Subscriber access provided by STEPHEN F AUSTIN STATE UNIV
Article
Surface chemistry dependence of mechanochemical reaction of adsorbed molecule – An experimental study on tribopolymerization of #-pinene on metal, metal oxide, and carbon surfaces Xin He, and Seong H. Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03763 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Surface chemistry dependence of mechanochemical reaction of adsorbed molecule – An experimental study on tribopolymerization of α-pinene on metal, metal oxide, and carbon surfaces
Xin He and Seong H. Kim* Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, PA 16802, USA * Corresponding author:
[email protected] Abstract Mechanochemical reactions between adsorbate molecules sheared at tribological interfaces can induce association of adsorbed molecules, forming oligomeric and polymeric products (often called tribopolymers). This study revealed the role or effect of surface chemistry of the solid substrate in mechanochemical polymerization reactions. As a model reactant, αpinene was chosen since it was known to readily form tribopolymers at the sliding interface of stainless steel under vapor phase lubrication (VPL) conditions. Eight different substrate materials were tested – palladium, nickel, copper, stainless steel, gold, silicon oxide, aluminum oxide, and diamond-like carbon (DLC). All metal substrates and DLC were initially covered with surface oxide species formed naturally in air or during the oxidative sample cleaning. It was found that the tribopolymerization yield of α-pinene is much higher on the substrates that can chemisorb αpinene, compared to the ones on which only physisorption occurs. From the load-dependence of 1 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
the tribopolymerization yield, it was found that the surfaces capable of chemisorption give a smaller critical activation volume for the mechanochemical reaction, compared to the ones capable of physisorption only. Based on these observations and infrared spectroscopy analyses of the adsorbed molecules and the produced polymers, it was concluded the mechanochemical reaction mechanisms might be different between chemically-reactive and inert surfaces and the chemical reactivity of the substrate surface greatly influences the tribochemical polymerization reactions of adsorbed molecules.
Keywords: Mechanochemistry, Tribochemcal polymerization, Vapor phase lubrication, Interfacial reaction, Chemisorption,
2 ACS Paragon Plus Environment
Page 2 of 31
Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Introduction Mechanochemistry refers to reactions activated by mechanical forces.1-3 Compared to conventional thermochemistry, electrochemistry, and photochemistry where reactions are induced by heat, potential bias, or light irradiation, the molecular mechanisms or key parameters governing yields or selectivity of mechanochemical reactions are much less studied and understood. When a substantial amount of frictional heat is generated at high sliding speeds and not dissipated fast enough through conduction to the solid substrate, then thermal activations of interfacial molecules can be considered to play important roles.4-5 When severe wear of solid materials occurs at high applied loads or in poorly-lubricated conditions, then reactive surface sites such as dangling bonds or emission of charged particles (electrons or ions) or photons could be involved in initiation of chemical reactions of molecules present at the wearing surface.6-8 At relatively low load and/or slow sliding speed conditions, frictional heating and tribo-emissions are insignificant; but, mechanochemical reactions can still take place.9-14 In the case where frictional heating and tribo-emissions are negligible, one can hypothesize that perturbation of the potential energy of a molecular system under the action of external mechanical force could substantially lower the activation energy of a chemical reaction, allowing reactions to take place even at low temperature where typical thermal reactions do not occur.11-13, 15 In this scheme, the decrease in activation barrier by a mechanical action can be modeled with a modified Arrhenius-type equation:11, 13-18 Reaction rate or yield = ∙
(1)
where A is the pre-exponential factor which will vary depending on the unit of the left-hand side of the equation, is the activation barrier of thermal reaction without any external mechanical 3 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 31
shear, is the amount of decrease in the activation barrier due to the mechanical shear, is the Boltzmann constant (1.38 × 10 / ), and
is the (average) temperature of the sliding
interface. When the mechanical force is applied directly to the molecule along the reaction coordinate, then the mechanical term can be expressed as = ! ∙ ∆ ∗ where ! is the force applied to the specific bond and ∆ ∗ is the difference in bond length at the equilibrium and transition states.17-20 However, it would be impossible to experimentally measure how much force is applied directly along the specific bond of a molecule. Practically, it is more feasible to control the applied pressure or estimate the shear stress in experiments. Alternatively, the mechanical term can be expressed as = $ ∙ ∆% ∗ where $ is the applied stress and the proportionality constant (∆% ∗ ) has the dimension of volume; so, it is often called a critical activation volume.11, 13-15, 21-25 Recently, we have studied tribopolymerization reactions of three hydrocarbons − αpinene, pinane, and n-decane.12 These three molecules consists of 10 carbon atoms with different molecular structures. α-Pinene is a bicyclic terpene with 4-membered ring and one C=C doublebond in the 6-membered ring; thus, it has a very high internal strain.26-27 Pinane is the hydrogenated form of pinene; without the C=C bond in the 6-membered ring, its internal strain is reduced compared to α-pinene. N-decane is a linear alkane without any internal strain in its equilibrium state. When these molecules are adsorbed on the native oxide layer (mostly consisting of Cr2O3) of AISI 440C stainless steel (SS) and sheared at an applied load of 0.4 ~ 1.5 GPa at room temperature, all three molecules (even, n-decane) produced polymeric products.12-13 From the applied pressure dependence and the measured friction coefficient, the shear stress was estimated. By analyzing the shear stress dependence of the tribopolymerization yield, it was found that the internal strain of the molecule has a great impact on the critical activation volume 4 ACS Paragon Plus Environment
Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
− α-pinene has the smallest ∆% ∗ (~3% of its molar volume, %&' ), pinane has an intermediate value (~8% of %&' ), and n-decane has the largest value (~10% of %&' ). In equilibrium state, molecules are not compressible; then, what is the physical meaning of the magnitude of ∆% ∗ ? In order to study this question, a computational approach was necessary. In another mechanochemical system studied using molecular dynamics (MD) simulations with a reactive force field called ReaxFF, it was found that tribopolymerization reactions of allyl alcohol at a sliding interface of silicon oxide appear to be associated with deformation of adsorbed molecules.15 This result implies that the physical process governing ∆% ∗ must be shear-induced deformation of the adsorbed molecules.15 It was also found that such deformations are facilitated when molecules are chemisorbed on the solid surface. This finding suggests that the substrate and counter-surface are not just an inert boundary or wall that confines the molecules and delivers the mechanical energy from the external mechanical actuator to the molecule being sheared at the interface; they are involved in mechanochemical reactions as an active participant. The involvement of the solid substrate is expected when mechanochemical reactions result in removal of substrate atoms such as wear of silicon, silicon oxide, silicate glass, and gallium arsenide in humid environments or wear of carbon materials upon rubbing against silicon in vacuum.28-33 However, it is not straightforward to see the involvement of the solid substrate in association reactions among adsorbed molecules by mechanical shear.9, 11-13, 24 So, this study designed and performed an experimental investigation to test if the solid surface chemistry plays a critical role in mechanochemical polymerization of adsorbate molecules. αPinene was chosen as a model adsorbate system since it was found to have a good mechanochemical reactivity in previous studies.12-13 Eight different substrates were chosen based 5 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
on their activity or inertness in various catalytic reactions – Pd, Ni, Co, SS, Au, SiO2, Al2O3, and diamond-like carbon (DLC). All experimental works were carried out in ambient pressure conditions; thus, Ni, Co, and SS are covered with native oxide layers (NiO, CuO, and Cr2O3, respectively).34-36 Pd is also covered with a monolayer of oxide.37-38 Even the Au surface could contain oxide species when cleaned with UV/O3.39 The DLC surface is also partially oxidized in ambient air.40 The comparison of tribopolymer yield of α-pinene on these surfaces under vapor phase lubrication (VPL) conditions clearly revealed the critical role of the surface chemistry in the activation of adsorbed molecules in mechanochemical reactions.
Experimental method A custom-built reciprocating ball-on-flat tribometer with an environment control capability was employed to conduct all tribo-tests. Details of the system was described previously elsewhere.41 Eight substrates were tested in this study. A silicon (100) wafer was purchased from Wafer World, Inc. (West Palm Beach, FL, USA). The silicon wafer surface was covered with 1-2 nm thick native oxide.42-43 A hydrogenated diamond-like carbon (DLC) film (approximately 1 µm thick) was deposited on a silicon wafer at Argonne National Laboratory.44 Details of the DLC deposition was described in a previous paper.45 An aluminum oxide wafer (with the (0001) c-plane) was purchased from University Wafer (Boston, MA, USA). Polycrystalline gold and palladium foils (~200 µm thick) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Thick copper, nickel, and AISI 440C stainless steel (SS) substrates were purchased from McMaster-Carr (Elmhurst, IL, USA). The Cu, Ni, and SS substrates were polished with fine grit sandpapers and then polishing slurry containing 1 µm colloidal alumina.
6 ACS Paragon Plus Environment
Page 6 of 31
Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Using optical profilometry (Zygo NewView 7300), the polished surfaces were found to have a root-mean-square (rms) roughness less than 30 nm. All the substrates were cleaned with ethanol followed by a UV/O3 treatment to make sure there is no organic residue before tribo-tests.46 The counter ball was made of silicon nitride (diameter = 3/32 inch; purchased from McMaster-Carr). Optical profilometry analysis showed that the rms roughness of the ball surface was around 10 nm after removal of the sphere curvature. Silicon nitride was chosen because its surface is relatively inert in the absence of water.47 It also has high elastic modulus so that its deformation would be small during tribo-tests. In friction tests, the normal load was adjusted to keep the Hertzian contact pressure at around 0.5 GPa for all materials. The Hertzian deformation depth varied in the range of 20 ~ 140 nm, depending on the substrate. For the load dependence test, the normal load was varied from ~0.4 GPa up to ~0.9 GPa. The Hertzian deformation depth of each sample was larger than the surface roughness of the sample; thus, two solid surfaces within the nominal contact area are indeed in intimate and conformal contact.48 The sliding speed was 0.4 cm/s and the sliding span was 2.5 mm. At this low speed, the flash temperature rise was estimated to be less than 5 ℃.12 The sliding contact region during the tribo-test was continuously purged with a mixture of dry nitrogen stream (with a dew point of -80oC and
