Formaldehyde Generation in Photooxidation of Isoprene on Iron Oxide

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C: Physical Processes in Nanomaterials and Nanostructures

Formaldehyde Generation in Photooxidation of Isoprene on Iron Oxide Nanoclusters Shi-Ying Lv, Qing-Yu Liu, Yan-Xia Zhao, Mei-Qi Zhang, Li-Xue Jiang, and Sheng-Gui He J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12471 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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The Journal of Physical Chemistry

Formaldehyde Generation in Photooxidation of Isoprene on Iron Oxide Nanoclusters Shi-Ying Lv,†,‡,§ Qing-Yu Liu,*,†,§ Yan-Xia Zhao,†,§ Mei-Qi Zhang,†,‡,§ Li-Xue Jiang,†,‡,§ and Sheng-Gui He*,†,‡,§ †State

Key Laboratory for Structural Chemistry of Unstable and Stable Species,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡University §Beijing

of Chinese Academy of Sciences, Beijing 100049, P. R. China

National Laboratory for Molecular Sciences, CAS Research/Education Center

of Excellence in Molecular Sciences, Beijing 100190, P. R. China

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The photooxidation of isoprene (C5H8) dominates the source of formaldehyde (CH2O) in the atmosphere. The isoprene degradation initiated by atmospheric radicals has been well understood. However, the potential role of metal oxide particles, which are important components of mineral dust, has not been well studied. Herein, the generation of CH2O through photooxidation of isoprene on iron oxide clusters (FexOy+, 3 ≤ x ≤ 23, 3 ≤ y ≤ 35) up to a nano-size has been identified successfully by mass spectrometric experiments. Theoretical studies indicate that the production of CH2O from the C5H8 adsorption complex has to overcome a significant barrier and the photo irradiation can accelerate this process. The product selectivity of CH2O can be enhanced by visible light irradiation with respect to ultra-violet irradiation. The large-sized clusters can be more efficient to generate CH2O in a gas-particle interaction system. This study can provide a new source of formaldehyde from photooxidation of isoprene on iron oxide-based mineral dust in the atmosphere.


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1. INTRODUCTION Isoprene (C5H8), the largest source of nonmethane hydrocarbons to the atmosphere,1 makes significant contributions to the photooxidative processes in the troposphere.2-4 Formaldehyde (CH2O) is one of the major first-generation photooxidation products of isoprene5 and it can be measured as a column integral from satellite observation.6 Several studies have applied the CH2O column measurements to quantify the isoprene emissions.7-12 This application requires an accurate understanding of isoprene oxidation processes.13 The photooxidation of isoprene generating CH2O has been described systematically in several atmosphere chemistry models, such as the global Goddard Earth Observing System-Chemistry (GEOS-CHEM) model and the Master Chemical Mechanism (MCM) model.14-15 The isoprene degradation mechanisms in atmospheric chemistry models mainly rely on homogeneous gasphase processes involving OH radicals,16 NO3 radicals,17 and ozone.18 There are still considerable uncertainties when using the CH2O column as a proxy for the determination of isoprene emissions.19-20 Therefore, it is important to investigate other possible species and mechanisms which may also play critical roles within the photooxidation of gas phase isoprene. In the atmosphere, mineral dust particles including various metal oxides could appreciably affect tropospheric photooxidation capacity through providing facile adsorption and photoreaction sites for volatile organic compounds (VOCs).21-23 The interactions between dust particles and isoprene should be well understood in order



to have an accurate description of atmospheric isoprene chemistry. The adsorption of isoprene on natural Gobi dust was investigated under dark condition in a previous study.24 The reactions of isoprene photooxidation on the surface of ZnO and TiO2 were also reported and the CH2O product was detected.25-26 Iron oxides, which account for the major part of transition metal oxides in the atmospheric mineral dust (2~5%),27 serve as a key factor enhancing the light absorption of dust at UV and visible wavelengths.28 Studies have shown that photoreactions of iron oxides have potential to impact the transformation of VOCs in the atmosphere.29-30 Therefore, the performance of isoprene on iron oxide particles will be quite interesting and it is very beneficial for atmospheric chemistry models to obtain the mechanistic insights. The surfaces of particles are complex, while gas-phase atomic clusters are considered as ideal models of active sites on related solid surfaces.31-38 Nano-sized atomic clusters are of great importance to bridge atoms and their bulk counterpart.39-42 In this study, by using a homemade time-of-flight mass spectrometer (TOF-MS), we studied the interaction of isoprene with iron oxide cluster cations. The formation of CH2O has been evidenced and the competing of CH2O generation and isoprene desorption has been discussed. The photon energy dependent product selectivity and the size-dependent reactivity have been identified. The density functional theory (DFT) calculations were used to study the reaction mechanism of CH2O generation at the molecular level. This study provides new insights into CH2O formation through oxidation of C5H8 on iron oxide particles in the atmosphere.


