Abstract
Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was used in this work to analyze the physical and chemical properties of plasma blacks and carbon deposits produced by thermal cracking of natural gas using different types of plasma reactors. In a typical configuration with a double-chamber reactor, N2 or Ar was injected as plasma working gas in the first chamber and natural gas was injected in the second chamber, inside the arc column. The solid residue was collected at different points throughout the plasma apparatus and analyzed by 13C solid-state NMR spectroscopy, using either cross polarization (CP) or direct polarization (DP), combined with magic angle spinning (MAS). The 13C CP/MAS NMR spectra of a number of plasma blacks produced in the N2 plasma reactor showed two resonance bands, broadly identified as associated with aromatic and aliphatic groups, with indication of the presence of oxygen- and nitrogen-containing groups in the aliphatic region of the spectrum. In contrast to DP experiments, only a small fraction of 13C nuclei in the plasma blacks are effectively cross-polarized from nearby 1H nuclei and are thus observed in spectra recorded with CP. 13C NMR spectra are thus useful to distinguish between different types of carbon species in plasma blacks and allow a selective study of groups spatially close to hydrogen in the material.
Highlights
Plasma pyrolysis of natural gas is a promising way of producing high-purity carbon blacks without generation of environmentally harmful products
These products were first characterized by thermogravimetry (TG), using a Shimadzu TGA-50H instrument, with constant heating-rate of 20∘C/min up to 1000∘C under O2 flow (50 mL/min); Xray diffraction (XRD), using a Shimadzu XRD-6000 powder diffractometer operating with a Cu-Kα X-ray source; scanning electron microscopy (SEM), using a Shimadzu SSX-550 electron microscope; and elemental analysis, performed with a Leco CHNS932 instrument
In the case of direct polarization (DP) experiments, a pulse sequence is composed of a 13C π/2 pulse immediately followed by a pair of π pulses and the subsequent detection of the free induction decay (FID) was employed, in order to suppress spurious background nuclear magnetic resonance (NMR) signals coming from some carbon-containing parts of the NMR probe [17]; the recycle delay was 15.0 s in the DP experiments
Summary
Plasma pyrolysis of natural gas is a promising way of producing high-purity carbon blacks without generation of environmentally harmful products. Methane decomposes to produce hydrogen and a solid, carbon-rich residue commonly designed as “plasma black” [1, 2] These materials are promising for several applications, including the production of inks, electrodes, and catalyst supports [3,4,5], which means that their profitable use would aggregate value to the process of plasma conversion of natural gas. There are many technologically important types of carbon materials with low (or even zero) hydrogen content, including carbon blacks, carbon nanotubes, nanodiamonds, nanographites, graphene, and amorphous carbon films For such materials, it is certainly more advantageous to record the 13C NMR spectra using DP, even if these experiments are more timeconsuming, allowing a larger portion of carbon atoms to be observed in DP experiments as compared to the CP method. Both DP and CP approaches were employed, allowing, on the one hand, a thorough characterization of the chemically distinct carbon atomic sites and, on the other hand, the study of minor contributions due to hydrogencontaining groups eventually present in the material
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
