CATALYTIC OXIDATION O F HYDROGEN IN SOLUTIONS

OF COBALT-HISTIDINE COMPLEXES N. I. II'chenko and N. V. Shvydak

UDC 541.128

Solutions of complexes of cobaltous ion with histidine are capable of the reversible addition of oxygen at low temperatures [ 1 ]. It was of interest to clarify whether such addition is sufficient for the catalytic oxidation of molecular oxygen and the initiation of the oxidation of hydrogen by oxygen and, if it is sufficient, what is the mechanism for the catalytic reaction. The resolution of these questions is important for a chemical model for the action of oxidative metalloenzymes. The kinetic measurements were carried out at 30.0 +_ 0.1~ by our previously described method [2]. The gaseous mixtures consisted of hydrogen, oxygen, and argon in various ratios for total pressure 1 atm. The partial pressures of the reactants (PH and Po2) were usually varied in the range from 0.1 to 0.6 atm. The experiments were run for virtually constant P. ir~ each experiment since the maximal conversion of the gaseous reagents did not exceed 0.2. The error in the volume determinations was 0.05 ml. We used Co(NO a) 2, histidine, NaOH, and distilled water to prepare solutions of the catalyst. In typical experiments, the total concentration of cobalt C M was 0.02 M, the total concentration of histidine Ch was 0.04 M, pH = 9.5. The solution volume was 5 ml. In the absence of histidine, cobaltous ions do not react with hydrogen, oxygen, or a mixture of these gases. Molecular hydrogen in the absence of 02 does not react with the catalyst: at PH~ = 0.3 atm, the volume of the H 2 + Ar gas mixture in contact with a stirred solution of the catalyst does not change over 3 h within experimental error. On the other hand, oxygen under the same conditions reacts vigorously with the c o b a l t - h i s t i d i n e complex. Rather rapid oxygen absorption is observed over ~ 3 0 rain and then the process virtually ceases (Fig. 1, curve 1). The total volume of oxygen absorbed by the catalyst ZXVo2 1.1 ml and the initial absorption rate r ~ 0.06 ml/min. A typical kinetic curve reflecting the reaction of the gaseous t-Iz + 02 mixture with the catalyst is given in Fig. 1, curve 2. Similar curves were obtained for other Pi" Each such curve consists of two clearly pronounced segments. The initial segment virtually coincides with the corresponding segment of curve 1 and, thus, characterizes the addition of molecular oxygen to the c o b a l t - h i s t i d i n e complex. This conclusion is supported by the virtual independence of the initial reaction rate in this segment from PHz in a broad range of variation of PH2 and various concentrations of cobalt, histidine, and oxygen (Figs. 2 and 3). The value for ro~2 increases with increasing oxygen partial pressure (Fig. 2a) and has a weak dependence on the solution pH in the range from pFI 8 to 1 I. F o r ratios of the histidine and cobalt concentrations Ch:C~ /> 2, the initial rate of oxygen absorption increases with increasing CM and is independent of Ch (Fig. 3). F o r an insufficiency of histidine (relative to the Ch:CMratio of 2), r g increases with increasing Ch (Fig. 3b). In this region, the proportionality between r~ 2 and CM breaks down with a decrease in r ~ (Fig. 3a). The second, less-steep segment of the kinetic curve (see Fig. 1, curve 2) is characterized by constant rate r which is significantly less than r~02" The value for r is directly proportional to PH2 and has a weak dependence on P%(Fig. 4). Thus, not only the magnitude but the nature of the changes in r and r 02 ~ upon variation in the composition of the gas mixture differ sharply. There are two possibilities for the process which characterizes the value of r. 1) The values for r are related to a reaction o f hydrogen with the oxygenated c o b a l t - h i s t i d i n e complex which is completed by its irreversible decomposition. This may be either the addition of hydrogen or the reduction of the complex. 2) The term r expresses the rate of the catalytic (cyclic) process, during which this oxygen complex is constantly regenerated. The. calculation shows that if the first variant is true, then the concentration of the oxygen complex and, thus, the value of r should decrease within the second segment at least by a few times. In fact, the value of r is constant, which forces us to eliminate this proposal. Thus, r characterizes a catalytic process in which the loss of oxygen added to the complex upon the reaction of the complex with H~ is constantly replaced by oxygen from the gas phase. Extensive reduction of oxygen to the 0 2. oxidation state occurs in the reaction of the oxygenated cobalt=histidine complexes with various reducing agents. Hydrogen peroxide is not formed [3]. Thus, we may propose that the final product of the catalytic reaction between hydrogen and oxygen is water. Let us examine the mechanism of this reaction. L. V. Pisarzhevskji Institute of Physical Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 16, No. 3, pp. 321-326, May-June, 1980. Original article submitted July 8, 1979.

