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The electrocatalytic reactions of oxidation and evolution of hydrogen on iridium electrodes modified by sulphur adsorption
ELSEVIER
Journal of Electroanalytical
Chemistry
416 ( 1996) 47-5 1
The electrocatalytic reactions of oxidation and evolution of hydrogen on iridium electrodes modified by sulphur adsorption E. Lamy-Pitara, J. Barbier LACCO,
URA
CNRS
350
Uniuersirr’
Received
de Poiriers
9 October
40, nuenue
du Rrcteur
1995; revised
1 April
Pineau,
F-86022
Poifirrs.
France
1996
Abstract Iridium electrodes were modified by adsorption of sulphur species produced by reaction with H,S, and tested for the electrocatalytic reactions of oxidation and cathodic evolution of hydrogen. At low sulphur coverages ( Bs < 0.4-0.5) the current densities for the hydrogen evolution and oxidation reactions decrease slightly (at low overpotentials) or increase (at larger overpotentials), resulting in a maximum for 0, = 0.20. At higher sulphur coverages (0, > 0.50) a stronger decrease of current densities is seen. These effects of sulphur obtained in the case of iridium electrodes, are quite comparable with those obtained in the case of platinum electrodes. The Tafel slope increases with increasing sulphur coverage from 35 mV, for OS = 0, to 60 mV, for 8, = 0.95, indicating a change of mechanism at high 8,, whereas in the case of platinum electrodes this parameter remains unchanged in the presence of adsorbed sulphur. Keywords:
Iridium
electrodes;
Sulphur
adsorption;
Electmcatalysis;
Hydrogen
1. Introduction
Sulphur is a well known and largely studied poison of metallic catalysts [l-4]. Sulphur-containing compounds are present in natural sourcesof hydrocarbons and, therefore, are found in industrial feedstocks. Moreover, industrial hydrogen, mostly produced by catalytic or steam reforming, can contain somesulphur compounds as impurities. Most work in the literature is related to the poisoning of platinum, nickel and palladium catalysts by sulphur [l-4] but only in somepapers [5-91 have iridium catalysts been examined. However, iridium catalysts alone or in a bimetallic state are used in various chemical processes (hydrogenation, isomerization, hydrocarbon synthesis,etc.> [lo-131. In the field of electrocatalysis, this metal has been mainly studied in the state of oxide films (IrG,), which are considered as good electrocatalysts for the 0, and Cl, evolution reactions [ 14,151. Although the electrocatalytic reactions of oxidation and evolution of hydrogen are highly interesting and extensively studied, there is very little work concerning the effect of sulphur poisonson these reactions [ 16-181. 0022.0728/96/$15.00 PII
SOO22-0728(96)047
Copyright 17.
0 1996 Elsevier I
Science
evolution
In the case of platinum electrocatalysts, it was found that adsorbedsulphur speciescan affect their activity in a complex way: they can inhibit or enhance these reactions, depending on the quantity of sulphur deposited and on the potential of the electrode [ 181. In this work, the effect of adsorbedsulphur specieson iridium electrodes, for the electrocatalytic reactions of oxidation and evolution of hydrogen, is studied, in order to compare the results with those obtained on platinum electrodes. Some results concerning the adsorption of sulphur specieson iridium from H,S are also presented.
2. Experimental
A conventional three-electrode arrangement cell and Wenking electronic equipment (an LB 75M potentiostat and a VSG 72 voltage scan generator) were used [19,20]. The working electrode was, in most experiments, an iridium wire (diameter 0.5 mm, H.P. 99.98%, LyonAlemand-Louyot), and in some experiments, an iridized platinum wire (deposition of iridium from a 3% chloroiridic acid solution, at a potential Ed = - 0.3 V (SCE) and at 80°C). An iridium wire was also used as a counter elec-
S.A. All rights reserved.
48
E. Lamy-Pitnra,
J. Borhier/Journal
r?fElectroamlyticaI
trode and the supporting electrolyte was a 0.5 M H,SO, solution. Deposition of sulphur was achieved by reaction with H,S produced in situ, by addition of Na,S in a 0.5 M H,SO, solution. The true surface area was evaluated by means of cyclic voltammetry in the potential region of adsorption and desorption of hydrogen, following the method proposed by Woods [21 I. The degree of coverage by sulphur species was determined by the decrease of the quantity of adsorbed hydrogen, measured also by cyclic voltammetry, as in the case of sulphur adsorbed on platinum [19,20]. After sulphur deposition and characterization, the modified iridium surface was examined in the potential range of the reaction of hydrogen by varying the potential (sweep rate 5 mVs-‘) in the negative direction between + 80 and - 60mV (RHE) or in the positive direction between - 20 and + 120 mV @HE). In the second case, a solution saturated by hydrogen bubbled through a glass frit was used.
