19Multiphase chemical reactions (gas+solid/liquid) involve a complex interplay between 20 bulk and interface chemistry, diffusion, evaporation, and condensation. Reactions of 21 atmospheric aerosols are an important example of this type of chemistry: the rich array of 22 particle phase states and multiphase transformation pathways produce diverse but poorly 23 understood interactions between chemistry and transport. Their chemistry is of intrinsic 24 interest because of their role in controlling climate. Their characteristics also make them 25 useful models for study of principles of reactivity of condensed materials under confined 26 conditions. In previous work, we have reported a computational study of the oxidation 27 chemistry of a liquid aliphatic aerosol. In this study, we extend the calculations to 28 investigate nearly the same reactions at a semisolid gas-aerosol interface. A reaction-29 diffusion model for heterogeneous oxidation of triacontane by hydroxyl radicals (OH) is 30 described, and its predictions are compared to measurements of aerosol size and 31 composition, which evolve continuously during oxidation. These results are also 32 2 explicitly compared to those obtained for the corresponding liquid system, squalane, to 33 pinpoint salient elements controlling reactivity. The diffusive confinement of the free 34 radical intermediates at the interface results in enhanced importance of a few specific 35 chemical processes such as the involvement of aldehydes in fragmentation and 36 evaporation, and a significant role of radical-radical reactions in product formation. The 37 simulations show that under typical laboratory conditions semisolid aerosols have highly 38 oxidized nanometer-scale interfaces that encapsulate an unreacted core and may confer 39 distinct optical properties and enhanced hygroscopicity. This highly oxidized layer 40 dynamically evolves with reaction, which we propose to result in plasticization. The 41 validated model is used to predict chemistry under atmospheric conditions, where the OH 42 radical concentration is much lower. The oxidation reactions are more strongly 43 influenced by diffusion in the particle, resulting in a more liquid-like character. 44 3

The electrochemical nitrate reduction reaction (NO 3 RR) on titanium introduces significant surface reconstruction and forms titanium hydride (TiH x , 0 < x ≤ 2). With ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH 2 enrichment with increasing NO 3 RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH 2 /Ti electrodes for use in NO 3 RR, thereby decoupling hydride formation from NO 3 RR performance. A wide range of NO 3 RR activity and selectivity on TiH 2 /Ti electrodes between −0.4 and −1.0 V RHE was observed and analyzed with density functional theory (DFT) calculations on TiH 2 (111). This work underscores the importance of relating NO 3 RR performance with near-surface electrode structure to advance catalyst design and operation.