Efficient alkane oxidation under combustion engine and atmospheric conditions

Autoxidation, a chemical process driven by molecular oxygen (O2), plays a significant role in various systems, including food spoilage, internal combustion engine ignition, and the formation of atmospheric organic aerosol (OA). This process involves the formation of peroxy radicals (ROO•) and their propagation through H-atom transfers (H-shift isomerization), ultimately leading to the creation of progressively more oxygenated species. The extent of multi-step autoxidation, a crucial factor in determining fuel ignition and OA formation, is influenced by molecular structure, temperature, pressure, and the presence of other reactants. Previous understanding suggested that atmospheric autoxidation requires precursors with specific structural features, such as double bonds or oxygen-containing moieties. Alkanes, the primary fuel in combustion engines and a major component of urban trace gases, were considered less susceptible to extensive autoxidation due to lacking these features. This study challenges this conventional wisdom. Recent advancements in chemical ionization mass spectrometry (CIMS), particularly CI-API-TOF, enable the sensitive detection of autoxidation products, including peroxy radicals and highly oxygenated organic molecules (HOM). Furthermore, recent research has shown that alkane fuel autoxidation in combustion extends to multiple O2 addition steps, and observed variations in secondary organic aerosol (SOA) yields from different alkane types (cyclic > linear > branched) suggest the potential role of autoxidation, which has been largely overlooked. This study aims to investigate the autoxidation of alkanes under both atmospheric and combustion conditions using state-of-the-art mass spectrometry, directly addressing the limitations of previous research and advancing the understanding of alkane oxidation in diverse systems.

The paper cites numerous studies on autoxidation in different contexts. Studies on atmospheric autoxidation (e.g., Claeys et al., 2004; Ehn et al., 2014; Crounse et al., 2013) established the importance of this process in secondary organic aerosol (SOA) formation, particularly for biogenic and anthropogenic volatile organic compounds (VOCs). Research on combustion autoxidation (e.g., Cox & Cole, 1985; Wang et al., 2017) highlighted its role in fuel ignition. Studies on alkane oxidation (e.g., Tkacik et al., 2012; Lim & Ziemann, 2009) reported varying SOA yields depending on the alkane structure, suggesting a potentially significant, yet unexplained, role for autoxidation. The authors also reference work on highly oxygenated organic molecules (HOM) (e.g., Bianchi et al., 2019; Molteni et al., 2018; Jokinen et al., 2014) and their role in SOA formation. The reviewed literature provides the background for the current study, highlighting existing knowledge gaps regarding alkane autoxidation and its implications for both combustion and atmospheric chemistry.

The study employed a multi-faceted approach involving three different experimental setups: a jet-stirred reactor (JSR), a flow reactor at the University of Helsinki, and a free-jet flow reactor at the Leibniz Institute for Tropospheric Research (TROPOS). The JSR, operating at combustion-relevant temperatures (545 K), was coupled with a CI-API-TOF mass spectrometer using NO3− reagent ions for detecting oxygenated species and HOM. Experiments were performed on n-decane, 2,7-dimethyloctane, n-butylcyclohexane, 2-decanone, and decanal. The JSR temperature was progressively lowered to simulate the transition from combustion to atmospheric conditions, utilizing TME + O3 reactions to generate OH radicals. The Helsinki flow reactor, operated at atmospheric conditions (~300 K) with a 3-second residence time, was used to study the oxidation of six alkanes and three oxygenates at high VOC concentrations (10 ppm). OH radicals were produced via TME + O3 reactions, and the products were analyzed using NO3− CI-API-TOF. The Leipzig flow reactor, operating at 295 K with a residence time of 7.9 s and low precursor loadings, allowed for the study of alkane oxidation under varying NOx conditions. OH radicals were generated using both TME + O3 and isopropyl nitrite photolysis. C2H5NH3+ CI-API-TOF was used to detect a broad range of oxidation products, including peroxy radicals. The researchers also used a kinetic model to support the interpretation of experimental results. In summary, the research utilized complementary techniques and advanced mass spectrometry to obtain comprehensive data on alkane autoxidation across a range of conditions.

The study revealed several key findings challenging the conventional understanding of alkane autoxidation. First, under combustion conditions (JSR at 520 K), decanal exhibited autoxidation extending to a fourth and even fifth O2 addition, forming highly oxygenated intermediates not previously reported. This finding significantly extends our understanding of autoignition chemistry. As the JSR temperature decreased (392 K to 334 K), the production of highly oxygenated species also decreased, while accretion products and radical species with odd hydrogen numbers emerged. At atmospheric conditions (300 K, Helsinki flow reactor), highly oxygenated products were observed from all tested VOCs except for n-decane and 2,7-dimethyloctane, even within the short 3-second residence time. Molar HOM yields at 300 K revealed substantial differences between different alkane types, with cycloalkanes showing the highest yields, followed by oxygenated VOCs, while linear and branched alkanes produced no observable HOMs. Further, the Leipzig flow reactor experiments with added NO showed a striking increase in the yield of highly oxygenated products for decalin, even at high NO concentrations (up to 20% molar HOM yield), contrasting with observations from other VOCs where NO suppresses autoxidation. This increase in HOM yields highlights the importance of RO isomerization steps and fast RO2 isomerization competing with termination reactions. Analysis of mass spectra revealed that products from all alkane types exhibited significantly higher oxygen content than previously assumed, even under high NO conditions. In summary, the study demonstrated that multi-step autoxidation of long-chain alkanes is significant under both atmospheric and combustion conditions, with RO and RO2 isomerization reactions playing crucial roles, particularly in the formation of highly oxygenated products. These highly oxygenated products are formed rapidly, on the timescale of seconds. This finding is noteworthy because it suggests that the assumption that multiple generations of OH oxidation are required for HOM formation is incorrect.

The findings of this study directly address the research question of alkane autoxidation under atmospheric and combustion conditions. The observed efficient autoxidation of alkanes, even at high NOx concentrations, has significant implications for both combustion engine efficiency and urban air quality. The high yields of highly oxygenated products from alkane autoxidation contribute substantially to the formation of secondary organic aerosol (SOA), a major component of air pollution with harmful health effects. The results provide a mechanistic explanation for the observed differences in SOA yields from various alkane types, highlighting the crucial role of RO isomerization reactions alongside RO2 isomerization. The surprising observation of high HOM yields even under high NOx conditions suggests that alkane autoxidation is a more important process than previously thought, even in polluted urban environments. The study's comprehensive approach, combining experimental data with kinetic modeling, contributes a significant advancement in our understanding of atmospheric and combustion chemistry.

This research demonstrates that multi-step autoxidation of alkanes is a significant process under both combustion and atmospheric conditions, contributing substantially to SOA formation, even under high NOx concentrations. The study highlights the importance of RO isomerization alongside RO2 isomerization in driving autoxidation and provides a mechanistic explanation for previously observed differences in SOA yields from different alkane types. This work fundamentally alters our understanding of alkane oxidation and its atmospheric implications. Future research could focus on further investigating the specific isomerization pathways involved in alkane autoxidation, expanding the range of alkane structures studied, and exploring the role of other atmospheric oxidants and environmental conditions on the process.

While this study provides significant advances in our understanding of alkane autoxidation, some limitations should be noted. The kinetic model used to interpret the results simplifies the complex chemistry involved, potentially overlooking some minor reactions or isomerization pathways. Furthermore, the experiments were conducted under specific, controlled conditions, and the generalizability of the findings to the full range of atmospheric conditions remains to be fully explored. Additional research is needed to confirm these findings across a wider range of environmental variables.