Role Of Active Sites Research Articles

Non-radical oxidation driven by iron-based materials without energy assistance in wastewater treatment.

Chemical oxidation is extensively utilized to mitigate the impact of organic pollutants in wastewater. The non-radical oxidation driven by iron-based materials is noted for its environmental friendliness and resistance to wastewater matrix, and it is a promising approach for practical wastewater treatment. However, the complexity of heterogeneous systems and the diversity of evolutionary pathways make the mechanisms of non-radical oxidation driven by iron-based materials elusive. This work provides a systematic review of various non-radical oxidation systems driven by iron-based materials, including singlet oxygen (1O2), reactive iron species (RFeS), and interfacial electron transfer. The unique mechanisms by which iron-based materials activate different oxidants (ozone, hydrogen peroxide, persulfate, periodate, and peracetic acid) to produce non-radical oxidation are described. The roles of active sites and the unique structures of iron-based materials in facilitating non-radical oxidation are discussed. Commonly employed identification methods in wastewater treatment are compared, such as quenching, chemical probes, spectroscopy, mass spectrometry, and electrochemical testing. According to the process of iron-based materials driving non-radical oxidation to remove organic pollutants, the driving factors at different stages are summarized. Finally, challenges and countermeasures are proposed in terms of mechanism exploration, detection methods and practical applications of non-radical oxidation driven by iron-based materials. This work provides valuable insights for understanding and developing non-radical oxidation systems.

Insights into Nitrogen-doped Carbon for Oxygen Reduction: The Role of Graphitic and Pyridinic Nitrogen Species.

Nitrogen-doped carbons (N/Cs) manifest good catalytic performance for oxygen reduction reaction (ORR) for fuel cell systems. However, to date, controversies remain on the role of active sites in N/Cs. In the present study, ORR test was conducted on three N/Cs in O2 -saturated 0.1 M KOH aqueous solution, where apparent linear correlation between graphitic N contents and ORR activity was observed. Theoretical calculations demonstrated that graphitic N doping is energetically more favorable than that of pyridinic N doping for ORR and the pyridinic N leads to more preferential with 2 e- ORR pathway. These results reveal that graphitic N plays a key role in N/Cs mediated ORR activity. This work lays a solid foundation on identifying the active sites in heteroatom-doped carbons and can be exploited for rational design and engineering of effective carbon-based ORR catalysts.

Visible-light-driven photocatalysis over nano-TiO2 with different morphologies: From morphology through active site to photocatalytic performance

Enormous efforts have been devoted to exploring the influence of the materials’ morphology on photocatalytic performance, while the role of active site between them attracts less attention. Herein, three types of anatase nano-TiO2 with different morphologies were successfully prepared for visible-light-driven photocatalytic selective oxidation of benzylamine. The material properties of the samples were systematically studied by various characterization techniques such as XRD, SEM, TEM, AFM, XPS, UV–vis DRS, ESR and in situ FTIR. Due to possessing clear advantages especially for having more active sites (coordinatively unsaturated Ti atoms and oxygen vacancies), the TiO2 with porous structure shows more excellent photocatalytic performance than the other samples. Coordinatively unsaturated Ti atoms can chemisorb and activate benzylamine molecules via forming H-N…Ti coordination species, whose absorption property of visible light guarantees that TiO2 samples can achieve the photocatalytic process under visible light. Oxygen vacancies can chemisorb and activate oxygen molecules to form reactive oxygen species (superoxide radical) by the reduction of photogenerated electrons. Finally, a possible synergetic mechanism based on the interaction of reactants and catalyst interfaces was proposed at the molecular level to illustrate the photocatalytic process.

