Catalytic Active Centers Research Articles

Iron Vacancies Induced Bifunctionality in Ultrathin Feroxyhyte Nanosheets for Overall Water Splitting.

Exploring of new catalyst activation principle holds a key to unlock catalytic powers of cheap and earth-abundant materials for large-scale applications. In this regard, the vacancy defects have been proven to be effective to initiate catalytic active sites and endow high electrocatalytic activities. However, such electrocatalytically active defects reported to date have been mostly formed by anion vacancies. Herein, it is demonstrated for the first time that iron cation vacancies induce superb water splitting bifunctionality in alkaline media. A simple wet-chemistry method is developed to grow ultrathin feroxyhyte (δ-FeOOH) nanosheets with rich Fe vacancies on Ni foam substrate. The theoretical and experimental results confirm that, in contrast to anion vacancies, the formation of rich second neighboring Fe to Fe vacancies in δ-FeOOH nanosheets can create catalytic active centers for both hydrogen and oxygen evolution reactions. The atomic level insight into the new catalyst activation principle based on metal vacancies is adaptable for developing other transition metal electrocatalysts, including Fe-based ones.

Catalytic Motifs in Bio-Inspired Materials As New Generation Electrocatalysts

Bio-inspired and bioorganic materials gain outmost of interest due to a wide application area. In the past years, hydrogen-bonded organic semiconductors relying on indigoid pigment family are applied in various device geometries e.g. organic solar cells, organic field effect transistors but also as thin film electrodes in catalysis [1,2]. Currently, the integration of the bio-inspired materials in sustainable catalysis is powerful. Especially, organic functionalities present in the chemical structure of this new material class provide these compounds outstanding properties in terms of using these functional themes as catalytic active centers to substitute state-of-the-art metallic catalysts. Herein, we report on the suitability of a bio-originated polymer – Polydopamine (PDA) - as an effective electrocatalyst [3]. Polydopamine is a major pigment of the eumelanin family. Due to its hydrogen-bonded sequences in the conjugated structure, stabilized over hydroxyl, amine and carbonyl groups, PDA is a prominent competitor for the new generation of sustainable catalysis. We engineered a new synthetic pathway for the polydopamine electrode prepraration [4]. We apply the chemical vapor deposition technique in the presence of the monomer dopaminhydrochlorid together with sulfuric acid as an oxidant in order to form polydopamine films directly on 3D, sponge-like carbon felt electrodes, creating an increased catalytically active surface area. Electrochemical investigations exhibit the interaction and interplay of the functionalities in catalytic processes resulting in high yields and current densities and thus reducing energy losses. [1] Glowacki E. D.; Irimia-Vladu M.; Kaltenbrunner M.; Gasiorowski J.; White M. S.; Monkowius U.; G. Romanazzi G.; Suranna G. P.; Mastrorilli P.; Sekitani T.; Bauer S.; Someya T.; Torsi L.; Sariciftci N. S.; Adv. Mater., 2013, 25, 1563–1569. [2] Glowacki E. D.; Romanazzi G.; Yumusak C.; Coskun, H.; Monkowius U.; Voss G.; Burian M.; Lechner R. T.; Demitri N.; Redhammer G. J.; Sünger N.; Suranna G. P.; Sariciftci N. S.; Adv. Funct. Mater.; 2014, DOI: 10.1002/adfm.201402539. [3] Coskun, H.; Aljabour, A.; De Luna, P.; Farka, D.; Greunz, T.; Stifter, D.; Kus, M.; Zheng, X.; Liu, M.; Hassel, A. W.; Schöfberger, W.; Sargent, E. H.; Sariciftci, N. S.; Stadler, P.; Sci. Adv. 2017, 3 (8), e1700686 DOI: 10.1126/sciadv.1700686. [4] Coskun, H.; Aljabour, A.; Uiberlacker, L.; Strobel, M.; Hild, S.; Cobet, C.; Farka, D.; Stadler, P.; Sariciftci, N. S.; 2018, 645 (August 2017), 320–325 DOI: 10.1016/j.tsf.2017.10.063.

