Steam reforming of ethanol is a prospective method for the production of hydrogen from renewable feedstock for subsequent use as a motor fuel or in fuel cells. In the full conversion of ethanol in the SRE process by the reaction

$$ {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+{3\mathrm{H}}_2\mathrm{O}={2\mathrm{CO}}_2+6{\mathrm{H}}_2 $$
(I)

the hydrogen is formed both from the ethanol (50 mole %) and from water (50 mole %), which makes the process particularly attractive. It is also important that if bioethanol obtained from plant feedstock is used carbon dioxide does not accumulate in the atmosphere since the release of CO2 into the atmosphere in reaction (I) is fully compensated by its consumption during growth of the plants.

According to the reviews [1,2,3,4,5] a large number of papers have been devoted to investigation of the steam reforming of ethanol, and in most of them supported metallic catalysts were used for SRE. Only a limited number of other types of catalysts have been studied in this reaction, and they include simple and complex oxides. The catalytic properties of MgO, Al2O3, ZnO, V2O5, La2O3, CeO2, and Sm2O3 were investigated in [6]. Most of the investigated simple oxides were active in the conversion of ethanol. Here products from steam reforming of ethanol were observed at the oxides V2O5, La2O3, CeO2, and Sm2O3 (the oxides of metals with variable valence), which have oxidation–reduction characteristics. Most effective was the oxide ZnO, which has redox characteristics. Complex oxides with a spinel structure MAl2O4 (M = Cu, Zn, or Ni) were highly effective in SRE [7]. The catalytic characteristics of complex oxides NiM2O4 (M = Al, Mn, Fe) were investigated in [8]. Spinel NiAl2O4 exhibited high activity and selectivity at 823 K; the crystal structure of NiAl2O4 remained practically unchanged under the reaction conditions, whereas the spinels NiMn2O4 and NiFe2O4 decomposed to some degree or other [8]. Catalysts based on the perovskites La2NiO4, LaFeyNi1–yO3, and LaCo1–xZnxO3 also proved fairly active and selective although they decomposed in the course of catalysis [9, 10]. High activity in SRE was exhibited by a catalyst based on cobalt hydrotalcite [11]; it is notable that only traces of metallic cobalt were identified after the reaction, i.e., cobalt in the oxidized state is the active particle in the mixed oxide catalyst. The FexCo3–xO4 catalysts, for which metal phases were not detected after catalysis with x > 0.15, were highly effective in SRE [12].

In the present work we present the results of our investigations of the steam reforming of ethanol on complex oxide catalysts: ferrites MFe2O4 (M = Mg, Mn, Fe, Co, Ni, Cu, Zn) [13,14,15,16,17,18,19,20,21]. Samples of Mn, Co, and Ni ferrites were prepared by thermal decomposition of [MFe2O(CH3COO)6(H2O)3]·2H2O (M= Mn, Co, Ni) by the method described in [22]. According to TEM and XRD the initial samples consisted of nanoparticles of the respective ferrites with an average diameter of about 8 nm and a narrow particle size distribution [15]. Manganese ferrite was also prepared by coprecipitation, as also were the ferrites of Fe, Mg, and Zn [15, 21]. Electron diffraction data for the samples prepared in this way showed the presence of crystalline phases in the corresponding ferrites with spinel structures.

The catalytic properties of the ferrites in the SRE process were investigated in a flow-type quartz reactor with molar ratio H2O/C2H5OH = 19 (2.7 mole % C2H5OH, 50 mole % H2O, remainder N2), which is close to the water/ethanol ratio in bioethanol produced by the fermentation of biomass, and the reaction mixture flow rate was 0.17 mol/h. All the obtained ferrites exhibited catalytic activity in SRE. Figure 1a and b shows the temperature dependence of the yield of hydrogen for the ferrite catalysts. The yield of hydrogen was taken as 100% if six moles of hydrogen were formed for one mole of ethanol added to the reactor in accordance with the stoichiometry of reaction (I).

Fig. 1.
figure 1

Temperature dependence of yield of hydrogen at nanocatalysts NiFe2O4 (1), CoFe2O4 (2), and MnFe2O4 (3) obtained by thermal decomposition of the heteronuclear complexes (a) and at the catalysts CuFe2O4 (1), FeFe2O4 (2), MnFe2O4 (3), MgFe2O4 (4), and ZnFe2O4 (5) obtained by the coprecipitation method (b).

