The autothermal behavior of platinum catalyzed hydrogen oxidation: experiments and modeling

The Autothermal Behavior of Platinum Catalyzed Hydrogen Oxidation: Experiments and Modeling N. E. FERNANDES, Y. K. PARK, and D. G. VLACHOS*

Department of Chemical Engineering, University of Massachusetts Amherst, Amherst, MA 01003, USA The autothermal behavior of H2/O2 mixtures over a platinum foil catalyst is investigated experimentally and theoretically. Experiments were conducted at atmospheric conditions, at levels of nitrogen dilution ranging between 80% and 92% by volume. The stagnation flow geometry was modeled using simplified multicomponent transport and detailed reaction mechanisms, both in the gas phase and on the catalyst surface. A maximum autothermal temperature was found at the unexpected, nonstoichiometric, H2/O2 volume ratio of approximately unity. Experimental and model results for the effect of dilution on autothermal temperatures and the flammability limits are presented and discussed. Sensitivity analyses indicate that the upper flammability limit depends on the relative adsorption rates of H2 and O2 onto the platinum surface, and the desorption of adsorbed H*, while the location of the maximum in autothermal temperature depends on the transport of H2 and O2. It is shown that repulsive H*-H* interactions on the surface may be essential for accurate prediction of the upper flammability limit. © 1999 by The Combustion Institute

INTRODUCTION The promising vistas of partial oxidation [1] and pollution-free combustion [2–5] have spurred investigators to study in detail the interplay between reactions occurring on a catalyst surface and in the gas phase. However, even though in many cases the gas-phase reactions are fairly well understood, our knowledge of how the surface interacts with the gas reactants is still limited at a quantitative level. Catalytic combustion reactions almost universally yield complex bifurcation behavior sensitive to the surface mechanisms. Hence, by detailed modeling of reaction kinetics and transport phenomena, and the aid of data collected from carefully designed yet essentially simple catalytic combustion experiments, it is possible to validate proposed surface reaction mechanisms. The obvious bifurcation features that can be practically exploited in such studies are catalyst ignition and extinction. Prior to ignition, reactions are believed to be very slow and concentration gradients are small. Ignition is therefore generally considered to be sensitive only to the surface reaction kinetics (kinetically controlled). However, our recent studies have shown that the diffusive transport also plays a role in catalyst ignition near the critical point [6]. Upon ignition, the catalyst temperature is *Corresponding author. E-mail: [email protected] 0010-2180/99/$–see front matter PII S0010-2180(98)00162-X

generally high enough for the reaction to be transport limited. This behavior is expected in the catalytically ignited branch, up to temperatures where the gas-phase ignites. However, near extinction, the chemistry should become slow, and the inherent coupling between transport and surface kinetics should be important. Of relevance on the ignited catalyst branch is the locus of catalytic autothermal points. An autothermal point is defined as a composition and temperature at which the ignited catalyst operates with zero power input; the catalytic reaction can still be extinguished, but only through energy extraction or thermal quenching of the catalyst. Typically, at very fuel-lean and very fuel-rich compositions, the heat generated by the reacting mixture is not sufficient to self-sustain combustion, and autothermal behavior is lost. The loss of autothermal stability coincides with an extinction point, and being of special significance, these extinction points are often referred to as flammability limits. Flammability limits are typically defined for flame propagation in tubes [7]. However, for flow reactors, the autothermal limits with respect to composition define the range of self-sustained combustion, and are often referred to as flammability limits (e.g., [8 –10]). Experimentally and theoretically, H2/O2 oxidation on platinum has been studied in quite some depth, since the homogeneous and surface chemistry of this simple fuel is relatively well COMBUSTION AND FLAME 118:164 –178 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.

AUTOTHERMAL BEHAVIOR OF H2/O2 ON PLATINUM known, compared to other catalysts and fuels. There is a vast literature on H2 combustion over platinum surfaces, with perhaps the first detailed work presented by Faraday [11]. A review of earlier experimental studies of the H2/O2 reaction on platinum was given several years ago by Norton [12]. Recent work has focused on catalytic ignition, such as the work by Kasemo and coworkers [13–15], Ikeda et al. [16], and Deutschmann et al. [17, 18]. The oxidation of H2/O2 on platinum has also been studied extensively through detailed modeling by Vlachos and coworkers. In previous work, Vlachos and Bui [19] were able to successfully model the ignition data of Cho and Law [20], using a surface reaction mechanism first proposed by Williams et al. [21] for H2/O2 oxidation on platinum. Through various numerical analyses, they were able to elucidate the essential surface and gas-phase chemistry for understanding the catalyst-induced inhibition of homogeneous ignition [22]. Following the apparent success of this surface mechanism, Bui et al. [6] have presented parametric studies of the roles of strain rate, pressure, and preheating, in catalytic ignition temperature. The various bifurcation features have been summarized in engineering maps to identify operating windows of catalytic reactors and start-up control strategies [10]. Reduced surface reaction mechanisms, applicable to different regimes of operation, were also developed [23] for more complex simulations and practical combustor design studies. Compared to catalytic ignition, the study of autothermal behavior has not been as extensive, with main contributions being the work of Schmidt and coworkers [8, 24] for alkanes and olefins. For H2/O2 on platinum, no previous work has studied catalytic autothermal behavior, and this reason motivates the present work. Here, we present for the first time, an experimental investigation of the catalytic autothermal behavior of H2/O2 mixtures over a polycrystalline platinum foil. This data is compared with simulation results obtained using the detailed model of Bui et al. [6]. The main features of the results, such as the flammability limits and the maximum in autothermal temperature, are interpreted using numerical experiments and analyses.

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Fig. 1. The experimental setup. Inlet flow of reactants is controlled by mass flow controllers. The platinum catalyst foil is spot-welded onto brass leads, and heated resistively by direct current. A ceramic glaze on the back of the foil prevents back reactivity. A chromel/alumel thermocouple is used to obtain temperature measurements.

