Thermal Stability of Cyclopentane as an Organic Rankine Cycle

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Thermal Stability of Cyclopentane as an Organic Rankine Cycle Working Fluid Daniel M. Ginosar, Lucia M. Petkovic, and Donna Post Guillen* Idaho National Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415, United States ABSTRACT: Laboratory experiments were performed to determine the maximum operating temperature for cyclopentane as an organic Rankine cycle working fluid. The thermochemical decomposition of cyclopentane was measured in a recirculation loop at 240, 300, and 350 °C at 43 bar in a glass-lined heated tube. It was determined that, in the absence of air at the two lower temperatures, decomposition was minor after more than 12 days of continuous operation. At 240 °C, the total cyclopentane decomposition products were approximately 65 ppm, and at 300 °C, the total decomposition products were on the order of 270 ppm at the end of the experiment. At 350 °C, the decomposition products were significantly higher and reached 1500 ppm. When the feed was saturated with air under prevailing atmospheric conditions, the decomposition rate increased dramatically. Residues found in the reactor after the decomposition experiments were examined by a number of different techniques. The mass of the residues increased with experimental temperature but was lower at the same temperature when the feed was saturated with air. Analysis of the residues suggested that the residues were primarily heavy saturated hydrocarbons.

’ INTRODUCTION The Rankine cycle is a thermodynamic cycle that converts heat into work.1 The organic Rankine cycle (ORC) is a thermodynamic cycle that operates using the same principles as the steam Rankine cycle, except that a working fluid with a lower boiling point and higher molecular weight than those of water is used. The hydrocarbon or refrigerant working fluid is evaporated to run through a turbine to generate electricity. This enables cycle operation at much lower temperatures (typically from 100 to 500 °C) compared to those of a steam Rankine cycle. Thus, the ORC can utilize the energy from waste heat sources to produce emission-free power with no additional fuel required. Because of the rather low molecular weight of water, the use of steam Rankine cycles for waste heat temperatures below 400 °C is inefficient and requires the use of expensive multistage expanders. The thermal efficiency of the cycle depends upon the respective reservoir temperatures.2 Because the low-temperature reservoir is fixed by the environment and cannot readily be lowered, it is most effective to increase the high-temperature source. Thus, it is desired to elevate the ORC working fluid to as high a temperature as can be tolerated without excessive thermochemical decomposition and cycle performance losses. Because the working fluid circulates within a closed loop, decomposition products (such as noncondensables) within the fluid or fouling of the heat exchange surfaces will reduce the heat transfer efficiency. The working fluid under examination in this research is cyclopentane. Cyclopentane (C5H10) is a cyclic alkane that exhibits a “puckered” (i.e., one carbon atom tends to jut out above the others to relieve ring stress) ring system of carbon hydrogen (C H) bonds and carbon carbon (C C) single bonds.3 Thermochemical decomposition progressively occurs as the temperature of the cyclopentane is raised. This is due to the breaking of chemical bonds between the molecules and the forming of smaller hydrocarbon molecules, which can in turn react to form other hydrocarbons. The literature 4,5 shows that, from previous experiments performed at 574 and 600 °C, the r 2011 American Chemical Society

Figure 1. Primary decomposition products of cyclopentane from experiments conducted around 600 °C.4,5

primary decomposition of cyclopentane is described by two reaction pathways (shown in Figure 1): (1) a dehydrogenation reaction to form cyclopentadiene (C5H6) and (2) a ring cleavage reaction to form propylene (C3H6) and ethylene (C2H4). Studies on the stability of cyclopentane under temperatures and pressures appropriate for ORCs were not found in the open literature. Only a few studies were found on the pyrolytic decomposition of cyclopentane, mostly at much lower pressures, higher temperatures, and/or shorter residence times. For example, Rice and Murphy6 reported that 11.4% of cyclopentane decomposed and produced ethylene, hydrogen, cyclopentene, cyclopentadiene, propylene, and C3H4 at 900 °C, 10 mmHg, and a contact time of 0.02 s. Frey5 reported that 22.1% of cyclopentane decomposed in an experiment at 574 °C, 76 mmHg, and a contact time of 10 min. Tsang7 performed single-pulse shock tube experiments to characterize the decomposition of cyclopentane at pressures up to 0.6 MPa and temperatures in the range 1000 1200 K. A review8 indicated that rates of reactions in paraffin pyrolysis are affected by a number of factors, including trace amounts of impurities (particularly oxygen) and the presence of surfaces. Some vessel surfaces accelerate the detrimental effect of oxygen, whereas others inhibit it.8 Although temperatures and pressures in an ORC are very different from the ones mentioned in these reports, the presence of oxygen and the types Received: April 25, 2011 Revised: July 19, 2011 Published: August 02, 2011 4138

