Article pubs.acs.org/JPCC
Dissociative Hydrogen Adsorption on Close-Packed Cobalt Nanoparticle Surfaces Emily A. Lewis,† Duy Le,‡ Colin J. Murphy,† April D. Jewell,† Michael F. G. Mattera,† Melissa L. Liriano,† Talat S. Rahman,‡ and E. Charles H. Sykes*,† †
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States Department of Physics, University of Central Florida, Orlando, Florida 32816, United States
‡
ABSTRACT: The dissociative adsorption of hydrogen on cobalt is central to a number of catalytic reactions, yet to date there are relatively few studies examining this important process. Here we utilize Co nanoparticles grown on Cu(111), instead of the traditional planar Co single crystals, to study a more catalytically relevant form of Co. We present scanning tunneling microscopy images of different phases of H on the close-packed Co nanoparticle surfaces with a range of densities. Our data reveal a so-far unreported high coverage phase of H with a (1 × 1) structure and elucidate the importance of spillover from step edges in H adsorption. We also illustrate that, in contrast to the low density phases, the H-(1 × 1) structure can only be formed at an intermediate temperature, indicating that compression to this higher-density phase is activated. Density functional theory calculations yield energies for each of the H overlayer structures, as well as their preferred geometries. This work is the first to report on higher coverage (>0.75 ML) phases of H on Co, which are undoubtedly important in catalytic systems at elevated pressure. Finally, through the use of epitaxial Co nanoparticle growth on Cu(111), we illustrate the importance of step edges in H2 activation and the formation of dense H phases.
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INTRODUCTION
In an effort to elucidate the effect of step edges in H2 activation and overcome problems associated with Co single crystal experiments (facile phase changes and refaceting at relatively low temperatures during standard cleaning procedures), we have studied H2 adsorption on Co nanoparticles grown on Cu(111). This approach enables us to examine the adsorption of H on close-packed Co surfaces and gain insight into the effect of Co in nanoparticle form on the H2 activation and uptake processes. Co deposited on Cu(111) forms welldefined nanoparticles of close-packed Co13−15 that have been studied extensively for their electronic and magnetic properties,16−23 but little work has been done to examine adsorbates on these surfaces.24−26 These Co nanoparticles are electronically very similar to bulk Co(0001);19−23 therefore, they serve as an excellent surface with which to model the bulk single crystal. Since molecules are typically only sensitive to the surface structure in their vicinity up to a few lattice sites away, adsorbates at the island centers should exhibit adsorption behavior very similar to bulk Co. Simultaneously, the nanoparticles’ high step-to-terrace ratio makes them excellent models for catalytic Co nanoparticles. Using a low temperature scanning tunneling microscope (LT-STM), we are able to visualize the structure of the Co nanoparticles and obtain atomic-scale resolution of H overlayers as a function of
Cobalt is the active element in many heterogeneous catalytic processes, including Fischer−Tropsch synthesis, which involves reaction of H2 with CO to form hydrocarbons.1 Despite its importance, surface science studies of Co are limited, especially those involving H2. This deficiency stems from the difficulty in reproducibly preparing single crystal Co samples, which are subject to phase changes and refaceting at relatively low temperatures. A few groups have examined the adsorption of H2 (D2) on Co single crystals,2−7 as well as Co thin films and foils.8−12 These studies show that adsorption of H2 on Co is dissociative down to 90 K and that defects and steps in the surface enhance uptake. The maximum reported coverage of H (D) on flat Co(0001) is ∼0.5 ML, and at this coverage a (2 × 2) structure is visible with low energy electron diffraction (LEED).3,5 Recently, van Helden et al. predicted with density functional theory (DFT) that the maximum coverage of H on Co(0001) could be as high as 1 ML, given the presence of defects in the surface to facilitate dissociation.5 By sputtering their Co(0001) surface, the group was able to achieve a H coverage of 0.75 ML, in which the facile dissociation of H2 at Co step edges mediated spillover to the terraces.5 This defective structure more accurately models Co nanoparticles, which due to their small size have a high ratio of steps to terraces, and indicates that spillover may be an important mechanism in delivering H to the Co terraces for subsequent reaction. © 2012 American Chemical Society
Received: September 11, 2012 Revised: October 1, 2012 Published: October 3, 2012 25868
dx.doi.org/10.1021/jp3090414 | J. Phys. Chem. C 2012, 116, 25868−25873
The Journal of Physical Chemistry C
Article
temperature and H exposure. We also employ DFT calculations to confirm that our structural assignments made with STM are the lowest energy structures. Our data strongly support the suggestion that spillover from step edges plays a key role in the formation of higher-coverage H phases not seen on flat closepacked single crystals. H2 adsorption was studied on the Co/Cu(111) system by one group;24,25 however, the primary purpose of that work was to examine STM tip-induced desorption and the effect of H adsorption/desorption on the Co electronic structure. We demonstrate herein that since those experiments24,25 were performed at a very low temperature (10 K), higher-density H phases that require activation were not observed. We thoroughly examine the adsorption phases of H on model Co nanoparticles grown on Cu(111) at both 78 and 5 K and use DFT to elucidate the energetics and structure of the various observed H phases.
monolayers of face-centered cubic (FCC) close packed Co on Cu(111), which is known to be the majority configuration.14,16,17,32 Our model of hexagonally-close packed (HCP) Co on Cu(111) showed no significant difference in adsorption behavior, in agreement with our experiments. We then studied the adsorption of five H structures on the FCC bilayer Co system: H-(2 × 2), 2H-(2 × 2), 5H-(3 × 3), 6H-(3 × 3), and H-(1 × 1). The notation nH-(m × m) indicates the number of H atoms (n) per unit cell (m × m). Through evaluation of the Gibbs free energy, the stability of H adsorbate structures in thermal equilibrium with a reservoir (here it was H2, and characterized by chemical potential of hydrogen μH) was determined. More specifically, the surface free energy with respect to that of the clean Co/Cu(111) surface defined as
METHODS Experimental. All STM imaging was performed in ultrahigh vacuum (UHV) using a low temperature scanning tunneling microscope (LT-STM, Omicron Nanotechnology) at 78 or 5 K, as noted in the text. The Cu(111) crystal (MaTecK) was prepared by cycles of sputtering with Ar+ (1 kV/14 μA) for 30 min followed by annealing to 1000 K in the preparation chamber (P = 2 × 10−10 mbar). The sample was then transferred to the separate STM chamber (P = 1 × 10−11 mbar) where it was cooled within ∼1 h to the cryogenic temperature and imaged to check for cleanliness. For Co depositions, the clean sample was warmed to room temperature in the STM chamber and then transferred to the preparation chamber where Co was deposited by use of an EFM 3 electron beam evaporator (Focus GmbH) with a flux between 0.02 and 0.10 ML min−1 to obtain a Co surface coverage of 25−35%. After Co deposition, the sample was immediately transferred back to the precooled scanning stage (
