Single Atom Cocatalysts in Photocatalysis

An Adaptive Palladium Single-Atom Catalyst Enabling Reactivity Switching between Borylation and C-C Coupling

The development of single-atom catalysts (SACs) with site-specific and tunable catalytic functionalities remains a highly desirable yet challenging goal in catalysis. In this study, we report a SAC featuring anisotropic coordination cavities synthesized via a one-step polymerization of 2,6-diaminopyridine and cyanuric chloride. These cavities provide a robust framework for anchoring isolated Pd single atoms with exceptional stability. The unique broken symmetry of the catalyst's local structure enables precise control over reaction pathways, allowing reactivity to be switched between distinct catalytic outcomes. Specifically, under tailored reaction conditions, the catalyst can either halt at the borylation step or proceed seamlessly to Suzuki coupling in a self-cascade process. Mechanistic studies unveil the pivotal role of Pd single atoms in driving key steps, including oxidative addition, base exchange, and reductive elimination. Furthermore, green metrics demonstrate the process's sustainability, with minimized waste generation and reduced reliance on hazardous reagents in the self-cascade transformation. This work establishes an innovative benchmark in the field of single-atom catalysis: by enabling complex, multistep transformations via strategic activation of multiple functional groups, this catalyst exemplifies the potential of self-cascade processes to revolutionize synthetic chemistry via catalysis engineering.

Self-Trapped Excitons Activate Pseudo-Inert Basal Planes of 2D Organic Semiconductors for Improved Photocatalysis

2D organic semiconductors are widely considered superior photocatalysts due to their large basal planes, which host abundant and tunable reaction sites. However, here, it is discovered that these basal planes can be pseudo-inert, fundamentally challenging conventional design strategies that assume uniform activity on the surface of 2D organic semiconductors. Using 2D potassium-poly (heptazine imide) (KPHI) for hydrogen peroxide photocatalysis as a model, it is demonstrated that the pseudo-inertness of basal planes stems from preferential exciton transport to edges, instead of interlayer transport in highly ordered structures. Thus, their dimension reduction enables controlled localization of exciton due to the self-trapping mechanism, whereby the basal planes can transform from pseudo-inert state into active catalytic sites. With this knowledge, a modified 2D KPHI capable of generating 35 mmol g-1 h-1 of H2O2, which is over 350% increase compared to pristine KPHI, is reported. More interestingly, the activated basal planes promote H2O2 production through a reaction pathway distinct from that of pseudo-inert basal planes. These findings establish fundamental principles connecting crystal structure, exciton dynamics, and reactive site distribution, providing new insights into the design of high-performance photocatalysts.

Pt Single Atom Deposition by Direct Current Sputtering in the Gas-Scattering Regime-A Simple Approach to Controlled Single Atom Loading

Supported single atoms represent a new frontier in many catalytic fields, and noble metal single atoms in particular have been reported to be a highly effective cocatalyst in photocatalysis or electrocatalysis. Herein, it is described that direct current sputtering of Pt, when operated under plain gas-scattering conditions, can be leveraged for single atom deposition on various substrates (herein TiO2 and graphene are used as examples). The approach allows a uniform single atom deposition with a high level of control over single atom density and loading amount on both surfaces. Such Pt single atoms on TiO2 can be used directly (without further treatments) as cocatalysts for photocatalytic H2 generation. Remarkably, single atom loading and H2 generation activity correlate linearly over a wide range of Pt loading (0.16-1.41 at%). The findings not only open a new avenue for the synthesis and use of single atoms in photocatalysis but also highlight the use of gas-phase scattering as a simple, scalable, and versatile approach for single-atom catalyst fabrication.

Pd single atoms on g-C3N4 photocatalysts: minimum loading for maximum activity

Noble metal single atoms (SAs) on semiconductors are increasingly explored as co-catalysts to enhance the efficiency of photocatalytic hydrogen production. In this study, we introduce a "spontaneous deposition" approach to anchor Pd SAs onto graphitic carbon nitride (g-C3N4) using a highly dilute tetraaminepalladium(ii) chloride precursor. Maximized photocatalytic activity and significantly reduced charge transfer resistance can be achieved with a remarkably low Pd loading of 0.05 wt% using this approach. The resulting Pd SA-modified g-C3N4 demonstrates a remarkable hydrogen production efficiency of 0.24 mmol h-1 mg-1 Pd, which is >50 times larger than that of Pd nanoparticles deposited on g-C3N4 via conventional photodeposition. This significant enhancement in catalytic performance is attributed to improved electron transfer facilitated by the optimal coordination of Pd SAs within the g-C3N4 structure.

Comparison of Single Atoms vs. Sub-Nanoclusters as Co-Catalysts in Perovskites and Metal Oxides for Photocatalytic Technologies

Adatoms as co-catalysts may play a key role in photocatalysis, yet control of their exact configuration remains challenging. Specifically, there is converging evidence that ultra-small structures may be optimal as co-catalysts; however, a comprehensive distinction between single atoms (SAs), sub-nanoclusters (SNCs), and quantum-sized small particles (QSSPs) has yet to be established. Herein, we present a critical review addressing these distinctions, along with challenges related to the controlled synthesis of SAs, SNCs, and QSSPs; their detection methods; and their functional benefits in photocatalysis. Our discussion focuses on perovskite oxides (e.g., such as ABO3, where A and B are cations) and metal oxides (MxOy, where M is a metal) decorated with adatoms, which demonstrate superior photocatalytic performance compared to their unmodified counterparts. Finally, we highlight cases of misinterpretation between SA, SNC, and QSSP configurations emerging from limitations in high-resolution detection techniques and synthesis methods.