Ni Metal Centers Research Articles

Catalytic charge synergy on Ni/Sr@rTiO2 system to promote hydrogen evolution for the implementation of green energy technologies

This study demonstrates an effective and sustainable approach for hydrogen production to the practical implementation of green energy technologies. For this purpose, extremely stable catalysts Ni/Sr@rTiO2 have been synthesized and evaluated for hydrogen evolution reactions. Optical and structural characteristics of catalysts have been evaluated via advanced analytical techniques. These techniques include XRD, UV–Vis/DRS, PL, FT-IR, Raman, SEM, AFM and TEM. Surface area and porosity were obtained with BET while, chemical compositions of catalysts were authenticated via EDX, and XPS. All photoreaction was done in 150 mL reactor (Quartz). Hydrogen evolution activities were examined on GC-TCD (Shimadzu-2010/Japan). Results indicate that bare rutile titanium dioxide (rTiO2) and Ni1.5/Sr0.5@rTiO2 have delivered 0.20 mmol g−1h−1 and 35.14 mmol g−1h−1 of hydrogen respectively. Higher activities (i.e. in case of Ni1.5/Sr0.5@rTiO2) were attributed to the rapid charge transfer on active sites i.e. Ni metal centers. It has been predicted that presence of strontium promotes electrons to the conduction bands of rTiO2; hence favor to elevate the electronic pools so that they can progressively involve for water reduction. The results revealed that Ni1.5/Sr0.5@rTiO2 exhibit higher charge separation relative to the bare rTiO2. It has been noted that formation of synergy for the electron quenching and utilization is main cause of higher hydrogen evolution that is due to presence of nickel and strontium. On the basis of results, it has been concluded that catalyst reported herein has potential to replace the expensive and traditional catalysts associated with hydrogen generation technologies.

Metal-ligand dual-site single-atom nanozyme mimicking urate oxidase with high substrates specificity

In nature, coenzyme-independent oxidases have evolved in selective catalysis using isolated substrate-binding pockets. Single-atom nanozymes (SAzymes), an emerging type of non-protein artificial enzymes, are promising to simulate enzyme active centers, but owing to the lack of recognition sites, realizing substrate specificity is a formidable task. Here we report a metal-ligand dual-site SAzyme (Ni-DAB) that exhibited selectivity in uric acid (UA) oxidation. Ni-DAB mimics the dual-site catalytic mechanism of urate oxidase, in which the Ni metal center and the C atom in the ligand serve as the specific UA and O2 binding sites, respectively, characterized by synchrotron soft X-ray absorption spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, and isotope labeling. The theoretical calculations reveal the high catalytic specificity is derived from not only the delicate interaction between UA and the Ni center but also the complementary oxygen reduction at the beta C site in the ligand. As a potential application, a Ni-DAB-based biofuel cell using human urine is constructed. This work unlocks an approach of enzyme-like isolated dual sites in boosting the selectivity of non-protein artificial enzymes.

Operando Investigations of Reversible and Irreversible Transformations of Metal Organic Framework Based Catalysts during the Oxygen Evolution Reaction