EXPERIMENTAL SETUP The experimental setup is illustrated in Fig. 1. Polycrystalline platinum (99.99% purity) was obtained from Aldrich Chemical Company. A foil of dimension 25 3 6 3 0.025 mm was mounted in a stagnation flow geometry by spotwelding its ends to two brass leads 1/80 in diameter. The catalyst could then be resistively heated to its ignition temperature by the passage of a direct current. The temperature of the foil was measured using a chromel/alumel thermocouple spot-welded to the back of the foil. The lower part of the reactor (;20 cm) was filled with glass Raschig rings to ensure complete mixing of the gases. The catalyst foil was held 1 cm from the outlet of this mixing section, and the nominal gas velocity in the reaction chamber was maintained at ;2.5 cm/s in all experiments.

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Fig. 2. Scanning electron micrographs of a fresh platinum foil (a) and a platinum foil after 10 hr of catalytic oxidation at ;873 K, H2/O2 5 0.5, and 88% N2 dilution at two different magnifications (b, c). Chemical oxidation causes morphological evolution of the catalyst.

This type of stagnation flow assembly has been used by several other researchers (e.g., [8, 16, 17, 24, 25]), but due to the type of investigation we have undertaken, namely the measurement of autothermal temperatures, we have incorporated the following modification. The back of the foil was thinly coated (;10 mm) with a low-curing-temperature, silica-based glaze, masking the rear surface from the reactants so that catalytic reaction only occurs on the surface facing the flow. This modification allows for more quantitative comparison of simulations with experiments. In particular, the stagnation flow model assumes that only the surface facing the flow is relevant in the calculations, and is inherently incapable of modeling the fluid flow and reaction at the rear surface of the experimental foil. The best approximation one can make to account for such an experimental difficulty is to assume that the rate of reaction on the rear surface is the same as, or some fraction of, that occurring on the front stagnation surface, the requisite modeling modification being to increase the reaction rate by an appropriate factor. However, the use of such an approximation would nullify the quantitative credibility of the model. In addition, the masking slightly increases the heat capacity of the system, making the foil temperature less sensitive to small fluctuations in the reaction rate, and increases

the radiative emissivity compared to a bare platinum foil. Experiments have been performed both with and without a ceramic glaze on the rear surface. Figure 2a shows a scanning electron microscopy (SEM) picture of a fresh foil, which shows a relatively uniform surface with no particular patterns. Figures 2b and 2c show different magnification SEM pictures of the front of a platinum foil which has been used for H2/O2 catalytic autotherm measurements. The micrographs show that chemical reactions cause morphological evolution of the platinum catalyst, as shown by the contrasting light and dark patterns. When experiments were conducted without a ceramic glaze on the rear of the catalyst surface, both the front and the rear of the catalyst showed similar evolution, indicating that reactivity occurs on both surfaces. We have also applied much thicker layers of glaze (up to ;1 mm), with almost identical results being obtained, indicating that varying the thickness of the glaze (at least in the range of ;10 mm to ;1 mm) has minimal influence on heat loss and inhibition of reactivity at the back of the surface. The difference in autothermal temperatures between the masked and the unmasked catalyst foils is significant. As an example, for 12% total H2 and O2 mixture diluted with N2, the unmasked catalyst exhibited auto-

AUTOTHERMAL BEHAVIOR OF H2/O2 ON PLATINUM thermal temperatures that were up to 100 K higher than the masked catalyst. The fuel-rich flammability limit was also larger for the unmasked catalyst. From these comparisons, it was evident that an unmasked catalyst exhibits an increased catalyst reactivity. Hence, all the results presented below are for the masked catalyst, to allow for better quantitative comparison with model predictions. The influence of the distance of the catalyst foil from the inlet of reactants was also investigated. Most of the previous investigators have kept this distance fairly large (.5 cm), with the exception of Ikeda et al. [16]. We have studied two different distances between the catalyst and the inlet, namely 15 cm and 1 cm, and found that the autothermal temperatures between the two cases were very close, the differences being within the range of experimental error. All the results presented below are for the catalyst to inlet distance of 1 cm, since this setup more closely represents the actual stagnation flow geometry. We have used N2 diluted gas mixtures in these experiments, primarily for reasons of safety. However for improved selectivity, partial oxidation reactors often run with pure oxygen instead of air. Thus, understanding the role of dilution in reactor performance is important and provides further validation for detailed models. The total reactant (O2 and H2) composition was varied between 8% and 20%, on a volume basis, with the remainder being N2. To avoid the formation of high concentration reactant streams, high-purity (99.99%) gas flows of N2 and H2 were first combined, and the mixture subsequently blended with air. In early experiments, the flows were controlled using calibrated rotameters. Identical results were obtained when these control elements were upgraded to mass flow controllers (MKS Instruments), and it is the results from these latter experiments which are presented here. To obtain the autothermal points, the catalyst was first ignited through resistive heating. The power dissipated in the foil can be calculated from ammeter measurements of the current, and estimates of the temperature-dependent platinum resistivity via the equation [18] R~T s! 5 R 300@1 1 3.57 3 10 23~T s 2 300!

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2 5.25 3 10 27~T s 2 300! 2#, where R is the platinum resistivity in V.cm and T s is the foil temperature in K. Once the system was catalytically ignited, the power was gradually turned off, and the first autothermal point was obtained (if one existed for this composition). Subsequent autothermal points were tracked by slowly changing the H2/O2 ratio, while maintaining the total reactant concentration constant. Strict criteria were used for ascertaining the autothermal temperatures. After changing the composition, it appeared that approximately 2 minutes were satisfactory for the steady-state temperature to be reached. Near the limits of autothermal behavior, however, at least 5 minutes were allowed for each measurement, due to longer transients observed there. Experiments using different foils at identical conditions showed differences in autothermal temperatures of up to 15 K, and the larger differences were usually found at higher temperatures. These differences can be reasonably attributed to the slight changes in glaze thickness, the size of the platinum foil, and the variance in inlet reactant gas temperatures from day to day. The mass flow controllers used were calibrated using a bubble flow meter. The relative uncertainties in the concentration of each reactant were then estimated to be on the order of 3%. Temperature fluctuations were also observed during the experiments. For each dilution ratio, the largest fluctuations were seen close to the maximum in autothermal temperatures. For example, the maximum fluctuation was ;6 K for 92% dilution, ;10 K for 88%, ;15 K for 84%, and ;22 K for 80%. Away from the maximum in autothermal temperature, the fluctuations in temperature were relatively low, at less than 4 K. For autotherm experiments at higher flow rates, these temperature fluctuations decreased, indicating that they were due mainly to fluctuations in the flow at low flow rates.

