Electrochemical promotion of novel catalysts with alkaline conductors for hydrogen production from methanol
- Autores: Jesús González Cobos
- Directores de la Tesis: José Luis Valverde Palomino (dir. tes.), Antonio de Lucas Consuegra (dir. tes.)
- Lectura: En la Universidad de Castilla-La Mancha ( España ) en 2015
- Idioma: inglés
- Tribunal Calificador de la Tesis: G. E. Marnellos (presid.), Fernando Dorado Fernández (secret.), Atsushi Urakawa (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: RUIdeRA
- Resumen
Hydrogen is a very important feedstock in the chemical industry and a promising energy carrier with main application in internal combustion engines and fuel cell technology as an alternative to the massive consumption of fossil fuels. H2 presents a high gravimetric energy density and can be considered as a clean synthetic fuel depending on the sustainability of the energy and raw material employed for its production. Hydrogen is currently obtained mainly via methane steam reforming. However, the use of liquid hydrogen carriers such as methanol is acquiring increasing interest. H2 production from methanol is typically carried out through its decomposition (MD, CH3OH ¿ 2H2 + CO), steam reforming (SRM, CH3OH + H2O ¿ 3H2 + CO2) or partial oxidation (POM, CH3OH + 1/2O2 ¿ 2H2 + CO2), by using catalysts based on Cu or Group VIII metals (Pt, Pd, Ni). A high catalytic activity, a low CO selectivity and a good durability of the catalyst are the main targets. Other valuable byproducts such as formaldehyde or methyl formate can also be obtained from these processes. On the other hand, the low volumetric energy density of gaseous H2 makes the development of efficient hydrogen storage systems to be of paramount importance. The coupling of catalysis and electrochemistry has been widely investigated for the last years for H2 production through electrolysis processes. In this thesis, the electrochemistry has been used to activate and tune different catalysts for H2 production from methanol through the Electrochemical Promotion Of Catalysis (EPOC). This phenomenon is based on the modification of the chemisorption properties of a metal catalyst by the electrochemical migration of promoter ions from a solid electrolyte support (via application of an electric current or potential). Hence, while in classical chemical promotion a specific amount of a promoter is added during the preparation step of the catalyst, in the case of the electrochemical promotion, promoter ions are electrochemically pumped between the metal catalyst and the solid electrolyte in a controlled way during the reaction step. Then, the electrochemical promotion presents several additional advantages, such as the possibility of optimizing the promoter coverage on the catalyst surface at different reaction conditions and the in-situ enhancement of the catalytic activity and selectivity. In this sense, previous studies have reported the interest of the phenomenon of electrochemical promotion on the methanol decomposition and partial oxidation reactions over Pt and Ag catalysts, by using yttria-stabilized zirconia (YSZ) as the solid electrolyte, which is an O2- conductor. However, these works were only focused on formaldehyde production (not H2) and were carried out at relatively high reaction temperatures. With respect to the application of the EPOC phenomenon to the trapping and storage of surface compounds, previous works have demonstrated the interest of using cationic electrochemical catalysts for NOx storage-reduction process (NSR) and CO2 capture. On the other hand, although a large number of studies have demonstrated the electrochemical promotion mechanism and this has been applied in a wide variety of catalytic reactions of industrial and environmental interest, a further technological progress is necessary for its practical application. Nowadays, some of the main challenges of EPOC are the development of compact and efficient reactors, and more competitive catalysts with a higher dispersion (composed of metal nanoparticles) or based in non noble metals such as Ni or Cu. Hence, in view of the mentioned above, this doctoral thesis aims to the study of the phenomenon of electrochemical promotion of catalysis (EPOC) in H2 production processes from methanol by using alkaline conductors. Furthermore, the possibility of applying the EPOC in the field of H2 storage was also investigated. For these purposes, novel catalyst films were developed by means of different preparation techniques in collaboration with other research groups specialized in the deposition of thin metallic films. Firstly, the effect of electrochemical promotion was studied in the H2 production via steam reforming and partial oxidation of methanol, being formaldehyde simultaneously obtained in the latter case. Two kinds of Pt catalyst films were deposited on K-ßAl2O3 (K+-conductor material) by two different techniques: impregnation and cathodic arc deposition (CAD). Both electrochemical catalysts were compared under electrochemical promotion conditions for the methanol partial oxidation reaction. It was found that in alkali-based solid electrolyte cells, an excess of electrocatalytic activity of the catalyst-working electrode (the case of the impregnated Pt film) can be detrimental for the electropromotional effect due to the excessive formation of promoter-derived surface compounds, which may block catalytic active sites. On the other hand, the catalytic activity and selectivity of the thin Pt film of low metal loading prepared by cathodic arc deposition were strongly enhanced by EPOC, i.e., by the electrochemical transfer of K+ ions to the catalyst under negative potentials. In this way, the Pt catalyst film prepared by this technique allowed to produce both H2 and H2CO at mild reaction conditions in a single reaction step. This electrochemical catalyst presented much less catalytic activity in the steam reforming of methanol, although it was also electrochemically promoted under these reaction conditions. The K+ promotional effect was attributed to the decrease in the catalyst work function and, hence, the strengthening of the Pt chemical bond with the electron acceptor (O2 or H2O) against that with the electron donor (CH3OH). Furthermore, the Pt catalyst film showed to be stable for long working times and the EPOC effect was found to be fully reversible in all the experiments, given the good reproducibility observed in the catalytic activity under every positive