Cobalt-Based Catalysts for Ethanol Steam Reforming: An Overview

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Cobalt-based Catalysts for Ethanol Steam Reforming: An Overview Hyuntae Sohn, and Umit S. Ozkan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00577 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on May 2, 2016

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Cobalt/Ceria-based Catalysts for Ethanol Steam Reforming: An Overview

Hyuntae Sohn1 and Umit S. Ozkan1*

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William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 151 W. Woodruff Avenue, Columbus, OH 43210

* Corresponding author Umit S. Ozkan E-mail: [email protected] Tel: (614)-292-6623

ACS Paragon Plus Environment

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Abstract Cobalt has been extensively studied for its use as an alternative catalyst to noble metals for ethanol steam reforming. Among the many other catalyst supports tested, cerium oxide contributed significantly to the catalytic activity and stability of the Co/CeO2 catalyst due to its high oxygen mobility and storage capacity. In this paper, an overview of the research conducted in our laboratory during the past decade on ethanol steam reforming using Co/CeO2 catalyst is presented. The role of the support oxygen mobility and storage capacity of the Co/CeO2 catalyst and how the addition of calcium to the ceria support changes the oxygen vacancies in the catalyst structure are discussed. Results from in-situ characterization techniques such as XRD, Raman, XANES and EXAFS are summarized, showing the evolution of the cobalt phases during synthesis, pre-treatment and ethanol steam reforming reaction. Also, how the synthesis parameters such as synthesis method, impregnation medium, choice of cobalt precursor, support morphology and particle size could alter the catalytic activity and selectivity have been outlined.

Keywords: ceria, cobalt, ethanol steam reforming


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1. Introduction The rapid increase in the world population and industrialization has led to a significant rise in energy consumption demands. As most of the energy is generated by combustion of fossil fuels today, the amount of greenhouse gases and toxic pollutants produced continues to rise as well. For these reasons, there have been attempts to utilize alternative and renewable energy sources as the next-generation of energy carriers. Among many other sources of energy, hydrogen has garnered a lot of attention because of its high energy storage capacity (120.7 kJ/mol) [1] and environmentally friendly combustion process which generates only water as the product. Especially use of hydrogen as a feedstock for proton exchange membrane (PEM) fuel cell is an especially attractive technology which converts chemical energy to electrical energy with higher efficiency and without producing any pollutants [2]. Currently, a large fraction of hydrogen is produced from natural gas steam reforming [3]. However, natural gas is also a fossil fuel and contributes to the carbon dioxide emissions. In order to have a closed carbon loop cycle, it is essential to use reactants originating from renewable sources such as biomass. In that sense, ethanol, which can be obtained through fermentation of biomass, is a good candidate. Although ethanol steam reforming produces carbon dioxide, it can be consumed by photosynthesis of plants, which then are used as a feedstock for biomass production, potentially completing the carbon loop cycle. Also, its non-toxicity and high solubility in water makes ethanol an attractive reactant for steam reforming processes [4]. In order to obtain high hydrogen yield and ethanol conversion, precious metals such as Pd, Pt, Rh, Ru, Re and Ir [5-16] have been tested and have shown promising catalytic activity for ethanol steam reforming. Especially over Rh-based catalyst, many studies have indicated the superior catalytic performance of Rh compared to other metals [6, 17-20]. For example, Breen et


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al. reported the order of catalytic activity to be Rh > Pd > Ni = Pt, supported on either ceria or zirconia for ethanol steam reforming [6]. Similarly, Aupretre and coworkers compared the catalytic activity of Rh, Pt, Ni, Cu, Zn and Fe and demonstrated that Rh is the most active catalyst for ethanol steam reforming [17]. Bimetallic systems such as Rh-Pt [21, 22], Rh-Pd [9, 23], Rh-Ni [24, 25], and Ru-Co [26, 27] have been widely studied for ethanol steam reforming as well. Although higher activities can be attained with precious metals, the catalyst cost limits the feasibility of this technology. Hence, non-precious metals such as Co, Ni and Cu [18, 28-34] have also been investigated at much higher metal loadings. Among these metals, cobalt has emerged as an active metal catalyst for C-C bond scission, which is an important characteristic for ethanol steam reforming [35]. Among the earlier studies performed regarding ethanol steam reforming, Llorca et al. have published studies on the catalytic activity of cobalt metal-based catalysts [28, 36-39]. High ethanol conversions over Co catalysts supported on different supports have also been reported [40-43]. The choice of support strongly affects the cobalt dispersion, particle size and reducibility, thereby altering the catalytic activity. Many metal oxide materials have been used as catalyst supports for cobalt including Al2O3 [44], ZrO2 [45, 46], SiO2 [47], CeO2 [48, 49] MgO [50] and ZnO [51, 52]. Based on a comparison study by Haga et al. [53], Co supported on Al2O3 revealed the highest catalytic activity compared to Co supported on SiO2, MgO, ZrO2 and carbon supports. They concluded that the change in support greatly influenced the catalytic performance mostly due to suppression of methanation and decomposition of ethanol reactions. Another study by Llorca and coworkers [28] demonstrated promising catalytic activity of cobalt over ZnO and CeO2 supports with high hydrogen selectivity compared to Co supported on MgO, γ -Al2O3, SiO2,


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TiO2, V2O5, La2O3 and Sm2O3. More recently, Moura et al. investigated the effect of support for RhCo on Al2O3, MgO and Mg-Al oxide catalysts. They reported the highest hydrogen yield and low ethylene selectivity over the RhCo/Mg-Al oxide catalyst [54]. In this paper, a series of studies performed in our laboratory, focusing on ethanol steam reforming over cobalt supported on cerium oxide (Co/CeO2) catalyst, are summarized. This includes investigations of catalytic activity and stability of the Co/CeO2 catalysts, effect of support oxygen mobility and storage capacity, evolution of cobalt phases during synthesis and reaction conditions, cobalt reduction and re-oxidation kinetics, effect of synthesis parameters, effect of support morphology and particle size, and a detailed adsorption/desorption mechanism under ethanol steam reforming conditions. Some of the characterization techniques implemented in these studies involve mass spectrometry (MS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray powder diffraction (XRD) and laser Raman spectroscopy.

2. Thermodynamics vs Kinetics The chemical reaction for ethanol steam reforming can be represented as follows: C2H5OH + 3H2O → 6H2 + 2CO2

(complete reforming of ethanol)

Ethanol steam reforming is an endothermic process which requires energy input in the form of heat during the reaction (∆H = 348 kJ/mol at 25 °C), with the two major products being H2 and CO2.


