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

Abstract

A few studies have claimed that Pt single‐atom catalysts (SACs) can catalyze the oxygen reduction reaction (ORR) following a four‐electron pathway (O2+4H++4e−→2H2O). Here, we show that, in the presence of a seemingly negligible amount of Pt nanoparticles, the ORR can mistakenly be thought to occur via a four‐electron pathway on Pt SACs. Various weight percentages (1, 2, 4, 8 wt%) of Pt SACs were prepared on C3N4 layers deposited on a carbon support (C@C3N4). Through a combination of H2/CO uptakes and high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) results, the amount of Pt nanoparticles could be estimated when they were mixed with Pt single atoms. Particularly, most of Pt in the 4 wt% Pt catalyst existed as single atoms, but a small number of Pt nanoparticles co‐existed. Although this catalyst seemed to follow the four‐electron pathway, the reaction actually occurred on Pt nanoparticles, not Pt single‐atoms.

Heterogeneous Atomic Catalysts Overcoming the Limitations of Single-Atom Catalysts

“…We showed that if seemingly negligible Pt nanoparticles coexist with Pt single atoms, the ORR reaction pathway can mistakenly be thought to follow a four-electron pathway on Pt SAC. 108 We prepared 1, 2, 4, and 8 wt % Pt supported on a carbon support with thin C 3 N 4 shell (C@C 3 N 4 ). Whereas 1 and 2 wt % Pt/C@C 3 N 4 consisted of Pt single atoms only, the 4 wt % catalyst contained a small amount (∼0.6 wt %) of Pt nanoparticles as well.…”

“…However, Pt nanoparticles coexisted with Pt single atoms in these catalysts, and it is unclear whether the four-electron pathway ORR really occurred on the Pt single atoms. We showed that if seemingly negligible Pt nanoparticles coexist with Pt single atoms, the ORR reaction pathway can mistakenly be thought to follow a four-electron pathway on Pt SAC . We prepared 1, 2, 4, and 8 wt % Pt supported on a carbon support with thin C 3 N 4 shell (C@C 3 N 4 ).…”

Highly Durable Heterogeneous Atomic Catalysts

“…The seemingly negligible number of nanoparticles can cause a misinterpretation of the reaction pathway. 20 Chronoamperometry (which measures the current change at a fixed potential for a long time) or chronopotentiometry (which measures the potential change at a fixed current for a long time) is usually used to test the durability of various electrochemical reactions, but these techniques are usually considered to be mild. Cyclic voltammetry (CV, which measures i−V curves repeatedly in a fixed potential range) might be a more reliable method for showing the actual durability.…”

Quasi‐Paired Pt Atomic Sites on Mo2C Promoting Selective Four‐Electron Oxygen Reduction

“…[ 16 ] However, in recent endeavors to develop atomically dispersed Pt catalysts to maximize noble metal utilization, many have demonstrated a high selectivity toward the 2 e ORR instead, and concluded that isolated Pt single atoms cannot effectively break the O─O bond and therefore exhibit poor selectivity toward the 4 e ORR process due to the absence of Pt–Pt ensemble sites. [ 7,13,17 ]…”

“…Nevertheless, SACs may not be a panacea for all catalytic reactions, especially not for those that require multiple neighboring active atoms to work in tandem. [ 6,7 ] The absence of such ensemble sites in SACs often compromises the activity and/or selectivity toward certain reactions, such as the low‐temperature oxidation of CO, C 3 H 6 , and C 3 H 8 , [ 8,9 ] vinyl acetate synthesis, [ 10 ] selective conversion of acetylene to ethylene, [ 11 ] and ethylene hydrogenation. [ 12 ]…”