Utilization of renewable resources: Investigation on role of active sites in zeolite catalyst for transformation of furfuryl alcohol into alkyl levulinate

A bio-derived furfuryl alcohol transformation into various high-value chemicals is a growing field of interest among researchers. This study reports an exclusive investigation of the porosity and active sites responsible for the efficient alcoholysis of furfuryl alcohol to alkyl levulinate by the aid of zeolite catalyst. Alkyl levulinate is a promising platform chemical potentially used as a fuel additive and also for the production of chemicals. A detailed study using well-characterized HZSM-5 catalyst on the influence of acidity and post synthesis modification like desilication, dealumination, metal ion exchange and phosphate modification revealed the most desired type of acid sites required to catalyze this reaction. Among the HZSM-5 catalysts tested, HZSM-5 (SAR 95) showed the best performance of ≥ 99 % furfuryl alcohol conversion and 85 % butyl levulinate selectivity under optimum conditions. The catalyst exhibited good recyclability additionally addressing all the challenges reported in the previous literature fulfilling the green chemistry principles.

Activation and conversion of alkanes in the confined space of zeolite-type materials.

Microporous zeolite-type materials, with crystalline porous structures formed by well-defined channels and cages of molecular dimensions, have been widely employed as heterogeneous catalysts since the early 1960s, due to their wide variety of framework topologies, compositional flexibility and hydrothermal stability. The possible selection of the microporous structure and of the elements located in framework and extraframework positions enables the design of highly selective catalysts with well-defined active sites of acidic, basic or redox character, opening the path to their application in a wide range of catalytic processes. This versatility and high catalytic efficiency is the key factor enabling their use in the activation and conversion of different alkanes, ranging from methane to long chain n-paraffins. Alkanes are highly stable molecules, but their abundance and low cost have been two main driving forces for the development of processes directed to their upgrading over the last 50 years. However, the availability of advanced characterization tools combined with molecular modelling has enabled a more fundamental approach to the activation and conversion of alkanes, with most of the recent research being focused on the functionalization of methane and light alkanes, where their selective transformation at reasonable conversions remains, even nowadays, an important challenge. In this review, we will cover the use of microporous zeolite-type materials as components of mono- and bifunctional catalysts in the catalytic activation and conversion of C1+ alkanes under non-oxidative or oxidative conditions. In each case, the alkane activation will be approached from a fundamental perspective, with the aim of understanding, at the molecular level, the role of the active sites involved in the activation and transformation of the different molecules and the contribution of shape-selective or confinement effects imposed by the microporous structure.

Comparison of Au-functionalized semiconductor metal oxides in sensitivity to VOC

Metal oxide semiconductors (MOS) in pristine form and functionalized by noble metals are extensively researched for gas detection. There is a lack of fundamental understanding of the relations between materials composition and sensitivity to volatile organic compounds (VOC). In this work, we compared the sensitivities of different pristine metal oxides and Au-functionalized metal oxides to methanol and acetone. Nanostructured MOS of p-type (NiO, CuO, Co3O4) and n-type (In2O3, ZnO, SnO2, TiO2, and WO3) were synthesized by common aqueous deposition techniques. The oxides differ in metal-oxygen bond energy (EM-O) which was chosen as a parameter for the comparison of sensitivity to methanol and acetone vapors. Metal oxides were functionalized by Au nanoparticles via colloid adsorption, and the effect of gold on the sensitivity to VOC was investigated. Acid sites and surface oxygen sites at the materials surfaces were determined by temperature-programmed probe molecule techniques. The interaction routes of methanol and acetone with materials surfaces were examined by diffuse-reflectance infrared spectroscopy (DRIFT). It was found that with the increase of metal-oxygen bond energy the surface acidity of MOS strengthened, which favored the adsorption and improved the sensitivity to methanol. Oxidation of methanol and acetone to formate and acetate species on the materials surfaces was revealed, and the roles of active sites in these reactions were rationalized. An enhanced sensitivity to VOC was observed for Au-functionalized metal oxides, with the most prominence for TiO2/Au nanocomposite. It was rationalized as a combination of a proper TiO bond energy and the catalytic effect of gold.

