Ludo Juurlink, PhD: Employing curved single crystal surfaces in surface science and gas-surface reaction dynamics studies

For decades, defects have been known to alter the chemical reactivity of surfaces [1, 2]. Systematic DFT calculations predict for stepped surfaces a universal trend in lowering activation barriers to dissociation for diatomics, e.g. O2, NO and CO, as compared to the (111) 'perfect' surfaces [3]. To date, there has not been a similar systematic experimental approach to investigating how different types of steps or other 'defects' at the surface affect energy transfer and (reactive) scattering of molecules.

Here, I will show that the predicted universal trend has exceptions based on our most recent supersonic molecular beam (SMB) study of H2 dissociation on Cu(211) and Cu(111). Then, I will introduce other recent work investigating effect of defects on reaction dynamics. We uniquely combine surface science and SMB methods with the use of curved single crystal metallic surfaces. These samples allow us to probe the effects of different types of defects over 3 orders of magnitude of a continuous variation of defect density. We now use STM and LEED, AES, TPD, RAIRS, and SMB techniques to study the surfaces with varying degrees of spatial resolution4.

To exemplify unique opportunities that curved crystals offer, I will address a 30-year old controversy. Two mutually exclusive models exist that quantitatively explain dissociative adsorption of H2 impinging onto Pt. The first model relies on resonant scattering into a weakly bound molecular state and frictionless diffusion to monoatomic steps where the molecule either dissociates or scatters5. The second model assumes that the molecule dissociates or scatters upon impact without any significant diffusion6. Using our curved single crystal approach, we can now definitively rule out the relevance of one of these models to actual catalysis.

References

[1] S.L. Rernasek, W.J. Siekhaus and G.A. Somorjai, Phys.Rev.Lett. 30, 1202 (1973)

[2] L. Vattuonea, L. Savioa, Ma.Rocca, Surf. Sci. Rep. 63, 101 (2008)

[3] J.K. Norskov et al. J. Catal. 209, 275 (2002)

[4] C. Hahn et al. J. Chem. Phys. 136, 114201 (2012); Mom et al., Surf. Sci. 615, 015 (2013); Janlamool et al., Molecules 19, 10845 (2014); Walsh et al., J. Vac. Sci. Technol A 35, 03E102 (2017)

[5] B. Poelsema, K. Lenz, and G. Comsa, J. Chem. Phys. 134, 074703 (2011); ibid. J. Phys. Condens. Matter 22, 304006 (2010).

[6] A. T. Gee et al., J. Phys. Chem. 112, 7660 (2000); D.A. McCormack et al. J. Chem. Phys. 122, 194708 (2005); I.M.N. Groot et al. J. Phys. Chem. C 117, 9266 (2013)