AVS2012 Session SS-WeM: Chemisorption on Metallic Surfaces

Hydrogen is green fuel capable of producing electricity. Methanol is a promising hydrogen storage molecule with a high hydrogen-to-carbon ratio (4:1). My research examines methanol oxidation and decomposition on model catalysts via variable temperature scanning tunneling microscopy (VT-STM) and temperature-programmed reaction (TPR).

Methanol desorption on Cu(111) was studied from 130 K (multilayer desorption) to 165 K (monolayer desorption) with STM. Analysis of STM images reveals several structures that are governed by hydrogen-bonding interactions. From the STM images, hexamers are the most thermodynamically stable structure on Cu(111). Hydroxyl-proton localization on the hexamers leads to two enantiomers.

The Cu surface was chemically modified in two ways to alter methanol reactivity. First, the effect of oxygen at various temperatures on Cu(111) was studied with STM. STM and XPS of a Cu(111) surface pre-adsorbed with oxygen after exposure to methanol show methoxy forms at the interface of the oxide-like domains and bare Cu.

The second avenue was a Cu-based bimetallic alloy containing small amounts of Pd. The TPD studies on model Pd/Cu catalysts interestingly reveal partial decomposition of methanol to formaldehyde and hydrogen. STM images acquired reveal that at temperatures above MeOH desorption, molecules only occupy sites near Pd atoms, suggesting these are the active sites.

We use STM to investigate the adsorption, self assembly and metal-coordination of isocyano- and cyano-based aromatic species on Cu (111). Despite their structural similarity, cyano and isocyano groups are chemically quite different. We investigate the effect of this distinction on the structures on the metal coordination structures they form and on the resultant geometric and electronic state of the metal coordination center. In particular we find very different structures for dicyano- and diisocyano-anthracene molecules. While the former generates a network consisting of trigonally coordinated metal centers, the latter generates molecular rows, i.e. more linear arrangement. Cyano- and Isocyano-naphtalene also generate different distributions of coordination compounds on Cu(111). The combination of these studies highlights the impact of the chemical structure of the ligand on the coordination center. Organic ligands are a common tool for affecting activity in homogeneous catalysis. Our study is aimed at exploring the capabilities of lateral coordination in controlling the activity of a surface.

The interaction of NH₃ with chemisorbed molecular O₂ on a Pt(111) surface has been studied at the single-molecule level with low temperature scanning tunneling microscopy. Chemisorbed O₂ molecules are found to form an ordered network at high coverages for adsorption temperatures below 50 K. Sites unoccupied by O₂ molecules on Pt(111) appear as holes in the network. Various hole-hole distances among nearest neighbors are observed reflecting variations in the arrangement of O₂ molecules. A hole-hole distance of 0.74 nm is found to be predominant on the surface and is assigned to be the most favorable one as it maintains the 3-fold symmetry of underlying platinum lattice. Ammonia molecules are observed to adsorb in the holes within the ordered network of O₂ molecules, which is also the a-top site with respect to the Pt(111) substrate. Further annealing of the ammonia-oxygen overlayer to 400 K results in the formation of a mixed p(2×2) overlayer of N, O and NH. This work provides new insights into the ammonia oxydehydrogenation reaction on platinum surface, which is an important catalytic reaction in the industrial production of nitric acid.

Many emerging microfabrication technologies such as microfluidic devices rely on the deposition of metal features onto polymeric substrates. Au and Pt thin film metallization are particularly important for electrodes, IR reflectors, interconnects and catalytic surfaces in these devices. However, due to the inert nature of Au and Pt, adhesion to polymeric substrates is generally poor. We report on the use of halogenated organic solvents as a pre-treatment onto poly(methyl methacrylate) (PMMA) substrates to improve the adhesion of an array of 121 electron-beam or magnetron sputter deposited 1.5 mm diameter metal dots by up to a factor of five compared to deposition on cleaned PMMA substrates. We have observed improvement by both spin-casting and vapor phase pre-treatment of the PMMA surface. Nearly 90% of the Au dots remain after a standard tape pull-test for samples pre-treated with CHCl₃ compared with only 17% remaining on the untreated samples. The adhesion of CHCl₃, CH₂Cl₂ and CHBr₃ pre-treated samples are also significantly improved compared to remote O₂ plasma pre-treatment (26% adhesion). Atomic force microscopy roughness data shows that this is not due to surface roughening, and we have shown through X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) studies that the halogenated solvent molecules form a Lewis acid-base adduct with the polymer chains leaving a halogen-rich surface. A covalent bond is then formed between the Cr metal adhesion layer and the surface terminated Cl which results in the strong metal adhesion. Molecular modeling has been performed to understand the origin of the enhanced adhesion. DFT calculations are consistent with the presence of adduct formation having a interaction enthalpy of ~35 kJ/mol. The mechanism of improved chemical metal bonding will be discussed.

Factors such as support type and nanoparticle morphology are known to impact catalytic activity, but detailed studies are required to elucidate the effect of nanoparticle size on the mechanism of dehydrogenation over nanoparticles supported on oxides. In this work, high resolution electron energy loss spectroscopy (HREELS) was used to characterize the dehydrogenation of hydrocarbons on platinum nanoparticles supported on an Al₂O₃ film grown by oxidation of a NiAl(110) single crystal. The alumina film is a good representation of the high surface area oxides used as catalyst supports in industry, and the flat surface enables investigation by electron inelastic scattering techniques. The platinum nanoparticles were deposited on a fresh alumina film at cryogenic temperatures, and platinum deposition was monitored by the reduction in elastic peak intensity in the HREEL spectra. Cyclohexane was used as a model hydrocarbon because the dehydrogenation on Pt(111) has been studied extensively and thus provides a good point of comparison for the nanoparticle studies. The HREELS spectra of cyclohexane on Pt(111) show a distinct low frequency peak at 2600 cm^-1 which can be monitored to determine the temperature at which dehydrogenation begins.^1,2 The HREELS annealing profile from an alumina film does not exhibit cyclohexane adsorption at 170 K, or upon successive annealing. In contrast, the HREEL spectra from platinum nanoparticles on alumina exhibit cyclohexane adsorption at 170 K with the ν_C-H stretch energy loss peak observed at 2930 cm^–1. Interestingly, HREEL spectra from cyclohexane on platinum nanoparticles do not resemble spectra from cyclohexane on Pt(111). Specifically, the additional ν_C-H energy loss peak observed at 2600 cm^-1 for cyclohexane adsorbed on Pt(111) is not seen in the case of platinum nanoparticles.^1,2 In addition, the ν_C-H energy loss peak from cyclohexane on platinum nanoparticles does not shift or decrease in intensity upon annealing to room temperature. Our results indicate that cyclohexane dehydrogenates upon adsorption to platinum nanoparticles at 170 K. This dehydrogenation temperature is lower than that on Pt(111), indicating that platinum nanoparticles supported on an alumina film are more catalytically active for the initial stages of dehydrogenation than Pt(111).

1. Land, D. P.; Erley, W.; Ibach, H., HREELS Investigation of the Orientation and Dehydrogenation of Cyclohexane on Pt(111). Surface Science 1993, 289 (3), 237-246.

2. Saeys, M.; Reyniers, M. F.; Neurock, M.; Marin, G. B., Adsorption of cyclohexadiene, cyclohexene and cyclohexane on Pt(111). Surface Science 2006, 600 (16), 3121-3134.