Textural properties
The DRM catalytic performance is affected by the textural properties of the catalyst. As a result, Fig. 1 depicts textural qualities in terms of surface area and porosity (N2 adsorption-desorption) as well as crystal phase (XRD). The N2 adsorption-desorption isotherm of NiO/AS catalysts was depicted in Fig. 1A. All of the catalysts exhibit a type III isotherm with an H3 hysteresis loop. This shows that all NiO/AS catalysts are nonporous or macroporous solids with weak adsorbent-adsorbate interactions (Thommes et al. 2015). Thus, the H3 form of the hysteresis loop reveals that these macroporous solids are made up of pore networks that are only partially filled with pore condensate (Thommes et al. 2015).
However, nickel impregnation boosted the produced catalysts' ability to adsorb, while also increasing BET surface area, SBET; pore size, and pore volume (see Table 1). The following orders caused an increase in pore volume and SBET: 15% NiO/AS, 10% NiO/AS, and 5% NiO/AS It was discovered that increasing the NiO loading lowered SBET and pore volume due to pore obstruction by NiO particles (Shamsuddin et al. 2021a). Compared to 5%NiO/AS and 15%NiO/AS, 10%NiO/AS had the largest pore size because it formed larger pores (5–50 nm). The pore size distribution of xNiO/AS catalysts is shown in Fig. 1B as bimodular porosity with small mesopore (4nm) and wide mesopore (5–50 nm) (Groen et al. 2003). Furthermore, when compared to others, 15%NiO/AS (refer to Fig. 1B*) has the largest distribution of narrow mesopore type. This demonstrates that metal addition displayed good sustentation of catalyst porosity (Kaydouh et al. 2015).
The phase composition of crystallized NiO/AS catalysts was shown in Fig. 1C. All AS-supported catalysts produce SiO (JCPDS 01-0378) and 3CaO.2SiO2 (JCPDS 02-0323). The intensity peaks of NiO (ICDD 00-022-1189) correspond to the planes of cubic NiO species and grow as NiO loading increases at 2θ (h,k,l) = 37°(111), 43°(200), 62°(220), 75°(311), and 79°(222). Thus, the creation of intermetallic CaNi5 (JCPDS 19–0244) demonstrates a significant interaction between active metal (Nickel) and support (Penner and Armbrüster 2015). Additionally, NiO crystallite sizes (see Table 1) obtained using the Debye-Scherrer equation at 2θ = 43° show that NiO/AS catalysts grew in the following orders: 5% NiO/AS, 15% NiO/AS, and 10% NiO/AS. As a result, Table 1 reveals that 15%NiO/AS has the most extensive NiO dispersion percentages when compared to 5%NiO/AS and 10%NiO/AS. In contrast to 15%NiO/AS, 10%NiO/AS showed a somewhat greater intensity of intermetallic CaNi5, which resulted in a lesser dispersion of NiO. In essence, the development of narrow mesopore types leads to an improvement in the size of the NiO crystallized phase and an increase in catalyst NiO dispersion.
Table 1
Physicochemical properties of NiO/AS catalysts
| Properties | SBET (m2g− 1) | Pore Size (nm) | Pore Volume (cm3g− 1) | NiO Crystallite size (nm)* | NiO Dispersions (%)** | TPR-H2 | TPD-CO2 |
| Catalysts | Temp (℃) | Amount (µmolg− 1) | Temp (℃) | Amount (µmolg− 1) |
| AS | 4.99 | 15.33 | 0.0065 | n/a | n/a | n/a | n/a | 571 851 | 109.72 6.32 |
| 5%NiO/AS | 15.84 | 15.31 | 0.0349 | 39.49 | 22.4 | 403 601 | 198.32 1709.74 | 307 693 | 26.88 133.56 |
| 10%NiO/AS | 13.21 | 25.30 | 0.0345 | 66.98 | 37.9 | 439 603 | 267.14 2562.45 | 402 693 824 | 207.59 317.29 107.48 |
| 15%NiO/AS | 12.60 | 17.28 | 0.0282 | 47.73 | 42.2 | 425 519 | 1547.82 2045.53 | 399 723 784 | 18.41 154.61 35.74 |
| * derived from Debye-Scherrer equation obtained by NiO peaks at 2θ = 43° |
** derived from H2-TPR results as suggested by Fadoni et al. (1999).
Surface morphology by FESEM-EDX
The xNiO/AS catalyst's surface morphological characteristics have been examined to ascertain the impact of the surface structure on the physicochemical properties of the developed catalyst. In accordance with Fig. 2, the images retrieved from the AS support (Fig. 2A) showed a coral-like formation with several brunches all around the catalyst. It's interesting to note that the produced image displays hierarchical units attached together to form a coral shape made up of a number of aggregates resembling dendrites. As the quantity of NiO loading on support increases, it is added gradually to AS modifications.
