Hydrogen Peroxide Induced Periodical Catalysis on Copper‐Containing Sulfides

Abstract

Ladung der Cut -1onen wird dadurch soweit reduziert, daL3 der Elektronentransfer vom Perhydroxyl-Radikal energetisch unmoglich wird. Ein autokatalytischer Reaktionsmechanismus, der zwei benachbarte Reaktanten umfaot, ist dann die einzige verbleibende katalytische Reaktionsmoglichkeit. Die Rolle von Cu(1)-Oxyd, welches fur die ausgepragte Lichtabhangigkeit von Oszillationsfrequenz und -amplitude verantwortlich ist, wird weiter studiert. Die Untersuchungen lassen schlieBen, daR Kupferionen in Sulfidoberflachen bei der katalytischen Zersetzung von Wasserstoffperoxyd eine analoge Rolle als Redox-Partner spielen wie Kupferionen in Losungen. Das Elektrodenpotential erlaubt es, ihre katalytische Reaktivitat zu kontrollieren und energetisch so anzupassen, daL3 periodisches katalytisches Verhalten eintritt.

Temporal Instabilities in Cathodic Processes at Liquid and Solid Electrodes

“…This effect was explained in terms of the role of Cu(I)-oxide species in the mechanism of oscillations. The crucial role of Cu(I) ions was discussed in spite of the fact that oscillatory reduction of H 2 O 2 was not observed on metallic Cu and semiconducting [108].…”

“…It is further considered that upon cathodic polarization of Cu 2 S electrode the positive charge of Cu(I) ions is reduced, i.e., the electrons are transferred back from sulfur ions, when a Fermi level in the sulfide is sufficiently raised to fill electrons into antibonding 4s orbitals of Cu þ surface atoms. Thus, the strength of ionic bonding will be significantly affected, whereas the contribution from the covalent bond is expected to remain Reprinted from [108] with kind permission of Deutsche Bunsen-Gesellschaft f€ ur Physikalische Chemie rather independent of the electrode potential. It is important to note that the oscillations set in when the extent of Cu(I) ions reduction exceeds certain critical level, i.e., when the electrode potential reaches a sufficiently negative value.…”

“…However, when a positive character of surface bonded copper ions becomes gradually reduced through the increase of the negative electrode potential, a point is reached at which electron transfer to the copper ions ceases to be energetically favorable and therefore the ordinary heterogeneous catalytic decomposition of H 2 O 2 stops to proceed. Instead, a less efficient, but energetically possible oscillatory catalytic reaction path can manifest Reprinted from [108] with kind permission of Deutsche Bunsen-Gesellschaft f€ ur Physikalische Chemie itself. At this point it is useful to indicate that the absence of electrochemical oscillations in the case of reduction of H 2 O 2 at FeS, FeS 2 , MoS 2 , CdS, and PbS was explained by the fact that for these compounds the ordinary, nonoscillatory catalytic activity could not be stopped and transformed into a periodical one through a cathodic potential.…”

“…We shall here briefly summarize the main features of the proposed oscillatory mechanism [95,108]. This requires first the description of two possible paths of decomposition of H 2 O 2 in contact with Cu(I) containing heterogeneous catalyst, the one which is "trivial," i.e., nonoscillatory and the other one, which occurs in an oscillatory manner.…”

“…With respect to catalytic decomposition of H 2 O 2 , experimental evidences were found for the following reaction steps (with M ¼ Cu þ ions) [99,107,108]:…”