Diatomites are naturally abundant materials consisting mainly of SiO2 (~60–80 wt.%) associated with organic matter (Hao et al., Reference Hao, Dai, Guan, Dang, Wang, Yan and Li2021). Thus, they are environmentally friendly biogenic sediments, and they qualify as ecological materials (Fraine et al., Reference Fraine, Seladji and Aït-Mokhtar2019; Shen et al., Reference Shen, Wang, Yu, Li, Lu and Kong2021). Diatomite is mainly composed of amorphous silica with an abundant silica hydroxyl group consisting of fossil remains of diatom frustules (Shen et al., Reference Shen, Wang, Yu, Li, Lu and Kong2021). In its pure state, it appears as a fine, white, very porous, lightweight, friable sediment (Taoukil et al., Reference Taoukil, El Meski, Lahlaouti, Djedjig and El Bouardi2021). In the Sig region (Mascara) of north-west Algeria, a diatomite deposit of Miocene age has formed in a lacustrine environment. The diatomite reserves are estimated to be several million tons (Meradi et al., Reference Meradi, Bahloul, Boubendira, Bouazdia and Ismail2015). The Algerian diatomite, also called kieselguhr, has a whitish colour and a high porosity, exceeding 72%.
Diatomite is commonly used in engineering and environmental applications (Xia et al., Reference Xia, Liu, Chen, Fang, Du and Zhang2020) as an adsorbent, filter and catalyst support (Yusan et al., Reference Yusan, Bampaiti, Aytas, Erenturk and Aslani2016; Hao et al., Reference Hao, Dai, Guan, Dang, Wang, Yan and Li2021). Previous studies have shown that diatomite is an important support for catalysts that remove heavy metal ions and dyes (Van Viet et al., Reference Van Viet, Van Chuyen, Hien, Duy and Thi2020), in explosives as a stabilizer (Guhr dynamite), as a material for tubular filters, as an insecticide and recently as a building material in the production of bricks (Lauermannová et al., Reference Lauermannová, Lojka, Jankovský, Faltysová, Pavlíková and Pivák2021). The periodic pores of diatomite give it advanced characteristics, such as the adsorption capacity and characteristics of photonic crystals at the nanoscale (Shen et al., Reference Shen, Wang, Yu, Li, Lu and Kong2021), and it can be used for thermal insulation due to its low thermal conductivity, as well as for the manufacture of antibiotics and some pharmaceutical syrups (Fraine et al., Reference Fraine, Seladji and Aït-Mokhtar2019). Algerian diatomite has many important industrial applications in sustainable and environmentally friendly development (Mansour et al., Reference Mansour, Bessedik, Saint Martin and Belkebir2008; Meradi et al., Reference Meradi, Bahloul, Boubendira, Bouazdia and Ismail2015). These applications include the purification of drinking water (Hadjar et al., Reference Hadjar, Hamdi, Jaber, Brendlé, Kessaissia, Balard and Donnet2008; Cherrak & Hadjel, Reference Cherrak and Hadjel2016), filtration (Hadjar et al., Reference Hadjar, Hamdi and Ania2011), as abrasive agents and anti-explosive agents (Balaska et al., Reference Balaska, Samar, Meradi, Abbess and Leksir2008; Meradi et al., 2016) and as ceramic and insulating materials (Benayache et al., Reference Benayache, Alleg, Mebrek and Sunol2018; Aouadja et al., Reference Aouadja, Bouzerara, Guvenc and Demir2022).
