scholarly journals Carbon Formation at High Temperatures (550–1400 °C): Kinetics, Alternative Mechanisms and Growth Modes

Catalysts ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 465 ◽  
Author(s):  
Luís Sousa Lobo ◽  
Sónia A. C. Carabineiro
Keyword(s):  
Catalyst Surface ◽  
Bulk Diffusion ◽  
Carbon Formation ◽  
Carbon Structure ◽  
Graphene Layers ◽  
Rate Determining Step ◽  
Growth Modes ◽  
Higher Temperature ◽  
Hybrid Carbon ◽  
Gas Pressures

This Note aims at clarifying the alternative mechanisms of carbon formation from gases at temperatures above 550 °C. Both the growth of carbon nanotubes (CNTs) by a hybrid route, and of graphene layers deposition by a pyrolytic route are analyzed: the transition had no influence in apparent kinetics, but the carbon structure was totally different. The transition temperature from hybrid to pyrolytic growth varies with the gas pressure: higher temperature transition was possible using lower active gas pressures. The rate-determining step concept is essential to understanding the behavior. In catalytic and hybrid carbon formation, the slower step controls and determines kinetics. In the pyrolytic region, the faster step dominates, and carbon bulk diffusion is blocked: layers of graphene cover the external catalyst surface. It is easier to optimize CNTs growth (rate, shape, properties) knowing the details of the alternative mechanisms operating.

scholarly journals Explaining Bamboo-Like Carbon Fiber Growth Mechanism: Catalyst Shape Adjustments above Tammann Temperature

2020 ◽  
Vol 6 (2) ◽  
pp. 18 ◽  
Author(s):  
Luís Sousa Lobo ◽  
Sónia A.C. Carabineiro
Keyword(s):  
Melting Point ◽  
Metal Nanoparticles ◽  
Bulk Diffusion ◽  
Pyrolytic Carbon ◽  
Metal Nanoparticle ◽  
Carbon Formation ◽  
Graphene Layers ◽  
Nanoparticle Shape ◽  
C Fibers ◽  
Nano Catalyst

The mechanism of bamboo-like growth behavior of carbon fibers is discussed. We propose that there is a requirement to have this type of growth: operation above the Tammann temperature of the catalyst (defined as half of the melting point). The metal nanoparticle shape can then change during reaction (sintering-like behavior) facilitating carbon nanotube (CNT) growth, adjusting geometry. Using metal nanoparticles with a diameter below 20 nm, some reduction of the melting point (mp) and Tammann temperature (TTa) is observed. Fick’s laws still apply at nano scale. In that range, distances are short and so bulk diffusion of carbon (C) atoms through metal nanoparticles is quick. Growth occurs under catalytic and hybrid carbon formation routes. Better knowledge of the mechanism is an important basis to optimize growth rates and the shape of bamboo-like C fibers. Bamboo-like growth, occurring under pyrolytic carbon formation, is excluded: the nano-catalyst surface in contact with the gas gets quickly “poisoned”, covered by graphene layers. The bamboo-like growth of boron nitride (BN) nanotubes is also briefly discussed.

scholarly journals Mechanism of Catalytic CNTs Growth in 400–650 °C Range: Explaining Volcano Shape Arrhenius Plot and Catalytic Synergism Using both Pt (or Pd) and Ni, Co or Fe

2019 ◽  
Vol 5 (3) ◽  
pp. 42 ◽  
Author(s):  
Luis Sousa Lobo
Keyword(s):  
Surface Reaction ◽  
Arrhenius Plot ◽  
Bulk Diffusion ◽  
Initial Surface ◽  
Carbon Formation ◽  
Reaction Orders ◽  
Higher Temperature ◽  
Carbon Nanotube Growth ◽  
Nanotube Growth ◽  
Surface Decomposition

The Arrhenius plot of catalytic carbon formation from olefins on Ni, Co, and Fe has a volcano shape in the range 400–550 °C with reaction orders 0 (at lower T: Below ~500 °C) and one (at higher T: Above ~500 °C) at each side of the maximum rate. The reaction follows a catalytic route with surface decomposition of the gas (olefin) on the catalyst nanoparticle, followed by the bulk diffusion of carbon atoms and carbon nanotube growth on the opposite side. At the higher temperature region (500–550 °C), the initial surface reaction step controls the rate and the reaction order is one, both in olefins and hydrogen (H). This confirms that H is essential for the surface reaction to occur. This is very valuable information to get faster CNT growth rate at relatively low temperatures. The apparent activation energy observed must correspond with the surface reaction Ea corrected for the temperature dependence of the two molecules involved (olefin and H). Adding a noble metal (Pt, Pd) to the carbon formation catalyst is frequently found to increase the reaction rate further. This effect has been described as an H spillover since 1964. However, there is evidence that the bulk diffusion of H atoms prevails and does not “spillover” the surface diffusion. Diffusion of H atoms through the solids involved is easy, and the H atoms remain single (“independent”) until emerging on a surface.

