PdRuIr ternary alloy as an effective NO reduction catalyst: insights from first-principles calculation

NO dissociation is an important reaction step in the NO reduction reaction, particularly in the three-way catalyst conversion system for automotive gas exhaust purification. In this study, we used first-principles calculations based on density functional theory to analyze the interaction and dissociation of NO on the PdRuIr ternary alloy. The electronic properties of the atomic combination of the PdRuIr ternary alloy create an effective catalyst that is active for NO dissociation and relatively stable against the formation of volatile RuO_x through a weakened O adsorption. This study also shows that for an alloyed system, the strength of NO adsorption may not necessarily predict the dissociation activity. This tendency is observed in the PdRuIr ternary alloy where Ru top is the active site for NO adsorption albeit not an effective site for dissociation. It is presumed that NO dissociation is mediated by its molecular diffusion to active sites for dissociation, which are usually high Ru- and/or Ir-coordinated hollow or bridge sites. These active sites allow high charge transfer from the surface to NO, which fills the NO anti-bonding state and facilitates dissociation. This therefore assumes that the strength of NO molecular adsorption is not a descriptor for NO dissociation on metal alloys but rather the ability of the surface to transfer charge to NO and homogeneity of the strength of adsorption. Furthermore, O adsorption on the ternary alloy, particularly near the Ru sites, is relatively weaker as compared to the pure Ru surface. This weakened O adsorption is attributed to charge re-distribution through alloying, particularly charge transfer from the Ru atom to the Ir and Pd atoms.