Formation, conductivity and activity of zinc chromite catalyst

Abstract

The kinetics of ZnCr 2O 4 formation was followed by calcining a powder mixture of ZnCrO 4 and CrO 3 (1:1 molar ratio) in the temperature range 150–1000 °C in air. The structural changes were identified using DTA, XRD and IR analysis. Surface Cr 6+ concentration and electrical conductivity measurements (between 100 and 400 °C) were performed. It was found that ZnCr 2O 4 formation started at 275 °C, became a major phase at 400 °C and reached its maximum formation at 500 °C. The fraction of reaction completed (α) vs. calcination temperature ( T c ) curve was constructed. The kinetic analysis was performed using the Coats-Redfern equation. The reaction order follows first order kinetics and the activation energy was determined as 19.65 kJ mol −1. The lowest conduction activation energy ( E σ = 0.172 eV) was observed at T c = 600 ° C. The sharp decrease of the built-in positive hole concentration with annealing the ZnCr 2O 4 spinel phase at T c > 600 ° C is reflected in a corresponding sharp lowering of the E σ values. The catalytic activity of the calcination products was investigated kinetically using the decomposition of H 2O 2 as a model catalytic process. The study revealed that the early stages of the reaction are heterogeneous, especially at T c ⩽ 500 ° C. Whenever surface Cr 6+ ions go into solution they catalyse the reaction homogeneously. A mobile electron zener phase on the surface (coupled Cr 3+-Cr 6+ ions) optimizes heterogeneous activity. The parameter which best characterises the intrinsic order of catalytic activity is the activation energy of the reaction.