Experimental Study On Effect Of Oxygen Functional Groups On Diesel Soot Oxidation Reactivity

Diesel Particulate Filter(DPF)is a technology to effectively reduce the emission of diesel soot.The deposited soot needs periodic oxidation and removal to regenerate DPF,so the key to DPF regeneration is soot oxidation.Oxygen functional groups on the surface of soot have an important influence on its oxidation reactivity.Currently,most studies have qualitatively analyzed the correlation between some functional groups and soot oxidation based on XPS and other spectroscopic measurement methods.In this paper,the Boehm titration method was used to quantitatively analyze the oxygen functional groups,and the effects of different oxygen functional groups on the soot oxidation reactivity were further explored to provide theoretical guidance for the efficient regeneration of DPF.The surface of diesel soot usually contains soluble organic matter(SOF,Soluble Organic Fraction),which may hinder the reaction between reaction base and oxygen functional groups,and interfere with the quantitative measurement of oxygen functional groups by Boehm titration.The study used simulated soot mixed with diesel to make soot samples with different SOF content,and investigated the effect of physical package on the measurement.It was found that different content of SOF had little effect on the measurement results.The measurement of modified and real soot samples based on Boehm titration method showed that this method can better reflect the changes of oxygen functional groups of different samples.The content of hydroxyl groups,lactone groups and carboxyl groups in the diesel soot were 942 umol/g、377 umol/g 、903 umol/g,respectively.During the soot oxidation process,SOF and dry soot are oxidized at different temperature intervals,and the oxidation reactivity between different soot samples is difficult to compare.Based on thermogravimetric analysis,Coats-Redfern and DAEM methods were used to systematically compare the analysis methods for the oxidation activity of different soot samples.The results showed that for soot samples without SOF,Coats-Redfern or DAEM method could be used to directly calculate the activation energy of the reaction,and the average value was taken as the comparison benchmark.For the soot samples containing SOF,the Coats-Redfern method would cause a deviation in the calculation of the activation energy.It was necessary to analyze the activation energy of dry soot and SOF in stages by the DAEM method,and take the average value of dry soot as a benchmark.Through chemical modification and temperature-programmed heat treatment to change the content of oxygen functional groups in soot samples,and the influence of different oxygen functional groups on the soot oxidation reactivity was explored.It was found that carboxyl groups(-COOH)acted before 400℃ and hydroxyl groups(C-OH)acted at 400-600℃.They could reduce the activation energy of soot oxidation and promote the oxidation at low temperatures.The effect of carboxyl groups on oxidation reactivity was greater than that of hydroxyl groups.The microstructure of different soot during oxidation processes were further analysised based on TEM,it was found that carboxyl and hydroxyl groups can increase the soot reaction rate of the external combustion mode,but did not change the soot combustion mode.