Palladium Single‐Atom Catalysts Supported on C@C₃N₄ for Electrochemical Reactions

Abstract

Single atom catalysts (SACs) maximize the utilization of noble metal whereas nanoparticle catalysts have inner metal atoms unavailable. In this study, various electrocatalytic reactions were investigated for Pd and Pt SACs. The single atoms were immobilized on thin layers of graphitic carbon nitride with carbon black (for simplicity, C@C3N4) to produce an electrochemically efficient and stable SACs. Single atomic structure was confirmed by high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) and extended X‐ray absorption fine structure (EXAFS) analyses. Oxygen reduction reaction (ORR) and CO stripping experiments were conducted, and the results were compared with the corresponding nanoparticle catalysts. Lack of ensemble sites in the SACs resulted in two‐electron pathway for ORR; single atomic Pd on C@C3N4 (C@C3N4−Pd1) showed high activity and selectivity for H2O2 formation. DFT calculations showed that C@C3N4−Pd1 follows a downhill path for H2O2 formation unlike single atomic Pt on C@C3N4 (C@C3N4‐Pt1), resulting in enhanced H2O2 selectivity. Weaker adsorption of oxygen intermediates on C@C3N4−Pd1 resulted in enhanced ORR activity. The SACs showed no interaction with CO as confirmed by no CO stripping peak. This resulted in no activity for formic acid oxidation following indirect pathway or methanol oxidation, which necessitates COads as reaction intermediates. SACs can be efficient electrocatalysts with high activity and unique selectivity.

Recent Progress of Single‐atom Catalysts in the Electrocatalytic Reduction of Oxygen to Hydrogen Peroxide

“…DFT calculations show that when the single atoms of Pt and Pd were fixed within graphitic carbon nitride (g‐C 3 N 4 ) thin layers with carbon black (C@C 3 N 4 ) by taking advantage of the strong coordination of the pyridinic N in g‐C 3 N 4 , O 2 could adsorb onto the surface. Yet the O−O bond length was shorter on the Pd atom (1.31 Å) than that on Pt (1.34 Å) [60]. This implies that Pt is more conducive to the breaking of the O−O bond, has a higher O 2 binding energy (Figure 2d), and is more likely to produce H 2 O.…”

Local structure engineering for active sites in fuel cell electrocatalysts

“…Through the change of coordination number (N) or the introduction of defects, the electronic structure of the Pt site is changed, and the d-band center of the metal atom is also migrated, thus weakening the Pt-O bond [86]. But a monolecular catalyst is not a completely isolated system; it still has to be attached to some base [87]. The breaking of the O-O bond at the isolated atom Pt is not thermodynamically desirable [88].…”

Seemingly Negligible Amounts of Platinum Nanoparticles Mislead Electrochemical Oxygen Reduction Reaction Pathway on Platinum Single‐Atom Catalysts

“…prepared 3.8 wt% of Pt SAC on N‐doped porous carbon support, claiming the ORR following the four‐electron pathway, but the difference in reactivity between the Pt‐free support material and Pt SAC was not significant in acidic conditions . Because Pt clusters or small Pt nanoparticles are inevitably formed as the weight percentage of Pt increases, the effects of these Pt nanoparticles should be carefully considered.…”

“…The detailed procedure for the syntheses and characterizations can be found in the supporting information. Briefly, C 3 N 4 layers were formed on a carbon support (Ketjenblack EC‐600JD) with 25 wt% (Figure S1), and different amounts of Pt were deposited on the C@C 3 N 4 support using H 2 PtCl 6 ⋅ 6H 2 O as a Pt precursor . Figure shows HAADF‐STEM images of the Pt/C@C 3 N 4 catalysts with various Pt contents.…”

“…Highly oxidic Pt single‐atoms formed on N‐doped carbon support do not show high activity for ORR, following the two‐electron pathway. Many other works have also confirmed that Pt single‐atoms on doped carbon supports followed the two‐electron pathway, while the corresponding Pt nanoparticles followed the four‐electron pathway . However, we do not intend to insist that ORR will not follow the four‐electron pathway for all Pt single‐atoms.…”