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The composition of the Mg, Mn, Fe, and Zn ferrites did not change in the course of catalysis [13, 20, 21] (only sintering of the nanoparticles occurred), while the Co, Ni, and Cu ferrites were partly or completely reduced to the respective metals by the action of the reaction mixture [15, 18, 21].

The highest yield of hydrogen (94.6%) was obtained for the nanosized MnFe2O4 obtained by decomposition of the heteronuclear complex at 650 °C (Fig. 1a). The second highest yield of hydrogen (84.3%) was obtained at 550 ºC with MnFe2O4 obtained by coprecipitation (Fig. 1b). It is not impossible that a higher yield could be obtained with this catalyst if the catalysis temperature was increased since, as seen from Fig. 1b, the yield of hydrogen has a tendency to increase with increase of temperature.

The main side products observed during SRE on ferrites were acetaldehyde, acetone, and carbon oxides. Exceptions were the nickel and cobalt ferrites, where the formation of acetone was not observed during catalysis. The yield of hydrogen was reduced by the formation of side products. From our data and from the published data [1, 3, 23, 24] it is possible to propose the following scheme for the conversion of ethanol in the SRE process on ferrites (Fig. 2, the subscript “a” signifies the adsorbed state).

Fig. 2.
figure 2

Scheme of the conversion of ethanol at ferrites.

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The scheme includes the target reaction (I) and reactions leading to the formation of the main side products:

$$ {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}={\mathrm{C}\mathrm{H}}_3\mathrm{CHO}+{\mathrm{H}}_2, $$
(II)
$$ {2\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}={\mathrm{CH}}_3{\mathrm{CO}\mathrm{CH}}_3+{\mathrm{CO}}_2+{4\mathrm{H}}_2, $$
(III)
$$ {2\mathrm{CH}}_3\mathrm{CHO}+{\mathrm{H}}_2\mathrm{O}={\mathrm{CH}}_3{\mathrm{CO}\mathrm{CH}}_3+{\mathrm{CO}}_2+{2\mathrm{H}}_2, $$
(IV)
$$ {\mathrm{CH}}_3\mathrm{CHO}+3{\mathrm{H}}_2\mathrm{O}=2{\mathrm{CO}}_2+{5\mathrm{H}}_2. $$
(V)

At higher temperatures the formation of CO was also observed at NiFe2O4 (550-700 °C), CoFe2O4 (600-700 °C), and MnFe2O4 (700 °C); the formation of CO was not observed on the other ferrites. The side products include C1-C3 hydrocarbons, which in most cases were formed in relatively small amounts.

The mechanism of the SRE process as a whole includes mechanisms of two types: oxidation–reduction and acid–base [19,20,21]. The oxidation–reduction mechanism is confirmed convincingly by the data obtained for FeFe2O4 and NiFe2O4 ferrites in [25, 26]. In these papers the SRE process was conducted by delivering the ethanol vapor and steam alternately to the ferrite catalyst at 450 °C. The delivery times of the reagents to the catalyst varied from 1 h to 2 min. In the long reduction stage of the cycle (1 h) the conversion of ethanol was close to 100%, and the products were CO2 and H2 and also CO,H2O, CH4, carbon, and acetone (observed in the first 10-15 min). The ferrites were reduced in the ethanol atmosphere with the formation of metallic iron and nickel. In the following oxidation stage the structure of the ferrites was restored under the influence of the steam. Apart from hydrogen CO and CO2 were also formed at this stage as a result of interaction between the water molecules and the surface carbon. Cyclic delivery of ethanol vapor and steam with short reaction times (the reactions were conducted with FeFe2O4 [26]) did not lead to change in the structure of the ferrite. Such behavior provides an important argument in favor of the oxidation–reduction mechanism for the SRE process at ferrites under stationary conditions.

At the same time, the individual reactions of the SRE process can include an acid–base stage. Such a reaction may include dehydrogenation of the ethanol to acetaldehyde (II).Ascheme for the probable heterolytic mechanism of the reaction on ferrites is presented in Fig. 3. The main features of this mechanism were taken from [27,28,29], in which a mechanism involving the dehydrogenation of propane to propylene at single MII centers (M = Zn, Co, Fe) in a matrix of SiO2 was established.

Fig. 3.
figure 3

Scheme of the probable mechanism of dehydrogenation of ethanol to acetaldehyde on the ferrites.

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The surface cations of the metals in the ferrites exhibit the characteristics of Lewis acid centers with oxygen anions as basic centers. Interaction of the ethanol with the acid–base pair of the centers leads to heterolytic cleavage of the O–H bond in the ethanol with the formation of an ethoxy intermediate localized at the metal ion (the Lewis acid center) and a proton localized on a surface oxygen ion (the basic center).

