The ‘Green’ Ni-UGSO Catalyst for Hydrogen Production under Various Reforming Regimes

Abstract

A new spinelized Ni catalyst (Ni-UGSO) using Ni(NO₃)₂·6H₂O as the Ni precursor was prepared according to a less material intensive protocol. The support of this catalyst is a negative-value mining residue, UpGraded Slag Oxide (UGSO), produced from a TiO₂ slag production unit. Applied to dry reforming of methane (DRM) at atmospheric pressure, T = 810 °C, space velocity of 3400 mL/(h·g) and molar CO₂/CH₄ = 1.2, Ni-UGSO gives a stable over 168 h time-on-stream methane conversion of 92%. In this DRM reaction optimization study: (1) the best performance is obtained with the 10–13 wt% Ni load; (2) the Ni-UGSO catalysts obtained from two different batches of UGSO demonstrated equivalent performances despite their slight differences in composition; (3) the sulfur-poisoning resistance study shows that at up to 5.5 ppm no Ni-UGSO deactivation is observed. In steam reforming of methane (SRM), Ni-UGSO was tested at 900 °C and a molar ratio of H₂O/CH₄ = 1.7. In this experimental range, CH₄ conversion rapidly reached 98% and remained stable over 168 h time-on-stream (TOS). The same stability is observed for H₂ and CO yields, at around 92% and 91%, respectively, while H₂/CO was close to 3. In mixed (dry and steam) methane reforming using a ratio of H₂O/CH₄ = 0.15 and CO₂/CH₄ = 0.97 for 74 h and three reaction temperature levels (828 °C, 847 °C and 896 °C), CH₄ conversion remains stable; 80% at 828 °C (26 h), 85% at 847 °C (24 h) and 95% at 896 °C (24 h). All gaseous streams have been analyzed by gas chromatography. Both fresh and used catalysts are analyzed by scanning electron microscopy-electron dispersive X-ray spectroscopy (SEM-EDXS), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) coupled with mass spectroscopy (MS) and BET Specific surface. In the reducing environment of reforming, such catalytic activity is mainly attributed to (a) alloys such as FeNi, FeNi₃ and Fe₃Ni₂ (reduction of NiFe₂O₄, FeNiAlO₄) and (b) to the solid solution NiO-MgO. The latter is characterized by a molecular distribution of the catalytically active Ni phase while offering an environment that prevents C deposition due to its alkalinity.

1. Introduction

Several research works on nickel-based reforming catalysts are devoted to the development of their supports. The catalyst presented in this work valorizes a mining residue by using it as a support, allowing the production of renewable hydrogen from green catalysts and responding to the different spheres of sustainable development.

Worldwide, H₂ is produced mainly from hydrocarbons using steam reforming of methane (SRM) (Reaction (R1)). Partial oxidation (POX) (Reaction (R2)) or dry reforming (DRM) (Reaction (R3)) are good candidates too. However, their use in the industry is limited due to certain drawbacks, namely, the relatively fast catalyst deactivation [1].

$${\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{CO}+3{\mathrm{H}}_2 (\text{∆}\mathrm{H}{\text{°}}_{298}=206 \mathrm{MJ}/\mathrm{kmol})$$

(R1)

$${\mathrm{CH}}_4+1/2{\mathrm{O}}_2\leftrightarrow \mathrm{CO}+2{\mathrm{H}}_2 (\text{∆}\mathrm{H}{\text{°}}_{298}=-38 \mathrm{MJ}/\mathrm{kmol})$$

(R2)

$${\mathrm{CH}}_4+{\mathrm{CO}}_2\leftrightarrow 2\mathrm{CO}+2{\mathrm{H}}_2 (\text{∆}\mathrm{H}{\text{°}}_{298}=247 \mathrm{MJ}/\mathrm{kmol})$$

(R3)

SRM is the largest scale process for the production of H₂ by natural gas conversion. CH₄ is converted with steam on supported Ni catalysts in a highly endothermic reaction. The SRM reaction is conducted in heated tubular reactors to achieve the high temperatures needed, making steam reforming a major energy consumer.

Secondary reactions of carbon deposition also occur such as:

$$2\mathrm{CO}\leftrightarrow \mathrm{C}+{\mathrm{CO}}_2 \mathrm{Boudouard}$$

(R4)

$${\mathrm{CH}}_4\leftrightarrow \mathrm{C}+2{\mathrm{H}}_2 {\mathrm{CH}}_4 \mathrm{dissociation}$$

(R5)

$${\mathrm{H}}_2+\mathrm{CO}\leftrightarrow \mathrm{C}+{\mathrm{H}}_2\mathrm{O} {\mathrm{H}}_2\mathrm{O} \mathrm{production}$$

(R6)

$$2{\mathrm{H}}_2+{\mathrm{CO}}_2\leftrightarrow \mathrm{C}+2{\mathrm{H}}_2\mathrm{O} {\mathrm{H}}_2\mathrm{O} \mathrm{production}$$

(R7)

An excess of water makes it possible to drive Reaction (R6) towards the formation of synthesis gas and minimize carbon formation. In fact, by reacting CO with H₂O according to the water-gas-shift (WGS) Reaction (R8), the CO becomes CO₂, thus producing more hydrogen and increasing the H₂/CO ratio:

$$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{CO}}_2+{\mathrm{H}}_2 (\text{∆}\mathrm{H}{\text{°}}_{298}=-41 \mathrm{MJ}/\mathrm{kmol})$$

(R8)

Because the SRM reaction is endothermic, it requires high-temperature conditions (T > 800 °C) to increase conversion and decrease carbon formation while increasing H₂ and CO yields. Relatively high working pressures (20–40 bar) are also used industrially [2,3]. This process, despite its importance, has the major disadvantage of being energy-intensive, a situation which is made worse by the use of excess water.

