Enhanced charge separation of CuS and CdS quantum-dot-cosensitized porous TiO2-based photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using high-performance catalysts shows considerable promise in generating environment-friendly hydrogen energy. Its practical applications, however, suffer from several shortcomings, such as low photocurrent density, large onset-voltage value, and poor durability. In this study, CuS and CdS quantum-dot-cosensitized porous TiO₂-based PEC catalysts (CuS-CT) have been successfully synthesized via in situ sulfuration of CuO and CdO coexisting inside a porous TiO₂ monolith by a hydrothermal method. Compared to porous TiO₂, CuS-sensitized porous TiO₂ (CuS-TiO₂), and CdS-sensitized porous TiO₂ (CdS-TiO₂) in terms of PEC performance, the CuS-CT photoanode exhibited a significantly high anodic photocurrent for water splitting under simulated sunlight radiation. The photocurrent produced by the optimized sample of 7% CuS-5% CdS-TiO₂ (7% CuS-CT) was nearly 2.7 times higher than that of pure porous TiO₂ at 1.0 V versus a reversible hydrogen electrode (RHE). Porous TiO₂ possesses large surface areas that can drive fast electrolyte transport and afford more surface reaction active sites. On the other hand, CuS and CdS quantum dots not only broaden the visible light absorption range, but also improve photoinduced electron-hole separation efficiency. The co-sensitized multi-nanostructures photoanodes lead to a remarkable and promising application in PEC water splitting reactions.     

Acetylene Hydrogenation on Au-Based Catalysts

Hydrogenation of acetylene has been investigated on Au/TiO₂, Pd/TiO₂ and Au-Pd/TiO₂ catalysts at high acetylene conversion levels. The Au/TiO₂ catalyst (avg. particle size: 4.6 nm) synthesized by the temperature-programmed reduction-oxidation of an Au-phosphine complex on TiO₂ showed a remarkably high selectivity to ethylene formation even at 100% acetylene conversion. Au/TiO₂ prepared by the conventional incipient wet impregnation method (avg. particle size: 30 nm), on the other hand, showed negligible activity for acetylene hydrogenation. Although the Au catalysts showed a high selectivity for ethylene, the acetylene conversion activity and catalyst stability were inferior to the Pd-based catalysts. Au-Pd catalysts prepared by the redox method showed high acetylene conversions as well as high selectivity for ethylene. Interestingly Au-Pd catalysts prepared by depositing Pd via the incipient wetness method on Au/TiO₂ showed very poor selectivity (comparable to mono-metallic Pd catalysts) for ethylene. High-resolution transmission electron microscopy (TEM) studies coupled with energy dispersive X-ray spectroscopy (EDS) showed that while the redox method produced bimetallic Au-Pd catalysts, the latter method produced individual Pd and Au particles on the support.     

Enhancement of catalytic activity of Au/TiO2 by thermal and plasma treatment

A significant enhancement in the catalytic activity of Au/TiO₂ in CO oxidation and preferential oxidation reaction by creating the active sites on the catalyst surface by thermal treatment as well as by producing small gold particles by plasma treatment has been studied. Au/TiO₂ catalyst (Au (1 wt%) supported on TiO₂) was prepared by conventional deposition-precipitation method with NaOH (DP NaOH) followed by washing, drying and calcination in air at 400 °C for 4 h. Thermal treatment of Au/TiO₂ was carried out at 550 °C under 0.05 mTorr. A small amount of Au/TiO₂ catalyst was taken from the untreated and thermally treated Au/TiO₂ and both kinds of catalysts were treated with plasma sputtering at room temperature. The activity of the catalysts has been examined in the reaction of CO oxidation and preferential oxidation (PROX) at 25–250 °C. Thermally treated Au/TiO₂ showed better catalytic activity as compared to the untreated catalyst. There is also an additional enhancement in the catalytic activity due to plasma sputtering on the both kinds of catalysts. Thermally treated Au/TiO₂ followed by plasma sputtering Au/TiO₂ showed higher conversion rates for CO oxidation reaction compared with untreated, thermally treated and plasma sputtered Au/TiO₂ catalysts. It may be concluded that the enhancement of catalytic activity of thermally treated Au/TiO₂ followed by plasma sputtering is owing to the generation of active sites such as oxygen vacancies/defects in TiO₂ support using thermal treatment as well as by producing small gold particles using plasma treatment.     

