ZrO2 catalyst

Catalysis Today 242 (2015) 50–59

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Comparative study on steam and oxidative steam reforming of ethanol over 2KCo/ZrO2 catalyst Magdalena Greluk ∗ , Piotr Rybak, Grzegorz Słowik, Marek Rotko, Andrzej Machocki Marie Curie-Sklodowska University, Faculty of Chemistry, Department of Chemical Technology, Maria Curie-Sklodowska Sq. 3, 20-031 Lublin, Poland

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 27 February 2014 Received in revised form 15 July 2014 Accepted 18 July 2014 Available online 20 August 2014

The steam rreforming and oxidative steam reforming of ethanol were studied over 2KCo/ZrO2 catalyst which was prepared by support impregnation with cobalt nitrate-citric acid solution. The catalyst was characterized by N2 adsorption, H2 chemisorption, X-ray fluorescence, Raman spectroscopy, temperature-programmed reduction, temperature-programmed oxidation. The results indicate that the optimum EtOH/O2 molar ratio of 1/0.9 corresponds to the oxygen amount which prevents coke formation on the catalyst, when a constant molar ratio of H2 O/EtOH = 9/1. However, the presence of oxygen causes oxidation of metallic particles which leads to oxidized cobalt active phases that behave poorly in the reformation reactions. Thus, compared to the SRE, both activity and selectivity to desired products over the 2KCo/ZrO2 catalyst in the OSRE process is definitely lower and the higher temperature for complete conversion of ethanol is required. © 2014 Elsevier B.V. All rights reserved.

Keywords: Hydrogen production Oxidative ethanol steam reforming Steam reforming of ethanol Cobalt-based catalyst Zirconia support

1. Introduction Hydrogen is potentially, a clean energy source for the highlyefficient generation of electricity through fuel cell systems. Among various renewable sources, bio-ethanol produced by fermentation of biomasses seems to represent an excellent candidate owing to its high hydrogen content, availability, low toxicity and ease storage and handling. Hydrogen can be produced directly from ethanol by various processes such as steam reforming of ethanol (SRE, Eq. (1)), partial oxidation of ethanol (POE, Eq. (2)) and oxidative steam reforming of ethanol (OSRE, or autothermal reforming, Eq. (3))): C2 H5 OH + 3H2 O → 6H2 + 2CO2 ,

H298◦ = +347.7 kJ/mol

(1)

C2 H5 OH + 1.5O2 → 3H2 + 2CO2 ,

H298◦ = −557.2 kJ/mol

(2)

C2 H5 OH + 1.78H2 O + 0.61O2 → 4.78H2 + 2CO2 ,

H298◦ = 0 kJ/mol

(3)

Taking into account the endothermic nature of the SRE reaction (1), a significant energy consumption is required for performing this process. However, it gives the highest yield of hydrogen in comparison to other two processes. By supplying oxygen, SRE energy

∗ Corresponding author. Tel.: +48 81 537 55 25; fax: +48 81 537 55 65. E-mail address: [email protected] (M. Greluk). http://dx.doi.org/10.1016/j.cattod.2014.07.032 0920-5861/© 2014 Elsevier B.V. All rights reserved.

consumption can be decreased. Because POE is highly exothermic reaction (2), it does not require any external heat supply. But the major drawback of POE is that hydrogen yield in this process is much lower in comparison to the SRE. Moreover, because of the high heat generation, hot spot formation can occur resulting in catalyst damage. The OSRE reaction combines the benefits of both SRE and POE reactions by co-feeding oxygen, steam and ethanol. Moreover, the addition of oxygen allows to avoid carbon deposition [1–4]. Many authors studied the OSRE reaction, paying particular attention to properties of different catalyst. Several studies [5–10] have evaluated the performance of the cobalt-based catalysts in the OSRE. Silva et al. [5–7] observed that the addition of oxygen to the feed helps to clean surface of the cobalt supported on ceria, keeping it active for a longer period of time. Also, in the case of the Co/carbon nanofibers catalyst [8], the presence of oxygen in the feed significantly improved the catalyst stability and lowered amount of carbon deposited on the surface in comparison with those in the SRE reaction. However, oxidation of cobalt particles by oxygen was observed. Similarly, Pereira et al. [9] demonstrated that, on the Co/SiO2 system, oxidation of the surface of cobaltbased particles may occur. Hung et al. [10] mainly indicate that the dehydration of ethanol preferentially occur on Co/Al2 O3 during OSRE reaction. As can be seen, the used support strongly affects the catalytic performance of supported cobalt catalysts in the OSRE reaction. There is still lack of the information about the catalytic performance of cobalt supported on zirconia under OSRE

