Dry Reforming of Methane over Ni-based catalysts

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H2 production from biomass

With the increasing global concern about the climate change and CO2 emissions, it is urgent to produce H2 in a CO2 neutral route. Renewable resources offer a long-term potential for sustainable hydrogen production with low environmental impact. It is worth mentioning that biomass has the potential to accelerate the realization of hydrogen as a major fuel of the future. In general, Hydrogen can be produced from biomass by pyrolysis, gasification, and enzymatic decomposition of sugars [15,16]. There are two types of biomass feedstock available to produce hydrogen [20], specific bioenergy crops, and less expensive biomass residues, such as organic waste from regular agricultural processing. Though CO2 is produced as a by-product in the process using biomass as a raw material, it can be fixed by the growing plants during photosynthesis to produce new biomass, which brings a small net CO2 impact compared to fossil fuels.
Biomass has tremendous potential to accelerate the realization and utilization of H2 as a primary fuel of the future, as biomass is the fourth largest source of energy in the global context. It accounts for 15% of world‘s primary energy consumption and about 38% of the primary energy consumption in developing countries such as India, China, and Brazil [1].

H2 production from Catalytic Reforming of CH4

As introduced above, the reforming of biogas is in the same principle of methane reforming, which is widely studied worldwide. Methane is a well-developed energy source in our daily life, it can be directly used for combustion thus generating heat and/or power [36]. The attention on catalytic conversion and sequestration of methane is increasing rapidly in recent years with receiving growing attention from the view point of global-warming issue as well as the development of shale gas and methane clathrate [37,38].
Many strategies are developed for the conversion of methane to more valuable chemicals and fuels. There are two strategies for conversion of methane, which are described as an indirect and direct method. In the direct processes, methane may be converted to methanol, formaldehyde, ethylene, aromatics or acetic-acid [39]. The indirect method is based on the formation of synthesis gas (CO and H2). Particularly, the steam reforming of methane source such as natural gas and byproduct of petroleum refining, which is the most commonly used method for hydrogen production.
As we know, the average bond energy of methane is 414 kJ/mol, and 435 kJ/mol for CH3-H bond, it is difficult to break this stable structure. Therefore, achieving effective activation and conversion of methane molecules are crucial topic worldwide. In current studies, There are mainly three mainstream reactions to convert methane to produce H2, namely Steam Reforming (SRM), Dry Reforming (DRM) and Partial Oxidation of methane (POM). Moreover, combining two or three reforming processes mentioned above, people proposed many new concepts as bi-reforming or tri-reforming of methane.

H2 production from steam reforming of methane

Steam reforming of methane (SRM) is the most widespread method of producing hydrogen, with an energy consumption rate of about 1.23–1.35 GJ-NG/GJ-H2 [43]. In the steam reforming of methane, the natural gas or other methane streams, such as biogas or landfill gas reacts with water vapor in the presence of catalyst under 3–25 atm pressure to produce H2, CO and a small amount of CO2 (Eq.1-2). According to DOE of USA (2010), SMR is approximately 72% efficient in hydrogen generation when starting with natural gas. It is worth mentioning that, natural gas reforming produces about half of the global supply of hydrogen and remains the most commonly used method for hydrogen production. However, as an endothermic process, SRM is still not a flawless route with defects in the attempts of high energy consumption and carbon formation. Also, side reactions need to be inhibited. Recently, many new catalysts and strategies are studied by researchers as follow.
CH4 + H2O → 3H2 + CO ΔH298K = 206 kJ/mol (1-2)
Antzara et al. [44] evaluated the SMR performance of NiO-based oxygen carriers supported on ZrO2, TiO2, SiO2, Al2O3 and NiAl2O4 as conventional reforming catalysts at low temperature (650C). NiO/SiO2 and NiO/TiO2 were found to have low activity (<50% initial CH4 conversion) and deactivated rapidly, eventually dropping to less than 10% CH4 conversion after 2 h on stream. NiO/ZrO2 exhibited good activity with initial CH4 conversion higher than 80% and had excellent stability. Min-Ho et al. [45] investigated the methane steam in microchannel reformers by using porous-membrane-type catalysts and shifting a combustion point onto the top of the catalysts. Improved heat transfer efficiency has been obtained and methane conversion has increased by 14.7% at the same supply rate of fuel. In addition, the long-term stability of the microchannel reformers was verified by methane reforming tests for 500 h.


