The kinetics of ethylene hydrogenation catalyzed by metallic palladium

The activity of Pd(111) for ethylene hydrogenation is measured using a high-pressure reactor incorporated into an ultrahigh vacuum chamber for temperatures between 300 and 475 K, ethylene pressures between 50 and 300 Torr and hydrogen pressures from 45 to 600 Torr. The reaction rate is found to be rapid with turnover frequencies up to 400 reactions/site/s (where rates are referenced to the atom site density on the (111) face of palladium). The measured activation energy is 35 kJ/mol. A hydrogen reaction order of 1.02 was found at a reaction temperature of 300 K and an ethylene pressure of 100 Torr, where the hydrogen reaction order was found to depend on temperature. A negative reaction order of –0.22 was found in ethylene pressure at a reaction temperature of 320 K and a hydrogen pressure of 100 Torr. The reaction rates are in good agreement with values obtained on silica-supported palladium and with other work on palladium single crystals.     

Molecular surface studies of adsorption and catalytic reaction on crystal (Pt,Rh) surfaces under high pressure conditions (atmospheres) using sum frequency generation (SFG) – surface vibrational spectroscopy and scanning tunneling microscopy (STM)

Sum frequency generation (SFG) – surface vibrational spectroscopy and the scanning tunneling microscope (STM) have been used to study adsorption and catalyzed surface reactions at high pressures and temperatures using (111) crystal surfaces of platinum and rhodium. The two techniques and the reaction chambers that were constructed to make these studies possible are described. STM and SFG studies of CO at high pressures reveal the high mobility of metal atoms, metal surface reconstruction, ordering in the adsorbed molecular layer, and new binding states for the molecule. CO oxidation occurs at high turnover rates on Pt(111). Different adsorbed species are observed above and below the ignition temperature. Some inhibit the reaction, and others are reaction intermediates since their surface concentration is proportional to the reaction rate. The dehydrogenation of cyclohexene on Pt(100) and Pt(111) proceeds through a 1,3‐cyclohexadiene surface intermediate. The higher dehydrogenation rate is related to the higher surface concentration of these molecules on the (100) crystal face. This revised version was published online in August 2006 with corrections to the Cover Date.     

Surface photochemistry: 7. Synthesis on Pt(111) of surface ethyl groups and their rearrangement to ethylidyne

The vibrational spectra of photochemically produced adsorbed ethyl fragments and their thermal rearrangement products have been measured on Pt(111). Heating ethyl fragments, in the presence of adsorbed chlorine, leads to ethylidyne, but not directly; di--bonded ethylene is formed as a relatively stable intermediate.     

Electrothermal fluidized bed chlorination of zircon

Zircon sand, ZrSiO₄, is chlorinated in an electrothermally-heated fluidized bed reactor using calcined petroleum coke as the reducing agent and chlorine as the chlorinating agent. The electrothermally-heated fluidized bed reactor proved to be versatile and effective for the chlorination of zircon sand. The rate of the reaction can be expressed by the formula The reaction was found to be of zero order with respect to the chlorine concentration, which suggests that chlorine is strongly adsorbed on the solid particles; as such, the concentration of chlorine in the gas phase has no effect on the reaction rate. The activation energy for the reaction was found to be 10.55 Kcal/gram mole which is indicative of control by the rate of a surface reaction or adsorption step.     

Heterogeneous reductions of (homohypostrophene)dialkylplatinum(II) complexes provide a useful system for the study of intermediate surface alkyls on platinum [1]

This paper reports a comparison of the ethyl moieties generated on the surface of platinum black during the heterogeneous, platinum-catalyzed hydrogenations of (1,5-cyclooctadiene)diethylplatinum(II) ((COD)PtEt₂), of (homohypostrophene)diethylplatinum(II) ((HOP)PtEt₂), and of ethylene using D₂ in ethyl alcohol-d. In reductions of the platinum complexes, the -olefin and -alkyl organic ligands are converted to alkanes by reaction of the intermediate surface alkyls with surface hydrides, and the platinum(II) becomes part of the platinum(0) surface. These reductions are characterized by two kinetic regimes: one in which the rate of reaction is limited by the mass transport of hydrogen to the surface of the catalyst (the mass transport limited regime, MTL), and one in which the rate is limited by a reaction on the surface of the catalyst (the reaction rate limited regime, RRL). In the RRL regime, the isotopic compositions of the ethanes-d _n produced from the reductions of the platinum complexes and of ethylene suggest that the surface ethyls (Et *) generated from each precursor have similar relative rates of isotopic exchange (and thus of C-H bond activation) and of reductive elimination as ethane. In the MTL regime, the Et * derived from each precursor have different reactivities: the Et* generated from (HOP)PtEt₂ have reactivities that are more similar to the Et * generated from ethylene than do the Et * generated from (COD)PtEt₂.     

