Liquid-Phase Hydrogenation of 1-Phenyl-1-propyne on the Pd(1)Ag(3)/Al(2)O(3) Single-Atom Alloy Catalyst: Kinetic Modeling and the Reaction Mechanism

This research was focused on studying the performance of the Pd(1)Ag(3)/Al(2)O(3) single-atom alloy (SAA) in the liquid-phase hydrogenation of di-substituted alkyne (1-phenyl-1-propyne), and development of a kinetic model adequately describing the reaction kinetic being also consistent with the reaction mechanism suggested for alkyne hydrogenation on SAA catalysts. Formation of the SAA structure on the surface of PdAg(3) nanoparticles was confirmed by DRIFTS-CO, revealing the presence of single-atom Pd(1) sites surrounded by Ag atoms (characteristic symmetrical band at 2046 cm(−1)) and almost complete absence of multiatomic Pd(n) surface sites (<0.2%). The catalyst demonstrated excellent selectivity in alkyne formation (95–97%), which is essentially independent of P(H(2)) and alkyne concentration. It is remarkable that selectivity remains almost constant upon variation of 1-phenyl-1-propyne (1-Ph-1-Pr) conversion from 5 to 95–98%, which indicates that a direct alkyne to alkane hydrogenation is negligible over Pd(1)Ag(3) catalyst. The kinetics of 1-phenyl-1-propyne hydrogenation on Pd(1)Ag(3)/Al(2)O(3) was adequately described by the Langmuir-Hinshelwood type of model developed on the basis of the reaction mechanism, which suggests competitive H(2) and alkyne/alkene adsorption on single atom Pd(1) centers surrounded by inactive Ag atoms. The model is capable to describe kinetic characteristics of 1-phenyl-1-propyne hydrogenation on SAA Pd(1)Ag(3)/Al(2)O(3) catalyst with the excellent explanation degree (98.9%).