Repulsive Adsorbate-adsorbate Interactions Research Articles

NH3 adsorption on anatase-TiO2(101).

The adsorption of ammonia on anatase TiO2 is of fundamental importance for several catalytic applications of TiO2 and for probing acid-base interactions. Utilizing high-resolution scanning tunneling microscopy (STM), synchrotron X-ray photoelectron spectroscopy, temperature-programmed desorption (TPD), and density functional theory (DFT), we identify the adsorption mode and quantify the adsorption strength on the anatase TiO2(101) surface. It was found that ammonia adsorbs non-dissociatively as NH3 on regular five-fold coordinated titanium surface sites (5f-Ti) with an estimated exothermic adsorption energy of 1.2 eV for an isolated ammonia molecule. For higher adsorbate coverages, the adsorption energy progressively shifts to smaller values, due to repulsive intermolecular interactions. The repulsive adsorbate-adsorbate interactions are quantified using DFT and autocorrelation analysis of STM images, which both showed a repulsive energy of ∼50 meV for nearest neighbor sites and a lowering in binding energy for an ammonia molecule in a full monolayer of 0.28 eV, which is in agreement with TPD spectra.

The effect of fluorination on the bonding of alkyl ethers to the Al(110) surface

Temperature-programmed desorption (TPD) has been used to investigate the effect of fluorination on the surface chemistry of several model ethers on the Al(110) metal surface. The model compounds studied were dioxolane ( ▪), diethylether [(CH 3CH 2) 2O], dimethoxymethane [(CH 3O) 2CH 2], dimethylether [(CH 3) 2O] and their fully fluorinated analogs. Perfluorination of these ethers decreases their desorption energies from ∼13 to ∼7 kcal/mol. At coverages in the range 0.1 < θ <1.0 the heat of adsorption of the fluorinated ethers is indistinguishable from that of the multilayers. The hydrogenated ethers were found to have stronger repulsive adsorbate-adsorbate interactions than the perfluorinated ethers. By comparing the surface chemistry of the ethers on Al(110) surface with that on the Cu(111) surface, it was found that the two different metals simply shift the magnitude of the adsorbate-adsorbate interactions by a constant. None of the ethers studied (either hydrogenated or fluorinated) were found to decompose on the Al(110) metal surface.

The effect of fluorination on the surface chemistry of alkyl ethers on Cu(111)

By using small fluorinated ethers as models for perfluoropolyalkylether (PFPAE) lubricants, we have been able to determine the effect of fluorination on the bonding of the alkyl ethers adsorbed on the Cu(111) surface. The desorption energies have been determined by using temperature programmed desorption (TPD). The model compounds studied were dioxolane , \(({\text{H}}_{\text{2}} )\)diethyl ether ((CH3CH2)2O), dimethoxymethane ((CH3O)2CH2), dimethyl ether ((CH3)2O) and their perfluorinated analogues. All of the molecules studied adsorb molecularly and reversibly on the Cu(111) surface exhibiting first-order desorption kinetics. Upon fluorination of the alkyl ethers, the adsorbate-metal bond was weakened by ∽14 to ∽8 kcal/mol. The intermolecular interaction parameters for the hydrogenated ethers indicated repulsive adsorbate-adsorbate interactions, while the fluorinated ethers have attractive adsorbate-adsorbate interactions.

CuCl adlayer formation and Cl induced surface alloying: An in situ STM study on Cu underpotential deposition on Au(110) electrode surfaces

Cu underpotential deposition on Au(110), in pure and Cl− containing H2SO4 solution, was investigated by in situ STM. In the absence of Cl− the general surface topography and the (1 × 1) structure of the nonreconstructed surface are maintained over the entire upd potential range. This is interpreted as formation of a (1 × 1) Cu adlayer, with weak and presumably slightly repulsive adsorbate-adsorbate interactions. In Cl− containing electrolyte a (2 × 1) structure is reversibly formed at potentials below 0.27 Vsce, coincident with a sharp peak in the cyclic voltammograms and the appearance of anisotropic, [001] oriented Au adlayer islands and monolayer holes on the previously flat terraces. A model based on the CuCl(111) bilayer is suggested for the (2 × 1) structure and compared to adlayer structures on Au(111) and (100). The roughening is interpreted as anion induced surface alloying.

