Different Types Of Active Sites Research Articles

Selectivity effects related to the diversity of active sites operating on nanostructured multifunctional catalysts

The goal of the present study was to gain a better understanding of the selectivity of processes over multifunctional catalysts exhibiting diversity of operating active sites. The concept is that the concurrent performance of different types of active sites may provoke effects on the process selectivity comparable to the effects resulting from the kinetic regularities and activation energies of the occurring reactions. Accordingly, in the kinetic model the authors introduce specific parameters reflecting the contribution of distinct types of active sites, facilitating different reaction routes. Reasons are adduced how suchlike parameters serve to account the impact of various reaction routes occurring on different types of sites. The suggested approach links the deactivation-caused selectivity changes to dissimilarities in the vulnerability of different types of active sites. This work relates the probabilities for action of different types of sites to the size of active-phase islands. Various reaction mechanism patterns are modeled to examine relevant selectivity effects.

Reactions of 2-octanol on Amberlyst® 15: concurrent Eley–Rideal and Langmuir–Hinshelwood reactions on two different types of active sites

On Amberlyst® 15 (dry) at 75 °C, 2-octanol undergoes concurrent esterification and dehydration in the presence of heptanoic acid in the liquid phase. Kinetic data and 18O labelling studies show that the esterification reaction most likely occurs by an Eley–Rideal (ER) mechanism between adsorbed carboxylic acid and bulk phase alcohol, while the dehydration reaction occurs by a Langmuir–Hinshelwood (LH) mechanism requiring interaction of adsorbed alcohol with a vacant site. The two reactions occur on two different types of catalytic sites: while all the sites are sulfonic acid groups, kinetic data suggest that not all of the acidic sites are positioned appropriately to a neighboring site to assist with dehydration. A kinetic model fits the observed data for the simultaneous ER and LH reactions over at least 23 h of observation.

Evidences of the presence of different types of active sites for the oxygen reduction reaction with Fe/N/C based catalysts

Abstract Fe/N/C catalysts are very active for the oxygen reduction reaction (ORR); however, the nature of the active site(s) is not fully understood. In this work, we study the performance of different types of N/C and Fe/N/C catalysts for the ORR, and the effect of the addition of NaCN. Phthalocyanine and graphene have been studied as model metal-free catalysts for the ORR. Fe-phthalocyanine (FePhcy), Fe-phthalocyanine dispersed in graphene (FePhcy/G) and Fe/N/G, have been used as model Fe-containing catalysts. FePhcy and FePhcy/G only contain Fe atoms coordinated to 4 nitrogen atoms. On the other hand, different species such as Fe-Nx and Fe3C coexist in Fe/N/G. In addition, C C ensembles are present in the graphene present in FePhcy/G and Fe/N/G. The ORR activity is characteristic of each catalyst, being the highest for the catalysts containing FeN4 ensembles. The addition of CN− results in the selective poisoning of the Fe-containing sites but it does not suppress the ORR activity of the Graphene containing samples. In-situ infrared spectroscopy studies during the ORR reveal that CN− poisoning of the Fe sites is reversible, desorbing at potentials less positive than ca. 600 mV. As a consequence, the ORR activity of the Fe-containing sites is recovered gradually.

Synthesis and Structural Analysis of Double Layered Ni-Mg-Al Hydrotalcite Like Catalyst

Hydrotalcites are double layered lamellar clays. It has high surface area and different types of active sites can be fused on its surface this make hydrotalcites an ideal structure for hetero generous catalytic reactions. In this article synthesis of Ni-Mg-Al hydrotalcite by coprecipitation is discussed alongwith its characterization. The hydrotalcite formation is verified by powder XRD. FTIR and BET characterization is carried out to understand the structural characteristics of the material and its possible use as a catalyst. High surface area with large porosity and a consistent atomic percentages was measured during these analysis.

KMAD: knowledge-based multiple sequence alignment for intrinsically disordered proteins.