Images of a 5%NiO/AS catalyst in Fig. 2B demonstrate very minor alterations to the catalyst's structural integrity. Images at higher magnification show the development of nodules at a specific location on the surface of the catalysts. These nodules' presence is evidence that AS's surface was effectively impregnated with NiO. This impregnation is therefore demonstrated by an elevated plot level (approximately 5%) of EDX. The surface morphology of the catalyst is significantly altered when the NiO loading is increased by 10% (Fig. 2C). A towel-like structure has formed on the catalyst's surface subsequent to the coral-like structure collapsing. Apart from that, larger NiO particles started to emerge between the AS structure's gaps. Numerous nodule particles (grainy-like) were scattered on these towel-like structures as a result of higher impregnation of NiO up to 15%. A high-magnification image demonstrates evenly distributed rough particles, supporting the conclusion of the NiO dispersion degree (Table 1). As a result, these exceptional qualities are anticipated to enhance DRM catalytic performance.
Reduction behavior
To explore reduction behaviors, the estimated oxygen storage capacity of each metal oxide was measured using the TPR-H2 analysis (Samsuri et al. 2020). The reducibility. The reducibility of active metals is crucial in the DRM reaction because it offers vacancy active sides, which oxygen abandons throughout the reduction process (Puigdollers et al. 2017). According to TPR-H2 analysis conditions, Fig. 3a depicts the reduction behavior of NiO/AS catalysts. The flat line on the TPR-H2 spectra indicates that, as would be anticipated, oxides like SiO2 and CaO in AS support were not reducible (Puigdollers et al. 2017). Significantly distinct peaks are produced when nickel is impregnated into an AS support, and these peaks correlate to the reduction behavior of NiO species on catalysts. Most NiO/AS catalysts displayed two dominating peaks, each of which indicated two reduction processes. The first notable peak occurs between 300oC and 450oC owing to the reduction of Ni2+ to Ni0 species on the surface of catalysts (Shamsuddin et al. 2021b). Evidently, 15%NiO/AS exhibits the maximum quantity and intensity (see Fig. 3a) at lower temperatures when compared to 5%NiO/AS and 10%NiO/AS. Metal-support interaction has been found to be substantially linked with catalyst reducibility. At lower temperatures, a decreased peak was created by weak contact between NiO and the AS support. However, significant interaction results in a greater temperature peak decrease. The above finding is consistent with(Lokteva and Golubina (2019) that lower metal loading would result in less metal-support interaction.
Additionally, the larger dispersion of NiO species in catalyst surfaces also had an impact on the low reduction temperature. As NiO loadings were increased, the percentages of dispersion of NiO in produced NiO/AS catalysts increased. Furthermore, the lowest peak intensity of 10%NiO/AS indicates that the NiO species in this catalyst is in a bulky form (largest NiO crystallite size), which is harder to reduce and may interfere with the catalyst's catalytic behaviours. As a result, the size of the crystallites increases in the following order: 5% NiO/AS, 15% NiO/AS, and 10% NiO/AS. According to (Soled et al. 2003), the strong association of metal to support makes it more difficult to decrease than related bulk metal oxides(Soled et al. 2003). Consequently, greater reduction temperatures must be achieved to provide an active metal phase, resulting in poor metal dispersion. In contrast, poor metal-support interactions result in outstanding metal oxide dispersion and decreased crystallite size, resulting in reductions at lower temperatures (Soled et al. 2003). Therefore, an appropriate compromise between the support and the precursor could be reached by optimal metal loading, which improved metal oxide dispersion, crystallite size, and reducibility behaviors.
The subsequent findings (Fig. 3B) were obtained by XRD scanning of reduced-xNiO/AS catalysts under the same conditions. The comparative investigation of NiO/AS with reduced-xNiO/AS catalysts reveals the production of new metallic Ni (ICDD 01-087-0712) phases at 2θ = 44.5° (111), 51.8 (200), and 76.4 (220), which greatly obliterate the NiO phase in the diffractogram. SiO2 (ICDD 01-0378), 3CaO.2SiO2 (ICDD 02-0323), and CaNi5 (ICDD 19–0244) phases all show significant modifications. The peak intensities of 3CaO.2SiO2 and CaNi5 were slightly less intense than those of the SiO2 peaks due to the high-temperature impact observed during TPR-H2 analysis compared to the NiO/AS catalyst. It has been established that ranging materials' response to temperature affected the change of lattice parameters and the decomposed (Shen et al. 2018).