M. Greluk et al. / Catalysis Today 242 (2015) 50–59

conditions, although zirconia is known to have an excellent thermal stability and well resistance against coke deposition [11]. The main drawback in the application of ZrO2 as supporting material is only its low surface area and poor mechanical stability at high reaction temperatures [12]. It is worth to mention that according to Youn et al. [13], nickel catalyst supported on ZrO2 showed a better catalytic performance than that supported on ZnO, MgO, TiO2 or Al2 O3 in hydrogen production in the OSRE reforming, because of the favorable modification of electronic structure of nickel species supported on ZrO2 . Previous studies [14,15] also show that excellent thermal stability, high water adsorption ability and efficient CO oxidation ability of zirconia influences on improvement of selectivity to hydrogen and on decrease of selectivity towards by-products over zirconia-supported catalysts in the OSRE reaction [1,14]. Doping of supported metal catalysts with potassium has been presented as a promising way to improve catalytic selectivity towards hydrogen and carbon dioxide and to reduce coke deposition at low temperatures by neutralizing the acid sites of the zirconia, which are likely responsible for ethanol dehydration into ethylene and other coke precursors [16,17]. Many papers reported the promoting effect of alkali metals on the acidic character of ZrO2 . It is well known that alkali metal addition on zirconia decreases its Lewis acidity and increases its basicity, i.e. modification of ZrO2 highly decreases the strength and the number of Lewis acidic sites [18–21]. Moreover, doping the cobalt based catalyst with potassium decreases the contribution of disproportionation of carbon monoxide which is also the source of coke formation (see Supporting information, Figs. S1 and S2, Table S1). The goal of the paper is provide us with experimental results for comparison of both, the steam and oxidative steam reforming of ethanol, processes which were proceeding over the same 2KCo/ZrO2 catalyst. Thus, cobalt catalyst doped with potassium supported on commercial zirconia was prepared by an impregnation method to test its catalytic activity toward SRE and OSRE. The influence of the presence of oxygen on oxidation of carbon deposits and cobalt particles was also determined. In this aim, the obtained results for the catalyst under OSRE conditions were compared to those obtained in the SRE reaction. Effects of the reaction temperature on the product composition were also determined.

2. Experimental 2.1. Catalyst preparation The 2KCo/ZrO2 catalyst was prepared by the impregnation method. Prior to the impregnation ZrO2 support (Aldrich, 46 m2 /g) was dried at 110 ◦ C for 3 h. The solution of cobalt nitrate and citric acid CA (the relative molar concentrations of Co and CA were 1/1) was used for impregnation and deposition of ca. 9 wt% of cobalt on the support. After impregnation, the catalyst precursor was dried at 110 ◦ C for 12 h, then calcined at 400 ◦ C with heating rate of 2 ◦ C min−1 up to the calcination set point and maintained for 1 h at this temperature. Next, the obtained catalyst precursor was impregnated with potassium nitrate solution in order to introduce 2 wt% of potassium promoter to the catalyst and again it was dried at 110 ◦ C for 12 h, then calcined at 400 ◦ C with heating rate of 2 ◦ C min−1 up to the calcination set point and maintained for 1 h at this temperature. We had looked in described studies, among other issues, for the lowest temperature of the SRE and OSRE which enabled complete conversion of ethanol and the most appropriate product composition for application of reformate gas to supply fuel cells. Therefore, the temperatures of all pretreatment steps were chosen as low as possible in order to avoid intensive sintering of the catalyst. The applied calcination temperature of 400 ◦ C was low enough and

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sufficient for the decomposition of the supported cobalt nitrate and obtaining supported cobalt oxide phase. 2.2. Catalyst characterization The cobalt content in the catalyst was determined by X-ray fluorescence analysis using Canberra 1510 fluorescence spectrometer equipped with a liquid N2 -cooled Si(Li) detector. The AXIL software package for spectra deconvolution and for calculation of Co content was used. Before measurements the catalyst was reduced in situ with hydrogen at 400 ◦ C for 1 h. The Brunauer–Emmett–Teller (BET) surface area of the catalysts was determined by low-temperature (−196 ◦ C) nitrogen adsorption in the ASAP 2405 N v1.0 analyser (Micromeritics). The pore volume and their average diameter were evaluated applying the Barrett–Joyner–Halenda (BJH) method. The active metal surface area, dispersion of cobalt phase and mean size of cobalt particle in in-situ pre-reduced catalyst were calculated from hydrogen chemisorption data obtained in AUTOSORB-1CMS apparatus (Quantachrome Instruments) at 40 ◦ C. The amount of surface metal atoms was calculated from the amount of hydrogen chemisorbed, assuming that one hydrogen atom is adsorbed on the area occupied by one surface cobalt atom (the stoichiometry of chemisorptions is Co/H = 1/1) and that the surface area occupied by one atom of hydrogen is equal to 0.065 nm2 [22]. The total H2 uptake was determined by extrapolation of the linear part of the isotherm to zero pressure. The Raman spectra were recorded with the resolution of 2 cm−1 in the Raman microscope (inVia Reflex, Renishaw) with Raman dispersive system, using a semiconducting laser 785 nm, working in a back scattered confocal arrangement. In order to avoid sample overheating the low, 3 mW of laser power was used. Temperature-programmed reduction (TPR) experiment was carried out with the Altamira Instruments apparatus equipped with a thermal conductivity detector. The reduction profile was obtained by passing 6% H2 /Ar flow at the rate of 30 mL min−1 through 0.05 g of the catalyst (0.1–0.2 mm). The temperature was increased from RT to 750 ◦ C at the rate of 10 ◦ C min−1 . The water vapor formed during reaction was removed in a cold trap (immersed in a liquid nitrogen-methanol slush) placed before the thermal conductivity detector. The temperature-programmed oxidation (TPO) experiment was carried out in the AutoChem II 2920 system supplied by Micromeritics Instrument Corporation and coupled with quadrupole mass spectrometer (HPR-20 with triple filter, Hiden Analitical) to study the susceptibility of active metal phase to its oxidation. Prior to the main experiment, the catalyst sample (0.1 g) was treated with the 6% H2 /Ar mixture, at the temperature of 400 ◦ C for 1 h and next, it was cooled down up to −70 ◦ C. The TPO measurement was performed using 5 %O2 /He and heating of the catalyst sample from −70 ◦ C to 600 ◦ C with the temperature ramp of 10 ◦ C min−1 . In all cases, i.e. during the pre-treating and the main experiment, the total flow rate of the reaction mixture was 45 mL min−1 . The thermogravimetric method was used to determine the amount of oxygen preventing from the formation of carbon on the catalyst during the ethanol conversion by using the TG121 microbalance system (CAHN), under dynamic conditions in a quartz reactor with a continuous flow of ethanol–water vapors mixed with helium (1:1) or ethanol–water vapors mixed with air and diluted with He at 420 ◦ C for 21 h. The molar ratio of EtOH/H2 O was 1/9 and EtOH/H2 O/O2 was equaled to 1/9/0.8–1.2. Prior to reaction, catalyst sample (0.01 g, 0.15–0.3 mm) was reduced by passing 10% H2 /He flow at the temperature of 400 ◦ C for 1 h. The temperature of 400 ◦ C, as it is shown by TPR experiment, was sufficient for reduction of a majority of cobalt oxide to metallic cobalt. In all cases,

