Formation Of Filamentous Carbon Research Articles

Synergetic effects of Al addition on the performance of CoFe2O4 catalyst for hydrogen production and filamentous carbon formation from direct cracking of methane

Improving methane catalytic decomposition process was deemed an effective approach for hydrogen production. Here, a novel catalyst AlxCo1-xFe2O4 with a variable aluminum content was synthesized via a co-precipitation route for the direct cracking of methane. The catalytic activity of AlxCo1-xFe2O4 was performed in a fixed bed reactor at temperature of 800°C and 20mL/min of fed gas. A sequence of characterizations, including SEM, BET, XRD, XPS, Raman and TGA for fresh and spent catalysts, was exploited to examine the influence of Alx content on the morphology, porosity, structure, chemical composition, and cation distribution of AlxCo1-xFe2O4. As the Alx content increased in AlxCo1-xFe2O4 their morphology became more fractal and decreased in particle size, lattice parameter, lattice size, while their pore surface increased. Further cation re-distributions from octahedral to tetrahedral sites as Fe3+ and Al3+ relocated in the Tetrahedral site and Oxygen lattice decreased as observed from Raman and XPS results. Catalytic activity studies showed that Alx content of 0.6 in AlxCo1-xFe2O4 lead maximum methane conversion up to ~ 91.10% and hydrogen formation rate of 1.780 ×10-3mol H2 g-1 s-1. The catalytic activity behavior was explained in correlation with the characteristics of spent catalysts.

Insight into role of Co species on NiCo/CeMgAl for ethanol steam reforming: Enhanced activity and resistance to coke

Bimetallic catalysts with unique geometric and electronic structure exhibit high performance compared to that of monometallic counterparts in the ethanol steam reforming (ESR). Herein, NiCo bimetals supported on CeO2 doped MgAl2O4 (NiCo/CeMgAl) with different ratio of Ni/Co were prepared for ESR. The synergies between Ni and Co in catalytic activities and ability to anti-coking were focused. The results demonstrate Co atoms trend to segregate toward the surface and are oxidized synchronously, forming the Ni-CoO hybrid surface. The reduced Ni particles are beneficial to maintain a stable catalytic activity for the C-C bonds scission, and the CoO particles formed by segregation and irreversible oxidation of Co0 are more conducive to activation of H2O, the resulting OH promotes the transformation of CHx species, which significantly reduces the selectivity of CH4 and prevents the formation of filamentous carbon. Controlling Co content and maintaining a stable M/Mδ+ are critical to balance the C-C bond scission reaction and the CH3* conversion reaction on the NiCo bimetallic catalysts. The optimal catalyst 9Ni1Co/CeMgAl shows the ethanol conversion of 93.9 % at TOS of 100 h, and the average rate of coke formation is 0.50 mgc·gcat-1·h−1.

Dry reforming of methane over Ni/Al2O3 catalysts: Support morphological effect on the coke resistance

Flower-like (Al2O3-F), wire-like (Al2O3-W) and sphere-like (Al2O3-S) alumina supported nickel catalysts (designated as Ni/Al2O3-F, Ni/Al2O3-W and Ni/Al2O3-S) were prepared for catalytic dry reforming of methane (DRM). The resultant catalysts exhibited variable catalytic activities and coke resistance performances in the reaction, which was closely dependent on their various physicochemical properties from the morphological effect of Al2O3 supports. To be precise, Ni/Al2O3-W presented adequately high catalytic activity and promising anti-coke behavior, which was attributed to its strong metal-support interaction, the surface dispersion of small-sized Ni nanoparticles and noticeable activation ability towards CO2. Ni/Al2O3-F possessed high anti-coke performance as well in terms of its significant CO2 activation ability. However, the encapsulation effect of Al2O3-F nanostructure resulted in insufficient abundance of metallic Ni active sites on the surface of Ni/Al2O3-F, thus giving rise to poor initial catalytic activity. The weak CO2 activation ability and the large particle size of metallic Ni on Ni/Al2O3-S induced the considerable formation of filamentous carbon after long-term DRM test. With regard to in situ DRIFTs analysis, the existence of OH* as the coke precursor scavenger and its reaction with the CHx* intermediate species from CH4 cracking were demonstrated to play crucial roles in dominating the overall anti-coke performances of Ni/Al2O3 catalysts. This work provides new insights into the development of high coke-resistance Ni-based catalysts from the perspective of support morphology modulation.

