C<sub>2</sub> Vacancy-Mediated N<sub>2</sub> Activation over Ni-Loaded Rare-Earth Dicarbides for Ammonia Synthesis
Yiliguma Yiliguma, Sang‐Won Park, Jiang Li, Masato Sasase, Masaaki Kitano, Hideo Hosono
Abstract
Present in various forms, diatomic carbon (C2) unfolds fascinating science, bridging organic and inorganic and molecular and crystalline chemistry. Rare-earth dicarbides have the simplest rock-salt-like crystalline structure that contains C2 units and have rarely been studied for catalysis. Here, we present N2 activation on the C2 vacancy of rare-earth dicarbides for ammonia (NH3) synthesis under mild conditions. We show both experimentally and theoretically that C2 vacancies can be generated on CeC2 by H2 treatment, which could be further promoted by Ni loading, thanks to its superior H2 activation feature. The infrared spectroscopic study confirms that N2 adsorption on the C2 vacancy leads to partial reduction of N2. Adsorbed nitrogen species on Ni-loaded CeC2 catalyst (Ni/CeC2) after NH3 synthesis are identified to be reactive with H2 at NH3 synthesis temperature, and their entity was identified as N2 and NHx at the site of the C2 vacancy of CeC2 by spectroscopic and spectrometric methods. Ni/CeC2 exhibits a catalytic activity of 3.4 mmol g–1 h–1, comparable to state-of-the-art Ni-loaded CeN (Ni/CeN) under 0.1 MPa, 400 °C. The catalytic mechanism is elucidated by DFT calculations to reveal the synergy between CeC2 and Ni as well as each stage of the NH3 synthesis catalytic cycle mediated by the C2 vacancy. Further, Ni/CeC2 and Ni/CeN were compared to find their differences in N2 adsorption and the NH3 synthesis catalytic pathway.