Novel mesoporous aluminosilicate supported palladium-rhodium catalysts for diesel upgrading: I. Preparation and characterisation

Bimetallic PdRh (with Pd/Rh mole ratio = 2/1) catalysts supported on a mesoporous aluminosilicate have been prepared and three methods of metal incorporation in the support have been compared: direct incorporation into the synthesis gel, impregnation and ion-exchange. Physico-chemical characterisations (nitrogen adsorption–desorption, hydrogen chemisorption, transmission electron microscopy, X-ray photoelectron and extended X-ray absorption fine structure spectroscopies), as well as simulated coking and regeneration tests are described. The dispersion, metal particle size and the formation of bimetallic particles within the material depend on the method of metal incorporation. Direct incorporation and ion-exchange methods lead to co-existence of PdRh intermetallic aggregates and segregated Pd; on the other hand, the impregnation method leads to the formation of very small and well dispersed PdRh alloy particles, with Pd preferentially located on the alloy particle surface.     

Gold catalysts supported on mesoporous titania for low-temperature water–gas shift reaction

Mesoporous titania with high surface area and uniform pore size distribution was synthesized using surfactant templating method through a neutral [C₁₃(EO)₆–Ti(OC₃H₇)₄] assembly pathway. The different gold content (1–5 wt.%) was supported on the mesoporous titania by deposition–precipitation (DP) method. The catalysts were characterized by X-ray diffraction, TEM, SEM, N₂ adsorption analysis and TPR. The catalytic activity of gold supported mesoporous titania was evaluated for the first time in water–gas shift reaction (WGSR). The influence of gold content and particle size on the catalytic performance was investigated. The catalytic activity was tested at a wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. It is clearly revealed that the mesoporous titania is of much interest as potential support for gold-based catalyst. The gold/mesoporous titania catalytic system is found to be effective catalyst for WGSR.     

Gold catalysts supported on mesoporous zirconia for low-temperature water–gas shift reaction

Mesoporous ZrO₂ with high surface area and uniform pore size distribution, synthesized by surfactant templating through a neutral [C₁₃(EO)₆–Zr(OC₃H₇)₄] assembly pathway, was used as a support of gold catalysts prepared by deposition–precipitation method. The supports and the catalysts were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, N₂ adsorption analysis, temperature programmed reduction and desorption. The catalytic activity of gold supported on mesoporous zirconia was evaluated in water–gas shift (WGS) reaction at wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. The catalytic behaviour and the reasons for а reversible deactivation of Au/mesoporous zirconia catalysts were studied. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new Au/mesoporous zirconia catalyst was compared to the reference Au/TiO₂ type A (World Gold Council), revealing significantly higher catalytic activity of Au/mesoporous zirconia catalyst. It is found that the mesoporous zirconia is a very efficient support of gold-based catalyst for the WGS reaction.     

Gold nanoparticles supported on ceria-modified mesoporous titania as highly active catalysts for low-temperature water-gas shift reaction

New gold catalytic system prepared on ceria-modified mesoporous titania (CeMTi) used as water-gas shift (WGS) reaction catalyst is reported. Mesoporous titania (MTi) was synthesized using surfactant templating method through a neutral [C₁₃(EO)₆–Ti(OC₃H₇)₄] assembly pathway. Ceria modifying additive was deposited on MTi by deposition precipitation (DP) method. Gold-based catalysts with different gold content (1–5 wt.%) were synthesized by DP of gold hydroxide on mixed metal oxide support. The supports and the catalysts were characterized by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), N₂ adsorption analysis and temperature-programmed reduction (TPR). The catalytic behavior of the gold-based catalysts was evaluated in WGS reaction in a wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new gold/ceria-modified mesoporous titania catalysts was compared with that of gold catalysts supported on simple oxides CeO₂ and mesoporous TiO₂, as well as gold/ceria-modified titania and reference catalyst Au/TiO₂ type A (World Gold Council). A high degree of synergistic interaction between ceria and mesoporous titania and a positive modification of structural and catalytic properties by ceria has been achieved. It is clearly revealed that the ceria-modified mesoporous titania is of much interest as potential support for gold-based catalyst. The Au/ceria-modified mesoporous titania catalytic system is found to be efficient catalyst for WGSR.     

Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders

Bimodal nanocrystalline mesoporous TiO₂ powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C₄H₉)₄, TBOT) as precursor. The as-prepared TiO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption–desorption measurements. The photocatalytic activity of the as-prepared TiO₂ powders was evaluated by the photocatalytic degradation of acetone (CH₃COCH₃) under UV-light irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO₂ powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO₂ powders from amorphous to anatase and crystallization of anatase. All TiO₂ powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4–8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 45–50 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 °C for 10 h) was determined. The photocatalytic activity of the prepared TiO₂ powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25.     

Role of platinum-like tungsten carbide as cocatalyst of CdS photocatalyst for hydrogen production under visible light irradiation

To investigate the role of platinum-like tungsten carbide as a cocatalyst for photocatalytic hydrogen production under visible light irradiation, we fabricated for the first time a composite material consisting of WC and CdS by a precipitation/hydrothermal method. The composite prepared by this procedure consisted of well-dispersed WC and CdS, as confirmed by TEM mapping analysis. The WC/CdS composite photocatalyst exhibited a high rate of hydrogen production, comparable to that of Pt/CdS, under visible light irradiation (λ ≥ 420 nm) from water containing sulfide and sulfite ions as hole scavengers. Like Pt cocatalyst loaded on the surface of CdS, WC provides active sites for proton reduction and causes fast diffusion of photoelectrons generated from CdS toward WC, leading to high photocatalytic activity of hydrogen production.     

Surface photochemistry: efficient photocatalysis by CdS supported on a solid surface

The photocatalytic efficiency of a series of CdS samples: (i) a colloidal solution in aqueous CH₃CN; (ii) suspensions of (a) supported colloidal particles, (b) bulk CdS, and (c) commercial powders, was compared with respect to stilbenec-t isomerization. The samples of type (ii a), prepared from cadmium chloride and H₂S showed a high photocatalytic activity, attributable to the combined effects of the catalyst particle size and the presence of chloride ions. Increased activity over the commercial sample was also shown in the conversion of hexamethyl(dewar)benzene to hexamethylbenzene, and in the cleavage of anthracene dimer. It has been found that bulk CdS is activated by grinding with < 10% of certain metal chlorides and bromides.     

Structure Sensitivity of Nano-structured CdS/SBA-15 Containing Au and Pt Co-catalysts for the Photocatalytic Splitting of Water

Photocatalysts containing nanocrystallites of cadmium sulphide dispersed in the channels of mesoporous silicate SBA-15 exhibited enhanced activity for visible-light mediated splitting of water, as compared to bulk CdS. Whereas the incorporation of 1 wt.% Pt co-catalyst in CdS/SBA-15 led to a substantial increase in H₂ yield, Au at similar loading resulted in marginally decreased activity. The results indicate that the photocatalytic activity of CdS crystallites is structure sensitive. Similarly, the role of a noble metal may be associated with the particle size dependent micro-structural features, rather than its electronic properties such as the work function or the heat of adsorption of reactant or product molecules.     

The role of carbon deposition on precious metal catalyst activity during dry reforming of biogas

Gold and palladium were supported on a mesoporous TiO₂ for total oxidation of volatile organic compounds (VOCs). Mesoporous high surface area titania support was synthesised using of Ti(OC₂H₅)₂ in the presence of CTMABr surfactant. After removing the surfactant molecules, 0.5 or 1.5 wt% of palladium and 1 wt% of gold were precipitated on the support by, respectively, wet impregnation and deposition–precipitation methods. The activity for toluene and propene total oxidation of the prereduced samples follows the same order: 0.5%Pd-1%Au/TiO₂ > 1.5%Pd/TiO₂ > 0.5%Pd/TiO₂ > 1%Au-0.5%Pd/TiO₂ > 1%Au/TiO₂ > TiO₂. Moreover, a catalytic comparison with samples based on a conventional TiO₂, shows the catalytic advantage of the mesoporous TiO₂ support. The promotional effect of gold added to palladium could be partly explained by small metallic particles (TEM), but meanly by metallic particles made up of Au-rich core with a Pd-rich shell. Moreover, the hydrogen TPR profile of 0.5%Pd-1%Au/TiO₂ shows only the signal attributed to small PdO particles. Gold also implies a protecting effect of the support under reduction atmosphere. Operando diffuse reflectance infrared fourier transform (DRIFT) spectroscopy was carried on and allowed to follow the VOCs oxidation and the formation of coke molecules, but also a metallic electrodonor effect to the adsorbed molecule which increases in the same order as the activity for oxidation reaction. The presence of coke after test was also shown by DTA–TGA by exothermic signals between 300 and 500 °C and by EPR (g = 2.003).     

