Strong Metal-metal Interaction Research Articles

Efficient electrocatalysts with strong core-shell interaction for water splitting: The modulation of selectivity and activity

Optimum of electrocatalysts plays a key role in the design and development of cost-effective and environmentally benign technologies for energy conversion and storage. In this work, we study two kinds of core-shell structured electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acid and alkali electrolyte. The electrocatalyst of cobalt@cobalt phosphide nanoparticles is grown on carbon nanotubes (Co@CoP/CNT) and shows a current density of 10 mA cm−2 at an overpotential of 67 mV in acid and 125 mV in alkali, demonstrating the strong metal-metal phosphide interaction facilitating hydrogen reduction. However, the in situ formed cobalt@cobalt oxide (Co@Co3O4/CNT) electrocatalysts from the Co@CoP/CNT can retard both the HER and OER performances through the metal-metal oxide interaction. The underlying mechanisms of the activity and selectivity of the core-shell catalysts towards water splitting reactions is further unveiled by density functional theory (DFT) simulation. The different catalytic behaviors of the core-shell structured catalysts in HER and OER are attributed to the different interactions between the metallic cores, shells and the adsorbed intermediate species. This study not only promises a facile and general strategy to fabricate high performance electrocatalysts but also sheds new light on crucial design principles of catalyst materials for intermediate-selective electrocatalytic reactions.

Tuning crystal-phase of bimetallic single-nanoparticle for catalytic hydrogenation

Bimetallic nanoparticles afford geometric variation and electron redistribution via strong metal-metal interactions that substantially promote the activity and selectivity in catalysis. Quantitatively describing the atomic configuration of the catalytically active sites, however, is experimentally challenged by the averaging ensemble effect that is caused by the interplay between particle size and crystal-phase at elevated temperatures and under reactive gases. Here, we report that the intrinsic activity of the body-centered cubic PdCu nanoparticle, for acetylene hydrogenation, is one order of magnitude greater than that of the face-centered cubic one. This finding is based on precisely identifying the atomic structures of the active sites over the same-sized but crystal-phase-varied single-particles. The densely-populated Pd-Cu bond on the chemically ordered nanoparticle possesses isolated Pd site with a lower coordination number and a high-lying valence d-band center, and thus greatly expedites the dissociation of H2 over Pd atom and efficiently accommodates the activated H atoms on the particle top/subsurfaces.

Boosting the deep oxidation of propane over zeolite encapsulated Rh-Mn bimetallic nanoclusters: Elucidating the role of confinement and synergy effects

Metal nanocluster catalysts with superior activity and stability are urgently needed in the deep oxidation of volatile organic compounds (VOCs). In this study, a novel silicalite-1 zeolite confined rhodium-manganese bimetallic nano-cluster (1.6 nm) catalyst (Rh-MnOx@S-1) was designed and prepared by a simple one-pot method using a dual confinement strategy (shell confinement and strong metal-metal oxide interaction) and applied in the deep oxidation of propane. The results revealed that the temperature at 90% conversion (T90) for propane over Rh-MnOx@S-1 was as low as 264 °C, which is evidently higher than the conventional supported (Rh-MnOx/S-1, T90 = 369 °C) and the MnOx-free (Rh@S-1, T90 = 285 °C) catalysts. Moreover, the turnover frequency (TOF) value of Rh-MnOx@S-1 at 220 °C was up to 16 × 10−3 s−1, which is approximately 5–6 times higher than those of comparable catalysts. Most importantly, Rh-MnOx@S-1 also exhibited excellent high temperature thermal stability, water resistance and recyclability, which can be attributed to the confinement effect of the zeolite shell and the synergistic electronic-structure interaction between the Rh and MnOx species. The in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) analysis indicates that the C3H8 deep oxidation mechanism over Rh-MnOx@S-1 catalyst follows the Mars-van Krevelen (MvK) mechanism, i.e., the adsorbed C3H8 species reacted with the activated oxygen species from Rh-MnOx interfaces. The dual confinement strategy developed in this study provides a new way to design high performance catalysts for the removal of VOCs under harsh reaction conditions.

