React. Kinet. Catal. Lett., Vol. 8, No. 2,241-247 (1978) CATALYTIC OXIDATION OF HYDROGEN IN SOLUTIONS OF METAL COMPLEXES G. I. Golodets and N. I. Ilchenko Pisarzhevskii InstRute of Physical Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev, USSR Received May 15, 1977 Accepted October 25, 1977

T h e kinetics of H2 oxidation at 25 ~ in solutions of Pd-histidine complexes has been studied and a reaction mechanism suggested for the explanation of the experimental data. In terms of this mechanism, the correlation between the activities of homogeneous catalysts a n d r e a c t a n t - c a t a l y s t bond energies are considered. The oxygen-catalyst bond energy proves to be a determining factor similarly as in heterogeneous c a t a l y t i c oxidation. H3yqeHa KtlHeTHKaOKHCJIeHIISIBO~Opo~a IlpH 25 ~ C B pacrBopax KOMIIHeKCOBPd c rH~OM, Hpeg~rlo~Kclt MeXaHH3M peaKttnH; COOTBeTCTByIOIJ~eeeMy KHHeTHtleCKOe ypaBHeHHeOIIHCbIBaeT3aBHCHMOCTbcKopOCTHOTKOHIIettTpaKv~BCeXyqaCTHHKOBKaTanaTHqecKoro npoaecca.

Complexes of transition metals c a t a l y z e the reaction (I)

2Hg+O 2=2H20 in solutions at room t e m p e r a t u r e / 1 , 2 / ; these systems can serve as interesting c h e m i c a l models for metalloermymes activating H2 and 09. In the present work the kinetics and mechanism of process (I) and the structure reactivity correlation have been studied. Rate measurements were m a d e at 9.5 ~

and 1 atom in a gas-volumetric appa-

r a t u s / 8 / . During reaction (1) the volume V decreases with t i m e . From the initial slopes ofzlV vs. t i m e curves, the rate of the process (r) was c a l c u l a t e d . The partial pressures of reactants (Pi) were varied by changing the composition of a 241

GOLODETS, ILCHENKO: CATALYTIC OXIDATION 0.6

(a) 0

0.4 r

0.2

0.2

0./-.

0.6

0.8

0.6

0.8

PH2 0.5

{b) 0

r

0.2

0

0.2

0.4

Po2 Fig. 1. Dependence of r (ml NTP/min) on PH2 (atm) at P~ Po2 at PH2 = 0.3 (b); CM = I0 -3 M, Ch =2x10"~r

= 0.8 (a), and on

H2 + 0 2 + N 2 mixture. An aqueous solution (5 ml) of a Pd(0)-complex with histidine / 1 / was used as a catalyst. In each run a new portion of the catalyst was taken, metallic Pd was not formed. The rate is u n a f f e c t e d b y an increase in pH and CI" ion concentration from 8 to 11 and from 10 -3 to 10 -2 M, respectively. The dependence o~ r on Pi and the total concentrations of histidine (Ch) and palladium (CM) are given in Figs. 1 and 2. In the absence of H2 no noticeable consumption of 0 2 by the catalyst is observed, however, in the absence of 0 2, hydrogen reacts readily with the Pal-complex finally reducing it to the m e t a l Hence interaction between H2 and the catalyst is supposed to be the first step of catalysis. It is assumed that [ M ( L ' L " ) ] , [M(L'L") 2] and [ M ( L ' L " ) 3] species are present in the solution. HereM is the metal and L'L" a bidentate ligand (histidine), each of the L' and L" donor groups being bonded to 242

GOLODETS, ILCHENKO: CATALYTIC OXIDATION (a) 0.6

0

0.4 r

0.2

2

4

6 ChxlO3

8

(b)

0.6

10

o

O.t. r

0.2

0

2

/.

6 CM*IO &

8

10

Fig. 2. Dependence of r on C h at CM = 10 -3 M (a), and on CM at Ch = 2 x 10-3 M (b); stoichiometric mixture of 2H 2 + 02

one coordination site. The [M(L' L") 2] complex is the catalytically active species as the maximal rate corresponds to Ch:C M = 2 (Fig. 2a). The proposed first stage of reaction (1) is similar to the fast reversible addition of H 2 / 4 / to triphenylphosphine complexes such as [IrCI(CO)(PhgP)2], where the metals are in a low oxidation state with a coordination number of 4. The addition of 02 to the H2-complex is assumed to be the next step, since adducts containing H2 and 02 (1:1) are k n o w n / 2 / . The rate-determining step should be the interaction of the two intermediates mentioned. Thus, the following mechanism is suggested: K1

I)-[M(L'L")2] +2

[M (Li L, t )2(H2)]

~

K2 2) [M(L'L")2(H2)] + 0 2

.

[M(L' L")2(H2)(O2>]

243

GOLODETS, ILCHENKO: CATALYTIC OXIDATION k 3 L"" "H 3) [M(L'L")2(H2)(O2) ] + EM(L' )2~ 2)] > 2 H20 + 2 [M(L' L")2] K' v------x4 [M(L, a,,)2 ]

4') [M(L'L") ] + L ' L "

4")

(II)

[M(L'L") 2] + L ' L "

, 4

[M(L,L,,)3]

2 H2 + 02 = 2 H20

Here K1, K2, K~ and (K4') are the equilibrium constants for steps 1, 2, 4' and 4 " , respectively and k3 is the rate constant for the slow step 3. The rate equation corresponding to mechanism (II), k C 2 K2K p2 p 3 M ' I 2 H2 02 (+

KIPH2

+

KIK2PH2PO2

+

,

,

+

1

K4C6, L,,

(1)

