Tracing oxygen incorporation in flame-formed soot from oxygenated fuels

Abstract

The influence of oxygenated fuels on soot oxidation chemistry was investigated to clarify the role of acetaldehyde (CH₃CHO) in oxygen incorporation onto soot particles. Premixed fuel-rich flames of ethylene (C₂H₄) and six oxygenated blends – ethanol (EtOH), oxymethylene ether-3 (OME₃), and four butanol isomers – were studied under identical conditions (Φ = 2.46). Soot samples were characterized using infrared spectroscopy (FTIR), UV–visible (UV–vis) spectroscopy, and thermogravimetric analysis (TGA), while CH₃CHO concentrations were measured in the exhaust. Specifically, to quantify oxygen in the collected particles, TGA in an inert environment was combined with UV–vis spectroscopy. UV–vis measurements were used to estimate the volatile organic fraction, enabling attribution of the remaining TGA mass loss to oxygen-containing surface groups. A consistent correlation between soot oxygen content and CH₃CHO concentration was observed across all fuels, identifying CH₃CHO as a key oxidizing intermediate in particle growth. Building on this evidence, a surface reaction model inspired by analogous gas-phase kinetics was developed and implemented within a sectional framework that accounts for particle-size-dependent reactivity. Using a single effective rate constant, the model accurately reproduced the experimentally measured oxygen content within 15 wt.% of relative error for all cases. These results establish a quantitative link between gas-phase oxidation intermediates and soot surface functionalization, providing a framework to integrate oxygen-addition chemistry into predictive soot models and flame synthesis applications.

Novelty and significance statement

Despite the recognized potential of oxygenated fuels to lower soot emissions, the mechanism by which oxygen becomes incorporated into soot has remained unresolved. This work identifies acetaldehyde (CH₃CHO) as a key gas-phase driver of soot oxygenation and establishes, for the first time, a quantitative correlation between CH₃CHO concentration and soot oxygen content across a broad set of oxygenated-fuel flames. By combining complementary diagnostics, soot oxygenation was independently assessed and linked to gas-phase chemistry. A lumped kinetic scheme, inspired by the analogous gas-phase C₆H₅• + CH₃CHO reaction and implemented in a sectional soot model, reproduced the measured oxygen content using a single effective rate constant. These results move soot oxygenation from a qualitative observation to a predictive framework, enabling integration of oxygen-functionalization pathways into combustion and flame-synthesis models. Beyond emission control, the study opens a route to tailoring the surface chemistry of flame-formed carbon nanostructures through fuel-dependent oxidation chemistry.