Heterogeneity Of Active Sites Research Articles

A molecular view of single-atom catalysis toward carbon dioxide conversion.

Carbon dioxide (CO2) conversion has attracted much interest recently owing to its importance in both scientific research and practical applications, but still faces a bottleneck in selectivity control and mechanism understanding owing to diversified active sites. Single-atom catalysts (SACs) featuring isolated and well-defined active centers are proved to not only exhibit unparalleled performances in various processes of CO2 conversion but also provide excellent research paradigms by circumventing the heterogeneity of active sites. Herein, we will not only critically review recent progress on the application of SACs in chemical CO2 conversion based on previous comprehension of general thermodynamics and kinetics, but also try to offer a multi-level understanding of SACs from a molecular point of view in terms of the central atom, coordination environment, support effect and synergy with other active centers. Meanwhile, crucial scientific issues of research methods will be also identified and highlighted, followed by a future outlook that is expected to present potential aspects of further developments.

Heterogeneity of Active Sites in the Polymer Chain Transfer Reactions at Olefin Polymerization over Multisite Supported Ziegler-Natta Catalysts.

In this review, we summarize and discuss our experimental data published in a number of papers on the transfer reactions of polymer chains in the polymerization of ethylene, propylene, and hexene-1, and the copolymerization of ethylene with α-olefins over multisite supported titanium-magnesium catalysts (TMC). Three groups of transfer reactions are discussed in the review: (1) transfer reactions with AlEt3 cocatalyst, (2) transfer reactions with hydrogen, and (3) transfer reactions with participation of α-olefins in the case of ethylene copolymerization with α-olefins. We have found polymerization conditions where it is possible to observe heterogeneity of active sites of TMC for all three groups of the indicated reactions. It is shown that (1) the transfer reaction with AlEt3 proceeds with higher reactivity on the active sites that produce polymers with low molecular weight; (2) the transfer reaction with hydrogen, in the case of α-olefin polymerization and copolymerization of ethylene with α-olefins, proceeds with higher reactivity on the active sites which produce polymers with high molecular weight; (3) the transfer reaction with α-olefins proceed with higher reactivity on the active sites that produce high molecular weight polymers.

Fe3O4 Templated Pyrolyzed Fe−N−C Catalysts. Understanding the role of N‐Functions and Fe3C on the ORR Activity and Mechanism

AbstractPyrolyzed non‐precious metal catalysts have been proposed as an alternative to substitute the expensive and scarce noble metal catalysts in several conversion energy reactions. For the oxygen reduction reaction (ORR), the pyrolyzed catalyst M−N−C (M: Fe or Co) presents remarkable catalytic activity in acid and alkaline media. These pyrolyzed materials show a high heterogeneity of active sites being the most active in the MNx moieties. The activity and stability of these catalysts are also conditioned by other structural parameters such as the area, the N‐doping, and by the presence of metal particles. In this study, we explore the use of Fe3O4 nanoparticles as templates and as iron sources to synthesize Fe−N−C. The best performance for the ORR in acidic media was reached with the catalysts using nanoparticles covered by PANI and iron salts as the precursor, with an onset potential of 0.85 vs. RHE and a direct 4‐electrons mechanism. We corroborated the use of the catalysts’ redox potential as reactivity descriptors and discussed the detrimental role of the presence of Fe3C metallic particles in the mechanism. Based on the experimental results, we performed DFT calculations to explore the influence of N‐doped species on the electronic density of the iron centers of FeN4 active sites, and we propose a theoretical model for increasing the activity based on the distance and ratio of N‐doping to iron center.

Heterogeneity of active sites in recombinant betaine aldehyde dehydrogenase is modulated by potassium.

