Dry reforming of methane (DRM) has been demonstrated to help in greenhouse gas mitigation. However, a proper catalyst is required for catalytic activity and stability during DRM. Research published in the MDPI journal molecules investigated the effect of strontium incorporation into cerium- and nickel-based nanocrystalline perovskites for greenhouse gas mitigation strategies.
Image Credit: Zdenek Sasek/Shutterstock.com
Dry reforming of methane (DRM), among the other CO2 conversion, has garnered attention majorly because DRM converts major greenhouse gases—methane and carbon dioxide—to produce hydrogen and carbon monoxide, which is then used to produce liquid hydrocarbons.
The DRM process plays a role in greenhouse gas mitigation and serves as a cause of climate change. However, it also produces synthesis gas, a mixture of equimolar hydrogen and carbon monoxide appropriate for the synthesis of hydrocarbon with the help of the Fischer–Tropsch synthesis process.
Both transition metal-based and noble metal-based catalysts are utilized, but transition metal-based catalysts like Nickel (Ni)- and cobalt (Co)-based catalysts are most analyzed for DRM as they are inexpensive, abundant, and provide quick turnover rates. Ni-based catalysts become deactivated during DRM, a major challenge to be overcome.
Metallic nanoparticles supported on basic oxide supports or promoters, show enhanced catalytic activity and benefit CO2 chemisorption. Nickel-based double-layered hydroxides (LDHs), in addition to the metallic nanoparticle-supported oxides, have demonstrated better catalytic performances towards DRM reaction. However, they eventually deactivate owing to the rapid growth of randomly sized impregnated nanoparticles.
The current study shows the effect of incorporation of strontium (Sr) into cerium- and nickel-based nanocrystalline perovskites (CeNiO3) in both dry and/or CO2 reforming of methane. The research concentrates on analyzing the insights of the replacement of cerium with strontium and the impact of this replacement on catalytic conversions and durability.
The thermal decomposition curves, which include thermogravimetric (TG) and the differentials (DTG) of the precursors as a measurement of their calcination temperatures, are depicted in Figure 1. It can be observed that each precursor decomposed differently.
Figure 1. TG−DTG curves versus temperature of CexSr1−xNiO3 (x = 0.6–1) perovskites. Image Credit: Ahmad, et al., 2022
The XRD patterns of the as-prepared nanocrystalline perovskites without and with the incorporation are illustrated in Figure 2. MDI Jade® software was employed to analyze XRD data and a substantial number of peaks corresponding to CeNiO3 were recorded.
Figure 2. XRD patterns of CexSr1−xNiO3 (x = 0.6–1) perovskites. Image Credit: Ahmad, et al., 2022
Figure 3 reveals the N2 adsorption–desorption isotherms and the pore-size distributions of air-calcined precursors (700 °C, 5 hours). Figure 3 depicts that all of the samples showed a type II isotherm with an H3 hysteresis loop, indicating that these samples had macro-pores.
Figure 3. N2 adsorption–desorption isotherms of CexSr1−xNiO3 (x = 0.6–1) perovskites. Image Credit: Ahmad, et al., 2022
The BET (Braunner, Emmet, and Teller) surface areas and pore parameters are listed in Table 1.
Table 1. Textural properties and deactivation factors of CexSr1−xNiO3(x = 0.6–1) perovskites. Source: Ahmad, et al., 2022
||Ni/Ce/Sr Content (%) a
|Coke (wt%) d
||7.7 b (0.88) c
||−62.6 b (1.71) c
||−13.4 b (1.15) c
a Determined from ICP-OES. b Deactivation factor (D.F., %) = 100 × (CH4conversioninitial − CH4conversionfinal)/(CH4conversioninitial). c D.F based on first-order deactivation, % = ln(1 − CH4conversionfinal)/ln(1 − CH4conversioninitial). d Calculated from TPO data.
The morphology of the nanocrystalline perovskite catalysts (fresh, reduced, and used) were analyzed using transmission electron microscopy (TEM). Figure 4 shows the images of the catalysts before and after the reaction. The particles were spherical in shape. Insignificant sintering was noted for reduced catalysts.
Figure 4. TEM images of CexSr1−xNiO3 (x = 0.6–1) perovskites; fresh catalysts: (a) CeNiO3, (b) Ce0.8Sr0.2NiO3, (c) Ce0.6Sr0.4NiO3; reduced catalysts: (d) CeNiO3, (e) Ce0.8Sr0.2NiO3, (f) Ce0.6Sr0.4NiO3; spent catalysts: (g) CeNiO3, (h) Ce0.8Sr0.2NiO3, (i) Ce0.6Sr0.4NiO3. Image Credit: Ahmad, et al., 2022
Temperature-Programmed Reduction (TPR) is majorly used to investigate the metal–support interaction, the reducibility, and to identify the activation and/or reduction temperature. Figure 5 shows the reduction profiles, denoting the differences in metal–support interaction and/or the reducibility of CeNiO3 after Sr addition.
