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Title: n-alkanes on Pt(111) and on C(0001)/Pt(111): Chain Length Dependence of Kinetic Desorption Parameters

Abstract

We have measured the desorption of seven small n-alkanes (CNH2N+2, N = 1-4, 6, 8, 10) from the Pt(111) and C(0001) surfaces by temperature programmed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption pre-exponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups havemore » yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
901763
Report Number(s):
PNNL-SA-52516
Journal ID: ISSN 0021-9606; JCPSA6; 2652a; KC0301020; TRN: US200715%%57
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics, 125(23):Art. No. 234308; Journal Volume: 125; Journal Issue: 23
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ALKANES; DESORPTION; ENTROPY; GRAPHITE; PLATINUM; SUBSTRATES; KINETICS; MOLECULAR WEIGHT; MATHEMATICAL MODELS; Environmental Molecular Sciences Laboratory

Citation Formats

Tait, Steven L., Dohnalek, Zdenek, Campbell, Charles T., and Kay, Bruce D. n-alkanes on Pt(111) and on C(0001)/Pt(111): Chain Length Dependence of Kinetic Desorption Parameters. United States: N. p., 2006. Web. doi:10.1063/1.2400235.
Tait, Steven L., Dohnalek, Zdenek, Campbell, Charles T., & Kay, Bruce D. n-alkanes on Pt(111) and on C(0001)/Pt(111): Chain Length Dependence of Kinetic Desorption Parameters. United States. doi:10.1063/1.2400235.
Tait, Steven L., Dohnalek, Zdenek, Campbell, Charles T., and Kay, Bruce D. Thu . "n-alkanes on Pt(111) and on C(0001)/Pt(111): Chain Length Dependence of Kinetic Desorption Parameters". United States. doi:10.1063/1.2400235.
@article{osti_901763,
title = {n-alkanes on Pt(111) and on C(0001)/Pt(111): Chain Length Dependence of Kinetic Desorption Parameters},
author = {Tait, Steven L. and Dohnalek, Zdenek and Campbell, Charles T. and Kay, Bruce D.},
abstractNote = {We have measured the desorption of seven small n-alkanes (CNH2N+2, N = 1-4, 6, 8, 10) from the Pt(111) and C(0001) surfaces by temperature programmed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption pre-exponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.},
doi = {10.1063/1.2400235},
journal = {Journal of Chemical Physics, 125(23):Art. No. 234308},
number = 23,
volume = 125,
place = {United States},
year = {Thu Dec 21 00:00:00 EST 2006},
month = {Thu Dec 21 00:00:00 EST 2006}
}
  • Coverage-dependent desorption kinetics parameters are obtained from high quality temperature programmed desorption (TPD) data for seven small n-alkane molecules on MgO(100). The molecules, CNH2N+2 (N = 1-4, 6, 8, 10), were each studied for a set of 29 initial coverages at a heating ramp rate of 0.6 K/s as well as at a set of nine ramp rates in the range 0.3 to 10.0 K/s. The inversion analysis method with its least-squares prefactor optimization discussed in the accompanying article is applied to these data. This method allows for accurate determination of prefactors and coverage-dependent desorption energies. The pre-exponential factor formore » desorption increases dramatically with chain length from 1013.1 to 1019.1 s-1 over the range N = 1-10. We show that this increase can be physically justified by considering the increase in rotational entropy available to the molecules in the gas-like transition state for desorption. The desorption energy increases with chain length as Ed(N) = 6.5 + 7.1 N, which implies an incremental increase of 7.1?0.2 kJ/mol per CH2.« less
  • The initial probabilities of precursor-mediated, dissociative chemisorption of the saturated hydrocarbons {sup 13}C-labeled ethane, propane, isobutane, {ital n}-butane, and neopentane on the close-packed Ir(111) surface have been measured. The selective activation of primary (1{degree}), secondary (2{degree}), and tertiary (3{degree}) C--H bonds has been quantified by examining the reactivities of the selectively deuterated isotopomers of propane, C{sub 3}H{sub 8}, CH{sub 3}CD{sub 2}CH{sub 3}, and C{sub 3}D{sub 8}, and of isobutane, (CH{sub 3}){sub 3}CH, (CH{sub 3}){sub 3}CD, and (CD{sub 3}){sub 3}CH. With respect to the bottom of the physically adsorbed well for each hydrocarbon, the apparent C--H bond activation energies have beenmore » found to be 10.4{plus_minus}0.3 kcal/mol (ethane), 