Lisa D. Pfefferle
Catalysis and combustion have long been linked. In fact, the science of catalysis stems from Davy's discovery (1840) that platinum wires could promote flameless combustion. Despite the long-known capability of catalysts to oxidize hydrocarbons without significant production of carbon monoxide, soot or thermal NOx, the volumetric heat release rates of mass-transfer-limited catalytic oxidation reactors are far too low to supplant conventional flame burners. Catalytically stabilized combustion surmounts this limitation by using catalytic surface reactions to promote gas phase combustion even at temperatures well below those possible in conventional flame combustors. This is an interesting process from a chemical engineering point of view, involving interactions between surface heterogeneous reactions and gas phase homogeneous reactions, with both kinetics and transport influencing the system performance.
We are combining modeling work on CST combustion with experimental measurements using laser-induced fluorescence (LIF), photoionization mass spectrometry and interferometric holography to provide information on the interaction between homogeneous and heterogeneous processes in these systems. More recently we are applying UV-vis reflection spectroscopy to monitor changes in catalyst morphology under actual combustor operating conditions. The purpose of these studies is to gain insight into how catalytically active surfaces can be used to affect homogeneous reaction mechanisms. This coupling can occur through thermal processes, for example by affecting where energy from the exothermic reaction is released in the system, or through chemical interaction for example through intermediates produced on the surface which initiate chain reactions in the gas phase.
The fundamental experiments and numerical models have yielded information which is being used to better understand processes such as ignition and heat transfer in flow over heated surfaces, selectivity of gas phase reactions and pollutant formation kinetics. In ultra-lean methane and ethane combustion over a Pt catalyst, LIF studies of OH radical production from the catalyst surface have been used to demonstrate how chemical coupling between surface and gas phase reactions can be important in combustion stabilization. A boundary layer model of CST combustion including finite rate chemistry for methane combustion in the gas phase coupled with methane oxidation and radical production/recombination on the surface has been developed (in collaboration with M. Smooke, Yale ME) and is being used to analyze results from our experiments. A full 2-D model has been developed to enable modeling of catalytic ignitors with arbitrary geometries Model development is important to guide development of applications especially in this type of system where both surface and gas phase reaction kinetics are important and where transport properties can not be decoupled from the chemical kinetic processes.
Our most recent work in focuses on the in-situ characterization of supported precious-metal catalysts under actual catalytic combustion operating conditions. We believe that precious metal catalysts will be valuable in the long term for both low temperature ignitor and hybrid catalytic combustor applications and to bootstrap less active materials used for high temperature applications. Precious metal catalysts already can be used without preheaters at the 12-15 atm design outlet temperatures and will likely be available for the lower temperature of the 10 atm turbine combustors. A small section of precious metal catalyst can be used to initiate the reaction in many design applications such as the traditional staged monolith combustor demonstrated by OSAKA Gas Company. Two approaches that may allow elimination of precious metal catalysts include exhaust gas recirculation and rich catalytic combustion followed by lean combustion. This last approach is less favored, however, because it is thought that mixing cannot be fast enough to avoid "diffusion-flame-like" burning and consequent NOx formation. Hybrid applications ( all applications where most of the combustion occurs in the gas phase) are becoming increasingly important because the catalyst can be maintained at cooler temperatures and systems can be designed for retro-fit in existing combustors. This may lead to more near-term applications of catalysts in engine designs and increases the incentive for understanding the reactivity and stability of precious metal catalysts under catalytic combustion conditions. Experimental studies of the reactivity and structure of Pt and Pd metal catalysts on various support materials are helping us understand how to introduce the catalytic chemistry into our detailed catalytically stabilized combustor models and in defining high temperature support effects as a function of substrate material as well as changes that occur in catalyst morphology and reactivity.
