Chapter 4: Results and Discussions
4.5 Catalytic Tests and Correlation with Physicochemical Characteristics
The catalysts were tested in partial oxidation of methane at 700°C for 80 hrs on-stream, and the CH4 conversion, the main products’ selectivity, and the H2/CO ratio are shown in Figure 15. In addition to these products, small amounts of CO2 were detected with an average selectivity around 3.5% for NiMgAl-CM and around 1% for the other catalysts.
The sol-gel based catalysts have always shown stable performance with methane conversion to syngas around 89%, H2 selectivity close to 100%, and a H2/CO ratio around 2.05. The catalyst based on coprecipitation support showed somewhat similar behavior except a noticeable decrease in the conversion at the end of the reaction.
However, NiMgAl-CM showed an unstable and declining conversion, and an average H2/CO ratio of 2.2. The stable performance of the sol-gel-based catalysts is referred to their significantly higher coking resistance compared to the NiMgAl-CM and NiMgAl- CP as was indicated from the different characterizations of the spent catalysts presented in Figure 16 and Table 5. The enhanced coke formation over NiMgAl-CM correlates with its higher H2/CO ratio indicating lower rate of oxidation of the carbonaceous species that result from CH4 cracking to H2 and carbon. Moreover, its higher CO2
selectivity correlates with its lower density of basic sites, which would promote CO2
activation and further reaction with carbon deposit as discussed above. Coke deposits on NiMgAl-CM have always resulted in clogging the reaction tube leading to increased pressures on the feed side and unstable conversion.
41 Figure 15: CH4 conversion (A) and averages of conversion, products’ selectivity, and H2/CO ratio (B) during 80 hrs on-stream at 700°C, 1 atm, and 7600 mL CH4 g-1h-1 over the different catalysts.
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The XRD pattern of spent NiMgAl-CM, presented in Figure 16, showed an intense peak for crystalline carbon at 2q angle of 25.8°, with a crystallite size of 7 nm, while the patterns of the other catalysts showed no evidence for the presence of crystalline carbon.
Carbon accumulation on NiMgAl-CM was found to start at earlier stages of the reaction as was observed in other reactions conducted for 12 and 24 hrs. The TGA profiles and the corresponding results presented in Table 4 correlate with the XRD results. The small amounts of coke observed over NiMgAl-SG catalysts are mainly soft carbon that was removed at temperatures in the range of 200-400°C, while carbon over NiMgAl-CM was removed at temperatures >500°C, indicating the dominance of crystalline carbon.
NiMgAl-CP, on the hand, showed small mass loss in the temperature range of 600- 750°C, as shown in the TGA profile inset, indicating the presence of some crystalline carbon that was not detected by XRD due to its small concentration. The observed small amounts of carbon on NiMgAl-CP explains the small decrease in CH4 conversion
towards the end of the reaction. The mass loss in the temperature range below 200°C is usually referred to removal of adsorbed water and other volatile species on the surface and therefore, was not included in the reported amounts of coke [79].
Figure 16 illustrates the characterization of carbon deposits on the spent catalysts using Raman spectroscopy, which supports the TGA and the XRD results. While the spectrum of NiMgAl-CM showed the typical G and D bands of graphitic and disordered carbon, these bands disappeared in the spectra of the other catalysts. The carbon bands that were observed at 1345 cm−1 and 1580 cm−1 are usually linked to D (1345 cm-1) and G (1580 cm-1) vibration modes of carbon-containing species [80, 81]. While D band is referred to amorphous or disordered carbon filaments and nanoparticles, the G band is usually assigned to the sp2 C–C vibrations of ordered graphitic carbon. The spectrum of spent NiMgAl-CM indicates the formation of significant amounts of both types of carbon with IG/ID ratio of 1.1. The peak around 1485 cm-1 can be assigned to C60. [82].
43 Figure 16: Characteristics of the spent catalysts: (A) XRD patterns, (B) TGA profiles, and (C) Raman spectra.
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Table 5: Amounts of carbon deposits based on TGA results.
