Fuel Cell Grade Hydrogen Production from the Steam Reforming of Bio-Ethanol Over Co-based Catalysts: An Investigation of Reaction Networks and Active Sites
Honors Thesis for Graduation with Distinction Submitted May 2005 By Drew J. Braden
The Ohio State University Department of Chemical and Biomolecular Engineering
140 West 19th Avenue Columbus, OH 43210
Honors Committee: Professor Umit S. Ozkan, Advisor
Professor Kurt Koelling
Acknowledgements
I would like to cordially thank each person that has invested his or her time and
confidence in me during this project. This has been a truly unforgettable experience and I
have you all to thank. First and foremost, I would like to thank my family. Their
inconceivable love and support has and will enable me to achieve my potential in
everything I do. I would also like to thank my second family, the Heterogeneous
Catalysis Research Group. Dr. Umit S. Ozkan wholeheartedly welcomed me into her
group and provided me with invaluable direction and experience; I could never thank her
enough. My mentor and inspiration from the beginning was Paul H. Matter. Dr. Rick B.
Watson is responsible for the continued success of this project and I sincerely thank him
for his time, wisdom, and style. A special thank you also goes out to my roommates and
their capacity to entertain my constant mental and physical involvement in this project.
They were my dose of reality at the end of every day.
Abstract
The catalytic steam reforming of bio-ethanol offers a highly attractive route for
catalytically converting biomass to hydrogen. A cost-effective, non-precious metal,
supported cobalt catalyst system has been developed that is effective for ethanol
reforming to produce fuel cell grade hydrogen. A series of cobalt catalysts have been
synthesized using zirconia as a support. Catalyst testing on a lab scale continuous flow
reaction system with a packed catalyst bed showed the best performance for the 10% Co-
Zr catalyst at a reaction temperature of 450°C. The optimal catalyst parameters were
determined using the characterization techniques BET surface area analysis, temperature
programmed reduction (TPR), Laser Raman Spectroscopy, thermal gravimetric analysis
(TGA), and Diffuse Reflectance Infra-red Fourier Transform Spectroscopy (DRIFTS).
Table of Contents List of Figures................................................................................................................5
1. Introduction..............................................................................................................6 2. Literature Review...................................................................................................13
3. Experimental Methods ...........................................................................................17 3.1 Catalyst Preparation ....................................................................................................17 3.2 Activity Testing ............................................................................................................19 3.3 BET Surface Area Measurements ...............................................................................22 3.4 Temperature Programmed Reduction.........................................................................22 3.5 Thermal Gravimetric Analysis ....................................................................................23 3.6 Diffuse Reflectance Infra-red Fourier Transform Spectroscopy................................23 3.7 Laser Raman Spectroscopy...........................................................................................24
4. Results and Discussion ...........................................................................................25 4.1 Activity Testing ............................................................................................................25 4.2 BET Surface Area Measurements ...............................................................................32 4.3 Temperature Programmed Reduction.........................................................................36 4.4 Thermal Gravimetric Analysis ....................................................................................38 4.5 Laser Raman Spectroscopy..........................................................................................40 4.6 Diffuse Reflectance Infra-red Fourier Transform Spectroscopy................................42
5. Summary.................................................................................................................45
6. Future Work ...........................................................................................................47 Bibliography ................................................................................................................48
Appendix......................................................................................................................52
List of Figures
Figure 1 The electrochemistry of a fuel cell ....................................................................8
Figure 2 The overall process of producing hydrogen from biomass...............................10
Figure 3 Schematic of the reactant humidifying vessel..................................................20
Figure 4 Ethanol steam reforming of supported cobalt catalysts ....................................26
Figure 5 Normalized reaction results for cobalt dispersion on varying supports ............28
Figure 6 Reaction testing for the effect of cobalt loading on zirconia ............................29
Figure 7 Comparison of 10% Co/Zr reaction results with literature values ....................31
Figure 8 Cobalt dispersion measurements for varying supports.....................................33
Figure 9 Cobalt dispersion for Co/Zr catalysts for various Co loading ..........................34
Figure 10 Dispersion of 10% Co/Zr at varying reduction temperatures .........................35
Figure 11 TPR profile of zirconia supported cobalt catalysts.........................................37
Figure 12 TGA calcinations of 10% cobalt catalysts on various supports ......................39
Figure 13 Laser Raman spectroscopy of zirconia supported cobalt................................41
Figure 14 Mass spectrometer results for 10% Co/Zr during ethanol reforming reaction44
Figure 15 DRIFTS results for 10% Co/Zr catalyst during ethanol reforming reaction....44
Figure 16 The effect of preparation parameters on catalytic properties..........................45
1. Introduction
The fossil fuel era that has fathered the globalization of industry and
transportation is coming to a close. Petroleum resources are running dry and the
vulnerability of our economy and, more importantly, our environment has been exposed.
With each passing day the need for an alternative source of �clean� energy becomes more
pronounced.
