Effect of the addition of Na to Pt/Al2O3 catalysts for the reduction of NO by C3H8 and C3H6 under lean-burn conditions

A series of Pt/γ-Al2O3 catalysts promoted by various concentrations of Na were prepared and tested for the SCR of NO by propene and propane under lean-burn conditions. The addition of Na markedly modifies the Pt catalytic behaviors. Under a C3H8/NO/O2 atmosphere, the presence of Na leads to a drastic decrease of the C3H8 and NO conversions. In contrast, under C3H6/NO/O2, Na promotion improves the Pt catalytic performance. For high Na loadings (5 wt%), we observed an enhancement of the selectivity to N2 and a widening of the operating temperature window. The promotional phenomena observed have been explained in terms of modification of the adsorption strengths of the various reactants on the metal surface. The Na effects should be to strengthen the Pt–electron-acceptor (O and NO) bonds and to weaken the Pt–electron-donor (C3H8 and C3H6) bonds. The opposite effects observed by using propane or propene have been attributed to their different mechanisms of adsorption on Pt.