Special attention is focused on changes in catalyst dispersion, morphology and reactivity of the Pd/alumina system under actual high-pressure combustor operating conditions. Interestingly, this system shows at least three activity states as a function of temperature and fuel oxidizer partial pressures involving at least two forms of PdO and a phase change to Pd. Transformations between these states occur on time scales that may present problems in actual combustor designs. At the recent International Meeting on Catalytic Combustion Dr. Furuya from Toshiba Corporation reported that in an actual prototype catalytic combustor using an alumina supported PdO catalyst where two forms of PdO with different reactivities were observed before the transition point from PdO to Pd. At high fuel concentrations the activity decrease noted for the "higher temperature PdO-state observed at leaner conditions does not form. This decrease in activity before PdO transformation to Pd has been observed by previous investigators but the fuel concentration dependence was not expected. In addition, oscillations in combustor performance using PdO catalysts related to oscillation in catalyst temperature on the minute time-scaler have been observed by many investigators especially at higher pressure conditions expected under actual engine conditions (for example Griffin, ABB Quick, Engelhard, Fukuzawa, CRIEPI). These oscillations pose problems in real engine designs. The observations that the magnitude of these oscillations increase with pressure point to changes in the surface rather than gas-surface transport. We have already completed qualitative studies using in-situ EXAFs to investigate the environment of the Pd catalyst during methane oxidation up to 900K. We are currently investigating the use of Raman and UV-vis spectroscopy to follow the reduction/reoxidation and morphology changes of PdO under reaction conditions.
Chlorinated hydrocarbon combustion is a challenging problem in part due to the importance of attaining complete destruction of the toxic fuel and combustion intermediates despite combustion inhibition via radical scavenging by chlorine atoms. Catalytic stabilization offers an approach for achieving plug flow incineration, resulting in improved destruction effectiveness for a given average residence time. Catalytic stabilization can also be used in new chemical processes for the production of chlorinated hydrocarbons or the production of C2 chemicals from methane.
We have shown that a catalytically stabilized burner (which acts as a virtual plug flow reactor with self heating walls) can achieve high destruction effectiveness (99.999%+) of methyl chloride and methylene chloride at low residence times (5 ms or less). This enables the design of an effective yet small combustor for destroying gaseous or liquid chlorinated hydrocarbons. We have developed a Mn-based catalyst, which is active for methyl chloride and methylene chloride decomposition and is stable to temperatures of 1700K in the presence of chlorine. Analysis of reaction products at low methyl chloride conversions shows that complex homogeneous/ heterogeneous reaction mechanism coupling, initiated by the reactivity of the catalyst, is a primary contributor to stabilization of CST combustion of methyl chloride. We have simulated the low conversion product distributions from the catalytic combustion of methyl chloride over the Mn-based catalyst using a laminar flow model combined with a detailed chemical model for methyl chloride oxidation in the gas phase and an experimentally derived rate constant for decomposition of methyl chloride on the catalyst to ethane and chlorine radicals. This analysis is being used to design combustors for toxic fume incineration, and investigate a methane coupling process via methyl chloride. We have demonstrated a process for conversion of methane/methyl chloride conversion to ethylene and/or vinyl chloride with both high conversion and selectivity.
The role of surface-generated hydrocarbon radicals in catalytic mechanisms for production of C2 hydrocarbons from methane is an important, unresolved question. Our research couples a molecular-beam-sampled packed-bed catalytic microreactor with photoionization MS and GC techniques to study the kinetics governing the activation of methane and methyl chloride over selected catalysts. Direct detection of radicals allows more confident testing of reaction models. Resonantly Enhanced Multiphoton Ionization (REMPI) is used to study methyl radicals with single photon or Electron Impact (EI) ionization for detection of major reaction products. Since the concentration levels are low, the high sensitivity of the VUV and REMPI photoionization techniques are important.
A promising system that we have studied involves the dissociation of methyl chloride to methyl and chlorine radicals on a Mn-based catalyst and subsequent reaction in the gas phase with the predominant initial products being ethylene and vinyl chloride. The C2 product selectivity can be adjusted by varying reaction conditions and oxygen concentration.