Role of active sites in N-coordinated Fe-Co dual-metal doped graphene for oxygen reduction and evolution reactions: A theoretical insight

Identifying the atomic-scale structures of active sites is crucial to develop Fe-Co dual-metal doped N-coordinated graphene (Fe-Co-N-C) as a promising electrode material for fuel cell or metal-air batteries but remains debatable. Here the catalytic performance of Fe-Co-N-C with different active sites, including FeCoN6, FeCoN7, and FeCoN8, towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is systematically studied by density functional theory calculations. We show that the stability and catalytic activity of Fe-Co-N-C for ORR and OER would be affected by the atomic-structures of active sites, and Fe-Co-N-C with active sites of FeCoN7 and FeCoN8 possess a better catalytic performance compared with FeCoN6. The top site of Co atom is the main active center responsible for ORR and OER in active sites of FeCoN7 and FeCoN8, in which the extremely low overpotential of 0.22 V for both ORR and OER can be obtained by regulating the atomic-structure of the active site. Importantly, we plot the relationship curve between the d-band center of Co atoms and overpotentials of ORR and OER on Fe-Co-N-C, which can link the atomic-structure of active sites to catalytic performance. These findings are great significance for the scientific design of efficient bifunctional catalysts.

CO2 Hydrogenation to CH3OH on Supported Cu Nanoparticles: Nature and Role of Ti in Bulk Oxides vs Isolated Surface Sites

The selective hydrogenation of CO2 to CH3OH is a crucial part of efforts to mitigate climate change via the methanol economy. Understanding the nature and role of active sites is essential for desi...

Individual effect of charge balance defects on the photocatalytic activity of Cr3+-modified TiO2

Loss in the photocatalytic activity of Cr3+: TiO2 powders upon hydrogen annealing shows that charge-compensating oxygen vacancies VO and Cr6+ ions can have opposite effects on the kinetics of photocatalytic decolorization of methyl orange solution. The former ones inhibit this reaction acting as electron–hole recombination centers, and the latter ones promote it playing the role of active sites. This conclusion is verified by an XPS technique for a Cr3+-doped TiO2 sample and by 121Sb Mossbauer spectroscopy for a co-doped (Cr3+,Sb5+): TiO2 sample.

Nanodisperse gold catalysts in oxidation of benzyl alcohol: comparison of various supports under different conditions

Monodisperse gold particles (ca. 2 nm) were prepared and deposited on various supports (SiO2, Al2O3, HAP, MgAl2O4 and MgO). The acid/base properties of supports were characterized by NH3 and CO2 sorption. The size of the gold nanoparticles spans in the 1.7–6.5 nm mean diameter range after calcination as determined from TEM measurements. The amounts of accessible surface sites were estimated by binary concentration pulse chromatography of CO with Kr adsorption. The data are in agreement with the results of CO adsorption obtained by DRIFT spectroscopy. The activities of the catalysts were compared in the oxidation of benzyl alcohol in stirred batch reactors under two different conditions: in xylene solvent with atmospheric oxygen at 60 °C (in presence and in absence of K2CO3), and in a solvent-free mixture at elevated pressure and temperature (5 bar O2, 150 °C, 5 h). The activities of catalysts in benzyl alcohol conversion are described in two variants, namely related to (i) active catalytic sites (ASNA), and (ii) number of Au atoms on the geometric surface of particles (GSNA). The activities of catalysts in xylene solvent at 60 °C were excellent, with 0.28–1.11 s−1 characteristic GSNAini values (initial reaction rates related to surface Au atoms, Ausurf) in presence of K2CO3. The observed order of activities under these conditions is Au/SiO2 < Au/Al2O3 < Au/HAP < Au/MgAl2O4 < Au/MgO. In the experiments performed at 150 °C under solvent-free conditions, the reaction partners are depleted in greater extents (with the exception of Au/Al2O3), thus the obtained average GSNAave (average reaction rate during 5 h reaction related to Ausurf) values are less reliable, however selectivity data provide useful information as well. These estimated average GSNAave values (0.14–0.83 s−1) attest still good activities. For the interpretation of the obtained data, the roles of active sites on gold nanoparticles of various dispersion and the accessibility of their surfaces as well as the acid–base properties and surface hydroxyl concentration of supports, water ad- and desorption phenomena are considered simultaneously.

