Coke Accumulation Research Articles

Efficient Catalysts of Ethanol Steam Reforming Based on Perovskite-Fluorite Nanocomposites with Supported Ni: Effect of the Synthesis Methods on the Activity and Stability

Catalysts based on perovskite—fluorite nanocomposites with supported nickel 5%Ni/[Pr0.15Sm0.15Ce0.35Zr0.35O2 + LaMn0.9Ru0.1O3] were synthesized by three different methods. Structural and surface features of as-prepared samples were elucidated by N2 adsorption, XRD, HR TEM with EDX; reducibility and reactivity were estimated by H2-TPR, and catalytic properties were studied in ethanol steam reforming in the 500–700 °C temperature range. The best catalytic activity without coke accumulation was demonstrated for the 5%Ni/[Pr0.15Sm0.15Ce0.35Zr0.35O2+ LaMn0.9Ru0.1O3] catalyst with nanocomposite support obtained by a simple sequential polymeric preparation method. Highly dispersed particles of metallic nickel strongly fixed on the support after the reaction were shown by the HR-TEM and H2 –TPR data. The catalyst provides stable full conversion of ethanol and hydrogen yield above 60% at 600 °C for at least 6 h.

High activity and stability of nano‐nickel catalyst based on LaNiO3 perovskite for methane bireforming

AbstractIn this study, a nano‐nickel Ni0 catalyst for bireforming of methane was in‐situ synthesized from the sol‐gel fabricating LaNiO3 perovskite. The various physico‐chemical methods such as powder X‐ray diffraction (XRD), energy‐dispersive X‐ray spectroscopy (EDS), H2 temperature‐programmed reduction (H2‐TPR), scanning electron microscopy (SEM), and CO2 temperature‐programmed desorption (CO2‐TPD) were used for characterization of the obtained samples. The coke accumulation was investigated via the temperature‐programmed oxidation (TPO) method. The results confirmed the formation of both LaNiO3 perovskite structure and a small amount of NiO with high crystallinity. H2‐TPR profiles and XRD patterns showed that the highly dispersed nanocrystals Ni0 on La2O3 catalysts were formed by in‐situ reduction of LaNiO3 at 800°C for as short as 1 hour or more. The crystal sizes of Ni0 and La2O3 crystals are in the range of 17‐18 nm and 18‐19 nm, respectively. The activity of the catalysts was studied in bireforming of methane by the micro‐flow method in a temperature range of 550‐800°C. LaNiO3‐based catalyst showed excellent performance for methane bireforming reaction. Of which, LaNiO3 sample calcined at 800°C for 1.5 hours and reduced at 800°C for 1.5 hours showed the highest activity when it reached the CH4 conversion of 87% and abnormally high CO2 conversion, reaching 97 % at 700°C and approximately 100 % at 800°C with an ideal H2/CO ratio of approximately 2. At 800°C, the activity of the most outstanding catalyst remained stable for 100 hrs thanks to the high sintering resistance of excellent dispersion of Ni in LaNiO3 lattice and high coke tolerance of the catalyst. The catalytic activity remained stable even when the amount of inert γ‐carbon reached 8.58 mgC/gcat.

Investigation of defect-rich CeO2 catalysts for super low-temperature catalytic oxidation and durable styrene removal.

Spherical cerium dioxide (CeO2-S) nanoparticles were successfully prepared using a solvothermal method, and their performances in catalytic oxidation reactions were studied. The CeO2-S catalyst showed superior low-temperature catalytic activity for styrene removal (T90=118°C, GHSV=18,000 h-1) compared with commercial CeO2. The characterization results showed that there were numerous oxygen defects in CeO2-S that were key to its catalytic performance at low temperatures, high redox properties, and high adsorption capacity for the reaction gases (O2 and styrene). Moreover, the catalytic performance of CeO2-S was highly stable (132h), and the particles were reusable. FTIR and in-situ DRIFTS results showed that the type of intermediates formed during the oxidation of styrene determined the CeO2 catalytic stability, and the main intermediates were bidentate carbonate species that accumulated on the surface of deactivated CeO2-S and were not thermally stable. Moreover, the soft carbon that also deposited on CeO2-S during the reaction was easily decomposed at higher temperatures. The role of the oxygen vacancies on the CeO2-S catalyst was further revealed by correlating the concentration of oxygen vacancies and the accumulation of coke on the catalyst surface.

