Synthesis, Characterization, and Catalytic Activity of Nickel Sulfided Catalysts for the Dehydrogenation of Propane: Effect of Sulfiding Agent and Sulfidation Temperature

Abstract

The effect of sulfiding agent and sulfidation temperature on nickel catalysts supported on MgAl₂O₄ were investigated for propane dehydrogenation. The catalysts were prepared by reduction of NiO/MgAl₂O₄, followed by sulfidation using (NH₄)₂SO₄ (S1), (NH₄)₂S (S2), and DMSO (S3) as sulfiding agents. The catalysts were sulfided at 200 °C, 400 °C, and 550 °C to form Ni/MgAl₂O₄-Sx-y, where x and y represent the sulfiding agent and sulfidation temperature, respectively. Physiochemical properties of the catalysts were characterized by XRD, BET, SEM, TEM, and TGA to investigate the type of nickel-sulfur species, surface area, morphology, particle size, and stability of the catalysts. Structural and textural properties revealed that the anion present on the sulfiding agent as well as the sulfidation temperature affect both the type and the strength of the Ni-S species. For the S1 catalysts, the SO₄²⁻ ion interacted with the support to form MgSO₄, while the S²⁻ ion on the S2 and S3 catalysts was responsible for the formation of the Ni₃S₂ phase. The sulfidation temperature contributed to the %S present on each catalyst. Although the catalysts sulfided by S3 contained the least %S, Ni/MgAl₂O₄-S3-550 displayed the best catalytic performance as a result of the higher particle dispersion and stronger Ni-S interaction compared to S1 and S2 catalysts.

1. Introduction

With an increasing demand for olefins, processes for alkane dehydrogenation have gained considerable attention. The dehydrogenation of light alkanes serves as a fundamental reaction for the petrochemical industry and refining, since it is a selective process used to produce short-chain alkenes [1,2,3,4,5,6,7]. In recent years, metal sulfided catalysts have gained considerable attention in the academic and patent literature [8,9,10,11]. For example, sulfided nickel catalysts have been identified as an alternative type of catalyst in dehydrogenation reactions. Wang et al. found that the introduction of sulfur on nickel oxide catalysts decreased coking and increased the dehydrogenation activity of the catalysts significantly [4,8]. Transition metal sulfides are usually prepared by direct sulfiding of a metal salt or by decomposition of a sulfur-containing precursor [12].

H₂S is decomposed to H₂ and S²⁻, which is the source of sulfur on a range of metal components. However, H₂S is highly toxic, and a high corrosion resistance is required for industrial equipment, leading to a high investment cost [13,14,15,16]. Various alternative sulfiding agents have been used in order to introduce sulfur into metal sulfided catalysts to improve catalytic activity for dehydrogenation reactions [4,17,18].

Wang et al. prepared sulfur modified catalysts through the addition (NH₄)₂SO₄ of Ni/MgAl₂O₄ and Mo/MgAl₂O₄ catalysts for the dehydrogenation of isobutane [4,19]. The adsorption of various anions, such as sulfate ions onto oxides, have been investigated as a means of improving their catalytic activity [20]. The addition of relatively safe ammonium sulfate to metal catalysts has been explored in previous research studies [4,19,21,22].

Liu and co-workers used (NH₄)₂S, as a sulfiding agent, to sulfide a CoMo/Al₂O₃ catalyst. They speculated that due to the high sulfidation activity of S²⁻ the transition metals of the catalyst would react with (NH₄)₂S, leading to the full sulfidation of the active components [17]. With this reasoning, (NH₄)₂S was used as a sulfiding agent in their study through a simple sulfiding procedure in low temperature and ambient pressure conditions.

Additionally, DMSO is a sulfur containing organic solvent used as a sulfiding agent for refineries due to its ease of handling and relatively fewer toxic properties [23,24]. The Ni/Al₂O₃ catalysts were sulfided with DMSO by Resasco for the dehydrogenation of isobutane [18]. Three different procedures have been reported for the conventional sulfiding of oxidic catalyst precursors: (1) reduction followed by sulfiding of the reduced catalyst, (2) simultaneous reduction and sulfiding, and (3) sulfiding followed by reduction. The first two routines are more typically applied, where H₂ is used for the in-situ reduction [25,26].

Yu et al. studied the intrinsic effect of various anion precursors on nickel catalysts. It was reported that the properties of the anions might affect the dispersion of particles during drying, calcination, and reduction of the catalyst. It has been suggested that the anion might be involved in modifying catalytic behaviors of the catalyst. This could be due to the different anions, which have different properties and distinct decomposition temperatures. The anion might interact with the nickel cation or the support in various ways, which could potentially affect the dispersion of the nickel species, reduction of the nickel oxide, and the interaction of the nickel with the support. Therefore, a fundamental understanding about the effects of precursor anions on the catalyst is required for the development of effective catalysts [27].

Previous studies demonstrate that catalyst textural properties and catalytic performance were greatly influenced by treatment conditions such as sulfidation temperature on catalysts for hydrodesulfurization reactions [28,29,30]. Jiang et al. found that Mo-based catalysts sulfidation depended on temperature. They reported that lower sulfidation temperatures (<500 °C) had little effect on the catalyst morphology or catalytic activity and catalytic stability increased when sulfidation temperature was >500 °C [28]. Farag and co-workers reported complete sulfidation to a highly crystalline MoS₂-2H structure at 800 °C and sulfidation at 400 °C produced an amorphous MoS₂ state [29]. Therefore, it is clearly observed from literature that temperature can influence the degree of sulfidation, catalyst morphology, and catalytic activity. However, to the best of our knowledge, not much attention has been paid to the sulfidation temperature on dehydrogenation catalysts.

The investigation of the effect of these three sulfiding agents, containing different anions, namely (NH₄)₂SO₄ (S1), (NH₄)₂S (S2), and DMSO (S3) for the sulfidation of Ni/MgAl₂O₄ catalyst, which has not been reported on before. Moreover, the decomposition of the sulfiding agent is important to determine which sulfiding agent is most effective in the sulfidation of the nickel catalyst.

Herein, we investigate the effect of the sulfiding agent and sulfidation temperature on the morphology, textural properties, and catalytic activity of the sulfided catalysts. Initially, NiO/MgAl₂O₄ catalysts were reduced to Ni/MgAl₂O₄ before being sulfided to Ni/MgAl₂O₄-Sx-y, where x represents the sulfiding agent and y represents the temperature at which sulfidation took place.

