ORIGINAL CONTRIBUTION Preparation of metal and metal oxide doped silica hollow spheres and the evaluation of their catalytic performance Aurel Diacon 1 & Edina Rusen 1 & Adrian Trifan 1 & Raluca Șomoghi 2 & Oana Tutunaru 3 & Gabriel Crăciun 3 & Cristina Busuioc 1 & Georgeta Voicu 1 Received: 21 April 2020 /Revised: 6 July 2020 /Accepted: 24 July 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The aim of this study was the synthesis of silica hollow spheres-based materials doped with metal nanoparticles or metal oxides. Two different strategies based on the use of polymer colloids and cetyltrimethylammonium bromide (CTAB) as dual templates were developed. The first strategy involves the use of soap-free emulsion polymerization for the trapping of metal nanoparticles (Ag and Ni) and in the polymer structure. The second strategy exploited carboxyl groups present on the surface of the polymer particles for the adsorption of metal salts (Ni and Fe). A complete porous SiO 2 shell was generated around the polymer colloids using a Stöber method and CTAB to guide the silica shell growth. The obtained silica hollow sphere displayed either yolk-shell distribution of the doping elements or a uniform oxide deposit on the interior surface of the silica capsules. The versatility of the synthesis method and the catalytic performance of the materials were demonstrated in the case of Fe@SiO 2 for a Fischer-Tropsch process. Keywords Silica hollow spheres . Metal nanoparticles and metal oxides . Soap-free polymerization . Adsorption . Catalyst Introduction Inorganic nanospheres with hollow interior have attracted ever-increasing attention as a result of their many remarkable properties, such as low density, low toxicity, good chemical and thermal stability, and large surface area [1–4]. The hollow spheres are now used in the preparation of materials specially designed for catalysis and medicine, as supports of catalytically active species and for the controlled release of molecules with biological activity used in pharmaceutical for- mulations. Thus, the hollow interior of these spheres enables the encapsulation of a wide range of molecules [5, 6]. Numerous techniques have been developed to prepare hol- low silica, including template-assisted synthesis [1, 5, 7–9], self-templating method [7, 10, 11], and spray drying [6, 12]. The templates could be removed by chemical etching or by calcination. As templates, carbonaceous spheres have been usually considered as sacrificial templates for the synthesis of hollow spheres due to the facile removal [6]. Template choice has a decisive influence on cavity shape control and volume of hollow silica. Depending on their phys- ical characteristics, templates can be further divided into hard templates [13] and soft templates [14, 15]. Hard templates usu- ally have a clear definite shape and a structural rigidity. Many substances, including polymeric, inorganic, biological, and me- tallic components, can be used as hard templates. In contrast, soft templates are susceptible to deformation during the silica- coating process. Soft templates examples include emulsion droplets, polymer aggregates, and surfactant micelles [6]. When considering hard sacrificial templating, the critical aspects are the size and size distribution of the template as well as the method for its removal. Polymer templates can address Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00396-020-04722-4) contains supplementary material, which is available to authorized users. * Edina Rusen edina_rusen@yahoo.com 1 Faculty of Applied Chemistry and Materials Science, University POLITEHNICA of Bucharest, Gh. Polizu Street 1-7, 011061 Bucharest, Romania 2 National Research and Development Institute for Chemistry and Petrochemistry—ICECHIM, 202 Splaiul Independenţei, 060021 Bucharest, Romania 3 National Institute for Research and Development in Microtechnologies—IMT Bucharest, Erou Iancu Nicohlae Street 126A, 077190 Bucharest, Romania Colloid and Polymer Science https://doi.org/10.1007/s00396-020-04722-4