Mechanism Of Carbon Deposition Research Articles

Study on the mechanism of carbon nanotube-like carbon deposition in tar catalytic reforming over Ni-based catalysts

The use of Ni-based catalysts is a common method for eliminating tar through catalytic cracking. Carbon deposition is the main cause of deactivation in Ni/ZSM-5 catalysts, with filamentous MWCNTs being the primary form of carbon deposits. This study investigates the formation and evolution of CNTs during the catalytic process of biomass tar to explore the mechanism behind carbon deposition. The effect of the 9Ni/10MWCNTs/81ZSM-5 on toluene reforming was investigated through a vertical furnace. Gases produced by tar catalysis were evaluated through GC analysis. The physicochemical structure, properties and catalytic performance of the catalyst were also tested. TG analysis was used to assess the accumulation and oxidation reactivity of carbon on the catalyst surface. An analysis was conducted on the mechanism of carbon deposition during catalyst deactivation in tar catalysis. The results showed that the 9Ni/91ZSM-5 had a superior toluene conversion of 60.49%, but also experienced rapid and substantial carbon deposition up to 52.69%. Carbon is mainly deposited as curved filaments on both the surface and pore channels of the catalyst. In some cases, tip growth occurs where both carbon deposition and Ni coexist. Furthermore, specific surface area and micropore volume are reduced to varying degrees due to carbon deposition. With the time increased, the amount of carbon deposited on the catalyst surface increased to 62.81%, which gradually approached saturation, and the overall performance of the catalyst was stabilized. This situation causes toluene molecules to detach from the active sites within the catalyst, hindering gas release, which leads to reduced catalytic activity and further carbon deposition. It provides both a basis for the development of new catalysts and an economically feasible solution for practical tar reduction and removal.

Influence of pyrolysis temperature on carbon deposition from the perspective of volatile evolution during the ex-situ pyrolysis-catalysis of plastic

To better understand the mechanism of carbon deposition, the ex-situ catalytic pyrolysis of polyethylene (PE) was explored over a FeNi/Al2O3 catalyst, and the evolution of volatiles as well as the yield and structure of carbon products at different temperatures were analyzed in depth by an in-situ atmospheric pressure photoionization high-resolution mass spectrometry (APPI HRMS) combined with a fixed-bed reactor. The results show that raising the pyrolysis temperature (from 500 to 800 °C) reduces carbon product yield and H2 production, and the proportion of carbon nanotubes (CNTs) in the carbon products decreases. The source of carbon products gradually shifts from liquid-phase carbon sources to gaseous-phase carbon sources. Below 700 °C, both the outer diameter and carbon interlayer spacing of CNTs decrease, while above 700 °C, they both increase along with increased distortion of carbon layers. It is also observed that the carbon layers are connected to the catalyst lattice stripes, and the interlayer spacing of carbon layers is greater than the catalyst lattice spacing. In situ mass spectrometry reveals a significant increase in the carbon number and double bond equivalent (DBE) values of volatiles with increasing pyrolysis temperature, but the rate of increase slows down after 700 °C. Volatiles from lower pyrolysis temperatures undergo severe polymerization, upon entering the catalytic zone at higher temperatures, minimizing temperature-induced distinctions. Nevertheless, it remains evident that volatiles from lower pyrolysis temperatures exhibit less pronounced polymerization when exposed to higher catalytic temperatures.

Numerical Simulation of the Carbon Deposition Mechanism of Solid Oxide Fuel Cells Based on Total Harmonic Distortion Analysis

Identifying the corresponding degradation mechanisms at the early stage of operation is important for the long-term stable operation of solid oxide fuel cells (SOFCs). This paper uses total harmonic distortion analysis (THDA) to identify the anode carbon deposition mechanism in the early stage of SOFC operation with carbon-containing fuels. An accelerated calculation model for the electrochemical impedance spectrum (EIS) and a total harmonic distortion calculation model were constructed to study the carbon deposition mechanism and carbon removal strategy of SOFC anodes based on a one-dimensional transient elementary reaction kinetic model of an SOFC fueled by a CH4/H2O mixture. It is shown that the THDA calculation model can identify the primary stages of carbon deposition on SOFC anodes at specific frequencies. The THDA profiles at frequencies greater than 2 kHz contain essentially the same key information as at lower frequencies, suggesting that THDA can significantly reduce the test time for identifying performance degradation in the early stages of SOFC operation compared to EIS. Using a mixture of hydrogen and water vapor to remove the solid carbon generated by the cell anode and restore it to its initial performance, the THDA computational model can successfully respond to this process.

