Affinity of lead for noble metals on different supports

Abstract

AbstractDirect association of lead compounds with Pt, Pd and Rh sputtered onto ZrO2, TiO2 and Al2O3 wafer supports was observed. The samples were exposed to combustion products of iso‐octane containing 1.5 g Pb gal−1 as Motor Mix, 0.8 mg P gal−1 and 0.03 wt% S at 700°C in an overall lean exhaust; however, the samples experienced alternating rich and lean exhaust pulses. A semi‐quantitative electron microprobe study showed that the association of lead compounds to the surface of Pt, Pd and Rh was at least 2 or 3 orders of magnitude higher compared to the amount found on the supports. On Pt lead was present predominantly as PbSO4, on Rh as PbO or Pb metal, and as solid solution in Pd. These large differences in the form of Pb compounds could explain the known fact that Pd and Rh are more sensitive to Pb poisoning than Pt. The higher activity of Pt in catalyzing the oxidation of SO2 to SO3 results in the formation of stable but inert PbSO4 on Pt. On the other hand, Rh and Pd are poor catalysts for SO2 oxidation and therefore Pb remains as PbO or Pb on Rh and forms PdPb solid solutions with Pd. In both cases (Pd and Rh) the active sites for catalytic reactions are poisoned. Halide‐containing lead species were more specific than volatile PbO in poisoning noble metals as indicated by various independent experimental observations.