Optimization of a fluidized bed reactor for methane decomposition over Fe/Al2O3 catalysts: Activity and regeneration studies

Catalytic methane decomposition was investigated over 40 wt% Fe/Al₂O₃ catalyst in fluidized bed reactor (FLBR). After optimization of FLBR conditions in terms of catalyst bulk density, particle size, minimum fluidization velocity, and the catalyst bed height, the catalyst activity and stability tests were conducted by comparison with a fixed bed reactor (FBR). Although a similar stable methane conversion was obtained over both reactors, the pressure drop during 35 min operation of FBR was 9 times higher than that of FLBR, which indicated the possibility of continuous operation of methane decomposition process over FLBR. Further, the influence of the space velocity, feed dilution and regeneration on catalysts reactivity was studied in FLBR to conclude that a reaction condition of 12 L/g_cat∙h, feed of 20%H₂–80%CH₄ and CO₂-regeneration of deactivated catalysts may be favourable for operating methane decomposition in FLBR continually and effectively to provide stable hydrogen.     

Production of hydrogen and carbon nanofibers through the decomposition of methane over activated carbon supported Ni catalysts

Hydrogen has been produced by decomposing methane thermocatalytically at 1123 K and volume hourly space velocity (VHSV) of 1.62 L/h g in the presence of activated carbon supported Ni catalysts of different compositions (Samples coded as Ni10, Ni20, Ni30 and Ni40 respectively). The studies indicated that the sample coded Ni30 catalyst (with Ni content of 23.33 wt.%) has the highest catalytic activity among all the catalysts tested. The initial methane decomposition rate, accumulated carbon in 4 h and sustainability factor (SF) of Ni30 catalyst are 0.89 mmol/min.g, 7.92 g C/g Ni and 0.7 respectively. SEM image of Ni30 catalyst after methane decomposition reaction showed formation of degenerated carbon fibers. XRD of the above catalyst revealed, moderately crystalline peaks of Ni which may be responsible for the increase in the catalytic life and the formation of carbon fibers.     

Lean methane catalytic combustion over the mesoporous MnOx-Ni/MgAl2O4 catalysts: Effects of Mn loading

In the current investigation, the textural features and catalytic efficiency of the Mn-promoted Ni/MgAl₂O₄ catalysts were evaluated in the catalytic combustion of lean methane. The results demonstrated that adding manganese oxide up to 5 wt.% to the catalyst improved the structural features and light-off temperatures due to the increase of the reduction degree and oxygen vacancies. The role of the T_calcination and processing factors on the methane conversion of the 5%Mn-20%Ni/MgAl₂O₄ catalyst was also studied. The results indicated that the CH₄ conversion decreased with the increment of O₂/CH₄ molar ratio and GHSV value. The stability test revealed that the prepared sample exhibited high stability during 10 h time on stream. Furthermore, the obtained results indicated that the BET area, reduction degree, oxygen mobility, and catalytic performance increased with decreasing the calcination temperature from 700 to 500 °C.     

Hydrogen production from COx-Free thermocatalytic decomposition of methane over the mesoporous iron aluminate spinel (FeAl2O4) nanopowder supported nickel catalysts

In the present study, the thermocatalytic decomposition of methane (TDM) was performed over the NiO(x)/FeAl₂O₄ catalysts with various contents of nickel oxide. The FeAl₂O₄ catalyst support with mesoporous structure and high S_BET (80.26 m² g^?1) was synthesized according to the carbonate-based mechanochemical method. The calcined catalysts were characterized by the XRD, BET, H₂-TPR, CO₂-TPD, TPO, and FESEM analyses. The obtained results demonstrated that the S_BET of the synthesized catalysts reduced from 62 to 26 m² g^?1 by increasing the nickel loading from 20 to 60 wt.%, which is ascribed to the blockage of FeAl₂O₄ support pores. Furthermore, the activity results showed that the catalytic activity improved by increasing the nickel content from 20 to 50 wt.% due to the rise of active site concentration. However, the methane conversion was reached to 40% at 600 °C over the NiO(50)/FeAl₂O₄ catalyst. The more increment of nickel content decreased the catalytic efficiency due to the decline in active phase dispersion. Moreover, the increment amount of deposited carbon was seen by increasing the weight percentage of NiO. Therefore, the catalyst with 50 wt.% of NiO possessed excellent catalytic potential in the TDM process under the hard operating conditions (GHSV = 50,000 ml.h^?1.g^?1_cat). The influence of GHSV, feed ratio (CH₄:N₂), calcination temperature, and reduction temperature on the textural properties and catalytic activity of the NiO(50)/FeAl₂O₄ catalyst were also evaluated in detail.     

