Local Structure Of Active Sites Research Articles

Carbon-Conjugated Co Complexes as Model Electrocatalysts for Oxygen Reduction Reaction

Single-atom catalysts are a family of heterogeneous electrocatalysts widely used in energy storage and conversion. The determination of the local structure of the active metal sites is challenging, which limits the establishment of the reliable structure-property relationship of single-atom catalysts. A carbon black-conjugated complex can be used as the model catalyst to probe the intrinsic activity of metal sites with certain local structures. In this work, we prepared carbon black-conjugated [Co(phenanthroline)Cl2], [Co(o-phenylenediamine)Cl2] and [Co(salophen)]. In these catalysts, the Co complexes with well-defined structures are anchored on the edge of carbon black by pyrazine moieties. The number of electrochemical accessible Co sites can be measured from the area of the redox peaks of pyrazine linkers in the cyclic voltammetry curve. Then, the intrinsic electrocatalytic activity of one Co site can be obtained. The catalytic performances of the three catalysts towards oxygen reduction reaction in alkaline conditions were measured. Carbon black-conjugated [Co(salophen)] showed the highest intrinsic activity with the turnover frequency of 0.72 s−1 at 0.75 V vs. the reversible hydrogen electrode. The strategy developed in this work can be used to explore and verify the possible local structure of active sites proposed for single-atom catalysts.

Local structure engineering for active sites in fuel cell electrocatalysts

In this review, we surveyed the significance of local structure engineering on electrocatalysts and electrodes for the performance of oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Both on precious metal catalysts (PMC) and non-precious metal catalysts (NPMC), the main methods to modulate local structure of active sites have been summarized. By change of atomic coordination, modulation of bonding distortion and synergy effect from hierarchical structure, local structure engineering has influence on the intrinsic activity and stability of electrocatalysts. Moreover, we emphasized the intimate correlation between lyophobicity of electrocatalysts and membrane electrodes by local structure engineering. Our review aimed to inspire the exploration of advanced electrocatalysts and mechanism study for PEMFCs based on local structure engineering.

New Nitrogen-Doped Graphene/MOF-modified catalyst for Fuel Cell Systems

Metal-organic frameworks (MOFs), which have advantages such as large surface area and high porosity, are recently investigated as a component of electrochemical catalysts to enhance the catalytic performance. But the influences of MOFs on the properties of catalysts such as surface structure, local structure of active sites, catalytic activity, and selectivity are still under investigation. This research is focused on the new MOF-modified nitrogen-doped graphene (N-G/MOF) catalyst, which is expected to have better ORR activity than N-G catalysts, for PEM fuel cell systems. The primary research objective is to understand the catalytic reaction mechanism of the N-G/MOF. It also includes the study of the physical structure & chemical composition of the N-G/MOF and their influence on the ORR activity and selectivity. The experimental efforts to realize this objective include the controllable synthesis of the N-G/MOF, imaging of the microstructure, analyzing the chemical bonding distribution of active sites, and the electrochemical characterization at the acid electrolyte.

Metric learning for enzyme active-site search

Motivation: Finding functionally analogous enzymes based on the local structures of active sites is an important problem. Conventional methods use templates of local structures to search for analogous sites, but their performance depends on the selection of atoms for inclusion in the templates.Results: The automatic selection of atoms so that site matches can be discriminated from mismatches. The algorithm provides not only good predictions, but also some insights into which atoms are important for the prediction. Our experimental results suggest that the metric learning automatically provides more effective templates than those whose atoms are selected manually.Availability: Online software is available at http://www.net-machine.net/∼kato/lpmetric1/Contact: kato-tsuyoshi@k.u-tokyo.ac.jpSupplementary information: Supplementary data are available at Bioinformatics online.

Local Structures of Active Sites on Ti-MCM-41 and Their Photocatalytic Reactivity for the Decomposition of NO

The characterization of mesoporous Ti-MCM-41 prepared at ambient temperature, with Ti contents in the 0.15–2.00 wt% range, was carried out using XAFS, FT-IR and photoluminescence methods. For Ti contents up to 0.60 wt%, isolated tetrahedrally coordinated Ti-oxide species was found to be formed in the MCM-41 network structures, while at higher titanium contents, dimeric or oligomeric Ti-oxide species, with Ti (IV) in tetrahedral coordination, became overwhelming. It was found that only the highly dispersed isolated tetrahedrally coordinated Ti-oxide species act as active sites in the photocatalytic decomposition of NO into N2 and O2.

Polarization-dependent total-reflection fluorescence EXAFS study about active structures on single crystal oxides as model catalyst surfaces

We review recent development of a polarization-dependent total-reflection fluorescence EXAFS technique applied to Co3O4/Al2O3(0001), Pt/ Al2O3(0001), and Mo/TiO2(110). We would like to stress that the polarization-dependent total-reflection fluorescence EXAFS has become a practical tool for a characterization of three-dimensional local structure of active sites in model supports.

Molecular mechanisms of catalytic isomerization and hydrogen exchange of olefins over the MoS 2 single crystal catalysts: Regulation of catalytic processes by the conformation of active sites

Reaction mechanisms and crystallographic specification of active sites were attained by using deuterium tracer for the reactions taking place on MoS 2 single crystal catalysts. The isomerization reaction as well as the hydrogenation reaction of olefins via alkyl intermediates were found to proceed on the edge surface of the MoS 2 crystal, while the isomerization of 2-methyl 1-butene via tertiary carbonium cation occurred on the basal plane of the MoS 2 crystal. Both the hydrogenation and the hydrogen exchange or the isomerization reactions of olefins occurred on the edge surface of the MoS 2 crystal, but the active sites for these two reactions were entirely different in catalytic ability. Such different catalytic ability of the active sites exposed on the edge surface may be referable to the different degrees of coordinative unsaturation. Some reactions via alkyl intermediates on the edge surface accompanied certain induction times. It was shown that the edge surface has the ability to produce alkyl species in the period of the induction time for the isomerization and certain hydrogen exchange of olefins. It was deduced that the rotational motion of bulky groups in these alkyl species is restricted during the induction time. The coordination of bulky groups in these alkyl species seems to undergo the reconstruction of the local structures of active sites, which results in being off the latch for the rotation of bulky groups being one elementary step in the catalysis.

Synchrotron X-ray studies of biological preparations at low temperatures with optical monitoring of sample integrity

Increasing use of X-ray absorption spectroscopy (edge and EXAFS) to determine the local structure of active sites of biological molecules has caused greater interest in and attention to the nature of X-ray damage and the integrity of the sample. In a complementary way, efficient X-ray photon collection from the sample is required in order to maximize data acquisition and minimize sample damage. This report describes systems currently in use at Stanford Synchrotron Radiation Laboratory and Cornell High Energy Synchrotron Source, comprising two types of low-temperature techniques for sample protection and fast X-ray photon detection of large solid-angle collection.