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Hydrogen sulfide formation is suppressed by a three-way conversion catalyst having an underlayer and a topcoat overlying the under layer. The under layer is prepared by dispersing a Group IIa metal oxide such as an oxide of magnesium, calcium, barium or strontium on a carrier such as a refractory me

Hydrogen sulfide formation is suppressed by a three-way conversion catalyst having an underlayer and a topcoat overlying the under layer. The under layer is prepared by dispersing a Group IIa metal oxide such as an oxide of magnesium, calcium, barium or strontium on a carrier such as a refractory metal oxide monolith. A topcoat overlying the undercoat is comprised of a three-way conversion catalyst material such as a platinum-group metal catalytic component, e.g., platinum, palladium, rhodium or mixtures thereof.

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What is claimed is: 1. A hydrogen sulfide suppressing catalyst comprising a Group IIa metal oxide and a lanthanum oxide undercoat disposed on a carrier, and a topcoat discrete layer segregated from, and disposed on, the undercoat comprising a three-way conversion catalyst material overlying the und

What is claimed is: 1. A hydrogen sulfide suppressing catalyst comprising a Group IIa metal oxide and a lanthanum oxide undercoat disposed on a carrier, and a topcoat discrete layer segregated from, and disposed on, the undercoat comprising a three-way conversion catalyst material overlying the undercoat. 2. The catalyst of claim 1 wherein the metal oxide comprises strontium oxide. 3. The catalyst of claim 1 wherein the carrier comprises a refractive ceramic or metal monolith having a honeycomb structure. 4. The catalyst of claim 3 wherein the ceramic monolith is selected from the group consisting of cordierite, cordierite-alpha alumina, silicon nitride, zircon mullite, spodumene, alumina-silica magnesia, zircon silicate, sillimanite, magnesium silicates, zircon petalite, alpha alumina and aluminosilicates. 5. The catalyst of claim 1 wherein the Group IIa metal oxide is dispersed on the carrier in a loading of about 0.005 to about 1.0 g/in3 of carrier. 6. The catalyst of claim 5 wherein the Group IIa metal oxide is dispersed on the carrier in a loading of 0.1 to 0.6 g/in3 of carrier. 7. The catalyst of claim 1 wherein the lanthanum oxide is present in a loading of about 0.005 to about 1.0 g/in3 of carrier. 8. The catalyst of claim 7 wherein the lanthanum oxide is present in a loading of 0.2 to 0.6 g/in3 of carrier. 9. The catalyst of claim 1 wherein the top coat comprises a middle layer overlying the undercoat and an upper layer overlying the middle layer. 10. The catalyst of claim 1 wherein the three-way conversion catalyst material comprises a platinum-group metal catalytic component. 11. The catalyst of claim 10 wherein the platinum-group metal catalytic component is selected from the group consisting of platinum, palladium, rhodium and mixtures thereof. 12. The catalyst of claim 11 wherein the platinum-group metal catalytic component comprises a mixture of platinum and rhodium. 13. The catalyst of claim 12 wherein the platinum and rhodium are present in the mixture in a molar ratio of about 0.2 to about 20 moles of platinum per mole of rhodium. 14. The catalyst of claim 10 wherein the platinum-group metal catalytic component is present in a loading of about 10 to about 200 g/ft3 of carrier. 15. The catalyst of claim 1 wherein the three-way conversion catalyst material comprises a refractory metal oxide support. 16. The catalyst of claim 15 wherein the support comprises gamma alumina. 17. The catalyst of claim 1 wherein the topcoat further comprises a binder.