Showing papers by "Tanya Tsoncheva published in 2021"

Formation of Catalytic Active Sites in Hydrothermally Obtained Binary Ceria-Iron Oxides: Composition and Preparation Effects

Abstract: A series of mesoporous cerium-iron binary oxides was prepared by a hydrothermal technique using CTAB as a template. The influence of the Fe/Ce ratio and the variations in the preparation techniques such as the type of solvent and the precipitation agent, the approach of the template release, and the temperature of calcination on the phase composition, textural, structural, surface, and redox properties of the obtained materials was studied in details by XRD, nitrogen physisorption, TPR, FTIR, UV-vis, XPS, Raman, and Moessbauer spectroscopies. The materials were tested as catalysts in methanol decomposition and total oxidation of ethyl acetate. It was assumed that the binary materials represented a complex mixture of differently substituted ceria- and hematite-like phases. Critical assessment of their formation on the base of a common mechanism scheme was proposed. This scheme declares the key role of the formation of shared Ce-O-Fe structures by insertion of Fe3+ in the ceria lattice and further competitive compensation of the lattice charge balance by the existing in the system ions, which could be controlled by the Fe/Ce ratio and the hydrothermal synthesis procedure used. This mechanism provides proper understanding and regulation of the catalytic behavior of cerium-iron oxide composites in methanol decomposition with a potential for hydrogen production and total oxidation of ethyl acetate as a model of VOCs.

Ni0.5M0.5Fe2O4 (M = Cu, Zn) Ferrites Hosted in Nanoporous Carbon from Waste Materials as Catalysts for Hydrogen Production

Abstract: This work is focused on the preparation of Ni0.5M0.5Fe2O4 mixed ferrites (M = Zn or Cu), supported on nanoporous carbons materials. The carbon supports are obtained from various waste residues, such as peach stones from the canning industry and by-products from the low rank coals pyrolysis. X-ray diffraction (XRD), nitrogen physisorption, high-resolution transmission electron microscopy (HRTEM), Moessbauer spectroscopy and temperature programmed reduction (TPR) analyses as well as mesoporous silica with tridimensional structure type KIT-6 silica based reference samples are used for detail characterization of the obtained materials. It was established that the loaded on the carbon supports phase is a complex mixture of finely dispersed ferrite, substituted magnetite, metal (Cu, Fe, FeNi alloy) and ZnO particles. Their dispersion and composition depend on the texture characteristics of the carbon support, which could be easily controlled by the waste precursor used. The existence of mesoporosity in the carbon host matrix provokes the formation of more finely dispersed and easily reducible spinel particles, which ensures higher initial catalytic activity, but fast deactivation of the catalysts. The formation of activated carbon mesoporosity is facilitated by the presence of cellulose and hemicellulose in the biomass or the addition of furfural to the coal tar pitch precursor. The Ni0.5Zn0.5Fe2O4 modifications demonstrate higher potential as catalysts for hydrogen production via methanol decomposition.

Design and Catalytic Behaviour of Hosted in Activated Carbon Foam CoxZn1−xFe2O4 Ferrites

Abstract: Carbon foams with different surface functionality and tailored texture characteristics were prepared from mixtures containing coal tar pitch and furfural in different proportions. The obtained materials were used as a host matrix for the preparation of zinc- and cobalt-mixed ferrite nanoparticles. The texture, morphology, phase composition, and the related redox and catalytic properties of the obtained composites were characterized by low-temperature nitrogen physisorption, XRD, SEM, HRTEM, FTIR, Mossbauer spectroscopy, TPR and catalytic decomposition of methanol to syngas. The impact of the carbon support on the formation of Co- and Zn-mixed ferrites was discussed in detail using KIT-6 silica-based modifications as reference samples. The catalytic behavior of the ferrites was considered in a complex relation to their composition, morphology, location in the porous matrix and metal ions distribution in the spinel sub-lattices. The higher amount of furfural in the carbon foam precursor promoted the formation of cobalt-rich, more accessible and highly active methanol decomposition to syngas spinel particles.

Synthesis, Mössbauer study and catalytic properties of Cu-Ni-Fe- oxide/nitride mixed-phase materials

Abstract: Materials with cation ratio Cu:Ni:Fe = 0.5:0.5:2 were prepared by sol-gel auto-combustion technique using corresponding metal nitrates and citric acid as complexating/fuel agent and ammonia for pH adjustment. For comparison, one sample was synthesised by the same method, but without pH adjustment, and one sample with nominal composition Cu0.5Ni0.5Fe2O4 was synthesised by co-precipitation method. The methods of X-Ray Diffraction, Mossbauer spectroscopy at room (RT) and liquid nitrogen (LNT) temperature, UV-Vis, FTIR, nitrogen physisorption, and H2-TPR were applied for structural characterization of the materials. X-Ray Diffraction and Mossbauer spectroscopy proved formation of spinel phase, Fe4-xNixN and minor quantity CuO phase. Presence of doublets with high quadrupole splitting (Δ = 1.02–1.07 mm/s) at RT Mossbauer spectra, which are partly preserved at LNT is evidenced that nitride phase has high nickel content. Specific surface of oxide/nitride samples was determined in the range 19–24 m2/g. The catalytic behaviour of the Cu-Ni-Fe- oxide/nitride materials is studied in reaction of methanol decomposition. Mossbauer spectra reveal significant phase transformations of the samples to Hagg carbide (χ-Fe5C2) and Fe,Ni-alloy under the reaction medium, which ensures good catalytic activity and selectivity to H2 and CO. The photocatalytic oxidation activity of samples was tested in degradation of Alizarin model contaminant dissolved in water under visible light and UV-C light irradiation.