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Solid ‘oxygen reservoirs’ for selective hydrogen oxidation
Beckers, J.
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‘oxygen reservoirs’ for selective hydrogen oxidation.
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ABOUT THE AUTHORBefore starting his PhD at the University of
Amsterdam in 2005, Jurriaan Beckers (Reimerswaal, 1972) obtained
his bachelor's degree at the Hogeschool Zeeland in Vlissingen and
worked at the Agrotechnologisch Onderzoeksinstituut (ATO-DLO) in
Wageningen, Lyckeby Starch in Kristianstad, Sweden, GE Plastics in
Bergen op Zoom and at the University of Amsterdam. Jurriaan
currently lives in Amsterdam with his wife and nine guitars. His
hobbies are music and writing. On Sunday afternoon you can often fi
nd him at the Crea Café in Amsterdam.
Solid 'oxygen reservoirs' for selective hydrogen oxidation
Jurriaan BeckersISBN/EAN:978-90-9024476-1Cover design: S.J. de
Vet
Solid 'oxygen reservoirs' for selective hydrogen oxidation
Jurriaan Beckers 2009
Solid 'oxygen reservoirs' for
selective hydrogen oxidation
UITNODIGING
Voor het bijwonen van de openbare verdediging van
mijn proefschrift:
op dinsdag 22 septemberom 12.00 uur
in de Agnietenkapel van deUniversiteit van AmsterdamOudezijds
Voorburgwal 231
1012 EZ Amsterdam
na afl oop bent u van hartewelkom bij de receptie
ter plaatse
Paranimfen:Arjen Boogaard
[email protected]
Lars van der [email protected]
Jurriaan BeckersMuntendamstraat 421091DV Amsterdam
[email protected]
-
Solid ‘oxygen reservoirs’ for selective hydrogen oxidation
Jurriaan Beckers
-
Solid ‘oxygen reservoirs’ for selective hydrogen oxidation
ACADEMISCH PROEFSCHRIFT ter verkrijging van de graad van
doctor
aan de Universiteit van Amsterdam, op gezag van de Rector
Magnificus
prof. dr. D.C. van den Boom ten overstaan van een door het
college voor promoties ingestelde commissie,
in het openbaar verdedigd in de Agnietenkapel
op dinsdag 22 september 2009 om 12 uur
door
Jurriaan Beckers geboren te Reimerswaal
-
Promotiecommissie Promotor: Prof. dr. G. Rothenberg Promotor:
Prof. dr. C.J. Elsevier Overige leden: Prof. dr. K.J. Hellingwerf
Prof. dr. F. Kapteijn Prof. dr. ir. B.M. Weckhuysen Dr. A.F. Lee
Dr. M. Ruitenbeek Dr. G. Zwanenburg
Faculteit der Natuurwetenschappen, Wiskunde en informatica The
research reported in this thesis was carried out at the Van 't Hoff
Institute for Molecular Sciences, Faculty of Science, University of
Amsterdam (Nieuwe Achtergracht 166, 1018 WV Amsterdam), with
financial support of the Advanced Sustainable Processes by Engaging
Catalytic Technologies (ASPECT) programme, part of the Advanced
Chemical Technologies for Sustainability (ACTS) platform of the
Netherlands Organisation for Scientific Research (NWO).
-
Opgedragen aan mijn grootvader Drs. Hubert Maria Beckers
-
Contents
Chapter 1 Introduction 1.1 General introduction 11 1.2 What are
we dealing with? Some properties and pitfalls 12 1.3 Oxidative
dehydrogenation (ODH) using ceria based materials 16
1.3.1 Oxidative dehydrogenation of ethane 17 1.3.2 Oxidative
dehydrogenation of propane 21 1.3.3 Oxidative dehydrogenation of
other hydrocarbons 24 1.3.4 Combined dehydrogenation and selective
hydrogen combustion 26
Chapter 2 Selective hydrogen oxidation reactions using solid
‘oxygen reservoirs’ 2.1 Ceria-based selective hydrogen oxidation
catalysts via genetic
algorithms 37
2.2 Perovskites as solid oxygen reservoirs for selective
hydrogen oxidation 77 2.3 Lead-containing solid oxygen reservoirs
99
Chapter 3 Characterisation of solid oxygen reservoirs
3.1 Redox kinetics of ceria-based catalysts 119 3.2 Redox
properties of doped and supported copper-ceria catalysts 145 3.3
The optimisation and characterisation of bismuth doped ceria
catalysts 173 3.4 Particle size and dopant concentration effects on
the catalytic
properties of ceria based solid oxygen reservoirs 199
Summary 237 Samenvatting 241 List of publications 245 List of
abbreviations 249 Dankwoord 251 Appendix I Success of doping 257
Appendix II Catalyst activity 267
-
9
Chapter 1 Introduction
This work has been submitted to a peer-reviewed journal.
-
10
-
Chapter 1 Introduction
11
1.1 General introduction Cerium was discovered in 1803, by both
Jöns Jakob Berzelius and Wilhelm Hisinger in Sweden, and Martin
Heinrich Klaproth in Germany.[1] It was named after the dwarf
planet Ceres, discovered in 1801, which was in turn named after the
Roman goddess of agriculture (particularly the growth of
cereals).[2] Cerium is part of the ceric or light rare-earth
elements, and is the main component of the Bastnasite (USA, China),
Monazite (Australia) and Loparite (Russia) rare-earth minerals.[3]
Cerium or cerium-oxide (‘ceria’) is used in various applications,
such as the removal of free oxygen and sulphur from the melt in the
casting of iron, as a polishing agent for glass and for the
decolourisation of glass.[3, 4] Due to their good ionic conduction,
ceria-based materials can also be applied as electrolytes in solid
oxide fuel cells.[5] One of the most successful industrial
applications of ceria is as oxygen storage material in automotive
three way (catalytic) converters (TWC).[6-9] Plain ceria has been
used from the eighties onwards, and, in 2003, TWC sales accounted
for one quarter of the global catalytic market.[10] The successful
application of ceria in TWCs is due to its temperature stability
and its facile Ce3+ Ce4+ + e– redox reaction, allowing the ceria to
easily store and release oxygen.[11] In the catalytic converter,
this aids the hydrocarbon combustion in the fuel-rich mode, and the
NO reduction in the fuel-lean mode. The application of ceria-based
materials in TWCs has been reviewed by several authors.[7, 10, 12]
An excellent review on the physical and (electro-) chemical
properties of ceria-based oxides was published by Mogensen et al.
in 2001.[13] The group of Trovarelli has performed a lot of work on
the (redox) chemistry and catalysis of ceria and ceria-based
materials,[11, 14, 15] and a book on the subject was published in
2002.[3] The TWC is related to combustion engines running on
conventional fuel, with the ceria aiding full combustion and NO
reduction. In the last ten years, however, the research focus has
shifted to alternative power sources, such as fuel cells, and
alternative fuels, such as hydrogen. Furthermore, alternatives for
crude oil for the production of (fine) chemicals are sought, such
as the production of syngas from (bio-)methane, and the conversion
of these to larger molecules by the Fischer-Tropsch process.
Interestingly, in this field ceria-based materials have also gained
a lot of attention, but contrary to the TWC, most recent literature
focuses on selective oxidation applications rather than on full
combustion. Many of these
-
Chapter 1 Introduction
12
recent papers deal with selective oxidation of CO from
CO/hydrogen (preferential oxidation, PROX), often using
copper-ceria, to clean up the hydrogen used in fuel cells.[16-18]
The CO removal is needed since it poisons the Pt-catalyst at the
low operating temperatures used. Secondly, in the search for
alternative fuels and chemical building blocks, the generation of
syngas by partial oxidation of methane (POM), or methane steam
reforming has gained a lot of attention.[19-22] Ceria-based
materials have also been used for selective oxidations of small
molecules such as H2S to S or H2 to H2O2, and for selective
oxidations of various hydrocarbons.[23-28] In view of the topic of
this thesis, I will discuss the performance of ceria-based
materials in one type of selective oxidation, namely oxidative
dehydrogenation (ODH). This yields valuable small alkenes such as
ethene and propene. These can be used either as building blocks for
various chemicals or high purity monomers for plastics such as
polypropene and polyethene. Ceria-based materials come in many
forms. Ceria can act as support for ‘active metals’, but at low
concentrations these metals can also be doped in the ceria's
fluorite lattice, forming ‘solid solutions’. Ceria can also form
‘mixed oxides’ with other metal oxides, drastically changing its
catalytic properties. Importantly, the ceria's facile redox cycle
often results in a strong metal-support interaction (SMSI), which
can have a pronounced effect on the catalysis. I therefore start by
explaining this interaction, as well as outlining some basic
properties relevant to catalysis, before discussing ceria's role in
ODH.
