The interactions of coal with CO2 and its effects on coal structure

The interactions of coal with CO2 at pressures of up to 30 bar concerning mechanisms of diffusion, the strength of interactions, and the irreversibility of uptake for the permanent disposal of CO2 into coal fields have been studied. Differential scanning calorimetry was used to investigate coal/CO2 interactions for North Dakota, Wyodak, Illinois No. 6, and Pittsburgh No. 8 coals. It was found that the first interactions of CO2 with coals led to strongly bound carbon dioxide on coal. Energy values attributed to the irreversible storage capacity for CO2 on coals were determined. The lowest irreversible sorption energy was found for North Dakota coal (0.44 J/g), and the highest value was for the Illinois No. 6 coal (8.93 J/g). The effect of high-pressure CO2 on the macromolecular structure of coal was also studied by means of differential scanning calorimetry. It was found that the temperature of the second-order phase transition of Wyodak coal decreases with an increase in CO2 pressure significantly, indicating that high-pressure CO2 diffuses through the coal matrix, causes significant plasticization effects, and changes the macromolecular structure of the Wyodak coal. Desorption characteristics of CO2 from the Pittsburgh No. 8 coal were studied by temperature-programmed desorption mass spectrometry. It was found that CO2 desorption from the coal is an activated process and follows a first-order kinetic model. The activation energy for CO2 desorption from the Pittsburgh No. 8 coal increased with the preadsorbed CO2 pressure, indicating that CO2 binds more strongly and demands more energy to desorb from the Pittsburgh No. 8 coal at higher pressures.

Thermodynamical studies of irreversible sorption of CO2 by Wyodak coal

Differential scanning calorimetry (DSC), temperature programmed desorption mass spectrometry (TPD-MS) and small angle neutron scattering (SANS) were used to investigate CO2 uptake by the Wyodak coal. The adsorption of carbon dioxide on Wyodak coal was studied by DSC. The exotherms evident at low temperatures are associated with the uptake of CO2 suggesting that carbon dioxide interacts strongly with the coal surface. The reduction in the value of the exotherms between the first and second runs for the Wyodak coal suggests that some CO2 is irreversibly bound to the structure even after heating to 200 °C DSC results also showed that adsorption of CO2 on the coal surface is an activated process and presumably at the temperature of the exotherms there is enough thermal energy to overcome the activation energy for adsorption. The adsorption process is instantly pursued by much slower diffusion of the gas molecules into the coal matrix (absorption). Structural rearrangement in coal by CO2 is examined by change in the glass transition temperature of coal after CO2 uptake at different pressures. The amount of gas dissolved in the coal increases with increasing CO2 pressure. TPD-MS showed that CO2 desorption from the Wyodak coal follows a first order kinetic model. Increase in the activation energy for desorption with pre-adsorbed CO2 pressure suggests that higher pressures facilitate the transport of CO2 molecules through the barriers therefore the amount of CO2 uptake by the coal is greater at higher pressures and more attempts are required to desorb CO2 molecules sorbed at elevated pressures. These conclusions were further confirmed by examining the Wyodak coal structure in high pressure CO 2 by SANS.

Acidity Characterization of Amorphous Silica-Alumina

Surface characterization of amorphous silica-alumina (ASA) by COads IR, pyridine(ads) IR, alkylamine temperature-programmed desorption (TPD), Cs+ and Cu(EDA)(2)(2+) exchange, H-1 NMR, and m-xylene isomerization points to the presence of a broad range of Bronsted and Lewis acid sites. Careful interpretation of IR spectra of adsorbed CO or pyridine confirms the presence of a few very strong Bronsted acid sites (BAS), typically at concentrations lower than 10 mu mol/g. The general procedure for alkylamine TPD, which probes both Bronsted and Lewis acidity, is modified to increase the selectivity to strong Bronsted acid sites. Poisoning of the m-xylene isomerization reaction by a base is presented as a novel method to quantify strong BAS. The surface also contains a weaker form of BAS, in concentrations between 50 and 150 mu mol/g, which can be quantified by COads IR Cu(EDA)(2)(2+) exchange also probes these sites. The structure of these sites remains unclear, but they might arise from the interaction of silanol groups with strong Lewis acid Al3+ sites. The surface also contains nonacidic aluminol and silanol sites (200-400 mu mol/g) and two forms of Lewis acid sites: (i) a weaker form associated with segregated alumina domains containing five-coordinated Al, which make up the interface between these domains and the ASA phase and (ii) a stronger form, which are undercoordinated Al sites grafted onto the silica surface. The acid catalytic activity in bifunctional n-heptane hydroconversion correlates with the concentration of strong BAS. The influence of the support electronegativity on the neopentane hydrogenolysis activity of supported Pt catalysts is considerably larger than that of the support Bronsted acidity. It is argued that strong Lewis acid sites, which are present in ASA but not in gamma-alumina, are essential to transmit the Sanderson electronegativity of the oxide support to the active Pt phase.