Basic sites properties
The core of the catalytic DRM process is initiated by acid-base interactions and is sustained by reaction cycles. Particularly, CO2 reagents are strong acidic molecules and only interact with the catalyst with basic sites. Precisely as a result, basic sites play an essential part in enhancing acidic CO2 activation, inhibiting carbon formation on active sites, and enhancing catalytic durability [19]. TPD-CO2 may therefore be used to evaluate these basic sites and their unique characteristics in both quantities and qualitatively. Corresponding to the CO2 desorption temperature, Fig. 4 showed three types of basic strength, which were weak (250°C), medium (250–500°C), and strong (> 500°C) (Lahuri et al. 2017). The strongest basic sites are typically found at temperatures of 571°C and 851°C, by AS support. The formation of CO2 ligands such as monodentate, bidentate, polydentate, and bicarbonate species between the CO2 reactant and CaO basic sites matched these strong basic sites [9]. The availability of CaO as a basic site is less abundant than SiO2, which results in a somewhat decreased CO2 adsorption capacity in this support (see Table 1). Consequently, XRD analysis demonstrated that the production of intermediate species of 3CaO.2SiO2 reduced the creation of CO2 ligands.
In comparison to AS support, the TPD-CO2 profiles of xNiO/AS catalysts exhibit substantial variations due to the resulting formation of moderate basic sites and peak shifting of strong basic sites. The presence of free O2− anions from the NiO species, which are provided by surface defects on the catalysts, strengthens the basic side nature as NiO is impregnated [15,20]. Based on the findings, the qualitative analysis of CO2 desorption shown in Table 1 revealed that 10% NiO/AS (207.6 mol g− 1) had the maximum CO2 adsorption capacity of medium basic sites, followed by 5% NiO (26.9 mol g− 1) and 15% NiO/AS (18.4 mol g− 1). Strong basic sites expand as well in the following sequences: 15%NiO/AS (190.3 mol g− 1), 10%NiO/AS (424.8 mol g− 1), and 5%NiO/AS (133.6 mol g− 1) accordingly. It was discovered that strong basic sites had a significant impact on DRM reaction because the temperature of the reaction was remarkably closest to as temperature at CO2 desorbed. Yet 10% NiO/AS had a greater possibility for DRM reaction because of its advanced strong basic sites rather than 5% and 15% NiO/AS, excessive strong CO2 bonding with AS support resulted in large thermodynamics that necessary more energy to break these intermolecular bond [9]. Based on the qualities stated, 15%NiO/AS might be potential candidates for DRM reaction.
Catalytic Performance of NiO/AS Catalysts
Catalytic evaluations must be performed to determine reactant conversion, selectivity of desired product, and/or yield of product generated from the catalytic process in order to evaluate the superiority of produced catalysts. The stability and reusability of the catalyst toward deactivation are therefore equally important when investigating those attributes (Thybaut and Martin 2010). Figure 5 depicts the catalytic performance as it relates to NiO concentration, reaction temperature, catalyst loading, and gas hourly space velocity (GHSV). Initially, the impact of NiO concentration is investigated in order to determine the optimal xNiO/AS catalysts for the DRM process. As a result, only the best xNiO/AS catalysts were evaluated for the succeeding reaction temperature, catalyst loading, and GHSV.
As previously described in the section on the physicochemical characteristics of xNiO/AS catalysts, NiO concentration is a critical parameter to consider in this work. As a result, the catalytic activity of 5–15% NiO concentration was found at optimal conditions previously described(Shamsuddin et al. 2021) CH4:CO2 (50:50), 750°C, 2g catalyst, and GHSV 9000 h− 1 for 10 h reaction. Following that, Fig. 5a depicted the conversion of CH4 and CO2 as well as the selectivity of H2 and CO towards NiO concentration. Clearly, the 15%NiO/AS catalyst outperforms the others in terms of reactant conversion (90% CH4; 80% CO2) and product selectivity (50% H2; 40% CO), with 5%NiO/AS (40% CH4 & CO2; 25% H2 & CO) and 10%NiO/AS (30% CH4 & 40%CO2; 25%H2 & 30% CO). The higher reactant conversions and selectivity of the product by 15%NiO/AS are attributed to the catalyst's physicochemical characteristics, which play a crucial part in boosting catalytic performance during the DRM process. It should be noted that the chemical properties of DRM govern its catalytic effectiveness. The rapid reducibility and strong basic sites of 15%NiO/AS correlated to better reactant conversion and product selectivity. Despite SBET and porosity being low compared to 10% NiO/AS, the percentage of NiO dispersion has a greater influence on DRM response. Therefore, it is possible that the reaction happened on the surface of the catalyst rather than in the constructed pore, where the greater distribution of NiO active sides boosted catalytic efficiency.