STAGNATION FLOW MODELING To complement the experimental results with model prediction and analysis, a stagnation point flow is modeled. The corresponding two-

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N. E. FERNANDES ET AL. TABLE 1 The 13 Reactions/5 Species H2/O2 on Pt Surface Reaction Mechanisma

Reaction H* 1 O* 3 OH* 1 * OH* 1 * 3 H* 1 O* H* 1 OH* 3 H2O* 1 * H2O* 1 * 3 H* 1 OH* 2OH* 3 H2O* 1 O* H2O* 1 O* 3 2OH* H2 1 2* 3 2H* 2H* 3 H2 1 2* O2 1 2* 3 2O* 2O* 3 O2 1 2* H2O 1 * 3 H2O* H2O* 3 H2O 1 * OH* 3 OH 1 *

ko

Ea

Reaction No.

1.000E115 1.000E108 9.000E116 1.800E113 1.000E115 0.000E100 1.000E100 1.000E113 2.790E-02 or 8/Tb 1.000E113 0.100E100 1.000E113 1.500E113

2,500 5,000 15,000 37,000 12,300 31,800 0 18,000-AuHc 0 52,000-AuOc 0 10,800 48,000

1 2 3 4 5 6 7 8 9 10 11 12 13

From Williams et al. [21]. k o is the sticking coefficient for adsorption or the reaction preexponential in (cm2/mol)n s21 where n is the reaction order, and E a is the activation energy in (cal/mol). b See text for details. c See text for values of repulsive interactions. a

dimensional governing equations of momentum, continuity, energy, and species conservation are first converted into a one-dimensional problem using a similarity transformation [6]. The transformed steady-state equations for species, energy, and stream function are discretized along the axial centerline using a finite difference method, and the resulting set of algebraic equations is solved using Newton’s technique. Steady-state solutions are obtained through a robust, dynamically adaptive, multiple-weight arc-length continuation algorithm, capable of passing around turning points. The details of the model as well as the solution algorithm are discussed elsewhere [6, 26]. Detailed chemistry for both the gas-phase and the surface chemistry is used to compute reaction rates. In particular, the 20 reversible reactions/9 species homogeneous H2 oxidation chemistry [6], taken from Miller and Bowman [27], and a modified 13 reactions/5 species surface reaction mechanism of H2/O2 on platinum by Williams et al. [21] are employed. The modifications to this surface reaction mechanism are based on comparison to experimental data and theoretical analysis, as elaborated below. The homogeneous chemistry does not play any role at the relatively low temperatures encountered in our experiments, as the temperature for gas-phase ignition is considerably higher [22,

26]. The surface reaction mechanism, which is of importance to this study, is listed in Table 1. For the modeling results presented, the adsorbate–adsorbate interaction parameter, A, for H* and O* desorption is taken to be zero, and the sticking coefficient of O2 is fixed at a constant value of 0.0279, unless otherwise specified. The diffusive velocities are computed using the Wilke expressions. The CHEMKIN formalism is used to calculate multicomponent transport properties, equilibrium constants of gas reactions, and the thermodynamic properties of reacting mixtures [28, 29]. Previous work by Vlachos and coworkers comparing the model predictions of catalyst ignition [19, 30] and homogeneous ignition [22, 31] to experimental data has shown reasonable agreement.

IGNITION, EXTINCTION, AND AUTOTHERMAL BEHAVIOR All of the results presented herein pertain to experiments performed using the same catalyst foil. As mentioned previously, autotherm measurements were also conducted on other platinum foils, and identical results within experimental error were obtained. Figure 3 shows

AUTOTHERMAL BEHAVIOR OF H2/O2 ON PLATINUM

Fig. 3. Typical response curves generated experimentally, for a flow velocity of 2.5 cm/s, 88% N2 dilution, and atmospheric conditions. Circles connected with a solid line indicate temperature measurements for increasing current; diamonds connected with a dotted line indicate temperature measurements for decreasing current. At high H2/O2 ratios (5.0), reactivity is observed without hysteresis. For intermediate H2/O2 ratios (2.4), hysteresis is seen, with corresponding ignition and extinction points. For lower H2/O2 ratios (1.5), a larger region of hysteresis is observed, and the reacting mixture generates enough heat to operate autothermally.

typical bifurcation behavior observed during our investigation, for a 88% N2 diluted mixture. To better illustrate the bifurcation behavior of the system, two sets of experimental data are shown for each H2/O2 ratio in Fig. 3. The circles with solid lines connecting each experimental data point are the result from increasing the current input to the foil from zero. The diamonds with dotted lines connecting each experimental data point are the result from decreasing the current input to the foil from high values (10 A) from the previous experiment. For very fuel-rich mixtures (e.g., H2/O2 5 5) there are no turning points or hysteresis. As the current input to the foil is increased, the surface temperature increases gradually from ambient to ;500 K at 10 A. Decreasing the current from this point retraces the previous experimental data in which the current was being increased. An increase in slope is observed at ;6 A, indicative of the onset of surface reactions. As the H2/O2 ratio is reduced, however, the temperature– current curve begins to fold, resulting in ignition and extinction points. As an example, for H2/O2 5 2.4, a small region of hysteresis is observed at a current input of ;4.8

