Synthesis of functionalized mesoporous TiO2 molecular sieves and their application in photocatalysis

Functionalized mesoporous TiO₂ molecular sieves were prepared by treating ordered mesoporous TiO₂ with phosphoric acid or ammonium sulfate at high temperature. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption–desorption measurement, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometer (FT-IR). The photocatalytic activity of the samples was evaluated by photocatalytic decomposition of bromomethane (CH₃Br) in air. Results revealed that the functionalized TiO₂ samples preserved ordered mesostructure and exhibited enhanced physicochemical properties. The photocatalytic activity of the functionalized mesoporous TiO₂ sample was about three times higher than that of the pure mesoporous TiO₂. The concentrations of phosphoric acid and ammonium sulfate solutions used for the functionalization of TiO₂ greatly influenced the photocatalytic activity of the resultants materials. The optimal concentrations of phosphoric acid and ammonium sulfate solutions were 0.05 and 0.10 M, respectively. The enhanced photocatalytic performance of the functionalized mesoporous TiO₂ could be attributed to large specific surface area, high hydroxyl density, and enhanced surface chemical state.     

Efficient adsorption and photocatalytic degradation of organic pollutants diluted in water using the fluoride-modified hydrophobic titanium oxide photocatalysts: Ti-containing Beta zeolite and TiO2 loaded on HMS mesoporous silica

Using the F⁻ media, the hydrophobic zeolite and mesoporous silica can be synthesized. These hydrophobic porous materials exhibit the high ability for the adsorption of organic compounds diluted in water and become the useful supports of photocatalyst. The hydrophobic Ti-Beta(F) zeolite prepared in the F⁻ media exhibited high efficiency than the hydrophilic Ti-Beta(OH) zeolite prepared in OH⁻ media for the liquid-phase photocatalytic degradation of 2-propanol diluted in water to produce CO₂ and H₂O. The TiO₂ loaded on the hydrophobic mesoporous silica HMS(F) (TiO₂/HMS(F)), which was synthesized using tetraethyl orthosilicate, tetraethylammonium fluoride as the source of the fluoride and dodecylamine as templates, also exhibited the efficient photocatalytic performance for the degradation. The amount of adsorption of 2-propanol and the photocatalytic reactivity for the degradation increased with increasing the content of fluoride ions on these photocatalysts. The efficient photocatalytic degradation of 2-propanol diluted in water on Ti-Beta(F) zeolite and TiO₂/HMS(F) mesoporous silica can be attributed to the larger affinity for the adsorption of propanol molecules on the titanium oxide species depending on the hydrophobic surface properties of these photocatalysts.     

Photocatalytic reduction of CO2 with H2O on Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts

Titanium oxide species included within the framework of mesoporous zeolites (Ti-MCM-41 and Ti-MCM-48) prepared by a hydrothermal synthesis exhibited high and unique photocatalytic reactivity for the reduction of CO₂ with H₂O at 328 K to produce CH₄ and CH₃OH in the gas phase. In situ photoluminescence, diffuse reflectance absorption, ESR and XAFS investigations indicated that the titanium oxide species are highly dispersed within the zeolite framework and exist in tetrahedral coordination. The charge transfer excited state of the highly dispersed titanium oxide species played a significant role in the reduction of CO₂ with H₂O exhibiting a high selectivity for the formation of CH₃OH.     

NO decomposition and reduction by C3H6 over transition metal oxides supported on MgF2

The monooxides copper, manganese, molybdenum and chromium catalysts supported on MgF₂ were tested in NO decomposition and reduction by propene. The effect of the oxides content, time on stream and O₂ concentration in reaction mixture during NO reduction on their catalytic activity was investigated. All the catalysts showed the optimum active phase concentration corresponding to 2–4 wt.% of the metal. For the best copper catalyst an effect of introduction of another oxide (manganese or chromium oxide) on the catalytic performance was studied. The double copper-manganese oxide sample containing 2 wt.% Cu and 4 wt.% Mn was proved to ensure the best catalytic performance.     

