- Email: [email protected]
On the anodic dissolution of molybdenum in acidic and alkaline electrolytes
Electroanalytical Chemistry and Interfacial Electrochemistry
143
Elsevier Seouoia S.A., L a u s a n n e - Printed in The Netherlands
ON THE ANODIC DISSOLUTION OF MOLYBDENUM IN ACIDIC AND ALKALINE ELECTROLYTES
MICHAEL N. HULL
ESB Incorporated, Research Center, 19 W. College Avenue, Yardley, Pa. 19067 (U.S.A.) (Received 30th August 1971 ; in revised form 17th December 1971)
INTRODUCTION
Relatively few studies have been conducted concerning the electrochemical behavior of molybdenum anodes in aqueous electrolytes although the general chemical behavior of this metal is quite well understood. Qualitative observations concerning the general conditions under which molybdenum shows passive behavior were described in early papers by Kuessner I and by Becket and Hilberg z. More recently Besson and Drautzburg 3 found that in both acid and alkaline media the metal dissolved quantitatively in the + 6 state and that the electrode surface was covered with colored films of unknown nature. It was further noted that semilogarithmic plots of the current-voltage curves were Tafel lines independent of the electrolyte pH which determined only their relative position in the potential scale. On a more quantitative basis the kinetic behavior of the metal in acid electrolytes has been studied by three main groups 4'~. K6nig and G6hr 4 found that the current-potential curves for molybdenum electrodes in such solutions could be divided into three distinct potential regions. In the first of these, (E< -0.1 V v s . NHE), hydrogen was evolved from the electrode while in the potential range -0.1 V < E < + 0.4 V it was suggested that a layer of molybdenum dioxide was formed on the electrode surface and molybdenum ions were produced in solution in both the + 3 and + 5 oxidation state. In the region + 0.4 V < E < + 4.0 V the surface was presumed to be covered with a layer of the corresponding trioxide from which molybdenum passed into solution in the + 6 state as the molybdate ion. Heumann and Hauck 5, on the other hand, suggested that although molybdenum dissolves in the + 6 state in sulphuric, nitric and hydrochloric acids the rate determining step in this overall process was the transition from the + 4 to the + 6 oxidation state. They also proposed that an amorphous layer of the dioxide was present on the electrode surface under all conditions of oxidation and that molybdenum went into solution through the passive state. More recently Wikstrom and Nobe 6 have investigated the steady state behavior of molybdenum in 1 N H2SO 4 and have interpreted their results as being in agreement with those of Heumann and Hauck. The polarographic reduction of Mo (VI) in acid solution proceeds in two steps to the Mo(V) and then the Mo(III) state 7- ao. Reduction to the + 5 state in sulphuric acid solutions begins at a potential of + 0.2 V (SCE) and to the + 3 state at approximately -0.2 V 7'8 although, depending on the relative concentrations of the background electrolyte and of the Mo(VI) species, each of these reduction waves may be split into J. Electroanal. Chem., 38 (1972)
144
M.N. HULL
two further steps. This latter phenomenon has been attributed s to the presence of two different forms of both Mo(VI) and Mo(V) which are present to a greater or lesser degree in solution depending on the pH of the electrolyte11,12. Using a platinum or gold ring electrode to detect the production of soluble intermediates and products generated at an anodically dissolving molybdenum disc electrode, it should therefore be possible to detect the reduction of Mo(VI) only to the + 5 state since hydrogen evolution will prevent detection of the additional reduction step to the + 3 state. In neutral or alkaline media the reduction of Mo(VI) is not possible 1°. As a general statement therefore it appears to be well established that molybdenum electrodes are covered with some form of an oxide layer during their anodic dissolution in acidic electrolytes. However, the nature of the intermediate oxidation states which are produced during the overall oxidation reaction to the final + 6 state are still the subject of discussion. In addition to these considerations, the electrochemical behavior of molybdenum electrodes in alkaline media has been studied to a lesser degree although it might be anticipated that the behavior of molybdenum under these conditions would show some important differences from that exhibited in acid media. The present study was undertaken to extend the previous knowledge of the electrochemical behavior of molybdenum in acidic electrolytes to alkaline media and also to gain some further insight into the oxidation state of the possible intermediate species which are produced in the overall oxidation process in both media. To do this, the stationary behavior of molybdenum wire and disc electrodes was investigated in acidic and basic electrolytes under both quiescent and convective diffusion conditions, employing the techniques of linear sweep voltammetry and the rotating ring-disc. EXPERIMENTAL
The molybdenum wire electrodes were produced by sheathing a 20 mil* diameter wire in heat shrinkable Teflon tubing to leave an area of 0.42 cm 2 exposed to the electrolyte. For the experiments under convective diffusion conditions the molybdenum disc was machined from a molybdenum rod to give an electrode area of 0.1 cm 2. In this latter case the disc was surrounded by a gold ring to give an electrode configuration whose radial dimensions were r 1= 1.78, r 2 = 1.99, r 3 = 2.27 mm, yielding a value for the collection efficiency13 N of 0.265. The molybdenum material was supplied by Alfa Inorganics and was of m3N5 purity, the principal impurity being tungsten in a concentration of 100 p.p.m. The electrical arrangement used for the linear sweep experiments and that for the experiments with the rotating ring~zlisc electrode have been described in detail previously14'15. In the latter case, rotation of the electrode was performed by an ESB Depak Rotator which will permit continuous variation of the rotation speed from 50-30,000 rev. min-1. Electrolyte solutions were prepared from AnalaR reagents and doubly distilled water and were continuously purged with pure nitrogen except during an actual experimental determination during which time the gas was passed oyer the solution surface. A cylindrical platinum gauze was used as the counter electrode while a Hg - Hg O electrode was used as the reference in alkaline, and a saturated calomel electrode in acidic, electrolytes. * lmil=2.54x10
5m.
J. Electroanal. Chem., 38 (1972)
ANODIC
DISSOLUTION
145
OF Mo IN ACID AND ALKALI
RESULTS
A. General behavior in quiescent electrolytes The stationary current potential (i-E) curves of a molybdenum wire electrode in 1.0 M and 3.88 M K O H are shown in Fig. 1. For all scan rates investigated (0.005-1.0 V s- l) three distinct oxidation waves (A, B, and C of Fig. 1) could be identified on the anodic sweep 16. Wave C, however, appears to consist of two or more closely spaced steps since in most cases an inflection is observed on the rising side of this wave (see for example the 3.88 M case shown in Fig. 1). Each of these oxidation waves could be visibly correlated with the formation of a different film on the electrode surface. At A the electrode becomes covered with a thin light-colored film which undergoes a sudden darkening in appearance at the foot of wave B. Once type C film begins to form at approximately +0.75 V (Hg-HgO) the surface layer becomes completely black, at which point the film thickens appreciably and peels from the electrode in the form of a stream of fine particles. (CI
'
'
'
"
20
+l.O
+0.5
0.0
ELECTRODEPOTENTIAL/VOLTS vs. Hg-HgO
Fig. 1. C u r r e n t - p o t e n t i a l b e h a v i o r o f a m o l y b d e n u m w i r e e l e c t r o d e r e c o r d e d in 1.0 M a n d 3.88 M q u i e s c e n t K O H soln. at a s c a n r a t e o f 0.05 V s - 1 . E l e c t r o d e a r e a = 0 . 4 2 c m 2.
After passing through the maximum at C the current continues to decline monotonically in the absence of reversal of the sweep direction at + 1.5 V. This passive behavior of the electrode is maintained to anodic potentials as high as +4.0 V at which point steady state oxygen evolution begins to occur causing disruption of the passive layer resulting in sharp oscillations in the magnitude of the anodic current. On the return sweep, the potential at which reactivation of the electrode occurs due to removal.of the type C film from the electrode surface, shifts to more cathodic potentials as the scan rate is increased or the electrolyte concentration is decreased. In Fig. 1 it is evident that at a scan rate of 50 mV s- 1 removal of the type C film in J. Electroanal. Chem., 38 (1972)
146
M.N. HULL
3.88 M K O H is complete at a potential of +0.95 V while in 1.0 M K O H the film remains on the electrode surface until a potential of + 0.45 V is attained. It was observed that, in general, for each electrolyte concentration a certain fixed time was required after the electrode potential had become more cathodic than +0.95 V before reactivation was noted. This time was quite independent of the scan rate but decreased by an approximate factor of two for each factor of two increase in the K O H concentration. This behavior indicates that chemical dissolution of the type C film is responsible for its removal rather than a step of electrochemical reduction of the oxide film to a lower oxidation state and that the potential dependence of the point of reactivation on the scan rate has no particular electrochemical significance. As a general statement the magnitude of wave A undergoes a marked increase relative to that of waves B and C as the electrolyte concentration or the scan rate is increased. In Fig. 1 it is evident that wave C is predominant in 1.0 M K O H while at the higher concentration of 8.7 M K O H and at the higher scan rate of 0.5 V s -1 shown in Fig. 2, wave A is predominant and wave C can no longer be distinguished. In addition, the magnitude of wave A is linearly dependent on both the square root of the scan rate and also, up to 2.0 M, on the electrolyte concentration, indicating that under these conditions the diffusive transport of hydroxyl ions to the electrode surface is rate controlling. Further evidence in support of this conclusion will be presented below. On the other hand, the dependence of wave B and particularly that of wave C is
,-,,
8 . ...7. . .M. K O H
3.9MKOH
r• /
~
SC/~N RATE : 500mY sec -I
\ , _, \
.
