Determination of intermediates and mechanism for soot combustion with NOx/O₂ on potassium-supported Mg-Al hydrotalcite mixed oxides by in situ FTIR

The soot combustion with NO(x) and/or O(2) on potassium-supported Mg-Al hydrotalcite mixed oxides under tight contact condition was studied using temperature-programmed oxidation (TPO), isothermal reaction and in situ FTIR techniques. The presence of NO(x) in O(2) favors the soot combustion at lower temperatures (<300 °C). However, a little suppression was observed at higher temperatures (>300 °C), which was accompanied by a substantial NO(x) reduction. The ketene (C═C═O) and isocyanate (NCO(-)) species were determined as the reaction intermediates. In NO(x) + O(2), NO(2) directly interacts with the free carbon sites (C═C*) through two parallel reactions: (1) NO(2) + C═C* → C═C═O + NO; (2) NO(2) + C═C* → NCO(-) + CO(2). The two reactions can proceed easily, which accounts for the promotion effect of NO(x) on soot combustion at lower temperatures. The further oxidation of NCO(-) by NO(2) or O(2) is responsible for the simultaneous reduction of NO(x). However, the reactions between NO(2) and C═C* are limited by the amount of free carbon sites, which can be provided by the oxidation of soot by O(2) at higher temperatures. The interaction of NO(x) and catalyst results in the formation of nitrates and nitrites, which poisoned the active K sites.     

Studies on the surface chemistry based on competive adsorption of NOx-SO2 onto a KOH impregnated activated carbon in excess O2

This study identifies surface chemistry characteristics based on competitive behavior in the simultaneous adsorption behavior of NOx (NO rich) and SO2 using KOH impregnated activated carbon (K-IAC) in excess O2. The NOx and SO2 adsorption on K-IAC occurred mainly through the acid-base reaction. The high surface area with many pores of activated carbon acted as storage places of oxide crystal produced from NOx and SO2 adsorption. KOH, an impregnant, provided the selective adsorption sites to NOx and SO2, enabling simultaneous adsorption. However, larger amounts of SO2, with higher adsorption affinity to K-IAC compared to NOx, were adsorbed in a NOx/SO2 coexistent atmosphere. Oxygen was chemisorbed to K-IAC, which enhanced the selective adsorptivity for NO. In binary-component adsorption of NOx and SO2 on K-IAC, oxide crystals such as KNO, (x = 2,3) and K2SOx (x = 3,4) were dominantly formed through two different adsorption mechanisms by chemical reacton. Depending on the extent that oxide crystals blocked pores, compositions of oxide crystals were distributed differently according to depth.     

Mechanistic characterization of adsorption and slow desorption of phenanthrene aged in soils

Long-term adsorption of phenanthrene to soils was characterized in a silt-loam (LHS), a sandy soil (SBS), and a podzolized soil (CNS) by use of the Polanyi-Manes model, a Langmuir-type model, and a black carbon-water distribution coefficient (K(BC)) at a relative aqueous concentration (C(e)/S(w)) of 0.002-0.32. Aqueous desorption kinetic tests and temperature-programmed desorption (TPD) were also used to evaluate phenanthrene diffusivities and desorption activation energies. Adsorption contribution in soils was 48-70% after 30 days and 64-95% after 270 days. Significant increases in adsorption capacity with aging suggest that accessibility of phenanthrene to fractions of SBS soil matrix was controlled by sorptive diffusion at narrow meso- and micropore constrictions. Similar trends were not significant for LHS silt-loam or CNS podzol. Analysis of TPD profiles reveal desorption activation energies of 35-53 kJ/mol and diffusivities of 1.6 x 10(-7-)9.7 x 10(-8) cm2/s. TPD tests also indicate that the fraction of phenanthrene mass not diffusing from soils was located within micropores and narrow width mesopores with a corresponding volume of 1.83 x 10(-5-)6.37 x 10(-5) cm3/g. These values were consistent with the modeled adsorption contributions, thus demonstrating the need for such complimentary analytical approach in the risk assessment of organic contaminants.     