Kinetics of carbon monoxide methanation on a Ni/SiO2 catalyst

1990 ◽  
Vol 55 (7) ◽  
pp. 1678-1685
Author(s):  
Vladimír Stuchlý ◽  
Karel Klusáček
Keyword(s):  
Carbon Monoxide ◽  
Reaction Rate ◽  
Catalyst Activity ◽  
Adsorbed Hydrogen ◽  
Reaction Products ◽  
Co Methanation ◽  
Molar Ratios ◽  
Rate Determining Step ◽  
Higher Temperature ◽  
Kinetics Of

Kinetics of CO methanation on a commercial Ni/SiO2 catalyst was evaluated at atmospheric pressure, between 528 and 550 K and for hydrogen to carbon monoxide molar ratios ranging from 3 : 1 to 200 : 1. The effect of reaction products on the reaction rate was also examined. Below 550 K, only methane was selectively formed. Above this temperature, the formation of carbon dioxide was also observed. The experimental data could be described by two modified Langmuir-Hinshelwood kinetic models, based on hydrogenation of surface CO by molecularly or by dissociatively adsorbed hydrogen in the rate-determining step. Water reversibly lowered catalyst activity and its effect was more pronounced at higher temperature.

Carbon formation and gasification on metals. Bulk diffusion mechanism: A reassessment

2011 ◽  
Vol 178 (1) ◽  
pp. 110-116 ◽  
Author(s):  
L.S. Lobo ◽  
J.L. Figueiredo ◽  
C.A. Bernardo
Keyword(s):  
Diffusion Mechanism ◽  
Bulk Diffusion ◽  
Carbon Formation

ChemInform Abstract: Carbon Formation and Gasification on Metals. Bulk Diffusion Mechanism: A Reassessment

ChemInform ◽  
2012 ◽  
Vol 43 (12) ◽  
pp. no-no
Author(s):  
L. S. Lobo ◽  
J. L. Figueiredo ◽  
C. A. Bernardo
Keyword(s):  
Diffusion Mechanism ◽  
Bulk Diffusion ◽  
Carbon Formation

Enlargement of Crystal Grain Size and Significant Inclusion of Hexagonal Diamond Phase in Ultra Pure Microcrystalline Silicon

2000 ◽  
Vol 638 ◽  
Author(s):  
Toshihiro Kamei
Keyword(s):  
Grain Size ◽  
Growth Mode ◽  
Growth Surface ◽  
X Ray Diffraction ◽  
Microcrystalline Silicon ◽  
Preferential Growth ◽  
Temperature Growth ◽  
Hexagonal Diamond ◽  
Growth Modes ◽  
Higher Temperature

AbstractWe have shown significant effects of atmospheric impurities on crystal grain sizes of hydrogenated microcrystalline Si (μc-Si:H), the growth surface of which is totally covered by hydrogen. By reducing impurity concentrations, two growth modes emerge, bordered at 250°C. In particular, at higher temperature growth mode, strong (220) preferential growth takes place, resulting in increased grain size. Appearance of this growth mode seems to be related to surface monohydride. Sharp x-ray diffraction peak in lower angle than (111) is observed, which stems from (10-10) crystal plane of hexagonal diamond Si. Its formation is related to (220) oriented crystallites. Implication of these results is the possibility of Si polytype control.

