Electrochemical Denitrification and Oxidative Dehydrogenation of Ethylbenzene over N-doped Mesoporous Carbon: Atomic Level Understanding of Catalytic Activity by ¹⁵N NMR Spectroscopy

Abstract

Spherical mesoporous carbon (with a particle size in the range of 40–75 μm) was synthesized by nanoreplication of a hard silica template using sucrose as the carbon precursor. The mesoporous carbon with BET surface areas higher than 1200 m2/g was doped with N by a treatment in an aqueous solution of nitric acid and/or in a flow of gaseous ammonia. The highest N content (3.2 wt % of N in bulk) was obtained when both modification methods were combined. Complementary physicochemical characterization techniques, including scanning electron microscopy (SEM), low-temperature N2 adsorption, powder X-ray diffraction (XRD), and Raman spectroscopy revealed the morphology, structure, and textural properties of the synthesized N-loaded carbon materials. For the identification of the detailed chemical structure on the surface of the carbons, 1H, 13C, and 15N solid-state nuclear magnetic resonance (NMR) measurements were performed, and the data were supported by chemical shift calculations with accurate quantum chemistry methods and X-ray photoelectron spectroscopic (XPS) analyses. All NMR experiments were performed at natural isotope abundance. The verified experimental data clearly showed that after the introduction of the N-containing moieties by the combined methods of treatment, a high concentration of pyridinic N at the edge, and pyrrolic N being external to the edge, was achieved for the mesoporous carbon. The distributed N surface species promoted the catalytic activity in the oxidative dehydrogenation of ethylbenzene to styrene but did not significantly influence the efficiency of the carbon materials in the electrochemical reduction of nitrate ions.

Graphitic nitrogen in carbon catalysts is important for the reduction of nitrite as revealed by naturally abundant15N NMR spectroscopy

“…Our pre-vious report on another type of N-doped carbon catalyst, containing only pyridinic N and pyrrolic N, showed no specific influence on the NH 3 selectivity during the electrochemical reduction of nitrate. 49 The reduction of nitrites is found to follow a similar reduction mechanism to that for nitrates, i.e. energetically-favored reduction from nitrate over N 2 and/or NO x directly to NH 3 .…”

“…The acquisition of the 1 H MAS NMR spectrum involved the use of a rotor-synchronized, double-adiabatic spin-echo sequence with a 90°excitation pulse of 1.25 µs followed by a pair of 50.0 µs tanh/tan short high-power adiabatic pulses (SHAPs) with a 5 MHz frequency sweep. 49,60,61 Such a pulse sequence was applied to prevent proton background signals and to achieve a flat baseline in the acquired 1 H MAS NMR spectrum. All pulses operated at a nutation frequency of 200 kHz, and 64 signal transients were acquired using a relaxation delay of 5 s. Directlyexcited 13 C and 15 N MAS NMR spectra were recorded using a 7 mm probehead and a 7 kHz MAS rate.…”

“…Only a few 15 N ssNMR studies using N-doped carbon materials at the natural abundance of 15 N have been reported in the literature including the study by Fang et al, and our very recent study. 48,49 In the latter, we reported the synthesis of N-doped mesoporous carbon by using porous silica as a template and sugar as a carbon precursor. 49 Upon treatment with HNO 3 and carbonization under a flow of NH 3 , followed by the dissolution of the silica template in hydrofluoric acid (HF), an N-doped carbon catalyst with 3.7 wt% of N was produced.…”

“…48,49 In the latter, we reported the synthesis of N-doped mesoporous carbon by using porous silica as a template and sugar as a carbon precursor. 49 Upon treatment with HNO 3 and carbonization under a flow of NH 3 , followed by the dissolution of the silica template in hydrofluoric acid (HF), an N-doped carbon catalyst with 3.7 wt% of N was produced. 15 N ssNMR spectroscopy at natural 15 N isotope abundance revealed that N was present in pyrrolic and pyridinic moieties.…”

Tailoring Properties of Resol Resin-Derived Spherical Carbons for Adsorption of Phenol from Aqueous Solution

“…The 52_PVA_600 material was the most saturated with these groups. Forming pyridinic and pyrrolic functionalities by the reduction of NO 2 groups dispersed on mesoporous carbon obtained by the nanoreplication of silica gel was previously reported by Szewczyk et al [ 49 ].…”