Cu-ZrO2/multi-walled carbon nanotubes catalyst for the hydrogenation of carbon dioxide to methanol
Date
2021
Authors
Modisane, Kamogelo
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Abstract
Carbon dioxide, is known as a greenhouse gas (heat-absorbing and heat-radiating gas), is critical to the preservation of life and habitable environmental temperatures. However, human-caused increases in CO2 concentrations in the atmosphere, such as deforestation, the industrial revolution, and the use of fossil fuels, have
resulted in catastrophic climate changes. According to research, converting carbon dioxide to value-added fuels is one method that can play in reducing CO2 emissions while also providing economic and environmental benefits. In this study, an attempt has been made to investigate a specific catalytic system entailing theCO2 conversion into fuels and chemicals like MeOH and DME. The system studied is a Cu-ZrO2 catalyst supported in/on carbon nanotubes(CNTs). In particular, the impact of confinement utilizing the inner channels of CNTs as a nano-reactor for the encapsulation of Copper, Zirconia, and Cu-ZrO2 for the conversion of CO2 via the hydrogenation reaction has been studied. Encapsulation of copper, Zirconia, and Cu-ZrO2 was accomplished using novel and new techniques and principles
described in the literature. As a result, the synthesis, characterization, and application of the obtained catalyst with distinct properties were described.
The synthesis of carbon nanotubes using acetylene as a carbon source resulted in CNTs in high yields, multiple walls and a spaghetti-like nature, and with inner diameter ranges of approximately 27 ± 7 nm. Investigations revealed that purifying the materials in concentrated HNO3 at 140oC was critical in producing MWCNTs with open-cut tips, which served as entry points of the metal nanoparticles.
A simple wetness impregnation technique assisted by sonication was used to selectively deposit the metal/metal oxide inside the tubes. Because of their low surface tensions, tetrahydrofuran and ethanol was used as solvents. The solvents allowed for easy wetting and penetration of the nanoparticles into the nanotube cavity. Water was used after the reaction to wash away and remove any metal/metal oxide residue from on the outside of the tube. The prepared Cu@CNTs, ZrO2@CNTs, and Cu-ZrO2@CNTs materials that were formed were analyzed and characterized using various analytical techniques such as powder X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, transmission electron microscopy (TEM), temperature programmed reduction (TPR) and thermo-gravimetric analysis (TGA) technique.
The analysis revealed that the synthesis methods were able to encapsulate the metals inside and to evaluate the amount placed inside/outside the MWCNT cavity as not all of the particles were encapsulated.
These techniques also revealed that the deposition produced small particles of varying sizes depending on the metal/metal oxide used and the location of the particles. According to the BET analysis, the presence of metal or metal oxide increased the specific surface area and pore volume, of the samples resulting in a mesoporous pore structure with surface areas ranging from 80-166 m2/g. TEM analysis displayed that an increase in loading led to more particles being located outside the tubes and also agglomeration, and this was confirmed by the pore blockage and the reduction temperatures observed in the TPR data. TEM data also revealed that confinement had an effect on the particles grown inside the tubes, resulting in differences in particle sizes and shapes between the particles found outside and inside the tube.
Because previous reports on confinement demonstrated an increase in activity with reference to the confined metals, the synthesized catalyst was used in the CO2 hydrogenation to methanol in reactor studies which were performed at 250–300oC (p = 15 bar) and the results showed that all the materials were capable of acting as a catalyst at the lower temperatures. However, the Cu-ZrO2@CNTs displayed better activities than studies performed on the MWCNTs (blank reaction) or the metal loaded MWCNTs. The CO2 conversions for Cu-ZrO2@CNTs ranged from 5 –40%, and this catalyst had higher stability and activity, but lower selectivity as compared to the Cu@CNTs and ZrO2@CNTs catalysts. The catalysts demonstrated minimal reverse gas shift reactions with formations of traces of methane and carbon monoxide. This could be attributed to the synergistic relationship between copper and zirconia, as well as the synergy of the Cu-ZrO2 with the MWCNT inner walls
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment for the degree of Master of Science in Chemistry, 2021