Redox cocatalysts play crucial roles in photosynthetic reactions, yet simultaneous loading of oxidative and reductive cocatalysts often leads to enhanced charge recombination that is detrimental to photosynthesis. This study introduces an approach to simultaneously load two redox cocatalysts, atomically dispersed cobalt for improving oxidation activity and anthraquinone for improving reduction selectivity, onto graphitic carbon nitride (C3N4) nanosheets for photocatalytic H2O2production. Spatial separation of oxidative and reductive cocatalysts was achieved on a two-dimensional (2D) photocatalyst, by coordinating cobalt single atom above the void center of C3N4and anchoring anthraquinone at the edges of C3N4nanosheets. Such spatial separation, experimentally confirmed and computationally simulated, was found to be critical for enhancing surface charge separation and achieving efficient H2O2production. This center/edge strategy for spatial separation of cocatalysts may be applied on other 2D photocatalysts that are increasingly studied in photosynthetic reactions.

Single atom catalysts have been found to exhibit superior selectivity over nanoparticulate catalysts for catalytic reactions such as hydrogenation due to their single-site nature. However, improved selectively is often accompanied by loss of activity and slow kinetics. Here we demonstrate that neighboring Pd single atom catalysts retain the high selectivity merit of sparsely isolated single atom catalysts, while the cooperative interactions between neighboring atoms greatly enhance the activity for hydrogenation of carbon-halogen bonds. Experimental results and computational calculations suggest that neighboring Pd atoms work in synergy to lower the energy of key meta-stable reactions steps, i.e., initial water desorption and final hydrogenated product desorption. The placement of neighboring Pd atoms also contribute to nearly exclusive hydrogenation of carbon-chlorine bond without altering any other bonds in organohalogens. The promising hydrogenation performance achieved by neighboring single atoms sheds light on a new approach for manipulating the activity and selectivity of single atom catalysts that are increasingly studied in multiple applications.

Transition-metal catalysts that can efficiently activate peroxide bonds have been extensively pursued for various applications including environmental remediation, chemical synthesis, and sensing. Here, we present pyridine-coordinated Co single atoms embedded in a polyaromatic macrostructure as a highly efficient peroxide-activation catalyst. The efficient catalytic production of reactive radicals through peroxymonosulfate activation was demonstrated by the rapid removal of model aqueous pollutants of environmental and public health concerns such as bisphenol A, without pH limitation and Co2+ leaching. The turnover frequency of the newly synthesized Co single-atom catalyst bound to tetrapyridomacrocyclic ligands was found to be 2 to 4 orders of magnitude greater than that of benchmark homogeneous (Co2+) and nanoparticulate (Co3O4) catalysts. Experimental results and density functional theory simulation suggest that the abundant π-conjugation in the polyaromatic support and strong metal–support electronic interaction allow the catalysts to effectively adsorb and activate the peroxide precursor. We further loaded the catalysts onto a widely used poly­(vinylidene fluoride) microfiltration membrane and demonstrated that the model pollutants were oxidatively removed as they simply passed through the filter, suggesting the promise of utilizing this novel catalyst for realistic applications.

Nanotechnology has driven scientific advances in catalytic materials and processes over the past few decades. Unique physicochemical and electronic properties that emerge when materials are engineered from the bulk to the nanoscale have been exploited for a wide range of applications, including environmental remediation such as catalytic pollutant destruction. Recent advances in the catalytic synthesis of fuels and value-added chemicals explore the properties of materials, noble and transition metal catalysts in particular, when they are engineered to be below nanoscale and at the single-atom limit. In addition to the maximized efficiency of atomic utilization due to size reduction, significantly reduced costs and the potential to achieve highly selective catalysis are particularly appealing to the environmental application of single-atom catalysts, overcoming certain limitations that the field has been unable to address with nanotechnology. This critical review, built upon a comprehensive discussion of fundamental properties, synthesis methods, and application examples, evaluates in depth the opportunities and challenges of single-atom catalysts as new frontier materials for environmental remediation applications beyond nanomaterials.