Surface acidity studies metal oxides

Bolis, V., Cerrato, G., Magnacca, G., and Morterra, C. Surface acidity of metal oxides. Combined microcalorimetric and IR-spectroscopic studies of variously dehydrated systems. Thermochim. Acta 1998, 312, 63-11. 

Some Examples of the Application of FTIR Spectroscopy to the Study of Surface Acidity in Metal Oxides... 

V. Bolis, G. Cerrato, G. Magnacca, C. Morterra, Surface acidity of metal oxides. Combined microcalorimetric and IR-spectroscopic studies of variously dehydrated systems. Ther-mochim. Acta312(1-2), 63-77 (1998). doi 10.1016/S0040-6031(97)00440-l Fubini B, Bolis V, Bailes M, Stone FS (1989) The reactivity of oxides with water vapor. Solid State Ionics 32-33, (Part 1 (0))258-272. doi 10.1016/0167-2738(89)90230-0 J.M. Newsam, M.M.J. Treacy, W.T. Koetsier, C.B.D. Gruyter, Structural characterization of zeolite beta. Proc. R. Soc. Lond. Math. Phys. Sci. 420(1859), 375 05 (1988). doi 10.1098/ rspa.1988.0131... 

Using resonant effects in core-level spectroscopic investigations of model chromophore adsorbates, such as bi-isonicotinic acid, on metal-oxide surfaces under UHV condition, even faster injection times have been tentatively proposed [85]. The injection time is observed to be comparable to the core-hole decay time of ca. 5 fs. It is also possible to resolve different injection times for different adsorbate electronic excited states with this technique. While the core-excitations themselves provide a perturbation to the system, and it cannot be ruled out that this influences the detailed interactions, the studies provide some of the first local molecular, state-specific injection time analysis with good temporal resolution in the low femtosecond regime. The results provide information about which factors determine the injection time on a molecular level. 

Acid-base characterizations of metal oxide surfaces are often used to explain their catalytic behavior. Extensive studies have been performed on the interaction of acid-base probe molecules with powders or supported metal oxides. The adsorption of NH3 at cation sites has been used to characterize the Lewis acidity of metal oxides. The systematic use of CO adsorption at room temperature as a probe for... 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Abstract The surfaces of model metal oxides offer many fundamental examples where the outcome of a specific chemical reaction might be linked to the surface structure and local electronic properties. In this work the reaction of simple molecules such as ammonia, alcohols, carboxylic and amino acids is studied on two metal oxide single crystals rutile TiO CllO) and (001) and fluorite UOj(l 11). Studies are conducted with XPS, TPD, and Plane Wave Density Functional Theory (DFT). The effect of surface structure is outlined by comparing the TiOj(llO) rutile surface to those of TiOjCOOl), while the effect of surface point defects is mainly discussed in the case of stoichiometric and substoichiometric UOjClll). 

Many examples of surface-surfactant interactions which promote self-assembly are known. Apart from gold-thiol monolayers which are formed because of the creation of the strong S—Au bond, other commonly studied monolayers include alkyltrichlorosilane layers on hydroxylated surfaces (such as SiC>2)6, fatty acids on metal oxide surfaces7 8 and alkyl phosphonate salts on zirconium9. 

Adsorption on Oxide/Hydroxide Suifrices Since the metal species at oxide or hydroxide surfaces are present in form of isolated cations (i.e. nonzerovalent species), separated from each other by 0 or OH anions, they resemble the metal centers of mononuclear metal complexes in solution more than metal atoms in a bare metal surface. Consequently, concepts from coordination chemistry of the corresponding solute complexes can be applied more readily in this case. The exposed cations and anions on oxide surfaces can be regarded as hard acids and bases, respectively. Studies of oxide single crystals under ultrahigh vacuum (UHV) conditions identified three key concepts to describe the surface chemistry of metal oxides [27] ... 

What are the factors that determine the acid-base properties of solid surfaces such as metal oxides On the basis of the discussion thus far it seems appropriate to relate the appearance of Lewis acidity and disappearance of Bronsted acidity to the increase in the degree of dehydroxylation. Indeed, the interconversion of Lewis and Bronsted acid sites has been demonstrated for some oxides, such as ZnO or supported Mo03 Cr203, or WO3, by IR studies of pyridine or ammonia adsorption [59]. But which factors determine the strength of acid sites ... 

Hagaman et al. (2012) studied interaction of benzoic acid with metal oxides using solid-state O NMR spectroscopy. Complexes formed by dry benzoic acid with mesoporous silica and nonporous titania and alumina were analyzed. Chemical reactions with silica were not observed, but the behavior of benzoic acid on silica was a function of the water content. The acid was characterized by high mobility as evidenced by a liquid-like, Lorentzian NMR resonance. Excess benzoic acid remained as the crystalline hydrogen-bonded dimer. Benzoic acid reacted with titania and alumina surfaces in equilibrium with air to form the corresponding titanium and aluminum benzoates. In both materials, the oxygen of the O-labeled acid was bound to the metal, showing the bond... 

The surface properties of metal oxides can be studied by a variety of methods. Different characterization methods can be used to give different information about the surface properties. No one method can be used to give a complete understanding of a surface, but integration of results gained from different techniques can lead to an understanding of the structure, reactivity, strengths, and amount of acidic and basic sites on the surface of metal oxides. 

It is convenient to use metal oxides as reference points to compare the surface acidity of metal fluorides as oxides have been studied very widely and much of the methodology described for fluorides below was developed from studies of high surface area oxides. Some examples of studies made using FTIR spectroscopy serve to introduce this aspect of the subject. 

In choosing a SAM system for surface engineering, there are several options. Silane monolayers on hydroxylated surfaces are an option where transparent or nonconductive systems are needed. However, trichlorosilane compounds are moisture-sensitive and polymeri2e in solution. The resulting polymers contaminate the monolayer surface, which occasionally has to be cleaned mechanically. CarboxyUc acids adsorb on metal oxide, eg, AI2O2, AgO through acid—base interactions. These are not specific therefore, it would be impossible to adsorb a carboxyUc acid selectively in the presence of, for example, a terminal phosphonic acid group. In many studies SAMs of thiolates on Au(lll) are the system of choice. 

First there are the physical chemists, chemical engineers, and surface scientists, who study mainly nonpolar hydrocarbon reactions on clean and relatively clean metals and metal oxides. These have been the traditional studies formerly driven by the petroleum industry and now driven by environmental concerns. These workers typically treat the surface as a real entity composed of active sites (usually not identified, but believed in). These investigators typically, although not always, interpret mechanisms in terms of radical reactions on metals and in terms of acid-base reactions on metal oxides. 

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