Synthesis of Ti-containing SBA-15 materials and studies on their photocatalytic decomposition of orange II

Titanium substituted SBA-15 mesoporous materials have been successfully prepared by conventional hydrothermal method and they were also used as support on TiO₂ loaded SBA-15 photocatalysts. The synthesized materials were characterized by XRD, UV–vis DRS, FT-IR, BET and TEM. We also examined the activity of these materials as photocatalysts for the decomposition of orange II. The incorporation of titanium into framework of SBA-15 makes the pore diameter and pore volume to decrease and slightly decreases the surface area compared to SBA-15. In addition, the pore size distribution becomes broaden with an increase of titanium amount in the SBA-15 framework. For Ti-SBA-15 and TiO₂ loaded Ti-SBA-15 photocatalysts, the IR absorption at 960 cm⁻¹ commonly accepted the characteristic vibration of Ti–O–Si bond. From the TEM images, the regular silica morphology is maintained in the case of Ti-SBA-15(Si/Ti = 50) but the Ti-SBA-15 sample having Si/Ti ratio = 10 partially destroys the hexagonal highly ordered structure and the mesopore structure is disappeared by the clogging of mesopore channels by the titanium dioxides particles for the 50 wt.% TiO₂/Ti-SBA-15 samples. The photocatalytic activity increases with an increase of Ti content (decrease of Si/Ti ratio) and with an increase of TiO₂ loading content.     

Mesoporous and nanostructured CeO2 as supports of nano-sized gold catalysts for low-temperature water-gas shift reaction

Mesoporous particles and 1D nanorods of cerium oxides have been prepared by modifying the hydrothermal route of a surfactant-assisted controllable synthesis. Mesoporous cerias were obtained in a sealed glass vessel under continuous stirring, while ceria nanorods were obtained in a Teflon-lined autoclave without stirring. The mesoporous cerias did not show long-range mesoscopic organization, exhibiting a broad mesopore size distribution in the region 8–15 nm. A BET surface area of 100 m²/g with a total pore volume of 0.33 cm³/g is obtained for as-synthesized mesoporous ceria. The ceria nanorods exhibit a cubic crystalline structure after calcination, having the lengths in the range of 150–300 nm and diameters in the range of 10–25 nm. The growth direction of ceria nanorods is along [1 1 0]. A surface area of above 50 m²/g is obtained in the calcined nanorods. These synthesized ceria materials were used as supports of nano-sized gold catalysts, prepared by deposition–precipitation method. Their catalytic activity was evaluated by the low-temperature water-gas shift reaction. The gold/mesoporous ceria catalytic system exhibited higher catalytic activity than gold/ceria nanorods. It is revealed that the mesoporous and nanostructured cerias are of much interest as potential supports for gold-based catalysts that are effective for low-temperature water-gas shift reaction.     

