Low-Temperature Oxidation of CO and Volatile Organic Compounds over Supported Platinum Catalysts

Doktorsavhandling, 2004

The catalytic oxidation of carbon monoxide (CO) and volatile organic compounds (VOC) is important for the reduction of harmful emissions from both vehicles and from stationary sources. Both these applications suffer from poor catalytic efficiency at lower temperatures. The objective of this work was to gain a deeper understanding of the low-temperature oxidation of CO and VOC over alumina supported platinum catalysts. The work has been focused on the preparation of platinum-based model catalysts, extensive flow reactor studies and thorough sample characterisation. A new preparation method was used to prepare supported catalysts with platinum distributed locally in high concentrations on the alumina support. The purpose was to prepare catalysts that would retain the evolved reaction heat to a higher extent and thereby become more active at low temperatures than conventionally prepared catalysts. The catalysts prepared using this new method showed an improved low-temperature activity for the oxidation of CO, however, not for the oxidation of propene or propane. When considering heat transfer, calculations of both temporary effects and heat accumulation showed that the enhanced activity for CO oxidation can not be explained by a heat effect. Instead, mass transfer limitations probably improved the activity to a certain extent, and structural effects can also be of importance. The oxidation of propane over Pt/γ-Al2O3 was shown to be highly influenced by the oxygen concentration. Maximum activity was observed close to stoichiometric conditions. At net-oxidising conditions the catalysts were most likely passivated due to formation of platinum oxide. In a deactivation study, the influence of hexamethyldisiloxane (HMDS) on the oxidation of ethyl acetate over iron-promoted Pt/γ-Al2O3 samples was investigated. The catalytic deactivation was found to proceed as HMDS decomposed into silicate (SixOy), which blocks the active sites on the catalyst surface. The poisoning was found to be selective as silicon rather attached to platinum and iron sites than to alumina. Moreover, depending on the catalyst composition the deactivation was either reversible or irreversible. Promoting the Pt/γ-Al2O3 catalyst with iron increased the tolerance towards HMDS as fewer platinum sites were blocked by silicate.