Nonoxidative conversion of methane and <Emphasis Type="Italic">n</Emphasis>-pentane over a platinum/alumina catalyst

Methane adsorption on the Pt–H/Al₂O₃ and Pt/Al₂O₃ catalysts begins at Т = 475°C and is accompanied by the appearance of hydrogen in the reaction medium. At a higher temperature is raised to 550°C, the amount of adsorbed hydrogen increases to 1.1 and 0.8 mol/(mol Pt), respectively. According to the calculated degree of methane dehydrogenation on platinum sites at Т = 550°C, the Н/C ratio is 1.3 (at/at) for the Pt–H/Al₂O₃ catalyst and 1.5 (at/at) for the Pt/Al₂O₃ catalyst. The introduction of n-pentane into the reaction medium increases the yield of aromatic hydrocarbons (benzene and toluene) by a factor of 8.8 over the arene yield observed in individual n-pentane conversion. A mass spectrometric analysis of the arenes obtained with the Pt/Al₂O₃ catalyst has demonstrated that 37.5% of the adsorbed methane is involved in the methane–n-pentane coaromatization yielding benzene and toluene.     

Microstructure and ferroelectric properties of r.f. magnetron sputtering derived PZT thin films deposited on interlayer (PbO/TiO2)

Lead zirconate titanate (PZT) thin films were deposited on Pt/Ti/SiO₂/Si and interlayer/Pt/Ti/SiO₂/Si substrate by radio frequency (r.f.) magnetron sputtering with a Pb_1.1Zr_0.53Ti_0.47O₃ target. The crystallization of the PZT thin films was formed only by substrate temperature. When interlayer (PbO/TiO₂) was inserted between the PZT thin film and the Pt electrode, the grain growth and processing temperature of the PZT thin films were considerably improved. Compared to PZT/Pt structure, the dielectric constant and polarization properties of the PZT/interlayer/Pt structure were fairly improved. In particular, PZT/interlayer/Pt at the substrate temperature of 400 °C showed prevalent ferroelectric properties (_r=475.97, tanδ=0.0591, P_r=23 μC/cm²). As a result of an X-ray photoelectron spectroscopy (XPS) depth-profile analysis, it was found that PZT/interlayer/Pt deposited only by substrate temperature without the post-annealing process via r.f. magnetron sputtering method remained independent of each other regardless of substrate temperatures.     

Effect of Thermal Activation of Supported Catalysts Pt/MeOx,Where MOx = Al2O3, CeO2, La2O3, and ZrO2, for Complete Oxidation

A sharp increase in the atomic catalytic activity (ACA) of supported platinum catalysts in the model reaction of n-pentane complete oxidation is found on going from the preliminary calcination temperature of 500–600°C to a temperature of 700°C. ACA increases by an order of magnitude for the Pt/-Al₂O₃ system, 3 times for Pt/ZrO₂, and 1.5 times for Pt/CeO₂. The per-gram activities of all catalysts decrease because of a decrease in the dispersion of supported platinum with an increase in the temperature of preliminary calcination.     

An XPS and STM Study of Oxidized Platinum Particles Formed by the Interaction between Pt/HOPG with NO<Subscript>2</Subscript>

X-ray photoelectron spectroscopy (XPS) (with AlK_α and AgL_α radiations) and scanning tunneling microscopy (STM) were used to study the interaction of two model samples prepared by vacuum evaporation of platinum on highly oriented pyrolytic graphite (HOPG) with NO₂ at room temperature. According to STM data, platinum evaporation on the graphite surface produced particles of a flattened shape. In the Pt/HOPGS1 sample with a lower concentration of platinum, the average diameter of particles d and the height-to-diameter ratio h/d were 2.8 nm and 0.29, respectively. In the Pt/HOPG-S2 sample with a higher concentration of platinum, the average values of d and h/d were 5.1 nm and 0.32. When the samples interacted with NO₂ (P ≈ 3 × 10^–6 mbar), the particles of metallic platinum completely converted to the particles of PtO_2· Upon oxidation, the shape of larger platinum particles in the Pt/HOPG-S2 sample did not change, although for the dispersed particles in the Pt/HOPG-S1 samples under these conditions, the h/d ratio increases. The reduction of oxide to metal particles on heating the Pt/HOPG-S1 sample in vacuum at 460°С is accompanied by an increase in the size of particles. Their shape became more round compared to the initial one. It was found that X-ray radiation affects the state of platinum in the oxidized sample by reducing the surface layer of PtO₂ to PtO.     

