The Effects Of Core Structure And Shell Pore Volume Of Pd-M@HCS (M=Sn, Fe, Co, Ni) Catalysts On The Direct Synthesis Of H₂O₂

The direct synthesis of H₂O₂（DSHP）from hydrogen and oxygen has the advantages of simple process and environmental friendliness,which is a green and economically feasible method.At present,Pd-based catalysts are the best catalysts for DSHP.However,the conventional Pd-based catalyst has low selectivity and productivity for DSHP.The main reason is that,although the active component of the Pd-based catalyst is beneficial to the synthesis of H₂O₂,it is also beneficial to the easy dissociation of the O-O bond in O₂ and H₂O₂,resulting in low H₂O₂ selectivity and productivity.In addition,the synthesis reaction of H₂O₂ only proceeds on the surface of Pd,the Pd in the bulk phase cannot be fully utilized,resulting in great waste.In order to improve the selectivity and productivity of H₂O₂,Pd nanoparticles not only have a smaller particle size and provide more active sites,they should also have fewer unsaturated sites to reduce the dissociation of O-O bonds.The synthesis of nanoparticles in inverse microemulsion is beneficial to its morphology control,which not only limits the growth of particles,but also improves the dispersibility of metals.The doping of non-precious metals with different reduction potentials will affect the content of Pd O and change the adsorption of the active components to the reactant molecules,while the core-shell structure of M@Pd active components with non-precious metals as the core and Pd as the shell can effectively reduce the cost and increase Pd utilization.In this study,based on the difference of metal reduction potential,core-shell catalysts Pd-M@HCS（M=Sn,Fe,Co,Ni）and（Sn@Pd）@HCS were prepared in reverse microemulsion by co-reduction method and electric replacement reaction（GRR）,which are consist of non-noble metal doped bimetallic Pd-M,Pd-coated Sn bimetallic nanoparticles with cores and porous carbon as shells.The catalysts were characterized by TEM,XPS,XRD,N₂ adsorption/desorption and H₂/O₂-TPD,and tested for the direct synthesis of H₂O₂.The effects of particle size and composition of active components of the catalyst and H₂/O₂ adsorption capacity on the DSHP were discussed.The specific research content is as follows:1.Co-reduction method to prepare Pd-M@HCS catalyst and the effect of n _Pd/n _Snon the DSHP.Core-shell catalyst Pd-M@HCS（M=Sn,Fe,Co,Ni）with non-noble metal doped bimetallic Pd-M bimetallic nanoparticles as the core and porous carbon as the shell was prepared in the reverse microemulsion by co-reduction method.The structure of the catalyst and its performance for the synthesis of H₂O₂ were studied.The research results shown,the doping of non-noble metal M（M=Sn,Fe,Co,Ni）with different reduction potentials increases the amount of H₂ adsorbed by the active components and reduces the dissociation of O-O bonds in O₂.The order of the H₂adsorption capacity of each catalyst is Pd-Sn@HCS>Pd-Fe@HCS>Pd-Co@HCS>Pd-Ni@HCS>Pd@HCS,which was consistent with the order of the H₂O₂productivity,selectivity and yield.This is because the doping of non-precious metals effectively improved the adsorption of the reactant molecules by the active components,the more reactants adsorbed at the active site,the better the catalytic transformation of the reactants.The effect of n _Pd/n _Sn on the synthesis of H₂O₂ over Pd-Sn@HCS catalyst was also studied.The research results shown,with the decrease of n _Pd/n _Sn,the Pd particle size decreases first and then increases.When n _Pd/n _Sn is 2,the active components are well dispersed and the grain size is smaller,and the average particle size is 5.77±0.97 nm,the H₂O₂selectivity and productivity are the highest,97%and 3961 mmol/g _Pd·h^-1,respectively.After 10 cycles,H₂O₂ the productivity decreased from 3961 mmol/g _Pd·h^-1 to 3650 mmol/g _Pd·h^-1,down to 92.1%of the original value,indicating that its stability was well.2.The effect of the cavity and shell pore volume of the Pd-Sn@HCS catalyst on the DSHP was studied.The catalyst cavity and shell pore volume were adjusted by changing the TEOS content of the prepared seed solution Pd-Sn@Si O₂and the CTAB content of the prepared Pd-Sn@HCS carbon shell.The research results show that,with the increase of TEOS and CTAB addition,the cavity and shell pore volume of the catalyst increased first and then decreased.When the TEOS addition was 2 ml,the cavity pore volume of the catalyst Pd-Sn@HCS-2 was the largest,0.66 cm³·g^-1.When the amount of CTAB added is 3.3 ml,the shell pore volume of the catalyst Pd-Sn@HCS-3.3 reaches the maximum,which is 0.16 cm³·g^-1.The catalyst Pd-Sn@HCS（y=2,z=3.3）has the best cavity size and shell pore volume,and the H₂conversity and H₂O₂ production of the DSHP are the highest,which are 40%and4685 mmol/g _Pd·h^-1,respectively.It shows that the increase of the catalyst cavity and shell pore volume is beneficial to the mass transfer of reactant molecules and product molecules,thereby promoting the synthesis of H₂O₂.3.The preparation of（Sn@Pd）@HCS catalyst and the effect of nanoparticle size on the DSHP.Core-shell structured catalyst（Sn@Pd）@HCS with Pd-core-Sn-shell structured nanoparticles as the core and porous carbon as the shell was prepared in the reverse microemulsion by the galvanic replacement reaction（GRR）.Studies have shown,by controlling the proportion and concentration of the metal precursor solution,the particle size of Sn@Pd can be adjusted through the electric displacement reaction in the inverse microemulsion.As the ratio of n _Sn/n _Pd increases（3:1→3:3）,the size of Sn@Pd nanoparticles first decreases and then increases（5.71±1.54 nm→5.15±0.81nm→6.12±1.48 nm）.With the increases of Pd²⁺（0.2 ml→0.8 ml）,the particle size of Sn@Pd nanoparticles gradually increases（4.85±1.14 nm→7.77±2.85 nm）.The activity evaluation results show that,with the increase of the particle size of Sn@Pd nanoparticles,the H₂O₂ selectivity and productivity first increase and then decrease.When the particle size of Sn@Pd nanoparticles is 5.15±0.83 nm(n _Sn/n _Pd ratio is 3:2,Pd²⁺is 0.4 ml),the H₂O₂ selectivity and productivity are 98%and 4899 mmol/g _Pd·h^-1,respectively.In addition,after 5 cycles of the catalyst,H₂O₂ productivity decreased from 4899 mmol/g _Pd·h^-1 to 4677 mmol/g _Pd·h^-1,decreased to 95%of the original value,indicating that the catalyst had well stability and circulation ability.