Reactant conversion and product selectivity were shown in Fig. 5B as a function of reaction temperature. Typically, the results show that around 90% of CH4 and 80% of CO2 had been converted, resulting in approximately 40–60% H2 and CO selectivity. Yet, the reaction temperature of 800°C produced an unusual result due to a large drop in reactant conversion (approximately 85% CH4 and 70% CO2) and product selectivity (about 50% H2 and 40% CO). It is expected that increasing the temperature of the reaction from 700°C to 900°C considerably improved catalytic performance. Nevertheless, the catalytic activity of 15%NiO/AS at 800°C was significantly reduced due to parallel reactions such as the water-gas shift (WGS) reaction Eq. (1–3) and CO disproportionation (Bourdouard reaction) Eq. (4) given in the following equation;
water-gas shift (WGS) CO + H2O ⇌ CO2 + H2, ∆H 298K = − 41 kJ mol− 1 (1)
C + 2H2O ⇌ CO2 + H2, ∆H 298K = + 90 kJ mol− 1 (2)
H2O + C ⇌ CO + H2, ∆H 298K = + 131.3 kJ mol− 1 (3)
CO disproportionation 2CO ⇌ C + CO2, ∆H 298K = − 172.4 kJ mol− 1 (4)
Notably, WGS reactions that produced improved CO2 utilizing CO as the reactant had worse catalytic performance primarily on CO2 conversion and CO selectivity (refer to Eq. (1)). Hassan Amin and Bhargava confirmed that the WGS and Boudouard reactions occurred at temperatures lower than 817°C (1090K) and 701°C (974K), respectively. Since the ratio of H2/CO is almost unity over 817°C, WGS above that point may be ignorant (Hassan Amin 2018). In addition, this reversed trend was induced by outstanding decarbonization of CaCO3 by CaCO3/CaO system on SiO2 ideally at 800°C, as confirmed by Khosa and Zhao (2019).
Figure 5C was produced by conducting 15%NiO/AS evaluations with varied catalyst loadings packed on a reactor system. In accordance with the catalytic performance, increasing the catalyst loading reduces reactant conversions and selectivity of the products in the following order: 0.1 g > 0.2 g > 0.3g > 0.4g > 0.5g. The most favorable catalytic performance was achieved with 0.1 g catalyst loading, with > 95% CH4 and CO2 conversion and > 50% H2 and CO selectivity. By adding 0.2 g 15NiO/AS catalyst with up to 60% H2 selectivity, excellent H2 production was achieved. Since catalyst loading correlated with the surface reaction of the reactant to the catalyst, too little loading may result in a lack of active sites on the catalyst. On the other hand, excessive loading might inhibit surface interaction between the reactant and active sides because the 'packed' state between particles limits the exposed surface on the catalyst. With respect to the results, it can be concluded that moderate catalyst loading increases catalytic performance in the DRM process, as agreed by Sulaiman et al. (2018) and Inga and Morsi (1997).
The effect of GHSV on the catalytic performance of 15%NiO/AS is shown in Fig. 5D, with reactant conversion ranging from 80–98% (CH4 and CO2); 60–65% for H2 and 38–42% for CO selectivity, respectively. Despite GHSV 6000h− 1 demonstrating a modest decrease in reactant conversion, with around 82% CH4 and 75% CO2 conversion, this minor decline had no effect on total catalytic performance. Conceptually, GHSV is associated with the contact period between the reactant and catalyst, which was determined by the changing flow rates of all reactant mixtures (Yaghobi 2013). The extended contact durations resulted in increased product production by approaching thermodynamic equilibrium. Elkhalifa and Friedrich (2019) clarify that a longer duration of contact times which means low GHSV results in the higher formation of the product by reaching thermodynamic equilibrium. On the contrary, shorter contact times (greater GHSV) resulted in a sufficient period for the reactant and catalyst to react, followed by a decrease in catalytic performance. As a result, elevated GHSV likely lowered product selectivity by favouring homogeneous reactions. As a result of the outstanding physicochemical properties of the catalyst, 15%NiO/AS generated remarkable performance on GHSV from 3000 to 15000 h− 1 based on the GHSV plot.
