Nickel Supported on AlCeO₃ as a Highly Selective and Stable Catalyst for Hydrogen Production via the Glycerol Steam Reforming Reaction

Abstract

In this study, a critical comparison between two low metal (Ni) loading catalysts is presented, namely Ni/Al₂O₃ and Ni/AlCeO₃ for the glycerol steam reforming (GSR) reaction. The surface and bulk properties of the catalysts were evaluated using a plethora of techniques, such as N₂ adsorption/desorption, Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP–AES), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy (SEM/EDX, Transmission Electron Microscopy (TEM), CO₂ and NH₃– Temperature Programmed Desorption (TPD), and Temperature Programmed Reduction (H₂–TPR). Carbon deposited on the catalyst’s surfaces was probed using Temperature Programmed Oxidation (TPO), SEM, and TEM. It is demonstrated that Ce-modification of Al₂O₃ induces an increase of the surface basicity and Ni dispersion. These features lead to a higher conversion of glycerol to gaseous products (60% to 80%), particularly H₂ and CO₂, enhancement of WGS reaction, and a higher resistance to coke deposition. Allyl alcohol was found to be the main liquid product for the Ni/AlCeO₃ catalyst, the production of which ceases over 700 °C. It is also highly significant that the Ni/AlCeO₃ catalyst demonstrated stable values for H₂ yield (2.9–2.3) and selectivity (89–81%), in addition to CO₂ (75–67%) and CO (23–29%) selectivity during a (20 h) long time-on-stream study. Following the reaction, SEM/EDX and TEM analysis showed heavy coke deposition over the Ni/Al₂O₃ catalyst, whereas for the Ni/AlCeO₃ catalyst TPO studies showed the formation of more defective coke, the latter being more easily oxidized.

1. Introduction

Biofuels are expected to play an important role in meeting the combined challenge of providing adequate energy supplies, while simultaneously combating the threat posed by climate change, with projections estimating that their use will grow from 1.3 million barrels of oil equivalent (BOE) in 2012 to 4.6 BOE in 2040 [1]. Although biodiesel had captured approximately 25% of total biofuel output in 2015 [2], the high costs associated with its production means that the sector remains uncompetitive, with favourable government policies underpinning much of its growth [3]. The industry is also facing the challenge of dealing, in a sustainable way, with the co-production of crude glycerol (C₃H₈O₃), which constitutes its major by-product and amounts to approximately 10 wt % of the oil undergoing transesterification [4].

However, glycerol can be converted into hydrogen, a fuel that finds a variety of present-day applications in transport, building and other industry sectors, while at the same time is expected to have a leading role in a future carbon-free energy mix [5]. Although different thermochemical processes may be used for the conversion of C₃H₈O₃ into H_2—e.g., aqueous phase reforming (APR), autothermal reforming (ATR) and super critical water reforming (SCWR)—the process that appears most promising is that of the steam reforming of glycerol (GSR) [6,7,8]. This is because GSR has a high H₂ production capacity per mol of C₃H₈O₃ reformed (Equation (1)) and is a mature industrial technology unlikely to require major technical adjustments in switching feedstocks [9,10]. The GSR (Equation (1)), a strongly endothermic reaction (ΔH^Θ = 123 kJ/mole), combines the decomposition of glycerol (Equation (2)) and the water-gas shift reaction (WGS, Equation (3)), but a number of parallel reactions that include methanation (Equation (4)), methane dry reforming (Equation (5)) and carbon formation reactions (Equations (6)–(9)) also take place [11,12]. According to the thermodynamic studies undertaken, the GSR should be undertaken at high temperature (>630 °C), high water to glycerol feed ratio (WGFR < 9:1, molar) and at atmospheric pressure [13,14].

C₃H₈O₃ + 3H₂O → 3CO₂ + 7H₂,

(1)

C₃H₈O₃ → 3CO + 4H₂,

(2)

CO + H₂O ↔ CO₂ + H₂,

(3)

CO + 3H₂ ↔ CH₄ + H₂O,

(4)

CH₄ + CO₂ ↔ 2H₂ + 2CO,

(5)

2CO ↔ CO₂ + C,

(6)

CH₄ ↔ 2H₂ + C,

(7)

H₂O + C ↔ CO + H₂,

(8)

C₃H₈O₃ → H₂ + 3H₂O + 3C,

(9)

To be effective, the catalysts for use in the GSR should not promote C–O cleavage and CO or CO₂ hydrogenation, but they should favour the cleavage of the C–H, C–C and O–H bonds [15,16]. As Ni-based systems are known to be highly active in reforming reactions, most efforts currently found in the GSR related literature have focused on the development of such catalysts, using a variety of metal oxides (e.g., Al₂O₃, ZrO₂, SiO₂) as supports [17,18,19,20]. The most commonly used support is Al₂O₃ as it possesses a number of desirable properties, such as chemical and mechanical resistance under reaction conditions and a high specific surface area, which favours the dispersion of the active phase on the support [20,21]. However, there are two main disadvantages associated with the use of alumina; firstly, the Lewis type acid sites that it possesses promote acid-base-catalyzed reactions, which in turn favour the formation of a filamentous type of carbon, and secondly, it fosters the sintering of metallic particles [22,23]. For example, Chiodo et al. [24], using a low steam to glycerol ratio (3 mol/mol) and testing the stability at 800 °C for 20 h, reported rapid deactivation for a Ni/Al catalyst (30 wt % Ni). Cheng et al. [25], reported significant carbon deposition for a Ni/Al (15 wt % Ni) at 550 °C tested for approximately 4 h, even under excess steam conditions. Previous work by our group [15] showed severe deactivation for a Ni/Al (8 wt % Ni) during 20 h time-on-stream at 600 °C (WGFR of 9/1, molar), which was attributed to the high degree of crystallinity of the carbon deposits. Thus, much effort has been spent on overcoming the disadvantages associated with the use of alumina by inducing support-mediated promotional effects using different alkaline earth metals (e.g., Mg, Ca), lanthanide metals (e.g., Ce, La), and/or transition metals (e.g., Zr, Cu) as promoters for Ni/Al catalysts [26,27,28].

