An Elevated N₂-to-NO Transition Temperature in Ammonia Oxidation on Polycrystalline Palladium: A Pulsed Time-Resolved Ambient-Pressure XPS Study

Sammanfattning

Catalytic oxidation of ammonia (NH₃) is the key reaction in the Ostwald process for industrial nitric acid production. The reaction yields three nitrogen-containing products, N₂, N₂O, and NO, and the selectivity between them depends on the catalyst and the operating conditions. Most of what is known comes from steady-state studies on platinum and rhodium. This thesis studies what happens when the reactant supply is varied over time rather than held constant. Time-resolved ambient-pressure X-ray photoelectron spectroscopy (tr-APXPS) is used to follow NH₃ oxidation on a polycrystalline palladium catalyst at 500°C and 600°C. The catalyst is held under a 1 mbar O₂ flow, and short NH₃ pulses are repeatedly injected over 120 cycles to enable event-averaging. Three findings emerge. First, the Pd surface composition differs sharply between the two temperatures, dominated by Pd oxide at 500°C and largely metallic at 600°C. Second, the product selectivity shifts toward NO at the higher temperature, in the same direction as the Pt/Rh literature. Third, the NH₃ conversion drops from 94% to 80% with increasing temperature, while the peak oxygen conversion stays around 40% at both temperatures. We propose that the conversion drop reflects the temperature-driven change in surface O coverage: by analogy with DFT results reported on Pt, adsorbed O and OH species at 500°C may assist NH₃ dehydrogenation more than the metallic surface at 600°C. The transition to NO-dominated selectivity also occurs at a higher temperature than reported for steady-state Pt/Rh (below 600 K), with N₂ and N₂O still accounting for nearly half of the products at 500°C. Two factors may contribute: the catalyst (Pd rather than Pt or Rh, possibly with longer surface N residence) and the pulsed delivery, which may briefly raise the surface N coverage.