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

Abstract

Oxide-supported single-atom catalysts are commonly modeled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Herein, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and X-ray photoelectron spectroscopy (XPS) to show that high densities of single Rh adatoms are stabilized on α-Fe 2 O 3 (11̅02) in the presence of 2 × 10 –8 mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 °C in UHV desorbs all water from the substrate leaving only the OH groups coordinated to Rh, and high-resolution ncAFM images provide a direct view into the internal structure. We provide direct evidence of the importance of OH ligands in the stability of single atoms and argue that their presence should be assumed when modeling single-atom catalysis systems.

Single-Atom Catalysis: Insights from Model Systems

“…Finally, we have recently begun investigating the α-Fe 2 O 3 (102) surface as a SAC model system and found it to be highly promising. This facet is much easier to prepare than the (0001), and single Rh adatoms were stabilized at room temperature by coadsorbed water . Since the support in most powder catalyst works is hematite, rather than magnetite, we find this to currently be the most promising iron oxide model system for bridging high surface area studies and theory.…”

“…Orange arrow highlights a third protrusion that is sometimes present, which was attributed to an additional water molecule atop a surface Fe and hydrogen bonded to one of the OH groups. Reproduced with permission from ref . Copyright 2022 American Chemical Society.…”

“…One of the major recommendations to emerge from this analysis is that the barrier for diffusion should be calculated in addition to the adsorption energy before such a geometry is claimed. Then, there is the question of environment: The adsorption of reactants can destabilize otherwise stable metal adatoms, − and there is growing evidence that interactions with water/OH groups can lead to stabilization. , It may therefore be necessary to revisit some of the assumptions made in theoretical modeling, testing both more complex adsorption environments and possible coadsorbates before attempting to predict reaction barriers. Efforts to find true global minima for both surface termination and adatom coordination may be aided by recent advances in applying machine learning methods for surface science.…”

“…For this we need to develop model systems based on model photocatalysts. The lack of stability of metal adatoms on titania seems problematic, and we suggest that Fe 2 O 3 (102) − (band gap ≈ 2 eV) can be a fertile model system for fundamental photocatalysis work. Other systems where the surface structures are understood, such as the perovskite SrTiO 3 , could be ideal if single-metal adatoms can be stabilized there.…”

“…While Rh on α-Fe 2 O 3 (1102) forms clusters at room temperature in UHV, it can be stabilized by coadsorption with water, which is stable on this surface up to 345 K. 103 This results in Rh(OH) 2 species (Figure 8), which are mobile at room temperature but do not agglomerate to clusters. 104 This is interesting because water and hydroxyl groups are omnipresent on metal oxides in atmospheric conditions, so their presence should be generally taken into account in the modeling of oxidesupported SAC systems.…”