Reduction of NO by hydrogen versus reduction by CO over Pt, Pd and Rh clusters in NaX zeolite

The activation of hydrogen and CO was examined using D₂+H₂ equilibration at room temperature and by ¹³CO+C¹⁸O scrambling at 170°C, respectively, the adsorption of NO at room temperature and its TPD were used to characterise the activation of NO. These reactions were compared with the NO reduction by carbon monoxide and by hydrogen. It appeared that the M/NaX clusters (M=Pt, Rh or Pd) exhibit opposite behaviour in the NO reduction by these two reductants: with CO the sequence was RhPdPt, while platinum (Pt/NaX) was the most active catalyst (PtPd>Rh) when hydrogen was employed.

The CO scrambling was found to be most rapid over Pt, while the adsorption and dissociation of NO was most extensive over Rh; in the NO reduction by CO the weak CO activation over Rh was overwhelmed by the strong NO dissociation. On the other hand, the extensive NO adsorption and dissociation over Rh hindered the dissociation of hydrogen, which resulted in the lowest activity in the NO reduction by H₂ accompanied by an intermediate formation of N₂O. This was not the case with Pt, over which easily dissociated hydrogen reacted with probably molecularly adsorbed NO.

The reduction of N₂O by hydrogen proceeded readily over all metallic clusters at room temperature, being thus, either of the same activity as that of NO+H₂ reaction, or even of higher activity over Pd and especially over Rh. The easy reduction of N₂O by hydrogen does not agree with the reduction by CO, which was found to proceed worse than that of NO.

In some cases, also bimetallic species (PtRh/NaX, PtPd/NaX, PdRh/NaX) were employed, as well as oxidized M clusters.