EPR Spectroscopy of Iron- and Nickel-Doped [ZnAl]-Layered Double Hydroxides: Modeling Active Sites in Heterogeneous Water Oxidation Catalysts

Abstract

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to inter-spin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spinspin broadening, iron and nickel atoms were doped into non-magnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites was solvent-exposed (i.e. at an edge, corner, or defect site). These solvent-exposed sites featured an axial ZFS of 0.21 cm -1 when hydrated. The ZFS increased dramatically upon dehydration (to -1.5 cm -1 ), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other ("inert") Fe(III) site maintained an axial ZFS of 0.19-0.20 cm -1 under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (0.30 cm -1 ) in hydrated material, whereas two distinct Ni(II) ZFS values (0.30 and 1.1 cm -1 ) were observed in the dehydrated samples. Although the magnetically-dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g. by simple vacuum drying) is an important step towards identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials.

Heterogeneous Catalytic Hydrogenation of Carbon Dioxide Into Hydrocarbons: Achievements and Prospects

“…The EMR spectra shown in Fig. 1, are characteristic to superpara / ferromagnetic particles of Fe, Ni -containing catalysts with an oxide base [73,74]. As seen from Fig.…”

“…The given XRD patterns indicate the presence and absence, respectively, of a metallic phase in hydrogen-reduced Fe, Ni / Al and Fe, Zr / Al -oxide catalysts. Thus, it can be argued that while the magnetism of the former is due to particles of Fe, Ni, and Fe(Ni), Fe(Al)2O4, the latter is exclusively due to oxide structures [73][74][75]. It is not possible to say about any connection between the magnetic and catalytic properties of the studied systems in the hydrogenation of CO2 into hydrocarbons.…”

“…Note that the intensity of the signal with a g-factor equal to 4.21 and the shoulder with g ~ 7.2 practically does not change when the catalyst is treated in a flow of hydrogen at 573, 673, 773 K, as well as with an increase in the iron content in the samples. This gives grounds to believe that signals with g = 4.21 and 7.20 can be attributed to isolated Fe 3+ ions in crystal fields of rhombic and axial symmetry, respectively, while resonance with g ~2.0 is due to Fe 3+ ions associated with exchange interactions in the magnetically concentrated phase [74]. Thus, the experimental data allow us to conclude that the catalysts before and after the methanation reaction contain superpara / ferromagnetic particles, most likely nickel, iron oxides FeOx, possibly also NixFe1-xOy.…”

Doubly-Charged Negative Ions as Novel Tunable Catalysts: Graphene and Fullerene Molecules Versus Atomic Metals

“…These authors also pursue the development of new artificial water oxidation catalysts for artificial photosynthetic water oxidation [ 2 ]. In recent years, research on water oxidation catalyzed by various systems, processes and mechanisms has intensified [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. The advent of the COVID-19 pandemic has also accelerated a vigorous search for and synthesis of atomic nanoparticles and catalysts for various applications (see for examples [ 17 , 18 ]).…”

Doubly-charged Negative Ions as Novel Tunable Catalysts: Graphene and Fullerene Molecules Versus Atomic Metals

“…Also pursued by these authors is the development of new artificial water oxidation catalysts for artificial photosynthetic water oxidation [2]. In recent years research on water oxidation catalyzed by various systems, processes and mechanisms has intensified [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The advent of the COVID-19 pandemic has also accelerated the vigorous search for and synthesis of atomic nanoparticles and catalysts for various applications, see for examples [17,18].…”