Dependence of gas-phase crotonaldehyde hydrogenation selectivity and activity on the size of Pt nanoparticles (1.7-7.1 nm) supported on SBA-15

The selectivity and activity for the hydrogenation of crotonaldehyde to crotyl alcohol and butyraldehyde was studied over a series of Pt nanoparticles (diameter of 1.7, 2.9, 3.6, and 7.1 nm). The nanoparticles were synthesized by alcohol reduction of a Pt salt in the presence of poly(vinylpyrrolidone) (PVP), followed by incorporation into mesoporous SBA-15 silica. The rate of crotonaldehyde hydrogenation and selectivity towards crotyl alcohol both increase with increasing particle size. With an increase in particle size from 1.7 nm to 7.1 nm, the selectivity towards crotyl alcohol increases from 13.7% to 33.9% (8 Torr crotonaldehyde, 160 Torr H₂ and 353 K). The turnover frequency increases from 2.1 × 10^-2 s^-1 to 4.8 × 10^-2 s^-1 with increasing particle size. Additionally, the decarbonylation pathway to form propene and CO is enhanced over smaller nanoparticles. The apparent activation energy remains constant (~16 kcal mol^-1 for the formation of butyraldehyde and ~8 kcal mol ^-1 for the formation of crotyl alcohol) as a function of particle size as does the reaction order in H₂, which is unity. In the presence of 130-260 mTorr CO, the reaction rate decreases for all products with a CO reaction order of -1 to -1.4 for crotyl alcohol and butyraldehyde. Hydrogen reduction at 673-723 K results in increased activity and selectivity relative to reduction at either higher or lower temperature; this is discussed with respect to the organic capping agent, PVP.