Digging Its Own Site: Linear Coordination Stabilizes a Pt1/Fe2O3 Single-Atom Catalyst.

Determining the local coordination of the active site is a prerequisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems and much emphasis is placed on density functional theory computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (11̅02) facet of α-Fe2O3; a common support material in SACs. Using a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudolinear coordination to surface oxygen atoms. Despite breaking three surface Fe-O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O-Pt-O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase. Moreover, we conclude that extensive structural searches are necessary to determine realistic active site geometries in single-atom catalysis.

Effect of Different In2O3(111) Surface Terminations on CO2 Adsorption.

In2O3-based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2O3 is still unclear. To elucidate the initial steps of CO2 hydrogenation over In2O3, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2O3(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.

Actinide Coordination Chemistry on Surfaces: Synthesis, Manipulation, and Properties of Thorium Bis(porphyrinato) Complexes.

Actinide-based metal-organic complexes and coordination architectures encompass intriguing properties and functionalities but are still largely unexplored on surfaces. We introduce the in situ synthesis of actinide tetrapyrrole complexes under ultrahigh-vacuum conditions, on both a metallic support and a 2D material. Specifically, exposure of a tetraphenylporphyrin (TPP) multilayer to an elemental beam of thorium followed by a temperature-programmed reaction and desorption of surplus molecules yields bis(porphyrinato)thorium (Th(TPP)2) assemblies on Ag(111) and hexagonal boron nitride/Cu(111). A multimethod characterization including X-ray photoelectron spectroscopy, scanning tunneling microscopy, temperature-programmed desorption, and complementary density functional theory modeling provides insights into conformational and electronic properties. Supramolecular assemblies of Th(TPP)2 as well as individual double-deckers are addressed with submolecular precision, e.g., demonstrating the reversible rotation of the top porphyrin in Th(TPP)2 by molecular manipulation. Our findings thus demonstrate prospects for actinide-based functional nanoarchitectures.