Control of H2S Emissions from High-Tech TWC Converters

Environmental Catalysis: Mobile Sources

“…Nevertheless, the promise of reduced levels of Pb and S in reformulated fuel stimulated development of Pd-Rh (Yamada et al, 1993; and Pd catalysts (Summers and Williamson, 1993;Dettling et al, 1995). Pd, however, is less resistant to deactivation by poisons such as Pb and S than Pt and Rh.…”

“…Sulfur is then liberated as S02/S03 when the exhaust again encounters lean conditions (Dettling et al, 1990;Lox er al., Chapter 10: Environmental Catalysis: Mobile Sources 7 17 1989). For example, at A/F > 14.6 (slightly lean) SO2 is oxidized to SO3, which adsorbs and is stored on the high-surface-area CeO2, which serves to store oxygen.…”

“…Figure 10.9 illustrates an example of a strategy for (i) minimizing Rh-AI203 interaction by isolating the Rh in the pores of a non-reactive Zr02 support in a phase separate from that of Pt/La203-BaO/AI2O3, (ii) maximizing oxygen storage capacity, CO oxidation and the water-gas shift reaction by stabilizing CeOt through incorporation in a solid solution with Zr02, (iii) placing PdO and the Ce02/Zr02 OSM in catalyst layer 1, thereby protecting them from poisoning by SO2 through the use of the two upper layers (layer 2 and the A1203 layer) that serve as reactioddiffusion barriers. Addition of a low-mass "close-coupled" catalyst, located very close to the engine exhaust ports, can greatly alleviate this problem, since it is rapidly warmed by the hot exhaust gases and reaches hydrocarbon light-off temperatures within 8-10 s (Ball, 1992;Summers and Williamson, 1993;Hepburn et af., 1994;Dettling et al, 1995). Note that Pd is supported directly on CeO2-ZrO2; the principle is to facilitate exchange of oxygen between the OSM and the PdO surface while also promoting CO oxidation and the water-gas-shift reaction on Ce02.…”