Density functional theory studies of biomass conversion on metal surfaces: from small to large molecules
- Autores: Qiang Li
- Directores de la Tesis: Nuria Lopez Alonso (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Javier Pérez Ramírez (presid.), Hai-Jun Jiao (secret.), Jordi Ribas-Ariño (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Química por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: hdl.handle.net TDX
- Resumen
In this thesis, the purposes are mainly consisted of two parts: energy and material substitutions. The author focused on the conversion of a series of biomass derived molecules: C1, C2 and C3 alcohols (methanol, ethanol, ethylene glycol, and glycerol); Cellulose related products (levulinic acid (LA), γ-valerolatone (GVL), glucose, and mannose); and represented lignin dimer. All calculations were performed by using The Vienna Ab initio Simulation Package (VASP, Version 5.3.3, 5.3.5) 1 Hydrogen production from small alcohols 1.1 Objectives: 1.1.1 Create a rich database based on the complete decomposition networks.
1.1.2 Investigate the adsorption properties of the intermediates in the complex reaction network 1.1.3 Define the reaction profile via transition state theory.
1.1.4 Study the linear scaling method such as Brøsted-Evans-Polanyi relationship (BEP) and transition state scaling (TSS).
1.1.5 Combine the database with the linear scaling method and ab initio microkinetic study to expand the DFT calculations into a larger scale, both time and molecule size.
1.1.6 Interpret some experiment observations both in surface studies and reforming techniques.
1.1.7 Above all, deepen our understanding on surface reactions and promotes the catalysts’ design process.
1.2 Calculations: 1.2.1 Gas phase of the related close-shell molecules.
1.2.2 Complete C1 and C2 alcohols decomposition intermediates’ optimization on the Cu(111) and Ru(0001) surfaces. Complete glycerol decomposition species optimization on Ru(0001) surface.
1.2.3 Complete transition states searching in the C1 and C2 alcohols decomposition network.
1.3 Conclusions 1.3.1 On Cu(111), H2 and CH2O are the products and the decomposition process goes as: CH3OH → CH3O + H → CH2O + 2H → CH2Ogas +2H Further decomposition are not favored because of the weak adsorption energy of formaldehyde. Similarly, methanol formation process (reverse reaction steps) would stop when formaldehyde forms because of its trends to desorb. Hence, high H coverage is likely to promote the formaldehyde hydrogenations to methanol. From CH2O to CH3OH, CH3O + H → CH3OH is the rate determining step.
1.3.2 On Ru(0001), CO and H2 are the finally products and the process goes as: CH3OH → CH3O + H → CH2O + 2H → CHO + 3H →CO + 4H. It is impossible to produce methanol as C–O bond breaks much easier from the intermediates than their hydrogenations.
1.3.3 Biased initial C–H and O–H competition in the first decomposition step between DFT calculations and experiment observations have been discussed by taking account of the lateral hydrogen bond inter-actions between two methanol molecules. Inter molecule hydrogen bond formation stabilizes both the initial and transition states for O–H bond breaking. In the meanwhile, it also increases the rigidity for the methyl group bending to interaction with the surface and leads to larger C–H bond breaking barriers.
1.3.4 Brøsted-Evans-Polanyi (BEP) and Transition state scaling (TSS) relationships have been extrapolated and analyzed. O–H, C–H, and C–O bond breakings exhibit the IS, FS, and FS-like relationships, and C–O has the worst scaling results due to the significant structure changes.
1.3.5 Complete decomposition network of ethanol, ethylene glycol, and glycerol have been calculated. 350 structures including C0 (7), C1 (18), C2 (55), and C3 (270) have been optimized on Ru(0001) and C0−2 (80) on Cu(111) and 250 transition states for C1 and C2 species on each metal have been found in this work.
1.3.6 An empirical principle description of the stability for O or OH terminated structures based on the metal oxophility was displayed.
