A putative mechanism for capture of carbon dioxide by quartz, mediated by triboelectric charging

Mechanical activation of quartz grains in a dry atmosphere causes triboelectric charging that can capture CO 2 quantitatively (Th ø gersen et al ., https://doi.org/10.1016/j.cplett.2021.139069). Based on electron structure calculations, we propose a mechanism for this process. According to the mechanism, CO 2 is inserted in the quartz lattice to form an anchored CO 3 moiety. Predicted 13 C chemical shifts of this product agree with solid-state 13 C NMR measurements.


Introduction
It has been shown [1] that triboelectric charging of quartz grains can capture CO 2 from an atmosphere consisting of a mixture of 20.8 % O 2 , 78.7 % N 2 and 0.5 % CO 2 . In view of the current interest in mitigating global warming, it seems useful to understand this process in more detail. Here we use electron structure calculations to propose a reaction mechanism for capturing CO 2 by triboelectric charged quartz (SiO 2 ) grains. For simplicity, we consider an atmosphere consisting of CO 2 only. The proposed mechanism is supported by experimental 13 C NMR chemical shifts.

Model and calculation details
The model for the quartz grain is based on the normal α-quartz structure [2] augmented with OH groups on surface Si-atoms to ensure that all Si-atoms have the normal valence. The bulk formula for the quartz model is Si 20 O 46 H 12 .
The principle in our calculations is to find potential energy minima for the model and from these derive thermodynamic properties using statistical mechanics relations [3]. This is done for the neutral grains and for mono anions and mono cations as well. The potential minima are found using the geometry optimization techniques incorporated in the software package, Gaussian09 [4]. The calculations are performed at the density functional level of theory using the implementation B3LYP (or UB3LYP). Most calculations use a basis set with diffuse functions, 6-31 + G(d,p), useful for anions and open shell configurations. Thermodynamic properties are calculated at 298 K using this basis set. The calculations of chemical shifts use the more extensive basis set, B3LYP/ 6-311 + G(2d,p) [5].

NMR experiments
The 13 C{ 1 H} magic-angle spinning (MAS) NMR spectra were acquired at 100.49 MHz on a Bruker Avance 400 NMR (9.39 T) spectrometer using a Bruker 4 mm CP/MAS NMR probe. The single-pulse spectrum employed a ~ 45 • excitation pulse (2.5 µs for γB 1 /2π = 50 kHz), 1 H decoupling during acquisition (γB 2 /2π = 100 kHz), a spinning frequency of 10.0 kHz, a relaxation delay of 15 s and 8192 scans. The 13 C { 1 H} CP/MAS spectrum used a spinning frequency of 10.0 kHz, a CP contact-time of 3.0 ms, and a 13 C rf field strength of γB 1 /2π = 48 kHz. A linear ramp of γB 2 /2π = 28-55 kHz was used for the 1 H rf field during the CP contact time. The 1 H rf field was γB 2 /2π = 100 kHz for the initial 90 • pulse and 1 H decoupling during acquisition. 32 k scans were accumulated with a relaxation delay of 4.0 s. 13 C chemical shifts are relative to tetramethyl silane (TMS). The sample was sieved prior to the analysis and the fine fraction (<100 μm) of the tumbled quartz grains were loaded into a 4 mm zirconia NMR rotor inside a nitrogen filled glove box.

Putative mechanism for capture of CO 2 in quartz
The mechanism builds on a picture of the triboelectric effect [6][7][8] in our experiments. When quartz sand is mechanical activated (tumbled) in a closed container, the grains become electrically charged and there is an exchange of charge between the grains. In a related investigation [8], it was shown that energies as high as 21 eV are involved, indicating the grains can be highly excited. When the grains relax, they may not necessarily return to the ground state in one step but may proceed via intermediate states with unusual geometries that may facilitate reactions, which do not occur in the ground state.
The proposed mechanism consists of three elementary reactions. The first one is creation of an anionic quartz grain: The experimental estimate of the electron affinity of quartz is 0.8 eV [9]. The calculated structure of the anionic quartz grain is visualized using the package GaussView6 [10] and shown in Fig. 1A. The structure is similar to that of the neutral grain in the sense that all Si-atoms have four bonds to O-atoms. However, the triboelectric process creates a manifold of anionic structures. Fig. 1B shows one of such structures. It is unusual as one Si-atom has five bonds while another (nearby) Si-atom has three bonds. These sites present a dislocation, where chemical reactions are likely to take place.
The second elementary reaction is a reaction of CO 2 with the quartz anion to create a CO 2 -quartz anion complex, [CO 2 , Si 20 O 46 H 12 ] − . Specifically, CO 2 reacts with the Si-atom with three bonds (Fig. 1B).
ΔG • for this reaction is − 30 kJ/mol. A picture of the CO 2 -quartz anion complex is shown in Fig. 2A.
In the third elementary reaction, the CO 2 -quartz anion complex loses its excess charge to become neutral. For example, this can occur by collision with a positively charged quartz grain: not in a potential minimum. However, ΔG • for the reaction between the opposite charged two ions is expected to be large and negative, especially as the ionization energy for quartz is high, 9.1 eV [9]. The loss of an electron in [CO 2 Fig. 2B.
The overall reaction of CO 2 with quartz may be considered as an insertion of a CO fragment (from CO 2 ) in a Si-O bond mediated by the triboelectric charging. In the process, the fragment grabs an O-atom from the quartz lattice with a concurrent reduction in the number of bonds at a neighboring Si-atom from five to four.
Geometry optimizations on the structure in Fig. 2B at the level UB3LYP/6-311 + G(2d,p) and subsequent NMR calculations leads to a prediction of a 13  However, the C-atom is close to an H-atom (2.339 Å), which may not be representative of the experimental case. To estimate the effect of a nearby H-atom on the 13 C chemical shift we have simulated a quartz grain with two CO 3 units, shown in Fig. 3. In this grain, the distances from the C-atom to the nearest H-atom are 4.645 Å and 2.919 Å, respectively and the corresponding chemical shifts are 143.7 and 149.8 ppm.

