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Kinetics and mechanism of the electrochemical oxidation of the NO−2 ion on platinum in AgNO2-acetonitrile solution
KINETICS AND MECHANISM OF THE ELECTROCHEMICAL OXIDATION OF THE NO, ION ON PLATINUM IN AgNO,-ACETONITRILE C. E. CASTELLANO.
SOLUTION
A. J. CALANUKA and A. J. AKV~A
Instituto de Investlgaciones Fisicoquimicas Tebricas y Aplicadas, Diviskn Electroquimica, Facultad de Ciencias Exactas, Universidad National de La Plats. La Plata, Argentina
Abstract-The anodic oxidation of NO; ion dissolved as AgNO, in ACN, in the range - 12” to 68°C. is investigated using platinumelectrodes. Single and multiple linear potential sweep and rotating disc electrode techniques are used. The electrochemical reaction is interpreted with the following sequence of reactions: rds NOI
=
NO,+@
[2 NO2
+
N2041
(2)
NO2
+ NO,
=
NO
(3)
NO
=
NO++@
(4)
NO+
+ NO2
7
N,O,
(5)
CN,O,
e
NO
(1)
+ NO,
+ NO,]
(6)
Step (1) behaves as an irreversible step which at high potentials is diffusion controlled, while step (4) corresponds to a reversible charge transfer step. The chemical reactions are fast processes. The postulated reaction mechanism for the anodic discharge of the NO; ion implies the preferential reaction of the mttral product with a NO, ion instead of a solvent molecule as it occurred with solutions employing other aprotic solvents.
in ACN initiates as a first order rate process followed by a series of chemical and electrochemical steps. The information reported in this paper supports the idea of the cxistencc of a common formal mechanism for the anodic oxidation of NO; ion on platinum both in aqueous and non-aqueous solutions[6].
The anodic oxidation of NO; ion has been studied in aqueous solution[l], and in aprotic solvents such as nitromethane[2]. and dimethylsulphoxide[3, 41, particularly on platinum electrodes. The course of the anodic reaction apparently depended both on the solvent employed and on the presence of water in the aprotic solvents. The solvent plays an important role on the fate of intermediates formed at the initial stage of the anodic oxidation of NO; ion. Thus, for a better understanding of this effect the electrochemistry of the NO, ion dissolved as AgNOz in ACN has been investigated. This solvent is stable over a relatively large potential range[5]. The reactivity of ACN towards the reaction intermediates is closer to that of nitromethane than to DMSO, therefore one would expect the anodic reaction to follow the same pathway both in ACN and nitromethane[2]. For the latter the electrode reaction was interpreted as a second order rate process. The present results obtained with the linear potential sweep and the rotating disc electrode techniques indicate, however, that the anodic NO, ion oxidation
EXPERIMENLAL
The electrolysis cell including the platinum working electrodes (stationary plane plate electrode, 1 cm’, and rotating disc electrode, O-071 cm”) wcrc the same dcscribed in previous pubhcations[3,4]. An Ag/Ag+ (0.5 M) reference electrode was used. Acetonitrile (C. Erba R.P.) was purified as indicated in the literature[7]. Its final water content, as revealed by the K. Fischer method, was between 40 and 60 ppm. AgNO, was prepared by reacting AgNO, and KNO, in aqueous solution. The reaction product was isolated and purified up to 99-98 per cent by successive recrystallisations using triple distilled water. All these processes were made in darkness[g]. AgClO, and 701
C.
702
E. CASTELLANO.
A.
.J. CALANURAANI) A. J. ARviA
LiCIO, were indistinctly employed as supporting electrolytes. The former was obtained by reacting Ag,O in a perchloric acid aqueous solution. The product was purified by successive recrystallisations from alcoholic solutions[9]. LiCIO, (BDH) was dried in vacua at 15~160°C during 60 hr[lO]. The electrolytic solutions were prepared and filled into the cell under a N, controlled atmosphere. Experiments were made in the range - 12” to 68 (&0.2)“C with AgNO, solutions (5 x lo-’ to 2.5 x IO- .’ M) and 0.5 M supporting elcctrolytc. Density and viscosity of solutions wcrc detcrmincd as usual. Stationary and non-stationary potentiostatic E/I curves with static and rotating disc electrodes. E/I voltammograms under linear potential sweep with ohmic drop compensation. and constant potential electrolysis were made with conventional techniques. The reaction products were analyscd by spectrommetry (UD and ir), gas chromatography and standard chemical methods.
RESULTS
AND
INTERPRETATION
The anodic oxidation of a 0.5 M AgNOz solution in ACN, on platinum, at O&O.7 V, yields N,O, which is easily swept out of the cell with N, streaming. The analysis of the solution after the electrolysis revealed the formation of NO< ion, with the corresponding decrease of NO; ion concentration. Blank experiments performed by bubbling NO, gas into a AgNOz solution showed also the formation of N,O, and NO; ion as reaction products[6]. The current efficiencies for both the decrease of NO; ion concentration and the formation of NO; ion calculated with (2/3)F per mole of NO, ion are close to 100 per cent. The reproducibility of these results depends upon the water content of the solutions. Thus,
0
current efficiencies increase slightly about this figure when the water content is larger than 500 ppm. (2) E/I chtrructrri~tic.s Blank single sweep voltammograms were run between 0 and 2-2 V using a 0.5 M LiClO* solution at different potential sweep rates. At 200 mV/s and - 12’C the voltammogram exhibits an anodic current peak at 2-4 V. In the &2-2 V range the maximum anodie and cathodic currents are, respectively, 30 fi and 8 PA. The characteristics of the blank voltammograms correspond to the anodic discharge of ClO, ion and to the reduction of their corresponding reaction products[5. 1 l-l 31. The products anodically formed at 24 V are reduced at potentials lower than 0.3 V. Figures la and I b show single sweep voltammograms swept at various potential sweep rates, v, in the range (t-2.4 V with different concentrations of AgNOz in @5 M LiC104, at two different temperatures. Rnodic current peaks are observed at about 0.45, 0.75 (as a shoulder at those temperatures), 1.87 and 2.4V. An appreciable current flows between 0.45 and 0.75 V, while beyond 2.1 V the current increases abruptly. During the returning half-cycle a cathodic current is only observed at potentials lower than 0.3 V. The current peaks observed at low anodic potentials are directly related to the discharge of NO, ion. The anodic current peak at about I.87 V corresponds to the discharge of NO, ion and its height increases linearly with the NO; ion concentration (added as AgNO,), (Fig. lc). Without NO; ion addition, the heights of the anodic current peaks corresponding to 0.45V and I-87 V, after correction for the residual current, are practically equal. This, in principle, indicates that the NO, ion formation is related to the initial discharge of NO; ion. Amoredetailedanalysisofthesinglesweepvoltammograms run between 0 and I-I V, (Figs. Pa-c) at different ti, evidenced that the NO, ion oxidation current peak
Potentlffl
0
Fig. I. Single sweep voltammograms. (a) 2.16 x IO-’ M AgNO, + O-5 M Lic‘IO,; 0 C: different potential sweep rates. (b) IO-’ M AgN0? + O-5 M LICIO,; - 12 C: dittercnt potential sweep rates. (c) 2-16 x JO-’ M + O-5 M LiCIO, at.200 mV/s and different additions ofAgN0, ; (a) lW2 M AgN03 ;(b) 6.64 x 1OF3 M AgNO, ;(c) 3.32 x JOm3 M AgNO, : (dl without addition of AgNO, : 256°C.
Kinetics
and mechamsm
of the electrochemical
703
oxidation
(b)(
0
0
0
0
J
Potential Fig. 2. Single sweep voltammograms run at diRerent potential swrep rates. (a) 1.7 x IO-’ M AgNO, + + O-5 M 0.5 M LiCIO,: 68 C.. (b) 5.0 x 10. ’ M AgNO, + 0-S M LiC‘IO I: 75.2 (‘. (c) IOY M AgNO, LiClO,: ~ I I.5 C. is relatively complex. Thus, while at low temperature only one broad anodic peak is observed, it splits into two well defined anodic current peaks at high temperatures. Hence, within that potential range, there are at least two processes which apparently have different temperature coefficients. Under constant conditions, the height and location of the anodic current peaks at
cu 0.45 V depend on v, while apparently, only the height of the anodic peak at CLI0.75 V changes appreciably with v. The E/I curves at the platinum rdc were obtained both at low linear potential sweep rates and by stepwise change of potential. Curves run by changing the potential upwards at a constant v (Fig. 3a). exhibit a
“‘L”
Potential
Fig. 3. Currentipotential curves after correctmn for the pseudo-ohmic drop. (a) 2 mVjs: different rotation speeds: 5 x IO-’ M AgNO, + 0.5M LiCIO,: 0 C. (b) 2mV/s; 300rpm: potential chenged either upwards or downwards; 5 x IO-’ M AgNO, + O-S M LiCIO,;O’C. (c)Single sweep voltammograms run at 2. 5. 10 and 20 mV,‘s: 2.16 x IO-’ M AgNO, + 0.5 M Lic‘IO,: 24.4 C. (d) E/l curves run between @1 and 1-O V by changing the potential In 20 mV steps: IO- 2 M AgNO, + 0 5 M A&LO,: 571 rev/min: + 0.5 M AgCIO,; - 1 “C. (e) E/l curves at different rotation speeds run as in (d); 2.5 I x 10 ’ M AgNO, 9-5’C. (f) E/I curves run as (e). at - 6‘C.
