Washcoat overlayer for improved activity and stability of natural gas vehicle monolith catalysts operating in the presence of h2o and so2

Abstract

A γ-AlOOH/γ-Al2O3washcoat overlayer, applied to a PdO(Pt/CeO2)/ γ-AlOOH/γ-Al2O3on cordierite (2MgO 2Al2O3 5SiO2) mini-monolith catalyst (400 cells per square inch, 1 cm diameter × 2.5 cm length), is shown to reduce the impact of H2O and SO2 on the catalyst activity and stability during CH4 oxidation at low temperatures (<500°C). The washcoat overlayer covered the Pd (and Ce and Pt when present) and decreased the catalyst CH4 oxidation activity in a dry feed gas (0.07 vol % CH4, 8.5 vol % O2, 0.06 vol % CO, 8 vol % CO2) at a gas hourly space velocity (GHSV) of 36 000 h-1. However, when H2O (2, 5, and 10 vol %) and SO2 (2 and 5 ppm) were added to the feed gas, the loss in catalyst activity was reduced and the catalyst stability was improved. The overlayer provided additional sites for both SOx and H2O adsorption, thereby reducing their adsorption on the active phase of the catalysts, and consequently, the catalysts maintained higher activity in the presence of both SO2 and H2O over periods of up to 200 h.