1.2 What are we dealing with? Some properties and pitfalls There
are various routes for synthesising ceria,[3] but the simplest is
by calcining ceriumnitrate, Ce(NO3)3.[29, 30] At about 65 °C the
ceriumnitrate melts, followed by dehydration and, from about 200 °C
onwards, nitrate decomposition.[31, 32] No further weight loss
occurs above 400 °C, and this temperature is sufficient to form the
ceria fluorite structure.[32] At these low calcination
temperatures, the ceria's crystallite size is small, and the
surface area high. Ceria catalysts are often used at higher
temperatures, however, and increasing the calcination temperature
will increase the crystallite size, decreasing the specific surface
area. Typical values obtained when preparing ceria by calcining
ceriumnitrate are a crystallite size of 10 nm and surface area of
85 m2/g when
-
Chapter 1 Introduction
13
calcining at 550 °C, and a crystallite size of 25 nm and surface
area of 30-50 m2/g when calcining at 700 °C.[30, 33] High surface
area cerias can be obtained by applying sol-gel or surfactant
assisted synthesis methods.[34-36] With these techniques, surface
areas ranging from about 125 - 230 m2/g can be achieved, at
calcination temperatures of 800 °C and 450 °C, respectively. The
sintering behaviour of ceria is dependent on the gas atmosphere.
Practically, this means that when a ceria catalyst is used in a
reducing atmosphere, it can still sinter, even at temperatures
below its calcination temperature. The sintering behaviour of ceria
under various atmospheres is shown in Figure 1. All data was
obtained starting from the same batch of ceria.[37]
0
2040
6080100120
140
400 500 600 700 800 900
Temperature (°C)
Surfa
ce a
rea
(m2 /g
)
H2Vacuum
CO2H2OCO
0
2040
6080100120
140
400 500 600 700 800 900
Temperature (°C)
Surfa
ce a
rea
(m2 /g
)
H2Vacuum
CO2H2OCO
Figure 1. The specific surface area of plain ceria when heated
in various gasses (at atmospheric pressure) and under vacuum.[37]
All data is obtained starting from the same batch of ceria. Data of
the treatment in air is not added for clarity, but their trend is
similar to those obtained in vacuum. Reproduced with permission of
the author and the publisher. The high temperatures encountered in
automotive catalysis, typically 1000-1100 °C, sparked the search
for ceria-based materials with a higher temperature stability.
Ceria-zirconia mixed oxides have higher thermal stability and
excellent redox behaviour.[10, 33, 38-40] Indeed, ceria-based
materials are versatile since the ceria can be used not only as
support for active metals, but these metals can also be doped into
the ceria lattice itself, or form ceria containing mixed oxides.
Incorporation of dopant atoms in the ceria bulk allows for tuning
the oxygen
-
Chapter 1 Introduction
14
conduction, the electronic conduction, and with them the
catalytic properties.[13, 41, 42] Importantly, the distinction
between an active metal supported on ceria or doped into the ceria
lattice is not always clear, especially at elevated temperatures
and/or in the presence of reducing gasses. The facile redox of the
ceria can result in strong metal-support interaction when ceria is
used as support.[43-45] Phase segregation or a change in phase
composition can occur for ceria-based solid solutions and mixed
oxides.[39, 46] Thus, the active site can change during catalysis.
For example, when nickel supported on ceria is reduced at 750 °C,
the nickel crystallites can spread over the reduced ceria support
(see Scheme 1).[43] Conversely, at too high doping levels or
temperatures, a dopant can segregate from the ceria as a separate
oxide, or the catalyst's surface can be enriched in dopant
atoms.[47-50] Importantly, the spreading of nickel crystallites
over the ceria surface in case of a ceria support, and the surface
enrichment in case of the doped ceria could lead to similar surface
structures. These effects also complicate catalyst
characterisation. In case of XRD, for example, the absence of
dopant oxide crystals does not prove dopant incorporation in the
bulk. Indeed, several groups observed that when impregnating copper
on ceria supports, no copper oxide phases were detected, provided
that copper loading and calcination temperatures were kept low.[51]
Lattice doping can be demonstrated by using XRD, EXAFS or EPR.[48,
52-55] This does not exclude, however, that the surface of the
material, where catalysis takes place, is enriched with the dopant.
A small amount of dopant or dopant oxide clusters, or a dopant
enriched surface phase will not be detected by bulk techniques.
Surface sensitive techniques such as LEIS and XPS can detect
surface enrichment, and in case of XPS, the oxidation states of
surface components.[49] Their signal however, is still the average
of the entire catalyst surface, which can complicate things if
multiple types of surface species are present.
-
Chapter 1 Introduction
15
O Ce NiO Ce NiO Ce Ni
Scheme 1. Proposed spreading of nickel supported on ceria upon
reduction to 750 °C, drawn after Gonzalez-DelaCruz.[43] Another
type of strong metal-support interaction is the so-called
‘decoration’ of Pd, Pt and Rh by ceria observed upon reduction to
600-700 °C. Contrary to nickel, the supported metal crystals stay
intact, but are decorated with a layer of ceria upon the
high-temperature reduction, shielding the noble metal's surface and
thereby affecting the catalysis (see Figure 2).[44] Note that this
shielding effect can not be observed by, for example, XRD.
-
Chapter 1 Introduction
16
Figure 2. Metal decoration of 4 wt% Pt supported on ceria upon
reduction in hydrogen at 700 °C.[44] Reproduced with permission of
the author and the publisher. 1.3 Oxidative dehydrogenation (ODH)
using ceria-based materials The demand for small alkenes is high.
Propene demand, for example, is expected to rise to 80 million
tonnes in 2010 worldwide.[56-58] The main routes to propene are
steam cracking, fluid catalytic cracking, and catalytic
dehydrogenation. All these processes are endothermic. Advantageous
on-demand production of alkenes is achieved by catalytic
dehydrogenation, but it is equilibrium limited and deactivation of
the catalyst occurs due to coking (the formation of a carbon-rich
solid on the catalyst's surface).[59-61] Oxidative dehydrogenation
(ODH), where oxygen or an oxygen containing molecule such as N2O or
CO2 is added to the gas feed, can overcome these limitations,
allowing exothermic, non equilibrium-limited, and on-demand
production of the alkenes (see Scheme 2).[61] Furthermore, the
addition of oxygen limits catalyst deactivation due to coking. It
can, however, result in over-oxidation of the hydrocarbons to CO
and CO2.[62] This is a big challenge, since the alkene product is
more reactive than the alkane starting material. Thus, specific ODH
catalysts have to be developed, and up till now, (supported)
vanadium or molybdenum oxides have gained most attention.[61] For
ethane ODH, yields comparable to those of steam cracking are
obtained, but propane ODH yields are still far from being
interesting for industry. In both cases, little is reported on
catalyst lifetime.[61] In the search for better ODH catalysts,
-
Chapter 1 Introduction
17
various ceria-based materials have also been investigated. The
results are summarised in the following sections. Dehydrogenation
C3H8 ↔ C3H6 + H2 ΔHo460 °C = +130 kJ/mol Oxidative dehydrogenation
C3H8 + ½ O2 ↔ C3H6 + H2O ΔHo420 °C = –117 kJ/mol Scheme 2. Propane
dehydrogenation (top)[63] and oxidative propane dehydrogenation
(bottom)[64]
1.3.1 Oxidative dehydrogenation of ethane Table 1 gives the
catalyst composition and catalytic performance of ceria-based
materials as catalysts for ethane ODH (note that some data is taken
from figures, not tables). Doped ceria's (or solid solutions, where
the crystal structure of the ceria remains unchanged by the
addition of the dopant), or mixed oxides containing metal M and
ceria are denoted as ‘Ce–M–O’. Metal M supported on ceria is
denoted as ‘M/CeO2’. The catalysts generally give a high
selectivity towards ethene at low conversion, and lower
selectivities at high conversion. I have therefore incorporated the
data of the highest selectivity, highest activity, and highest
overall performance in the table, where available. The activity and
selectivity data in the table is presented graphically in Figure
3.