704
C. E. CAS~~LLANO.A. J. CALANDRAAND A. J. ARV~A
Potential Fig. 4. Single sweep voltammograms run after bubbling NO, into the solutions. (a) Blank (curve 1): 2.16 x IO-‘M AgNO, + @5M LiClO J, 22’C. Sdme solution (cwve 2) after NO2 bubbling. (b) Effect of I’ on voltammograms run with a 0.5 M LiCIO, soILltlon saturated with NO2 : 25’ C.
net first anodic limiting current hetween 0.9 and 1.5 V, and a second ant: heyond 2.0 V. The latter, which corresponds to the NO; ion discharge, turns to a current peak at the highest rotation speeds. The half-wave potential for the NO; ion discharge lies at 1-83 V (Fig. 3a). The E/I curves show a net hysteresis (Fig. 3b). At a constant rotation speed, w, and low u, the E/I curves exhibit a second anodic limiting current at low anodic potentials (Fig. 3~). This limiting current turns to a current peak as u increases and approaches the E/I characteristics already described for the single sweep voltammograms. The E/I curves obtained by changing the potential downwardly are practically independent on I:. They present two inflexions, at O-4 V and 0.75 V, respectively. The effect of hysteresis is more marked when stepwise potential changes of longer duration are used (Fig. 3d). Figures 3e and 3f correspond to E/i curves run at 230 2143 rev/min in the 0.2 1.5 V range. At potentials lower than 0.3 V the curves remam practically unchanged at different rotation speed. At higher potentials the current increases as w increases. The various features of the E/I curves run at the rde are in good correspondence with the observations already made in the voltammetric runs. (i) I~fluencr of NO,. The addition of NO, to a AgNO, + supporting electrolyte solution suppresses the first anodiccurrentpcak observed in the single sweep voltammogram (Fig. 4a), but it produces a new broad anodic current peak which extends from 0.5 to ca 1.5 V thereabouts. The returning half-cycle exhibits an appreciable cathodic current at potentials lower than 0.3 V. Both the anodic current peak and the cathodic residual current increase with NO2 concentration in the solution. When runs are made with a 0.5 M
LlClO, + NO? solution, the current peak is located at UI 1-2 V (Fig. 4h). The height of the current peak increases with 0, while the peak potential apparently becomesmoreanodic as u increases. The oxidation current peak observed in the presence of AgNO> + LiClO, + NOz solution disappears when N, is bubbled through thesolution, thecurrentpeak ofNO; ion oxidation being the only one left in the voltammogram. The height of the latter is practically equal to the height of the current peak due to NO; ion oxidation initially present in the solution. Consequently, one concludes that the broad anodic current peak observed in the presence of NO, is a contribution of the electrochemical oxidation of NO, and the oxidation ofa reaction product formed between NO, and AgN02. (ii) InJuencr of NO. To determine the nature of the anodic peak at ca 0.9 V voltammograms were run with containing AgN02 + NO + supporting solutions electrolyte (Fig. 5). The peak height at 0.9 V increases with respect to that observed with solutions without NO addition; the returning voltammogram shows a smaller anodic current. These results suggest the occurrence of an electrode process associated to the redox couple NO=NO++e Then, the NO+ reactions: NO; NO;
ion initiates
(I) the following
+ NO+
=
N,03
CNzO3
=
NO
+ NO2
=
NO;
sequence
of (2)
+ NOJ + NO
(3) (4)
Therefore, the electrochemical oxidation of NO contributes to decrease the NO; ion concentration. A series of comparative voltammograms run in the presence of
Kinetics
and mechanism
of the electrochemical
oxidation
705
peak observed in the presence of NO? is actually due to the chemical formation of NO through reaction (4). (iii) Repetitive mhunmoyrarr~s. Repetitive voltammograms run with AgNO, + supporting electrolyte solutions (Fig. 6) exhibit the following Interesting features. The first sweep towards anodic potentials shows the NO; ion current peak is higher than that of NO, but in the following cycles hoth peaks attain the samt: height. The charge comprised, however, decreases until a stationary situation is reached at a cycling which depends on ti. At large potential sweep rates the returning E/I curves, in the range 2.%OV, exhibits a relatively low residual current (Figs. 6a and 6c), but at low. r. within 1.5 to 0.6V. a relatively high anodic current flows (Fig. 6b). This effect is related to the amplitude of the potential sweep and the probable anodic oxidation of NO originated during the NO, ion discharge (Fig.
I c). Potenhcl tig. 5. Single sweep voltammograms run at 64 C with a I.7 x 10.’ M A&NO, + 0.5 M LiC‘lO, solution saturated with NO. NO confirms that the anodic peak observed at 0.9 V is actually due to the anodic oxidation of NO. Consequently the occurrence of this current peak during the anodic oxidation of NO, Ion indicates that NO is formed after the initial discharge of NO, ion. These results also indicate that the anodic current
(b)
Repetitive voltammograms run with solutions added of NO show a larger decrease of the NO, ion current peak than that observed for solutions without NO addition. This effect increases with the number of repetitive cycles due to reaction (2). No concentration change after repetitive experiments was noticed in the electrolytic solutions employed. The reproducibility of the voltammograms both in the single sweep and in the repetitive sweep experiments as far as the peak current heights and their corresponding potentials are concerned. is within I per cent.
The following nomenclature is used in the quantitative evaluation of results: I, refers to the NO; ion discharge; II, corresponds to the electrochemical oxidation of NO; III identifies NO; ion discharge and IV. indicates the current peak due to the anodic oxidation of Clue Ion. (i) C~rre~~r prak I. At any temperature and AgNOz concentration a reasonable linear relationship between the peak height and I” is obtained (Fig. 7). The
60
-
40-
0
Fig. 6. Cyclic voltammograms run rrom 0 V upwards. C‘ycle number is indicated in the figure. (:I) 300 mV,s: 5 x IO-’ M AgNO, + 0.5 M LiC‘lO,;O CY(h) 50 mV;s: 2.16 x 1W’M + 05LiC‘lO,; 24-4 c‘. (c) 3WmV/s; 7-16 x IO-” M AgNO, + 05 M LiC‘lO,: 24-4 C. ’
5
IO
I5
20
706
C. F. CASIE.I.LANO.A. _I. CAI.ANI)KA AND A. J. ARviA
T= 25 2’C 150 -
l-• - 12.5 OC
100 -
peak height/z.‘,’ ratio is independent on I’. The peak potential apparentty depends linearly with log li (Fig. 8) the slope of the straightline. at 25’C, is close to the 2-3(RT/F) ratio. It dccrcascs as T incrcascs. At constant temperature and u, the peak height increases linearly with the AgNOz concentration (Fig. 9). Otherwise at constant composition and u, the peak height changes with temperature according to an Arrhenius law (Fig. 10). The apparent activation energy is 1.27 + 0.5 kcal/mole. (ii) Current peak II. The relationship between peaks I and TI has been already established. Nevertheless only at high temperature some partial quantitative data ofpeak II can be obtained. Under constant conditions the peak height is directly propor-tional to VI ’ (Fig. 11) and no appreciable shift of the peak potential with u is found. The same results arc obtained with solutions added with NO. (iii) Current prczk ITI. At constant ionic strength and temperature the height of current peak III also increases linearly with L’I” (Fig. 12) although the best straightline does not Intersect the origin of coordinates. This behaviour coincides with the catalytic type of reaction recently reported for the NO_; ion discharge on Pt in ACN[14]. The peak height increases linearly with the amount of AgNO, added to the solution (Fig. 13). The straightline extrapolated at null concentration yields the NO; ion concentration produced during NO; ion discharge. The peak potential incrcascs also linearly with log r (Fig. 14), the slope of the straight line being 2-3 (RTtF).
0
to
5
Concentration
20
15
x 103,
PA
(iv) C~rer~rpeak IV. For a solution without AgNOz addition the peak height, under a fixed set of conditions, depends linearly on v (Fig. 15). The straightline does not intercept the origin of coordinates suggesting
I
I
I
I
34
35
36
I03/T.