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i.e. during pre-treating and main experiment, the total flow rate of the reaction mixture was 70 mL min−1 . 2.3. Catalytic performance test The reactions of ethanol conversion (SRE and OSRE) were carried out in a Microactivity Reference unit (PID Eng & Tech.) under atmospheric pressure in a fixed-bed continuous-flow quartz reactor over the catalyst (0.1 g. 0.15–0.3 mm) reduced in situ with hydrogen at 400 ◦ C for 1 h, prior to the reaction. The catalyst was diluted (at the weight ratio of 1/10) with 0.15–0.3 mm grains of quartz in order to ensure the constant temperature in the catalytic layer. The aqueous solution of ethanol (H2 O/ethanol = 9/1) was supplied by a mass controller (Bronkhorst) to an evaporator (150 ◦ C) and the reactant vapors, without diluting with any inert gas, were fed to the reactor at a flow rate of 100 mL min−1 . In the case of the OSRE process, the reactant vapors were additionally mixed with the air flow before being fed to the reactor providing the EtOH/H2 O/O2 mixture with molar ratio equal to 9/1/0.9. The stoichiometry of the OSRE reaction (3) shows that the H2 O/EtOH ratio of 1.78 and the O2 /EtOH ratio of 0.61 are required to achieve its thermal neutrality. However, the higher O2 /EtOH molar ratio of 0.9 was applied in further OSRE experiments in order to compensate also inevitable heat loss form a catalytic reactor. The catalytic performance was tested in the temperature range of 350–600 ◦ C in order to choose the optimum temperature for SRE and OSRE processes. The temperature was measured in the center of the catalyst + quartz bed. The temperature was increased step-by-step and the analyses of the reaction products were carried out several times for about 1 h of the reaction at each temperature. The stability tests were conducted at 420 ◦ C and 540 ◦ C for 24 h. The analysis of the reaction mixture and the reaction products (all in the gas phase) were carried out on-line by means of two gas chromatographs. One of them, Bruker 450-GC was equipped with two columns, the first filled with a porous polymer Porapak Q (for all organics, CO2 and H2 O vapor) and the other one–capillary column CP-Molsieve 5 A˚ (for CH4 and CO analysis). Helium was used as a carrier gas and a TCD detector was employed. The hydrogen concentration was analyzed by the second gas chromatograph, ˚ argon as a carrier gas and a Bruker 430-GC, using a Molsieve 5 A, TCD detector. The total conversion of ethanol XEtOH was calculated on the basis of its concentrations before and after the reaction, with a correction introduced for the change in gas volume during the reaction, from the equation: XEtOH =

out × K in − CEtOH CEtOH in CEtOH

× 100%

(3)

Table 1 Physicochemical properties of 2KCo/ZrO2 catalyst. Cobalt content (wt.%)a SBET (m2 /g)b Pore volume (mL/g)b Average pore diameter (nm)b Total H2 uptake (␮mol/gcat. )c Cobalt surface area (m2 /gcat )c Dispersion of cobalt (%)c Average cobalt crystallite size (nm)c a b c

8.8 18.1 0.06 12 24 1.9 3.2 31

Determined by the XRF technique. Determined by the low-temperature N2 adsorption. Determined by the hydrogen chemisorption measurement.

