The porous structure of all calcined catalysts was determined using the BET-Micromeritics ASAP 2020 surface area and porosity analyzer. Both the adsorption and desorption isotherm branches, and nitrogen gas adsorbed at 77 K were taken into consideration in the analysis. The Brunauer-Emmett-Teller (BET) method was used in calculating the surface areas whereas, the total pore volume and pore size distribution were determined by the Barett-Joyner-Halenda (BJH) method. Approximately 0.1 g of each sample was outgassed at 200 °C to a residual pressure of <6.6 × 10⁻⁴ Pa prior to adsorption.

Thermal stabilities of catalyst samples that had been dried and calcined were analyzed using a TA Q500 series instrument with simultaneous thermogravimetric analysis (TGA)/differential scanning calorimeter (DSC) application. The analysis was carried out by heat treating approximately 15 mg of sample from room temperature to 700 °C at 10°C/min ramp rate and the flow rate of nitrogen used was 60 ml/min.

The structure, crystallinity and phase of catalyst samples were examined by XRD analysis with the Bruker D8 Advance powder X-ray diffractometer using CuKα radiation (λ = 0.1541 nm). The X-ray gun was powered at 30 mA and 40 kv. Bragg angle (2θ) was scanned from 1.5 to 10° for low angle X-ray diffraction, and from 10 to 80° for wide angle X-ray diffraction. The sample weight for analysis was about 0.25 g.

An ASAP 2020 micrometrics instrument was used in conducting CO chemisorption analysis and this enabled the percentage metal dispersion to be calculated in relation to carbon monoxide uptake measured on each catalyst. About 0.10 g of each catalyst sample which had previously undergone a degassing step for 60 min at 110°C was further reduced in situ under hydrogen flow for 2 h at 350°C. Thereafter, the reduced sample was cooled to 35°C and consequently evacuated after attaining a static pressure of less than 1.3 × 10⁻⁵ Pa. CO chemisorption measurements corresponding to CO uptake at 35°C were done after CO pulses had passed over the sample. For bimetallic catalysts, Eq. (1) was used in calculating the percentage metal dispersion of the active metals that are available and interact with the adsorbate. %M D I S P =   1   22414 ∗   × V   x   SF C A L C w t . f r a c t i o n   M O W A T O M I C , M O +   w t .   f r a c t i o n   N i W A T O M I C , N i (1) where, %M _DISP = percentage metal dispersion, SF _CALC = calculated stoichiometry factor, W_{ATOMIC, Mo} = atomic weight of first metal, Mo (g/mole), W_{ATOMIC, Ni} = atomic weight of second metal, Ni (g/mole), V = volume intercept that was obtained from the line of best fit to the differences in volume between the selected points of the first analysis and the repeated analysis (cm³/g STP), *The volume occupied by 1 mol of gas (cm³ STP/mole of gas) (Micromeritics ASAP, 2020).

The morphology of each catalyst was analyzed using a JEOL 2011 (200 kv) scanning transmission electron microscope (STEM). In preparation for the analysis, catalyst samples were ground to fine particle size and then a minute quantity was dispersed onto a 200-mesh holey carbon-coated TEM support grid. For every sample, about 5–10 representative images were taken at low to high magnification ranging from 20–250 k times magnification.

H₂-temperature programmed reduction (H₂–TPR) analysis was conducted in a TPD/TPR Quantachrome AutosorbiQ (United States) equipment to evaluate how the reducibility of the metal species in the catalyst behaved. The gas mixture used in conducting the analysis comprised of 3% H₂/N₂ (v/v). A catalyst sample weight of 0.1 g was used for the analysis. The TPR analysis procedure involved these steps i.e., purging the sample with helium (He) at 400 °C for an hour to pre-treat the sample, cooling to room temperature, followed by ramping the program from room temperature to 800°C at a ramp rate of 10°C/min as the reducing gas mixture flowed at 30 ml/min. A thermal conductivity detector enabled the monitoring of the hydrogen consumption within the temperature range of interest.

