An Investigation of Promoter Effects in the Reduction of NO by H2 under Lean-Burn Conditions

The reduction of NO by H2 has been investigated under lean conditions at temperatures representative of automotive “cold-start” conditions (<200°C) using MoO3- and Na2O-modified Pt/Al2O3 and Pt/SiO2 catalysts. It has been found that small additions of sodium significantly increase the NO conversion while larger loadings of sodium have a severe poisoning effect. However, in the presence of excess O2 no enhancement in nitrogen selectivity at low temperatures has been observed for all loadings of Na. Indeed, an adverse effect has been found at higher temperatures. Addition of molybdenum as a promoter results in increases in NO conversion and nitrogen selectivity for all loadings tested. The optimal formulation was determined to be 1% Pt/10% MoO3/0.27% Na2O/Al2O3. Steady-state isotopic-transient kinetic (SSITK) experiments were performed on this and a “model” Pt/MoO3/Na2O/SiO2 catalyst using labelled nitric oxide in order to estimate the surface concentrations of species leading to N2, N2O, and retained NO. The data reveal significantly greater surface concentrations of N2 precursors over the modified catalysts for both the Pt/Al2O3 and Pt/SiO2 systems and this has been used to rationalise the increased selectivity to N2. An additional significant effect of the molybdenum promoter has been proposed because when the concentrations of N2 intermediates and the amount of available platinum in the modified catalysts are calculated, the “storage” of N2 precursors on the MoO3 seems to occur. This effect has been further explored using the modified SiO2 catalyst in non-steady-state transient experiments where the reductant supply (i.e., the H2) is cut off. It has been found that the decay in the production of N2 is very significantly delayed in comparison with the unmodified catalysts, and it is proposed that this is consistent with the trapping on the Mo of a reduced intermediate that can form N2 even in the absence of the normal H2 reductant. The mechanistic consequences of these novel results are discussed.