CO2 conversion into value-added chemicals and fuels is considered as one of the great challenges of the 21st century. Due to the limitations of the traditional thermal approaches, several novel technologies are being developed. One promising approach in this field, which has received little attention to date, is plasma technology. Its advantages include mild operating conditions, easy upscaling, and gas activation by energetic electrons instead of heat. This allows thermodynamically difficult reactions, such as CO2 splitting and the dry reformation of methane, to occur with reasonable energy cost. In this review, after exploring the traditional thermal approaches, we have provided a brief overview of the fierce competition between various novel approaches in a quest to find the most effective and efficient CO2 conversion technology. This is needed to critically assess whether plasma technology can be successful in an already crowded arena. The following questions need to be answered in this regard: are there key advantages to using plasma technology over other novel approaches, and if so, what is the flip side to the use of this technology? Can plasma technology be successful on its own, or can synergies be achieved by combining it with other technologies? To answer these specific questions and to evaluate the potentials and limitations of plasma technology in general, this review presents the current state-of-the-art and a critical assessment of plasma-based CO2 conversion, as well as the future challenges for its practical implementation.

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Plasma technology - a novel solution for CO2 conversion?

Thermal-catalytic gas processing is integral to many current industrial processes. Ever-increasing demands on conversion and energy efficiencies are a strong driving force for the development of alternative approaches. Similarly, synthesis of several functional materials (such as nanowires and nanotubes) demands special processing conditions. Plasma catalysis provides such an alternative, where the catalytic process is complemented by the use of plasmas that activate the source gas. This combination is often observed to result in a synergy between plasma and catalyst. This Review introduces the current state-of-the-art in plasma catalysis, including numerous examples where plasma catalysis has demonstrated its benefits or shows future potential, including CO2 conversion, hydrocarbon reforming, synthesis of nanomaterials, ammonia production, and abatement of toxic waste gases. The underlying mechanisms governing these applications, as resulting from the interaction between the plasma and the catalyst, rend...

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Plasma Catalysis: Synergistic Effects at the Nanoscale

This review describes the history and development of plasma-assisted catalysis focussing mainly on the use of atmospheric pressure, non-thermal plasma. It identifies the various interactions between the plasma and the catalyst that can modify and activate the catalytic surface and also describes how the catalyst affects the properties of the discharge. Techniques for in situ diagnostics of species adsorbed onto the surface and present in the gas-phase over a range of timescales are described. The effect of temperature on plasma–catalysis can assist in determining differences between thermal catalysis and plasma-activated catalysis and focuses on the meaning of temperature in a system involving non-equilibrium plasma. It can also help to develop an understanding of the gas-phase and surface mechanism of the plasma–catalysis at a molecular level. Our current state of knowledge and ignorance is highlighted and future directions suggested.

http://iopscience.iop.org/article/10.1088/0022-3727/49/24/243001/pdf

Plasma–catalysis: the known knowns, the known unknowns and the unknown unknowns

A cylindrical dielectric barrier discharge (DBD) reactor has been developed for the conversion of undiluted CO2 into CO and O2 at atmospheric pressure and low temperatures. Both the physical and chemical effects on reaction performance have been investigated for the addition of BaTiO3 and glass beads into the discharge gap. The presence of these packing materials in the DBD reactor changes the physical characteristics of the discharge and leads to a shift of the discharge mode from a typical filamentary discharge with no packing to a combination of filamentary discharge and surface discharge with packing. Highest CO2 conversion and energy efficiency are achieved when the BaTiO3 beads are fully packed into the discharge gap. It is found that adding the BaTiO3 beads into the plasma system enhances the average electric field and mean electron energy of the CO2 discharge by a factor of two, which significantly contributes to the enhancement of CO2 conversion, CO yield, and energy efficiency of the plasma process. In addition, the highly energetic electrons (>3.0 eV) generated by the discharge could activate the BaTiO3 photocatalyst to form electron–hole pairs on its surface, which contributes to the enhanced conversion of CO2.

https://iopscience.iop.org/article/10.1088/0963-0252/24/1/015011/pdf

Plasma-assisted conversion of CO2 in a dielectric barrier discharge reactor: understanding the effect of packing materials

