Structure-insensitive Reaction Research Articles

Activity and structure-sensitivity of the water-gas shift reaction over Cu [sbnd]Zn [sbnd]Al mixed oxide catalysts

The activity and structure-sensitivity of the water-gas shift (WGS) reaction over Cu Zn Al mixed oxide catalysts were studied. Three sets of samples with different Cu/Zn and (Cu+Zn)/Al atomic ratios were prepared by coprecipitation. Depending on the cation ratio, the ternary hydroxycarbonate precursors contained hydrotalcite, aurichalcite and/or rosasite phases. The decomposed precursors contained CuO, ZnO, ZnAl 2O 4, and Al 2O 3. The relative proportion of these phases depended on both the chemical composition of the sample and the calcination temperature employed for decomposing the precursor. After activation with hydrogen, samples were tested for the WGS reaction at 503 K. The turnover frequency of the eighteen samples tested was essentially the same (0.2–0.3 s −1) irrespective of changing the copper metal surface area between 3 and 35 m 2/g Cu and the metallic copper dispersion between 0.5 and 5.0%. This indicated that the WGS reaction is a structure-insensitive reaction, as the specific reaction rate r 0 (mol CO/h/g Cu) is always proportional to the copper metal surface area. Preparation of mixed oxides with a high copper dispersion is therefore required for obtaining more active catalysts. It was found that the value of the metallic copper dispersion is related to the amount of hydrotalcite contained in the hydroxycarbonate precursor: the higher the hydrotalcite content in the precursor, the higher the copper metal dispersion in the resulting catalyst and, as a consequence, the higher the catalyst activity. Ternary Cu/ZnO/Al 2O 3 catalysts exhibited a substantially faster WGS activity than binary Cu/ZnO catalysts. The addition of aluminium, although inactive for the WGS reaction, is required for improving the catalyst performance.

The Effect of Pretreatment on Pd/C Catalysts: II. Catalytic Behavior

Pd catalysts were anaerobically prepared using a Pd acetylacetonate precursor and a high surface area turbostratic carbon black which had been pretreated to change its surface functionalities and remove sulfur impurities. These Pd/C catalysts exhibited suppressed chemisorption and hydride formation, which was attributed to the presence of carbon atoms on the surface and in the bulk of Pd particles. The effect of these carbon atoms on three probe reactions-benzene hydrogenation, CO hydrogenation, and CO oxidation-was examined. If sulfur were not removed from the carbon, it contaminated the Pd surface and caused a drastic decrease in benzene hydrogenation activity and a decrease in both activity and activation energy in CO hydrogenation. Coverage of the Pd surface by carbon atoms had different effects on the three probe reactions. The turnover frequency (TOF) in benzene hydrogenation, a structure-insensitive reaction, was not affected for Pd supported on the clean carbon blacks, although specific activity was lower than expected based on crystallite size, and the apparent activation energies (8.5-10 kcal/mole) were slightly lower than anticipated. In contrast, the TOFs for methanation were substantially decreased, and the apparent activation energies varied over a wide range (5-30 kcal/mole). Under an oxidizing environment, these Pd/C catalysts had TOFs for CO oxidation similar to or higher than those for Pd/Al2O3 catalysts, and similar activation energies near 24 kcal/mole were obtained, thus indicating that the Pd surfaces present were alike.

Transformations of n-heptane over Pt/activated carbon catalysts

A series of carbon-supported platinum catalysts with different metallic dispersions (from 0.05 to 0.68) was tested for the n-heptane aromatization reaction. The influence of a basic function (MgO) on the catalytic behaviour was also examined. It is found that, as the metal particle size decreases, the turnover frequency for hydrogenolysis increases, while those for isomerization and aromatization remain unchanged. Moreover, the selectivities for isomerization and hydrogenolysis change with the metal dispersion while that for aromatization remains constant. In addition, no modification in catalytic properties is observed by adding base sites to a Pt/carbon catalyst. These results suggest that aromatization is a structure-insensitive reaction and that it is independent of the other reactions.

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H2 was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H2 on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.

Structure insensitivity: Application of the surface electronic gas model

Structure sensitive and insensitive reactions with nonuniformly reactive adsorbates are discussed in terms of the surface electronic gas model using the Polanyi parameter (α). For the liquid-phase hydrogenation it was demonstrated, that the reaction is structure insensitive whenα is equal to unity, which in terms of the surface electronic gas model means that effective charges of adsorbed species and activated complexes in transition state are equal.

