Article pubs.acs.org/JPCC
Enhanced Hydrogen Dissociation by Individual Co Atoms Supported on Ag(111) David Serrate,*,†,‡ Maria Moro-Lagares,†,‡ Marten Piantek,‡,§ Jose I. Pascual,†,∥,⊥ and M. Ricardo Ibarra†,‡ †
Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragón, University of Zaraqoza, E-50018 Zaragoza, Spain Dpto. Física Materia Condensada, University of Zaraqoza, E-50009 Zaragoza, Spain § Instituto de Ciencia de Materiales de Aragón, CSIC-University of Zaragoza, 50009 Zaragoza, Spain ∥ IKERBASQUE, Basque Foundation for Science, E-48011 Bilbao, Spain, and ⊥ CIC NanoGUNE, E-20018 Donostia-San Sebastián, Spain ‡
S Supporting Information *
ABSTRACT: By means of scanning tunneling microscopy, individual Co atoms adsorbed on Ag(111) are found to behave as a model catalyst for the hydrogen oxidation reaction. The dosing of H2 in a cryogenic environment produces the otherwise unstable CoH3 molecule, which results in the complete suppression of the Kondo resonance of the host Co atom. Short voltage pulses over Co hydrides permit reversible dehydrogenation and so, the identification of the intermediate compounds. The electric polarizability of CoH3 allows controlling molecular diffusion via an external electric field, from the range of tens of nm down to the assembly of larger hydride complexes at the molecular scale.
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INTRODUCTION
Here, we show that individual Co atoms supported on a Ag(111) surface behave as an efficient catalyzer for hydrogen dissociation. We find that at temperatures as low as 5 K Co atoms react spontaneously with H2 molecules to form cobalthydrides. The reaction stops when the end product CoH3 is formed. Using low temperature scanning tunneling microscopy (STM), we resolve the structure of this hydride and demonstrate that molecular diffusion can be controlled by external electric fields. Interestingly, the electric fields at the junction can also dehydrogenate these hydride molecules, which allowed us to follow step by step the posterior rehydrogenation reaction. Both the catalytic ratio of three H per Co atoms found in our work, and the low temperature at which the reaction takes place, are exceptional in terms of catalytic activity, supporting that this system be considered as a highly efficient model catalyst.
Understanding the mechanisms driving the enhanced catalytical activity of atomic scale metal clusters1,2 is challenging because, under conventional reaction conditions, size selection of clusters becomes compromised.3,4 Identification of the catalytically active sites calls for a local approach using spatially resolved techniques. Scanning Tunneling Microscopy (STM) is an excellent tool to image subnanometric clusters with potential catalytical activity and probe their properties as a function of the environment (temperature, atmospheric composition, and crystalline structure).5 STM can also detach molecular ligands and act controllably on the position and conformation at the single molecular level.6,7 The combination of space and energy resolution has triggered numerous studies on model catalytic reactions of isolated atoms and molecules in contact with a metal support.8 In this work, we focus on the dissociation of molecular hydrogen, a reaction of primary technological interest taking place at the anode of fuel cells. It is commonly catalyzed by Pt nanoparticles immersed in a carbonous matrix. In order to have actual impact in the automotive power industry, it is highly desirable to replace the costly precious Pt with a less noble and less strategic metal such as Co.9,10 H2 molecules generally dissociate when enter in contact with transition metals. In fact, it has been shown that thin films of Co incorporate atomic H when exposed to H2.11,12 However, the catalytic activity of atomic scale Co clusters remains unexplored. © 2014 American Chemical Society
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EXPERIMENTAL SECTION The experiments were performed in a SPECS Joule-Thompson STM under ultrahigh-vacuum conditions (chamber pressure
