Potassium stabilization in β-K2Fe22O34 by Cr and Ce doping studied by field reversal method

The stabilization of potassium in β-K2Fe22O34, the active phase of styrene catalyst, by chromium and cerium doping (2 wt.%) is investigated by means of potassium ion thermal desorption. The kinetics for K+ emission from undoped and Ce-, Cr-doped β-K2Fe22O34 is studied with the field reversal technique in high vacuum conditions. Rate constants for K+ desorption are determined at 600 °C (typical styrene production temperature) from the K+ flux changes upon rapid electric modulation of the sample potential. The strong stabilization effect of both surface (Ce) and bulk (Cr) additives are observed by decrease in the rate constant of potassium ion desorption from 24.5 ± 3.6 s− 1 for undoped to 11.4 ± 2.1 s− 1 and 8.2 ± 2.8 s− 1 for Ce- and Cr-doped β-K2Fe22O34, respectively. The stabilization effect is also reflected in the previously found increase in K+ desorption energy from 2.33 ± 0.03 eV for β-K2Fe22O34 to 2.86 ± 0.01 eV for Ce–β-K2Fe22O34 and 3.47 ± 0.01 eV for Cr–β-K2Fe22O34 . A model of the adsorbed sites for K atoms including the potential energy surfaces for their desorption account reasonably well for the observed changes in desorption rates with addition of Ce and Cr. The stabilization effect is discussed in terms of different distribution of Ce and Cr dopants in the material and blocking of different K desorption sites on the surface.