TRANSFORMATION OF GLASSY PALLADIUM-ZIRCONIUM ALLOYS TO HIGHLY-ACTIVE COOXIDATION CATALYSTS DURING INSITU ACTIVATION STUDIED BY THERMOANALYTICAL METHODS AND X-RAY-DIFFRACTION

Palladium/zirconia catalysts highly active for the oxidation of CO can be prepared by exposing amorphous Pd-Zr alloys to CO oxidation condit ions at 280-degrees-C. The bulk chemical and structural changes occuri ng under these conditions have been studied using thermoanalytical met hods (TG, DTA) combined with mass spectrometry and in-situ powder XRD. Amorphous PdZr2 and PdZr3 alloys exhibit virtually no activity when e xposed to CO oxidation conditions, mainly due to their low specific su rface area (approximately 0.01 m2/g). The activity develops with time on stream, passes through a maximum and reaches a stable state only af ter 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 amo rphous Pd-Zr alloys, which results in a drastic increase of the specif ic surface area of the samples, starts at significantly lower temperat ure than the crystallization temperatures of the alloys. The stable ca talysts contain poorly crystalline monoclinic and tetragonal ZrO2, met allic palladium and PdO as bulk phases. The concentration of these pha ses is influenced by a series of simultaneously occurring reactions, i ncluding: the oxidation of the alloy constituents by 02 Which results in PdO and ZrO2, the oxidation by CO2 resulting in Pd and ZrO2, and th e 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 lo ng as metallic Zr is abundant in the alloys.