Carbon-Supported Silver Catalysts for Electrocatalytic Reduction of CO2 to CO

The work described in this thesis was aimed at understanding the influence of structural properties of silver catalysts supported on carbon for the electrocatalytic reduction of CO2 to CO. This goal was achieved by rationally designing, characterizing and testing cathode materials. This enabled a correlation between material properties and the catalytic performance. Chapter 1 describes the potential benefits of electrochemistry and electrocatalysis in the context of global warming. The CO2 electrocatalytic reduction to value-added chemicals was described, including the effect of different metal electrodes and buffer electrolytes. Specifically, a background is given on the CO2RR to CO over silver electrocatalysts, and the properties of carbon electrodes, based on literature. In chapter 2, the effect of surface-modification of carbon-based electrodes on the reduction of CO2 to CO is systematically treated. The surface chemistry of the electrodes was characterized with acid-base titration, potentiometric titration and XPS. The basic surface properties (high point of zero charge) of the N functionalized carbon catalyst led to an increased CO production with respect to the O-functionalized and reduced carbon materials. The CO turnover frequency per surface group for pyridinic groups was higher than for O-containing groups. This study demonstrated the possibility to tune the surface properties of carbon materials to enhance the ability of the electrocatalyst to reduce CO2 to CO. In chapter 3, the effect of silver nanoparticle size on the CO2 reduction to CO is discussed. Using the surface modification methods described in chapter 2, control over the ligand-free silver particle size was achieved by tuning the surface properties of the carbon supports. It was demonstrated that the silver particle size, in the range of 10 to 30 nm, decreased by increasing the density of O-containing group on the support. The small nanoparticles (11 nm diameter) effectively steered the selectivity towards CO, even greater than the selectivity achieved by bulk silver electrodes under the same conditions. In chapter 4, the aim was to suppress the hydrogen formation over the high surface area carbon support by functionalizing the surface of the support with alkylamines. The effect of the number of carbon atoms in the alkyl chain on the HER suppression and CO selectivity was investigated. Alkylamine functionalization successfully suppressed H2 evolution, while at the same time promoting CO production. This resulted in a 1 to 2 H2 to CO ratio for the catalyst functionalized with hexylamine, more favorable than for the pristine carbon-based catalyst (benchmark), able to generate only a 3.3 to 1 H2 to CO ratio. In chapter 5, the catalytic properties of oxide-derived silver nanowires, are explored. XRD and XPS analysis confirmed that by selecting the final potential during the oxidation procedure, both different silver oxidations states and different nanowires roughness were achieved as a function of the oxidation potential. This surface-modification procedure enhanced the catalytic properties of the nanowires. The active surface-normalized CO partial current density increased 3.7-times when the pristine nanowires were oxidized to 1.0 V vs Ag/AgCl in basic electrolyte solution.