At the next stage β-elimination of a H– ion from a C–H bond of the ethoxy particle occurs, and the H– combines with a surface metal ion with desorption of an acetaldehyde molecule. The catalytic cycle closes with recombination of the hydride ion and proton to form a molecule of hydrogen in the gas phase.

In our opinion such a mechanism is to be expected for the Mg, Mn, Fe, and Zn ferrites, the chemical composition of which did not change under the action of the reaction mixture in the SRE process. It seems less likely for the CuFe2O4 ferrite. This ferrite proved much more active and selective with respect to acetaldehyde than the other investigated ferrites [18, 21]. With this ferrite at an initial temperature of 300 °C in the catalytic experiments the yield of acetaldehyde amounted to 90%, which is not much lower than the thermodynamically possible yield. The high activity of CuFe2O4 may be due to the readily occurring reversible Cu2+↔Cu+ transition on the surface of the copper ferrite, and a homolytic (redox) mechanism of transformation of ethanol into acetaldehyde is thus realized. In any case the copper ferrite can be included among prospective catalysts for the important dehydrogenation of ethanol to acetaldehyde.

The desorption of acetaldehyde most likely includes intermediate formation of an adsorbed particle of CH3CHO(a), which initiates the subsequent reaction paths of the SRE process and particularly transformation into acetone by the aldol condensation mechanism as in the following mechanism for oxide catalysts proposed in [30] and confirmed in [31, 32]:

$$ {2\mathrm{CH}}_3-{\mathrm{CH}\mathrm{O}}_{\left(\mathrm{a}\right)}+{\mathrm{O}}_{\left(\mathrm{s}\right)}\leftrightarrow {\mathrm{CH}}_3-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_2-{\mathrm{COO}}_{\left(\mathrm{a}\right)}+{\mathrm{H}}_{\left(\mathrm{a}\right)}, $$
(VI)
$$ {\mathrm{CH}}_3-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_2-{\mathrm{CO}\mathrm{O}}_{\left(\mathrm{a}\right)}+{\mathrm{H}}_{\left(\mathrm{a}\right)}\leftrightarrow {\mathrm{CH}}_3-\mathrm{CO}-{\mathrm{CH}}_3+{\mathrm{CO}}_2+{\mathrm{H}}_2 $$
(VII)

where O(s) is the surface oxygen in the crystal lattice of the ferrite.

As known, the aldol condensation is catalyzed by acids or bases; in the case of the SRE process on ferrites condensation takes place with participation of the acid or base centers of the catalyst. The overall conversion of acetaldehyde or ethanol into acetone also includes a redox reaction involving surface oxygen. Removal of the surface oxygen in stage (VI) leads to the formation of surface oxygen vacancies. The loss of oxygen is compensated by interaction of the water with the reduced surface centers according to reaction (VIII), as was observed for CeO2 [31] and for certain other catalysts [32]:

$$ {\mathrm{H}}_2\mathrm{O}+{}_{\left(\mathrm{s}\right)}\leftrightarrow {\mathrm{O}}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2 $$
(VIII)

where (s) represents surface oxygen vacancies.

The oxygen vacancy can be represented in greater detail as (Fe2+ Fe2+)(s), which is transformed by reaction (VIII) into [Fe3+O2–Fe3+](s). We note also that reaction (VIII) plays an important role in the SRE process since it is possible from it to obtain up to 50 mole % of hydrogen from such a cheap feedstock as water.

Oxygen vacancies appear in the ferrites not only during the formation of acetone but also as a result of reduction of the surface ions of the metals by other gaseous organic compounds or by their fragments (CH3(a), CH2(a), CO(a)) and by hydrogen (which is the main reducing agent at elevated temperatures). The overall reduction stage in the SRE process can be represented as follows:

$$ {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}+3{\mathrm{O}}_{\left(\mathrm{s}\right)}\leftrightarrow 2{\mathrm{CO}}_2+{3}_{\left(\mathrm{s}\right)}+3{\mathrm{H}}_2. $$
(IX)

The concentration of oxygen vacancies in the ferrites can vary within certain limits without change in the crystalline phase of the ferrite. However, the crystalline phase is destroyed if it becomes too high, as observed in the ferrites NiFe2O4, CoFe2O4, and CuFe2O4.

The selectivity toward acetone for the ferrites MgFe2O4, MnFe2O4, FeFe2O4, and ZnFe2O4 that did not undergo reduction during catalysis is presented in Fig. 4.

