Emissions from fossil-fuel combustion contribute significantly to smog, acid rain, and global warming problems, and are subject to stringent environmental regulations. These regulations are expected to become more stringent as state and regional authorities become more involved in addressing these environmental problems. Better systems are needed for catalytic control.
In general, existing catalytic converters used for NOx and He emission control use precious metal (PM) or their combinations as wash coats with various architectures over alumina on ceramic substrates to effect catalytic conversion. Some of the more common are coatings of Pd, Pd/Rh, or Pt/Rh. Existing catalytic converters are less effective for removal of methane HC emissions due to the high light-off temperatures for methane on these catalysts.
The present invention utilizes at least two PMs with at least two different metaloxides (for example, tin-oxide plus one or more promoters) in a layered matrix to convert CO, HCs, and NOx to CO2 and N2 by oxidation of the first two components (CO, HCs) and reduction of the third (NOx) in a moderately high-temperature gaseous environment (for example, between about 200 to about 500 °C) containing excess oxygen.
Preparation of ruthenium/platinumtin-oxide-based catalyst coatings for pellets, beads, granules, fabrics, and especially ceramic honeycomb monoliths can be accomplished by successive layering of the desired components, as follows:
- A clean, dry substrate is deaerated in a solution containing tin (II) 2-ethylhexanoate (SnEH). The substrate is removed from the solution, and excess solution is removed from the substrate. Residual solution components are evaporated leaving an SnEH layer on the substrate that is thermally decomposed in air to tin-oxide at 300 °C. Several layers are applied in the same manner to achieve the desired loading of tin-oxide.
- The promoters are added to the catalyst matrix in a similar fashion. For example, an iron oxide promoter is added to an existing tin-oxide-coated substrate by deaerating in an iron nitrate solution, removing excess solution, evaporating the solvent, and finally thermally decomposing the nitrate to oxide.
- Platinum is added to the coated substrate as above using an aqueous solution of tetraamine platinum (II) dihydroxide or other platinum salt, and then thermally decomposing the salt. Instead of the thermal decomposition, a reductive decomposition can be used. For example, the catalystcoated substrate is heated in an atmosphere containing a reducing gas, such as carbon monoxide or hydrogen, to induce reduction of the platinum salt to platinum. A similar process can be used to add the second precious metal (i.e., ruthenium), for example, by starting with an appropriate salt, or the mixed PMs may be applied in one step.
The instant catalyst can absorb the NOx species and convert them to NO. As such, nitrosyl complexation takes place with a noble metal in order to allow it to react with a reducing agent and therefore be converted to nitrogen. Preferred metal-oxide promoters are Fe2O3, NiO, and CO2O3. The metal-oxide adduct with NOx is converted to NO on desorption. The NO is subsequently transferred and bound to the PM until reduced by CO and HCs to N2. The CO and HCs are similarly oxidized by NO or O2 and SnO2 at the PM interface site.
The advantages of the present invention include:
- Lower light-off temperatures can enable oxidation of methane emissions to CO2 for natural gas fueled vehicles at lower exhaust gas temperatures.
- Lower light-off temperatures for CO and HCs enable more efficient catalytic conversion to CO2 at lower cost.
- A tin-oxide base wash-coat on a ceramic substrate minimizes loss of coating through cracking, peeling, or dusting mechanisms.
- The PM coatings are on the top surface and are enabled to be more efficiently used, thus requiring less PM resulting in lower PM costs.
- The mixed PMs result in a more efficient oxidation/reduction catalyst and may be applied in one step.
This work was done by David Schryer and Billy Upchurch of Langley Research Center. LAR-16001-1