Title: A study of dispersed Ru + alkaline oxides in dual function materials (DFM) for direct air capture of carbon dioxide and from natural gas power plants with subsequent methanation using renewable hydrogen
Description: The rise of anthropogenic CO₂ emissions and the associated increasing levels of CO₂ in the atmosphere are expected to bring uninhabitable conditions on earth due to global climate change and numerous associated environmental crises. To reduce these impacts, warming must be kept to 1.5°C above pre-industrial levels. With the slow transition to more favorable energy generation methods with lower carbon emissions, it is clear that power plants utilizing fossil fuel combustion for electricity will not be reduced to an acceptable level. Thus, the deployment of negative emission and carbon capture, utilization, and storage (CCUS) technologies will be crucial to meet the 1.5°C target. The current state-of-the-art carbon capture technology is point source amine scrubbing, in which diluted aqueous amine solutions are used to absorb CO₂ from power plant flue gases. The CO₂ is captured at ~40°C by the formation of an amine/H₂O—CO₂ complexes. The absorbent solution, now containing CO₂, must be heated to release the CO₂ for further processing and to regenerate the amine for recycling.

Though this technology is well-engineered and commercially available, there are some major drawbacks such as energy intensity mainly due to vaporization of the water during CO₂ separation from the amine, corrosivity of the amine material, and the need to transport the released CO₂ for further utilization or sequestration. To this end, dual function materials (DFM) were developed to address these issues. DFMs eliminate the need for the energy intensive regeneration of liquid amine solutions and transportation of CO₂. Comprised of both a capture and catalytic component co-dispersed on the same high surface area carrier, the DFM is able to selectively capture CO₂ from the effluent flue gas and catalytically convert it to methane (or renewable natural gas) with the introduction of preferably renewable H₂ in the same reactor. The DFM can operate isothermally at around 320°C by harnessing the sensible heat of typical power plant effluent flue gases. 5% Ru, 6.1% “Na₂O”/Al₂O₃ was shown to be a very robust, demonstrating stable performance after 50 cycles of capture and catalytic conversion with simulated flue gas.

In addition to point-source capture, negative emission technologies like direct air capture (DAC) are required to mitigate climate change. Thus, we investigate the use of DFM for a new application – the direct air capture of CO₂ and subsequent catalytic methanation. Furthermore, for such applications, the loading of Ru was dramatically decreased to alleviate the economic burden for commercialization and wide-scale deployment. This thesis demonstrates the flexibility of the DFM as a carbon capture technology for direct air capture of CO₂ at ambient air temperatures and subsequent methanation (DAC-M) at temperatures in excess of 200°C. Recognizing the energy intensity of isothermal DAC-M operation, the capture and conversion cycles were modified for temperature-swing operation, with adsorption occurring at 25°C, followed by heating up to 300°C in H₂ for methanation. Short-term aging was conducted on 1% Ru, 10% Na₂O/Al₂O₃ in a packed bed configuration for 10 cycles of adsorption in dry air conditions (400ppm CO₂ in air) and methanation.

The sample was also tested in humid adsorption conditions (400ppm CO₂, ~2% H₂O/air) to better simulate ambient environments. These tests showed that the DFM is able to operate in a temperature swing mode and exhibits a higher, stable CO₂ adsorption capacity in humid conditions unlike other capture technologies using amines and physical adsorption methods, which show a significant decline in capture capacity. We were able to establish that the DFM has great potential for DAC-M. Consequently, these materials are moving towards advanced process development with our engineering partners under the sponsorship of DOE. Critical parameters such as pressure drop, heating rate, and methanation temperature are primary parameters that must be optimized. New low Ru loading DFMs, 0.5% Ru and 1% Ru DFM, were aged with simulated power plant flue gas (7.5% CO₂, 4.5% O₂, 15% steam, balance N2) for over 50 cycles of capture and catalytic conversion to CH4 (and water) in a packed bed configuration. These conditions of continuous operation at 320°C with 15% steam and 4.5% O₂ are far more severe than for DAC which adsorbs low levels of CO₂ from air at ambient air conditions (0-40°C with 2-5% moisture).

Therefore, these power plant effluent test conditions can be considered accelerated aging for DAC-M. A reduced level of 0.5-1% Ru DFM was tested under simulated power plant effluent conditions on several Al₂O₃ structures, particularly tablets and ring tablets for scale-up of the technology. These tests showed a subtle but gradual deactivation of the material. Characterization with CO chemisorption and in-situ FT-IR indicated that the Ru component is deactivated – most likely by sintering – due to the presence of O₂ and H₂O in the flue gas. Microreactor studies show that in the presence of O₂ and H₂O, adsorption capacity is reduced and the rate of methanation is decreased. Upon removing O₂ and H₂O from the adsorption step, the adsorption capacity is restored but the rate and selectivity of methanation declines steadily, indicating that the deactivation is irreversible. Interestingly, the DFM with higher Ru loading showed more stable performance suggesting that higher catalytic content is required for more improved stability.

Fortunately, Ru can be leased and recycled, reducing the capital economic burden of higher Ru loadings. Additionally, we expect that given the milder conditions for capture in DAC scenarios, low Ru loaded DFMs will be more stable. Initial DAC-M data substantiates this stability, but longer aging times are required for confirmation. Furthermore, stability may be favored with the use of higher concentration H₂. Finally, this thesis also investigates the use of other Ru+sorbent/carrier combinations for DAC and the apparent enhancement of adsorption arising from the use of a reactive sorbent (e.g., addition of Ru). After screening Al₂O₃-supported Na₂O, CaO, MgO, and BaO in combination with 1% Ru, we are able to show that Ru+Na₂O/Al₂O3 has the best adsorption capacity. This material, relative to the other alkaline oxides studied, shows a unique enhancement in the CO₂ adsorption capacity compared to the bare supported sorbent (Na₂O/Al₂O₃). The enhancement effect is shown to be an asymptotic function of an increasing Ru loading, plateauing after 3% Ru.

ZrO₂-supported Ru+Na₂O is also tested but does not show favorable adsorption capacity, indicating that Al₂O₃ is also a crucial component of the DFM formulation. This technology is the subject of a provisional patent application.