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Electrocatalysis of transition-metal oxides for reduction and oxidation of nitrite ions
161
J. Electroanul.
Chem., 354 (1993) 161-171
ElsevierSequoiaS.A.,Lausanne JEC 02690
Electrocatalysis of transition-metal and oxidation of nitrite ions
oxides for reduction
Seiji Sunohara a, Katsunori Nishimura a,*, Kiyochika Yahikozawa a, Mitsuo Ueno ‘, Michio Enyo b and Yoshio Takasu a,** a Department of Textile Science and Technology, Shimhu Universi@, Ueda 386 Japan b Catalysis Research Center, Hokkaido
University, Sapporo 060 Japan
(Received23 October 1992)
Electrodes of Mn, Fe, Co, Ni and Cu oxides supported on graphite substrates were prepared by pyrolysis of the metal nitrates. Ex situ X-ray photoelectron spectroscopy (XPS) revealed that Mn, Fe and Ni oxides at different chemical states were present at the electrode surfaces. The electrodes catalyzed the reduction and oxidation of NO; in a weakly acidic medium (pH 2). All of the electrodes were deactivated by increasing the amount of metal oxide deposited, probably because of an increase in electric resistance of the deposited electrodes. The Fe oxide electrode with a small amount of deposited oxide exhibited the highest activity for the reduction and oxidation of NO;.
INTRODUCTION
The electrochemistry of nitrogen oxides is of importance in cleaning and monitoring the pollution of air and water, in removing nitrates from concentrated acid solutions of radioactive elements, etc. Much work has been devoted to clarifying the reaction mechanisms of electroreduction of nitrogen oxides (N,O [l-3], NO [4-91, NO; [4,10-161, NO; [10,15,16,17-231) at metal electrodes (Pt, Ni, Hg, Fe) and to designing highly active or selective electrocatalysts by modification with metal complexes [24-261 and electroconducting polymers [23,27,28]. No attempt has been made to use transition metal oxide electrodes in these reactions, although metal oxides (TiO,, V,O,, Fe,O,, NiO, MnO,, CuO, etc.) are known as heterogeneous catalysts for the reduction of NO and decomposition of N,O.
Present address: Hitachi Research Laboratory, Hitachi Ltd., Hitachi 319-12, Japan. * To whom correspondence should be addressed.
l
l
0022-0728/93/$06.00
0 1993 - Elsevier Sequoia S.A. All rights resewed
162
We believe that metal oxide electrodes possess some unique properties different from those of pure metals in the electroreduction of nitrogen-containing compounds: (1) hydrogen species are weakly adsorbed or entirely excluded on an oxide surface; (2) a redox couple of oxide species at two different states may enhance the reaction rate; (3) enlargement of the effective electrode surface is feasible. In particular, the weak affinities of hydrogen for metal oxides (1) may suppress the formation of highly reduced products and realize the selective conversion of nitrogen oxides into a certain substance. This article explores the possibility of 3d-transition metal (Mn, Fe, Co, Ni, Cu) oxides as electrocatalysts for electrochemical reduction and oxidation of NO; in aqueous media. EXPERIMENTAL
Substrates for the electrodes studied were graphite sheets (99.8% in purity, 0.5 mm in thickness, Nilaco Corp.), of which the surfaces were washed supersonically with acetone and deionized water. Aqueous solutions containing nitrate salts of 3d-transition metal ions (Mn ‘+ Fe3+ Co3+, Ni*+, Cu*+) were prepared at the concentration of 5 x lo-* mol’dm-3’using Mn(NO,), - 6H2O (99.9% in purity, Wako Pure Chemical Industry, Ltd.), Fe(NO,), .9H,O (99.9%), Co(NO,),
163
NaCIO, solution (pH 2) but unstable in a strongly acidic medium such as 1 mol dmp3 HClO, (pH 0). The reference electrode was an Ag/AgCl electrode (HS-205S, TOA Electronics Ltd.), but all the electrode potentials were referred to the reversible hydrogen electrode (RHE) scale; 0 V vs. RHE = - 0.320 V vs. Ag/AgCl for the HClO, + NaClO, solutions. RESULTS
AND DISCUSSION
1. Su$ace analysis by ex situ XPS
The surface composition of graphite-supported metal oxide electrodes was analyzed by ex-situ XPS. Figure 1 shows XPS spectra of specimens of graphite-supported Mn, Fe, Co, Ni and Cu oxides. The Mn oxide specimen showed XPS peaks of 2p,,, and 2pr,* at 641.4 and 652.8 eV respectively (Fig. l(a)). These peaks indicate the presence of Mn,O, [35,361 or Mn,O, [35,361. As the 2p,,, p eak is not symmetrical, it probably includes a sub-peak around 642-643 eV. This peak is consistent with that of MnO, (653.8 eV [35]). The 0 1s band is composed of several peaks, at least two broad peaks located at 529.6 and 531-533 eV. The former suggests the presence of lattice oxygen atoms of the Mn oxides mentioned above, all of which have an 0 1s peak around 529-530 eV [35]. However, the latter may be attributed to absorbed oxygen species (533 eV [35]). The Fe ZP,,, (711.0 eV) and Fe 2p,,, (724.6 eV> peaks of the Fe oxide electrode indicate the presence of Fe,O, or Fe,O, species (Fig. l(b)) [35,37,38].
Fig. 1. X-ray photoelectron spectra of 0 1s peak and 2p peaks of metal oxides for graphite-supported oxide electrodes of (a) Mn, (b) Fe, (c) Co, (d) Ni and (e) Cu. All electrodes were prepared by the pyrolytic deposition of metal oxides at 773 K for 1 h, fwe times.
