Syngas production from CH4 and CO2 was investigated over bimetallic nickel-cobalt catalysts, promoted by a CeZrO2 redox component. The appropriate design of active sites responsible for methane and CO2 activation (bimetallic clusters below 45 nm, adjacent to oxygen vacancy sites of the CeZrO2 solid solution) enabled kinetic balancing of both reaction halves, producing catalysts that are highly resistant to carbon accumulation in a wide range of CH4-CO2 feed compositions. With the employed approach, carbon accumulation can be prevented over metal clusters that are 2-fold larger compared to state-of-the-art. By anchoring the active NiCo bimetallic and CeZrO2 redox components over a high surface area ß-SiC carrier (3NiCo/CeZrO2/S catalyst), the redox promoter is diluted and sintering of bimetallic NiCo clusters is reduced. At ambient pressure, a remarkably stable catalytic performance for 550 h was recorded with a produced H2/CO ratio of 0.82, methane reforming rate of 0.18 mol/gcat h and negligible carbon accumulation. Stable operation is maintained for 60 h during reforming at 20 bar, producing syngas with a H2/CO ratio of 0.33. Importantly, accumulated carbon yield is 2–3 orders of magnitude lower compared to state-of-the-art. These results constitute a promising basis for the design of a prospective technology for CO-rich syngas production through CH4-CO2 reforming.
COBISS.SI-ID: 6097946
This study explores CeZrO2 deposited over commercial ß-SiC, and a highly ordered 3D ß-SiC synthesised in the laboratory via electrophoretic deposition, as well as ?-Al2O3 in order to prepare three types of dual support for NiCo bimetallic catalyst in CH4–CO2 dry reforming (DR). CeZrO2 was deposited over ?-Al2O3 and ß-SiC by dry impregnation (DI), wet impregnation (WI) and 2-step deposition precipitation (DP). XRD analysis indicated that the constituents of the dual supports were retained after calcination, as well as before and after the DR reaction. CeZrO2 remained as a mixed oxide solid solution, whilst alumina formed spinel structures with Ni and Co before the catalysts were reduced in H2 during the pretreatment step prior to the activity tests. During 550 h stability tests, WI, 2-step SICAT/CeZrO2 and 2-step ?-Al2O3/CeZrO2 solids were identified as the most promising catalysts, maintaining high DR activities without deactivation. Notably, 2-step SiC(SICAT) and 2-step ?-Al2O3/CeZrO2 samples recorded the highest yield (H2 = 77%, CO = 90%; H2 = 71%, CO = 81%), with a coke content of 7.7 and 0.6 wt.%, respectively. Carbon deposition for the former is high; contrarily, for WI SiC(SICAT) solid, it accumulated a lower amount of 2.6 wt.%. No agglomeration of CeZrO2 and NiCo phases was observed, evidencing excellent robustness and thermal resistance of these dual supports.
COBISS.SI-ID: 37574917
Stable catalytic performance is one of the most important aspects of heterogeneous catalysis. Synthesized supported transition metal catalysts were thoroughly characterized using advanced surface-sensitive and bulk in-situ and operando techniques. Changes in physical and chemical properties of tested materials were correlated to the observed oscilation in their catalytic activity. It was discovered that oxidation of NiCo bimetallic clusters is strongly size-dependent and as such represents the primary reason for the observed catalyst deactivation. Catalyst regeneration was connected partly to transition of growing (in)active oxide clusters back to their initial metallic state and recrystallization of the ceria-zirconia redox support.
COBISS.SI-ID: 37294341
Wet hydrogen peroxide catalytic oxidation (WHPCO) is one of the most importantindustrially applicable advanced oxidation processes (AOPs) for the decomposition of organic pollutants in water. It is demonstrated that manganese functionalized silicate nanoparticles with interparticle porosity act as a superior Fenton-type nanocatalyst in WHPCO as they can decompose 80% of a test organic compound in 30 minutes at neutral pH and room temperature. By using X-ray absorption spectroscopic techniques it is also shown that the superior activity of the nanocatalyst can be attributed uniquely to framework manganese, which decomposes H2O2 to reactive hydroxyls and, unlike manganese in Mn3O4 or Mn2O3 nanoparticles, does not promote the simultaneous decomposition of hydrogen peroxide. The presented material thus introduces a new family of Fenton nanocatalysts, which are environmentally friendly, cost-effective, and possess superior efficiency for the decomposition of H2O2 to reactive hydroxyls (AOP), which in turn readily decompose organic pollutants dissolved in water.
