Chapter 2 ~ Experimental
2.7 Ethanol pulse reaction
Ethanol pulse reaction experiments were carried out using a Micromeritics AutoChem Ⅱ 2920 instrument. A Thermo SCIENTFIC ProLab mass spectrometer was attached and used as the detector. Prior to the experiment, a weight of approximately 0.1 g of sample was first reduced under 20 ml/min of 10% H2 in Ar flow at 600 oC for 20 min (the temperature was increased to 600 oC at a ramp rate of 30 oC/min, then hold for 20 min). After reduction, the system was purged with 50 ml/min of He flow to remove residual hydrogen and cooled down to room temperature, subsequently ramped to the reaction temperature and sent pulse of an ethanol/helium mixture, which was obtained by flowing He through a saturator containing ethanol at 70 oC. The pulse reaction was carried out at 200 oC, 300 oC, 400
oC, 500 oC, and 600 oC respectively.
Chapter 3
Results and Discussion
3.1 Catalyst preparation
A series of CeO2- and SiO2- supported catalysts was prepared and investigated in order to further understand about ethanol reforming process. For CeO2- supported catalysts (CeO2, Pt/CeO2, high surface area CeO2, and Pt/high surface area CeO2), two kinds of CeO2 supports were used in this work. One is purchased CeO2, and the other is high surface area CeO2. Pt was loaded to these supports by incipient wetness impregnation method. High surface area CeO2 provided a contrast with more available active site.
For SiO2- supported catalysts (SiO2, Ce/SiO2, Pt/SiO2, and Pt/Ce/SiO2), the support was commercial fume SiO2. Ce and Pt were loaded to the support by incipient wetness impregnation method.Ce/SiO2 and Pt/Ce/SiO2 provided another alternative to investigate the effect of Pt addition on ceria while Pt/SiO2 was taken as a ceria-free contrast. The diluted Pt (mixture of PtO2 and SiO2) was also prepared for comparing with Pt/SiO2 catalyst.
3.2 X-ray diffraction (XRD)
X-ray diffraction measurements provide the information about crystallization and crystal size of samples. The XRD patterns of the catalysts are shown in Figure 3 ~ 12.
For CeO2 (Figure 3) and high surface area CeO2 (Figure 5), the patterns exhibited the characteristic peaks of ceria at 2θ= 28.6 o, 33.2 o, 47.5 o, 56.3 o, 59.1 o, 69.3 o, 76.7 o, 79.0 o, and 88.3 o. CeO2 exhibits very sharp and strong peaks while high surface area CeO2 shows broader peaks because of its smaller crystal size. The pure SiO2 (Figure 7) itself exhibited no peaks (amorphous structure), while Ce/SiO2 (Figure 8) revealed the peaks corresponding to ceria (2θ= 28.6 o, 33.2 o, 47.5 o, 56.3 o), indicating crystallized ceria was formed on the surface of SiO2. The peaks observed for Ce/SiO2
are much weaker and broader than which observed for CeO2 because of its smaller crystal size and less amount of ceria. For all the samples with Pt loading (Pt/CeO2, Pt/high surface area CeO2, Pt/SiO2, and Pt/Ce/SiO2), no Pt characteristic peaks was observed. This may due to the low loading of Pt. Similarly, no characteristic peaks of PtO2 (Figure 12) was observed on the diluted Pt (Figure 11).
10 20 30 40 50 60 70 80 90 100
2θ CeO2
Figure 3. XRD profile of CeO2.
10 20 30 40 50 60 70 80 90 100
Pt/CeO2
2θ
Figure 4. XRD profile of Pt/CeO2.
10 20 30 40 50 60 70 80 90 100
2θ High surface area CeO2
Figure 5. XRD profile of high surface area CeO2.
10 20 30 40 50 60 70 80 90 100
2θ Pt / High surface area CeO2
Figure 6. XRD profile of Pt/high surface area CeO2.
10 20 30 40 50 60 70 80 90 100
2θ SiO2
Figure 7. XRD profile of SiO2.
10 20 30 40 50 60 70 80 90 100
2θ Ce / SiO2
Figure 8. XRD profile of Ce/SiO2.
10 20 30 40 50 60 70 80 90 100
2θ Pt / SiO2
Figure 9. XRD profile of Pt/SiO2.
10 20 30 40 50 60 70 80 90 100
2θ Pt / Ce / SiO2
Figure 10. XRD profile of Pt/Ce/SiO2.
10 20 30 40 50 60 70 80 90 100
2θ PtO2 SiO2
Figure 11. XRD profile of diluted Pt.
