Anodic reactions of Ni3S2, β-NiS and nickel matte

The anodic reactions of Ni₃S₂,-NiS and a commercial nickel matte have been investigated galvanostatically. It is shown that the matte as well as both compounds decompose with loss of nickel ions through the series of phases: Ni_1.5S₂ Ni_1.2S₂ NiS₂ Ni²⁺ + 2S⁰     

Reduction of CuO in H2: In Situ Time-Resolved XRD Studies

CuO is used as a catalyst or catalyst precursor in many chemical reactions that involve hydrogen as a reactant or product. A systematic study of the reaction of H₂ with pure powders and films of CuO was carried out using in situ time-resolved X-ray diffraction (XRD) and surface science techniques. Oxide reduction was observed at atmospheric H₂ pressures and elevated temperatures (150-300 °C), but only after an induction period. High temperature or H₂ pressure and a large concentration of defects in the oxide substrate lead to a decrease in the magnitude of the induction time. Under normal process conditions, in situ time-resolved XRD shows that Cu¹⁺ is not a stable intermediate in the reduction of CuO. Instead of a sequential reduction (CuO Cu₄O₃ Cu₂O Cu), a direct CuO Cu transformation occurs. To facilitate the generation of Cu¹⁺ in a catalytic process one can limit the supply of H₂ or mix this molecule with molecules that can act as oxidant agents (O₂, H₂O). The behavior of CuO-based catalysts in the synthesis of methanol and methanol steam reforming is discussed in the light of these results.     

Isotopic assessment of CO<Subscript>2</Subscript> production through soil organic matter decomposition in the tropics

Values of ¹³C and ¹⁵N of soil organic matter (SOM) under different land cover in Pasir Mayang, Jambi Province, Sumatra Island, Indonesia were examined to apply them as indicators of SOM dynamics and related CO₂ production. The ¹³C and ¹⁵N values of SOM increased with depth in the 0–30 cm layer in the preserved forest, reflecting ¹³C and ¹⁵N richment in SOM through mineralization and immobilization. The degree of vertical enrichment, difference between 0–5 cm and 10–15 cm SOM, was more pronounced in ¹⁵N than in ¹³C at all sites in Pasir Mayang. The ¹³C -SOM profiles fluctuated through clear-cutting the forest and subsequent burning, which was due to input of biomass with higher C/N molar ratio and lower ¹³C value than the original SOM. However, the ¹⁵N-SOM profiles before and after such a drastic event did not change appreciably. The ¹⁵N-SOM became higher as the C/N ratio decreased and as soil sugar content decreased. These observations suggest that ¹⁵N-SOM is a variable that changes with the amount of easily decomposable organic matter (EDOM) in soil. Soil incubation experiments demonstrated a correlation between CO₂ production rate and degree of vertical ¹⁵N-enrichment in SOM, which was applied to field data to estimate CO₂ production through SOM decomposition. A similar analysis was performed with the soils collected at 27 locations in other districts in Jambi Province than Pasir Mayang. In five locations covered by oil palm plantation, CO₂ production through SOM decomposition controlled 70 of variation in CO₂ emission among the locations. In the remaining 22 locations, however, the CO₂ emission was neither related to CO₂ production from SOM nor to ground litter amount. This observation indicated that mechanisms other than dead organic matter decomposition such as root respiration were dominant sources for CO₂ emission in these sites.     

Heterogeneous asymmetric reactions. 23. Enantioselective hydrogenation of ethyl pyruvate over cinchonine- and α-isocinchonine-modified platinum catalysts

The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and -isocinchonine-modified Pt/Al₂O₃ catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmoldm^–3, 1 bar H₂, 23°C, 6% AcOH in toluene), and 76% ee in the case of -isocinchonine (0.14 mmoldm^–3, 1 bar H₂, –10°C, 6% AcOH in toluene). Since -isocinchonine of rigid structure exists only in anti-open conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of anti-open conformation in these cinchona-modified enantioselective hydrogenations.     

Reduction of Zr (IV) in (KCl/NaCl) eutectic (50/50 mol%) containing KF/ZrCl4 in molar ratios of 6/1 or 4/1 at 750° C. Characterization of the dissolved species by IR spectroscopy

Infrared analysis of baths which are mixtures of (KCl/NaCl) (50/50) with KF and ZrCl₄ in the molar ratio 4 or 6, indicates that the species present at 750° C are ZrF₄ and K₃ZrF₇. Voltammetric study of these melts containing 1 wt % Zr shows that ZrF₄ reduces in three steps: ZrF₄ ZrF₂, ZrF₄ Zr, ZrF₂ Zr, whilst K₃ZrF₇ is reduced via: K₃ZrF₇ Zr or K₃ZrF₇ ZrF₂, ZrF₂ Zr. It appears that ZrF₄ vanishes completely by increasing the KF/ZrCl₄ ratio until the KF concentration is able to stabilize K₃ZrF₇ in the melt.     

