INFRARED-SPECTROSCOPY AND TEMPERATURE-PROGRAMMED DESORPTION STUDY OF CO ON RH AL2O3 CATALYSTS - PROBING OVERLAYER AND SUPPORT SITES

The adsorption of CO on reduced Rh/Al2O3 catalysts has been investigated using infrared (IR) spectroscopy and temperature programmed desorption (TPD). At approximately 120 K, carbon monoxide absorbs on Al3+ sites of the gamma-Al2O3 support and in the form of gem-dicarbonyl, linear and bridge bound CO species on the Rh overlayer as identified by IR spectroscopy. TPD of CO from a 5% Rh/Al2O3 catalyst reveals two desorption features with maximum rates of desorption at 170-185 K and 490-500 K. The former peak is assigned to CO desorbing from Al3+ sites while the latter is assigned to CO desorbing from the Rh overlayer. The CO adsorption capacity of the Rh overlayer has been quantified as a function of anneal temperature in ultrahigh vacuum using TPD. Sintering of the Rh overlayer has been identified as the primary mechanism by which exposed Rh0 is lost from the catalyst surface. Indirect evidence is provided which suggests that loss of catalyst surface area (T greater-than-or-equal-to 1200 K) may also be a mechanism by which surface Rh is lost, possibly due to encapsulation of Rh particles. Structural changes in the Rh overlayer have been found to occur during CO TPD with oxidative disruption occurring at low temperatures and reductive desorption occurring at high temperatures.