Effect of the rhenium additive to VOx/TiO2 and MoOx/TiO2 catalysts on their properties in oxidative dehydrogenation of propane

VO_x/TiO₂ and MoO_x/TiO₂ catalysts with the addition of Re (Re/V or Mo = 0.5) were synthetized and tested in oxidative dehydrogenation of propane and in reduction by propane. XPS measurements showed depletion of the surface in Re. The Re additive does not affect the total conversion of propane, but increases the selectivity to propene. The effect is more pronounced for the MoO_x/TiO₂ catalyst. The increase in the selectivity to propene is accompanied with the increase in the reducibility of the catalysts.     

Propane combustion over supported Pt catalysts

We prepared Pt catalysts supported on various metal oxides, viz., ZrO₂, CeO₂, TiO₂, yttria-stabilized zirconia (YSZ), SiO₂, SiO₂–Al₂O₃, and γ-Al₂O₃, using an incipient wetness method and applied them to propane combustion. In the cases of ZrO₂-, CeO₂-, and TiO₂-supported Pt catalysts, supports with different surface areas were also used. The Pt dispersion in Pt catalysts supported on metal oxides increased with increasing surface area of the support for the same metal oxide. Pt catalysts on supports with lower surface areas (ZrO₂, CeO₂, and TiO₂) showed higher catalytic activities for propane combustion than did Pt catalysts on supports with higher surface areas. The catalytic activity decreased in the following order: Pt/ZrO₂ (2) > Pt/CeO₂ (9) > Pt/TiO₂ (1) = Pt/SiO₂ (350) > Pt/ZrO₂ (18) = Pt/YSZ > Pt/TiO₂ (330) > Pt/SiO₂–Al₂O₃ (350) > Pt/ZrO₂ (73) > Pt/γ-Al₂O₃ (180) > Pt/CeO₂ (160). The catalytic activity is inversely proportional to the amount of O₂ chemisorbed up to the reaction temperature. It can be concluded that metallic Pt is essential for propane combustion and is maintained for the Pt catalysts with large Pt metal particles, which can be prepared by using a support with a low surface area.     

Propane oxidation with chemically bound oxygen on Pt/TiO2/Al2O3 and Pt/CeO2/Al2O3 catalysts

Possible mechanisms are suggested for propane oxidation on Pt/TiO₂/Al₂O₃ and Pt/CeO₂/Al₂O₃ catalysts in the cyclic reactant supply mode. As compared to the steady-state process, the process conducted as catalyst oxidation-reduction cycles results in a very different product composition: it is more selective toward partial oxidation products and yields much smaller amounts of complete oxidation products. It is established by isothermal and temperature-programmed oxygen desorption that, under the reaction conditions examined, the oxygen desorbed from the catalyst surface into the gas phase makes a negligible contribution to propane oxidation. It is proved by XPS that propane oxidation is due to the chemically bound oxygen of the catalyst. The hypothetical mechanism of the process includes propane activation on Pt followed by the transfer of the activated species to the oxygen-storing component (TiO₂ or CeO₂), where the intermediates are oxidized by chemically bound oxygen.     

Investigation of Br–N Co-doped TiO<Subscript>2</Subscript> photocatalysts: preparation and photocatalytic activities under visible light

Bromine (Br) and nitrogen (N) co-doped TiO₂ ((Br–N–TiO₂) photocatalysts were prepared by a sol–gel method. The catalysts were characterized by X-ray Diffraction (XRD), N₂ adsorption and desorption isotherms, X-ray Photoelectron Spectra (XPS), UV-Vis Diffraction Spectra and Electron Spin Response (ESR) Spectra. Experiments on photodegradation of Methylene Blue (MB) and Sulfosalicylic Acid (SSA) under visible light were carried out to evaluate the photocatalytic activities of the catalysts. Chemical Oxygen Demand (COD) analysis was also conducted to evaluate the mineralization degrees of the catalysts in MB photodegradation. Enhanced photocatalytic activities were observed for the Br–N–TiO₂ catalysts in the experiments of MB and SSA photodegradation. A possible mechanism was proposed to explain the improved photocatalytic activities of the Br–N–TiO₂ catalysts.     

