Step and Pulse Transient Studies of Ir-Observable Adsorbates During NO and CO Reaction on Rh/SiO2

The dynamics and reactivity of infrared-observable adsorbates for the reaction of NO with CO over a 44% Rh/SiO2 catalyst have been studied by in situ infrared spectroscopy combined with steady-state isotopic and pulse NO-CO transient techniques. Steady-state isotopic transient results reveal that (i) a rapid exchange between the gaseous CO and adsorbed CO occurs on the Rh surface; (ii) Rh+(12CO)(13CO) is an intermediate for the exchange between Rh+(12CO)2 and gaseous 13CO; (iii) the adsorbed NCO species is not directly involved in the formation of CO2; (iv) the residence time of the intermediate for CO2 formation decreases with an increase in temperature from 473 to 573 K. At 573 K, rapid NO dissociation led to the high availability of surface oxygen, which results in CO2 response leading the CO response during steady-state isotopic transient studies. Pulse NO-CO transients further confirm that the NCO species is not a reaction intermediate in the formation of CO2. Pulse NO-CO studies show that Rh-NO−, Rh-NCO, and CO2 are formed prior to gem-dicarbonyl; CO2 can be produced without involving the gem-dicarbonyl as an intermediate. Steady-state isotopic transient and pulse studies were excellent complementary techniques that provided an insight into the reactivity and dynamics of adsorbates under reaction condition.