Development of Methane Oxidation Catalysts
for Different Gas Turbine Combustor Concepts
Sara Eriksson
Licentiate Thesis 2005
KTH - The Royal Institute of Technology
Department of Chemical Engineering and Technology
Chemical Technology
SE-100 44 Stockholm, Sweden
TRITA-KET R211
ISSN 1104-3466
ISRN KTH/KET/R--211--SE
ABSTRACT
Due to continuously stricter regulations regarding emissions from power
generation processes, development of existing gas turbine combustors is
essential. A promising alternative to conventional flame combustion in gas
turbines is catalytic combustion, which can result in ultra low emission levels
of NOx, CO and unburned hydrocarbons. The work presented in this thesis
concerns the development of methane oxidation catalysts for gas turbine
combustors. The application of catalytic combustion to different combustor
concepts is addressed in particular.
The first part of the thesis (Paper I) reports on catalyst development for fuellean methane combustion. The effect on catalytic activity of diluting the
reaction mixture with water and carbon dioxide was studied in order to
simulate a combustion process with exhaust gas recirculation. Palladium-based
catalysts were found to exhibit the highest activity for methane oxidation under
fuel-lean conditions. However, the catalytic activity was significantly
decreased by adding water and CO2, resulting in unacceptably high ignition
temperatures of the fuel.
In the second part of this thesis (Paper II), the development of rhodium
catalysts for fuel-rich methane combustion is addressed. The effect of water
addition on the methane conversion and the product gas composition was
studied. A significant influence of the support material and Rh loading on the
catalytic behavior was found. The addition of water influenced both the lowtemperature activity and the product gas composition.
Keywords: catalytic combustion, methane oxidation, ceria, palladium,
platinum, rhodium, TPO
SAMMANFATTNING
Eftersom utsläppskraven för energiproducerande anläggningar kontinuerligt
skärps krävs utveckling av dagens förbränningsteknik. Katalytisk förbränning
är
ett
attraktivt
alternativ
till
konventionell
flamförbränning
då
utsläppsnivåerna av NOx, CO och oförbrända kolväten kan minskas avsevärt.
Arbetet som presenteras i denna avhandling behandlar utveckling av
katalysatorer för förbränning av metan. Tillämpningen av katalytisk
förbränning i olika förbränningskoncept för gasturbiner har särskilt undersökts.
Första delen av avhandlingen (artikel I) behandlar katalysatorutveckling för
förbränning av metan i luftöverskott. Inverkan på metanomsättningen av att
späda ut reaktionsblandningen med vatten och koldioxid studerades i syfte att
simulera en förbränningsprocess med recirkulation av förbränningsprodukter.
Palladiumbaserade katalysatorer uppvisade den högsta aktiviteten vid
luftöverskott. Emellertid minskade aktiviteten drastiskt då vatten och koldioxid
tillsattes.
Den andra delen av avhandlingen (artikel II) rapporterar om utvecklingen av
rodiumkatalysatorer för metanförbränning i bränsleöverskott. Effekten av att
tillsätta vatten till reaktionsblandningen undersöktes. Signifikanta skillnader för
metanomsättning
och
sammansättning
av
produktgas
upptäcktes
för
katalysatorer bestående av olika bärarmaterial och innehållandes olika halter av
rodium. Tillsatsen av vatten påverkade metanomsättningen särskilt vid låga
temperaturer liksom sammansättningen av produktgasen.
Nyckelord: katalytisk förbränning, oxidation av metan, ceriumoxid, palladium,
platina, rodium, TPO
PUBLICATIONS REFERRRED TO IN THE THESIS
The work presented in this thesis is based on the following publications,
referred to in the text by their Roman numerals. The papers are appended at the
end of the thesis.
I. Catalytic combustion of methane in steam and carbon dioxide-diluted
reaction mixtures
S. Eriksson, M. Boutonnet and S. Järås, submitted to Appl. Catal. A.
II. Partial oxidation of methane over rhodium catalysts for power generation
applications
S. Eriksson, M. Nilsson, M. Boutonnet and S. Järås, Catal. Today (2005), in
press.
OTHER PUBLICATIONS
1. Microemulsion: an alternative route to preparing supported catalysts
S. Rojas, S. Eriksson, M. Boutonnet, Catal. Special. Period. Rep. R. Soc.
Chem. 17 (2004) 258.
2. Preparation of catalysts from microemulsions and their applications in
heterogeneous catalysis
S. Eriksson, U. Nylen, S. Rojas, M. Boutonnet, Appl. Catal. A 265 (2004)
207.
3. Catalytic combustion of methane over bimetallic catalysts
A. Ersson, H. Kusar, S. Eriksson, M. Boutonnet, S. Järås, presented at the
5th International Workshop on Catalytic Combustion, Seoul, Korea, 2002.
4. Stability tests of catalysts for methane combustion
S. Eriksson, K. Persson, M. Boutonnet and S. Järås, presented at the 10th
Nordic Symposium on Catalysis, Helsingør, Denmark, 2002.
5. Catalytic combustion of methane in humid gas turbine cycles
K. Persson, S. Eriksson, P.O. Thevenin, S.G. Järås, presented at the 10th
Nordic Symposium on Catalysis, Helsingør, Denmark, 2002.
6. Stability tests of catalysts for methane combustion
S. Eriksson, K. Persson, M. Boutonnet and S. Järås, presented at the 2nd
EFCATS School on Catalysis, Tihany, Hungary, 2002.
7. Combustion of methane over ceria based catalysts
S. Eriksson, M. Boutonnet and S. Järås, presented at the 6th EuropaCat,
Innsbruck, Austria, 2003.
8. Partial oxidation of methane over rhodium catalysts for power generation
applications
S. Eriksson, M. Nilsson, M. Boutonnet and S. Järås, presented at the 11th
Nordic Symposium on Catalysis, Oulu, Finland, 2004.