Interface Engineering-Inspired Electron Regulation in Pt/Pd Hetero-Metallene for Methanol-Assisted Hydrogen Evolution

The small molecule oxidation reaction instead of oxygen evolution reaction coupled with hydrogen evolution reaction can greatly reduce the reaction overpotential of electrochemical water splitting, which is a very efficient and energy-saving hydrogen evolution strategy. Herein, we report an interface engineering constructed two-dimensional ultrathin curled Pt/Pd hetero-metallene for efficient electrocatalytic hydrogen evolution assisted by methanol. The thin-sheet structure of Pt/Pd hetero-metallene provides a large specific surface area and exposes numerous surface atoms that could act as reactive sites, thus accelerating the reaction mass transfer process. More importantly, the constructed Pt/Pd hetero-metallene possesses abundant Pt/Pd heterointerface, which can maximize the strong metal-metal interaction and increase the utilization of metal atoms, thereby optimizing the adsorption and activation of reactants during the reaction. Pt/Pd hetero-metallene can produce hydrogen stably and efficiently in 1 M KOH + 1 M CH3OH, and the voltage only needs 0.83 V at @100 mA cm-2 when used in electrocatalytic hydrogen evolution, which is much lower than the voltage required for the traditional electrochemical water splitting process (1.94 V). This work not only provides a powerful approach to rational design and construction of hetero-metallene through interface engineering, but also builds a bridge between hetero-metallene and methanol-assisted hydrogen evolution.

Realizing a redox-robust Ag/MnO2 catalyst for efficient wet catalytic ozonation of S-VOCs: Promotional role of Ag(0)/Ag(I)-Mn based redox shuttle

A novel process for S-VOCs degradation using wet scrubbing coupled with catalytic ozonation was developed for first time. We report redox-robust catalysts consisting of Ag(0)/Ag(I) species decorated rambutan-like MnO2 hollow microspheres (Ag/R-MnO2) with strong metal-metal oxide interaction (MMOI) exhibited superior catalytic ozonation performance for CH3SH elimination. The optimum Ag/R-MnO2 reached a significant improvement in CH3SH elimination of 96.9% conversion over pristine R-MnO2 under GHSV of 75,000 mL h−1 g−1, and O3 utilization rate of 92.3%. An outstanding stability of Ag/R-MnO2 under wet catalytic ozonation process was demonstrated, which outperformed that in gaseous system. CH3SH was captured by aqueous solution and preferentially chemisorbed on Ag, then deeply oxidized to the final products of SO42-/CO32- via catalytic ozonation by multivalent R-MnO2. The excellent performance can be ascribed to efficient electron replenishing interaction between Ag(0)/Ag(I) and multivalent R-MnO2, efficient O3 activation through oxygen vacancies-rich R-MnO2, and enhanced mass diffusion in wet scrubbing process.

Effect of copper on NiCu bimetallic catalyst supported on SBA-16 for the catalytic hydrogenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran

As a consequence of the rapid utilization of fossil resources, the effective, sustainable biomass transformation will get great importance towards the production of valuable chemicals. Here, we developed 5 wt% Ni loaded Cu(x) (where x = 8,10,12, &14%) supported on SBA-16and evaluated its catalytic performance in the hydrogenation of 5-hydroxymethylfurfural (5-HMF), which was carried out a bench top stainless steel autoclave reactor. The major product in the present reaction is 2,5-dimethylfuran (DMF). A number of characterization techniques were used to clarify the effective loading of bimetals on SBA-16 and the novelty of catalysts. The prepared bimetallic catalysts reduction profile shifted towards lower temperatures compared to a monometallic catalyst, indicating stronger metal-metal interaction. X-ray photoelectron spectroscopy (XPS) was performed to distinguish between oxide and metallic states of Ni and Cu. Among the prepared catalysts, 5Ni–12Cu/SBA-16 exhibited superior performance regarding the entire conversion of 5-HMF on account of better synergy between Cu and Ni species, well dispersion of the metal in the support, favorable surface acidity. Also, this catalyst shows a favorable DMF yield of 60.7%, and DMF is the major hydrogenation product in all the reactions. The reaction was thoroughly optimized and developed by a thorough investigation of various reaction parameters such as Ni/Cu ratio, temperature, pressure, reaction time, and catalyst recyclability.