0 10 20 30 40 50 60 70 80 90
2θ PtO2
Figure 12. XRD profile of PtO2.
3.3 N2 physisorption
In order to understand the difference in surface area between the catalysts, N2
physisorption analyses were performed and the results are presented in Table 1. The surface area of the purchased CeO2 is very low (9.9 m2/g), while the high surface area CeO2 prepared via homogeneous precipitation method revealed much higher surface area (129.2 m2/g). Loading of Pt (Pt/CeO2 and Pt/high surface area CeO2) didn’t cause considerable change in surface area of these supports. The surface area of SiO2 is 179.2 m2/g and the Ce and Pt loaded samples (Ce/SiO2, Pt/SiO2, and Pt/Ce/SiO2) also didn’t reveal considerable change in surface area.
Table 1.
BET surface area, pore volume, and pore size of catalysts.
Catalyst Surface area Pore volume Pore size
(m2/g) (cm3/g) (Ǻ)
The TPR profiles of catalysts are shown in Figure 13 ~ Figure 30. As proposed by Jacobs et al. [38], the TPR profiles of CeO2 (Figure 13) presented peaks around 500 oC. This peak can be assigned to the reduction of the ceria layers on and close to the surface. Signal around 800 oC are bulk ceria reduction. After loaded with Pt (Figure 15), the reduction peaks of the surface ceria layers was shifted to lower temperature, leaving the bulk ceria reduction peak remained unchanged. This observation indicated that the reduction of surface ceria layers was catalyzed by the Pt loading.
For high surface area CeO2 and Pt/high surface area CeO2 (Figure 17 and Figure 19), similar results were observed, except the much greater reduction peaks of the surface ceria layers due to their high surface area. The shifting of surface ceria layers
reduction peaks to lower temperature by Pt addition was observed, and the reduction peak of bulk ceria remained unchanged.
For SiO2 and metal-loaded SiO2 samples (Ce/SiO2, Pt/SiO2, and Pt/Ce/SiO2), bare SiO2 (Figure 21) exhibited no reduction peak at all. After loaded with Ce (Ce/SiO2), the reduction peaks similar to CeO2 but with smaller scale was observed (Figure 23). Further Pt loading shifted the reduction peaks of surface ceria layers to lower temperature (Figure 27). Because the loading of the Pt metal was very low (1 wt%), Pt/SiO2 (Figure 23) exhibited very small and ambiguous reduction peaks around 100 oC and 400 oC. No peak can be identified on diluted Pt (Figure 29).
The signal of hydrogen (m/z = 2) obtained by mass spectrometer provides a way to make sure that if the response of TCD signal actually attributed to the change of hydrogen in the flow. The interference from the instability of TCD or other component in the flow could be easily recognized by comparing the signal of mass spectrometer with TCD signal. The results of TPR analysis shows that the surface shell ceria of all samples could be completely reduced below 600 oC. This temperature was used for reduction pretreatment in the following experiments.
0 200 400 600 800 1000
Temperature ( oC ) 483 525 H 2 comsumption
CeO2
Figure 13. TPR profile of CeO2 obtained by TCD.
Figure 14. TPR profile of CeO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
Temperature ( oC )
188 517
Pt / CeO2
385
Figure 15. TPR profile of Pt/CeO2 obtained by TCD.
Figure 16. TPR profile of Pt/CeO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
Temperature ( oC ) 475
High surface area CeO2
Figure 17. TPR profile of high surface area CeO2 obtained by TCD.
Figure 18. TPR profile of high surface area CeO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
Temperature ( oC ) 217
422 260
Pt / High surface area CeO2
Figure 19. TPR profile of Pt/high surface area CeO2 obtained by TCD.
Figure 20. TPR profile of Pt/high surface area CeO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
Temperature ( oC ) SiO2
Figure 21. TPR profile of SiO2 obtained by TCD.
Figure 22. TPR profile of SiO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
Temperature ( oC ) 557
Ce / SiO2
Figure 23. TPR profile of Ce/SiO2 obtained by TCD.
Figure 24. TPR profile of Ce/SiO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
100 409
Temperature ( oC ) Pt / SiO2
Figure 25. TPR profile of Pt/SiO2 obtained by TCD.
Figure 26. TPR profile of Pt/SiO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
142 353
Pt / Ce / SiO2
Temperature ( oC )
Figure 27. TPR profile of Pt/Ce/SiO2 obtained by TCD.
Figure 28. TPR profile of Pt/Ce/SiO2 obtained by mass spectrometer.
