Self-regeneration of three-way catalyst rhodium supported on La-containing ZrO2 in an oxidative atmosphere

Abstract

Rhodium supported on lanthanoid (La, Ce, Pr, or Nd)-containing ZrO₂ was investigated as a three-way catalyst following an aging treatment (oxidation at 1273 K) that simulates 80 000 km of mileage in real vehicles. The properties of rhodium were assessed by transmission electron microscopy, CO chemisorption, and temperature-programmed reduction using CO. The oxidation states of rhodium before and after the aging treatment were evaluated using X-ray photoelectron spectroscopy. Rhodium supported on lanthanum-containing ZrO₂ (Rh/Zr–La–O) was highly active for the removal of NO_x and hydrocarbons from a synthetic auto exhaust. The support determined the oxidation state of rhodium after the aging treatment: rhodium supported on Zr–La–O maintained its low oxidation state during the three-way catalytic reaction and after the aging treatment, whereas rhodium supported on ZrO₂ and other lanthanoid-containing ZrO₂ was converted to the higher oxidation states. The Rh/Zr–La–O catalyst, following the aging treatment, exhibited superior activity for the steam reforming reaction. The hydrogen produced from the steam reforming reaction reduced the previously oxidized Rh in Rh/Zr–La–O, thereby regenerating the catalyst that was previously deactivated by an oxidation treatment. Self-regeneration of the Rh/Zr–La–O catalyst by the steam reforming reaction was more efficient when compared with that of the other lanthanoid-containing ZrO₂ catalysts. These results highlight the potential of the present strategy for developing active three-way catalysts with high tolerance to oxidative conditions.