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2. METHODS 2.1. Experimental Method. The thermal reactions of FexOy+ with isoprene were performed by a homemade reflectron TOF-MS equipped with a laser ablation cluster source, a quadrupole mass filter (QMF), and a linear ion trap (LIT) reactor (Figure 1a).43 The iron oxide cluster cations were generated by laser ablation of a rotating and translating metal disk compressed with isotope-enriched

56Fe

powder (99.92%,

Isoflex) in the presence of about 5% O2 seeded in a helium (99.99% purity) carrier gas with a backing pressure of about 4.5 atm. The cluster ions of interest were massselected by the QMF and then delivered into the LIT reactor where they were confined and thermalized through the collisions with the cooling gas He (5~11 Pa) for about 1.0 ms. The thermalized ions then interacted with a pulse of reactant isoprene (C5H8) seeded in He gas for about 2.0 ms. The partial pressure of the reactant C5H8 molecules ranged from 0.4 mPa to 4.0 mPa. The temperature of the cooling gas (He), the reactant gas (C5H8), and the LIT reactor was around 298 K. The reactant and product ions ejected from the LIT were detected by the reflectron TOF-MS. Furthermore, the photo- and collision- induced reactions of the adsorption products FexOyC5H8+ were investigated by a tandem TOF-MS coupled with a laser ablation cluster source and a fast flow reactor (Figure 1b).44 The laser ablation generated FexOy+ ions were expanded and reacted with C5H8 in the fast flow reactor (length ~60 mm) for about 60 μs. The instantaneous total effective pressure in the fast flow reactor was estimated to be around 50 Pa at T = 298 K. The partial pressure of the reactant



C5H8 was about 60 mPa. The reactant and product ions in the reaction of FexOy+ with C5H8 passed through the reflector in the primary TOF-MS. The FexOyC5H8+ ions of interest were mass-selected and irradiated by a crossed laser beam of which the laser wavelength was tunable from 355 nm to 600 nm to induce photoreactions. The tunable laser was generated by an optical parametric oscillator pumped by the third harmonic output of a Nd:YAG laser. The mass-selected FexOyC5H8+ ions were also interacted with a gas pulse (He) to proceed collision-induced dissociation (CID).45 After photo- or collision-induced reactions, the daughter (fragment) and the parent ions passed through the reflector of the secondary TOF-MS and then were detected by a dual microchannel plate detector. 2.2. Theoretical Method. The DFT calculations using Gaussian 09 program46 were carried out to investigate the structures of particular iron oxide cluster Fe6O8+ and the mechanisms of reaction with C5H8. The hybrid B3LYP functional,47 which performed well for iron-containing complexes48 was used in this work. The 6-311+G* basis set49-50 was used for all atoms. The reaction mechanism calculations involved geometry optimizations of reaction intermediates (IMs) and transition states (TSs). The initial guess structures of the TS species were obtained through relaxed potential energy surface (PES) scans using single or multiple internal coordinates (MCD).51 Vibrational frequency calculations were carried out to check that each of the IMs and TSs has zero and only one imaginary frequency, respectively. Intrinsic reaction coordinate (IRC) calculations52-53 were also performed to make sure that a TS connects two


