Evidence for a Two Step-One Electron Mechanism

N O TIZEN 135

Evidence for a Two Step-One Electron Mechanism

in the Heterogeneous Catalytic Decomposition of Hydrogen Peroxide by

Copper(II) -Peroxide

Ge r h a r d Vi e r k e

I n s titu t für physikalische Biochem ie der U n iv ersität F ra n k fu rt

(Z. Naturforsch. 29b, 135-136 [1974]; eingegangen am 27. September 1973)

H eterogeneous ca ta ly tic decom position, H 20 2, copper- (II )-peroxide, electron radical m echanism Only very little is know n in detail on th e m echa­

nism of th e heterogeneous cataly tic decom position of HoCV’2. This process can be regarded as an oxidation of H , 02 to th e stage of oxygen. Among various com pounds copper(II)-hydroxide in col­

loidal solution an d in suspension was found to be catalytically a c tiv e1. Since C u(O H)2 reacts w ith H , 02 under form ation of copper peroxide com ­ pounds3-7, it has been arg u ed1-6-8 th a t these com ­ pounds are involved in H202 decom position. B u t nothing is know n on th e m echanism of this process.

The E P R investigations rep o rted here yield evi­

dence th a t th e oxidation of H202 by th e copper peroxide p recip itate has to be described by a two step-one electron tran sfer m echanism involving H 02 an d HO radicals as interm ed iate products.

Copper hydroxide m ay be prepared by precipi­

ta tio n from solutions of a copper(II)-salt using excess am ounts of alkali a t tem p eratu res below 5 °C3>9>10. T hen an excess am o u n t of conc. H , 02 (11 m) was dropped to th e copper hydroxide suspension which results in th e form ation of dark brow n copper(II) peroxide [e.^.3-7]. An intensive 02 evolution set in im m ediately. The suspended precipitate was tran sferred to th e E P R sample tube an d frozen a t 77 °K. This sam ple only showed th e intensive signal w ith axial sym m etry represented on Fig. 1. I n a second experim ent th e copper p er­

oxide precip itate was filtered off, im m ediately tra n s ­ ferred to th e sam ple tu b e, resuspended in m ethanol, a n d suddenly frozen a t 77 °K. In addition to the strong signal a w eaker th ree line spectrum appeared (Fig- !)•

The E P R spectra were recorded by th e V arian E -1 2 spectrom eter under usual conditions. All chemicals used were of an aly tical grade. Only trace am ounts of heavy m etals were present which can­

no t account for th e intensive effects observed.

W ith th e simple freezing m ethod described above th e existence of a t least th ree unstable interm ediate reaction products form ed d uring th e heterogeneous

R equests for re p rin ts should be sent to Dr. G.

Vi e r k e, I n s titu t f ü r Physikalische Biochemie der U n iv ersität, D -6000 F r a n k fu r t a. M .-N iederracl I , S andhofstraße.

catalytic decom position of H202 in alkaline Cu- (OH)2-suspensions has been established which will now be discussed in some detail.

Fig. 1. E P R spectrum of an alkaline suspension of copper hydroxide in m ethanol a n d H 20 2 a t th e beginning of oxygen evolution. Mole ra tio H 2Oa/

Cu(OH)2 = 100; 0,2 m K O H , m od u latio n am p litu d e:

2 G, m icrowave pow er: 200 m W , te m p e ra tu re : 93 °K, D P P H : 14,0534 MHz.

1. The E P R spectrum of th e copper peroxide precipitate in m ethanol a t 90 °K reveals th e fo r­

m ation of two radical species (Fig. 1). The intensive signal w ith axial sym m etry can be a ttrib u te d to th e 0 2~ radical tra p p e d in m ethanol. The g values were found to be in agreem ent w ith those o btained by

Be n n e t t et. al.11 in pure m ethanol w ithin experi­

m ental error. This shows th a t 0 2_ is n o t com plexed w ith copper or adsorbed on th e surface of copper peroxide. The dem o n stratio n of 0 2_ as a precursor of 02 reveals th a t th e decom position of H202 by copper peroxide occurs via a two stejp-one electron mechanism.

No E P R signal due to a copper(III)-com pound could be observed th e form ation of w hich is highly im probable anyw ay because of th e low oxidation p otential of H202 in strongly alkaline solutions12.

Hence, we m ust conclude th a t it is th e H 02 radical which is prim arily generated. Because of its pK - value of 4,513’14 it dissociates in alkaline solutions into 0 2_ and H+.

2. The w eak three-line signal w ith th e line sp lit­

tings of 16 G a n d 15 G (after correction for th e superposition of th e th ird line on th e g±-absorption of 0 2_) is identified w ith th e C H 2OH radical. The in ten sity ratio of th e th ree lines was found to be approx. 1 : 2.3 : 1.2 (Fig. 1) which is fairly consistent w ith th e th eoretical value expected for C H2OH.