Characterization of self-standing Ti-containing porous silica thin films and their reactivity for the photocatalytic reduction of CO2 with H2O

Self-standing porous silica thin films with different pore structures were synthesized by a solvent evaporation method and used as photocatalysts for the photocatalytic reduction of CO₂ with H₂O at 323 K. UV irradiation of these Ti-containing porous silica thin films in the presence of CO₂ and H₂O led to the formation of CH₄ and CH₃OH as well as CO and O₂ as minor products. Such thin films having hexagonal pore structure exhibited higher photocatalytic reactivity than the Ti-MCM-41 powder catalyst even with the same pore structure. From FTIR investigations, it was found that these Ti-containing porous silica thin films had different concentrations of surface OH groups and showed different adsorption properties for the H₂O molecules toward the catalyst surface. Furthermore, the concentration of the surface OH groups was found to play a role in the selectivity for the formation of CH₃OH.     

Cr-doped Zr, Si-mesoporous molecular sieves as catalysts of CH2Cl2 oxidation

Zirconium-containing mesoporous silica of nominal Si/Zr ratio in the range 40–5 have been synthesized using dodecylamine as a structure-directing surfactant. The samples were characterized with PXRD, SEM/EDX, BET, CEC measurement and chemical analysis. The materials were used as supports for chromium species incorporated by means of cation exchange procedure. Gradual substitution of Si by Zr in the mesoporous framework results in an increasing structural disorder, increasing cation exchange capacity and increasing capacity for incorporation of Cr. All Cr-doped ZrMMSx catalysts are active in the deep oxidation of methylene chloride reaching 100% conversion at temperature ≥400°C. The catalytic performance depends strongly on the catalyst composition. The role of particular components in determining the catalytic activity and selectivities to various products is discussed.     

Decomposition of nitric oxide over perovskite oxide catalysts: effect of CO2, H2O and CH4

Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La_0.87Sr_0.13Mn_0.2Ni_0.8O_3−δ, La_0.66Sr_0.34Ni_0.3Co_0.7O_3−δ and La_0.8Sr_0.2Cu_0.15Fe_0.85O_3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO₂, H₂O and CH₄ on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H₂O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO₂ and H₂O inhibit the NO decomposition, but inhibition by CO₂ is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO₂ and H₂O over the three perovskites were determined. Addition of methane to the feed (NO/CH₄=4) increases conversion of NO to N₂ about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane.     

Wilson parameters for the system H2, N2, CO,CO2, CH4, H2S,CH3OH,and H2O

Parameters for the Wilson equation have been determined for 24 of the 28 binary pairs in the system: H₂, N₂, CO, CO₂, CH₄, H₂S, CH₃OH, and H₂O. The data for eleven pairs were fit using the symmetric convention, with the remaining pairs satisfying the unsymmetric convention. Coefficients for the missing pairs could be estimated from Henry's Law constants. References have been included for the heat capacities of liquid methanol and carbon dioxide. Heats of mixing were also found in the literature. This information, plus readily available gas heat capacities, provides sufficient information to calculate multicomponent material and energy balances for the columns used in the separation of H₂S and CO₂ by cold methanol absorption.     

Preparation of egg-shell type Al2O3-supported CdS photocatalysts for reduction of H2O to H2

Alumina-supported cadmium sulfide photocatalysts were prepared for the photocatalytic reduction of water to hydrogen using visible light. The activity depends on the nature of interaction between alumina and cadmium and also on the distribution of CdS on the support. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen desorption and temperature-programmed oxidation. The impregnation of alumina hydrogel with an ammoniacal solution results into a chemical interaction between cadmium sulfide and alumina and also yields a preferential distribution of cadmium sulfide on the surface which give a better activity to the photocatalyst. The possible role of ammonia in causing the solute segregation to the surface during the drying stage of the catalyst preparation has been explained.     

Selective catalytic reduction of NO by C2H2 over MoO3/HZSM-5

Molybdenum impregnated HZSM-5 zeolite catalysts with MoO₃ loading from 1 to 8 wt.% were studied in detail for the selective catalytic reduction (C₂H₂-SCR) of NO by acetylene. A 83.9% of NO could be removed by the reductant at 350 °C under 1600 ppm of NO, 800 ppm of C₂H₂ and 9.95% of O₂ in He over 2%MoO₃/HZSM-5 catalyst with a specific activity of in NO elimination and the competitiveness factor (c.f.) of 33.6% for the reductant. The NO elimination level and the c.f. value were ca. 3–4 times as high as those using methane or propene as reductant over the catalyst in the same reaction condition. About same reaction rate was estimated in NO oxidation as that in the NO reduction over each xMoO₃/HZSM-5 (x = 0–8%) catalyst, which confirms that NO₂ is a crucial intermediate for the aimed reaction over the catalysts. Appropriate amount of Mo incorporation to HZSM-5 considerably enhanced the title reaction, both by accelerating the intermediate formation and by strengthening the adsorption NO_x on the catalyst surface under the reaction conditions. Rather lower adsorption tendency of acetylene compared with propene on the catalysts explains the catalyst's steady performance in the C₂H₂-SCR of NO and rapid deactivation in the C₃H₆-SCR of NO.     

