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Mechanistic insights into the preferential CO oxidation in H2-rich gas (PROX) over supported noble metal catalysts [Elektronische Ressource] / Markus Matthias Schubert

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218 pages
Universität UlmAbteilung Oberflächenchemie & KatalyseMechanistic Insights into thePreferential CO Oxidation in H -rich Gas (PROX)2over Supported Noble Metal CatalystsDissertationvorgelegt vonMarkus Matthias Schubert2000:PaX]Abteilung Oberflächenchemie & KatalyseUniversität UlmMechanistic Insights into thePreferential CO Oxidation in H -rich Gas (PROX)2over Supported Noble Metal CatalystsDissertationzur Erlangung des Doktorgrades Dr. rer. nat.der Fakultät für Naturwissenschaftender Universität Ulmvorgelegt vonMarkus Matthias Schubertaus MünchenUlm 2000Der experimentelle Teil dieser Arbeit wurde in der Zeit von Oktober 1996 bis September1999 in der Abteilung Oberflächenchemie und Katalyse der Universität Ulm angefertigt.Wissenschaftlicher Betreuer: Prof. Dr. R. J. Behm (Leiter der Abteilung Oberflächenchemie & Katalyse)Amtierender Dekan: Prof. Dr. O. Marti1. Gutachter: Prof. Dr. R. J. Behm2. Gutachter: Prof. Dr. J. GarcheTag der Promotion: 11. Mai 2000CONTENTSI. Introduction1. The selective CO oxidation - reactions..............................................................................12. Technical application: PROX............................................................................................33. Intention of this work.........................................................................................................5II. Experimental1. Setup..........................................................................
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Universität Ulm
Abteilung Oberflächenchemie & Katalyse
Mechanistic Insights into the
Preferential CO Oxidation in H -rich Gas (PROX)2
over Supported Noble Metal Catalysts
Dissertation
vorgelegt von
Markus Matthias Schubert
2000:PaX]Abteilung Oberflächenchemie & Katalyse
Universität Ulm
Mechanistic Insights into the
Preferential CO Oxidation in H -rich Gas (PROX)2
over Supported Noble Metal Catalysts
Dissertation
zur Erlangung des Doktorgrades Dr. rer. nat.
der Fakultät für Naturwissenschaften
der Universität Ulm
vorgelegt von
Markus Matthias Schubert
aus München
Ulm 2000Der experimentelle Teil dieser Arbeit wurde in der Zeit von Oktober 1996 bis September
1999 in der Abteilung Oberflächenchemie und Katalyse der Universität Ulm angefertigt.
Wissenschaftlicher Betreuer: Prof. Dr. R. J. Behm
(Leiter der Abteilung Oberflächenchemie & Katalyse)
Amtierender Dekan: Prof. Dr. O. Marti
1. Gutachter: Prof. Dr. R. J. Behm
2. Gutachter: Prof. Dr. J. Garche
Tag der Promotion: 11. Mai 2000CONTENTS
I. Introduction
1. The selective CO oxidation - reactions..............................................................................1
2. Technical application: PROX............................................................................................3
3. Intention of this work.........................................................................................................5
II. Experimental
1. Setup....................................................................................................................................6
1.1 Reaction gases & mixture facility....................................................................................6
1.2 The plug-flow reactor.......................................................................................................9
1.3 Infrared spectroscopy - DRIFTS.....................................................................................12
1.3.1 DRIFTS-theory.......................................................................................................13
1.3.2 DRIFTS-setup.........................................................................................................15
1.3.3 Processing of DRIFTS spectra...............................................................................17
1.4 On-line GC analysis.......................................................................................................20
1.5 Other methods used........................................................................................................22
1.5.1 XPS.........................................................................................................................22
1.5.2 On-line IMR-MS analysis.......................................................................................22
1.5.3 Isotope studies / pulse experiments.........................................................................23
1.5.4 XRD.........................................................................................................................23
2. Catalyst characterization.................................................................................................24
2.1 Platinum catalysts...........................................................................................................24
2.2 Pt Sn catalyst..................................................................................................................243
2.3 Gold catalysts.................................................................................................................25
iIII. Results & discussion
1. The standard catalyst - Pt/γ-Al O ..................................................................................262 3
1.1 The influence of the CO coverage on the activity / selectivity.......................................27
1.1.1 Selectivity & hydrogen induced rate enhancement.................................................27
1.1.2 Experimental limitations - neglecting the reaction rate.........................................29
1.1.3 Adsorption of CO in pure N ..................................................................................322
1.1.4 Adsorption of CO in H / N ...................................................................................362 2
1.1.5 Coadsorption behaviour of CO and H ..................................................................392
1.2 From idealized to more realistic conditions...................................................................46
1.2.1 Self-poisoning by CO ...........................................................................................46ad
1.2.2 Influence of water...................................................................................................48
1.2.3 Performance in CO -rich gas.................................................................................562
1.3 Other support materials..................................................................................................63
1.3.1. Comparison of activity / selectivity........................................................................63
1.3.2. The water-gas shift - limiting the CO conversion..................................................66
1.4 Summary on platinum catalysts......................................................................................71
2. Improved performance on bimetallic systems - Pt Sn/Vulcan.....................................733
2.1 Low-temperature CO oxidation over Pt Sn/Vulcan.......................................................753
2.1.1 Superior performance on Pt Sn/Vulcan vs Pt/ -Al O ...........................................753 2 3
