Atmospheric chemistry of the self-reaction of HO₂ radicals: stepwise mechanism versus one-step process in the presence of (H₂O)_n (n = 1–3) clusters

Abstract

In the incorporation of the catalyst (H2O)n (n = 1–3) into the HO2 + HO2 → H2O2 + 3O2 reaction, the catalytic effect of water, water dimers and water trimers is mainly derived from the contribution of a single water vapor molecule by a stepwise route.

Effect of ammonia, ammonia‐water, and sulfuric acid on the HO2 + HO2 → H2O2 + 3O2 reaction in troposphere: Competition between stepwise and one‐step mechanisms

“…As for the H 3 N···H 2 O···HO 2 + HO 2 reaction, the stepwise Channel AW_s is more favorable than the one‐step Channel AW_o as the rate constant of the stepwise reaction is larger by one order of magnitude than the one‐step reaction. This is similar to the H 2 O‐ and (H 2 O) 2 ‐catalyzed HO 2 + HO 2 → H 2 O 2 + 3 O 2 reactions, [ 18 ] and NH 3 assisted the reaction above where a stepwise mechanism is more favorable than its corresponding one‐step process. Here, only stepwise routes, namely, Channel AW_s, have been mainly discussed in the H 3 N···H 2 O···HO 2 + HO 2 reaction.…”

“…Potential energy profiles for NH 3 -catalyzed Channel AM_o and Channel AM_s ( Figure 2) are similar to that of H 2 O-catalyzed Channel WM_o and Channel WM_s ( Figure S2) (Figure 3) and H 2 OÁÁÁH 3 NÁÁÁHO 2 + HO 2 ( Figure S3) reactions. As seen in Table S5, for these two reactions, the bimolecular rate constant (k b ) for the former reaction was found to be 10 1 to 10 2 times higher compared to the latter one, which indicates that the [18] and NH 3 assisted the reaction above where a stepwise mechanism is more favorable than its corresponding one-step process. Similar to our previous work on water-catalyzed HO 2 + HO 2 !…”

“…The NH 3 ‐catalyzed HO 2 + HO 2 → H 2 O 2 + 3 O 2 reaction proceeds in a very similar way as that of the H 2 O‐catalyzed route [ 18 ] (Figure S2). We also observed that the reaction could proceed through two channels for the NH 3 ‐catalyzed HO 2 + HO 2 → H 2 O 2 + 3 O 2 reaction, namely, a one‐step process (Channel AM_o) and stepwise mechanism (Channel AM_s).…”

“…[1,2] This reaction has been proven to be a key process controlling steady-state concentrations of HO x radicals in the troposphere [3,4] and the main source of H 2 O 2 in the atmosphere. [5][6][7] The kinetic of the HO 2 + HO 2 reaction, especially in the presence of a water molecule, has been studied experimentally [8][9][10][11][12][13] and theoretically [6,[14][15][16][17][18] for a range of temperatures. Experimentally, Kanno et al [10] determined the rate constant (1.5 × 10 −11 ± 0.1 × 10 −11 cm 3 Ámolecule −1 Ás −1 at 297 K) for the HO 2 + HO 2 reaction with H 2 O using continuous-wave near-IR two-tone frequency modulation absorption.…”

“…Through Zhu's [ 5,14 ] investigation, our earlier calculated results [ 17 ] show that, for the HO 2 + HO 2 → H 2 O 2 + 3 O 2 reaction, the calculated barrier with H 2 O compared to the value without H 2 O reduces by 6.72 kcal·mol −1 . Furthermore, in one of our previous works, [ 18 ] we have explored both one‐step and stepwise mechanisms in the HO 2 + HO 2 → H 2 O 2 + 3 O 2 reaction with the H 2 O catalyst, where the stepwise mechanism is more dominant.…”