Study On Modification Of Pt/TiO₂ Nanosheets Catalysts And Their Applications In CO₂ Hydrogenation

CO₂is one of the main greenhouse gases,which has caused a series of environmental,economic and political problems due to its large emission.Using abudant,cheap and non-toxic CO₂as raw material to react with H₂to make fuels and high-value-added chemicals,will not only alleviate the emission of CO₂,but also is important for mitigating environmental damage and energy problems.Among them,the research on CO₂conversion to CO and methanol through reverse water gas conversion reaction?RWGS?and CO₂hydrogenation reaction has attracted much attention,RWGS reaction is also a key step in methanol synthesis and other reactions.Furthermore,it has been reported that CO is the carbon source of CO₂hydrogenation to methanol.Aiming at the mechanism of CO₂hydrogenation to methanol may be the RWGS+CO hydrogenation path;therefore,an efficient RGWS catalyst was studied and designed first in this thesis and then improved to obtain high efficiency for methanol from CO₂hydrogenation.At the same time,the feasibility of designing and preparing stable and efficient catalysts for CO₂hydrogenation to methanol based on RWGS+CO hydrogenation path mechanism was also investigated.The specific research content and conclusions are as follows:TiO₂nanosheets?TN?with high exposure{001}crystal surface were prepared through hydrothermal method by using HF acid as morphology control agent.Then it was pretreated in dry air,hydrogen?5%H₂+Ar?and pure ammonia at 450^oC?marked as TN-A,TN-H,TN-N respectively?to tune the defects of the nanosheets;in comparison,the nanosheets heat treated with hydrogen followed by pure ammonia were labelled as TN-HN.Then Pt/TN catalyst was prepared with impregnation method loaded with Pt;the catalytic performance of RWGS reaction was also evaluated under the conditions of atmospheric pressure 300-600^oC,gas ratio V?H₂?:V?CO₂?=3:1,weight hourly space velocity?WHSV?=300000ml·g^-1_cat·h^-1.The effects of heat treatment on the structure and distribution of Ti O₂defects in different atmospheres were investigated,as well as the influences of defect sites on the interactions between Pt and Ti O₂during the catalyst preparation process;finally,the mechanism of the carrier defects on the catalyst performance was revealed.The main conclusions of this part are as follows:?1?TN-A has no defects after heat treatment;TN-H has surface oxygen vacancy?O_v-?and unstable bulk defects(Ti³⁺);but TN-N and TN-HN have both O_v-and stable Ti³⁺accompanied with N doping as well.After loading Pt,the O_v-in the carrier can anchor Pt species and be oxidized,leading to Pt⁰;the content of Pt⁰in N-doped catalyst is the most.For all Pt/TN catalysts,O_v-and some Ti³⁺disappeared after calcination,especially Ti³⁺in Pt/TN-H catalyst,accompanied with the Ti³⁺structure transformation;however,the introduction of Pt will also facilitate the formation of various defects during the reduction;after reduction,O_v-and Ti³⁺are largely regenerated and even higher than the corresponding carriers,especially for N-doped catalysts.?2?Both the defects on the carrier and N-doped species have an effect on the anchoring of Pt,so the Pt/TN-HN catalyst with abundant defects and N content has the highest Pt dispersion and the best catalytic performance.The CO₂conversion reached 43.5%,the CO selectivity was 97.8%at 600^oC;and the catalyst still maintained good performance after reaction for 30 hours at this temperature.?3?The TOF_Ptvalue indicates that Pt particle size is not the only determinant of catalytic performance and the defects such as O_v-/Ti³⁺in the catalysts are also important.The N-doped catalysts,Pt/TN-N and Pt-TN-HN,have more defects and display stronger metal-support interaction effects,thus forming more active sites Pt-O_v-Ti³⁺at the junction between the surface defects of the carrier and Pt particles;and stable Ti³⁺increases the stability of the surface defects.Therefore,the N-doped catalysts showed better activity and stability.The highly exposed{001}crystal plane Ti O₂nanosheets prepared by hydrothermal method was used as the carrier,Cu and Pt were loaded over Ti O₂{001}nanosheet through co-reduction method;the obtained catalysts were tested for CO₂hydrogenation to methanol under the conditions of P=3.0 MPa,T=200-300^oC,V?N₂?:V?H₂?:V?CO₂?=8:69:23,WHSV=3600 m L·g^-1_cat·h^-1.Among them,the catalyst with 0.5wt%Pt and 10wt%Cu supported on Ti O₂had the best catalytic performance at 260^oC,the CO₂conversion reached 10%and the yield of methanol was 3.5%.XRD,XPS,EPR,and CO₂-TPD etc.characterizations show that compared to Pt/Ti O₂{001}and Cu/Ti O₂{001}catalysts,Pt-Cu/Ti O₂{001}catalysts display stronger interactions between the metals and the support due to the electron promotion effects of Pt.This is helpful to stabilize the Cu particle size and to the form more defect sites in the carries(such as O_v-/Ti³⁺);so more active sites on the interface between the metal and the support of the Pt-Cu/Ti O₂{001}catalyst have been produced,thus showing better CO₂activation ability and methanol formation rate.