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

Abstract

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}o{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 @9. Autoreduction in helium leads to multiple steps in the reduction. Around 210 and 240}o{C, NO{x} released due to the decomposition of NH{4}NO{3}, formed during heating up to 180-200}o{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 @9. Calcination-reduction results in large particles via a similar decomposition of NH{4}NO{3}. 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 [Pt}2{}+{(NH{3}){4}](NO{3}}-{){2}, this is best achieved by autoreduction.