Enhanced dry reforming of methane over mesostructured fibrous Ni/MFI zeolite: Influence of preparation methods

Dry reforming of methane is acknowledged to be an environmentally benign route for conversion of CO₂ and CH₄ into syngas (CO and H₂). Herein, unique mesostructured fibrous MFI support was synthesized by microemulsion method, and Ni incorporation via double solvent, physical mixing and wetness impregnation methods. Results revealed wetness impregnation catalyst had the highest activity and stability. Activation energy of reactants showed a reliance on acidity, where moderate acidity impeded deactivation by CH₄ cracking. Furthermore, degree of catalyst deactivation was negligible compared to what is attainable on conventional zeolite catalysts. Thus, fibrous morphology, microscopic dispersion and moderate acidity played a positive role in boosting reactants accessibility to active Ni sites which results in preservation of activity under the harsh conditions of DRM process.     

Mo-promoted Ni/Al2O3 catalyst for dry reforming of methane

Mo-promoted alumina supported Ni catalysts were prepared through a conventional impregnation method and tested in dry reforming of methane (DRM) at temperatures from 550 to 850 °C. The catalysts were characterized by means of H₂-temperature programmed reduction (H₂-TPR), CO₂-temperature programmed desorption (CO₂-TPD), X-ray diffraction (XRD), N₂ physisorption and Raman spectroscopy. Mo-promotion caused a reduction in the DRM catalytic activity. The weaker interaction between NiO species and the alumina support, the formation of a MoNi₄ phase, and the lower basicity of this Ni-Mo/Al₂O₃ catalyst were identified as the main causes for its lower activity. However, pre-reducing the Ni-Mo/Al₂O₃ catalyst at temperatures lower than 700 °C, instead of 900 °C, resulted in a considerable increase of its catalytic activity. This was mainly due to the formation of a separate Ni⁰ phase that did not interact with Mo and to an increase in medium strength basicity.     

Effect of O2 and temperature on the catalytic performance of Ni/Al2O3 and Ni/MgAl2O4 for the dry reforming of methane (DRM)

Al₂O₃ and MgAl₂O₄ supported 10% (w/w) Ni catalysts having a dispersion of 1.5 and 2.0% are active for DRM at 600 and 750 °C. High temperature reduction of both the calcined catalysts resulted in metallic Ni being formed, suggesting strong support metal interactions. The CH₄ and CO₂ conversion during DRM are relatively constant with time-on-stream, and are higher for Ni/MgAl₂O₄ than Ni/Al₂O₃. Carbon-whiskers are also detected on both catalysts. O₂ co-feed of 2.6% (v/v) and increasing reaction temperature to 750 °C helped in decreasing the amount of carbon deposited, except for Ni/MgAl₂O₄ at 600 °C. Furthermore, higher conversions and H₂/CO ratios are achieved. It appears that on spent Ni/MgAl₂O₄ a different type of carbon species was formed, and this carbon species was difficult to remove by oxygen at 600 °C. Thus, co-feeding O₂, using an appropriate temperature, and choosing a suitable support can reduce the carbon present on the nickel catalysts during DRM.     

Syngas production from glycerol-dry(CO2) reforming over La-promoted Ni/Al2O3 catalyst

A 3 wt% La-promoted Ni/Al₂O₃ catalyst was prepared via wet co-impregnation technique and physicochemically-characterized. Lanthanum was responsible for better metal dispersion; hence higher BET specific surface area (96.0 m² g⁻¹) as compared to the unpromoted Ni/Al₂O₃ catalyst (85.0 m² g⁻¹). In addition, the La-promoted catalyst possessed finer crystallite size (9.1 nm) whilst the unpromoted catalyst measured 12.8 nm. Subsequently, glycerol dry reforming was performed at atmospheric pressure and temperatures ranging from 923 to 1123 K employing CO₂-to-glycerol ratio from zero to five. Significantly, the reaction results have yielded syngas as main gaseous products with H₂:CO ratios always below than 2.0 with concomitant maximum 96% glycerol conversion obtained at the CO₂-to-glycerol ratio of 1.67. In addition, the glycerol consumption rate can be adequately captured using power law modelling with the order of reactions equal 0.72 and 0.14 with respect to glycerol and CO₂ whilst the activation energy was 35.0 kJ mol⁻¹. A 72 h longevity run moreover revealed that the catalyst gave a stable catalytic performance.     

