Surface Reduction State Determines Stabilization and Incorporation of Rh on α‐Fe₂O₃(11¯02)

Abstract

Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as‐synthesized catalyst prior to use. Here, Rh adsorption and incorporation on the (11true¯02) surface of hematite (α‐Fe2O3) are studied, which switches from a stoichiometric (1 × 1) termination to a reduced (2 × 1) reconstruction in reducing conditions. Rh atoms form clusters at room temperature on both surface terminations, but Rh atoms incorporate into the support lattice as isolated atoms upon annealing above 400 °C. Under mildly oxidizing conditions, the incorporation process is so strongly favored that even large Rh clusters containing hundreds of atoms dissolve into the surface. Based on a combination of low‐energy ion scattering (LEIS), X‐ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) data, as well as density functional theory (DFT), it is concluded that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer.

Single Rh Adatoms Stabilized on α-Fe2O3(11̅02) by Coadsorbed Water

“…In the case of room-temperature deposition in UHV ( Figure 1 a), this surface does not stabilize single Rh adatoms, which instead form small clusters after deposition. 20 In contrast, after depositing with background H 2 O, the majority of features in Figure 1 b,c appear as uniform isolated protrusions. To determine how many Rh atoms each feature contains, we performed a separate experiment in which we again deposited Rh in background H 2 O, evaluated the feature density in STM, then annealed the surface in oxygen at 520 °C.…”

“…The Rh 3d peak ( Figure 3 b) initially has its maximum at ∼308.9 eV, significantly higher than when Rh is deposited without water (307.8 eV, dashed gray line). 20 The maximum remains at this position while heating to 100 and 150 °C. At higher temperatures, the behavior closely resembles that previously observed in the absence of water: A component corresponding to clusters (307.6 eV) starts developing at 150 °C and reaches a maximum at 300 °C, and a component corresponding to incorporated Rh (309.3 eV) first appears at 200 °C and finally accounts for almost the entire Rh peak at 400 °C.…”

“… 15 , 16 We instead utilize the (11̅02) facet, on which a monophase bulk-truncated (1 × 1) termination can reproducibly be prepared. 17 − 20 Moreover, the surface structure and its interaction with water are well-understood. 17 , 18 The surface termination 17 and room-temperature water adsorption sites 18 are introduced in Figure S1 .…”

“…At higher temperatures, the behavior closely resembles that previously observed in the absence of water: A component corresponding to clusters (307.6 eV) starts developing at 150 °C and reaches a maximum at 300 °C, and a component corresponding to incorporated Rh (309.3 eV) first appears at 200 °C and finally accounts for almost the entire Rh peak at 400 °C. 20 …”

Techniques for the characterization of single atom catalysts

“…The lateral resolution of STM is 0.1 nm, which makes it applicable to investigate the singly dispersed atoms in SACs. [42][43][44][45] For example, Flytzani-Stephanopoulos et al utilized highresolution STM to characterize the distribution of Pt atoms on Cu(111) surface. The results in Fig.…”