Nature Of Carbon Deposits Research Articles

The nature of carbon deposits and their formation pathways during catalytic supercritical water gasification of glycerol

Supercritical water gasification (SCWG) is a promising technology to convert wet biomass to renewable natural gas. The present study sheds light on the chemical composition and the formation mechanism of coke deposits on a Ru/CNF catalyst during continuous SCWG of glycerol in a fixed-bed reactor. SCWG experiments were performed using pristine carbon nanofibers (CNF) and 5 wt% Ru/CNF catalysts, which suffered from partial deactivation. Transmission electron microscopy (TEM) indicated a 34–42 % dispersion loss independently of the conditions, only partially explaining the activity loss observed during the test at very high space velocity. When operated under thermodynamic conditions, no deposits could be observed. A nanometric layer of deposits on the catalysts was present under intermediate conditions, while in the kinetic regime (conversion of glycerol down to 4 %), significant carbon deposition was observed. The chemical compositions of aqueous phases and organic deposits extracted from the catalysts and support showed that the presence of Ru enhanced the formation of unsaturated compounds from glycerol. In particular, molecules containing aromatic rings were detected. The space velocity significantly affected the composition of the coke deposits. The number of carbons in the molecules increased from 5–25 to 10–45 when moving away from the thermodynamic regime (full conversion), and the O/C ratio significantly decreased from 0.3–0.6 to 0.05–0.3, maintaining a very broad H/C ratio. The extracted organic deposits contained various aromatic compounds but were mostly represented by unsaturated aliphatic compounds. All these results were used to elucidate a reaction pathway for the formation of organic deposits from glycerol.

Ni/Y2O3-ZrO2 catalysts for dry reforming of methane: Increased Y content boosted the performance via enhancing metal-support interaction and surface oxygen species

Dry reforming of methane is an alternative route to realize the efficient recycling of carbon resources. This work aims to unravel the effects of the Y/Zr molar ratio in the Y2O3-ZrO2 supports on metal-support interaction (MSI) and surface oxygen species, where the former is related to Ni dispersion and the anti-sintering ability and the latter is corresponded to CO2 activation and anti-carbon deposition ability. Interestingly, the characterization results reveal that increasing Y content in the catalyst effectively enhances MSI, improves Ni dispersion, and enriches the surface oxygen species. Also, the higher Y content in the catalyst makes carbon deposition more active and easier to be removed, and the nature of carbon deposition, rather than the amount, is the crucial factor for catalyst deactivation. As a result, the catalyst with high Y content shows the superior performance. Specially, Ni/Y2O3 exhibits the highest CH4 and CO2 conversion (∼80 % and 85 %, respectively) among the representative catalysts at 700 °C, and its activity (CH4 conversion: ∼88 %, CO2 conversion: ∼93 %) can maintain at least 3600 min at 750 °C. Moreover, the origin of the excellent performance over the catalyst with high Y content is elucidated and the reaction mechanisms are discussed using in situ experiment techniques. The metallic Ni species are active sites to dissociate CH4 and the oxygen species react with the carbonaceous species derived from CH4 cracking to produce CO or CO2. After consuming the adsorbed oxygen species, the re-exposed oxygen vacancies and hydroxyl groups adsorb CO2 to form the active intermediates again, including the bicarbonate and carbonate species. Furthermore, the bicarbonate species are verified to be more active than the other carbonates, and they can react with the carbonaceous species to produce CO or transform into the carbonates.

Producing Hydrogen from Steam Reforming of Bio-oil Derived Oxygenated Model Compounds by Utilizing Ce-Modified Ni/Attapulgite Catalysts

Steam reforming (SR) of bio-oil has been considered as one of the promising methods for hydrogen production. In this paper, cerium modified Ni/Attapulgite (ATP) catalysts were successfully synthesized by traditional chemical precipitation method and utilized for SR of various bio-oil derived oxygenated model compounds. The effect of Ce content on the structural properties of Ni/ATP was investigated by ICP-OES, N2 adsorption–desorption, XRD, H2-TPR and HRTEM characterizations. The results exhibited that 10Ce–Ni/ATP catalyst had the strongest metal-support interaction and highly dispersion of Ni species with smallest particle size of 6.21 nm. Therefore, it presented the maximum ethanol conversion of 97.21% and H2 yield of 83.15% at relative lower reaction temperature 500 °C during 4 h of SR of ethanol (SRE). In addition, it unfolded promising performance for SR of acetic acid (SRAA) and acetone (SRA). Furthermore, the influence of different oxygenates on the amount and nature of carbon deposits in used 10Ce–Ni/ATP catalyst was revealed by TG and Raman analyses. It found that the types of feedstock significantly affected the carbon species and amount and the formation of graphic layers around the Ni metal sites was the mainly factor for rapid deactivation of 10Ce–Ni/ATP during 4 h of SRA reaction.

