M . Kelm, J. Lilie, A. Henglein, and E. Janata
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Pulse Radiolytic Study of Ni+ Nickel-Carbon Bond Formation M. Kelm, J. Lilie, A. Henglein,* and E. Janata Hahn-Meitner-lnstitut fur Kernforschung Berlin GmbH, Bereich Strahlenchemfe, 7 Berlin 39, Germany (Received November 20, 1973) Publication costs assisted by Hahn-Meffner-lnstitut fur Kerntorschung Berlin
Deaerated aqueous solutions of NiS04 were irradiated with 6oCo y rays or by single pulses of high-energy electrons. The solutions contained aliphatic alcohols, diethyl ether, e-pentane, or formate as scavengers for OH and H radicals. Metallic nickel was precipitated from some of these solutions upon yirradiation. Reactions between Ni+ and organic radicals leading to organic nickel compounds were observed in the pulse experiments using both optical and conductimetric detection methods. The rate constants of reactions of Ni+ with organic radicals are of the order of lo9 M - I sec-l. The reaction 2Ni+ Ni Ni2+ was not observed under these conditions. Nickel formation is attributed to the attack of radicals on the organic nickel compounds. These compounds can be oxidized by H202 with rate constants of 104-106 M - l sec-I. They react slowly with water, the lifetime being somewhat less than 1 sec in the cases of methanol and ethanol and longer in the other cases. Only the .CH2C(CH3)20H radical does not form a complex but is rapidly reduced to yield isobutene. The complex formed from Ni+ and C02- rapidly picks up a proton. +
Introduction Abnormal valency states of metals which play an important role as inhibitors or catalysts in chemical reactions can be produced by reactions of the hydrated electron with metal ions. Divalent metal cations such as Ni2+ are reduced this way to form unstable monovalent cations such as Ni+. These monovalent ions have an absorption band a t 300 nm and act as powerful reductants. Ni+, for example, was found to react with oxygen, nitrous oxide, hydrogen peroxide, and haloaliphatic compounds. The rate constants of such processes were measured in pulse radiolytic experiments by recording the decay of the Ni+ absorption in solutions containing both Ni2+ and the reactant.1-8 The products of these reactions have not yet been investigated, however. Irradiation of a solution containing a divalent'metal ion often leads to the precipitation of the corresponding metal.g The yield is generally significant only if an organic OH scavenger is simultaneously present. Some metal ions such as, Co2+, Ni2+, and Cu2+, were found to increase the yield of formaldehyde a t the expense of glycol during the radiolysis of methanol. This effect was ascribed to complex formation between the divalent metal ions and methanol radicals, CH20H.I0 An interaction of Ni+ ions also formed in these solutions with methanol radicals was not considered. In the present investigations, steady y irradiation and pulse radiolysis experiments were carried out to obtain more detailed information about the mechanisms of the reduction of Ni2+ in aqueous solutions and of reactions of Ni+ with various radicals produced simultaneously from dissolved organic solutes. In the y irradiations, the amount of nickel precipitated under various conditions was determined. In the pulse radiolysis experiments, conductimetric observation of the intermediates in addition to the conventional optical absorption measurements yielded decisive information. Earlier conductimetric observations had confirmed the reaction of the hydrated electron with Ni2+, but no conclusion about the mechanism of formation of metallic nickel could be drawn from these measurements.ll The Journal of Physical Chemistry, Voi. 78, No. 9, 7974
+
Experimental Section All solutions were prepared with triply distilled water. NiS04, methanol, c-pentane, ethanol, 2-propanol, and diethyl ether (Merck p.a.) and sodium formate (Riedel pea.) were used without further purification. The solutions were flushed with purified argon for about 2 hr to remove air. The solutions (1.925 1.) were irradiated in the field of a 20,000 Ci 6oCo source a t a dose rate of 5.5 X lo5 rads/hr. The precipitated nickel was filtered under exclusion of air, washed with water and with acetone, dried under vacuum, and its weight determined. In order to check whether the precipitate was pure metallic nickel, the precipitate was dissolved in dilute hydrochloric acid and titrated complexometrically with EDTA (titriplex 111). In some of the experiments, the pH of the solution was kept constant during irradiation by automatic addition of small amounts of 1 M NaOH solution. The pH of the solution was continuously measured with a glass electrode. The output voltage of the pH meter (Knick, Model 26) was fed into a comparator controlling a small pump which automatically injected the NaOH solution. The solution was flushed with argon during irradiation in order to assure rapid distribution of the injected liquid. The pulse radiolysis equipment has already been described12 (1.6-MeV electrons from a Van de Graaff generator; pulse duration: 0.5-5 mec; beam current: 25 mA; dose per Fsec of pulse: about 1500 rads). Since the base conductivity of the NiS04 solution used was rather high, the dc method13 for recording the conductivity could not be applied. We used the 10-MHz method, details of which have recently been reported,14 with a different cell because of the lower electron energy. Figure l shows the dimensions of the double cell and the scheme of the ac bridge. The upper part of the cell was irradiated while the lower part served as a blank. The cell was shielded such that the electron beam penetrated only the space between the upper two platinum electrodes. The analyzing light passed through at right angles to the beam and to the electric field. The electron beam penetrated the solution to a depth of about 5 mm. The cell constant and absorbed dose were always determined before and after a run of ex-
-rIF+
Pulse Radiolysis Study of Ni+
solution out
LOO
t
- 200
-E
0,
quartz cell
t
t
10 MHz
solution in
0
rlcm--i
time Ih]
Figure 1. The double cell for conductivity measurements in pulse radiolysis and the main components of the ac bridge.
periments using a 10- M tetranitromethane solution containing 5% 2-propan01.l~Knowing the yield and extinction coefficient of the C(N02)3- ion formed ( G = 6.0; €350nm 1.4 x IO4 M - l cm-l) and the molar conductivity of the H+C(NO2)3- ion pair (390 mol-l ohm-l cm2 a t 25") the cell constant and absorbed dose could be calculated from the observed signals after the pulse.