3. Results and discussion The physicochemical properties of 2KCo/ZrO2 catalyst are summarized in Table 1. Fig. 1 displays the Raman spectra of Co3 O4 , ZrO2 support and 2KCo/ZrO2 catalyst. Five Raman modes (A1g + Eg + 3F2g ) were found for Co3 O4 . The peaks correspond to the Eg (487 cm−1 ), F2g (524 and 621 cm−1 ) and A1g (695 cm−1 ) modes of Co3 O4 crystalline phase. The band at 197 cm−1 is attributed to the characteristics of the tetrahedral sites which corresponded to the F2g mode [23]. Furthermore, the pure zirconium oxide exhibits peaks at 146 (B1g ), 267 (Eg ), 310 (B1g ) and 618 cm−1 (B1g ) predicted theoretically for tetragonal ZrO2 (t-ZrO2 ) and Raman bands at 181 (Ag ), 225 (Bg ), 336 (Bg ), 385 (Ag ), 480 (Ag ), 561 (Ag ) and 638 cm−1 (Ag ) which are characteristic for monoclinic ZrO2 (m-ZrO2 ) [24–27]. The highly strong Raman intensities of bands assigned to m-ZrO2 in comparison with weak signals of t-ZrO2 suggests that monoclinic ZrO2 is the main phase. The Raman spectrum of the calcined 2KCo/CeO2 catalyst clearly exhibits intense Raman bands of Co3 O4 . There are no signals derived from any of the support. The similar results were obtained by Rybak et al. for CoOx /ZrO2 catalyst [24]. Fig. 2a shows the temperature-programmed reduction of the 2KCo/ZrO2 catalyst. It is known that reduction of Co3 O4 is a twosteps process which corresponds to the reduction of Co3 O4 to CoO and then reduction of CoO to metallic cobalt [24,28–30]. Thus, the first step of reduction peak located at the low temperature region from 225 ◦ C to 320 ◦ C was identified as the reduction of Co3 O4 to CoO whereas the second step of reduction peaks at temperature from 320 ◦ C to 490 ◦ C was assigned to the reduction of CoO to metallic cobalt. As can be seen this second step reduction peak consists of two peaks. The presence of two peaks of the second step on TPR profiles of Co/ZrO2 catalysts was also observed by Zhang et al. [28] and indicated that two CoO phases are present in the catalyst: surface phase CoO and bulk phase CoO. By analogy with published

in is the molar concentration of C2 H5 OH in the reacwhere CEtOH out is the molar concentration of C H OH in tion mixture (mol%), CEtOH 2 5 the post-reaction mixture (mol%), and K is the volume contraction factor (K = CCin /CCout /CCout where CCin and CCout are the molar concentrations of carbon in the C2 H5 OH feed to the reaction and in all carbon-containing compounds which were present in post reaction gases, respectively). The distribution (Xp ) of the reaction products was determined from the equation:

XP =

Cpout



out

Cp

× 100%

(4)

where Cpout is the molar concentration of the given product in the



out

post-reaction mixture (mol%) and C p is the sum of molar concentrations of all products in the post-reaction mixture (mol%).

Fig. 1. Raman spectra of Co3 O4 , 2KCo/ZrO2 catalyst and ZrO2 support.

M. Greluk et al. / Catalysis Today 242 (2015) 50–59

(a)

(b)

Fig. 2. (a) Temperature-programmed reduction and (b) temperature-programmed oxidation of the 2KCo/ZrO2 catalyst.

results [28] it could be concluded that the former peak could be assigned to the reduction of the surface phase CoO in the catalyst and the latter peak could be attributed to the reduction of the bulk phase CoO in the catalyst. On the TPR profile of 2KCo/ZrO2 , it is also observed a small peak with the maximum of hydrogen consumption at 640 ◦ C which could be attributed to the reduction of barely reducible cobalt oxide forms most probably very strongly interacting with the support. According to literature [31,32], it suggests the formation of a solid solution or cobalt zirconate phase, between unreduced cobalt oxides and zirconia. Undoubtedly, the temperature-programmed reduction profile proves the most of cobalt oxides present in the 2KCo/ZrO2 catalyst are reduced to metallic cobalt at the pre-reduction step at 420 ◦ C before the beginning of the steam reforming and oxidative steam reforming of ethanol. The TPR results proved that not all cobalt active phase is reduced at 420 ◦ C to metallic cobalt, the estimated reduction degree is ca. 82–83%. Even if under isothermal pre-reduction at 420 ◦ C for 1 h final reduction degree is higher than under programmed temperature regime in TPR conditions some part of cobalt active phase remains in oxide form; as was discussed above most probably not as easily reduced Co3+ (to Co2+ ) but in the form of Co2+ . However, those Co2+ sites may not be at intimate contact with metallic cobalt atoms and we cannot expect presence of Co2+ /Co sites which are discussed further, important for the SRE and OSRE. Cobalt oxide CoO and a solid solution or cobalt zirconate phase most probably forms separated crystallites, strongly interacting with the support and therefore hard to further reduction to metallic Co.

