New efficient catalytic system for the photocatalytic reduction of CO2 to hydrocarbons
04 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.
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.—Kang et al.
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.
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
Can it be competitive with petrol. Im glad that they make progress and I am willing to put ch4 in my car instead of polluting petrol gasoline. That might be a better method than batteries or hydrogen as it apply to all kind of vehicles like cars, trucks, planes, etc. Batteries and hydrogen only apply to small short distance cars.
Posted by: gorr | 04 December 2014 at 10:20 AM