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appropriate local minima. Furthermore, the diffuse and polarization functions for hydrogen were considered with 6-311++G** basis set for the stationary points on the PES. The zero-point-vibrations-corrected energies (ΔH0) and some Gibbs free energies at 298 K (ΔG298) were reported in the unit of eV in this work. 3. RESULTS 3.1. Experimental Results. 3.1.1. Thermal and photo-reactions of Fe6O8+ with C5H8. The reaction of the Fe6O8+ cluster which is the smallest iron oxide cation with good reactivity with C5H8 was studied. The laser ablation generated Fe6O8+ cations were mass-selected and the TOF mass spectra is shown in Figure 2a. Upon interaction with 2.5 mPa C5H8 in the ion trap reactor for about 2.0 ms, in addition to the molecular association product ions Fe6O8C5H8+, additional product ions assigned as Fe6O7C4H6+ were generated (Figure 2b). The relative intensities of these products increased as the C5H8 pressure increased and the secondary products Fe6O8(C5H8)2+ and Fe6O7C9H14+ became relatively stronger (Figure 2c), which suggests the following reactions: Fe6O8+ + C5H8 → Fe6O8C5H8+ Fe6O8+ + C5H8 → Fe6O7C4H6+ + CH2O

(1) (2)

The absolute rate constant of Fe6O8+ with C5H8 is (8.7 ± 2.6) × 10−10 cm3 molecule−1 s−1. When the cooling gas pressure (He) is 5 Pa, the branching ratios (BRs) of reaction channels (1) and (2) are 65% and 35%, respectively. If the cooling gas pressure increases to 11 Pa (Figure 2d), the BR of reaction channel (1) increases to 86%



(Figure S1). It indicates that the association product Fe6O8C5H8+ can be stabilized by collisional cooling to prevent further transformation such as reaction channel (2). The isotopic labeling experiment of Fe618O8+ with C5H8 confirmed the above reaction channels (Figure 2e). The association product ions Fe6O8C5H8+ formed by the interaction of Fe6O8+ with C5H8 in the fast flow reactor were mass-selected for photo and collisional excitations and the TOF mass spectra are shown in Figure 3a-e. It should be noted that the instantaneous total effective pressure in the fast flow reactor (about 50 Pa) was much higher than the pressure in the LIT reactor (5~11 Pa). As a result, the major products from the interactions between isoprene and iron oxide cations in the fast flow reactor were the association products (Figure S2). Upon interaction with the laser at 355 nm, two major product channels of Fe6O8C5H8+ were observed (Figure 3a). The absolute intensities of Fe6O8C5H8+ decreased while the signals at the position of ∆mass1 = −30 amu (P1, Fe6O7C4H6+) and ∆mass2 = −68 amu (P2, Fe6O8+) appeared. P1 can be formed through dissociation of CH2O and P2 represents the molecular desorption of C5H8. Significantly, photo-excitation of Fe6O8C5H8+ at longer wavelength (425 nm and 500 nm) generates less C5H8 loss product and relatively more CH2O loss product (Figure 3b and c). It indicates that the photon energy can manipulate the product selectivity. The visible light excitation can produce CH2O very selectively (Figure 3c). The assignments of the products in Figure 3a were further confirmed in the isotopic labeling experiment with

18O 2

in place of

16O 2

in the carrier gas to generate the Fex18Oy+


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clusters (Figure 3d). A minor H2O loss channel was also observed at 355 nm and 425 nm photo-excitations (Figure 3a and b). Relatively low laser fluences (about 8 mJ/cm2) were applied to avoid multiphoton processes. The linear fit for the product yield of P1 and the laser fluence demonstrates that the P1 channel is a single-photon absorption process at both 355 nm and 500 nm (Figure S3). The collisional excitation of Fe6O8C5H8+ cluster was further conducted to compare with the photo-excitation. The center-of-mass collisional energy between Fe6O8C5H8+ and He is calculated to be 16.81 eV.45 The two major products are the same in both excitations, while the relative intensities of P1 (CH2O loss) versus P2 (C5H8 loss) are different as shown in Figure 3e. It turns out that the collisional excitation (Figure 3e) generated relatively more C5H8 loss product (P2) than the photo-excitations (Figure 3a-c). As a result, the photo-excitation is more selective than the collisional excitation in generating CH2O loss product (P1). 3.1.2. Photo-reactions of other FexOy+ with C5H8. In addition to Fe6O8+, many other FexOy+ (3 ≤ x ≤ 23, 3 ≤ y ≤ 35) cluster cations could also pick up C5H8 molecule in the fast flow reactor to form FexOyC5H8+. The FexOy+ clusters can be classified into different series by their oxygen deficiencies (∆):54

∆ = 2y + 1 – 3x

(3)

