RESUMO
In the pursuit of enhancing the applications of hydrogen as an energy carrier, this research delved into the utilization of a singular hybrid catalyst capable of performing both dehydrogenation and hydrogenation processes for Liquid Organic Hydrogen Carriers (LOHCs). This study presents the synthesis and characterization of a hybrid catalyst, combining an organometallic pincer complex with Pd-Ru heterostructures supported on γ-alumina. Unlike conventional transition and noble metal nanoparticles, the use of a pincer complex offers exceptional thermal stability due to its aryl backbone, which is advantageous for various endothermic dehydrogenation reactions of hydrocarbons in LOHCs. This pioneering hybrid catalyst is a novel approach, demonstrating a proof of concept. In this study, we utilized the hybrid catalyst to investigate the dehydrogenation and hydrogenation of a lower enthalpic system, specifically the cyclooctane-cyclooctene system. The dehydrogenation of cyclooctane was conducted at 443â K using tertiary butyl ethylene as a sacrificial hydrogen acceptor, while the hydrogenation of cyclooctene reaction catalyzed by Pd-Ru nanostructures occurred at 298â K and 1â atm H2. The results showed successful tandem dehydrogenation-hydrogenation reactions. However, challenges were noted in terms of catalytic activity and recyclability, providing valuable insights for further refinement and optimization.
RESUMO
Digestive ripening (DR) is a postsynthetic protocol for the transformation of a colloid consisting of polydisperse metal nanoparticles (NPs) into a colloid composed of nearly monodisperse metal nanoparticles. This process is brought about by the digestive ripening agent, typically an organic ligand with a long alkyl chain at one end and a functional group at the other, at the boiling point of the solvent in which it is carried out, requiring long periods of time. In this work, digestive ripening of polydisperse Ag and Au nanoparticles brought about by ionic liquids (ILs) under very mild conditions (â¼273 K, â¼30 min) to obtain nearly monodisperse nanoparticles has been demonstrated. Herein, the ionic liquid plays a dual role, as a digestive ripening and a stabilizing agent for the nanoparticles. Ionic liquid-assisted digestive ripening under such mild temperatures and short period of time has hitherto not been reported.
RESUMO
A reaction of fundamental and commercial importance is acetylene semi-hydrogenation. Acetylene impurity in the ethylene feedstock used in the polyethylene industry poisons the Ziegler-Natta catalyst which adversely affects the polymer quality. Pd based catalysts are most often employed for converting acetylene into the main reactant, ethylene, however, it often involves a tradeoff between the conversion and the selectivity and generally requires high temperatures. In this work, bimetallic Pd-Zn nanoparticles capped by hexadecylamine (HDA) have been synthesized by co-digestive ripening of Pd and Zn nanoparticles and studied for semi-hydrogenation of acetylene. The catalyst showed a high selectivity of ~85 % towards ethylene with a high ethylene productivity to the tune of ~4341â µmol g-1 min-1 , at room temperature and atmospheric pressure. It also exhibited excellent stability with ethylene selectivity remaining greater than 85 % even after 70â h on stream. To the best of the authors' knowledge, this is the first report of room temperature acetylene semi-hydrogenation, with the catalyst effecting high amount of acetylene conversion to ethylene retaining excellent selectivity and stability among all the reported catalysts thus far. DFT calculations show that the disordered Pd-Zn nanocatalyst prepared by a low temperature route exhibits a change in the d-band center of Pd and Zn which in turn enhances the selectivity towards ethylene. TPD, XPS and a range of catalysis experiments provided in-depth insights into the reaction mechanism, indicating the key role of particle size, surface area, Pd-Zn interactions, and the capping agent.
RESUMO
Development of heterogeneous metal catalysts with superior activity and high selectivity towards the semi-hydrogenation of alkynes is of significant industrial importance. Bimetallic Pd-Ru nanocomposites were synthesized over the entire composition range using a capping-agent assisted solution-based approach. The as-prepared nanocomposites demonstrated excellent catalytic activity towards the hydrogenation of a wide range of olefinic substrates and exceptional selectivity towards styrene in a phenylacetylene semi-hydrogenation reaction under mild conditions. In addition, hydrogen sorption studies were performed on the as-prepared bimetallic Pd-Ru nanomaterials, where an increase in the incorporation of Ru atoms into the bimetallic structure resulted in a decrease in the hydrogen storage capacity of the resulting material. Comprehensive structural characterization and insights obtained from the hydrogen sorption studies revealed that the presence of weakly bound hydrogen atoms in the hydride phase of the bimetallic species contributes to its excellent selectivity in hydrogenation reactions.
