Kinetics of the uninhibited and ethanol-inhibited CoO, Co2O3 and Ni2O3 catalyzed autoxidation of sulfur(IV) in alkaline medium

For getting an insight into the mechanism of atmospheric autoxidation of sulfur(IV), the kinetics of this autoxidation reaction catalyzed by CoO, Co2O3 and Ni2O3 in buffered alkaline medium has been studied, and found to be defined by Eqs. I and II for catalysis by cobalt oxides and Ni2O3, respectively. $$ - {\text{d}}\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]/{\text{d}}t = AK_{ 2} \left[ {{\text{Catalyst}}} \right]\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right] \left[ {{\text{O}}_{ 2} } \right] /\left( { 1+ K_{ 2} \left[ {{\text{O}}_{ 2} } \right]} \right) $$ (I) $$ - {\text{d}}\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]/{\text{d}}t = k_{ 1} K_{ 1} \left[ {{\text{Ni}}_{ 2} {\text{O}}_{ 3} } \right]\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]\left[ {{\text{O}}_{ 2} } \right]/\left\{ { 1+ K_{ 1} \left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]} \right\} $$ (II) The values of empirical rate parameters were: A{0.22(CoO), 0.8 L mol−1s−1 (Co2O3)}, K1{2.5 × 102 (Ni2O3)}, K2{2.5 × 102(CoO), 0.6 × 102 (Co2O3)} and k1{5.0 × 10−2(Ni2O3), 1.0 × 10−6(CoO), 1.7 × 10−5 s−1(Co2O3)} at pH 8.20 (CoO and Co2O3) and pH 7.05 (Ni2O3) and 30 °C. This is perhaps the first study in which the detailed kinetics in the presence of ethanol, a well known free radical scavenger for oxysulfur radicals, has been carried out, and the rate laws for catalysis by cobalt oxides and Ni2O3 in the presence of ethanol were Eqs. III and IV, respectively. $$ - {\text{d}}\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]/{\text{d}}t = AK_{ 2} \left[ {{\text{CoO}}} \right]\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]\left[ {{\text{O}}_{ 2} } \right]/\left\{ {\left( { 1+ C\left[ {{\text{Ethanol}}} \right]} \right)\left( { 1+ K_{ 2} \left[ {{\text{O}}_{ 2} } \right]} \right)} \right\} $$ (III) $$ - {\text{d}}\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]/{\text{d}}t = {k_1}{K_1}\left[ {{\text{Ni}}_{ 2} {\text{O}}_{ 3} } \right]\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]\left[ {{\text{O}}_{ 2} } \right]/\left\{ {\left( { 1+ {K_1}\left[ {{\text{S}}\left( {{\text{IV}}} \right)} \right]} \right)\left( { 1+ C\left[ {{\text{Ethanol}}} \right]} \right)} \right\} $$ (IV) For comparison, the effect of ethanol on these catalytic reactions was studied in acidic medium also. In addition, alkaline medium, the values of the inhibition factor C were 1.9 × 104 and 4.0 × 103 L mol−1 s for CoO and Co2O3, respectively; for Ni2O3, C was only 3.0 × 102 only. On the other hand, in acidic medium, the values of this factor were all low: 20 (CoO), 0.7 (Co2O3) and 1.4 (Ni2O3). Based on these results, a radical mechanism for CoO and Co2O3 catalysis in alkaline medium, and a nonradical mechanism for Ni2O3 in both alkaline and acidic media and for cobalt oxides in acidic media are proposed.