Betaine aldehyde dehydrogenase (BADH EC 1.2.1.8) catalyzes the irreversible oxidation of betaine aldehyde to glycine betaine using NAD+ as a coenzyme. Porcine kidney BADH (pkBADH) follows a bi-bi ordered mechanism in which NAD+ binds to the enzyme before the aldehyde. Previous studies showed that NAD+ induces complex and unusual conformational changes on pkBADH and that potassium is required to maintain its quaternary structure. The aim of this work was to analyze the structural changes in pkBADH caused by NAD+ binding and the role played by potassium in those changes. The pkBADH cDNA was cloned and overexpressed in Escherichia coli, and the protein was purified by affinity chromatography using a chitin matrix. The pkBADH/NAD+ interaction was analyzed by circular dichroism (CD) and by isothermal titration calorimetry (ITC) by titrating the enzyme with NAD+ . The cDNA has an open reading frame of 1485 bp and encodes a protein of 494 amino acids, with a predicted molecular mass of 53.9 kDa. CD data showed that the binding of NAD+ to the enzyme caused changes in its secondary structure, whereas the presence of K+ helps maintain its α-helix content. K+ increased the thermal stability of the pkBADH-NAD+ complex by 5.3°C. ITC data showed that NAD+ binding occurs with different association constants for each active site between 37.5 and 8.6 μM. All the results support previous data in which the enzyme incubation with NAD+ provoked changes in reactivity, which is an indication of slow conformational rearrangements of the active site.

Role of potassium levels in pkBADH heterogeneity of NAD+ binding site.

Betaine aldehyde dehydrogenase (BADH) catalyzes the oxidation of betaine aldehyde to glycine betaine using NAD+ as a coenzyme. Studies in porcine kidney BADH (pkBADH) suggested that the enzyme exhibits heterogeneity of active sites and undergoes potassium-induced conformational changes. This study aimed to analyze if potassium concentration plays a role in the heterogeneity of pkBADH active sites through changes in NAD+ affinity constants, in its secondary structure content and stability. The enzyme was titrated with NAD+ 1mM at fixed-variable KCl concentration, and the interaction measured by Isothermal Titration Calorimetry (ITC) and Circular Dichroism (CD). ITC data showed that K+ increased the first active site affinity in a manner dependent on its concentration; KD values to the first site were 14.4, 13.1, and 10.4μM, at 25, 50, and 75mM KCl. ΔG values showed that the coenzyme binding is a spontaneous reaction without changes between active sites or depending on KCl concentration. ΔH and TΔSb values showed that NAD+ binding to the active site is an endothermic process and is carried out at the expense of changes in entropy. α-Helix content increased as KCl increased, enzyme (Tm)app values were 2.6°C and 3.3°C higher at 20mM and 200mMK+. PkBADH molecular model showed three different interaction K+ sites. Results suggested K+ can interact with pkBADH and cause changes in the secondary structure, it provokes changes in the enzyme affinity by the coenzyme, and in the thermostability.

Direct synthesis of H2O2 on PdZn nanoparticles: The impact of electronic modifications and heterogeneity of active sites

The direct synthesis of H2O2 (H2 + O2 → H2O2) would reduce the cost of H2O2, but few catalytic materials demonstrably provide the necessary combination of high H2O2 selectivity and stability over long periods of continuous reaction. Here, we show that silica supported PdZnx nanoparticles (where x indicates the bulk composition, 0 ≤ x ≤ 30) are stable and selective catalysts for the continuous production of H2O2 in the absence of liquid-phase promoters. Increasing the ratio of Zn to Pd precursors in the synthesis lead to distributions of nanoparticles that contain greater fractions of β1-Pd1Zn1 and lower fractions of face-centered cubic (fcc) Pd nanoparticles. A combination of the measured functional dependence of steady-state H2O2 and H2O formation rates on H2 and O2 pressures and the need for protic solvents indicate that H2O2 forms on all members of the PdZnx series by proton-electron transfer steps to dioxygen and hydroperoxy surface intermediates that saturate active sites during catalysis. Primary H2O2 selectivities increase systematically with the Zn to Pd ratio and range from 26% on fcc Pd nanoparticles to 69% on PdZn30 catalysts, which contains β1-Pd1Zn1 nanoparticles and lacks detectable amounts of fcc Pd. The differences in H2O2 selectivities among the PdZnx catalysts reflect changes in activation enthalpies (ΔH‡) for H2O2 (from -3 to 14 kJ mol−1) and H2O (11 to 44 kJ mol−1) formation. These results demonstrate that pathways for H2O2 formation are less sensitive than H2O formation to electronic differences between active sites on fcc Pd and intermetallic β1-Pd1Zn1 nanoparticles, whose densities of states differ significantly near the Fermi level. Comparisons of the ratios of H2O2 and H2O formation rates to ΔH‡ values among Pd and PdZnx catalysts show that the increases in H2O2 selectivities are less than predicted from the differences in ΔH‡ values alone. This analysis shows that the current synthesis method introduces at least two distinguishable catalytically active sites, and a portion of the sites formed predominantly cleave O-O bonds in primary and secondary reaction pathways that form H2O and lead to lower H2O2 selectivities than would be achieved in their absence. These findings demonstrate that differences in electronic structure of active sites cause β-Pd1Zn1 nanoparticles to give greater H2O2 selectivities than Pd catalysts and suggest that a synthesis procedure that uniformly creates β-Pd1Zn1 and alloys all Pd on the support may lead to increasingly selective conversion of O2 and H2 to H2O2.

