Shape of catalytic curve affected by catalytic rate constant k cat and effective concentrations of catalyst and substrate.

Transcription

1 Electrocatalysis: CVs Cat Cat + + e - Cat + + A Cat + B E C Blue: No Catalysis Green: Limited Catalysis Black: Ideal Catalysis Shape of catalytic cure affected by catalytic rate constant k cat and effectie concentrations of catalyst and substrate.

2 Cat Cat + + e - Cat + + A Cat + B k e Saéant, J.-M. Chem. Re. 2008, 108, J. Electroanal. Chem. 1984, 171, 341

3 Zone D: No Catalysis Limiting or no substrate results in no catalysis. Only original redox wae of catalyst is obsered. k e

4 Zone KT2: Total Catalysis Rapid catalysis, k e, and limiting substrate results in complete depletion of substrate. Catalysis happens so quickly in comparison to CV time scale that original redox wae of catalyst is still obsered. k e

5 Zone K: Pure Kinetic Conditions Competition between consumption and diffusion of the substrate. Depletion of substrate at the electrode surface. k e

6 Zone KS: S-Shaped Substrate concentration at electrode and in bulk are equal. Typically achieed by rapid scan rate and high substrate concentration. k e

7 Zone KD: No Substrate Consumption Substrate concentration at electrode and in bulk are equal. Slower kinetic parameter preents ideal S-shape. 00 k e

8 Zone KG/KG*: Substrate Consumption Substrate consumption and diffusion compete. Slower kinetic parameter results in return wae. 00 k e

9 Zone KS: S-Shaped Substrate concentration at electrode and in bulk are equal. Typically achieed by rapid scan rate and high substrate concentration. k e

10 i c /i p Equations Solution Catalyst Surface Bound Catalyst i p = (0.446AC 3/2 n p 3/2 D 1/2 1/2 )/(R 1/2 T 1/2 ) i p = (n p2 2 nag)/(4) i c = n c ACD 1/2 k obs 1/2 i c = n c AGk obs i c /i p = (R 1/2 T 1/2 n c /0.446n p 3/2 1/2 )k obs 1/2 n -1/2 i c /i p = (4n c /n p2 )k obs (1/n) k obs = k cat [substrate] or both i c /i p equations, there is no account for diffusional processes, hence for i c /i p to apply, competition between substrate consumption and diffusion cannot exist. Depletion of substrate at surface causes unknown substrate concentrations, inaccurate k cat. Background oxidation of substrate at electrode surface and catalyst deactiation add complications.

11 How to deal with non-ideal catalytic current If there is any change in concentration of catalyst, substrate or considerable background current, Tafel analysis or oot of the Wae must be performed Both are extrapolation techniques. Both analyze the idealized catalyst reactiity by considering only the initial portion of the catalytic wae in the CV where it is expected that little to no substrate consumption, catalyst degradation, or other side phenomena such as background current hae yet occurred.

12 OW (oot of the Wae) Analysis Recommended for usage when there is bend oer in catalytic current in CV due to substrate or catalyst depletion. S-shape Catalysis: Ideal catalytic response results in perfectly linear OW plot. Zone K: Substrate depletion at the electrode surface as competition of consumption and diffusion of the substrate occurs. Deiation from linearity in OW plot. Can still fit linear region to obtain k obs alues. Costentin, C. J. Am. Chem. Soc. 2012, 134,