TPD studies of ethylene adsorption over platinum black

TPD studies of ethylene adsorption over Pt-blacks of various dispersity with and without hydrogen at 298 K indicate that the metal dispersity and the presence of hydrogen on the surface influence the direction of hydrocarbon transformation on platinum.

---

298 Pt- . .

     

Reactivity of chemisorbed hydrogen species on Pt towards acetylene

In the presence of both molecular and atomic hydrogen on catalyst surface, C₂H₂ is hydrogenated directly to C₂H₆. In the presence of atomic hydrogen only, the reaction can also have another route to form C₂H₄.

---

, C₂H₂ C₂H₆, , C₂H₄.

     

Interaction of oxygen adsorbed on silver film with hydrogen and ethylene

Kinetic data on hydrogen interaction with adsorbed oxygen testify to the uniformity and constancy of the oxygen bond with the silver surface. The surface reaction of ethylene epoxidation proceeds when the surface coverage by oxygen is higher than a certain threshold value, which is close to 0.4 for silver films at 333 K.     

Reactivity of allylic hydrogen in cyclohexadiene towards OH radicals

The yield of benzene in the reaction of 1,4- and 1,3-cyclohexadiene with OH radicals in the presence of oxygen was determined using H₂O₂ and CH₃ONO as OH radical sources. Both in the H₂O₂ and the CH₃ONO systems, the yield of benzene from 1,4-cyclohexadiene was 15.3% and the yield from 1,3-cyclohexadiene was 8.9%. On the basis of the obtained yields, the rate constant for allylic hydrogen abstraction per C? H in cyclohexadiene was determined to be 3.8 × 10^?12 cm³ molecule^?1 s^?1. The branching ratio of the hydrogen abstraction to overall reaction for 1-butene and 1-pentene was estimated to be (25–14)% by applying the obtained rate constants. The result was in good agreement with the branching ratio determined directly by use of the discharge flow photoionization mass spectrometer by Biermann, Harris, and Pitts [4].     

TPD studies of hydrogen sorption on platinum powder catalysts modified with adsorbed gold

It has been shown that hydrogen determined from the TPD curves of platinum saturated with hydrogen sorbed above room temperature is chiefly absorbed and not adsorbed hydrogen; this absorbed hydrogen has no effect on the charging curve.

---

, , , , ; .

     

Investigation of lead adsorption via the ionization of hydrogen adsorbed on platinized platinum

In the case of lead adsorption via the ionization of hydrogen adsorbed on platinized platinum, adsorbed Pb⁺ ions are also formed on, and partly desorbed from, the surface. The site requirement of an adsorbed lead atom is S=2.3 and 1.9.     

Mechanism of interaction of molecular hydrogen with adsorbed oxygen on platinum electrodes under open circuit conditions

Transients of open-circuit potential observed at the reaction of hydrogen molecules with oxygen preliminarily adsorbed (O_ads) on the smooth polycrystalline (pc Pt) and platinized platinum (Pt/Pt) electrodes are measured under conditions of controlled stirring of solution (0.5 M H₂ SO₄). The dependence of the surface coverage with O_ads(θ_O) on the potential in the cause of the potential decay on pc Pt are determined. It is found that for Pt/Pt, the reaction kinetics is largely determined by diffusion of H₂. For pc Pt in the range of high θ_O, the Eley-Ridiel mechanism is realized. For medium θ_O, the regions where the reaction obeys the mechanisms of Eley-Ridiel, “conjugated reactions”, and diffusion control are observed to overlap (even at the most intense stirring possible). The rate of H₂ reaction with O_ads is substantially higher compared with analogous reactions of CO, HCOOH, and CH₃OH.     

Catalytic decomposition of hydrogen peroxide by a flow-through self-regulating platinum black heater

A flow-through reactor where a fine platinized platinum wire (Pt black wire) runs through the entire length of the PTFE tube was fabricated for the rapid decomposition of hydrogen peroxide (H₂O₂) at concentrations up to 1 M. Since the Pt black wire is directly electrically heated and the exterior of the tubular reactor is well insulated, the energy efficiency of this heater reactor is excellent. The temperature coefficient of resistance of the wire is significant (3280±40 ppm/°C). This provides a means to ascertain the mean temperature by measuring the resistance of the wire. The concentration of residual H₂O₂ was determined by the hematin-catalyzed oxidation of nonfluorescent thiamine to fluorescent thiochrome by H₂O₂. Essentially, quantitative (99.997%) decomposition of 1 M H₂O₂ could be achieved at a mean reactor temperature of 108 °C from a neutral aqueous solution with a mean residence time of ∼270 s.     

