The effect of chloride complexes on Pd surface structure, dispersion and CO hydrogenation over Pd-Mg(II)/SiO 2 catalysts

Abstract

The effect of chloride complexes has been investigated over Cl-containing and Cl-free Pd-Mg(II)/Silica gel catalysts for CO hydrogenation. By using TPD-MS, XPS and ESR techniques, a Pd 0 x-Pd(I) (Cl, O)Mg(II) (SiO y) bridge structure has been determined on the Cl-containing catalyst. Complexing causes the Cl-containing catalyst to have about three times better Pd dispersion and much more positive Pd valency than that of the Cl-free catalyst. Moreover, the bridge structure can easily react with adsorbed CO and H , forming a Pd 0 x-Pd(I) (Cl, HCOO) Mg(II) (SiO y) species, which gives rise to a doubly-broad Pd(I) ESR signal. By using a chemical trapping method, it is also found that the bridge structure makes the Cl-containing catalyst have about twice as abundant H δ+ species as that of the Cl-free one. As a result, the turnover frequencies (TOF) and selectivity for methanol have been found to be respectively about 100% and 19% higher for the Cl-containing than those for the Cl-free one. In addition, oxygen pretreatments are found to destroy the bridge structure and hence to largely decrease the TOF and selectivity for methanol.