Hydrogenation of the α,β-Unsaturated Aldehydes Acrolein, Crotonaldehyde, and Prenal over Pt Single Crystals: A Kinetic and Sum-Frequency Generation Vibrational Spectroscopy Study

Sum-frequency generation vibrational spectroscopy (SFG-VS) and kinetic measurements using gas chromatography have been used to study the surface reaction intermediates during the hydrogenation of three α,β-unsaturated aldehydes, acrolein, crotonaldehyde, and prenal, over Pt(111) at Torr pressures (1 Torr of aldehyde, 100 Torr of hydrogen) in the temperature range of 295−415 K. SFG-VS data showed that acrolein has mixed adsorption species of η 2 -di-σ(CC)-trans, η 2 -di-σ(CC)-cis as well as highly coordinated η 3 or η 4 species. Crotonaldehyde adsorbed to Pt(111) as η 2 surface intermediates. SFG-VS during prenal hydrogenation also suggested the presence of the η 2 adsorption species and became more highly coordinated as the temperature was raised to 415 K, in agreement with its enhanced C═O hydrogenation. The effect of catalyst surface structure was clarified by carrying out the hydrogenation of crotonaldehyde over both Pt(111) and Pt(100) single crystals while acquiring the SFG-VS spectra in situ. Both the kinetics and SFG-VS showed little structure sensitivity. Pt(100) generated more decarbonylation “cracking” product while Pt(111) had a higher selectivity for the formation of the desired unsaturated alcohol, crotyl alcohol.