164
The satellite peak seen around 719 eV is characteristic of Fe,O, species [36]. The 0 1s peak at 530 eV is in agreement with the lattice oxygen of Fe,O, and Fe,O,, while that at higher binding energy is derived from the adsorbed oxygen. FeOOH species had XPS peaks of Fe positions similar to those of Fe,O,. However, the content of this species should be minor or negligible, as the 0 1s peak of FeOOH (531.0 eV [37]) is not apparent. The Co oxide specimen had 2p3,2 and 2p,,, peaks of Co at 780.3 and 795.7 eV respectively (Fig. l(c)>. These correspond to those of Co,O, (780.3 and 795.4 eV [36]). The two broad satellite peaks at 788 and 803 eV and the 0 1s peak at 529.9 eV [39] support our assignment. Two Ni 2p,,, p eaks at 854.0 and 855.6 eV suggest that there were two types of Ni oxide at lower and higher oxidized states respectively (Fig. l(d)). The former is considered to be NiO (854.0 eV [39,401), which is supported again by the Ni 2~i,~ peak (873.3 eV) and the two satellite peaks (861 and 879 eV). The highly oxidized Ni oxide is assigned to Ni,O, (854.0 eV [40]) or Ni(OH), (855.6-856.6 eV [39,41]). The 0 1s peak of the high binding energy peak (531.2 eV) may be derived from the oxygen species which is bound to Ni3+ (as Ni,O, [41]). The lower energy 0 1s peak at 529.3 eV indicates the presence of lattice oxygen of NiO (529.6 eV [39]). For the Cu oxide electrode (Fig. l(e)), the CuO species was found from the Cu 2P 3,2 peak observed at 934.0 eV, of which the binding energy is in fairly good agreement with the literature value (933.6 eV [391X Furthermore, two shake-up peaks at 942 and 962 eV are characteristic of CuO species. As the Cu 2p,,, peak for Cu,O (932.4 eV [39]) is not seen in Fig. l(e), the content of Cu,O species should be negligible. The 0 1s peak at 529.9 eV indicates the presence of lattice oxygen of CuO (529.6 eV [39]). The broad 0 1s peak around 531-532 eV may be derived from contaminated layers on the Cu oxide electrode. 2. Voltammetric behavior of metal oxide electrodes in NO,
containing solutions
Figure 2 shows cyclic voltammograms of (a) the heat-treated graphite electrode and (b)-(f) a series of graphite-supported metal oxide electrodes in 1 x lop3 mol dme3 HClO, + 1 mol dm -3 NaClO, solutions containing 1 X lo-* mol dmA3 NaNO, (shown as solid curves); the broken curves were obtained in the base solution. In the absence of NO; the simple graphite electrode showed the oxidation current rising at E > 1.4 V (broken curve (a)>. This is the current of oxygen evolution. At the other potentials double-layer charging currents were seen. After adding NaNO, to the solution (solid curve (a)> a very small reduction current of NO, appeared at E < 0.25 V in the negative-going sweep and E < 1 V in the positive-going sweep. The oxidation current of NO; for the production of NO; was noticeable at E > 0.9 V, irrespective of the direction of potential sweep. The onset potentials of NO; oxidation at the graphite electrode are almost the same as those at a polycrystalline Pt electrode in an acidic medium [42].
165
(a)
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___--
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----
7
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,-
:\ I :.._____ --______.__ ----7 i---- ,’,____ * _I
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0
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I
0.5 1 E/V(vs.RHE)
I
1.5
I
I
0
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1
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fYVbd3HE)
Fig. 2. Cyclic voltammograms of (a) a heat-treated graphite electrode and graphite-supported oxide electrodes of (b) Mn, (c) Fe, (d) Co, (e) Ni and (f) Cu in 1 X 10e3 mol dmm3 HCIO, + 1 mol dmm3 NaClO, containing 0 mol dmA3 NaNO, (- - -_) and 1 X lo-’ mol dmm3 NaNO, ( -1; sweep rate, 50 mV s-‘; geometric area of the electrodes, 0.5 cm*. The graphite electrode was heat treated at 773 K for 1 h, five times, and the metal oxide electrodes were prepared by the pyrolytic deposition of metal nitrate at 773 K for 1 h, five times. The solid and broken curves in (a) are shown at 2 times magnification.