As is well understood, ceria can affect the surface acidity of alumina due to its higher point of zero charge (PZC) [29]. It also has the ability to act as an O₂ buffer, storing/releasing O₂ via the Ce³⁺/Ce⁴⁺ redox couple in CeO₂ [30], and thus promote water dissociation and the water gas shift reaction [31]. Recent research also suggests that a [O^δ−, δ⁺] dipolar layer is created on the surface of Ni particles that can protect them against thermal sintering [32,33,34]. For the GSR, a handful of works exist where ceria was used as promoter to alumina for Ni-based catalysts [35,36,37,38]. The general conclusion from these papers is that the addition of CeO₂ as alumina modifier enhances the activity of the non-promoted catalyst [35,36,37]. This is mainly related to the ability of ceria to stabilize the nickel particles and to promote steam reforming of the oxygenated hydrocarbons intermediates, leading to a reduction in coke deposition. It has also been suggested that ceria hinders secondary dehydration reactions (favoured by the presence of acid sites in the support), and lead to the formation of hydrocarbons that are coke precursors (and thus, responsible for catalyst deactivation) [38]. It has also been suggested that the formation of a CeAlO₃ perovskite structure can suppress the interaction between Ni and Al₂O₃ and increase the number of active Brønsted acid sites. This, in turn, improves the bifunctional metal-acid properties of Ni/CeO₂–Al₂O₃ and favours the hydrogenolysis and dehydrogenation-dehydration of condensable intermediate products that produce more H₂ [39].

In the present investigation, low metal loading (8 wt %) Ni catalysts supported on γ-Al₂O₃ and γ-Al₂O₃ modified with CeO₂ were investigated in terms of catalytic activity and time-on-stream stability during the GSR. Different characterization techniques, i.e., N₂-physisorption, ICP, XRD, temperature programmed reduction (H₂–TPR), CO₂–TPD, NH₃–TPD, XPS, TPO, SEM and TEM were used in an effort to identify the catalyst surface and bulk properties which affect the reaction and its products. The performance of the catalysts was investigated with the aim of identifying the effect of temperature on the total conversion of glycerol and the conversion of glycerol to gaseous products, the selectivity towards H₂, CO₂, CO, CH₄ and the liquid effluents produced during the reaction and for the determination of the H₂/CO and CO/CO₂ molar ratio in the gaseous products mixture. Time-on-stream experiments were conducted for 20 h under harsh reaction conditions in order to induce carbon deposition and the main liquid effluents were quantified.

2. Results and Discussion

2.1. Characterization Results

2.1.1. Chemical Analysis and Textural Properties

The physicochemical, structural and textural properties of the calcined and reduced catalysts are summarized in

Table 1. Physicochemical, structural and textural properties of calcined and reduced catalysts.

Catalyst	Calcined Samples			Reduced Samples
	Metal Loading (Ni, wt %)	SSA (m2g−1)	Pore Volume (cm3g−1)	SSA (m2g−1)	Pore Volume (cm3g−1)	Av. Pore Width (nm)	Ni0 Mean Crystallite Size (nm)
Ni/Al2O3	7.14	158	0.58	136	0.32	20.1	16.8
Ni/AlCeO3	7.69	43	0.27	26	0.18	23.5	14.2

. As a first observation, both catalysts have a similar Ni metal loading, which was measured for the calcined samples at 7.14 and 7.69 wt % for Ni/Al₂O₃ and Ni/AlCeO₃, respectively. Regarding the properties of the calcined supports used herein, N₂-physisorption measurements showed that Al₂O₃ has a much higher specific surface area (SSA) than AlCeO₃ (195 m² g⁻¹ compared to 48 m² g⁻¹) and pore volume (0.65 cm g⁻¹ compared to 0.24 cm g⁻¹). Following Ni impregnation and then catalyst calcination, the SSA of the Ni/Al₂O₃ catalyst dropped to 158 m² g⁻¹ and then further to 136 m² g⁻¹ after the reduction procedure. For Ni/AlCeO₃, this drop was less pronounced after calcination, but somewhat sharper after reduction (43 m² g⁻¹ after calcination and 26 m² g⁻¹ after reduction).

Also, for both catalysts, the pore volume remained almost unchanged following calcination, but dropped substantially after reduction. However, the doping of Al₂O₃ with CeO₂ resulted in enhanced basicity, as shown from the potentiometric titration curves obtained for the Al₂O₃ and AlCeO₃ suspensions, with the PZC values recorded at 6.8 and 8.2, respectively (Figure 1). It is noted that the PZC is defined as the pH value where the basic and acidic sites on the surface are in equilibrium [40].

The N₂ adsorption—desorption isotherms of both catalysts, displayed as an inset in Figure 2, are of Type IV with an H4-type hysteresis loop [41], and show major adsorption at high pressures (0.7 < P/P0 < 1.0) and only minor adsorption at low pressure (P/P₀ < 0.05). This is indicative of mainly mesoporous material with some macroporosity [42] and is corroborated by the corresponding pore size distributions.