3‐D X‐ray Nanotomography Reveals Different Carbon Deposition Mechanisms in a Single Catalyst Particle

AbstractCatalyst deactivation involves a complex interplay of processes taking place at different length and time scales. Understanding this phenomenon is one of the grand challenges in solid catalyst characterization. A process contributing to deactivation is carbon deposition (i. e., coking), which reduces catalyst activity by limiting diffusion and blocking active sites. However, characterizing coke formation and its effects remains challenging as it involves both the organic and inorganic phase of the catalytic process and length scales from the atomic scale to the scale of the catalyst body. Here we present a combination of hard X‐ray imaging techniques able to visualize in 3‐D the distribution, effect and nature of carbon deposits in the macro‐pore space of an entire industrially used catalyst particle. Our findings provide direct evidence for coke promoting effects of metal poisons, pore clogging by coke, and a correlation between carbon nature and its location. These results provide a better understanding of the coking process, its relation to catalyst deactivation and new insights into the efficiency of the industrial scale process of fluid catalytic cracking.

Methane decomposition and carbon deposition over Ni/ZrO2 catalysts: Comparison of amorphous, tetragonal, and monoclinic zirconia phase

The influence of crystal phase of ZrO2 on the catalytic performance of methane decomposition and the properties of deposited carbon on Ni/ZrO2 catalysts was investigated. Ni/ZrO2 catalysts were prepared by incipient-wetness impregnation method with 5% Ni loading, using amorphous, monoclinic, and tetragonal ZrO2 as supports. It was found that Ni/am-ZrO2 exhibited high activity and superior stability for carbon dioxide reforming of methane during 50 h of TOS, which could be attributed to the smaller Ni particle size and the lower coking rate. Additionally, the results of O2-TPO/TPH/CO2-TPO showed that the nature of carbon deposition via CH4 decomposition on Ni/ZrO2 catalysts was strongly influenced by both the Ni particle size and the crystal phase of zirconia. The lowest coking rate on Ni/am-ZrO2 for carbon dioxide reforming of methane was due to the lower CH4 decomposition rate and the higher gasification rate of carbon species by CO2. Carbon deposited on Ni/am-ZrO2 could be greatly removed by CO2, due to the presence of large amount of adsorbed oxygen species on the surface of amorphous ZrO2. For comparison, the imbalance of CH4 cracking and removal of coke by CO2 resulted in the accumulation of coke, leading to the deactivation of catalysts.

Hydrogen production from crude oil with fine iron particles through microwave-initiated catalytic dehydrogenation promoted by emulsified feed

With increasing concerns about climate change caused by carbon dioxide emissions, decarbonization is rapidly becoming an important and attractive challenge for continuing fossil fuel utilization. This paper reports an entirely new route of producing hydrogen directly from crude oil through microwave-initiated, iron-catalyzed deep dehydrogenation. Importantly, there is very little carbon dioxide emission in this process, a key element towards stabilising global mean temperature. The analytical techniques of Scanning Electron Microscope, Transmission Electron Microscopy, Temperature Programmed Oxidation and Raman spectroscopy were employed to interrogate the catalytic process and the nature of carbon deposition over the catalyst which was brought about by the dehydrogenation process. The presence of emulsified feed significantly promotes hydrogen production, owing to its presence depressing carbonaceous deposition on the catalyst together with the occurrence of the micro-explosion phenomenon.

Role of Sn in the Regeneration of Pt/γ-Al2O3 Light Alkane Dehydrogenation Catalysts.

Alumina-supported Pt is one of the major industrial catalysts for light alkane dehydrogenation. This catalyst loses activity during reaction, with coke formation often considered as the reason for deactivation. As we show in this study, the amount and nature of carbon deposits do not directly correlate with the loss of activity. Rather, it is the transformation of subnanometer Pt species into larger Pt nanoparticles that appears to be responsible for the loss of catalytic activity. Surprisingly, a portion of the Sn remains atomically dispersed on the alumina surface in the spent catalyst and helps in the redispersion of the Pt. In the absence of Sn on the alumina support, the larger Pt nanoparticles formed during reaction are not redispersed during oxidative regeneration. It is known that Sn is added as a promoter in the industrial catalyst to help in achieving high propene selectivity and to minimize coke formation. This work shows that an important role of Sn is to help in the regeneration of Pt, by providing nucleation sites on the alumina surface. Aberration-corrected scanning transmission electron microscopy helps to provide unique insights into the operating characteristics of an industrially important catalyst by demonstrating the role of promoter elements, such as Sn, in the oxidative regeneration of Pt on γ-Al2O3.