Intermittency issues due to the growth of renewable energy usage lead to an increasing interest in energy storage systems. Thereby, one can use the excess energy for the production of hydrogen from water by anion exchange membrane (AEM) water electrolysis using earth-abundant, non-noble metal catalysts. However, the kinetics of the oxygen evolution reaction (OER) limit the activity of the catalyst and hence, the development of performance stable and active OER catalysts is of great importance for the commercialization of AEM water electrolyzers. In metal organic frameworks (MOFs), a porous structure is created by linking metal atoms/clusters with other metal centres using organic ligands. The resulting structure with a high surface area and dispersed metal centres is a promising catalyst for OER.MOF catalysts with Ni and Co metal centres show impressive OER activity. Both, Co-MOF-74 and Ni-MOF-74 exhibit higher OER activity than their oxide counterparts produced by flame spray synthesis in form of nanoparticles (Fabbri (2017) and Abbott (2018)).1,2 Thereby, an increasing OER activity of Ni-MOF-74 during rotating disk electrode stability tests, indicates that the catalytic species undergoes favorable electronic and structural transformations.Using an in-house-developed spectro-electrochemical flow cell, these transformations were studied by operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Operando XAS enabled us to monitor the changes occurring in the Ni metal centers while performing cyclic voltammetry (CV) from 1 VRHE up to potentials above the OER onset potential with a time resolution of 5 sec. The operando XAS measurements show that Ni-MOF-74 develops into a highly OER active and stable catalytic species. However, operando XRD measurements prove that the crystalline MOF structure changes into an amorphous structure during OER. Monitoring the electronic and structural transformations due to potential cycling, three different structures were extracted: an initial state, which disappeared within the first eight CV cycles, and two novel electronic/local structures appearing at low and high potential, respectively. The transformations in the electronic and local structure of the Ni metal centers occurring between low and high potential appear to be reversible, as the Ni metal centers return to their initial state during long-term storage in air. However, XRD measurements indicate that this further transformation does not affect the structure of the catalyst: Even though the Ni centers return to their original local structure and oxidation state after long-term storage in air, the long-range structure stays amorphous.The study focuses on elucidating the structure-performance relations of Ni-MOF-74 based OER catalysts providing the key structural parameters that lead to the formation of the highly OER active species. The mechanism of reversible and irreversible transformations occurring in Ni-MOF-74 catalysts allows the extraction of activity descriptors for the development of highly OER active and stable non-noble metal catalysts for AEM water electrolysis.

Applied machine learning to analyze and predict CO2 adsorption behavior of metal-organic frameworks

Machine learning provides new insights for designing MOFs with high CO2 adsorption capacity and understanding the CO2 adsorption mechanism. In this work, 348 data points from published reports were collected and four tree-based models were designed to predict the CO2 adsorption capacity of MOFs by machine learning. The results showed that the Random Forest (RF) had the best prediction performance (R2train = 0.970, R2test = 0.896). Feature importance analysis revealed the relative importance of CO2 adsorption parameters (73 %), textures (23 %) and metal centers of MOFs (4 %) for the CO2 adsorption process. Single and synergistic effects of different features were observed through partial dependence analysis. MOFs with Cu, Fe, Co, and Ni metal centers exhibited a promoting effect on CO2 adsorption. In addition, under high pressure, well-developed textures had significant positive impact on CO2 adsorption capacity, while under medium and low pressure, textures were not determining factors.

Ni-single atom decorated mesoporous carbon electrocatalysts for hydrogen evolution reaction

A single atom catalyst (SAC) is a type of heterogeneous catalyst in which individual metal atoms are dispersed on a support material, such as carbon, metal oxides, or zeolites, rather than forming nanoparticles or clusters. SACs have unique catalytic properties, such as high selectivity and activity, due to their high surface area and the exposed active metal sites, allowing them to interact more efficiently with the reactants. SACs also have the advantage of being more environmentally friendly and cost-effective than traditional catalysts, as they require smaller amounts of metal and can be easily recycled. In this study, we report the synthetic method of mesoporous single atom catalysts (MSACs) containing nickel (Ni), platinum (Pt), iridium (Ir), cobalt (Co), and iron (Fe) as metal-centered atoms for hydrogen evolution reaction (HER). MSACs are successfully synthesized with metal-chelation and soft-templating methods. The results show that highly uniform MSACs can be fabricated with high reproducibility. The metal contents in MSACs can be reduced to under 1wt%, keeping good HER reactivity. For HER, the Ni mesoporous single atom catalysts (Ni-MSACs) show 270 mV overpotential at 10 mA/cm2 at the initial polarization curve and reduces to 190 mV after 2,000 cycles, which outperforms Ni bulk catalyst (Ni plate). In contrast, porous pure carbon without Ni atoms shows inferior performance, indicating that Ni metal center plays an important role in the catalytic activity. The computational calculation using density functional theory (DFT) tells that Volmer-Heyrovsky pathway is dominant for HER using the Ni-MSACs. The performance of MSACs has a high potential for improvement of performance and extreme reduction of metal usage by doping nitrogen, phosphorous, and two or more metal atoms into MSACs.

