Electrochemical and photochemical applications of multifunctional metal phosphonates
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2020-07-21
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Salcedo, Inés R.
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UMA Editorial
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In this PhD thesis, proton conductivity, photocatalytic and luminescence properties of synthesised
multifunctional metal phosphonates have been studied.
Iron and zinc derivatives of the R,S hydroxyphosphonoacetic acid (HPAA), M[HO3PCH(OH)CO2]2.5H2O, have been synthesised as well as the corresponding iron-containing bimetallic solid solutions, FexM1-x[HO3PCH(OH)CO2]2.5H2O (M= Mn2+, Co2+ and Zn2+), showing remarkable properties as proton conductors and Photo-Fenton catalyst. These layered solids exhibited NH3/H2O-mediated proton conductivity, which increases with higher amounts of these two guest species (up to 1.8·10-3 S·cm-1). Upon ammonia adsorption, solids become increasingly amorphous, but PDF studies revealed that the local Fe2+ environment was still preserved. Regarding to photocatalytic properties, Fe-HPAA and FexM1-x-HPAA (x > 0.5) are efficient photo-Fenton catalysts for the degradation of recalcitrant pollutants, such as phenol and 4-chlorophenol (~90% TOC removal) and methylene blue (75%).
Transition metals and lanthanide 2-[bis(phosphonomethyl)amino]-ethanesulfonate compounds have been obtained by solvo-, hydrothermal or microwave-assisted synthesis: Mn2[(O3PCH2)2-NH-(CH2)2-SO3]·3H2O, o-Mn2HSP, Zn2[(O3PCH2)2-NH-(CH2)2-SO3]/Zn0.4[O3PCH2-NH-CH3]0.4(NH3)2·4H2O, m-Zn2HSP, Zn2[(O3PCH2)2-NH-(CH2)2-SO3]·7H2O, h-Zn2HSP, and Ln[H(O3PCH2)2-NH-(CH2)2-SO3]·2H2O, m-LaH2SP and o-LnH2SP (Ln= Pr, Nd, Sm, Eu, Gd and Tb).
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The sulfonate moiety can play a different role, being uncoordinated (o-Mn2HSP and o-LnH2SP) or coordinated to the lanthanide ion (m-LaH2SP). These solids were compared to establish valuable structure-properties relationships. Zinc derivatives showed the highest proton conductivity values, ~ 10-2 S·cm-1 (80 ºC and 95% RH), notwithstanding, their structures remain unsolved. o-Mn2HSP and o-SmH2SP also exhibited high proton conductivity values (~ 10-3 S·cm-1). In these solids, the proton conductivity was most a result of structural features, but other various factors also contributed. Two polymorph series of sulfate-containing lanthanide aminotris(methylenephosphonates) have been isolated by controlled crystallization at 25 ºC: {Ln2[HN(CH2)3(PO3H)3]2(H2O)4}(HSO4)2·nH2O (Ln= Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb, Series Ln-I) and Ln[HN(CH2)3(PO3H2)(PO3H)2]SO4·2H2O (Ln= Pr, Nd, Eu, Gd, Tb, Series Ln-II). These series differ most significantly in the structural role played by the sulfate anion, which can be coordinated or uncoordinated to the lanthanide ion. Solids of Series Ln-I evolve to sulfate-deficient networks upon heating at 80 ºC and 95%, while solids of Series Ln-II remain stable. In addition, upon heating at 230ºC, solids of Series Ln-I transform into a crystalline anhydrous phase. All of them exhibit high proton conductivity values, ranging from 1.1·10-3 to 3.6·10-2 S·cm-1. Bimetallic solid solution of Series Ln-I showed luminescence properties due to the characteristic f-f transitions of the lanthanide ions.
Alkali and magnesium derivatives of the hexamethylenediamine-N,N,N’,N’-tetrakis(methylenephosphonic acid) have been synthesised by crystallization at RT and hydrothermal or microwave-assisted synthesis, respectively. These compounds showed a rich structural diversity, from 1D to 3D, induced by the cation size. Other crystalline anhydrous phases were obtained by dehydration of Li+ and Mg2+ derivatives. This reversible process enhanced their proton conductivity almost one order of magnitude due to subtle changes in the H-bond networks. In addition, CO2 adsorption studies were carried out for Mg-HDTMP-1deh.
Finally, preparation and characterisation of Nafion-based composite membranes have been carried out employing selected metal phosphonates (o-SmH2SP, Eu-I-230, SD-Eu-I and Tb-II). These composite membranes showed excellent performances in MEAs, even higher than pure Nafion membranes, under operating conditions of PEMFCs.
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