Evaluation of hybrid neutralization/biosorption process for zinc ions removal from automotive battery effluent by dolomite and fish scales

ABSTRACTThis work focused in the evaluation of Oreochromis niloticus fish scales (FS) as biosorbent material in the removal of Zn from a synthetic effluent based on automotive battery industry effluent and, further, a hybrid neutralization/biosorption process, aiming at a high-quality treated effluent, by a cooperative use of dolomite and FS. For this, a physicochemical and morphological characterization (i.e. SEM–EDX, FTIR, XRD, and TXRF) was performed, which helped to clarify a great heterogeneity of active sites (phosphate, carbonate, amide, and hydroxyl) on the biosorbent; also the inorganic constituents (apatites) leaching from the FS was identified. Biosorption results pointed out to a pH-dependent process due to changes in the functional group’s anionic character (i.e. electrostatic interactions), where an initial pH = 3 favored the Zn uptake. Kinetic and equilibrium studies confirmed the heterogeneous surface and cooperative sorption, wherein experimental data were described by Generalized Elovich kinetic model and the favorable isotherm profile by Langmuir–Freundlich isotherm ( = 15.38 mg g–1 and ). Speciation diagram of Zn species along with the leached species demonstrated that, for the studied pH range, the biosorption was the most likely phenomena rather than precipitation. Finally, the hybrid neutralization/biosorption process showed great potential since both the Zn concentration levels and the pH reached the legislation standards (CZn = 4 mg L–1; pH = 5). Hence, based on the characterization and biosorption results, a comprehensive evaluation of the involved mechanisms in such complex system helped to verify the prospective of FS biosorbent for the Zn treatment from solution, in both individual and hybrid processes.

Consequence of heterogeneity of active sites for reactivity mechanism of n-butane isomerization over SO42−/ZrO2-Al2O3 catalyst

The relationship between the heterogeneity of active sites and isomerization mechanism of n-butane over alumina-promoted sulfated zirconia (SZA) was elucidated by two series of catalysts with different surface properties. The incomplete removal of coke deposition leads to a decrease in overall activity without the reduction of the rate of isobutane formation. Subjecting the fresh SZA catalyst to the thermal treatment at 650°C causes a striking decrease in activity, resulting from the decrease in the superacidity and oxidizability of the catalyst. While the thermal treatment at 600°C only selectively suppresses the side reactions. It is found that the peaks in NH3-TPD profiles at 500–700°C are caused by a redox reaction between strongly adsorbed ammonia and superacid sites by in situ NH3-TPD-MS measurements. Thus, this peak reflects the strength of superacidity and oxidizability. At the beginning of the reaction, the effective oxidative and protonative activation routes result in the higher concentration of intermediates, which facilitates the “dimerization-isomerization-cracking” reactions. The incomplete removal of coke and the appropriate decrease in the density of sulfate species on the surface selectively promoted the monomolecular pathway.

Pd/ZnO catalysts with different origins for high chemoselectivity in acetylene semi-hydrogenation

The heterogeneity of active sites is the main obstacle for selectivity control in heterogeneous catalysis. Single atom catalysts (SACs) with homogeneous isolated active sites are highly desired in chemoselective transformations. In this work, a Pd1/ZnO catalyst with single-atom dispersion of Pd active sites was achieved by decreasing the Pd loading and reducing the sample at a relatively low temperature. The Pd1/ZnO SAC exhibited excellent catalytic performance in the chemoselective hydrogenation of acetylene with comparable chemoselectivity to that of PdZn intermetallic catalysts and a greatly enhanced utilization of Pd metal. Such unusual behaviors of the Pd1/ZnO SAC in acetylene semi-hydrogenation were ascribed to the high-valent single Pd active sites, which could promote electrostatic interactions with acetylene but restrain undesired ethylene hydrogenation via the spatial restrictions of σ-chemical bonding toward ethylene.

