MODIFICATION OF TiO2 SUPPORTS FOR Pd/TiO2 CATALYSTS IN HYDROGENATION OF HEXYNOL AND OXIDATION OF BENZYL ALCOHOL

Abstract

The characteristics of surface modification of TiO2 support with various concentrations of 3-(aminopropyl)triethoxysilane (APTES) were investigated. As revealed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) results, monolayer APTES grafting was obtained using 0.1mM of APTES on 1.5 g of TiO2 support. Excess APTES concentrations (e.g. 1, 10 mM) resulted in both multilayer and reversed attachment, which NH2-groups attached on TiO2 surface rather than giving free NH2 termination. In addition, three different preparation methods namely electroless deposition (ED), sonochemical (SN), and sol immobilization (IM) were used to prepare Pd catalysts supported on the TiO2 and APTES modified TiO2. The 1%Pd/TiO2-0.1APTES catalysts prepared by ED exhibited the best catalytic performances in both liquid-phase selective oxidation of benzyl alcohol and liquid-phase selective hydrogenation of 3-hexyn-1-ol because high amounts of Pd were deposited on catalyst surface. In addition, the presence of both metallic Pd0 and PdOx (1≤X≤2) gave high catalytic activity in the benzyl alcohol oxidation, suggesting the formation of highly metallic Pd0 sites via the in-situ reduction of PdO/PdO2 by the adsorbed benzyl alcohol. Moreover, the hydrogenation rate was found to increase when electron donating from NH2 termination of APTES molecules to the metallic Pd0 species occured in case of Pd supported on APTES modified TIO2 catalysts derived by ED and SN methods. For 1%Pd/TiO2(S) catalyst prepared by the sonochemical method with H2 reduction, the SMSI effect occurred during high temperature reduction at 500oC whereas the conventional catalyst derived by incipient wetness impregnation method showed the absence of SMSI and sintering of metallic Pd0 particles was observed instead.