Fig. 1. Catalytic performance of Fe-based catalysts. (a) Conversion at 500 °C over 4 h shows different induction periods for the catalyst with and without Pt addition. (b) Plot for the quantification of the nitride formation rate, where k' is the slope of the fitted linear lines and represents the nitride formation rate. Reaction conditions: 500 °C, 12000 mL gcat-1 h-1.

Fig. 2. (a?c) STEM-HAADF imaging and the corresponding elemental mappings of as-prepared 0.1Pt-10Fe/SiO2 (passivated) using EDS. (d?f) Representative STEM-HAADF images of spent 0.1Pt-10Fe/SiO2 catalyst after 20 h of reaction, indicating the stability of Pt species on Fe particles. (g) A representative STEM-HAADF image and the corresponding EELS elemental maps of a catalyst particle from the spent catalyst, show the formation of iron nitride for spent catalysts.

Fig. 3. Formation and identification of iron nitride species. (a) XRD of catalysts before reaction and spent catalysts after 20 h of NH3 decomposition reaction at 500 °C. Reference cards of Fe (PDF #06-0696), Fe2O3 (PDF #39-1346), Fe3N (PDF #49-1662), Fe2N (PDF #72-2126 and FeN (PDF #88-2153) are also presented. (b) Fourier transform of k3-weighted χ EXAFS in situ spectra at different stages of the nitridation process performed at 500 °C with 40 mL min-1 5%NH3 (i) and close-up views of the time-series Fourier transform k3-weighted χ EXAFS in situ spectra overlaid (ii).

Fig. 4. Role of Pt in facilitating H reverse spillover shown through H2-TPD (a), whose peaks have been deconvoluted using a Bi-Gaussian fitting. H spillover is proven by investigating reduction behaviour through H2-TPR (b) and XANES (c)(i) during in situ reduction that can be quantitively compared using LCF of the experimental spectra (ii) with the reference spectra of Fe2O3, FeO, and Fe foil.

Fig. 5. The role of H2 reverse spillover in facilitating nitride formation rate. (a) Effect of H2 co-feeding on the nitride formation rate of the catalysts. Reactions were performed at 500 °C with a GHSV of 12000 mL g-1cat h-1 and P(NH3) = P(H2) = 0.5. (b) XRD of spent catalysts after 4 h of reaction with H2 co-feeding.

Fig. 6. Proposed mechanism for the role of Pt (single atoms or clusters) in facilitating reverse H spillover and enhancing the rate of nitride formation in iron catalysts.