3724
Energy & Fuels 2008, 22, 3724–3735
Characterization of Early-stage Coal Oxidation by Temperature-programmed Desorption Wei-Yin Chen,* Guang Shi, and Shaolong Wan Department of Chemical Engineering, Anderson Hall, P.O. Box 1848, UniVersity of Mississippi, UniVersity, Mississippi 38677-9740 ReceiVed May 29, 2008. ReVised Manuscript ReceiVed August 6, 2008
To obtain representative temperature-programmed desorption (TPD) profiles of young oxidized chars up to 1650 °C with minimal reactor wall interferences, the chemistry and physics of four ceramic materials has been critically reviewed. A two-staged experimental apparatus is then uniquely designed to produce chars in an Al2O3 flow reactor with 1-21% O2 followed by in situ TPD with a SiC tube. Comparison of TPD profiles of oxidized chars with those from pyrolyzed chars and ashes suggests early-stage char oxidation is profoundly influenced by oxygen from three sources: organics oxygen, mineral matters, and gas phase O2. Young chars oxidized at 1000 °C with less than 0.3 s residence time shows CO desorption peaks during TPD at three distinct temperatures: 730, 1280, and 1560 °C. The peaks at 730 °C are mainly caused by incomplete devolatilization. The peaks at 1280 °C mainly represent desorption of stable surface oxides and incomplete devolatilization. Increasing the gas phase oxidants notably increases the amount of stable surface oxides. The broad peaks between 1400 and 1650 °C are attributed to the reactions of oxidants decomposed from minerals and carbon in the char or SiC tube. Gas-phase oxygen shifts these reactions to lower temperatures. Detailed oxygen balance based on the CO and CO2 yields and elemental compositions of both pyrolysis and oxidized chars reveals that oxygen uptakes are very high, +0.056 mg O per mg of carbon, in chars derived from bituminous coal, whereas lignite chars show negative oxygen uptake, -0.020 mg O per mg of carbon, in char. Indeed, lignite char seems to possess little amount of stable surface oxides other than those contributed by the minerals. The extensive emissions of CO from lignite chars during TPD seem to suggest that either O2 or minerals promotes the oxygen transfer on char surface and subsequent carbon oxidation.
Introduction Gasification of coal-derived chars has been extensively investigated and reviewed in the last few decades.1-9 Nevertheless, young chars’ reactivities in flame environments have just started receiving attention and are in the infant stage in several scientifically and technologically important aspects. Chars produced under low thermal severities are usually called young chars. To obtain “super-cleaned” chars, that is, chars without adsorbed species on their surfaces, chars have traditionally been pyrolyzed in inert gas at 1000∼1100 °C for several hours before reactivity studies. Evidences suggest that this level of thermal treatment destroys many important chemical and physical * Corresponding author. Phone: (662) 915-5651; fax: (662) 915-7023; e-mail:
[email protected]. (1) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221–270. (2) Essenhigh, R. H. Chemistry of Coal Utilization; Wiley: New York, 1981; Vol 2, pp 1278-1285. (3) Smith, I. W. Proc. Combust. Inst. 1982, 1045–1065. (4) Yang, R. T. In Chemistry and Physics of Carbon, Walker, P. L. Ed.; M. Dekker: New York, 1984; Vol 4, pp 163-210. (5) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1985. (6) Kapteijn, F.; Mulijn, J. A. Carbon and Coal Gasification: Science and Technology; Martinus Nijhoff Publishers: Dordrecht/Boston/Lancaster, 1986; pp 291-360. (7) Walker, P. L., Jr.; Taylor, R. L.; Ranish, J. M. Carbon. 1991, 29, 411–421. (8) Smoot, L. D. In Fossil Fuel Combustion, Bartok, W.; Sarofim, A. F., Eds.; Wiley Interscience: New York, 1991; pp 653-782. (9) Smith, L. K.; Douglas, L. D.; Fletcher, T. H.; Pugmire, R. L. The Structure and Reaction Processes of Coal; Plenum Press: New York, 1994.