Summary: Intrinsically disordered proteins (IDPs) lack tertiary structure and thus differ from globular proteins in terms of their sequence–structure–function relations. IDPs have lower sequence conservation, different types of active sites and a different distribution of functionally important regions, which altogether make their multiple sequence alignment (MSA) difficult. The KMAD MSA software has been written specifically for the alignment and annotation of IDPs. It augments the substitution matrix with knowledge about post-translational modifications, functional domains and short linear motifs.Results: MSAs produced with KMAD describe well-conserved features among IDPs, tend to agree well with biological intuition, and are a good basis for designing new experiments to shed light on this large, understudied class of proteins.Availability and implementation: KMAD web server is accessible at http://www.cmbi.ru.nl/kmad/. A standalone version is freely available.Contact: vriend@cmbi.ru.nl

Superresolution fluorescence mapping of single-nanoparticle catalysts reveals spatiotemporal variations in surface reactivity

For the practical application of nanocatalysts, it is desirable to understand the spatiotemporal fluctuations of nanocatalytic activity at the single-nanoparticle level. Here we use time-lapsed superresolution mapping of single-molecule catalysis events on individual nanoparticles to observe time-varying changes in the spatial distribution of catalysis events on Sb-doped TiO2 nanorods and Au triangle nanoplates. Compared with the active sites on well-defined surface facets, the defects of the nanoparticle catalysts possess higher intrinsic reactivity but lower stability. Corners and ends are more reactive but also less stable than flat surfaces. Averaged over time, the most stable sites dominate the total apparent activity of single nanocatalysts. However, the active sites with higher intrinsic activity but lower stability show activity at earlier time points before deactivating. Unexpectedly, some active sites are found to recover their activity ("self-healing") after deactivation, which is probably due to desorption of the adsorbate. Our superresolution measurement of different types of active catalytic sites, over both space and time, leads to a more comprehensive understanding of reactivity patterns and may enable the design of new and more productive heterogeneous catalysts.

Multifunctionality of Crystalline MoV(TeNb) M1 Oxide Catalysts in Selective Oxidation of Propane and Benzyl Alcohol

Propane oxidation at 653–673 K and benzyl alcohol oxidation at 393 K over phase-pure MoV(TeNb) M1 oxide catalysts were studied to gain insight into the multiple catalytic functions of the surface of the M1 structure. Electron microscopy and X-ray diffraction confirmed the phase purity of the M1 catalysts. Propane oxidation yields acrylic acid via propene as intermediate, while benzyl alcohol oxidation gives benzaldehyde, benzoic acid, benzyl benzoate, and toluene. The consumption rates of benzyl alcohol and propane level in the same range despite huge difference in reaction temperature, suggesting high activity of M1 for alcohol oxidation. Metal–oxygen sites on the M1 surface are responsible for the conversion of the two reactants. However, different types of active sites and reaction mechanisms may be involved. Omitting Te and Nb from the M1 framework eliminates acrylic acid selectivity in propane oxidation, while the product distribution in benzyl alcohol oxidation remains unchanged. The results suggest...

Surface reaction of 2‐propanol on modified Keggin type polyoxometalates: In situ IR spectroscopic investigation of the surface acid–base properties

AbstractIn order to investigate the influence of modified Keggin‐polyoxometalates and supported gold particles on reactivity and reaction pathways in thermal 2‐propanol oxidation, titanium‐substituted, insoluble Cs+‐salts of the Keggin‐heteropolytungstate (H3PW12O40) Cs3PW12O40 1, Cs5PW11TiO40 · nH2O 2, and supported gold composite Au/Cs5PW11TiO40 · nH2O 3 were synthesized and characterized as molecular model catalysts. The Ti‐substituted polyoxometalate 2 did not show any observable activity in the gas phase oxidation of 2‐propanol. The presence of gold nano‐particles activates the supporting polyoxometalate 3 and enhances formation of different oxidation products (acetone, diisopropyl ether) depending on the presence of different types of active sites. The formation of the diisopropyl ether involves two adjacent 2‐propanol molecules adsorbed on two different types of active sites, whereas the dehydrogenation reaction for the acetone formation involves the initial adsorption of 2‐propanol on one type of active site.

