High Entropy Oxides Based Materials as Active Catalysts for CO2 Hydrogenation

  • Shaima Hamad Albedwawi

Student thesis: Master's Thesis

Abstract

The exploration of the rare earth high entropy oxides based catalysts and the transition metal high entropy oxides was conducted in this research. Four rare earth high entropy oxides catalysts with different nickel loadings were investigated thoroughly: 5%Ni-500/CeGdLaPrSmO-CP, 10%Ni-500/CeGdLaPrSmO-CP, 10%Ni-900/CeGdLaPrSmO-CP and 15%Ni-500/CeGdLaPrSmO-CP in terms of their morphology, microstructure and their textural properties. Shedding the light on investigating their catalytic activity and stability for CO2 hydrogenation to methane reaction and explaining their performance based on their surface properties was also one of the pillars of this work along with comparing the effectiveness of producing methane by the rare earth high entropy oxides and the transition metal high entropy oxides. Different synthesis methods were proposed, but among them, coprecipitation synthetic technique succeeded in making a high entropy oxide support from both the rare earths oxides (CeGdLaPrSmO) and the transition metals oxides (NiCuCoMgZnO). The nickel was added to the rare earth high entropy oxides support by the wet impregnation to be tested for methanation. It was shown that the addition of more dopants to Ce-M-O lattice (M=La, Sm, Gd and Pr) induces the population of oxygen vacancies which enhanced the conversion of CO2 to start at low temperature (200 °C). The nickel loaded high entropy oxide based catalysts exhibited maximum activity that ranged between 44.6% up to 51.2 % of converting CO2 at 500 °C, along with a maximum CH4 selectivity of 79.4% recorded by 5%Ni-500/CeGdLaPrSmO-CP at 350 °C and a maximum CH4 yield of 37.2 % exhibited by 15%Ni-500/CeGdLaPrSmO-CP at 500 °C. On the other hand, the transition metal high entropy oxides could not convert CO2 to CH4. A simple DFT model was proposed to mimic the nature of the high entropy oxides by doping CeO2 (111) surface with La and Cu and subjecting it to different strains such as tensile and compressive with the presence of oxygen vacancies. The model showed that doping induces the creation of oxygen vacancies with low formation energies. Besides, tensile strain in CeO2 and Ce-La-O (111) surfaces facilitates the generation of the oxygen vacancies. The opposite phenomenon was recorded on the Ce-La-Cu-O (111) surface where the compressive strain was better for stabilizing the system in the presence of oxygen vacancies. Also, the CO2 adsorption energy was studied in the presence of the previous mentioned defects (e.g.: oxygen vacancies, strain and dopants), where the presence of dopants and oxygen vacancies strengthened the adsorption energies due to the weakening of the M-O bonds that led to a better activation of the carbon dioxide adsorbate in the form of carbonate species.
Date of AwardJul 2021
Original languageAmerican English

Keywords

  • High entropy oxides; CO2 methanation; co-precipitation; nickel supported catalysts; oxygen vacancy; rare earth oxide
  • transition metal oxides; DFT; Strain effect.

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