WOx/ZrO2 Catalysts. Part 3. Surface coverage as investigated by low temperature CO adsorption: FT-IR and volumetric studies

The adsorption of CO at 77K on zirconia-supported tungsten oxide (ZW) samples prepared by equilibrium adsorption or by impregnation and with tungsten content up to 12.4 W atoms nm-2 was studied with the aim of determining the coverage of ZrO2 surface by the supported tungsten oxospecies. As an essential background for quantitative volumetric measurements,ZW samples were investigated by FT-IR spectroscopy to identify the surface sites responsible for theCOadsorption. FT-IR showed that CO adsorbed selectively and irreversibly on Zr4+ cus centers of the support surface, but not on W6+ cus sites. By volumetric determinations, the coverage of the zirconia surface by tungsten oxospecies was about 50% at maximum, irrespective of the preparation procedure and of the support crystallographic modification (monoclinic or tetragonal). Zirconia coverage remained significantly unchanged after leaching with a NH3 solution that removed tungsten oxospecies not directly interacting with the zirconia surface. This finding indicates that ZrO2 support coverage depends mainly on the tungsten species strongly anchored to the zirconia surface.