Structure of Carbon-Coated C12A7 Electride via Solid-State NMR and

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Structure of Carbon-Coated C12A7 Electride via Solid-State NMR and DFT Calculations Ilya V. Yakovlev, Alexander M. Volodin, Evgeniy S. Papulovskiy, Andrey S. Andreev, and Olga B. Lapina J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08132 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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The Journal of Physical Chemistry

Structure of Carbon-Coated C12A7 Electride via Solid-State NMR and DFT Calculations Ilya V. Yakovlev‡†, Alexander M. Volodin‡, Evgeniy S. Papulovskiy‡, Andrey S. Andreev‡, Olga B. Lapina‡†* ‡ Boreskov Institute of Catalysis, Novosibirsk 630090, Russia † Novosibirsk State University, Novosibirsk 630090, Russia

ABSTRACT

C12A7 calcium aluminate electride (C12A7:e-) is a novel inorganic functional material with mayenite crystal structure. Due to its physical and chemical properties, C12A7:e - can find application as a catalyst carrier in various reactions, including ammonia synthesis. However, the low specific area of this material imposes limitations on its use in catalysis. This limitation can be circumvented by synthesis within carbon nanoreactors as the carbon coated particles resist sintering. Here, we investigate by solid state

27

Al Nuclear Magnetic Resonance (NMR)

spectroscopy the structure of C12A7 electride prepared by the carbon nanoreactor process. An accurate determination of NMR parameters via simultaneous application of both DFT calculations and solid state NMR proved the structure of carbon-coated C12A7 electride to be identical to the one of materials synthesized by more conventional routes. It was shown that the conducting electrons of C12A7:e- do not alter the cage structure thus leading to similar NMR

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parameters of electride and insulating (cement) samples. Finally, the carbon nanoreactor process led to a significant synthesis temperature decrease of C12A7 structure formation.

INTRODUCTION Until recently, calcium aluminate Ca 12Al14O33 (C12A7 in cement notation) also known as mayenite was used only as an aluminate cement constituent. In the past decade, its unusual crystal structure has attracted many studies. The group led by Hideo Hosono has shown that C12A7 can find a lot of applications as a functional material: catalyst and a catalytic support 1-4; an ion beam source5; an insulator with photoinduced conductivity6. A variety of applications arises from the crystal structure of C12A7. Its unit cell can be denoted as [Ca24Al28O64]4+ + 2O2-, where [Ca24Al28O64]4+ is a stable rigid lattice consisting of 12 “cages”, and the two oxide ions form a relatively free anion sublattice that occupies 2 out of 12 cages. The oxide O2- ions occupying the cage can be substituted with other anions such as OH-, H-, O-, NH2-, F-, Cl-, or even e-, etc.5 The composition of the anionic sublattice determines the electrical and chemical properties of C12A7. In particular, the material containing caged electrons is of especially high interest due to its unusual stability at room-temperature for an electride, i.e. an ionic compound with the electrons playing the role of anions. It can be potentially utilized as a heterogeneous catalyst support or an electron donor due to its low work function. Applicability of C12A7 electride (C12A7:e -) to industrial catalytic processes depends on the upscaling of highly dispersed powder material. The generally proposed synthesis method includes a high-temperature (1600 °C) sintering stage provoking a formation of large particles of low specific surface area.2 Recently, the size stability of C12A7 nanoparticles has been achieved by covering their surface with a carbon shell.7 A set of characterization techniques, such as X-ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR) and Transmission Electron


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Microscopy (TEM), shows that the material synthesized inside the carbon coating constitutes C12A7:e- nanoparticles with the size of ~ 100 nm.7 However, the detailed structure of carbon coated electride still remains to be established. High resolution solid-state NMR is one of the most informative experimental techniques of nanoparticle characterization. Solid-state NMR, being a method especially sensitive to the shortrange order of the material, provides information regarding the local environment of the observed nucleus through the different NMR parameters, in particular the anisotropic chemical shift and quadrupolar interaction tensors, and the relative orientation of their principal axes.8 This can be achieved by the use of high magnetic fields and modern experimental techniques, such as magic angle spinning (MAS) and Multi-Quantum MAS (MQMAS)9,10, in combination with simulation software and ab initio calculations.11 However, traditional experiments under static conditions remain invaluable for accurate determination of the angles that describe relative orientation of these tensors. The structure of C12A7 includes two inequivalent tetrahedral aluminum sites: Al1 bonded to three bridging oxygen atoms and one non-bridging oxygen (NBO) atom (often denoted as Q3 group), and Al2 bonded to four bridging oxygen atoms (often denoted as Q 4 group). As the local environment of Al1 site is more asymmetric compared to the one of Al2, it is reasonable to expect higher electric field gradient (EFG) values at the Al1 site, which in turn lead to a higher quadrupole coupling constant CQ given by (in Hz)

