Bruce J. Tatarchuk

Review of Experimental Characterization of Active Sites and Determination of Molecular Mechanisms of Adsorption, Desorption and Regeneration of the Deep and Ultradeep Desulfurization Sorbents for Liquid Fuels

This review analyzes the recent literature on the direct experimental determination of molecular and atomic-level nature of adsorption sites, mechanisms of adsorption under the mild (close to ambient) conditions, desorption of sulfur-aromatic compounds, and surface chemical reactions upon regeneration of the “spent” sorbents for desulfurization of the liquid fuels. Supported noble, transition metals and metal oxides, binary, and ternary metal oxides, activated carbons, zeolites, supported polymers, pervaporation membranes and other advanced adsorptive desulfurization materials are discussed. The recent trends in developing the deep and ultradeep desulfurization sorbents are discussed, and challenges of the direct determination of molecular mechanisms are described.

Pillared-clay catalysts containing mixed-metal complexes I. Preparation and characterization

Abstract High-surface-area pillared clays were prepared from naturally occurring montmorillonites by exchanging interlayer ions with polyoxocations containing (i) iron, (ii) aluminum, (iii) discrete mixtures of (i) and (ii), or (iv) iron and aluminum located within the same complex. The valence state, solid-state properties, and stability of these pillars were determined following reduction and oxidation using Mossbauer spectroscopy, X-ray diffraction, and BET surface area measurements. Controlled atmosphere electron microscopy and transmission electron microscopy were also used to follow the nucleation and sintering behavior of the pillars during reduction. Mossbauer data suggested interlayer formation of metallic iron domains following reduction of types (i) and (iii) pillared systems. The magnetic properties and the oxidation behavior deduced from Mossbauer analysis and the complementary insights provided by XRD strongly indicated that these crystallites were in the form of thin-film/pancake-shape islands most likely conforming to the geometry of the interlayer region. Reduced domains remained accessible to the gas phase and in some cases resisted sintering during reduction/oxidation cycles. Reduction of the iron phase could be enhanced by addition of platinum to the sample. The absence of Mossbauer features attributable to FePt alloys and the onset of iron reduction, from Fe3− to Fe2+, at room temperature suggested that reduction was facilitated by hydrogen spillover from platinum. The expanded structures of types (ii) and (iii) pillared systems were found to be relatively stable following reduction up to 723 K due to the irreducible nature of discrete aluminum pillars under these conditions. At appropriate iron pillar to aluminum pillar ratios, results obtained from type (iii) pillared systems also indicated that at least one monolayer of Fe2+ was preferentially decorated/accommodated at the surfaces of the aluminum oxide pillars. This behavior was attributed to the relatively stronger interaction of iron with alumina than with silica and was triggered at temperatures ≤673 K by introducing platinum, and presumably hydrogen atoms, to the specimen. On the basis of the findings noted above, intercalation of clays with mixtures of chemically distinct pillars appears to provide a unique method for preparing highly dispersed metallic or even bimetallic catalysts possessing two-dimensional sieve-like behavior with high overall surface areas and high loadings of the active metal.

Modeling double-layer capacitor behavior using ladder circuits

The double-layer capacitor (DLC) is a very complex device that is best represented by a distributed parameter system. Many different lumped-parameter equivalent circuits have been proposed for the DLC. An examination into utilizing a ladder circuit to model a DLC is presented. Parameters for different ladder circuits are determined from AC impedance data. Variations in circuit parameters with DC bias and manufacturing have been investigated. The performance of the ladder circuit has been evaluated in slow discharge and pulse load applications.

Physical characterization of FeTiO2 model supported catalysts: I. Electron microscopic studies of reduction behavior

The morphology of FeTiO2 model supported catalysts has been monitored using transmission electron microscopy following hydrogen reduction at progressively higher temperatures. Nucleation and growth of iron particles from an initially contiguous iron overlayer was observed in the temperature range 608 to 707 K. Following reduction at 773 K, the smaller iron crystallites (less than 10 nm in size) were observed to spread over or wet the support surface, and reduction at temperatures of 875 K or higher extended this spreading behavior to larger crystallites. In addition, reduction at these latter temperatures caused decreases in the average iron particle size and the amount of iron present on the support as distinct iron crystallites. This can be attributed to a diffuse spreading of iron over the support or a diffusion of iron into the support.