Carbon Deposition on a Toluene Disproportionation and Transalkylation Catalyst

The nature of carbon deposition on a toluene disproportionation and transalkylation catalyst was characterized using thermogravimetry and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, N2 adsorption/desorption and NH3-temperature programmed desorption (NH3-TPD). Result showed that the kinetics of hard coke combustion approximately fitted a first order reaction with respect to the amount of coke and, therefore, most of the coke existed in a monolayer state and the corresponding apparent activation energy was about 110 kJ·mol-1. There were three kinds of carbon species on the surface of the Cat-1000 catalyst, which presented the catalyst after 1000 h reaction. The C atom fractions in the presence of C—O, C=O, and —C—C— were 22.7%, 9.1%, 68.1%, respectively. The formed carbon species on the Cat-1000 catalyst surface was mostly a graphite structure. The results showed that three quarters of the total carbon deposition on Cat-1000 was hard coke and the rest was soft coke. The specific surface area of the Cat-1000 catalyst decreased obviously compared with the catalyst without reaction (Cat-0). The Cat-1000 had almost unchanged acid intensity as well as high activity and stability. This shows that the cokes mainly deposit on the binder.

Study of catalyst deactivation and reaction mechanism of steam reforming, partial oxidation, and oxidative steam reforming of ethanol over Co/CeO 2 catalyst

The mechanisms of Co/ceria catalyst deactivation during steam reforming, oxidative steam reforming, and partial oxidation of ethanol were explored by comparing the results from different characterization techniques with those obtained from catalytic testing in a fixed-bed reactor. The nature of carbon deposition and the reaction conditions played critical roles in determining the extent of a catalyst deactivation. To shed light on the modes of carbon deposition under different reaction conditions, the mechanisms by which the adsorbed surface species turned over on the catalyst surface were evaluated using diffuse reflectance infrared spectroscopy under reaction conditions and temperature-programed desorption of adsorbed ethanol. In steam reforming, ethoxy species were converted to acetate and steam promoted forward acetate demethanation. The resulting methane decomposed on Co metal particles. In this case, carbon diffused through the Co particle, nucleating growth sites for filamentous carbon behind it, with the resulting filaments lifting Co from the support. High H 2O/ethanol ratios and oxygen promoted cleaning of the cobalt surface.

N-Dodecane reforming over monolith-based Ni catalysts: SEM study of axial carbon distribution profile

The extent and nature of carbon deposition that occurs during catalytic reforming reactions of hydrocarbons depends strongly on the reaction conditions and the surface properties of the catalyst. In conventional packed bed reactors it is difficult to track the spatial distribution of carbon and link it to the location of individual catalyst particles. The present work uses monolith-based nickel catalysts to investigate the axial carbon gradient post n-dodecane reforming. The carbon, which was deposited during autothermal reforming (ATR), partial oxidation (POX), and steam reforming (SR) over Ni/cerium-zirconium oxide (CZO)/monolith and Ni/monolith catalysts, was characterized by a combination of temperature-programmed oxidation (TPO), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The amount, location, and type of carbon deposited strongly depended on the type of reforming reaction. The majority of carbon deposition was concentrated inside cordierite pores of the monolith. In general, the catalysts showed the expected trend of increased filamentous carbon deposition down the length of the monolith, reflecting the axial variation in temperature and oxygen concentration, except for POX over Ni/monolith catalyst where massive coating carbon deposition occurred in the upstream section of the monolith.

Coke Characterization on Pt/Al2O3−β-Zeolite Reforming Catalysts

The amount and nature of carbon deposits on platinum catalysts supported on a mixture of alumina−β-zeolite, non neutralized and neutralized with cesium, were investigated and compared with those of a commercial catalyst (Pt/Al2O3). The catalysts were deactivated in the methylcyclopentane reaction. Coke characterization was performed by elemental analysis, temperature-programmed oxidation (TPO), Fourier transform infrared (FTIR), and 13C (CP/MAS) NMR. The significant difference between Pt/Al2O3 and Pt supported on a mixture of γ-alumina and β-zeolite is the nature of the carbon deposits. Elemental analysis, TPO, FTIR, and 13C (CP/MAS) NMR pointed out different features of the deposits. The zeolitic catalyst neutralized by Cs presented similar amounts of carbon residues but was the most deactivated. The results point out acid sites with different responses to carbonaceous residues.