Andreev Andrey S, d'Espinose de Lacaillerie Jean-Baptiste, Lapina Olga B, Gerashenko Alexander
Novosibirsk State University (NSU), Pirogova str. 2, Novosibirsk, 630090, Russia.
Phys Chem Chem Phys. 2015 Jun 14;17(22):14598-604. doi: 10.1039/c4cp05327c.
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.
尽管钴基催化剂已在工业规模上用于费托合成，但在实际操作条件下究竟是哪种钴金属相在起作用以及其分散状态如何仍不清楚。事实证明，金属钴的不同相，即面心立方（fcc）相和六方密堆积（hcp）相，会产生不同的铁磁共振。此外，在一个钴金属颗粒内，多畴颗粒中钴的特定共振可以证明存在几个尺寸有限的铁磁畴。因此，通过铁磁共振，人们可以在接近实际操作的条件下定量跟踪负载型催化剂中分散的钴金属颗粒的烧结和相变。铁磁钴核磁共振探测的最小尺寸尚不完全清楚，但对于室温下负载的钴颗粒，该尺寸被认为约为10纳米，在850 K时增加到约35纳米。在此，在负载于β-SiC上的钴金属费托合成催化剂中，面心立方多畴、面心立方单畴和六方密堆积钴的共振得到了清晰区分。仔细分析它们的频率和宽度对温度的依赖性，使得能够在感兴趣的温度范围内对光谱进行定量分析，从而反映接近实际操作条件下催化剂的状态，即没有与预先淬火相关的不确定性。发现同素异形体转变温度始于600 - 650 K，这比体相转变温度低约50 K。相变是完全可逆的，并且发现很大一部分六方密堆积相在高达850 K时都是稳定的。在不进行淬火的情况下观察到的这种异常行为可能对于理解和模拟不仅是催化剂而且还有电池材料中的活性物种至关重要。