Deactivation and Regeneration of Nitrogen Doped Carbon Catalyst for Acetylene Hydrochlorination.

The poor stability of carbon materials doped with nitrogen limited their development in acetylene hydrochlorination. Therefore, investigating the deactivation reasons of carbon catalysts and researching regeneration methods became the research focus. Herein, carbon-nitrogen materials were synthesized by one-step pyrolysis, which using biomass materials with high nitrogen content, the synthesized material was used in an acetylene hydrochlorination reaction. The acetylene conversion rate of D-GH-800 catalyst was up to 99%, but the catalytic activity decreased by 30% after 60 h reaction. Thermogravimetric analysis results showed that the coke content was 5.87%, resulting in catalyst deactivation. Temperature-programmed desorption verified that the deactivation was due to the strong adsorption and difficult desorption of acetylene by the D-GH-800 catalyst, resulting in the accumulation of acetylene on the catalyst surface to form carbon polymers and leading to the pore blockage phenomenon. Furthermore, based on the catalyst deactivation by carbon accumulation, we proposed a new idea of regeneration by ZnCl2 activation to eliminate carbon deposition in the pores of the deactivated catalyst. As a result, the activity of D-GH-800 was recovered, and lifetime was also extended. Our strategy illustrated the mechanism of carbon deposition, and the recoverability of the catalyst has promising applications.

DFT study on the chemisorption and reforming of naphthalene over bio-char: The detailed mechanism of carbon deposition and hydrogen production

Tar removal over char bed is a potential method for syngas cleaning. The heterogeneous interaction mechanism between char surface and tar molecule is crucial and still not fully revealed. In this work, the detailed interaction between bio-char surface and naphthalene was studied by experiment and density functional theory (DFT) at molecular level. Three bio-char models and corresponding reaction paths were proposed. The geometric optimization and frequency calculation of each species were conducted. Mulliken charges and electrostatic potential results of char models indicate that oxygenated functional groups could weaken the electron cloud density of adjacent carbon atoms and make the carbon atoms positively charged, which appears as a “Lewis acid center” and benefit the adsorption of tar molecule. The orbital electron results show that the band gaps of three char models are 1.2965 eV, 2.7903 eV and 3.4366 eV, respectively, indicating the unpaired electrons on char surface will enhance the reactivity of char compared with oxygenated functional groups. The potential energy evolution shows that the initial adsorption of naphthalene on char surface is an exothermic reaction. The activation energies for naphthalene decomposition over char surface are 207.83 kJ/mole, 164.01 kJ/mole, and 159.17 kJ/mole, which are much lower than that of thermal decomposition (446 kJ/mole). The chemisorption of naphthalene on char surface changes the electron density distribution, which greatly enhance tar decomposition. Experimental results proved that the oxidative pyrolysis char is rich in micropores and oxygenated functional groups, which largely enhanced the naphthalene reduction efficiency over bio-char surface.

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.

Understanding the mechanism of carbon deposition of Ni3Co catalysts for methane dry reforming

• Linear relationships between activation energy and reaction entropy was identified. • Reaction pathways of methane reforming reaction on Ni 3 Co (1 1 1) surface were obtained. • Mechanism of carbon deposition on Ni 3 Co (1 1 1) surface was revealed in detail. • Anti-coking strategies on methane reforming reaction were proposed.

Insights into Coke Formation and Removal under Operating Conditions with a Quantum Nanoreactor Approach.

The in situ formation and removal of coke is a critical problem in heterogeneous catalysis, but its mechanism is not well understood. This work investigates the mechanism of carbon deposition and hydrogenation on an Fe cluster under high-temperature conditions with the density functional tight-binding (DFTB) based nanoreactor molecular dynamics (NMD) method. Our study shows that successive formation of carbon chains, rings, and fused rings occurred during the carbon deposition on Fe clusters. Hydrogenation of activated carbon happens through direct C-H coupling, while the hydrogenation of graphitic carbon involves hydrogenation of the edge carbon, ring-opening reaction, and dealkylation reaction. The main function of the Fe catalyst is to provide the active sites for H2 dissociation and dissociated H spillover, while its activity toward C-C bond breaking is limited. These results highlight the role of the DFTB-NMD method as an effective tool to investigate reaction mechanisms under operating conditions in heterogeneous catalysis.