Zur Adsorptionspolymerisation auf porösen Feststoffen. 1. Kinetische Untersuchungen der Adsorptionspolymerisation auf der Oberfläche von porösem Silicagel

The polymerization of different monomers adsorbed from the vapour-phase onto the internal surface of porous silica containing vinyl groups has been investigated. The macromolecules were covalently bound to the inorganic solid. Polymerization was initiated by radicals generated by thermal decomposition of adsorbed AIBN at 60°C. Adsorption and simultaneous polymerization has been measured by a sensitive spring balance. The rate of polymerization in the adsorbed state is given by the equation: A strict first-order reaction related to the adsorbed monomer concentration has been obtained in the multimolecular adsorption range, while a nearly second order reaction could be measured below the monolayer capacity of silica. A formal kinetic scheme of the elementary polymerization steps in the adsorbed state including the reactive centers of the solid surface is proposed to explain the experimental results. The thermal degradation and the structure of porous silica containing polystyrene has been described.     

Comments on “Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces”

This letter is a re-examination of the ethylene oxide (EO) temperature programmed desorption (tpd) peak shapes (reported in references 1 and 2) which were obtained by tpd after having adsorbed EO at 250 K on to Ag(110) and Ag(111) surfaces. In these papers, the peaks were deemed to originate from a unimolecular rearrangement of an adsorbed oxametallacycle, producing EO, which desorbed on formation. Consequently, on the basis of this thesis, the activation energy of the composite process of internal rearrangement of the oxametallacycle and desorption of EO was determined by solution of the first order Redhead equation [3] at the peak maximum temperature, using an assumed value of 10¹³ s^?1 for the desorption pre-exponential term. The conclusions of this re-examination are that the m/z = 29 desorption peaks shown in Fig. 1 derive from a second order surface reaction of an adsorbed O atom and adsorbed ethylene molecule and that this is the rate determining step in the desorption of EO. An important corollary of these conclusions is that an oxametallacycle is not involved in the ethylene epoxidation reaction coordinate as the papers suggest.

Acetate formation and explosive decomposition during ethanol oxidation on Rh

The adsorption and reaction of ethanol with the Rh(110) surface has been studied using a thermal molecular beam system and temperature programmed desorption. On the clean surface, ethanol shows a very simple dehydrogenation, producing hydrogen in the gas phase, adsorbed CO (which is desorbed by heating to 550 K) and carbon. Since in alcohol synthesis reactions it is likely that the surface will be partially oxidised, the reaction with predosed oxygen was also investigated. The reaction pathway then becomes much more complex. The main changes are (i) CH₄ and H₂O evolution during adsorption, and (ii)Acetate formation by oxygen insertion in the molecule. The acetate shows very unusual decomposition kinetics — a surface explosion with a very narrow peak-yielding CO₂ and H₂ in the gas phase and adsorbed C. The acetate is always seen on Rh catalysts which are selective for alcohol synthesis from CO and H₂, and it is proposed that oxidic promoters such as vanadia may act to stabilise this intermediate.     

HREELS study: low-temperature reaction of NO with isolated carbon atoms adsorbed on Pt(111) surface

The reaction between isolated carbon atoms and nitrogen oxide molecules in the adlayer on Pt(111) surface has been studied. Carbon atoms have been deposited on the surface from the special source. The reaction was found to proceed atT 100 K and to provide, at least, two intermediate surface species, which have been assigned to adsorbed isocyanate NCO_ads and fulminate CNO_ads particles. Both intermediates dissociated into on-top state of CO_ads and N_ads under heating toT 300 K.     

