Quantifying the photocatalytic role and activity at the edge and surface of Pd co-catalysts using N2 fixation as a case

Abstract

To quantitatively evaluate the catalytic role and activity of different active sites of catalysts in photocatalytic reactions is of great significance for the design and development of high-performance photocatalysts. However, it remains a substantial challenge due to the complex structure of catalysts as well as the complicated processes in the generation, transfer and consumption of photoexcited charge carriers. Herein, using TiO₂-Pd catalysts in photocatalytic N₂ fixation as a case, a novel protocol to quantitatively determine the atom-specific activity of the edge and surface atoms in metal co-catalysts has been developed in the present work. One key to this strategy is synthesizing well-defined metal nanocrystals with tunable size and a specific exposed facet; another is to guarantee the approximate number of Pd atoms exposed for surface reaction and contact with the TiO₂ support via precise control of the loading amount of Pd of different sizes. Correspondingly, the effects of charge separation and transfer can be excluded, and the number of edge atoms is the only controllable variable contributing to the size-dependent photocatalysis. Based on accurate calculation of the number of surface and edge atoms in the Pd nanocrystals, quantitative correlation shows that the NH₃ formation turnover frequencies (TOFs) of the edge and surface atoms are 18.9 and 0.0436 h⁻¹ on nanocubes, and 41.04 and 0.914 h⁻¹ on nanotetrahedrons, respectively. The superior activity of edge atoms is in agreement with DFT calculations, in which electrons are enriched on the edge of Pd nanocrystals and the d center of the Pd atoms at the edge shifts up to the Fermi energy level, leading to stronger interactions between N₂ and Pd atoms at the edge as well as a lower barrier to N₂ hydrogenation. This work is the first to quantitatively discriminate the catalytic role of metal atoms at the edge and on the surface, and unambiguously prove that the Pd atoms at the edge play a dominant role in photocatalytic N₂ fixation.