2. Results

2.1. X-ray Diffraction

The X-ray diffraction (XRD) pattern of Ni/MgAl₂O₄ is shown in Figure 1a below. The reflections corresponding to the MgAl₂O₄ phase occur at 19°, 31°, 37°, 45°, 56°, 59°, 65°, and 77° 2θ angles. The diffraction peaks corresponding to MgAl₂O₄ can be indexed to the cubic spinel structure of the MgAl₂O₄ support [31]. The possibility of intermediate products has not been detected in the support, as indicated by the absence of MgO and Al₂O₃ peaks, which consequently confirms the single phase of the as-synthesized MgAl₂O₄ support. The peak present at 52° 2θ is associated with the reduced Ni metal on the catalyst. The peaks present in the 2θ regions 43° and 63° correspond to NiO, which was not fully reduced during the reduction step of the catalyst. According to MATCH software, using semi-quantitative phase analysis, the % of NiO still present on the catalyst was 6.4%. The Scherrer equation was applied to calculate the particle size at 52° 2θ of the unmodified Ni and was equal to 35 nm.

The XRD patterns of Ni/MgAl₂O₄ sulfided with (NH₄)₂SO₄ (S1) to form Ni/MgAl₂O₄-S1-y, where y is the temperature of sulfidation, as shown in Figure 1b. At 200 °C, the XRD pattern of Ni/MgAl₂O₄-S1-200 matched that of (NH₄)₂SO₄, with traces of reduced Ni and the MgAl₂O₄ support. At sulfidation temperatures of 400 °C and 550 °C, the XRD pattern of Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 contain peaks corresponding to MgSO₄, indicating that the SO₄²⁻ anion from the sulfiding agent has a stronger interaction with the support than the Ni metal on the catalyst. As seen in the XRD patterns of all three catalysts that were sulfided with (NH₄)₂SO₄, the presence of nickel-sulfur species was absent, or the intensity was insufficient to be detected by XRD analysis. The % of unreacted Ni with sulfur is 2%, 11%, and 5% for Ni/MgAl₂O₄-S1-200, Ni/MgAl₂O₄-S1-400, and Ni/MgAl₂O₄-S1-550 catalysts, detected by MATCH software. This could imply that there is more interaction between sulfur and the MgAl₂O₄ support in Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 catalysts, leading to a higher % of unreacted Ni metal.

The XRD patterns of Ni/MgAl₂O₄ sulfided with (NH₄)₂S (S2) to form Ni/MgAl₂O₄-S2-y, where y is the temperature of sulfidation, is shown in Figure 1c. For the Ni/MgAl₂O₄-S2-200 catalyst, the peaks present in the 2θ regions 30°, 34°, 53°, and 56° correspond to NiS on the catalyst. Studies have shown that nickel-sulfide can exist as several phases. The reaction of nickel and sulfur at temperatures between 177 °C–477 °C under steady state conditions could produce the NiS, Ni₃S₂, Ni₆S₅, and NiS₂ phases.

The formation of a specific nickel-sulfide phase depends on various factors, such as the role of starting precursors and temperature [32,33]. The Ni/MgAl₂O₄-S2-400 and Ni/MgAl₂O₄-S2-550 catalysts contained Ni₃S₂ phase after sulfidation. It can be noticed that when using the (NH₄)₂S precursor nickel sulfides are easily formed at all temperatures compared to the (NH₄)₂SO₄ sulfiding agent. Furthermore, an increase in sulfidation temperature resulted in the formation of a different nickel-sulfur species compared to the lower temperature of 200 °C, indicating that the sulfidation temperature could influence the type of nickel-sulfur species that form.

It was noted that as the sulfidation temperature is increased, the intensity of the NiO peak decreased, and there is an increase in the metal sulfided phases (Ni₃S₂ and NiS) when using (NH₄)₂S as the sulfiding agent.

The % of unreacted Ni according to MATCH is 1% and 0.5% for Ni/MgAl₂O₄-S2-200 and Ni/MgAl₂O₄-S2-400 catalysts. This could suggest that (NH₄)₂S is an effective sulfiding agent, as complete sulfidation can be obtained at low temperatures, which is probably due to the low decomposition temperature of (NH₄)₂S and the presence of the S²⁻ ion on (NH₄)₂S. The S²⁻ ion is known to be involved in the formation of nickel-sulfide species, which is the active phase for dehydrogenation [4]. The crystallite size of nickel-sulfur particles was calculated using the Scherrer equation. The peak at 53° was used to calculate the crystallite size of NiS and was equal to 9 nm.

The peaks present in the 2θ regions 21°, 50°, and 55° correspond to Ni₃S₂ on the Ni/MgAl₂O₄-S2-400 and Ni/MgAl₂O₄-S2-550 catalysts. The crystallite size of Ni₃S₂ on Ni/MgAl₂O₄-S2-400 and Ni/MgAl₂O₄-S2-550 was calculated using the peak at 55° and equaled 11 nm and 12 nm, respectively. The crystallite sizes of nickel sulfides are smaller compared to the unsulfided nickel. This indicates that the sulfur acts as a structural promoter and reduces the particle size of nickel. However, the crystallite sizes, although still smaller than pure nickel, do increase in size with the increase in sulfidation temperature as expected.

The XRD patterns of the catalysts sulfided with DMSO (S3) to form Ni/MgAl₂O₄-S3-y, where y is the temperature of sulfidation, is shown in Figure 1d below. The catalysts sulfided at the three temperatures all contained peaks corresponding to the presence of a Ni-S_x species in the form of the Ni₃S₂ phase. In addition, the presence of NiO was also observed, which seems to be slightly more prevalent compared to catalysts sulfided with agents S1 and S2. According to MATCH, the % NiO present on Ni/MgAl₂O₄-S3-200, Ni/MgAl₂O₄-S3-400, and Ni/MgAl₂O₄-S3-550 is 6.4%, 6%, and 3.7%, respectively. This indicates that not all the metallic nickel was sulfided by DMSO. The % NiO present on the Ni/MgAl₂O₄-S3-200 and Ni/MgAl₂O₄-S3-400 catalysts was also more than the unmodified catalyst.

Furthermore, the EDS data (

Table 1. EDS data showing wt% S on catalysts sulfided at 200 °C, 400 °C, and 550 °C.

