2.1. Catalysts Characterization
The surface area measured for both powdered catalysts is about 330 m
2·g
−1, and the average Barrett-Joyner-Halenda (BJH) pore size is 9.2 nm. These values are characteristic of aerogel structures [
41].The XRD profile of the powdered
aer-CoFeSi catalyst is shown in
Figure 1, along with that of
aer-CoSi [
41] for comparison. Both profiles are similar and show diffraction peaks characteristic of the talc structure [
45]. The talc structure corresponds to a layered metal silicate hydroxide, (Co,Fe)
3[Si
2O
5]
2(OH)
2, which is synthesized under supercritical drying between SiO
2 and the metal precursors [
41]. The peak measured at low angles corresponds to the basal plane (001) of the layered structure. This peak exhibits a very low intensity, which means that the number of layers in the talc crystallites is low, consistent with a good dispersion of the catalyst particles in the aerogel host. The atomic ratio Co:Fe:Si measured by X-ray fluorescence (XRF) is 0.31:0.04:1. The amount of metal added is similar to that observed in the sample without iron, Co:Si = 0.37:1 [
39]. Taking into account the stoichiometry of the talc structure and the chemical analysis, the resulting catalyst is a composite material constituted of layered metal silicate hydroxide particles dispersed in an aerogel matrix. The amount of catalyst particles with respect to the aerogel is about 20% w/w.
Figure 1.
X-ray diffraction (XRD) profiles of catalysts aer-CoFeSi (a) and aer-CoSi (b).
Figure 1.
X-ray diffraction (XRD) profiles of catalysts aer-CoFeSi (a) and aer-CoSi (b).
The characterization of the catalyst coatings onto the honeycomb structures was carried out by electron microscopy and X-ray photoelectron spectroscopy.
Figure 2 shows a picture of the cordierite honeycomb before and after deposition of the
aer-CoFeSi catalyst.
Figure 3 shows scanning electron microscopy (SEM) images recorded over individual channels of
aer-CoFeSi and
aer-CoSi catalytic honeycombs as well as of a cordierite channel before deposition. An excellent deposition of the catalysts is achieved, covering all the surface of the channels, including the interior of the wall pores. At high magnification, the particles of the talc structure are clearly observed which, according to the XRD results, exhibit a well-defined layered morphology. For both samples the morphology of the talc layers is similar and measures about 1.5 μm × 300 nm × 50 nm. The catalytic particles are strongly anchored over the aerogel host. No weight loss is observed when the catalytic honeycombs are exposed to mechanical vibration stress tests up to 15 G and 100 Hz for 30 min (NTP conditions).
Figure 4 shows a SEM image corresponding to the
aer-CoFeSi catalytic honeycomb recorded after a FIB cut perpendicular to the channel direction. In this way the thickness of the catalyst layer can be directly measured, which is about 8 μm. Interestingly, several pores of the honeycomb structure are also visible and are coated with catalyst as well. The layered structure of the cobalt talc structure doped with iron is clearly visible within the catalytic layer along with the classical cluster-of-grape morphology of SiO
2 aerogel. The X-ray maps corresponding to the area enclosed within the square for Co, Fe, Si and Al (
Kα signals) are depicted also in
Figure 4. Clearly, Co and Fe signals are located within the catalyst layer, both at the channel’s surface and at the interior of honeycomb pores, where the Co signal is more intense, as expected from the catalyst formulation. Contrarily, the Al signal is restricted to the cordierite substrate and the Si signal exhibits contribution both from the cordierite support (more intense) and the catalyst. The Fe/Co atomic ratio of the catalyst layer obtained by energy dispersive X-ray analysis (EDX) is about 0.12, very close to the bulk value determined by XRF.
Figure 2.
Images of the honeycomb structures (400 cpsi) used in this work (a) and loaded with the aer-CoFeSi catalyst (b).
Figure 2.
Images of the honeycomb structures (400 cpsi) used in this work (a) and loaded with the aer-CoFeSi catalyst (b).
Figure 3.
SEM images taken at several magnifications of the interior of a monolith channel (a,b) and loaded with aer-CoFeSi (c,d) and aer-CoSi (e,f) catalysts.
