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Ind. Eng. Chem. Res. 2007, 46, 1114-1119
Methane Partial Oxidation Catalyzed by Platinum and Rhodium in a High-Temperature Stagnation Flow Reactor Steven F. Rice,* Anthony H. McDaniel, Ethan S. Hecht, and Alicia J. J. Hardy† Combustion Research Facility, Sandia National Laboratories P.O. Box 969, LiVermore, California 94551-0969
A series of measurements investigating the partial oxidation chemistry of methane and oxygen over platinum and rhodium foils was performed in a stagnation flow reactor at a pressure of 30 Torr. Products from the reactions on the catalytic substrates were quantitatively analyzed via mass spectrometry for consumption of the reactants, methane and oxygen, and for production of hydrogen, carbon monoxide, carbon dioxide, and water. Reactivity for fractional methane conversion, along with hydrogen and carbon monoxide selectivities versus water and carbon dioxide, respectively, were determined over a temperature range of 700-1350 °C. Three reactant stoichiometries were investigated, corresponding to methane-to-oxygen equivalence ratios of 3, 4, and 6. The results highlight differences in the behavior of the two metals over a range of conditions. In general agreement with earlier work, rhodium seems to exhibit greater reactivity. At low temperatures, heavy coking was observed on platinum over time, and rhodium exhibited a susceptibility to deactivation at lean equivalence ratios. Introduction The catalytic partial oxidation (CPOX) of methane over metals such as platinum and rhodium has been of continued interest for over a decade since the observation of their excellent reforming performance as partial oxidation catalysts.1 This discovery, coupled with the inherent autothermal nature of the process, has driven much of the research on platinum and rhodium in short contact time reactors (SCTRs).2-4 More recently, several engineered reactor-catalyst coupled designs have been developed, with the intention to optimize selectivity and conversion.5-8 Bharadwaj and Schmidt4 have noted that, under autothermal conditions, rhodium seems to outperform platinum, with respect to optimum synthesis gas product composition; they attributed the poorer performance of platinum to the more facile formation of surface OH, leading to the creation of water. Along with many reports of reacting flow characteristics in these systems, there have been many attempts to conduct measurements specifically designed to validate models of this chemistry, using both global and elementary reaction mechanisms.9-17 Generally, these mechanisms have been directly compared with flow reactor data and are capable of reproducing a subset of the quantitative observations. As microkinetic mechanisms have continued to evolve in complexity and thermodynamic rigor, two extensive thermodynamically consistent mechanisms have been reported by the Vlachos group for the CPOX of methane over both platinum18 and rhodium.19 However, even in this work, a need still exists to calibrate or refine specifics within the overall scheme by adjusting one or more key thermodynamic or kinetic parameters to match selected flow reactor data. After calibration, these mechanisms successfully reproduce an array of results. Nevertheless, the experimental data available from flow reactor measurements in CPOX configurations is usually limited by the autothermal nature of the fixed-bed flow systems, wherein * To whom correspondence should be addressed. Tel: (925) 2941353, Fax: (925) 294-2999. Email:
[email protected]. † Present address: Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA.
there are limits to the range of temperature profiles available to test the completeness of these mechanisms. To move the development of these mechanisms forward, it is valuable to obtain a more expansive database, regarding the generic reactivity of these catalysts. A low-pressure stagnation flow configuration permits a much wider range of reaction temperatures to explore the validity of these mechanisms and provides measurements to test the fundamental nature of the detailed expressions under conditions far from the calibration range. If these mechanisms are truly elementary in nature and are independent of reactor configuration and catalyst support, they should be applicable to a data set obtained at radically different pressure and metal catalyst aggregation. This paper reports the results from a series of measurements in a low-pressure (30 Torr) stagnation flow reactor that determine the reactivity of methane under CPOX conditions. Building on the results reported in the work of Taylor et al.,20 the methane conversion and synthesis gas selectivity of both rhodium and platinum catalysts are examined via real-time mass spectrometry (RTMS) at temperatures of >1300 °C and at several equivalence ratios (φe). The results are analyzed in terms of relative conversion efficiencies and selectivities for hydrogen and carbon monoxide production versus water and carbon dioxide. Unusual particulars regarding reaction initiation and coking are also described for rhodium and platinum. These data contribute to the body of information regarding partial oxidation reactivity over Group VIII metals and are especially useful for validation of future modeling efforts, given the controlled flow and temperature conditions in this experimental configuration. Equipment Schematic representations of an earlier version of the stagnation flow apparatus used in these experiments is presented in McDaniel et al.21 and in Taylor et al.,20 along with a detailed description of the reactor operation. In this work, the reactor configuration has been changed to a down-flow orientation, shown in Figure 1, in which the reactive substrate rests on the actively heated surface and is not held in place by supports or clips. Detailed, two-dimensional finite-element analysis of the
10.1021/ie060892x CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007
Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1115 Table 1. Gas Feed Conditions at Various Equivalence Ratios (Oe) Gas Flow (SLPMa) feed gas
at φe ) 3
at φe ) 4
at φe ) 6
CH4 O2 argon helium (as purge)
2.94 1.96 0.1 1.0
3.25 1.63 0.1 1.0
3.68 1.23 0.1 1.0
a
Standard liters per minute.