,\
"'.
+t.O
+0.5
ANOD,C
0.0
-0.5
ELECTRODEPOTENTIAL/VOLTSvs. Hg-HgO Fig. 2. Current potential behavior of a m o l y b d e n u m wire electrode in 3.9 M and 8.7 M quiescent K O H soln. recorded at a scan rate of 0.5 V s 1. Other exptl, conditions as for Fig. 1.
J. Electroanal. Chem., 38 (1972)
147
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI
o
T
80 ANODIC CURRENT
:mA
SCA~. MfATH~05~ 2E 0mV sec"
I
+2.0
I
+1.5
I
+1.0
I
+0.5
ELECTRODEPOTENTIAL/VOLTS vs. S C E
Fig. 3. Current-potential behavior of a molybdenum wire electrode in 1.0 M quiescent H2SO 4 recorded at a scan rate of 0.005 V s-~.
less marked than that of A which indicates that in the potential regions encompassed by these waves heterogeneous processes such as mass transport in a continuously thickening surface film play an increasingly greater role in determining the overall dissolution rate. In contrast to the behavior exhibited in potassium hydroxide electrolytes the stationary i-E curve for a molybdenum wire electrode in sulphuric acid solutions exhibits only one current maximum. This is clearly shown in Fig. 3 where it may be seen that molybdenum exhibits active behavior between points P and Q on the Figure (+ 0.35 V and + 0.8 V vs. SCE respectively), after which a sharp drop in the dissolution current occurs and the electrode may be regarded as having entered its region of passivity. In the region of "active" behavior the surface is covered with a black film which forms immediately oxidation commences at P and which spalls from the electrode surface in a continuous stream of black particles. At Q, however, as the electrode becomes passive this stream of particles becomes less intense and is no longer visible for electrode potentials more anodic than approximately + 1.2 V (point S). The electrode remains covered with a thick black film though the composition of this film must be distinctly different from that present on the surface prior to point Q. No further spalling is observed until electrode potentials in excess of + 3.0 V are attained when disruption of the surface layer by oxygen bubble formation becomes apparent. In addition, in contrast to the behavior in alkaline media, there is no evidence of reactivation of the electrode on the return sweep in quiescent H2SO 4 regardless of the scan rate or concentration of the electrolyte employed. The passive layer on the electrode has thus a very low solubility in sulphuric acid and remains on the surface for the whole of the return sweep,
B. Rotating ring-disc studies--general behavior 1. Alkaline electrolytes. Although the production of molybdenum intermediates having valence states less than Mo(VI) has been proposed in previous studies of the J. Electroanal. Chem., 38 (1972)
148
M.N. HULL
anodic behavior of this electrode in acidic media4'5, no firm experimental verification of the existence of such intermediates has been obtained to date. Accordingly, rotating ring-disc studies were performed in which the production of both oxidizable and reducible intermediates generated at an anodically dissolving molybdenum disc electrode were monitored by means of a gold ring electrode held at various fixed negative and positive potential values while the disc electrode was scanned at a sweep rate of 100 mV s-~. This rather high sweep rate was chosen in order that no significant recession of the electrode surface would occur in the time span required for a single scan of the disc. I
I
I
I
ELECTROLYTE IM KOH
i1"'~ iI
,.2,
/
II
MO DISC
- -
AU R I N G
.....
i
t CURRENT
•
"
i
.;,
\ \ ,,, ,, \ \\\,\ , \
"\\
\
-...
I +I.5
+I.0
+0.5 EDISC / VOLTS vs. Hg - H9 0
0.0
-0,5
Fig. 4. ( ) ID--ED, (..... ) I,-E D traces recorded with a M ~ A u ring-disc electrode rotated at a rate of 1000 rev. m i n - ~ in 1.0 M K O H . Scan rate of disc potential = 0.1 V s - ~ ; ER = + 0.65 V.
The i-E behavior of a molybdenum disc electrode in 1.0 M KOH and under convective diffusion conditions is traced by the solid line in Fig. 4. Initially the current rises to a limiting value between + 0.6 and + 1.0 V (Hg-HgO) within which potential range the surface is covered with type B film. In this, and for all electrolyte concentrations less than 2.0 M, linear behavior is exhibited between the limiting current in this region and the square root power of the rotation rate for values of co up to 10,000 rev. min- ~.A typical plot of this type is presented in Fig. 5 from data recorded in 0.1 M KOH. From the Levich equation I L = 0.62nFA c b D ~ v - +co½
(l)
in which the terms have their usual electrochemical significance, such plots are found J. Electroanal. Chem., 38 (1972)
A N O D I C D I S S O L U T I O N OF Mo IN A C I D A N D A L K A L I
149
y 2O
II /mA
•
I 20
H
I
I 6o
co I/2/rev I/2 m~nl/~ Fig. 5. Plot of limiting current recorded at a Mo disc electrode for Eo < + 1.0 V as a function of square root power of the rotation rate. Electrolyte=0.1 M KOH.
to be in excellent agreement with those calculated theoretically for the diffusive transport of hydroxyl ions up to the electrode surface assuming D = 5.23 x 10-5 cm 2 s- 1 and v = 1.6 x 10- 2 cm 2 s- 1. Under convective diffusion conditions the formation of type A film is not apparent from the disc i-E trace at + 0.2 V though, as will be shown later, analysis of the response of the surrounding gold ring electrode held at a positive electrode potential reveals the formation of this film. For electrode potentials more positive than + 1.0 V, a change in the electrode mechanism occurs and the disc current undergoes a further increase passing through a broad maximum at approximately + 1.6 V. Thereafter, in the absence of a reversal in the sweep direction, the current continues to decline as the heterogenous formation and growth of the black type C film occurs. Control of the overall dissolution rate then becomes governed by transport processes in this continuously thickening surface layer. For a sweep reversal at a potential of + 2.0 V a sharp reactivation of the electrode is observed at + 0.95 V since chemical removal of type C film is completed at this point and the surface again becomes covered with type B film. In contrast to the behavior exhibited under quiescent conditions, where two reactivation steps may be identified corresponding to the removal of the type C followed by the type B film, only the removal of the former film can be identified from the i-E curve recorded under these conditions. The dotted line plotted on Fig. 4 traces the response of a gold ring electrode as a function of the disc potential when the ring is held at a fixed potential of +0.65 V. It may be observed that the presence of a solution-soluble oxidizable intermediate is immediately detected at the ring electrode once anodic oxidation of the molybdenum J. Electroanal. Chem., 38 (1972)
150
M.N. HULL
disc commences at - 0.3 V. At a disc potential of + 0.2 V, which corresponds closely to the peak potential at which type A film is observed to form at slow scan rates in the linear sweep voltammetric studies, the ring current passes through a maximum and thereafter declines steadily while the disc current continues to rise to the diffusion limited value. Following this, the onset of formation of the type C film at a disc potential of + 1.0 V is initially accompanied by a slight fall in the oxidation current registered at the ring until replacement of the type B film is complete. At this point, (ED = + 1.45 V), the presence of a further solution-soluble oxidizable intermediate is again detected at the ring electrode. During the reverse potential scan, two distinct oxidation peaks are observed in the Ir-ED trace. The first and most pronounced of these accompanies the disappearance of the type C film from the disc and reaches a maximum just prior to the abrupt reactivation of this electrode at + 0.95 V. The second reaches a maximum at a disc potential of + 0.05 V though the dissolution current is continuously declining in this potential region. These observations may be understood as resulting from a release of oxidizable material from the disc as one type of film disappears from the electrode surface and is replaced by another of different chemical composition.
!
i
\
M00,sc Au-..R,.0
----
A!0DIC CURRENT
+1.~
+1.0
+0.5
0.0
EDIsc/VOLTS vs. Hg-HgO
Fig. 6. ID--ED and I r E D traces recorded under the same exptl, conditions as Fig. 4 except ER= --0.9 V.
The ir-EDtrace recorded with an amalgamated gold ring electrode held at a fixed potential of - 0.9 V is shown by, the dotted line in Fig. 6. All other experimental conditions were the same as for Fig. 4. In this instance, no reducible species are detected by the ring, in agreement with the known polarographic behavior of molybdate solutions mentioned previously. However, with decreasing electrolyte concentration the onset of a reduction process becomes apparent for disc potentials in excess of approximately + 1.2 V. This is illustrated in Fig. 7 for ring-disc data recorded J. Electroanal. Chem.,
38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI J
l
151
i
i
0
/ -~
/ // I
~o,sc
RING CURRE~
CLIR~ENI /mA
Au RING . . . . . .
I i \
'
+ ,5
~
0.~MK0.