Pioneering practical direct sea water splitting via an intrinsically-selective chlorine-phobic nickel polysulphide nanostructured electrocatalyst for pure oxygen evolution

Direct sea water splitting as a source of clean and uninterrupted energy source is an unparalleled approach to sustainable development. Chlorine oxidation, however, strongly limits its usage and thus requests novel avenues to create chlorine-repelling electrocatalysts for pure oxygen evolution under direct-sea water conditions. Herein, for the first time, a NiS2pSxsurface (catenated sulphur type Ni polysulfide)-based binder-free, 3D electrocatalyst is established as an excellent oxygen evolution reaction (OER) catalyst under un-buffered neutral water conditions, exhibiting a remarkably decreased overpotential (Δη∼320 mV) in comparison to nowadays accepted noble metal IrO2 catalyst. This overpotential value (∼ 360 mV, lowest ever reported in the literature) is lower than the limit for chlorine evolution in neutral conditions, thus furnish a potential platform for intrinsically chlorine-phobic OER electrocatalysis for direct sea water splitting. Surprisingly, quantitative analysis of electro-oxidation products in 0.5 M NaCl using our NiS2pSxsurface electrocatalyst demonstrates the sole formation of pure oxygen, without any chlorine, which is inevitable when using IrO2 and Pt as catalysts. The intrinsic ion-selective behavior of our electrocatayst is related to limited exposure of the sterically and electrostatically hindered Ni metal centers to large Cl- ions, thus selectively evolving oxygen. Furthermore, the fast electrochemical evolution of the NiS2pSxsurface surface to form catenated sulphur type polysulphides species (pSn2-/ S2-=2.1) further enhances the intrinsic chlorine-phobicity of the catalyst, displaying pure oxygen evolution even at noteworthy current densities up to 300 mA/cm2. This is the first demonstration of intrinsically chlorine-phobic catalytic electrodes for the “direct” sea water splitting, displaying unprecedented electrochemical performances and stability, which further opens new paths towards engineering the electrocatalysts surface aiming for intrinsically ion-selective electrodes for various applications.

From Ligand- to Metal-centered Reactivity: Metal Substitution Effect in Thiosemicarbazone-based Complexes for H2 Production.

The quest to develop and optimize catalysts for H2 production requires a thorough understanding in the possible catalytic mechanisms involved. Transition metals are very often the centers of reactivity in the catalysis, although this can change in the presence of a redox-active ligand. Investigating the differences in catalysis when considering ligand- and metal-centered reactivity is important to find the most optimal mechanisms for hydrogen evolution reaction. Here, we investigated this change of reactivity in two versions of a thiosemicarbazone-based complex, using Co and Ni metal centers. While the Ni version has a ligand-centered reactivity, Co switches it toward a metal-centered one. Comparison between the mechanisms show differences in rate-limiting steps, and shows the importance of identifying those steps in order to optimize the system for hydrogen production.

Spin Crossover in Nickel(II) Tetraphenylporphyrinate via Forced Axial Coordination at the Air/Water Interface.

Coordination-induced spin crossover (CISCO) in nickel(II) porphyrinates is an intriguing phenomenon that is interesting from both fundamental and practical standpoints. However, in most cases, realization of this effect requires extensive synthetic protocols or extreme concentrations of extra-ligands. Herein we show that CISCO effect can be prompted for the commonly available nickel(II) tetraphenylporphyrinate, NiTPP, upon deposition of this complex at the air/water interface together with a ruthenium(II) phthalocyaninate, CRPcRu(pyz)2, bearing two axial pyrazine ligands. The latter was used as a molecular guiderail to align Ni···Ru···Ni metal centers for pyrazine coordination upon lateral compression of the system, which helps bring the two macrocycles closer together and forces the formation of Ni–pyz bonds. The fact of Ni(II) porphyrinate switching from low- to high-spin state upon acquiring additional ligands can be conveniently observed in situ via reflection-absorption UV-vis spectroscopy. The reversible nature of this interaction allows for dissociation of Ni–pyz bonds, and thus, change of nickel cation spin state, upon expansion of the monolayer.