Figure 5. H2-TPR profiles of (a) CeNiO3, (b) Ce0.8Sr0.2NiO3, and (c) Ce0.6Sr0.4NiO3 perovskites. Image Credit: Ahmad, et al., 2022
The carbon dioxide (CO2) and methane (CH4) conversions versus time-on-stream are depicted in Figure 6. It was noted that CO2 conversions greater than CH4 conversions were responsible for side reactions including reverse water–gas-shift reaction and reverse CO disproportionation.
Figure 6. (a) CH4 conversion, (b) CO2 conversion, and (c) H2/CO ratios versus time-on-stream (TOS) of CexSr1−xNiO3 (x = 0.6–1) perovskites. Image Credit: Ahmad, et al., 2022
The likelihood of carbon formation over the catalyst’s surface after a dry-reforming reaction was shown by temperature-programmed oxidation (TPO) analysis (see Figure 7). A single broad peak was observed in the temperature range of 110 °C to 500 °C for each catalyst.
Figure 7. TPO profiles of (a) CeNiO3, (b) Ce0.8Sr0.2NiO3, and (c) Ce0.6Sr0.4NiO3 perovskites. Image Credit: Ahmad, et al., 2022
The TEM microscopic images were recorded after a dry-reforming reaction to comprehend the morphological changes and also the formation of deactivating coke above the surface of spent catalysts.
Ahead of the dry methane-reforming reaction analysis, the as-prepared nanocrystalline perovskites were characterized. The mechanism connected with dry methane reforming needs reactant adsorption over the active sites of the catalyst. However, in spite of higher activity, CeNiO3 perovskite deactivated over time, these observations were substantiated by TEM and TPO.
But strontium-incorporated perovskites did not deactivate in spite of carbon deposition and sintering. The degree of agglomeration or sintering was the same for all perovskites. This indicates that methane decomposition, a prevalent DRM side reaction at the higher reaction temperature, is the major source of carbon deposition resulting in catalyst deactivation. Figure 8 illustrates the reaction mechanism and the impact of Sr incorporation.
Figure 8. Schematic diagram of the reaction mechanism over Sr-free and Sr-incorporated perovskites. Image Credit: Ahmad, et al., 2022
Earlier studies inferred that Sr incorporation in lesser amounts showed less-active, but stable, catalytic performance, and Table 2 compares the observations of this study with earlier works. It was seen that the current conversions outperformed similar perovskites.
Table 2. Comparison of current work with previously reported work. DF refers to the deactivation factor. TOS indicates time-on-stream. Source: Ahmad, et al., 2022
||Reaction Temp. (°C)/
a Deactivation Factor (D.F., %) = 100 × (CH4conversioninitial − CH4conversionfinal)/(CH4conversioninitial).
Nanocrystalline perovskites—CexSr1−xNiO3 (x = 0.6–1)—were produced through the self-combustion method. The thermogravimetric analysis (TGA) of the perovskite precursors was documented with a thermal analyzer. X-ray diffraction profiles were recorded and BET specific surface areas of the samples were evaluated.
The morphology of both fresh and used perovskite catalysts was examined by transmission electron microscopy, and TPO and TPR profiles were conducted. The as-synthesized catalysts’ chemical compositions were measured by inductively coupled plasma optical emission spectroscopy.
The dry methane-reforming reaction was carried out in a tubular fixed-bed reactor at 700 °C and 1 atm pressure. The feed gas was flown at 70 mL/min and the catalysts were subjected to activation/reduction, before the reaction. The hydrogen mixture was replaced with helium after activation, to remove any leftover hydrogen.
The current study showed the catalytic performance results of strontium-incorporated nanocrystalline perovskites for dry methane reforming. The observations reveal the major role of strontium incorporation in CexSr1−xNiO3 (x = 0.6–1) perovskites to ascertain stable catalytic performance for prolonged periods to avoid deactivation.
Ahmad, N., Wahab, R., Manoharadas, S., Alrayes, B. F., Alam, M., Alharthi, F. A., (2022) The Role of Strontium in CeNiO3 Nano-Crystalline Perovskites for Greenhouse Gas Mitigation to Produce Syngas. Molecules, 27(2), p.356. Available Online: https://www.mdpi.com/1420-3049/27/2/356/htm.
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