2.1.2 Alloying effects........................................................................................................78
2.1.3 Coverage dependence of the selectivity..................................................................82
2.1.4 Catalyst state during conditioning & reaction.......................................................84
2.1.5 Mechanistic proposal - avoiding the CO poisoning...............................................89
2.2 Other important characteristics for an application in a PROX-stage.............................93
2.2.1 Long-term stability.................................................................................................93
2.2.2 Performance in more realistic reformates..............................................................99
2.2.3 The water-gas shift activitiy of Pt Sn/Vulcan.......................................................1033
2.3 Summary on bimetallic Pt Sn catalyst..........................................................................1053
ii
g
g3. An alternative for the low temperature conversion - Au/α-Fe O ............................1072 3
3.1 Mechanistic insight into the CO oxidation over gold catalysts....................................109
3.1.1 A short review.......................................................................................................109
3.1.2 The „support-effect“.............................................................................................111
3.1.3 Oxygen supply from the FeO -support (Transient response measurements).......116x
3.1.4 Proposed reaction scheme....................................................................................121
3.2 The influence of structural parameters on the activity / selectivity..............................123
3.2.1 The dependence of the activity on the particle size..............................................123
3.2.2 Influence of the calcination temperature..............................................................129
3.2.3 The dependence of the particle size on the sodium content..................................132
3.3 The CO coverage on Au and its influence on the kinetics...........................................134
.............................................................................................1343.3.1 Kinetic observations
....................1373.3.2 Comparison of the CO coverage on Pt/ -Al O versus Au/ -Fe O2 3 2 3
3.3.3 Temperature dependence......................................................................................143
3.4 Deactivation behaviour.................................................................................................147
3.4.1 „Off-line deactivation“.........................................................................................147
3.4.2 „On-line deactivation“ during CO oxidation......................................................148
a) Irreversible poisoning of more crystalline samples...........................................150
b) Reversible deactivation on amorphous samples................................................155
c) Deactivation model.............................................................................................157
3.5 Effects of CO and H O on the activity / selectivity....................................................1602 2
3.5.1 Selectivity enhancement by water.........................................................................160
3.5.2 Performance in CO -containing mixtures............................................................1632
3.6 Reverse water-gas shift over Au/ -Fe O ....................................................................1702 3
3.7 Other support materials................................................................................................172
3.8. Summary on gold catalysts..........................................................................................177
IV. Summary & future perspectives
iii
a
a g
aV. References
VI. Acknowledgement
Appendix
A) Improved DRIFTS-cell design......................................................................................198
B) Additional figures..........................................................................................................204
C) English abstract.............................................................................................................205
D) Deutsche Zusammenfassung (german abstract).........................................................207
E) List of publications........................................................................................................209
F) Lebenslauf (curriculum vitae).......................................................................................210
ivThe slective CO oxidation - reactions
I. INTRODUCTION
1. The selective CO oxidation - reactions
The following work is concerned with a detailed mechanistic investigation of the selective
(preferential) oxidation of CO over supported noble metal catalysts in hydrogen-rich gases. As
will be specified in the following chapter, this reaction has an important commercial
application for the purification of hydrogen feed gas streams for low temperature fuel cells
produced by methanol steam reforming.
Although in the past years numerous studies on the pure CO oxidation (i. e., in UHV, a
nitrogen or a helium background) on noble metal surfaces have appeared (e. g., [1-3] and
many others) only a few pulications were concerned with effects arising from coadsorbates
and parallely occuring surface reactions (e. g., [4-7]). Moreover all the latter studies use a
phenomenological approach by kinetics, but in most cases provide no mechanistic
explanations for the observed reaction orders and rates, which consequently is one of the
central intentions of this work.
For the preferential CO oxidation in H -rich gases two competing, parallely occuring2
processes have to be considered, the CO oxidation reaction
0
CO + ½ O fi CO ; H = -283 kJ/mol [8] (I-1.1)2 2
and the H oxidation reaction2
0H + ½ O fi H O ; H = -242 kJ/mol [8] (I-1.2)2 2 2
The selectivity, S, indicates, which of the two reactions is favoured by the kinetics. For
practical reasons it has been defined via the oxygen consumption:
CO-−oxidationCO ΔOr 2
S == == (I-1.3)−-CO H H oxidationCO-−oxidation2 2r + r ΔO + ΔO2 2
X X
r = oxidation rate of CO and H , resp.; D O = oxygen consumed by oxidation reactions2 2
In principle for our complex reaction mixture, which contains CO, O , H and the products2 2
CO and H O, also a few other reactions must be taken into consideration, e. g., methanation2 2
or methanol formation. Fortunately, on the catalyst systems, which are investigated in the
following chapters, most of these secondary reactions occur only above the applied operation
temperatures at a significant rate, e. g., over Pt/ -Al O methanation sets in only for2 3
1
g
D D
D
D
DThe slective CO oxidation - reactions
temperatures above 250°C, but the optimum temperature range for the selective CO oxidation
is around 200°C [9]. The only interference worth noticing arises from the water-gas shift
(denoted as „WGS“ in the following) reaction:
0CO + H CO + H O ; H = +41 kJ/mol [8] (I-1.4)2 2 2
Although the measured rates for the WGS reaction (forward and reverse direction) are at least
2 - 3 orders of magnitude smaller than the CO oxidation rates (see, e. g., chapter III-1.3.2), the
reverse WGS sets the limit for the minimum CO concentration, which may be achieved by the
selective CO Oxidation [10-12].
2
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