Requisitos para decretar las medidas cautelares en general

The redox properties for catalytic materials were measured electrochemically using a coulometric-titration cell. This technique measures the sample composition as a function of pO2 for the oxides and provides redox isotherms of the oxides as a function of oxygen fugacity from equilibrium constants.

For example, for a given redox reaction,

MOx + O2 MOx+2 Equation 2.5

where M is a metal, the equilibrium constant is a function only of the oxygen fugacity since the activity of solids is equal to 1.

_{Equation 2.6}

The differential Gibbs free energy of the oxidation reaction can then be calculated using:

ΔG = RTln(Keq) Equation 2.7

This can be calculated if the P(O2) can be measured experimentally. As the experimental P(O2) values are very low, that the oxygen fugacity must be established by an equilibrium of buffer gasses, such as H2-H2O and CO-CO2, as shown in following reactions:

H2 + O2 H2O Equation 2.8

CO + O2 CO2 Equation 2.9

If the values of the initial concentrations of the buffer gases are known, the P(O2) can be determined by the following equations:

log(P(O2)) = -2log(Keq) + 2log(P(H2O)/P(H2)) Equation 2.10 log(P(O2)) = -2log(Keq) + 2log(P(CO2)/P(CO)) Equation 2.11

41 The experimental setup used to measure the redox properties of materials used in this dissertation is shown in Figure 2.433.

Figure 2.4:Schematic of Coulometric Titration apparatus33.

The cell consisted of a YSZ tube with Ag electrodes painted onto the inner and outer surfaces. Pt wire was attached to the inner electrode with a Ag gauze that was also pasted onto the inner electrode using Ag paste. The other end of the inner electrode connected Pt wire was then spot-welded to an Ultra-Torr fitting (Swagelok) which will eventually fit over the end of the tube. Approximately 0.1g of sample was placed in the cell and then pre-reduced in flowing 10%H2:90%N2 at 973 K for 4 h. The H2 treatment insured that the measurements were started from the reduced state of the sample. After passing the gas buffer over the sample, the YSZ tube was immediately sealed using a glass stoppers into the Ultra-Torr fittings.

Precise amounts of oxygen were added to the cell in a stepwise fashion by pumping oxygen through the YSZ electrolyte by applying a voltage across the two electrodes using a Gamry Instruments potentiostat. After each O2 addition the sample was allowed to equilibrate (where the potential varied less than 0.1 mV/h which could take up

42 to two days), the pO2 in the cell was determined from the voltage across the two electrodes via the Nernst equation (equation 2.12).

Equation 2.12

The oxygen stoichiometry of the sample was determined from the known starting stoichiometry and the amount of O2 pumped into the cell. All coulometric titration experiments were performed with the sample at 973 K.

43

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