The Study On Catalytic Performances Of Pd Based Bi-component Catalysts For Low-temperature CO Oxidation Reaction

The noble metal catalysts?Pd,Pt,etc.?are the commonly used catalysts in low temperature CO oxidation.The loading of noble metals is high,and the activity and stability of the catalysts need to be improved.In this paper,the preparation method and active center of noble metal catalysts with high activity,high stability and low loading were studied from the perspectives of the structure-activity relationship and the CO oxidation mechanism.The work is as follows:1.Pd-Fe catalysts for low temperature CO oxidation reaction were prepared via the co-precipitation technique.The effect of calcination temperature,support,metal loading on the catalytic performances of CO oxidation was investigated in detail.It is suggested that Pd-Fe supported on carbon black shows the higher activity with CO complete conversion at nearly room temperature when calcined at 200 ^oC and 300 ^oC.Detailed characterizations were carried out with transmission electron microscopy,X-ray photoelectron spectroscopy,X-ray powder diffraction and CO chemisorption.Results reveal that the high activity could be attributed to the strong interaction between palladium and iron,which causes electron transfer from Pd to the interfacial Fe³⁺?Fe₂O₃?to form the reduced Fe^x+sites.PdO and reduced Fe^x+sites present as the active sites for CO and O₂ adsorption respectively,which are the main reasons for the superior performance of CO oxidation.2.Pd-?-Fe₂O₃ and Pd-?-Fe₂O₃ catalysts were acquired by co-precipitation technique,treating Pd-Fe catalysts with redox pretreatment.These catalysts were characterized by various techniques and applied in the CO oxidation reaction.Characterization reveals that the Pd-?-Fe₂O₃ catalyst is obtained after calcination in air at 400 ^oC.The following reduction at lower temperature forms the Pd-?-Fe₂O₃ catalyst.In the case of larger Pd?or PdO?and Fe₂O₃ nanoparticles,the Pd-?-Fe₂O₃ catalyst is highly active for low temperature CO oxidation compared with the Pd-?-Fe₂O₃ catalyst,which may originate from the high oxygen storage properties of?-Fe₂O₃ and stronger interaction between Pd and?-Fe₂O₃.Higher reduction temperature results in much larger particle sizes and decreased activity.Stability tests also indicate that the highly active Pd-?-Fe₂O₃ catalyst could transform to Pd-?-Fe₂O₃ in reactive atmosphere,leading to catalyst deactivation.Re-reduction treatment of the inactivated catalyst results in reproduction of the activity.3.Pd/SnO_x and Pd/VO_x catalysts were synthesized by fractional precipitation method and co-impregnation method respectively.The effect of pretreatment atmosphere and temperature on the catalytic performances of CO oxidation was investigated in detail.These catalysts were characterized by BET,XRD,HRTEM and H₂-TPR technologies.The results show that all the catalysts contain mesoporous structures,and the precious metal Pd is highly dispersed on the carrier surface.After 600 ^oC calcination,Pd/SnO_x can form Pd/SnO₂.The following reduction at lower temperature leads the activity to be improved,which may be caused by the increased specific surface area of the reduced catalyst.Pd/VO_x can form Pd/V₂O₅ after 300 ^oC calcination.The results of H₂-TPR show that there is strong interaction between Pd species and V₂O₅,which may be the reason for its high activity.After reduction of Pd/V₂O₅ at lower temperature,the activity decreases,possibly due to partial reduction of V₂O₅ and the change of carrier structure.4.Leached Pt-Fe and Pt-Co catalysts were prepared by acid leaching the reduced catalysts in acid solution.Oxidation treatments of leached catalysts produced the structure of metal oxides decorating the surface of nanoparticles.The fully oxidized Fe₂O₃ and Co₃O₄ species on Pt nanoparticle surfaces result in the low performance of the CO complete oxidation?COOX?reaction.In contrast,unsaturated FeO and CoO surface species can be formed during exposure to the CO preferential oxidation?PROX?reaction with an excess of H₂,leading to a high O₂ activation ability and enhancing the PROX activity.The Fe O_x surface structures can be transformed between these two states by varying reactive gas environments,exhibiting oscillating activity in these two reactions.Conversely,the CoO surface structure formed in the H₂-rich atmosphere is stable when exposed to the COOX reaction and exhibits similar activity in these two reactions.