Transient kinetics of carbon monoxide oxidation by oxygen over supported palladium/ceria/zirconia three-way catalysts in the absence and presence of water and carbon dioxide

摘要

The transient kinetics of CO oxidation by O_2 over alumina-supported Pd/CeO_2/ZrO_2 three-way catalysts is described in the absence and presence of H_2O and CO_2 in the feed.Experiments were carried out in a laboratory fixed-bed reactor at 573 K,while periodically switching between a feed of 1 vol% CO in He and a feed of 0.5 vol% O_2 in He with a frequency of 1/30 Hz.Separate experiments were performed with 14 vol% water in both feeds,and,as is the case with real engine exhaust gas,with 14 vol% water and 14 vol% CO_2 in both feeds.The presence of water largely enhanced the reaction rate,while it was inhibited by CO_2- A transient kinetic model has been developed for this catalyst,based on the experimental data.It was found that the reaction in the absence of H_2O and CO_2 proceeds via the same elementary steps as reported for a Pt/Rh/CeO_2/gamma-Al_2O_3 catalyst (R.H.Nibbelke,A.J.L.Nievergeld,J.H.BJ.Hoebink,G.B.Marin,Appl.Catal.B 19 (1998) 245).Only the rate coefficient for CO desorption from Pd was significantly lower,in line with literature data.Additional elementary steps were combined with the above model in order to describe quantitatively the effect of water.The present study also shows that bulk diffusion of oxygen in ceria plays a major role,when the rate is enhanced by water.The inhibition effect of CO_2 could be described by a lower amount of oxygen-storage sites.