The Theoretical Study Of Reaction Mechanism For Direct Synthesis Of H₂O₂ From H₂ And O₂ On Palladium Based Catalysts

Using the Pd-based catalysts to catalyze the direct synthesis of H₂O₂ from H₂ and O₂is a very promising new process for the preparation of H₂O₂.It has the advantages of green,environmentally friendly,high efficient and atomic economy.However,because Pd-based catalysts can also catalyze H₂ and O₂ to form H₂O,there is a problem of low selectivity in the reaction.It seriously restricts the industrialization development of direct synthesis of H₂O₂.If we can understand the reaction mechanism from the theoretical level and analyze the factors of selectivity limitations,it would provide feasibility suggestions for the rational modification of Pd-based catalysts.This is of great significance for the industrialization of direct synthesis of H₂O₂.In this thesis,the density functional theory?DFT?and the micro-kinetics method?MKM?are firstly used to study the reaction mechanism of direct synthesis of H₂O₂ on pure Pd catalysts.DFT results show that the adsorptions of O-containing substances are very strong on clean Pd?111?and Pd?100?surfaces,especially O and O₂;and the calculated coverage of O by MKM on clean Pd surfaces is close to 100%,which is extremely high.Too high coverage of O makes the Pd surfaces poisoned by O,so the pure Pd catalysts show lower selectivity.However,we find that the presence of co-adsorbed O on the Pd-O surfaces not only greatly improves the reactivity,but also significantly increases the selectivity.The co-adsorbed O can weaken the adsorptions of O-containing substances.On the Pd?111?-O surface,the selectivity of H₂O₂ is over 99%;while on Pd?100?-O surface,more H₂O is generated,and its selectivity also exceeds 99%.This indicates that the Pd?111?surface with co-adsorbed O is the main active surface for the direct synthesis of H₂O₂.On the basis of the conclusion of Pd-O surfaces,the research models of Pd-Au bimetallic catalysts are established because Au can also weaken the adsorptions of O-containing substances.The results indicate that H₂ is difficult to be activated and dissociated to generate H atoms on pure Au catalysts,so its activity and selectivity for generating H₂O₂ are very low.The Pd-Au bimetallic catalysts can activate H₂ to generate H atoms by means of Pd.At the same time,the Au atoms on the alloy surfaces have a similar effect as co-adsorbed O,which can greatly weaken the adsorptions of O-containing substances,thereby solving the the problem of O poisoning on the pure Pd surfaces.So it shows excellent catalytic performance for the direct synthesis of H₂O₂.