Deactivation of Pd/Ce0.5Zr0.5O2 model three-way catalyst by P, Ca and Zn deposition

Abstract

The negative effects of P, P-Ca and P-Zn deposition on the surface of a model 1wt% Pd/Ce0.5Zr0.5O2 “three-way” catalyst (TWC) on its structural, morphological, textural, oxygen storage and release, and catalytic performance have been investigated for the first time. The aim of this research work is to provide fundamental understanding on the multiple functions of a TWC when P, Ca and Zn contaminants are accumulated on its surface. Towards this aim, a model Pd/Ce0.5Zr0.5O2 TWC and P, P-Ca and P-Zn contaminated ones were prepared and ischaracterised by means of BET, XRD, HR-TEM, XPS and H2/O2 pulse injection techniques. The catalytic performance of the non-contaminated and contaminated Pd/Ce0.5Zr0.5O2 solids was evaluated for CO and C3H6 oxidation and NO reduction by H2 probe reactions under oxygen lean conditions. Incorporation of P, Ca and Zn contaminants was found to cause a significant decline (16–40%) in the concentration (μmol O/g) of labile active (OSC) and total (OSCC) oxygen species of Pd/Ce0.5Zr0.5O2 catalyst aged in air at 850°C. After P incorporation followed by calcination at 850°C, CePO4 was detected, while in the presence of Ca or Zn the formation of Ca3(PO4)2 and ZnO, respectively, was verified. At low reaction temperatures, co-addition of Zn and P was found to further deteriorate the OSC (μmol O/g), while at high temperatures no appreciable differences were observed. This is ascribed to the formation of CePO4 which decreases significantly the degree of reduction of Ce(IV) oxide. CePO4 physically covers catalyst particles (e.g., pore mouth and internal surface) leading to the substantial loss of BET area, pores volume, and Pd surface sites. More importantly, it is shown for the first time that the presence of Pd was found to diminish to a great extent the deteriorating effect of P, Ca and Zn when deposited on Ce0.5Zr0.5O2 on its oxygen storage and release properties. High concentrations (mg/g) of P, P-Ca and P-Zn contaminants on the catalyst surface was found to cause a considerable decrease in catalyst's activity for CO and C3H6 oxidation, and NO reduction by H2 under oxygen lean conditions. The deterioration of catalytic performance is largely due to the presence of P, and to a lesser extent to the presence of Ca or Zn.