The Mn oxide electrode provided oxidation and reduction currents at E > 1.1 V in the base solution (broken curve (b)). These currents are probably derived from oxidation and reduction of surface Mn species respectively; the oxidation current wave may also involve oxygen evolution. In this potential region, two redox reactions can take place [43,44]: 2Mn0, + 2H++ 2e-+ MnO, + 4H++ 2e-+
Mn,O, + H,O Mn2++ 2H,O
E” = 1.147 V
(1)
E” = 1.233-1.241 V
(2)
where E” denotes the standard electrode potential with respect to NHE. The ex situ XPS observation (Fig. l(a)) revealed the presence of Mn,O, and MnO,. Since the redox currents remained during continuous potential cycling between 0 and 1.5 V, the Mn2+ species was formed only at the early stage of cyclic voltammetry or was strongly bound to the graphite substrate. Therefore, graphite substrates may stabilize incorporated Mn oxides and suppress the corrosion of MnO, in acidic solutions. The double-layer region of this electrode seems to be located at E < 1 V where no significant current was observed. In the NaNO,-containing solution (solid curve (b)), reduction currents of NO; appeared at E < 0.8 V in the negative- and positive-going sweeps. Much larger oxidation currents of NO; are
166
seen at E > 0.9 V. A unique feature of the Mn oxide electrode is a shoulder accompanied by the large current wave at 0.8-l V. In the case of the graphite-supported Fe oxide electrode, a pair of reduction and oxidation currents appeared reversibly at 0.8-1.1 V in the background solution (broken curve (c)). They are probably due to the redox reaction of Fe,0,/Fe2+ (E” = 0.728 V [45]) or Fe,0,/Fe2+ (E” = 0.980 V [45]). The XPS observation (Fig. l(b)) revealed Fe,O, or Fe,O, to be present at the electrode surface. However, the species Fe,O, may be rejected, because of its low thermodynamic stability at pH 2 [45]. As this electrode exhibited steady cyclic voltammograms, we judge that the Fe2+ species was intimately bound to the graphite substrate. After the addition of NO; into the solution (solid curve (c)), NO; reduction currents appeared at E < 0.7 V in the positive- and negative-going sweeps, being very weakly dependent on the electrode potential. The NO; oxidation reaction also took place at E > 1 V, similarly to the Mn oxide electrode. The graphite-supported Co oxide electrode had very small redox peaks at 0.5-0.8 V in the absence of NaNO, (broken curve (d)). For Co oxide species, some redox reactions are operative between 0 and 1.5 V [43]: Co(OH),/Co (E” = 0.095 V), Coo/Co (0.166 V), Co,O,/Co(OH), (0.993 V), Co,O,/CoO (0.777 V), Co,O,/cO,O, (1.018 V) and cO02/c020, (1.477 VI. According to the standard equilibrium potentials listed above, the redox current peaks observed at 0.5-0.8 V may be attributed to the reaction between C&O, and COO; Co,O, was identified by XPS (Fig. l(c)). The addition of NaNO, into the base solution resulted in la’rge oxidation currents at E > 1 V in the positive- and negative-going sweeps (solid curve (d)). A small reduction current also appeared at E < 0.6 V. The graphite-supported Ni oxide electrode exhibited no prominent redox current peaks in the cyclic voltammogram in the base solution (broken curve (e)). Redox reactions such as NiO/Ni (E” = 0.110 V), Ni,O,/NiO (0.876 V) and Ni,O,/NiO (1.032 V) [43] may take place in the potential region studied, but any intrinsic current for these reactions was not observed. This electrode was also active toward NO, oxidation, like the Mn, Fe and Co oxide electrodes, but apparently less active toward reduction (solid curve (e)). In the absence of NaNO,, the cyclic voltammogram of the graphite-supported Cu oxide electrode clearly showed oxidation and reduction current peaks of nearly the same size at 0.4-0.6 V and 0.15-0.5 V respectively (broken curve (f)). These are judged to be currents of the Cu,O/CuO and Cu/Cu,O redox reactions, of which the standard potentials are 0.645 and 0.471 V respectively, with respect to the SHE [44]. The XPS observation (Fig. l(e)) indicated the presence of CuO species at the surface of this electrode. In the potential regions below the positions of the coupled peaks, the Cu oxides or at least their surfaces were reduced to metallic Cu. A further oxidation state of Cu oxide was indicated at E > 0.6 V. We found that Cu oxides incorporated into the graphite substrate were quite stable under the action of acid (pH 2). In the same background solution, a Cu metal electrode dissolved into the solution and formed brown, thick Cu oxide layers at E > 0.5 V (data not shown). In the NaNO,-containing solutions (solid curve (f)),
167
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a \E 0.2
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< 7
i
oMn
(0)
Fe
Co
Ni
Cu
I
I
I
I
I
Mn
Fe
Co
Ni
Cu
(b)
Fig. 3. Apparent current densities of (a) reduction and (b) oxidation of NO; at the graphite-supported metal oxide electrodes in 1 mol dmm3 NaClO, + 1 X 10m3 mol dmm3 HClO, containing 1 X lo-’ mol dmm3 NaNO,. The currents were measured at (a) 0 and (b) 1.2 V (RHE) in the negative- and positive-going potential sweep curves respectively, sweep rate, 50 mV s-‘. The metal oxide electrodes were prepared by the pyrolysis of metal nitrates, five times. The graphite electrode was heat treated at 773 K for 1 h, five times and is shown as a reference at the right-hand ordinate. Geometric areas of the electrodes were 0.5 cm’.
the oxidation current peak of Cu species diminished appreciably, whereas the cathodic peak did not change. Current waves due to NO; reduction then appeared at 0.3-0.7 and O-O.2 V in the negative-going sweep. In the positive-going sweep NO; reduction took place at E < 0.9 V. Oxidation of NO; proceeded noticeably at E > 1 V. Apparent current densities of the reduction and oxidation of NO; for the graphite-supported metal oxide electrodes are summarized in Fig. 3. The metal oxide electrodes underwent pyrolytic deposition five times, and the graphite electrode was similarly heat treated. At 0 V, the reduction current of NO; on each electrode was greater than that of the simple graphite electrode shown at the right-hand ordinate (Fig. 3(a)). The apparent activity was greater in the order Co > Mn > Cu, Fe > Ni. However, the metal oxide electrodes exhibited higher activities toward NO; oxidation at 1.2 V than the heat-treated graphite electrode (Fig. 3(b)). The apparent activity order in this case is Co > Mn, Cu > Ni, which is analogous to that for NO; reduction. Specific activities of the metal oxide electrodes studied with respect to NO; reduction and oxidation are compared at unit true surface area in the next section. 3. Injluence of the amount of deposited metal oxide on the catalytic activity of the metal oxide electrode Figure 4 demonstrates the dependence of the current densities of reduction and oxidation of NO; at a true unit area of the electrode on the number of pyrolytic
168
Fig. 4. Relationship between current densities of (a) reduction and (b) oxidation of NO; on graphite-supported metal oxide electrodes and the electrode surface area: 0 Mn, 0 Fe, A Co, A Ni, 0 Cu in 1 mol dmm3 NaClO, + 1 x 10m3 mol dme3 HCIO, containing1 X lo-* mol dmm3 NaNO,. The currents were measured at (a) 0 and (b) 1.2 V (vs. RHE) in the negative- and positive-going potential sweep curves respectively; sweep rate, 50 mV s- l. The pyrolytic deposition of metal oxides was conducted l-4 times.
