Using transition metal tetraphosphonates as precursor of phosphorus-containing electrocatalysts

Coordination polymers (CPs) are widely studied due to their applicability in many fields. Among them, metal phosphonates (MPs) are attractive materials due to their versatile structural diversity and functionality, with interesting properties as proton conductors and electrocatalyst precursors. In this work, we report the synthesis and crystal structures of several MPs derived from the combination of hexamethylenediamine-N,N,N’,N’-tetrakis(methylenephosphonic acid) (HDTMP) with different transition metals (M2+= Mn, Fe, Co, and Ni). The resulting solids, M[(HO3PCH2)2N(CH2)6N(CH2PO3H)2]·2H2O, show pillared-layered structures with capabilities of ammonia adsorption (Co2+ and Ni2+ derivatives). The ammonia-containing solids are crystalline, with a composition M[(HO3PCH2)2N(CH2)6N(CH2PO3H)2(H2O)2](NH3)4(H2O)12. The catalytic activities toward Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR) and Hydrogen Evolution Reaction (HER) of the corresponding (5% H2-Ar)-pyrolyzed materials, as well as the crystal structure of non-pyrolyzed precursor solids, will be discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Electrocatalytic properties of cobalt phosphides and pyrophosphates derived from phosphonate-based-MOFs

As a class of coordination polymers (CPs), metal phosphonates (MPs) are constructed by coordination bonds connecting metal sites and phosphonate (RPO32−) ligands, where the metal sites are dispersed uniformly at the atomic level. This feature facilitates the construction of metalphosphorous-based/nano-carbon composites by one-step pyrolysis, making them very attractive precursors of Non-Precious Metal Electrocatalysts (NPMCs) [1, 2] In this work, we report the synthesis, characterization and electrochemical properties of three cobalt(II) coordination polymers erived from the N,N-bis(phosphonomethyl)glycine (BPMGLY), Co(C4H9O8NP2·nH2O (n=2-4). These MPs, with different frameworks according to the crystallographic data, are used as precursors of new NPMCs by pyrolytic treatment under 5%-H2/Ar at different temperatures. The electrochemical behavior of the resulting compounds, mainly crystalline cobalt pyrophosphates and/or phosphides, is fully investigated regarding to the Oxygen Evolution and Reduction Reactions (OER and ORR, respectively) as well as Hydrogen Evolution Reaction (HER). In general, cobalt phosphides (CoP) derived from compound Co-BPMGLY-I (n=4), displayed better performances for the HER with an overpotential of 156 mVUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Tuning the activity of cobalt 2-hydroxyphosphonoacetates-derived electrocatalysts for water splitting and oxygen reduction: insights into the local order by pair distribution function analysis

Pyrophosphate- or phosphide-based iron/cobalt electrocatalysts were prepared from the metal (R,S)−2-hydroxyphosphonoacetates to evaluate the effects of metal composition, N-doping and P-enrichment on the electrocatalytic activity. Rietveld and Pair Distribution Function analysis were used to determine phase composition. Irrespectively of the amorphous or crystalline nature, all pyrolyzed solids transformed under OER operation into biphasic Fe/CoO(OH), composed of discrete clusters (size ≤ 20 Å). Carbon paper-supported Fe0.2Co0.8O(OH) electrocatalysts displayed the best OER performances (overpotentials of 270–279 mV at 10 mA·cm−2), attributable to the formation of highly active bimetallic intermediate species. For HER, increased concentration of o-CoP in phosphide-based electrocatalysts resulted in improved performance, up to an overpotential of 140 mV. Employed as anode in alkaline water splitting, amorphous Fe-doped cobalt pyrophosphate and phosphide-derived electrocatalysts showed a cell voltage of 1.58 V at 10 mA·cm−2, with comparable stability to that of RuO2 and requiring lower voltage demand at high current densities.This work was funded by the PID2019-110249RB-I00/AEI/10.13039/501100011033; TED2021–129836B-I00/AEI/10.13039/501100011033/Unión Europea NextGenerationEU/PRTR (MICIU, Spain) and P20-00416 (Junta de Andalucia, Spain/FEDER) research projects. Synchrotron X-ray powder diffraction studies were performed at MSPD04 beamline at ALBA Synchrotron Light with the collaboration of ALBA staff. A.V.C. thanks MICIU for PRE2020-094459 student grant. R.M.P.C. acknowledges funding by project acknowledges B1_2022-23 (Plan Propio UMA). M.B.G. thanks PAIDI2020-DOC_00272 research grant (Junta de Andalucia, Spain). Funding for open access charge: Universidad de Málaga / CBUA

