Partha Sarathi Roy

Facile synthesis of Pd nanostructures in hexagonal mesophases as a promising electrocatalyst for ethanol oxidation

One of the significant challenges for the commercialization of direct ethanol fuel cells (DEFCs) is the preparation of active, robust, and low-cost catalysts. In this work, a facile and reproducible method is demonstrated for the synthesis of Pd assembled nanostructures in a hexagonal mesophase formed by a quaternary system (Pd-doped water, surfactant, oil, and cosurfactant) via photoirradiation. The formation of Pd nanostructures in the confined region of hexagonal mesophases was further supported by water relaxation dynamics study using a solvation probe. The mesophases can be doped with high concentrations of a palladium salt (0.1 M) without any disturbance to the structure of the mesophases which results in a high yield and facilitates the clean synthesis of Pd nanostructures without using any toxic chemicals. Electrochemical measurement confirms that the as-prepared catalysts exhibit significant electrocatalytic activity for ethanol oxidation in alkaline solution. Additionally, we present an alternative strategy using reduced graphene oxide nanosheets in combination with Nafion (a proton conducting phase) as a support, revealing the pronounced impact on dramatically enhanced electrocatalytic activity and stability of Pd nanostructures compared to Nafion alone. This unique combination allowed the effective dispersion of the Pd nanostructures that is responsible for the enhancement of the catalytic activity. Our approach paves the way towards the rational design of practically relevant catalysts with both enhanced activity and durability for fuel cell applications.

The size-dependent anode-catalytic activity of nickel-supported palladium nanoparticles for ethanol alkaline fuel cells

Spherical nanoparticles of palladium with varying particle diameters have been prepared from PdCl2 by wet chemical single pot synthesis using citric acid as reducing agent in the presence of PVA. The size of the nanoparticles has been tuned by changing the duration of reflux. The resulting nanoparticles have been dip-coated on Ni-foil, and evaluated as anode catalysts for oxidation of ethanol under alkaline conditions. The morphology and surface characteristics of the Pd nano-catalyst have been investigated by TEM and FE-SEM in conjugation with EDS. Measurements of catalytic activity by electrochemical methods (cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy) reveal that the majority of the nanoparticle-embedded anodes, despite less Pd0 loading, act as superior electrocatalysts as compared to the nickel-supported Pd electrode constructed electrochemically. In this study, the effects of the different extent of catalyst-loading on the experimental parameters have been rationalized to obtain the size-dependence of the electrocatalytic activity. The experimental results clearly show that there is noticeable development in the intrinsic catalytic activity and poisoning resistance of the anode catalysts. In addition, the intrinsic electrocatalytic activity of the nano-palladium is found to be size-dependent, which increases with decrease in particle size particularly below the diameter of 19 nm.

Size-controlled synthesis and characterization of polyvinyl alcohol-coated platinum nanoparticles: role of particle size and capping polymer on the electrocatalytic activity

Electrocatalytically active polymer–platinum nanocomposites have been synthesized in one pot by refluxing an aqueous solution consisting of chloroplatinic acid, PVA and formic acid. The as-synthesized platinum nanoparticles have large variations in their average size from 2.9 to 60.0 nm. The size of the nanoparticles is tuned by changing the concentration of the stabilizer as well as that of the reducing agent. In addition, the particle size is also controlled by varying the reflux time for a particular precursor solution. In this study, the nanoparticles have been characterized by UV-visible spectroscopy, XRD, TEM, FE-SEM and cyclic voltammetry. The study also shows that nanoparticles have size-dependent electrocatalytic behavior for anodic oxidation of alkaline methanol when the particle surface bears almost the same concentration of PVA. But the expected catalytic activity based on the particle size is found to alter when the nanoparticles possess a different surface concentration of PVA.

