An infrared study of CO and NO adsorption on Pt, Rh, Pd 3-way catalysts

Adsorption of CO, NO, CO-air, CO-NO, and CO-NO-air on Al2O3 and CeO2-Al2O3 loaded with Pt, Rh, or Pd was studied using an in-situ infrared chamber installed under a DRIFT-measuring unit. The chamber allowed heating of a sample and introduction of various gases. The noble metal loading (Pt, Rh, Pd) on the washcoat varied in the range of 1.4 to 2.9 %. The samples were reduced at 300 degrees C by 5 % H-2/Ar prior to the FT-IR measurements. The reaction gases were introduced into the chamber in different sequences and at different temperatures. According to the CO adsorption results the adsorbed gem-dicarbonyl species (Rh1(CO)2) on Rh and linearly bonded species on both Pt and Pd and bridged CO species on Pd were clearly identified. When CO pressure was increased stepwise no adsorbed CO was seen on the noble metal free washcoat, whereas linearly adsorbed CO was seen on Pt/Al2O3. Assignment of bands to different species of NO was difficult due to the numerous NO adsorption states. On Rh/Al2O3 and Rh/CeO2-Al2O3 adsorbed NO showed a band caused by Rh-NO+ at around 1910 cm-1. Rh-NO and Rh-NO- species could not be seen clearly. NO had a strong interaction with alumina giving adsorption bands in the range of 1600 to 1200 cm-1 Furthermore, NO was the dominant species compared to CO in occupying the adsorption sites on the surface of the noble metals examined. In kinetic studies the oxidation of CO in a CO-air mixture over a Rh/Al2O3 catalyst started at around 200 degrees C. Mixtures of CO and NO or CO, NO, and air led to the formation of an isocyanate complex. In the presence of CeO2 the formation of an isocyanate complex required that the catalyst surface was first occupied by NO. It is concluded that the carbonyls on the noble metals probably reacted with adsorbed nitrogen and oxygen to isocyanate complexes and CO2, respectively.