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A thermodynamic equilibrium analysis of the product distribution for different feed conditions was performed by using HSC 5.1 software [55] and is presented in Figure 1. Reaction parameters such as temperature, pressure and molar ratio of ethanol to water were altered and the resulting hydrogen yield and ethanol conversion were calculated. Regarding the reaction temperature and pressure, it was found that increase in both parameters improve the hydrogen yield. However, at higher temperatures above 500 °C, the formation of hydrogen was limited due to reverse watergas shift reaction (H2 + CO2 → CO + H2O). Below 400 °C, methane was the major product obtained. This was attributed to the methanation reaction which was thermodynamically favorable at lower temperatures due to the exothermicity of the reaction. The ethanol-to-water ratio under equilibrium conditions showed a significant impact on formation of carbon deposits on the catalyst surface. When the ratio was less than 1:5, no catalyst coking was predicted. Although the ethanol steam reforming reaction appears to be quite simple, and under thermodynamic equilibrium conditions, the number of possible products are few, when the reaction is governed by kinetics, there are many side and intermediate reactions that occur simultaneously, hence decreasing the hydrogen yield and forming many by-products such as CH4, CO, acetone, acetaldehyde, ethylene and coke. The following is a list of possible reactions that can take place during ethanol steam reforming. CH3CH2OH → CH4 + CO + H2

(ethanol decomposition)

CH3CH2OH + H2O → 2CO + 4H2

(incomplete reforming)

CH3CH2OH → CH3CHO + H2

(dehydrogenation)

CH3CH2OH → C2H4 + H2O

(dehydration)

C2H4 → Coke

(coke formation)

2CH3CH2OH → (C2H5)2O + H2O

(diethyl ether formation)


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CO + 3H2 → CH4 + H2O

(methanation)

CH4 → C + 2H2

(methane decomposition)

CH4 + 2H2O → 4H2 + CO2

(methane steam reforming)

CH3CHO → CH4 + CO

(acetaldehyde decomposition)

CH3CHO + 3H2O → 2CO2 + 5H2

(acetaldehyde steam reforming)

2CH3CHO → CH3COCH3 + CO + H2

(acetone formation)

C2H5OH + H2O → CH3COOH + 2H2

(acetic acid formation)

CO+H2O CO2+H2

(water-gas shift, reverse water-gas shift)

2 CO → CO2 + C

(Boudouard reaction)

2.1 Reaction network Ethanol steam reforming involves a complex reaction network. The adsorption of ethanol and water on the catalyst surface undergoes multiple side reactions thereby forming different intermediate surface species, which eventually lead to production of hydrogen and carbon dioxide. Figure 2 [56] presents a reaction scheme for ethanol steam reforming. This network was proposed based on a detailed reaction mechanistic study using in-situ DRIFTS, temperature programmed desorption (TPD) and thermogravimetric analysis (TGA) along with isotopic labeling techniques [56]. In Scheme 1, water adsorbs dissociatively, forming OH* and H* species on the surface, primarily on the surface of the support. Ethanol molecules can adsorb either molecularly and dissociatively on the Co sites (Scheme 2). Adsorbed ethanol molecules can decompose to form single C-species, (i.e., CH4 and CO) along with H2 (Scheme 3). The dissociatively adsorbed ethanol forms adsorbed ethoxide species (Scheme 2). The first H abstracted from ethanol can


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either form OH groups with the surface O* or combine with hydrogen from a surface OH* and form H2. Ethoxide species can move to the interface of metal and oxide support and be oxidized by an additional hydrogen abstraction forming acetaldehyde (Scheme 4). Acetaldehyde can desorb to the gas phase, decompose to CH4 and CO, or further oxidize to form acetic acid and surface acetate species. The formation of acetone might be derived from the aldol condensation of acetaldehyde, followed by a dehydrogenation and decarboxylation (scheme 4), as proposed by T. Nishiguchi, et al. [53]. Reactions between acetic acid and acetaldehyde are also possible. There are multiple routes for the acetate species once they are formed. In one of the routes, the metal may be involved in C-C bond cleavage leading to the formation of single carbon species (Scheme 5). It is also possible that acetate species dissociate to form surface methyl groups and CO2 (Scheme 5). The carbon-oxygen surface species may desorb or further oxidize to give carbonate species, especially on supports with high oxygen storage capacity, eventually desorbing as CO2. In an alternate route, the CH3 fragment (which can also form through hydrogen abstraction from methane) will undergo oxidation (through H abstraction and O addition) to form formate, possibly through a formaldehyde intermediate, and carbonate. The catalyst surface is then regenerated through CO2 desorption following carbonate decomposition and ready for the next catalysis cycle regardless of the route followed. Reforming of methane and water-gas and reverse water gas shift reactions are also possible steps over these catalysts (scheme 7). The acetate species can also accumulate on the catalyst surface and lead to coke formation [14, 15], resulting in deactivation if O* supplied through catalyst surface is insufficient. If the surface is highly acidic, ethanol dehydration may dominate the reaction pathway and result in the


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formation of H2O and C2H4, which is the major precursor to coke through polymerization (Scheme 6).

3. Support Effects The activity and the stability of Co catalysts were found to vary significantly based on the support materials used. Among the ZrO2, TiO2 and γ -Al2O3 supports tested, Co catalysts supported on ZrO2 were found to have higher activity and higher H2 yields [55]. However, these catalysts suffered from rapid deactivation due to coking as shown in the TEM images obtained over the post-reaction samples (Fig 3). With respect to the Co supported on ZrO2 catalyst, although a promising catalytic activity was achieved for ethanol steam reforming, significant carbon deposition was observed which degraded the catalytic activity over time [57]. The transmission electron microscopy (TEM) images of the spent Co/ZrO2 catalyst in Figure 3 (a) and (b) clearly show carbon fibers deposited on the cobalt particles. Figure 3(c) and (d) are the scanning transmission electron microscope (STEM) images taken for the same area. The verification of the carbon was done using energy-dispersive X-ray analysis (EDX), and the spectra are shown in the inserted images. A strong carbon signal was acquired for both areas which demonstrates that carbon deposition was significant. Depending on the cobalt particle size, the diameter of the carbon fibers varies a lot from approximately 15 nm to 150 nm. Among the support tested, ceria provided the best balance of activity, selectivity and stability. The next sections will summarize our studies where ceria was used as the support.