Fe-Promoted Pt-Fe/Al2O3 Catalyst Prepared by Microemulsion Technique for m-Chloronitrobenzene Hydrogenation

A Fe-promoted Pt-Fe/Al2O3 catalyst was prepared by water/oil microemulsion. The physicochemical properties of the Pt/Al2O3 and Pt-Fe/Al2O3 catalyst were characterized by TEM, SAED, XRD, XPS, BET, ICP, and CO-chemisorption. The introduction of Fe could promote the formation of a catalytic active center Pt, enhance electron enrichment of Pt, and improve the degree of Pt dispersion. The effect of Fe on the catalytic performance of the Pt/Al2O3 catalyst was investigated by the liquid-phase hydrogenation of m-chloronitrobenzene to m-chloroaniline. The Fe-promoted Pt-Fe/Al2O3 catalyst was extremely active and more selective than the Pt/Al2O3 catalyst (conversion from 57.7 to 96.3%, selectivity from 98.5 to 99.2%).

Cu‐induced assembly of methanobactin‐modified gold nanoparticles and its peroxidase mimic activity

Methanobactin (Mb) is a small copper-chelating molecule that functions as an agent for copper acquisition, uptake and copper-containing methane monooxygenase catalysis in methane-oxidising bacteria. The UV-visible spectral and fluorescence spectral suggested that Mb/Cu coordination complex as a monomer (Mb-Cu), dimmer (Mb2-Cu) and tetramer (Mb4-Cu) could be obtained at different ratios of Mb to Cu (II). The kinetics of the oxidation of hydroquinone with hydrogen peroxide catalysed by the different Mb/Cu coordination complex were investigated. The results suggested that Mb2-Cu coordination form has highest catalytic capacity. Further, Mb-modified gold nanoparticles (AuNPs) were obtained by ligand exchange and assembled into two- and three-D nanocluster structure by metal-organic coordination as driving force. It has been found that AuNPs increased the catalytic activity of Mb2-Cu on AuNPs. The more significant catalytic activity was exhibited by the nanocluster assembly with multi-catalytic centres. This may be attributed to the multivalent collaborative characteristics of the catalytic active centres in the nanocluster network assembly. The assembly of Mb-modified AuNPs can act as excellent nanoenzyme models for imitating peroxidase.

Synthesis of well-ordered MCM-41 containing highly-dispersed NiO nanoparticles and efficient catalytic epoxidation of styrene

Well-ordered MCM-41 materials with different loadings of NiO were prepared combining hydrothermal method using cetyltrimethylammonia bromide as the structure-directing agent in an ammonia aqueous solution with calcination. A series of characterization of the resulting samples revealed that the materials maintained ordered mesostructure of MCM-41 after loading NiO and NiO nanoparticles were highly dispersed in the mesoporous wall and on the external surface of MCM-41. Characterization results also unveiled that $$\hbox {Ni}^{2+ }$$ prefer to enter the pores that formed by silicone gel during the hydrothermal reaction and incorporated into the pore wall and then formed NiO during the calcination process. And the TEM images indicated that smaller size NiO nanoparticles are easier to be formed on the MCM-41 materials. Catalytic results revealed that NiO should be the catalytic active centers for the oxidation of styrene and Ni-based materials showed the efficient catalytic property. Synopsis Owing to high dispersion of nickel oxide on the MCM-41 support and synergistic effect between NiO nanoparticles and the support, 6Ni-MCM-41 gave higher conversion and selectivity compared to pure NiO nanoparticles.

Efficient Rare-Earth-Based Coordination Polymers as Green Photocatalysts for the Synthesis of Imines at Room Temperature.

