In Situ Diagnostics and Modeling of Methane ... - ACS Publications

measured reactivity with that predicted by the CHEMKIN SPIN stagnation-flow code when combined with recently developed partial oxidation elementary ...
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Ind. Eng. Chem. Res. 2003, 42, 6559-6566

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In Situ Diagnostics and Modeling of Methane Catalytic Partial Oxidation on Pt in a Stagnation-Flow Reactor Joshua D. Taylor, Mark D. Allendorf, Anthony H. McDaniel, and Steven F. Rice* Combustion Research Facility, Sandia National Laboratories, MS 9052, P.O. Box 969, Livermore, California 94551-0969

The effect of catalyst temperature on the partial oxidation reaction pathways of methane over platinum is investigated in a stagnation-flow reactor. A new experimental method is described that uses Raman spectroscopy to measure the concentration of CH4 along the centerline of the reactor with the Pt surface at 900-1100 °C. The method permits the direct comparison of the measured reactivity with that predicted by the CHEMKIN SPIN stagnation-flow code when combined with recently developed partial oxidation elementary reaction mechanisms. A significant increase in the reactivity of CH4 on Pt is observed between 1000 and 1100 °C, concurrent with an increase in the selectivity to H2 and CO. No single mechanism available in the literature correctly models the increase in reactivity or the change in selectivity in this temperature range. The experimental results are interpreted by examining the competitive adsorption between CH4 and O2 and the two pathways by which CH4 can undergo dissociative adsorption. The temperature dependence of the sticking coefficient for the direct dissociative adsorption of CH4 is specifically identified as an important yet highly uncertain parameter in the reaction mechanism. Introduction The conversion of methane to synthesis gas is of growing importance to the petrochemical industry. This process is typically conducted in large steam reforming units and consumes significant amounts of energy because of the highly endothermic reaction involved (∆Hrxn ) 206 kJ/mol of CH4). Catalytic partial oxidation of methane over noble metals (Pt or Rh) in short-contacttime reactors (SCTRs) has been proposed as an advantageous pathway for syngas production because of the compact design of SCTRs. In such a configuration, energy savings result from the efficient use of the net exothermic reaction of partial oxidation (∆Hrxn ) -36 kJ/mol of CH4), and the product syngas has a desirable H2/CO ratio of 2:1.1 Such a composition is ideal for subsequent Fischer-Tropsch processing or methanol synthesis. Furthermore, conversions and selectivities comparable to those of steam reformers have been achieved in SCTRs.2 Experimental autothermal SCTRs for methane conversion to synthesis gas applications have a typical operating temperature range of 900-1200 °C, depending on the feed composition, geometry, and preheat conditions.2-4 This is an especially interesting temperature range because it lies between the purely heterogeneous and homogeneous ignition/extinction branches for a range of methane-oxygen stoichiometries.5 In an effort to better understand the fundamental chemistry that occurs in SCTRs, a number of detailed surface reaction mechanisms have been proposed for the partial oxidation of methane on both platinum2,6-9 and rhodium.2 These mechanisms have been primarily optimized to end-of-pipe composition measurements in reactors with porous catalytic monoliths in the 8001200 °C range or to ignition phenomena in the 500* To whom correspondence should be addressed. Tel.: (925) 294-1353. Fax: (925) 294-2276. E-mail: [email protected].

700 °C range. In the end-of-pipe experiments, complex transport and steep temperature gradients exist. Because the temperature profiles through the foam monoliths cannot be measured accurately, temperaturedependent changes in the surface reaction pathways cannot be quantitatively assessed. Although temperature sensitivity can be assessed in the lower temperature ignition data, these results may not be easily related to the higher temperature steady-state autothermal reforming conditions. In fact, recent experiments in high vacuum reveal significant incident energy and vibrational energy dependences for CH4 adsorption kinetics on platinum both with and without the presence of previously adsorbed oxygen.10-13 Therefore, steadystate catalytic experiments under well-defined flow conditions with known temperature profiles in the range of interest are desirable to better probe the surface chemistry. Stagnation-flow reactors (SFRs) can be used to study complex heterogeneous chemical reaction systems under well-understood temperature and flow conditions. Several authors have previously used SFRs to study catalytic combustion systems in order to probe and refine elementary reaction models.7,14-19 These studies are based on ignition temperature measurements or downstream composition analysis. In a previous publication,20 we show the utility of the stagnation-flow configuration as an ideal platform to compare mechanistic predictions within a computationally simplified flow field by examining the downstream reaction product profile with mass spectrometry. Recently, Park et al.21 demonstrated that Raman spectroscopy can be used to examine temperature profiles in the stagnation-flow geometry with a noncatalytic substrate and that these results can be compared to the predictions of a computational flow dynamics model. In this paper, we extend our use of a SFR by using Raman spectroscopy to measure directly the centerline composi-

10.1021/ie020934r CCC: $25.00 © 2003 American Chemical Society Published on Web 11/06/2003

