Monometallic 50 wt?% Cu/Al2O3 catalyst and bimetallic catalysts containing 25 wt?% Co/25 wt?% Cu, 25 wt?% Co/25 wt?% Fe, and 25 wt?% Cu/25 wt?% Fe, supported on Al2O3, were prepared by impregnation and coimpregnation methods. For bimetallic catalysts, metal oxides were in the form of spinel oxides, which exhibited a strong metal–support interaction. The decomposition of methane over these catalysts led to the formation of pure hydrogen and carbon nanotubes on their surfaces. The activation energy, total carbon yield, and amount of hydrogen formed, by using the prepared catalysts, were in agreement with the metal dispersion and acid–base site ratio on the surface of the catalysts. Cu-Fe/Al2O3 catalyst exhibited a stable hydrogen formation rate of 58 mmol?min-1?g-1 at a temperature of 650?°C. All catalysts exhibited deactivation after 500 min, which occurred due to the formation of carbon on the surface of the catalysts. The carbon material deposited predominantly assumed the form of multiwalled carbon nanotubes, as evidenced by high-resolution TEM and Raman spectroscopy. Thermogravimetric analysis finally confirmed that Cu-Fe/Al2O3 exhibited a higher yield of multiwalled carbon nanotubes.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 6028314Nanostructured inorganic fluorides are a relatively new class of nanomaterials, i.e. broader research of these fluorides commenced approximately 15 years ago. Since then, research in this field resulted in the preparation of a variety of fluoride-based nanostructured materials with unprecedented characteristics. These achievements are closely connected with the development of novel synthetic approaches that allow a better control of the key nanoscopic features of the fluorides formed. Earlier, our contribution to this field was the development of an alternative and completely inorganic route to fluorides with unconventionally high surface areas (HS-fluorides). More recently, we developed a modified solvothermal methodology in non-aqueous media for the preparation of very porous AlF3-based materials. This approach allowed, for the first time, a direct preparation of extremely voluminous AlF3-based aerogels. For both types of fluoride materials, the uncommon characteristics of the underlying fluorides can be associated with their distinctive nanostructure. Invited presentation gave a brief overview of the related preparation methodologies. Focus was on the solvothermal approach that proved t obe highly versatile and allows the preparation of fluorides with variable porosity, i.e. in the form of compact xerogels, weakly agglomerated powders or voluminous aerogels. As determined, bulk structure of theAlF3-based products strongly depends on the shape, size and spatial arrangement of the primary nanoparticles. These characteristics of fluoride nanoparticles can be effectively controlled by a proper combination of solvent(s) and temperature regimes applied throughout the preparation.Under optimal conditions, this procedure allows the preparation of regularly shaped and uniformly sized fluoride nanoparticles. Some key preparation features that determine the acidity of these nanomaterials,i.e. strength and type of the surface acidic sites, were also outlined.
B.04 Guest lecture
COBISS.SI-ID: 29673255The effect of the FePO4 material phase transformation in the direct selective oxidation of methane to methanol was studied using various oxidants, i.e. O2, H2O and N2O. The phases of the heterogeneous catalyst applied, before and after the reactions, were characterized by Mossbauer spectroscopy. The main reaction products were methanol, carbon monoxide and carbon dioxide, whereas formaldehyde was produced in rather minute amounts. The Mössbauer spectra showed the change of the initial catalyst material, FePO4 (tridymite-like phase (tdm)), to the reduced metal form, iron(II) pyrophosphate, Fe2P2O7, and thereafter, the material phase change was governed by the oxidation with individual oxidizing species.Mössbauer spectroscopy measurements applied along with X-ray diffraction (XRD) studies on fresh, reduced and spent catalytic materials demonstrated a transformation of the catalyst to a mixture of phases which depended on operating process conditions. Generally, activity was low and should be a subject of further material optimization and engineering, while the selectivity towards methanol at low temperatures applied was adequate. The proceeding redox mechanism should thus play a key role in catalytic material design, while the advantage of iron-based heterogeneous catalysts primarily lies in them being comparably inexpensive and comprising non-critical raw materials only.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 30319143In our search for fluoride materials exhibiting both, high surface area and high porosity, we combined a slightly modified fluoride sol-gel synthesis with solvothermal treatment in non-aqueous media below and above the critical conditions. This approach proved to be very adaptable and allowed the preparation of fluorides with significant differences in porosities. Depending on the specific preparation conditions, fluoride materials ranging from dense xerogels to extremely voluminous aerogels could be obtained. This combined methodology enabled a first direct preparation of very voluminous aerogels based on AlF3 which represent an entirely new class of inorganic aerogels. It was found that the bulk structure of AlF3-based products strongly depends on the spatial arrangement of primary nanoparticles which is determined by their shape, size and aspect ratio. These features of fluoride nanoparticles can be controlled to some extent by a proper combination of solvent(s) and temperature regimes applied throughout the preparation. Fluoride aerogels based on AlF3 are obtained only in methanol-containing solvents above critical conditions where relatively uniform and elongated nanoparticles are formed which are loosely entangled in very open but sufficiently rigid structures. Other geometries of basic nanoparticles yield collapsed or more compact products, e.g. xerogels or weakly agglomerated powders. During the optimisation of the solvothermal processing, structure development during solvothermal treatment was tracked by in-situ sampling followed by TEM analyses. In some cases, additional insights in the structure on the atomic scale were obtained by advanced STEM techniques.
F.01 Acquisition of new practical knowledge, information and skills
COBISS.SI-ID: 31307815The present work contains the studies of the effect of the phases, dispersion and reduction of bimetallic Cu–Fe transition metal oxides, supported on the commercial and synthesized carbon nanotubes (CNT) and ?-Al2O3, which were prepared by various procedures, as well as their effect in the catalytic reduction of NO with CO. All catalysts contained the monocrystalline phases of CuO and Fe2O3 alongside with CuFe2O4, as well as Cu and Fe oxides. The chemisorbed NO and CO amounts were in agreement with the respective metal dispersion. The catalytic activity kinetics for NO reduction reaction mechanism was improved over the synthesized CNT when compared to their commercial analogues and alumina support. This was due to the strong interaction of Cu–Fe with the supporting carbon nanotubes, as well as acidic and basic surface sites, to accelerate NO reduction. CO conversion profiles paralleled those of NO on these catalysts, while all the materials showed an unchanged stability over the period of 10 h. The apparent activation energies of the selective catalytic reduction of NO with CO over Cu–Fe, supported by the synthesized CNT and ?-Al2O3, were 8.7 kJ mol-1 and 11.4 kJ mol-1, respectively. The in situ FTIR studies under NO/CO flow indicated that metallic copper and iron were the active species for N2O decomposition; thus, Cu+/Cu0 and Fe2+/Fe0 redox cycle evidently played an important role in NO decomposition. The present results suggest that CO can be used as a reducing agent to selectively reduce NO without oxygen.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 6182170