Abstract
Inverse oxide/metal model systems are frequently used to investigate catalytic structure-function relationships at an atomic level. By means of a novel atomic layer deposition process, growth of single-site Fe1Ox on a Pt(111) single crystal surface was achieved, as confirmed by scanning tunneling microscopy (STM). The redox properties of the catalyst were characterized by synchrotron radiation based ambient pressure X-ray photoelectron spectroscopy (AP-XPS). After calcination treatment at 373 K in 1 mbar O2 the chemical state of the catalyst was determined as Fe3+. Reduction in 1 mbar H2 at 373 K demonstrates a facile reduction to Fe2+ and complete hydroxylation at significantly lower temperatures than what has been reported for iron oxide nanoparticles. At reaction conditions relevant for preferential oxidation of CO in H2 (PROX), the catalyst exhibits a Fe3+ state (ferric hydroxide) at 298 K while re-oxidation of iron oxide clusters does not occur under the same condition. CO oxidation proceeds on the single-site Fe1(OH)3 through a mechanism including the loss of hydroxyl groups in the temperature range of 373 to 473 K, but no reaction is observed on iron oxide clusters. The results highlight the high flexibility of the single iron atom catalyst in switching oxidation states, not observed for iron oxide nanoparticles under similar reaction conditions, which may indicate a higher intrinsic activity of such single interfacial sites than the conventional metal-oxide interfaces. In summary, our findings of the redox properties on inverse single-site iron oxide model catalyst may provide new insights into applied Fe-Pt catalysis.
Highlights
Supported nanoparticle catalysts have immense societal impact due to their critical role for heterogeneous catalysts in industrial chemical production [1, 2]
We surprisingly find that larger nanoparticles are formed, a considerable fraction of the monodispersed iron oxide protrusions still survive after such high temperature treatment, distinctly different from the FeOx film prepared by physical vapor deposition (PVD) where triangular FeOx inslands with above 10 nm size are usually formed after the same annealing treatment [40]
We conducted in-situ ambient pressure X-ray photoelectron spectroscopy (AP-X-ray photoemission spectroscopy (XPS)) studies of the surface to monitor the dynamic changes of this model catalyst in the presence of reactive gas streams
Summary
Supported nanoparticle catalysts have immense societal impact due to their critical role for heterogeneous catalysts in industrial chemical production [1, 2]. A successful strategy for achieving atomic level understanding of catalysts is the application of simplified model catalysts with uniform and well-defined surface structures [8, 16]. Formation of atomically dispersed iron oxide species supported on Pt(111), the structurally most simple inverse model catalyst, by conventional PVD methods remains challenging. The results indicate that the high catalytic efficiency can be traced from the unusually high flexibility in changing the oxidation state of the iron center
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