Characterizing Complex Gas–Solid Interfaces with in Situ Spectroscopy: Oxygen Adsorption Behavior on Fe–N–C Catalysts

Abstract

Electrocatalysts for the oxygen reduction reaction (ORR) within polymer electrolyte membrane fuel cells (PEMFCs) based on iron, nitrogen, and carbon elements (Fe-N-C) have received significant research attention as they offer an inexpensive alternative to catalysts based on platinum-group metals (PGMs). While both the performance and the fundamental understanding of Fe-N-C catalysts have improved over the past decade, there remains a need to differentiate the relative activity of different active sites. Towards this goal, our study is focused on characterizing the interactions between O 2 and a set of five structurally-different Fe-N-C materials. Detailed characterization of the Fe speciation was performed with 57 Fe Mössbauer spectroscopy and soft X-ray absorption spectroscopy (sXAS) of the Fe L 3,2 -edge, while nitrogen chemical states were investigated with X-ray photoelectron spectroscopy (XPS). In addition to initial sXAS and XPS measurements performed in ultra-high vacuum (UHV), measurements were also collected (at the identical location) in an atmosphere of 100 mTorr of O 2 at 80 °C (O 2rich). XPS and sXAS results reveal the presence of several types of FeN x C y adsorption sites.FeN x C y sites that are proposed as the most active ones do not show significant change (based on the techniques used in this study) when their environment is changed from UHV to O 2 -rich.Correlation with Mössbauer and sXAS results suggest this is most likely due to the persistence of strongly adsorbed O2 molecules from their previous exposure to air. However, other species do show spectroscopic changes from UHV conditions to O 2 -rich. This implies these sites have a weaker interaction with O 2 that results in its desorption in vacuum conditions, and re-adsorption when re-exposed to the O 2 -rich environment. The nature of these weakly and strongly O 2adsorbing FeN x C y sites is discussed in the context of different synthetic and processing parameters employed to fabricate each of these five Fe-N-C materials.EE0008419 "Active and Durable PGM-free Cathodic Electrocatalysts for Fuel Cell Application").

Microenvironment Alters the Oxygen Reduction Activity of Metal/N/C Catalysts at the Triple-Phase Boundary

“…On the weak OH* binding branch, step 1 (O 2 to OOH*) is the PDS. According to research on the interactions between Fe/N/C catalysts and oxygen under ultrahigh vacuum, the Fe/N/C catalysts strongly bind to oxygen. Therefore, we analyzed our Fe/N/C catalysts at the strong OH* binding branch of the “volcano.” This analysis revealed that step 4 (OH* to H 2 O) is the PDS.…”

Structural Evolution of Atomically Dispersed Fe Species in Fe–N/C Catalysts Probed by X-ray Absorption and 57Fe Mössbauer Spectroscopies

“…The doublet D1 represents Fe 2+ coordinated by four pyrrolic N groups connected to a carbon support, and it is assigned to low-spin Fe 2+ –N 4 . The doublets D2 and D3 consist of ferrous intermediate-spin sites, and the main difference between the two doublets is their Fe local environments. ,,,,,− The δ iso and Δ E Q values of D2 are close to those of FePc, while the values of D3 are similar to those of Fe porphyrins. , The singlet (Sing) represents superparamagnetic Fe, which has a smaller size than the Weiss domain; thus, it does not have a magnetic splitting value ( H 0 ).…”

Using Computational Chemistry To Reveal Nature’s Blueprints for Single-Site Catalysis of C–H Activation

“…Various experimental characterization techniques have been used to investigate SAC structures and reactivity, especially in the case of iron. For example, the properties of Fe are characterized by 57 Fe Mössbauer spectroscopy, , X-ray absorption near-edge spectroscopy (XANES), and EXAFS. , High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) , is frequently utilized to confirm that metals are atomically dispersed in SACs. These spectroscopic analyses suggest that the support and primary coordination sphere are essential in determining catalytic reactivity, , consistent with conclusions drawn from DFT simulations. , Joint experimental characterization and computational studies have highlighted the roles of the support and primary coordination sphere for SACs in catalyzing HER, ORR, − CO 2 RR, ,, and selective C–H bond oxidation. , …”

“…Gu et al found that Fe­(III) does not get oxidized or reduced during the electrocatalytic reduction of CO 2 to CO, which contributed to the greater activity of Fe­(III) relative to conventional Fe­(II) sites . The 57 Fe Mössbauer spectroscopic studies necessary to assign the precise oxidation/spin states of the FeN x C y moiety remain challenging due to the distribution of sites that must be obtained from spectral fits. , Typically, SACs are modeled with plane-wave, semilocal DFT, in which the metal oxidation and spin states are either erroneously predicted or not precisely defined (i.e., due to fractional occupation of frontier states with smearing using semilocal DFT). To reveal the electronic structure of the metal and its relationship to the catalytic reactivity, efforts to understand SAC reactivity via molecular mimics have probed the effects of nitrogen type (e.g., pyridinic or pyrrolic), typically embedded in 14- or 16-membered macrocycles. ,, DFT modeling of finite size nanoflakes that represent the infinite system also help address the challenges in periodic systems …”