Formation of Pd nanoparticles in surfactant-mesoporous silica composites and surfactant solutions

Highly dispersed palladium nanoparticles containing mesoporous silicas MCM-41 and MCM-48 were prepared by one-pot synthesis. The method consists of the simultaneous formation of CTA⁺ surfactant templating MCM-41 mesophase and CTA⁺ micelle-capped PdO, which was reduced by hydrogen to Pd metal with particle size ≈ 2 nm and was observed to stay inside the mesochannels of MCM-41 (pore size ≈ 3.8 nm) by TEM, XAS, and PXRD. During hydrothermal synthesis of Pd/MCM-48, Pd nanoparticles of average size ≈ 6–7 nm were deposited on the MCM-48 of pore size = 4 nm. The deposition is probably derived from ethanol reduction of Pd(II) complex generated from PdCl₂ precursor by hydrolysis of TEOS and C₁₂H₂₅(OCH₂CH₂)₄OH surfactant. The formation of Pd(0) from Pd(II) species in solid mesoporous silicas by hydrogen reduction was monitored by in situ XAS, and compared with the formation of Pd(0) from [PdCl₄]²⁻, [PdCl₃(H₂O)], and Pd(OH)₂ by sodium dodecyl sulfate surfactant and alcohol reduction in aqueous solutions.     

CdS/TiO2复合纳米粒子的制备及其产氢性能研究

采用化学沉淀法制备了CdS包覆TiO2（CdS/TiO2）复合纳米粒子,利用XRD、TEM、SEM、UV-vis吸收光谱等对其进行了表征分析,并以可见光分解水制氢为探针反应考察了复合纳米粒子的活性。结果表明,CdS/TiO2 复合纳米粒子的颗粒大小约为40nm,TiO2 以锐钛矿型存在,CdS以六方相存在;复合纳米粒子的吸收光谱较TiO2 发生“红移”,大幅拓宽了对可见光区的吸收范围。光解水制氢实验表明,CdS/TiO2 复合纳米粒子具有良好的可见光释氢活性和光学稳定性。     

Preparation of Au/TiO2 catalysts from Au(I)–thiosulfate complex and study of their photocatalytic activity for the degradation of methyl orange

Gold loaded on TiO₂ (Au/TiO₂) catalysts were prepared using Au(I)–thiosulfate complex (Au(S₂O₃)₂³⁻) as the gold precursor for the first time. The samples were characterized by UV–vis diffuse reflectance spectra, X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption flame emission spectroscopy (AAS), and X-ray photoelectron spectroscopy (XPS) methods. Using Au(S₂O₃)₂³⁻ as gold precursor, ultra-fine gold nanoparticles with a highly disperse state can be successfully formed on the surface of TiO₂. The diameter of Au nanoparticles increases from 1.8 to 3.0 nm with increasing the nominal Au loading from 1% to 8%. The photocatalytic activity of Au/TiO₂ catalysts was evaluated from the analysis of the photodegradation of methyl orange (MO). With the similar Au loading, the catalysts prepared with Au(S₂O₃)₂³⁻ precursor exhibit higher photocatalytic activity for methyl orange degradation when compared with the Au/TiO₂ catalysts prepared with the methods of deposition–precipitation (DP) and impregnation (IMP). The preparation method has decisive influences on the morphology, size and number of Au nanoparticles loaded on the surface of TiO₂ and further affects the photocatalytic activity of the obtained catalysts.     

Characterization and performance of Pt-USY in the SCR of NOx with hydrocarbons under lean-burn conditions

Pt-USY was used for the selective catalytic reduction of NO_x with hydrocarbons in the presence of excess oxygen. The catalyst was prepared by an ion-exchange method and characterized by XRD, TEM, CO chemisorption, and Ar adsorption at 87 K. The platinum particle size distribution was found to be broad (2–20 nm), with no apparent sintering of the active phase during the HC-SCR process after 25 h time-on-stream. Generally, large metal clusters (>15 nm) are situated at the external surface of the zeolite, while the smaller ones are located in the pores of the support. Pt-USY shows an excellent activity in the deNO_x reaction (molar NO_x conversion 90% at 475 K) with propene as the reductant in 5 kPa O₂, as well as stable operation during time-on-stream. Propane only yields a low NO_x conversion compared to propene. The presence of high oxygen contents (5–10 kPa O₂) slightly inhibits the reaction. No significant decrease in deNO_x activity was observed at high space velocities (up to 100,000 h⁻¹). The presence of SO₂ and H₂O in the feed stream did not significantly affect the deNO_x activity. Pt-USY performs better under lean-burn conditions than other Pt-catalysts supported on e.g. ZSM-5, Al₂O₃, or SiO₂. The selectivity to N₂ was similar to the other Pt-based catalysts (30%), the other major product being N₂O.     

