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An automobile exhaust system includes a catalytic converter, a NOx trap, and a sulfur oxide trap. The sulfur oxide trap improves the efficiency of the NOx trap. Sulfur oxide has a deleterious effect on the performance of nitrogen oxide traps. The sulfur oxide trap comprises a monolithic substrate wh

An automobile exhaust system includes a catalytic converter, a NOx trap, and a sulfur oxide trap. The sulfur oxide trap improves the efficiency of the NOx trap. Sulfur oxide has a deleterious effect on the performance of nitrogen oxide traps. The sulfur oxide trap comprises a monolithic substrate which is over-coated with an aluminum oxide layer and a mixed oxide layer of calcium oxide and magnesium oxide. In a variation, a sulfur oxide trap is integrated with a nitrogen oxide trap by coating the catalyst contained within a nitrogen oxide trap with a mixed oxide layer of calcium oxide and magnesium oxide. In each embodiment, the sulfur oxide trap can be regenerated by heating at elevated temperature for a short time period.

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What is claimed: 1. An internal combustion engine exhaust system having: a catalytic converter; a NOx trap; and a sulfur oxide trap useful for increasing the efficiency of a nitrogen oxide trap, said sulfur oxide trap comprising: a monolithic substrate; an aluminum oxide layer deposited on said sub

What is claimed: 1. An internal combustion engine exhaust system having: a catalytic converter; a NOx trap; and a sulfur oxide trap useful for increasing the efficiency of a nitrogen oxide trap, said sulfur oxide trap comprising: a monolithic substrate; an aluminum oxide layer deposited on said substrate; and a mixed oxide layer comprising a mixture of calcium oxide and magnesium oxide deposited on said aluminum oxide layer; wherein the sulfur oxide trap is located upstream of the NOx trap and downstream of said catalytic converter and the sulfur oxide trap is regenerable by operating the internal combustion engine system under stoichiometric or rich fuel conditions for a sufficient time that the sulfur oxide trap achieves a temperature of at least 450째 C. 2. The internal combustion engine exhaust system of claim 1 wherein the sulfur oxide trap is regenerable by operating the internal combustion engine system under stoichiometric or rich fuel conditions for a sufficient time that the sulfur oxide trap achieves a temperature of at least 500째 C. 3. The internal combustion engine exhaust system of claim 1 wherein the sulfur oxide trap is regenerable by operating the internal combustion engine system under stoichiometric or rich fuel conditions for a sufficient time that the sulfur oxide trap achieves a temperature of at least 600째 C. 4. The internal combustion engine exhaust system of claim 1, wherein the combined weight of the aluminum oxide layer and the mixed oxide layer is about 10% to 50% of the weight of the sulfur oxide trap and the weight of the mixed oxide is about 2% to 20% of the combined weight of the aluminum oxide layer and the mixed oxide layer. 5. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises: calcium oxide in an amount of about 20% to 80% of the weight of the mixed oxide layer; and magnesium oxide in an amount of about 20% to 80% of the weight of the mixed oxide layer. 6. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises: calcium oxide in an amount of about 30% to 70% of the weight of the mixed oxide layer; and magnesium oxide in an amount of about 30% to 70% of the weight of the mixed oxide layer. 7. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises: calcium oxide in an amount of about 50% of the weight of the mixed oxide layer; and magnesium oxide in an amount of about 50% of the weight of the mixed oxide layer. 8. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises: calcium oxide in an amount of about 40% of the weight of the mixed oxide layer; and magnesium oxide in an amount of about 60% of the weight of the mixed oxide layer. 9. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises calcium oxide and magnesium oxide in a weight ratio of about 0.25 to about 4. 10. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises calcium oxide and magnesium oxide in a weight ratio of about 0.43 to about 2.3. 11. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises calcium oxide and magnesium oxide in a weight ratio of about 1. 12. The internal combustion engine exhaust system of claim 1 wherein the mixed oxide layer comprises calcium oxide and magnesium oxide in a weight ratio of about 0.67. 13. The internal combustion engine exhaust system of claim 1, wherein the aluminum oxide layer is deposited by: a) coating said substrate with an aluminum oxide slurry to form an aluminum oxide coating; b) drying said aluminum oxide coating at a first elevated temperature to form a dried aluminum oxide coating; and c) calcining said dried aluminum oxide coating at a second elevated temperature to form the aluminum oxide layer deposited on said substrate. 14. The internal combustion engine exhaust system of claim 1, wherein the aluminum oxide layer is deposited by: a) coating said substrate with a solution containing a water soluble aluminum compound to form a first coating; b) drying said first coating at a first elevated temperature to form a dried coating; and c) calcining said dried coating at a second elevated temperature to form the aluminum oxide layer deposited on said substrate. 15. The internal combustion engine exhaust system of claim 14, wherein the water soluble aluminum compound is aluminum nitrate. 16. The internal combustion engine exhaust system of claim 1, wherein the mixed oxide layer is prepared by: a) coating said aluminum oxide layer with a mixed slurry comprising a calcium oxide slurry and magnesium oxide slurry to form a mixed oxide coating; b) drying said mixed oxide coating at a first elevated temperature to form a dried mixed oxide coating; and c) calcining said dried mixed oxide coating at a second elevated temperature to form the layer comprising a mixture of calcium oxide and magnesium oxide deposited on said aluminum oxide layer. 17. The internal combustion engine exhaust system of claim 1, wherein the mixed oxide layer is deposited by: a) coating said aluminum oxide layer with a mixed solution containing a water soluble calcium compound and a water soluble magnesium compound to form a mixed coating; b) drying said mixed coating at a first elevated temperature to form a dried mixed coating; and c) calcining said dried mixed coating at a second elevated temperature to form the layer comprising a mixture of calcium oxide and magnesium oxide deposited on said aluminum oxide layer. 18. The internal combustion engine exhaust system of claim 17, wherein the water soluble calcium compound is calcium nitrate and the water soluble magnesium compound is magnesium nitrate. 19. A method of preparing an internal combustion exhaust engine system, the method comprising: a) coating a monolithic substrate with a solution containing a water soluble aluminum compound to form a first coating; b) drying said first coating at a first elevated temperature to form a dried first coating; c) calcining said dried first coating at a second elevated temperature to form an aluminum oxide layer, wherein the second elevated temperature is higher than the first elevated temperature; d) coating onto said aluminum oxide layer a mixed solution containing a water soluble calcium compound and a water soluble magnesium compound to form a second coating; e) drying said second coating at a third elevated temperature to form a dried second coating; f) calcining said dried second coating at a fourth elevated temperature to form a sulfur oxide trap having a layer comprising a mixture of calcium oxide and magnesium oxide, wherein the fourth elevated temperature is higher than the third elevated temperature; and g) locating the sulfur oxide trap in a vehicle exhaust system downstream from a catalytic converter and upstream of an NOx trap; wherein the sulfur oxide trap is regenerable by operating the internal combustion engine system under stoichiometric or rich fuel conditions for a sufficient time that the sulfur oxide trap achieves a temperature of at least 450째 C. 20. The method of claim 19, wherein, said first coating is dried at a temperature of 80째 C. for about 6 hours; said dried first coating is calcined at a temperature of 600째 C. for about 6 hours; said second coating is dried at a temperature of 80째 C. for about 6 hours; and said dried second coating is calcined at a temperature of 600째 C. for about 6 hours.