Chemistry
0
416
0.4
(1996)
47-51
0.8
1.2
E/V(P.liEl
Fig. 2. Successive vohammetric cycles of an Ir electrode modified by adsorption of sulphur (0.5M H,SO,; 5OmVsC’; 298K; es = 0.76): ) partially sulphurized iridium; (---I pure iridium (after ( regeneration by vohammetric cycling).
3. Results and discussion 3.1. Voltammetric ium electrodes
behaviour of sulphur deposited on irid-
The adsorption of sulphur species on iridium electrodes was achieved by reactive deposition from H,S (produced in situ in an acid solution, by addition of a Na,S solution), at a potential fixed at 0.4V @HE). Different degrees of coverage of the iridium by sulphur were obtained by varying the concentration of Na,S (10e4 to lop6 M) and the adsorption time, or by a partial oxidative desorption of the initially deposited quantity of sulphur. Under such
I/Ill4
A
0
0.16
0.32
E/V(P.HEl
Fig. 1. Effect of adsorbed sulphur on the underpotential adsorption and desorption of hydrogen on iridium (0.5 M H,SO,; SOmVs- ‘, 298K): ( -) pure iridium (absence of adsorbed sulphur); (---I iridium modified by adsorption of sulphur.
conditions, during the adsorption of H,S there arises a positive current, due to its oxidation to a degree of oxidation higher than - 2. The characterization of deposited sulphur species was carried out by cyclic voltammetry in another cell containing only supporting electrolyte (0.5 M H,SO,) or in the same cell after washing and refilling with previously degassed pure electrolyte. These two methods gave the same results, indicating that the deposited layer of sulphur species is stable in air. In the potential region of adsorption of hydrogen a decrease of the two main peaks for adsorption-desorption of hydrogen occurs (proportionally to their heights) (Fig. l), indicating that a fraction of the surface of the Ir electrode is occupied by sulphur species which inhibit the adsorption of hydrogen. This allows the quantity of adsorbed hydrogen, displaced by the adsorption of sulphur, to be determined by the difference A Qn = (&!, Q”,), where Qi and Qi?, are the quantities of electricity associated with the adsorption (or desorption) of hydrogen, in the absence and in the presence of adsorbed sulphur respectively. The degree of coverage by sulphur 8, can also be calculated: OS = en/Q!. In the determination of AQn, it is taken into account that at 0.06V (RHE) about 65% of a monolayer of hydrogen is adsorbed, following the work of Woods [21]. In contrast, the deposited sulphur species can be electro-oxidized in the potential region of oxygen adsorption, as in the case of S,,-Pt [19,20]. Indeed, the increase of the anodic current, observed at potentials higher than 0.9 V (Fig. 2), can be attributed to the oxidation of adsorbed sulphur species into sulphates predominantly, as was found by Sutyagina et al. [22]; some other products (SO,, SOi- ) being formed in minor quantities.
E. Lamy-Pitam,
J. Burhier/Jourd
of Electroanalytical
However, for high sulphur coverages, several potential cycles are needed in order to oxidize completely all the deposited sulphur and to regenerate the initial surface area of the iridium electrodes (Fig. 2). A preliminary study of the adsorbed sulphur was carried out based on the comparison of the quantities of electricity involved in the adsorption and the oxidation of sulphur and also in the displacement of adsorbed hydrogen (following the same method as in the case of a sulphurized platinum electrode, see Refs. [ 19,20,23]. The results obtained show that the mean oxidation state of adsorbed sulphur species (at 8, < 0.3) is more negative than - 1, indicating that predominantly it is adsorbed in the state of SH, or as polysulphides, the adsorption of neutral sulphur being less probable. The formation of a bulk sulphide (Ir,S,), at low 8, is not favoured thermodynamically [5]. Negative oxidation states were also observed in the case of platinum electrodes at low sulphur coverages [20,231).
High inhibition of the underpotentially adsorbed hydrogen (at 8s < 0.3) was observed, giving high values of the mean stoichiometry of sulphur species deposited on iridium: about six to ten platinum or hydrogen atoms (with H/Pt = 1) were inhibited by one sulphur atom. This result could be ascribed to important changes in the electronic structure of the substrate, induced by sulphur chemisorption, as was proved by McCarty and Wise [5] by angle-resolved photoemission spectra. This stronger effect of adsorbed sulphur on iridium compared with a platinum substrate could be correlated to the lower electronic affinity of the former (160eV) compared with that of the latter (2.12eV). 3.2. The electrocatalytic modified by deposition
evolution of sulphur
of hydrogen species
on iridium
An iridium electrode (true surface area 2cm’) was modified by adsorption of sulphur species (by H,S produced in situ), at different surface coverages, and characterized by cyclic voltammetry. This electrode was then tested in the potential region between + 20mV and - 60 mV (RHE) by a slow potential sweep (5 mV s- ’ ) and the j-V curves corresponding to the reaction 2H+ + 2ewere traced. +I&, The effect of sulphur on this reaction depends on the potential value (Fig. 3). At low overpotentials (0 to - 4OmV (RHE)) a very slow decrease of current densities is observed for low sulphur coverages (0, < 0.30-0.40), while at higher overpotentials (-50mV (RHE)) a slight increase of the current density, with a maximum for 8, = 0.20, is noted. For higher sulphur coverages (0, > 0.500.60), a stronger decrease, leading to current densities equal to zero for 8, = 1, is observed. Similar results were obtained in the case of a platinum electrode modified by sulphur adsorption [ 181, and explained by the formation of polysulphides or multilayers of sulphur [ 18,231.