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A. As the current is increased from zero, the surface temperature shows a sharp increase from ;358 K to ;428 K at ignition. When the current is decreased along the ignited branch (from high values), the ignited branch is retraced again down to ;5 A. However, further decrease in current to ;4.8 A still keeps the system ignited (hysteresis). The system extinguishes as the current is further decreased, at around 4.75 A. As the ratio of H2/O2 is further decreased, the region of hysteresis increases. As shown in Fig. 3, at H2/O2 5 1.5, the system exhibits a large region of hysteresis at current input of 0 to ;2.5 A. In fact, the reacting mixture generates so much heat, that the system does not extinguish at zero current (self-sustained combustion), as the extinction point has crossed over into the negative power quadrant. This ignited temperature at zero power is termed the catalytic autothermal temperature, and will be the main focus of the remainder of this paper. CATALYST ACTIVATION AND DYNAMICS OF CATALYST REACTIVITY Previous experimental work on wires and foils has shown that the catalytic activity of platinum is quite complicated, depending on the temperature, chemical environment, and the duration of use. In general, investigators have observed that a fresh wire or foil of platinum catalyst is relatively inert, and requires pretreatment to activate it [32, 33]. For H2/O2 oxidation on platinum, Ljungstro ¨m et al. [25] and Deutschmann et al. [17] have indicated that the reproducibility of results depends on how clean the catalyst foil is, and that decontamination procedures prior to experiments are crucial. Ljungstro ¨m et al. have also observed through SEM, recrystallization, grain boundaries, and facets on used platinum catalysts, which could affect reproducibility of results over the duration of catalyst use. We also have found that the catalytic activity of platinum depends strongly on the oxidizing history. Our experience indicates that a fresh platinum foil is relatively inert (or has only minor catalytic activity) and requires a period of activation to increase its catalytic activity. This

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Fig. 4. Experimental autothermal temperature as a function of H2/O2 ratio for conditions identical to Fig. 3, at various time intervals of catalyst activation. The catalyst exhibits reproducible autothermal temperatures between 20 to 60 hr of activation. Slight loss in catalyst activity is seen after 70 hr, indicated by the lower autothermal temperature and fuel-rich flammability limit.

was done by heating the platinum foil at elevated temperatures under a reacting environment of H2/O2/N2 for a period of time, producing an active catalyst. Since impurities on the surface can also influence catalytic activity, an activated catalyst was always decontaminated by heating it under reactive conditions for a short time, prior to experiments. However, we found that the activity of the catalyst also depends strongly on the duration, temperature, and chemical environment during catalyst activation and decontamination. Changes in surface morphology (e.g., Fig. 2), as well as in the catalytic ignition and autothermal temperatures were observed for different catalyst activation and decontamination procedures, stressing the importance of maintaining a stringent activation and decontamination procedure for data consistency. We have found the catalyst to be quite stable, and the data reproducible, if it was activated and decontaminated at ;873 K, under an oxidizing condition of H2/O2 5 0.5 in 88% N2 dilution. The reproducibility of the autothermal temperature is shown in Fig. 4. Every 10 hours of catalyst use under activation/decontamination conditions, autothermal temperatures were recorded for various H2/O2 ratios, for up to 80 hours of catalyst use. The results show autothermal temperature measurements every 20 hours

N. E. FERNANDES ET AL.

Fig. 5. Autothermal temperature as a function of H2/O2 ratio for conditions identical to Fig. 3, at four different dilutions. Circles indicate experimental data with the associated error bars; solid lines indicate model prediction. As dilution decreases, autothermal temperatures increase, and the upper flammability limit is extended. A sharp maximum in autothermal temperature, corresponding to the surface stoichiometric point, is seen at H2/O2 , 1.0.

from the 10th hour of activation. Figure 4 shows that the catalyst activity is fairly consistent from 10 to 80 hours, with a slight decrease in activity after the 70th hour. Therefore, for all the results presented, the catalyst was activated for 20 hours, and all the experimental data were taken within a total of 60 hours of catalyst use (which includes activation, decontamination, and actual experiment time). Prior to each experiment, the catalyst was decontaminated under the same conditions for 15 minutes. The limits of flammability as well as the autothermal temperatures were also checked after all data acquisition, to ensure data reproducibility. EFFECT OF DILUTION ON AUTOTHERMAL TEMPERATURES AND FLAMMABILITY LIMITS The experimental autothermal temperatures as a function of H2/O2 ratio are shown in Fig. 5 (circles), for different dilution ratios ranging from 92% to 80% N2. As expected, the autothermal temperature increases with decreasing dilution, since more heat is generated by higher concentrations of H2 and O2. An interesting feature that is seen independent of dilution is a maximum in autothermal temperature, at H2/O2 , 1. At first glance, the location of this

AUTOTHERMAL BEHAVIOR OF H2/O2 ON PLATINUM maximum is unexpected; for homogeneous combustion the maximum flame temperatures are generally observed to be near the stoichiometric points [34] which for the reaction 2H2 1 O2 3 2H2O is H2/O2 ;2. Another interesting feature seen in Fig. 5 is that as dilution decreases, the surface stoichiometric point shifts to higher H2/O2 ratios. For example, at 92% dilution, the maximum in autothermal temperature occurs at H2/O2 ;0.7, but shifts to H2/O2 ;0.94 for 80% N2 dilution. Maxima in autothermal temperatures, at nonstoichiometric mixtures, have been also observed by Veser and Schmidt [8] for hydrocarbon catalyzed combustion over platinum. Maxima in gas-phase ignition temperatures have been previously observed experimentally in combustion near platinum of natural gas/air mixtures [35] and diluted H2/air mixtures [36], and in simulations of H2/air mixtures [22]. For CH4/air mixtures, the maximum autothermal temperature and the maximum gas ignition temperature occur near the stoichiometric point. In contrast, for H2/air mixtures, we have predicted that the homogeneous ignition temperature over platinum would be a maximum near H2/O2 ;1, in close agreement with recently published data [36]. An explanation for this apparent anomaly for H2 was first suggested by Bui et al. [22], to be a result of multicomponent transport. In a catalytic reactor with spatial concentration gradients, the concentration of reactants at the catalyst is generally not the same as the bulk concentration. Hence, the maximum in catalytic autothermal temperature does not occur at the homogeneous or bulk stoichiometric point, but at the stoichiometric point just above the catalytic surface, which is dictated by the relative diffusivities of the fuel and oxygen in the mixture [37]. The diffusivities of H2 and O2 in N2 are approximately 47 and 12 cm2 min21, respectively, at 25°C and 1 atm. Thus, it is at an approximately equimolar reactant mixture in the bulk gas, that results in a maximum reaction rate at the catalyst surface. This point has been defined as the surface stoichiometric point, and it occurs at a bulk H2/O2 ratio of ;0.88 [22]. The results in Fig. 5

