PULSE REACTION STUDIES OF TRANSIENT NATURE OF ADSORBATES DURING NO-CO REACTION OVER RH/SIO2

The transient nature of adsorbates for the reaction of NO with CO over a 4 wt % Rh/SiO2 catalyst has been studied by in situ infrared spectroscopy combined with pulse transient techniques. Dynamic behavior of infrared-observable Rh-NO-, Rh-NO+, Rh+(CO)(2), and linear and bridged CO reveals that dissociation of Rh-NO- oxidizes Rh-0 to Rh+; Rh+ chemisorbs CO and NO as Rh+(CO)(2) and Rh-NO+, respectively; and Rh+(CO)(2) can either undergo reductive agglomeration or react with adsorbed oxygen to produce CO2 at 473-523 K. At 573 K, Rh is in the reduced state where linear CO and bridged CO react with the oxygen produced from dissociation of Rh-NO- to produce CO2. The catalyst in the reduced surface state is more active for the formation of N2O and CO2 products, and the response of N2O formation leads that of CO2 formation at temperatures below the light-off temperature. The rapid CO2 response during the pulse reaction studies indicates that the reaction can quickly reach steady state at temperatures above 473 K when the reaction conditions are altered.