Fig. 4.
figure 4

Temperature dependence of the selectivity to acetone on ferrites FeFe2O4 (1), MnFe2O4 (2), MgFe2O4 (3), and ZnFe2O4 (4).

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As seen from Fig. 4, the selectivity to acetone passes through a maximum with increase of temperature. If the overall scheme for the transformation of ethanol is taken into account (Fig. 2), this indicates that the SRE process takes place in consecutive stages:

$$ {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\leftrightarrow {\mathrm{CH}}_3\mathrm{CHO}\leftrightarrow {\mathrm{CH}}_3{\mathrm{CO}\mathrm{CH}}_3\leftrightarrow {\mathrm{CO}}_2,{\mathrm{H}}_2 $$

or, which is particularly characteristic of FeFe2O4, according to the reaction path:

$$ {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}\leftrightarrow {\mathrm{CH}}_3{\mathrm{CO}\mathrm{CH}}_3\leftrightarrow {\mathrm{CO}}_2,{\mathrm{H}}_2 $$

(i.e., without the intermediate formation of acetaldehyde in the gas phase).

Maximum selectivity to acetone was observed with FeFe2O4 at 400 °C (Fig. 4). According to the stoichiometry of reaction (III) the maximum selectivity is 75% to acetone and 25% to CO2 since 3/4 of the ethanol molecules are converted into acetone and 1/4 into CO2. As seen from Table 1, the experimental values of the selectivity to acetone and CO2 for FeFe2O4 at 400 °C are close to their theoretical values; according to data in [16, 19] the same was observed for iron oxide Fe2O3. (In the course of the SRE process Fe2O3 is converted into FeFe2O4.)

Table 1. Experimental and Equilibrium Values for the Conversion of Ethanol and Product Selectivity at 400 °C
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For comparison Table 1 gives the equilibrium values for the conversion of ethanol and the selectivity to the products calculated for the conversion of ethanol into acetaldehyde and acetone with the composition of the initial reaction mixture, 2.7 mole % C2H5OH, 50 mole % H2O, remainder N2, that was used in the experiments. The results show that the catalytic conversion of ethanol into acetone is close to equilibrium, and further conversion of acetone into CO2 and H2 is still greatly retarded at 400 °C.

It can also be concluded that iron oxides have proved themselves as prospective catalysts for the production of acetone (and at the same time hydrogen) from ethanol under certain conditions.

Above 400 °C the conversion of acetone into CO2 and H2 becomes more significant both for FeFe2O4 and for MgFe2O4, MnFe2O4, and ZnFe2O4 (the selectivities toward CO2 and H2 increase with decrease in the selectivity to acetone) [21].

As seen from Fig. 1b, the highest and closest values for the yield of hydrogen at the ferrites MnFe2O4, MgFe2O4, and FeFe2O4 were obtained at the maximum experimental temperature of 550 °C. Since magnesium is a metal with constant valence the proximity of the yields of hydrogen at MnFe2O4 and MgFe2O4 indicates that the Fe3+ ions, which are capable of redox transitions in oxidation–reduction pairs Fe3+↔Fe2+, have a key role in the catalytic action of ferrites in the SRE process. The iron ions are located at least partly at the octahedral positions in the structure of the ferrites, and according to [34,35,36] the surface of the oxides in the spinel structure consists largely of sites of octahedral coordination, and their catalytic activity is actually determined by the octahedral cations.

The nature of the metal in the composition of the ferrospinels has a significant effect on the ability of the Fe3+ cations of the spinel crystal lattice to be reduced, and this is confirmed by our data on thermoprogrammed reduction with hydrogen [21]. The lowest temperature of maximum reduction (characterizing the strength of the bond between the oxygen and the surface of the catalyst) in the investigated series of ferrites was observed for the Mn ferrite. Consequently, MnFe2O4 can easily give up oxygen for oxidation of the surface organic intermediate compounds, and the selectivity to the most oxidized reaction product CO2 and to hydrogen is higher for MnFe2O4 than for the other ferrites.

As known, the distribution of the products of steam reforming of ethanol also depends on the acid–base characteristics of the catalysts [37,38,39]. According to [40,41,42], in the presence of a sufficient amount of weak surface acid centers the selectivity toward the aldehydes is closely related to the strength of the basic centers on the surface of the oxides; the surface weak acid centers play a key role in dehydrogenation of the alcohols. The scheme of the probable mechanism for the ferrites presented above in Fig. 3 agrees with the conclusions in the literature about the important role of both base and acid centers in the dehydrogenation of ethanol to acetaldehyde.

