Stability of a NiAl₂O₄ Derived Catalyst in the Ethanol Steam Reforming in Reaction-Regeneration Cycles: Effect of Reduction Temperature

Abstract

The catalyst regeneration is still a challenge to make the ethanol steam reforming (ESR) process feasible for sustainable H₂ production. NiAl₂O₄ spinel derived catalysts are highly active and selective for ESR, but they require avoiding irreversible deactivation to ensure their regeneration. Their stability depends on the catalyst structure, and herein we report different Ni/Al₂O₃-NiAl₂O₄ catalysts obtained upon reduction of a NiAl₂O₄ spinel at 700, 750, or 850 °C. The catalysts were tested in ESR reaction-regeneration cycles, with reaction at 600 °C and regeneration by coke combustion at 850 °C followed by reduction at the corresponding temperature. The fresh, spent, and regenerated catalysts were characterized using X-ray diffraction, N₂ physisorption, temperature programmed reduction and oxidation, and scanning electron microscopy. The irreversible deactivation is due to Ni volatilization and catalyst particle fragmentation. These phenomena are prompted by a high filamentous carbon deposition favored by the Al₂O₃ content in the catalyst. The reduction in the 700–750 °C range is optimum for controlling the Al₂O₃ content, increasing the NiAl₂O₄/Al₂O₃ ratio in the resulting catalyst. These catalysts show a period of partial reversible deactivation by coke with a change in the H₂ formation mechanism reaching a pseudo-stable state with a H₂ yield of 40% and a reproducible performance in successive reaction-regeneration cycles.

1. Introduction

Hydrogen is a promising energy vector [1] and raw material for the synthesis of commodities [2], complying with sustainability precepts when it is obtained from renewable raw materials. H₂ production from ethanol via steam reforming (ESR) is a suitable alternative since ethanol can be obtained sustainably from biomass (bio-ethanol) by fermentation/hydrolysis with no need to separate the water and is easy to handle with relative low risks in comparison with other feedstock for the H₂ production [3,4]. Moreover, the ESR fundamentals are similar to those of CH₄ steam reforming (MSR) which is the basis of the largest technology for H₂ production from natural gas extended worldwide [5], representing an advantage for making an easier adaptation and transition to sustainable H₂ production from ethanol.

The ESR process involves several reactions in a complex series-parallel scheme, in which ethanol reforming (Equation (1)) and water gas shift (Equation (2)) are the main reactions giving the global steam reforming reaction represented by Equation (3) [3,6,7,8]. However, ethanol may undergo dehydrogenation (Equation (4)), dehydration (Equation (5)), and decomposition (Equation (6)) yielding up more H₂ or intermediates (ethylene and acetaldehyde). Acetaldehyde and ethylene may undergo steam reforming (Equations (7) and (8), respectively) and decomposition (Equations (9) and (10), respectively). CH₄ may be formed from some decomposition reactions (Equations (6) and (9)) or methanation (Equation (11)), and simultaneously may undergo steam reforming (reverse of Equation (11)) and decomposition (Equation (12)). CO may undergo disproportionation (Boudouard reaction, Equation (13)). The carbon formed from decomposition (Equations (10) and (12)) or CO disproportionation may undergo gasification (Equation (14)).

C₂H₆O + H₂O → 2CO + 4H₂

(1)

CO + H₂O ⇌ CO₂ + H₂

(2)

C₂H₆O + 3H₂O → 2CO₂ + 6H₂

(3)

C₂H₆O → C₂H₄O + H₂

(4)

C₂H₆O ⇌ C₂H₄ + H₂O

(5)

C₂H₆O → CH₄ + CO + H₂

(6)

C₂H₄O + H₂O → 2CO + 3H₂

(7)

C₂H₄ + 2H₂O → 2CO + 3H₂

(8)

C₂H₄O → CH₄ + CO

(9)

C₂H₄ → 2C + H₂

(10)

CO + 3H₂ ⇌ CH₄ + H₂O

(11)

CH₄ → C + 2H₂

(12)

2CO ⇌ C + CO₂

(13)

C + H₂O → CO + H₂

(14)

The most suitable catalysts in terms of activity and costs for the ESR process are based on Co or Ni supported on resilient oxides, being Ni/Al₂O₃ catalysts the widest studied [7]. Ni is highly active for breaking C-C bonds and absorbing and dissociating water [9,10]. On the other hand, Al₂O₃, often combined with La₂O₃ or CeO₂, is an appropriate support for a high Ni dispersion, which improves the catalytic performance and provides good mechanical strength to the catalyst particles for operation in most types of reactors. However, Ni catalysts are prone to suffer deactivation due mainly to carbon (coke) formation and deposition that blocks the access to active sites, and sintering of active sites that decreases the surface area or amount of active sites [11,12]. The deactivation by carbon deposition might be reversible since carbon deposits are eliminated by combustion and the catalyst might recover its activity. This catalyst regeneration capacity is a key factor to make the ESR processes feasible for large-scale operations.

