E. Garrone

An experimental and theoretical evaluation of surface states in MgO and other alkaline earth oxides

Abstract Ultra-violet (u.v.) diffuse reflectance spectra of microcrystalline MgO, CaO, SrO and BaO reveal bands at energies below those of bulk excitonic transitions. Correlation of the spectra permit three distinctive absorptions (I, II, III) to be identified, each with similar characteristics in the respective oxides. The absorptions are ascribed to surface states, and the transitions have the characteristics of excitons. Excitons II and III obey the Mollwo–Ivey relation typical of bound excitons, but excition I with the highest energy of transition behaves similarly to a free exciton of the bulk. The four oxides are isostructural and ionic. The Levine–Mark theory of the surface states of ionic insulators, not hitherto tested for oxides, is examined. The numerical values of the parameters appropriate to the oxides are evaluated by using experimental data on bulk excitons. It is shown that the theory accounts satisfactorily for the energies of exciton I in MgO, CaO and SrO on the basis that it is an exci...

Reactions of silica strained rings: an experimental and ab-initio study

Abstract Ab-initio techniques inclusive of electron correlation (accounted for through both the perturbative MP2 method and the local and gradient-corrected density functional methods) are used in conjunction with IR spectroscopy of surface species to study the reaction of a few molecules with the strained four-atom rings present at the surface of amorphous silica outgassed at high temperature, mimicked in the calculations by cyclodisiloxane. Slightly acidic molecules like water are known to react under mild conditions; attempts have been made to activate N 2 , CO 2 , H 2 and H 2 CO in a microwave discharge. Only with H 2 reaction has been observed. In full agreement with the experiment, calculated standard free enthalpies of reaction and activation energies show that reaction is: (i) facile with water; (ii) possible though somewhat activated with hydrogen; (iii) thermodynamically not allowed with carbon dioxide; (iv) by no means possible with molecular nitrogen.

22-P-21-Spectroscopic properties of dye-loaded mesoporous silicas of the structural type MCM-41

Publisher Summary This chapter discusses the spectroscopic properties of dye-loaded mesoporous silicas of the structural type MCM-41. Congo Red and Curcumin, the two pH indicators, have been incorporated in MCM-41 precursors, which have been characterised by means of X-ray diffraction, ultraviolet (UV)-Visible, and Fourier transform infrared spectroscopy (FTIR) spectroscopy. Dyes are located in the micellar phase of the silica-surfactant mesophase, and their spectroscopic properties confirm that they are in a solvated state, where both surfactant and silica wall can act as a solvent. Dyes maintain their pH indicator properties and are accessible to gases, such as hydrogen chloride (HCl) and ammonia (NH 3 ).

29-P-31-Comparison, of 3-aminopropylsilane linked to MCM-41 and HMS type silics synthesised under biphasic and monophasic conditions

Publisher Summary This chapter presents a comparison of 3-aminopropyisilane linked to MCM-41- and HMS-type silicas synthesised under biphasic and monophasic conditions. Functionalized mesoporous silicas prepared by either grafting by silylation of MCM-41 or by self-assembly co-condensation of organically functionalized silica precursors and silica precursor (RO) 4 Si in presence of nonionic surfactants feature different properties. Materials are characterized by N 2 sorption isotherms, Fourier transform infrared (FTIR), and magic angle spinning nuclear magnetic resonance (MAS-NMR). Imine formation during contacting the different samples with probes, such as acetone and benzaldehyde, demonstrate different environments of the anchored amine groups.

Isomerization of K(CO)+ and K(OC)+ carbonyls formed upon CO adsorption on the faujasite-type K-Y zeolite

Abstract When adsorbed on the faujasite-type K-Y zeolite, carbon monoxide forms cationic C-bonded K(CO)+ carbonyls and also O-bonded K(OC)+ isocarbonyls. The carbonyl species gives a characteristic C-O stretching band at 2158 cm−1, while that of the isocarbonyl appears at 2124 cm−1. By means of variable-temperature infrared spectroscopy, these K(CO)+ and K(OC)+ species were found to be in a temperature-dependent isomerization equilibrium, where the relative proportion of the isocarbonyl increases as temperature is raised. The corresponding isomerization enthalpy resulted to be 1.6(±0.5) kJ mol−1. Comparison with data available for other alkali-metal-exchanged zeolites shows that, for most cases, this thermodynamic parameter is an approximately linear function of the electrostatic field at the cation site. Lithium-exchanged zeolites, however, do not seem to conform to this rule; presumably because of the strong polarizing power of the Li+ ion.

Acidity of ITQ-2 zeolite as studied by FT-IR spectroscopy of adsorbed molecules in comparison with that of MCM-22

Publisher Summary This chapter presents the acidity of ITQ-2 zeolite as studied by the Fourier transform infrared (FT-IR) spectroscopy of adsorbed molecules in comparison with that of MCM-22. ITQ-2, a novel zeolitic structure prepared by swelling and delaminating a MWW precursor, has been studied by IR spectroscopy. The same precursor yields, when calcined, the zeolite MCM-22. BrOnsted acidity has been measured as the propensity either to engage in H-bonds or to transfer the proton to unsaturated hydrocarbons. Comparison with MCM-22 shows that dealumination accompanies the process of delamination, but no appreciable change in residual BrOnsted acidity takes place. Reaction of propene with BrOnsted sites to branched oligomers occurs mainly on the external surface. Oligomers show no tendency to evolve to allylic cationic species, in contrast with MCM-22.

Possible formation of Cu+2(CO)2(H2O)n complexes in a ZSM-5 zeolite prepared by direct synthesis: evidence for the occurrence of Cu+-Cu+ pairs?

Publisher Summary This chapter discusses the possible formation of Cu + 2 (CO) 2 (H 2 O) n complexes in a ZSM-5 zeolite prepared by direct synthesis. Copper (Cu)-ZSM-5 zeolite prepared by direct synthesis through a methylamine route shows Cu + species as defined as those present in samples prepared by cuprous chloride (CuCl) vapour exchange. The infrared (IR) study of the reversible interaction of water with presorbed carbon monoxide (CO) shows a variety of bands, the most plausible interpretation of which is the formation of species (H 2 O) n COCu + (OH)Cu + CO(H 2 O) m , which suggests the occurrence of Cu + species in pairs.