deposition treatments of the metal oxides. The abscissa represents the surface area, which was evaluated from a double-layer charging current of each metal oxide electrode at a potential between 0 and 0.2 V (Fig. 2), assuming that the double-layer capacitance of the electrode at a true unit area is equal to that of Hg (18 PC cmW2 [43]) or that the double-layer charging current density at a unit area is 0.9 PA crn2 at the sweep rate of 50 mV s-l. The apparent surface area of all the electrodes was 0.5 cm2. True surface areas of the oxide electrodes of Mn (open circles) and Co (open triangles) greatly increased with the number of pyrolysis treatments, while those of the other electrodes did not increase dramatically. The heat-treated graphite electrode used had an apparent surface area of 0.5 cm2 and a true surface area of 3.7 cm2 (not shown). True current densities of NO; reduction for the Mn, Fe, Co and Ni oxide electrodes decreased monotonically with an increase in the surface area (Fig. 4(a)). This deactivation phenomenon probably resulted from an increase in the electric resistance of the oxides or the diffusive resistance of NO; ions near the electrode surface with increasing thickness of the oxide overlayers. Only the Cu oxide electrode (open squares) was unique in NO; reduction, as the reduction current density remained almost constant irrespective of the electrode surface area. For NO; oxidation, the current densities of all the electrodes decreased appreciably with an increase in the electrode area (Fig. 4(b)). Figure 5 shows the current densities of (a) reduction and (b) oxidation of NO; on the metal oxide electrodes at unit true surface area of the electrodes. The electrodes shown in this figure were prepared by pyrolysis one and five times. The true surface areas were estimated from the double-layer charging currents at O-O.2 V. For the pyrolytic deposition of five times (open circles), the most active electrode toward NO; reduction was the Ni oxide electrode (Fig. 5(a)). It also exhibited the highest activity toward NO; oxidation (Fig. 5(b)). True current
169 IO
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30
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OL
’ hk
I Fe
(0)
I CO
I Ni
I Cu
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OL---J-h
,
Fe
Co
Ni
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Fig. 5. Current densities of (a) reduction and (b) oxidation of NO, at the graphite-supported metal oxide electrodes in 1 mol dme3 NaClO, + 1 X 10m3 mol drnm3 HClO, containing 1 X lo-* mol dmm3 NaNO,. The current densities are measured at (a) 0 and (b) 1.2 V (vs. RHE) in the negative- and positive-going potential sweep curves respectively, and are given at unit surface area of electrodes; sweep rate, 50 mV s-l. True surface areas of the metal oxide deposited were estimated from the double-layer charging currents, assuming the capacitance at a unit area of 18 PC cm-‘. All electrodes were prepared by the pyrolytic deposition of metal oxides at 773 K for 1 h, either once (01, or five times (0).
densities of reduction and oxidation of NO, on all the oxide electrodes were greater at lower amounts of metal oxide deposited. For the single pyrolysis (closed circles), the Fe oxide electrode was the most active toward both reduction and oxidation of NO;. We have demonstrated by on-line mass spectroscopy that the NO; ion was reduced to some nitrogen-containing gaseous products at metal oxide electrodes prepared similarly to those used in this work. A report of the preliminary results is in press [46]. CONCLUSION
Graphite-supported Mn, Fe, Co, Ni and Cu oxide electrodes were prepared by the pyrolytic decomposition of metal nitrates. Observation by XPS revealed that some types of metal oxides remained on the graphite substrates for the Mn, Fe and Ni oxide electrodes. The prepared electrodes exhibited activities toward the reduction and oxidation of NO; in HClO, + NaClO, solutions of pH 2. The true current density of NO; reduction at the Cu oxide electrode was not affected by the amount of Cu oxide deposited. The reduction current densities at the other electrodes and the oxidation current densities at all oxide electrodes were strongly dependent on the number of pyrolysis treatments, i.e. they became greater at lower amounts of metal oxide deposited. The Fe oxide electrode with a small amount of Fe oxide was found to be the most active toward the reduction and oxidation of NO,.
