Investigation of Intermediates by Electron Photoemission from Metal into Electrolyte Solution BY Yu. V. PLESKOV, Z. A. ROTENBERG, V. V. ELETSKY, AND V. I. LAKOMOV Institute of Electrochemistry, Academy of Sciences of the U.S.S.R., Moscow Received 11 th June, 1973 During electron photoemission from an illuminated electrode into an electrolyte solution con- taining scavengers of hydrated electrons, stable and unstable products from reduction of the scaven- ger are formed in the vicinity of the electrode, which, at the same time, are intermediates of some electrochemical reactions. Thus, electron photoemission into acid solutions produces atomic hydrogen, that into nitrate solutions-the ion-radical NO;-. In this type of experiment photo- emission can be used, on the one hand, as a convenient source of intermediates and, on the other, as an instrument for measuring the rates of their chemical and electrochemical reactions.Investigation of the simultaneous ionization and reduction of atomic hydrogen at a mercury electrode has shown one of the two reactions to be activationless (the transfer coefficient is zero). The rate constant of the decomposition of the unstable ion-radical NO$- in the solution bulk has been measured, as well as the transfer coefficient of its anodic oxidation on mercury. The photoemission of electrons from metal into solution is accompanied by a number of processes associated with formation and further transformation of the solvated Among these, of great interest are the homogeneous and het- erogeneous reactions involving free radicals.The kinetics of these reaction affect the value of the measured photocurrent so that photoemission may be used as a convenient technique of quantitative study of such processes. The experiments described below were conducted, mainly using a mercury electrode which was illuminated by ultra-violet light from a high-pressure mercury lamp. The experimental techniques are described in detail in ref. (2) and (3). KINETICS OF ELECTROCHEMICAL TRANSFORMATION OF ATOMIC HYDROGEN The properties of atomic hydrogen have attracted the attention of scientists for a long time. This is mainly due to the fact that the atomic hydrogen is an intermediate product of one of the best-studied reactions : electrochemical hydrogen evolution. Hydrogen atoms are formed during photoelectron emission into acidic aqueous solutions as a result of a capture by hydrogen ions of hydrated electrons : Hydrogen atoms either recombine or approach the metal surface and enter into electrochemical reactions.The first mechanism of hydrogen removal from solution is, apparently, not effective since recombination, being a second-order reaction, is very slow when the concentration of hydrogen atoms is low. Therefore, practically all the atomic hydrogen is removed by an electrochemical mechanism. show that, in a wide range of potentials, e,; + H,O++H + H20. (A) Studies using a mercury electrode 52YU. V. PLESKOV, Z. A. ROTENBERG, V. V. ELETSKY, V. I. LAKOMOV 53 hydrogen atoms preferentially enter into a reaction of the electrochemical desorption type H + HS + e--+H, + S-.(B) In the above equation, HS is a source of protons, that is, hydrogen ions and water molecules in aqueous solutions. At more positive potentials, as well as this process, there is a possibility of a reaction of atomic hydrogen involving ionization H + H20+H,0++e- (C) which brings about a decrease of the measured photocurrent. This reaction was detected in experimental studies of photoemission from mercury by Barker et al. and quantitatively studied in detail in ref. (4). It should be noted that, in this case, photoemission is an effective source of atomic hydrogen near the electrode surface and that such a source proved to be more convenient than the earlier ones 5-7 because of its simplicity and the possibility of con- trol of hydrogen influx rate which it provided.KINETIC EQUATIONS FOR THE ATOMIC HYDROGEN REMOVAL Taking into account the reactions (B) and (C), we may define the experimentally measured photocurrent j by the following equation : + t j = h + j ~ - j ~ wherej, is the emission current Iless the current I.. produced by return to the electrode of the electrons which were not trapped by scavengers in the solution ; jH is the cathode current of the reaction (B) andjH is the anode current of ionization (C). Essentially, there are two possible mechanisms for the reactions (B) and (C)-one involving adsorption of hydrogen atoms on the surface, and the other directly from solution, by-passing the adsorption stage. If the reaction proceeds simultaneously by both mechanisms the following relations are valid under stationary conditions : -* 4- - t i - + c j = j , + (k, - kI)cH(0) + (k2 - k2)0. (24 In the above relations, kads is the adsorption rate constant expressed, as the other kinetic constants, in electrical units; to a first approximation it does not depend on the potential; k, and k, are, respectively, rate constants for cathode and anode removal of atomic hydrogen directly from the solution; k , and k2 are the same constants for adsorbed hydrogen ; ~ ( 0 ) is concentration of hydrogen atoms near the electrode, and 0 is the surface coverage by adsorbed hydrogen atoms.The rela- tions (2) do not take into account recombination of adsorbed hydrogen atoms on the electrode surface : this is justifiable for a mercury electrode5 If the reaction (B) proceeds with simultaneous participation of water molecules and hydrogen ions, the rate constants k, and k2 may formally be divided into two components: ki = kl+ Xky, where kf and ky are, respectively, rate constants for hydrogen removal + 4- 4- -+ -+ -+ -b + - + + +54 ELECTRON PHOTOEMlSSlON involving water molecules and hydrogen ions and Xis the molar fraction of H30+ in the solution.By eliminating 8 and cH(0) from the relations (2) we obtain the following equation for the measured photocurrent : r + -+ 1 kl kzkads I + + + -+ c + c j = 2j0 (3) In the limiting case, when the oxidation reactions may be neglected, all the atomic hydrogen is reduced on the electrode so that the measured photocurrent is twice the emission current ( j = 2j0) and its relationship with potential 4 is defined by the “ 5/2 law ” : joc($-$o)5/2, where #o is the photoemission threshold.In the general case, since the constants in eqn (3) are themselves functions of the potential, the relationship between j and 4 is complicated and does not allow for direct experi- mental verification. It is expedient to consider the limiting cases for comparison between theory and experiment. (a) The reactions (B) and (C) proceed by-passing the adsorption stage (/cads = 0). Then -+ k1 j = 2j0 . t kI+ kI + t The relationship between constants ki, ki and potential may be represented by the following equations : ki = klo exp( - a,F+/RT), & = kc,, exp(PiF4/RT), + + where ai and Pr are the transfer coefficients for the appropriate reactions. Then the relationship between j and 4 may be written as (b) Atomic hydrogen enters into both reactions kl = 0).From the eqn (3) we obtain 4- (4) -+ only as an adsorbed particle (k, = (c) The reaction (B) proceeds through the adsorption stage while the reaction (C) proceeds directly from the solution (k, = k2 = 0), then - + c + = - 2.3RT “og7+log*]. 2jo-j k1o PIF ( d ) The reaction (B) proceeds bypassing the adsorption stage while the oxidation of hydrogen proceeds via the absorption stage (k, = kI = 0). 4 4 - In this case we obtainYU. V. PLESKOV, Z . A. ROTENBERG, V . V. ELETSKY, V . I. LAKOMOV 55 The eqn (4)-(7) exhaust the real mechanisms for removal of atomic hydrogen. These equations are similar to the kinetic equations for slow discharge, with the only difference that instead of discharge current we have dimensionless parameter (2j0 -j)/’ which is equal to the ratio of oxidation current jH to reduction current j,.The structure of the eqn (4)-(7) provides for linear relationship between 4 and 10g(2j0 - j ) l j with the line slope defining transfer coefficients for the corresponding reactions. -+ c RELATIONSHIP BETWEEN T H E KINETICS OF HYDROGEN REMOVAL, POTENTIAL A N D COMPOSITION OF THE SOLUTION Fig. 1 presents the (photocurrent, potential) relationship for a mercury electrode plotted injoa4, 4 coordinates for three solutions with various acid concentrations, but with constant (1 N) overall electrolyte concentration. For more acidic solutions the potential, which corresponds to the beginning of the departure from linearity arising from hydrogen oxidation, is somewhat shifted to the more positive values.With specific adsorption of halogen ions, particularly iodine, the reaction of hydrogen oxidation is accelerated in relation to the reaction of its cathode reduction. Adsorp- tion of tetrabutylammonium cation is accompanied by a reverse effect: drop of photocurrent in the potential region studied disappears. 