Lucie Obalová

Photocatalytic reduction of CO2 over TiO2 based catalysts

At present, carbon dioxide is considered the largest contributor among greenhouse gases. This review covers the current state of problem of carbon dioxide emissions from industrial and combustion processes, the principle of photocatalysis, existing literature related to photocatalytic CO2 reduction over TiO2 based catalysts and the effects of important parameters on the process performance including light wavelength and intensity, type of reductant, metal-modified surface, temperature and pressure.

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.

Simulation of N2O Abatement in Waste Gases by Its Decomposition over a K-Promoted Co-Mn-Al Mixed Oxide Catalyst

Abstract Intrinsic data of N 2 O catalytic decomposition over a K-promoted Co-Mn-Al mixed oxide prepared by the thermal treatment of a layered double hydroxide was used for the design of a pilot reactor for the abatement of N 2 O emissions from the off-gases in HNO 3 production. A pseudo-homogeneous one-dimensional model of an ideal plug flow reactor under an isothermal regime (450°C) was used for reactor design. A catalyst particle diameter of 3 mm is a compromise size because increasing the size of the catalyst particle leads to a decrease in the reaction rate because of an internal diffusion limitation, and particles with a smaller diameter cause a large pressure drop. A catalyst bed of 11.5 m 3 was estimated for the target N 2 O conversion of 90% upon the treatment of 30000 m 3 /h of exhaust gas (0.1 mol% N 2 O, 0.005 mol% NO, 0.9 mol% H 2 O, 5 mol% O 2 ) at 450°C and 130 kPa.

N2O catalytic decomposition — effect of pelleting pressure on activity of Co-Mn-Al mixed oxide catalysts

The effect of pelleting pressure (0–10 MPa) during the preparation of Co-Mn-Al mixed oxide catalyst on its texture and activity for N2O catalytic decomposition was examined for small grain sizes used in laboratory experiments, and for model industry catalyst particles. Adsorption/desorption measurements of nitrogen, mercury porosimetry and helium pycnometry were used for detail characterization of porous structure. A volume of micropores of about 20 mm3 g−1 was evaluated using modified BET equation. This value did practically not change with the increasing pelletization pressure except that of the sample formed at the pressure of 10 MPa. Although an increase of pelleting pressure caused an increase in bulk density and a decrease in pore size and pore volume of the prepared catalyst (resulting in lower values of N2O effective diffusion coefficient), no direct correlation between pelleting pressure used and catalyst activity has been found. In contrary, estimation of the internal diffusion limitation according to the Weisz-Prater criterion indicated that even laboratory experimental data obtained for catalyst grains with particle size lower than 0.315 mm pelletized at higher pressures could be influenced by internal diffusion. Estimation of the internal mass transfer limitation in industrial catalyst particles described by the effectiveness factor showed that effectiveness factor of about 0.07 and 0.2 can be obtained for spheres with the radius of 1.5 mm and 0.5 mm, respectively, if pelleting pressure of about 6 MPa was used for the catalyst preparation.

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.

Cobalt oxides supported over ceria-zirconia coated cordierite monoliths as catalysts for deep oxidation of ethanol and N2O decomposition

Cordierite monoliths coated with ceria–zirconia supporting cobalt oxide were prepared, examined in the deep oxidation of ethanol and N2O decomposition, and compared with pelletized commercial cobalt oxide catalyst. Interaction of Co3O4 with ceria–zirconia washcoat led to formation of Co3O4 particles with slightly worse structure ordering resulting in better reducibility than that observed for the commercial Co3O4 catalyst. In oxidation of ethanol, activity of the Co3O4-containing monoliths was comparable with that of pelletized cobalt oxide catalyst with nearly seven times higher content of active components. However, conversions of N2O over the monolith catalysts were lower. Nevertheless, incorporation of Co3O4 onto ZrO2–CeO2 washcoat increased rate of both catalytic reactions, i.e., N2O decomposition and deep ethanol oxidation.Graphical Abstract

The balancing of VOC concentration fluctuations by adsorption/desorption process on activated carbon

Volatile organic compounds (VOCs) are mostly toxic and carcinogenic substances. The technologies for cleaning of exhaust gases containing the constant concentrations of VOCs are commercially available. However, if concentration fluctuations occur in the range of several orders of magnitude, it can cause problems for a subsequent gas cleaning e.g. by thermal or catalytic oxidation. The balancing of VOC concentrations in flue gases can be a great simplification of a subsequent reduction of VOC emissions from sources with time-variable concentrations. Paint shops belong to the important sources of VOCs and are an example of periodic processes with time-variable concentrations of VOCs. One of the main aims was to experimentally determine the conditions, such as the minimal mean residence time, to balance out the fluctuations of inlet VOC concentrations at the laboratory model. After that, the verification of obtained results was applied for a real exhaust gas from a paint shop.

