Ligand-Free Noble Metal Nanocluster Catalysts on Carbon Supports via “Soft” Nitriding

Abstract

We report a robust, universal “soft” nitriding method to grow in situ ligand-free ultrasmall noble metal nanocatalysts (UNMN; e.g., Au, Pd, and Pt) onto carbon. Using low-temperature urea pretreatment at 300 °C, soft nitriding enriches nitrogen-containing species on the surface of carbon supports and enhances the affinity of noble metal precursors onto these supports. We demonstrated sub-2-nm, ligand-free UNMNs grown in situ on seven different types of nitrided carbons with no organic ligands via chemical reduction or thermolysis. Ligand-free UNMNs supported on carbon showed superior electrocatalytic activity for methanol oxidation compared to counterparts with surface capping agents or larger nanocrystals on the same carbon supports. Our method is expected to provide guidelines for the preparation of ligand-free UNMNs on a variety of supports and, additionally, to broaden their applications in energy conversion and electrochemical catalysis.

Engineering Surface Ligands of Noble Metal Nanocatalysts in Tuning the Product Selectivity

“…Using CO oxidation as a model reaction, Wu et al studied the catalytic activity Surface ligands are usually unwanted in electrocatalysis, since the charge transfer of catalysts can be significantly slowed down or completely shut down with a ligand length of >2 nm. Electrocatalytic activity of noble metal catalysts can be suppressed by ligands in various reactions [26,27]. However, when surface ligands are short and they can strongly interact with surface atoms, electrocatalytic selectivity can be modulated similar to other chemical reactions.…”

“…In the presence of excess surface ligands (essentially necessary during the wet-chemical synthesis), ligands can form a densely packed self-assembled monolayer (SAM) in the thickness of 0.5-3 nm by maximizing van der Waals interactions of alkyl chains [18]. The surface ligands are demonstrated to be detrimental to the catalytic activity in many examples [20][21][22][23][24][25][26][27]. The formation of SAMs is often considered to block the surface accessibility, leading to a decrease in the accessibility of catalytic sites [20,23], although there are many reported examples showing that surface ligands do not depress catalytic activity of metal NPs [28][29][30][31].…”

Synthesis of well-dispersive 2.0 nm Pd–Pt bimetallic nanoclusters supported on β-cyclodextrin functionalized graphene with excellent electrocatalytic activity

“…It is well known that the catalytic activities of noble metal particles are highly size dependent. 4 For instance, ultrane Pd nanoparticles with a size of 3.5 nm supported on graphene oxide (GO) reported by Xie's group have displayed enhanced electrocatalytic activity for formic acid and ethanol electrooxidation.…”

“…4 However, it is still highly desirable to develop a facile and general approach for the fabrication of ultrane bimetallic nanocatalysts such as Pd-Pt clusters with ultrasmall sizes (#2 nm) though some monometallic clusters supported on carbon materials have been synthesized by different approaches if considering the potential industrial applications.…”

“…2,3 In particular, ultrasmall noble metal nanoclusters (#2 nm) with several to hundreds of atoms, have received signicant attention. 4 However, the surface energy increases with decreasing particle size, which usually leads to serious aggregation of the small particles in order to minimize the total surface energy. 2,5 To overcome this aggregation, the metal particles must be anchored to suitable supports.…”