Total S and N compositions for both untreated and treated gas oil samples were determined using the Antek 9000 NS analyzer. A combustion/fluorescence technique together with the ASTM D5463 standard procedure was used in analyzing the sulfur removal whereas, a combustion/chemiluminescence technique and the ASTM D4629 standard procedure was used to analyze the nitrogen removal.

Table 1 shows the surface area, pore volume and pore diameter of as-synthesized OCP_f, functionalized OCP_f, and NiMo/OCP_f catalysts with varying Ni and Mo compositions. The functionalized OCP_f material was found to be of higher surface area and pore volume than the pristine OCP_f due to the presence of defects after functionalization. Contrarily, a decrease in pore diameter was rather observed as the pristine OCP_f material was functionalized, and this was due to the breaking of bigger pores, with the average pore diameter being more representative of smaller pores (Karousis et al., 2016). For the different catalysts it was observed that the surface area and pore volume of all the catalysts decreased from the functionalized material due to the plugging of pores by metals. An increment in pore diameter however, occurred after the functionalized OCP_f samples had been impregnated with metals to form different catalysts.

There was no significant change in surface area at constant Ni composition of 2.5 wt% as Mo composition was raised from 13 wt% to 19 wt%. On the other hand, as the Ni loading was kept constant at higher compositions (either 3.5 wt% or 5.0 wt%), increased in Mo composition from 13 wt% to 19 wt% resulted in ∼16% decrease in surface area. This is because more pores were blocked as higher Mo loadings were impregnated on the support. Keeping the Mo loading constant at a lower composition of 13 wt% did not bring about any significant change in surface area and pore diameter at increasing Ni composition from 2.5 wt% to 3.5 wt%. However, at a constant Mo loading of 19 wt% both the surface area and pore diameter decreased steadily as Ni compositions increased from 2.5 to 3.5 wt%. Also, the decrease in surface area was significant under this condition. Overall, almost all the catalysts exhibited mesoporous pore diameters and constant pore volume. Unlike all other catalysts, the 5.0wt%Ni26wt%Mo/OCP_f catalyst with the highest Ni and Mo compositions exhibited the lowest surface area since its pores were filled with metals having the highest weight compositions during catalyst formulation. Consequently, its pore diameter was high since it was more likely for its micropores to be blocked making its mesopores the major representative of its pore diameter.

The adsorption-desorption isotherms of pristine OCP_f, functionalized OCP_f and all the NiMo/OCP_f catalysts used in this study are shown in Figure 1.

From Figure 1 all the samples tested displayed Type IV isotherm which is typical of mesoporous materials but with H3 hysteresis loop. Among all the samples, the functionalized OCP_f exhibited higher adsorption capabilities because of the nanowindows created on the support material as well as the consequent increase in surface area after acid treatment. The pristine OCP_f sample showed minimal adsorption capabilities compared to the rest of the samples. From Figure 1, the shapes of the functionalized adsorption-desorption isotherm and that of the various catalysts were similar and this implies that the pore structure of the functionalized OCP_f material remained intact in all the catalysts (Sing et al., 1985). The highly loaded catalysts are practically a mixture of well dispersed catalysts and some remnants of crystalline NiMo phase.

Since all OCP_f catalysts were prepared from a by-product support and there is an instant negativity surrounding by-products, the thermal stability of the catalysts had to be tested with TGA at the hydrotreating temperature conditions (330, 350 and 370°C) being experimented to ensure that the catalysts would not burn out during the hydrotreating process. Thus, Figure 2 displays the TGA profiles of all the calcined OCP_f-supported NiMo catalysts utilized in this study.