The MnOx and CeOx were in situ supported on carbon nanotubes (CNTs) by a poly(sodium 4-styrenesulfonate) assisted reflux route for the low-temperature selective catalytic reduction (SCR) of NO with NH3. X-Ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR) and NH3 temperature-programmed desorption (NH3-TPD) have been used to elucidate the structure and surface properties of the obtained catalysts. It was found that the in situ prepared catalyst exhibited the highest activity and the most extensive operating-temperature window, compared to the catalysts prepared by impregnation or mechanically mixed methods. The XRD and TEM results indicated that the manganese oxide and cerium oxide species had a good dispersion on the CNT surface. The XPS results demonstrated that the higher atomic concentration of Mn existed on the surface of CNTs and the more chemisorbed oxygen species exist. The H2-TPR results suggested that there was a strong interaction between the manganese oxide and cerium oxide on the surface of CNTs. The NH3-TPD results demonstrated that the catalysts presented a larger acid amount and stronger acid strength. In addition, the obtained catalysts exhibited much higher SO2-tolerance and improved the water-resistance as compared to that prepared by impregnation or mechanically mixed methods.

In situ supported MnO(x)-CeO(x) on carbon nanotubes for the low-temperature selective catalytic reduction of NO with NH3.

The decomposition of CO2 in a dielectric packed-bed plasma reactor has been studied. It was found that the dielectric properties and morphology of packing dielectric pellets play important roles in the reaction due to their influence on the electron energy distribution in the plasma. The acid–base properties of the packing materials also affect the reaction through the chemisorption of CO2 on basic sites of the materials. Heterogeneous reactions on the solid surfaces of the dielectric materials also play a role in the reaction, which was also confirmed through the investigation of the influence of the discharge length on the reaction. The reverse reaction of CO2 decomposition, the oxidation of CO, was also investigated to further understand the role of dielectric materials in the plasma and their effect on plasma reactions. Both the decomposition of CO2 and the oxidation of CO in non-packed or dielectric packed reactors are first-ordered.

Characteristics of the Decomposition of CO 2 in a Dielectric Packed-Bed Plasma Reactor

A combined adsorption-discharge plasma catalytic process was used for the removal of NOx using zeolites as catalysts without external heating. It was found that the types of plasma carrier gases exert great effect on the conversion of adsorbed NOx. The conversion of adsorbed NOx is much lower in N2 plasma than in Ar plasma, which is attributed to the reverse reaction, NOx formation reaction. The momentary increase of oxygen species derived from the decomposition of adsorbed NOx is considered to be the main cause as their collisions with nitrogen species can generate NOx again. Thus, solid carbon was added to the catalyst to act as a scavenger for active oxygen species to improve the conversion of adsorbed NOx in N2 plasma. A NOx removal rate of 97.8% was obtained on 8.5wt.% carbon mixed H-ZSM-5 at an energy efficiency of 0.758 mmol NOx/W·h.

High-efficiency removal of NOx using a combined adsorption-discharge plasma catalytic process.

NOx storage and reduction with CH4 by a plasma process was proposed for NOx removal at ambient temperature. The efficiency of this new process for NOx removal could achieve 95% at ambient temperature. NOx removal via cyclic operation has also been investigated, maintaining efficieniency above 90%.

NOx storage and reduction with methane by plasma at ambient temperature

Non-thermal plasma assisted CO2 reforming of propane over Ni/γ-Al2O3 catalyst

Natural mordenite (NMOR), modified by acid treatment and ion-exchange, was employed for NO adsorption in the present study. The NO storage capacity of modified NMOR was greatly improved compared with its original correspondents, mainly due to the preservation of crystalline structure and the improvement of surface area of NMOR. Among all the modified NMOR, Ni-NMOR exhibited the highest adsorption capacity for NO (1.20 mmol·g−1) in the presence of 10% O2 at 308 K. The influence of the main ingredients in flue gas on the storage capacity of NMOR for NO had also been investigated. In general, H2O, CO2 and SO2 all displayed negative impact on NO adsorption due to their competitive adsorption on the surface of NMOR with NO, while the presence of O2 greatly improved the adsorption of NO because of the formation of NO2 and N2O3. Moreover, Ni-NMOR exhibited high efficiency for NOx removal through the NOx adsorption-plasma discharge process.

Tong Liu

Papers

Characteristics of the Decomposition of CO 2 in a Dielectric Packed-Bed Plasma Reactor

High-efficiency removal of NOx using a combined adsorption-discharge plasma catalytic process.

NOx storage and reduction with methane by plasma at ambient temperature

Non-thermal plasma assisted CO2 reforming of propane over Ni/γ-Al2O3 catalyst

Superior Storage Performance for NO in Modified Natural Mordenite