Natural gas has become one of the most attractive alternative fuels for gasoline and diesel due to its competitive price resulting from its abundant availability and lower GHG emissions. In stoichiometric natural gas engines, three-way catalysts (TWCs) are widely employed to convert the exhaust pollutants such as CO, HC and NOx to CO2, H2O and N2. One of the main components of TWC is ceria-zirconia (CeZrOx) which provides oxygen storage capacity (OSC) that is required for achieving high CO, HC and NOx conversion over a wide air-to-fuel ratio (AFR) window. OSC is typically measured using H2 or CO as a reductant because they are typical components of gasoline exhaust. However in natural gas engine applications a major reductant in the exhaust gas is methane (CH4) and to our knowledge, there are very limited studies focusing on OSC measurement using CH4 as the reductant.
In this work, a method of measuring the OSC of a bimetallic Pd/Rh on ceria-zirconia supported alumina was developed. The OSC measurement using H2, CO and CH4 reductants in varying oxygen concentrations and at varying catalyst temperatures are presented. It was found that the nature of the reductant along with other process conditions has a significant impact on the measured OSC. Below 450°C the OSC is limited by the number of reduced sites, which are controlled by the reactivity of the reductant (CH4<CO<H2) under fuel rich conditions. Above 450°C, the effect of the nature of the reductant has minimal impact on OSC measurement due to higher rate of reactions (CeO2+reductants) as well as the generation of more active reductants due to co-existing reactions (water-gas shift and CH4 steam reforming). Furthermore, the measured OSC is very sensitive to the lean/rich gas environment. For example, both O2 and reductant concentration could affect the OSC measurement. The observed oxygen storage during fuel lean can be explained by kinetic and mass transfer controlled processes that occur on surface and sub-surface oxygen storage sites, respectively. The higher reductant concentration during fuel rich results in more reduced sites (Ce2O3), which are able to store more oxygen during fuel lean.