Effect of oxychlorination treatment on the regeneration of Pt–Sn/Al2O3 catalyst for propane dehydrogenation

Research on Chemical Intermediates - Propane dehydrogenation and regeneration of Pt–Sn/θ-Al2O3 catalysts was evaluated with oxychlorination treatment. It was observed that the catalytic...     

Pt–Ni alloy nanoparticles supported on multiwalled carbon nanotubes for methanol oxidation in alkaline media

The Pt–Ni alloy nanoparticles with different Pt/Ni atomic ratios supported on functionalized multiwalled carbon nanotubes surface were synthesized via an impregnation-reduction method. The nanocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical techniques. XRD demonstrated that Pt was alloyed with Ni. TEM showed that the Pt–Ni alloy nanoparticles were uniformly dispersed on the multiwalled carbon nanotubes (MWCNTs) surface, indicating appropriate amount of Ni in Pt–Ni alloy which facilitates the dispersion of nanoparticles on the MWCNT surface. XPS revealed that the Pt 4f peak in Pt–Ni/MWCNT (4:1) catalyst shifted to a lower binding energy compared with Pt/MWCNT catalyst, and nickel oxides/hydroxides such as NiO, Ni(OH)₂, and NiOOH were on the surface of Pt–Ni nanoparticles. Electrochemical data based on cyclic voltammetry and chronoamperometric curves indicated that Pt–Ni (4:1) alloy nanoparticles exhibited distinctly higher activity and better stability than those of Pt/MWCNTs toward methanol oxidation in alkaline media.     

Simultaneous effect of carbon and water on NOx adsorption on a stabilized Pt–Ba/Al2O3 catalyst

The influence of the presence of H₂O on the contact between carbon, used as model soot, and a model four-way catalyst (1% Pt–10% BaO/Al₂O₃) was investigated. NO_x adsorption/TPD cycles at 300 °C together with XRD, XPS and DRIFTS characterizations showed that only surface nitrate species are destabilized by the carbon present in the catalytic bed, leading to a decrease of the NO_x storage capacity and carbonate species formation. In another way, injection of water in the reactive gas flow decreases also the NO_x storage capacity of the catalyst, but promotes the formation of stable nitrate species. A non-cumulative effect of carbon and water was observed. It was proposed that a competition between the destabilization, by carbon, of weakly bonded surface nitrate species and the enhancement of bulk nitrate species formation in the presence of water occurs.     

Alkylation of aniline with methanol in the presence of FeCl3 · 6H2O in carbon tetrachloride

The reaction of aniline with methanol in the presence of FeCl₃ · 6 H₂O in carbon tetrachloride leads to the formation of N-methyl- and N,N-dimethylanilines and 4,4′-methylenebis(N,N-dimethylaniline).     

Keggin-type phosphotungstic acid supported on mesoporous SiO2–Al2O3 aerogel like beads and their application in the isopropylation of naphthalene

Spherical mesoporous silica–alumina aerogel like beads based on sol–gel technology and the drop wise addition have been synthesized and used as catalyst support for phosphotungstic acid (PWA). Their catalytic performances in the isopropylation of naphthalene with isopropanol were investigated in a batch reactor. It was found that PWA was highly dispersed on the silica–alumina support and their Keggin structure can be retained. In addition, PWA/SiO₂–Al₂O₃ catalyst showed high surface area, both of Lewis acid sites and Brönsted acid sites. Because of having more Brönsted acid sites, silica–alumina supported acid catalysts showed much higher conversion (87.97 %) and selectivity to diisopropylnaphthalenes (41.41 %) and β,β-products (59.82 %) than pure acid and reactive supports in the isopropylation of naphthalene. The catalytic behavior has been discussed in relation with the physical chemical properties of catalysts, reaction and activation temperature and reaction time.     

