An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.