Effect of Ni particle size on the production of renewable methane from CO2 over Ni/CeO2 catalyst

Production of ‘renewable Methane’ has attracted renewed research interest as a fundamental probe reaction and process for CO2 utilization through potential use in C1 fuel production and even for future space exploration technologies. CO2 methanation is a structure sensitive reaction on Ni/CeO2 catalysts. To precisely elucidate the size effect of the Ni metal center on the CO2 methanation performance, we prepared 2%Ni/CeO2 catalysts with pre-synthesized uniform Ni particles (2, 4 and 8 nm) on a high surface area CeO2 support. Transmission electron microscopy (TEM) and ambient pressure X-ray photo spectroscopy (AP-XPS) characterization have confirmed that the catalyst structure and chemical state was uniform and stable under reaction conditions. The 8 nm sized catalyst showed superior methanation selectivity over the 4 and 2 nm counterparts, and the methanation activity in term of TOF is 10 times and 70 times higher than for the 4 and 2 nm counterparts, respectively. The DRIFTS studies revealed that the larger Ni (8 nm particles) over CeO2 efficiently facilitated the hydrogenation of the surface formate intermediates, which is proposed as the rate determining step accounting for the excellent CO2 methanation performance.

9,9'-Spirobifluorene Based MOFs: Synthesis, Structure, Catalytic Properties

The 9,9'-spirobi[fluorene]-3,3',6,6'-tetracarboxylic acid (H4SBF), a ligand based on the spirobifluorene (SBF) core, was used as a ligand for the synthesis of the MOFs [Co(H2SBF)Phen(H2O)]n (1), and [M(H2SBF)(4,4´-Bipy)]n (M = Co, Ni, Cu) (2). The ligand H4SBF is based on the spirobifluorene (SBF) core: the interest in the SBF platform is due to its application as chiral ligand in organic electronics, as solid state laser or in third-order nonlinear optics. The MOFs with H4SBF have been obtained by hydrothermal reaction of H4SBF, the corresponding MCl2 and the base (Phen or 4,4´-Bipy) in a molar ratio H4SBF : M2+ : base = 1:1:1. The obtained MOFs are stable in air and thermally stable up to ~400°C. The MOFs 2 (M = Co, Ni, Cu) are isostructural (proved by DRXP data). Only for 1 and 2 (M=Ni) the crystals were obtained apt for crystallographic study. The Ni compound crystallizes in monoclinic system, Cc space group with cell parameters: 40.8251 Å, 12.0031 Å, 11.8383 Å and β = 104.3980, vol. 5618.89 Å3. Ni metal center is hexacoordinated in an octahedral environment with five O atoms of two ligands and one N atom from 4,4- Bipy, which means that there is another N atom not coordinated to metal center. The structure of 2 is 2D with interlayer H bonds, which built up a supramolecular 3D net. The Co compound is triclinic, P-1 space group with cell parameters: 10.9775 Å, 12.7231 Å, 14.2381 Å, α = 65.2030, β = 87.5280, γ = 72.9470, vol. 1718.6 Å3. Like Ni compound, Co metal center is hexacoordinated in an octahedral environment with three O atoms of one ligand, two N atoms from Phen and one water coordinated molecule, which builds up a 2D structure. To test the catalytic ability, the obtained MOFs were used to oxidize styrene molecules with tert-butyl hydroperoxide oxidant. Styrene was fully oxidized into its epoxidized product by 1 in CH2Cl2 in 6 h. Under the same conditions, 1 can oxidize the most inert cyclohexane substrate at 60°C with 80.6% substrate conversion.

Metal derivatives of N1-substituted thiosemicarbazones: Synthesis, structures and spectroscopy of nickel(II) and cobalt(III) complexes

Reactions of nickel(II) and cobalt(II) salts with various N1-substituted thiosemicarbazones [R1R2C2N3–N2H–C1(S)–N1HR3, H2L; R1=C6H5-, R2=H, H2L1(R3=Me); H2L2(R3=Ph), R1=2-OHC6H4-, R2=H, H2L3(R3=Me), H2L4(R3=Et), H2L5(R3=Ph); R2=Me, H2L6(R3=Me), R2=Me, H2L7(R3=Et) and R2, R3=H, Me, H2L8(R1=C4H3S), H2L9(R1=C4H3O)] are described. Reactions of Ni(OAc)2 with the ligands having R1 as phenyl group at C2 carbon (H2L1, H2L2) gave complexes, [Ni(κ2-N3, S-HL)2] (HL−=HL1, 1; HL2, 2). Other ligands with R1 as 2-hydroxyphenyl groups, namely, H2L3–H2L7, with Ni(OAc)2 yielded rust colored compounds, {Ni(κ3-O, N3, S–L)} (L2−=L3–L7) which after the addition of PPh2-CH2-PPh2 yielded dinuclear, [Ni2(κ3-O, N3, S–L)2(μ-P, P–PPh2–CH2–PPh2)] (L2−=L3, 3; L4, 4; L5, 5) and mononuclear, [Ni(κ3-O, N3, S–L)(κ1-P–PPh2–CH2–PPh2)] (L2−=L6, 6; L7, 7) complexes. Likewise, {Ni(κ3-O, N3, S-L5} with 2-phenylpyridine(2-Phpy) has yielded a mononuclear complex, [Ni(κ3-O, N3, S–L5)(κ1-N-2-Phpy)] 8. The geometry around Ni metal center can be formally described as square planar in each of complexes 1–8. Reactions of CoCl2 with H2L8 and H2L9 involved oxidation of CoII to CoIII and yielded octahedral complexes, [Co(κ2-N, S–HL)3] (HL−=HL8, 9; HL9, 10).

