J. Phys. Chem. 1993, 97, 10783-10789
10783
Fluorine Substituent Effects on Alkoxide Chemistry and Orientation on the Cu( 100) Surface Qing h i Department of Chemistry, University of Illinois, Urbana, Illinois 61801 Andrew J. Cellman’ Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 1521 3 Received: May 25, 1993; In Final Form: July 28, 1993”
The straight chain hydrocarbon alcohols (CH3(CH2),OH, n = 0-4)and fluorocarbon alcohols ( C F ~ ( C F ~ ) , C H Z O H , n = 0-2) all adsorb reversibly on the clean Cu(100) surface. As the alkyl chain is lengthened, the heats of adsorption of the hydrocarbon alcohols are incremented by 1.2 kcal/mol per CH2 group while the heats of adsorption of the fluorinated alcohols are incremented by 0.7 kcal/mol per CF2 group. On the preoxidized Cu(100) surface the alcohols are all deprotonated to form alkoxides. The intensity of the vco mode in the H R E E L spectra suggests that the C - 0 bond lies along the surface normal in methoxide and the propoxides but is tilted toward the surface in ethoxides, butoxides, and pentoxide. During heating the alkoxides decompose by &hydride elimination to yield aldehydes. Fluorination of the alkyl chain in the r-position has a dramatic influence on the reaction kinetics, resulting in an increase in the &hydride elimination barrier from 26 kcal/mol in hydrocarbon alkoxides to 42 kcal/mol in the fluorinated alkoxides. This is consistent with a description of the transition state in which charge separation is of the form C$+”’H” and the influence of the fluoroalkyl group is to energetically destabilize the cationic @-carbon in the transition state.
1. Introduction
The chemistry of short chain alcohols on Cu surfaces has been studied in some depth as a result of their importance in catalytic chemistry.Iq2 Longer straight chain alcohols are also of interest because they are representative of amphiphiles which have numerous technological applications. The initial interest which stimulated the work reported in this paper was their use in tribological applications. The straight chain alcohols are representative of the amphiphilic materials used as boundary layer additives in lubricant f l ~ i d s . ~These - ~ amphiphiles adsorb from solution onto metal surfaces to form surfactant-like films which prevent direct contact between metal surfaces during sliding. The use of fluorocarbons as high-temperature lubricant fluids requires the development of fluorocarbon amphiphiles for use as boundary layer additives under high temperature conditions.&* One of the goals of this work has been to understand the influence of alkyl chain fluorinationon the surface chemistry of the alcohols. From a more fundamental perspective, comparison of the fluorocarbon and hydrocarbon alcohols provides insight into the surface chemistry of the alcohols in general. The strong dipole moment of theC-Fbondenables us tousevibrational spectroscopy to determine the orientation of the CF3 group in some of the alkoxides. Furthermore, the electronegativity of the CF3 group causes it to have a strong influence on the kinetics of reactions involving ionic transition states or intermediates. We have found that fluorination of the alkyl chain in alcohols does not change the mechanisms of their surface reactions however it does have a profound effect on some reaction kinetics. As a result we are able to use the substituenteffects of fluorine in trying tounderstand reaction kinetics. Specifically, in this paper we address the structure of the alkoxides generated from straight chain alcohols and the transition state for the @-hydrideelimination reaction which initiates alkoxide decomposition. The chemistry of methanol and ethanol on the Cu( 100) surface has been studied using high resolution electron energy loss spectroscopy (HREELS)and is quite similar to their surface chemistry on the other Cu surfaces and on Ag.l,239-11The 0
Author to whom correspondence should be addressed. Abstract published in Advance ACS Absrracrs. September 15, 1993.
