Abstract
A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface.
Highlights
Low-temperature Fischer-Tropsch Synthesis (FTS) is a non-reversible, highly exothermic, and complex reaction (Equation (1)), which has been industrially applied for many decades for using syngas (CO and H2 ), which is mainly derived from natural gas, biogas, and coal, towards the formation of chemicals and fuels [1,2,3].n (CO + 2H2 ) → (−CH2 −)n + n H2 O (1)Great efforts have been made toward understanding the FTS mechanism and kinetically relevant elementary reaction steps in order to design new and improved catalytic materials via different preparation methods with optimum metal loading, particle size, and chemical and structural promoters’composition
Prior to any catalytic and transient kinetic measurements, the catalyst was in situ reduced in pure H2 (1 bar, 50 NmL min−1 ) at 425 ◦ C for 10 h, which was followed by an He-purge and cooling to 230 ◦ C for FTS
The present study aimed at providing deeper fundamental understanding on how the CO partial pressure affects important kinetic parameters of the methanation reaction path at 230 ◦ C and 1.2 bar total pressure over a commercially relevant 20 wt% Co-0.05 wt% Pt/γ-Al2 O3 catalyst
Summary
Low-temperature Fischer-Tropsch Synthesis (FTS) is a non-reversible, highly exothermic, and complex reaction (Equation (1)), which has been industrially applied for many decades for using syngas (CO and H2 ), which is mainly derived from natural gas, biogas, and coal, towards the formation of chemicals and fuels [1,2,3].n (CO + 2H2 ) → (−CH2 −)n + n H2 O (1)Great efforts have been made toward understanding the FTS mechanism and kinetically relevant elementary reaction steps in order to design new and improved catalytic materials via different preparation methods with optimum metal loading, particle size, and chemical and structural promoters’composition. Steady State Isotopic Transient Kinetic Analysis (SSITKA) has been recognized as one of the most advanced techniques providing important kinetic information for a heterogeneous catalytic reaction under working reaction conditions, such as the concentration (mol g−1 ), surface coverage (θ), mean residence time (τ, s), and intrinsic site activity (k) of truly active reaction intermediates [14,24,25,26,27,28,29,30]. The intrinsic turnover frequency (TOFITK , s−1 ) based on the concentration of active reaction intermediates measured by SSITKA (not on the total exposed metal surface sites measured by selective chemisorption) leads to the given reaction product and can be estimated [11,13,14,15,16,18,19]
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