ABSTRACT
The magnetic properties of cobalt metal nanowires grown by electrodeposition in porous membranes depend largely on the synthesis conditions. Here, we focus on the role of electrolyte additives on the magnetic anisotropy of the electrodeposited nanowires. Through magnetometry and internal field nuclear magnetic resonance (IF NMR) studies, we compared both the magnetic and crystalline structures of 50 and 200 nm diameter Co nanowires synthesized in the presence or absence of organic additives. The spectral characteristics of IF NMR were compared structurally to X-ray diffraction patterns, and the anisotropy of the NMR enhancement factor in ferromagnetic multidomain structures to magnetometry results. While the magnetic behavior of the 50 nm nanowires was dominated, as expected, by shape anisotropy with magnetic domains oriented on axis, the analysis of the 200 nm proved to be more complex. 59Co IF NMR revealed that the determining difference between the samples electrodeposited in the presence or in absence of organic additives was not the dominant crystalline system (fcc or hcp) but the coherent domain sizes and boundaries. In the presence of organic additives, the cobalt crystal domains are smaller and with defective grain boundaries, as revealed by resonances below 210 MHz. This prevented the development in the Co hcp part of the sample of the strong magnetocrystalline anisotropy that was observed in the absence of organic additives. In the presence of organic additives, even in nanowires as wide as 200 nm, the magnetic behavior remained determined by the shape anisotropy with a positive effective magnetic anisotropy and strong anisotropy of the NMR enhancement factor.
ABSTRACT
Carbon hydrogasification is the slowest reaction among all carbon-involved small-molecule transformations. Here, we demonstrate a mechanochemical method that results in both a faster reaction rate and a new synthesis route. The reaction rate was dramatically enhanced by up to 4 orders of magnitude compared to the traditional thermal method. Simultaneously, the reaction exhibited very high selectivity (99.8 % CH4 , versus 80 % under thermal conditions) with a cobalt catalyst. Our study demonstrated that this extreme increase in reaction rate originates from the continuous activation of reactive carbon species via mechanochemistry. The high selectivity is intimately related to the activation at low temperature, at which higher hydrocarbons are difficult to form. This work is expected to advance studies of carbon hydrogasification, and other solid-gas reactions.
ABSTRACT
Investigating the size distributions of Co nanoparticle ensembles is an important problem, which has no straightforward solution. In this work, we use the combination of 59Co internal field nuclear magnetic resonance (59Co IF NMR) and ferromagnetic resonance (FMR) spectroscopies on a metallic Co nanoparticle sample with a narrow Co nanoparticle size distribution due to encapsulation within the inner channels of carbon nanotubes. High-resolution transmission electron microscopy (TEM) images showed that the nanoparticles can be represented as prolate spheroids, with the majority of particles having an aspect ratio between 1 and 2. This observation has increased the accuracy of superparamagnetic blocking size calculations from Néel relaxation model by introducing the actual volume of the ellipsoids taken from the image processing. 59Co IF NMR and FMR experiments conducted under different temperatures allowed us to observe the thermal blocking of superparamagnetic particles in full accordance with the TEM particle volume distribution. This proved that these magnetic resonance techniques can be used jointly for characterization of Co nanoparticles in the bulk of the sample.
ABSTRACT
Silicate hydration is prevalent in natural and technological processes, such as, mineral weathering, glass alteration, zeolite syntheses and cement hydration. Tricalcium silicate (Ca3SiO5), the main constituent of Portland cement, is amongst the most reactive silicates in water. Despite its widespread industrial use, the reaction of Ca3SiO5 with water to form calcium-silicate-hydrates (C-S-H) still hosts many open questions. Here, we show that solid-state nuclear magnetic resonance measurements of (29)Si-enriched triclinic Ca3SiO5 enable the quantitative monitoring of the hydration process in terms of transient local molecular composition, extent of silicate hydration and polymerization. This provides insights on the relative influence of surface hydroxylation and hydrate precipitation on the hydration rate. When the rate drops, the amount of hydroxylated Ca3SiO5 decreases, thus demonstrating the partial passivation of the surface during the deceleration stage. Moreover, the relative quantities of monomers, dimers, pentamers and octamers in the C-S-H structure are measured.
ABSTRACT
Despite the fact that cobalt based catalysts are used at the industrial scale for Fischer-Tropsch synthesis, it is not yet clear which cobalt metallic phase is actually at work under operando conditions and what is its state of dispersion. As it turns out, the different phases of metallic cobalt, fcc and hcp, give rise to distinct ferromagnetic nuclear magnetic resonance. Furthermore, within one Co metal particle, the occurrence of several ferromagnetic domains of limited sizes can be evidenced by the specific resonance of Co in multi-domain particles. Consequently, by ferromagnetic NMR, one can follow quantitatively the sintering and phase transitions of dispersed Co metal particles in supported catalysts under near operando conditions. The minimal size probed by ferromagnetic Co NMR is not precisely known but is considered to be in the order of 10 nm for supported Co particles at room temperature and increases to about 35 nm at 850 K. Here, in Co metal Fischer-Tropsch synthesis catalysts supported on ß-SiC, the resonances of the fcc multi-domain, fcc single-domain and hcp Co were clearly distinguished. A careful rationalization of their frequency and width dependence on temperature allowed a quantitative analysis of the spectra in the temperature range of interest, thus reflecting the state of the catalysts under near operando conditions that is without the uncertainty associated with prior quenching. The allotropic transition temperature was found to start at 600-650 K, which is about 50 K below the bulk transition temperature. The phase transition was fully reversible and a significant part of the hcp phase was found to be stable up to 850 K. This anomalous behavior that was observed without quenching might prove to be crucial to understand and model active species not only in catalysts but also in battery materials.