RESUMO
From an ingenious methodology for obtaining monodispersity, digestive ripening has advanced to become an outstanding solution-based synthesis route to realizing various bimetallic heterostructures. This feature article attempts to provide an overview of the various facets of the codigestive ripening process and the array of heterostructures that could be achieved by this technique. We briefly discuss the mechanism of digestive ripening in the case of monometallic elements and use that understanding to elucidate the mechanism of the less well established codigestive ripening strategy for designing bimetallic nanostructures. The systems studied by our group in the past decade for the fabrication of diverse heterostructures are highlighted in this article. The exploitation of digestive ripening to realize monodisperse bimetallic nanostructures by several other groups is also featured. In addition to digestive ripening agents, the significance of tuning various reaction parameters and its consequences on the final structure and morphology have also been discussed. Additionally, efforts based on theoretical studies to gain insight into the factors which dominate the mechanism of the digestive ripening process have also been covered. This article is a contribution to the understanding of the codigestive ripening methodology and a demonstration of its tremendous potential in achieving the desired bimetallic heteronanostructures.
RESUMO
Here we present digestive ripening facilitated interatomic diffusion for the phase controlled synthesis of homogeneous intermetallic nanocrystals of Au-Sn system. Au and Sn metal nanoparticles synthesized by a solvated metal atom dispersion (SMAD) method are employed as precursors for the fabrication of AuSn and Au5Sn which are Au-rich Au-Sn intermetallic nanocrystals. By optimizing the stoichiometry of Au and Sn in the reaction mixture, and by employing growth directing agents, the formation of phase pure intermetallic AuSn and Au5Sn nanocrystals could be realized. The as-prepared Au and Sn colloidal nanoparticles and the resulting intermetallic nanocrystals are thoroughly characterized by powder X-ray diffraction, transmission electron microscopy (TEM and STEM-EDS), and optical spectroscopy. The results obtained here demonstrate the potential of solution chemistry which allows synthesizing phase pure Au-Sn intermetallics with tailored morphology.
RESUMO
CdS-based materials are extensively studied for photocatalytic water splitting. By incorporating Ag+ into CdS nanomaterials, the catalyst's charge carrier dynamic can be tuned for photo-electrochemical devices. However, photo-corrosion and air-stability of the heterostructures limit the photocatalytic device's performance. Here, a one-pot, single molecular source synthesis of the air-stable AgCdS ternary semiconductor alloy nanostructures by heat-up method is reported. Monoclinic and hexagonal phases of the alloy are tuned by judicious choice of dodecane thiol (DDT), octadecyl amine (ODA), and oleyl amine (OLA) as capping agents. Transmission electron microscope (TEM) and powder X-ray diffraction characterization of the AgCdS alloy confirm the monoclinic and hexagonal phase (wurtzite) formation. The high-resolution TEM studies confirm the formation of AgCdS@DDT alloy nanorods and their shape transformation into nano-triangles. The nanoparticle coalescence is observed for ODA-capped alloys in the wurtzite phase. Moreover, OLA directs mixed crystal phases and anisotropic growth of alloy. Optical processes in AgCdS@DDT nano-triangles show mono-exponential decay (3.97 ± 0.01 ns). The monoclinic phase of the AgCdS@DDT nanorods exhibits higher electrochemical hydrogen evolution activity in neutral media as compared to the AgCdS@ODA/OLA alloy nanocrystals. DDT and OLA-capped alloys display current densities of 14.1 and 14.7 mA cm-2 , respectively, at 0.8 V (vs RHE).
RESUMO
A highly electrophilic ruthenium center in the [RuCl(dppe)(2)][OTf] complex brings about the activation of the B-H bond in ammonia borane (H(3)N·BH(3), AB) and dimethylamine borane (Me(2)HN·BH(3), DMAB). At room temperature, the reaction between [RuCl(dppe)(2)][OTf] and AB or DMAB results in trans-[RuH(η(2)-H(2))(dppe)(2)][OTf], trans-[RuCl(η(2)-H(2))(dppe)(2)][OTf], and trans-[RuH(Cl)(dppe)(2)], as noted in the NMR spectra. Mixing the ruthenium complex and AB or DMAB at low temperature (198/193 K) followed by NMR spectral measurements as the reaction mixture was warmed up to room temperature allowed the observation of various species formed enroute to the final products that were obtained at room temperature. On the basis of the variable-temperature multinuclear NMR spectroscopic studies of these two reactions, the mechanistic insights for B-H bond activation were obtained. In both cases, the reaction proceeds via an η(1)-B-H moiety bound to the metal center. The detailed mechanistic pathways of these two reactions as studied by NMR spectroscopy are described.
