INFLUENCE OF CHLORINE ON THE SURFACE AREA AND MORPHOLOGY OF Ti02

Changes in BET surface area and morphology of Ti02 (anatase) were studied as a function of temperature and level of chlorine contamination. The objective was to study the behavior of TiO2 supports under typical catalyst preparation and reaction conditions, using adsorption, wide angle X-ray diffraction and scanning electron microscopy. Chlorinated samples had lower surface areas compared to blank Ti02 and showed a significant loss of resistance to sintering. Chlorine also retarded the phase transformation of anatase to rutile. INTRODUCTION Catalyst preparation by impregnation techniques often results in incorporation of chlorine onto the support. Chlorine comes mainly from the precursor metal salt solutions used to impregnate the support. Another source for chlorine contamination is HCl that is added in order to adjust the pH of the precursor metal salt solutions. Furthermore, the addition of HCl is of importance to achieve appropriate control over the surface concentration of acidic sites on the support and to facilitate metal deposition. Part of the chlorine is subsequently lost depending on the drying and reduction conditions during catalyst preparation. The residual chlorine content influences the acidity of the support and has an important bearing on the catalytic properties as shown for naphtha reforming Pt/A1203 catalysts [l-3]. For some support materials, chlorine also influences the BET surface area and pore structure. This has been observed previously for MgO [4] which is known to exhibit support effects on Ru, Au and bimetallic Ru-Au catalysts [5-71. MgO undergoes bulk hydration to Mg(OH)2 during the impregnation step in aqueous phase. Subsequent heating to 673°K restores the oxide, resulting in the formation of small pores and an increase in surface area from 15 to 350 m2/gm. This surface area increase was found to be strongly dependent on the amount of chlorine remaining on the support. Chlorine contaminations up to 1 wt% resulted in steep reductions in MgO surface area. On samples containing more than 1 wt% chlorine, the surface area remained (Q 25 m*/g) irrespective of the thermal treatment. Chlorine seemed to cause an 0166-9834/85/$03.30