Adsorption of acetonitrile and its reaction with oxygen on TiO2-supported Au — Rh bimetallic catalysts

The adsorption of acetonitrile and its surface reactions with oxygen on TiO₂-supported Au — Rh bimetallic catalysts were studied by FTIR and mass spectrometry at 300–673 K. The band due to CH₃CN on Lewis acid centres shifted to lower wavenumbers with the increase of the Rh content, which shows that the strength of Lewis acid sites weakens with the increase of the Rh content of the catalysts. CH₃CN, on the other hand, dissociates producing CN_(a) species. From the shift to higher wavenumbers of the band due to CN_(a), strengthening of the C — N bond with increasing Rh content has been established. During the heat treatments of the adsorbed CH₃CN layer, methylamine (CH₃NH₂) was detected in the gas phase, the amount of which depended on the composition of the catalysts. Dehydrogenation of the adsorbed acetonitrile also depends on the Rh content: the higher the Rh content of the sample, the lower the temperature of the first appearance of gaseous H₂, and the higher the amount of H₂ evolved. The presence of gaseous oxygen affects the formation of isocyanate (NCO) surface species only on 1% Au/TiO₂ and on 1% (0.75Au–0.25Rh)/TiO₂ catalysts.     

Low temperature complete combustion of dilute propane over Mn-doped ZrO2 (cubic) catalysts

Combustion of dilute propane (0.9 mol%) over Mn-doped ZrO₂ catalysts prepared using different precipitating agents (viz. TMAOH, TEAOH, TPAOH, TBAOH and NH₄OH), having different Mn/Zr ratios (0.05—0.67) and calcined at different temperatures (500—800°C), has been thoroughly investigated at different temperatures (300—500°C) and space velocities (25,000–100,000 cm³ g⁻¹ h⁻¹) for controlling propane emissions from LPG-fuelled vehicles. Mn-doped ZrO₂ catalyst shows high propane combustion activity, particularly when its ZrO₂ is in the cubic form, when its Mn/Zr ratio is close to 0.2 and when it is prepared using TMAOH as a precipitating agent and calcined at 500—600°C. Pulse reaction of propane in the absence of free-O₂ over Mn-doped ZrO₂ (cubic) and Mn-impregnated ZrO₂ (monoclinic) catalysts has also been investigated for studying the relative reactivity and mobility of the lattice oxygen of the two catalysts. Both reactivity and mobility of the lattice oxygen of Mn-doped ZrO₂ are found to be much higher than that of Mnimpregnated ZrO₂. Propane combustion over Mn-doped ZrO₂ catalyst involves a redox mechanism     

Facet Dependence of Photochemistry of Methanol on Single Crystalline Rutile Titania

The crystal phase, morphology and facet significantly influence the catalytic and photocatalytic activity of TiO₂. In view of optimizing the performance of catalysts, extensive efforts have been devoted to designing new sophisticate TiO₂ structures with desired facet exposure, necessitating the understanding of chemical properties of individual surface. In this work, we have examined the photooxidation of methanol on TiO₂(011)-(2×1) and TiO₂(110)-(1×1) by two-photon photoemission spectroscopy (2PPE). An excited state at 2.5 eV above the Fermi level (E_F) on methanol covered (011) and (110) interface has been detected. The excited state is an indicator of reduction of TiO₂ interface. Irradiation dependence of the excited resonance signal during the photochemistry of methanol on TiO₂(011)-(2×1) and TiO₂(110)-(1×1) is ascribed to the interface reduction by producing surface hydroxyls. The reaction rate of photooxidation of methanol on TiO₂(110)-(1×1) is about 11.4 times faster than that on TiO₂(011)-(2×1), which is tentatively explained by the difference in the surface atomic configuration. This work not only provides a detailed characterization of the electronic structure of methanol/TiO₂ interface by 2PPE, but also shows the importance of the surface structure in the photoreactivity on TiO₂.     

Effect of TiO2 crystallite size on the dispersion of Co on nanocrystalline TiO2

Nanocrystalline TiO₂ powders with average crystallite sizes of 9–15 nm were synthesized by the solvothermal method and employed as supports for Co catalysts. The value of H₂ chemisorption/specific surface area of Co/TiO₂ increased significantly with increasing TiO₂ crystallite size. It was suggested that the higher amount of Ti³⁺ surface defects on the larger crystalline TiO₂ resulted in a stronger interaction between Co and TiO₂, hence, higher dispersion of Co was obtained.     