Reactivity of olefin an acetylene complexes of platinum(0) towards tetrachloro-o-benzoquinone

Tetracloro-o-benzoquinone reacts with (diphenylacetylene)bis(tirphenylphosphine)platinum(0) to give the novel platinum(II) diphenylacetylene complex, Pt(C₆Cl₄O₂)PhCCPh)(PPh₃), (I), which reacts with hydrogen halides to give the compelexes cis-PtX₂(PhCCPh((PPh₃), (X = Cl or Br). Hydrogen chloride also readily removes the tetrachloro-o-benzoquinoneligand from the adducts Ni(C₆Cl₄O₂)(Ph₂PCH₂CH₂PPh₂) and M(C₆Cl₄O₂)(PPh₃)₂, (M = Pd or Pt) but it has no reaction upon Ir(Cl)(C₆Cl₄O₂)(CO)(PPh₃)₂ at room temperature. The acetylene in (1) is susceptible to nucleophilic attact and reaction with diethylamine gives the vinyl adduct Pt(C₆Cl₄O₂)(CPhCPh)NHEt₂)(PPh₃). Other reactions of (I) have also been studied. Attemps to prepare other olefin or acetylene complexes of platinum(II) by the action of tetrachlor-o-benzoquinone on the complexes Pt(L)(PPh₃)₂, (L = PhCCH,(Et)(Me)(HO)CCCC(OH)(Me)(Et), HOCH₂OH, CF₃CCCF₃, CF₂CF₂, CF₂CH₂ or trans-PhCHCHPh) are also described.     

Investigations of copper adsorption in hydrochloric acid media,via the ionization of hydrogen adsorbed on platinized platinum

Copper deposition in hydrochloric acid media, via the ionization of hydrogen adsorbed on platinized platinum, results in an adsorbed metal layer similar to that formed potentiostatically. However, the adsorbed hydrogen is replaced by adsorbed copper only to a certain extent, owing to the formation of Cu⁺ ions. This discrepancy depends on the chloride ion activity and the structure of the platinum black.     

Orientational transitions of aromatic molecules adsorbed on platinum electrodes

A method to determine the orientation of molecules adsorbed at solid-liquid interfaces, based on thin-layer electrochemical techniques, is described. This method has been applied to determine orientational changes of molecules adsorbed from solution onto smooth platinum electrodes as a function of adsorbate concentration. Twenty-six diphenols and quinones, representing a variety of structures and chemical properties, were studied. In general, these compounds are adsorbed with the diphenol or quinonoid ring parallel to the surface at low concentrations and re-orient irreversibly to non-random edgewise-orientations as the concentration is increased. A possible explanation for these edgewise-orientations is discussed.     

Catalytic oxidation of hydrogen on platinum

Using the thermochemical approach to interpret the kinetics of heterogeneous reactions and the mechanism of congruent dissociative decomposition of solids developed in the 1980s and (re)analyzing the experimental data available in the literature over the last 90 years, a novel mechanism for the catalytic oxidation of H₂ by PtO₂ is proposed. In place of the conventional Langmuir–Hinshelwood and Eley–Rideal adsorption reaction mechanisms, our model is based on the reactions: PtO₂(s) + 2H₂ ? Pt(g) + 2H₂O and Pt(g) + O₂ ? PtO₂(g) → PtO₂(s). The first reaction determines the kinetics of H₂ oxidation and the second determines the kinetics of restoration of the PtO₂ layer. Thermochemical consideration of kinetic features of this model enables (for first time in the history of this reaction) the enthalpy and equilibrium constants for H₂ oxidation on platinum to be calculated. The results are in good agreement with experimental data. In addition, the proposed mechanism explains the origin of the surface-retexturing effect, the impact of autocatalysis, the influence of H₂O vapor on oxidation rate, and the three-fold variation of the Arrhenius E parameter with temperature. This all convincingly demonstrates the value of the thermochemical approach in interpreting heterogeneous reactions.     

Low-temperature oxidation of hydrogen on platinum

The interaction of oxygen adsorbed on platinum with hydrogen at low temperatures has been studied. The molecular surface oxygen is shown to be involved in the reaction with hydrogen. The steady-state rate of hydrogen oxidation has been measured at 170–350 K. The activation energy is 3 kcal/mol.

---

, , , , . 170–350° , 3 /.

     

An infrared study of species from ethylene,hydrogen and carbon monoxide adsorbed on palladium/silica catalysts

Two different procedures, A and B, for preparing palladium on silica catalysts give different infrared spectra, and to a large measure different surface species, from the adsorption of ethylene, hydrogen or carbon monoxide. A π-species (I) from ethylene is found on both catalyst preparations. On preparation A the other dominant species is probably the σ-diadsorbed species (II). On preparation B additional strong bands are tentatively interpreted in terms of a doubly π - bonded non-associatively adsorbed species (III). Only preparation B gives absorptions in the accessible region above 1300 cm^?1 which can be attributed to adsorbed hydrogen.