Kinetic and autocatalytic effects during the hydrogen production by methane decomposition over carbonaceous catalysts

The reaction kinetics of methane decomposition to yield hydrogen and carbon has been investigated comparing different types of carbonaceous catalysts: two ordered mesoporous carbons (CMK-3 and CMK-5) and two commercial carbon blacks (CB-bp and CB-v). The evolution of the reaction rate along the time has been analyzed concluding that it is governed by different and opposite events: reduction of active sites by carbon deposition, autocatalytic effects of the carbon deposits and pore blockage and diffusional constraints. A relatively simple kinetic model has been developed that fits quite well the experimental reaction rate curves in spite of the complexity of the involved phenomena.     

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.     

Numerical investigation of hydrogen production via autothermal reforming of steam and methane over Ni/Al2O3 and Pt/Al2O3 patterned catalytic layers

In this study a numerical analysis of hydrogen production via an autothermal reforming reactor is presented. The endothermic reaction of steam methane reforming and the exothermic combustion of methane were activated with patterned Ni/Al₂O₃ catalytic layer and patterned Pt/Al₂O₃ catalytic layer, respectively. Aiming to achieve a more compacted process, a novel design of a reactor was proposed in which the reforming and the combustion catalysts were modeled as patterned thin layers. This configuration is analyzed and compared with two configurations. In the first configuration, the catalysts are modeled as continuous thin layers in parallel, while, in the second configuration the catalysts are modeled as continuous thin layers in series (conventional catalytic autothermal reactor). The results show that the pattern of the catalyst layers improves slightly the hydrogen yield, i.e. 3.6%. Furthermore, for the same concentration of hydrogen produced, the activated zone length can be decreased by 38% and 15% compared to the conventional catalytic autothermal reforming and the configuration where the catalysts are fitted in parallel, respectively. Besides, the oxygen consumption is lowered by 5%. The decrement of the catalyst amount and the oxygen feedstock in the novel studied design lead to lower costs and compact process.     

Hydrogen production from methanol steam reforming over Al2O3- and ZrO2-modified CuOZnOGa2O3 catalysts

New CuOZnOxGa₂O₃–Al₂O₃ and CuOZnOxGa₂O₃–ZrO₂ (CuZnxGaAl, CuZnxGaZr) catalysts with different Ga contents were prepared and tested in the methanol steam reforming reaction (MSR) under stoichiometric methanol/water = 1 mol ratio (S/C = 1) at 523 K and 548 K. Addition of Al₂O₃ or ZrO₂ components increases the surface area and modifies the reducibility of CuOZnOGa₂O₃ catalysts; the CuZnxGaZr systems showed the highest reducibility. The performance of CuOZnOGa₂O₃-based catalysts for MSR is improved by the presence of ZrO₂ promoter. CuZn3GaZr catalyst showed a high performance for MSR at 523 K and 548 K under stoichiometric conditions (S/C = 1). The catalyst resulted highly stable and selective for H₂ production, with formation of less than 0.3% mol of CO at 523 K. CO is produced as a secondary by-product through the reverse water gas shift reaction. The new catalysts show high resistance to carbon formation at the temperatures analyzed under stoichiometric conditions (S/C = 1).     

