Understanding the interaction between platinum and ceria in platinum/cerium oxide/aluminum oxide-catalyzed oxidations

• Main Author: Oliviero, Andrew
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1996
• Subjects: Chemical engineering|Automotive materials
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9639014

CO, propylene, and propane are common components in automotive exhaust. These pollutants are converted to CO$\sb2$ and H$\sb2$O by three-way catalysts contained in the catalytic converter of your automobile. These catalysts (Pt, Rh, CeO$\sb2$) are very effective in converting the pollutants under varying exhaust conditions; however, the method by which these catalysts achieve their high activity is not fully understood. The activities and kinetics for the oxidation of carbon monoxide, propylene, and propane oxidations were compared over Pt/Al$\rm\sb2O\sb3$ and Pt/CeO$\rm\sb2/Al\sb2O\sb3$ catalysts in an attempt to correlate catalyst activity with the chemistry of the reactants. Additional experiments involving CO, H$\sb2$ and O$\sb2$ chemisorption, the extent of reduction and oxidation of CeO$\sb2$ and CeO$\rm\sb2/Al\sb2O\sb3$, spillover studies, and the activity of Pt/Nd$\rm\sb2O\sb3/Al\sb2$O catalysts were performed in order to correlate catalyst activity with the strength of adsorption of the reactants and the mobility of lattice oxygen in CeO$\sb2$. Experiments were also performed for methanol oxidation over Pt/Al$\rm\sb2O\sb3$, Pt/CeO$\rm\sb2/Al\sb2O\sb3$, and Pt/K$\rm\sb2O/Al\sb2O\sb3$ catalysts in order to further understand the effects of ceria on the complete oxidation of alternate fuels. The Pt/CeO$\rm\sb2/Al\sb2O\sb3$ catalyst was more active than the PT/Al$\rm\sb2O\sb3$ catalyst for CO and propylene oxidation but less active for propane oxidation. It appears that an Eley-Rideal mechanism whereby gaseous oxygen reacts with CO adsorbed on Pt is consistent with the kinetic results for CO oxidation on Pt/Al$\rm\sb2O\sb3$. Based on the kinetic results and studies reported in the literature, it was suggested that CO and propylene adsorbed on Pt reacted with adsorbed oxygen species (O$\sb2\sp-$ and O$\sp-$) at low temperatures ($<$170$\sp\circ$C) and lattice oxide ions (O$\sp{-2}$) at higher temperatures ($>$200$\sp\circ$C) over Pt/CeO$\rm\sb2/Al\sb2O\sb3$ catalysts. The adsorption experiments showed that CO adsorbed only on the Pt sites, whereas O$\sb2$ adsorption was enhanced on the ceria phase of a Pt/CeO$\rm\sb2/Al\sb2O\sb3$ catalyst. Therefore, a Pt/CeO$\rm\sb2/Al\sb2O\sb3$ catalyst can accommodate a large amount of CO and O$\sb2$. On the other hand, it was shown by the activity and kinetic results that oxygen inhibited the adsorption of propane on Pt sites thereby rendering the Pt/CeO$\rm\sb2/Al\sb2O\sb3$ catalyst less effective for propane oxidation. The presence of ceria reduced the partial oxidation product, CO, for both propylene and propane oxidations. The additional experiments suggest that H$\sb2$ and CO adsorbed only on the Pt sites, O$\sb2$ adsorption is enhanced on Pt/CeO$\rm\sb2/Al\sb2O\sb3$ catalysts, Ce$\rm\sb2O\sb3$ and Ce$\rm\sb2O\sb3/Al\sb2O\sb3$ were not produced by H$\sb2$ reduction at 800$\sp\circ$C, Pt/Nd$\rm\sb2O\sb3/Al\sb2O\sb3$ was more active than Pt/Al$\rm\sb2O\sb3$, and H$\sb2$ spillover with Pt present may be responsible for the activation of ceria. The Pt/Al$\rm\sb2O\sb3$ and Al$\rm\sb2O\sb3$ catalysts produced dimethyl ether as the main reaction product for methanol oxidation. However, the dimethyl ether yield was reduced over Pt/CeO$\rm\sb2/Al\sb2O\sb3$ and Pt/K$\rm\sb2O/Al\sb2O\sb3$ catalysts.