Reconnoitring the nature of interaction and effect of electric field on Pd/Pt/Ni decorated 5-8-5/55–77 defected graphene sheet for hydrogen storage

There is plenty of graphene based Hydrogen storage technologies and studies still few questions like ‘what kind of interaction present between Metal-Metal, Metal-Graphene, Metal-Hydrogen and Graphene-Metal and a possible way of controlling it to enhance H2 adsorption’ are not revealed properly. Similarly, the chosen metal atoms Pd, Pt and Ni are widely reported as a promising catalyst yet there is no conclusive evidence to show the best among three atoms. Thus, in this present work 5-8-5 and 55–77 defected graphene is decorated with the Pd, Pt and Ni metal atoms to adsorb Hydrogen molecules. The obtained results have shown that the better adsorption of H2 molecule depends on Metal-Metal and Metal-Graphene interaction. Similarly, the adhesive force between Pt and 5-8-5/55–77 sheets are slightly higher than the Pd and Ni atoms. Pd–Pd (−0.47 eV) and Pt–Pt (−1.99 eV) interaction values on the surface of 5-8-5 sheets are slightly lesser in magnitude than the Pd–C (−1.14 eV, −1.19 eV) and Pt–C (−2.42 eV, −2.55 eV) interactions. The topological analysis results exhibit the partially covalent nature of interaction and it confirms that the adhesive force between Metal-Graphene is higher than the cohesive force between Metal-Metal on 5-8-5 and 55–77 sheets. The electrophilicity results of Pd, Pt and Ni decorated sheets show that the two Pt decorated 5-8-5 sheet has higher electrophilicity value of 16.782 eV which is considerably higher than other sheets and this particular 5-8-5-Pt2 system has higher H2 adsorption energy value of −1.696 eV. The overall pattern of H2 adsorption on chosen three metals are Pt > Ni > Pd and our results show that both strong Metal-Metal and Metal-Graphene interactions lead to poor adsorption activity. The metals are strongly polarizing the H2 molecules which lead to good adsorption. Further, the results confirm that the π orbitals of Metal and Graphene play a major role in the adsorption of excessive H2 molecules. In order to enhance and control the H2 adsorption energy, a positive electric field is applied to the system. This applied electric field enhances the polarization which leads to H–H bond elongation and strong adsorption. From the obtained results, it is conclusive that the 5-8-5-Pt system has shown good response for the supplied electric field with the maximum adsorption energy value of −5.23 eV. Comparatively, the 5-8-5 systems are responding well for the applied electric field by increasing the adsorption energy than 55–77 systems.

Temperature dependent chiroptical response of sigmoidal gold clusters: probing the stability of chiral metal clusters.

The stability of chiral metal clusters is of great importance for their practical applications. Herein we select three structurally well-defined gold cluster compounds to probe how structural factors influence the stability of chiral metal clusters upon heating. Through monitoring the variation of CD, UV-vis and NMR spectra at elevated temperatures, the biased chiroptical response of three sigmoidal Au6 clusters is finally ascribed to the synergistic effect of the distinct structural tunability of central diamino ligands, inter-cluster aurophilic interactions and steric hindrance. The rigid skeleton of chiral ligands and the strong metal-metal interaction effectively enhance the stability of asymmetric structural motifs in chiral metal clusters. In addition, some central diamino ligands lead to a destructive decomposition of corresponding chiral clusters in the heating process due to the reduction of Au(i) to Au(0). The relationship between structural characteristics and the stability of chiral clusters addressed in this study will facilitate our understanding on how to achieve stable chiral metal clusters and potentiate their practical applications.

Synthesis, characterization and single crystal structures of chiral Schiff base and its tetranuclear palladium complex with Pd[sbnd]O[sbnd]Pd bridging and Pd[sbnd]Pd bonds