CONTENTS
1
2
3
4
INTRODUCTION
1
1.1 The Advanced Zero Emission Power (AZEP) concept
2
1.2 Scope of the work
2
GAS TURBINE COMBUSTION
5
2.1 Gas turbine configuration
5
2.2 Emissions from combustion
5
2.3 Zero emission combustion concepts
8
2.3.1 Post-combustion capture
9
2.3.2 Oxy-fuel combustion
10
2.3.3 Pre-combustion decarbonization
11
CATALYTIC COMBUSTION
13
3.1. Introduction
13
3.2. Catalytic combustor design
14
3.2.1 Fully catalytic design
14
3.2.2 Hybrid design
15
3.3 Commercial status of catalytic combustors
16
CATALYST DEVELOPMENT
17
4.1 Introduction
17
4.2 Fuel-lean catalytic combustion (Paper I)
17
4.2.1 Experimental set-up and testing conditions
17
4.2.2 Metal oxide catalysts
18
4.2.3 Noble metal catalysts
20
4.2.4 Temperature programmed oxidation (TPO) experiments
22
4.3 Fuel-rich catalytic combustion (Paper II)
22
4.3.1 Experimental set-up and testing conditions
23
4.3.2 Rhodium catalysts
24
4.4 The influence of combustion products on the catalyst performance
(Papers I and II)
5
26
4.4.1 Palladium and platinum catalysts
26
4.4.2 Rhodium catalysts
27
CONCLUSIONS
29
ACKNOWLEDGEMENTS
31
CONTRIBUTIONS TO THE PAPERS
32
REFERENCES
33
APPENDICES: Papers I and II
1
INTRODUCTION
Gas turbines that burn fuel to generate heat are widely used for the production
of electricity. Natural gas is commonly used as fuel for stationary gas turbines
due to low levels of impurities and long-term availability. However, the
combustion process produces emissions that are harmful for the environment,
i.e. oxides of nitrogen (NOx), unburned hydrocarbons (UHCs) and carbon
monoxide (CO). The effect of these emissions was noticed in the 1950s when
phenomena such as smog and acid rain appeared. This awareness resulted in
strict regulations regarding emissions of pollutants being issued. Today, the
control of emissions is one of the most important factors when designing a gas
turbine.
Catalytic combustion is an attractive alternative to conventional flame
combustion from an environmental point of view. The main advantages of this
technique are the opportunity to perform the combustion reaction over a wide
range of fuel-to-air ratios and the ability to operate at lower temperatures than
for conventional flame combustion. These features provide the possibility to
reduce emission levels of pollutants such as NOx, CO, UHC and soot
considerably. Since the concept of catalytically stabilized combustion was
introduced in the 1970s, extensive research on finding suitable catalyst
materials has been conducted.
Furthermore, the environmental effect of carbon dioxide (CO2), which is one of
the major combustion products, has been recognized. Carbon dioxide is a
greenhouse gas and contributes to 50 % of the global warming. The production
of CO2 cannot be avoided in combustion processes. However, a process can be
modified in order to facilitate the separation of carbon dioxide from the
exhaust. The separated CO2 could potentially be stored in geological reservoirs
preventing air pollution.
1
1.1 The Advanced Zero Emission Power (AZEP) concept
The Advanced Zero Emission Power (AZEP) concept has been a research
project within the European Union with the aim of developing a zero emission,
gas turbine-based power generation process. The AZEP concept enables NOx
elimination as well as cost reduction of CO2 separation compared to
conventional techniques (post-combustion capture). These targets can be
achieved by i) combusting natural gas in pure oxygen produced by a mixedconductive membrane in which O2 is separated from air and, ii) dilution of the
fuel/oxygen mixture with combustion products, i.e. water and carbon dioxide,
instead of nitrogen.
The division of Chemical Technology at KTH has been involved in the design
of a combustor for the AZEP project together with ALSTOM Power and Paul
Scherrer Institut. At an early stage, a catalytically stabilized combustion
process was found to be an attractive option. Therefore, our work has been
focused on the development of catalysts for AZEP mixtures, which contain
large amounts of water and carbon dioxide.
1.2 Scope of the work
The objective of the present thesis is to describe the development of catalysts
for combustion of methane in gas turbine applications. Special attention was
given to the influence of combustion products, i.e. water and carbon dioxide,
on the catalytic performance. Furthermore, different combustion concepts were
investigated.
Fuel-lean catalytic combustion of methane was studied in Paper I. A variety of
catalysts were prepared, with emphasis on palladium-based materials, and the
activity in both air and exhaust gas-diluted mixtures was investigated.
2
In Paper II, a fuel-rich catalytic combustion concept was studied. The
performance of rhodium-based catalysts was investigated with focus on the
effect of support material, Rh loading and presence of water vapor on the
methane conversion efficiency and the product gas composition.
3
4
2
GAS TURBINE COMBUSTION
2.1 Gas turbine configuration
The main components of a gas turbine are a compressor, a combustion chamber
and a turbine [1]. The configuration of a simple gas turbine system is presented
in Figure 1. The compressed air is fed to the combustion chamber together with
the fuel. The temperature increases during combustion resulting in an outlet
temperature of approximately 1800 ºC. Before entering the turbine, the
temperature of the exhaust must be reduced to avoid turbine material damage.
Part of the compressed air is therefore bypassed the combustor and used for
exhaust gas cooling. Power is then generated by expansion of the hot exhaust
gas in the turbine. The main factors influencing the performance of gas turbines
are the component efficiencies and the turbine working temperature.
Combustor
Fuel
1800 °C
Air
Exhaust
350 °C
Bypass air
Compressor
1400 °C
Turbine
Figure 1. Flow diagram of a conventional gas turbine combustor
2.2 Emissions from combustion
The major compounds formed in combustion reactions, using air or oxygen as
oxidant, are water and carbon dioxide. Furthermore, small amounts of nitrogen
oxides (NOx), unburned hydrocarbons (UHCs), sulfurous oxides (SOx) and
5
carbon monoxide (CO) can be produced [2]. Even though the formation of
these substances is limited, the high throughput rates in gas turbines will result
in substantial emission levels. The emission levels of UHCs, CO and NOx
depend on operating conditions, especially the air/fuel ratio as presented in
Figure 2. Since the negative environmental effects of these compounds were
noticed in the 1950s, continuously stricter regulations regarding emissions of
pollutants have emerged.
Figure 2. Dependence of emission levels of pollutants on air/fuel ratio. Adapted from
Thevenin [3]
When the combustion process is incomplete, only partial oxidation of the fuel
occurs resulting in the formation of UHCs and CO. The formation of CO is
favored by increasing the flame temperature (at constant air/fuel ratio),
reducing the amount of H2 in the fuel and a rapid exhaust gas cooling rate. The
main environmental concern regarding carbon monoxide is related to human
health since it is highly toxic and can cause suffocation. The formation of UHC
emissions is controlled by the same factors as CO and, therefore, they tend to
follow the same trends.
6
Sulfurous oxides (SOx) may be formed when combusting a sulfur-containing
fuel. This is generally the case for fuels such as heavy oils and coal. However,
gaseous fuels such as natural gas contain very small amounts of sulfur. The
main sulfurous oxide formed under combustion conditions is SO2. The primary
environmental effects of SOx emissions include acid rain and human
suffocation at high enough concentrations.
The minimization of NOx emissions is at present one of the major concerns of
gas turbine combustion. Environmental effects such as ozone depletion, acid
rain and smog formation have been identified.