Biosynthesized Au/TiO<Subscript>2</Subscript>@SBA-15 catalysts for selective oxidation of cyclohexane with O<Subscript>2</Subscript>

A variety of TiO₂@SBA-15 supporters with various TiO₂ loadings were synthesized using a facile sol-gel method. Gold (Au)-based catalysts were prepared with an environmentally benign and economical bioreduction method via Cacumen Platycladi (CP) leaf extract and immobilized on various TiO₂@SBA-15 supporters with different TiO₂ loadings. The as-prepared biosynthesized Au catalysts were applied in the liquid-phase cyclohexane oxidation. The results showed that the Au nanoparticles were well-dispersed on TiO₂@SBA-15, and the Au existed as Au⁰. These biosynthesized Au catalysts are promising for cyclohexane oxidation, achieving a turnover frequency up to 3,426 h^?1 with a 14.1% cyclohexane conversion rate.     

Aerobic oxidation of alcohols over Au/TiO2: An insight on the promotion effect of water on the catalytic activity of Au/TiO2

The unique and significant promotion effect of water has been evidenced by the selective oxidation of benzyl alcohol to benzaldehyde over Au/TiO₂ catalysts. Water has dual promotional functions in the reaction system: to help form unique microdroplets in a multiphase reaction system and to assist the oxygen adsorption and activation. The conversion of benzyl alcohol at a molar ratio of water to solvent (p-xylene) of 7 is 7 times higher than in the absence of water. The present work has highlighted the potential of Au/TiO₂ catalysts in aerobic oxidation of alcohols in the unique multiphase reaction system with water as promoting solvent.     

Reprint of: Oxidative dehydrogenation of cyclohexane and cyclohexene over supported gold, –palladium catalysts

Supported gold, palladium and gold–palladium catalysts have been used to oxidatively dehydrogenate cyclohexane and cyclohexenes to their aromatic counterpart. The supported metal nanoparticles decreased the activation temperature of the dehydrogenation reaction. We found that the order of reactivity was Pd ≥ Au–Pd > Au supported on TiO₂. Attempts were made to lower the reaction temperature whilst retaining high selectivity. The space-time yield of benzene from cyclohexane at 473 K was determined to be 53.7 mol/kg_cat/h rising to 87.3 mol/kg_cat/h at 673 K for the Pd catalyst. Increasing the temperature in this case improved conversion at a detriment to the benzene selectivity. Oxidative dehydrogenation of cyclohexene over AuPd/TiO₂ or Pd/TiO₂ catalysts was found to be very effective (conversion >99% at 423 K). These results indicate that the first step in the reaction sequence of cyclohexane to cyclohexene is the slowest step. These initial results suggest that in a fixed-bed reactor the oxidative dehydrogenation in the presence of oxygen, palladium and gold–palladium catalysts are readily able to surpass current literature examples and with further modification should yield even higher performance.     

Ultrafine Cu nanoparticles decorated porous TiO2 for high-efficient electrocatalytic reduction of NO to synthesize NH3

Low-temperature electrocatalytic reduction of nitric oxide (NORR) is promising for the low-cost, high-efficiency removal and utilization of nitric oxide (NO), which couples the two crucial industrial processes of denitration and synthetic ammonia. We reported a binary catalyst of Cu/P–TiO₂ for NORR process, which took advantages of the Cu electrocatalysts and porous TiO₂. The increase of surface areas and channels can improve the adsorption of NO molecules. The decoration of the ultrafine Cu nanoparticles on the porous TiO₂ may prevent the agglomeration of ultrafine Cu nanoparticles. The as-prepared Cu/P–TiO₂ electrode exhibits excellent electrocatalytic activity with a Faraday efficiency of 86.49% and NH₃ yield of 3520.80 μg·h^?1·mg^?1 in 0.1 M K₂SO₄ at ?0.3 V vs. RHE. The mechanism of the NORR reaction process with Cu/P–TiO2 catalysts was also discussed. This research may broaden the application areas of TiO₂-based materials in the electrocatalysis and provide a new sight in the treatment of NO pollutants.     

