In the experimental work leading to this contribution, the parameters of the ultrasound treatment (temperature, output power, emission periodicity) were varied to learn about the effects of the sonication on the crystallization of Ni nanoparticles during the hydrazine reduction technique. The solids were studied in detail by X-ray diffractometry, dynamic light scattering, thermogravimetry, specific surface area, pore size analysis, temperature-programmed CO2/NH3 desorption and scanning electron microscopy. It was found that the thermal behaviour, specific surface area, total pore volume and the acid-base character of the solids were mainly determined by the amount of the nickel hydroxide residues. The highest total acidity was recorded over the solid under low-power (30 W) continuous ultrasonic treatment. The catalytic behaviour of the nanoparticles was tested in a Suzuki-Miyaura cross-coupling reaction over five samples prepared in the conventional as well as the ultrasonic ways. The ultrasonically prepared catalysts usually performed better, and the highest catalytic activity was measured over the nanoparticles prepared under low-power (30 W) continuous sonication.

Effective oxidative transformation of toluene into valuable products was achieved under solvent-free reaction conditions with as-prepared nickel nanoparticles as heterogeneous catalysts in liquid phase. The crystalline structure and size of the asprepared nanoparticles were confirmed by X-ray diffractometry (XRD) and dynamic light scattering (DLS). The catalytic implications of the different crystalline forms (face-centred cubic: fcc; hexagonal close-packed: hcp) of these nanocatalysts were investigated. The product selectivity of toluene oxidation was found to vary depending on the crystalline forms of the catalyst. Fcc nanocatalysts showed remarkable chemoselectivity (83 mol %) for the product benzyl alcohol and were readily reusable. In contrast, the hcp Ni phase showed reasonable reusability but lower chemoselectivity (29 mol %) compared to its fcc counterpart. Moreover, the simple organic solvents used had a remarkable effect on the crystal structure and phase purity of the Ni nanocrystals, which also affected the catalytic process.