Characterization by CO/FTIR spectroscopy of Pd/silica catalysts and its correlation with syn-gas conversion

Well-dispersed Pd catalysts, supported on two morphologically different sil icas (meso- and microporous Davison G59 and G03 grades, respectively) and u sed for syn-gas activation at T = 493-523 K and P = 1-4 MPa (CO/H-2 = 1/2.5 ), have been studied by CO chemisorption using FTIR spectroscopy. The long- term exposure to 760 Torr CO(g) at 298 K produces deep changes on the surfa ce structure of Pd particles on both supports. The Pd particles become roug her and/or show more open crystal planes. This phenomenon of surface restru cturing seems to depend both on the exposed metal fraction (FE) of palladiu m and the morphology of the support. The rate of surface restructuring but not its extent, is a function of the superimposed CO(g) pressure. On the mi croporous G03 silica CO chemisorbs in multicoordinated or "hollow" sites (H band), but these signals are not shown by preparations of the supported me tal of comparable dispersity on mesoporous G59 grade. Terminal (L band) and di-coordinated COs (B band) appear in both types of catalysts. The high-lo ading preparations on the microporous support showed a higher proportion of Pd(111) planes than those of low Pd loading, which seems to contribute to the high TOFCH4 and high selectivity to methane in syn-gas activation of th ese catalysts, while the remaining ones show excellent capability for metha nol production.