Effect of atomic-layer-deposited TiO2 on carbon-supported Ni catalysts for electrocatalytic glycerol oxidation in alkaline media

Thin TiO₂ layers were deposited onto a carbon-supported Ni catalyst (Ni/C) through atomic layer deposition (ALD) and the resulting TiO₂-coated Ni/C (ALD(TiO₂)-Ni/C) was utilized for electrochemical glycerol oxidation in alkaline media. X-ray photoelectron spectroscopy analysis demonstrated that the Ni surface phase of ALD(TiO₂)-Ni/C mainly consisted of Ni(OH)₂ while that of uncoated Ni/C was a mixed phase of NiO and Ni(OH)₂. The ALD(TiO₂)-Ni/C exhibited electrocatalytic activity at least 2.4 times higher than that of Ni/C. Density functional theory calculations were used to investigate how the modified Ni surface with the TiO₂ coating affects the adsorption/desorption of glycerol.     

Comparison of CO2 Reduction Performance with NH3 and H2O between Cu/TiO2 and Pd/TiO2

The aim of this study is to clarify the effect of doped metal type on CO₂ reduction characteristics of TiO₂ with NH₃ and H₂O. Cu and Pd have been selected as dopants for TiO₂. In addition, the impact of molar ratio of CO₂ to reductants NH₃ and H₂O has been investigated. A TiO₂ photocatalyst was prepared by a sol-gel and dip-coating process, and then doped with Cu or Pd fine particles by using the pulse arc plasma gun method. The prepared Cu/TiO₂ film and Pd/TiO₂ film were characterized by SEM, EPMA, TEM, STEM, EDX, EDS and EELS. This study also has investigated the performance of CO₂ reduction under the illumination condition of Xe lamp with or without ultraviolet (UV) light. As a result, it is revealed that the CO₂ reduction performance with Cu/TiO₂ under the illumination condition of Xe lamp with UV light is the highest when the molar ratio of CO₂/NH₃/H₂O = 1:1:1 while that without UV light is the highest when the molar ratio of CO₂/NH₃/H₂O = 1:0.5:0.5. It is revealed that the CO₂ reduction performance of Pd/TiO₂ is the highest for the molar ratio of CO₂/NH₃/H₂O = 1:1:1 no matter the used Xe lamp was with or without UV light. The molar quantity of CO per unit weight of photocatalyst for Cu/TiO₂ produced under the illumination condition of Xe lamp with UV light was 10.2 μmol/g, while that for Pd/TiO₂ was 5.5 μmol/g. Meanwhile, the molar quantity of CO per unit weight of photocatalyst for Cu/TiO₂ produced under the illumination condition of Xe lamp without UV light was 2.5 μmol/g, while that for Pd/TiO₂ was 3.5 μmol/g. This study has concluded that Cu/TiO₂ is superior to Pd/TiO₂ from the viewpoint of the molar quantity of CO per unit weight of photocatalyst as well as the quantum efficiency.     

Superior Photostability and Photocatalytic Activity of ZnO Nanoparticles Coated with Ultrathin TiO2 Layers through Atomic‐Layer Deposition

Atomic‐layer deposition (ALD) is a thin‐film growth technology that allows for conformal growth of thin films with atomic‐level control over their thickness. Although ALD is successful in the semiconductor manufacturing industry, its feasibility for nanoparticle coating has been less explored. Herein, the ALD coating of TiO₂ layers on ZnO nanoparticles by employing a specialized rotary reactor is demonstrated. The photocatalytic activity and photostability of ZnO nanoparticles coated with TiO₂ layers by ALD and chemical methods were examined by the photodegradation of Rhodamine B dye under UV irradiation. Even though the photocatalytic activity of the presynthesized ZnO nanoparticles is higher than that of commercial P25 TiO₂ nanoparticles, their activity tends to decline due to severe photocorrosion. The chemically synthesized TiO₂ coating layer on ZnO resulted in severely declined photoactivity despite the improved photostability. However, ultrathin and conformal ALD TiO₂ coatings (≈0.75–1.5 nm) on ZnO improved its photostability without degradation of photocatalytic activity. Surprisingly, the photostability is comparable to that of pure TiO₂, and the photocatalytic activity to that of pure ZnO.     