I 37
I 38
1
P K)-’
Fig 10 Arrhenius plot of the anodic current peak related to NO; ion oxidation. Data are taken from voltammograms run :it diKcrent I’: I.7 x II) ’ M AgNU, + O-5 M L.i<‘lO,.
and mechanism
Kinetics
of the electrochemical
707
oxidation
40
20 -
0
5
IO
$4
,
20
I5
‘k?
5
mV
_‘/z
Fig.
Il. Dependence of the he@ of the anodic current peak 11 on I‘’ z 5.0 x IO-’ M AgNO, + 0.5 M LiCIO, at 0 c.
Fig.
; ”
I
I
I $2
,
I
I IO
5
0
mV
‘4
$4
Fig. 12. Dependence of the height of arm&c cur~cnt peak III on P’ ‘; 10m2 M AgNO, + 0.5 M LiCIO, at - 12 C.
15. Dependence of the height of the anodic peak IV on r ’ ’ , 0.5 M LiClO, at - 12 C.
again the occurrence of a rather complex electrode process. ‘The peak potential depends also approximately linearly on log IJ, with a slope close to 2.3 (RT/F). (v) Analysis of E/I curves run with the rde. The height of the limiting current plateau at a constant rotation speed depends linearly on the AgNOz concentration (Fig. 16a), and, at a constant concentration, it increases linearly with the square root of the rotation speed of the w&king electrode, w (Fig. 17a). The half-wave potential, Ellz, changes linearly with log w (Fig. 17b). the slopes of the best straight lines are
I
=I 0
current
I
0
5
Concentration
IO
Y 103,
M
Fig. 13. Dependence of the height of the anodic peak 111 on the AgNO, concentration. Experimental tlons arc indlcatcd in Fig. 3.
current condi-
0.01
Concentrotlon.
log
”
Fig. 14. Dependence of the potential of current peak III on log 1’; IO-’ M AgNO, + 0.5 M LiClO, at - 12’ C.
0 02
M
Fig. lb. Dependence of the anodic limiting current rclatcd to the NO; ion discharge on the AgNOl concentration. at 0 C and different rotation speeds. (A) 230, (+) 571, (m) 1 I 11, ( x ) I621 and (0) ?. 143 rev/min.
708
C
I (a)
I
E. CASTELLANO.
I
I
A. J. CALANDKA
I
MI,
A. J.
ARviA
1
i
Li
c)
0
“z2 (rev/min)
w 0.60 >
40
20
(
I
I
I
I
(b)
Fig. 19. Linear plot of xnodic staticmar> polari/atlon curves obrainrd at dilkrent rotation speeds: 2.51 x IO-’ M AgNO~ + O-5 M AgC‘lO, at 0 C‘. (,) 230. ( x ) 571. (0) II I I. (A, 1621 ;und (+)2143 rev.n,,n.
tog keg.
‘k
I7
(al
Dependence
w
of
the anodlc limiting current dated to the NOi ran osldation on
_
3- I
I
/
I
approaches a slope equal to 2.3 (2RT,‘F) while at higher potentials a greater slope is attained. The portion of the E/I curve lying in the potential range O-2-0,42V thereabouts, fits reasonable well a Tafel plot (Fig. 20) with a slope close to 2.3 (2RT/F). Data obtained from this potential range can be worked out. on the assumption that the anodic reaction is a first order process with an intermediate kinetics. Thus, any concentration polarization contribution is easily eliminated by plotting l/i US l/rr>‘:’ at a constant potential (Fig. 21). With the current extrapolated at w -+ 00, a concentration-polarization free Tafel line is obtained (Fig. 22). which also approaches a slope close to 2-3 (2RTjF). Conventional extrapolations of thcsc Tafcl lines at the rest potential yield values of the apparent exchange current densities (iO),,, assembled in Table 1. a first These data show that at a constmt ovcrvoltagc
I
o-3 T=
233 35
3-6 IO j/
37 T,
Pig IX. Arrhenius plot of the related to the NOi ion oxidation; 03 M AgCIO,. Different
0.2
IO
OK-1
anodic I-8 x
rotation
limiting
current +
IO- ’ M AgNOz
speeds.
Log
Fig.
- 6QC
I.5 I
20. Anodic Tafcl plots at different temperatures; 2.51 x IO .’ M AgNO> + 0.5 M Ag(‘10,.
Kinetics
and mechanism
of the electrochemical Table
1. Kinetic
c
CA,MJ: M 0.025 I O-01 80 O-01 80 0~0180 U-01 00
data derived T
h,
M
c
V
0
O-122 0.1 15 O- I20 0 I4t) 01211
I
t
A~
1
I
I
1
E=420mV
E = 440mV -
0 010
-X<460rnV -0
r_“n,
0,005
n
004
0.02 w-
21
I
‘12 ,
‘b
I I \ 0, ’ ’ plot of anodic crlrves: LO-’ AgNOl + 0.5 M AgC‘lO,.
order dependence of the anodic curlent on NO; concentration is reasonably obeyed (Fig. 23).
2.51 x
ivn
DISCUSSION
(1) The overall
reaction
The final products of the anodic oxidation of NO; ion as determined by both spectrometry and voltam-
0 10 0
= NO,
Then.
+ N,03
reaction
10-o 10 h 10 h 10mh lo-’
the overall
reac-
+ 2e
(5)
= NO1
+ P
(ha)
by the equilibrium: 2N0,
+
N,O,
(6b)
I -
-200
log
plots
X x X x x
pathways
NO;
Log I
7-l I.4 2.1 6.0 X.0
The voltammetric runs show that the anodic oxidation of NO2 ion, NO and NOA ion occur consecutively as the electrode potential becomes more anodic. As the two latter species are not initially present in the AgNOz 4ution, one concludes they ant: foulme &IIing the electrochemical oxidation of NO; ion. If the discussion is firstly restrained to the anodic potential region preceding the NO; ion oxidation, the overall process (5) can be explained in terms of a reaction pathway involving five stages. the inital step being the discharge of NO: ion:
followed
Fig. 22. Takl plots drawn with data obtained from cuch as that of Fig. II at two different temperatures.
(k&J A/cm’
The posed question is to find a common pathway related to reaction (5) for results obtained with the rde and the linear potential sweep technique. Within the potential range related to NO; ion oxidation, the latter shows the existence of two anodic current peaks which correspond to two limiting currents at the rde. However, as the limiting current occurring at the lower potential region is not well defined, data obtained from voltammetric experiments are preferentially considered to discusa the probahlt: reaLlion pathway. 2. Prdmhlr
0.06 Crev/mlnS
-5
ion and N,O,.
3NO7 I
from the Tafel plots
tec‘,o,
0.5 0.5 O-5 0.5 0.5
metry. are NO; tion is:
I
700
oxidation
Fig. 23. Log(current) constant overvoltage responds
I
I 75
1.9. log(NOJ ion concentration) (0.5 VI, and 0 C’. The stragbtline to the
I
c
Ist. order
law.
at u car-
710
C. E.
and the parallel according to :
reaction
NO; The
involving
+ NO2
= NO
NO is electrochemically
CASTELLANO.
NO;
A. J. CALANDRA
ion and NO,,
+ NO;
(64
oxidized:
NO=NO++e
(6d)
and finally the nitrosonium ion, which under the present circumstances has not been detected, should react as follows: NO,
+ NO*
= N,03
(&)
The potentials of the NO/NO+ and NO;/NOz couples, as derived from voltammetric measurements at 25°C are respectively O-45 and 0.82 V (GS Ag/Ag+). On the basis of reactions (6a) to (6e) a reasonable interpretation of the E/I voltammetric curves can be made. As earher described the first anodic current peak shifts towards more anodic potential when the potential sweep rate as well as the temperature are increased, while the position of the second anodic current peak related to NO oxidation lies practically unchanged. This suggests that the initial electrochemical oxidation of NO; ion is undoubtedly related to an irreversible process which at high anodic potentials is diffusion controlled, while the NO electrochemical oxidation appears as a reversible process. Let us further assume that the various chemical steps involved in the reaction scheme are fast processes. This implies that the reaction scheme approaches the condition of an PCP mechanism where the rate of the chemical process is much larger than the rates of the elt-ctrochcmical steps, and the first electron transfer step is much slower than the second one. According to the theory of vol-
Fig. 24. Expcrimcntal
and cslculated
AND
A. J. ARViA
tammetric E/I curves for an ece mechanism[lS], for the case just described, the observed single sweep voltammogram can be conceived as the addition of the voltammograms of the individual electron transfer processes. Therefore, the experimental voltammogram for the first anodic peak was theoretically calculated from the equation derived for an irreversible diffusioncontrolled process[ 163. I = nFAC1;(7c &b)“‘$(bt)
(7)
where II is the number of electrons per mole of reacting species; A is the electrode area; C$ is the concentration of the reacting species whose diffusion coefficient is D,; h is equal to the ratio m,Ft,/RT, where ~1, is the product of b, the transfer coeficent, and n,, the number of electrons entering the rate determining step and $(bt) corresponds to the current function of the irreversible diffusion controlled reaction[16]. The second anodic current peak was calculated according to the equation of a reversible diffusion-controlled reaction[16]: I = nFAC;(n
L+,a)“‘$(at)
(f-9
where a = nFv/RT and $(at) is the corresponding current function. Equations (7) and (8) were normalised at the peak current potential. Both equations (7) and (8) predict that the current peak heights depend on the square root of u as it was found experimentally. Otherwise, if 3 = 0.5, the potential of the first anodic current peak depends linearly on the logarithm of v with a slope equal to 2.3(RT/F), thus coinciding with the observed results. Figure 24 compares a single sweep experimental voltammogram with one calculated from equations (7) and (8) for a 1.7 x 10-l M AgNO, + 0.5 M LiCIOl solution at T = 25°C and taking c( = 0.50. The coincidence is certainly very good.
