Microkinetic model validation for Fischer-Tropsch synthesis at methanation conditions based on steady state isotopic transient kinetic analysis

The Effect of H2 Pressure on the Carbon Path of Methanation Reaction on Co/γ-Al2O3: Transient Isotopic and Operando Methodology Studies

“…Chain-growth steps related to C 2+ -hydrocarbon formation were reported to be much faster than CO dissociation and −CH x monomer formation steps . This is consistent (a) to the observed higher activation energy for CH 4 formation compared with that for higher hydrocarbons, as illustrated in the present work (see Figure S3, section ) and reported also in earlier studies, − and (b) to the more negative reaction orders for CH 4 formation with respect to the CO pressure than those for the CO conversion …”

“…Thus, there will be a small effect on this important θ CO parameter established on the active cobalt surface sites under FTS reaction conditions at 1 atm or higher pressure. Chain-growth steps related to C 2+ -hydrocarbon formation were reported to be much faster than CO dissociation and −CH x monomer formation steps . This is consistent (a) to the observed higher activation energy for CH 4 formation compared with that for higher hydrocarbons, as illustrated in the present work (see Figure S3, section ) and reported also in earlier studies, − and (b) to the more negative reaction orders for CH 4 formation with respect to the CO pressure than those for the CO conversion It was reported that, using the Langmuir CO adsorption/desorption equilibrium relationship, the fraction of free Co sites (θ v ) is strongly related to the CO pressure, and a plot of chain-growth probability versus θ v K CO (K CO (bar –1 ) is the equilibrium constant) provides a linear relationship.…”

“…The dynamic behavior of the rate of exchange of the active −CH x species during the 12 CO/H 2 → 13 CO/H 2 SSITKA switch as a function of H 2 pressure (Figure A) unambiguously provided more information related to the kinetics of −CH x formation (from CO-s) and sequential hydrogenation to CH 4 , as opposed to the typical dimensionless concentration ( Z ) transient response curve of 13 CH 4 (Figure B), which is commonly reported in the literature of SSITKA experimentation with little discussion about its significance, except when used in microkinetic modeling. , The Z ( 13 CH 4 ) response curve (Figure ) is usually used to estimate the amount of N CH x (Table ). It is very instructive to note also that integration of the R CH x exch rate profile with time (Figure A) must provide the amount of −CH x leading to CH 4 and which was formed under 12 CO/H 2 before the SSITKA switch.…”

“…These values suggest that, within the carbide mechanism, C–C bond forming reactions in chain-growth and termination steps are faster than those of CH 4 formation, in agreement with theoretical studies reported by van Santen et al The authors in their perspective paper reported that the overall activation energy to form CH 4 from CH-s is at least 100 kJ mol –1 and can increase to 140 kJ mol –1 , which is substantially higher than that of the lower activation energy (70 kJ mol –1 ) C–C bond forming reactions on Co surface terraces. A recent microkinetic model for FTS, mainly under methanation reaction conditions ( T = 210–230 °C, H 2 /CO = 3.3–15, P T = 1.85 bar, P CO = 37–167 mbar; S CH4 > 70%), and based on SSITKA experimental results, reported activation energies of 88 and 95 kJ mol –1 for chain-growth and alkanes formation, respectively . Furthermore, other microkinetic modeling studies of FTS on Co/Al 2 O 3 at high pressures (15–25 bar), T = 220–250 °C and for H 2 /CO = 1–2, reported activation energies for cobalt–alkyl hydrogenation in the range of 80 kJ mol –1 . , …”

Study on the tolerance of low-temperature CO methanation with single pulse experiments

“…Steady-State Isotopic Transient Kinetics Analysis (SSITKA) combines steady-state and transient techniques by step-or pulse-wise injecting an isotopically labeled species into the reactor and measuring the transient response via mass spectrometry (MS) [17,18]. Since only the isotopic composition of the gas phase and the adsorbed species changes [16], quasisteady-state reaction conditions are maintained.…”