Schematic of Ag-decorated blue TiO2/WO3 catalyst for reduction of carbon dioxide to useful chemicals like carbon monoxide.
Schematic of Ag-decorated blue TiO2/WO3 catalyst for reduction of carbon dioxide to useful chemicals like carbon monoxide.

Artificial photosynthesis could provide clean energy in the future while simultaneously removing anthropogenic CO2 from the atmosphere. Converting CO2 into useful chemicals like CO or other hydrocarbons is an attractive option but, because CO2 is so stable, a catalyst is needed to drive the reaction.

“Photocatalytic reduction of CO2 into solar fuels is regarded as a promising method to address global warming and energy crisis problems,” explains Hyoyoung Lee of Sungkyunkwan University in Korea. “Although heterostructured hybrid metal oxide catalysts have been used for CO2 reduction, selective control for CO production only remains the subject of debate.”

Lee and colleagues at Sungkyunkwan University believe that they have come up with a promising candidate catalyst in the form of Ag-decorated reduced titanium oxide (TiO2)/tungsten (WO3) hybrid nanoparticles [Nguyen et al., Materials Today (2019),]. The combination of TiO2, WO3, and Ag creates in a catalyst that is 100% selective for CO and increases the reaction rate.

Nanostructured TiO2 is a well-known attractive photocatalyst because of its high reactivity and stability paired with low toxicity and cost, but only absorbs 4% of the ultraviolet spectrum. Phase-selectively disordered TiO2 creates oxygen vacancies and Ti3+ states, leaving behind blue-colored TiO2 and reducing the band gap to ~2.7 eV, improving light and CO2 absorption. The combination of blue TiO2 and WO3 as a Z-scheme heterogeneous catalyst not only extends the solar spectrum response but also prevents the recombination of photoinduced charge carriers. Another promising strategy involves decorating the surface of blue TiO2 or WO3 metal oxide nanoparticles with noble metal nanoparticles like Ag or Au to utilize their plasmonic response to boost photoactivity at higher wavelengths.

The researchers brought together these various approaches, depositing Ag nanoparticles on the surface of blue TiO2 and WO3 catalytic nanoparticles, to make a hybrid photocatalyst capable of producing exclusively CO. The catalyst’s high surface area and large pore size provide ample active sites for the adsorption of CO2 molecules, which are photocatalytically reduced to CO. Effective separation of electron-hole pairs prevents recombination in the Z-scheme blue TiO2/WO3 system, so more electrons are transported to the Ag nanoparticles for CO2 reduction while holes remain on WO3 to oxidize H2O to O2. But the proportion of Ag nanoparticles has to be just right – too many increase light scattering and reduce photocatalytic activity.

“The 100% selectivity of CO2 reduction into CO has been an issue for a long time,” points out Lee. “We believe that our findings represent a new strategy for the development of 100% CO-only production via durable photocatalysts for CO2 reduction.”

The boost to photocatalytic activity and selectivity for CO could help industry develop methods to convert anthropogenic CO2 into useful products.

Click here to read the article in the journal.