Oxidation during the production of FGH4095 superalloy powders by electrode induction-melt inert gas atomization

Super-clean and super-spherical FGH4095 superalloy powder is produced by the ceramic-free electrode induction-melt inert gas atomization (EIGA) technique. A continuous and steady-state liquid metal flow is achieved at high-frequency (350 kHz) alternating current and high electric power (100 kW). The superalloy is immersed in a high-frequency induction coil, and the liquid metal falling into a supersonic nozzle is atomized by an Ar gas of high kinetic gas energy. Numerical calculations are performed to optimize the structure parameters for the nozzle tip. The undesired oxidation reaction of alloying elements starts at 1000 °C with the reaction originating from the active sites on the powder surfaces, leading to the formation of oxides, MexOy. The role of active sites and kinetic factors associated with the diffusion of oxygen present in the atomization gas streams are also examined. The observed results reveal that the oxidation process occurring at the surface of the produced powders gradually moves toward the core, and that there exists a clear interface between the product layer and the reactant. The present study lays a theoretical foundation for controlling the oxidation of nickel-based superalloy powders from the powder process step.

Protein Film Electrochemistry of Iron-Sulfur Enzymes.

A suite of dynamic electrochemical techniques known as protein film electrochemistry (PFE) offers important insight into the roles of active sites in enzymes, including properties of electron-transfer centers (individually or collectively), rates and dependences of catalytic electron transport, and binding and dissociation of inhibitors. In this chapter, we explain how PFE is used to investigate the properties of FeS clusters-centers lacking distinctive or convenient spectroscopic signatures that are often very sensitive to O2. We see that PFE allows simultaneous detection and control of the reactions of individual FeS clusters, and measurement of their relaying efficiency in long-range electron transfer.

ChemInform Abstract: Aerobic Oxidation of Alcohols on Au/TiO2 Catalyst: New Insights on the Role of Active Sites in the Oxidation of Primary and Secondary Alcohols.

AbstractEfficient and simple methods to oxidize primary, benzylic, or secondary alcohols by atmospheric oxygen on a commercial Au—TiO2 catalyst are developed.

Atomically-thin two-dimensional sheets for understanding active sites in catalysis.

Catalysis can speed up chemical reactions and it usually occurs on the low coordinated steps, edges, terraces, kinks and corner atoms that are often called "active sites". However, the atomic level interplay between active sites and catalytic activity is still an open question, owing to the large difference between idealized models and real catalysts. This stimulates us to pursue a suitable material model for studying the active sites-catalytic activity relationship, in which the atomically-thin two-dimensional sheets could serve as an ideal model, owing to their relatively simple type of active site and the ultrahigh fraction of active sites that are comparable to the overall atoms. In this tutorial review, we focus on the recent progress in disclosing the factors that affect the activity of reactive sites, including characterization of atomic coordination number, structural defects and disorder in ultrathin two-dimensional sheets by X-ray absorption fine structure spectroscopy, positron annihilation spectroscopy, electron spin resonance and high resolution transmission electron microscopy. Also, we overview their applications in CO catalytic oxidation, photocatalytic water splitting, electrocatalytic oxygen and hydrogen evolution reactions, and hence highlight the atomic level interplay among coordination number, structural defects/disorder, active sites and catalytic activity in the two-dimensional sheets with atomic thickness. Finally, we also present the major challenges and opportunities regarding the role of active sites in catalysis. We believe that this review provides critical insights for understanding the catalysis and hence helps to develop new catalysts with high catalytic activity.

Effect of Modification with Vanadium or Carbon on Destructive Sorption of Halocarbons over Nanocrystalline MgO: The Role of Active Sites in Initiation of the Solid-State Reaction

Small amounts of vanadium or carbon added to nanocrystalline MgO aerogels were shown to promote their activity in destructive sorption of CF2Cl2 and CFCl3 halocarbons. This reaction is characterized by a prolonged induction period, which is considerably shortened after the addition of the studied promoters. It was demonstrated that the promoting effect of finely dispersed carbon did not depend on its location relative to the MgO nanoparticles. Approximately the same results were obtained when carbon was deposited as a thin layer on the surface of the MgO nanoparticles, when activated carbon was physically mixed with the nanocrystalline MgO or when it was located separately in the reactor. The modifying agents are believed to have a catalytic effect on the reaction of nanocrystalline MgO with halocarbons, accelerating the formation of active sites on the surface of the MgO nanoparticles. Electron-acceptor sites are shown to be possible candidates for the role of such active sites. The concentration of weak...

