Abstract
Carbon dioxide (CO2) is considered as the prime reason for the global warming effect and one of the useful ways to transform it into an array of valuable products is through electrochemical reduction of CO2 (ERC). This process requires an efficient electrocatalyst with high faradaic efficiency at low overpotential and enhanced reaction rate. Herein, we report an innovative way of reducing CO2 using copper-metal supported on titanium oxide nanotubes (TNT) electrocatalysts. The TNT support material was synthesized using alkaline hydrothermal process with Degussa (P-25) as a starting material. Copper nanoparticles were anchored on the TNT by homogeneous deposition-precipitation method (HDP) with urea as precipitating agent. The prepared catalysts were tested in a home-made H-cell with 0.5 M NaHCO3 aqueous solution in order to examine their activity for ERC and the optimum copper loading. Continuous gas-phase ERC was carried out in a solid polymer electrolyte (SPE) reactor. The 10% Cu/TNT catalysts were employed in the gas diffusion layer (GDL) on the cathode side with Pt-Ru/C on the anode side. Faradaic efficiencies for the three major products namely methanol, methane, and CO were found to be 4%, 3%, and 10%, respectively at −2.5 V with an overall current density of 120 mA/cm2. The addition of TNT significantly increased the catalytic activity of electrocatalyst for ERC. It is mainly attributed to their better stability towards oxidation, increased CO2 adsorption capacity and stabilization of the reaction intermediate, layered titanates, and larger surface area (400 m2/g) as compared with other support materials. Considering the low cost of TNT, it is anticipated that TNT support electrocatalyst for ECR will gain popularity.
Highlights
It has been scientifically well established that carbon dioxide (CO2 ), a major greenhouse gas, is the prime reason for the climate change and global warming phenomena in recent decades [1,2].A substantial share of the anthropological CO2 comes from coal and natural gas-fired power plants, Catalysts 2019, 9, 298; doi:10.3390/catal9030298 www.mdpi.com/journal/catalystsCatalysts 2019, 9, 298 combustion of liquid fuels in automobiles, and other industrial activities [3,4,5]
There are some practical challenges for the large-scale utilization of the electrochemical reduction of CO2 (ERC) process which include large over potentials for the hydrocarbon product(s), low overall current density, and small faradaic efficiency for the desired product(s) [19,20]
Titanium oxide nanotubes are synthesized using the alkaline hydrothermal method and used as support material for electrocatalyst for the electrochemical reduction of CO2. It is clear from the characterization results that titanium oxide nanotubes with the exceptionally high surface area (395 m2 /g) are produced by the alkaline hydrothermal process
Summary
It has been scientifically well established that carbon dioxide (CO2 ), a major greenhouse gas, is the prime reason for the climate change and global warming phenomena in recent decades [1,2]. 0.5 M K2 SO4 solution standard H-cell to understand the role of TiO2 as support material and the effect of Ag loading and particle size on the performance of Ag/TiO2 catalyst for the conversion CO2 to CO They were able to produce CO with faradaic efficiency higher than 90% at partial current density 101 mA/cm with 40 wt.% Ag/TiO2 which is two times higher than 40% Ag/C. For the ERC process to be economically viable for commercialization, the reaction should be carried out in a continuous flow reactor at high current densities (~200 mA/cm2 ) and in the gas phase to overcome the mass transfer issue originating from the limited solubility of CO2 in aqueous and non-aqueous mediums [54,55]. Basu et al reported an overall faradaic efficiency of 39.6% for methane, methanol, and ethane as the major product in the ERC on gas phase reaction over copper nanoparticles [26]. In the aqueous sodium bicarbonate solution and in a continuous gas-phase SPE reactor
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