Five new rare-earth coordination polymers (CPs) were designed in order to offer a remarkable platform that contains light-harvesting antennas and catalytic active centers to achieve solar-energy conversion as green alternatives in the synthesis of imines. These five new spirobifluorene-containing Ln-CPs, named [Er3(Hsfdc)3(sfdc)3(H2O)]· xH2O (RPF-30-Er), [Ln(Hsfdc)(sfdc)(EtOH)]·S (RPF-31-Ln, where Ln = La, Nd, and Sm and S = H2O or EtOH), and [Ho(Hsfdc)(sfdc)(H2O)] (RPF-32-Ho) (RPF = rare-earth polymeric framework and H2sfdc = 9,9'-spirobi[9 H-fluorene]-2,2'-dicarboxylic acid), have been solvothermally synthesized, and their structural features can be described as follows: (i) RPF-30-Er shows a 3D framework in which the inorganic trimers (secondary building units) are cross-linked by Hsfdc- and sfdc2- linkers displaying a pcu topology. (ii) The isostructural RPF-31-Ln series of materials, together with RPF-32-Ho, exhibit a 1D network of chains growing along the a axis with a ribbon-of-rings topology type. The photocatalytic activity of the RPF- n materials was tested in the oxidative coupling of amines using molecular oxygen and air as oxidizing agents under warm light. Among the materials investigated, RPF-31-Nd was chosen to further investigate the approach in the selectivity of different amine derivates.

Polyoxometalate-Supported Bis(2,2'-bipyridine)mono(aqua)nickel(II) Coordination Complex: an Efficient Electrocatalyst for Water Oxidation.

A polyoxometalate (POM)-supported nickel(II) coordination complex, [NiII(2,2'-bpy)3]3[{NiII(2,2'-bpy)2(H2O)}{HCoIIWVI12O40}]2·3H2O (1; 2,2'-bpy = 2,2'-bipyridine), has been synthesized and structurally characterized. We could obtain a relatively better resolved structure from dried crystals of 1, NiII(2,2'-bpy)3]3[{NiII(2,2'-bpy)2(H2O)}{HCoIIWVI12O40}]2·H2O (D1). Because the title compound has been characterized with a {NiII(2,2'-bpy)2(H2O)}2+ fragment coordinated to the surface of the Keggin anion ([H(CoIIW12O40]5-) via a terminal oxo group of tungsten and the [NiII(2,2'-bpy)3]2+ coordination complex cation sitting as the lattice component in the concerned crystals, the electronic spectroscopy of compound 1 has been described by comparing its electronic spectral features with those of [NiII(2,2'-bpy)2(H2O)Cl]Cl, [NiII(2,2'-bpy)3]Cl2, and K6[CoIIW12O40]·6H2O. Most importantly, compound 1 can function as a heterogeneous and robust electrochemical water oxidation catalyst (WOC). To gain insights into the water oxidation (WO) protocol and to interpret the nature of the active catalyst, diverse electrochemical experiments have been conducted. The mode of action of the WOC during the electrochemical process is accounted for by confirmation that there was no formation/participation of metal oxide during various controlled experiments. It is found that the title compound acts as a true catalyst that has NiII (coordinated to POM surface) acting as the active catalytic center. It is also found to follow a proton-coupled electron-transfer pathway (two electrons and one proton) for WO catalysis with a high turnover frequency of 18.49 (mol of O2)(mol of NiII)-1 s-1.

The catalytic activity for ginkgolic acid biodegradation, homology modeling and molecular dynamic simulation of salicylic acid decarboxylase

The toxic ginkgolic acids are the main safety concern for the application of Ginkgo biloba. In this study, the degradation ability of salicylic acid decarboxylase (SDC) for ginkgolic acids was examined using ginkgolic acid C15:1 as a substrate. The results indicated that the content of ginkgolic acid C15:1 in Ginkgo biloba seeds was significantly decreased after 5 h treatment with SDC at 40 °Cand pH 5.5. In order to explore the structure of SDC and the interaction between SDC and substrates, homology modeling, molecular docking and molecular dynamics were performed. The results showed that SDC might also have a catalytic active center containing a Zn2+. Compared with the template structure of 2,6-dihydroxybenzoate decarboxylase, the residues surrounding the binding pocket, His10, Phe23 and Phe290, were replaced by Ala10, Tyr27 and Tyr301 in the homology constructed structure of SDC, respectively. These differences may significantly affect the substrates adaptability of SDC for salicylic acid derivatives.

New Insights into Sensitization Mechanism of the Doped Ce (IV) into Strontium Titanate.