Relationship of the microheterogeneity of isoprene-butadiene copolymers with the kinetic heterogeneity of active sites

Relationship of the microheterogeneity of isoprene-butadiene copolymers with the kinetic heterogeneity of active sites

Heterogeneity of active sites of Ziegler‐Natta catalysts: The effect of catalyst composition on the MWD of polyethylene

AbstractExperimental data on the molecular weight distribution (MWD) of polyethylene (PE) produced over a broad number of Ziegler‐Natta catalysts differing in composition and preparation procedure are presented. These catalysts include nonsupported TiCl3 catalyst, four types of supported titanium‐magnesium catalysts (TMC) differing in the content of titanium and the presence of various modifiers in the composition of the support, and a supported catalyst containing VCl4 as an active component instead of TiCl4. The studied catalysts produce PE with different molecular weights within a broad range of polydispersity (Mw/Mn = 2.8–16) under the same polymerization conditions. The heterogeneity of active sites of these catalysts was studied by deconvolution of experimental MWD curves into Flory components assuming a correlation between the number of Flory components and the number of active site types. Five Flory components were found for PE produced over nonsupported TiCl3 catalysts (Mw/Mn = 6.8), and three–four Flory components were found for PE produced over TMC of different composition. A minimal number of Flory components (three) was found for PE samples (Mw/Mn values from 2.8 to 3.3) produced over TMC with a very low titanium content (0.07 wt %) and TMC modified with dibutylphtalate. It was shown that five Flory components are sufficient to fit the experimental MWD curve for bimodal PE (Mw/Mn = 16) produced over VMC. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Hydrodynamic effect on the heterogeneity of active sites of butadiene polymerization upon formation of titanium catalyst in situ

The effect of hydrodynamic conditions of the reaction mixture flow during formation of active sites sites of butadiene polymerization on their distribution by kinetic and stereospecific heterogeneity was studied. The enhanced intensity of the turbulent mixing at the stage of formation of active sites results in lowering of the number of types of active sites of chain propagation and the polydispersity coefficient with changes in stereoregular composition of polybutadiene.

The nature of the organoaluminum compound and the kinetic heterogeneity of active sites in lanthanide‐based diene polymerization

AbstractThe influence of the organoaluminum compound on the kinetic heterogeneity of active sites in lanthanide‐based diene polymerization was investigated. It was found that the heterogeneity of investigated catalytic systems was shown in the existence of four types of active centers. They were formed at the beginning of the polymerization process and produced macromolecules with lengths that were definite for each type of active center. The nature of the organoaluminum compound greatly influenced the kinetic activity of the polymerization centers. A method for analyzing the curves' maximums of distribution on kinetic activity and the change in kinetic activity of every type of active centers is proposed. This method allows calculating the concentration values for each type of active center separately. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 674–685, 2003

The influence of the nature of organoaluminum compound on kinetic heterogeneity of active sites in lanthanide-based diene polymerization

The influence of organoaluminum compound on kinetic heterogeneity of active sites in lanthanide-based diene polymerization was investigated. It was shown that heterogeneity of investigated catalytic systems was displayed in the existence of four types of active centers. They are formed at the beginning of the polymerization process and produce macromolecules with lengths that are definite for each type of active centers. The organoaluminum compound's nature greatly influences the kinetic activity of polymerization centers. The method of analysis of the curves' maximum distribution on kinetic activity and the change of kinetic activity of every active centers' type were proposed, which permits the calculation of values of concentration for each type of active center separately. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 358–368, 2003

The chemical heterogeneity of active surface of solid catalysts

The chemical adsorption behaviors of three adsorbates on the Ni-W/F-Al2O3-SiO2 and the supported Ni catalysts have been investigated. On the basis of fractal geometry, the roughness fractal dimension (Dr) and the chemisorption fractal dimension (Dc) of the catalysts are estimated. We use the difference between Dc and Dr to characterize the chemical heterogeneity of active sites on the catalytic surface. It is found that the chemical heterogeneity of the acid sites of the Ni-W/F-Al2O3-SiO2 catalyst is very large, while it decrease with increasing of impregnating operation times in preparation of the supported Ni catalyst. After 4 times impregnation, the chemical heterogeneity of its active surface for the supported Ni catalyst is eliminated.