Chemistry j/mA
416 (1996)47-5I
49
cm-”
IO-
0.0
0.2
0.4
0.6
0.8
1.0 0s
Fig. 3. Effect of adsorbed sulphur on the electrocatalytic evolution of hydrogen on iridium, at different potentials: (+) -30; (0) -40; (A) - 50mV @HE).
The Tafel slope increases with increasing sulphur coverages from 35 mV, for 8, = 0, to 60mV, for 8, = 0.95 (Fig. 4, Table 1). In the case of a platinum electrode, no change of the Tafel slope was observed in the presence of adsorbed sulphur [ 181. This result shows that a change of the mechanism occurs at high 8, (in the case of an Ir electrode) indicating that a stronger modification of the surface properties of this metal, than in the case of a Pt electrode, is induced by sulphur. A sulphidation of the surface of Ir at high 8, cannot be excluded on a thermodynamic basis [S]. Moreover, other metal sulphides, such as silver sulphide [24] and nickel sulphide [25], are known as good electrocatalysts for the hydrogen evolution reaction, giving Tafel slopes equal to 60mV [25]. The forms of the j=fles) curves obtained can be explained by the occurrence of two opposite effects: a geometrical inhibiting effect, which is predominant at high 8,) and an electronic promoting effect, predominant at low 8, and high overpotentials, in a similar way to the case of platinum [18]. In the latter case, a mechanism first proposed by Ijzermans [26], based on the presence of HS,, and on the increase of H+ concentration at the electrode (favoured by the negative polarization of adsorbed sulphur species) was suggested [ 181. 2HS,, + 2H+ + 2H,S,,, 2H,S,,, + 2e-+ ZHS;, + 2H,,, 25~s --, 4 2H++ 2e-+ This mechanism electrode.
H,
could also apply to the case of an Ir
3.3. Effect of sulphur deposited catalytic oxidation of hydrogen
on iridium
on the electro-
Current vs. potential curves were first recorded, in the absence and in the presence of adsorbed sulphur species on
50
E. Lamy-Pituru,
log (j /mA
of Electrounulytical
J. Burbier/Jourd
Chemistry
416 (1996)
47-51
cm-*) 2’ 4 l-
O-
-1 -
-80
-60
-40
Fig. 4. Tafel plots for the cathodic evolution of hydrogen on iridium (W) 8, = 0.43; (0) 8, = 0.62; (0) 8, s 0.77; (+) Bs = 0.95.
-20 modified
iridium for the reaction H, + 2Hf+ 2e-, and then the resulting current densities at different potentials were plotted as a function of the degree of coverage by sulphur (Fig. 5). The current density at 8s = 0, measured before the deposition of sulphur, was lower than that measured after deposition of sulphur, followed by oxidative desorption and regeneration of the surface of iridium. This result could be explained by a promotional effect of some traces of sulphur, which could have remained on the surface of Ir after elimination of most of the deposited sulphur, or by an irreversible modification or restructuring of the surface of iridium induced by sulphur. However, the voltammogram of iridium did not change significantly after deposition of sulphur and regeneration of the Ir surface (at the end of the experiment) when compared with that obtained before sulphur deposition (at the beginning of the experiment). A similar behaviour, but
Table 1 Exchange evolution
current densities and Tafel slopes for the reaction on partially sulphurized iridium
4
j, /mAcm-’
0 0.19 0.43 0.62 0.77 0.95
0.25 0.18 0.16 0.16 0.10 0.02
by adsorption
0 of sulphur
E/mV(RHE)
20 at different
coverages:
(0)
8, = 0; (0 ) 8s = 0.19;
to a smaller extent, was observed in the case of a platinum electrode [ 181. An increase of current densities appears at 0s = 0.25 compared with the initial current densities, which could be connected with the catalytic or restructuring effect explained previously. This effect is larger for higher overpotentials, the greatest one being obtained for the diffusion current. The current densities decrease for quite high sulphur coverages (0, > 0.5-0.6) and become very low (near zero) for 0, = 1. In this case also, the results obtained can be explained by assuming two opposing effects of deposited sulphur species: a geometric inhibiting effect and an elecj/mA
cm?
