Metallic Membranes | CO2 Capture | Hydrogen Production | Nitrogen Separation |
Material Synthesis and Characterization | Atomic Scale Simulation (Density Functional Theory)
Anthropogenic carbon dioxide (CO2) emissions must be controlled in order to mitigate the impacts of global climate change. While the largest CO2 emissions source is attributed to the use of fossil fuels, they still play a substantial role in the global energy portfolio due to the continuously increasing energy demand and world economy associated with a secure energy supply. CO2 capture, therefore, has been suggested as an indispensible option for clean utilization of fossil fuels. Among various approaches in CO2 capture technology, this study is closely related to enhancing the pre- and post-combustion capture systems. In this study, a theoretical approach on hydrogen-metal systems will be presented for application to pre-combustion capture, followed by an experimental approach on nitrogen-metal systems to be applied to post-combustion capture.
In the pre-combustion capture process, coal or natural gas is partially oxidized (gasified) by steam to produce synthesis gas (mixture of carbon monoxide and hydrogen), followed by the formation of additional hydrogen and CO2 via the water-gas shift reaction. Similar chemical reactions occur in hydrogen production from natural gas, which would increase CO2 emissions substantially in the near term when natural gas becomes more prevalent. In these two systems, CO2 capture is essentially a gas separation process for a CO2 and hydrogen mixture. Hydrogen-selective metallic membranes are promising since high-temperature resistance is required for pre-combustion capture. Hydrogen solubility is an important parameter that affects hydrogen-selective membrane properties because the hydrogen permeability through a metallic membrane is half the product of the solubility and diffusivity of atomic hydrogen in the bulk metal. Therefore, accurate solubility evaluation is crucial for the prediction of overall membrane properties. The first part of this study is focused on the predictions of hydrogen solubility in metals from first principles, which may provide useful information in the search for new materials for enhanced hydrogen-selective membranes. Additionally, this study can be applied to find hydrogen storage materials using metal hydrides.
Hydrogen solubilities in ten transition metals (V, Nb, Ta, W, Ni, Pd, Pt, Cu, Ag, and Au) have been predicted by first principles based on density functional theory (DFT) combined with chemical potential equilibrium between hydrogen in the gas and solid-solution phases. Using a very simple model, the solubility predictions in this study match experimental data within a factor of two in the cases of V, Nb, Ta and W, and within a factor of three in the cases of Ni, Cu, and Ag. Deviations of an order of magnitude are obtained in the cases of Pd, Pt, and Au. In the case of Pd, the deviation in solubility predictions is mainly attributed to the errors involved in the calculated vibrational frequencies of dissolved hydrogen. In Pt and Au, hydrogen in the octahedral interstitial site is less stable than in the tetrahedral site, contradicting the predictions based on the hard-sphere model. Potential energy surface analysis reveals a slightly downward concavity near the center of the octahedral sites in Pt and Au, which may explain the calculated imaginary vibrational frequencies in these sites and lead to unreliable solubility predictions. More complex models, e.g., including metal vacancies and other defect structures, may improve the solubility predictions to better match experiments.
In the post-combustion capture process, CO2 is captured from coal- or gas-fired flue gases after fuel combustion. Conventional technologies have experienced inefficiencies associated with low CO2 concentration (5–12 mol%) in flue gases. On the other hand, nitrogen consists of more than 70% of the flue gas mixture, so nitrogen separation at an early stage in power plants may render CO2 capture more efficient. Additional benefits of nitrogen separation may come from the volume reduction for subsequent pollutant control devices in conventional power plants. Another co-benefit of a nitrogen-selective membrane is the potential for ammonia synthesis by sweeping hydrogen on the permeate side of the membrane, where reactive nitrogen species are offered from the membrane performance.
It has been known that diffusion of nitrogen through the bulk metal is limited due to a large activation barrier, resulting in a low nitrogen flux through the metals. A thin metal-alloy composite membrane is desired to enhance the permeation properties. The aim of the second part of this study is (1) to produce thin metal and metal alloy films on porous ceramics, (2) to confirm the stability of the composites under thermal and flue gas conditions, and (3) to evaluate the separation performance. Here, the results of the first two aims will be discussed, and the last will be continued in future studies. Metals chosen are Group V and VI metals, and the substrates are lab-made yittria-stabilized zirconia (YSZ) and commercial anodized alumina.
Porous YSZ discs were prepared by pressing YSZ powder, followed by sintering at 1050 – 1250˚C in a controlled oxygen environment. High-resolution scanning electron microscopy (HR-SEM) has confirmed that both the initial particle size and sintering temperature are critical in determining the porosity as well as the size of the surface defects. Local surface roughness was achieved within 40 nm after polishing, confirmed by atomic force microscopy (AFM). Nevertheless, macro-defects have been observed due to the lack of cohesion between large and small grains. In order to avoid such macro-defects, consistency in the size of YSZ nanoparticles before pressing is important. Nb thin film has been deposited by e-beam evaporation. The film quality and adhesion have been tested under various thermal (50 – 600 ˚C) and gas conditions (Ar, N2, CO2).
Thin films of Nb and Mo 100 nm-thick have been deposited via e-beam evaporation on anodized alumina. The presence of dense grain boundaries with random hillocks were confirmed by HR-SEM. Due to a large number of local pinhole defects, the membrane does not exhibit an appreciable selectivity toward nitrogen. While improving the film quality by modifying the support surface structure and varying deposition parameters, we first have investigated the impact of thermal (20, 150, 300, 450, 500, 550, and 600 ˚C) and gas conditions (Ar, N2, CO2) on Mo-alumina composite membrane samples. Grain growth was predominant at and above 450 ˚C in all gases. The continuity of the Mo film became completely destroyed at 600 ˚C upon nitrogen exposure maybe due to further phase transformation. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have shown various extent of Mo oxidation states and potential nitride species formed on the surface. While Mo is chosen as a reference case of Group VI metals, which are less reactive to nitrogen, surface nitride formation took place after gas exposure without added pressure. Nb thin films are expected to be more reactive toward nitrogen and the tests with Nb films are in progress. In future studies, pinhole-free thin film will be subsequently developed and nitrogen permeation will be measured.
In summary, hydrogen absorption into metals has been understood by theoretical predictions of solubility, which may lead to the development of enhanced metallic membranes for pre-combustion CO2 capture. The results of hydrogen-metal interactions are also applicable for advancing materials for improved hydrogen storage. Nitrogen surface reactions with metals have been investigated by fabricating composite thin membranes, and the current results with Mo indicate surface nitrogen absorption, promising to the nitrogen-permeable metallic membranes. Provided that further improvement of pinhole-free continuous metal film would be successful, this technology may have a significant impact on effective post-combustion CO2 capture.
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