Effects of High-Temperature Aging on the Dispersion of Rh/Al 2O 3

Abstract

The effects of high-temperature treatment in relatively simple gas mixtures of 1% O 2/N 2, 1% H 2/N 2, and an environment which alternates between these two gases on the dispersion of 0.5 wt% Rh/Al 2O 3 were characterized using temperature-programmed desorption (TPD) of D 2. The TPD data was used to determine the relative changes in the Rh metal dispersion as a result of treatment. These treatments were selected as they are believed to model the oxidation and reduction environments in the typical automotive exhaust. When the fresh catalyst was aged in reducing environments at 600, 700, or 800°C, the dispersion decreased rapidly within the first 2 h of treatment, but more slowly thereafter. The rate of this decrease is directly related to the treatment temperature. When the catalyst was aged in oxygen at 600, 700, or 800°C, the Rh was rapidly and completely oxidized. At the lower aging temperatures, most of the Rh could be recovered to the metallic state using a reduction treatment at 200°C, but at 800°C, only a small amount of the Rh was recovered to the metal using this mild reduction treatment. Subsequent reduction treatments at high temperature resulted in near complete recovery of Rh in the form of large metal particles. Treatment in alternating oxygen/hydrogen environments at low frequency produced Rh 2O 3 after each oxidation treatment and Rh metal particles after each reduction treatment. The higher temperature treatments produced sintered Rh metal particles, even in the alternating environment. Treatment in alternating oxygen/ hydrogen environments at high frequency resulted in incomplete oxidation of Rh after the oxidation cycle, but did result in more rapid sintering of the Rh metal as the treatment temperature was increased. This sintering rate, however, was slower than for treatments in the pure gases at the same temperature.