170 ACKNOWLEDGMENT
This work is financially supported by the association of Nagano for science development. The authors acknowledge gratefully Professor Touhara, of their faculty, for the XPS experiments. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
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171 41 KS. Kim, W.E. Baitinger, J. W. Amy and N. Winograd, J. Electron Spectrosc. Relat. Phenom., 5 (1974) 351. 42 K. Nishimura, K. Machida and M. Enyo, Electrochim. Acta, 36 (1991) 877. 43 A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, Vol. 1, pp. 349-403, 1973; Vol. 9, pp. l-227, 1982; Vol. 3, pp. 43-210, 1975; Vol. 3, pp. 211-421, 1973. 44 A.J. Bard, R. Parsons and J. Jordan (Eds.), Standard Potentials in Aqueous Solution, Marcel Dekker, New York, 1985, pp. 429-439, 287-293. 45 M. Pourbaiz (Ed.), Atlas d’Equilibres Electrochimiques a 25 “C, Gauthier-Villars, Paris, 1963, pp. 307-321. 46 S. Sunohara, K. Nishimura, R. Ohnishi, M. Enyo, K. Yahikozawa and Y. Takaru, J. Electroanal. Chem., in press (JEC 2770).
J. Electroanul.
Chem., 354 (1993) 161-171
ElsevierSequoiaS.A.,Lausanne JEC 02690
Electrocatalysis of transition-metal and oxidation of nitrite ions
oxides for reduction
Seiji Sunohara a, Katsunori Nishimura a,*, Kiyochika Yahikozawa a, Mitsuo Ueno ‘, Michio Enyo b and Yoshio Takasu a,** a Department of Textile Science and Technology, Shimhu Universi@, Ueda 386 Japan b Catalysis Research Center, Hokkaido
University, Sapporo 060 Japan
(Received23 October 1992)
Electrodes of Mn, Fe, Co, Ni and Cu oxides supported on graphite substrates were prepared by pyrolysis of the metal nitrates. Ex situ X-ray photoelectron spectroscopy (XPS) revealed that Mn, Fe and Ni oxides at different chemical states were present at the electrode surfaces. The electrodes catalyzed the reduction and oxidation of NO; in a weakly acidic medium (pH 2). All of the electrodes were deactivated by increasing the amount of metal oxide deposited, probably because of an increase in electric resistance of the deposited electrodes. The Fe oxide electrode with a small amount of deposited oxide exhibited the highest activity for the reduction and oxidation of NO;.
INTRODUCTION
The electrochemistry of nitrogen oxides is of importance in cleaning and monitoring the pollution of air and water, in removing nitrates from concentrated acid solutions of radioactive elements, etc. Much work has been devoted to clarifying the reaction mechanisms of electroreduction of nitrogen oxides (N,O [l-3], NO [4-91, NO; [4,10-161, NO; [10,15,16,17-231) at metal electrodes (Pt, Ni, Hg, Fe) and to designing highly active or selective electrocatalysts by modification with metal complexes [24-261 and electroconducting polymers [23,27,28]. No attempt has been made to use transition metal oxide electrodes in these reactions, although metal oxides (TiO,, V,O,, Fe,O,, NiO, MnO,, CuO, etc.) are known as heterogeneous catalysts for the reduction of NO and decomposition of N,O.
Present address: Hitachi Research Laboratory, Hitachi Ltd., Hitachi 319-12, Japan. * To whom correspondence should be addressed.
l
l
0022-0728/93/$06.00
0 1993 - Elsevier Sequoia S.A. All rights resewed
162
We believe that metal oxide electrodes possess some unique properties different from those of pure metals in the electroreduction of nitrogen-containing compounds: (1) hydrogen species are weakly adsorbed or entirely excluded on an oxide surface; (2) a redox couple of oxide species at two different states may enhance the reaction rate; (3) enlargement of the effective electrode surface is feasible. In particular, the weak affinities of hydrogen for metal oxides (1) may suppress the formation of highly reduced products and realize the selective conversion of nitrogen oxides into a certain substance. This article explores the possibility of 3d-transition metal (Mn, Fe, Co, Ni, Cu) oxides as electrocatalysts for electrochemical reduction and oxidation of NO; in aqueous media. EXPERIMENTAL
Substrates for the electrodes studied were graphite sheets (99.8% in purity, 0.5 mm in thickness, Nilaco Corp.), of which the surfaces were washed supersonically with acetone and deionized water. Aqueous solutions containing nitrate salts of 3d-transition metal ions (Mn ‘+ Fe3+ Co3+, Ni*+, Cu*+) were prepared at the concentration of 5 x lo-* mol’dm-3’using Mn(NO,), - 6H2O (99.9% in purity, Wako Pure Chemical Industry, Ltd.), Fe(NO,), .9H,O (99.9%), Co(NO,),
163
NaCIO, solution (pH 2) but unstable in a strongly acidic medium such as 1 mol dmp3 HClO, (pH 0). The reference electrode was an Ag/AgCl electrode (HS-205S, TOA Electronics Ltd.), but all the electrode potentials were referred to the reversible hydrogen electrode (RHE) scale; 0 V vs. RHE = - 0.320 V vs. Ag/AgCl for the HClO, + NaClO, solutions. RESULTS
AND DISCUSSION
1. Su$ace analysis by ex situ XPS
The surface composition of graphite-supported metal oxide electrodes was analyzed by ex-situ XPS. Figure 1 shows XPS spectra of specimens of graphite-supported Mn, Fe, Co, Ni and Cu oxides. The Mn oxide specimen showed XPS peaks of 2p,,, and 2pr,* at 641.4 and 652.8 eV respectively (Fig. l(a)). These peaks indicate the presence of Mn,O, [35,361 or Mn,O, [35,361. As the 2p,,, p eak is not symmetrical, it probably includes a sub-peak around 642-643 eV. This peak is consistent with that of MnO, (653.8 eV [35]). The 0 1s band is composed of several peaks, at least two broad peaks located at 529.6 and 531-533 eV. The former suggests the presence of lattice oxygen atoms of the Mn oxides mentioned above, all of which have an 0 1s peak around 529-530 eV [35]. However, the latter may be attributed to absorbed oxygen species (533 eV [35]). The Fe ZP,,, (711.0 eV) and Fe 2p,,, (724.6 eV> peaks of the Fe oxide electrode indicate the presence of Fe,O, or Fe,O, species (Fig. l(b)) [35,37,38].
