In this paper, we present a comprehensive study on how stacking faults, crystallite size, crystallite size distribution as well as shape and strain dictate the nature of the X-ray powder diffraction patterns of small (<20 nm) and large (>20 nm) cobalt (Co) nanoparticles. We provide a unique library of simulated diffractograms which can be used for fingerprint analysis. Likewise, the simulated data are used as a basis for structural refinements of experimentally obtained Xray powder diffractograms. We provide examples of using the library for fingerprint analysis and for full structural analysis of synthesized Co nanoparticles. Structural refinements presented in this study allow to reveal fine structural details that directly correlate to different behavior upon heating in a CO atmosphere relative to a H 2 or He atmosphere. All calculations were performed using the Discus package and the Debye scattering equation.

Using a MEMS nanoreactor in combination with a specially designed in situ Transmission Electron Microscope (TEM) holder and gas supply system, we imaged the formation of multiple layers of graphene encapsulating a cobalt nanoparticle, at 1 bar CO : N (1 : 1) and 500 °C. The cobalt nanoparticle was imaged live in a TEM during the Boudouard reaction. The in situ/operando TEM studies give insight into the behaviour of the catalyst at the nanometer-scale, under industrially relevant conditions. When switching from Fischer-Tropsch syngas conditions (CO : H : N 1 : 2 : 3 at 1 bar) to CO-rich conditions (CO : N 1 : 1 at 1 bar), we observed the formation of multi-layered graphene on Co nanoparticles at 500 °C. Due to the high temperature, the surface of the Co nanoparticles facilitated the Boudouard reaction, causing CO dissociation and the formation of layers of graphene. After the formation of the first patches of graphene at the surface of the nanoparticle, more and more layers grew over the course of about 40 minutes. In its final state, around 10 layers of carbon capped the nanoparticle. During this process, the carbon shell caused mechanical stress in the nanoparticle, inducing permanent deformation.

In the past few decades, advances in colloidal nanoparticle synthesis have created new possibilities for the preparation of supported model catalysts. However, effective removal of surfactants is a prerequisite to evaluate the catalytic properties of these catalysts in any reaction of interest. Here we report on the colloidal preparation of surfactant-free Ni/AlO model catalysts. Monodisperse Ni nanoparticles (NPs) with mean particle size ranging from 4 to 9 nm were synthesized via thermal decomposition of a zerovalent precursor in the presence of oleic acid. Five weight percent Ni/AlO catalysts were produced by direct deposition of the presynthesized NPs on an alumina support, followed by thermal activation (oxidation-reduction cycle) for complete surfactant removal and surface cleaning. Structural and morphological characteristics of the nanoscale catalysts are described in detail following the propagation of the bulk and surface Ni species at the different treatment stages. Powder X-ray diffraction, electron microscopy, and temperature-programmed reduction experiments as well as infrared spectroscopy of CO adsorption and magnetic measurements were conducted. The applied thermal treatments are proven to be fully adequate for complete surfactant removal while preserving the metal particle size and the size distribution at the level attained by the colloidal synthesis. Compared with standard impregnated Ni/AlO catalysts, the current model materials display narrowed Ni particle size distributions and increased reducibility with a higher fraction of the metallic nickel atoms exposed at the catalyst surface.

BackgroundReproducible growth of narrow size distributed ε-Co nanoparticles with a specific size requires full understanding and identification of the role of essential synthesis parameters for the applied synthesis method. For the hot injection methodology, a significant discrepancy with respect to obtained sizes and applied reaction conditions is reported. Currently, a systematic investigation controlling key synthesis parameters as injection-temperature and time, metal to surfactant ratio and reaction holding time in terms of their impact on mean (mean) and median (median) particle diameter using dichlorobenzene (DCB), Co2(CO)8 and oleic acid (OA) as the reactant matrix is lacking.MethodsA series of solution-based ε-Co nanoparticles were synthesized using the hot injection method. Suspensions and obtained particles were analyzed by DLS, ICP-OES, (synchrotron)XRD and TEM. Rietveld refinements were used for structural analysis. Mean (mean) and median (median) particle diameters were calculated with basis in measurements of 250–500 particles for each synthesis. 95 % bias corrected confidence intervals using bootstrapping were calculated for syntheses with three or four replicas.Resultsε-Co NPs in the size range ~4–10 nm with a narrow size distribution are obtained via the hot injection method, using OA as the sole surfactant. Typically the synthesis yield is ~75 %, and the particles form stable colloidal solutions when redispersed in hexane. Reproducibility of the adopted synthesis procedure on replicate syntheses was confirmed. We describe in detail the effects of essential synthesis parameters, such as injection-temperature and time, metal to surfactant ratio and reaction holding time in terms of their impact on mean (mean) and median (median) particle diameter.ConclusionsThe described synthesis procedure towards ε-Co nanoparticles (NPs) is concluded to be robust when controlling key synthesis parameters, giving targeted particle diameters with a narrow size distribution. We have identified two major synthesis parameters which control particle size, i.e., the metal to surfactant molar ratio and the injection temperature of the hot OA–DCB solution into which the cobalt precursor is injected. By increasing the metal to surfactant molar ratio, the mean particle diameter of the ε-Co NPs has been found to increase. Furthermore, an increase in the injection temperature of the hot OA-DCB solution into which the cobalt precursor is injected, results in a decrease in the mean particle diameter of the ε-Co NPs, when the metal to surfactant molar ratio is fixed at ~12.9.Electronic supplementary materialThe online version of this article (doi:10.1186/s13065-016-0156-1) contains supplementary material, which is available to authorized users.