Carbon-based monolithic supports for palladium catalysts: The role of the porosity in the gas-phase total combustion of m-xylene

Three different carbon-based monoliths have been studied in their performance as Pd catalyst supports in the total gas-phase combustion of m-xylene at low temperatures. The first monolithic support (HPM) was a classical square channel cordierite modified with -Al₂O₃, blocking the macroporosity of the cordierite and rounding the channel cross-section, on which a carbon layer was applied by carbonization of a polyfurfuryl alcohol coating obtained by dipcoating. The other two monolithic supports were composite carbon/ceramic monoliths (MeadWestvaco Corporation, USA), microporous (WA) and a mesoporous (WB) sample.

All the catalysts have a comparable total Pd loading and very similar Pd particle size (around 5–6 nm). In sample Pd/WA the Pd is situated only in the macropores, while in the case of Pd/WB the Pd is distributed throughout the mesoporous texture. In the case of Pd/HPM, Pd particles are clearly situated at the external surface of the carbon layer.

The catalytic activities of the samples were very different, decreasing in the order: Pd/WB > Pd/WA > Pd/HPM. These results show that the carbon external surface area, the macropores and mainly mesopores, play an important role in this kind of gas-phase reactions, improving the contact between the Pd particles and the m-xylene molecules. The catalytic activity of the Pd supported on carbon-based monoliths correlates with the surface area developed in macro- and mesopores of the monolithic support.     

Production of hydrogen via partial oxidation of methanol over Au/TiO2 catalysts

Selective production of hydrogen by partial oxidation of methanol (CH₃OH + (1/2)O₂ → 2H₂ + CO₂) over Au/TiO₂ catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO₂ catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au⁰), non-metallic gold (Au^δ+) and Au₂O₃, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O₂/CH₃OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming.     

Methanol partial oxidation on carbon-supported Pt and Pd catalysts

Different Pt and Pd catalysts supported on an activated carbon were prepared by using different metal precursors. Prepared catalysts were pretreated at 400 °C under different atmospheres to decompose the precursor compound and reduce the metal. After pretreatments, the supported catalysts were characterized by H₂ chemisorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy to know their metal dispersion, particle size, distribution and oxidation state. Afterwards, the catalysts were tested in methanol partial oxidation with two different O₂/CH₃OH molar ratios. Results obtained in this reaction were compared with those obtained for methanol decomposition in inert atmosphere. For Pt catalysts, there was an increase in methanol conversion and hydrogen production and a decrease in carbon monoxide production under oxidizing conditions. Both methanol conversion and partial oxidation reactions appear to be sensitive to Pt particle structure in the particle size range studied. Results obtained under oxidizing conditions differed between Pd and Pt catalysts. Finally, catalytic activity in methanol partial oxidation was more affected by Pt than Pd particle size in the size range studied.     

Production of hydrogen for fuel cells by steam reforming of ethanol over supported noble metal catalysts

The catalytic performance of supported noble metal catalysts for the steam reforming (SR) of ethanol has been investigated in the temperature range of 600–850 °C with respect to the nature of the active metallic phase (Rh, Ru, Pt, Pd), the nature of the support (Al₂O₃, MgO, TiO₂) and the metal loading (0–5 wt.%). It is found that for low-loaded catalysts, Rh is significantly more active and selective toward hydrogen formation compared to Ru, Pt and Pd, which show a similar behavior. The catalytic performance of Rh and, particularly, Ru is significantly improved with increasing metal loading, leading to higher ethanol conversions and hydrogen selectivities at given reaction temperatures. The catalytic activity and selectivity of high-loaded Ru catalysts is comparable to that of Rh and, therefore, ruthenium was further investigated as a less costly alternative. It was found that, under certain reaction conditions, the 5% Ru/Al₂O₃ catalyst is able to completely convert ethanol with selectivities toward hydrogen above 95%, the only byproduct being methane. Long-term tests conducted under severe conditions showed that the catalyst is acceptably stable and could be a good candidate for the production of hydrogen by steam reforming of ethanol for fuel cell applications.