Nitrogen oxides include nitric oxide (NO) and nitrogen dioxide (NO2) with NO
being the predominant form in combustion emissions. Nitric oxide can be
produced according to three different mechanisms: fuel, prompt and thermal.
Fuel NOx is produced by oxidation of fuel-bound nitrogen whereas prompt
NOx is formed by high-speed reactions of nitrogen, oxygen and hydrocarbon
radicals. Prompt NOx is usually formed in combustion chambers operating with
hydrocarbon fuel-rich flames. Thermal NOx is formed by oxidation of
atmospheric nitrogen and is generally the main mechanism above 1100 Cº. The
formation rate of thermal NOx has been found to increase exponentially with
temperature making a reduction of the flame temperature the key factor for
achieving low NOx emissions. For gas turbine systems, this means that a
decrease of the adiabatic flame temperature is required.
Emissions of nitrogen oxides can be drastically reduced by water injection,
exhaust gas clean up by selective catalytic reduction (SCR) or dry low NOx
technologies. The injection of water, or steam, results in a decrease of the flame
temperature and, therefore, lower levels of NOx emissions. In dry low NOx
technologies, combustor designs resulting in a lower flame temperature are
used. A common approach is to use a lean premixed combustor. However, this
can result in problems such as poor combustion efficiency and flame instability
7
and, therefore, higher emission levels of CO and UHC. An example of dry low
NOx technologies is catalytic combustion, which allows a stable combustion
reaction to take place at significantly lower temperatures than for conventional
flame combustion. Therefore, ultra-low levels of NOx emissions (< 3 ppm) can
be achieved without the formation of UHC and CO.
2.3 Zero emission combustion concepts
More recently, the environmental effect of carbon dioxide has been noticed.
Although some controversy exists, most agree that emissions of CO2 are
connected to global warming. The high CO2 levels present today are related to
both fossil-fuel combustion and forest depletion. The production of CO2 cannot
be avoided for combustion processes since it is a natural by-product. However,
the process can be modified in order to facilitate the separation of carbon
dioxide from the exhaust. The captured CO2 can potentially be stored in oceans
or geological reservoirs, i.e. depleted oil and gas reservoirs or deep saline
aquifers. Another alternative is to use carbon dioxide for enhanced oil
recovery, which is already occurring today.
To facilitate the reduction of greenhouse gases, the Kyoto Protocol was
adopted in 1997 [4]. The aim of the protocol is to prevent dangerous
anthropogenic interference with the climate system and contains legally
binding emission targets for developed countries for the post-2000 period. A
total reduction of greenhouse gas emissions from 1990 levels by 5 % over the
period 2008-2012 should be achieved. Within the European Community, an
emission trade system for CO2, according to the Kyoto Protocol, started in
February 2005.
8
Technologies for carbon dioxide capture from fossil fuel combustion processes
can be divided into the following main groups [5]:
• Post-combustion capture
• Oxy-fuel combustion
• Pre-combustion decarbonization
2.3.1 Post-combustion capture
In the post-combustion capture concept, CO2 is separated from the exhaust gas
without modifying the gas turbine system as presented in Figure 3.
To atmosphere
(N2, H2O, O2)
CO2
capture
CO2 for storage
Fuel
Air
Steam
Generator
Gas Turbine
Steam
Turbine
Figure 3. Post-combustion capture of CO2
Several different techniques can be employed to capture the carbon dioxide
depending on gas composition and flow rates. Absorption into an alkaline
solution, such as an amine, is the most suitable technique for fossil fuel power
plants where the partial pressure of CO2 normally is in the range of
0.15-0.3 bar.
Problems related to the use of this technology for gas turbines are high oxygen
content in the flue gas, which could cause oxidation of amines to carboxylic
9
acids, and the presence of NO2 resulting in formation of amine salts. Both
processes result in amine losses and corrosion problems. Another important
parameter to consider is the cost of installation and operation of this type of
system.
2.3.2 Oxy-fuel combustion
In oxy-fuel combustion, the fuel is combusted in CO2-diluted reaction mixtures
instead of nitrogen. A unit separating air into O2 and N2 is required prior to the
combustion process as depicted in Figure 4. Recycling of CO2 is necessary in
order to lower the flame temperature to values similar to conventional
combustors. The steam produced in the combustion reaction can easily be
separated from the exhaust gas by condensation.
N2
Air
Air
separation
CO2 for storage
O2
Condenser
H2O
Fuel
Combustor
CO2
Gas Turbine
Figure 4. Oxy-fuel combustion with CO2 capture
The main advantage of this combustion technique is a reduced cost of CO2
separation due to the absence of nitrogen. Furthermore, the formation of NOx
emissions is avoided. A challenge with this process is to separate oxygen from
air in a cost-effective way. Pure oxygen could be produced by cryogenic air
separation or membrane technology.
10
An alternative approach is to recycle both CO2 and water. This concept is
applied in the Advanced Zero Emission Power (AZEP) process together with
membrane technology for air separation [6, 7]. Here, the exhaust gas mixture is
recycled to a unit consisting of an air separation membrane and a combustor.
This unit is described in Figure 5 and is here referred to as MCM (mixed
conductive membrane) reactor. A major advantage with the AZEP concept is
that conventional gas turbine equipment can be utilized and the installation of a
CO2 turbine can be avoided. A challenge with this concept is to achieve a
stable combustion process with the high amounts of water present. Preliminary
investigations have shown that catalytic combustion may be an attractive
alternative.
CO2 and H2O to
separation unit
Fuel
CH4 + O2
CO2 + H2O
---------------------------------
Air
O2
Q
O2
Q
MCM reactor
Air compressor
Air turbine
Figure 5. Description of the AZEP concept with the MCM reactor unit
2.3.3 Pre-combustion decarbonization
The pre-combustion decarbonization process is described in Figure 6. The fuel
is first converted to syngas, i.e. H2 and CO, by partial oxidation and/or steam
reforming [8]. Partial oxidation is preferable when combusting coal, heavy oils
and high molecular weight hydrocarbons, which are difficult to convert by
steam reforming. The partial oxidation process uses air from the gas turbine
compressor.
11
In the steam reforming route, approximately 25 % of the exhaust gas is
recycled and used in order to form CO and H2. Thereafter, CO2 is formed by
the water gas shift reaction, CO + H2O → CO2 + H2, and separated from the
hydrogen. Separation of carbon dioxide by physical absorption is preferred
under the prevailing conditions.
Challenges with this combustion concept are related to the combustion of
hydrogen mixtures. Modifications of existing burners might be necessary in
order to obtain a reliable combustion process.