Mesoporous nanostructured Ni/MgAl2O4 catalysts: Highly active and stable catalysts for syngas production in combined dry reforming and partial oxidation

In this paper, the combination of dry reforming and partial oxidation of methane on nickel catalysts supported on mesoporous MgAl₂O₄ was investigated. The support was prepared by a facile sol-gel route using propylene oxide as a gelation agent. The characterizations of the catalysts were performed by BET, XRD, TPR, TPO, TPH, UV–vis, CO-dispersion, SEM and TEM techniques. In addition, the effects of nickel content, reaction and reduction temperatures, feed ratio and the GHSV value on the physicochemical and catalytic properties were studied. The results revealed that the nickel content had an optimum value of 7.5 wt% and the catalyst with this content of nickel exhibited the highest activity. Furthermore, the results demonstrated that the increase in reaction temperature enhanced the rate of the dry reforming reaction and led to obtain a H₂/CO ratio around unity. The 7.5 wt% nickel catalyst showed a 5% decline in activity within 15 h in combined reforming. The TPO analysis showed that there was no deposited carbon on the catalyst surface in combined reforming and the SEM analysis confirmed the results of TPO analysis.     

Methane tri-reforming for synthesis gas production using Ni/CeZrO2/MgAl2O4 catalysts: Effect of Zr/Ce molar ratio

Three Ni/CeZrO₂/MgAl₂O₄ catalysts synthesized using different Zr/Ce molar ratios (0.25, 1, and 4) were studied for methane tri-reforming. The catalysts were characterized using XRD, ²⁷Al-NMR, H₂-TPR, CO₂-TPD, XPS, and in situ techniques (XPD and XANES). The addition of CeZrO₂ at Zr/Ce = 0.25 on the MgAl₂O₄ spinel support considerably reduced the amount of carbon deposits, because the methane decomposition reaction was attenuated by the presence of less agglomerated Ni⁰ species produced after the reduction process. The highest CO₂ adsorption capacity (basicity) was associated with the participation of medium-strength basic sites, which facilitated coke gasification and led to higher CO₂ conversions. A syngas with quality (H₂/CO ratio) of 1.8 was produced, suitable for use in Fischer-Tropsch reactions.     

Syngas production from methane dry reforming over SmCoO3 perovskite catalyst: Kinetics and mechanistic studies

The kinetics of the methane dry (CO₂) reforming over the SmCoO₃ was investigated in the temperature ranged 973–1073 K by varying the CH₄ and CO₂ partial pressures. Based on detailed study of the reaction mechanism, a mechanistic model is proposed from which a kinetic model is derived. The mechanistic pattern assumes adsorption of CH₄ on reduced Co, followed by methane cracking and carbon deposition. CO₂ reacts with Sm₂O₃ to form Sm₂O₂CO₃ and the oxycarbonates react with carbon to produce CO. The power law and Langmuir–Hinshelwood kinetic model which is established on this mechanism were able to forecast the kinetic results.     

CO2 reforming of methane over Ni/ZnAl2O4 catalysts: Influence of Ce addition on activity and stability

The catalytic performance of Ni supported on Ce-promoted ZnAl₂O₄ was evaluated in methane dry reforming. The effect of different nominal loadings of cerium (3, 5 and 7 wt%) in the activity, product yield and stability was studied. Ce presented a promote effect in catalytic activity, product yield and especially in stability. However the catalytic performance was considerably influenced by the amount of cerium. SEM images presented smaller particles and TPR profiles revealed stronger active phase/support interaction by Ce addition which led to increasing methane conversion and decreasing coke deposition. Although high amount of Ce was not in favor of its promoting effect due to aggregation of CeO₂ on the catalyst surface. Among the catalysts investigated, the optimal catalytic activity and stability was achieved over the sample with 5 wt% of cerium.     