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Also, the susceptibility to oxidation of the 2KCo/CeO2 catalyst was studied in the temperature-programmed oxidation experiment in order to understand how this catalyst is affected by an oxidative atmosphere under ethanol conversion reactions. TPO profile is presented in Fig. 2b. During the heating of catalyst sample the oxidation of cobalt started at c.a. room temperature and two oxygen consumption peaks below 500 ◦ C are observed due to the formation of cobalt oxide or cobalt oxides. The major consumption of oxygen occurs at c.a. 255 ◦ C. But oxygen-consumption peak appears also at 105 ◦ C. The TPO profile of the 2KCo/CeO2 catalyst suggests sequential oxidation of metallic cobalt via CoO to Co3 O4 [33,34]. However, oxidation of cobalt metal particles to a mixture of CoO and Co3 O4 could not be excluded [33,35]. It could be stated that 2KCo/ZrO2 catalyst can be oxidized (at least in part) under oxidative conditions of the OSRE process. The nature of the SRE and OSRE active sites remains an important issue under debate. The coexistence of metallic Co and CoOx phases was observed in many supported cobalt catalysts during SRE [37–44] and OSRE [45] processes. The degree of reduction of Co under reaction is determined by the nature of supports and the Co2+ /Co ratio is strongly sensitive to the degree of Co reduction obtained in activation of catalysts and to the presence of oxidant reactants (O2 , H2 O) in the feed stream as well as to the temperature of reaction [45]. According to Ávila-Neto et al. [45], the control of the Co2+ /Co ratio by manipulating the interaction of Co oxides with the support and by the composition of reactants can equilibrate steps of ethanol activation and carbon oxidation, resulting in stable catalysts. In our experiments the surface Co2+ /Co ratios in the SRE and OSRE are undoubtedly different, what differentiate product composition and coke deposition in both processes. To determine the sufficient oxygen-to-ethanol molar ratio for the OSRE process over 2KCo/ZrO2 , the thermogravimetric method was used. With this aim, some amount of oxygen was added to the feed stream with a constant molar ratio of H2 O/EtOH = 9/1 at a constant temperature of 420 ◦ C and the catalyst weight change was estimated as a function of time. The oxygen/ethanol molar ratio was gradually increased to determine the sufficient one and the effect of oxygen addition to the feed stream on the 2KCo/ZrO2 catalyst weight change is shown in Fig. 3a and b. As can be seen in Fig. 3a, the certain change of the catalyst weight is observed for all O2 /EtOH molar ratios and it could be attributed to the equilibrium of coke formation and/or the oxidation of cobalt species. Because the catalyst weight slightly decreases with the increase of O2 /EtOH molar ratio from 0.8 to 1.2, it could suggest that the regeneration of the

Fig. 3. Changes of the 2KCo/ZrO2 catalyst weight under ethanol conversion conditions.

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Fig. 4. Raman spectra of 2KCo/ZrO2 catalyst after 21 h of the SRE (EtOH/H2 O = 1/9) and OSRE (EtOH/H2 O/O2 = 1/9/0.9) processes at 420 ◦ C.

catalyst in situ by oxidation of the carbonoceous species on the catalyst surface is improved with the increase of the oxygen amount in the feed. On the basis of these data, the oxygen/ethanol molar ratio of 1.0 was chosen as optimum for the OSRE reaction over 2KCo/ZrO2 catalyst, i.e. sufficient to prevent the carbon formation but small enough to not oxidize significantly the cobalt active phase. However, the thermogravimetric data for the studied catalyst obtained for this molar ratio of the oxygen to ethanol during 21 h of the OSRE process (Fig. 3b) shows that increase of the catalyst weight, probably mainly because of oxidation of cobalt particles, is significant. Thus, the molar ratio of oxygen to ethanol was reduced from 1.0 to 0.9 and as can be seen in Fig. 3b, the change of catalyst weight under OSRE conditions corresponding to this molar ratio was lesser in comparison with the catalyst change weight observed for molar ratio of O2 /EtOH = 1.0. In the case of both OSRE processes with molar ratio of O2 /EtOH = 0.9 and 1.0, the increase of the catalyst weight was rapid only in the first minutes of the process. However, the change of weight at this time was lesser for molar ratio of O2 /EtOH = 0.9 than O2 /EtOH = 1.0 which suggests that the increase of the catalyst weight in the first minutes of the OSRE process is the result of partial oxidation of cobalt particles. The Raman results showed (Fig. 4) that their surface was oxidized to Co3 O4 . The results obtained for the SRE process, which was carried out in the same way but without the presence of oxygen (Fig. 3b), indicates that the catalyst change weight is significantly greater in the case of the SRE than that under OSRE process after 21 h. Because the change of the catalyst weight under SRE conditions is not observed during the first 2 h as opposed to the results obtained for OSRE processes, it could suggest that the 2KCo/ZrO2 catalyst is resistant to coking at the initial stage of this process. However, after the first 2 h of this process, the increase of the weight of the catalyst is very rapid which is undoubtedly the result of fast coking of the catalyst. The initial very slow coking of the catalyst in the SRE results from formation of a new phase of carbon deposit and it is called ‘induction period’. After formation of stable nucleus of carbon their grow is faster and faster and finally the rate of catalyst coking become to be almost constant (the almost straight part of the coking curve seen in Fig. 3b). Such course of catalyst coking through nucleation of the new phase of carbon was frequently observed in many metal-hydrocarbons (or carbon monoxide) systems when filamentous carbon deposit with metal particle at the end of growing whiskers was formed. The references can be monograph of Rostrup-Nielsen [46] or more general monograph of Delmon [47]. In the OSRE, the mechanism of carbonaceous deposit formation is different. The coke is formed rather on the surface as a polymer (polymerization of ethylene or aldol condensation products) encapsulating crystallites of the active phase or the whole particles of the catalyst. The presence of oxygen

