Modifications Of Pd/Al₂O₃ Catalyst Support And Their Effects On The Performance Of Ethanol Oxidation

Volatile organic compounds（VOCs）are recognized as one of the major contributors to air pollutions and harmful to the environment and human health.Catalytic oxidation is the most effective technique for the elimination of VOCs and the core task is the choice of catalysts.The catalysts used for catalytic oxidation are classed into two types:supported noble metal catalysts and non-noble metal oxide catalysts.Furthermore,the noble metal catalysts are proved the more effective ones.For the sake of stability and economy,Pd is often adopted as the active components.For supported catalysts,the intrinsic properties of support would profoundly affect the catalytic performance.For Pd and Pt catalysts,the traditional single oxide supports of Al₂O₃,ZrO₂,CeO₂,SiO₂ and TiO₂ were limited for applications because of some defects which existed in themselves.The composite oxides attracted widespread attention for their advantages to utilize properties associated with multiple materials so as to overcome the disadvantages observed while using a single oxide as support.With the increasing energy crisis,ethanol fuel has been used as an additive or substitute for gasoline.However,the unburned ethanol and released acetaldehyde in the exhaust gases were still harmful.The previous work investigated the effects of support composition on the performance of ethanol oxidation for Pd/Al₂O₃-TiO₂ catalysts and found that the two catalysts with different compositions showed the higher ethanol conversion.The above description was called the double peaks which stood that the two supports with different composition would possess the similar catalytic activity.It was more important when the cost or the preparation complexity of compositions was in large difference,because the similar result was achieved by the lower cost or the easier way.The concept of double peaks was only proposed without the detailed discussion and reasons in the previous work.Therefore,the Pd/Al₂O₃ was chosen as the basic catalyst and the Pd/Al₂O₃-TiO₂ catalyst was investigated,meanwhile,the Pd/Al₂O₃-CuO,Pd/Al₂O₃-CeO₂ and Pd/Al₂O₃-NiO catalysts were prepared by different means.The specific effects of support composition for these catalytic systems were investigated.Based on the above work,we got an initial understanding about the ethanol oxidation reaction and the research contents were displayed as following:1.Pd/Al₂O₃-TiO₂ catalysts with different TiO₂ contents were prepared by the sol-gel method and were tested in the ethanol oxidation reaction.Double peaks and symmetric path phenomena were observed at certain temperatures with the increase in TiO₂ contents.The higher contents of low-temperature acid and the larger difference between Pd⁰ and Pd²⁺were the origins of the double peaks for Pd/AlTi20（Pd/Al₂O₃-TiO₂（20wt%））and Pd/AlTi80 catalysts.The symmetric path phenomena illustrated the diverse activity fluctuations,which were ascribed to the different factors controlling the course of the catalytic paths.The large surface area and high adsorbed oxygen contents were conducive to the catalytic performance,while the larger acid contents were not beneficial to ethanol oxidation.With the increase in TiO₂,the surface area,adsorbed oxygen contents and the amount of surface acid decreased gradually.At 150 and 175℃,Pd/AT（X）（X<50）catalysts performed better in ethanol oxidation because of the larger surface area and higher adsorbed oxygen contents.Owing to the larger acid amount,Pd/AT（X）（X<50）catalysts showed lower performance at 200 and225℃.2.The Pd/Al₂O₃-CuO catalysts which possessed different CuO contents were prepared by diverse methods.The activity test results showed that the Pd/AlCu50（Pd/Al₂O₃-CuO（50wt%））catalyst performed best.For Pd/AlCu50sample,the more PdO and less PdCl₂ were conducive to the oxidation reaction.The larger quantity of Al³⁺could provide more adsorption sites for reaction and make more gaseous oxygen transform into active oxygen species on the catalyst surface.The formation of Pd-Cu alloy decreased the reduction temperatures of Pd and Cu oxides and enhanced the interaction between Pd and Cu species.Furthermore,the large content of high crystalline degree lattice plane（-111）might be beneficial for the higher reactivity.3.The small content of CuO was loaded by impregnation method and the Pd/Al₂O₃-1.0%CuO catalyst showed the best performance.The T₅₀ and T₉₀decreased at least 50℃in comparison with Pd/Al₂O₃,meanwhile,the activity of Pd/Al₂O₃-1.0%CuO catalyst was also superior to Pd/AlCu50 sample.When compared to Pd/Al₂O₃ catalyst,the enhancement of diffraction peak intensity and the disappearance of decomposition of palladium hydride for Pd/Al₂O₃-CuO catalysts stated that the Pd-Cu alloy structure was formed and it was conducive to the synergistic effect between Pd and Cu species.For the Pd/Al₂O₃-1.0%CuO catalyst,the shift to lower temperatures of reduction peak and the increased area demonstrated the easier reduction and larger quantity of oxidizing substances which were beneficial to the oxidation reaction.The new reduction peak illustrated that the interaction of Pd and Cu produced some new species.The higher content of low temperature acid was conducive to the enhancement of reactivity and the new NH₃ desorption peak stated that the new acidic sites were formed.4.The series of Pd/AlCe（Pd/Al₂O₃-CeO₂）and Pd/AlNi（Pd/Al₂O₃-NiO）catalysts which possessed different CeO₂ and NiO contents were prepared by different ways and used for ethanol oxidation reaction.For Pd/AlCe catalysts,the reactivity enhanced with the increased CeO₂ contents,while the activity of Pd/AlNi catalysts did not show obvious change rule.Although there was difference in the by-products yield of Pd/AlCe and Pd/AlNi catalysts,the change trend and the peak temperatures were almost the same.The above results illustrated that this phenomenon might belong to the characteristics of ethanol oxidation reaction or Pd catalyst and was less affected by metal oxides.