Study Of The Structure Of Novel Pd-Ce/γ-Al₂O₃-TiO₂Catalysts And Its Catalytic Performance For Total Oxidation Of Ethanol And Acetaldehyde

The content of CHx and CO in emission of ethanol fuel cars is much lower than traditonal gasoline vehicles, so ethanol fuel is known as "clean fuel". However, with the raising of auto emissions control, people have realized that unburned ethanol and its oxidation intermediates, such as acetaldehyde, ether, ethyl acetate and carbon monoxide, etc. from alcohol fueled car exhaust emission are harmful to the environment and human health. As a purifying catalyst of ethanol fuel automotive exhaust gases, the catalyst must have the ability to convert all the by-products completely to CO2and H2O at low temperature. So it is necessary to develop a low temperature and high efficiency catalyst.In our previous work, Pd-V/y-Al2O3-TiO2has been synthesis, and the ethanol conversion reach to100%at200℃over the catalyst. However, the selectivity of CO2and yield of intermediate by-products which were generated by oxidation of ethanol or acetaldehyde, were not clearly studied yet. In this paper, rare earth element Ce was added in1%Pd/y-Al203-Ti02and1%Pd-(1-3%)V/y-Al2O3-TiO2catalysts, the novel catalyst aimed at converting ethanol (or acetaldehyde) completely to CO2at less than200℃was studied. It was also investigated that whether the adsorbed oxygen or the lattice oxygen play an important role in the total oxidation of ethanol. Finally the relationship of catalytic activity, selectivity and the structure of the catalyst were characterized by TEM, XRD, O2-TPD and H2-TPR. The conclusions are as follows:(1) When1%Ce was added in Pd-V/y-Al2O3-TiO2catalysts, the catalytic activity to ethanol conversion was not improved significantly. Whereas catalytic activity was significantly higher than that of1%Pd-3%V/y-Al203-Ti02catalyst when1wt.%Ce added into1%Pd/y-Al203-Ti02catalysts, no matter prepared in co-impregnation method or step wise-impregnation method.(2) The catalytic activity of Pd-Ce/y-Al2O3-TiO2prepared by stepwise-impregnation method was obviously higher than that prepared by co-impregnation, the conversion of ethanol was100%at175℃(3) Cerium content has an obvious effect on catalytic performance for ethanol oxidation over Pd-Ce/y-Al2O3-TiO2prepared by stepwise-impregnation method. The sample with1%Ce content has the best activity for ethanol oxidation. The possible reason is that the introduction of appropriate amount of Ce produce strong interaction between the carrier and the metal atom, even prevent the sintering of active component. Thus its ability to oxide ethanol completely is improved.(4) The selectivity of CO2reached to about87%at175℃for ethanol oxidation over Pd-Ce/γ-Al2O3-TiO2catalyst, comparatively the CO2selectivity was only17.3%on Pd/γ-Al2O3-TiO2catalyst. And the amount of by-products (acetaldehyde, ethyl acetate and methane) was significantly reduced. This indicated that the catalyst loaded by1%Ce could achieve the deep oxidation of ethanol at low temperature.(5) Besides the high selectivity of CO2and less byproduct, CO was not detected when ethanol was oxidized over Pd-Ce/γ-Al2O3-TiO2catalyst. The results indicate that in the presence of O2, the ethanol is oxidized by1%Pd-1%Ce/γ-Al2O3-TiO2, In this process, the ethoxy is produced in dehydrogenation reaction at first, some of which can be decomposed into CH4, H2and CO, while others are conversed to acetaldehyde. Then the produced CO is oxidized to CO2. There are two possible ways to form ethyl acetate. The one is the esterification between the adsorbed ethanol and acetic acid which is produced in the oxidation reaction of the adsorbed acetaldehyde. The other is the direct reaction between the condensation of acetaldehyde and the remaining ethanol.(6) Temperature of acetaldehyde oxidation reaction was higher than the ethanol over1%Pd-1%Ce/γ-Al2O3-TiO2catalyst, because the acyl in acetaldehyde was more difficult to break than hydroxy in ethanol at the low temperature. Acetaldehyde conversion and selectivity of CO2were the best on the sample.(7) The XRD results indicate that γ-Al2O3-TiO2compound supporter is composed of anatase structure. A small amount of Ce (≦1%) is dispersed on the supporter in the micro or amorphous state, which is propitious to generate a bi-metallic synergy with Pd. When the content of Ce is higher, Ce with the cubic fluorite structure is deposited on the surface of the supporter and CeOCl structure form easily, so the bond between Pd and Ce is weaker, the catalytic activity is lower sequentially.(8) The O2-TPD results show that the catalyst modified by V and Ce play a catalytic role to ethanol or acetaldehyde oxidation activity. On1%Pd-1%Ce/γ-Al2O3-TiO2catalyst, proper Pd2+particle may enter in the lattice of CeO2due to form lattice defect and provide more oxygen vacancies, which improves the deep oxidation activity of catalyst.(9) The H2-TPR results demonstrate that the addition of V or Ce in single palladium catalyst improve the conversion of ethanol and acetaldehyde, which is mainly because the content of Pd2+on catalyst surface increase. The addition of Ce can make PdO has better dispersity. Furthermore, a new interaction among palladium, cerium and carrier is formed. So1%Pd-1%Ce/γ-Al2O3-TiO2possesses the best activity to ethanol and acetaldehyde.(10) The results of the experiment of adsorbed oxygen and lattice oxygen show that adsorbed oxygen plays an important role for ethanol complete oxidation at low temperature over Pd-M(M=V, Ce)/Al2O3-TiO2series of catalysts. Reaction mechanism of oxidation of ethanol in the absence of O2is totally different from the presence of O2.(11) When oxidized for10h at600℃over1%Pd-1%Ce/y-Al203-TiO2catalyst, ethanol or acetaldehyde conversion was still100%, selectivity of CO2was maintained at about87%, methane was not detected when the temperature was higher than400℃. Results showed that the catalyst had good thermal stability. Thus this study laid the foundation for the industrialization of Pd-Ce/y-Al2O3-TiO2.