Accelerated Photolysis of H2O2 at the Air-Water Interface of a Microdroplet

Catalyst-Free Oxidation of Styrene to Styrene Oxide Using Circulating Microdroplets in an Oxygen Atmosphere

Water microdroplets possess unique interfacial properties that enable chemical reactions to occur spontaneously and increase the reaction rate by orders of magnitude. In this study, water containing styrene (SY) was cyclically sprayed into the air to form microdroplets with an average diameter of 6.7 μm. These microdroplets allowed SY to be oxidized into styrene oxide (SO) without catalysts. No oxidation products of SY were observed in the bulk solution under the same conditions, while in microdroplet reactions 4.2% conversion of SY with approximately 3.1 mM SO was detected. Compared with the traditional spraying microdroplet method, the oxidation product concentration was enhanced by 1000 times. Experiments proved that an aerobic environment boosts SY oxidation, leading to a proposed dual-path hydrogen peroxide (H2O2) oxidation mechanism at the droplet interface. This was confirmed by density functional theory calculations (DFT). Furthermore, in the presence of additional ultrasound, the SY oxidation process initiated by water droplets can be further enhanced, and 7.0% conversion of SY with approximately 5.2 mM SO was detected. The cyclic spraying method greatly enhanced the oxidation product concentration, showing the potential for large scale chemical production using microdroplets.

Crystal Water in Minerals Modulates Oxygen Activation for Hydrogen Peroxide Photosynthesis

Sunlight-responsive minerals contribute significantly to biogeochemical cycles by activating oxygen (O2) to generate reactive oxygen species (ROS). However, the role of crystal water, incorporated into minerals through hydration during rock cycles, in O2 activation remains largely unexplored. Here, we construct tungstite models containing oxygen vacancies to elucidate the modulation of mineral-based ROS dynamics by the synergy between oxygen vacancy and crystal water. Crystal water promotes the protonation process of superoxide anion radicals to produce hydrogen peroxide (H2O2) and alleviates its decomposition. This mineral-based H2O2 photosynthesis system efficiently eliminates organic pollutants in a sequential light-dark reaction. Furthermore, this synergy effect can extend to other metal oxide minerals such as TiO2, SnO2, CuO, ZnO, and Bi2O3. Our results illuminate an overlooked pathway for modulating the protonation process by immobilized water in hydrous minerals, playing a crucial role in ROS storage and migration and pollutant dynamics in a natural environment throughout the day/night cycle.

Valence Electronic Structure of Interfacial Phenol in Water Droplets

Biochemistry and a large part of atmospheric chemistry occur in aqueous environments or at aqueous interfaces, where (photo)chemical reaction rates can be increased by up to several orders of magnitude. The key to understanding the chemistry and photoresponse of molecules in and "on" water lies in their valence electronic structure, with a sensitive probe being photoelectron spectroscopy. This work reports velocity-map photoelectron imaging of submicrometer-sized aqueous phenol droplets in the valence region after nonresonant (288 nm) and resonance-enhanced (274 nm) two-photon ionization with femtosecond ultraviolet light, complementing previous liquid microjet studies. For nonresonant photoionization, our concentration-dependent study reveals a systematic decrease in the vertical binding energy (VBE) of aqueous phenol from 8.0 ± 0.1 eV at low concentration (0.01 M) to 7.6 ± 0.1 eV at high concentration (0.8 M). We attribute this shift to a systematic lowering of the energy of the lowest cationic state with increasing concentration caused by the phenol dimer and aggregate formation at the droplet surface. Contrary to nonresonant photoionization, no significant concentration dependence of the VBE was observed for resonance-enhanced photoionization. We explain the concentration-independent VBE of ∼8.1 eV observed upon resonant ionization by ultrafast intermediate state relaxation and changes in the accessible Franck-Condon region as a consequence of the lowering of the intermediate state potential energy due to the formation of phenol excimers and excited phenol aggregates. Correcting for the influence of electron transport scattering in the droplets reduced the measured VBEs by 0.1-0.2 eV.