In the catalytic POX (Reaction (R2)), CH₄ is converted with oxygen or air to form H₂ and CO with a ratio of H₂/CO = 2. This ratio of 2 is ideal for most downstream processes, making partial oxidation of methane a simple, one-step process. Unlike SRM, the POX of methane is slightly exothermic.

In the last decades, the constant development of DRM (Reaction (R3)) is mainly due to:

(a)

the use 2 GHGs (CH₄ and CO₂); the technology is under study and making its first steps at industrial demonstration scale [4];

(b)

the production of a valuable syngas (CO + H₂) with a molar H₂/CO = 1, which is essential, among other things, for the production of methanol and wax according to Fischer–Tropsch [5,6]. More recently, Topsoe developed a technology, called ReShift™ that uses a significant amount of CO₂ for making synthesis gas (syngas; H₂ + CO) without the traditional challenge of carbon formation [7].

DRM, being an endothermic reaction, requires high temperatures (higher than 650 °C). Otherwise, the side reactions below may take place [8].

$${\mathrm{CO}}_2+{\mathrm{H}}_2\leftrightarrow \mathrm{CO}+{\mathrm{H}}_2\mathrm{O} \mathrm{Reverse} \mathrm{WGS}$$

(R9)

$$2\mathrm{CO}\leftrightarrow \mathrm{C}+{\mathrm{CO}}_2 \mathrm{Boudouard}$$

(R4)

$${\mathrm{CH}}_4\leftrightarrow \mathrm{C}+2{\mathrm{H}}_2 {\mathrm{CH}}_4 \mathrm{dissociation}$$

(R5)

$$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{CO}+{\mathrm{H}}_2 \mathrm{C} \mathrm{gasification}$$

(R10)

The production of hydrogen H₂ by reforming reactions mainly uses Ni-based catalysts under conditions that prevent their deactivation by the formation of carbon (C) [9,10,11,12]. Carbon formation results mainly from the Boudouard reaction (Reaction (R4)) and the decomposition of CH₄ (Reaction (R5)) [10,13].

One way to overcome the drawback of coking is the strong interaction between the active metal (Ni) and the support that prevents the formation of carbon and the sintering of Ni [14].

For catalysts supported by alumina (Al₂O₃), the spinel phase formed (NiAl₂O₄) allows high conversion of CH₄ with low formation of C during SRM reactions [15]. Likewise, the nickel-alumina spinel catalyst supported on alumina and yttria-stabilized zirconia (NiAl₂O₄-/Al₂O₃-YSZ), developed by our research group, has demonstrated significant resistance to poisoning when compared to standard Ni/Al₂O₃-YSZ formulations in the case of diesel steam reforming [16,17,18].

The addition of promoters in the formulation of the catalyst has also been the subject of several research studies [19,20,21]. Luna et al. [22] studied the effect of K, Ca, Sn and Mn on a Ni/Al₂O₃ catalyst. The authors indicated that the presence of K improved the stability significantly with a slight decrease in activity after 30 h of TOS. Siahvashi et al. [23] studied the DR of propane and showed that the addition of K to the Ni–Mo catalyst supported by Al₂O₃ increased the CO yield with a significant decrease in carbon deposition.

In addition, several studies have shown the beneficial effect of the basicity of supports. Thus, the addition of MgO to a Ni/Al₂O₃ catalyst improved the stability of the catalyst at high temperatures by forming MgAl₂O₄ spinel. The strong adsorption of CO₂ on the basic surface of MgO is considered the main mechanism of carbon formation prevention [24]. The work carried out by Ranjbar et al. [25], on the addition of Ca, shows an increase in the activity of the catalyst and its resistance to carbon deposition.

Rio Tinto Iron and Titanium (RTIT), a wholly-owned subsidiary of Rio Tinto, has been operating for several years at the Tio mine, the largest ilmenite deposit (FeTiO₃) in the world, located 43 km northeast of Havre-Saint-Pierre, Quebec. A world leader in the industry, RTIT has developed and implemented a proprietary process called UpGraded Slag (UGS) on the site of the Sorel-Tracy metallurgical complex (Quebec). UGS produces titanium slag with the highest TiO₂ content (94.5%) on the market from the Ilmenite of the North Shore. However, this process generates a large amount of UGS oxide residue (noted UGSO), sent directly to the tailings deposit to end up in the landfill. Concerned by the accumulation of large quantities of this residue and the importance of its footprint, RTIT has for several years made significant efforts to find other solutions. Avoiding depositing these UGSOs in landfill tailings ponds by upgrading them has become a major and urgent challenge.

This waste, whose chemical composition has been presented in our previous paper [26] has been considered an excellent nickel-based catalyst support candidate. Indeed, UGSO already contains oxides recognized for their ability to avoid carbon formation, the main cause of these catalysts’ deactivation.

