Reference LPI-ARTICLE-1999-017View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

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Metal Oxide Surfaces and Their Interactions with Aqueous Solutions and Microbial Organisms.

One of the first subjects introduced to students of environmental chemistry is the aqueous chemistry of aluminum. This metal is the third most abundant element in the shallow Earth, where it hydrolyzes in water to produce a rich array of solute molecules and solids, including clays and aluminum hydroxide phases. Although these materials are ubiquitous, we are just beginning to understand the kinetic properties of their surfaces at the molecular scale. The problem is experimental sthe solids are too unwieldy, even as colloids, for detailed spectroscopy. One recent approach has been to use 1 -2 nm aqueous Al(III) molecules as experimental models to determine reaction rates and pathways at a fundamental level. The 1 -2nm-sized clusters are useful because they expose functional groups that resemble those found on the minerals, yet reactions at these functional groups can be studied at the molecular scale using relatively simple methods of solution spectroscopy, such as NMR. The 1 -2 nm ions are sufficiently small that reactions can then be simulated at a high * E-mail: whcasey@ucdavis.edu. Phone: 530-752-3211. William Casey (born 1955) received his Ph.D. degree in Mineralogy and Geochemistry from The Pennsylvania State University in 1986 under the direction of Prof. Antonio Lasaga. After graduating, he worked as a research geochemist at Sandia National Laboratories in Albuquerque, New Mexico, for several years and jointed the faculty of the University of California in 1991. He has published over 130 scientific articles on subjects relating to aqueous solution chemistry of natural waters, mineral surface chemistry, and reaction kinetics. Volume 106, Number 1

Large aqueous aluminum hydroxide molecules.

It remains difficult to understand the surface of solid acid catalysts at the molecular level, despite their importance for industrial catalytic applications. A sulfated zirconium-based metal-organic framework, MOF-808-SO4, was previously shown to be a strong solid Bronsted acid material. In this report, we probe the origin of its acidity through an array of spectroscopic, crystallographic and computational characterization techniques. The strongest Bronsted acid site is shown to consist of a specific arrangement of adsorbed water and sulfate moieties on the zirconium clusters. When a water molecule adsorbs to one zirconium atom, it participates in a hydrogen bond with a sulfate moiety that is chelated to a neighbouring zirconium atom; this motif, in turn, results in the presence of a strongly acidic proton. On dehydration, the material loses its acidity. The hydrated sulfated MOF exhibits a good catalytic performance for the dimerization of isobutene (2-methyl-1-propene), and achieves a 100% selectivity for C8 products with a good conversion efficiency.

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Identification of the strong Brønsted acid site in a metal–organic framework solid acid catalyst

The kinetic stability of oxide surfaces affects a broad range of physical phenomena, including mineral dissolution1,2,3 and sorption reactions4, stable-isotope fractionation5, and catalyst support degradation6. Our knowledge of the rates of these processes derives mostly from the rates of net mass transfer between the bulk solid and fluid phases. But from such data it is difficult to determine rates of elementary steps that are needed to test theoretical models. Here we determine the rates of oxygen exchange between an aqueous fluid and specific sites on the ‘Al13’ polyoxocation—AlO4Al12(OH)24(H2O)7+12—the structure of which closely resembles the surfaces of some Al-(hydr)oxide minerals in soils and catalyst supports. Extrapolation of these data to 298 K (and near pH 5.3) yields half-lives for oxygen on the complex that range from ∼0.6 milliseconds for bound water to 41 seconds and 13 hours for the two distinct, but structurally similar, bridging hydroxyls. This surprisingly large range of labilities (∼107) indicates that reactivity is very sensitive to molecular structure. Moreover, these results indicate that well chosen aqueous complexes provide important information to relate bonding to reactivity at mineral surfaces.

Bonding and reactivity at oxide mineral surfaces from model aqueous complexes

Rates and mechanisms of oxygen exchanges between sites in the AlO4Al12(OH)24(H2O)127+(aq) complex and water: implications for mineral surface chemistry

Publisher Summary Polynuclear complexes with hydroxide (or oxide) as bridging ligands constitute an important class of complexes. They are formed by hydrolysis of mononuclear aqua complexes of most metal ions and, therefore, constitute an important aspect of the hydrolytic chemistry of metal ions. They display a chemistry which is interesting and is also relevant in relation to applied chemistry and to biochemistry. This chapter reviews the chemistry of hydroxo-bridged chromium(III), cobalt(III) complexes, and includes studies of the related rhodium(III) and iridium(III)systems. The review focuses the chemical properties of these complexes, and special emphasis has been made on reactions that involve the cleavage or formation of a hydroxo bridge. Reactions of bridging groups other than hydroxide and reactions of nonbridging ligands are also discussed in the chapter. A large number of X-ray crystal-structure determinations have been made during the past two decades. The hydroxo-bridged chromium(III) oligomers constitute a very interesting class of complexes from a spectroscopic and magnetochemical point of view. The most important methods for obtaining structural information about solutions are probably potentiometric pH measurements and visible/ultraviolet spectroscopy, but many other techniques have been applied, including bridge-cleavage experiments, magnetic measurements, and electron spin resonance (ESR) spectroscopy.

Johan Springborg

Papers

Hydroxo-Bridged Complexes of Chromium (III), Cobalt (III), Rhodium (III), and Iridium (III)

Hydroxo-Bridged Complexes of Chromium(III), Cobalt(III), Rhodium(III), and Iridium(III)