Flexible Mono‐ and Bis‐tetraazamacrocyclic Complexes of Copper and Nickel Stabilizing Different Oxidation States

AbstractThe synthesis and structural properties of new mono‐ and bis‐macrocyclic cyclidene complexes of copper(II) andnickel(II) are presented. The compounds with aliphatic ligands are flexible enough to stabilize metal centers in the three oxidation states +1, +2, and +3. All the electron‐transfer processes in the 16‐membered mono‐ and binuclear macrocyclic complexes of copper(II) and nickel(II) were fast and reversible. Two polymorphic forms of the 16‐membered mononuclear complex of nickel(II) were isolated in the solid state and characterized. The 16‐membered macrocyclic complexes show both U‐ and Z‐shape conformations. The deviation of the macrocyclic ligand from planarity affects the electrochemical properties of the complexes and leads to stabilization of different oxidation states of the Cu and Ni metal centers. Substitution of the exocyclic nitrogen atoms with a second alkyl group increases the deviation from planarity and induces rotation of the functional groups at the meso position relative to the macrocyclic ring. In the bis‐macrocyclic complexes, the electron‐donating properties are improved and the formal potential of the metal(II)/metal(III) system is less positive with increasing linker length between the macrocyclic units. The acceptor properties of the metal(II) complexes are weaker, the formal potential of the metal(II)/metal(I) couple being shifted to more negative values.

Support effect of anode catalysts using an organic metal complex for fuel cells

The carbon support effect of Pt–Ni(mqph) electrocatalysts on the performance of CO tolerant anode catalysts for polymer electrolyte fuel cells (PEFCs) was investigated using carbon black and multi-walled carbon nanotubes (MWCNTs), with and without defect preparation. 20%Pt–Ni(mqph)/defect-free CNTs showed a very high CO tolerance (75% compared to the CO-free H 2 case) under 100 ppm CO level in the half-cell system of the hydrogen oxidation reaction. On the other hand, the hydrogen oxidation current on Pt–Ni(mqph)/defective CNTs, Pt–Ni(mqph)/VulcanXC-72R and Pt–Ru/VulcanXC-72R significantly decreased with increasing concentration of CO up to 100 ppm (25–47% compared to the CO-free H 2 case). It is thus considered that the carbon support materials strongly affect the CO tolerance of anode catalysts. This is ascribed to a change in the electronic structure of the Pt particles due to the interaction with the graphene surface, leading to a reduction in the adsorption energy of CO. Ni(mqph) also mitigates CO poisoning due to its ability of CO coordination on Ni metal center.

A Bench-Stable Homodinuclear Ni2−Schiff Base Complex for Catalytic Asymmetric Synthesis of α-Tetrasubstituted anti-α,β-Diamino Acid Surrogates

Catalytic asymmetric direct Mannich-type reactions of α-substituted nitroacetates using a new bench-stable homodinuclear Ni2−Schiff base 1b complex are described. The Ni2−1b complex gave Mannich products, precursors for anti-α,β-diamino acids with an α-tetrasubstituted carbon stereocenter, in >99−91% ee. The Ni2−1b complex was also applicable to direct Mannich-type reactions of malonates and β-keto ester, giving products in high stereoselectivity (up to 99% ee, dr >97:3). Preliminary mechanistic studies suggested that the dinuclear Ni metal centers had key roles to achieve good reactivity and high stereoselectivity.