11.4{plus_minus}0.3 kcal/mol (propane), 11.5{plus_minus}0.3 kcal/mol ({ital n}-butane), 11.3{plus_minus}0.3 kcal/mol ({ital i}-butane), and 11.3{plus_minus}0.3 kcal/mol (neopentane). For all the alkanes examined, the ratios of the preexponential factors of the rate coefficients of reaction and desorption are 1{times}10{sup {minus}2}. The C--D bond activation energies are higher than the corresponding C--H bond activation energies by 480 cal/mol (ethane), 630 cal/mol (propane), and 660 cal/mol ({ital i}-butane). By analyzing the primary kinetic isotope effects for the selectively deuterated isotopomers of propane and isobutane, the 2{degree} C--H bond activation energy is found to be 310{plus_minus}160 cal/mol less than the 1{degree} C--H bond activation energy on this surface, and similarly, 3{degree} C--H bond cleavage is less by 80{plus_minus}70 cal/mol. The quantification of the branching ratios within the C--H bond activation channel for propane and isobutane on this surface shows that the formation of 1{degree}-alkyl intermediates is, in general, favored over the formation of either 2{degree}- or 3{degree}-alkyl intermediates. (Abstract Truncated)« less
  • A grazing incidence x-ray diffraction study of CH[sub 3](CH[sub 2])[sub [ital n][minus]1]SH self-assembled on a Au(111) surface shows that self-assembly results in a nonequilibrium state with a specific defect structure. We also explore the global ([ital n],[ital T]) phase diagram whose phases are distinguished by the tilt direction and the two-dimensional periodicity. These phases result in distinct long ([ital n][gt]14) and short ([ital n][lt]14) chain-length regimes, and reflect the relative importance of the hydrocarbon and interface interactions in these systems.
  • The adsorption/desorption behavior of n-butane and isobutane on Pt(111) and the p(2[times]2) Sn/Pr(111) and ([radical]3[times] [radical]3)R30[degrees] Sn/Pt(111) surface alloys has been examine using a combination of adsorption kinetics measurements utilizing a collimated molecular beam and temperature programmed desorption (TPD) mass spectroscopy. Initial sticking probabilities for both molecules on Pt(111) and the surface alloys at temperatures below the monolayer desorption threshold are essentially unity (S[sub 0] [>=] 0.95). The monolayer saturation coverages of n-butane and isobutane were also independent of the amount of Sn in the surface layer. The desorption activation energies measured by TPD for the monolayer states of bothmore » n-butane and isobutane progressively decrease by 5-8 kJ/mol compared to Pt(111) as the surface concentration of Sn increases from 0.25 to 0.33 atom fraction in the respective surface alloys. The decrease in the desorption activation energy scales linearly with the Sn concentration. No thermal decomposition of either molecule on any surface occurred during TPD measurements. Molecular interactions probed by adsorption and desorption of saturated C[sub 4] hydrocarbons are not influenced as strongly by the presence of Sn in the Pt(111) surface as previously observed for unsaturated molecules, such as ethylene and isobutylene. 30 refs., 9 figs.« less
  • We compare the growth dynamics of the three n-alkanes C{sub 36}H{sub 74}, C{sub 40}H{sub 82}, and C{sub 44}H{sub 90} on SiO{sub 2} using real-time and in situ energy-dispersive x-ray reflectivity. All molecules investigated align in an upright-standing orientation on the substrate and exhibit a transition from layer-by-layer growth to island growth after about 4 monolayers under the conditions employed. Simultaneous fits of the reflected intensity at five distinct points in reciprocal space show that films formed by longer n-alkanes roughen faster during growth. This behavior can be explained by a chain-length dependent height of the Ehrlich-Schwoebel barrier. Further x-ray diffractionmore » measurements after growth indicate that films consisting of longer n-alkanes also incorporate more lying-down molecules in the top region. While the results reveal behavior typical for chain-like molecules, the findings can also be useful for the optimization of organic field effect transistors where smooth interlayers of n-alkanes without coexistence of two or more molecular orientations are required.« less