0040-5760/80/1603-0255507.50

9 1981 P l e n u m P u b l i s h i n g C o r p o r a t i o n

255

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o

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Fig. 1. Change in the volume of the gas phase V depending on the time of reactionr at CM= 0.02 M, C h = 0 . 0 4 M , p H = 9 . 5 : 1) Poz = 0 . 3 , Pro = 0 . 7 a t m ; 2 ) Po2 = 0 . 3 , PH2 = 0.3, PAr = 0.4 atm. Fig. 2. Dependence of the initial rate of oxygen absorption on the composition of the gas mixture at CM= 0.02 M, Ch = 0.04 M, pH = 9.5 (the open circles represent mixtures of O 2 + Ar and the filled circles represent mixtures of O z + tI~ + Ar): a) PH2 = 0.3 atm; b) Po2 = 0.3 atm (curve 1), Po~ = 0.1 atm (curve 2). The curve in Fig. 2a is calculated using Eq. (6). The following equilibria are established in the cobalt-histidine system in alkaline medium:

Co2§ + L - ~--Co L+, Co L + + L - ~.~-CoL2, where L- is the histidine anion. At p H / > 9.5, almost all the cobalt is found as CoL 2 [4]. Thus, the observed catalytic effect should be attributed to the CoL 2 complex. This complex is unable to activate H2, i.e., hydrogen absorption in the absence of 02 does not occur, while molecular oxygen actively adds to the catalyst. The mechanism for the reversible addition of oxygen by the cobalt-histidine complexes in solution entails the formation of a molecular complex [CoLz'O 2] and binuclear complex [L2Co'O2"CoL21, in which oxygen acts as a bridge joining the CoL 2 species. This mechanism is expressed as follows [1 ]:

1) [Z] +O,.~--[ZO,], (I) 2) IZO~l + [Zl ~ [Z~O~],

where [Z] indicates CoL z, [ZO 2] indicates [Co~'O2], and [ZzO =,] indicates [L2Co'Oz'CoL2]. Upon reaching equilibrium, the corresponding constants in the two steps have the form

K, = Ctzod/Ctz]P o,,

( 1)

K, = Ctz,od/Ctzo,]CtzI

(2)

(C i are equilibrium concentration). Furthermore, K, = KoK', , where K o characterizes the equilibrium dissolution of molecular oxygen in water and K~ characterizes the reaction of [Z] with dissolved oxygen. Substituting for Ctzo d and C[z,o, r found from Eqs. (1) and (2) into the cobalt balance equation Ctz I 4-Ctzo d + -+- 2Ctz,o d = CM, we obtain the equation

2KIK2Po C~z] .q- (I q- KtPo,) C[z] -- CM ----O. Solving this equation taking account of Eqs. (1) and (2), we find the following expressions for the equilibrium concentrations of cobalt-containing complexes:

Ctz]= [V'(I + KtPo.)z + 8CMK,K2Po.-(I + K,Po)I/4K,K2Po.