-
Chapter 1 Introduction
18
V 4
Ca 8
V 5
Sr 1
V 6
Ca 9
CeO2 10
Sr 2
CeO2 7
Sr 3
0
25
50
75
100
25 50 75 100
Activity (% ethane conversion)
Sele
ctiv
ity to
war
ds e
then
e (%
)
11
V 4
Ca 8
V 5
Sr 1
V 6
Ca 9
CeO2 10
Sr 2
CeO2 7
Sr 3
0
25
50
75
100
25 50 75 100
Activity (% ethane conversion)
Sele
ctiv
ity to
war
ds e
then
e (%
)
11
Figure 3. Activity and selectivity in ethane ODH. The labels
show the type of metal added to the ceria, and the catalyst number.
A Mo-V-Te-Nb-mixed oxide catalysts (11) is added for reference. The
data show that the best catalysts, achieving highest selectivity
and activity, are the calcium doped 9, the supported strontium 2
and 3 and the plain ceria 10. Interestingly, most of these
catalysts were tested under special conditions: steam was added in
case of 2, increasing the activity by dilution and the selectivity
by prevention of coking, and the ODH was performed with CO2 instead
of oxygen in case of 9 and 10.[65-67] The ODH with CO2 was
performed at rather high temperatures (≥750 °C), which is higher
than the temperature needed for the (endothermic) catalytic
dehydrogenation.[68] The vanadium-containing catalysts 4–6 run
below 600 °C, but also with lower activity and selectivity. The
supported Sr 3 has rather high selectivity and activity at 660 °C,
running without addition of steam, nor using CO2 as the
oxidant.[69] Their performance falls short, however, compared to
the best reference catalyst 11, a V and Mo containing mixed oxide,
which also operates at lower temperature (400 °C) and without steam
and/or CO2. (see Table 1 and Figure 3). Note however, that catalyst
11 is one of three catalysts, out of 70 tested, which displays this
good performance,[61] and that the other 67 catalysts have
substantially lower activity and selectivity. Far less research has
been performed on ceria-based materials. No data is available, for
example, for molybdenum-containing ceria-based materials, although
the data on Ce-V-O shows
-
Chapter 1 Introduction
19
that using components of the benchmark reference catalysts does
not guarantee good performance in ceria-based materials (vide
infra). Indeed, in a pre-screening for a combinatorial catalysis
approach, Ni-Ce-Nb and Ni-Ce-Ta mixed oxides were found to be the
most interesting leads, outperforming the best Mo-V-Nb oxide in
ethane ODH.[70] Up till now, no Ce-Ni-O or Ce-(Ni, Nb, Ta)-O
catalyst was tested for the ethane ODH. Interestingly, operando
studies with vanadia supported on ceria show that this system is
highly interactive, as was seen for nickel and noble metals
supported on ceria.[71-73] Starting from vanadia supported on
ceria, the vanadia interacts strongly with the ceria, eventually
forming a CeVO4 phase. Surprisingly, this process does not affect
the selectivity and activation energy in the ethane ODH.[73]
Possibly, the active phase consists of Ce3+-O-V5+, which is present
in both supported vanadia and the CeVO4. This also explains why the
catalytic properties of Ce-V-O materials differ from other vanadium
containing ODH catalysts (which are the ODH benchmark).
-
Cha
pter
1
Intr
oduc
tion
20
Ta
ble
1. C
eria
-bas
ed m
ater
ials
use
d fo
r eth
ane
OD
H.
Cat
alys
t nu
mbe
r C
atal
yst c
omp.
C
once
ntra
tion
adde
d m
etal
A
lkan
e:O
2[a]
Sp
ace
velo
city
(m
l/g.h
)[b]
Tem
pera
ture
(°
C)
Etha
ne
conv
ersi
on
(%)
Sele
ctiv
ity
tow
ards
eth
ene
(%)
Ref
eren
ce
1 Sr
/CeO
2 10
mol
%
6:1
(mol
ar)
1020
0 70
0 18
56
[c]
[74]
2
800
50
88[c
]
3 Sr
Cl 2/
CeO
2 30
mol
%
2:1
6000
66
0 73
69
[7
5]
4 V
/CeO
2 3
wt%
1:
2 90
000
510
1 69
[7
1-73
] 5
3
wt%
59
0 9
38
6
1
wt%
59
0 19
20
7 C
eO2 re
f
550[
d]
66
4
U
sing
CO
2 ins
tead
of o
xyge
n:
8 C
e-C
a-O
10
mol
%
1:2
(CO
2) 12
000
650
3 98
[e]
[68]
9
750
25
90
10
C
eO2 re
f.
750
41
71
Non
-cer
ia-b
ased
refe
renc
e ca
taly
st:
11
Mo-
V-T
e-N
b 1:
0.2:
0.17
:0.1
7 9:
6 10
0 40
0 87
84
[7
6]
[a] C
O2 w
hen
appl
icab
le. [
b] R
eact
ions
wer
e pe
rfor
med
at a
tmos
pher
ic p
ress
ure.
[c] S
team
was
add
ed a
t H2O
:C2H
6 = 1
:1 m
olar
ratio
. [d
] Sam
e pe
rfor
man
ce a
t 510
and
590
°C
. [e] T
his
is th
e va
lue
obta
ined
afte
r pre
-trea
ting
the
cata
lyst
at 7
50 °
C u
nder
the
reac
tion
cond
ition
s. Th
e fr
esh
cata
lyst
has
a se
lect
ivity
of ~
55%
, and
the
incr
ease
in se
lect
ivity
is ir
reve
rsib
le.
-
Chapter 1 Introduction
21
1.3.2 Oxidative dehydrogenation of propane The success of
nickel-containing catalysts in propane and isobutane ODH, has led
to the testing of nickel-containing ceria-based catalysts in
propane ODH.[77, 78] The group of Barbaux showed that using
nickel-containing ceria-based catalysts, ODH can be performed at
lower temperatures, as compared to when the other nickel catalysts
are used (300 °C).[77] Upon comparing ceria-nickel mixed oxides
with nickel supported on ceria, it was found that the supported
catalysts gave the highest selectivity, but low conversion (see
Table 2, 1). The mixed oxides give higher conversion, but lower
selectivity (2 and 3, all data taken at 300 °C). Nickel-containing
mixed oxides were found to be superior in yield as compared to
mixed oxides with either Cr, Co, Cu or Zn (catalysts 4, 5, and
6).[79] Note that the mixed oxides from both studies, at the same
composition and reaction temperature, differ strongly in activity
and selectivity (compare 2 and 5, note that both space velocity and
conversion of 5 are higher). The preparation methods of the
catalysts are very similar, except for the calcination temperature
(700 °C for 2, 500 °C for 5). As was the case for ethane ODH, the
supported vanadium catalysts perform less well than other
ceria-based mixed oxides (catalysts 7-9, Table 2).[78] Contrary to
the ethane ODH experiments shown in Table 1, the propane ODH is
performed at temperatures well below that of the catalytic propane
dehydrogenation, albeit at rather low conversion. No data is
available on the use of other oxidants, such as CO2, but the
selectivity of plain ceria is seen to increase substantially when
adding trichloromethane gas (10-12).[80] It is worthwhile to
investigate the effect of halogen addition on the catalyst
performance in propane ODH via a more practically applicable route,
such as using supported metal-halogens, as was done for the ethane
ODH.[75] As was the case in ethane ODH, the non-ceria-based
reference catalyst (13) outperforms the ceria-based ones. But
again, much more non-ceria-based catalysts have been tested, and
the large majority of these perform less well than 13.
-
Chapter 1 Introduction
22
Ni 3Ni 4
Ni 5 Ni 6
V 7
V 8
V 9CeO2 10
CeO2 TCM 12
13Ni(K) 1
Ni 2CeO2 TCM 11
25
50
75
100
0 25 50 75Activity (% propane conversion)
Sele
ctiv
ity to
war
ds p
rope
ne (%
)
Ni 3Ni 4
Ni 5 Ni 6
V 7
V 8
V 9CeO2 10
CeO2 TCM 12
13Ni(K) 1
Ni 2CeO2 TCM 11
25
50
75
100
0 25 50 75Activity (% propane conversion)
Sele
ctiv
ity to
war
ds p
rope
ne (%
)
Figure 4. Activity and selectivity in propane ODH. The labels
show the type of metal added to the ceria, and the catalyst number.
A supported vanadia catalyst (13) is added for reference.