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3.1 Oxygen Storage Capacity and Mobility of Ceria Support Ceria, or cerium oxide possess high number of oxygen vacancies which facilitate oxygen mobility and storage capacity within its structure [58]. Moreover, the Ce4+/Ce3+ redox cycle promotes redox reactions. Thus, ceria has been utilized for many different applications as a catalyst support especially in three-way catalysts [59] and for the water-gas shift reaction [60, 61]. For ethanol steam reforming, it has been reported that the abundant oxygen source of the ceria support plays an important role for reducing the amount of carbon deposited on the catalyst surface [48, 62-65]. Thus, to improve the catalytic activity and hydrogen selectivity of the cobalt catalyst, the active metal was supported on ceria support [57]. The oxygen exchange capacities of 10% Co/ZrO2 and 10% Co/CeO2 catalysts were calculated using oxygen pulse chemisorption. Among all samples, as demonstrated in Figure 4 (a), the Co/CeO2 consumed the most oxygen compared to other catalysts. Also, when cobalt was present on the catalyst surface (Co/ZrO2 and Co/CeO2), the oxygen uptake was significantly larger than their bare support counterparts. This was mostly attributed to the re-oxidation of the reduced metallic cobalt. The oxygen consumption over the Co/CeO2 catalyst was greater than the Co/ZrO2 catalyst in comparison which infers that Co/CeO2 sample contains higher oxygen storage capacity. Surprisingly, over the ZrO2 support, there was no oxygen consumed. In addition to probing the oxygen exchange capacity, isotopic

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O2/18O2 exchange experiments

were performed to investigate the oxygen mobility of the samples. Samples were first pre-treated under helium at 400 °C for 30 min to remove any impurities left over on the catalyst surface. The reactor temperature was then brought down to 300 °C where

18

O2 was first introduced to the

sample. This corresponds to a gradual rise in signal 36 (18O18O) shown in Figure 4 (b). At the same time, signal 34 (16O18O) reaches to its maximum point and drops down rapidly. The 16O18O


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signal is attributed to the recombination of 16O located on the catalyst surface and 18O generated from dissociation of 18O18O molecules. It is reported that the intensity of the 16O18O oxygen gas can be related to the oxygen accessibility and mobility of the sample [57]. After all the signals reached a steady state, the reverse switch was made where

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O16O was sent to the reactor. The

gas-phase holdup time was estimated by flowing 10% Ar in the reactant stream. Similar to oxygen pulse chemisorption results, Co/CeO2 catalyst exhibited a higher oxygen mobility among all samples. The Co/ZrO2 sample showed a slightly lower amount of exchanged oxygen compared to bare CeO2. The lowest amount was obtained for the bare ZrO2 support. In-situ laser Raman spectroscopy was also utilized to conduct the isotopically-labeled oxygen exchange experiment. This is shown in Figure 4 (c). Among all the sample spectra, a peak shift due to the isotopic effect (16O2 to 18O2) was observed only over the Co/CeO2 catalyst. This again shows that there was a significant oxygen exchange present on the Co/CeO2 catalyst surface whereas the peak shift was not observed for Co/ZrO2. The increase in oxygen mobility observed over the catalysts supported on ceria translated to higher H2 yields and much higher stability [57].

3.2. Incorporation of Ca in the CeO2 Support To further increase the oxygen mobility of the Co/CeO2 catalyst, calcium (Ca) was doped into the CeO2 support [66]. Similar effects have been reported in the literature [67-70]. Ceria supports with a composition of Ca0.1Ce0.9O1.9 were prepared and cobalt impregnation was then done by incipient wetness impregnation (IWI). Oxygen exchange rates were evaluated by 16

O2/18O2 and H216O/H218O switching experiments, which showed that oxygen in the matrix of

Ca-incorporated support was much more readily accessible and could exchange with O atoms in


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molecular O2 or H2O (Fig 5) [66]. One likely explanation for this observation is that the replacement of Ce ions by a lower valence cation such as Ca creates a charge imbalance, which is compensated by increased oxygen vacancies. These vacancies, in turn, facilitate the oxygen mobility.

3.3. Surface Acidity Another factor that was found to impact the activity and stability of Co-based catalysts was the surface acidity. There is ample evidence in the literature showing that surface sites with higher basicity catalyze the ethanol dehydrogenation forming acetaldehyde, which is an important intermediate leading to hydrogen production [71, 72]. On the other hand, ethanol dehydration takes place on acidic surface sites, producing ethylene which facilitates catalyst coking. Co catalysts supported on ZrO2 versus CeO2 were compared for their surface acidity. Pyridine, NH3 and CO2 were used to probe the acidic and basic sites on the surface [73] in pulse chemisorption and in-situ DRIFTS experiments. Figure 6, which presents a comparison of NH3 and CO2 uptakes for the two catalysts shows a much higher NH3 uptake over the ZrO2-supported catalysts, signaling a highly acidic surface whereas CO2 uptake is found to be higher over the ceriasupported catalyst, signaling a higher basicity. Figure 7 compares the in-situ DRIFTS spectra taken over Co/ZrO2 and Co/CeO2 catalysts during pyridine desorption [73]. First of all, the hydrogen bonded pyridine was observed over both samples (1606 cm-1 for Co/ZrO2, 1591 cm-1 for Co/CeO2). The intensity of these peaks were maintained even at high temperature regions. The 1549, 1349 and 1286 cm-1 peaks for Co/ZrO2 indicates pyridinium ions, which are associated with Brønsted acid sites. Similarly, over


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Co/CeO2 sample, 1564, 1354 and 1284 cm-1 peaks were visible, however the peak 1354 cm-1 peak appeared with a much lower intensity especially temperatures above 400 °C, suggesting a weaker adsorption on these sites. The peak appearing at 1439 cm-1 over the Co/ZrO2 catalyst was assigned to Lewis acid-coordinated pyridine. This was also observable in the Co/CeO2 spectra at 1437 cm-1. The relative intensity of this peak was also stronger in the case of Co/ZrO2 when compared to Co/CeO2, indicating a stronger adsorption. The 1479 cm-1 peak in Co/ZrO2 spectra was linked to both Brønsted and Lewis acid sites and this was also detected for the Co/CeO2 sample. Overall, both samples encompass Brønsted and Lewis acid sites; however, when compared, Co/ZrO2 catalyst was more acidic than Co/CeO2. The differences in surface acidity were reflected in the much higher activity and H2 yield observed over the ceria-supported catalysts as well as in their much higher stability [73].

