The Dubinin-Radushkevitch (D-R) equation, which was originally proposed as an empirical adaptation ~d'the Polanyi adsorption potential theory, has been the fundamental equation to quantitatively describe the adsorption td gases and vapors by microporous sorbents. The equation, based on the postulate that the mechanism for adsorption in micropores is that of pore-filling rather than layer-by-layer surface coverage, generally applies well to adsorption systems involving only van der Waals forces and is especially useful to describe adsorption on activated carbon. The ability of the D-R equation to describe gas adsorption on porous materials has inspired many to undertake studies, both experimental and theoretical, to explain the source of the success of the D-R equation in terms of molecular properties at the gas-solid interface. In many cases, these studies have led to extensions or modifications of the original D-R equation. Many of these attempts and the resulting extensions are reviewed and discussed here. Recently, an isotherm equation was derived for adsorption of gases and vapors on microporous solids from statistical mechanical principles. It was shown that the D-R equation is an approximated form of this potential theory isotherm. This development is also reviewed and discussed.

Hydrotalcite-like compounds (HTlcs) are solid sorbents that may potentially be used for high-temperature separation and capture of CO2. The high-temperature adsorption of CO2 on Mg−Al−CO3 HTlc is affected by structural changes that take place upon heating of the material. The structural changes of a synthetic HTlc upon heating to 200 and 400 °C in a vacuum were characterized using various analytical techniques. These structural changes were then related to observed behavior with respect to the physisorption and chemisorption of CO2 at 200 °C. Upon heating to 200 °C, the material retains its layered structure, though the interlayer spacing is decreased by ∼0.6 Å due to loss of interlayer water. Chemisorption of CO2 at 200 °C represents more than half of the total adsorption capacity (at 107 kPa) due to increased availability of the framework Mg2+ cation and the subsequent formation of MgCO3. There is no significant increase of surface area or pore volume after heating to 200 °C. Upon heating to 400 °C the CO3 2- in the interlayer is decomposed and the material is completely dehydrated and partially dehydroxylated. The resulting amorphous 3-D structure with increased surface area and pore volume and decreased availability of the Mg2+ cation favors physisorption over chemisorption for these samples. An increased understanding of structure−property relationships will help in the further development of HTlcs as viable CO2 solid sorbents.

No abstract

Brominated powdered activated carbon sorbents have been shown to be quite effective for mercury capture when injected into the flue gas duct at coal-fired power plants and are especially useful when burning Western low-chlorine subbituminous coals. X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) have been used to determine information about the speciation and binding of mercury on two commercially available brominated activated carbons. The results are compared with similar analysis of a conventional (non-halogenated) and chlorinated activated carbon. Both the XAS and XPS results indicate that the mercury, though introduced as elemental vapor, is consistently bound on the carbon in the oxidized form. The conventional and chlorinated activated carbons appeared to contain mercury bound to chlorinated sites and possibly to sulfate species that have been incorporated onto the carbon from adsorbed SO2. The mercury-containing brominated sorbents appear to contain mercury bound primarily at bromination sites. The mechanism of capture for the sorbents likely consists of surface-enhanced oxidation of the elemental mercury vapor via interaction with surface-bound halide species with subsequent binding by surface halide or sulfate species.

A bench-scale study was conducted on the simultaneous removal of SO2, NOX, and mercury (both Hg0 and Hg2+) from a simulated coal flue gas using a wet calcium carbonate scrubber. The multipollutant capacity of the scrubber was enhanced with the addition of the oxidizing salt, sodium chlorite. The results showed a maximum scrubbing of 100% for SO2 and Hg species and near complete NO oxidation with about 60% scrubbing of the resulting NOX species. The chlorite additive was less effective as an oxidant in the absence of SO2 and NO in the flue gas. Oxidation of NO and mercury were only about 50% and 80%, respectively, in the case of no SO2 in the simulated flue gas. The mercury oxidation was similarly affected by the absence of NO in the flue gas.

Silver is known to strongly affect the adsorptive properties of some zeolites. It is also known that thermal vacuum dehydration of some argentiferous zeolites leads to the formation of charged silver clusters within the zeolite. In this work we have synthesized silver zeolites of the types Y, X, and low-silica X. The zeolites were treated in such a way as to promote the formation of intracrystalline charged silver clusters. Equilibrium room-temperature isotherms were measured for adsorption of nitrogen for each of the zeolites after various heat treatments and dehydration. These materials were structurally characterized via Rietveld refinement using neutron powder diffraction data. Color changes upon heat treatment and subsequent X-ray photoemission spectroscopy confirmed some reduction of Ag+ → Ag0. The effects of various dehydration conditions, including the time, temperature, and atmosphere, on the room-temperature adsorption of nitrogen are discussed. Structural characterization, along with valence bond calculations, revealed the presence of cations in site II*, which are more active in Ag−LSX samples that were vacuum dehydrated at 450 °C as compared to those that were vacuum dehydrated at 350 °C.

A study was conducted to describe and quantify how substitution of the divalent cation and interlayer charge compensating anions affect the CO 2 adsorptive capacity of various hydrotalcite-like compounds (HTlcs). Physical and chemical properties of the HTlcs were evaluated using a number of methods and the CO 2 adsorption rate and capacity were measured at elevated temperature (603 K). The results showed that the synthetic analogue of the naturally occurring hydrotalcite mineral, [Mg 0.73 Al 0.27 (OH) 2 ](CO 3 ) 0.13 · xH 2 O, had the best overall adsorption capacity and kinetics. The stability of the adsorption capacity was tested by subjecting the model HTlc to 10 equilibrium adsorption and desorption cycles. At the end of the cycle, the HTlc had maintained approximately sixty-five percent of its initial capacity. Temperature programmed desorption of CO 2 was used to quantify the surface basicity of the various HTlcs. The results showed that the reversible physisorption portion of the CO 2 isotherm was correlated to the number of surface basic sites on the HTlcs.

Li-X zeolite SirAls 1.0 is currently the best sorbent for use in the separation of air by adsorption processes. Sil®er is also known to strongly affect the adsorpti®e properties of zeolites and thermal ®acuum dehydration of sil®er zeolites leads to the formation of sil®er clusters within the zeolite. In this work we synthesized type X zeolites containing ®arying mixtures of Li and Ag. Adding ®ery small amounts of Ag at the proper dehydration conditions formed sil®er clusters and enhanced adsorpti®e characteristics and energetic heterogeneity, as compared to those of the near fully exchanged Li q -zeolites. The performance for air separation by the best of these sorbents, containing, on a®erage, only one Ag per unit cell, was compared to that of the near fully Li q -exchanged zeolite using a standard PSA cycle by numerical simulation. The results show that the new ( ) sorbent pro®ides a significantly higher ) 10% product throughput at the same product purity and reco®ery when compared to that of the near fully Li q -exchanged zeolite. from the LDF expression. As shown below, Eq. 9 satisfies this requirement only when ns 2 Ž . and 5. Moreover, an additional requirement for A t is that the negative portion of the concentration profile near r s 0 at short times is minimized.