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

Abstract

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)}, K ₁{2.5 × 10² (Ni₂O₃)}, K ₂{2.5 × 10²(CoO), 0.6 × 10² (Co₂O₃)} and k ₁{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.