Solid catalysts for the production of fine chemicals: the use of ALPON and hydrotalcite base catalysts for the synthesis of arylsulfones

α#x2010;phenylsulfonylcinnamononitrile and derivatives as well as α-phenylsulfonylchalcone have been obtained by condensation of phenylsulfonylacetonitrile and phenylsulfonylacetophenone with benzaldehyde and 4-substituted benzaldehydes, using solid base catalysts with different basic strengths. Cs-exchanged X zeolite showed low activity for the above reactions, while high-surface-area MgO and MgO–Al2O3 mixed oxides derived from hydrotalcite were active and selective catalysts. An optimum basicity was observed with MgO–Al2O3 with the atomic ratio Al/(Al+Mg) between 0 and 0.20, showing the increase in basicity when introducing Al in the MgO structure. Nitridation of a high-surface-area aluminophosphate gives active and selective catalysts for the above reactions. The activity per unit surface area for the less demanding reaction between sulfonylacetonitrile and benzaldehyde is directly proportional to the level of nitridation. However, formation of α-phenylsulfonylchalcone, which is a more demanding reaction, does not increase linearly but goes through a maximum with the nitrogen content of the solid, showing the heterogeneity of active sites on ALPONs and the decrease in the activity coefficient of the basic sites when the density of N atoms increases.

A Study on the Chemical Heterogeneity of Active Surface of Solid Catalysts

A method that can be used to characterize the chemical heterogeneity of active surface of solid catalyst is given. The chemical adsorption behavior of three adsorbates on Ni–W/F–Al2O3–SiO2catalyst and supported Ni catalyst has been investigated. On the basis of fractal geometry, the roughness fractal dimension (Dr) and the chemisorption fractal dimension (Dc) of two kinds of catalyst are estimated. We use the difference betweenDcandDrto characterize the chemical heterogeneity of active sites on the catalytic surface. As a result, the chemical heterogeneity of the acid sites of Ni–W/F–Al2O3–SiO2catalyst is very great. As for supported Ni catalyst, the chemical heterogeneity of active sites decreased when impregnating operation times in preparation of the catalyst increased. After impregnated four times, the chemical heterogeneity of its active surface of supported Ni catalyst is eliminated.

Equilibrium and Rate Study of Analyte−Matrix Interactions in Supercritical Fluid Extraction

Variation in analyte extraction efficiencies from environmental samples using supercritical carbon dioxide (SC CO2) have often been attributed to “matrix effects” including strong matrix−analyte binding. To examine this hypothesis, the equilibrium distribution of phenanthrene between SC CO2 and five well-characterized natural materials was measured as a function of temperature, pressure, and methanol modifier concentration. Dynamic extraction efficiencies and rates with and without methanol modifier were measured from the same materials. Energetic heterogeneity of active sites on dry soils was suggested by nonlinear isotherms displaying significant capacity differences across soil types. Removal efficiencies were highest from low organic matter sorbents, and these materials exhibited the lowest phenanthrene capacities and most linear isotherms. Modifier addition decreased the sorbent's equilibrium capacity for phenanthrene sorption by as much as an order of magnitude, resulting in linear isotherms and off...

Sepharose-bound trypsin and activated sepharose-bound trypsinogen: Studies with small and large substrates

Several enzymic and physical properties of Sepharose-bound trypsin and activated Sepharose-bound trypsinogen have been compared to those of the soluble enzyme. Sepharose-bound trypsinogen could be activated to the same extent as soluble trypsinogen; the release of the activation peptide and formation of the active site occurred as expected in the presence of catalytic amounts of trypsin. With synthetic substrates, the relative activity and pH dependence of both immobilized trypsin preparations were essentially identical and nearly the same as the soluble enzyme. Sepharose-trypsin also formed an inactive complex with soybean trypsin inhibitor, with 85% of the active sites participating. In contrast, the activity of Sepharose-trypsin with chymotrypsinogen and with trypsinogen as substrates was only 40% that of soluble trypsin. There is evidence for some catalytic heterogeneity of active sites of bound trypsin; probably those sites buried within the gel have a limited catalytic efficiency with macromolecular substrates. The immobilized enzyme is more stable than the soluble enzyme at elevated temperatures and to concentrated urea, and denaturation by urea at pH 8 is fully reversible since the loss of molecules by autolysis is eliminated.