Ammonia decomposition on polycrystalline Pt at pressures between 1.3 and 93 kPa

The kinetics of ammonia decomposition on polycrystalline Pt at temperatures between 600 and 1700 K and at pressures between 1.3 and 93 kPa were measured and correlated with a Langmuir-Hinshelwood unimolecular reaction rate expression. This rate expression had previously been shown to fit data in the same temperature range for pressures between 2 and 1400 Pa.

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Pt 600–1700 1,3 93 , - . , , , 2 1400 .

     

Low-pressure decomposition of ammonia on rhodium

Kinetics of ammonia decomposition and adsorption of reactants on a polycrystalline rhodium wire have been studied by thermal desorption and mass-spectrometric methods. Qualitive discussion of the mechanism of ammonia interaction with rhodium is presented.

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Oxygen interaction with rhodium at low pressures II. Oxygen desorption

Oxygen desorption on polycrystalline Rh and single crystal Rh(100) at 400–1600 K has been studied using thermal desorption and numerical calculation methods. With low surface coverages (<0.3) the adsorbed oxygen particles pentrate the metal and the diffuse back to the surface (peak ₂). At >0.3 the processes of formation/decomposition of surface oxide Rh₂O₃ takes place (peak ₁).

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- O₂ Rh Rh(100) 400–1600 . (<0,3) O ë ( ₂), >0,3 - - (Rh₂O₃) ( ₁).

     

Oxygen interaction with rhodium at low pressures I. Oxygen adsorption

Oxygen adsorption and desorption on polycrystalline Rh and Rh(100) at PO₂<10^–5 Pa and 400–1600 K have been studied using TDS and AES methods. Adsorption kinetics for both samples is described by the King model accounting for the effect on O₂ adsorption of both precursor state and lateral interaction.

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O₂ Rh(100) 10^–5 400–1600 . - , O₂ , -.

     

Effect of traces of water vapor on the rate of ammonia decomposition on polycrystalline platinum

The influence of small (lower than 0.02) molar fractions of water on the rate of ammonia decomposition on platinum wires was studied for ammonia pressures varying between 6.7 and 93.3 kPa and temperatures between 800 and 1700 K. The presence of water strongly inhibited the rate of reaction in all experiments.

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( 0,02) , 6,7 93,3 , 800 1700 K. .

     

The rate of methyl radical decomposition at high temperatures and pressures

Rate constants are calculated for CH₃ (+ Ar) ? CH₂ + H (+ Ar) at the limiting low-pressure, the limiting high-pressure, as well as the intermediate fall-off ranges. The results show that published experimental rate constants for methyl dissociation correspond to the fall-off region close to the low-pressure limit. At the low-pressure limit the activation energy is less than the bond dissociation energy, in agreement with experimental results. Forward and backward rate coefficients at the high-pressure limit are compared with other theoretical calculations. More theoretical and experimental work is necessary to understand the reverse reaction and its competing reactions, as well as the decomposition channel leading to CH + H₂. © 1994 John Wiley & Sons, Inc.     

Electrocrystallization of rhodium clusters on thiolate-covered polycrystalline gold