Carbon deposition mechanism of molten salt cleaning and optimization of multicomponent molten salt formula for remanufacturing.

In remanufacturing engineering, cleaning is the key factor for subsequent blank inspection and parts repair. Molten salt has the characteristics of low viscosity, good fluidity, and strong chemical stability. Salt bath cleaning can be comprehensively applied to remove various organic pollutants. Molten salt has the function of self-cleaning. The dirt in the cleaning pool can be decomposed and reused. Moreover, the waste molten salt is massive, and the waste residue is easy to treat. The formation and adsorption mechanism of carbon deposition was explored, and the formation mechanism of carbon deposition was verified by experiments. Then, the existing formula was improved by mixing experiment and compared with the current cleaning method; the cleaning effect was excellent. A new molten salt formula is proposed on the basis of the research on solar thermal storage medium. The composition percentage and the optimal process parameters of the new molten salt formula were determined on the basis of the experimental design of mixture. The cleaning effect of the new formula is good.

Improving the activity and stability of Ni-based electrodes for solid oxide cells through surface engineering: Recent progress and future perspectives

Solid oxide cells (SOCs) have attracted great attention in the past decades because of their high conversion efficiency, low environmental pollution and diversified fuel options. Nickel-based catalysts are the most widely used fuel electrode materials for SOCs due to the low price and high activity. However, when hydrocarbon fuels are employed, nickel-based electrodes face serious carbon deposition challenges, leading to a rapid decline of cell performance. Great efforts have been devoted to understanding the occurrence of the coking reaction, and to improving the stability of the electrodes in hydrocarbon fuels. In this review, we summarize recent research progress of utilizing surface modification to improve the stability and activity of Ni-based electrodes for SOCs by preventing carbon coking. The review starts with a briefly introduction about the reaction mechanism of carbon deposition, followed by listing several surface modification technologies and their working principles. Then we introduce representative works using surface modification strategies to prevent carbon coking on Ni-based electrodes. Finally, we highlight future direction of improving electrode catalytic activity and anti-coking performance through surface engineering.

Simultaneous production of aromatics-rich bio-oil and carbon nanomaterials from catalytic co-pyrolysis of biomass/plastic wastes and in-line catalytic upgrading of pyrolysis gas

An integrated process that includes catalytic co-pyrolysis of biomass/plastic wastes and in-line catalytic upgrading of pyrolysis gas were conducted to simultaneously produce aromatics-rich bio-oil and carbon nanotubes (CNTs). The influences of feedstocks blending ratio on the characteristics of bio-oil and CNTs were established. The reaction mechanism of carbon deposition during the system was also probed. The results showed that co-feeding plastic to biomass siginificantly enhanced the selectivity of monoaromatics (benzene, toluene, and xylene) from 5.6% for pure biomass to the maximum yield of 44.4% for 75.0% plastic ratio, and decreased naphthalene and its derivates from 85.9 to 41.7% correspondingly. The most synergistic effect on BTX selectivity occurred at 25% of plastic ratio. The multi-walled CNTs were successfully synethsized on Ni catalyst by utilizing prolysis gas as feedstocks. For pure biomass, the least CNTs yield with ultrafine diameters of 3.9–8.5 nm was generated via disproportionation reaction of CO which was derived from decarboxylation and decarbonylation of oxygenates on the ZSM-5 acid sites. With the rise of plastic ratio, sufficient hydrocarbons were produced for CNTs growth, endowing CNTs with long and straight tube walls, along with uniform diameters (~16 nm). The CNTs yield increased as high as 139 mg/g-cata. In addition, the decreased CO2 inhibited dry reforming with C1-C4 hydrocarbons and deposited carbon, avoiding excessive etching of CNTs. Thereby, high-purity CNTs with less defects were fabricated when plastic ratio was beyond 50% in the feedstock. The strategy is expected to improve the sustainability and economic viability of biomass pyrolysis.

