Showing papers on "Dehydrogenation published in 1994"
TL;DR: In this article, the authors measured the kinetics of methanol electro-oxidation on well-characterized Pt-Ru alloy surfaces as a function of temperature and found that the activity of Ru towards the dissociative adsorption of Methanol is a strong function of the temperature.
Abstract: The kinetics of methanol electro-oxidation on well-characterized Pt-Ru alloy surfaces were measured in sulfuric acid solution as a function of temperature. The alloy surfaces were prepared in ultrahigh vacuum with the surface composition determined by low energy ion scattering. It was found that the activity of Ru towards the dissociative adsorption of methanol is a strong function of temperature. This change in the adsorptive nature of the Ru sites with temperature produced a variation in the optimum surface composition with temperature. The optimum surface had an Ru content which increased with increasing temperature, from close to [approximately]10 atomic percent (a/o) Ru at 25 C to a value in the vicinity of [approximately]30 a/o at 60 C. The shift in optimum composition with temperature was attributed to a shift in the rate-determining step from methanol adsorption/dehydrogenation at low temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen at high temperature. The apparent activation energies were consistent with this change in the rate-determining step.
551 citations
TL;DR: In this article, an ion-induced compression of sp2 a-C into the denser sp3 phase is described, where the sp3 bonding arises from C ions entering subsurface atomic sites and producing a quenched-in density increase.
373 citations
Abstract: Publisher Summary This chapter summarizes the data and current understanding regarding the oxidative dehydrogenation reaction of alkanes. The reaction mechanism, the nature of the catalysts, and factors that determine selectivity for dehydrogenation versus formation of oxygen-containing products are discussed in the chapter. From the pattern of product distribution in the oxidation of C 2 to C 6 alkanes obtained with supported vanadium oxide, orthovanadates of cations of different reduction potentials, and vanadates of different bonding units of VO x in the active sites, it is shown that the selectivities can be explained by the probability of the surface alkyl species (or the surface alkene formed from the alkyl) to react with a reactive surface lattice oxygen. Catalysts for which this occurs with a high probability would show low selectivities. This probability increases for vanadates that have low heats of removal of lattice oxygen, which are those that contain easily reducible cations in the active sites and for vanadates whose active sites can bind the surface alkyl species (or alkene) in a way that bring the surface intermediate close to the reactive lattice oxygen. The dependence of selectivity for dehydrogenation on the conversion of alkane shows that for the more selective catalysts known, the reaction proceeds with a sequential mechanism.
271 citations
TL;DR: In this article, the authors investigate the factors controlling the selectivity of isobutane conversion over silica-supported Pt and Pt/Sn catalysts at temperatures near 700 K.
211 citations
TL;DR: In this article, the preparation and characterization of co-deposited Pt/WO[sub 3] electrodes are presented, and it is shown that WO[s are amorphous and that [approximately]40 [angstrom] Pt crystallites are uniformly dispersed in the deposit.
Abstract: The details of the preparation and characterization of co-deposited Pt/WO[sub 3] electrodes are presented. X-ray, scanning, and transmission electron microscopic studies revealed that WO[sub 3] is amorphous and that [approximately]40 [angstrom] Pt crystallites are uniformly dispersed in the deposit. The influence of the deposition conditions and the effect of the solution acidity on the activity of such electrodes for methanol oxidation have been studied. The results demonstrated that the Pt/WO[sub 3] electrodes are much more active and more resistant to poisoning than Pt or Pt/Ru alloy catalysts. The reaction mechanism was studied by various electrochemical and surface analysis techniques. A reaction scheme that involves successive stepwise dehydrogenation of methanol, formation/oxidation of hydrogen tungsten bronze, and oxidation of organic intermediates and CO at the Pt/WO[sub 3] interface is proposed.
209 citations
TL;DR: In this article, the vanadium coordination was investigated by UV-visible, near infrared, Raman, 51V NMR, and ESR spectroscopies, respectively after drying, calcination and catalytic oxidative dehydrogenation of propane in the 300-450°C temperature range.