Removal of nitrates, ammonia and water in an impregnated catalyst sample is prevalent during TGA analysis, and nitrates present in carbon-supported catalysts could cause oxidation of carbon support. It is also worth noting that with respect to calcination, carbon supports are prone to oxidation as NH₃ is removed during a calcination step which is catalyzed by Mo deposits. Thus, oxidation of carbon supports can be prevented by low temperature calcination of the catalyst in air or by calcination in an inert environment. Another way to prevent oxidation of the catalyst is to use a presulfided catalyst that does not decompose (NH4)₆Mo₇O₂₄. Additionally, oxidation of carbon supports can be prevented by impregnation of the support with a MoO₃/water slurry in place of (NH4)₆Mo₇O₂₄/water solution. For the MoO₃/water slurry or slurry, impregnation method calcination is not required because although the solubility of MoO₃ is low, molybdena species are gradually transported and adsorbed onto the carbon surface because of electrostatic attraction between molybdate anions and positively charged carbon surfaces (Vissers et al., 1987; Kaluža, and Zdražil, 2001). Work by Vissers et al., 1987 confirmed that direct sulfidation of an uncalcined carbon-supported catalyst with a sulfiding agent preferably H₂S/H₂ mixture impedes carbon destruction due to the lack of gasification of the carbon support to methane or CO (Vissers et al., 1987).

From Figure 2 the desorption of water that had been physically adsorbed led to a weight loss of about 1% up to 120°C. Afterwards, the weight loss was almost constant up to about 400°C and mostly stable until ∼600°C. The stability of the thermal properties from 120–400°C may be due to less concentration of surface defects on the catalysts. A slight loss in weight from 120–400°C can be attributed to the loss of impurities that remain on the sample after catalyst preparation, or loss due to MoO₃ decomposition into MoO₂ or Mo4O₁₁ intermediate compounds (Spevack and Mcintyre, 1992; Serp et al., 2003). Beyond 500°C–700°C carbothermal reductive decomposition of NiO or NiMoO₄ may contribute to the weight loss (Lebukhova and Karpovich, 2008). The drastic drop in weight for all the NiMo/OCP_f catalyst samples at about 700°C can also be attributed to the findings from Bekyarova et al. (2002) which showed that at higher temperatures of at least 500°C, carbon nanomaterials are prone to burn-off due to instability. Although, an inert gas (nitrogen) was used to carry out the TGA experiments the thermal stability of the catalyst in terms of thermal sintering is the same regardless of the reactive gas environment (H₂) used during hydrotreating since, exposure to higher (>500°C) gas-phase temperatures is the main factor that influences catalyst deactivation by thermal sintering (Bartholomew, 2001; Bekyarova et al., 2002; Fogler, 2006). Based on the above information the thermal stability of all the catalysts in terms of thermal sintering is assured for the hydrotreating temperatures (330–370°C) being studied.

Low angle XRD profile patterns of all the different NiMo/OCP_f catalysts displayed in Figure 3 shows a hexagonal mesoporous structure with well resolved strong (2θ = 0.9) and weak (2θ = 1.7) peaks that were identical for every catalyst used in this study. The peaks were indexed at d (100) and d (110). This identical characteristic implied that variation in metal loading composition did not influence the hexagonal mesoporous structure.

Figure 4 shows the high angle XRD profile patterns of all the NiMo/OCP_f catalysts, and the XRD diffractograms revealed crystalline phases for all samples. The 2θ diffraction peak at ∼ 26° represents, (002) plane characteristic of a hexagonal carbon structure. This 2θ diffraction peak at ∼26° also corresponds to an intense MoO₃ and NiMoO₄ peak. Thus, the peak at this position can be associated to an overlap of carbon (OCP_f), MoO₃ and NiMoO₄ material. Other small intensity peaks occurred at ∼37° and ∼54° on all the catalysts which corresponded to NiO and NiMoO₄, respectively. The appearance of these characteristic crystalline peaks relating to MoO₃, NiO and NiMoO₄ on the OCP_f-supported NiMo catalysts implied that the metals were not well dispersed on the support. A small diffraction peak relating to molybdenum carbide (Mo₂C) was also observed at 2θ = ∼61° (Quanli et al., 2003; Gattia et al., 2006; Liu et al., 2011; Aryee et al., 2021). The crystal sizes of all the catalysts were calculated from all XRD patterns using the Derbye-Scherrer equation. These values ranged from 8 to 14 nm. For the 5.0wt%Ni19wt%Mo/OCP_f catalyst, the crystal sizes were calculated using peaks at two theta values of 26°, 37°, 54° and 61° in Figure 4. Also, the crystal sizes of the NiO phase for 2.5wt%Ni13wt%Mo/OCP_f, 2.5wt%Ni19wt%Mo/OCP_f, 3.5wt%Ni13wt%Mo/OCP_f, 3.5wt%Ni19wt%Mo/OCP_f, 5.0wt%Ni13wt%Mo/OCP_f, 5.0wt%Ni19wt%Mo/OCP_f and 5.0wt%Ni26wt%Mo/OCP_f catalysts were calculated, which varied from 11–13 nm.