-
Cha
pter
1
Intr
oduc
tion
23
T
able
2. C
eria
-bas
ed m
ater
ials
use
d fo
r pro
pane
OD
H.
Cat
alys
t nu
mbe
r C
atal
yst
com
p.
Con
cent
ratio
n ad
ded
met
al
Alk
ane:
O2
Alk
ane,
O2
conc
. (%
v/v)
Spac
e ve
loci
ty
(ml/g
.h)[a
]
Tem
pera
ture
(°
C)
Prop
ane
conv
ersi
on
(%)
Sele
ctiv
ity
tow
ards
pr
open
e (%
)
Ref
.
1 N
i-K/C
eO2
Ni:C
e =1
, K
:Ni =
0.05
[b]
1:2
4, 8
54
5 30
0 8
72
[77]
2 C
e-N
i-O
Ni:C
e =0
.5[b
]
15
58
3 C
e-N
i-O
Ni:C
e =1
[b]
19
60
4
Ce-
Ni-O
[c,d
] N
i:Ce
=0.5
[b]
1:3
5, 1
5 30
000
200
2 50
[7
9]
5
300
25
12
6
37
5 62
10
7 V
/CeO
2 12
wt%
1:
3 5,
15
5000
30
0 2
85
[78]
8
6
wt%
300
14
34
9
6
wt%
400
24
20
10
C
eO2
1:
1 14
, 13[
e]
3600
45
0 17
5
[80]
11
C
eO2+
TCM
17
% v
/v T
CM
[f]
1:1
14, 1
3[e]
23
52
12
CeO
2+TC
M
17%
v/v
TC
M
3.5:
1 14
, 4[e
]
17
70
[g]
N
on-c
eria
-bas
ed re
fere
nce
cata
lyst
:
13
V/M
CF[
h]
4.2
wt %
1:
1 10
, 10
7200
0 55
0 31
84
[8
1]
[a] R
eact
ions
wer
e pe
rfor
med
at a
tmos
pher
ic p
ress
ure.
[b]
The
se a
re a
tom
ic r
atio
s. [c
] Ref
eren
ce m
easu
rem
ents
wer
e pe
rfor
med
on
ceria
and
nic
kel o
xide
. C
eria
: 3%
con
vers
ion,
2%
sel
ectiv
ity (
300
°C),
10%
con
vers
ion,
6%
, se
lect
ivity
(40
0 °C
, no
te:
quite
clo
se t
o en
try 1
0).
NiO
10%
con
vers
ion,
17%
, se
lect
ivity
(35
0 °C
). [d
] Ni o
utpe
rfor
ms
sim
ilar
cata
lyst
s co
ntai
ning
Cr,
Co,
Cu
or Z
n. [
e] T
he c
once
ntra
tion
is in
kPa
inst
ead
of %
v/v.
[f] T
CM
sta
nds
for
trich
loro
met
hane
. [g] A
t thi
s oxy
gen
pres
sure
the
valu
es fo
r pla
in c
eria
are
: 7%
con
vers
ion,
10%
sele
ctiv
ity. [h
] MC
F st
ands
for M
isoc
ello
us S
ilica
Foa
ms.
-
Chapter 1 Introduction
24
1.3.3 Oxidative dehydrogenation of other hydrocarbons Besides
ethane and propane, ceria-based materials have been applied in
isobutane and ethylbenzene ODH (see Table 3). High selectivity and
conversion, at temperatures lower than those at which commercial
catalysts are used, were obtained for ethylbenzene ODH over plain
ceria, using N2O as oxidant (1).[82] The high activity was
attributed to a high concentration of Ce4+-O--Ce3+ defect sites.
Both doped and supported chromium-ceria catalysts were applied in
the isobutane ODH (see Figure 5).[83, 84] The chromium containing
catalysts show better results than plain ceria. The activity of
chromium supported on ceria is higher than plain chromium oxide and
the chromium ceria mixed oxide (this is the case at both 270 °C and
300 °C). Conversely, the selectivity of the chromium supported on
ceria is somewhat lower as compared to the ceria chromium mixed
oxides and plain Cr2O3. In case of the chromium-ceria systems, well
dispersed Cr6+-Ox, and not Cr2O3 aggregates, was proposed as the
active site, which was poisoned by the presence of
potassium.[84]
Cr/CeO2 2
Cr/CeO2 3
Ce-Cr-O 4
Ce-Cr-O 5
CeO2 6 CeO2 7
Cr2O3 8
Cr2O3 9
25
50
75
100
0 5 10 15 20 25
Activity (% isobutane conversion)
Sele
ctiv
ity to
war
ds is
obut
ene
(%)
Cr/CeO2 2
Cr/CeO2 3
Ce-Cr-O 4
Ce-Cr-O 5
CeO2 6 CeO2 7
Cr2O3 8
Cr2O3 9
25
50
75
100
0 5 10 15 20 25
Activity (% isobutane conversion)
Sele
ctiv
ity to
war
ds is
obut
ene
(%)
Figure 5. Isobutane ODH over chromium-ceria catalysts at 270 °C
(open circles) and 300 °C (full circles). The labels show the type
of metal added to the ceria, and the catalyst number. Plain ceria
and Cr2O3 are added for reference.
-
Cha
pter
1
Intr
oduc
tion
25
Ta
ble
3. C
eria
-bas
ed m
ater
ials
use
d fo
r eth
ylbe
nzen
e an
d is
obut
ane
OD
H.
Cat
alys
t nu
mbe
r C
atal
yst
com
p.
Con
cent
ratio
n ad
ded
met
al
Rea
ctan
t Sp
ace
velo
city
(m
l/g.h
)
Tem
pera
ture
(°
C)
Alk
ane
conv
ersi
on
(%)
Sele
ctiv
ity
tow
ards
al
kene
(%)
Ref
.
1 C
eO2
Et
hylb
enze
ne[a
] -
325
45
94
[82]
2
Cr/C
eO2
5-40
Cr a
tom
s / n
m2
Isob
utan
e[b]
-
270
10
54
[83]
3
300
20
36
4
Ce-
Cr-
O
Cr:C
e =
0.2-
1.8[
c]
270
5 60
5
Cr:C
e =
0.2-
1.8[
c]
300
10
45
6
CeO
2
270
5 20
7
30
0 10
20
8 C
r 2O
3
270
5 68
9
30
0 15
40
[a] N
2O w
as u
sed
inst
ead
of O
2. N
o da
ta o
n th
e re
acta
nt c
once
ntra
tions
is g
iven
. [b]
The
isob
utan
e : O
2 rat
io is
1 :
1, a
t 6.5
% v
/v.
[c] T
hese
are
(bul
k) a
tom
ic ra
tios.
-
Chapter 1 Introduction
26
1.3.4 Combined dehydrogenation and selective hydrogen combustion
Another type of ODH has been industrially implemented. Here, the
dehydrogenation is performed over a conventional dehydrogenation
catalyst, and a second catalyst is added to selectively combust
part of the hydrogen formed. The process may be therefore viewed as
‘two-step ODH’ (Scheme 3). The selective hydrogen combustion
generates heat and shifts the equilibrium to the products side,
yielding the same benefits as the ‘conventional’ ODH. The use of
two catalysts or two reactors, however, allows for separate tuning
of the dehydrogenation and the hydrogen oxidation reactions, and
the advantage of this over conventional ODH is proven by the
industrial implementation of the process. For example, the STAR
(Steam Active Reforming) oxydehydrogenation process is implemented
in two plants for the ODH of isobutane to MTBE. In this process, an
oxydehydrogenation reactor is placed after a STAR-dehydrogenation
reactor, both using the same catalyst.[85] The SMART process
(Styrene Monomer Advanced Reheat Technology) is in operation in
five plants for the ODH of ethylbenzene to styrene. This process
uses two catalysts in one reactor (the selective hydrogen
combustion catalyst is Pt-based).[86, 87] In both processes, steam
is added to minimise coking and dilute the feed. These processes
use a co-fed approach, where small amounts of oxygen are added to
the gas feed (Scheme 4, left). The mixing of gaseous oxygen with
hydrogen and hydrocarbons at elevated temperatures is, however, a
safety risk, which is avoided in the redox-mode (Scheme 4, right).