196 citations
TL;DR: In this article, the authors used the citrate method to prepare catalysts with the Mg/V atomic ratio varying from 1/4 to 9/1, and found that alpha-Mg2V2O7 (pyrovanadate) was the most selective, while MgV 2O6 (metavanadate), the worst one.
164 citations
TL;DR: In this article, a complete reaction network for the isopropanol decomposition on these catalysts has been established, and it was shown that the concentration of acetone in the products cannot always be used for a direct estimation of the rate of dehydrogenation.
162 citations
TL;DR: In this article, the catalytic activity of CrOx/SiO2 (SC, 0.08 to 0.99% chromium by weight), Cr Ox/Al2O3 (AC,0.16 to 1.55%), and α-Cr2O2 for the dehydrogenation of propane has been investigated at 723 to 823 K in a flow apparatus.
136 citations
TL;DR: In this article, the decomposition reaction of formic acid (DCOOD) was examined on a rutile TiO2(110) surface in catalytic and non-catalytic conditions.
126 citations
TL;DR: In this paper, the autothermal production of olefins from propane or n-butane by oxidative dehydrogenation and cracking in air or oxygen at atmospheric pressure over noble metal coated ceramic foam monoliths at contact times of approximately 5 milliseconds has been studied.
TL;DR: In this paper, a kinetic model for ethene and propene aromatization over HZSM-5 and GaHZ SM-5 was developed, which describes olefin oligomerization and cracking on zeolite catalytic sites (ZCS), diene formation via hydrogen transfer on ZCS and via dehydrogenation on gallium species.
Abstract: A kinetic model for ethene and propene aromatization over HZSM-5 and GaHZSM-5 is developed. This model describes olefin oligomerization and cracking on zeolite catalytic sites (ZCS), diene formation via hydrogen transfer on ZCS and via dehydrogenation on gallium species, diene cyclization on ZCS, and formation of cyclic diolefins and aromatics via hydrogen transfer on ZCS and via dehydrogenation on gallium species. The rate constants of various reaction steps are compared, and the contribution of gallium in formation of dienes and aromatics is estimated. It is shown that aromatics formation accelerates olefin conversion due to the olefin consumption, on one hand, and inhibits olefin conversion due to the partial blocking of the zeolite catalytic sites, on the other hand. Because of this, both the increase and the decrease in olefin conversion over GaHZSM-5 can be observed (in comparison with HZSM-5), depending on the feed olefin and on the reaction conditions.
TL;DR: In this paper, the adsorption and dehydrogenation of cyclohexane and benzene on Pt(111) and two ordered Sn/Pt(1) surface alloys using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopic (HREELS), and sticking coefficient measurements were studied.
Abstract: Cyclohexane conversion to benzene over bimetallic Pt catalysts is an important prototypical reaction for fundamental studies of selective catalytic dehydrogenation catalysis. We have studied the adsorption and dehydrogenation of cyclohexane and benzene on Pt(111) and two ordered Sn/Pt(111) surface alloys using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and sticking coefficient measurements. Vapor-depositing Sn on the Pt(111) surface and annealing gives a (2×2) or (√3×√3)R30 o LEED pattern, producing well-defined surfaces identified as the (111) face of Pt 3 Sn and a substitutional alloy of composition Pt 2 Sn
TL;DR: In this paper, different transition states for the Bronsted acid catalyzed reactions of alkanes involving carbonium ions as intermediates are proposed based on data of n-butane conversion over HZSM5.
Abstract: Based on data ofn-butane conversion over HZSM5, different transition states are proposed for the Bronsted acid catalyzed reactions of alkanes involving carbonium ions as intermediates. Hydrogen-deuterium exchange, dehydrogenation and cracking are proposed to proceed via pentacoordinated carbonium ions that are stabilized by the zeolite lattice.
TL;DR: The main industrial methods for the production of styrene and α-methylstyrene are the dehydrogenation of ethylbenzene and isopropylbenzenes using mixed oxide catalysts as discussed by the authors.
