Synthesis gas production from partial oxidation of methane over highly dispersed Pd/SiO_2 catalyst

摘要

Pd/SiO_2 catalyst prepared by impregnation method using (NH_4)_2PdCl_4, Pd(NO_3)_2 and Pd(acac)_2 as precursors was used for the synthesis gas production by partial oxidation of methane (POM). Compared to the catalysts prepared from other two precursors, the catalyst prepared from Pd(acac)_2 demonstrated better activity and stability for the reaction at 700 ℃. Methane conversion above 90% with H_2 yield higher than 80% was achieved during 7 h on stream. The catalysts were characterized by N_2 physical adsorption, CO chemisorption, Transmission electron microscopy and in situ X-ray diffraction techniques. The Pd/SiO_2 catalyst prepared from Pd(acac)_2 was found to have a superior performance against sintering during air-calcination at 800 ℃. The size of Pd particles remained at 4-5 nm even after the POM reaction at 700 ℃ for 7 h. The results of mass spectroscopy characterization on the thermal decomposition of a SiO_2 supported Pd(acac)_2 sample under air suggest that formation of highly dispersed (=SiO_s)_2Pd intermediate through the H-transfer from silica surface hydroxyl groups to the acac ligand of Pd(acac)_2 followed by a fast reduction of the (=SiO_s)_2Pd at ~200 ℃ by acacH species plays an important role in the formation of highly dispersed Pd particles on SiO_2 with excellent thermal stability.