In the field of catalysis, diatomite is used as a catalyst support because of its large specific surface area, uniform pore diameter, regular shape and pore sizes ranging from the nanometre to the micrometre scale (Hadjadj-Aoul et al., Reference Hadjadj-Aoul, Belabbes, Belkadi and Guermouche2005; Liu et al., Reference Liu, Yu, Deng, Yuan, Ma, Yuan and He2016; Dekkar et al., Reference Dekkar, Tezkratt, Sellam, Ikkour, Parkhomenko, Martinez-Martin and Roger2020; Taoukil et al., Reference Taoukil, El Meski, Lahlaouti, Djedjig and El Bouardi2021). All of these properties of diatoms make this material a very promising candidate for the preparation and synthesis of hierarchical porosity catalyst supports applicable in the field of catalysis and chromatography (Li et al., Reference Li, Wang, Le, Qin, Zhang and Zeng2022). The choice of natural resources (aluminosilicates) as support materials is based on their interesting textural properties (specific surface area, pore volume, pore diameter), good catalytic properties, high availability and low cost. The Ni–SiO2-based catalysts are suitable due to their mechanical resistance, good chemical and thermal stability, low cost and high availability (Dekkar et al., Reference Dekkar, Tezkratt, Sellam, Ikkour, Parkhomenko, Martinez-Martin and Roger2020).
The dry reforming of methane (DRM) reaction consumes two main greenhouse gases (methane and carbon dioxide) and produces hydrogen and carbon monoxide in a 1:1 ratio. They can be used directly as raw materials to replace traditional petroleum synthetic chemical products in downstream Fischer–Tropsch synthesis (Raje et al., Reference Raje, O'Brien and Davis1998). Therefore, this appears to be a suitable path for the disposal of both gases, with direct energetic and environmental benefits (Mendoza-Nieto et al., Reference Mendoza-Nieto, Duan and Pfeiffer2018; Abdullah et al., Reference Abdullah, Ainirazali and Ellapan2021; Sophiana et al., Reference Sophiana, Iskandar, Devianto, Nishiyama and Budhi2022). Although noble metal catalysts, such as Ru, Rh, Pd, Ir and Pt, can achieve better catalytic activity and stability (Pakhare & Spivey, Reference Pakhare and Spivey2014; Qin et al., Reference Qin, Chen, Xie, Luo, Su and Ji2020; Moreno et al., Reference Moreno, Ramirez-Reina, Ivanova, Roger, Centeno and Odriozola2021; Niu et al., Reference Niu, Wang, Liland, Regli, Yang and Rout2021), their high cost limits their applicability. At present, the active components of methane dry reforming catalysts are mainly transition metals, such as Ni, Co and Fe, and Ni-based catalysts have been the most widely studied in recent decades due to their low cost and high activity.
In this work, a diatomite soil was used as a support for Ni-based catalysts to study its influence on the catalytic reactivity of the generated oxide particles and to test their performance and the possibility of their application in the field of catalysis. The diatomite powder was physically and chemically characterized and the <63 μm fraction was used as a support to remove all impurities that negatively affect the catalyst support. The catalyst was prepared via the method of impregnation of Ni on the <63 μm diatomite support, and the whole catalyst system was calcined at 700°C for 2 h. The results of converting methane and CO2 to synthesis gas (CO and H2) via the gas chromatography method showed that the catalyst was not deactivated after 6 h of catalytic testing, and the conversion rate was estimated at 12%.
Materials and methods
Materials
The Algerian diatomite (commercial product obtained from the Sig mine in Mascara, north-west of Algeria, a diatomite processing factory) was naturally dried for 48 h, crushed with a FRITSCH laboratory-type jaw crusher and then sieved with a series of standardized sieves (2, 1, 0.5, 0.25, 0.125 and 0.063 mm) with a Retsch sieve shaker. The fine fraction of <63 μm was selected as a support to prepare the Ni catalyst. Nickel (II) nitrate hexahydrate (Ni(NO3)2.6H2O) was used to prepare a catalyst based on nickel (15% Ni), supported by the diatomite.
To prepare 3 g of 15% Ni catalyst, a suitable mass of Ni(NO3)2.6H2O was dissolved in an appropriate volume of water and then impregnated onto the support. The resulting product was then dried at 100°C for 24 h. At the end of the drying process, a 15% Ni-doped sample was obtained. The resulting material was ground manually in an agate mortar and subsequently calcined at 700°C using a tunnel furnace to produce a catalyst named 15Ni/Ds-700.
