Syngas production by electrocatalytic reduction of CO2 using Ag-decorated TiO2 nanotubes

The CO2 that comes from the use of fossil fuels accounts for about 65% of the global greenhouse gas emission, and it plays a critical role in global climate changes. Among the different strategies that have been used to address the storage and reutilization of CO2, the transformation of CO2 into chemicals or fuels with a high added-value has been considered a winning approach. This transformation is capable of reducing the carbon emission and induce a “fuel switching” that exploits renewable energy sources. This work aims to focus on electrocatalytic reduction of CO2 to produce syngas as a raw material for many other chemical industries. The chosen electrocatalyst for this purpose is Ag decorated Titania nanotubes. The presented work has been divided in 3 parts: Initially, a thorough analysis of state of the art about syngas production by electrocatalysis of CO2 has been presented which includes current status, the catalyst used so far and the parameter that effects the process and also how to improve them. Next chapter is about the synthesis of catalyst. As for this work, Ag decorated Titania nanotubes has been chosen as the electrocatalyst. The synthesis also divided in two sub sessions. First, how to synthesize the substrate which is Titania nanotubes and the next sub session is concerned with Ag deposition methods used in this work. After a brief description of existing methods for the growth of Titania nanotubes, anodization method has been described and used for the fast and well oriented vertically aligned nanotubes regarding the aforementioned sub session. In the next sub session, two different deposition methods have been described and used. One of the methods described in the current study is UV deposition in which Ag particles are being deposited on nanotubes by UV illumination of Silver nitrate solution. Furthermore, the next method has been carried out using sputtering of Ag nanoparticles on nanotubes in which the deposition time and applied currents for sputtering has been changed to achieve the best dispersed Ag nanoparticles. In the third chapter, different Ag decorated Titania nanotubes has been evaluated and tested in order to analyze the performance of each electrocatalyst. Additionally, different electrochemical tests from chronoamperometry to cyclic voltammetry and electrochemical surface area measurement has been taken into account so as to evaluate the performance of the catalysts. All the mentioned analysis has been carried out in ambient conditions. In these tests the maximum current density of 60mA.cm-2 with H2/CO ratio of 3 to 1 has been achieved by Titania nanotube decorated with Ag nanoparticles by sputtering at 60mA for 90s. Furthermore, to investigate the effect of pressure, chronoamperometry analyzes has been done in 7 bar which showed to increase the production rate due to higher solubility of CO2 into electrolyte. Moreover, for stability analysis of electrocatalyst, Titania nanotube which decorated by Ag nanoparticles by sputtering method at 60mA for 90s has been chosen for a 21-hour chronoamperometry test. Surface analysis by Transmission Electron Microscopy (TEM) and Electrochemically Active Surface Area (ECSA), before and after the reaction, confirmed that the decay in the activity of this electrocatalyst happened due to agglomeration and dissolution of some nanoparticles. Also, in order to investigate the effect of Titania nanotubes as a co-catalyst for Ag nanoparticles in CO2 reduction, different substrates with the same amount of Ag loading have been tested and analyzed. Consequently, it is concluded that Titania can act as a facilitating medium for stabilizing rate determining step radicals. Also, using Titania in the form of nanotubes proved to be beneficial by increasing the active surface area of electrocatalyst and decreasing the resistance of mass transfer and electrical transportation in the electrodes’ surface.