single awecp voltammograms. LiClO,; 15.4 C.
l-7 x IO-’
M
AgNOz
+ 0.5M
Kinetics
and mechanism
of the electrochemical
If the precedent description is correct it should also explain the results obtained with the rotating disk electrode. Obviously the limiting current plateau should correspond to the overall electrode process. The characteristics of the anodic half-wave potential already described indlcatc an appreciable contribution of the irreversible process alrttady at potentials much lower than those of the current plateau. Therefore, the kinetic analysis of the E/I curves must be restricted to anodic potentials lower than 0.5 V depending on T, where the interference of the reversible anodic oxidation of NO is still negligible. Thus, the calculation based upon the first order rate process leads to a concentration-polariration-free-Tafel line involving a slope equal 10 2.3 (2Rz.F) (Fig. 22). The apparent exchange current density under isothermal conditions. increases linearly with NO, ion concentration. Analogously, the reaction order with respect to NOT ion concentration obtained at a constant potential is also practically one. The transfer coefliclent derived from these calculations is again O-5 in a good correspondcncc with that employed in equation (71 to draw the first peak of the voltammogram. Therefore it is concluded that the postulated rcaction pathway explain satisfactorily all the E/I curves differently measured.
To verify the coherence of the interpretation given above, the calculation of the NO; ion diffusion coefficient using data from both types of experiments is attempted. If the first anodic current peak corresponds to the NO; ion discharge according to steps (6a) to (6c), the NO; ion diffusion coefficient can he immediately obtained from the slope of the plot shown in Fig. 7, assuming rz in equation (7) is equal to l/2, since the reaction is in this case: ZNO, = NO + NO, + c (9) Otherwise, if the calculation is based upon the well defined limiting current dcrivcd from the rdr, then the overall reaction (5) should be considered and. in this case. II. in the rdv limiting current equation[l7], should be taken equal to 213. Results of both calculations, as Any other shown in Table 2, are in good agreement. set of II values for calculating the diffusion coefficient yields mconsistent results. Table 2. Diffusion coefficients
-
T C
D(rdc)
D(CI )
cm2;h
ctn~ 3
II
5-l x lo-”
4-s x 10 ” 4.x X 106
0
25
6.1 x IO * 8.2 x
10-b
x-7 x
10-f’
4. The, NO, iota d$fir.kr~ roeficic~nt in twriou.s aprotic solur,ll.s The NO, ion diffusion coefficient given above, was checked by comparing it with the values reported in
Table
711
oxidation
3. Comparative
data
for diflerent
solvents
DMS0[3, 41, through the evaluation of the EinsteinStokes ratio Dq/T. where q is the solvent viscosity after assuming that the solvation properties of NO, ion in both solvents are about the same. Table 3 shows the Dr//T ratios obtained with different values of PT.In spite of the different compositions of the ionic media. the best agreement is achieved when r& data is used with II = 2/3 in ACN and II = 1 in DMSO. Therefore. the apparent solvodynamic radius of NO; ion both in ACN and DMSO obtained from the Einstein-Stokes ratio is about 4 A.
~1he anodlc current peaks and the limiting current increase with temperature according to arl Arrherriustype law. Xhe apparent activation energies derived from the first anodic voltammetric current peak, AH,*, can be compared with the predictions of equations (7) and (8) since the apparent activation energies AH: for diffusion and AH:, for viscous flow are known. Thus, from (8) one obtains: AH; and from rcl417]:
the equation AH;
= 4 AH:, for the limiting
= i AH:, -
:, AH;.
(10) current
at the (11)
As AH; = 2.81 kcal/mole and AH: = 1.50 kcal/mole, equations (10) and (I 1) yields AH: = (1.40 + 05) kcal/ mole and AH: = 2.12 ( + 0.5) kcal/mole. Figures calculated with equations (107and (1 I) coincide reasonably well with those derived from the temperature dependence of the peak current and limiting current, respectively On the other hand, the apparent activation energy related to the discharge of NO; ion, as derived from the Arrhenius plot in the @4>O+XV range, is 7.69 kcal/mole. This figure indicates that the activation energy related to the charge transfer process (probably step 6a) is larger than the activation energy of any mass transport process.
I he anodlc oxidation of NO.7 ion is characterised by a current peak at nearly 1.8 V. The peak height depends approximately linearly on c”‘. but the line does not intercept the orlgln of coordinates, as it is expected for a catalytic electrochemical reaction. The catalytic nature of this reaction was already considered through chronopotentiometric and stationary E/I
C. E. CASTELLANO.A. J. CALANDRAANC)A. J.
712
curves determined with a rde, using-AgNO;-ACN solutions[14]. Those results were interpreted in terms of the following reaction scheme: NO,
+ M = M(N0,)
M(N03)
+ Ag+
+ P
(lib)
The anodic oxidation of NO; ion dissolved in nitromethane at platinum electrodes had been studied a few years ago by running potcntiostatic polarization curves[2]. Four anodic waves were recorded, at 0.77 V, I.38 V. 1.72 V and 2.30 V (vs Ag/AgCI). The first step of the reaction was interpreted as a second order rate process, according to :
and the NO; 2N01
= N,O,
ion formation + NO1
+ 2r
occurring
= N,03
(12) as follows:
+ NC&
National de lnvestigaciones Cientificas y Tecnicas and the Comisi6n de lnvcstigaciones Cientificas (Provincia de Buenos Aires]. C. E. Castellano thanks the fellowship granted by ~ht- Univcrsidad de Tucunriir~ (1970-l 972).
(lla)
= NOzAg+
where the Ag’ + ion is a reaction intermediate. The voltammetric results are. in principle, in agreement with the catalytic mechanism, although the reduction of the Ag *+ ion which one would expect to occur at a high anodic potential, is not observed at the potential sweep rates employed. The reaction Ag’+ - Ag’, however, may occur as a homogeneous process in solution.
2NO;
ARViA
+ 2e
(13)
In ACN solutions, as discussed above, there is no evidence of a second order process such as step (12). Moreover, the mechanism of reaction proposed in the present paper explains the absence ofeithzr any rt-duction wave or cathodic current related either to NO+ ion or NO,. It should also be mentioned that any oxidation reaction of NO, yielding NO: ion must occur at a potential more anodic than that of the NO electrochemical oxidation. The anodic oxidation of NOz, as inferred from its behaviour on Pt in 98% H,S04, is much more irreversible than the NO oxidation[ 18, I’)]. In conclusion, the anodic oxidation of NO; ion in ACN solution fits the general mechanism for this reaction recently postuiatedl6, 201.
Arknowiedgrmw- This work is part of the research program oT the Electrochemistry DI~ISIOII of INIFTA, sponsored by the Universidad National de La Plats, the Consejo
REFERENCES I.
K. Vetter, Z. and K. Kate, Raspi and F. Lyalikov and Nauk. Turkm.
phys. Chem.194,
199 (1950);
N. Tanaka
Japan 29, 837 (1956); G. Pergola. Chrm. Ind. 45. 1398 (1963); Y. S. 0 M. Makhamendnaxarova, 1z11. Akad. S.S.R. Ser. Fiz. Tekhn. Khim. i Grol. Nnuk Bull. Chem. Sot.
5,45 (1963). 2. Cr. Cauquis
and D. Serve, C. r. lrebd S&mc. Acnd. Sci. Paris 266, 1591 (1968); ibid. 270, 1773 (1970). Acta 16, 1619 3. J. A. Wargon and A. J. Arvia, Electrochim. {1971). and A 1. Arwa, ElPctrochim. Acta 17, 649 4. J. A. Wargon (1972). Chem. 1, 486 (1960). 5. J. P. Billon, J. elrctronml. 6. C. E. Castellano, J. A. Wargon and A. J, Arvia, J. electroanal. Chern. 47, 37 I (1973). J. T. Ayres and Ch. K. Mann. Anal. 7. J. F. O’Donell,
Chem. 37, 1161 (1965).