The association products of C5H8 with ∆ = −2 [(Fe2O3)nFe+, n = 3, 4], ∆ = −1 [(Fe2O3)nFe2O2+, n = 1−4], ∆ = 0 [(Fe2O3)nFeO+, n = 2−4], ∆ = 1 [(Fe2O3)n+, n = 2−11], and ∆ = 2 [(Fe2O3)nFeO2+, n = 4−11] were mass-selected to interact with 355 nm laser



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photons. The largest cluster mass-selected in the photo-reaction experiments was Fe23O35C5H8+ cation (~1.04 nm). The diameters of FexOy+ clusters were estimated for a round ball with the mass density of the bulk Fe2O3 material (5.242 g/cm3). The CH2O loss (P1, FexOy−1C4H6+) products were detected for all the clusters. The C5H8 loss (P2, FexOy+) products were detected for most of the clusters except the (Fe2O3)2C5H8+ and (Fe2O3)3C5H8+ clusters. The photo-reaction channels generally observed can be listed below: FexOyC5H8+ + hv → FexOy−1C4H6+ + CH2O

(4)

FexOyC5H8+ + hv → FexOy+ + C5H8

(5)

The ∆ = 1 cluster series [(Fe2O3)n+], which usually have high oxidative reactivity in transition metal oxide clusters according to the previous studies,55-56 were chosen to explore the size-dependent photo-reactivity. One of the typical photo-reaction mass spectra of the mass-selected Fe22O33C5H8+ cluster (~1.02 nm) is shown in Figure 3f. The relative intensities of the different products (P1 and P2) with respect to the reactants [R, (Fe2O3)nC5H8+, n = 2−11] are shown in Figure 4. The total product yields of P1 and P2 generally increase with the size of the clusters. Competing with the C5H8 desorption, the CH2O loss process is dominant in small-sized region. 3.2. Theoretical Results. In order to investigate the mechanism of CH2O production from C5H8 oxidation on iron oxide clusters, the DFT calculations were performed on Fe6O8+ cluster which is the smallest iron oxide cation with good reactivity. The structure of Fe6O8+ cluster has been reported and the most stable isomer of Fe6O8+ is


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a cage-like structure with Cs symmetry.57 The calculated reaction pathway of Fe6O8+ with isoprene is shown in Figure 5. It is noteworthy that the mechanism of C5H8 oxidation on Fe6O8+ cluster did not involve the activation of C−H bonds. Therefore, the relative energies with the diffuse and polarization functions for hydrogen (B3LYP/6311++G**, Figure S4) are very close (within 0.04 eV) to the energies (B3LYP/6311+G*) shown in Figure 5. To differentiate the C atoms in isoprene molecule, the C atoms of C5H8 are marked as CαH2=Cβ(CH3)CγH=CδH2. The C5H8 prefers to adsorb on the most positively charged Fe atom in Fe6O8+ cluster through the Fe−Cδ interaction to form encounter complex I1 with a binding energy of 1.24 eV. Then the Cα atom of isoprene approaches a bridging oxygen atom to form a Cα−O bond in I2 and this step has a relatively high barrier of 0.96 eV. The formation of I2 from I1 involves the change of spin multiplicity from 30 to 28 (Figure S5). The I2 has sufficient energy (1.49 eV) to overcome the barriers of subsequent steps. From the I2, the weakening of Fe−Cδ bond and the formation of a Cβ−O bond result in the generation of I3 with a five-membered −Fe−O−Cα−Cβ−O− ring. The formation of the five-membered ring structures has also been demonstrated for the C=C bond cleavage step in the previous studies.58-60 The Cα−Cβ bond is significantly activated in I3 (Cα−Cβ distance = 156 pm) compared with the free C5H8 molecule (Cα−Cβ distance = 134 pm) and then cleaves to form I4 (Cα−Cβ distance = 298 pm). The cleavage of the Cα−Cβ bond has a barrier of 0.90 eV (I3 → TS3). After that, the CαH2O unit is formed in I4 and the reaction then proceeds through