Methods
Approximately 50 mg of Ni catalyst supported on diatomite powder was introduced into the middle of a tubular quartz reactor with an internal diameter of 6 mm and intercalated with quartz wool. The catalyst was introduced into the tubular furnace fixed vertically so that the catalyst zone corresponded to the central zone of the furnace under reduced conditions with a pure hydrogen flow (40 mL min–1). The catalytic tests were carried out under a gas stream containing CH4 and CO2 (CH4:CO2 ratio = 1) with a volume flow rate of 50 mL min–1 at 700°C (i.e. 25 mL min–1 for each gas). At the end of the reaction, the final quantities of products and reagents were detected using a gas chromatograph (Shimadzu GC-2014).
In the DRM, methane (CH4) and carbon dioxide (CO2) react to produce carbon monoxide (CO) and hydrogen (H2) according to Equation 1:
The conversions of methane and carbon dioxide and H2:CO ratio were calculated according to Equations 2 & 3:
This main reaction of Equation 1 is accompanied by intermediate reactions such as methane dissociation, the Boudouard reaction and the water–gas shift reaction. These secondary reactions influence the final CO and H2 yields according to Equation 4:
where n (H2)out represents the moles of hydrogen produced and 2n (CH4)in represents the maximum theoretical moles of hydrogen that could be produced from the consumed methane.
As CO is produced from the reaction of methane and carbon dioxide, the CO yield is given according to Equation 5:
where n (CH4)in + n (CO2)in represents the total of the reactants (methane and carbon dioxide) consumed in the reaction.
The H2:CO molar ratio is calculated according to Equation 6:
In addition, the carbon balance (X C) is calculated according to Equation 7:
where n expresses the number of moles of CH4, CO2, CO and H2 at the inlet and outlet of the reactor, X CH4 and X CO2 are the conversion rates of CH4 and CO2, respectively, and Y H2 and Y CO are the yields of H2 and CO, respectively.
The H2:CO– molar ratio is calculated according to Equation 8:
where
and A H2 is the activity of H2 (μM/g.s), A CO is the activity of CO (μM/g.s), m cata is the mass of the catalyst (g), $F_{( {CH_4} ) int}$
is the CH4 flow rate at the reactor inlet (μM s–1), $( {n_{{\rm C}{\rm H}_4}} ) _{{\rm int}}$
is the number of moles of CH4 at the reactor inlet, $( {n_{{\rm H}_2}} ) _{{\rm out}}$
is the number of moles of H2 at the outlet of the reactor and (n CO)out is the number of moles of CO at the outlet of the reactor.
The following experimental conditions of the DRM reaction were chosen according to the Challiwala et al. (Reference Challiwala, Ghouri, Linke, El-Halwagi and Elbashir2017): reaction temperatures higher than 650°C, atmospheric pressure and CH4:CO2 ratio equal to 1. The evolutions of CH4 and CO2 conversions over time for the 15Ni/Ds-700 catalysts were recorded under the following conditions: gas hourly space velocity (GHSV) 30 Lh–1 g–1, reaction temperature 700°C and reaction time 6 h. The evolutions of CO and H2 yields and of the H2:CO ratio over time for the 15 Ni/Ds-700 catalysts were recorded under the following conditions: GHSV 30 Lh–1 g–1, reaction temperature 700°C and reaction time 6 h.
Granulometric analysis of crude diatomite
The particle-size distribution of the crude diatomite was determined by passing the material through standardized sieves (2, 1, 0.5, 0.25, 0.125 and 0.063 mm). The <0.063 mm size fraction of diatomite was than analysed using a laser diffraction particle-size analyser (Fritsch Analysette 22 NanoTec Plus).
Chemical and mineralogical analysis
The chemical composition of the diatomite powder support was determined using X-ray fluorescence (XRF; Skyray Explorer7000). The mineralogical compositions of the crude diatomite (Dc), the 2 mm diatomite fraction (D2mm), support diatomite (Ds) and the catalyst before and after the reduction process were determined using X-ray diffraction (XRD; Riyaku Miniflex diffractometer; Cu-Kα radiation; scanning range: 10–70°2θ; scanning step: 0.02°2θ).
Fourier-transform infrared spectroscopy
Fourier-transform infrared (FTIR) spectra were obtained using a Perkin Elmer spectrometer connected to a computer in the mid-infrared (MIR) range of 400–4000 cm–1. The spectra were collected from samples Ds (<63 μm) and D2mm (2 mm diatomite fraction).