characteristics of young chars, leading to underestimations of the complexity of char oxidation and erroneous estimation of the char’s reactivity in flame, where char has a residence time on the order of seconds.10-19 It is generally believed that young char undergoes at least four simultaneous classes of processes in the flame: devolatilization, reactions of adsorbed oxygen on char surface, thermal annealing of carbon, and reactions of mineral constituents. They induce a complex interrelated network of rapid chemical and physical changes to the char structure and therefore to its reactivity in flame. Literature contains scattered works on the characterization of young chars. Radovic et al.10,11 examined the effects of pyrolysis conditions on the reactivity of demineralized and cation-exchanged lignite and CaO dispersion, where the pyrolysis condition varied from 0.3 s to 1 h at different (10) Radovic, L. R.; Walker, J. P. L.; Jenkins, R. G. J. Catal. 1983, 82, 382–394. (11) Radovic, L. R.; Walker, J. P. L.; Jenkins, R. G. Fuel 1983, 62, 849–856. (12) Hurt, R. H.; Gibbins, J. R. Fuel 1995, 74, 471–480. (13) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; Headley, T. J.; Mitchell, G. D. Fuel 1995, 74, 1297–1306. (14) Beeley, T.; Crelling, J.; Gibbins, J.; Hurt, R.; Lunden, M.; Man, C.; Williamson, J.; Yang, N. Proc. Combust. Inst. 1996, 26, 3103–3110. (15) Russell, N. V.; Gibbins, J. R.; Man, C. K.; Williamson, J. Energy Fuels 2000, 14, 883–888. (16) Chen, W. Y.; Tang, L. AIChE J. 2001, 47, 2781–2797. (17) Molina, A.; Eddings, E. C.; Pershing, D. W.; Sarofim, A. F. Proc. Combust. Inst. 2002, 29, 2275–2281. (18) Garijo, E. G.; Jenson, A. D.; Glarborg, P. Combust. Flame 2004, 136, 249–253. (19) Chen, W. Y.; Wan, S. L.; Shi, G. Energy Fuels 2007, 21, 778– 792.
10.1021/ef800405h CCC: $40.75 2008 American Chemical Society Published on Web 09/23/2008
Early-stage Coal Oxidation by TPD
temperatures. They observed that CaO crystallites grew during heat treatment and caused char deactivation. De Soete20 reported that the “partially” devolatilized chars produced at temperatures not exceeding 800-900 K and a short residence time have much higher NO reduction capabilities than those produced at 1300 K for 2 h. He speculated that the hydrogen on the char surface plays a role. Solid state 13C and 1H NMR studies of various carbon and hydrogen functional groups of chars derived from short devolatilization times at temperatures up to 1250 K21-23 revealed that, at the end of devolatilization, the carbon skeletal structures of chars from five coals of different rank are remarkably similar. This similarity in the carbon structure of the chars, however, is in marked contrast to the observed differences in their char reactivity. Thus, the differences in char reactivity are likely be caused by the second-order variations in the carbon skeletal structure that produce variations in active sites, surface area, pore structure, and mineral contents. The residual carbon samples extracted from commercial and pilotscale coal combustion fly ash samples have been found to have lower oxidation reactivities and more fully developed turbostratic crystallinity.12,13 It was later discovered that the chars produced from pyrolysis with a residence time and temperatures similar to those in the industrial boilers have essentially the same oxidation reactivities as the residual carbons.14 Thermal annealing, or graphitization, of the carbon structure at high flame temperatures has been considered the principal contributor to char deactivation at temperatures above 1500 °C. Russell et al.15 examined the reactivity of char produced from pyrolysis up to 1800 °C with holding time in the range 0∼5 s and claimed that structural changes within the char matrix is the main cause of char deactivation. No oxidant was introduced into their pyrolysis procedure. Chen and Tang16 and Molina et al.17 reported that young chars have one order-of-magnitude higher NO reduction rates than those of old chars. Garijo et al.18 compared a char produced in situ with short residence time with that pyrolyzed for 15 min and found that the initial NO reduction rate of the young char is about 2.7 times higher than that of the old char at 1123 K. The reactivities of chars produced from long pyrolysis times have been extensively characterized by thermal desorption of surface oxides at temperatures below 1100 °C.24-49 These temperatures are much lower than those in typical coal flames, (20) De Soete, G. G. In PulVerized Coal Combustion: Pollutant Formation and Control, 1970-1980, EPA-600/8-90-049; EPA: North Carolina, 1990; Ch. 8, pp 1-59. (21) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Critchfield, S.; Pugmire, R. J. Proc. Combust. Inst. 1991, 1231–1237. (22) Pugmire, R. J.; Solum, M. S.; Grant, D. M.; Critchfield, S.; Fletcher, T. H. Fuel 1991, 70, 414–423. (23) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels 1992, 6, 643–650. (24) Bonnetain, L. J. Chim. Phys. 1961, 58, 34–46. (25) Laine, N. R.; Vastola, F. J.; Walker, P. L. J. Phys. Chem. 1963, 67, 2030–2034. (26) Vastolazp, F. J.; Hart, P. J.; Walker jr., P. L. Carbon 1964, 2, 65– 71. (27) Tucker, B. G.; Mulcahy, M. F. R. Trans. Farad. Soc. 1969, 65, 274–286. (28) Kelemen, S. R.; Feeund, H. Carbon. 1985, 23, 619–625. (29) Zhang, Z. G.; Kyotani, T.; Tomita, A. Energy Fuels. 1988, 2, 679– 684. (30) Lizzio, A. A. The Concept of ReactiVe Surface Area Applied to Uncatalyzed and Catalyzed Carbon (Char) Gasification in Carbon Dioxide and Oxygen; Ph.D. Thesis, Department of Material Science and Engineering, The Pennsylvania State University, University Park, PA, 1990. (31) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990, 28, 7–19. (32) Lear, A. E.; Brown, T. C.; Haynes, B. S. Proc. Combust. Inst. 1990, 23, 1191–1197. (33) Du, Z.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1990, 4, 296– 302.