Mutation Studies in the Active Site of β-glycosidase from Pyrococcus furiosus DSM 3638

Sequence alignments and homology modeling of Pyrococcus furiosus thermostable glycosidase (PFTG) showed that the residue 150 is conserved as tryptophan in β-glycosidase and in other related enzymes such as β- mannosidase and β-galactosidase. To elucidate the relationship between the substrate size and geometric shape of the catalytic site of thermophilic β-glycosidase and category of PFTG, the Q77, Q150 and D206 located at the interface of the dimer were replaced with Trp and Asn. Also, to confirm the role of active sites of PFTG, the Q77R/Q150W double mutant was created through subcloning. Temperature and pH optima of both mutants and native enzyme were same at 100 °C and pH 5.0 in sodium citrate buffer, respectively. The catalytic efficiencies (k(cat)/K(m)) of the mutants on synthetic and natural substrates by Isothermal Titration Calorimetry were slightly changed, but indicated the characteristics of β-glycosidase activity. Kinetic parameters of the mutant enzymes indicated that they possess characteristics of both β- galactosidase and β-mannosidase activities. Although the mutant enzymes showed similar substrate specificities compared to the recombinant enzyme, they had more affinity (K(m)) to substrates with low turnover number (k(cat)).

Investigation on Process Mechanism on Cu–Cr Catalysts for Ethanol Dehydrogenation to Ethyl Acetate

The Cu–Cr catalysts were prepared by co-precipitation method,and the role of active sites on Cu–Cr catalyst for direct synthesis of ethyl acetate from ethanol was investigated. The catalysts were characterized by means of XRD, TEM, XPS, ethanol-TPSR, EA-TPD and FT-IR etc., respectively. The results show that the outermost surface layer of Cu–Cr catalysts after reduction is composed of Cu0, Cr2O3 and CuCr2O4 phases. The ethanol dehydrogenation step is carried out on Cu0 active sites, but the selectivity of ethyl acetate is not remarkably influenced by the properties of Cu0 active sites, the proper performances of Lewis acidic sites on the Cu–Cr catalyst might play a key role to improve the selectivity of ethyl acetate.

Active sites of heterogeneous nucleation understood as chemical reaction sites

The role of active sites in heterogeneous nucleation can be understood on the basis of chemical thermodynamics as chemical reaction sites. The basic assumption is the formation of a chemical bond between the atoms (or molecules) of the nuclei with the atoms (or molecules) of the active site. This view on nucleation will be applicable to all cases where the nucleating phase has a sufficient chemical affinity to the active site.

Adsorption equilibriums in a silica-poly(vinyl alcohol)-gelatin system

The interaction of gelatin with nonporous nanosized amorphous silica containing poly(vinyl alcohol) (PVA) preliminarily adsorbed on its surface is studied. It is established that the modification of the nanosilica via PVA adsorption does not influence its ability to adsorb the protein. It is shown that, during adsorption, gelatin partly displaces PVA molecules from the silica surface. It is assumed that hydroxyl groups of strongly retained PVA macromolecules may play the role of active sites of protein adsorption along with free silanol groups.

The Role of Active Sites in the Non-Catalytic Oxidation of Carbon Particulate Matter: A Theoretical Approach

The oxidation of carbon particulate matter is a complex process involving many different surface compounds; however, it is clear that there is a direct relationship between the inherent structure of the carbon and the oxidation reaction rate. This reaction occurs on surface sites which are on the periphery of the crystallites that make up carbon particles. These surface sites can be described as active sites where the reaction occurs and spectator sites that do not participate in the reaction. A model has been constructed that calculates the distribution of these types of surface sites during oxidation to show their dynamic behavior, and is compared to experimental data.