Preparation and Characterization of Copper Catalysts Supported on Mesoporous Al2O3 Nanofibers for N2O Reduction to N2

The gas-phase hydrogenation of benzene to cyclohexane over Ce_{1 - x} Pt _x O_{2 -} (x = 0.01, 0.02) catalyst was investigated in the temperature range 80-200 °C. A 42% conversion of benzene to cyclohexane with 100% specificity was observed at 100 °C over Ce_0.98Pt_0.02O_{2 -} with a catalyst residence time of 1.22 × 10⁴ g s/mol of benzene. The activity of the catalyst was compared with those of Pt metal, combustion-synthesized Pt/-Al₂O₃ and Pt/-Al₂O₃. The turnover frequency value of Ce_0.98Pt_0.02O_{2 -} is 0.292, which is an order of magnitude higher than those of the other Pt catalysts investigated. The kinetics of reaction and the deactivation behavior of the catalyst were studied and a regeneration methodology was suggested. The deactivation kinetics and structural evidence from XRD, XPS, TGA and H₂ uptake studies suggest that the oxidized Pt in Ce_0.98Pt_0.02O_{2 -} is responsible for the high catalytic activity towards benzene hydrogenation.     

Phase transitions and dimensional and morphological changes in an “Aluminum Hydroxide-Alumina” system under a shock-wave action

A method is proposed to study the sequence of phase transitions in powdered materials under a shock-wave action. It is shown that the aluminum hydroxide-alumina system has the following sequence of phase transformations under a shock-wave action: bayerite boehmite -Al₂O₃ -Al₂O₃. It is found that there are no transitional high-temperature modifications of alumina. A method is developed for obtaining a submicron alumina powder, which allows obtaining materials with a controlled phase composition, including a thermodynamically stable -modification of Al₂O₃. The specific features of the morphological structure and the phase and structural characteristics of powders after a shock-wave action are considered.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 110–119, January–February, 2005.     

New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Pd and Pt supported on ZnO, Ga₂O₃ and In₂O₃ exhibit high catalytic performance for the steam reforming of methanol, CH₃OH+H₂OCO₂+3HH₂, and the dehydrogenation of methanol to HCOOCH₃, 2CH₃OHHCOOCH₃+2HH₂. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.     

Effect of the crystalline from of Al2O3 on the interaction and sintering of large-crystal quartzite and disperse alumina

It is established that the amount of mullite and the value of the open porosity of specimens increase in the sequence -Al₂O₃(+)-Al₂O₃-Al₂O₃Al₂O₃ · 3H₂O. The apparent density and the ultimate compressive strength increase in the reverse direction. Active modifications of alumina (hydrargillite, -Al₂O₃) stimulate mullite formation, which is accompanied by an increase in the open porosity to 40% and a decrease of the mechanical strength to 8–12.5 MPa, and high (up to 12%) linear shrinkage. High-quality, dense, strong refractories can be produced in a single firing from coarse-grained quartzite and finely disperse corundum and alumina in the form of -Al₂O₃.Translated from Ogneupory, No. 10, pp. 18 – 20, October, 1994.Eastern Institute of Refractories.     

Surface characterization of zirconium titanate (ZrTiO4) powder by measurements of electrical photoconductance and photoassisted oxygen isotope exchange

The photosensitivity of a ZrTiO₄ sample (33 m²/g) prepared by a sol-gel method has been assessed in the presence of O₂ by both photoconductance and oxygen isotope exchange (OIE) measurements at room temperature at wavelengths > 290 nm. For oxygen pressures < ca. 13.3 Pa, the steady-state photoconductance of ZrTiO₄ was unaffected by , which indicated that the direct recombination of the photoproduced charges played the dominant role. At higher pressures, varied as the reciprocal of , which was consistent with the fact that the electronic equilibrium was then governed by O₂ + e^– O ₂ ^– . OIE over ZrTiO₄ occurred predominantly via the overall mechanism which involves the exchange of two surface oxygen atoms for each exchange act. It was very slow as compared with OIE over photocatalytically active anatase samples which, in addition, occurs via another mechanism. These results allow one to predict that this ZrTiO₄ sample is a poorly active photocatalyst for oxidations involving gaseous oxygen, and further illustrate the interest of and OIE measurements to evaluate the photosensitivity of semiconductor oxide samples.     

