The Controllable Synthesis And “structure-activity” Relationship Study Of Bi-doped Silica-Supported Metallic Catalysts

With the development of science and the innovation of technology,the material needs of human beings are increasing.As a consequent,the energy crisis and environmental pollution are also becoming more and more serious.Therefore,all kinds of catalysts show their unique performance in above problems,not only speeding up production but also effectively alleviating environmental pollution.Among them,supported-metal catalysts have been widely used in various catalytic reactions due to low loadings of metal and high catalytic activity.However,there is an upper limit of catalytic performance for every catalyst.To pursue higher catalytic activity,it is an efficient way to modify and regulate the active site via doping the secondary inactive metal through constructing active interface and forming metal alloy.Bismuth（Bi）is an oxyphilic main group element,and its oxides can provide abundant surface migrating oxygen species.Therefore,bismuth element can be used as a dopant to modify active sites of supported metal catalysts and improve the catalytic activity in oxidation reactions.Based on the above research objectives,bismuth element was doped into silica-supported Pt,Pd and Cu-based catalysts by incipient-wetness impregnation method in this dissertation.We have carefully detected the doping effect of bismuth in these three catalysts and observed the promotion of activity in CO oxidation.With the comprehensive and in-depth characterization results,we identified precise structure of active sites for three Bi-doped catalysts and determined the reliable"structure-activity"relationship in CO oxidation reaction.The main research findings are summarized as following:（1）we have synthesized a series of Pt Bi-Si O₂ catalysts with different doping amount of bismuth element.The X-ray diffraction（XRD）,aberration-corrected high angle annular dark field-scan transmission electron microscope（ac-HAADF-STEM）and extend x-ray adsorption fine structure（EXAFS）results indicate that the formation of Pt_xBi_yO_z binary metal oxide clusters could prevent the formation of huge metallic Pt particles.After the regulation of active site with hydrogen reduction,we identify the active site structure:metallic Pt cluster with surface Pt-[O]_x-Bi interface,which provides highly reducible oxygen species to activate CO at low temperature（～50°C）with a high CO₂ production rate of 487μmol_CO2·g _Pt^-1·s^-1 at 110°C.Experiment data combined with density functional calculation（DFT）results demonstrate that Pt cluster with surface Pt-O-Bi structure is the active site for CO oxidation via providing moderate CO adsorption and activating CO molecules with electron transformation between platinum atom and carbon monoxide.（2）We further prepared silica-supported palladium-bismuth catalysts via incipient wetness impregnation method,possessing excellent sinter resistance due to the formation of oxidized Pd_xBi_yO_z clusters（<2 nm）with Pd-O-Pd and Pd-O-Bi local coordination structure through bismuth atoms partial replacing palladium atoms.After hydrogen reduction before CO oxidation,restructuration of active site occurred in Bi-promoted catalysts:from Pd_xBi_yO_z clusters to Pd Bi₁ single atom alloy with bismuth at atomic level as confirmed by aberration-corrected HAADF-STEM and EXAFS results.This Pd Bi₁ single atom alloy exhibits a good performance in CO oxidation with a reaction rate at 3.3μmol _CO·g^-1_cat·s^-1 at 100?C and lower E_a value（～35 k J/mol）,which is about ten times of that(0.4μmol _CO·g^-1_cat·s^-1 at 100?C)for pure palladium-silica catalyst.Meanwhile,this single atom alloy only adsorbs CO molecule in linear method and strengthens the interaction between palladium atoms and CO molecule according to in-situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）results.（3）Silica-supported copper-bismuth oxide catalysts also were prepared with incipient-wetness impregnation method.This Bi-doped catalyst exhibits an excellent performance in CO oxidation with a reaction rate at 0.6μmol _CO·g^-1_cat·s^-1 at 170?C,which is about ten times of that for pure copper-silica catalyst.Furthermore,we found a unique active site structure:Bi single atom anchored on the surface of Cu O_x cluster via Bi-O-Cu structure with the help of HAADF-STEM and EXAFS fitting results.In one hand,the Bi-O-Cu structure can significantly prevent the aggregation of copper oxide cluster into huge particle（50-100 nm）and provide more available Cu active sites with similar apparent activation energy（～40 k J/mol）.In another hand,this structure not only increases surface active oxygen species confirmed by CO-temperature programmed reduction（CO-TPR）,but also significantly strengthens the adsorption of CO molecule by CO-temperature programmed desorption（CO-TPD）to promote the activity of CO oxidation.