of hydrogen
Tafel slope/mV 35 33.5 34 39 43 60
0.0
0.2
0.4
0.6
0.8
1.0
e,
Fig. 5. Effect of adsorbed sulphur on the electrocatalytic hydrogen on iridium, at different potentials: (*I + 10mV; (a) maximum diffusion limited current.
oxidation of (0) + 30mV;
E. Lamy-Pitara,
J. Burhier/Journd
of Elecrroamlyricul
Chemistry
tronic promoting effect occurring mostly at low sulphur coverages. However, this last promoting effect is contradictory to the high inhibition effect of deposited sulphur observed in the underpotential adsorption of hydrogen. It could be suggested that some more weakly adsorbed hydrogen species may be active in these reactions, as was proposed by other authors [27,28]. Indeed Nichols and Bewick [27] found a new weakly adsorbed hydrogen state near OV (from + 180 to 0 mV (RI-IE)) by in situ IR reflection spectroscopy, which was suggested to be the active hydrogen for the hydrogen evolution reaction, while Conway and Bai 1281 proposed that some hydrogen adsorbed overpotentially (but not underpotentially) could be involved in this reaction. A promoting effect of sulphur at low 8, was also reported in the case of the Hz-D, equilibration reaction at a (111) monocrystalline platinum catalyst [29], and in the ethylene hydrogenation reaction [30] with polycrystalline platinum. Jackson et al. [3 11 explained the effect of sulphur on the activity and selectivity of Pt-Al,O,, for propane dehydrogenation at high temperature, by the presence of a hydrogen reservoir on the catalyst.
References
4. Conclusions
[I81
The results obtained indicate that the effect of adsorbed sulphur on the cathodic evolution and on the electro-oxidation of hydrogen, on an iridium electrode, depends on the sulphur coverage and on the potential value. At low sulphur coverages (0, < 0.30) and low overpotentials, a slight poisoning or a promoting effect of these reactions occurs, whereas at higher overpotentials higher promoting effects are observed, as in the case of a platinum electrode. At high sulphur coverages (0, > 0.5) a quite strong deactivation of iridium, for these reactions, is noted, and could be ascribed to the formation of polysulphides, as in the case of a platinum electrode. Larger current densities are obtained for the electrochemical oxidation of hydrogen after sulphur deposition, recording of stationary j-V curves, and surface regeneration by potential cycling, than before sulphur deposition. This result could be explained by a strong modification or a surface restructuring effect, induced by some remaining traces of sulphur on the surface of the iridium electrode.
[I] [2]
[3] [4] [5] [6] [7] [S] [9] [lo] [1 l] [12] [13] [14] [15] [16] [ 171
[19] [20] [21]
[22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
416 (1996)
47-51
51
C.H. Bartholomew, P.K. Agrawal and J.R. Katzer, Adv. Catal., 31 (1982) 13.5. (a) J. Oudar, Catal. Rev., 22 (1) (1980) 171. (b) .I. Oudar, in B. Imelik et al. (Eds.), Metal-Support and Metal-Additive Effects in Catalysis, Elsevier, Amsterdam, 1982, p. 2.55. J. Barbier, in B. Imelik et al. (Eds.), Metal-Support and Metal-Additive Effects in Catalysis, Elsevier, Amsterdam, 1982, p. 293. J. Barbier, E. Lamy-Pitara. P. Marecot, J.P. Boitiaux, J. Cosyns and F. Vema, Adv. Catal., 37 (1990) 279. J.G. McCarty and H. Wise, J. Catal., 94 (1985) 543. C.M. Chan and M.A. Van Hove, Surf. Sci., 183 (1987) 303. R. Frety, P.N. Da Silva and M. Guenin, Catal. Lett.. 3 (1989) 9. R. Frety, P.N. Da Silva and M. GuCnin, Appl. Catal.. 57 (1990) 99. M. Guenin, P.N. Da Silva, J. Massardier and R. Frety. Proc. 9th Int. Congress on Catalysis, Calgary, 1988, Vol. 3, p. 1522. K. Foger and J.R. Anderson, J. Catal.. 