Glycine-assisted preparation of highly dispersed Ni/SiO2 catalyst for low-temperature dry reforming of methane

Ni-based catalysts have been widely studied in reforming methane with carbon dioxide. However, Ni-based catalysts tends to form carbon deposition at low temperatures (≤600 °C), compared with high temperatures. In this paper, a series of Ni/SiO₂-XG catalysts were prepared by the glycine-assisted incipient wetness impregnation method, in which X means the molar ratio of glycine to nitrate. XRD, H₂-TPR, TEM and XPS results confirmed that the addition of glycine can increase Ni dispersion and enhance the metal-support interaction. When X ≥ 0.3, these catalysts have strong metal-support interaction and small Ni particle size. The Ni/SiO₂-0.7G catalyst has the best catalytic performance in dry reforming of methane (DRM) test at 600 °C, and its CH₄ conversion is 3.7 times that of Ni/SiO₂-0G catalyst. After 20 h reaction under high GHSV (6 × 10⁵ ml/g_cat/h), the carbon deposition of Ni/SiO₂-0.7G catalyst is obviously lower than that of Ni/SiO₂-0G catalyst. Glycine-assisted impregnation method can enhance the metal-support interaction and decrease the metal particle size,which is a method to prepare highly dispersed and stable Ni-based catalyst.     

Highly-efficient hydroxyapatite-supported nickel catalysts for dry reforming of methane

Ni-based catalysts were prepared using two hydroxyapatites (Ca-HA1, S_BET = 7 m²/g and Ca-HA2, S_BET = 60 m²/g) with different physico-chemical properties. The objective of the study was to evaluate the performance of these two materials as promising supports for dry reforming reaction (DRM) as well as to investigate the influence of different process parameters, such as temperature, pressure, time and catalyst pretreatment on the performance of these two catalysts. Thermodynamic calculations were performed to determine the conditions that would limit solid carbon deposit and favor the reactants conversion. Then, an experimental parametric study was carried out to investigate the impact of the temperature, pressure, catalyst pretreatment and support thermal treatment on the catalysts performance. The results showed that the catalyst pretreatment allowed the reduction of the nickel particles in a higher extent, which resulted in better catalytic performance when compared to the catalysts without pretreatment. High temperatures around 700 °C and low pressures around 1.6 bar were required to attain high CH₄ and CO₂ conversions around 70–80% as well as high H₂ and CO selectivity around 90% for 90 h of time on stream. In all cases, Ni/Ca-HA2 catalyst presented better catalytic performance than Ni/Ca-HA1 due to the presence of smaller nickel particles (10–20 nm), stronger basicity, higher density of basic sites (0.23 mmol g⁻¹) as well as higher specific surface area (S_BET = 60 m²/g) of the Ca-HA2 support. Ni/Ca-HA2 catalyst was highly active (initial methane conversion: 75%) and relatively stable during 90 h of TOS and its catalytic behavior was comparable with the performance of Ni-based catalysts prepared with conventional supports reported in the literature.     

Syngas production via dry reforming of methane over CeO2 modified Ni/Al2O3 catalysts

Syngas production via dry reforming of methane (DRM) was experimentally investigated using Ni-based catalyst. Ni/Al₂O₃ modification with CeO₂ addition and O₂ addition in the reactant were employed in this study to suppress carbon deposition and to enhance catalyst activity. It was found that DRM performance can be enhanced using CeO₂ modified Ni/Al₂O₃ catalyst due to CeAlO₃ formation. However, an optimum amount of CeO₂ loading exists to obtain the best DRM performance due to the decrease in specific surface area as the CeO₂ loading increases. Without O₂ addition, the reverse water-gas shift reaction plays an important role in DRM. It was found that CH₄ conversion and CO yield were enhanced while CO₂ conversion and H₂ yield are decreased as the CO₂ amount in feedstock increased in DRM. With O₂ addition in the fed reactant, it was found that the methane oxidation reaction plays an important role in DRM. CH₄ conversion can be enhanced by O₂ addition. However, decreases in CO₂ conversion and H₂ and CO yields occurred due to greater H₂O and CO₂ productions from the methane oxidation reaction. The thermogravimetric analysis (TGA) results showed that CeO₂ modified Ni/Al₂O₃ catalyst would have the lowest amount of carbon deposition when O₂ is introduced into the reaction.     