Fig. 1. X-ray photoelectron spectra of 0 1s peak and 2p peaks of metal oxides for graphite-supported oxide electrodes of (a) Mn, (b) Fe, (c) Co, (d) Ni and (e) Cu. All electrodes were prepared by the pyrolytic deposition of metal oxides at 773 K for 1 h, fwe times.
164
The satellite peak seen around 719 eV is characteristic of Fe,O, species [36]. The 0 1s peak at 530 eV is in agreement with the lattice oxygen of Fe,O, and Fe,O,, while that at higher binding energy is derived from the adsorbed oxygen. FeOOH species had XPS peaks of Fe positions similar to those of Fe,O,. However, the content of this species should be minor or negligible, as the 0 1s peak of FeOOH (531.0 eV [37]) is not apparent. The Co oxide specimen had 2p3,2 and 2p,,, peaks of Co at 780.3 and 795.7 eV respectively (Fig. l(c)>. These correspond to those of Co,O, (780.3 and 795.4 eV [36]). The two broad satellite peaks at 788 and 803 eV and the 0 1s peak at 529.9 eV [39] support our assignment. Two Ni 2p,,, p eaks at 854.0 and 855.6 eV suggest that there were two types of Ni oxide at lower and higher oxidized states respectively (Fig. l(d)). The former is considered to be NiO (854.0 eV [39,401), which is supported again by the Ni 2~i,~ peak (873.3 eV) and the two satellite peaks (861 and 879 eV). The highly oxidized Ni oxide is assigned to Ni,O, (854.0 eV [40]) or Ni(OH), (855.6-856.6 eV [39,41]). The 0 1s peak of the high binding energy peak (531.2 eV) may be derived from the oxygen species which is bound to Ni3+ (as Ni,O, [41]). The lower energy 0 1s peak at 529.3 eV indicates the presence of lattice oxygen of NiO (529.6 eV [39]). For the Cu oxide electrode (Fig. l(e)), the CuO species was found from the Cu 2P 3,2 peak observed at 934.0 eV, of which the binding energy is in fairly good agreement with the literature value (933.6 eV [391X Furthermore, two shake-up peaks at 942 and 962 eV are characteristic of CuO species. As the Cu 2p,,, peak for Cu,O (932.4 eV [39]) is not seen in Fig. l(e), the content of Cu,O species should be negligible. The 0 1s peak at 529.9 eV indicates the presence of lattice oxygen of CuO (529.6 eV [39]). The broad 0 1s peak around 531-532 eV may be derived from contaminated layers on the Cu oxide electrode. 2. Voltammetric behavior of metal oxide electrodes in NO,
containing solutions
Figure 2 shows cyclic voltammograms of (a) the heat-treated graphite electrode and (b)-(f) a series of graphite-supported metal oxide electrodes in 1 x lop3 mol dme3 HClO, + 1 mol dm -3 NaClO, solutions containing 1 X lo-* mol dmA3 NaNO, (shown as solid curves); the broken curves were obtained in the base solution. In the absence of NO; the simple graphite electrode showed the oxidation current rising at E > 1.4 V (broken curve (a)>. This is the current of oxygen evolution. At the other potentials double-layer charging currents were seen. After adding NaNO, to the solution (solid curve (a)> a very small reduction current of NO, appeared at E < 0.25 V in the negative-going sweep and E < 1 V in the positive-going sweep. The oxidation current of NO; for the production of NO; was noticeable at E > 0.9 V, irrespective of the direction of potential sweep. The onset potentials of NO; oxidation at the graphite electrode are almost the same as those at a polycrystalline Pt electrode in an acidic medium [42].
165
(a)
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,-
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I
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I
0.5 1 E/V(vs.RHE)
I
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Fig. 2. Cyclic voltammograms of (a) a heat-treated graphite electrode and graphite-supported oxide electrodes of (b) Mn, (c) Fe, (d) Co, (e) Ni and (f) Cu in 1 X 10e3 mol dmm3 HCIO, + 1 mol dmm3 NaClO, containing 0 mol dmA3 NaNO, (- - -_) and 1 X lo-’ mol dmm3 NaNO, ( -1; sweep rate, 50 mV s-‘; geometric area of the electrodes, 0.5 cm*. The graphite electrode was heat treated at 773 K for 1 h, five times, and the metal oxide electrodes were prepared by the pyrolytic deposition of metal nitrate at 773 K for 1 h, five times. The solid and broken curves in (a) are shown at 2 times magnification.