Hydrogen, the lightest and most ubiquitous element in the universe, is a �cleaner�
energy source since its combustion produces only water and energy. A more efficient use
of hydrogen is as a fuel for fuel cells. Using electrochemistry, the chemical energy of
hydrogen can silently be converted to electricity without the excessive thermal energy
loss observed in combustion engines. However, hydrogen rarely exists in its free form in
nature. In order for hydrogen energy to fulfill its potential for protecting the environment
and decreasing our nation�s dependence on foreign oil, development of efficient
technologies for hydrogen production from renewable energy sources are essential.
Fuel cells are becoming a reality as a means of generating clean energy. Their
prospective utility for automobiles is being enthusiastically recognized as a possible
candidate for revolutionizing the world and fundamentally reinventing the traditional
automobile. Automakers around the globe have spent more than $2 billion on research
and development of fuel-cell-powered cars, trucks and buses with hopes of mass
producing the environmental friendly cars by the end of our current decade (23). Today�s
internal combustion engine cars are only 25 to 30 percent efficient in converting the
energy content of fuels into drive-wheel power while fuel cells can yield up to 80 percent
efficiency (15).
A fuel cell is a device that generates electricity through the electrolytic reaction of
hydrogen with oxygen to form water. A single fuel cell is comprised two oppositely
charged sides, the negative being the anode and the positive being the cathode. Between
the oppositely charged plates is an electrolyte membrane center that allows only the
permeation of protons. The electrolyte middle can be a polymer membrane as found in
PEM fuel cells or it can be a watery acidic or alkaline solution (15). Pressurized
hydrogen is fed to the anode at which point a catalyst directs the splitting of the molecule
hydrogen into two protons and two electrons. On the cathode pressurized hydrogen is
split into negatively charged atomic oxygen species on the surface of specialized
catalysts. The electrons must travel from the anode to the cathode through an external
circuit while the protons permeate through the electrolyte to meet up with the oxygen
atoms. As the electrons travel through the external circuit their electronic potential may
be harnessed and used to do work. Figure 1 depicts how a fuel cell works. The
individual reactions and the net reaction are shown below.
2H2 ! 4H+ +4e- (anode reaction)
O2 + 4H+ +4e- ! 2H2O (cathode reaction)
2H2 + O2 ! 2H2O (net reaction)
Main fuel cell plates may be linked together to form a �stack� capable of producing
increasing large amounts of energy. The development of fuel cell vehicles is well
advanced, but is not matched by the progress in producing the hydrogen necessary for the
fuel cell.
Figure 1 The electrochemistry of a fuel cell
Fuel reforming is a necessary step for the integration of fuel cells into today�s
society. It is likely that the main source of hydrogen will vary with the geographical
location of its demand. For example, in Texas natural gas reforming may be used to
produce hydrogen where as in Iceland the hydrolysis of water using thermal energy
would be preferred (6). For Ohio and the surrounding Midwest and Central states
bioethanol would be an ideal choice for a hydrogen carrier because of fruitful agriculture.
The catalytic steam reforming of bio-ethanol offers a highly attractive route for
catalytically converting biomass to hydrogen. Bioethanol is easier to reform than
gasoline or natural gas based on reaction temperature, it is safer to handle than methanol,
and already has the ethanol-to-water ratios required for the reforming reaction, 10% to
25% ethanol.
Ethanol can be produced renewably from several biomass sources, including
plants, waste materials from agro-industries, and even organic fractions of municipal
solid waste. Biomass is the intermediate step in harnessing solar energy and converting it
to electricity. Photosynthesis uses solar energy to convert carbon dioxide from the
atmosphere to carbohydrates. In comparison to fossil-fuel-based systems, the bioethanol-
to-hydrogen system has a significant advantage of being nearly CO2 neutral, since the
CO2 produced from the reforming reaction is consumed for biomass growth. This forms
a nearly closed CO2 loop that is a noteworthy advantage over methanol, which is
primarily produced from non-renewable fossil fuels creating fossil carbon pollution. The
overall process of converting the solar energy absorbed by biomass into usable hydrogen
is depicted in figure 2.
Figure 2 The overall process of producing hydrogen from biomass
The use of ethanol as a transportation feedstock would greatly lower our Nation�s
dependence on imported foreign oil that continues to increase in price. Currently, the
United States produces about 2.8 billion gallons of industrial alcohol a year from bio-
mass. Ethanol has more qualified advantages over methanol for transportation
applications since it is much less toxic and offers a high octane number, a high heat of
vaporization, and a low photochemical reactivity (27). Ethanol is most commonly
converted directly to hydrogen through two main reactions, steam reforming and partial
oxidation. The two reforming techniques are described by the respective reactions:
C2H5OH + 3H2O = 6H2 + 2CO2 (steam reforming of ethanol)
C2H5OH + 3/2O2 = 3H2 + 2CO2 (partial oxidation of ethanol)
The steam reforming reaction is highly endothermic and requires a reaction temperature
of 300°C for the reaction to take place. The partial oxidation reaction is exothermic and
may reach a reaction temperature in excess of 400°C. Previous studies on ethanol
reforming have shown that the reactions are accompanied by side reactions that produce
unwanted byproducts such as carbon monoxide and methane (19-22). This poses a
problem for fuel cells because the catalysts used in the fuel cell anodes are very sensitive
to CO, which chemisorbs to the active sites of the catalyst. If the fuel cell�s feed stream
contains more than 10 ppm carbon monoxide it will chemisorb deactivating the catalysts,
reducing the number of active sites for H2 and which, in turn, will decrease the energy
efficiency of the fuel cell (15). Therefore, it is imperative that the catalyst for the
reforming reaction must have a high selectivity towards producing H2 with minimal side
reactions. In automobiles there would be an additional CO reformer that further reduces
the amount of CO in the feed stream before it reaches the fuel cell. This is accomplished
using the water-gas shift reaction that is described by the following reaction:
CO + H2O = CO2 + H2 (water-gas shift reaction)
Research in this area is also well aligned with Governor Bob Taft�s Third Frontier
Fuel Cell Coalition project which is a $100 million, three-year initiative that will position
Ohio as a national leader in the growing fuel cell industry through investment in research,
project demonstration, and job creation for Ohio citizens. Ohio State is well positioned
to contribute and become a leader in the fuel cell industry as it has strong research and
development capabilities, is a leader in academic fuel cell research, and has a rich
collaborative network from several backgrounds.