Syngas production from methane dry reforming over Ni/SBA-15 catalyst: Effect of operating parameters

The influence of operating conditions including reactant partial pressure and reaction temperature on the catalytic performance of 10%Ni/SBA-15 catalyst for methane dry reforming (MDR) reaction has been investigated in this study. MDR reaction was carried out under atmospheric pressure at varying CH₄/CO₂ volume ratios of 3:1 to 1:3 and 923–1023 K in a tubular fixed-bed reactor. SBA-15 supported Ni catalyst exhibited high specific surface area of 444.96 m² g^?1 and NiO phase with average crystallite size of 27 nm was detected on catalyst surface by X-ray diffraction and Raman measurements. H₂ temperature-programmed reaction shows that NiO particles were reduced to metallic Ni⁰ phase with degree of reduction of about 90.1% and the reduction temperature depended on the extent of metal-support interaction and confinement effect of mesoporous silica support. Catalytic activity appeared to be stable for 4 h on-stream at 973–1023 K whilst a slight drop in activity was observed at 923 K probably due to deposited carbon formed by thermodynamically favored CH₄ decomposition reaction. Both CH₄ and CO₂ conversions increased with rising reaction temperature and reaching about 91% and 94%, respectively at 1023 K with CO₂ and CH₄ partial pressure of 20 kPa. CH₄ conversion improved with increasing CO₂ partial pressure, ${\mathit{P}}_{\mathit{C}{\mathit{O}}_{\mathit{2}}}$ and exhibited an optimum at ${\mathit{P}}_{\mathit{C}{\mathit{O}}_{\mathit{2}}}$ of 30–50 kPa depending on reaction temperature whilst a substantial decline in CO₂ conversion was observed with growing ${\mathit{P}}_{\mathit{C}{\mathit{O}}_{\mathit{2}}}$. Additionally, CH₄ and CO₂ conversions decreased significantly with rising CH₄ partial pressure because of increasing carbon formation rate via CH₄ cracking in CH₄-rich feed. Post-reaction characterization shows that active Ni metal phase was not re-oxidized to inactive metal oxide during MDR reaction. The heterogeneous nature of deposited carbons including carbon nanofilament and graphite was detected on catalyst surface by Raman measurement.     

Hydrogen production from glycerol dry reforming over Ag-promoted Ni/Al2O3

In recent times, glycerol has been employed as feedstock for the production of syngas (H₂ and CO) with H₂ as its main constituent. This study centers on dry reforming of glycerol over Ag-promoted Ni/Al₂O₃ catalysts. Prior to characterization, the catalysts were synthesized using the wet impregnation method. The reforming process was carried out using a fixed bed reactor at reactor operating conditions; 873–1173 K, carbon dioxide to glycerol ratio of 0.5 and gas hourly space velocity (WHSV) in the range of 14.4 ≤ 72 L g_cat⁻¹ h⁻¹). Ag (3)-Ni/Al₂O₃ gave highest glycerol conversion and hydrogen yield of 40.7% and 32%, respectively. The optimum conditions which gave highest H₂ production, minimized methane production and carbon deposition were reaction temperature of 1073 K and carbon dioxide to glycerol ratio of 1:1. This result can attributed to the small metal crystallite size characteristics possessed by Ag (3)–Ni/Al₂O₃, which enhanced metal dispersion in the catalyst matrix. Characterization of the spent catalyst revealed the formation of two types of carbon species; encapsulating and filamentous carbon which can be oxidized by O₂.     

Microwave-assisted dry reforming of methane

The microwave-assisted dry reforming of methane over an activated carbon, which acted as catalyst and microwave receptor, was investigated. As a preliminary study, the CO₂ reforming of CH₄ was carried out using conventional heating and microwave heating in order to compare both heating devices. Higher conversions of CH₄ and CO₂ were achieved by microwave heating. Under microwave heating, various operating variables were studied in order to determine the best conditions for performing dry reforming with high conversions and the most suitable H₂/CO ratio. Thus, the dry reforming reaction was studied at different temperatures. An optimum range of working temperatures (between 700 °C and 800 °C) was established. In this range of temperatures, the dry reforming reaction is believed to take place as a combination of CH₄ decomposition and CO₂ gasification. Carbonaceous deposits from CH₄ decomposition are gasified by CO₂ and, as a result, active centres for the dry reforming reaction are constantly regenerated. The effect of the proportion of CO₂ fed in on the CH₄ and CO₂ conversions was also investigated. Small increases in the percentage of CO₂ fed in gave rise to large increases in both conversions, but especially in the case of CH₄. The volumetric hourly space velocity was also studied. It was found that the lower the space velocity, the higher the conversions obtained.     

