Understanding the Influence of the Pretreatment Procedure on Platinum Particle Size and Particle-Size Distribution for SiO2 Impregnated with [Pt2+(NH3)4](NO3−)2: A Combination of HRTEM, Mass Spectrometry, and Quick EXAFS

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 [Pt2+(NH3)4](NO3−)2 impregnated on high-surface-area SiO2 (400 m3/g). Direct reduction in hydrogen leads to the formation of Pt metal particles in the temperature range of 150–200°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 Å. Autoreduction in helium leads to multiple steps in the reduction. Around 210 and 240°C, NOx released due to the decomposition of NH4NO3, formed during heating up to 180–200°C, reduces the catalyst precursor at a high rate. At higher temperatures, the reduction continues slowly through an autoreduction of the Pt(NH3)x2+ complex. The slow reduction rate suggests a nonmobile species. Accordingly, the final metal-particle size is small, with particles of 10–12 Å. Calcination–reduction results in large particles via a similar decomposition of NH4NO3. Particle-size distribution after autoreduction is considerably smaller than after direct reduction. The key to obtaining small particles with a relatively narrow-sized distribution is to avoid the formation of mobile species. With impregnated [Pt2+(NH3)4](NO3−)2, this is best achieved by autoreduction.