Effect of natural organic substances on the surface and adsorptive properties of environmental black carbon (char): pseudo pore blockage by model lipid components and its implications for N2-probed surface properties of natural sorbents

Black carbon (BC; char and soot) particles emitted to the environment typically are formed with high microporosity and surface area, properties that are responsible for their presumed important role in adsorption of anthropogenic organic compounds in soils and sediments. An issue that has received little direct attention is the possibility that naturally occurring organic matter attenuates the surface activity of BC. We found that simulated "aging" of prepared wood char particles in a soil-water suspension leads to a strong decline in char total surface area (TSA) by N2 adsorption at 77 K with BET analysis and a more modest decline in affinity for dissolved benzene. To help determine the underlying cause, we measured the effects of adsorbed natural lipids or lipid fractions of humic substances, modeled by triglycerides of a commercial vegetable oil. With increasing lipid loading (up to 40% by char weight) from aqueous mixtures, N2 TSA was strongly suppressed (up to 100-fold), while CO2 cumulative surface area (CSA, 0-1.4 nm) at 273 K and benzene adsorption at 293 K were hardly affected. In addition, the rate of CO2 adsorption was retarded. We propose that externally adsorbed lipid molecules occupy pore throats with access to interior pore networks. At 77 K, as opposed to the higher temperatures, lipid chains are too inflexible to allow passage of adsorbate. It is concluded that benzene adsorption to char predominates at interior pore sites and does not correlate with N2-probed micropore properties when the char accrues pore-blocking substances from the surroundings. The findings question the suitability of N2 for probing hydrophobic microporosity of BC in soils and sediments.     

Removal of PAH compounds from liquid fuels by Pd catalysts

Palladium catalysts supported on gamma-alumina (AN, AS), amorphous silica-alumina (ASA), and beta-zeolite (betaZ) were prepared with the aim to reduce the content of polycyclic aromatic hydrocarbons (PAHs) in diesel fuels. The removal of PAH compounds was evaluated with a model feed (toluene, naphthalene, and dibenzothiophene)that approached the composition of diesel fuel. The catalysts were characterized by N2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy, transmission electron microscopy, temperature-programmed reduction/temperature-programmed oxidation, Fourier transform (FT) IR of absorbed CO, and diffuse reflectance FT spectroscopy of adsorbed NH3. When palladium was supported on ASA instead of AN or betaZ, the intrinsic activity improved considerably. Such behavior is discussed in terms of larger Br?nsted acidity and the degree of reduction of the Pd species on Pd/ASA. For the 2.2 Pd/betaZ catalyst, interaction of the Pd clusters with zeolite protons led to formation of Pddelta+ species, which are not active in the hydrogenation of aromatics. All catalysts were resistant to poisoning by S compounds.     

Modeling adsorption-desorption processes of Cu on edge and planar sites of montmorillonite

The effect of the ionic strength on adsorption of Cu on calcium montmorillonite was studied at concentrations ranging from 31 to 516 microM. An adsorption model was employed in the analysis of the data. When the background electrolyte was NaClO4, the ionic exchange was suppressed at 0.5 M concentration, and Cu adsorption took place on edge sites, reaching a plateau at about 24 mmol/kg. A further increase in ionic strength did not have any effect on Cu adsorption, suggesting that the heavy metal was adsorbed by inner-sphere complexes on the edge sites of the clay. A binding coefficient for Cu2+ on the edge sites K = 2 x 10(4) M(-1) was determined, indicating very high affinity of Cu2+ for these sites. When the electrolyte used was NaCl, the amounts of Cu adsorbed were reduced. The model predicted well the adsorption data by considering the adsorption of CuCl+ species. Adsorption-desorption processes of Cu on calcium montmorillonite in media of 0.01 and 0.1 M NaCl showed hysteresis. Model calculations also predict the desorption fairly well. According to the model, the hysteresis is mainly attributed to the heterogeneity of sites for the adsorption of Cu. The hysteresis arising from the planar sites is largely due to reduced competition of ion species for adsorption and enhancement in the magnitude of the surface potential.     

Effects of surface features on sulfur dioxide adsorption on calcined NiAl hydrotalcite-like compounds

The hydrotalcite-based NiAl mixed oxides were synthesized by coprecipitation and urea hydrolysis approaches and employed for SO? removal. The samples were well characterized by inductively coupled plasma (ICP) elemental analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and N? adsorption/desorption isotherm analyses. The acid-base properties were characterized by pyridine chemisorption and CO? temperature-programmed desorption (TPD). The calcined NiAlO from the urea method showed excellent SO? adsorption and its adsorption equilibrium showed a type I isotherm, which significantly improved the adsorption performance for low-concentration SO?. Both the physical structure and the acidic-basic sites were found to play important roles in the SO? adsorption process. In situ Fourier transform infrared spectroscopy (FTIR) investigation revealed that adsorbed SO? molecules formed surface bisulfite, sulfite, and bidentate binuclear sulfate. The mechanisms for SO? adsorption and transformation are discussed in detail.     

