(Invited) Mechanism of H2 Synthesis Due to Plasma-Induced Water Splitting Via Free-Electron Impact
Authors: Mota-Lima Andressa 
Published: September 2023

The electrified plasma-liquid interface (EPLI) has two major local reactors, the plasma phase and the liquid phase. Both local reactors enable water splitting, but their synthesis mechanism is different. While the mechanism of H2 synthesis in a liquid phase (plasma electrochemistry) was reasonably deciphered, the respective mechanism in the plasma phase does not (plasma chemistry). In this present contribution, the Chemical Reaction Network (CRN) analysis serves as a tool to decipher the H2 synthesis mechanism in the plasma reactor. For that, the datum from the laboratory of Sankaran serves as grounds for the CNR analysis. Sankaran team identified that the equilibrium vapor pressure of water introduces water vapor in the plasma phase; and that, fundamentally, there were eight elementary reactions involved in the plasma-chemistry mechanism of H2 synthesis, i.e., those reactions containing the largest constant rates (or electron rate coefficients). [1] The authors obtained the reaction rate constants via a numerical solution of the Boltzmann equation (BE) for electrons in weakly ionized gases considering the uniform electrical field. [2] Together, those eight elementary reactions form the following global reaction for the water splitting via electron-impact

8 H2O → ●H+ ●OH + ●O + O– + O+ + OH– + OH+ + H– + H+

that is also rationalized in terms of global CRN, shown in figure 1. The water molecule splits into two major classes of fragments, the radical fragment set {●H, ●OH and ●O} and the ionic fragment set {O–, O+, OH–, OH+, H–, H+}. These two classes of fragments receive different treatments in this work because their kinetic relaxation is supposed to follow different time scales. The radical fragment is supposed to relax toward final products much faster than the ionic fragments. For this reason, only the chemical dynamics of the radical fragment are considered in the relaxation step toward the final products. The chemical dynamics of relaxation toward final products, thus, take into account only the set of radical fragments. This talk will present and discuss the chemical reaction network (CRN) for the relaxation of the radical fragment toward final products like hydrogen (H2), hydrogen peroxide (H2O2), water (H2O), and eventually oxygen (O2). The radical-relaxation CNR is not shown in this abstract but will be present/discussed in the talk.

Roughly speaking, the radical-relaxation dynamics must deliver hydrogen as the major product and hydrogen peroxide as the major side-product, besides a stochiometric ratio of 4 to 1, respectively. Specifics on whether the radicals •OH relaxes toward H2O2 or H2O depend on the local dynamics, meaning that it mostly relies on the parameter set values that drive the CRN. Unfortunately, there is no experimental evidence already reporting the stoichiometry of products, which makes it unfeasible to evaluate the adherence of our analysis to the experimental pieces of evidence. References

[1] Toth, J.R., et al., On the Non-Faradaic Hydrogen Gas Evolution from Electrolytic Reactions at the Interface of a Cathodic Atmospheric-Pressure Microplasma and Liquid Water Surface. Journal of The Electrochemical Society, 2020. 167(11): p. 116504.Society, 2020. 167(11): p. 116504.

[2] Hagelaar, G.J.M. and L.C. Pitchford, Solving the Boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models. Plasma Sources Science and Technology, 2005. 14(4): p. 722-733.

Figure
1
. Global chemical reaction network (CRN) for the water splitting due to the free-electron collision with the molecules of water vapor in the plasma phase formed in a noble-gas jet; such CRN also represents the global mechanism of hydrogen-radical formation, which is the key water-fragment allowing for the hydrogen synthesis in the plasma reactor.

Figure 1