Comparative study of coke deposition on catalysts in reactions with and without oxygen

Two types of catalysts, i.e. Pt/γ Al₂O₃ and Cu/Na-ZSM-5, were used to investigate the catalyst activity and amount of coke formation on the spent catalysts. The reactions of particular interest were the hydrocarbon oxidation and the SCR of NO with and without O₂. Propane and propene were used as the hydrocarbon sources. The reaction conditions were as follows: reaction temperature =170–500°C, GHSV=4,000 hr⁻¹, TOS=2 hr, feed composition depending on each reaction, but the composition of gases were fixed as HC=3,000 ppm, NO=1,000 ppm and O₂=2.5%, using He balance. It was found that both the case of Pt/γ Al₂O₃ and the case of Cu/Na-ZSM-5, propene provided higher conversion and coke deposition than propane in the presence or the absence of O₂ and/or NO. For Pt/γ Al₂O₃ catalyst, in case of the absence of oxygen reactions, the propene conversion dropped more rapidly than the propane conversion. Finally the reaction of propene gave a lower percent of hydrocarbon conversion than the reaction of propane. Additionally, propene had a higher percent selectivity of coke formation for the reaction with the absence of oxygen, but propane had a higher percent selectivity of coke formation for the reaction with the presence of oxygen. For Cu/Na-ZSM-5, in the system with absence and presence of oxygen, the addition of oxygen caused a significant change in % coke selectivity. With the presence of NO_x, the percent conversion of both propane and propene decreased and that the % coke selectivity of propane decreased, whereas that of in propene increased.     

Chemical synthesis of Ni/TiO<Subscript>2</Subscript> nanophotocatalyst for UV/visible light assisted degradation of organic dye in aqueous solution

The Ni/TiO₂ nanoparticles with different Ni dopant content were prepared by a modified sol–gel method. The structure and photoinduced charge properties of the as-prepared catalysts were determined using X-ray diffraction, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy and surface photovoltage spectroscopy techniques, and the photocatalytic efficiency of these catalysts was tested using an organic dye. It was shown that Ni modification could greatly enhance the photocatalytic efficiency of these nanocomposite catalysts by taking the photodegradation of methyl orange as a model reaction. With appropriate ratio of Ni and TiO₂, Ni/TiO₂ nanocomposites showed the superior photocatalytic activity than the single TiO₂ nanoparticles. Surface photovoltage spectra demonstrated that Ni modification could effectively inhibit the recombination of the photoinduced electron and holes of TiO₂. This electron–hole pair separation conditions are responsible for the higher photocatalytic performance of Ni/TiO₂ nanocomposites in the visible region of electromagnetic spectrum.     

CuO Catalysts Supported on Porous TiO2 Microspheres for Low-Temperature CO Oxidation

The porous TiO₂ microspheres were prepared by the reversed-phase suspension polymerization and sol-gel method using reversed-phase suspension droplets as the templates. The CO oxidation catalytic properties of the CuO/TiO₂ catalysts prepared by hydrothermal method and impregnation method were extensively investigated. The structure of CuO/TiO₂ catalysts was determined by TG-DTA, XRD, TEM, and XPS. The results indicated that the redox capacity of CuO/TiO₂ was greatly depended on the aqueous solution concentration of Cu(NO₃)₂ used in the preparation of CuO/TiO₂ and the calcination temperature of the CuO/TiO₂ catalysts.     

Effects of titanium silicalite and TiO2 nanocomposites on supported Co‐based catalysts for Fischer–Tropsch synthesis

Titanium silicalite (TS) and TiO₂ nanocomposites were prepared by mixing TS and TiO₂ with different ratios in ethanol. They were impregnated with 15 wt% Co loading to afford Co‐based catalysts. Fischer–Tropsch synthesis (FTS) performance of these TS–TiO₂ nanocomposite‐supported Co‐based catalysts was studied in a fixed‐bed tubular reactor. The results reveal that the Co/TS–TiO₂ catalysts have better catalytic performance than Co/TS or Co/TiO₂ each with a single support, showing the synergistic effect of the binary TS–TiO₂ support. Among the TS–TiO₂ nanocomposite‐supported Co‐based catalysts, Co/TS–TiO₂‐1 presents the highest activity. These catalysts were characterized using N₂ adsorption–desorption measurements, X‐ray diffraction, X‐ray photoelectron spectroscopy, H₂ temperature‐programmed reduction, H₂ temperature‐programmed desorption and transmission electron microscopy. It was found that the position of the active component has a significant effect on the catalytic activity. In the TS–TiO₂ nanocomposites, cobalt oxides located at the new pores developed between TS and TiO₂ can exhibit better catalytic activity. Also, a positive relationship is observed between Co dispersion and FTS catalytic performance for all catalysts. The catalytic activity is improved on increasing the dispersion of Co.     