CQ 

e2 qQ h ,

where eq is the largest principal value of the EFG and Q is the quadrupolar moment of the nucleus.8


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In their works concerning calcium aluminate cements, Müller et al.12 and Skibsted et al.13 have shown that the 27Al NMR spectra of the C12A7 phase consist of two lines corresponding to two aluminum sites with chemical shifts typical for tetrahedral environment and distinctive difference in CQ. These observations led to the conclusion that the line with the largest CQ (10-11 MHz) corresponds to the Al1 site and the line with the smaller CQ (~4 MHz) corresponds to the Al2 site. Oppositely, in an amorphous glass with the stoichiometry of C12A7, Neuville et al. 27

reported the occurrence of only one

Al NMR resonance line even when using MQMAS. This

line broadened by a Gaussian distribution of CQ corresponds to a single tetrahedral site, so there is no indication of cage structures in the glassy C12A7.14 The only work dedicated to the 27Al NMR study of the electride form of C12A7 was published by Matsuda et al15 who investigated the influence of the caged electron density on the structure of C12A7:e- using static 27Al NMR. At high electron densities, they observed for the Al1 site a paramagnetic shift outside the typical range for tetrahedral coordination (~140 ppm) caused by interaction with the electrons of the cage-conducting band. Thus, the existing literature demonstrates that the structure and NMR parameters of C12A7:X - depend strongly on the synthesis method and the kind of caged anions. Herein we present the results of

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Al solid-state NMR study of the C12A7 electride and how

its structure is affected by the carbon shell at different calcination temperatures up to its melting point (~1450 °C). Even though the quadrupolar interaction is considered to give the main contribution to the line width of

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Al NMR spectra, we show that in the case of C12A7, the

chemical shift anisotropy (CSA) also must be taken into consideration for a correct description of the line shapes. The NMR data are supported by ab initio DFT calculations of NMR parameters whose results are in a good agreement with the experimental spectra.


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EXPERIMENTAL C12A7 was synthesized with calcium oxide obtained from thermal decomposition of calcium carbonate (Reachim, special purity) and aluminum hydroxide (Condea Pural SB-1) mixed together in the C12A7 stoichiometric ratio (12CaO : 14Al(OH)3) in distilled water. The wet mixture was then calcined in a furnace under Ar flow in a graphite crucible at temperatures ranging from 550 to 1450 °C, thus providing the first series of samples labeled as C12A7-T, where T is the calcination temperature. A second series of carbon-coated samples was obtained by mixing the C12A7-550 powder with polyvinyl alcohol (Reachim, Russian state standard specification SSS 10779-78) in 7:3 ratio and subsequent calcination of the mixture in Ar flow (0.5 l/hour flow rate) at the required temperature in the same range. The samples of the second series were labeled C12A7@C-T, where @C denotes the carbon coating and T is the calcination temperature. A more detailed description of the synthesis protocol can be found elsewhere.7 27

Al NMR spectra were recorded under ambient conditions using Bruker Avance 400 and

Bruker Avance 500 spectrometers at Larmor frequencies of ν0(27Al)=104.31 MHz and ν0(27Al)=130.39 MHz, respectively. Magic angle spinning experiments were conducted on the Bruker Avance 400 spectrometer with magic angle (MAS) spinning frequencies of 15 kHz and 30 kHz. The samples were placed inside a 4 mm zirconia rotor for static and 15 kHz MAS experiments and 2.5 mm zirconia rotor for 30 kHz MAS experiments. Both static and MAS

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Al

NMR spectra were recorded by a simple one-pulse sequence with a short π/20 pulse duration of 0.2 μs and a recycle delay of 1 s. [Al(H2O)6]3+ was used as an external reference with isotropic chemical shift of 0 ppm. Computer simulations of NMR parameters were performed using the NMR5 program developed at the Boreskov Institute of Catalysis.16 This program considers anisotropic chemical