Thermal analysis
The thermal behaviour of diatomite was determined by thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) using a Mettler-Toledo SDT-Q600 TA instrument from ambient temperature to 1100°C at a heating rate of 10°C min–1.
Structural and morphological characteristics of the diatomite supports and the catalyst
The structural and morphological characteristics of the support and the catalyst were examined using scanning electron microscopy (SEM; Sirion 200, FEI Co.).
Textural properties of the support and the catalyst
The textural characterization of the support and the catalyst was carried out using adsorption isotherms; the specific surface area was obtained according to the Brunauer–Emmett–Teller (BET) method using a NOVA 1994-2010 instrument (Quantachrome Instruments version 11.0). The pore size and the pore volume of the samples, previously degassed at 250°C, were determined from the low-temperature nitrogen adsorption.
Results and discussion
Particle-size distribution of the crude diatomite
The particle-size distribution of the crude diatomite powder (Dc; Fig. 1) showed a D 50 value (50 wt.% of the material) of <0.98 mm, a D 10 value of ~0.14 mm and a D 90 value of 1.8 mm. The particle-size distribution as assessed using a laser scattering particle-size distribution analyser (Fig. 2) to analyse the fine fraction of diatomite shows that almost 50% of the particles have diameters <4.22 μm, with 10% being <1.13 μm and 90% being <12.40 μm. Two main granulometric populations were noted: the first one at 3.2 μm and the second one at 11 μm, with volume percentages of 3.50% and 3.72%, respectively. The most dominant fraction is of micrometre size, associated with small amounts of particles sized in the nanometre range (<100 nm).
Figure 1. Particle-size distribution of the crude diatomite by dry sieving.
Figure 2. Particle-size distribution of the diatomite support.
Chemical composition of the diatomite support
The crude diatomite powder (Dc; Table 1) is mainly composed of SiO2 (79.38 wt.%), with lesser amounts of CaO and Al2O3 (8.55 and 2.02 wt.%, respectively). Minor amounts of K2O, Na2O and Fe2O3, considered to be fluxing agents, are also present (0.78, 0.59 and 0.97 wt.%, respectively). The SiO2 and Fe2O3 contents of the support diatomite decreased from 79.38 and 0.97 wt.% to 66.35 and 0.24 wt.%, respectively. The CaO content increased from 8.55 to 16.81 wt.%, and the loss on ignition (LOI) values increased from 5.67 to 7.45 wt.% for the Ds and Dc samples, respectively. The LOI of the diatomite support (Ds; 7.45 wt.%) is attributed to the combustion of organic matters and the release of CO2 from the decomposition of the carbonate minerals.
Table 1. Chemical composition (wt.%) of the crude diatomite, support diatomite and 15Ni/Ds-700 catalyst.
The chemical composition of the 15Ni/Ds-700 catalyst (Table 1) showed lower oxide mass ratios compared with the support diatomite. Specifically, the SiO2, Al2O3 and CaO contents decreased from 79.98, 7.13 and 16.81 wt.% to 64.60, 1.92 and 13.24 wt.%, respectively, for the support and the catalyst, respectively. On the other hand, the content of NiO in the catalyst was 15 wt.%, confirming that impregnation was successful. The LOI value of the 15Ni/Ds-700 catalyst (2.16 wt.%) was significantly reduced, suggesting that combustion of organic matter was taking place during the catalyst calcination at 700°C.
Mineralogical analysis of the diatomite
The XRD traces (Fig. 3) show that the crude diatomite (Dc) consists of amorphous SiO2, characterized by a hump at 20–25°2θ. However, this hump diminished in the Ds support and disappeared in D2mm. Quartz was identified in all of the samples (main peak at 26.85°2θ); quartz seems to have been better crystallized in the Ds sample compared to the Dc and D2mm samples. Calcite (CaCO3) and ankerite ((Ca,Mg,Fe)CO3; Hadjar et al., 2008; Aouadja et al., Reference Aouadja, Bouzerara, Guvenc and Demir2022) were identified by peaks at 29.40 and 30.74°2θ, respectively, in all of the samples. The abundance of these phases diminished in the diatomite support (Ds).
