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Reactions of cationic gold clusters in a radio frequency ion trap under multi-collision conditions [Elektronische Ressource] : coadsorption phenomena and activation of molecular oxygen and methane / vorgelegt von Sandra Marianne Lang

179 pages
Reactions of Cationic Gold Clusters in aRadio Frequency Ion Trap underMulti-Collision Conditions: CoadsorptionPhenomena and Activation of MolecularOxygen and MethaneDissertationzur Erlangung des Doktorgrades Dr. rer. nat.der Fakult¨at fu¨r Naturwissenschaftender Universit¨at Ulmvorgelegt vonSandra Marianne Langaus Wu¨rzburg2009amtierender Dekan: Prof. Dr. Peter B¨auerle1. Gutachter: Prof. Dr. Thorsten M. Bernhardt2. Gutachter: Prof. Dr. Harold JonesTag der Promotion: 17.07.2009Man sieht oft etwas hundert Mal, tausend Mal,ehe man es zum allerersten Mal wirklich sieht.Christian MorgensternSummaryThe aim of the present work was to determine reactivity patterns and general conceptsthatareimportanttotheanalysisofthecatalyticC H andCH oxidationreactionson3 6 4gold particles by experimentally investigating small mass selected gold cluster cationsin the gas-phase under well defined reaction conditions.+InafirstsetofexperimentsthetemperaturedependentreactivityofAu (x=2-7)xtowards several for the hydrocarbon oxidation relevant small molecules (O , H , N ,2 2 2CH , and C H ) were investigated separately and unexpected reactant and cluster size4 3 6dependent reactivity patterns were identified. While the gold cations were found to+be completely unreactive towards molecular oxygen, Au show a distinct temperaturexdependent reactivity towards H and N .
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Reactions of Cationic Gold Clusters in a
Radio Frequency Ion Trap under
Multi-Collision Conditions: Coadsorption
Phenomena and Activation of Molecular
Oxygen and Methane
Dissertation
zur Erlangung des Doktorgrades Dr. rer. nat.
der Fakult¨at fu¨r Naturwissenschaften
der Universit¨at Ulm
vorgelegt von
Sandra Marianne Lang
aus Wu¨rzburg
2009amtierender Dekan: Prof. Dr. Peter B¨auerle
1. Gutachter: Prof. Dr. Thorsten M. Bernhardt
2. Gutachter: Prof. Dr. Harold Jones
Tag der Promotion: 17.07.2009Man sieht oft etwas hundert Mal, tausend Mal,
ehe man es zum allerersten Mal wirklich sieht.
Christian MorgensternSummary
The aim of the present work was to determine reactivity patterns and general concepts
thatareimportanttotheanalysisofthecatalyticC H andCH oxidationreactionson3 6 4
gold particles by experimentally investigating small mass selected gold cluster cations
in the gas-phase under well defined reaction conditions.
+InafirstsetofexperimentsthetemperaturedependentreactivityofAu (x=2-7)x
towards several for the hydrocarbon oxidation relevant small molecules (O , H , N ,2 2 2
CH , and C H ) were investigated separately and unexpected reactant and cluster size4 3 6
dependent reactivity patterns were identified. While the gold cations were found to
+be completely unreactive towards molecular oxygen, Au show a distinct temperaturex
dependent reactivity towards H and N . Low temperature saturation measurements2 2
allowed for the determination of active adsorption sites on the different cluster sizes.
+Au was found to be the only investigated cluster size which yielded reaction products2
with methane that prove the activation and dehydrogenation of CH . The measure-4
+ments of temperature dependent reaction kinetics of all Au with CH permitted the4x
+determination of Au -CH binding energies for the first time. In contrast to the mod-4x
erate reaction behavior towards H , N , and CH , the C H adsorption was found to2 2 4 3 6
proceed extremely fast even at very low C H partial pressures and at room tempera-3 6
ture.
The conflicting results of the facile adsorption of propylene and methane on the
one hand and the lack of adsorption of molecular oxygen on the other hand, seemed
to preclude the potential successful partial oxidation of hydrocarbons on cationic gold
clusters. The strategy pursued in the present investigations to nevertheless initiate the
adsorption of molecular oxygen was to take advantage of coadsorption effects. In this
context, the first comprehensive study on coadsorption effects on gold cations was per-
formed, revealing reactant, cluster size, and temperature dependent competitive and
cooperative adsorption effects. Additionally, a hitherto unknown new coadsorption
phenomenononfreemetalclusters, termed’permissivecoadsorption’wasfound, which
describes the mutually cooperative adsorption of two reactants on the same adsorp-
tion site. Among the performed studies, the investigation of the H /O coadsorption2 2
turned out to be of special interest. Most surprisingly, the cooperative coadsorption
and activation of molecular oxygen on even size gold clusters after preadsorption of
molecular hydrogen was observed. With the help of molecular dynamics simulations,
performed by U. Landman and coworkers, it was even possible to determine a reaction
path of this cluster size selective reaction.