Reactions of nitrogen and oxygen surface groups in nanoporous carbons under inert and reducing atmospheres

The reactions of surface functional groups have an important role in controlling conversion of char nitrogen to NOx during coal combustion. This study involved an investigation of the thermal stability and reactions of nitrogen surface functional groups in nanoporous carbons. Four suites of carbons, which were used as models for coal chars, were prepared with a wide range of nitrogen and oxygen contents and types of functional groups. The porous structures of the carbons were characterized by gas adsorption methods while chemical analysis, X-ray photoelectron spectroscopy, and X-ray near edge structure spectroscopy were used to characterize the surface functional groups. Temperature programmed desorption and temperature programmed reduction methods were used to study the reactivity of the surface functional groups during heat treatment under inert and reducing conditions. Heat treatment studies show that the order of stability of the functional groups is quaternary nitrogen > pyridinic > pyrrolic > pyridine N-oxide. Pyridine N-oxide surface groups desorb NO and form N-2 via surface reactions at low temperature. Pyrrolic and pyridinic functional groups decompose and react with surface species to give NH3, HCN, and N-2 as desorption products, but most pyrrolic groups are preferentially converted to pyridinic and quaternary nitrogen. The main desorption product is N-2. Approximately 15-40 wt % of the original nitrogen was retained in the carbons mainly as quaternary nitrogen after heat treatment to 1673 K. The results are discussed in terms of decomposition ranges for surface functional groups and reaction mechanisms of surface species.

Competitive adsorption of aqueous metal ions on an oxidized nanoporous activated carbon\

Competitive adsorption is the usual situation in real applications, and it is of critical importance in determining the overall performance of an adsorbent. In this study, the competitive adsorption characteristics of all the combinations of binary mixtures of aqueous metal ion species Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) on a functionalized activated carbon were investigated. The porous structure of the functionalized active carbon was characterized using N-2 (77 K) and CO2 (273 K) adsorption. The surface group characteristics were examined by temperature-programmed desorption, Fourier transform infrared spectroscopy, Raman spectroscopy, acid/base titrations, and measurement of the point of zero charge (pH(PZC)). The adsorption of aqueous metal ion species M2+(aq), on acidic oxygen functional group sites mainly involves an ion exchange mechanism. The ratios of protons displaced to the amount of M2+(aq) metal species adsorbed have a linear relationship for both single-ion and binary mixtures of these species. Hydrolysis of metal species in solution may affect the adsorption, and this is the case for adsorption of Hg2+(aq) and Pb2+(aq). Competitive adsorption decreases the amounts of individual metal ions adsorbed, but the maximum amounts adsorbed still follow the order Hg2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq) obtained for single metal ion adsorption. The adsorption isotherms for single metal ion species were used to develop a model for competitive adsorption in binary mixtures, involving exchange of ions in solution with surface proton sites and adsorbed metal ions, with the species having different accessibilities to the porous structure. The model was validated against the experimental data.

Adsorption of metal ions on nitrogen surface functional groups in activated carbons

A commercially available coconut-shell-derived active carbon was oxidized with nitric acid, and both the original and oxidized active carbons were treated with ammonia at 1073 K to incorporate nitrogen functional groups into the carbon. An active carbon with very high nitrogen content (similar to9.4 wt % daf) was also prepared from a nitrogen-rich precursor, polyacrylonitrile (PAN). These nitrogen-rich carbons had points of zero charge (pH(pzc)) similar to H-type active carbons. X-ray absorption near-edge structure (XANES) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and temperature-programmed desorption (TPD) were used to characterize the nitrogen functional groups in the carbons. The nitrogen functional groups present on the carbon surface were pyridinic, pyrrolic (or indolic), and pyridonic structures. The adsorption of transition metal cations Cd2+, Ni2+, and Cu2+ from aqueous solution on the suite of active carbons showed that adsorption was markedly higher for carbons with nitrogen functional groups present on the surface than for carbons with similar pH(pzc) values. In contrast, the adsorption characteristics of Ca2+ from aqueous solution were similar for all the carbons studied. Flow microcalorimetry (FMC) studies showed that the enthalpies of adsorption of Cd2+(aq) on the active carbons with high nitrogen contents were much higher than for nitric acid oxidized carbons studied previously, which also had enhanced adsorption characteristics for metal ion species. The enthalpies of adsorption of Cu2+ were similar to those obtained for Cd2+ for specific active carbons. The nitrogen functional groups in the carbons act as surface coordination sites for the adsorption of transition metal ions from aqueous solution. The adsorption characteristics of these carbons are compared with those of oxidized carbons.