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tion in a catalytically reactive SFR. Using the SPIN stagnation-flow simulation code,22 we compare the measured concentration profiles with those predicted by elementary reaction models available in the literature. Experimental Methods A schematic representation of the SFR used in these experiments is presented in work of McDaniel et al. along with a detailed description of the reactor operation.20 In general, the apparatus is a stainless steel vacuum chamber equipped with a gas-handling manifold and a controlled downstream butterfly valve with a mechanical pump that maintains the reactor pressure at 0-100 Torr. The catalytic substrate is a 250-µm-thick piece of polycrystalline platinum foil (99.99% purity) fastened to a Boralectric (Advanced Ceramics) heater. Thermocouples described in ref 20 for temperature measurement were supplemented with measurements from a two-color pyrometer. In the experiments presented here, only the reactants are fed into the system, without a diluent in contrast to ref 20, at a total flow rate of 4.5 SLPM with a composition of 2:1 CH4/O2. This results in a uniform velocity of approximately 350 cm/s across the entire injector. The injector is radiatively heated by the hot catalyst surface to approximately 500 °C. The optical path shown in the apparatus figure in ref 20 is used to measure the concentration of CH4 along the centerline of the SFR using Raman spectroscopy. A manual micrometer translates the injector and catalytic substrate vertically within the vacuum chamber, allowing concentration measurements at distances of 0.519.5 mm from the platinum surface. The light source for the Raman apparatus is a pulsed Nd:YAG laser (doubled to 532 nm) operating at 20 Hz and 4 W output (measured after the reactor). The Raman spectra are measured with an intensified change coupled device (CCD) array (Roper Scientific) with a gate width of 50 ns to minimize interference of blackbody radiation from the substrate. Scattered light from 5000 laser pulses is accumulated, and standard background subtraction and correction methods are used to condition the signal. The concentration of CH4 is determined by the integration of the recorded intensity of the symmetric C-H stretching Raman band, located at 2914 cm-1.23 The relationship between the spontaneous Raman signal and the species concentration is developed in many texts on the subject. To be used as a measurement of the concentration in this instance, the integrated intensity recorded for the 2914 cm-1 band is recognized to be directly proportional to the species concentration, the input laser power, the Raman cross section for scattering at this particular Raman shift, and a number of geometric and polarization factors associated with the collection optics and detection apparatus. If all of the experimental parameters other than concentrations are held constant throughout the experiment, the direct integration of the recorded intensity can be calibrated to a known concentration and used as a reference for a complete set of measurements. Calibration for the concentration of methane is done by measuring the signal of the feed mixture (2:1 CH4/ O2) immediately above the injector at reaction conditions (Tin ∼ 500 °C and 30 Torr). The system is calibrated while the reactor is hot because the alignment of the collection lens shifts as the system heats and therefore must be reoptimized at steady-state conditions. This

calibration technique assumes that no gas-phase reaction occurs at the inlet conditions and the signal corresponds to a 20 Torr partial pressure of CH4. This was verified by shutting off the oxygen feed and measuring the concentration of pure CH4 at 30 Torr, and good agreement was found. The integrated signal is directly proportional to the concentration of CH4 and can therefore be linearized from this single-point calibration. Additionally, an empirically determined geometry correction factor is applied to the measured concentration to account for the reduction in the solid collection angle of scattered light near the catalytic surface. A typical experimental measurement involves initially flowing a rich H2/O2 mixture (4:1) at 3.0 SLPM with the system at 10 Torr while the platinum substrate is heated. This “water chemistry” ensures that the surface is catalytically active and clean prior to the partial oxidation experiments. When the system is at a temperature greater than 800 °C, the methane flow is initiated and the hydrogen flow is discontinued. The flows are adjusted to the proper feed mixture (pure CH4 or CH4:O2 ) 2:1) and flow rate (4.5 SLPM). The pressure is then increased to 30 Torr, and the power input is adjusted manually to obtain the desired surface temperature. The system is allowed to equilibrate for approximately 1 h, and then the Raman signal is calibrated as described previously. The concentration of CH4 is measured at distances of 0.5-19.5 mm from the platinum surface at intervals of 1 mm. The temperatures of the surface and the inlet were logged with each measurement and did not change significantly over the course of the measurements. Experiments were conducted at surface temperatures of 900, 1000, and 1100 °C for this work. Additionally, the concentration profile of pure CH4 was measured with a surface temperature of 900 °C to validate the technique under experimental conditions. In general, the platinum surface remained clean. At 900 °C, over time, a thin carbon deposit accumulates on the edges of the substrate. At lower temperatures (e.g., 800 °C), deposit formation is more rapid and will eventually coat the entire surface. The deposit is easily removed by a brief exposure of 4:1 H2/O2 at 10 Torr. Above 1000 °C, coking was not an issue. Care was taken to record data only with a clean substrate. The subsequent figures in this paper that present methane concentration versus position do not include error bars in the interest of clarity. There are two main sources of measurement error that produce the scatter in the data. The first source of error originates from the statistical noise on the Raman signal itself, as can be seen in the raw Raman data. This background noise makes a contribution that is independent of the signal magnitude and corresponds to a 1σ value of about (2% of the full-scale signal at a point far from the substrate. The second and more significant measurement error originates from drift in the optical detection system over the course of a day’s measurements. Experiments were conducted by starting at a position far from the substrate, followed by translating to 0.5 mm, and then translating back again. As the windows of the reactor increased in temperature over a number of hours, small deviations in the laser position and collection imaging efficiency could continue to develop. These drifts in alignment appear as scatter in the data because points

