Gary R. Burns

Proton insertion and lithium-proton exchange in spinel lithium manganates

Abstract The protonic species formed in spinel λ-MnO 2 by proton exchange for lithium ions in Li 1+ x Mn 2− x O 4 precursors have been characterised by infrared and inelastic neutron scattering spectroscopies. Proton diffusion into the lattice appears to depend on the presence of octahedral manganese vacancies, with formation of lattice hydroxyl co-existing with water. Mechanisms of proton transfer are considered in relation to lithium-proton exchange during reinsertion of lithium into the materials, and for insertion of additional protons by chemical reduction.

An infrared study of H2O and D2O on HZSM-5 and DZSM-5

Abstract Distinct differences are observed in the FTi.r. spectra of HZSM-5 + H2O, HZSM-5 + D2O, DZSM-5 + H2O, and DZSM-5 + D2O when water is sorbed in a ratio of one molecule of water per zeolite Al, or less, at 80°C. A peak at 3700 cm−1 and broad bands with maxima at ∼ 2900 and ∼ 2470 cm−1 were the same for HZSM-5 with either H2O or D2O sorbed and have been assigned to the spectrum of the zeolite Bronsted hydroxyl. A broad band at ∼ 3600 cm−1 was observed for sorbed H2O on HZSM-5 and DZSM-5, but not for D2O. These differences imply that hydrogen exchange between the zeolite Bronsted hydroxyls and sorbed water is slow under the conditions used here. To explain these observations, it is proposed that each sorbed water molecule is both partially protonated and is interacting with the zeolite lattice.

Interaction of water with the zeolite HY, studied by FT i.r.

The interaction of water with the Bronsted acid sites of zeolite HY at 25 and 80 °C, and subsequent decomposition of HY upon thermal desorption of the water, has been studied by FT i.r. If less than one molecule of water was sorbed per Al atom in the zeolite, the acid sites were fully restored after water desorption. When the amount of sorbed water was increased, permanent loss of Bronsted acidity occurred on water desorption, along with collapse of the zeolite framework. Upon water sorption, the formation of new i.r. hydroxyl peaks was observed at 3710, 3675, and 3615 cm−1. These may be due to partial removal of Al from the lattice to form AlOH species, accompanied by the formation of internal silanol groups.

Fourier-transform infrared study of pyridine sorbed on zeolite HY

Adsorption of pyridine on zeolites can show three modes of sorption: physisorption, sorption on the LF hydroxyls and sorption on the HF hydroxyls. In this study a separate IR spectrum was obtained for each of the three sorbed pyridine species and major differences were observed, especially in the ring-bending and stretching region (2000 to 1200 cm–1). A comparison of the spectra showed that pyridine sorbed on the LF hydroxyls is not physisorbed, but protonated to some degree. H/D exchange enabled the IR spectrum of the bonded Bronsted proton to be obtained for pyridine sorbed on the HF hydroxyl sites which proved to be a broad band from ca. 3600 to ca. 2600 cm–1. For pyridine sorbed on the LF hydroxyls, a broader more complex band was observed from ca. 3700 to ca. 2200 cm–1. Reversible sorption/desorption occurred for physisorbed pyridine and pyridine sorbed on the LF hydroxyls, but complete desorption of pyridine from the HF hydroxyls resulted in loss of acid sites.

Birnessite-type manganese oxide–alkylamine mesophases obtained by intercalation and their thermal behaviour

Layered birnessite-type manganese oxide inserts alkylamines (decylamine–octadecylamine) by ion-exchange and intercalation mechanisms and the concomitant expansion is compatible with formation of a head-to-tail layer of interdigitated amine. The structural arrangement of the MnO2–alkylamine mesophases is sensitive to temperature, and at a temperature dependent on the chain length of the intercalated alkylamine, undergoes a phase transition to a more highly expanded phase. The basal spacing of the more expanded phases formed at higher temperature indicates that de-interdigitation occurs to give a bilayer organisation of neutral and protonated alkylamine species. The phase transition temperature is identified by differential scanning calorimetry. X-Ray absorption spectroscopy shows that while the local structure around manganese is not affected by intercalation, it provides evidence that this transition is accompanied by partial lowering of the oxidation state of manganese and migration of manganese into the interlayer region. This phase transition is reminiscent of some of those observed in phospholipid membranes, but seems to represent a new phenomenon for inorganic intercalates.

