Abstract
The participation of the water-gas shift reaction in the transient response of Rh-containing catalysts in exhaust was demonstrated by comparing the responses of a Pt/Rh/Ce/Al2O3 catalyst, a Rh-free Pt/Ce/Al2O3catalyst, and a Ce-free Pt/Rh/Al2O3 catalyst. Each of the bead-type catalysts was mounted in a converter in the exhaust system of a gasoline engine. Infrared diode laser spectroscopy was used to measure CO concentrations in exhaust simultaneously at the converter inlet and outlet with high time-resolution. In the first series of experiments, a catalyst was stabilized under rich exhaust and then given a 1-s exposure to lean exhaust. Following the subsequent lean-to-rich transition, a transient reduction in CO emission was observed over all three catalysts. The reduction in CO emission over the PtCe catalyst could be explained by reaction of CO with oxygen “stored” in the catalyst and CO adsorption and accumulation in the catalyst. The reductions in CO emissions over the two Rh-containing catalysts were greater than the amounts that could be explained by CO and oxygen accumulation and reaction but were consistent with a transient enhancement of the water-gas shift reaction over Rh. Lean-to-rich step response experiments provided further evidence of the participation of water-gas shift in the dynamic behavior of three-way automotive catalysts. During air-fuel ratio cycling and the warmed-up portion of the Federal Test Procedure, time-averaged CO conversions decreased in the order Pt/Rh/Ce>PtCe>PtRh. This ranking suggests that oxygen accumulation and reaction over Ce contributes somewhat more to CO conversion during driving than water-gas shift over Rh.
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