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, Ph, Ir and Ru) for the selective reduction of nitrogen oxides by hydrocarbons un der oxidising conditions is reviewed. Kinetic and mechanistic studies on these catalysts are assessed. Many of the factors influencing de-NO x behavior are considered. These include the choice of metal, the meta l loading and the metal particle size, the carbon number and hydrocarb on type of the reductant used and the type of support material. It is found that highest de-NOx activity is obtained with platinum-based cat alysts although large quantities of nitrous oxide are formed in additi on to nitrogen. Rhodium also exhibits a significant but lower activity although much less N2O is produced. It is seen that NOx eduction is c oincident with hydrocarbon combustion. Although metal particle size ha s little effect on overall de-NOx activity, large particles have highe r turnover frequencies. For a given carbon number, it is found that th e de-NOx, efficiency increases in the order i-paraffins < aromatics < n-paraffins < olefins approximate to alcohols and that, in general, ac tivity increases with increasing carbon number. When using olefins as the reductant, varying the support material has little effect on activ ity although large differences can be seen when using saturated hydroc arbons. Mechanistic studies, including FT-IR and TAP experiments, are reviewed and the various mechanisms that have been proposed for this r eaction are discussed.