Trevor J. Dines

Infrared and Raman study of the adsorption of NH3, pyridine, NO and NO2 on anatase

An anatase precursor of V2O5/TiO2 catalysts contains nitrate and sulphate impurities, the former being destroyed by calcination at 673 K but surface sulphate remaining after calcination at 723 K. Two types of Lewisacidic site were distinguished by ammonia and pyridine adsorption. Slight surface Bronsted acidity was considerably enhanced by deliberate doping with additional surface sulphate. Both nitric oxide and nitrogen dioxide were adsorbed to form bridged and bidentate nitrato species, with monodentate nitrate also formed from the dioxide. Ammonia did not react with the nitrogen oxides on the anatase surface. Co-adsorption was noncompetitive, the two gases being adsorbed at different surface sites.

Raman spectroscopic study of the Pt–CeO2 interaction in the Pt/Al2O3–CeO2 catalyst

Raman spectra have been obtained from a series of Pt/Al2O3 catalysts containing from 0 to 20% CeO2, and a Pt/CeO2 catalyst, prepared by sol–gel processing. Spectra were recorded following calcination of the catalyst precursors at various temperatures up to 600 °C and compared with thermogravimetric data. The results clearly present evidence for a strong interaction between Pt and CeO2. The surface mixed oxide species persists in an oxidising environment up to 600 °C and in a reducing environment up to 170 °C.

Vibrational Spectra of α-Amino Acids in the Zwitterionic State in Aqueous Solution and the Solid State: DFT Calculations and the Influence of Hydrogen Bonding

The zwitterionic forms of the two simplest alpha-amino acids, glycine and l-alanine, in aqueous solution and the solid state have been modeled by DFT calculations. Calculations of the structures in the solid state, using PW91 or PBE functionals, are in good agreement with the reported crystal structures, and the vibrational spectra computed at the optimized geometries provide a good fit to the observed IR and Raman spectra in the solid state. DFT calculations of the structures and vibrational spectra of the zwitterions in aqueous solution at the B3-LYP/cc-pVDZ level were found to require both explicit and implicit solvation models. Explicit solvation was modeled by inclusion of five hydrogen-bonded water molecules attached to each of the five possible hydrogen-bonding sites in the zwitterion and the integration equation formalism polarizable continuum model (IEF-PCM) was employed, providing a satisfactory fit to observed IR and Raman spectra. Band assignments are reported in terms of potential-energy distributions, which differ in some respects to those previously reported for glycine and l-alanine.

Raman spectroscopic study of supported chromium(VI) oxide catalysts

Raman spectroscopy has been used to characterise Cr(VI) oxide species supported on the surfaces of SiO2, Al2O3, SiO2–Al2O3 and TiO2, as a function of Cr loading and calcination temperature. The interpretation of the spectra has been facilitated by Lorentzian band deconvolution and ab initio calculations employing the hybrid SCF-DFT method B3-LYP, in conjunction with the effective core potential LanL2DZ basis set. These calculations have enabled the elucidation of the geometries of surface chromate species by comparison with reference Cr(VI) compounds, and also detailed force constant calculations and vibrational assignments in terms of potential energy distributions. For all of the surface chromate(VI) species only Raman bands due to stretching vibrations involving CrO bonds are observed. Other chromate vibrations are predicted to be strongly mixed with vibrational coordinates of the support, giving rise to composite modes which have much lower Raman intensity.

Adsorption of 2-chloropyridine on oxides—an infrared spectroscopic study

The adsorption of 2-chloropyridine on SiO(2), TiO(2), ZrO(2), SiO(2)-Al(2)O(3) and H-mordenite has been studied by IR spectroscopy. The different modes of interaction with oxide surfaces, i.e. hydrogen-bonding and adsorption at Brønsted or Lewis acid sites, was modelled by ab initio calculations at the B3LYP/DZ+(d) level. Adsorption on SiO(2) results in hydrogen bonding to surface hydroxyl groups, whereas the spectra obtained following adsorption on TiO(2) and ZrO(2) display evidence for electron transfer at Lewis acidic surface sites. Protonation of 2-chloropyridine at Brønsted acidic sites was detected only for adsorption on SiO(2)-Al(2)O(3) and H-mordenite, indicating the presence of Brønsted acidic sites on these oxide surfaces with pK(a) values </=0.70. IR studies of 2-chloropyridine adsorption can provide useful information regarding the nature of surface acidity on oxides which is complementary to that obtained from other probe molecules such as NHs and pyridine.

