Reactivity of steam in exhaust gas catalysis I. Steam and oxygen/steam conversions of carbon monoxide and of propane over PtRh catalysts

Abstract

Pt (1 wt.-%), Rh (0.2 wt.-%) and Pt (1%) Rh (0.2%) catalysts supported on A 2O 3 (A) and on 12 wt.-% CeO 2/Al 2O 3 (CeA) were prepared and characterized by the chemisorption and the titration of probe molecules. Their catalytic behavior in steam and oxygen/steam conversions of carbon monoxide and of propane was investigated. The reactions were carried out in oxygen-deficient medium, under isothermal or temperature-programmed reaction (TPR) conditions. Light-off temperatures as well as turnover frequency ( N t) deduced from the low-conversion branch of the TPR curves were used to evaluate the catalytic activities. In oxygen/steam conversion of carbon monoxide, the relative N t values were: PtCeA, 2500 > PtA, 240 > RhCeA, 75 > RhA, 1 for carbon monoxide oxidation at 100°C and PtCeA, 43500 > RhCeA, 100 > PtA, 75 > RhA, 1 for the water-gas shift (WGS) at 200°C. This reaction, on Al 2O 3 and to a lesser extent on CeO 2/Al 2O 3, was inhibited by oxygen, WGS activities being one to three orders of magnitude higher in the CO+H 2O mixture. In oxygen/steam conversion of propane, the relative N t values were: PtA, 76 > PtRhA, 5.4 > PtCeA, 3.6 > RhCeA, 1.8 > RhA, 1 > PtRhCeA, 0.8 for propane oxidation at 300°C while the activity order was: PtRhCeA > PtRhA > RhCeA > PtA > RhA for the propane steam reforming by 400°C. This reaction started when oxygen was consumed and was inhibited by carbon monoxide, which explains why there was a definite cooperation effect between RhCeA intrinsically active in steam reforming and platinum active in oxidation and in WGS.