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confirm the existence of a surface stoichiometric point as predicted by Bui et al. [22]. In the remainder of the discussion, the terms fuel-lean and fuel-rich will be used relative to the surface stoichiometric point. From Fig. 5, we also note that as the dilution decreases, the upper flammability limit is extended significantly to more fuel-rich mixtures, with a corresponding temperature of ;420 K. On the fuel-lean side, autothermal behavior is still apparent for the fuel-leanest mixtures that can be reliably measured in our system, H2/O2 ;0.02. It is interesting to compare these flammability limits to those measured for methane. Using a similar stagnation geometry, Veser and Schmidt [8] reported the lower and upper catalytic flammability limits for undiluted methane/ air mixtures to be relatively narrow, CH4/O2 ;0.53 and ;0.98, respectively. The relatively narrow flammability regime implies that for methane, good process control is required for any industrially implemented device, in order to avoid extinction. In comparison to simulated inert surface autotherms at identical conditions, the catalyst is found to expand the fuel-lean flammability limit. For example, experimental data for 80% N2 dilution shows that the fuel-lean flammability limit occurs below H2/O2 ;0.02, whereas for the simulated inert surface, it occurs at H2/O2 ;0.77. This is a significant advantage of catalytic combustion over homogeneous combustion alone, regarding flame stability. STAGNATION FLOW MODEL PREDICTIONS The results of the stagnation flow model are also shown in Fig. 5. Most of the parameters in the model were fixed by the reactor conditions, such as the pressure, inlet H2/O2 ratio, and dilution. However, two parameters, heat loss and strain rate, are specific to the details of the reactor setup, and had to be estimated. The strain rate denotes the velocity gradient outside the boundary layer and quantifies how fast the reactants flow against the catalytic stagnation plane. For diluted reacting mixtures in which displacement effects can be ignored, the strain rate can be approximated as 2u/L

172 [38], where u is the velocity of the reactant mixture at the inlet, and L is the distance from the inlet to the catalytic surface. For our setup, u is ;2.5 cm/s, and L is ;1 cm, giving an approximate strain rate of around 5 s21. Heat loss from the platinum foil is assumed to be manifested as radiation from the surface, convective and conductive losses due to flow at the back of the platinum surface, and conduction through the leads. Typical emissivity of platinum is ;0.1, while the emissivity of ceramic glaze may be higher [39]. Hence, for the simulations, the emissivity of the combined platinum and glaze system was kept at a constant of 0.3. The conduction term from the front of the catalyst was calculated as a result of the energy boundary condition for stagnation flow. All the other heat losses were lumped into an effective convective heat transfer coefficient, h L. This heat loss term, h L, was determined by matching the maximum experimental autothermal temperature for a single dilution of 88% N2, yielding a value of 4.0 3 1024 cal/cm2 s K. Subsequently, this value of h L was kept constant, and used to predict autothermal temperatures for the entire range of H2/O2 ratios and dilutions. The model predictions show good agreement with experiments, for low dilutions such as 84% and 80%. However, as the dilution increases, the fuel-lean autothermal temperatures are slightly underpredicted, whereas the fuel-rich autothermal temperatures are slightly overpredicted. The surface stoichiometric point (the maximum in autothermal temperatures) is also well predicted by the model, but the agreement is better for higher dilutions. Finally, for all dilutions, the fuel-rich flammability limit is underpredicted by the simulations. The differences in the surface stoichiometric point, as well as the underprediction of the fuel-rich flammability limit will be discussed below in more detail. Figure 6 illustrates, for the ignited catalyst, the predicted coverages of various surface species as a function of the H2/O2 ratio along the autotherm, when the dilution level is 88% N2. Most striking is the dramatic transition from an essentially O* covered surface, to a primarily H* covered surface as the surface stoichiometric point is crossed by increasing the H2/O2 ratio. This behavior was seen, in general, for all

N. E. FERNANDES ET AL.

Fig. 6. Model prediction of coverages of surface species along the autotherm corresponding to 88% N2 dilution, 5 s21 strain rate, 4.0 3 1024 cal/cm2 s K heat loss term, and atmospheric pressure. For fuel-lean conditions, the surface is dominated by O*; H* dominates for fuel-rich conditions. At the surface stoichiometric point of H2/O2 ;0.9, there is a sharp transition in the dominant surface species.

of the autotherms calculated using the stagnation flow model. We also remark that the local maximum in vacant sites (*) is indicative of higher reactivity on the catalyst resulting in a local maximum in H2O formation, justifying our definition of a surface stoichiometric point. It should be noted that numerically, we have also observed a very small second set of catalytic autotherms very close to the stoichiometric point, which is not further discussed here. On the fuel-rich side (H2/O2 . 1), OH* coverage is very low because of excess H*, which rapidly consumes OH* (* denotes vacancies or an adsorbed species). Hence, the primary surface reaction pathway for H2O is H* H* 1 O* 3 OH* O ¡ H2O* 3 H2O. However, on the surface fuel-lean side, O* dominates. This leads to a different surface reaction path to form H2O, namely, the recombination of OH* with itself: 2OH* 3 H2O* 1 O* 3 H2O 1 O*. Since this reaction is slow, the coverage of OH* is relatively high. This high coverage of OH* has important ramifications for gas-phase combustion near platinum, since at higher temperatures, platinum will act as a source of OH radicals