Effect of Ce promotion on catalytic activity of Ni-Al catalysts in dry reforming of methane

The Ni-Al, Ni-Ce and Ni-Ce-Al catalysts tested in the dry methane reforming (DRM) were studied. Catalysts were synthesized by solution combustion synthesis and characterized by BET, XRD and TEM. Catalytic activity was studied at 600–900 °C with a 33%CH4:33%CO2:34%Ar (vol.%) fed with total flow rate of 100 ml/ min (3000 h−1). The CH4 and CO2 conversion increased with the increasing of Ce up to 15 wt.%, however, with further increase in Ce content conversion of gases decreased. Carbon was formed as filaments when catalysts worked at high temperature. CeAlO3 species could prevent the formation of filamentous carbon during DRM. Solution combustion synthesis is attractive method of preparation of catalysts, due to high dispersion of Ni particles, thus, surface area is small, diminishing the coke deposition and enhancing the stability.

Carbon Deposit Analysis in Catalyst Deactivation, Regeneration, and Rejuvenation

AbstractHydrocarbon conversion catalysts suffer from deactivation by deposition or formation of carbon deposits. Carbon deposit formation is thermodynamically favored above 350 °C, even in some hydrogen‐rich environments. We discuss four basic mechanisms: a carbenium‐ion based mechanism taking place on acid sites of zeolites or bifunctional catalysts, a metal‐induced formation of soft coke (i.e., oligomers of small olefins) on bifunctional catalysts, a radical‐mediated mechanism in higher‐temperature processes, and fast‐growing carbon filament formation. Catalysts deactivate because carbon deposits block pores at different length scales, or directly block active sites. Some deactivated catalysts can be re‐used, others can be regenerated or have to be discarded. Catalyst and process design can mitigate the effects of deactivation. New analytical tools allow for the direct observation (in some cases even under in situ or operando conditions) of the 3D‐distribution of coke‐type species as a function of catalyst structure and lifetime.

Carbon Deposit Analysis in Catalyst Deactivation, Regeneration, and Rejuvenation.

Hydrocarbon conversion catalysts suffer from deactivation by deposition or formation of carbon deposits. Carbon deposit formation is thermodynamically favored above 350 °C, even in some hydrogen-rich environments. We discuss four basic mechanisms: a carbenium-ion based mechanism taking place on acid sites of zeolites or bifunctional catalysts, a metal-induced formation of soft coke (i.e., oligomers of small olefins) on bifunctional catalysts, a radical-mediated mechanism in higher-temperature processes, and fast-growing carbon filament formation. Catalysts deactivate because carbon deposits block pores at different length scales, or directly block active sites. Some deactivated catalysts can be re-used, others can be regenerated or have to be discarded. Catalyst and process design can mitigate the effects of deactivation. New analytical tools allow for the direct observation (in some cases even under in situ or operando conditions) of the 3D-distribution of coke-type species as a function of catalyst structure and lifetime.

Effect of multi-component gases on the behavior and mechanism of carbon deposition in hydrogen-rich blast furnaces

Hydrogen-rich blast furnace is considered an advanced technology in the iron and steel industry for reducing carbon emissions at the source. The CO utilization is instead reduced due to the severe carbon deposition behavior after hydrogen enrichment. This paper quantitatively investigated the effect of CO–H2–H2O–N2 multi-component on carbon deposition behavior, and a formation mechanism of deposited carbon was proposed finally. The results revealed that the optimum temperature range for carbon deposition was about 470–900 °C, and the maximum rate of carbon deposition was reached at 550–700 °C. At 600 °C, the carbon deposition rate for 100%CO and 40%CO–60%N2 were 0.54 g/d and 0.13 g/d, respectively. The aggravating effect of H2 to carbon deposition was 5.2 times that of H2O. Whereas the carbon deposition will be constrained in low CO and high H2 environment. The SEM showed that the morphology of the deposited carbon was filamentous, its formation mechanism on the iron surface is: (a) Adsorption of gases on the iron surface, (b) Formation of Fe3C layer, (c) Deposition of graphite layer, (d) Decomposition of Fe3C layer, (e) Formation of Fe3C–Fe + C (diss.) catalytic particles, (f) Formation of filamentous carbon, (g) Carbon activity in metallic Fe-filamentous carbon.

