Influence of pre-treatment on the low-temperature activity of Pt/ceria

The transient, isothermal CO oxidation activity of ceria and Pt/ceria was s tudied in the temperature range 40-230 degrees C. The influence of oxidativ e and reductive pre-treatments, at different temperatures, on the low-tempe rature activity was investigated. Reductive pre-treatment of Pt/ceria was f ound to-increase the activity; the temperature required to reach a certain activity was lowered by about 100-140 degrees C by reductive pre-treatment. The temperature of reduction, 200 degrees C, 300 degrees C or 500 degrees C, had no dramatic effect on this activity gain. However, for a moderate-di spersion sample, the activity was higher after reduction at 200 degrees C a nd 300 degrees C than after reduction at 500 degrees C. XPS measurements sh owed that Pt was not completely reduced at 200 degrees C. The high activity obtained after a reductive pre-treatment was almost completely diminished by a mild oxidation at room temperature. The high activity is discussed in terms of reduction of ceria in the vicinity of Pt particles. Isotopically labeled oxygen was used to discriminate between possible react ion schemes. The isotope distribution indicates that initially, CO mainly r eacts with oxygen from the catalyst, even on the pre-reduced sample. Howeve r, it was difficult to draw unambiguous conclusions from the isotope distri bution, since there is likely also isotope exchange. The presence of water in the feed had no large influence on the total activ ity for CO2 formation at low temperatures, but it altered the isotope compo sition of the CO2 formed, since some CO is oxidized by water instead of by oxygen. At temperatures above 140 degrees C, water did increase the total C O oxidation activity by the WGS reaction. (C) 1999 Elsevier Science B.V. Al l rights reserved.