1.3.7 BEP and TSS relationships have been extrapolated and the factors that affect the scaling behaviors have been analyzed in detail.
1.3.8 O–H has the best IS-like TSS results, and the rest bond breaking (C–H, C–C, C–OH and C–O) have FS-like types, the scaling behaviors are followed by the C–H, C–O, C–C, and C–OH.
1.3.9 Reaction network of glycerol decomposition (1698 reactions) have been mapped and the transition states’ energy were predicted via the scaling method (TSS) from the optimized intermediates’ energies.
1.3.10 ab initial microkinetics study of hydrogen production from ethanol, ethylene glycol and glycerol have been performed.
1.3.11 For ethanol and EG on Cu(111), direct decomposition is the best way to produce H2 , and CCO is the most abundant intermediate. For the other reforming techniques, surface oxygen species O and OH are the most abundant species.
1.3.12 For ethanol on Ru(0001), direct decomposition has the best hydrogen production behavior, CCH , CO, and CCO are the main surface species with percentage of 68%, 16%, and 7%. CO and H2 are the main products with approximate stoichiometric ratio of 2:3.
1.3.13 APR has lower H2 production rate than DD, but much higher than other methods, the most rich species in sequence are CO, CxHy and CCO .
1.3.14 For ethanol ATR and SR, surface O and CxHy are the main species respectively.
1.3.15 For EG Ru(0001), the H2 production rates in sequence are SR, DD, APR and ATR. In DD and APR, the surface is almost covered by CO. In ATR, surface is covered mostly by O. In SR, CxHy, CO, and CCO are the most abundant surface species.
1.3.16 Both EG and Glycerol have the same hydrogen production sequence of SR, DD, APR and ATR. In steam reforming, the activity of EG is 3.5 order of magnitude smaller than that of glycerol. And the selectivity towards H2 for EG and glycerol steam reforming on Ru(0001) surface are 84% and 72%, respectively.
2 LA conversion to GVL on Ru(0001) and HHDMA decorated Ru(0001) surfaces.
2.1 Objective: 2.1.1 Study the mechanism of LA conversion to GVL on Ru(0001) surface 2.1.2 Elucidate the reason why HHDMA decorated Ru catalysts have higher activity and stability than the commercialized Ru catalyst.
2.2 Calculations: 2.2.1 Close shell molecules’ optimization in gas phase: reactants, intermediates and products 2.2.2 Solvation energy calculations of reactants and products.
2.2.3 Optimization of related species in the proposed reaction network on the Ru(0001) surface 2.2.4 Identification of the transition states related with species above 2.2.5 HHDMA-Ru(0001) structure optimization 2.2.6 Acid molecules adsorption on HHDMA-Ru(0001) 2.2.7 Oxygen adsorption on HHDMA-Ru(0001) 2.3 Conclusion 2.3.1 The mechanisms starting from LA and its levulate form have been calculated in detail, our results show that when conversion starts from the surface LA, it is more kinetic preferable than that from levulate form.
2.3.2 By using a series of probe acid molecules, the estimated interfacial acidity (pH) is around 1.0, and this is strong enough to protonate the levulate into LA and change the reaction path, thus making the HHDMA-Ru(0001) better than pure Ru(0001) for LA conversion.
2.3.3 The anti-oxidation ability of the interfacial has been studied by adsorption 6 and 8 oxygen atoms on HHDMA-Ru(0001) and pure Ru(0001) surface. Our results imply that oxygen adsorption on HHDMA decorated Ru(0001) surface is less thermodynamic preferable than on pure Ru(0001).
2.3.4 Interfacial acidity and the coverage effect from the HHDMA ligands also inhibits the oxygen adsorption and insertion into the Ru bulk, which results into the higher stability towards anti oxidation.
3 Glucose and mannose hydrogenation to sugar alcohols on Ru(0001) surface 3.1 Objectives 3.1.1 Study the mechanisms of glucose and mannose hydrogenation to their sugar alcohols 3.1.2 Interpret the experiment results that mannose has the faster hydrogenation rate than glucose.
