3.2.3. BET Analysis of Catalysts
After loading Mn or Mn-Ce on the TiO
2 powder, the specific surface area of the catalysts was improved to varying degrees compared with that of TiO
2. The pore volume and pore size of the catalyst loaded with a single Mn element were lower than those of TiO
2, and the pore volume of the catalyst loaded with Mn and Ce was higher than that of TiO
2. The pore size of the Mn-Ce catalysts, except for M10C5, significantly increased compared with that of TiO
2. The surface parameters of TiO
2 and various catalysts are shown in
Table 3. It can be seen from
Table 5 that the specific surface area of the TiO
2 powder was only 12.53 m
2·g
−1. After adding 10% Mn, the specific surface area slightly increased to 13.10 m
2·g
−1. After adding Mn and Ce at the same time, the specific surface area was significantly increased. The catalyst with 10% Mn and 3% Ce content had the largest specific surface area, which was 55.9% higher than that of TiO
2. As the Ce content continued to increase, the specific surface area of the catalyst began to decrease.
After loading Mn on TiO
2, the pore volume decreased. It may be that the oxide of Mn blocked the mesopores between some TiO
2 particles. The addition of Ce significantly increased the pore volume of Mn-based catalysts, which was larger than the pore volume of TiO
2. It is believed that the simultaneous addition of Ce and Mn inhibits the aggregation of Mn species, making them well dispersed on the carrier surface in an amorphous state [
24], thus improving the pore volume of the catalyst. The pore volume of the M10C3 catalyst was the largest, increasing by about 83.1% compared with that of TiO
2. As the Ce content continued to increase, the pore volume of the catalyst began to decrease. When the Ce content reached 5%, the pore volume of the catalyst decreased significantly.
Similar to the change in pore volume, the addition of Mn reduced the average aperture of TiO2, which may have been due to the adhesion of Mn oxide on the surface of TiO2 particles, making the pore size between particles decrease or even disappear. After Ce was added, the pore size increased significantly, and the average aperture of the M10C3 catalyst with the best catalytic efficiency was 3.2% higher than that of TiO2. However, from the data, there was no obvious relationship between the change in the average aperture and the amount of Ce added.
Figure 8 shows the change trend of the surface parameters of TiO
2 and various catalysts. It can be seen from the figure that, when the Ce content increased from 1% to 3%, the average aperture of the catalyst decreased; when the Ce content increased from 3% to 4%, the average aperture of the catalyst increased; and when the Ce content increased to 5%, the average aperture of the catalyst decreased significantly. Combined with the change analysis of the total pore volume, the simultaneous addition of Mn and a small amount of Ce created more larger pores and improved the average aperture of the catalyst. With the increase in the Ce content, the average aperture of the catalyst began to decrease gradually, but the total pore volume still increased, indicating that the number of macropores in the catalyst decreased, and the number of small pores increased. When the Ce content was 3%, the denitration efficiency of the catalyst reached the highest value. At this time, the average aperture was the smallest among the Mn-Ce catalysts. However, the total pore volume was the largest. Therefore, the efficiency of the catalyst was not directly related to the size of the average aperture. The large number of evenly distributed pores in the catalyst was more conducive to the denitration reaction.
The isothermal adsorption and desorption diagram and pore size distribution curve of the sample are shown in
Figure 9 and
Figure 10. The isotherms of the samples were all type IV isotherms with obvious H3 mesoporous hysteresis loops, which proved that the mesopores in the samples were mainly formed by the stacking of TiO
2 particles [
25], and the addition of Mn blocked some large pores, thus leading to the reduction in pore volume. After Ce was added, the area of the hysteresis loop increased significantly, indicating that the number of mesopores in the sample increased, and that the joint addition of Mn and Ce increased the number of mesopores in the catalyst, thus enhancing the gas adsorption capacity of the sample. When the Ce content was greater than 3%, the area of the hysteresis loop decreased slightly, and at this time, the denitration efficiency of the catalyst also began to decrease, indicating that the addition of excessive Ce reduces the number of mesopores in the catalyst. This would affect the denitration performance of the catalyst.
From
Figure 10, it can be seen that the addition of Mn increased the number of mesopores in the range of 2~4 nm and decreased the number of large pores above 50 nm, thus decreasing the average pore size of TiO
2. After the addition of Ce, the number of mesopores from 2 to 4 nm started to decrease, and the number of mesopores from 10 to 50 nm increased. Therefore, the average pore size of the catalyst increased. At a Ce content of 3%, the number of mesopores from 10 to 50 nm increased, the number of pores around 20 nm was the highest, and the average pore size of the catalyst at this time was 20.47 nm, which indicated that the distribution of pores was more uniform in the M10C3 catalyst. As the Ce content continued to increase, the number of pores in the 10–50 nm range began to decrease gradually, and the denitration effect of the catalyst began to deteriorate gradually at this time.
