Destruction of SO2on Au and Cu Nanoparticles Dispersed on MgO(100) and CeO2(111)

Abstract

When going from periodic surfaces to isolated clusters or nanoparticles, there is a big increase in the reactivity of Au and Cu toward SO(2). Density functional calculations indicate that the enhancement in the SO(2) adsorption energy is due to the presence of corner sites (i.e., metal atoms with a low coordination number) and the fluxionality of the nanoparticles. Therefore, small Au particles bind SO(2) stronger than a periodic Au(100) surface. However, the S <--> Au and O <--> Au interactions are not strong enough to induce the rupture of the S-O bonds. In contrast, the dissociation of SO(2) on Cu particles is a very exothermic process, even more exothermic than on a periodic Cu(100) surface. Experiments of synchrotron-based high-resolution photoemission and X-ray absorption spectroscopy show big differences in the DeSOx activity of Au and Cu nanoparticles dispersed on MgO(100) and CeO(2)(111). The heat of adsorption of the SO(2) on Au nanoparticles supported on MgO(100) or CeO(2)(111) was 0.2 to 0.4 eV larger than on Au(100) with negligible dissociation of the molecule. The full decomposition of SO(2) was observed only after O vacancies were introduced in the ceria support. The O vacancies in ceria either played a direct role in the dissociation of SO(2) (cracking of the molecule at the oxide-metal interface) or enhanced the chemical activity of the supported Au nanoparticles. The addition of Cu particles to MgO(100) or CeO(2)(111) generates systems that are extremely active for the destruction of SO(2). At 100-150 K, the SO(2) adsorbs molecularly on the supported Cu particles. Heating to temperatures above 200 K leads to massive dissociation of the SO(2). A comparison of the behavior of SO(2) on Cu/MgO(100) and Cu/CeO(2-x)(111) shows how important the reducibility of the oxide support in DeSOx operations can be.