8. J. A. Ketelaar. Z. Kristulloyr. (A) 95, 383 (1936). 9. J. S. Serullas. /lwz. Chirn. Phys. 46, 307 (1931). 10. J. W. Mellor, A Conrprehrnsive Treatisr on Inoryanic
und Theorrticul Chemistry. Vol. II. Longmans. Green & Co. London (1927). 11. H. Schmidt and J. Noack. Z. &urg. nIl+rn. Chew 2%, 262 ( 1958). 12. C. D. Rusell.
An&. Chmt. 35, 1291 (1963). 13. M. Fleischmann and D. Pletcher, in Reactions qf Moleculrr UT Elrcrrodus (edited by N. S. Hush), Chap. 8. p. 359,Wilcy. New York (1971). 14. H. Mishima, T. Iwasita, V. A. Macagno and M. C. Giordano, Elrctrorhirn. Actu 18, 2R7 (1973). 15. C. P. Andrieux, L. Nadjo and J. M. Saveant, J. rlectroanal. Chem 26, 147 (1970). I 6. R. S. Nicholson and I. Shain. Anal. Chrm. 36, 706 (1964). 17 V. Levich. PhJsicochrmical Hydclrodprwnics PrenticeHall, Englewood Clitfs, N. J. (1963). 18. A. J. Calandra, C. Tamayo. J. Herrcra and A. J. Arvia, Electrochirn. Acta 17, 2035 (I 972). 19 C. Tamayo Garcia, A. J. Calandra and A. J. Arvia, Electrochirn. Acta 17, 2181 (1972). 20 C. E. Castellano, J. A. Wargon and A. J. Arvla, Arral. .4soc. Quirlr. Arc/.61, 187 (1973).
SOLUTION
A. J. CALANUKA and A. J. AKV~A
Instituto de Investlgaciones Fisicoquimicas Tebricas y Aplicadas, Diviskn Electroquimica, Facultad de Ciencias Exactas, Universidad National de La Plats. La Plata, Argentina
Abstract-The anodic oxidation of NO; ion dissolved as AgNO, in ACN, in the range - 12” to 68°C. is investigated using platinumelectrodes. Single and multiple linear potential sweep and rotating disc electrode techniques are used. The electrochemical reaction is interpreted with the following sequence of reactions: rds NOI
=
NO,+@
[2 NO2
+
N2041
(2)
NO2
+ NO,
=
NO
(3)
NO
=
NO++@
(4)
NO+
+ NO2
7
N,O,
(5)
CN,O,
e
NO
(1)
+ NO,
+ NO,]
(6)
Step (1) behaves as an irreversible step which at high potentials is diffusion controlled, while step (4) corresponds to a reversible charge transfer step. The chemical reactions are fast processes. The postulated reaction mechanism for the anodic discharge of the NO; ion implies the preferential reaction of the mttral product with a NO, ion instead of a solvent molecule as it occurred with solutions employing other aprotic solvents.
in ACN initiates as a first order rate process followed by a series of chemical and electrochemical steps. The information reported in this paper supports the idea of the cxistencc of a common formal mechanism for the anodic oxidation of NO; ion on platinum both in aqueous and non-aqueous solutions[6].
The anodic oxidation of NO; ion has been studied in aqueous solution[l], and in aprotic solvents such as nitromethane[2]. and dimethylsulphoxide[3, 41, particularly on platinum electrodes. The course of the anodic reaction apparently depended both on the solvent employed and on the presence of water in the aprotic solvents. The solvent plays an important role on the fate of intermediates formed at the initial stage of the anodic oxidation of NO; ion. Thus, for a better understanding of this effect the electrochemistry of the NO, ion dissolved as AgNOz in ACN has been investigated. This solvent is stable over a relatively large potential range[5]. The reactivity of ACN towards the reaction intermediates is closer to that of nitromethane than to DMSO, therefore one would expect the anodic reaction to follow the same pathway both in ACN and nitromethane[2]. For the latter the electrode reaction was interpreted as a second order rate process. The present results obtained with the linear potential sweep and the rotating disc electrode techniques indicate, however, that the anodic NO, ion oxidation
EXPERIMENLAL
The electrolysis cell including the platinum working electrodes (stationary plane plate electrode, 1 cm’, and rotating disc electrode, O-071 cm”) wcrc the same dcscribed in previous pubhcations[3,4]. An Ag/Ag+ (0.5 M) reference electrode was used. Acetonitrile (C. Erba R.P.) was purified as indicated in the literature[7]. Its final water content, as revealed by the K. Fischer method, was between 40 and 60 ppm. AgNO, was prepared by reacting AgNO, and KNO, in aqueous solution. The reaction product was isolated and purified up to 99-98 per cent by successive recrystallisations using triple distilled water. All these processes were made in darkness[g]. AgClO, and 701
C.
702
E. CASTELLANO.
A.
.J. CALANURAANI) A. J. ARviA
LiCIO, were indistinctly employed as supporting electrolytes. The former was obtained by reacting Ag,O in a perchloric acid aqueous solution. The product was purified by successive recrystallisations from alcoholic solutions[9]. LiCIO, (BDH) was dried in vacua at 15~160°C during 60 hr[lO]. The electrolytic solutions were prepared and filled into the cell under a N, controlled atmosphere. Experiments were made in the range - 12” to 68 (&0.2)“C with AgNO, solutions (5 x lo-’ to 2.5 x IO- .’ M) and 0.5 M supporting elcctrolytc. Density and viscosity of solutions wcrc detcrmincd as usual. Stationary and non-stationary potentiostatic E/I curves with static and rotating disc electrodes. E/I voltammograms under linear potential sweep with ohmic drop compensation. and constant potential electrolysis were made with conventional techniques. The reaction products were analyscd by spectrommetry (UD and ir), gas chromatography and standard chemical methods.
RESULTS
AND
INTERPRETATION
The anodic oxidation of a 0.5 M AgNOz solution in ACN, on platinum, at O&O.7 V, yields N,O, which is easily swept out of the cell with N, streaming. The analysis of the solution after the electrolysis revealed the formation of NO< ion, with the corresponding decrease of NO; ion concentration. Blank experiments performed by bubbling NO, gas into a AgNOz solution showed also the formation of N,O, and NO; ion as reaction products[6]. The current efficiencies for both the decrease of NO; ion concentration and the formation of NO; ion calculated with (2/3)F per mole of NO, ion are close to 100 per cent. The reproducibility of these results depends upon the water content of the solutions. Thus,
0
current efficiencies increase slightly about this figure when the water content is larger than 500 ppm. (2) E/I chtrructrri~tic.s Blank single sweep voltammograms were run between 0 and 2-2 V using a 0.5 M LiClO* solution at different potential sweep rates. At 200 mV/s and - 12’C the voltammogram exhibits an anodic current peak at 2-4 V. In the &2-2 V range the maximum anodie and cathodic currents are, respectively, 30 fi and 8 PA. The characteristics of the blank voltammograms correspond to the anodic discharge of ClO, ion and to the reduction of their corresponding reaction products[5. 1 l-l 31. The products anodically formed at 24 V are reduced at potentials lower than 0.3 V. Figures la and I b show single sweep voltammograms swept at various potential sweep rates, v, in the range (t-2.4 V with different concentrations of AgNOz in @5 M LiC104, at two different temperatures. Rnodic current peaks are observed at about 0.45, 0.75 (as a shoulder at those temperatures), 1.87 and 2.4V. An appreciable current flows between 0.45 and 0.75 V, while beyond 2.1 V the current increases abruptly. During the returning half-cycle a cathodic current is only observed at potentials lower than 0.3 V. The current peaks observed at low anodic potentials are directly related to the discharge of NO, ion. The anodic current peak at about I.87 V corresponds to the discharge of NO, ion and its height increases linearly with the NO; ion concentration (added as AgNO,), (Fig. lc). Without NO; ion addition, the heights of the anodic current peaks corresponding to 0.45V and I-87 V, after correction for the residual current, are practically equal. This, in principle, indicates that the NO, ion formation is related to the initial discharge of NO; ion. Amoredetailedanalysisofthesinglesweepvoltammograms run between 0 and I-I V, (Figs. Pa-c) at different ti, evidenced that the NO, ion oxidation current peak
Potentlffl
0
Fig. I. Single sweep voltammograms. (a) 2.16 x IO-’ M AgNO, + O-5 M Lic‘IO,; 0 C: different potential sweep rates. (b) IO-’ M AgN0? + O-5 M LICIO,; - 12 C: dittercnt potential sweep rates. (c) 2-16 x JO-’ M + O-5 M LiCIO, at.200 mV/s and different additions ofAgN0, ; (a) lW2 M AgN03 ;(b) 6.64 x 1OF3 M AgNO, ;(c) 3.32 x JOm3 M AgNO, : (dl without addition of AgNO, : 256°C.
