Research Article pubs.acs.org/journal/ascecg
Simultaneous Generation of Syngas and Multiwalled Carbon Nanotube via CH4/CO2 Reforming with Spark Discharge Wei-Chieh Chung and Moo-Been Chang*
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Graduate Institute of Environmental Engineering, National Central University, Zhongli, Taoyuan City, 320, Taiwan ABSTRACT: Application of atmospheric-pressure spark discharge plasma for conversion of the two most important anthropogenic greenhouse gases (GHGs) into valuable products, including syngas and multiwalled carbon nanotubes (MWCNTs), is experimentally evaluated. Syngas with different CO/H2 ratios and MWCNTs with different physicochemical properties can be generated by varying the CO2/CH4 feeding ratio from 0 to 3. Operating spark discharge with pure CH4, the reactor generates syngas with the lowest rate and MWCNT without oxygen-containing functional groups. Introducing an appropriate amount of CO2 into plasma results in higher syngas and MWCNT generation rates. To achieve the highest syngas generation rate, a CO2/CH4 ratio of 1 is suggested, while for the highest MWCNTs generation, a CO2/CH4 ratio of 1/3 is favored. Moreover, the physicochemical properties of the MWCNTs generated are influenced via controlling the CO2/CH4 ratio, and various characteristic properties of MWCNTs may provide different applications. Overall, simultaneous generation of MWCNTs and syngas as demonstrated in this work can be a promising technique for carbon capture, utilization, and storage (CCUS). KEYWORDS: Dry reforming of methane, CCUS, Nonthermal plasma, Syngas, Carbon nanotube, CO2/CH4 ratio, XPS
1. INTRODUCTION
tubes formed and deposited on electrodes or a quartz reactor surface can be easily collected as a solid product.16
Emission of anthropogenic greenhouse gases (GHGs) into the atmosphere has been greatly increasing after the industrial revolution, and global CO2-equivalent GHG emission reached 50 Gt/yr in 2012.1 Carbon capture, utilization, and storage (CCUS) stands for the combination of various technologies for reducing GHG emissions. Via CCUS technologies, CO2 can be either directly utilized or converted into other useful products, such as urea,2−4 methanol,5,6 formic acid,7,8 cyclic carbonates9,10 and syngas, and this approach is called carbon utilization (CU).11,12 For syngas production, reforming of CH4 with CO2 is a promising process to simultaneously convert two GHGs, and the syngas generated can be used as an energy source or feedstock of a Fischer−Tropsch process to synthesize straight-chain hydrocarbons.13 Methane reforming with CO2, which is also called dry reforming of methane (DRM, Reaction 1), can be realized via either catalysis or plasma to generate syngas. For catalysis, noble metal catalysts and nickel-based catalysts are proved to have good activity on syngas generation. In addition to syngas generation, carbon nanotubes, including single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWNCTs), are also formed as solid products. However, carbon nanotubes deposited on a catalyst surface lead to catalyst deactivation and are difficult to collect, resulting in a low value of solid product.14,15 Nonthermal plasma may also be applied for DRM and can work as plasma-enhanced chemical vapor deposition (PECVD). During discharge, carbon nano© 2016 American Chemical Society
CO2 + CH4 → 2CO + 2H 2
(1)
For effective generation of MWCNTs, PECVD processes are generally operated at a low pressure (
