80 81 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Well defined single-site catalysts enabled by surface coordination chemistry Related publication: Kang L.,Wang B.,Thetford A.,Wu K., DanaieM., He Q., Gibson E. K., Sun L. D., Asakura H., CatlowC. R. A. &Wang F. R. Design, Identification, and Evolution of a Surface Ruthenium(II/III) Single Site for CO Activation. Angew. Chemie - Int. Ed. 60 , 1212–1219 (2021). DOI: 10.1002/anie.202008370 Publication keywords: Single-site; Coordination chemistry; Catalysis F irst discovered in the 19 th century, Ru II compounds are widely used in catalysis, photocatalysis, and medical applications. They are usually obtained in a reductive environment, as molecular oxygen can oxidise Ru II to Ru III and Ru IV . Researchers designed an air‐stable Ru II site that is coordinated on the polyphenylene-bipyridine (PPhen-bipy) based system. The teamused the Electron Physical Science Imaging Centre (ePSIC) at Diamond Light Source to confirm the presence of Ru single-site. They performed their study at atomic resolution with High Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM). X-ray Absorption Spectroscopy (XAS) at Ru K-edge carried out at beamline B18 followed the evolution of the geometric structures of the Ru single-site during CO oxidation. In addition, the HAADF-STEM study confirmed the successful synthesis of eight other metal single-sites coordinated with the PPhen-bipy framework. Those single-sites have well-defined geometric and electronic structures for catalytic applications. Such single-site catalysts exhibit great potential in life sciences and the pharmaceutical industry. The PPhen-bipy framework also enables the study of coordination chemistry at solid/gas interface, helping to understand structure-performance relationship of the surface single-sites. Single-sites are the smallest catalytic active centres. They have discrete molecular orbitals that can be controlled via the geometric coordination environment, enabling the selective bond breaking and formation at the surface. Here a series of surface single-sites are designed at the surface of the polyphenylene-bipyridine (PPhen-bipy) framework (Fig. 1a). The bidentate bipy group coordinate single metal cations, forming well-defined and dispersed single-sites on the surface. High Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) enables the direct observation of those single-sites with atomic resolution (Fig. 1b and Fig. 2). The images confirm the uniform dispersion of the single-site with very high density (from 2-5wt%). The X-ray Absorption Fine Structure (XAFS) study of those single-sites reveals their local coordination environment. Due to the fact that bidentate nitrogen coordinative environment in bipy is well preserved, the formation of surface -[bipy-MX n ] site is predictable according to coordination chemistry. Across the thirteen metal cations that is studied, nine single-sites are successfully prepared and validated. They are -[bipy-Ru(III)Cl 4 ] - (Fig. 1b), -[bipy-FeCl 4 ] - , -[bipy-CoCl 2 ], -[bipy-Ni(H 2 O) 4 ] 2+ , -[bipy-CuCl 2 ], -[bipy-ZnCl 2 ] (Fig. 2a), -[bipy-RhCl 4 ] - (Fig. 2b), -[bipy-PdCl 2 ] (Fig. 2c) and -[bipy-PtCl 4 ] (Fig. 2d). Ag, Sn, Ir and Au form clusters instead of single-sites, due to their low coordination strength with bipy ligands. In addition to metal cations, the selection of ligands is also controllable. -[bipy-RuCl 4 ]-, -[bipy-RuBr 4 ]-, -[bipy- CuCl 2 ] and -[bipy-Cu(H 2 O) 4 ] 2+ are achieved accordingly. The diverse selection of metal cations and ligands provides a single-site library with controllable geometric and electronic structures for a target catalytic reaction. The -[bipy-Ru(III)Cl 4 ] - site is further studied as a model system for catalytic CO oxidation. Upon heating in the presence of CO and O 2 , a release of HCl is observed starting from 433 K. A surface adsorbed C ≡ O vibration is found in the Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) at 2057 cm -1 (Fig. 3a). This vibration corresponds well to the FTIR spectrum of molecular mer(Cl)-[Ru(bipy)(CO)Cl 3 ] 1 . The fitted extended XAFS spectrum shows the decrease of Ru-Cl coordination number from 4.0 ± 0.4 to 3.2 ± 0.2 while maintaining the bond distance at 2.35 ± 0.01 Å (Fig. 3b). A new Ru- CO scattering is observed at 1.88 ± 0.02 Å with a coordination number of 1.1 ± 0.2. The bond lengths are the same as the values reported for molecular mer (Cl)-[Ru(bipy)(CO)Cl 3 ] (Ru-N 2.08 Å, Ru-Cl 2.33 Å and Ru-C 1.89 Å) 1 . Both -[bipy-Ru(III)Cl 4 ] - and mer (Cl)-[bipy-Ru(III)(CO)Cl 3 ] are not active for CO oxidation. H 2 is then used to convert mer (Cl)-[bipy-Ru(III)(CO)Cl 3 ] into its active Ru(II) form. HCl is detected in the online mass spectrometry, suggesting the reaction between H 2 and the Cl - ligand. DRIFTS and FTIR spectra show a second CºO stretching at 1996 cm -1 (Fig. 2c). This result matches the FTIR spectrum of molecular cis (CO)- trans (Cl)-[Ru(II)(bipy)(CO) 2 Cl 2 ] 2 but is different from cis (CO)- cis (Cl)-[Ru(II)(bipy)(CO) 2 Cl 2 ] (2040 and 1980 cm -1 ) 3 . In the EXAFS spectra, the Ru-Cl coordination number is reduced to 2.2 ± 0.3 with a slight increase of bond length to 2.39 ± 0.01 Å, confirming the ligand exchange of Cl - via CO with the help of H 2 (Fig. 2d). The coordination number of Ru-CO at 1.88 ± 0.01 Å increases to 1.9 ± 0.3. The Ru oxidation states is further confirmed in Ru L-edge and Cl K-edge XAFS spectra.The newly formed cis (CO)- trans (Cl)-[bipy-Ru(CO) 2 Cl 2 ] site does not have the Ru 2p to 4d t 2g and Cl 1s to Ru 4d t 2g transition. This indicates that the Ru has the t 2g 6 configuration with oxidation state of +2. Once transformed into the cis (CO)- trans (Cl)-[bipy-Ru(II)(CO) 2 Cl 2 ] form, the single-site is then active for CO oxidation. An initial light off is shown at 388 K and 35% conversion at 453 K, P CO = 0.01 bar andWHSV 60,000 ml × h -1 g -1 . An activation energy (Ea) of 90 kJ × mol -1 is determined at the WHSV of 600,000 ml × h -1 g -1 . The corresponding Turnover Frequency (TOF) is then calculated as 3.89 x 10 -2 s -1 at 498 K, which is similar to the state-of-the-art Cu and Pt single- site over metal oxide support for CO oxidation. HAADF-STEM images show the high density of Ru 2+ cations well dispersed on the polymer framework after catalysis.There is no cluster or particle formation.The distinct differences in CO oxidation activity between Ru(III) and Ru(II) single-site are resulted from their geometric and electronic properties. The former has three or four Cl- ligands, which are strong binding ligands in metal complexes. In comparison, the latter offers two exchangeable CO ligands in cis configuration, enabling the local activation of CO within the Ru coordination environment. Single-site identification and characterisation is of great importance. The ePSIC and the Spectroscopy Group at Diamond provide state-of-the-art characterisation techniques and enable the accurate measurement of the single-site electronically and geometrically. More importantly, the rapid access and block allocationmode at these facilities enable the proof of concept design of single-site experiments, accelerating the materials, innovations for catalysis and energy applications. References: 1. Pruchnik F. P. et al. Carbonyl ruthenium(III) complexes with 1,10-phenanthroline and 2,2'-bipyridine. Polyhedron 18 , 2091–2097 (1999). DOI: 10.1016/S0277-5387(99)00079-0 2. Kelly J. M. et al. Synthesis and spectroscopic characterisation of [Ru(bpy)2(CO)2](PF)6)2. Inorganica Chim. Acta 64 , L75–L76 (1982). DOI: 10.1016/S0020-1693(00)90282-2 3. Chardon-Noblat S. et al. Selective Synthesis and Electrochemical Behavior of trans(Cl)- and cis(Cl)-[Ru(bpy)(CO)2Cl2] Complexes (bpy = 2,2'-Bipyridine). Comparative Studies of Their Electrocatalytic Activity toward the Reduction of Carbon Dioxide. Inorg. Chem. 36 , 5384–5389 (1997). DOI: 10.1021/ic9701975 Funding acknowledgement: EPSRC (EP/P02467X/1 and EP/S018204/1), Royal Society (RG160661, IES\ R3\170097, IES\R1\191035, IES\R3\193038) and the Newton International Fellowship (NF170761). Corresponding author: Dr Feng RyanWang, Department of Chemical Engineering, University College London,
[email protected] Imaging andMicroscopy Group ePSIC (and beamline B18 from the Spectroscopy Group) Figure 1: (a) Schematic of the general approach in creating the single-site coordination environment; (b) HAADF-STEM images of PPhen-[bipy-Ru(III)Cl 4 ]H; (c) Experimental and fitted EXAFS data of PPhen-[bipy-Ru(III)Cl 4 ]H. The inlet is the DFT simulation of the -[bipy- Ru(III)Cl 4 ] - structure. Figure 2: HAADF-STEM and EXAFS identifications of single-site PPhen-[bipy-MXn] structure. Cs-corrected HAADF-STEM images showing the single metal ions supported on PPhen-bipy for (a) Zn 2+ , (b) Rh 3+ , (c) Pd 2+ and (d) Pt 4+ . Corresponded experimental and fitted EXAFS result of (a) -[bipy-ZnCl 2 ], (b) -[bipy-RhCl 4 ] - , (c) -[bipy-PdCl 2 ] and (d) -[bipy-PtCl 4 ], respectively. Figure 3: (a,c) In situ DRIFTS spectra, showing the formation of the CO stretching from -[bipy-Ru(III)Cl 4 ]- to -[bipy-Ru(III)(CO)Cl 3 ] and frommer(Cl)-[bipy-Ru(III)(CO)Cl 3 ] to cis(CO)-trans(Cl)-[bipy- Ru(II)(CO) 2 Cl 2 ], respectively; (b,d) Corresponded experimental and fitted EXAFS data, respectively.