Anti-coking NiCex/HAP catalyst with well-balanced carbon formation and gasification in methane dry reforming

Methane dry reforming (MDR) is a promising process to convert two major greenhouse gases into syngas. However, the major drawback of the process is the deactivation of Ni-based catalysts due to carbon deposition and metal sintering. Herein, we prepared hydroxyapatite (HAP) supported Ni catalysts modified by CeO2, which can efficiently promote MDR reaction with the excellent anti-coking property. The strong interaction between Ni and HAP guarantees the stability of highly dispersed Ni particles during the MDR reaction. The neighboring CeO2 over Ni particles could facilitate the mobility of oxygen, which assists the activation of CH4 to CH3O and in some extent hinders the deep cracking of CH4 into deposited carbon. Meanwhile, the kinetic studies indicate that the existence of reactive oxygen species is beneficial for enhancing carbon gasification. Accordingly, the addition of CeO2 balanced the carbon formation and gasification over Ni and therefore efficiently prevented coke accumulation. The exploration of NiCex/HAP catalysts would provide inspiration for future MDR catalyst design.

Roles of AAEMs in catalytic reforming of biomass pyrolysis tar and coke accumulation characteristics over biochar surface for H2 production

Coke formation is a significant challenge in catalytic tar reforming. AAEMs are essential in the conversion and decomposition of tar catalyzed by biochar. In this paper, four biochar catalysts with different K and Ca contents were prepared by acid washing and loading, and the coke accumulation characteristics in catalytic tar reforming at 650 °C were investigated using a single-stage reaction system. The gas-liquid-solid products were characterized by GC-MS, Raman, N2 adsorption, FTIR and TG. The results suggest that K-loaded biochar has a maximum tar reforming capacity of 94.9%, while H-form biochar has a tar removal efficiency of only 27.8%. The micropore area in biochar is considerably reduced and the average pore size is increased after coke deposition. While K-loaded biochar retains the highest micropore area, it also exhibits a smaller increase in average pore size. The loading of K/Ca affects the growth structure of the coke, resulting in an increased number of O-containing structures in it. The coke on the Raw biochar surface is mainly small aromatic ring structures and aliphatic structures, thus increasing the intensity of the vibrational peaks corresponding to aromatic = C–H and aliphatic C–H on it. The coke on K-loaded biochar has a large proportion of aliphatic structures, which also contributes to the reduced graphitization of it after reforming. The AAEMs-free biochar surface preferentially removes tar components carrying O-containing groups. K-loaded biochar preferentially catalyzes the reforming of mono-aromatic ring components in tar. Ca-loaded biochar preferentially removes the mono-aromatic ring components, while being less selective for the removal of tar components containing hydroxyl groups and polyaromatic ring components. The loading of K/Ca promotes the dehydrogenation of the tar fraction during reforming, while only K catalyzes the deoxygenation of tar components. H-form biochar has no appreciable catalytic activity on CH4 cracking. AAEMs have a catalytic activity on CH4 cracking. K is particularly effective in improving tar conversion and hydrogen production of biochar.

Improving catalytic production of aromatic hydrocarbons with a mesoporous ZSM-5 modified with nanocellulose as a green template

ZSM-5 is an important class of acid catalyst for producing bio-oil rich in petrochemicals; however, continuous efforts are required to improve the conversion efficiency of the biomass-derived oxygenates with large molecular sizes. The mesoporous ZSM-5 (N-ZSM-5) was prepared in a post-treatment method with nanocellulose as a green template in this study to induce additional surface and porous features. The post-treatment promoted the mesoporous properties in N-ZSM-5, which effectively converted lignocellulosic biomass to deoxygenated aromatic products in catalytic pyrolysis. Moreover, compared to the conventional or alkali-treated mesoporous ZSM-5, higher selectivity of monocyclic aromatic hydrocarbons (MAHs) was attained in catalytic pyrolysis of lignin while mitigating the coke accumulation. The product distribution was evaluated using the response surface methodology with reaction temperature (°C) and catalyst loading (g/g feedstock). The compositions of upgraded volatile products measured MAHs (~46.55 area%) and monophenols (~98.09 area%) depending on the process conditions.

