An investigation of promoter effects in the reduction of NO by H2 under lean-burn conditions

The reduction of NO by H-2 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 O-2 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 N-2, N2O, and retained NO. The data reveal significantly greater surface concentrations of N-2 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 N-2. An additional significant effect of the molybdenum promoter has been proposed because when the concentrations of N-2 intermediates and the amount of available platinum in the modified catalysts are calculated, the "storage" of N-2 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 H-2) is cut off. It has been found that the decay in the production of N-2 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 N-2 even in the absence of the normal H-2 reductant. The mechanistic consequences of these novel results are discussed. (C) 2002 Elsevier Science (USA).