+10
+0.5
EDISC/
0,0
-
VOLTSvs. Ng-HgO)
Fig. 7. ID--ED and lr-EDtraces recorded with a Mo~Auring~tisc electrode in 0.1 M K OH at a rotation rate of 400 rev, min-x. All other conditions as for Fig. 6. in 0.1 M K O H solution and is believed to reflect the concurrent production of a small amount of molecular oxygen during the initial stages of formation of type C film in the weaker electrolytes. This behavior may be explained in terms of the relative thickness of the type C film between strong and dilute K O H solutions. In the former case the film thickness is much greater, and hence the electronic resistance o'f the film is higher than in the more dilute electrolyte, and oxygen evolution is suppressed. 2. Acidic electrolytes. The general form of the i-E curve recorded with a molybdenum disc electrode under convective diffusion conditions in acidic media is quite similar to that exhibited under quiescent conditions. A typical curve recorded in 1.0 M H2SO 4 and at a rotation rate of 1000 rev. m i n - 1 is traced by the solid line in Fig. 8. It m a y be seen that the electrode exhibits an "active" region between +0.35 and + 1.22 V (SCE) followed by a relatively sharp decrease of the current to the "passive" region. On the return sweep, removal of the passivating film is observed by the slight rise in current which occurs at E o = + 1.2 V. In acidic electrolytes, similarly to the behavior observed in the alkaline case, intermediate species are produced during oxidation of the m o l y b d e n u m disc electrode and these may be made to undergo further oxidation at the ring as shown by the dotted trace of Fig. 8. It is evident that the ring current trace shows three distinct regions in the potential range encompassing the onset of oxidation of the disc at +0.35 V and the onset of the disc passivity at + 1.34 V. In this range the ring current exhibits two main waves at ED= +0.7 V and Eo = 1.34 V, respectively, in addition to a third less prominent wave present as a region of inflection at ED = + 1.22 V on the rising portion of the more anodic of the two main waves. On the return cathodic sweep of the disc, reactivation of the electrode is accompanied, as in alkaline solutions also, by an increasing production of a soluble oxidizable intermediate species. Further, J. Elec~roanal. Chem., 38 (1972)
152
M.N. HULL
120 °
t" 40
I
8o] DISC
RING
CURRENT /mA
4O
0
I
I
I
I
+2.5
+2.0
+1.~
+l.O
0
*0.5
EDIsc/VOLTS vs, S C E
Fig. 8. ID-E D a n d Ir-E o traces recorded in 1.0 M H 2 S O 4 w i t h E R = + 1.0 V. co = 1000 rev. m i n - 1. Scan rate = 0.1 V s -1.
120
CURRENT /mA
DISC CURRENT /mA
-12
I 42.5
~1 ..... +2.0 EDISC /
Fig. 9.
]D
/~
I +1,5
I +l.O
I ~ll, ÷0,5
VOLTSVS. S C E
ED a n d Ir-E D traces recorded with E R = --0.1 V. All other exptl, conditions as in Fig. 8.
release of the oxidizable material from the disc continues for a substantial time after completion of the potential scan even though the current flow through the disc has fallen to zero. In Fig. 9 the corresponding response curve of the gold ring electrode when held at a negative potential of -0.1 V is presented. As was indicated above, previous studies concerning the polarographic behaviour of molybdate solutions show that one should observe only the reduction of Mo(VI) to Mo (V) at this potential. The Figure shows two clearly discernible regions in the ring response curve. In the potential range + 0.3 5 < ED < + 1.15 V only a small fraction of the disc current gives rise to the J. Electroanal. Chem., 38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID A N D ALKALI
153
production of soluble Mo (VI) species and the majority of the current flow passing through the disc is being utilized in heterogeneous processes associated with, for example, the growth and replenishment of the surface film which is being swept off the surface in the form of a stream of relatively insoluble black particles. If one calculates the experimental collection efficiency at ED----1.15 V (i.e. just prior to the onset of formation of the second and passivating film) one obtains a value of No~s = 0.017. (Owing to the time required for transit of a species between the disc and the ring there is a displacement in the position of the iR-E o response curve relative to that of the iD-Eo curve. At high scan rates of the disc electrode this shift can lead to an incorrect value of NoBs when both iR and iD are read at the same value of ED. At the scan rates employed in this study, however, a maximum error of 3 ~ is introduced in the calculation of No~s from this effect.) Comparing this with the theoretical collection efficiency of N = 0.044, which would be observed assuming a 6-electron oxidation of the molybdenum disc with a 1-electron reduction step to the Mo(V) state at the ring, it is evident that at this point about 6 0 ~ of the disc current produces product which leaves the electrode surface in the form of undissolved particles of oxide. At a value of Eo = + 1.15 V, at which point a change in the nature of the surface film from an "active" to a "passive" layer is taking place, a rapidly increasing reduction current is recorded by the ring which reaches a maximum value at Eo = + 1.8 V. Here the experimental collection efficiency has a value of NoBs = 0.118 which is the highest value recorded at any point of the complete trace. Comparing this latter value with the theoretical value of N = 0.044 (again assuming Mo ~ Mo (VI) + 6e- and Mo (VI) + e- --, Mo (V) are the electrode reactions at the disc and ring respectively) it is evident that at this point in the trace a second parallel oxidation process must be proceeding at the disc and which is giving rise to the species readily reduced at the ring. The high value of the experimentally observed collection efficiency may be explained by suggesting that oxygen is being produced at the disc during the early stages of formation of the trioxide film. In contrast to the behavior exhibited at positive ring potentials (Fig. 8) one finds that the transition from the "passive" to"active" behavior on the return scan of the disc at approximately + 1.2 V is not accompanied by any increased production of soluble and reducible molybdate species. Rather, in line with the behavior exhibited during the forward sweep, the reduction current at the ring tends to fall offwith return of the electrode to the "active" region of dissolution. Thus the oxidation intermediate detected in this region with positive ring potentials cannot be reduced (if at all) at a ring potential of -0.1 V. DISCUSSION
It has been well established x- 6 that the final product of the oxidation of molybdenum in acid and in alkaline electrolytes is Mo(VI) which, in the former case exists as molybdic or polymolybdic species (depending on the pH) and in the latter case as molybdate 18. Lower oxidation states of molybdenum in solution have been well characterised, particularly Mo(III) and Mo(V) which, according to Latimer 19, in strongly acid solutions exist as M o O + and MoO~- or even MoO 3 + respectively. Of the various possible oxides of molybdenum, both the dioxide M o O 2 and the trioxide MoO 3 are readily formed and are well known. There also exists a series of non-stoichiod. EtectroanaL Chem., 38 (1972)
154
M.N. HULL
metric oxides between the dioxide and the trioxide, notably M o 4 0 1 1 , M°8028 and M o 9 0 2 6 , referred to as 7,/~,/~', respectively z° in addition to the stoichiometric oxide Mo205. Various other solid molybdenum compounds have been recognised which are precipitated by the addition of alkali to acidic solutions of molybdenum in one of its lower valence states and particular mention may be made of M o (OH)3 which is formed as a precipitate by the addition of alkali to deaerated acidic solutions containing molybdenum in its + 3 valence state. The results of the anodic behavior of molybdenum in alkaline electrolytes show that three well defined films are formed which have been designated as type A, B, and C respectively. Further, there are distinct indications that additional heterogeneous processes are occurring on the electrode surface in the potential region between completion of the formation of type B film and the formation of type C, which reflect the successive formation of various oxides having a stoichiometric composition between t h ~ of type B and C films. This latter film, which is the final oxide formed at high anodic potentials, is M o O 3 and the type B film, M o O 2.One can thus assign an oxidation state of + 5 to the soluble species undergoing oxidation at the ring during the early stages of formation of the trioxide film on the disc. Moreover, the first of the marked oxidation waves which are registered at the ring during the reverse scan of the disc must also reflect a rapid increase in the rate of release of Mo(V) as production of the trioxide ceases and transient formation of the 7, fi,/~' occurs at the disc prior to reformation of the dioxide (type B) film on the surface. The remaining oxidizable species which appears at the ring for relatively low anodic overvoltages reflects the production of Mo(III) as an intermediate in this region. Further, the potential at which the concentration of Mo(III) reaches a maximum corresponds to that at which formation of type A film is observed under quiescent conditions. Thus, type A film contains molybdenum in the + 3 oxidation state and has a composition of Mo2 O3 or, more likely, Mo(OH)3. This is supported by the fact that the maximum ring current which appears from oxidation of this intermediate (at ED = + 0.2 V of Fig. 4) is linearly proportional to the square root of the rotation rate. Thus at the point of formation of type A film, a fixed limiting concentration of Mo (III) has been attained at the molybdenum anode. This maximum ring current may be related to the steady state concentration of Mo(III) at the disc electrode by the equation ir = n F A N D c S / 6