The carbon elimination by combustion is a critical step for a successful catalyst regeneration since the uncontrolled combustion may sinter Ni sites due to the large amount of heat released [13], causing the irreversible deactivation of the catalyst. Thus, many catalyst formulations have been proposed to the end of decreasing the carbon formation, increasing the catalyst lifetime and making possible its regeneration. Accordingly, Montero et al. [14] demonstrated the reproducible performance of a Ni/αAl₂O₃-La₂O₃ catalyst in ESR reaction-regeneration cycles (reaction at 500 and 700 °C) upon an equilibration treatment consisting of a reaction-regeneration cycle with a reaction temperature of 700 °C. Campos et al. [15] showed that a 1.0%Rh10%Ni/15%La₂O₃10%CeO₂ alumina supported catalyst can increase the H₂ selectivity with a stable operation and regeneration capacity for two ESR reaction-regeneration cycles at 500 °C. Contreras et al. [16] demonstrated that various Ni, Co and Ni-Co catalysts supported on hydrotalcite (Mg and Al oxides) stabilized with W oxides have a good regeneration capacity in various ESR reaction-regeneration cycles at 600 °C. Boudadi et al. [17] found a good activity recovery of various Ni catalysts with modified supports, including Ni/La-Al₂O₃, Ni/La/TiO₂-Al₂O₃, and Ni/La-clay, in ESR reaction-regeneration cycles at 500 °C. Di Michele et al. [18] proved Ni/MgAl₂O₄ is a stable catalyst for the ESR reaction with low carbon formation leading to an easy regeneration.

A remarkable and reproducible Ni catalyst easily prepared from the reduction of NiAl₂O₄ spinel is quite active for the steam reforming of bio-oil [19,20] and ethanol [21]. Accordingly, the NiAl₂O₄ spinel precursor is obtained by co-precipitation of Ni and Al sources followed by calcination, and then the synthesized spinel is completely reduced in H₂ to obtain a Ni/Al₂O₃ catalyst with high Ni dispersion. This catalyst has been proven to have a moderate carbon deposition in the bio-oil steam reforming (BSR) and recovers its activity when it is used in reaction-regeneration cycles [22]. The regeneration of this catalyst consists of two steps: (i) coke elimination by combustion at 850 °C in air, which also reconstructs the NiAl₂O₄ spinel structure; and (ii) reduction of the spinel at 850 °C in H₂ resulting in the highly dispersed Ni/Al₂O₃ catalyst. This performance in the stability and regeneration capacity makes it a promising alternative over others for the steam reforming of oxygenates, since the formulation is simple and reproducible. However, the use of this catalyst in the ESR leads to a high carbon formation due to the presence of acidic Al₂O₃ that favors the ethanol dehydration reaction yielding ethylene (Equation (5)), and favoring the subsequent reactions of oligomerization, aromatization and condensation into carbon structures (coke) (Equation (10)) [21]. This high carbon formation and nature may affect the regeneration capacity of this catalyst due to the possible uncontrollable combustion and other issues that cause an irreversible deactivation.

In this work, we propose to attenuate the deactivation and achieve the regeneration of the catalyst derived from NiAl₂O₄ spinel, by decreasing the Al₂O₃ presence to control the carbon formation. The simple and reproducible strategy to this end is decreasing the reduction temperature of the spinel, which leads to an incomplete reduction of the Ni species resulting in a Ni/Al₂O₃-NiAl₂O₃ catalyst with lower Al₂O₃ content than in the Ni/Al₂O₃ catalyst prepared from the spinel reduction at 850 °C. This strategy has been effective for the catalyst equilibration in the glycerol aqueous-phase reforming [23], and it is studied in this work for the ESR in reaction-regeneration cycles.

2. Results

2.1. Fresh Catalyst Properties

Figure 1a shows the temperature programmed reduction (TPR) profile of the NiAl₂O₄ synthetized. The TPR profile evidences that the Ni reduction takes place between 500 and 900 °C with a maximum at 800 °C, characteristic of the reduction of Ni species in the spinel structure. Likewise, the absence of reduction peaks at lower temperatures indicates that no free nickel oxides are significantly present. This is verified by the XRD pattern (plotted in Figure 1b), evidencing the absence of NiO crystalline phases and the only presence of NiAl₂O₄ spinel. Thus, the reduction of all Ni species results in Ni crystals supported on Al₂O₃, which is possible at reduction temperatures above 850 °C for 4 h [19]. Likewise, the Ni content calculated from the TPR profile is 34.7 wt% in NiAl₂O₄ or 38.7 wt% in Ni/Al₂O₃, being very close to the stoichiometric values of 33.2 and 36.5 wt%, respectively.