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Figure 2. Schematic of the organic fluid decomposition test system.

of surfaces in contact with the fluid are expected to play a major role in the fluid decomposition rate. The literature suggests that the presence of transition metals, especially nickel, can cause significant decomposition of the cyclopentane under certain conditions.9,10 While no decomposition was evident over a Ni(111) surface, on a Ni(755) surface, cyclopentane decomposed almost completely into adsorbed carbon and gaseous hydrogen while heating from 150 to 300 °C. Kuramochi et al.9 concluded that cyclopentane will decompose readily over catalysts that contained active step sites. Catalytic reactions should be considered during component specification, and the use of materials containing catalysts, such as nickel, should be avoided. A glass-lined reactor tube was used in the present work to decouple the thermochemical decomposition from possible catalytic effects. In this work, a recycle test loop was constructed and tests were performed to examine the stability of cyclopentane at 43 bar (P/Pc = 0.97) for three temperatures between 240 and 350 °C (T/Tc >1). Three tests were conducted where the circulating fluid feed was purged with helium to remove dissolved air. A fourth experiment was conducted at 300 °C where the feed was saturated with air at prevailing atmospheric conditions prior to introduction to the test loop. Analyses of the circulating fluid at different times on stream and of residues found at the end of the experiments were performed to obtain a preliminary evaluation of the cyclopentane thermal stability relative to its potential application as an ORC fluid.

’ EXPERIMENTAL SECTION Chemicals. Cyclopentane, ReagentPlus grade, was obtained from Sigma-Aldrich (Milwaukee, WI). Helium, UHP grade, was obtained from Norco (Boise, ID). Both chemicals were used without further purification other than purging the cyclopentane with helium to remove dissolved air or contacting it with air purposely, as described below. Test System. A recirculation test system was used to examine the stability of this prospective ORC fluid. A schematic of the experimental system is shown in Figure 2. The main component of the system was a heated glass-lined reactor tube connected to auxiliary equipment for fluid circulation and fluid sample extraction. Considering the possibility that cyclopentane decomposition also might take place downstream of the reactor (i.e., in auxiliary tubing and equipment), house air at ambient temperature was blown over these parts to quickly reduce the temperature of the cyclopentane exiting the reactor. Where possible, PEEK tubing was used. Two switching valves (SV1 and SV2) were placed in the system. SV1 was used to select helium or cyclopentane. SV2 was used to allow the system to operate in either a purging mode or a recirculation mode. A micro gear pump (Micropump Inc., Vancouver, WA, model 1805/56C) was used to recirculate the cyclopentane. The pump was followed by a relief valve (RV), used to protect the system from overpressure, and a flow meter. The fluid leaving the flow meter traveled through a high-pressure sample valve (SV) equipped with a 1-mL sample loop and then past a pressure transducer (PT) (Omega, Stamford, CT, model PX615) before entering the heated glass-lined reactor tube. 4139

