Lisa D. Pfefferle

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.

Catalysis in Combustion

Abstract Catalysis and combustion have long been linked. In fact, the science of catalysis stems from Davys discovery [1] that platinum wires could promote the flameless combustion of flammable fuel-air mixtures. Today, catalysis is a mainstay of our modern chemical industry. Oxidation catalysts are used not only for the complete oxidation of fuels to carbon dioxide and water, as in radiant catalytic tent heaters and fume abatement devices, but also for the selective partial oxidation of hydrocarbons or other “fuels” to produce basic chemicals such as ethylene oxide (from ethylene), terephthalic acid (from p-xylene), and nitric acid (from ammonia). However, despite the long-known capability of catalysts to oxidize hydrocarbons without significant production of carbon monoxide, soot, or thermal NOx, there seemed little possibility that catalytic oxidation reactors could ever displace conventional flame combustors as primary fuel combustors. This is because the volumetric heat release rates of conventional...

Computational and experimental study of soot formation in a coflow, laminar diffusion flame

Abstract A detailed soot growth model in which the equations for particle production have been coupled to the flow and gaseous species conservation equations has been developed for an axisymmetric, laminar, coflow diffusion flame. Results from the model have been compared to experimental data for a confined methane–air flame. The two-dimensional system couples detailed transport and finite rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation includes detailed treatment of the transport, inception, surface growth, oxidation, and coalescence of soot particulates. Effects of thermal radiation and particle scrubbing of gas-phase growth and oxidation species are also included. Predictions and measurements of temperature, soot volume fractions, and selected species are compared over a range of heights and as a function of radius. Flame heights are somewhat overpredicted and local temperatures and volume fractions are underpredicted. We believe the inability to reproduce accurately bulk flame parameters directly inhibits the ability to predict soot volume fractions and these differences are likely a result of uncertainties in the experimental inlet conditions. Predictions of the distributions of particle sizes indicate the existence of (relatively) low-molecular-weight species along the centerline of the burner and trace amounts of the particles that escape from the flame, unoxidized. Oxidation of particulates is dominated by reactions with hydroxyl radicals which attain levels approximately 10 times higher than calculated equilibrium levels. Gas cooling effects due to radiative loss are shown to have a very significant effect on predicted soot concentrations.

Influence of Biomacromolecules and Humic Acid on the Aggregation Kinetics of Single-Walled Carbon Nanotubes

The initial aggregation kinetics of single-walled carbon nanotubes (SWNTs) were studied using time-resolved dynamic light scattering. Aggregation of SWNTs was evaluated in the presence of natural organic matter [Suwannee River humic acid (SRHA)], polysaccharide (alginate), protein [bovine serum albumin (BSA)], and cell culture medium [Luria-Bertani (LB) broth] with varying solution concentrations of monovalent (NaCl) and divalent (CaCl(2)) salts. Increasing salt concentration and adding divalent calcium ions induced SWNT aggregation by screening electrostatic charge and thereby suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The presence of biomacromolecules significantly retarded the SWNT aggregation rate. BSA protein molecules were most effective in reducing the rate of aggregation followed by SRHA, LB, and alginate. The slowing of the SWNT aggregation rate in the presence of the biomacromolecules and SRHA can be attributed to steric repulsion originating from the adsorbed macromolecular layer. The remarkably enhanced SWNT stability in the presence of BSA, compared to that with the other biomacromolecules and SRHA, is ascribed to the BSA globular molecular structure that enhances steric repulsion. The results have direct implications for the fate and behavior of SWNTs in aquatic environments and biological media.

Complete methane oxidation over Pd catalyst supported on α-alumina. Influence of temperature and oxygen pressure on the catalyst activity

Abstract The influence of the reaction parameters including temperature, oxygen concentration, and of in situ hydrogen reduction on the Pd catalyst activity towards complete methane oxidation is studied experimentally. Zero porosity α-alumina plates are used as a support for Pd catalyst. This lowers the influence of metal–support interaction on the catalyst state as confirmed by UV–visible spectroscopy. A plug flow reactor with a high linear gas velocity is used to measure the reaction rate. Overall conversion is kept low for most of the experiments so that the reaction is in the kinetically limited regime. The oxidation state of the catalyst before and after the reaction is determined using UV–visible reflectance spectroscopy of the plate surface. Changes in the catalyst activity with time are monitored after stepwise changes in the reaction parameters. Activity was found to decrease with time at low temperatures and high oxygen concentrations (condition when PdO phase is stable) and to increase with time at high temperatures and low oxygen concentrations (conditions when Pd is stable). A sharp increase in conversion was observed after the in situ hydrogen reduction of the sample. The experimental data is consistent with the reduced Pd form of the catalyst being more active towards methane oxidation than the oxidized PdO form at high temperatures. Possible particle size and morphology effects are discussed.

Soot volume fraction and temperature measurements in laminar nonpremixed flames using thermocouples

Abstract Thermocouple particle densitometry (TPD), a new method for measuring absolute soot volume fraction in flames which was suggested by Eisner and Rosner, has been successfully implemented in several laminar nonpremixed flames. This diagnostic relies on measuring the junction temperature history of a thermocouple rapidly inserted into a soot-containing flame region, then optimizing the fit between this history and one calculated from the principles of thermophoretic mass transfer. The TPD method is very simple to implement experimentally, yields spatially resolved volume fractions directly, can easily measure small volume fractions, and does not depend on the prevailing soot particle size, morphology, or optical characteristics. p]In a series of methane and ethylene counterflow flames whose soot volume fractions varied by more than an order of magnitude, the TPD results agreed to within experimental error with our own laser extinction measurements. In axisymmetric methane and ethylene co-flowing flames, the shape of TPD profiles agreed well with published laser extinction measurements, but the TPD concentrations were significantly larger in the early regions of the ethylene flame and throughout the methane flame; these discrepancies are probably attributable to visible light-transparent particles that are detectable with TPD but not with laser extinction. The TPD method is not applicable to the upper regions of these co-flowing flames since OH concentrations there suffice to rapidly oxidize any soot particles that deposit. Gas temperatures were obtained simultaneously with volume fraction by averaging the junction temperature history shortly after insertion. The error in these temperatures due to soot deposition-imposed changes in the junction diameter and emissivity were assessed and found to be moderate, e.g., less than 60 K near the centerline of the ethylene coflowing flame where the volume fraction was 6 ppm and the gas temperature was 1550 K.

