Cyrille Costentin, Samuel Drouet, Marc Robert and Jean-Michel Savéant

Size: px

Start display at page:

Download "Cyrille Costentin, Samuel Drouet, Marc Robert and Jean-Michel Savéant"

Moses Bates
5 years ago
Views:

Supporting Information Turnover numbers, Turnover Frequencies and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and reparative-scale Electrolysis.

1 Supporting Information Turnover numbers, Turnover Frequencies and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and reparative-scale Electrolysis. Cyrille Costentin, Samuel Drouet, Marc Robert and Jean-Michel Savéant Experimental Details Chemicals. Dimethylformamide (Sigma-Aldrich, >99.8 %, over molecular sieves), the supporting electrolyte NBu 4 F 6 (Fluka, purriss.), phenol (Merck), meso-tetraphenylporphyrin iron (III) chloride (Aldrich) were used as received. Methods and Instrumentation Cyclic voltammetry. The working electrode was a mercury drop hung to a 1 mm diameter gold disk. The counter-electrode was a platinum wire and the reference electrode an aqueous SCE electrode. All experiments were carried out under argon or carbon dioxide at 1 C, the double-wall jacketed cell being thermostated by circulation of water. Cyclic voltammograms were obtained by use of a Metrohm AUTOLAB instrument. Ohmic drop was compensated using the positive feedback compensation implemented in the instrument. Electrolysis. Electrolyses were performed using a rinceton Applied Research (ARSTAT 7) potentiostat. The experiments were carried out in a cell (figure 1S) with a mercury pool as working electrode. The reference electrode was an aqueous SCE electrode and the counter electrode a platinum wire in a bridge separated from the cathodic compartment by a glass frit, containing a.4m EtNCO CH.1M NBu 4 F 6 DMF solution. The electrolysis solution was purged with CO during min prior to electrolysis. articular care was exerted to minimize the ohmic drop between working and reference electrodes. This was done as follows: the reference electrode was directly immerged in the solution (without separated bridge) and put progressively closer to the working electrode until sustained oscillations appear. It is then slightly moved away until the remaining oscillations are compatible with recording of the catalytic current-potential curve (figure 7). The appearance of oscillations in this cell configuration does not require positive feedback compensation as it does with micro-electrodes. 1S The potentiostat is equivalent to a self inductance. 1S Oscillations thus appear as soon as the resistance that is not compensated by the potentiostat comes close to zero as the reference electrode comes closer and closer to the working electrode surface. Gaz detection. Gas chromatography analyses of gas evolved in the course of electrolysis were performed with a H 689 series equipped with a thermal conductivity detector (TCD). CO and H production was quantitatively detected using a carbosieve 5 III 6-8 Mesh column m in length and 1/8 inch in diameter. Temperatures were held at ºC for the detector and 4 ºC for the oven. The carrier gas was helium flowing at constant pressure with a flow of ml/min. Injection was performed via a syringe (5 µl) previously degazed with CO. The retention time of CO was 7 min. Calibration curves for H and CO were determined separately by injecting known quantities of pure gas. CE WE RE EV Fig. 1S. Electrolysis cell. WE: mercury pool working electrode, CE: platinum grid counter-electrode, RE: aqueous saturated calomel electrode, EV: expansion vessel Determination of E CO CO,DMF We are first looking for E CO CO,S,AH acid, AH: CO (S) HA (S) e -, the standard potential for the conversion of CO into CO in a solvent S and in the presence of an CO (S) H O (S) A - (S), referred to the aqueous standard hydrogen electrode (SHE). In practice, 1S