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appearing adjacent to each other were generally not collected adjacent in time. Computational Methods and Modeling SFR Modeling. The reactor system was modeled using a one-dimensional stagnation-flow code (SPIN) developed at Sandia National Laboratories as part of the CHEMKIN collection of software tools.22 The SPIN program approximates an SFR as an inlet flow (uniform velocity) impinging perpendicularly on an infinite plate, creating a stagnation point along the centerline. SPIN utilizes algorithms for gas-phase chemistry mechanisms, heterogeneous reaction mechanisms, and mixtureaveraged transport properties. The user supplies the feed temperature, composition, and velocity, the separation from inlet to surface, the surface temperature, and the system pressure. Given a set of input conditions, the program solves the one-dimensional steady-state mass, transport, and energy equations and outputs the velocity, temperature, and mole fraction of each species along the centerline of the reactor. Using this information and the ideal gas law, the concentration of each species can then be calculated along the centerline. The calculated concentration profile includes two contributions: (1) a decrease in concentration due to the temperature increase approaching the substrate and (2) a change in concentration due to reactions that either produce or consume a particular species. These model predictions of the centerline concentration can be compared directly to the Raman spectroscopy measurements. The output from the SPIN code can also be used to predict the bulk composition of the effluent gas using a treatment developed by Takeno and Nishioka24 and described in more detail previously.18,20 This method allows model predictions to be compared with measurements obtained from the mass spectrometer. The approach assumes that no downstream reactions, such as steam reforming or water-gas shift, occur that would change the product composition. In a SFR, most of the input gases do not interact with the platinum surface but rather flow around the catalyst and out the exhaust. The result is that the total conversion of CH4 is less than 10% and is not a very sensitive measure of the CH4 reactivity on the Pt catalyst. Instead, it is useful to define an SFR conversion parameter, χs, which represents the effective centerline conversion of CH4 at the Pt surface:

χs )

|

[CH4]no rxn - [CH4]0 [CH4]no rxn

(1) at Pt surface

where [CH4]no rxn is the concentration of methane at the surface assuming no reaction occurs. (Note that [CH4]no rxn is always e[CH4]feed because of the increase in gas temperature close to the Pt surface.) Similarly, [CH4]0 is either the extrapolation of the Raman CH4 profile to the Pt surface (recall that optical measurements cannot be made closer than 0.5 mm from the surface) or the concentration predicted by the model in the computational cell immediately adjacent to the Pt surface. With this definition, χs ranges from 0 to 1, where χs ) 0 corresponds to the case where no reaction occurs and χs ) 1 corresponds to complete consumption of CH4 at the Pt surface. Reaction Mechanisms. The SPIN code was used to compare three elementary surface reaction mechanisms

available in the literature. The first mechanism was published by Zerkle et al. and was developed to model both C1 and C2 partial oxidation reactions in SCTRs.9 This mechanism was chosen because it is the most recently published that addresses this system and has been shown to model ethane partial oxidation in SCTRs accurately. The mechanism is thermodynamically consistent and consists of 19 surface species and 41 reversible reaction steps, of which 9 species and 19 reactions involve C2 species. Although this mechanism was originally applied to ethane dehydrogenation, it has a complete C1 surface mechanism embedded within. The second mechanism was published by Deutschmann et al. for the partial oxidation of methane to syngas and consists of 10 surface species, 9 reversible reactions, and 8 irreversible reactions.6 This mechanism was chosen because it has been widely used (in several variations) over the past decade. The modification of this mechanism’s surface site dependence for methane adsorption that was employed by Raja et al.25 was not included. The third mechanism chosen is a recent version presented by Veser and Frauhammer that has its roots in the original work in this field by Hickman and coworkers.8 Homogeneous chemistry is modeled by a modified hydrocarbon oxidation mechanism from Glarborg et al. in which reactions involving species with more than two carbon atoms are omitted.26 A second gas-phase mechanism developed to model combustion of natural gas (GRI-Mech 3.0) is also used for comparison.27 However, gas-phase reactions are not significant under these conditions, and the two homogeneous mechanisms yield identical results when combined with any of these surface mechanisms. Results Modeling Results. A comparison of the SPIN output for two of the three surface mechanisms is shown in Figure 1 using a surface temperature of 1000 °C for illustration. To orient the reader, these plots show the feed composition on the right side and the composition at the Pt surface on the left side. The shape of the intermediate region on the plot results from diffusion of reactants to the surface and upstream diffusion of products formed on the Pt surface. The qualitative results from the mechanisms do not change significantly over the temperature range investigated (900-1100 °C). The Zerkle mechanism shows a slight decrease in reactivity with increasing temperature due to slower O2 adsorption. The Deutschmann mechanism shows a significant accumulation of adsorbed atomic carbon [C(s)] at 1100°C, which reduces the reactivity of the Pt surface. All results are unaffected by “turning off” the homogeneous chemistry, which confirms that all chemistry occurs on the surface. The Veser mechanism shows very little reaction in this temperature range and under these reaction conditions. The results of the two models that show reactivity differ significantly. The Zerkle model predicts a much lower reactivity of CH4 on the platinum surface (χs ) 0.24) than the Deutschmann model (χs ) 0.76). Additionally, the product distribution is clearly very different for the two models. While the Zerkle model predicts that nearly all of the reacted hydrogen atoms (from CH4) are converted to H2O (SH2 ) 3.5%), the Deutschmann model predicts primarily H2 formation

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Figure 2. Sample of experimentally measured CH4 Raman spectra with the Pt surface at 1100 °C and a feed composition of CH4:O2 ) 2:1. Temperatures at each distance are predicted by SPIN.

results in the production of H2O rather than H2 as shown in eqs 5-7.

