40 41 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 1 / 2 2 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 1 / 2 2 Investigating hydrogen spillover to improve hydrogenation catalysts Related publication: Wu, S.; Tseng, K. Y.; Kato, R.;Wu, T. S.; Large, A.; Peng, Y. K.; Xiang,W.; Fang, H.; Mo, J.;Wilkinson, I.; Soo, Y. L.; Held, G.; Suenaga, K.; Li, T.; Chen, H. Y. T.; Tsang, S. C. E. Rapid interchangeable hydrogen, hydride, and proton species at the interface of transition metal atom on oxide surface. Journal of the American Chemical Society 143 , 9105–9112 (2021). DOI: 10.1021/jacs.1c02859 Publication keywords : Hydrogen spillover; Polar metal oxide; In situ characterization; Frustrated Lewis Pair H ydrogen spillover is an atomic migration phenomenon on the catalyst surface that is crucial to the development of catalytic hydrogenation and dehydrogenation reactions at mild conditions. Typically, this takes place over a metal on the support structure where a dihydrogen molecule undergoes dissociative chemisorption to form hydrogen atoms on the metal active centre, followed by the migration of atomic hydrogen from the metal surface to the catalytic support. Despite researchers having spent tremendous efforts investigating the hydrogen spillover mechanism, there is so far no direct visualisation of this at an atomic level, and the interchangeable pathway between the various hydrogen species on the metal/support is still unknown. In this respect, researchers from the Wolfson Catalysis Centre at the University of Oxford have precisely engineered an atomic [Ru 2+ -O 2- ]/ MgO(111) solid-state Frustrated Lewis Pair catalyst and investigated the H spillover pathway through this material interface. They used Diamond Light Source’s Versatile Soft X-ray (VerSoX) beamline (B07), which can probe the oxidation state (O.S.) change of the ruthenium in real-time as well as monitor the change in concentration of the surface hydrogenic species on the support under reaction conditions. This is critical to determining the electronic structure and the surface composition of the catalyst. The results demonstrate a spontaneous reduction in the Ru O.S. fromoxidised tometallic and oxidation of H 2 to protons whereas the surface hydroxyl species concentration is found to growmassively on MgO(111) upon passing hydrogen to the catalyst. This provides the key evidence for mechanistic derivation. To achieve fast reaction rates for hydrogenation reactions at mild conditions such as low temperature ammonia synthesis, it is essential to alleviate the hydrogen poisoning effect, which is to prevent the surface hydrogen atoms from blocking the metal active sites 1,2 . One effective means to do so is via hydrogen spillover, where the H atoms are transported from themetal sites to the catalytic support 3 . In this pioneering work, the researchers have employed a Frustrated Lewis Pair approach to construct [Ru 2+ -O 2- ] pairs on the polar MgO(111) support to enable labile dihydrogen activation and provide highly energetic surface oxygen sites for H atoms to migrate to. The Transmission Electron Microscopy (TEM) image in Fig. 1a shows a high dispersion of Ru single atoms on the polar MgO(111) surface despite a high metal loading of 3.4 wt %. This is probably due to the strong surface polarity of the oxygen-terminated surface of MgO(111) that stabilises the Ru species. Atom Probe Tomography (APT) then reveals the Ru is selectively located over magnesium on the oxygen-terminated surface (Fig. 1b, c). To reconstruct the exact position of Ru on the surface, High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) is employed in conjunction with Density Function Theory (DFT) modelling to derive a structure of a single Ru sitting on top of a 3-O hollow site with a Mg atom underneath at the next layer (Fig. 1d-g). This is further confirmed by Extended X-Ray Absorption Fine Structure (EXAFS) analysis that shows a tri-coordinated Ru-O bond for the Ru/ MgO(111) sample (Fig. 1h). While imaging and X-ray absorption techniques can provide clues on structural information, high resolution in-situ ambient pressure X-ray Photoelectron Spectroscopy (XPS) sheds light on dynamic charge states and real-time changes of surface support compositions at reaction conditions 5 . To identify the oxidation state of Ru on MgO(111) at reaction conditions, ambient- pressure XPS was carried out at 300 o C under an Ar atmosphere of 1 mbar after hydrogen pre-reduction at the same temperature. Ru is found to have an oxidation state of around +2 when it is supported on the polar