0 200 400 600 800 1000
H 2 comsumption
Temperature ( oC ) PtO2 SiO2 TPR
Figure 29. TPR profile of diluted Pt obtained by TCD.
Figure 30. TPR profile of diluted Pt obtained by mass spectrometer.
3.5 Ethanol pulse chemisorption
The ethanol pulse chemisorption experiments were performed in order to investigate the ethanol adsorption ability of samples. The results of samples pretreated (reduction and oxidation) at 600 oC are shown in Table 2. For CeO2, the oxidation pretreated sample revealed greater ethanol adsorption than that of reduction pretreated sample. For Pt/CeO2, addition of Pt to CeO2 increased the ethanol adsorption of reduction pretreated sample but has no effect (even slightly decreased its ethanol adsorption) on oxidation pretreated sample. For high surface area CeO2, the oxidation pretreated sample showed greater ethanol adsorption than that of reduction pretreated one. Addition of Pt (Pt/high surface area CeO2) still increased the ethanol adsorption of reduction pretreated sample. However, the effect of Pt loading for oxidation pretreated sample can not be identified in this experiment, since all of the ten ethanol pulses were exhausted in both cases (Figure 33 and Figure 34).
Table 2.
Results of catalyst ethanol pulse chemisorption measurements. (R) stands for the reduction pretreatment sample and (O) stands for the oxidation pretreatment sample.
Catalyst Surface area Ethanol uptake (R) Ethanol uptake (O) (m2/g) (µmol/gcat) (µmol/gcat) diluted Pt, signals of pulse ethanol did not exhibit distinct peaks appropriate for quantitative determination (Figure 35 ~ Figure 39). However, we can still obtain some clues from the results of TPD experiments (see in 3.6). The TPD results of SiO2
(Figure 48 and Figure 49) revealed almost no signal of desorption for ethanol or any other compound, implying that ethanol can barely adsorb on the surface of SiO2. By contrast, TPD results of Ce/SiO2 (Figure 50 and Figure 51) and Pt/SiO2 (Figure 52 and Figure 53) revealed considerable amount of ethanol adsorption, indicating that the adsorption of ethanol was attributed to the existence of Ce and Pt. In addition, the
TPD results of Diluted Pt (Figure 56 and Figure 57) showed no signal of ethanol or any other compound.
From the observation mentioned above, ceria itself showed ability of ethanol adsorption. Considerable quantity of ethanol adsorbed on Pt/SiO2 while both of SiO2
and Diluted Pt showed no sign of adsorption. These results imply that ethanol adsorption on Pt/SiO2 associates with the interaction between Pt and SiO2. However, the lacking of ethanol adsorption on Diluted Ptmay due to the poor dispersion of Pt.
The possibility that ethanol may adsorbs on Pt still can’t be totally excluded.
In order to obtain more information (ethanol adsorption quantities of all samples) and reduce measurement error, better experimental design should be carried out for the future work (e.g. lower vapor concentration with more pulse).
Figure 31. Ethanol pulse chemisorption profile of CeO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 32. Ethanol pulse chemisorption profile of Pt/CeO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 33. Ethanol pulse chemisorption profile of high surface area CeO2. (Left:
sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 34. Ethanol pulse chemisorption profile of Pt/high surface area CeO2. (Left:
sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 35. Ethanol pulse chemisorption profile of SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 36. Ethanol pulse chemisorption profile of Ce/SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 37. Ethanol pulse chemisorption profile of Pt/SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 38. Ethanol pulse chemisorption profile of Pt/Ce/SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment)
Figure 39. Ethanol pulse chemisorption profile of diluted Pt. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment)
3.6 Temperature-programmed desorption (TPD) of ethanol
Temperature-programmed desorption of adsorbed ethanol was performed in order to investigate the desorption composition at different temperature. So that conversion of adsorptive surface species can be studied. Experimental results of 600
oC pretreated (reduction and oxidation) samples are mentioned as following.
CeO2
The TPD profiles of adsorbed ethanol over CeO2 are shown in Figure 40 (reduction pretreated) and Figure 41 (oxidation pretreated). All signals were weak due to the small surface area of CeO2 and low adsorption on its surface. For reduction pretreated sample, broad signals for ethanol and a acetaldehyde between 80 and 300
oC were observed. A small peak corresponding to hydrogen exhibited around 300 oC.
Peaks corresponding to CO2 between 200 and 600 oC were witnessed. For oxidation pretreated sample, the results are almost the same, except the slight increasing in CO2
signal at higher temperature.
