Synthesis of Ag nanoparticles on oxide and carbon supports from Ag diammine precursor

Abstract

To determine whether the method of "strong electrostatic adsorption" (SEA) can be extended to the preparation of uniform and highly dispersed supported Ag catalysts, the adsorption of silver diammine, Ag(NH 3) 2 + , also known as Tollen's reagent, has been examined over five supports (i.e., SiO 2 , γ-Al 2 O 3 , ZrO 2 , Nb 2 O 5 , and carbon) with different surface areas and PZCs. The speciation of Ag in solution was followed by UV-vis spectroscopy, Ag uptake as a function of the solution pH was determined by atomic absorption, and Ag particle size was determined by powder XRD and STEM. At the Ag concentrations, pH, and surface loadings employed, silver diammine complexes convert into soluble Ag(H 2 O) 2 + aquo complexes below pH 11 and hydrolyze to insoluble Ag 2 O above pH 11. The deposition of either Ag species over carbon at any pH appears to be reactive and results in large particles. The silver aquo complexes appear to adsorb via ion exchangenear the PZC of alumina, zirconia, and niobia but not silica, while the silver diammine complexes appear to adsorb electrostatically at high pH over all the oxides tested-niobia, silica, zirconia, and alumina in the order of ascending PZC. Niobia, which has the lowest PZC, adsorbs the highest surface density of Ag via both mechanisms. The particles obtained via electrostatic adsorption of Ag(NH 3) 2 + at high pH are somewhat smaller than those formed from Ag(H 2 O) 2 + ions at the lower pH values. In the absence of ammonia in solution at high pH, deposition of Ag aquo complexes occurs viasurface precipitation and gives large particles. In sum, SEA of silver ammine is demonstrated to be a simple, reproducible way to synthesize small particles on all supports but carbon.

Interfacial coordination chemistry for catalyst preparation

“…Molybdenum has only 25% of isotopes with a non-null nuclear spin (i.e. I=5/2), 97 Mo and 95 Mo, but the Mo precursors can be 95 Mo-enriched. Through the implementing of EPR at two frequencies (X-and Q-bands) and the simulation of spectra, it has been possible to precisely depict the local environment of Mo by full analysis of Mo 5+ EPR signals, namely g tensor values of the anisotropic signal, and coupling constants A of the hyperfine tensor (coupling between the spin of the unpaired electron and the 95 Mo nuclear spin).…”

“…In this case, the Mo 5+ EPR signal showed an axial symmetry with g^ >g // . Again, thanks to the use of 95 Mo-enriched Mo precursors, the g tensor values and coupling constants were determined [25]. From their comparison with those of a series of reference molybdenyl [26].…”

Nb2O5‐Based Photocatalysts

“…Thus, Ag/Nb 2 O 5 can be synthesized via the interaction between the Ag(NH 3 ) 2 + and Nb 2 O 5 at high pH (>5). [ 96 ]…”

Heterogeneous catalysis by ultra-small bimetallic nanoparticles surpassing homogeneous catalysis for carbon–carbon bond forming reactions

“…The hump present at ∼22°for all the samples is characteristic of the amorphous structure of SiO 2 . 55,58 The absence of any other peaks indicates the absence of crystalline materials and the formation of ultrasmall particles with no signs of particle aggregation as typically XRD has a 2-2.5 nm size limit. 59 XRD analysis was also performed on recycled CuPd/SiO 2 as shown in Fig.…”