Nanostructured Fe-N-C materials represent a new type of "platinum-like" non-noble-metal catalyst for various electrochemical reactions and organic transformations. However, no consensus has been reached on the active sites of the Fe-N-C catalysts because of their heterogeneity in particle size and composition. In this contribution, we have successfully prepared atomically dispersed Fe-N-C catalyst, which exhibited high activity and excellent reusability for the selective oxidation of the C-H bond. A wide scope of substrates, including aromatic, heterocyclic, and aliphatic alkanes, were smoothly oxidized at room temperature, and the selectivity of corresponding products reached as high as 99%. By using sub-ångström-resolution HAADF-STEM in combination with XPS, XAS, ESR, and Mössbauer spectroscopy, we have provided solid evidence that Fe is exclusively dispersed as single atoms via forming FeN (x = 4-6) and that the relative concentration of each FeN species is critically dependent on the pyrolysis temperature. Among them, the medium-spin FeN affords the highest turnover frequency (6455 h), which is at least 1 order of magnitude more active than the high-spin and low-spin FeN structures and 3 times more active than the FeN structure, although its relative concentration in the catalysts is much lower than that of the FeN structures.

Semihydrogenation of acetylene in an ethylene-rich stream is an industrially important process. Conventional supported monometallic Pd catalysts offer high acetylene conversion, but they suffer from very low selectivity to ethylene due to overhydrogenation and the formation of carbonaceous deposits. Herein, a series of Ag alloyed Pd single-atom catalysts, possessing only ppm levels of Pd, supported on silica gel were prepared by a simple incipient wetness coimpregnation method and applied to the selective hydrogenation of acetylene in an ethylene-rich stream under conditions close to the front-end employed by industry. High acetylene conversion and simultaneous selectivity to ethylene was attained over a wide temperature window, surpassing an analogous Au alloyed Pd single-atom system we previously reported. Restructuring of AgPd nanoparticles and electron transfer from Ag to Pd were evidenced by in situ FTIR and in situ XPS as a function of increasing reduction temperature. Microcalorimetry and XANES measurements support both geometric and electronic synergetic effects between the alloyed Pd and Ag. Kinetic studies provide valuable insight into the nature of the active sites within these AgPd/SiO2 catalysts, and hence, they provide evidence for the key factors underpinning the excellent performance of these bimetallic catalysts toward the selective hydrogenation of acetylene under ethylene-rich conditions while minimizing precious metal usage.