Kinetics and Structural Changes in CO₂ Capture of K₂CO₃ under a Moist Condition

The capacity and kinetics of CO₂ capture of K₂CO₃ were studied to determine the mechanism for CO₂ sequestration under ambient conditions. Bicarbonate formation of K₂CO₃ was examined by thermogravimetric analysis under various CO₂ concentrations in the presence of water vapor, and the accompanying structural changes of K₂CO₃ were demonstrated by X-ray diffraction (XRD). Morphological variations were observed during the reaction in the presence of different CO₂ concentrations through scanning electron microscopy (SEM). Structural changes and morphological variations, which occurred during the course of the reaction, were then connected to the kinetic and exothermic properties of the CO₂ capture process from XRD and SEM measurements. The XRD results showed that the bicarbonate formation process of K₂CO₃ could be divided into three reactions, such as the formation of K₂CO₃·1.5H₂O from K₂CO₃, the subsequent formation of K₄H₂(CO₃)₃·1.5H₂O from K₂CO₃·1.5H₂O, and the slow formation of KHCO₃ from K₄H₂(CO₃)₃·1.5H₂O. The SEM observations showed that the morphology of the particles at all three stages played a crucial role in the kinetic behavior for CO₂ sorptivity of K₂CO₃. CO₂ capture of K₂CO₃ was inhibited under a concentrated CO₂ atmosphere during the initial stage, consisting of the first and second reactions, but the formation of KHCO₃ from K₄H₂(CO₃)₃·1.5H₂O was thermodynamically favorable upon the increase of the CO₂ concentration

Ti³⁺ Aqueous Solution: Hybridization and Electronic Relaxation Probed by State-Dependent Electron Spectroscopy

The electronic structure of a Ti³⁺ aqueous solution is studied by liquid-jet soft X-ray photoelectron (PE) spectroscopy. Measured valence and Ti 2p core-level binding energies, together with the Ti 2p resonant photoelectron (RPE) spectra and the derived partial electron-yield L-edge X-ray absorption (PEY-XA) spectra, reveal mixing between metal 3d and water orbitals. Specifically, ligand states with metal character are identified through the enhancement of signal intensities in the RPE spectra. An observed satellite 3d peak structure is assigned to several different metal–ligand states. Experimental energies and the delocalized nature of the respective orbitals are supported by ground-state electronic structure calculations. We also show that by choice of the detected Auger-electron-decay channel, from which different PEY-XA spectra are obtained, the experimental sensitivity to the interactions of the metal 3d electrons with the solvent can be varied. The effect of such a state-dependent electronic relaxation on the shape of the PEY-XA spectra is discussed in terms of different degrees of electron delocalization

Ultrafast Proton and Electron Dynamics in Core-Ionized Hydrated Hydrogen Peroxide: Photoemission Measurements with Isotopically Substituted Hydrogen Peroxide

Auger-electron spectroscopy is applied to hydrogen peroxide aqueous solution to identify ultrafast electronic relaxation processes, specifically those involving a proton transfer between core-ionized hydrogen peroxide and solvating water molecules (proton transfer mediated-charge separation, PTM-CS). Such processes yield dications where the two positive charges resulting from the Auger decay are delocalized over the two molecules. These species contribute to the high-energy tail of the Auger-electron spectrum as do also species resulting from charge delocalization in the ground-state geometry. However, the immediate and secondary transient species are different for ground-state and proton-transferred structures. Here we show that it is possible to experimentally distinguish the species by studying the H₂O₂/D₂O₂ isotope effect on the Auger spectra. To interpret the measured Auger-electron spectra, we complement the experiment with ab initio based dynamical calculations

Origin of Dark-Channel X-ray Fluorescence from Transition-Metal Ions in Water

The nonradiative dark channels in the L-edge fluorescence spectra from transition-metal aqueous solution identify the ultrafast charge-transfer processes playing an important role in many biological and chemical systems. Yet, the exact origin of such spectral dips with respect to the X-ray transmission spectrum has remained unclear. In the present study we explore the nature of the underlying decay mechanism of 2p core-excited Co²⁺ in water by probing the nonradiative Auger-type electron emission channel using photoelectron spectroscopy from a liquid microjet. Our measurements demonstrate unequivocally that metal-to-water charge transfer quenches fluorescence and will inevitably lead to a dip in the total-fluorescence-yield X-ray absorption spectrum. This is directly revealed from the resonant enhancement of valence signal intensity arising from the interference of two identical final states created by a direct and Auger-electron emission, respectively