Resumen de Fotosíntesis Artificial: Influencia de la química superficial y los procesos optoelectrónicos en la reducción fotocatalítica de CO2
Laura Collado Brunete
One of the greatest challenges of our times in the Energy research field is the search for a sustainable, competitive and secure energy supply. Conventional energy sources, based on fossil fuels, have caused an increase in the atmospheric concentrations of greenhouse gases, directly related to global warming. Carbon dioxide (CO2) accounts for more than 70% of anthropogenic greenhouse gases emissions, mostly derived from the energy and transport sectors. This situation has fostered the establishment of targets and policies focused on the reduction of anthropogenic emissions, and the promotion of renewable energy technologies. Within this context, CO2 valorisation technologies are an appealing alternative for the chemical recycling of this greenhouse gas into products of energy and industrial demand. Specifically, Artificial Photosynthesis is a photocatalytic route of great scientific interest, which can be used to recycle CO2 into fuels and other value-added products. This process is based on the photoinduced conversion of CO2 and water to reduced compounds, mainly C1 and C2. Chapter 1 comprises further descriptions and provides an overview of the state of the art to place in context the operation modes and the relevant advances in this field. Within this context, the main objective of this Thesis (chapter 2) has been the investigation of the photocatalytic reduction of CO2 and water vapour, under mild conditions, for the production of fuels and other value-added products. This reaction is a multi-electronic process that requires the use of photocatalysts capable of efficiently driving the three main stages: (1) light absorption and photogeneration of e-CB/h+VB pairs; (2) charge transport to the surface of the photocatalysts and charge transfer to adsorbed species; and (3) conversion of adsorbed surface species into products. Besides the inherent complexity of a multi-electron transfer process, the photocatalytic CO2 reduction must cope with other challenges, such as: high thermodynamic stability of the CO2 molecule, limited adsorption on the photocatalyst surface, competitive H2O adsorption and slow desorption kinetics of intermediates and/or reaction products from the surface. All these factors hinder the CO2 activation/conversion and result in low production yields. Furthermore,at such low concentrations of CO2-derived products, even small amounts of carbon contamination can play part of the product yields. Thus, the identification of the origin of the carbon products is one of the main questions raised when evaluating CO2 photoreduction performance. In order to shed some light into these uncertainties, the specific objectives of this Thesis (chapter 2) are focused on the study of the optoelectronic properties of the catalysts and the influence of the adsorbed surface species on the catalytic reactivity. Chapter 3 comprises the synthesis procedures for the preparation of the catalysts, as well as the operation conditions of the photocatalytic tests, which were performed in a continuous gas-phase photoreactor using H2O as electron donor. Moreover, it presents the characterisation techniques employed in this work that include in-situ characterisation based on synchrotron-based NAP ¿ XPS analyses and isotope labelled (13C and 1H) NMR measurements. These studies aimed to gain a deeper insight into the physicochemical and optoelectronic properties of the catalysts, and their influence on the reaction mechanism. To achieve the proposed objectives in this Thesis, different catalysts were selected to study specific aspects of the photocatalytic process. In particular, TiO2 was employed to study the CO2 photoreduction from the surface reactivity point of view. Noble metal nanoparticles facilitated the understanding of the charge dynamics, while the use of non-conventional semiconductors, such as Bi2WO6, aimed at studying band gap engineering methods to improve the photocatalytic activity. The discussion block is divided into four chapters with the following sections: (1) design and synthesis of photocatalysts; (2) characterisation of the physicochemical and optoelectronic properties, and analysis of the surface chemistry of prepared materials; (3) evaluation of the gas-phase CO2 photoreduction performance under UV and visible light irradiation; and (4) in-situ characterisation based on synchrotron-based NAP ¿ XPS analyses and isotope labelled (13C and 1H) NMR measurements. The first section of results of this Thesis is focused on the study of titanium dioxide (TiO2) as a reference catalyst for the CO2 photoreduction process (chapter 4). This study comprises an evaluation and comparison of two different TiO2 samples; one of them is entirely composed of anatase crystal phase and the other one by a mixture of anatase/rutile phases. Both materials showed activity towards CO2 photoreduction, yielding H2 and CO as well as minor amounts of CH4 and CH3OH, under UV irradiation. With the aim to further investigate the photocatalytic behaviour of TiO2, a series of in-situ characterisation measurements were performed to try to elucidate the origin of the evolved products and the reaction mechanism, as well as to investigate the deactivation mechanism. The next block of results investigates the modification of the surface of TiO2 by depositing silver (chapter 5) and gold (chapter 6) nanoparticles (NPs) with surface plasmon resonance effect (SPR). This modification aims to obtain visible-light driven photocatalysts and improve charge separation and charge transfer processes. Noble metal NPs exhibiting SPR effect were chosen because they can act as effective charge trapping centres and as co-catalysts, which directly improves the photocatalytic activity. Besides, both chapters include a detailed characterisation study of prepared photocatalysts, and an evaluation of the photocatalytic activity under UV and visible light irradiation. Moreover, the influence of other factors such as particle size, metal content and interaction metal ¿ support are also considered. In this case, the photocatalytic tests showed an enhanced selectivity towards hydrocarbons, mainly CH4. Transient absorption spectroscopy (TAS) measurements confirmed that metal NPs act as efficient charge traps, thus decreasing recombination processes and promoting the formation of more electron demanding products. The study of the photocatalytic activity of these materials was complemented by an evaluation of the surface chemistry evolution under reaction conditions, using synchrotron-based NAP ¿ XPS measurements. The last chapter describing results (chapter 7) deals with the design and development of a non-conventional visible-light driven photocatalyst for CO2 photoreduction. The semiconductor oxide chosen for this purpose was bismuth tungsten (Bi2WO6), which was active towards CH3OH formation. In addition to photocatalytic tests, this chapter also contains a detailed characterisation study that includes transient absorption measurements and synchrotron-based NAP ¿ XPS experiments. Finally, chapter 8 of this Thesis summarises the conclusions drawn from the most relevant results attained in the present investigation. In addition to this, some suggestions for further studies are also given at the end of the same Chapter.