Here, no gaseous oxygen is added, but the lattice oxygen of the
selective hydrogen combustion catalyst is used. At one point in
time, however, the lattice oxygen is depleted and has to be
refilled. That is, the reactor has to be purged and an oxygen
containing feed has to be applied to the catalyst bed, resulting in
a cyclic process. Note that the conventional DH catalyst also has
to be regenerated periodically to burn off the coke accumulated on
its surface. In redox mode ODH of ethane and propane, high
selectivities towards hydrogen combustion can be obtained for
several supported oxides (e.g. Sb2O4, In2O3, WO3, PbO and
Bi2O3).[88-91] These are, however, unstable under the high
temperature redox cycling. The melting point of most of these
metals lies below the operating temperature, and when the supported
metal oxide is reduced to metal(0), it liquefies, causing sintering
and deactivation. Conversely,
-
Chapter 1 Introduction
27
ceria is stable under the redox cycling conditions, and has a
good oxygen storage capacity. The selectivity of plain ceria is
low, but in a screening experiment, working with
hydrogen/ethane/ethene mixtures, we showed that doping the ceria
lattice can overcome both the problems of low selectivity and low
stability.[92] In this thesis, we investigated the fundaments of
this oxidative dehydrogenation process.
Energy H2
H2O + Ce2O3 2 CeO2
Dehydrogenationcatalyst
Propane Propene
Energy H2
H2O + Ce2O3 2 CeO2
Dehydrogenationcatalyst
Propane Propene
Scheme 3. Catalytic cycle for redox mode oxidative
dehydrogenation using ceria as solid oxygen reservoir.
-
Chapter 1 Introduction
28
CnH(2n+2)
A
N2O2N2
B C D
N2O2COx
N2CnH2nH2O
Fresh SHC
Fresh DH
Spent SHC
Spent DH
ReoxidationReduction
CnH(2n+2) + O2
CnH2nH2O
Co-fed process Redox process
Purge Purge
CnH(2n+2)
A
N2O2N2
B C D
N2O2COx
N2CnH2nH2O
Fresh SHC
Fresh DH
Spent SHC
Spent DH
ReoxidationReduction
CnH(2n+2) + O2
CnH2nH2O
Co-fed process Redox process
Purge Purge
Scheme 4. Left: scheme of a co-fed mode oxidative
dehydrogenation process. Right: scheme of a redox-mode
dehydrogenation process (SHC: selective hydrogen combustion
catalyst). After the dehydrogenation step A, the bed is flushed
with nitrogen (B), and the catalysts are regenerated through
reoxidation (C). This burns coke from the dehydrogenation catalyst
and restores the lattice oxygen of the selective hydrogen
combustion catalyst. After another nitrogen flush (D) the reactor
is ready for the next redox cycle.
-
Chapter 1 Introduction
29
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Chapter 1 Introduction
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Chapter 1 Introduction
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-
34
-
35
Chapter 2 Selective hydrogen oxidation reactions using solid
‘oxygen reservoirs’
Experimental setup used to screen the catalysts performance in
the selective hydrogen oxidation reaction.
-
36
-
37
2.1 Ceria-based selective hydrogen oxidation catalysts via
genetic algorithms
Mg
Ca Cr MnTiK Fe Cu
Al
Pd SnRuZrYSr
Ta W Pt Pb Bi
YbSmNdPrLa Ce
Energy H2
H2O Ce1-x-yM1xM2yO2
Dehydrogenationcatalyst
Propane Propene
Gd
Mg
Ca Cr MnTiK Fe Cu
Al
Pd SnRuZrYSr
Ta W Pt Pb Bi
YbSmNdPrLa Ce
Energy H2
H2O Ce1-x-yM1xM2yO2
Dehydrogenationcatalyst
Propane Propene
Gd
Part of this work has been published as: - 'A “green route” to
propene through selective hydrogen oxidation', Jan Hendrik Blank,
Jurriaan Beckers, Paul F. Collignon, Frédéric Clerc, and Gadi
Rothenberg, Chem. Eur. J. 2007, 13, 5121. - 'Selective Hydrogen
Oxidation Catalysts via Genetic Algorithms', Jurriaan Beckers,
Frédéric Clerc, Jan Hendrik Blank, and Gadi Rothenberg, Adv. Synth.
Catal. 2008, 350, 2237.
-
Chapter 2.1 Catalysis
38
Abstract Solid ‘oxygen reservoirs’, such as doped ceria, can be
successfully applied in a novel process for propane oxidative
dehydrogenation. The ceria lattice oxygen selectively burns
hydrogen from the dehydrogenation mixture at 550 °C. This gives
three key advantages: it shifts the dehydrogenation equilibrium to
the desired products side, generates heat in situ, which aids the
endothermic dehydrogenation, and simplifies product separation. We
have applied a genetic algorithm to screen doped cerias for their
performance in the selective hydrogen oxidation. Five generations
of doped ceria catalysts (97 catalysts in total), were synthesised.
Dopants were chosen from a set of 26 elements, and with a maximum
of two dopants per catalyst, at five different concentrations. The
catalyst performance (activity and selectivity), is expressed by a
fitness value. The average fitness value increases from generation
one to three, and then stabilises. That is, the system converges
after three generations. The dopant type has a large effect on the
catalyst fitness. We identified six dopant atoms which lead to
selective hydrogen combustion catalysts, namely Bi, Cr, Cu, K, Mn,
Pb and Sn (‘good’ dopants). Analysis of the effect of
electronegativity, ionic radius and dopant concentration shows that
most elements yielding a high fitness have an electronegativity in
the range of 1.5–1.9. Generally, the properties of catalysts
containing two dopants can be predicted from the behaviour of
singly doped ones. Synergy does occur for certain copper and iron
containing catalysts. The addition of Ca or Mg to Cu doped
catalysts doubles the activity, and the selectivity of iron doped
catalysts can be improved by adding Cr, Mn or Zr. Importantly, the
doped cerias show a high stability in the redox cycling, much
higher than that of supported oxides. A Cr and Zr doped catalyst
(Ce0.90Cr0.05Zr0.05O2) was highly selective and active over 250
redox cycles (a total of 148 hours on stream), with no phase
segregation or change in particle size.
-
Chapter 2.1 Catalysis
39
Introduction Selective oxidation is applied in the production of
many important bulk chemicals and intermediates, such as acrolein,
acrylic acid, MTBE, and maleic anhydride.[1] In these processes,
the commercial value of the hydrocarbons is increased by selective
addition of oxygen atoms, and the greatest challenge is to prevent
over-oxidation. Selective oxidation can also, however, remove
certain species by combusting them. This is the case in oxidative
dehydrogenation, which can be used to obtain propene from propane
(Scheme 1).[2-9] Propane dehydrogenation is an endothermic
reaction, but oxidative dehydrogenation can overcome this
limitation. Selectively combusting the formed hydrogen into water
generates heat in situ and shifts the equilibrium towards the
products side.
Oxidation
Latticerecharged
Propane
Propene + H2O
O2
CO2
Energy H2
H2O + Ce2O3 2 CeO2
Dehydrogenationcatalyst
Propane Propene Oxidation
Latticerecharged
Propane
Propene + H2O
O2
CO2
Energy H2
H2O + Ce2O3 2 CeO2
Dehydrogenationcatalyst
Propane Propene
Scheme 1. Left: the reactions occurring during the combined
propane dehydrogenation and selective hydrogen combustion. The
dehydrogenation consumes energy and yields hydrogen. The hydrogen
combustion consumes hydrogen and yields energy. Right: Cartoon
showing a proposed reactor configuration for the redox process,
enabling continuous production of high purity propene. Whilst the
propane dehydrogenation and selective hydrogen combustion are
performed in the left hand reactor, the catalysts in the right hand
reactor are being regenerated (coke combustion and refilling of the
lattice oxygen for the solid oxygen reservoir).
-
Chapter 2.1 Catalysis
40
We recently introduced a new type of oxidative dehydrogenation
system, employing doped cerias as solid oxygen reservoirs
(SORs).[10, 11] The dehydrogenation step is performed over a
conventional Pt-Sn-Al2O3 catalyst, and the hydrogen is combusted
using the oxygen of the ceria lattice (Scheme 1, left).[12] After
the ceria is reduced, the lattice oxygen vacancies are re-filled
using air, creating a cyclic redox process (Scheme 1, right) and
simultaneously burning off any coke. This is safer than mixing
gaseous O2 and H2 at high temperatures (typically 500–600 °C).