(2)
6 = 1.61 D ~ v ~ o - ~
(3)
where In these equations c s is the concentration o f M o (III) at the disc surface, v the kinematic viscosity of the electrolyte and D the diffusion coefficient of the Mo(III) species. Assuming a value for the diffusion coefficient of Mo(III) of 0.5 × 10 -5 cm 2 s-1 and with v = 1.6 × 10 -2 cm 2 s-1, a value of 1.14 × 10-3 M may be calculated for c s. This constancy of the concentration of Mo(III) species at that potential (ED) for which the ring current attains a maximum value could be interpreted as indicating that a "dissolution-precipitation" mechanism 21"22 is operative for the formation of type A film though, as Armstrong and West 23 have pointed out, such a result may also be interpreted in terms of a nucleation overvoltage due to a solid state mechanism with parallel metal dissolution. J. Electroanal. Chem., 38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI
155
Based on the above conclusions the following sequence of reaction steps constitute a plausible mechanistic path for the anodic dissolution of molybdenum in potassium hydroxide electrolytes at low overvoltages ( - 0.3 V to + 0.2 V vs. Hg-HgO). Mo + 2OH- --* Mo(OH) + + 3 e-
(4)
Mo(OH)] + OH- ~ Mo(OH)3 (s)
(5)
slow
Mo(OH)3+5 OH
-----~MoO ] + 4 H z O + 3 e
(6)
Here the initial charge transfer step occurs in the presence of two hydroxyl ions and yields the singly charged Mo(III) species Mo(OH)~- which, with the addition of a further hydroxyl species, tends to precipitate as the hydroxide Mo(OH)3. In the absence of diffusional limitations on the rate of arrival of hydroxyl ions at the electrode surface, further charge transfer reactions can then occur to yield the final Mo(VI) state. In the intermediate potential range +0.2V < E D< q- 1.0 V, production of the Mo (III) intermediate declines and the surface becomes covered with a layer of Mo 02 which is dissolved in a further charge transfer step with the production of molybdate as follows: (7a)
-* MoO2 +2 H 2 0
Mo(OH)3+ OH-
M o + 4 OH- ~ M o O ; +2 I - { 2 0 + 3 e-
(Tb)
MoO2 + MoO2(s)+e-
(8) (9) (10)
MOO2+2 OH- -* MoO/(OH)2+2 eMoOz(OH)z+2 OH- --+ MoO 2- +2 H 2 O
Since MoO z is unstable in aqueous solution 19 a parallel disproportionation reaction is also possible according to 3 MOO:+4 OH- --+ 2 M o O ] - + M o + 2 H 2 0
(11)
At high overvoltages (ED > + 1.0 V) the electrode becomes covered with the oxide MoO3 and Mo(V) species are released in the solution. The production of Mo(V) species from a dioxide-covered electrode would be readily caused through the extraction of a further electron from the dioxide at sufficiently anodic electrode potentials MoO 2 --+ MoO~ We-
(12a)
or directly from the under!ying molybdenum metal via M o + 4 OH- --+ MoO~ +2 H 2 0 + 5
e-
(12b)
followed by fast
MoO~ +2 OH- - - - * MoO3 + H 2 0
(13)
to yield the negatively charged MoO~- ion. This may then undergo further charge transfer to produce the trioxide layer via M o O ; --, MoO3(s)+eMOO3+2 OH-
--+ MOO]- + H 2 0
J. Electroanal. Chem., 38 (1972)
(14) (15)
156
M.N. HULL
In acidic media the surface is continuously covered with a thick layer of oxide and visually one cannot observe any physical change in the nature of this layer during the complete potential scan. However, the presence of a sharp active-passive transition point must reflect a change in the chemical composition of the surface layer in this region which, according to the potential -pH diagram of molybdenum presented by Pourbaix 18, is a transition between a MoOz-covered surface to one covered with M o O 3.
For positive ring potentials three separate regions are evident in the iR--ED curve during "active" dissolution of the disc electrode. In the first 18 of these Mo0II ) gives rise to the wave which reaches a maximum at ED = + 0.7 V while oxidation of Mo(V) species is responsible for both waves appearing at + 1.22 V and + 1.34 V respectively. In the former case the Mo (V) produced in the active region of dissolution results from an increase in the rate of production of this species at the disc electrode with increasing anodic potential and is accompanied by a corresponding decline in the production of Mo(III). In the latter case a quantity of M o(V) is suddenly generated heterogeneously at the disc by oxidation of the surface layer of M o O 2 to the passive layer of MoO 3. In the "active" region of dissolution therefore the following reaction scheme may be written to represent the overall reaction path at low overvoltages (ED<
+0.85 v) Mo+HzO~MoO
+ +214 + 3 e -
MOO/'+ + H 2 0 -* MoO2(s)+2 H + + e MOO2+2
H20
--* H 2 M o O 4 + 2
H + +2 e-
(16) (17) (18)
while with increasing overvoltage in the active region, prior to the onset of passivity, the production of Mo(III) species declines, to be replaced by that of Mo(V). MoO + ~ MoO 3÷ +2 e-
(19)
Finally, at a sufficiently anodic potential heterogeneous oxidation of the dioxide layer on the surface commences with liberation of Mo(V) species and with the production of a layer of MOO3. MoO 2 ~
(20)
MoOI +e-
MoO + --* MoO 2+ + e -
(21)
fast MoO~ + +H20
4
MoO3(s)+2 H +
(22)
In agreement, therefore, with the work of K6nig and G6hr 4, the results of linear sweep voltammetric studies under both quiescent and convective diffusion conditions support the view that both Mo(III) and Mo(V) are produced as intermediates in the overall dissolution reaction in sulphuric acid electrolytes. In addition, two different oxide films appear to be present on the electrode surface, one of which permits "active" dissolution of the electrode while the other causes passivity. The suggestion that these films have the composition MoO2 and MoO3 respectively4 is probably correct. In alkaline solution, at least three films may be identified on the electrode and these are suggested to have the composition Mo(OH)3, MoO 2 and MoO3 while, in agreement with the conclusions drawn concerning the nature of the J. Electroanal. Chem., 38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI
157
intermediate species produced in acidic electrolytes, both Mo(III) and Mo(V) are also produced as intermediates in this electrolyte. Further work is now in progress to extend and verify these conclusions from studies on the behavior of molybdenum electrodes in hydrochloric and perchloric acid media and also from studies conducted in solutions of neutral pH. SUMMARY
The electrochemical oxidation of molybdenum wire and disc electrodes has been studied in 0.1-8.7 M KOH and H/SO4 electrolytes under both quiescent and convective diffusion conditions. In alkaline media three different films may be identified on the electrode surface during a potential sweep between the open circuit rest potential of -0.52 V (Hg-HgO) and the onset of steady state oxygen evolution at approximately + 4.0 V. On the other hand, only two distinctly separate films could be identified on an anodically dissolving molybdenum electrode in acidic electrolytes. In both media the mechanism of the anodic dissolution proceeds through the formation of Mo(III) at low overvoltages and Mo(V) at higher values. REFERENCES 1 H. Kuesner, Z. Electrochem., 16 (1910) 754. 2 E. Becker and H. Hilberg, Z. Elektrochem., 31 (1925) 31. 3 J. Besson and G. Drautzburg, Electrochim. Acta, 3 (1960) 158. 4 M. K6nig and H. G6hr, Ber. Bunsenges. Phys. Chem., 67 (1963) 837. 5 T. Heumann and G. Hauck, Z. Metallkd., 56 (1965) 75. 6 L. L. Wikstrom and K. Nobe, J. Electrochem. Soc., 116 (1969) 525. 7 M. G. Johnson and R. J. Robinson, Anal. Chem., 24 (1952) 366. 8 I. M. Kolthoffand I. Hodara, J. Electroanal. Chem., 4 (1962) 369. 9 J. J. Wittick and G. A. Rechnitz, Anal. Chem., 37 (1965) 816. 10 M. Walter, D. O. Wolf and M. von Stackelberg, J. Electroanal Chem., 22 (1969) 221. 11 L. Sacconi and R. Cini, J. Amer. Chem. Soc., 76 (1954) 4239. 12 H. M. Neumann and N. C. Cook, J. Amer. Chem. Soc., 79 (1957) 3026. 13 W. J. Albery and S. Bruckenstein, Trans. Faraday Soc., 62 (1966) 1920. 14 M. N. Hull, J. E. EUison and J. E. Toni, J. Electrochem. Soc., 117 (1970) 192. 15 M. N. Hull and J. E. Toni, Trans. Faraday Soc., 67 (1971) 1128. 16 M. N. Hull, J. Electroanal. Chem., 30 (1971) App. 1. 17 R. Parsons, Handbook of Electrochemical Constants, Academic Press, New York, 1959. 18 M. Pourbaix, Atlas d'Equilibres Electrochimiques d 25°C, Gauthier-Villars, Paris, 1963. 19 W. M. Latimer, Oxidation Potentials, Prentice Hall, New York, 2nd ed., 1964. 20 A. F. Wells, Structural Inorganic Chemistry, Oxford Clarendon Press, 3rd ed., 1962. 21 W. J. Miiller, Z. Elektrochem., 33 (1927) 401. 22 W. J. Miiller, Trans. Faraday Soc., 27 (1931) 737. 23 R. D. Armstrong and G. D. West, J. Electroanal Chem., 30 (1971) 385.