2 x ‘I I -0.5 - 1.0 4lV FIG. l.-(joe4, 4) plot for the mercury electrode in K2S04+ H2S04 solutions. Concentrations of H30+ : 1-10-3, 2-10-2, 3-lo-’ N. Current is expressed in arbitrary units, potential is determined with respect to the saturated calomel electrode. On a bismuth electrode, the drop of photocurrent associated with hydrogen oxidation begins at more negative potentials (-0.8 V) which may be due to a higher energy of hydrogen adsorption than in the case of mercury.Photocurrent decay was not found on a lead electrode (at 4 < -0.7 V) and on a cadmium electrode (at For the experimental verification of the eqn (4)-(7) it is necessary to know, besides current j , also current 2j0 in the absence of atomic hydrogen oxidation. This current may be determined by extrapolating the linear portion of the joS4, 4 plot into the potential range where experiments show departure from linearity, or by measuring photocurrents with other scavengers whose products of interaction with hydrated electrons are not oxidized on the electrode. This requirement is met, for example, by 4 < -0.9 V).56 ELECTRON PHOTOEMISSION nitrous oxide which produces after electron capture, the OH radical.This radical is reduced on the electrode in the whole potential range under consideration. Fig. 2 presents the experimental (4, 10g[(2j0 -jib]) plots which are, actually, straight lines for all the solutions studied thus indicating the validity of the eqn (4)-(7). > 2 -0.5 - 1 L I 0 I log(2jo -.M FIG. 2.-( 4, 10g(2j0-j)/j) plot for the mercury electrode in various solutions. The concentration of H30+ is 0.01 N. 1,0.9 N K,SO,; 2,O.l N KCI; 3,O.l N KBr; 4,O.l N KT. THE MECHANISM OF ATOMIC HYDROGEN REMOVAL On the basis of the above information we concluded that the mechanism of hydro- gen removal via the adsorption stage is the most realistic since the reactions of ioniza- tion and reduction of hydrogen directly from the solution in the studied potential range should be activationless (that is, their rates do not depend on the potential) and, thus, cannot explain the photocurrent decay observed in the experiments.A similar conclusion was made by Barker who studied photocurrents in acidic solu- tions in the presence of ethanol. We now consider the influence of pH on the photocurrent. For the quantitative study of the effect of pH it is convenient to introduce the potential 4* at which cathodic current is equal to anodic current, that is, (2j0 - j ) / j = 1. The potential 4* may be determined by the point of intersection of the nonlinear portion of the experi- mental curve ( j o e 4 , 4) with a straight line passing through the extrapolation point (&) with slope which is 20a4 times less than the slope of the experimental curve.Fig. 3 presents $* as a function of concentration of hydrogen atoms c ~ ~ ~ + in the concentra- tion range from 0.001 to 1 N. The potential 4* is practically independent of pH in solutions with low acidity (<0.01 N). When acidity of the solution is increased, the potential 4* rises steadily, and its greatest change is observed for the high values of concentration cH30+. The marked influence of 4* on pH indicates that the reaction of electrochemical desorption simultaneously involves both hydrogen ions and water molecules. Krishtalik lo was the first to suggest the possibility of adsorbed hydrogen removal by electrochemical mechanism via water molecules in acidic solutions. The observed ($*, pH) dependence might formally be explained by suggesting intermediate formation of H i radical in acidic solutions ; this radical is then electro- chemically reduced.However, the reaction rate constant for the interaction between H and H30+ is so low that this reaction’s contribution may be neglected.YU. V. PLESKOV, Z . A. ROTENBERG, V . V. ELETSKY, V . I. LAKOMOV 57 - 0.61 I i * I * I - 0.5 I > 1 ,c2 16’ t CH30+ /mol I.-’ FIG. 3.