the cleavage of Fe−O bond to form I5. Finally, the CαH2O unit can be evaporated from I5. The DFT calculations indicate that the CH2O production through C5H8 oxidation on Fe6O8+ is exothermic (−0.84 eV). However, it should be noted that the energy (ΔH0) of TS1 is slightly lower than the separated reactants and the absolute energy barrier to overcome TS1 is high (0.96 eV). Thus, some of the Fe6O8C5H8+ ions with the I1 structure could be stabilized in the experiment through collisions with the bath gas (He). As a result, both Fe6O8C5H8+ and Fe6O7C4H6+ (+ CH2O) were observed in the experiments (Figure 2b). The mechanism of CH2O generation on the small-sized Fe6O8+ cluster may give a general picture of isoprene oxidation on iron oxide cations. 4. DISCUSSION 4.1. CH2O formation versus C5H8 desorption on Fe6O8C5H8+. Our experimental observation indicates that the CH2O generation and C5H8 desorption are competing processes in photo or collisional excitation on Fe6O8+. The CH2O loss product (P1) can be formed selectively in visible light excitation (Figure 3b and c), while the C5H8 desorption to form P2 is more selective during 355 nm photo-excitation and CID experiments (Figure 3a and e). During the gas-phase experiment, the adsorption complex I1 can be stabilized through the collisions with the bath gas. Alternatively, it can transform through TS1 with a significant energy barrier (ΔH0 = 0.96 eV) to produce CH2O (I1 → TS1) or dissociate back to Fe6O8+ and C5H8 (I1 → R). The BR of the CH2O formation is directly linked to the relative reaction rates of the two-competing


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processes which probably depend on the photon61 or collisional energy. The RiceRamsperger-Kassel-Marcus (RRKM) theory62 and the RRKM-based variational TS theory (VTST)63 were used to estimate the internal conversion rate (kIC) from I1 to TS1 of the CH2O production and the desorption rate (kD) of C5H8 from I1 with different extra energy (Eex) obtained from the photon or collisional excitation processes:

kRRKM = gN‡ (E − Ea) / [ρ(E) × h]

(6)

in which g is the symmetry factor (g = 1 for Fe6O8+ + C5H8), ρ(E) denotes the density of the states of the reaction intermediate at the energy E, N‡ (E − Ea) represents the total number of states of the transition state with the barrier Ea (∆H0), and h is the Planck constant. Taking into account that the mass-selected Fe6O8C5H8+ cluster generated in the high-pressure (50 Pa) fast flow reactor can be completely thermalized (T = 298 K), the total energy (E) consists of the vibrational energy (Evib) of Fe6O8C5H8+ complex at 298 K and the Eex (E = Evib + Eex, Evib = 0.62 eV at DFT level). The VTST which involved geometry optimizations of Fe6O8C5H8+ by fixing the distance between the Fe6O8+ and C5H8 moieties at various values was used to obtain the kD values. The RRKM based theory estimated that both the kIC and the kD increase with the increase of the Eex, while the kD changes more significantly than the kIC. The calculated BR of the CH2O generation [BR = kIC / (kIC + kD)] was estimated to reduce with the increase of the Eex (Figure 6a). The reaction rates by RRKM based theory are sensitive to the calculated energies (reaction barrier and binding energy), while the tendency of the BR of the CH2O production from the RRKM estimation can be qualitatively correct.



As shown in Figure 6b, if the barrier height of I1 → TS1 is reduced by 0.4 eV which can be the uncertainty of the DFT energies, the BR of CH2O formation product is estimated to be 0.47 with the single-photon excitation at 355 nm (Figure S3, photon energy = 3.49 eV). This BR value (0.47) is in good agreement with the experimental observation. When the wavelength of photo-excitation changes from 355 nm to the visible wavelengths (3.49 eV → 2.48 eV), the estimated result is generally consistent with the experiment. In the CID reaction, the center-of-mass collisional energy between Fe6O8C5H8+ and He is calculated to be 16.81 eV which is much larger than the photon energy, so the C5H8 desorption can be the major process. 4.2. Reactivity on nanoclusters and the correlation with atmospheric process. Nanosized metal oxide clusters can help to bridge the gap between cluster behavior and the relevant atmospheric particle behavior.64 Herein, the CH2O production from isoprene photooxidation on the nano-sized iron oxide clusters up to 1 nm has been detected successfully with a high-resolution mass spectrometer (Figure 3f). The total product yields of P1 and P2 increase distinctly in large-sized clusters (Figure 4). The photon absorption can be more efficient when the cluster size increases.65 It is also noteworthy that the BR of C5H8 desorption generally increases with the increase of the cluster size (Figure 4). It is difficult to perform theoretical studies on large-sized iron oxide clusters. We suppose that the mechanism of isoprene oxidation on other iron oxide clusters can be similar to that on Fe6O8+. The size-dependent BRs should mainly depend on the relative generation rates of P1 and P2 which can be altered by the