Simultaneous measurements of soot volume fraction and particle size/microstructure in flames using a thermophoretic sampling technique

A new particle volume fraction measurement technique was developed using electron microscope analysis of thermophoretically sampled particles/aggregates based on a theoretical treatment of particle deposition to a cold surface immersed in a flame. This experimental method, referred to as the thermophoretic sampling particle diagnostic (TSPD), can yield all particle parameters of principal interest (particle volume fraction, particle and aggregate sizes, and fractal properties) without requiring knowledge of particle bulk density and refractive index. To assess its reliability, the TSPD technique was implemented at various heights on the centerline of a soot-containing coflowing ethylene/air nonpremixed laminar flame. Inferred soot volume fractions agreed with previous laser extinction and thermocouple particle densitometry measurements within experimental uncertainties at sampling positions where only aggregates of mature particles were present. However, TSPD-soot volume fractions were about a factor of 3 higher than light extinction results in the lower part of the flame. This significant difference was evidently a result of the presence of translucent precursor soot particles, which do not absorb as much visible light as mature particles, but can be quantified with the electron microscope. Clearly, this ability of TSPD to separately measure the concentration and morphology of each type of soot is a significant advantage over other available diagnostics, making it extremely valuable for studying particle formation in flames.

Computational and experimental study of axisymmetric coflow partially premixed ethylene/air flames

Abstract Six coflowing laminar, partially premixed methane/air flames, varying in primary equivalence ratio from ∞ (nonpremixed) to 2.464, have been studied both computationally and experimentally to determine the fundamental effects of partial premixing. Computationally, the local rectangular refinement solution–adaptive gridding method incorporates a damped modified Newton’s method to solve the system of coupled nonlinear elliptic partial differential equations for each flame. The model includes a C2 chemical mechanism, multicomponent transport, and an optically thin radiation submodel. Experimentally, both probe and optical diagnostic methods are used to measure the temperature and species concentrations along each flame’s centerline. Most experimentally measured trends are well predicted by the computational model. Because partial premixing decreases the flame height when the fuel flowrate is held constant, computational and experimental centerline profiles have been plotted against nondimensional axial position to reveal additional effects of partial premixing. Heat release profiles, as well as those of several species, indicate that the majority of the partially premixed flames contain two flame fronts: an inner premixed front whose strength grows with decreasing primary equivalence ratio; and an outer nonpremixed front. As the amount of partial premixing increases, computational results predict a continual reduction in the amount of flow radially inward; the resulting decrease in radial transport is responsible for various effects observed both computationally and experimentally, including a cooling of the gases near the burner surface. At the same time, radiative losses decrease with increasing amounts of premixing, resulting in higher flame temperatures.

Methane combustion over the α-alumina supported Pd catalyst: Activity of the mixed Pd/PdO state

Abstract The effect of variations in temperature and oxygen partial pressure on the methane oxidation activity of α-alumina supported Pd catalysts was studied experimentally. It was found that after pretreatment at temperatures above 800°C, conditions where the metallic state of the catalyst is stable, “negative activation” occurred on cooling cycle leading to increase in conversion with decreasing temperature. The pretreatment at temperatures exceeding 800°C was found to be a necessary condition in order for this effect to take place. This phenomenon is attributed to appearance of highly dispersed PdO clusters in thermodynamic equilibrium with the previously formed metallic Pd surface. The contradictory results on the activity of Pd/PdO phase of the catalyst towards methane oxidation reported in the literature are explained by the proposed hypothesis.

Experimental study of nonfuel hydrocarbons and soot in coflowing partially premixed ethylene/air flames

Abstract Centerline profiles of gas temperature, C1 to C12 nonfuel hydrocarbon concentrations, polycyclic aromatic hydrocarbon (PAH) laser-induced fluorescence (LIF), and soot volume fraction are reported for coflowing ethylene nonpremixed and partially premixed flames with primary equivalence ratios ranging from 24 to 3. Concentrations of acetylene and C4 hydrocarbons were lower in nearly all of the partially premixed flames than in the nonpremixed flame, whereas concentrations of methane and C3H4 were larger in all of the partially premixed flames than in the nonpremixed flame. These results indicate that the primary effect of partial premixing is not to uniformly increase the concentrations of pyrolysis products, but to shift the pyrolysis mechanism towards odd-carbon species. The concentration of benzene was larger in several of the richer partially premixed flames than in the nonpremixed flame, probably because the shift in pyrolysis mechanism enhances self-reaction of C3H3 radicals. Increases in soot volume fraction and other aromatics were observed that matched the increases in benzene. Profiles of PAH fluorescence agreed closely with those for specific gas-phase PAH such as naphthalene, and the maximum PAH signals were a good predictor of the eventual maximum soot volume fractions. Concentrations of oxygenated hydrocarbons such as formaldehyde and ketene were dramatically increased in the partially premixed flames; for formaldehyde this trend was confirmed with in situ LIF measurements.