Enhancing chemoselectivity via strong metal-support interaction: Selective hydrogenation of 4-chloronitrobenzene over Pd/TiO2 catalysts

Abstract

Supported metal catalysts play an important role in various chemical reactions, including hydrogenation and dehydrogenation. Their activity and selectivity depend on factors such as the metal particle size and surface area and interaction between the metal and the support. Among these factors, “strong metal-support interaction” (SMSI) is a phenomenon by which metal particles supported on an oxide support are embedded under high-temperature reduction conditions, causing important changes in their surface adsorption properties and electronic states. Such electronic state changes reportedly alter the d-band centers of the metal and the adsorption energy of intermediates, thereby strongly influencing catalytic performance. For this study, we investigated SMSI effects on the selective hydrogenation of 4-chloronitrobenzene (4-ClNB) to 4-chloroaniline (4-ClAN) using a Pd/TiO₂ catalyst. This reaction requires reduction of the -NO₂ group while preventing dissociation of the -Cl group. Actually, Pd catalysts are known to have high reducing activity toward the -NO₂ group, but they also cause undesirable C-Cl bond dissociation, which reduces their selectivity. By contrast, we found that controlling the electronic state of Pd via SMSI can enhance the selectivity to 4-ClAN. The underlying mechanisms were elucidated using TPR, TEM, CO adsorption, DRIFTS, XAFS, XPS, and DFT calculation.