3.2 Calculations 3.2.1 Close shell molecules’ optimization in gas phase: reactants, intermediates and products.
3.2.2 Solvation energy calculations of reactants and products.
3.2.3 Linear and ring sugar molecules, intermediates and products geometry optimization on Ru(0001) surface.
3.2.4 Search transition states in the related reaction network.
3.3 Conclusions 3.3.1 Glucose and mannose hydrogenation to their alcohols over Ru(0001) surface have been calculated.
3.3.2 Reactions from linear isomers of glucose and mannose have been studied and the differences in the reaction coordinate are small. Given the low concentration of them less than 1% in the liquid phase, the different reaction rates cannot be explained from the linear structures.
3.3.3 Cyclic isomers of glucose and mannose adsorption have been calculated and exist obvious adsorption sequence of α-mammnose > β-mammnose > α-glucose > β-glucose.
3.3.4 Water associates in the ring opening steps and β-glucose has lower ring opening barrier than α-mannose.
3.3.5 α-glucose and β-mannose have much higher reaction barriers than their isomers.
3.3.6 The rate determining step is hydrogenation reaction (C+H) from β-glucose and is the ring opening from α-mannose.
3.3.7 Different solution concentrations, adsorption energies and different rate determining steps control the overall hydrogenations rates.
3.3.8 Surface kinetic analysis based on Langmuir-Hinshelwood mechanism agrees well the experiment studies. Surface kinetic analysis based on Langmuir-Hinshelwood mechanism have been employed and the outcome qualitatively agrees with experiment results well.
4 Lignin decomposition on pure and Ru doped Ni(111) surfaces 4.1 Objective 4.1.1 Study the mechanism of lignin de-polymerization mechanisms by using more complex dimer models than previous reported.
4.1.2 Investigate the stereocenter’s role in lignin conversions from different isomers 4.1.3 Ru doping effects in promoting the catalysts behaviors.
4.2 Calculations 4.2.1 Close shell molecules’ optimization in gas phase: reactants, intermediates and products.
4.2.2 Solvation energy of reactants and products 4.2.3 Intermediate geometries related in this investigation, both on Ni(111) and Ru doped Ni(111) 4.3 Conclusions 4.3.1 More complex coniferyl alcohol dimers models containing the sterocenters at α and β carbon positions have been built.
4.3.2 Adsorption of these models have been calculated and the stability were compared.
4.3.3 Complete decomposition and hydrogenation of lignin conversion have been calculated. The α-C--OH or α-C--H bond breaking firstly, followed by the β-O4 bond breaking and then the hydrogenation steps.
4.3.4 Sterocenter effects on reaction kinetics have been discussed, changing the OH and H positions in the α-C would promote the reaction by lower reaction barriers.
4.3.5 Ru doping effects in promoting the Ni catalysts have been investigated. In the doping model, one surface Ni atom was replaced bu Ru atom and the stability of this model has been verified by the segregation and near surface energies, and island formation energy.
4.3.6 Ru doping effects in promoting the Ni catalysts have been investigated. On one side, Ru facilitates the β-O$_4$ bond breaking via lowering the barrier by 0.20 eV. On the other side, as Ru has stronger binding ability towards the species than Ni, doping too much Ru would deteriorate the catalyst behavior by the poisoning effects. Thus, will increasing the doping amount of Ru into the Ni catalyst, experimentalist observed a volcano curve.
Above all, all these three objectives are tightly related with available experiment observations. By performing DFT calculations, the main goals are to explain and interpret the phenomenon that cannot be easily understood by the limited experiment techniques and formalize the reaction mechanisms. The author believes that those calculations would expand our view sights in heterogeneous catalyzed biomass conversions and provide rich atomic information (adsorption and reaction mechanisms) and guidelines for the catalysts design.
