The world continues to rely heavily on energy supplied by fossil fuels, and the forecast for future energy supply and demand does not indicate that fossil fuel use will diminish substantially in the coming years. The use of these energy resources releases huge amounts of CO 2 into the atmosphere every year and there is increased pressure throughout the world to limit these releases as a consequence of the impact of CO 2 on global climate change. A primary method to limit the release of CO 2 is to trap it at its release point (carbon capture) for storage via one of several potential storage technologies (sequestration).[1] A key roadblock is the development of cost-effective CO 2 capture/separation technologies, because these represent the majority of the costs in oceanic or geologic sequestration scenarios.[2] In capturing CO 2 for beneficial uses, such as feeding algae farms for biofuel production, the capture costs represent the entire cost of the CO 2 supply.The current benchmark method for CO 2 capture from gas streams uses aqueous amine solutions (e.g., ca. 40 % monoethanolamine and ca. 60 % water) to absorb CO 2 , and this approach has been suggested for CO 2 capture from power plant effluents.[3] In parallel, solid adsorbents based on supported amines have been evaluated and show promising CO 2 adsorption properties. Among the various classes of solid CO 2 adsorbents, supported amines have many promising features, such as operation at low temperatures (ambient-120 8C). In addition, they have strong CO 2 -sorbent interactions (50-105 kJ mol À1 ), acting as unique, low-temperature chemisorbants.[4] In contrast, most other low-temperature adsorbents such as zeolites, carbons, and (some) metal-organic frameworks (MOFs) rely on weaker physisorption interactions, making water, a common component in flue gas, out-compete CO 2 for adsorption cites in many cases. Indeed, there are over 70 publications in the open literature that explore the CO 2 -adsorption properties of supported amine adsorbents. [2, However, to be a practical CO 2 sorbent, the cost-effective regeneration of the material must be demonstrated. Unfortunately, the singular focus of the academic community to date has been on design of high capacity adsorbents, with nearly all published studies focusing entirely on the capture step. [4] Adsorbents are routinely shown to be at least partly regenerable, but almost always via temperature swing with an inert gas purge, which ultimately does not result in a separation.[75] Of course, this regeneration method is entirely impractical in a real process and it is used in the laboratory simply for convenience. Despite this, it is noteworthy that nearly all previous published research, including our own, has not effectively concentrated the CO 2 , or performed a useful separation. Thus, a critical missing link in the development of supported amine CO 2 -adsorbents are studies of practical desorption processes.Supported amine CO 2 sorbents are most effectively regenerated in a temperature swing process, as signif...

This review focuses on the use of designed reactants, consisting of ultrathin elemental layers sequentially deposited in ultrahigh vacuum, to control reaction pathways and the structure of compounds formed. Two reaction pathways are discussed. The first uses controlled nucleation of an amorphous intermediate to kinetically access compounds under conditions where they may be thermodynamically unstable. The second approach takes advantage of the controlled diffusion lengths in elementally modulated reactants and slow diffusion rates at low temperatures to prepare superlattice and heterostructure compounds with controlled composition and structure.

A new metastable binary compound and a series of new, metastable ternary crystalline compounds with the skutterudite crystal structure have been prepared through controlled crystallization of amorphous reaction intermediates formed by low-temperature interdiffusion of modulated elemental reactants. Discussed in this paper are La1 - x Fe4Sb12, FeSb3, Hf1 - x Fe4Sb12, and Y1 - x Fe4Sb12. The amorphous reaction intermediate for each system crystallizes exothermically near 200 °C forming the desired skutterudite structure. At temperatures above 500 °C, they decompose exothermically forming a thermodynamically more stable mixture of binary compounds and elemental components. We propose that the desired compounds nucleate from the amorphous precursor because slow solid state diffusion rates hinder disproportionation into the more stable mix of binary compounds and elements. This general synthetic approach to prepare new metastable compounds gives control of both the composition and structure of the product compounds.

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Modulated elemental reactants were used to form amorphous ternary reaction intermediates of the desired compositions. The amorphous reaction intermediate for each system crystallizes exothermically below 200 °C forming kinetically stable "filled" skutterudites of general formula M 1-x Fe 4 Sb 12 where M ) Sn, Al, Ga, In, and Zn. These metastable compounds can only be prepared by controlling the reaction intermediates, avoiding the formation of more thermodynamically stable binary compounds. We propose that the desired compounds nucleate from the amorphous precursor because slow solid-state diffusion rates hinder disproportionation into the more stable mix of binary compounds and elements. If modulated elemental reactants are prepared with layer thicknesses above a critical value (∼40 Å), the thermodynamically stable binary compound FeSb 2 interfacially nucleates, preventing formation of the desired metastable ternary compounds. The use of low-angle X-ray diffraction and calorimetry provide the data necessary to develop an understanding of the reaction mechanism.