Fine particulate matter (PM) produced, e.g., by burning fossil fuels, decreases air quality. Due to their small size, fine particulates can easily enter the lungs and bloodstream and cause health problems. Solutions such as air filters can alleviate this problem. However, filters need to be replaced frequently, which can be expensive. Thermal decomposition of the filtered pollutants could help to solve this issue and reduce costs.

Xiang Ji, Jing Kong, Massachusetts Institute of Technology (MIT), Cambridge, USA, and colleagues created an all-thermal-catalyst air filter (ATCAF) using a bottom-up synthesis method. The team created the air filter to collect fine particles efficiently and degrade them with heat to regenerate the filter. Na₂Ti₈O₁₇ nanowires were produced in a hydrothermal reaction of TiO₂ particles and NaOH at a temperature of 250 °C and self-assembled into hydrogels. Na₂Ti₈O₁₇ was transformed into H₂Ti₈O₁₇ by ion exchange and the resulting hydrogels were converted to aerogels by removing the solvent. Finally, the material was transformed into TiO₂ aerogels by calcination at 600 °C.

The ATCAF has a filtration efficiency of over 99.999 % for PM_2.5, i.e., PM with an aerodynamic diameter below 2.5 μm. The thin fibers and 3D fibrous structure of the ATCAF help lower the pressure drop that traditional filters like HEPA air filters cause. Since the filter can be regenerated by decomposing the pollutants using heat, the cost of filter replacements can be reduced. Due to the high melting point of TiO₂, the filters can directly use the heat of the combustion process to decompose the pollutants created.

• Bottom-Up Synthesized All-Thermal-Catalyst Aerogels for Heat-Regenerative Air Filtration,
  Xiang Ji, Jiayuan Zhao, Sung Mi Jung, Amy I. H. Hrdina, Martin J. Wolf, Xiulin Yang, Geoffrey Vaartstra, Helen Xie, Shao-Xiong Lennon Luo, Ang-yu Lu, Roy E. Welsch, Evelyn N. Wang, Lain-Jong Li, Jing Kong,
  Nano Letters 2021.
  https://doi.org/10.1021/acs.nanolett.1c02593