Periodic Reporting for period 1 - HEO4CAT (Development of new high-entropy oxide catalysts by the anodic oxidation of high-entropy alloys)
Okres sprawozdawczy: 2022-10-01 do 2025-09-30
The European Green Deal should ensure that Europe becomes the first climate neutral continent by 2050. This will mean a cleaner environment, more affordable energy, smarter transport, new jobs and overall a better quality of life. Ensuring the quality of water is an important part of this ambitious target. Conventional water- and wastewater-treatment plants (WWTPs) were originally designed for the removal of suspended solids and biodegradable organic matter. As such they exhibit widely varying efficiencies (from 0 to 100%) for the removal of “new pollutants” like pharmaceuticals, pesticides and hormones. Several new technologies have been investigated to reduce the potential risks associated with these pollutants and their impact on the environment and human health, including photocatalysis. There is an urgent need for an effective, safe, low-cost, high-technology-readiness-level (TRL) strategy for removing emerging pollutants from water and wastewater.
In addition, ensuring the production of green hydrogen is also an important part of the European Green Deal, since after its use in fuel cells only water vapour is produced. The green hydrogen is generated when the process of obtaining hydrogen fuel involves the use of renewable energy sources like solar, wind, or hydroelectric power. Breaking the water molecule to generate H2 and O2, i.e. the water splitting process, is a good strategy to generate green hydrogen. However, the current state-of-the-art catalysts for water splitting (Pt, RuO2, IrO2, NiO) are critical raw materials and/or doesn’t present long-term stability. As a consequence, the search for highly efficient and high-performance catalyst for the water splitting process is essential to address the sustainable energy production challenge.
High-entropy oxides (HEOs) are a new category of materials constituted of five or more elements that are randomly distributed in a single phase. Such materials can have better properties than conventional oxides, related to the lattice distortion and synergistic effects of the components. Due to this lattice distortion and the uneven electron-cloud distribution between the metals and the oxygen, HEOs can impact different catalytic reactions, such as CO2 reduction, H2S removal and the degradation of pollutants. It is well known that doping or co-doping of conventional catalysts can promote changes in the crystalline structure and improve the photocatalytic activity. Thus, the use of new, multi-element materials such as HEOs appears to be an excellent, innovative alternative to overcome these drawbacks. HEOs can be synthesised by the anodic oxidation of high-entropy alloys (HEAs), creating strongly attached nanostructures with enhanced photo(electro)catalysts properties. Using this strategy, our ambition is to develop a new and highly efficient (photo)(electro)catalyst to be applied in the water splitting process and/or pollutants degradation, which will contribute to a major step forward in catalysis.
In addition, ensuring the production of green hydrogen is also an important part of the European Green Deal, since after its use in fuel cells only water vapour is produced. The green hydrogen is generated when the process of obtaining hydrogen fuel involves the use of renewable energy sources like solar, wind, or hydroelectric power. Breaking the water molecule to generate H2 and O2, i.e. the water splitting process, is a good strategy to generate green hydrogen. However, the current state-of-the-art catalysts for water splitting (Pt, RuO2, IrO2, NiO) are critical raw materials and/or doesn’t present long-term stability. As a consequence, the search for highly efficient and high-performance catalyst for the water splitting process is essential to address the sustainable energy production challenge.
High-entropy oxides (HEOs) are a new category of materials constituted of five or more elements that are randomly distributed in a single phase. Such materials can have better properties than conventional oxides, related to the lattice distortion and synergistic effects of the components. Due to this lattice distortion and the uneven electron-cloud distribution between the metals and the oxygen, HEOs can impact different catalytic reactions, such as CO2 reduction, H2S removal and the degradation of pollutants. It is well known that doping or co-doping of conventional catalysts can promote changes in the crystalline structure and improve the photocatalytic activity. Thus, the use of new, multi-element materials such as HEOs appears to be an excellent, innovative alternative to overcome these drawbacks. HEOs can be synthesised by the anodic oxidation of high-entropy alloys (HEAs), creating strongly attached nanostructures with enhanced photo(electro)catalysts properties. Using this strategy, our ambition is to develop a new and highly efficient (photo)(electro)catalyst to be applied in the water splitting process and/or pollutants degradation, which will contribute to a major step forward in catalysis.
- Preparation of HEAs: synthesis of CoFeNiMnCr high entropy alloy via arc melting; obtention of other HEAs: TiNbZrHfTa, CoFeNiCrCo, CoFeNiMnCu, CoFeNiCuTi. Implementing the cleaning protocol of HEAs for the next step characterisation or anodisation.