CH3(s) + 3Pt(s) f f f C(s) + 3H(s)

(5)

H(s) + O(s) f OH(s) + Pt(s)

(6)

H(s) + OH(s) f H2O(s) + Pt(s) f H2O + 2Pt(s) Figure 1. Predictions of mole fraction profiles in SFR with the Pt surface at 1000 °C and a feed composition of CH4:O2 ) 2:1 using two surface reaction mechanisms: (top) Zerkle et al.;9 (bottom) Deutschmann et al.6

(SH2 > 99.5%). Here, the selectivity, is defined as

SH2 )

xH2 xH2 + xH2O

C(s) + O(s) f CO(s) + Pt(s) f CO + 2Pt(s) (8) In the Deutschmann model, the only pathway by which CH4 adsorbs onto the Pt surface is the direct dissociative adsorption:

CH4 + 2 Pt(s) f CH3(s) + H(s) × 100%

(2)

where xi is the mole fraction of species i in the effluent gas.20 Both models predict very high carbon selectivities to CO (SCO > 97%), where

xCO × 100% SCO ) xCO + xCO2

This difference in selectivity for hydrogen is a result of the different pathways by which CH4 adsorbs onto the platinum surface. In the Zerkle model, the primary pathway for CH4 adsorption (as determined by a sensitivity analysis) under these conditions is

CH4 + O(s) + Pt(s) f CH3(s) + OH(s)

(4)

where (s) indicates a surface species. This oxygenassisted adsorption requires surface oxygen to be present prior to CH4 adsorption. In this case, the steady-state coverage of O(s) reaches approximately 15% of the Pt surface sites. This abundance of O(s) on the Pt surface

(9)

Here, the rate of CH4 adsorption is faster than that of O2 adsorption. The result is that adsorbed hydrogen atoms rapidly combine to form H2, which desorbs (eqs 10 and 11). Simultaneously, as O2 adsorbs, the accumulated surface carbon is converted to CO via eq 8.

CH3(s) + H(s) + 3Pt(s) f f f C(s) + 4H(s) (3)

(7)

2H(s) f H2 + 2Pt(s)

(10) (11)

These very different reaction pathways motivate this experimental study, which aims to differentiate between the two mechanisms. The differences in the reaction pathways will be discussed as they apply to the experimental results. Experimental Results. A sample of CH4 spectra measured at several distances from the catalytic substrate (with a surface temperature of 1100 °C) is shown in Figure 2. The dominant vibrational band, with a peak at 2914 cm-1, corresponds to the symmetric stretch, ν1(a1), of CH4.22 A smaller band can be seen at 3020 cm-1 that corresponds to the asymmetric stretch, ν3(t2).22 This smaller band was not included in the area integration used to determine the concentration of CH4. The signal-

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Figure 3. Experimentally measured concentration profile of pure CH4 relative to the feed concentration with the Pt surface at 900 °C and the predicted concentration from the SPIN temperature profile.

Figure 4. Experimentally measured concentration profile of CH4 relative to the feed concentration with the Pt surface at 900 °C of a feed mixture of 2:1 CH4/O2. Also shown are SPIN predictions assuming (1) no surface reaction, (2) a Zerkle surface mechanism, (3) a Deutschmann surface mechanism, and (4) a Veser surface mechanism.

to-noise ratio is approximately 50:1 at the inlet and decreases to 4:1 at the point nearest the Pt surface. A plot of the relative methane concentration versus distance from the platinum is shown in Figure 3 for the control experiment, where pure methane was fed into the reactor with a catalyst temperature of 900 °C. In this case, no reaction occurred (as verified by the mass spectrometer) and the decrease in concentration is only due to the temperature profile in the reactor. The solid line represents the predicted concentration based on the temperature profile calculated with SPIN. Fairly good agreement is seen between the measured concentration profile and that predicted by SPIN. This result verifies that the temperature profile predicted by SPIN is consistent with that observed experimentally. Furthermore, any measurement yielding smaller values of the CH4 concentration in the experiments with oxygen present will indicate consumption due to reaction. Figure 4 shows the results for the experiment with the surface at 900 °C and a feed mixture of CH4:O2 ) 2:1. Also shown in this figure are the modeling results for three mechanisms and a curve assuming no reaction occurs on the substrate. The “No Rxn” line simply corresponds to a curve of Tfeed/T and is shown to distinguish between the contributions of temperature and reaction on the concentration of CH4. Note that the “No Rxn” curve is different from that in Figure 3. This

Figure 5. Experimentally measured and calculated concentration profiles of CH4 relative to the feed concentration with the Pt surface at 1000 °C, analogous to Figure 4.