In our previous papers [26,27,28], we have developed a simple and easy solid-state method of preparing a new generation of Ni-UGSO catalyst using UGSO as a support and Ni as the active metal.

The first results obtained, when applying the Ni-UGSO to DRM, show good catalytic performances with no carbon deposit [26].

The objective of this work consists of, on the one hand, an optimization of the DRM, and on the other hand, the study of the catalytic activity of this new Ni-UGSO catalyst in other types of CH₄ reforming, namely SRM, mixed (dry and steam) methane reforming and its sulfur-poisoning resistance. In this work, Ni-UGSO BT designates the fresh catalyst before treatment reforming (BT) and Ni-UGSO AT designates the catalyst after treatment reforming.

2. Results

2.1. Characterization of Fresh Catalysts Ni-UGSO BT

As presented in our previous article [26], by ICP-MS analysis, the Ni-UGSO BT catalyst shows a Ni content of 13.9 ± 1.6 wt%; BET analysis gave a specific surface area of 3.94 ± 0.26 m²/g and SEM analyses have shown that the catalyst particles have a lamellar morphology and average size of 168 ± 59 nm.

Compared to oxide residue before testing (UGSO-BT), XRD patterns [26] of the new catalyst sample before catalytic testing (Ni-UGSO-BT) showed, in addition to the spinel peaks present in UGSO-BT, the appearance of new peaks corresponding to Ni-based spinels (NiFe₂O₄ and FeNiAlO₄). XRD patterns also showed three other components at the same 2θ = 37.31, 43.30, 63.40, 75.5 and 79.1° [26]. These peaks corresponded to the remaining MgO, NiO, and solid solution NiO-MgO. These results agree with those of Requies et al. [29], who study the partial oxidation of methane; the diffraction patterns of Ni/MgO catalysts containing 20 and 30 wt% showed the formation of a solid solution between NiO and MgO at 2θ of 37.2, 43.1, 62.6, 75.0 and 78.9°.

2.2. Thermodynamics of DRM and SMR Reactions

CH₄ and CO₂ conversions (DRM), CH₄ and H₂O conversion (SRM), and the thermodynamic equilibrium composition of the products, under a pressure of 1 atm, were calculated using the FactSage 7.3 software (FactSage 7.3; Thermfact/CRCT, Montreal, Canada; www.crct.polymtl.ca and GTT-Technologies, Aachen, Germany, www.gtt-technologies.de; 2019). Full data from the thermodynamic study are provided in Part A of the Supplementary Materials.

The study showed that, at thermodynamic equilibrium, starting at 850 °C, the DRM reaction showed a molar ratio of H₂/CO = 1 and conversion rates of 97% for CH₄ and 95% for CO₂. However, it was only at or above 950 °C that the formation of carbon was fully inhibited.

The SRM reaction showed, at or above 850 °C, a molar ratio of H₂/CO = 3 and CH₄ and H₂O conversion rates of 94% and 95%, respectively. At or above this temperature (850 °C), carbon formation no longer occurred.

Taking all presented data into consideration, it would be more reasonable to study the catalytic performance of the Ni-UGSO catalyst in the temperature range of 800–900 °C, where conversion was practically complete and carbon formation was still marginally thermodynamically possible.

2.3. Ni-UGSO Performance as a Catalyst for DRM

2.3.1. Experimental Evaluation of Catalyst Activity and Stability

In the DRM of our previous work [26], a molar CO₂/CH₄ was heated at 810 °C in the presence of 0.3 g of catalyst. A comparison of the catalytic activity of Ni-UGSO (Q_CO2 = 7.08 mL/min, Q_CH4 = 7.26 mL/min and GHSV = 2867 mL/(h·g)) with UGSO (Q_CO2 = 7.63 mL/min, Q_CH4 = 7.35 mL/min and GHSV = 2996 mL/(h·g)) showed that a maximum CH₄ conversion of 30% was obtained after 30 min of reaction in the presence of UGSO, which gradually decreased and reached 18% after 4 h of reaction. Under the same experimental conditions in the presence of the new Ni-UGSO catalyst, the conversion of CH₄ quickly reached 87% and remains stable during the 4 h of reaction.

Activity and stability of the new Ni-UGSO catalyst were assessed for 7 days of DRM in the same conditions as above at a molar ratio of CO₂/CH₄ = 1.2 (Q_CO2 = 9.30 mL/min and Q_CH4 = 7.76 mL/min and GHSV = 3412 mL/(h·g)) [26]. CH₄ conversion rapidly reached 92% and remains stable over 168 h TOS. The same stability was observed for H₂ and CO yields, which were at around 82% and 87%, respectively, with a molar H₂/CO ratio close to 1, which was closer to the thermodynamic equilibrium. An activity loss of approximately 5% over seven days TOS was observed.

The various techniques for characterizing spent catalysts (XRD, SEM-EDX, XPS, TGA-MS) showed no carbon deposition.

