The effects of NO on the CO-induced structural changes of Rh deposited
on SiO2, MgO and TiO2 are investigated in the temperature range 110-5
03 K by means of infrared spectroscopy. It was observed that the addit
ion of NO to CO dramatically enhanced the development of gem-dicarbony
l, Rh-I(CO)(2), from Rh-x-CO species - indicative of the occurrence of
the oxidative disruption of the Rh-x cluster - for all the three samp
les at 233-300 K. This is explained by the formation of a strong bond
between NO and Rh-x crystallites which weakens the Rh-Rh bond. It is a
ssumed that the NO and its dissociation product, the adsorbed O, parti
cipate in the oxidation of isolated Rh-0 atoms to Rh-I ions. On the ot
her band, the presence of NO slowed down the conversion of gem-dicarbo
nyl into Rh-x-CO at and above 448 K to a great extent, indicating that
NO retards the reductive agglomerization of Rh-I to Rh-x crystallites
on these supports.