Size-controlled synthesis, characterization and electrocatalytic behaviors of polymer-protected nickel nanoparticles: a comparison with respect to two polymers

A facile reduction of nickel chloride in aqueous solution with sodium borohydride leads to fairly monodisperse, and electrochemically active nickel nanoparticles by the separate use of a capping polymer like polyvinyl pyrrolidone (PVP) and polyacrylic acid (PAA). The resulting nanoparticles have been characterized by transmission electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, and cyclic voltammetry. The size of the Ni nanoparticles is very small (1.2–6.1 nm) and can be readily tuned by changing the polymer as well as its concentration in each case. PVP bearing bulky pyrrolidone moieties leads to the formation of smaller nanoparticles than PAA at the chosen concentrations of polymers providing similar numbers of monomer units. In alkaline medium graphite-supported Ni nanoparticles form Ni(OH)2 and then NiOOH which are electrocatalytically active towards the electro-oxidation of methanol. The study reveals that at high positive potential the polyacrylate anion interacts more with the catalyst nanoparticles as compared to PVP resulting in lowering of the current density at higher concentrations of polymer. Thus the catalyst nanoparticles are capable of exhibiting competing effects of size- and particle-surface-environment in electrocatalysis and possess alcohol sensing properties for alkaline oxidation of methanol.

Reductive Nitrosylation of ReO− 4 and Synthesis of an Azido Nitrosyl Complex of Rhenium and its 1,10-Phenanthroline and 2,2 ' -Bipyridine Derivatives Directly from ReO− 4

Abstract Reductive nitrosylation of ReO- 4 by any of the known nitrosylating agents1 and subsequent synthesis of rhenium nitrosyl derivatives have not yet been reported. The known nitrosyl compounds of this metal are very few and were prepared from low valent rhenium compounds (cyano, chloro or carbonyl complexes) by the treatment with HNO3 2.3 or NO4,5 or NOX6. We have recently reported7 the reduction and subsequent nitrosylation of ReOi using NH2OHHCI and NCS- in alkaline medium. Herein is described the direct reductive nitrosylation of ReO- 4 using NH2OHHCl and N- 3 as evidenced by synthesing the complex ion [Re(NO)(N3)3H2O]- and its 1,10-phenanthroline and 2,2′-bipyridine derivatives from an aqueous-aerobic medium. It may be mentioned that azido nitrosyl complexes of any metal are extremely rare (only one is known).6

A very simple synthesis of K3[Re(NO)(CN)5] · 2 H2O directly from [ReO4]−: Existence of two structural isomers in aqueous medium as indicated by13C N.m.r. spectroscopy

SummaryPerrhenate(VII) was reductively nitrosylated using an excess of CN−, OH− and NH2OH · HCl, and from the reaction mixture K3[Re(NO)(CN)5] · 2H2O has been isolated. Its aqueous solution behaves as a 3∶1 electrolyte and its13C n.m.r. spectrum in D2O solution suggests that the complex molecule forms two types of isomeric structure arising from the two different modes of intramolecular hydrogen (deuterium) bonding of the two lattice water (D2O in the bulk solvent) molecules.

Reductive nitrosylation of tetraoxometallates: XIII. Generation of {V(NO)2}2+ moiety: Synthesis of [V(NO)2(CN)4]2− and [V(NO)2(CN)2(L-L)] (L-L = 2,2′-bipyridine and 1,10-phenanthroline) directly from VO43− in aqueous-aerobic media☆

Abstract The vanadium(0) dinitrosyl complex [V(NO) 2 (CN) 4 ] 2− containing {V(NO) 2 } 2+ moiety is generated for the first time via reductive nitrosylation of VO 4 3− with NH 2 OH and CN − in an alkaline medium. The complex anion reacts with L-L [L-L = 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen)] in an aqueous medium to give [V(NO) 2 (CN) 2 (L-L)]. The complexes show characteristic IR bands and paramagnetism, as expected for a system containing a single unpaired electron. They give interesting EPR signals, with g av close to 2.0.