is because the experimentally measured steady-state inlet temperature was approximately 100 °C higher at a given catalyst temperature when a CH4/O2 feed was present relative to the same flow rate of CH4 alone as a result of the much lower heat capacity of O2. The Figure 3 “No Rxn” curve corresponds to a 412 °C inlet temperature. The inlet temperature in Figure 4 is 515 °C. The experimental concentration measurements are clearly below the values of the “no reaction” case, showing a χs of 0.21, and are in good agreement with the modeling results from the Zerkle mechanism, where χs ) 0.25. The Deutschmann mechanism shows a much greater consumption of CH4 at the platinum surface (χs ) 0.76) than is observed experimentally. The results from the mass spectrometer show that the selectivity to H2 is only 10% and the selectivity to CO is approximately 60%. The results from the experiments with the platinum surface at 1000 °C are shown in Figure 5. The concentration measurements are still in fairly good agreement with the SPIN predictions using the Zerkle mechanism but show a more reactive catalyst surface than predicted. In this case, the experimental χs of 0.36 is slightly greater than the value predicted by the Zerkle mechanism (χs ) 0.24) and significantly less than that predicted by the Deutschmann mechanism (χs ) 0.77). Under these conditions, the selectivity to H2 was 11% and the selectivity to CO increased to 65%. When the temperature of the platinum is increased to 1100 °C, a significant increase in the reactivity occurs, with χs ) 0.64, as shown in Figure 6. At this surface temperature, the concentration profile is in good agreement with that predicted by the Deutschmann mechanism (χs ) 0.64), which is significantly more reactive than that predicted by the Zerkle mechanism (χs ) 0.23). Concurrent with the increase in reactivity is a shift in the selectivity. The H-atom selectivity to H2 increased to 35%, and the C-atom selectivity to CO jumped to 84%. The experimentally determined values of the SFR conversion parameter, χs, and the selectivities to H2 and CO are tabulated in Table 1 for each temperature. Discussion The experimental concentration profiles show a significant increase in reactivity with increasing temperature that is not predicted by the surface mechanisms that we have explored. Furthermore, the change in the composition of the product stream as the temperature

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Figure 6. Experimentally measured and calculated concentration profiles of CH4 relative to the feed concentration with the Pt surface at 1100 °C as in Figure 4. Table 1. Values of the SFR Conversion Parameter (χs) and Selectivities (Si) χs exptl χs Zerkle χs Deutschmann χs Veser SH2 exptl SH2 Zerkle SH2 Deutschmann SH2 Veser SCO exptl SCO Zerkle SCO Deutschmann SCO Veser

900 °C

1000 °C

1100 °C

0.21 0.256 0.758 0.001 10% 2.3% 97.4% 0.0% 60% 92.1% 99.9% 0.01%

0.36 0.249 0.769 0.002 11% 2.3% 99.5% 0.0% 65% 97.1% 99.9% 0.03%

0.64 0.242 0.769 0.002 35% 2.2% 99.6% 0.0% 84% 98.77% 99.9% 0.10%

is increased indicates that the relative importance of certain reaction pathways in the “true” surface mechanism changes at higher temperatures. At 900 °C, the experimental concentration profile of CH4 is modeled well by the Zerkle surface mechanism. This model also does fairly well at 1000 °C, although the surface is slightly more reactive than predicted. In the Zerkle mechanism, the primary reaction pathway under the conditions investigated is oxygen-assisted adsorption of CH4 (eq 4). Accordingly, the rate of CH4 adsorption depends on the prerequisite adsorption of O2 onto the Pt surface, creating a partial coverage of O(s). At steady-state conditions, the fractional surface coverage of O atoms reaches approximately 0.15 according to the predictions of SPIN. Because of the abundance of O atoms on the surface, as the CH4 adsorbs via eq 4, the released H atoms rapidly react with O(s) to form OH(s) (eq 6) or with OH(s) to form H2O (eq 7). This preferential formation of H2O is consistent with the effluent concentration measured by the mass spectrometer in the experiments at both 900 and 1000 °C. At 1100 °C, the reactivity increases significantly and the product composition shifts to produce more H2 and CO. This increase in both reactivity and selectivity (SH2 and SCO) at higher temperature is in agreement with experimental observations in SCTRs by Hickman and Schmidt.28 In those experiments, preheating the reactants to higher temperatures resulted in higher conversion and selectivity to H2 and CO. However, neither of the mechanisms tested here correctly captures the experimentally observed effect of increasing the temperature on reactivity and selectivity. According to the Zerkle mechanism, O2 adsorption decreases with increasing temperature, which slows the reaction of CH4 slightly because the presence of O(s) is necessary for CH4 adsorption. This is clearly not consistent with the

