Rm. Gabr et al., EFFECT OF SPINEL (ZNCR2O4) FORMATION ON THE TEXTURE, ELECTRICAL-CONDUCTION AND CATALYTIC BEHAVIOR OF THE ZNO-CR2O3 SYSTEM, Materials chemistry and physics, 39(1), 1994, pp. 53-62
Structural and phase changes accompanying the calcination in air, up t
o 1000-degrees-C, of the ZnO-Cr2O3 system were monitored using differe
ntial thermal analysis, differential scanning calorimetry, infrared sp
ectrophotometry, X-ray diffractometry and chromium ion estimation. The
texture was assessed by analyzing nitrogen sorption isotherms measure
d at -196-degrees-C. The results indicate that ZnO-Cr2O3 solid solutio
n becomes a major product at 500-degrees-C, and the ZnCr2O4 spinel pha
se starts to form at 400-degrees-C. The electrical conductance propert
ies of the samples were investigated before and after admission of 2-p
ropanol in the temperature range 100-400-degrees-C. The electrical con
duction is attributed to the existence of a surface mobile-electron Ze
ner phase that maximizes the conductivity. The obtained interstitial Z
n2+ cations and free electrons (charge carriers) are considered to be
localized at the ions or vacant sites, and the conduction occurs via a
hopping-type process, which implies a thermally activated electronic
mobility. The catalytic activity data of the vapor-phase decomposition
of 2-propanol were obtained in the temperature range 200-400-degrees-
C, using a flow system technique. It was found that propylene is the m
ain reaction product, with a minor yield of acetone. In the solid solu
tion of ZnO in Cr2O3, the Cr ion sites will be electronically more iso
lated than either disordered or ordered spinel phases. The neutralizat
ion of these sites will not occur by bulk electron transfer, but they
tend to trap electrons from the oxygen end of the 2-propanol molecule,
leading to a high dehydration activity. Correlations were attempted b
etween the structure of the catalysts and their catalytic activity.