Understanding the influence of the pretreatment procedure on platinum particle size and particle-size distribution for SiO_2 impreganted with [Pt~(2+)(NH_3)_4](NO_3~-)_2

摘要

Using a combination of mass spectrometry, in situ quick extended X-ray absorption fine structure, high-resolution transmission electron microscopy, and hydrogen chemisorption, we studied the reactions taking place during different pretreatments of the catalyst precursor [Pt~(2+)(NH_3)_4](NO_3~-)_2 impregnated on high-surface-area SiO_2 (400 m~3/g). Direct reduction in hydrogen leads to the formation of Pt metal particles in the temperature range of 150-200 deg C in a fast process. The reduction is accompanied by sintering of the platinum particles, leading to relatively large particles, with an average particle size of approximately 14-16 A. Autoreduction in helium leads to multiple steps in the reduction. Around 210 and 240 deg C, NO_x released due to the decomposition of NH_4NO_3, formed during heating up to 180-200 deg C, reduces the catalyst precursor at a high rate. At higher temperatures, the reduction continues slowly through an autoreduction of the Pt(NH_3)_x~(2+) complex. The slow reduction rate suggests a nonmobile species. Accordingly, the final metal-particle size is small, with particles of 10-12 A. Calcination-reduction results in large particles via a similar decomposition of NH_4NH_3. Particle-size distribution after autoreduction is considerably smaller than after direct reduction. The key to obtaining small particles with a relatively narrowsized distribution is to avoid the formation of mobile species. With impregnated [Pt~(2+)(NH_3)_4](NO_3~-)_2, this is best achieved by autoreduction.