2.3.2. Influence of the Active Phase Content on the Catalytic Activity

This study aimed to optimize the content of the active phase (Ni) necessary to carry out the DRM while obtaining a suitable conversion. Five nickel mass contents were chosen: 3%, 6%, 9%, 10% and 13%. Catalysts were calcined at 900 °C for 12 h. The DRM reaction was carried out at 840 °C with a CO₂/CH₄ ratio of 1.25 (Q_CO2 = 9.62 mL/min and Q_CH4 = 7.71 mL/min) without Ar and a GHSV of 3500 mL/(h·gcat).

Figure 1 shows the evolution of CH₄ conversion along the reforming tests. Part B of the supplementary information presents figures of CO₂ conversions (Figure S6, Supplementary Materials), H₂ yield (Figure S7, Supplementary Materials) and the H₂/CO ratio (Figure S8, Supplementary Materials).

At 3% Ni, the catalyst had the lowest catalytic activity compared to the others because the CH₄ and CO₂ conversions did not exceed 50% and 60%. The same applied to the H₂ yield, which was in the order of 65%. The best performance was obtained with the 10% Ni catalyst; it was equivalent to the performance of the 13% Ni catalyst. The data obtained from H₂/CO (0.5) and X-CO₂/X-CH₄ (1.3) conversion rates indicated the significant role of the reverse-water-gas-shift (RWGS) reaction. However, these two ratios tended towards thermodynamic values (H₂/CO = 1, X-CO₂/X-CH₄ = 1) as the Ni content increased.

These results corroborated those obtained from the XRD analyses (Figure 2), where the intensity of the peaks attributed to different species of Ni oxides (spinel NiFe₂O₄, FeNiAlO₄ and solid solution NiO-MgO (red arrow) was a function of the Ni content.

Average catalyst surface BET as a function of Ni-loading (

Table 1. Average catalyst surface BET as function of Ni-loading.

Ni (wt%)	3	6	9	10	13
SBET (m2/g)	5.4 ± 0.5	5.0 ± 0.2	4.5 ± 0.2	4.4 ± 0.4	3.8 ± 0.4

) shows that the BET decreased slightly as a function of Ni-loading. This could be due to some agglomeration of the particles. The same was also observed on the UGSO support (without Ni), whose surface area decreased from 38 m²/g to 6 m²/g after calcination at 900 °C for 12 h.

Figure 3 below shows SEM photos of the Ni-UGSO catalyst as a function of Ni-loading. These figures show that, whatever the Ni content, the particles kept their lamellar shape.

The mapping study shows, in all cases, a good dispersion of the Ni (Figure S9, Supplementary Materials).

2.3.3. Influence of Catalyst Calcination Time

Three catalysts were obtained by calcination at 900 °C for 1 h, 3 h and 12 h. The catalytic performances of these three solids was studied with 0.3 g of Ni-UGSO catalyst at 840 °C using a CO₂/CH₄ molar ratio = 1.2 (Q_CO2 = 9.75 mL/min and Q_CH4 = 8.20 mL/min) without Ar. The results obtained, presented in Figure 4, showed that the performances of the catalyst calcined for 3 h at 900 °C was similar to that of the catalyst treated for 12 h at the same temperature. Moreover, the catalyst with 1 h of heat treatment at 900 °C gave the best performance. Consequently, 1 h of calcination would be sufficient to obtain a catalyst giving CH₄ conversions exceeding 95%.

Examination of the XRDs of these catalysts (Figure S10, Supplementary Materials) indicated that the longer the calcination time, the more the peaks at 2θ = 43° and 63° (two of the four representative peaks of the catalyst) became sharper. These indications and those of the specific surfaces presented in

Table 2. Average catalyst surface BET as function of calcination time of Ni-UGSO catalyst with 13% Ni (wt%).

Cal. Time (h)	1	3	12
SBET (m2/g)	5.0 ± 0.2	4.7 ± 0.2	3.8 ± 0.4

went in the same direction, thus confirming the effect of prolonged exposure of the catalyst to the heat treatment on particle size. The best performances obtained by the catalyst calcined for 1 h was, therefore, related to its higher specific surface area compared to the other two. This is an important result because it is related to the energy consumption during the eventual commercial manufacturing of the catalyst.

2.3.4. Representativeness of UGSO Mining Residue Lots

A second batch (L2) of UGSO was compared with the first (L1) to evaluate the impact of a variation of the UGSO composition on the Ni-UGSO catalyst performance.

As shown in

Table 3. ICP-MS analysis of two batches of UGSO (wt%).

	Fe	Mg	Al3	Ca	Mn	V5	Ti2	Cr	Zn
UGSO-L1	31.11	17.98	5.23	1.28	1.68	1.46	0.49	0.47	0.01
UGSO-L2	34.04	21.09	6.84	1.58	1.82	1.71	0.52	0.56	0.01
Variation	9.44%	17.32%	30.70%	24.13%	8.24%	16.74%	5.38%	18.50%	0.00%

, the ICP-MS analysis shows that the constitutive elements of the residue remain unchanged despite the variation in their contents. The balance is oxygen.

XRD patterns (Figure S11, Supplementary Materials) showed that, although the structures of UGSO-L1 and UGSO-L2 seemed different, UGSO-L2 was identical to that of the first batch calcined at 900 °C/12 h (UGSO-L1_Cal.). This suggests that the second batch may have undergone additional calcination before its disposal in the RTIT Landfill [30]. It should be noted that only the L1 batch was slightly different in terms of phases. All other batches received subsequently had the same phases, and their calcination did not bring any change to the material, as shown by the calcination carried out on L3 (Figure S12, Supplementary Materials). Otherwise, despite the slight difference in composition, the catalytic activity of the different batches used in the reforming tests showed similar catalytic activities.

