Highly Efficient Metal-Free Nitrogen-Doped Nanocarbons with Unexpected Active Sites for Aerobic Catalytic Reactions Yangming Lin,* ,†,‡ Zigeng Liu, ‡ Yiming Niu, † Bingsen Zhang, † Qing Lu, ‡ Shuchang Wu, † Gabriele Centi, § Siglinda Perathoner, § Saskia Heumann, ‡ Linhui Yu,* ,‡,∥ and Dang Sheng Su †,⊥ † Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P.R. China ‡ Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34−36, Mü lheim an der Ruhr 45470, Germany § University of Messina, V.le F. Stagno D’Alcontres 31, 98166 Messina, Italy ∥ Fuzhou University, Fuzhou 350002, P.R. China ⊥ Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, Berlin 14195, Germany *S Supporting Information ABSTRACT: Nitrogen (N)-doped nanocarbons (NDN) as metal-free catalysts have elicited considerable attention toward selective oxidation of alcohols with easily oxidizable groups to aldehydes in the past few years. However, finding a new NDN catalytic material that can meet the require- ment of the feasibility on the aerobic catalytics for other complicated alcohols is a big challenge. The real active sites and the corresponding mechanisms on NDN are still unambiguous because of inevitable coexistence of diverse edge sites and N species based on recently reported doping methods. Here, four NDN catalysts with enriched pyridinic N species and without any graphitic N species are simply fabricated via a chemical-vapor-deposition-like method. The results of X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectra suggest that the dominating N species on NDN are pyridinic N. It is demonstrated that NDN catalysts perform impressive reactivity for aerobic oxidation of complicated alcohols at an atmospheric pressure. Eleven kinds of aromatic molecules with single N species and tunable π conjugation systems are used as model catalysts to experimentally identify the actual role of each N species at a real molecular level. It is suggested that pyridinic N species play an unexpected role in catalytic reactions. Neighboring carbon atoms in pyridinic N species are responsible for facilitating the rate-determining step process clarified by kinetic isotope effects, in situ nuclear magnetic resonance, in situ attenuated total reflectance infrared, and theoretical calculation. Moreover, NDN catalysts exhibit a good catalytic feasibility on the synthesis of important natural products (e.g., intermediates of vitamin E and K3) from phenol oxidation. KEYWORDS: nanodiamond, carbon materials, metal-free, nitrogen-doped, catalysis, model catalyst, kinetic isotope effect C atalytic oxidation of alcohols to value-added products is an important reaction in catalytic synthesis. 1,2 Metal-based materials are the most common catalysts for highlighting that reaction. Although doped nanocarbon materials, such as nitrogen-doped nanocarbon (NDN) or phosphorus-doped carbon, as the representative metal-free catalysts, have shown promising ability in the catalytic oxidation of alcohols with easily oxidizable functionalities using O 2 as oxidant, the validity for other complicated alcohols (e.g., unsaturated, heterocyclic, and aliphatic alcohols) has been rarely revealed due to the inactivity of the reported carbon catalysts. 3,4 This inevitably hinders their practicality. It can be expected that finding an efficient and green heterogeneous nanocatalyst to enable aerobic oxidation of diverse alcohols at an atmospheric pressure is highly desired. Moreover, understanding the detailed activation process of the alcohols on metal-free catalysts is still a big challenge. Some important achievements on the investigation of active sites have been made using various research means; however, difficulties still focus on understanding the intrinsic mechanism of NDN catalysts. 3,5 The reasons are (1) the inevitable coexistence of diverse N functional groups at the carbon edge (pyrrolic, OC−NH, pyridinic, −NH 2 etc.) during the preparation process and (2) multifarious edge defect (heteroatom-free) configurations. Taking aerobic oxidation of the alcohols as an example, graphitic N (GN) species have Received: July 25, 2019 Accepted: November 25, 2019 Published: November 25, 2019 Article www.acsnano.org Cite This: ACS Nano XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acsnano.9b05856 ACS Nano XXXX, XXX, XXX−XXX This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Downloaded via 23.89.236.7 on December 7, 2019 at 15:23:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.