CO Oxidation Over Pt Catalysts And Their Resistance To Water

Catalytic oxidation of CO is a very important reaction in catalysis.It has been conducted as a probe reaction to investigate some fundamental issues in catalysis science such as structure-performance relation,active sites.On the other hand,this reaction is of realisitic importance is applications such as proton exchange membrane fuel cell（PEMFC）and abatement of mobile exhaust.Pt-based catalysts are active in CO oxidation and their behaviors are closely related to the nature of the support.Thus,the invesitigation on the roles of Pt-support interaction on the properties of the Pt species and the reaction behaviors of the catalysts is beneficial for the development of highly efficient catalysts for CO oxidation,particularly under practical reaction conditions.In this thesis,we prepared Pt catalysts supported on spinel and mixed oxides,and investigated their catalytic performance for CO oxidation and their resistance to water.Various characterizations（XRD,XPS,H₂-TPR,CO-TPD,O₂-TPD and CO-DRIFTS,etc.）were used to analyze the properties of the catalysts and explore the reasons for the change of catalytic activity.The effects of H₂O in the reaction atmosphere on the kinetic behavior of the catalyst were investigated.These findings provide some new information for the design of efficient catalysts for CO oxidation,especially under practical conditions.The main contents of this thesis are as follows:1.Low temperature oxidation of CO over Pt/MFe₂O₄ catalysts and their tolerance to water.A series of catalysts（Pt/CoFe₂O₄,Pt/NiFe₂O₄,Pt/Fe₂O₃,Pt/Co₃O₄ and Pt/Ni O）were prepared and tested for CO oxidation under different conditions.It was found that compared with the Pt/Fe₂O₃,Pt/Co₃O₄ and Pt/Ni O,the Pt/CoFe₂O₄ and Pt/NiFe₂O₄catalysts have better low-temperature CO oxidation activities,which could be maintained even with the presence of 10%H₂O in the feed gas,and thus have good water resistance.Various characterizations show that the CoFe₂O₄ and NiFe₂O₄supports are a mixture of normal spinel and inverse spinel structure,which could improve the caability of oxygen activation.Also,the Pt species in the Pt/CoFe₂O₄catalyst have higher dispersion compared with those in the Pt/Fe₂O₃ catalyst,resulting more adsorbed CO on the catalyst surface.Additionally,CO on the Pt/CoFe₂O₄ catalyst could more easily desorb due to the Pt-support interaction.The two facts（facile oxygen activation and CO desorption）synergistically account for the enhanced activity of the Pt/CoFe₂O₄ catalyst.The kinetic results suggest that the reaction may follow a typical Mars-van Krevelen（M-K）mechanism.The addition of H₂O in the feed gas results in decline of the apparent rate constant and reaction order of CO,indicating competitive adsorption of CO and H₂O and the the lowered CO surface coverage.2.CO oxidation and water resistance of Pt/Ce_1-xFe_xO₂ catalyst.A series of Pt catalysts with Pt cotents of 0.1 wt.%supportd on Ce_1-xFe_xO₂ oxides（x=0,0.1,0.3,0.5,0.7,1）were prepared by a sol-gel method.The catalytic activities of these catalysts for CO oxidation and their resistance to H₂O were investigated.The Pt/Ce O₂ has the highest activity under the condition of 1%CO+1%O₂,and the catalytic activity gradually decreases with increasing Fe doping;under the condition of1%CO+1%O₂+10%H₂O,the Pt/Ce_0.3Fe_0.7O₂ has the highest activity,but the catalytic activity of Pt/Ce O₂ is suppressed.For the Pt/Ce_1-xFe_xO₂（x≥0.1）catalysts,the H₂O addition could greatly promote the activities,and the promotion of water is more pronounced with increasing Fe doping.XPS results show that the interaction between Pt and Fe doped support is weakened,which makes Pt⁰ easier to be oxidized to Pt²⁺,resulted in the decrease of catalytic activity.For the Pt/Ce O₂ catalyst,the apparent rate constant(K_app)and reaction order of CO decrease with the addition of H₂O,which indicated that H₂O and CO may compete for adsorption on the catalyst surface and inhibit its activity.The increase of Fe content results in the formation of Fe₂O₃,which leads to H₂O dissociation into hydroxyl groups on the Fe₂O₃surface and react with CO,thus reducing the activation energy and promoting the reaction activity.