4. Evolution of Cobalt Species 4.1 In-situ Calcination The calcination step is essential in catalyst synthesis. The formation of the desired crystalline phase of the metal as well as the thermal stabilization of the support matrix for its use at higher reaction temperatures takes place during calcination. Several in-situ techniques were employed in order to understand the evolution of the Co/CeO2 catalysts during calcination [74]. It should be noted that the Co/CeO2 sample used herein was synthesized using Co(NO3)2 precursor. Figure 8 (a) shows the TPO results using air as an oxidant. As the catalyst went through an oxidation process with a temperature ramp rate of 10 °C/min, the exhaust stream was analyzed by on-line mass spectroscopy. The collected mass fractions were assigned to water (m/z=18), oxygen


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(m/z=32), NOx (m/z=30), NO2 (m/z=46) and CO2 (m/z=44). With significant oxygen consumption, the emission of NOx and NO2 compounds between 150 °C to 300 °C were observed, signaling the decomposition of the nitrate species. This was in good agreement with the result obtained by the in-situ Raman method (Figure 8 (b)). The two Raman bands attributed to nitrate species were shown to decrease in intensity as the temperature was increased from 200 °C to 300 °C. The production of carbon dioxide seen in the TPO profile can be attributed to the oxidation of acetate groups originating from the ethanol impregnation medium. A more detailed discussion about the effect of cobalt precursor and impregnation medium is provided in the next sections. Figure 9 shows both in-situ XRD patterns and Raman spectra collected during calcination under air environment. As the nitrate species leave the cobalt surface, formation of Co3O4 was observed in a cubic phase. In the Raman spectra, the appearance of the Co3O4 band at 300 °C coincides with the disappearance of the nitrate peaks in Figure 9 (b). Regarding the XRD patterns, the formation of crystalline Co3O4 is only observable above 250 °C. From 250 to 450 °C, the area for this peak increases with increasing temperature. Above 450 °C, the peak area and intensity does not change indicating a fully oxidized cobalt oxide phase.

4.2 In-situ Reduction It is well established that the reduction of cobalt oxide to metallic cobalt occurs in two consecutive steps: 1) Co3O4 to CoO and 2) CoO to metallic Co [74-78]. Thus, a typical TPR profile for Co3O4 shows two distinctive peaks. The reduction of the calcined Co/CeO2 sample was carried out in-situ in an enclosed TGA reactor chamber using 5% H2/He. Temperature was


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increased from 100 °C to 600 °C at a ramp rate of 5 °C/min. The concentration of the effluent hydrogen gas was analyzed using a thermal conductivity detector (TCD). As in Figure 10 (a), the appearance of the first peak corresponds to the hydrogen consumption due to reduction of Co3O4 to CoO. Likewise, the second peak is related to change from CoO to metallic Co. Concurrently, with the change in oxidation state of cobalt, a significant decrease in mass of the catalyst was observed. As expected, the loss of mass was more predominant in the case of the transition from CoO to metallic Co. The reduction of cobalt was reconfirmed by utilizing in-situ Raman spectroscopy technique where data were collected while 5% H2/He flowed through the in-situ reaction cell. The standard spectra for Co3O4 and CoO were also obtained for the purpose of comparison. As it can be seen in Figure 10 (b), the peaks assigned to Co3O4 (481 cm-1, 521 cm-1, 619 cm-1 and 688 cm-1) hold the intensity until a reduction temperature of 300 °C is reached. When reaching to 400 °C, all four bands disappear with the appearance of a small band near 680 cm-1 which is associated with CoO. However, this band also disappears when the temperature is increased to 450 °C. Thus, it was demonstrated that only metallic cobalt was present above 450 °C.

4.3 In-situ XANES and EXAFS during ethanol steam reforming Although the change in the cobalt species in either an oxidizing (O2) or reducing (H2) environment showed the primary phases at the end of those processes, under ethanol steam reforming conditions, the behavior of the cobalt phases can be considerably different. For instance, during the reaction, the reactant H2O may behave as an oxidizer thereby oxidizing the reduced cobalt species. On the other hand, the H2 produced from ethanol steam reforming acts as


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a reducing agent, which reduces the cobalt particles. In addition, ethanol can be adsorbed on the cobalt surface interacting with surface oxygen also leading to reduction of cobalt. Understanding the oxidation state changes of cobalt species during ethanol steam reforming is important since it reveals information about the active cobalt phase when correlated with the catalytic activity data. In that sense, in-situ X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques are powerful characterization tools since they provide the oxidation state of the metal, as well as coordination number and bond distances to the neighboring atoms. With the use of these techniques, it is possible to gain insight into the catalyst phase distribution under steady-state conditions. The XANES and EXAFS experiments were performed at the bending magnet beamline (5BM-D) of the Dow-Northwestern-DuPont Collaborative Access Team (DND-CAT) of the Advanced Photon Source, Argonne National Laboratories. Prior to the actual reaction, the Co/CeO2 sample was either pre-reduced or pre-oxidized in order to study the effect of pretreatment on the extent of reduction of cobalt during ethanol steam reforming. Figure 11 shows the XANES spectra obtained under ethanol steam reforming conditions at different temperatures [79]. Quantification of the XANES spectra was performed applying a linear relationship among the reference spectra of Co3O4 (red), CoO (green) and Co0 (blue). Figure 11 (a) shows the XANES spectra collected over the pre-oxidized Co/CeO2 catalyst for ethanol steam reforming. The composition of the cobalt phase was acquired as 100% Co3O4 after the oxidation pretreatment. As the ethanol and water reactants were sent to the catalyst bed at 350°C, significant reduction of the cobalt species was observed, indicated by the evolution of CoO species. With the increase in temperature, more CoO species were transformed to metallic Co. At 450 °C, no Co3O4 was observed in the catalyst. As mentioned previously, the reduction of


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cobalt species during ethanol steam reforming is likely ascribed to the hydrogen gases produced from the reaction. Thus, when the reaction temperature increases, the amount of hydrogen significantly rises thereby reducing the cobalt species furthermore. Surfaces, however, can be reduced by ethanol as well. The pre-reduction treatment on the Co/CeO2 catalyst led to the formation of metallic Co particles. Only CoO and Co0 were obtained after the reduction pretreatment. When the ethanol and water mixture were introduced to the reactor, the pre-reduced Co/CeO2 sample was first slightly reoxidized by H2O. An interesting observation was that regardless of the initial oxidation state of cobalt, the composition of cobalt phase approached to a similar value at 500 °C. This infers that the extent of reduction of cobalt is more dependent on the reaction temperature than the initial phase of cobalt. Lin et al.[80] also performed a similar study where they found that the ratio of water to ethanol in the reactant stream strongly affects oxidation state of cobalt at a fixed temperature. The EXAFS spectra obtained over the two Co/CeO2 sample for ethanol steam reforming are shown in Figure 12 [79]. The starting spectra for the two catalysts are very different at 350°C (Fig. 12 (a)). Over the pre-oxidized Co/CeO2 catalyst, only Co-O-Co bridge was observed at 350 °C whereas all Co-O, Co-Co and Co-O-Co bonds were found over the pre-reduced sample. At 500 °C, only Co-Co bond was observed for both samples with similar bond distance and coordination number. This indicates that the local coordination environment for cobalt under ethanol steam reforming condition converges to the same composition regardless of the pretreatment that the catalyst went through. These results were in good agreement with the XANES data.


