Abstract
Hydrogen has the potential to fuel the energy needs of a more sustainable society. As hydrogen is not found in nature in any appreciable quantities, this energy carrier needs to be produced from a primary energy source. Biomass can serve as a source for sustainable hydrogen production. In principle, it
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is possible to produce hydrogen via gasification of whole biomass, but the imbalance between the cost of the process and associated energy losses have spurred the development of new techniques for hydrogen production from biomass or its components. Aqueous-Phase Reforming (APR), one of these alternative technologies, has gained considerable prominence and is the topic of the work described in this PhD thesis. APR entails the conversion of biomass-derived oxygenates in water to H2 and CO2 over an appropriate heterogeneous catalyst material. The scientific approach is put in context in Chapter 1, which focuses on the use of supported metal catalysts for APR showing that both the choice of the (bi-) metal as well as the support have a pronounced influence on activity and selectivity. Chapter 2 is concerned with the development of an active and selective system for the production of H2 from glycerol, which stands out as a readily available, practical source for hydrogen production as it is obtained in large volumes as the major by-product of biodiesel production. The mixed oxides were chosen as support materials for the preparation of Pt-based APR catalysts and obtained by calcination of parent Layered Double Hydroxides (LDHs). Next to the influence of the support oxide on APR activity, the influence of alloying was also studied. The addition of Cu as a second metal to the Pt/Mg(Al)O was shown to improve the selectivity towards H2 by suppressing the methane production. In Chapter 3 the influence of support composition of the catalyst, in particular a study of the variation of the nature of divalent metal ion in the mixed oxide, on the APR of glycerol is presented. Catalysts consisting of Pt supported on LDH-derived mixed oxides, in which the magnesium ions had been (partly) substituted for cobalt or copper have been tested. The acidity/basicity of the support material as well as the redox properties of the metals introduced were shown to play an important role in determining the product selectivity. Chapter 4 deals with the consequences of using crude glycerol , obtained from an industrial plant, for the APR process and the effect of its components on catalytic activity. The use of synthetic mixtures, mimicking (part of) the composition of crude glycerol, allowed the cause of deactivation to be elucidated. Finally, in Chapter 5, APR of xylitol, the second most abundant polyol, is presented. Xylitol is a promising substrate for the production of H2 via the APR process and is readily available by hydrogenation of the hemicellulose fraction of lignocellulose. Xylitol APR was studied both using a semi-batch setup to initially assess the selectivity as well as in a continuous flow reactor system to study the activity of the catalyst materials in more detail.
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