polarization (unpromoted or reference state). On the basis of the obtained results, the catalyst films used in the following studies were prepared by cathodic arc deposition or by other kind of physical vapor deposition (PVD) technique, such as sputtering or oblique angle deposition (OAD). Then, in other study, the phenomenon of electrochemical promotion (EPOC) was applied on a catalyst film composed of Pt nanoparticles (of around 3 nm) dispersed in a diamond-like carbon (DLC) matrix which was prepared by the cathodic arc deposition technique. In first place, a temperature-programmed pretreatment was carried out in order to achieve a suitable electrical conductivity for the electrochemical promotion experiments. The decrease in the surface electrical resistance was due to the transition of the sp3-hybridized carbon form into a more graphitic structure (sp2-hybridized) as confirmed by EELS. Moreover, the stability of the Pt nanoparticles was verified by STEM. The catalytic performance of the Pt-DLC film in the methanol partial oxidation (POM) and steam reforming (SRM) reactions for H2 production was promoted by K+ ions electrochemically transferred from a K-ßAl2O3 solid electrolyte. Hence, it was demonstrated that the EPOC phenomenon may be applied to catalysts based on metal nanoparticles dispersed in an electronic non-ionic conductor support. Moreover, two different electropromotional effects were found under POM conditions depending on the applied potential, which were attributed to the formation of different kinds of promoter phases on this catalyst. The higher catalytic activity of Pt-DLC, compared to that of the pure Pt film, demonstrated the practical interest of this kind of dispersed catalyst films with lower metal loading. Other novel electrochemical catalyst based on Au nanoparticles dispersed in a Yttria-Stabilized Zirconia (YSZ) matrix was deposited by reactive co-sputtering of Zirconium-Yttrium and Au targets on a K-ßAl2O3 solid electrolyte. The Au-YSZ film was electrically non-conductive and a silver current collector was used to polarize the catalyst film that would be used in the electrochemical promotion experiments. The Au nanoparticles showed to be active in the partial oxidation of methanol with a high selectivity toward methyl formate production. This configuration allowed to decrease the amount of metal used in the solid electrolyte cell, and to activate a highly dispersed Au catalyst via electrochemical promotion (EPOC), by in-situ controlling and optimizing the supplied amount of K+ ions. A number of experiments confirmed that the observed electropromotional effect did depend on neither the rate of K+ supply nor the operation mode (galvanostatic or potentiostatic). It only depended on the achieved final promoter coverage. The stability of the Au nanoparticles under the explored conditions was also confirmed under EPOC reaction conditions. Then, a novel Cu catalyst film was prepared by oblique angle physical vapour deposition (OAPVD) on a K-ßAl2O3 solid electrolyte. This technique allowed to obtain a highly porous and electrically conductive Cu catalyst electrode composed of metal nanocolumns, which was electrochemically promoted in the partial oxidation of methanol (POM). The production rates of hydrogen, carbon dioxide and methyl formate were in-situ enhanced in a reversible and reproducible way, by means of the controlled migration of electropositive potassium ions. Moreover, the enhancement ratios were comparable to those obtained with the Pt-DLC catalyst. Under the studied reaction conditions, these promoter ions also formed potassium-derived surface compounds as demonstrated by post-reaction characterization analysis. Some nitrogen functional groups and carbonaceous compounds were also detected. The obtained results demonstrate the interest of the used catalyst-electrode preparation technique for the electrochemical activation of non-noble metal catalyst films with high gas-exposed surface area. Then, three possible applications of the phenomenon of electrochemical promotion of catalysis (EPOC) were demonstrated with a Ni catalyst for methanol conversion processes: activation of the catalyst, modification of the catalytic selectivity and partial oxidation of the catalyst. The Ni catalyst film was prepared by cathodic arc deposition and electrochemically promoted by K+, upon negative polarization, in the methanol decomposition (MD) and steam reforming (SRM) reactions, showing an electrophilic EPOC behavior in both cases. In the presence of water, the K+ ions promotional effect also attenuated the Ni deactivation by carbon deposition. On the other hand, under methanol partial oxidation conditions (POM), the K+ ions caused a sharp decrease in the catalytic selectivity toward H2 and CO while the production rates of CO2 and H2CO slightly enhanced, due to an increase in the Ni oxidation state by the alkali-induced O2 activation. All the potassium-derived effects were fully reversible between the negative and positive polarizations, which showed different interesting possibilities of the EPOC phenomena in heterogeneous catalysis. Finally, the possibility of applying the EPOC phenomenon with alkaline conductors in the fields of H2 production and storage was also explored. In this case, a porous Ni catalyst film was deposited on K-ßAl2O3 by the oblique angle deposition technique. Under steam reforming conditions and negative polarization, this electrocatalytic system allowed to produce and store H2 with a very high yield per amount of metal (up to 19 g H2 x 100 g Ni-1) due to the promotional effect of K+ ions. Moreover, it was possible to release the stored, highly pure, H2 in a controlled way under positive polarization and mild (constant) reaction conditions (280 ºC, 1 atm), which represents a new application of the EPOC phenomenon of great interest. The influence of the catalyst microstructure, the applied negative current and the reaction atmosphere was studied. A H2 storage mechanism was proposed on the basis of the obtained results. Although some K+-derived surface compounds such as bicarbonates were detected by post-reaction characterization of the Ni catalyst surface, the very high observed H2 storage capacity was mainly attributed to the chemisorption of H atoms on Ni active sites and their spillover onto carbonaceous surface compounds. These carbonaceous species were simultaneously formed under K+-promoted reaction conditions and showed to be mainly composed of graphene oxide.