AUTOTHERMAL BEHAVIOR OF H2/O2 ON PLATINUM

Fig. 7. Model prediction of profiles of major species along the length of the reactor, at specific H2/O2 ratios indicated for conditions identical to Fig. 6. For fuel-lean mixtures (a), transport of H2 to the surface is rate-limiting. At the surface stoichiometric point (b), both H2 and O2 are completely consumed at the surface. For fuel-rich mixtures (c), transport of O2 to the surface is rate-limiting. Close to the upper flammability limit (d), chemistry becomes slower, and both transport and chemistry are important.

on the surface fuel-lean side. In contrast, this catalyst-assisted homogeneous combustion behavior through OH desorption is not important on the surface fuel-rich side. If these results apply to hydrocarbons as well, then use of laser-induced fluorescence (LIF) to measure OH may be limited to fuel-lean mixtures of interest in catalyst-assisted homogeneous combustion but not to fuel-rich mixtures of interest in partial oxidation. Figure 7 shows the calculated mole fraction profiles for various stable species along the length of the reactor at different H2/O2 ratios at autothermal conditions, for 88% N2 dilution. The results show that at the entrance of the reactor, the mole fractions of H2 and O2 remain unchanged. Near the surface of the catalyst, H2 and O2 mole fractions steadily decrease (due to reaction on the catalyst), primarily forming H2O. For a surface fuel-lean mixture, H2 is the limiting reactant, and just above the surface (at ;0 cm from the surface), H2 is completely depleted, while O2 shows finite concentrations (panel a). This indicates that for fuel-lean conditions, transport of H2 controls the autothermal temperature. Near the surface stoichiometric point of H2/O2 ;0.88 (Fig. 7b), H2 and O2 are both completely depleted at the catalytic surface, indicating that the transport of both H2 and O2 controls the autothermal temperature (panel b). The same

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trend is seen for a fuel-rich composition of H2/O2 5 1.10 (Fig. 7c), for which the transport of O2 to the surface is now rate-limiting. So far, results of Figs. 7a– c are consistent with the general understanding that phenomena that occur after ignition (such as autothermal behavior) are transport-controlled. However, an interesting feature has been found for mixtures close to the flammability limits. For example, for a mixture of H2/O2 5 1.25 (Fig. 7d), both the mole fractions of H2 and O2 are finite, indicating that surface reactions are not infinitely fast. In fact, as the H2/O2 ratio increases along the fuel-rich side of the autotherm, the relative concentration of O2 compared to the bulk increases. This result is consistent with our understanding of bifurcation behavior. Autothermal behavior emerges from an extinction point crossing from the subspace of positive to negative power. The flammability limits are, in fact, two autothermal points that coincide with extinction points. Extinction occurs at low reactivity, which can be viewed as a combined transport-kinetic phenomenon. In this context, at surface fuel-rich conditions close to the flammability limit, one should expect the system to behave as it would at an extinction point. In other words, the surface reactions should also become quite important compared to transport. The relative importance of transport versus surface kinetics is compared in more detail in the following sensitivity analysis section. INFLUENCE OF KINETIC AND TRANSPORT PARAMETERS ON AUTOTHERMAL BEHAVIOR The influence of reaction kinetics and transport properties on autothermal behavior was investigated numerically through sensitivity analysis (SA), by perturbing the reaction preexponential of each surface reaction or the Schmidt number of each species, respectively. Calculation of the Schmidt numbers along the length of the reactor indicates that changes of up to 5% can occur. Three different responses to perturbations in reaction kinetics and Schmidt numbers were studied: the autothermal temperature at fuel-lean and fuel-rich conditions, the fuel-rich flammability limit, and the H2/O2 ratio at which

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Fig. 8. Sensitivity of autothermal temperatures with respect to surface reaction preexponentials (a) and species Schmidt numbers (b) for fuel-lean and fuel-rich mixtures, for conditions identical to Fig. 6. Autothermal temperature is most sensitive to the adsorption of O2 and H2, the desorption of H*, and to the transport of O2 and H2, depending on the conditions.

the maximum autothermal temperature occurs. For all the results presented, the reaction preexponential was halved for SA on kinetic parameters, and the Schmidt number was increased by 10% for SA on transport parameters. Figure 8 shows the influence of kinetic and transport parameters on autothermal temperature, for two different fuel compositions, a surface fuel-lean case of H2/O2 5 0.6 and a surface fuel-rich case of H2/O2 5 1.10. The response is defined as Relative Change in Autothermal Temperature 5

T perturbed 2 T nominal . T nominal

For reaction preexponentials, the results show that only two reactions significantly influence autothermal temperature for fuel-lean conditions, namely the adsorption of H2 and O2. In particular, decreasing the preexponential of H2