Effect of metal additives on the catalytic performance of Ni/Al2O3 catalyst in thermocatalytic decomposition of methane

Thermocatalytic decomposition of methane is proposed to be an economical and green method to produce CO_x-free hydrogen and carbon nanomaterial. In present work, 60 wt% Ni/Al₂O₃ catalysts with different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by co-impregnation method to investigate promotional effects of these metal additives on the activity and stability of 60 wt% Ni/Al₂O₃ and find out a really effective promoter for decomposition of methane. The catalyst was characterized by N₂ adsorption/desorption, X-ray diffraction, scanning electron microscopy, inductively coupled plasma optical emission spectrometer and hydrogen temperature programmed reduction. While metal additives (5 wt%) were added into 60 wt% Ni/Al₂O₃, the activity stability of 60 wt% Ni/Al₂O₃ was improved and the CH₄ conversion of 60 wt% Ni/Al₂O₃ was also improved except Zn addition. Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al₂O₃ significantly and the CH₄ conversion of 5 wt% Mn-60 wt% Ni/Al₂O₃ was ∼80%. Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al₂O₃ and the CH₄ conversion of 5 wt% Cu-60 wt% Ni/Al₂O₃ decreased from 61% to 45% after 250 min of reaction time. Carbon nanomaterials formed in the thermocatalytic decomposition process were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer and Raman spectroscopy. Carbon deposits consist of amorphous carbon and carbon nanofibers.     

Ni/Ce-MCM-41 mesostructured catalysts for simultaneous production of hydrogen and nanocarbon via methane decomposition

For the first time, simultaneous production of hydrogen and nanocarbon via catalytic decomposition of methane over Ni-loaded mesoporous Ce-MCM-41 catalysts was investigated. The catalytic performance of the Ni/Ce-MCM-41 catalysts is very stable and the reaction activity remained almost unchanged during 1400 min steam on time at temperatures 540, 560 and 580 °C, respectively. The methane conversion level over these catalysts reached 60–75% with a 100% selectivity towards hydrogen. TEM observations revealed that most of the Ni particles located on the tip of the carbon nanofibers/nanotubes in the used catalysts, keeping their exposed surface clean during the test and thus remaining active for continuous reaction without obvious deactivation. Two kinds of carbon materials, graphitic carbon (C_g) as major and amorphous carbon (C_A) as minor were produced in the reaction, as confirmed by XRD analysis and TEM observations. Carbon nanofibers/nanotubes had an average diameter of approximately 30–50 nm and tens micrometers in length, depending on the reaction temperature, reaction time and Ni particle diameter. Four types of carbon nanofibers/nanotubes were detected and their formations greatly depend on the reaction temperature, time on steam and degree of the interaction between the metallic Ni and support. The respective mechanisms of the formation of nanocarbons were postulated and discussed.     

Hydrogen production by catalytic methane decomposition over yttria doped nickel based catalysts

Catalytic methane decomposition can become a green process for hydrogen production. In the present study, yttria doped nickel based catalysts were investigated for catalytic thermal decomposition of methane. All catalysts were prepared by sol-gel citrate method and structurally characterized with X-ray powder diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and Brunauer, Emmet and Teller (BET) surface analysis techniques. Activity tests of synthesized catalysts were performed in a tubular reactor at 500 ml/min total flow rate and in a temperature range between 390 °C and 845 °C. In the non-catalytic reaction, decomposition of methane did not start until 880 °C was reached. In the presence of the catalyst with higher nickel content, methane conversion of 14% was achieved at the temperature of 500 °C. Increasing the reaction temperature led to higher coke formation. Lower nickel content in the catalyst reduced the carbon formation. Consequently, with this type of catalyst methane conversion of 50% has been realized at the temperature of 800 °C.     

Hydrogen production via decomposition of methane over activated carbons as catalysts: Full factorial design

The thermo-catalytic decomposition of methane is proposed as an alternative for producing hydrogen without CO₂ emissions. The present study was divided into three parts. First, a screening study of the rate of methane decomposition (RCH₄)$\left(R_{{\text{CH}}_4}\right)$ was performed using two types of activated carbons as catalysts with progressive time of methane decomposition at four different temperatures. The catalysts differed in textural properties. A full factorial design consisting of 20 experimental points for each catalyst was applied in the second part. Quadratic RCH₄$R_{{\text{CH}}_4}$ models as functions of the relative time of catalyst deactivation and decomposition temperature were developed by regression analysis of variance. The results of the RCH₄$R_{{\text{CH}}_4}$ models showed that the relative time had twice as much influence as temperature. Finally, a general RCH₄$R_{{\text{CH}}_4}$ model was then developed representing both catalysts regardless of their textural properties. All the empirical models were consistent with experimental results and were adequate for designing the methane decomposition process.     

