Suppression of the oxygen storage and release kinetics in Ce0.5Zr0.5O2 induced by P, Ca and Zn chemical poisoning

Abstract

The present investigation concerns for the first time the understanding of the individual and combined effects of phosphorous with calcium or zinc deposited on a Ce0.5Zr0.5O2 solid followed by calcination in 20%O2/He at 850°C, on the kinetics of oxygen storage and release of the thus derived contaminated Ce0.5Zr0.5O2 solids. Towards this aim, physicochemical and redox properties of Ce0.5Zr0.5O2, P/Ce0.5Zr0.5O2, P-Ca/Ce0.5Zr0.5O2 and P-Zn/Ce0.5Zr0.5O2 solids were investigated by means of BET, XRD, SEM, XPS, TPR, OSC and TPIE techniques. P, Ca or Zn deposition on Ce0.5Zr0.5O2 solid was found to result in a drastic decrease in the population of reactive oxygen species present on the surface and within the subsurface/bulk region of the solid. The latter is attributed to the formation of cerium and calcium phosphates in the subsurface region of the solid, as well as to the formation of zinc oxide on the surface of the solid, clogging pores lower than 10nm in size as revealed by BJH pores size distribution measurements. Another reason for the severe reduction in the OSC (up to 70%) of contaminated by phosphorous Ce0.5Zr0.5O2 solid is the rearrangement of its surface composition resulting in the lowering of the Ce/Zr surface ratio. The latter is linked to the lowering of the concentration of reactive oxygen associated with the strength of Ce-O-Zr bonding. The incorporation of P within the subsurface region of Ce0.5Zr0.5O2 crystals and its reaction with the solid producing CePO4 causes the formation of a significant amount of Ce3+ which is very difficult to get re-oxidised, and locks the Ce(III)/Ce(IV) redox couple. Phosphorus incorporation within the subsurface/bulk of Ce0.5Zr0.5O2 solid oxide particles was found to significantly increase the activation energy of bulk oxygen diffusion from 126.5 to ~169.0kJmol-1, while the concurrent deposition of Ca or Zn along with P was found to strongly suppress surface oxygen diffusion. © 2011 Elsevier B.V.