Two approaches for the design of low-NOx gas turbine combustors are two stage rich/lean combustion and ultra lean low temperature combustion. In two-stage combustion, the fuel is partially combusted under oxygen starved conditions to prevent NOx formation. Energy is removed to lower the temperature, and then the partially reacted mixture is burned in excess air. A major problem with current two-stage rich-lean combustor designs is that soot formed in the fuel-rich combustion zone might not be completely destroyed in the fuel-lean combustion zone. Lean burn designs combust the fuel in excess air, maintaining the temperature in a range low enough to minimize NOx production. To operate well these burners must quickly mix the fuel/air inlet streams which is not easily accomplished. The ability of fuel-rich catalytically stabilized combustion to preheat and break down hydrocarbon fuels with minimal soot formation suggests that it would be attractive as a rich stage burner for low-NOx engine design or as a way to pre-mix/preactivate fuel and air without the stability problems of lean burn engine designs.
We are studying fuel-rich catalytic combustion of hydrocarbons using flow-tube and microreactor kinetics studies. The goal of these experiments is to be able to predict how the catalyst can be used to modify rich combustion product distribution and to clean up the emissions from rich-burn engines. Because rich catalytic combustion of hydrocarbons results in a complex array of large and small hydrocarbons and radicals which are important in higher hydrocarbon growth processes, VUV mass spectrometry is being used along with conventional GC/MS for determining stable and unstable species production for use in reaction mechanism analysis. These studies tie in with our work on hydrocarbon growth processes in flames described in section 2 below. Results to date have been highly encouraging. C6 fuels (benzene, cyclohexane, hexane) and surrogate jet fuels have been studied at several fuel/air ratios and a range of dilutions over a supported Pt catalyst, a Mn-based catalyst and Pt doped Mn-based catalyst. Product distributions are different for each system and the surrogate fuel performance prior to gas phase reaction can be predicted from the single fuel data. The Mn-based catalyst alone shows similar activity to Pt under these reducing conditions and coke and higher hydrocarbon production was not a problem
This work focuses on exploring catalytically-stabilized reaction mechanisms that can be applied in either primary burner designs or in post-process clean-up devices to reduce NOx emissions from combustion devices. Thermal-NOx emissions increase strongly with both burner temperature and residence time while CO and hydrocarbon emissions exhibit the opposite behavior. A primary effect of catalytic combustion on NOx emissions is to allow lower temperature complete combustion (and thus lower thermal-NOx) than in the absence of catalytic stabilization. Yet the low temperature suppression of NOx formation does not fully explain experimentally achievable NOx reductions, highlighting the observation that there are many mechanisms dependent on both catalyst and reaction conditions that can result in reduction of burner NOx emissions from catalytically stabilized combustors. Two promising approaches for NOx reduction that do not involve direct reduction of NOx on the catalyst surface are the use of catalysts to reduce CO/HC emissions from quench zones, allowing air flow designs that reduce NOx production, and the use of catalysts that significantly reduce production of oxidizing radical concentration in the gas phase depleting reactants necessary for NOx production. Our reactive-surface boundary layer numerical model results have shown that thermal-NOx levels formed in a catalytic combustion system can either be higher or lower than over a heated nonreactive surface at the same temperature and residence time. For a system where most of the oxidation occurs on the surface, gas phase O and OH levels are significantly lower than when combustion takes place solely in the gas phase over a non-catalytic surface and thermal-NOx production is consequently reduced. Current modeling work shows how transport effects can be important in determining selectivity for the NO + CO reaction over precious metal catalysts in the presence of oxygen.
A new project (AFOSR, 1/94) using in-situ measurement of physical and thermodynamic properties is being initiated to characterize the reactivity of supercritical fuels.
In order to design a "deposit-controlling" fuel system, we need to understand how to minimize formation of carbonaceous deposit precursors, how to control the transformation of precursors to less soluble products and deposits, and to understand the relative solubility of various precursor classes as a function of temperature so that production of less soluble forms can be minimized.
Our approach is unique in that it employs direct in-situ measurement of physical and thermodynamic properties, an isothermal cell design, on-line detection of large hydrocarbon species, and theoretical underpinning to guide technique calibration and testing of system parameter dependence. This concerted approach is essential if the data is to be used for design purposes, because at near-supercritical conditions physical properties of the mixture (density, viscosity, dielectric, solubilities, thermal and ionic conductivities, etc.) are strongly affected by small changes in pressure and temperature.