COBISS.SI-ID: 4863514
Titanate nanotube-based catalysts were prepared via alkaline hydrothermal synthesis route followed by heat-treatment at different temperatures, ranging from 300 to 700 oC. The resulting metal-free solids were then applied as a catalyst in a three-phase trickle-bed reactor, where catalytic wet air oxidation (CWAO) reactions of model aqueous bisphenol A (BPA) solution were performed. Mainly, the CWAO experiments were conducted at 200 oC with oxygen partial pressure of 10 bar over 300 mg of a catalyst. It was observed in the given range of operating conditions that BPA undergoes both non-catalytic as well as catalytic oxidation routes, while the latter is far more pronounced. At 210 oC and in the presence of 0.5 g of titanate nanotube-based catalyst, which was annealed at 600 oC, complete BPA removal was obtained. From TOC point of view, approximately 70 % conversion was achieved indicating the persistence of refractory intermediates of lower carboxylic acids. The cross-section of results derived from various analytical techniques, which were used to identify surface, textural and morphological properties, revealed that balanced physicochemical properties are required to achieve meaningful extent of BPA removal. During 2-4 day time on stream, no catalyst deactivation occurred that could be attributed to the dissolution of active powders, or to the carbonaceous deposits accumulated on the catalyst surface. Therefore, these nanotubular materials can be regarded as innocuous and efficient long-term catalysts for oxidation of hazardous organic compounds (such as BPA) in the CWAO process.
COBISS.SI-ID: 5149722
Zinc(II) oxide nanoparticles were used for the stabilization of dicyclopentadiene (DCPD)-water-based high internal phase emulsions (HIPEs), which were subsequently cured using ring-opening metathesis polymerization (ROMP). The morphology of the resulting ZnO-pDCPD nanocomposite foams was investigated in correlation to the nanoparticle loading and nanoparticle surface chemistry. While hydrophilic ZnO nanoparticles were found to be unsuitable for stabilizing the HIPE, oleic acid coated, yet hydrophobic, ZnO nanoparticles were effective HIPE stabilizers, yielding polymer foams with ZnO nanoparticles located predominately at their surface. These inorganic/organic hybrid foam-materials were subsequently calcined at 550 °C for 15 min to obtain inorganic macroporous ZnO foams with morphology reminiscent to the original hybrid foam, and a specific surface area of 1.5 m2 g-1. Longer calcination time (550 °C, 15 h) resulted in a sea urchin like morphology of the ZnO foams, characterized by higher specific surface area of 5.5 m2 g-1. The latter foam type showed an appealing catalytic performance in the catalytic wet air oxidation (CWAO) process for the destruction of bisphenol A.
COBISS.SI-ID: 5593626
In this study, testing of TiO2 polymorphs (anatase, rutile, brookite) and their mixtures (anatase/rutile, anatase/TiO2-B) in heterogeneous photocatalytic oxidation process was conducted at ambient conditions in a batch slurry reactor. The efficiency of bare TiO2catalysts was evaluated based on the degree of bisphenol A (BPA) removal, which is a well-known endocrine disrupting compound (EDC). The obtained results indisputably show that BPA removal is strongly affected by catalyst morphology, crystallite size, structure and specific surface area. Detailed interpretation of catalyst properties combined with BPA removal rates leads to the conclusion that photocatalytic oxidation is the most prominent either by using pure anatase particles or high surface area anatase/TiO2-B nanocomposite. However, the highest extent of mineralization was observed in the presence of high specific surface area nanotubular anatase/TiO2-B nanocomposite. Interestingly, when anatase and rutile particles were physically mixed, an additional beneficial effect on BPA degradation was observed. Interpretation of the obtained results shows that a synergistic effect between the respective phases takes place, and consequently enhances the overall activity. This phenomenon was explained by the proposed mechanism of overall hydroxyl radicals concentration increment due to transfer of OH formed on the surface of anatase particles (via H2O oxidation with photogenerated holes in the valence band) to rutile particles.
