ARTICLE pubs.acs.org/JPCC
Hydrogen-Bonded Networks in Surface-Bound Methanol Ashleigh E. Baber, Timothy J. Lawton, and E. Charles H. Sykes* Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States ABSTRACT: Fundamentally, methanol represents one of the simplest molecules for understanding extended hydrogenbonded networks, and from a practical perspective it constitutes a very important chemical feedstock. The interaction of methanol with transition metal surfaces is of relevance to many catalytic processes. Gold-based catalysts show high selectivity for the partial oxidation of several species including, most recently, methanol. However, due to its weak interaction, it has not yet been possible to study intact methanol adsorption and ordering on any metal surface with molecular resolution. Using careful annealing treatments and low tunneling current, variable-temperature scanning tunneling microscopy, we show the basic bonding geometries of a range of hydrogen-bonded methanol structures as a function of surface coverage and temperature on Au(111). Like ice, methanol forms hexamer units; however, at all but very low coverages, chain structures dominate. Unlike the 2D honeycomb structure of ice, which is interconnected by hydrogen bonds on wettable metal surfaces, methanol chains interact weakly and actually repel one another until forced into close contact at high coverages. We report the three basic geometrical units of methanol that form the basis of all the observed structures. Annealing experiments reveal the reversibility of the assembly and demonstrate that the reported structures are thermodynamically formed. To demonstrate the generality of these results, we also present similar data for methanol on Cu(111), which has a very different lattice constant than Au.
’ INTRODUCTION Hydrogen-bonded networks form the backbone of many complex and functional structures in nature. While the transfer of geometrical order from the molecular to the macroscopic level is not often well understood, this effect clearly dominates structural properties at many length scales; for example, the complex macroscale structure of snowflakes ultimately derives from nanoscale ice crystallinity. Additionally, many biological systems maintain their unique shape, crucial for functionality, with a dynamical network of hydrogen bonds. Pioneering scanning probe research has allowed the adsorption and ordering of H2O on surfaces to be examined with high resolution.122 These studies√ revealed √ that water generally forms ice-like hexagonal bilayers with ( 3 3)R30° periodicity on close-packed hexagonal metal surfaces.26,17,18,23 These bilayers are based on a hexamer unit in which water molecules may bond to atop atomic sites on the surface via a lone pair of the H2O oxygen.7,2426 Hexamers, in which each water molecule acts as a single H-bond acceptor and single H-bond donor, were shown to be the most stable water structure at low coverages.46,12,24,25,27 While MeOH hydrogen-bonded networks have not yet been imaged with high-resolution techniques, they are believed to be more complicated than water H-bonded networks due to steric repulsion between neighboring methyl groups.28 The structure of liquid MeOH is dependent upon H-bond interactions as well as hydrophobic interactions due to the methyl groups.28 There is a long-running debate over the nature of H-bonded networks in liquid MeOH. Early molecular dynamics and Monte Carlo calculations predicted that long, winding chains are the dominant structure of liquid MeOH,29,30 while density functional theory r 2011 American Chemical Society
(DFT) calculations indicated that rings are more stable than chains in liquid MeOH.28,31 X-ray emission spectroscopy in combination with DFT calculations revealed that both ring and chain structures were present in liquid MeOH, with a predominance of structures containing 68 molecules.32 Recently, Raman spectroscopy combined with DFT calculations suggested that the most stable structures of liquid MeOH are trimer, tetramer, and pentamer rings.33 The crystal structure for solid MeOH is described as hydrogen-bonded zigzag chains with the methyl groups pointed outward from the HO 3 3 3 H backbone.34 In terms of its surface chemistry, MeOH adsorbs nondissociatively on clean noble metal single crystals (Au(111), Cu(111), Ag(111)).3538 Studies of MeOH on Au indicated that Au(111) does not activate MeOH decomposition, unless it is predosed with atomic oxygen.35,38 Highly kinked Au surfaces and particles were also active for MeOH decomposition.3948 Oxygen-covered Au nanoparticles on Au(111) selectively coupled MeOH and aldehydes for methyl ester formation40 and catalyzed MeOH esterification reactions41 below room temperature. Nanoporous Au catalyzed the selective oxidative coupling of MeOH to methyl formate with high selectivity.42 Au and Cu nanoparticles supported on oxides have been shown to be active catalysts for MeOH decomposition, indicating their potential use for MeOH steam reformation.4349 In addition, methanol has shown importance as a biofuel and in direct methanol fuel cells.5054 Received: February 14, 2011 Revised: April 7, 2011 Published: April 19, 2011 9157
dx.doi.org/10.1021/jp201465d | J. Phys. Chem. C 2011, 115, 9157–9163
The Journal of Physical Chemistry C
ARTICLE
Here we report the first direct observation by scanning tunneling microscopy (STM) of the assembly of intact MeOH in a number of unique structures from low coverage up to a full monolayer. Snapshots of molecular-scale structure were attained at the range of temperatures over which temperature programmed desorption (TPD) experiments indicate multi- and monolayer desorption. While all extended water honeycomb structures are derived of cyclic hexamers, we show that MeOH, although forming a variety of rings, critically adopts chain structures at all but very low surface coverages. To interrogate the effect of the substrate and demonstrate the generality of these results, we compare MeOH assembly data on surfaces with very different lattice constants (Cu and Au). DFT calculations predict that MeOH binds to atop sites on Au(111) and Cu(111) via donation of a lone pair of electrons from the oxygen atom and that the OH bond is almost parallel to the plane of the surface.55,56 Therefore, as one of the oxygen lone pairs is involved in bonding, surface-bound MeOH can only form two H-bonds per molecule in any extended network, in contrast to H2O, which can form up to three H-bonds when bound to metal surfaces (in accordance with Salmeron’s 2D ice rules4,5).
’ EXPERIMENTAL SECTION All experiments were performed in an Omicron NanoTechnology variable-temperature ultrahigh vacuum (VT-UHV) STM. Before the STM stage was cooled with liquid helium via a flow cryostat, the base pressure in the STM chamber was approximately 1 1010 mbar. Precooling the STM before the sample was loaded decreased the pressure in the STM chamber (
