Metal oxide-based compounds surface defects

Metal oxides belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate systems to be used as supports for highly dispersed metal catalysts or as precursors of a metal phase or sulfide, chloride, etc. Simple metal oxides range from essentially ionic compounds with the electropositive elements to covalent compounds with the nonmetals. However, taking into account the large variety of metal oxides, the principal objective of this book is to examine only metal oxides that are more attractive from the catalytic point of view, and most specifically transition metal oxides (TMO). In particular, TMO usually exhibit nonstoichiometry as a consequence of the presence of defective structures. The interaction of TMO with surfaces of the appropriate carriers develop monolayer structures of these oxides. The crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character and cation valence sates are major ingredients of the chemistry investigated in the first part of the book. New approaches to the preparation of ordered TMO with extended structure of texturally well defined systems are also included. 

The inhibiting effect of the soluble hexavalent compounds in the film provides another protective action. Chromate films commonly exhibit an iridescent color. When a treated metal is exposed in a natural or salt spray atmosphere the color of the surface changes from yellow to green, or becomes clear. This indicates the reduction of soluble hexavalent chromium to insoluble trivalent chromium. When in contact with moisture the hexavalent chromium slowly leaches, providing a self-healing action for defects in the coatings. The leached hexavalent chromium acts as an anodic inhibitor. Dissolved hexavalent chromiums are adsorbed by the defects and form passive films that are composed of mixtures of insoluble trivalent chromium compounds and oxides of the base metal. 

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