Study On Oxidation Performance Of Pd-V/Al₂O₃-TiO₂ Catalyst For Alcohols And Aldehyds

The study on synthesis alcohol from coal has caused the attention of the domestic research worker because of abundant coal resources in China. They have begun to explore that makes automobile fuel with the alcohol instead of petrol. The low carbon alcohol has already been very long history as the automobile fuel. With the increasing of the price of petroleum, our country pays close attention to the development of the alcohol automobile day by day. At present,ethanol-petrol automobile have been popularized in some areas. The elimination technology of alcohol fuel car pollutes is one of key problems so that pollution problem of the alcohol fuel car must be mentioned and come up on the schedule. The target of alcohol exhaust purification is mainly on low carbon alcohol and aldehyde. Because of the lower temperature of exhaust, the catalyst should have better performance for deep oxidation and low-temperature catalytic activation. The catalysts including noble metal such as Pd, Pt etc.are studied mostly at present.but there are much mid-products produced over these catalysts. So we add the second component and attempt to reduce midproduct and low the reaction temperature in the oxidizing performance for multi-pollutants. On the basis of earlier study in this subject,γ-Al₂O₃-TiO₂ composite are prepared by the co-sol-gel method and a series of palladium catalysts with transition metal are prepared by impregnation respectively. The effects of different carriers, second component, impregnation order, Pd content, V content, impregnation time, calcinations temperature, calcinations time and different react conditions on catalysis performance were investigated through the evaluation of catalysts for the ethanol. At the same time Support and catalysts were characterized by XRD, TEM, FT-IR, NH₃-TPD and BET. The conclusions are as follows:(1) The impregnation sequence and carrier had an obvious effect on the activity of the catalyst for ethanol oxidation. Among four series of catalysts with different impregnation sequence and carrier, the activity of Pd-M/A catalyst for ethanol oxidation is the best, which is with the carrier of Al₂O₃-TiO₂ (0.95) and impregnated first with Pd and then with second component. Pd-V /A catalyst has the highest activity;(2) The light-off temperature of catalysts with V is lowest ,in which the Pd-V/A catalyst is the minimum one.It demonstrates that the high activity at low temperature is favorable to cold start for vehicle;(3) The catalysts sample Pd-M(C)/A, which was prepared by co-impregnation with Pd and M, has lower conversion than the single component catalyst Pd/A, so the second component don't make the activity enhanced;(4) The contection curvses of the enthalpy -â–³H_f^o with T₅₀ and T₉₅ which can exhibt the activity of catalyst,take on the similer change tendency. Moreover the different catalysts had a " volcano" type of curve relationship when the enthalpy is lower than that of Cr₂O₃;(5) The bicomponents catalysts Pd-V/A with different Pd content have various activity for ethanol oxidation. Among the catalysts ,Pd-V/A catalyst exhibits the best activity with an optium loading of about 1% than others. And it has T₉₅=150℃and has narrow range acetaldehyde yield (T_s-T_e);(6) The content of V have influence for the bicomponents catalysts Pd-V/A on ethanol and acetaldehyde oxidation. When the content is lower than 0.4%,the conversion is enhancing with the increasing V content. While the content is higher than 0.4%, the condition of conversion is verse. Moreover we can conclude that the bicomponents catalysts Pd-V/A with 0.4% V content has the best low temperature activity and good deep oxidation.That is charactered by XRD;(7) The impregnation time had great effect on the activity of the catalyst for ethanol oxidation.When the impregnation time of V is 12 hours ,the sequence of activity is Pd-V(24,12)> Pd-V(12,12) >Pd-V(6,12)> Pd-V(2,12).While the impregnation time of Pd is 24 hours ,the sequence of activity is Pd-V(24,12)> Pd-V(24,6) >Pd-V(24,2)> Pd-V(24,24).The activity has relation with the ICP-AES result that the actual loading is effected by the impregnation time.Among these catalyst with different impregnation time, Pd-V(24,12) catalyst has the highest conversion and the lowest acetaldehyde yield;(8) The influence of calcination condition including time and temperature was investigated.The results showed that when the activity of catalyst calcinated 5 hours in 50℃is the best. When the temperature is too low, the presuor is not completely decomposed, while the calcined temperature is too high that the Pd particle will agglomerate.The catalysts in the same temperature 500℃but different calcination time differ in catavity for ethanol oxidation. The catalyst with the calcination time 5h has T₃₀=80℃,while the catalysts with the calcination time 3h or 7h have T₃₀=140℃.Thereas the acetaldehyde yield is less affected by calcination time;(9) The bicomponents catalysts Pd-V/A with Pd content 1% and V content 0.4% has good low temperature oxidation for methanol, ethanol and acetaldehyde. The conversion of methanol and ethanol on Pd-V/A catalyst is 40% at 100℃and higher than 90% at 110℃.Methanol, ethanol and acetaldehyde are all conversed at 150℃simultaneously;(10) The second component V make the activity of Pd catalyst improve because of the interaction of V and Pd. Pd-V/A catalyst demonstrates the better low-temperature activity than single Pd catalyst Pd/A.We determined that The second component V is additive element; (11) At low temperature,space velocity has a great influence on the performance of Pd-V/A catalyst because of outside spread,as 24000h^-1 the conversion is higher than 12000 h^-1 or 48000 h^-1.The performance in different space velocity are similar as higher than 150℃.At 150℃the conversion is all about 90% and the acetaldehyde yield is near zero. So we conclude that space velocity has a less effect on the activity of Pd-V/A catalyst;(12) Different particle size demonstrates different activity for ethanol oxidation. Catalyst with smaller particle can descrease the inside spread and help to improve the surface utilization ratio ,so it improve the activity. However too tiny particle make space rate of bed layer reduce and obstruction coefficient increase,and the lowered pressure increase,so that the activity of the catalyst decline. In conclusion,the particle size 40 mesh～60 mesh is relatively suitable;(13) Catalyst Pd-V/A has good cold start and higher complete conversion performance for ethanol oxidation. when the concentration of ethanol is higher than 1500ppm,the conversion tends towards stability at 55% and acetaldehyde yield is only 10%;(14) With the increase of oxygen content,the conversion of the catalyst Pd-V/A increase firstly and than decrease. It is concluded that the increasing of oxygen partial pressure helps to improve the activity of the catalyst as lower 8% and when oxygen content exceeds 8% its increasing is unfavorable to the complete transformation for the ethanol because of the phenomenon of oxygen inhibition;(15) Pd-V/A catalyst has high stability. After 100 hours reaction, the activity is also high.