Electrochemical characterization of a new system for detection of superoxide ion in alkaline solution

A new medium system containing dimethyldistearylammonium bromide (DSAB) was developed for the electrochemical detection of superoxide ion in alkaline solution. The reductions of molecular oxygen in alkaline media as a function of the electrode material were evaluated for Pt, Ag, Au and glassy carbon (GC) electrode. And a quasi-reversible redox process for the O₂/O₂⁻ couple was only obtained at the Pt electrode. The electrochemical responses of the O₂/O₂⁻ couple at a platinum electrode and a platinized platinum electrode were compared, which suggesting that the electrochemical behavior of the O₂/O₂⁻ couple was improved greatly at a platinized Pt electrode. The frequency change (mass change) on the surface of Pt electrode was characterized by the electrochemical quartz crystal microbalance. The reduction of the dissolved oxygen at a platinized Pt electrode in the presence of DSAB was also studied by using chronocoulometry and the result indicated that a one-electron reduction was involved. In addition, the scavenging activity of cysteine towards superoxide ion was evaluated by cyclic voltammetry.     

Substitution of [M(bpyMe)Cl3] and [M(bpyMe-H)Cl2] (M = Pd or Pt; bpyMe =N-methyl-2,2′-bipyridylium ion) withN-heterocycles

Summary The complex Pt(bpyMe)Cl₃ (bpyMe = N-methyl-2,2-bipyridylium cation) reacts with pyridine(py) to give cis-[Pt(bpy-Me) (py)Cl₂]⁺, which on heating cyclometallates with loss of py to give Pt(bpyMe-H)Cl₂; some Pt(py)₂Cl₂ is also formed. Pt(bpyMe)Cl₃ reacts with 2,2-bipyridyl (bpy) to yield a mixture of [Pt(bpyMe-H)(bpy)]²⁺ and [Pt(bpy)₂]²⁺. The analogous reactions with Pd(bpyMe)Cl₃ proceed under very mild conditions to afford PdL₂Cl₂ (L₂ = 2py, bpy).     

High surface area silicon carbide from rice husk: A support material for catalysts

Ultrafine -SiC with high surface area (150 m² g^–1) has been synthesized by inflight processing of charred rice husk in a r.f. plasma reactor operating at atmospheric pressure. The plasma-synthesized particles were doped with platinum (1%) and tested as a catalytic support material. The catalyst (1% Pt doped -SiC) showed 100% conversion of CO to CO₂ at a temperature as low as 175°C.     

Hydrogenation of Benzonitrile on Sn-Pt/SiO2 Catalysts Prepared by Introducing SnEt4 to Pt/SiO2: Role of Tin

Liquid phase hydrogenation of benzonitrile was studied over Sn-Pt/SiO₂ catalysts prepared by introducing tetraethyl tin onto the 3 wt.% Pt/SiO₂ catalyst. Tin content of the catalysts ranged from 0.05 to 0.63 wt.%, whereas Sn/Pt surface atomic ratios determined by chemisorption measurements were between 0.1 to 3.5. Dibenzylamine selectivity influenced to a small extent by the level of conversion and the Sn/Pt ratio wasca. 75 %. The addition of tin to Pt in the range of (Sn/Pt)_surface = 0.50–1.25 led to an increase in the turnover frequency (TOF) by a factor of 2. TOF showed a maximum at a surface atomic ratio of Sn/Pt = 1. The enhancement of catalyst activity upon the addition of tin is explained by the formation of Sn⁺-Pt ensemble sites on the surface of bimetallic nanoclusters. It is suggested that highly dispersed positively charged tin species, by polarizing the triple bond, enhance the reactivity of the -CN group. Calcination at 300°C followed by re-reduction of the catalysts resulted in a monotonic decrease of specific activity with increasing Sn/Pt ratio.     

Effect of properties of aluminum oxide on selectivity of Pt/Al2O3 in dehydrogenation of n-alkanes

Conclusions The selectivity of Al-Pt catalysts (0.25% Pt) in the dehydrogenation of n-decane and n-hexane to n-alkenes depends on the properties of the aluminum oxide and increases in the order: Pt/-Al₂O₃2O₃s > Pt/-Al₂O₃.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2615–2618, November, 1978.