Characterization of chromia-promoted $gamma;-iron oxide catalysts and their CO conversion efficiency

Dynamics of CrO3–Fe2O3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

“…The initial oxidized chromium–iron oxide catalysts possess a surface chromia phase primarily present as dioxo (O) 2 Cr 6+ O 2 surface species, but a small amount of surface mono-oxo OCrO 4 species are also present (see Figure S1 in the Supporting Information). The separation of ∼14 cm –1 between the symmetric and asymmetric vibrations of OCrO observed in the Raman and IR spectra, respectively, also matches the vibrational rules for dioxo metal oxides. ,− ,, The surface chromia species on Fe 2 O 3 are metastable, since their total concentration decreases at elevated temperatures in oxidizing environments (see Figure ). There appears to be an equilibrium ratio of Cr 6+ /Cr 3+ in the surface region, as indicated by nearly identical atomic fractions observed in the in situ XPS (Figure S4 in the Supporting Information).…”

“…The dissolution of Cr 3+ into the iron oxide bulk lattice at elevated temperatures and WGS/RWGS reaction conditions is indicated by the increased Cr 3+ signal (EXAFS, see Figure ). Reoxidation with O 2 after RWGS only partially reoxidizes the chromia to surface Cr 6+ (EXAFS and Raman, see Figures and and Figure S13) and is consistent with the trapping of Cr 3+ in the iron oxide bulk lattice. ,− The Cr 3+ sites in the iron oxide bulk lattice during the WGS/RWGS reactions are responsible for stabilization of the Fe 3 O 4 phase (minimization of metallic Fe 0 phase) and the enhanced BET surface area (see Table ). A diagram of the bulk structures of the chromium–iron oxide catalysts during the WGS/RWGS reactions is given in Scheme .…”

“…In addition, just heating chromium–iron oxide catalysts under oxidizing conditions increases the concentration of Cr 3+ in the bulk Fe 2 O 3 lattice (see Figure ). Dissolution of chromium oxide into the iron oxide bulk lattice as Cr 3+ is known to displace Fe 2+ /Fe 3+ from octahedral sites and expose the iron oxide surface. ,− A diagram of the bulk structure of the initial oxidized chromium–iron oxide catalyst is given in Scheme .…”

“…It is now also well established that catalyst surfaces are dynamic under reaction conditions, a trait that cannot be fully appreciated and understood without in situ and operando spectroscopy analysis . Ambient and ex situ characterizations of the iron–chromium oxide HTS catalyst have been performed with many techniques, including X-ray diffraction (XRD), IR spectroscopy, electron microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy. − The results of these studies have reached the following conclusions: (1) the fresh catalyst is a Fe 2 O 3 phase, typically either α or γ depending on synthesis and calcination conditions, (2) both Cr 3+ and Cr 6+ exist in the fresh catalyst, (3) Cr 3+ is substituted preferentially into the iron oxide lattice octahedral sites ( O h ) but not tetrahedral sites ( T d ), (4) there is some surface segregation of chromium, (5) the reduced catalyst contains Fe 3 O 4 (magnetite), and (6) discrete Cr 2 O 3 particles are present above ∼14 wt % Cr 2 O 3 /Fe 2 O 3 .…”

Water Gas Shift Reaction Activity on Fe (110): A DFT Study

“…WGSR can be divided into low-temperature (190-250 • C) WGSR and high-temperature (300-450 • C) WGSR under different catalysts according to the temperature needed for the reaction [1]. The hightemperature WGSR catalyst is mainly ferric oxide, which is widely used in industry due to its low price and excellent catalytic performance [4][5][6]. Generally, the active phase of the iron oxide catalyst is supposed to be Fe 3 O 4 [7,8].…”