The corresponding Ni-UGSO-L1 and Ni-UGSO-L2 catalysts, both at 13% Ni (wt%), were prepared according to the same protocol of improved solid-state reaction and calcined at 900 °C for 1 h.

The XRDs of the two catalysts (Figure S13, Supplementary Materials) showed that both catalysts obtained by adding Ni as active metal had the same structure (a slight difference was observed for the peak at 2θ = 63.45°).

The reforming reactions were carried out at 840 °C on 0.3 g of Ni-UGSO-L1 or L2 calcined for 1 h using a CO₂/CH₄ molar ratio = 1.2 (Q_CO2 = 9.45 mL/min and Q_CH4 = 7.68 mL/min) without Ar and a GHSV = 3400 mL/(h·gcat). The results obtained (Figure 5) showed that the two catalysts had the same CH₄ conversions. Despite a slight difference of the composition, the Ni-UGSO catalysts obtained from these two batches of tested residues demonstrated equivalent performance during DRM.

Comparison of Ni-UGSO-L1 (batch 1) and Ni-UGSO-L2 (batch 2) calcined at 900 °C for 1 h.

2.3.5. Ni-UGSO Sulfur Poisoning Resistance

Ni-UGSO sulfur poisoning resistance was assessed for 5 h of DRM with a molar ratio of CO₂/CH₄ = 0.96 at 810 °C under 1.5 ppm, 5.5 ppm and 12 ppm of H₂S.

Up to 5.5 ppm, the catalyst showed no deactivation. A slight deactivation (3%) was observed at 12 ppm and remained stable throughout the 1 h exposure of the catalyst to H₂S at the reported concentrations (Figure 6).

As shown in the XRD of used catalysts (Figure S14, Supplementary Materials), the amount of S was so low that the peaks corresponding to Ni-S were not detected. However, a more in-depth study of the resistance of Ni-UGSO to sulfides was carried out, by our group research, at higher concentrations (275 ppm of H₂S) on dry autothermal ethane reforming [31]. XRD, BET, SEM and TGA analysis are used. XRD showed the formation of new phases such as NiFe, NiS and FeS. In TGA analysis, weight losses observed were attributed to the endothermal decomposition of NiSO₄, which might be formed by oxidation of the NixSy species; this decomposition was described by the following equation: NiSO₄(s) = NiO(s) + SO₂(g) + ½ O₂(g).

2.4. Ni-UGSO Performance as a Catalyst for SRM

SRM on Ni-based catalysts is currently the only industrially used method for hydrogen production [32]. S/C ratios more than 3 [32] are used at the expense of energy to avoid catalyst deactivation by carbon deposition. The excess of steam is beneficial in gasifying a portion of the carbonaceous deposits on the catalyst surface and, therefore, improving the carbon gasification [32,33]. Nevertheless, the higher amounts of H₂O also lead to an enrichment of the product in H₂ due to the enhancing the WGS reaction.

Ni-UGSO stability in SRM was evaluated for seven days at 900 °C with a molar ratio of H₂O/CH₄ = 1.7 (Q_H2O = 13.94 mL/min, Q_CH4 = 8.20 mL/min and Q_Ar = 17.15 mL/min), well below industrial ratios.

The results obtained showed that, during the 7 days of reaction, CH₄ conversion remained stable at around 98%, with a molar H₂/CO ratio in the products close to 3. This is a clear indication that, at these experimental conditions, the reaction reached thermodynamic equilibrium (Figure 7).

It is important to note that no trace of carbon was detected on the spent catalyst, as shown by the XRD analysis (Figure S15, Supplementary Materials) and SEM-EDX analyses (Figure 8). It should also be noted that the small peak of carbon observed in the EDX of the spent catalyst (Ni-UGSO AT) was identical to that of the fresh catalyst (Ni-UGSO-BT). As mentioned in our previous work [26], this carbon peak is attributed to the CaCO₃ carbonates, the remaining carbon traces used during the UGS process reduction step. It can also be attributed to atmospheric CO₂ absorbed or adsorbed. However, the SEM photos (Figure 8) showed the appearance of spherical particles in the spent catalyst compared to the fresh catalyst.

The mapping analysis of the fresh catalyst (Figure 9) and used catalyst (Figure 10) showed that the former has a homogeneous morphology as well as a good dispersion of the active metal (Ni) as well as those of the support (Mg, Al and Fe). After 7 days of reaction, the spent catalyst (Figure 10) showed the appearance of certain spherical particles consisting mainly of Ni and Fe. As described in the interpretation proposed during the DRM, these spherical particles were attributed to Ni–Fe alloys formed by reduction during the DRM.

2.5. Ni-UGSO Performance as a Catalyst for Mixed Methane Dry and Steam Reforming

For this study, Ni-UGSO stability was validated in DRM using a Q_CO2 = 7.29 mL/min, Q_CH4 = 7.55 mL/min and Q_H2O = 1.04 mL/min) without Ar (H₂O/CH₄ = 0.14, CO₂/_CH4 = 0.97 and CH₄/(CO₂ + H₂O) = 0.91) for 74 h. Three reaction temperature levels (828 °C, 847 °C and 896 °C) were used over these 74 h TOS.

