Development of heterogeneous catalysts for clean hydrogen production from biomass resources

Abstract

Chapter I is focused in the current energy crisis and provides a brief introduction to the use of hydrogen as an energy carrier, mentioning the different methods that can be used for production/purification of hydrogen from renewable resources. This chapter also includes a description of the role that biomass can play as an alternative to fossil fuels, and its conversion into biofuels and value-added chemicals. Catalytic reforming of glycerol for hydrogen-rich streams or synthesis gas production is presented as a potential, promising alternative route that has attracted attention in recent years. This reaction is often carried out in metal-based catalysts supported on stable materials. Chapter II explores Sn addition effect on the properties and stability of Pt catalysts supported on carbon in glycerol steam reforming reaction. To this end, a series of samples with different Pt/Sn atomic ratios were prepared and characterized. The high price of noble metals motivates the search and use of cheaper and more abundant metals that also provide a good catalytic behaviour in this reaction. Therefore, in Chapter III Ni-based catalyst promoted by ceria for glycerol steam reforming were used. Furthermore, it is necessary to optimize the use of the CeO2 due to its limited availability and extensive applications. Thus, in this Chapter CeO2 was dispersed on activated carbon of high surface area, obtaining great cerium oxide surface exposed with a remarkable reduction on the ceria loading. Also the effect of tin was studied in these catalysts. Several advantages are obtained when glycerol reforming is carried out in liquid phase. For instance, the produced gas obtained is rich in H2 and poor in CO. This is due to the moderate temperatures and high pressures employed, favouring the water gas shift reaction. Furthermore, the overall energy consumption is reduced by the fact that glycerol is converted into hydrogen in an aqueous liquid phase rather than in the gas phase, eliminating the need to vaporize the high-boiling biomass derived oxygenated. Moreover, glycerol upgrading at low temperature prevents the undesirable thermal decomposition reaction of glycerol when high temperatures are used. 2 In this context, in Chapter IV, a comparative study of the catalytic properties of three samples, Pt/CeO2, Ni/CeO2 and Pt-Ni/CeO2 is tested in glycerol aqueous phase reforming reaction. In addition, in-situ attenuated total reflectance spectroscopy was employed to obtain relevant information about reaction intermediates and the evolution of the catalysts during the reaction. This study allowed to propose the more likely reaction pathways. For clean energy production, pure hydrogen is required as a feed gas for electricity generation in low temperature fuel cells. To obtain clean hydrogen, the reforming stream generated must be processed in several steps, including CO elimination via water-gas shift reaction. In Chapter V the same series of Ni catalysts promoted by CeO2 supported in carbon were tested in the low temperature water gas shift. Two different feed gas mixtures were used in this study: an idealized one (only CO and H2O) and, a post-reforming surrogate stream (CO, CO2, H2 and H2O). To conclude, in Chapter VI, the catalyst which presented the best catalytic behaviour in the previous chapter was studied in greater depth, linking its properties with catalytic activity. Finally this sample was submitted to several stability tests under more demanding reaction conditions aiming to check its potential application in an integrated fuel processor.