Promotion Of Adding Gas On Direct Synthesis Of Hydrogen Peroxide Via Plasma Reaction

Hydrogen peroxide?H₂O₂?,the most efficient green oxidant,is widely used in electronic industry,chemical synthesis,aerospace and military and some other fields.Based on the previous research work in our laboratory,this paper studied the effects of adding gas on the direct synthesis of hydrogen peroxide from hydrogen and oxygen via plasma reaction.The experimental results and conclusions are as follows:In the H₂-O₂-X?X=He,Ne,Ar,Kr,Xe?plasma,the results showed that the H₂-O₂-Xe plasma exhibited the highest O₂ conversion?88.2%?and H₂O₂ yield?40.1%?.However,the H₂-O₂-Ar plasma showed the highest H₂O₂ selectivity?69%?,in which the H₂O₂ yield reached29.6%.The influence of Ar flow rate on the reaction was further studied,and the results showed that the O₂ conversion and the H₂O₂ yield increased with the increase of Ar flow,while the H₂O₂ selectivity was maintained around 78%.Under the condition of 60 ml/min Ar,the O₂ conversion and the H₂O₂ selectivity reached 37.3%and 77.0%,respectively.The results of plasma diagnosis show that abundant metastable species existed in the discharge zoon when the rare gases were introduced into the plasma,and the improvement of H₂O₂ synthesis could be attributed to these metastable species.It was also found that the intensity of H atomic spectrum and O atom spectrum increases with the increase of Ar flow rate,and the intensity of H atom spectrum intensities was larger than that of O atomic spectrum,indicating the function of Ar on selectively promoting H₂ dissociation,which contributed to the increased H₂O₂ yield.In the H₂-O₂-H₂O plasma,the optimum conditions for H₂O₂ synthesis were 40W discharge power and 16ml/min water vapor flow,under which 59.5%O₂ conversion,89.2%H₂O₂ selectivity and 53.1%H₂O₂ yield were achieved.Plasma diagnosis results show that the addition of water vapor enhanced the electron density in the plasma.Compared with the H₂-O₂plasma,the intensity of the emission spectrum was obviously enhanced by adding H₂O vapor.In the gas phase reaction,H₂O did not generate H₂O₂ itself,and H₂O₂ was formed by dissociation of H₂ and O₂.H₂O molecule in the H₂-O₂-H₂O plasma played two roles in promoting H₂O₂ synthesis.The one role is that H₂O molecule enhanced the electron density which is beneficial to the activation of H₂ and O₂.The another role is that H₂O molecule accelerated the radical reactions of H+O₂?HO₂ and HO₂+HO₂?H₂O₂+O₂ to promote H₂O₂ synthesis.In the H₂-O₂-H₂O-X?X=He,Ne,Ar,Kr,Xe?plasma,the O₂ conversion in H₂-O₂-H₂O-Xe plasma was the highest?80.99%?,and the H₂O₂ selectivity in H₂-O₂-H₂O-Ar plasma was the best?76.38%?.Plasma diagnosis results indicate that the discharge current value of H₂-O₂-H₂O-X?X=He,Ne,Ar,Kr,Xe?plasma was smaller than that of H₂-O₂-H₂O plasma.And the intensity of optical emission spectrum in the H₂-O₂-H₂O-X?X=He,Ne,Ar,Kr,Xe?plasma was stronger than that of H₂-O₂-X?X=He,Ne,Ar,Ne?plasma,but the components of the active species remain unchanged.The synergistic effect of H₂O and Ar on H₂O₂ synthesis showed that the H₂-O₂-H₂O reaction effect was better than that of H₂-O₂-H₂O-Ar and H₂-O₂-Ar.Plasma diagnosis result indicate that the discharge current was the main reason affecting O₂ conversion,H₂O₂ selectivity and yield.