Electrochemical Promotion Using Na Conductors

A typical electrochemical promotion experiment utilizing (3 -Al203, a Na+ conductor, as the promoter donor is shown in Fig. 4.15. The reaction under study is the oxidation of CO on Pt.51 

The corresponding Na coverage on the Pt surface, 0Na, can be computed from Faraday s law, i.e.  

As shown in Fig. 4.15, increasing 0wa up to 0.02 causes a linear decrease in Uwr and a concomitant 230% increase in catalytic rate. The rate increase Ar 5,5T0 7 mol O/s is 2600 times larger than -I/F. Upon further increasing 0Na in the interval O.O2 0Na O.O6, Uwr remains practically constant while r decreases sharply and reaches values below the initial unpromoted value ro. When 0Na exceeds 0.06, UWr starts decreasing sharply while r decreases more slowly. The system cannot reach steady state since 0Na is constantly increasing with time due to the applied constant current. 

Setting 1=0 gradually restores Uwr to -0.3 V while r remains practically unaffected. Restoration of the initial r value requires potentiostatic setting of UWr at 0.4 V, and thus removal of Na(Pt) from the Pt catalyst surface. 

The complex transient r vs t, or equivalently r vs 0Na or r vs Uwr behaviour of Fig. 4.15 parallels the steady-state rvs UWr behaviour shown in Fig. 4.16, where for each point UWr has been imposed potentiostatically, until the current I has vanished and the corresponding rate value, r, has been measured. This shows that the catalyst surface readjusts fairly fast to the galvanostatically imposed transient 0Na values (Fig. 4.15). The dashed and dotted line transients on the same figure were obtained with the same gaseous composition but with initial Uwr values of 0 and -0.3 V respectively. It is noteworthy that the three transients are practically identical which shows the reversibility of the system. 

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