Boron-promoted Ni/MgO catalysts for enhanced activity and coke suppression during dry reforming of methane

The dry reforming of methane (DRM) provides an attractive solution for the utilization of CH4 and CO2 and its performance highly depends on the catalyst. In this study, DRM to syngas was studied using a series of Ni/MgO catalysts modified by B2O3 (0.1–2.0 wt%). The results demonstrated that introducing only 0.25 wt% B2O3 to 4Ni/MgO catalyst significantly improved its activity, stability, and carbon resistance. After 40 h time on stream (TOS), the activity of 4Ni0.25B/MgO catalyst almost unchanged and was approximately 20% higher than that of the unmodified 4Ni/MgO catalyst. The 4Ni0.25B/MgO catalyst also showed the least carbon deposits (0.3 wt%) among all samples. The characterization tests suggest that B2O3 substantially enhanced the Ni reducibility during DRM without weakening the Ni–MgO interaction, thereby improving the activity without sacrificing the dispersion and stability of the Ni nanoparticles (NPs). Intermetallic nickel boride was formed when B2O3 interacted with metallic Ni, which suppressed the formation of filamentous and graphitic carbon. In addition, excess B2O3 has a slight influence on the formation of amorphous carbon.

Facile modification of cobalt ferrite by SiO 2 and H‐ZSM‐5 support for hydrogen and filamentous carbon production from methane decomposition

In this work, new catalyst composed of cobalt ferrite CoFe2O4 supported on amorphous silica (SiO2) and zeolite (H-ZSM-5) were prepared using impregnation method with loading ratio 1:1. The catalytic activity of the new 50 wt% CoFe2O4/SiO2 and 50 wt% CoFe2O4/H-ZSM-5 catalysts were evaluated for direct methane decomposition to hydrogen production at fixed operating temperature of 800°C. The physiochemical properties of the fresh and spent catalysts were characterized by several techniques such as scanning electron microscope (SEM), X-ray diffractometer (XRD), Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy (XPS), thermogravimetric (TGA), and Raman spectroscopy. The CoFe2O4/H-ZSM-5 catalyst exhibited higher activity and stability than CoFe2O4/SiO2 catalyst. Maximum methane conversion reached 37.35% and hydrogen formation rate of 76.48 × 10−5 mol H2 g−1 min−1 at time on stream (TOS) of 300 min for CoFe2O4/H-ZSM-5, while its 5.80% with 15.82 × 10−5 mol H2 g−1 min−1 at TOS of 30 min. The XRD, Raman, and XPS results confirm the presence of CoFe2O4 particles on the CoFe2O4/H-ZSM-5 catalyst surface, while the CoFe2O4 particles partially shield with SiO2 in CoFe2O4/SiO2 catalyst. SEM images of spent catalysts elucidated the formation of filamentous carbon which indirectly confirm the higher distribution of CoFe2O4 in CoFe2O4/H-ZSM-5 than that of CoFe2O4/SiO2. The deposited carbon on spent CoFe2O4/H-ZSM-5 catalyst was evaluated using TGA, which was found to be ca. 50 wt%. Highlights CoFe2O4/SiO2 and CoFe2O4/H-ZSM-5 catalysts synthesized and characterized by various techniques. H-ZSM-5 and SiO2 support material were found strongly affects the physiochemical properties and the methane decomposition activity of CoFe2O4 catalyst. CoFe2O4/H-ZSM-5 catalyst performance reached 37.4% and the hydrogen formation rate was 76.5 × 10−5 mol H2 g−1 min−1. CoFe2O4/SiO2 catalyst demonstrated a negligible activity toward methane cracking and producing hydrogen. The estimated deposited carbon on the spent catalysts were 1 wt% and 50 wt% for CoFe2O4/SiO2 and CoFe2O4/H-ZSM-5, respectively.

