CORRELATION BETWEEN THE PREPARATION METHODS AND PHYSICOCHEMICAL CHARACTERISTICS OF MgAl2O4 AS SUPPORT FOR NI CATALYSTS IN PARTIAL OXIDATION OF. This thesis is submitted in part to fulfill the requirements for obtaining the degree of Master of Science in Chemistry. I further declare that there is no potential conflict of interest in relation to the research, data collection, authorship, presentation and/or publication of this thesis.
All known routes for methane conversion are catalytic processes, where performance depends mainly on physico-chem. Catalysts that have shown promising performance in methane reforming are based on expensive metals such as Pt, Pd and Ru. Ni, on the other hand, is a cost-effective metal and has shown promising catalytic activity.
For example, MgAl2O4, which is the support investigated in this research, has been found to be a promising support for Ni catalysts where it is. Since the solid characteristics very often depend on the preparation method and conditions, the main aim of the research here is the preparation of MgAl2O4 as a support for Ni catalysts from.
Introduction
Therefore, the main objective of the research reported in this thesis was to correlate the various physicochemical properties of MgAl2O4 and its Surface OH groups can provide advantages for the Ni catalyst including dispersion through coordination of Ni ions with these groups and enhanced coke formation by adsorbing CO2 molecules promoting their subsequent reaction with the deposited carbon yielding CO according to an opposite . Ni supported on prepared powders, as well as commercial ones, MgAl2O4 were prepared by wet impregnation and its physico-chemical properties were related to the properties and preparation method of the support.
Literature Review
- Overview
- Methane Conversion Routes
- Overview
- Steam Reforming of Methane
- Dry Reforming of Methane
- Auto-thermal Reforming of Methane
- Methane Pyrolysis
- Gas-to-Liquid Technology
- Catalyst Preparation Methods
- Sol-Gel Process
- Co-Precipitation
- Wet Impregnation
- Catalyst Deactivation
- Sintering
- Carbon Deposits
- Partial Oxidation of Methane
- Possible Reaction Mechanisms
- Catalysts for POM
- Catalyst Supports
- Objective of the Present Thesis
The most expensive of the three processes is the production of syngas, mainly because of the high energy consumption. The choice of preparation method and conditions can have a significant impact on the physico-chemical properties of the catalyst, as well as its catalytic performance. During this process, a homogeneous sol develops from the dissolved precursors, followed by the formation of the gel over some time.
According to numerous studies, the sol-gel method is one of the best methods for In this case, the active metal precursor and the salts of the supporting precursors are dissolved in a solvent and with the addition of the precipitating agent, the simultaneous precipitation of all. Deactivation of catalyst nanoparticles due to thermal sintering is one of the main challenges in supported metal catalyst applications.
In addition, the uniform distribution of Ni species on the surface also reduces the sintering of the carrier particles. Depending on the efficiency and performance of the catalyst, both soft and hard carbon can accumulate during the reaction. The reaction mechanism and product formation in the catalyst layer is an important and ongoing topic in research on the catalytic partial oxidation of methane.
The performance of the catalyst and its resistance to coke are very often influenced by the physico-chemical characteristics of the support. Therefore, the composition of the catalyst support as well as its textural properties can be modified to improve coke resistance and overall performance [58, 60]. The support is an important component of the catalyst composition as it allows the distribution of the active phase of the catalyst on its surface in the form of nanoparticles.
The interaction of the active metal particles with the support surface and thus their distribution on the support prevents sintering and provides greater surface area of the active phase. In addition, these promoters can improve the thermal stability of the alumina-based supports and prevent the support and the active phase from sintering acting as textural, structural and chemical modifiers [62]. 17 preparation methods affect the surface acid-base properties and textural properties of the support, as well as the performance and coke resistance of the resulting catalysts during partial oxidation of methane reaction.
![Figure 1: The various methane conversion routes [4].](https://thumb-ap.123doks.com/thumbv2/azpdfco/10574639.0/20.892.105.778.110.524/figure-1-various-methane-conversion-routes-4.webp)
Experimental Methods
- Introduction
- Materials and Catalysts’ Preparation
- Sol-Gel Synthesis
- Co-precipitation Synthesis
- Preparation of the Catalyst Ni/MgAl₂O₄
- Catalysts Characterization
- X-ray Diffraction
- Nitrogen-Sorption Analysis
- TEM Analysis
- Elemental Analysis
- XPS Analysis
- H 2 -Temperature Programmed Reduction
- H 2 Pulse Chemisorption
- DRIFTS Analysis
- CO 2 Temperature Programmed Desorption, CO 2 -TDP
- Thermogravimetric Analysis
- Raman Spectra
- Catalytic Activity Tests
Information about the phase type, particle size and uniformity of the metal distribution is obtained via the electronic beam. H2-Temperature Programmed Reduction (H2-TPR) is an efficient method for determining the reducibility of the oxide of the active metal on the surface of the support. A U-shaped quartz tube reactor was used in which 50 mg of the sample under investigation was packed between two plugs of quartz wool.
