Hydrogen chemisorption and related anion effects on Pt(110) electrodes

J. Electroanal. Chem., 151 (1983) 109-131 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

HYDROGEN CHEMISORPTION Pt(ll0) ELECTRODES

109

AND RELATED ANION EFFECTS ON

F.E. WOODARD *, C.L. SCORTICHINI ** and C.N. REILLEY*** Department of Chemtstry, Umversity of North Carohna, Chapel Hdl, NC 27514 (U.S.A.)

(Received 8th February 1982; in revised form 9th November 1982)

ABSTRACT The four primary states of hydrogen adsorption on polycrystalline platinum electrodes are assigned to sites characteristic of (100), (110), and (111) single crystal surfaces. The coupling between hydrogen adsorption and adsorbed sulfuric and hydrochloric acid anions is investigated. Time resolved staircase voltammetry is used to show the absence of a cathodic counterpart to the "third anodic peak". The results of electrochemical relaxation measurements following small potential step perturbations are given for Pt(110) in sulfuric acid.

INTRODUCTION F o r s o m e time, there has b e e n a c o n t r o v e r s y r e g a r d i n g the relative influence of a d s o r p t i o n site geometries a n d a d s o r b e d anions o n h y d r o g e n c h e m i s o r p t i o n at p l a t i n u m electrodes [ 1-4]. I n this p a p e r it is shown that b o t h factors are i m p o r t a n t . Investigations of h y d r o g e n a d s o r p t i o n on single crystal p l a t i n u m surfaces [5-18] i n d i c a t e t h a t to a large extent the c o n f i g u r a t i o n of surface p l a t i n u m a t o m s governs which states of h y d r o g e n a d s o r p t i o n are possible. T h e results r e p o r t e d here i n d i c a t e t h a t at least three different a d s o r p t i o n sites are p r e s e n t on p o l y c r y s t a l l i n e Pt electrodes. However, this does n o t m e a n that every p e a k o b s e r v e d in cyclic v o l t a m m e t r y necessarily c o r r e s p o n d s to a different a d s o r p t i o n site. In fact, we show that a single site can give two p e a k s if that site is b l o c k e d b y two different species. EXPERIMENTAL Single-crystal p l a t i n u m electrodes were p r e p a r e d b y melting Pt wire into a single-crystal b e a d a n d then etching, cutting, polishing, a n d a n n e a l i n g that b e a d as d e s c r i b e d in ref. 15. I m m e d i a t e l y after annealing, the electrodes were q u e n c h e d in * To whom correspondence should be addressed. Present address: Corporate Research Laboratories, Monsanto Company, St. Louis, MO 63167, U.S.A. ** Present address: Dow Chemical Company, Midland, MI 48640, U.S.A. *** Deceased. 0022-0728/83/$03.00

© 1983 Elsevier Sequoia S.A.

110 water and placed in an N2-saturated, unstirred supported electrolyte. The electrode potential was then held at 0.06 V for approximately 1 min prior to potential cycling. (Generally large cathodic currents were observed when the electrode was first inserted into the solution.) The electrode potential was then cycled into the oxide formation region several times to clean the electrode and obtain a reproducible current-potential profile in the hydrogen adsorption region. Most experiments were carried out in a glass cell previously described [15]. Experiments using H F as the supporting electrolyte were done in a two-compartment Teflon cell. The compartment containing the hydrogen reference electrode was connected to the main compartment via a Luggin capillary. The working electrode and a platinum foil counter electrode were located in the main compartment. The electrochemical cells were cleaned in hot H N O 3 vapors followed by several soaking periods in boiling distilled water. The soaking periods were effective in removing trace amounts of chloride and sulfate anions. Contamination by these anions was particularly troublesome when doing experiments in 0.01 M H F or HClO 4. Electrolytes were prepared in deionized distilled water using concentrated HC1, HF, H2SO 4 (Mallinckrodt Analytical Reagent), and HC104 (70%, double distilled from Vycor, G. Frederick Smith). Data acquisition was performed using a U.N.C. microcomputer equipped with high speed analog I / O modules: Track-and-hold amplifiers, Fast analog-to-digital converter, Quad digital-to-analog converter, and an i n p u t / o u t p u t sequencer [19]. The design of the sweep generator was published previously [20]; however, the circuit was modified to allow the computer to initiate and terminate potential sweeps. The three-electrode potentiostat was constructed "in house" using Teledyne Philbrick 1011 operational amplifiers. Potentials were measured relative to a hydrogen electrode in the same solution as the working electrode. When appropriate, peak potentials were determined by averaging the values obtained during negative and positive scans. RESULTS AND DISCUSSION

Significance of electrode surface structure in determining hydrogen adsorption states on Pt As shown in Fig. l(a), four hydrogen adsorption peaks can be distinguished in CVs of polycrystalline Pt in 0.005 M H z S O 4. Further reduction of the acid concentration to 0.0005 M or using acids with more weakly adsorbing anions (e.g., 0.01 M HC104 or 0.01 M HF) did not reveal additional peaks. These four peaks were observed by Huang et al. [2]. Conway et al. [1] also found four primary hydrogen adsorption peaks, but they noted that two more peaks could be observed under certain conditions. The contribution of various crystal faces to polycrystalline cyclic voltammograms can be ascertained by comparing peak potentials in steady state, hydrogen region

111

CVs for various Pt electrodes (Fig. 1). Peaks II and III can be attributed to a (110) crystal face and peak IV to a (100) crystal face. Peak I appears to be the primary peak found on (I I I) electrodes, but it is also present in CVs of (1 I0) electrodes (cf. with work by Ross [6-9], Clavilier [10-12], Hubbard [13], Yamamoto [14], Yeager [ 18] and their co-workers). Peak III was found in cyclic voltammograms of Pt(110) electrodes but not in CVs of Pt(100) or P t ( l l l ) electrodes, unless those electrodes had been subjected to extensive cycling in the oxide formation region. These observations are in agreement with the results reported by Yamamoto et al. [14] and Clavilier et al. [10.21]. Will [5] suggested that peak III was due to hydrogen adsorption on a (111) crystal face, but

~/pAcm -2 _

•

S~

-5~

)

Poly

II

Pt

IV

i/pAcm-2 75

IV Pt(t00)

5~

-

-25 -5~

-I~D

~o'

[bl

I/pA C m -2

Pt (110)

d#Acrn-2 I I

Pt (111)

1~

E 5~

2S

"" IV

~E/V vsRHE - 25

I~ i ~

O~

-5~ -7S

I!

