Prandtl–Tomlinson-Type Models for Coupled Molecular Sliding Friction: Chain-Length Dependence of Friction of Self-assembled Monolayers

Tribology Letters - Most asteroids with a diameter larger than $$\sim 300 \ {\mathrm{m}}$$ are rubble piles, i.e., consisting of more than one solid object. All asteroids are rotating but almost...     

On the reaction pathway for the formation of benzene from acetylene catalyzed by palladium

The reaction between gas‐phase acetylene and alumina‐supported palladium saturated with ¹³C‐labelled vinylidene is studied using both one‐pulse, ¹³C magic‐angle spinning, nuclear magnetic resonance (NMR) spectroscopy and by mass spectroscopic analysis of the reaction products to probe the reaction pathway. The presence of vinylidene on alumina‐supported palladium is confirmed by comparing the infrared spectra of the species formed on the supported sample with those found on a Pd(111) single crystal. It is shown using NMR that a high pressure (∼350 Torr) of gas‐phase acetylene reacts with adsorbed vinylidene at the same rate at which benzene is formed catalytically on the same sample. The resulting benzene incorporates two ¹³C atoms. This indicates that benzene is formed by a slow reaction between gas‐phase (¹²C‐labelled) acetylene and adsorbed vinylidene (¹³CH₂=¹³C=) to form a C₄ intermediate which reacts rapidly with further acetylene to yield benzene. There are precedents for such reactions in homogeneous phase. The proposed reaction pathway differs from that elucidated previously from ultrahigh vacuum studies on clean Pd(111), where it was found that benzene synthesis also proceeds via a C₄ intermediate, in this case formed from two adsorbed acetylenes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molybdenum metal catalyzed reaction of ethylene

The reaction of ethylene catalyzed by clean molybdenum foil is examined between 800 and 880 K. Products up to C₇ are detected in measurable quantities and hydrocarbons up to C₈ are observed. The product distribution is well described by a Schulz-Flory function where a plot of ln (W_n/n) versus n, where W_n is the yield of each C_n hydrocarbon, is a good straight line. Analysis of the slope and intercept of the distribution shows that the slope has a zero-order ethylene pressure dependence and varies with temperature with an activation energy of 4.6±0.6 kcal/mol, while the intercept has a pressure dependence of 1.3±0.3 and an activation energy of 46±4 kcal/mol. A kinetic model is proposed that successfully accounts for these pressure dependences. These data suggest that the activation energy for degenerate ethylene metathesis should be 55±5 kcal/mol and that the reaction should be first order in ethylene. These kinetic parameters are in good agreement with those found experimentally for the non-degenerate meta thesis of propylene.     

Understanding the tribological chemistry of chlorine-, sulfur- and phosphorus-containing additives

It is demonstrated that the surface chemistry of simple-model extreme-pressure lubricant additives, measured in ultrahigh vacuum, corresponds to that measured at higher pressures, where film growth rates are monitored using a microbalance. This chemistry and reaction kinetics are used to explain the extreme-pressure lubrication behavior by successfully modeling the measured seizure load vs. additive concentration curves. It is also demonstrated, by growing ferrous chloride films on iron substrates in ultrahigh vacuum, that these have the same friction coefficients as those found for model extreme-pressure lubricants. It is found that a monolayer of a solid boundary lubricant film is sufficient to lower the interfacial friction coefficient to its minimum value. These results demonstrate that the chemistry measured under conditions of thermodynamic equilibrium at some temperature can be successfully applied to the formation of a boundary film, in the extreme-pressure regime.     

Determination of interfacial temperatures under extreme pressure conditions

Interfacial temperatures attained in a pin and v-block apparatus under extreme pressure (EP) conditions were measured using pins made from either copper or an aluminum alloy from the asymptotes in the curve of removal rate versus applied load since these have been shown to correspond to the temperatures at which the interfacial material melts. The interfacial temperature rise was proportional to the applied load, where the proportionality constant = A where is the interfacial friction coefficient and A a geometrical constant which has been previously measured for steel pins and v-blocks lubricated by chlorinated hydrocarbons dissolved in a poly -olefin as 2.3 ± 0.3 K/N. Values of A measured when using the aluminum alloy (2.4 ± 0.1) and for copper (2.1 ± 0.2) were in good agreement with this measurement and indicated that interfacial temperatures in excess of 1000 K can be attained during EP lubrication. Finally, the rate of material removal in the pin and v-block apparatus can be related to the metallurgical properties of the pins.     

