Catalysis mineral surfaces

Contact with a mineral surface can in many cases allow a redox reaction to proceed at a rate considerably greater than attainable within an aqueous solution itself. The catalyzing mineral sorbs the electron donating and accepting species, then, within its structure or along its surface, conducts electrons from one to the other. Where electron transfer by this pathway proceeds more rapidly than via a direct transfer in solution between colliding molecules, the redox reaction proceeds preferentially by heterogeneous catalysis. 

Mineral surfaces may accelerate the rate of ester hydrolysis (Stone, 1989 Hoffmann, 1990 Torrents and Stone, 1991). One plausible scheme for this heterogeneous catalysis assumes a nucleophilic addition of the ester to the surface functional group, e.g., in case of a carboxylic acid ester... 

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

Chemical reactions of adsorbed species are of importance in vast areas of science, the involvement of adsorbed metal ions in catalysis being one example of great economic value. In addition reactions involving adsorbed species can sometimes produce products that may be either difficult or impossible to prepare away from the mineral surface. Therefore, an understanding of the chemical processes that occur in such systems is of potential economic benefit to industrial operations. Such knowledge is also of much wider significance, however, because the movement of ions in most environmental situations is controlled by sorption processes, and aluminosilicate minerals play a major role in many situations. 

Abiotic, for example by adsorption of reactants onto mineral surfaces, distinguished from biotic catalysis by the absence of a temperature optimum. 

As reviewed by Alexander (1985) and Morris Pritchard (1994), many studies suggest that bioavailability represents one of the most important factors influencing PAH biodegradability in the environment. This is because PAHs, especially the HMW, hydrophobic compounds, possess intrinsically low water solubilities and therefore tend to partition onto soil mineral surfaces and to sorb tightly to available organic materials (Hites et al., 1977 Means et al., 1980 Weber etal., 1993). When PAHs are adsorbed in this way they become physically unavailable to resident microorganisms and are therefore protected from microbial catalysis. 

Applying the foregoing thermodynamic and kinetic information to manganese behavior in natural water systems is considerably limited because the manganese system exemplifies the difficulties discussed earlier. On the thermodynamic side, the kinds of oxide phases in natural waters may not correspond to those for which equilibrium data are available. Also, cation exchange reactions are probably important (21). On the kinetic side, the role of catalysis by various mineral surfaces in suspension or in sediments is not really known. Of considerable importance may be microbial catalysis of the oxidation or reduction processes, as described by Ehrlich (7). With respect to the real systems, relatively... 

Even so, it is clear that certain structural features and special entities do influence catalysis by mineral surfaces, as well. The most common means by which they operate fall into four general classes, three of which have been extensively investigated. Covalent linkages in clays are less common than the others, being dependent on grafting to SiOH, which entities are not numerous in clays (10). 

Weyl, W. A., New Approach to Surface Chemistry and to Heterogeneous Catalysis, Mineral Industries Experiment Station, Pennsylvania State University, Bull. 57 (1951). 

Weyl, W.A., 1951. A new approach to surface chemistry and to heterogeneous catalysis. Mineral Industries Exp. Sta., Bull., 57 47-58. 

The application of IR spectroscopy to catalysis and surface chemistry was later developed in the fifties by Eischens and coworkers at Texaco laboratories (Beacon, New York) in the USA [7] and, almost simultaneously, by Sheppard and Yates at Cambridge University in the UK [8]. Mapes and Eischens published the spectra of ammonia chemisorbed on a silica-alumina cracking catalyst in 1954 [6], showing the presence of Lewis acid sites and also the likely presence of Br0nsted acid sites. Eischens, Francis and Pliskin published the IR spectra of carbon monoxide adsorbed on nickel and its oxide in 1956 [9]. Later they presented the results of an IR study of the catalyzed oxidation of CO on nickel at the First International Congress on Catalysis, held in Philadelphia in 1956 [10]. Eischens and Pliskin also published a quite extensive review on the subject of Infrared spectra of adsorbed molecules in Advances in Catalysis in 1958, where data on hydrocarbons, CO, ammonia and water adsorbed on metals, oxides and minerals were reviewed [11]. These papers evidence clearly the two tendencies observed in subsequent spectroscopic research in the field of catalysis. They are the use of probes to test the surface chemistry of solids and the use of spectroscopy to reveal the mechanism of the surface reactions. They used an in situ cell where the catalyst sample was... 

Vorlicek T. P. and Helz G. R. (2002) Catalysis by mineral surfaces implications for Mo geochemistry in anoxic environments. Geochim. Cosmochim. Acta 66, 3679-3692. 