Wet layup and sintering of metal-containing microfibrous composites for chemical processing opportunities

Abstract A new class of composite materials is prepared using traditional high speed and low cost paper making equipment and techniques. In this process, μm diameter metal fibers in a variety of compositions and alloys are slurried in an aqueous suspension with cellulose fibers and other selected particulates and/or fibers. The resulting mixture is then cast into a preform sheet using a wetlay process and dried to create a sheet or roll of preform material. Subsequent sintering of the preform at elevated temperatures (ca. 1000°C) removes the cellulosic binder/pore former and entraps the selected particulates/fibers within a sinter-locked network of conductive metal fibers. Unique physical properties are obtained in terms of: void volume, thermal/electrical conductivity, porosity, surface area, permeability, particle size, layer thickness, etc. To a first approximation these composites possess averaged physical properties over heretofore unavailable regions located between those of the entrapped component and those of the high void volume sintered metal carrier. For chemical processing applications the high void volume of the metallic binder/carrier (i.e. 20–99%) facilitates intralayer heat and mass transport while the ability to trap very small particulates (using the novel pore size-void volume relationship of microfibrous carriers) greatly reduces intraparticle heat and mass transport. A description of the unique and fundamental structure-property relationships and behaviors of these materials will be presented and contrasted with those of the more traditional engineering approaches and practices using fused particulates and pasted-carriers. Opportunities for significant steady-state volumetric processing improvements result when one must balance the competing demands of chemical kinetics (e.g. at the entrapped particulates) with those of the required transport processes (i.e. via the interparticle and intralayer voidage). Examples of beneficial processing applications and opportunities will be discussed in: (a) heat transfer materials, (b) catalysts and sorbents, (c) electrochemical processing and (d) filtration.

Permeability of sintered microfibrous composites for heterogeneous catalysis and other chemical processing opportunities

Abstract Microstructured materials have potential for enhanced mass and heat transfer compared to typical catalyst particulates used in industrial processes. The pressure drop through catalyst-containing materials is a very important reactor design consideration. A model equation to predict porous media permeability (PMP) over the entire range of possible bed voidages is extended to predict properties of sintered metal meshes. A correlation of data for sintered meshes of nickel fibers is presented in the form of a Kozeny constant form drag plot. Comparison of predictions by the PMP equation with data taken on a sintered composite fiber/particle mesh is presented. Use of the PMP equation as a design tool for optimization of media for adsorbents, catalysts, and filters is discussed.

Activated chemisorption of hydrogen on supported ruthenium: II. Effects of crystallite size and adsorbed chlorine on accurate surface area measurements

Abstract A crystallite size effect has been observed during the activated adsorption of hydrogen on supported ruthenium catalysts. This effect does not occur in the absence of adsorbed chlorine and is increasingly more pronounced as metal dispersion is increased. Activation energies for adsorption, at constant hydrogen coverage, may be as high as 16 kcal/mole and have been measured utilizing (i) Al 2 O 3 and SiO 2 support materials, (ii) crystallites which range in size from 2.6 to 22.0 nm, and (iii) surface chlorine coverages which vary from 0.01 to 0.35 of the number of surface ruthenium atoms. Results of these studies suggest that preferential adsorption of chlorine atoms at high coordination sites reduces electron density at adjacent low coordination sites, thereby increasing the activation energy barrier for electron donation to, and dissociative chemisorption of, incoming hydrogen molecules. While this effect may be viewed as primarily electronic in origin, a structure-sensitive (i.e., crystallite size-dependent) mechanism has been proposed to account for the observed adsorption behavior based on the number of high coordination-chlorine adsorption sites in close proximity to low coordination-hydrogen adsorption sites. Activated adsorption sites, attributed to the above-noted mechanism, can be responsible for a greater than two-fold underestimation of the number of surface ruthenium atoms measured by irreversible hydrogen adsorption at 298 K. Significant differences in adsorption behavior between silica- and alumina-supported crystallites of equal dispersion also suggest that deliberate addition of chlorine adatoms may provide a sensitive probe for discriminating differences in crystallite shape and surface texture.

Physical characterization of FeTiO2 model supported catalysts: II. Electron spectroscopic studies of reduction behavior

X-Ray photoelectron spectroscopy (XPS) and conversion electron Mossbauer spectroscopy (CEMS) have been used to characterize FeTiO2 model supported catalysts following hydrogen reductions at progressively higher temperatures. Reduction at temperatures near 707 K converted the initially present Fe3+ and Fe2+ into metallic iron. After further treatment at 773 K, the metallic iron peaks in the Mossbauer spectrum were observed to broaden, consistent with the spreading of iron crystallites over the support. Upon reduction at temperatures of ca. 875 K or higher, decreases were seen in the XPS and CEMS spectral iron areas. In addition, CEMS indicated the formation of a new, metallic species that did not show magnetic hyperfine splitting at room temperature, and XPS indicated that part of the Ti4+ was reduced to lower valence states. These results suggest that hydrogen treatment at these temperatures leads to a reduction of the support and to the diffusion of iron into the support, as a dispersed and strongly interacting species (e.g., γ-Fe; FexTi, 1 ≲ x ≲ 2).