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The effect of phase composition of the catalyst on the catalytic performance was examined [79]. Figure 13 (a) shows how the H2 and CO2 formation rates changed with temperature for the two catalysts that went through oxidation and reduction pre-treatments. Although the performance of the two catalysts is very different at the lower temperatures, the reaction rates are seen to converge to the same value as the temperature was increased. This observation is consistent with the results which showed that the two catalysts end up with the same phase composition regardless of the pretreatment they went through. The TOF values for Co0 (metallic Co) and Co2+ (CoO) were also calculated at different temperatures. As expected, the TOF values for metallic Co are significantly higher than the ones for Co2+. The trend is true at all temperature range from 300 to 550 °C. This is in good agreement with several studies performed in the literature where metallic cobalt sites were shown to be the active sites for ethanol steam reforming [39, 41, 78, 80]. It is also important to note that the partially oxidized cobalt species also have some intrinsic activity. The inserted figure in Figure 13, shows the Arrhenius plot in order to calculate the activation energy of the two cobalt phases.

4.4 Kinetics of Cobalt Reduction and Re-oxidation Kinetic analysis was conducted to calculate the activation energies and rate constants for reduction and re-oxidation processes of cobalt particles. Three different approaches were used to perform an accurate analysis: 1) Kissinger Method 2) Quantitative Isothermal Reduction 3) XANES [81]. In the Kissinger experiments, TPR and TPO were performed at different ramp rates to obtain the apparent activation energies of hydrogen reduction and oxygen re-oxidation, respectively. The Kissinger plot is shown in the inserted graph in Figure 14 (a). The calculated apparent activation energy for hydrogen reduction was 29 kJ/mol. Additionally, the activation


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energy for re-oxidation of Co/CeO2 catalyst under 5% O2/He was also studied. Unlike the reduction process, only one peak appeared in the TPO spectra which was assigned to Co0 to Co3O4. The calculated apparent activation energy for re-oxidation of Co/CeO2 sample was 20 kJ/mol which was lower of that of the reduction. The activation energy for reduction of CoO to Co0 was also estimated employing a quantitative isothermal reduction (QIR) technique (Fig 15). The sample was reduced for 1h at each temperature of 250, 275, 300 and 350 °C under 5% H2/He. The formation of water due to cobalt reduction was monitored using MS (signal m/z=18) and was used to quantify the extent of reduction reaction. The data was analyzed assuming a first order reaction rate, which was verified in Figure 15 (b). Figure 15 (c) shows the Arrhenius plot, where the activation energy for reduction of CoO to Co0 with hydrogen was calculated to be 29 kJ/mol. The activation energy obtained from QIR analysis was the same value achieved from the Kissinger experiment. Lastly, the activation energy for re-oxidation of cobalt with water was calculated. This study was also performed to understand how water affects the oxidation state of cobalt during ethanol steam reforming. The catalyst was exposed to reduction (with H2) and oxidation (with H2O) cycles. XANES spectra were collected continuously during the cycles. The overall reduction and the subsequent oxidation steps were repeated at each temperature of 350, 400 and 450 °. In Figure 16, the oxidation states of cobalt quantified from the XANES results are shown. The first reduction reaction due to hydrogen significantly increases the extent of reduction of cobalt. This phenomenon is more predominant at 450 °C in which lower oxidation states of cobalt were achieved compared to other temperatures. The re-oxidation process due to water increased the oxidation state of cobalt at all temperatures. However, once cobalt is reduced, the change in oxidation state was quite stable as it can be seen in the second and third switches. Overall, the


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data clearly indicates that the water reactant for ethanol steam reforming oxidizes the cobalt species. In order to calculate the activation energy of the re-oxidation process, first order kinetic was assumed. Through the Arrhenius plot, an apparent activation energy 37 kJ/mol was obtained, which showed a higher activation energy barrier for oxidation with water than oxidation with O2.

5. Synthesis of Cobalt Catalyst The synthesis technique for catalyst preparation is important since it could lead to differences in pore size, pore volume and metal dispersion of a catalyst. Depending on what type of synthesis method was utilized, the structural, electronic and chemisorptive properties of the catalyst change, thus influencing the catalytic activity. In this work, several different synthesis techniques and were used for preparing Co/CeO2 catalyst, the effect of many synthesis parameters such as choice of cobalt precursor and precipitation medium was examined.

5.1 Novel Synthesis Methods Three different novel synthesis methods were used to prepare Co/CeO2 and the effect on the catalytic activity of Co/CeO2 sample was investigated for ethanol steam reforming [82]. 1) Solvothermal decomposition. CeO2 and CoO were synthesized by thermal decomposition of Ce or Co oleate in a mixture of ethanol, water and hexane. The resulting CeO2 and CoO nanocrystals were then mixed in hexane which was gradually heated to 450 °C. As the solvent evaporated, Co/CeO2 catalyst was obtained as black powders. 2) Colloidal crystal templating (CCT) method. This was used to prepare CeO2 support containing a three-dimensionally ordered macroporous (3DOM) structure. Polymethylmethacrylate (PMMA)


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nanospheres beads were incorporated as a template to create the macro porous structure. The cobalt particles were then placed on the 3DOM CeO2 sample using wet impregnation. 3) Reverse micro-emulsion technique. The Co/CeO2 catalyst prepared by this technique is expected to have a homogenous cobalt particle size distribution throughout the sample. Furthermore, the addition of surfactants prevents agglomeration of the cobalt particles, thus stabilizing the cobalt dispersion on the ceria surface. The details of the synthesis techniques that are mentioned herein is reported previously [82]. All three catalyst synthesis techniques were seen to lead to higher activity compared to the ones prepared by incipient wetness impregnation techniques. However, among the three synthesis techniques, reverse micro-emulsion technique was seen to provide the best catalytic performance. The superior behavior of the Co/CeO2 prepared by reverse microemulsion was ascribed to a combination of many factors including higher dispersion of cobalt metal on the catalyst surface, narrower cobalt particle size distribution, and better reducibility of cobalt [82].