Figure 1. Schematic of the stagnation flow reactor used in this work.
down-flow configuration confirmed that, at the prescribed gas composition, flow rates, and operating pressure, buoyancydriven vortexes are not expected to impact the stagnation flow. The heater was changed from a boron nitride-coated graphite resistive element to a custom silicon carbide heater (Morgan Advanced Ceramics). In addition, an 8.6-cm-diameter quartz cap was supported directly over the heater, such that the catalytic substrate was separated from the silicon carbide by an ∼0.1cm gap and 0.25 cm of quartz. The metal foil was placed directly on the cap and heated primarily by radiant flux from the silicon carbide. The volume under the cap that contained the heater was purged with 1.0 SLPM (standard liters per minute) of helium. The catalytic substrates of platinum and rhodium consisted of 0.025-cm-thick high-purity polycrystalline foils (platinum, 99.99%, Goodfellow; rhodium, 99.8%, Alfa Aesar), cut to dimensions of 4.5 × 3.0 cm. The samples were polished prior to each use. Auger analysis of the “as-received” samples exhibited evidence of considerable surface oxygen, carbon, and iron that could be removed mechanically. The typical surface site density for clean Group VIII metals is 1015 sites/cm2. The temperature of the substrate was monitored with a dualwavelength optical pyrometer (Accufiber, model 100C). Using ratioed dual-wavelength pyrometry, the temperature is measured largely independent of variations in the emissivity of the substrate, due to surface finish. The pyrometer was focused on a 0.3-cm-diameter spot in the center of the catalyst. A thermocouple on the inlet flow disperser monitored the temperature of the reactants. A typical measurement on platinum consisted of exposing the catalytic substrate at 900 °C to a 5:1 mixture of H2 and O2 for 15 min, followed by establishing a maximum stable temperature on the substrate under a flow of helium set to the predetermined flow rate for methane in the experiment. A closed-loop control valve was used to maintain system pressure at 30 Torr, independent of the input flow rate. Methane was then substituted for helium, followed by the addition of oxygen to the desired premixed flow condition. Experiments on rhodium were more difficult. Rhodium substrates exhibited no reactivity to a surface preparation of H2/O2 at 900 °C. However, after exposure to 2/1 CH4/O2 mixtures at 1200 °C, a reactive substrate could be established, as indicated by a fairly abrupt exotherm. The required exposure times varied. The rhodium catalysts were either inactive or very
Figure 2. Real-time mass spectroscopy (RTMS) ion current response for a typical dataset at a CH4/O2 equivalence ratio of φe ) 3 over platinum.