A spectroscopic study of the chemistry and reactivity of SO2 on Pt{111}: Reactions with O-2, CO and C3H6

The chemisorption and reactivity of SO2 on Pt{111} have been studied by HREELS, XPS, NEXAFS and temperature-programmed desorption. At 160 K SO2 adsorbs intact at high coverages, with eta(2) S-O coordination to the surface. On annealing to 270 K, NEXAFS indicates the SO2 molecular plane essentially perpendicular to the surface. Preadsorbed O-a reacts with SO2 to yield adsorbed SO4, identified as the key surface species responsible for SO2-promoted catalytic alkane oxidation. Coadsorbed CO or propene efficiently reduce SO2 overlayers to deposit S-a, and the implications of this for catalytic systems are discussed.

Assessing the surface modifications following the mechanochemical preparation of a Ag/Al2O3 selective catalytic reduction catalyst

The surface modification of a mechanochemically prepared Ag/Al O catalyst compared with catalysts prepared by standard wet impregnated methods has been probed using two-dimensional T -T NMR correlations, HO temperature programmed desorption (TPD) and DRIFTS. The catalysts were examined for the selective catalytic reduction of NO using n-octane in the presence and absence of H. Higher activities were observed for the ball milled catalysts irrespective of whether H was added. This higher activity is thought to be related to the increased affinity of the catalyst surface towards the hydrocarbon relative to water, following mechanochemical preparation, resulting in higher concentrations of the hydrocarbon and lower concentrations of water at the surface. DRIFTS experiments demonstrated that surface isocyanate was formed significantly quicker and had a higher surface concentration in the case of the ball milled catalyst which has been correlated with the stronger interaction of the n-octane with the surface. This increased interaction may also be the cause of the reduced activation barrier measured for this catalyst compared with the wet impregnated system. The decreased interaction of water with the surface on ball milling is thought to reduce the effect of site blocking whilst still providing a sufficiently high surface concentration of water to enable effective hydrolysis of the isocyanate to form ammonia and, thereafter, N. This journal is © The Royal Society of Chemistry.

Sputtering of oxygen ice by low energy ions

Naturally occurring ices lie on both interstellar dust grains and on celestial objects, such as those in the outer Solar system. These ices are continuously subjected to irradiation by ions from the solar wind and/or cosmic rays, which modify their surfaces. As a result, new molecular species may form which can be sputtered off into space or planetary atmospheres. We determined the experimental values of sputtering yields for irradiation of oxygen ice at 10 K by singly (He+, C+, N+, O+ and Ar+) and doubly (C2 +, N2 + and O2 +) charged ions with 4 keV kinetic energy. In these laboratory experiments, oxygen ice was deposited and irradiated by ions in an ultra high vacuum chamber at low temperature to simulate the environment of space. The number of molecules removed by sputtering was observed by measurement of the ice thickness using laser interferometry. Preliminary mass spectra were taken of sputtered species and of molecules formed in the ice by temperature programmed desorption (TPD). We find that the experimental sputtering yields increase approximately linearly with the projectile ion mass (or momentum squared) for all ions studied. No difference was found between the sputtering yields for singly and doubly charged ions of the same atom within the experimental uncertainty, as expected for a process dominated by momentum transfer. The experimental sputter yields are in good agreement with values calculated using a theoretical model except in the case of oxygen ions. Preliminary studies have shown molecular oxygen as the dominant species sputtered and TPD measurements indicate ozone formation.

A laboratory study of water ice erosion by low-energy ions

Water ice covers the surface of various objects in the outer Solar system.Within the heliopause, surface ice is constantly bombarded and sputtered by energetic particles from the solar wind and magnetospheres. We report a laboratory investigation of the sputtering yield of water ice when irradiated at 10 K by 4 keV singly (13C+, N+, O+, Ar+) and doubly charged ions (13C2+, N2+, O2+). The experimental values for the sputtering yields are in good agreement with the prediction of a theoretical model. There is no significant difference in the yield for singly and doubly charged ions. Using these yields, we estimate the rate of water ice erosion in the outer Solar system objects due to solar wind sputtering. Temperature-programmed desorption of the ice after irradiation with 13C+ and 13C2+ demonstrated the formation of 13CO and 13CO2, with 13CO being the dominant formed species.

