Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides.

A. General Introduction Among the transition metal ions of the 3d series, Cr takes a particular position because of its variability in oxidation state, coordination numbers and molecular structure.1,2 The elucidation of these Cr species on inorganic oxidic surfaces is a complex task, which is of fundamental importance to understanding the behavior of Cr in the environment, colloids, and Cr-based heterogeneous catalysts.3-6 The environmental behavior of Cr is illustrated in Figure 1. Chromium is frequently encountered in minerals and in geochemical deposits,7 and due to erosion and weathering, chromium becomes a surface species or can be released into the environment. On the other hand, Cr compounds are used in industries such as leather tanning, electroplating, and pigment production and, therefore, are found in solid wastes and in waste waters. All these Cr oxides are highly soluble and susceptible to various reactions at the solid-water interface: redox processes and homogeneous and heterogeneous reactions. Chromiummight be oxidized or reduced by soil constituents and such redox reactions have dramatic influences on the behavior of Cr.8,9 The water soluble and mobile Cr6+ is toxic, while the hazard of the less mobile Cr3+ is relatively low. Manganese oxides are the known naturally occurring oxidants of Cr3+, while Cr6+ is reduced to the less mobile Cr3+ by organic soil constituents (amino, humic, and fulvic acids) and Fe2+. Chromium-based catalysts are composed of Cr oxides supported on inorganic oxides, such as silica, alumina, and molecular sieves. Chromium on silica (Cr/SiO2), as illustrated in Figure 2, is the famous Phillips catalyst for the polymerization of ethylene at relatively low pressures.10-13 This catalyst is the basis for the Phillips particle form process in the production of high-density polyethylene (HDPE), one of the most extensively used polymers. Other important catalytic activities are hydrogenation-dehydrogenation, oxidation, isomerization, aromatization, and DeNOx reactions.14-28 The basis for the activity of Cr in such a wide spectrum of reactions lies in the variability of oxidation states, of coordination envi* Auhor to whom correspondence should be addressed. † Katholieke Universiteit Leuven. ‡ Lehigh University. Figure 1. Behavior of Cr in the environment. Chromium ions can be released from natural chromium sources or by man in the environment, where it is susceptible to redox and homogeneous and/or heterogeneous reactions. 3327 Chem. Rev. 1996, 96, 3327−3349