Ni/MgAl2O4 Sulfiding Conditions:	200 °C	400 °C	550 °C
(NH4)2SO4 (S1)	33.7	24	12.87
(NH4)2S (S2)	9.42	13.81	18.17
DMSO (S3)	2.55	3.18	5.91

) show that the catalysts sulfided with DMSO contained the least %S compared to the catalysts sulfided with (NH₄)₂SO₄ and (NH₄)₂S as sulfiding agents. This could be due to the loss of sulfur by H₂S gas. The S-O bond of DMSO (chemical formula C₂H₆OS) is weak, thus DMSO can easily be reduced to CH₃SH. With a further increase in temperature CH₃SH bonds will break and some H₂S gas will be produced [34].

It was speculated that some of the oxygen lost from DMSO may oxidize the reduced Ni, hence a combination of the smaller presence of sulfur and availability of oxygen may have led to an increased amount of NiO on the sulfided catalysts. The peak at 2θ 55° was used to calculate the crystallite size of Ni₃S₂ on Ni/MgAl₂O₄-S3-200, Ni/MgAl₂O₄-S3-400, and Ni/MgAl₂O₄-S3-550, which equaled 4.5 nm, 12 nm, and 14 nm, respectively.

The various sulfiding agents, as well as sulfidation temperature, influence the Ni-S species formed, crystallite size, and degree of sulfidation. The sulfur in (NH₄)₂SO₄ (S1) interacts mostly with the support, as indicated by the peaks corresponding to MgSO₄. Under the reaction conditions in our study, the (NH₄)₂SO₄ did not reduce to a sulfide phase as with (NH₄)₂S (S2) and DMSO (S3). For S2 and S3 catalysts, the Ni₃S₂ phase is present at sulfiding temperatures of 400 °C and 550 °C. The crystallite size of Ni₃S₂ increased with an increase in temperature for the S2 sulfiding agent. The formation of NiS at low sulfidation temperature and Ni₃S₂ at high sulfidation temperature could be due to the reduction of the nickel-sulfided species at the higher temperature according to the equations listed below [35].

7NiS + 2e⁻ →Ni₇S₆ + S²⁻

(1)

3Ni₇S₆ + 8e⁻ → 7Ni₃S₂ + 4S²⁻

(2)

These results are consistent with that found in the literature. Increasing temperatures usually cause the sintering of metal particles. The crystallites move over the support and collide to form larger particles [36,37,38]. As the reaction temperature increases, so does the crystallite size of a given material.

The % of unreacted Ni decreased with an increase in temperature for both S2 and S3 sulfiding agents, which could indicate increased decomposition of the sulfiding agents, hence an increase in the nickel-sulfur phase. Evidence given by EDS analysis highlights the increase in wt% S with an increase in sulfidation temperature, implying increased decomposition from low to high temperature conditions. This is consistent with previous studies that show complete sulfidation of catalysts is likely to occur at higher temperatures [28,29,30]. The exception to the trend is S1, which could be due to loosely bound sulfur being lost with an increase in sulfidation temperature.

2.2. Thermogravimetric Analysis

Thermogravimetric analysis (TGA), shown in Figure 2, was performed on S1, S2, and S3 catalysts to determine which sulfiding agent was the best for sulfiding the nickel supported catalyst as well as the strength of the sulfur interaction with the catalyst. As sulfidation temperature increased from 200 °C–550 °C, the sulfur loss from the catalyst decreased, indicating a stronger nickel-sulfur interaction as a consequence of the higher sulfidation temperature. This trend was observed for the S1 and S2 catalysts, while S3 catalysts showed no wt% mass loss, suggesting that sulfur has the strongest interaction with the catalysts sulfided with DMSO.

The TGA profile of Ni/MgAl₂O₄-S1-200 is typical for (NH₄)₂SO₄, with multiple mass loss steps. The wt% loss at the higher temperatures i.e., above 600 °C, was assigned to the decomposition of the sulfate groups on Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550, respectively [20]. The results obtained from TGA correlate with EDS data, which show that Ni/MgAl₂O₄-S1-200 catalyst had the highest sulfur content, with a total mass loss of 75% due to decomposition of the ammonium sulfate. This was followed by wt% mass loss of Ni/MgAl₂O₄-S1-400 (23%) > Ni/MgAl₂O₄-S1-550 (20%) > Ni/MgAl₂O₄-S2-200 (10%) > Ni/MgAl₂O₄-S2-400 (6%) > Ni/MgAl₂O₄-S2-550 (5%). The mass loss observed at approximately 800 °C could be associated with the loss of sulfur from the catalysts [39].

TGA data shows that loosely bound sulfur, with mass loss occurring <200 °C, were present on the catalysts sulfided with (NH₄)₂SO₄. The sulfur interaction with the catalyst was stronger using (NH₄)₂S as the sulfiding agent compared to (NH₄)₂SO₄, and the interaction strengthened with an increase in sulfidation temperature as observed for the smaller wt% mass loss on S2 catalysts. The catalysts sulfided with DMSO showed no losses of loosely bound sulfur. This is most likely due to the smaller amount of sulfur on the catalyst which interacts strongly with the nickel particles. Interestingly, DMSO is identified as the most stable, further highlighting the strong nickel-sulfur interaction using this sulfiding agent, due to no or negligible mass loss at temperatures as high as 800 °C. TGA analysis clearly highlights DMSO as an excellent sulfiding agent, resulting in highly stable metal sulfide species.

2.3. Scanning Electron Microscopy, Transmission Electron Microscopy, and Surface Area Analysis

SEM analysis was used to determine the morphology and dispersion of NiS_x particles on the support material. The morphology of the catalysts sulfided with (NH₄)₂SO₄ (S1), (NH₄)₂S (S2), and DMSO (S3) at 200 °C, 400 °C, and 550 °C can be observed from the SEM micrographs shown in Figure 3. The morphology of the catalysts sulfided with S1 (Figure 3) varied gradually from low sulfidation temperature to high sulfidation temperature. The appearance of Ni/MgAl₂O₄-S1-200 (Figure 3a, 200 °C) was composed of needle-like agglomerates, which could be due to the catalyst consisting of (NH₄)₂SO₄ as indicated by XRD analysis. The surface of the Ni/MgAl₂O₄-S1-400 (Figure 3a, 400 °C) catalyst is characterized by floccules, which could be attributed to some decomposition of (NH₄)₂SO₄ on the catalyst. When the sulfidation temperature is increased to 550 °C, the flocculant-shaped surface morphology disappeared, as observed for the Ni/MgAl₂O₄-S1-550 catalyst (Figure 3a, 550 °C).

