When sum frequency generation (SFG) spectroscopy is applied to charged solid/liquid interfaces, the observed SFG signals include both the second-order and third-order polarizations. The latter is called the χ effect, which mainly includes induced molecular orientation by electric fields at charged interfaces. We theoretically evaluate the χ effect on the SFG spectroscopy of liquid water using molecular dynamics (MD) simulations. The MD simulations enable us to definitely calculate the χ susceptibility as a bulk property, and thereby separating it from the usual χ effect shown in the SFG spectra. The calculated results of χ for liquid water are fairly consistent with the experimental estimates. The present finding is utilized to analyze the spectral change of SFG at the air/water interface under electric fields and at the charged silica/water interface. The present analysis of the spectral changes allows for distinguishing the intrinsic change of the interface structure and the χ effect from bulk liquid.

Sea-spray aerosol particles have major yet poorly understood influence on the state of the atmosphere. Although non-linear vibrational spectroscopy is a reliable technique for probing the nature of aerosol interfaces, resolving the spectral features into specific structural and dynamical properties of the interface poses substantial difficulties. Here, computer simulations are used to disentangle strictly surface-sensitive contributions from bulk-dependent effects at a model sea-spray aerosol, which allows for a detailed, molecular-level characterization of the interfacial properties.

Sum-frequency generation (SFG) spectra from charged solid–liquid interfaces include significant contribution from third-order susceptibility χ(3), which mainly originates from induced water orientation in the electric double layer. We quantitatively evaluate the χ(3) susceptibility by molecular dynamics simulation in aqueous electrolyte solutions with varying concentrations and temperatures. We found that the value of χ(3) decreases with increasing concentration or temperature and that the perturbation on χ(3) is quite well-correlated with that on the dielectric constant ϵ of the solution. This correlation is understood as both quantities are commonly governed by the response of molecular orientation to the electric field. Accurate evaluation of χ(3) in various conditions is important in quantitative estimate of the third-order effect on the SFG spectroscopy, particularly in conditions of high surface charge and ion concentration.

Recently, Zn x Zr1–x O2–x catalysts have attracted attention as next-generation CO2-to-methanol hydrogenation catalysts. In this study, we examined the effect of the Zn content on CO2-to-methanol hydrogenation over Zn x Zr1–x O2–x catalysts and determined the active-site structure through both calculations and experiments. When the Zn content was low, Zn x Zr1–x O2–x contained Zn clusters (isolated [ZnO a ] clusters and [Zn b O c ] oligomers). The presence of clusters indicates the formation of Zn–O–Zr sites. Interestingly, our calculations revealed that the Zn species in the clusters are easily exposed on the Zn x Zr1–x O2–x surface. This result is in line with the experimental results, suggesting that Zn species were unevenly distributed on the Zn x Zr1–x O2–x surface and deposited near the surface. The addition of excess Zn to ZrO2 led to the formation of both Zn-containing clusters and ZnO nanoparticles. During the reactions, the Zn–O–Zr sites derived from the clusters showed specific activity for CO2-to-methanol hydrogenation. Understanding the active-site structure will lead to the future development of Zn x Zr1–x O2–x catalysts.