No abstract

The critical role of the Ag–Pd ligand effect (which is tuned by changing the number of Pd atomic layers) in determining the dehydrogenation and dehydration of HCOOH on the bimetallic Pd/Ag catalysts was elucidated by using the spin-polarized density functional theory (DFT) calculations. Our calculations suggest that the selectivity to H2 production from HCOOH on the bimetallic Pd/Ag catalysts strongly depends on the Pd atomic layer thickness at near surface. In particular, the thinnest Pd monolayer in the Pd/Ag system is responsible for enhancing the selectivity of HCOOH decomposition toward H2 production by reducing the surface binding strength of specific intermediates such as HCOO and HCO. The dominant Ag–Pd ligand effect by the substantial charge donation to the Pd surface from the subsurface Ag [which significantly reduce the density of state (particularly, d z 2 –r 2 orbital) near the Fermi level] proves to be a key factor for the selective hydrogen production from HCOOH decomposition, whereas the expansive (tensile) strain imposed by the underlying Ag substrate plays a minor role. This work hints on the importance of properly engineering the surface activity of the Ag–Pd core–shell catalysts by the interplay between ligand and strain effects.

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.

Carbon-supported Pd 3 Au nanoparticle catalysts were synthesized via chemical reduction. Surface segregation of Pd in Pd 3 Au catalyst was achieved via heat treatment under air, Ar, CO−Ar, and CO atmospheres, in order to obtain a surface with changed structures and composition. The surface composition was analyzed by electrochemical methods, and the Pd surface composition was observed to increase from 67.2% (air (asp) sample) to 80.6% (CO sample) after heat treatment under a CO atmosphere. The CO-induced surface segregation of the Pd 3 Au electrocatalyst resulted in a significantly improved formic acid oxidation (from 6.93 to 18.11 mA cm −2 ) and stability (from 0.66 to 2.01 mA cm −2 ) compared to the sample prepared in air, as well as increased mass activity (from 199.20 to 520.55 A g −1 ), electrochemical surface area (ESA Pd ) (from 61.08 to 95.68 m 2 g Pd −1), and specific activity (from 0.33 to 0.55 mA cm Pd −1 ), respectively. The electrochemical activities were significantly increased because of changes in the structure and composition of the surface due to the increased surface ratio of Pd promoted by CO heat treatment. The exchange of Pd and Au atoms between the surface and the bulk material was observed to influence formic acid oxidation and stable performance. The CO-induced surface segregation has the potential to greatly enhance the electrochemical activities and surface control of Pd−Au alloy nanoparticle with lower Pd contents.