We believe that a careful study of the mechanisms and kinetics of reacions that lead to the deposition of carbonaceous species, coupled with a more thorough understanding of transport phenomena near the mixture critical point of advanced fuels, will allow advanced engine designs and fuel formulations that inherently control deposit formation.
Hydrocarbon growth processes in flames are important both to the understanding of soot production from combustion systems and for the development of continuous processes for the manufacture of hydrocarbon/carbon films. We study hydrocarbon growth processes in oxidative environments and analysis techniques for measurement and characterization of intermediate species. Our experimental procedure employs the direct observation of precursor ions, radicals and neutral molecules emanating from a molecular-beam-sampled microjet reactor. Two detection configurations are currently being used. A quadrupole-MS and VUV photoionization mass spectrometry (VUV-MS) which allows nondestructive real-time mass analysis of the pyrolysis and combustion species down to sub-ppm levels.
The VUV-MS studies are of importance in providing detailed species and precursor development information, allowing more confident prediction of major reaction pathways to soot nucleation than is possible at the present time employing typical electron-impact mass spectrometer fragmentation or GC/MS data. For example, our studies comparing the nonoxidative pyrolysis of methyl acetylene and allene show that depending on reaction conditions, these two isomers can have different reaction pathways for formation of the first aromatic ring. Allene also produces significantly larger quantities of higher hydrocarbons at lower temperatures for equivalent reaction times when compared to methyl acetylene. Direct allene dimerization was shown to provide a relatively low temperature channel for higher hydrocarbon formation not available to methyl acetylene or other hydrocarbons and also provides a route to PAH production that does not proceed through benzene. Benzene formation through C3H3(A) recombination was also studied for this system. Microreactor VUV-MS studies are currently underway for the pyrolysis of methyl acetylene, allene, butadiene, cyclohexane and several C5 ring species to resolve some of the mechanistic issues brought to light by this research. Theoretical analysis of the pyrolysis data using reaction rate analysis techniques such as QRRK analysis is also being carried out to explore mechanistic pathway dependence on pressure and temperature and to extrapolate the rate constants to the conditions typical in process reactors and engines.
Another important issue where this work has already provided new insights is in the transition in flames from large PAH to soot nuclei. We have confirmed that for a large range of fuels PAH growth in the range from 228-378 amu occurs predominantly through small molecule addition to PAH radicals. This process, however, is not fast enough to account for the appearance of soot at a 2 ms time frame in a flame. Our new data shows that reactive dimerization is fast between some species in the 200-400 amu mass range leading to new dimer species possibly with CH2 bridges. It is proposed that this dimer formation takes place through reaction of PAH benzylic type radicals (or diradicals) with other large planar PAH. This process is theoretically estimated to be fast enough to explain our measured dimer formation at amu 408, 468, 483, 528, and 600 as well as providing a new explanation for how precursor species leading to the formation of soot nuclei are born in flames.
The mechanism for benzene oxidation in excess oxygen at moderate temperatures is important both in cleanup of low-concentration emissions and for understanding hydrocarbon emissions from combustors. This mechanism changes significantly as temperature is incresased. Because of the large number of unknowns, even at ultralow conversions, kinetic model verification requires an experimental strategy which allows measurements of relevant stable and readical intermediates. This system is an ideal candidate for the application of the VUV-MS technique. Both reactant amd initial reaction products are easily accessible to single-photon ionization, and because of their uinsaturated character, remain relatively unaffected by photo fragmentation during the analysis.
We have obtained Product profiles for lean benzene oxidation and pressures from 150 torr to 650 torr, temperatures from 1000-1450 K and varying surface/volume ratios. Phenoxy, phenol, C5H5 (cyclic) and CO are important intermediates at incipient benzene conversion conditions. We are currently using this data along with molecularly-based parameter estimation techniques to evaluate mechanisitic pathways. One early conclusion is that the reaction of benzene (TI, ground state is singlet ) with O2 is likely an important initiation step at low/moderate temperatures and stoichiometric to lean conditions.