Results and Discussion y Radiolysis. Propanol Containing Solutions. Figure 2a shows the amount of nickel which precipitated from a soM NiS04 and 5 X 10-l M 2lution containing 5 x propanol. After a short induction period, the amount of nickel increases with dose. A G value of 0.7 atoms per 100 eV of absorbed energy is calculated from the initial slope of the curve after the induction period. The curve starts to level off toward the horizontal axis after rather short irradiation times. This effect can be attributed to the formation of protons during irradiation which dissolve part of the precipitated nickel. As can be recognized from Figure 2b, a linear relationship exists between the amount of precipitated nickel and the time of irradiation if the pH of the solution is kept constant by automatic addition of NaOH. The G value calculated from the slope of curve b is equal to 1.7 nickel atoms per 100 eV. The nickel precipitate partially dissolved if oxygen was bubbled through the solution after irradiation. However, after filtration and drying under exclusion of air, the metal was no longer reactive toward oxygen. Analysis showed the precipitate to be practically pure metal. The following equations describe the processes which are expected to occur upon y irradiation
6~ + (CHJ,CHOH H
+
(CHXHOH eaq-
+
--
Ni2+
H,O H, --+
+
+ Ni'
( C H J ~ ~ O H (1) (cH,),~oH
(2)
(3)
The rate constants of reactions 1 and 2 are known as 1.2 x lo9 and 5 X lo7 M - l sec-l, re~pective1y.l~ The hydrated electron reacts with the Ni2+ ion with h = 2.3 X 1O1O M - l ~ e c - l .eaq~ scarcely reacts with 2-propanol and OH is not reactive toward Ni2+. Reaction 3 is expected to occur with the yield of the hydrated electron which is 2.7/100 eV in dilute solutions. At the high nickel ion concentration used, additonal electrons may be scavenged in the spurs. It thus appears from the observed yield of 1.7 atoms of nicke1/100 eV, that the yield of Ni is half of that of intermediate Ni+ . The dismutation reaction 2Ni'
-+
NiZ+
+
Ni
(4)
which was already proposed by other authors2 would explain the yield of 1.7/100 eV. However, reasons against the occurrence of reaction 4 will be given below. Experiments were also carried out with a solution con-
-
5
Figure 2. Amount of precipitated nickel as a function of time: solution 1.925 I . of 0.05 M NiS04 plus 0.5 M 2-propanol; dose rate 5.5 X lo5 rads/hr: curve a, without compensation of pH changes during irradiation (initial p H 5 . 5 ) ; curve b, with compensation by automatic NaOH injection (pH 5.5).
taining 0.5 M 2-propanol as OH and H scavenger, 0.5 M acetone as q,- scavenger, and M NiS04. Under these conditions, practically all the hydrated electrons react with acetone to form the 2-propanol radical eaq-
+
(CH,XCO
H+
(cH,),~oH
(5)
which is therefore the only species that could subsequently react with the nickel ion. No nickel, however, was precipitated from this solution. It is concluded that the 2propanol radical does not react with Ni2+ under our conditions. The pulse radiolysis experiments described below led to the same conclusion. Other Organic Solutes. Similar experiments were carried out with solutions containing the organic compounds listed in Table I. Integral G values for 3 hr of irradiation are shown in the table. Practically no nickel was formed when methanol, 2-methyl-2-propano1, and diethyl ether were present. In the absence of an organic additive, the formation of nickel was also suppressed. A rather high yield was observed for the sodium formate containing solution. The precipitated nickel was extremely fine and could be filtered only after addition of silicic acid. The nickel redissolved rapidly if oxygen was admitted to the irradiated solution. These results indicate that the formation of nickel from N i t must be more complicated than described by eq 4. If the organic additive only scavenged OH radicals and H atoms and did not interfere with reactions 4, the yield should be independent of the nature of the organic additive. The results, however, indicate a strong influence of the nature of the organic solute. One can only understand this influence, if one assumes an interaction of the radicals formed by OH and H attack on the organic solute with the Ni+ ions formed in reaction 3. Depending on the redox behavior of the organic radical, this interaction may result in the formation of Ni or Ni2+ from Ni+. This problem will further be examined in the pulse radiolysis experiments described below. In solutions containing formate, the C02- radicals formed by OH attack may in part be able to reduce Ni2+ ions a t the high nickel ion concentration used.8 This reduction may contribute to the rather large yield of precipitated nickel of Table I. Isobutene was traced as a final product of radiolysis of the 2-methyl-2-propanol containing solution. The solution was flushed with a stream of argon and the volatile compounds of the gas stream were frozen out a t -190". They were afterwards dissolved in CS2 and analyzed gas chromatographically and by measuring the consumption of added bromine. A G value of 1.3 for isobutene was found which has to be regarded as lower limit, since isobutene The Journal of Physical Chemistry. Vol. 78. No. 9. 1974
M. Kelm. J. Lilie. A. Henglein. and E. Janala
884
'TABLEI: The 100-eV Yields of Nickel Formed in Solutions Containing Various O n a n i c Additives Owanie additive
lW+V yield of Ni