Figure 3. XRD traces of Dc, D2mm and Ds. An = ankerite ((Ca,Mg,Fe)CO3); Ca = calcite (CaCO3); Q = quartz (SiO2).
FTIR analysis
All of the functional groups assigned to calcite, ankerite, quartz and water were present in both Ds and D2mm samples but at different band intensities (Fig. 4). The FTIR spectra of the Ds and D2mm diatomite samples show the presence of CO32– bands at 1431, 876 and 727 cm–1 from calcite and ankerite. The deformation band centred at 876 cm–1 is more accentuated in the Ds sample than in the D2mm sample. The bands related to silica (Si–O–Si) occur at 1068 and 484 cm–1 for both samples at the same intensity. A large band centred at 3435 cm–1 and a weak one at 1637 cm–1 are attributed to the stretching and bending vibrations of the OH bond of the adsorbed water; they are slightlyS visible in the Ds sample. The silanol group SiO–H centred at 730 cm–1 is attributed to O–H deformation of the free silica, which is accentuated in the Ds sample.
Figure 4. FTIR spectra of the D2mm and Ds diatomite.
Thermal analysis of the diatomite support
The results of coupled TGA/DSC are presented on Fig. 5. A total weight loss of 18.39% at 1180°C was recorded in three main steps. The first weight loss event (2.57 wt.%) associated with an endothermic peak at 62°C is due to dehydration of diatomite (absorbed moisture). The second weight loss event (2.68 wt.%) is associated with a large exothermic peak at 255°C due to the combustion of organic matter. The third weight loss event (13.14 wt.%), associated with a large endothermic peak at 720°C, is due to both dehydroxylation of silanol groups and carbonate decomposition (Giles et al., Reference Giles, Smith and Huitson1974). Finally, a weak exothermic peak observed at 930°C is caused by the crystallization of diatomite.
Figure 5. TGA/DSC analysis of the diatomite support.
Structural analysis of the diatomite support
The nitrogen adsorption–desorption isotherms of the diatomite support (Ds; Fig. 6) are type-IV isotherms, indicating the presence of both micropores and mesopores (Sun et al., Reference Sun, Chen, Zhao, Wang and Su2018). Based on the De Boer classification (De Boer, Reference De Boer1958), Ds is defined as a mesoporous solid incorporating cylindrical pores (Rabie et al., Reference Rabie, Shaban, Abukhadra, Hosny, Ahmed and Negm2019; Liu et al., Reference Liu, Zheng, Lin, Ni and Song2020).
Figure 6. Nitrogen adsorption–desorption isotherms of the diatomite support. Squares represent results from the adsorption process and circles represent results from the desorption process.
The diatomite support had specific surface area, total pore volume and average pore diameter values of 46.33 m2 g–1, 0.579 cm3 g–1 and 26.55 nm, respectively. The diatomite support is a mesoporous material because the average pore diameter is between 2 and 50 nm (Chen et al., Reference Chen, Li, Li, Chen, Chen, Yang and Liu2021). The specific surface area of the diatomite used in this work was larger (46.33 m2 g–1) than that determined by Chen et al. (Reference Chen, Li, Li, Chen, Chen, Yang and Liu2021) for a Chinese diatomite (20.50 m2 g–1). In addition, the pore volume of the Algerian diatomite support (0.579 cm3 g–1) was larger and the pore size (26.55 nm) was smaller than those of the Chinese diatomite used by Chen et al. (Reference Chen, Li, Li, Chen, Chen, Yang and Liu2021; 0.084 cm3 g–1 and 164.29 nm, respectively).
