Isoprene Heterogeneous Uptake and Reactivity on TiO2 : A Kinetic and Product Study

Inhibition Effect of H2O on the Heterogeneous Reaction between Isoprene and Fe-Substituted Cryptomelane

The transportation and transformation of biogenic isoprene are vital for the organic carbon cycle in the troposphere. As a typical mineral with high oxidation potential, Fe-substituted cryptomelane oxidizes the surface monolayer of isoprene into formic and acetic acids, and simultaneously, the Mn4+ ions in the structure are reduced to Mn3+ and Mn2+. The flow of H2O in isoprene decreases the adsorption and oxidation of isoprene significantly, even at low relative humidity (10%). As physisorbed H2O retains Fe-substituted cryptomelane's crystal structure and oxidation ability, the adsorption and oxidation capacity recovers when H2O is absent in the isoprene flow. Theoretical calculations on (001) surfaces show that isoprene prefers to be adsorbed by the Fe3+ site and H2O tends to form hydrogen bonds. Due to the decrease in total adsorption energy of H2O and isoprene, Fe-substituted cryptomelane favors the adsorption of H2O in the flow of humid isoprene. The low oxidation performance at ambient relative humidity suggests that direct oxidation by aerosols of mineral dust might not be the transformation pathway of biogenic isoprene at night.

Effect of iron substitution in cryptomelane on the heterogeneous reaction with isoprene

Biogenic isoprene is an important pollutant for regional air quality. Being ubiquitously distributed on the earth surface, manganese (hydr)oxides should play a vital role in the transformation of isoprene. Cryptomelane is a typical manganese oxide with isomorphous substitution of Fe for Mn, but less attention has been paid to its heterogeneous reaction with isoprene. When Fe³⁺ replaces Mn³⁺, K⁺ is depleted and Mn³⁺ is oxidized to Mn⁴⁺. In contrast, oxygen vacancies are formed when Fe³⁺ substitutes Mn⁴⁺. Fe substitution creates weak crystallites and abundant mesopores, resulting in the increase of isoprene adsorption. As found by theoretical calculations, the Mn⁴⁺-O^2- bonds at the cross sections of the tunnels is more active than that on the outer wall of the tunnels. After the adsorption of isoprene, bridging carboxylate species and hydrogen-bonding water are produced and the surface octahedra are distorted, i.e., Mn⁴⁺O₆ → Mn³⁺O_6-δ. As the heat facilitates the breakage of Mn⁴⁺-O^2-, the increase of environmental temperature enhances the oxidation of isoprene. The above findings shed light on the effect of Fe substitution in cryptomelane to enhance the oxidation of isoprene, and illustrates that heterogeneous reaction with isoprene impairs the transformation of other environmental substances on cryptomelane.

Photooxidation of Isoprene by Titanium Oxide Cluster Anions with Dimensions up to a Nanosize

Titania (TiO₂) nanoparticles are active photocatalysts, and isoprene (C₅H₈) is a biogenic volatile organic compound that contributes crucially to global particulate matter generation. Herein, the direct photooxidation of isoprene by titanium oxide cluster anions with dimensions up to a nanosize by both ultraviolet (UV) and visible (Vis) light excitations has been successfully identified through mass spectrometric experiments combined with quantum chemistry calculations. The potential role of "dry" titania in atmospheric isoprene oxidation has been revealed, and a clear picture of the photooxidation mechanism on titanium oxide nanoparticles has been provided explicitly at the molecular level. The adsorption of isoprene on the atomic oxygen radicals (O^•-) of titanium oxide clusters leads to the formation of the crucial interfacial state (IS) within the band gap of titanium oxides. This IS is demonstrated to be the significant factor in delivering the electron from the π orbital of C₅H₈ to the Ti_3d orbital in the photooxidation process (C₅H₈ + Ti⁴⁺-O^•- → C₅H₈O + Ti³⁺) and creating photoactivity in the Vis region. It is revealed that after the photogeneration of the O^•- radicals by UV excitation on the TiO₂ particle surface, the subsequent reactions can be induced by Vis excitation through the IS. This multicolor strategy in both the UV and Vis regions can enhance the efficiency of solar energy harvesting and improve the product yield of the photocatalysis on TiO₂ nanoparticles. New insights have been provided into both the atmospheric chemistry of isoprene and the photochemistry of TiO₂ nanoparticles.

Oxidation of isoprene by titanium oxide cluster cations in the gas phase

The heterogeneous oxidation of isoprene (C5H8) by metal-oxide particles, such as the typical mineral aerosols TiO2, plays an important role in the isoprene atmospheric chemistry. However, the underlying mechanism of C5H8 oxidation remains elusive owing to the complexities of aerosol surfaces and reaction channels. Herein, we report the gas-phase reactions of TixOy+ (x = 1-7, y = 1-14) cations with isoprene by using mass spectrometry and density functional theory (DFT) calculations. Five types of reaction channels were observed: association, hydrogen atom transfer (HAT), C-C bond cleavage, combined oxygen atom transfer (OAT) and HAT and combined OAT and C-C bond cleavage. It is noteworthy that formaldehyde is known as the major oxidation product of isoprene/hydroxyl radicals in the atmosphere. In addition, CO has not been observed in the reactions of isoprene with gas-phase ions. Therefore, the reaction mechanisms of CH2O and CO generation observed in Ti2O5+/C5H8 and Ti4O8+/C5H8 systems were further investigated by DFT calculations, and the calculated results are in agreement with the experimental observations. In these two reactions, both Ti and O atoms can be the adsorption sites for C5H8. The reaction channels and mechanistic information gained in these gas-phase model reactions may offer fundamental insights relevant to the corresponding oxidation processes over titanium oxide aerosols in the atmosphere.

Reactive uptake of NO2 on volcanic particles: A possible source of HONO in the atmosphere

The heterogeneous degradation of nitrogen dioxide (NO₂) on five samples of natural Icelandic volcanic particles has been investigated. Laboratory experiments were carried out under simulated atmospheric conditions using a coated wall flow tube (CWFT). The CWFT reactor was coupled to a blue light nitrogen oxides analyzer (NO_x analyzer), and a long path absorption photometer (LOPAP) to monitor in real time the concentrations of NO₂, NO and HONO, respectively. Under dark and ambient relative humidity conditions, the steady state uptake coefficients of NO₂ varied significantly between the volcanic samples probably due to differences in magma composition and morphological variation related with the density of surface OH groups. The irradiation of the surface with simulated sunlight enhanced the uptake coefficients by a factor of three indicating that photo-induced processes on the surface of the dust occur. Furthermore, the product yields of NO and HONO were determined under both dark and simulated sunlight conditions. The relative humidity was found to influence the distribution of gaseous products, promoting the formation of gaseous HONO. A detailed reaction mechanism is proposed that supports our experimental observations. Regarding the atmospheric implications, our results suggest that the NO₂ degradation on volcanic particles and the corresponding formation of HONO is expected to be significant during volcanic dust storms or after a volcanic eruption.