Partial oxidation of methane

Partial oxidation of methane (POM) is an alternative method to produce H2 with reduced energy costs, since the reaction is moderately exothermic (Eq. (1-3)), contrary to SRM which is highly endothermic. Methane is partially oxidized to CO and H2 (H2/CO ratio close to 2) and reduce carbon formation. However, a slight decrease in CO selectivity causes the methane to react with oxygen to form CO2 (Eq. 1-4), leading to combustion (strong exothermic reaction), which results in high reaction temperature increase. It can form hot-spots in the reactor bed and carbon occurs simultaneously on the catalyst surface. According to the literature [46], the CH4 conversion increases with increasing space velocity because of the presence of hot spots.

H2 production from oxidative dry reforming of methane

A comprehensive strategy to evade the aforementioned defects and improve the conversion at a lower temperature is combining the DRM and POM in a single reactor (Eq.1-8). It has been reported to be effective in compensating energy consumption of dry reforming and adjusting the H2/CO ratio of syngas. Co-feeding O2 with CH4 and CO2 provides additional advantages such as: reducing the global energy requirement, enhanced catalyst stability, increased deactivation resistance [4] and inhibit the carbon deposition rate by gasifying carbon species, as demonstrated by Lucredio et al. [58]. Furthermore, the H2/CO ratio could be manipulable for downstream. Another advantage is compensating the energy of endothermic reforming reactions, significantly reducing the global requirements of energy.

Table of contents :

1 General introduction
1.1 Hydrogen: energy for the future
1.2 Global manufacture of H2 production
1.3 H2 production from biomass
1.4 Biogas production
1.5 H2 production from Catalytic Reforming of CH4
1.5.1 H2 production from steam reforming of methane
1.5.2 Partial oxidation of methane
1.5.3 Dry reforming of methane
1.5.4 H2 production from oxidative dry reforming of methane
1.5.5 H2 production from other reforming processes of methane
1.6 Catalysts for reforming of methane
1.6.1 Catalysts for dry reforming
1.6.2 Catalysts for oxidative dry reforming of methane
1.7 Catalyst deactivation
1.7.1 Carbon formation
1.7.2 Sintering
1.8 Objective of the thesis
2 Preparation and characterization of catalysts
2.1 CeNiX(AlZ)OY catalysts
2.2 NiXMg2AlOY catalysts
2.3 Ni_x/SBA-
2.4 Conclusion
3 Dry Reforming of Methane over Ni-based catalysts
3.1 Dry reforming over CeNiX(AlZ)OY mixed oxides
3.1.1 Influence of reaction temperature over CeNiXOY
3.1.2 Influence of pretreatment
3.1.3 Influence of Ni content
3.1.4 Influence of reactants concentration
3.1.5 Stability test
3.2 Dry reforming over CeNiXAl0.5OY mixed oxides
3.2.1 Influence of reaction temperature and pretreatments
3.2.2 Influence of Ni content
3.2.3 Performance in harsh condition
3.2.4 Stability test
3.2.5 The influence of Al in CeNiX(AlZ)OY mixed oxides
3.2.6 Conclusion for CeNiX(AlZ)OY in DRM
3.3 Dry reforming over NiXMg2AlOY mixed oxides
3.3.1 The influence of reaction temperature
3.3.2 Influence of pretreatment in H2
3.3.1 Catalytic performance in harsh conditions
3.3.2 Stability test
3.3.3 Conclusion for NiXMg2AlOY in DRM
3.4 Dry reforming over Ni_x/SBA-15 catalysts
3.4.1 Influence of pretreatment
3.4.2 Influence of Ni loading
3.4.3 Catalytic performance in harsh condition
3.4.4 Conclusion for Ni_x/SBA-15 in DRM
3.5 Conclusion for this chapter
4 Oxidative dry reforming of methane over CeNiX(AlZ)OY
4.1 Oxidative dry reforming over CeNiXOY mixed oxides
4.1.1 Catalytic tests at low-temperature range
4.1.2 The effect of nickel content on CeNiXOY catalysts
4.1.3 Stability test
4.2 Oxidative dry reforming over CeNiXAl0.5OY mixed oxides
4.2.1 Temperature influence
4.2.2 Influence of Ni content
4.2.3 Influence of O2 concentration
4.2.4 Stability test
4.2.5 The influence of Al in the catalysts
5 General discussion
5.1 Comparison and discussion
5.2 Proposal of active site and possible mechanism
6 General conclusion
References .


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