Spatial Profiling of a Pd/Al2O3 Catalyst during Selective Ammonia Oxidation

Abstract

The utilization of operando spectroscopy has allowed us to watch the dynamic nature of supported metal nanoparticles. However, the realization that subtle changes to environmental conditions affect the form of the catalyst necessitates that we assess the structure of the catalyst across the reactant/product gradient that exists across a fixed bed reactor. In this study, we have performed spatial profiling of a Pd/Al₂O₃ catalyst during NH₃ oxidation, simultaneously collecting mass spectrometry and X-ray absorption spectroscopy data at discrete axial positions along the length of the catalyst bed. The spatial analysis has provided unique insights into the structure-activity relationships that govern selective NH₃ oxidation-(i) our data is consistent with the presence of PdN _x after the spectroscopic signatures for bulk PdN _x disappear and that there is a direct correlation to the presence of this structure and the selectivity toward N₂; (ii) at high temperatures, ≥400 °C, we propose that there are two simultaneous reaction pathways-the oxidation of NH₃ to NO _x by PdO and the subsequent catalytic reduction of NO _x by NH₃ to produce N₂. The results in this study confirm the structural and catalytic diversity that exists during catalysis and the need for such an understanding if improvements to important emission control technologies, such as the selective catalytic oxidation of NH₃, are to be made.

Figure 1

XANES spectra of Pd/Al₂O₃, after reduction, at axial position 0 within the bed compared to Pd foil and PdO reference.

Figure 2

Pd/Al₂O₃ at 100 °C under reaction conditions. (a) XANES spectra of Pd/Al₂O₃ under reaction condition, 100 °C, for the front and the end of the bed, compared to Pd foil and PdO reference; (b) intensity of XANES at the whiteline (24 368 eV) and at the PdN_x peak (24 388 eV) along the catalyst bed; (c) Pd–Pd and Pd–N coordination number obtained from EXAFS fit; and (d) Pd–Pd and Pd–N/O distances obtained from EXAFS fit (the error is present but smaller than the symbol size). Mass spectrometry data can be found in Figure S2.

Figure 3

(a) MCR of the identified spectral components: oxidic Pd, PdO (black), metallic Pd⁰ (red), and PdN_x (blue). See Figure S4 for a comparison with reference spectra (b) spatial profile of the component percentage for Pd/Al₂O₃ under reaction conditions, 100 °C.

Figure 4

Pd/Al₂O₃ at 175 °C. (a) XANES spectra of Pd/Al₂O₃ under reaction condition, 175 °C, for the front and the end of the bed, compared to Pd foil and PdO reference; (b) component percentage, obtained from MCR, for Pd/Al₂O₃ under reaction conditions, 175 °C, at various positions along the bed; (c) Pd–Pd, Pd–N, and Pd–N/O coordination number obtained from EXAFS fit; and (d) normalized mass spectrometry signal along the bed; the intensity of XANES at the whiteline (24 368 eV) and at the PdN_x peak (24 388 eV) along the catalyst bed is shown in Figure S6.

Figure 5

Spatial analysis of Pd/Al₂O₃ at 300 °C under reaction conditions. (a) XANES spectra; (b) component percentage, obtained from MCR, for Pd/Al₂O₃ under reaction conditions, 300 °C, at various positions along the bed. (c) Pd–Pd, Pd–N/O, Pd–O (in PdO), and Pd–PdO coordination number obtained from EXAFS fit; and (d) Pd–Pd normalized mass spectrometry signal along the bed; the intensity of XANES at the whiteline (24 368 eV) and at the PdN_x peak (24 388 eV) along the catalyst bed is shown in Figure S9.

Figure 6

(a) Component percentage, obtained from MCR, for Pd/Al₂O₃ under reaction conditions, 400 °C, at various positions along the bed; (b) normalized mass spectrometry signal along the bed.