Kinetics and mechanism of the oxidation of sulfide by oxygen: catalysis by homogeneous metal-phthalocyanine complexes

The kinetics of the catalyzed autoxidation of hydrogen sulfide to sulfur, thiosulfate, and sulfate in aqueous solution has been investigated potentiometrically over the pH range 5-12. The catalytic effects of cobalt(II), nickel(II), and copper(I1) 4,4’,4”,4”'- tetrasulfophthalocyanine have been determined, and differences in catalytic ability have been explained in terms of relative capacity of the octahedral complexes to reversibly bind oxygen. Kinetic data have been analyzed in terms of a bisubstrate Michaelis-Menton rate law. The rate law and other data indicate that the catalyzed reaction proceeds via the formation of a tertiary-activated complex in which HS^- and O_2 reversibly bind with CoTSP^(2-). pH variations have been explained in terms of the acid dissociation of H_2S, general base catalysis, and deprotonation of pyrrole nitrogens within the phthalocyanine ring