J. Electroanal. Chem., 38 (1972)
143
Elsevier Seouoia S.A., L a u s a n n e - Printed in The Netherlands
ON THE ANODIC DISSOLUTION OF MOLYBDENUM IN ACIDIC AND ALKALINE ELECTROLYTES
MICHAEL N. HULL
ESB Incorporated, Research Center, 19 W. College Avenue, Yardley, Pa. 19067 (U.S.A.) (Received 30th August 1971 ; in revised form 17th December 1971)
INTRODUCTION
Relatively few studies have been conducted concerning the electrochemical behavior of molybdenum anodes in aqueous electrolytes although the general chemical behavior of this metal is quite well understood. Qualitative observations concerning the general conditions under which molybdenum shows passive behavior were described in early papers by Kuessner I and by Becket and Hilberg z. More recently Besson and Drautzburg 3 found that in both acid and alkaline media the metal dissolved quantitatively in the + 6 state and that the electrode surface was covered with colored films of unknown nature. It was further noted that semilogarithmic plots of the current-voltage curves were Tafel lines independent of the electrolyte pH which determined only their relative position in the potential scale. On a more quantitative basis the kinetic behavior of the metal in acid electrolytes has been studied by three main groups 4'~. K6nig and G6hr 4 found that the current-potential curves for molybdenum electrodes in such solutions could be divided into three distinct potential regions. In the first of these, (E< -0.1 V v s . NHE), hydrogen was evolved from the electrode while in the potential range -0.1 V < E < + 0.4 V it was suggested that a layer of molybdenum dioxide was formed on the electrode surface and molybdenum ions were produced in solution in both the + 3 and + 5 oxidation state. In the region + 0.4 V < E < + 4.0 V the surface was presumed to be covered with a layer of the corresponding trioxide from which molybdenum passed into solution in the + 6 state as the molybdate ion. Heumann and Hauck 5, on the other hand, suggested that although molybdenum dissolves in the + 6 state in sulphuric, nitric and hydrochloric acids the rate determining step in this overall process was the transition from the + 4 to the + 6 oxidation state. They also proposed that an amorphous layer of the dioxide was present on the electrode surface under all conditions of oxidation and that molybdenum went into solution through the passive state. More recently Wikstrom and Nobe 6 have investigated the steady state behavior of molybdenum in 1 N H2SO 4 and have interpreted their results as being in agreement with those of Heumann and Hauck. The polarographic reduction of Mo (VI) in acid solution proceeds in two steps to the Mo(V) and then the Mo(III) state 7- ao. Reduction to the + 5 state in sulphuric acid solutions begins at a potential of + 0.2 V (SCE) and to the + 3 state at approximately -0.2 V 7'8 although, depending on the relative concentrations of the background electrolyte and of the Mo(VI) species, each of these reduction waves may be split into J. Electroanal. Chem., 38 (1972)
144
M.N. HULL
two further steps. This latter phenomenon has been attributed s to the presence of two different forms of both Mo(VI) and Mo(V) which are present to a greater or lesser degree in solution depending on the pH of the electrolyte11,12. Using a platinum or gold ring electrode to detect the production of soluble intermediates and products generated at an anodically dissolving molybdenum disc electrode, it should therefore be possible to detect the reduction of Mo(VI) only to the + 5 state since hydrogen evolution will prevent detection of the additional reduction step to the + 3 state. In neutral or alkaline media the reduction of Mo(VI) is not possible 1°. As a general statement therefore it appears to be well established that molybdenum electrodes are covered with some form of an oxide layer during their anodic dissolution in acidic electrolytes. However, the nature of the intermediate oxidation states which are produced during the overall oxidation reaction to the final + 6 state are still the subject of discussion. In addition to these considerations, the electrochemical behavior of molybdenum electrodes in alkaline media has been studied to a lesser degree although it might be anticipated that the behavior of molybdenum under these conditions would show some important differences from that exhibited in acid media. The present study was undertaken to extend the previous knowledge of the electrochemical behavior of molybdenum in acidic electrolytes to alkaline media and also to gain some further insight into the oxidation state of the possible intermediate species which are produced in the overall oxidation process in both media. To do this, the stationary behavior of molybdenum wire and disc electrodes was investigated in acidic and basic electrolytes under both quiescent and convective diffusion conditions, employing the techniques of linear sweep voltammetry and the rotating ring-disc. EXPERIMENTAL
The molybdenum wire electrodes were produced by sheathing a 20 mil* diameter wire in heat shrinkable Teflon tubing to leave an area of 0.42 cm 2 exposed to the electrolyte. For the experiments under convective diffusion conditions the molybdenum disc was machined from a molybdenum rod to give an electrode area of 0.1 cm 2. In this latter case the disc was surrounded by a gold ring to give an electrode configuration whose radial dimensions were r 1= 1.78, r 2 = 1.99, r 3 = 2.27 mm, yielding a value for the collection efficiency13 N of 0.265. The molybdenum material was supplied by Alfa Inorganics and was of m3N5 purity, the principal impurity being tungsten in a concentration of 100 p.p.m. The electrical arrangement used for the linear sweep experiments and that for the experiments with the rotating ring~zlisc electrode have been described in detail previously14'15. In the latter case, rotation of the electrode was performed by an ESB Depak Rotator which will permit continuous variation of the rotation speed from 50-30,000 rev. min-1. Electrolyte solutions were prepared from AnalaR reagents and doubly distilled water and were continuously purged with pure nitrogen except during an actual experimental determination during which time the gas was passed oyer the solution surface. A cylindrical platinum gauze was used as the counter electrode while a Hg - Hg O electrode was used as the reference in alkaline, and a saturated calomel electrode in acidic, electrolytes. * lmil=2.54x10
5m.
J. Electroanal. Chem., 38 (1972)
ANODIC
DISSOLUTION
145
OF Mo IN ACID AND ALKALI
RESULTS
A. General behavior in quiescent electrolytes The stationary current potential (i-E) curves of a molybdenum wire electrode in 1.0 M and 3.88 M K O H are shown in Fig. 1. For all scan rates investigated (0.005-1.0 V s- l) three distinct oxidation waves (A, B, and C of Fig. 1) could be identified on the anodic sweep 16. Wave C, however, appears to consist of two or more closely spaced steps since in most cases an inflection is observed on the rising side of this wave (see for example the 3.88 M case shown in Fig. 1). Each of these oxidation waves could be visibly correlated with the formation of a different film on the electrode surface. At A the electrode becomes covered with a thin light-colored film which undergoes a sudden darkening in appearance at the foot of wave B. Once type C film begins to form at approximately +0.75 V (Hg-HgO) the surface layer becomes completely black, at which point the film thickens appreciably and peels from the electrode in the form of a stream of fine particles. (CI
'
'
'
"
20
+l.O
+0.5
0.0
ELECTRODEPOTENTIAL/VOLTS vs. Hg-HgO
Fig. 1. C u r r e n t - p o t e n t i a l b e h a v i o r o f a m o l y b d e n u m w i r e e l e c t r o d e r e c o r d e d in 1.0 M a n d 3.88 M q u i e s c e n t K O H soln. at a s c a n r a t e o f 0.05 V s - 1 . E l e c t r o d e a r e a = 0 . 4 2 c m 2.
After passing through the maximum at C the current continues to decline monotonically in the absence of reversal of the sweep direction at + 1.5 V. This passive behavior of the electrode is maintained to anodic potentials as high as +4.0 V at which point steady state oxygen evolution begins to occur causing disruption of the passive layer resulting in sharp oscillations in the magnitude of the anodic current. On the return sweep, the potential at which reactivation of the electrode occurs due to removal.of the type C film from the electrode surface, shifts to more cathodic potentials as the scan rate is increased or the electrolyte concentration is decreased. In Fig. 1 it is evident that at a scan rate of 50 mV s- 1 removal of the type C film in J. Electroanal. Chem., 38 (1972)
146
M.N. HULL
3.88 M K O H is complete at a potential of +0.95 V while in 1.0 M K O H the film remains on the electrode surface until a potential of + 0.45 V is attained. It was observed that, in general, for each electrolyte concentration a certain fixed time was required after the electrode potential had become more cathodic than +0.95 V before reactivation was noted. This time was quite independent of the scan rate but decreased by an approximate factor of two for each factor of two increase in the K O H concentration. This behavior indicates that chemical dissolution of the type C film is responsible for its removal rather than a step of electrochemical reduction of the oxide film to a lower oxidation state and that the potential dependence of the point of reactivation on the scan rate has no particular electrochemical significance. As a general statement the magnitude of wave A undergoes a marked increase relative to that of waves B and C as the electrolyte concentration or the scan rate is increased. In Fig. 1 it is evident that wave C is predominant in 1.0 M K O H while at the higher concentration of 8.7 M K O H and at the higher scan rate of 0.5 V s -1 shown in Fig. 2, wave A is predominant and wave C can no longer be distinguished. In addition, the magnitude of wave A is linearly dependent on both the square root of the scan rate and also, up to 2.0 M, on the electrolyte concentration, indicating that under these conditions the diffusive transport of hydroxyl ions to the electrode surface is rate controlling. Further evidence in support of this conclusion will be presented below. On the other hand, the dependence of wave B and particularly that of wave C is
,-,,
8 . ...7. . .M. K O H
3.9MKOH
r• /
~
SC/~N RATE : 500mY sec -I
\ , _, \
.