Evidence that carbon formation from acetylene on nickel involves bulk diffusion

Carbon ◽  
1976 ◽  
Vol 14 (5) ◽  
pp. 287-288 ◽  
Author(s):  
Carlos Bernardo ◽  
Luis Sousa Lobo
Keyword(s):  
Bulk Diffusion ◽  
Carbon Formation

scholarly journals Experimental investigation and computer simulation of diffusion in Pt-coated bulk Ni

2010 ◽  
Vol 46 (2) ◽  
pp. 153-160 ◽  
Author(s):  
W. Gong ◽  
L. Zhang ◽  
M. Ode ◽  
H. Murakami ◽  
C. Zhou
Keyword(s):  
Thin Film ◽  
Experimental Investigation ◽  
Electron Probe ◽  
Bulk Diffusion ◽  
Concentration Profiles ◽  
Atomic Mobility ◽  
Measured Concentration ◽  
Calculated Concentration ◽  
Higher Temperature ◽  
Interdiffusion Coefficients

The concentration profiles of thin-film Pt/bulk Ni coatings annealed at 1150, 1250 and 1300?C for different time were measured by means of electron probe microanalysis. The corresponding interdiffusion coefficients were then determined using the thin-film solution. The calculated concentration profiles based on the presently obtained interdiffusion coefficients agree well with the experimental ones, but better at a higher temperature or a longer time. The comparison between the presently measured concentration profiles and the DICTRA simulated ones indicates that it is promising to apply the well-established atomic mobility databases due to bulk diffusion information in coating systems with some simple modifications for diffusivities.

scholarly journals HDO REACTION OF THF USING Pt/γ-AL2O3 CATALYST ENRICHED BY ALUMINA : EFFECT OF TEMPERATURE TO PRODUCT DISTRIBUTION, RATE OF REACTION AND DEACTIVATON OF CATALYST

2016 ◽  
Vol 10 (1) ◽  
pp. 94
Author(s):  
Yuniawan Hidayat ◽  
Idul Fitri Nurcahyo ◽  
Ana Sofiana ◽  
Arifin Dwi Saputro
Keyword(s):  
Catalyst Deactivation ◽  
Catalyst Surface ◽  
Product Distribution ◽  
Effect Of Temperature ◽  
Exponential Regression ◽  
Rate Of Reaction ◽  
Deactivation Of Catalyst ◽  
Higher Temperature ◽  
Deactivation Process ◽  
Al2o3 Catalyst

<p>Effect of temperature variation to product distribution, rate and deactivation of catalyst of tetrahydrifuran hydrodeoxygenation have been conducted using Pt/gAl<sub>2</sub>O<sub>3</sub> with aluminum enrichmen. Reaction was conducted by flow system. Product of reaction were analyzed as propane (C<sub>3</sub>) and buthene derivate (C<sub>4</sub>). At 350ºC, reaction product and rate constant were optimum. At higher temperature, product distribution was shift from C<sub>4</sub> to C<sub>3</sub>. Lowering pore size catalyst, surface area and acidity of catalyst were responsible to catalyst deactivation. Deactivation process was follow exponential regression.</p>

scholarly journals Evolution in size and structural order for incipient soot formed at flame temperatures greater than 2100 K

2021 ◽  
Author(s):  
Joaquin Camacho ◽  
Shruthi Dasappa
Keyword(s):  
Equivalence Ratio ◽  
Carbon Materials ◽  
High Surface ◽  
Flame Temperature ◽  
High Surface Area ◽  
Bimodal Distribution ◽  
Carbon Structure ◽  
Combustion Chemistry ◽  
Polycyclic Aromatic ◽  
Higher Temperature

Soot formed in flames hotter than conventional combustion applications is expected to undergo unique formation processes and develop a carbon structure distinct from typical soot. A complementary experimental and modeling approach is reported here to assess flame temperature and equivalence ratio effects for soot formed in the higher-temperature regime (1950 K &lt; Tf &lt; 2250 K). Computations using three separate combustion chemistry models show that predictions of polycyclic aromatic hydrocarbon (PAH) concentration profiles for the higher-temperature flames are more sensitive to the choice in mechanism rather than specific flame conditions. As for material properties, the Raman signatures transition from a typical soot spectrum to features observed in disordered sp2 carbon materials. The defect distance extracted from the Raman bands nearly doubles from values typically reported for soot as the flame temperature exceeds 2200 K. Higher concentrations of gas-phase precursors may facilitate development of an ordered carbon structure as indicated by the relatively high defect distance observed for the highest equivalence ratio series. Particle size distributions measured by mobility sizing show size and yield of soot decreases with increasing flame temperature and the bimodal distribution falls within the ultra-fine range for all flame conditions. This is especially promising if the significant transformation in carbon structure inferred from the evolution in Raman spectra enables development of functional high-surface area sp2 carbon materials. Namely, the current observations indicate that the flame-formed carbon structure evolves towards high-defect sp2 carbon with size and carbon structure that can be tuned to some extent.

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