Adsorption on oxides

In addition to the collector, polyvalent ions may show sufficiently strong adsorption on oxide, sulfide, and other minerals to act as potential-determining ions (see Ref. 98). Judicious addition of various salts, then, as well as pH control, can permit a considerable amount of selectivity. 

Some aspects of adsorption on oxides and other semiconductors can be treated in terms of the electrical properties of the solid, and these are reviewed briefly here. More details can be found in Refs. 84 and 182. 

There is much room for further study of various importaut categories of materials oue promiueut example is oxides aud other compouuds (carbides, nitrides,. . . ) another is all types of adsorption on oxides and other compounds. 

Early studies on oxide films stripped from iron showed the presence of chromium after inhibition in chromate solutionand of crystals of ferric phosphate after inhibition in phosphate solutions. More recently, radio-tracer studies using labelled anions have provided more detailed information on the uptake of anions. These measurements of irreversible uptake have shown that some inhibitive anions, e.g. chromateand phosphate are taken up to a considerable extent on the oxide film. However, other equally effective inhibitive anions, e.g. benzoate" pertechnetate and azelate , are taken up to a comparatively small extent. Anions may be adsorbed on the oxide surface by interactions similar to those described above in connection with adsorption on oxide-free metal surfaces. On the oxide surface there is the additional possibility that the adsorbed anions may undergo a process of ion exchange whereby... 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

Temperature dependence of cadmium adsorption on oxides. 1. Experimental observations and model analysis. J. Coll. Int. Sd. 135 118-131... 

Spark, KM. Johnson, B.B. Wells, J.D. (1995) Characterizing heavy-metal adsorption on oxides and oxyhydroxides. Eur. J. Soil Sd. 46 621-631... 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

The summary of the single-crystal results is followed by the presentation and illustration (with permission from the authors and publishers) of significant results from the literature on finely divided metals, mostly infrared spectra from adsorption on oxide-supported metal catalysts. Particular emphasis is given to results obtained at room temperature or below, where the structures present are likely to be the best-defined. 

Acetic acid chemisorption has been previously studied using IETS by Lewis, Mosesman and Weinberg for oxide covered aluminum surfaces. Using reflection IR Tompkins and Allara have reported spectra for adsorption on oxidized copper and Hebard, Arthur and Allara S for adsorption on oxidized indium. All these studies demonstrate that chemisorption from the gas phase involves proton dissociation since the observed spectra are those of acetate ion species. 

In contrast, recent work (4-12) has shown that Raman spectroscopy can be used to study Ti) adsorption on oxides, oxide supported metals and on bulk metals [including an unusual effect sometimes termed "enhanced Raman scattering" wherein signals of the order of 10 - 106 more intense than anticipated have been reported for certain molecules adsorbed on silver], (ii) catalytic processes on zeolites, and (iii) the surface properties of supported molybdenum oxide desulfurization catalysts. Further, the technique is unique in its ability to obtain vibrational data for adsorbed species at the water-solid interface. It is to these topics that we will turn our attention. We will mainly confine our discussion to work since 1977 (including unpublished work from our laboratory) because two early reviews (13,14) have covered work before 1974 and two short recent reviews have discussed work up to 1977 (15,16). 

The use of centrifugation to separate the liquid from solid phases in traditional batch or tube techniques has several disadvantages. Centrifugation could create electrokinetic effects close to soil constituent surfaces that would alter the ion distribution (van Olphen, 1977). Additionally, unless filtration is used, centrifugation may require up to 5 min to separate the solid from the liquid phases. Many reactions on soil constituents are complete by this time or less (Harter and Lehmann, 1983 Jardine and Sparks, 1984 Sparks, 1985). For example, many ion exchange reactions on organic matter and clay minerals are complete after a few minutes, or even seconds (Sparks, 1986). Moreover, some reactions involving metal adsorption on oxides are too rapid to be observed with any batch or, for that matter, flow technique. For these reactions, one must employ one of the rapid kinetic techniques discussed in Chapter 4. 

Frank, M. and Baumer, M. (2000) From atoms to crystallites adsorption on oxide-supported metal particles. Phys. Chem. Chem. Phys., 2, 3723. 

Modification of Vibrational Spectra of Diatomic Molecules Induced by the Adsorption on Oxide and Halide surfaces A method for Probing the Structures of the Adsorption Sites and the Surface Morphologies of Sintered Materials... 

As fax as we know, only the bis-ethylenediamine (or 1,2-diaminoethane) Aum cation, [Au(en)2]3+, has been used to prepare gold catalysts by cation exchange of zeolites, or cation adsorption on oxide supports synthesis of the complex with chloride counter-ions is easy.98... 

The acid strength of the catalyst was probed by CO adsorption and IR spectroscopy.23 Low-temperature CO adsorption on oxidized WZ indicates the presence of both Lewis and Bronsted acid sites.20,21 Lewis acidity is attributed to coordinatively unsaturated Zr4+ sites, present both on pure Zr02 and on WZ. The acid strengths of these centers are enhanced by the presence of polytungstate species.20... 

L. G. J. Fokkink, Ion Adsorption on Oxides, PhD Thesis, Agricultural University Wageningen, 1987. 

Henrich, V. E. (1979). Ultraviolet photoemission studies of molecular adsorption on oxide surfaces. Prog. Surf. Sci. 9, 143-64. 

We finish with a brief look at surface adsorption on oxides of catalytic importance. Among the most challenging problems are those in which redox activity of an oxide is required. 

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