Exhaust-catalyst development for methanol-fueled vehicles: III. Formaldehyde oxidation

Abstract

Formaldehyde oxidation was examined in a laboratory reactor over alumina-supported Pt, Pd, Rh, Ag, Cu-Cr, and Pd-Ag catalysts as part of a program to develop catalysts for lean-exhaust methanol-fueled vehicles. In general, the H 2CO oxidation behavior of the catalysts paralleled the CH 3OH oxidation behavior reported in Parts 1 and 2 of this series [Appl. Catal. 27 (1986) 83; J. Catal. 103 (1987) 419]. Principal similarities were: (i) high activity of Pt when there was no carbon monoxide (CO) in the feed, but strong inhibition by added CO, (ii) lower activity of Ag and Cu-Cr catalysts, but only weak inhibition due to added CO, (iii) enhanced activity of the Pd-Ag catalyst compared to Pd and Ag alone for feeds containing CO in addition to H 2CO, and (iv) relatively poor activity of the Rh catalyst. The most unusual behavior was observed with the Pd catalyst, which showed local minima in the H 2CO conversion vs. temperature profiles. These saddles were similar to, but more pronounced than, those observed in CH 3OH oxidation. The depths of the saddles in the H 2CO and CH 3OH conversion profiles correlated with the feed equivalence ratio, with the deepest saddles occurring in feeds containing a large excess of O 2. A mathematical model was developed which accounts for the local minima in terms of changes in the oxidation state of Pd with both temperature and feed composition. These results indicate that catalyst design must be tailored to the particular operating characteristics (exhaust composition, temperature, etc.) of the vehicle in question.