-i~I~

Fig. 1. Cyclic voltammogramsfor: (a) polycrystalline,(b) (100), (c) (111), and (d) (110) Pt electrodes in 0.005 M H2SO4; sweep rate 50 mV s - l . ( . . . . . . ) less than 15 cycles to positive linut (1.5 V) since single-crystalelectrodes were annealed. Then (. • ) second or third cycle after reducing positive limit to approximately0.6 V (except for the polycrystallineelectrode: 15th cycle to 0.6 V).

112 his single crystal electrodes apparently restructured as a consequence of potential cycling. Hubbard et al. [13] also observed peak II1 with a P t ( l l l ) electrode; but we suspect this peak was due to sites not characteristic of the (111) surface. Those sites may have been on the edges of their parallelopiped shaped electrode or on the supporting polycrystalline wire. In this work the dipping technique [22] was used to insure minimal edge effects. Another way to compare a single-crystal electrode to a polycrystalline one is to cycle the potential of the single crystal electrode into the oxide formation region many times. As shown in Fig. 2 for the Pt(ll0) electrode, it appears that the hydrogen adsorption peaks measured during consecutive cycles reflect the transformation of the single-crystal surface into a polycrystalline state. This polycrystalline state is apparently the consequence of a superficial restructuring because annealing the electrode in a natural gas-air flame for 10 min, essentially restores the electrode to its original state. Similar behavior is also observed for P t ( l l l ) and Pt(100) electrodes. From the data given above, it appears that adsorption states I, II, and IV require different electrode surface geometries (i.e., different adsorption sites); however, it does not follow that surface geometry is the only difference between the adsorption states (e.g., see ref. 23). The assignment of states II and III to separate adsorption sites is not so straightforward because those two states seem to be interrelated. This is discussed later.

Influence of anions on peaks I and I1 In this section, anion adsorption is studied indirectly by noting the influence of anions on two hydrogen adsorption states evident in cyclic voltammograms for i/~uAcm-2

II i~

Fig. 2. Cyclicvoltammogramsfor Pt(110)in 0.005 M H2SO4; sweep rate 50 mV s- I; positivelimit 1.5 V. ( ) after 10 cycles; (. . . . . . ) after 100 cycles; ( . . . . . . ) after 350 cycles.

113 i/pAcm-2

2~

i,,--~

~ V v s RHE

Fig. 3. Cyclicvoltammogramfor Pt(110) in 0.01 M HF; sweep rate 20 mV s ~. Tenth cycleafter reducing positive limit from 1.5 V to 0.5 V.

Pt(ll0) in acidic supporting electrolytes (i.e., peaks I and II in Fig. 3). Several comparisons are made between this work and previous studies of anion adsorption on polycrystalline electrodes. However, it seems likely that contact anion adsorption is crystal-face dependent just as hydrogen chemisorption is (see work by Horhnyi et al. [24,25] and Schuldiner et al. [26]). Therefore those comparisons must be viewed with a critical eye, since the same anion adsorption sites may not be present on both electrodes. Throughout this paper, the anion adsorbed from sulfuric acid solutions is considered as sulfate. The species actually adsorbed may be bisulfate, sulfate, or both. Peak I - - i n s e n s i t i v e to anions

I n numerous supporting electrolytes the potential of peak I is not affected by anions. This peak occurred at 0.128 V (steady state, hydrogen region CVs) in: 0.01 M, 0.1 M, and 1 M HC104; 0.01 M and 0.1 M HF; and 0.005 M H2SO 4. In 0.05 M and 0.5 M H2SO 4 and low concentrations of HC1 (e.g., 10 -6 M ) the overlap of peaks I and II made an exact potential determination difficult, but peak I did not appear to be shifting. This implies that these anions are not adsorbed on peak I adsorption sites at potentials where hydrogen adsorbs. (In the electrolytes listed above, the potential of peak I (vs. RHE) is also independent of pH.)

114

Hydrogen-hydrogen interactions evident from peak I The widths of hydrogen adsorption peaks in cyclic voltammograms reflect (among other things) the degree of lateral interactions between chemisorbed hydrogen species [27-29]. Peak I in 0.01 M H F (Fig. 3) or 0.01 M HC104 has two characteristics which makes it ideally suited for determining the extent of those interactions: (1) the hydrogen adsorption process does not have anion desorption coupled to it, and (2) peak I is a well resolved peak corresponding to a single adsorption state. After correcting for double layer charging (approximated with a linear extrapolation from the double-layer region), peak widths at half height were between 75 and 90 mV for 0.01 M HC104 and 0.01 M HF. Since a width of 90 mV indicates no interactions [28], the influence of other adsorbed species on a hydrogen species adsorbed in state I, must be very weak. This confirms the generally accepted notion that hydrogen ions are essentially completely discharged upon adsorption [29,30].

Peak H--contact adsorption The potential of peak II is dependent on the type and concentrations of anions in the supporting electrolyte. In the experiment shown in Fig. 4 the adsorption of sulfate from a solution of 0.01 M HC104 plus 5 x 10 -7 M H2SO 4 caused a - 5 0 mV shift in the potential of peak II. The quantity of sulfate adsorbed was regulated by varying the time the electrode was potentiostatted at 0.6 V prior to the negative scan shown in the figure. (The adsorption rate was limited by diffusion.) These results show that sulfate is adsorbed in preference to perchlorate. This implies that the attraction between the electrode surface and adsorbed sulfuric acid anions is probably more intimate than the electrostatic attraction between the electrode and anions in the ionic part of the double layer (i.e., sulfuric acid anions must be contact

104i/A cm-2

-OS

-2

3

_

~

1

-3 . . . . . . . . o'~ . . . . . . . o'5 . . . . . . . o ' 5 " o ' 4

.... " o ' £ "

....

E / V vs. RHE

Fig. 4. Cyclic voltammogramsfor Pt(110) in 0.01 M HCIO4 plus 5 × 10 7 M H2SO4; sweep rate 200 mV s- 1. Electrode pretreatment prior to cathodic scans shown in figure: 1.3 V for 10 s, 0.02 V for 10 s; 0.6 V for 10 s (curve 1), 40 s (curve 2), or 120 s (curve 3).

