Probing The Active Sites Of Pd-based Catalysts For Direct Synthesis Of H₂O₂

Hydrogen peroxide(H2O2),as a high-performance and environmentally benign oxidant,is widely applied in selective oxidation of organic molecules,pulp/paper bleaching,metallurgy,cosmetic and wastewater treatment.The global utilization of H2O2 has been increasing for the last three decades.The domestic demand for H2O2 has ranked the first of the world.Currently,H2O2 is produced primarily by the Riedl-Pfleiderer process.However,this process inherently requires heavy capital and operating costs because of the energy-intensive separation,concentration and transportation of H2O2,and it inevitably results in the generation of high amounts of waste,thus being competitive only at large scale.H2O2 synthesis directly from H2 and O2 is a promising atom-efficient alternative,due to the advantages such as the absence of organic substrates,simplified separation process,and operation in decentralized plants at any scale.Albeit so many above-mentioned benefits,the industrial implementation of the direct process has not been realized because of the limited performance of catalysts.How to improve the selectivity toward H2O2 is the biggest challenge for this process.Due to the lack of the understanding of the mechanism and active sites for direct H2O2 synthesis,the high-performance catalyst is difficult to develop.Using theoretical methods to study the structure and performance of surface sites,we can get clear understanding of correlation between catalysts structure and properties,thus guiding the design and preparation of the desired catalytic materials containing the optimum active sties to realize highly effective production of H2O2.In addition,the fundamental understanding of catalytic reaction of H2 and O2 could give new inspiration to the development of fuel cells and other researches involving catalytic oxidation reduction reactions(ORR).With the aid of first-principles electronic structure calculations based on density functional theory(DFT),this study focused on the structure of active sites and mechanism involved in the direct synthesis of H2O2 from H2 and O2 over Pd-based catalysts.Structures and performance of typical surface sites of Pd catalysts were studied to clarify the effects of surface geometry,particle size,alloy and functional support.Consequently,highly efficient catalysts were prepared with the guidance of theoretical results.Firstly,periodical slab and cluster models were built to understand size effects and the correlation between structure and properties of Pd catalysts on inert support.Then,performance of Pd-based alloy catalysts with nonprecious second metals,Te,Sn,and Bi,were studied with the combination of theoretical and experimental methods.Finally,the promotional effects of functional support hydroxyapatite(HAp)on the monometallic Pd catalyst were studied.The main results and conclusions are as follows:(1)By calculating the elementary reactions of direct H2O2 synthesis over Pd(111)、Pd(100),Pd(110),and Pd(211)surfaces,we found that Pd(111)surface sites with the highest density of Pd atoms show the lowest reactivity for O2 dissociation and the highest selectivity toward H2O2 among all the surface sites.Step sites of Pd(211)have the highest capacity for O2 activation;the lowest H2O2 selectivity suggested that step sites of Pd catalysts were unfavorable for H2O2 production.Therefore,the capacity of Pd catalysts for non-dissociative hydrogenation of O2 is the crucial factor in high-selectivity H2O2 synthesis.In addition,the high coverage of reactants on surfaces at high pressures could effectively suppress the side reactions,thus achieving higher H2O2 selectivity.(2)For Pd subnano clusters,the low-coordinated edge sites showed inferior reactivity for the aimed reaction,while the low-coordinated terrace sites exhibited higher activity and remarkably lower H2O2 selectivity than high-coordinated surface sites with the same site geometry of large particles and bulk materials.Accordingly,H2O2 selectivity would decrease with the reduction in Pd particle sizes.Pd(111)terrace sites have better performance than Pd(100)terrace sites for catalysts with the same size.(100)terrace sites of Pd subnano clusters,over which only byproduct(water)could be produced at low temperature,showed the worst performance among all the Pd sites.Thus,the performance of active sites on Pd catalysts supported by inert supports is mainly determined by site coordination and site geometry.For Pd sites with the same structures,the sites with lower coordination number have higher capacity for the adsorption and dissociative activation of O2,and thus possess worse performance for direct H2O2 synthesis.For Pd sites with similar coordination numbers,site geometry is the dominative factor in catalytic performance.This study indicates that H2O2 selectivity over Pd catalysts on inert supports could hardly be increased by adjusting the particle size and surface structure alone.(3)Compared to Pd-Sn and Pd-Bi catalysts,Pd-Te catalysts showed the best performance for H2O2 synthesis under ambient pressure.A near-100%H2 selectivity towards H2O2 was acquired by finely tuing the Pd/Te atomic ratio.Te is inert to the adsorption and activation of surface species containing O-O bonds,and it thus has a dilute effect on continuous Pd sites.Meanwhile.the addition of Te can also alter the electronic structure of active sites remarkably.DFT calculations indicates that Pd-Te sites and Pd-Pd sites adjacent to Te atoms have lower capacity for adsorption and dissociative activation of O2.and they thus are highly selective active sites for direct H2O2 synthesis.For high-coordinated terrace sites of Pd-Te catalysts,H2O2 selectivity increases with the rise in Te concentration.The activation barriers of all the side reactions are higher than those of the related main reactions for H2O2 synthesis over the Pd-Te catalyst with a surface Te/Pd ratio of 1/3.Accordingly,this catalyst showed the highest H2O2 selectivity among all catalysts.The low-coordinated terrace and corner sites of Pd-Te clusters with small sizes showed low selectivities toward H2O2.Therefore,the particle size should be deliberately controlled in application.In addition,excessive ratio of Te would lead to the deactivation of catalysts,due to the low ability of Te to the adsorption and dissociative activation of H2.(4)Pd particles supported on HAp were positive charged,and there was a remarkable reconstruction of the Pd surface sites adjacent to the HAp surface,and a disordered Pd surface structure thus was formed.Therefore,new active sites that are completely different from ordered Pd0 sites were created on supported Pd particles,over which the non-dissociative hydrogenation of O2 was significantly improved,be responsible for the high H2O2 selectivity.Since the effects of HAp on subnano Pd clusters is much higher than those on larger particles,active sites of subnano clusters showed the best performance for H2O2 synthesis.However,single Pd sites showed no ability to adsorb and activate H2 because of the lack of adsorption sites and their extremely high positive charge,and the Pd/HAp catalyst only composed of single sites is inert to this reaction.The stronger effects of HAp on smaller Pd particles lead to higher H2O2 selectivity over catalysts with lower sizes.The size effects of Pd/HAp catalysts are different from that of Pd catalysts on inert supports and should be correlated to support effects.