Catalyst Amount of Carbon (mgC.g-1catalyst)
Soft Hard
NiMgAl-SG-AN 1.2 0.4
NiMgAl-SG-ASB 1.2 0.5
NiMgAl-CP 1.0 1.0
NiMgAl-CM 0.09 12.3
The TEM images of the spent catalysts are shown in Figure 17. The image of NiMgAl- CM show carbon that accumulates around the Ni particles as well as on the support surface. Some carbon filaments are observed in the image of NiMgAl-CP supporting the results of the other characterizations of the spent catalysts discussed above. The images also indicate that no significant sintering of the particles can be observed.
Figure 17: TEM images of spent catalysts. (a) NiMgAl-CM, (b) NiMgAl-CP, and (c) NiMgAl-SG-AN.
The results above suggest that the textural characteristics of the catalyst's support can significantly influence how well a catalyst performs. The concentration of active surface sites, such as coordinatively unsaturated oxide ions and hydroxyl groups, is strongly influenced by the texture's surface area and pore features. They are essential in eliminating carbonaceous species that develop during reactions. It is clear that high surface areas and sufficient interparticle porosity of the catalyst support are two crucial factors in coking resistance because they allow for the constant elimination of newly
45 formed carbonaceous species. The observation of some crystalline carbon on NiMgAl- CP, a material with relatively low pore volume and diameter, and the observation of a sizable coke deposit on NiMgAl-CM, a material with the lowest pore volume and
smallest pore diameter as well, allow the suggestion that the interparticle porosity plays a significant role in coking resistance. The coke formation over NiMgAl-CM and
NiMgAl-CP may be attributed to the presence of amorphous MgO and/or Al2O3
impurities in trace concentrations on their surfaces. Two observations allow us to rule out this hypothesis. Initially, with an H2/CO ratio of roughly 2, all investigated catalysts showed a comparable high initial CH4 conversion, H2 as well as CO selectivity,
demonstrating that they were all active at the beginning with little influence from the support. The gradual deactivation of the NiMgAl-CM suggests that methane breakdown began to predominate as a result of the loss of catalytic reforming activity without adequate regeneration. Second, the rapid and significant coke development on NiMgAl- CM, the material with the smallest pore volume and diameter. While MgO and Al2O3, along with their mixtures, have already been investigated as potential supports and have demonstrated pretty good activity, their effects, if any, would not be as significant and quick as those that were found [83]. Because of this, the obtained results allow for the hypothesis that the interparticle porosity had a significant impact here and that the significant coke formation is mostly due to the NiMgAl-tiny CM's pore widths and low pore volume. Since larger pores predominate and have lesser portions of their pores below 5 nm, as illustrated in Figure 9b, the impact of tiny pore widths was not as
significant in the case of NiMgAl-CP. An established process for the partial oxidation of methane involves the activation of the C-H bond by reduced Ni particles, which
encourages the breakdown of methane into hydrogen and carbon [84]. If the
carbonaceous species that arise in this stage are not subjected to the proper oxidation conditions, they develop into larger crystallites that collect into larger particles or filaments around the Ni particles and in the support's interparticle pores. If the support's pores are too small, as they are in NiMgAl-CM, carbonaceous species may quickly fill them, preventing molecules of oxygen and carbon dioxide from entering to oxidize and remove the species as they form. Moreover, the built-up cocaine blocks some active sites, which results in deactivation. On the other hand, bigger, broader pores, such those
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found in NiMgAl-SG catalysts, inhibit such carbon buildup and pore blockage, allowing for continuous gas diffusion over the carbonaceous species and, as a result, their
continual oxidation and removal as they form. Furthermore, NiMgAl-SG and NiMgAl- CP displayed larger basic site densities, which was attributed to the smaller crystallites of their supports that revealed higher concentrations of complementary unsaturated ions, including significant basic sites. According to what was previously mentioned, surface basic sites encourage the oxidation and removal of carbon deposits by adsorbing and activating CO2 molecules, which encourages their subsequent interaction with
intermediate carbon species to produce CO [85, 86]. Furthermore, the distinct textural properties of NiMgAl-SG catalysts also made it possible for the existence of additional surface OH groups, which can operate as a source of oxygen for further oxidation of carbon deposits. The clear significance of the physicochemical characteristics of the supports in the studied catalysts suggests that carbonaceous species resulting from
methane decomposition over Ni particles migrate to the Ni-support interface, where they interact with the reactive sites of the support, including the basic O2- and OH groups, to produce CO and H2 as the main products.
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