X-ray Photoelectron Spectroscopy (XPS) of fresh and spent catalysts
XPS comparison studies for fresh and spent 15%NiO/AS to evaluate chemical states and surface species (element oxidation states) involved during DRM and created after the reaction. Figure 6A depicted a wide deconvoluted peak for components found on pristine and spent catalysts. The elements discovered by the wide range scan from 0 to 1200 eV are Ni, O, C, Ca, and Si, which coincide with XRF analysis. All elements showed significant variations in peak intensity; Ni, O, Ca, and Si display low-intensity peaks due to low concentration after the DRM reaction except C. As a result, other elements were impeded by the high concentration of carbon, as shown by the high intensity of C peaks.
Figure 6B indicates a narrow scan of C1s that was implemented for peak correction as the reference value at a binding energy of 284.5 eV (Moulder and Chastain 1992). The observed spectra were fitted with three carbon species, including C1s (from carbon tape during sample preparation), C = O (287 eV), and CO2 (290 eV) predicted from adsorbed-CO2 on the catalyst surface. In contrast, the spent catalyst spectrum has identical peaks as the fresh catalyst spectrum together with two distinct peaks reflecting coke production from isolated carbon (C) at binding energy 285.1 eV and carbide species, Ni3C (283.8 eV). (Favaro et al. 2017) agreed that at 25°C (298K), CO2 binds physically (physisorption) with a thin suboxide structure (in this instance CaO), as demonstrated by APXPS analysis. (Favaro et al. 2017). Thus, the isolated carbon and carbide species formation from XPS analysis was proven by (Zhang and Koka 1998).
Figure 6C shows a narrow scan from the core level element of Ni 2p with an energy binding range of 845 to 890 eV. The fresh catalyst exhibited significant peaks that corresponded to three Ni chemical states: NiO 3/2 (854.9 eV), Ni(OH)2 3/2 (856.9 eV), and NiO satellite, correspondingly. The formation of nickel carbides (Ni3C) peaks at energy bound 852 eV caused substantial changes on 15%NiO/AS spent catalyst. Browning and Emmert (1952) recognized that the formation of Ni3C during the DRM reaction was associated with the CH4 dissociation mechanism as the following equation (Browning and Emmett 1952).
3Ni + CH4 ⇌ Ni3C + 2H2 K1 = (H2)2/CH4 (5)
Thus, the free energy generated by Ni3C species demonstrates that it is a favourable reaction; nevertheless, the influence on the H2/CH4 ratio is unaffected by this formation since free carbon formation is decreased or occurs gradually at low temperatures and is only affected by thermal diffusion and contaminants in the equilibrated gas (Browning and Emmett 1952). This Ni3C formation is consistence with C 1s peaks as discussed before.
Evaluation of the support generates Figs. 6D and 6E, which were ascribed to the narrow scan of Si 2p and Ca 2p at binding energies of 98–110 eV and 342–354 eV, respectively. Based on the peaks observed, the pristine catalyst only shows SiO2 peaks of 103.7 eV, which is consistent with those reported by (Senna et al. 2018). The formation of SiO4 at BE = 106.2 eV, together with accessible SiO2 peaks, considerably affects Si 2p binding energy. As a result, the drop in electron density near Si was followed by silicon oxidation from its normal state of Si4+ to higher oxidation states. Additionally, the Ca 2p narrow scan reveals two noteworthy CaO and CaCO3 peaks at binding energies of 346 eV and 347 eV, respectively. The formation of CaCO3 is caused by adsorbed-CO2 bonded into CaO, as seen by C1s peaks. Nevertheless, peak deconvolution of spent catalyst for Ca 2p is not feasible due to insufficient element concentration to discriminate between peaks and background scans. Meanwhile, as shown in Fig. 6F, a narrow scan of O1s reveals the existence of three primary species. In accordance with the results, peaks of SiO (531.6 eV), CaO (530 eV), and NiO (529 eV) were identified. However, there are considerable changes adhering to the DRM reaction, particularly on SiO2 peaks (533.6 eV), which were found on the spent catalyst. Hashimoto et al. concurred that this shifting can be attributed to the decrease of SiO2 species caused by ion bombardment, which can result in changes in FWHM (full width at half-maximum) and chemical transformations (Hashimoto et al. 1992).
