Stabilization of Single Metal Atoms on Graphitic Carbon Nitride

Abstract

Graphitic carbon nitride (g-C3N4) exhibits unique properties as a support for single-atom heterogeneous catalysts (SAHCs). Understanding how the synthesis method, carrier properties, and metal identity impact the isolation of metal centers is essential to guide their design. This study compares the effectiveness of direct and postsynthetic routes to prepare SAHCs by incorporating palladium, silver, iridium, platinum, or gold in g-C3N4 of distinct morphology (bulk, mesoporous and exfoliated). The speciation (single atoms, dimers, clusters, or nanoparticles), distribution, and oxidation state of the supported metals are characterized by multiple techniques including extensive use of aberration-corrected electron microscopy. SAHCs are most readily attained via direct approaches applying copolymerizable metal precursors and employing high surface area carriers. In contrast, although post-synthetic routes enable improved control over the metal loading, nanoparticle formation is more prevalent. Comparison of the carrier morphologies also points toward the involvement of defects in stabilizing single atoms. The distinct metal dispersions are rationalized by density functional theory and kinetic Monte Carlo simulations, highlighting the interplay between the adsorption energetics and diffusion kinetics. Evaluation in the continuous three-phase semihydrogenation of 1-hexyne identifies controlling the metal–carrier interaction and exposing the metal sites at the surface layer as key challenges in designing efficient SAHCs

The Multifaceted Reactivity of Single‐Atom Heterogeneous Catalysts

“…[39] Furthermore,itisknown that the active metal can go into sites beneath the top-most layer (subsurface) [40] and become inaccessible in amanner similar to bulk atoms in NPs. [42] Thechoice of host can be expected to have amajor impact on the electronic characteristics of the metal. [26][27][28][29][30][31][32] Theh igh level of control over the heteroatom content (for example,nitrogen can be varied from 0-60 wt %) provides immense scope for tuning the density and structure of the coordination sites within functionalized carbons.…”

Charge Transfer Modulated Activity of Carbon‐Based Electrocatalysts

“…Wang and co‐workers have reported the FeN 4 in edge site can tailor the bonding structures of N which can significantly increase the overall ORR activity 52f. Besides, due to the high‐proportion pyridine‐N which provides abundant electron lone pairs to embed metal ions, g‐C 3 N 4 reveals unique properties as single‐atom support and shows excellent electrocatalytic activity . In general, for MN x C single‐atom catalysts, the metal atoms, nitrogen coordination number, and the supports property are essential for the activity of SACs.…”

Tailoring Nitrogen‐Doped Carbons as Hosts for Single‐Atom Catalysts

“…Detailed analysis by multiple techniques confirms the incorporation of different amount of nitrogen with distinct species, which influences the dispersion (e. g. single atoms or nanoparticles) and average oxidation state of palladium. To elaborate and compare synthesis‐property‐performance relationships the catalysts are subsequently evaluated in the semi‐hydrogenation of 2‐methyl‐3‐butyn‐2‐ol, a relevant reaction known to discriminate single atoms due to an increased barrier for hydrogen splitting below a minimum ensemble size of three, and compared to a palladium SAC based on exfoliated carbon nitride (ECN) …”