Catalytic combustion of benzene over supported metal oxides catalysts

Catalytic combustion of benzene over supported metal oxides has been investigated. The catalysts have been prepared by incipient wetness method and characterized by XRD, FT-Raman, ESR and TPR. Among supported metal oxides, CuO_x, supported on TiO₂ is found to have the highest activity for benzene oxidation. In addition, among the catalysts of copper oxide supported on TiO₂, A1₂O₃ and SiO₂, titania-supported catalyst (CuO_x/TiO₂) gives the highest catalytic activity. CuO_x/TiO₂ (Cu loading 5.5 wt%) shows the total oxidation of benzene at about 250 °C. From the ESR and FT-Raman results, the CuO dispersed on the TiO₂ surface acts as an active site of CuO_x/TiO₂ catalysts on the oxidative decomposition of benzene. The catalytic activity gradually increases with an increase of Cu loading on TiO₂. When Cu loading reaches 5.5 wt%, the total conversion temperature is lowered to 300 °C. However, the catalytic activity considerably decreases at 7 wt% Cu loading. The catalytic activity increased with an increase of oxygen concentration but the concentration of benzene showed no difference in the benzene conversion. This result suggests that the rate determining step is the adsorption of oxygen.     

Partial oxidation of methane to synthesis gas via the direct reaction scheme over Ru/TiO2 catalyst

The partial oxidation of methane to synthesis gas has been investigated over various supported metal catalysts. The effects of operational variables on mass and heat transport resistances were investigated for defining the kinetic regime. It is observed that, in the absence of significant mass and heat transfer resistances, high selectivity (up to 65%) to synthesis gas is obtained over Ru/TiO₂ catalysts in the low methane conversion range ( ) whereas only negligibly small selectivity to synthesis gas is observed over all other catalysts investigated under similar conditions. This indicates that the Ru/TiO₂ catalyst possesses unique properties, offering high selectivity to synthesis gas formation via the direct reaction scheme, whereas the other catalysts promote the sequence of total oxidation of methane to CO₂ and H₂O, followed by reforming reactions to synthesis gas. An increase of selectivity to synthesis gas, in the presence of oxygen, is achieved over the Ru/TiO₂ catalyst by multi-feeding oxygen, which is attributed to suppression of deep oxidation of H₂ and CO.     

Effect of Nickel as Dopant in Mn/TiO<Subscript>2</Subscript> Catalysts for the Low-Temperature Selective Reduction of NO with NH<Subscript>3</Subscript>

Nickel doped manganese oxide supported on titania materials were investigated for the low-temperature NH₃-SCR. For this purpose, a series of Ni modified Mn/TiO₂ catalysts were prepared and evaluated for the low-temperature SCR of NO with ammonia in the presence of excess oxygen. The catalytic performance of these materials was compared with respect to the nickel weight percentage in order to examine the correlation between physicochemical characteristics and reactivity of optimized materials. It was found that the 5% Mn–2% Ni/TiO₂ catalyst showed the highest activity and yielded 100% NO conversion at 200 °C. XRD results reveal highly dispersed manganese–nickel species on TiO₂ support for the Mn–Ni/TiO₂ catalysts. Our TPR data results suggested an increase in reducibility of manganese species in Mn–Ni/TiO₂ catalysts. The absence of the high-temperature (736 K) peak indicates that the dominant phase is MnO₂. This increase of reducibility and dominant MnO₂ phase seems to be the reason for the enhanced activity and time on stream patterns of nickel-promoted titania-supported manganese catalysts. BET results illustrate that the high NO conversion is strongly dependant on the specific surface area and pore volume of this catalyst. All the physicochemical techniques we used suggested that the composition of manganese and nickel oxides on the support surface is playing an important role for the enhancement of NO conversion and the prominent time on stream stability.     

