Hydropyrolysis: towards fluorescence-free Raman spectroscopy analysis of internal diesel injector deposits

Micro-Raman spectroscopy (μRS) is an established analytical tool enabling facile determination of the structural ordering within carbonaceous materials, including the particulate-matter found in vehicle deposits and environmental matrices. However, if fluorescent species are present within the material under examination, μRS analysis can be at best complicated and at worst impossible. Common methods to circumvent this issue, such as changing the excitation laser wavelength and/or photobleaching, are not always possible. In this study, we demonstrate pyrolysis under high hydrogen pressures (hydropyrolysis, HyPy) as an effective thermal treatment for the removal of fluorescent species at relatively low temperatures (350 °C), without significantly altering the structure of the parent carbonaceous material. This was illustrated through μRS investigation of a series of six carbon reference samples, whereby, after HyPy, the interference from fluorescence was significantly reduced, whilst the positions, widths and intensity ratios of the diagnostic D and G bands remained largely unchanged. Application of hydropyrolysis to a series of five internal diesel injector deposits (IDIDs) enabled a μRS investigation of the physicochemical structure of the deposited carbons for the first time. Moreover, mass spectrometry analysis of the volatile species removed during HyPy of IDIDs where engine failures had occurred suggested that linear polyunsaturated n-C₁₆ and n-C₁₈ alkenes were likely responsible for the fluorescence. As HyPy can be readily applied to a variety of carbonaceous materials, for example, petroleum source rocks contaminated with drilling muds, the approach we describe here represents a general strategy for fluorescence suppression enabling structural investigation of carbons by μRS.