Resumen de Electrochemical promotion of methane oxidation over pd based catalyst-electrodes
This Doctoral Thesis aims at the development of suitable materials to be used as catalyst-electrodes for the exploitation of EPOC concept applied for the catalytic oxidation of methane at low temperatures.
The effect of non-faradaic electrochemical modification of catalytic activity (NEMCA effect) or electrochemical promotion of catalysis (EPOC) phenomenon is an innovative concept and a unique tool for the improvement of the behaviour of a heterogeneous catalyst by electrochemistry. This process is based on the control, by applied current or potential, of the catalytic work function due to the electrochemical pumping of ions between the solid electrolyte and the surface of a porous catalyst. From the discovery of this phenomenon in 1981, it has been extensively applied in environmental and energetic catalysis. The application of the EPOC phenomenon to the catalytic total oxidation of methane could embrace a couple of the principles considered in the assessment of the NEMCA application for its commercialisation.
Palladium based catalyst-electrodes supported on YSZ and prepared by different techniques were characterized. It was found that Pd film preparation technique resulted to have a significant role on both the palladium particle sizes and the active surface area of the catalyst-electrodes, and therefore, on the metal dispersion. Hence, the use of the impregnation technique led to better dispersed Pd films and it was the most suitable catalyst for the electrochemically promoted methane oxidation at low temperature.
EPOC studies were carried out on the selected catalyst-electrode. It was found that the slight reduction of PdO achieved allowed the electrochemical promotion at temperatures as low as 320 ºC. Operating parameters such as O2 partial pressure, temperature, polarization time and applied current or potential were investigated by different techniques such as TPC, CV, current-potential curves and NEMCA transients. Enhancements in the reaction rate up to 120 % and Faradaic efficiencies up to 800 were obtained. Besides, the feasibility of electrochemically promoted methane oxidation while cofeeding ethylene to the reaction mixture was investigated. Hence, the electrochemical promotion behaviour was studied with a long term potentiostatic transient together with AC impedance measurements during the polarization. It was observed a slow oxidation process of the palladium film. By this approach, higher values of the rate enhancement ratio and of the Faradaic efficiency towards methane oxidation were found. Then, it was investigated the kinetic of the electrochemically promoted methane oxidation reaction. Six different models belonging to Eley-Rideal, Langmuir-Hinshelwood and Mars van Krevelen mechanisms were fitted to the experimental data. It was found that Langmuir-Hinshelwood mechanism considering dissociative chemisorption of O2 and only one oxygen surface specie consumed in the rate determining step resulted in a satisfactory prediction of the kinetic of the system, both under open circuit state and electrochemical promotion conditions. By using the modified electrochemical Langmuir or effective double layer isotherm, it was demonstrated that anodic polarizations led to an increase of the partial charge transfer parameter for methane, and, thus, to an increase in its adsorption equilibrium constant.
In addition, it was explored the electrochemically promoted methane oxidation reaction over Pd deposited on a combination of porous-dense YSZ. An electro-active palladium catalyst on a highly porous support was prepared for the first time and was successfully used to enhance the catalytic activity by electrochemical promotion. The Pd catalyst impregnated on porous YSZ turned to be much more active in methane combustion than that prepared on dense YSZ. However, it could be electrochemically promoted yet by anodic polarizations, providing a reaction rate enhancement up to 50 %. Finally, the study was extended to the electrochemical promotion of the combustion of natural gas. Rate enhancement ratios around 250 % on the CO2 formation upon positive polarizations were obtained at temperatures as low as 340 ºC. Different electropromoted behaviours were observed for each of the reactants (methane, ethane and propane) and they were explained with regard to the bond strength between these reactants and the catalyst.