Catalyst characterization
XRD results of the catalyst before and after the reduction process
The XRD results of the catalyst material before the reduction process (Fig. 7) show that it consists mainly of calcite (CaCO3), nickel oxide (NiO), feldspar and quartz. NiO was observed with an intense 020 peak at 43.22°2θ, which indicates impregnation of Ni in the pores and tunnels of the diatomite structure (Merkouri et al., Reference Merkouri, Le Saché, Pastor-Pérez, Duyar and Reina2022). After reduction with H2 for 4 h, the catalyst 15Ni/Ds-700 consisted of quartz (SiO2), wollastonite (CaSiO3), NiO and metallic nickel (Ni0; Fig. 7). The presence of wollastonite is due to a reaction between CaO and SiO2 during calcination at 700°C. Ni0 is observed at ~44.5 °2θ (111 plane), characterizing the active phase of the catalyst. By contrast, the incomplete reduction of Ni2+ in the catalyst leads to the presence of the NiO phase, which is identified by the peak at ~43.6°2θ (Terekhova et al., Reference Terekhova, Belskaya, Trenikhin, Babenko, Muromtzev and Likholobov2023).
Figure 7. XRD traces of the catalyst material before and after the reduction process. Ca = calcite; Ca-Si = wollastonite (CaSiO3); Ni0 = metallic nickel; NiO = nickel oxide; Q = quartz.
These results are in agreement with the results obtained by Zhang et al. (Reference Zhang, Feng, Liu, Zhao, Song, Zhang and Gao2018), who prepared a Ni/MgOSiO2 catalyst to improve the coking resistance of catalysts in DRM reactions. XRD analysis indicated a strong presence of NiO (peaks at ~39.7, 43.3 and 53.2°2θ). In addition, Ni0 was identified from the peaks at ~44.5 and 51.8°2θ.
Textural analysis of the catalyst
The nitrogen adsorption–desorption isotherms of the catalyst supported on the diatomite powder are typical of type IV isotherms, indicating the presence of both micropores and mesopores (Fig. 8). The catalyst exhibited H3 hysteresis loops in the relative pressure (P/P 0) range 0.2–1.0. This is due to the filling of micropores belonging to the calcined diatomite support (Yao et al., Reference Yao, Lei, Zhang, Sun and Zheng2018; Zijie et al., Reference Zijie, Yuhao, Renji, Zhengzheng, Huimin and Xiangliang2022). Consequently, the specific surface area (25.69 m2 g–1), pore volume (0.20 cm3 g–1) and pore size (15.72 nm) of the catalyst decreased compared to the support before impregnation. This is due to the calcination of the catalyst, which affects the structural characteristics blocking the pores after impregnation, and to the incorporation of NiO groups into the tunnels of the diatomite support (Sun et al., Reference Sun, Chen, Zhao, Wang and Su2018; Liu et al., Reference Liu, Zheng, Lin, Ni and Song2020).
Figure 8. Nitrogen adsorption–desorption isotherms of the prepared catalyst. Squares represent results from the adsorption process and circles represent results from the desorption process.
Structural and morphological characteristics of the diatomite supports and the catalyst
The SEM images of the Dc diatomite powder showed the presence of continuous circular rings closely interconnected, forming a porous network structure with connected tunnels (Fig. 9a). The diatoms are of marine origin with a secondary, regular, circle-shaped structure (Hadjadj-Aoul et al., Reference Hadjadj-Aoul, Belabbes, Belkadi and Guermouche2005). However, the SEM images of the diatomite support Ds before the reduction process revealed the heterogeneous morphology of the grains within the diatomite (Fig. 9b). The pores are circular and have a regular-shaped structure with nanosized dimensions, yielding a high porosity of the support Ds. In addition, impurities surrounding the holes were observed. According to energy-dispersive X-ray spectroscopy (EDS) spectral analysis of a selected zone in the support Ds (Fig. 9c), Si, Ca and O are the main elements in the support. The small amounts of Ni, Mg and Al detected belong to the support, and they are considered to be impurities.
Figure 9. SEM images of (a) the diatomite support Ds and (b) the catalyst before the reduction process. (c) EDS spectrum of the diatomite support. (d) SEM image of the catalyst after the reaction process and (e) EDS spectrum of the catalyst after reaction.
