Article pubs.acs.org/JPCC
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*,‡,† ‡
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. Because of 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. We investigate by solid-state 27Al 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 parameters of electride and insulating (cement) samples. Finally, the carbon nanoreactor process led to a significant synthesis temperature decrease of C12A7 structure formation.
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surface area (0.1 m2/g).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 Xray diffraction (XRD), electron paramagnetic resonance (EPR), and transmission electron 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 short-range 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 multiquantum 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.
INTRODUCTION Until recently, calcium aluminate Ca12Al14O33 (C12A7 in cement notation), also known as mayenite, was used solely 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 many of applications as a functional material: catalyst and a catalytic support,1−4 an ion beam source,5 and an insulator with photoinduced conductivity.6 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−, and so on.5 The composition of the anionic sublattice determines the electrical and chemical properties of C12A7. Particularly, an electride, being an ionic compound with the electrons playing the role of anions, containing caged electrons is of a special interest due to its unusual stability at room-temperature. 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 the formation of large particles of low specific © 2017 American Chemical Society
Received: August 15, 2017 Revised: September 19, 2017 Published: September 19, 2017 22268
DOI: 10.1021/acs.jpcc.7b08132 J. Phys. Chem. C 2017, 121, 22268−22273
Article
The Journal of Physical Chemistry C
calcination of the mixture in Ar flow (0.5 L/h 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. MAS experiments were conducted on the Bruker Avance 400 spectrometer with MAS spinning frequencies of 15 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 27Al 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 shielding, first and second orders of quadrupolar interaction, CQ 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 (20 × 20 × 20 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