MgO(111) surface whereas it is localised to a metallic state when supported on the non- polar MgO(110) surface (Fig. 2a, b). Combining this result with Bader charge calculations, a [Ru 2+ -O 2- ] Frustrated Lewis Pair is identified. By switching the gas feed from Ar to H 2 , the Ru 3d peaks exhibit a very significant shift of -1.1 eV that implies a spontaneous reduction from Ru (II) to the metallic state (Fig. 2c). Simultaneously, the high binding energy shoulder in the O1s spectrum, which is associated with the [OH] component, is found to increase from16.4%to24.8%underflowinghydrogen (Fig. 2d).This signifies the migration of H species from themetal to the MgO support as it is experimentally proven that dihydrogen cannot be dissociated on the bare support due to kinetic stability. Meanwhile, Nuclear Magnetic Resonance (NMR) has identified the co- existence of a hydridic Ru-H peak and a surface hydroxyl peak (with H-bonding) that affirms the postulation. When the gas stream is switched from H 2 back to Ar, Ru is returned to an oxidised state while the OH concentration of the support returns to its starting position, showing the reversibility of this process. All the information combined with the DFT calculations, a reversible reaction pathway for the hydrogen spillover on the refractory oxide is proposed (Fig. 3). The dihydrogen is first activated by the surface [Ru 2+ -O 2- ] FLP pair on the MgO(111), followed by redox transfer of Ru-hydride to proton on O 2- at the metal-support interface. The fast migration is further supported by Grotthuss proton hopping with low activation barriers for the H spillover. References: 1. Wu, S. et al. Removal of hydrogen poisoning by electrostatically polar MgO support for low-pressure NH 3 synthesis at a high rate over the Ru catalyst. ACS Catalysis 10 , 5614– 5622 (2020). DOI: 10.1021/acscatal.0c00954 2. Jiang, L. et al. Facet engineering accelerates spillover hydrogenation on highly diluted metal nanocatalysts. Nature Nanotechnology 15 , 848–853 (2020). DOI: 10.1038/s41565-020-0746-x 3. Prins, R. Hydrogen spillover. Facts and fiction. Chemical Reviews 112 , 2714–2738 (2012). DOI: 10.1021/cr200346z 4. Wu, S. et al. Rapid interchangeable hydrogen, hydride, and proton species at the interface of transition metal atom on oxidesSurface. Journal of the American Chemical Society 143 , 9105–9112 (2021). DOI: 10.1021/ jacs.1c02859 5. Held, G. et al . Ambient-pressure endstation of the Versatile Soft X-ray (VerSoX) beamline at Diamond Light Source. Journal of Synchrotron Radiation 27 , 1153–1166 (2020). DOI: 10.1107/S1600577520009157 Funding acknowledgement: The financial support of this work from the EPSRC research council of UK and Siemens, plc are acknowledged. S.W. would like to thank Siemens, plc, and EPSRC for a joint DPhil Studentship.We thank Diamond for the access to beamline B07. Corresponding author: Professor Shik Chi Edman Tsang, University of Oxford,
[email protected] Structures and Surfaces Group Beamline B07 Figure 1: (a) TEM image of 3.4 wt % Ru/MgO(111); (b) APT atommap of 3.4 wt % Ru/MgO(111) (blue, Ru; red, O; orange, Mg); (c) Composition profile of Ru, Mg, and O of 3.4 wt % Ru/MgO(111) along the black line in b; (d) HAADF-STEM image of 3.4 wt % Ru/MgO(111) observed from the [110] direction; (e and f ) Simultaneous acquisition (e) EELS extracted on oxygen atom (green circle in d), Ru atom (blue circle in d); and (f ) HAADF acquired along the line in d; (g) Simulation of STEM images of two Ru atoms supported on different positions of MgO(111) from [111] and [110] directions. Atomic model was also provided for reference; (h) Fourier transform of k 3 -weighted Ru K-edge of X-ray absorption fine structure spectroscopy (EXAFS) spectra of the post hydrogen-reduced 3.4 wt % Ru/MgO(111) measured at 300 °C. Reprinted with permission from ref [4]. Copyright 2022 American Chemical Society. Figure 2 : Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) Ru 3d spectra for (a) Ru/ MgO(111) and; (b) Ru/MgO(110). Measurement was performed under 1 mbar Ar at 350 °C inside an airtight sample cell after pretreatment at 1 mbar H 2 at 350 °C; (c) Ru 3d and (d) corresponding O 1s of AP-XPS spectra at 350 °C under alternative sweeping between 1 mbar Ar and 1 mbar H 2 for the Ru/MgO(111) sample. All the spectra are calibrated with reference to the internal standard Mg 2s of 88.1 eV. Reprinted with permission from ref [4]. Copyright 2022 American Chemical Society. Figure 3: Proposed mechanism for hydrogen spillover on Ru/MgO(111). Reprinted with permission from ref [4]. Copyright 2022 American Chemical Society.