Libor Čapek

Wavelength Effect on Photocatalytic Reduction of CO2 by Ag/TiO2 Catalyst

Photocatalytic reduction of CO2 by water was performed in the presence of a Ag/TiO2 catalyst under illumination by lamps with different wavelengths (254, 365, and 400 nm). The yields of the main products (methane and methanol) were higher with the 254 nm lamp than with the 365 lamp while no products were observed with the 400 nm lamp. This was because the electron-hole generation rate increased with increasing energy of irradiation (decreasing wavelength) and there were higher densities of electron states at higher energies in TiO2. The increased efficiency of electron-hole generation with a shorter wavelength irradiation increased the efficiency of the catalyst. The energy of the electrons excited by visible light (400 nm) was too low for CO2 photocatalytic reduction.

Quantitative LIBS analysis of vanadium in samples of hexagonal mesoporous silica catalysts.

The method for the analysis of vanadium in hexagonal mesoporous silica (V-HMS) catalysts using Laser Induced Breakdown Spectrometry (LIBS) was suggested. Commercially available LIBS spectrometer was calibrated with the aid of authentic V-HMS samples previously analyzed by ICP OES after microwave digestion. Deposition of the sample on the surface of adhesive tape was adopted as a sample preparation method. Strong matrix effect connected with the catalyst preparation technique (1st vanadium added in the process of HMS synthesis, 2nd already synthesised silica matrix was impregnated by vanadium) was observed. The concentration range of V in the set of nine calibration standards was 1.3-4.5% (w/w). Limit of detection was 0.13% (w/w) and it was calculated as a triple standard deviation from five replicated determinations of vanadium in the real sample with a very low vanadium concentration. Comparable results of LIBS and ED XRF were obtained if the same set of standards was used for calibration of both methods and vanadium was measured in the same type of real samples. LIBS calibration constructed using V-HMS-impregnated samples failed for measuring of V-HMS-synthesized samples. LIBS measurements seem to be strongly influenced with different chemical forms of vanadium in impregnated and synthesised samples. The combination of LIBS and ED XRF is able to provide new information about measured samples (in our case for example about procedure of catalyst preparation).

Dehydrogenation of ethane over vanadium, cobalt and nickel based catalysts

Abstract The contribution deals with the catalytic performance of vanadium-, cobalt- and nickel-based catalysts in oxidative dehydrogenation of ethane. Vanadium, cobalt and nickel species were loaded to microporous (MFI) and mesoporous (HMS) materials, and supported on inorganic support (alumina). The activity of catalysts was compared at 600 °C (9 % ethane and 2.5 % oxygen in helium, and W/F 0.12 g.s.cm −3 ). The most effective catalytic system was Ni−Al 2 O 3 , which also was tested at varied oxygen concentration, reaction temperature, and W/F. The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, and 0.91 g(C 2 =).g cat −1 .h −1 in the ethene productivity for Ni−Al 2 O 3 . The activity of Ni−Al 2 O 3 was stable for 6 hours in time-on-stream.

Precursors of active Ni species in Ni/Al2O3 catalysts for oxidative dehydrogenation of ethane

Ni/Al2O3 catalysts for oxidative dehydrogenation (ODH) of ethane were prepared by impregnation of Al2O3 with nickel acetate or nickel nitrate, and by mechanical mixing of NiO and Al2O3. The Ni-based catalysts were characterized by N2 adsorption-desorption, X-ray diffraction, diffuse reflectance UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction of hydrogen. The results showed that formation of crystalline NiO particles with a size of < 8 nm and/or non-stoichiometric NiO species in the Ni/Al2O3 catalysts led to more active species in ODH of ethane under the investigated reaction conditions. In contrast, tetrahedral Ni species present in the catalysts led to higher selectivity for ethene. Formation of large crystalline NiO particles (22–32 nm) over Ni/Al2O3 catalysts decreased the selectivity for ethene.

TiO2 and nitrogen doped TiO2 prepared by different methods; on the (micro)structure and photocatalytic activity in CO2 reduction and N2O decomposition

TiO2 as nanostructured powders were prepared by (1) sol-gel process and (2) hydrothermal method in combination with (A) the processing by pressurized hot water and methanol or (B) calcination. The subsequent synthesis step was the modification of prepared nanostructured TiO2 with nitrogen using commercial urea. Textural, structural, surface and optical properties of prepared TiO2 and N/TiO2 were characterized by nitrogen physisorption, powder X-ray diffraction, X-ray photoelectron spectroscopy and DR UV-vis spectroscopy. It was revealed that TiO2 and N/TiO2 processed by pressurized fluids showed the highest surface areas. Furthermore, all prepared materials were the mixtures of major anatase phase and minor brookite phase, which was in nanocrystalline or amorphous (as nuclei) form depending on the applied preparation method. All the N/TiO2 materials exhibited enhanced crystallinity with a larger anatase crystallite-size than undoped parent TiO2. The photocatalytic activity of the prepared TiO2 and N/TiO2 was tested in the photocatalytic reduction of CO2 and the photocatalytic decomposition of N2O. The key parameters influencing the photocatalytic activity was the ratio of anatase-to-brookite and character of brookite. The optimum ratio of anatase-to-brookite for the CO2 photocatalytic reduction was determined to be about 83 wt.% of anatase and 17 wt.% of brookite (amorphous-like) (TiO2-SG-C). The presence of nitrogen decreased a bit the photocatalytic activity of tested materials. On the other hand, TiO2-SG-C was the least active in the N2O photocatalytic decomposition. In the case of N2O photocatalytic decomposition, the modification of TiO2 crystallites surface by nitrogen increased the photocatalytic activity of all investigated materials. The maximum N2O conversion (about 63 % after 18 h of illumination) in inert gas was reached over all N/TiO2.

