Crystal facet engineering of Pd/TiO2 to boost the activity and selectivity for nitroarenes hydrogenation

Palladium-based catalysts have been intensively investigated for the hydrogenation of nitroarenes due to their high activity. However, it remains a challenge to achieve high selectivity when reducing or halo groups are present. Herein, we propose a facet engineering strategy to regulate the activity and selectivity of Pd/TiO₂ and a series of Pd/TiO₂ catalysts with adjustable proportions of (101) and (001) facet of TiO₂ were synthesized for the selective hydrogenation of nitroarenes. With increased percentage of (101) facet, better dispersed Pd species and more oxygen vacancies (O_V) can be generated on the Pd/TiO₂ catalyst, thereby providing increasing hydrogenation reaction centers with strongly adsorbed nitro groups. As a result, the Pd/TiO₂ with dominant (101) facet and 0.1 wt% of Pd loading (Pd/TiO₂-c) delivers an outstanding conversion (100 %) of m-chloronitrobenzene, high selectivity (96.8 %) toward m-chloroaniline, and excellent reusability. In situ characterizations and density functional theory (DFT) simulation reveal that the Pd/TiO₂-c exhibits much stronger adsorption toward nitro groups than chlorine group, while the catalyst with more (001) facet shows similar adsorption capability toward these two groups. Thereby, nitro group is more preferentially adsorbed and hydrogenated during the reaction on Pd/TiO₂-c, which explains the excellent selectivity. Remarkably, Pd/TiO₂-c can also realize excellent conversion (100 %) and selectivity (> 96 %) toward other functional nitroarenes containing −F, –OH, –CHO, –COCH₂- groups. This work highlights the facet engineering of supports to tune the physicochemical properties of loaded active metal and the adsorption behavior of reactants, and provides an effective strategy for the further development of efficient Pd-based catalysts for selective nitroarenes hydrogenation.