CO2 for storage
Partial oxidation
and/or reforming
CO
shift
CO2
separation
H2
Fuel
Compressed air
(partial oxidation)
Combustor
Gas Turbine
Air
Gas turbine exhaust
(steam reforming)
Figure 6. Pre-combustion decarbonization
12
N 2, H 2O
3
CATALYTIC COMBUSTION
3.1. Introduction
Catalytic combustion can be divided into primary and secondary combustion
processes. In primary processes, the objective is to produce heat while limiting
the amount of pollution formation. Secondary processes typically deal with
abatement of volatile organic compounds (VOC). Secondary processes are also
called low-temperature catalytic combustion since they occur at temperatures
below 400 ºC. In primary processes, higher concentrations of hydrocarbons are
present and, therefore, considerably higher temperatures (> 1000 ºC) can be
achieved. Only catalytic combustion for gas turbine applications, which of
course is a primary process, will be discussed here.
Catalytic combustion for gas turbines is a promising alternative to conventional
flame combustion that has been extensively investigated for more than 30 years
[9, 10]. This technique utilizes a catalyst, which is placed in the combustion
chamber as presented in Figure 7. The fuel is oxidized at the catalyst surface
without requiring a flame. Since complete combustion can occur outside the
flammability limits of the fuel/oxygen mixture, lower combustor outlet
temperatures are achieved and the need for by pass air cooling can be avoided
[11].
The main advantages of this technique are the opportunity to perform the
combustion reaction over a wide range of fuel-to-air ratios and the ability to
operate at lower temperatures than possible for conventional flame combustion.
These qualities provide the possibility to reduce emission levels of pollutants
such as NOx, CO, soot and unburned hydrocarbons significantly.
13
Catalytic
combustor
Fuel
<1400 °C
Air
Exhaust
350 °C
Compressor
Turbine
Figure 7. Flow diagram of catalytic gas turbine combustor
The major challenge with this combustion method is to find a catalytic material
that can withstand the harsh combustor environment, i.e. temperatures up to
1400 ºC, high gas velocities (10-40 m/s) and the possibility of thermal shocks,
for longer periods of time. The activity of catalysts for gas turbine combustors
should also be stable and last for at least one year of operation without
problems.
3.2. Catalytic combustor design
Ideally, a catalyst with both a high activity and long-term stability under all
conditions relevant for gas turbine combustors, i.e. over a broad power load
range, is preferred. In reality, it is a great challenge to obtain such a catalyst.
Several combustor designs have been proposed in order to overcome this
problem. Some of these configurations will be summarized here. More detailed
information on this subject can be found elsewhere [12].
3.2.1 Fully catalytic design
In this design, all of the fuel and compressed air are mixed before entering the
catalytic combustor. This configuration relies on a highly active ignition
catalyst at the entrance and thermally stable catalysts further down stream. The
14
catalytic end segment should be active to some extent but, most importantly,
stable at temperatures up to 1300 ºC, which is very difficult to obtain.
3.2.2 Hybrid design
This two-stage combustion configuration consists of a catalytic segment
followed by homogeneous gas phase combustion, i.e. flame combustion. Thus,
a high temperature catalytic end segment can be avoided. The fuel is partly
combusted over the catalyst, resulting in a moderate temperature increase, and
complete conversion is obtained in the homogeneous zone. The catalyst outlet
temperature is generally below 1000 ºC. Several different designs resulting in
partial conversion of the fuel have been proposed.
A catalyst of monolith structure with alternately coated channels can be utilized
to control the catalyst temperature. The fuel/air mixture entering the inactive
channels without catalyst coating will not be combusted before the high
temperature homogeneous zone is reached. An advantage with this catalyst
configuration is that the heat formed by the combustion reaction in the active
channels is transported to the inactive channels. Thus, the formation of hot
spots, which can damage the catalyst, can be avoided.
If a fully coated catalyst structure is used, secondary fuel or secondary air
configurations can be used to limit the catalyst temperature. In the secondary
fuel configuration, part of the fuel is completely combusted over the catalyst
and the remaining fuel is introduced directly to the subsequent homogeneous
zone. In the secondary air concept, fuel-rich catalytic combustion occurs in the
first stage producing hydrogen and carbon monoxide in addition to products of
complete combustion. Additional air is added after the catalyst to fully oxidize
the partial oxidation products, i.e. H2 and CO, in the homogeneous zone.
15
3.3 Commercial status of catalytic combustors
The first commercial catalytic combustor for natural gas-fired gas turbines was
recently introduced. In 2002, the XONON cool combustion system, which
utilizes a two-stage lean premix preburner configuration, was commercialized
by Catalytica Energy Systems [13, 14]. Since then, a 1.4 MW Kawasaki M1A13X gas turbine equipped with a XONON combustor has been operating in
California. The combustor has a guaranteed life of 8000 h with emission levels
of NOx < 3 ppm and CO and UHC < 6 ppm.
Development of hybrid catalytic combustors has been carried out by Siemens
Westinghouse Power Corp. for large industrial engines (> 200 MW) and by
Solar Turbines for smaller engines (< 20 MW) [15].
However, further development of catalytic combustors is required to improve
this technique and expand the field of application. The development of catalytic
combustors has until now mainly been focused on fuel-lean methane
combustion. This configuration usually requires a pre-burner in order to
achieve low-temperature ignition, which result in higher levels of NOx
emissions. Also, the flexibility of the combustor should be improved to meet
variations in load, ambient conditions and gas turbine size.
Moreover, other combustion concepts have emerged lately resulting in the need
for different catalytic systems. The combustor concept utilizing secondary air
resulting in fuel-rich catalytic combustion is one example. Benefits such as
low-temperature ignition have been observed [16]. Therefore, pre-heaters could
potentially be avoided. The development of catalytic materials for this concept
together with testing under gas turbine conditions are of interest. Another
example is the zero emission combustion concept utilizing exhaust gas
recirculation as discussed previously.
16
4
CATALYST DEVELOPMENT
4.1 Introduction
In general, combustion catalysts consist of a substrate, a washcoat (or support)
and an active phase. The substrate, which can be metallic or ceramic, is usually
of monolith structure. The washcoat is typically a metal oxide of relatively high
surface area facilitating a high dispersion of the active phase.
Several important factors should be considered when designing a catalyst. For
example, the support material can play an important role. Common support
materials for oxidation catalysts are Al2O3, ZrO2, SiO2 and mixed metal oxides.
Properties such as surface area, acidity, crystal structure and support-noble
metal interaction can influence the activity significantly. Aspects to consider
related to the active phase include dispersion, particle size and oxidation state.