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Energy & Fuels The reactor tube was fabricated from 316 stainless steel with an outside diameter of 12.7 mm with a glass-lined inside diameter of 9.5 mm and a length of 60 cm. The heated length of the glass-lined tube was 50 cm. A thick-walled quartz tube was centered inside the tube to act as a thermowell. It had an outside diameter of 6 mm and an inside diameter of 2 mm. Six narrow-gauge type K thermocouples (Omega) evenly spaced from 17 to 47 cm from the front of the reactor were placed inside the quartz thermowell to measure the temperature within the fluid stream. The first 15 cm of the tubing was packed with quartz chips to enhance heat transfer. The total system volume was 64.8 mL, and the reactor volume, subtracting the quartz chips and the thermowell volume, was 24.3 mL. The mass averaged residence times in the heated zone are provided in the Results and Discussion section. A high-pressure (HP) syringe pump (Teledyne Isco, Lincoln, NE, model 260D) was used to supply cyclopentane, and a back pressure regulator (Tescom, Elk River, MN, series 26-1700) was used to control system pressure. The cyclopentane was received in glass bottles with a purity of 99.6 wt %. Ultrahigh purity (UHP) helium was used to purge dissolved oxygen from the cyclopentane for a minimum of 16 h prior to introduction into the test system. UHP helium was also used to purge the experimental system. In one experiment, the cyclopentane was not purged with helium, but instead, it was saturated with air at ambient conditions (i.e., 20 °C and 0.84 bar). This was accomplished by purging the cyclopentane with air for approximately 15 min. Next, the cyclopentane was allowed to settle in contact with air for approximately 30 min to ensure that no air bubbles remained in the sample. To start the experiment, the system was initially set to the purge mode. Helium flowed for a minimum of 30 min to remove any oxygen from the system. Additionally, helium pressure was cycled from atmospheric pressure to approximately 20 bar five times to ensure that tubing and parts not directly in the flow path were purged of oxygen. The system was then purged with a minimum of five volumes of cyclopentane at the experimental pressure of 43 bar. After the system was well purged with helium and cyclopentane, the system was switched to the recirculation mode. A sample was taken, and then, the system was heated to the desired experimental temperature. The system was operated continuously for a minimum of 12 days. In all experiments, the operating temperature was automatically controlled while the pressure was manually controlled at the set points established. The pressure set point was 43 bar, but fluctuations occurred during the experiments causing the reduced pressure to occasionally be greater than one. Reported temperatures are averages of the six thermocouple measurements. Liquid samples were collected up to two times for most days of operation. A liquid sample was trapped within the sample loop and then recovered by flowing helium into the loop to expel the sample. The liquid was then replaced with deoxygenated cyclopentane. Analytical Methods. Samples were analyzed by gas chromatography (GC) using an Agilent 6890 GC equipped with an autoinjector, a 30-m J&W DB-1 column, and a flame ionization detector (FID). The temperature program consisted of an initial temperature of 35 °C for 15 min, a 1 °C/min ramp to 50 °C, a 2 °C/min ramp to 130 °C, and then a 4 °C/min ramp to 250 °C. Peaks were identified by retention time match and analysis by GC/mass spectrometry (MS). Samples of the deposits on the solid surfaces within the reactor were examined by three techniques: temperature programmed oxidation (TPO), diffuse reflectance infrared spectroscopy (DRIFTS), and diffuse reflectance UV vis analyses. Temperature programmed oxidation studies: The spent quartz chips from the experiments were thoroughly mixed and divided in half to obtain a representative sample (about 4 g). This sample of the spent quartz chips was subjected to TPO under 100 sccm flowing air while the temperature was ramped at 15 °C/min from room temperature to

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950 °C. The spent quartz chips were placed in a U-tube quartz reactor. The temperature was measured with a type K thermocouple (Omega) positioned at the top surface of the bed. The exit stream was monitored continuously by quadrupole mass spectrometry (MS) in a Cirrus MS instrument. Mass/charge ratios m/z = 44 and m/z = 18 were monitored for carbon dioxide and water formation. Carbon monoxide (m/z = 28) formation, if any, could not be accurately monitored as a result of the overlap of its signal with N2 from the air present in the system. However, the possibility of carbon monoxide formation was low because of the presence of excess air. The quartz chips were weighed before and after the temperature programmed experiment. An estimate of the total weight of the residue on the quartz chips was calculated from the measured weight loss. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies: Aliquots of the quartz wool recovered from the reactor at the end of the experiments were analyzed by DRIFTS on a Magna 750 FTIR system (Nicolet). Two thousand scans at 4 cm 1 resolution were collected in the Kubelka Munk format using a KBr spectrum as the background. A sample of fresh quartz wool was also analyzed for comparison. All the DRIFTS spectra are shown offset for clarity. Diffuse reflectance UV vis analyses: Kimwipe papers containing particles of residues recovered from the inner walls of the reactor were subjected to UV vis analysis on a Shimadzu UV-3101 PC scanning spectrophotometer equipped with an integrating sphere attachment. A sample of clean Kimwipe paper was also analyzed for comparison.