CH 3I and C 2H 5I on Au(100): adsorption and reaction

The adsorption and reaction of CH 3I and C 2H 5I on a Au(100) surface have been studied by a combination of temperature-programmed desorption/reaction (TPD/R), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), reflection-absorption infrared spectroscopy (RAIRS), high-resolution electron energy loss spectroscopy (HREELS) and work function change (Δф) measurements. Both alkyl halides adsorb and desorb mainly in molecular form without decomposition. Only 4% of the first layer CH 3I molecules dissociate on the surface; the methyl groups formed by this reaction couple to form ethane at ∼ 360 K. By contrast, 20% of the first layer C 2H 5I molecules decompose and the final surface reaction products are ethylene, ethane and butane. The dominant product is C 4H 10 which is formed at ∼ 270 K. Repulsive adsorbate-adsorbate interactions are observed in the first adsorption layer for both alkyl iodides. For CH 3I (but not for C 2H 5I), a high coverage metastable adsorption state characterized by a desorption temperature lower than the multilayer desorption temperature is observed. The structure of the metastable state has yet to be determined, but the results suggest the presence of a CH 3I bilayer in which the CH 3I molecules in the first layer are oriented with their CI bonds approximately parallel to the surface and the molecules in the second, metastable layer have a tilted orientation similar to the average orientation in the multilayer. To the best of our knowledge, this is the first observation of a metastable adsorption state for alkyl halides on metal surfaces.

The coadsorption of ethene and chlorine on Ag(100)

We have characterised the low temperature adsorption of ethene on both clean and chlorine-precovered Ag(100) surfaces. On the clean surface at temperatures ca. 100 K ethene is found to be weakly π-bonded at all coverages with dynamic pressures of the gas being required to saturate the monolayer adsorption state. At high coverages, around the monolayer point, repulsive adsorbate-adsorbate interactions induce a reorientation of the ethene which rotates the molecular plane towards the surface normal. The interaction of ethene with a surface pretreated with Cl 2 is strongly dependent on the Cl coverage. At low values an enhancement of the ethene-surface interaction via the formation of ordered ethene-chlorine structures is observed, although no substantial re-hybridisation of the ethene is induced, while at high Cl coverage a progressive passivation of the surface is produced. The results presented are consistent with the formation of a simple chemisorbed layer of chlorine.

A comparative study of PF 3, PH 3 and P(CH 3) 3 on clean, K-covered, and Cl-covered Ag(111)

Abstract The adsorption and desorption of PF3, PH3 and P(CH3)3 on clean, K-covered, and Cl-covered Ag(111) has been studied using temperature programmed desorption (TPD), work function change measurement (Δφ), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). All three compounds adsorb molecularly at 105 K, adsorb through their P atoms, and desorb reversibly with no detectable decomposition. The adsorptions are accompanied by a maximum surface work function change of −0.15 eV (PF3), −1.4 eV (PH3) and −2.45 eV (P(CH3)3). The desorption energies increase in the order PF3 E d = 7.5 kcal mol . For PH3 and P(CH3)3, there appear to be strong repulsive adsorbate-adsorbate interactions. The bonding modes of PH3 and P(CH3)3 only involve σ-donation. For PF3, σ-donation and π-back-donation are involved. TPD and Δφ measurements indicate that the σ-donation capability follows the order P(CH3)3 > PH3 > PF3. This order is directly related to the electronegativity of the group attached to the phosphorus. Δφ, UPS and XPS measurements indicate that there is some structural rearrangement taking place in the multilayer P(CH3)3 when the surface temperature is increased. Surface Cl stabilizes adsorbed PF3, PH3 and P(CH3)3. Surface K also stabilizes PF3, but destabilizes PH3 and P(CH3)3. We compare our results with the corresponding coordination chemistry.