SrTiO3 and Ce4+ doped SrTiO3 were synthesized by a modified sol–gel process. The optimization synthesis parameters were obtained by a series of single factor experiments. Interesting phenomena are observable in Ce4+ doped SrTiO3 systems. Sr2+ in SrTiO3 system was replaced by Ce4+, which reduced the surface segregation of Ti4+, ameliorated agglomeration, increased specific surface area more than four times compared with pure SrTiO3, and enhanced quantum efficiency for SrTiO3. Results showed that Ce4+ doping increased the physical adsorption of H2O and adsorbed oxygen on the surface of SrTiO3, which produced additional catalytic active centers. Electrons on the 4f energy level for Ce4+ produced new energy states in the band gap of SrTiO3, which not only realized the use of visible light but also led to an easier separation between the photogenerated electrons and holes. Ce4+ repeatedly captured photoelectrons to produce Ce3+, which inhibited the recombination between photogenerated electrons and holes as well as prolonged their lifetime; it also enhanced quantum efficiency for SrTiO3. The methylene blue (MB) degradation efficiency reached 98.7% using 3 mol % Ce4+ doped SrTiO3 as a photocatalyst, indicating highly photocatalytic activity.

Tailoring TiO2 Nanotube-Interlaced Graphite Carbon Nitride Nanosheets for Improving Visible-Light-Driven Photocatalytic Performance.

Rapid recombination of photoinduced electron–hole pairs is one of the major defects in graphitic carbon nitride (g‐C3N4)‐based photocatalysts. To address this issue, perforated ultralong TiO2 nanotube‐interlaced g‐C3N4 nanosheets (PGCN/TNTs) are prepared via a template‐based process by treating g‐C3N4 and TiO2 nanotubes polymerized hybrids in alkali solution. Shortened migration distance of charge transfer is achieved from perforated PGCN/TNTs on account of cutting redundant g‐C3N4 nanosheets, leading to subdued electron–hole recombination. When PGCN/TNTs are employed as photocatalysts for H2 generation, their in‐plane holes and high hydrophilicity accelerate cross‐plane diffusion to dramatically promote the photocatalytic reaction in kinetics and supply plentiful catalytic active centers. By having these unique features, PGCN/TNTs exhibit superb visible‐light H2‐generation activity of 1364 µmol h−1 g−1 (λ > 400 nm) and a notable quantum yield of 6.32% at 420 nm, which are much higher than that of bulk g‐C3N4 photocatalysts. This study demonstrates an ingenious design to weaken the electron recombination in g‐C3N4 for significantly enhancing its photocatalytic capability.

Efficient Nitrogen Fixation via a Redox-Flexible Single-Iron Site with Reverse-Dative Iron → Boron σ Bonding.

Model systems of the FeMo cofactor of nitrogenase have been explored extensively in catalysis to gain insights into their ability for nitrogen fixation that is of vital importance to the human society. Here we investigate the trigonal pyramidal borane-ligand Fe complex by first-principles calculations, and find that the variation of oxidation state of Fe along the reaction path correlates with that of the reverse-dative Fe → B bonding. The redox-flexibility of the reverse-dative Fe → B bonding helps to provide an electron reservoir that buffers and stabilizes the evolution of Fe oxidation state, which is essential for forming the key intermediates of N2 activation. Our work provides insights for understanding and optimizing homogeneous and surface single-atom catalysts with reverse-dative donating ligands for efficient dinitrogen fixation. The extension of this kind of molecular catalytic active center to heterogeneous catalysts with surface single-clusters is also discussed.

Direct Synthesis of Nano‐Ferrierite along the 10‐Ring‐Channel Direction Boosts Their Catalytic Behavior

AbstractFerrierite zeolites with nanosized crystals and external surface areas higher than 250 m2 g−1 have been prepared at relatively low synthesis temperature (120 °C) by means of the collaborative effect of two organic structure directing agents (OSDA). In this way, hierarchical porosity is achieved without the use of post‐synthesis treatments that usually involve leaching of T atoms and solid loss. Adjusting the synthesis conditions it is possible to decrease the crystallite size in the directions of the 8‐ and 10‐ring channels, [010] and [001] respectively, reducing their average pore length to 10–30 nm and increasing the number of pores accessible. The small crystal size of the nano‐ferrierites results in an improved accessibility of reactants to the catalytic active centers and enhanced product diffusion, leading to higher conversion and selectivity with lower deactivation rates for the oligomerization of 1‐pentene into longer‐chain olefins.