Heptane Dehydrocyclization Over Pt/KL Catalysts Doped with Barium or Lanthanum

Several 1 wt% Pt/KL catalysts doped with different concentrations of either BaO or La₂O₃ were prepared by successive impregnation of a zeolite KL and then characterized by H₂-O₂ titration, TPR, CO–FTIR, TEM-XEDS and XPS. Catalytic activity measurements in the hydroconversion of n-heptane showed that barium highly enhances the aromatization activity of Pt/KL, the more so the higher the barium concentration. The yield to aromatics increases to a lesser degree by adding 1 wt% La, but it is little modified at higher La concentrations. The CO–FTIR results suggest that the promoter effect of barium is related to an electron enrichment of Pt produced by the BaO Pt⁰ interaction which, according to the TEM–XEDS and XPS results, is more favored than the La₂O₃ Pt interaction.     

Theoretical Analysis of N2O to N2 Conversion During the Catalytic Decomposition of NO by Cu-Zeolites

Catalytic reactions of N₂O in Cu-exchanged silica zeolites (ZSM-5) have been investigated theoretically using first-principles density functional theory (DFT). We consider four possible reaction paths for the production of N₂, including (i) ZCu+N₂OZCuO+N₂, (ii) ZCuO+N₂OZCuO₂+N₂, (iii) ZCu+NO+N₂OZCuNO₂+N₂ and (iv) ZCu+NO₂+N₂OZCuNO₃+N₂ (Z refers to zeolites). Reactions (i) and (iii) are found to be the most favorable, whereas reactions (ii) and (iv) have much larger barriers. The implication for N₂O reactions in non-selective reduction of NO by CO is also discussed.     

A chemically regenerative redox fuel cell

A novel chemically regenerative redox fuel cell is described. The electrode reactions are based on the following redox reactions: cathodic reaction: anodic reaction: VO ₂ ⁺ +2H⁺+e VO²⁺+H₂O (E ₀ +1V), SiW₁₂O ₄₀ ^5– SiW₁₂O ₄₀ ^4– +e (E ₀ 0V). Regeneration of the oxidant by direct oxidation with O₂ was achieved by using the soluble heteropoly acid catalysts, H₃PMo₁₂O₄₀ or H₅PMo₁₀V₂O₄₀, whereas regeneration of the tungstosilicic acid, H₃SiW₁₂O₄₀, was accomplished by direct reduction with H₂ utilizing small amounts of Pt, Pd, Rh, Ru or the soluble Pd-4, 4, 4, 4'-tetrasulphophthalocyanine complex as catalysts. Some aspects of the regeneration kinetics and their influence on the overall performance of the redox fuel cell are discussed.     

Selective vapor phase hydroformylation of ethylene over cluster-derived cobalt catalyst

Vapor phase hydroformylation of ethylene was studied with silica-supported metal catalysts. A cobalt metal catalyst derived from Co₂(CO)₈ gave propanal and its derivatives in as high selectivity of about 36% as Rh/SiO₂ catalyst under the reaction conditions of 1.1 MPa of a gas-mixture of ArCOC₂H₄H₂ = 1333 at 423–503 K. On the other hand, conventional cobalt catalysts derived from cobalt nitrate, chloride, or acetate, and other noble metal catalysts (Pd/SiO₂ and Ir/SiO₂) produced mainly ethane.     

Preparation of nanosize Pt clusters using ion exchange of Pt(NH3) 4 2+ inside mesoporous channel of MCM-41

Mesoporous molecular sieve MCM-41 with a Si/Al ratio of 35 was obtained by hydrothermal synthesis using a gel mixture with a molar composition of 6 SiO₂0.1 Al₂O₃1 hexadecyltrimethylammonium chloride 0.25 dodecyltrimethylammonium bromide 0.25 tetrapropylammonium bromide0.15 (NH₄)₂O1.5 Na₂O300 H₂O. The MCM-41 sample was calcined in O₂ flow at 813 K and subsequently ion exchanged with Ca²⁺. A small Pt cluster has been supported on the MCM-41 sample following a procedure using ion exchange of Pt(NH₃) ₄ ²⁺ . The Pt(NH₃) ₄ ²⁺ ion supported on MCM-41 has been activated in O₂ flow at 593 K and subsequently reduced with Fh flow at 573 K, in the same way used for the preparation of a Pt cluster entrapped inside the supercage of zeolite NaY. The resulting Pt cluster supported on the MCM-41 shows hydrogen chemisorption oftotal two H atoms per Pt at 296 K (based on the total amount of Pt) and high catalytic activity for hydrogenolysis of ethane. The chemical shift in¹²⁹Xe NMR spectroscopy of adsorbed xenon indicates that the Pt cluster is located inside the mesoporous molecular sieve.     