Effect of Support on Stability and Coke Resistance of Ni-Based Catalyst in Combined Steam and CO2 Reforming of CH4.

Ni-based catalysts dispersed on different supports (MgO-α-Al2O3, CeO2, SBA-15, and MgO-SBA-15) were prepared by the impregnation method. Characteristics of the catalysts, including specific surface areas (N2 physisorption), crystalline phase compositions (powder X-ray diffraction, Raman spectroscopy), reducibility (hydrogen temperature-programmed reduction, H2-TPR), and morphology (scanning electron microscopy (SEM) and transmission electron microscopy, TEM)) were investigated. The activity and stability of the catalysts were tested for the combined steam and CO2 reforming of methane at 700 °C in a microflow system. The results show that the catalysts exhibit high activity in the BRM reaction. At 700 °C, the conversion of CH4 and CO2 reached 86–99% and 67–80%, respectively, in which the Ni/Mg–SBA catalyst is the best with conversions of CH4 and CO2 reaching 99% and 80%. Coke accumulation on the surface of the catalysts for 100 h time on stream (TOS) was evaluated by the temperature-programmed oxidation (TPO) technique. The major cause of the catalytic deactivation was elucidated by combining the determination of the amount and type of deposited coke with the changes in the physicochemical properties of the catalysts after the long-term reaction. Almost complete loss of activity was observed on Ni/Mg–Al catalyst after 100 h TOS, while the activity drop was slow on the Ni/Mg–SBA sample, about 15–20% of the total value. Otherwise, the Ni/CeO2 and Ni/SBA catalysts firmly retained their stable activity for 100 h TOS due to the minimal carbon deposition and stability of these catalysts’ structure. The highly considerable formation of inert Cγ carbon and sintering over Ni catalyst supported on MgO-α-Al2O3 were responsible for the lower stability of this catalyst compared to those supported on CeO2 and SBA-15.

Induction Heating of Magnetically Susceptible Nanoparticles for Enhanced Hydrogenation of Oleic Acid.

Radio frequency (RF) induction heating was compared to conventional thermal heating for the hydrogenation of oleic acid to stearic acid. The RF reaction demonstrated decreased coke accumulation and increased product selectivity at comparable temperatures over mesoporous Fe3O4 catalysts composed of 28–32 nm diameter nanoparticles. The Fe3O4 supports were decorated with Pd and Pt active sites and served as the local heat generators when subjected to an alternating magnetic field. For hydrogenation over Pd/Fe3O4, both heating methods gave similar liquid product selectivities, but thermogravimetric analysis–differential scanning calorimetry measurements showed no coke accumulation for the RF-heated catalyst versus 6.5 wt % for the conventionally heated catalyst. A different trend emerged when hydrogenation over Pt/Fe3O4 was performed. Compared to conventional heating, the RF increased the selectivity to stearic acid by an additional 15%. Based on these results, RF heating acting upon a magnetically susceptible nanoparticle catalyst would also be expected to positively impact systems with high coking rates, for example, nonoxidative dehydrogenations.

Mesostructured Zn/ZSM‐5 Zeolite as Catalyst for Furan Deoxygenation,

AbstractThe furan conversion into aromatics over zinc promoted ZSM‐5 zeolites was studied in a continuous‐flow fixed‐bed reactor. Two series of microporous and mesoporous catalysts were prepared at different zinc loadings of 2 and 5 wt.%. It was intended to optimize the overall aromatics production and control coke accumulation over novel synthesized mesoporous ZSM‐5 catalyst, in a continuous flow reactor. The catalysts were characterized using X‐ray diffraction (XRD), nitrogen adsorption/desorption, ammonia temperature programmed desorption (TPD) and X‐ray photoelectron spectroscopy (XPS). The zinc loaded mesoporous materials exhibit an XRD pattern that matches with the ZSM‐5 XRD reference pattern. No extra peaks were observed in the XRD results indicating the high dispersion of zinc species. The initial furan conversion rate was higher over the microporous catalysts, and increased upon increasing metal loading, however contrary to the mesoporous samples, the microporous ones deactivated rapidly over 2 hours of time on‐stream. The results indicated that around 40 % of carbon in the fed furan, was recovered in aromatics when the new mesostructured zeolite catalyst was used, whereas this could be further increased to 50 % by the addition of zinc in the catalyst, benzene being the major product.

