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Considering climate change and the environmental pollution crisis, CO2-emission-free H2 production based on green, low-energy processes is critically important. In this article, we report the accidental discovery and subsequent investigation of room-temperature thermochemical water splitting in a planetary ball mill. H2 yields of ∼20–1600% were obtained by milling six metals (Al, Zn, Fe, Ti, Mn, and Sn) and their oxides in water at 30–38 °C. Remarkably, when Ti was milled, H2 was generated continuously until the water was consumed. Experimental and theoretical investigations—based on redox potentials, Gibbs energies, collision energies, and local temperatures and pressures—revealed that the milling medium acts as a mechano-cocatalyst, regenerating the catalyst (Ti and/or titanium oxides). Thus, thermochemical water splitting, via a cyclic mechanocatalytic TiO2/Ti–water reaction, was responsible for the continuous H2 production. The high H2-production rate was attributed to reactions occurring in supercritical water at impact sites between balls, where the instantaneous local pressure and temperature were 11 GPa and 1300 °C, respectively. The production of high-purity hydrogen (>99%) using seawater feedstocks in a compact (∼50 cm), low-power (∼0.26 kW) reactor indicates that on-site, on-demand H2 production is achievable. [ABSTRACT FROM AUTHOR]

The time evolution of the density fluctuation of molecules inhomogenously dispersing in a mesoscopic volume is investigated by dynamic light scattering in several fluids in supercritical states. The obtained time correlation functions were analyzed on the basis of the Kawasaki theory and the Landau-Placzek theory and the theoretical relation between the two studies is well accomplished.

Carbonyl compounds are solutes that are highly soluble in supercritical CO2 (scCO2). Their solubility governs the efficiency of chemical reactions, and is significantly increased by changing a chromophore. To effectively use scCO2 as solvent, it is crucial to understand the high solubility of carbonyl compounds, the solvation structure, and the solute-solvent intermolecular interactions. We report Raman spectroscopic data, for three prototypical ketones dissolved in scCO2, and four theoretical analyses. The vibrational Raman spectra of the C=O stretching modes of ketones (acetone, acetophenone, and benzophenone) were measured in scCO2 along the reduced temperature Tr = T/Tc = 1.02 isotherm as a function of the reduced density ρr = ρ/ρc in the range 0.05-1.5. The peak frequencies of the C=O stretching modes shifted toward lower energies as the fluid density increased. The density dependence was analyzed by using perturbed hard-sphere theory, and the shift was decomposed into attractive and repulsive energy components. The attractive energy between the ketones and CO2 was up to nine times higher than the repulsive energy, and its magnitude increased in the following order: acetone < acetophenone < benzophenone. The Mulliken charges of the three solutes and CO2 molecules obtained by using quantum chemistry calculations described the order of the magnitude of the attractive energy and optimized the relative configuration between each solute and CO2. According to theoretical calculations for the dispersion energy, the dipole-induced-dipole interaction energy, and the frequency shift due to their interactions, the experimentally determined attractive energy differences in the three solutes were attributed to the dispersion energies that depended on a chromophore attached to the carbonyl groups. It was found that the major intermolecular interaction with the attractive shift varied from dipole-induced dipole to dispersion depending on the chromophore in the ketones in scCO2. As the common conclusion for the Raman spectral measurements and the four theoretical calculations, solute polarizability, modified by the chromophore, was at the core of the solute-solvent interactions of the ketones in scCO2. [ABSTRACT FROM AUTHOR]

Vibrational Raman spectra of C=C stretching modes of ethylene derivates (cis-C2H2Cl2, cis-stilbene, and trans-stilbene) were measured in supercritical fluids along an isotherm as functions of their densities. The substitution effect of the Raman shift is so significant that a difference among three solutes can be 20 times and is observed similarly in dipolar (CHF3) and non-dipolar (CO2) fluids. In particular, the shifts of trans-stilbene were enormously large among all systems for studies of vibrational spectroscopies of supercritical fluids and were equivalent to those of typical hydrogen-bonded fluids. Such large shifts arising from the significant attractive energy between solute and solvent molecules were attributed to a site-selective solvation around a phenyl group, which was driven by a dispersion force in the absence of steric hindrance. We found that the absence of steric hindrance causes the significant local density augmentation. To the best of our knowledge, Raman experiments and their theoretical analysis are the first ones quantifying how the difference of steric hindrance produces solvation structures in solution as well as supercritical solutions. [ABSTRACT FROM AUTHOR]

The short-range structure of supercritical methanol (CH3OH) is investigated by measuring the spontaneous Raman spectra of the C–O stretching mode. The spectra are obtained at a reduced temperature, Tr=T/Tc=1.02 (522.9 K), which permits the neat fluid to be studied isothermally as a function of density. As the density increases, the spectral peaks shift toward the lower energy side and the spectra broaden. In the supercritical region, the amount of shifting shows nonlinear density dependence and the width becomes anomalously large. We use the perturbed hard-sphere model to analyze these density dependencies along the vibrational coordinate. The amount of shifting is decomposed into attractive and repulsive components, and the changes in attractive and repulsive energies are evaluated as functions of density and packing fraction, both of which are continuously varied by a factor of 120. Here we show that the shift amount consists principally of the attractive component at all densities, since the attractive energy is about eight times the repulsive energy. The density dependence of the widths is analyzed by calculating homogeneous and inhomogeneous widths as a function of density. The results show that, although vibrational dephasing and density inhomogeneity contribute similarly to the width at low and middle densities, at high density the main contributor turns out to be the vibrational dephasing. We estimate the local density enhancements of supercritical CH3OH as function of bulk density by two methods. The results of these analyses show common features, and both the estimated local density enhancements of CH3OH are considerably larger than the local density enhancements of simple fluids, i.e., those having nonhydrogen bonding. It is revealed that the local density of supercritical CH3OH is 40%–60% greater than the local densities of the simple fluids. We also estimate the local density fluctuation using the obtained values of attractive shift, inhomogeneous width, and local density. The density fluctuation in the vicinity of a vibrating molecule is compared to the fluctuation of bulk density, which is obtained from the thermodynamic calculation. [ABSTRACT FROM AUTHOR]

Time dependence of density fluctuation for neat supercritical ethylene (C[sub 2]H[sub 4]) is investigated by dynamic light scattering at various densities in isothermal conditions of reduced temperatures T[sub r] = T/T[sub c]= 1.02, 1.04, and 1.06. Time correlations of the density fluctuation for all thermodynamic states decay as single exponential functions with the time constant of submicrosecond. Critical slowing down of diffusive motions is observed in the time domain, and the correlation time of the density fluctuation becomes maximum at the extension of the gas-liquid coexistence curve on the P-T phase diagram. It is revealed that the time dependence of the density fluctuation just corresponds to the magnitude of static density fluctuation obtained by small-angle x-ray scattering measurements. By measuring correlation times as a function of scattering angle, the critical slowing down is thermodynamically discussed. It is elucidated experimentally that the critical slowing down of diffusive motions considerably correlates to the increase of specific heat capacity, the decrease of the thermal diffusivity, and rotational relaxation time in the supercritical state of the neat fluid. [ABSTRACT FROM AUTHOR]