2) Inst. Nacional de Tecnologia - Rio de Janeiro

The use of ethanol blended with gasoline lowers the amount of olefins, complex hydrocarbons and SO₂ on the exhaust systems of automobiles. However, it increases the amount of aldehydes and alcohol emitted to the atmosphere. These products may be harmful to the environment and it is necessary to use adequate catalysts.

The automotive three-way catalysts have in their basic formulations noble metals (rhodium, platinum or palladium) dispersed over wash coated g -Al₂O₃ and are used to control NO_x , CO and hydrocarbon emissions. However, the effect of oxigenated organic compounds on the catalytic activity of Pd-Mo catalysts for reduction of NO by CO has not been studied.

This work investigates the adsorption properties of ethanol and acetaldehyde through Temperature Programmed Desorption (TPD) and Oxidation (TPO) technique on 1%Pd/Al₂O₃, 8%Mo/Al₂O₃ and 1%Pd-8%Mo/Al₂O₃ catalysts.

TPD results of ethanol adsorption on alumina showed great formation of ethylene around 550 K. This high selectivity towards dehydration of ethanol may be attributed to the presence of strong acid sites. The addition of Pd changed the reaction pathway decreasing dehydration and the increasing dehydrogenation of ethanol, evidenced by a peak of acetaldehyde at 530 K. The presence of Pd also favored the decomposition of ethanol with formation of CO, CH₄ and H₂ at 495 K.

For the catalysts containing Mo, TPD results showed a further reduction in the formation of ethylene, which suggests that the molybdenum layer covered great part of the surface of the support. Another feature of the addition of Mo is the presence of two peaks of acetaldehyde around 473 and 520 K, which suggests that molybdenum oxide is active for the oxidative dehydrogenation of ethanol.

TPD results of acetaldehyde adsorption on alumina showed that part of it desorbed at low temperature (around 380 K) while part remained on the surface and decomposed at higher temperatures (between 773 and 823 K), as evidenced by the peaks of CH₄, CO and H₂ at 813 K. A peak of CO₂ was also seen at around 803 K. This may be due to the reaction of acetaldehyde with surface hydroxyls. This result is consistent with the explanation given for the desorption of CO, CO₂ and H₂, at high temperature during TPD of ethanol on the catalysts that were active for the dehydrogenation of ethanol. Since alumina alone was not active for this reaction, no acetaldehyde remained adsorbed and, consequentially, the formation of CO, CO₂, CH₄ and H₂ at higher temperature was not observed.

TPO of adsorbed ethanol on alumina was very similar to TPD results since alumina is no a good oxidation catalyst. This was also observed for the 8Mo/Al₂O₃ sample, at lower temperatures. For the Pd/Al₂O₃ catalyst, there was great formation of CO₂ in two peaks (376 and 627 K). The first peak is due to oxidation of ethanol and the second peak is probably due to the oxidation of an intermediate compound that remained adsorbed. The addition of Mo favored CO₂ formation at lower temperature.

The results of TPO of adsorbed acetaldehyde were very similar to the results obtained for TPO of adsorbed ethanol. Once again, the Pd/Al₂O₃ catalysts showed great formation of CO₂ in two peak and the addition of Mo increased the selectivity for CO₂ formation at lower temperature.