Sulfur Deactivation of Ceria-Based Oxygen Storage Materials

A well-known application of heterogeneous catalysis is the three-way catalyst (TWC), used to eliminate pollutant gases (such as hydrocarbons, carbon monoxide and nitric oxide) from automobile exhaust. However, the performance of TWCs depends on the air-to-fuel ratio that the catalyst “sees”. Ceria (CeO₂)-based oxygen storage materials (OSMs) are added to TWCs to moderate the effects of changes in the air-to-fuel ratio, so that high conversions can be maintained. However, sulfur, always present in exhaust gas, can deactivate OSMs. The primary goal of our study is to investigate the relationship between physical properties (such as surface area, crystal size) and sulfur deactivation on pure ceria exposed to different temperatures that would change their physical properties, i.e. exposed to air at 500 through 800^oC, and ceria-zirconia mixed oxides (CexZr1-x, x from 1 to 0.5, exposed to air at 650^oC). X-ray diffraction (XRD) and N₂ physisorption were used to characterize the crystal sizes and surface areas of the materials. The oxygen storage capacity (OSC) was measured between 200 and 600^oC by introducing a 2-minute CO pulse and measuring the CO₂ production, and the degree of deactivation was evaluated by comparing the OSC before and after SO₂ adsorption. The degree of deactivation was also evaluated by comparing the total amount of sulfur adsorbed and desorbed, and the decomposition and desorption temperature of sulfur species formed, using temperature programmed desorption (TPD). Understanding the relationship between physical properties and sulfur deactivation can help us design a more effective and durable TWC.