Effect of the extent of reduction of V2O5 on the adsorption of propylene and the oxidative dehydrogenation of propane to give propylene on V2O5 and V2O5/TiO2-SiO2

An increase in the propylene output in the oxidative dehydration of propane on V₂O₅/TiO₂-SiO₂ was observed after prior reduction of V₂O₅ in the reaction mixture to V₂O₄, which reduces the destructive chemisorption of propylene. A low titanium dioxide content in TiO₂-SiO₂ hinders the deep reduction of V₂O₅ to V₂O₃, which reduces the conversion of propane. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 6, pp. 373–378, November–December, 2007.     

Surface species and gas phase products in the preferential oxidation of CO on TiO2-supported Au-Rh bimetallic catalysts

The oxidation of CO in the presence of hydrogen (PROX process) was investigated on bimetallic Au-Rh catalysts at 300–373 K by Fourier transform infrared spectroscopy and mass spectroscopy. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied. The IR studies revealed the formation of bi- and monodentate carbonates, bicarbonates and hydrocarbonates on the catalysts surfaces; these surface species proved to be not involved in the surface reactions. The formation of adsorbed formaldehyde was observed on all surfaces, except 1% (0.25Au+0.75Rh)/TiO₂. Adsorbed CO₂ (as the surface product of CO oxidation) was not detected on any surface. The presence of both O₂ and H₂ reduced the surface concentration of CO adsorbed on the metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO₂ was formed in the highest amount in the CO+O₂ mixture, the presence of H₂ suppressed the amount of CO₂ produced. This negative effect of H₂ was the lowest on the 1% Rh/TiO₂ and 1% (0.25Au+0.75Rh)/TiO₂ catalysts.     

Light expanded clay aggregate (LECA) as a support for TiO<Subscript>2</Subscript>, Fe/TiO<Subscript>2</Subscript>, and Cu/TiO<Subscript>2</Subscript> nanocrystalline photocatalysts: a comparative study on the structure,morphology, and activity

In this work, TiO₂ and doped TiO₂ photocatalysts (Fe/TiO₂ and Cu/TiO₂) were synthesized by the sol–gel method. The main objective of this study was to investigate the influence of dopants on the structure, morphology, and activity of the catalysts in powder and immobilized states. XRF, XRD, and SEM methods were used to characterize the catalysts. The structure and phase distribution of the nanocrystalline powders were identified by XRD. Nanoparticles crystallite size and the degree of crystallinity were affected by doping. The anatase contents of catalysts were achieved as follows: TiO₂ (5.89 %) < Fe/TiO₂ (42.17 %) < Cu/TiO₂ (70.28 %). It was indicated that the activity of the catalysts strongly depends on the anatase content. Under the same circumstances, copper-modified TiO₂ exhibited a twofold higher photocatalytic activity compared with TiO₂. The nanostructured catalysts were immobilized on light expanded clay aggregate (LECA) granules in order to investigate the effect of a novel support on the activity of the catalysts. Morphological changes are recognizable in the SEM images. Activity tests indicated that the best catalytic performance was assigned to Cu/TiO₂/LECA. After 120 min of irradiation, 61 % degradation of phenol in synthetic wastewater was achieved. The high photocatalytic activity of Cu/TiO₂/LECA confirms that LECA is as an excellent support.     

Correlation of the Catalytic Activity for Oxidation Taking Place on Various TiO2 Surfaces with Surface OH Groups and Surface Oxygen Vacancies

Three catalytic oxidation reactions have been studied: The ultraviolet (UV) light induced photocatalytic decomposition of the synthetic dye sulforhodamine B (SRB) in the presence of TiO₂ nanostructures in water, together with two reactions employing Au/TiO₂ nanostructure catalysts, namely, CO oxidation in air and the decomposition of formaldehyde under visible light irradiation. Four kinds of TiO₂ nanotubes and nanorods with different phases and compositions were prepared for this study, and gold nanoparticle (Au‐NP) catalysts were supported on some of these TiO₂ nanostructures (to form Au/TiO₂ catalysts). FTIR emission spectroscopy (IES) measurements provided evidence that the order of the surface OH regeneration ability of the four types of TiO₂ nanostructures studied gave the same trend as the catalytic activities of the TiO₂ nanostructures or their respective Au/TiO₂ catalysts for the three oxidation reactions. Both IES and X‐ray photoelectron spectroscopy (XPS) proved that anatase TiO₂ had the strongest OH regeneration ability among the four types of TiO₂ phases or compositions. Based on these results, a model for the surface OH group generation, absorption, and activation of molecular oxygen has been proposed: The oxygen vacancies at the bridging O^2? sites on TiO₂ surfaces dissociatively absorb water molecules to form OH groups that facilitate adsorption and activation of O₂ molecules in nearby oxygen vacancies by lowering the absorption energy of molecular O₂. A new mechanism for the photocatalytic formaldehyde decomposition with the Au/TiO₂ catalysts is also proposed, based on the photocatalytic activity of the Au‐NPs under visible light. The Au‐NPs absorb the light owing to the surface plasmon resonance effect and mediate the electron transfers that the reaction needs.     