prevents the catalyst from nucleating road of filamentous carbonaceous deposit formation. The thermodynamic analysis performed in work of Mas et al. [48] indicates that if temperatures higher than 230 ◦ C are used for the SRE process, a water-to-ethanol molar ratio higher than 3 is required to avoid coke formation. However, in their consideration it was assumed that sources of carbon are only disproportionation of carbon monoxide and methane decomposition, carbon formed is elemental and in the graphitic form. They did not take other sources of carbonaceous deposit formation into account, such as polymerization and dehydrogenation of ethylene, aldol condensation of acetaldehyde. Furthermore, the ethanol/water vapor ratio on the catalyst surface may be, and most probably it is different than that at the gas phase, used for thermodynamic considerations. Although the formation of carbon was not predicted thermodynamically for SRE conditions (T = 420 ◦ C, H2 O/EtOH = 9/1 applied in this work) [48,49], the catalyst weight change under SRE conditions must be interpreted as caused by its coking. The addition of oxygen to the stream of ethanol and water vapors in amount corresponding to molar ratio of O2 /EtOH = 0.9 allowed significantly decrease the rate and amount of carbonaceous deposit formation. Because the changes of 2KCo/ZrO2 catalyst weight were observed in the SRE and in the OSRE processes in thermogravimetric studies (however the thermogravimertic method does not give any information about the reasons of these changes) the Raman spectra of the catalyst after both reactions of ethanol conversion were taken over to find the characteristic bands for coke and/or cobalt oxide. The Raman spectra are shown in Fig. 4. In the case of Raman spectrum of the catalyst used in the SRE process, two bands are observed around 1320 and 1600 cm−1 which are characteristic for disordered carbonaceous and ordered graphitic species and only three small peaks corresponded to the stronger bands of Co3 O4 can be seen. These results suggest that the surface of the catalyst after SRE process (H2 O/EtOH = 9/1) was covered by the coke [24] and only in minor extend it was oxidized. On the other hand, the Raman spectrum obtained for the catalyst used in the OSRE process displays all bands attributed to the Co3 O4 phase and only one very small signal which can derive from disordered carbonaceous species is observed around 1350 cm−1 . It means that the cobalt active phase of the 2KCo/ZrO2 catalyst underwent the oxidation to a large degree during OSRE process (H2 O/EtOH/O2 = 9/1/0.9). These results reveal that the coke formation and oxidation of the metallic particles are the main reasons of the observed catalyst weight change on thermogravimetric profiles under the SRE and OSRE processes, respectively. Coke formation and oxidation of metallic particles during ethanol is significantly affected by the size of the cobalt particles. According to da Silva et al. [36] studies, the rate of formation of carbon deposit during SRE process is significantly lower on the smallest cobalt particles (<3 nm) than on the larger ones. On the other hand, other studies of da Silva et al. [8] shows that the catalyst with the small cobalt particle size (<4 nm) easier deactivates due to the oxidation of surface atoms by the oxygen from the feed during OSRE process. These studies reveal that the controlling of the size of the cobalt nanoparticles plays a key role in obtaining stable catalyst during SRE and OSRE processes. The cobalt particles in our catalyst are rather large (31 nm) and these large-size cobalt particles are most probably responsible for carbon deposition in the SRE. However, the catalyst with relatively large particles of cobalt active size, which are usually more stable in the SRE and OSRE conditions than smaller crystallites, enabled us to clearly monitor influence of oxygen addition to the reaction mixture on effects of both processes, on the catalyst coking and on its oxidation. Presented results show that even large cobalt particles do not prevent surface of the cobalt active phase from intensive oxidation by oxygen.

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Fig. 5. Effect of temperature on ethanol conversion and products distribution under SRE (EtOH/H2 O = 1/9) and OSRE (EtOH/H2 O/O2 = 1/9/0.9) conditions over the 2KCo/ZrO2 catalyst.

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Table 2 Effect of temperature on ethanol conversion and products distribution under SRE (EtOH/H2 O = 1/9) and OSRE (EtOH/H2 O/O2 = 1/9/0.9) conditions over the 2KCo/ZrO2 catalyst. Temp. (◦ C)

XEtOH (%)