Effect of Stefan Flow on Combustion Characteristics of a Moving Carbon Particle

The effect of Stefan flow on the diameter of a moving carbon particle is analyzed. The stable (combustion) and critical (ignition, spontaneous and forced extinction) regimes of heat and mass transfer and the kinetics of parallel chemical reactions (C+O₂ CO₂ and 2C+O₂ 2CO) are determined with allowance for radiant heat transfer to the cold walls of the reaction device.     

Activity and selectivity of PtNi/TiO2 catalysts for hydrogenation of crotonaldehyde

The hydrogenation of crotonaldehyde over a series of titania supported PtNi catalysts was investigated. Pt/TiO₂ had the highest activity to hydrogenate the bond of crotonaldehyde. With the addition of Ni to Pt the activity for carbonyl group hydrogenation decreased, while that for the bond hydrogenation increased. Two different types of sites were established to be active for bond hydrogenation, located at (i) the Pt-TiO₂ interface and (ii) the PtNi bimetallic phase. For the Pt-TiO₂ interface the presence of accessible Ti ^x+ cations in close distance to Pt and for the PtNi phase a slightly positively charged Ni on the surface of these alloy particles were concluded to be responsible for the enhanced selectivity to crotylalcohol. The hydrogenation of the bond occurs mainly on the clean metal surfaces of Ni and Pt.     

From Bifunctional Site to Metal–Proton Adduct Site in Alkane Reforming Reactions on Sulphated-Zirconia-Supported Pt or Pd or Ir Catalysts

Isomerization reactions of n-heptane, n-octane and n-nonane are studied on sulphated-zirconia-supported 0.2 wt% Pt, Pd or Ir catalysts. Evolutions of isomer selectivity versus total conversion and reaction temperatures are analysed. When total conversion (_T) is increased, isomer selectivity (%S _isom) is decreased and the slope of the curve %S _isom=f(_T) is more pronounced when the carbon number in the alkane is more important. At isoconversion, around 20%, below 473 K, cracking is favoured over isomerization reaction, and above 473 K it is the reverse. Moreover, with n-heptane, when the catalytic reaction occurred at 423 K and at low conversion, _T20%, we observed a large decrease in the isomer selectivity percentages on Pd/SZ and Ir/SZ compared to Pt/SZ. What is remarkable is that, at this low temperature, both metals are inactive in the carbon–carbon bond rupture. To explain these results the following points are raised: (i) an associative mechanism is proposed for the adsorption step of the alkane involving an agostic intermediate species where the carbon–hydrogen bonds act as ligands to the transition metal centres forming covalent C–HM systems, and (ii) a metal–proton adduct site, which gathers metallic and acidic sites is suggested. This approach seems to better explain our results than the traditional bifunctional mechanism.     

Synthesis and Photocatalytic Properties of HTaWO<Subscript>6</Subscript> Intercalated with Oxide Materials

Transition metal oxides (TiO₂, MnO₂, Cr₂O₃, Fe₂O₃, NiO, CuO) were intercalated into the interlayer of HTaWO₆ by (1) hydrothermal reaction on HTaWO₆ with 1 M soluble metal nitrate aqueous solution at 130C for 12 h followed by calcination at 250C for 3 h and (2) sol reaction on HTaWO₆/n-C₃H₇NH₂ precursor with metal oxide sol, followed by UV irradiation. The gallery height of transition metal oxide in the interlayer of HTaWO₆ is changed from 0.42 to 0.71 nm, the band gap energies of intercalated materials are less than 3 eV. HTaWO₆/Cr₂O₃, HTaWO₆/MnO, HTaWO₆/Fe₂O₃, HTaWO₆/NiO and HTaWO₆/CuO porous materials are capable of photocatalytic decomposing methyl orange, and the photocatalytic activity of HTaWO₆/TiO₂ is superior to that of unsupported TiO₂.     

Electrochemical reduction of niobium ions in molten LiF-NaF

The mechanism of the electrochemical reduction of niobium ions in molten LiF-NaF (11 mol) has been studied in detail at 750 and 800° C by the use of cyclic voltametry, chronoamperometry and chronopotentiometry. In the solution of niobium ions in LiF-NaF, it is concluded that the mechanism for the electrochemical reduction of fluoroniobate is: Nb^v+eNb^IV Nb^IV+4eNb⁰ at potentials of about –0.06 and –0.17 V, respectively (referred to the reliable Ni/NiF₂ (1 mol%) electrode [1]. The electrochemical reaction Nb^IV+4eNb⁰ is a (quasi)reversible and diffusion-controlled process.The conclusion has also been confirmed by analysis of the cathodic product obtained at constant potential with a scanning electron microscope.