59 (1979) 325. S. Taniguchi, T. Mori, Y. Mori, J. Hattori and Y. Murakami, J. Catal., 116 (1989) 108. J.C. Rasser, W.H. Beindorff and J.J.F. Scholten, J. Catal., 59 (1979) 211. M.J. Dees and V. Ponec. J. Catal., 115 (1989) 347. D.N. Buckley and L.D. Burke, Chem. Sot. Faraday Trans. I, 72 (1976) 2431. J. Mozota and B.E. Conway, J. Electrochem. Sot., 128 (1981) 2142. D.T. Chin and P.D. Howard, J. Electrochem. Sot., 133 (1986) 2441. E. Protopopoff and P. Marcus, J. Vat. Sci. Technol. A, 5 (1987) 94.4. E. Lamy-Pitara, L. Lghouzouani, Y. Tainon and J. Barbier. J. Electroanal. Chem., 260 (1989) 157. E. Lamy-Pitara, L. Bencharif and J. Barbier, Appl. Catal., 18 (1985) 117. E. Lamy-Pitara, L. Bencharif and J. Barbier, Electrochim. Acta, 30 (1985) 971. R. Woods, Chemisorption at electrodes; hydrogen and oxygen on noble metals and their alloys, in A.J. Bard @XL), Electroanalytical Chemistry, Vol. 9, Marcel Dekker, New York, 1976, p. I. A.A. Sutyagina, V.A. Perepelitsa and M.N. Semenenko, Russ. J. Phys. Chem.. 57 (3) (1983) 408; 411. E. Lamy-Pitara, Y. Tainon and J. Barbier, J. Chim. Phys., 85 (1988) 861. C. Nguhen Van Huong, R. Parsons, P. Marcus, S. Montes and J. Oudar, J. Electroanal. Chem., 199 (1981) 137. H. Vandenborre. Ph. Venneiren and R. Leysen, Electrochim. Acta, 26 (1984) 297. A.B. Ijzermans, Rec. Trav. Chim. Pays-Bas, 88 (1969) 335. R.J. Nichols and A. Bewick, J. Electroanal. Chem., 243 (1988) 445. B.E. Conway and L. Bai, J. Electroanal. Chem., 198 (1986) 149. C.M. Pradier and Y. Berthier, Bull. Sot. Chim. Fr., 20 (3) (1985) 317. N. Barbouth and M. Salame, J. Cat& 104 (1987) 240. S.D. Jackson, P. Leeming and J. Grenfell. J. Catal., 150 (1994) 170.
Journal of Electroanalytical
Chemistry
416 ( 1996) 47-5 1
The electrocatalytic reactions of oxidation and evolution of hydrogen on iridium electrodes modified by sulphur adsorption E. Lamy-Pitara, J. Barbier LACCO,
URA
CNRS
350
Uniuersirr’
Received
de Poiriers
9 October
40, nuenue
du Rrcteur
1995; revised
1 April
Pineau,
F-86022
Poifirrs.
France
1996
Abstract Iridium electrodes were modified by adsorption of sulphur species produced by reaction with H,S, and tested for the electrocatalytic reactions of oxidation and cathodic evolution of hydrogen. At low sulphur coverages ( Bs < 0.4-0.5) the current densities for the hydrogen evolution and oxidation reactions decrease slightly (at low overpotentials) or increase (at larger overpotentials), resulting in a maximum for 0, = 0.20. At higher sulphur coverages (0, > 0.50) a stronger decrease of current densities is seen. These effects of sulphur obtained in the case of iridium electrodes, are quite comparable with those obtained in the case of platinum electrodes. The Tafel slope increases with increasing sulphur coverage from 35 mV, for OS = 0, to 60 mV, for 8, = 0.95, indicating a change of mechanism at high 8,, whereas in the case of platinum electrodes this parameter remains unchanged in the presence of adsorbed sulphur. Keywords:
Iridium
electrodes;
Sulphur
adsorption;
Electmcatalysis;
Hydrogen
1. Introduction
Sulphur is a well known and largely studied poison of metallic catalysts [l-4]. Sulphur-containing compounds are present in natural sourcesof hydrocarbons and, therefore, are found in industrial feedstocks. Moreover, industrial hydrogen, mostly produced by catalytic or steam reforming, can contain somesulphur compounds as impurities. Most work in the literature is related to the poisoning of platinum, nickel and palladium catalysts by sulphur [l-4] but only in somepapers [5-91 have iridium catalysts been examined. However, iridium catalysts alone or in a bimetallic state are used in various chemical processes (hydrogenation, isomerization, hydrocarbon synthesis,etc.> [lo-131. In the field of electrocatalysis, this metal has been mainly studied in the state of oxide films (IrG,), which are considered as good electrocatalysts for the 0, and Cl, evolution reactions [ 14,151. Although the electrocatalytic reactions of oxidation and evolution of hydrogen are highly interesting and extensively studied, there is very little work concerning the effect of sulphur poisonson these reactions [ 16-181. 0022.0728/96/$15.00 PII
SOO22-0728(96)047
Copyright 17.
