Studies on the behavior of mixed-metal oxides and desulfurization:: Reaction of H2S and SO2 with 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 H2S and SO2 with Cr2O3(0001), MgO(100) and CrxMg1-x(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 Cr2O3, MgO and CrxMg1-xO catalysts. The activity toward the breaking of S-H and S-O bonds was found to increase in the sequence: MgO < Cr2O3 < CrxMg1-xO. 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 + SO2 --> 2H(2)O + 3S(solid)) and the reduction of; SO2 by CO (S-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 ina matrix of MgO. These Cr atoms have a lower oxidation state than the atoms in Cr2O3, and exhibit occupied 3d levels that are less stable than the valence bands of MgO. Both properties favor interactions with the LUMO of SO2 (S-O antibonding) and the subsequent dissociation of the molecule. The behavior of the CrxMg1-xO system illustrates a fundamental principle for the design of mixed-metal oxide catalysts.