Reduced Precious Metal Catalysts for CH<sub>4</sub> and NO<sub>x</sub> Emission Control of Natural Gas Vehicles

Abstract

The abundant domestic natural gas resources coupled with the desire for clean air and reduced greenhouse gas emissions motivates the accelerated development of natural gas vehicles (NGVs) and engine. Methane (CH4), a potent greenhouse gas, is the major component of natural gas. The current emission catalysts fall short in meeting the desired exhaust CH4 emission standards for NGVs. In this project we have developed a class of novel Platinum Group Metal (PGM) based-monolith catalysts with enhanced activity. The overall aim of this project was to develop cost-effective PGM catalysts with enhanced activity for abatement of CH4, non-CH4 hydrocarbons (NMHC), CO, and NOx conversion in the exhaust of vehicles fueled by natural gas in compressed or liquefied forms (CNG, LNG). Over three budget periods (four years with no-cost extension) we focused on the discovery and development of a group of mixed metal oxide spinels that, in combination with Platinum Group Metals (PGM; Pt and Pd), exhibit enhanced methane oxidation activity during modulation. In addition to achieving improved performance with these materials, including a significant reduction in the requisite PGM loading, we established a molecular level understanding and developed a predictive model for catalyst optimization. To this end, we made many fundamental advances that are of practical value. Two mechanisms were identified that underpin the observed conversion enhancement achieved with modulation and spinel addition. These include (i) achieving a partially oxidized PGM surface for enhanced methane activation, and (ii) mitigating methane steam reforming inhibition by products CO and H2 through their oxidation by the spinel during the lean-to-rich part of the cycle. The project findings included the development of a monolith reactor model comprising a set of overall reactions with global kinetics and the relevant mass and heat transport processes. The project identified several mixed metal oxide spinels that show good performance when included as an additional washcoat layer next to a Pt-based layer: NiCo2O4, NiFe2O4, CoMn2O4, MnCo2O4, and MnFe2O4. The dual-layer PGM-spinel catalysts are poisoned by sulfur but the extent is mitigated in part by the spinel. Feeds containing CO, H2, and NO lead to diminished detrimental impact of SO2. The results suggest it is the poisoning of the OSC function that is leading to the methane conversion loss. Regeneration of the catalyst can be restored in part through high temperature hydrothermal treatment although care must be taken to avoid PGM sintering. The project led to 6 peer reviewed publications with several more at various stages of completion by project end. Finally, the project funds supported three graduate students (two at UH, one at UVa) and one post-doctoral associate (at ORNL).