Abstract: The main industrial methods for the production of styrene and α-methylstyrene are the dehydrogenation of ethylbenzene and isopropylbenzene using mixed oxide catalysts: These reactions are endothermic and reversible, they take place at high temperatures (853–923 K), and their reversibility thermodynamically limits the yields of the products. Displacement of the equilibrium toward the formation of the vinylbenzenes is generally achieved by lowering the partial pressure of the reacting alkylbenzenes by diluting them in steam, heated to 973 K.
TL;DR: In this article, the role of modifying metals has been ascertained from initial rate and selectivity determinations, from initial selectivity determination, and from initial catalysts with and without metal-free catalysts.
TL;DR: In this paper, extended Huckel calculations were performed to study the rate-limiting step in the oxidative dehydrogenation of methanol as catalyzed by transition-metal oxides and polyoxometalates.
Abstract: Extended Huckel calculations were performed to study the rate-limiting step in the oxidative dehydrogenation of methanol as catalyzed by transition-metal oxides and polyoxometalates. The reaction was modeled as the transfer of hydrogen from a methyl adduct to the surface of a transition-metal oxide fragment whose valence electronic structure was shown to resemble those of the Keggin ions which are known to catalyze the reaction
TL;DR: Experimental observations of the effects of solvation on the dehydrogenation reaction of Mg+(H2O)n to produce MgOH+1 are presented for n ≥ 6 as mentioned in this paper.
Abstract: Experimental observations of the effects of solvation on the dehydrogenation reaction of Mg+(H2O)n to produce MgOH+(H2O)n−1 are presented for n≤6. The reaction is seen to occur spontaneously at room temperature for n≳4. Ligand switching reactions are used to show the Mg+–OH bonds are stronger than Mg+H2O bonds. The results show the energy required to lose an H atom decreases with the number of water molecules attached because the magnesium changes oxidation state and this results in stronger interactions with the water ligands. Ab initio calculations are used to explain these observations.
TL;DR: A submitted manuscript is the version of the article upon submission and before peer-review as mentioned in this paper, while a published version is the final layout of the paper including the volume, issue and page numbers.
Abstract: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.
TL;DR: In this article, a high-temperature catalytic membrane was used for the dehydrogenation of ethane to ethylene using a hightemperature membrane reactor under isothermal conditions, and the membrane used in this study was a thin layer of Pd-23 wt% Ag on porous Vycor glass.
TL;DR: In this paper, the acid sites of the aluminosilicate sample were investigated and the effect of the pretreatment temperature on the acid and dehydrogenating activity of the 30 Ga MFI sample was investigated.
TL;DR: In this article, it was shown that if a hydride ion is split off from the paraffin, then the transition state resembles the adsorbed carbonium ion and the reaction results in molecular hydrogen and in formation of the surface alkoxy group.
Abstract: HF-21G quantum-chemical analysis of the protolytic attack of acid protons in zeolites at the C-H bonds in methane and ethane indicated that the resulting transition states depend on the sign of the bond polarization. If a hydride ion is split off from the paraffin, then the transition state resembles the adsorbed carbonium ion and the reaction results in molecular hydrogen and in formation of the surface alkoxy group. The case, when a proton tends to split off from the paraffin, corresponds to the hetero-isotope exchange of paraffins with surface OH groups. This is a concerted acid-base reaction with a transition state different from adsorbed carbonium ion.
TL;DR: A catalytic membrane reactor with a tubular palladium-silver (PdAg) membrane sealed in the centre to separate in situ the hydrogen produced, was used to exceed equilibrium limitations in the dehydrogenation of methylcyclohexane as discussed by the authors.
TL;DR: In this paper, three vanadate phases, i.e., MgV2O6 (metavanadate), α-Mg2V 2O7 (pyrovana-date), and Mgs V2O8 (orthovanadate) have been successfully prepared with high purity by the citrate method at a relatively low temperature (550°C).