In situ fabrication of dual coating structured SiO/1D-C/a-C composite as high-performance lithium ion battery anode by fluidized bed chemical vapor deposition

To notably improve the cycling stability of high-capacity SiO anode, an ingenious carbon dual coating structure was proposed, utilizing the high conductivity of 1D carbon (1D-C) and the excellent buffering of amorphous carbon (a-C). The carbon deposition pattern by regulating process temperatures was explored. An evolutive carbon deposition mechanism between 1D-C and a-C was deduced, which was a competitive mode between low-temperature catalytic growth and high-temperature pyrolysis deposition. Motivated by the deduced carbon deposition mechanism, a novel two-step coating process via fluidized bed chemical vapor deposition was proposed to fabricate the SiO/1D-C/a-C composite. The grown thickened 1D-C and deposited a-C were heterogeneously coated on the SiO particle surface, forming an ingenious dual coating structure. The synthesized SiO/1D-C/a-C exhibited extremely significant enhanced cycling stability, which showed a reversible capacity of 1012 mAh g−1 (capacity retention of 88.3%) after 120 cycles. The thickened 1D-C is entangled with each other to form a three-dimensional conductive network, while the deposited a-C formed a shell-like coating to buffer the volume expansion during cycling. The unique carbon dual coating structure achieved synergistic strengthening, which guarantees the superior electrochemical performance of the SiO/1D-C/a-C.

New insights into carbon deposition mechanism of nickel/yttrium-stabilized zirconia cermet from methane by in situ investigation

In situ thermal expansion and in situ X-ray powder diffraction (XRD) are used to investigate the dynamic process of the interaction between Ni and methane at high temperature. Thermal expansion results show that it takes 68 min for the Ni and yttrium-stabilized zirconia (Ni-YSZ) cermet at 650 °C to collapse, while it takes only 14 min at 800 °C when exposed to methane. The crystal structure change of Ni, when it interacts with methane at high temperature, is directly observed by in situ XRD characterization. The results of scanning electron microscopy and transmission electron microscopy show a large number of carbon fibers on Ni particles in methane at 650 °C, whereas a considerable amount of encapsulating carbon and few carbon fibers are noted at 800 °C. On the basis of the experimental results, the deactivation mechanisms of Ni-YSZ cermet in methane at 650 °C and 800 °C are elucidated, respectively.

Phase transformation and carbon precipitation of coal fly ash magnetospheres during a CVD process for microwave adsorption

Abstract Coal fly ash is industrial waste produced by coal power plants, containing large amounts of magnetospheres (MSs), where Fe3O4 is abundant and magnetite is the major composition. However, the functional reuses of MSs are always restricted by the impurities of aluminosilicate and other forms of iron oxide including maghemite, magnesioferrite, as well as hematite. In this paper, a chemical vapor deposition (CVD) process was proposed to convert MSs into a magnetic core@ carbon shell structure, which would found its application in microwave adsorption field. The prepared samples were characterized by using SEM, HRTEM, EDX, BET, XRD, Raman spectroscopy and TGA analysis. Results showed that MSs burst during the CVD process and transformed into a loose agglomeration of submicron-sized particles with a typical core@ shell structure. Crucially, instead of single sphere of MSs, the ultrafine grains of ferrospinel within MSs served as the core. The mechanism of carbon deposition of graphite shells and phase transformation of ferrospinel cores during the CVD process were revealed. Magnetism and microwave adsorption properties were tested, and the structural dependence relationship between these properties and the core@ shell structure was clearly identified. For a sample time of 60 min, the minimum absorption peak was observed as −32.4 dB at 12 GHz, with the minimum frequency bandwidth below −10 dB at 4.3 GHz. We propose a low-cost, one-step preparation of cementite core@ graphite shell composites with high-performing microwave adsorption. Our method provides an alternative option for the practical reuse of the industrial waste.

Effect of Ni incorporation in cobalt oxide lattice on carbon formation during ethanol decomposition reaction