0 1996 Elsevier I
Science
evolution
In the case of platinum electrocatalysts, it was found that adsorbedsulphur speciescan affect their activity in a complex way: they can inhibit or enhance these reactions, depending on the quantity of sulphur deposited and on the potential of the electrode [ 181. In this work, the effect of adsorbedsulphur specieson iridium electrodes, for the electrocatalytic reactions of oxidation and evolution of hydrogen, is studied, in order to compare the results with those obtained on platinum electrodes. Some results concerning the adsorption of sulphur specieson iridium from H,S are also presented.
2. Experimental
A conventional three-electrode arrangement cell and Wenking electronic equipment (an LB 75M potentiostat and a VSG 72 voltage scan generator) were used [19,20]. The working electrode was, in most experiments, an iridium wire (diameter 0.5 mm, H.P. 99.98%, LyonAlemand-Louyot), and in some experiments, an iridized platinum wire (deposition of iridium from a 3% chloroiridic acid solution, at a potential Ed = - 0.3 V (SCE) and at 80°C). An iridium wire was also used as a counter elec-
S.A. All rights reserved.
48
E. Lamy-Pitnra,
J. Borhier/Journal
r?fElectroamlyticaI
trode and the supporting electrolyte was a 0.5 M H,SO, solution. Deposition of sulphur was achieved by reaction with H,S produced in situ, by addition of Na,S in a 0.5 M H,SO, solution. The true surface area was evaluated by means of cyclic voltammetry in the potential region of adsorption and desorption of hydrogen, following the method proposed by Woods [21 I. The degree of coverage by sulphur species was determined by the decrease of the quantity of adsorbed hydrogen, measured also by cyclic voltammetry, as in the case of sulphur adsorbed on platinum [19,20]. After sulphur deposition and characterization, the modified iridium surface was examined in the potential range of the reaction of hydrogen by varying the potential (sweep rate 5 mVs-‘) in the negative direction between + 80 and - 60mV (RHE) or in the positive direction between - 20 and + 120 mV @HE). In the second case, a solution saturated by hydrogen bubbled through a glass frit was used.
Chemistry
0
416
0.4
(1996)
47-51
0.8
1.2
E/V(P.liEl
Fig. 2. Successive vohammetric cycles of an Ir electrode modified by adsorption of sulphur (0.5M H,SO,; 5OmVsC’; 298K; es = 0.76): ) partially sulphurized iridium; (---I pure iridium (after ( regeneration by vohammetric cycling).
3. Results and discussion 3.1. Voltammetric ium electrodes
behaviour of sulphur deposited on irid-
The adsorption of sulphur species on iridium electrodes was achieved by reactive deposition from H,S (produced in situ in an acid solution, by addition of a Na,S solution), at a potential fixed at 0.4V @HE). Different degrees of coverage of the iridium by sulphur were obtained by varying the concentration of Na,S (10e4 to lop6 M) and the adsorption time, or by a partial oxidative desorption of the initially deposited quantity of sulphur. Under such
I/Ill4
A
0
0.16
0.32
E/V(P.HEl
Fig. 1. Effect of adsorbed sulphur on the underpotential adsorption and desorption of hydrogen on iridium (0.5 M H,SO,; SOmVs- ‘, 298K): ( -) pure iridium (absence of adsorbed sulphur); (---I iridium modified by adsorption of sulphur.
conditions, during the adsorption of H,S there arises a positive current, due to its oxidation to a degree of oxidation higher than - 2. The characterization of deposited sulphur species was carried out by cyclic voltammetry in another cell containing only supporting electrolyte (0.5 M H,SO,) or in the same cell after washing and refilling with previously degassed pure electrolyte. These two methods gave the same results, indicating that the deposited layer of sulphur species is stable in air. In the potential region of adsorption of hydrogen a decrease of the two main peaks for adsorption-desorption of hydrogen occurs (proportionally to their heights) (Fig. l), indicating that a fraction of the surface of the Ir electrode is occupied by sulphur species which inhibit the adsorption of hydrogen. This allows the quantity of adsorbed hydrogen, displaced by the adsorption of sulphur, to be determined by the difference A Qn = (&!, Q”,), where Qi and Qi?, are the quantities of electricity associated with the adsorption (or desorption) of hydrogen, in the absence and in the presence of adsorbed sulphur respectively. The degree of coverage by sulphur 8, can also be calculated: OS = en/Q!. In the determination of AQn, it is taken into account that at 0.06V (RHE) about 65% of a monolayer of hydrogen is adsorbed, following the work of Woods [21]. In contrast, the deposited sulphur species can be electro-oxidized in the potential region of oxygen adsorption, as in the case of S,,-Pt [19,20]. Indeed, the increase of the anodic current, observed at potentials higher than 0.9 V (Fig. 2), can be attributed to the oxidation of adsorbed sulphur species into sulphates predominantly, as was found by Sutyagina et al. [22]; some other products (SO,, SOi- ) being formed in minor quantities.
