Application of Boehm Titration for the Quantitative Measurement of Soot Oxygen Functional Groups

The oxygen functional groups on diesel soot have potential effects on its oxidation reactivity, which is of great importance for diesel particulate filter (DPF) operation. For the quantitative measurement of oxygen functional groups on diesel soot, Boehm titration was adopted and procedures considering CO2 removal and sample characteristics involving sample mass as well as the interferences of soluble organic fractions (SOFs) were evaluated to ensure the titration precision possible. Titration was also conducted on samples of nitric acid (HNO3)-modified soot as well as real soot samples and the results were combined with X-ray photoelectron spectroscopy (XPS) analysis to verify the reliability of titration procedures. Results showed that heating at 105 °C for 90 min can eliminate CO2 interference to an acceptable extent. Integrated with mixing and titration error, a sample mass of 70 mg showed the best repeatability. Very little oxygen functional groups were detected for pure diesel oil. Titration with samples varied in SOF content showed a comparable amount of oxygen functional groups, indicating that SOF wrapped around soot surface had little physical and chemical effect on titration. Titration tests with HNO3-modified samples correctly reflect the changing trend of oxygen functional groups after acid treatment, which was further verified by XPS analysis. However, due to the relatively large standard deviation and error propagation, the concentration value of oxygen functional groups calculated by subtraction often showed a large error range, especially for low-concentration samples. Titration error was much lower for high-concentration samples. For real diesel soot, much higher concentrations of 941 ± 119, 376 ± 151, and 903 ± 106 μmol/g were measured for phenolic hydroxyl, lactones, and carboxyl groups, respectively. In all, the Boehm titration process conducted in this article showed the capability and reliability of the quantitative measurement of oxygen functional groups on soot surface. Coupled with XPS analysis, more information about the current state and possible transformation process of carbonyl groups and the possible distribution of functional groups between internal and external surfaces can be inferred.