Coulostatic study of intermediates formed by pulse radiolysis

J Electroanal. Chem., 97 (1979) 127--133

127

© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

Preliminary note COULOSTATIC STUDY OF INTERMEDIATES FORMED BY PULSE RADIOLYSIS

F. BARRIGELLETTI, F. BUSI and M. CIANO

C.N.R. Laboratorio di Fotochimica e Radiazioni d 'Alta Energia, Bologna (Italy) V. CONCIALINI and O. TUBERTINI

Istituto Chimico "G. Ciamician", Via F. Selmi 2, Bologna (Italy) G.C. BARKER

School o f Chemistry, Cantock's Close, Bristol (England) (Received 8th December 1978)

Galvanostatic methods have been used extensively in the pioneering work of Henglein and co-workers [ 1 ] to study the properties of unstable intermediates formed by pulse radiolysis. In such work at values of elapsed time below 50--100 ps it is difficult to avoid error in the measurement of faradaic current caused by imperfect control of the interfacial potential. Also it is difficult to obtain adequate signal:noise ratio without the use of radiation doses so large that intermediates are largely lost by reaction in pairs while diffusing to the test electrode. Thus it has proved difficult by elementary galvanostatic methods to study the properties of the simplest and most basic products of irradiation (eg H, OH) though valiant attempts (see ref. lc) to do this have been made. In the present note we describe briefly a new coulostatic method which, although not changing the situation dramatically, does permit measurements to be made with significantly smaller doses, and at smaller values of elapsed time, than when a galvanostatic (strictly quasi-galvanostatic) method is employed. The method involves essentially the study of the gradual change in interfacial potential under quasi-coulostatic conditions produced by faradaic processes involving radiation-produced intermediates in the solution, employing as the test electrode a DME (to avoid error due to surface contamination). Thus far the method has been used only with pulsed X-rays but there is no obvious reason why the radiation source should not be a Lineac. EXPERIMENTAL

A slowly dropping (~>12 s drop-life) DME, part of a large three-electrode cell, is irradiated with single 50 ns duration X-ray pulses of high intensity supplied by the thin Ta target of a 2-MeV Febetron electron accelerator. The cell geometry is such that the X-ray source (of small diameter) is close to the DME but far from the SCE reference electrode and the large mercury pool counter-electrode. Nevertheless, despite the close proximity of the DME to the X-ray source the two are separated by the wall of a large aluminium

128

box which encloses the entire cell and contains also a lead-shielded preamplifier used to enhance the cell signal prior to transmission to the Febetron control room. The box as far as is possible is thick-walled and serves to isolate the cell from interference radiated by the circuits of the Febetron. The preamplifier has a gain of ca 300 and a finite input impedance (ca 1 k~2 ) and, with the cell connected, the overall noise factor throughout most of the passband is better than 10 dbs. Irradiation takes place at a fixed time in the drop-life and the amplified signal is displayed on the screen of a storage oscilloscope and recorded photographically. The potential of the DME relative to the SCE is controlled manually from the control room, and all electrical links between the control room and the aluminium box are appropriately screened and filtered, to minimise the addition to the studied signal of electrical interference. Despite these precautions interference makes measurements at elapsed times smaller than 2--3 ps impossible at the present time. As to obtain a good signal:noise ratio the input impedance of the solid state preamplifier (specially designed) is made finite, measurements are made under quasi-coulostatic conditions and for large t the signal seen on the oscilloscope screen gradually decays. Data have first to be corrected for this distortion and this is done using a Hewlett Packard Desk Calculator, employing a deconvolution program making use of relevant electrical parameters such as the differential capacity of the DME and the effective leakage resistance at the time of the X-ray pulse. Specific capacity data are taken from the literature or obtained using the integrating m o d e of a modified differential pulse polarograph. Subsequently corrected coulostatic potential change--time data may be analysed more quantitatively using the calculator and iterative finite difference programs which take account of linear diffusion of electrochemically active intermediates in the presence of complications introduced by the slow reaction of intermediates in pairs within the solution. Quite generally the coulostatic change in interfacial potential is given by t

dE = (1/C~l)

f ~ 0

I=A,B etc.

