We have broadened the scope of the aminophosphine precursor chemistry that has been developed for InP quantum dots to the synthesis of cadmium, zinc, cobalt, and nickel phosphide nanocrystals. The generalized synthetic conditions involve thermolysis of the appropriate MX 2 salt with tris-diethylaminophosphine in a long chain primary amine. The resulting Cd 3 P 2 nanocrystals exhibit size tuning effects based on the metal halide reactivity. 31 P NMR studies show that the II-V materials form via the previously described mechanism observed for InP, demonstrating the invariance of this chemistry to the metal valence. We also demonstrate that electrocatalytically active transition metal phosphides, specifically Co 2 P, CoP, and Ni 2 P, can be produced using this synthetic method at relatively mild temperatures and in high yields.
The activity of colloidally synthesized CoP for the hydrogen evolution reaction (HER) was studied to determine the impact of surface ligands on catalysis. The as-synthesized CoP nanocrystals were stripped with trialkyloxonium tetrafluoroborate (Meerwein’s reagent) to create a “blank template” that was then re-ligated with carboxylates (oleate, octanoate, acetate) and amines (oleylamine, octylamine, dioctylamine, trioctylamine butylamine). Carboxylates and amines were chosen due to their prevalence in colloidal syntheses, and the range of ligands was chosen to study the impact of sterics and hydrophobicity of the surface ligands on catalysis. Long chain carboxylates were found to result in a larger increase in overpotential when compared to the equivalent amine (e.g., oleate vs oleylamine), due to higher ligand density and stronger coordination with carboxylates. Increased carbon chain length resulted in increased overpotential with carboxylates; however, the range of 1° amines studied had similar overpotentials. This is due to the lower ligand density and therefore sparse ligand packing for the 1° amines. These results suggest that the mechanism by which surface ligands impede catalysis on CoP for HER is primarily through inhibiting substrate access to surface active sites rather than poisoning the active sites.
Cobalt phosphide (CoP) is one of the most promising earthabundant replacements for noble metal catalysts for the hydrogen evolution reaction (HER). Critical to HER is the binding of H atoms. While theoretical studies have computed preferred sites and energetics of hydrogen bound to transition metal phosphide surfaces, direct experimental studies are scarce. Herein, we describe measurements of stoichiometry and thermochemistry for hydrogen bound to CoP. We studied both mesoscale CoP particles, exhibiting phosphide surfaces after an acidic pretreatment, and colloidal CoP nanoparticles. Treatment with H 2 introduced large amounts of reactive hydrogen to CoP, ca. 0.2 H per CoP unit, and on the order of one H per Co or P surface atom. This was quantified using alkyne hydrogenation and Hatom transfer reactions with phenoxy radicals. Reactive H atoms were even present on the as-prepared materials. On the basis of the reactivity of CoP with various molecular hydrogen donating and accepting reagents, the distribution of binding free energies for H atoms on CoP was estimated to be roughly 51−66 kcal mol −1 (ΔG°H ≅ 0 to −0.7 eV vs H 2 ). Operando X-ray absorption spectroscopy gave preliminary indications about the structure of hydrogenated CoP, showing a slight lattice expansion and no significant change of the effective nuclear charge of Co under H 2flow. These results provide a new picture of catalytically active CoP, with a substantial amount of reactive H atoms. This is likely of fundamental relevance for its catalytic and electrocatalytic properties. Additionally, the approach developed here provides a roadmap to examine hydrogen on other materials.
Interfacial chemistry dramatically impacts the activity (performance) and reactivity (mechanism) of nanoparticle catalysts.
A series of four new bis-P 2 N 2 (P 2 N 2 = 1,5-diaza-3,7-diphosphacyclooctane) Pd(II) complexes were synthesized and characterized by spectroscopy, electrochemistry, and X-ray crystallography. The compounds crystallize in square planar or tetrahedrally distorted geometries and exhibit a single quasireversible 2e − Pd(II/0) redox couple in voltammetric studies.[Pd(P Ph 2 N Bn 2 ) 2 ] 2+ and [Pd(P Me 2 N Ph 2 ) 2 ] 2+ were tested for electrochemical CO 2 reduction in the presence of excess protons and found to preferentially produce H 2 .
We are interested in achieving control of heterogeneous interfaces to promote inner-sphere electron-proton-transfer reactions using insights from molecular chemistry. Toward this end, we are developing solution-phase methods for the synthesis and post-synthetic modification of colloidal nanostructures with low overpotentials and high activity for multi-electron, multi-proton electrocatalytic reactions. Ligand exchange, surface etching, and covalent modification will be presented as complementary methods for altering the catalytic activity of colloidal transition metal phosphide electrocatalysts. A combination of electrochemical characterization, structural analysis, and electronic structure characterization will be presented. Together with stoichiometric probes of inner-sphere reactivity, these data will serve to reveal mechanistic details and design principles for catalysis in these systems.
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