Figure 3 presents the XRD patterns for the supports (after calcination) and catalytic samples (after calcination and after reduction). Regarding the Al₂O₃ support and Ni/Al₂O₃ catalyst, the characteristic peaks of γ-Al₂O₃ are clearly identified at 2θ = 37.2°, 47.2° and 67.2°. The nickel aluminate phase (NiAl₂O₄) was observed at 2θ = 19.0°, 32.0°, 37.0°, 45.0°, 60.2° and 65.9° for both calcined and reduced catalysts [43]. However, it is also clear from the diffractograms that the Al₂O₃ and NiAl₂O₄ peaks are not as intense on the reduced sample in comparison to the calcined one. Moreover, on the reduced catalyst, two small peaks corresponding to metallic nickel (Ni⁰), at 2θ = 44.0° and 51.2° [43], can be observed. For the calcined AlCeO₃ support, γ-Al₂O₃ was detected only at 2θ = 67.2° and the crystal phase of CeO₂ dominated with peaks at 2θ = 28.5°, 33.0°, 47.5°, 56.5° and 59.0° (Figure 3b) [44]. The XRD patterns of the Ni/AlCeO₃ catalyst after calcination and after reduction (Figure 3b) are nearly identical to the pattern of the calcined AlCeO₃ support with the only differences being two small peaks corresponding to NiAl₂O₄ detected on the calcined catalyst (2θ = 32.0° and 37.0°), and two peaks, corresponding to Ni⁰, detected at 2θ = 43.5^ο and 50.8° on the reduced sample. It is noted that the reaction 4CeAlO₃ + O₂ ↔ 4CeO₂ + 2Al₂O₃ is reversible, i.e., CeAlO₃ can be oxidised to CeO₂ by heating in air and CeO₂ can be reduced by heating in H₂ flow [45]. The Ni species mean particle size (Table 1), for the reduced samples, was determined from the XRD spectra using Scherrer analysis and was found at 16.8 nm for the Ni/Al₂O₃ and at 14.2 nm for the Ni/AlCeO₃ catalyst.

2.1.2. Surface Acidity-Basicity Estimation

Figure 4a presents the CO₂–TPD profiles obtained over the Ni/Al₂O₃ and Ni/AlCeO₃ catalysts. The Ni/Al₂O₃ catalyst presents mainly two types of basic sites of weak and medium strength. This is reflected by the peaks that appeared in the temperature regimes of 30 °C < T_max < 220 °C and 220 °C < T_max < 450 °C. The strong basic site population is very limited. On the other hand, it seems that the modification of Al₂O₃ support with Ce leads to a catalyst with a limited population of weak basic sites and a wider distribution of medium strength basic sites, whereas the population of strong basic sites is enhanced significantly. This result is in good agreement with the PZC studies, discussed earlier.

Figure 4b presents the NH₃–TPD profiles obtained over the Ni/Al₂O₃ and Ni/AlCeO₃ catalysts. The Ni/Al₂O₃ presents a rather rich surface in terms of acid sites of weak, medium and high strength. Incorporation of Ce into the catalyst support leads to a drastic re-distribution of the acid sites of the catalyst, with only the weak sites dominating the surface. It is commonly accepted that on the surface of pure Al₂O₃ only the Lewis-type acid centres are observed. Miranda et al. [46] attributed desorption peaks at 77–277 °C and at 277–627 °C to the presence of two kinds of acid sites having different strengths. According to the authors, the lower desorption peak is linked to desorption of NH₃ bridge species which are bonded to penta-coordinated aluminum (Lewis weak sites), while the higher desorption peak to terminally bonded NH₃ on tri-coordinated aluminum (Lewis strong sites) [47]. Thus, it can be concluded that the high temperature acid sites correspond to the Brønsted acid sites of the solid surface [48] and the strong Lewis acid sites are associated with Al₂O₃ [47,49], whereas the low temperature acid sites are weak acid sites and mainly associated with CeO₂ [50] due to its generally weak basicity [39,51].

2.1.3. Ni species Reducibility

Metal-support interactions (electronic or geometrical) are crucial in defining the catalytic properties. In general, NiO interaction with the support, as well as the size of NiO, regulates the extent of its reduction. Weak interaction with the support is considered to give rise to reduction peaks at low temperatures, whereas peaks at high temperatures are due to a strong NiO-support interaction. In the H₂-TPR profile of the Ni/Al₂O₃ catalyst (Figure 5), two predominant peaks at 420 °C and 690 °C are apparent (Figure 5). The peak at 690 °C is due to the reduction of nickel aluminate (NiAl₂O₄), while the peak at 420 °C is due to small NiO crystallites interacting with the support (supported NiO). The small hump at 390 °C for the Ni/Al₂O₃ catalyst can be assigned to the reduction of bulk NiO [52]. The addition of Ce causes a redistribution of species and most likely changes the support size thus affecting the NiO-support contact area, with the latter being increased. This causes a reduction profile with peaks at lower temperatures compared with the Ni/Al₂O₃ catalyst.

These TPR reduction profiles are in agreement with the ones presented by Yang et al. [53] where Ni/Al₂O₃ reduction takes place between 400–900 °C, whereas the addition of Ce gave rise to another reduction peak in the 200–400 °C region. This is due to the excellent redox properties of the Ce-related phases formed [54]. Thus, it can be concluded that the nickel species on the AlCeO₃ support retained a higher reducibility than Al₂O₃; the higher reducibility being a result of the incorporation of ceria into the alumina lattice [39]. Also, the complex profile of NiO/AlCeO₃ catalyst can be due to the fact that the nucleation and growth of the NiO over the Ce-modified Al₂O₃ is different compared to that of Al₂O₃. This could be due to the presence of new sites (e.g., Ce³⁺, Al–O–Ce site, defects), which can act as anchoring sites for the growth of NiO, which affect the kinetics of the growth and ultimately the NiO size (distribution of NiO sizes). Furthermore, a support that exhibits strong metal-support interactions is anticipated to show less sintering during oxidation/reduction.