Furthermore, the use of two catalysts allows for separate tuning of
the hydrogen combustion and the dehydrogenation. Supported metal
oxides can also perform this selective hydrogenation, but they
sinter under redox cycling.[13-17] Ceria has high temperature
stability and a facile Ce3+ Ce4+ + e– reaction, making it a good
SOR.[18] CeO2 itself, however, is not selective, but we showed that
selectivity, activity and stability can be tuned by doping the
ceria lattice with different cations.[10] In a preliminary
screening, we tested ten catalysts for their selectivity towards
hydrogen combustion from a mix with ethane and ethene, with the
dopant type as the only variable. A tungsten-doped catalyst showed
excellent selectivity and stability for this reaction.[10] In this
study, we screen doped ceria catalysts for hydrogen combustion from
a mix with propane and propene. Twenty-six different dopants are
used, at five possible concentrations, with a maximum of two
dopants per catalysts. This yields a huge amount of catalyst
candidates. Synthesising and testing all of the combinations is not
practical. Instead, we employ a genetic algorithm (GA), to find the
optimal catalyst using an iterative approach. GAs mimic
evolutionary biology in silico.[19-21] Compared with the almost
ubiquitous application of GAs in other scientific fields, few
researchers have used an evolutionary approach for screening
heterogeneous catalysts. Baerns et al. have studied propane
oxidative dehydrogenation, using gaseous oxygen,[22-24] total
propane oxidation,[25] and the production of hydrocyanic acid.[26]
Yamada et al. have investigated methanol synthesis,[27-29] and
others have performed studies on (selective) oxidation,[30-33]
reduction,[34] methane reforming,[35] and isomerisation.[36] Most
of the catalysts used in these studies are mixed oxides containing
four to five different metals. Kim et al. have used doped cerias
for the reforming of methane.[35]
-
Chapter 2.1 Catalysis
41
In this paper, we present the results of a genetic
algorithm-based catalyst optimisation. A total of 97 doped-cerias
are evaluated for their performance in selective hydrogen
combustion. The evolution of the fitness value over five
generations is evaluated, and the performance of the catalysts is
correlated to their composition.
Results and Discussion Catalyst preparation and
characterisation. Catalysts were prepared by co–melting a mixture
of the metal nitrate hydrate precursors (chlorides or ammonium
metallates were used when nitrates were not available).[11, 37]
After the precursor has liquefied, the pressure was lowered and a
solid mixed metal nitrate formed. This was converted into the mixed
oxide by calcining in static air at 700 °C for 5 h. The following
notation is used: Gn–m, where n is the generation number and m is
the catalyst number. The activity, selectivity and fitness value of
all 97 catalysts are given in Table 1, together with the catalyst
composition and characterisation data. The activity is determined
as the percentage of the hydrogen feed combusted by each catalyst
(labelled ‘hydrogen activity’). The fitness function is defined as:
F = [selectivity + (0.2 × activity)]/120 × 100, and ranges from
0–100. Note that the ‘hydrogen activity’ presented in Table 1 is
converted into a 0 – 100 scale prior to the calculation of the
fitness function. An activity-value of either 0, 33, 66 or 100 is
given to catalysts with hydrogen activities ranging from 0%,
0.1–7%, 7.1–21.2%, or are higher than 21.3%, respectively (these
values appear strange since they are based on a different unit of
activity, which we have replaced with the more informative hydrogen
activity). Specific attributes of some of the catalysts are
presented in detail thereafter. Figure 1 shows pictures of three
catalysts after heating at reduced pressure (top, mixed nitrates),
and after calcination (bottom, mixed oxides). Whereas pure cerium
nitrate is white, and CeO2 is pale yellow, the mixed oxides show a
variety of colours. These ceria based mixed oxides have the general
catalyst formula Ce1-x-yM1xM2yO2. The metals M1 and M2 are added at
zero, two, five, eight or ten mol%, and chosen from 26 candidates
(vide infra). Each catalyst was characterised using powder X-ray
diffraction, to ensure it consists of a uniform phase. That is,
-
Chapter 2.1 Catalysis
42
only the diffraction lines of the cerias fluorite structure are
observed, and no separate oxides of the dopants are present.
Importantly, the catalysts were not prepared by impregnating CeO2
supports. The co-melting of the cerium nitrate with the nitrates of
the appropriate metals yields a well mixed liquid catalyst
precursor. This allows for incorporation of the dopants into the
ceria fluorite structure after calcination. However, dopant
enriched surface phases can occur for these type of catalysts, and
cannot be detected by XRD.[38-40] Indeed, in case of a copper-ceria
mixed oxide, Bera et al. observed both surface enrichment and bulk
incorporation of the copper.[40]
Figure 1. Photos of three catalysts (G3–09 Ce0.96Mn0.02Cu0.02O2,
G3–10 Ce0.96W0.02Sn0.02O2 and G3–15 Ce0.90Bi0.08Cu0.02O2) after
heating under reduced pressure (top, mixed nitrates) and calcining
(bottom, mixed oxides).
-
Cha
pter
2.1
Cata
lysi
s
43
Ta
ble
1. C
ompo
sitio
n, c
hara
cter
isat
ion
data
and
cat
alyt
ic p
erfo
rman
ce o
f dop
ed c
eria
s 1–9
7.
Cat
alys
t C
ompo
sitio
n[a]
Su
rfac
e ar
ea
(m2 /g
) C
ryst
allit
e si
ze
(nm
)[b]
Latti
ce
para
met
er (Å
)[c]
Hyd
roge
n ac
tivity
(%
H2 c
ombu
sted
) H
2 oxi
datio
n se
lect
ivity
(%)
Fitn
ess
valu
e[d]
G
1–01
C
e 0.8
7Al 0.
08Ta
0.05
O2
58
n.d.
[e]
n.d.
0
0 0
G1–
02
Ce 0
.96C
a 0.0
2Sr 0
.02O
2 42
n.
d.
n.d.
0.
5 18
21
G
1–03
C
e 0.8
9Cr 0
.02F
e 0.0
9O2
28
n.d.
n.
d.
9 85
82
G
1–04
C
e 0.9
6Pd 0
.04O
2 53
n.
d.
n.d.
0
0 0
G1–
05
Ce 0
.98S
n 0.0
2O2
67
12
5.40
8 8
77
75
G1–
06
Ce 0
.89P
t 0.02
Mn 0
.09O
2 57
n.
d.
n.d.
0
0 0
G1–
07
Ce 0
.90T
a 0.0
5Ti 0.
05O
2 46
n.
d.
n.d.
0
0 0
G1–
08
Ce 0
.92Z
r 0.0
2Mg 0
.08O
2 26
n.
d.
n.d.
0
0 0
G1–
09
Ce 0
.90N
d 0.1
0O2
61
14
5.43
0 0
0 0
G1–
10
Ce 0
.90Y
b 0.0
8Gd 0
.02O
2 24
n.
d.
n.d.
1
40
39
G1–
11
Ce 0
.90R
u 0.0
5Sm
0.05
O2
65
n.d.
n.
d.
0 0
0 G
1–12
C
e 0.9
0Y0.
05Sr
0.05
O2
27
n.d.
n.
d.
0.4
44
42
G1–
13
Ce 0
.90B
i 0.05
K0.
05O
2 17
n.
d.
n.d.
18
[f]
91
87
G1–
14
Ce 0
.91L
a 0.0
9O2
49
n.d.
n.
d.
0 0
0 G
1–15
C
e 0.9
0W0.
10O
2 25
26
5.
411
0 0
0 G
1–16
C
e 0.9
2Cr 0
.08O
2 24
26
5.
414
15
86
83
G1–
17
Ce 0
.90F
e 0.1
0O2
50
14
5.40
4 0
0 0
G1–
18
Ce 0
.90C
u 0.1
0O2
47
15
5.41
1 7
89
85
G1–
19
Ce 0
.90B
i 0.10
O2
33
18
5.41
6 33
[f]
77
81
G1–
20
Ce 0
.91M
n 0.0
9O2
56
11
5.40
7 5
93
83
-
Cha
pter
2.1
Cata
lysi
s
44
Ta
ble
1, c
ontin
ued.
Cat
alys
t C
ompo
sitio
n[a]
Su
rfac
e ar
ea
(m2 /g
) C
ryst
allit
e si
ze
(nm
)[b]
Latti
ce
para
met
er (Å
)[c]
Hyd
roge
n ac
tivity
(%
H2 c
ombu
sted
) H
2 oxi
datio
n se
lect
ivity
(%)
Fitn
ess
valu
e[d]
G
1–21
C
e 0.9
1Ca 0
.09O
2 22
28
5.
