Preparation Of Supported Palladium Catalysts By Liquid Phase Reduction And Their Catalytic Performance For CO Oxidation

The reaction of carbon monoxide oxidation has received considerable attention because of its gerat potential application in energy production and environmental protection.Much effort has been devoted to the development of catalysts for CO oxidation;however,the developed catalysts including supported noble metal catalysts(e.g.Au)and transion metal oxides have some drawbacks such as high sensitivity to preparation conditions,low reproducity,poor stability,and poor water-resistance etc.Therefore,the design and preparation of novel and effective catalysts with high dispersion,high activity and stability is still a challeng for its practical application.In this thesis,various supported Pd catalysts have been preprared by liquid-phase reduction and by using hydroxides such as Al(OH)3 and hydrotalcite as support,and their catalytic performances for CO oxidation are investigated.The catalysts are characterized by using X-ray diffraction(XRD),N2 physical adsorption,CO temperature-programmed reduction(CO-TPR),Fourier transform infrared spectroscopy(FT-IR),in situ diffuse reflectance infrared Fourier transform spectroscopy(insitu-DRIFTS),transmission electron microscopy(TEM),scanning electron microscopy and energy spectrometer(SEM-EDS),inductively coupled plasma(ICP),and hydrogen-oxygen titration(H2-O2 titration).The influence of preparation method,nature of hydroxide support,and Pd loading on the catalytic performance is studied,and the role of surface hydroxyl in the CO oxidation is also discussed.The obtained results are summarized as follows:(1)The Pd/Al(OH)3 catalyst prepared by coprecipitation-liquid phase reduction(CL)shows higher Pd dispersion than those prepared by traditional preparation methods including impregnation(IMP),coprecipitation(CP),and deposition precipitation(DP).The Pd metal dispersion decreased in the order of 1.5Pd/Al(OH)3-CL>1.5Pd/Al2O3-DP>1.5Pd/Al2O3-CP>1.5Pd/Al2O3-IMP.The Pd/Al(OH)3 catalyst shows higher reducibility,smaller Pd particle size(?3.0 nm),and high specific metal surface area(?21,.4 m2 g-1),which can provide more active sites,leading to higher activity.(2)To investigate the influence of support,Al(OH)3,Mg(OH)2,and Mg3Al(OH)8CO3·4H2O hydrotalcite(HT)supported Pd catalysts are prepared by the improved deposition precipitation-liquid phase reduction method(DL).It is found that Pd/MgAl-HT shows much higher activity than Pd/Al(OH)3 and Pd/Mg(OH)2.The IR spectroscopy suggests that the presence of large amounts of hydroxyl groups and the hydrogen bonding on the hydrotalcite may inhibit the CO adsorption on the active Pd metal,and reduce the CO coverage on Pd,so as to promote the O2 activation on the Pd surface and enhance the catalytic activity.The optimum Pd loading on Pd/MgAl-HT catalyst is 1 wt.%,which leads to smaller Pd particles(-2.4 nm),higher metal dispersion(-47.1%),and better activity.(3)To further investigate the influence of the composition of hydrotalcite,MgAl-HT,ZnAl-HT,and NiAl-HT supported Pd catalysts are prepared.It is found that Pd/NiAl-HT exhibits higher Pd dispersion and higher activity than Pd/MgAl-HT and Pd/ZnAl-HT.The Pd particle size is 1.4 nm for Pd/NiAl-HT,indicating that highly dispersed Pd nanoparticles catalyst can be obtained by the liquid phase reduction method with NiAl-HT as the catalyst support.The IR spectroscopy suggests that the CO adsorption and CO coverage are affected by the composition of hydrotalcite,probably due to the different properties of surface hydroxyl groups related to the hydrotalcite.Furthermore,it is found that the surface hydroxyl groups of hydrotalcite may be involved in the CO oxidation reaction process,in which hydroxyl groups react with CO to produce formate/bicarbonate intermediates,thus promiting the catalytic reaction.