Kinetics
and mechamsm
of the electrochemical
703
oxidation
(b)(
0
0
0
0
J
Potential Fig. 2. Single sweep voltammograms run at diRerent potential swrep rates. (a) 1.7 x IO-’ M AgNO, + + O-5 M 0.5 M LiCIO,: 68 C.. (b) 5.0 x 10. ’ M AgNO, + 0-S M LiC‘IO I: 75.2 (‘. (c) IOY M AgNO, LiClO,: ~ I I.5 C. is relatively complex. Thus, while at low temperature only one broad anodic peak is observed, it splits into two well defined anodic current peaks at high temperatures. Hence, within that potential range, there are at least two processes which apparently have different temperature coefficients. Under constant conditions, the height and location of the anodic current peaks at
cu 0.45 V depend on v, while apparently, only the height of the anodic peak at CLI0.75 V changes appreciably with v. The E/I curves at the platinum rdc were obtained both at low linear potential sweep rates and by stepwise change of potential. Curves run by changing the potential upwards at a constant v (Fig. 3a). exhibit a
“‘L”
Potential
Fig. 3. Currentipotential curves after correctmn for the pseudo-ohmic drop. (a) 2 mVjs: different rotation speeds: 5 x IO-’ M AgNO, + 0.5M LiCIO,: 0 C. (b) 2mV/s; 300rpm: potential chenged either upwards or downwards; 5 x IO-’ M AgNO, + O-S M LiCIO,;O’C. (c)Single sweep voltammograms run at 2. 5. 10 and 20 mV,‘s: 2.16 x IO-’ M AgNO, + 0.5 M Lic‘IO,: 24.4 C. (d) E/l curves run between @1 and 1-O V by changing the potential In 20 mV steps: IO- 2 M AgNO, + 0 5 M A&LO,: 571 rev/min: + 0.5 M AgCIO,; - 1 “C. (e) E/l curves at different rotation speeds run as in (d); 2.5 I x 10 ’ M AgNO, 9-5’C. (f) E/I curves run as (e). at - 6‘C.
704
C. E. CAS~~LLANO.A. J. CALANDRAAND A. J. ARV~A
Potential Fig. 4. Single sweep voltammograms run after bubbling NO, into the solutions. (a) Blank (curve 1): 2.16 x IO-‘M AgNO, + @5M LiClO J, 22’C. Sdme solution (cwve 2) after NO2 bubbling. (b) Effect of I’ on voltammograms run with a 0.5 M LiCIO, soILltlon saturated with NO2 : 25’ C.
net first anodic limiting current hetween 0.9 and 1.5 V, and a second ant: heyond 2.0 V. The latter, which corresponds to the NO; ion discharge, turns to a current peak at the highest rotation speeds. The half-wave potential for the NO; ion discharge lies at 1-83 V (Fig. 3a). The E/I curves show a net hysteresis (Fig. 3b). At a constant rotation speed, w, and low u, the E/I curves exhibit a second anodic limiting current at low anodic potentials (Fig. 3~). This limiting current turns to a current peak as u increases and approaches the E/I characteristics already described for the single sweep voltammograms. The E/I curves obtained by changing the potential downwardly are practically independent on I:. They present two inflexions, at O-4 V and 0.75 V, respectively. The effect of hysteresis is more marked when stepwise potential changes of longer duration are used (Fig. 3d). Figures 3e and 3f correspond to E/i curves run at 230 2143 rev/min in the 0.2 1.5 V range. At potentials lower than 0.3 V the curves remam practically unchanged at different rotation speed. At higher potentials the current increases as w increases. The various features of the E/I curves run at the rde are in good correspondence with the observations already made in the voltammetric runs. (i) I~fluencr of NO,. The addition of NO, to a AgNO, + supporting electrolyte solution suppresses the first anodiccurrentpcak observed in the single sweep voltammogram (Fig. 4a), but it produces a new broad anodic current peak which extends from 0.5 to ca 1.5 V thereabouts. The returning half-cycle exhibits an appreciable cathodic current at potentials lower than 0.3 V. Both the anodic current peak and the cathodic residual current increase with NO2 concentration in the solution. When runs are made with a 0.5 M
LlClO, + NO? solution, the current peak is located at UI 1-2 V (Fig. 4h). The height of the current peak increases with 0, while the peak potential apparently becomesmoreanodic as u increases. The oxidation current peak observed in the presence of AgNO> + LiClO, + NOz solution disappears when N, is bubbled through thesolution, thecurrentpeak ofNO; ion oxidation being the only one left in the voltammogram. The height of the latter is practically equal to the height of the current peak due to NO; ion oxidation initially present in the solution. Consequently, one concludes that the broad anodic current peak observed in the presence of NO, is a contribution of the electrochemical oxidation of NO, and the oxidation ofa reaction product formed between NO, and AgN02. (ii) InJuencr of NO. To determine the nature of the anodic peak at ca 0.9 V voltammograms were run with containing AgN02 + NO + supporting solutions electrolyte (Fig. 5). The peak height at 0.9 V increases with respect to that observed with solutions without NO addition; the returning voltammogram shows a smaller anodic current. These results suggest the occurrence of an electrode process associated to the redox couple NO=NO++e Then, the NO+ reactions: NO; NO;
ion initiates
(I) the following
+ NO+
=
N,03
CNzO3
=
NO
+ NO2
=
NO;
sequence
of (2)
+ NOJ + NO
(3) (4)
Therefore, the electrochemical oxidation of NO contributes to decrease the NO; ion concentration. A series of comparative voltammograms run in the presence of
Kinetics
and mechanism
of the electrochemical
oxidation
705
peak observed in the presence of NO? is actually due to the chemical formation of NO through reaction (4). (iii) Repetitive mhunmoyrarr~s. Repetitive voltammograms run with AgNO, + supporting electrolyte solutions (Fig. 6) exhibit the following Interesting features. The first sweep towards anodic potentials shows the NO; ion current peak is higher than that of NO, but in the following cycles hoth peaks attain the samt: height. The charge comprised, however, decreases until a stationary situation is reached at a cycling which depends on ti. At large potential sweep rates the returning E/I curves, in the range 2.%OV, exhibits a relatively low residual current (Figs. 6a and 6c), but at low. r. within 1.5 to 0.6V. a relatively high anodic current flows (Fig. 6b). This effect is related to the amplitude of the potential sweep and the probable anodic oxidation of NO originated during the NO, ion discharge (Fig.
I c). Potenhcl tig. 5. Single sweep voltammograms run at 64 C with a I.7 x 10.’ M A&NO, + 0.5 M LiC‘lO, solution saturated with NO. NO confirms that the anodic peak observed at 0.9 V is actually due to the anodic oxidation of NO. Consequently the occurrence of this current peak during the anodic oxidation of NO, Ion indicates that NO is formed after the initial discharge of NO, ion. These results also indicate that the anodic current
(b)
Repetitive voltammograms run with solutions added of NO show a larger decrease of the NO, ion current peak than that observed for solutions without NO addition. This effect increases with the number of repetitive cycles due to reaction (2). No concentration change after repetitive experiments was noticed in the electrolytic solutions employed. The reproducibility of the voltammograms both in the single sweep and in the repetitive sweep experiments as far as the peak current heights and their corresponding potentials are concerned. is within I per cent.
The following nomenclature is used in the quantitative evaluation of results: I, refers to the NO; ion discharge; II, corresponds to the electrochemical oxidation of NO; III identifies NO; ion discharge and IV. indicates the current peak due to the anodic oxidation of Clue Ion. (i) C~rre~~r prak I. At any temperature and AgNOz concentration a reasonable linear relationship between the peak height and I” is obtained (Fig. 7). The
60
-
40-
0
Fig. 6. Cyclic voltammograms run rrom 0 V upwards. C‘ycle number is indicated in the figure. (:I) 300 mV,s: 5 x IO-’ M AgNO, + 0.5 M LiC‘lO,;O CY(h) 50 mV;s: 2.16 x 1W’M + 05LiC‘lO,; 24-4 c‘. (c) 3WmV/s; 7-16 x IO-” M AgNO, + 05 M LiC‘lO,: 24-4 C. ’
5
IO
I5
20
706
C. F. CASIE.I.LANO.A. _I. CAI.ANI)KA AND A. J. ARviA
T= 25 2’C 150 -
l-• - 12.5 OC
100 -
peak height/z.‘,’ ratio is independent on I’. The peak potential apparentty depends linearly with log li (Fig. 8) the slope of the straightline. at 25’C, is close to the 2-3(RT/F) ratio. It dccrcascs as T incrcascs. At constant temperature and u, the peak height increases linearly with the AgNOz concentration (Fig. 9). Otherwise at constant composition and u, the peak height changes with temperature according to an Arrhenius law (Fig. 10). The apparent activation energy is 1.27 + 0.5 kcal/mole. (ii) Current peak II. The relationship between peaks I and TI has been already established. Nevertheless only at high temperature some partial quantitative data ofpeak II can be obtained. Under constant conditions the peak height is directly propor-tional to VI ’ (Fig. 11) and no appreciable shift of the peak potential with u is found. The same results arc obtained with solutions added with NO. (iii) Current prczk ITI. At constant ionic strength and temperature the height of current peak III also increases linearly with L’I” (Fig. 12) although the best straightline does not Intersect the origin of coordinates. This behaviour coincides with the catalytic type of reaction recently reported for the NO_; ion discharge on Pt in ACN[14]. The peak height increases linearly with the amount of AgNO, added to the solution (Fig. 13). The straightline extrapolated at null concentration yields the NO; ion concentration produced during NO; ion discharge. The peak potential incrcascs also linearly with log r (Fig. 14), the slope of the straight line being 2-3 (RTtF).
