Kinetic and Thermodynamic Considerations in Thermocatalytic CO2 Hydrogenation

Abstract

^aStoichiometric amounts of H₂ and H₂O were omitted. How can the reaction network connectivity between CO₂, CO, and hydrocarbon/oxygenate products be determined to assess pathway-specific rates? How can thermodynamic contributions to observed trends in rates and selectivity be identified and decoupled from kinetic contributions? How can relationships between and constraints imposed by thermodynamic driving forces be illustrated in energy diagrams? Figure 1. (a) Hypothetical reaction networks where A converts to B, C, D, and E following two cases: (i) C is a secondary product with no product inhibition present, and (ii) all species are primary products with E inhibiting the formation rates of B and D. Here, C_E denotes the concentration of species E. (b, c) Simulated first rank delplots and (d, e) yields as a function of contact time for the two cases. Inset in (c) shows the selectivity to product C over the 0–3% conversion region of the first rank plot for case (ii). (f) Simulated TOFs as a function of contact time for case (i). Values of rate and equilibrium constants were chosen for illustrative purposes and do not impact the general trends shown. Arbitrary units are abbreviated as a.u. Simulation details are provided in section S1 (Supporting Information (SI)). Figure 2. Simulated (a) overall reversibility and (b) carbon selectivity as a function of contact time during CO₂ hydrogenation to methanol, CO, and ethanol on a hypothetical catalyst kinetically selective toward methanol (30 bar; 503 K; 1 a.u. total inlet flow rate; H₂:CO₂ = 3). (c) Calculated equilibrium carbon selectivity (30 bar; H₂:CO₂ = 3) as a function of temperature, where the methanol selectivity is too low to be observed in the figure. The result at 503 K is highlighted and expectedly consistent with selectivity from (b) in the limit of infinite contact time. Simulation details are provided in section S3 (SI). Figure 3. (a) Free energy diagrams at the standard state (C₀ = 1 a.u.) and under reaction conditions (initial C_A,0 = 10 a.u.; X_A = 0.23) for single-path reaction sequence A ⇄ B ⇄ C ⇄ D. (b) Free energy diagrams for CO₂ hydrogenation at 523 K referenced to CO₂ + 3H₂ at the standard state (P₀ = 1 bar), hypothetical reaction condition 1 (P_tot = 10 bar; inlet CO₂:H₂ = 1:3; X_CO2 = 0.03; S_CH3OH = 0.5), and hypothetical reaction condition 2 (P_tot = 10 bar; inlet CO₂:H₂ = 1:3; X_CO2 = 0.03; S_CH3OH = 0.05) calculated based on values from ref (10). The values of the initial concentration of reactants and conversion in both examples were chosen to outline an illustrative example (details in section S4, SI). Figure 4. (a) Reaction condition free energy diagram with increasing extent of reaction, (b) z_i as a function of conversion of A, and (c) X_ZC,i as a function of conversion of A for single-path reaction sequence A ⇄ B ⇄ C ⇄ D. Simulation details are provided in section S4 (SI). combining delplot analysis, Wojciechowski’s criterion, product cofeed experiments, reversibility considerations, and isotopic tracing to elucidate the reaction network of CO₂ hydrogenation such that observed rates can be attributed to specific reaction pathways, deconvoluting thermodynamic and kinetic driving forces for each reaction pathway through reversibility formalisms with consistent length-scales (differential vs integral models) such that forward rates and selectivity can be related to catalyst properties, and acknowledging the dependence of kinetic and thermodynamic driving forces on the extent of reaction, which can be illustrated through reaction condition energy diagrams as opposed to standard-state energy diagrams. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.4c05805. Simulation details and MATLAB code for delplot analysis and Wojciechowski’s criterion, for evaluating equilibrium selectivity, and for standard and reaction condition energy diagrams, as well as simulated delplots for nonfirst order reactions (PDF) Kinetic and Thermodynamic Considerations in Thermocatalytic CO₂ Hydrogenation 0 views 0 shares 0 downloads Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html. T.C.L. and E.E.B.R. contributed equally to this work. The authors acknowledge financial support from the U.S. Department of Energy, Office of Basic Energy Science, Catalysis Science Program under Award DE-SC0025321 and from the University of Minnesota, College of Science and Engineering. This material is also based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant 2237827. This article references 19 other publications. This article has not yet been cited by other publications.