Seeing the 'impossible' – transition-state Oxygen and Carbon intermediates observed for the first time with X-ray Laser

Create: 02/17/2015 - 09:47

Knowledge about actual chemical transition states has been mostly theoretical until now, and has been considered as being one of several 'holy grail(s)' of chemistry.  Using ultra-fast femtosecond (10-15) spectroscopy to probe a CO oxidation reaction on Ruthenium, the laboratory of Nilsson and colleagues at the Stanford Linear Accelerator (SLAC) observed 'new electronic states in the Oxygen 'K-edge x-ray absorption spectrum'.

The transition state of a chemical reaction was probed using an Ruthenium (Ru) catalyst, that not only facilitates the reaction but also brings 'the reacting molecular fragments in close proximity'. Using free-electron x-ray lasers for optical excitation and the atom-specific tools of K-edge X-ray absorption spectroscopy (XAS), a 'short-lived intermediate precursor state was detected in CO desorption from Ru'.

Understanding chemical reactions and reactivity requires an understanding of the step-wise process by which reactants are converted to products. As discussed by Polyani & Zewail, reagents approach one another, cross an energy barrier, and then descend the barrier as products form, all in a time-domain that is almost unimaginably short and only recently approachable with modern ultra-fast spectroscopic methods.

Nilsson and colleagues probed the transition state in a Ru-surface catalyzed bimolecular reaction using 'time-resolved snapshots of the valence electronic structure' wherein O and CO formed CO2. The element-specific K-edge XAS enabled these scientists to observe alterations in the electronic structure of the unoccupied valence 'shell' during the reaction process.  Results indicated very rapid activation of adsorbed CO and O on the Ru surface, with O activation occurring first; followed by the appearance of novel adsorbed species with electronically distinct spectra.

One of the new resonance species was identified as 'the formation of a bond between CO and O that is substantially elongated in comparison to the CO2 final product'. This 'weak OC—O bond' had an 'elongated bond distance of 1.7 Å compared to 1.2 Å in the CO2 molecule'.Computational analysis indicates that the fragments observed were transition state species 'attempting to form CO2'.

The concatenation of the Ru-catalyst surface with 'ultrafast pump-probe x-ray spectroscopy' facilitated observation of this reaction far more readily than would have occurred for a gas-phase experimental process. The work by Nilsson and colleagues provides 'new insights into the electronic states of reacting molecules at surfaces', as well as providing new benchmarks for theoretical understanding of processes catalyzed at surfaces.

Related: The Next Big Step Toward Atom-Specific Dynamical Chemistry

Science news reference:

Probing the transition state region in catalytic CO oxidation on Ru. H. Ostrom, H., H. Xin, J. LaRue, M. Beye, M. Dell' Angela, J. Gladh, M.L. Ng, J.A. Sellberg, S. Kaya, G. Mercurio, D. Nordlund, M. Hantschmann, F. Hieke, D. Kuhn, W.F. Schlotter, G. L. Dakovski, J.J. Turner, M.P. Minitti, A, Mitra, S,P. Moeller, A. Fohlisch, > Wolf W. Wurth, M. Persson, J.K. Norskov, F. Abild-Pedersen, H. Ogasawara, L.G.M. Pettersson, A. Nilsson. Science, 2015. DOI: 10.1126/science.1261747

Direct Observation of the Transition State. J.C. Polanyi, A.H. Zewail. Acc. Chem. Res. 1995, 28, 119-132. DOI:10.1021/ar00051a005

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