-Variation of (b* with concentration of H30+ (mercury electrode). Thus, the reactions involving atomic hydrogen are represented by the following scheme : It follows from the slope of the 4, 10g[(2j0-j)/’] straight lines of fig. 2, that is equal to 100-120 mV, that a+p = + for two simultaneous reactions (B’) and (C’) if eqn ( 5 ) is valid.The transfer coefficients for electrochemical reactions involving adsorbed atomic hydrogen are usually equal to 3. Hence, the observed value of 3 for the sum of transfer coefficients for the two reactions indicates that one of these reac- tions is activationless (transfer coefficient is zero), that is, its rate is potential-independ- ent. It has not yet been decided definitely which one of the reactions is activationless. There is some evidence suggesting the activationless character of the first as well as of the second reaction. Thus, it should be noted that the ionization of adsorbed hydrogen is the reverse reaction in relation to the discharge of hydrogen ions. If the ionization is activation- less, the discharge of H,O+ should proceed in the same potential range as a barrierless reaction. According to Kri~htalik,~.O who studied the barrierless discharge of hydrogen on mercury, the transfer from the usual discharge to a barrierless one occurs at the potentials more positive than -0.5 V. Therefore, in the potential range where we observe the photocurrent decay, the ionization should be an activation reaction, so that the electrochemical desorption should be activationless. However, direct comparison of experimental data for hydrogen ion discharge with the results of photoemission experiments is not rigorous since in these two cases the hydrogen at oms are probably not energetically equivalent. The comparison of photocurrent magnitudes at -0.4 V for the two reactions studied, shows that at this potential the electrochemical desorption is tens of times slower than the hydrogen oxidation.At the more positive potentials the photocurrent58 ELECTRON PHOTOEMISSION practically disappears, that is, all the atomic hydrogen is oxidized. This ratio of rates for the two reactions indicates that the activationless reaction is the hydrogen ionization rather than desorption. The specific adsorption of ions does not influence the rate of activationless ionization since the latter with p = 0 does not depend on the structure of the electric double layer. For the activated electrochemical desorption, the change of $'- potential due to adsorption of, for instance, iodine, gives rise to two opposite effects : decrease of the discharge rate and increase in the vicinity of the electrode of hydrogen ion concentration involved in the process.The experimental results demonstrate the prevalence of the former effect. This is due, particularly, to the participation of water molecules in the reaction of electrochemical desorption. Although we were not able to establish which one of the two above reactions is activationless-the ionization of atomic hydrogen or its reduction, the very fact of experimental observation of an activationless process is of great importance by itself. It should be noted that the concept of activationless processes has existed in electro- chemistry for a comparatively long time 11* l 2 but up till now it was not possible to observe directly the limiting currents of activationless discharge. For this observation it is necessary to overcome diffusion limitations which are essential for high discharge rates.The photoemission permits one to by-pass these difficulties, since in this case we measure not the rate of an individual process, but the ratio of rates for two con- current processes involving the same material (atomic hydrogen) so that the diffusion difficulties are mutually cancelled out. ELECT R 00x1 D AT1 ON AND H 0 M 0 GENE 0 US D E CO M PO S I T TO N The chemical reactions accompanying capture by anion NO, of a hydrated OF ION-RADICAL NO$- electron may be represented by a following scheme : k r N O j +e&-+NO:- (in solution) kv NO:-+ H,O-+NO; + OH+OH- (in solution) keA NO $ -+NO, + e- (on electrode) OH + e-+OH- (on electrode). (GI The oxidation of NO:- is accompanied by a photocurrent decrease.'