Figure 3.
SEM images taken at several magnifications of the interior of a monolith channel (a,b) and loaded with aer-CoFeSi (c,d) and aer-CoSi (e,f) catalysts.
Figure 4.
SEM image corresponding to the interior of a monolith channel loaded with aer-CoFeSi catalysts after a perpendicular focus ion beam (FIB) cut and X-ray maps recorded over the same area for Co, Fe, Si and Al (Kα lines).
Figure 4.
SEM image corresponding to the interior of a monolith channel loaded with aer-CoFeSi catalysts after a perpendicular focus ion beam (FIB) cut and X-ray maps recorded over the same area for Co, Fe, Si and Al (Kα lines).
Several channels were examined by X-ray photoelectron spectroscopy (XPS) to study the chemical homogeneity of the catalytic surface.
Figure 5 shows a representative relative atomic concentration line scan along one channel of the
aer-CoFeSi honeycomb. XPS line scan analyses reveal that (Co,Fe)
3[Si
2O
5]
2(OH)
2 is distributed along all the channel, as expected. However, the distribution is not completely homogeneous, as a higher amount of Co and Fe is found at the edges, while a higher content of Si and Al from silica aerogel and cordierite (Mg
2Al
4Si
5O
18) is observed at the center of the channels. This phenomenon could be generated during the free impregnation of the silica alcogel with the metal precursor salts and/or during supercritical drying. Interestingly, the atomic ratio Fe/Co is maintained at an approximately constant value. A surface segregation of Fe is observed (Fe/Co~0.6–0.7) with respect to the bulk value of Fe/Co = 0.13. In all cases, the Co 2p
3/2 and Fe 2p
3/2 binding energy values recorded (780.6 and 711 eV, respectively) correspond to both oxidized Co and Fe surface species.
Figure 5.
Atomic concentration line scan along one channel of an aer-CoFeSi honeycomb determined by XPS.
Figure 5.
Atomic concentration line scan along one channel of an aer-CoFeSi honeycomb determined by XPS.
2.2. Catalytic Behavior
Ethanol conversion and selectivity values obtained over
aer-CoFeSi and
aer-CoSi catalytic honeycombs under low load of ethanol (diluted conditions) are reported in
Table 1 at different temperatures. The
aer-CoSi catalyst is more active for the ethanol steam reforming reaction than the
aer-CoFeSi catalyst. Accordingly, the amount of acetaldehyde, which is an intermediate of the reaction (Equation 5), is higher in the case of the
aer-CoFeSi sample compared to
aer-CoSi. Also, the amount of dimethyl ketone is higher for
aer-CoFeSi because it is formed by condensation of acetaldehyde [
12]. For both catalytic honeycombs, as the temperature increases so does the amount of the reforming products, H
2 and CO
2, according to the endothermic character of the reaction. At 673 K ethanol conversion is total and there are no traces of acetaldehyde or dimethyl ketone among the reaction products for both catalysts. Under these conditions, the amount of hydrogen obtained is higher with the
aer-CoFeSi sample, 72.8%
vs. 70.7% for
aer-CoSi, with the theoretical maximum value at 75% (Equation 1). This is due to a better WGS activity (Equation 3) and, more interestingly, to a lower selectivity towards CH
4. The promoting effect of Fe for the WGS reaction during the ethanol steam reforming has been shown to occur over Co/ZnO and Rh/Ca-Al
2O
3 catalysts doped with Fe [
37,
46]. Methane can only be reformed at high temperature, so for an ethanol steam reforming process operating at moderate temperature it is important to avoid it, since methane formation sharply decreases the hydrogen yield. It can be concluded that doping cobalt talc with Fe results in a certain loss of activity for ESR but, conversely, to an important enhancement of hydrogen selectivity due to methane suppression. No signs of deactivation were observed after 80 h on stream for both samples.
Table 1.
Ethanol conversion and product distribution (dry basis) over aer-CoFeSi and aer-CoSi catalytic honeycombs under ethanol steam reforming (ESR) conditions at different temperature. S/C = 3, W/F = 104 g·min·molEtOH−1, VHSV = 680 h−1.
Table 1.