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barriers of the two transformation processes. It is noteworthy that the formation of CH2O may also take place in the electronically excited state (the dash line in Figure 5). The efficiency of the process mediated by the electronically excited state can also be sensitive to the cluster size. Nonetheless, in a gas-particle interaction system, the desorbed C5H8 molecule will be re-adsorbed and the generation of CH2O can be enhanced on the large iron oxide clusters which can both absorb photon and adsorb C5H8 molecule with high efficiencies. The heterogeneous chemistry of isoprene plays a significant role in the atmosphere. Thévenet et al. found that the adsorption of isoprene on Gobi dust (containing ~2.6% Fe3O4) was intense even at dark condition within a lifetime of 13 h.24 When isoprene reacted on the surface of TiO2, the gas-phase product CH2O was detected at UV light irradiation.26 It was proposed that the isoprene was oxidized by the highly reactive OH radicals produced on the TiO2 surface in the presence of H2O. In our study, the direct photooxidation of isoprene into CH2O on iron oxide clusters has been detected successfully without the participation of H2O to generate OH radicals. Although the direct transformation of isoprene to CH2O has to overcome a significant barrier, the photo irradiation could accelerate this process. In the photooxidation study of isoprene on the iron oxide clusters, such as Fe6O8+, the product selectivity of CH2O enhanced when the light changed from ultraviolet to visible. This reactivity parallels the behavior of ZnO particle system, on which the CH2O can also be generated selectively under visible light rather than ultraviolet.25 It indicates that the visible light can be beneficial



in isoprene oxidation on metal oxides particles to form gas phase CH2O. This photooxidation study of isoprene on iron oxide clusters not only interprets the mechanism of related isoprene degradation but also provides a new pathway of CH2O generation on the surface of iron oxide-based mineral dust particles in the atmosphere. 5. CONCLUSION In summary, the photooxidation of isoprene into CH2O on FexOy+ (3 ≤ x ≤ 23, 3 ≤ y ≤ 35) up to 1 nm has been demonstrated successfully with mass spectrometry. The reaction mechanism of CH2O generation on Fe6O8+ cluster has been explored by the density functional theory calculations. It indicates that the direct oxidation of isoprene into CH2O has to overcome a significant barrier and the photo irradiation can accelerate the conversion of adsorption products to generate CH2O. The visible light can lead to more selective CH2O generation. In a gas-particle system, with the readsorption of C5H8, the CH2O production should be enhanced on large iron oxide particles that can have higher efficiencies in both photon absorption and isoprene adsorption. This study provides a new source of CH2O formation from photooxidation of isoprene on iron oxide-based mineral dust particles in the atmosphere.


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ASSOCIATED CONTENT Supporting

Information.

Figures

S1-S6

giving

additional

experimental

and

computational results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Qing-Yu Liu: 0000-0001-9387-4310 Sheng-Gui He: 0000-0002-9919-6909 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21773253, 21803073, and 21573247), National Key Research and Development Program of China (No. 2017YFC0209403), Beijing Natural Science Foundation (2182092), and the Youth Innovation Promotion Association, Chinese Academy of Sciences (2018041).



6. REFERENCES (1) Guenther, A.; Karl, T.; Harley, P.; Wiedinmyer, C.; Palmer, P. I.; Geron, C., Estimates of Global Terrestrial Isoprene Emissions Using MEGAN (Model of Emissions of Gases and Aerosols from Nature). Atmos. Chem. Phys. 2006, 6, 31813210. (2) Claeys, M.; Graham, B.; Vas, G.; Wang, W.; Vermeylen, R.; Pashynska, V.; Cafmeyer, J.; Guyon, P.; Andreae, M. O.; Artaxo, P., et al., Formation of Secondary Organic Aerosols Through Photooxidation of Isoprene. Science 2004, 303, 11731176. (3) Carlton, A. G.; Wiedinmyer, C.; Kroll, J. H., A Review of Secondary Organic Aerosol (SOA) Formation from Isoprene. Atmos. Chem. Phys. 2009, 9, 4987-5005. (4) Paulot, F.; Henze, D. K.; Wennberg, P. O., Impact of the Isoprene Photochemical Cascade on Tropical Ozone. Atmos. Chem. Phys. 2012, 12, 1307-1325. (5) Sprengnether, M.; Demerjian, K. L.; Donahue, N. M.; Anderson, J. G., Product Analysis of the OH Oxidation of Isoprene and 1,3-Butadiene in the Presence of NO. J.

