Effects of Graphitic and Pyridinic Nitrogen Defects on Transition Metal Nucleation and Nanoparticle Formation on N-Doped Carbon Supports: Implications for Catalysis

Abstract

Functionalization of carbon supports with heteroatom dopants is now widely regarded as a promising route for stabilizing and strengthening the interactions between the support and metal catalysts. Tuning the type and density of heteroatom dopants allows for the tailoring of nanoscale catalyst−support interactions; however, an understanding of these phenomena has not yet been fully realized because of the complexity of the system. In this work, computational modeling, materials synthesis, and advanced nanomaterial characterization are used to systematically investigate the intriguing effect of the two most common nitrogen functionalities in the carbon-based supports on the interactions with selected transition metals toward realizing catalytic applications. Specifically, this study utilized density functional theory to evaluate adsorption energies and modes of adsorption for 12 metals located in groups 8−11 and periods 4−6 with pyridinic and graphitic N defects. Based on these results, further electronic structure investigation of the period 4 metals was conducted to elucidate periodic group trends. Experimental work included synthesis and nanomaterial characterization of a subset of materials featuring three metals each supported on two types of N-doped carbon supports and undoped graphene. Characterization of nanomaterials with scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed that N functionalities enhanced the interactions with the selected transition metals when compared to the undoped support and demonstrated that the nature of the defect influences these interactions. Both computations and experiments agreed that Fe and Co are biased toward the graphitic sites over pyridinic sites, while Ni has an affinity to both defects without a statistically significant preference. This work established a correlation between computational and experimental work and a framework that can be expanded to other metals and alternative dopants beyond nitrogen in tailoring nanoscale catalyst−support interactions for a breadth of catalytic applications.

Effect of Nitrogen Defects on Pt Nanoparticle Dispersion and Stability Studied by Electron Microscopy Paired with Machine Learning Image Processing for Probing Catalyst–Support Interactions

“…Interestingly, while initial dispersion was improved in the pyridinic-rich samples, the graphitic-rich N demonstrated greater NP stability over 3600 degradation cycles from 0.4 to 1.4 V RHE at 1 V s –1 . Several other reports also attribute better initial NP nucleation and dispersion when using pyridinic N rich supports. , However, a recent report calculated greater energy of adsorption for Pt when placed around a graphitic N environment, which was confirmed with experimental evidence . This agrees with a study by Chen et al that demonstrated much smaller, and better distributed Pt NPs on N-doped, carbon nanotubes which had a large graphitic N content .…”

“…23 Several other reports also attribute better initial NP nucleation and dispersion when using pyridinic N rich supports. 24,25 However, a recent report calculated greater energy of adsorption for Pt when placed around a graphitic N environment, which was confirmed with experimental evidence. 26 This agrees with a study by Chen et al.…”

“…Typically, PEMFC literature reports featuring Pt nucleation on NC supports from a solution have found that pyridinic N is a more favorable site for Pt nucleation. − Since pyridinic N is seen predominately in carbon lattice defects, Pt ions may have a greater ability to access pyridinic N sites, while Pt NPs are less likely to be affected by the same chemistry due to the physical size of the NP . The results observed for samples investigated in this work, especially those produced at lower temperatures, emphasize the inherent differences in the chemical interaction between NC supports and the Pt nanoparticles.…”

Cobalt–platinum intermetallic composite loaded on pyridinic N-enriched carbon for acidic hydrogen evolution catalysis with ultralow overpotential

“…It is well known that for a carbon-substrate supported catalyst, the optimal interaction between metal species and the carbon supports is crucial to achieving increased performance and durability. 40 Thus, asides from the CoPt 3 active sites, the NCS substrate should also play an important role in the promising HER activity of NCS-CoPt. For comparison, CoPc-Pt was directly calcined without adding dicyandiamide to obtain NCB-CoPt.…”