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 CALCINATION TEMPERATURE AND CALCINATION TIME ON THE KAOLINITE/TIO2 COMPOSITE FOR PHOTOCATALYTIC REDUCTION OF CO2 VLIV KALCINAČNÍ TEPLOTY A DOBY KALCINACE NA KOMPOZIT KAOLINIT/TIO2 PRO FOTOKATALYTICKOU REDUKCI CO2

Abstract The kaolinite/TiO2 composite (60 wt% of TiO2) was prepared by thermal hydrolysis of a raw kaolin suspension in titanyl sulphate and calcined at different temperatures (600, 650 and 700°C) and for different times (1, 2 and 3 h). The obtained samples were characterized by XRPD, N2 physical adsorption and SEM, and tested for photocatalytic reduction of CO2. The different calcination conditions did not influence TiO2 phase composition, only slightly changed the specific surface area, and significantly affected crystallite size of kaolinite/TiO2 composite. A higher temperature and longer duration of calcination lead to higher crystallinity of the powder. The photocatalytic results showed that the crystallite size determined the efficiency of kaolinite/TiO2 photocatalysts Abstrakt Kompozit kaolinit/TiO2 (60 hm% TiO2) byl připraven termální hydrolýzou suspenze surového kaolinu v síranu titanylu a kalcinován při různých teplotách (600, 650 a 700°C) a po různou dobu (1, 2 a 3 h). Získaný vzorek byl charakterizován pomocí XRPD, N2 fyzikální adsorpcí a SEM, a testován na fotokatalytickou redukci CO2. Rozdíl kalcinačních podmínek neovlivnil složení fáze TiO2, pouze se mírně pozměnila specifická povrchová plocha a výrazně byla ovlivněna velikost krystalitu kompozitu kaolinit/TiO2. Vyšší teplota a delší doba kalcinace vedly k vyšší krystalitě prášku. Fotokatalytické výsledky ukázaly, že velikost krystality určuje účinnost fotokatalyzítoru kaolinit/TiO2

Influence of Reaction Medium on CO2 Photocatalytic Reduction Yields Over Zns-MMT / Vliv Reakčního Prostření Na Výtěžky Fotokatalytické Redukce CO2 V Přítomnosti Zns-MMT

Abstract The reduction of CO2 by photocatalysts is one of the most promising methods since CO2 can be reduced to useful compounds by irradiating it with UV light at room temperature and ambient pressure. The aim of this work was to assess the effect of a reaction media on CO2 photocatalytic reduction yields over ZnS nanoparticles deposited on montmorillonite (ZnS-MMT). Four different reaction media, such as NaOH, NaOH+Na2SO3 (1:1), NH4OH, NH4OH+Na2SO3 (1:1), were tested. The pure sodium hydroxide was better than ammonium hydroxide for the yields of the both gas phase (CH4 and CO) and liquid phase (CH3OH). The addition of Na2SO3 improved methanol yields due to the oxidation prevention of incipient methanol to carbon dioxide. The gas phase yields were decreased by the Na2SO3 addition. The best tested reaction medium for the photocatalytic reduction of CO2 was the solution of sodium hydroxide. Abstrakt Redukce CO2 pomocí fotokatalyzátorů je jedna z nejslibnějších metod, jelikož CO2 může být redukován na užitečné sloučeniny ozařováním UV zářením při pokojové teplotě a tlaku. Tato práce byla zaměřena na posouzení vlivu reakčních prostředí na výtěžky fotokatalytické redukce CO2 v přítomnosti nanočástic ZnS nanesených na montmorillonit (ZnS-MMT). Byla testována čtyři různá reakční prostředí, NaOH, NaOH+Na2SO3 (1:1), NH4OH, NH4OH+Na2SO3 (1:1). Výtěžky v obou fázích, plynné (CH4 a CO) i kapalné (CH3OH), byly vyšší v čistém hydroxidu sodném než v hydroxidu amonném. Přídavek Na2SO3 zvýšil výtěžky methanolu a to díky zamezení oxidace vznikajícího methanolu zpět na oxid uhličitý. Výtěžky plynné fáze se však po přídavku Na2SO3 snížily. Nejlepším z testovaných reakčních prostředí pro fotokatalytickou redukci CO2 byl roztok hydroxidu sodného.