,\
"'.
+t.O
+0.5
ANOD,C
0.0
-0.5
ELECTRODEPOTENTIAL/VOLTSvs. Hg-HgO Fig. 2. Current potential behavior of a m o l y b d e n u m wire electrode in 3.9 M and 8.7 M quiescent K O H soln. recorded at a scan rate of 0.5 V s 1. Other exptl, conditions as for Fig. 1.
J. Electroanal. Chem., 38 (1972)
147
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI
o
T
80 ANODIC CURRENT
:mA
SCA~. MfATH~05~ 2E 0mV sec"
I
+2.0
I
+1.5
I
+1.0
I
+0.5
ELECTRODEPOTENTIAL/VOLTS vs. S C E
Fig. 3. Current-potential behavior of a molybdenum wire electrode in 1.0 M quiescent H2SO 4 recorded at a scan rate of 0.005 V s-~.
less marked than that of A which indicates that in the potential regions encompassed by these waves heterogeneous processes such as mass transport in a continuously thickening surface film play an increasingly greater role in determining the overall dissolution rate. In contrast to the behavior exhibited in potassium hydroxide electrolytes the stationary i-E curve for a molybdenum wire electrode in sulphuric acid solutions exhibits only one current maximum. This is clearly shown in Fig. 3 where it may be seen that molybdenum exhibits active behavior between points P and Q on the Figure (+ 0.35 V and + 0.8 V vs. SCE respectively), after which a sharp drop in the dissolution current occurs and the electrode may be regarded as having entered its region of passivity. In the region of "active" behavior the surface is covered with a black film which forms immediately oxidation commences at P and which spalls from the electrode surface in a continuous stream of black particles. At Q, however, as the electrode becomes passive this stream of particles becomes less intense and is no longer visible for electrode potentials more anodic than approximately + 1.2 V (point S). The electrode remains covered with a thick black film though the composition of this film must be distinctly different from that present on the surface prior to point Q. No further spalling is observed until electrode potentials in excess of + 3.0 V are attained when disruption of the surface layer by oxygen bubble formation becomes apparent. In addition, in contrast to the behavior in alkaline media, there is no evidence of reactivation of the electrode on the return sweep in quiescent H2SO 4 regardless of the scan rate or concentration of the electrolyte employed. The passive layer on the electrode has thus a very low solubility in sulphuric acid and remains on the surface for the whole of the return sweep,
B. Rotating ring-disc studies--general behavior 1. Alkaline electrolytes. Although the production of molybdenum intermediates having valence states less than Mo(VI) has been proposed in previous studies of the J. Electroanal. Chem., 38 (1972)
148
M.N. HULL
anodic behavior of this electrode in acidic media4'5, no firm experimental verification of the existence of such intermediates has been obtained to date. Accordingly, rotating ring-disc studies were performed in which the production of both oxidizable and reducible intermediates generated at an anodically dissolving molybdenum disc electrode were monitored by means of a gold ring electrode held at various fixed negative and positive potential values while the disc electrode was scanned at a sweep rate of 100 mV s-~. This rather high sweep rate was chosen in order that no significant recession of the electrode surface would occur in the time span required for a single scan of the disc. I
I
I
I
ELECTROLYTE IM KOH
i1"'~ iI
,.2,
/
II
MO DISC
- -
AU R I N G
.....
i
t CURRENT
•
"
i
.;,
\ \ ,,, ,, \ \\\,\ , \
"\\
\
-...
I +I.5
+I.0
+0.5 EDISC / VOLTS vs. Hg - H9 0
0.0
-0,5
Fig. 4. ( ) ID--ED, (..... ) I,-E D traces recorded with a M ~ A u ring-disc electrode rotated at a rate of 1000 rev. m i n - ~ in 1.0 M K O H . Scan rate of disc potential = 0.1 V s - ~ ; ER = + 0.65 V.
The i-E behavior of a molybdenum disc electrode in 1.0 M KOH and under convective diffusion conditions is traced by the solid line in Fig. 4. Initially the current rises to a limiting value between + 0.6 and + 1.0 V (Hg-HgO) within which potential range the surface is covered with type B film. In this, and for all electrolyte concentrations less than 2.0 M, linear behavior is exhibited between the limiting current in this region and the square root power of the rotation rate for values of co up to 10,000 rev. min- ~.A typical plot of this type is presented in Fig. 5 from data recorded in 0.1 M KOH. From the Levich equation I L = 0.62nFA c b D ~ v - +co½
(l)
in which the terms have their usual electrochemical significance, such plots are found J. Electroanal. Chem., 38 (1972)
A N O D I C D I S S O L U T I O N OF Mo IN A C I D A N D A L K A L I
149
y 2O
II /mA
•
I 20
H
I
I 6o
co I/2/rev I/2 m~nl/~ Fig. 5. Plot of limiting current recorded at a Mo disc electrode for Eo < + 1.0 V as a function of square root power of the rotation rate. Electrolyte=0.1 M KOH.
to be in excellent agreement with those calculated theoretically for the diffusive transport of hydroxyl ions up to the electrode surface assuming D = 5.23 x 10-5 cm 2 s- 1 and v = 1.6 x 10- 2 cm 2 s- 1. Under convective diffusion conditions the formation of type A film is not apparent from the disc i-E trace at + 0.2 V though, as will be shown later, analysis of the response of the surrounding gold ring electrode held at a positive electrode potential reveals the formation of this film. For electrode potentials more positive than + 1.0 V, a change in the electrode mechanism occurs and the disc current undergoes a further increase passing through a broad maximum at approximately + 1.6 V. Thereafter, in the absence of a reversal in the sweep direction, the current continues to decline as the heterogenous formation and growth of the black type C film occurs. Control of the overall dissolution rate then becomes governed by transport processes in this continuously thickening surface layer. For a sweep reversal at a potential of + 2.0 V a sharp reactivation of the electrode is observed at + 0.95 V since chemical removal of type C film is completed at this point and the surface again becomes covered with type B film. In contrast to the behavior exhibited under quiescent conditions, where two reactivation steps may be identified corresponding to the removal of the type C followed by the type B film, only the removal of the former film can be identified from the i-E curve recorded under these conditions. The dotted line plotted on Fig. 4 traces the response of a gold ring electrode as a function of the disc potential when the ring is held at a fixed potential of +0.65 V. It may be observed that the presence of a solution-soluble oxidizable intermediate is immediately detected at the ring electrode once anodic oxidation of the molybdenum J. Electroanal. Chem., 38 (1972)
150
M.N. HULL
disc commences at - 0.3 V. At a disc potential of + 0.2 V, which corresponds closely to the peak potential at which type A film is observed to form at slow scan rates in the linear sweep voltammetric studies, the ring current passes through a maximum and thereafter declines steadily while the disc current continues to rise to the diffusion limited value. Following this, the onset of formation of the type C film at a disc potential of + 1.0 V is initially accompanied by a slight fall in the oxidation current registered at the ring until replacement of the type B film is complete. At this point, (ED = + 1.45 V), the presence of a further solution-soluble oxidizable intermediate is again detected at the ring electrode. During the reverse potential scan, two distinct oxidation peaks are observed in the Ir-ED trace. The first and most pronounced of these accompanies the disappearance of the type C film from the disc and reaches a maximum just prior to the abrupt reactivation of this electrode at + 0.95 V. The second reaches a maximum at a disc potential of + 0.05 V though the dissolution current is continuously declining in this potential region. These observations may be understood as resulting from a release of oxidizable material from the disc as one type of film disappears from the electrode surface and is replaced by another of different chemical composition.
!
i
\
M00,sc Au-..R,.0
----
A!0DIC CURRENT
+1.~
+1.0
+0.5
0.0
EDIsc/VOLTS vs. Hg-HgO
Fig. 6. ID--ED and I r E D traces recorded under the same exptl, conditions as Fig. 4 except ER= --0.9 V.
The ir-EDtrace recorded with an amalgamated gold ring electrode held at a fixed potential of - 0.9 V is shown by, the dotted line in Fig. 6. All other experimental conditions were the same as for Fig. 4. In this instance, no reducible species are detected by the ring, in agreement with the known polarographic behavior of molybdate solutions mentioned previously. However, with decreasing electrolyte concentration the onset of a reduction process becomes apparent for disc potentials in excess of approximately + 1.2 V. This is illustrated in Fig. 7 for ring-disc data recorded J. Electroanal. Chem.,
38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI J
l
151
i
i
0
/ -~
/ // I
~o,sc
RING CURRE~
CLIR~ENI /mA
Au RING . . . . . .
I i \
'
+ ,5
~
0.~MK0.