0022-365419312097- 10783$04.00/0
dominant reaction path on the clean Cu( 100) surface is molecular adsorption at low temperatures and desorption during heating. Y,~CHZR
H,O,CH~R
\H
‘0’
CHzR
/”
/ / / / / / / / / /
On preoxidized surfaces methanol and ethanol are deprotonated to form water and adsorbed methoxide and ethoxide, respectively. During heating water desorbs at -200 K leaving the alkoxide on the surface. The alkoxides then decompose by &hydride elimination in the temperature range 370-400 K to form aldehydes which desorb into vacuum. R
HZO f
R
/
f H
/ / / / / / / / / / / / / / / / / /
In discussing @-hydrideelimination, we refer to the breaking of the C-H bond which is in the /3 position with respect to the surface and is adjacent to the oxygen atom in the alkoxide. Decomposition of alkoxides by this mechanism is observed quite commonly on metal surfaces and is discussed in ref 12. In spite of the many important applications of fluorocarbons, very little has been done to understand their surface chemistry. Quite recently we have been able to show that fluorine substitution into adsorbed molecules serves as a means to probe surface transition states. In a number of cases we have found that fluorine does not change the mechanisms of adsorbate reactions. However, it does influence reaction kinetics by destabilizing cationic transitions states and stabilizing anionic transition states. The influence of fluorine has been observed in @-hydrideelimination reactions of ethoxides and propyl groups on the C u ( l l 1 ) surface,13J4deprotonationofaceticacids on the Ag( 110)surface,I5 and alkyl group coupling reactions on the Ag( 1 11) surface.I6 In addition to using fluorine substituent effects to understand surface reaction kinetics, we are able to address issues of surface structure of the alkoxides. In this paper we are able to use the 0 1993 American Chemical Society
Dai and Gellman
10784 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 V C F and ~ vco intensities to examine the structure of the alkoxides on the Cu( 100) surface. The strong dipole moment of the C-F bond results in strong dipole active losses from the fluorinated alkyl chain. As a result their intensity depends upon the orientation of the alkyl chain with respect to the surface.
2. Experimental Section The experiments were all performed in a single ion pumped UHV chamber equipped with an Ar+ ion sputter gun, a retarding field analyzer for Auger electron spectroscopy (AES), and low energy electron diffraction (LEED), a quadrupole mass spectrometer for desorption measurements, and a high-resolution electron energy loss spectrometer. Two capillary array gas dosers attached to leak valves were used for introduction of gases into the chamber. The sample was a Cu crystal cut and polished to expose the (100) surface. It was attached to a sample holder by spot welding between two Ta wires. The sample holder was then mounted in contact with two liquid nitrogen reservoirs at the end of a manipulator. In a UHV chamber the sample could be cooled to 1000 K. The temperature was measured by a thermocouple attached to the side of the crystal and was controlled during heating by a computer. The surface was cleaned by cycles of Ar+ ion sputtering followed by annealing to 900 K in vacuum. The oxidized surface was prepared by holding thecrystal at 470 K while being exposed t o ~ x y g e n A . ~ 1-langmuir oxygen exposure was used in order to maximize the amount of alkoxide generated by subsequent reaction with the alcohols. The alcohols and fluorinated alcohols were high-purity CH3(CH2),0H (n = 0-4)and CF3(CF2),CH20H (n= &2) obtained commercially, The alcohols were purified by multiple cycles of freezing, pumping, and thawing to remove high vapor pressure contaminants. Adsorption onto the clean surface was accomplished by introducing the vapor into the chamber through one of the capillary array dosers. Adsorption on the clean Cu( 100) surface was always done with the temperature at < 130 K and the sample immediately in front of the doser. The alkoxides were formed by adsorption of alcohols onto surfaces that had first been oxidized and then heating to 250 K to desorb water and leave the alkoxide. Desorption experiments were performed using a shrouded quadrupole mass spectrometer. The sample was positioned within 2-3 mm of the aperture to the ionizer and then heated at 5 K/s with the temperature ramp controlled by computer. The mass spectrometer was capable of recording signals for up to five m / q ratios simultaneously. The vibrational spectra were obtained by using an LK-200010 HREEL spectrometer. Following surface preparation the sample was cooled to 100K before obtaining the spectrum. Spectra were collected in multiple scans with collection times of 1 s/point and an incident beam energy of 4.1 eV. 3. Results
3.1. Alcohol Adsorption on the Cu(100) Surface. All the hydrocarbon and fluorocarbon alcohols used in this work adsorb molecularly a t low temperature (