Kinetic Study of Propylene Hydrogenation over Pt/Al2O3 by Parahydrogen-Induced Polarization

Parahydrogen-induced polarization has been successfully used for a kinetic study of propylene hydrogenation over a Pt/Al2O3 catalyst. It was shown that the reaction orders with respect to hydrogen are different for the pairwise and the non-pairwise hydrogen addition and are equal to 0.7 and 0.1, respectively. This observation of different reaction orders confirms the coexistence of different types of active sites which are responsible for the overall and the pairwise hydrogen addition to the propylene C=C double bond. Moreover, 0.7 reaction order with respect to H2 for pairwise hydrogen addition indicates that the contribution of pairwise addition depends on the concentration of molecular hydrogen. Therefore, this observation can be developed into a practical tool for producing fluids with highly polarized nuclear spins by changing the hydrogen concentration.

ChemInform Abstract: Genesis of Highly Active Cobalt Sulfide Catalyst for Hydrogenation and Isomerization: Effect of the Calcination Temperature of Precursor Cobalt Oxide Upon the Catalytic Properties of Cobalt Sulfide.

It was found that the catalytic activities and selectivities of cobalt sulfide catalysts are significantly affected by the calcination temperature of precursor cobalt oxide for the hydrogenation of butadiene and the isomerization of 1-butene. When cobalt sulfide was prepared from cobalt oxide calcined at 1473–1573 K, the specific activities per surface area of the sulfide were enhanced by 30–40 times those of cobalt sulfide prepared from cobalt oxide calcined at a conventional temperature, 573–973 K. The reaction selectivities were also strongly modified by the calcination temperature of precursor oxide. It is suggested that two different types of active sites are formed on cobalt sulfide, depending on the calcination temperature of the precursor. On the basis of XPS, XRD, SEM, and NO adsorption techniques, it is concluded that the particle size of the precursor cobalt oxide and sulfiding conditions determine the surface sulfur content and hence the catalytic properties of the sulfide catalysts. The sulfi...

Multiple active site Monte Carlo model for heterogeneous Ziegler‐Natta propylene polymerization

AbstractIn this study, the kinetics of propylene polymerization catalyzed with the fourth heterogeneous Ziegler‐Natta catalyst is studied. More than one type of active site is present in the propylene polymerization based on an analysis of the GPC curves. A multiple active site kinetic model (MSmodel) is proposed by using Monte Carlo technique. Good agreements in the polymerization kinetics are achieved for fitting the kinetic profiles with the MSmodel. In addition, the MSmodel is used to describe the dynamic evolutions of the active sites and their effects on the propylene polymerization. The simulated results indicate that different types of active sites have different polymerization kinetics and the site type can affect the propylene polymerization kinetics. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Chlorotitanium (IV) tetradentate Schiff‐base complex immobilized on inorganic supports: Support type and other factors having effect on ethylene polymerization activity

AbstractA titanium complex with [O,N,N,O]‐type tetradentate Schiff base (LTiCl2), never used before in polymerization of olefins, was immobilized on silica‐ and magnesium‐type carriers, and it was used in ethylene polymerization. The conducted research revealed that the catalytic properties of the complex LTiCl2 supported on those carriers were different for both the catalytic systems studied, and simultaneously they turned out different from those of the unsupported system. The supported catalysts require the use of Me3Al, Et3Al, or MAO as the activator to be able to offer high catalytic activities, whereas Et2AlCl is needed for the nonsupported catalyst. This finding, together with considerable changes in polymerization yields and in properties of polymers versus composition of the catalytic system, suggest that there are different types of active sites in the studied catalysts. The catalyst anchored on the carrier produced in the reaction of MgCl2·3.4EtOH with Et2AlCl is definitely the most active one within the support systems tested. Its activity remarkably increases with the increasing reaction temperature. Moreover, that catalyst does not undergo deactivation over the studied period of time, irrespective of the type of the activator used and of the process temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4811–4821, 2009