2 the potential measurements were made against the aqueous standard calomel electrode (SCE) of an electrode reaction that takes place in the solvent S, here DMF (potentials referred to aq SHE are.41 V more positive than when referred to the aq. SCE). Thus, after consideration of the following thermodynamical cycle: the change of solvent introduces an the interliquid potential E L, S between the aqueous SHE and DMF solution of a.1 M tetraalkylammonium supporting electrolyte in the equation relating E CO, the standard potential corresponding to the top reaction CO,S,AH to, E CO CO,aq, the standard potential corresponding to the bottom reaction: ( G G t,h,s->aq t,,h O,S->aq ) RT ln1 RT Kh,CO,S->g Kh,CO,aq->g ECO CO,S,AH = EL,S ECO CO, aq pka,ha,s ln F F Kh,CO,aq->g K h,co,s-> g F This problem has been previously addressed in the framework of a concerted dissociative electron transfer to organic halide S and generalized to any redox couple in any solvent. S E L,S can be estimated through the following relationship: 1 E E E G F, SHE, aq, SHE, aq L,S = Ag Ag,S Ag Ag,aq t, Ag,aq->S the various parameters being obtained as follows:, SHE, aq E.799 Ag = V Ag,aq, SHE, aq E Ag =.778 V 4S Ag, DMF G t, Ag,aq->DMF leading to t,h,dmf->aq = -.16 ev 5S E L,DMF =.141 V. G =.186 ev, 5S G = -. ev (considering that it is about the same as for HO t,ho,dmf->aq, itself calculated quantum mechanically. 6S E = -.16 V vs. SHE. 7S CO CO,aq K ( CO ) ([ CO ] C ) = [CO ] being the solubility of CO in S at h,co,s->g CO = 1 bar with =1 bar and C = 1 M. S

3 ( CO ) ([ CO] C ) Kh,CO,S->g = [CO] being the solubility of CO in S at CO = 1 bar with =1 bar and C = 1 M. [CO ] (M) [CO] (M) K h,co,s->g K h,co,s->g H O.8 8S.96 8S 9 14 DMF. 9S.5 1S 5 4 leading to: E CO CO,DMF,AH = RT ln1 pka F,HA,DMF V vs. SHE (1) In order to obtain the standard potential E CO CO,S in the presence of CO, it should be considered that CO in presence of water is the strongest acid present. In other words, the redox reaction to be considered is: CO (S) CO (S) H O (S) e - CO (S) H O (S) HCO - (S) Thus, replacing HA/A - - by CO H O / HCO, an estimation of pk a,co HO,DMF is needed to apply equation (1) to this case In water: Thus, for the reaction: 6.7 a,co,aq a,hco,aq h 1 K = K K = 8S We then consider the following cycle: leading to : pk a,co,s a,co,aq - h,co,aq->g G G G t,ho,aq->s t,hco,aq->s t,h,aq->s K = pk log K h,co,s->g RT ln1 RT ln1 RT ln1 In DMF, we have seen earlier that G t,h,dmf->aq =.186 ev and t,ho,dmf->aq G = -. ev. In addition, G,HCO -.4 ev t,aq->dmf assuming a comparable transfer free energy as other monoanions, see reference 5S. Finally: pk = 7.7 and thus : E CO a,co,dmf = V. vs. SHE CO,DMF Determination of iron meso-tetraphenylporphyrin diffusion coefficient. Iron meso-tetraphenylporphyrin diffusion coefficient was obtained from the cyclic voltammetry peak current recorded in a 1 mm solution on a mm diameter glassy carbon electrode, at.1 V/s: S

4 .446 Fv i = FSC D = p 9.8 µ A RT leading to : D = cm s -1. Foot of the wave analysis and TOF determination for second order catalytic reactions. As discussed in the main text, the following mechanism, second order in catalyst, is worth considering, Scheme 1S leading to the following diffusion-reaction equation for Q: CQ CQ = D k 1CACQ k 1CB t x Application of the steady-state approximation to B yields: ( ) k1c A CQ = k 1CB k CB and thus: kk1c ACQ k 1 k ( k 1) CB = 4k Second order kinetics in catalyst is obtained when kk1c ACQ / k 1 << 1 (corresponding to a pre-equilibrium followed by a bimolecular rate determining state). Then, under pure kinetic conditions (steady-state established by mutual compensation of diffusion and catalytic reaction 11S ): C Q D k apcq = with x Integration, taking into account: k1 kap = k CA k 1 CQ i= FSD x x= we obtain: CQ and CQ, x= = F 1 exp ( E E Q) CQ = x 4 k ap D ( C ) Q and, at the electrode surface: 4 C D kapc i = FS F 1 exp ( E E Q) 4S

