Effect of TiO2 supporting layer on Fe2O3 photoanode for efficient water splitting

For an electrochemical water splitting system, titanate nanotubular particles with a thickness of ∼700 nm produced by a hydrothermal process were repetitively coated on fluorine-doped tin oxide (FTO) glass via layer-by-layer self-assembly method. The obtained titanate/FTO films were dipped in aqueous Fe solution, followed by heat treatment for crystallization at 500 °C for 10 min in air. The UV–vis absorbance of the Fe-oxide/titanate/FTO film showed a red-shifted spectrum compared with the TiO₂/FTO coated film; this red shift was achieved by the formation of thin hematite-Fe₂O₃ and anatase-TiO₂ phases verified using X-ray diffraction and Raman results. The cyclic voltammetry results of the Fe₂O₃/TiO₂/FTO films showed distinct reversible cycle characteristics with large oxidation–reduction peaks with low onset voltage of I–V characteristics under UV–vis light illumination. The prepared Fe₂O₃/TiO₂/FTO film showed much higher photocurrent densities for more efficient water splitting under UV–vis light illumination than did the Fe₂O₃/FTO film. Its maximum photocurrent was almost 3.5 times higher than that obtained with Fe₂O₃/FTO film because of the easy electron collection in the current collector. The large current collection was due to the existence of a TiO₂ base layer beneath the Fe₂O₃ layer.     

Zeolite beta synthesized with acid-treated metakaolin and its application in diesel hydrodesulfurization

A new type of zeolite beta (denoted as MB) with multi-pore system was synthesized by using in situ synthesized method from kaolin mineral in this study. NiW/Al₂O₃–MB and NiW/TiO₂–Al₂O₃–MB catalysts were prepared and the hydrodesulfurization (HDS) activities of these catalysts were evaluated with FCC diesel feed. The samples were characterized by N₂ physisorption, XRD, SEM, TPR, FT-IR spectroscopy of pyridine adsorption, HRTEM and XPS techniques. The HDS results showed that the MB-containing catalyst exhibited much higher HDS conversion (98.7%) than that of NiW/γ-Al₂O₃ (97.5%). The incorporation of TiO₂ into the composite supports further increased the HDS conversion (99.3%) of NiW/TiO₂–Al₂O₃–MB. The higher HDS activity was mainly associated with the appropriate ratio of B/L (Brönsted acid/Lewis acid) and the enhanced hydrogenation activity.     

Effect of the hydrogen spillover on the selectivity of dibenzothiophene hydrodesulfurization over CoSx/γ-Al2O3, NiSx/γ-Al2O3 and MoS2/γ-Al2O3 catalysts

Dibenzothiophene (DBT) hydrodesulphurization (HDS) reaction at 3 MPa and 325–375 °C on Mo/γ-Al₂O₃ single-bed and Me/γ-Al₂O₃//SiO₂//Mo/γ-Al₂O₃ (Me = Co or Ni) double-bed catalysts were investigated. Results indicate that ratio cyclohexylbenzene (CHB)/biphenyl (BP) or selectivity is higher when using double-beds rather than a single-bed. Synergy in dibenzothiophene hydrodesulphurization on Co//Mo and Ni//Mo double-beds is also detected. Changes in selectivity and conversion are attributed to the action of spillover hydrogen (Hso) formed in the first bed that reaches the second bed.     

Synergism Between Ni and W in the Niw/ γ-Al2O3 Hydrotreating Catalysts

A series of NiW/ γ-Al₂O₃ catalysts (20 and 30 wt% W and 1–5 wt% Ni) have been prepared and studied by TPR and XPS. HDS activity has been tested in the thiophene conversion. The effect of Ni and W loadings on the formation of different structures is presented. In the calcined catalysts several phases coexist, concentrations of which depend on the Ni/(Ni+W) atomic ratio. The Ni synergistic effect in the HDS reaction is confirmed by the increase in the HDS activity (~10–15 times). This effect is ascribed to the formation of an active NiWS phase of high dispersion from the mixed NiW oxide precursors. At higher Ni/Ni+W ratio a redistribution of active components in additional amount ofNiWS phase during sulfidation is suggested.     