Infrared study of the reaction between nitrogen oxides and ammonia on titania-supported vanadia catalysts

Infrared spectroscopy has been used to study sequential adsorption of ammonia and nitric oxide and of ammonia and nitrogen dioxide on three anatase-supported vanadia catalysts with different vanadia loadings. Facile reaction occurred between ammonium ions at Bronsted-acidic surface sites on vanadia and nitrato species formed by the adsorption of nitrogen dioxide or nitric oxide. Nitric oxide was less reactive because of depletion of surface oxygen during reaction. Pre-reduction of catalyst in hydrogen not only removed surface oxygen but also destroyed catalytically active Bronsted-acidic sites. A reaction mechanism involving an intermediate V—O—NN species is tentatively proposed.

Raman and IR spectroscopic studies of fenamates--conformational differences in polymorphs of flufenamic acid, mefenamic acid and tolfenamic acid.

Solid-state Raman and IR spectra of two polymorphic forms of each of three fenamates (flufenamic acid, mefenamic acid and tolfenamic acid) display subtle but highly reproducible differences. Many of these spectral differences can be ascribed to different conformations of these molecules, involving two of four possible orientations of one substituted benzene ring with respect to the other. Interpretation of the vibrational spectra in terms of conformational differences has been facilitated by DFT calculations at the B3LYP/cc-pVDZ level for each conformer. The calculated spectra are compared with the experimental spectra in order to identify the conformers present in two polymorphic forms in each case, and detailed band assignments are obtained from the DFT calculations.

Infrared and Raman study of the adsorption of nitrogen oxides on titania-supported vanadia catalysts

Infrared and Raman spectra are reported of vanadia–anatase catalysts with three vanadia loadings exposed to nitric oxide and nitrogen dioxide at ambient temperature. Bridging, bidentate and monodentate nitrato species were identified. The surface species were less readily formed from nitric oxide than nitrogen dioxide and a smaller variety of adsorbed products was formed on catalysts with high rather than low vanadia loading. Reduced catalysts were reoxidised by nitrogen dioxide but not nitric oxide. The latter generated mono- and di-nitrosyl vanadium complexes on adsorption. Nitrous oxide failed to give adsorbed species on the catalyst surfaces.

Surface-enhanced Raman scattering studies of rhodanines: evidence for substrate surface-induced dimerization

The surface-enhanced Raman scattering (SERS) spectra of rhodanine adsorbed on silver nanoparticles have been examined using 514.5 and 632.8 nm excitation. There is evidence that, under the experimental conditions used, rhodanine undergoes a nanoparticle surface-induced reaction resulting in the formation of a dimeric species via the active methylene group in a process which is analogous to the Knoevenagel reaction. The experimental observations are supported by DFT calculations at the B3-LYP/cc-pVDZ level. Calculated energies for the interaction of the E and Z isomers of the dimers of rhodanine with silver nanoparticles support a model in which the (intra-molecular hydrogen bonded) E isomer dimer is of lower energy than the Z isomer. A strong band, at 1566 cm(-1), in the SERS spectrum of rhodanine is assigned to the nu(C double bond C) mode of the dimer species.

The surface acidity of oxides probed by IR spectroscopy of adsorbed diazines

The IR spectra of the three diazines, pyrazine, pyrimidine and pyridazine have been measured following adsorption on, and subsequent desorption from, a variety of oxides: SiO2, TiO2, ZrO2, SiO2–Al2O3 and H-mordenite. Three modes of adsorption have been observed, (i) hydrogen bonding to surface hydroxy groups, (ii) electron transfer at Lewis acidic surface sites, and (iii) proton transfer at Bronsted acidic surface sites. In the latter case only monoprotonation was detected for SiO2–Al2O3 and H-mordenite, indicating the presence of Bronsted acidic sites with pKa values in the range 0.65 to 2.24. There was no evidence for diprotonation, which would indicate the presence of acidic sites with pKa values below 5.75. These results establish that the adsorption of diazines can provide useful information regarding the nature of surface acidity on oxides which is complementary to that obtained from other probe molecules such as NH3 and pyridine.