A Multitechnique Study of C2H4 Adsorption on a Model Single-Atom Rh1 Catalyst

Abstract

Single-atom catalysts are potentially ideal model systems to investigate structure-function relationships in catalysis if the active sites can be uniquely determined. In this work, we study the interaction of C₂H₄ with a model Rh/Fe₃O₄(001) catalyst that features 2-, 5-, and 6-fold coordinated Rh adatoms, as well as Rh clusters. Using multiple surface-sensitive techniques in combination with calculations of density functional theory (DFT), we follow the thermal evolution of the system and disentangle the behavior of the different species. C₂H₄ adsorption is strongest at the 2-fold coordinated Rh₁ with a DFT-determined adsorption energy of -2.26 eV. However, desorption occurs at lower temperatures than expected because the Rh migrates into substitutional sites within the support, where the molecule is more weakly bound. The adsorption energy at the 5-fold coordinated Rh sites is predicated to be -1.49 eV, but the superposition of this signal with that from small Rh clusters and additional heterogeneity leads to a broad C₂H₄ desorption shoulder in TPD above room temperature.

Figure 1

DFT-determined structure models for the C₂H₄/Rh₁/Fe₃O₄(001) system. (a) Perspective and (b) top view models of a 2-fold oxygen coordinated Rh₁ on the Fe₃O₄(001) support. The two dashed circles in (b) indicate two equivalent subsurface oxygen atoms in the support, with which the Rh atoms can form a weak bond. (c) A 5-fold coordinated Rh₁ atom in a substitutional cation site and (d) a subsurface Rh site (pink arrow) with 6-fold coordination to lattice oxygen. Oxygen atoms are red in the models, while surface 5-fold coordinated Fe_oct atoms are dark blue. Rh is shown as cyan.

Figure 2

C₂H₄adsorption on the Rh₁/Fe₃O₄(001) surface. STM images of the as-prepared 0.2 ML Rh₁/Fe₃O₄(001) surface (a) before and (b) after 3.4 L of C₂H₄ adsorption. The yellow arrows indicate 2-fold coordinated Rh₁ atoms, which are located between the iron rows. The pink arrows indicate Rh dimer species, identified in previous work. In image (b) acquired at a different position after adsorption of C₂H₄, the red arrows indicate protrusions within the surface iron rows, which are due to C₂H₄ adsorbed on 5-fold coordinated Rh₁ atoms. (c) C1s XPS acquired from the 0.2 ML Rh₁/Fe₃O₄(001) sample before (black curve) and after exposure to 3.4 L C₂H₄ (pink curve), the curves have been shifted vertically for clarity. (d,e) DFT-derived minimum energy structures for C₂H₄ on 2-fold Rh₁ and 5-fold Rh₁, respectively. The oxygen atoms are red in the models, while surface 5-fold coordinated Fe_oct atoms are dark blue. Rh is shown as cyan. The carbon and hydrogen atoms of the ethylene molecule are colored black and white, respectively.

Figure 3

C₂H₄ desorption and Rh evolution. (a) A series of C₂H₄ TPD spectra obtained from 0.2 ML Rh/Fe₃O₄(001) sample following exposure to 10 L C₂H₄ at 293 K. The TPD was run from 250 to 500 K in the first round. The sample was then cooled to 293 K, and a further 10 L of C₂H₄ adsorbed. In the second and third TPD rounds, the temperature was ramped from 250 to 650 K. (b,c) XPS of C 1s and Rh 3d collected after different flashing temperatures. The spectra are collected after sample cooling down to room temperature. (d–f) STM images of the sample in Figure 2b, followed by annealing at 400, 500, and 600 K. The red arrows in panel (e) indicate 5-fold coordinated Rh₁, which may have either formed from 2-fold coordinated Rh₁ during annealing or was present already previously as 5-fold Rh. In any case, it has now lost adsorbed C₂H₄. After annealing at 600 K, panel (f) shows that the sample is similar to a clean Fe₃O₄(001) surface as shown in Figure S1.