Catalytic selective ethane dehydrogenation at low-temperature with low coke formation.

Catalytic ethane dehydrogenation (EDH) was investigated to improve the efficient production of ethylene, an extremely important chemical feedstock. The perovskite oxide YCrO3 was found to be more suitable than earlier reported catalysts because it exhibits greater activity and C2H4 selectivity (94.3%) in the presence of steam at 973 K. This catalyst shows the highest activity than ever under kinetic conditions, and shows very high ethane conversion under integral reaction conditions. Comparison with EDH performance under conditions without steam revealed that steam plays an important role in stabilizing the high activity. Raman spectra of spent catalysts indicated that steam prevents coke formation, which is responsible for deactivating YCrO3. Transmission IR and XPS measurements also revealed a mechanism by which H2O forms surface oxygen species on YCrO3, consequently removing C2H6-derived coke precursors rapidly and inhibiting coke accumulation.

Role of reforming agent in filamentous coke deposition on Ni/bio-char catalyst during non-oxygenates tar reforming

The filamentous coke deposition limits the application of Ni/bio-char in the tar reforming process during biomass gasification. The addition of H2O and CO2 can improve tar conversion during the gasification process, while the effect of H2O and CO2 on filamentous coke deposition during non-oxygenates tar reforming process has not been extensively explored. To reduce the filamentous coke deposition by optimizing reforming agent, this work investigated the effect of reforming agent on filamentous coke deposition on Ni/bio-char catalyst during reforming of non-oxygenates tar. During CO2 reforming process, the accumulation of filamentous coke resulted in the decline of catalyst activity. In terms of steam reforming process, the promotion effect of steam atmosphere on mesopores of Ni/bio-char contributed to improving the conversion of non-oxygenates tar. The free radicals O* from the dissociation of H2O could oxidize the filamentous coke on Ni/bio-char, which resulted in the filamentous coke deposition (38%) in steam reforming lower than CO2 reforming (57%). Overall, the addition of steam in reforming agent contributed to improving non-oxygenates tar conversion and inhibiting filamentous coke formation during the reforming of non-oxygenates tar over Ni/bio-char.

Integrating molten carbonates pyrolysis and catalytic reforming with modified HZSM-5 for monophenols produced from digestate

Digestate (DE), the residue following anaerobic digestion, has been well utilized as a potential feedstock of molten carbonates pyrolysis and catalytic reforming. The lignin of DE was converted into phenolics, guaiacols and furfural with molten carbonates pyrolysis, and further upgraded to bio-based monophenols by coupling with modified HZSM-5 catalyst. The maximum monophenols yield could achieve 7.22 g/100 g of digestate, and 81.34% of the organic phase of bio-oil at the optimum reaction conditions. The alkali-treated and Fe-loaded on HZSM-5 catalyst, with enhanced pore diameter distinctly and mild acidity, has an outstanding selectivity for monophenols and preferable anti-coking ability in integrated pyrolysis system, while with quite a low catalyst coke yield of 3.97%. The results indicated that the formation of mesopores might minimize the diffusion hindrance of the reactant molecules and reduce coke accumulation. In addition, according to the experiment conducted on a self-designed thermogravimetric analysis device, the kinetic study of the digestate with molten carbonates certificated that the reaction rate of pyrolysis had been promoted by both decreasing activation energy and intensification of heat transfer.

