Chemical kinetic considerations for postflame synthesis of carbon nanotubes in premixed flames using a support catalyst

Abstract

Multiwalled carbon nanotubes (MWCNTs) on a grid supported cobalt nanocatalyst were grown, by exposing it to combustion gases from ethylene/air rich premixed flames. Ten equivalence ratios (φ) were investigated, as follows: 1.37, 1.44, 1.47, 1.50, 1.55, 1.57, 1.62, 1.75, 1.82, and 1.91. MWCNT growth could be observed for the range of equivalence ratios between 1.45 and 1.75, with the best yield restricted to the range 1.5-1.6. A one-dimensional premixed flame code with a postflame heat loss model, including detailed chemistry, was used to estimate the gas phase chemical composition that favors MWCNT growth. The results of the calculations show that the mixture, including the water gas shift reaction, is not even in partial chemical equilibrium. Therefore, past discussions of compositional parameters that relate to optimum carbon nanotube (CNT) growth are revised to include chemical kinetic effects. Specifically, rapid departures of the water gas shift reaction from partial equilibrium and changes in mole fraction ratios of unburned C 2 hydrocarbons to hydrogen correlate well with experimentally observed CNT yields.

Simulation and modeling of carbon nanotube synthesis: current trends and investigations

“…Lumped cluster model -If CO reacts with metal clusters and removes atoms from them by forming MeCO, this has the effect of enhancing the evaporation rate and reducing SWCNT production [1] Iron catalyst chemistry in modeling a high-pressure carbon monoxide CNT reactor Gopinath and Gore (2007) [5] Gas-phase synthesis of SWCNTs on iron catalyst particles…”

“…Chemical kinetics considerations reported by Gopinath and Gore [5] for post-flame synthesis of CNTs in premixed flames using a support catalyst showed that the yield of multi-wall carbon nanotubes (MWCNTs) varied with the equivalence ratio of the fuel-air mixture. Chemical kinetics calculations also revealed that the gas phase mixture leading to CNT growth was not in partial equilibrium even for the water-gas shift reaction [87] .…”

Flame Synthesis of Carbon Nanotubes

“…Schematics of various flame synthesis processes are shown in figure 4. (Gopinath & Gore, 2007;Height et al, 2004;) (b) counter-flow diffusion flame (Li et al, 2007;Merchan-Merchan et al, 2003;)(c) co-flow diffusion flame Vander Wal, 2000;)(d) inverse diffusion flame (Lee et al, 2004;Unrau et al, 2007;Xu et al, 2006) In a flame, fuel (generally hydrocarbons such as methane (CH 4 ), ethylene (C 2 H 4 ), and acetylene (C 2 H 2 ) etc.) reacts with oxidizer (O 2 from air) to produce gaseous mixture that includes carbon dioxide (CO 2 ), water vapor (H 2 O), carbon monoxide (CO), hydrogen (H 2 ), saturated and unsaturated hydrocarbons(C 2 H 2 , C 2 H 4 , C 2 H 6 etc.)…”

“…The first evidence for filamentous carbon growth in flames was established using premixed flames (Singer & Grumer, 1959). In the last two decades premixed flat flames have been extensively studied for synthesis of carbon nanotubes (Diener et al, 2000;Gopinath & Gore, 2007;Grieco et al, 2000;Howard et al, 1992;Howard et al, 1991;. Formation of C60 and C70 fullerenes was first observed in premixed flames by Howard et al In their studies, sooty discharge from premixed laminar flames of benzene, oxygen and argon at low pressures (1.60 to 13.35 kPa) were analyzed using electron impact mass spectroscopy.…”

“…In a premixed flat flame the equivalence ratio is uniform. Therefore, premixed flames have been studied to understand the effect of equivalence ratio on nanotube growth (Gopinath & Gore, 2007;Height et al, 2004;Vander Wal, 2000;. Vander Waal et al characterized the equivalence ratio range that may be ideal for CNT growth for different fuels.…”