Spent Catalysts Analysis
In order to improve the catalyst's catalytic performance and stability/reusability, it is crucial to take into account coke production, sintering, and changes in chemical composition. Thus, Fig. 7 depicts the evaluation of the spent 15%NiO/AS catalyst using FESEM and TGA to monitor potential occurrences during and after a 10-hour DRM process. The empirical comparison of fresh (Fig. 7A) and spent (Fig. 7B) samples demonstrate evidence of carbon deposition and surface morphological changes that occur throughout the DRM reaction. Pristine catalyst surface morphology exhibits a 'towel-like' structure with nodules, as previously described. Yet adhering to the DRM reaction, the spent catalyst transforms into a fibrous structure with no nodules on the surface. The appearance of a nanotube structure in a higher magnification image is caused by the production of carbon nanotubes (CNTs) embedded in the catalyst surface with a diameter of around 60nm. It is widely acknowledged that the process of carbon or coke generation on the catalyst's surface during DRM involves two side reactions, namely CH4 breakdown (Eq. (5)) and the Boudouard reaction (Eq. 6), as shown by the following equation (Arora and Prasad 2016);
CH4 ⇌ C + 2H2 ∆Ho = 75 kJ mol− 1, ∆G = 2190-26.45T kJ mol− 1 (5)
2CO ⇌ C + CO2 ∆Ho = -171 kJ mol− 1, ∆G = -39810 + 40.87T kJ mol− 1 (6)
Based on the thermodynamic calculation, the equilibrium constant for this two sides reaction is given by Ki = exp (- ∆Goi /RT). Thus, the coke formation were thermodynamically favoured if K6 > α and K7 > β which α = (\({p}_{{H}_{2}}\)^2 / \({p}_{{CH}_{4}}\)^)/ P0, β = (\({p}_{{CO}_{2}}\)^ / pCO ^2)/ P0, P0 = 105 Pa and p = partial pressures (Ginsburg et al. 2005).
TGA and DTG thermograms were shown in Fig. 7C, which are ascribed to material/chemical generated during the breakdown of 15%NiO/AS fresh and spent catalyst. The fresh catalyst exhibits three distinct degradation phases, beginning with the elimination of moisture content at Tmax=72°C (2.77% wt). Since SiO2 and Ni(OH)2 concentration increased water molecule adsorption, the 15%NiO/AS catalyst generated moister peaks in the TGA thermogram. Increases in temperature of up to 500°C resulted in a second degradation phase at Tmax=604°C caused by CaCO3 breakdown on the catalyst's surface. As demonstrated by the XPS Ca 2p narrow scan, the appearance of CaCO3 was removed as shown by Eq. (7);
CaCO3 (s) \(\underrightarrow{\varDelta }\) + CaO (s) + CO2 (7)
According to Eq. (7), the decomposition of CaCO3 produced CaO and CO2. (Bilton et al. 2012) found that the breakdown of CaCO3 began at 600oC and is supported by traces of CaO at the same temperature (Bilton et al. 2012).
In contrast, the spent catalyst begins to undergo degradation phases at Tmax=536°C, followed by Tmax=597°C. The formation of two distinct peaks might be attributed to the development of two different types of coke deposited on the catalyst's surface. There are several forms of carbon species deposited during the DRM process, including adsorbed/atomic carbon in surface carbide form (C); polymers/amorphous films (C); Ni carbide in bulk form (C); vermicular filaments or whiskers (Cv); and graphite (crystalline) platelets films (Cc) (Arora and Prasad 2016). It is anticipated that the decomposition of Cv types of carbon species, which corresponded to CNT, was observed at Tmax=536oC with 21.06%wt found based on TGA and DTG thermogram. Cc forms of carbon are estimated to be degraded with 20.34%wt during TGA evaluation at Tmax=597oC. Images obtained from FESEM (Fig. 7B) demonstrate the production of filaments or whiskers and graphite kinds of carbon as a result of the filamentous uneven surface morphology identified on the spent catalyst. Increasing the temperature over Tmax=938oC produced the final degradation caused by the SiO2 breakdown or flaws in the support material. At the measured temperature, a substantial value of 3.16%wt was recorded on fresh catalysts but insignificant weight percentages on wasted catalysts due to extremely low weight loss. A previous study byHoffman et al. (1987) proved that at temperatures between 900°C and 1000°C, the oxide decomposition reaction of Si + SiO2 → 2SiO caused a defect or decomposition in the SiO2 structure during high-temperature annealing.