Direct Synthesis of Nano-Ferrierite along the 10-Ring-Channel Direction Boosts Their Catalytic Behavior.

Ferrierite zeolites with nanosized crystals and external surface areas higher than 250 m2 g-1 have been prepared at relatively low synthesis temperature (120 °C) by means of the collaborative effect of two organic structure directing agents (OSDA). In this way, hierarchical porosity is achieved without the use of post-synthesis treatments that usually involve leaching of T atoms and solid loss. Adjusting the synthesis conditions it is possible to decrease the crystallite size in the directions of the 8- and 10-ring channels, [010] and [001] respectively, reducing their average pore length to 10-30 nm and increasing the number of pores accessible. The small crystal size of the nano-ferrierites results in an improved accessibility of reactants to the catalytic active centers and enhanced product diffusion, leading to higher conversion and selectivity with lower deactivation rates for the oligomerization of 1-pentene into longer-chain olefins.

Research Progress of Pre - transition Metal Olefin Polymerization Catalyst for Salicylaldehyde Imine

Salicylaldehyde imine transition metal catalyst is a kind of olefin polymerization catalyst which is widely used in the coordination of salicylaldehyde imine ligand and pre-transition metal. Salicylaldehyde imine ligands have the characteristics of easily insert different substituents via organic synthesize. Therefore, the regulation of the polymerization activity, polymerization product and product distribution can be achieved by changing the steric hindrance effect, electronic effect and the number of metal active sites which near the catalytic active center. The development status of the transition metal catalyst of salicylaldehyde imide was summarized in this paper. The influence of the ligand structure of salicylaldehyde imide transition metal catalyst on the catalytic performance which involved the high selectivity ethylene trimerization, ethylene / α-olefin, / Polar monomer copolymerization, ethylene polymerization production of ultra-high molecular weight polyethylene and many other areas of olefin polymerization was elaborated and providing references for further study and industrial applications of this catalyst.

Preparation and Characterization of Mesoporous Nickel derived from Liquid crystalline Template and Evaluation of its Electro catalytic activity towards Methanol Oxidation

Mesoporous Nickel has been prepared by electrodeposition using non-ionic surfactant based liquid crystalline template under optimized processing conditions. Physico-chemical properties of mesoporous nickel is systematically characterized through XRD, SEM and AFM analyses. Comparison of electrocatalytic activity of mesoporous nickel with smooth nickel was interrogated using cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) analyses. Distinctly enhanced electrocatalytic activity with improved surface poisoning resistance related to mesoporous nickel electrode towards methanol oxidation stems from unique mesoporous morphology. This mesoporous morphology with high surface to volume ratio is highly beneficial to promote active catalytic centers to offer readily accessible Pt catalytic sites for MOR, through facilitating mass and electron transports.

Hammett Parameter in Microporous Solids as Macroligands for Heterogenized Photocatalysts

Here we present a series of heterogeneous catalysts based on metal–organic frameworks and microporous polymers used as macroligands for heterogenized organometallic complexes. We show that both homogeneous and heterogenized catalysts follow the same linear correlation between the electronic effect of the ligand, described by the Hammett parameter, and the catalytic activity. This correlation highlights the crucial impact of the local electronic environment surrounding the active catalytic center over the long-range framework structure of the porous support. The rational design of heterogenized catalysts can thus be guided by molecular chemistry rules. The conception of highly efficient heterogeneous catalyst based on porous polymer support and driven by the Hammett parameter of bipyridine-chelating macroligand is demonstrated here for the Rh-catalyzed photoreduction of carbon dioxide with turnover frequencies up to 28 h–1, among the highest reported for heterogeneous photocatalytic formate production.