Efficient catalytic decomposition of CO2 to CO and O2 over Pd/ mixed-conducting oxide catalyst in an oxygen-permeable membrane reactor

The thermal decomposition of CO2 to CO and O2 is a potential route for the consumption and utilization of CO2. However, this reaction is limited by both the thermodynamic equilibrium and the kinetic barrier. In this study, we reported an innovative catalytic process to decompose CO2 in an oxygen-permeation membrane reactor packed with a mixed-conducting oxide supported noble metal catalyst, or Pd/SrCo0.4Fe0.5Zr0.1O3-delta (Pd/ SCFZ), which is of high activity in the decomposition of CO2 into CO and O2. Pd/SCFZ catalyst was prepared by incipient wetness impregnation of the SCFZ powders with an aqueous solution of PdCl2, and the CO2 sorption/desorption property was examined by in situ Fourier transform infrared spectroscopy and temperature-programmed desorption-mass spectrometry technologies. It was shown that there appeared a typical of bridged carbonyls (Pd-CO) on the surface of the Pd/SCFZ catalyst formed after CO2 decomposition. Both CO2 and CO could be detected in the species desorbed from Pd/SCFZ catalyst, which implied that the Pd/SCFZ catalyst could effectively activate the CO2 molecule. During the catalytic process, furthermore, the activity of the Pd/SCFZ catalyst can self-regenerate by removing the produced lattice oxygen through the dense oxygen-permeable ceramic membrane. At 900 degrees C, this catalytic process attains 100% of CO formation selectivity at 15.8% of CO2 conversions.     

Remote identification and quantification of industrial smokestack effluents via imaging Fourier-transform spectroscopy

Industrial smokestack plume emissions were remotely measured with a midwave infrared (1800-3000 cm(-1)) imaging Fourier-transform spectrometer operating at moderate spatial (128 × 64 with 19.4 × 19.4 cm(2) per pixel) and high spectral (0.25 cm(-1)) resolution over a 20 min period. Strong emissions from CO(2), H(2)O, SO(2), NO, HCl, and CO were observed. A single-layer plume radiative transfer model was used to estimate temperature T and effluent column densities q(i) for each pixel's spectrum immediately above the smokestack exit. Across the stack, temperature was uniform with T = 396.3 ± 1.3 K (mean ± stdev), and each q(i) varied in accordance with the plume path length defined by its cylindrical geometry. Estimated CO(2) and SO(2) volume fractions of 8.6 ± 0.4% and 380 ± 23 ppm(v), respectively, compared favorably with in situ measurements of 9.40 ± 0.03% and 383 ± 2 ppm(v). Total in situ NO(x) concentration (NO + NO(2)) was reported at 120 ± 1 ppm(v). While NO(2) was not spectrally detected, NO was remotely observed with a concentration of 104 ± 7 ppm(v). Concentration estimates for the unmonitored species CO, HCl, and H(2)O were 14.4 ± 0.3 ppm(v), 88 ± 1 ppm(v), and 4.7 ± 0.1%, respectively.     

Carbon dioxide capture by functionalized solid amine sorbents with simulated flue gas conditions

A novel solid amine sorbent was prepared using KIT-6-type mesoporous silica modified with tetraethylenepentamine (TEPA). Its adsorption behavior toward CO(2) from simulated flue gases is investigated using an adsorption column. The adsorption capacities at temperatures of 303, 313, 333, 343, and 353 K are 2.10, 2.29, 2.58, 2.85, and 2.71 mmol g(-1), respectively. Experimental adsorption isotherms were obtained, and the average isosteric heat of adsorption was 43.8 kJ/mol. The adsorption capacity increases to 3.2 mmol g(-1) when the relative humidity (RH) of the simulated flue gas reaches 37%. The adsorption capacity is inhibited slightly by the presence of SO(2) at concentrations lower than 300 ppm but is not significantly influenced by NO at concentrations up to 400 ppm. The adsorbent is completely regenerated in 10 min at 393 K and a pressure of 5 KPa, with expected consumption energy of about 1.41 MJ kg(-1) CO(2). The adsorption capacity remains almost the same after 10 cycles of adsorption/regeneration with adsorption conditions of 10 vol % CO(2), 100 ppm SO(2), 200 ppm NO, 100% relative humidity, and a temperature of 393 K. The solid amine sorbent, KIT-6(TEPA), performs excellently for CO(2) capture and its separation from flue gas.