Local Structure and Coordination Define Adsorption in a Model Ir₁/Fe₃O₄ Single‐Atom Catalyst

Abstract

Single‐atom catalysts (SACs) bridge homo‐ and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus strongly influence how reactants adsorb. Now, atomically resolved scanning‐probe microscopy, X‐ray photoelectron spectroscopy, temperature‐programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3O4(001) support. The two‐ and five‐fold‐coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square‐planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism.

Adsorbate-induced structural evolution changes the mechanism of CO oxidation on a Rh/Fe3O4(001) model catalyst

“…S5. † similar to those observed in the previously published study of Ir 1 /Fe 3 O 4 (001), 14 and one would indeed expect similar behavior for these two metals. As in the Ir case, the concentration of the dicarbonyl species never exceeded 25% of the total, which we attribute to the low pressures used in our experiments.…”

“…2a and b. STM images taken after annealing to 500 K show predominantly elongated (orange circle) and circular ( purple circle) features on the Fe oct rows, which is consistent with Rh 1 incorporated into the subsurface 6-fold site or surface 5-fold site, respectively (STM simulations of different metal adatoms in the same sites are provided in ref. 14,15 and Fig. S2 †).…”

“…Such a phenomenon has been observed for several different metal adatoms on Fe 3 O 4 (001) before and the foreign adatom can either substitute a 5-fold coordinated surface Fe oct or incorporate in the vacant octahedral site of the SCV reconstruction. [13][14][15]29 Schematic representations of the three possible sites of Rh 1 near the surface are shown in insets in Fig. 2a.…”

“…We identify the new species as Rh 1 incorporated in the octahedral cation sites near the surface, consistent with previously reported studies of various metal adatoms on Fe 3 O 4 (001). 14,15 Schematic representations of these sites are shown in the insets and labelled with the color of the corresponding circle. (b) Rh 3d region of the XPS spectra (monochromatized Al Kα, grazing emission) taken after heating to different temperatures (measured after cooling back to 300 K).…”

“…14,[16][17][18][19] Metal-support interactions are not always beneficial, however, and encapsulation by the support can completely deactivate the catalyst. 13,14,20,21 Improving a catalyst's activity, selectivity, and stability requires the optimization of atomic-scale processes that are very difficult to ascertain on working catalysts. This motivates the use of welldefined model systems studied by the surface science approach, which can provide atomic-scale insights.…”