In this chapter, results regarding catalysts for both fuel-lean (Paper I) and fuelrich (Paper II) combustion of methane are summarized. Issues related to
combustion in AZEP mixtures, i.e. high amounts of water and CO2, were
addressed in particular.
4.2 Fuel-lean catalytic combustion (Paper I)
The catalytic activity of a variety of materials has been investigated for
methane combustion under fuel-lean conditions [17]. Noble metal catalysts are
generally found to be suitable ignition catalysts whereas metal oxide catalysts
are active at higher temperatures.
4.2.1 Experimental set-up and testing conditions
The activity tests were carried out in a laboratory-scale reactor consisting of a
quartz tube placed in a cylindrical furnace as presented in Figure 8.
17
Thermocouples were inserted at the catalyst inlet and inside the furnace to
monitor the temperature. Reactants and products were analyzed by
paramagnetic and IR-spectroscopy techniques. Catalysts of monolith structure,
each containing about 120 mg of catalyst material, were placed in the test rig.
Furnace
CH4
Quartz-glas
reactor
Catalyst
H2O
trap
Exhaust
Gas
analyser
Pump
Evaporator
H2O
air/N2
CO2
Thermocouples
Figure 8. Experimental set-up
The experiments were carried out at atmospheric pressure under fuel-lean
conditions (2.6 vol.% CH4 in air) at a gas hourly space velocity (GHSV) of
110 000 h-1. Repeated heating and cooling cycles in the temperature range
300-900 ˚C were performed for each catalyst whilst measuring the activity.
4.2.2 Metal oxide catalysts
The catalytic properties of ceria-based materials have been widely investigated
mainly due to the important role they play in the three-way catalysts (TWCs)
used for pollution abatement but these materials are also of interest for gas
turbine combustors. The properties that make ceria a promising material for use
in catalysis are mainly (i) the ability to shift easily between reduced and
oxidized state (i.e. Ce3+/Ce4+) and (ii) a high oxygen transport capacity [18].
18
However, pure cerium oxide suffers from poor thermal resistance and inferior
low-temperature
activity
for
combustion
applications.
The
catalytic
performance of cerium oxide can be improved by addition of another metal
oxide or a noble metal [19, 20]. In this section, the catalytic behavior of La or
Zr doped cerium oxide is addressed.
The methane conversion of doped cerium oxide materials is presented in
Figure 9. A strong dependence of catalytic activity on catalyst inlet temperature
can be observed. The mixed metal oxide containing lanthanum showed a higher
activity in the entire temperature range investigated compared to the Zr-doped
material. Ignition temperatures of 750 °C and 840 °C were obtained for
Ce0.9La0.1O2 and Ce0.9Zr0.1O2, respectively. Here, the ignition temperature (or
light-off temperature) is defined as the temperature required for 10 %
conversion of methane. In contrast to the Zr-doped catalyst, complete
conversion could be obtained for the La-doped catalyst at approximately
850 °C.
100
CH4 conversion (%)
80
60
40
20
0
600
650
700
750
800
850
900
Catalyst inlet temperature (°C)
Figure 9. Methane conversion versus catalyst inlet temperature for Ce0.9La0.1O2 (■) and
Ce0.9Zr0.1O2 (●)
19
Since the catalysts are active in the temperature range 750-900 °C, they are not
suitable as light-off catalysts. For this application ignition should occur around
400 °C. However, materials of this type could be utilized as mid-temperature
catalysts in a multiple-stage catalytic combustor.
4.2.3 Noble metal catalysts
The high activity of palladium-based catalysts for methane oxidation has been
well recognized for years [21]. In general, Pd is considered to be the most
active ignition catalyst for fuel-lean combustion of methane. Platinum is also
an efficient oxidation catalyst, especially for higher alkanes.
In order to increase the activity of the La-doped ceria catalyst, 5 wt.% Pd or Pt
was deposited on Ce0.9La0.1O2. The methane conversion was significantly
improved in the entire temperature range as shown in Figure 10. A Pd/ZrO2
catalyst was also investigated to elucidate the influence of the support. The
lowest ignition temperature was obtained for the supported Pd catalysts with
light-off occurring at 400 °C. Significant differences in activity could be
observed for different support materials. Both Pd catalysts showed similar
activities in the low-temperature range; however, an improvement in
conversion can be observed at higher temperatures when using Ce0.9La0.1O2.
The active phase of palladium catalysts in fuel-lean methane oxidation is still
an issue of debate. However, PdO is generally considered to be more active
than metallic Pd [22-24]. The phase transformation PdO → Pd0 occurs during
heating in the temperature range 680-800 °C depending on catalyst
composition and reaction conditions [25, 26].
20
100
CH4 conversion (%)
80
60
40
20
0
300
400
500
600
700
800
900
Catalyst inlet temperature (°C)
Figure 10. Methane conversion versus catalyst inlet temperature during heating (closed
symbols) and cooling (open symbols) for 5 % Pd/Ce0.9La0.1O2 (■ □), 5 % Pd/ZrO2 (▲ ∆) and
5 % Pt/Ce0.9La0.1O2 (● ○) in air
A decrease in methane conversion of the palladium catalysts at higher
temperatures related to this phase transformation can be observed in Figure 10.
The activity is recovered upon cooling since re-oxidation of the metallic
palladium occurs. The re-oxidation occurs at a lower temperature resulting in a
PdO decomposition-reformation hysteresis.
The activity of the supported platinum catalyst was considerably lower than for
the Pd catalysts with light-off occurring at 660 °C. The activation of the C-H
bond in methane is the first critical step in the combustion reaction. This
activation occurs according to different mechanisms for Pd and Pt [27]. For Pd,
a completely oxidized surface is generally preferred whereas a partially
oxidized surface results in the highest activity for Pt. Since fuel-lean reaction
conditions produce completely oxidized surfaces, this explains why the activity
of the palladium catalysts is significantly higher than for the platinum catalyst
in this study.
21
4.2.4 Temperature programmed oxidation (TPO) experiments
In order to study the redox behavior of palladium deposited on different
support materials more in detail, TPO experiments were performed. The results
are presented in Figure 11. The decomposition of PdO occurs in approximately
the same temperature range for both catalysts during heating. A temperature
gap between reduction and re-oxidation of palladium can be observed. The reoxidation of palladium takes place at a higher temperature for the Ce0.9La0.1O2supported catalyst, which is probably related to the high oxygen mobility of the
support, resulting in a stabilization of the PdO phase. These observations are in
agreement with activity test results assuming that PdO is the active phase of the
O2 concentration (a.u.)
catalyst.
