Linda I. McLaughlin

What do dissolution experiments tell us about natural weathering

Abstract Much information about the earliest stages of weathering comes from laboratory experiments where the mineral surface chemistry is rigorously controlled and the solution composition is maintained far from equilibrium with the solid. The experiments show that the pathways for dissolution are similar to those for ligand exchange around dissolved metal complexes. One difference is that the rates of mineral dissolution are controlled by the concentrations of surface species while rates of ligand exchange are proportional to bulk concentration. Nevertheless, the correlation is so strong that rates of olivine dissolution are predictable from the rates of solvent exchange around the corresponding divalent metal in solution. However, this level of resolution is achieved by maintaining unnaturally high fluid/mineral ratios and by taking great care to avoid precipitation of secondary minerals. Natural olivine weathers in intimate contact with secondary minerals. Reactions commonly proceed in clay-filled channels only a few tens of Angstroms in diameter and secondary minerals are topotactic with olivine. Primary and secondary minerals are so intimately intergrown in these channels that the olivine surface chemistry is undoubtably influenced by overlapping electrostatic double layers. Smectite growth may have proceeded at near-saturation with the fluid, but the solution is in gross disequilibrium with the olivine. There nevertheless remains some qualitative consistency between weathering in the laboratory and in the field. The ligand-exchange model for bond cleavage, for example, predicts that the presence of ferric iron at the mineral surface will retard weathering rates. Correspondingly, defect planes of oxidized olivine (to produce Fe 3+ and a vacancy) weather at a much slower rate than unoxidized material. Thus, the appropriate level of comparison of field and laboratory weathering is at the scale of reactivity trends; it is unreasonable to expect quantitative similarity in reaction rates.

Chain transfer processes in dichlorosilane reductive polymerization and their control: A simplified route to high molecular weight polysilylenes

Polysilylenes are usually synthesized by a reductive coupling of diorganodichlorosilanes with molten sodium dispersions in an inert solvent, typically neat toluene or toluene mixed with a cosolvent such as heptane. The surface nature of this reaction leads to highly nonstatistical molecular weight distributions which are broad and poorly reproducible in the absence of rigid control over reaction parameters. Because the electronic and physical properties of polysilylenes are sensitive to molecular weight distribution, it is important to understand the factors which influence the molecular weight distribution obtained during preparation. In this work, we have examined the chain transfer processes in the Na-mediated reductive coupling of representative alkyl and aryl diorganodichlorosilanes by comparing rigorously controlled reactions carried out in toluene, a solvent normally thought to be non-chain-transferring in these reactions, to reactions carried out in benzene, which is thermodynamically incapable of undergoing chain transfer via hydrogen abstraction by a terminal polysilyl radical. We have identified for the first time a low-yield back-biting hydrogen abstraction process on the polysilylene side chain C-H bonds which generates an Si-H terminated polysilylene chain and a reactive site alpha to the silicon. The data also suggest the presence of a second back-biting reaction on the polysilylene Si-Si backbone bonds which produces cyclosilane by-products. An outgrowth of the work is a new modification of the Wurtz coupling process which routinely provides very high molecular weight polysilylenes by the safer and more convenient “normal” addition procedure.

Evaluation of hydrous titanium oxide-supported NiMo catalysts for pyrene hydrogenation and upgrading coal-derived liquids

Abstract This study focuses on the synthesis of NiMo-based catalysts in both bulk and coated forms, using ion-exchangeable silica-doped hydrous titanium oxide (HTO:Si) supports. These catalysts were evaluated and compared against commercial NiMo-based catalysts (Amocat 1C and Shell 324) with respect to the hydrogenation of pyrene and the hydrodesulfurization/hydrodenitrogenation of coal-derived liquids. For pyrene hydrogenation, both bulk and supported (coated) NiMo/HTO:Si catalysts performed better than commercial benchmark catalysts on either a catalyst weight or an active metals basis. For both hydrodesulfurization and hydrodenitrogenation of coal-derived liquids in a trickle-bed reactor, the supported and bulk forms of the NiMo/HTO:Si catalysts nearly equalled the overall performance of the commercial catalysts at 3.45, 6.89 and 10.3 MPa and were superior on a total active metals basis. Extensive catalyst characterization was performed to explain the enhanced activity of the NiMo/HTO:Si materials.