2. Literature Review
Technologies are currently available for the production of hydrogen from
hydrocarbon sources. Though there is no agreement on the most effective and cost
efficient hydrogen production catalytic system, three principle pathways have been
elucidated: steam reforming, partial oxidation, and autothermal reforming (1, 24). Brown
(4) examined six possible �primary� fuels for hydrogen production�methanol, natural
gas, gasoline, diesel fuel, aviation jet fuel, and ethanol. The study concluded that the
combination of steam reforming and partial oxidation of methanol is theoretically the
most qualified in terms of energy inputs and possible by-products. However, the toxicity
and current infrastructure for the production of methanol make it an unlikely candidate
for commercial use. Ethanol is much less hazardous and can be produced renewably
from biomass thus making it a more attractive fuel for hydrogen production (27). The
steam reforming reaction is frequently used for industrial applications and produces a
high concentration of hydrogen.
The stoichiometric H2 yield from the steam reforming of ethanol is 6 moles H2 for
every 1 mole of ethanol reacted. Using sequential quadratic programming (SQP)
Vasudeva and coworkers (25) determined the equilibrium product distributions of the
ethanol reforming reaction and compared them to similar calculations performed by
Garcia (12). Both groups determined that the H2 yield increased with increasing
temperature as well as increasing the water to ethanol feed ratio. Vasudeva took into
consideration carbon deposition on the surface of the catalyst via the Boudard reaction,
shown below, and 9 reaction products � ethanol, acetaldehyde, methane, carbon
monoxide, carbon dioxide, hydrogen, water, ethylene, and carbon deposition.
2CO ! CO2 + C (Boudard reaction)
Garcia only considered six reaction products�ethanol, water, carbon monoxide,
methane, carbon dioxide, and hydrogen. The more realistic calculations by Vasudeva
determined that the equilibrium yield of H2 for a 10:1 water to ethanol feed ratio starts at
4.4 moles per mole of ethanol fed at 527°C and increases up to 5.3 moles per mole of
ethanol fed at 927°C. These high temperatures are not feasible for commercial
application however the observed trends are important for reaction parameter
considerations.
The existing research that has been performed on the catalytic steam reforming of
ethanol has focused on Ni and Cu catalyst systems. Marino and coworkers (19-22) have
investigated the use of Cu-Ni supported catalysts. The group investigated the effect of
Cu and Ni loading and determined that the Cu phase present on the catalyst�Copper
basic nitrate, CuAl, and/or CuNiAl�was highly dependent upon thermal pretreatment
conditions whereas the nickel was always found as NiAl2O4. Higher calcinations
temperature caused a greater Cu-Ni interaction and decreased the reducibility of nickel
thus hindering the catalysts performance (21). In their later work (22), Marino and
coworkers proposed a possible mechanism for the reforming reaction over the Cu-Ni/γ-
Al2O3 catalysts. The group also determined that for the catalysts tested a greater
residence time and lower water to ethanol ratio is favored. In work similar to Marino,
Velu and coworkers (26) examined the CuNiZnAl and CoNiZnAl catalysts for the
reforming of ethanol. It was determined that nickel was involved in the rupture of the C-
C bond of ethanol to produce CO, CO2, and CH4. The group also determined that CoNi
based catalysts exhibited reaction performance with fewer unwanted byproducts such as
CO, CH4, and CH3CHO.
The use of nickel supported catalysts has also been investigated. Freni and
coworkers (9) examined Ni/MgO catalysts for the steam reforming of ethanol. It was
determined that the basic character of the MgO support improved the electronics of the
nickel and decreased the amount of carbon deposition on the catalyst. In later work (10)
the group compared the catalytic performance of Ni/MgO and Co/MgO catalysts. It was
determined that the Ni/MgO catalysts had a greater activity and selectivity for H2. The
difference was attributed to the oxidation of Co during the reaction, resulting in carbon
monoxide methanation. In a further study (11) the group compared MgO supported
noble metals, Pd and Rh, as well as Ni and Co. Through deactivation testing it was
determined that Rh/MgO was the most active and had the greatest stability for the steam
reforming of ethanol. A study by Fatsikostas (8) investigated the Ni catalysts supported
on La2O3, Al2O3, YSZ, and MgO. The La2O3 supported Ni catalyst was determined to be
the most active.