Ethanol conversion and product distribution (dry basis) over aer-CoFeSi and aer-CoSi catalytic honeycombs under ethanol steam reforming (ESR) conditions at different temperature. S/C = 3, W/F = 104 g·min·molEtOH−1, VHSV = 680 h−1.
Catalyst | T/K | EtOH conv./% | Selectivity/% |
---|
H2 | CO2 | CO | CH4 | CH3CHO | (CH3)2CO |
---|
aer-CoFeSi | 573 | 86 | 49.7 | 3.1 | 3.4 | 3.2 | 40.6 | - |
| 598 | 93 | 57.8 | 6.5 | 2.0 | 3.9 | 28.6 | 1.2 |
| 623 | 99 | 69.8 | 14.5 | 1.1 | 5.8 | 7.1 | 1.7 |
| 673 | 100 | 72.8 | 23.0 | 0.9 | 3.3 | - | - |
aer-CoSi | 573 | 92 | 66.7 | 16.6 | 5.2 | 7.0 | 4.1 | 0.4 |
| 598 | 99 | 68.7 | 23.0 | 0.9 | 7.1 | 0.2 | 0.1 |
| 623 | 100 | 68.7 | 23.2 | 1.0 | 7.1 | - | - |
| 673 | 100 | 70.7 | 22.7 | 1.9 | 4.7 | - | - |
The catalytic performance of the
aer-CoFeSi catalytic honeycomb was studied in detail under high load of ethanol (undiluted conditions) at several temperature and pressure conditions. Operation under moderate pressure is advantageous for practical application since it allows for compact fuel processors [
1]. However, from the thermodynamics point of view, an increase of reactor pressure is always unfavorable for steam reforming reactions [
42]. Therefore, the study of the catalytic performance under various pressure values is important.
Figure 6a shows the yields for the different products attained at 673 K over the
aer-CoFeSi catalytic honeycomb by varying the pressure between 1 and 7 bar using a pure ethanol-water mixture with no diluents (S/C = 3) taking into account both the ethanol conversion and product selectivity. As the pressure is increased above 4 bar, the ethanol conversion decreases progressively, as expected, from about 85–87% down to 74%. Interestingly, the amount of acetaldehyde decreases when the pressure is increased (from 4.4% of selectivity on a dry basis at 1 bar down to 1.0% at 7 bar), suggesting that pressure affects less negatively the reforming of acetaldehyde (Equation 6) with respect to the dehydrogenation of ethanol into acetaldehyde (Equation 5), which is the first step of the reforming process over Co-based catalysts. Dimethyl ketone is kept constant at a selectivity value of ca. 0.3% for all pressure values. The decrease of ethanol transformation is accompanied by a significant decrease of hydrogen selectivity, whereas the amount of methane among the reaction products increases strongly with pressure, from 8% at 1 bar up to 18.4% at 7 bar. Methane formation is directly correlated with pressure, since reaction between carbon oxides and hydrogen to yield methane is progressively favored as pressure increases [
42]. The effect of pressure is also observed for the WGS equilibrium; the higher the pressure the more carbon dioxide is obtained at the expense of CO. All these processes results in a different net amount of hydrogen generated; up to 3 bar the amount of hydrogen generated is kept approximately constant at about 1.6 NL
H2·s
−1, whereas at pressures higher than 3 bar the production of hydrogen decreases progressively down to 1.0 NL
H2·s
−1 at 7 bar.
Figure 6b shows the product yield obtained at 7 bar by varying the temperature from 598 to 673 K. The reaction temperature has a strong effect on ethanol conversion at high pressure and, consequently, on hydrogen yield. At 7 bar, ethanol conversion drops from 74% at 673 K to 20% at 598 K, whereas the amount of acetaldehyde increases. The maximum hydrogen yield is obtained at 648–673 K. At 648 K the ethanol conversion is lower than at 673 K (66
vs. 74%), but the selectivity is better because an increase of reaction temperature results in a higher selectivity towards methane.
Figure 6.
Ethanol conversion and product distribution (dry basis) over
aer-CoFeSi catalytic honeycomb under ethanol steam reforming (ESR) conditions at 673 K and different pressure (
a); and at 7 bar and different temperature (
b). S/C = 3, W/F = 390 g·min·mol
EtOH−1, volume hourly space velocity (VHSV) = 1800 h
−1.

H
2,

CO
2,

CO,

CH
4,

CH
3CHO.
Figure 6.
Ethanol conversion and product distribution (dry basis) over
aer-CoFeSi catalytic honeycomb under ethanol steam reforming (ESR) conditions at 673 K and different pressure (
a); and at 7 bar and different temperature (
b). S/C = 3, W/F = 390 g·min·mol
EtOH−1, volume hourly space velocity (VHSV) = 1800 h
−1.

H
2,

CO
2,

CO,

CH
4,

CH
3CHO.