’ RESULTS AND DISCUSSION The cyclopentane feed composition was measured by GC to have a purity of 99.6 wt %. Impurities were found to consist of npentane (0.32 wt %) and a mixture of isopentane compounds with a total concentration of approximately 1000 ppm. During each experiment, several samples of the circulating fluid were taken and analyzed. Averaged temperatures at six different points along the reactor volume are shown in Figure 3. Total decomposition products are shown in Figures 4 7. End-of-run cyclopentane decomposition products and concentrations are summarized in Table 1. 240 °C Experiment. The experiment with set points at 240 °C and 43 bar ran for 309 h. The cyclopentane was purged with helium to remove any dissolved oxygen. The temperature averaged 239.5 °C with a standard deviation of 1.9 °C based on 21 884 measurements. The pressure averaged 43.4 bar with a standard deviation of 0.4 bar based on 28 recorded measurements. Over the course of the experiment, the cyclopentane concentration did not appreciably decrease. Decomposition products increased from 0 to 65 ppm at the end of the 309-h experiment (Figure 4). The end-of-run decomposition product concentration had a standard deviation of 4.4 ppm based on five independent samples. The measured concentrations of the three decomposition products detected—propane, butane, and cyclopentene— are provided in Table 1. As a result of the difference in the volume of cyclopentane inside the heated zone and the total volume of the system, the average residence time within the reactor for the 309-h experiment was 32.7 h. 300 °C Experiment. The experiment with set points at 300 °C and 43 bar ran for a total of 308 h. The cyclopentane was purged with helium to remove any dissolved oxygen. The temperature averaged 300.8 °C with a standard deviation of 16.9 °C based on 22 128 measurements. The pressure averaged 45.5 bar with a standard deviation of 6.2 bar based on 13 recorded measurements. In this experiment, the reduced pressure (P/Pc) varied 4140

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Figure 3. Average temperatures as a function of reactor axial position. Temperature set points were 240 °C (diamonds), 300 °C (squares), and 350 °C (triangles).

Figure 4. Total decomposition products as a function of time on stream. Experimental set points: 240 °C, 43 bar.

Figure 5. Total decomposition products as a function of time on stream. Experimental set points: 300 °C, 43 bar.

slightly from subcritical to supercritical. The average residence time within the reactor for this experiment was 20.3 h. During the course of the reaction, the cyclopentane concentration did not decrease appreciably. The decomposition products increased from 0 to 274 ppm at the end of the 308-h experiment (Figure 5). Twelve decomposition products, grouped into eight categories in Table 1, were detected by the end of this experiment. 350 °C Experiment. The experiment with set points at 350 °C and 43 bar ran for 314 h. The cyclopentane was purged with helium to remove any dissolved oxygen. The temperature averaged 351 °C with a standard deviation of 32 °C based on 21 290 measurements. The pressure averaged 43.4 bar with a standard deviation of 0.7 bar based on 31 recorded measurements. The average residence time within the reactor was 17.3 h. Differently from the experiments at 240 and 300 °C, during the course of the reaction at 350 °C, the cyclopentane concentration did decrease slightly to a measured value of 99.4 wt %. Decomposition products increased from 0 to 1516 ppm at the

Figure 6. Total decomposition products as a function of time on stream. Experimental set points: 350 °C, 43 bar. 4141

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Decomposition products increased from 0 to 504 ppm at the end of the 308-h experiment (Figure 7). However, the trend of increasing decomposition products with time was significantly different from those observed in the air-free experiments. Within 1 h of the start of the experiment, the level of decomposition products increased to over 300 ppm, and by 5 h, the level increased to over 500 ppm. By comparison, the 300 °C air-free experiment slowly reached a decomposition product level of 274 ppm over 308 h of continuous operation. The decomposition products detected by GC/MS in this experiment included the hydrocarbons observed in the 300 °C air-free experiment with the addition of minor quantities of C5 ketones, ethers, and aldehydes. Additionally, bicyclopentyl was detected at quantities of approximately 100 ppm. End-of-Run Deposits. At the end of each experiment, a small amount of deposits was observed on the quartz chips at the front of the reactor, on the quartz wool holding the quartz chips in place, and on the inside wall of the reactor. The reactor wall deposits were collected by wiping the inside of the reactor with a cyclopentane-wetted tissue. The rest of the tubing and accessories, which were maintained at ambient temperature, did not present any noticeable presence of residue buildup. The end-of-run quartz chips were weighed before and after the 240, 300, and 350 °C experiments and were determined to have a weight increase of 3.8, 16.4, and 54.4 mg, respectively, for the airfree experiments. No weight change was determined for the quartz wool or for the tissue paper. On the basis of a total cyclopentane mass of approximately 32 g at experimental conditions, the mass of the deposits on the quartz chips would

end of the 314-h experiment (Figure 6). Twenty decomposition products, grouped in 10 categories in Table 1, were detected by the end of this experiment. 300 °C Experiment, Cyclopentane Contacted with Air. The experiment with set points at 300 °C and 43 bar performed on cyclopentane contacted with air ran for 308 h. The average temperature within the reactor was 303 °C with a standard deviation of 34 °C. The pressure averaged 43.6 bar. The average residence time within the reactor was 20.3 h.

