Spatially resolving CO and C3H6 oxidation reactions in a Pt/Al2O3 model oxidation catalyst

Abstract

Spatially resolved Fourier transform infrared spectroscopy (Spaci-IR) was used to measure gas-phase concentration profiles during CO and C3H6 oxidation reactions over a Pt/Al2O3 monolith-supported catalyst. The reaction conditions were selected as representative of certain low temperature combustion (LTC) engine exhaust conditions, where in this study higher concentrations of CO and C3H6 and lower concentrations of NOX were used relative to standard engine exhaust. CO and C3H6 oxidation and NOX reduction reactions were examined individually and in combination via temperature programmed oxidation (TPO) experiments. Significant NOX reduction occurred right at CO and C3H6 light off, and NO oxidation only occurred after the oxidation of CO and C3H6. C3H6 oxidation was not observed until after most of the CO oxidized, as CO was more strongly adsorbed to the active site surface at low temperature. During the TPO of CO and C3H6, the conversion versus temperature profiles did not monotonically increase; two inflections were observed where the rate of conversion change as a function of temperature slowed over a small temperature range before again accelerating with temperature. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used in order to characterize intermediates that were present on the surface at the temperatures where these steps were noted. Surface ethylene and formate species were present during the first step, with acetate and formate in the second step. The inhibition steps were therefore attributed to the partial oxidation of propylene to ethylene, and then the subsequent partial oxidation of ethylene to acetate.