experimental observations. The Deutschmann mechanism models the CH4 concentration well only at 1100 °C and does not predict the presence of H2O in the effluent under any conditions. These experiments show that neither mechanism correctly models the system well over the entire temperature range of 900-1100 °C. The experimentally observed effects can be described by a model with competitive rates of adsorption between CH4 and O2 on the Pt surface. At the lower temperatures (900-1000 °C), the oxygen adsorbs more effectively on the surface. The result is that the surface is partially covered by O(s) and the rate of conversion is limited by the rate of CH4 adsorption. CH4 adsorption could occur by either oxygen-assisted adsorption (eq 4) or direct dissociative adsorption (eq 9). The actual pathway for CH4 adsorption at lower temperatures is not important, so long as O(s) is abundant on the Pt surface, resulting in H2O production. However, at temperatures above 1000 °C, the experimental measurements suggest that CH4 adsorbs more effectively than O2 onto the Pt surface because the CH4 concentration decreases in the experimental data from Figures 4-6 and H2 production increases (Table 1). Therefore, the high-temperature CH4 adsorption pathway must not require the presence of O(s) and is likely the direct adsorption pathway (eq 9). Because O(s) is no longer as abundant on the surface, some of the H(s) atoms recombine to form H2 as a competitive product. As the temperature is increased above 1100 °C, the selectivity to H2 should continue to increase according to this model. Veser and co-workers discussed the competition between the rates of O2 and CH4 adsorption in modeling their SCTR data.29 They describe the preferential adsorption of O2 near the SCTR inlet (at low temperature) and prior to ignition, resulting in a partial surface coverage of O(s). However, they argue that CH4 adsorption becomes competitive further downstream in the SCTR because O2 is depleted from the gas phase, thereby decreasing the O(s) concentration. Once O2 is depleted, they suggest that the partial oxidation pathway producing syngas becomes dominant. In contrast, in the model proposed above, the shift to selective partial oxidation is not due to oxygen depletion but occurs because the rate of direct CH4 dissociative adsorption increases as the temperature of the SCTR increases. At the high temperatures that are reached shortly after the entrance to the SCTR, CH4 adsorption is competitive or faster than the adsorption of O2. This difference is difficult to observe in an SCTR because the depletion of O2 and the increase in temperature occur simultaneously. However, gas-phase depletion of O2 cannot occur in our SFR experiment (recall that all reaction occurs on the surface), allowing us to isolate the effect of temperature on reaction pathways. Because the sticking coefficient for oxygen adsorption is nearly identical in the mechanisms investigated here, it is reasonable to assume that the difference in reactivity is due to the rate of CH4 adsorption. A sensitivity analysis shows that the CH4 adsorption rate has the largest effect on the CH4 concentration near the Pt surface in all mechanisms (oxygen-assisted for Zerkle and direct for Deutschmann and Veser). Although the direct dissociative adsorption pathway is not significant in the Zerkle mechanism, this pathway likely becomes more important at higher temperature, as discussed previously. Analysis of the three reaction mechanisms reveals a large disparity in the sticking coefficient (γCH4) for the

Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6565 Table 2. Values of the Sticking Coefficient (γCH4) for the Direct Dissociative Adsorption of Methane Deutschmann et al.3 Zerkle et al.6 Wolf et al.20 Veser and Frauhammer8

900 °C

1000 °C

1100 °C

10-2 5.6 × 10-7 8.6 × 10-5 10-4

10-2 1.0 × 10-6 1.9 × 10-4 10-4

10-2 1.6 × 10-6 3.6 × 10-4 10-4

direct dissociative adsorption of methane (eq 9). The value of γCH4 controls the rate of direct CH4 adsorption and could account for the differences observed in both reactivity and selectivity. In the Deutschmann model, dissociative adsorption is the only pathway by which CH4 is consumed and γCH4 has a large value of 0.01 (constant for all T ). In the Zerkle mechanism, γCH4 has small values, ranging from 5.6 × 10-7 at 900 °C to 1.6 × 10-6 at 1100 °C (eq 12).

kJ/mol [-72RT ]

γCH4 ) 9 × 10-4 exp

(12)

The Veser mechanism8 that we have used in this comparison shows no reactivity in the stagnation-flow configuration at 30 Torr. This is because of the low value of the methane sticking coefficient (1.0 × 10-4). That group has an earlier paper that presents a different view.30 In that paper, the methane sticking coefficient is 1.0 × 10-2. With this higher value, considerable reactivity is observed and results similar to those generated by Deutschmann are obtained. In an earlier work by Wolf et al.,31 which is referenced by Zerkle et al.9 for this particular reaction, the parameter values are different from those shown in eq 12. Applying the parameters for γCH4 from Wolf et al.31 results in values that are more than 2 orders of magnitude larger than those in ref 9, ranging from 8.6 × 10-5 at 900 °C to 3.6 × 10-4 at 1100 °C (eq 13).

[-72.2RTkJ/mol]

γCH4 ) 1.23 × 10-8T 2.3 exp

(13)

However, these values are still much too small to produce much reactivity on this path in the SPIN calculation. The various values for γCH4 are summarized in Table 2. On the basis of the large disparity of γCH4 values in Table 2 and the observed effect these values have on CH4 reactivity and product selectivity, it appears the sticking coefficient for this reaction step should be the main focus for improving any mechanism intended to predict methane partial oxidation on noble metal catalysts. Because γCH4 has no temperature dependence in the Deutschmann or Veser mechanisms, it is not possible to reproduce the experimentally observed trends without adding temperature dependence. We therefore focus on determining how much the direct dissociative adsorption sticking coefficient should change in the Zerkle mechanism from 900 to 1100 °C to produce the conversion temperature dependence observed in the experimental data. Acceptable agreement with the experimental measurements can be obtained with values of γCH4 that range from 5.3 × 10-4 at 900 °C to 2.3 × 10-3 at 1100 °C. In 1990, Sun and Weinberg32 published an experimentally determined expression describing the kinetics of activated dissociative adsorption on Pt(110) appropriate for the 240-470 °C range. Although the data in that work suggest that this expression may not extrapolate well to 900 °C and above, that expression produces