Preparation Effects on the Performance of Silica-Doped Hydrous Titanium Oxide (HTO:Si)-Supported Pt Catalysts for Lean-Burn NOx Reduction by Hydrocarbons

This report describes the development of bulk hydrous titanium oxide (HTO)- and silica-doped hydrous titanium oxide (HTO:Si)-supported Pt catalysts for lean-burn NOx catalyst applications. The effects of various preparation methods, including both anion and cation exchange, and specifically the effect of Na content on the performance of Pt/HTO:Si catalysts, were evaluated. Pt/HTO:Si catalysts with low Na content (< 0.5 wt.%) were found to be very active for NOx reduction in simulated lean-burn exhaust environments utilizing propylene as the major reductant species. The activity and performance of these low Na Pt/HTO:Si catalysts were comparable to supported Pt catalysts prepared using conventional oxide or zeolite supports. In ramp down temperature profile test conditions, Pt/HTO:Si catalysts with Na contents in the range of 3-5 wt.% showed a wide temperature window of appreciable NOx conversion relative to low Na Pt/HTO:Si catalysts. Full reactant species analysis using both ramp up and isothermal test conditions with the high Na Pt/HTO:Si catalysts, as well as diffuse reflectance FTIR studies, showed that this phenomenon was related to transient NOx storage effects associated with NaNO{sub 2}/NaNO{sub 3} formation. These nitrite/nitrate species were found to decompose and release NOx at temperatures above 300 C in the reaction environment (ramp up profile). A separate NOx uptake experiment at 275 C in NO/N{sub 2}/O{sub 2} showed that the Na phase was inefficiently utilized for NOx storage. Steady state tests showed that the effect of increased Na content was to delay NOx light-off and to decrease the maximum NOx conversion. Similar results were observed for high K Pt/HTO:Si catalysts, and the effects of high alkali content were found to be independent of the sample preparation technique. Catalyst characterization (BET surface area, H{sub 2} chemisorption, and transmission electron microscopy) was performed to elucidate differences between the HTO- and HTO:Si-supported Pt catalysts and conventional oxide- or zeolite-supported Pt catalysts.

Preparation and Evaluation of Novel Hydrous Metal Oxide (HMO)-Supported Noble Metal Catalysts

Hydrous Metal Oxides (HMOs) are chemically synthesized materials that, because of their high cation exchange capacity, possess a unique ability to allow the preparation of highly dispersed supported-metal catalyst precursors with high metal loadings. This study evaluates high weight loading Rh/HMO catalysts with a wide range of HMO support compositions, including hydrous titanium oxide (HTO), silica-doped hydrous titanium oxide (HTO:Si), hydrous zirconium oxide (HZO), and silica-doped hydrous zirconium oxide (HZO:Si), against conventional oxide-supported Rh catalysts with similar weight loadings and support chemistries. Catalyst activity measurements for a structure-sensitive model reaction (n-butane hydrogenolysis) as a function of catalyst activation conditions show superior activity and stability for the ZrO{sub 2}, HZO, and HZO:Si supports, although all of the Rh/HMO catalysts have high ethane selectivity indicative of high Rh dispersion. For the TiO{sub 2}-, HTO-, and HTO:Si supported Rh catalysts, a significant loss of both catalyst activity and Rh dispersion is observed at more aggressive activation conditions, consistent with TiO{sub x} migration associated with SMSI phenomena. Of all the Rh/HMO catalysts, the Rh/HZO:Si catalysts appear to offer the best tradeoff in terms of high Rh dispersion, high activity, and high selectivity.