CO2 is the final product of combustion of all fossil fuels. CO2 itself has little value by far, but it contributes more than 50% to the man-made greenhouse effect among all the greenhouse gases. There is still no proven technology for the chemical utilization of such a plentiful carbon resource. Recently, non-thermal plasmas have been found to be effective in the activation of CO2 for the formation of more valuable hydrocarbons. The non-thermal plasma approaches can even be performed at ambient condition. In this review, the present state of carbon dioxide utilization via non-thermal plasmas is addressed.

A density functional theory (DFT) study has been conducted in this work to investigate the reaction mechanism of synthesis of oxygenates and higher hydrocarbons from methane (CH4) and carbon dioxide (CO2), using cold plasmas. The main dissociation routes of the reactants were analyzed. The feasibility of the formation of various products, including syngas, higher hydrocarbons, and oxygenates, was discussed. The DFT study confirmed that the major obstacle of the synthesis is the dissociation of the reactants CH4 and CO2. After the cold plasma has supplied the necessary energy for the dissociation of CH4 and CO2, syngas, higher hydrocarbons, and oxygenates can be then easily produced. The present DFT study also demonstrates that the plasma synthesis will normally lead to a formation of a mixture of syngas, higher hydrocarbons, and oxygenates.

In this work, a direct conversion of methane in the presence of carbon dioxide using dielectricbarrier discharge plasmas has been conducted. The product includes syngas (H2 and CO), gaseous hydrocarbons (C2 to C5), liquid hydrocarbons (C5 to C11+), and oxygenates. The liquid hydrocarbons are highly branched, representing a high octane number, while the oxygenates mainly consist of series of alcohols and acids. A detailed analysis of product distribution has been performed under variable feed conditions with different reactor configurations. At the high CH4/CO2 feed ratio, the wider discharge gap (1.8mm) is more favored for the formation of methanol and ethanol. For the production of acetic acid, the narrower discharge gap (1.1mm) is better, especially, with the existence of after-glow zones. Conditions favored for the production of acetic acid are also good for the production of liquid fuels.

The effect of electrode materials on the co-generation of syngas and higher hydrocarbons from CO2 and CH4 using dielectric-barrier discharge has been investigated. The electrode materials tested here include aluminum, copper, steel, and titanium. For the feed of methane in the absence of CO2, the order of activity of methane conversion from high to low was Ti≈ Al > Fe > Cu, while the order of the activity of CO2 conversion was Al > Cu > Ti > Fe for the case of CO2 feed without methane. Regarding the co-feed of methane and CO2, the titanium electrode shows the best activity for the conversions, while the other three materials show a similar performance for the conversions. The effect of dilution gas, helium, on the conversions has also been discussed.

We have previously reported an experimental investigation on synthesis of acetic acid directly from CH4 and CO2 via dielectric-barrier discharge. In this work, a DFT study was conducted using three hybrid DFT methods in order to understand the mechanism of such direct synthesis. It suggests that the synthesis is via two pathways with CO 2 and CO as key intermediates. The energy requirement with CO 2 pathway is much less than that with CO. The methyl radical formation and the dissociation of CO2 are two limiting steps for the synthesis of acetic acid directly from CH4 and CO2.