We studied electrochemical nitrogen reduction reactions (NRR) to ammonia on single atom catalysts (SACs) anchored on defective graphene derivatives by density functional calculations. We find significantly improved NRR selectivity on SACs compared to that on the existing bulk metal surface due to the great suppression of the hydrogen evolution reaction (HER) on SACs with the help of the ensemble effect. In addition, several SACs, including Ti@N4 (0.69 eV) and V@N4 (0.87 eV), are shown to exhibit lower free energy for NRR than that of the Ru(0001) stepped surface (0.98 eV) due to a strong back-bonding between the hybridized d-orbital metal atom in SAC and π* orbital in *N2. Formation energies as a function of nitrogen chemical potential suggest that Ti@N4 and V@N4 are also synthesizable under experimental conditions.

Shifting electrochemical oxygen reduction towards 2e – pathway to hydrogen peroxide (H 2 O 2 ), instead of the traditional 4e – to water, becomes increasingly important as a green method for H 2 O 2 generation. Here, through a flexible control of oxygen reduction pathways on different transition metal single atom coordination in carbon nanotube, we discovered Fe-C-O as an efficient H 2 O 2 catalyst, with an unprecedented onset of 0.822 V versus reversible hydrogen electrode in 0.1 M KOH to deliver 0.1 mA cm −2 H 2 O 2 current, and a high H 2 O 2 selectivity of above 95% in both alkaline and neutral pH. A wide range tuning of 2e – /4e – ORR pathways was achieved via different metal centers or neighboring metalloid coordination. Density functional theory calculations indicate that the Fe-C-O motifs, in a sharp contrast to the well-known Fe-C-N for 4e – , are responsible for the H 2 O 2 pathway. This iron single atom catalyst demonstrated an effective water disinfection as a representative application.

Highly active and selective CO 2 conversion into useful chemicals is desirable to generate valuable products out of greenhouse gases. To date, various metal-based heterogeneous catalysts have shown promising electrochemical catalytic activities for CO 2 reduction, yet there have been no systematic studies of the active sites of these metal catalysts that can guide further experiments. In this study, we use first-principles calculations to identify active sites for the CO 2 reduction reaction for Ag and Au metals, the two metals that have been shown to be the most active in producing CO. We compare the catalytic activity and selectivity of three reaction sites of nanoparticles, namely, low-index surfaces, edge sites, and corner sites of these metals. For nanoparticle corner sites, in particular, we find that the size effect is critical and 309-atom (or larger) nanoparticles should be used to appropriately describe realistic metal nano-catalysts, whereas a 55-atom cluster model is often used in the literature to model nanoparticles. From a comparative study, we reveal that corner sites are the most active for the CO 2 reduction reaction in the case of Au, while edge sites are the most active in the case of Ag. An underlying reason for this is intrinsically stronger binding of *C-species on Au than Ag makes polycrystalline Au a very good catalyst for the CO 2 reduction reaction. However, our results indicate that reducing the size of Au nanoparticles up to 2 nm enhances the unwanted H 2 evolution reaction, as observed in a recent experiment, while reducing the size of Ag nanoparticles up to 2 nm enhances the CO 2 reduction reaction without suffering from the H 2 evolution reaction, and on this basis Ag nanoparticles are a comparable or even better performing, inexpensive catalyst than Au for electrochemical CO production. Our findings suggest that the catalyst design principle (elemental composition, morphology, and size) is metal-dependent and should be tailored for each system carefully.

We propose the great potential of single atom catalysts (SACs) for CO2 electroreduction with high activity and selectivity predictions over a competitive H2 evolution reaction. We find the lack of an atomic ensemble for adsorbate binding and unique electronic structure of the single atom catalysts play an important role.