+ +On the basis of the H /O coadsorption experiments, Au and Au were identified2 2 2 4
as suitable model systems for the very first gas-phase experiments performed in the
presence of a mixture of three reactants, namely H , O , and CH . Most interestingly,2 2 4
both cluster sizes are able to cooperatively and competitively coadsorb all three reac-
+tants. Due to the ability of Au to activate and dehydrogenate methane, fundamental2
+ +differences in the reaction mechanisms of Au and Au were revealed and potential2 4
partial oxidation products were identified.Contents
1 Introduction 1
2 Concepts of Gas-Phase Reaction Kinetics in an Ion Trap 5
2.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Kinetic Evaluation Procedure . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Low Pressure Reaction Kinetics . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Langevin Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.5 RRK- and RRKM-Theory . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 Computational Procedure to Obtain Binding Energies 15
3.1 RRKM Fitting Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1.1 Experimental Kinetic Data . . . . . . . . . . . . . . . . . . . . . 15
3.1.2 ’MassKinetics’ Input . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.3 Energized Molecule and Transition State Models . . . . . . . . . 19
3.1.4 Error Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2 Experimental O Binding Energies . . . . . . . . . . . . . . . . . . . . 222
3.2.1 Cluster Composition Dependence . . . . . . . . . . . . . . . . . 22
3.2.2 Cluster Size Dependence . . . . . . . . . . . . . . . . . . . . . . 28
3.3 Experimental CO Binding Energies . . . . . . . . . . . . . . . . . . . . 29
+4 Reactions of Au with Small Molecules 33x
4.1 Reactions with O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
4.2 Reactions with H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
4.2.1 Temperature Dependent H Adsorption . . . . . . . . . . . . . . 352
4.2.2 Molecular H Adsorption . . . . . . . . . . . . . . . . . . . . . . 382
+4.2.3 Au Binding Sites . . . . . . . . . . . . . . . . . . . . . . . . . 38x
4.3 Reactions with N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432
4.4 Reactions with C H . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 6
4.5 Reactions with CH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454
+4.5.1 Methane Activation on Au . . . . . . . . . . . . . . . . . . . . 472
4.5.2 Temperature and Pressure Dependent CH Adsorption . . . . . 524
4.5.3 Reaction Kinetics of CH Adsorption . . . . . . . . . . . . . . . 544
4.5.4 CH Binding Energies . . . . . . . . . . . . . . . . . . . . . . . 624
4.6 Reactant Dependent Reactivity . . . . . . . . . . . . . . . . . . . . . . 68
III Contents
5 Coadsorption Phenomena 71
5.1 Hindered Coadsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.2 Competitive Coadsorption . . . . . . . . . . . . . . . . . . . . . . . . . 75
+ +5.2.1 H /N Coadsorption on Au and Au . . . . . . . . . . . . . . 752 2 3 5
5.2.2 Low Temperature N /O Coadsorption . . . . . . . . . . . . . . 822 2
5.2.3 Competitive H /H O and H O/CH Cl Coadsorption . . . . . . 852 2 2 3
5.3 Permissive H /CH Coadsorption . . . . . . . . . . . . . . . . . . . . . 902 4
5.4 Cooperative H /O Coadsorption . . . . . . . . . . . . . . . . . . . . . 962 2
5.4.1 Low Temperature Cooperative H /O Coadsorption . . . . . . . 962 2
5.4.2 Oxygen Activation on Even x Cluster Sizes . . . . . . . . . . . . 98
5.4.3 Theoretical Treatment . . . . . . . . . . . . . . . . . . . . . . . 104
6 Influence of Methane Activation on H /O /CH Coadsorption 1132 2 4
+6.1 Reactivity of Au towards a Mixture of H , O , and CH . . . . . . . . 1142 2 42
+6.2 Reactivity of Au towards a Mixture of H , O , and CH . . . . . . . . 1242 2 44
6.3 Implications for Real Catalysis . . . . . . . . . . . . . . . . . . . . . . . 130
7 Conclusion 135
8 Experimental 139
8.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
8.2 Cold Reflex Discharge Ion Source (CORDIS) . . . . . . . . . . . . . . . 142
8.3 Octopole Ion Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
8.3.1 Design and Thermalization . . . . . . . . . . . . . . . . . . . . . 145
8.3.2 Trapping Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
Bibliography 149
Zusammenfassung 161
List of Publications 163
Acknowledgment 166
Curriculum Vitae 168

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