Our TOF mass spectrometer has recently been equipped with a second pulsed mass gate so that we can obtain structural information in-situ from individual mass cohorts and surface ion dissociation system.
In studies of combustion chemistry, a large range of chemical species must be measured in a particularly hostile environment (high temperature, reaction/flow interactions, many stable and labile species present). This has led us to use, improve, and develop laser-based, and mass spectrometric techniques for detection of both stable and labile combustion intermediates in complex mixtures.
Laser-induced fluorescence (LIF) has proved a powerful technique for probing combustion propagation over heated catalytically active surfaces. We use laser-induced fluorescence of OH radicals (a standard technique in the combustion community) to monitor combustion propagation over a hot igniting surface, and have improved a planar LIF method to monitor OH during combustion in a confined geometry with significant measurement interference due to light scattering (collaboration with M. Winter and M. Long, ME). Currently we are working to improve an interferometric holography technique for imaging temperature in the reacting boundary layer. These studies are important for investigating ignition processes as well as for calibrating the LIF species measurements. In addition, working jointly with D. Crosley of SRI, our group developed and used two-photon LIF detection of O atoms in a combustion boundary layer close to a heated surface. These results have proved instrumental in showing that a hot catalytic surface can promote the production of high enough levels of oxidizing radicals to affect gas phase ignition.
The impetus for our development research on VUV-ionization mass spectrometry for analysis of high temperature pyrolysis products was the recognition of the limitations of electron impact (EI) and multiphoton ionization (MPI) as a means for investigating reacting, complex flows (such as with soot precursor formation and chlorinated hydrocarbon combustion). Most nonfragmenting ionization techniques are species or species-class specific, which prohibits the simultaneous detection of a broad range of compounds. This is a serious deficiency in complex reaction systems where simultaneous measurement of only a few pre-selected species can lead to misinterpretation of the actual mechanistic processes. VUV photoionization can minimize fragmentation of a wide range of sampled molecules, allowing simultaneous mass analysis. We are working to develop and demonstrate tunable vacuum ultraviolet single photon photoionization time-of-flight mass spectrometry (VUV-MS) as a simple, highly accurate, sensitive, real-time method for the simultaneous mass detection and measurement of a wide range of labile and stable species produced in hydrocarbon pyrolysis and combustion reactions. For 118 nm ionization (10.5 eV), the accessible species include many molecules of interest in combustion studies including hydrocarbons larger than ethane (and substituted hydrocarbons) and radicals, C1 radicals, and NO. We have demonstrated the attractive potential of this technique in nonoxidative pyrolysis studies of ethyl acetylene, butadiene, methyl acetylene, and allene. High sensitivity and minimal product fragmentation was observed. We are currently developing theoretically-based correlations for VUV photoionization yields for targeted stable species and radicals. For many of the larger species the direct normalized intensity gives approximate relative concentrations.
Tuning the ionization wavelength in the VUV aids in species structure identification. In addition our newly implemented pulsed mass gate and surface induced dissociation (SID) features allows a single mass cohort to be separated and dissociated at a fixed energy which also allows structural analysis. These features provide a 3-dimensional mass spectrometer diagnostic tool for probing combustion and pyrolysis kinetics, as well as for process applications such as real time incinerator monitoring. The power of this technique has been demonstrated dramatically in our studies of aromatic hydrocarbon oxidation and pyrolysis and soot precursor formation discussed in sections 2-4 above.
We have also developed a dual ionization source based on simultaneous VUV photoionization and laser driven electron impact ionization (LDEI). The simultaneous use of these two techniques permits identification of both parent molecular weights and structural information from EI fragmentation patterns. This technique won a runner-up award for the graduate student (James Boyle) from the ACS analytical division's 1990 competition for most innovative new analytic method involving mass spectrometry.
[Return to Lisa D. Pfefferle.]
Updated: 03/11/97
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