The SEM image of the catalyst after the reaction showed agglomerated and heterogeneously dispersed Ni particles distributed within the diatomite matrix, with almost 13% Ni on the sections and inside the orifices of selected grains (Fig. 9d). Elemental EDS analysis of the selected zone of the catalyst showed that the main elements were Si, Ca, O and Ni. The small amount of Au detected is attributed to contamination (Fig. 9e).
Catalytic performance tests
The XRD traces of the 15Ni/Ds-700 catalyst after the reaction demonstrated the presence of the Ni0 metal phase obtained during the sintering of the catalyst identified from the peaks at 44.51 and 51.83°2θ belonging to the (111) and (200) planes, respectively (Fig. 10). Moreover, peaks of NiO were detected at 43 and 47.1°2θ, belonging to the (111) and (200) planes, respectively. A low-graphite carbon peak with a hexagonal structure appears at 26.38°2θ. The presence of Ni0 and the limited formation of carbon after the reaction explains the high stability of the catalyst (Wang et al., Reference Wang, Yu, Shen, Chu and Qian2013; Bian et al., Reference Bian, Suryawinata and Kawi2016; Zhang et al., Reference Zhang, Feng, Liu, Zhao, Song, Zhang and Gao2018). Finally, peaks of quartz and calcite were still observed on the catalyst at 20.85 and 29.40°2θ, respectively; despite the reaction process and the sintering temperature, no phase transformations occurred.
Figure 10. XRD trace of the catalyst (15Ni-Ds700) after the reaction process.
The catalytic performance of the synthesized 15Ni/Ds-700 catalyst assessed according to the conversion rate of CH4 and CO2 was studied in the DRM process (Figs 11 & 12). The numerical results regarding conversion, product selectivity and H2:CO ratio are given in Table 2, and Table 3 shows the theoretical carbon balance products and the activity of reactants and products.
Figure 11. Conversion to CH4 (%) as a function of reaction time.
Figure 12. Conversion to CO2 (%) as a function of reaction time.
Table 2. Conversion rate of the reagents (CH4 and CO2), selectivity of the products and the H2:CO ratio as function of time of reaction of 15Ni/ Ds-700.
Table 3. Theoretical balance of the carbonaceous products and the activities of the reagents and the products as a function of time of reaction of 15Ni/Ds-700.
The catalyst deposited on diatomite (15Ni/Ds-700) is active (Figs 11 & 12), showing CH4 conversion of 14% and CO2 conversion of 12%, with H2 yields of 17%, CO yields of 37% and a H2:CO ratio of 0.45 (Tables 2 & 3). Despite the physicochemical properties of the Ni catalyst supported on diatomite (fine Ni particle size, good dispersion of the active phase on the support and controlled pore volume), the conversion rates of CO2 and CH4 remained low at 700°C. This is probably due to the low reduction of NiO to Ni0, which is the active phase, and the low specific surface area of the Ni/diatomite catalyst (25.69 m2 g–1). A low rate of reduction of NiO was detected after XRD analysis of the samples reduced at 700°C. The XRD results of the catalyst after reduction with pure H2 at 700°C (Fig. 10) showed the presence of the NiO phase; consequently, Ni2+ was not entirely reduced to Ni0, thus explaining the low conversion rates of CH4 and CO2. The molar ratios of CO2:CH4 and H2:CO could be used as indicators to determine the activity of the 15Ni/Ds-700 catalyst (Liu et al., Reference Liu, Yao, Taief, Benzina, Da Costa and Gálvez2018). These ratios did not significantly vary for the catalyst, and they should be 1 for the DRM. The main disadvantage of Ni-based catalysts is the deposition of coke during the DRM process (Kristiani & Takeishi, Reference Kristiani and Takeishi2022). However, the CO2 conversions are higher than those of methane, which can be explained by the occurrence of the reverse secondary reaction of gas to water (Equation 9):
The presence of this reaction was confirmed by the low H2:CO molar ratio, which yields some carbon deposition by either the Boudouard reaction (below 700°C) and/or CH4 cracking reactions at high temperatures (Wang et al., Reference Wang, Lu and Millar1996; Zhang et al., Reference Zhang, Zhu, Li, Wu, Ma, Wang and Gong2013).
