Carbon-Encapsulated Pd/TiO<sub>2</sub> for Photocatalytic H<sub>2</sub> Evolution Integrated with Photodehydrogenative Coupling of Amines to Imines

Abstract: Supported metal nanocatalysts are promising candidates for heterogenous photocatalysis because the metal nanoparticles (e.g., Au, Pt, or Pd) loaded on the semiconductor surface not only act as a reductive cocatalyst, which accelerates the kinetics of reactions such as H+ reduction, but also trap the photoelectrons, which allows charge separation. Owing to these unique benefits, supported metal photocatalysts have been extensively studied for green H2 production at the reductive side integrated with organic selective oxidation at the oxidative side in a closed photocatalytic redox cycle. Imines and their derivatives are important chemicals in the industrial production of functional polymers, agrochemicals, and pharmaceuticals. Recently, imines have been successfully produced via the photocatalytic dehydrogenative coupling of amines over supported metal nanocatalysts. However, owing to the strong adsorption of H atoms and imines on the metal surface, the produced imines are converted to secondary amines via a self-hydrogenation process, thus greatly decreasing the selectivity toward the desired imines. Herein, we demonstrate that the construction of an ultrathin carbon layer on a Pd/TiO2 photocatalyst (Pd/TiO2@C) via the thermal annealing of self-assembled polydopamine layers is a simple yet effective strategy to address this issue. Temperature-programmed reduction of hydro-oxygen titration and cyclic voltammetry curves for Pd/TiO2 vs. Pd/TiO2@C indicate that the conformable coating of the carbon layer on the catalyst surface facilitates kinetic control of H atom adsorption on the supported Pd nanoparticles. Furthermore, in situ Fourier-transform infrared spectroscopy demonstrates that the conformably coated ultrathin carbon layer also decreases the adsorption of substrate molecules such as N-benzylidenebenzylamine on the catalyst surface, which weakens their interaction with the supported Pd nanoparticles. Thus, the construction of an ultrathin conformable carbon coating on Pd/TiO2 is a facile strategy to kinetically control the adsorption behavior of H atoms and imines on the Pd surface during photocatalytic redox reactions, which can suppress the excessive hydrogenation of imines toward selectivity improvement. In addition, owing to the strong electronic interaction between the Pd nanoparticles and the carbon layer, the encapsulated Pd nanoparticles retain their unique catalytic properties toward the H2evolution reaction. As a result, Pd/TiO2@C with an optimized carbon layer thickness facilitates improved photocatalytic synthesis of imines, with conversion and selectivity as high as 95% and 99%, respectively. This study provides an effective strategy to develop high-performance supported metal nanocatalysts for integrated photocatalytic systems to produce H2 and valuable organic chemicals.