The g- value of th e radical observed in th e suspension

136 N O TIZ E N (g = 2,045), however, differs from th a t o btained in a homogeneous solution w hich w'as determ ined to be 2,00115. P resu m ab ly th e radicals are adsorbed on th e surface of th e solid copper peroxide precipi­

ta te . T hus th e highly reactive C H 2OH radical is stabilized to some ex te n t. This w ould account for its d etectio n by th e ra th e r slow freezing technique applied. The fo rm atio n of th e C H 2OH radical im plies th e generation of OH as a secondary radical by th e reaction of H 02 writh H20 2, since H O , does n o t react directly w ith m eth an o l16-17.

3. I n alkaline suspensions of freshly precipitated copper peroxide no E P R signal due to a c o p p e r(II)- compound was found a t th e beginning of 0 2 evo­

lu tio n in th e range 0-11000 G. T his resu lt can be m ost readily explained on th e assum ption th a t a d iam agnetic copper(I)-com pound has been form ed18.

This w ould be consistent w ith th e view th a t p ri­

m arily H O , radicals are formed.

B u t during th e course of th e 02 evolution an intensive signal a t g — 2,173 {A H1>2 = 182 G) ap peared to g eth er w ith th e 0 2~ spectrum (Fig. 2).

I t is n o t identical w ith th a t of Cu(O H )2, or cop- per(II)-peroxide, or CuO eith e r18.

I t has been also observed, however, in alkaline C u(O H)2 suspensions w hich h ad been aged for four days a t 4 °C an d in a copper peroxide powder sam ple (mole ra tio H ,0 ,/C u (0 H ), = 1) after glowing. This indicates th a t th e unknow n signal m ust be related to a specially stru c tu re d copper(ll)- hydroxide or to a decom position p ro d u ct of it which m ay be also form ed on copper(II) peroxide decom ­ position.

1 Gm elins H an d b u c h der anorganischen Chemie.

8. A uflage, S ystem N r. 60; Sauerstoff, L ieferung 7.

p. 2387, Verlag Chemie, W einheim 1966.

2 M. Ar d o n, Oxygen, W . A. B enjam in, New Y o rk - A m sterd am 1965.

3 Gmelins H a n d b u c h d er anorganischen Chemie.

8. A uflage, System N r. 60, K upfer, Teil B, p. 133 137, V erlag Chemie, W einheim 1966.

4

5 V Fr e i, Coll. Czech. Chem. Com m un. 27, 179 [1962].

6 P . Pi e r r o n, Bull. s o c . c h i m . F r a n c e 1950, 291.

7

V ysshikh. U chebn. Zavedenii, K him . i K him . Tekhnol. 7, 551 [1964],

8 V. Ko h l s c h ü t t e r a n d H . Ni t s c h m a n n, Helv.

chim . A cta 14, 1215 [1931].

9 I. M. Va s s e r m a n n a n d N. I. Si l a n t e v a. Zh. Neorg.

K him . 13, 2008 [1968].

Fig. 2. E P R spectrum of th e sam e suspension during th e course of oxygen evolution. M odulation am p li­

tu d e : 10 G, m icrowave pow er: 200 m W , m icrow ave frequency: 9,069143 G H^z, te m p e ra tu re : 93 °K.

A fter cessation of 02 evolution th e signal a t g —

2.173 disappeared.

These observations d em o n strate th a t th e de­

com position of H202 is accom panied by a reversible decom position of the cataly tically active copper compound.

The E P R results are clearly incom patible w ith the view th a t 0 2 evolution is sim ply caused by a decay of copper peroxide un d er form ation of CuO.

I am indebted to Prof. J . S ta u ff for in itiatin g th is work.

I t was supponted by th e D eutsche Forschungsgem ein­

schaft.

10 V . G. Bo b r y s h e v, Zh. N eorgan. K him . 14, 810 [1969],

II J. E . Be n n e t t. B . Mi l e, an d A. Th o m a s, Trans.

F arad a y Soc. (»4, 3200 [1968].

12 W. M. L a t i m e r, The oxid atio n sta te s of th e ele­

m ents and th e ir p o te n tia ls in aqueous solutions.

P rentice H all. Inc., Englewood Cliffs, N. J . 1952.

13 G. Sz a p s k i and B . H. J. Bi e l s k i. J. physic. Chem.

G7, 2180 [1963].

14 G. Sz a p s k i an d L. H . Do r f m a n n, J. physic. Chem.

68, 1169 [1964].

15 M. Fu j i m o t o and D. J. E. In g r a m, T rans. F a ra d a v Soc. 54. 1303 [1958].

16 ^{J .} H . B a x e n d a l e , R ad. Res. Suppl. 4, 114 [1964],

17 T. Sh i g a, A. Bo n k h o r s, an d P. D o u zo u , ed. by S. F u jiw ara and L. H . P iette, R ecent D evelopm ents of M agnetic R esonance in Biological System s, p. 146, H irokaw a P ubl. Co., Tokyo 1968.

18 G. Vi e r k e, J . chem. Soc. [London], F a ra d a y T ransact. I 69, 1523 [1973].