Study On Performance-Improvement And Mechanism Of Oxygen Vacancy-Assisted Photo-,Thermal Catalytic Hydrogen Generation On Precious Metal Catalysts

The dissociation of water molecules is a rate-determining step in coal chemistry and renewable hydrogen energy conversion.Therefore,it is of great significance to design catalysts reasonably and fully clarify the structure-activity relationship for obtaining high-performance water decomposition catalysts.As the most common anion vacancy in metal oxides,oxygen vacancy(Vo)effectively alters the localization charge distribution and electronic energy levels near defect sites.In the catalytic reaction,Vo provides more catalytic active sites and has a significant effect on electron transfer and electronic interaction between active metal and support.VO plays an increasingly important role in catalyzing the dissociation of water molecules though its unique physical and chemical properties.By accurately regulating the Vo content in the catalyst at the nanoscale,the intrinsic activity of the noble metal catalysts can be significantly improved,thus improving the dissociation ability of water molecules in noble metal catalysts.It has been fully confirmed that Vo can greatly improve the ability of catalyst to dissociate water molecules in high temperature water-gas shift reaction,photolysis,and electrolysis of water.However,whether Vo can promote the efficient dissociation of water molecules under mild conditions remains to be further explored.The dissociation of water molecules in the hydrolysis reaction of ammonia borane(NH3BH3,AB)is the rate-determining step.In this paper,the mechanism and structure-activity relationship of Vo to improve the performance of noble metal catalyst for hydrogen production were studied using AB hydrolytic hydrogen production reaction as probe.Specific research ideas and results are listed as follows.(1).The mechanism of Vo on single crystal surface in Titanium dioxide(TiO2)as electron promoter to improve the catalytic activity of Ru based catalyst for AB hydrolysis was studied.The experimental results show that the catalyst with Vo(RTVo)has an excellent turnover frequency(TOF)in AB hydrolysis,which is higher than that of RTN catalyst.Besides,the relative concentration of Vo is positively correlated with the TOF of the catalysts.The calculation results of density functional theory(DFT)show that the dissociation energy barrier of water molecules in RTVo is lower than that of RTN.Under photocatalysis,the additional photo-generating electron furtherer enhances the ability of RTVo to dissociate water molecules.Vo,as electron promoter,transfers electrons to Ru,leading to the electron-rich Ru,which is the origin of the high activity of the catalyst.(2).The regulation of Vo as the active site of water dissociation on AB hydrolysis catalyzed by Pd-based catalyst was studied.The results show that the TOF of catalyst with Vo(1.5-PdTVo)is higher than that of catalyst without Vo(1.5-PdT)in AB hydrolysis.The high TOF is due to the Vo on TiO2 providing more active sites for the dissociation of water molecules.DFT calculations show that the dissociation energy barrier of water molecules in 1.5-PdTVo is lower than that of 1.5-PdT.Vo in TiO2 synergies with Pd to dissociate water molecules and promote the hydrolysis of AB to hydrogen production.(3).The regulatory mechanism of Vo constructed dimension on Ru’s ability to dissociate water molecules was investigated.A series of supported Ru-based catalysts were constructed on spherical anatase TiO2(TA),spherical rutile titanium dioxide TiO2(TR)and flake anatase TiO2(T),and the same concentration of VO was introduced.The TOF of RuTAVo and RuTVo increased by 48.0%and 281.1%compared with RuTA and RuT,respectively.The experimental results show that the two-dimensional structure is more helpful for Vo to regulate the electronic properties of Ru than twodimensional structure.(4).Based on the different effects of Vo,the effect of multi-effect Vo on the catalytic activity of Pt-based catalyst for hydrogen production from AB hydrolysis was studied.The experimental results and DFT simulation show that the Vo in TiO2-Vo synergistic with Pt toward water dissociation in AB hydrolysis.In highly dispersed NiOx nanoclusters,Vo acts as an electron promoter transfers electrons to Pt,resulting in electron-rich Pt,which enhances the ability of Pt to dissociate water molecules and AB molecules.Under photocatalysis,active site Vo generates additional photogenic electrons,which further enhances the ability of Vo and Pt to dissociate water molecules.In this paper,the effect of Vo in the catalyst was selectively designed,and the multieffect VO(electron promoter VO and active site VO)was realized to enhance the ability of noble metal catalyst to dissociate water molecules in photo-,thermal catalyzed AB hydrolysis.Theoretical simulation and experimental results reveal the mechanism of multi-effect Vo in photocatalytic and thermal dissociation of water molecules.This work provides valuable reference for the rational design of efficient dissociated water molecule catalysts in energy conversion by Vo.