416
0 0
0 G
1–22
C
e 0.9
2Pb 0
.08O
2 56
13
5.
411
46[f
] 92
93
G
1–23
C
e 0.9
0Pd 0
.10O
2 72
13
5.
411
0 0
0 G
1–24
C
eO2
36[g
] 25
[g]
5.40
9[g]
0
0 0
G1–
25
Ce 0
.90Z
r 0.1
0O2
71
n.d.
n.
d.
1 36
36
G
2–01
C
e 0.9
0Yb 0
.10O
2 n.
d.
18
5.40
6 0
0 0
G2–
02
Ce 0
.86C
a 0.0
9Cu 0
.05O
2 n.
d.
24
5.41
1 18
87
84
G
2–03
C
e 0.9
0Cr 0
.05B
i 0.05
O2
31
28
5.41
2 38
[f]
84
87
G2–
04
Ce 0
.87M
g 0.0
5Cu 0
.08O
2 n.
d.
18
5.40
9 17
87
84
G
2–05
C
e 0.9
0Mn 0
.02F
e 0.0
8O2
n.d.
16
5.
405
4 87
78
G
2–06
C
e 0.8
4Zr 0
.08C
u 0.0
8O2
54
17
5.41
1 13
92
88
G
2–07
C
e 0.8
7Bi 0.
08Sn
0.05
O2
55
14
5.41
1 45
[f]
84
87
G2–
08
Ce 0
.96G
d 0.0
2Bi 0.
02O
2 n.
d.
19
5.41
2 7
68
68
G2–
09
Ce 0
.88C
r 0.0
2W0.
10O
2 n.
d.
21
5.40
9 1
36
36
G2–
10
Ce 0
.90Z
r 0.0
2Fe 0
.08O
2 n.
d.
14
5.40
2 2
78
71
G2–
11
Ce 0
.90A
l 0.10
O2
n.d.
13
5.
408
0 0
0 G
2–12
C
e 0.9
0Al 0.
02C
u 0.0
5O2
52
14
5.40
9 11
89
85
G
2–13
C
e 0.9
0K0.
10O
2 n.
d.
93
5.41
1 3
94
84
G2–
14
Ce 0
.96P
r 0.0
2Gd 0
.02O
2 n.
d.
17
5.41
3 0
0 0
G2–
15
Ce 0
.94M
n 0.0
4Sr 0
.02O
2 n.
d.
13
5.40
9 0
0 0
-
Cha
pter
2.1
Cata
lysi
s
45
Ta
ble
1, c
ontin
ued.
Cat
alys
t C
ompo
sitio
n[a]
Su
rfac
e ar
ea
(m2 /g
) C
ryst
allit
e si
ze
(nm
)[b]
Latti
ce
para
met
er (Å
)[c]
Hyd
roge
n ac
tivity
(%
H2 c
ombu
sted
) H
2 oxi
datio
n se
lect
ivity
(%)
Fitn
ess
valu
e[d]
G
2–16
C
e 0.9
2Ti 0.
08O
2 n.
d.
16
5.40
8 0
0 0
G2–
17
Ce 0
.98L
a 0.0
2O2
n.d.
17
5.
415
0 0
0 G
2–18
C
e 0.9
6Pr 0
.02W
0.02
O2
n.d.
17
5.
412
0 0
0 G
3–01
C
e 0.9
8K0.
02O
2 n.
d.
53
5.41
1 2
88
79
G3–
02
Ce 0
.98Z
r 0.0
2O2
n.d.
21
5.
414
1 65
60
G
3–03
C
e 0.9
8Pr 0
.02O
2 n.
d.
18
5.41
1 0.
3 50
47
G
3–04
C
e 0.9
6Mn 0
.04O
2 59
13
5.
408
4 85
76
G
3–05
C
e 0.9
8Al 0.
02O
2 n.
d.
13
5.40
7 1
35
35
G3–
06
Ce 0
.93A
l 0.02
Yb 0
.05O
2 n.
d.
12
5.40
8 1
41
40
G3–
07
Ce 0
.96Z
r 0.0
2Cu 0
.02O
2 56
16
5.
409
6 95
85
G
3–08
C
e 0.9
6La 0
.02B
i 0.02
O2
n.d.
17
5.
418
9 87
84
G
3–09
C
e 0.9
6Mn 0
.02C
u 0.0
2O2
n.d.
13
5.
407
5 75
68
G
3–10
C
e 0.9
6W0.
02Sn
0.02
O2
n.d.
17
5.
402
6 92
82
G
3–11
C
e 0.9
6Gd 0
.05O
2 n.
d.
20
5.41
4 1
66
61
G3–
12
Ce 0
.98M
n 0.0
2O2
60
14
5.40
8 3
95
85
G3–
13
Ce 0
.98R
u 0.0
2O2
n.d.
16
5.
408
0 0
0 G
3–14
C
e 0.9
0Y0.
05Fe
0.05
O2
n.d.
14
5.
407
0 0
0 G
3–15
C
e 0.9
0Bi 0.
08C
u 0.0
2O2
28
25
5.41
5 29
[f]
83
86
G3–
16
Ce 0
.98P
t 0.02
O2
n.d.
16
5.
411
0 0
0 G
3–17
C
e 0.9
0Cr 0
.05Z
r 0.0
5O2
29
22
5.40
5 9
95
90
-
Cha
pter
2.1
Cata
lysi
s
46
Ta
ble
1, c
ontin
ued.
Cat
alys
t C
ompo
sitio
n[a]
Su
rfac
e ar
ea
(m2 /g
) C
ryst
allit
e si
ze
(nm
)[b]
Latti
ce
para
met
er (Å
)[c]
Hyd
roge
n ac
tivity
(%
H2 c
ombu
sted
) H
2 oxi
datio
n se
lect
ivity
(%)
Fitn
ess
valu
e[d]
G
3–18
C
e 0.9
6Sn 0
.02P
d 0.0
2O2
n.d.
16
5.
411
0 0
0 G
4–01
C
e 0.9
8Nd 0
.02O
2 n.
d.
19
5.41
4 1
74
67
G4–
02
Ce 0
.98Y
0.02
O2
n.d.
22
5.
410
2 72
66
G
4–03
C
e 0.9
8Sr 0
.02O
2 n.
d.
20
5.41
2 1
87
78
G4–
04
Ce 0
.98B
i 0.02
O2
n.d.
18
5.
411
10
94
89
G4–
05
Ce 0
.93K
0.02
Yb 0
.05O
2 n.
d.
36
5.41
0 3
81
73
G4–
06
Ce 0
.98W
0.02
O2
n.d.
19
5.
411
0 0
0 G
4–07
C
e 0.9
6Al 0.
02C
u 0.0
2O2
n.d.
13
5.
408
7 90
81
G
4–08
C
e 0.9
6Zr 0
.02F
e 0.0
2O2
n.d.
13
5.
405
0 0
0 G
4–09
C
e 0.9
5Al 0.
05O
2 n.
d.
11
5.40
9 2
56
52
G4–
10
Ce 0
.98C
a 0.0
2O2
n.d.
23
5.
413
1 93
83
G
4–11
C
e 0.9
5Ru 0
.05O
2 n.
d.
14
5.41
0 0
0 0
G4–
12
Ce 0
.93B
i 0.07
O2
n.d.
17
5.
416
26[f
] 82
85
G
4–13
C
e 0.9
3Al 0.
02La
0.05
O2
n.d.
12
5.
425
2 62
57
G
4–14
C
e 0.9
3Gd 0
.02Y
b 0.0
5O2
n.d.
17
5.
409
2 10
0 89
G
4–15
C
e 0.9
2Cu 0
.08O
2 n.
d.
14
5.41
1 6
90
81
G4–
16
Ce 0
.88M
n 0.0
2Cu 0
.10O
2 n.
d.
14
5.41
0 7
95
85
G4–
17
Ce 0
.90N
d 0.0
8Fe 0
.02O
2 n.
d.
14
5.42
2 0
0 0
G4–
18
Ce 0
.90B
i 0.07
Al 0.
02O
2 n.
d.
13
5.41
6 25
[f]
85
88
G5–
01
Ce 0
.98G
d 0.0
2O2
n.d.
20
5.
411
1 65
60
-
Cha
pter
2.1
Cata
lysi
s
47
Ta
ble
1, c
ontin
ued.
