Effect of Transition Metals on the Oxygen Reduction Reaction Activity at Metal‐N₃/C Active Sites

Abstract

The roles of various transition and post‐transition metals in model non‐precious‐metal catalysts for the oxygen reduction reaction (ORR) are reported after being prepared via a molecularly defined terpyridine unit covalently attached to a carbon black support. We previously reported the use of a terpyridine‐modified iron‐based catalyst that allowed for the controlled deposition of highly active nitrogen functionalities on the carbon support, which adopts a Fe−N3/C active site formation. In this work, we expand on this idea by altering the metal center in the predefined active sites M−N3/C and compare the ORR reactivity of the isostructural set of catalysts, where M=Fe, Co, Ni, Mn, and Sn. The results show that the iron‐based material was the most active catalyst in acid, whereas the cobalt‐based catalyst was most active in base. In addition, nickel‐ and manganese‐based materials showed promising activity for the ORR in both acidic and basic media. We demonstrate that, with a suitable templating bis‐chelating nitrogenous ligand, the M−N3/C active site geometry is adopted with a wide range of non‐precious‐metal centers on a Vulcan carbon surface, and the resulting catalysts are ORR active in a range of conditions, further confirming the tunability and versatility of the N3 site. As expected, post‐transition metals, such as tin, do not coordinate to the N3 nitrogenous ligand under synthetic conditions and deposits tin oxides(s). This study confirms the generality of the phenomenon of M−N3/C as an ORR catalytic site.

A post-functionalization toolbox for diazonium (electro)-grafted surfaces: review of the coupling methods

“…the MeCN ligands in [Ru(bpy) 2 (MeCN) 2 ](OTf) 2 . 263 The method has been successfully used for the grafting of Mn, 264,265 Fe, [265][266][267][268][269] Co, 262,265 Ni, 265,270,271 Cu, 160,272,273 Zn, 103 Ru, 103,263,274,275 Rh 153,276 and Ir 263 complexes. Most of the corresponding ligands were anchored following two different approaches; namely, the direct reduction of their diazonium precursors or the reaction of the conveniently modified ligand, (through one of the other post-functionalization processes examined in this review), with a previously immobilized organic coating.…”

“…16). 262,263,[265][266][267]271,[274][275][276][277][278][279][280][281] However, these compounds are generally prepared ''in situ'' from diazotization of their amine precursors, as the diazonium species are only stable at low temperature (E0 1C) in the timescale of minutes, preventing their isolation. On the other hand, in some cases, it is convenient to include an alkyl chain as spacer between the coordination moiety of the ligand and the anchoring group.…”

Carbon–nitrogen–metal material as a high performing oxygen evolution catalyst

“…See Table 1 for the summary of the physical characterization of the materials. 18 From our work we had determined that the Sn is not coordinating to the tpy-moieties, and forms Sn oxides on the surface of the carbon support instead. This was confirmed through intensive XPS and STEM analysis of the catalysts.…”

“…We had coordinated various metals (Ni, Co, and Sn) to the tpy ligand on the surface and assessed the ability of the materials to catalyze the oxygen reduction reaction. 15 In the work presented here we used the previously synthesized by us Vulcan carbon-based catalysts modified with a terpyridine ligand and various non-noble transition metals (Ni, Co, Sn) to assess their activity and stability for the oxygen evolution reaction (OER) in alkaline media, as Ni and Co are known to show some favourable OER activity in alkaline media. 16,17 The synthesis of V-tpy-Ni, V-tpy-Co, and V-tpy/Sn catalysts for the oxygen reduction reaction is described in detail in our previously published works.…”

“…16,17 The synthesis of V-tpy-Ni, V-tpy-Co, and V-tpy/Sn catalysts for the oxygen reduction reaction is described in detail in our previously published works. 18,19 Briefly, these materials were prepared at ambient temperature by covalent attachment of the terpyridine (tpy) ligand to the commercial Vulcan carbon support via diazonium coupling, followed by reaction with the Ni, Co or Sn precursor to obtain V-tpy-Ni, V-tpy-Co, and V-tpy/Sn materials, respectively. The diazonium coupling reaction leads to the loss of N 2 gas and forms a carboncarbon bond to the surface where the terminal NH 2 was.…”