Comparison with NMR data
Single-pulse (SP) 13 C and 13 C{ 1 H} cross-polarization (CP) MAS NMR spectra (Fig. 4) have been obtained for a quartz sample tumbled in a 13 C enriched CO 2 atmosphere (815 mbar) for 174 days. The SP 13 C NMR spectrum is dominated by resonances at 141.7 ppm and 124.6 ppm whereas a low-intensity peak is observed at 110.8 ppm. The 13 C{ 1 H} CP/ MAS spectrum is quite similar, however, the high-frequency peak is shifted to 146.8 ppm, indicating that this broadened resonance contains a least two 13 C species in different environments. The resonance at 124.6 ppm can be assigned to molecular CO 2 confined to the quartz lattice and with 1 H atoms in its near vicinity, following earlier studies of tumbled quartz in borate glass flasks [11]. The apparent chemical shifts of 141.7 ppm and 146.8 ppm for the two components of the broad resonance agree very well with the calculated 13 C chemical shift for the two CO 3 complexes shown in Fig. 3. The 13 C{ 1 H} CP/MAS NMR experiment transfers magnetization from 1 H to 13 C via 1 H -13 C dipolar spin-spin couplings, which are inversely proportional to the cube of the internuclear distance [12]. Thus, a shorter 1 H -13 C distance will result in a stronger intensity enhancement for relatively short CP contact times. The component at 146.8 ppm is identified by its stronger intensity enhancement compared to the peak at 141.7 ppm, implying that the 1 H -13 C distance is shorter for the 146.8 ppm peak as compared to the 141.7 ppm resonance. This is in full agreement with the calculated values for the quartz cluster in Fig. 3, adding support to the assignment of the resonances at 141.7 ppm and 146.8 ppm to different CO 3 moieties of the type shown in Fig. 3. It is noted that carbonate ions (CO 3 2− ) in inorganic compounds resonate in the 13 C chemical shift range of approx. 175-160 ppm, whereas bicarbonate ions (HCO 3 − ) possess resonances at slightly lower frequencies, 155-165 ppm [13][14][15]. Thus, the calculated and observed resonances in the range 141-147 ppm represent a quite different type of CO 3 units, as described above. Finally, the minor resonance, observed at 110.8 ppm, is not identified so far.

Discussion
The proposed mechanism for incorporating CO 2 into the quartz lattice rests on the creation of a reactive site and on charge exchange A relevant question is, if the mechanism will describe the kinetic isotope effect [3], for example, the possible change in the reaction rate on replacing 13 CO 2 by 12 CO 2 . The three elementary reactions are not expected to be appreciably sensitive to isotope exchange. However, a second-order isotope effect may be incorporated into the mechanism by considering the fate of the reaction product [CO 3 , Si 20 O 45 H 12 ]. Once it is produced, it may become an anion by triboelectric charging. Its geometry will essentially be the same as that of the neutral species indicating that oscillations between the reaction product and its anionic state are irrelevant for an isotope effect. However, the free energy of [CO 3 , Si 20 O 45 H 12 ] − is 31 kJ/mol higher than that of the anion in Fig. 2A, which is the channel leading to the reaction product. A transition state will connect these two anions. The transition state is not identified, but if the vibration mode in the transition state involves a C-atom then we expect a difference in the reaction rate for capture of 13 CO 2 and 12 CO 2 .

Summary
A mechanism for capturing CO 2 by quartz, exposed to triboelectric charging of the grains, is proposed. The triboelectric effect creates an environment where the grains can exchange charges. An anionic quartz grain with a dislocation serves as an active site for capturing CO 2 to form an anionic complex. This complex rearranges to a product with an anchored CO 3 moiety when the excess negative charge is removed by the triboelectric effect. The calculated standard free energy change for the overall reaction of capturing CO 2 into the quartz lattice is − 40 kJ/mol.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability
Data will be made available on request. The sharp resonance at 125 ppm is assigned to 13 CO 2 confined to the quartz lattice and the broader resonance at 141-147 ppm to 13 CO 3 moieties created as a CO moiety (from CO 2 ) is inserted in a Si -O bond in a triboelectric activated quartz grain. The minor resonance at 110.8 ppm is not identified so far.