Adsorbent/catalyst bi-functional Fe-ZSM-5 prepared by a simple CVD process for exhaust gas treatment

In exhaust gas purification systems, the use of an inorganic porous ‘pre-filter’ in combination with a conventional catalytic converter with Pt-group metal nanoparticles is considered to be effective for reducing cold-start emissions. Organic compounds that are difficult to catalytically oxidize at low temperatures can be captured to this ‘pre-filter’ until the Pt-group catalyst has reached its operating temperature. In the present work, a simple chemical vapor deposition (CVD) method was applied to load catalytically active Fe species (Fe(III) ions, Fe(III)xOy clusters, and Fe2O3 nanoparticles) on a commercial MFI type H-ZSM-5 zeolite. The CVD could maintain the adsorption performance of H-ZSM-5 zeolite, but more importantly enhance the catalytic oxidation activity of acetaldehyde. The adsorbed acetaldehyde was oxidized with high efficiency on Fe ion & oxide loaded H-ZSM-5 (Fe-ZSM-5), which eventually can reduce cold-start emissions and improve the catalytic conversion efficiency of exhaust purification systems. Furthermore, based on in-situ DRIFTS analyses, surface poisoning of H-ZSM-5 that was induced by accumulation of coke could be eliminated by CVD of Fe ion & oxide.

Role of filamentous coke in deactivation of Ni/bio-char catalyst during dry reforming of non-oxygenates tar

During tar reforming process, the coke deposition limits the catalyst activity. The encapsulating coke has been confirmed to be responsible for catalyst deactivation in steam reforming of oxygenates tar, while the role of filamentous coke in catalyst deactivation in dry reforming of non-oxygenates tar remains unclear. In this work, the conventional impregnation method and one-pot hydrothermal carbonization method were used to synthesize Ni/bio-char. The catalyst activity and coke deposition on catalysts in dry reforming of toluene were explored. The characteristics of fresh catalysts revealed that the high pressure and temperature in one-pot synthesis method improved the nickel content (6.72–55.20 %), promoting the catalyst activity in tar conversion. However, due to the coke accumulation on Ni/bio-char, the hydrogen conversion declined with the reaction proceeding. The characterizations of spent catalysts indicated that the dominant coke deposition on Ni/bio-char was filamentous coke in dry reforming of tar, which accounted for 7.35–36.16 %. The filamentous coke produced by the Boudouard reaction consumed the CO in gaseous products, which reduced the syngas quality. Overall, large amounts of filamentous coke deposition can result in the deactivation of Ni/bio-char in dry reforming of non-oxygenates tar.

Pore diameters of Ni/ZrO2 catalysts affect properties of the coke in steam reforming of acetic acid

Pore structures of support affect not only the dispersion of metals species, but also the catalytic stabilities and the properties of the generated coke in steam reforming reaction of acetic acid. This has been demonstrated in this study with Ni/ZrO2 catalysts of varied pore diameters. The results showed that the Ni/ZrO2 catalyst with the support calcined at 700 °C had the largest diameter, which facilitated the entering of nickel species into the pores, suppressing the migration and agglomerate in reduction of the catalysts. Although the catalysts with the varied pore diameters showed little impacts on the catalytic performances, the properties of the coke varied substantially. The catalyst with the large pore diameter favored the growth of the crystal carbon in the coke to a bigger size and was more tolerant to coke accumulation while maintained the catalytic stability. In addition, thermal stability, resistivity to oxidation, aromaticity, functionalities and the morphologies of the coke (i.e. carbon nanofibers, carbon nanotubes and bamboo-like structure) were all connected with the porous structures of the Ni/ZrO2 catalysts.

Inhibitor, co-catalyst, or intermetallic promoter? Probing the sulfur-tolerance of MoOx surface decoration on Ni/SiO2 during methane dry reforming

As for the application of CH4dry reforming(DRM), primary concern was focused on solving the deactivation ofsupported Ni catalystscaused byparticle sintering and coke accumulation, while little attention has been paid to negative effect resulted from sulfur poisoning. The present work gave an account of the effect of H2S on the DRM over MoOx decorated Ni/SiO2 catalyst. It showed that Mo modified samples presented better sulfur resistance performance. Moreover, the interesting discovery was that different forms of Mo species were worked as special roles during H2S-induced DRM. The suitable amount of Mo, involved as the inhibitor and/or the co-catalyst, could increase the competitive adsorption capacity for H2S, thus slowing down sulfuration rate of the active Ni sites. Meanwhile, as for the specially pre-reduced catalysts, the formed intermetallic metal of Ni-Mo component contributed to a continuous sulfur-adsorption behavior for H2S, thus protecting the active Ni sites from sulfuration. Consequently, probing the sulfur-tolerance of MoOxsurface decoration on Ni/SiO2could elucidate some useful keys for the development of high sulfur-resistance catalysts, which, as a consequence, would be adequate catalysts for the H2S-induced DRM process.