Several other studies have focused on using noble metals for reforming reactions.
A study by Liguras and coworkers (16) investigated the performance of the noble metals
Rh, Ru, Pt, and Pd on the supports Al2O3, MgO, and TiO2. Rh was determined to have
the greatest activity. The overall order of ethanol conversion efficiency was determined
to be Rh > Pt > Ru = Pd. Similar results were reported by Breen (3). However, Rh, as
well as other noble metals, is very expensive and it would not be feasible for the
commercialization of hydrogen production via steam reforming.
The focus of current research has shifted towards finding a transition metal
alternative that can perform as well as the noble metal catalysts. Cavallaro and
coworkers (5) developed a Co/MgO catalyst that showed comparable catalytic
performance to Rh/Al2O3. It was determined that the acidic nature of the Al2O3 support
caused an increase in coke formation on the Co/Al2O3 catalyst. Studies by Haga and
coworkers (13, 14) examined cobalt catalysts on various supports. In contrast to the
findings of Cavallaro, Haga determined that Co/Al2O3 had a greater activity and
selectivity than Co/MgO in the steam reforming reaction (14).
Upon reviewing existing studies over the various reforming catalysts it is evident
that the role of the support is influential in the overall performance of a reforming
catalyst. Llorca and coworkers (17) investigated the performance of 1% cobalt reforming
catalysts using various supports � MgO, g-Al2O3, SiO2, TiO2, V2O5, ZnO, La2O3, CeO2,
and SmO2 � prepared by impregnation. The catalysts were tested for the steam reforming
of ethanol with a water to ethanol ratio of 13 at a reaction temperature of 500°C and a gas
hour space velocity of 5000 h-1. The group determined that Co/ZnO was selective only
for H2 and CO2 while the other oxygenated supports produced unwanted byproducts such
as acetaldehyde and dimethyl ether (17, 18). In a study by Aupretre (2) the influence of
metal used as well as the role of the support was examined. The catalytic activity for the
steam reforming catalysts increased with increasing hydroxyl mobility but decreased for
catalysts that promoted the water gas shift reaction. Aupretre reported that these findings
were in agreement with the bifunctional mechanism proposed by Duprez (7).
3. Experimental Methods
3.1 Catalyst Preparation
The Co-Zr reforming catalysts were prepared in-house using purchased precursor
materials. Two techniques utilized were sol-gel chemistry and incipient wetness.
The precursor cobalt material used for both catalyst preparation techniques was cobalt
(II) nitrate hexahydrate. The ziroconia support material differed for the individual
preparation techniques.
The first catalyst preparation technique used was sol-gel chemistry. The
precursor material used was zirconia (IV) propoxide which is 70% weight in propanol.
First, zirconia support was prepared in two trials without addition of cobalt solution in
order to exam the effect of the nonpolar solvent hexane on the rate of hydrolization and
branching. For the first trial that did not include hexane, the stoichiometric amount of
water was added drop wise using a syringe to the zirconia (IV) propoxide solution while
at room temperature. The solution was continuously stirred using a stir bar. White
clumps approximately 2 mm in diameter formed immediately upon the addition of water.
Once all the water was added the solution was stirred for an additional 20-30 min. the
solution was dried overnight and a white granular solid resulted. For the second trial 100
mL of hexane was added to 20 mL of the zirconia (IV) propoxide solution.
Stoichiometric amounts of water was then added drop wise using a syringe. The solution,
at room temperature, was continuously stirred using a stir bar. No precipitate was
immediately observed upon addition of water. Approximately 5 minutes after all the
water had been added a white gel began to form. Stirring was discontinued at the
resulted gel was allowed to dry overnight. The resulting support was a low density white
powder.
Cobalt catalysts prepared using the sol gel method were performed using hexane
as a dilettante, the method described above. The desired cobalt loading was achieved by
dissolving cobalt (II) nitrate hexahydrate in the stoichiometric amount of water. The
aqueous cobalt solution was then added to the determined amount of zirconium (IV)
propoxide at room temperature while being continuously stirred by a stir bar. The
resulting gel was allowed to dry over night. The pinkish-purple powered Co-Zr material
was then calcined. A black fine powdered catalyst resulted.
In the incipient wetness preparation technique cobalt containing solution is added
drop wise onto a precalcined zirconia support powder. The cobalt solution is added so
that the pores of the zirconia support are filled with the cobalt solution while the surface
of the particles remains dry. A pelletized zirconia purchased from Saint Gobain was used
as the starting material for the support. The zirconia pellets were ground into powder
using an electric grinder. The powder zirconia support was then sifted through a 100-150
mesh. The sifted zirconia was then calcined at 350°C for 3 hours under atmospheric
conditions. BET surface measurements were performed and the pore dimensions of the
support were determined. The desired amount of cobalt (II) nitrate hexahydrate was
dissolved in distilled water and added drop wise to a determined amount of zirconia
powder. Once the pores became filled to capacity the Co-Zr material was dried in an
oven for 30 min at approximately 105°C. The process of adding the cobalt solution was
then repeated until the solution had been depleted. The resulting Co-Zr material was then
dried in the atmosphere overnight. A deep pinkish-purple color resulted from the
addition of cobalt. The catalyst precursor was then calcined. A black fine powdered
catalyst resulted.
