How Surface Species Drive Product Distribution during Ammonia Oxidation: An STM and Operando APXPS Study

The oxidation of ammonia is a key reaction for the production of artificial fertilizers and for environmental protection. Depending on the area of application, the catalytic reaction needs to be tuned toward the production of either N-2 or NOx and this selectivity is controlled by temperature, pressure, reactant ratio, and the type of catalyst. PtRh alloys are highly useful catalytic materials for the oxidation of ammonia, and they can be employed at different reaction conditions. In contrast to pure Pt and Rh catalysts, for which a large number of studies of ammonia oxidation reaction mechanism are available, for PtRh alloys, direct spectroscopic evidence for structure-performance relationship is still lacking. To understand the behavior of PtRh alloys, namely, what is their active phase under reaction conditions and how the alloy composition leads to a particular product distribution, we study the oxidation of ammonia over PtRh/Pt(111) surfaces by simultaneous operando ambient pressure X-ray photoelectron spectroscopy and mass spectrometry at 1 mbar total reaction pressure. These data are complemented by a catalyst surface characterization by scanning tunneling microscopy in ultrahigh vacuum. We establish that the predominant surface structure during NH3 oxidation strongly depends on the degree of Pt enrichment and the O-2/NH3 mixing ratio. At the nanoscale, the selectivity toward N-2 or NO production is driven by the surface populations of N and O species. These, in turn, are controlled by the nature of the alloying of Pt with Rh.

Automated summary

The oxidation of ammonia is a key reaction for the production of artificial fertilizers and for environmental protection. Depending on the application area, the catalytic reaction needs to be tuned towards the production of either N-2 or NOx, and PtRh alloys are effective catalytic materials for this purpose.