C
atal
yst
Com
posi
tion[
a]
Surf
ace
area
(m
2 /g)
Cry
stal
lite
size
(n
m)[b
] La
ttice
pa
ram
eter
(Å)[c
] H
ydro
gen
activ
ity
(% H
2 com
bust
ed)
H2 o
xida
tion
sele
ctiv
ity (%
) Fi
tnes
s va
lue[
d]
G5–
02
Ce 0
.96A
l 0.02
Pt0.
02O
2 n.
d.
15
5.41
0 0
0 0
G5–
03
Ce 0
.98T
i 0.02
O2
n.d.
19
5.
409
0 0
0 G
5–04
C
e 0.9
6K0.
02C
u 0.0
2O2
n.d.
45
5.
410
2 10
0 89
G
5–05
C
e 0.9
6Nd 0
.02S
n 0.0
2O2
n.d.
13
5.
409
8 89
85
G
5–06
C
e 0.9
6Cr 0
.02A
l 0.02
O2
n.d.
n.
d.
n.d.
1
76
69
G5–
07
Ce 0
.93M
n 0.0
2Cu 0
.05O
2 n.
d.
n.d.
n.
d.
7 86
77
G
5–08
C
e 0.9
5Y0.
05O
2 n.
d.
n.d.
n.
d.
1 54
51
G
5–09
C
e 0.9
3Gd 0
.02M
n 0.0
5O2
n.d.
14
5.
408
4 74
67
G
5–10
C
e 0.9
8Yb 0
.02O
2 n.
d.
18
5.40
8 2
55
51
G5–
11
Ce 0
.93P
r 0.0
2Zr 0
.05O
2 n.
d.
15
5.41
0 2
50
47
G5–
12
Ce 0
.92Z
r 0.0
8O
n.d.
13
5.
405
2 52
49
G
5–13
C
e 0.9
1Mn 0
.04S
r 0.0
5O2
n.d.
13
5.
411
0 0
0 G
5–14
C
e 0.8
8Cr 0
.08B
i 0.04
O2
n.d.
25
5.
410
36[f
] 83
86
G
5–15
C
e 0.8
6Gd 0
.08C
u 0.0
6O2
n.d.
18
5.
417
5 93
83
G
5–16
C
e 0.8
9Sn 0
.04L
a 0.0
7O2
n.d.
12
5.
431
16
92
88
G5–
17
Ce 0
.95C
a 0.0
3Pt 0.
02O
2 n.
d.
17
5.41
2 0
0 0
G5–
18
Ce 0
.96M
n 0.0
2Bi 0.
02O
2 n.
d.
13
5.40
9 13
83
80
[a
] Not
e th
at in
the
GA
, con
cent
ratio
ns o
f 2, 5
, 8, a
nd 1
0 m
ol%
are
use
d. [b
] Cer
ia h
as a
n av
erag
e cr
ysta
llite
siz
e of
25
nm (s
tand
ard
devi
atio
n =
4, n
= 4
). D
opin
g w
ith p
otas
sium
yie
lds
larg
er c
ryst
allit
es
(93
nm fo
r G2–
13, C
e 0.9
0K0.
10O
2, an
d 53
nm
for G
3–01
, Ce 0
.98K
0.02
O2)
. In
gene
ral,
how
ever
, dop
ing
decr
ease
s th
e cr
ysta
llite
siz
e. T
he a
vera
ge c
ryst
allit
e si
ze o
f the
rest
of t
he d
oped
cat
alys
ts is
17
nm
(sta
ndar
d de
viat
ion
= 6,
n =
78)
. [c]
Cer
ia h
as a
n av
erag
e la
ttice
par
amet
er o
f 5.4
094
Å (s
tand
ard
devi
atio
n =
0.00
08, n
= 4
). D
opin
g w
ith n
eody
miu
m y
ield
s a la
rger
latti
ce p
aram
eter
(5.4
30 Å
for G
1–09
, C
e 0.9
0Nd 0
.10O
2, an
d 5.
422
for G
4–17
, Ce 0
.90N
d 0.0
8Fe 0
.02O
2). A
lso,
cat
alys
t G4–
13, C
e 0.9
3Al 0.
02La
0.05
O2,
has a
val
ue o
f 5.4
25. T
here
are
no
trend
s, ho
wev
er, u
pon
dopi
ng, t
he a
vera
ge la
ttice
par
amet
er o
f the
re
st o
f the
dop
ed c
atal
ysts
is 5
.410
4 nm
(sta
ndar
d de
viat
ion
= 0.
0040
, n =
77)
. [d]
The
fitn
ess
valu
e is
def
ined
as:
F =
[sel
ectiv
ity +
(0.2
× a
ctiv
ity)]
/120
× 1
00, a
nd ra
nges
from
0 –
100
. Not
e th
at th
e ‘h
ydro
gen
activ
ity’ i
s co
nver
ted
into
a 0
– 1
00 s
cale
prio
r to
the
calc
ulat
ion
of th
e fit
ness
func
tion.
An
activ
ity v
alue
of e
ither
0, 3
3, 6
6 or
100
is g
iven
to c
atal
ysts
with
hyd
roge
n ac
tiviti
es ra
ngin
g fr
om
0%, 0
.1–7
%, 7
.1–2
1.2%
, or a
re h
ighe
r tha
n 21
.3%
, res
pect
ivel
y. [e
] Not
det
erm
ined
. [f]
The
se c
atal
ysts
con
vert
100%
of t
he h
ydro
gen
feed
at t
he b
egin
ning
of t
he re
duct
ive
cycl
e. T
his d
oes
not a
ffec
t the
to
tal a
ctiv
ity, h
owev
er, s
ince
all
of th
ese
cata
lyst
s are
dep
lete
d be
fore
the
end
of th
e re
duct
ion
cycl
e. [g
] Ave
rage
of 4
sam
ples
.
-
Chapter 2.1 Catalysis
48
Selectivity towards hydrogen oxidation. In a typical reaction,
(Scheme 1) 250 mg of SOR catalyst was placed on a quartz wool plug
in a quartz reactor and heated to 550 °C in 1% v/v O2 in Ar. The
selectivity and activity were assessed over nine redox cycles, each
consisting of an 18 min oxidation step (1% v/v O2 in Ar), a 4 min
purge in pure Ar, a 10 min reduction step (4:1:1% v/v C3H8:C3H6:H2
in Ar), and a second 3 min purge in pure Ar. The reductive gas feed
simulates the effluent from industrial propane dehydrogenation.[14]
The selectivity and activity are assessed during this step using
the data of 15 GC measurements, spread over
the 10 min interval. The selectivity is determined as the ratio
1002 total
H
conversionconversion
.
The ‘total conversion’ is the conversion of hydrogen, propane
and propene. A selective catalyst will convert only hydrogen,
yielding a selectivity of 100%. Conversion of propene and/or
propane will lower the selectivity. Note that several interactions
between these hydrocarbons and the catalyst can occur which result
in hydrocarbon conversion (see Scheme 2). The activity of the
catalyst is determined as the percentage of the hydrogen feed which
is combusted during the reduction cycle, and is labelled ‘hydrogen
activity’. Note that the oxygen source for this combustion is the
catalyst's lattice oxygen, which has to be refilled once depleted,
hence the redox cycling. The lattice oxygen of all of the catalysts
tested was depleted before the end of the reduction cycle (i.e.
within 10 minutes).
H2
SOR-O
(De)hydrogenation
C CC
SOR-O
Coking
COx
SOR-
Hydrocarboncombustion
H2O
SOR-
Hydrogencombustion
H2H2 CH3
Cracking
SOR-O
H2
SOR-O
(De)hydrogenation
C CC
SOR-O
Coking
COx
SOR-
Hydrocarboncombustion
H2O
SOR-
Hydrogencombustion
H2COx
SOR-SOR-
Hydrocarboncombustion
H2O
SOR-
Hydrogencombustion
H2H2 CH3
Cracking
SOR-O
CH3
Cracking
SOR-O
Scheme 2. Cartoon showing possible interactions between the
dehydrogenation gas mixture and the SOR catalyst. The so-called
oxygen demand is the total amount of catalyst oxygen used by the
processes.
-
Chapter 2.1 Catalysis
49
Figure 2 shows a scheme of the proposed redox process, where the
reactor contains both the SOR (black) and dehydrogenation catalyst
(white).[41] The alkane is fed over the reactor bed and is
dehydrogenated by the dehydrogenation catalyst. The formed H2 is
selectively burned from the gas mixture by the SOR (Figure 2A).
Since the colour of ceria changes from yellow to black when
reduced, the proc