Pt/CeO2
The TPD profiles of adsorbed ethanol over Pt/CeO2 are shown in Figure 42 (reduction pretreated) and Figure 43 (oxidation pretreated). All signals were weak due to the small surface area and low adsorption. For reduction pretreated sample, peak corresponding to ethanol exhibited between about 80 and 200 oC. Signal corresponding to acetaldehyde exhibited between 80 and 200 oC, and seems weaker than that observed in CeO2. Peaks between 300 and 500 oC were observed for both hydrogen and CH4 formation. Peaks of CO2 exhibited between 300 and 600 oC. For oxidation pretreated sample, the positions of peaks basically remain unchanged.
Signals corresponding to CH4, acetaldehyde, and CO2 were stronger, while peak corresponding to hydrogen at lower temperature seems weaker than those observed in Figure 42.
High surface area CeO2
The TPD profiles of adsorbed ethanol over high surface area CeO2 are shown in Figure 44 (reduction pretreated) and Figure 45 (oxidation pretreated). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 400 oC.
Peaks corresponding to hydrogen exhibited around 100 oC, 300 oC, and 500 oC.
Signals corresponding to ethene and acetaldehyde exhibited between 80 and 400 oC, with a higher peak at 330 oC. CO also exhibited a maxima peak at 330 oC, with a minor signal detected around 500 oC. Signal corresponding to CH4 was observed between 300 and 500 oC, and CO2 exhibited two broad signals around 200 oC and 500
oC respectively. For oxidation pretreated sample, signals for most of compounds became stronger without much change in position.
Pt/ high surface area CeO2
The TPD profiles of adsorbed ethanol over Pt/high surface area CeO2 are shown in Figure 46 (reduction pretreated) and Figure 47 (oxidation pretreated). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 200 oC, the signal around higher temperature was remarkably reduced (comparing to high surface area CeO2). The peaks corresponding to hydrogen could be detected all the way from 100 to 600 oC, with several maxima peaks in this range. Signals corresponding to ethene and acetaldehyde exhibited mainly between 80 and 400 oC, but much weaker than those observed in high surface area CeO2. Signal corresponding to CH4 was observed between 100 and 450 oC, while CO exhibited between 400 and 600 oC. The signals corresponding to CO2 exhibited from 100 to 600
oC, with the maxima at about 400 oC. For oxidation pretreated sample, the signals for most of compounds became stronger without much change in position.
SiO2
The TPD profiles of adsorbed ethanol over SiO2 are shown in Figure 48 (reduction pretreated) and Figure 49 (oxidation pretreated). For both reduction and oxidation pretreated sample, it shows very small and ambiguous ethanol signal without signal of any other compound, implying that there should be almost no adsorption of ethanol on the surface of SiO2.
Ce/SiO2
The TPD profiles of adsorbed ethanol over Ce/SiO2 are shown in Figure 50 (reduction pretreated) and Figure 51 (oxidation pretreated). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 300 oC. Peaks corresponding to hydrogen were observed around 100 oC and 300 oC. Signals corresponding to ethene and acetaldehyde exhibited between 80 and 300 oC. CO2
signal could be detected all the way from 100 to 600 oC, with maxima at 100 oC and
500 oC. For oxidation pretreated sample, signals corresponding to hydrogen, ethene, and acetaldehyde at higher temperature were reduced. CO2 signal around 500 oC decreases and exhibited a signal around 600 oC.
Pt/SiO2
The TPD profiles of adsorbed ethanol over Pt/SiO2 are shown in Figure 52 (reduction pretreatment) and Figure 53 (oxidation pretreatment). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 200 oC.
Peaks corresponding to hydrogen exhibited two minor peaks around 100 oC and 200
oC with a major peak at 550 oC. Signals corresponding to ethene and acetaldehyde were observed between 80 and 200 oC, while signal corresponding to CH4 exhibited between 100 and 300 oC. CO2 signal could be detected all the way from 100 to 600 oC, with maxima at 100 oC and 400 oC. For oxidation pretreated sample, signals corresponding to CH4, ethene, and CO2 were weaker than those observed in Figure 52.
Pt/Ce/SiO2
The TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 are shown in Figure 54 (reduction pretreatment) and Figure 55 (oxidation pretreatment). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 200 oC, signal around higher temperature remarkably reduced (comparing to Ce/SiO2). The peaks corresponding to hydrogen could be detected all the way from 100 to 600 oC, with several maxima peaks in this range. Signals corresponding to ethene and acetaldehyde exhibited between 80 and 200 oC, while signal corresponding to CH4
was observed between 100 and 400 oC. Signals corresponding to CO2 exhibited from 100 to 600 oC, with the maxima at about 300 oC. For oxidation pretreated sample, only small variations were observed in signals corresponding to hydrogen and CO2.