115

or specifically adsorbed). (Similar results were obtained when a trace amount of HC1 was used instead of H2SO 4, except the II(CI~ peak was approximately 40 mV more negative than the II(SO 2 - ) peak.) The contact absorption of sulfate and chloride anions on polycrystalline electrodes has been reported previously. On the basis of radiotracer studies of sulfate and chloride adsorption, conducted in the presence of an excess of a more weakly adsorbing anion, Hor~nyi et al. [25,31] concluded that both sulfate and chloride anions were contact adsorbed. In other radiotracer work [32], those anions were found to adsorb superequivalently--also implying contact adsorption.

Peak H--anion blocking Hubbard [13] and Huang [2] and their co-workers have suggested that anions affect hydrogen adsorption by competing with hydrogen ions for adsorption sites (i.e., a blocking effect). The results shown in Fig. 5 support this view. In a solution of 0.01 M HC104 plus 5 × 10 -7 M H 2 S O 4 , sulfate adsorbed on a Pt(110) electrode during a wait at 0.6 V, was removed as hydrogen was adsorbed. This is indicated by the absence of an anodic counterpart to peak II(SO4 2) in the CV obtained after waiting at 0.6 V. The positive scan in that CV is not identical to the steady state CV because the desorption of sulfate (accumulated on the electrode during the wait at 0.6 V) caused a temporary increase in the sulfate concentration in the vicinity of the electrode. The fact that sulfate was desorbed as hydrogen adsorbed strongly suggests that the two processes are coupled. Similar conclusions can be made regarding chloride adsorption. (Note that hydrogen adsorption is not the only driving force for the desorption of anions. Increased electrostatic repulsion at more negative potentials also plays a role.)

Peak II-effect of sulfate concentration Anion concentration has a rather peculiar effect on coupled hydrogen adsorption peaks [1]. When the sulfuric acid concentration was low, the potential of peak II(SO 2 - ) remained constant despite increases in the sulfate surface coverage (Fig. 4) i /~A cn#2

~ . E/V I--, ~.. vs RHE

-2~

q".tJ(c,o ) II(SO4)

Fig. 5. Cyclic voltammograms for Pt(110) in 0.01 M HC104 plus 5 × 10 -7 M H2SO4; sweep rate 200 mV s-1. ( . . . . . . ) steady state CV with limits 0.16 V and 1.5 V; ( ) electrode potential held at 0.6 V for l rain prior to negative scan shown in figure.

116

and there were significant differences in the charge under peak II(C104) and peak II(SO]-) during positive and negative scans (Fig. 5). On the other hand, at higher acid concentrations peak II(SO~-) shifted negatively as the sulfate or bisulfate concentration was increased (Fig. 6a). This indicates that in the solutions containing trace amounts of sulfate, the anion surface coverage was not in equilibrium with the bulk anion concentration during potential scans at 50 mV s-L

Sulfate desorption/ adsorption kinetics To get an indication of how rapidly sulfate adsorbs (or desorbs), the following experiments were done for both (110) and polycrystalline Pt electrodes: I /pAcm 2

1~

i / ~ A c m -2 2~ _

'hl

"'iij (C tO4)

S~

1~

--:,,_,

" ~'~..

E/v

b%~

E / V v s RHE , h, ,Tl'-~-h4-,-~J-, ,_,-,4

,,/

-1~1~

-%~ ,~v

Ca]

(b)

-2~

Fig. 6. Cyclic voltammograms for Pt(110) in HC104 and H2SO4; sweep rate 50 mV sq; potential limits 0.06 V and 1.5 V. Assuming that the second dissociation constant for sulfuric acid is 1.2x 10 -2, the concentrations of various species are determined to be:

Plot (a): pH approxtmately c o n s t a n t Cycle

[HCIO4]

[H2 SO4]

[H + ]

[SO~- ]

[HSO4- ]

Epkli

( ) (. . . . . . ) ( ...... ) ( ..... )

0.010 0.0099 0.0090 0

0 0.000050 0.00050 0.0050

0.010 0.010 0.0098 0.0080

0 0.000027 0.00028 0.0030

0 0.000023 0.00022 0.0020

0.244 0.190 0.181 0.171

P~t(b):pHvaHes Cycle

[H2SO4]

[H +]

[SO~- ]

[HSO 4 ]

Epkll

( ) (. . . . . . ) ( ...... ) not shown

0.50 0.050 0.0050 0.00050

0.51 0.059 0.0080 0.00096

0.011 0.0085 0.0030 0.00046

0.49 0.041 0.0020 0.000037

0.132 0.150 0.171 0.195

where [H2SO4] and [HC104] refer to molar acid concentrations before dissooation.

117

(1) After waiting several minutes at 0.6 V in 0.005 M or 0.5 M H2SO4 to allow sulfate anions to adsorb, the electrode potential was ramped negatively at 50 mV s-1. The resulting current-potential profiles were identical to those obtained with continuous potential cycling between 0.06 V and 0.6 V. (2) Waiting 1 rain at 0.6 V (anions adsorb) or 0.06 V (anions desorb) in 0.5 M H z S O 4 followed by a potential sweep to 1.5 V at 100 V s -1 gave identical current-potential profiles in the oxide formation region. These results, which are in disagreement with work by Labkovskaya et al. [33], indicate that sulfate adsorption responds rapidly to changes in potential. (This does not refer to processes associated with peak III.) Angerstein-Kozlowska et al. [34] also report that cyclic voltammetry experiments show sulfate adsorption and desorption to be rapid.

Influence of p H on peaks I and H The solution pH affects the relationship between anion concentration and the potential of peak II(SO2-). When the pH was held constant, the potential of peak II(SO42-) was linearly related to the logarithm of the concentration of sulfate and bisulfate (Figs. 6(a) and 7). A decade increase in the anion concentration caused a shift of approximately - 10 mV in the potential of peak II. (Because it is not known whether the sulfate or bisulfate anion is responsible for blocking hydrogen adsorption, both possibilities were considered.) When pH variations accompanied changes in anion concentration, a different relationship was observed (Figs. 6(b) and 7). Thus Ell / V vs R HE 0.19

",% '~ " ~ " \\\ \\

018

017

016

x\ \\ \\

QIS \

\ \ \

0.14

\ \

QI3

-5.0

. . . .