Effect of the carburization of MoO3-based catalysts on the activity for butane hydroisomerization

Catalysts based on molybdena (MoO3) reduced at mild temperatures are highly active and selective for the hydroisomerization of alkanes: however, further catalyst development has been hampered by the structural complexity of the material and the controversy regarding the nature of the active phase. The present work is aimed at determining the relationship between the content of carbon present in an oxycarbide phase and the activity for n-butane hydroisomerization. A series of temperature-programmed oxidation (TPO) and temporal analysis of product (TAP) data showed that the oxycarbidic carbon content is not related to the activity of the sample for the isomerization of n-butane to isobutane. The formation of a carbon-containing phase is, therefore, not crucial to obtain an active catalyst. This study also highlights the capability of the multi-pulse TAP technique to investigate structure-activity relationships over materials with readily variable atomic composition. (C) 2008 Elsevier B.V. All rights reserved.

New Cu-Based Catalysts Supported on TiO2 Films for Ullmann SNAr-Type C-O Coupling Reactions

New routes for the preparation of highly active TiO2-supported Cu and CuZn catalysts have been developed for C-O coupling reactions. Slurries of a titania precursor were dip-coated onto glass beads to obtain either structured mesoporous or non-porous titania thin films. The Cu and CuZn nanoparticles, synthesized using a reduction by solvent method, were deposited onto calcined films to obtain a Cu loading of 2 wt%. The catalysts were characterized by inductively coupled plasma (ICP) spectroscopy, temperature-programmed oxidation/reduction (TPO/TPR) techniques, Cu-63 nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy (S/TEM-EDX) and X-ray photo-electron spectroscopy (XPS). The activity and stability of the catalysts obtained have been studied in the C-O Ullmann coupling of 4-chloropyridine and potassium phenolate. The titania-supported nanoparticles retained catalyst activity for up to 12 h. However, catalyst deactivation was observed for longer operation times due to oxidation of the Cu nanoparticles. The oxidation rate could be significantly reduced over the CuZn/TiO2 catalytic films due to the presence of Zn. The 4-phenoxypyridine yield was 64% on the Cu/nonporous TiO2 at 120 degrees C. The highest product yield of 84% was obtained on the Cu/mesoporous TiO2 at 140 degrees C, corresponding to an initial reaction rate of 104 mmol g(cat)(-1) s(-1). The activation energy on the Cu/mesoporous TiO2 catalyst was found to be (144 +/- 5) kJ mol(-1), which is close to the value obtained for the reaction over unsupported CuZn nanoparticles (123 +/- 3 kJ mol(-1)) and almost twice the value observed over the catalysts deposited onto the non-porous TiO2 support (75 +/- 2 kJ mol(-1)).

Sulfated Zirconia Modified SBA-15 Catalysts for Cellobiose Hydrolysis

Zirconia modified SBA-15 becomes a very active catalyst for the selective hydrolysis of cellobiose to glucose after sulfation. Spectroscopic investigations indicate the presence of Bronsted acid sites with similar properties to those present in conventional sulfated zirconia. Indications are found that the sulfate groups attached to zirconia interact with silanol groups of SBA-15. The catalytic activity in cellobiose hydrolysis correlates well with results for temperature-programmed decomposition of i-propylamine for a range of sulfated ZrO2/SBA-15 catalysts. A glucose yield of 60% during cellobiose hydrolysis at a reaction time of 90 min at 160 degrees C is obtained.

Electronic, structural, and reactive properties of ultrathin aluminum oxide films on Pt(111)

Low-energy electron diffraction, X-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction spectrometry results are reported for the structural and reactive behavior of alumina films grown on Pt(111) as a function of thickness and oxidation temperature. Submonolayer Al films undergo compete oxidation at 300 K, annealing at 1100 K resulting in formation of somewhat distorted crystalline gamma-alumina, Thicker deposits require 800 K oxidation to produce Al2O3, and these too undergo crystallization at 800 K, yielding islands of apparently undistorted gamma-alumina on the Pt(111) surface. Oxidation of a p(2 x 2) Pt3Al surface alloy occurs only at>800 K, resulting in Al extraction, These alumina films on Pt(lll) markedly increase the coverage of adsorbed SO4 resulting from SO2 chemisorption onto oxygen-precovered surfaces. This results in enhanced propane uptake and subsequent reactivity relative to SO4/Pt(111). A bifunctional mechanism is proposed to account for our observations, and the relevance of these to an understanding of the corresponding dispersed systems is discussed.