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.

We here present an innovative approach to increase the electron density of metallic Pd nanoparticles loaded on TiO2 photocatalysts by coordinating Pd with surface-anchored organic ligands. X-ray photoelectron spectroscopy and X-ray absorption near edge structure measurements confirm the negative charge on the Pd nanoparticle induced by electron donation from amine groups of the ligands. The electronically modified Pd on TiO2 exhibits unprecedentedly high photocatalytic H2O2 production from O2 reduction. Mechanistic investigations suggest that the enhanced performance results from electronic tuning of Pd nanoparticles, leading to enhanced charge separation on the TiO2 support, improved activity of Pd nanoparticles as an oxygen reduction center, and improved selectivity for O2 reduction to produce H2O2.

This study presents a single-atom Pt catalyst that achieves efficient C–F bond activation, which is a challenging reaction in both chemical synthesis and environmental remediation of recalcitrant fluorinated hydrocarbons. Up to 1.6 wt % Pt was loaded as a single-atom on SiC substrate (Pt1/SiC) using a facile, scalable wet-chemical method developed based on anchor-site and photoreduction techniques. The high catalytic activity of Pt1/SiC for hydrodefluorination of perfluorooctanoic acid, which is a select perfluorinated pollutant of significant environmental concern, is attributed to the effective hydrogen spillover from isolated Pt onto the SiC surface where the resulting Si–H bond further redistributes with the C–F bond to accomplish hydrodefluorination.

In this study, we loaded Pd catalysts onto a reduced graphene oxide (rGO) support in an atomically dispersed fashion [i.e., Pd single-atom catalysts (SACs) on rGO or Pd 1 /rGO] via a facile and scalable synthesis based on anchor-site and photoreduction techniques. The as-synthesized Pd 1 /rGO significantly outperformed the Pd nanoparticle (Pd nano ) counterparts in the electrocatalytic hydrodechlorination of chlorinated phenols. Downsizing Pd nano to Pd 1 leads to a substantially higher Pd atomic efficiency (14 times that of Pd nano ), remarkably reducing the cost for practical applications. The unique single-atom architecture of Pd 1 additionally affects the desorption energy of the intermediate, suppressing the catalyst poisoning by Cl − , which is a prevalent challenge with Pd nano . Characterization and experimental results demonstrate that the superior performance of Pd 1 /rGO originates from (1) enhanced interfacial electron transfer through Pd−O bonds due to the electronic metal−support interaction and (2) increased atomic H (H*) utilization efficiency by inhibiting H 2 evolution on Pd 1 . This work presents an important example of how the unique geometric and electronic structure of SACs can tune their catalytic performance toward beneficial use in environmental remediation applications.

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