Timothy J. Gardner

An Asymptotic Model of Nonadiabatic Catalytic Flames in Stagnation-Point Flow

A formal asymptotic model is derived for a nonadiabatic catalytic flame in stagnation-point flow. In the present context, the premixed reaction in the bulk gas is augmented by a surface catalytic reaction on the stagnation plane, where conductive heat losses are allowed to occur. In addition, the thermal effects of a finite-volume combustor are accounted for by allowing for volumetric heat losses from the bulk gas. The analysis exploits the near-equidiffusional limit corresponding to near-unity Lewis numbers, and yields a general nonsteady nonplanar model for the reactionless outer flow subject to boundary conditions that reflect both surface catalysis and distributed chemical reaction in a thin boundary layer. For the case of steady planar combustion, the surface-temperature response indicates the possibility of multiple solution branches, which are shown to be linearly stable, and a corresponding extension of the extinction limit that demonstrates how the presence of a surface catalyst can counterbalanc...

Activation behavior of Ni/hydrous titanium oxide (HTO) catalysts

The activation of Ni-TiO2 catalysts prepared via both impregnation on commercial TiO2 and by ion exchange on hydrous titanium oxide (HTO) supports was investigated. The reactivity of these catalysts for a structure-sensitive reaction (n-butane hydrogenolysis) was investigated as a function of different activation treatments (temperature, time) in a H2 atmosphere. Complete activation of the ion exchanged Ni/HTO catalyst in H2 required temperatures > 623 K and times > 18 h. Low temperature (573 K) activation of the ion exchanged Ni/HTO catalyst in H2 was not possible without performing a mild (573 K) reduction/oxidation/reduction cycle. Observations of the ion exchanged Ni/HTO catalyst microstructures by transmission electron microscopy revealed that inactive samples contained a distinct film which covered Ni particle surfaces. The exact nature of this film has not yet been determined; experimental evidence is consistent with the film being composed of either carbonaceous residue or TiOx which has migrated to the Ni particle surfaces from the bulk support phase.

Extinction limits of nonadiabatic, catalyst-assisted flames in stagnation-point flow

An idealized geometry corresponding to a premixed flame in stagnation-point flow is used to investigate the effects of catalysis on extending the extinction limits of nonadiabatic stretched flames. Specifically, a surface catalytic reaction is assumed to occur on the stagnation plane, thereby augmenting combustion in the bulk gas with an exothermic surface reaction characterized by a reduced activation energy. Assuming the activation energies remain large, an asymptotic analysis of the resulting flame structure yields a formula for the extinction limit as a function of various parameters. In particular, it is demonstrated that the presence of a surface catalyst can extend the burning regime, thus counterbalancing the effects of heat loss and flame stretch that tend to shrink it. The analysis is relevant to small-volume combustors, where the increased surface-to-volume ratio can lead to extinction of the nonadiabatic flame in the absence of a catalyst.

Catalytic Membrane Sensors. A Thin Film Modified H2 Resistive Sensor for Multi-Molecular Detection

Abstract A so-called “catalytic membrane sensor” (CMS) is being developed to impart selectivity and reactivity to the surface of an existing sensor by modifying it with a series of thin films. The proposed “sandwich-type” modification involves deposition of a catalyst layer between two size selective sol-gel layers on a Pd/Ni resistive H2 sensor. The role of the catalyst is to convert organic materials to H2 and organic by-products by a dehydrogenation mechanism. The roles of the membranes are to impart chemical specificity by molecular sieving of the analyte and the converted product streams as well as to control access to the underlying Pd/Ni sensor. The “sandwich” modification will mediate the sensor response and avoid potential poisoning effects. Ultimately, an array of these CMS elements encompassing different catalysts and membranes will further enable improvements in selectivity and specificity via pattern recognition methodologies. This report details the synthesis of the various thin film solutio...

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.

Extinction Limits of Nonadiabatic, Catalyst-Assisted Flames in Stagnation-Point Flow

An idealized geometry corresponding to a premixed flame in stagnation-point flow is used to investigate the effects of catalysis on extending the extinction limits of on adiabatic stretched flames. Specifically, a surface catalytic reaction is assumed to occur on the stagnation plane, thereby augmenting combustion in the bulk gas with a exothermic surface reaction characterized by a reduced activation energy. Assuming the activation energies remain large, an asymptotic analysis of the resulting flame structure yields a formula for the extinction limit as a function of various parameters. In particular, it is demonstrated that the presence of a surface catalyst can extend the burning regime, thus counterbalancing the effects of heat loss and flame stretch that tend to shrink it. The analysis is relevant to small-volume combustors, where the increased surface-to-volume ratio can lead to extinction of the nonadiabatic flame in the absence of a catalyst.

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.

Platinum Catalyzed Decomposition of Activated Carbon: 1. Initial Studies

Carbon is an important support for heterogeneous catalysts, such as platinum supported on activated carbon (AC). An important property of these catalysts is that they decompose upon heating in air. Consequently, Pt/AC catalysts can be used in applications requiring rapid decomposition of a material, leaving little residue. This report describes the catalytic effects of platinum on carbon decomposition in an attempt to maximize decomposition rates. Catalysts were prepared by impregnating the AC with two different Pt precursors, Pt(NH{sub 3}){sub 4}(NO{sub 3}){sub 2} and H{sub 2}PtCl{sub 6}. Some catalysts were treated in flowing N{sub 2} or H{sub 2} at elevated temperatures to decompose the Pt precursor. The catalysts were analyzed for weight loss in air at temperatures ranging from 375 to 450 C, using thermogravimetric analysis (TGA). The following results were obtained: (1) Pt/AC decomposes much faster than pure carbon; (2) treatment of the as-prepared 1% Pt/AC samples in N{sub 2} or H{sub 2} enhances decomposition; (3) autocatalytic behavior is observed for 1% Pt/AC samples at temperatures {ge} 425 C; (4) oxygen is needed for decomposition to occur. Overall, the Pt/AC catalyst with the highest activity was impregnated with H{sub 2}PtCl{sub 6} dissolved in acetone, and then treated in H{sub 2}. However, further research and development should produce a more active Pt/AC material.

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.