Study On The Supports And Catalytic Performance Of Au-Pd Bimetallic Catalysts For Hydrogen Production From Methanol

Partial oxidation of methanol has been suggested as a suitable route for hydrogen extraction from methanol.The supported palladium based catalyst is reported to be active for partial oxidation of methanol to produce hydrogen,but also produce considerable amount of CO as a by-product.CO is a poison to fuel cell anode.If the level of CO exceeds a few ppm,the precious metal-based anode electrocatalyst of the fuel cell will be deactivated.It is reported that the supported Au catalysts exhibited high activity for CO oxidation in the low-temperature.The supports play an important role in the catalytic performance of supported catalysts.It could enhance the metal dispersion,decrease the cost of the catalysts,act as a chemical promoter,structural promoter and as a cocatalyst.Therefore,further development of new efficient catalyst systems that exhibit an improved long-term stability and selectivity towards hydrogen production are highly desired.In the present work,we have investigated the catalytic performances of the different metal oxides and composite metal oxides(such as ZnO,CeO₂,TiO₂,Al₂O₃, Fe₂O₃,Cr₂O₃,ZnO-Fe₂O₃ and ZnO-CeO₂) suppored Au-Pd bimetallic catalysts for partial oxidation of methanol.The effects of the sorts of metal oxides MO_x(Zn,Ce, Ti,Al,Fe and Cr),second constituent MO_x(M=Ce and Fe),different molar ratio of Zn/Ce and calcination temperature on the performance of Au-Pd catalysts for partial oxidation of methanol(POM) are studied.We have slso successfully compared the catalytic performance between the Au-Pd bimetallic catalysts and the Au or Pd monometallic catalysts.The prepared samples were characterized by XRD,ICP, nitrogen adsorption-desorption,H₂-TPR,CH₃OH(H₂,CO₂)-TPD and TG analyses.1.Hydrogen production by POM was investigated over Au-Pd/MO_x(M=Zn,Ce, Fe,Ti,Cr and Al) bimetallic catalysts,prepared by PVP-stabilized reduction method by C₂H₅OH.The results indicated that the supports play an important role in the catalytic performance of catalysts.Compared with titanium,chromium and aluminum oxides,the alkalescent supports,such as zinc,cerium and iron oxides supported Au-Pd bimetallic catalysts obtained the methanol conversion above 80%at 448 K, and the amount of carbon deposited can be decreased on alkalescent supports.The Au-Pd/ZnO catalyst showed the best catalytic performance of all the catalysts.A high methanol conversion of 99.0%is attained at 523 K with 45.6%hydrogen selectivity for Au-Pd/ZnO catalyst,and the amount of carbon deposited was only 0.0232 g/g cat after reacting 20 h.2.The comparison of the activities and selectivity for partial oxidation of methanol over Au/ZnO,Pd/ZnO and Au-Pd/ZnO were evaluated.The results indicated,that Au-Pd/ZnO catalyst shown higher methanol conversion and hydrogen selectivity,lower CO selectivity and well stability for partial oxidation of methanol reaction compared with the Au/ZnO and Pd/ZnO catalysts.For Au-Pd/ZnO catalyst,it had the synergetic effect of gold and palladium species,higher dispersion and smaller particles of gold,all these were advantageous to the catalyst active and hydrogen selectivity enhancement.3.The effects of MO_x(M=Ce,Fe) on the catalytic performance of Au-Pd/ZnO catalyst for partial oxidation of methanol were studied.The results showed that the addition of CeO₂ or Fe₂O₃ could improve the activity and hydrogen selectivity of the Au-Pd/ZnO catalyst for partial oxidation of methanol.The Au-Pd/ZnO-CeO₂ catalyst was more active and exhibit higher hydrogen selectivity compared to other catalysts. The enhanced activity of Au-Pd/ZnO-CeO₂ catalyst was due to the addition of CeO₂ could improve the dispersion of the Au₂O₃,PdO and Au_xPd_yO on the support and increase the adsorption of Au-Pd/ZnO catalyst for methanol,in addition,reduce the desorption of the catalyst for hydrogen.4.The catalytic performances of the Au-Pd/ZnO-CeO₂ catalysts with different molar ratio of Zn/Ce for partial oxidation of methanol were studied.The results showed that the addition of ceria could improve the activity,hydrogen selectivity and stability of Au-Pd/ZnO catalyst for partial oxidation of methanol.When the molar ratio of Zn/Ce was 7:3,the activity and hydrogen selectivity of Au-Pd/ZnO-CeO₂ catalyst was the best,the methanol conversion of 99%and hydrogen selectivity of 64%were obtained at 598 K.The methanol conversion remained above 95%after reacting 20 h.It was due to the surface area of the supports was the largest,the interaction of the active species and the support was the strongest,the alkali sites and absorption of methanol of the catalyst were the most.5.The influences of the calcination temperature on the performance of Au-Pd bimetallic catalysts for partial oxidation of methanol reaction were studied.The results showed that the Au-Pd/ZnO-CeO₂ catalyst calcined at 773K showed higher methanol conversion and hydrogen selectivity and well stability for partial oxidation of methanol reaction.The methonal convesion of 99.9%and hydrogen selectivity of 67.3%is observed at 598 K.For Au-Pd/ZnO-CeO₂(773K) catalyst,the enhanced activity and stability is attributed in terms of the stronger interaction between active metal species and support,higher dispersion and smaller particles of active metal species,and increased the absorption of Au-Pd/ZnO-CeO₂ catalyst for methanol.