Selective catalytic reduction of NO with NH3 at low temperatures over iron and manganese oxides supported on mesoporous silica

A series of catalysts of iron–manganese oxide supported on mesoporous silica (MPS) with different Mn/Fe ratio were studied for low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. Effects of amounts of iron–manganese oxide and calcination temperatures on NO conversion were also investigated. It was found that the Mn–Fe/MPS with Mn/Fe = 1 at the calcination temperature of 673 K showed the highest activity. The results showed that this catalyst yielded 99.1% NO conversion at 433 K at a space velocity of 20,000 h⁻¹. H₂O has no adverse impact on the activity when the SCR reaction temperature is above 413 K. In addition, the SCR activity was suppressed gradually in the presence of SO₂ and H₂O, while such effect was reversible after heating treatment.     

Direct decomposition of NO into N2 and O2 on BaMnO3-based perovskite oxides

Effects of dopant in BaMnO₃ perovskite oxide on the NO direct decomposition activity were investigated. NO direct decomposition activity was greatly elevated by doping La and Mg for Ba and Mn site in BaMnO₃, respectively. The highest N₂ yield was achieved on Ba_0.8La_0.2Mn_0.8Mg_0.2O₃. The NO decomposition rate increased with increasing NO partial pressure with P_NO^1.19. Coexistence of oxygen lowered the N₂ yield with P_O2^−0.18; however, N₂ yield of 40% was sustained even under coexisting of 5% O₂ at 1123 K. Adsorption state of oxygen was also studied with temperature programmed desorption (TPD) method and the desorption temperature of oxygen was lowered by doping Mg for Mn site in BaMnO₃.     

Synthesis of thermally stable mesoporous TiO2 and investigation of its photocatalytic activity

The effects of heat treatment, dopant type and thickness of sol layer, on the formation and stability of ordered TiO₂ mesostructure were investigated. Higher ageing temperature facilitates the separation of organic phase and inorganic phase, while thinner sol layer facilitates the formation of homogeneous system and improves the mesostructural order of TiO₂. Lanthanum dopant, which is more electropositive than Ti, can improve the thermal stability of mesostructure by enhancing the strength of Ti–O bond. On the contrary, Fe and Pd dopants, which are more electronegative than Ti, decrease the thermal stability of mesostructure. Furthermore, La doped mesoporous TiO₂ shows high activity in the photocatalytic degradation of Rhodamine B under the irradiation of visible light.     

Structure,stability and permeation properties of NaA zeolite membranes for H2O/H2 and CH3OH/H2 separations

NaA zeolite membranes were synthesised in the secondary growth hydrothermal method based on the seeding of the inner surface of a ceramic α-alumina tube. The impacts of crystallisation time and zeolite precursor concentration (in H₂O) were investigated. The structure and stability of the prepared NaA zeolite membranes were also investigated with operating temperatures, times and pressures. The results indicate that the optimal synthesis gel molar composition was 3Na₂O: 2SiO₂: Al₂O₃: 200H₂O. This led to cubic-shaped NaA zeolite which showed good stability. The optimal NaA zeolite membrane had H₂O and CH₃OH fluxes of 2.77 and 0.19 kg/m²h, with H₂O/H₂ and CH₃OH/H₂ separation factors of ∞ and 0.09 at a temperature of 30 °C. The NaA zeolite membrane had high thermal stability, but poor separation performance at high temperature (240 °C). The results suggested that the H₂ permeation flux is significantly influenced by preferential adsorption of vapour in the NaA zeolite membrane.     

