| Hydrogen peroxide is widely used in daily life and industry because of its high oxidation activity.The global demand for hydrogen peroxide is increasing day by day.The main production method of hydrogen peroxide is anthraquinone method,which has disadvantages such as complicated process,excessive consumption of resources and unfavorable to sustainable development.The direct synthesis of hydrogen peroxide from hydrogen and oxygen is considered to have great potential to replace anthraquinone because of its simple operation and low pollution.In the process of industrialization of direct synthesis of hydrogen peroxide,the selectivity and yield of hydrogen peroxide are found to be low,the key to solve the problem is to develop a stable and efficient catalyst.Palladium-based catalysts have high catalytic activity and selectivity for the direct synthesis of hydrogen peroxide.However,the catalytic mechanism of palladium-based catalysts in terms of the size of palladium-based catalysts,the evolution of valence state under in-situ reaction conditions,and the role of the support need to be further studied.In this paper,the effects of the size and valence of Pd nanoclusters and the titanium dioxide support on the direct synthesis of hydrogen peroxide were studied by density functional theory.The main results are as follows:(1)Most stable geometric structures of Pdn,(PdO0.5)n,and(PdO)n(n=4,12 and 19)were searched by minimum hopping method.It is found that the larger palladium nanoclusters are more difficult to be highly oxidized.The adsorption properties of H,O,H2,O2,OH,OOH and H2O2 on Pd(111)and Pd(111)with different O coverage(θ=0.16 MLand 0.32 ML)was investigated.The adsorption energy of O2 is higher than that of H2 on all the Pd catalysts except for Pd4O4.With the increase of oxidation degree of Pd nanoclusters,the adsorption strength of H decreases.The adsorption strengths of H2,O2,OH,and OOH on clusters of size 12 and 19 are enhanced when they are oxidized from(PdO0.5)n to(PdO)n due to geometric restructuring induced by oxidation.On the larger Pd(111)surface,the adsorption strength of all adsorbents decreases with the increase of oxygen atom coverage.According to the activation energy barrier of elementary reaction of hydrogen peroxide direct synthesis,it is found that the ratedetermining step of H2O2 formaiton is the first O2 hydrogenation on pure palladium nanoclusters,and it is OOH hydrogenation on oxidized nanoclusters.The size and valence state of palladium clusters have a significant effect on the energy barrier.For smaller nanoclusters,Pd4O2 and Pd12O6 can reduce the energy barrier of O2 hydrogenation reaction.For the larger catalysts,Pd19O10 and 0.16 ML O/Pd(111)surface slightly inhibits the dissociation of O2 and accelerates the hydrogenation of OOH into H2O2.(2)The global optimal structures of supported nanoclusters Pdn/TiO2 and(PdO0.5)n/TiO2 are determined by minimum hopping method.It is found that the Pd atoms near TiO2 support are more easily oxidized.After supported,the Pd12 and Pd19 nanoclusters are more difficult to be highly oxidized.Compared with free Pd nanoclusters,the adsorption strengths of H2 and O2 are enhanced on the TiO2 supported nanocluster except for Pd4.And the adsorption strength of H2O2 is enhanced on the TiO2 supported smaller nanoclusters.The adsorption energy of O2 molecule is higher than that of H2 molecule and the adsorption strengths of H2,H,O and OH on PdO2/TiO2 and Pd19/TiO2 are weakened after oxidation.The adsorption strength of H2O2 on all supported nanoclusters is enhanced after oxidation due to the formation of hydrogen bond.By studying the reaction paths of direct synthesis of hydrogen peroxide,it is found that the dissociation of H2 and hydrogenation of O2 and OOH are accelerated on Pd12O6 nanocluster supported on TiO2,but the dissociation energy barriers of O2 and OOH increased,which promotes the formation of hydrogen peroxide.But Pd19O10/TiO2 is more conducive to the formation of water as a byproduct.Among supported nanocluster catalysts,(PdO0.5)n/TiO2 are beneficial to the formation of hydrogen peroxide.It is found that the decomposition energy barriers of O2,OOH and H2O2 are increased.Meanwhile,the activation energy barriers of the two hydrogenation reactions in the synthesis of hydrogen peroxide are decreased. |
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