Ethylenebis(5‐chlorosalicylideneiminato)vanadium dichloride immobilized on MgCl2‐based supports as a highly effective precursor for ethylene polymerization

AbstractEthylenebis(5‐chlorosalicylideneiminato)vanadium dichloride supported on MgCl2(THF)2 or on the same carrier modified by EtnAlCl3−n, where n = 1–3, was used in ethylene polymerization in the presence of MAO or a common alkylaluminium compounds as a cocatalyst. The support type alter vanadium loading and also change the characteristic of the catalytic active sites. Et2AlCl is the best activator for a catalyst which has been immobilized on a nonmodified support, whereas the systems which contain a carrier which has been modified by an organoaluminium compound reveal the highest activity in conjunction with MAO. That difference, together with different temperature effects on polymerization efficiency (i.e., decrease and increase of catalytic activity for increasing temperatures, respectively) suggest the formation of different types of active sites in the catalytic systems supported on modified and nonmodified magnesium carrier. However, all supported precatalysts possess a long lifetime, still being active towards ethylene polymerization after 2 h. All the systems yield wide MWD polyethylene, while bimodal MWD is found for some part of analyzed samples. Polyethylene with bimodal particle size distribution is formed with the system which contain modified carriers at higher temperatures. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3480–3489, 2009

Electrooxidation of hydrazine catalyzed by noncovalently functionalized single-walled carbon nanotubes with CoPc

We report on the electrooxidation of hydrazine catalyzed by single-walled carbon nanotube (SWCNT) functionalized with cobalt phthalocyanine (CoPc) which shows that the presence of the single-walled carbon nanotubes enhances the catalytic activity of the CoPc itself without any change in the reaction mechanism. A synergistic effect, in terms of reactivity when the new nanocomposite material was adsorbed on the GC electrode, was observed. The obtained hybrid electrodes were tested under hydrodynamic conditions, showing two different oxidation processes, which suggest the presence of two different types of active sites on the electrode surface catalyzing the reaction. Electrochemical impedance spectroscopy (EIS) analyses in the presence of [Fe(CN) 6] 3−/4− as a redox probe revealed that the GC/SWCNT + CoPc showed much lower electron-resistance ( R et) confirming the synergistic effect of the composite mentioned above. Atomic force microscopy (AFM) images showed the clear differences in surface roughness for each film, confirming the different compositions of the hybrid electrodes used in this study.

Characterization of Ethylene‐1‐Hexene Copolymers Made with Supported Metallocene Catalysts: Influence of Support Type

AbstractSummary: It is known that the nature of the support, as well as the technique used to anchor the metallocene onto it, play important roles on catalytic activity and on the properties of the polymers produced with supported metallocenes. In the present work, the effect of different support types on the microstructure of ethylene/1‐hexene copolymers made with supported metallocene catalysts has been investigated through the analysis of molecular weight and chemical composition distributions using high temperature gel permeation chromatography (GPC) and crystallization analysis fractionation (Crystaf). The copolymer samples obtained using commercial carriers (silica and silica‐alumina) had unimodal chemical composition distributions and were used to create a linear calibration curve relating the peak crystallization temperature from Crystaf and the comonomer content as determined by 13C NMR. This calibration curve is useful to determine the 1‐hexene fractions for each peak in the resins showing bimodal chemical composition distributions, such as those obtained with catalysts supported on MCM‐41 and SBA‐15 materials. The structure and chemistry of the support used had a large influence on comonomer incorporation and the shape of the chemical composition distribution of the polymer, which suggests that the supporting process creates different types of active sites.