- Anodisation of the produced HEAs (HEOs synthesis): anodisation of CoFeNiMnCr and TiNbZrHfTa. A nanostructured thin film high entropy oxide was formed on the top TiNbZrHfTa. For the CoFeNiMnCr HEA, the anodisation led to pit formation on the surface of the material, increasing the roughness.
- Post-treatment of anodised HEAs (crystallisation): both anodised CoFeNiMnCr and TiNbZrHfTa were submitted to thermal treatment on a muffle furnace at different times and temperatures. For anodised CoFeNiMnCr it was possible to obtain HEOs formation ((CoFeNiMnCr)3O4) after 1 hour at 650 °C. The thin film formed after anodization on TiNbZrHfTa HEA was amorphous and remained so at temperatures below 1000 °C. However, after 1000 °C the HEO started to segregate into different mixed oxides (not necessarily HEOs).
- Materials characterisation: all materials were submitted to morphological, structural and chemical characterisation, including XRD, XPS, SEM-EDS, HRTEM, STEM, UV-Vis, linear sweep voltammetry, cyclic voltammetry, open circuit potential, and chronoamperometry in the dark and under illumination. From the results, it was possible to correlate the HEAs and HEOs structures with their catalytic properties. TiNbZrHfTa composition was pointed out as promising material for photocatalysis for pollutants degradation and/or H2 generation, while CoFeNiMnCr/(CoFeNiMnCr)3O4 presented great potential for electrocatalytic water-splitting process.
- Catalytic studies: TiNbZrHfTa HEOs were tested for the photo(electro)catalytic degradation of the antibiotic tetracycline. The (CoFeNiMnCr)3O4 HEOs were tested for oxygen evolution reaction.
- Anodisation of the produced HEAs (HEOs synthesis): anodisation of CoFeNiMnCr and TiNbZrHfTa. A nanostructured thin film high entropy oxide was formed on the top TiNbZrHfTa. For the CoFeNiMnCr HEA, the anodisation led to pit formation on the surface of the material, increasing the roughness.
- Post-treatment of anodised HEAs (crystallisation): both anodised CoFeNiMnCr and TiNbZrHfTa were submitted to thermal treatment on a muffle furnace at different times and temperatures. For anodised CoFeNiMnCr it was possible to obtain HEOs formation ((CoFeNiMnCr)3O4) after 1 hour at 650 °C. The thin film formed after anodization on TiNbZrHfTa HEA was amorphous and remained so at temperatures below 1000 °C. However, after 1000 °C the HEO started to segregate into different mixed oxides (not necessarily HEOs).
- Materials characterisation: all materials were submitted to morphological, structural and chemical characterisation, including XRD, XPS, SEM-EDS, HRTEM, STEM, UV-Vis, linear sweep voltammetry, cyclic voltammetry, open circuit potential, and chronoamperometry in the dark and under illumination. From the results, it was possible to correlate the HEAs and HEOs structures with their catalytic properties. TiNbZrHfTa composition was pointed out as promising material for photocatalysis for pollutants degradation and/or H2 generation, while CoFeNiMnCr/(CoFeNiMnCr)3O4 presented great potential for electrocatalytic water-splitting process.
- Catalytic studies: TiNbZrHfTa HEOs were tested for the photo(electro)catalytic degradation of the antibiotic tetracycline. The (CoFeNiMnCr)3O4 HEOs were tested for oxygen evolution reaction.
- Synthesis of (CoFeNiMnCr)3O4 high entropy oxide via anodisation and thermal treatment of high entropy alloy (CoFeNiMnCr), reported for the first time in the literature. The obtained material exhibits a low overpotential for the oxygen evolution reaction, 341 mV at 10 mA/cm2, a Tafel slope of 50 mV/dec, and an unchanged surface after a long-term stability test (10 h, at 1.6 V) in alkaline media.
- Synthesis of Ti-Nb-Zr-Hf-Ta-O high entropy oxide via anodisation and thermal treatment of high entropy alloy (TiNbZrHfTa). The material presented nanotubular shape and was tested on photo(electro)catalytic experiments for organic pollutants degradation. The results showed 86% of tetracycline removal in 3 h, even after 5 reuse cycles during photoelectrocatalysis process (unpublished results).
- Synthesis of Ti-Nb-Zr-Hf-Ta-O high entropy oxide via anodisation and thermal treatment of high entropy alloy (TiNbZrHfTa). The material presented nanotubular shape and was tested on photo(electro)catalytic experiments for organic pollutants degradation. The results showed 86% of tetracycline removal in 3 h, even after 5 reuse cycles during photoelectrocatalysis process (unpublished results).