Solar Thermochemical Recycling of CO2 Using ALD Deposited CoFe2O4 On Alumina Supports

Fuels production through metal oxide redox cycling can be completely renewable if reduction temperatures are achieved using concentrated solar energy. Many combinations of metal oxides have been investigated over the years and cobalt ferrites show much promise as they have good thermodynamic properties and high melting points. However, bulk ferrites degrade when cycled due to sintering of starting materials and resulting diffusion limitations. Recent work has found that coating the ferrites on rigid supports helps to reduce sintering. When using alumina as a support several Co/Fe/Al spinel species are formed which are capable of being cycled at lower temperatures. Cobalt ferrite has been coated on alumina nano powder and high surface area alumina shells. Large, high surface area polymer particles (~600µm, 43.5 m²/g) were coated via atomic layer deposition (ALD) with several layers of alumina. The alumina layers were applied using TMA/water ALD chemistry. The particles were then heated to 800°C in air to burn out the polymer substrate. The alumina substrates were then coated with alternating layers of Fe₂O₃ and Co­O using ferrocene and cobaltocene ALD chemistries. ICP results also show metal oxide mass loadings up to 27%. Thermo-chemical cycling was performed in a Netzch DIL 402 thermogravimetric analyzer with CO production measured using an in-situ Netzch QMS 403C mass spectrometer. Particles with alumina supports maintain cycling efficiency as compared to unsupported particles. Testing has been conducted on sun in a multi-tube reflective cavity reactor at the National Renewable Energy Laboratory in Golden, Colorado. Cycling results help to estimate reactor efficiencies.