I

,

,

,

,

I

. . . .

i , i i [ , t l l l

-~.0 -3.0 -2.0 -1.0 log(an ion c o n c e n t r a t i o n / M

)

Fig. 7. The potential of peak II(SO 2 - ) as a function of the concentration of bisulfate (©, e) and sulfate ([3, m). Curve ( ) is based on data from Fig. 6(a) 0.e., pH is approximately constant). Curve ( . . . . . . ) is based on data from Fig. 6(b) (i.e., pH varies).

118

EII/V vs RHE Q19 o.19 0.17

0.16

0.15

O.I'F

0.13

.

.

.

.

.

.

1.!~

.

.

.

.

.

2.~

.

.

J

3.0

pH Fig. 8. The potential of peak II(SO~ ) as a function of pH. Potentials calculated from the data in Fig. 6(b), assuming a - 10 mV shift in peak II per decade increase in the blocking anion concentration. Points ([3) assume sulfate is the blocking species and the sulfate concentration is held constant at 0.011 M. Points (©) assume bisulfate is the blocking species and the bisulfate concentration is 0.49 M. apparently the potential of peak II (SO 2 )-is a function of both blocking anion concentration and solution pH. By first assuming either sulfate or bisulfate to be the blocking anion, shifts in the potential of peak II(SO42-) due to variations in the anion concentration can be calculated (using data obtained at constant pH) and subtracted from the peak potentials tabulated in Fig. 6(b). As shown in Fig. 8, the resulting peak potentials are linearly related to pH. The peak shift per unit increase in p H depends on which anion is assumed to be the blocking species: sulfate (18 mV) or bisulfate (9 mV). On the other hand, in many acidic supporting electrolytes the potential of peak I appears to be independent of solution p H and independent of the type of anion present. These data suggest that the solution p H affects the potential of the hydrogen adsorption reaction in an indirect fashion. As the solution p H increases the potential of zero charge becomes more positive relative to the H 2 reversible potential. This affects ion adsorption processes which in turn influence coupled hydrogen adsorption (cf. work by Sveshnikova et al. [35]). This explanation of the p H effect, which was given previously by Ho and coworkers [36], is also in agreement with the linear relationship found between the potential of peak II and the solution p H (Fig. 8). Frumkin and Petrii [37] reported that in acidified solutions of 0.05 M Na2SO 4, the potential of zero charge shifts by about + 15 mV (vs. R H E ) for each unit increase in pH.

119

Peak III The remainder of this paper is devoted to a discussion of peak III (i.e., the anomalous peak or third anodic peak). Numerous explanations for the phenomena responsible for that peak have been published [4,5,13,27,28,38-40]. In recent publications, the authors have noted a structural dependence for hydrogen adsorption in that state. This work confirms their observations; however, it seems that anion adsorption also plays a role.

Peculiar properties of peak III There are two properties associated with the anomalous peak which allow it to be distinguished from other states of hydrogen adsorption: slow adsorption/desorption kinetics and certain cathodic ageing effects. These are discussed below.

Ageing at negative potentials As shown in Fig. 9 for polycrystalline and (110) Pt electrodes, ageing at potentials less than 0.15 V caused the charge associated with peak III to increase [1,41]. Moreover, from Fig. 9(b) it is evident that peak II (i.e., the precursor peak) lost charge as the charge under peak III increased. This ageing effect allowed peaks II and III in CVs of polycrystalline and (110) Pt electrodes to be matched with increased confidence. Ageing of freshly annealed (100) or (111) electrodes did not produce the anomalous peak. However, if those electrodes were cycled into the oxide formation region hundreds of times, the surfaces restructured and both the anomalous and precursor peaks became evident. There are processes other than those associated with the anomalous peak, which occur when a polycrystalline Pt electrode is aged in 0.5 M H2SO 4 at a potential near

[ / ~ A c m -2

i/pA c rrf 2 II

(a) I

Poly Pt

i~

(b)

A

Pt(110)

li,

$O

[] -j I J ] I

0 1 0.2 QB 0.4 Q~ E / V vs RFIE

] J I I

I I I

"lml

I' j ]

0.1 0.2 Q[3 O.+ 0.5 E I V vs RHE

Fig. 9. Cyclic voltammograms for polycrystalline platinum (a) and Pt(110) (b) in 0.005 M H2SO4; sweep rate 50 mV s - l ; potential limits 0.06 V and 0.6 V. First ( ) and second (. . . . . . ) positive scans following ageing at 0.06 V for 15 s.

120

~ / p A c m -2 P o l y Pt 2~

I,II

._,

IV

~.vs

b 1 o 2 o.B

E/V RHE

~---us.~.~

-1~I~

i~ -

.....

21~121

Fig. I0. Cyclic voltammograms for polycrystalline Pt in 0.5 M H2SO4; sweep rate 100 mV s - i. ( . . . . . . ) steady state CV with limits 0.06 V and 1.5 V; ( ) second cycle between limits 0.06 V and 0.6 V after ageing for 1 min at 0.06 V.

the H 2 reversible potential. As shown in Fig. 10, after ageing the electrode at 0.06 V for 1 rain, if the electrode was then cycled between the limits 0.06 V and 0.6 V the charge under peak IV was larger and the charge under the weakly bound peak was smaller (especially during the negative scan) than found in CVs which included the oxide formation region. (The first scan after cathodic ageing is not shown because it was complicated by the presence of the anomalous peak.) If either of the CVs shown in Fig. 10 was interrupted at 0.6 V during a negative scan and held at that potential for 30 s to allow sulfate anions to adsorb, then afterwards as the potential was scanned negatively from 0.6 V the resulting current-potential profile was identical to that obtained without an interruption. This suggests that this effect is not the result of a sluggish anion adsorption process. Two reasonable explanations for this "side effect" of cathodic ageing are surface restructuring a n d / o r a hard-to-reduce surface oxide [3]. Whatever the explanation is, it appears that steady state, hydrogen region CVs correspond to hydrogen adsorption processes closer to equilibrium than do CVs which include the oxide formation region. This is indicated by more symmetrical anodic and cathodic current-potential profiles. Slow kinetics In some acids (e.g., 0.5 M H2SO4, 0.1 M HC1, and to a lesser extent in 1 M HC104), the desorption processes associated with the anomalous peak have a step