A new chiral Schiff base ligand 2-{N-[(2S)-(1-hydroxy-3-phenylpropan-2-yl]ethanimidoyl} phenol ((S)H2L) was obtained by acid catalyzed condensation of (2S)-(−)-2-amino-3-phenyl-1-propanol with 2′-hydroxyacetophenone. The palladium complex was prepared by treating a solution of (S)H2L in acetone with a solution of Na2PdCl4 in water in 1:1 M ratio. The new ligand and its complex were characterized by FT-IR, 1H, 13C{1H} NMR spectroscopy, polarimetry and elemental analysis and their molecular structures were determined by single crystal X-ray diffraction. Both the compounds crystallizes in monoclinic system in the space group P21. There exists an intra [OH⋯N (1.62(5) Å)] and intermolecular [OH⋯O (1.53(5) Å) and CH⋯O (2.59 Å)] hydrogen bonding and secondary interactions in the crystal of (S)H2L. The structure of the palladium complex was found very interesting wherein the ligand coordinated to metal center as tridentate dianionic (O−, N, O−) fashion, (S)-L, resulting in a tetranuclear palladium cluster, [Pd4((S)-L)4]. In these supramolecular structures phenolate oxygen coordinated to Pd(II) ion as PdO terminal bonds [1.934(12) - 1.977(11) Å] and the alkoxide oxygen coordinated as PdOPd bridging bonds [1.993(11) - 2.012(12) Å]. The PdN bond lengths found were in the range of 1.949(13) to 1.919(12) Å. There exists two asymmetric tetranuclear complex molecules in its crystal lattice. There exists very strong metal-metal bond interaction, Pd(2)Pd(3) [3.0410(18) Å] and Pd(6)Pd(7) [3.0517(19) Å] respectively in the two asymmetric units.

Half-Metallicity in One-Dimensional Metal Trihydride Molecular Nanowires.

The development of one-dimensional (1D) molecular nanowires with high spin-polarization is important for both fundamental research and practical applications in nanoscale spintronics. Herein, we report new 1D metal trihydride molecular nanowires MH3 (M = Sc, Cr, Mn, and Co) with versatile magnetic properties on the basis of first-principles calculations and molecular assembly of their molecular motifs. Among the 1D nanowires considered, CrH3, MnH3, and CoH3 are either antiferromagnetic or ferromagnetic in their ground states. In particular, CoH3 nanowire is a half-metal, which ideally could provide 100% spin-polarized currents. Moreover, carrier doping in MnH3 nanowire can induce a transition from ferromagnetic metal to half-metal. Strong metal-metal interaction in 1D MH3 nanowires is responsible to versatile magnetic properties and high Curie temperature. Born-Oppenheimer molecular dynamics simulation indicates that these nanowires are stable at elevated temperature. In particular, the ScH3 nanowire appears to have the highest structural stability at temperature 1200 K. These novel properties of 1D MH3 nanowires render their potential applications in nanoscale spintronics.

Effect of Strong Metal Support Interactions of Supported Ni and Ni-Co Catalyst on Metal Dispersion and Catalytic Activity toward Dry Methane Reforming Reaction

This work studied the effect of interaction between active metal species (Ni and Ni-Co species) and different support (Al2 O3-MgO and SiO2) on the catalyst properties and catalytic performance in dry methane reforming reaction. Supports were synthesized by sol-gel method, then the catalyst samples were prepared by incipient wetness impregnation with a salt solution of active metal(s). The fresh catalysts were characterized via Scanning Electron Microscopy (SEM), Temperature Programmed Reduction (TPR) and Temperature Programmed Desorption (TPD) techniques. Dry Methane Reforming (DMR) activities were tested in a conventional fixed-bed reactor at 620°C. The SEM images and H2-TPR results reveal that the Strong Metal Support Interaction (SMSI) takes place in the catalysts based on Al2 O3-MgO support. The metal dispersion and average metal particle size were evaluated using TPD results. It was found that a Ni-Co bimetallic catalyst provides higher metal dispersion and smaller average metal particle size than Ni monometallic catalysts due to the effect of metal-metal interaction. The bimetallic catalyst with weaker metal-support interaction improve the metal-metal interaction resulting in increasing higher metal dispersion. DMR test presented that sites of strong metal-support interaction enhanced CH4 activity. The stronger metal-metal interaction in Ni-Co bimetallic catalyst with SiO2 support promoted CO2 activity.

