(222b) Influence of ZrO2 Nanoligands on the Catalytic Performance of Supported Pt/ZrO2/SiO2 Catalysts

A series of double-supported Pt/ZrO₂/SiO₂ catalysts were prepared to determine the influence of ZrO₂ nanoparticle (NP) domain size on the reactivity of the catalytic active Pt component. The catalysts were synthesized by first impregnating zirconium tert-butoxide, Zr[OC(CH₃)₃]₄, in toluene, drying and then calcining at 500 °C to form ZrO₂. In a second step, aqueous platinum tetra-ammine nitrate, Pt(NH₃)₄(NO₃)₂, was impregnated, dried and calcined in air at 500 °C to form the final double-supported Pt/ZrO₂/SiO₂ catalyst. The ZrO2 loading was varied between 1% and 50% and the Pt loading was maintained constant at 0.1%. In situ Raman and UV-vis spectroscopy and TEM microscopic characterization revealed that the supported ZrO₂ phase varied its domain size from isolated surface species to polymeric surface species to NPs (1-3 nm). TEM microscopy revealed that the supported Pt phase was present as 10-70 nm NPs for 1-20% ZrO₂/SiO₂, where the surface ZrOx species was present (1-12%) and a combination of surface ZrOx species and ZrO₂ NPs (15-20%). The Pt NPs, however, were completely absent for higher zirconia loading where ZrO₂ NPs are present and reflect the presence of a highly dispersed Pt phase on the ZrO₂ NPs. The reactivity of the supported Pt phase was chemically probed with CH₃OH oxidation (both steady-state and CH₃OH-temperature programmed surface reaction (TPSR) spectroscopy). The reactivity of the methanol oxidation reaction was found to increase with the dimension of the Pt NPs, which also corresponds to lower ZrO₂ domain size. Thus, the reactivity of the Pt catalytic active sites could be tuned by the domain size of the ZrO₂ nanoligands.