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 silicas (meso- and microporous Davison G59 and G03 grades, respectively) and used 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 surface structure of Pd particles on both supports. The Pd particles become rougher and/or show more open crystal planes. This phenomenon of surface restructuring seems to depend both on the exposed metal fraction (FE) of palladium 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 microporous G03 silica CO chemisorbs in multicoordinated or "hollow" sites (H band), but these signals are not shown by preparations of the supported metal of comparable dispersity on mesoporous G59 grade. Terminal (L band) and di-coordinated COs (B band) appear in both types of catalysts. The high-loading 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 these catalysts, while the remaining ones show excellent capability for methanol production.