The calcinations of the catalyst precursors were performed in a tube furnace. The
temperature at which the catalysts were calcined varied between 350°C to 550°C
depending on the desired experimental parameters. The tube furnace was open to the
atmosphere. The resulting black catalysts were labeled and stored in glass vials until
experimental testing.
3.2 Activity Testing
A laboratory scale continuous flow reaction system was built for the experimental
testing of the reforming catalysts. A schematic of the reaction system is included in the
appendices. The system was built using 1/8� and 1/4� 316 stainless steel tubing and
Swagelock fittings. A furnace was constructed using high temperature cement, metal
wire of the desired resistance, and a metal hinged casing. The catalyst samples were
loaded into a removable 1/4" 316 stainless steel plug flow reactor. A metal grating fixed
in the center of the reactor supported a wad of silica wool on which the catalyst samples
were implanted. The catalyst loading varied between 20 to 100 mg depending on the
experimental parameters. A thermocouple was inserted in the reactor below the catalyst
bed. The temperature of the reactor was controlled by an Omega CN76000 temperature
controller. A pressure gauge was located just upstream of the reactor.
Argon and Nitrogen were used as carrier gases for the reaction system. The flow
rates of the respective gases were controlled using individual flow controllers. The flow
controllers were calibrated regularly using a bubble flow meter. The reactants, ethanol
and water, were introduced into the system using �bubblers�. The bubblers were
stainless steel vessels that contained the individual reactants, water and ethanol. The
temperatures of the bubblers were controlled using external heating tapes that were
connected to Omega temperature controllers. The inner workings of the reactant
bubblers are depicted in figure 3.
Figure 3 Schematic of the reactant humidifying vessel
The reactant vapors are mixed and travel through heated metal tubing to a six-port
valve. The six-port valve has two modes: reaction and pretreatment/bypass. In the
reaction mode the six-port valve direct the reactant vapor stream into the reactor. The
products from the reactor travel back into the six-port valve and then flow through
another heated metal tube to the gas chromatograph to be analyzed. In the
pretreatment/bypass mode the six-port valve direct the reactant vapor stream directly to
the gas chromatograph, bypassing the reactor.
Pretreatment gases were used to prepare a catalyst before performing a reaction
experiment. Hydrogen diluted in nitrogen reduced the catalyst before a reaction. In the
pretreatment/bypass mode the pretreatment gas was flown through the six-port valve into
the reactor. The flow from the reactor travels back into the six-port valve and is then
vented out of the system. In the reaction mode the pretreatment gas was immediately
vented after the six-port valve without entering the reactor.
The products of the reaction were flown into a gas chromatograph and analyzed
via a thermal conductivity detector (TCD) and a flame ionization detector (FID). A
schematic of the GC valve configure is included in the appendices. The expected
reaction products were determined by examining existing research literature. A table of
possible reaction products is included in the appendices. The GC contained a porapack-Q
column that was connected in series to a molecular sieve. An additional column was
required to effectively separate larger organic molecules that are produced during the
catalytic reforming of ethanol. A carbowax column was purchased and installed in the
GC. The final configuration has the porapak-Q and molecular sieve in series and those
two columns in parallel with the carbowax column. A pressure buffer valve was installed
to compensate for the pressure variation when switching the flow between the molecular
sieve and bypass mode.
Preparatory measurements were taken once the reforming system was in operating
order. Spreadsheets were constructed and utilized to calculate desired flows of the
reactants based on the flow of the inert gas, temperature of the bubbler containing the
reactant, and system pressure. TCD and FID response factors and retention times were
calculated for the GC using gas standards for the expected reactants and products.
3.3 BET Surface Area Measurements
The surface characteristics of the catalysts were determined using a Micromeritics
ASAP 2010. This system was used for surface area measurements, pore size studies, and
chemisorption analysis. Carbon monoxide chemisorption was used to determine the
cobalt dispersion on the support, the ratio of exposed metal atoms to total metal atoms.
The catalysts were reduced in situ for 3 hours using 5% H2/He. The catalyst sample were
analyzed pre-calcination as well as post-calcination to examine the change in surface
properties.
3.4 Temperature Programmed Reduction
A TPR/TPD system was used to characterize the reduction characteristics of the
catalysts. The system is capable of using different adsorbates and reducing agents to treat
the catalyst samples and is equipped with a vacuum system (10-8 torr). Catalyst samples
(0.1 g) were loaded into a 1/8� quartz U-tube reactor and supported by wads of quartz
wool. Nitrogen was used to flush the system after loading the sample. The catalysts
were calcined in situ for 30 minutes at 300°C while flowing oxygen at approximately 10
sccm. 5% H2/N2 was then introduced to the system as the reducing agent. The catalysts
were reduced at a controlled ramping rate of 10°C per minute from 50°C to 800°C. The
effluent from the reactor was measured online via a Hewlett Packard gas chromatograph
and mass spectrometer.