Recent advances in the application of nanoparticles in the selective reduction of carbon–carbon double bonds

Herein, we present recent advances in the application of metal nanoparticles in the selective hydrogenation of C–C double bonds. The review focuses on reduction methods of alkenes, arenes, and aromatic heterocycles, which were classified according to transition metals used as catalysts. The majority of described systems concern direct hydrogenation, which is of particular importance to industrial processes. Nonetheless, interesting transfer hydrogenation protocols were also developed, which may be incredibly convenient for laboratory purposes. Some of the methods are distinguished with excellent chemoselectivity making them the perfect tool for the synthesis of compounds containing reducible functional groups. Apart from noble metals, the application of earth-abundant ones as catalysts was a subject of studies, and the related methods were highlighted.     

Oxidation of carbon monoxide and formic acid on bulk and nanosized Pt–Co alloys

Bulk Pt₃Co and nanosized Pt₃Co and PtCo alloys supported on high area carbon were investigated as the electrocatalysts for the CO_ads and HCOOH oxidation. Pt₃Co alloy with Co electrochemically leached from the surface (Pt skeleton) was employed to separate electronic from ensemble and bifunctional effects of Co. Cyclic voltammetry in 0.1 M HClO₄ showed reduced amount of adsorbed hydrogen on Pt sites on Pt₃Co alloy compared to pure Pt. However, no significant difference in hydrogen adsorption/desorption and Pt-oxide reduction features between Pt₃Co with Pt skeleton structure and bulk Pt was observed. The oxidation of CO_ads on Pt₃Co alloy commenced earlier than on Pt, but this effect on Pt₃Co with Pt skeleton structure was minor indicating that bifunctional mechanism is stronger than the electronic modification of Pt by Co. The HCOOH oxidation rate on Pt₃Co alloy was about seven times higher than on bulk Pt when the reaction rates were compared at 0.4 V, i.e., in the middle of the potential range for the HCOOH oxidation. Like in the case of CO_ads oxidation, Pt skeleton showed similar activity as bulk Pt indicating that the ensemble effect is responsible for the enhanced activity of Pt₃Co alloy toward HCOOH oxidation. The comparison of CO_ads and HCOOH oxidation on Pt₃Co/C and PtCo/C with the same reaction on Pt/C were qualitatively the same as on bulk materials.     

Structural evolution of Pt/ceria–zirconia TWC catalysts during the oxidation of carbon monoxide

The structural evolution of two Pt/ceria–zirconia catalysts, characterized by different amounts of supported Pt, was monitored by in situ X-ray diffraction during the anaerobic oxidation of CO at different temperatures. In a first phase, oxygen coming from the surface layers of the ceria–zirconia mixed oxide is consumed and no structural variation of the support is observed. After this induction time, bulk reduction of Pt/ceria–zirconia takes place as a step-like process, while the CO₂ production continues at a nearly constant rate. This behavior is totally different from that of the metal-free support in similar reaction conditions, that show a gradual bulk reduction. In repeated oxidation–reduction cycles, it was observed that the induction time in Pt/ceria–zirconia is a function of the thermal history, of the amount of supported Pt and of the structural evolution of the samples.     

In situ coarsening study of inverse micelle-prepared Pt nanoparticles supported on γ-Al2O3: pretreatment and environmental effects

The thermal stability of inverse micelle prepared Pt nanoparticles (NPs) supported on nanocrystalline γ-Al(2)O(3) was monitored in situ under different chemical environments (H(2), O(2), H(2)O) via extended X-ray absorption fine-structure spectroscopy (EXAFS) and ex situ via scanning transmission electron microscopy (STEM). Drastic differences in the stability of identically synthesized NP samples were observed upon exposure to two different pre-treatments. In particular, exposure to O(2) at 400 °C before high temperature annealing in H(2) (800 °C) was found to result in the stabilization of the inverse micelle prepared Pt NPs, reaching a maximum overall size after moderate coarsening of ～1 nm. Interestingly, when an analogous sample was pre-treated in H(2) at ～400 °C, a final size of ～5 nm was reached at 800 °C. The beneficial role of oxygen in the stabilization of small Pt NPs was also observed in situ during annealing treatments in O(2) at 450 °C for several hours. In particular, while NPs of 0.5 ± 0.1 nm initial average size did not display any significant sintering (0.6 ± 0.2 nm final size), an analogous thermal treatment in hydrogen leads to NP coarsening (1.2 ± 0.3 nm). The same sample pre-dosed and annealed in an atmosphere containing water only displayed moderate sintering (0.8 ± 0.3 nm). Our data suggest that PtO(x) species, possibly modifying the NP/support interface, play a role in the stabilization of small Pt NPs. Our study reveals the enhanced thermal stability of inverse micelle prepared Pt NPs and the importance of the sample pre-treatment and annealing environment in the minimization of undesired sintering processes affecting the catalytic performance of nanosized particles.     

