Preparation Of Ag/C Hybrid Catalysts With Chemical Reduction Method And Study Of Their Electrocatalytic Properties

Recently, Platinum, palladium, gold and their alloys nanoparticlessupported on the carbon materials as carbon-based composite electrocatalystsare widely studied, such as Au/C,Pt/C, Pd/C, Pt-Ru/C, Au-MnO2/C, Pd-Ni/C.These catalysts have been applied for oxygen reduction reaction in alkalinemedia, oxidation of hydrazine in alkaline media and oxidation of methanol inalkaline media. However, on one hand the high cost associated to these noblemetals is still a drawback for full commercial application; on the other hand,Especially in the case of oxygen reduction reaction in harsh alkaline media,the catalysts mentioned above are not suitable for this bad environment. It isimportant to develop a new catalyst with low cost, high catalytic activity andcorrosion resistance for oxygen reduction reaction, oxidation of hydrazine andoxidation of methanol. Silver is a potential candidate due to its highconductivity and corrosion resistance in alkaline media. Hence, it is significantto develop ultra-high loading Ag/C hybrid catalysts. Because ultra-high silverloading not only improves corrosion resistance and stability of the catalyst inalkaline media, but also increases the electrocatalytic activity of oxygenreduction reaction, oxidation of hydrazine and oxidation of methanol. Based on the background above, this thesis mainly focuses on thesynthesis of ultra-high loading Ag/C hybrid catalysts and study of theirelectrocatalytic activity for oxygen reduction reaction, oxidation of hydrazine,oxidation of methanol in alkaline media. Firstly, ultra-high loading Ag/Chybrid catalysts were synthesized by ethylene glycol as reducing agent usingchemical reduction method. Then, various measurements,such as XRD,HRTEM, XPS, SEM, TG, UV, IR, were used to investigate the physicalproperties and reaction mechanisms of catalysts in detail. Finally, theelectrocatalytic activities and mechanism of catalysts applied for oxygenreduction reaction, oxidation of hydrazine and oxidation of methanol inalkaline media were studied. The main creative achievements are showed asfollows:(1) An easy route to prepare ultra-high loading Ag/C hybrid catalysts isdeveloped. A novel reaction bath containing ethylene glycol as reducing agent,silver nitrate as silver source, ammonia as chelating agent, and sodiumhydroxide as adjusting pH value, has been successfully developed to prepareto ultra-high loading Ag nanoparticles supported on the carbon blacks namedVulcan-72. Silver nanoparticles which sizes10nm are highly dispersed oncarbon blacks even though the silver loading is400wt.%.(2) Synthesis mechanism of ultra-high loading Ag/C hybrid catalysts isstudied. After the systematic research on the synthesis mechanisms of catalysts,we know that Ag[NH3]2+in the solution of silver ammonia are absorbed on carboxyl groups of carbon blacks at first. Then, silver ions are reduce byethylene glycol to obtain the silver nanoparticles supported on the carbonblacks. The functions of sodium hydroxide enhance the reducing ability ofethylene glycol and accelerate reaction rate in order to make nucleation rate ofsilver faster than nucleus growth rate. Finally, high dispersed silvernanoparticles supported on the carbon blacks are obtained.(3) Electrocatalytic activity and kinetic mechanism of ultra-high loadingAg/C hybrid catalysts are studied for oxygen reduction reaction in alkalinemedia. After the systematic research on the electrochemical properties ofcatalysts for oxygen reduction reaction, we know that ultra-high loading Ag/Chybrid catalysts exhibit high electrocatalytic activity of oxygen reductionreaction. Moreover, oxygen reduction reaction is controlled by diffusionprocess and proceeded by four electron pathway. Compared withelectrochemical properties of catalyst for oxygen reduction reaction, gasdiffusion electrode modified with catalyst improves mass transfer rate ofoxygen gas and increases electrochemical properties of oxygen reductionreaction.(4) Electrocatalytic activity and kinetic mechanism of ultra-high loadingAg/C hybrid catalysts are studied for oxidation of hydrazine in alkaline media.After the systematic research on the electrochemical properties of catalysts foroxidation of hydrazine, we know that oxidation of hydrazine is controlled bydiffusion process and proceeded by four electron pathway. Ultra-high loading Ag/C hybrid catalysts exhibit high electrocatalytic activity of oxidation ofhydrazine. Hence, this catalyst is used for detection of hydrazine to estimatethe detection limit, linear limit, response time and sensitivity of hydrazinedetection. the catalyst as hydrazine sensor exhibits a linear response in theconcentration range from20μM~220μM. The detection limit, response timeand ensitivity of hydrazine detection are12.67μM,2s and0.8983μA·μM-1,respectively.(5) Electrocatalytic activity and kinetic mechanism of ultra-high loadingAg/C hybrid catalysts are studied for oxidation of methanol in alkaline media.After the systematic research on the electrochemical properties of catalysts foroxidation of methanol, we know that the peak currents and potentials ofoxidation of methanol are increased with increase of concentration ofmethanol. While the concentration of methanol is unchanged, the peakcurrents of oxidation of methanol are increased with increase of concentrationof sodium hydroxide. Meanwhile, oxidation of methanol is controlled bydiffusion process.