The Atomic Level Structure of Layered Double Hydroxides

Anders Bruhn Arndal Andersen

Research output: ThesisPh.D. thesis

Abstract

The aim of this Ph.D. project was to investigate “aluminum hydroxide” type layered double hydroxides (LDH), which has the general chemical formula of [M2+Al4(OH)12A2/xx−·nH2O] and referred to as MAl4-LDH. These consist of positively charged cation layers counterbalanced by interlayer anions, Ax−, accompanied by water. However, synthesis of MAl4-LDH is challenging, and their properties such as thermal degradation and magnetism are unexplored, which will be highly relevant in relation to potential applications within catalysis and quantum materials important for both industry and in future electronic components.


Extensive synthesis optimization of MAl4-LDH using bayerite as Al(OH)3 source with M = Co2+, Ni2+, and Cu2+ and Ax− = SO4 2−, yielded pure samples except for CuAl4-LDH, which contained 10-12 %w/w unreacted bayerite. Increased reaction temperature (>120 °C) to promote full bayerite conversion led to formation of boehmite, AlO(OH). Moreover, a pH adjustment (≈ 2) of the reactant solutions was necessary to prevent formation of NiOH2 and Cu3(OH)4(SO4) impurities for M = Ni2+ and Cu2+, respectively. The thermal degradation was studied on samples heated ex situ (also for M = Zn2+), where the end products are MAl2O4 (spinel) and α-Al2O3 at 1200 °C for M = Co2+ and Zn2+, whereas a defect spinel-like NiAl4O7 was obtained for NiAl4-LDH. The CuAl4-LDH degraded in a redox reaction yielding Cu(I)AlO2 and α-Al2O3. This was obtained by a combination of PXRD, SEM/TEM, EDS, ICP-OES, FTIR, TGA, and solid-state NMR spectroscopy. For the magnetic properties of MAl4-LDH, 1D spin systems were predicted form the crystal structure and confirmed by magnetic susceptibility, heat capacity, and neutron scattering experiments combined with computational chemistry. Here, no long-range magnetic order down to 2 K were detected and only weak ferromagnetic nearest neighbor interactions of J = 0.07, 0.63 and 0.74 cm−1 is observed for M = Co2+ (S = 3/2), Ni2+ (S = 1), and Cu2+ (S = 1), respectively, as expected for both 1D spin systems and single ion magnets. Moreover, Co2+ and Ni2+ displayed zero field splittings of D = 95 cm−1 and D = −10 cm−1 (E = 2.8 cm−1), respectively. Finally, a combined experimental and computational approach led to 13C MAS NMR assignment of paramagnetic metalorganic acetylacetonate complexes, [M(acac)n], where a reversal in δ(13C) for CO and CH3 when going from [VO(acac)2], [V(acac)3] to [Ni(acac)2(H2O)2], [Cu(acac)2] was observed. Thus, the presented work contributed to the knowledge of MAl4-LDH synthesis and their properties of thermal degradation and magnetism, together with contributions to the challenging field of paramagnetic NMR.
Original languageEnglish
Awarding Institution
  • University of Southern Denmark
Supervisors/Advisors
  • Nielsen, Ulla Gro, Principal supervisor
  • Jensen, Hans Jørgen Aagaard, Co-supervisor
Publisher
DOIs
Publication statusPublished - 11. Apr 2022

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