In this work, we report the results of catalytic decomposition of ethanol over a porous mixed metal oxide of Ni-Co (NiCoO2) and analyze the effect of Ni incorporation in Co3O4 lattice on the activity and hydrogen selectivity from ethanol decomposition reaction. In-situ FTIR analysis was conducted between 50 °C to 400 °C for both, Co3O4 and NiCoO2, catalysts to understand the reaction mechanism leading to gas phase product distribution. On cobalt surface the reaction pathway proceeds via the formation of surface ethoxy intermediate that subsequently transforms to aldehyde and acetate intermediates before decomposing to release CO2, H2 and CH4 gases at high temperature. A similar pathway is followed on NiCoO2 surface, leading to aldehyde intermediate, which is relatively unstable and decomposes to release CO along with CH4 and H2 at 400 °C. The NiCoO2 catalyst shows relatively low selectivity for CO2, possibly due to the unstable aldehyde species providing little acetate intermediate that decomposes to release gaseous CO2. The addition of Ni improves the activity for ethanol decomposition by achieving a complete ethanol conversion at 350 °C as compared to 420 °C for cobalt alone. The crystallinity, morphology and particle analysis of the spent catalyst after the reaction was studied using XRD, SEM, and TEM respectively. The XRD shows a phase change of porous NiCoO2 to NiCo alloy, whereas SEM indicates the presence of fibrous structure on the surface with 91.7% of carbon while keeping 1:1 ratio of Ni and Co after the reaction. The detailed analysis of carbon structure using HRTEM-STEM shows the simultaneous growth of carbon nanofibers (CNFs) and multiwalled carbon nanotubes (MWCNTs) that were favored by larger and smaller crystallites respectively. A detailed mechanism of carbon deposition is also proposed.

Gas Switching Reforming for syngas production with iron-based oxygen carrier-the performance under pressurized conditions

A four-stage Gas Switching Reforming for syngas production with integrated CO2 capture using an iron-based oxygen carrier was investigated in this study. The oxygen carrier was first reduced using dry methane, where high methane conversion rate was achieved producing CO2 and steam. Following the reduction stage is a transition to syngas production in an intermediate stage that begins with partial oxidation of methane while methane cracking dominates the rest of the stage. This results in substantial carbon deposition that gasifies in a subsequent reforming stage by cofeeding steam and methane, contributing to more syngas yield. Some of the deposited carbon that could not gasify during the reforming stage slip to the oxidation stage and get combusted by oxygen in the air feed to release CO2, thereby reducing the CO2 capture efficiency of the process. It is in this oxidation stage that heat is being generated for the whole cycle given the high exothermicity nature of this reaction. Methane conversion was found to drop substantially in the reforming stage as the pressure increases driven by the negative effect of pressure on both carbon gasification by steam and on the steam methane reforming. The intermediate stage (after reduction) was found less sensitive to the pressure in terms of methane conversion, but the mechanism of carbon deposition tends to change from methane cracking in the POX stage to Boudouard reaction in the reforming stage. However, methane cracking shows a tendency to reduce substantially at higher pressures. This is could be a promising result indicating that high-pressure operation would remove the need for the reforming stage with steam as no carbon would have been deposited in the POX stage.

Insight into carbon formation from acetic acid decomposition over Pd(100) via density functional theory calculations

The mechanism of carbon deposition in acetic acid/palladium system is of great research significance in the catalytic field. In order to illustrate the plausible carbon formation routes, a systematic survey on the stepwise decomposition from adsorbed acetic acid to atomic carbon on Pd(100) was conducted via density functional theory calculations. A complex reaction network including OH bond scission reaction and various CH and CC bond scission reactions was built and the relevant structural and energetic properties were calculated. The results show that OH bond breaking is very possible for CH3COOH, that CC bond breaking is always more favorable than CH bond breaking for CHxCOO (x = 1–3), and the dehydrogenation of CHx (x = 1–3) is more likely to proceed than most of other reactions. The most possible pathway for the formation of carbon monomer was proposed based on the analysis of the reaction network and it features the decarbonation of CH3COO to CH3 as the rate-limiting step.

In situ Imaging and Spectroscopy of the Carbon Deposition Mechanism on Ni/CeO2 Solid Oxide Fuel Cell Anode Catalyst

<i>In situ</i> Imaging and Spectroscopy of the Carbon Deposition Mechanism on Ni/CeO₂ Solid Oxide Fuel Cell Anode Catalyst

The Research on the Formation Mechanism of Carbon Deposition Based on Remanufacture Cleaning

In the process of remanufacturing cleaning, the carbon deposition was difficult to remove, this paper wants to provide the basic theory about the carbon removal. In this paper, the chemical composition and the microstructure of carbon deposition were analyzed, so as to get the microscopic structure of carbon deposition. Analyzing the formation reaction, adhesion process and the contribution of different electrostatic forces to the formation of carbon deposition. At last, explaining the reasons of the formation of carbon deposition.

Investigation of carbon deposition induced by pyrolytic decomposition of ethylene

We employed Raman spectroscopy to characterize the formed carbon deposits under different conditions inside a built-in U-shape quartz tube for unravelling mechanism of carbon deposition on stainless steel.