The catalysts have been prepared by reducing the NiAl₂O₄ spinel precursor at 700, 750 and 850 °C, and they are named according to their reduction temperature, as R-700, R-750 and R-850, respectively. These catalysts have been characterized by several techniques to determine their more relevant properties. Figure 1b shows the X-ray diffraction (XRD) patterns of the catalysts and the NiAl₂O₄ precursor to determine the crystalline phases present in each material. The NiAl₂O₄ shows diffraction peaks at 2θ = 37.2, 45.3, 59.9 and 65.7° corresponding to a typical spinel cubic structure (PDF 01-071-0965) [21,23]. Upon reduction at 850 °C, the NiAl₂O₄ is almost completely converted into reduced Ni crystals (2θ = 44.6, 51.9 and 76.5°) (PDF 04-010-6148) and Al₂O₃ (2θ = 37.5, 45.9 and 66.9°) (PDF 04-005-4662), being a Ni/Al₂O₃ catalyst (R-850). In contrast, the catalysts obtained upon reduction at 700 °C (R-700) or 750 °C (R-750) show peaks of the NiAl₂O₄, reduced Ni crystals and Al₂O₃, indicating that a partial Ni reduction takes place since the reduction temperature is insufficient for a complete reduction according to the TPR data (Figure 1a). Thus, these catalysts are composed of reduced Ni crystals supported on NiAl₂O₄ and Al₂O₃ (Ni/Al₂O₃-NiAl₂O₄ catalysts). The average Ni crystal size was determined from the XRD data using the Scherrer equation (with diffraction peak at 51.9°), and the results (listed in

Table 1. Main properties of the NiAl₂O₄ spinel and derived catalysts at different reduction temperatures.

Sample	Ni Content (wt%)	Reduced Ni 1 (wt%)	SBET (m2 g−1)	Vpore (cm3 g−1)	Dpore (nm)	Ni Crystal Size (nm)	Acidity (mmol g−1)
NiAl2O4 spinel	34.7 **	-	78.3	0.177	8.50	-	-
R-700 catalyst	38.7 *	11.8	76.7	0.197	10.5	9.10	0.026
R-750 catalyst	38.7 *	17.7	74.0	0.205	10.9	12.0	0.029
R-850 catalyst	38.7 *	38.7	70.3	0.212	12.6	17.0	0.044

S_BET, specific surface area calculated using the Brunauer-Emmett-Teller (BET) theory; V_pore, total volume of pores; D_pore, average pore diameter. * Amount of reduced Ni in the Ni/Al₂O₃ or Ni/Al₂O₃-NiAl₂O₄ catalyst. ** Amount of Ni in the NiAl₂O₄ spinel. ¹ Estimated from the TPR data.

) show that the Ni crystal size increases with increasing reduction temperatures. Likewise, the content of reduced Ni crystals was estimated from the TPR data according to the reduction temperature employed, and the results (listed in Table 1) confirm that this content increases with increasing reduction temperatures, as expected. Therefore, the decrease in the reduction temperature leads to a lower content of reduced Ni crystals but it also shortens the extent of Ni sintering, which results in lower average Ni crystal sizes.

The textural properties listed in Table 1 also evidence the effect of the transition of the NiAl₂O₄ spinel to Ni/Al₂O₃-NiAl₂O₄ catalysts. Accordingly, the specific surface area (S_BET) decreases with increasing reduction temperatures, whereas the total pore volume (V_pore) and average pore diameter (D_pore) increase. The catalyst acidity was determined by means of NH₃ adsorption, and the results are listed in Table 1. As seen, the acidity increases with increasing reduction temperatures from 0.026 mmol g⁻¹ at 700 °C to 0.044 mmol g⁻¹ at 850 °C, which should be related to the increasing Al₂O₃ content, with the presence of acidic γ-Al₂O₃ phases [21].

In summary, the reduction at 850 °C is sufficient to reduce all of the Ni species in the NiAl₂O₄ spinel and obtain a Ni/Al₂O₃ catalyst (R-850 catalyst), whereas the reduction at 700 and 750 °C partially reduces the Ni species in the NiAl₂O₄ spinel leading to obtain Ni/Al₂O₃-NiAl₂O₄ catalysts (R-700 and R-750 catalysts). Therefore, the contents of reduced Ni species (active for the ESR reaction) and acidic Al₂O₃ decrease as the reduction temperature decreases. In consequence, for the ESR reaction-regeneration cyclic tests, the space time is adjusted by using higher catalyst amounts when the reduction temperature is lower, for providing similar amount of reduced Ni species in the catalytic bed with the purpose of obtaining comparable values of ethanol conversion and H₂ yield.

2.2. Catalyst Performance in ESR Reaction-Regeneration Cycles

The performance of the catalysts in reaction-regeneration cycles was studied quantifying the time on stream (TOS) evolution of the ethanol conversion and product yields for the ESR using the R-850 (Figure 2), R-750 (Figure 3) and R-700 (Figure 4) catalysts. The main products observed are H₂, CO, CO₂, CH₄, and C₂H₄ and carbon deposited on the catalyst. The carbon yield was estimated from C balance taking into account the carbonaceous components in the feed and effluent streams.

