Supported Pt/Cr_xFe_2-xO₃ Catalysts For CO Oxidation And Their Influence To CO₂ And H₂O

In recent decades,catalytic CO oxidation has attracted much attention in catalysis.On one hand,CO oxidation is an ideal model reaction to investigate some important scientific questions such as structural sensitivity and active sites;on the other hand,the reaction is widely applied in practical processes such as automotive exhaust gas emission reduction and proton exchange membrane fuel cells?PEMFC?.Most employed noble metal catalysts are very active and stable in the presence of CO+O₂ and a small amount of H₂O,but they suffer severe deactivation under actual reaction conditions?in the presence of large amounts of CO₂ and H₂O?.Therefore,it is important to develop catalytic systems that are highly resistant to H₂O and CO₂.In this work,we reported CO oxidation over a series of supported Pt/Cr_xFe_2-xO₃ catalysts and their performance in the presence of high-concentration H₂O and CO₂.The catalysts were characterized in detail by various techniques such as X-ray diffraction?XRD?,transmission electronic microgragh?TEM?,X-ray photoelectron spectroscopy?XPS?,hydrogen temperature-programmed reduction?H₂-TPR?,oxygen temperature-programmed desorption,CO temperature-programmed desorption?CO-TPD?.Combined with their catalytic behaviors and kinetics,the influences of H₂O and CO₂ on the reaction was discussed and a reasonable structure-activity relationship was established.These findings provide important experimental evidence for an in-depth understanding of the reaction behavior of this type of catalyst system in the presence of H₂O and CO₂,and provide an important reference for the development of high-efficiency catalysts.The main contents of this thesis are as follows:1.CO oxidation over Pt/Cr_xFe_2-x-x O₃ catalysts and their tolerance to H₂O and CO₂.A series of Cr_xFe_2-xO₃ compos(Pt/Cr_1.6Fe_0.4O₃,Pt/Cr_1.3Fe_0.7O₃,Pt/Cr_1.0Fe_1.0O₃,Pt/Cr_0.4Fe_1.6O₃ and Pt/Fe₂O₃,with Pt contents of 2 wt.%)were prepared by an incipient wetness impregnation method.Their catalytic behaviors under different reaction conditions were thoroughly investigated.The main findings are as follows:1,The catalytic activity under CO+O₂ condition is closely related to the reducibility of catalyst.Among the catalysts,the Pt/Cr_1.3Fe_0.7O₃ catalyst has the lowest H₂ initial reduction temperature and the highest active oxygen content?maximum hydrogen consumption?due to its corundum structure of the support and its interaction with Pt species,and therefore the best activity;2,In the presence of 10%CO₂ and 10%H₂O in the reaction atmosphere,the catalysts with Fe-containing supports show different degrees of promoting effect.The promotion is the most prominent on the Pt/Cr_1.3Fe_0.7O₃ catalyst,which shows promising potential in practical application;3,The effects of CO₂ and H₂O on the reaction were investigated on the representative Pt/Cr_1.3Fe_0.7O₃ catalyst,based on detailed characterizations and kinetic tests.Staged reaction reveals that the addition of CO₂ in the reactants results in the formation of surface carbonate species which blocks the active sites in the catalyst and thus deactivates the catalysts.In addition,kinetic results indicates that the addition of CO₂causes an increasing reaction order of CO and a decreasing CO adsorption equilibrium constant,which suggests competitive adsorption of CO and CO₂ on the catalyst surface and accounts for the inhibiting role of CO₂ in the reaction;4,The promoting role of H₂O in the reaction was investigated by staged reaction,which clearly show that the addition of H₂O could effectively decompose the surface carbonates.Meanwhile,the presence of H₂O in the reactants could maintain the metallic Pt species in the catalyst even in the oxidizing environment,as revealed by the XPS results.Also,the kinetic results indicate that the addition of H₂O leads to an increasing reaction order of CO and a decreasing CO adsorption equilibrium constant.However,the CO-TPD results indicate that the addition of H₂O could greatly promote the formation of CO₂,which is probably resulted from the reaction between CO and hydroxyl group formed by H₂O decomposition as evidenced by the XPS results.Therefore,the addition of H₂O may open up a new reaction path,which is responsible for the enhanced activity.2.CO oxidation over Pt/Cr_1.3Fe_0.7O₃ catalysts with different Pt contents and their tolerance to H₂O and CO₂.We further investigated the CO oxidation over Pt/Cr_1.3Fe_0.7O₃ catalysts with different Pt contents.The results show all the catalysts show good tolerance to H₂O and CO₂,and it is interesting that the promoting effect of H₂O is more prominent on the catalyst with lower Pt content.For example,the presence of 10%H₂O in the reactants reduces the T50 temperature?the temperature at which the CO conversion is50%?by 70 ^oC?from 120 ^oC to 50 ^oC?on 0.2Pt/Cr_1.3Fe_0.7O₃ catalyst.Furthermore,the catalyst remains active in the presence of 10%H₂O+10%CO₂?comparable to 10%H₂O alone?.The kinetic results show that the reaction order of CO was 0.52 under the condition of CO+O₂ reaction,indicating that the adsorption of CO on Pt atoms was weak;the addition of water vapor and CO₂ increased the CO reaction order to0.80-0.90,implying that the addition of H₂O and CO₂ forms a competitive adsorption with CO to reduce its surface coverage.The reaction order of O₂ keeps at zero regardless of the oxygen partial pressure,indicating that the lattice oxygen in the support may directly participate in the reaction.The addition of H₂O reduced the apparent activation energy of the reaction from 39.2±3.4 kJ mol^-11 under CO+O₂ to16.4±1.4 kJ mol^-1,implying that its addition may trigger a new reaction pathway.