5 % Pd/Ce0.9La0.1O2
5 % Pd/ZrO2
300
400
500
600
700
800
900
Temperature (°C)
Figure 11. TPO profiles for the palladium catalysts
4.3 Fuel-rich catalytic combustion (Paper II)
In the hybrid combustor design utilizing secondary air as described previously,
a catalytic fuel-rich stage where partial oxidation of the fuel occurs is followed
by lean homogeneous combustion. Syngas, i.e. H2 and CO, is produced under
fuel-rich conditions, resulting in a hydrogen-stabilized second stage
22
combustion process. Only limited studies of this combustion concept have been
published in the past. Lyubovsky et al. investigated methane combustion under
various fuel-to-air ratios for supported Pd, Pt and Rh catalysts demonstrating
benefits such as low light-off temperature during fuel-rich operation [16].
Various metals, such as Rh, Pd, Pt, Ru, Ni, Co and Fe, have been investigated
as catalysts for partial oxidation of methane to syngas [28]:
CH4 + 1/2 O2 = CO + 2 H2
(1)
Different reaction mechanisms for partial oxidation of methane have been
proposed [29-34]. The reaction can either proceed according to i) complete
combustion of some methane forming CO2 and H2O (reaction 2, below),
followed by methane reforming by steam (reaction 3) and CO2 (reaction 4)
yielding syngas or ii) direct partial oxidation forming CO and H2 as primary
products (reaction 1).
CH4 + 2 O2 = CO2 + 2 H2O
(2)
CH4 + H2O = CO + 3 H2
(3)
CH4 + CO2 = 2 CO + 2 H2
(4)
Several studies have reported that especially Rh-based catalysts are suitable for
partial oxidation of methane due to their high activity, syngas selectivity and
resistance to carbon deposition [35, 36]. Here, the behavior of supported
rhodium catalysts for combustion applications is discussed.
4.3.1 Experimental set-up and testing conditions
The experimental set up presented in Figure 8 was also used for investigating
the fuel-rich combustion concept. The experiments were carried out at
atmospheric pressure with nitrogen diluted fuel-rich gas mixtures (3.3 vol.%
CH4, 1.7 vol.% O2, balance N2) at a gas hourly space velocity (GHSV) of
99 000 h-1. Repeated heating and cooling cycles in the temperature range
200-750 ˚C were performed for each catalyst whilst measuring the activity.
23
4.3.2 Rhodium catalysts
The catalytic behavior of various supported rhodium catalysts is presented in
Figure 12. It is evident that both the support material and Rh loading can
influence the catalytic performance significantly.
100
Conversion (%)
80
60
40
20
0
200
300
400
500
600
700
800
Temperature catalyst inlet (°C)
Figure 12. Conversion of methane (closed symbols) and oxygen (open symbols) in N2-diluted
air versus catalyst inlet temperature for 0.5 % Rh/Ce-ZrO2 (■ □), 0.5 % Rh/Ce0.9La0.1O2 (0
1),
0.2 % Rh/ZrO2 (▲ ∆), 0.5 % Rh/ZrO2 (▼ &) and 1 % Rh/ZrO2 (● ○)
The presence of ceria in the support improved the activity, especially at higher
temperatures, and complete conversion could be obtained. This behavior is
probably related to the high oxygen mobility of ceria. Previous studies have
shown that the addition of Ce to ZrO2 increases the activity and stability of Pt
and Ni catalysts for partial oxidation of methane due to an increased CH4
dissociation and enhanced carbon elimination from the surface [37, 38].
The activity can be significantly enhanced by increasing the Rh loading. Here,
increasing the amount of rhodium from 0.2 to 1 wt% resulted in a decrease of
the ignition temperature by 160 °C. The high-temperature activity was also
considerably improved when increasing the Rh loading. These results show
that the noble metal loading is an important factor to consider when preparing
24
combustion catalysts. If the amount of noble metal is too high, the dispersion
can decrease resulting in fewer active sites. Therefore, an optimal metal
loading should be found.
For the fuel-rich combustion concept, the absolute values of the product
concentrations after the catalyst are of importance in order to evaluate the
stability of the subsequent homogeneous combustion stage. Simultaneous H2,
CO and CO2 production can be observed as shown in Figure 13 (closed
symbols). A reaction mechanism consisting of complete oxidation followed by
steam and dry reforming is implied since formation of CO2 occurs mainly at
lower temperatures whereas CO is produced at higher temperatures.
3,5
8
a
7
CO production (vol.%)
H2 production (vol.%)
b
3,0
6
5
4
3
2
2,5
2,0
1,5
1,0
0,5
1
0,0
0
200
300
400
500
600
700
200
800
300
CO2 production (vol.%)
2,0
400
500
600
700
800
Temperature catalyst inlet (°C)
Temperature catalyst inlet (°C)
c
1,5
1,0
0,5
0,0
200
300
400
500
600
700
800
Temperature catalyst inlet (°C)
Figure 13. a) H2 production, b) CO production and c) CO2 production in N2-diluted air
(closed symbols) and in 10 % H2O/N2-air (open symbols) versus catalyst inlet temperature for
0.5 % Rh/Ce-ZrO2 (■ □), 0.5 % Rh/Ce0.9La0.1O2 (0), 0.2 % Rh/ZrO2 (▲ ), 0.5 % Rh/ZrO2
(▼) and 1 % Rh/ZrO2 (● ○)
25
4.4 The influence of combustion products on the catalyst performance
(Papers I and II)
The combustion products, i.e. water and carbon dioxide, may influence the
reaction significantly depending on catalyst composition and reaction
conditions. The product gas composition can be altered, which can be an
advantage in some cases. A major drawback of large amounts of water is that it
may promote sintering of the catalyst at high temperatures resulting in a
decreased dispersion of the active phase on the catalyst surface.
In this section, results from Papers I and II regarding the influence of
combustion products, i.e. water and CO2, on the catalytic activity are discussed.
4.4.1 Palladium and platinum catalysts
The influence of water and carbon dioxide on palladium catalysts has been
investigated in several studies [39-42]. Most authors agree that water has a
negative effect on methane conversion whereas different opinions regarding the
influence of CO2 can be found. A reaction order of approximately –1 has been
identified for water. Both zero order and strong inhibition of order –2 have
been reported for CO2. Furthermore, the effect of water was found to be
dominant when both combustion products were present in the feed. Water may
affect the activity according to the following reaction [43]:
PdO + H2O = Pd(OH)2
Most studies have investigated the effect of relatively low amounts of water on
the activity of palladium catalysts. However, the results presented in this work
have been obtained in reaction mixtures consisting of 1.2 vol.% CH4, 2.4 vol.%
O2, 11.3 vol.% CO2, 27.3 vol.% H2O, 57.8 vol.% N2 at a gas hourly space
velocity (GHSV) of 110 000 h–1. The high concentration of combustion
products has been investigated since these conditions are relevant in exhaust
gas-diluted combustion processes such as the AZEP concept.