121 which is slower than hydrogen desorption from any other state. For example, in 0.5 M H2SO 4 this can be determined with cyclic voltammetry from the shift in the potential of peak I I I as the sweep rate is increased from 50 to 200 mV s - ~. The slow kinetics leads one to suspect that the cathodic counterpart to the anomalous peak is overlapped by peaks I and II. Time-resolved staircase voltammetry allows this question to be answered. In that experiment the potential of the electrode is varied in a series of steps, just as in conventional staircase voltammetry [42-46]. During the interval between consecutive potential steps, multiple current measurements (e.g., 256) are made, providing a complete characterization of the intervening current decays (cf. potential step experiments described in refs 47-49). The current flowing during a selected period following each potential step is digitally integrated and plotted as a function of the potential-stepped-to. If the entire current decay is integrated, the resulting plot is similar to results obtained in linear sweep voltammetry. If only a later portion of each current decay is integrated, the resulting time resolved staircase voltammogram emphasizes slow processes. Figure l l(b) shows a slow process for a polycrystalline Pt electrode that was aged for 30 s at 0.06 V in 0.5 M H2SO 4. A similar result was obtained with a Pt(110)

q//JC c m -2 ~

I

Poly Pt (a)

1

i~,,

0

-10

3

(b) III

1

~

ll..

r .....

E / V vs RHE

Fig. 11. Time resolved staircase voltammogramsfor polycrystallineplatinum in 0.5 M H2SO4; effective sweep rate 1.061 V s-l; potential step excitation 9.62 mV; interval between current measurements60 ~s; width of current sampling window 9 ms; integration period: (a) 0-9 ms, (b) 3-9 ms. The electrode potential was held at 0.06 V for 30 s prior to acquiring the data.

122

electrode. In neither case was a cathodic counterpart to the anomalous peak apparent (cf. work by Hanafey [50]).

Studies linking peak I I I to anion adsorption Factors which affect the cathodic ageing process in H 2 S O 4 and HC1 are investigated in this section. Contrary to the conclusions made by Loo and Furtak [4], the results of these investigations strongly suggest that the desorption of anions is somehow related to the transference of charge from peak II to peak III. In order to quantitate the effect of ageing under various conditions, the charge under the anomalous peak was determined by cycling the potential of the electrode between the ageing potential and a potential in the double-layer region (e.g., 0.4 V). The method used to obtain an integration baseline depended on the sweep rate. For sweep rates of at least 1 V s - 1 the baseline was obtained from the second positive scan following ageing because in that scan the anomalous peak was virtually absent. At slower sweep rates the mirror image of the ensuing negative scan was used as a baseline.

Ageing at various negative potentials The effect of potential on ageing is of some interest because at more negative potentials the electrostatic repulsion between the electrode and contact adsorbed anions increases. This was investigated for a Pt(110) electrode in 0.5 M H2SO 4. As shown in Fig. 12(a), after ageing at 0.02 V or 0.06 V for approximately 100 s, the charge under the anomalous peak reached a maximum. A larger charge was obtained at the lower ageing potential, presumably indicating that the equilibrium sulfate

qlll/PC cm -2

• 5~ ~ I i .

613

1

5~ (a)

413

+s

(b) I

13

. . . . . . . . .

I

i.~

,

,

,

. . . . . .

I

,

2.~

,

log ( t / s )

,

31Z]

I

a

129

a

i

n

i

i

,

i

i

I

an

1.1~

n

a

I

a

I

'

i

[

i

,

n

2.0

log ( t / s )

Fig. 12. Charge associated with peak III as a function of the ageing period at 0.06 V (O) and 0.02 V (v). Pt(ll0) in 0.5 M HESO 4 (a) and 0.1 M HC1 (b). Electrode pretreatment: 5 s at 1.3 V, various periods at 0.02 V or 0.06 V as indicated in plots, followed by potential cycling at 5 V s-~ to determine the charge under peak III.

123

Cllll/NCcrn-2 55

58

45

4.8

35 -C~6 -q~÷

-Q.82

8

082

E/V

08~" 086

vs

RHE

Fig. 13, Charge due to peak III as a function of the ageing potential. Pt(110) m 0.5 M H2SO 4 (0) and 0.1 M HC1 (,). Electrode pretreatment: 5 s at 1.3 V, 10 s at various potentials as indicated in plots, 20 s at 0.06 V, followed by potential cycling at 1 V s - 1 to determine the charge under peak III.

coverage was less at that potential. As indicated in Fig. 12(b), similar behavior was observed in 0.1 M HC1. The results of another experiment designed to probe the potential dependence of the ageing process are given in Fig. 13. In that experiment the electrode was potentiostatted at 1.3 V for 3 s to remove surface contaminants by oxidation, at 0.6 V for 3 s to adsorb anions, at variable low pretreatment potentials for 10 s to desorb anions, and at 0.06 V for 20 s to allow time for the hydrogen gas generated at the preceding potential to diffuse away from the electrode. Then the anomalous peak charge was determined. That charge increased (or the anion surface coverage decreased) as the variable pretreatment potential was made more negative. (During the final 20 s wait at 0.06 V, some anions may have readsorbed on the surface.)

Desorbing anions at positive potentials If the ageing process corresponds to the removal of contact adsorbed anions, then a similar effect might be obtained by ageing the electrode at 1.3 V. At that potential oxide formation drives some anions off of the surface. To investigate this, two slightly different potential pretreatment schemes were used. In one case the electrode potential was held at 1.3 V for 5 s and then at 0.06 V for varying periods. The second potential pretreatment scheme differed only in that the electrode was held for 1 s at 0.6 V immediately prior to ageing. This allowed some anions to readsorb on the surface. The results of those experiments are shown for 0.005 M H2SO4, 0.5 H : S O 4, and

124

qlll/pC cm-2 4-1Zl

(a) @IZI

31ZI

212~

~

)

1,1,1,1,ii

,

,

,

i

,

,

.........

,

,

,

i.

I

L

.

c~

.

.

.

.

.

.

i

.

i

.i

,

,

,

2 . ~

log(t/s) Fig 14. Charge associated with peak III as a function of the ageing period. Pt(110) in (a) 0.005 M H2SO 4, (b) 0.5 M H2SO4, and (c) 0.1 M HC1. Electrode pretreaments: for points (11) 5 s at 1.3 V and various periods at 0.6 V (see plots); for points (O) 5 s at 1.3 V, 1 s at 0.6 V, and various periods at 0.06 V. After ageing the charge under peak III was determined from CVs.