N. E. FERNANDES ET AL. adsorption (i.e., the sticking coefficient) decreases the autothermal temperature, whereas decreasing the preexponential of O2 adsorption has the opposite effect on autothermal temperature. This is consistent with our understanding of the system. For fuel-lean conditions, H2 is the limiting species. Since H2 and O2 adsorb competitively on platinum, the surface is covered primarily by O*. Therefore, any reaction that facilitates the adsorption of H2 over that of O2, will, in general, increase the reactivity of the platinum surface, resulting in a higher autothermal temperature. It is interesting to note, however, that neither the desorption preexponential of H* or O* is important to autothermal temperature. For the surface fuel-rich case, the opposite situation from the surface fuel-lean case occurs. Because H* now blocks the surface, decreasing the sticking coefficient of H2 increases autothermal temperature, whereas decreasing the sticking coefficient of O2 decreases autothermal temperature. An interesting difference from the surface fuel-lean case is that in addition to the adsorption of both H2 and O2, the desorption of H* from the surface also significantly influences autothermal temperature: decreasing the desorption rate of H* from the surface inhibits O2 adsorption, leading to lower autothermal temperatures. The difference compared to the surface fuel-lean side is that the desorption of H* is much faster than that of O* due to its significantly lower activation energy of desorption (see also Table 1). At these relatively low autothermal temperatures, desorption of O* is so slow that a small change in the preexponential of desorption does not affect the results. The autothermal temperature response to SA on Schmidt numbers of all species (Fig. 8b) shows results consistent with Fig. 8a. The results indicate that the transport of only H2 and O2 is important to autothermal temperature; in particular, autothermal temperature is most sensitive to H2 transport for fuel-lean conditions and to O2 for fuel-rich conditions. The results of Fig. 8 indicate that indeed, in the bulk of the flammability range, transport (or adsorption) of H2 and O2 dominates autothermal temperature. However, although transport of reactants is important for autothermal behavior, the results of Fig. 7d suggest that the limits of flammability would behave similar to extinction points, which could be sensitive to kinetic parameters. Hence,

AUTOTHERMAL BEHAVIOR OF H2/O2 ON PLATINUM

Fig. 9. Sensitivity of the upper flammability limit (a) and the maximum autothermal point (b) with respect to surface reaction preexponentials and species Schmidt numbers for conditions identical to Fig. 6. The upper flammability limit is sensitive only to the adsorption of O2 and H2, the desorption of H*, and the transport of H2 and O2. The maximum in autothermal temperature is sensitive primarily to the transport of H2 and O2.

Fig. 9 examines the sensitivity of the fuel-rich flammability limit to both reaction preexponentials and Schmidt numbers. In addition, the location of the surface stoichiometric point (the maximum in autothermal temperature) is studied in this SA, because it is expected to be a strong function of multicomponent transport, and insensitive to kinetic parameters. In Fig. 9, the response is defined in terms of the change in H2/O2 ratio, Relative Change in H2/O2 Ratio

5

H2 H2 2 O 2perturbed O 2nominal H2

.

O 2nominal For reaction preexponentials (Fig. 9a), similar trends are seen as in Fig. 8a. Adsorption of both O2 and H2, and the desorption of H2 control the

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upper flammability limit. Decreasing the sticking coefficient of H2 increases the fuel-rich flammability limit, whereas the opposite holds for decreasing the sticking coefficient of O2 or the desorption preexponential of H*. In the response to Schmidt number SA, only the transport of H2 and O2 is important, with the upper flammability limit being more sensitive to the O2 Schmidt number than that of H2. In comparison, the surface stoichiometric point is much more sensitive to the Schmidt numbers of H2 and O2. An increase in O2 Schmidt number (or decrease in O2 diffusivity) shifts the maximum autothermal temperature to more fuel-lean ratios, and vice-versa for H2. As seen in Fig. 5, the model-predicted location of the maximum in autothermal temperature (the surface stoichiometric point) is independent of dilution. In contrast, the experimental data shows that the location of the maximum in autothermal temperature shifts to higher H2/O2 ratios with decreasing N2 dilution. The SA results indicate that transport of H2 and O2 dictates the location of the maximum autothermal temperature. For computational efficiency, Schmidt numbers are assumed constant throughout the boundary layer and are evaluated at inlet conditions. Upon closer investigation, we have found that the ratio of the diffusivities of H2 to O2 can decrease by as much as 10% from inlet to surface conditions. Furthermore, the ratio of H2 to O2 diffusivities at the surface decreases systematically with decreasing dilution under reactive conditions, which is the opposite trend found for the ratio computed at inlet conditions. A lower ratio of H2/O2 diffusivities implies a more fuel-rich surface stoichiometric point, since more H2 is needed to maintain surface stoichiometric conditions. Hence, the change in the location of the surface stoichiometric point with dilution seen experimentally, is most probably a manifestation of the changing H2/O2 diffusivities along the boundary layer with dilution, under reactive conditions. Another factor that could lead to differences in predicted and experimental surface stoichiometric points is the limitations of the experimental geometry. For example, the stagnation model assumes an infinite catalytic surface with no physical boundaries, while in the experimental setup, the finite size of the catalyst causes edge

176 effects. In particular, the size of the catalyst in relation to its physical boundaries might alter the local velocity just above and around the catalyst, by blocking a portion of the channel diameter, influencing transport of both H2 and O2. For typical autothermal temperatures and species concentrations, calculations indicate that the mass flux of H2 due to temperature gradients is quite small in comparison to the H2 mass flux due to concentration gradients. However, it is expected that thermal diffusion might be significant when large gradients in temperature occur near the surface, such as for a homogeneously burning flame stabilized near a cooled catalytic surface. Another difference between modeling and experiment shown in Fig. 5 is that the model consistently underpredicts the upper flammability limit for all dilution levels. The results of Fig. 9a show that a couple of kinetic parameters might be responsible for this discrepancy: the sticking coefficient of O2 or H2, or the desorption of H*. As an example, the SA results indicate that if the desorption of H* is increased, then the upper flammability limit will increase. Studies of H* desorption from platinum surfaces by other researchers have indicated that for high surface coverages of H*, there is H*-H* repulsion, facilitating desorption of H*. This interaction can be approximated as a linear relationship [40, 41] E a~desorption! 5 E oa 2 A u , where E 0a is the nominal activation energy of desorption at zero surface coverage, A is the adsorbate–adsorbate interaction parameter, and u is the surface coverage. Norton et al. [40] have reported an interaction parameter of ;1.7 kcal/mol for deuterium on Pt (111) through experimental investigation of the isosteric heat of adsorption as a function of surface coverage. Previous investigators have found good agreement in catalytic ignition temperature of H2 on platinum between modeling and experiments, for H*-H* interaction varying from 1.38 kcal/ mol [14, 17] to 7.84 kcal/mol [15]. The results of varying the A factor on the upper flammability limit are shown in Fig. 10a, for 88% dilution. A H*-H* interaction parameter of ;2.5 kcal/mol shows good agreement between the experimental results and model prediction, both in autothermal temperature as