This result could indicate that the catalyst is loose and porous with an increase in sulfidation temperature [40]. Additionally, at 550 °C, the (NH₄)₂SO₄ sulfiding agent could be completely decomposed, with enhanced dispersion of particles and interaction with the MgAl₂O₄ support to form MgSO₄ as shown by XRD analysis.

Figure 3b displays the surface morphology of catalysts sulfided with S2 as the sulfiding agent. It was observed that a coating was present on the catalyst, which became more prominent with an increase in sulfidation temperature, which could possibly be due to the increased decomposition of the sulfiding agent. The coating was not uniformly distributed over the support but rather appeared as agglomerations, indicating a lower dispersion of nickel-sulfide species when using the (NH₄)₂S sulfiding agent. According to the literature, nickel sulfide prepared by the addition of (NH₄)₂S to a nickel salt usually contains a non-stoichiometric excess of sulfur [41]. The data obtained from EDS show that, in fact, the %S on the catalysts increased with sulfidation temperature, thereby correlating with the result obtained from SEM. Nickel sulfide is a complex compound with various valence states and it is difficult to obtain nickel sulfide with pure phase, uniform size, and structural morphology [41]. The excess sulfur could react with the nickel to form the dense areas of aggregated nickel-sulfided species as seen in the SEM images.

The surface morphology of Ni/MgAl₂O₄ sulfided with S3 at the various temperatures is shown in Figure 3a. The Ni/MgAl₂O₄-S3-550 catalyst consists of distinctive clusters of small particles with a spherical morphology, which is suspected to be Ni₃S₂ particles, absent from the catalysts sulfided with S1 and S2 sulfiding agents. This morphology is consistent at all sulfidation temperatures for S3 catalysts.

It is clear from SEM imaging that the morphology of the catalysts is affected by the type of sulfiding agent. The morphology changes with an increase in temperature, in particular for (NH₄)₂SO₄ sulfiding agent, however, for DMSO the morphology is maintained at all sulfiding temperatures. The increase in sulfidation temperature from 200 °C–550 °C seem to result in a more uniform distribution of particles for S1 and S3, however with S2 larger clusters of particles are observed. Furthermore, SEM analysis provides evidence that the sulfiding agents produce distinct morphologies. For example, S1 consists of closely packed particles, S2 is distinguished by the appearance of the “coating” on the catalyst surface due to agglomeration of NiS_x particles, and S3 is composed of aggregates of small spherical particles. The dense coating of nickel-sulfided complexes, observed for the S2 catalysts, is absent in the catalyst sulfided with DMSO.

To gain more information about the level of dispersion of nickel and sulfur over the MgAl₂O₄, support electron mapping of the sulfided catalysts was conducted. The images are shown in Figure 4. The sulfur on Ni/MgAl₂O₄-S1-200 (Figure 4a) seems to be present as a thick mass over the catalyst. This is noticed by the map of sulfur (green), which shows sulfur to be dense and well dispersed on the support. This is due to the (NH₄)₂SO₄ crystals that completely cover the support. EDS analysis also confirmed the high sulfur content of 33%, the highest of all compared catalysts.

Visual evidence of sulfur interacting with MgAl₂O₄ can be observed in Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 (Figure 4b,c). There is a greater interaction with Mg than with Ni, as evidenced by the electron maps of the elements. This is consistent with XRD analysis, which indicated the presence of MgSO₄ for the Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 catalysts. In industry, the MgAl₂O₄ spinel can be used as a sulfur-transfer catalyst in fluid catalytic cracking units for SO_x emission control, and it has been reported that sulfur oxide species have a high reactivity with MgAl₂O₄ spinel.

Gerle and co-workers found that the chemical reaction of MgAl₂O₄ with sulfur oxides to form MgSO₄ occurred between reaction temperatures of 400 °C–800 °C [42,43]. This result corresponds with our system, where the MgSO₄ phase was present at sulfidation temperatures of 400 °C and 500 °C.

The electron mapping images for S2 catalysts (Figure 5) indicate that the “coating” observed from SEM analysis could be attributed to nickel-sulfided particles that are distributed on the catalyst. The %S is equal to 9.42%, 13.81%, and 18.17% for Ni/MgAl₂O₄-S2-200, Ni/MgAl₂O₄-S2-400, and Ni/MgAl₂O₄-S2-550, respectively, as indicated from EDS analysis.

It is suggested that the nickel-sulfided species increase with an increase in sulfidation temperature (Figure 5a–c) due to greater decomposition of the sulfiding agent. The electron maps of the individual elements confirm a greater interaction with Ni and S compared to S1, however aggregates of Ni and S are present on the support with a low dispersion.

Figure 6 shows the electron mapping of the catalysts sulfided with DMSO. Both the nickel and sulfur particles are evenly dispersed on the Ni/MgAl₂O₄-S3-200 (Figure 6a) and Ni/MgAl₂O₄-S3-400 (Figure 6b) catalysts, with some clusters of nickel-sulfided species forming on the Ni/MgAl₂O₄-S3-550 catalyst as indicated by the white arrows in Figure 6c. DMSO has been used previously in the preparation of highly dispersed silica-supported nanocopper heterogeneous catalysts and dispersible palladium nanoparticles. The advantages of using DMSO were attributed to (i) sufficient interaction with the surface of the metal nanoparticles to effectively stabilize nanoparticle dispersion and (ii) the absence of agglomeration [44,45]. In our study, overall sulfiding with DMSO at all temperatures led to a high dispersion of nickel and sulfur compared to the S1 and S2 catalysts, correlating well with the results obtained from XRD analysis. The high dispersion may be due to a stronger interaction between Ni and S, as sulfur acts as a structural promoter and reduces the nickel particle size.

The sulfur content of S3, however, is much lower compared to S1 and S2, as shown in the EDS results. The interaction between nickel and sulfur on the catalysts sulfided with DMSO seem to have a high consistency, high dispersion, and small particle size, which could be promising for catalytic activity.

SEM and electron mapping indicated that the surface morphology and dispersion varies when using S1, S2, and S3 sulfiding agents. The sulfur content on the S1 catalysts contained the highest %S, which was densely distributed as seen for Ni/MgAl₂O₄-S1-200 and Ni/MgAl₂O₄-S1-400 catalysts. For Ni/MgAl₂O₄-S1-550, the sulfiding agent is decomposed, as suggested from SEM analysis, and sulfur interacts strongly with the MgAl₂O₄ support, seen in Figure 4c. The same trend is observed for S2 catalysts, where the increase in sulfidation temperature could lead to higher decomposition of the sulfiding agent.

