Thermal stability and hcp–fcc allotropic transformation in supported Co metal catalysts probed near operando by ferromagnetic NMR

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 β-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.

Editors' Choice—Review—Cobalt Thin Films: Trends in Processing Technologies and Emerging Applications

“…The data presented should be viewed by the reader as guide, but should not be taken as specific to any particular Co thin film formed by any of the techniques discussed herein. The information is meant to represent relevant, albeit not absolute, properties of Co films obtained at temperatures in the range of 20 • -100 • C, with the obvious The data reported here is compiled from various sources 68,[70][71][72][73][74] and should be viewed as guideline data for Co properties rather than absolute values. * Film resistivity/dielectric properties highly dependent on film thickness and purity level.…”

HDO of Methyl Palmitate over Silica-Supported Ni Phosphides: Insight into Ni/P Effect

“…The sequence repetition delay was 100 ms due to very short 31 P relaxation time in Ni-P intermetallic compounds [47]. Due to very large spectra width, all spectra were collected by frequency stepped technique with a step of 600 ppm (with respect to 31 P NMR frequency,~100 kHz) covering~5000-6000 ppm spectral width, the details can be found elsewhere [48]. The spectra were collected at room temperature and referenced to 85% water solution of orthophosphoric acid (for 31 P experiments).…”

Structure–Performance Relationships on Co-Based Fischer–Tropsch Synthesis Catalysts: The More Defect-Free, the Better

“…Exposure of bulk hcp-Co to temperatures above ~420°C has been shown to initiate/trigger transformation to fcc-Co 47,48 . The topic is still a matter of scientific debate and various deviations from this transformation temperature have been reported and related to various factors such as NP size, metal-support interactions and pre-existing concentration of stacking faults [49][50][51] . Here, with in situ TPR-XRD, the transformation from hcp-Co to fcc-Co is followed.…”