3.1. NOx conversion performances of catalysts
The operating temperature range of the tests conducted to investigate the catalytic activity was between 200 and 280°C. The results of tests performed to investigate the effect of phosphorus on the activity of the CeMn/TiO2 catalyst were shown in Fig. 2.
It was observed that the activity of all catalysts increased depending on the temperature increase. Maximum NOx conversion ratios of CeMn/TiO2, 0.5P-CeMn/TiO2 and 1P-CeMn/TiO2 catalysts at 280°C were 84.6%, 80.3% and 78.5%, respectively. In our previous studies, we proved that the positive effect of temperature increases on catalytic activity (Keskina, 2021; Keskin et al. 2021; Keskinb, 2021). The lowest NOx conversion ratios were obtained at 200°C and were 67.8%, 67.5% and 64.5% for the CeMn/TiO2, 0.5P-CeMn/TiO2 and 1P-CeMn/TiO2 catalysts, respectively.
Previous research has shown that phosphorus leads to a decrease in catalytic activity due to its negative effect on the chemical properties of the catalyst. You et al. (2017) found that the CeMo/Ti catalyst exhibited 100% NO conversion at 400°C, whereas the 1.32P-CeMo/Ti catalyst showed 57% NO conversion. Chen et al. (2018) proved that the NO conversion of Cu-S catalyst was 83% at 250 ℃, while the NO conversions of 0.1, 0.2 and 0.4 P loaded Cu-S catalysts were 73%, 65% and 45%, respectively.
It was observed that the NOx conversion of the CeMn/TiO2 and 0.5P-CeMn/TiO2 catalysts at low temperatures was close. However, the increase in CeMn/TiO2 catalyst activity was greater with the increase in temperature. It was determined that the 1P-CeMn/TiO2 catalyst activity was lower than the activity of other catalysts at all temperatures. In the tests, it was seen that phosphorus negatively affected the catalytic activity. As the phosphorus content in the catalyst increased, the catalyst activity decreased significantly. The decrease in NOx conversion ratios of poisoned catalysts was higher at temperatures between 240 and 280°C. When the NOx conversion ratios of the CeMn/TiO2 and the poisoned catalysts were compared at 280°C and 1 kW engine load, the decrease in the NOx conversion ratio for the 0.5P-CeMn/TiO2 and 1P-CeMn/TiO2 catalysts was 5.4% and 8.9%, respectively. The results proved that the increase in the catalysts’ phosphorus content remarkably affects the catalytic activity.
When the NOx conversion ratios at 1 kW and 3 kW engine loads were compared, a difference was found approximately 1.3% with CeMn/TiO2 catalyst, 1.1% with 0.5P-CeMn/TiO2 catalyst, and 1.5% with 1P-CeMn/TiO2 catalyst. It was seen that the difference in NOx conversion ratios obtained at different engine loads was not much. As a result, it was concluded that the catalysts maintain their activity at all engine loads and the catalyst production method was successful. Although the difference between the two engine loads is not much, it was seen that the increase in engine load caused an increase in catalytic activity and higher NOx conversion ratios were obtained at 3 kW engine load. As can be seen in Table 1, the oxygen concentration decreased due to the increase in engine loads. The reason for this was thought that the decrease in oxygen concentration due to the increase in engine load and the decrease in the effect of DOC. Similar results were also found by researchers (Keskinb, 2021 Ahmad et al. 2020; Yaşar et al. 2019).
3.2. Characterization of catalysts
The surface morphology and structure of the fresh catalyst and poisoned catalysts were observed by SEM analysis. SEM images of CeMn/TiO2, 0.5P-CeMn/TiO2 and 1P-CeMn/TiO2 catalysts at 10000 times magnification were shown in Fig. 3. SEM images showed that the particles were very small size and were homogeneously dispersed over the both pores and also surface of the catalysts. The crystallization in both fresh and also poisoned catalysts was seen. It could be considered that this crystallization is the result of the calcination of catalytic elements. As seen in Fig. 3, the crystallization of the poisoned catalysts was affected by the phosphorus ratios and increased on the 1P-CeMn/TiO2 catalyst. The homogeneous distribution in small particles has an increasing effect on the catalytic activity. For this reason, the activity of 1P-CeMn/TiO2 catalyst may have been lower than other catalysts. In addition, pores of the 1P-CeMn/TiO2 catalyst were not as clear as other catalysts. It was thought that some pores were blocked by the phosphorus elements. The decrease in catalytic activity may be due to pores blockage.
The EDX mapping was performed to examine the distributions of elements on the catalysts and was shown in Fig. 4. Oxygen, magnesium, aluminum and silicon were present in all catalysts. These elements existed the structure of cordierite, which is used as the main material in catalyst synthesis. The TiO
2 compounds show both catalytic effect and increase the surface area. Therefore, its distribution on the surface significantly affects the catalytic activity. In EDX mapping, it was seen that TiO
2 had the highest ratio in all catalysts. However, the increase in the amount of phosphorus negatively affected the distribution of TiO
2 on the catalyst surface. The reduction of TiO
2 on the surface of the 1P-CeMn/TiO
2 catalyst may have caused the decrease catalytic activity.