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shielding, first and second orders of quadrupolar interaction, C Q distribution and line broadening, as well as the angles between the chemical shift and quadrupolar interaction tensors principal axes. Satellite transitions were not observed in static experiments, so only the central transition spectra were used for fitting. All ab initio calculations were carried out using the periodic DFT method as implemented in the CASTEP software package.17 Exchange-correlation PW91 (Perdew-Wang) functional18 was used in conjunction with ultrasoft pseudopotentials19, 20 and a plane-wave basis set. Geometry optimizations were performed with a basis set cutoff energy of 450 eV using the BFGS algorithm.21 Calculations with higher cutoff energy did not result in any significant changes. Only the Γ-point was used for the Brillouin zone integration as it was considered sufficient due to the large supercells used in the calculations (20x20x20 Å). Self-consistent electron density calculations were performed until a convergence of 10-6 eV was reached and the structure optimization calculations were stopped when the forces were less than 0.02 eV/Å. Calculations of 27Al NMR parameters following the structure relaxation were carried out employing the PBE exchange-correlation functional22 and the GIPAW method11. RESULTS AND DISCUSSION The line shape in solid-state NMR of

27

Al is generally described with 8 parameters.

Anisotropic chemical shift tensor that corresponds to the shielding of the nucleus by electrons and currents is described with 3 parameters: isotropic chemical shift δiso, chemical shift anisotropy Δδ and chemical shift asymmetry ηδ. Since the

27

Al nucleus has a non-zero electric

quadrupole moment, it also interacts with EFG created by electrons and surrounding atoms. This interaction is often the most dominant in 27Al spectra and cannot be completely averaged to zero using MAS experimental technique.23 Quadrupolar coupling is usually characterized by 2


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parameters: the quadrupole coupling constant (QCC) CQ and the asymmetry parameter 0 ≤ η ≤ 1. The remaining 3 parameters are Euler angles α, β and γ that describe the relative orientation of chemical shift and quadrupolar interaction tensors. Figure 1 displays a comparison of 15 kHz MAS spectra of C12A7 and C12A7@C samples calcined at different temperatures. All spectra contain a relatively narrow and intensive peak at ~70 ppm and a broad resonance stretching from -25 ppm to 100 ppm corresponding to Al2 and Al1 of C12A7 structure, respectively. Calcination of the samples at 1030 °C (Fig. 1a) led to the appearance of an additional signal at ~10 ppm. Based on the strong dependence of 27Al chemical shift on the coordination number of Al site, this signal can be assigned to an octahedral site in residual alumina or in other calcium aluminate phases. Both are considered as impurities. Increasing the calcination temperature up to 1250 °C (Fig. 1b) led to the complete disappearance of the impurities thus leading to a pure C12A7 phase at this stage of the synthesis. The spectrum of the sample without carbon coating includes several peaks corresponding to 4-, 5- and 6coordinated aluminum sites, displaying the presence of different amorphous transition phases. The spectra of samples calcined at 1380 °C (Fig. 1c) are almost identical, thus stating the lesser importance of carbon shell for C12A7 synthesis at this temperature. Comparing the effect of carbon shells at different temperatures, the main conclusion to be drawn is that the carbon coating reduced the temperature required for the synthesis of C12A7. This decrease of the synthesis temperature may happen due to a mechanism described by Kim et al24. Briefly, C12A7 needs template ions to compensate the positive charge of framework and facilitate cage formation. As the samples described in this paper were synthesized under inert gas atmosphere, it is possible that the carbon coating served as a source of C 2- template anions.


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Figure 1. 27Al MAS NMR spectra of C12A7 (bottom, blue) and C12A7@C (top, red) calcined at 1030 °C (a), 1250 °C (b), 1380 °C (c). Spinning sidebands are denoted with *.

Measurement of chemical shift and quadrupolar tensor parameters of C12A7 is necessary to determine how the carbon coating affects the structure. The fact that the line of Al1 is spread over several hundred ppm leading to an overlap with the line of Al2 hampers the determination of NMR parameters based only on the experimental spectra. To provide guidance, we used ab initio calculations of NMR parameters.


