Steric Effects in the Chemisorption of Vibrationally Excited Methane on Nickel

185 Pages·2012·4.6 MB·English

by  Bruce L. Yoder (auth.)

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Bruce L. Yoder
Steric Effects in the
Chemisorption of
Vibrationally Excited
Methane on Nickel
Doctoral Thesis accepted by
the École Polytechnique
Fédérale de Lausanne, Switzerland
123
Author Supervisor
Dr. BruceL. Yoder Prof.Rainer D.Beck
Universityof BritishColumbia Laboratory ofMolecular
MainMall 2036 Physical Chemistry
Vancouver Swiss Federale InstituteofTechnology
BC V6T1Z1
(EPFL), Station6
Canada
1015Lausanne, Switzerland
ISSN 2190-5053 e-ISSN 2190-5061
ISBN 978-3-642-27678-1 e-ISBN978-3-642-27679-8
DOI 10.1007/978-3-642-27679-8
SpringerHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2011946087
(cid:2)Springer-VerlagBerlinHeidelberg2012
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Parts of this thesis have been published in the following articles:
B.L. Yoder, R. Bisson, R.D. Beck, Steric effects in the chemisorption of vibra-
tionally excited methane on Ni(100). Science 329(5991), 553–556 (2010)
B.L. Yoder, R. Bisson, P.M. Hundt, R.D. Beck, Alignment dependent chemi-
sorption of vibrationally excited CH (v ) on Ni(100), Ni(110), and Ni(111).
4 3
J. Chem. Phys. 135, p. 224703, (2011)
The article published in 2011 should be considered more up to date than the data
contained in this thesis. The reader should defer to the JCP article for any minor
discrepencies found between it and this thesis.
Supervisor’s Foreword
Stericeffectsdescribehowtherateofachemicalreactiondependsontherelative
orientationofthereactionpartners.Thedetectionandexplorationofstericeffects
are central to the field of chemical reaction dynamics, which seeks to describe
chemicalreactionsatamicroscopiclevelintermsofindividualreactivecollisions
between reagent molecules. This thesis uncovers and explores a previously
unknown steric effect in the dissociative chemisorption of methane on a nickel
surface. The reaction is of critical importance for the chemical industry because
the dissociation of methane on the surface of a nickel catalyst is the rate limiting
step in the large scale transformation of natural gas into hydrogen used as clean
fuel and as starting material for many other synthesis processes such as ammonia
synthesis leading to synthetic fertilizers.
The experiments described in this thesis probe the reactivity of methane under
highly defined conditions which specify most of the collision parameters such as
speed of the incident molecules, their angle of incidence, as well as the compo-
sition and structure of the target surface, using a molecular beam of methane
colliding with different faces of a Ni-single crystal in ultrahigh vacuum. Infrared
laser excitation of the surface incident methane makes it possible to control the
reagent’s rotational and vibrational quantum state and probe how the methane
reactivity depends on vibrational and rotational excitation.
In his thesis research described here, Bruce Yoder goes beyond what has been
done in previous quantum state resolved gas/surface reaction dynamics experi-
ments and also exerts control over the steric conditions. An intense molecular
beam of vibrationally excited methane was produced by rapid adiabatic passage
exploiting a new, continuous-wave, high-power, single-mode infrared optical
parametric oscillator. During the infrared excitation of the antisymmetric (m )
3
stretch vibration of CH or the C–H (m ) stretch of CD H by linearly polarized
4 1 3
radiation, the angular momentum and vibrational transition dipole moment of
methanearealignedinthelaboratoryframe.Theexcited,alignedmolecularbeam
was used to probe the stereodynamics of the chemisorption reaction of vibra-
tionally excited methane.
vii
viii Supervisor’sForeword
For methane dissociation on Ni(100), an increase in methane reactivity of
nearly 60% is observed when the laser polarization is changed from normal to