26
The results presented in Figure 14 show a drastic decrease in methane
conversion for the Pd-based catalysts when water and CO2 are present in the
feed, see Figure 10 for comparison. An influence of the support material on the
activity can be observed with Ce0.9La0.1O2 exhibiting a significantly higher
activity than ZrO2. Ciuparu et al. [44] propose that support materials with high
oxygen mobility, such as ceria, will reduce the effect of water during methane
combustion by oxygen transfer from the bulk to the PdO particles. This process
will reduce the formation of surface hydroxyls, which probably prevent
methane activation resulting in a decreased reaction rate. The activity of the
supported platinum catalyst was only slightly decreased in the presence of
water and CO2.
100
CH4 conversion (%)
80
60
40
20
0
300
400
500
600
700
800
900
Catalyst inlet temperature (°C)
Figure 14. Methane conversion versus catalyst inlet temperature during heating (closed
symbols) and cooling (open symbols) for 5 % Pd/Ce0.9La0.1O2 (■ □), 5 % Pd/ZrO2 (▲ ∆) and
5 % Pt/Ce0.9La0.1O2 (● ○) in exhaust gas-diluted mixtures
4.4.2 Rhodium catalysts
The effect of water (10 vol.%) on supported rhodium catalysts for fuel-rich
methane combustion was studied. The results presented in Figure 15 show that
water has a negative effect on the catalytic performance in the low-temperature
range with increased light-off temperatures, see Figure 12 for comparison. The
27
delay in ignition is presumably due to water adsorbed on the catalyst surface
preventing methane C-H bond activation. However, advantages of adding
water could be observed at higher temperatures. For example, the conversion of
methane increased in this temperature range for all ZrO2-supported catalysts.
100
Conversion (%)
80
60
40
20
0
200
300
400
500
600
700
800
Temperature catalyst inlet (°C)
Figure 15. Conversion of methane (closed symbols) and oxygen (open symbols) in 10 vol.%
H2O/N2-air versus catalyst inlet temperature for 0.5 % Rh/Ce-ZrO2 (■ □), 0.5 %
Rh/Ce0.9La0.1O2 (0 1), 0.2 % Rh/ZrO2 (▲ ∆), 0.5 % Rh/ZrO2 (▼ &) and 1 % Rh/ZrO2 (● ○)
Furthermore, the product gas composition is altered when water vapor is added
to the reaction mixture as observed in Figure 13. Higher amounts of CO2 are
formed in the entire temperature range whereas less CO is formed. Also, at
higher temperatures, the formation of H2 was enhanced by adding water
resulting in H2/CO ratios higher than 2. The product gas composition is altered
due to the water gas shift reaction taking place (reaction 5). The large amount
of water vapor in the reaction mixture shifts the equilibrium favoring the
products H2 and CO2.
CO + H2O = CO2 + H2
(5)
28
5 CONCLUSIONS
Two different catalytic combustion concepts have been investigated, i.e. fuellean and fuel-rich combustion of methane. In order to simulate a combustion
process utilizing exhaust gas recirculation, the effect of water and CO2 on the
catalyst performance was studied in particular. Significant differences in
activity for methane oxidation and product gas composition could be detected
for the different configurations.
Palladium-based catalysts were found to exhibit the highest activity for
methane oxidation under fuel-lean conditions. In fuel-air reaction mixtures,
both supported Pd catalysts, i.e. Pd/Ce0.9La0.1O2 and Pd/ZrO2, exhibited similar
light-off behaviour. The high temperature activity was however improved when
using Ce0.9La0.1O2. The catalytic activity was significantly decreased by adding
water and CO2 resulting in unacceptably high ignition temperatures of the fuel.
Furthermore, the high-temperature activity was decreased due to the
PdO → Pd0 phase transition.
Even though the activity of the platinum catalyst was only slightly affected by
the addition of combustion products, the initial activity was significantly lower
than for Pd catalysts making this catalyst unsuitable for the first light-off stage.
A variety of supported rhodium catalysts were prepared and tested for the fuelrich combustion concept. The results from the activity tests revealed a
significant importance of the support material and Rh loading on the catalytic
behavior. The catalysts containing ceria in the support material showed the
highest activity and production of H2 and CO.
The addition of water was found to have both positive and negative effects on
the catalytic performance of rhodium under fuel-rich conditions. A negative
effect on the activity in the low-temperature range with increased light-off
29
temperatures could be observed. However, the formation of hydrogen was
increased in the presence of water at higher temperatures, which is favorable
for a combustion configuration with a homogeneous final stage.
The results presented here show that a hybrid combustor design consisting of a
first fuel-rich catalytic stage followed by H2-stabilized homogeneous
combustion is an attractive alternative to the more traditional fuel-lean catalytic
combustion approach. The fuel-rich concept is particularly of interest for oxyfuel combustion processes utilizing exhaust gas recirculation.
30
ACKNOWLEDGEMENTS
I would like to thank Prof. Sven Järås for giving me the opportunity to carry
out my graduate studies at the Department of Chemical Engineering and
Technology at KTH. I also thank Ass. Prof. Magali Boutonnet for her
supervision.
I want to thank my colleagues, former and present, at Chemical Technology
and Chemical Reaction Engineering for providing a positive working
environment and good friendship. Special thanks to Katarina, Ulf, Erik, Sergio,
Anders and Stelios for stimulating collaboration. Also thanks to Marita and
Bård for being good room-mates.
I am grateful to Christina Hörnell for improving the linguistic quality of this
thesis and Inga Groth for assistance with BET and XRD analysis.
I would also like to thank the undergraduate students who I had the opportunity
to work with, Marita Nilsson and Andrea Scarabello, for doing an excellent
job.
The financial support from the European Commission in FP5, contract number
ENK5-CT-2001-00514, and the Swiss Government (BBW01.0054-1) is
gratefully acknowledged. I want to thank the partners in the AZEP project with
whom I had the pleasure to work with, especially Markus Wolf and Raffaella
Villa at ALSTOM Power and Ioannis Mantzaras and Adrian Schneider at Paul
Scherrer Institut.
Last, but not least, I would like to thank my family and Jonas for support and
encouragement during the years.
31
CONTRIBUTIONS TO THE PAPERS
I.
I am the main author of Paper I and the experimental work was performed
by myself.
II.
I am the main author of Paper II. Marita Nilsson performed most of the
experimental work under my guidance.