Selective catalytic Knoevenagel condensation by Ni–SiO2 supported heterogeneous catalysts: An environmentally benign approach

Selective catalytic Knoevenagel condensation is achieved with catalytic amounts of Ni impregnated on SiO₂ or TiO₂ supports. In this liquid phase reaction, quantitative yields were obtained under mild reaction conditions. Ni–SiO₂, Ni–TiO₂ catalysts showed efficiency of 100% conversion and 100% selectivity. With smaller quantities of 10% Ni–SiO₂ took less duration for the completion of reaction than TiO₂ catalysts in this environmentally benign process.     

Selective Ring-Opening of Methylcyclopentane Over Titania-Supported Monometallic (Pt, Ir) and Bimetallic (Pt–Ir) Catalysts

An investigation of the selective ring-opening of methylcyclopentane (MCP) was conducted for the first time on Pt/TiO₂, Ir/TiO₂ and Pt?CIr/TiO₂ catalysts with low amounts of noble metals (0.5?wt%) over a temperature range of 180?C400?°C under hydrogen at atmospheric pressure. The catalysts were prepared by impregnation or co-impregnation and characterized by different physico-chemical techniques, including SEM, XRD, H₂-TPR, N₂-sorption, TEM and elemental analysis. The metallic particles were highly dispersed on the TiO₂ support at isodispersion of ~1?nm. The particles exhibited icosahedral Mackay structures limited by (111) planes. The catalytic results show that the activity in the MCP was strongly influenced by the intrinsic nature of the metal and by the temperature. The most active catalyst was Ir/TiO₂. The order of the reactivity as a function of the temperature and total conversion was Ir/TiO₂ 180?°C (???=?2?%)?>?Pt?CIr/TiO₂ 220?°C (???=?27.8?%)?>?Pt/TiO₂ 260?°C (???=?9.9?%). Under these conditions, all of the catalysts exhibited the ability to open the ring of MCP with an atom economy, without unwanted products of cracking and ring-enlargement reactions. The synergy between Pt?CIr bimetallic particles was assessed by the total conversion of MCP, whereas the ring-opening results indicated that the reaction took place on Ir sites. These results suggest that the bimetallic catalyst contained separate entities of two metals. Increased reaction temperatures led to reduced reaction selectivity with respect to ring-opening of MCP versus the cracking side reaction.     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

CO Oxidation Activity of Ag/TiO2 Catalysts Prepared via Oxalate Co-precipitation

Ag/TiO₂ catalysts with different Ag loadings (2, 4, 7 and 10% (w/w)) have been prepared by means of co-precipitation of Ag- and TiO-oxalates followed by temperature programmed oxidation (TPO). The catalysts were subjected to CO oxidation in a flow reactor at atmospheric pressure and temperatures up to 573 K. Best conversion performance was obtained in a CO/O₂ = 1:1 mixture over 10% Ag/TiO₂ for which the temperature of 50% CO conversion was T ₅₀ = 333 K. The initial reaction rates were determined in a circulation reactor at low conversions and apparent activation energies between 13 and 25 kJ/mol were found for all catalysts. Transmission electron microscopy shows a broad range of nano-sized Ag particles on TiO₂ (nearly pure anatase).     