Comparing the performance of the catalysts in the first reaction, the initial ethanol conversion and H₂, CO, CO₂, CH₄ and C₂H₄ yields are similar. The TOS evolution of the H₂, CO, CO₂ and CH₄ yields is also quite similar for the three catalysts, showing a significant decrease in the first 2 h on stream and more stable behavior afterwards, which indicates a partial catalyst deactivation for the reactions forming these products. The ethanol conversion decreases faster and reaches lower values in the pseudo-stationary state as the reduction temperature decreases (in this catalyst order: R700 > R750 > R850). Above 4 h on stream, the three catalysts reach a pseudo-stable state with constant product yields, in particular that of H₂. The data evidence that the deactivation rate for the H₂ formation reactions is similar for the three catalysts. However, the TOS evolution of the C₂H₄ and carbon yields is different among the three catalysts. For the R-850 catalyst, the C₂H₄ and carbon yields increase with TOS and keep a stable behavior up to 4 h on stream and afterwards the C₂H₄ yield slightly increases coinciding with the decrease in the carbon yield. In contrast, for the R-700 and R-750 catalysts, the C₂H₄ yield increases, reaches a maximum value (much lower than the values observed for the R-850 catalyst) and decreases afterwards, whereas the carbon yield is negligible. The slower decrease and higher values in the ethanol conversion with increasing reduction temperatures is due to the higher presence of acidic Al₂O₃ (Table 1) that catalyzes the ethanol dehydration yielding more C₂H₄ [21].

Upon the first reaction, the corresponding spent catalyst was subjected to the regeneration procedure described in Section 4 (combustion with air at 850 °C for coke removal and reconstruction of the spinel, followed by spinel reduction with H₂-N₂ stream at different temperatures of 700, 750 or 850 °C for the R-700, R-750 and R-850 catalysts, respectively). The whole procedure is repeated for a third reaction and the results are also shown in Figure 2, Figure 3 and Figure 4. For the R-850 catalyst, the TOS evolution of the product yields completely changes from the first to the second and third reactions, with the product distribution of the second and third reactions being almost identical (reproducible behavior upon the regenerations). Accordingly, the initial H₂, CO, CO₂ and CH₄ yields are almost comparable with those of the fresh catalyst, but they rapidly decrease with TOS reaching much lower values in the second and third reactions in comparison with the first reaction. Likewise, the C₂H₄ and carbon yields notably increase in the second and third reactions, making the ethanol conversion also increase in comparison with the first use of the catalyst.

In contrast, for the R-700 and R-750 catalysts, the initial values of H₂, CO, CO₂, and CH₄ yields and their TOS evolutions in the second and third reactions are quite comparable with those of the first reaction for each catalyst. However, the TOS evolution of the C₂H₄ yield notably differs, with an increasing trend in the values and apparent prolonged period of the maximum value in the successive cycles, which makes the ethanol conversion also increase compared with the first reaction for each catalyst.

It should be noted that the H₂ yield is almost constant upon 4 h on stream, whereas the yields of other products change as a consequence of a partial catalyst deactivation for the extent of various reactions (Equations (1)–(14)). This observation is consistent with the H₂ formation mechanism on catalysts derived from a NiAl₂O₄ spinel proposed in a previous work [21]. Accordingly, the partial catalyst deactivation sequentially affects the H₂ and carbon formation from ethylene (Equation (10)) that explains the increase in the C₂H₄ yield over TOS, and the ethanol dehydration (Equation (5)) that explains the maximum observed in the C₂H₄ yield in the R-700 and R-750 catalysts. The reforming reactions catalyzed by Ni sites keep constant the H₂ yield, and these sites keep their accessibility for these reactions since they also catalyze the carbon gasification [24].

2.3. Catalyst Characterization after Reaction-Regeneration Cycles

The catalysts were characterized after the third reaction using several techniques with the purpose of evaluating the carbon deposition and the recovery of the catalyst properties.

2.3.1. Carbon Deposition

The content and combustion characteristics of deposited carbon were determined by subjecting samples of the spent catalysts to temperature programmed oxidation (TPO). Figure 5 shows the TPO profiles of the three spent catalysts after the third reaction, in which a unique combustion peak is observed for each catalyst. The deposited carbon starts to burn above 400 °C and most of the carbon species burn at 525 °C for the R-850 catalyst and at 545 °C for the R-700 and R-750 catalysts. Thus, the carbon content (estimated from the area of each TPO profile) is notoriously higher for the R-850 catalyst (2.37 g (g catalyst)⁻¹) than for the R-700 (0.12 g (g catalyst)⁻¹) or R-750 (0.15 g (g catalyst)⁻¹) catalysts. The values of the average carbon formation rate (r_C) [25,26], (also indicated in Figure 5) are significantly lower for R-700 and R-750 catalysts than for R-850 catalyst. Likewise, the high combustion temperature at the maximum combustion rate (maximum peak position in the TPO profile) indicates that the carbon deposited on the three catalysts is highly structured and condensed with a low H/C ratio, expectedly composed of carbon filaments [21,27]. The shift to higher combustion temperatures for the carbon formed on the R-700 and R-750 catalysts indicates that the carbon structures are more refractory than those formed on the R-850 catalyst [28].