values of 1.2 × 10-3 at 900 °C and 3.06 × 10-3 at 1100 °C. When substituted into the Zerkle mechanism, conversions (χs) of 0.35, 0.44, and 0.52 are calculated at 900, 1000, and 1100 °C, respectively. This modification based on the experimental results in ref 31 captures the increase in H2 selectivity with increasing temperature, although the model overpredicts the magnitude of the increase, yielding 23% at 900 °C, 49% at 1000 °C, and 69% at 1100 °C compared to the experimental values of 10%, 11%, and 35%, respectively. Both mechanism and data analysis suggest that increasing the selectivity to hydrogen is coupled with an increase in the overall conversion. However, the lack of quantitative agreement points out that the competition between water and hydrogen formation is still more complicated than what the mechanisms can capture. The body of literature focused on the details of water chemistry on platinum is extensive and remains a complicated and active field of research within both the surface science and catalytic combustion communities. That the present mechanism can only qualitatively reproduce the competition between water formation and hydrogen formation in the presence of adsorbed carbon on a polycrystalline substrate is not surprising, given the uncertainties in many of surface parameters. Nonetheless, understanding hydrogen selectivity is equally as important as the improvements in modeling conversion, and this topic merits some discussion. Over the past decade, Schmidt and co-workers have led research in the catalytic partial oxidation of methane, motivated by the promise the SCTR concept holds for natural gas conversion. They have explored this process over a number of metal substrates and have observed a variety of results ranging from excellent performance in the case of rhodium to extreme coking in the case of palladium.1 In the course of that work, they suggested that the propensity for platinum to form water rather than hydrogen originated from a low (10 kJ/mol) activation barrier for O(s) + H(s) w OH(s) and identified this as the reason for reduced performance as a synthesis gas catalyst relative to rhodium, given that rhodium’s Ea ) 84 kJ/mol for this recombination.2 Recent work now points to a much more complicated description involving selected active sites for oxygen and coverage-dependent adsorption thermodynamics.33,34 The new method presented here, offering the ability to examine hydrogen/water selectivity under controlled flow conditions and a wide temperature range, suggests that more experiments on both platinum and rhodium should aid in sorting out these remaining discrepancies between elementary mechanisms and observations. Summary and Conclusions The objective of this work is to investigate the effects of temperature on the reaction pathways that dominate the partial oxidation of methane over a platinum catalyst. Experimentally measured concentration profiles of CH4 along the centerline of a SFR are consistent with the predictions of the Zerkle et al. reaction mechanism9 for 900 and 1000 °C. However, the experimentally determined CH4 reactivity increases dramatically at 1100 °C, and the selectivity shifts to favor higher production of H2 and CO. This shift is not predicted by the Zerkle et al. surface mechanism9 or the other mechanisms. The experimentally observed shift at 1100 °C can be explained by the competitive adsorption of CH4 and O2 onto the platinum surface. At and below 1000 °C, oxygen

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readily adsorbs onto the surface, resulting in high H2O production. Above 1000 °C, CH4 adsorption is competitive with O2 adsorption, depleting the surface of O(s) and resulting in a higher production of H2. These results are consistent with prior studies on the effect of reactant preheating on selectivity in SCTRs.2 This temperature dependence on the adsorption pathways of CH4 must be incorporated into existing models. The sticking coefficient for the dissociative adsorption of CH4 was identified as a highly uncertain parameter, showing values that range over 5 orders of magnitude in the various models. A detailed investigation of the rate constant is recommended to ensure that the adjusted value successfully models other experimental results. Acknowledgment The authors thank Dr. David Zerkle for helpful discussions. This work was supported by the U.S. Department of Energy, Office of Industrial Technologies, Chemical Industries of the Future Team. Sandia National Laboratories is a multiprogram laboratory, operated by the Sandia Corp., a Lockheed Martin company, for the United States Department of Energy under Contract No. DE-AC-04-94AL85000. Literature Cited (1) Bharadwaj, S. S.; Schmidt, L. D. Calalytic Partial Oxidation of Natural Gas to Syngas. Fuel Process. Technol. 1995, 42, 109. (2) Hickman, D. A.; Schmidt, L. D. Steps in CH4 Oxidation on Pt and Rh Surfaces: High-Temperature Reactor Simulations. AIChE J. 1993, 39, 1164. (3) Fathi, M.; Monnet, F.; Schuurman, Y.; Holmen, A.; Mirodatos, C. Reactive Oxygen Species on Platinum Gauzes during Partial Oxidation of Methane into Synthesis Gas. J. Catal. 2000, 190, 439. (4) Hohn, K. L.; Schmidt, L. D. Partial Oxidation of Methane to Syngas at High Space Velocities over Rh-coated Spheres. Appl. Catal. A 2001, 211, 53. (5) Song, X.; Williams, W. R.; Schmidt, L. D.; Aris, R. Ignition and Extinction of Homogeneous-Heterogeneous Combustion: CH4 and C3H8 Oxidation on Pt. Proc. Combust. Inst. 1990, 23, 1129. (6) Deutschmann, O.; Schmidt, R.; Behrendt, F.; Warnatz, J. Numerical Modeling of Catalytic Ignition. Proc. Combust. Inst. 1996, 26, 1747. (7) Bui, P.-A.; Vlachos, D. G.; Westmoreland, P. R. Catalytic Ignition of Methane/Oxygen Mixtures over Platinum Surfaces: Comparison of Detailed Simulations and Experiments. Surf. Sci. 1997, 385, L1029. (8) Veser, G.; Frauhammer, J. Modelling Steady State and Ignition during Catalytic Methane Oxidation in a Monolith Reactor. Chem. Eng. Sci. 2000, 55, 2271. (9) Zerkle, D. K.; Allendorf, M. A.; Wolf, M.; Deutschmann, O. Understanding Homogeneous and Heterogeneous Contributions to the Platinum-Catalyzed Partial Oxidation of Ethane in a Short Contact Time Reactor. J. Catal. 2000, 196, 18. (10) Walker, A. V.; King, D. A. Dynamics of the Dissociative Adsorption of Methane on Pt{110} (1 × 2). Phys. Rev Lett. 1999, 82, 5156. (11) Watson, D. T. P.; van Dijk, J.; Harris, J. J. W.; King, D. A. Coverage Dependence of the Dissociative Sticking Probability of Methane on Pt{110}-(1 × 2). Surf. Sci. 2002, 506, 243. (12) Walker, A. V.; King, D. A. A Molecular-Beam Study of Methane Dissociative Adsorption on Oxygen-Precovered Pt{110} (1 × 2). Surf. Sci. 2000, 444, 1. (13) Valden, M.; Xiang, N.; Pere, J.; Pessa, M. Dissociative Chemisorption of Methane on Clean and Oxygen Precovered Pt(111). Appl. Surf. Sci. 1996, 99, 83. (14) Warnatz, J.; Allendorf, M. D.; Kee, R. J.; Coltrin, M. E. A Model of Elementary Chemistry and Fluid Mechanics in the Combustion of Hydrogen on Platinum Surfaces. Combust. Flame 1994, 96, 393.