5 lotting i / i p vs. F 1 / 1 exp ( E E ) RT Q in cyclic voltammetry gives thus rise to a straight line, the slope of which, 4 k 1.4 ( RT / Fv) k C C, provides immediate access to k k A ap. -1 The same expressions also apply, under pure kinetic conditions, to preparative-state electrolysis, leading, after a second integration, to: kapc = x C Q CQ, x= C D It thus appears, as in the first order case, that the Q-concentration profile is contained a thin reaction-diffusion layer adjacent to the electrode surface. Its thickness is however different: D µ =. kapc The total amount of catalyst, including both forms, per unit surface area in the reaction layer, may thus be expressed as: D mol( Q) = S C, µ k C ap It follows that: i b t k C apc mol C C V TON = = = F = t mol (Q) D D S C S C F 1 exp ( E EQ k ) apc kapc leading to the following expression of the turnover frequency (s -1 ) : C k1 C TOF = k ap = k CA F k 1 F 1 exp ( E EQ) 1 exp ( E EQ) After introduction of the overpotential, η = EAC E, the above equation may be recast as : k1 C TOF = k CA k 1 F 1 exp ( E AC E Q η) For low values of the overpotential the equation may be simplified, leading to a Tafel-like expression, which relates the turnover frequency to the overpotential: k1 F Fη logtof = log k CA C ( EAC EQ ) k 1 RT ln1 RT ln1 from which the turnover frequency at zero overpotential is obtained as: k1 F logtof = log k CA C EAC EQ k 1 RT ln1 It is thus seen that TOF depends on ( ) C unlike to the first order case (Scheme 1). Moreover, the inverse slope of the logtof η plot is different ( RT ln1 / F instead of RT ln1 / F, 4 mv instead of 6 mv, at room temperature). These features may be used as diagnostic criteria to distinguish the first order and the second order mechanisms. 5S

6 References 1S. Savéant, J.-M. Elements of Molecular and Biomolecular Electrochemistry, Wiley-Interscience, New York, 6. Chap. 1, pp. 14-; Chap. 6, pp S. (a) Andrieux, C..; Gallardo, I. ; Savéant, J-M.; Su, K. B. J. Am. Chem. Soc. 1986, 18, 68. (b) Andrieux, C..; LeGorande, A. ; Savéant, J- M. J. Am. Chem. Soc. 199, 114, 689. S. Isse, A. A.; Gennaro, A. J. hys. Chem. B 1, 114, S. Matsuura, N.; Umemoto, K.; Takeuchi, Z. Bull. Chem. Soc. Jpn. 1974, 47, 81. 5S. Marcus, Y. In Ion roperties. Marcel Dekker, NY p 14. 6S. Costentin, C.; Evans, D. H.; Robert, M.; Savéant, J-M.; Singh,. S. J. Am. Chem. Soc. 5, 17, S. Standard otentials in Aqueous Solution, Ed. Bard, A. J. ; arsons, R. ; Jordan, J., Marcel Dekker, Inc., New York, p S. Handbook of Chemistry and hysics, 76th Edition, Lide, D. R. and Frederikse H.. R., Ed. CRC ress, Inc., Boca Raton, FL, S. Gennaro, A.; Isse, A. A.; Vianello, E. J. Electroanal. Chem., 199, 89,. 1S. Kutal, C.; Weber, M. A.; Ferraudi, G.; Geiger, D. Organometallics, 1985, 4, S. Savéant, J.-M. Elements of Molecular and Biomolecular Electrochemistry, Wiley-Interscience, New York, 6. Chap., pp S

Supporting Information. Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential

Supporting Information. Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential Supporting Information Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential Guillaume Passard, Andrew M. Ullman, Casey N. Brodsky and Daniel G. Nocera*