Al. Diaz et al., INFRARED-SPECTROSCOPY AND TEMPERATURE-PROGRAMMED DESORPTION STUDY OF CO ON RH AL2O3 CATALYSTS - PROBING OVERLAYER AND SUPPORT SITES, Langmuir, 10(5), 1994, pp. 1461-1471
The adsorption of CO on reduced Rh/Al2O3 catalysts has been investigat
ed using infrared (IR) spectroscopy and temperature programmed desorpt
ion (TPD). At approximately 120 K, carbon monoxide absorbs on Al3+ sit
es of the gamma-Al2O3 support and in the form of gem-dicarbonyl, linea
r 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 desorp
tion features with maximum rates of desorption at 170-185 K and 490-50
0 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 funct
ion 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 p
rovided which suggests that loss of catalyst surface area (T greater-t
han-or-equal-to 1200 K) may also be a mechanism by which surface Rh is
lost, possibly due to encapsulation of Rh particles. Structural chang
es in the Rh overlayer have been found to occur during CO TPD with oxi
dative disruption occurring at low temperatures and reductive desorpti
on occurring at high temperatures.