niFDi|

"

|

\ Ox /x =0

.t

(i)

where the summation embraces all species involved in faradaic reactions at the electrode surface, ci is the concentration of species i and Di its diffusion coefficient, ni is the number of electrons involved in the destruction of reactant i by the electrode and C~ is the specific differential capacity. If the dose is uniform, if the one or more faradaic processes are virtually diffusioncontrolled and if there is no loss of any intermediate by reaction in pairs, or by reaction with any other intermediate, the signal is proportional to t 1~. Unfortunately this case is the exception rather than the rule. In the work done so far the radiation dose has been adjusted to give at t = 0 initial concentration of the main primary radiation products in the vicinity of 1.5× 10 -6 M. The observed yields {calculable when Fe(CN)~- is present in N2 O-saturated 0.5 M Na2 SO4 ) are somewhat larger than those estimated from dose measurements made using a LiF crystal fluorimeter (with the crystal in the position of the DME). In view of the difficulty in mea-

129 suring doses accurately at high X-ray intensity and geometrical uncertainties we do not attach great significance to this discrepancy, but there is a possibility nevertheless that the dose in the thin layer of solution bounded by the electrode surface, in which originate the intermediates important in the present context, may be enhanced by the degradation within the mercury drop of high energy X-rays and the concomitant emission into the solution of readily-absorbed low energy X-rays. However, there is currently no evidence to suggest that for the thin layer of importance the dose is appreciably nonuniform (see Fig.3 curve b where the signal is proportional to t lh with fair accuracy). Of course, some radiation-induced interfacial charge transfer during the X-ray pulse must occur (such as emission into solution [2] of excited conduction band electrons and electron transfer to the electrode resulting from photo-ionisation of highly charged or easily ionised anions). With Fe(CN)~- present in solution there is indeed some slight evidence for such photoionisation, but it seems unlikely that radiation-induced interracial charge transfer produces after-effects of importance at elapsed times larger than 10 ps. Though this topic is a slightly grey area calling for clarification, we have assumed that charge transfer during the X-ray pulse can be ignored in the analysis and interpretation of data for t > 10 #s. RESULTS AND DISCUSSION Figure 1 shows results for 0.5 M Na2 SO4 solution saturated with N~ O employing timebase velocities of 20 and 5 ps/divn. For this system e~q is rapidly converted to OH by reaction with N 2 0 and thus the observed potential change is caused almost entirely by the diffusion of OH to the electrode surface, where it is instantaneously (for the present measurement timescale) reduced to OH-. A little H2 O2 is formed as a primary product [3] by OH combination within the "spurs", and more is formed subsequently by the homogeneous combination in pairs of OH radicals, but the contribution of H2 02 to the observed potential change only becomes important at potentials more negative than -- 0.9 V vs. SCE. However the homogeneous loss of H~ 02 is the main cause of the slow rate of change of potential in Fig. 1 at large elapsed time. Electronic noise for the conditions to which Fig.1 refers is barely detectable and the use of somewhat smaller doses should prove feasible in the future. Measurements are possible at elapsed times down to about 3 ps, the limiting factor here being not low signal:noise ratio but residual interference from the Febetron circuits. Figure 2 shows the distortion of the waveform produced by the use of quasi-coulostatic rather than perfect coulostatic conditions. The two curves refer to potentials at which the differential capacities are almost identical and, in addition to the ultimate slow exponential decay of the waveforms connected with the finite input impedance of the preamplifier, there is an appreciable difference in signal size caused by the much more rapid reduction at --1.25 V of H2 02 formed initially and more gradually by OH reaction in pairs. The signals observed at potentials more negative than -- 0.9 V tend to be too complex in origin for quantitative analysis.

130

I (b.) I - - -

_

/-1.25V

....,.+.-----

~ .,/f'

2

(al Time

-o.9~

~

~

~,,..

Time

Fig.1. Oscilloscope traces for 0.5 M Na 2 SO, saturated with pure N 20. 20 mV/vert, divn.; (a) 5, (b) 20 us/hor, divn.; potential -- 0.8 V vs. SCE. Fig.2. Oscilloscope traces for the Fig. 1 system. 20 mV/vert.divn.; 200 ~s/hor. divn.; slightly smaller dose than for Fig.1 data, potentials of -- 0.9 and -- 1.25 V vs. SCE.

