Discriminating Catalytically Active FeN_x Species of Atomically Dispersed Fe–N–C Catalyst for Selective Oxidation of the C–H Bond

Abstract

Nanostructured Fe-N-C materials represent a new type of "platinum-like" non-noble-metal catalyst for various electrochemical reactions and organic transformations. However, no consensus has been reached on the active sites of the Fe-N-C catalysts because of their heterogeneity in particle size and composition. In this contribution, we have successfully prepared atomically dispersed Fe-N-C catalyst, which exhibited high activity and excellent reusability for the selective oxidation of the C-H bond. A wide scope of substrates, including aromatic, heterocyclic, and aliphatic alkanes, were smoothly oxidized at room temperature, and the selectivity of corresponding products reached as high as 99%. By using sub-ångström-resolution HAADF-STEM in combination with XPS, XAS, ESR, and Mössbauer spectroscopy, we have provided solid evidence that Fe is exclusively dispersed as single atoms via forming FeN (x = 4-6) and that the relative concentration of each FeN species is critically dependent on the pyrolysis temperature. Among them, the medium-spin FeN affords the highest turnover frequency (6455 h), which is at least 1 order of magnitude more active than the high-spin and low-spin FeN structures and 3 times more active than the FeN structure, although its relative concentration in the catalysts is much lower than that of the FeN structures.

The Multifaceted Reactivity of Single‐Atom Heterogeneous Catalysts

“…Au-SACs based on functionalized carbon nanotubes were also shown to be inactive in the oxidation of thiophenol, but gold clusters formed during the reaction exhibited comparable activity to that of sulfhydryl oxidase enzymes. [142] [144] Moving away from precious metals,e xciting selective oxidation activity has also been reported for transition metals supported on N-doped carbons.…”

“…[137] This was linked to distinct adsorption configurations,w here the trimeric site could bond to both the hydroxy and amino groups of the substrate,t hereby facilitating the reaction. [139][140][141][142][143] Va riation of the average coordination number of FeN x (x = 4-6) species in N-doped graphene was shown to yield up to an order of magnitude difference in tof for the selective oxidation of hydrocarbons. [136] On the other hand, ac omparative study of Pd n /CeO 2 (n = 1o r6 ) catalysts evidenced the remarkably enhanced activity of SAs in the oxidation of benzyl alcohols,w hich was attributed to the more intimate interaction with the host compared to the clusters.…”

Engineering the Coordination Environment of Single‐Atom Platinum Anchored on Graphdiyne for Optimizing Electrocatalytic Hydrogen Evolution

“…[9][10][11][12][13] More recently,P tS ACsw ith individual Pt atoms anchored on the supports of carbon nanotube and CoP have also been developed as efficient HER electrocatalysts. [17] Wu and co-workers constructed aseries of Co SACs with different nitrogen coordination numbers,a nd they found that atomically dispersed Co with two coordinate nitrogen atoms displays the best CO 2 electroreduction catalytic performance. [16] Forexample,Zhang et al found that Fe III N 5 species in Fe SACs displays the highest catalytic activity for selective oxidation of the CÀH bond, which is one order magnitude more active than Fe III N 6 species and three times more active than Fe II N 4 species.…”

Mono‐Atomic Fe Centers in Nitrogen/Carbon Monolayers for Liquid‐Phase Selective Oxidation Reaction

“…[11] The formation of FeÀN coordination was also unambiguously confirmed by the similarity of absorption edges and white lines in the extended X-ray absorption fine structure (EXAFS) spectra of 2D-FeNC when compared to Fe 2 + -containing iron phthalocyanine (FeÀPc) (Figures 3c and S11). [12] Further Fourier-transformed EXAFS spectra (Figure 3d) indicates that a Fe/N coordination (R = 1.47 Å ), rather than FeÀFe coordination (R = 2.14 Å ) or Fe/O coordination (R = 1.41 Å ), is the major connection as shown in the scheme of FeÀX bonds in 2D-FeNC ( Figure S12). [12] Further Fourier-transformed EXAFS spectra (Figure 3d) indicates that a Fe/N coordination (R = 1.47 Å ), rather than FeÀFe coordination (R = 2.14 Å ) or Fe/O coordination (R = 1.41 Å ), is the major connection as shown in the scheme of FeÀX bonds in 2D-FeNC ( Figure S12).…”