INTERACTION OF CERIUM OXIDE WITH NOBLE-METALS

Ce oxide interacts with noble metals to affect greatly the metal dispersions and CO oxidation and three-way conversion activities. For fresh Pt catalysts the apparent metal dispersions decrease with increasing Ce content. For fresh Pd catalysts, on the other hand, the metal dispersions are independent of Ce loading. Thermal aging of Pt Ce · Al2O3 catalysts results in serious losses in apparent Pt dispersion. The apparent dispersion of the aged Pt Ce · Al2O3 catalysts depends on the gaseous environment (air or H2) in which the catalysts are aged. Infrared spectroscopic investigations of CO chemisorption over Pt and Pt Ce · Al2O3 catalysts revealed that Ce promotes the oxidation of Pt. Ce did not affect CO chemisorption over the fresh Pt Ce · Al2O3 catalysts. To determine if bulk compound formation between the noble metals and Ce oxide could occur during thermal aging, mixtures of Pt CeO2 and Pd CeO2 were heated (600 and 900 °C, 6 hr) in air and H2. None were observed for the Pd system. However, at 900 °C in H2, Pt reacted to form Pt5Ce. The Pt5Ce was identified by X-ray diffraction. Small quantities of Ce oxide (0.6-1.3 wt% Ce) enhance the oxidation of CO over fresh Pt and Pd catalysts. With increasing quantities of Ce the CO conversion activity of the fresh Pt catalysts greatly deteriorates while no such deterioration is observed for Pd. The effect of thermal aging on catalytic activity was studied under oxidizing (air), inert (N2), and reducing (H2) conditions. When aged in air, the activity of Ce-containing Pt and Rh catalysts greatly deteriorated. However, thermal aging in H2 resulted in little or no activity loss under the conditions employed. The three-way activity of an air-aged Rh catalyst could be mostly regenerated upon high-temperature exposure to H2. © 1979.