Chlorine reduction on platinum and ruthenium: the effect of oxide coverage

The rate and mechanism of the electroreduction of chlorine on electrochemically oxidised Pt and Ru electrodes has been investigated relative to the state of oxide formation. Current/potential curves for the reduction process in 1 M HCl solution saturated with Cl₂ have been obtained for electrode surfaces in various states of preoxidation with the use of the rotating disc electrode technique (RDE). In the case of chlorine reduction on platinum, the results indicate that adsorption of chlorine molecules with a subsequent rate determining electrochemical adsorption step is the dominant mechanism. The exchange current density seems to decrease linearly with the logarithm of the amount of surface oxide.Chlorine reduction on ruthenium is best described by a Heyrovsky-Volmer mechanism with the first charge transfer reaction as the rate determining step. The Krishtalik mechanism incorporating adsorbed OCl⁺ intermediates is also able to describe the reaction successfully. The reaction order is constant for all oxide coverages while the exchange current density apparently moves through a maximum at intermediate oxide coverages (∼100 mC cm⁻²).The results show that the electrocatalysis of the cathodic reduction of chlorine is very sensitive to the state of the oxide film on the surface.     

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10¹ to 10^-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10² to 10⁰ nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.     

Recent advances in selective acetylene hydrogenation using palladium containing catalysts

Recent advances with Pd containing catalysts for the selective hydrogenation of acetylene are described. The overview classifies enhancement of catalytic properties for monometallic and bimetallic Pd catalysts. Activity/selectivity of Pd catalysts can be modified by controlling particle shape/morphology or immobilisation on a support which interacts strongly with Pd particles. In both cases enhanced ethylene selectivity is generally associated with modifying ethylene adsorption strength and/or changes to hydride formation. Inorganic and organic selectivity modifiers (i.e., species adsorbed onto Pd particle surface) have also been shown to enhance ethylene selectivity. Inorganic modifiers such as TiO₂ change Pd ensemble size and modify ethylene adsorption strength whereas organic modifiers such as diphenylsulfide are thought to create a surface template effect which favours acetylene adsorption with respect to ethylene. A number of metals and synthetic approaches have been explored to prepare Pd bimetallic catalysts. Examples where enhanced selectivity is observed are generally associated with decreased Pd ensemble size and/or hindering of the ease with which an unselective hydride phase is formed for Pd. A final class of bimetallic catalysts are discussed where Pd is not thought to be the primary reaction site but merely acts as a site where hydrogen dissociation and spillover occurs onto a second metal (Cu or Au) where the reaction takes place more selectively.

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Reactivity of Ethyl Groups on a Sn/Pt(111) Surface Alloy

In order to probe the thermal stability and reactivity of ethyl intermediates on Pt-Sn alloy catalysts, we have synthesized these species by reaction of H atoms with adsorbed ethylene on a ( ) R30°-Sn/Pt(111) surface alloy. Adsorbed ethyl groups are stable until 376 are they react to evolve ethane, ethylene, and H₂. The activation energy for ethyl dehydrogenation is E_dehyd* 97 kJ mol, which is twice that reported on Pt(111). In addition, we place a lower limit of E_dehyd* 97 kJ mol on the barrier to ethyl hydrogenation on this alloy.     

n-butane hydrogenolysis on planar and faceted Pt/W(111) surfaces

The Relationship between surface structure and reactivity is investigated by means ofn-butane hydrogenolysis, a known structure sensitive reaction, for planar and faceted Pt/ W(111) surfaces. The W(111) surface reconstructs to form pyramidal facets with [211] orientation upon vapor deposition of Pt (>1.3 ML) and annealing above 750 K. The hydrogenolysis kinetics over the planar and the faceted surface are found to be quite different. The planar surface has a higher selectivity towards ethane formation and a higher reaction rate. The apparent activation energies are found to be 33 ± 4 kJ/mol for the planar surface and 76 ± 6 kJ/mol for a surface covered with 20 nm facets. There appears to be a correlation between the concentration of fourfold coordination (C₄) sites on the surface and the amount of ethane produced. The C₄ concentration is altered by changing the facet size (annealing temperature). The results indicate the presence of a different intermediate on the C₄ sites as evidenced by the differences in the apparent activation energy, the reaction rate and the overall selectivity.     

A simple method of estimating surface reaction rates by moment analysis of tap reactor pulse experiments; application to benzene hydrogenation

A method of estimating the lifetime of surface reaction intermediates by moment analysis of TAP reactor pulse response curves has been developed. The average surface lifetime of the adsorbed reaction intermediate in the hydrogenation of benzene to cyclohexane over Pt/SiO₂ at 140°C and 92 kPa of hydrogen is 38 ± 4 ms, as measured by a TAP pulse experiment. The steady-state turnover frequency for this reaction under the same conditions is 1.0 ± 0.2 s^?l. This implies that the surface coverage of Pt by benzene is low under the steady-state conditions.     

