Studies on the Behavior of Mixed-Metal Oxides and Desulfurization:  Reaction of H₂S and SO₂with Cr₂O₃(0001), MgO(100), and CrxMg₁₋xO(100)

In environmental catalysis the removal or destruction of S-containing molecules is a very important issue nowadays. The interaction of H 2 S and SO 2 with Cr 2 O 3 (0001), MgO(100) and Cr x Mg 1 - x O(100) surfaces ( x = 0.05−0.08) was studied using synchrotron-based high-resolution photoemission. X-ray absorption near-edge spectroscopy (XANES) was used to examine the chemistry of the molecules on powders of Cr 2 O 3, MgO and Cr x Mg 1 - x O catalysts. The activity toward the breaking of S−H and S−O bonds was found to increase in the sequence: MgO < Cr 2 O 3 < Cr x Mg 1 - x O. The mixed-metal oxide displays a unique ability for breaking S−O bonds that makes it the best catalyst for the Claus process (2H 2 S + SO 2 → 2H 2 O + 3S solid ) and the reduction of SO 2 by CO (SO 2 + 2CO → 2CO 2 + S solid ). First-principles density-functional (DF-GGA) calculations revealed that the superior catalytic properties of the mixed-metal oxide are due to the special electronic properties of Cr cations contained in a matrix of MgO. These Cr atoms have a lower oxidation state than the atoms in Cr 2 O 3, and exhibit occupied 3d levels that are less stable than the valence bands of MgO. Both properties favor interactions with the LUMO of SO 2 (S−O antibonding) and the subsequent dissociation of the molecule. The behavior of the Cr x Mg 1 - x O system illustrates a fundamental principle for the design of mixed-metal oxide catalysts.