A short review on bimetallic Co-based catalysts for carbon dioxide reforming of methane

Abstract The development of catalysts that afford excellent catalytic performance along with high resistance toward coke accumulation is fundamental in carbon dioxide reforming of methane (CDRM). Apart from Ni-based catalysts, the Co-based catalysts gained significant attention in CDRM accredited to the Co’s capability in improving catalytic stability and lowering the coke formation. However, the lower catalytic activities of Co-based catalysts when compared to Ni-based catalysts in reforming works are the real challenges that need to be solved. In this study, a short review of various approaches that have been implemented by researchers for improving the catalytic performance issues related to Co-based catalysts is presented. This paper also presents recent Co-bimetallic catalysts approached, covers the catalyst activity as well as issues related to catalyst deactivation when compared to Co monometallic catalysts. In addition, the outlook of the related bimetallic Co-based catalysts has been proposed to provide more critical information.

Investigation of coke deposition inside catalyst with heterogeneous active component distribution

Coke deposition over catalytic particles is not only associated with pore structure and pore distribution, but also depends on the active component distribution. A particle-resolved model is employed to explore the coke deposition process inside catalyst particles with heterogeneous active component distribution. The spatial and instantaneous distribution of coke deposition and its influence on catalytic performance in a single particle and a packed bed are evaluated. The results show that there is a weaker internal resistance of mass and heat transfer inside the egg-shell type catalyst so as to lead to a faster coke accumulation and catalyst deactivation, compared to the egg-yolk type catalyst. Meanwhile, the wall effect on coke accumulation is more significant for egg-yolk type catalysts.

Preventing of flooding phenomena on vacuum distillation trays column via controlling coking value factor

The occurrence of flooding phenomena (FP) on the vacuum distillation trays column (VDTC) is investigated on a refinery in the middle region of Iran. Accumulation of coke sediment that is occasionally observed in refinery processing using coal tar (CT) feeding is the main reason of the FP on VDTC. This usually occurs at the down-comer areas and the seal pans of the column. Quinolone insoluble (QI) and toluene insoluble (TI) parameters, two routine methods used to measure coke containing sediments, are employed to determine probable critical areas of FP at the down-comers and the seal pans of CT feed in this study. Analyzing the obtained results of QI and TI tests demonstrated that FP at the column is directly related to the quality of CT feeding. For instance, when the QI of the inlet CT in battery 1 increases from 0.56%W in the first sample to 6.5%W in the 25th sample, the vacuum pressure at the top of the distillation column is reduced from − 230 mbar to − 420 mbar. To prevent sediment accumulation of coke and the occurrence of FP, the CT feeds are centrifuged to reduce coke sedimentation at the down-comer areas and the seal pans of VDTC. Moreover, the seal pan trays are replaced by normal ones and all moveable valves with 25 mm diameter on trays 15 and higher in the column, where coke sediment accumulation is observed the most, are modified by installing wider fixed valve (40 mm diameter) trays.

Evaluation of coke deposition in catalyst particles using particle-resolved CFD model

Catalyst deactivation by carbon deposition is an important phenomenon of catalytic industrial processes, which will lead to the decrease of reaction rates and even the blockage of the reactor. In this work, coke deposition process is investigated by means of a particle-resolved CFD model considering the change of catalyst physical property and activity caused by carbon formation. The spatial and instantaneous distributions of coke deposition in the single catalyst particle and packed bed are evaluated. Meanwhile, the interaction of reaction performance and catalyst activity under coke deposition is further analyzed. The results reveal that the local catalyst activity is different from the core to the shell zone. In addition, the wall effect on coke accumulation in a fixed bed is revealed. The results indicate that more coke is formed near the wall with lower catalyst activity.