Mechanism Research For The Direct Synthesis Of H₂O₂via Hydrogen-Oxygen Plasma Reaction

Hydrogen peroxide (H2O2) is a widely used green oxidant. Direct reaction of H2and O2became a research focus in the field of H2O2synthesis. Based on our group’s previou research work about direct synthesis of H2O2via H2/O2plasma method, this dissertation fucused on the reaction mechanism of H2/O2mixture in the explosive limits via optical emission spectra and isotopic tracing technology. In addition, single molecule catalysis of Ar for H2O2synthesis and scale-up synthesis of H2O2with H2/O2mixture of high O2content have also been studied. Main results and conclusions abtained are as followings:1. Structure of reactor has obvious effect on safety of H2/O2plasma reaction. The self-cooling DDBD reactor has the highest safety and the permitted O2content is up to30mol%. This reactor is featured by weak discharge current, homogeneous and diffusive discharge behavior, and low gas temperature. However, low electron density is the foundamental reason for high safety of the self-cooling DDBD reactor. This reactor can use H2/O2mixture with O2content at10mol%-25mol%(in explosive limits) to synthesize H2O2. Because of higher O2content, the yield and energy efficiency of H2O2synthesis have been increased.2. In the H2/O2plasma, H2O2is formed through chain termination reactions H+O2â†'HO2and HO2+HO2â†'H2O2+O2. H2O is formed through chain branching reactions H+O2â†'OH+O and O+H2â†'OH+H, and chain transfer reaction H2+OHâ†'H2O+H. The electron density of the self-cooling DDBD reactor is low, which results in low activation degree of H2and O2, thus chain termination reactions dominate to synthesize H2O2. However, the activation degree of H2and O2in the self-cooling SDBD reactor is high, and chian branching and chain transfer reactions dominate, which lead to formation of H2O and chain explosion.3. Energy of electron has great influence on the activation of H2and O2molecule. Electrons with energy of E<1eV and E>2eV can activate O2. However, electrons with energy of1eV<E<2eV can activate H2selectively, which in favor of chain termination reactions to synthesize H2O2. This conclusion supplies possibility to improve the selectivity and security of H2O2synthesis in the future. 4. Inert gases (Ar, He and N2) have obvious effect on H2/O2mixture discharge and plasma reaction. The addition of Ar can make the discharge more homogeneous, decrease discharge voltage, and increase the yield and energy efficiency of H2O2synthesis. When flow rate of Ar increased to100ml/min, yield of H2O2synthesis reached57.8gH2O2/l-h and energy efficiency reached16.1g/kW.h. Based on pure H2/O2plasma in the absence of Ar, these two parameter have increased about158%and156%, respectively. It was found that metastable Ar*(1s3and1s5) species can promote dissociation of H2selectively through forming van der Waals complex with H2. Therefore, Ar inhibited single molecule catalysis for direct synthesis of H2O2via H2/O2plasma.5. A new generation metal powder high voltage electrode double dielectric barrier discharge (MP-DDBD) reactor has been invented, which make the direct synthesis of H2O2via H2/O2plasma realize safe and uninterrupted operation. In addition, a multitube parallel device based on the MP-DDBD reactor has been designed for scale-up synthesis of H2O2. In which,150hours continuous synthesis of H2O2was carried out successful with15mol%O2content H2/O2mixture as feed. In the process, operation of the device was safe, reaction results was stable. The product was concentrated (65wt%) and high purity electronic grade (SEMI Grade1) H2O2.