COLLOIDS A AND Colloids and Surfaces SURFACES ELSEVIER A: Physicochemicaland Engineering Aspects97 (1995) 109 117 Mechanistic aspects of the deposition of the Cr(VI) species on the surface of TiO 2 and SiO 2 N. Spanos, S. Slavov 1, Ch. Kordulis, A. Lycourghiotis * Department of Chemistry, Institute of Chemical Engineering and High Temperature Chemical Processes, University of Patras, P.O. Box 1414, GR-26500, Patras, Greece Received 3 May 1994; accepted 12 January 1995 Abstract The deposition of the negatively charged Cr(VI) species on TiO2 and SiO 2 surfaces suspended in aqueous electrolyte media has been studied in the pH range 3.0 8.0. The investigation included deposition isotherm and microelectro- phoretic mobility measurements. From the experimental results and a theoretical analysis of the deposition isotherms, it was concluded that the Cr~O~- ions taken up were located in energetically equivalent sites at the inner Helmholtz plane (IHP) of the electric double layer that had developed between the support particles and the impregnating solution. The deposition sites were mainly created by protonated surface hydroxyl groups in the case of SiO2 and by protonated and undissociated surface hydroxyl groups in the case of TiO 2. The lateral interactions operative among the deposited Cr(VI! species were, in general, strong, being stronger in SiO 2 supports in comparison with those of TiO2. Moreover, it was found that upon a pH decrease from 6.0 to 3.0, the extent of deposition of the Cr(VI) species on silica increased from 0.01 to 0.13 gmol Cr(VI) m -2. The extent of deposition of the Cr(VI) species on titania increased also from 0.23 to 1.68 gmol Cr(VI) m 2 with a pH decrease from 8.0 to 4.8. Further decrease in the pH in the case of titania resulted in a considerable decrease in the extent of deposition. Keywords: Chromia catalysts; Equilibrium deposition filtration; Microelectrophoresis; Preparation of supported catalysts; Silica catalysts; Titania catalysts I. Introduction Supported catalysts based on chromia are widely employed in numerous catalytic processes. The supports commonly used are 7-alumina, titania and silica. Chromia/alumina catalysts are used for the conversion of paraffins to olefinic hydro- carbons, in the hydrodealkylation of aromatics, and, to some extent, in the catalytic reforming * Corresponding author. On leave from the Department of Chemistry, "K. Preslavski" University, 9700 Shumen, Bulgaria. 0927-7757/95/$09.50 © 1995 ElsevierScience B.V. All rights reserved SSDI 0927-7757(95 )03087-5 process 1-1]. Chromia/silica catalysts are mainly used for the polymerization of ethylene into linear chains [2]. Finally, chromia-supported titania cat- alysts are of interest for the selective catalytic reduction of nitric oxide by ammonia in excess oxygen [ 3 ]. A relatively high dispersity of the supported chromium may be attained by using equilibrium deposition followed by filtration (EDF) for the preparation of chromium-supported catalysts. An increasing number of studies dealing with the preparation of supported catalysts by EDF have appeared recently in the literature [4-32]. Despite