Geophys. Res. 2002, 107, 4268. (6) Chance, K.; Palmer, P. I.; Spurr, R. J. D.; Martin, R. V.; Kurosu, T. P.; Jacob, D. J., Satellite Observations of Formaldehyde over North America from GOME. Geophys.

Res. Lett. 2000, 27, 3461-3464. (7) Shim, C.; Wang, Y. H.; Choi, Y.; Palmer, P. I.; Abbot, D. S.; Chance, K., Constraining Global Isoprene Emissions with Global Ozone Monitoring Experiment


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7. FIGURES Figure 1. A schematic diagram of the reflectron TOF-MS equipped with a laser ablation cluster source, a QMF, and a LIT (a) and the tandem TOF-MS coupled with a laser ablation cluster source and a fast flow reactor (b). Adapted from References 43 and 44. Figure 2. TOF mass spectra for reactions in the LIT reactor of mass-selected Fe6O8+ (a), with 0.7 mPa C5H8 (b), 2.5 mPa C5H8 (c) under 5 Pa cooling gas pressure, and with 2.5 mPa C5H8 under 11 Pa cooling gas pressure (d). The mass selected Fe618O8+ with 2.5 mPa C5H8 under 5 Pa cooling gas pressure in (e). The time periods for the reactions were about 2.0 ms. The FexOyZ+ (Z = C4H6, C5H8, etc.) species are labeled as x, y, Z. Peaks marked with ▲, ▼ and * are due to water and O2 impurities. Figure 3. TOF mass spectra for the photoreaction of mass-selected Fe6O8C5H8+ at 355 nm (a), 425 nm (b), 500 nm (c), Fe618O8C5H8+ at 355 nm (d), the CID reaction of Fe6O8C5H8+ with He (e), and the photoreaction of Fe22O33C5H8+ at 355nm (f). The relative intensities around the product peaks are amplified by a factor of 20 for (a-e) and 10 for (f). The cluster FexOyC5H8+ is labeled as x, y, 1. The P1 and P2 denote products from losses of CH2O and C5H8 from FexOyC5H8+, respectively. The peaks marked with −H2O, −O2, and * denote losses of H2O, O2, and C5H8O respectively. The laser fluence is 7.62 ± 0.90 mJ/cm2.


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Figure 4. The photoreaction signal intensities of P1 [I(P1)] and P2 [I(P2)] relative to the signal intensities of reactant ions [(Fe2O3)nC5H8+, n = 2−11] at 355 nm. The laser fluence is 6.67 ± 0.28 mJ/cm2. Figure 5. DFT calculated ground-state PES for the oxidation of isoprene into CH2O by Fe6O8+. The relative ∆H0 energies of the reaction IMs (I1−I6), TSs (TS1−TS5), and product (P) with respect to the separate reactants (R, Fe6O8+ + C5H8) are given in eV. The relative ΔG298 energies are given in parentheses. The superscript numbers indicate the spin multiplicities. There is a spin conversion point (CP) from TS1 to I2 (Figure S5). A lot of effort has been made to optimize the structure of TS2 by using initial guess structures from the standard relaxed PES scans as well as the MCD method51 but a fully optimized structure could not be obtained. The structure and energy provided for TS2 were determined with the energy-maximum point generated with the MCD method (partially optimized, Figure S6). Figure 6. The BR of CH2O generation [kIC / (kIC + kD)] with respect to the extra energy: (a) the barrier height of I1 → TS1 by DFT is used, and (b) the barrier height of I1 → TS1 is reduced by 0.4 eV relative to the DFT value.



Figure 1.


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Figure 2.



Figure 3.


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Figure 4.



Figure 5.


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Figure 6.



7. TOC Graphic.


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Formaldehyde Generation in Photooxidation of Isoprene on Iron Oxide

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