Unveiling the Roles of Pt and CeO₂ during Solvent-Free Amide Hydrogenation Utilizing <i>Operando</i> Photoelectron Photoionization Coincidence Spectroscopy

The hydrogenation of amides to amines is an important reaction in the synthesis of building blocks in organic chemistry. However, the solvent-free hydrogenation of amides (requiring heterogeneous catalysts and high reaction temperatures) is challenging, with low selectivity obtained due to further hydrogenolysis of the alcohol and amine products. Such behavior was observed for a CeO₂-supported Pt catalyst (Pt/CeO₂). Although it is known that the support promotes the adsorption of the amide on the catalyst, and Pt facilitates hydrogenation, their individual roles in amide conversion and how they impact product selectivity remain unclear. Using <i>operando</i> photoelectron photoionization coincidence (PEPICO) spectroscopy, the conversion of <i>N</i>-ethylpropionamide was studied over both Pt/CeO₂ and CeO₂. The experiments reveal that Pt is active for amide hydrogenation below 150 °C to produce amines with high selectivity. The Lewis acidic support (CeO₂) is also active at higher temperatures, producing ethylamine and propanal, while being vulnerable to coking. Although Pt nanoparticles mitigate coke formation, they also catalyze the hydrogenolysis of the amine products above 150 °C. This study provides insights on bifunctional metal oxide--metal nanoparticle catalysts in amide hydrogenation to facilitate the design of superior catalysts.