A Diode Laser Study of the Catalytic Oxidation Dynamics of Acetaldehyde on Polycrystalline Platinum

Abstract

The catalytic oxidation of acetaldehyde on platinum was studied using a flow reactor equipped with a tunable diode laser absorption spectrometer and a quadrupole mass spectrometer. Reaction mixtures containing this molecule in varying proportion with oxygen and with argon as a carrier gas were flowed over a polycrystalline platinum mesh, which was resistively heated to different temperatures between 700 and 1000 K. The products of these reactions were monitored using mass spectrometry and the state-resolved spectra of CO$_{2}$ produced were collected using high-resolution tunable diode laser absorption spectroscopy. These data were analyzed to yield information about the dynamics of the reaction.

Results indicate that production of CO and CO$_{2}$ by this reaction proceeds via two distinct pathways. Acetaldehyde adsorbed on the surface decomposes to acetyl, which in turn decomposes CO and CH$_{x}$. The adsorbed CO so prepared desorbs to yield the bulk of CO generated across all reaction conditions and also yields CO$_{2}$ with a relatively deactivated asymmetric stretching mode under conditions of high temperature and low oxygen coverage. The acetyl-derived CH$_{x}$ dehydrogenates to yield surface carbon and H adatoms. Total oxidation of this surface carbon is the primary source of CO$_{2}$ produced under all reaction conditions except those mentioned previously and is found to yield products with a preferentially excited asymmetric stretch. Combination of the CH$_{x}$-derived H adatoms with surface oxygen drives the production of water by this reaction.

During the course of the work described here, two notable improvements were made to our experimental apparatus. The first of these was the modification of the data acquisition process to significantly improve the signal-to-noise ratio achievable by our laser spectrometer with no increase in data collection time. The second was the development of data analysis software which significantly improved the efficiency and thoroughness of the process by which state-resolved laser spectra are analyzed and interpreted.