Combining the above surface parameter changes with the pore size distribution analysis, it can be seen that the addition of Mn created a large number of small pores and also consumed some of the larger pores, thus reducing the average pore size and pore volume compared to those of TiO2. As the Ce content increased, the combination of Mn and Ce tended to be more uniform, and the M10C3 catalyst had a large number of mesopores around 20 nm. Although the average pore size was reduced, the distribution of the pores was more uniform, the pore capacity of the catalyst surface had a greater influence on the catalyst, and the uniformly distributed mesopores of around 20 nm were more conducive to the reaction. In addition, the addition of excess Ce would block the pores and reduce the number of these mesopores, resulting in the poor denitration performance of the catalyst.
3.2.5. XPS Analysis of Catalysts
Figure 12 shows the O1s XPS spectra of some catalysts. The O1s spectra of the catalysts were fitted as two characteristic peaks representing the O
α and O
β of chemisorbed oxygen on the surface of the catalysts [
2], which were located at 529.9 eV and 531.4 eV, respectively. It can be seen from the diagram that the peak area of O
α increased significantly with the increase in the Ce content. When the Ce content reached 2%, the peak area of O
α began to decrease.
Table 6 shows the relative content of O on the surface of the catalyst. The ratio of O
α/O
α+ O
β increased first and then decreased with the increase in the Ce content. The literature review shows that O
α is more active than O
β and more conducive to the reaction [
26]. The changes reflected by the XPS spectra are in agreement with the experimental results.
Figure 13 shows the XPS spectrum of the Mn2p orbital of the M10~M10C5 catalysts. The Mn2p orbital of the catalysts has two characteristic peaks, Mn2p1/2 and Mn2p3/2 [
27], which are located at 638.6 eV and 649.9 eV, respectively. Mn exists as Mn
3+, Mn
4+ and Mn-sat [
28]. It is known from the literature that Mn
4+ is more active in the catalytic reaction [
29].
Table 7 shows the relative content of Mn on the surface of the catalysts. The relative content of Mn
4+ was the highest before the addition of the Ce element. After the addition of 1% Ce, the relative content of Mn
4+ rapidly decreased. By continuing to add Ce, the relative content of Mn
4+ began to rise, and the Ce content reached 3%. The further addition of Ce would reduce the Mn
4+ content on the surface of the catalysts.
The results show that the M10 catalyst without Ce had a higher content of Mn4+ but a lower denitration efficiency than the Mn-Ce catalyst. Although the content of Mn4+ decreased after adding Ce, the catalytic effect was improved to a certain extent, indicating that the relative content of Mn4+ was not the only factor affecting the catalytic effect. From the data trend, as the content of Mn4+ or Ce added to the Mn-Ce catalyst becomes higher, the catalytic effect becomes better. This indicates that the different valence states of Ce on the surface of the catalyst also had an important influence on the catalytic effect.
Figure 14 shows the XPS spectrum of the Ce3d orbital of the M10C1~M10C5 catalyst. The peaks at u1 and v1 represent the peaks of Ce
3+3d3/2 and Ce
3+3d5/2, respectively. The peaks at u0, u2 and u3 and v0, v2 and v3 are Ce
4+3d3/2 and Ce
4+3d5/2, respectively. The coexistence of Ce
3+ and Ce
4+ facilitates the storage and release of oxygen [
30]. The presence of Ce
3+ is more conducive to the generation of oxygen voids and can improve the denitration effect of the catalyst and the mercury oxidation rate [
1]. The relative content of Ce
3+ in the sample increased first and then decreased with the increase in the Ce content, which was the highest in M10C3 (25.31%).
The relative content of Ce on the surface of the catalysts is shown in
Table 8. The change trend of Ce
3+ in the Mn-Ce catalyst was similar to that of Mn
4+ in that it increased first and then decreased. The highest Ce content was 3%. Based on the analysis of the experimental results, although the O
α content in the M10C2 catalyst was high, the catalytic effect of the M10C3 catalyst was the best. Therefore, the relative content of O
α in the Mn-Ce catalyst had a greater influence on the catalytic effect than that of Mn
4+ and Ce
3+.