parallel to the surface. The dependence of the alignment effect on the rotational
branch used for excitation (P-, Q-, or R-branch) indicates that alignment of the
vibrational dipole moment rather than the angular momentum is responsible for
the alignment dependent reactivity. Dephasing of the initially prepared alignment
duetohyperfinecouplingisobservedtobeonthetimescaleof5–15 lswhichdoes
notprecludethestudyofalignmentdependentreactivityinourexperimentalsetup.
Reactivity decreased monotonically from parallel to perpendicular alignment for
both methane isotopologues studied. The alignment effect is shown to be inde-
pendent of incident velocity for CH (m ) and appears to decrease with increasing
4 3
velocity of CD H(m ).
3 1
For the Ni(110) surface, which consists of parallel rows of closely spaced Ni
atomsseparatedbyone-layerdeeptroughs,Yoderprobedifthereactivitydepends
on the methane alignment relative to the direction of the surface rows. The
CH (m ) reactivity increases by a factor of two when the laser polarization
4 3
directionischangedfromnormaltoparalleltothesurface.Furthermore,alignment
ofthevibrationperpendiculartothesurfacerowsproducedahigherreactivitythan
alignment parallel to the surface rows.
The detailed reactivity data described in this thesis provides stringent tests for
theoretical models of methane/surface reactivity and thereby will aid the devel-
opmentofa better predictive theoretical descriptionof methane/surface reactivity
which in turn may lead to better catalysts and more energy efficient synthesis
processes.
Switzerland, November 2011 Prof. Rainer D. Beck
Acknowledgments
MytimeinLausannehasbeenawonderfulexperienceandIowemuchofittomy
friends and colleagues. I would like to address my sincere gratitude to all men-
tioned here for the valuable contributions they brought to this thesis work and to
me.
To my thesis advisor, Prof. Rainer Beck: Thank you for taking me into your
group,foralwayspushingmetodobetter,foryouravailability,yourinsight,your
example, and your patience. I have learned a lot through working with you.
To Prof. Thomas Rizzo: Thank you for accepting me into the Laboratoire de
Chimie Physique Moléculaire and for helpful advice during my doctoral work.
A Dr. Régis Bisson: Merci de m’avoir enseigné comment fonctionne notre
laboratoire,pourtonenthousiasme,dem’avoirapprisàparlerfrançais,etpourton
amitié.
A Marianne Dang: Merci beaucoup. Tu m’as tellement aidé.
To Dr. Thanh Tung Dang, Dr. Marco Sacchi, Li Chen and Morten Hundt:
Thankyoufor making ourgroup afriendly,positive place toworkand forall the
constructive input.
ToDr.MathieuSchmidandDr.PlinioMaroni:Thankyouforconstructingthe
surface science machine and developing the experimental setup to such a nice
form.
To my thesis committee members Prof. Fleming Crim, Prof. Andreas
Osterwalder, Prof. Bret Jackson and Prof. Vassily Hatzimanikatis: Thank you.
To the entire group at the Laboratoire de Chimie Physique Moléculaire: I am
fortunatetohaveworkedwithsuchadynamic,internationalgroup.Iwouldliketo
particularlyacknowledgeDr.MarcelDrabbels,GiorgosPapadopoulos,Dr.Monia
Guidi, Dr. Annette Svendsen, Ulrich Lorenz, Dr. Evgeniy Loginov, Maxim
Grechko, Dr. Jaime Stearns, Prof. Tino Gäumann and Dr. Oleg Boyarkine.
Al’ateliermécanique:Mercipourtouslesprojetsbienréussis.Caatoujoursété
unplaisirdetravailleravecvous.J’aimeraisremercierenparticulierAndréFattet,
Yves Morier, Gil Corbaz, Christophe Clement, Roger Mottier et Cédric Mora.
ix
x Acknowledgments
A l’atelier électronique: Merci pour votre aide. Essentiellement Patrick Favre,
Supardi Sujito, Frederic Gumy, Gabriel Roch et Pierre-André Perroud.
A mon amour, Maya Chelmis: Merci. Tu m’as donné plus que tu ne le sais.
Sans toi, je n’y serait jamais arrivé. Je t’aime.
And finally, to my family- Beverly (mom), Dwight (dad), Derrick (Pete), Beth
and Theodore: Thank you for your unconditional love and support. You are the
stability of my life.
Switzerland, July 2010 Bruce L. Yoder