Recently published work on EUROPT-1, a 6% Pt/SiO 2 reference catalyst

EUROPT-1 is a 6% Pt/SiO 2 catalyst which has been thoroughly characterised with respect to its composition, structure, chemisorption behaviour and catalytic properties. Extensive further work on this catalyst has been published and this is now reviewed. Debye function analysis of X-ray diffraction spectra suggest platinum particles to be cubooctahedral, although preliminary extended X-ray absorption fine structure results do not accord with this model. Further isotherms for carbon monoxide, hydrogen and oxygen chemisorption have been reported and enthalpies of adsorption for the last two gases measured. Limited results on structure-insensitive reactions such as exchange of methane with deuterium and alkene hydrogenation are available. Methane homologation as well as skeletal reactions of alkanes in the presence of hydrogen represent structure-sensitive reactions. Some further work on ethane, propane and n-butane is reported; detailed results on n-hexane and methylcyclopentane include changes in rates and selectivities as a function of hydrogen and hydrocarbon pressure. They are interpreted in terms of availability of surface hydrogen and hydrogen effects on the formation of carbonaceous deposits of various degree of dehydrogenation. Effects of sintering have also been studied, and active centres for various reactions have been tentatively suggested. On the whole, EUROPT-1 shows behaviour which is characteristic of platinum catalysts containing small platinum particles but in some limited respects it shows unusual and unexpected properties. Reactions of some oxygenated compounds are described briefly. Catalytic properties of EUROPT-1 can be modified by metals, oxides, sulphur and nitrogen; treatment with cinchonidine converts it into an enantioselective catalyst for the hydrogenation of methyl pyruvate. Finally the importance of selecting appropriate reference supported metal catalyst (s) is stressed and the future role for EUROPT-1 as a possible candidate is pointed out; results must however be compared under carefully defined conditions if they are to have real value.

Effect of catalyst preparation on NiAl 2O 3 poisoning by thiophene

The influence of experimental conditions such as the temperature of calcination and pelleting pressure on NiAl 2O 3 deactivation by thiophene in benzene hydrogenation is outlined and discussed. It was found that benzene hydrogenation over unpoisoned nickel catalyst is a structure-insensitive reaction. However, its inhibition by thiophene produced structure sensitivity: the reaction rate was less sensitive to thiophene for large nickel crystallites (formed during catalyst calcination at high temperatures followed by reduction), than for small crystallites. Thus, the structure sensitivity of poison adsorption imparted an apparent structure sensitivity to the main reaction in the presence of a poison in the feedstream (secondary structure sensitivity). The pore structure of the catalyst pellet directly influences the degree of diffusion resistance of both the main and the poisoning reactions. It was found that higher mass transfer resistances increase the life-time of the pellets while a lower mass transfer resistance increases the overall effectiveness factor.

Oxidation of CO by oxygen on a stepped platinum surface: Identification of the reaction site

The coadsorption of oxygen and carbon monoxide on the stepped Pt(112) surface has been studied using electron stimulated desorption–ion angular distribution (ESDIAD), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). It has been possible to preferentially adsorb different isotopic CO molecules on step and terrace sites, respectively, following oxygen adsorption on step sites to partial coverage. Transient kinetic experiments show that below ∼200 K, isotopic CO present exclusively on terrace sites is more effectively involved in CO2 production, compared to less reactive CO on the step sites. Above ∼200 K, site exchange between step and terrace CO species prevents the measurement of the relative reactivity of the two kinds of chemisorbed CO. The results show that the elementary step producing CO2 from adsorbed CO and adsorbed oxygen is structure sensitive, even though the overall catalytic reaction between CO and O2 is generally classed as a structure insensitive reaction.

Catalytic properties of new Cu based catalysts containing Zr and/or V for methanol synthesis from a carbon dioxide and hydrogen mixture

Pure unsupported copper is a poor catalyst for CH3OH synthesis from CO2+H2 when it is compared to Cu coprecipitated with Zr or Zr+V in which case its selectivity and yield in methanol are strongly enhanced. The two Zr and V components added to pure Cu are shown to be textural promoters towards zerovalent Cu which participate to the building of the active sites. Methanol formation on these catalysts is a structure insensitive reaction with respect to the metallic Cu dosed by N2O surface decomposition.