A Large-Area Patterned TiO2/SnO2 Bilayer Type Photocatalyst Prepared by Gravure Printing

Anatase TiO₂ films (thickness = 50 nm) were formed in shape of stripes (width = 1.6 mm, interval = 0.4 mm) by gravure printing on commercially available SnO₂ coated soda-lime glass substrates (dimension = 300 × 300 mm). Its photocatalytic activity was examined for the gas-phase oxidation of CH₃CHO in comparison with a simple TiO₂ photocatalyst formed on a silica glass. The patterned TiO₂/SnO₂ bilayer type photocatalyst showed a high photocatalytic activity in an H₂O bearing atmosphere. On the other hand, neither the patterning nor stacking effect was observed for the same reaction under dry conditions. These results could be explained in terms of the reducing potential of the electrons in the conduction band of the SnO₂ layer.     

Effect of PEG with different <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> as template direction reagent on preparation of porous TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> with assistance of supercritical CO<Subscript>2</Subscript>

Titania–silica composite have been prepared using polyethylene glycol (PEG) with different molecular weights (M _w), PEG20000, PEG10000, and PEG2000, as template in supercritical carbon dioxide (SC CO₂). The composite precursors were dissolved in SC CO₂ and impregnated into PEG templates using SC CO₂ as swelling agent and carrier. After removing the template by calcination at suitable temperature, the titania–silica composite were obtained. The composite were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen sorption–desorption experiment. Photocatalytic activity of the samples has been investigated by photodegradation of methyl orange. Results indicate that there are many Si–O–Ti linkages in the TiO₂/SiO₂ composite; the PEG template has a significant influence on the structure of TiO₂/SiO₂. In addition, the TiO₂/SiO₂ prepared with PEG10000 exhibited high photocatalytic efficiency. So this work supplies a clue to control and obtain the TiO₂/SiO₂ composite with different photocatalytic reactivity with the aid of suitable PEG template in supercritical CO₂.     

Avoiding Self‐Poisoning: A Key Feature for the High Activity of Au/Mg(OH)2 Catalysts in Continuous Low‐Temperature CO Oxidation

Au/Mg(OH)₂ catalysts have been reported to be far more active in the catalytic low‐temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO₂ catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)₂ with CO₂ formed during the low‐temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO₂ also catalyzes CO₂ formation, but does not allow for CO₂ desorption, which results in self‐poisoning. At higher temperatures (above 0 °C), however, CO₂ formation is rate‐limiting, which results in a much higher activity for Au/TiO₂ under these reaction conditions.     

FT-IR study on CO<Subscript>2</Subscript> adsorbed species of CO<Subscript>2</Subscript> sorbents

The stability of amine-functionalized silica sorbents prepared through the incipient wetness technique with primary, secondary, and tertiary amino organosilanes was investigated. The prepared sorbents were exposed to different gaseous streams including CO₂/N₂, dry CO₂/air with varying concentration, and humid CO₂/air mixtures to demonstrate the effect of the gas conditions on the CO₂ adsorption capacity and the stability of the different amine structures. The primary and secondary amine-functionalized adsorbents exhibited CO₂ sorption capacity, while tertiary amine adsorbent hardly adsorbed any CO₂. The secondary amine adsorbent showed better stability than the primary amine sorbent in all the gas conditions, especially dry conditions. Deactivation species were evaluated using FT-IR spectra, and the presence of urea was confirmed to be the main deactivation product of the primary amine adsorbent under dry condition. Furthermore, it was found that the CO₂ concentration can affect the CO₂ sorption capacity as well as the extent of degradation of sorbents.     

TiO2/clay as a heterogeneous catalyst in photocatalytic/photochemical oxidation of anionic reactive blue 19

A TiO₂-coated Tunisian clay (TiO₂–clay) was synthesized by a typical impregnation method. The physicochemical characterization points to a successful impregnation of titania on the clay surface. The activity of this structured catalyst was studied in the photocatalytic/photochemical oxidation of anionic reactive blue 19 (RB 19). The effect of UVA and solar irradiation (UV-solar) was studied at room temperature. TiO₂–clay demonstrated an effective degradation of RB 19 under both types of irradiation. Moreover, in this study, the effects of various oxidants such as hydrogen peroxide (H₂O₂), potassium peroxodisulfate (K₂S₂O₈) and sodium carbonate (Na₂CO₃) were thoroughly investigated. H₂O₂ was a promising oxidant for promoting RB 19 degradation under UV_A. The kinetics of discoloration of RB 19 followed a pseudo-first-order rate law. We can remark that 20 min of UV irradiation was enough to achieve 100% discoloration of the aqueous solution. However, under UV–Vis, HPLC and chemical oxygen demand measurements indicated, that a longer reaction time (of around 45 min) was required for achieving the complete dye mineralization. The findings clearly demonstrated the applicability of this TiO₂/clay catalyst for the photocatalytic oxidation of RB 19.     