Formation of Isolated Single-Atom Pd1 Sites on the Surface of Pd–Ag/Al2O3 Bimetallic Catalysts

Kinetics and Catalysis - The structure of Pd–Ag/Al2O3 samples with different Ag/Pd ratios has been studied by a complex of physicochemical methods (H2-TPR, XRD, H2-TPD, TEM). Catalysts have...     

Influence of SrO content on microstructure and crystallization of glazes in the SiO2–Al2O3–CaO–MgO–K2O system

The effect of the SrO addition on the microstructure and structure of the glazes from the SiO₂–Al₂O₃–CaO–MgO–K₂O system was investigated in this study. The results were obtained by testing the ability of the frits crystallization, the stability of the crystallizing phases during the single-step fast-firing cycle depending on their chemical composition and the effect of addition of strontium oxide. Differential scanning calorimetry (DSC) curves showed that all glazes crystallized, and diopside and anorthite were mainly identified as dominant phases in the obtained glazes, while the size and amount of each depended on the amount of SrO introduced. The thermal characteristic of the frits was carried out using DSC, and crystalline phases were determined by X-ray diffractometry. The glaze microstructure was investigated by scanning electron microscopy and transmission electron microscopy. Additional information on the microstructure of frits was derived from spectroscopic studies in the mid-infrared range.

     

Surface organometallic chemistry: Reductive carbonylation of OsCl3·3H2O supported on silica

The reductive carbonylation of silica-supported OsCl₃·3H₂O was investigated under atmospheric pressure of CO or of a mixture of CO and H₂O at relatively mild temperatures. Starting from OsCl₃·3H₂O a mixture of physisorbed α-[Os(CO)₃Cl₂]₂, cis-[Os(CO)₄Cl₂] and a species bound to the surface silanol groups, [Os(CO)₃Cl₂(HOSi)], is formed working at 100°C under CO. At higher temperatures [Os(CO)₃Cl₂(HOSi)] is the major surface species. Attempts to reduce further on the surface these Os^II chlorocarbonyl species failed due to their easy sublimation and to the difficult removal of the chloro ligands. However, when the silica is treated with a weak base such as NaHCO₃, α- or β-[Os(CO)₃Cl₂]₂ supported on silica may be reduced with CO to [Os₃(CO)₁₂], with CO and H₂O to a mixture of [OS₃(CO)₁₂] and [H₄Os₄(CO)₁₂], and finally with H₂ to [H₄Os₄(CO)₁₂]. These reduction processes occur via the anchored [Os(CO)₃(OSi)₂]_n species, as intermediate surface species.     

The effect of mechanical activation on the thermal reactions of P2O5–Al2O3–Fe2O3–Na2O phosphate glasses

The effect of mechanical activation on the structure and thermal reactions of glasses has been studied on the example of Na–Al–Fe phosphate glasses. These glasses are used in nuclear technology for immobilization of radioactive waste. The glasses were activated by grinding in a planetary mill. Mechanical activation causes a decrease of the T _g temperature as well as of the glass crystallization temperature. The type of crystalline phases formed and the quantitative proportions between them are changing. Analysis of inter-atomic interactions in the structure of glass was applied to explain the observed regularities governing the crystallization of the activated glasses.     

