Reduction and Oxidation Behavior of Ni<i>_x</i>Fe_3–<i>x</i>O_4−δ Spinels Probed by Reactive in Situ XRD

A semiempirical crystal model based on the hard sphere model is proposed to determine the oxygen deviation from stoichiometry (δ) of a mixed metal spinel of general formula A<i>_x</i>B_3–<i>x</i>O_4−δ from its lattice parameter. The model was calibrated with data for Ni- and Mn-ferrites taken from the literature. We demonstrate that the lattice parameter of a Ni<i>_x</i>Fe_3–<i>x</i>O_4−δ spinel can be predicted within a precision of 0.01 Å. This model was used to monitor the value of <i>x</i> and δ of Ni<i>_x</i>Fe_3–<i>x</i>O_4−δ nanopowders (with initial <i>x</i> = 0, 0.25, 0.5, and 1) during reactive in situ X-ray diffraction H₂ reduction and CO₂ oxidation at 400 °C. Results show that H₂ reduction occurs in two steps: (i) transition from a γ-type (δ < 0) to a regular (δ ≈ 0) spinel and (ii) preferential reduction of nickel from the spinel lattice to form a (Ni,Fe) solid solution. The face-centered cubic configuration for this alloy is favored in cases of high initial contents of nickel (<i>x</i> = 0.5, 1), and body-centered cubic for samples with low initial nickel content (<i>x</i> = 0, 0.25, 0.5). A subsequent CO₂ reoxidation of the samples shows that the process is partly reversible: iron will first be preferentially reintegrated into the lattice, and the initial excess of oxygen will be partially replenished. In addition to providing a thorough description of the phases and their evolution during reaction, these results describe the thermochemical behavior of nonstoichiometric nickel ferrites for the first time

Carbon Nanofilaments Functionalized with Iron Oxide Nanoparticles for in-Depth Hydrogen Sulfide Adsorption

The purification of hydrogen prior of its use in various applications, such as fuel cells, is of paramount importance. Although there are many commercial ways to obtain hydrogen sulfide, the need to reach very low concentration values, at the ppm or even at the ppb level, is the main motivation behind this work. This work examines the production and utilization of a new, low H₂S breakthrough and high capacity adsorbent, made of iron nanoparticles embedded in carbon nanofilaments. It is produced by a 2-step functionalization methodology: acid pretreatment and iron wet impregnation. This novel adsorbent was characterized by scanning transmission electron microscope, X-ray absorption near edge structure, Brunauer Emmet and Teller calculations, and thermogravimetric analysis, and the adsorption efficiency was measured for different iron-loadings, temperatures, and H₂S breakthrough values. Operating conditions and metal-loading that allow a decrease of H₂S concentration from 500 ppm to below 1.5 ppm are reported. It has also been found that acid treatment influences metal dispersion and, due to the nanometric nature of adsorbents, the process is not controlled by mass diffusion phenomena

Synthesis and Characterization of Co/C and Fe/C Nanocatalysts for Fischer–Tropsch Synthesis: A Comparative Study Using a Fixed-Bed Reactor

Production of Fischer–Tropsch catalysts is challenging because it involves controlling and optimizing multiple parameters in numerous technical steps. Here, we present C-supported nanometric Fe and Co catalysts synthesized by plasma spraying, a method that contracts catalyst production into a single step, in contrast to traditional multistep catalyst production by precipitation or impregnation. The catalysts were reduced <i>in situ</i> and then tested for Fischer–Tropsch synthesis in a gas–solid fixed-bed reactor at 230 °C and 30-bar pressure for 24 h. The performance of plasma-synthesized catalysts was superior at a gas hourly space velocity of 6,000 mL·g_<i>cat</i>^–1·h^–1, with Fe/C catalysts showing about 30% CO conversion per pass while Co/C catalysts yielded about 20% CO conversion. Identical C-supported Co and Fe catalysts prepared by impregnation or precipitation gave CO conversions of about 7% under similar reaction conditions

Atomic-Scale Faceting in CoPt Nanoparticles Epitaxially Grown on NaCl

Sub-10 nm CoPt nanoparticles were slowly grown at 400 °C in epitaxy on a NaCl substrate. Their faceted shape was analyzed using state-of-the-art TEM techniques: aberration-corrected imaging, electron tomography, and probe-aberration-corrected scanning transmission electron microscopy. These nanoparticles consist in truncated octahedrons with a chemically disordered face-centered cubic (FCC) structure. We evidenced slight variations of the truncation of these nano-octahedrons depending on their size: the largest particles are less truncated than the smallest particles. We also highlighted the up–down symmetry of the NPs, suggesting that the adhesion energy of FCC-CoPt on NaCl is negligible. Energy descriptions of these NPs were made by using quenched molecular dynamics in the framework of the second moment approximation of the tight-binding formalism, while taking into account the random distribution of Co and Pt atoms. In a general manner, this original energy approach for studying faceting in chemically disordered nanoalloys is consistent with experimental results, particularly for small-size clusters. However, as the experimentally observed size-effect on the NPs truncation was not theoretically predicted, this phenomenon could originate from kinetic effects inherent to nanocrystal growth