The activation and conversion of carbon dioxide and methane by novel catalysts
Creators: name: Bajaj, Nardev
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400403
subject: Chemical engineering design
classificationCode: 400408
subject: Reaction engineering (excl. nuclear reactions)

description: The thermal decomposition and reduction of greenhouse gases have been an essential issue in tackling the global warming effect for several years. However, among all greenhouse gases, carbon dioxide (CO2) and methane (CH4) are the most significant contributors to greenhouse effects due to human activity. Several technologies have been developed to convert these gases into valuable products. Still, catalyst deactivation, high temperature, and complex synthesis and reaction methodology are the most significant hurdles that limit their applicability. One catalyst (ferrite based) that was extensively investigated could convert greenhouse gases (CO2 and CH4) into a value-added product. However, a limiting factor in the available studies is the harmful by-product (for example CO, CO2) that requires additional filtration. This by-product can interact with the catalyst and impact its overall activity and conversion efficiency. Therefore, strategies that can directly decompose these greenhouse gases into carbon need to be developed, yielding an economically beneficial process. 

This research project aims to design a thermocatalytic system using a new or modified catalyst that can directly decompose greenhouse gases (CO2 and CH4) into carbon at low temperatures without any harmful by-products. To this end, we develop a reactor and synthesise highly active catalysts to reduce the decomposition temperature of the gases with high efficiency. 

This research identifies and characterises a highly effective catalyst for the direct decomposition of the greenhouse gases CO2 and CH4. As a result, the essential tasks include the following: selecting a highly active catalyst, building a reactor, determining the catalyst’s activity, and analysing the reaction’s by-products to determine the reaction mechanism. The entire study was separated into three distinct phases.

First, CO2 decomposition to carbon is conducted at a low temperature. NaY zeolite was selected as the most suitable catalyst based on its activity towards CO2, thermal stability, non-toxicity, ease of modification, and increased loading capacity. It is further doped with zinc to enhance the system’s efficiency and lower the reaction temperature to 275–550°C. A series of reactions are carried out on the catalyst. The by-product is then analysed using several characterisation tools to validate the presence of the carbon after the reaction and evaluate its physical and chemical properties. The findings show that zinc-modified NaY zeolite decomposed CO2 at temperatures ranging from 275 to 550°C. The maximum conversion efficiency (70%) was reached at a reaction temperature of 450°C with negligible carbon monoxide generation. The presence of the carbon after the reaction was verified by scanning electron microscopy–energy dispersive X-ray analysis (SEM-EDX) and Fourier transform infrared spectroscopy (FTIR) of the fresh and spent catalysts. Additionally, CHN analysis corroborated these findings by providing the weight percent (0.73%) of carbon remaining after the reaction, and X-ray diffraction (XRD) showed the catalyst’s stability.

The second phase of this research focuses on directly converting CH4 into carbon utilising a novel catalyst. According to preliminary research and the first phase of this project, an active catalyst is needed to decompose CH4 efficiently and readily separate the carbon after the reaction. Therefore, we review liquid metal–based systems and perform preliminary studies to select a highly efficient catalyst (gallium) that can meet all the requirements of our existing system. As a result, the addition of molybdenum to gallium is found to reduce the reaction temperature of CH4 decomposition to as low as 400°C, resulting in the production of hydrogen and carbon, which experience significant global demand. Gallium (Ga) and molybdenum-doped gallium (Mo-Ga) were utilised for CH4 decomposition at 400 to 700°C. This study examined the effect of reaction temperature and metal loading on CH4 breakdown, hydrogen production, and solid carbon properties. Of note, the catalyst’s pre-treatment (oxidation) was found to be a critical aspect in catalyst activation for continuous CH4 decomposition. Blank gallium did not form significant amounts of solid carbon; however, the addition of molybdenum to gallium produced solid carbon and hydrogen at 400°C. The increased temperature significantly enhanced the conversion of CH4 to hydrogen, as validated by gas chromatography. The physical properties (e.g. shape, size, and weight per cent) of the by-product (carbon) were investigated using SEM-EDX. In contrast, the chemical properties (surface and functional groups) were confirmed using FTIR and Raman spectroscopy.

The third and final phase of this project entails the conversion of CO2 and CH4 (combined) into carbon. This work serves as a continuation of our second phase, where molybdenum-doped gallium is used for the present reaction and studied further to determine the mechanism of the reaction. The following sections offer an overview of the approach used to develop each component. Gallium (Ga) and molybdenum-doped gallium (2%Mo-Ga) were used in a decomposition reaction at a temperature of 700°C (determined based onto the preliminary studied reactions). We were able to determine the influence of reaction temperature on CH4 decomposition, hydrogen generation, and characterization of the generated solid carbon. It is important to note that the incorporation of CO2 gas into the methane proved to be both successful and an essential component in the generation of carbon throughout the continuous decomposition reaction. In spite of this, in addition to the primary decomposition reaction, four alternative sets of reactions were investigated in order to get a better understanding of the catalytic activity of the catalyst towards CO2 and CH4 (in separate studies), as well as the generated products that result from these reactions. It was discovered that the blank gallium did not show any substantial catalytic activity towards the conversion of CH4 and CO2 gases. On the other hand, there was a noticeable conversion when CH4-CO2 (mixed) gases were utilised for decomposition in presence of Ga; nevertheless, it is important to note that there was no formation of solid carbon during this reaction. However, 2%Mo-Ga was also examined to determine its activity towards the same gases under the same reaction conditions. It was observed that 2% Mo-Ga did not show substantial CH4 conversion, but that its activity towards CO2 was quite high, and it reached up to 99% conversion during the course of the reaction. Although the rate of CO2 conversion was extremely high, the only product that was produced throughout the reaction was CO; there was no production of elemental oxygen, nor was there any detectable formation of solid carbon. In contrast, when CH4-CO2 (mixture) was used for the reaction, the same catalyst performed well and generated solid carbon. This provides significant evidence for the positive effect of adding molybdenum to the catalyst and CO2 in the decomposition of methane. The scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDX) technique was used to analyse the by-product (carbon) in order to determine its structural characteristics (such as its morphology, size, and weight%). The FTIR and RAMAN spectroscopy, on the other hand, were used to verify the chemical characteristics (surface and functional groups). Along with this, recovered catalyst after reaction was also analysed using XRD and XPS to determine the possible reaction mechanism and to understand the effect of reaction onto catalyst.