2.1.4. Surface Analysis

The XPS Ni 2p and Ce 3d peaks for both the calcined and reduced Ni/Al₂O₃ and Ni/AlCeO₃ catalysts are shown in Figure 6a–f. The Ni 2p_3/2 peak for the calcined Ni/Al₂O₃ sample shows a peak at a binding energy of 856.1 eV, associated with the presence of NiAl₂O₄ (Figure 6a). The reduced catalyst also shows the presence of a main Ni 2p_3/2 peak at a binding energy of 856.1 eV due to NiAl₂O₄/Ni₂O₃ but also a shoulder at lower binding energies at around 853 eV, due to the presence of metallic Ni (Figure 6b). For the Ni/AlCeO₃ catalysts, only the Ni 2p_3/2 peak and satellite is shown in the Figure 6c,d as the 2p_1/2 peak and satellite overlap with the Ce 3d peaks.

The calcined Ni/AlCeO₃ catalyst exhibits a Ni 2p_3/2 peak at a binding energy of 856.2 eV due to NiAl₂O₄. The reduced Ni/AlCeO₃ catalyst shows a more complex peak shape with 3 components at binding energies of 856.2, 854.1 and around 852.6 eV. The complexity of the Ni 2p spectra in terms of their photoelectron and satellite features (particularly Ni oxide) precludes a sensible peak fit of the Ni 2p envelope [55]. The different components can be tentatively assigned to NiAl₂O₄/Ni₂O₃ (856.2 eV), NiO (854.1 eV) and Ni metal (852.6 eV). XPS Ce 3d photoelectron spectra for CeO₂ (Ce⁴⁺) and Ce₂O₃ (Ce³⁺) (Figure 6e,f) are also well known for their complex photoelectron and satellite peak structure [56,57,58]. Furthermore, for the catalyst surfaces examined here, overlap with the main Ni 2p_1/2 satellite further complicates the spectral shape. Nevertheless, using standard spectra for CeO₂ and Ce₂O₃ recorded on this XPS instrument as a reference, and with the assistance of other XPS work on cerium oxides [56,57,58] the Ce 3d spectra have been peak fitted with a reasonable degree of certainty. The photoelectron and satellite peaks for the calcined Ni/AlCeO₃ catalyst are considered first (Figure 6e). The calcined catalyst surface shows peaks generally representative of Ce⁴⁺ (CeO₂). Using the nomenclature employed in previous XPS work on cerium oxide [56,57], the peaks are labelled v and u, corresponding to transitions associated with the Ce 3d_5/2 and Ce 3d_3/2 respectively. The peaks observed are those corresponding to the v, v’’, v’’’, u, u’’ and u’’’, corresponding to the Ce⁴⁺ species, hence CeO₂ is clearly the predominant oxide present. However, the peak fit shows the presence of weak peaks associated with the Ce³⁺ species (v₀, v’, u₀ and u’) and approximate fractions of the two oxide forms is 85% CeO₂ and 15% Ce₂O₃, corresponding to a stoichiometry of CeO_1.94. The peak shape agrees well with that given by Henderson et al., for a reported stoichiometry of CeO_1.95 [58]. For the reduced catalyst (Figure 6f), the Ce 3d spectrum is very different, with clear peaks present for both the Ce⁴⁺ (v, v’’’, u, u’’ and u’’’) and Ce³⁺ (v’ and u’) species. The peak fit gives a ratio of 41% CeO₂ and 59% Ce₂O₃, corresponding to a stoichiometry of CeO_1.80. The peak shape recorded for the reduced Ni/AlCeO₃ catalyst agrees very well with the Ce 3d peak shapes shown for samples with reported stoichiometries of CeO_1.78 [56] and CeO_1.82 [58], having relative peak intensities in-between those observed for CeO_1.78 and CeO_1.82, as would be expected for a stoichiometry of CeO_1.80.

As is widely accepted, CeO₂ increases the dispersion of the metallic phase [59] and this is also observed for the catalysts tested herein with the results shown in

Table 2. Ni2p XPS data of calcined and reduced Ni/Al₂O₃ and Ni/AlCeO₃ catalysts.

Catalyst	Calcined Samples		Reduced Samples
	Peak (BE)	Atomic (at. %)	Peak (BE)	Atomic (at. %)
Ni/Al2O3	856.05	2.69	856.04	2.76
Ni/AlCeO3	856.16	3.86	855.74	3.54

. It has also been suggested that a high dispersion causes a specific interaction between the nickel and ceria species, were the Ni particles close to CeO₂ activate the dissociation of H₂ and by spillover favour the reduction of the ceria surface [60].

2.2. Catalytic Performance

2.2.1. Total Conversion and Conversion to Gaseous Products

Figure 7a presents the results obtained following the catalytic tests, performed under experimental protocol #1, in terms of glycerol total conversion (X_C3H8O3, %) and glycerol conversion to gaseous products (X_{C3H8O3 gaseous}, %) in relation to reaction temperature. It is clear that a very high conversion of glycerol was achieved by both catalysts, but also for the AlCeO₃ support, over the whole temperature range (from ≈85 at 400 °C to > 95% at 750 °C). In contrast, the total conversion of glycerol was relatively low for the bare alumina support, ranging from approximately 70% at 400 °C to 80% at 750 °C. The high conversion values recorded are due not only to the reforming of glycerol but also to its thermal decomposition, which can take place simultaneously during the GSR process [61,62,63]. The endothermic nature of the GSR reaction can be clearly deduced from the conversion of glycerol to gaseous products, which increases sharply with increasing temperature (Figure 7a).