0
to
5
Concentration
20
15
x 103,
PA
(iv) C~rer~rpeak IV. For a solution without AgNOz addition the peak height, under a fixed set of conditions, depends linearly on v (Fig. 15). The straightline does not intercept the origin of coordinates suggesting
I
I
I
I
34
35
36
I03/T.
I 37
I 38
1
P K)-’
Fig 10 Arrhenius plot of the anodic current peak related to NO; ion oxidation. Data are taken from voltammograms run :it diKcrent I’: I.7 x II) ’ M AgNU, + O-5 M L.i<‘lO,.
and mechanism
Kinetics
of the electrochemical
707
oxidation
40
20 -
0
5
IO
$4
,
20
I5
‘k?
5
mV
_‘/z
Fig.
Il. Dependence of the he@ of the anodic current peak 11 on I‘’ z 5.0 x IO-’ M AgNO, + 0.5 M LiCIO, at 0 c.
Fig.
; ”
I
I
I $2
,
I
I IO
5
0
mV
‘4
$4
Fig. 12. Dependence of the height of arm&c cur~cnt peak III on P’ ‘; 10m2 M AgNO, + 0.5 M LiCIO, at - 12 C.
15. Dependence of the height of the anodic peak IV on r ’ ’ , 0.5 M LiClO, at - 12 C.
again the occurrence of a rather complex electrode process. ‘The peak potential depends also approximately linearly on log IJ, with a slope close to 2.3 (RT/F). (v) Analysis of E/I curves run with the rde. The height of the limiting current plateau at a constant rotation speed depends linearly on the AgNOz concentration (Fig. 16a), and, at a constant concentration, it increases linearly with the square root of the rotation speed of the w&king electrode, w (Fig. 17a). The half-wave potential, Ellz, changes linearly with log w (Fig. 17b). the slopes of the best straight lines are
I
=I 0
current
I
0
5
Concentration
IO
Y 103,
M
Fig. 13. Dependence of the height of the anodic peak 111 on the AgNO, concentration. Experimental tlons arc indlcatcd in Fig. 3.
current condi-
0.01
Concentrotlon.
log
”
Fig. 14. Dependence of the potential of current peak III on log 1’; IO-’ M AgNO, + 0.5 M LiClO, at - 12’ C.
0 02
M
Fig. lb. Dependence of the anodic limiting current rclatcd to the NO; ion discharge on the AgNOl concentration. at 0 C and different rotation speeds. (A) 230, (+) 571, (m) 1 I 11, ( x ) I621 and (0) ?. 143 rev/min.
708
C
I (a)
I
E. CASTELLANO.
I
I
A. J. CALANDKA
I
MI,
A. J.
ARviA
1
i
Li
c)
0
“z2 (rev/min)
w 0.60 >
40
20
(
I
I
I
I
(b)
Fig. 19. Linear plot of xnodic staticmar> polari/atlon curves obrainrd at dilkrent rotation speeds: 2.51 x IO-’ M AgNO~ + O-5 M AgC‘lO, at 0 C‘. (,) 230. ( x ) 571. (0) II I I. (A, 1621 ;und (+)2143 rev.n,,n.
tog keg.
‘k
I7
(al
Dependence
w
of
the anodlc limiting current dated to the NOi ran osldation on
_
3- I
I
/
I
approaches a slope equal to 2.3 (2RT,‘F) while at higher potentials a greater slope is attained. The portion of the E/I curve lying in the potential range O-2-0,42V thereabouts, fits reasonable well a Tafel plot (Fig. 20) with a slope close to 2.3 (2RT/F). Data obtained from this potential range can be worked out. on the assumption that the anodic reaction is a first order process with an intermediate kinetics. Thus, any concentration polarization contribution is easily eliminated by plotting l/i US l/rr>‘:’ at a constant potential (Fig. 21). With the current extrapolated at w -+ 00, a concentration-polarization free Tafel line is obtained (Fig. 22). which also approaches a slope close to 2-3 (2RTjF). Conventional extrapolations of thcsc Tafcl lines at the rest potential yield values of the apparent exchange current densities (iO),,, assembled in Table 1. a first These data show that at a constmt ovcrvoltagc
I
o-3 T=
233 35
3-6 IO j/
37 T,
Pig IX. Arrhenius plot of the related to the NOi ion oxidation; 03 M AgCIO,. Different
0.2
IO
OK-1
anodic I-8 x
rotation
limiting
current +
IO- ’ M AgNOz
speeds.
Log
Fig.
- 6QC
I.5 I
20. Anodic Tafcl plots at different temperatures; 2.51 x IO .’ M AgNO> + 0.5 M Ag(‘10,.
Kinetics
and mechanism
of the electrochemical Table
1. Kinetic
c
CA,MJ: M 0.025 I O-01 80 O-01 80 0~0180 U-01 00
data derived T
h,
M
c
V
0
O-122 0.1 15 O- I20 0 I4t) 01211
I
t
A~
1
I
I
1
E=420mV
E = 440mV -
0 010
-X<460rnV -0
r_“n,
0,005
n
004
0.02 w-
21
I
‘12 ,
‘b
I I \ 0, ’ ’ plot of anodic crlrves: LO-’ AgNOl + 0.5 M AgC‘lO,.
order dependence of the anodic curlent on NO; concentration is reasonably obeyed (Fig. 23).
2.51 x
ivn
DISCUSSION
(1) The overall
reaction
The final products of the anodic oxidation of NO; ion as determined by both spectrometry and voltam-
0 10 0
= NO,
Then.
+ N,03
reaction
10-o 10 h 10 h 10mh lo-’
the overall
reac-
+ 2e
(5)
= NO1
+ P
(ha)
by the equilibrium: 2N0,
+
N,O,
(6b)
I -
-200
log
plots
X x X x x
pathways
NO;
Log I
7-l I.4 2.1 6.0 X.0
The voltammetric runs show that the anodic oxidation of NO2 ion, NO and NOA ion occur consecutively as the electrode potential becomes more anodic. As the two latter species are not initially present in the AgNOz 4ution, one concludes they ant: foulme &IIing the electrochemical oxidation of NO; ion. If the discussion is firstly restrained to the anodic potential region preceding the NO; ion oxidation, the overall process (5) can be explained in terms of a reaction pathway involving five stages. the inital step being the discharge of NO: ion:
followed
Fig. 22. Takl plots drawn with data obtained from cuch as that of Fig. II at two different temperatures.
(k&J A/cm’
The posed question is to find a common pathway related to reaction (5) for results obtained with the rde and the linear potential sweep technique. Within the potential range related to NO; ion oxidation, the latter shows the existence of two anodic current peaks which correspond to two limiting currents at the rde. However, as the limiting current occurring at the lower potential region is not well defined, data obtained from voltammetric experiments are preferentially considered to discusa the probahlt: reaLlion pathway. 2. Prdmhlr
0.06 Crev/mlnS
-5
ion and N,O,.
3NO7 I
from the Tafel plots
tec‘,o,
0.5 0.5 O-5 0.5 0.5
metry. are NO; tion is:
I
700
oxidation
Fig. 23. Log(current) constant overvoltage responds
I
I 75
1.9. log(NOJ ion concentration) (0.5 VI, and 0 C’. The stragbtline to the
I
c
Ist. order
law.
at u car-
710
C. E.
and the parallel according to :
reaction
NO; The
involving
+ NO2
= NO
NO is electrochemically
CASTELLANO.