The data obtained by thermal programmed desorption of CO2 (TPD-CO2) by the method in [21] indicate the presence of base centers on the surface of the ferrites (Table 2). The desorption maxima with temperature \( {T}_{\mathrm{m}1}^{{\mathrm{CO}}_2} \) in the region of 86-121 °C can be attributed to desorption of CO2 from weak base centers. Here a contribution from desorption of physically adsorbed CO2 cannot be entirely ruled out. The temperatures of the desorption maxima \( {T}_{\mathrm{m}2}^{{\mathrm{CO}}_2} \) in the region of 150-203 °C correspond to base centers of moderate strength.

Table 2. Temperatures of the Desorption Maxima of Carbon Dioxide \( {T}_{\mathrm{m}2}^{{\mathrm{CO}}_2},{}^{{}^{\circ}}\mathrm{C} \) and Ammonia \( {T}_{\mathrm{m}}^{{\mathrm{NH}}_3} \) during TPD-CO2 and TPD-NH3 from the Surface of Ferrite Catalysts
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Of the ferrites stable in catalysis that we studied at low degrees of conversion of ethanol (300 °C) FeFe2O4 has the lowest selectivity to acetaldehyde. We note that this ferrite has the highest strength for the surface acid centers among the studied ferrites. According to the data that we obtained by thermoprogrammed desorption of ammonia (TPD-NH3) by the method in [21], the temperature of the NH3 desorption maximum, which characterizes the strength of the acid centers, is substantially higher for FeFe2O4 than \( {T}_{\mathrm{m}}^{{\mathrm{NH}}_3} \) for the other ferrites (Table 2). The Zn and Mg ferrites, which have relatively weak acid centers as evidenced by low values of \( {T}_{\mathrm{m}}^{{\mathrm{NH}}_3} \) in a series of studied ferrites, have high initial selectivity to acetaldehyde.

If the temperature is increased to 400 °C the conversion of ethanol and acetaldehyde into acetone is increased, and the highest selectivity to acetone is achieved with FeFe2O4 and MnFe2O4 (Fig. 4). The aldol condensation includes the intermediate formation of carbanionic intermediates localized at an acid center–base center pair.

The higher selectivity to acetone in the steam reforming of ethanol on FeFe2O4 and MnFe2O4 can be explained by the presence on their surface of base centers of sufficient strength and acid centers that are stronger than the acid centers of the Zn and Mg ferrites, as we have demonstrated by the TPD-CO2 and TPD-NH3 methods.

The oxidation–reduction stages of the SRE process are characteristic primarily of the reactions in the steam conversion of ethanol (I), acetaldehyde (V), and acetone to H2 and CO2, which take place through the formation of surface particles CHx(a) and CO(a), and other intermediate compounds [23, 43]. As mentioned above, at not too high temperatures (up to 550 °C) the formation of CO in the gas phase was not observed at any of the ferrites except the Co and Ni ferrites, which were reduced to the metals during SRE. The absence of CO may be due to the fact that the adsorbed particles of CO are oxidized much more quickly to CO2 (CO(a) + O(s) → CO2) than they are desorbed. It is also clear that Mg, Mn, Fe, Cu, and Zn ferrites are relatively inactive in water-gas shift reaction, otherwise the appearance of CO as a result of the reaction CO2 + H2 → CO + H2O would be observed. (According to thermodynamic calculations for the conditions used in the present work [5] the equilibrium content of CO in the products of the SRE reaction becomes appreciable above 400 °C.)

In conclusion we will briefly summarize the obtained results. The Mn, Fe, and Mg ferrites exhibit high catalytic activity in the production of hydrogen by steam reforming of ethanol. Over MnFe2O4 a close to stoichiometric yield of hydrogen (94.6%) (5.68 mol H2/mol of initial ethanol) is obtained at 650 °C. On Mg, Mn, Fe, Cu, and Zn ferrites it is possible to realize the reaction with complete conversion of ethanol without the formation of CO, which is an important factor for later use of the hydrogen in low-temperature fuel cells. Copper ferrite is a promising catalyst for the production of acetaldehyde from ethanol, while iron ferrite (Fe3O4) is promising for acetone. These catalysts can be used in particular in two-stage catalytic processes that include the intermediate formation of acetaldehyde or acetone. The mechanism of the steam reforming of ethanol on ferrites includes both oxidation–reduction and acid–base stages. To obtain high selectivity in the process it is necessary to have a specific balance of oxidation–reduction and acid–base properties of oxide catalysts.