La-doped ATaO3 for the Photocatalytic Degradation of PFAS

 

Rachael K. Matthews, Cameron J. Shearer

 

Per- or Polyfluoroalkyl Substances (PFAS)

• Group of fluorinated chemicals with useful properties.
• Both hydrophobic and hydrophilic with strong C-F bonds that are resistant to biodegradation.
• Current destruction techniques (combustion) are ineffective for PFAS degradation.¹
• 8,300 tonnes of PFAS firefighting foams stored in Australia alone.²
• Detrimental effects on health and environment.
• 3 compounds listed as Persistent Organic Pollutants in the Stockholm Convention.

 

Photocatalysis for PFAS Degradation

• Materials that absorb light can be used to drive chemical reactions.
• Typically use semiconductors due to energy difference between occupied conduction band and vacant valence band (e.g. TiO₂).
• 100 % PFAS conversion and 34.8 % mineralisation can be achieved with TiO₂ under UV irradiation.³

 

Tantalum-Based Metal Oxides (ATaO₃,  A = Na, Li, K)

• Perovskite ABO₃ structure.
• Band gap of 3.6 – 4.7 eV (UV active).⁴
• La-doping at the A-site reduces recombination rates and therefore improves photocatalytic activity.^4,5

 

Effect of La doping – HR-STEM EDS Mapping

• Increasing La mol % results in greater La spread throughout nanoparticle.
• La exists in a consistent single atomic layer at the surface of doped nanoparticles.
• Likely explains plateau in activity at high La mol % doping.
• La-doping likely the cause of terrace-like features on nanoparticles.

 

Model Pollutant Degradation

• Doping with La improves methyl orange degradation for all ATaO₃ significantly.
• NaTaO₃ activity improved the most by La-doping.
• No reduction in activity at high La mol % as seen with other metal oxides (e.g. Al-doped SrTiO₃).
• Trend matches observed La mol % doping in SEM-EDS.

 

PFOA Photocatalysis Reactions

• % PFOA conversion capability is dependent on the redox potential of ATaO₃ (LiTaO₃ > NaTaO₃ > KTaO₃).
• % mineralisation based on reduction in total organic fluorine content as measured by combustion ion chromatography (CIC).
• Greater redox potential leads to greater extent of mineralisation (LiTaO₃ > NaTaO₃ > KTaO₃).