0.1 M HC1 in Fig. 14. In each case the a n i o n surface coverage was less (i.e., the a n o m a l o u s p e a k charge was greater) if the p o t e n t i a l of the electrode was c h a n g e d directly f r o m 1.3 V to 0.06 V. R e g a r d l e s s of w h e t h e r or n o t the e l e c t r o d e was p r e t r e a t e d at 0.6 V, given sufficient ageing time, the a n i o n surface coverage r e a c h e d e q u i l i b r i u m a n d the a n o m a l o u s p e a k charge reached a c o m m o n value (within e x p e r i m e n t a l error). Even when the p o t e n t i a l was c h a n g e d directly f r o m 1.3 V to 0.06 V, some anions were p r e s e n t on the surface. P e r h a p s anions were a d s o r b e d as the p o t e n t i a l was c h a n g i n g f r o m 1.3 V to 0.06 V or p e r h a p s all sulfate species were n o t d e s o r b e d d u r i n g the wait at 1.3 V. (Based on r a d i o t r a c e r experiments, B a l a s h o v a [32] suggested that specifically a d s o r b e d sulfate a n d b r o m i d e a n i o n s m a y n o t b e fully d e s o r b e d d u r i n g o x i d e f o r m a t i o n . )

Readsorbing anions after ageing T o investigate the rate at which sulfate a n i o n s were r e a d s o r b e d f r o m a 0.5 M H 2 SO 4 solution, ageing followed b y r a p i d p o t e n t i a l cycling (100 V s - l ) b e t w e e n 0.06

125 1 10 i / A c m -2 2 i.II

-1

-2 0

.... I.... I.... I.... I.... I.... I.... I.... I.... l .... I.... Q1 02 03 0.4 05 E / V vs. R H E

Fig. 15. Cyclic voltammograms for Pt(ll0) in 0.5 M H2SO4; sweep rate 100 V s -l. Electrode pretreatment: 30 s at 1.3 V, 20 s at 0 V, 30 s at 0.06 V, followed by potential cycling between 0.06 V and 0.5 V. First and second scans following ageing are labeled 1 and 2, respectively.

V a n d 0.6 V was used. A s shown in Fig. 15, after the first positive scan in which the a n o m a l o u s p e a k was present, n o further i n d i c a t i o n of the p e a k was found. This m u s t m e a n that sulfate anions were r e a d s o r b e d r a p i d l y as h y d r o g e n was d e s o r b e d f r o m the a n o m a l o u s state (i.e., in m u c h less than 10 ms since that was the time r e q u i r e d for a c o m p l e t e l y cycle). T o d e t e r m i n e h o w r a p i d l y sulfate anions were r e a d s o r b e d at a p o t e n t i a l slightly negative of p e a k III, a P t ( l l 0 ) electrode was held at 0 V for 20 s a n d then h e l d at 0.17 V or 0.06 V for a period. T h e results of those e x p e r i m e n t s (Fig. 16) i n d i c a t e that sulfate a d s o r p t i o n at 0.17 V is m u c h slower t h a n t h a t discussed in the p r e c e d i n g p a r a g r a p h . I n fact, the time scale is similar to that f o u n d for the sulfate d e s o r p t i o n process discussed earlier. These d a t a can be e x p l a i n e d b y a s s u m i n g h y d r o g e n species a n d sulfuric acid a n i o n s are in c o m p e t i t i o n for certain " a n o m a l o u s " a d s o r p t i o n sites. W h e n h y d r o g e n is a d s o r b e d o n these sites a n d the e l e c t r o d e p o t e n t i a l is h e l d at 0.17 V, the a d s o r p t i o n of a n i o n s is kinetically hindered. Prior to ageing at 0.06 V, a similar s i t u a t i o n exists except the roles o f h y d r o g e n a n d sulfate are reversed. (The relationship b e t w e e n the a d s o r p t i o n sites r e s p o n s i b l e for p e a k s II a n d I I I is n o t clear.)

126 i/lJAcrr~ 2 21;;3121 _ I,II

I

i "k

0.1 0 . 2 0 . B 0.~- 0.5 E / V vs R H E

Fig. 16. Cyclic voltammograms of Pt(110) in 0.5 M H2SO4; sweep rate 50 mV s i. ( ) steady state cyclic voltammogram with limits 0.06 V and 1.5 V. Electrode pretreatment before displayed positive scans: (. . . . . . ) 20 s at 0 V then 60 s at 0.17 V; ( . . . . . . ) 20 s at 0 V then 20 s at 0.17 V; (. . . . . ) 20 s at 0 V then 60 s at 0.06 V.

Effect of cycling into the oxide region Several authors [4,38] have noted that cycling the potential of polycrystalline platinum electrodes into the oxide formation region increases the charge under the anomalous peak. This was also observed in our work with a P t ( l l 0 ) electrode. Cycling just in the hydrogen region caused a gradual reduction in the anomalous peak charge while the precursor peak charge increased. As suggested by Loo and Furtak [4], it m a y be that certain adsorption sites (i.e., anomalous sites) are generated during cycles into the oxide formation region and that those sites are lost due to surface restructuring when cycling just in the hydrogen adsorption region. However, one must also consider that possibility that the anomalous sites are deactivated by the adsorption of contaminants which are oxidatively desorbed at positive potentials.

Ratio of peak H charge loss to peak 111 charge gain W h e n charge was transferred from peak II to peak III as a result of ageing, the total charge measured in the hydrogen adsorption region increased [41]. In particular, the ratio of charge lost from peak II to charge gained under peak I I I was 2 / 3 for 0.5 M H 2 S O 4 and 0.1 M HC1. One explanation for the increase in charge is that it is due to the adsorption of anions that were removed during ageing. However, we cannot rule out the possibility that the total hydrogen coverage is larger when hydrogen species are adsorbed in the anomalous state.

127

Electrochemical relaxation studies of peak 111 To elucidate further the mechanism for hydrogen desorption from the anomalous state, a kinetic study of Pt(ll0) in 0.5 M H2SO 4 was done. After cycling the potential of a freshly annealed electrode for 5 min at 200 mV s - 1 between the limits 1.5 V and 0.06 V, the cycling was stopped at 1.5 V. The electrode potential was immediately changed to 0.06 V where the electrode was aged for 20 s. Afterward, the potential of the electrode was made more positive in a series of 10.1 mV steps. The electrochemical relaxation was monitored by recording the current flowing after each step (i.e., during a current sampling window). In Fig. 17, In (current) vs. time profiles are plotted for every other potentialstepped-to. The curves obtained by fitting one or more exponentials (computer assisted peeling) to each current decay are also shown. Figure 18(a) shows the total charge (obtained by digitally integrating the current that flowed during each current sampling window), as a function of the potentialstepped-to. If the electrochemical system did not completely relax during a particular current sampling window, the remaining charge spilled over into the following window.