E. Lamy-Pitam,
J. Burhier/Jourd
of Electroanalytical
However, for high sulphur coverages, several potential cycles are needed in order to oxidize completely all the deposited sulphur and to regenerate the initial surface area of the iridium electrodes (Fig. 2). A preliminary study of the adsorbed sulphur was carried out based on the comparison of the quantities of electricity involved in the adsorption and the oxidation of sulphur and also in the displacement of adsorbed hydrogen (following the same method as in the case of a sulphurized platinum electrode, see Refs. [ 19,20,23]. The results obtained show that the mean oxidation state of adsorbed sulphur species (at 8, < 0.3) is more negative than - 1, indicating that predominantly it is adsorbed in the state of SH, or as polysulphides, the adsorption of neutral sulphur being less probable. The formation of a bulk sulphide (Ir,S,), at low 8, is not favoured thermodynamically [5]. Negative oxidation states were also observed in the case of platinum electrodes at low sulphur coverages [20,231).
High inhibition of the underpotentially adsorbed hydrogen (at 8s < 0.3) was observed, giving high values of the mean stoichiometry of sulphur species deposited on iridium: about six to ten platinum or hydrogen atoms (with H/Pt = 1) were inhibited by one sulphur atom. This result could be ascribed to important changes in the electronic structure of the substrate, induced by sulphur chemisorption, as was proved by McCarty and Wise [5] by angle-resolved photoemission spectra. This stronger effect of adsorbed sulphur on iridium compared with a platinum substrate could be correlated to the lower electronic affinity of the former (160eV) compared with that of the latter (2.12eV). 3.2. The electrocatalytic modified by deposition
evolution of sulphur
of hydrogen species
on iridium
An iridium electrode (true surface area 2cm’) was modified by adsorption of sulphur species (by H,S produced in situ), at different surface coverages, and characterized by cyclic voltammetry. This electrode was then tested in the potential region between + 20mV and - 60 mV (RHE) by a slow potential sweep (5 mV s- ’ ) and the j-V curves corresponding to the reaction 2H+ + 2ewere traced. +I&, The effect of sulphur on this reaction depends on the potential value (Fig. 3). At low overpotentials (0 to - 4OmV (RHE)) a very slow decrease of current densities is observed for low sulphur coverages (0, < 0.30-0.40), while at higher overpotentials (-50mV (RHE)) a slight increase of the current density, with a maximum for 8, = 0.20, is noted. For higher sulphur coverages (0, > 0.500.60), a stronger decrease, leading to current densities equal to zero for 8, = 1, is observed. Similar results were obtained in the case of a platinum electrode modified by sulphur adsorption [ 181, and explained by the formation of polysulphides or multilayers of sulphur [ 18,231.
Chemistry j/mA
416 (1996)47-5I
49
cm-”
IO-
0.0
0.2
0.4
0.6
0.8
1.0 0s
Fig. 3. Effect of adsorbed sulphur on the electrocatalytic evolution of hydrogen on iridium, at different potentials: (+) -30; (0) -40; (A) - 50mV @HE).
The Tafel slope increases with increasing sulphur coverages from 35 mV, for 8, = 0, to 60mV, for 8, = 0.95 (Fig. 4, Table 1). In the case of a platinum electrode, no change of the Tafel slope was observed in the presence of adsorbed sulphur [ 181. This result shows that a change of the mechanism occurs at high 8, (in the case of an Ir electrode) indicating that a stronger modification of the surface properties of this metal, than in the case of a Pt electrode, is induced by sulphur. A sulphidation of the surface of Ir at high 8, cannot be excluded on a thermodynamic basis [S]. Moreover, other metal sulphides, such as silver sulphide [24] and nickel sulphide [25], are known as good electrocatalysts for the hydrogen evolution reaction, giving Tafel slopes equal to 60mV [25]. The forms of the j=fles) curves obtained can be explained by the occurrence of two opposite effects: a geometrical inhibiting effect, which is predominant at high 8,) and an electronic promoting effect, predominant at low 8, and high overpotentials, in a similar way to the case of platinum [18]. In the latter case, a mechanism first proposed by Ijzermans [26], based on the presence of HS,, and on the increase of H+ concentration at the electrode (favoured by the negative polarization of adsorbed sulphur species) was suggested [ 181. 2HS,, + 2H+ + 2H,S,,, 2H,S,,, + 2e-+ ZHS;, + 2H,,, 25~s --, 4 2H++ 2e-+ This mechanism electrode.