32
REFERENCES
[1] H. Cohen, G.F.C. Rogers, H.I.H. Saravanamuttoo, Gas turbine theory 4th
ed., Addison Wesley Longman Limited, Essex, 1996.
[2] C.E. Baukal, Industrial combustion pollution and control, Marcel Dekker,
New York, 2004.
[3] P. Thevenin, Catalytic combustion of methane, PhD thesis, KTHKungliga Tekniska Högskolan, Department of Chemical Engineering and
Technology, Stockholm, 2002.
[4] European Commission,
http://europa.eu.int/comm/environment/climat/kyoto.htm, accessed
2004-09-06.
[5]
Carbon Dioxide Capture and Storage, Report of DTI International
Technology Service Mission to the USA and Canada from 27th October to
7th November 2002.
[6] M. Reinke, R. Carroni, D. Winkler, T. Griffin, Experimental Investigation
of Natural Gas Combustion in Oxygen / Exhaust Gas Mixtures for Zero
Emissions Power Generation, 6th Int. Conf. on Technologies and
Combustion for a Clean Environment, 9-12 July, 2001, Oporto, Portugal,
1:15.
[7] T. Griffin, S.G. Sundkvist, K. Åsen, T. Bruun, Advanced Zero Emissions
Gas Turbine Power Plant, ASME TURBO EXPO, 16-19 June, 2003,
Atlanta, Georgia, USA.
[8] H. Audus, O. Kaarstad, G. Skinner, CO2 Capture by Pre-Combustion
Decarbonisation of Natural Gas, IEA GHG Paper presented at GHGT-4,
Interlaken, Switzerland, 1998.
[9] W.C. Pfefferle, Belgian Patent 814 (1974), to Engelhard Corporation.
[10] W.C. Pfefferle, J. Energy 2 (1978) 142.
[11] R.E. Hayes, S.T. Kolaczkowski, Introduction to catalytic combustion,
Gordan and Breach Science Publishers, Amsterdam, 1997.
33
[12] E.M. Johansson, D. Papadias, P.O. Thevenin, A.G. Ersson, R.
Gabrielsson, P.G. Menon, P.H. Björnbom, S.G. Järås, Catal. Special.
Period. Rep. R. Soc. Chem. 14 (1999) 183.
[13] D.Y. Yee, K. Lundberg, C.K. Weakley, ASME Paper 2000-GT-0088,
2000.
[14] Catalytica Energy Systems,
http://www.catalyticaenergy.com/xonon/index.html, accessed 2005-01-13.
[15] D.B. Fant, G.S. Jackson, H. Karim, D.M. Newburry, P. Dutta, K.O.
Smith, R.W. Dibble, ASME Paper 99-GT-57.
[16] M. Lyubovsky, L.L. Smith, M. Castaldi, H. Karim, B. Nentwick, S.
Etemad, R. LaPierre, W.C. Pfefferle, Catal. Today 83 (2003) 71.
[17] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Appl. Catal. A 234
(2002) 1.
[18] A. Trovarelli, M. Boaro, E. Rocchini, C. de Leitenburg, G. Dolcetti, J.
Alloys Compd. 323-324 (2001) 584.
[19] C. de Leitenburg, A. Trovarelli, J. Llorca, F. Cavani, G. Bini, Appl. Catal.
A 139 (1996) 161.
[20] C. Bozo, N. Guilhaume, E. Garbowski, M. Primet, Catal. Today 59 (2000)
33.
[21] R.B. Anderson, K.C. Stein, J.J. Feenan, L.J.E. Hofer, Ind. Eng. Chem. 53
(1961) 809.
[22] R.J. Farrauto, M.C. Hobson, T. Kennelly, E.M. Waterman, Appl. Catal. A
81 (1992) 227.
[23] R. Burch, F.J. Urbano, Appl. Catal. A 124 (1995) 121.
[24] J.G. McCarty, Catal. Today 26 (1995) 283.
[25] R.J. Farrauto, J.K. Lampert, M.C. Hobson, E.M. Waterman, Appl. Catal.
B 6 (1995) 263.
[26] G. Groppi, C. Cristiani, L. Lietti, P. Forzatti, Stud. Surf. Sci. Catal. 130
(2000) 3801.
[27] R. Burch, D.J. Crittle, M.J. Hayes, Catal. Today 47 (1999) 229.
[28] S.C. Tsang, J.B. Claridge, M.L.H. Green, Catal. Today 23 (1995) 3.
34
[29] E.P.J. Mallens, J.H.B.J. Hoebink, G. B. Marin, J. Catal. 167 (1997) 43.
[30] J. C. Slaa, R.J. Berger, G.B. Marin, Catal. Lett. 43 (1997) 63.
[31] W.Z. Weng, M.S. Chen, Q.G. Yan, T.H. Wu, Z.S. Chao, Y.Y. Liao, H.L.
Wan, Catal. Today 63 (2000) 317.
[32] W.Z. Weng, C.R. Luo, J.J. Huang, Y.Y. Liao, H.L. Wan, Top. Catal. 22
(2003) 87.
[33] O.V. Buyevskaya, D. Wolf, M. Baerns, Catal. Lett. 29 (1994) 249.
[34] O.V. Buyevskaya, K. Walter, D. Wolf, M. Baerns, Catal. Lett. 38 (1996)
81.
[35] K. Heitnes Hofstad, J.H.B.J. Hoebink, A. Holmen, G.B. Marin, Catal.
Today 40 (1998) 157.
[36] F. Basile, G. Fornasari, F. Trifirò, A. Vaccari, Catal. Today 64 (2001) 21.
[37] W-S. Dong, K-W. Jun, H-S. Roh, Z-W. Liu, S-E. Park, Catal. Lett. 78
(2002) 215.
[38] L.V. Mattos, E. Rodino, D.E. Resasco, F.B. Passos, F.B. Noronha, Fuel
Process. Technol. 83 (2003) 147.
[39] R. Burch, F.J. Urbano, P.K. Loader, Appl. Catal. A 140 (1996) 17.
[40] J.C. van Giezen, F.R. van den Berg, J.L. Kleinen, A.J. van Dillen, J.W.
Geus, Catal. Today 47 (1999) 287.
[41] K. Fujimoto, F.H. Ribeiro, M. Avalos-Borja, E. Iglesia, J. Catal. 179
(1998) 431.
[42] D. Ciuparu, N. Katsikis, L. Pfefferle, Appl. Catal. A 216 (2001) 209.
[43] R. Burch, P.K. Loader, F.J. Urbano, Catal. Today 27 (1996) 243.
[44] D. Ciuparu, E. Perkins, L. Pfefferle, Appl. Catal. A 263 (2004) 145.
35