However, it is clear that the Ni/AlCeO₃ catalyst produces a far greater number of gaseous products in comparison to Ni/Al₂O₃, especially for temperatures lower than 600 °C. Interestingly, the values of X_gaseous for the Ni/AlCeO₃ sample ranged from 60% to 80% for the whole temperature range. Furthermore, the AlCeO₃ supporting material seems to produce more or less the same number of gaseous products as the Ni/Al₂O₃ catalytic sample. Previously published works [39] argued that the formation of CeAlO₃ perovskite structures suppresses any strong interaction between the nickel species and the Al₂O₃ support, thereby increasing the number of active sites and the reducibility of the active nickel species. From the results presented herein, the superior behaviour of the Ni/AlCeO₃ can be related to its higher dispersion (XPS), i.e., smaller Ni crystallites, and also to higher electron density and accessibility of the active sites caused by the close contact between nickel and the cerium species.

2.2.2. Gaseous Products’ Selectivity

The influence of reaction temperature on H₂ selectivity (S_H2) and yield (Y_H2), following experimental protocol #1, for both catalysts and supports is presented in Figure 7b. As a first observation, the H₂ production of the Ni/AlCeO₃ catalyst is markedly higher in comparison to the Ni/Al₂O₃ sample for the whole temperature range, as its S_H2 and Y_H2 take the values of 80–90% and 4–6 mol/mol (6 mol/mol is very close to the value predicted by thermodynamics). This improved performance can also be observed for the AlCeO₃ support in comparison to the pure alumina. Thus, it can be suggested that the Ni active sites combined with the Brønsted acid sites promote the hydrogenolysis and dehydrogenation-dehydration of the bifunctional metal-acid Ni/AlCeO₃ catalyst, thereby converting more condensable intermediates into gaseous products, as well as leading to higher glycerol conversion and H₂ yield.

The influence of reaction temperature on the selectivity to CO₂ and CO (S_CO2 and S_CO) for all samples is presented in Figure 7c. It is obvious that the Ni/AlCeO₃ sample is more selective towards CO₂ and less selective towards CO for the whole temperature range. This is in contrast to the trend observed for the Ni/Al₂O₃ catalyst and supporting materials, as these materials appear more selective towards CO and less selective towards CO₂. Thus, it seems that the ability to transform OHCs into H₂ and CO₂ is significantly higher for the Ni/AlCeO₃ catalyst probably due to its enhanced basic character (higher PZC value), which was introduced by the addition of CeO₂ to the alumina. Moreover, from Figure 7d it can be seen that both catalytic samples exhibit very low S_CH4 for the whole temperature range; in contrast, S_CH4 increases for the supports with increasing temperature. This was rather expected as according to the literature [36] at high water to glycerol feed ratios and at temperatures higher than 650 °C, the formation of CH₄ is strongly inhibited due to the methane steam reforming reaction being catalysed by the metallic active phase. Keeping in mind that glycerol decomposition to CH₄ during the reforming process is highly favoured [64], it appears that both catalysts have the capacity to reform the produced CH₄ into H₂ and CO.

Finally, the influence of reaction temperature on the H₂/CO and the CO/CO₂ molar ratios in the gaseous products’ mixture is presented in Figure 7e. For the Ni/AlCeO₃ catalyst, the CO/CO₂ molar ratio is close to zero for the whole temperature range, while the H₂/CO molar ratio value decreases with increasing temperature from a value of about 17 (450 °C) to a value of about 10 (550–750 °C). In contrast, for the Ni/Al₂O₃ catalyst, both ratios are equal to 2–3 and appear relatively stable over the whole temperature range under investigation. The H₂/CO molar ratio value is more or less negligible for the supporting materials, whereas the CO/CO₂ molar ratio values range from about 2 between 400 and 500 °C, increases from 500 to 600 °C and decreases again for 650 °C < T < 750 °C (its maximum value was ~5.5 for the Al and ~4 for the AlCeO₃ sample). It is known that in the reaction process, dehydrogenation of the adsorbed glycerol molecule first takes place on the metal surface of catalyst to give adsorbed intermediates for the cleavage of C–C or C–O bonds.

From the results presented above it is obvious that the presence of ceria in the alumina supporting material has an important effect on the conversion and the gaseous product distribution, mainly by increasing H₂ and CO₂ production to the detriment of CH₄ and CO formation. It is likely that for the Ni/AlCeO₃ catalyst there is a first reaction step that involves rapid C–C breaking and CO formation, which is then followed by the WGS reaction, enhancing H₂ and CO₂ production. Sad et al. [65] has argued that acidic supports (such as alumina) favour the production of oxygenates (such as acrolein, acetol, acetaldehyde and acetic acid) via dehydration and dehydrogenation reactions, and that H₂ production is favoured by the use of non-acidic supports. However, the GSR process is also influenced by pyrolysis phenomena, given that the glycerol molecule is not thermally stable, which means that the intermediates formed by glycerol cracking are reformed on the catalyst surface [66].

2.2.3. Liquid Products’ Selectivity

In accordance with previous research (e.g., [10,12,31]), a variety of liquid products were identified for the GSR however, only the main ones, i.e., acetone, acrolein, acetaldehyde, acetic acid, allyl alcohol, and acetol were quantified. The influence of temperature on liquid product selectivity for the Ni/Al₂O₃ and Ni/AlCeO₃ catalysts and the corresponding supports is shown in Figure 8. For the Ni/AlCeO₃ catalyst and the AlCeO₃ support, production of liquid effluents ceases over 700 °C, 50 °C higher compared to the Ni/Al₂O₃ sample. In contrast, pure alumina produces liquid effluents even at temperatures as high as 750 °C. Another important observation is the differences in the liquid product distribution between modified and unmodified samples. Specifically, for the Ni/AlCeO₃ and AlCeO₃, allyl alcohol seems to be the main product (at least for high temperatures), with acetaldehyde and acetone the secondary products. On the other hand, acetone is the main product at the high temperature range for the Ni/Al₂O₃ sample and acetic acid and acetol for pure alumina.