Slurry-phase hydrogenation of aromatic compounds over supported noble metal catalysts

The hydrogenation of benzene, naphthalene and biphenyl over monometallic (Pt, Rh, Ru) and bimetallic (Pt, Rh, Ru-Pd, Ir, Re ) catalysts supported on γ-Al 2O 3 or TiO 2 is investigated in a three-phase slurry reactor at a temperature of 300 °C and a hydrogen pressure of 66 atm. Under the conditions employed in this study, of the primary metals investigated platinum was found to possess the highest specific hydrogenation activity while biphenyl was found to be the most reactive molecule. The hydrogenation of benzene over platinum and naphthalene over platinum and rhodium were found to be structure insensitive reactions. In most cases, bimetallic formulations were found to exhibit significantly higher hydrogenation activity as compared to their monometallic counterparts. The enhancement factor was found to be larger when the bimetallic formations were supported on TiO 2 as opposed to Al 2O 3 carriers. Bimetallic catalysts, especially those containing rhenium were also found to exhibit enhanced resistance to sulfur poisoning.

Adsorbate bonding and the selection of partial and total oxidation pathways

The oxidation reaction of ethane has been studied on a series of well-characterized samples of vanadium oxide supported on silica. At low concentrations the vanadium species are isolated vanadyl groups, while at high concentrations they form small V 2O 5 crystallites. Oxygen chemisorption is used to measure the number of surface vanadium atoms on both types of species. At low conversions the oxidation of ethane is selective, producing mostly ethylene and acetaldehyde via structure-insensitive reactions. Intermediates for these are probably bonded to vanadium through an oxygen atom. In contrast the CO x reaction is structure-sensitive, and probably involves intermediates bonded directly to vanadium.

Adsorption and catalytic properties of Pd/SiO2, Cu/SiO2, and Pd-Cu/SiO2 systems : III. Carbon monoxide and benzene hydrogenation over Pd-Cu/SiO2 catalysts

The addition of copper to a well dispersed 2.48% Pd/SiO 2 catalyst gave reasonably homogeneous, stable bimetallic systems under mild reaction conditions. Benzene hydrogenation exhibited a relatively constant activation energy around 11.5 kcal mol -1 (1 cal=4.1868 J) and a reversible activity maximum with temperature which shifted to higher values with increasing copper content. The turnover frequency (TOF) decreased non-linearly with copper content and implied the active site was an ensemble of three surface palladium atoms. Activation energies for methane did not vary with copper content when palladium was present, and TOFs at 0.1 MPa changed little as copper was added although a weak maximum was obtained with 42% Cu. This behavior showed that methanation is a structure-insensitive reaction over palladium. At 1.5 MPa, another weak maximum was observed for methane formation and a sharper maximum for methanol synthesis was obtained near a composition of 59% Cu. Selectivity to methanol increased wit hincreasing copper content. At 1.5 MPa and 548K these SiO 2-supported catalysts sintered significantly during the first 24 h on stream. This tended to increase methanol selectivity, but it made the determination of actual metal surface compositions much more difficult.

Catalytic hydrogenation of cyclohexene: liquid-phase reaction on rhodium

The turnover rate vt for the liquid-phase hydrogenation of cyclohexene on Al2O3-supported clusters of rhodium containing ca. 12 atoms was found to be the same as that on supported rhodium clusters containing 150 atoms or more. The smallest rhodium clusters were examined by chemisorption of H2 and CO, by infrared spectroscopy of adsorbed CO, and by extended X-ray absorption fine structure, which indicated an average coordination number of five for the first shell. The results show that ligand effects for metallic clusters containing only a few atoms have no observable effect on the measured rate of the structure-insensitive reaction under study.

Effect of structure in selective oxide catalysis: oxidation reactions of ethanol and ethane on vanadium oxide

The catalytic activity of well-characterized samples of vanadium oxide supported on silica were studied for the partial oxidation of ethanol and ethane. Surface vanadium atoms were counted by a new oxygen chemisorption technique which allowed measurements of sample dispersions and turnover rates. Ethanol oxidation was found to be a structure-insensitive reaction that produced mostly acetaldehyde. Ethane oxidation was different; while its overall turnover rate changed little with vanadium oxide dispersion, its selectivity did vary with catalyst structure. The products of this reaction were mostly ethylene and acetaldehyde, but at low temperatures on the highest dispersion catalysts there was excess carbon dioxide formation

Liquid phase hydrogenation of benzonitrile over pt and pt-sn catalysts

Hydrogenation of benzonitrile has been studied over supported Pt and Pt-Sn catalysts. The effect of platinum particle size on the rate of Hydrogenation has been also investigated. Dibenzylamine is preferentially formed on all catalyst samples with a selectivity which is little influenced by the level of conversion, platinum particle size and Pt/Sn ratio. In the range of experimental conditions used, the rate of reaction does not depend on the platinum particle size and therefore the Hydrogenation of benzonitrile can be considered a structure-insensitive reaction. Addition of tin ions to platinum leads to an increase in the rate of reaction at lower tin content, whereas a decrease in rate is observed at the highest Sn/Pt ratios. It is suggested that the presence of tin ions enhances the reactivity of the CN group.