Effect of surface hydroxyl groups of pure TiO2 and modified TiO2 on the photocatalytic oxidation of aqueous cyanide

It is still debatable whether the photocatalytic oxidation of cyanide proceeds via hydroxyl radicals or by photogenerated holes. We synthesized pure TiO₂ catalysts via sol-gel process. In order to elucidate the oxidation pathway of cyanide, we used hydroxyl radical scavengers and controlled the concentration of surface hydroxyl group on the catalysts adopting fluoride-exchange. The degree of fluoride-exchange of TiO₂ catalysts was independent of the pH of suspension. We also adopted a polyoxometalate, tungstophosphoric acid (TPA, H₃PW₁₂O₄₀) which is well known for high charge transfer ability and hydrolytic stability. TPA-modified TiO₂ catalysts were prepared with sol-gel technique to overcome the high solubility of TPA in water. As another attempt for the insoluble TPA, proton of TPA supported on TiO₂ catalysts was replaced by cesium ion to form Cs-TPA/TiO₂ catalysts. Both attempts were successful in immobilizing TPA on TiO₂ catalysts. Commercially available TiO₂ catalysts such as P25 from Degussa AG were also used as catalysts. XRD analysis revealed that pure TiO₂ and TPA-modified TiO₂ catalysts prepared by sol-gel process were composed of well-developed anatase crystalline structure. In the presence of hydroxyl radical scavengers, the photoactivity of TPA-modified TiO₂ catalysts was retarded much less than that of pure TiO₂ catalysts. The concentration of surface hydroxyl group was effectively suppressed by the fluoride-exchange causing the decrease of the activity of the catalysts. In the case of fluoride-exchanged catalysts, the drop in activity was obvious for the pure TiO₂ catalysts in the presence of iodide as a hydroxyl radical scavenger suggesting that indirect oxidation via hydroxyl radicals was the preferential reaction pathway. For the TPA-modified TiO₂ catalysts, meanwhile, the diminution was such a small extent suggesting that direct oxidation by photogenerated holes was the main reaction pathway. The activity arising from TPA in the catalysts was due to the Keggin structured anion (PW₁₂O ₄₀ ^3- ) which acted as an electron relay with the aid of dissolved oxygen in the reaction system. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts

A series of vanadia doped TiO₂-pillared clay (TiO₂-PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V₂O₅/TiO₂-PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V₂O₅/TiO₂ catalyst and was the same as the commercial V₂O₅ + WO₃/TiO₂ catalyst. The V₂O₅/TiO₂-PILC catalysts also had higher N₂/N₂O product selectivities as compared to V₂O₅ doped TiO₂ catalysts. In addition, H₂O + SO₂ slightly increased the activities at high temperatures (>350°C) for the V₂O₅/TiO₂-PILC catalysts. Addition of WO₃ to V₂O₅ further increased the activities of the PILC catalysts. These results indicate that TiO₂-PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Brønsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Brønsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH₃ on the V₂O₅/TiO₂-PILC is similar to that on V₂O₅/TiO₂ catalyst, i.e., N₂ originates from the reaction between gaseous NO and NH₃ adspecies.     

WO3 and MoO3 addition effect on V2O5/TiO2 as promoters for removal of NOx and SOx from stationary sources

As an attempt to improve the catalytic activity at higher reaction temperatures between 300-450°C, various mole ratios of WO₃ were added to V₂O₅/TiO₂ catalytic systems. And also, in order to suggest a new mixed oxide catalyst system for simultaneous removal of NO_x and SO_x, from stationary sources, MoO₃-V₂O₅/TiO₂catalysts were prepared by a conventional impregnation method together with a newly introduced method of surface fixation (non-aqueous solution method). In case of WO₃ addition, at higher reaction temperature range (300–450°C), WO₃ and WO₃-V₂O₅/TiO₂ catalysts showed significant high conversion in NO reduction with NH₃ while V₂O₅/TiO₂ catalyst showed a significant change in selectivity mainly due to the excess side reaction of NH₃ oxidation. This difference in selectivity due to NH₃ oxidation at high temperature is supposed to be associated with the difference in values of surface excess oxygen between WO₃ and V₂O₅ on titania. The surface acidities of tested catalysts were relatively well correlated with the % conversion of NO at 400°C. In case of MoO₃ addition, the catalytic activity for the simultaneous removal of NO_x and SO_x were quite enhanced by the addition of MoO₃ into V₂O₅/TiO₂ catalysts. The enhanced activities were responsible for the formation of Mo=O bond on the intermediate species produced by solid solutions on MoO₃-V₂O₅/TiO₂ (aqueous). However, in the case of MoO₃-V₂O₅/TiO₂ (non-aqueous), the exact source of active site was not able to detect in IR spectra in spite of more enhanced activity was obtained in this study. After SO₂ contact, VOSO₄ is newly formed on the surface of catalyst, which supposed to be associated with the activity enhancement.     