Figure 7. Total decomposition products as a function of time on stream for air-contacted feed. Experimental set points: 300 °C, 43 bar.

Table 1. Decomposition Products Found in the Cyclopentane Recovered at the End of the Experiments conc (ppm)a cmpd

240 °Cb

300 °Cb

350 °Cb 4

2

4

59

11

methane C2 hydrocarbons

300 °C airc

C3 hydrocarbons

4

21

202

35

C4 hydrocarbons d C5 hydrocarbons

27 34

62 50

99 335

15 120

24

138

C5 oxygenates

43

C6 hydrocarbons C7 hydrocarbons

27

330

10

C8 hydrocarbons

53

232

26

C9 hydrocarbons

11

C10 hydrocarbons

45

159

104

C14 hydrocarbons

72

a

Based on GC area fraction. Hydrogen formation was not quantified. Experiment where cyclopentane was purged with helium. c Experiment where cyclopentane was contacted with air. d Cyclopentane not included.

Figure 8. TPO analysis of quartz chips obtained from the (a) 300 °C, air; (b) 240 °C, no air; (c) 300 °C, no air; and (d) 350 °C, no air experiments.

b

Table 2. Estimate of Total Residue on the Quartz Chips by Weight Gain Following the Cyclopentane Experiments and Based on TPO Results 240 °C air-free

300 °C air-free

350 °C air-free

experiment

experiment

experiment

300 °C air experiment

weight gain after cyclopentane decomposition experiments (mg)

3.8

16.4

54.4

11.2

estimated residue weight based on TPO experiments (mg)

7.4

21.3

69.4

3.8

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Figure 9. Diffuse reflectance infrared spectra of quartz wool. Samples are (a) fresh; (b) spent at 240 °C, no air; (c) spent at 300 °C, no air; (d) spent at 350 °C, no air; and (e) spent at 300 °C, with air.

account for a hydrocarbon deposition of 110, 510, and 1700 ppm at 240, 300, and 350 °C, respectively. For the 300 °C experiment where the cyclopentane feed was contacted with air, the quartz chips were determined to have a weight increase of 11.2 mg, corresponding to 350 ppm of the total cyclopentane mass. Samples of the deposits on the solid surfaces within the reactor were examined by three techniques: TPO, DRIFTS, and diffuse reflectance UV vis analyses. 1. Temperature Programmed Oxidation (TPO). As a result of the difficulties associated with accurately measuring the amount of residue formed in the reactor, two methods were applied to estimate the amount. One method was the direct weighing of quartz chips before and after the experiments. The other was based on the weight change produced during the TPO analysis. From the weight change measured in the TPO experiment and the known total quantity of spent quartz chips, the total weight of residue on the quartz chips was estimated. These two estimates are shown in Table 2. Although not a quantitative match, the two estimates of the residue on the quartz chips follow the same trend: the residue mass increased with temperature for the airfree experiments, and the residue on the 300 °C air-contacted feed was lower than that of the air-free experiment. Note that the quartz chips were located at the front 15 cm of the reactor where the actual temperature increased from room temperature to the working temperature. The TPO profiles obtained are shown in Figure 8. (Smoother profiles are the result of performing a higher number of scans.) Carbon dioxide and water (water signal not shown in Figure 8) were produced between 200 and 600 °C. It is interesting to note that the residue from the 350 °C air-free experiment produced decomposition products that oxidized at the lowest temperatures during the TPO experiment. It is worth mentioning that different peaks in a TPO profile are representative of different reactions taking place between the residue and oxygen.

2. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The infrared spectra of the quartz wool from the three experiments are shown in Figure 9. A blank infrared analysis on fresh quartz wool was done for comparison. The only features that could be discriminated on the spent quartz wool from the blank analysis were the vibrations at approximately 2954 and 2873 cm 1. These vibrations indicate saturated aliphatic C—H stretching.11 Vibrations above 3000 cm 1 or around 1580 cm 1, usually associated with unsaturation, were absent. Obtaining a better compound identification was not possible as a result of the strong silica vibration in the 1300 1500 cm 1 range that masked both C—H deformations and C—C and CdC stretching, if present. A preliminary supposition for the type of compound would be a relatively heavy (e.g., a material that is solid at room temperature) saturated hydrocarbon. The possibility that the C—H stretching found in the spent quartz wool was simply cyclopentane (reactant) instead of decomposition products was investigated. Clean quartz wool was soaked in cyclopentane, dried at ambient conditions for a few minutes, and then analyzed by infrared spectroscopy. No C—H vibrations were evident, which confirms that the C—H vibrations from the spent quartz wool were indeed an indication of the presence of cyclopentane decomposition products. The lack of significant differences in the DRIFTS features shown by the spent samples suggests that the residues on the solid surfaces are of similar chemical nature. 3. Diffuse Reflectance UV Vis Spectroscopy. Diffuse reflectance UV vis analyses were performed on Kimwipe papers containing particles of residue (not shown). Compared to the UV vis spectra of clean Kimwipe paper, only the paper from the 350 °C experiment showed a minor band at 350 nm. Comparing band assignments for carbonaceous materials found in the literature,12 the band at 350 nm may be assigned to diene materials. Higher degrees of unsaturation above 400 nm were not observed. This suggests that the unsaturation of the residue is not important, in agreement with the DRIFTS results that showed mostly paraffinic contribution. 4143

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’ CONCLUSIONS The thermochemical decomposition of cyclopentane in a recirculating flow loop in the absence of catalysts has been quantified. In the absence of air, cyclopentane decomposition increased with increasing temperature. At 240 and 300 °C, decomposition was minor after 12 days online. At 350 °C, the decomposition rate increased significantly. When the cyclopentane feed was saturated with air, the rate of decomposition increased dramatically. In less than one hour online, the decomposition products from the air-contacted feed exceeded the amount measured after 308 h at the same temperature for the helium-purged feed. It is clear from examination of the types of decomposition products listed in Table 1 that the primary decomposition products, in turn, form other products. In all cases, residues were deposited on the inside of the reactor. For the most part, these residues increased with increased reaction temperature and were primarily heavy paraffinic hydrocarbons. Cyclopentane appears to be a viable candidate as an ORC working fluid as long as temperatures are maintained below 300 °C and air is excluded from the system. It is worth emphasizing that the experimental work reported in this article was performed in a laboratory setting using a glass-lined reactor and high-purity cyclopentane. The data help determine an upper limit for the ORC working fluid temperature under idealized conditions. Impurities, the presence of oxygen, and materials of construction can affect the fluid decomposition rates. Longer duration testing using industrial grade cyclopentane in a prototypic ORC loop is required to obtain a realistic estimate of the working fluid degradation in an operational environment.

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(2) Distributed Generation: The Power Paradigm for the New Millennium; Borbely, A. M., Kreider, J. F., Eds.; CRC Press LLC: Boca Raton, FL, 2001; pp 146. (3) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function, 4th ed.; W. H. Freeman and Company: New York, 2003; pp 134. (4) Vanas, D. W.; Lodge, N. M.; Walters, W. D. J. Am. Chem. Soc. 1952, 74 (2), 451–455. (5) Frey, F. E. Ind. Eng. Chem. 1934, 26, 198–203. (6) Rice, F. O.; Murphy, M. T. J. Am. Chem. Soc. 1942, 64, 896–899. (7) Tsang, W. Int. J. Chem. Kinet. 1978, 10 (6), 599–617. (8) Leathard, D. A.; Purnell, J. H. Annu. Rev. Phys. Chem. 1970, 21, 197–224. (9) Kuramochi, H.; et al. Surf. Sci. 1993, 287/288, 217–221. (10) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425–445. (11) Nyquist, R. A. The Interpretation of Vapor-Phase Infrared Spectra; Sadtler Research Laboratories: Philadelphia, PA, 1984. (12) Petkovic, L. M.; Ginosar, D. M.; Burch, K. C. J. Catal. 2005, 234, 328–339.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ DISCLOSURE Disclosure: This paper was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy, Industrial Technologies Program, under Contract No. DE-PS36-08GO98014. ’ REFERENCES (1) Reynolds, W. C.; Perkins, H. C. Engineering Thermodynamics; McGraw-Hill Book Company: New York, 1977; pp 293 311. 4144

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