Diluted Pt
The TPD profiles of adsorbed ethanol over Diluted Pt are shown in Figure 56 (reduction pretreatment) and Figure 57 (oxidation pretreatment). For both reduction and oxidation pretreated sample, it shows very small and ambiguous ethanol signal without signal of any other compound. Similar to SiO2, it shows no adsorption of ethanol.
Figure 40. TPD profiles of adsorbed ethanol over CeO2 with reduction pretreatment.
Figure 41. TPD profiles of adsorbed ethanol over CeO2 with oxidation pretreatment.
Figure 42. TPD profiles of adsorbed ethanol over Pt/CeO2 with reduction pretreatment.
Figure 43. TPD profiles of adsorbed ethanol over Pt/CeO2 with oxidation pretreatment.
Figure 44. TPD profiles of adsorbed ethanol over high surface area CeO2 with reduction pretreatment.
Figure 45. TPD profiles of adsorbed ethanol over high surface area CeO2 with oxidation pretreatment.
Figure 46. TPD profiles of adsorbed ethanol over Pt/high surface area CeO2 with reduction pretreatment.
Figure 47. TPD profiles of adsorbed ethanol over Pt/high surface area CeO2 with oxidation pretreatment.
Figure 48. TPD profiles of adsorbed ethanol over SiO2 with reduction pretreatment.
Figure 49. TPD profiles of adsorbed ethanol over SiO2 with oxidation pretreatment.
Figure 50. TPD profiles of adsorbed ethanol over Ce/SiO2 with reduction pretreatment.
Figure 51. TPD profiles of adsorbed ethanol over Ce/SiO2 with oxidation pretreatment.
Figure 52. TPD profiles of adsorbed ethanol over Pt/SiO2 with reduction pretreatment.
Figure 53. TPD profiles of adsorbed ethanol over Pt/SiO2 with oxidation pretreatment.
Figure 54. TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 with reduction pretreatment.
Figure 55. TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 with oxidation pretreatment.
Figure 56. TPD profiles of adsorbed ethanol over diluted Pt with reduction pretreatment.
Figure 57. TPD profiles of adsorbed ethanol over diluted Pt with oxidation pretreatment.
3.7 Ethanol pulse reaction
Ethanol pulse reaction experiments were performed in order to estimate the ethanol conversion ability of samples. The experimental results of samples reduced at 600 oC are displayed as follow.
Blank
The ethanol pulse reaction profiles of blank at different temperature are shown in Figure 58. No significant change for ethanol signals was observed from 200 to 600 oC, which means the conversion ethanol was very low (or no conversion at all) with absence of catalyst. The small hydrogen signal appeared at low temperature was the noise coming with ethanol pulse, which shall be ignored in the following discussion.
The increasing in hydrogen signal revealed at 500 oC and 600 oC may due to the slightly decomposition of ethanol at high temperature.
CeO2
The ethanol pulse reaction profiles of CeO2 at different temperature are shown in Figure 59. Considerable decreasing in ethanol signals began to reveal at 500 oC with increasing in signals of hydrogen observed at 400 oC. At higher temperature, the reaction process was accelerated, which caused the further improvement in ethanol conversion.
Pt/CeO2
The ethanol pulse reaction profiles of Pt/CeO2 at different temperature are shown in Figure 60. Both of slight decreasing in ethanol signals and slight increasing in hydrogen signals began to reveal at 300 oC. At higher temperature, the overall conversion of ethanol was further increased. For all temperatures, Pt/CeO2 shows better conversion than bare CeO2 in the beginning (first few peaks). However, the conversion of ethanol was gradually weakened. These observations implying that addition of Pt promoted the conversion of ethanol but also cause deactivation of catalyst, which are in agreement with the results proposed by Jacobs et al. [35].
High surface area CeO2
The ethanol pulse reaction profiles of high surface area CeO2 at different
temperature are shown in Figure 61. At 300 oC, slight decreasing in ethanol signals and slight increasing in hydrogen signals for the first few peaks were observed.
Referring to the reaction mechanism proposed by Jacobs et al. [26-27] [34-35], the small amount of ethanol consumption observed at 300 oC may attribute to ethanol
Referring to the reaction mechanism proposed by Jacobs et al. [26-27] [34-35], the small amount of ethanol consumption observed at 300 oC may attribute to ethanol