Catalysis Assignment: The effect of support on redox properties and methanol-oxidation activity of vanadia catalysts

Basic Principle of   Coulometric Titration Technique

Coulometric titration technique is a technique that provides a more quantitative measure

of oxygen binding in metal oxide catalyse. To be more specific, it is based on

measurement of chemical potential of oxygen in a gaseous phase which able to indicates

the composition of the oxides.

In this technique, a sample is placed in a sealed container, which is an

experimental apparatus used, that is equipped with one or two electrochemical element

for oxygen sensing and pumping. In addition, the example of the electrochemical element

which is commonly used is Yttria-Stabilized Zirconia (YSZ) tube. YSZ tube is painted

with Ag electrodes on the inner and outer surfaces.

To analyse the sample, a ceramic crucible is filled with the sample and inserted

into the center of the YSZ tube. Then, the sample size which contains of approximately

100 mol vanadium is used. With The Cajon fittings attached to the end of YSZ tube, the

tube and catalyst sample is allowed to be heated by placing the center portion of the tube

in a tube furnace. In order to insure that the sample can fully oxidized, a mixture of 2.5%

O2, 50% CO2 and 50% Ar is allowed to flow over the sample for 1 hour at the

temperature of 748 K (510.85 oC).

Then, the Cajon fitting is plugged and allow the sample to equilibrate with the gas

phase until the potential across the Ag electrodes become constant. The duration for this

usually took in between one to five days. The constant potential is then used to calculate

the oxygen partial pressure inside the cell by using the Nernst equation.

Next, a controlled amount of oxygen is removed or added by using a Gamry

Instruments potentiostat to pass a certain amount of charge across the Ag electrodes.

Gamry instrument is the company that created instruments with uncommon value and

performance and one of it is potentiostat that is used in this experiment (Gamry.com,

2015). This instrument is shown in Figure 1.

Figure 1: Gamry Instruments’ Potentiostat

According to Gamry.com (2015), a potentiostat is an electronic instrument that

used to control the potential difference between a reference electrode and a working

electrode where both of the electrodes contained in an electrochemical cell. The control

of the potential difference is done by injecting current into the cell through an electrode

and measures the current flow between both of the electrodes as well. Thus, a potentiostat

control the cell potential whereby the cell current is varied.

Next, in this experiment, most of the oxygen is removed with the conversion of

CO2 to CO. The removal of oxygen is then caused the reduction of the sample. And since

the amount of oxygen added or removed from cell is measured by using the Gamry

Instruments’ potentiostat as well as the starting composition of the sample is known, the

sample composition as a function of the oxygen partial pressure, P(O2) can be calculated

by using the mole balance.

Reference

Gamry.com, (2015). Potentiostat-Instrument Between Working Electrode/Reference

Electrode. [online] Available at:

http://www.gamry.com/application-notes/instrumentation/potentiostat-fundamentals/

[Accessed 1 Aug. 2015].