A study of thermal aging of [formula omitted] using temperature-programmed desorption spectroscopy

Abstract

The metal dispersion of 1.0% Pt Al 2O 3 was characterized using temperature-programmed desorption (TPD) of D 2 in an ultrahigh vacuum apparatus. On the nonaged catalyst desorption of D 2 gives rise to two peaks, one which can be identified as weak desorption from the Al 2O 3 support surface and the other as D 2 desorption from the supported Pt particle surfaces. The Pt Al 2O 3 sample was thermally aged in 5% O 2 N 2 or in 5% H 2 N 2 at a selected aging temperature for several hours. At intervals as small as 1 min, the Pt dispersion was obtained by D 2 TPD. For aging temperatures of 700, 800, and 900 °C, the Pt dispersion decreased rapidly within the first few minutes of treatment and then more gradually afterward, consistent with the change in the dominant sintering mechanism from particle migration to interparticle transport. For sintering temperatures of 700, 800, and 900 °C, the D 2 TPD peak shape changed as a function of aging time; this change suggests that faceting of the Pt particles occurs during sintering. The data also indicate that aging in an oxygen environment does not redisperse Pt Al 2 O 3 , contrary to other reports. For all aging temperatures, sintering was less severe in the H 2 environment than in the O 2 environment. The aging data from both environments were fitted to a simple kinetic sintering model.