5.2 Organic vs Aqueous Impregnation Medium Another synthesis parameter which affects the catalytic activity of the Co/CeO2 catalyst is the choice of impregnation medium during IWI synthesis. When the cobalt precursor solids were dissolved in an organic medium such as ethanol instead of an aqueous medium, it was found that the hydrogen yield and selectivity as well as the catalyst stability was greatly improved [74]. A possible explanation was that the presence of oxygenated carbon species on the catalyst surface which originated from ethanol impregnation medium restricted the cobalt re-agglomeration and blocked the sites that produce side products during ethanol steam reforming. These oxygenated


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carbon species were found to be stable through calcination and reduction processes, thereby playing an important role during the catalyst synthesis as well as under reaction. A related note to this is that a better reducibility of cobalt was obtained in the case of Co/CeO2 catalyst using ethanol impregnation medium. This was confirmed by using an in-situ reactor cell where both Co/CeO2 samples prepared either in ethanol and water were placed inside the cell and Raman spectra were collected simultaneously while 5% H2/He was introduced for reduction. Based on the result, it was concluded that the cobalt species in Co/CeO2 sample prepared in ethanol impregnation medium was significantly more reducible at lower temperatures with faster kinetics. When these samples were analyzed using the XPS technique, the obtained ratio of Co/Ce was higher for the Co/CeO2 sample with ethanol as a medium, which exhibited greater cobalt dispersion. XPS data also showed presence of carbon species on the surface of the catalysts prepared in an organic medium. These catalysts also had a much higher catalytic activity for ethanol steam reforming. As a possible explanation of the superior catalytic behavior a “molecular imprinting” phenomenon was also discussed [74].

5.3 Cobalt Precursors The effect of using different cobalt precursors on the catalytic activity of Co/CeO2 catalyst was also examined [83]. The cobalt precursors used included inorganic chemicals such as CoCl2, CoSO4, Co(NO3)2, and CoCO3 and organometallic salts such as Co(C8H15O2)2, Co(C5H7O2)2, Co2(CO)8. Ethanol steam reforming was conducted over these catalysts under the same reaction conditions. Co/CeO2 samples synthesized using inorganic cobalt precursors showed lower catalyst performance compared to the ones prepared with organometallic cobalt compounds.


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Especially, over the Co/CeO2 catalyst synthesized using cobalt acetyl acetonate, the hydrogen yield was the highest among all the samples. On the other hand, the Co/CeO2 catalysts prepared with inorganic salts produced a lot of acetaldehyde and ethylene. The stability test at 450 °C conducted for 70 h over the two samples, Co/CeO2-cobalt acetyl acetonate and Co/CeO2-nitrate, showed a significantly better stability for the catalyst prepared using cobalt acetyl acetonate. The catalyst performance of Co/CeO2-nitrate degraded significantly due to severe catalyst coking. The promising catalytic activity and stability of the Co/CeO2-cobalt acetyl acetonate were thought to be due to several reasons. First, N2O chemisorption results showed a better cobalt dispersion on the catalyst surface. Thus, the increase in cobalt surface area clearly led to an enhancement of the catalytic activity for ethanol steam reforming. Secondly, higher oxygen mobility of the catalyst was observed from ethanol TPD using in-situ DRIFTS technique. Also, the reducibility of the cobalt species was considerably better in case of Co/CeO2-cobalt acetyl acetonate which exhibited a decrease in reduction temperature compared to that of Co/CeO2nitrate catalyst. The improved activity and stability observed when organic precursors were used are believed to be related to the presence organo-ligands attached to the cobalt metal sites, segregating the cobalt particles, and thereby increasing the dispersion and decreasing the particle size. Furthermore, the organic ligand species may provide some sort of “imprinting effect” on the cobalt active sites, thereby making them more accessible to the ethanol and water reactants.

6. Effect of Ceria Support Morphology and Particle Size The change in support morphology and particle size are known to indirectly influence the metal dispersion, metal reducibility and catalytic activity and have been reported in several studies.


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Our more recent studies, which showed a strong correlation between the catalytic activity and support morphology and particle size of the ceria support are briefly summarized below [84-86].

6.1 Cobalt supported on ceria Nanorods (NR) vs Nanocubes (NC) Two different shapes of ceria supports were prepared using hydrothermal synthesis method. The particle sizes for NR were 22 ± 6 nm (length) and 6 ± 1.3 nm (width). The NC were observed as bigger particles with 21 ± 7 nm particle sizes. It was found that the morphology of the ceria support significantly affected the reduction characteristics of cobalt species. In-situ XANES experiments showed that the extent of reduction cobalt species supported on nano-cubes was higher than that of those supported on nano-rods under the same reducing environment. Moreover, surface acidity of the two samples were quite different. Using methanol as a probe molecule in methanol oxidation pulse experiments, where the production of HCHO are linked to redox sites and CO2 formation is associated with basic sites, catalyst supported on nano-cubes was shown to be more basic compared to its counterpart. These results were further confirmed by the pulse CO2 chemisorption experiments. The significant difference in properties of the Co/CeO2-NC compared to the Co/CeO2-NR led to a pronounced increase in catalytic performance for ethanol steam reforming. Over the Co/CeO2NC catalyst, 4.5 times greater hydrogen yield was achieved compared to Co/CeO2-NR.

6.2 Cobalt supported on Nano-ceria (NP) vs Micro-ceria (MP)


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Two different ceria supports were prepared, one in nano size the other in micron size, to examine the effect of support particle size on the catalytic activity of the Co/CeO2 catalyst. Similar to the morphology study, both Co/CeO2 samples were characterized regarding their cobalt reducibility and surface acidity. The particle sizes of nano-ceria were in the range of 5-8 nm whereas the micro-ceria contained 0.1-0.2 µm particles. Both in-situ XRD and in-situ XANES studies showed a higher reducibility of Co/CeO2-NP in similar reducing environments compared Co/CeO2-MP. Using methanol pulse oxidation experiments, catalysts supported on nano-ceria was seen to contain a higher density of redox sites. Co/CeO2-NP was also shown to lead to a higher H2 yield and higher stability. After 13 h of ethanol steam reforming, a significant catalyst coking was observed for Co/CeO2-MP. Our more recent studies, which focused on the reduction characteristics of the ceria support as well as those that reported the ethanol steam reforming activity of the bare ceria support are not included in this overview article.