3.5 Thermal Gravimetric Analysis
The thermal gravimetric analysis experiments were performed in house on a
Perkin-Elmer TGA7. The system is capable of quantitatively measuring the change in
mass of a sample as a function of temperature up to 1000°C. The change is weight is
then related to surface phenomena on the catalyst. The catalysts analyzed in the TGA
were 10% Co loaded on aluminum oxide, titanium oxide, and zirconium oxide supports.
Air was flown through the TGA at 25 mL/min as the temperature was ramped at
10°C/min. The effect of calcination on the catalysts was measured.
3.6 Diffuse Reflectance Infra-red Fourier Transform Spectroscopy
Fourier Transform Infra-red Spectroscopy is a technique which allows
characterization of catalysts and/or adsorbed species under reaction conditions. The
radiation that reflects from an absorbing material is composed of surface-reflected and
bulk re-emitted components, which summed are the diffuse reflectance of the sample.
The DRIFTS experiments were performed on a Bruker IFS66 DRIFT spectrometer
equipped with a MCT detector.
A mass spectrometer is an instrument that can separate charged atoms or
molecules according to their mass-to-charge ratio. Mass Spectrometry is an analytical
technique that is used to identify unknown compounds, quantify known materials, and
elucidate the structural and physical properties of ions. A Shimadzu QP5050 quadrupole
mass spectrometer with splt/splitless injector was used to collect the MS data.
The catalyst used for the DRIFTS experiment was the 10% Co/ZrO
calcined at 350°C/12 hrs. The catalyst was pre-reduced in situ at 400°C while flowing
5% H2/He for 1 hour. An ethanol and water mixture was then allowed to adsorb onto the
catalyst surface for one hour. The system was then flushed with helium. The reaction
temperature was ramped at 10°C/min under helium flowing at 30 mL/min. The sample
was measured at a split ratio of 5 for a total of 50 scans.
3.7 Laser Raman Spectroscopy In Raman spectroscopy light is focused on a surface and the incident photons are
subsequently scattered by the molecules. Inelastically scattered light is called Raman
scatter. The energy difference between the incident light and the Raman scattered light is
equal to the energy involved in changing the molecule's vibrational state. This energy
difference, called the Raman shift, is unique for a given molecule. Based on the Raman
Shift bands, molecular species can be determined. The Raman spectroscopy was
performed using a 514.5nm argon ion laser on a Laser Raman spectrometer (Kaiser) with
a 1000x microprobe. The three catalysts tested were 10% cobalt loaded on alumina,
titania, and zirconia supports.
4 Results and Discussion
4.1 Activity Testing
The catalyst activities were measured according to their extent of ethanol
conversion, H2 yield, ethanol conversion rate, and gas hour space velocity. These terms
are defined as follows:
The initial goal for the reaction testing of the catalysts was to investigate the role
of the support. Catalysts were prepared using Al2O3, TiO2, and ZrO2 supporting 10% Co
by mass. The catalysts were tested for the steam reforming of ethanol at a 10:1 water to
ethanol ratio. Figure 4 displays the results of reaction testing using various supports. For
the same catalyst weight loading, reduction procedure, and reaction conditions the
ethanol conversion for the Co/Zr catalyst was essentially the same as the ethanol
conversion for the Co/Al catalyst. It was initially determined that the Co/Zr catalyst had
the highest H2 yield. The Co/Ti catalyst showed the least activity for ethanol steam
reforming.
Figure 4 Ethanol steam reforming of supported cobalt catalysts
Using the CO chemisorption results for the cobalt dispersion on the three different
supported catalysts the reaction results were normalized. The amount of exposed cobalt
available for participating in the reaction was determined for each catalyst based on the
amount of catalyst used. The normalized reaction results are shown in figure 5. The
ethanol conversion rate was determined to be approximately the same for all three
supported catalysts. This suggests that the cobalt metal is the active component of the
steam reforming reaction. However, it should be noted that the zirconia support provided
the largest cobalt dispersion for equal catalyst weight. Therefore, less materials are
necessary when zirconia is used as a support. For these reasons the zirconia supported
catalyst was chosen for further investigation.
The next reaction experiment investigated the effect of cobalt loading on the
zirconia support. Three catalyst�5%, 10%, and 15% cobalt loading�were tested for the
steam reforming of ethanol. The results from the experiments are shown in figure 6. The
three catalysts displayed activity proportional to their respective cobalt loadings at 350°C
and 400°C. As the temperature was increased to 450°C and 500°C the 10% cobalt on
zirconia catalyst showed the greatest increase in activity and was determined to have the
largest hydrogen yield.