Characterization of transient response curves in heterogeneous catalysis—II Estimation of the reaction mechanism in the oxidation of ethylene over a silver catalyst from the mode of the transient response curves

The reaction mechanism and the kinetic structure of the oxidation of ethylene over a silver catalyst at 91°C have roughly been elucidated from the mode of the transient response curves of ethylene oxide, carbon dioxide and water, due to the concentration jump of reactants. The typical overshoot mode with an instantaneous maximum for C₂H₄O clearly indicates two things. First that the surface reaction controlling in the reaction between adsorbed diatomic oxygen species and gaseous C₂H₄ (an Eley—Rideal type mechanism) with the rapid desorption of C₂H₄O formed and, second, that the slower regeneration of the adsorbed oxygen species. The typical S-shape mode with an overshoot for CO₂ strongly suggests the existence of an intermediate (In) and, the presence of the (In) has been actually confirmed by the transient response of its decomposition in an O₂-He stream. The (In) can desorb as acetic acid only when the surface is reduced by H₂. Much transient data seems to suggest that the (In) is formed from the reaction of adsorbed monoatomic oxygen species with C₂H₄ and, is decomposed by the reaction with diatomic oxygen species. The false start mode for the response curve of (In) decomposition indicates the inhibitory effect of adsorbed ethylene on the reaction.     

Marked Role of Carbon Monoxide in the Formation of Benzene During CH4-CO Reaction Over Rh/SiO2 Catalysts

Temperature-programmed desorption (He-TPD) and temperature-programmed reaction with hydrogen (H₂-TPR), carbon monoxide (CO-TPR) or methane (CH₄-TPR) were carried out to elucidate the benzene formation mechanism as well as the role of CO during CH₄-CO reaction over SiO₂-supported Rh catalysts. The steady-state surface for the CH₄-CO reaction was different from that of the CH₄ decomposition reaction. The existence of benzene-like adsorbed species as building blocks was demonstrated on the CH₄-CO reaction surface, while no such higher hydrocarbon adsorbed species was detected in the case of the CH₄ decomposition surface. On the contrary, in CO-TPR experiments various unsaturated hydrocarbons were released from the steady-state CH₄ decomposition surface, which was not the case from the CH₄-CO reaction surface. It is concluded that adsorbed CO may play an important role to enhance the C-C bond formation of carbonaceous species, which correlates deeply with the novel phenomenon of selective benzene formation in the CH₄-CO reaction.     

Poly(ethylene naphthalate) formation 1. transesterification of dimethylnaphthalate with ethylene glycol

The transesterification of dimethyl naphthalate (DMN) with ethylene glycol (EG) was kinetically investigated in the presence of various catalysts at 185 °C. The transesterification was assumed to obey first-order kinetics with respect to DMN and EG, and a rate equation was derived. The rate constant of transesterification which calculated from the quantity of methanol distilled from the reaction vessel was used to evaluate each metal compound in its activity. The first-order dependence on the catalyst concentration is valid below a critical concentration which was found to be dependent on the catalyst type. The order of decreasing catalytic activity of various metal ions was found to be: Pb Zn > Co > Mg > Ni Sb. But in the case of highly basic metal salts, the rate constants were found to be extremely large at the initial stage of the reaction, and then rapidly decreased with the progress of the reaction. Effects of reaction temperature were also discussed. The activation energies for zinc acetate and lead acetate were 97.84 and 97.2 KJ/mol, respectively, which were calculated from Arrhenius equation.     

载体硅胶表面氟改性对Phillips铬系催化剂诱导期内引发乙烯聚合行为的影响

通过密度泛函理论考察了载体表面氟改性对吸附不同数目甲醛分子的Phillips催化剂诱导期内引发乙烯聚合反应的影响。结果表明,当乙烯还原六价铬酸酯形成的二价铬前驱体模型上吸附两分子甲醛时,其位阻效应阻碍了任何反应的进一步发生;当二价铬前驱体模型上吸附一分子甲醛时,只能通过先形成铬金属五元环进而发生乙烯二聚反应和易位反应,但是氟改性对两者的影响很小;当二价铬前驱体模型上吸附的甲醛分子完全脱附后,则可以进一步环增长生成铬金属七元环,并且氟改性对这一步反应有促进作用;而氟改性对铬金属七元环进一步开环生成1-己烯则是不利的。研究还表明,氟改性对于三价铬-烷基聚合活性中心模型上的链增长是有利的。