Reactivity of the Re-NO centre: Proton induced oxidation of Re(NO)2+ to Re(NO)3+. Synthesis and characterisation of some Re(II) thiocyanato-halogenonitrosyl complexes

The Re(NO)2+ moiety as [Re(NO)(NCS)3H2O]− or [Re(NO)(NCS)2(L-L)H2O]· [L-L = phen (1,10-phenanthroline) or bipy (2,2′-bipyridine)] undergoes proton-induced oxidation reaction with HX (X = Cl, Br) to produce a Re(NO)3+ moiety. The spectral and physico-chemical data suggest that the anionic complex is 5 coordinate and the neutral one is 6 coordinate with axial NO group and two NCS ligands intrans-equatorial positions. The complex, [Re(NO)(NCS)2(phen)Br]·H2O shows complicated magnetic behaviour which is discussed in the paper. The ESR spectrum of this compound shows typical rhenium hyperfines and -tensor anisotropy compatible with the loss of axial symmetry. However, the spectrum of [Re(NO)(NCS)2Br2]− quite reasonably shows axial symmetry, other features being grossly comparable to the L-L compounds. The anionic species and the neutral L-L complex show irreversible one-electron oxidation waves at different voltages. This may correspond to a conversion of Re(NO)3+ to Re(NO)4+ in both the cases. Interestingly enough, only the neutral complexes exhibit an irreversible reduction wave due probably to a conversion of Re(NO)3+ to Re(NO)2+.

Reductive nitrosylation of tetraoxometallates. Part 14. Generation and proton induced disproportionation of the Re(NO)3+ moiety. Synthesis, characterisation, and electrochemistry of novel hydroxo- and halogenonitrosyl complexes of rhenium

In an aqueous alkaline reaction medium, [ReO4]– undergoes reductive nitrosylation by NH2OH·HCl producing [Re(NO)(OH)4]–, isolated as APh4(A = P or As) salts or as a neutral complex [Re(NO)(OH)3(L–L)][L–L = 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy)]. The complexes show v(NO) vibrations at ca. 1 680 cm–1 and their e.s.r. spectra suggest that, besides the hydroxo complex (C4v), the L–L derivatives also possess axially symmetric structures. These complexes contain the Re(NO)3+[or{Re(NO)}5] group and show spin-only magnetic moments and almost identical e.s.r: profiles in the polycrystalline condition, both at 298 and 77 K, showing 〈gav〉≈ 2.0. A well defined sextet due to metal hyperfine structure is observed of which the two outermost peaks are further split to a triplet arising from 14N (of NO) superhyperfine interaction. However, in frozen acetonitrile or dimethylformamide (dmf) the hyperfine structures collapse. These complexes of Re(NO)3+ disproportionate when boiled with HX (X = Cl or Br but not I, which forms only [ReI6]3– species), producing [Re(NO)X5]– or [Re(NO)X4(phen)][i.e. Re(NO)4+], [Re(NO)2X4]–[i.e. Re(NO)23+], and [Re2X8]4–(Re24+). While a square-pyramidal structure (C4v) can be suggested for the mononitrosyl halogeno complex anions from i.r. and electronic absorption spectra, these do not distinguish between axial–equatorial (Cs) or equatorial–equatorial (C2v) dispositions of the two nitrosyl ligands in the dinitrosyl complexes. The seven-co-ordinate complexes [Re(NO)X4(phen)] undergo thermal and electrochemical (irreversible) reduction corresponding to two one-electron steps. The six-co-ordinate Re(NO)4+ species, [Re(NO)Cl5]–, shows a similar electrochemical reduction process but the five-co-ordinate [Re(NO)(OH)4]– is electro-inactive. The cyclic voltammogram of the Re(NO)23+ species, [Re(NO)2Cl4]–, reveals a reversible, Re(NO)23+–Re(NO)22+ couple at a formal potential of –0.04 V vs. a saturated calomel electrode {–0.42 V vs.[Fe(η-C5H5)2]–[Fe(η-C5H5)2]+}.