Isoprene Heterogeneous Uptake and Reactivity on TiO2: A Kinetic and Product Study

The heterogeneous interaction of isoprene with TiO₂ surfaces was studied under dark and UV light irradiation conditions. The experiments were conducted at room temperature, using zero air as bath gas, in a flow reactor coupled with a SIFT‐MS (selected‐ion flow‐tube mass spectrometer) and a FTIR spectrometer for the gas‐phase monitoring of reactants and products. The steady‐state uptake coefficient and the yields of the products formed were measured as a function of TiO₂ mass (9–120 mg), light intensity (37–112 W m⁻²), isoprene concentration (36–12000 ppb), and relative humidity (0.01–90% of RH). Under dark and dry conditions, isoprene was efficiently and reversibly adsorbed on TiO₂. In contrast, under humid conditions, isoprene uptake was diminished, pointing to competitive adsorption with water molecules. In the presence of UV light irradiation, isoprene reacted on the surface of TiO₂. The reactive steady‐state uptake coefficient, γ_ss, was independent of RH under most ambient relative humidity conditions (>50%). However, γ_ss was strongly dependent on isoprene initial concentration according to the empirical expression: γ_ss = (2.0 × 10⁻⁴) × [isoprene] with n = 0.35 and 0.28 for 37 and 112 W m⁻² irradiation conditions, respectively. In addition to the kinetics, a detailed product study was performed. The gas‐phase oxidation products were mostly CO₂ (ca. 90% of the carbon mass balance) and a large variety of carbonyl compounds (methyl vinyl ketone, acetone, methacrolein, formaldehyde, acetaldehyde, propanal, traces of butanal, and pentanal), the distribution of which was investigated as a function of mineral oxide mass, isoprene concentration, and RH. Furthermore, the surface‐adsorbed products were determined employing off‐line HPLC chromatography; their concentrations were inversely dependent on RH and decreased to background levels at RH greater than 30%. Finally, the reaction mechanism and possible implications of isoprene reaction on TiO₂ are briefly discussed.     

Effect of La2O3 on Catalytic Performance of Au/TiO2 for CO Oxidation

Pure TiO₂ and La-doped TiO₂ were prepared by the sol-gel method. Au was supported on TiO₂ by the deposition-precipitation (DP) method, and its catalytic activity for CO oxidation was tested. The results showed that doping La in Au/TiO₂ could improve its catalytic activity obviously for CO oxidation. The analyses of X-ray diffraction (XRD), temperature-programmed desorption (TPD), and Brunauer-Emmett-Teller (BET) surface area further showed that the presence of La in TiO₂ not only increased its surface area and restrained the growth of TiO₂ crystallites, but could also enhance the microstrain of TiO₂. In terms of O₂-TPD, a new adsorbed species O⁻ appeared on the surface of La-doped TiO₂. The results of in-situ Fourier transform-infrared (FT-IR) spectroscopy illustrated that the high activity of Au/La₂O₃-TiO₂ was attributed to the presence of La promoting the reactivity of CO adsorbed on the Au site and the formation of the second active site on the surface of TiO₂     

Studies on TiO2/SiO2 and Pd/TiO2/SiO2 Catalysts in Photoreduction of CO2 with H2O to Methanol

The development of industry induced a massive increase in the emission of carbon dioxide into the atmosphere. A large amount of CO₂ and its general availability causes that it could be a cheap reactant in a reaction that runs in a way similar to photosynthesis in plants. Pure TiO₂ and metal doped TiO₂ are the most studied semiconductor catalysts for photoreduction of CO₂. The TiO₂/SiO₂ and Pd/TiO₂/SiO₂ catalysts were prepared and studied by temperature-programmed desorption, X-ray diffraction analysis, SEM-EDS, temperature-programmed reduction and then used for the methanol synthesis. The photoactivity of Pd/TiO₂/SiO₂ catalysts in the reduction of CO₂ with H₂O was tested at room temperature using photoreactor equipped with 16 lamps. The wavelength was characteristic of near ultraviolet. Post-reaction products were identified with gas chromatograph equipped with the flame ionization detector. Pd doping made the catalysts photoactive and the photoactivity of catalysts was changing as follows: 1%Pd/5%TiO₂/SiO₂ > 1% Pd/10% TiO₂/SiO₂ > 1% Pd/15% TiO₂/SiO₂. Optimum ultraviolet radiation time in the photoreduction of CO₂ to methanol was 7 h. An addition of Pd does not change the surface of the carrier.     