Unveiling the deactivation by coke of NiAl2O4 spinel derived catalysts in the bio-oil steam reforming: Role of individual oxygenates

The catalyst stability, mainly affected by coke deposition, remains being a challenge for the development of a sustainable process for hydrogen production by steam reforming (SR) of bio-oil. In this work, the influence of oxygenates composition in bio-oil on the deactivation by coke of a NiAl2O4 spinel derived catalyst has been approached by studying the SR of a wide range of model oxygenates with different functionalities, including acetic acid, acetone, ethanol, acetaldehyde, acetol, catechol, guaiacol and levoglucosan. A fluidized bed reactor was used in the following conditions: 600 and 700 °C; steam/carbon ratio, 3 (6 for levoglucosan); space–time, 0.034 gcatalysth/gbio-oil (low enough to favor the rapid catalyst deactivation), and; time on stream, 5 h. The spent catalysts were analyzed with several techniques, including Temperature Programed Oxidation (TPO), X-ray Diffraction (XRD), N2 adsorption–desorption, Scanning and Transmission Electron Microscopy (SEM, TEM) and Raman Spectroscopy. The main factors affecting the catalyst stability are the morphology, structure and location of coke, rather than its content, that depend on the nature of the oxygenate feed. The deposition of pyrolytic and amorphous coke that blocks the Ni sites inhibiting the formation of filamentous carbon causes a rapid deactivation in the guaiacol SR. Conversely, the large amount of carbon nanotubes (CNTs) giving rise to a filamentous coke deposited in the SR of aliphatic oxygenates only causes a slight deactivation. The increase in the temperature significantly reduces coke deposition, but has low impact on deactivation.

Interaction of chlorinated hydrocarbons with nichrome alloy: From surface transformations to complete dusting

The process of a metal dusting of the nichrome alloy in the atmosphere of chlorinated hydrocarbon was investigated. A series of samples with different exposure times was prepared to study the bulk NiCr evolution. The X-ray diffraction data revealed no changes in the phase composition of the nichrome alloy after 1-h treatment, since the amount of deposited carbon does not exceed 1 wt%. The formation of carbon filaments was confirmed by scanning electron microscopy. According to the transmission electron microscopy results, the 4-stage schematic mechanism describing the metal dusting process of the bulk nichrome alloy was proposed and discussed.

Synthesis-Gas Production from Methane over Ni/CeO2 Catalysts Synthesized by Co-Precipitation Method in Different Solvents

Ni/CeO2 catalysts were synthesized by the coprecipitation method in a basic medium, using different solvents: water, methanol, ethanol, and isopropanol (Ni content, 10% wt.). These catalysts were tested in the production of syngas through the oxidative reforming of methane (ORM), and partial oxidation of methane (POM). The results of this research demonstrated that the use of alcohols (methanol, ethanol, and isopropanol) during the preparation of the Ni/CeO2 catalysts by the coprecipitation method, improved their characteristics such as crystallite size (nm), surface area (m2·g−1), and reducibility (measured by H2-TPR) that influenced on their catalytic performance in ORM and POM reactions. The best solvent of this study was isopropanol. The use of alcohols (methanol, ethanol, isopropanol) in the co-precipitation method led to the formation of filamentous carbon on the catalyst after the reactions. The catalyst synthesized in the water proved to be inefficient in the POM and ORM reactions and led to the formation of amorphous carbon after the reactions.

Effect of different simulated metal dusting environments on austenitic Alloys 602CA and 800

Metal dusting processes and associated filamentous carbon formation were studied on austenitic Alloys 602CA and 800 after exposure to 18.9%CO-79.1%H2-2%H2O and 25%CO-70%H2-5%H2O (vol.%) gas mixtures at 650°C. The alloys were studied by visual examination, optical microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), Raman spectroscopy and X-ray diffraction (XRD). Alloy 602CA was more resistant to metal dusting than Alloy 800. SEM imaging of the surfaces showed that Alloy 800 suffered metal attack at 168 h, whereas Alloy 602CA suffered minor attack only after 336 h. Coke deposits increased on Alloy 800 with increased exposure, and there was little coke deposited on Alloys 602CA from after 336 h. Raman spectroscopy confirmed graphite by the characteristic D- and G-peaks.