7. Conclusions This paper summarizes the ethanol steam reforming studies performed in our laboratory over the past decade using Co-based catalysts. These studies showed a strong effect of the support material on the catalytic activity, selectivity as well as stability. High oxygen mobility of the support was seen to suppress coking and contribute to higher H2 yields. Many catalyst synthesis parameters, such as cobalt precursors, preparation method, and impregnation medium were seen to directly impact the catalytic performance. It was also shown that catalytic performance can be affected by changing particle morphology and shape of the support or by doping it with a lower-


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valence cation.

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Catalyst synthesis parameters as well as support characteristics had a

pronounced effect on the surface acidity and reduction/reoxidation characteristics of cobalt species, which in turn affected the catalytic behavior. The change in oxidation state of cobalt was investigated under various conditions such as calcination, pre-reduction and ethanol steam reforming. Co-existence of metallic Co and CoO was observed under steady-state reaction conditions. Both sites had ethanol steam reforming activity, but the metallic sites provided a much higher turn-over frequencies. Moreover, regardless of the pre-treatment step, similar oxidation states of cobalt were obtained, indicating that the oxidation state of cobalt at steady state during ethanol steam reforming is independent of its initial oxidation state. Our more recent studies showed that ceria, which is a commonly used as a support in many catalyst systems has significant reducibility as well as catalytic activity in its own right and is likely to contribute to the observed activities of many catalysts supported on ceria, including the cobalt catalysts used in this study.

Acknowledgements This research was based upon work supported by the U.S. Department of Energy with the Grant DE-FG36-05GO15033 for our funding. XAFS experiments were performed at the Dupont−Northwestern− Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). E.I. DuPont de Nemours & Co., The Dow Chemical Company, and the State of Illinois supported DND-CAT. The utilization of APS was supported by the U.S. Department of Energy, Oﬃce of Science, Oﬃce of Basic Energy Sciences under Contract No.


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DE-ACO206CH11357. We thank Dr. Jeffrey Miller for helping us conduct the XAS experiments at Argonne National Laboratories and also for his valuable insight to the data analysis. We would like to acknowledge many former members of the Heterogeneous Catalysis Research Group (HCRG) at the Ohio State University for their contributions to this research, especially Dr. Hua Song, who performed most of the studies summarized here. We would also like to thank Dr. I. Ilgaz Soykal, Dr. Lingzhi Zhang, Dr. Burcu Mirkelamoglu, Dr. Rick B. Watson and Dr. Bing Tan, for their valuable contribution.

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List of Captions Figure 1. Product distribution in ethanol steam reforming in thermodynamic equilibrium (a) conversion and yields (EtOH:H2O=1:10 (molar), CEtOH=2.8%) (b) H2 yield and C selectivity at different EtOH-to-water ratios (Reprinted from [43] with permission from Elsevier) Figure 2. Reaction network of ethanol steam reforming (Adapted from [44] with kind permission from Springer Science and Business Media) Figure 3. (a-b) TEM images of the used Co/ZrO2 catalyst showing carbon fiber growth on cobalt metal particles (c-d) STEM images with EDX analysis in the same area range (Reprinted from [45] with permission from Elsevier) Figure 4. Quantified (a) oxygen storage capacity from oxygen pulse chemisorption and (b) oxygen mobility using isotopic oxygen exchange experiment (c) Raman spectra showing isotopic peak shift in Co/CeO2 (18O2) spectra (For (a) and (b), Reprinted from [45] with permission from Elsevier) Figure 5. Isotopic labeling experiments over Co/CeO2 and Co/Ca0.1Ce0.9O1.9. (a) 16O2/18O2 switching experiment (b) H216O/H218O switching experiments (c) quantified amounts of exchanged oxygen (Reprinted from [54] with permission from American Chemical Society) Figure 6. TPD profiles showing (a) NH3 and (b) CO2 desorption over the Co supported on CeO2 and ZrO2 catalysts (Adapted from [60] with kind permission from Springer Science and Business Media) Figure 7. in-situ DRIFTS spectra showing desorption of pyridine at different temperatures over (a) Co/ZrO2 and (b) Co/CeO2 catalysts (Adapted from [60] with kind permission from Springer Science and Business Media) Figure 8. Removal of nitrate species from the Co/CeO2 catalyst (a) TPO (b) in-situ Raman (Reprinted from [61] with permission from Elsevier) Figure 9. Formation of Co3O4 phase observed during in-situ (a) XRD (b) Raman results (Reprinted from [61] with permission from Elsevier) Figure 10. in-situ (a) TGA profiles and (b) Raman spectra showing reduction of Co3O4 to CoO and CoO to metallic Co under using 5% H2 in helium (For (a), Reproduced from [87] with permission from the Royal Society of Chemistry, For (b), Reprinted from [61] with permission from Elsevier)


30

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Figure 11. Co K-edge XANES spectra obtained over (a) pre-oxidized and (b) pre-reduced Co/CeO2 catalyst (Reprinted from [66] with permission from Elsevier) Figure 12. Co K-edge EXAFS spectra obtained over pre-oxidized and pre-reduced Co/CeO2 catalyst at (a) 350 °C (b) 500 °C (Reprinted from [66] with permission from Elsevier) Figure 13. (a) Effect of oxidation and reduction pretreatment on H2 and CO2 production rate as a function of temperature over Co/CeO2. (b) Change of TOFs with temperature for Co0 and Co+2 species based on XANES data. Insets: Arrhenius plots obtained using the TOFs. (Reprinted from [66] with permission from Elsevier) Figure 14. Kissinger experiments with different ramp rates for (a) reduction and (b) re-oxidation of the Co/CeO2 catalyst (Adapted from [68] with permission of Springer) Figure 15. Quantitative isothermal reduction (QIR), (a) m/z=18 signal monitored by MS and integration of the area under the curve (b) Kinetic analysis based on a first order reduction reaction rate (c) Arrhenius plot to calculate activation energy of CoO to Co reduction (Adapted from [68] with permission of Springer) Figure 16. Change in oxidation states of cobalt in Co/CeO2 catalyst during hydrogen reduction and water re-oxidation at (a) 350 °C (b) 400 °C and (c) 450 °C (Adapted from [68] with permission of Springer)


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(a) Production Distribution 100 100 80 80 60 60 40 40 20 20 0 0

