Tandem CO₂ Valorization and Ethane Dehydrogenation: Elucidating the Nature of Highly Selective Iron Oxide Active Sites

Abstract

Iron oxide-based catalysts can selectively covert CO 2 to added-value CO in parallel with on-purpose ethylene production via ethane dehydrogenation (CO 2 -EDH). The 5Fe/10NiMgZr catalyst, calcined at 700 °C for 5 h, reached 90% selectivity, at a single-pass C 2 H 6 conversion of ∼23.3%, during CO 2 -EDH at 650 °C. X-ray characterization techniques (XRD, XPS, XAS, and XRS), along with structural modeling were used to elucidate the nature of highly selective active sites toward C−H bond scission. Under reaction conditions, Fe atoms are incorporated into the support lattice, as it was exemplified by XRS at Mg L 2,3 -and O K-edges. The dynamic structural modifications, occurring during CO 2 -EDH, highlighted the pivotal role of the support, reaching an ethylene yield of 21%. Mg coordination altered from octahedral to tetrahedral. The latter was accompanied by an electronic modification, which had a strong impact on the catalyst acidity, based on Fourier transform infrared spectroscopy combined with in situ adsorption of pyridine, resulting in an unpresented ethylene selectivity for Fe oxide-based catalysts.

Evolution of Low Z-Elements in a Ni/MgFeAlO4 Catalyst during Reaction: Insight from In Situ XRS

“…In order to understand the observed results, the O K-edge has been analyzed by simulation, involving the use of screening as a semiempirical parameter. ,,, The FDMNES code was applied to quantify the electronic changes occurring due to reduction, reoxidation, and DRM treatment. In FDMNES, the screening parameter can range from 0 (i.e., the photoelectron is fully promoted to an unoccupied state and the core-hole is not screened, hence the absorbing atom has an oxidation state close to +1) to 1 (i.e., the photoelectron is fully promoted to an unoccupied state and its core-hole is fully screened, making the absorbing atom appear neutral).…”

“…Therefore, an increase in the screening value refers to an increase in electron density, provided by the neighboring cations, on the absorbing atom leading to better screening of the core-hole. 24,46 To simulate the fresh spectrum, contributions related to support MgFeAlO 4 and NiO have been considered as they both hold O in fresh condition. Simulations of NiO with variable screening values all lead to spectra with a distinct preedge feature in the energy range 525−530 eV (Figure S6).…”

“…Hence, these NiO and FeO x phases were omitted from the simulation, keeping only the MgFeAlO 4 support as the contributor to the O K-edge simulations. Reproduction of the main features of the O K-edge for the sample in fresh, reduced, reoxidized, and DRM-treated state requires using a linear combination of 2 screening values 1 and 0.1 24 (Figure 9A), reflecting two different electronic states of oxygen in MgFeAlO 4 . 25 The high screening value of 1 points to adequate screening of the O core-hole by the neighboring cations; the lower screening value 0.1 indicates that only 10% of the positive core-hole is screened, hence less electron density on those oxygen atoms because of less charge transfer from the neighboring 3d transition metal ions.…”

“…25 The high screening value of 1 points to adequate screening of the O core-hole by the neighboring cations; the lower screening value 0.1 indicates that only 10% of the positive core-hole is screened, hence less electron density on those oxygen atoms because of less charge transfer from the neighboring 3d transition metal ions. 24 Figure 9B shows the impact of the value for the screening parameter upon the simulated K-edge spectrum. Each of the screening values refers to a particular electronic structure of the sample.…”

“…In this way, XRS provides direct observation of soft X-ray edges by means of hard X-rays scattered from the bulk of the sample. The shape of the O K-edge reflects the local charge distribution and depicts the interaction with the neighboring metal cations. , Also, Al is a crucial component of the support spinel structure that provides a high surface area for dispersing the active species as small particles on its surface. Moreover, it brings stability to the catalyst at high temperature and prevents Ni from sintering or agglomerating. , …”