The Mn oxide electrode provided oxidation and reduction currents at E > 1.1 V in the base solution (broken curve (b)). These currents are probably derived from oxidation and reduction of surface Mn species respectively; the oxidation current wave may also involve oxygen evolution. In this potential region, two redox reactions can take place [43,44]: 2Mn0, + 2H++ 2e-+ MnO, + 4H++ 2e-+
Mn,O, + H,O Mn2++ 2H,O
E” = 1.147 V
(1)
E” = 1.233-1.241 V
(2)
where E” denotes the standard electrode potential with respect to NHE. The ex situ XPS observation (Fig. l(a)) revealed the presence of Mn,O, and MnO,. Since the redox currents remained during continuous potential cycling between 0 and 1.5 V, the Mn2+ species was formed only at the early stage of cyclic voltammetry or was strongly bound to the graphite substrate. Therefore, graphite substrates may stabilize incorporated Mn oxides and suppress the corrosion of MnO, in acidic solutions. The double-layer region of this electrode seems to be located at E < 1 V where no significant current was observed. In the NaNO,-containing solution (solid curve (b)), reduction currents of NO; appeared at E < 0.8 V in the negative- and positive-going sweeps. Much larger oxidation currents of NO; are
166
seen at E > 0.9 V. A unique feature of the Mn oxide electrode is a shoulder accompanied by the large current wave at 0.8-l V. In the case of the graphite-supported Fe oxide electrode, a pair of reduction and oxidation currents appeared reversibly at 0.8-1.1 V in the background solution (broken curve (c)). They are probably due to the redox reaction of Fe,0,/Fe2+ (E” = 0.728 V [45]) or Fe,0,/Fe2+ (E” = 0.980 V [45]). The XPS observation (Fig. l(b)) revealed Fe,O, or Fe,O, to be present at the electrode surface. However, the species Fe,O, may be rejected, because of its low thermodynamic stability at pH 2 [45]. As this electrode exhibited steady cyclic voltammograms, we judge that the Fe2+ species was intimately bound to the graphite substrate. After the addition of NO; into the solution (solid curve (c)), NO; reduction currents appeared at E < 0.7 V in the positive- and negative-going sweeps, being very weakly dependent on the electrode potential. The NO; oxidation reaction also took place at E > 1 V, similarly to the Mn oxide electrode. The graphite-supported Co oxide electrode had very small redox peaks at 0.5-0.8 V in the absence of NaNO, (broken curve (d)). For Co oxide species, some redox reactions are operative between 0 and 1.5 V [43]: Co(OH),/Co (E” = 0.095 V), Coo/Co (0.166 V), Co,O,/Co(OH), (0.993 V), Co,O,/CoO (0.777 V), Co,O,/cO,O, (1.018 V) and cO02/c020, (1.477 VI. According to the standard equilibrium potentials listed above, the redox current peaks observed at 0.5-0.8 V may be attributed to the reaction between C&O, and COO; Co,O, was identified by XPS (Fig. l(c)). The addition of NaNO, into the base solution resulted in la’rge oxidation currents at E > 1 V in the positive- and negative-going sweeps (solid curve (d)). A small reduction current also appeared at E < 0.6 V. The graphite-supported Ni oxide electrode exhibited no prominent redox current peaks in the cyclic voltammogram in the base solution (broken curve (e)). Redox reactions such as NiO/Ni (E” = 0.110 V), Ni,O,/NiO (0.876 V) and Ni,O,/NiO (1.032 V) [43] may take place in the potential region studied, but any intrinsic current for these reactions was not observed. This electrode was also active toward NO, oxidation, like the Mn, Fe and Co oxide electrodes, but apparently less active toward reduction (solid curve (e)). In the absence of NaNO,, the cyclic voltammogram of the graphite-supported Cu oxide electrode clearly showed oxidation and reduction current peaks of nearly the same size at 0.4-0.6 V and 0.15-0.5 V respectively (broken curve (f)). These are judged to be currents of the Cu,O/CuO and Cu/Cu,O redox reactions, of which the standard potentials are 0.645 and 0.471 V respectively, with respect to the SHE [44]. The XPS observation (Fig. l(e)) indicated the presence of CuO species at the surface of this electrode. In the potential regions below the positions of the coupled peaks, the Cu oxides or at least their surfaces were reduced to metallic Cu. A further oxidation state of Cu oxide was indicated at E > 0.6 V. We found that Cu oxides incorporated into the graphite substrate were quite stable under the action of acid (pH 2). In the same background solution, a Cu metal electrode dissolved into the solution and formed brown, thick Cu oxide layers at E > 0.5 V (data not shown). In the NaNO,-containing solutions (solid curve (f)),
167
0.4
1
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0
2 z?
0
0
0
0 0
0
a \E 0.2
0
< 7
i
oMn
(0)
Fe
Co
Ni
Cu
I
I
I
I
I
Mn
Fe
Co
Ni
Cu
(b)
Fig. 3. Apparent current densities of (a) reduction and (b) oxidation of NO; at the graphite-supported metal oxide electrodes in 1 mol dmm3 NaClO, + 1 X 10m3 mol dmm3 HClO, containing 1 X lo-’ mol dmm3 NaNO,. The currents were measured at (a) 0 and (b) 1.2 V (RHE) in the negative- and positive-going potential sweep curves respectively, sweep rate, 50 mV s-‘. The metal oxide electrodes were prepared by the pyrolysis of metal nitrates, five times. The graphite electrode was heat treated at 773 K for 1 h, five times and is shown as a reference at the right-hand ordinate. Geometric areas of the electrodes were 0.5 cm’.