Selectivity enhancement of Co-Mo/Al2O3 FCC gasoline hydrodesulfurization catalysts via incorporation of mesoporous Si-SBA-15

Mesoporous Si-SBA-15 was applied to enhance the FCC gasoline selective hydrodesulfurization (HDS) performance of conventional Co-Mo/Al₂O₃ catalysts and the physicochemical properties of the resulting catalyst were compared with those of Co-Mo/Al₂O₃ catalysts incorporated with macroporous kaolin, mesoporous Si-MCM-41 and microporous Si-ZSM-5. The selective HDS performances of all the catalysts were assessed with different FCC gasolines as feedstocks. The results showed that the HDS selectivity of the catalysts was closely related to the Mo sulfidation that depends on catalyst surface area and metal-support interaction. With the superior Mo sulfidation, the Co-Mo/Si-SBA-15-Al₂O₃ catalyst had the optimal HDS selectivity for not only the full-range FCC gasolines but also the heavy fractions thereof. The present article demonstrates the significance of enhancing Mo sulfidation in improving HDS selectivity and thus sheds a light on the development of highly selective HDS catalysts.     

The functionalities of Pt/γ-Al2O3 catalysts in simultaneous HDS and HDA reactions

A Pt/γ-Al₂O₃ catalyst was tested in simultaneous hydrodesulfurization (HDS) of dibenzothiophene and hydrodearomatization (HDA) of naphthalene reactions. Samples of it were subjected to different pretreatments: reduction, reduction–sulfidation, sulfidation with pure H₂S and non-activation. The reduced catalyst presented the best performance, even comparable to that of Co(Ni)Mo catalysts. All catalyst samples were selective to the HDS reaction over HDA, and to the direct desulfurization pathway of dibenzothiophene HDS over the hydrogenation reaction pathway of HDS. The effect of H₂S partial pressure on the functionalities of the reduced Pt/γ-Al₂O₃ catalyst was studied. The results showed that an increase in H₂S partial pressure does not cause poisoning, but an inhibition effect, without changing the catalyst selectivity. Accordingly, the activity trends were ascribed to adsorption differences between the different reactive molecules over the same catalytic active site. TPR characterization along with a thermodynamics analysis showed that the active phase of reduced Pt/γ-Al₂O₃ is constituted by Pt⁰ particles. However, presulfidation of the catalyst leads to a mixture of PtS and Pt⁰ which has a negative effect on the catalytic performance without changing catalyst functionalities.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.     

Synthesis and characterization of W-SBA-15 and W-HMS as supports for HDS

W-modified HMS and SBA-15 mesoporous materials (Si/W molar ratio equal to 40) were synthesized using sodium tungstate as tungsten source. In order to prepare NiW catalysts these mesoporous materials were impregnated with an aqueous solution of nickel salt of 12-tungstophosphoric acid Ni_3/2PW₁₂O₄₀. The synthesized W-HMS, W-SBA-15 materials and NiW catalysts have been characterized by S_BET, XRD, UV–Vis DRS, FT-IR, TPD of NH₃, ²⁹Si MAS NMR, SEM and HRTEM. The influence of these particular supports on catalytic activity of NiW catalysts was studied in the reaction of hydrodesulfurization (HDS) of thiophene. The results from the FT-IR and UV–Vis DR spectroscopy confirm incorporation of W into the HMS and SBA-15 structures. Additionally ²⁹Si MAS NMR measurements revealed relatively stronger effect of W ion incorporation in HMS structure on degree of silica cross-linking as compared to the effect of W ion incorporation in SBA-15 structure. The catalytic study showed that both W-HMS and W-SBA-15 materials modified with W are good supports for NiW catalysts in the HDS reaction of thiophene. The catalysts show lower selectivity for butanes than a reference NiW/γ-Al₂O₃ catalyst leveling of about 10% for chosen experimental conditions.     

Synthesis, characterization and hydrotreating performance of supported tungsten phosphide catalysts

Supported tungsten phosphide catalysts were prepared by temperature-programmed reduction of their precursors (supported phospho-tungstate catalysts) in H₂ and characterized by X-ray diffraction (XRD), BET, temperature-programmed desorption of ammonia (NH₃-TPD) and X-ray photoelectron spectroscopy (XPS). The reduction-phosphiding processes of the precursors were investigated by thermogravimetry and differential thermal analysis (TG-DTA) and the suitable phosphiding temperatures were defined. The hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities of the catalysts were tested by using thiophene, pyridine, dibenzothiophene, carbazole and diesel oil as the feedstock. The TiO₂, γ-Al₂O₃ supports and the Ni, Co promoters could remarkably increase and stabilize active W species on the catalyst surface. A suitable amount of Ni (3%–5%), Co (5%–7%) and V (1%–3%) could increase dispersivity of the W species and the BET surface area of the WP/γ-Al₂O₃ catalyst. The WP/γ-Al₂O₃ catalyst possesses much higher thiophene HDS and carbazole HDN activities and the WP/TiO₂ catalyst has much higher dibenzothiophene (DBT) HDS and pyridine HDN activities. The Ni, Co and V can obviously promote the HDS activity and inhibit the HDN activity of the WP/γ-Al₂O₃ catalyst. The G-Ni5 catalyst possesses a much higher diesel oil HDS activity than the sulphided industrial NiW/γ-Al₂O₃ catalyst. In general, a support or promoter in the WP/γ-Al₂O₃ catalyst which can increase the amount and dispersivity of the active W species can promote its HDS and HDN activities.     

Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

Effects of the preparation methods on the performance of the Cu–Cr–Fe/γ-Al2O3 catalysts for the synthesis of 2-methylpiperazine

Co-precipitation, impregnation and ultrasonic sol–gel (USG) methods have been used to prepare Cu–Cr–Fe/γ-Al₂O₃ catalysts, which were further used to synthesize 2-methylpiperazine. The catalysts were characterized by XRD, XPS, TG/DSC, BET, TPR, AAS and TEM. It is found that preparation method can greatly impact the catalytic performance of the catalysts, the Cu–Cr–Fe/γ-Al₂O₃ catalyst prepared by the ultrasonic sol–gel method proved to be the most active and stable for this reaction. The dispersion and stabilization of Cu⁰ in the reduced catalysts are attributed to the existence of CuCr₂O₄ and Fe₂O₃. A surprising copper migration was detected by XPS analysis for the Cu–Cr–Fe/γ-Al₂O₃-USG catalyst after the calcination process, which may be crucial to the high activity and stability of this catalyst.     

Reduction of NO by CO on Cu/ZrO2/Al2O3 catalysts: Characterization and catalytic activities

ZrO₂, γ-Al₂O₃ and ZrO₂/γ-Al₂O₃-supported copper catalysts have been prepared, each with three different copper loads (1, 2 and 5 wt%), by the impregnation method. The catalysts were characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂, Raman spectroscopy and electronic paramagnetic resonance (EPR). The reduction of NO by CO was studied in a fixed-bed reactor packed with these catalysts and fed with a mixture of 1% CO and 1% NO in helium. The catalyst with 5 wt% copper supported on the ZrO₂/γ-Al₂O₃ matrix achieved 80% reduction of NO. Approximately the same rate of conversion was obtained on the catalyst with 2 wt% copper on ZrO₂. Characterization of these catalysts indicated that the active copper species for the reduction of NO are those in direct contact with the oxygen vacancies found in ZrO₂.     

Simple fabrication and photocatalytic activity of S-doped TiO2 under low power LED visible light irradiation

A series of S-doped TiO₂ with visible-light photocatalytic activity were prepared by a simple hydrolysis method using titanium tetrachloride (TiCl₄) and sodium sulfate (Na₂SO₄) as precursors. The photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV–vis diffuse reflectance spectrophotometer (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). With the doping of S, photocatalysts with small crystal size, high content of anatase phase were obtained. The result showed that S-doped TiO₂ demonstrate considerably high photoactivity under low power visible LED light irradiation, while undoped TiO₂ and the Degussa P25 have nearly no activity at all. The possible mechanism of S-doped for the visible-light activity was discussed.     

Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Influence of Ru segregation on the activity of Ru–Co/γ-Al2O3 during FT synthesis: A comparison with that of Ru–Co/SiO2 catalysts

γ-Al₂O₃ and SiO₂ supported Co catalysts, with varying amounts of Ru, were prepared and evaluated for Fischer–Tropsch synthesis (FTS). The composition of Ru for optimum activity was found to be support-dependent. The reducible Co₃O₄ was high in the region of 0–1.64 wt.% of Ru in Co/SiO₂ catalysts. Co/γ-Al₂O₃ displayed a maximum for reducible Co species at 0.42 wt.% Ru. Segregation of Ru occurred beyond this composition decreasing the extent of reduction. Co/γ-Al₂O₃ catalysts showed lower activity and olefin selectivity, in spite of higher Co dispersion, than Co/SiO₂ catalysts. The catalytic performance depends on the amount of reducible Co species, which again depends upon the optimum content of Ru.     

La and Ce loaded TiO2-ZSM-5 catalysts: Comparative characterization and photocatalytic activity investigations

Lanthanide ions (Ln:La³⁺ or Ce³⁺) loaded TiO₂ supported ZSM-5 catalysts were prepared and characterized by using X-ray diffraction (XRD), surface area (BET), UV–vis diffuse reflectance spectra (UV–vis DRS), atomic force microscopy (AFM) and scanning electron microscopy SEM with energy dispersive X-ray analysis (EDX) techniques. Photocatalytic activities of the supported catalysts were examined in the presence of methyl orange (MO) molecule under UV irradiation. La³⁺ loaded catalysts revealed higher dark adsorption capacities and decolorization percentages in comparison to Ce³⁺ incorporated ones. Effect of lanthanide ion contents and reuse performances of the catalysts were controlled. A tentative degradation mechanism of MO was postulated based on the electron-accepting ability of the lanthanide ions and hole-trapping duty of ZSM-5.     

