MOO3 ZRO2 AND CRO3/ZRON CATALYSTS - STUDY OF THE PRECURSOR FORMATION/

Equilibrium adsorption was used to study the influence of some paramet ers on the precursor formation of catalysts consisting of zirconia-sup ported chromia (ZC) or molybdena (ZMo). Uptake capacities of zirconia treated at various temperatures before adsorption (from 673 to 1023 K) showed little dependence on temperature. The Me uptake (Me = Cr or Mo ) as a function of the equilibrium concentration of Me species in solu tion obeyed the Langmuir law for limited ranges. The Langmuir plots th erefore yielded saturation coverage values, n(m), that depended on the concentration range. The meaning of ''monolayer coverage'' is discuss ed. The n(m) value measured at a range of solution pH from 1 to 8 depe nded little on pH. The values ranged from 1.6 to 2 atoms nm(-2) for Cr , and were around 3.6 atoms nm(-2) for Mo. The adsorption data suggest ed three adsorption regimes: (i) a strong, apparently irreversible ads orption, (ii) a partly reversible, direct adsorbate-surface interactio n, corresponding to the initial stage in the Langmuir plots, which eve ntually merges in (iii), a weaker adsorption regime, showing an adsorb ate-adsorbate interaction. We compare our data with reported values fo r Cr and Mo equilibrium adsorption on alumina. (C) 1998 Academic Press .