Abstract: Three magnesium vanadate phases, i.e., MgV2O6 (metavanadate), α-Mg2V2O7 (pyrovana-date) and Mgs V2O8 (orthovanadate), have been successfully prepared with high purity by the citrate method at a relatively low temperature (550°C). FT-IR, LRS, XRD and SEM techniques have been used to characterize these vanadate phases. The effect of calcination temperature has also been investigated. It was found that the particle size and morphology of the MgV2O6 phase, which is a function of calcination temperature, appear to have a strong effect on the infrared spectra. Furthermore, the catalytic properties of the three phases were examined in the oxidative dehydrogenation of propane. The propene selectivity follows the order: α-Mg2V2O7 > Mg3 Vg2O8 > MgV2O6, which is consistent with their redox properties. This fact suggests that there is some correlation between the catalytic and redox properties of these magnesium vanadate phases.
TL;DR: In this article, the formation of highly dispersed vanadyl species is indicated by Raman and XPS measurements, and a strong increase in V2O5 formation at 10 and 15 wt.-% V is shown.
Abstract: Vanadium oxide supported on amorphous aluminium phosphate (0.2–15 wt.-% V) has been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), surface area, Fourier transform (FT) Raman and FT-IR spectroscopy, and used for catalytic oxidation of propane, n-butane and toluene. At low vanadium concentrations, ⩽ 2 wt.-% V, formation of highly dispersed vanadyl species is indicated by Raman and XPS measurements. The vanadyl species is after dehydration/calcination characterized by a Raman band at 1031 cm−1, At 6 wt.-% V the amount of highly dispersed vanadyl species is lower and tetrahedral vanadium species and crystalline V2O5 are formed. The tetrahedral vanadium species is characterized by an infrared band at 929 cm−1 , and the amount appears to increase somewhat up to 15 wt.-% V. Both Raman and XRD indicate a strong increase in V2O5 formation at 10 and 15 wt.-% V. XRD also reveals partial crystallization of A1PO4 affected by high vanadium concentrations. FT-IR studies of carbon monoxide adsorbed on pretreated catalysts (oxidation (O2) at 500°C/ reduction (CO) at 500°C) indicated that vanadium species had reacted with aluminium and phosphorus surface hydroxyl groups, forming Al-O-bonded and P-O-bonded species. The latter is abundant at low vanadium loading, but after a more severe pretreatment involving oxidation at 800°C followed by reduction at 500°C, the Al-O bonded species is dominating. Further evidence for formation of Al-O-bonded and P-O-bonded vanadium species is the decrease of P-OH and Al-OH IR band intensities. The activities for oxidation of propane, n-butane and toluene increase with the vanadium loading. Acidic support sites show activity for dealkylation of both toluene and propane. Oxidative dehydrogenation of propane is best performed at 6 wt.-% V, which is associated with the presence of tetrahedral vanadium species. A lower selectivity at higher loading is associated with more active and less selective sites on V2O5.
TL;DR: In this paper, the adsorption and dehydrogenation of cyclohexene on Pt(111) and two Sn/Pt(1) surface alloys has been studied.
TL;DR: In this article, the structural decomposition of (NH4)3−xPMo12O40 heteropoly compounds was investigated at increasing temperatures to evaluate the thermal behavior, and was characterized by means of X-ray diffraction, FTIR spectroscopy, surface area measurements, ionic chromatography, EPR spectroscopic, and reactivity in the oxidative dehydrogenation of isobutyric acid to methacrylic acid.
TL;DR: The effect of rhenium and sulfur on the catalytic properties of platinum and on its ability to resist deactivation by carbonaceous deposits was studied in this paper, where the reactions of neopentane (2,2-dimethylpropane), n -hexane, and cyclohexane in excess H 2 2 were studied on model bimetallic catalysts prepared by the deposition of platinum on a Rhenium foil or vice versa.
TL;DR: In this paper, the authors studied the degradation of ethylbenzene to styrene over unpromoted and potassium-promoted iron catalysts using kinetic measurements and infrared spectroscopy.