Preparation Of Model Oxide Thin Films On Metal Surfaces And Their Catalytical Performance For CO Conversion

The works in this thesis were mainly performed in a home-built model catalysis facility with Auger spectroscopy(AES),low energy electron diffraction(LEED),infrared reflection adsorption spectroscopy(IRAS),and other basic surface science techniques.Several model surfaces,TiO_x/Pt(111),MnO_x/Rh(111)and O&TiO_x/Pd(100),were prepared to mimic strong metal support interaction(SMSI)surfaces.Catalytic performances for CO oxidation or hydrogenation were tested in the combined in situ reaction cell.In situ wide spectral range IRAS was used to monitor the model surfaces under reaction conditions.The works aim at better understanding the promotion effects of SMSI.At first,several types of well-ordered ultrathin TiO_x films were grown on the Pt(111)surface,among which we focused on TiO_x-Re/Pt(111)and TiO_x-Ox/Pt(111)for CO oxidation.The results show that both two model surfaces could enhance catalytic activities for CO oxidation over bare Pt(111)surfaces.The catalytic activity for CO oxidation on the TiO_x-Re/Pt(111)model surfaces,which mainly consists of Ti3+,increases nearly linearly as increasing the TiO_x coverage up to 1 ML.For the TiO_x-Ox/Pt(111),a volcano curve between 0?1 ML with a highest value at about 0.4 ML was observed.The in situ wide spectral range IRAS confirms the catalytic efficiency and stability of the TiOOx/Pt(111)model surfaces under reaction condition.By covering Pt(111)with SiOx film,the catalytic activities for different surfaces can be determined.A more reliable apparent activation energy of 97 kJ/mol was obtained for the clean Pt(111)surface for CO oxidation.The apparent activation energies are 51 kJ/mol and 54 kJ/mol for the 1 ML TiO_x-Re/Pt(111)and 2 ML TiO_x-Ox/Pt(111),respectively,both were much lower than that for the clean Pt(111).The active sites are the boundary of the zigzag domains for the TiO_x-Re/Pt(111)and the perimeter of the TiO_x-Pt interface for the TiO_x-Ox/Pt(111),respectively.Secondly,MnO_x films were grown on the Rh(111)surface as model catalysts for CO hydrogenation.The stability of the model surface was examined under various conditions.The ethanol selectivity for CO hydrogenation over 0?1 ML MnO_x/Rh(111)shows a volcano curve,with a highest value at about 0.75 ML.It is indicated that the presence of MnO increases the selectivity of ethanol,possibly because of the interface effect of MnO-Rh which lowers the dissociation energy of CO.Thirdly,three O/Pd(100)model surfaces were prepared and examined for CO oxidation by in situ IRAS.The results reveal that Pd surface covered by either chemisorbed O or((?)×(?))R27°-0 surface oxide shows high CO reactivity,while the bulk-like PdO is less active.All the three surfaces are reduced to metallic Pd under CO oxidation conditions.Ultrathin TiO_x films were also grown on the Pd(100)surface following a layer by three-dimensional cluster model.Different from the TiO_x/Pt(111),TiO_x suppresses the activity for CO catalytic oxidation.The results demonstrate that TiO_x thin films on different substrates behave significantly different.