Ni-modified Fe3O4(001) surface as a simple model system for understanding the oxygen evolution reaction

Adatom Bonding Sites in a Nickel‐Fe3O4(001) Single‐Atom Model Catalyst and O2 Reactivity Unveiled by Surface Action Spectroscopy with Infrared Free‐Electron Laser Light

“…Figure S3 shows STM images for higher Ni coverages, but also here STM does not reveal atomic details of the Ni adatom bonding sites. STM images of Fe 3 O 4 (001) single crystal surfaces and Ni adatoms on such surfaces as published by the Diebold and Parkinson group [18, 22, 23] are very similar to those shown in Figure 2a, which shows that Fe 3 O 4 (001) thin film and single crystal surfaces have very similar properties.…”

“…[30][31][32] Spectrum (iv) was recorded with 18 O 2 to verify that the observed peaks are due to OÀ O vibrations. In the 18 O 2 spectrum peaks at 790 and 995 cm À 1 are found, which correspond to those at 840 and 1054 cm À 1 in the 16 O 2 spectra according to the energy shift predicted by the harmonic approximation: v( 16 O 2 )/v( 18 O 2 ) = 1.06. [33] The spectra (iii) and (iv) were measured after the O 2 layer had been quickly flashed to 250 K, a temperature between the two TPD peaks, see Figure 3a and b.…”

“…Fe−Ni mixed oxides are promising catalysts for the oxygen evolution reaction (OER), [18, 19] and therefore we have selected Fe 3 O 4 (001) with Ni adatoms to demonstrate the suitability of this approach. The (√2×√2)R45° surface reconstruction of Fe 3 O 4 (001) has been studied with IV‐LEED, which revealed that it is caused by a rearrangement of the iron‐sublattice, leading to subsurface cation vacancies (SCVs) [20] .…”

The Effect of Fe Dopant Location in Co(Fe)OOHx Nanoparticles for the Oxygen Evolution Reaction

“…A set of materials with very prominent catalytic dopant effects are represented by mixed transition metal oxide systems consisting of Fe added to Co or Ni oxides. − These materials are considered among the best earth-abundant catalyst candidates to replace the high cost and scarce noble metal-based oxygen evolution reaction (OER) catalysts like Ir and Ru for alkaline conditions. − The active form of the mixed oxides is associated with an oxyhydroxide phase (denoted as Co­(Fe)­OOH x and Ni­(Fe)­OOH x ), shown to be present as hexagonal platelet nanoparticles under OER conditions. , According to the literature, the observed OER activity is highly sensitive to the Fe concentration. However, the role of Fe dopants remains debated and an optimal Fe-doping synthesis procedure remains to be developed, exemplified by the wide range of OER activities observed for a given Fe concentration. ,− Moreover, a wide range of Fe concentrations from 3% to 70% have been reported as an optimal doping level for OER activity. ,− Extensive characterization efforts have utilized microscopy and spectroscopy techniques and computational modeling for Co­(Fe)­OOH x and Ni­(Fe)­OOH x to investigate the promoting effect of Fe. − Several active sites in Fe-containing oxides/(oxy)­hydroxides have been suggested, including the possibility of under-coordinated edge sites involving alternating Fe and Co sites being the best sites for OER. ,− Atom-resolved scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations have, for example, directly shown that hexagonal cobalt oxide nanoparticles on Au(111) exhibit different electrocatalytic activity at the under-coordinated edges of the nanoparticles as compared to the basal planes, in line with edge reactivity reported for exfoliated CoOOH x . , For catalyst systems where Fe has been found to have a promotional effect, it has been proposed that the Fe dopants embedded in the Co oxyhydroxide result in an Fe–O bond shortening and a high oxidation state of Fe, which then has lead authors to conclude that Fe is the active site for OER. , Operando X-ray photoemission and absorption spectroscopies have shed light on the chemical state of Fe dopants, −…”

Improved kinetics of reduction of alkaline water on the g‐CN‐ supported transition metal oxide/boride hetero‐interface: A case study

“…Moreover, as per XRD and HRTEM analysis, lattice defects are generated due to lattice mismatch on the composite surfaces would provide more active sites. The valency of Ni 2+ increased in the presence of ferric oxide and CN in the composite that would further improve the adsorption of substrate, 13 on the other hand, the intermediate‐spinel Fe center interaction was promoted in the presence of Ni 2+ 80 . Moreover, the inter‐connected nanosheet array morphology acted as efficient catalysis bed.…”