Complete oxidation of egg albumin on photoirradiated TiO2: factors determining catalytic performance in solid�Csolid heterogeneous systems

Solid?Csolid heterogeneous photooxidation of egg albumin with TiO₂ was carried out under near UV-light irradiation (?? > 370 nm) and the oxidation behavior was investigated. Egg albumin was photooxidized to form CO₂ without evolution of CO and organic byproducts. The activity for TiO₂-catalyzed albumin photooxidation strongly depended on the albumin and TiO₂ mixing conditions, indicating that the contact conditions between TiO₂ and albumin were important for complete oxidation of albumin. In-situ diffuse reflectance FTIR studies revealed that the peptide bond of albumin was decomposed with the alkyl side chains and that decomposition of the peptide bond followed pseudo first-order kinetics.     

Preparation and Characterization of Bismuth Doped TiO2 Thin Films

A series of Bismuth-doped titanium oxide (Bi-doped TiO₂) thin films on glass substrates have been prepared by sol-gel dip coating process. The prepared catalysts were characterized by XRD and XPS. The photocatlytic activity of the thin film catalysts was evaluated through the photodegradation of aqueous methyl orange under UV illumination. The experiments demonstrated that the Bi-doped TiO₂ prepared was anatase phase. The doped bismuth was in the 3⁺ oxidation state. The presence of Bi significantly enhanced the photocatalytic activity of TiO₂ films. At calcination temperature of 500°C, with doping concentration of 2 wt %, Bi-doped TiO₂ thin film showed the highest photocatalyic activity.     

掺杂纳米金粒子的火焰氧化TiO2膜的光电化学性质的研究

A nano-Au modified TiO2 electrode was prepared via the oxidation of Ti sheet in flame and subsequent modification with gold nanoparticles. The results of SEM and TEM measurements show that the Au nanoparticles are well dispersed on TiO2 surface. A near 2-fold enhancement in photocurrent was achieved upon the modification with Au nanoparticles. From the results of photocurrent and electrochemical impedance experiments it was found that the flatband potential of nano-Au/TiO2 electrode negatively shifted about 100 mV in 0.5 mol/L Na2SO4 solutions compared with that of bare TiO2 electrode. The improvement of photoelectrochemical performance was explained by the inhibition for charge recombination of photo-induced electrons and holes, and the promotion for interracial charge-transfer kinetics at nano-Au/TiO2 composite film. Such nanometal-semiconductor composite films have the potential application in improving the performance of photoelectrochemical solar cells.     

Thermal and hydrothermal stability of flame hydrolytically synthesized SiO2/TiO2 mixed oxides

The phase stability of the two TiO₂ modifications (anatase and rutile) in fumed SiO₂/TiO₂ nano-composites (0–24.8 wt-% silica) under thermal and hydrothermal conditions was investigated by X-ray powder diffraction, transmission electron microscopy (TEM) and gas adsorption methods (BET). The results show that the phase transformation from anatase to rutile type of structure and the growth of anatase crystallites are significantly retarded by mixing small amounts of SiO₂ into TiO₂, while the specific surface area is maintained. The SiO₂/TiO₂-composites reveal a remarkable shift in the anatase to rutile transformation temperature from approx. 500 °C (pure TiO₂) to approx. 1000 °C (samples with SiO₂ contents of more than 10%). The rate of phase transformation from anatase to rutile is enhanced under hydrothermal conditions compared to conventional thermal treatment, e.g. pure titania (AEROXIDE^® TiO₂ P25) annealed under hydrothermal conditions (100 g/m³ absolute humidity, 4 h at 600 °C) had a rutile content of 85%, while the same specimens annealed in absence of humidity contained only 46% rutile. However, the difference in rate of phase transformation became less pronounced when the silica content in SiO₂/TiO₂-composites was further increased.TEM results showed that the surface of the anatase crystallites was covered with silica. This averts coalescence of anatase crystallites and keeps them under a critical size during the annealing process. When the crystal domains grew larger, a rapid conversion to rutile took place. The critical size of anatase crystallites for the phase transformation was estimated to be 15–20 nm.     