0.1

01~-

02~1

0,221

?5

4-1

32

0.12

0.181

T 'E L)

3~

~5

0.2~- i

7~

25~

0.3~1

0281

0261

vX e4 v C-

256

174-

4-6

C) l

time

,

Fig. 17. Current decays (ln domain) following a series of potential step excitations. Experiment parameters: Pt(110) in 0.5 M H 2 S O 4 ; effective sweep rate 2.61 V s - 1 ; potential step excitation 10.1 mV; interval between current measurements 15 #s; width of current sampling window 3.84 ms; number of current measurements in each window 256. Electrode pretreatment: aged for 20 s at 0.06 V prior to acquiring data. Explanation of displayed frames: the potential-stepped-to is given in the upper right corner. The number of points displayed is given in the lower left comer. (Once the current has decayed below the noise level, the remaining points in a current sampling window are not displayed.) Points represent the actual data and the solid line represent the result of an exponential fitting procedure. The ln t axes (labeled in the first frame) have the same scale in all frames.

128 q l n t / / J C c m -2 2s

(a)

J-O

.o.~1

.........

o~2. . . . . . . . . E/V

or~ '

.

vs RHE

qc~ /JJC c m -2 (b) 2~

,'flllH~ _ 01

015

02

025

031

E/Vvs

RHE

Fig. 18. Integrated (a) and calculated (b) charge vs. potential-stepped-to plots based on the data shown in Fig. 17. In (b) the calculated charge for each exponential decay is shown with (e) and the total calculated charge is drawn with a solid line. Part (a) is a time resolved staircase voltammogram with an integration period of 0-3.84 ms (i.e., the entire current sampling window).

In Fig. 19, log (relaxation time constant) is plotted as a function of the potentialstepped-to. Also included in that figure is that charge calculated from the following equation: qcal = A0~'(1 -

e-b/r)

(1)

where A 0 (the initial current amplitude) and ~- (the time constant) are obtained from the exponential fitting procedure. The duration of the current sampling window is given by b. Only the charge that flowed during a single current sampling window is included in qc~J. The calculated charge associated with each relaxation time constant and the total calculated charge (i.e., a sum of calculated charges if multiple exponentials are present) at each potential-stepped-to are plotted in Fig. 18(b). This calculated total charge plot can be c o m p a r e d to the integrated total charge plot shown in Fig. 18(a). Agreement between these plots is one test of the success of the exponential fitting process. There are several different adsorption sites associated with the series of fast time constants shown in Fig. 19; however, the fastly decaying currents were fairly well

129

log ('El s ) -2 /

I

L SlOW

-3

fast

t £ &

-S '

'

'

I

0.1 '

i

'

'

0 15

. . . .

I

0.2

. . . .

i

0.25

. . . .

I

'

'

0.3

E / V v s RHE Fig. 19. Log (relaxation time constant) vs. potential-stepped-to plots based on the data in Fig. 17. The values of time constants are indicated by horizontal bars. The height of the spike resting on each horizontal bar is proportional to the calculated charge associated with that relaxation process.

described with a single exponential. (For a discussion of the fast hydrogen adsorption states, see chapters II and III in ref. 51.) In this experiment, the solution resistance may have affected the rate measured for the fast adsorption processes; however, the solution resistance must have been less than or equal to 0.78 £ cm 2 because that was the smallest resistance observed for any relaxation process. Since the faradaic resistance due to peak III R varied from 2.59 to 8.10 £ cm 2, the solution resistance had little effect on that data. (The faradaic resistance was obtained from Ao/AE where AE refers to the amplitude of the potential step.) In Fig. 18, it is apparent that two peaks are associated with the anomalous peak. III H corresponds to a slow process. Peak IIIA is associated with a rapid process. Speculation regarding the phenomena associated with each peak is given below.

The slow step Immediately after acquiring the relaxation data shown in Fig. 17, the Pt(ll0) electrode was cleaned by cycling several times between 1.5 V and 0.06 V. Then it was aged just as done prior to the relaxation measurements. After ageing, the total charge associated with the anomalous peak was determined using conventional cyclic voltammetry as described earlier. The charge was 51 /~C cm-2. The charge under peak III n in Fig. 18 is 34/~C cm -2 or two-thirds of the total anomalous peak charge. As discussed earlier, the total charge loss under peak II is approximately two-thirds of the charge gained under the anomalous peak; so it appears that during ageing, the adsorbed hydrogen species which were initially adsorbed in state II, become adsorbed in a different state (i.e., the anomalous state) which gives rise to peak III H.

130

The fast step Some charge may be associated with the readsorption of sulfate anions desorbed from the electrode during ageing. If so, the charge under peak IIIA might be attributed to anion adsorption. Alternatively, if very little charge is passed when the anions are readsorbed, then the fast process might result from the desorption of hydrogen from a state that exists only when hydrogen is coadsorbed in the anomalous state.

Theoretical consideration of the slow hydrogen desorption step If it is assumed that the readsorption of hydrogen into the anomalous state is negligible, then the rate at which hydrogen is desorbed from that state can be given as;

d n H / d t = - kn H

(2)

where n H is the number of hydrogen species per unit area adsorbed in the anomalous state and k is a potential dependent rate constant given by: k = k ° exp[(1 - f l ) F E / ( RT)]

(3)

F r o m eqn. (2) it can be shown that the current flowing after a potential step is given by:

i = QnlHk exp( -- t/'r)

(4)

where Q is the charge required to desorb a single hydrogen species from the anomalous state, n~ is the number of hydrogen species per unit area adsorbed in the anomalous state at the time of the step, and • is a relaxation time constant equal to 1/k. From eqn. (3) it can be shown that: log ~- = log(1/k °) - (1 - fl)E/O.05916

(5)

This equation correctly predicts the linear relationship observed between log ~- and the potential-stepped-to (Fig. 19). The slope of the log ~- vs. potential plot gives a value of 0.070 for ft. This value for fl is much lower than that found for other states of hydrogen adsorption [51,52]. However, in general the behavior of hydrogen species adsorbed in the anomalous state differs drastically from that observed for hydrogen in other states (e.g., slower kinetics and no participation in the surface equilibration discussed in ref. 51.) REFERENCES 1 B.E. Conway, H. Angerstein-Kozlowska,D.M. Novak and M. de Smet in S. Bruckenstein, J.D.E. Mclntyre, B. Miller and E. Yeager (Eds.), Proc. Symp. Electrode Processes, Boston, 1979, The Electrochemical Society, Princeton, NJ, 1980, p. 271. 2 J.C. Huang, W.E. O'Grady and E. Yeager, J. Electrochem. Soc., 124 (1977) 1732. 3 E. Yeager, J. Electrochem. Soc., 128 (1981) 160C. 4 B.H. Loo and T.E. Furtak, Electrochim. Acta, 25 (1980) 505. 5 F.G. Will, J. Electrochem. Soc., 112 (1965) 451. 6 P.N. Ross, J. Electroanal. Chem., 76 (1977) 139.