To investigate the extent of carbon deposition and the carbon morphology, samples of the spent catalyst were analyzed by means of scanning electron microscopy (SEM) using a backscattered electron (BSE) or secondary electron (SE) detectors, with the equipment described in Section 4. Figure 6 shows the BSE-SEM images of the spent catalysts after the third reaction, which provide insights into the extent of carbon deposition on the catalyst particles according to the brightness intensity levels. All of the spent catalyst particles show a strong carbon deposition on the external surface based on the almost homogeneous dark color of all samples. Remarkably, the size and textural appearance of some R-850 catalyst particles (Figure 6a) notably changed in comparison with those of the R-700 and R-750 catalysts (Figure 6c,d). Some R-850 catalyst particles are larger than the fresh catalyst (0.15–0.25 mm) and with a highly rough and meso- and macro-porous texture characteristic of carbon structures. Likewise, some bright small fragments can be seen on the external surface (magnified in Figure 6b), which is indicative of the presence of catalyst fragments (Ni and Al would give this brightness intensity) over the carbon deposited. The particle sizes of the spent R-750 (Figure 6c) and R-700 (Figure 6d) catalysts did not significantly change by the carbon deposition.

Figure 7 shows the SE-SEM images providing more details of the carbon morphology by magnifying some areas of representative particles of each sample. The photos evidence that the formed and deposited carbon is mostly composed of carbon filaments as typically expected for the ESR [21,27,28,29], being particularly abundant in all of the spent R-850 catalyst particles (Figure 7a,b). An apparent catalyst fragment is captured in Figure 7b, suggesting these fragments are on the tip of carbon filaments. On the other hand, in the particles of the spent R-700 and R-750 catalysts, two different surface morphologies can be distinguished, one similar to that of the R-850 catalyst with abundant carbon filaments (Figure 7c,e) and another with an incipient presence of carbon filaments and apparent amorphous mass of carbon (Figure 7d,f).

2.3.2. Regenerated Catalyst Properties

To verify the spinel reconstruction ability from the spent catalysts by combustion/calcination at 850 °C, samples of the reconstructed NiAl₂O₄ spinel from the used R-700, R-750 and R-850 catalysts were characterized by using TPR and XRD (Figure 8). The TPR profiles (Figure 8a) are compared with that of the NiAl₂O₄ precursor. In the reconstructed NiAl₂O₄ spinels, particularly from the used R-850 catalyst, the Ni reduction takes place in two temperature ranges, 400–600 and 600–950 °C, indicating the presence of two Ni species. The XRD patterns (Figure 8b) verify the presence of two crystalline phases, NiAl₂O₄ spinel (PDF 01-071-0965) and NiO (PDF 01-080-5508), being the spinel structure predominant in all of the samples and the presence of NiO more notorious in the spinel reconstructed from the spent R-850 catalyst. Thus, the Ni reduction at 400–600 °C may be associated to the reduction of NiO, whereas the Ni reduction in the NiAl₂O₄ spinel takes place at 600–950 °C with maxima at around 810–820 °C. Interestingly, the reduction peak and XRD peaks associated to the NiAl₂O₄ spinel tend to be narrower for the all of the reconstructed NiAl₂O₄ spinels than for the spinel precursor, which is indicative of more homogeneous NiAl₂O₄ structures. Likewise, the TPR profiles evidences that the total amount of reduced Ni (listed in

Table 2. Main properties of the regenerated catalysts.

Regenerated Catalysts	Total Ni 1 (wt%)	Reduced Ni 1,2 (wt%)	SBET (m2 g−1)	Vpore (cm3 g−1)	Dpore (nm)	Ni crystal Size (nm)
R-700	37.2	8.30	49.0	0.232	19.0	7.10
R-750	35.0	12.5	46.4	0.178	15.3	8.70
R-850	33.1	30.1	42.7	0.162	15.1	10.9

¹ Amount of Ni in the Ni/Al₂O₃ or Ni/Al₂O₃-NiAl₂O₄ catalyst. ² Estimated from the TPR data.

) is lower for the reconstructed NiAl₂O₄ spinels than for the spinel precursor. Therefore, the decrease in the Ni content with respect to the fresh catalyst indicates a partial Ni loss after the ESR reaction cycles. These losses are 3.90, 9.50, and 14.5% for the R-700, R-750, and R-850 catalysts, respectively. Presumably, the Ni loss is due to the formation and volatilization of Ni(CO)₄ during the combustion and spinel reconstruction at 850 °C. This carbonyl species formation is favored at this high combustion temperature and mainly affects the Ni crystals separated from the Al₂O₃ support that are on the tips or dispersed over the carbon filaments [30].

