Figures

Water Vapor Dependence of the β-Hydroxyethyl Peroxy Self Reaction

4.5 Figures

Figure 1: Schematic of oxidation chemistry studied in this chapter to investigate the water vapor dependence of the β-HEP self reaction. Circled species can be detected by the multiplexed synchrotron photoionization mass spectrometry technique. For most of the species, time-resolved ion signals can also be measured. The reaction numbers correspond to the values in the text. Not shown is the HO2 self reaction (R8), which also occurs.

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Figure 2: (a) Example image of ion counts as a function of 𝑚/𝑧 (time-of-flight) and reaction time collected in these experiments at a photoionization energy of 11.75 eV.

Streaks in the image correspond to time-resolved ion signals. Various masses of relevance to these experiments are labeled. We acquire a series of these images by stepping the photoionization energy. (b) Example time-resolved ion signal of formaldehyde (𝑚/𝑧 = 30) acquired by integrating across a subset of the time-of-flight axis in (a).

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Figure 3: Example time-resolved ion signals (blue dots) of (a) H2O2, (b) HO2, and (c) OH measured in experiments calibrating the initial radical concentration, [OH]0. Example dataset was acquired by photolyzing a mixture of solely [H2O2] = 4.18 x 10¹⁴ molc cm^-3 in a balance of He at P = 30 Torr and T = 26.6 °C. The three ion signals were simultaneously fit to a chemical kinetic model (orange curves) and optimized, which yielded [OH]0 = 4.0 x 10¹² molc cm^-3 (0.48% photolysis).

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Figure 4: Summary of experiments used to determine the vinyl alcohol yield. (a) The relative photoionization spectrum of vinyl alcohol measured in this work (recorded up to 12.0 eV), scaled to match the known absolute spectrum (recorded up to 10.5 eV).²³ The cross section at 10.75 eV was determined by averaging over 10.675–10.825 eV.

(b) Kinetics of vinyl alcohol measured at 10.75 eV. The vinyl alcohol yield in ion signal units was obtained by averaging over 50–80 ms. The vinyl alcohol yield in concentration units of [VA] = 6.4 x 10¹⁰ molc cm^-3 was determined in reference to the known pre- photolysis concentration and cross section of ¹³C ethene. These experiments were conducted with [OH]0 = 4.0 x 10¹² molc cm^-3, leading to a vinyl alcohol yield of 1.6%. The ion signal axis has been transformed to absolute concentration units.

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Figure 5: Relative photoionization efficiency curves of 𝑚/𝑧 = 45, integrated over 0–80 ms of reaction time after photolysis. The blue trace was collected with solely ethene as the hydrocarbon precursor and the initial peroxy radicals were all β-HEP. The orange trace was collected with a mixture of [C2H4]:[CH3OH] = 0.14:0.86 that yielded [β-HEP]0:[HO2]0

= 0.52:0.48. The spectra are essentially identical, particularly up to 10.75 eV, suggesting that only one of either β-HEP or β-hydroxyethyl hydroperoxide is contributing to the ion signal. Based on the shape of the 𝑚/𝑧 = 45 kinetics, we conclude that only β-HEP contributes to the 𝑚/𝑧 = 45 ion signal at 10.75 eV.

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Figure 6: Representative time-resolved ion signal of β-HEP. Data (blue circles) was collected at [H2O] = 2.8 x 10¹⁶ molc cm^-3 and T = 3.6 °C (RH = 14%). A fit (red curve) was performed to Eqn. 2 (second order loss with baseline) to determine the bimolecular decay constant, which was divided by the calibrated initial radical concentration to determine 𝑘_𝑜𝑏𝑠. We began fits at 15 ms to avoid complications by the source chemistry and the temporal instrument response function.

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Figure 7: Values of 𝑘_𝑜𝑏𝑠 measured in this work as a function of [H2O] at temperatures of 3.6 and 26.6 °C. 𝑘_𝑜𝑏𝑠 is insensitive to [H2O] at 26.6 °C, but increases by a factor of ~3 between [H2O] = 0 and 3.7 x 10¹⁶ molc cm^-3 (RH = 18%) at 3.6 °C. Linear fits (solid lines) were performed to data collected at each temperature in accordance with Eqn. 3. The parameterization determined by Kumbhani et al.¹ is also plotted (dashed lines) and exhibits excellent agreement with our results, under the assumption that their definition of 𝑘_𝑜𝑏𝑠 matches ours and encompasses loss of β-HEP by secondary HO2.

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Figure 8: Representative time-resolved ion signals for the products of the β-HEP self reaction: (a) ethylene glycol, (b) HO2, (c) glycoaldehyde, and (d) formaldehyde. Data (blue circles) was collected at [H2O] = 2.8 x 10¹⁶ molc cm^-3 and T = 3.6 °C (RH = 14%).

Fits (red curves) were performed through numerical integration of Eqn. 6 to determine the yield of each product from the self reaction expressed in ion signal units. The decays of ethylene glycol and glycoaldehyde at longer reaction times are attributed to heterogeneous wall loss, which was more prevalent at colder temperatures and greater relative humidity.

The decay of HO2 is attributed to its loss reactions (R7 and R8). The positive slope in the formaldehyde signal at longer reaction times arises from divergence of the photolysis laser beam and a gradient in the initial radical concentration; this effect is empirically captured by fitting a negative loss rate for this species.

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Figure 9: (a) Relative change in the HO2 to ethylene glycol product yield ratio from the β-HEP self reaction at T = 3.6 °C. Calculated from the ratio of their yields expressed in ion signal units, relative to the [H2O] = 0 case. The ratio decreased by ~90% over the range of [H2O] used in this work. (b) Measured values of the branching fraction to the radical propagating channel α as a function of [H2O] at T = 3.6 °C. Determined from the relative data in (a) and Eqn. 4 by assuming that α = 0.5 at [H2O] = 0, as measured by Barnes et al.²⁵ Our results for α are therefore relative to the [H2O] = 0 case. α decreases as additional water vapor is added indicating that the radical terminating channel of the self reaction is being enhanced relative to the radical propagating channel.

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Figure 10: Relative change in the formaldehyde to glycoaldehyde product yield ratio from the β-HEP self reaction at T = 3.6 °C. Calculated from the ratio of their yields expressed in ion signal units, relative to the [H2O] = 0 case. The ratio increased by ~250% over the range of [H2O] used in this work. The change in this ratio is partially influenced by the change in α, although correction for this effect is not straightforward since the absolute concentrations are unknown. Our data suggests that decomposition of the alkoxy radical is enhanced relative to its reaction with O2 under humid conditions. The fitted trendline is empirical.

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