N. E. FERNANDES ET AL.

Fig. 10. Model prediction of autotherms for different H*-H* interaction parameters (a), for conditions identical to Fig. 6. As the H*-H* interaction energy increases, the upper flammability limit extends and the autothermal temperature near the upper flammability limit increases. An interaction parameter of 2.5 kcal/mol (thick dotted lines in panel b) results in better agreement between experimental data and model predictions. A temperature-dependent O2 sticking coefficient with a 3.7 kcal/mol H*-H* interaction energy (thin dashed lines in panel b) exhibits similar results.

well as the upper flammability limit. Higher values of H*-H* interaction lead to better agreement in upper flammability limit, but a larger discrepancy in autothermal temperature. Results using this H*-H* interaction parameter of 2.5 kcal/mol to different dilution levels are shown in Fig. 10b (thick dotted lines). Good agreement is found for all dilutions. The H*-H* interaction parameter is one way to interpret the higher upper flammability limits seen experimentally and is probably not unique. Kasemo and coworkers, for example, have obtained good agreement between experimental catalytic ignition data and modeling using two quite different sets of H2 sticking coefficient and H*-H* interaction parameter [15], as well as a temperature-dependent sticking coefficient of

AUTOTHERMAL BEHAVIOR OF H2/O2 ON PLATINUM O2. As shown in the SA of Fig. 9a, the upper flammability limit will also increase with increasing sticking coefficient of O2 or decreasing sticking coefficient of H2. Changes in these parameters (not shown) indicate similar effects on the upper flammability limit as seen by implementing H*-H* interaction. Further analysis of the autothermal temperature has been performed, by varying the sticking coefficients of H2 and O2. The results indicate that it is not so much the individual values of the sticking coefficients that are important, but rather their relative ratio. For fuel-lean conditions, the catalytic autothermal temperature is sensitive only to the ratio of the sticking coefficients of H2 and O2, whereas for fuel-rich conditions, the desorption of H* also becomes important. A similar dependence is also found for the catalytic ignition, and we believe that this behavior arises from the competitive nature of H2 and O2 adsorption onto the platinum surface. These results suggest that with autothermal data alone, only the ratio of the sticking coefficients of H2 and O2, relative to the desorption of H* can be estimated, but not the absolute values of these parameters. Careful study of other experimental data, such as the direct measurement of species desorption from polycrystalline platinum as a function of reactor conditions, is needed to further elucidate this matter. Finally, the influence of several other surface reaction parameters on catalytic autothermal behavior has also been investigated. For O*-O* interaction, a linear decrease in the activation energy of desorption as a function of O* coverage has been observed, on the order of ;8 –10 kcal/mol for 0.25 monolayer coverage [42, 43]. However, incorporation of a linear O*-O* interaction parameter ranging from 8 to 40 kcal/ mol has no influence on the catalytic autotherm, because on the fuel-rich side, the O* coverage is very low. On the other hand, for fuel-lean conditions, where the O* coverage is significant, one would expect to see an influence from O*-O* interactions. However, because the nominal activation energy of desorption for O* from an empty surface is quite high at 52 kcal/mol, typical values for O*-O* interaction energy have no influence on the autothermal temperature. Another parameter of consideration has been the sticking coefficient of O2. Its dependence on

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gas-phase temperature, surface temperature, and surface coverage of species has been under investigation and debate for some time. Using a 1/T dependence for the O2 sticking coefficient, Rinnemo et al. [15] have reasonably modeled H2 catalytic ignition temperatures. Figure 10b (thin dashed lines) shows the model-predicted autotherms, using this temperature dependence of O2 sticking coefficient (see Table 1) and a higher H*-H* interaction energy of 3.7 kcal/mol. More specifically, the sticking coefficient is taken to be 8/T, which corresponds to reported literature values for the sticking coefficient of O2 on a clean platinum surface at room temperature. As seen from Fig. 10b, the fuel-rich flammability limits are slightly better predicted at lower dilutions, but overpredicted at higher dilutions when compared to the previous case of constant O2 sticking coefficient and H*-H* interaction of 2.5 kcal/mol (thick dotted lines). Otherwise, no differences are seen in the predicted autotherms. CONCLUSIONS The intent of this paper was to experimentally obtain the autothermal behavior of H2/O2 mixtures over a platinum foil catalyst, for the first time, and by comparison with our detailed simulations, learn more about the fundamental coupling between transport and reaction on the catalyst surface. Our results show that the platinum catalyst significantly extends the limits of flammability beyond those for noncatalyzed H2/O2 mixtures. In our experiments, the lower flammability limit was below the limit that can be accurately measured, even for the most diluted mixtures. As the level of N2 dilution increases, the autothermal temperatures decrease, and the upper flammability limit decreases. The maximum in autothermal temperature, located at the surface stoichiometric point of H2/O2 , 1, first proposed and explained by Bui et al. [22], was confirmed experimentally. The detailed stagnation flow model was able to quantitatively reproduce most of the experimental data, including the maximum at the surface stoichiometric point. However, the fuel-rich flammability limit was underpredicted for all dilution levels. Sensitivity analysis on the fuel-rich

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flammability limit indicated that the desorption of H* from the catalytic surface could be a factor for this disagreement between experiments and simulations. Incorporating a reasonable value for the H*-H* interaction (within the range previously used by other investigators to model catalyst ignition) resulted in better agreement between the model and experiment. Acknowledgment is made for partial support of this work to the Office of Naval Research with Dr. G. D. Roy through a Young Investigator Award under Contract N00014-96-1-0786.

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Received 2 June 1998; revised 5 November 1998; accepted 20 November 1998