A sample of each reconstructed NiAl₂O₄ spinel was reduced at the corresponding temperature to obtain each regenerated catalyst. Figure 9 shows the XRD patterns of the regenerated catalysts after the third ESR reaction, which should be compared with those of Figure 1b (fresh catalysts). As seen, the R-700 and R-750 regenerated catalysts have phases of Al₂O₃ (PDF 04-005-4662), NiAl₂O₄ (PDF 01-071-0965), and reduced Ni crystals (PDF 04-010-6148), similar to the corresponding fresh catalysts (Figure 1b). The regenerated R-850 catalyst has Al₂O₃ and reduced Ni phases as in the corresponding fresh catalyst (in Figure 1b). Likewise, the peaks associated with NiAl₂O₄ in the XRD patterns of all of the regenerated catalysts (Figure 9) are more intense than those of the corresponding fresh catalysts (Figure 1b), suggesting that the reconstructed NiAl₂O₄ is more resistant to be reduced. This fact is also verified with the TPR profiles of the reconstructed NiAl₂O₄ spinels (Figure 8a), which shows that the maximum of the main reduction peak is shifted to higher temperatures in comparison with that of the NiAl₂O₄ spinel used as original precursor. Thus, it is reasonably expected that the amounts of reduced Ni in the regenerated catalysts (estimated from the TPR data and listed in Table 2) are slightly lower than those in the fresh catalysts (Table 1).

The textural properties (mainly S_BET and V_pore) and the reduced Ni average crystal size are slightly lower for the regenerated catalysts with respect to the corresponding fresh catalysts. However, the slight change in these properties does not seem to affect the activity of the R-700 and R-750 catalysts upon regeneration and neither explains the low regeneration capacity of the R-850 catalyst.

To further investigate the state of the catalyst particles upon regeneration, we analyzed a sample of each regenerated catalyst using SE-SEM (Figure 10). The SE-SEM images reveal that the regenerated R-700 and R-750 catalysts (Figure 10a,b) keep the initial average catalyst particle size (between 0.15 and 0.25 mm), whereas the particles of the regenerated R-850 catalyst (Figure 10c) slightly decreased in size and some of them are powdered, which is clearly seen in the zoom of Figure 10d. This may be due to the catalyst fragmentation observed in this catalyst covered with abundant carbon species after the ESR reaction-regeneration cycles (Figure 6b and Figure 7b).

3. Discussion

The preparation of Ni catalysts from NiAl₂O₄ spinel using different reduction temperatures reports different performances in ESR reaction-regeneration cycles. At comparable ethanol conversion and H₂ yield levels, the catalysts obtained by reducing at 700 and 750 °C (R-700 and R-750 catalysts, respectively) show a remarkable regeneration capacity as the performance in the second and third reactions is almost identical to that of the first reaction. Conversely, the R-850 catalyst obtained by reducing at 850 °C does not show this stability since the kinetic performance in the second and third reactions greatly differs from the first one, with a lower H₂ yield. However, the R-850 catalyst is equilibrated upon the first reaction-regeneration cycle making its performance in the second and third reactions be outstandingly reproducible among them.

To investigate this behavior, we characterized the catalysts after the ESR reaction-regeneration cycles. The TPO results indicate that the R-850 catalyst produced much more carbon than the R-700 and R-750 catalysts, and the SE-SEM analysis shows that this carbon mainly grows in the form of filaments creating some fragmentation of the R-850 catalyst particles. This observation is reasonably explained by the lower acidity of the R-700 and R-750 catalysts, since it is well stablished that acidic supports promote the carbon formation and development of filamentous carbon structures in the ESR reaction [3].

Additionally, the reconstruction of the NiAl₂O₄ spinel from the used R-850 catalyst is notoriously incomplete resulting in a mixture of NiO and NiAl₂O₄ phases according to the XRD analysis, with a Ni content about 14.5% lower than in the synthesized spinel. The Ni loss also occurs in the reconstruction of the NiAl₂O₄ spinel from the R-700 and R-750 catalysts but to a lesser extent. This loss may be attributable to the formation of Ni(CO)₄ that is volatized during the carbon combustion step [30], which may be expectedly favored at higher carbon contents. This is favored since some Ni crystals are separated from the Al₂O₃ support and swept along by the carbon filaments on the external surface of the catalyst particles, facilitating its volatilization by the Ni(CO)₄ formation. However, the R-850 catalyst regeneration upon reduction of the reconstructed spinel results in a Ni/Al₂O₃ catalyst with little presence of NiAl₂O₄ (absence in the fresh catalyst), which is similar to the fresh R-850 catalyst in terms of the crystallographic analysis. Likewise, the textural properties and average size of reduced Ni crystals slightly decrease for the regenerated catalysts in comparison with the values for the corresponding fresh catalysts. However, none of these changes in the catalyst properties explains the low regeneration capacity of the R-850 catalyst, since the properties of the R-700 and R-750 catalyst are also affected to a similar extent and they showed an outstanding regeneration capacity.