According to the literature, there are four main reaction routes of glycerol consisting of three types of reactions, i.e., dehydrogenation, hydrogenolysis, and dehydration. These reactions represent the effects of the bifunctional metal–acid properties of the catalysts [47]. The first and the second routes of glycerol dehydrogenation and hydrogenolysis generate a common product, such as ethylene glycol, which is converted to acetaldehyde, glycolaldehyde, and ethanol. The hydrogenolysis of glycerol also produces methanol. The third route is glycerol dehydration to hydroxyacetone (acetol), which can be converted simultaneously in three ways to form a variety of products such as acetaldehyde, acetone, ethanol, acetic acid, propionic acid, formic acid, phenol, 1,2-propanediol, propanal, 1-propanol, and 2-cyclo-pentanone. This pathway is caused by the activation of the terminal OH of glycerol on the Lewis acid sites, which is represented by the strong acid sites of the catalysts. The fourth pathway is another glycerol dehydration reaction producing 3-hydroxypropanal, which is a starting reactant for the production of several chemicals. The species observed and confirming this pathway include allyl alcohol, acetaldehyde, methanol, formic acid, C₂H₄, and C₂H₆. This pathway involves the protonation of the secondary OH of glycerol to form acrolein, which favours the Brønsted acid sites mainly represented by the moderate acid sites [67].

2.3. Catalytic Stability

The results for catalytic stability are presented in Figure 9 and

Table 3. Catalytic performance of the Ni/Al₂O₃ and Ni/AlCeO₃ catalysts during time-on-stream at 600 °C (first and last measurement).

Reaction Metric	Ni/Al2O3		Ni/AlCeO3
	1st Measure	Last Measure	1st Measure	Last Measure
X(C3H8O3), %	95.03	70.42	93.58	76.69
X(C3H8O3), % into gaseous products	53.84	22.33	47.76	37.32
Y(H2), %	3.13	0.75	2.98	2.30
S(H2), %	89.85	19.74	89.19	81.47
S(CO2), %	76.17	8.78	75.72	67.26
S(CO), %	22.03	70.70	23.08	29.23
S(CH4), %	1.79	20.52	1.20	3.51
S(acetol), %	30.59	39.75	30.09	39.63
S(acetone), %	16.09	15.80	16.30	7.48
S(allyl alcohol), %	20.21	14.81	22.19	30.51
S(acetaldehyde), %	18.73	17.09	13.95	12.19
S(acetic acid), %	14.38	12.55	17.45	10.17
H2/CO	9.51	0.65	9.02	6.51
CO/CO2	0.29	8.06	0.31	0.44

. It is noted that these experiments were carried out under more severe conditions (experimental protocol #2) in order to provoke carbon deposition and catalyst deactivation. From the results presented herein it is clear that although the Ni/AlCeO₃ catalyst suffers a slight decrease concerning glycerol total conversion (94–77%) and glycerol conversion to gaseous products (48–37%), it maintains remarkably stable values for H₂ yield (2.9–2.3) and selectivity (89–81%), as well as CO₂ (75–67%) and CO (23–29%) selectivity. As a result, the H₂/CO and CO/CO₂ molar ratios remain reasonably constant for the duration of the experiment, with values ranging between 9 to 6.5 and 0.3 to 0.4, respectively. The catalytic performance observed indicated that the effect of the WGS reaction do not weaken with time. It is accepted that the addition of basic modifiers, such as CeO₂ to Ni/Al₂O₃ catalysts can prevent carbon formation by favouring both the adsorption of H₂O, O₂, CO₂ or–OH fragments and the spillover of such fragments from the support to the metal particles [67], and thus facilitate carbon gasification.

In contrast, although the decrease in total X_C3H8O3 and X_C3H8O3 gaseous products for the Ni/Al₂O₃ catalyst was similar to the Ni/AlCeO₃, S_H2, Y_H2 and S_CO2 decreased substantially; this decrease was accompanied by an increase in the S_CO values. As a result, for the Ni/Al₂O₃ catalyst, the H₂/CO molar ratio decreased from 9.5 to 0.6 and the CO/CO₂ ratio increased from 0.3 to 8.0 with time on stream; a significant increase in methane production was also observed. The deactivation of Ni catalysts supported on pure alumina during the GSR reaction has also been reported by other research groups and has been attributed to carbon formation and sintering [68,69,70].

The difference in the catalytic behavior of the samples can be strongly affected by the nature of the carrier, suggesting that the activity and the rate of deactivation is likely to be related to the different extent of electronic interaction between supported metal and support, influencing the bonding and reactivity of the chemisorbed species. The presence of Ni²⁺/Ni⁰ and Ce⁴⁺/Ce³⁺ couples means that different species can participate in the activation of the glycerol molecules. According to Bazin et al. [71] an increase of the local electron density is expected with increased metal dispersion, as has been proven for the Ni/AlCeO₃ catalyst; it exhibits a great number of active sites accessible to the reactant molecules compared to that observed for the Ni/Al₂O₃ catalyst.

2.4. Characterization of Used Catalysts

The nature of the carbonaceous deposits formed on to the spent catalytic samples following the time-on-stream experiments (experimental protocol #2) was examined using TPO, SEM, and TEM.

