Alkali metals have been widely used as promoters of metal catalysts in various applications, yet the atomic understanding of the promotional mechanism remains elusive. In this work, we for the first time report the significant promotional effect of potassium to the Co-N-C single-atom catalyst for the direct dehydrogenation of ethylbenzene. K cation was introduced by impregnation of Co-N-C with KCl followed by calcination at 600 °C, which resulted in the bonding of K to the Co-O moiety of the Co-N-C catalyst as revealed by X-ray absorption spectroscopy in combination with theoretical calculations. The formation of Co-O-K moiety led to the increase of electron density at the O atom due to electron transfer of K to O, and consequently facilitated the heterolytic cleavage of the C–H bond of ethylbenzene across the Co-O moiety. The promotional effect of K was found to be a volcano-function with the K/Co ratio and became the greatest at the K/Co ratio of 1.36, which resulted in the highest steady-state reaction rate of 9.7 mmol_EB·g_cat⁻¹·h⁻¹ reported thus far. Moreover, the catalyst exhibited excellent stability during 100 h time on stream.

Fine-tuning of the coordination environment of single-atom catalysts (SACs) is effective to optimize their catalytic performances, yet it remains challenging due to the vulnerability of SACs. Herein, we report a new approach to engineering the coordination environment of M-N-C (M = Fe, Co, and Ni) SACs by using glutamic acid as the N/C source and pyrolysis atmosphere as a regulator. Compared with that in N₂, NH₃ was able to promote the doping of N at T < 700 ℃ yet etch the N-species at higher temperatures, by which the M-N coordination number (CN) and the electronic structure were delicately tuned. It was found that the electron density of Ni single atoms increased with the decrease of Ni-N CN. As a consequence, the capability of Ni-N-C to dissociate H₂ was greatly enhanced and a higher catalytic activity in chemoselective hydrogenation of functionalized nitroarenes was achieved. Moreover, this modulation method could be applied to other transition metals including Fe and Co. In particular, the as-synthesized Co-N-C SAC afforded a turnover frequency of 152.3 h⁻¹ with 99% selectivity to 3-vinylaniline in the hydrogenation of 3-nitrostyrene, which was the highest ever reported thus far and was at least one order of magnitude more active than state-of-the-art noble-metal-free M-N-C catalysts, demonstrating the great potential of engineering the coordination environment of SACs.