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Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

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10.2 <strong>Kinetics</strong> of heterogeneous reactions (surface reactions) 225<br />

I Limiting cases: It is often the case that one species is adsorbed much more strongly<br />

than the other.<br />

• Assume, for instance, that ¿ . Then,<br />

[P]<br />

<br />

= 5 <br />

(1 + ) 2 (10.70)<br />

(1) À 1:<br />

[P]<br />

<br />

= 5 <br />

<br />

(10.71)<br />

The reaction is inhibited by B, which is strongly adsorbed: At high ,all<br />

active sites are blocked by B.<br />

(2) ¿ 1:<br />

[P]<br />

= 5 (10.72)<br />

<br />

The reaction is truly second order.<br />

⇒ We can see from these two cases that, as increases, the rate [P]<br />

<br />

increases, reaches a maximum, and then decreases again.<br />

first<br />

d) Example 1: Heterogeneous oxidation of CO<br />

I<br />

Figure 10.7: Heterogeneous oxidation of CO on Pt. (a) Langmuir-Hinshelwood and<br />

Eley-Rideal mechanisms, (b) potential energy diagram (values in kJ mol −1 ).

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