Transformation of Glassy Palladium‐Zirconium Alloys to Highly Active CO‐Oxidation Catalysts During In Situ Activation Studied by Thermoanalytical Methods and X‐Ray Diffraction

Abstract

AbstractPalladium/zirconia catalysts highly active for the oxidation of CO can be prepared by exposing amorphous Pd‐Zr alloys to CO oxidation conditions at 280°C. The bulk chemical and structural changes occuring under these conditions have been studied using thermoanalytical methods (TG, DTA) combined with mass spectrometry and in‐situ powder XRD. Amorphous PdZr2 and PdZr3 alloys exhibit virtually no activity when exposed to CO oxidation conditions, mainly due to their low specific surface area (˜ 0.01 m2/g). The activity develops with time on stream, passes through a maximum and reaches a stable state only after several hours. The maximum in the activity is observed when about 50–70% of the amount of oxygen necessary for complete oxidation of the precursor to PdO and ZrO2 has been consumed. The oxidation of the amorphous Pd‐Zr alloys, which results in a drastic increase of the specific surface area of the samples, starts at significantly lower temperature than the crystallization temperatures of the alloys. The stable catalysts contain poorly crystalline monoclinic and tetragonal ZrO2, metallic palladium and PdO as bulk phases. The concentration of these phases is influenced by a series of simultaneously occurring reactions, including: the oxidation of the alloy constituents by O2 which results in PdO and ZrO2, the oxidation by CO2 resulting in Pd and ZrO2, and the reduction of the PdO formed by CO and by metallic Zr present in the unreacted part of the alloy. The solid state reduction 2 PdO + Zr → Pd + ZrO2 contributes significantly to the reduction of the PdO as long as metallic Zr is abundant in the alloys.