131 7 P.N. Ross in J.D.E. Mclntyre, S. Srinivasan and F.G. Wdl (Eds.), Proc. Symp. Electrode Materials and Processes for Energy Conversion and Storage, Philadelphia, 1977 The Electrochemical Society, Princeton, NJ, 1977, p. 290. 8 P.N. Ross, J. Electrochem. Soc., 126 (1979) 67. 9 P.N. Ross, Surf. Sci., 102 (1981) 463. 10 J. Clavilier, R. Durand, G. Guinet and R. Faure, J. Electroanal. Chem., 127 (1981) 281. 11 J. Clavilier, R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem., 107 (1980) 205. 12 J. Clavilier, J. Electroanal. Chem., 107 (1980) 211. 13 A.T. Hubbard, R.M. Ishikawa and J. Katekaru, J. Electroanal. Chem., 86 (1978) 271. 14 K. Yamamoto, D.M. Kolb, R. Kotz and G. Lehmpfuhl, J. Electroanal. Chem., 96 (1979) 233. 15 C.L. Scortlchini and C.N. Reilley, J. Electroanal. Chem., 139 (1982) 233. 16 C.L. Scortichini and C.N. ReiUey, J. Electroanal. Chem., 139 (1982) 247. 17 C.L. Scortichini, F.E. Woodard and C.N. Reilley, J. Electroanal. Chem., 139 (1982) 265. 18 E. Yeager, W.E. O'Grady, M.Y.C. Woo and P. Hagans, J. Electrochem. Soc., 125 (1978) 348. 19 F.E. Woodard, W.S. Woodward and C.N. Reilley, Anal. Chem., 53 (1981) 1251A. 20 W.S. Woodward, R.D. Rocklin and R.W. Murray, Chem. Blomed. and Environ. Instrumentation, 9 (1979) 95. 21 R. Parsons, J. Electroanal. Chem., 118 (1981) 3. 22 D. Dickertmann, F.D. Koppitz and J.W. Schultze, Electrochim. Acta, 21 (1976) 967. 23 A. Bewick and J.W. Russell, J. Electroanal. Chem., 132 (1982) 329. 24 G. Horanyi and E.M. Rizmayer, J. Electroanal. Chem., 83 (1977) 367. 25 J. Solt, G. Horanyi and F. Nagy, Acta Chim. Acad. Sci. Hung., 63 (1970) 385. 26 S. Schuldiner and M. Rosen, J. Electrochem. Soc., 118 (1971) 1138. 27 P. Stonehart, Electrochim. Acta, 15 (1970) 1853. 28 K. Kinoshita, J. Lundquist and P. Stonehart, J. Catal., 31 (1973) 325. 29 B.E. Conway and H. Angerstein-Kozlowska, J. Electroanal. Chem., 113 (1980) 63. 30 B.E. Conway, H. Angerstein-Kozlowska and W.B.A. Sharp, Z. Physlk. Chem. Neue Folge, 98 (1975) 61. 31 G. Horanyi and G. Inzelt, J. Electroanal. Chem., 86 (1978) 215. 32 N.A. Balashova and V.E. Kazarinov, Russ. Chem. Rev., 34 (1965) 730. 33 I.I. Labkovskaya, V.I. Luk'yanycheva and V.S. Bagotskii, Elektrokhimiya, 5 (1969) 580. 34 H. Angerstein-Kozlowska and B.E. Conway, J. Electroanal. Chem., 95 (1979) 1. 35 D.A. Sveshnikova, V.E. Kazarinov and O.A. Petni, Elektrokhimiya, 13 (1977) 1505. 36 F.C. Ho, B.E. Conway and H. Angerstem-Kozlowska, Surf. Sci., 81 (1979) 125. 37 A.N. Frumkin and O.A. Petrii, Electrochim. Acta, 20 (1975) 347. 38 R. Woods, J. Electroanal. Chem., 49 (1974) 217. 39 M.W. Breiter, Electrochemical Processes in Fuel Ceils, Springer-Verlag, New York, 1969. 40 K. Kinoshita and P. Stonehart, Electrochim. Acta, 20 (1975) 101. 41 M. Nakamura and H. Kita, J. Electroanal. Chem., 68 (1976) 49. 42 J.H. Christie and P.J. Lingane, J. Electroanal. Chem., 10 (1965) 176. 43 D.R. Ferrier, D.H. Chidester and R.R. Schroeder, J. Electroanal. Chem., 45 (1973) 361. 44 D.R. Ferrier and R.R. Schroeder, J. Electroanal. Chem., 45 (1973) 343. 45 J.J. Zipper and S.P. Perone, Anal. Chem., 45 (1973) 452. 46 H.L. Surprenant, T.H. Ridgway and C.N Redley, J. Electroanal. Chem., 75 (1977) 125. 47 B.E. Conway, M.A. Sattar and D. Gilroy, Electrochlm. Acta, 14 (1969) 711. 48 D.E. Icenhower, H.B. Urbach and J.H. Harrison, J. Electrochem. Soc., 117 (1970) 1500. 49 A.H. Thompson, J. Electrochem. Soc., 126 (1979) 608. 50 M. Hanafey, Ph.D. Dissertation, University of North Carolina, Chapel Hill, NC, 1982. 51 F.E. Woodard, Ph.D. Dissertation, Umversity of North Carolina, Chapel Hill, NC, 1982. 52 J. Honz and L. Nemec, Collect. Czech. Chem. Commun., 34 (1969) 2030.