X-ray absorption near-edge structure analysis of the chemical environment of zinc in the tribological film formed by zinc dialkyl dithiophosphate decomposition on steel

Calculation of X-ray absorption near-edge structure (XANES) has been used to investigate the chemical nature of zinc in thermal films formed on the surfaces of steel coupons by the additive zinc dialkyl dithiophosphate (ZDDP) in oil solution. Carbon K, oxygen K, and zinc L edge XANES spectra were used to characterize the thermal films and a model film formed by addition of cumene hydroperoxide (CHP) to the ZDDP. Thermal films formed by heating at up to 100 °C for 5 h showed little change from unreacted ZDDP. At 160 °C for 5 h, the ZDDP underwent thermal solution decomposition with similarities to the oxidation induced by CHP. The Zn L edge XANES spectra were compared to calculated spectra of the model surface species ZnO and ZnS in the wurtzite structure and ZnS in the zinc blende structure. Agreement between experiment and simulation is found for the 160 °C thermal film with a roughly two monolayer film of ZnO and for oxidation by CHP with a three to four monolayer film of ZnS.     

Evidence for olefin metathesis catalyzed by metallic molybdenum proceeding via an associative mechanism at high temperature

The reaction of ethylene catalyzed by metallic molybdenum at 875 K yields higher hydrocarbons (>C₂) with a distribution that is well described by a Schulz-Flory plot suggesting that their formation arises from the polymerization of surface C₁ species. This observation is in accord with results from surface science experiments that show facile C=C cleavage in alkenes. A similar molybdenum-catalyzed reaction of propylene yields metathesis and hydrogenolysis products. In view of the product distribution found with ethylene, these products are proposed to arise from the recombination of surface species formed by propylene dissociation. This result is confirmed by modifying the Schulz-Flory function assuming equal surface coverages of carbenes and methyl carbenes which adequately predicts the measured product distribution.     

Low-temperature, shear-induced tribofilm formation from dimethyl disulfide on copper

The frictional properties of a sliding copper-copper interface exposed to dimethyl disulfide (DMDS) are measured in UHV under conditions at which the interfacial temperature rise is <1 K. A significant reduction in friction is found from the clean-surface values and sulfur is found on the surface and below the surface in the wear scar region by Auger spectroscopy. Because the interfacial temperature rise under the experimental conditions used to measure friction is very small, tribofilm formation is not thermally induced. The novel, low-temperature tribofilm formation observed here is ascribed to a shear-induced intermixing of the surface layer(s) with the subsurface region as suggested using previous molecular dynamics simulations. Although the tribofilm contains predominantly sulfur, a small amount of carbon is also found in the film.     

The Frictional Properties of Thin Inorganic Halide Films on Iron Measured in Ultrahigh Vacuum

Friction coefficients are measured in ultrahigh vacuum using a tungsten carbide tribopin against thin films of sodium chloride and potassium iodide deposited onto clean iron. It is found, in accordance with previous measurements for potassium chloride on iron, that the friction coefficient falls from an initial value of 2 for the clean iron surface to a minimum value after a few tens of Ångstroms of the halide have been deposited onto the surface, and remains constant for thicker films. The minimum friction coefficient is independent of applied load and therefore obeys Amontons' law. Simple theories for the effect of a low-shear-strength film suggest that the friction coefficient should depend on the shear strength of the film. This idea is tested by plotting the minimum friction coefficient versus the hardness of the film material, which is proportional to its shear strength, where a linear correlation was found. The lack of dependence of friction coefficient with film thickness for thicker films implies that ploughing forces do not contribute significantly to the friction coefficient.     

Creation of Low-Coordination Gold Sites on Au(111) Surface by 1,4-phenylene Diisocyanide Adsorption

The adsorption of CO on a saturated overlayer of 1,4-phenylene diisocyanide (PDI) adsorbed on a Au(111) surface at 300 K is studied using scanning tunneling microscopy (STM), density functional theory (DFT) calculations and reflection absorption infrared spectroscopy (RAIRS). The PDI forms closed-packed rows of gold-PDI chains by extracting gold atoms from the Au(111) substrate. They are imaged by STM and the structure calculated by DFT. The adsorption of CO is studied on the low-coordination gold sites formed on the PDI-covered surface where it adsorbs exhibiting a CO stretching frequency of 2004 cm^?1, consistent with adsorption on an atop site. It is found that CO is stable on heating the sample to ~150 K and is only removed from the surface by heating to ~180 K. Since low-coordination gold atoms are suggested to be the active catalytic sites on supported gold nanoclusters, ??embossing?? the surface to form similar low-coordination sites using PDI might offer a strategy for tailoring the catalytic activity of gold.