Figure 5 Normalized reaction results for cobalt dispersion on varying supports
Figure 6 Reaction testing for the effect of cobalt loading on zirconia
It was determined that the 10% cobalt supported on zirconia catalyst was the
optimal ethanol steam reforming catalyst based on reaction experimentation. To further
investigate its catalytic properties the ethanol steam reforming reaction was run at various
gas hour space velocities (GHSV). This was accomplished by adjusting the feed flows as
well as the amount of catalyst loaded into the reactor. The data obtained from the
reaction testing was then used for comparison with literature values for the steam
reforming of ethanol over different catalysts. The result of the comparison with literature
values can be found in figure 7.
The catalysts used in the comparison were magnesia supported cobalt and
alumina supported rhodium, a noble metal. Cavallaro and coworkers (5) prepared and
tested the catalysts and reported the mol hydrogen produced/mol ethanol fed. The
greatest possible theoretical value for this ratio is 6 meaning that a total of 6 hydrogen
molecules can be produced from the conversion of 1 mole of ethanol. At the lower
GHSV the rhodium catalyst produced a higher ratio of hydrogen to ethanol than the 10%
Co/Zr catalyst. However, this GHSV is too low for practical purposes as the amount of
hydrogen produced per time is not sufficient to fulfill commercial hydrogen needs.
Looking at the higher end of the GHSV tested it can be seen that the 10% Co/Zr catalyst
shows increasing activity and comparable results to the rhodium catalyst. The higher end
of the GHSV is the region of interest for hydrogen production on a larger scale. It should
be noted that the 10% Co/Zr consistently performed better than the Co/Mg catalyst from
the Cavallaro group.
Figure 7 Comparison of 10% Co/Zr reaction results with literature values
4.2 BET Surface Area Measurements
CO chemisorption measurements were first performed on the three cobalt catalysts of
varying supports. The catalysts tested were 10% Co on alumina, titania, and zirconia.
The results for the characterization of the effect of the support are shown in figure 8. It
was determined that the zirconia supported catalyst by far had the greatest cobalt
dispersion. The least cobalt dispersion was observed for the titania supported catalyst.
The zirconia supported cobalt catalyst was investigated further because of the large
dispersion found from the support characterizations. The next set of CO chemisorption
experiments examined the effect of cobalt loading. The three catalysts tested were 5%,
10%, and 15% cobalt loadings by weight on the zirconia support. The results from the
testing are shown in figure 9. It was observed that the cobalt dispersion has a maximum
value for the 10% cobalt on zirconia catalyst.
The next catalyst characterization experiments performed examined the effect of the
reduction temperature on the catalyst. The 10% Co/ZrO catalyst was chosen as the most
promising catalyst based on the previous CO chemisorption results and was thus used for
the reduction temperature investigation. Catalysts were reduced at 350°C, 400°C, 450°C,
and 500°C and were then measured for cobalt dispersion using CO chemisorption. The
results are organized in figure 10. The graph clearly shows that the catalyst reduced at
400°C possessed the greatest cobalt dispersion. These results suggest that the 10%
Co/ZrO catalyst reduced at 400°C is the optimal catalyst for reaction testing.
Figure 8 Cobalt dispersion measurements for varying supports
Figure 9 Cobalt dispersion for Co/Zr catalysts for various Co loading
Figure 10 Dispersion of 10% Co/Zr at varying reduction temperatures
4.3 Temperature Programmed Reduction
Temperature programmed reduction experiments were performed over three zirconia
supported cobalt catalysts. The effect of cobalt loading on the reduction of the catalyst
was examined for 5 wt%, 10 wt%, and 15 wt%. The catalysts, previously calcined at
350°C, were reduced at a controlled temperature ramp from room temperature up to 650
°C. A graph comparing the results of the three cobalt loadings can be found in figure 11.
The TPR results show that the 10 wt% cobalt loading is most easily reduced. This
corresponds to the maximum cobalt dispersion at 10 wt% cobalt loading as was
determined from the CO chemisorption using the Micrometrics ASAP machine discussed
in section 4.2. The change in oxidation state of cobalt is directly dependent upon the
temperature of the reaction as is expected. The first small peak observed for the
reduction corresponds to the combustion of residual nitrates remaining of the catalyst
surface. The larger, second peak positioned at approximately 300°C corresponds to the
reduction of Co3+ to Co2+. The third and final peak observed at approximately 475°C-
500°C corresponds to the reduction of Co2+ to Co0, metallic cobalt. It is this metallic
cobalt, Co0, that is believed to be related to the active catalyst species.
Figure 11 TPR profile of zirconia supported cobalt catalysts
4.4 Thermal Gravimetric Analysis
Thermal gravimetric analyses were performed on 10 wt% cobalt supported on the
three supports of interest, aluminum oxide, titanium oxide, and zirconium oxide. The
catalysts were calcined in the TGA system in order to determine the effect of the support.
A graph comparing the TGA results for all three catalysts can be found in figure 12.
As can be seen from the graph, the calcinations of the titania supported and alumina
supported cobalt catalyst are very similar. They each only have one sharp peak
corresponding to the weight loss associated with combustion of nitrates and precursor
materials from the catalyst. However, the calcinations of the zirconia supported catalyst
resulted in a unique graph consisting of three �shoulders� depicted by the arrows inserted
on the graph. These shoulders suggest that zirconia has a greater interaction with cobalt
than alumina and titania supports do.
