Coated silica as support for platinum catalyst: II. Hydrogen chemisorption and activity for combustion

Abstract

Samples of 0.5 wt.-% platinum supported on silica, alumina, titania and lanthana, and on coated supports (Al 2O 3/SiO 2, TiO 2/SiO 2 and La 2O 3/SiO 2) with threshold monolayer loadings, were characterized by hydrogen-oxygen titration, temperature-programmed desorption (TPD) of hydrogen and catalytic combustion of methane. All the samples show depressed chemisorption when reduced at 773 K, compared to reduction at 573 K. Surface platinum atom concentrations for Pt/TiO 2 and Pt/La 2O 3 are around one order of magnitude higher than those for Pt/SiO 2 and Pt/Al 2O 3. But when coated on silica, the oxide coatings caused greatly depressed surface platinum atom concentrations even lower than that on pure silica support. With Pt/Al 2O 3/SiO 2 and Pt/La 2O 3/SiO 2, hydrogen desorption profiles can be attributed to hydrogen spilt over onto alumina and lanthana, respectively. But for Pt/TiO 2/SiO 2, the desorbed hydrogen seems to originate from spilt over hydrogen on silica. This may imply full or nearly full coverage of silica by the oxides in the former two cases, and partial coverage in the latter case. Differences in TPD profiles of LTR and HTR samples agree well with chemisorption data. Titania and lanthana supported catalysts have much higher specific activity and TOF than silica and alumina supported platinum catalysts for methane combustion. When coated on silica, TOFs were increased while specific activities were lowered relative to those of pure oxides. Titania and titania coated silica, when used as supports for platinum, yield the most active catalysts for catalytic combustion of methane.