Negative apparent kinetic order in steady-state kinetics of the water-gas shift reaction over a Pt-CeO2 catalyst

The kinetics of the water-gas shift reaction were studied on a 0.2% Pt/CeO₂ catalyst between 177 and 300 °C over a range of CO and steam pressures. A rate decrease with increasing partial pressure of CO was experimentally observed over this sample, confirming that a negative order in CO can occur under certain conditions at low temperatures. The apparent reaction order of CO measured at 197 °C was about -0.27. This value is significantly larger than that (i.e. -0.03) reported by Ribeiro and co-workers [A.A. Phatak, N. Koryabkina, S. Rai, J.L. Ratts, W. Ruettinger, R.J. Farrauto, G.E. Blau, W.N. Delgass, F.H. Ribeiro, Catal. Today 123 (2007) 224] at a similar temperature. A kinetic peculiarity was also evidenced, i.e. a maximum of the reaction rate as a function of the CO concentration or possibly a kinetic break, which is sometimes observed in the oxidation of simple molecules. These observations support the idea that competitive adsorption of CO and H₂O play an essential role in the reaction mechanism.