Mixed valent manganese-based NOx adsorber
Herein disclosed are compositions for passive NOx adsorption and oxidation that include at least a manganese-based oxide and one or more promoter materials and methods for making and using said compositions. The promotor materials may include a rare earth, transition, or main group metal. The compositions may be used in NOx emission control system and adsorbs NOx compounds at low temperatures and then release NOx at higher temperatures, where the NOx can be oxidized, without the hybridized MnOX composition breaking down. The compositions are capable of maintaining a sufficiently large surface area at high temperatures found in the emissions gas streams of internal combustion engines necessary for the complete elimination of NOx.
1. A composition of matter comprising:
a first promoter, comprising one of: aluminum (Al), barium (Ba), cerium (Ce), lanthanum (La), copper (Cu), iron (Fe), magnesium (Mg), titanium (Ti), yttrium (Y), zirconium (Zr), and zinc (Zn);
a second promoter, comprising one of: aluminum (Al), barium (Ba), cerium (Ce), lanthanum (La), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), titanium (Ti), yttrium (Y), zirconium (Zr), and zinc (Zn), but different from the first promoter;
a manganese oxidizer; and
oxygen, according to the formula A a B b Mn y O x , wherein A is the first promoter, B is the second promoter, the additive value of a, b, and y is 1, a is in a range of 0.05-0.25, b is in a range of 0.05-0.25, and y is in a range of 0.50-0.90, x is selected to balance the valences of the formula, wherein said composition exhibits a NOx adsorption value of greater than 70 micromoles of NOx per gram at 300° C., and wherein the composition does not contain platinum group metals.
2. The composition of claim 1 , wherein the Mn y O x component is 50-70 wt % of the composition.
3. The composition of claim 1 , wherein said composition exhibits a specific surface area of between 150 square meters per gram and 250 square meters per gram at room temperature.
4. The composition of claim 1 , wherein said composition exhibits a NOx adsorption value of greater than 50 micromoles of NOx per gram at 100° C.
5. The composition of claim 1 , wherein said composition exhibits a pore diameter between 5 nanometers and 20 nanometers.
6. The composition of claim 1 , wherein said composition exhibits a pore volume between 0.3 mL/g and 0.5 mL/g.
7. The composition of claim 1 , wherein said composition exhibits sulfur content below 0.5%.
8. The composition of claim 1 , wherein said composition does not produce N 2 O in an exhaust gas stream in a temperature range of 100 to 350° C.
9. The composition of claim 1 , wherein said composition, when in an exhaust gas stream with a temperature range of 200 to 350° C., is either adsorbing or releasing NOx continuously.
10. The composition of claim 1 , wherein said composition has a total alkali metal content of less than 3%.