Kinetics of the NO/H2/O2 reactions on natural gas vehicle catalysts—Influence of Rh addition to Pd

This paper reports an extensive kinetic study of the NO/H2 reaction on monometallic and bimetallic Pd and Rh based catalysts. The presence of oxygen in the feed has been examined and found to strongly alter the kinetic behaviour of noble metals according to the extent of the H2/O2 reaction which competes with the NO/H2 reaction. Rate measurements have been discussed based on two different mechanisms which essentially differ from the nature of the NO dissociation step. On Pd/Al2O3, the predominant H2/O2 reaction depletes the surface hydrogen concentration inducing a dissociation step on a nearest-neighbour vacant site whereas hydrogen assists the dissociation of NO on Rh/Al2O3. Regarding Pd-Rh/Al2O3, none competitive adsorptions would likely occur with NO preferentially adsorbed on Rh. Such a result is supported by the weak partial pressure dependencies of the selectivity to N2O formation similarly to Rh/Al2O3 which demonstrates that only Rh is involved in the formation of N2 and N2O on Pd-Rh/Al2O3. Nevertheless, it was found that Rh incorporation to Pd has a strong detrimental effect on the formation of nitrogen which suggests that structural might alter the adsorptive properties of rhodium in interaction with palladium.

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► Competition in favour of the NO/H2 reaction on Rh/Al2O3 under lean conditions.
► Rh addition to Pd favours the NO/H2 reaction but has a detrimental effect on the production of N2.
► Importance to avoid direct interactions of Pd and Rh in NGV catalysts to get optimal efficiencies.