(15) Veser, G.; Schmidt, L. D. Ignition and Extinction in the Catalytic Oxidation of Hydrocarbons over Platinum. AIChE J. 1996, 42, 1077. (16) Fernandes, N. E.; Park, Y. K.; Vlachos, D. G. The Autothermal Behavior of Platinum Catalyzed Hydrogen Oxidation: Experiments and Modeling. Combust. Flame 1999, 118, 164. (17) Bui, P.-A.; Vlachos, D. G.; Westmoreland, P. R. Homogeneous Ignition of Hydrogen-Air Mixtures over Platinum. Proc. Combust. Inst. 1996, 26, 1763. (18) Dupont, V.; Zhang, S.-H.; Williams, A. Experiments and Simulations of Methane Oxidation on a Platinum Surface. Chem. Eng. Sci. 2001, 56, 2659. (19) Deutschmann, O.; Maier, L. I.; Riedel, U.; Stroemman, A. H.; Dibble, R. W. Hydrogen Assisted Catalytic Combustion of Methane on Platinum. Catal. Today 2000, 59, 141. (20) McDaniel, A. H.; Lutz, A. E.; Allendorf, M. D.; Rice, S. F. Effects of Methane and Ethane on the Heterogeneous Production of Water from Hydrogen and Oxygen over Platinum in Stagnation Flow. J. Catal. 2002, 208, 21. (21) Park, C.; Hwang, J. Y.; Huang, M.; Anderson, T. J. Investigation of an Upflow Cold-Wall CVD Reactor by Gas Phase Raman Spectrscopy. Thin Solid Films 2002, 409, 88. (22) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixon-Lewis, G.; Smooke, M. D.; Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.; Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.; Adigun, O. CHEMKIN Collection, Release 3.6; Reaction Design, Inc.: San Diego, CA, 2000. (23) Herzberg, G. Molecular Spectra and Molecular Structure. Infrared and Raman Spectra of Polyatomic Molecules; Krieger Publishing Co.: Malabar, FL, 1991; Vol. II. (24) Takeno, T.; Nishioka, M. Species Conservation and Emission Indices for Flames Described by Similarity Solutions. Combust. Flame 1993, 92, 465. (25) Raja, L. L.; Kee, R. J.; Petzold, L. R. Simulation of the Transient, Compressible, Gas-Dynamic Behavior of CatalyticCombustion Ignition in Stagnation Flows. Proc. Combust. Inst. 1998, 27, 2249. (26) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Kinetic Modeling of Hydrocarbon/Nitric Oxide Interactions in a Flow Reactor. Combust. Flame 1998, 115, 1. (27) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner, W. C., Jr.; Lissianski, V. V.; Qin, Z. http:// www.me.berkeley.edu/gri_mech. (28) Hickman, D. A.; Schmidt, L. D. Synthesis Gas Formation by Direct Oxidation of Methane over Pt Monoliths. J. Catal. 1992, 138, 267. (29) Veser, G.; Frauhammer, J.; Friedle, U. Syngas formation by Direct Oxidation of Methane. Reaction Mechanisms and New Reactor Concepts. Catal. Today 2000, 61, 55. (30) Veser, G.; Frauhammer, J.; Schmidt, L. D.; Eigenberger, G. Catalytic Ignition During Methane Oxidation on Platinum: Experiments and Modeling. In Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Studies in Surface Science and Catalysis; Froment, G. F., Waugh, K. C., Eds.; Elsevier: Amsterdam, The Netherlands, 1997; Vol. 109, p 273. (31) Wolf, M.; Deutschmann, O.; Behrendt, F.; Warnatz, J. Kinetic Model of an Oxygen-free Methane Conversion on a Platinum Catalyst. Catal. Lett. 1999, 61, 15. (32) Sun, Y.-K.; Weinberg, W. H. Kinetics of Dissociative Chemisorption of Methane and Ethane on Pt(110)-(1 × 2) J. Vac. Sci. Technol. A 1990, 8, 2445. (33) Wilke, S.; Natoli, V.; Cohen, M. H. Theoretical Investigation of Water Formation on Rh and Pt Surfaces. J. Chem. Phys. 2000, 112, 9986. (34) Verheij, L. K. Kinetic Modelling of Hydrogen-Oxygen Reaction on Pt(111) at Low Temperature (