The presence of nickel and sulfur clusters grow and increase at the sulfidation temperature of 550 °C for Ni/MgAl₂O₄-S2-550 and there is evidence of strong nickel-sulfur interactions present on the catalyst. The catalysts sulfided with S3 at 200 °C, 400 °C, and 550 °C have the “best” dispersion compared to S1 and S2 sulfiding agents. For the Ni/MgAl₂O₄-S3-550 catalyst, the nickel and sulfur form small clusters less dense than S2 catalysts, indicating strong nickel-sulfur interactions at this sulfidation temperature. Since the catalysts sulfided at 550 °C showed improved dispersion and strong sulfur interactions, this could imply that the optimum temperature for sulfidation is 550 °C for all three sulfiding agents.

TEM analysis was used to measure particle size of the catalysts. Firstly, the unmodified catalyst (Figure S1, Supplementary materials) contained large particles measuring 59 nm. There was an expected change in particle size with the addition of sulfur, due to the geometric effect, which dilutes aggregated Ni sites on the catalyst and dissociates large Ni ensembles responsible for hydrogenolysis reactions [46]. The sulfided nickel particles are depicted in each image by white arrows. For the S1 catalysts (Figure 7), the nickel-sulfur species was not present in the XRD analysis, due to the sulfiding agent (NH₄)₂SO₄ being present on the surface of the catalyst (low temperatures-200 °C) and interacting mostly with the Mg in the support (high temperatures-550 °C). It was observed that there was a wider particle size distribution for Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 compared to the Ni/MgAl₂O₄-S1-200 catalyst (Figure 8). The particle sizes of the catalysts sulfided at 200 °C, 400 °C, and 550 °C was calculated from the program Image J to be 5 nm, 11 nm, and 9 nm, respectively. The increase in temperature led to an increase in particle size.

The TEM imaging and particle size distribution for S2 catalysts is shown in Figure 9 and Figure 10, respectively. The particle cluster size for Ni/MgAl₂O₄-S2-200, Ni/MgAl₂O₄-S2-400, and Ni/MgAl₂O₄-S2-550 was calculated as 11 nm, 17 nm, and 48 nm, respectively. The images for Ni/MgAl₂O₄-S2-400 and Ni/MgAl₂O₄-S2-550 show that the catalysts consisted of small particles clustered together. As sulfidation temperature increased, the interaction of the Ni and S particles increased as well as sulfur content, leading to the formation of larger sized and increased number of clusters (Ni₃S₂) as seen from the TEM images (Figure 9c). This correlates with the results from SEM analysis and electron imaging, showing that the nickel sulfided particles become denser with an increase in sulfidation temperature.

The TEM imaging and particle size distribution for S3 catalysts is shown in Figure 11 and Figure 12, respectively. The TEM images show that the catalyst sulfided with DMSO displays the best dispersion of the nickel particles. Particle size for Ni/MgAl₂O₄-S3-200, Ni/MgAl₂O₄-S3-400, and Ni/MgAl₂O₄-S3-550 was calculated as 6 nm, 11 nm, and 15 nm.

The same trend was observed for the catalyst sulfided with (NH₄)₂S as the sulfiding agent. It was observed that Ni/MgAl₂O₄-S2-400 and Ni/MgAl₂O₄-S2-550 had a similar particle size to Ni/MgAl₂O₄-S3-550. However, the particles were better dispersed on Ni/MgAl₂O₄-S3-550 compared to the S2 catalysts. The strongest interaction between nickel and sulfur occurs in S3 catalysts, as indicated by TGA. The beneficial geometric effects of sulfur, causing the increased dispersion, is most notable in S3 catalysts. Since the interactions of Ni and S increase with sulfidation temperature (200 °C, 400 °C, and 550 °C), we would expect that Ni/MgAl₂O₄-S3-550 has the highest dispersion of particles on the catalyst.

Nitrogen adsorption–desorption isotherms of the catalysts sulfided with (NH₄)₂SO₄, (NH₄)₂S and DMSO, at 200 °C, 400 °C, and 550 °C are shown in Figure 13a–c, respectively. All the catalysts, with the exception of Ni/MgAl₂O₄-S1-200, display type IV isotherms and type H₁ hysteric loops, which is indicative of mesopores present in the catalysts [47,48]. The largest decrease in surface area observed for the Ni/MgAl₂O₄-S1-200 could be due to the covering of (NH₄)₂SO₄ crystals on the surface and within the pores of the support.

An increase in surface area is observed with an increase in sulfidation temperature for S1 catalysts, which is consistent with the findings from SEM analysis that indicated a change in morphology to a more porous structure with an increase in sulfidation temperature. This could be due to the decomposition of (NH₄)₂SO₄ and reduction of pore blockages caused by sulfur [49]. The surface areas of the S1 catalysts at higher temperatures were also the largest compared to the other sulfiding agents.

The N₂-physisorption results for the catalysts are displayed in

Table 2. Surface area, pore volume, pore size, and crystallite size of supported catalysts.

Sample	SBET (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
MgAl2O4	85	0.190	7.9
Ni/MgAl2O4	58	0.156	9.2
Ni/MgAl2O4-S1-200	4	0.022	22.4
Ni/MgAl2O4-S1-400	119	0.340	8.3
Ni/MgAl2O4-S1-550	90	0.400	16
Ni/MgAl2O4-S2-200	30	0.119	14
Ni/MgAl2O4-S2-400	63	0.188	11
Ni/MgAl2O4-S2-550	56	0.194	12
Ni/MgAl2O4-S3-200	49	0.200	14
Ni/MgAl2O4-S3-400	50	0.200	15
Ni/MgAl2O4-S3-550	51	0.200	15

. The S1 catalysts showed the highest surface area of 90–120 m²/g compared to S2 and S3 catalysts. The surface areas increased for the S2 catalysts as the sulfiding temperature increased above 200 °C. This could be due to the difference in surface areas of the NiS and Ni₃S₂ species observed at different sulfiding temperatures. The surface areas of the catalysts sulfided with DMSO, however, all had the same surface area, likely due to the high dispersion observed at all sulfiding temperatures. The N₂ physisorption studies indicate that differences in the textural properties, in particular the surface areas, are obtained when using different sulfiding agents. This was dependent on the decomposition of the sulfiding agent in which temperature has an effect, as well as dispersion of Ni-S species on the catalyst.