Figure 12 TGA calcinations of 10% cobalt catalysts on various supports
4.5 Laser Raman Spectroscopy
Three zirconia supported catalysts with 5%, 10%, and 15% cobalt loading by weight
were examined using laser Raman spectroscopy. A cobalt oxide sample as well as a
blank zirconia support sample was also tested for comparison with the zirconia supported
cobalt catalysts. The results from the testing are organized in figure 13. The cobalt oxide
spectrum is the predominate spectrum observed for the three cobalt loaded catalysts.
There is little variation between the three different cobalt loadings. The blank zirconia
spectrum is not visible in the other samples as it is masked by the cobalt oxide.
Figure 13 Laser Raman spectroscopy of zirconia supported cobalt
4.6 Diffuse Reflectance Infra-red Fourier Transform Spectroscopy
The reaction mechanism was investigated using Diffuse Reflectance Infra-red
Fourier Transform Spectroscopy (DRIFTS) coupled with Mass Spectrometry (MS). The
results for the in situ reaction over the 10% Co/Zr catalyst are shown in figures 14 and
15. Figure 14 shows the data collected from the mass spectrometer while figure 15
shows the data collected from the DRIFTS instrument. The mass spectrometer data
elucidates what the products of the ethanol reforming reaction are as a function of
temperature. The data obtained from the DRIFTS instrument provides insight into the
surface species present on that catalyst as a function of temperature. Correlating the
experimental results from the two instruments provides insight into the reaction
intermediates and reaction pathway as a function of the reaction temperature.
At the low temperature, between 25°C and 250°C, there is little ethanol conversion.
In this region figure MS shows that the primary products from the reaction are ethanol
and acetaldehyde as well as the unreacted water that is fed in excess. Looking at figure
15 in this same temperature region it can be seen that on the catalyst surface are adsorbed
ethanol species and eth-oxy species. As the adsorbed ethanol and eth-oxy species
diminish with increasing temperature an adsorbed bidentate eth-oxy species begins to
appear in the DRIFTS spectra at approximately 150°C and are present up to a reaction
temperature of approximately 325°C. Acetaldehyde is likely formed from these adsorbed
species following their dehydrogenation.
The cleavage of the C-C bond of ethanol does not begin to occur until approximately
275°C. This temperature point marks the beginning of a sharp decrease in ethanol
produced from the reaction as shown in figure MS. At the corresponding temperature on
figure DF it can be seen that carbonato species begin to appear on the surface of the
catalyst following the cleavage of the C-C bond of ethanol. Looking at both figures in
the temperature range between 325°C and 450°C it is observed that only C1 species exist
on the surface of the catalyst while CO2 and CO are prevalent in the effluent of the
reactor. The amount of acetaldehyde formed also decreases within this temperature
range.
Figure 14 Mass spectrometer results for 10% Co/Zr during ethanol reforming reaction
Figure 15 DRIFTS results for 10% Co/Zr catalyst during ethanol reforming reaction
5 Summary
Cobalt is an active transition metal for the steam reforming of ethanol and has
promising catalytic properties that are competitive with precious metals. The reaction
performance and stability of the catalyst is a function of many parameters from the
precursor stage to the steady-state reaction. A flowchart of different catalyst stages and
their interdependence is shown below in figure 16. Each of the four catalyst stages were
investigated and optimal conditions were determined.
Figure 16 The effect of preparation parameters on catalytic properties
Zirconium oxide provides the best dispersion for the cobalt metal at the optimal 10
wt% cobalt loading. The amount of reduced cobalt is a function of cobalt loading and
dispersion and affects the overall catalytic performance. Calcination and reduction
parameters elucidated through catalyst characterization provide the optimal reaction
performance for the 10% Co/Zr catalyst studied. The mechanistic study of the surface
species on the catalyst showed that ethanol�s C-C bond is broken at approximately 300°C
and selectively forms carbon dioxide over other C1 species.
6 Future Work The completion of this thesis merely marks the beginning of the bio-ethanol
reforming project. The Heterogeneous Catalysis Research Group was selected by the
United States Department of Energy under George W. Bush�s Hydrogen Program to
conduct a 4-year $1.2 million project for hydrogen production from bio-ethanol. The
project is scheduled to begin in June 2005 and will involve the work of two graduate
students, one post doctorate, and several more undergraduate students.
A systematic and detailed study of bioethanol reforming has been proposed that
will provide fundamental answers that are not readily solved in an industrial setting.
More advanced catalysts will be prepared to enhance catalyst stability and selectivity in
accordance with the structure, dispersion, and active site distribution. The effect of
oxidative and autothermal reforming will also be determined. Reaction mechanisms will
be investigated using in situ IR spectroscopy for adsorption and desorption studies.
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Appendix
Appendix 1. Reaction system schematic
Appendix 2. Gas chromatograph configuration
Appendix 3. Possible ethanol reforming products
Appendix 1. Reaction system schematic
Appendix 2. Gas chromatograph configuration
Appendix 3. Possible ethanol reforming products