+10
+0.5
EDISC/
0,0
-
VOLTSvs. Ng-HgO)
Fig. 7. ID--ED and lr-EDtraces recorded with a Mo~Auring~tisc electrode in 0.1 M K OH at a rotation rate of 400 rev, min-x. All other conditions as for Fig. 6. in 0.1 M K O H solution and is believed to reflect the concurrent production of a small amount of molecular oxygen during the initial stages of formation of type C film in the weaker electrolytes. This behavior may be explained in terms of the relative thickness of the type C film between strong and dilute K O H solutions. In the former case the film thickness is much greater, and hence the electronic resistance o'f the film is higher than in the more dilute electrolyte, and oxygen evolution is suppressed. 2. Acidic electrolytes. The general form of the i-E curve recorded with a molybdenum disc electrode under convective diffusion conditions in acidic media is quite similar to that exhibited under quiescent conditions. A typical curve recorded in 1.0 M H2SO 4 and at a rotation rate of 1000 rev. m i n - 1 is traced by the solid line in Fig. 8. It m a y be seen that the electrode exhibits an "active" region between +0.35 and + 1.22 V (SCE) followed by a relatively sharp decrease of the current to the "passive" region. On the return sweep, removal of the passivating film is observed by the slight rise in current which occurs at E o = + 1.2 V. In acidic electrolytes, similarly to the behavior observed in the alkaline case, intermediate species are produced during oxidation of the m o l y b d e n u m disc electrode and these may be made to undergo further oxidation at the ring as shown by the dotted trace of Fig. 8. It is evident that the ring current trace shows three distinct regions in the potential range encompassing the onset of oxidation of the disc at +0.35 V and the onset of the disc passivity at + 1.34 V. In this range the ring current exhibits two main waves at ED= +0.7 V and Eo = 1.34 V, respectively, in addition to a third less prominent wave present as a region of inflection at ED = + 1.22 V on the rising portion of the more anodic of the two main waves. On the return cathodic sweep of the disc, reactivation of the electrode is accompanied, as in alkaline solutions also, by an increasing production of a soluble oxidizable intermediate species. Further, J. Elec~roanal. Chem., 38 (1972)
152
M.N. HULL
120 °
t" 40
I
8o] DISC
RING
CURRENT /mA
4O
0
I
I
I
I
+2.5
+2.0
+1.~
+l.O
0
*0.5
EDIsc/VOLTS vs, S C E
Fig. 8. ID-E D a n d Ir-E o traces recorded in 1.0 M H 2 S O 4 w i t h E R = + 1.0 V. co = 1000 rev. m i n - 1. Scan rate = 0.1 V s -1.
120
CURRENT /mA
DISC CURRENT /mA
-12
I 42.5
~1 ..... +2.0 EDISC /
Fig. 9.
]D
/~
I +1,5
I +l.O
I ~ll, ÷0,5
VOLTSVS. S C E
ED a n d Ir-E D traces recorded with E R = --0.1 V. All other exptl, conditions as in Fig. 8.
release of the oxidizable material from the disc continues for a substantial time after completion of the potential scan even though the current flow through the disc has fallen to zero. In Fig. 9 the corresponding response curve of the gold ring electrode when held at a negative potential of -0.1 V is presented. As was indicated above, previous studies concerning the polarographic behaviour of molybdate solutions show that one should observe only the reduction of Mo(VI) to Mo (V) at this potential. The Figure shows two clearly discernible regions in the ring response curve. In the potential range + 0.3 5 < ED < + 1.15 V only a small fraction of the disc current gives rise to the J. Electroanal. Chem., 38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID A N D ALKALI
153
production of soluble Mo (VI) species and the majority of the current flow passing through the disc is being utilized in heterogeneous processes associated with, for example, the growth and replenishment of the surface film which is being swept off the surface in the form of a stream of relatively insoluble black particles. If one calculates the experimental collection efficiency at ED----1.15 V (i.e. just prior to the onset of formation of the second and passivating film) one obtains a value of No~s = 0.017. (Owing to the time required for transit of a species between the disc and the ring there is a displacement in the position of the iR-E o response curve relative to that of the iD-Eo curve. At high scan rates of the disc electrode this shift can lead to an incorrect value of NoBs when both iR and iD are read at the same value of ED. At the scan rates employed in this study, however, a maximum error of 3 ~ is introduced in the calculation of No~s from this effect.) Comparing this with the theoretical collection efficiency of N = 0.044, which would be observed assuming a 6-electron oxidation of the molybdenum disc with a 1-electron reduction step to the Mo(V) state at the ring, it is evident that at this point about 6 0 ~ of the disc current produces product which leaves the electrode surface in the form of undissolved particles of oxide. At a value of Eo = + 1.15 V, at which point a change in the nature of the surface film from an "active" to a "passive" layer is taking place, a rapidly increasing reduction current is recorded by the ring which reaches a maximum value at Eo = + 1.8 V. Here the experimental collection efficiency has a value of NoBs = 0.118 which is the highest value recorded at any point of the complete trace. Comparing this latter value with the theoretical value of N = 0.044 (again assuming Mo ~ Mo (VI) + 6e- and Mo (VI) + e- --, Mo (V) are the electrode reactions at the disc and ring respectively) it is evident that at this point in the trace a second parallel oxidation process must be proceeding at the disc and which is giving rise to the species readily reduced at the ring. The high value of the experimentally observed collection efficiency may be explained by suggesting that oxygen is being produced at the disc during the early stages of formation of the trioxide film. In contrast to the behavior exhibited at positive ring potentials (Fig. 8) one finds that the transition from the "passive" to"active" behavior on the return scan of the disc at approximately + 1.2 V is not accompanied by any increased production of soluble and reducible molybdate species. Rather, in line with the behavior exhibited during the forward sweep, the reduction current at the ring tends to fall offwith return of the electrode to the "active" region of dissolution. Thus the oxidation intermediate detected in this region with positive ring potentials cannot be reduced (if at all) at a ring potential of -0.1 V. DISCUSSION
It has been well established x- 6 that the final product of the oxidation of molybdenum in acid and in alkaline electrolytes is Mo(VI) which, in the former case exists as molybdic or polymolybdic species (depending on the pH) and in the latter case as molybdate 18. Lower oxidation states of molybdenum in solution have been well characterised, particularly Mo(III) and Mo(V) which, according to Latimer 19, in strongly acid solutions exist as M o O + and MoO~- or even MoO 3 + respectively. Of the various possible oxides of molybdenum, both the dioxide M o O 2 and the trioxide MoO 3 are readily formed and are well known. There also exists a series of non-stoichiod. EtectroanaL Chem., 38 (1972)
154
M.N. HULL
metric oxides between the dioxide and the trioxide, notably M o 4 0 1 1 , M°8028 and M o 9 0 2 6 , referred to as 7,/~,/~', respectively z° in addition to the stoichiometric oxide Mo205. Various other solid molybdenum compounds have been recognised which are precipitated by the addition of alkali to acidic solutions of molybdenum in one of its lower valence states and particular mention may be made of M o (OH)3 which is formed as a precipitate by the addition of alkali to deaerated acidic solutions containing molybdenum in its + 3 valence state. The results of the anodic behavior of molybdenum in alkaline electrolytes show that three well defined films are formed which have been designated as type A, B, and C respectively. Further, there are distinct indications that additional heterogeneous processes are occurring on the electrode surface in the potential region between completion of the formation of type B film and the formation of type C, which reflect the successive formation of various oxides having a stoichiometric composition between t h ~ of type B and C films. This latter film, which is the final oxide formed at high anodic potentials, is M o O 3 and the type B film, M o O 2.One can thus assign an oxidation state of + 5 to the soluble species undergoing oxidation at the ring during the early stages of formation of the trioxide film on the disc. Moreover, the first of the marked oxidation waves which are registered at the ring during the reverse scan of the disc must also reflect a rapid increase in the rate of release of Mo(V) as production of the trioxide ceases and transient formation of the 7, fi,/~' occurs at the disc prior to reformation of the dioxide (type B) film on the surface. The remaining oxidizable species which appears at the ring for relatively low anodic overvoltages reflects the production of Mo(III) as an intermediate in this region. Further, the potential at which the concentration of Mo(III) reaches a maximum corresponds to that at which formation of type A film is observed under quiescent conditions. Thus, type A film contains molybdenum in the + 3 oxidation state and has a composition of Mo2 O3 or, more likely, Mo(OH)3. This is supported by the fact that the maximum ring current which appears from oxidation of this intermediate (at ED = + 0.2 V of Fig. 4) is linearly proportional to the square root of the rotation rate. Thus at the point of formation of type A film, a fixed limiting concentration of Mo (III) has been attained at the molybdenum anode. This maximum ring current may be related to the steady state concentration of Mo(III) at the disc electrode by the equation ir = n F A N D c S / 6
