Ultra-stable and highly reactive colloidal gold nanoparticle catalysts protected using multi-dentate metal oxide nanoclusters

Owing to their remarkable properties, gold nanoparticles are applied in diverse fields, including catalysis, electronics, energy conversion and sensors. However, for catalytic applications of colloidal gold nanoparticles, the trade-off between their reactivity and stability is a significant concern. Here we report a universal approach for preparing stable and reactive colloidal small (~3 nm) gold nanoparticles by using multi-dentate polyoxometalates as protecting agents in non-polar solvents. These nanoparticles exhibit exceptional stability even under conditions of high concentration, long-term storage, heating and addition of bases. Moreover, they display excellent catalytic performance in various oxidation reactions of organic substrates using molecular oxygen as the sole oxidant. Our findings highlight the ability of inorganic multi-dentate ligands with structural stability and robust steric and electronic effects to confer stability and reactivity upon gold nanoparticles. This approach can be extended to prepare metal nanoparticles other than gold, enabling the design of novel nanomaterials with promising applications.


Synthesis Preparation of supported gold nanoparticle catalysts
Gold nanoparticle catalyst supported on manganese oxide octahedral molecular sieve (Au/OMS-2) was prepared according to the reported procedure: 1 an aqueous solution of HAuCl4•4H2O (8.3 mM, 60 mL) containing OMS-2 (2.0 g) was vigorously stirred at room temperature.After 15 min, the pH of the solution was quickly adjusted to 10 by addition of an aqueous solution of NaOH (1.0 M), and the resulting slurry was further stirred for 24 h.The solid was then filtered, and the residue was washed with a large amount of water (4 L), and dried in vacuo to afford the supported hydroxide catalyst precursor.Then, the hydroxide precursor was calcined at 300 °C for 2 h to give Au/OMS-2 as a dark brown powder (Au content: 3.6 wt%).
Gold nanoparticle catalyst supported on hydroxyapatite (Au/HAP) was prepared according to the reported procedure: 2 HAP (2.0 g) was added to an aqueous solution of HAuCl4 (2 mM, 100 mL).After vigorously stirring the mixture for 2 min, aqueous NH3 (10%, 240 μL) was added, and the resulting mixture was stirred at room temperature for 14 h.The resulting slurry was filtered, washed with deionised water (1 L) and dried at room temperature in vacuo to give the HAP-supported Au precursor.The resulting species was dispersed in deionised water (100 mL) and treated with NaBH4 (80 mg) at room temperature for 1 h.The mixture was then filtered, and the residue was washed with water (1 L) and dried to afford Au/HAP as a reddishpurple powder (Au content as determined by ICP-AES: 1.5 wt%).

Preparation of TOASiW9 and TDASiW9 as reference materials
TOASiW9 and TDASiW9 were prepared through phase transfer of SiW9 using tetabutylammonium bromide (TOAB) and tetadecylammonium bromide (TDAB), respectively, as follows: an aqueous solution of NaSiW9 (20 mL, 5 mM) was mixed with a solution of TOAB or TDAB in toluene (20 mL, 50 mM).The two-phase mixture was vigorously stirred for 30 min, followed by a phase-separation to yield the organic layer as the toluene solution of )-/ × 6.02 × 10 0' = 834.Based on total concentration of gold atoms (from chloroauric acid as 5 ´ 10 −4 M, gold nanoparticle concentration is 6 ´ 10 −7 mol/L.Thus, the number of dodecanethiol molecules in one gold nanoparticle is calculated as ,×)* !" = 260.Based on previous knowledges that an average ratio as 8.5 during the ligand exchange process from mono-lacunary POMs ([AlW11O39] 9− ) to 11mercaptoundecanoate (J.Am.Chem.Soc.2009, 131, 17412−17422; ACS Nano 2012, 6, 629−640), there should be around 30 POM ligands surrounding a 3 nm gold particle in this case, and surface coverage ratio can be also estimated as 47%.During the optimization of amount of TOAB and NaBH4 (Table S1), most of POMs in toluene phase were found to be reversed back to aqueous phase according the elemental analysis results (Table S1, Entry 5).Unsurprisingly, as-obtained gold nanoparticles agglomerated and aggregated within one month; whereas, this "undesired" finding can be utilized here to confirm POM structures during synthesis.In the IR spectra, the characteristic peaks of SiW9 in the region of 500 − 1000 cm −1 were well consistent between NaSiW9 and POMs after mixing with gold precursors and sodium borohydride respectively, indicating their intact structures in this method.Supplementary Table 1   a Reaction conditions: 1a (0.25 mmol), 3 mL toluene solution of colloidal gold nanoparticles (Au: 4 mol%), K2CO3 (0.5 mmol), room temperature (~25 °C), O2 (1 atm), 24 h.b Formed by reaction of 3a with octylhalide, decomposition product of TOAB.c Cs2CO3 (0.5 mmol) was used instead of K2CO3 (0.5 mmol).d Freeze-pump-thaw cycles were carried out and the reactor was connected to a balloon filled with an Ar gas.