CH₄ conversion remained stable at each temperature level: 80% at 828 °C (26 h), 85% at 847 °C (24 h) and 95% at 896 °C (24 h) (Figure 11).

Referring to Figure S16 (Supplementary Materials), XRD of the Ni-UGSO catalyst used during 74 h of reaction showed that it had the same structure as that used for 4 h under the same conditions without any carbon deposition as illustrated by the SEM-EDX analyses (Figure S17, Supplementary Materials).

The results were the same order of magnitude as those obtained by Nakhaei et al. [34] in the mixed CO₂ and steam reforming of methane. Experimental conditions were close to ours except for the temperature, which was slightly lower, and the GHSV, which was higher: 750 °C, CH₄/(CO₂ + H₂O) = 0.87, CH₄/CO₂ = 1 and CO₂/H₂O = 6.4 and a GHSV = 20 L/(h·g). Different Ni/MgO/α-Al₂O₃ catalysts were prepared with Mg 1 wt% and Ni 1 to 5 wt%. Unlike our catalyst, which was used as it was produced, the Nakhaei et al. [34] catalyst underwent a pre-reduction. Results showed an increase of CH₄ conversion from 54% to 74% upon increasing Ni loading from 1% to 5%. The authors indicated that such performance is attributed to solid solution NiO–MgO and that the reactions reach conversions and yields close to thermodynamic equilibrium.

By studying a mixed DRM and SRM, at a ratio (CO₂ + H₂O/CH₄) closer to 1, at atmospheric pressure and at temperature greater than or equal to 800 °C, Jabbour et al. [35] recently discovered that the use of MgO (or CaO) basic promoters in Ni 5% Mg 5% Al₂O₃ or Ni 5% Ca 5% Al₂O₃ catalysts provided remarkable activity, stability and selectivity in syngas. Compared to unpromoted Ni 5% and Ni 10% Al₂O₃, after 40 h of reaction, a significant reduction in deposited carbon was observed with improvement in the reactivity and purity of the synthesis gases obtained.

Similarly, Koo et al. [24] and Mehr et al. [36] led mixed steam and dry reforming of methane without and in the presence of MgO-assisted, Ni-impregnated on Al₂O₃ catalysts. The authors deduced that the addition of MgO had a positive effect on the eliminating carbon deposition and on reactivity and selectivity.

3. Discussion

The new Ni-UGSO catalyst, resulting from upgrading of a mining residue with a negative UGSO value, has demonstrated catalytic performance at least equal to that of the various homologous nickel-based catalysts described in the literature. Besides the catalytic performance in DRM, SRM and MRM occurring near thermodynamic equilibrium, no carbon deposit was detected, especially as it demonstrated good resistance to sulfides (low quantity).

All of these promising results are attributed to the different constituent elements of the UGSO mining residue despite not being catalytically active by themselves (Fe, Mg, Al, Ca, Mn, V, Ti, Cr, Na, Si, K, P, Zr and Zn oxides and spinels). Indeed, the addition of Ni allows different interactions to be triggered synergistically between the components of UGSO and the active metal (Ni). It should be remembered that most of these elements, used as supports or promoters, have been the subject of several recommendations from studies aimed at reducing or eliminating the formation of coke on Ni-based catalysts. As demonstrated in our previous work [28], the reducing medium during the DRM causes a reduction of NiFe₂O₄ spinel compound into FeO, NiO and metallic Ni, Fe and some other alloys such as FeNi, FeNi₃ and Fe₃Ni₂. As described by Benrabaa et al. [37,38], the fact that Ni is well dispersed in this alloy minimizes the formation of carbon deposit as much as possible.

Regarding the other elements constituting the residue used as a promoter, our interest has focused on magnesium oxide (MgO) without neglecting the role of other elements whose effect is described in several articles in the literature.

In fact, several studies have demonstrated the positive effects of MgO as promoters with significant Lewis basicity and as an oxide easily forming a solid solution with Ni oxide (MgO–NiO). On the one hand, the presence of basic Lewis sites (O²⁻) [39] will reinforce the chemisorption of CO₂ on these sites, and the adsorbed CO₂ reacts with carbon. This reaction shifts the equilibrium of the reaction from Boudouard to CO (CO₂ + C → 2CO), promoting the coke elimination reaction, which considerably reduces the deposition of surface carbon [40]. On the other hand, Son et al. [41] reported that, with its oxygen storage capacity, MgO could remove carbon formed on the surface of a catalyst by its oxidation with liberated oxygen. The high nickel activity and carbon resistance are also attributed to the formation of a solid solution of NiO in MgO.

