CO2 capture and utilization using dual-functional catalysts

Abstract

The excessive atmospheric CO2 emission is one of the main contributions to the global climate change. However, with the growth of global economics, more fossil fuels will be consumed to feed the global activity, especially in developing countries. Thus, carbon capture storage and utilization draws intensive attentions in recent years. This thesis not only presents a fundamental study on the mechanisms of fast stage during CaO carbonation and sequentially proposed a unique three-dimensional supported structure for the improvement of carbon capture capacity and cyclic stability, but also converts the captured CO2 into value added products or uses the carbonation of CaO to provide energy for low-temperature biomass gasification. Firstly, we study the formation of a CaCO3 product layer on the outside surface of CaO grains during the fast reaction stage for carbon capture using two types of CaO adsorbents. The carbonation at 400 ˚C filled the small pores in the commercial CaO grains and no distinct product layer of CaCO3 was observed. However, a distinct layer of CaCO3 with a thickness around 90 nm was observed on the outside surface of the commercial CaO grains after the carbonation at 600 ˚C because the internal pores in the CaO grain had been filled and a layer of CaCO3 product was deposited on the outside surface of the CaO grain. For sol-gel CaO, the carbonation reaction is limited by the availability of useful porosity for the growth of CaCO3 product (confinement effect), instead of by the diffusion of ions in the critical layer of the CaCO3 product. No surface product layer was observed. Sequentially, we focus on enhancing the capture capacity and cyclic stability of CaO, using KIT-6 as an inert support structure. At the low CaO loading, the three-dimensional mesoporous structure was filled with CaO species. The further increase of CaO loading resulted in the CaO aggregation on the external surface of the support material, as identified by the electron microscopy. These adsorbents show excellent high-temperature CO2 carbonation/calcination stability over multiple cycles. This enhancement is attributed to the interaction between CaO and the silica skeleton of KIT-6 through the formation of interfacial CaSiO3 and Ca2SiO4 which enhance the CaO sintering resistance. Thirdly, we propose and demonstrate a one-pot method synthesized dual functional materials (DFMs), which contain a CaO sorbent coupled with a Ni-based catalyst component, allowing the sorbent regeneration and CO2 conversion to CO are performed simultaneously in a single reactor without any additional thermal energy. In addition, CeO2 is incorporated into the DFMs to largely enhance the stability of the materials for the process, and the influence of different Ce loadings on the performance of integrated CO2 capture and utilization (ICCU) process is studied. It is found that the DFMs with a Ca/Ni/Ce molar ratio of 1:0.1:0.033 displays almost 100% CO selectivity and 51.8% CO2 conversion in the reverse water-gas shift (RWGS) reaction and a remarkable cyclic stability after 20 cycles of ICCU process. Therefore, the incorporation of Ce into DFMs has two profits, for one thing, the oxygen vacancies generated by CeO2 directly reduces the dissociated CO2 regenerated from the DFMs, demonstrating the high CO yield; for another, the well-dispersed CeO2, which could act as a physical barrier, effectively prevents the growth and agglomeration of CaO crystallite and NiO species. In addition, we also propose and demonstrate the intermediate temperature DFMs, which are the physical mixture of an inexpensive high-capacity MgO adsorbent and CO2 methanation catalyst (Ru supported on the CeO2 nanorods) by the mass ratio of 2:1. The synthesized DFMs allow the sorbent regeneration and CO2 conversion to perform simultaneously in a single reactor at the same temperature (300 ˚C). It is found that compared to other DFMs, the 5Ru/CeO2-MgO exhibits a better stability and the CH4 yield and CO2 conversion are 3.36 mmol g-1 and 79% after 10 cycles of ICCU, which are much higher than that of 2.5Ru/CeO2-MgO (1.14 mmol g-1 and 39%) and 10Ru/CeO2-MgO (2.31 mmol g-1 and 69%). It is mainly attributed to that more oxygen vacancies are remained in 5Ru/CeO2-MgO resulted from the metal-support interaction. At the end, we propose a novel auto-thermal CaO looping biomass gasification (Auto-CaL-Gas) technology, in which CaO-based materials react with flue gas with a high concentration of CO2 (>30 vol.%) to produce heat inside the gasifier, simultaneously providing energy for low-temperature biomass gasification using CO2 as the gasification agent. The syngas production exhibits a significant increase from 0.21kg/h to 0.90 kg/h in the Aspen simulation results and more than threefold improvement in the experimental results. We believe this thesis will provide a fundamental and systematic understanding of ICCU process.

Date of Award	Jul 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Chinese Scholarship Council (CSC)
Supervisor	Chunfei Wu (Supervisor) & Nancy Artioli (Supervisor)