H,
could also apply to the case of an Ir
3.3. Effect of sulphur deposited catalytic oxidation of hydrogen
on iridium
on the electro-
Current vs. potential curves were first recorded, in the absence and in the presence of adsorbed sulphur species on
50
E. Lamy-Pituru,
log (j /mA
of Electrounulytical
J. Burbier/Jourd
Chemistry
416 (1996)
47-51
cm-*) 2’ 4 l-
O-
-1 -
-80
-60
-40
Fig. 4. Tafel plots for the cathodic evolution of hydrogen on iridium (W) 8, = 0.43; (0) 8, = 0.62; (0) 8, s 0.77; (+) Bs = 0.95.
-20 modified
iridium for the reaction H, + 2Hf+ 2e-, and then the resulting current densities at different potentials were plotted as a function of the degree of coverage by sulphur (Fig. 5). The current density at 8s = 0, measured before the deposition of sulphur, was lower than that measured after deposition of sulphur, followed by oxidative desorption and regeneration of the surface of iridium. This result could be explained by a promotional effect of some traces of sulphur, which could have remained on the surface of Ir after elimination of most of the deposited sulphur, or by an irreversible modification or restructuring of the surface of iridium induced by sulphur. However, the voltammogram of iridium did not change significantly after deposition of sulphur and regeneration of the Ir surface (at the end of the experiment) when compared with that obtained before sulphur deposition (at the beginning of the experiment). A similar behaviour, but
Table 1 Exchange evolution
current densities and Tafel slopes for the reaction on partially sulphurized iridium
4
j, /mAcm-’
0 0.19 0.43 0.62 0.77 0.95
0.25 0.18 0.16 0.16 0.10 0.02
by adsorption
0 of sulphur
E/mV(RHE)
20 at different
coverages:
(0)
8, = 0; (0 ) 8s = 0.19;
to a smaller extent, was observed in the case of a platinum electrode [ 181. An increase of current densities appears at 0s = 0.25 compared with the initial current densities, which could be connected with the catalytic or restructuring effect explained previously. This effect is larger for higher overpotentials, the greatest one being obtained for the diffusion current. The current densities decrease for quite high sulphur coverages (0, > 0.5-0.6) and become very low (near zero) for 0, = 1. In this case also, the results obtained can be explained by assuming two opposing effects of deposited sulphur species: a geometric inhibiting effect and an elecj/mA
cm?
of hydrogen
Tafel slope/mV 35 33.5 34 39 43 60
0.0
0.2
0.4
0.6
0.8
1.0
e,
Fig. 5. Effect of adsorbed sulphur on the electrocatalytic hydrogen on iridium, at different potentials: (*I + 10mV; (a) maximum diffusion limited current.
oxidation of (0) + 30mV;
E. Lamy-Pitara,
J. Burhier/Journd
of Elecrroamlyricul
Chemistry
tronic promoting effect occurring mostly at low sulphur coverages. However, this last promoting effect is contradictory to the high inhibition effect of deposited sulphur observed in the underpotential adsorption of hydrogen. It could be suggested that some more weakly adsorbed hydrogen species may be active in these reactions, as was proposed by other authors [27,28]. Indeed Nichols and Bewick [27] found a new weakly adsorbed hydrogen state near OV (from + 180 to 0 mV (RI-IE)) by in situ IR reflection spectroscopy, which was suggested to be the active hydrogen for the hydrogen evolution reaction, while Conway and Bai 1281 proposed that some hydrogen adsorbed overpotentially (but not underpotentially) could be involved in this reaction. A promoting effect of sulphur at low 8, was also reported in the case of the Hz-D, equilibration reaction at a (111) monocrystalline platinum catalyst [29], and in the ethylene hydrogenation reaction [30] with polycrystalline platinum. Jackson et al. [3 11 explained the effect of sulphur on the activity and selectivity of Pt-Al,O,, for propane dehydrogenation at high temperature, by the presence of a hydrogen reservoir on the catalyst.
References
4. Conclusions
[I81
The results obtained indicate that the effect of adsorbed sulphur on the cathodic evolution and on the electro-oxidation of hydrogen, on an iridium electrode, depends on the sulphur coverage and on the potential value. At low sulphur coverages (0, < 0.30) and low overpotentials, a slight poisoning or a promoting effect of these reactions occurs, whereas at higher overpotentials higher promoting effects are observed, as in the case of a platinum electrode. At high sulphur coverages (0, > 0.5) a quite strong deactivation of iridium, for these reactions, is noted, and could be ascribed to the formation of polysulphides, as in the case of a platinum electrode. Larger current densities are obtained for the electrochemical oxidation of hydrogen after sulphur deposition, recording of stationary j-V curves, and surface regeneration by potential cycling, than before sulphur deposition. This result could be explained by a strong modification or a surface restructuring effect, induced by some remaining traces of sulphur on the surface of the iridium electrode.
[I] [2]
[3] [4] [5] [6] [7] [S] [9] [lo] [1 l] [12] [13] [14] [15] [16] [ 171
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416 (1996)
47-51
51
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