Determination of DOH The diffusion coefficient of the OH radical can be determined by comparing the waveform obtained with N2 O-saturated solution with the corresponding waveform obtained after stoichiometric conversion of OH to Fe(CN)~- by adding to the solution a sufficiently large amount of Fe(CN}~- ion to produce rapid conversion, b u t insufficient to affect appreciably the differential capacity of the DME. Due allowance must be made for OH combination in pairs when making such comparisons. In Fig.3 are given experimental points and c o m p u t e d potential change curves before and after the addition of 10 -3 M Fe(CN)~-. The experimental data have been corrected for waveform distortion of instrumental origin. In the case of the c o m p u t e d curves were taken cOH = CFe(CN)53- = 2 . 8 5 1 0 -~ M at t = 0, DFe(CN)~- = 0.89× 10 -s cm 2 s-' [4], DOH = 2.1× 10 -s cm 2 s-' and for the reaction OH + OH

ka

H2 02

2ka = 1.1× 101° M -1 s-1 for 0.5 M Na2 SO4 medium, a value determined by us using the normal pulse radiolysis m e t h o d which agrees well with the low ionic strength values given in the literature [5]. It will be noted that, using the calculated values of COH and CFe(CN)~- , and an assumed value for DOH, the agreement between experiment and theory is quite good. The results of a large number of experiments employing potentials in the range 0.0 to -- 1.0 V vs. SCE show that good agreement between experiment and theory only can be obtained by using values for DOH appreciably larger than the photoelectrochemical value of (1_+0.3)× 10 -s cm 2 s-1 reported recently by Benderskii and co-workers [6]. The present work suggests that DOH = (2.0+ 0.3)× 10 -s cm 2 s - ' , but it should be mentioned that this conclusion rests heavily on the accuracy of the values taken for COH at t = 0 and ka. Figure 4 shows the dependence on potential of the change in double layer charge (corrected only for waveform distortion} for t = 25 ps, supposedly constant dose, and with, and without, Fe(CN)~- present in 0.5 M Na2 SO4 solution saturated with pure N 2 0 . There is some scatter in the experimental points, especially with Fe(CN)~- present, b u t the results for OH reduction

(a)

131

500

.~

> 2so

0

,

,

,

=

I 50

,

,

~ ( a )

,

,

I 100

,

,

,

,

t//Js

I 150

,

,

,

,

,I 200

Fig.3. Observed and calculated changes in interfacial potential with time for 0.5 M Na= SO, saturated with pure N: O. (a) before, (b) after making the solution 10 -3 M in K 4 Fe(CN) 6 .

0

Fe (CN}s

z-

=,OH e.,

~6 •

0 v

A

0

•

0

o

W/* n," <3 "r C)

•

o

o

o

0

0

n o

0

2

I

0

I

i

I

I

-0

i

~

5

E/V

I

J

I

-1 vs.

J

0

SeE

Fig.4. Change in double-layer charge for t = 50 ps before and after making the solution 0.5 M in Na~ SO 4 (saturated with pure N~ O) and 10-3M with respect to K 4 Fe(CN)~. Data only corrected for signal decay caused by finite input impedance of preamplifier.

support the view that this process is diffusion-controlled t h r o u g h o u t the studied potential range, a point not made clear by earlier experimental work [ l b ] on radiation-produced radicals. It also can be shown that the apparent rate constant for Fe(CN)36- at potentials in the vicinity of -- 0.05 V vs. SCE must be of the order of 1 cm s -1 , if the results in Fig.4 are to be believed. It seems likely that the quasi-coulostatic m e t h o d in the future should prove a powerful tool for the study of the electrochemical properties of radicals readily formed by OH radical reactions (e.g. CO2-, TI÷, CNS etc.), especially if electrical interference can be reduced (possibly by using a Lineac rather than a Febetron).

132

Iteterogeneous destruction o f H at mercury Of particular interest are the results obtained with acidified solutions as the present method, despite difficulty connected with OH radical formation, makes it possible to study the kinetics of the oxidation (ionisation) on mercury of adsorbed H atoms formed homogeneously initially by the reaction of e~q with hydrogen ions. Photoelectrochemical methods [2,6,7,8] cast little light on this topic as electron emission induced by light is difficult to produce at a positively charged mercury surface. In Fig.5 are shown some preliminary results obtained with 0.5 M Na2 SO4, 2× 10-2M H2 SO4 solution at potentials in the range -- 0.1 to --0.8 V vs. SCE. Throughout this potential range the coulostatic potential change is a composite signal containing a positive c o m p o n e n t due to OH radical reduction as well as the c o m p o n e n t produced by charge transfer reactions involving H atoms diffusing from the bulk of the solution. In harmony with conclusions drawn from photocurrent studies [2,7] in recent years is the obvious change in the sense of the latter c o m p o n e n t as the potential is made more positive, connected with the gradual change in the fate of the H atoms from reduction [2] (electrochemical desorption [7] ) to molecular hydrogen by

