery is presently under way.
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AUTHOR INFORMATION
Corresponding Author
*(A.E.M.) Telephone: 1-407-374-3783. E-mail: amasunov@ucf. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the Department of Energy (Grant Number DE-FE0025260). The authors acknowledge the National Energy Research Scientific Computing Center (NERSC), and the University of Central Florida Advanced Research Computing Center (https://arcc.ist.ucf.edu) for providing computational resources and support. A.E.M. is grateful to the Russian Science Foundation, Contract No. 1443-00052, administered by Center of Photochemistry, Russian Academy of Science.
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