Effects of the CePO4 on the oxygen storage and release properties of CeO2 and Ce0.8Zr0.2O2 solid solution

In an attempt to gain fundamental information about the role of P on the chemical deactivation of a three-way catalyst (TWC), P was deliberatively incorporated in CeO2 and Ce0.8Zr0.2O2 solid solution. XRD, Raman, XPS, and N2 physisorption studies have indicated that monazite, a Ce(III) phosphate (CePO4) solid phase, is located at the surface/subsurface region of both solids. Different experimental approaches were used to investigate the alteration by P incorporation of the redox chemistry of CeO2 and Ce0.8Zr0.2O2 solids that is relevant to the oxygen storage and release properties of the current commercial TWCs. In particular, H2-TPR, dynamic OSC measurements using CO and H2 pulses, 18O2 temperature-programmed isotopic exchange (TPIE), and 18O2 transient isothermal isotopic exchange have been used. These studies revealed that the amount of labile surface and bulk oxygen and the bulk oxygen diffusion rates are significantly reduced when the solids are contaminated with phosphorus. The monazite present in the surface and subsurface region of the solid is proposed to be responsible for these results. The reduction of Ce(IV) present in the CeO2 and Ce0.8Zr0.2O2 solids is also affected. Ce(III) in monazite is very stable and cannot be oxidized to Ce(IV). Therefore, it cannot participate in the redox Ce(IV)/Ce(III) couple needed for the reduction of Ce(IV). Moreover, the formation of bulk and surface O vacant sites needed for the O storage and release steps involved in the OSC process was also found to be affected. The results of the present work provide support to the hypothesis that formation of monazite is one of the main chemical deactivation mechanisms of real TWCs since CePO4 was found in deactivated vehicle-aged TWCs.