Methane decomposition over Ni supported on palm oil fuel ash (Ni-POFA) catalyst

The performance of Ni supported on palm oil fuel ash (Ni-POFA) catalyst was evaluated for methane decomposition reaction. The effects of Ni loading (5, 10, 15 and 20 wt.%) and gas hourly space velocity (GHSV) (5000, 7000, 15,000 and 25,000 mL/g h) were investigated at 550 °C for 6 h. The results show that increasing the Ni loading from 5 to 15 wt.% increased the initial H2 yield. However, the highest Ni loading (20 wt.% Ni) resulted in the lowest CH4 conversion and H2 yield, which has been associated to Ni particles agglomeration and formation of large filamentous carbon. The increase of GHSV from 5000 to 7000 mL/g h increased the initial CH4 conversion and initial H2 yield. Nonetheless, the catalytic performance of Ni-POFA degraded rapidly at higher GHSVs (15,000 and 25,000 mL/g h). This has been linked to a shorter contact time between CH4 gas and the catalyst at higher CH4 flow rates. The current study proposes the 15 wt.% Ni and 7000 mL/g h as the optimum Ni loading and GHSV for Ni-POFA catalyst, respectively. Under these conditions, methane decomposition performed admirably at 87.0% initial CH4 conversion and 27.0% initial H2 yield. Under the same conditions, the highest stable value of H2 production was achieved at ∼2.0% over the 6-h reaction time.

Effect of nanostructured grains in co-sputtered Ni-GDC thin-film anode on methane conversion kinetics for low temperature solid oxide fuel cells operating on nearly dry methane

Direct-methane solid oxide fuel cells (DMSOFCs) have recently attracted substantial attention due to their simplified system, reduced cost, and the direct availability of methane fuel obtained from natural gas. Among oxygen-ion conductive materials, doped-ceria such as gadolinium-doped ceria (GDC) or samarium-doped ceria can be incorporated into Ni-based anodes to reinforce their coking resistance, enlarge their electrochemical reaction area, and improve the kinetics of the internal reforming/electrochemical oxidation of methane. To reduce the range of operating temperatures of DMSOFCs while maintaining their performance, the thin film deposition technique of magnetron sputtering was adopted in this work. An Ni-GDC thin-film anode and a Pt thin-film cathode were deposited on scandia-stabilized zirconia (ScSZ) electrolyte supports. This fuel cell was tested with directly supplied methane fuel (3% H2O) at 500 °C. The results demonstrated the effects of the GDC volume fraction in the anode—which was controlled by co-sputtering power—on open circuit voltage and electrochemical performance. The co-sputtered Ni-GDC anode was able to survive through 36-h operation, although there was some performance degradation. Field-emission scanning electron microscopy results revealed no formation of filamentous carbon on the Ni catalysts, despite the fact that both X-ray photoelectron spectroscopy and Raman spectroscopy analyses detected carbon coking. The relatively high performance and resistance to carbon coking of co-sputtered thin-film anode were attributed to its intrinsic small grain size.

Tuning Metal‐Support Interaction and Surface Acidic Sites of Ni/Al2O3 by Dopamine Modification for Glycerol Steam Reforming

AbstractIn this work, a universal dopamine modification strategy is used to improve the catalytic performance of Ni/Al2O3 for hydrogen production in glycerol steam reforming (GSR). Ni/Al2O3 showed poor catalytic stability with rapid deactivation after 8 h of reaction in GSR. Glycerol conversion and hydrogen selectivity were significantly higher for dopamine‐modified Ni/Al2O3 (by 98 % and 95 %, respectively) and this improved performance was maintained well for 21 h in the stability test. The protective carbon shell derived from dopamine was formed easily on the Ni/Al2O3 surface, the aggregation of Ni nanoparticles was inhibited, and the dispersion of Ni nanoparticles was improved. The electron‐donating effect of amino group to Ni2+ weakened the Ni‐Al2O3 interaction, and then promoted the reduction of the Ni species. In addition, the decrease in the number of strong acidic sites reduced the amount of carbon deposition on dopamine‐modified Ni/Al2O3. The moderate metal‐support interaction was more conducive to the formation of filamentous carbon, which collectively contributed to the increased catalytic stability of dopamine‐modified Ni/Al2O3. The facile dopamine‐modified method significantly improves the catalytic performance and can be generalized to other catalytic reaction that involve sintering and carbon deposition issues.

