Adsorption of constitutional isomers of cyclic monoterpenes on hydroxylated silica surfaces

Heterogeneous interactions and transformations of dibasic esters with indoor relevant surfaces

Dibasic esters (DBEs) have recently become emerging indoor air pollutants due to their usage as a solvent for mixtures of paints and coatings. In this study, we explored the adsorption/desorption kinetics, heterogeneous interactions, and chemical transformations of dimethyl succinate (DMS, C6H10O4), a component of commercial dibasic ester solvent mixtures, on indoor relevant surfaces using transmission Fourier-transform infrared (FTIR) spectroscopy and high-resolution mass spectrometry (HRMS). Silica (SiO2) and rutile (TiO2) were used as proxies for window glass, and an active component in paint and self-cleaning surfaces, respectively. FTIR spectroscopy of these surfaces shows that DMS can interact with SiO2 and TiO2 through hydrogen bonding between the carbonyl oxygen and surface hydroxyl groups. The kinetics show fast adsorption of DMS onto these surfaces followed by slow desorption. Furthermore, new products formed observed on TiO2 surfaces in addition to molecularly adsorbed DMS. In particular, succinate (C5H7O) was observed binding to the surface in a bidentate chelating coordination mode as indicated by the appearance of νas(COO-) and νs(COO-) bands in the FTIR spectra. These absorption bands grow in intensity over time and the resulting product remains strongly adsorbed on the surface. The formation of adsorbed succinate is a result of a reaction with DMS on Lewis acid sites of the TiO2 surface. Overall, the slow desorption of these adsorbed species indicates that indoor surfaces can become long term reservoirs for dibasic esters and their surface products. Moreover, in the presence of ∼50% relative humidity, water displaces outer layers of adsorbed DMS on SiO2 and TiO2, while having no impact on the more strongly bound surface species.

Uptake and reactivity of NO2 on the hydroxylated silica surface: A source of reactive oxygen species

We report state-of-the-art first-principles molecular dynamics results on the heterogeneous chemical uptake of NO2, a major anthropogenic pollutant, on the dry and wet hydroxylated surface of α-quartz, which is a significant component of silica-based catalysts and atmospheric dust aerosols. Our investigation spotlights an unexpected chemical pathway by which NO2 (i) can be adsorbed as HONO by deprotonation of interfacial silanols (i.e., -Si-OH group) on silica, (ii) can be barrierless converted to nitric acid, and (iii) can finally dissociated to surface bounded NO and hydroxyl gas phase radicals. This chemical pathway does not invoke any previously experimentally postulated NO2 dimerization, dimerization that is less likely to occur at low NO2 concentrations. Moreover, water significantly catalyzes the HONO formation and the dissociation of nitric acid into surface-bounded NO and OH radicals, while visible light adsorption can further promote these chemical transformations. This work highlights how water-restricted solvation regimes on common mineral substrates are likely to be a source of reactive oxygen species, and it offers a theoretical framework for further and desirable experimental efforts, aiming to better constrain trace gases/mineral interactions at different relative humidity conditions.

Gas-Phase and Surface-Initiated Reactions of Household Bleach and Terpene-Containing Cleaning Products Yield Chlorination and Oxidation Products Adsorbed onto Indoor Relevant Surfaces

The use of household bleach cleaning products results in emissions of highly oxidative gaseous species, such as hypochlorous acid (HOCl) and chlorine (Cl2). These species readily react with volatile organic compounds (VOCs), such as limonene, one of the most abundant compounds found in indoor enviroments. In this study, reactions of HOCl/Cl2 with limonene in the gas phase and on indoor relevant surfaces were investigated. Using an environmental Teflon chamber, we show that silica (SiO2), a proxy for window glass, and rutile (TiO2), a component of paint and self-cleaning surfaces, act as a reservoir for adsorption of gas-phase products formed between HOCl/Cl2 and limonene. Furthermore, high-resolution mass spectrometry (HRMS) shows that the gas-phase reaction products of HOCl/Cl2 and limonene readily adsorb on both SiO2 and TiO2. Surface-mediated reactions can also occur, leading to the formation of new chlorine- and oxygen-containing products. Transmission Fourier-transform infrared (FTIR) spectroscopy of adsorption and desorption of bleach and terpene oxidation products indicates that these chlorine- and oxygen-containing products strongly adsorb on both SiO2 and TiO2 surfaces for days, providing potential sources of human exposure and sinks for additional heterogeneous reactions.