Product distribution (%) H2

CO

CO2

CH3 CHO

(CH3 )2 CO

CH4

C2 H4

CO2 /CO

H2 /CO

218 32.5 16.8 14.2 13.0

SRE 350 420 480 540 600

29.1 100 100 100 100

73.1 74.0 72.5 73.6 73.8

0.33 2.28 4.32 5.18 5.72

22.9 22.4 21.5 20.0 19.3

0.08 – – – –

– – – – –

0.36 1.38 1.66 1.22 1.25

– – – – –

68.4 9.82 4.98 3.86 3.37

OSRE 350 420 480 540 600

59.2 66.5 86.3 100 100

21.1 33.8 53.6 63.9 61.8

2.83 0.94 1.69 2.72 3.86

46.2 49.8 36.7 31.2 30.9

0.33 0.21 0.15 – –

1.14 3.13 2.41 0.40 –

0.99 0.53 0.87 1.43 3.38

0.65 0.48 0.53 0.31 0.07

16.3 52.7 21.8 11.5 8.01

Fig. 5 and Table 2 compare ethanol conversions and product compositions for the SRE and OSRE processes over 2KCo/ZrO2 catalyst obtained at different temperatures (350–600 ◦ C). For the SRE, only the temperature of 350 ◦ C was too low for the complete conversion of ethanol. In the case of OSRE process, 100% conversion of ethanol was not reached at temperature lower than 540 ◦ C. For both processes, the main reaction products were hydrogen and carbon dioxide. Under SRE conditions, the temperature does not influence significantly their production and the concentrations of hydrogen and carbon dioxide are ca. 72% and 22%, respectively. In the OSRE, the amount of hydrogen was below 25% at 350 ◦ C but it increased to 64% at 540 ◦ C and above. The simultaneous decrease of carbon dioxide concentration to 30% at the temperature of 540–600 ◦ C was observed. In the whole range of temperatures, the higher amounts of hydrogen are produced under the SRE than in the OSRE process. Probably, the presence of oxygen intensifies oxidation of hydrogen, what lead to the decrease of hydrogen production. This trend is inverted in the case of carbon dioxide production, i.e. in comparison with the SRE, higher amount of this product is formed in the OSRE process. The more intensive formation of carbon dioxide in the OSRE can be explained with the additional oxidation reactions that may occur when oxygen is cofed in the reactant mixture (ethanol, organic by-products, carbon monoxide, carbon) + O2 → CO2 Because the presence even traces of CO poisons the PEM fuel cells, it is highly undesirable product of the ethanol conversion. It is formed in both studied reforming but carbon monoxide oxidation is favored in the presence of oxygen, resulting in its smaller amounts in the OSRE process in comparison with those in the SRE. In the SRE process, the traces of acetaldehyde were formed at 350 ◦ C because of dehydrogenation of ethanol. At higher temperatures the presence of acetaldehyde among all products was eliminated. In the case of the OSRE process, small amounts of acetaldehyde were detected below 540 ◦ C. Moreover, small amounts of acetone were formed in this process below 600 ◦ C whereas this product was not formed in the SRE reaction. Therefore, it is expected that the oxidized surface of cobalt-based particles favors production of acetaldehyde and acetone. Similar results were obtained for cobalt-based catalyst by Pereira et al. [9] and Rybak et al. [24]. The acetaldehyde was almost sole product which was obtained from the partial oxidation of ethanol carried out over Co/CeO2 catalyst by Pereira et al. [9]. Whereas Rybak et al. [24] observed high selectivity to acetone over unreduced cobalt oxidesbased catalysts. Methane is an undesirable product because it suppresses the hydrogen yield. It is present among products of both reforming, but its concentration is insignificant. Also, small amounts of ethylene appear among OSRE products whereas it was absent in the case

7.46 35.8 31.8 23.5 16.0

of the second process. Because it is known that ethylene is one of the key factors of catalysts deactivation because its polymerization is followed by formation of carbonaceous deposit on the catalyst surface, the presence of ethylene among–by-products of ethanol conversion is highly undesirable. Taking into account the activity and selectivity of the 2KCo/ZrO2 catalyst, it could be concluded that the optimum, the lowest temperature of the SRE over this catalyst is 420 ◦ C whereas the OSRE process requires significantly higher reaction temperature at least 540 ◦ C or above. In order to highlight the different state of the 2KCo/ZrO2 catalyst under SRE and OSRE conditions, concluded from gravimetric studies shown in Fig. 3b, the comparisons of the stability of the SRE and OSRE processes at 420 ◦ C and 540 ◦ C were carried out. The performance of the catalyst under different conditions is shown in Figs. 6 and 7. The results obtained for the temperature of 420 ◦ C are in the good agreement with those obtained in thermogravimetric studies and show that oxygen addition to the feed significantly improves the stability of 2KCo/ZrO2 catalyst activity at this temperature. At 540 ◦ C the activity of the catalyst and its stability was very high and comparable in both processes. As can be seen in Fig. 6, at 420 ◦ C the catalyst is deactivated with time on the stream under SRE conditions very fast and the ethanol conversion decreased considerably within 24 h from 100% to 27%, due to intensive coking of the catalyst (Figs. 3b and 4). In the OSRE process, the temperature of 420 ◦ C was too low for the complete conversion of ethanol (see Fig. 5) and ethanol was converted only in the limited but stable extent, with slightly increasing conversion from 54% to 61% during 24 h. This picture was also expected, taking into account results of gravimetric and Raman measurements, shown in Figs. 3 and 4. At 540 ◦ C, the final conversion of ethanol in SRE and OSRE processes remained around 100% after 24 h. For both processes, the main reaction products are H2 and CO2 during the whole reaction period at both temperatures. At 420 ◦ C, hydrogen production during OSRE was rather low (∼30%) in comparison with the concentration of hydrogen in the outlet stream of the SRE process (∼71%). The increase of the reaction temperature to 540 ◦ C did not remarkably influence the hydrogen production during SRE process (∼73% at 24 h). However, significant increase in hydrogen production from 30% to 67% with the increase of the temperature from 420 ◦ C to 540 ◦ C, respectively in the OSRE process was observed. The production of the second main product, carbon dioxide, was favored under OSRE conditions at both temperatures. The concentration of carbon dioxide in the outlet during this process was especially high at the temperature of 420 ◦ C (∼45%) and decreased to 28% at 540 ◦ C. In the case of the SRE process, the carbon dioxide concentration was about 17% and 21% at 420 ◦ C and

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Fig. 6. Effect of time-on-stream on C2 H5 OH conversion and products composition over 2KCo/ZrO2 catalyst under SRE and OSRE conditions (420 ◦ C).