Considering the aforementioned results, the explanation for the unsuccessful reactivation of the R-850 catalyst is the abundant filamentous carbon formation favored by the Al₂O₃ acidity. The growth of carbon filaments fragments the catalyst particles and favors the Ni loss. This fragmentation was evidenced by SE-SEM analysis of the regenerated R-850 catalyst particles, showing an overall reduction in the particle sizes and even the presence of catalyst powder. Therefore, the size of the regenerated catalyst particles ranges from powder (below 0.1 μm) to 0.25 mm. The collapse of the catalyst particles redistributes the Ni sites in the catalyst surface, causing a large amount of Ni sites to be exposed on the external surface of the multiple sized particles. These sites prone the formation and growth of carbon unconstrainedly, even trapping and possibly encapsulating the powder particles in the second reaction (after the first use of the catalyst). In fact, this is verified by the TOS evolution of carbon yield (Figure 2) in the second and third reaction, which evidences a higher and sustained carbon formation. Consequently, the amount of Ni sites available for the reaction is significantly lower since carbon formation reactions are favored, and therefore making impossible the complete recovery of the catalytic activity for steam reforming reactions.

The low presence of Al₂O₃ in the R-750 and R-700 catalyst explains the lower carbon formation and development of filaments on these catalysts, whose regeneration is feasible without a significant fragmentation of the catalyst particles. The lower coke deposition favors to reach a stable state in the activity of these catalysts since the carbon formation is partially suppressed and equilibrated with its gasification [20,21], avoiding the uncontrolled development of long carbon filaments.

These findings reveal that the amount of carbon formed and deposited on the catalyst determines the regeneration capacity of the Ni catalyst derived from NiAl₂O₄ spinel. In this work, the carbon formation was controlled by decreasing the catalyst acidity achieved by changing the reduction temperature at which the catalyst is obtained. In this way, the resulting catalyst consists of reduced Ni supported on Al₂O₃-NiAl₂O₄ at reduction temperatures of 700 or 750 °C, instead of reduced Ni on a pure Al₂O₃ support when using a reduction temperature of 850 °C, which would have more acidity. Thus, further strategies targeting at minimizing the carbon formation may be effective to improve the regeneration capacity of the catalyst for the ESR reaction-regeneration cyclic operation.

4. Materials and Methods

The catalysts were prepared by reduction of a NiAl₂O₄ spinel precursor with H₂ at different temperatures. The NiAl₂O₄ spinel was synthesized by the co-precipitation of Ni(NO₃)₂ and Al(NO₃)₃ while dosing a NH₄OH solution dropwise to increase the pH of the medium up to 8, following the procedure described in previous works [19,20,21]. The resulting precipitate is calcined at 850 °C for 4 h in a static air atmosphere to obtain the NiAl₂O₄ spinel and then crushed and sieved at 0.15–0.25 mm. The catalyst is obtained by reduction of the NiAl₂O₄ spinel at 700, 750, and 850 °C for 1 h in H₂-N₂ flow (10 mol% H₂) with a heating rate of 10 °C min⁻¹ in the reaction system. The resulting catalysts are named according to the reduction temperatures as R-700, R-750, and R-850.

The NiAl₂O₄ spinel and catalysts were characterized using X-ray diffraction (XRD), temperature programed reduction (TPR), N₂ physisorption and NH₃ adsorption, whose experimental procedures are described elsewhere [20,21]. The structural phases detected by XRD were identified using the database of the International Center for Diffraction Data by matching with the appropriate Powder Diffraction File version 4 (PDF-4). Additionally, the spent catalysts were characterized using temperature programed oxidation (TPO) as described elsewhere [21], and scanning electron microscopy (SEM) using a secondary electron (SE) detector in a Hitachi S-4800 N field emission gun scanning electron microscope (FEGSEM), and using a backscattered electron detector in a Hitachi S-3400 N microscope.

The reduction treatment and ESR reaction-regeneration cyclic tests were carried out in the reaction system (Microactivity reference-PID Eng and Tech) described in a previous work [21]. Briefly, the setup is provided with an isothermal fluidized bed reactor (22 mm internal diameter and total length of 460 mm) inside a furnace, and this arrangement (reactor and furnace) is inside a hotbox kept at 150 °C. The catalytic bed in the reactor consists of a mixture of an inert material (SiC from VWR Chemicals sieved at 105 μm) and the needed catalyst amount, keeping an initial height/diameter ratio above 2 for all of the experiments. The feed consists of a mixture of various gas streams (N₂, H₂ or air), each one controlled with mass flow meters, and a liquid stream (ethanol-water mixture) provided with a piston pump (Gilson 307). The mixing of the different feed components (gas or liquid streams) takes place in the hotbox kept at 150 °C to allow the evaporation of liquid components and preheating of the feed. The outlet stream from the reactor is sampled by taking a small amount through a capillary and the rest of the flow goes to a separator with a Peltier cooler where the vapor components are condensed and collected and the gas components are safely vented. The sample is mixed and carried with He to an Agilent 3000 micro gas chromatograph (micro-GC) through a thermally insulated line, for the component analysis. The micro-GC has four column modules for the detection and quantification of the reaction components: (1) molecular sieve capillary column for separating O₂, N₂, H₂, CO, and CH₄; (2) PLOT Q capillary column for separating light oxygenates and hydrocarbons (C₁-C₃), CO₂, and water; (3) alumina capillary column for separating C₂-C₄ hydrocarbons; (4) Stabilwax type column for separating oxygenates (C₂₊) and water.