A comparative study of ZrO2, Y2O3 and Sm2O3 promoted Ni/SBA-15 catalysts for evaluation of CO2/methane reforming performance

Ni-based/SBA-15 catalysts, were promoted by 3wt % of samaria (Sm₂O₃), Yttria (Y₂O₃) and Zirconia (ZrO₂), by two-solvent impregnation method. The catalysts characterization was performed by N2 adsorption–desorption, X-ray Diffraction (XRD), X-ray Fluorescence (XRF), High Resolution Transmission Electron Microscopy (HRTEM), Field Emission Electron Scanning Microscopy (FESEM), Temperature Programmed Oxidation/Reduction (TPO/TPR) and NH₃-Temperature Programmed Desorption (NH₃-TPD) techniques. Then, evaluated by CO₂/methane reforming.The CO₂/methane reforming outcomes revealed that samaria-promoted catalyst showed excellent activity, stability and cock resistance, while yttria-promoted catalyst just illustrated good activity at high temperature and zirconia-promoted catalyst didn't show any modification in catalytic performance in comparison to Ni-based catalyst with no promoter. Samaria-promoted TEM and TPR analysis, indicated adding samaria improved the NiO particles interaction with SBA-15 support pores wall and NiO dispersion. The TPO analysis displayed that coke deposition in samaria-promoted sample after 12 h reaction is less than yttria-promoted during stream of 5 h. Also, it is suggested that for samaria containing catalyst, cock deposition occurred on the support. Therefore, nickel active sites were preserved for time on stream of 12 h, which is the main reason for samaria-promoted catalyst superior stability than other's.     

Process optimization of DBD plasma dry reforming of methane over Ni/La2O3MgAl2O4 using multiple response surface methodology

In this study, 10% Ni/La₂O₃MgAl₂O₄ nano-flake catalyst was synthesized, characterized and tested in a catalytic dielectric barrier discharge (DBD) plasma for dry reforming of methane (DRM). With design of experiment (DoE), the influence of process parameters namely (1) total feed flow rate (ml min⁻¹), (2) feed ratio (CO₂/CH₄), (3) input power (W) and (4) catalyst loading (g) were examined using multiple response surface methodology (RSM) through a four-factor, five-level central composite design (CCD). Second-order regression models were applied for evaluating the interaction between the process parameters and responses. Input power (X₃) and total feed flow rate (X₁) were the two most influential process parameters followed by catalyst loading (X₄) and feed ratio (X₂). The experimental and predicted results from the optimum conditions fitted-well with less than ±5% margin of error. The possible dynamic interactions between the process variables were elucidated. The optimum values are feed flow rate = 18.8 ml min⁻¹, feed ratio = 1.05, input power = 125.6 W and catalyst loading = 0.6 g. At these conditions, the predicted CH₄ and CO₂ conversions are 79.86% and 84.03%, respectively. The H₂ and CO yields are predicted as 41.37% and 40.47%, respectively while H₂/CO ratio is above unity. The calculated EE from the RSM model is predicted as 0.135 mmol kJ⁻¹. Low carbon deposition observed on the spent catalyst is attributed to the highly basic and oxidative nature of the La₂O₃ co-supported catalyst.     

Pt supported on doped CeO2/Al2O3 as catalyst for dry reforming of methane

This work investigated the effect of the nature of dopant (Pr, Nb and Zr) on the performance of Pt supported on cerium-based oxides deposited on alumina for dry reforming of methane. in situ XRD and XANES analyses showed that the sample doped with Pr exhibited the highest redutibility of ceria (23%). Furthermore, the cyclohexane dehydrogenation reaction revealed that the addition of Pr improved the resistance to metal sintering during the dry reforming reaction. In the absence of doped-ceria oxide, a strong deactivation took place on Pt/Al₂O₃ catalyst during reaction, which was due to the absence of support reducibility and the highest Pt sintering. Among the doped-ceria samples, Pt/CePr/Al₂O₃ exhibited the highest activity and stability. These results were attributed to: (i) the oxygen mobility of the supports containing ceria, mainly for the sample doped with Pr, which favors the carbon removal mechanism; and (ii) the absence of Pt sintering during the reaction.