Assuntos
Amônia/química , Boranos/química , Dimetilaminas/química , Compostos Organometálicos/química , Rutênio/química , Espectroscopia de Ressonância Magnética , Compostos Organometálicos/síntese química , TemperaturaRESUMO
Diluting the precious metal to be used for catalysis with an abundant and non-precious metal to form alloy/intermetallic nanostructures is of great interest owing to the cost-effectiveness of the catalyst. The physicochemical properties of such bimetallic nanostructures are influenced by the atomic ordering of different atoms in the system, generally enhancing the catalytic activity, selectivity, and durability compared to their monometallic counterparts. To understand the relationship between the structure and the activity of the catalyst, phase-controlled synthesis of alloy/intermetallic nanostructures is crucial. Easy and scalable synthesis of such nanostructures with phase control presents a true challenge. We exploited a colloidal-based synthetic route termed 'co-digestive ripening' to prepare Pd-Sn alloy/intermetallic nanostructures. Oleylamine capped Pd and Sn colloids were utilized to obtain network-like Pd3 Sn and grape-like Pd2 Sn nanostructures. Temperature and the stoichiometric ratio between Pd and Sn played significant roles in achieving phase control. The mixture of ligands (oleylamine and trioctylphosphine) in the synthetic procedure resulted in the formation of well-separated nanoparticles (2.9±0.5â nm) in the case of Pd3 Sn and few nm-sized particles along with aggregates in the case of Pd2 Sn. Pd-Sn nanostructures showed enhanced activity and selectivity as compared to their monometallic counterparts for the catalytic performance towards oxidation of benzyl alcohol.
RESUMO
Treatment of trans-[Ir(H)(N)2(iPr)4(POCOP)(DMAP)][BAr4f] (2) with H2 (1 bar) under ambient conditions (298 K) results in the formation of a trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(DMAP)][BAr4f] (3) complex. Complex 3 exhibits H-atom site exchange between the bound H2 and the hydride ligands which are mutually trans to one another. A plausible mechanism of this exchange involves metal-ligand cooperativity as studied by computations.
RESUMO
A new solid state synthetic route has been developed toward metal and bimetallic alloy nanoparticles from metal salts employing amine-boranes as the reducing agent. During the reduction, amine-borane plays a dual role: acts as a reducing agent and reduces the metal salts to their elemental form and simultaneously generates a stabilizing agent in situ which controls the growth of the particles and stabilizes them in the nanosize regime. Employing different amine-boranes with differing reducing ability (ammonia borane (AB), dimethylamine borane (DMAB), and triethylamine borane (TMAB)) was found to have a profound effect on the particle size and the size distribution. Usage of AB as the reducing agent provided the smallest possible size with best size distribution. Employment of TMAB also afforded similar results; however, when DMAB was used as the reducing agent it resulted in larger sized nanoparticles that are polydisperse too. In the AB mediated reduction, BNH(x) polymer generated in situ acts as a capping agent whereas, the complexing amine of the other amine-boranes (DMAB and TMAB) play the same role. Employing the solid state route described herein, monometallic Au, Ag, Cu, Pd, and Ir and bimetallic CuAg and CuAu alloy nanoparticles of <10 nm were successfully prepared. Nucleation and growth processes that control the size and the size distribution of the resulting nanoparticles have been elucidated in these systems.
Assuntos
Ligas/química , Aminas/química , Boranos/química , Nanopartículas Metálicas/química , Metais/química , Oxirredução , Tamanho da Partícula , Sais/química , Propriedades de SuperfícieRESUMO
The Ag-Sn based system in the nano-size regime is one of the strongest candidates for lead-free solders. Besides, the investigation of several other applications of Ag-Sn nanostructures, especially in catalysis, remains scarce which makes it an interesting system to synthesize and explore its chemistry. In this report, nearly monodisperse ε-Ag3Sn intermetallic nanoparticles were prepared by a simple and convenient solution-based process of co-digestive ripening using Ag and Sn colloids obtained by the solvated metal atom dispersion (SMAD) method. Optimization of the temperature and stoichiometric ratio between the metal elements and the use of an appropriate capping agent are crucial factors to realise phase pure intermetallic nanoparticles. Ag3Sn nanoparticles with a size of 3.8 nm ± 0.6 nm were obtained within 12 h of reaction when tri-n-octylphosphine/tri-n-octylphosphine oxide was used as the capping agent at 205 °C. Interestingly, Ag3Sn@SnOx core-shell nanostructures were obtained by changing the capping agent to palmitic acid. These nanostructures were thoroughly characterized by powder X-ray diffraction, transmission electron microscopy (TEM and STEM-EDS), X-ray photoelectron spectroscopy and optical spectroscopy. Thereafter, Ag and Ag3Sn nanoparticles were utilized for photocatalytic degradation of methylene blue, methyl orange and a mixture of both the dyes.
