Thermal perturbation of NMR properties in small polar and non-polar molecules

A fast ceramic mixed OH-/H+ ionic conductor for low temperature fuel cells

Low temperature ionic conducting materials such as OH- and H+ ionic conductors are important electrolytes for electrochemical devices. Here we show the discovery of mixed OH-/H+ conduction in ceramic materials. SrZr0.8Y0.2O3-δ exhibits a high ionic conductivity of approximately 0.01 S cm-1 at 90 °C in both water and wet air, which has been demonstrated by direct ammonia fuel cells. Neutron diffraction confirms the presence of OD bonds in the lattice of deuterated SrZr0.8Y0.2O3-δ. The OH- ionic conduction of CaZr0.8Y0.2O3-δ in water was demonstrated by electrolysis of both H218O and D2O. The ionic conductivity of CaZr0.8Y0.2O3-δ in 6 M KOH solution is around 0.1 S cm-1 at 90 °C, 100 times higher than that in pure water, indicating increased OH- ionic conductivity with a higher concentration of feed OH- ions. Density functional theory calculations suggest the diffusion of OH- ions relies on oxygen vacancies and temporarily formed hydrogen bonds. This opens a window to discovering new ceramic ionic conducting materials for near ambient temperature fuel cells, electrolysers and other electrochemical devices.

Elucidation of the Roles of Water on the Reactivity of Surface Intermediates in Carboxylic Acid Ketonization on TiO2

The effects of water on the carboxylic acid ketonization reaction over solid Lewis-acid catalysts were examined by nuclear magnetic resonance (NMR) spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature-programmed desorption (TPD), and kinetic measurements. Acetic acid and propanoic acid were used as model compounds, and P25 TiO₂ was used as a model catalyst to represent the anatase TiO₂ since the rutile phase only contributes to <2.5% of the overall ketonization activity of P25 TiO₂. The kinetic measurement showed that introducing H₂O vapor in gaseous feed decreases the ketonization reaction rate by increasing the intrinsic activation barrier of gas-phase acetic acid on anatase TiO₂. Quantitative TPD of acetic acid indicated that H₂O does not compete with acetic acid for Lewis sites. Instead, as indicated by combined approaches of NMR and DRIFTS, H₂O associates with the adsorbed acetate or acetic acid intermediates on the catalyst surface and alters their reactivities for the ketonization reaction. There are multiple species present on the anatase TiO₂ surface upon carboxylic acid adsorption, including molecular carboxylic acid, monodentate carboxylate, and chelating/bridging bidentate carboxylates. The presence of H₂O vapor increases the coverage of the less reactive bridging bidentate carboxylate associated with adsorbed H₂O, leading to lower ketonization activity on hydrated anatase TiO₂. Surface hydroxyl groups, which are consumed by interaction with carboxylic acid upon the formation of surface acetate species, do not impact the ketonization reaction.

Low-temperature (< 200 °C) degradation of electronic nicotine delivery system liquids generates toxic aldehydes

Electronic cigarette usage has spiked in popularity over recent years. The enhanced prevalence has consequently resulted in new health concerns associated with the use of these devices. Degradation of the liquids used in vaping have been identified as a concern due to the presence of toxic compounds such as aldehydes in the aerosols. Typically, such thermochemical conversions are reported to occur between 300 and 400 °C. Herein, the low-temperature thermal degradation of propylene glycol and glycerol constituents of e-cigarette vapors are explored for the first time by natural abundance ¹³C NMR and ¹H NMR, enabling in situ detection of intact molecules from decomposition. The results demonstrate that the degradation of electronic nicotine delivery system (ENDS) liquids is strongly reliant upon the oxygen availability, both in the presence and absence of a material surface. When oxygen is available, propylene glycol and glycerol readily decompose at temperatures between 133 and 175 °C over an extended time period. Among the generated chemical species, formic and acrylic acids are observed which can negatively affect the kidneys and lungs of those who inhale the toxin during ENDS vapor inhalation. Further, the formation of hemi- and formal acetals is noted from both glycerol and propylene glycol, signifying the generation of both formaldehyde and acetaldehyde, highly toxic compounds, which, as a biocide, can lead to numerous health ailments. The results also reveal a retardation in decomposition rate when material surfaces are prevalent with no directly observed unique surface spectator or intermediate species as well as potentially slower conversions in mixtures of the two components. The generation of toxic species in ENDS liquids at low temperatures highlights the dangers of low-temperature ENDS use.

High-Temperature and High-Pressure In situ Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy represents an important technique to understand the structure and bonding environments of molecules. There exists a drive to characterize materials under conditions relevant to the chemical process of interest. To address this, in situ high-temperature, high-pressure MAS NMR methods have been developed to enable the observation of chemical interactions over a range of pressures (vacuum to several hundred bar) and temperatures (well below 0 °C to 250 °C). Further, the chemical identity of the samples can be comprised of solids, liquids, and gases or mixtures of the three. The method incorporates all-zirconia NMR rotors (sample holder for MAS NMR) which can be sealed using a threaded cap to compress an O-ring. This rotor exhibits great chemical resistance, temperature compatibility, low NMR background, and can withstand high pressures. These combined factors enable it to be utilized in a wide range of system combinations, which in turn permit its use in diverse fields as carbon sequestration, catalysis, material science, geochemistry, and biology. The flexibility of this technique makes it an attractive option for scientists from numerous disciplines.