Small molecule binding to surface-supported single-site transition-metal reaction centres

Abstract

Despite dominating industrial processes, heterogeneous catalysts remain challenging to characterize and control. This is largely attributable to the diversity of potentially active sites at the catalyst-reactant interface and the complex behaviour that can arise from interactions between active sites. Surface-supported, single-site molecular catalysts aim to bring together benefits of both heterogeneous and homogeneous catalysts, offering easy separability while exploiting molecular design of reactivity, though the presence of a surface is likely to influence reaction mechanisms. Here, we use metal-organic coordination to build reactive Fe-terpyridine sites on the Ag(111) surface and study their activity towards CO and C₂H₄ gaseous reactants using low-temperature ultrahigh-vacuum scanning tunnelling microscopy, scanning tunnelling spectroscopy, and atomic force microscopy supported by density-functional theory models. Using a site-by-site approach at low temperature to visualize the reaction pathway, we find that reactants bond to the Fe-tpy active sites via surface-bound intermediates, and investigate the role of the substrate in understanding and designing single-site catalysts on metallic supports.

Fig. 1. STM and ncAFM of Fe-tpy sites.

TPT molecule with one (a–c) and two (d–f) active Fe-tpy sites. STM images (a, d) acquired with CO-functionalized tips. STM topograph with single Fe-tpy site (a) acquired at V = 20 mV and I = 10 pA indicates an increase in apparent height at the Fe-tpy coordinate bond (a, red four-point star) compared to the non-metalated tpy group (a, blue circle). Laplace-filtered ncAFM ∆f images (A_vib~2 Å) show reduced ring definition at the Fe-tpy sites (b, e, red 4-point star) compared with the three distinguishable pyridines of the bare tpy group (b, blue circle) consistent with previous work.

Fig. 2. STM overviews of reaction progression.

STM topographs of the same region of a sample imaged after active Fe-tpy site preparation (a), after subsequent exposure to CO and C₂H₄ (maximum temperature during dosing 11.1 K) (b), and after successive incremental sample annealing up to T = 30 K for t = 5 min (c). Free C₂H₄ (yellow triangle) is visible on the surface, along with changes to active sites labelled “CO prebond” (grey pentagon), after gas exposure (b). After subsequent annealing of the sample at T = 30 K for t = 5 min (c) three additional distinct changes to active tpy-Fe sites are visible and labelled: “CO bond” (black five point star), “C₂H₄ prebond” (light orange hexagon), and “C₂H₄ bond” (orange six point star). After the anneal, C₂H₄ molecules can form pseudo hydrogen bonds with the nitrogen lone pairs of the distal, outward pointing pyridyl of non-metalated tpy groups (c, yellow triangle). Images shown were acquired with a metal tip at V = 20 mV and I = 20pA (a–c).

Fig. 3. STM and ncAFM of CO associated Fe-tpy structures.

Comparison of CO prebond (a–c) and CO bond (d–f) configurations. STM topographs (a, d) were imaged with CO-terminated tips at V = 20 mV and I = 10 pA. Constant height ncAFM images (Laplace-filtered) for the CO prebond (b) and CO bond (e) motifs show a distinct difference in the length of lines at the right side of the molecules where the CO is located. The proposed chemical structures for the CO prebond and CO bond configurations are shown in (c) and (f), respectively. Parameters for (b): V = −0.95 mV, A_vib = 2 Å, ∆z = −0.01 nm from setpoint V = 20 mV and I = 10 pA on Ag(111). Parameters for (e): V = −0.95 mV, A_vib = 8 Å, ∆z = −0.06 nm from setpoint V = 20 mV and I = 10 pA on Ag(111). Scalebars: 6.0 Å (a, d) 5.0 Å (b, e).

Fig. 4. STM and ncAFM of C₂H₄ associated Fe-tpy structures.

Topographs of the C₂H₄ prebond (a) and C₂H₄ bond (d) configurations imaged at V = 20 mV and I = 10pA. Laplace-filtered ncAFM images of the C₂H₄ prebond (b) and C₂H₄ bond (e) motifs imaged at constant height with A_vib = 2 Å from a setpoint of V = 20 mV and I = 10 pA on Ag(111). ∆z = −0.01 nm for (b), and ∆z = −0.022 nm for (e). Images (a, b, d, e) were all acquired with the same CO-terminated tip. Shear transformation performed on (b) to compensate for drift. Proposed chemical structures for the horizontal C₂H₄ prebond and vertical C₂H₄ bond motifs drawn in (c) and (f), respectively; note for the pre-bond structure, surface adsorption would indicate a flat-lying structure as shown, while DFT shows a weakly bonded side-orientation. Scalebars 5 Å (a, b, d, e).

Fig. 5. Tunnelling spectroscopy of Fe sites.

Averaged dI/dV comparing the bare active site signal (a, red solid line; position identified from comparing tpy-Fe relative to ligand) with the CO bond (b, black solid line), C₂H₄ prebond (c, orange dotted line), and C₂H₄ bond (c, orange solid line) tunnelling resonances. A clear shift in the Fe-centred state at V = −0.09 V (a) is observed upon CO bond formation at V = −0.16 V (b) and C₂H₄ bond formation at V = −0.30 V (c). The C₂H₄ prebond (c) does not exhibit deviation from the Fe-tpy resonance at V = −0.09 V. Spectra are spatially averaged from a grid spectroscopy measurement over a radius of 10 pixels (357 spectra) for the Ag(111) reference and 7 pixels (185 spectra) for all others.

Fig. 6. DFT modelling of 4 most commonly observed Fe-tpy bound structures.

Dispersion corrected DFT simulation results for 4 most common stable structures showing binding energies relative to bare Fe-tpy and dispersion and base functional contributions for each (a) with optimized structure (b–e) and simulated STM (f–i) and Laplace filtered nc-AFM (j–m) shown below for each to compare with data. Comparing CO vertical and CO horizontal simulation results to Fig. 3, these most closely match the CO pre-bond and bond structures respectively with corresponding increase in stability as expected from observations. C₂H₄ horizontal and C₂H₄ vertical simulation results compare well with Fig. 4 C₂H₄ pre-bond and C₂H₄ bond respectively, again with increase in stability, albeit smaller consistent with experimental observations. All structures except for the CO-horizontal were found to have a triplet ground state consistent with a weak ligand-field picture.