The measurement of the active metal surface area, metal dispersion and crystallite size is possible using the CO chemisorption technique. The ratio of total metal atoms on the surface of the active metal particles accessible to the adsorbate species to the total atoms of the active metal is known as metal dispersion. Since H2 molecules only bind to the atoms of active metals, the amount of molecules adsorbed can be used to calculate the metal surface area (m2/g metal) and the size of the active metal crystallite.
A quartz tube reactor was used in which 50 mg of the sample in the study was packed between two quartz wool plugs. The amount of H2 chemisorbed on the surface of the metal particles was quantified by the signal from the TCD detector. Raman spectroscopy is one of the effective vibrational spectroscopic techniques used to identify chemical species.
The average of three spectra recorded in different areas of a sample pallet was taken to ensure homogeneity. In each direction, 0.15 g of catalyst powder was used in the form of 180–350 μm granules packed between two quartz wool plugs. The reported results of the catalytic activity tests are averages of 3 runs under the same conditions.
CH4 conversion and product selectivities were calculated using molar flow rates (F) as follows:
Results and Discussions
- Structural and Textural Characterization
- XPS Results
- CO 2 -TPD and DRIFTS Studies
- H 2 Chemisorption and H 2 -TPR
- Catalytic Tests and Correlation with Physicochemical Characteristics
The N2 sorption isotherms of the different supports showed hysteresis loops typical of mesoporous powders, as shown in Figure 9. The isotherms and the corresponding pore size distributions of the sol-gel based supports showed relatively homogeneous mesopores in the range of 5-15 nm with maxima around 10 nm. The TEM images of the reduced catalysts showed spherical Ni particles with an average particle size close to 20 nm for all catalysts, as shown in Figure 10.
NiMgAl-CM and NiMgAl-CP showed denser particle aggregation of their support, resulting in more packed and less porous aggregates compared to the support of NiMgAl-SG catalysts, which showed less dense and smaller aggregates. The chemical state of the surface species and the elemental composition were studied by XPS. It is also noteworthy that the peaks of the commercial support catalyst (NiMgAl-CM) appeared at slightly lower BE values, which may indicate a slightly weaker interaction with the support.
The dominance of Ni2+ peaks and the absence of a Ni0 peak, which typically occurs around 852 eV, is referred to the oxidation of the surface of the Ni particles upon exposure to air [72]. The density of surface basic sites of the different supports was studied by CO2-TPD and the results are presented in Figure 12 and Table 3. These observations can be referred to the higher surface area of the support of NiMgAl-SG catalysts which allowed for better dispersion.
The catalyst based on co-precipitation support showed somewhat similar behavior except for a noticeable decrease in conversion at the end of the reaction. The stable performance of the sol-gel based catalysts refers to their significantly higher coking resistance compared to NiMgAl-CM and NiMgAl-CP as shown by the various characterizations of the spent catalysts shown in Figure 16 and Table 5 .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.
Some carbon filaments are observed in the image of NiMgAl-CP which supports the results of the other characterizations of the spent catalysts discussed above. The results above indicate that the textural properties of the catalyst's support can significantly affect how well a catalyst performs. The clear significance of the physicochemical properties of the supports in the studied catalysts indicates that carbonaceous species arise from
Conclusion
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![Figure 3: A scheme of the various sol-gel stages, from precursor to the final product [31]](https://thumb-ap.123doks.com/thumbv2/azpdfco/10574639.0/25.892.102.769.655.928/figure-scheme-various-sol-stages-precursor-final-product.webp)
![Figure 4: Schematic representation of the synthesis of supported metal catalysts by wet impregnation method [38]](https://thumb-ap.123doks.com/thumbv2/azpdfco/10574639.0/27.892.236.675.498.922/figure-schematic-representation-synthesis-supported-catalysts-impregnation-method.webp)
