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Hydrogen from Natural Gas
Published in Prasenjit Mondal, Ajay K. Dalai, Sustainable Utilization of Natural Resources, 2017
Mumtaj Shah, Prasenjit Mondal, Ameeya Kumar Nayak, Ankur Bordoloi
Depending on the relative values of KM* and KM, the following conclusions can be drawn: if KM > KM*, no coking occurs at all, and if the deposited carbon is graphite, KM* will be equal to KP. In case of encapsulated carbon or filamentous carbon, KM is smaller than KM*. However, KM* does not determine whether filamentous or encapsulated carbon will be produced. Hence, KM* does not give a clue regarding the life of the catalyst under filamentous carbon formation.
Experiments on Wetting by Catalysts
Published in Eli Ruckenstein, Gersh Berim, Wetting Experiments, 2018
Model catalysts formed of small crystallites of Ni, Co, or Fe supported on thin, electron transparent films of nonporous alumina were heated in chemical atmospheres having compositions in the range encountered in the steam-reforming reaction. Electron microscopy and electron diffraction were used to investigate the physical and chemical changes that occurred. For a better understanding of the phenomena involved, the effect of the mixture is compared to the effects caused by the individual components and by their combinations. It is shown that, depending upon the nature of the catalyst as well as upon the composition of the mixture, sintering or and coking can occur much more rapidly in mixtures than in single-component atmospheres. This suggests that a cooperative action of the reaction components is the cause of the early deactivation of the steam-reforming catalysts. Various phenomena such as carbon deposition (as films or patches on the surface of the catalyst and on the substrate or as filaments), deformation of the crystallites, severe sintering, and permanent loss of metal to the gas stream were observed. Explanations are provided for the behavior of the Ni/Al2O3, Co/A12O3, and Fe/Al2O3catalysts, in various atmospheres, by considering the competition between carbon deposition and its gasification, as well as the strength of the interactions between crystallites and substrate. Two kinds of filaments were observed: carbonaceous filaments and metal oxide filaments which probably contain some carbon. A thermodynamic condition for the formation of filamentous carbon is suggested. If the sum of the interfacial free energies between carbon and crystallite, and carbon and substrate is smaller than that between crystallite and substrate, then carbon could penetrate between crystallites and substrate and filaments would be generated. Indeed, carbonaceous filaments were identified only when the catalyst particles were present as metals. In such cases, the interfacial free energy between the metal crystallite and alumina is much greater than that between the oxidized metal and alumina and the above thermodynamic condition is more likely to be satisfied. Permanent loss of active metal to the gas stream occurred, either because of the disintegration of the crystallites caused by carbon precipitation inside the particles, and/or possibly because of the formation of volatile carbonyls. © 1987 Academic Press, Inc.
Comparison of preparation methods for improving coke resistance of Ni-Ru/MgAl2O4 catalysts in dry reforming of methane for syngas production
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Dahye Song, Unho Jung, Hyo Been Im, Tae Ho Lee, Young-Eun Kim, Ki Bong Lee, Kee Young Koo
The C‒H molar ratio for DRM is higher than that in other reforming processes, thus coke is deposited rapidly (Sehested 2006). Coke deposition can occur due to CO hydrogenation (eq. 4), CO2 hydrogenation (eq. 5), Boudouard reaction (eq. 6), and CH4 decomposition (eq. 7) (Nikoo and Amin 2011). In terms of thermodynamics, the endothermic CH4 decomposition reaction proceeds actively at a high temperature of 650°C or higher. In contrast, CO disproportionation, CO2 hydrogenation, and CO hydrogenation reactions have exothermic nature and therefore dominate at low temperatures (Zhang, Wang, and Dalai 2007). In addition, coke deposition essentially occurs below 870°C under CH4/CO2 ratio of 1, forming amorphous carbon, graphite, and filamentous carbon (Wang, Lu, and Millar 1996). Among these, filamentous carbon has the maximum influence on the deactivation (Alipour, Rezaei, and Meshkani 2014).
Elevated CO-free hydrogen productivity through ethanol steam reforming using cubic Co-Nanoparticles based MgO catalyst
Published in Environmental Technology, 2022
Radwa A. El-Salamony, Asmaa S. Morshedy, Ahmed M.A. El Naggar
For the spent catalyst which was collected after 10 h of SR, at 700°C, carbonaceous species were visually observed. The presence of such species or oxidation of metallic sites of the catalyst is responsible for its deactivation during the process of SR. This may consequently reduce the rates of hydrogen production [55]. Raman spectroscopic technique was used to investigate the crystallinity and graphitization degree of the as-produced filamentous carbon over the spent Co/MgO sample. As clearly observed in Figure 9, Raman spectra had displayed two main characteristic bands for carbon materials. The D-band (1346.34 cm−1) was attributed to the disordered carbon species with the vibration of the sp3 hybridized carbon atom [56]. The other peak (G-band) that was noted around 1598.34 cm−1 is due to the existence of carbon atom mode with sp2 hybridized carbon networks in the ordered graphite [57]. The peak ratio (ID/IG) of the D- and G- bands are identifying to the crystallinity (graphitization) degree as well as the present imperfection in the carbon species [58]. The ratio of ID/IG was found to be 0.86 which could indicate that the formed carbon onto the catalyst was nearly graphitized.
A Flame Structure Approach for Controlling Carbon Nanotube Growth in Flame Synthesis
Published in Combustion Science and Technology, 2021
M. T. Zainal, M. F. Mohd Yasin, M. Abdul Wahid, M. Mohd Sies
Carbon atoms that are catalytically decomposed from the fuel source adsorbs on the surface of a catalyst particle where a portion of the adsorbed carbon will desorb away from the catalyst. The remaining adsorbed carbons that are not desorbed will undergo bulk diffusion through the catalyst particle. The diffusion of single carbon atoms out of the catalyst particle contributes toward CNT nucleation and is evaluated at the saturation time as shown in Eq. (2) where and are the volume concentration of the carbon atoms at the substrate surface and the diameter of a carbon atom respectively. The variable is the carbon bulk diffusivity whose value depends on the experimental value for catalytic growth of filamentous carbon on cobalt catalyst (Zhang and Smith 2005).