Platinum nanoparticles supported on zirconia–carbon black nanocomposites for methanol oxidation reaction

Platinum (Pt) nanoparticles supported on zirconia–carbon black nanocomposites (Zr–C), which annealed at different temperatures, used as Pt/Zr–C electrocatalysts for methanol oxidation reaction (MOR) are prepared and characterized in this study. Transmission electron microscope images and X-ray diffraction analysis showed that the diameters of Pt nanoparticles are around 3–4 nm. Electrocatalytic MOR performances of these Pt/Zr–C electrocatalysts are investigated by cyclic voltammetry, CO-stripping voltammetry, and chronoamperometry. All the Pt/Zr–C electrocatalysts synthesized in this study exhibited higher MOR efficiency than that of the commercial E-TEK Pt/C electrocatalyst, and the electrocatalyst using Zr–C support annealed at 300 °C, achieving the highest MOR efficiency among all the electrocatalysts.     

Long-Chain Alkane Dehydrogenation over Hierarchically Porous Ti-Doped Pt–Sn–K/TiO2–Al2O3 Catalysts

Kinetics and Catalysis - Hierarchically porous γ-Al2O3, TiO2–Al2O3 composite supports, and Pt–Sn–K/Al2O3 and Pt–Sn–K/TiO2–Al2O3 catalysts were prepared...     

Co@Pt–Ru core-shell nanoparticles supported on multiwalled carbon nanotube for methanol oxidation

A novel synthesis route concerning reduction of cobalt core onto the surface of multiwalled carbon nanotubes (MWCNTs) and then substitution of part of Co core with Pt–Ru precursor was developed to synthesize the core-shell Co@Pt–Ru/MWCNTs catalyst. In this synthesis route, sodium borohydride and hydrazine hydrate were employed to reduce cobalt step by step in order to control the size of cobalt and the growth speed of cobalt crystal. The novel core-shell Co@Pt–Ru/MWCNTs catalyst shows good electrocatalysis towards methanol oxidation.     

IR spectroscopic investigation of structural hydroxyl groups in W−Si heteropolycompound supported on Al2O3

Structural hydroxyl and deuteroxyl groups within the K₄[SiW₁₂O₄₀]/Al₂O₃ and K₆[SiW₁₁PdO₃₉]/Al₂O₃ systems were studied by diffuse-reflectance FTIR spectroscopy in a spectral range of fundamental stretching vibrations, first overtones, and combination bands of stretching and bending vibrations. For hydroxyl groups, the region of combination vibrations is the most informative. The calculated frequencies of bending vibrations of hydroxyl groups (865 and 730 cm⁻¹) are characteristic of acidic OH groups. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2017–2020, October, 1999.     

Effects of nano-YAG (Y3Al5O12) crystallization on the structure and photoluminescence properties of Nd3+-doped K2O–SiO2–Y2O3–Al2O3 glasses

Nd³⁺-doped precursor glass in the K₂O–SiO₂–Y₂O₃–Al₂O₃ (KSYA) system was prepared by the melt-quench technique. The transparent Y₃Al₅O₁₂ (YAG) glass–ceramics were derived from this glass by a controlled crystallization process at 750 °C for 5–100 h. The formation of YAG crystal phase, size and morphology with progress of heat-treatment was examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Fourier transformed infrared reflectance spectroscopy (FT-IRRS). The crystallite sizes obtained from XRD are found to increase with heat-treatment time and vary in the range 25–40 nm. The measured photoluminescence spectra have exhibited emission transitions of ⁴F_3/2 → ⁴I_J (J = 9/2, 11/2 and 13/2) from Nd³⁺ ions upon excitation at 829 nm. It is observed that the photoluminescence intensity and excited state lifetime of Nd³⁺ ions decrease with increase in heat-treatment time. The present study indicates that the incorporation of Nd³⁺ ions into YAG crystal lattice enhance the fluorescence performance of the glass–ceramic nanocomposites.