Effect of ceria and zirconia supports on NO reduction over platinum-group metal catalysts: A DFT study with comparative experiments

Abstract

Understanding support effects at the molecular level is vital for improving the design of the platinum-group metal (PGM) three-way catalyst (TWC). As a first step, we performed density functional theory (DFT) calculations for tetrahedral M4 clusters (M = Pt, Pd, or Rh) supported on CeO2(111) and ZrO2(111) and examined their activity toward NO adsorption and reduction. We found that the M4 clusters are positively charged on ceria but close to neutral on zirconia. The site preference of NO adsorption is not affected much by changing ceria for zirconia. NO tends to gain slightly more negative charge from M4/ZrO2 than from M4/CeO2. The reaction of CO with adsorbed NO does not exhibit strong support dependence. In contrast, NO dissociation at the metal/oxide interface exhibits strong support dependence, with the transition state (TS) lowered by 0.7–1.1 eV by changing ceria for zirconia. This lowering occurs because Zr in ZrO2, carrying more positive charge than Ce in CeO2, interacts more strongly with the O end of NO to promote dissociation. The calculated TS energy for NO dissociation (as well as the dissociative adsorption energy) is strongly correlated with the NO-reduction activity obtained in our comparative TWC experiments. Thus, the ability of a catalyst to bind N and O, with the help of the oxide support, is key for the NO-reduction activity of PGM catalysts.