Had+H30 ÷+e-

-*

(b)

H2 + H 2 0

or

Had + H 2 0 + e-

-*

H2 + OH-

(c)

to oxidation [2] to hydrogen ions (ionisation [7] ) Had

H30 ÷+e-

-*

(d)

That this change in mechanism can be easily seen in the form of a change in signal sense, despite the presence of the OH reduction component, is due to

f

f

V vs. SOE -08

~

-07

-

/

-06

~f

"~ ---- -- -'- ~

-~ ~ , - -

~=--

-0.3 0-2

-- -01

I Time

Fig.5.

Oscilloscope traces

20 mV/vert,

for 0.5 M Na 2 SO 4 , 2x 10 -2 M Hz SO 4 saturated

divn.; 20~s/hor.

divn.

with argon.

133 the fortunate, but fortuitous, accident that DH is much larger than DOH. It has long been believed [2] that DH must be relatively large and the results in Fig.5, although n o t easy to interpret quantitatively due to the many complications of the system, suggest that DH must be a b o u t four times larger than DOH, assuming H atom destruction by the electrode to be diffusioncontrolled at the extremes of the potential range. This conclusion agrees roughly with results reported by Benderskii and co-workers [6]. H atom reduction clearly tends to be largely diffusion-controlled for potentials more negative than -- 0.6 V vs. SCE, but it would seem likely that at the more positive potentials the approach to diffusion-controlled oxidation (supposedly by (d)) is quite gradual. Thus at a potential such as -- 0.3 V, where it is known [2,7] that most of the H atoms are consumed by oxidation rather than reduction, the half-life of H at the electrode surface may be still quite large (> 10 ps) and the apparent transfer coefficient (ca. 0.2) finite. If these tentative conclusions, which seem difficult to reconcile with barrierless ionisation [9], stand the test of more detailed experimental study, it may be necessary to modify (d), introducing perhaps a rate-controlling chemical step Had + H 2 0

-~

H30

(e)

prior to ionisation of H 3 0 (homogeneous ionisation of H 3 0 may follow capture of eaq by H3 O+). We shall later be reporting the results of a more detailed study of the electrochemical reactivity of H at mercury. It should be mentioned that Henglein and co-workers [ l e ] have studied in detail, by a galvanostatie method, the electrochemistry of the H atom. Their results are seemingly somewhat different from those briefly discussed above, due we suspect to the use of relatively enormous radiation doses and the occurrence of appreciable faradaic current due to H202 reduction, despite the use of H2 at high concentration for the purpose of destroying OH. ACKNOWLEDGEMENTS

We are indebted to the C.N.R. (Italy) for financial support, to M. D'Aangelantonio for experimental assistance and to Prof. G. Semerano for active support of the work. One of use (G.C.B.) is indebted to the School of Chemistry, Bristol for its generous hospitality. REFERENCES l(a) (b) (c) 2 3 4 5 6 7 8 9

J. Lihe, G. Beck and A. Henglem, Ber. Bunsenges. Phys. Chem., 75 (1971) 458. M. Gr~itzel, K.M. Banzel and A. Henglein, in Radiation Research, O.F. Nygaard, H.I. Adler and W.K. Sinclair (Eds.), Academic Press, New York, 1975. P. Toffel and A. Henglein, Far. Soc. Disc., 63 (1976). G.C. Barker, D.C. S a m m o n and A.W. Gardner, J. Electrochem. Soc., 113 (1966) 1182. A. Kuppermann, in R a d l a t i o n Research 1966, G. Silini (Ed.), North-Holland, Amsterdam, 1967. I.M. Kohltoff and J.J. Lingane, Polarography, Interscience, New York, 1952, p.52. M. Anbar and P. Neta, Int. J. Appl. Rad. Isotopes, 18 (19677 493. S.D. Babenko, V.A. Benderskii and A.G. Krivenko, J. Electroanal. Chem., 84 (1977) 33. Z.A. Rotenberg, V.I. L a k o m o v and Yu.V. Pleskov, J. Electroanal. Chem., 27 (1970) 403. G.C. Barker, D. McKeown, M.G. Williams, O. Bottura and V. ConcialLni, Disc. Faraday Soc., 56 (19747 41. L.I. Kristalik, E l e k t r o k h i m i y a , 4 (1968) 877 (in Russian).