A comparative study of PF 3, PH 3 AND P(CH 3) 3 on clean, k-covered, and Cl-covered Ag(111)

The adsorption and desorption of PF 3, PH 3 and P(CH 3) 3 on clean, K-covered, and Cl-covered Ag(111) has been studied using temperature programmed desorption (TPD), work function change measurement (Δφ), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). All three compounds adsorb molecularly at 105 K, adsorb through their P atoms, and desorb reversibly with no detectable decomposition. The adsorptions are accompanied by a maximum surface work function change of −0.15 eV (PF 3), −1.4 eV (PH 3) and −2.45 eV (P(CH 3) 3). The desorption energies increase in the order PF 3 < PH 3 < P(CH 3) 3. The desorption of PF 3 follows simple first-order kinetics with E d = 7.5 kcal mol . For PH 3 and P(CH 3) 3, there appear to be strong repulsive adsorbate-adsorbate interactions. The bonding modes of PH 3 and P(CH 3) 3 only involve σ-donation. For PF 3, σ-donation and π-back-donation are involved. TPD and Δφ measurements indicate that the σ-donation capability follows the order P(CH 3) 3 > PH 3 > PF 3. This order is directly related to the electronegativity of the group attached to the phosphorus. Δφ, UPS and XPS measurements indicate that there is some structural rearrangement taking place in the multilayer P(CH 3) 3 when the surface temperature is increased. Surface Cl stabilizes adsorbed PF 3, PH 3 and P(CH 3) 3. Surface K also stabilizes PF 3, but destabilizes PH 3 and P(CH 3) 3. We compare our results with the corresponding coordination chemistry.

Bonding and chemistry of hydrocarbon monolayers on metal surfaces

Results from ultra-high vacuum surface analysis techniques over the last ten years have dramatically increased our understanding of how unsaturated hydrocarbons react with metal surfaces. We review here what has been learned, utilizing results from our laboratory on the bonding and chemistry of ethylene and benzene to illustrate general principles. Several points deserve emphasis. First, clean surfaces of most transition metals (as might be expected from their coordinative unsaturation) are highly reactive towards unsaturated hydrocarbons, sequentially dehydrogenating these adsorbates over the temperature range of ∼200–800K to evolve hydrogen and deposit carbon. Throughout this temperature range, partially dehydrogenated decomposition intermediates can be isolated and identified. The bonding (and also possibly the chemistry) of these surface fragments is strikingly similar to that of hydrocarbon ligands in multinuclear organometallic clusters. Studies which combine reactions at atmospheric pressure with surface analysis in UHV show that these stable surface fragments are present on active hydrocarbon catalysts but are not reaction intermediates. It appears, however, that these surface organometallic compounds are important for promoting catalytic turnover of reactive species by tempering the surface reactivity through adsorbate-adsorbate interactions. Studies (under ultra-high vacuum conditions) of adsorbates at high surface coverages and in the presence of coadsorbates show dramatic effects of attractive and repulsive adsorbate-adsorbate interactions on the surface bonding and chemistry.

Formation of islands consisting of repelling adsorbates

Clear evidence of adsorbate island formation in a system with only repulsive adsorbate-adsorbate interactions is presented. This evidence is independently obtained by thermal energy atom scattering (TEAS) and LEED studies of the H−CO/Pt(111) coadsorption system. Specularly reflected He intensity at hydrogen saturation coverage on the CO precovered surface is up to an order of magnitude greater than in the absence of hydrogen. A c(4×2) CO/Pt(111) LEED pattern is observed at hydrogen saturation even for CO precoverages of less than 0.1 monolayer.

Formation of islands consisting of repelling adsorbates

Clear evidence of adsorbate island formation in a system with only repulsive adsorbate-adsorbate interactions is presented. This evidence is independently obtained by thermal energy atom scattering (TEAS) and LEED studies of the H-CO/Pt(111) coadsorption system. Specularly reflected He intensity at hydrogen saturation coverage on the CO precovered surface is up to an order of magnitude greater than in the absence of hydrogen. A c(4 × 2) CO/Pt(111) LEED pattern is observed at hydrogen saturation even for CO precoverages of less than 0.1 monolayer.

Rates of desorption from solid surfaces: Coverage dependence

The recently developed Classical Stochastic Diffusion Theory is applied to obtain the coverage dependence of desorption rates for Xe on W(111). Using the attractive Xe-Xe potential from gas phase experiments, we find a strong coverage dependence in the desorption rates and calculate Temperature Programmed Desorption Spectra (for a potential with reduced attractiveness) that are in excellent qualitative agreement with experimental results. We also investigated the effect of purely repulsive adsorbate-adsorbate interactions where we find, for some coverage ranges, that two different adsorption configurations can be stable (at different temperatures) leading to a marked change in the corresponding desorption rates and to distinct non-Arrhenius behavior.