The TPO results for both catalysts are presented in Figure 10. As a first observation, significantly more carbon was deposited on to the Ni/Al₂O₃ catalyst (0.41 g_coke/g_catalyst) in comparison to the Ni/AlCeO₃ sample (0.14 g_coke/g_catalyst). The Ni/Al₂O₃ catalyst shows a weak peak at 470 °C and two broad peaks at 550 °C and 650 °C, which indicate the existence of different co-existing carbon allotropes. For the Ni/AlCeO₃ catalyst, there is a clear peak at ~375 °C, whereas a larger peak at 490 °C is observed; this points to the existence of coke that contains a higher fraction of defective carbon, which is known to be more easily oxidized. In addition, the high temperature decomposition peak appears as one single thermal event centred at 625 °C, i.e., slightly lower than the corresponding peak of the Ni/Al₂O₃ catalyst. It has been reported that the thermal decomposition of physisorbed carbonaceous products, termed as soft (or amorphous) coke, occurs at temperatures between 200 and 500 °C [39], while the gasification of hard coke (comprising of bulky carbonaceous species and often referred to as filamentous carbon) [72,73,74] takes place between 500 and 800 °C.

The hard coke species are 500–600 °C and can be associated with filamentous coke (carbon nanotubes, CNTs, and nanofibers, CNFs, that contain defects), whilst the coke gasified at over 600 °C can be associated with graphitic carbon species. This graphitic carbon is an inert coke that does not easily react with oxygen or steam [75]. Thus, not only was a higher quantity of coke deposited onto the spent Ni/Al₂O₃ catalyst, but also the fractions of graphitic carbon were greater and more difficult to oxidize than on the Ni/AlCeO₃ catalyst. These results explain the excellent stability observed for the Ni/AlCeO₃ catalyst. The different coke species on the catalyst surface can be formed by the Boudouard, methane decomposition and polymerization reactions [72,73]. Thus, the redox properties of CeO₂ and a higher dispersion of the active phase for the Ni/AlCeO₃ catalyst has resulted in markedly lower coke yields.

The morphology of the carbon deposits was initially examined using SEM/EDX; representative SEM micrographs and the carbon EDX images of the corresponding areas are shown in Figure 11. Although it is not straightforward to determine the precise nature of the coke deposits from this initial examination (i.e., whether these are nano-fibers, micro-fibers or carbon nanotubes), entangled tubular arrangements (filaments) can be discerned from the images. Also, the orientation of these filaments is quite random, which makes it difficult to estimate their length with any degree of accuracy. However, the EDX carbon images (red) show heavy coke deposition, particularly on the Ni/Al₂O₃ spent catalyst, in good agreement with the TPO results discussed above.

Figure 12 presents representative TEM images of the two spent catalytic samples and the corresponding particle size distribution histograms. The images confirm the formation of different carbon allotropes on the catalytic surface (indicated by black dashed circles), but also show heavy filament formation on the Ni/Al₂O₃ system, in excellent agreement with the discussion above. Encapsulating carbon can also be observed in the lower image for this system (examples of Ni particles are shown by red dashed circles), which is known to lead to a loss of activity as the Ni particle is no longer available during the reaction. Moreover, the particle size distribution histograms show that the mean Ni size of the Ni/AlCeO₃ catalyst is substantially smaller than that of the Ni/Al₂O₃, helping to explain the improved activity and stability characteristics of the former sample.

3. Materials and Methods

3.1. Catalyst Preparation

The alumina support (pellets) was acquired from Akzo, while the AlCeO₃ (powder) was sourced from Sigma Aldrich. The pelletized support was crushed and sieved to particle sizes in the range 350–500 μm, while the powder was first pelletized and then crushed to the same size. The catalysts were produced via the wet impregnation technique using an aqueous solution of Ni(NO₃)₂ 6H₂O, obtained from Sigma Aldrich, with a concentration of 0.17 M to obtain catalysts with metal content of 8 wt %. The water contained in the slurries was evaporated under continuous stirring at 75 °C over 5 h. After, the suspensions were dried for 12 h at 120 °C and the calcination of the catalysts was carried out at 800 °C for 4 h.

3.2. Catalyst Characterization

For the determination of the catalysts’ surface and bulk properties, different characterization techniques were employed. These included the calculation/determination of: (a) the Point of Zero Charge (PZC) for the calcined supports via Potentiometric Mass Titrations (PMTs), (b) the total specific surface area (SSA) of the catalytic materials and corresponding supports via the Brunauer–Emmet–Teller (BET) method, (c) the pore size distribution (PSD) of the catalytic materials and corresponding supports using the Barrett, Joyner, and Halenda (BJH) method, (d) the total metal loading (wt %) of the calcined catalysts using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), (e) the crystalline structure of the calcined supports, as well as the calcined and reduced catalysts by X-ray Diffraction (XRD) analysis, (f) the degree of Ni species reducibility using Temperature Programmed Reduction (H₂-TPR), (g) the acid and basic properties of the catalysts via CO₂- and NH₃-TPD experiments, (h) the nature of the various surface species and their oxidation states, on the reduced catalysts, using X-ray Photoelectron Spectroscopy (XPS) analysis, (i) the morphological characteristics of the spent catalysts using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), and (j) the amount and nature of the carbonaceous deposits on the spent catalysts using Temperature Programmed Oxidation (TPO). The methodology and equipment used has been described in detail in previous publications by our group, and in particular: PZC in [26], XRD in [11], ICP at [76], and N₂ adsorption-desorption, H₂–TPR, CO₂–TPD, NH₃–TPD, XPS, SEM, TEM and TPO in [17,23].

