Kinetics Of Direct Synthesis Of Hydrogen Peroxide From Hydrogen And Oxygen

Hydrogen peroxide?H₂O₂?,producing only water and oxygen in the reaction,is recognized as one of the substantial green and environmentally friendly chemical raw materials.At present,the most of hydrogen peroxide production industries in the world are synthesized by anthraquinone method,but this method consumes a large amount of raw materials and the process is complex,so there are greater safety risks.The direct synthesis of H₂ and O₂ into H₂O₂ is one of the most promising H₂O₂ production methods with the advantages of environmental protection,simple production process and atomic economy.However,direct synthesis of hydrogen and oxygen has not yet played a major role in industrial production,mainly because of the low yield and low selectivity of H₂O₂.Catalyst surface species may play a crucial role in the formation of H₂O₂,but the mechanism of surface species type,coverage rate and binding energy with catalyst surface on the formation of H₂O₂ or the activation of H₂ and O₂ has not been clarified.The reaction mechanism of direct synthesis of H₂O₂ by hydrogen and oxygen over palladium catalyst was studied under the control of intrinsic reaction kinetics.By adjusting the reaction conditions of H₂O₂ generation,it is found that the kinetic model of H₂O₂ generation is strictly dependent on the ratio of H₂/O₂,and presents two completely different kinetic intervals.In each kinetic interval,there are independent rate equations,speed control steps,isotope effects and catalyst surface adsorption states.When the H₂/O₂ ratio is less than 4.0,the generation rate of H₂O₂ increases linearly with the partial pressure of hydrogen,but not with the partial pressure of oxygen.The H₂-O₂ catalytic reaction is controlled by the adsorption and dissociation of H₂ on the pure Pd catalyst surface.When the H₂/O₂ ratio is greater than 4.0,the generation rate of H₂O₂ no longer changes with the change of partial pressure of hydrogen,but increases linearly with the increase of partial pressure of oxygen.The H₂-O₂ catalytic reaction is subject to the OOH*hydrogenation on the surface of the Pd catalyst covered with H*atom saturation.The generation and consumption rates of H*are coupled with H₂ activation and O₂ hydrogenation rate on Pd catalyst,so the H₂/O₂ ratio determines the coverage rate of H*.Therefore,the first-order rate coefficient of the formation of H₂O₂ on palladium catalyst is defined as the ratio of the formation rate of H₂O₂ to the partial pressure of O₂,which is only a single-valued function of H₂/O₂ and increases linearly with the ratio of H₂/O₂ and then remains unchanged.By investigating the Pd/Si O₂ catalysts with different particle sizes?5.9 nm,7.5nm and 12.0 nm?,it is found that the formation rate of H₂O₂ is not affected by the size of the Pd particles as the H₂-O₂ catalytic reaction is controlled by the adsorption and dissociation of H₂ on the pure Pd catalyst surface.The formation rate of H₂O₂ increases with the size of Pd particles increases when the H₂-O₂ catalytic reaction is subject to the OOH*hydrogenation on the surface of the Pd catalyst covered with H*atom saturation.The transiton in the surface coverages of Pd,from intermediates-free to saturated with H*in response to changes in H₂/O₂ ratios,the neglibible effects of particle diameter because kinetically relevant H₂ dissociative occurs on Pd sites via an early transition state with an extremely low activation barrier and high exothermicity.In the later stage of transition,H*atoms present on larger Pd particles and interact with the O in OOH*species with more weakly bound,thus decreasing activation barriers and increasing the formation rate of H₂O₂.