Analysis of graphene structure in particulate matter emitted from diesel engine

Abstract: Particulate matter emitted from diesel engine is a factor that makes PM2.5 (particulate matter with a diameter of lower than 2.5 μm) increase, which is related to several adverse health effects including respiratory tract inflammation and cancer. Particulate matter is classified to 3 size modes, i.e. the nucleation mode (<50 nm), the accumulation mode (100-1 000 nm) and the coarse mode (>1 000 nm). As is known to all, particulate matter with smaller size does more harm to human than particulate matter with larger size. It is very necessary to carry out research on reducing particulate matter emitted from diesel engine, especially particulate matter with smaller size. It has been demonstrated that there is graphene structure in particulate matter emitted from diesel engine and graphene structure is related to particulate matter removal. Micro-orifice uniform deposition impactor which was produced in MSP company in America was used to collect diesel particulate matter with 3 size ranges, which were 0.18-0.32 μm, >0.32-0.56 μm and >0.56-1 μm respectively. Raman spectroscopy, a fast and nondestructive method, was used to test crystal structure of carbon material. Near edge X-ray absorption spectra, a nondestructive method, was adopted to characterize molecular structure and valence state of carbon atom by using synchrotron radiation technique. DXR Raman spectrometer and soft X-ray microscopy beamline station were used to analyze the defect type of graphene structure, degree of graphitization, crystallite size of graphene, neighboring graphene spacing, molecular structure and valence state of carbon atom. The results showed that the ratio of D1 peak to D2 peak ranged from 3.34 to 4.01, which indicated that the defect type of graphene structure in diesel particulate matter mainly was graphene edge defect. With the size of particulate matter decreasing, the proportion of graphene edge defect increased. When the size of particulate matter increased, width at half maximum of D1 peak increased by 2.8 and 6.7 cm-1, indicating that the material type in particulate matter increased and the chemical heterogeneity of particulate matter was higher. The ratio of D1 peak to G peak decreased by 14.67% and 27.17% respectively with particulate matter changing from small size to the other 2 bigger sizes, which indicated that the proportion of order graphene in diesel particulate matter increased and led to higher graphitic-like structure and degree of graphitization. When the size of particulate matter increased, the relative intensity of D3 peak decreased by 13.73% and 39.22%, respectively. That was to say the proportion of amorphous carbon especially organic ingredients reduced. The length of C-C bond in graphene lattice had no relationship with the size of diesel particulate matter. The length of C-C bond in graphene lattice of diesel particulate matter with 3 size ranges was about 0.142 nm. When the size of particulate matter increased, the crystallite size of graphene increased and the neighboring graphene spacing decreased, which indicated the oxidative reactivity of particulate matter reduced and the energy in the process of oxidizing diesel particulate matter increased. The types of surface functional groups in particulate matter had no relationship with the size of diesel particulate matter. There were many types of surface functional groups which were "graphene" C=C, C=C, aliphatic C=C, phenolic C-OH, ketones C=O, aliphatic C-H and carboxy C=O respectively. With the size of particulate matter increasing, aliphatic C=C, aliphatic C-H, carboxy C=O in diesel particulate matter decreased and "graphite" C=C, phenolic C-OH, ketones C=O in diesel particulate matter increased. This study can provide reference for the perfection of the formation mechanism of diesel particulate matter with different size range and for the optimization of particulate matter removal device.