3.3. Catalytic Tests

For the catalytic testing a continuous flow fixed-bed reactor was employed; the exact system and procedure has been described in detail in previous publications (e.g., [17,23]). Succinctly, using glycerol with 99.5% purity (Sigma-Aldrich, St. Louis, MO, USA) and a Weight Hourly Space Velocity (WHSV) of 50,000 mL g⁻¹ h⁻¹, two different experimental protocols were employed. For the first protocol, the gas feed at the inlet of the reactor was a gas mixture of 73% H₂O, 4% C₃H₈O₃ and 23% He (i.e., 20 v.v. % of C₃H₈O₃ diluted in H₂O) and the catalytic performance was investigated between 400–750 °C. The second protocol was used for the investigation of catalytic stability during time-on-stream and was carried out for 20 h at 600 °C. The conditions chosen were more severe and the gas mixture at the reactor’s inlet consisted of 63% H₂O, 7% C₃H₈O₃, 30% He (31° of C₃H₈O₃ diluted in H₂O). Before commencing the experiments, catalytic activation was undertaken in situ using a flow (100 mL min⁻¹) of high purity H₂ (5.0) at 800 °C for 1h. The system was then purged with He and the temperature lowered according to the protocol that was to be followed. The liquid products were analysed via a combination of Gas Chromatography (Agilent, Santa Clara, CA, USA, 7890A) and Mass Spectroscopy (Agilent 5975C). The gaseous products were determined via an Agilent gas chromatographer (7890A). Detailed information regarding the analysis of liquid and gaseous products can be found in Reference [11].

3.4. Reaction Metrics

The investigation of catalytic performance necessitated the calculation of total glycerol conversion, conversion of glycerol into gaseous products, and determination of the H₂ yield, H₂, CH₄, CO₂ and CO selectivity. For the calculations Equations (10) and (14) shown below were used. The selectivity of acetone [(CH₃)₂CO], acetaldehyde (C₂H₄O), acetol (C₃H₆O₂), allyl alcohol (CH₂=CHCH₂OH), acrolein (C₃H₄O) and acetic acid (C₂H₄O) was calculated based on Equation (15):

$$\%glycero{l}_{} conversio{n}_{\left(tota{l}_{} conversion\right)}=\left(\frac{Glycero{l}_{in}-Glycero{l}_{out}}{Glycero{l}_{in}}\right)\times 100$$

(10)

$$\%glycero{l}_{} conversio{n}_{\left(gaseou{s}_{} products\right)}=\left(\frac{C_{} atom{\mathrm{s}}_{} i{n}_{} th{e}_{} ga{s}_{} products}{tota{l}_{} C_{} atom{\mathrm{s}}_{} i{n}_{} th{e}_{} feedstock}\right)\times 100$$

(11)

$$H_2 yield=\frac{H_2 mo{l}_{} produced}{mo{l}_{} o{f}_{} glycero{l}_{} i{n}_{} th{e}_{} feedstock}$$

(12)

$$\%H_2 selectivity=\left(\frac{H_2 mo{l}_{} produced}{C_{} atom{s}_{} produce{d}_{} i{n}_{} th{e}_{} ga{s}_{} phase}\right)\times \left(\frac{1}{RR}\right)\times 100$$

(13)

where, RR is the reforming ratio (7/3), defined as the ratio of moles of H₂ to CO₂ formed.

$${\%}_{} selectivit{y}_{} o{f}_{} i=\left(\frac{C_{} atom{s}_{} i{n}_{} specie{s}_{} i}{C_{} atom{s}_{} produce{d}_{} i{n}_{} th{e}_{} ga{s}_{} phase}\right)\times 100$$

(14)

where, species $i$ refers to CO, CO₂ and CH₄.

$${\%}_{} selectivit{y}_{} o{f}_{} i^{\prime }=\left(\frac{C_{} atom{s}_{} i{n}_{} specie{s}_{} i^{\prime }}{C_{} atom{s}_{} produce{d}_{} i{n}_{} th{e}_{} liqui{d}_{} phase}\right)\times 100$$

(15)

where, species $i^{\prime }$ refers to acetol, acetone, allyl alcohol, acetaldehyde, acrolein and acetic acid.

4. Conclusions

A critical assessment of the effect of the Ce-modification of Al₂O₃ on the catalytic performance of Ni/Al₂O₃ and Ni/AlCeO₃ catalysts towards glycerol steam reforming reaction and H₂ production has been performed. A thorough comparison of the AlCeO₃, Al₂O₃ (supports) as well as Ni/Al₂O₃ and Ni/AlCeO₃ (catalysts) has been presented.

The study has shown that Ce-modification of Al₂O₃ leads to a Ni catalyst with increased basicity (PZC and CO₂–TPD studies) and a higher Ni dispersion (H₂–TPR, XPS studies). The Ni/AlCeO₃ sample was more selective towards CO₂ and less selective towards CO. The opposite trend was observed for the Ni/Al₂O₃ catalyst. Importantly, both Ni/Al₂O₃ and Ni/AlCeO₃ had low CH₄ selectivity. For the Ni/AlCeO₃ catalyst the CO/CO₂ molar ratio was almost zero, while the H₂/CO molar ratio value decreases (from 17 to 10) with increasing temperature (450 °C to 750 °C). Regarding the liquid products for the Ni/AlCeO₃ and AlCeO₃, allyl alcohol was found to be the main product (at least for high temperatures), with acetaldehyde and acetone the secondary ones. On the other hand, acetone was the main product at the high T range for the Ni/Al₂O₃ sample and acetic acid and acetol for pure alumina. Stability studies performed for over than 20 h on stream showed that the Ni/AlCeO₃ catalyst experienced a slight decrease on glycerol total conversion (94–77%) and glycerol conversion to gaseous products (48–37%), but it maintained remarkably stable values for H₂ yield (2.9–2.3) and selectivity (89–81%), as well as for the CO₂ (75–67%) and CO (23–29%) selectivity. Characterization of the exhausted catalysts was performed in order to understand the nature of the deposited coke. It was found that Ni/AlCeO₃ was far more resistant to coke, while at the same time, it favored the formation of more defective carbon compared to the coke deposited onto Ni/Al₂O₃.