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

• Published in: Physical chemistry chemical physics : PCCP Vol. 22; no. 46; pp. 27357 - 27363
• Main Authors: Li, Shu-Qiang, Lv, Shi-Ying, Zhou, Hai-Yan, Ding, Yong-Qi, Liu, Qing-Yu, Ma, Jia-Bi
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 07.12.2020
• Subjects: Aerosols; Atmospheric chemistry; Carbon monoxide; Cations; Channels; Cleavage; Covalent bonds; Density functional theory; Hydroxyl radicals; Isoprene; Mass spectrometry; Metal oxides; Oxidation; Reaction mechanisms; Titanium dioxide; Titanium oxides; Vapor phases
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d0cp05472k

The heterogeneous oxidation of isoprene (C 5 H 8 ) by metal-oxide particles, such as the typical mineral aerosols TiO 2 , plays an important role in the isoprene atmospheric chemistry. However, the underlying mechanism of C 5 H 8 oxidation remains elusive owing to the complexities of aerosol surfaces and reaction channels. Herein, we report the gas-phase reactions of Ti x O y + ( 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 CH 2 O and CO generation observed in Ti 2 O 5 + /C 5 H 8 and Ti 4 O 8 + /C 5 H 8 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 C 5 H 8 . 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. The reactions of isoprene with titanium oxide cluster cations were investigated. Five reaction types were classified, and several neutral oxygenated products, including CO, CH 2 O (formaldehyde), C 3 H 4 O, C 5 H 6 O, C 5 H 7 O and so on, were generated.