The crystalline peak intensities exhibited by the 5.0wt%Ni13wt%Mo/OCP_f, 5.0wt%Ni19wt%Mo/OCP_f, 3.5wt%Ni13wt%Mo/OCP_f and 2.5wt%Ni13wt%Mo/OCP_f catalysts were smaller than the rest of the catalysts hence, it is expected that the dispersion of the metal components in these catalysts would be much improved compared to the remaining OCP_f-supported NiMo catalysts used in this study. Additionally, this result implies that an improved dispersion can be obtained if a catalyst is formulated using a combination of Ni compositions varying from 2.5 to 5.0 wt% and a low Mo composition of 13 wt%. However, for a high Mo composition of 19 wt% an improved dispersion can also be achieved with a high Ni composition of 5.0 wt%.

Results from CO-chemisorption analysis are displayed in Table 2, and these results show that at constant Ni composition of 2.5 and 5.0 wt%, an increase in Mo composition from 13 wt% to 19 wt% resulted in a decrease in dispersion and a corresponding increase in CO uptake. Nonetheless, no change in dispersion occurred at constant Ni composition of 3.5 wt% as the Mo varied from 13 to 19 wt%. However, at constant Mo loadings of 13 wt% or 19 wt%, metal dispersion and CO uptake increased steadily with increasing Ni compositions from 2.5 to 5.0 wt%. This positive impact is ascribed to the fact that increase in Ni loadings increases the amount of Ni release during sulfidation, and this effect simultaneously enhances the redistribution of MoO₃ which further results in a decrease in crystallite size with a corresponding increase in metal dispersion (Badoga et al., 2014). Results from Table 2 also indicate that of all the catalyst under study a significant improvement in both metal dispersion and CO uptake can be achieved by using a catalyst with high Ni composition (5.0 wt%) and an Mo composition of either 13 wt% or 19 wt%. Thus, the 5.0wt%Ni13wt%Mo/OCP_f and 5.0wt%Ni19wt%Mo/OCP_f catalysts exhibited maximum metal dispersions whereas, the 5.0wt%Ni26wt%Mo/OCP_f catalyst showed the least metal dispersion since, the Mo loadings of 26 wt% was too high for the 5.0 wt% Ni to have any positive influence on the redistribution of MoO₃ to form smaller crystallite size with enhanced dispersion.

The average diameter (nm) of the crystallites (assumption-spherical) is given by this equation d =   6.10 3 ρ M where, M = metallic surface area (m²/g), ρ = the specific mass of the metal (g/cm³), (Lynch, 2003).

The morphology of the different NiMo/OCP_f catalysts used for this study are displayed in Figure 5. The TEM micrographs of all the catalysts showed dark spots that signified that indeed metals have been impregnated on the support. However, the micrographs suggest that metals were not evenly dispersed on the support. Additionally, apart from CNH structures, some of the catalysts also revealed CNT and other embedded structures typical of an OCP_f material (Gattia et al., 2006; Zhang et al., 2019).