Exploring bulk and colloidal Mg/Al hydrotalcite–Au nanoparticles hybrid materials in aerobic olefin epoxidation

Au nanoparticles (AuNPs) were immobilized on S-containing amino acids (cysteine and methionine) Mg/Al hydrotalcite clay in both bulk and exfoliated (colloidal) forms. The Au nanoparticles were prepared by a biomimetic method using an anti-oxidant tea extract to reduce the Au salt solution. The performance of bulk and exfoliated clays with Au nanoparticles in aerobic olefin epoxidation was assessed with very interesting results. Both catalysts were very active towards the epoxide products; selectivity was found to be dependent on the catalyst form. The exfoliated catalysts yielded higher product stereoselectivity in the case of limonene epoxidation. These results can be explained by the existence of a confined environment around the AuNPs structures provided by the clay nanosheets wrapping the nanoparticles, modulating how substrates interact with the catalytic active centres. Based on experimental data mechanistic proposals were also rationalized for these catalysts showing how they assist the catalytic process.

Hydroxide Ligands Cooperate with Catalytic Centers in Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction.

Converting CO2 into fuels via photochemical reactions relies on highly efficient and selective catalysts. We demonstrate that the catalytic active metal center can cooperate with neighboring hydroxide ligands to boost the photocatalytic CO2 reduction. Six cobalt-based metal-organic frameworks (MOFs) with different coordination environments are studied at the same reaction condition (photosensitizer, electron donor, water/organic mixed solvent, and visible light). In pure CO2 at 1.0 atm, the MOFs bearing μ-OH- ligands neighboring the open Co centers showed CO selectivities and turnover frequencies (TOFs) up to 98.2% and 0.059 s-1, respectively. More importantly, their TOFs reduced only ca. 20% when the CO2 partial pressure was reduced to 0.1 atm, while other MOFs reduced by at least 90%. Periodic density functional theory calculations and isotope tracing experiments showed that the μ-OH- ligands serve not only as strong hydrogen-bonding donors to stabilize the initial Co-CO2 adduct but also local proton sources to facilitate the C-O bond breaking.

Experimental study on selective catalytic reduction of NO with propene over iron based catalysts supported on aluminum pillared clays

Iron based catalysts supported on aluminum pillared clays (Fe/Al-PILC) was prepared by impregnation method and the selective catalytic reduction of NO with propene by the Fe/Al-PILC catalysts was investigated in a fixed bed reactor. The physical and chemical properties of the catalysts were characterized by N2 adsorption/desorption, XRD, H2-TPR, UV-Vis, Py-IR, etc. Results showed that 9% Fe/Al-PILC reduced 98% of NO at 400-550°C. SO2 and water vapor slightly influenced the catalytic activity of the catalysts. XRD and N2 adsorption/desorption characterization results revealed that the iron oxides in the catalyst were highly dispersed on the surface of the carrier and the catalyst has a large specific surface area and pore volume. H2-TPR results indicated that the activity of the catalysts was mainly determined by the reduction performance of Fe2O3 phase. UV-Vis results showed that the activity of the catalysts was positively correlated with the iron oxide oligomer FexOy. Py-IR results indicated that both Lewis acid and Brønsted acid were formed on the catalyst surface and the Lewis acid sites were the main catalytic activity center of NO and C3H6 reaction.

H2 effect in Chevron–Phillips ethylene trimerization catalytic system: an experimental and theoretical investigation

Effect of H2 amount on the activity and selectivity of Chevron–Phillips trimerization system, i.e., chromium(III) tris(2-ethylhexanoate)/2,5-dimethylpyrrole/triethylaluminum (TEA)/tetrachloroethane (TCE), was examined. In this regard, in addition to catalytic ingredients and ethylene monomer, 2.5 and 5 bar H2 were utilized in the oligomerization reactor and the reaction results were compared with those without H2. Results showed that addition of hydrogen has a remarkable effect on polymer formation and catalyst activity so that 147% decrease in polymer formation and 38% increase in activity were observed upon addition of 5 bar H2. However, its effect on 1-hexene selectivity was not pronounced. Then, molecular modeling with DFT method was employed to simulate oligomerization process in molecular level. Energy results revealed that formation of 1-hexene has less energy barrier than the formation of 1-butene and 1-octene which confirms higher selectivity of Chevron–Phillips system toward 1-hexene formation. Furthermore, interaction of hydrogen with catalytic active center was studied via hydrogenolysis of metalacycles. Energy results revealed that it is energetically a facile process so that the bigger the circle, the easier hydrogenolysis.