A Simplified Model for Prediction of Molecular Weight Distributions in Ethylene‐Hexene Copolymerization Using Ziegler‐Natta Catalysts

AbstractA simplified steady‐state model has been developed to predict molecular weight distributions and average compositions of ethylene‐hexene copolymers produced using heterogeneous Ziegler‐Natta catalysts in gas‐phase reactors. The model uses a simplified reaction scheme to limit the number of parameters that must be estimated. The number of parameters is further reduced by assuming that different types of active sites share common rate constants for some reactions. Estimates of kinetic parameters are obtained using deconvolution analysis of industrial copolymer samples produced using a variety of isothermal steady‐state operating conditions. The parameter estimates should prove useful as initial guesses for future parameter estimation in a non‐isothermal model.magnified image

Kinetic Modeling of the Liquid-Phase Hydrogenation of Cinnamaldehyde on Copper-Based Catalysts

Fil: Marchi, Alberto Julio. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Santa Fe. Instituto de Investigaciones en Catalisis y Petroquimica Ing. Jose Miguel Parera. Universidad Nacional del Litoral. Instituto de Investigaciones en Catalisis y Petroquimica Ing. Jose Miguel Parera; Argentina

Catalytic characteristics of various rubber-reinforcing carbon blacks in decomposition of methane for hydrogen production

Carbon black has recently been reported to act as an effective catalyst for methane decomposition and to exhibit stable catalytic behavior despite carbon deposition, and thus it can be used for CO2-free production of hydrogen from natural gas. In this work, various carbon blacks with different primary particle size were investigated with respect to methane decomposition under atmospheric pressure from 1123 to 1223K. Catalytic characteristics, such as activity, activation energy and reaction order, were investigated and compared. It was observed that with decreasing primary particle size (or increasing specific surface area), the specific activity increased and the activation energy decreased. The reaction orders for various pelletized, rubber-reinforcing carbon blacks were 0.6–0.7, about the same regardless of the primary particle size, while they were near 1 for fluffy carbon blacks. Fluffy carbon black showed higher activity and activation energy than the pelletized carbon black of the same primary particle size. Changes of the surface morphology during carbon deposition were observed by TEM. Variations of the number of active sites were discussed in regard of the primary particle size, carbon deposition and binder. The presence of different types of active sites was also suggested.

Kinetics of CO hydrogenation on modified Cu/ZnO catalyst in a slurry reactor

CO hydrogenation activity and product selectivity of a modified Cu/ZnO catalyst was investigated in a wide range of temperatures, pressures, space velocities, and synthesis gas compositions. Except alcohols, the catalyst produced a significant amount of hydrocarbons and CO 2 . The formation of alcohols and hydrocarbons follows the Anderson–Schulz–Flory distribution. Based on the experimental results obtained in a stirred tank slurry reactor, a mechanistic-kinetic model was developed. The main assumption of the model was that formation of alcohols and hydrocarbons occurred independently on different types of active sites. The model successfully predicts product formation rates within the range of experimental conditions used in this study.

TPO–TPD study of an activated carbon-supported copper catalyst–sorbent used for catalytic dry oxidation of phenol

Adsorption of organic pollutants in wastewater by carbon-based catalyst–sorbents followed by in situ catalytic dry oxidation of the adsorbed pollutants to regenerate the catalyst–sorbents at low temperatures is promising for the treatment of toxic and/or biorefractory wastewater streams. This paper investigates an activated carbon-supported copper catalyst–sorbent (CuO/AC) for catalytic dry oxidation of phenol at low temperatures in a TG/MS system through temperature-programmed oxidation (TPO) and temperature-programmed desorption (TPD). Phenol oxidation activities and ignition characteristics of the catalyst–sorbent were mainly discussed. The results indicate that the CuO/AC catalyst–sorbent has high catalytic activities for phenol oxidation and for AC ignition compared to the AC support itself; the initial oxidation temperature for phenol is about 190 °C, which is 130 °C lower than that for the AC. Consecutive adsorption–oxidation experiments show that the phenol adsorption capacity of the CuO/AC catalyst–sorbent decreases from an initial value of about 140 mg / g to a stable value of about 70 mg / g in 9 cycles, which is much better than the performance of activated carbon reported in the literature. TPD results indicate that phenol molecules adsorb on different types of active sites on AC and CuO/AC catalyst–sorbent. CuO plays an important role both in enhancing the affinity between phenol and CuO/AC catalyst–sorbent during phenol adsorption and in promoting catalytic activity for phenol oxidation.