Perspective on Theoretical and Experimental Advances in Atmospheric Photochemistry

Research that explores the chemistry of Earth's atmosphere is central to the current understanding of global challenges such as climate change, stratospheric ozone depletion, and poor air quality in urban areas. This research is a synergistic combination of three established domains: earth observation, for example, using satellites, and in situ field measurements; computer modeling of the atmosphere and its chemistry; and laboratory measurements of the properties and reactivity of gas-phase molecules and aerosol particles. The complexity of the interconnected chemical and photochemical reactions which determine the composition of the atmosphere challenges the capacity of laboratory studies to provide the spectroscopic, photochemical, and kinetic data required for computer models. Here, we consider whether predictions from computational chemistry using modern electronic structure theory and nonadiabatic dynamics simulations are becoming sufficiently accurate to supplement quantitative laboratory data for wavelength-dependent absorption cross-sections, photochemical quantum yields, and reaction rate coefficients. Drawing on presentations and discussions from the CECAM workshop on Theoretical and Experimental Advances in Atmospheric Photochemistry held in March 2024, we describe key concepts in the theory of photochemistry, survey the state-of-the-art in computational photochemistry methods, and compare their capabilities with modern experimental laboratory techniques. From such considerations, we offer a perspective on the scope of computational (photo)chemistry methods based on rigorous electronic structure theory to become a fourth core domain of research in atmospheric chemistry.

Study on the Transport Properties of SO2 and NO at the Interface of H2O2 Solutions Using Molecular Dynamics

Gas-liquid interfaces have a unique structure different from the bulk phase due to the complex intermolecular interactions within them and are regarded as barriers that prevent gases from entering solution or as channels that affect gas reactions. In this study, the adsorption and mass-transfer mechanisms of sulfur dioxide and nitric oxide at the gas-liquid interface of a H2O2 solution were comprehensively analyzed using molecular dynamics (MD) simulations. The analysis on molecule angle showed that H2O molecules tended to align parallel to the solution surface on the surface of the H2O2 solution. Regardless of whether the gas was adsorbed on the surface of the solution or not, H2O2 molecules were always perpendicular to the interface of the solution. The analysis on molecule angle and radial distribution function revealed that the H atoms of H2O molecules had a corresponding turn, and SO2 molecules were greatly affected by the attraction of H2O2 molecules during the adsorption of gas molecules on the interface. Steered MD was utilized to investigate the mass-transfer process of SO2 and NO molecules across the gas-liquid interface. The S atoms of SO2 molecules were significantly influenced by H2O2 molecules, while the O atoms of NO molecules gradually transitioned from the gas phase to the liquid phase. The results provided information on how gas molecules interacted with the surface of the solution and the specific details of the molecular orientation at the solution surface.

Weakened Interfacial Hydrogen Bond Connectivity Drives Selective Photocatalytic Water Oxidation toward H2O2 at Water/Brookite-TiO2 Interface

The formation of H2O2 through the two-electron photocatalytic water oxidation reaction (WOR) is significant but encounters the competition with the four-electron O2 evolution reaction. Recent studies showed a crystal-phase dependence in H2O2 selectivity, where high purity brookite TiO2 (b-TiO2) exhibits remarkable H2O2 selectivity in contrast to the common rutile phase TiO2 (r-TiO2). However, the origin of such a structure-induced selectivity preference remains elusive, primarily due to the complexities associated with the solid-liquid interface system and excited-state chemistry. Herein, we conducted a comprehensive investigation into the selectivity mechanism of WOR at the water/b-TiO2(210) and water/r-TiO2(110) interfaces, employing first-principles molecular dynamics simulations and microkinetic analyses. Intriguingly, our results reveal that the intrinsic catalytic ability of the b-TiO2(210) itself does not enhance H2O2 selectivity compared to r-TiO2(110). Instead, it is the weakened interfacial hydrogen bond connectivity, modulated by the herringbone-like local atomic structure of the b-TiO2(210) surface, that determines the selectivity. Specifically, this weakened H-bond connectivity (i.e., local low water density) at the interface, owing to the strong water adsorption and distinct adsorption orientation, can stabilize the OH• radical and inhibit its deprotonation, leading to an improved H2O2 selectivity. By contrast, the relatively strong interface H-bond connectivity established over r-TiO2(110) accelerates the deprotonation of OH•, with the OH• coverage being 3 orders of magnitude lower than at the water/b-TiO2(210) interface. This study quantitatively demonstrates that the local H-bond structure (water density) at the liquid/solid interface significantly influences photocatalytic selectivity, and this insight may offer a rational approach to enhance the H2O2 selectivity.