*2.4. XPS Results Analysis*

### 2.4.1. Oxygen Element Analysis

XPS is often used to characterize the existence of oxygen, nitrogen and other heteroatoms in coal, which provides an important basis for the construction of macromolecular structure model of HLH [18]. XPS tests of HLH raw coal, microwave modified HLH and ultrasonic modified HLH were carried out, and the XPS maps of 3 samples were processed by peak fitting. The peak-splitting diagram is shown in Figure 3, and the form and content of nitrogen elements are shown in Table 8.

**Figure 3.** XPS peak fitting of oxygen atoms of HLH before and after modification.

The main forms of oxygen in HLH coal samples are C=O, C-O, -OH and COO-, and it can be found in Table 8 that oxygen exists in most of the four forms of C-O in structure. The content of C-O in HLH samples after microwave modification decreases 2%, but the content of COO-structure form decreases from 14.37% to 35.25% of raw coal. 12.37%. The reason for this change may be that microwave treatment destroys the oxygen structure of HLH raw coal. The content of oxygen-OH and C=O in HLH coal samples treated by ultrasonic wave also increased obviously, while the content of CO-form in opposite direction was still decreasing. Therefore, the microwave and ultrasonic treatment of HLH has different effects on the existing forms of oxygen elements. The stability of COO-and C-O structure forms is relatively poor and easy to be destroyed in the process of modification. On the contrary, the stability of C=O structure makes it not easy to be destroyed in the process of ultrasonic and microwave treatment, which increases the proportion of C=O in the structure.


**Table 8.** XPS detection and analysis of oxygen composition forms of HLH before and after modification.

### 2.4.2. Nitrogen Element Analysis

Nitrogen in coal mainly comes from coal-forming plants, and most of it exists in the form of organic matter, which mainly includes pyridine nitrogen (N-6), pyrrole nitrogen (N-5), nitrogen oxides (N-X) and proton nitrogen (N-Q) [19,20]. In order to characterize the forms of nitrogen elements in HLH before and after modification, the XPS spectra of HLH lignite were fitted by peak-splitting method. The peak-splitting diagram is shown in Figure 4, and the forms and contents of nitrogen elements are shown in Table 9.

**Figure 4.** XPS peak fitting of nitrogen atoms of HLH before and after modification.


**Table 9.** XPS detection and analysis of nitrogen composition forms of HLH before and after modification.

By comparing the XPS data of N element in HLH before and after modification, it is obvious that the percentages of N-5, N-6 and N-Q in HLH have changed significantly. The percentage of N-Q in pulverized coal decreased to 31.28% and 30.78% respectively after modification, and the corresponding proportion of N-5 and N-6 increased to varying degrees. The total amount of N-5 and N-6 of modified HLH is nearly 70%. It can be seen that microwave and ultrasonic modification methods mainly play a role in N-Q, while N-5 and N-6 form of nitrogen bond are relatively stable, and the above modification methods have little effect on it. Therefore, in order to make the model representative, N-5 and N-6 nitrogen bonds are often used in the construction of macromolecule HLH lignite structure model.

#### *2.5. Raman Results Analysis*

There are two relatively broad D and G peaks in the Raman spectra of HLH, and two vibration peaks have abundant information, the G peak in lignite does not really represent its crystal structure. It mainly reflects the strength of the stretching vibration bond of aromatic rings [21,22]. Raman spectra of coal samples are exhibited in Figure 5.

**Figure 5.** Raman spectra of HLH coal samples before and after modification.

There are many overlapping peaks between D and G peaks in Raman spectra of coal. In order to obtain more accurate functional group information of HLH lignite before and after modification, Position and area values of D and G peaks of coal samples obtained by deconvolution process of Raman spectra shown in Figure 6. Raman fitting parameters of HLH coal samples before and after modification are shown in Table 10.

**Figure 6.** Raman peak separation fitting charts of HLH coal samples before and after reformation.


**Table 10.** Raman spectrum structural parameters of different coal samples.

From the AD and AG values of coal samples before and after modification, it can be found that the D peak area of coal samples after microwave and ultrasonic treatment decreases, especially the D peak area of coal samples after microwave treatment decreases significantly, which also shows that microwave treatment makes HLH structure more complete. Compared with G peak area AG, the AG value of HLH structure after microwave modification is significantly smaller than that of HLH. It can be seen that the total number of aromatic rings in macromolecular structure of HLH treated by microwave is the smallest, and the enrichment degree of aromatic carbon is the lowest, followed by the macromolecular structure of HLH coal treated by ultrasound, while the content of aromatic rings in the macromolecular structure of HLH coal is the highest and the enrichment degree of aromatic carbon is the largest.

ID/IG is usually used to evaluate the disordering degree of carbon materials, and it decreases with the increase of graphitization degree. After microwave treatment, the ID/IG value of HLH was reduced from 0.81 to 0.78. Similarly, after ultrasonic treatment, the ID/IG value of HLH was also reduced to 0.75, and the ID/IG value was reduced. This indicated that the ordering degree of aromatic ring layers in the structure increased and the content of fat chain and side chain decreased after microwave and ultrasonic treatment.
