Decoding technical multi-promoted ammonia synthesis catalysts

Abstract

Ammonia is industrially produced by the Haber-Bosch process over a fused, multi-promoted iron-based catalyst. Current knowledge about the reaction has been derived from model systems of reduced structural complexity, impeding any clear-cut structure-activity correlation relevant for the industrial counterpart. Here, we unveil the structural evolution of complex, technical, multi-promoted ammonia synthesis catalysts by operando scanning electron microscopy and near-ambient pressure X-ray photoelectron spectroscopy. We highlight that the activation is the critical step in which the catalyst is formed and decode the pivotal role of the promoters. We discover that the active structure consists of a nanodispersion of Fe covered by mobile K-containing adsorbates, so called "ammonia K". The porous catalyst is stabilized by mineral cementitious phases containing oxides of Al, Si, Ca, and Fe. The synergism between the different promoters contributes simultaneously to the structural stability, hierarchical architecture, catalytic activity, and poisoning resistance. The confluence of these aspects is the key for the superior performance of technical catalyst formulations.

Fig. 1. SEM survey of the surface morphology of a technical multi-promoted ammonia synthesis catalyst.

a The overview image of the surface structure of the initial catalyst shows a brick wall architecture with iron-rich granules held together by a continuum of promoter material prior to catalytic reaction. b SEM image of the region of interest highlighted in (a) displaying the complex structure in a confluence point of several iron-rich granules. c The close-up look on an iron-rich granule from the region of interest highlighted in (b) denotes the surface morphology covered by a heterogeneously distributed material and a maze-like porous structure of the substrate. d SEM image of the surface of the spent catalyst after 96 h of ammonia synthesis at 90 bar showing platelet, needle-like, and crust-like materials. Conditions of acquisition of (a–c): 22 Pa, H₂:N₂:Ar = 3:1:0.1.

Fig. 2. Time series of temperature protocol and products during the OSEM experiment.

The time series shows the heating protocol and the ion currents of NH, NH₂, NH₃, and H₂O (normalized to Ar) with major contributions at m/z = 15, 16, 17, and 18, respectively. The ionic currents at m/z = 15, 16, and 17 were multiplied by 500, 100, and 10, respectively. The pink region marks the onset of the catalytic activation with simultaneous catalyst reduction. The blue region marks the phase transformation of the surface into a disordered material. Conditions of acquisition: 22 Pa, H₂:N₂:Ar = 3:1:0.1. Heating ramp: 12 Kh⁻¹.

Fig. 3. Consecutive in situ SEM images of the catalyst surface at iron-rich granules during heating and under reaction conditions.

a–c The evolution of the catalyst surface during the temperature ramp. a The segregation of material is already detectable at 377 °C at TOS = 64.5 h in the form of bright roundish nanoparticles on the catalyst surface. b The segregation of material continued at 428 °C and TOS = 68.8 h. c The treatment induced growth of the segregated phase at 449 °C and TOS = 70.5 h. Images extracted from the Supplementary Movies 1 and 2. d–i Evolution of the catalyst surface during the isothermal treatment at 500 °C showing platelet formation. Images in (d–f) were extracted from Supplementary Movie 3. g Overview image showing the external surface coverage by the segregated phase after TOS = 253 h. h, i Close-up views of the regions of interest marked in (g) showing the platelet-like morphology of the segregated material. Conditions of acquisition: 22 Pa, H₂:N₂:Ar = 3:1:0.1.

Fig. 4. Continuation of the in situ SEM observation of the catalyst surface at iron-rich granules under reaction conditions.

a–c Morphological transformation of the segregated material after TOS = 272 h. The images in (a–c) were extracted from Supplementary Movie 4. d–f Catalyst surface imaging during isothermal treatment at 500 °C in a second experiment shows phase exsolution and platelet segregation. Images in (d–f) were extracted from Supplementary Movie 5. Conditions of acquisition: 22 Pa, H₂:N₂:Ar = 3:1:0.1.

Fig. 5. Structural and compositional investigation of a thinned cross-section of an iron-rich granule of the spent catalyst after the OSEM experiment.

a A high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the cross-section. The arrow marks the direction of the plotline used for the analyses of the energy dispersive X-ray spectroscopy (EDX) measurements presented in (h). b–g EDX elemental maps corresponding to Fe, O, K, Al, Si, and Ca, respectively. h Plot of elemental ratios for O/Fe, Al/Fe, Si/Fe, K/Fe, Ca/Fe, Al/Si, and Ca/Si calculated by integration of EDX intensities of (b–g) along the plotline represented in (a). i A high resolution TEM (HR-TEM) image of a platelet particle showing an ordered structure (stacked nanofibers) surrounded by an amorphous layer. j, k HR-TEM images of the porous catalytic substrate revealing the presence of metallic iron crystallites held together in a porous arrangement by an amorphous material. The black arrows in (k) indicate surface steps of ammonia iron.

Fig. 6. Variation of chemical composition of the catalyst surface under reaction conditions.

NAP-XPS spectra of a Fe2p, b N1s, c K2p, d Al2p, e Si2p, and f Ca2p. From bottom to top, spectra display data collected at 250 °C, 500 °C, and after cooling to room temperature, respectively. Conditions of acquisition: 50 Pa, Deuterium:N₂:Kr = 3:1:0.1. Further details are given in Supplementary Table 2.

Fig. 7. A compilation of the integrated peak intensities corresponding to K, Al, Si, and Ca as a function of temperature as obtained from NAP-XPS measurements.

Conditions of acquisition: 50 Pa, H₂:N₂:Kr = 3:1:0.1.The data dispersion was given a contour map (95% density) for visualization.

Fig. 8. Schematic representations of the morphological evolution of the promoter phases, the activated catalyst architecture, and the catalytic production of ammonia.

a The chemical linking of Al, Si, Ca, and Fe oxidic phases produces a material similar to a cementitious phase. At early stages of hydration, this phase gives rise to nanofiber basic units. With ongoing aging (TOS) under decreasing water vapor the basic units assemble sequentially in platelets, needle-like aggregates, and densified/disordered material. b The activated catalyst exhibits a 3D nanodispersion of ammonia Fe crystallites structurally stabilized by a set of cementitious phases. Excess of the segregated promoter material accumulates at the external surface in the several forms depicted in (a). Ammonia K covers the ammonia Fe of the catalytic substrate at reaction conditions (500 °C), but preferentially accumulates on the external surface at low temperatures. c The dissociative chemisorption of N₂ is enhanced by in situ formed ammonia K/ammonia Fe phases due to electronic transfer effects. The atomic N species reacts either sequentially with atomic H to give rise to the ammonia product, or with the Fe surface to produce nitrides. The presence of alkaline KOH increases the desorption of the ammonia product.