The TPR results of all the NiMo/OCP_f catalysts are shown in Figure 6. Two Mo reduction peaks at low (320–394°C) and high (570–660°C) temperature values were detected. The first peak corresponded to a partial reduction of Mo⁶⁺ to Mo⁴⁺ whereas the second peak was from a combination of a complete reduction of Mo⁶⁺ to Mo⁴⁺ and Mo⁴⁺ to Mo⁰⁺. Reduction of Ni²⁺ also contributed to the low temperature reduction peak in the range of 250–375°C whereas, NiMoO₄ also contributed to the reduction peak that ranged from 375 to 480°C (Hurst et al., 1982; Brito et al., 1989; Calafat et al., 1996; Aryee et al., 2021). All the catalysts revealed the presence of crystalline MoO₃ in the shoulder of the first peak and on the high temperature side. This is a typical position for crystalline MoO₃ and normally the absence of this peak signifies that MoO₃ is amorphous and/or well dispersed. As a result, it is obvious that the most overloaded catalyst is 5.0wt%Ni26wt%Mo/OCP_f due to the high intensity of the MoO₃ crystalline peak and the shift of this peak towards a higher temperature. The preceding information correlates well with the XRD patterns, CO chemisorption dispersion results as well as the overall activity results from all catalysts (Regalbuto and Ha, 1994; Qu et al., 2003).

Overall, except for the 5.0wt%Ni26wt%Mo/OCP_f catalyst that revealed a first peak reduction temperature at 372°C, the first peak reduction temperature for all the remaining catalysts under study was centered around 362°C. Thus, the 5.0wt%Ni26wt%Mo/OCP_f catalyst with the highest Ni and Mo composition among all the catalysts under study exhibited the greatest difficulty in reduction. Also, for the remaining catalysts it can be inferred that variation of Ni and Mo in any of these catalyst combinations only resulted in a slight change in the first peak reduction temperature and consequently the effect of metal support interaction from these catalysts may have similar impact on hydrotreating performance. However, the hydrogen consumption intensity increased drastically as Ni compositions increased from 2.5 to 5.0 wt% as compared to the slight jump in hydrogen consumption intensity with increasing Ni composition from 2.5 to 3.5 wt%. From Figure 6, it was obvious that hydrogen consumption intensities were also higher for the catalysts with 19 wt% Mo compositions than catalysts with Mo compositions of 13 wt% since more active sites were present in the former than the later. (Grimblot, 1998; Afanasiev and Bezverkhyy, 2007). Additionally, the high MoS₂ active sites accompanying high compositions aided in shifting the peaks to lower temperatures as in the case of the 5.0wt%Ni13wt%Mo/OCP_f and 5.0wt%Ni19wt%Mo/OCP_f catalysts.

The catalysts with 5.0 wt% Ni composition are expected to have high HDS and HDN activities because, the highest hydrogen consumption intensities displayed by these catalysts especially in the case 5.0wt%Ni19wt%Mo/OCP_f catalyst is advantageous since this characteristic renders the MoO₃ present in these catalysts, to undergo rapid transformation to a Type II Ni-Mo-S phase associated with enhanced activity (Park et al., 1997; Yin et al., 2011). This assertion from TPR analysis implies that the hydrogen consumption intensities, which indicate higher extent of reducibility of MoO₃ species, can have a significant impact on creating Type II Ni-Mo-S phase and inducing higher hydrotreating performance. From the TPR study, it is also noted that the metal-support-interaction is slightly impacted by variation in metal loadings on OCP_f-supported catalysts due to similarities in the first peak reduction temperature values displayed by these catalysts in Figure 6.

Figure 7 and 8 show the results of the effects of temperature on the HDS and HDN activities of all the different catalysts after a rigorous long-lasting test. Overall, although small differences in activity results were realized the results are very meaningful. In all cases, both HDS and HDN activities increased with increasing temperature. A rise in HDS and HDN activities with increasing temperature is attributed to higher reaction rates and conversions accompanying an increase in temperature (Speight, 2000). Thus, maximum HDS and HDN activities were obtained at the highest operating temperature of 370°C. Overall, the activity results of all catalysts revealed that HDS processes exhibited higher activities than the HDN processes under the different temperature conditions. This trend is acceptable because, it is much easier to convert sulfur compounds with less refractory linkages than total nitrogen compounds comprising of the most refractory compounds. Total nitrogen compounds in gas oils exist in two forms i.e. basic and non-basic nitrogen compounds, and in terms of activity the non-basic nitrogen compounds contribute to the low activities exhibited by total nitrogen compounds since they are less reactive (Laredo et al., 2001; Botchwey et al., 2003; Sano et al., 2004).

