Preparation,Characterization And Application Of FeO_x,CoO And NiO Model Surface On Rh(111)

The oxidation dehydrogenation of propane to propene,as well as the CO hydrogenation to ethanol and C2 oxygenates have been considered as the effective paths for producing chemicals and fuel.The catalysts used in these two reactions have been extensively studied.However,the catalytic efficiencies are still not good enough.Moreover,the mechanism of the reactions and the relationships between the structure of the catalysts and catalytic performances are still far from being concluded.In this work,NiO/Rh(111)>CoO/Rh(111)model catalysts were constructed to reveal the absorption properties and the nature of active sites for propane activation on the catalysts by using XPS and LEIS.In addition,FeOx/Rh(111)model catalyst was prepared to investigate the relationships between the structure and catalytic performance and the promotion effect of Fe on Rh-based catalysts for CO hydrogenation to ethanol by using XPS、LEIS、LEED and IRAS.In the first part,the NiO films were prepared by the post oxidation method on Rh(111)and characterized by XPS and LEIS.The NiO oxide film is gown in a 2D mode at submonolayer,and bulk-like at higher coverage.The thick film is much more stable than the thin one upon oxidation and annealing.The reaction of propane with the NiO oxide of various coverages shows that the activation of propane need a suitable site on the surface and the dynamic mode of the reduction depended on the thickness of the NiO films.The reaction might proceed in O defects which then produce NiO-Niy sites for a thick film.Correspondingly,for the submonolayer nickel oxide film the reaction might take place in the interface of the substrate,and the substrate Rh also plays an important role.In the second part,the CoO films were found to be grown in a layer-by-layer mode by the reaction deposition method on Rh(111)and characterized by XPS and LEISS.The multilayer is much more stable than the monolayer one upon oxidation and annealing.The reaction of propane with the CoO films of various coverages shows that the activation of propane might proceed in the interface of the substrate,and the substrate Rh plays an important role independent of the thickness of the films.In the third part,FeO1-x/Rh(111)and FeO2_x/Rh(111)model catalysts were established.The combined results from XPS,LEIS,LEED and AES show that the FeO1_X can be grown on Rh(111)up to 1ML,while the FeO2-x can be continuously deposited.The FeO1-x and FeO2-x can interchange to each other in the oxygen-rich condition at the temperature between 623K and 1023K.The results of catalytic performance test on the model catalysts for CO hydrogenation(CO:H2=1:2)indicate that the Fe modification can improve the formation of ethanol while suppress the methane formation.The maximum ethanol selectivity occurs at the coverage of 1ML.The promotion effect of Fe can be due to the formation of Fe-Rh surface alloy according to the IRAS and AES results.The enhancement role of Fe-Rh alloy is the modification and/or blocking of Rh sites responsible for methane and/or other side reactions.On the other hand,the improvement in ethylene selectivity can be ascribe to the formation of FexCy which is reported to be an active species for C2-C4 olefins.In the case of high coverge(＞1ML)of FeO2-x/Rh(111)model catalysts,the abundance of FexCy limited the formation of ethanol while improved the selectivity to ethylene.