reactive; there did not seem to be conditions exhibiting intermediate reactivity. Table 1 shows the reactant feed conditions used. After stable conditions were achieved at the maximum experimental temperature, the voltage on the heater was reduced in small increments, subsequently producing steady conditions at lower temperatures in small steps. Figure 2 shows an example of a standard set of measurements. Calibration of the mass spectrometric response for CH4, CO, CO2, and H2 were obtained using a premixed calibration gas standard that consisted of 10% of each component in argon. In this way, the response of the RTMS to each gas was calibrated relative to argon. Calibration for the RTMS response to O2 was done by measuring the signal from the baseline feed mixture of 2/1 CH4/O2 + 1.7% argon under reaction conditions (Tin ≈ 900 °C and 30 Torr) over a nonreactive substrate. Analysis Methods The RTMS measures an ion current that is proportional to the mole fraction of a particular species in the downstream flow. Conversion of these raw data relies on the accuracy and stability of the RTMS response calibrations, relative to the fiducial 0.1 SLPM argon flow, as well as the stability of the input flow rate of CH4, O2, and argon. Water could not be calibrated accurately for two reasons: (i) it was not possible to establish a known water-to-argon ratio within the apparatus under experimental conditions, and (ii) the RTMS response to water had a tendency to be very slow and exhibit considerable hysteresis. Numerous RTMS response calibrations were performed over the course of the experimental work and small measurement calibration drifts (on the order of 5%) were identified. However, this small error presents a problem in determining the hydrogen selectivity, relative to water, because the H2O mole fraction cannot be determined directly by the RTMS. It must be
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Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007
calculated from a balance on either O atoms, as in eq 1, or H atoms, as in eq 2:
n˘ H2O ) 2[n˘ O2(in) - n˘ O2] - 2n˘ CO2 - n˘ CO
(1)
n˘ H2O ) 2[n˘ CH4(in) - n˘ CH4] - n˘ H2
(2)
or
Equations 1 and 2 were derived assuming H2, H2O, CO, and CO2 were the only products of the reaction. Because the input molar fluxes are known from the mass flow controller settings and not measured via the RTMS as the other values are, any error in the response of the RTMS, relative to the calibrated response, produces a large error and can be calculated depending on the atom balance that is used. These difficulties in obtaining quantitative results were mitigated using an analysis method that was based on the assumption that, once set, the input molar flow rates for oxygen, methane, and argon do not drift and that it is the response of the RTMS that varies slightly over a day-long experimental run. The assumption is validated by measurements on the fixedcomposition calibration gas, which show drift in the individual gas-to-argon response ratios as the RTMS warms up over the course of an 8-h experiment. The ion current to product composition conversion procedure is as follows. First, the raw ion currents are corrected for any small baseline offset in the RTMS. These are negligible for all species except hydrogen. Next, the output molar flow rate for the product species (H2, CO, and CO2) are calculated from the measured ion current, relative to the ion current for the known argon flow of 0.1 SLPM, and the individual relative response factors R(H2), R(CO), and R(CO2) are determined by the RTMS response to the calibration gas. The secondary ion current at m/e ) 28 generated by CO2 is subtracted at this stage, to correct for spectral overlap between CO and CO2. The total contribution to the effluent composition from H2, CO, and CO2 is small and, therefore, drift on the order of several percent in the RTMS response for them has only a small effect on the oxygen and hydrogen atomic total molar balances. Finally, the relative response of the RTMS to molecular oxygen and methane was determined explicitly by solving the series of equations that represented atom balances on hydrogen, carbon, and oxygen. The resultant expressions for R(CH4) and R(O2) are
R(CH4) )
n˘ CH4(in) - R(CO)ICO - R(CO2)ICO2 ICH4(out)
(3)
R(O2) ) n˘ O2(in) - 1.5R(CO)ICO - 2.0R(CO2)ICO2 + 0.5R(H2)IH2 IO2(out) (4) where n˘ j(in) is the inlet molar flow rate of species j, which is presumed to be known and without drift error, and Ij is the measured relative ion current of species j at the appropriate m/e value. These two equations form a unique solution for the amount of water produced under each experimental condition.
Figure 3. Selectivities for hydrogen and carbon monoxide production over platinum.
Figure 4. Selectivities for hydrogen and carbon monoxide production over rhodium.
this case, for platinum at φe ) 3. When helium in the reactant flow is replaced by methane in Scans 1-28, a small hydrogen background is seen, but relative to the CPOX conditions, methane by itself exhibits no appreciable reactivity, even at very high temperature. When oxygen is added at scan 28, the temperature increases to over 1200 °C and H2, CO, CO2, and H2O are detected. A few minutes (∼20 scans) are required for steady conditions to be established. The subsequent drops in hydrogen and carbon monoxide production correspond to reduction of the heater power in small steps. Figures 3 and 4 show the selectivity results for platinum and rhodium, respectively, at three different reactant stoichiometries. Hydrogen and carbon monoxide selectivities are defined as
Results Table 1 shows the feed conditions for all of the measurements presented here. A typical dataset is presented in Figure 2, in
SH2 )
n˘ H2 n˘ H2 + n˘ H2O
(5)
Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1117
and
SCO )
n˘ CO n˘ CO + n˘ CO2
(6)
Figure 5 shows the methane conversion percentage, which is defined as
XCH4 (%) )
n˘ CH4(in) - n˘ CH4(out) n˘ CH4(in)
× 100
(7)
An important observation during the course of data collection is that the platinum substrate had a tendency to coke as the temperature was reduced to
