With the purpose of investigating the reactivity of Fe carbide as active phase in Fischer-Tropsch catalysis, we study the formation of a well-defined Fe carbide surface structure resulting from carbon exposure of an Fe film on Au(111). Using two different sources of carbon (C), namely atomic carbon and ethylene gas, we use synchrotron X-ray photoelectron spectroscopy (XPS) to 2 show that a 6 ML Fe film readily converts into a well-defined and thermodynamically stable carbide phase. Scanning tunneling microscopy (STM) shows that the surface of the Fe carbide film is crystalline and dominated by Fe(110)-like facets perturbed into a (2×2) periodic structure due to insertion of C in the interstitial sites. The reactivity of the carbide film towards CO, H2 and O2 was furthermore probed by XPS under vacuum conditions. While the pristine Fe carbide surface was unreactive towards hydrogen gas at 500 K, we interestingly find that CO dissociation from a pre-adsorbed monolayer of CO takes place already at low temperature. This observation points to an intrinsic activity of the Fe carbide phase where additional carbon originating from CO can be placed in the Fe carbide surface. The catalytic significance of the model catalyst surface presented here is that it can be seen as a stable Fe-carbide phase with intrinsically vacant sites for additional C insertion at elevated pressure, and we propose that such additional C may act as active species in CC coupling reactions during FTS. The studies pave the way for a better understanding of FTS processes on Fe-based catalysts based on a welldefined model surface.
Atomically precise subnanometer catalysts are of significant interest because of their remarkable efficiency in a variety of catalytic reactions. However, the dynamic changes of active sites under reaction conditions, in particular, the transition of cluster–oxide interface structure have not yet been well-elucidated, lacking in situ measurements. By using multiple state-of-the-art in situ characterizations, here we show a dynamic interplay between copper tetramers and iron oxides in a single-size Cu4/Fe2O3 catalyst, yielding an enrichment of surface Cu4–Fe2+ species under reaction conditions that boosts CO2 hydrogenation at near-atmospheric pressures. During reaction, Cu4 clusters facilitate the reduction of Fe2O3 producing surface-rich Fe2+ species in the proximate sites. The as-formed Fe2+ species in return promotes CO2 activation and transformation over Cu4 cluster, resulting in strikingly high methanol synthesis at low temperatures and C1/C3 hydrocarbon production in a high-temperature regime. The discovery of highly active Cu4–Fe2+ sites thus provides new insights for the atomic-level design of copper catalyst toward high-efficiency CO2 conversion under mild conditions.
Model catalysts consisting of iron particles with similar size deposited on thin-film silica (Fe/SiO2) and on silicon (Fe/Si) were used to study iron carbidization in a CO atmosphere using in situ near-ambient-pressure X-ray photoelectron spectroscopy. Significant differences were observed for CO adsorption, CO dissociation, and iron carbidization when the support was changed from thin-film silica to silicon. Stronger adsorption of CO on Fe/Si than that on Fe/SiO2 was evident from the higher CO equilibrium coverage found at a given temperature in the presence of 1 mbar of CO gas. On thin-film silica, iron starts to carbidize at 150 °C, while the onset of carbidization is at 100 °C on the silicon support. The main reason for the different onset temperature for carbidization is the efficiency of removal of oxygen species after CO dissociation. On thin-film silica, oxygen species formed by CO dissociation block the iron surface until ∼150 °C, when CO2 formation removes surface oxygen. Instead, on the silicon support, oxygen species readily spill over to the silicon. As a consequence, oxygen removal is not rate-limiting anymore and carbidization of iron can proceed at a lower temperature.
The adhesive proteins secreted by marine mussels contain an unusual amino acid, 3,4-dihydroxyphenylalanine (DOPA), that is responsible for the cohesive and adhesive strength of this natural glue and gives mussels the ability to attach themselves to rocks, metals, and plastics. Here we report a detailed structural and spectroscopic investigation of the interface between N-stearoyldopamine and a single-crystalline Au(111) model surface and an amide-absent molecule, 4-stearylcatechol, also on Au(111), with the aim of understanding the role of the amide functionality in the packing, orientation, and fundamental interaction between the substrate and the monolayer formed from an aqueous environment by the Langmuir-Blodgett technique. The organization of monolayers on gold was observed directly and studied in detail by X-ray photoelectron spectroscopy (XPS), contact angle measurements (CA), surface-enhanced Raman spectroscopy (SERS), infrared reflection-absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). Our study shows that within the monolayer the catecholic oxygen atoms are coordinated to the gold surface, having a more perpendicular orientation with respect to the aromatic ring and the apparently tilted alkyl chains, whereas the amide functionality stabilizes the monolayer that is formed.
Using scanning tunneling microscopy (STM), we characterize the atomic-scale details of ultrathin films of iron carbide (Fe x C y ) on Au(111) synthesized as a potential model system for the active iron carbide phase in iron Fischer–Tropsch synthesis (FTS) catalysts. The experiments show that room-temperature exposure of Fe islands gas to C2H4 deposited on the clean Au(111) surface results in partly converted Fe/Fe x C y islands. Multistep flash-heating treatment of the partly converted Fe/Fe x C y islands at 523 and 773 K results in pure highly crystalline Fe x C y islands with in-plane nearest-neighbor distances of 0.315 ± 0.005 nm. On the basis of the atom-resolved STM data, we propose that C2H4 dissociates at Fe island edges, after which the carbon diffuses inward into the interstitial region between the Fe and the Au substrate to form an Fe x C y surface that may be a good starting point for the investigation of iron carbide surfaces present under FTS conditions.
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