Effect of metal-metal and metal-support interaction on activity and stability of Pd-Rh/alumina in CO oxidation

A series of Pd-Rh/alumina catalysts with total precious metals loading of 0.2wt.% was prepared by means of mechanical mixing of monometallic precursors and incipient wetness impregnation of the support with dual complex salt. Gamma and delta alumina supports were obtained from commercial aluminum hydroxide by calcinations at 720 and 1000°C respectively. Supports were examined by electron paramagnetic resonance spectroscopy, low-temperature nitrogen adsorption, X-ray diffraction analysis and photoluminescence spectroscopy. It was shown that concentration of donor sites of the supports differs according to distinctions between values of their specific surface areas. Finally, the strength of metal-support interaction is getting much weaker in the case of δ-Al2O3. Bimetallic Pd-Rh species with strong metal-metal interaction supported on γ-Al2O3 were found to be characterized by superior high-temperature stability. Pd/Rh ratios of 3:2 and 7:3 were shown to be the most appropriate for this purpose.

The effect of double impurity cluster of Ni and Co in TiO2 bulk; a DFT study

The geometrical structure, electronic properties and stability energy of different arrangements of double cluster of Ni-Ni, Co-Co and Ni-Co in rutile and anatase bulk were investigated. Our calculated results showed that Co-Co preferred to state in the nearest distance (nn arrangement) in both anatase and rutile unit cells. While, the Ni-Ni and Ni-Co clusters showed the most stability with the nn form in rutile structure, they preferred the n arrangement (next to the nearest neighbor) in anatas one. These results can be explained by the crystalline structure of rutile that provides the stronger metal-metal interaction by lower distance and also the activity of Co rather than Ni. The band structure results showed a high, moderate and slight decline in the band gap of anatase and rutile in the presence of Co-Co, Ni-Co and Ni-Ni clusters, respectively. An interesting result was shown by altering the direct band gap of pure rutile to the indirect band gap in effect of Co-Co cluster that introduces the Co/rutile structure with an obvious photo-activity. Ni-Ni and Ni-Co impurities show an inverse behavior in anatase phase by changing the indirect band gap of pure anatase to the direct band gap. This may be the main reason to explain the lower photo-efficiency of Ni/TiO2 than pure TiO2 under UV illumination. In the last section we show that the Co impurity energetically prefer to state in TiO2 bulk versus the Ni one by energy of 5 eV.

Can Multiconfigurational Self-Consistent Field Theory and Density Functional Theory Correctly Predict the Ground State of Metal-Metal-Bonded Complexes?

The electronic structure of a diiron (FeFe) complex with strong metal-metal interaction and those of analogous complexes (CoCo, CoMn, CoFe, and FeMn) with much weaker metal-metal bonding are investigated with wave function-based methods and density functional theory. The delocalization and bonding between the metal centers in the diiron complex is only fully captured after inclusion of the complete set of 3d and 4d orbitals in the active space, a situation best suited for restricted active space (RAS) approaches. Truncation of the included set of 4d orbitals results in inappropriate localization of some 3d orbitals, incorrect description of the ground spin state as well as wrong spin state energetics, as compared to experiment. Using density functional theory, some local functionals are able to predict the correct ground spin states, and describe the chemical bonding and structural properties of all the metal-metal complexes considered in this work. In contrast, the introduction of some exact exchange results in increased localization of 3d orbitals and wrong spin state energetics, a situation that is particularly troublesome for the diiron complex.

High catalytic activity for formaldehyde oxidation of AgCo/APTES@MCM-41 prepared by two steps method

We reported here a novel synthesis procedure to prepare a highly active bimetallic AgCo/APTES@MCM-41 catalyst. Complete oxidation of formaldehyde was achieved at temperature as low as 90°C over the bimetallic catalysts with Ag/Co=3/1 (mass ratio). N2 isotherms, small-angle XRD results suggested that both Ag and Co species were well dispersed on the support, and the pore structure of the support was not destroyed after introducing the active metal. The formation of the strong metal–metal interaction (SMMI) between Ag and Co in the bimetallic catalysts due to the electron transfer between each other resulted in the high Co3+/Co2+ ratio and good low-temperature reducibility. And more surface active oxygen species (Osurf) formed on the surface of the bimetallic catalysts. All these factors gave rise to the high formaldehyde catalytic oxidation activity in AgCo bimetallic catalysts. The possible structure changes of bimetallic catalysts and the reaction path for the complete oxidation of formaldehyde over AgCo bimetallic catalyst was also proposed.