     

Photoinduced surface dynamics of CO adsorbed on a platinum electrode

The surface dynamics of adsorbed CO molecules formed by dissociative adsorption of HCHO at a polycrystalline Pt electrode/electrolyte solution interface was studied by picosecond time-resolved sum-frequency generation (TR-SFG) spectroscopy. A SFG peak at 2050-2060 cm(-1) was observed at the Pt electrode in HClO(4) solution containing HCHO at 0-300 mV (vs Ag/AgCl), indicating the formation of adsorbed CO at an atop site of the Pt surface as a result of dissociative adsorption of HCHO. The peak position varied with potential by approximately 33 cm(-1)/V, as previously found in an infrared reflection absorption spectroscopy (IRAS) study. Irradiation of an intense picosecond visible pulse (25 ps, 532 nm) caused an instant intensity decrease and broadening of the CO peak accompanied by the emergence of a new broad peak at approximately 1980 cm(-1) within the time resolution of the system. These results suggest a decrease and increase in the populations of CO adsorbed on atop and bridge sites, respectively, upon visible pump pulse irradiation.     

Reversibly adsorbed hydrogen on a molybdena-alumina catalyst reduced with hydrogen

The elution of hydrogen and water desorbed from a MoO₃Al₂O₃ catalyst reduced with hydrogen at 550°C was investigated at 550°C and at O°C. Only hydrogen may be eluted at 550°C, and neither hydrogen may be eluted at 550°C, and neither hydrogen, nor water desorbs at O°C. The reversibly adsorbed hydrogen is responsible for the deactivation of the catalyst used in disproportion ation of propylene at 0–80°C without evacuation at elevated temperatures after reduction.

---

, MoO₃/Al₂O₃, 550°C, 550 0°C. 550°C , 0°C , . , 0–80°C , .

     

Reactivity of hydrogen species on oxide surfaces

Hydrogen species on oxides are widely involved in oxides-catalyzed reactions such as H_2/hydrocarbon oxidation, hydrogenation/dehydrogenation, water-gas shift, and water-splitting reactions. Thus identifications of hydrogen species on oxide surfaces and their reactivity are important for fundamental understanding of these oxides-catalyzed reactions. In this Feature Article, we briefly review our research progress on the reactivity of various hydrogen species on oxides, including surface hydroxyl species,hydride species and hydrated protons. We have successfully developed effective strategies of using gas-phase atomic H to controllably create oxygen vacancies and prepare various hydrogen species on oxide model catalysts under ultra-high vacuum(UHV) conditions and using well-defined oxide nanocrystals with different surface structures and oxygen vacancy concentrations to study the H_2-oxide interaction under ambient or even higher H_2 pressures. Reactivity of various hydrogen species on oxide surfaces has been identified, including local oxygen vacancy-controlled reactivity of OH species, oxygen vacancystabilized hydride species, homolytic dissociation of H2 at oxygen vacancies of reduced oxide surfaces into hydride species accompanied by surface oxidation, photoexcited holes-stimulated desorption of hydride species, electron-stimulated desorption of hydride and OH species, and photoexcited electrons-stimulated desorption of hydrated protons. Strong influences of oxygen vacancies in oxides on both stability and reactivity of various hydrogen species on oxide surfaces are highlighted.     

Cation effects on the vibrational frequencies of adsorbed thiocyanate on platinum

Infrared spectra of thiocyanate adsorbed on a platinum electrode surface were obtained in the presence of perchlorate electrolytes of various alkali metal cations. It was discovered that the vibrational frequency of the C-N stretching mode is dependent upon the nature of the supporting electrolyte cation. Two bands were observed in the 2050 to 2150 cm⁻¹ range; one band was attributed to nitrogen-bound thiocyanate, and the other to species adsorbed via the sulfur atom. Each of these bands demonstrated independent frequency dependencies on cation nature and on the applied electric field within the interfacial region. Differences were also observed in the intensity dependence of the bands on the applied potential. The results were explained in terms of changes in the distance between the outer Helmholtz plane (OHP) and the surface of the electrode, and also in terms of the possible influence of coadsorbed alkali metal cations on the vibrational frequency of thiocyanate species adsorbed through the nitrogen atom. The effects that variations in the OHP-electrode distance impart on the magnitude of the potential drop across the interface, and the influence of small changes in this potential field on the C-N stretching frequency of N- and S-adsorbed thiocyanate species, are discussed.     

Reactivity of polyhydroxyalkyl-heterocycles towards Lewis acids

We report the reactivity of different polyhydroxyalkyl-heterocycles towards ceric ammonium nitrate (CAN) and ferric chloride. The behaviour of 2-methyl-5-(tetritol-1-yl)-pyrroles and -furans is different towards CAN oxidation. Pyrroles afford 2,5-diformylheterocycles, while furans give access to 1,4-dicarbonyl compounds containing three stereogenic centres. Ferric chloride promotes an intramolecular C-arylation reaction on O-benzylated-polyhydroxyalkyl furans, yielding an isochroman moiety, which is the basic skeleton of a variety of natural products.