Three-Component Copper-Phosphonate-Auxiliary Ligand Systems: Proton Conductors and Efficient Catalysts in Mild Oxidative Functionalization of Cycloalkanes

The synthesis, structural characterization, topological analysis, proton conductivity, and catalytic properties are reported of two Cu(II)-based compounds, namely a dinuclear Cu(II) complex [Cu2(μ-VPA)2(phen)2(H2O)2]· 8H2O (1) (H2VPA = vinylphosphonic acid, phen = 1,10- phenanthroline) and a 1D coordination polymer [Cu(μ- SO4)(phen)(H2O)2]∞ (2). Their structural features and Hbonding interactions were investigated in detail, showing that the metal−organic structures of 1 and 2 are extended by multiple hydrogen bonds to more complex 2D or 1D Hbonded architectures with the kgd [Shubnikov plane net (3.6.3.6)/dual] and SP 1-periodic net (4,4)(0,2) topology, respectively. These nets are primarily driven by the H-bonding interactions involving water ligands and H2O molecules of crystallization; besides, the (H2O)4/(H2O)5 clusters were identified in 1. Both 1 and 2 are moderate proton conductors, with proton conductivity values, σ = 3.65 × 10−6 and 3.94 × 10−6 S·cm−1, respectively (measured at 80 °C and 95% relative humidity). Compounds 1 and 2 are also efficient homogeneous catalysts for the mild oxidative functionalization of C5−C8 cycloalkanes (cyclopentane, cyclohexane, cycloheptane, and cyclooctane), namely for the oxidation by H2O2 to give cyclic alcohols and ketones and the hydrocarboxylation by CO/H2O and S2O82− to the corresponding cycloalkanecarboxylic acids as major products. The catalytic reactions proceed under mild conditions (50−60 °C) in aqueous acetonitrile medium, resulting in up to 34% product yields based on cycloalkane substrate.Proyecto MAT2016-77648-R del MINECO y proyecto FQM-1656 de la Junta de Andalucí

Structural and Proton Conductivity Studies of Fibrous π-Ti2O(PO4)2·2H2O: Application in Chitosan- Based Composite Membranes

Although the fibrous polymorphic modification of titanium phosphate, π-Ti2O(PO4)2·2H2O (π-TiP) is known for decades, its crystal structure has remained unsolved. Herewith we report the crystal structure of π-TiP at a room temperature, determined from the synchrotron radiation powder X-ray diffraction, and corroborated by 31P solid state NMR and the accurate density functional theory calculations. In contrast to the previously reported ρ-TiP polymorph, the as-synthesized hydrated phase crystallizes in the monoclinic system (P21/c, a = 5.1121(2) Å, b = 14.4921(9) Å, c = 12.0450(11), β = 115.31(1)°, Z=4) and is composed of corner-sharing titanium octahedra and phosphate units arranged in a pattern distinct of ρ-TiP polymorph. The unit cell was confirmed by SAED, while the formation of planar packing imperfections and stacking faults along the [101] was revealed by HRTEM analysis. In situ dehydration study of π-TiP, monitored by high-temperature powder X-ray diffraction, led to a new anhydrous monoclinic [P21/c, a = 5.1187(13) Å, b = 11.0600(21) Å, c = 14.4556(26), β = 107.65(2)°, Z=4) phase that crystallizes at 500°C. The latter resembles the packing fashion of the parental π-TiP, albeit titanium atoms are present both in distorted tetrahedral and octahedral coordination environments. Anhydrous π-TiP was found to partially rehydrate at room temperature adopting reversibly the structure of the initial phase. The studies carried out under different conditions of leaching and impregnation with H3PO4 showed that π-TiP exhibits an extrinsic proton conductivity (1.3·10−3 S·cm−1 at 90 °C and 95% RH) due to the presence of protonated phosphate species bounded on the particles surface, as revealed by 31P MAS-NMR spectroscopy data. The composite membranes of Chitosan (CS) matrices filled with H3PO4-impregnated π-TiP solid show an increment of proton conductivity up to 4.5·10–3 S·cm–1, at 80 °C and 95% RH, 1.8-fold higher than those of bare CS membranesMAT2016-77648-R and PID2019-110249RB-I00), Junta deAndalucía (FQM-113) M. Bazaga-García thanks Junta de Andalucía for her Postdoc PAIDI grant and R. M. P. Colodrero thanks UMA Research Plan for her financial support