Growth kinetics of metal nanoparticles on solid surfaces

Dispersed metal nanoparticles (nps) in a polymer matrix are essential for many technological applications, including biological imaging, thin film technology, magnetic recording media, optoelectronics and sensors. Real time investigation of the evolution of nps size and shape during the in-situ metal deposition on polymer thin films enables a fine tune of magnetic and electric properties. Metals in their atomic state are deposited on several homopolymer and block copolymer films by DC magnetron system (Metwalli et al., 2013, Metwalli et al., 2008, Buffet et al., 2011). With the unprecedented time resolution of 10 milliseconds, the growth kinetics of the metal nps on the polymer surfaces is monitored using in-situ GISAXS. An exponential growth of nps size on all polymer surfaces is observed. Below a certain critical nps size, an initial fast particle growth is due to high particle mobility. A slower kinetics at concentrated metal dispersion is due to the strong metal-metal interactions. The metal growth kinetics study for many chemically different homopolymer films explains the long-time debated high selectivity characteristics of metals towards one block in block copolymer based nano-templates.

Bis(pentalene)di-titanium: a bent double-sandwich complex with a very short Ti–Ti bond

The novel bimetallic bis(pentalene) complex Ti2(μ:η(5),η(5)-Pn(†))2 (Pn(†) = C8H4{Si(i)Pr3-1,4}2) has been synthesised and structurally characterised. Structural data show a Ti-Ti distance of 2.399(2) Å, consistent with a strong metal-metal interaction, which DFT calculations best describe as a double bond with σ and π components.

Synthesis and Characterization of [M2(NCtBu2)5]− (M=Mn, Fe, Co): Metal Ketimide Complexes with Strong Metal–Metal Interactions

Hilfreiche Unterstützung: Die Zugabe von 2.5 Äquiv. Li(NCtBu2) zu MCl2 (M=Mn, Fe, Co) und der anschließende Zusatz von [12]Krone-4 führen zur Bildung von [Li([12]Krone-4)2][M2(NCtBu2)5] in guten Ausbeuten. Diese Komplexe weisen kleine M-M-Abstände und eine starke magnetische Wechselwirkung zwischen den Metallzentren auf, was belegt, dass Ketimide geeignete Hilfsliganden für den Aufbau von Metall-Metall-Bindungen sind.

Catalytic properties of Pt cluster-decorated CeO2 nanostructures

Uniform clusters of Pt have been deposited on the surface of capping-agent-free CeO2 nanooctahedra and nanorods using electron beam (e-beam) evaporation. The coverage of the Pt nanocluster layer can be controlled by adjusting the e-beam evaporation time. The resulting e-beam evaporated Pt nanocluster layers on the CeO2 surfaces have a clean surface and clean interface between Pt and CeO2. Different growth behaviors of Pt on the two types of CeO2 nanocrystals were observed, with epitaxial growth of Pt on CeO2 nanooctahedra and random growth of Pt on CeO2 nanorods. The structures of the Pt clusters on the two different types of CeO2 nanocrystals have been studied and compared by using them as catalysts for model reactions. The results of hydrogenation reactions clearly showed the clean and similar chemical surface of the Pt clusters in both catalysts. The support-dependent activity of these catalysts was demonstrated by CO oxidation. The Pt/CeO2 nanorods showed much higher activity compared with Pt/CeO2 nanooctahedra because of the higher concentration of oxygen vacancies in the CeO2 nanorods. The structure-dependent selectivity of dehydrogenation reactions indicates that the structures of the Pt on CeO2 nanorods and nanooctahedra are different. Thes differences arise because the metal deposition behaviors are modulated by the strong metal-metal oxide interactions.

Noncovalent Metal−Metal Interactions: The Crucial Role of London Dispersion in a Bimetallic Indenyl System

The nature of the "heterodox" metal-metal bond in bimetallic indenyl systems is revealed by a new high-level electronic structure analysis procedure based on perturbation theory within a localized molecular orbital basis. Surprisingly strong metal-metal interactions are found, and their origin in London dispersion effects has been uncovered. The importance of intramolecular van der Waals interactions, even for quite small organometallic systems, is highlighted.