Cerium-modified manganese octahedral molecular sieves as catalysts for selective catalytic reduction
Catalysts and articles useful for selective catalytic reduction (SCR) are disclosed. The catalyst comprises a cerium oxide and an octahedral molecular sieve (OMS) comprising manganese oxide. The catalysts, which comprise 0.1 to 50 wt. % of cerium, can be made by doping, ion-exchange, deposition, or other techniques. Also disclosed is an SCR process in which a gaseous mixture comprising nitrogen oxides is selectively reduced in the presence of a reductant and the cerium-modified OMS catalyst. The cerium-modified manganese oxide OMS catalysts offer unexpected advantages for selective catalytic reduction, especially NH 3 -SCR, including improved thermal stability and improved activity for NOx conversion, especially at relatively low temperatures (100° C. to 250° C.).
1. A catalyst useful for selective catalytic reduction, comprising:
(a) an octahedral molecular sieve (OMS) comprising manganese oxide; and
(b) a cerium oxide;
wherein the catalyst comprises from 0.1 to 50 wt. % of cerium,
wherein the OMS is formed in the presence of cerium oxide.
2. The catalyst of claim 1 made by adding a cerium compound during synthesis of the OMS.
3. The catalyst of claim 1 made by contacting the OMS with an aqueous solution of a cerium compound and drying the resulting ion-exchanged material.
4. The catalyst of claim 1 made by depositing a cerium compound on the OMS from a solution, dispersion, or suspension, and concentrating the resulting mixture.
5. The catalyst of claim 4 made by incipient wetness.
6. The catalyst of claim 1 calcined at a temperature within the range of 300° C. to 650° C. following its preparation.
7. The catalyst of claim 1 wherein the octahedral molecular sieve is OMS-2.
8. The catalyst of claim 1 wherein the cerium oxide is selected from the group consisting of cerium(III) oxide, cerium(IV) oxide, and mixtures thereof.
9. The catalyst of claim 1 comprising from 5 to 15 wt. % cerium.
10. A process which comprises selectively reducing a gaseous mixture comprising nitrogen oxides in the presence of a reductant and a catalyst useful for selective catalytic reduction, the catalyst comprising:
(a) an octahedral molecular sieve (OMS) comprising manganese oxide; and
(b) a cerium oxide;
wherein the catalyst comprises from 0.1 to 50 wt. % of cerium.
11. The process of claim 10 wherein the reductant is selected from the group consisting of ammonia and C 1 -C 8 hydrocarbons.
12. The process of claim 10 wherein the reductant is ammonia.
13. The process of claim 10 wherein the catalyst is a cerium-doped OMS-2.
14. The process of claim 10 wherein the catalyst is a cerium oxide on OMS-2.
15. The process of claim 10 wherein the catalyst is cerium-exchanged OMS-2.
16. The process of claim 10 wherein % conversion of nitrogen oxides at temperatures within the range of 100° C. to 200° C. improves compared with that of a similar process in which a cerium-free OMS catalyst is used.
17. The process of claim 10 wherein the catalyst demonstrates improved thermal stability compared with that of a cerium-free OMS catalyst.
18. An article for treating a gas containing nitrogen oxides, comprising a substrate and, deposited on the substrate, a catalyst useful for selective catalytic reduction, the catalyst comprising:
(a) an octahedral molecular sieve (OMS) comprising manganese oxide; and
(b) a cerium oxide;
wherein the catalyst comprises from 0.1 to 50 wt. % of cerium.