Search

Cobalt catalysts are involved in many chemical processes crucial to the energy transition and the shift from oil to biosourced chemicals. Thanks to the ferromagnetic character of cobalt metal, these catalysts can be studied by nuclear magnetic resonance (NMR) in the internal magnetic field of the particle. Using carbon hydrogasification as an example, this work illustrates the potential of 59Co internal field (IF) NMR to study the Co crystallite phase transitions and to distinguish quantitatively, within the limits of the skin effect, different Co–C intermediates present inside a sample containing cobalt and carbon. Here, the main Co–C intermediates evidenced are a Co/C solid solution and a Co3C phase. When using a ball milling process for carbon hydrogasification, a high amount of Co–C intermediates forms, and the reaction rate is increased by several orders of magnitude compared to a classic catalytic reaction (in a fixed-bed reactor). In this work, the time evolution of the 59Co IF NMR are compared to the reaction rate of carbon hydrogasification over the course of the ball milling process. We find a direct relationship between the amount of Co–C intermediates inside the sample and the reaction rate. This confirms that, like for a carbon hydrogasification reaction under classic conditions, the carbon dissociation and formation of Co–C bonds is the rate-determining step.

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.

A facile NMR approach based on dipolar filtering (DF) and spin echo (SE) is proposed to study commercially available dealuminated USY zeolites, such as H-CBV760, H-CBV720, and NH4-CBV712 of nominal Si/Al ratio of 30, 15, and 6, respectively. The proposed 1H DF-SE magic angle spinning (MAS) NMR approach provides the substantial suppression of water signal intensity in partly hydrated samples providing an opportunity to obtain the signal of other surface groups. 1H DF-SE MAS NMR technique has been demonstrated its usability for quantitative (semiquantitative) analysis of dried, i.e., partly hydrated, samples. It is essential to use this approach when calcination under vacuum used as a reference procedure leads to drastic surface changes. Moreover, the technique is applicable for qualitative analysis of fully hydrated samples. This method is found to be extremely sensitive to the residual ammonium content in zeolite structure even in transformed to H form by calcination. Finally, the framework stacking faults species are found to be more pronounced in 1H DF-SE MAS NMR spectra in hydrated state as ∼1 ppm peak that can be crucial for understanding of relationships of structure and performance. Additionally, the “standard” 27Al and 29Si MAS NMR approaches are also discussed in both hydrated and dried states of zeolites. 29Si MAS NMR spectra demonstrate that a dependence on hydration state and the highest quantity of crystalline part is achieved in dried samples, whereas the best resolution of 27Al MAS NMR spectra is obtained in a fully hydrated state. Finally, a local order of Si framework given by full width at half-maximum parameter of crystalline Q40peak correlates with increasing relative Al content, which is responsible for the distortion of zeolite structure.

For the first time, the detailed structure of χ-Al2O3and η-Al2O3surface has been established by implementing the NMR crystallography approach. The surface of η-Al2O3has been demonstrated to be formed primarily by the (111) facets, while the χ-Al2O3surface is a combination of (111) and (110) facets. This observation supports the block model of aluminum oxides previously proposed by Tsybulya and Kryukova [S. V. Tsybulya, G. N. Kryukova, Phys. Rev. B 77 (2008) 024112.]. The additional terminal OH groups, observed experimentally and not contributing to (111) and (110) theoretical calculations, are considered to be bonded to the tetrahedral aluminum sites. Their origin is related to the junctions of crystallographic faces of spinel building blocks, being a part of discussed model. Higher content of these terminal OH groups in χ-Al2O3is a result of more junctions in the case of its more mosaic structure compared to η-Al2O3.

Multinuclear solid-state 1H, 27Al, and 93Nb NMR experiments and DFT calculations were carried out for structural characterization of alumina-supported niobium oxide catalysts with high niobium content following an every stage in the catalyst preparation. It was found that the first stage of the impregnation procedure plays a key role in determining the catalyst structure and acidity. In order to monitor the presence in catalysts of aluminum niobate phase, AlNbO4, a series of 27Al and 93Nb NMR experiments was performed for several different individual AlNbO4samples. Aluminum and niobium NMR parameters were determined for AlNbO4, which crystal structure contains two different crystallographic sites for each element. The compound was investigated through a combination of experimental 93Nb and 27Al NMR spectroscopy methods at several magnetic field strengths (9.4, 11.7, 19.4, and 21.1 T) and complemented by ab initio quantum chemical calculations of NMR parameters for these nuclei. The chemical shielding and the quadrupole coupling tensor parameters were determined for both 93Nb and 27Al.

Porous ceramometal Al2O3/CoAlO/CoAl was studied by set of physicochemical techniques such as XRD, SEM, internal field 59Co and 27Al MAS NMR, and porosity measurements. They revealed the cermet containing three parts. First, cobalt-free large porous alumina particles which surrounded by the second oxide part representing spinel CoxAl3-xO4 (x=1,2,3) oxides. And third, oxygen-free metallic part consists of cobalt metal particles covered by Co-Al oxide protecting the metallic part from oxidation. Porosity measurements ascertained high porosity (60%) and good SSA (122 m2/g). Also the enhanced adsorption of microwaves due to metallic particles randomly distributed in oxides was found.