Surface And Interface Regulations Of Pd/MO_x（M=Ti And Zn）catalysts And Its Catalysis For CO₂ Hydrogenation Into Formic Acid/formate

Energy shortage and environment deterioration are two major problems in current society.In recent years,the"greenhouse effect"caused by CO₂ produced by the burning of fossil energy has led to climate changes and the increase of global temperature year by year,which is seriously endangering the living environment of human beings,so it is the common goal of governments around the world to reduce the concentration of CO₂ in the atmosphere.A net-zero-carbon fuel economy may be realized if it efficiently utilizes CO₂ to produce carbon-based fuels such as CO,hydrocarbons,alcohols,ethers,formic acid/formate.Among them,formic acid is an attractive organic chemical raw material as well as a new type of hydrogen storage material and thus exhibits excellent utilization value.Catalytic hydrogenation of CO₂ into formic acid/formate is one of the effective ways to promote the carbon cycle.Research shows that supported Pd catalysts show the best application prospect,and its excellent catalytic performance is mainly attributed to the synergistic effect of Pd and support.Therefore,regulating the structure of Pd and support has important guiding significance for the development of more efficient and stable supported Pd catalyst.The support plays a critical role in determining the catalytic performance of supported Pd catalysts in CO₂ hydrogenation into formic acid/formate.Metal oxides have attracted much attention because of their good stability,excellent redox properties and unique acid-base properties.It was found that the structure of metal oxides strongly affects the catalytic performance of supported Pd catalysts.Previously,Pd supported on inert supports such as Al₂O₃,Ca CO₃ and Ba SO₄ were widely used and their catalytic performance is better than that of Pd black catalyst but poorer than those of Pd supported on carbon-based supports.Recent research found that the active supports with high density of weak basic sites significangly affect the catalytic performance in CO₂ hydrogenation.Moreover,the density and strength of basic sites are dependent on not only the nature of support but also the crystal phase/crystal planes of support.In this paper,Pd/TiO₂ and Pd/ZnO catalysts were chosen as the research objects.By adjusting the morphology of TiO₂ and ZnO,a stable and efficient Pd/TiO₂{100}catalyst for CO₂ hydrogenation into formate under base conditions and a high-efficiency Pd/r-ZnO catalyst for CO₂ hydrogenation into formic acid under base-free conditions were obtained,respectively.The effects of the support basicity and the structure of Pd respectively on the adsorption and activation of CO₂ and H₂ as welle as the catalytic performance were conducted comprehensively.Moreover,the effects of the structure of Pd on the metal-support interactions and their catalytic stability was also explored.Combined with kinetic measurements and in situ diffuse reflectance infrared Fourier transformed spectroscopy,the surface reaction mechanism in CO₂ hydrogenation into formic acid/formate were further understanded.Furthermore,it also identified the key factors affecting the reaction behavior so as to provide theoretical guidance and design methods for the development of highly active supported Pd catalysts.Detailed contents of the research are as follows:1.Regulating the surface and interface structures of Pd/TiO₂ catalysts and its catalysis in CO₂ hydrogenation into formatePd/TiO₂ catalysts employing anatase TiO₂ nanocrystals with various morphologies as supports,mainly exposing either{100},{101},or{001}facets,have been prepared by deposition precipitation method for CO₂ hydrogenation into formate under mild conditions.A significant morphology-dependent of titanium oxide in the Pd/TiO₂ catalysts was observed.The order of catalytic activity is Pd/TiO₂{100}>Pd/TiO₂{101}>Pd/TiO₂{001}.Among them,2%Pd/TiO₂{100}is highly active and stable,affording an unprecedented turnover frequency of ca.1369 h^-1 and keeping stable after 6 cycles at 40 ^oC.This can be associated with,on the one hand,higher density of moderate basic site and more Pd（0）species over2%Pd/TiO₂{100},relatively contributing to the activations of CO₂ and H₂,which are favorable for the activity;On the other hand,higher oxygen vacancy concentrations in the TiO₂{100},promoting the Pd-TiO₂ interactions,contribute to the formation and stability of flat Pd particles over 2%Pd/TiO₂{100},which is beneficial to the stability.These results illustrate the importance of oxide morphology in formate formation and open up possibilities of oxide morphology engineering for developing highly active Pd-based catalysts for CO₂hydrogenation.2.Regulating the surface and interface structures of Pd/ZnO catalysts and its catalysis in CO₂ hydrogenation into formic acidPd/ZnO catalysts with ca.2 wt.%Pd loading employing ZnO nanocrystals with various morphologies as supports,including ZnO nanoplates（p-ZnO）predominantly exposing polar{002}facets,ZnO nanorods（r-ZnO）predominantly exposing nonpolar{100}and{101}facets,and commercial ZnO（c-ZnO）exposing random facets,have been successfully synthesized by modified precipitation-deposition method for the catalytic hydrogenation of CO₂ into formic acid（FA）in the absence of base under mild conditions.Remarkable crystal-plane-dependent catalytic performance was observed.The order of catalytic activities of various Pd/ZnO catalysts in CO₂ hydrogenation into FA is Pd/r-ZnO>Pd/p-ZnO>Pd/c-ZnO,and the catalyst can maintain 95%of initial activity after 4 cycles.Kinetic study,CO₂-TPD analysis,and in situ DRIFTS spectra results suggest that all the Pd/ZnO catalysts follow a similar catalytic mechanism that CO₂ activation is the rate-determining step.Consequently,r-ZnO that possesses a higher density of weak basic sites than p-ZnO and c-ZnO can act as an excellent support to prepare Pd/r-ZnO catalyst,which is more active than Pd/p-ZnO and Pd/c-ZnO catalysts for the production of FA.These findings clearly illustrate the crystal-plane engineering of the metal oxide support in tuning the catalytic performance of Pd-based catalysts for CO₂ hydrogenation into FA.