Catalytically induced flows : increasing productivity by activity contrast

Abstract

There is a general agreement on the diffusion only transport inside porous catalysts. In this thesis we challenge the narrative on mass transport for heterogeneous catalysis and demonstrate that convective transport inside catalytic dead-end pores is not only possible, but ubiquitous for higher kinetics. Surface flows induced by concentration gradients, i.e. diffusio-osmotic flows, form during catalysis inside the pores. This additional mass transport does not require any external input. It spontaneously arises when the configuration of the catalyst matrix facilitates the development of significant gradients with respect to the bulk solution. The reaction driven surface flow which originates in the osmotic pressure gradient and diffusion potential in case of charged species, replenishes the catalytic pores with fresh solution, having a positive impact on the conversion. We visualise and quantify the flow in 3D using the General Defocusing Particle Tracking technique. We analyse the phenomena using a model that includes the fluid dynamics actuated by the concentration gradients that arise due to the catalytic reaction. We are able to extract parameters revealing the interaction strength between the reactant/product chemical species and the catalytic surface. Diffusion is also the dominating mass transport mechanism close to the catalyst surface where external mixing loses its efficiency. We introduce the concept of enhancing external transport by patterning catalysts. The reactivity contrast designed by alternating active and inactive regions generates spontaneously in-plane gradients that drive a steady diffusio-osmotic flow. While the details of the numerical model may need to be adjusted for particular catalytic reactions, the approach is not pinned to a certain chemistry. Concentration gradients will develop as long as kinetics are fast enough to place the system in the mass transfer limiting regime. The diffusio-osmotic flow is redirected out of plane due to the symmetry of the system and mixes the otherwise diffusion dominated boundary layer which greatly enhances mass transport and thus impacts the overall conversion. Scaling laws provide a direct correlation between the catalytic chemistry, the dynamics of the system, and the conversion enhancement. Specific chemistries, with known reaction kinetics and interaction potentials, can now be tested against this predictive model.