Results from Figure 7 shows that at 370°C, the HDS activities for the 2.5wt%Ni19wt%Mo/OCP_f, 3.5wt%Ni13wt%Mo/OCP_f, and 3.5wt%Ni19wt%Mo/OCP_f catalysts were almost the same (∼85%). However, these catalysts exhibited a steady increase in HDN activities as depicted in Figure 8. Also, from Figure 7 doubling the compositions of Ni and Mo as in the case of the 2.5wt%Ni13wt%Mo/OCP_f catalyst and 5.0wt%Ni26wt%Mo/OCP_f catalyst did not double the conversions in a direct scale up manner. Activity results from these two catalysts indicated that a slight decrease (∼1%) in HDS activities occurred when the Ni and Mo compositions were doubled, whereas a slight increase (∼4%) in HDN activities occurred by doubling the metal compositions. This implies that doubling the metal compositions have insignificant impact on the conversions as it overloads the support with crystalline phase, therefore not a good choice in catalyst formulation.

For the 2.5wt%Ni13wt%Mo/OCP_f catalyst, although the use of this catalyst in the preliminary study mentioned in the abstract and reported elsewhere resulted in an HDS and HDN activity of 78 and 25%, respectively the HDS and HDN activities further increased to 89 and 42% using the same catalyst composition and light gas oil in this study. The enhancement of the catalyst activities in this study can be attributed to an improvement in textural properties, percentage dispersion and reducibility brought about by a reduction in particle size after sieving since, more fine materials with better characteristics were collected and used for catalyst formulation by using a particle cut-off size of 250 microns in this study versus a particle cut-off size of 297 microns used in the preliminary study. Additionally, the improvement in HDS and HDN activities for the 2.5wt%Ni13wt%Mo/OCP_f catalyst used in this study may be due to the LHSV value 1 h⁻¹, which was used in sulfiding the catalyst in this study prior to all hydrotreating experiments compared to LHSV value of 2 h⁻¹ used in sulfiding a similar catalyst composition in the preliminary study (Aryee et al., 2021). Less LHSV means more contact time between the catalyst and liquid during sulfidation, and this increases the active sites with subsequent increase in activities (Satterfield, 1991). Findings from this work also demonstrated that high HDS activities can be achieved with a NiMo/OCP_f catalyst having lower metals loading (2.5 wt% Ni and 13 wt% Mo) and/or higher metals loading (5.0 wt% Ni and 19 wt% Mo).

On the other hand, it was found that among all the catalyst used in this study use of the 5.0wt%Ni19wt%Mo/OCP_f catalyst led to the highest hydrotreating performance for both HDS (90%) and HDN (50%) activities due to its high ability to consume hydrogen (TPR results) that translates to easier formation to a Type II Ni-Mo-S phase with enhanced activity (Yin et al., 2011; Park et al., 1997). Additionally, this catalyst exhibited a high metal dispersion and optimal pore diameter from Co-chemisorption and XRD analysis. The minimum HDS (84%) and HDN (42%) conversions were also obtained with the 2.5wt%Ni19wt%Mo/OCP_f catalyst because, of its low percentage dispersion (4.4%) coupled with it having the highest pore diameter (∼13 nm). Even though this pore diameter was the highest amongst all the catalysts, it may not be the optimal. This is because, it would allow reactant molecules to access the catalytic sites without making maximum contact at the active sites for maximum activity results. Although the 5.0wt%Ni26wt%Mo/OCP_f catalyst had the lowest surface area of ∼78 m²/g and exhibited the lowest percentage metal dispersion of 3.8% and highest reducibility temperature, these properties did not have any negative impact on its corresponding HDS and HDN activities as the effect from its high hydrogen consumption intensity from TPR result in Figure 6 outweighed or nullified the effect from the other characteristics. High hydrogen consumption also implied that the 5.0wt%Ni26wt%Mo/OCP_f catalyst had a high propensity of forming a Type II Ni-Mo-S with enhanced activity as confirmed by Yin et al., 2011 (Park et al., 1997; Yin et al., 2011).