It was observed that cerium and manganese elements, which showed catalytic effects, showed good distribution in all catalysts. With EDX mappings, it was proven that phosphorus exists in poisoned catalysts and therefore reduces the catalytic effect.
The surface area has a significant effect on catalytic activity. Therefore, surface area analyzes of the catalysts were performed and the BET surface area and micropore volumes of the catalysts were given in Table 2. According to BET results, the surface area of CeMn/TiO2 catalyst was 25.55 m2/g and had the highest surface area. The surface area of the catalysts containing phosphorus was lower and the surface area decreased further as the phosphorus loading ratio increased. The reduction in the surface area of the 0.5P-CeMn/TiO2 catalyst was 47.2% while it was 51.7% for the 1P-CeMn/TiO2 catalyst. The reduction in surface area is one of the reasons for the reduced NOx conversion ratios of poisoned catalysts. Because the regions where NOx reduction reactions take place on the surface decreased. When the micropore volumes were compared, it was seen that the micropore volumes of the phosphorus containing catalysts were lower and these results were compatible with the SEM images. It is known that an increase in micropore volume positively affects catalytic activity. The best activity of the CeMn/TiO2 catalyst can be due to the higher surface and micropores volume than other catalysts.
Table 2
BET surface area and micropore volume of catalysts
|
CeMn/TiO2
|
0.5P-CeMn/TiO2
|
1P-CeMn/TiO2
|
BET surface area (m2/g)
|
25.55
|
13.48
|
12.33
|
Micropore volume (mm3/g)
|
6.03
|
4.6
|
2.28
|
The crystal structure of catalytic elements on the surface provides information about the catalyst activity. Therefore, XRD analysis was performed and results were presented in Fig. 5. The results of the analysis indicated that cordierite has the highest intensity peaks on the catalysts due to its use as the main structure. Peaks at 2θ = 10.42 °, 26.35 °, 29.43 ° were associated with the cordierite. It was seen that the catalytic elements were contributed to the crystal structure. The peaks at 2θ = 21.8 ° and 2θ = 28.1 ° were associated with MnO2 and CeO2, respectively. After phosphorus addition observed no difference in the shape and intensity of these peaks. The catalytic activities of the catalysts were close to each other, especially at 200°C, which could be attributed to the fact that the peak intensity of the cerium and manganese does not change. As can be seen in Fig. 5, no peaks of TiO2 were found. The no visible TiO2 peaks may have been due to their very good surface distribution or strong interaction with the coating elements. These results were consistent with EDX analysis. Phosphorus peaks were observed on 0.5P-CeMn/TiO2 and 1P-CeMn/TiO2 catalysts at 2θ = 35 °. It was concluded that the activity of the poisoned catalysts was lower than the CeMn/TiO2 catalyst due to the phosphorus element. In addition, the increase in the intensity of the phosphorus peaks with increasing phosphorus content explained that 0.5P-CeMn/TiO2 catalyst activity was higher than the 1P-CeMn/TiO2 catalyst activity.
The chemical state of catalytic element species was examined with UV-Vis spectra and results were shown in Fig. 6. The chemical state of catalytic elements plays an important role in catalytic reductions. It was observed that all catalysts showed intense absorption bands and the absorption edges were noticeable. TiO2 provides active sites for NOx reduction, thus increasing catalytic activity. The characteristic strong absorption bands were observed at approximately 380 nm, which is compatible with anatase TiO2 (Tian et al. 2015). The intensity of the adsorption band of TiO2 did not change in all catalysts. According to previous studies (Chen et al. 2019; Gupta et al. 2017) the bands at 340–400 nm can be attributed to CeO2 and absorption band at 316 nm, corresponded to MnO2.
The absorption band intensity of poisoned catalysts with an increase of phosphorus loading was not changed. However, a big difference was seen between the CeMn/TiO2 and poisoned catalysts in the range between 260 and 320 nm. Obvious changes were observed after the phosphorus was added to the catalyst, proving that phosphorus significantly affects the activity. 0.5P-CeMn/TiO2 and 1P-CeMn/TiO2 catalysts showed a higher absorption from 470 nm to 710 nm than CeMn/TiO2 catalyst, indicating that the absorption increases with the increase of the wavelength. Agglomeration of catalytic elements during sintering can affect catalytic activity. The absorption band increases as increase in wavelength due to aggregation in the UV spectrum. In 0.5P-CeMn/TiO2 and 1P-CeMn/TiO2 catalysts, it was thought that increase in the absorption band intensity at 470–710 nm wavelength range might be due to agglomeration. These results supported the reduction in the catalytic activity of the poisoned catalysts.