Hydrogen production takes a step forward with the help of chromium oxide

Photocatalysis vs electrocatalysis

There are two ways to produce zero-emissions hydrogen, electrocatalysis and photocatalysis.

Electrocatalysis has been studied for a while, and uses electricity to split water (H2O) molecules into hydrogen (H2) and oxygen (O2). Electricity works as a catalyst, which means it speeds up the reaction.

Photocatalysis does the same thing but uses light instead of electricity as the catalyst. So light and energy act as catalysts and speed up the reaction.

However, the problem with catalysts is that they can encourage the reaction both ways. This means that it can cause water to split up into hydrogen and oxygen, or it can make hydrogen and oxygen react to form water. The latter is known as a back reaction and needs to be avoided.

Sometimes additional compounds known as co-catalysts are added to a reaction to increase its efficiency and avoid certain things like back reactions from happening.

Chromium oxide for photocatalytic water splitting

Chromium oxide (Cr₂O₃) has been shown to prevent the back reaction from happening, but it is not very stable. The team, led by Prof. Gunther G. Andersson from Flinders University, photodeposited the chromium oxide onto a few catalysts and co-catalysts using a process known as annealing (heating followed by cooling) to study its stability. 

They found that the stability of the chromium oxide depended on the layers used underneath. They also observed that chromium oxide had no effect on the water-splitting reaction, making it an excellent candidate for use in photocatalysis.

Since chromium oxide is a leading candidate in photocatalysts, this study has important implications as it gives valuable insights into the properties of the chromium oxide coating. Thus, their findings can pave the way for further research in photocatalysis as a viable option for hydrogen production. 

Study abstract:

Chromium oxide (Cr₂O₃) is a beneficial metal oxide used to prevent the backward reaction in photocatalytic water splitting. The present work investigates the stability, oxidation state, and the bulk and surface electronic structure of Cr-oxide photodeposited onto P25, BaLa₄Ti₄O₁₅, and Al:SrTiO₃ particles as a function of the annealing process. The oxidation state of the Cr-oxide layer as deposited is found to be Cr₂O₃ on the surface of P25 and Al:SrTiO₃ particles and Cr(OH)₃ on BaLa₄Ti₄O₁₅. After annealing at 600 °C, for P25 (a mixture of rutile and anatase TiO₂), the Cr₂O₃ layer diffuses into the anatase phase but remains at the surface of the rutile phase. For BaLa₄Ti₄O₁₅, Cr(OH)₃ converts to Cr₂O₃ upon annealing and diffuses slightly into the particles. However, for Al:SrTiO₃, the Cr₂O₃ remains stable at the surface of the particles. The diffusion here is due to the strong metal–support interaction effect. In addition, some of the Cr₂O₃ on the P25, BaLa₄Ti₄O₁₅, and Al:SrTiO₃ particles is reduced to metallic Cr after annealing. The effect of Cr₂O₃ formation and diffusion into the bulk on the surface and bulk band gaps is investigated with electronic spectroscopy, electron diffraction, DRS, and high-resolution imaging. The implications of the stability and diffusion of Cr₂O₃ for photocatalytic water splitting are discussed.