Wang et al. [42] explain that the solid NiO–MgO solution could have two main phases: A solid NiO–MgO solution with a high Ni content in the outer layer and a solid NiO–MgO solution with a very low Ni content in the mass. Thus, Ni particles can diffuse from the outer layer to the deeper layer to form a more stable structure at high temperatures, which prevents the sintering of nickel particles for high-temperature reactions. In addition, Hu and Ruckenstein [39] indicate that, due to the strong interaction between NiO and MgO, the coalescence or aggregation of Ni, which stimulates carbon formation, is inhibited. This strong interaction between NiO and MgO could also weaken the donor character of Ni, thereby preventing disproportionation of CO. Bradford et al. [43] have shown that the Ni/MgO catalyst is both active and stable during 44 h of dry reforming of CH₄. Such performance and resistance to coke deposition were explained by the formation of a solid solution of MgO–NiO due to the strong dissolution of NiO in MgO. This partially reducible solution is reported to stabilize metal Ni and enhance resistance to carbon deposition. Likewise, the good catalytic performance of the Ni/MgAl₂O₄ spinel, obtained by the addition of MgO to the Ni/Al₂O₃ system, is attributed to a greater CO₂ adsorption capacity due to the increase in the density of the basic Lewis sites of on the surface of the catalyst [25,44].

4. Materials and Methods

4.1. Materials

4.1.1. Constitutive Elements of the UGSO Residue

Table 3 shows the constitutive elements of the UGSO residue obtained by ICP-MS analysis. Fe, Mg and Al are the main elements.

4.1.2. Particle Size Distribution

The first raw UGSO (UGSO-L1) received from our industrial partner was milled in a mortar and sieved to collect the fraction corresponding to sizes below 53 µm (smaller sieve); this fraction was chosen for the preparation of the catalytic formulations

Table 4. Particle size distribution of UGSO-L1.

	d (0.1)	d (0.2)	d (0.5)	d (0.8)	d (0.9)
UGSO (µm)	1.75	6.51	25.85	48.07	61.42

shows the particle size distribution of this fraction of UGSO.

4.2. Ni-UGSO Preparation via Improved Solid-State Reaction

Ni-UGSO was prepared following the improved solid-state reaction developed in a previous work [26,27,28]. To study the effect of the content of Ni, 3, 6, 9, 10 and 13 wt% were added.

Table 5. Particle size distribution of Ni-UGSO (13 wt%).

	d (0.1)	d (0.2)	d (0.5)	d (0.8)	d (0.9)
Ni-UGSO (µm)	1.24	3.71	20.34	46.83	56.97

shows particle size distribution and Ni-UGSO (13 wt%) compared to UGSO.

4.3. Catalysts Characterization

Our previous work [26] describes various characterization techniques used to analyze the new Ni-UGSO catalyst, such as X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), temperature-programmed reduction (TPR), X-ray photoemission spectrometry (XPS), thermogravimetric analysis (TGA) coupled with mass spectroscopy (MS) and BET specific surface.

4.4. Use of Ni-UGSO as a Catalyst

The produced Ni-UGSO was used as a catalyst for DRM, SRM and mixed methane reforming (MRM).

The experiments were conducted under the same conditions as those described in a previous work [27] at atmospheric pressure in a differential fixed-bed catalytic reactor (Figure 12) using 0.3–0.5 g of Ni-UGSO. Total flow rate and space velocity were 15 mL/min and 3000 mL/(h·gcat) respectively.

Gases were fed from commercial gas cylinders (supplied by Praxair, Sherbrooke, Québec, Canada): CH₄ (99%), CO₂ (99%) and Ar (99%). Flow rates of CO₂ and CH₄ were adjusted by the AALBORG-type mass flow controller (Model GFC17) (supplied by AALBORG, Orangeburg, NY, USA). Flow rates of the reactants and the reaction products were also measured by a bubble meter. In the case of SRM, a mixture of argon and methane is saturated in the bubbler, and the desired amount of water is regulated using the water temperature control. At saturation at 73 °C, the partial pressure of water vapour is 354.45 hPa. For 32% of CH₄ in Ar, the CH₄ partial pressure is 212.97 hPa corresponding to a H₂O/CH₄ ratio = 1.7. The effluents were analyzed using a gas chromatograph (Varian CP-3800) (supplied by Varian, Inc, Walnut Creek, CA, USA) equipped with Hayesep and Molsieve columns and with thermal conductivity (TCD) and flame ionization (FID) detectors. Helium and nitrogen were used as carrier gases, and hydrogen and air were used for the detector flame. The flow rates of gases are: 18.8 mL/min (He), 21.2 mL/min (N₂), 30 mL/min (H₂) and 300 mL/min (air). The columns and the detector were heated to 175 °C at a rate of 20 °C/min. H₂, CO, CH₄, and CO₂ are the main gases detected. Calibration was performed using gas mixture standards at different concentrations. The concentrations of reactants were added to the experimental conditions of each type of test.

4.5. Reaction Metrics

Conversion of CH₄ was calculated by Equation (1). H₂ and CO yields as defined by Equations (2) and (3) respectively:

$$X_i\left(\%\right)=\frac{F_{i, in}-F_{i, out}}{F_{i, in}}\times 100$$

(1)

$$Y_{H_2}\left(\%\right)=\frac{F_{H_2}}{2\times F_{C{H}_{4, in}}}\times 100$$

(2)

$$Y_{CO}\left(\%\right)=\frac{F_{CO}}{F_{C{H}_{4, in}}+F_{C{O}_{2, in}}}\times 100$$

(3)

where F_i,in or F_i,out is the flow rate of each component in the feed or effluent.

