Adsorption of Alkanethiols on Well Defined and Nanoparticle Pd Surfaces

ALDO ALBERTO RUBERT; J. AZCARATE; DE SIERVO, A.; FONTICELLI, M. H.

Campinas

Congreso; 26th RAU ? Annual Users Meeting LNLS/CNPEM; 2016

Laboratorio Nacional de Luz Síncrotron

It is known that, opposite to the model system -Au(111) surfaces-, the adsorption of thiols on Pd surfaces lead to mixed sulfide/thiolate interfaces. Furthermore, it was proposed that: (a) the ability of the metal surface to induce the S-C bound cleavage is related with changes in the surface density of states (DOS),[1] and (b) the disorder in ~ 3 nm thiolate-protected nanoparticles is promoted by the strong S-Pd bond in the sulfide layer that surrounds the NPs.[2] Nowadays, the needing of a deeper understanding of the thiol/Pd interfaces is evident because of the interest that thiolate Self-Assembled Monolayers (SAMs) have on catalysis. Medlin et al. have recently demonstrated that alkanethiol SAMs improve the selectivity of Pd surfaces (both planar and nanoparticles) towards a variety of reactions.[3,4] However, the SAMs? role on the catalytic activity is not analyzed in close correlation with their actual composition. Furthermore, two recent works renewed the interest of thiol SAMs? on platinum group metals (PGMs). Esaulov et al. recently claimed that the component at ~162eV belongs to thiolates, while that at 163eV to atomic sulfur[5], which is contrary to previous assignments and the predictions based on the electronegativity of sulphur species. This is, for itself, an interesting puzzling problem to resolve. On the other hand, recent high resolution XPS measurements of thiol SAMs on Ni(111) showed that sulfide is also a coadsorbate for this metal.[6]We have studied Pd(111) and (100) surfaces modified by dodecanethiol and by mercaptobenzene in the PGM beamline at LNLS. We were able to resolve the component at BE Sp3/2~162.6-162.8eV, which could not be separated from that assigned to molecular species (BE Sp3/2~163.4eV) in previous studies. The data support our previous assignment (i.e. the component at ~162eV belongs to atomic sulfur, while that at 163eV to thiolates), and shows that the thermal treatment increases the relative sulfide contribution to the S2p signal. Also, we used laboratory XPS and standard characterization techniques (UV-visible spectroscopy and transmission electron microscopy (TEM) ?for nanoparticles- and LEED -for single crystals-) to characterize thiol modified surfaces.Our study of Self-Assembled Monolayers of thiols on Pd surfaces (polycrystalline, Pd(111), Pd(100) and 3nm in diameter nanoparticles), which were built-up in liquid media, demonstrate that sulfide species are present irrespective of the chemical nature of the thiol (aliphatic or aromatic), or the atomic arrangement of the metal surface. Furthermore, we demonstrate that the surface composition under the in operando conditions (T=463 K [4]) of a relevant catalytic processes correspond to mixed thiolate-palladium(II) sulfide layer.[1] P. Carro, G. Corthey, A. A. Rubert, G. Benitez, M. H. Fonticelli, R. C. Salvarezza, Langmuir 26, 14655 (2010).[2] G. Corthey, et. Al. J Phys Chem C 118, 24641 (2014).[3] Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W., Acc. Chem. Res. 47, 1438 (2014).[4] S. H. Pang, C. A Schoenbaum, D. K. Schwartz, J. W. Medlin, J. W. ACS Catalysis 4, 3123 (2014).[5] J. Jia, A. Bendounan, K. Chaouchi, S. Kubsky, F. Sirotti, L. Pasquali, V. Esaulov, J Phys Chem C 118, 24983 (2014)[6] F. Blobner, et. Al. J Phys Chem C 119, 15455 (2015)Acknowledgements: This work was supported by LNLS (Proposal SGM - 17848), CONICET and UNLP.