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The original structure for DFT calculations was taken from Palacios et al. (ICSD #241241). 25 The structure contains two aluminum atoms, Al1 and Al2. Two spherical clusters were created by removal of all the atoms at a distance longer than 5.5 Å from them (Fig. 2). This radius was considered sufficient to minimize edge effect. Hydrogen atoms were added to compensate the charge.

Figure 2. Two spherical clusters with 5.5 Å radius centered around the Q3 group of Al1 (left) and the Q4 group of Al2 (right) sites used in DFT calculations. All atoms are depicted with colored spheres according to the table in the top right.

As assumed, both aluminum species exhibited an isotropic chemical shifts in the range typical for tetrahedral coordination. Moreover, the quadrupolar coupling constant CQ of Al1 was significantly larger than the one of Al2 due to the asymmetry of its local environment (see Table 1). The calculations also predicted that both Al1 and Al2 sites should exhibit chemical shift anisotropy that can cause some changes in the static line shapes, even though they would be severely broadened by second order quadrupolar interaction. These parameters were used as a


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starting point for the fitting of the experimental NMR spectra (both static and MAS obtained in the field of the Avance 400 spectrometer) of C12A7 and C12A7@C electrides calcined at 1450 °C. The best fits for C12A7-1450 and for the C12A7@C-1450 electrides are presented in Figure 3.

Figure 3.

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Al NMR central transition spectra of conventionally synthesized C12A7-1450 (a –

30 kHz MAS and b – static) and of C12A7@C-1450 with carbon coating (c – 30 kHz MAS and d - static). Separate simulated lines are depicted with dashed line (red). The sum of the simulated lines (solid, blue) is the result of a best fit analysis of experimental spectra (solid, black). Fit parameters of the experimental spectra are presented in the Table 1.


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The experimental spectra were apparently featureless due to the parameter distribution caused by a lack of long-range order in the samples. The spectra discussed in this paper consist of two lines corresponding to Al1 and Al2 with the intensity ratio between them being in perfect agreement with the value 4:3 expected from the structure. This observation meant that the cage structure of C12A7 was completely preserved, but that the exact positions of atoms in the cage were not so well determined possibly due to the random arrangement of occupied and empty cages, which are different due to the coulombic interaction of the cage wall with the caged anion. The extended Czjzek Model (ECM) is a model that correctly describes parameter distribution of systems, in which the local environment of the studied nucleus is stable, but the long-range order is missing.26,

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Using an ECM distribution of the QCC, the CQ distribution width ΔCQ was

chosen to best fit the spectra. Theoretical and experimental NMR parameters as well as data from the literature are summarized in Table 1. The consistency of these results was established by the fact that the same set of NMR parameters could reproduce satisfactorily the experimental static spectra recorded with the Avance 500 spectrometer. NMR parameters that are not listed in the table (i.e. chemical shift asymmetry ηδ and Euler angles α and γ) seemed to have little or no effect on either MAS or static simulated spectra, so their determination in this work was impossible. A comparison of NMR parameters of C12A7 electride given in the present paper to parameters reported earlier revealed some discrepancies in the analysis of experimental data. Matsuda et al 15 observed a paramagnetic shift at Al1 site arising from the delocalized electrons of cage conducting band. However, no chemical shift higher than 84 ppm is observed. Moreover, the chemical shifts of both Al sites in all samples are within the typical range for 4-coordinated aluminum, similar to insulating (oxide anions) samples. This means that no additional


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paramagnetic shift is induced on Al1 sites by the electrons of the cage conducting band in our samples. This particularity remains unexplained but must be related to the different synthesis methods used here and in ref. 15. The difference in the observed CQ of Al1 can emerge from the fact that static

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Al NMR spectra of powder samples are not informative enough to discern the

broadening caused by second-order quadrupolar effect from chemical shift anisotropy. In such case MAS technique is indispensable, because fast spinning of the sample significantly decreases broadening caused by chemical shift anisotropy leaving mostly second-order quadrupolar broadening. Analyzing solely MAS spectra, as it was performed in ref. 12, may lead to a loss of information about the chemical shift anisotropy. 28-30 In the case of C12A7, chemical shift anisotropy indeed significantly contributes to the line width making impossible to describe both static and MAS spectra using only quadrupolar parameters. Therefore, a correct determination of all NMR parameters of such broad lines requires the analysis of both MAS and static spectra simultaneously as well as considering the chemical shift and the quadrupolar interactions concomitantly. Most importantly, the NMR parameters of samples synthesized under carbon nanoreactor conditions were very close to the parameters of samples prepared with a more conventionally method. Therefore, it is safe to assume that the structure, physical and chemical properties of nanoparticles did not vary significantly due to their inclusion into a carbon shell. It is worth noticing that a previous study via XRD and TEM has shown that the average size of the particles in carbon-coated samples is much smaller than usual7. This would result in a lesser probability for them to have long range ordering and this is most likely the reason for the greater ΔCQ of the carbon-coated C12A7 samples. Another interesting effect of the carbon coating arises most