Sulfide-mineral oxidation by microbial populations has been postulated to proceed via direct or indirect mechanisms (Tributsch and Bennett, 1981a,b Boon and Heijnen, 2001 Fowler, 2001 Sand et al., 2001 Tributsch, 2001). In the direct mechanism, it is assumed that the action taken by the attached cell or bacterium on a metal sulfide will solubilize the mineral surface through direct enzymatic oxidation reactions. The sulfur moiety on the mineral surface is oxidized to sulfate without the production of any detectable intermediates. The indirect mechanism assumes that the cell or bacteria do not act directly on the sulfide-mineral surface, but catalyze reactions proximal to the mineral surface. The products of these bacterially catalyzed reactions act on the mineral surfaces to promote oxidation of the dissolved Fe(II) and S° that are generated via chemical oxidative processes. Ferrous iron and S°, present at the mineral surface, are biologically oxidized to Fe(III) and sulfate. Physical attachment is not required for the bacterial catalysis to occur. The resulting catalysis promotes chemical oxidation of the sulfide-mineral surface, perpetuating the sulfide oxidation process (Figure 1). 

Sparks, D. L. (2005). Metal and oxyanion sorption on naturally occurring oxide and clay mineral surfaces. In Environmental Catalysis, ed. Grassian, V. H., Taylor Francis, Boca Raton, FL, 3-36. 

However, amino acids are unlikely to form themselves into polymers without the help of some form of catalyst (Bada, 2004). Possible natural catalysts are mineral surfaces such as in the regular, repeating structure of clays although once bound to a clay the polymer has to be released. This is achieved in the laboratory with salt solutions and may, in nature, reflect an evaporative marine environment. An alternative venue is at hydrothermal vents where peptide bond formation is favored, and where catalysis may take place on sulfide mineral surfaces (Bada, 2004). Such a process has been described by Holm and Charlou (2001) who found linear saturated hydrocarbons with chain lengths of 16 to 29 carbon atoms in high-temperature hydrothermal fluids from a vent in the Mid-Atlantic Ridge. 

Catalysts played an important role in the emergence of life on Earth nearly 4 billion years ago. Catalysis by mineral surfaces and small molecules enabled the emergence of a proto-metabolic network that, in turn, enabled the emergence of the RNA world. The first macromolecular catalysts may have been ribozymes, an idea first proposed by Carl Woese that gained credence with the discovery of catalytic RNAs by Cech and Altman. Subsequently, ribozymes generated by in vitro evolution methods have been shown to catalyze a wide range of reactions involved in metabolism, including amino acid activation formation of coenzyme A (CoA), nicotinamide adenine dinucleotide (NAD), and flavin adenine dinucleotide (FAD)... 

Bell, JS, Palmer DA, Barnes HL, Drammond SE (1994) Thermal decomposition of acetate 111 Catalysis by mineral surfaces. Geochim Cosmochim Acta 58 4155-4177 Benson SW (1960) The Foundations of Chemical Kinetics. McGraw-Hill, New York Bemdt ME, Allen DE, Seyfried WE (1996) Reduction of CO2 during serpentinization of olivine at 300°C and 500 bars. Geolo 24 351-354... 

Catalysis of hydrolytic reactions may also occur by surface-bound metals. Although there has been a greater focus on the study of metal catalysis by dissolved metal ions, there is increasing evidence to suggest that catalysis by surface-bound metals may be of greater importance in environmental systems such as groundwater aquifers. Stone (1989) postulates 3 mechanisms for catalysis at the mineral-water... 

The application of IR spectroscopy to organic sorbates has been primarily limited to gas-phase a orption kinetics and gas-phase catalysis (21). The usefrilness of IR for investigating sorption/desorption processes in situ, has been demonstrated for the selectivity of conformer adsorption at mineral surfaces (22). Strong absorption of H O vibrational modes by IR radiation has been a major hindrance in the application of IR spectroscopy to study organic sorption at aqueous-mineral interfaces. Attenuated total reflectance-IR (ATR-IR) spectroscopy and the use of DjO in the aqueous phase minimizes the water absorption problem. Figure 3 details the ATR setup used in this study. Clay pastes were loaded into Teflon plaques and clamped to both sides of a vertical ATR prism. Silicon sealant around the edges of the plaques prevented water evaporation during extended data collection times (up to 2 days). The area of the D O... 

Long paired regions would have been difficult to synthesise in early biology, however. A simple, general, and processive RNA polymerase ribozyme (i.e., capable of proceeding base-by-base to replicate an entire RNA sequence) has not yet been found. Early RNAs were likely short, perhaps synthesised relatively slowly on mineral surfaces [35]. Since most known ribozymes are tens or hundreds of nucleotides long, and since most protein enzymes are composed of chains of hundreds of amino acids, it may seem that short nucleic acids are incapable of biologically relevant catalysis, beyond simple template-driven reactions. 

Barteau MA, Bowker M, Madix RJ (1981) Formation and decomposition of acetate intermediates on the Ag(llO) surface. J Catal 67 118-128 Barth T (1987) Quantitative determination of volatile carboxylic acids in formation waters by isotachophoresis. Anal Chem 59 2232-2237 Bell JLS (1991) Acetate decomposition in hydrothermal solutions. PhD Thesis, The Pennsylvania State University, University Park, 228 pp Bell JLS, Palmer DA, Barnes HL, Drummond SE (1993) Thermal decomposition of acetate. Part III. Catalysis by mineral surfaces. Geochim Cosmochim Acta (in press) Benziger JB, Madix RJ (1979) The decomposition of formic acid on Ni(lOO). Surface Sci 79 394-412... 

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