Influence of the nature of the Ce-promoter on the behavior of Pd and Pd–Cr TWC systems

Abstract

A series of monometallic Pd and bimetallic Pd–Cr catalysts supported on CeO x /Al 2O 3 (CA) and (Ce, Zr)O x /Al 2O 3 (CZA) materials have been characterized using a combination of X-ray diffraction (XRD), electron microscopy, and Raman spectroscopy, and employing electron paramagnetic resonance and in situ diffuse reflectance Fourier transform infrared and X-ray near-edge structure spectroscopies to analyze the redox and chemical processes taking place under light-off conditions in a gaseous, stoichiometric mixture containing CO, NO, and O 2. The catalytic behavior of these mono and bimetallic systems was strongly affected by the nature of the cerium-containing promoter. The Pd–ceria interface appeared significantly more active than Pd–ceria/zirconia in both CO oxidation and NO reduction processes under stoichiometric conditions. This factor seems to dominate the differential behavior of Pd monometallic systems. In the case of Pd–Cr bimetallic systems, an additional strong influence of the base metal on the oxidation state of Pd appears of importance in explaining the catalytic behavior of the samples. For the CeO x /Al 2O 3 supported catalyst, the formation of Pd(0) at lower temperatures leads to an initial enhancement in activity, while formation of a Pd–Cr alloy above 473 K suppressed full NO conversion at high temperatures. These effects were absent in the (Ce, Zr)O x /Al 2O 3 supported system.