Modeling of heterogeneous reaction kinetics: A stochastic approach

A stochastic model representing the state of the catalyst surface is developed for several possible reaction mechanisms. With the pseudo-steady state assumption, the fresh catalyst solution was used for the initial value of deactivating system The deactivation functions obtained from the present study shows a good agreement with the model proposed by Nam and Froment[1]. When the reaction requires more than one site, the reaction becomes structure sensitive. However, for a single site reaction requiring one site only, the reaction belongs to the class of structure-insensitive reaction Deactivation reactions can also be classified by this manner.

Effect of promoters on benzene hydrogenation over fused iron catalysts

The hydrogenation of benzene to cyclohexane is a structure-insensitive reaction which, at atmospheric pressure, is thermodynamically favoured at temperatures below 550 K. The Group VIIIA metals are the most active catalysts for this reaction; studies prior to the 1960s showed zero (or very low) activity for iron. Badilla-Ohlbaum et al. reported the first thorough kinetic study of the reaction over a 7.95 mass% Al{sub 2}O{sub 3}-promoted fused iron catalyst; a rate maximum was observed at 443 K and the authors suggested that the rate-determining step was the surface reaction of adsorbed benzene with adsorbed hydrogen. Vannice and co-workers have reported the activities of the kinetic data over a series of supported iron catalysts. Rate maxima were observed near 473 K; activities were roughly one order of magnitude less than that reported by Badilla-Ohlbaum et al. under similar conditions. While it is apparent that the catalytic activity of iron for this reaction is too low to be of industrial significance, it was of interest to know to what extent the difference in the rates is due to chemical promotion. To this end, the activities of four multiply promoted ammonia synthesis catalysts were compared with that of a precipitated iron in anmore » effort to determine the effect of acidic and basic promoters on the reaction rate.« less

The role of adsorbate overlayers in thiophene hydrodesulfurization over molybdenum and rhenium single crystals

Thiophene hydrodesulfurization (HDS) has been investigated over model single crystal catalysts of molybdenum and rhenium. Thiophene HDS is a structure sensitive reaction over rhenium and a structure insensitive reaction over molybdenum. Adsorbed sulfur decreases the activity of both Re(0001) and Mo(100) surfaces while adsorbed carbon has distinctly different effects on the catalytic properties of the two metals. Carbon overlayers deactivate the Re(0001) surface but have no effect on the HDS activity of the Mo(100) surface. Radiotracer35S and14C studies indicate that HDS occurs on an adsorbate overlayer on molybdenum comprised primarily of carbon. HDS over rhenium, on the other hand, occurs on the bare metal surface. This appears to be responsible for the observed differences in the influence of surface structure on HDS activity.

Resistance to sulfur poisoning of metal catalysts: Dehydrogenation of cyclohexane on [formula omitted] catalysts

Model Pt catalysts, containing ~0.6 wt% Pt, were prepared by deposition of dinitrodiamino-platinum salt on a nonmicroporous alumina. The carrier has been modified by addition of K or Cl. The dispersion of the metallic phase was varied through different decomposition-reduction processes, but the final activation conditions were identical in all cases. The various catalysts have been tested in the dehydrogenation of cyclohexane which was confirmed to be a structure-insensitive reaction. The thiotolerance level of the various catalysts was measured with a feed contaminated with 0.6 ppm of thiophene. The thiotolerance level is more affected by the acidity of the support (presence of K or Cl) than by variations of Pt particle size. This has been confirmed by making sudden injections of CH 2Cl 2 or (C 2H 5) 3N into the feed. On removing thiophene from the reactant mixture partial decontamination occurred and the activity recovered to ~50% of the initial value.

Structure sensitivity of catalytic reactions

The rate and product distribution of chemical reactions catalyzed by metals can depend on the particle size of supported catalysts. This can be explained by the interaction between chemisorbed species and surface sites of different configuration in the metal associated with atoms in terrace, steps and kinks. Single-crystal catalysts are excellent models for studying these interactions. In this review, the contribution of single-crystal studies to the understanding of structure sensitivity in catalytic reaction is discussed. Reactions are classified in three types. Structure sensitive reactions, structure insensitive reactions and reactions that show both behaviors, depending on the experimental conditions. Several examples are discussed in each of the three categories. 72 refs., 13 figs.