This paper presents a review of the literature dealing with the formation of hydrogen peroxide from plasma processes. Energy yields for hydrogen peroxide generation by plasma from water span approximately three orders of magnitude from 4 ? 10?2 to 80?g?kWh?1. A wide range of plasma processes from rf to pulsed, ac, and dc discharges directly in the liquid phase have similar energy yields and may thus be limited by radical quenching processes at the plasma?liquid interface. Reactor modification using discharges in bubbles and discharges over the liquid phase can provide modest improvements in energy yield over direct discharge in the liquid, but the interpretation is complicated by additional chemical reactions of gas phase components such as ozone and nitrogen oxides. The highest efficiency plasma process utilizes liquid water droplets that may enhance efficiency by sequestering hydrogen peroxide in the liquid and by suppressing decomposition reactions by radicals from the gas and at the interface. Kinetic simulations of water vapor reported in the literature suggest that plasma generation of hydrogen peroxide should approach 45% of the thermodynamics limit, and this fact coupled with experimental studies demonstrating improvements with the presence of the condensed liquid phase suggest that further improvements in energy yield may be possible. Plasma generation of hydrogen peroxide directly from water compares favorably with a number of other methods including electron beam, ultrasound, electrochemical and photochemical methods, and other chemical processes.

https://iopscience.iop.org/article/10.1088/0963-0252/20/3/034006/pdf

Review of the methods to form hydrogen peroxide in electrical discharge plasma with liquid water

Saturn's moon Enceladus has a subsurface ocean covered by a layer of ice. Some liquid escapes into space through cracks in the ice, which is the source of one of Saturn's rings. In October 2015, the Cassini spacecraft flew directly through the plume of escaping material and sampled its chemical composition. Waite et al. found that the plume contains molecular hydrogen, H2, a sign that the water in Enceladus' ocean is reacting with rocks through hydrothermal processes (see the Perspective by Seewald). This drives the ocean out of chemical equilibrium, in a similar way to water around Earth's hydrothermal vents, potentially providing a source of chemical energy.

Science , this issue p. [155][1]; see also p. [132][2]

 [1]: /lookup/doi/10.1126/science.aai8703
 [2]: /lookup/doi/10.1126/science.aan0444

http://science.sciencemag.org/content/sci/356/6334/155.full.pdf

Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes.

We present a computational study for the conversion of CH4 and CO2 into value-added chemicals, i.e., the so-called “dry reforming of methane”, in a dielectric barrier discharge reactor. A zero-dimensional chemical kinetics model is applied to study the plasma chemistry in a 1:1 CH4/CO2 mixture. The calculations are first performed for one microdischarge pulse and its afterglow, to study in detail the chemical pathways of the conversion. Subsequently, long time-scale simulations are carried out, corresponding to real residence times in the plasma, assuming a large number of consecutive microdischarge pulses, to mimic the conditions of the filamentary discharge regime in a dielectric barrier discharge (DBD) reactor. The conversion of CH4 and CO2 as well as the selectivity of the formed products and the energy cost and energy efficiency of the process are calculated and compared to experiments for a range of different powers and gas flows, and reasonable agreement is reached.

Plasma-Based Dry Reforming: A Computational Study Ranging from the Nanoseconds to Seconds Time Scale

Global warming has received much public concern and carbon dioxide utilization has been considered as one of viable approaches to reduce the CO2 emissions and alleviate global warming. Dry reforming of methane (DRM) is regarded as potential technique to reduce anthropogenic (greenhouse gases) GHGs emissions. Both catalysis and plasma technologies have been applied for DRM to investigate the CO2 and CH4 conversion as well as syngas generation efficiency. For catalysis, noble metal catalysts exhibit good activity but the cost is too high. Ni-based catalysts are usually investigated and several methods of modifying are postulated to enhance their DRM performance including better metal-support interaction, basicity of catalyst and smaller metal cluster size. However, catalysis needs to be operated at a high temperature which results in high energy consumption. Moreover, coke deposition leads to deactivation of catalyst which also limits the lifetime of catalyst. Plasma reforming which can be operated at a wide range of temperature (from room temperature to over 1000 °C) is another technique for DRM. Both non-thermal plasma and thermal plasma are proved to effectively convert CO2 and CH4 into syngas. However, the energy utilization efficiency is still low and relatively low syngas selectivity results in low syngas generation efficiency. Thus, combination of catalysis and plasma can be an alternative to integrate the advantages of catalysis and plasma. Plasma catalysis is proved to have synergistic effects to improve the syngas generation efficiency, since catalysis and plasma can improve the performance of each other. Plasma can enhance catalysis activity and durability, while the existence of catalyst promotes electron density in plasma and energy utilizing efficiency is expected to improve. In this study, the mechanisms of catalysis promotion are described, the synergistic effects between catalyst and plasma are elucidated, and possible approaches to optimize DRM are proposed.

Review of catalysis and plasma performance on dry reforming of CH4 and possible synergistic effects

The main species and chemical processes in low-temperature atmospheric-pressure He?+?O2 plasmas are identified using a comprehensive global model. The simulation results highlight the significance of Penning processes at low oxygen concentration, and the increasingly important role of electron attachment as the oxygen concentration increases. With increasing the oxygen concentration, the electron energy dissipation shifts from elastic collisions with He to dissociative excitation and attachment of O2 molecules, and large ions (, ) become the dominant charged species. Generation and loss of ROS (O, O(1D), O(1S), O2(a1?g), O2(b1?), O3) relevant for biomedical applications are discussed.

C. Winstead

Papers

Electron impact dissociation of oxygen-containing molecules A critical review

Electron-impact excitation of the A^3Sigma^+_, B^3Pi_g, W^3Delta_u and a^1Pi_g states of N_2 at low energies