Decomposition of chlorofluorocarbons on TiO2ZrO2

The decomposition of chlorofluorocarbons (CFCs) in the presence of water was examined over a variety of solid acid catalysts. The TiO₂ZrO₂ catalyst was found to have the highest activity and longest life among the catalysts examined. The activity of the TiO₂ZrO₂ catalysts depended upon the content of TiO₂. At the contents of TiO₂ from 58 to 90 mole%, the TiO₂ZrO₂ catalysts exhibited high activity, and these catalysts were proven to contain TiZrO₄ crystal. From the study of the XRD peak intensity of the TiZrO₄ crystal, it was highest on the TZ-58 which contained 58 mole% of TiO₂, and decreased with increasing the content of TiO₂. Furthermore, the conversion of CFC113 measured at 673 K was highest at TZ-58, and decreased gradually with increasing TiO₂ content. Therefore, the TiZrO₄ crystal influences the activity of decomposition of CFC113. However, the TiO₂ZrO₂ catalyst was gradually deactivated during the reaction due to the elimination of titanium atoms. A good relationship was found between the activity on TiO₂ZrO₂ catalyst and bond energy of CCl in the compounds of chlorofluorocarbons and hydrochlorocarbons, suggesting that the rate controlling step was the cleavage of CCl bond.     

Direct epoxidation of propylene to propylene oxide on various catalytic systems: A combinatorial micro-reactor study

A combinatorial approach is used to investigate several bimetallic catalytic systems and the promoter effect on these catalysts to develop highly active and selective catalysts for direct epoxidation of propylene to propylene oxide (PO) using molecular oxygen. 2%Cu/5%Ru/c-SiO₂ catalyst yielded the highest performance with high propylene conversion and PO selectivity among the bimetallic catalytic systems including silver, ruthenium, manganese and copper metals. On the other hand, the most effective catalyst and promoter in the epoxidation reaction was determined to be sodium chloride promoted Cu–Ru catalyst supported over SiO₂ with 36% selectivity & 9.6% conversion (3.46% yield) at 300 °C and 0.5 feed gas ratio (propylene/oxygen).     

TiO<Subscript>2</Subscript> porous ceramic/Ag–AgCl composite for enhanced photocatalytic degradation of dyes under visible light irradiation

TiO₂ porous ceramic/Ag–AgCl composite was prepared by incorporating AgCl nanoparticles within the bulk of TiO₂ porous ceramic followed by reducing Ag⁺ in the AgCl particles to Ag⁰ species under visible light irradiation. The porous TiO₂ ceramic was physically robust and chemically durable, and the porous structure facilitated the implantation of AgCl NPs. Compared with the bare TiO₂ ceramic, TiO₂ porous ceramic/Ag–AgCl composite exhibited higher photocatalytic performance for the degradation of MO and RhB under visible light irradiation. The reaction rate constants k of MO and RhB degradation over TiO₂ porous ceramic/Ag–AgCl composite was respectively 6.25 times and 3.62 times higher than those recorded over the bare TiO₂ porous ceramic. The photocatalytic activity showed virtually no decline after four times cyclic experiments under visible light irradiation. Scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, UV–Vis diffuse reflectance spectroscopy, photoluminescence spectra and X-ray photoelectron spectroscopy were used to characterize the TiO₂ porous ceramic/Ag–AgCl composite.