The following procedure was applied for the ESR reaction-regeneration cyclic tests upon loading the desired amount of NiAl₂O₄ spinel in the reactor:

(1)

Reduction treatment by flowing a gas mixture of 10 mol% of H₂ in N₂ at 120 cm³ min⁻¹ measured at standard temperature and pressure (STP) while heating at 10 °C min⁻¹ up to the desired reduction temperature (700, 750, and 850 °C) and keeping isothermally for 1 h. Once the catalyst is reduced, the flow is switched to pure N₂ at the desired flowrate for the ESR reaction and the reactor is cooled down to 600 °C.

(2)

ESR reaction at 600 °C for 6 h on stream by feeding to the reactor a mixture of ethanol and water (steam/ethanol ratio of 3) pumped at 2 (g ethanol) h⁻¹ and diluted in N₂, so that ethanol concentration is 5 mol%. During the reaction run, the effluent stream is continuously sampled to analyze the composition. The ethanol conversion is calculated as:

$$X=\frac{F_{E0}-F_E}{F_{E0}}$$

(15)

where F_E0 is the ethanol flowrate in the feed and F_E is the ethanol flowrate in the effluent stream. Likewise, the product yields are calculated as:

$$Y_i=\frac{F_i}{{\mathsf{\nu }}_i{F}_{E0}}$$

(16)

where F_i is the molar flowrate of the product i in the effluent stream and ν_i is the stoichiometry coefficient (6 for H₂ according to Equation (3), 2 for CO, CO₂ and CH₄, and 1 for C₂H₄ and C₂H₄O). At the end of the reaction run, the ethanol-water feed is cut off and the reactor is cooled down to room temperature in N₂ flow.

(3)

Carbon elimination from the spent catalyst by flowing air to the reactor at 100 cm³ min⁻¹ (STP) at 850 °C for 4 h, which also reconstructs the NiAl₂O₄ spinel.

(4)

Cyclic tests by repeating steps 1 to 3. At the final cycle, the catalyst is removed from the reactor after the ESR reaction, without eliminating the carbon, to analyze the spent catalyst. The carbon content is analyzed using TPO as aforementioned, and the average carbon formation rate (r_C) is calculated as [25,26]:

$$r_C=\frac{m_{carbon}}{m_{catalyst}{m}_{C0}t}$$

(17)

where m_carbon is the mass of carbon formed during the reaction, m_catalyst is the mass of catalyst used in the experiment, m_C0 is the total mass of carbon fed in the reaction run, and t is the time on stream.

5. Conclusions

The reduction temperature used for developing the structure of Ni supported catalysts (Ni/Al₂O₃ or Ni/Al₂O₃-NiAl₂O₄) is a key condition when preparing catalysts from NiAl₂O₄ spinel precursors. At 850 °C, the Ni species are completely reduced and a catalyst consisting of Ni/Al₂O₃ is obtained, whereas at 700 or 750 °C, the Ni species are partially reduced resulting in Ni/Al₂O₃-NiAl₂O₄ catalysts with lower acidity due to the moderate presence of Al₂O₃ in the mixed support. The lower acidity in these catalysts decreases the ethylene formation rate and consequently the carbon formation, making these catalysts have an outstandingly regeneration capacity with a reproducible performance in successive reaction-regeneration cycles. Likewise, these catalysts reach a pseudo-stable state with a H₂ yield of 40% (at 600 °C; steam/ethanol ratio in the feed of 3, ethanol partial pressure in the feed of 0.05 bar and space time of 0.05 h). Contrarily, the catalyst obtained upon reduction at 850 °C shows a poor regeneration capacity. The irreversible deactivation is due to the activity of acid sites in the Al₂O₃ support for the ethylene formation from ethanol and subsequent ethylene conversion into carbon filaments. The excessive growth of these filaments contributes to the fragmentation of the catalyst particles with redistribution of part of the Ni sites to the external surface and the Ni loss by volatilization in the form of Ni carbonyl species.

The strategy of reducing the NiAl₂O₄ spinel at 700–750 °C to achieve the catalyst regeneration is easy and reproducible, keeping the catalyst formulation simple, which is technically and economically feasible for industrial applications. These reduction conditions allow the ESR process to be reproducible in reaction-regeneration cycles, which is a relevant advance for the industrial scale-up of this process.