COBISS.SI-ID: 5573914
In this study, we report a simple synthesis procedure of anatase/rutile/brookite TiO2 nanocomposite material, designed for efficient transformation of emerging water pollutants (e.g., bisphenol (A)) to CO2 and H2O as final products of complete photo-oxidation. Sol–gel procedure with a subsequent hydrothermal treatment carried out at mild temperature and in the presence of 3 M HCl led to the formation of TiO2 nanomaterial, which consists of anatase (43%), rutile (24%) and brookite (33%) polymorph phases within the same material. For the purpose of efficient evaluation of nanocomposite activity, individual polymorphs of anatase, rutile and brookite were also prepared using the same precursor material. Individual polymorph phases within the nanocomposite material crystallized separately and formed mixed agglomerates; the polymorphs were regularly shaped and randomly distributed in agglomerates, where some of the anatase particles exhibited truncated octahedron morphology, rutile was in the form of tetragonal prisms with pyramidal termination and brookite was shaped as blocky particles, which were found to be the smallest within the nanocomposite material (~20 nm). Newly synthesized TiO2 nanocomposite was highly active in terms of mineralization, since after 60 min of irradiation under UV light almost 60% of water dissolved pollutant bisphenol A was successfully transformed into CO2 in H2O. On the other hand, the benchmark TiO2 P25 Degussa catalyst reached a lower extent of mineralization, which is due to significantly less expressed resistance to accumulation of carbonaceous deposits on the catalyst surface.
COBISS.SI-ID: 5753626
In this article impregnation method was used to synthesize TiO2-WO3 composites with two different TiO2 morphologies (nanorods (R-TiO2-WO3) and polyhedral nanoparticles (P-TiO2-WO3)). Their structural, morphological, surface properties and electron trapping states were analyzed and correlated to performance in photocatalytic bisphenol A oxidation. TiO2 nanorods were prepared with alkaline hydrothermal digestion of commercially available high surface TiO2 nanopowder (DT- 51), which was used as a reference for TiO2 nanoparticles. TEM analysis showed that in R-TiO2-WO3 composites WO3 is dispersed over the surface of TiO2 nanorods in amorphous form and with increasing amount of WO3 the TiO2 crystal structure deteriorates. However, in the case of P-TiO2-WO3 composite, multiphase system with monoclinic WO3 intermixed with anatase TiO2 was observed. Moreover, P-TiO2-WO3 composite showed strong surface acidic sites, which were absent in R-TiO2-WO3 composites; this information is significant to understand the depth of the electron trapping states. UV light-induced electron–hole pair excitations and decay dynamics in both TiO2–WO3 composites were studied by infrared spectroscopy measurements and information about the conduction band (CB) electrons and surface trapping states of the composites were collected. In the case of composites with high density of shallow trapping states, enhanced photocatalytic activity was observed. On the contrary, lower photocatalytic activity of solids was observed in cases where deep trapping (TiO2 nanoparticles) or fast recombination (R-TiO2-WO3 composite) prevailed.
COBISS.SI-ID: 30278951
In this study the as-synthesized TiO2 nanorods a-TNR (amorphous TiO2 layer covering the crystalline anatase TiO2 core) and TNR (fully crystalline anatase TiO2) were decorated with reduced graphene oxide (rGO) to synthesize two series of TiO2+rGO composites with different nominal loadings of GO (from 4 to 20 wt%). The structural, surface and electronic properties of the obtained TiO2+rGO composites were analyzed and correlated to their performance in the photocatalytic oxidation of aqueous bisphenol A solution. X-ray photoelectron spectroscopy (XPS) analyses revealed that charge separation in TiO2+rGO composites is improved due to the perfect matching of TiO2 and rGO valence band maxima (VBM). Cyclic voltammetry (CV) experiments revealed that the peak-to-peak separations (?Ep) are the lowest and the oxidation current densities are the highest for composites with a nominal 10 wt% GO content, meaning that it is much easier for the charge carriers to percolate through the solid, resulting in improved charge migration. Due to the high charge carrier mobility in rGO and perfect VBM matching between TiO2 and rGO, the electron–hole recombination in composites was suppressed, resulting in more electrons and holes being able to participate in the photocatalytic reaction. rGO amounts above 10 wt% decreased the photocatalytic activity; thus, it is critical to optimize its amount in the TiO2+rGO composites for achieving the highest photocatalytic activity. BPA degradation rates correlated completely with the results of the CV measurements, which directly evidenced improved charge separation and migration as the crucial parameters governing photocatalysis.
COBISS.SI-ID: 30337063