2.4. Catalytic Performance

The catalysts sulfided with (NH₄)₂SO₄, (NH₄)₂S, and DMSO at the various temperatures were evaluated for the propane dehydrogenation. The conversion % of propane and selectivity % toward the various products for the catalyst sulfided with (NH₄)₂SO₄ is shown in Figure 14 and Figure 15, respectively. The initial conversion % of propane for the S1 catalysts increased in the following order: Ni/MgAl₂O₄-S1-200 (26%), Ni/MgAl₂O₄-S1-550 (37%), and Ni/MgAl₂O₄-S1-400 (51%) at three minutes of the reaction time. The S1 catalysts follow a trend of increasing stability with time, which could be due to the formation of Ni₃S₂ as the reaction proceeds due to reduction of (NH₄)₂SO₄ by propane. The presence of an active Ni₃S₂ was confirmed by XRD analysis of the spent catalysts at 120 min (Figure S2, Supplementary material). The S1-400 and S1-550, which both have higher initial conversions, then decreased slightly after 30 min TOS and then remained stable for the length of the reaction time.

Regarding selectivity of the catalysts, it was noted that as the reaction progressed, more C2 olefins were being produced with time on stream. The major side products for the reaction were CH₄, C₂H₄, and C₂H₆, indicating that hydrogenolysis as well as cracking reactions occur. The selectivity of Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 toward propylene was higher than Ni/MgAl₂O₄-S1-200. For example, at 3 min, the selectivity toward propylene was 76% and 72% for Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550, respectively, while Ni/MgAl₂O₄-S1-200 displayed a selectivity of 65% at 3 min. The Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 catalysts displayed stronger sulfur bonds being formed as indicated by TGA analysis, which could be the reason for the improved selectivity. At the sulfidation temperature of 200 °C, it was noted that (NH₄)₂SO₄ was deposited and present on the catalyst, as indicated by XRD analysis. Furthermore, the Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 catalysts had an increase in the sulfur interaction with the support compared to Ni/MgAl₂O₄-S1-200, indicated by the presence of MgSO₄. It was suggested that the improved interaction of sulfur on the support may have led to the enhanced selectivity.

In the case of the catalysts sulfided with (NH₄)₂S, the catalyst activity decreased over time (Figure 16). The S1 catalysts showed a decrease in the conversion between 3–30 min (S1-400 and S1-550), followed by the catalysts being more stable up to 120 min. This contrasts with S2 catalysts, whereby the catalysts are stable up to 60 min, followed by a decrease in conversion of propane. This contrast could be due to the phases present in the catalyst. Since fresh S2 catalysts contain NiS and Ni₃S₂, the initial conversion and stability is high, and activity gradually decreases as sulfur may be lost from the catalyst. However, the presence of Ni₃S₂ is only present in the spent S1 catalysts as the S1 catalysts experience an induction phase, which results in increase in conversion and stability as the Ni₃S₂ phase forms during the reaction due to reduction by propane. The conversion % at 3 min were in close range: Ni/MgAl₂O₄-S2-200 (50%), Ni/MgAl₂O₄-S2-400 (50%), and Ni/MgAl₂O₄-S2-550 (43%).

Overall, the catalyst activity is noticed to decrease with an increase in sulfidation temperature. It is possible that the decrease in activity is a result of an increase in particle size and lower dispersion as shown by the TEM and SEM analyses, which was observed when the sulfidation temperature was increased. The catalysts sulfided with (NH₄)₂S showed excellent selectivity toward propylene, 78% for Ni/MgAl₂O₄-S2-200, Ni/MgAl₂O₄-S2-400, and Ni/MgAl₂O₄-S2-550, indicating a high dehydrogenation activity of the catalyst as observed from Figure 17. This could be due to stronger nickel-sulfur bonds and better desorption of olefins on S2 catalysts compared to S1. Furthermore, S2 catalysts contains the S²⁻ species, which is part of the active phase (Ni₃S₂) for dehydrogenation.

Wang and co-workers reported that the introduction of sulfur facilitated the desorption of olefins by weakening the interaction between the surface metal atoms and the adsorbed alkene molecules. One reason is that sulfur caused a higher electron density on the surface atoms and led to more repulsive interactions with the olefins. It was proposed that sulfur addition, i.e., the presence of S²⁻ adjusted the electronic properties of Ni atoms, altered the nature of the surface species and decreased the adsorption heat of olefins, as well as the activation energy of its desorption, thereby leading to a higher dehydrogenation activity [4,46].

The dehydrogenation activity for the catalysts sulfided with DMSO is shown in Figure 18 and Figure 19. The conversion % of propane increased in the following order for the S3 catalysts: Ni/MgAl₂O₄-S3-400 (36%) < Ni/MgAl₂O₄-S3-200 (37%) < Ni/MgAl₂O₄-S3-550 (45%) in the first 3 min. The Ni/MgAl₂O₄-S3-550 catalyst displayed the highest stability compared to all the catalysts, including the catalysts sulfided with (NH₄)₂SO₄ and (NH₄)₂S as the sulfiding agents. The conversion of propane for Ni/MgAl₂O₄-S3-550 was 46%, the highest conversion value of all catalysts at 120 min. The enhanced stability of the S3 catalyst is likely due to the highest dispersion of Ni-S_x species observed by SEM electron mapping compared to S1 and S2 catalysts. Although sulfidation with DMSO resulted in the lowest wt% sulfur content, the high dispersion and small particle size may have resulted in the strong interaction between the metal and sulfur, thereby enhancing stability and preventing sulfur loss from the catalyst.

EDS analysis of the spent catalysts (Figure 20) further confirmed that S3 showed the lowest loss of sulfur compared to S1 and S2. The stability of the Ni/MgAl₂O₄-S3-550 catalyst could also be due to suppression of the release of S²⁻ in the metal sulfide, which is important for achieving stable dehydrogenation performance [50]. The TGA profile of S3 catalysts confirm the excellent stability of nickel-sulfur bonds when compared to S1 and S2. The increase in conversion with temperature for the S3 catalysts may be correlated with the slightly higher content of sulfur on the Ni/MgAl₂O₄-S3-550 catalyst compared to the Ni/MgAl₂O₄-S3-200 and Ni/MgAl₂O₄-S3-400 catalysts. This increase in activity may be attributed to the higher content of sulfur increasing the electronic charge distribution on the catalyst, thus acting as an electronic promoter. Furthermore, a growth in crystallite size was observed for (Table S1, Supplementary Material) for all the spent catalysts, which could indicate sintering and a possible reason for the decrease in catalytic activity.