Preparation and properties of a new composite photocatalyst based on nanosized titanium dioxide

The use of photocatalysts supported on adsorbents is receiving substantial attention. Supporting TiO₂ with zeolites is found to be one of the best solutions to increase the efficiency of TiO₂-based photocatalysts. This work was focused on simple preparation of a TiO₂/Na-ZSM-5 composite catalyst by the solid state dispersion (SSD) method and its modification with an organic photosensitizer — polythiophene (PT). Using the XRD diffractometry, structure of the new composite catalyst was proved. Beside this composite catalyst, mechanical mixtures of TiO₂-based catalysts with Na-ZSM-5 zeolite were prepared. The efficiency of all five available photocatalysts (TiO₂, TiO₂-PT, mechanical mixture of TiO₂ + Na-ZSM-5, mechanical mixture of TiO₂-PT + Na-ZSM-5, and the modified SSD-PT composite) on photodegradation of 4-chlorophenol was compared. By measuring the formation of chloride ions and decreasing the 4-chlorophenol concentration at two different initial concentrations of 4-chlorophenol in the basic aqueous solution, the photoefficiency and adsorption properties of our photocatalysts were determined. Presented at the 35th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 26–30 May 2008.     

Photocatalytic decomposition of liquid-water on the Pt-loaded TiO2 catalysts: Effects of the oxidation states of Pt species on the photocatalytic reactivity and the rate of the back reaction

The photocatalytic decomposition of liquid water on Pt-loaded TiO₂ (Pt/TiO₂) catalysts was investigated. The results obtained by XPS and XRD measurements of the catalysts as a function of the calcination temperature as well as the photocatalytic decomposition reactions of H₂O clearly indicate that controlling the oxidation state of Pt as well as the amount of loaded Pt species are both important factors in the design of water-splitting photocatalysts having high efficiency and stoichiometry.     

Synthesis of Fe<Superscript>3+</Superscript> doped ordered mesoporous TiO<Subscript>2</Subscript> with enhanced visible light photocatalytic activity and highly crystallized anatase wall

Fe³⁺ doped mesoporous TiO₂ with ordered mesoporous structure were successfully prepared by the solvent evaporation-induced self-assembly process using P123 as soft template. The properties and structure of Fe³⁺ doped mesoporous TiO₂ were characterized by means of XRD, EPR, BET, TEM, and UV–vis absorption spectra. The characteristic results clearly show that the amount of Fe³⁺ dopant affects the mesoporous structure as well as the visible light absorption of the catalysts. The photocatalytic activity of the prepared mesoporous TiO₂ was evaluated from an analysis of the photodegradation of methyl orange under visible light irradiation. The results indicate that the sample of 0.50%Fe–MTiO₂ exhibits the highest visible light photocatalytic activity compared with other catalysts.     

Stereochemistry of the initiation step in Ziegler-Natta catalysts containing dialkyl propane diethers: A tool for distinguishing the role of internal and external donors

The results of a study on a new generation of MgCl₂ supported Ziegler-Natta catalysts that contain dialkyl propane diethers as internal donors and produce highly stereospecific polypropene even without any external donor are presented. The results concerning three different catalysts containing 2,2-diisobutyl-1,3-dimethoxy propane (DIBDMP), 2,2-dicyclopentyl-1,3-dimethoxy propane (DCPDMP) and 2-ethyl,2-butyl-1,3-dimethoxy propane (EBDMP) as internal donors are compared. The same donors have also been studied by using them as external donors with a catalyst contaning diisobutyl phthalate as internal donor. The present results, along with previously obtained findings on traditional supported Ziegler-Natta catalysts, permit a general comprehension of the role of internal and external donors.