Kinetic model for the reversible deactivation of a Pt/Al2O3 catalyst during NO oxidation

Abstract

Microkinetic approach is used to develop a kinetic model for the reversible deactivation of a Pt/Al2O3 catalyst during NO oxidation. The model predicts the dependence of temperature on the transient evolution of NO conversion. Consistent with literature, it is shown that for low and high temperatures, it takes a few minutes to achieve steady state, whereas for intermediate temperatures (>130°C and <250°C), the time taken is of the order of a few hours. The model explains the inverse hysteresis phenomenon reported in the literature. In contrast to other works, the effect of NO2 as well as O2 on the catalyst deactivation is considered. The restoration of catalyst activity is explained by the reduction of Pt oxides by NO at low temperatures and the thermal decomposition of Pt oxides at high temperatures. The effect of inlet NO concentration, gas hourly space velocity, and temperature ramp rates on the light-off behavior is captured accurately by the model. The effect of gas composition during cooling, maximum temperature during heating, minimum temperature during cooling, and catalyst exposure to various gases at a fixed temperature on the NO oxidation activity in the subsequent runs is predicted. Consistent with the literature, the model predicts that cooling in the presence of N2 only can result in a significant restoration of the catalyst activity, provided that the catalyst has been exposed to high temperatures (>300°C) before the start of cooling. This has been attributed to the enhanced decomposition of Pt oxides at high temperatures. It is also shown that exposing the catalyst to NO at low temperatures for long time periods can result in a regain of the catalyst activity.