.9 * 13-15 the measured photocurrent with electron capture by NO; : where ZA is the oxidation current for NO;-. The factor 2 indicates that each act of decomposition of NO;- in the solution is accompanied by transfer across the interface of one additional electron due to OH reduction. The oxidation current is determined by the relation IA = k e A c ( 0 ) , where c(0) is the concentration of NO:- in the vicinity of the electrode, keA is the oxidation rate constant for NO:-, which is potential- dependent as for any electrochemical reaction : Diffusion and homogeneous reactions of formation and decomposition of the anion- radical NO : - may be represented by the following equation : Taking into account the reactions (D)-(G), we obtain the following equation for j = 2(I- 1, - I A ) (8) k e A = k t ~ exp(PF$/RT). Dd2c/dx2 - kvc + kACACe = O (9)Y U .V. PLESKOV, Z . A. ROTENBERG, V . V. ELETSKY, V . I. LAKOMOV 59 where D is the diffusion coefficient for NO:-, k , is the rate constant of NO$- homo- geneous decomposition, CA and kA are NO; concentration and its rate of reaction with hydrated electrons of concentration c,. Thus, the product kAcAce defines the rate of NO:- formation in solution. This equation, together with a similar equation for hydrated electrons, allows one to determine the current I A . Integration of eqn (9) with boundary conditions c(00) = 0, keAC(0) = D(dc/dx),,o yields the following expression : where Q = (kAcA/De)) and Q, = (kv/De)* ; the function @(x) defines deposition rate of hydrated electrons in solution, and De is the diffusion coefficient of hydrated electron.The final expression for the photocurrent is obtained ‘ 9 l6 by substituting (10) into (8) and taking into account the expression for I, (for the case of infinitely fast capture of hydrated electrons by the electrode surface) : At very negative potentials when keA < QvD (that is, NO:- oxidation may be practically neglected) j = 2(I-Ie)= In the reverse limiting case (keA>> QvD) we obtain dx} . In both cases the photocurrent is proportional to the emission current, that is, its potential dependence is defined by the “ 5/2 law ”, to a first approximation. The experimental study of this reaction demonstrated both of the discussed limiting cases on the actual (photocurrent, potential) curve for mercury and bismuth elec- trodes 13* l4 (fig.4 and 5). The linear portions at high and at low potentials in 4 FL - - 2 - - I - r I H’ - 0.5 - 1.0 - 1.5 4lV FIG. 4.-(j0-4, 4) plot for the mercury electrode in 1 N KN03 solution (1) and the same solution with addition of tetrabutylammonium bromide (2).60 E LECT R 0 N P H 0 TOE M I S S I 0 N ( j o a 4 , 4) coordinates are separated by the region of monotonous decay of the photo- current (from -0.7 to - 1.2 V) as may be seen on fig. 4. Addition to the solution of small amounts of tetrabutylammonium ( mol l.-I) produces a characteristic change of the shape of the curve. At potentials more positive than - 1.4 V (desorp- tion potential of tetrabutylammonium) the curve becomes practically a straight line.FIG. 5.-(jos4, - 0.5 -1.0 4lV 9) plot for the bismuth electrode in 0.5 N solution of KN03. It is evident that the adsorbed tetrabutylammonium produces two concurrent effects : emission current decrease and inhibition of NO:- oxidation (cf. inhibition of hydrogen oxidation discussed above), thus resulting in absence of photocurrent decay at positive potentials. Experimental data used with expression (12) made it possible to evaluate the rate constant k, for homogeneous decomposition of NO:- and the transfer coefficient p for its clectrooxidation. The rate conslant k , was deter- mined in sufficiently concentrated (0.5-1 .O N) nitrate solutions where practically all the hydrated electrons were captured. If the rate of NO:- decomposition in the solution is not too high, so that the inequality Qvxo< 1 is fulfilled (where xo is the mean length of electron hydration), the expression (12) yields : j = 21when+<-1.2V; j = 2xoQvZwhen~>-0.8V.p was found to be 0.25. By comparing the photocurrent values for these two limiting cases and, thus, determin- ing Q,, it is easy to obtain the rate constant k, which was found to be 5 x lo6 s-I. A value of k, of about the same order of magnitude was found using the photoemission technique with pulsed illumination of the electrode. According to ref. (17), if the NO, concentration is high, emitted “ dry ” electrons can react directly with this scavenger prior to their hydration. Therefore, if it is possible for the “ dry ” electrons to return to the electrode, the photocurrent in the NO, solution should be higher than in solutions of the scavenger that do not capture the “dry” electrons (for instance, H30+).However, this does not influence theYU. v. PLESKOV, z. A. ROTENBERG, V. v. ELETSKY, v. I. 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