No evidence for Evans’ holes in the A, B, C vibrational structure of potassium dihydrogen arsenate

Proton-Rich POM-Type K12Mo8O20(HPO4)8(PO4)Cl with Ion-Exchange Capabilities

A proton-rich POM-type molybdenum phosphate K₁₂Mo₈O₂₀(HPO₄)₈(PO₄)Cl was successfully obtained. It crystallizes in a noncentrosymmetric tetragonal space group of P-4 (No. 81) with the unit cell parameters of a = 9.6580(4) Å, c = 14.2607(10) Å, and Z = 1. The occurrence and positions of the light element H in the structure are inferred from single-crystal X-ray diffraction and confirmed by DFT calculations. The hydrogen atoms are found to form hydroxyl bonds with O atoms from P(2)O₄ and P(3)O₄ constituting the [Mo₄P₄O₂₆H₄]^4- layers but are only weakly bound to the isolated P(3)O₄ group through hydrogen bonds. The title compound presents a POM-type framework of corrugated [Mo₄P₄O₂₆H₄]^4- layers with four K⁺ ions and mixed ions (K₄Cl³⁺ and isolated PO₄^3-) orderly imbedding in the interlayer spaces with distances of 5.0396 (1) and 5.5966 (3) Å, respectively. The proton-rich nature and the structure feature were further verified by a series of experiments including ¹H, ⁷Li, and ³¹P MAS NMR spectra, IR spectroscopy, and thermal analysis. Moreover, the weak bonding and large interlayer spaces make K⁺ and H⁺ ions susceptible to exchange with ions of Cs⁺, Ba²⁺, Zn²⁺, Pb²⁺, Cu²⁺, and Ni²⁺ commonly presented in chemical pollutants or nuclear wastes. In addition, the title compound shows a small second-harmonic generation signal, consistent with its noncentrosymmetric structure.

A Spectroscopic Paradox: The Interaction of Methanol with ZSM-5 at Room Temperature

The adsorption of methanol in HZSM-5 at low temperatures has long been regarded as an associative process involving hydrogen bonding to the acidic zeolite hydroxyl groups. Recent studies employing inelastic neutron scattering spectroscopy (INS) have reported that complete dissociation to methoxylate the zeolite occurs at 298 K, and infrared evidence for a partial dissociation at 298 K has also been described. Here we investigate the apparent contradictions between different techniques, using a combination of INS, infrared spectroscopy and solid-state NMR spectroscopy, including isotopic substitution experiments. Different possible explanations are proposed and considered; we conclude that at room temperature methanol is very largely associatively adsorbed, although the presence of some small extent (>1%) of methoxylation cannot be ruled out.

Structure and Dynamics of the Superprotonic Conductor Caesium Hydrogen Sulfate, CsHSO4

We have investigated caesium hydrogen sulfate, CsHSO₄, in all three of its ambient pressure phases by total scattering neutron diffraction, inelastic neutron scattering (INS) and Raman spectroscopies and periodic density functional theory calculations. Above 140 °C, CsHSO₄ undergoes a phase transition to a superprotonic conductor that has potential application in intermediate temperature fuel cells. Total scattering neutron diffraction data clearly show that all the existing structures of this phase are unable to describe the local structure, because they have either partial occupancies of the atoms and/or non-physical O–H distances. Knowledge of the local structure is crucial because it is this that determines the conduction mechanism. Starting from one of the previous models, we have generated a new structure that has no partial occupancies and reasonable O–H distances. After geometry optimisation, the calculated radial distribution function is in reasonable agreement with the experimental data, as are the calculated and observed INS and Raman spectra. This work is particularly notable in that we have measured INS spectra in the O–H stretch region above room temperature, which is extremely rare. The INS spectra have the enormous advantage that the electrical anharmonicity that complicates the infrared spectra is absent and the stretch modes are plainly seen.

Experimental arrangements suitable for the acquisition of inelastic neutron scattering spectra of heterogeneous catalysts

Inelastic neutron scattering (INS) is increasingly being used for the characterization of heterogeneous catalysts. As the technique is uniquely sensitive to hydrogen atoms, vibrational spectra can be obtained that emphasize a hydrogenous component or hydrogen-containing moieties adsorbed on to an inorganic support. However, due to sensitivity constraints, the technique typically requires large sample masses (∼10 g catalyst). A reaction system is hereby described that enables suitable quantities of heterogeneous catalysts to be appropriately activated and operated under steady-state conditions for extended periods of time prior to acquisition of the INS spectrum. In addition to ex situ studies, a cell is described which negates the need for a sample transfer stage between reaction testing and INS measurement. This cell can operate up to temperatures of 823 K and pressures up to 20 bar. The apparatus is also amenable to adsorption experiments at the gas-solid interface.