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

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.208, Iss.2, 2002-06, pp. : 435-447

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Abstract

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. © 2002 Elsevier Science (USA).