Investigation of low Ce amount doped-TiO2 prepared by using pressurized fluids in photocatalytic N2O decomposition and CO2 reduction

Ce doped TiO2 anatase/brookite composites with 0.6–5.5 wt% of Ce, as well as parent TiO2 anatase/brookite were synthesized to be investigated in two environmentally-beneficial reactions, the photocatalytic decomposition of N2O and the photocatalytic reduction of CO2. Composites were prepared unconventionally, by using sol–gel method combined with the processing by pressurized hot fluids (in the sequence water/methanol/water). The physicochemical and electronic properties of all synthesized composites were characterized by organic elementary analysis, nitrogen physisorption, powder X-ray diffraction, X-ray fluorescence spectroscopy, diffuse reflectance UV-vis spectroscopy and work function measurements. It was revealed that all composites show comparable textural properties, crystallite size, as well as optical properties, except for the 5.5 wt% Ce/TiO2 composite which showed significantly lowered band gap energy due to the significantly higher population of Ce. Concerning the composite structural properties, the addition of different amounts of Ce in the range of 0.6–5.5 wt% affected markedly the phase composition of composites, namely the anatase-to-brokite weight ratio. Concerning the photocatalytic tests the 5.5 wt% Ce/TiO2 composite showed the highest photocatalytic performance. The highest photocatalytic performance of the 5.5 wt% Ce/TiO2 composite can be attributed to the lowest composite work function which is affected by both the amount of Ce, as well as the phase composition.Graphical Abstract

Preparation and Characterization of TiO2- ZrO2 Mixed Oxide Catalysts for Photocatalytic Reduction of Carbon Dioxide

Seven pure and mixed oxides TiO2, ZrO2 catalysts with the mole fraction of TiO2 in the range 0–1 were prepared by a sol-gel process controlled in the reverse micellar environment. Triton X114 was used as the nonionic surfactant and titanium(IV) isopropoxide and/or zirconium(IV) propoxide were used as the metal precursors. As treatment method the calcination at temperature of 400°C in air was applied. The effect of ZrO2 on photocatalytic properties of prepared mixed-oxide catalysts was evaluated. Textural properties, structural properties, morphology and purity of prepared catalysts were characterized by nitrogen physisorption at 77 K, advanced XRD analysis, TEM, XRF, UV-VIS and organic elemental analysis. For testing of photocatalytic activity the photocatalytic reduction of carbon dioxide was chosen with methane and methanol as the main reduction products.

Effect of particle size distribution in laser-induced breakdown spectroscopy analysis of mesoporous V–SiO2 catalysts

In this paper, the effect of particle size on Laser-Induced Breakdown Spectroscopy (LIBS) analysis of mesoporous V–SiO2 catalyst samples was investigated. The measurements were realized on three LIBS devices with different parameters. Concentrations of V in samples used for LIBS experiments previously determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) varied from 1.2 to 4.7 w/w%. Granulometry of silica samples was modified by two grinding methods (conventional vibration mill and cryogenic mill) and three sets of samples with different particle size distributions were obtained. Ground samples were then deposited in the form of a thin layer on the adhesive tape and analysed by LIBS. Curves describing the relationship between the vanadium concentration and the corresponding intensity of LIBS signal (C–I curves) were constructed for all three groups of samples, with different granulometry profiles measured with three different LIBS devices. The C–I curves for samples with narrow particle size distribution showed the highest values of slopes. Detection limits achieved were in the range 0.063–0.012% (w/w). The best LOD values were obtained for samples with the lowest mean particle size.

Aspects of stability of K/Al2O3 catalysts for the transesterification of rapeseed oil in batch and fixed-bed reactors

Abstract Catalytically active, stable, and mechanically durable solid K/Al 2 O 3 catalysts for the transesterification of rapeseed oil with methanol was studied. In a batch reactor, high catalytic activity was accompanied by leaching of K species, caused by glycerol, and mechanical destruction of the solid catalyst as a result of contact with the stirrer. In a fixed-bed reactor, some leaching of K species into the liquid phases was also observed, but approached 0 during 30 h of time-on-stream; the activity of the K/Al 2 O 3 catalyst (∼83% ester yield) was stable for 100 h of time-on-stream and no mechanical destruction of the catalyst was observed. The populations of K 2 O and K-O-Al species for fresh and used K/Al 2 O 3 catalysts were compared using Fourier transform infrared spectroscopy. It was found that some K 2 O species leached into the liquid phases at the beginning of the reaction.

Metallo-zeolites actives in the reduction of NO with decane—main component of diesel fuel

The performance of Cu-, Co-, Fe- and Ag-ZSM-5 zeolites in seletive catalytic reduction of NO with decane in the presence of water vapor has been investigated. The catalyst activity increased in the order of Ag-ZSM-5 ≪Fe-ZSM-5 ≪Co-ZSM-5 In-situ FTIR spectra and gas phase analysis of the reaction products evidenced that decane is in the first step cracked to low chain hydrocarbons, which afterward participate in the reduction of NO to N 2 . Reduction of NO with decane, propene and propane was compared over the most active Cu-ZSM-5 in order to analyze the differences in the reduction of NO employing olefins and paraffins. While the reduction of NO with propane is manifested in the surface covered mainly by NO x intermediates and adsorbed CO, with propene and decane the spectrum indicates high coverage by -C=O, -C=C species as well as -NH and -NCO groups.