This study reports on the electrochemical deposition of rhodium metal clusters on a polycrystalline gold electrode, modified with a monolayer of dodecanethiol through self-assembly from solution. The deposition process was investigated using cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance. It is shown that the presence of the thiol monolayer drastically alters the nucleation and growth mechanism compared with the mechanism on the bare gold electrode. The small uncovered gold domains, located at the imperfections in the thiolate monolayer which are induced by the gold nanoroughness, act as nucleation sites for small rhodium clusters. At longer times, these clusters can outgrow the organic monolayer. The resulting surface morphology was characterized by scanning electron microscopy. Rhodium electrocrystallization on the bare gold substrate resulted in an ensemble of a very large amount of very small clusters that are difficult to distinguish from the gold roughness. In contrast, in the presence of a self-assembled monolayer (SAM) of dodecanethiol covalently attached to the gold electrode, the resulting deposit consisted of an ensemble of hemispherical particles. The size distribution of the rhodium particles obtained by using double step chronoamperometry was compared to the ones obtained with cyclic voltammetry and "classical" chronoamperometry. It is shown by X-ray photoelectron spectroscopy that the SAM is still present after rhodium deposition on the thiolate-covered gold substrate. Because the rhodium clusters are directly attached to the gold substrate and can thus easily be electrified, the resulting interface could be used as a composite electrode consisting of a random array of gold supported rhodium nano/microparticles separated from each other by an organic phase. On the other hand, it is shown that the SAM is easily removed by electrochemical oxidation without dissolving the rhodium clusters and, thus, leaving a different array of rhodium clusters on the gold surface compared with the topography obtained in the absence of the SAM. From this point of view, substrate modification with such "removable" organic monolayers was found to be an interesting tool to tune the nano- or microtopography of electrochemically deposited rhodium.     

103Rh NMR spectroscopy and its application to rhodium chemistry

Rhodium is used for a number of large processes that rely on homogeneous rhodium-catalyzed reactions, for instance rhodium-catalyzed hydroformylation of alkenes, carbonylation of methanol to acetic acid and hydrodesulfurization of thiophene derivatives (in crude oil). Many laboratory applications in organometallic chemistry and catalysis involve organorhodium chemistry and a wealth of rhodium coordination compounds is known. For these and other areas, 103Rh NMR spectroscopy appears to be a very useful analytical tool. In this review, most of the literature concerning 103Rh NMR spectroscopy published from 1989 up to and including 2003 has been covered. After an introduction to several experimental methods for the detection of the insensitive 103Rh nucleus, a discussion of factors affecting the transition metal chemical shift is given. Computational aspects and calculations of chemical shifts are also briefly addressed. Next, the application of 103Rh NMR in coordination and organometallic chemistry is elaborated in more detail by highlighting recent developments in measurement and interpretation of 103Rh NMR data, in relation to rhodium-assisted reactions and homogeneous catalysis. The dependence of the 103Rh chemical shift on the ligands at rhodium in the first coordination sphere, on the complex geometry, oxidation state, temperature, solvent and concentration is treated. Several classes of compounds and special cases such as chiral rhodium compounds are reviewed. Finally, a section on scalar coupling to rhodium is provided.     

Formation and decomposition of thin rhodium oxide films

Interaction of O₂ with Rh (poly) and Rh (100) has been studied by Auger Electron Spectroscopy and thermal desorption method at O₂ pressures of 10^–5–10³ Pa and at 400–1600 K. At P(O₂)<10^–5 Pa chemisorption of O₂ occurs, at P(O₂)=10^–5–10^–1 Pa surface oxides are formed, at P(O₂)>1.0 Pa a bulk Rh₂O₃ oxide starts to grow. The growth of rhodium oxide film proceeds via the Cabrera-Mott mechanism. Its decomposition occurs via a mechanism including electron transfer across the oxide film, O₂ desorption from the surface layer and rearrangement of the oxide layer.     

SIMS studies of water vapor adsorption on polycrystalline rhodium

Studies of water vapor adsorption on polycrystalline Rh at T>315 K and P=(2–4)×10^–2 Pa indicate that water is adsorbed dissociatively to O_ads and O_ads through a molecularly adsorbed species. Desorption activation energy is 46 and 69 kJ/mol for molecular and dissociative species, respectively.

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Rh (2–4)·10^–2 . , O OH - . 46 /, -69 /.

     

Kinetics of the disproportionate of N-arylhydroxylamines at different hydrogen partial pressures

Conclusions Dependence of the reaction rate on the hydrogen partial pressure 400–150 hPa, was noted in the disproportionate of N-arylhydroxylamines in the presence of a platinum catalyst in a hydrogen-nitrogen atmosphere. This dependence is attributed to the increase in the amount of oxygen-containing products, which are strongly adsorbed on the catalyst and inhibit the reaction. At hydrogen pressures above 0.1 MPa, oxygen-containing products are not formed and the rate of disproportionation is not dependent on the hydrogen pressure.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2844–2846, 1988.The authors express their gratitude to V. G. Dorokhov for assistance in carrying out these experiments.     

