F2008-09-063
Pd-Perovskite Catalysts for Methane Emissions Abatement: Study of Pd Substitution Effects
Advanced compressed natural gas (CNG) engines entail considerable advantages over conventional gasoline and diesel engines. Natural gas (NG) is a largely available fossil fuel and therefore non-renewable. However, NG has some advantages compared to gasoline and diesel from an environmental perspective. Its emissions are lower. The low flame temperature of lean operated CNG engines helps to limit the formation of NOx. Furthermore, since NG contains only 75 wt% carbon versus 86-88 wt% for gasoline or diesel, it produces less CO2 per unit of energy released. Furthermore, soot particulate can hardly be formed from methane combustion. Other benefits lie in the fact that NG is neither toxic, carcinogenic, nor caustic. However, unconverted methane in CNG flue gases is much harder to oxidise than gasoline-derived unburned hydrocarbons (UHC). The strong greenhouse effect of methane (more than one order of magnitude higher than that of CO2) forces a higher and higher concern at a legislation level and, as a consequence, the development of new aftertreatment technologies to abate these emissions. Catalytic combustion of methane on honeycomb converters similar to those used for the treatment of gasoline engine exhaust gases is the way to go. Commercial catalysts are mostly based on gamma-Al2O3-supported Pd, having a at least three fold higher noble metal loading compared to that of conventional three-way catalysts (up to 300 g/ft3 against 80 g/ft3 ). A research line of ours is aimed at developing nanostructured Pd-perovskite-type-oxide catalysts employing an overall noble metal load significantly smaller than that used in conventional converters, the catalytic performance being the same. Several perovskite-type oxide catalysts (LaMnO3, LaMn0.9Pd0.1O3, LaFeO3, LaFe0.9Pd0.1O3, LaCrO3, LaCr0.9Pd0.1O3) were prepared by SCS, characterized, and tested as catalysts for methane combustion. The comparative analysis of the catalysts activity was carried out with pure perovskites in powder. The best catalyst was found to be LaMn0.9Pd0.1O3 (T50 = 425°C) and therefore it was selected to be deposited and tested on a cordierite monolith. Experimental tests on powders and on ad-hoc prepared CNG exhaust gas after-treatment converter demonstrated a superior activity towards methane conversion by inducing a partial substitution of the perovskite B site with Pd. This should entail a reduction of the overall catalyst costs compared to conventional Pd-only catalysts currently employed characterized by high Pd loads. Specific activities are currently in progress either to develop more active perovskite catalysts and to optimize the amount of Pd in order to get a further reduction of this costly noble metal in the catalyst formulation.
Session: Exhaust Gas Aftertreatment