Search

To obtain a clearer understanding of the catalytic bio-oil reforming mechanism, we performed density functional theory calculations on the decomposition of formic acid over a stepped Ni surface. Formic acid was selected as a model bio-oil compound. Fourteen elementary reactions were considered. All the zero-point-energy corrected activation energies and reaction energies were obtained. The kinetic parameters of the reaction rate constants and thermodynamic equilibrium constants in the temperature range of 300–1000 K were revealed. COOH formation and its subsequent dissociation are energetically easier than that of HCOO. When combined with kinetic modeling, the most preferable pathway for formic acid dissociation on the surface is HCOOH → COOH → CO. The rate-determining step is COOH dehydroxylation to CO, associating with an activation energy of 0.51 eV and a rate constant of 5.16 × 10 9 s −1 at 773 K. In addition, HCOO will be slowly accumulated on the surface as the temperature rises. [ABSTRACT FROM AUTHOR]

To clarify the understanding of the mechanism of bio-oil catalytic steam reforming, we selected acetic acid as a typical bio-oil model compound to study its detailed behavior in decomposition over an active stepped Ni surface by density functional theory calculations. The adsorption geometries and energies of various intermediates were reported. Linear correlations between the adsorption energy and the number of hydrogen atoms removed for CH x COOH, CH x COO, and CH x species ( x = 1–3) were found, with increments of −1.56, −0.81, and −1.80 eV, respectively. Thirty-seven possible elementary reactions of acetic acid decomposition were proposed, and their activation energies, reaction energies, rate constants, and equilibrium constants were calculated. Acetic acid dissociation likely started via α-carbon dehydrogenation, OH dehydrogenation, and dehydroxylation. Combined with microkinetic modeling, the most preferable decomposition pathway was suggested as CH 3 COOH → CH 3 CO → CO + CH 3 . The rate-determining step was CH 3 COOH dehydroxylation to CH 3 CO with an activation energy of 0.68 eV and a rate constant of 3.82 × 10 8 s −1 . The formation of CH 3 COO was dominant at high temperatures, whereas its decomposition occurred with difficulty. [ABSTRACT FROM AUTHOR]

Solvent effects on the decomposition of formic acid over a Co(111) surface were studied via density functional theory calculations combined with a continuum implicit solvation model. The solvents used here were water, methanol, and acetone. The adsorption energies of key intermediates, the activation barriers and the rate and equilibrium constants of various elementary reactions in vacuum and in the solvents were obtained. Solvent presences decrease the adsorption energies of species. Formic acid decomposition on the surface goes through HCOO rather than COOH both in vacuum and in the solvents. The most favorable decomposition pathways in vacuum and in acetone are HCOOH → HCOO → HCO → CO. The corresponding rate determining steps are HCOO deoxidation to HCO with activation barriers of 0.78 and 0.76 eV, respectively. In the presences of water and methanol, the preferred pathways are shifted to HCOOH → HCOO → HCOO- m → CO 2 below 750–800 K. Above those temperatures, the path of HCOOH → HCOO → HCO → CO becomes dominant again. [ABSTRACT FROM AUTHOR]

To elucidate bio-oil catalytic reforming mechanisms, acetic acid has been selected as a model compound to carry out density functional theory calculations on its decomposition pathways over an active Co stepped surface. The adsorption energies and stable geometries of the reactant and important reaction intermediates have been obtained. Preferential adsorption on a step as opposed to a flat surface has been discerned. The activation barriers and reaction enthalpies for various elementary reactions involved in acetic acid decomposition have also been obtained. Four likely parallel decomposition pathways with activation barriers of less than 0.9 eV have been identified. The most kinetically favorable decomposition pathway is CH 3 COOH → CH 3 CO → CH 2 CO → CH 2 → CH. The rate-determining step is CH 3 CO dehydrogenation, with a reaction barrier of 0.52 eV. In addition, the formation and primary decomposition pathways of the critical intermediate acetone have also been investigated. [ABSTRACT FROM AUTHOR]