Ultrafine metallic cobalt nanoparticle encapsulated in N-doped carbon as high-efficiency catalysts for reduction of 4-nitrophenol

Novel and efficient ultrafine metallic cobalt nanoparticles encapsulated in N-doped carbon catalysts are prepared via a facile method from pyrolysis of ZIF-67-x%CTAB, which is modified by CTAB (cetyltrimethyl ammonium bromide). The content of surfactant CTAB can control the crystal size of ZIF-67, which further determines the Co particle size of Co@CN-x%CTAB. The as-prepared ZIF-67-x%CTAB and Co@CN-x%CTAB were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS). Co@CN-0%CTAB could not maintain the primary morphologies of ZIF-67-0%CTAB with the collapse of skeleton. However, the Co@CN-3%CTAB and Co@CN-5%CTAB catalysts inherit the original morphologies with skeleton constriction. It indicated that Co@CN catalysts could maintain the skeleton structure of ZIF-67 by adding CTAB during the preparation process. The particle size of Co@CN decreased with the increasing of CTAB content. Moreover, the addition of CTAB facilitated the formation of Co-N-x species on catalyst surface. The kinetic rate constant of Co@CN-5%CTAB catalyst was 0.99 min(-1), which is 2.5 times as high as that of Co@CN-0%CTAB catalyst for 4-nitrophenol reduction. The excellent catalytic performance of Co@CN-5%CTAB may be due to the presence of Co (0) species generated in situ during the calcination process, the smaller Co nanoparticles and more Co-N-x site on the surface of the catalyst.