K"C ,,,2 4 L'L )

(CL' L" is the free ligand concentration) is consistent with the observed Kinetic behavior, At a fixed P_ the rate should increase with P

and at a fixed P

it should

pass through a :a~xlmum, which is expenmentally2observed (Fig. 1).H2At Ch/CM= = 2, the ligands are considered to be practically bonded only in the form of [ M(L'L") 2] , so in this case eq. (1) becomes k C 2 K2K P 2 P 3 M I 2 H2 O2

(2)

r =

(I + KIPH2 + KIK2PH2Po2 )2

which describes the experimental data if k3 = 8.1 x 106 ml min -1 mol'212, K1 = 0.7 atm "1 and K2 = 32.3 arm "1. The calculated curves are given in Fig. 1. 244

GOLODETS, ILCHENKO:CATALYTICOXIDATION According to eq. (1), with increasing histidine concentration the rate should pass through a maximum which is confirmed by the experiments (Fig. 2a). From the value of r for high histidine to metal ratios (i. e. CL, L,,~. Ch) one obtains 2 The data K4'~750 m o l - l l . At a fixed Pi the rate should be proportional to C M, in Fig. 2 b are in favor of this conclusion; the curve is evaluated using the above constants. A series of other mechanisms were found to be inconsistent with the experimental data. At constant Pi' Ci and temperature, the rate of process (I) depends on K1, K2 and k8, which change when passing from one catalyst to another. Using the dependence of K1 and K2 on the corresponding heat effects ql and q2 and applying the BrOnsted equation to k8, we obtain instead of eq. (2): A' exp(1-~)(2q I + q2) r

--

[1 + A " exp(q 1) + A " 'exp(q1 + q2)] where A', A", A"' and

are constants (O < ~ < 1). Since ql and q'2 are approxi~

mately propartional to the M-O bond energies (sM.(~) in MO molecules /5/, r can be expressed as a function of ~M-O" With eM_ O varying between wide limits a maximum of the catalytic activity should be expected. Using eM. O v a l u e s / 6 / and the data of Ref. / 2 / on the specific activities of similar phenylphosphine complexes we have plotted the rate against eM_ O (Fig. 3a). Only the Co-complex deviates from the expected correlation probably due to the differences in the reaction mechanism. Ligands may influence the catalyst-reactant bond energy and hence affect r. With halide ions (X') as ligands in the [MLm(X ) (02) ] intermediate, the catalystoxygen bond will be stronger when electron density shifts from X- to M more easily. Thus, upongoing from CI- to Br" to I-, the metal-oxygen bond energy should increase and the catalytic activity is expected .to decrease. The data of Ref. / 2 / support this conclusion (Fig. 3b). 245

GOLODETS,ILCHENKO:CATALYTICOXIDATION

(a)

15 10 5 r

80

100 120 140 EM-o(kcal/tool)

r~105CV 03

3.4 I

";"iA105 ~ICI-Br- (~~J_ ' 0

3.6

(eV)

3 4 5 6 7

P(~,~)

Fig. 3. (a) Dependence of the catalytic activity of [PtO(PhaP)9] (1), [ IrCI(CO)(Ph3P)9. ] (2), [ RhCI(CO)(PhaP) 2] (3), [auCl2(PhaP)2] (4), [ OsBr2(Ph3P)2] (5), [CoCl(Ph3P)a] (6) on the metal-oxygen bond energy; (b) Dependence of the catalytic activity of [IrX(CO)(Ph3P)2 ] complexes (X = CI', Br-, I') on the ionization potential/6/ and the polarizability/7/ of X-

The importance and limitations of the above approach for both homogeneous and heterogeneous catalyses are analogous. In oxfdation over solid oxides, metals, etc., the role of surface oxygen bond energy (qs) is determining/5,8/. Different additives (such as halogens to silver), changing qs' affect the catalytic activity. A correlation between r and sM-O can be expected only for systems with similar mechanisms, entropy factors, etc. Interactions during catalysis involve rupture and formation not only of M-O but also of other bonds with the catalyst.

~ M-O can be regarded as a decisive factor only if the other

bond energies are either proportional to, or much smaller than eM. O. It has been shown that the bond energy of our Pd-eomplex with 09. is significantly 246

GOLODETS, ILCHENKO:CATALYTICOXIDATION greater than that with H2. In general, deviations from the Br0nsted rule are possible due to different bond polarities or other reasons. Despite the limitations, the above theory is useful as a first approximation to predict the activity of both heterogeneous and homogeneous catalysts.

REFERI2qCES 1. N.N. Chemova, E.S. Dmitrieva, Yu.N. Kukushkin: Kinet. KataL, 1..~5, 1597 (1974); M.L. Khidekel, et al.: Izv. Akad. Nauk SSSR, Ser. Khim., 483 (1973). 2. L. Vaska, M.E. Tadros: I. Amer. Chem. Soc., 93, 7099 (1971). 3. N.M. Emanuel, E.T. Denisov, Z.K. Maizus: Chain oxidation of hydrocarbons in liquid phase (in Russian), p. 33. Nauka, Moskva 1965. 4. B.R. lames: Homogeneous Hydrogenation, New York and London 1973. 5. G.I. Golodets: Heterogeneous Catalytic Reactions Involving Molecular Oxygen (in Russian), Naukova dumka, Kiev 1977. 6. L, V. Gurvitch, et al. : Energies of Chemical Bonds (in Russian) Nauka, Moskva 1974. 7. Handbook of Chemistry (in Russian), Vol. 1. Leningrad, Moskva 1963. 8. G.K. Boreskov: Kinet. Katal., I4, 7 (1973).

247