Adsorption studies of camphene and eucalyptol molecules on orthorhombic germanane nanosheet - A first-principles investigation

In the present work, we deployed a novel orthorhombic germanane nanosheet (ortho-GeNS) as a sensing material to detect camphene and eucalyptol molecules, the indoor air pollutants in the ambient environment. In the beginning, the structural and dynamical permanency of ortho-GeNS is confirmed with cohesive energy (-4.164eV/atom) and phonon-band maps. Successively, the electronic features of ortho-GeNS are conferred using band structure along with the projected density of states maps. The energy gap of ortho-GeNS at the hybrid GGA/B3LYP level of theory is computed to be 3.948 eV. Mainly, the adsorption properties of terpinene molecules, namely camphene and eucalyptol on ortho-GeNS are investigated via ascertaining adsorption energy, Mulliken population analysis, and relative band gap variations. Besides, the scope of adsorption energy values (-0.405eVto-0.669eV) exemplifies that the target molecules are physisorbed on ortho-GeNS. Overall results suggested that the ortho-GeNS can be deployed as a worthy chemiresistive sensor to sense indoor air pollutants for monitoring indoor air quality.

Adsorption of 6-MHO on two indoor relevant surface materials: SiO2 and TiO2

The compound 6-methyl-5-hepten-2-one (6-MHO) is a product of skin oil ozonolysis and is of significance in understanding the role of human occupants in the indoor environment. We present a joint computational and experimental study investigating the adsorption of 6-MHO on two model indoor relevant surfaces, SiO₂, a model for a glass window, and TiO₂, a component of paint and self-cleaning surfaces. Our classical force field-based molecular dynamics, ab initio molecular dynamics simulations, and FTIR absorption spectra indicate 6-MHO can adsorb on to both of these surfaces via hydrogen and π-hydrogen bonds and is quite stable due to the linear geometry of 6-MHO. Detailed analysis of 6-MHO on the SiO₂ surface shows that relative humidity does not impact surface adsorption and adsorbed water does not displace 6-MHO from the hydroxylated SiO₂ surface. Additionally, the desorption kinetics of 6-MHO from the hydroxylated SiO₂ surface is compared to other compounds found in indoor environments and 6-MHO is shown to desorb with a first order rate constant that is approximately four times slower than that of limonene, but six times faster than that of carvone. In addition, our joint results indicate 6-MHO forms a stronger interaction with the TiO₂ surface compared to the SiO₂ surface. This study suggests that skin oil ozonolysis products can partition to indoor surfaces leading to the formation of organic films.

Heterogeneous Reactions of α-Pinene on Mineral Surfaces: Formation of Organonitrates and α-Pinene Oxidation Products

Organonitrates (ON) are important components of secondary organic aerosols (SOAs). α-Pinene (C₁₀H₁₆), the most abundant monoterpene in the troposphere, is a precursor for the formation of several of these compounds. ON from α-pinene can be produced in the gas phase via photochemical processes and/or following reactions with oxidizers including hydroxyl radical and ozone. Gas-phase nitrogen oxides (NO₂, NO₃) are N sources for ON formation. Although gas-phase reactions of α-pinene that yield ON are fairly well understood, little is known about their formation through heterogeneous and multiphase pathways. In the current study, surface reactions of α-pinene with nitrogen oxides on hematite (α-Fe₂O₃) and kaolinite (SiO₂Al₂O₃(OH)₄) surfaces, common components of mineral dust, have been investigated. α-Pinene oxidizes upon adsorption on kaolinite, forming pinonaldehyde, which then dimerizes on the surface. Furthermore, α-pinene is shown to react with adsorbed nitrate species on these mineral surfaces producing multiple ON and other oxidation products. Additionally, gas-phase oxidation products of α-pinene on mineral surfaces are shown to more strongly adsorb on the surface compared to α-pinene. Overall, this study reveals the complexity of reactions of prevalent organic compounds such as α-pinene with adsorbed nitrate and nitrogen dioxide, revealing new heterogeneous reaction pathways for SOA formation that is mineralogy specific.