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH₄ over 4%Sr/La₂O₃ in the absence and presence of O₂ were examined between 773 and 973 K, and N₂O decomposition was also studied. The presence of CH₄ greatly increased the conversion of NO to N₂ and this activity was further enhanced by co-fed O₂. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH₄ in the absence of O₂, and reduction with 1% O₂ in the feed were 8.3·10⁻⁴, 4.6·10⁻³, and 1.3·10⁻² μmol N₂/s m², respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N₂ formation by decomposition was inhibited by O₂ in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH₄ continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O₂ with equal activation energies of 26 kcal/mol. The addition of O₂ increased the reaction order in CH₄ at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O₂ was 0.26 up to 0.5% O₂ during which time the CH₄ concentration was not decreased significantly. N₂O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10⁻⁴ μmol N₂/s m² at 623 K and 1220 ppm N₂O and an activation energy of 24 kcal/mol. The addition of CH₄ inhibits this decomposition reaction. Finally, the use of either CO or H₂ as the reductant (no O₂) produced specific activities at 773 K that were almost 5 times greater than that with CH₄ and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H₂ to reduce NO to N₂ over these REO catalysts.     

Catalytic decomposition of N2O and catalytic reduction of N2O and N2O + NO by NH3 in the presence of O2 over Fe-zeolite

The decomposition of N₂O, and the catalytic reduction by NH₃ of N₂O and N₂O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of Fe^II-zeolite with N₂O (-oxygen), are formed in largest amounts with this material. The decomposition of N₂O is promoted by addition of NH₃, and even more with NH₃ + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N₂O originated from the fast surface reaction between -oxygen O^* and NO^* to yield NO₂^*, which in turn reacts immediately with NH₃.     

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N₂ and N₂O formation in the direct decomposition and reduction of NO by NH₃ were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying ¹⁵NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH₃–NO interaction, N₂O formation was most marked between 573 and 673 K, whereas N₂ formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the ¹⁵NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N₂O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.     

Structural and photocatalytic degradation characteristics of hydrothermally treated mesoporous TiO2

Mesoporous TiO₂ photocatalysts have been synthesized using polyethylene glycol (PEG) as a template direction agent in diluted acetic acid aqueous solution. This medium slows down the hydrolysis reaction of titanium sources due to the hydrolytic retardant and the strong chelating effects of acetic acid. A hydrothermal treatment process was introduced to better control the resultant mesoporous structures. The effects of PEG molecular weight and thermal treatment temperature on the resultant structure and photoactivity were investigated. Morphological, structural and phase compositional properties of the resultant photocatalysts were systematically characterized using transmission electron microscopy, X-ray diffraction and nitrogen adsorption/desorption analysis. The mesoporous structure with diameters between 13.3 and 17.0 nm and mean porous sizes that ranged from 9.6 to 13.3 nm were obtained when the molecular weight of PEG were varied from 200 to 20,000. The mesoporous diameters were changed significantly from 9.8 to 18.4 nm with mean porous sizes slightly increasing from 8.0 to 10.0 nm when the calcination temperature was varied from 350 to 550 °C. The activities of the resultant TiO₂ photocatalysts were evaluated using 2,4,6-tribrominated phenol as a testing compound that represents a class of toxic brominated flame retardants. The experimental results revealed that the photocatalytic activity depends on the phase and on the structural characteristics of the resultant photocatalysts.     

Preparation of multi-walled carbon nanotube supported TiO2 and its photocatalytic activity in the reduction of CO2 with H2O

Multi-walled carbon nanotube (MWCNT) supported TiO₂ composite catalysts were prepared by sol-gel and hydrothermal methods. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy and N₂-adsorption analysis were carried out to characterize the composite catalysts. In using the sol-gel method, the MWCNTs were coated with anatase TiO₂ nanoparticles, and by the hydrothermal method, rutile TiO₂ nanorods were uniformly deposited on the MWCNTs. The photocatalytic activities of the composite catalysts were evaluated by the reduction of CO₂ with H₂O. The results indicate that the addition of an appropriate amount of MWCNTs as supports for TiO₂ could remarkably improve the efficiency of the photocatalytic reaction. The composite catalysts prepared by the sol-gel method lead to the main formation of C₂H₅OH, while HCOOH is found to be the major product on the sample prepared by the hydrothermal method.     

Development of the efficient TiO2 photocatalyst in photoassisted selective catalytic reduction of NO with NH3

Photoassisted selective catalytic reduction (photo-SCR) of NO with NH₃ in the presence of O₂ takes place at room temperature over TiO₂ photocatalyst. From the results of photo-SCR reaction over various TiO₂, we found that JRC-TIO-11 exhibited the best activity. The reaction activity correlated to the amount of acid sites of TiO₂, but did not depend on the specific surface area and crystal diameter. The mixture of rutile and anatase shows higher activity than any of the corresponding TiO₂ single phase catalyst.