The reaction steps of the reduction of SO
2 and NO by CO on La
2O
2S, and the effect of H
2O on these reactions were studied using temperature-programmed reaction coupled with mass spectrometry, step-change method, steady-state kinetic measurements, XRD and XPS.
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The reaction steps of the reduction of SO
2 and NO by CO on La
2O
2S, and the effect of H
2O on these reactions were studied using temperature-programmed reaction coupled with mass spectrometry, step-change method, steady-state kinetic measurements, XRD and XPS.
The present study shows that the redox mechanism is not the major reaction route for the reduction of SO
2 by CO on La
2O
2S. Rather, the reduction follows a surface reaction between the reaction intermediate COS and pools of SO
2 already adsorbed on the oxysulfide.
The reduction of NO on La
2O
2S is similar to that on La
2O
3. It starts with the dissociation of NO to N and O. The removal of O can be a rate-limiting step but this step is facilitated by the oxidation reaction with labile sulfur to SO
2 and thus, promoting the reduction of NO to N
2. Upon the depletion of the labile sulfur, the removal process stops and so does the reduction. The NO reduction is fast. To sustain the reduction, SO
2 is needed to regenerate the labile sulfur on the oxysulfide formed by the catalytic reduction of SO
2 by CO.
The presence of H
2O not only shifts the selectivity of sulfur to H
2S but also deactivates the reduction of SO
2 and NO. When the ratio NO/SO
2 is low (~0.4) or when NO is absent, the catalyst is partially deactivated. The deactivation and shift in selectivity can be attributed to the competitive adsorption of H
2O, reverse Claus reaction between H
2O and sulfur in the oxysulfide and hydrolysis of the intermediate COS to H
2S. When the ratio is sufficiently high (~1), the oxysulfide catalyst is completely deactivated and the deactivation is irreversible. The surface of the oxysulfide catalyst is oxidized to oxysulfate, which is stable at the reaction conditions but inactive for the reduction of SO
2 and NO.
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