3
982
J. Am. Chem. Soc. 1996, 118, 3982-3983
Homogeneous Catalysis of Electrochemical
Hydrogen Evolution by Iron(0) Porphyrins
1
Iqbal Bhugun, Doris Lexa, and Jean-Michel Sav e´ ant*
Laboratoire d’Electrochimie Mol e´ culaire
de l’UniVersit e´ de Paris 7, Unit e´ Associ e´ e au
CNRS No. 438, 2 Place Jussieu
7
5251 Paris Cedex 05, France
ReceiVed December 29, 1995
Catalysis of electrochemical hydrogen evolution from acidic
solutions by various metal electrodes has attracted considerable
2
and continuous attention. Surprisingly, there are few examples
of catalysis of the reaction by molecules dispersed in the solution
or imbedded in a supporting phase coating the electrode surface.
The only example of homogeneous catalysis by organic
molecules involves pyridines and has been proposed to occur
3
according to the following reaction scheme:
+
-
+
-
Py + AH h PyH + A , 2PyH + 2e f 2Py + H
2
There have also been relatively few attempts to use, as
molecular catalysts, low-valent transition metal complexes,
forming hydrides upon reaction with acids and evolving
hydrogen heterolytically or homolytically. Heteropolytungstates
(
Si, P, Mo) electrodeposited on electrodes have been used
4
Figure 1. Cyclic voltammetry of TPPFe(III)Cl (a, 0.96 mM; b, 0.65
mM) in DMF + 0.1 M Et NClO at a mercury drop hung to a 1 mm
diameter gold disk in the presence of Et
mM). Scan rate: 0.1 V/s. Temperature: 25 °C. Top: experimental
curves. Bottom: simulated curves.
successfully in this purpose. Hydridocobaloxime has been
shown to react, albeit slowly, with acids.5 Co(I) porphyrins in
solution, or attached to the electrode surface, catalyze hydrogen
evolution in water.6 However, the catalyst rapidly deactivates.
Hydrogen evolution also occurs upon reaction of osmium and
ruthenium porphyrin hydrides with acids. It involves the attack
of the hydride by the acid.7 Several dihydrides of complexes
containing two metal centers, designed to promote facile
homolytic formation of hydrogen, do evolve hydrogen (see refs
4
4
+
-
3
NH Cl (a, 1.6 mM; b, 7.1
also note the presence at more negative potentials of a small
but distinct reversible wave (3/3′). At low acid/catalyst
concentration ratios, the catalytic wave occurs at a more positive
a
-
2-
potential than the PFe(I) , and PFe(0) couple which still gives
rise to a reversible wave (Figure 1a). Upon raising the acid/
catalyst concentration ratio, the catalytic wave increases in height
and shifts in the negative direction thus merging with the 2/2′
wave while the reversibility disappears. This behavior is typical
of a “total” catalysis situation where the catalytic reaction is so
fast that the current is controlled by the diffusion of the substrate
7
b,c and references therein).
We have found that iron porphyrins at the zero oxidation state
2-
+
(PFe(0) ), electrochemically generated from PFe(III) , PFe(II),
-
PFe(I) successively, are very efficient molecular catalysts of
hydrogen evolution. Figure 1 shows the catalytic currents that
are observed in cyclic voltammetry with iron tetraphenylpor-
phyrin (TPP) in N,N′-dimethylformamide (DMF) in the presence
of protonated triethylamine. In the absence of the acid, the iron
porphyrin, introduced as TPPFeCl, exhibits three successive
reversible one-electron waves corresponding to the successive
9
to the electrode surface. Very similar results were obtained
with CHF2COOH as the acid, showing both the appearance of
the catalytic wave 2C and the small wave 3/3′.
Figure 2 summarizes the results of a preparative-scale
catalytic reduction of the acid. The observed faradaic yields
show that the formation of hydrogen is totally selective.
Another noteworthy result is that there is no significant
degradation of the porphyrin catalyst after 1 h of electrolysis
and passage of 70 C.
-
2- 8
formation of PFe(II), PFe(I) , and PFe(0) . Upon addition
of the acid, the first two waves remain unchanged (this is shown
-
in Figure 1 for wave 1/1′, representing the Fe(II)/Fe(I) couple).
The formation of PFe(0)2 at wave 2/2′ triggers the appearance
-
of a catalytic irreversible wave, noted as 2C in Figure 1. We
(1) Present address: Laboratoire de Bio e´ nerg e´ tique and Ing e´ ni e´ rie des
What about the mechanism of the catalytic reaction? One
may think of two different types of catalysis. In one of them,
Proteines, UPR CNRS 9036, 31 Chemin J. Aiguier, 13009 Marseille, France.
(2) Concerning electrocatalysis of hydrogen evolution at metal electrodes,
2
-
addition of a proton to the PFe(0) catalyst would involve the
porphyrin ring, leading to the formation of the phlorin anion
which would then react with a second acid molecule to produce
H2. The second alternative would involve the iron coordination
sphere, namely the formation of the iron(II) hydride which
would then react with a second acid molecule hence evolving
H2. The first of these alternatives appears unlikely in view of
the observation we made that the complex resulting from the
addition of two electrons to TPPCu(II) does not catalyze
hydrogen evolution under the same conditions in spite of the
see: ref 2b and references therein. (b) Divisek, J.; Schimtz, H.; Steffen, B.
Electrochim. Acta 1994, 39, 1723.
(3) Mairanovskii, S. G. Catalytic and Kinetic WaVes in Polarography;
Plenum, New York, 1968; pp 245-261.
4) (a) Keita, B.; Nadjo,L.; Sav e´ ant., J-M. J. Electroanal. Chem. 1988,
43, 105. (b) Keita, B.; Nadjo, L. Mater. Chem. Phys. 1989, 22, 77.
5) (a) Chao, T-H.; Espenson, J. H. J. Am. Chem. Soc. 1978, 100, 129.
b) Connolly, P.; Espenson, J. H. Inorg. Chem. 1986, 25, 2684.
6) (a) Kellett, R. M.; Spiro, T. G. Inorg Chem. 1985, 24, 2373. (b)
Kellett, R. M.; Spiro, T. G. Inorg. Chem. 1985, 24, 2378.
7) (a) Collman, J. P.; Wagenknetcht, P. S.; Lewis, N. S. J. Am. Chem.
(
2
(
(
(
(
Soc. 1992 114, 5665. (b) Collman, J. P.; Ha, Y.; Wagenknetcht, P. S.; Lopez,
M-A.; Guilard, R. J. Am. Chem. Soc. 1993, 115, 9080. (c) Collman, J. P.;
Wagenknetcht, P. S.; Hutchison, J. E. Angew. Chem., Int. Ed. Engl. 1994,
-
2-
fact that the standard potential of the TPPCu(II) /TPPCu(II)
3
3, 1537.
(
8) (a) Lexa, D.; Rentien, P.; Sav e´ ant, J-M.;. Xu, F. J. Electroanal. Chem.
(9) (a) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J-M.; M’Halla,
F.; Sav e´ ant, J-M. J. Electroanal. Chem. 1980, 113, 19. (b) Sav e´ ant, J-M.;
Su, K. B. J. Electroanal. Chem. 1984, 171, 341.
1
985, 191, 253. (b) Gueutin, C.; Lexa, D.; Momenteau, M.; Sav e´ ant, J-M.;
Xu, F. Inorg. Chem. 1986, 25, 4294.
0
002-7863/96/1518-3982$12.00/0 © 1996 American Chemical Society