(3)

C[zo,]----[I/(I-5 KtPo,) z -5 8CMK,K2Po,- (I -I-KtPo,)]/4K~,

(4)

Ctz.od ---[I"(I -5 K,Po,)z q- 8C:~KtKaPo~ -- (I -5 K,Po,)]'~/16K,K2Po.

256

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Fig. 4

Fig. 3. Dependence of the initial rate of oxygen absorption on the cobalt concentration at Ch = 0.04 M (a) and on the histidine concentration at CM= 0.02 M (b), pH = 9.5; the open circles represent mixtures of O2 + Ar, Po2 = 0.3 atm and the filled circles represent mixtures of O2 + H2 + Ar, Po~ = P"2 = 0.3 atm. Fig. 4. Dependence on the rate of the catalytic oxidation of hydrogen on the composition of the gas mixture at CM= 0.02 M, Ch = 0.04 M, pH = 9.5. a) Po2 = 0.3 atm; b) PH2 = 0.3 atm. The curves given by the solid lines were calculated using Eq. (8). For a quantitative analysis of these equations, we must know the values for the equilibrium constants K~ and I(2. According to Powell and Nancollas [5], the equilibrium constant Koa of the reaction 2 [ZI + [Oz] ~-[Z2021

at 25~

is

4.2" 106 M-2 ([O2] is dissolved oxygen). Since the equilibrium constant for the dissolution of oxygen Ko = 1.I' 10-3 M'atm -~ [5], then K = K t K 2 = [ Z 2 Q I / [ Z I z Po, = Ko, Ko = 4,6.103 M -latin-l-

The heat effect of this overall process for the formation of at 30~

[Z2021 is equal

to 30 kcal/mole [5] such that K = 2"103 Mq.atm "~

There are no data for the constants K~ and K2 in the literature. Our results on the kinetics for the addition of oxygen permit us to evaluate these terms. If we assume that the rate-limiting step in scheme I is the first step, then its rate constant k, calculated using our data for r ~ is several orders of magnitude less than values for k~ found by the stopO 2

flow method [ 1]. Thus, it is natural to assume that the equilibrium is established in the first rapid step and the second step is rate-limiting. Then, as is easily shown (6)

r~O~ = le2C~KIPo:/(1 + KiPo,) z,

where k 2 is the rate constant of the second step. The experimentally determined dependence of r~ on Po2 (Fig. 2a) is satisfactorily described by this equation for KI = 0.1 atm -~ and k2 = 6' 103 ml/min.M-2. 'File solid curve in Fig. 2a and also the initial segment of the curve for ~o2VS CMin Fig. 3a were calculated using Eq. (6). Thus, we have K t = O, 1 ann "l , K2 = K / K t = 2. I0 ~ M -1.

Substituting these values and also CM= 2"10-a M into Eqs. (3), (4), and (5), we find that with increasing Po2 up to "~ 0.1 atm, the concentration of free species [Z] sharply decreases from CM= 0.02 M to 0.006 M and continues to drop, reaching 0 002 M at P 9

0 2

= 1 atm

The concentration of the binuclear complex [Z O ] increases sharply from zero to (71 n ~= 0 007 "

2

2

t

Z2v2J

'

M with an increase in Po from zero to 0.1 atm and then slowly increases, reaching % 0.009 M at Po = 1 atm. The concentration of mononuclcar c:omplexes [ZO2] is negligibly low in this region and very slowly increases to 89

"4 M at Po2 = 1 atm.

The calculation results are in good accord with the experimental values of AVo2. the maximum amount of oxygen absorbed in the initial segment at various Po2 (Table 1).

