Selective reduction of nitrogen oxides by hydrocarbons under lean-burn conditions using supported platinum group metal catalysts

The performance of supported platinum-group metals (Pt, Pd, Rh, Ir and Ru) for the selective reduction of nitrogen oxides by hydrocarbons under oxidising conditions is reviewed. Kinetic and mechanistic studies on these catalysts are assessed. Many of the factors influencing de-NOx behavior are considered. These include the choice of metal, the metal loading and the metal particle size, the carbon number and hydrocarbon type of the reductant used and the type of support material. It is found that highest de-NOx activity is obtained with platinum-based catalysts although large quantities of nitrous oxide are formed in addition to nitrogen. Rhodium also exhibits a significant but lower activity although much less N2O is produced. It is seen that NOx reduction is coincident with hydrocarbon combustion. Although metal particle size has little effect on overall de-NOx activity, large particles have higher turnover frequencies. For a given carbon number, it is found that the de-NOx efficiency increases in the order i-paraffins < aromatics < n-paraffins < olefins ≈ alcohols and that, in general, activity increases with increasing carbon number. When using olefins as the reductant, varying the support material has little effect on activity although large differences can be seen when using saturated hydrocarbons. Mechanistic studies, including FT-IR and TAP experiments, are reviewed and the various mechanisms that have been proposed for this reaction are discussed.