In situ growth of TiO2 in interlayers of expanded graphite for the fabrication of TiO2-graphene with enhanced photocatalytic activity

We present a facile route for the preparation of TiO₂–graphene composites by in situ growth of TiO₂ in the interlayer of inexpensive expanded graphite (EG) under solvothermal conditions. A vacuum‐assisted technique combined with the use of a surfactant (cetyltrimethylammonium bromide) plays a key role in the fabrication of such composites. Firstly, the vacuum environment promotes full infusion of the initial solution containing Ti(OBu)₄ and the surfactant into the interlayers of EG. Subsequently, numerous TiO₂ nanoparticles uniformly grow in situ in the interlayers with the help of the surfactant, which facilitates the exfoliation of EG under the solvothermal conditions in ethanol, eventually forming TiO₂–graphene composites. The as‐prepared samples have been characterized by Raman and FTIR spectroscopies, SEM, TEM, AFM, and thermogravimetic analysis. It is shown that a large number of TiO₂ nanoparticles homogeneously cover the surface of high‐quality graphene sheets. The graphene exhibits a multi‐layered structure (5–7 layers). Notably, the TiO₂–graphene composite (only 30 wt % of which is TiO₂) synthesized by subsequent thermal treatment at high temperature under nitrogen shows high photocatalytic activity in the degradation of phenol under visible and UV lights in comparison with bare Degussa P25. The enhanced photocatalytic performance is attributed to increased charge separation, improved light absorbance and light absorption width, and high adsorptivity for pollutants.     

Effect of the amine type on thermal stability of modified mesoporous silica used for CO2 adsorption

In this study, the preparation by grafting of amino-functionalized SBA-15 molecular sieves was carried out. Amino-functionalized molecular sieves were synthesized using a silane coupling agent and different types of amination reagents which react with modified SBA-15. These composites were characterized by FT-IR spectroscopy, X-ray diffraction at low angles, nitrogen physisorption at 77 K, and evaluated by the adsorption of CO₂ and its temperature-programmed desorption—TPD. Thermal stability was investigated by TGA and DTA methods. In the view of a possible use of these amino-functionalized molecular sieves as sorbents for CO₂ removal, their adsorption–desorption properties towards CO₂ were also investigated by the TPD method. The mass loss of amino-functionalized molecular sieves above 215 °C was due to the oxidation and decomposition of amino propyl functional groups. This means that these composites could be used for adsorption of CO₂ at temperatures below 215 °C. The adsorption of CO₂ and its temperature programmed desorption using thermogravimetry were studied for amino-functionalized molecular sieves at 60 °C. The evolved gases during the adsorption–desorption of CO₂ on amino-functionalized molecular sieves were identified by online mass spectrometry coupled with thermogravimetry. CO₂ adsorption isotherms of functionalized samples at 60 °C showed that both the adsorption capacity (mg CO₂/g adsorbent) and the efficiency of amino groups (mol CO₂/mol NH₂) depend on the type of amination reagents and the amount of organic compound used.

     

The Adsorption,Thermal Desorption and Photochemistry of Methyl Iodide on an Ag‐Covered TiO2(110) Surface

The thermal reactions and photochemistry of monolayer methyl iodide (CH₃I) on a silver covered TiO₂(110) surface have been studied using combinative techniques of temperature programmed desorption (TPD) and x‐ray photoelectron spectroscopy (XPS). About ? 60% of CH₃I at monolayer coverage on Ag/TiO₂(110) dissociates between 130 and 200 K yield adsorbed CH₃ and I, with the rest desorbing molecularly at a peak temperature of 200 K in a TPD study. Photochemistry of CH₃I on Ag/TiO₂(110) is wavelength dependent. Irradiation of monolayer CH₃I by 404 nm photon causes C‐I bond dissociation and CH₃ desorption. Upon 290 nm, UV irradiation, the depletion of CH₃I_(a) is dominated by photodesorption of molecular CH₃I.     

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.     

Effect of Pt loading on the photocatalytic reactivity of titanium oxide thin films prepared by ion engineering techniques

Platinum-loaded titanium oxide thin-film photocatalysts were prepared by using an ionized cluster beam (ICB) deposition method and a RF magnetron sputtering (RF-MS) deposition method as dry processes. From the results of the photocatalytic oxidation of acetaldehyde with O₂ under UV light irradiation, small amounts of Pt loading (less than 10 nm film thickness) were found to dramatically enhance the photocatalytic reactivity. However, when TiO₂ thin films were loaded with relatively larger amounts of Pt (more than 30 nm as the film thickness), the photocatalytic reactivity became lower than for the pure TiO₂ thin films. Moreover, investigations of the ratio of Pt loaded onto the surface of the thin film catalysts by XPS measurements revealed that the small amounts of Pt loaded exist as very small clusters working to efficiently enhance the charge separation, whereas, large amounts of Pt covers the entire surface of the TiO₂ thin films, resulting in a decrease of the photocatalytic reactivity.