In the present study, the 15Ni/Ds-700 recorded a low carbon deposition rate and stability during 6 h of the dry reforming reaction. The low conversion rate to reactive gases (CH4 and CO2) obtained (12%) is probably due to the partial reduction of NiO to Ni0 (Figs 11 & 12).
The Ni-diatomite catalysts in dry reforming reactions achieved 90% CH4 conversion when the Ni-diatomite was calcined at the 800°C (Jabbour et al., Reference Jabbour, El Hassan, Davidson, Massiani and Casale2015). In addition, the H2:CO molar ratio was close to unity. These catalysts were also relatively stable during catalytic measurements carried out at 650°C for 12 h (Jabbour et al., Reference Jabbour, El Hassan, Davidson, Massiani and Casale2015). This is probably due to the reduction state of NiO to Ni0 (Kristiani & Takeishi, Reference Kristiani and Takeishi2022). The activity and stability of catalysts supported by diatomite depend on the heat treatment of the diatomite. This also affects the type of carbon formed after the reaction test. However, Pan et al. (Reference Pan, Guo, Dai, Ren and Chu2020) obtained 58% and 44% conversion rates to CH4 and CO2, respectively, using mesoporous silica-supported Ni or Ni–Pd bimetallic catalysts. The presence of noble metals was beneficial for reducing the size of the active metal particles and promoting their dispersion (Pan et al., Reference Pan, Guo, Dai, Ren and Chu2020). Furthermore, the stability of conversion to CH4 and CO2 depends on the amount of carbon deposition and consumption, which affect the activity and stability of the catalyst (Li et al., Reference Li, Jiang, Wang, Wang, Song, Zhao and Ma2017).
Furthermore, the catalyst used has a low rate of carbon deposition (carbon balance = 93%) and high stability for 6 h. This result is mainly due to the strong Ni support that generates the formation of well-dispersed Ni metal particles on the silica support, thereby enabling good catalytic activity and stability by inhibiting the sintering of Ni particles and carbon deposition. The results obtained are in accordance with those from a similar work that used natural silica as a support for perovskite-based catalysts: 20% LaNiO3/SiO2 and 40% LaNiO3/SiO2 (Sellam et al., Reference Sellam, Ikkour, Dekkar, Messaoudi, Belaid and Roger2019).
Summary and conclusions
A diatomite support (Ds) obtained from sieving (<63 μm) of a crude Algerian diatomite (Dc) was used as a support for a Ni-based catalyst applied in the DRM reaction. The 15Ni/Ds-700 catalyst had lower SiO2, Al2O3 and CaO contents than the support diatomite (79.98, 7.13 and 16.81 wt.% and 64.60, 1.92 and 13.24 wt.%, respectively). The NiO content of the catalyst was 15 wt.%, confirming successful impregnation. In addition, the specific surface area of Ds (46.33 m2 g–1) was higher compared to Dc (26.47 m2 g–1).
The 15Ni/Ds-700-supported catalysts were prepared according to the wet impregnation procedure. Their activity was tested in the DRM at 700°C. XRD analysis of the supported catalyst showed the presence of an NiO phase. The reduction of the Ni-based catalyst (NiO) to Ni0 supported on diatomite was not complete because this process requires very high temperatures. The 15Ni/Ds-700 catalyst has a relatively low specific surface area (25.69 m2 g–1) and small particle size (15.72 nm). Consequently, this catalyst exhibits low catalytic activity, which is attributed to the low specific surface area and low reducibility of NiO to Ni0. A small NiO particle size, high dispersibility on the support and a successful reduction of NiO to Ni0 might increase the catalytic activity.
The fine powder of Algerian diatomite (<63 μm) is a promising low-cost material; it might be used as a support for a Ni-based catalyst for DRM applications. However, it should be optimized through the addition of promoters and by increasing the temperature of reduction to avoid the formation of excess water during the reaction, which would cause deactivation of Ni0. An increase in reduction temperature might lead to the total transformation of NiO to Ni0. Furthermore, the addition of promoters (alkaline, alkaline earth or rare earth elements) holds promise for enhancing the basicity of the catalyst support.
Conflicts of interest
The authors declare none.