Reaction gas treatment promoting activity and stability of PdO for lean methane oxidation over phosphorus modified Pd/Al2O3 catalysts

Pretreatment under specific atmosphere is a general strategy to activate catalysts. How physicochemical properties of supports are modulated during activation process and their influence on active sites are still the ongoing topic. Herein, the effect of reaction gas treatment on performance of phosphorus modified Pd/Al₂O₃ catalysts in lean methane oxidation was studied. The pretreated Pd/P-doped Al₂O₃ catalyst exhibited a full conversion of CH₄ at ∼400 °C, excellent stability under both dry and wet conditions. Characterization results reveal that the uniform and stable P species in Al₂O₃ enhanced the metal-support interaction and availability of oxygen in support. Under reaction gas condition, PdO-support connection was further promoted to favor generation of oxygen vacancies on support with the help of CH₄, leading to weakened Pd–O bond, facile reformation and increased fraction of PdO. These facilitated formation of intermediates and surface dehydroxylation and mitigated the deactivation caused by thermodynamic decomposition of PdO.     

Production of hydrogen by methane dry reforming over ruthenium-nickel based catalysts deposited on Al2O3, MgAl2O4, and YSZ

In this work, monometallic (1 wt% of Ru or 5 wt% of Ni) and bimetallic catalysts (1 wt% Ru-5 wt.% Ni) deposited on alumina (Al₂O₃), magnesium aluminate spinel (MgAl₂O₄), and yttria-stabilized zirconia (YSZ) were prepared by wet impregnation. The synthesis method of MgAl₂O₄ was optimized and a well crystallized phase with high specific surface area was obtained by using wet impregnation, as a simple and low cost route, at 800 °C for 2 h.The catalytic activity was compared at atmospheric pressure and 750 °C toward methane dry reforming (DRM) reaction with a molar ratio CH₄/CO₂ = 1/1 and a Weight Hourly Space Velocity (WHSV) of 60.000 mL g⁻¹.h⁻¹.Catalytic activity classification was obtained as the following: Ni/MgAl₂O₄ > Ru-Ni/Al₂O₃ > Ru-Ni/MgAl₂O₄ > Ru-Ni/YSZ > Ni/Al₂O₃ > Ni/YSZ > Ru/Al₂O₃ > Ru/YSZ » Ru/MgAl₂O₄. Between the different catalysts, 5 wt% Ni/MgAl₂O₄ catalyst exhibited excellent catalytic activity for DRM. Furthermore, this catalyst was found to be very stable without any deactivation after 50 h under reacting mixture with a low carbon formation rate (3.58 mg_C/g_cat/h). Such superior activity and stability of MgAl₂O₄ supported Ni catalyst is consistent with characterization results from BET, XRD, TPR, CO-pulse chemisorption and CHNS analysis. It can be due to a strong interaction between Ni and MgAl₂O₄ leading to the incorporation of Ni into the spinel lattice and the formation of oxygen vacancies offering a benefit for DRM reaction.Furthermore, it seems that the addition of ruthenium onto Ni/MgAl₂O₄ decreases the interaction between Ni and the spinel leading to a decrease in the catalyst performance. On the other side, the addition of ruthenium on Ni/Al₂O₃ leads to an increase in the catalyst stability and efficiency by inhibiting the formation of poorly active phase NiAl₂O₄ already observed in TPR.     

Production of hydrogen and carbon nanofibers through the decomposition of methane over activated carbon supported Pd catalysts

Hydrogen has been produced by decomposing methane thermocatalytically at 1123 K in the presence of activated carbon supported Pd catalysts (Samples coded as Pd5 and Pd10 respectively) procured from SRL Chemicals, India. The studies indicated that the Pd10 catalyst has higher catalytic activity and life for methane decomposition reaction at 1123 K and volume hourly space velocity (VHSV) of 1.62 L/hr?g. An average methane conversion of 50 mol % has been obtained for Pd10 catalyst at the above reaction conditions. SEM and TEM-EDXA images of Pd10 catalyst after methane decomposition showed formation of carbon nanofibers. XRD of the above catalyst revealed, moderately crystalline peaks of Pd which may be responsible for the increase in the catalytic life and the formation of carbon nanofibers.     