100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900

(b) H2 Yield and Coke Formation

Figure 1. Product distribution in ethanol steam reforming in thermodynamic equilibrium (a) conversion and yields (EtOH:H2O=1:10 (molar), CEtOH=2.8%) (b) H2 yield and C selectivity at different EtOH-to-water ratios (Reprinted from [43] with permission from Elsevier)


Page 33 of 47

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Figure 2. Reaction network of ethanol steam reforming (Adapted from [44] with kind permission from Springer Science and Business Media)


Energy & Fuels

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Page 34 of 47

(a)

(c)

(b)

(d)

Figure 3. (a-b) TEM images of the used Co/ZrO2 catalyst showing carbon fiber growth on cobalt metal particles (c-d) STEM images with EDX analysis in the same area range (Reprinted from [45] with permission from Elsevier)


Page 35 of 47

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Energy & Fuels

(c)

(a)

(b)

Figure 4. Quantified (a) oxygen storage capacity from oxygen pulse chemisorption and (b) oxygen mobility using isotopic oxygen exchange experiment (c) Raman spectra showing isotopic peak shift in Co/CeO2 (18O2) spectra (For (a) and (b), Reprinted from [45] with permission from Elsevier)


Energy & Fuels

Normalized MS Signal (a.u.)

(a)

1.2

Co/CeO2 Co/CaCeO2

1 0.8

16O

2

18O

2

0.6 0.4 16O 18O 2 2

0.2 0 5

10

15

20

25

30

35

40

Time (Minutes)

(b) Normalized MS Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2 1

H216O H218O

0.8 0.6 0.4

Co/CeO2 Co/CaCeO2

0.2 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Time (Minutes)

(c)

Exchanged O (mmol/g Cat) 16.48

Co/CeO2 Co/CaCeO2

9.42

9.01

2.52

1

Results from 16O – 18O 2 2 Switching experiments

2

Results from H216O – H218O Switching experiments

Figure 5. Isotopic labeling experiments over Co/CeO2 and Co/Ca0.1Ce0.9O1.9. (a) 16 O2/18O2 switching experiment (b) H216O/H218O switching experiments (c) quantified amounts of exchanged oxygen (Reprinted from [54] with permission from American Chemical Society)


Page 37 of 47

MS Signal (a.u.)

(a) Co/ZrO2

Co/CeO2

0

100

200

300

400

500

600

700

Temperature (°C)

(b)

Co/CeO2

MS Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Co/ZrO2

0

100

200

300

400

500

600

Temperature (°C)

700

Figure 6. TPD profiles showing (a) NH3 and (b) CO2 desorption over the Co supported on CeO2 and ZrO2 catalysts (Adapted from [60] with kind permission from Springer Science and Business Media)


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(a)

(b)

Figure 7. in-situ DRIFTS spectra showing desorption of pyridine at different temperatures over (a) Co/ZrO2 and (b) Co/CeO2 catalysts (Adapted from [60] with kind permission from Springer Science and Business Media)


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(a)

(b)

Figure 8. Removal of nitrate species from the Co/CeO2 catalyst (a) TPO (b) in-situ Raman (Reprinted from [61] with permission from Elsevier)


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(a)

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(b)

Figure 9. Formation of Co3O4 phase observed during in-situ (a) XRD (b) Raman results (Reprinted from [61] with permission from Elsevier)


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(a)

(b)

Figure 10. in-situ (a) TGA profiles and (b) Raman spectra showing reduction of Co3O4 to CoO and CoO to metallic Co under using 5% H2 in helium (For (a), Reproduced from [74] with permission from the Royal Society of Chemistry, For (b), Reprinted from [61] with permission from Elsevier)


Energy & Fuels

After Oxidation Pretreatment

(a)

(b)

After Reduction Pretreatment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 11. Co K-edge XANES spectra obtained over (a) pre-oxidized and (b) pre-reduced Co/CeO2 catalyst (Reprinted from [66] with permission from Elsevier)


0.04

(a)

Magnitude of FT [k2 * Chi(k)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Magnitude of FT [k2 * Chi(k)]

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350 °C

Co-Co

Co-O-Co

0.03

Pre-oxidized Pre-reduced

Co-O

0.02

0.01

0 0

1

2

3

4

5

0.05

(b)

500 °C

Co-Co

0.04 0.03

Pre-oxidized Pre-reduced

0.02 0.01 0

6

0

1

R [A]

2

3

4

5

6

R [A]

Figure 12. Co K-edge EXAFS spectra obtained over pre-oxidized and pre-reduced Co/CeO2 catalyst at (a) 350 °C (b) 500 °C (Reprinted from [66] with permission from Elsevier)


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(a)

(b)

Co0:Ea= 15 kJ/mol Co+2: Ea= 67kJ/mol

Figure 13. (a) Effect of oxidation and reduction pretreatment on H2 and CO2 production rate as a function of temperature over Co/CeO2. (b) Change of TOFs with temperature for Co0 and Co+2 species based on XANES data. Insets: Arrhenius plots obtained using the TOFs. (Reprinted from [66] with permission from Elsevier)


-9.70 -9.90

(b)

slope-3.4414

ln(β/Tmax2)

(a)

-10.10 -10.30

-9.40 -9.60

Slope= -2.35

-9.80 -10.00 -10.20

-10.50 1.60

1.70

1.80

1.80

1.90

1.90

2.00

2.10

1000/Tmax (K-1)

1000/Tmax (K-1)

CoO → Co

19 °C/min 19 °C/min

O2 Signal (a.u.)

Hydrogen Consumption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ln(β/Tmax2)

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Co3O4 → CoO

16 °C/min

13 °C/min

16 °C/min 13 °C/min 10 °C/min

10 °C/min

160

220

280

340

400

100

Temperature in °C

200

300

400

Temperature in (°C)

Figure 14. Kissinger experiments with different ramp rates for (a) reduction and (b) reoxidation of the Co/CeO2 catalyst (Adapted from [68] with permission of Springer)


Energy & Fuels

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(a)

(b)

(c)

Figure 15. Quantitative isothermal reduction (QIR), (a) m/z=18 signal monitored by MS and integration of the area under the curve (b) Kinetic analysis based on a first order reduction reaction rate (c) Arrhenius plot to calculate activation energy of CoO to Co reduction (Adapted from [68] with permission of Springer)


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Energy & Fuels

Figure 16. Change in oxidation states of cobalt in Co/CeO2 catalyst during hydrogen reduction and water re-oxidation at (a) 350 °C (b) 400 °C and (c) 450 °C (Adapted from [68] with permission of Springer)


Cobalt-Based Catalysts for Ethanol Steam Reforming: An Overview

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