Studies of direct decomposition and reduction of nitrogen oxide with ethylene by supported noble metal catalysts

The performances of platinum-group metals (Ru, Rh, Pd, and Pt supported on 'y-alumina - (5% weight metal)) were studied for the direct decomposition of NO (20 000 ppm) and the selective reduction of NO by ethylene (NO: C2H4 = 1 : 1, 4000 ppm) in a different oxidizing atmosphere (NO-C2H4-O2). The reactions were studied varying the temperature in the range 200-800°C for NO decomposition, and 150-500°C for NO reduction by ethylene. Isothermal activity measurements of NO reduction at 300°C were also performed with 0-4 vol.% oxygen to investigate the competition between the NO-C2H4 reduction versus the O2-C2H4 combustion reactions from a kinetic point of view. Pt and Pd catalysts had the highest NO decomposition activity (N2 production > 80%) among the tested metal catalysts. Kinetic analysis was realized in a suitable temperature range for each catalyst, and activation energies of 91, 55, 18, and 14 kJ mol-1 were found for Pd, Pt, Ru, and Rh, respectively. All the catalysts showed good activity in NO reduction by ethylene in a non-oxidizing atmosphere (NO-C2H4), giving around 70-90% N2 production. The following activity scale: Pt < Rh <Pd < Ru was individualized. Catalytic activity decreased with increasing O2 concentration for all the catalysts. A kinetic interpretation of the collected data permitted evaluating the ratio between the rate constants of the O2-C2H4 to NO-C2H4 reactions. On this basis, a scale of catalyst activity reflecting the ability of reducing NO in high oxidizing atmosphere was individualized: Ru << Pd <Pt < Rh.