RESUMO
Metal nanoparticles (NPs) of Cu (air-stable), Ag, and Au have been prepared using an atom-economy green approach. Simple mechanical stirring of solid mixtures (no solvent) of a metal salt and ammonia borane at 60 degrees C resulted in the formation of metal NPs. In this reaction, ammonia borane is transformed into a BNH(x) polymer, which protects the NPs formed and halts their growth. This results in the formation of the BNH(x) polymer protected monodisperse NPs. Thus, ammonia borane used in these reactions plays a dual role (reducing agent and precursor for the stabilizing agent).
Assuntos
Cobre/química , Ouro/química , Química Verde , Nanopartículas Metálicas/química , Prata/química , Tamanho da Partícula , Propriedades de SuperfícieRESUMO
Nanoclusters of 25 nm sized Mg-THF have been prepared by the solvated metal atom dispersion method. Room-temperature digestive ripening of these nanoclusters in the presence of hexadecylamine (HDA) resulted in highly monodisperse colloidal Mg-HDA nanoparticles of 2.8 +/- 0.2 nm. An insight into the room-temperature digestive ripening process was obtained by studying the disintegration of clusters for various Mg:HDA ratios. The Mg colloids are quite stable with respect to precipitation of particles under Ar atmosphere. Using this procedure, pure Mg(0) nanopowders were obtained in gram scale quantities. The Mg powder precipitated from the colloid was fully hydrided at 33 bar and 118 degrees C. Initial desorption of H(2) from samples of MgH(2) was achieved at a remarkably low temperature, 115 degrees C compared to >350 degrees C in bulk Mg, demonstrating the importance of the size on the desorption temperatures.
RESUMO
Ammonia-borane (AB) hydrolysis for the generation of hydrogen has been studied using first row transition metal ions, such as Co (2+), Ni (2+), and Cu (2+). In the cases of cobalt- and nickel-assisted AB hydrolysis, amorphous powders are formed that are highly catalytically active for hydrogen generation. Annealing of these amorphous powders followed by powder X-ray diffraction measurements revealed the presence of Co(0) and Co 2B and Ni(0) and Ni 3B, respectively. On the other hand, copper-assisted AB hydrolysis was catalyzed by in situ generated H (+) and Cu(0) nanoparticles. The reduction ability of AB for the realization of coinage metal nanoparticles from the respective metal salts has also been studied. These reduction reactions were found to be facile, affording colloids of pure metal nanoparticles. Nanoparticles prepared in this manner were characterized by UV-visible spectroscopy and high-resolution electron microscopy.
RESUMO
We report a general strategy for the synthesis of metal nanosponges (M = Ag, Au, Pt, Pd, and Cu) using a capping agent dissolution method where addition of water to the M@BNHx nanocomposite affords the metal nanosponges. The B-H bond of the BNHx polymer gets hydrolysed upon addition of water and produces hydrogen gas bubbles which act as dynamic templates leading to the formation of nanosponges. The rate of B-H bond hydrolysis has a direct impact on the final nanostructure of the materials. The metal nanosponges were characterized using powder XRD, electron microscopy, XPS, and BET surface area analyzer techniques. The porous structure of these nanosponges offers a large number of accessible surface sites for catalytic reactions. The catalytic activity of these metal nanosponges has been demonstrated for the reduction of 4-nitrophenol where palladium exhibits the highest catalytic activity (k = 0.314 min-1). The catalytic activity of palladium nanosponge was verified for the tandem dehydrogenation of ammonia borane and the hydrogenation of nitroarenes to arylamines in methanol at room temperature. The reduction of various substituted nitroarenes was proven to be functional group tolerant except for a few halogenated nitroarenes (X = Br and I) and >99% conversion was noted within 30-60 min with high turnover frequencies (TOF) at low catalyst loading (0.1 mol%). The catalyst could be easily separated out from the reaction mixture via centrifugation and was recyclable over several cycles, retaining its porous structure.
