Turnover frequency (TOF) and turnover number (TON) of such Ziegler-type catalysts based on Ni(acac)(2).nH(2)O (n = 0, 0.5, 3.0) and AlEt3, at different Al/Ni molar ratios, have been determined during their examination as styrene hydrogenation catalysts. It is found that the amount of crystallization water effects on the TOF and TON values of the nickel catalysts and defines their dependence on the Al/Ni ratio. It is shown that optimum concentration of proton-donating compounds ensures high values of TOF and TON. The nature of nickel-containing nanoparticles was determined using high-resolution electron microscopy. The size of the nickel clusters depended on the water content of the nickel precursor. The average particle size increased from 1.2 to 2.3 nm on changing the precursor from Ni(acac)(2).0.5H(2)O to Ni(acac)(2).3H(2)O. It was shown that organoaluminum compounds acted as Ni(II) -> Ni(0) reducing agents, nanoparticle stabilizers, and hydrogenation activity inhibitors. A nickel nanoparticle model is specified, one which explains the activating role of the proton-donating compounds in combination with the Al/Ni ratio.(C) 2015 Elsevier B.V. All rights reserved.