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probably due to the conductivity of carbon layers. They create additional diamagnetic shielding that leads to a small (~ 3 ppm) upfield shift in carbon-coated samples. Table 1. Theoretical and experimental 27Al NMR parameters of C12A7

Samples

DFT calculations

C12A7-1450:eC12A7@C1450:eC12A7:O2-

C12A7:eCA39 glass

Site

δiso, ppm

Δ δ, ppm

CQ MHz

Al1

77

-46

Al2

77

Al1

(ΔCQ),

ηQ

β, °

Ref.

10.74

0.005

3

-7

3.55

0.234

6

this work

84±1

-90

9.8 (2.0) ±0.2

0

25±2

Al2

82±1

-10

3.8 (1.5) ±0.2

0.55±0.1

5±2

Al1

81±1

-100

9.8 (3.0) ±0.2

0±0.1

25±2

Al2

79±1

-11

3.8 (2.0) ±0.2

0.5±0.1

5±2

Al1

85.9±1.0

-

9.7±0.2

0.4±0.1

-

Al2

80.2±0.3

-

3.8±0.2

0.7±0.1

-

Al1

144

-

10.87

0

-

Al2

76

-

3.20

0.26

-

Al1

80.4

10.1

6.4

-

-

this work this work [13]

[15] [14]

All NMR parameters are given according to Haeberlen convention. ΔCQ denotes the width of QCC distribution according to the Extended Czjzek Model


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CONCLUSION A detailed analysis of MAS and static

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Al NMR spectra of C12A7 electride supported by

DFT calculations led for the first time to the accurate determination of NMR parameters of both aluminum sites in the unit cell. The line shapes of these sites are broadened due to quadrupolar interaction and chemical shift anisotropy and display the effects of a parameter distribution that may arise from the random arrangement of empty and occupied cages. We have shown that the structure of C12A7:e - synthesized using the carbon nanoreactor process is almost identical to the structure of C12A7:e - synthesized in a conventional solid-state reaction. Moreover, similarity between the values of the aluminum chemical shift of electride and insulating (oxide) forms of C12A7 showed that the electrons of the cage conducting band do not induce a paramagnetic shift on Al1 site as was proposed in ref 15. The carbon nanoreactor process also led to a significant decrease (down to 1250 °C) in the temperature of C12A7 electride formation compared to the conventional calcination. These results agree well with the structural methods data described in the paper by Volodin et al.7 concerning the formation of C12A7 electride nanoparticles under carbon nanoreactor conditions. It should also be noted that the carbon coating is permeable to molecules in gas phase and allows conducting anion substitution without decomposing. We suggest that the carbon nanoreactor process is a prospective way of producing high specific area C12A7 electride suitable for catalytic application.

ACKNOWLEDGEMENTS Financial support from Russian Science Foundation (project No. 16-13-10168) is acknowledged with gratitude.


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The authors would also like to thank the late Prof. Nikolay Moroz for providing the possibility to record spectra with Avance 500 spectrometer in Nikolaev Institute of Inorganic Chemistry and Dr. Jalil Khabibulin for his assistance in NMR experiments.

AUTHOR INFORMATION Corresponding Author Olga B. Lapina, Professor, D. Chem. Sc., head of solid state NMR group at Boreskov Institute of Catalysis, Novosibirsk, 630090, Russia. +73833269505, [email protected] ABBREVIATIONS NMR, nuclear magnetic resonance; MAS, magic angle spinning; MQMAS, multi-quantum magic angle spinning; EFG, electric field gradient; QCC, quadrupole coupling constant; CSA, chemical shift anisotropy; DFT, density functional theory; GIPAW, gauge including projector augmented waves.


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Structure of Carbon-Coated C12A7 Electride via Solid-State NMR and

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