Catalytic abatement of industrial pollutants on 3D-Printed geopolymer/zeolite carriers

Catalytic combustion is an effective strategy for the mitigation of pollutant emissions from industrial sources. Appropriate catalysts can degrade harmful compounds at low temperatures and costs by converting them into safer species such as H₂O and CO₂ with significant health and environmental benefits. The overall effectiveness of a catalyst is affected by many factors. The chemical interactions between the catalyst and the compound of interest determine the intrinsic reaction kinetics, while the shape of the catalyst governs mass transfer phenomena ensuring efficient mixing of the reactants and sufficient contact time with the catalytic surface for the reaction to take place. Thus, the development of innovative catalysts for oxidative abatement of industrial pollutants was approached simultaneously from both a design and a materials perspective. Additive Manufacturing (AM) technologies have recently introduced unprecedented freedom in the design and shaping of porous bodies with intricate geometries and optimised properties for fluid flow and mass transfer. Direct Ink Writing (DIW) is a versatile and inexpensive AM process which is suitable for the production of ceramic bodies with a wide range of chemical compositions. The process is similar to the popular Fused Filament Fabrication (FFF) technology used for thermoplastic materials, but in comparison it presents a few additional constraints which pose some limitations on design freedom. The first part of this Thesis consists in a study based on Design for Additive Manufacturing (DfAM) principles specifically applied to the DIW technology. Generative algorithms were developed to automatically create porous designs suitable for DIW featuring a wide variety of different pore architectures and flow properties which were screened on the basis of pressure drop measurements. The second part deals with optimisation of the material. Geopolymers are highly porous aluminosilicates with several beneficial properties for applications in catalysis, such as moderate Specific Surface Area (SSA), good thermochemical stability and mechanical properties, low cost, and easy functionalisation routes. However, the traditional focus of geopolymer research has been as potential alternatives to cementitious binders and the formation of their intrinsic porosity is still poorly understood. Thus, a systematic study was conducted to identify trends correlating the composition and synthesis protocol of geopolymers to their textural properties. The results have allowed optimisation of synthesis conditions to obtain geopolymers with high SSA and strongly interconnected pore networks, which may be useful either in pure form or as binders for traditional powder catalysts such as zeolites. In the third part, the two approaches were combined to produce geopolymer and geopolymer-matrix composites containing zeolite ZSM-5 by DIW with optimised textural properties and intricate designs. The samples were fully characterised by X-Ray Diffractometry (XRD), pycnometry, Scanning Electron Microscopy (SEM), mechanical testing, physisorption and chemisorption measurements, displaying beneficial properties as catalytic carriers. Finally, after functionalisation with catalytically active silver clusters, the samples were tested for the catalytic oxidation of CO, a relevant reaction for pollution abatement from a variety of industrial processes, where they showed good activity with complete conversion of a model stream at temperatures as low as 150°C. Additionally, the reaction is a benchmark for the evaluation of mass transfer properties of catalytic carriers, allowing validation of the DIW-compliant designs developed over the course of the project.