Turn on the light: Diffusio-osmosis induced by photocatalytic reactions

Abstract

Catalysis is widely used in industry, where 90% of the industrial processes uses at least one catalyst in the system. Heterogeneous catalysts are most commonly used (80%) compared to homogeneous catalysts (15%) and biocatalysts (5%). A heterogeneous catalyst is a solid surface with active sites to accelerate a reaction without changing the thermodynamic equilibrium, and the reactant is in gas or liquid phase. The reactant is transported to the surface and active site to be able to convert to products. If the reaction is really fast compared to the mass transport towards the surface, then a boundary layer is formed. This boundary layer limits the effectiveness of the catalyst, as the concentration of reactant vanishes near the surface and conversion mainly depends on the diffusivity of the reactant.The boundary layer can thus be a limiting factor in heterogeneous catalysis. In this study, we aim to create a surface induced flow such that reactant is transported to the surface and product is transported from the surface. The surface flow of interest is diffusio-osmosis, originating from a solute concentration gradient parallel to the surface with which the solute interacts. The interaction of the solute with the wall creates a pressure on the fluid in the interaction layer. This pressure is higher for higher concentrations, so the concentration difference creates an osmotic pressure along the surface. This pressure difference is balanced by a viscous force, creating a flow within the interaction layer along the surface. This flow is defined as the diffusio-osmotic flow.A heterogeneous catalysis process suitable to study diffusio-osmosis involves the oxidation of organic compounds over titanium dioxide. Titanium dioxide is a photocatalyst, meaning that the catalytic properties are activated by light. This process can be used in wastewater treatment systems to remove organic micropollutants from water. An elaboration on this process and a general introduction of diffusio-osmosis are given in Chapter 1.The magnitude of the diffusio-osmotic velocity depends on the concentravii viii Summary ment. This was attributed to the dissociation constants of carboxylic acids, so the ionic gradients were actually lower, decreasing the driving force for diffusiophoretic movement. The H-shaped channel can be used to study diffusiophoretic movement of particles, especially to compare experimental work with theory.The diffusio-osmotic flow induced by a photocatalytic reaction at the surface is studied in Chapter 5. The reaction kinetics measured in Chapter 2 and 3 are used to determine the concentration gradient. The surface ζ-potential of titanium dioxide was measured under reactive and nonreactive conditions, resulting in a potential between 40 and -70 mV for pH 3 to 10. A numerical study showed how diffusio-osmosis can enhance mass transfer especially when the flow is directed from noncatalytic to catalytic surface. The numerical study was also used to determine the diffusio-osmotic velocity for methylene blue, formic acid, acetic ...