the oxidation current peak of Cu species diminished appreciably, whereas the cathodic peak did not change. Current waves due to NO; reduction then appeared at 0.3-0.7 and O-O.2 V in the negative-going sweep. In the positive-going sweep NO; reduction took place at E < 0.9 V. Oxidation of NO; proceeded noticeably at E > 1 V. Apparent current densities of the reduction and oxidation of NO; for the graphite-supported metal oxide electrodes are summarized in Fig. 3. The metal oxide electrodes underwent pyrolytic deposition five times, and the graphite electrode was similarly heat treated. At 0 V, the reduction current of NO; on each electrode was greater than that of the simple graphite electrode shown at the right-hand ordinate (Fig. 3(a)). The apparent activity was greater in the order Co > Mn > Cu, Fe > Ni. However, the metal oxide electrodes exhibited higher activities toward NO; oxidation at 1.2 V than the heat-treated graphite electrode (Fig. 3(b)). The apparent activity order in this case is Co > Mn, Cu > Ni, which is analogous to that for NO; reduction. Specific activities of the metal oxide electrodes studied with respect to NO; reduction and oxidation are compared at unit true surface area in the next section. 3. Injluence of the amount of deposited metal oxide on the catalytic activity of the metal oxide electrode Figure 4 demonstrates the dependence of the current densities of reduction and oxidation of NO; at a true unit area of the electrode on the number of pyrolytic
168
Fig. 4. Relationship between current densities of (a) reduction and (b) oxidation of NO; on graphite-supported metal oxide electrodes and the electrode surface area: 0 Mn, 0 Fe, A Co, A Ni, 0 Cu in 1 mol dmm3 NaClO, + 1 x 10m3 mol dme3 HCIO, containing1 X lo-* mol dmm3 NaNO,. The currents were measured at (a) 0 and (b) 1.2 V (vs. RHE) in the negative- and positive-going potential sweep curves respectively; sweep rate, 50 mV s- l. The pyrolytic deposition of metal oxides was conducted l-4 times.
deposition treatments of the metal oxides. The abscissa represents the surface area, which was evaluated from a double-layer charging current of each metal oxide electrode at a potential between 0 and 0.2 V (Fig. 2), assuming that the double-layer capacitance of the electrode at a true unit area is equal to that of Hg (18 PC cmW2 [43]) or that the double-layer charging current density at a unit area is 0.9 PA crn2 at the sweep rate of 50 mV s-l. The apparent surface area of all the electrodes was 0.5 cm2. True surface areas of the oxide electrodes of Mn (open circles) and Co (open triangles) greatly increased with the number of pyrolysis treatments, while those of the other electrodes did not increase dramatically. The heat-treated graphite electrode used had an apparent surface area of 0.5 cm2 and a true surface area of 3.7 cm2 (not shown). True current densities of NO; reduction for the Mn, Fe, Co and Ni oxide electrodes decreased monotonically with an increase in the surface area (Fig. 4(a)). This deactivation phenomenon probably resulted from an increase in the electric resistance of the oxides or the diffusive resistance of NO; ions near the electrode surface with increasing thickness of the oxide overlayers. Only the Cu oxide electrode (open squares) was unique in NO; reduction, as the reduction current density remained almost constant irrespective of the electrode surface area. For NO; oxidation, the current densities of all the electrodes decreased appreciably with an increase in the electrode area (Fig. 4(b)). Figure 5 shows the current densities of (a) reduction and (b) oxidation of NO; on the metal oxide electrodes at unit true surface area of the electrodes. The electrodes shown in this figure were prepared by pyrolysis one and five times. The true surface areas were estimated from the double-layer charging currents at O-O.2 V. For the pyrolytic deposition of five times (open circles), the most active electrode toward NO; reduction was the Ni oxide electrode (Fig. 5(a)). It also exhibited the highest activity toward NO; oxidation (Fig. 5(b)). True current
169 IO
2
30
0.5 -
OL
’ hk
I Fe
(0)
I CO
I Ni
I Cu
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OL---J-h
,
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Fig. 5. Current densities of (a) reduction and (b) oxidation of NO, at the graphite-supported metal oxide electrodes in 1 mol dme3 NaClO, + 1 X 10m3 mol drnm3 HClO, containing 1 X lo-* mol dmm3 NaNO,. The current densities are measured at (a) 0 and (b) 1.2 V (vs. RHE) in the negative- and positive-going potential sweep curves respectively, and are given at unit surface area of electrodes; sweep rate, 50 mV s-l. True surface areas of the metal oxide deposited were estimated from the double-layer charging currents, assuming the capacitance at a unit area of 18 PC cm-‘. All electrodes were prepared by the pyrolytic deposition of metal oxides at 773 K for 1 h, either once (01, or five times (0).
densities of reduction and oxidation of NO, on all the oxide electrodes were greater at lower amounts of metal oxide deposited. For the single pyrolysis (closed circles), the Fe oxide electrode was the most active toward both reduction and oxidation of NO;. We have demonstrated by on-line mass spectroscopy that the NO; ion was reduced to some nitrogen-containing gaseous products at metal oxide electrodes prepared similarly to those used in this work. A report of the preliminary results is in press [46]. CONCLUSION
Graphite-supported Mn, Fe, Co, Ni and Cu oxide electrodes were prepared by the pyrolytic decomposition of metal nitrates. Observation by XPS revealed that some types of metal oxides remained on the graphite substrates for the Mn, Fe and Ni oxide electrodes. The prepared electrodes exhibited activities toward the reduction and oxidation of NO; in HClO, + NaClO, solutions of pH 2. The true current density of NO; reduction at the Cu oxide electrode was not affected by the amount of Cu oxide deposited. The reduction current densities at the other electrodes and the oxidation current densities at all oxide electrodes were strongly dependent on the number of pyrolysis treatments, i.e. they became greater at lower amounts of metal oxide deposited. The Fe oxide electrode with a small amount of Fe oxide was found to be the most active toward the reduction and oxidation of NO,.
170 ACKNOWLEDGMENT
This work is financially supported by the association of Nagano for science development. The authors acknowledge gratefully Professor Touhara, of their faculty, for the XPS experiments. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
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