Physical characterization of FeTiO2 model supported catalysts: III. Combined electron microscopic and spectroscopic studies of reduction and oxidation behavior

The combination of results from transmission electron microscopy, X-ray photoelectron spectroscopy, and conversion electron Mossbauer spectroscopy are used to summarize and discuss the behavior of FeTiO2 model supported catalytic specimens following hydrogen and oxygen treatments at progressively higher temperatures. During hydrogen treatment, initial iron overlayers (ca. 5 nm thick) on TiO2 undergo reduction, nucleation, and growth to small metallic iron particles at temperatures from 608 to 707 K; these iron crystallites spread over (or wet) the titania support at 773 K, forming particles with a “thin-crystal” morphology; and at 875 K, iron facilitates reduction of titania, accompanied by the diffusion of iron into the support. Following this high-temperature reduction, samples were treated in oxygen at ca. 950 K. Some of the iron that had diffused into the support returned to the surface. This high-temperature oxidation does not, however, simply reverse the effect of high-temperature reduction. Instead, the iron is converted into large particles of FeTi2O5. The lack of reversibility during sequential hydrogen and oxygen treatments at high temperatures is attributed to strong interactions between iron and titanium, manifested by the formation of dispersed and strongly interacting iron (e.g., γ-Fe; FexTi, 1 ≲ x ≲ 2) or FeTi2O5 under reducing or oxidizing conditions, respectively.

Hydrogenation and hydrodesulfurization over sulfided ruthenium catalysts. II: Impact of surface phase behavior on activity and selectivity

Abstract Thiophene hydrodesulfurization (HDS) has been studied over sulfided Ru/γ-Al 2 O 3 and CoMo/γ-Al 2 O 3 catalysts using a microreactor operated at 101 kPa and 548–623 K. The activity and selectivity of thiophene HDS over ruthenium catalysts depended on the presulfidization procedures, yet similar trends were not observed over CoMo/γ-Al 2 O 3 catalysts. Ruthenium catalysts sulfided in 100% H 2 S at 673 K possessed ca. sevenfold higher thiophene conversion rates than CoMo/Al 2 O 3 when compared per square meter of active area. Thiophene HDS rates averaged over oxygen titratable sites were ca. twofold higher on Ru/Al 2 O 3 catalysts than on CoMo/Al 2 O 3 specimens when compared per oxygen titratable site. Mild presulfidization in 101 kPa of 10% H 2 S/H 2 at T ≤ 673 K or in 101 kPa of 100% H 2 S at T ≤ 523 K, provided surfaces (i) retaining partial monolayers of adsorbed sulfur as evidenced by microgravimetry, XPS, and pulse oxygen adsorption and (ii) catalyzing direct hydrogenolysis to C 4 products and H 2 S. Extensive presulfidization in 101 kPa of ≥ 80% H 2 S/H 2 at T ≥ 673 K leads to sulfur incorporation into the bulk and formation of crystalline RuS 2 at the surface. The RuS 2 -like surface produced approximately equal quantities of C 4 hydrogenolysis products and tetrahydrothiophene. S (a) /Ru (s) ratios observed by microgravimetry, XPS, and pulse oxygen adsorption were found to depend on the presulfidization conditions. S (a) /Ru (s) ratios could be reversibly altered by appropriate resulfidization/annealing procedures and were found to correlate well with thiophene HDS selectivity but not with the presence of RuS 2 in the bulk. The above-noted trends are believed to be the result of a phase transformation dictated by surface thermodynamic driving forces. Calculations and comparisons of thiophene selectivity versus presulfidization conditions indicate that Gibbs free energies of + 7 kJ/mole are required to form RuS 2 at the surface compared to values of −59 kJ/mole in the bulk. The instability of the RuS 2 surface may be indicative of a generalized surface thermodynamic criterion applicable to other pyrite sulfides. RuS 2 surfaces were observed to chemisorb much larger quantities of reversibly and irreversibly bound hydrogen which may affect the competition between direct hydrogenation and direct hydrogenolysis pathways, thereby controlling the selectivity of thiophene conversion.