TOASiW9 and TDASiW9. Supplementary Fig. 5 |SupplementaryFig. 9 |
Photographs and UV-vis spectra of the solutions of various ligand-protected gold nanoparticles in toluene before and after the addition of Cs2CO3 and stirring at room temperature for 24 h: a, Au-TOASiW9, b, Au-dodecanethiol, c, Au-TOAB.Au-dodecanethiol and Au-TOAB agglomerated and partially precipitated.Reaction conditions: 3 mL toluene solution of colloidal gold nanoparticles (Au: 0.01 mmol),Cs2CO3 (0.5 mmol), room temperature (~25 °C), O2 (1 atm), 24 h.Titration experiments using dodecanethiol in confirming surface coverage of Au-TOASiW9: a,b, UV-vis spectra of Au-TOASiW9 (0.5 mM) in toluene upon addition of toluene solution of dodecanethiol (5 mM).c, A plot showing the change of the absorbance of SPR band (524 nm) upon addition of a toluene solution of dodecanethiol.d, Illustration for Au-TOASiW9.The UV-vis spectra exhibit the continuous decrease of SPR band with showing isosbestic points upon addition of toluene solution of dodecanethiol, and then become constant after addition of 70 μL of the solution (dodecanethiol concentration, 1.56 ´ 10 −4 mol/L).Based on the differences in refractive indices (J.Phys.Chem.B 2005, 109, 21556−21565; Chem.Rev. 2008, 108, 462−493), these results indicate the ligand exchange from POMs to dodecanethiols.Based on the density and molar mass of Au (19.3 g/cm 3 and 197 g/mol ), assuming a spherical shape and a uniform face-centered cubic structure, the average number of gold atoms in 3 nm gold nanoparticle is calculated as

Supplementary Fig. 10 |Supplementary Fig. 14 |
XPS spectra of various ligand-protected gold nanoparticles: a, Au-TOASiW9.b, Au-TOAB.c, Au-TOASiW10.d, Au-TOASiW11.e, Au-TOASiW12.f, bulk Au (CAS No. 7440-57-5).Confirmation of structures of POMs in aqueous phase during synthesis after mixing with gold precursors and addition of sodium borohydride.a, Illustrative scheme of operative procedure.b, Photographs of solid samples by freeze-drying aqueous phase containing POMs and their IR spectra in comparison to NaSiW9.

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Reported synthetic routes/preparation methods of gold nanoparticles with various stabilizing agentsConcentration of initially added gold precursors in final solution used.b A = hydrophobic surface, B = hydrophilic surface.
a c Catalytic application was not mentioned in these reports.d Tetraoctylammonium bromide.e Cetyltrimethylammonium bromide.

Table 2 .
Results of elemental analysis for investigating percentages of transferred SiW9 into toluene phase (i.e., TOASiW9) from initial SiW9 in water phase (i.e., NaSiW9) under different adding amounts of TOAB and reductant (NaBH4) with respect to HAuCl4.