Oxygen dissolution in polycrystalline palladium at O2 pressures of 0.1 to 100 Pa

Oxygen dissolution in polycrystalline palladium Pd(poly) at O₂ pressures ( $P_{O_2 } $ ) of 0.1 to 100 Pa and a temperature of 600 K has been investigated by temperature-programmed desorption. The dissolution process under these conditions includes O₂ chemisorption on the oxide film surface, the insertion of Oads atoms under the oxide layer, and their diffusion into the subsurface layers of palladium. During chemisorption, a structure ensuring that the O_ads coverage of the surface increases with increasing $P_{O_2 } $ forms on the surface of the oxide film. This is favorable for O_ads penetration through the oxide film and increases the amount of absorbed oxygen. The O_ads coverage of the surface calculated via the Langmuir equation at an O₂ desorption activation energy of E _des = 125 kJ/mol correlates with the number of absorbed oxygen monolayers (n). At n ≥ 1, oxygen absorption by Pd(poly) is due to the diffusion of O atoms in the palladium lattice. After the accumulation of 14–18 oxygen monolayers in the subsurface layers of palladium, oxygen absorption practically stops depending on $P_{O_2 } $ . Thus, the acceleration of oxygen dissolution in palladium is due to the formation of the surface oxide film and the increase in the O_ads coverage of this film, which facilitates the insertion of O_ads atoms into the subsurface layers of palladium.     

Kinetics and rate-limiting mechanisms of dolomite dissolution at various CO_2 partial pressures

Techniques of rotating-disk and catalyst were used in investigating the kinetics of dolomite dissolution in flowing CO2-H2O system. Experiments run in the solutions equilibrated with various CO2 partial pressures (PCO2) from 30 to 100000 Pa. It shows that dissolution rates of dolomite are related with rotating speeds at conditions far from equilibrium. This was explained by modified diffusion boundary layer (DBL) model. In addition, the dissolution rates increase after addition of carbonic anhydrase (CA) to solutions, where the CA catalyzes CO2 conversion. However, great differences occur among various CO2 partial pressures. The experimental observations give a conclusion that the modified DBL model enables one to predict dissolution rates and their behaviour at various PCO2 with satisfactory precision at least far from equilibrium.     

Kinetics and rate-limiting mechanisms of dolomite dissolution at various CO2 partial pressures

Techniques of rotating-disk and catalyst were used in investigating the kinetics of dolomite dissolution in flowing CO₂-H₂O system. Experiments run in the solutions equilibrated with various CO₂ partial pressures (P_{CO 2} ) from 30 to 100000 Pa. It shows that dissolution rates of dolomite are related with rotating speeds at conditions far from equilibrium. This was explained by modified diffusion boundary layer (DBL) model. In addition, the dissolution rates increase after addition of carbonic anhydrase (CA) to solutions, where the CA catalyzes CO₂ conversion. However, great differences occur among various CO₂ partial pressures. The experimental observations give a conclusion that the modified DBL model enables one to predict dissolution rates and their behaviour at various P_{CO 2} with satisfactory precision at least far from equilibrium.     

Kinetics and mechanism for synthesis of iodo-containing telomeres and block copolymers at high pressures

Radical telomerization reactions of hexafluoropropylene, perfluoromethyl vinyl ether, and perfluoropropyl vinyl ether in the presence of alkyl iodides were studied at high pressures (1000 MPa). Iodo- and diiodo-containing telomeres were synthesized. Their preparation is almost impossible under usual conditions. The mechanism and kinetics of telomerization were studied. Block copolymers of different structures were synthesized from the obtained telomeres by the “living” radical telomerization method.