(2)
6 = 1.61 D ~ v ~ o - ~
(3)
where In these equations c s is the concentration o f M o (III) at the disc surface, v the kinematic viscosity of the electrolyte and D the diffusion coefficient of the Mo(III) species. Assuming a value for the diffusion coefficient of Mo(III) of 0.5 × 10 -5 cm 2 s-1 and with v = 1.6 × 10 -2 cm 2 s-1, a value of 1.14 × 10-3 M may be calculated for c s. This constancy of the concentration of Mo(III) species at that potential (ED) for which the ring current attains a maximum value could be interpreted as indicating that a "dissolution-precipitation" mechanism 21"22 is operative for the formation of type A film though, as Armstrong and West 23 have pointed out, such a result may also be interpreted in terms of a nucleation overvoltage due to a solid state mechanism with parallel metal dissolution. J. Electroanal. Chem., 38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI
155
Based on the above conclusions the following sequence of reaction steps constitute a plausible mechanistic path for the anodic dissolution of molybdenum in potassium hydroxide electrolytes at low overvoltages ( - 0.3 V to + 0.2 V vs. Hg-HgO). Mo + 2OH- --* Mo(OH) + + 3 e-
(4)
Mo(OH)] + OH- ~ Mo(OH)3 (s)
(5)
slow
Mo(OH)3+5 OH
-----~MoO ] + 4 H z O + 3 e
(6)
Here the initial charge transfer step occurs in the presence of two hydroxyl ions and yields the singly charged Mo(III) species Mo(OH)~- which, with the addition of a further hydroxyl species, tends to precipitate as the hydroxide Mo(OH)3. In the absence of diffusional limitations on the rate of arrival of hydroxyl ions at the electrode surface, further charge transfer reactions can then occur to yield the final Mo(VI) state. In the intermediate potential range +0.2V < E D< q- 1.0 V, production of the Mo (III) intermediate declines and the surface becomes covered with a layer of Mo 02 which is dissolved in a further charge transfer step with the production of molybdate as follows: (7a)
-* MoO2 +2 H 2 0
Mo(OH)3+ OH-
M o + 4 OH- ~ M o O ; +2 I - { 2 0 + 3 e-
(Tb)
MoO2 + MoO2(s)+e-
(8) (9) (10)
MOO2+2 OH- -* MoO/(OH)2+2 eMoOz(OH)z+2 OH- --+ MoO 2- +2 H 2 O
Since MoO z is unstable in aqueous solution 19 a parallel disproportionation reaction is also possible according to 3 MOO:+4 OH- --+ 2 M o O ] - + M o + 2 H 2 0
(11)
At high overvoltages (ED > + 1.0 V) the electrode becomes covered with the oxide MoO3 and Mo(V) species are released in the solution. The production of Mo(V) species from a dioxide-covered electrode would be readily caused through the extraction of a further electron from the dioxide at sufficiently anodic electrode potentials MoO 2 --+ MoO~ We-
(12a)
or directly from the under!ying molybdenum metal via M o + 4 OH- --+ MoO~ +2 H 2 0 + 5
e-
(12b)
followed by fast
MoO~ +2 OH- - - - * MoO3 + H 2 0
(13)
to yield the negatively charged MoO~- ion. This may then undergo further charge transfer to produce the trioxide layer via M o O ; --, MoO3(s)+eMOO3+2 OH-
--+ MOO]- + H 2 0
J. Electroanal. Chem., 38 (1972)
(14) (15)
156
M.N. HULL
In acidic media the surface is continuously covered with a thick layer of oxide and visually one cannot observe any physical change in the nature of this layer during the complete potential scan. However, the presence of a sharp active-passive transition point must reflect a change in the chemical composition of the surface layer in this region which, according to the potential -pH diagram of molybdenum presented by Pourbaix 18, is a transition between a MoOz-covered surface to one covered with M o O 3.
For positive ring potentials three separate regions are evident in the iR--ED curve during "active" dissolution of the disc electrode. In the first 18 of these Mo0II ) gives rise to the wave which reaches a maximum at ED = + 0.7 V while oxidation of Mo(V) species is responsible for both waves appearing at + 1.22 V and + 1.34 V respectively. In the former case the Mo (V) produced in the active region of dissolution results from an increase in the rate of production of this species at the disc electrode with increasing anodic potential and is accompanied by a corresponding decline in the production of Mo(III). In the latter case a quantity of M o(V) is suddenly generated heterogeneously at the disc by oxidation of the surface layer of M o O 2 to the passive layer of MoO 3. In the "active" region of dissolution therefore the following reaction scheme may be written to represent the overall reaction path at low overvoltages (ED<
+0.85 v) Mo+HzO~MoO
+ +214 + 3 e -
MOO/'+ + H 2 0 -* MoO2(s)+2 H + + e MOO2+2
H20
--* H 2 M o O 4 + 2
H + +2 e-
(16) (17) (18)
while with increasing overvoltage in the active region, prior to the onset of passivity, the production of Mo(III) species declines, to be replaced by that of Mo(V). MoO + ~ MoO 3÷ +2 e-
(19)
Finally, at a sufficiently anodic potential heterogeneous oxidation of the dioxide layer on the surface commences with liberation of Mo(V) species and with the production of a layer of MOO3. MoO 2 ~
(20)
MoOI +e-
MoO + --* MoO 2+ + e -
(21)
fast MoO~ + +H20
4
MoO3(s)+2 H +
(22)
In agreement, therefore, with the work of K6nig and G6hr 4, the results of linear sweep voltammetric studies under both quiescent and convective diffusion conditions support the view that both Mo(III) and Mo(V) are produced as intermediates in the overall dissolution reaction in sulphuric acid electrolytes. In addition, two different oxide films appear to be present on the electrode surface, one of which permits "active" dissolution of the electrode while the other causes passivity. The suggestion that these films have the composition MoO2 and MoO3 respectively4 is probably correct. In alkaline solution, at least three films may be identified on the electrode and these are suggested to have the composition Mo(OH)3, MoO 2 and MoO3 while, in agreement with the conclusions drawn concerning the nature of the J. Electroanal. Chem., 38 (1972)
ANODIC DISSOLUTION OF Mo IN ACID AND ALKALI
157
intermediate species produced in acidic electrolytes, both Mo(III) and Mo(V) are also produced as intermediates in this electrolyte. Further work is now in progress to extend and verify these conclusions from studies on the behavior of molybdenum electrodes in hydrochloric and perchloric acid media and also from studies conducted in solutions of neutral pH. SUMMARY
The electrochemical oxidation of molybdenum wire and disc electrodes has been studied in 0.1-8.7 M KOH and H/SO4 electrolytes under both quiescent and convective diffusion conditions. In alkaline media three different films may be identified on the electrode surface during a potential sweep between the open circuit rest potential of -0.52 V (Hg-HgO) and the onset of steady state oxygen evolution at approximately + 4.0 V. On the other hand, only two distinctly separate films could be identified on an anodically dissolving molybdenum electrode in acidic electrolytes. In both media the mechanism of the anodic dissolution proceeds through the formation of Mo(III) at low overvoltages and Mo(V) at higher values. REFERENCES 1 H. Kuesner, Z. Electrochem., 16 (1910) 754. 2 E. Becker and H. Hilberg, Z. Elektrochem., 31 (1925) 31. 3 J. Besson and G. Drautzburg, Electrochim. Acta, 3 (1960) 158. 4 M. K6nig and H. G6hr, Ber. Bunsenges. Phys. Chem., 67 (1963) 837. 5 T. Heumann and G. Hauck, Z. Metallkd., 56 (1965) 75. 6 L. L. Wikstrom and K. Nobe, J. Electrochem. Soc., 116 (1969) 525. 7 M. G. Johnson and R. J. Robinson, Anal. Chem., 24 (1952) 366. 8 I. M. Kolthoffand I. Hodara, J. Electroanal. Chem., 4 (1962) 369. 9 J. J. Wittick and G. A. Rechnitz, Anal. Chem., 37 (1965) 816. 10 M. Walter, D. O. Wolf and M. von Stackelberg, J. Electroanal Chem., 22 (1969) 221. 11 L. Sacconi and R. Cini, J. Amer. Chem. Soc., 76 (1954) 4239. 12 H. M. Neumann and N. C. Cook, J. Amer. Chem. Soc., 79 (1957) 3026. 13 W. J. Albery and S. Bruckenstein, Trans. Faraday Soc., 62 (1966) 1920. 14 M. N. Hull, J. E. EUison and J. E. Toni, J. Electrochem. Soc., 117 (1970) 192. 15 M. N. Hull and J. E. Toni, Trans. Faraday Soc., 67 (1971) 1128. 16 M. N. Hull, J. Electroanal. Chem., 30 (1971) App. 1. 17 R. Parsons, Handbook of Electrochemical Constants, Academic Press, New York, 1959. 18 M. Pourbaix, Atlas d'Equilibres Electrochimiques d 25°C, Gauthier-Villars, Paris, 1963. 19 W. M. Latimer, Oxidation Potentials, Prentice Hall, New York, 2nd ed., 1964. 20 A. F. Wells, Structural Inorganic Chemistry, Oxford Clarendon Press, 3rd ed., 1962. 21 W. J. Miiller, Z. Elektrochem., 33 (1927) 401. 22 W. J. Miiller, Trans. Faraday Soc., 27 (1931) 737. 23 R. D. Armstrong and G. D. West, J. Electroanal Chem., 30 (1971) 385.
J. Electroanal. Chem., 38 (1972)