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We conducted the SO 2 reduction with H 2 over Sn-Zr-based catalysts for the direct sulfur recovery process. The reaction temperature was varied from 250 to 550°C while using SnO 2 -only, ZrO 2 -only, and SnO 2 -ZrO 2 (Sn/Zr ) 2/1) catalysts. The highest reactivity was obtained using the SnO 2 -ZrO 2 (Sn/Zr ) 2/1) catalyst at 550°C, for which the SO 2 conversion and sulfur selectivity were 98 and 55%, respectively. Also, the following mechanistic pathway was suggested: (1) The elemental sulfur is produced by the direct conversion of SO 2 according to the redox mechanism (SO 2 + 2H 2 f S + 2H 2 O). (2) The produced sulfur is partially converted into H 2 S with the hydrogenation (H 2 + S f H 2 S). (3) Finally, the Claus reaction proceeds through Lewis and Brönsted acidic sites (SO 2 + 2H 2 S f 3S + 2H 2 O). It was estimated that the lattice oxygen vacancies might be active sites for the redox mechanism and the Lewis and Brönsted acidic sites might be related to the pathway of the Claus reaction.

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In this work, a ZrO2 catalyst was used to reduce SO2 using CO for the direct sulfur recovery process (DSRP), and a mechanistic investigation was performed. ZrO2 catalyst was prepared by a precipitation method. It was supposed that ZrO2 catalysts exhibit high activity in the SO2 reduction by CO at relatively high temperature because of their Lewis acidic sites and Brönsted acidic sites. In addition, the following mechanistic pathway could be suggested:  (1) In the first step initialized by the redox mechanism, the ZrO2 catalyst was reduced by CO and then sulfate groups, which have the effect of improving the Lewis acidic sites and Brönsted acidic sites, were formed on the surface. (2) In the second step, elemental sulfur was produced by the movement of lattice oxygen between SO2 and the lattice oxygen vacancies of the ZrO2 catalyst having redox catalytic properties. (3) In the third step, COS was formed by the reaction of S + CO → COS. (4) In the fourth step, SO2 and COS were adsorbed and reacted on the surface of the ZrO2 catalyst having Lewis acidic and Brönsted acidic sites, and then the abundant amount of elemental sulfur was produced. Consequently, we would like to suggest the mechanistic pathway corresponding to the modified COS intermediate mechanism involving the redox mechanism.

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