NO;
A. J. CALANDRA
ion and NO,,
+ NO;
(64
oxidized:
NO=NO++e
(6d)
and finally the nitrosonium ion, which under the present circumstances has not been detected, should react as follows: NO,
+ NO*
= N,03
(&)
The potentials of the NO/NO+ and NO;/NOz couples, as derived from voltammetric measurements at 25°C are respectively O-45 and 0.82 V (GS Ag/Ag+). On the basis of reactions (6a) to (6e) a reasonable interpretation of the E/I voltammetric curves can be made. As earher described the first anodic current peak shifts towards more anodic potential when the potential sweep rate as well as the temperature are increased, while the position of the second anodic current peak related to NO oxidation lies practically unchanged. This suggests that the initial electrochemical oxidation of NO; ion is undoubtedly related to an irreversible process which at high anodic potentials is diffusion controlled, while the NO electrochemical oxidation appears as a reversible process. Let us further assume that the various chemical steps involved in the reaction scheme are fast processes. This implies that the reaction scheme approaches the condition of an PCP mechanism where the rate of the chemical process is much larger than the rates of the elt-ctrochcmical steps, and the first electron transfer step is much slower than the second one. According to the theory of vol-
Fig. 24. Expcrimcntal
and cslculated
AND
A. J. ARViA
tammetric E/I curves for an ece mechanism[lS], for the case just described, the observed single sweep voltammogram can be conceived as the addition of the voltammograms of the individual electron transfer processes. Therefore, the experimental voltammogram for the first anodic peak was theoretically calculated from the equation derived for an irreversible diffusioncontrolled process[ 163. I = nFAC1;(7c &b)“‘$(bt)
(7)
where II is the number of electrons per mole of reacting species; A is the electrode area; C$ is the concentration of the reacting species whose diffusion coefficient is D,; h is equal to the ratio m,Ft,/RT, where ~1, is the product of b, the transfer coeficent, and n,, the number of electrons entering the rate determining step and $(bt) corresponds to the current function of the irreversible diffusion controlled reaction[16]. The second anodic current peak was calculated according to the equation of a reversible diffusion-controlled reaction[16]: I = nFAC;(n
L+,a)“‘$(at)
(f-9
where a = nFv/RT and $(at) is the corresponding current function. Equations (7) and (8) were normalised at the peak current potential. Both equations (7) and (8) predict that the current peak heights depend on the square root of u as it was found experimentally. Otherwise, if 3 = 0.5, the potential of the first anodic current peak depends linearly on the logarithm of v with a slope equal to 2.3(RT/F), thus coinciding with the observed results. Figure 24 compares a single sweep experimental voltammogram with one calculated from equations (7) and (8) for a 1.7 x 10-l M AgNO, + 0.5 M LiCIOl solution at T = 25°C and taking c( = 0.50. The coincidence is certainly very good.
single awecp voltammograms. LiClO,; 15.4 C.
l-7 x IO-’
M
AgNOz
+ 0.5M
Kinetics
and mechanism
of the electrochemical
If the precedent description is correct it should also explain the results obtained with the rotating disk electrode. Obviously the limiting current plateau should correspond to the overall electrode process. The characteristics of the anodic half-wave potential already described indlcatc an appreciable contribution of the irreversible process alrttady at potentials much lower than those of the current plateau. Therefore, the kinetic analysis of the E/I curves must be restricted to anodic potentials lower than 0.5 V depending on T, where the interference of the reversible anodic oxidation of NO is still negligible. Thus, the calculation based upon the first order rate process leads to a concentration-polariration-free-Tafel line involving a slope equal 10 2.3 (2Rz.F) (Fig. 22). The apparent exchange current density under isothermal conditions. increases linearly with NO, ion concentration. Analogously, the reaction order with respect to NOT ion concentration obtained at a constant potential is also practically one. The transfer coefliclent derived from these calculations is again O-5 in a good correspondcncc with that employed in equation (71 to draw the first peak of the voltammogram. Therefore it is concluded that the postulated rcaction pathway explain satisfactorily all the E/I curves differently measured.
To verify the coherence of the interpretation given above, the calculation of the NO; ion diffusion coefficient using data from both types of experiments is attempted. If the first anodic current peak corresponds to the NO; ion discharge according to steps (6a) to (6c), the NO; ion diffusion coefficient can he immediately obtained from the slope of the plot shown in Fig. 7, assuming rz in equation (7) is equal to l/2, since the reaction is in this case: ZNO, = NO + NO, + c (9) Otherwise, if the calculation is based upon the well defined limiting current dcrivcd from the rdr, then the overall reaction (5) should be considered and. in this case. II. in the rdv limiting current equation[l7], should be taken equal to 213. Results of both calculations, as Any other shown in Table 2, are in good agreement. set of II values for calculating the diffusion coefficient yields mconsistent results. Table 2. Diffusion coefficients
-
T C
D(rdc)
D(CI )
cm2;h
ctn~ 3
II
5-l x lo-”
4-s x 10 ” 4.x X 106
0
25
6.1 x IO * 8.2 x
10-b
x-7 x
10-f’
4. The, NO, iota d$fir.kr~ roeficic~nt in twriou.s aprotic solur,ll.s The NO, ion diffusion coefficient given above, was checked by comparing it with the values reported in
Table
711
oxidation
3. Comparative
data
for diflerent
solvents
DMS0[3, 41, through the evaluation of the EinsteinStokes ratio Dq/T. where q is the solvent viscosity after assuming that the solvation properties of NO, ion in both solvents are about the same. Table 3 shows the Dr//T ratios obtained with different values of PT.In spite of the different compositions of the ionic media. the best agreement is achieved when r& data is used with II = 2/3 in ACN and II = 1 in DMSO. Therefore. the apparent solvodynamic radius of NO; ion both in ACN and DMSO obtained from the Einstein-Stokes ratio is about 4 A.
~1he anodlc current peaks and the limiting current increase with temperature according to arl Arrherriustype law. Xhe apparent activation energies derived from the first anodic voltammetric current peak, AH,*, can be compared with the predictions of equations (7) and (8) since the apparent activation energies AH: for diffusion and AH:, for viscous flow are known. Thus, from (8) one obtains: AH; and from rcl417]:
the equation AH;
= 4 AH:, for the limiting
= i AH:, -
:, AH;.
(10) current
at the (11)
As AH; = 2.81 kcal/mole and AH: = 1.50 kcal/mole, equations (10) and (I 1) yields AH: = (1.40 + 05) kcal/ mole and AH: = 2.12 ( + 0.5) kcal/mole. Figures calculated with equations (107and (1 I) coincide reasonably well with those derived from the temperature dependence of the peak current and limiting current, respectively On the other hand, the apparent activation energy related to the discharge of NO; ion, as derived from the Arrhenius plot in the @4>O+XV range, is 7.69 kcal/mole. This figure indicates that the activation energy related to the charge transfer process (probably step 6a) is larger than the activation energy of any mass transport process.
I he anodlc oxidation of NO.7 ion is characterised by a current peak at nearly 1.8 V. The peak height depends approximately linearly on c”‘. but the line does not intercept the orlgln of coordinates, as it is expected for a catalytic electrochemical reaction. The catalytic nature of this reaction was already considered through chronopotentiometric and stationary E/I
C. E. CASTELLANO.A. J. CALANDRAANC)A. J.
712
curves determined with a rde, using-AgNO;-ACN solutions[14]. Those results were interpreted in terms of the following reaction scheme: NO,
+ M = M(N0,)
M(N03)
+ Ag+
+ P
(lib)
The anodic oxidation of NO; ion dissolved in nitromethane at platinum electrodes had been studied a few years ago by running potcntiostatic polarization curves[2]. Four anodic waves were recorded, at 0.77 V, I.38 V. 1.72 V and 2.30 V (vs Ag/AgCI). The first step of the reaction was interpreted as a second order rate process, according to :
and the NO; 2N01
= N,O,
ion formation + NO1
+ 2r
occurring
= N,03
(12) as follows:
+ NC&
National de lnvestigaciones Cientificas y Tecnicas and the Comisi6n de lnvcstigaciones Cientificas (Provincia de Buenos Aires]. C. E. Castellano thanks the fellowship granted by ~ht- Univcrsidad de Tucunriir~ (1970-l 972).
(lla)
= NOzAg+
where the Ag’ + ion is a reaction intermediate. The voltammetric results are. in principle, in agreement with the catalytic mechanism, although the reduction of the Ag *+ ion which one would expect to occur at a high anodic potential, is not observed at the potential sweep rates employed. The reaction Ag’+ - Ag’, however, may occur as a homogeneous process in solution.
2NO;
ARViA
+ 2e
(13)
In ACN solutions, as discussed above, there is no evidence of a second order process such as step (12). Moreover, the mechanism of reaction proposed in the present paper explains the absence ofeithzr any rt-duction wave or cathodic current related either to NO+ ion or NO,. It should also be mentioned that any oxidation reaction of NO, yielding NO: ion must occur at a potential more anodic than that of the NO electrochemical oxidation. The anodic oxidation of NOz, as inferred from its behaviour on Pt in 98% H,S04, is much more irreversible than the NO oxidation[ 18, I’)]. In conclusion, the anodic oxidation of NO; ion in ACN solution fits the general mechanism for this reaction recently postuiatedl6, 201.
Arknowiedgrmw- This work is part of the research program oT the Electrochemistry DI~ISIOII of INIFTA, sponsored by the Universidad National de La Plats, the Consejo
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