Figure 20 shows that sulfur is lost after 2 h of reaction time by the decrease in %S on the spent catalysts. The S3 catalysts, although they have the lowest sulfur content, also showed the least loss of sulfur compared to S1 and S2 catalysts. This indicates a strong interaction between the metal and sulfur, confirmed by TGA, possibly due the higher dispersion observed. The difference in catalytic activity between S1, S2, and S3 catalysts could be due to the strength of the sulfur bonds to the metal. Furthermore, the difference in decomposition of the sulfiding agent at different temperatures, i.e., either 200 °C, 400 °C, or 550 °C, may lead to different sulfur contents which change the net electronic charge distribution that affects the activity of the catalyst. Thus, the amount of sulfur present and a stronger metal-sulfur bond could lead to an enhanced activity and stability, as shown for the Ni/MgAl₂O₄-S3-550 catalyst.

It was noted that S1 catalysts (containing the SO₄²⁻ ion) had a higher selectivity toward C2 compared to S2 and S3 catalysts (containing the S²⁻ ion), indicative of more cracking reactions. It is known that the SO₄²⁻ ion promotes acidity on dehydrogenation catalysts. It has previously been reported that cracking reactions occur on dehydrogenation catalysts containing higher acid site density in comparison to moderate acid site density [22,51]. This is consistent in our study, whereby S1 contained the highest wt% S in the form of the SO₄²⁻ as indicated by XRD analysis and electron mapping imaging. For the catalysts sulfided with (NH₄)₂S and DMSO, although a high dehydrogenation activity existed, side products (CH₄, C₂H₄, and C₂H₆) were observed, an indication that propane cracking and propane hydrogenolysis reactions were not completely eliminated [50].

The selectivity of the catalysts sulfided with DMSO to propene, however, were still high, comparable with the S2 catalysts. Interestingly, it showed the best selectivity as the reaction proceeded. The selectivity increased to as high as 80% at 120 min of the reaction time, highlighting the beneficial effect of high dispersion of sulfur on the catalyst. The enhanced stability of the catalyst resulted in the high selectivity to propene with time on stream. The high dispersion may have also resulted in sulfur reducing the nickel ensemble size, and blocking sites responsible for the hydrogenolysis as the lowest methane selectivity was observed for the S3 catalysts.

3. Materials and Methods

3.1. Raw Materials

The following chemicals were used in the study: Magnesium nitrate hexahydrate (98–102%, Merck, Johannesburg, South Africa), aluminium nitrate nonahydrate (≥98%, Merck, Johannesburg, South Africa), oxalic acid (99.5%, LabChem, Johannesburg, South Africa), nickel nitrate hexahydrate (≥97%, Merck, Johannesburg, South Africa), ammonium sulfate (99%, LabChem, Johannesburg, South Africa), ammonium sulfide (20wt%, Merck, Johannesburg, South Africa), DMSO (≥99.9%, Merck, Johannesburg, South Africa), and propane gas. Distilled water is obtained by a house supply that produces Milli-Q water.

3.2. Catalyst Preparation: Synthesis of MgAl₂O₄ Support, Supported Ni Catalysts, and Sulfur Modified Catalysts

The synthesis of MgAl₂O₄ was performed according to our previous work and adapted by Nassar and co-workers [31,52]. The loading of Ni by weight was 13 wt% for all catalysts. Nickel oxide catalysts supported on MgAl₂O₄ was prepared by wetness impregnation, using Ni(NO₃)₂.6H₂O as the metal precursor. After impregnation of the supports, the catalysts were dried at 140 °C overnight and calcined at 700 °C in air for 2 h. The catalyst was then reduced under a flow of H₂ (50 mL/min) at 600 °C for 5 h.

Sulfur modified Ni/MgAl₂O₄ catalysts were prepared using three different sulfiding agents: (NH₄)₂SO₄ (S1), (NH₄)₂S (S2), and DMSO (S3). The molar ratio of Ni:S is 1:5 for each sulfiding agent. The catalysts were loaded onto the reactor, then sulfided with the calculated amount of sulfiding agent under a flow of nitrogen (50 mL/min) at 200 °C, 400 °C, and 550 °C for 3 h.

3.3. Characterization

X-ray diffraction (XRD) analysis of the samples was performed on a Bruker AXS D8 Advance Diffractometer (Cu-Kα radiation λKα1 = 1.5406 Å) 40 kV running from 5–90° in the 2θ range. The crystallite size (L) of the catalysts were determined using the Scherrer equation as follows: L = Kλ/βcosθ, where λ is the X-ray wavelength, β is the peak width of the diffraction peak, and K is the shape factor with a value of 0.94. The ICDD database available on MATCH software was used for peak identification. The surface area measurements and porosity analysis were determined by nitrogen adsorption/desorption isotherms at −196 °C using the Micromeritics 3 FLEX analyzer. Approximately 0.3 g of sample was degassed overnight with a constant flow of nitrogen gas over the sample in order to remove atmospheric moisture from the sample. The specific surface areas were determined by the Brunauer–Emmet–Teller (BET) and Barret–Joyner–Halenda (BJH) methods. Scanning electron microscopy (SEM) micrographs were obtained using a high-resolution SEM EHT 5.00 kV. Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai G2 F20 S-Twin HRTEM, operated at 200 kV. The powdered samples were dispersed in absolute ethanol, using ultrasound, pipetted onto carbon-coated copper grids, and allowed to dry for a few minutes under a lamp before imaging. A ZEISS MERLIN SEM instrument was used for STEM imaging and X-ray mapping of samples. The setting for the imaging was at a working distance of 9.5 mm with beam strength of 20 kV and probe current of 10 nA, using a backscattered electron detector. Samples were prepared by mounting the catalyst evenly on carbon tape and sputter coated with gold at 10 mm and 10 nm. The STEM detector was used at a 20 kV beam strength and 250 pA probe current with a 4 mm working distance. Samples were prepared by 10 x dilution, then pipetted onto the TEM grid and air dried. Urinyl acetate was used for negative staining and samples were left to dry in darkness before loading onto a STEM stub and into the MERLIN SEM instrument. Thermogravimetric analysis (TGA) was performed on a simultaneous analyzer (STA) 8000 in order to investigate the thermal stability of the catalysts. Approximately 0.03 g of sample was placed in the sample holder. The sample was then heated at a ramp rate of 10 °C/min from 30–1000 °C under a flow of argon gas.