540 ◦ C, respectively. The oxidation reactions may explain the more intensive formation of carbon dioxide in the OSRE process. The undesirable presence of carbon monoxide was found in the outlet stream for both processes. Especially, the high carbon monoxide amount (∼5%) was formed at 540 ◦ C. Also small amounts of methane and acetaldehyde were formed in the side reactions during SRE and OSRE process. Additionally, the ethylene and acetone were detected during OSRE process. Beside of very good conformity of changes in the conversion of alcohol observed during SRE and OSRE, carried out at 420 ◦ C, with results of coking and oxidation of the catalyst (Figs. 3b and 4) also the changes in selectivities of both processes are in good harmony with changes of the catalyst state.

In the case of the SRE, the greatest changes in the composition of all products take place in the initial period of time-on-stream, when the rate of catalyst coking increases (compare Figs. 3b and 6). In this period cobalt active phase crystallites are pull out of the support and carried with growing whiskers [46]. The formation of encapsulating carbon is not excluded at the same time; it diminishes the surface of active phase accessible for reactants and therefore conversion of alcohol. Cobalt crystallites located at the end of carbon whiskers are obviously a new catalyst in its nature, i.e. cobalt supported on carbon instead of initial cobalt supported on ceria. The selectivity of such new catalytic system is different from initial one and it does not changes significantly in the further period of the time-on-stream.

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Fig. 7. Effect of time-on-stream on C2 H5 OH conversion and products composition over 2KCo/ZrO2 catalyst under SRE and OSRE conditions (540 ◦ C).

Also the course of the OSRE is consisted with the course of catalyst oxidation, shown in Fig. 3b and confirmed by Raman spectroscopy (Fig. 4). Oxidation of the catalyst takes place in the initial period of the OSRE process–changes of products composition occurred also at the same time. An attention should be drawn to the stability and effects of the OSRE as well as of the SRE at the higher temperature, 540 ◦ C. The ethanol conversion and production of main products, hydrogen and carbon dioxide, were almost on unchanged high levels for many hours. However, changes in formation of by-products – acetaldehyde, carbon monoxide and methane – go with time-on-stream in the undesirable directions. The SRE has the certain advantage over OSRE in no ethylene nor acetone production.

4. Conclusions The chemical state of the zirconia supported cobalt catalyst promoted with 2 wt.% of potassium under SRE and OSRE conditions depends on the balance of redox environment in both processes. Under SRE cobalt mainly remains in metallic state while under OSRE the surface of the active phase is oxidized to cobalt oxide Co3 O4 . The different chemical state of the catalyst is reflected in different course and effects of both processes. In the SRE (EtOH/H2 O = 1/9 mol/mol) the catalyst undergoes strong formation of the carbonaceous deposit, and the odds are that it is mostly filamentous carbon with cobalt crystallites on the top of whiskers. However, thin layer of polymeric carbon encapsulating active phase

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and diminishing its accessibility for reactants, reflected in high decreasing alcohol conversion during time-on-stream, is also very probable. The EtOH/O2 molar ratio of 1/0.9 corresponds to the concentration of the oxygen which is sufficient to prevent significant carbon formation on the catalyst surface in the OSRE. In the OSRE (EtOH/H2 O/O2 = 1/9/0.9 mol/mol) the carbonaceous deposit forms (if any) rather only an encapsulating carbon. Different chemical state of the catalyst in both processes is the reason that the OSRE process requires significantly higher reaction temperature than that of the SRE to achieve a complete ethanol conversion. While the temperature of 420 ◦ C is sufficient for the SRE process, in the case of the OSRE, the temperature of 540 ◦ C is required. Moreover, at 420 ◦ C much smaller amount of hydrogen, which is the most desirable product of the ethanol conversion, is produced in the presence of oxygen. At 540 ◦ C the ethanol conversion is complete and hydrogen contents among all products are much similar in both processes, 73 and 67% at 24 h of the time-on-stream, respectively. Improving of the resistance of the catalyst to coking/oxidation under SRE/OSRE conditions is still the challenge to solution. The catalyst deactivation and accompanying changes in selectivity go parallel to its coking or oxidation. The catalyst deactivation is lesser in the OSRE than that in the SRE. However, in the OSRE except of smaller yield of hydrogen, small amount of ethylene and acetone are among products while in the SRE they are absent. The OSRE requires also more than one hundred degree higher temperature, where the concentration of carbon monoxide in the reformate gas is higher than that at the lower temperature of the SRE. This is an important issue if the hydrogen-containing gas is produced for applications in low or high temperature PEM fuel cells, which platinum anodes are poisoned by carbon monoxide. Supporting information Results of the TPD-CO from Co/ZrO2 and 2KCo/ZrO2 catalysts, showing influence of potassium promoter on disproportionation of chemisorbed carbon monoxide, being one of the sources of carbonaceous deposit formation during SRE and OSRE are presented. Acknowledgements The part of results has been achieved within the framework of the 1st call on Applied Catalysis carried out by ACENET ERA-NET (project ACE.07.009), with funding from the Ministry of Science and Higher Education of Poland. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-06-024/09 Center on Functional Nanomaterials). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cattod.2014.07.032. References [1] J.-L. Bi, Y.-Y. Hong, C.-C. Lee, C.-T. Yeh, C.-B. Wang, Catal. Today 129 (2007) 322–329. [2] A. Iulianelli, T. Longo, S. Liguori, P.K. Seelam, R.L. Keiski, A. Basile, Int. J. Hydrogen Energy 34 (2009) 8558–8565.

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