Enhancement of the visible light photocatalytic performance of C-doped TiO2 by loading with V2O5

V₂O₅ was loaded on the surface of C-doped TiO₂ (C-TiO₂) by incipient wetness impregnation in order to enhance the visible light photocatalytic performance. The physicochemical properties of the C-TiO₂/V₂O₅ composite were characterized by XRD, Raman, TEM, XPS, UV–vis diffuse reflectance spectra, and PL in detail. The result indicated that a heterojunction between C-TiO₂ and V₂O₅ was formed and the separation of excited electron–hole pairs on C-TiO₂/V₂O₅ is greatly promoted. Thus, this composite photocatalyst exhibited enhanced visible light photocatalytic activity in degradation of gas-phase toluene compared with the pristine C-TiO₂.     

Yttrium-doped TiO2 films prepared by means of DC reactive magnetron sputtering

Y₂O₃-doped TiO₂ films were prepared on glass substrates by means of pulsed DC reactive magnetron sputtering method using titanium and yttrium mixed target. XPS results showed that the films were composed of fully oxidation states of the two elements, Y₂O₃–TiO₂ composite oxides. The existence of yttrium inhibited the crystal growth of TiO₂ in the films and Y₂O₃ mainly presented in its amorphous state in the films. UV–vis transmittance of the films decreased whereas their reflectance increased slightly. Yttrium doping had detrimental effect on photocatalytic activity of the TiO₂ films. Photocatalytic degradation efficiency of methyl orange solution declined along with increasing yttrium concentration.     

Preparation of Co–Mo/B2O3/Al2O3 catalysts for hydrodesulfurization: Effect of citric acid addition

The effect of citric acid (CA) addition was studied on the HDS of thiophene over Co–Mo/(B)/Al₂O₃ catalysts. The catalysts were characterized by means of LRS, Mo K-edge EXAFS, NO adsorption capacity measurements, and UV–vis spectra. The catalysts were subjected to a chemical vapor deposition (CVD) technique using Co(CO)₃NO as a precursor of Co in order to get deeper insights into the effect of citric acid addition. It was shown that the HDS activity was enhanced by the citric acid addition up to the CA/Mo mole ratio of around 1 and leveled off with further addition. The amount of Co anchored by the CVD was increased by the addition of citric acid, suggesting an increase in the dispersion of MoS₂ particles on the catalyst by the simultaneous presence of Co, Mo and citric acid, in conformity with the increase in the NO adsorption capacity. In contrast to Co–Mo catalysts, the edge dispersion of MoS₂ particles in Mo/B/Al₂O₃ was not affected by the addition of citric acid. The LRS, UV–vis spectra and Mo K-edge EXAFS showed that Co–CA and Mo–CA surface complexes are formed by the addition of citric acid. The Co–CA surface complex is more preferentially formed on CoMo/Al than on CoMo/B/Al, in agreement with a greater promoting effect of citric acid at a lower CA/Mo mole ratio for CoMo/Al than for CoMo/B/Al.     

Investigation of the sulfidation of Mo/TiO2-Al2O3 catalysts by TPS and LRS

A series of Mo/Al₂O₃ and Mo/TiO₂-Al₂O₃ catalysts were investigated by temperature programmed sulfiding (TPS) and laser Raman spectroscopy (LRS). The effect of TiO₂ on the sulfidability of molybdena was studied in detail. It is found that Mo/Al₂O₃ catalysts can be partially sulfided by O-S exchange at low temperature, forming molybdenum oxysulfide. The Mo-S bond subsequently ruptures in the presence of H₂ to produce H₂S. At 530–550 K deep sulfiding of molybdenum oxysulfide occurs forming crystalline MoS₂. When the surface of Al₂O₃ was covered by a monolayer of TiO₂, the sulfiding rate of molybdena at low temperature was not only greatly increased, but H₂S produced in the reduction of Mo-S species caused deep sulfiding of the catalyst which resulted in a decrease of the TPS peak temperature by 80–100 K. The results indicate that this promotion of the sulfiding of molybdena is enhanced with TiO₂ loading. The function of TiO₂ is explained by the weakened interaction between MoO₃ and Al₂O₃ due to the coverage of the Al₂O₃ surface by TiO₂.