Effect of speciated HCs on the performance of modern commercial TWCs

• The CO oxidation activity was hardly affected by the presence of methane or propane.
• The NO reduction activity was significantly improved by unsaturated HCs in feed.
• The oxidation activity of HCs depended on the CH bonding energy except for ethanol.
• The superior oxidation activity of ethanol was due the dipole–dipole interaction.
• Each Pd and Rh TWC revealed an opposite trend in HC oxidation by O2 concentration.

Effects of speciated hydrocarbons on the TWC activity of modern commercial Pd and Rh catalysts were systematically investigated by using various HC species individually, selected from a group of eight HCs including methane, ethanol, propene, propane, toluene, m-xylene, n-octane and iso-octane. The oxidation activity of individual HCs decreased over both Pd and Rh catalysts in the following order: ethanol > propene ≈ toluene > m-xylene > n-octane > iso-octane > propane > methane, in a good correlation with the CH bonding energy of the HC species except for ethanol. The light-off temperature of CO (LOTCO) over the Pd catalyst shifted from 180 °C in the absence of HCs to a range of 181–215 °C in the individual presence of the HCs, a clear evidence of the inhibiting effect of HCs on the CO oxidation. However, the identical individual HCs exhibited little inhibiting effect on the CO oxidation activity over the Rh catalyst, mainly due to their negligible oxidation activities at the LOTCO of 214 °C over the Rh catalyst. The NO reduction activity over both Pd and Rh catalysts – while displaying volcano-type profiles as a function of reaction temperature – was improved by the presence of an individual HC, with the extent of the improvement depending on the HC and temperature. This promoting effect of individual HCs on the NO reduction was stronger over Rh than over Pd; at 400 °C for instance, the NO conversion increased from 20 to 65% due to the presence of propene over Rh, while it increased from 12 to 16% over Pd under the identical reaction condition. With the increasing O2 concentration from 1 to 21 vol.% in the feed stream, the Pd catalyst exhibited an increasing oxidation activity, while the Rh catalyst revealed the opposite trend. The concentrations of reaction intermediates formed on the surface of the Pd catalyst increased with the increasing O2 concentration, while decreasing on the Rh catalyst surface as confirmed in an in situ DRIFT study. These opposite trends of the TWC performance over the Pd and Rh catalysts may be partially attributable to the oxides of Pd and Rh formed on the surface thereof.

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