New efficient catalytic system for the photocatalytic reduction of CO2 to hydrocarbons

4 December 2014

Photocatalytic reduction products formed on various catalysts. The Au₃Cu@STO/TiO₂ array (red arrow) was the most reactive photocatalyst in this family to generate hydrocarbons from diluted CO₂. Kang et al. Click to enlarge.

Researchers from Japan’s National Institute for Materials Science (NIMS) and TU-NIMS Joint Research Center, Tianjin University, China have developed a new, particularly efficient photocatalytic system for the conversion of CO₂ into CO and hydrocarbons. The system, reported in a paper in the journal Angewandte Chemie, may be a step closer to CO₂-neutral hydrocarbon fuels.

More than 130 kinds of photocatalysts have been investigated to catalyze CO₂ reduction; of those, strontium titanate (SrTiO₃, STO) and titania (TiO2) are two of the most investigated materials. The research team headed by Dr. Jinhua Ye decided to use both, and devised a heteromaterial consisting of arrays of coaxially aligned STO/TiO₂ nanotubes.

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The researchers evenly loaded the nanotubes with nanoparticles of a gold-copper alloy to act as co-catalyst. Hydrazine hydrate (N₂H₄•H₂O) acted as the source of hydrogen and maintained the necessary reducing atmosphere. This system allowed the researchers to very efficiently convert CO₂ to CO and methane (CH₄), as well as other hydrocarbons.

Herein, we develop a new approach that is able to achieve high-rate UV/Vis-light-driven conversion of diluted CO₂ into CO and hydrocarbons in which STO/TiO₂ coaxial nanotube arrays loaded with an optimized combination of Au–Cu bimetallic NPs are used as the photocatalyst. Under UV/Vis-light illumination, a CO production rate of 138.6 ppm cm^-2 h^-1 (3.77 mmol g^-1 h^-1) and total hydrocarbon production rate of 26.68 ppm cm^-2 h^-1 (725.4 mmol g^-1 h^-1) are obtained on Au₃Cu@SrTiO³/TiO₂ nanotube arrays by using diluted CO₂ (33.3% in Ar). Generally the highest rate of production (e.g. methane) in previous reports does not exceed tens of mmol per hour of illumination per gram of photocatalyst.

On the nanoparticles, CO₂ is first reduced to CO, then to CH₄ and on to other hydrocarbons. A 3:1 ratio of gold to copper results in the largest amount of hydrocarbon product.

CH₄ is the main hydrocarbon product with an evolution rate of 15.49 ppm cm^-2 h^-1 (421.2 mmol g^-1 h^-1), and is 60% of the total hydrocarbon products. The other hydrocarbons are C₂H₆, C₂H4, and C₃H₆. The corresponding quantum efficiency for the photo-reduction is 2.51%. After five cycles measurement during a 34-hour test, the CH₄ gas-evolution rate decreases from 15.49 to 13.57 ppm cm^-2 h^-1, which is still 87.6 % of its original activity.

The researchers attributed the efficiency of their system to:

• employing high surface area nanotube array architectures, with holes in the tube walls to enhance the gas diffusion and increase the contact between photogenerated charge carriers and surface species;

• developing STO/TiO₂ heterostructures to facilitate the photogenerated charge separation;

• distributing noble bimetallic alloy NPs co-catalysts along the nanotube arrays to help the redox process; and

• choosing hydrous hydrazine as the hydrogen source and electron donor to provide a reductive atmosphere for keeping the alloying effect.

Irradiation with sunlight releases electrons within the semiconductor nanotubes. The STO/TiO₂ heterostructures allow the subsequent charge separation to be maintained better than in the pure substances. The electrons are transferred to the bimetallic precious metal nanoparticles and from there to the CO₂, the resulting CO, and other gaseous intermediates.

The large surface area of the nanotube bundles and the porosity of the nanotube walls facilitate a high degree of gas diffusion and ensure efficient charge transport. Special effects resulting from their alloyed state allow the gold-copper nanoparticles to stop the return of photogenerated electrons in the semiconductors much more effectively than the pure metals.

The hydrazine hydrate provides the necessary hydrogen, resupplies electrons to the catalyst, and forms a reducing atmosphere, which stabilizes the metal nanoparticles for a long time. If water is used as the hydrogen source instead, the catalytic system is rapidly deactivated.

Resources

• Qing Kang, Tao Wang, Peng Li, Lequan Liu, Kun Chang, Mu Li, and Jinhua Ye (2014) “Photocatalytic Reduction of Carbon Dioxide by Hydrous Hydrazine over Au–Cu Alloy Nanoparticles Supported on SrTiO3/TiO2 Coaxial Nanotube Arrays” Angewandte Chemie International Edition doi: 10.1002/anie.201409183