Concerning the experimental errors of the various reforming results, we proceeded, for brevity and figure-readability reasons, to the calculation of the standard deviations of the material balances (atomic: C, H, O) instead of error bars: C (1.4%), O (3.9%) and H (6.3%).

As shown in Table 6, the standard deviation was calculated from the average of the averages of each study below (Table 7):

•

influence of the active phase content on the catalytic activity;

•

influence of catalyst calcination time;

•

representativeness of UGSO mining residue lots;

•

Ni-UGSO sulfur poisoning resistance;

•

Ni-UGSO performance as a catalyst for steam methane reforming (SRM);

•

Ni-UGSO performance as a catalyst for mixed methane dry and steam reforming (MRM).

Table 6. Error mean and standard deviation.

	C	O	H
Error mean	2.59%	4.47%	3.99%
Standard deviation	1.35%	3.93%	6.28%

Table 6. Error mean and standard deviation.

	C	O	H
Error mean	2.59%	4.47%	3.99%
Standard deviation	1.35%	3.93%	6.28%

The following table shows the error mean and standard deviation.

Table 7. Average errors on atomic carbon, hydrogen and oxygen for of each study.

	C	O	H
Effect of Ni content	3.58%	8.03%	8.81%
Effect of calcination time	2.55%	3.54%	7.87%
Ni-UGSO sulfur poisoning resistance	2.54%	4.93%	−7.84%
Representativeness of UGSO mining residue lots	−0.18%	−1.35%	9.31%
Steam methane reforming (SMR)	4.07%	10.37%	6.63%
Mixed methane dry and steam reforming	2.97%	1.31%	−0.84%

Table 7. Average errors on atomic carbon, hydrogen and oxygen for of each study.

	C	O	H
Effect of Ni content	3.58%	8.03%	8.81%
Effect of calcination time	2.55%	3.54%	7.87%
Ni-UGSO sulfur poisoning resistance	2.54%	4.93%	−7.84%
Representativeness of UGSO mining residue lots	−0.18%	−1.35%	9.31%
Steam methane reforming (SMR)	4.07%	10.37%	6.63%
Mixed methane dry and steam reforming	2.97%	1.31%	−0.84%

This means that our confidence interval for carbon was 2.59 ± 1.35.

5. Conclusions

In this work, UGSO mining waste was doped with Ni using a recently published improved solid-state reaction protocol; the resulting spinelized structure proved to be a robust and highly active Ni-UGSO reforming catalyst. DRM, SRM, MRM and sulfur poisoning resistance were chosen to test the catalytic performance and robustness of this new catalyst. The DRM and SRM reactions demonstrated an excellent hydrocarbon conversion and hydrogen yield and the H₂/CO ratios obtained in the product were close to 1 (for DRM) and close to 3 (for SRM); these ratios are nearly those expected for thermodynamic equilibrium in the experimental conditions tested.

As mentioned before, the constituent elements of the UGSO had a positive effect on the performance of the catalyst, allowing a good dispersion and interaction of Ni in the UGSO. As in previously published works in this area, our results also show that, in addition to the effect of certain Ni_xFe_y alloys, the Ni inside the spinel structure leads to a high stability and a high dispersion of the particles of the active metal, limiting the growth of the Ni particles preventing the formation of carbon while maintaining a high catalytic activity. Likewise, the basic character of components like MgO acts as catalyst promoters by inhibiting the carbon formation on the catalyst surface and stimulating the solid carbon gasification. Furthermore, the protection of the metallic Ni (which is formed during reduction) by the solid solution Ni/NiO/MgO could explain the activity and the stability of the catalyst.

In conclusion, of these four types of investigation in this work, CH₄ conversion in SRM remained stable at around 98% during the 168 h TOS of reaction under 1 bar at 900 °C at a molar H₂O/CH₄ = 1.7. Likewise, in DRM, CH₄ conversion rapidly reaches 92% and remains stable over 168 h TOS under 1 bar at 810 °C and at molar CO₂/CH₄ = 1.2. For mixed CH₄ reforming using a molar ratio of H₂O/CH₄ = 0.15 and CO₂/CH₄ = 0.97 for 74 h under 1 bar and three reaction temperature levels (828 °C, 847 °C and 896 °C), CH₄ conversion remains stable at each temperature level: 80% at 828 °C (26 h), 85% at 847 °C (24 h) and 95% at 896 °C (24 h). Five levels of Ni mass contents (3%, 6%, 9%, 10% and 13%) in the Ni-UGSO formulation have been compared at 840 °C and a molar CO₂/CH₄ of 1.25. The best performances was observed with the 10% w/w Ni catalyst. Statistically similar performance was obtained with the 13% w/w Ni catalyst. The catalysts Ni-UGSO calcined at 900 °C for 3 h and 12 h give the same catalytic performances at 840 °C with a CO₂/CH₄ = 1.2. Even though the composition of the two batches of UGSO used to produce the Ni-UGSO catalytic formulation are slightly different, both Ni-UGSO catalysts demonstrated equivalent performance during DRM at 840 °C and molar CO₂/CH₄ = 1.2. The catalyst showed no deactivation up to 5.5 ppm of H₂S. A slight deactivation (3%) was observed at 12 ppm and remained stable throughout the 1 h exposure of the catalyst to H₂S.