RESUMO
Three dimensional porous structures offer high specific surface areas and large pore volumes, which enhance substrate diffusion within the porous structures and provide a large number of surface active sites. Such types of structures find applications in catalysis. Herein, we report a simple synthetic strategy for the preparation of iridium nanosponges by the capping agent dissolution method. An Ir@BNHx nanocomposite was prepared starting from different iridium precursors by a solid state reduction method using ammonia borane wherein iridium(0) nanoparticles are embedded in a BNHx polymer. Capping agent (here, the BNHx polymer) dissolution using water under ambient conditions resulted in the formation of a mesoporous iridium nanosponge. This iridium nanosponge exhibits a surface area of 33.5 m2 g-1. The iridium nanosponge was found to be catalytically active for hydrogenation of a variety of olefinic substrates including linear and cyclic alkenes and α,ß-unsaturated esters under relatively mild conditions and exhibits high turnover frequencies. It was also found to exhibit much better catalytic activity as compared to other iridium based heterogeneous catalysts for olefin hydrogenation reactions. Additionally, catalyst recovery was achieved via simple filtration from the hydrogenation reaction mixture. The catalytically active surface area of iridium nanosponge was estimated using H2-temperature programmed desorption (TPD) experiments. Moreover, the catalyst was found to be thermally quite robust. The catalyst was recyclable over seven cycles of styrene hydrogenation and was found to be capable of hydrogenating 99% of styrene to ethyl benzene after seven cycles.
RESUMO
Chemistry of lanthanide metals in their zerovalent state at the nanoscale remains unexplored due to the high chemical reactivity and difficulty in synthesizing nanoparticles by conventional reduction methods. In the present study, europium(0) nanoparticles, the most reactive of all the rare earth metals have been synthesized by solvated metal atom dispersion (SMAD) method using hexadecyl amine as the capping agent. The as-prepared europium nanoparticles show surface Plasmon resonance (SPR) band in the visible region of the electromagnetic spectrum. This lead to the investigation of its surface enhanced Raman scattering (SERS) using visible light excitation source. The SERS activity of europium nanoparticles has been followed using 4-aminothiophenol and biologically important molecules such as hemoglobin and Cyt-c as the analytes. This is the first example of lanthanide metal nanoparticles as SERS substrate which can possibly be extended to other rare-earth metals. Since hemoglobin absorbs in the visible region, the use of visible light excitation source leads to surface enhanced resonance Raman spectroscopy (SERRS). The interaction of biomolecules with Eu(0) has been followed using FT-IR and UV-visible spectroscopy techniques. The results indicate that there is no major irreversible change in the structure of biomolecules upon interaction with europium nanoparticles.
Assuntos
Európio/química , Nanopartículas Metálicas/química , Ressonância de Plasmônio de Superfície , Coloides/síntese química , Coloides/química , Tamanho da Partícula , Solubilidade , Análise Espectral Raman , Propriedades de SuperfícieRESUMO
A new ruthenium pincer complex [RuHCl(CO)(PNP)] (PNP = PhCH2N(CH2CH2PPh2)2) () was synthesized and characterized. The reactivity of complex with electrophilic reagents XOTf (X = H, CH3, and Me3Si; OTf = CF3SO3) was studied by variable temperature NMR spectroscopy with an aim to observe and characterize sigma complexes of type [Ru(η(2)-HX)Cl(CO)(PNP)][OTf] (X = H (), CH3 (), Me3Si ()). Reaction of complex with HOTf resulted in the formation of the dihydrogen complex, [Ru(η(2)-H2)Cl(CO)(PNP)[OTf] (). On the other hand, the reaction between complex and MeOTf and Me3SiOTf resulted in the direct elimination of MeCl and Me3SiCl via a SN2 type of reaction without the intermediacy of the respective sigma complexes and . This contrasting reactivity behaviour has been rationalized taking into consideration the approach of the relatively bulky electrophiles CH3(+) and Me3Si(+) onto the hydride moiety of the ruthenium fragment, which is sterically hindered.
RESUMO
Synthesis and characterization of cis,trans-[RuH(η(2)-H2)(PPh3)2(N-N)][OTf] (N-N = 2,2'-bipyridyl (bpy) 1a, 2,2'-bipyrimidine (bpm) 2a; OTf = trifluoromethane sulfonate (CF3SO3)) complexes are reported. The cis-H2/hydride ligands are involved in H-atom site exchange between the two moieties. This dynamics was investigated by variable temperature NMR spectral studies based on which the mechanism of the exchange process was deduced. The ΔG(≠) for the exchange of H-atoms between the η(2)-H2 and hydride ligands was determined to be around 8 and 13 kJ mol(-1), respectively, for 1a and 2a. The H-H distances (d(HH), Å) in complexes 1a and 2a have been calculated from the T1(minimum) and (1)J(H,D) and are found to be 1.07 Å (slow) and 0.95 Å for 1a and 1.04 Å (slow) and 0.94 Å for 2a, respectively. The molecular structure of 1a was determined by X-ray crystallography.