Hydrogen production with a low carbon monoxide content via methanol steam reforming over CuxCeyMgz/Al2O3 catalysts: Optimization and stability

Catalytic activities of Ce–Mg promoted Cu/Al₂O₃ catalysts via methanol steam reforming was investigated in terms of the methanol conversion level, carbon monoxide selectivity and hydrogen yield. The factors chosen were the reaction temperature, copper content, Mg/(Ce + Mg) weight-percentage and steam to carbon ratios. The catalysts were prepared by co-precipitation and characterized by means of XRD, BET, H₂-TPR, and FESEM. The Ce–Mg bi-promoter catalysts gave higher performance due to magnesium penetration into the cerium structure causing oxygen vacancy defects on the ceria. A response-surface-model was then designed to optimize the condition at a 95% confidence interval for complete methanol conversion to a high H₂ yield with a low CO content, and revealed an optimal copper level of 46–50 wt%, Mg/(Ce + Mg) of 16.2–18.0%, temperature of 245–250 °C and S/C ratio of 1.74–1.80. No deactivation of the Cu_0.5Ce_0.25Mg_0.05/Al catalyst was observed during a 72-h stability test.     

Production of hydrogen by steam reforming of phenol over Ni/Al2O3-ash catalysts

An improved method for hydrogen production by the steam reforming of phenol over novel fly ash-based catalysts is investigated. The Ni/Al₂O₃-ash catalysts are prepared by an equal-volume impregnation method and characterized by XRD, FESEM, BET and H₂-TPR techniques. The effects of various process parameters including mixing ratio of fly ash, temperature, support, gas hourly space velocity (GHSV) and steam-to-carbon molar ratio (S/C) on the catalytic activity are investigated. The results show that fly ash mixing at 50 wt% and choosing γ-Al₂O₃ as the support own the best performance. A maximum hydrogen yield of 83.8% is achieved at 450 °C with a S/C of 10 and a GHSV of 4968 h^?1 with a maximum phenol conversion of 98.6%. The stability of the Ni-ash1-γA1 catalyst is further investigated and it is shown to continuously and stably react for more than 20 h at 450 °C with excellent catalytic reaction stability.     

Non-supported bimetallic catalysts of Fe and Co for methane decomposition into H2 and a mixture of graphene nanosheets and carbon nanotubes

Herein, non-supported pure and mixed cobalt and iron oxide catalysts were synthesized from nitrate precursors using a simple, environmentally friendly preparation method in which water was the sole solvent. The prepared catalysts were then used to decompose methane into hydrogen and carbon (graphene nanosheets and carbon nanotubes). The fresh and spent catalysts were characterized by employing X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM) and N₂ adsorption-desorption techniques. In addition, the spent catalysts were subjected to thermo-gravimetric analysis (TGA) in order to measure the quantity of carbon deposits on the spent catalysts. The results indicated that the carbon deposited over these catalysts is a mixture of graphene nanosheets and carbon nanotubes (CNT). The results indicated that the mixed oxide catalysts exhibit higher catalytic activity than the pure oxides and that Fe: Co atomic ratio represents the key factor in the catalytic activity of these mixed oxides. After 420 min under the reaction feed, the 50Fe + 50Co catalyst shows the highest catalytic activity towards methane conversion of about 52.6% compared to 41.6% and 31.8% for 75Fe + 25Co and 25Fe + 75Co catalysts, respectively.     

Mild temperature hydrogen production by methane decomposition over cobalt catalysts prepared with different precipitating agents

Hydrogen production by methane decomposition has been studied using different cobalt catalysts obtained by reduction of cobalt oxide precursors synthesized in ethylene glycol and using three different precipitating agents: sodium carbonate, ammonium hydroxide and urea. The physicochemical properties of the catalysts precursors vary with the precipitating agent, which shows a significant influence in their catalytic performance. Thus, the catalysts obtained from precursors precipitated with Na₂CO₃ or CO(NH₂)₂ show remarkable catalytic activity at lower temperatures, which in both cases has been assigned to the lower particle size and aggregation degree of the final metallic Co phase. Accordingly, the use of urea as precipitating agent led to the catalyst with the highest H₂ production at 600 °C after 12 h of time on stream. Likewise, it is worth mentioning that the catalyst prepared using Na₂CO₃ shows significant activity in this reaction even at temperatures as low as 400 °C.