Advanced fabrication of structured catalysts for CO2 utilization: case studies on Ni/SiO2 and In2O3/ZrO2

Carbon dioxide utilization (CDU) processes offer solutions to the atmospheric CO2 crisis. Reactions like CO2 methanation and direct methanol synthesis enable the transformation of CO2 into valuable compounds via reduction with H2. However, to actually reduce the atmospheric CO2 level, these processes need to be carried out exclusively with waste CO2 and green H2, thereby significantly increasing raw material costs and thus requiring highly optimized processes, including very efficient catalysts. Currently, these reactions require large compressors that represent a significant part of the capital costs, as the reactors have to be filled with packed catalyst beds that maximize activity but generate large pressure losses, in addition to the desired products having increased yields at higher pressures. As such, the development of low-pressure-drop-catalysts that provide the same level of activity is necessary to turn CDU processes profitable and up-scalable. This thesis focuses on developing 3D-structured catalysts for CDU to explore the fabrication of highly active catalysts with improved gas flow properties, to maximize the activity across the catalyst surface. Materials such as ordered mesoporous silica (OMS) COK-12, a structural analog of SBA-15 with a more environmentally friendly synthesis, and sodium bentonite, an inexpensive, natural clay are used, avoiding more complex materials with larger associated CO2-emissions and energy costs. Robocasting, an additive manufacturing (AM) technique, is used to shape the catalysts, retaining high surface area and pore volume. AM offers design flexibility by decoupling pressure drop and feature size, allowing for novel catalyst geometries with unprecedented transport properties. A two-step method was developed to generate nickel nanoparticles (3-4 nm) inside the COK-12 pores, using only water, inorganic nickel salts, and ammonia — making the procedure simple and cost-effective. In-situ XRD revealed that these nanoparticles retained their size under operational conditions, and even harsher environments. The obtained catalyst was highly active towards the CO2 methanation reaction and comparable to other reported catalysts, with a CO2 conversion of over 80% and a selectivity towards methane of over 90%. Robocasting feedstock mixtures of COK-12 and sodium bentonite were tested for rheological properties, finding them shear-thinning and printable. In addition, their mechanical and textural characteristics after printing and thermal treatment were measured, finding the resultingii monoliths resistant enough to be handled, shaped and used as catalysts, with a compression strength of 1.0 MPa. Small-angle X-ray scattering revealed COK-12 pore enlargement (6 vs 8 nm) and maintained silica surface area due to the conditions of the mixture preparation process, particularly the alkaline pH. The developed Ni/COK-12 powder catalyst synthesis was successfully adapted to be able to be done on an AM support using two different Ni loading methods, resulting in distinct nickel particle morphologies, finding ion exchange to yield highly dispersed Ni particles. This difference was also evident in terms of their CO2 conversion rate, 0.14 vs 0.32 mol/(gNi*h). Challenges in characterization and performance measurement for CO2 methanation were addressed. The methods were extended to In2O3/ZrO2 systems, where a study of three geometries (the traditional cylindrical extrudate, FCC (AM, with “Zig-zag” channels), and BCC (AM, with straight channels)) showed that AM catalysts achieved comparable CO2 conversion rate (0.012, 0.025 and 0.009 mol/(h*gcat), respectively) and methanol production (0.17 gMeOH/(h*gcat), for all) to traditional extruded geometries while reducing pressure drops by nearly an order of magnitude (27.6, 3.3 and 2.2 Pa/m). In conclusion, this work demonstrated that AM catalysts based on metal oxides can exhibit high activity, and the capacity to utilize the catalyst surface more efficiently by routing the gas flow across the entirety of the structure. This thesis also provides a roadmap for applying AM to other catalytic systems, highlighting common challenges and advantages for future applications.