257

TABLE 1. Comparison of the Calculated and Experimental Oxygen Volume AVo2 Absorbed in the Initial Reaction Period Po,. alan 0,1 0,2 0,3 0,4 0,6

aVo. ml ealcu- experilated mental 0,78 0,87 0,89 0,93 0,97

0,75 0,85 0,90 0,95 1,oo

These findings permit us to formulate the following scheme for the mechanism of the catalytic oxidation of hydrogen in solutions of cobalt-histidine complexes: 1) [Zl + 02 ~ [ZOz], 2) [ZO2I + [Zl ~ [Z~O~l. 3) IZ2021 + t-12-,- {II l~:~- 2 IZ] + 2H20,

(II)

21-12-k 02 = 2H~O. The first two steps are equilibrium steps and the third step is rate-limiting. This follows from the much higher rate for the reaction of oxygen with the catalyst than for the catalytic reaction and the lack of marked effect of the reaction of the catalytic process on the reaction of the catalytic process on the reaction of oxygen with the catalyst. Since first-order kinetics is observed relative to hydrogen in the catalysis and the dependence of the reaction rate on Po, is analogous to the dependence of Clz o I on Po (mononuclear complexes are virtually absent), then it is obvious that molecular hydrogen 2 2 2 reacts in the rate-limiting step with the binuclcar oxygenated complex. This reaction is preceded by the dissolution of hydrogen in water such that the rate constant k 3 of the rate-limiting step is equal to the term k;Ks (similar to the case *

where K, =KoK I) , where K 0 characterizes the equilibrium dissolution of hydrogen in water and K~ characterizes the rate constant for the reaction between [Z2021 and dissolved hydrogen. The structure and characteristics of [Z202 ] complexes were described in earlier work [l ]. The intermediate complex [I] may be considered the addition product to hydrogen to [Z2021. Mechanism II corresponds to the kinetic equation r = r s = h3CIz,o,IPH:.

(7)

Substituting Eq. (5) into Eq. (7), we find k3PH,

r = 16KiKzPo,

l]/(1 -}- KtPo,) z + 8 C M K i K 2 P o , - (1 + KiPo:)] u-

(8)

Equation (8) is m accord with the experimental data as seen from Fig. 4, the curves in which were calculated using Eq. (8) for K, = 0.1 atm-', K 2 = 2"104 M-I k 3 = 0.64 ml/min'atm'M (at CM= 0.02 M and C h = 0.04 M). Thus, the activation of molecular oxygen by the cobalt-histidine complex is sufficient to pcrmit this complex to display catalytic activity relative to the oxidation of hydrogen by oxygen under mild conditions. This mechanism is similar to a certain extent to the typical mechanism for the heterogeneous catalytic oxidation of hydrogen [6] which feature the chemisorption of oxygen and the subsequent reaction of adsorbed oxygen with hydrogen. The difference lies in the fact that oxygen reacts in molecular form in homogeneous catalysis in solution, while the dominant intermediate in heterogeneous catalysis is the adsorbed atomic oxygen anion. The authors are grateful to G. I. Golodets for participating in a discussion of the results. LITERATURE CITED 1. 2. 3. 4. 5. 6.

258

Advances in the Chemistry of Coordination Compounds [in Russian], Naukova Dumka, Kiev (1975), Chap. 1, pp. 7-71. G . I . Golodets and N. I. ll'chenko, "Catalytic oxidation of hydrogen in solutions of metal complexes," React. Kinet. Catal. Lett., 8, No. 2, 241-247 (1978). Yu. I. Bratushko, I. L. Zatsny, and K. B. Yatsimirskii, "A study of" the kinetics and mechanism of the decomposition of" the Co(ll) -histidine complex with molecular oxygen," Zh. Neorg. Khim., 2__!,No. 11, 3014-3021 (1976). M.T. Beck, Chemistry of Complex Equilibria, Van Nostrand-Reinhold (1970). H . K . J . Powell and G. H. Nancollas, "Coordination of oxygen by cobalt(II) complexes in aqueous solution," J. Am. Chem. Soc., 94, No. 8, 2664-2667 (1972). G . I . Golodets, Heterogeneous Catalytic Reactions Involving Molecular Oxygen [in Russianl, Naukova Dumka, Kiev (1977).