Atomic Layer Deposition of TiO2 Thin Films on the Inner Walls of Steel Tubes Increases Anti-coking Properties.

To suppress catalytic coking, TiO2 passivation films were deposited on the inner walls of SS316 stainless steel tubes by atomic layer deposition (ALD). Indentation test results showed a platform on the indentation curve of TiO2 films grown over 2000 ALD cycles due to internal stress-induced microcracks. In coking experiments, the TiO2-coated tubes exhibited a higher heat flux and lower pressure difference than bare ones. Analysis of the coking surface revealed that TiO2 thin film passivation can reduce the size and number of particulate deposits. At the same time, passivation treatment inhibits the formation of filamentous carbon and improves anti-coking performance by reducing the ability of the tube to adsorb amorphous deposition products. The coking surfaces of TiO2-coated tubes had less graphitization, indicating that the coking products had fewer defects and lower activated carbon contents.

Sol-gel Ni-based/γ-Al2O3 as efficient catalysts for toluene reforming: Catalytic activity during long-term experiments and in presence of H2S

Last decades, the valorization of biomass became a topic of interest, and especially the production of syngas by the catalytic reforming of biomass. But one drawback for a large-scale development is the associated production of tars.This work focuses on the catalytic reforming of tars present in bio-syngas reactors. Efficient Ni-based/γ-Al2O3 catalysts were developed in previous studies by an aqueous sol-gel process. Here, four of those catalysts, exhibiting promising catalytic properties for toluene reforming (used as tar model molecule), were further evaluated on long-term experiments and under H2S. Indeed, long-term performance and resistance to poisoning are two key properties of catalysts for industrial applications. One commercial catalyst, Hifuel™, was also evaluated for comparison. The results showed that the pure Ni/γ-Al2O3 catalyst presented a low deactivation throughout 30 h of experiment with neither Ni particle size nor support crystallinity alteration. All co-doped Ni/γ-Al2O3 catalysts were more resistant against deactivation than the pure Ni/γ-Al2O3 catalyst. As only amorphous carbon was deposited, an easy regeneration was performed by heating the catalysts 2 h at 650 °C under air. The catalytic activity remained unchanged in similar 30 h-experiment.Contrarily, the commercial Hifuel™ catalyst underwent a strong and quick deactivation by the formation of filamentous carbon. The regeneration under air was not sufficient to regenerate the catalyst.The Ni/γ-Al2O3 catalyst activity was assessed in the presence of H2S. The results showed that the toluene conversion was only slightly affected by the presence of H2S in the syngas mixture, highlighting its very good resistance to poisoning.

Dynamics and Mechanism of Carbon Filament Formation during Methane Reforming on Supported Nickel Clusters

CH4–CO2 and CH4–H2O reforming on Ni-based catalysts can lead to the undesired formation of carbonaceous residues. The dynamics of the formation of carbon filaments and encapsulating layers on dispe...

Toluene steam reforming using nickel based catalysts made from mining residues

Two nickel-based catalysts are used for toluene steam reforming: Ni-Olivine and Ni-UGSO (Nickel-UpGraded Slug Oxides). Both catalysts are made from mining residues and prepared by incipient wetness impregnation (IWI). The activity and stability of the two catalysts in toluene steam reforming are investigated and compared, at atmospheric pressure and 800 °C in a fixed bed reactor. Reaction temperature and Ni Loading effects are evaluated for the Ni-UGSO catalyst. The physicochemical properties of the catalysts were analyzed using the following characterization techniques: X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX), X-ray absorption near edge spectroscopy (XANES), Temperature Programmed Reduction (TPR) and Brunauer-Emmett-Teller (BET) specific surface area. Results showed that toluene conversion, mainly into CO and H2, reached up to (98 w%). The catalyst activity was tested in multiple 24 h time-on-stream continuous isothermal runs and no signs of deactivation were observed. Ni in Ni-UGSO has a strong metal-support interaction, which, according to characterization results, is due to the formation of a NiO/MgO solid solution. This catalyst exhibited an elevated catalytic activity along with a high resistance to carbon formation. Ni in Ni-Olivine has shown a weak metal-support interaction leading to the formation of filamentous carbon at the catalyst’s surface.