3.4. Catalytic Evaluation

The catalysts were evaluated using a fixed bed quartz microreactor (i.d. 14 mm, length 600 mm) at atmospheric pressure and 600 °C. For the experiments, 1 g of the selected catalyst, diluted with silicon carbide beads, was loaded into the reactor. This was followed by flowing propane and nitrogen into the reactor (molar ratio of 1:1). The total flow rate equaled 30 mL/min. The catalyst particle size was in the range 200–250 µm and the WHSV equaled 3.7 g/h (residence time 3 min). The obtained products were then analyzed using a Bruker 450-GC. Gas products were analyzed on a BR-Alumina Na₂SO₄ column. Propane conversion and product selectivity were calculated using the formulas shown below. The conversion % was calculated with the assumption that feed (out) is unconverted propane.

$$\% \mathrm{Propane} \mathrm{conversion}=\frac{\mathrm{feed}\left(\mathrm{in}\right)- \mathrm{feed}\left(\mathrm{out}\right)}{\mathrm{feed}\left(\mathrm{in}\right)} \times 100$$

The product selectivity was calculated for each component $xi$

$$\% \mathrm{Product} \mathrm{selectivity}=\frac{\mathrm{mass} \mathrm{component} xi}{\sum xi} \times 100$$

4. Conclusions

Nickel catalysts supported on MgAl₂O₄ were successfully prepared using (NH₄)₂SO₄ (S1), (NH₄)₂S (S2), and DMSO (S3). The catalysts were sulfided at 200 °C, 400 °C, and 550 °C. For S1 catalysts, Ni/MgAl₂O₄-S1-200 contained the highest percentage of unreacted Ni with sulfur, and peaks associated with nickel-sulfur phases were absent, according to XRD analysis.

For the Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 catalysts, decomposition of the sulfiding agent increased with temperature above the decomposition temperature of 250 °C, leading better sulfur dispersion and interaction with the catalyst. XRD analysis displayed peaks for MgSO₄, indicating a stronger interaction between the support and SO₄^2- ion of the sulfiding agent when the temperature was increased.

Ni/MgAl₂O₄-S2-200, Ni/MgAl₂O₄-S2-400, and Ni/MgAl₂O₄-S2-550 catalysts contained NiS and Ni₃S₂, while Ni/MgAl₂O₄-S3-200, Ni/MgAl₂O₄-S3-400, and Ni/MgAl₂O₄-S3-550 catalysts contained only the Ni₃S₂ phase, indicating that the sulfiding agent and sulfiding conditions such as temperature may influence the NiS_x phase formed. Although Ni/MgAl₂O₄-Sx-y catalysts were reduced prior to sulfidation, S3 catalysts contained the presence of NiO, which decreased with an increase in sulfidation temperature. The presence of NiO could be an indication that the catalyst was not completely reduced.

This could indicate that the sulfiding agent decomposed to a higher extent at the elevated temperatures of 400 °C and 550 °C compared to the sulfidation temperature of 200 °C. Furthermore, it was noted that the increase in sulfidation temperature resulted in an increase in crystallite size of the nickel-sulfided particles, which could be due to sintering that commonly occurs at higher temperatures.

SEM analysis showed that the morphology of the catalysts was distinct with respect to sulfiding agent and sulfidation temperature. Electron mapping images confirmed the strong interaction between MgAl₂O₄ and S, with visual evidence of MgSO₄ as seen in Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 catalysts. The S2 catalysts displayed the presence of a “coating” that became more apparent with an increase in sulfidation temperature, resulting in a low dispersion. This coating was attributed to Ni₃S₂, highlighting the strong interaction between nickel and sulfur with an increase in temperature, as observed from TGA.

The S3 catalysts (Ni/MgAl₂O₄-S3-200, Ni/MgAl₂O₄-S3-400, and Ni/MgAl₂O₄-S3-550) had the lowest sulfur content and best dispersion compared to S1 and S2 catalysts, indicating that the type of sulfiding agent and sulfiding conditions used may influence the dispersion of NiS_x species. However, Ni/MgAl₂O₄-S3-550 showed the strongest interaction of nickel and sulfur, visible by clusters of Ni₃S₂ on the catalyst surface. TEM analysis indicate that the particle size was affected by increased sulfidation temperature on S1 catalysts S3 catalysts displayed improved dispersion, with clusters of smaller particles, highlighting the geometric effect of sulfur.

The sulfided catalysts were then tested for the dehydrogenation of propane. The Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 catalysts performed better than Ni/MgAl₂O₄-S1-200, as the former catalysts contained less unreacted Ni and had a higher impact by sulfur as the decomposition was more efficient at higher temperatures. The catalysts seemed to undergo an induction period, in particular Ni/MgAl₂O₄-S1-200, as the (NH₄)₂SO₄ crystals were reduced to Ni₃S₂ during the reaction. The conversion % for S2 catalysts were 50% with a selectivity toward propylene equal to 78%. This could be due to Ni₃S₂ present on the catalyst being the active phase, promoting better interaction between nickel and sulfur as sulfur content increased with higher sulfiding temperatures.

The high propene selectivities were attributed to increased desorption of olefins due to sulfur interaction with the Ni metal. Ni/MgAl₂O₄-S3-550 showed the highest conversion % of propane and selectivity % toward propylene at 120 min of the reaction. This catalyst displayed the best overall performance compared to all the catalysts, which could be attributed to the high dispersion of metal sulfide species. This led to strong Ni-S interactions and ultimately higher activity, selectivity, and enhanced stability.

By sulfiding catalysts at 200 °C, 400 °C, and 550 °C, the findings show that the sulfiding agent (containing the different anions) and the sulfidation conditions (temperature) have an influence on the catalyst structural and textural properties in terms of decomposition and sulfur content, which affect morphology, particle size, dispersion, and Ni-S interaction. The catalytic performance was shown to be most dependent on Ni-S interaction and particle dispersion, of which the best results were obtained with DMSO (S3). DMSO used as a sulfiding agent improved conversion, increased selectivity to propene (80%), and enhanced stability, especially when a high sulfiding temperature (550 °C) was utilized.