A Raman and resonance Raman study of polybromide anions and a study of the temperature dependence of the Raman-active phonons of tetrabutylaminonium tribromide

Abstract Raman and resonance spectroscopy are shown to be capable of characterizing the tribromide and pentabromide anions and interstitial dibromine. The anions have Raman-active phonons which display a broad maximum in their excitation profiles at 600 nm, whereas interstitial bromine has a maximum in its excitation profile shifted towards the blue. Large shifts in band wavenumber are observed for the symmetric and asymmetric bromine stretching vibrations in tribromide salts with different cations. The bromine stretch force constants for the tribromide anions, for dibromine and for the recently reported decabromide dianion, have been shown to obey the relationship, k=0.01r−6.7. The Raman-active phonons of tetrabutylammonium tribromide have been studied as a function of temperature, from 10 to 300 K. No first order phase changes were observed and remarkably small coefficients (∂ν/∂T)P, were measured for both the internal and the external modes.

Temperature dependence of the Raman spectrum of tetraphosphorus triselenide

Abstract The Raman spectrum of P 4 Se 3 has been measured as a function of temperature in the range 20–370 K, at ambient pressure. The internal and external first-order Raman-active phonons vary smoothly in wave number over the temperature interval 20–350 K, but the crystalline-plastic phase change at 355 K. results in the loss of the external mode manifold from the 370 K Raman spectrum, leaving only a wing to the Raleigh-scattered component. The temperature coefficients of most external phonons are larger than those of the energetically well-separated internal phonons. The lowest wave number rigid layer phonon displays soft behaviour (d ṽ /d T positive) whereas the remaining members of the external mode manifold have negative temperature coefficients which increase in magnitude approximately as the phonon wave number increases. Some of the internal mode Davydov components show near-zero or positive temperature coefficients, a behaviour similar to that observed for other chalcogenide semiconductors.

The use of Raman spectroscopy as a probe for the intermolecular bonding in crystalline tetraphosphorus trisulphide

Abstract The temperature and pressure dependences of the internal and external first-order Raman-active phonons of crystalline α-P 4 S 3 are shown to provide a sensitive probe of the intermolecular bonding. Comparison with a similar study of α-P 4 Se 3 leads to the conclusion that while intra-intermolecular coupling is present, it is weaker in crystalline α-P 4 S 3 . Unlike α-P 4 Se 3 , no rigid layer mode is observed nor is there any softening of the A 1 mode at 440 cm −1 analogous to the 361 cm −1 soft mode of α-P 4 Se 3 . Weak intra-intermolecular coupling is also supported by an analysis of the implicit-explicit contributions to the observed temperature coefficients. A pressure dependent increase in the intensity of the A 1 mode at 440 cm −1 is compared to similar pressure dependent changes in intensity for phonons in other inorganic molecular solids.

A 6Li and 7Li MAS NMR study of the spinel-type manganese oxide LiMn2O4 and the rock salt-type manganese oxide Li2MnO3

6 Li and 7Li MAS NMR spectroscopy is used to identify distinct lithium sites in two paramagnetic manganese oxides: spinel-type LiMn2O4 and rock salt-type Li2MnO3.

Pressure dependence of the Raman spectrum of crystalline tetraphosphorus triselenide

Abstract The Raman spectrum of crystalline α-P 4 Se 3 has been studied as a function of pressure up to 20.2 kbar at room temperature. Similarities between α-P 4 Se 3 and the polymorphs of elemental selenium are revealed in that internal phonons involving selenium-based coordinates show negative values of d v dP . This mode softening is attributed to strong intra-intermolecular coupling arising from the layer lattice structure established for crystalline α-P 4 Se 3 by an X-ray crystallographic study.