Energy & Fuels, Vol. 22, No. 6, 2008 3725
and these relatively weak oxides are likely playing only minor roles, if any, during pulverized coal combustion. On the other hand, the roles of possible strong surface oxides that desorb only above 1100 °C, as suggested by studies of graphite about 16 years ago,50 have not been receiving adequate attentions for char oxidation. Furthermore, the presence of stable oxides from young coal-derived chars was recently revealed in our study, suggesting their possible roles in the flame.19 Their structures appear more complex than those on graphite. Their formation and transformation, entangled with thermal annealing, in flames constitute a challenging research area that has critical importance to the design of coal-fired boilers since the turnover rates of these stable surface oxides are likely a real rate-controlling step of young char oxidation in flames. This paper seeks better understandings of the roles of surface oxides on young chars in flame environments by a new experimental procedure that produces young char in more representative conditions to flame. The oxidation was conducted with nominal residence times shorter than 0.3 s and with carbon burnout less than 20% for most of the experiments. The reactivities of these chars were characterized in situ by temperature-programmed desorption (TPD) in the temperature range of up to 1650 °C. TPD reveals the amounts and strengths of surface oxides on the young chars from two different coals at different burnout levels. To accomplish the objective stated above, we have successfully minimized the interferences of reactor wall that were reported in a previous paper of ours19 so that the CO productions from TPD can be properly interpreted and correlated to oxygen from different sources: gaseous, organics, and minerals. At the outset of this project, we assessed the thermodynamics properties of four commonly used tube materials in combustion laboratories: SiO2, Al2O3, fully yttria-stabilized ZrO2, and SiC. The tubes for the two-stage experiments were then selected based on their susceptibility to four kinds of possible interferences: decomposition, reactions with CO, carbon, O2, and oxygen selfdiffusion at 1900 K. Experimental Section Experimental Apparatus. The schematic drawing of the experimental apparatus was provided in an earlier paper.19 This apparatus has been designed for the study of amount and strength (34) Calo, J. M.; Hall, P. J. In Fundamental Issues in Control of Carbon Gasfication ReactiVity, Lahaye, J., Ehrburger, P. Eds.; Kluwer Academic Publishers: Dordrecht/Boston/London, 1991; pp 329-368. (35) Brown, T. C.; Haynes, B. S. Energy Fuels 1992, 6, 154–159. (36) Atamny, F.; Blocker, J.; Dubotzky, A.; Kurt, H.; Timpe, O.; Loose, G.; Mahdi, W.; Schlogl, R. Mol. Phys. 1992, 76, 851–886. (37) Chen, S. G.; Yang, R. T.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1993, 32, 2835–2840. (38) Crick, T. M.; Silveston, P. L.; Miura, K.; Hashimito, K. Energy Fuels 1993, 7, 1054–1061. (39) Fanning, P. E.; Vannice, M. A. Carbon 1993, 31, 721–730. (40) Zhuang, Q. L.; Kyotani, T.; Tomita, A. Energy Fuels 1994, 8, 714– 718. (41) Zhuang, Q. L.; Kyotani, T.; Tomita, A. Energy Fuels 1995, 9, 630– 634. (42) Tsai, W. T.; Chang, C. Y. Proc. Natl. Sci. Counc. 1995, 19, 258– 262. (43) Zhuang, Q. L.; Kyotani, T.; Tomita, A. Energy Fuels 1996, 10, 169–172. (44) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez de Lecea, C. Energy Fuels 1996, 10, 158–168. (45) Illan-Gomez, M. J.; Salinas-Martinez de Lecea, C.; Linares-Solano, A.; Phillips, J.; Radovic, L. R. Preprints of DiV. of Fuel Chem., Am. Chem. Soc 1996, 41, 174–178. (46) Sibraa, A.; Newbury, T.; Haynes, B. S. Combust. Flame 2000, 120, 515–525. (47) Haynes, B. S. Combust. Flame 2001, 126, 1421–1432.
3726 Energy & Fuels, Vol. 22, No. 6, 2008
Chen et al.
Figure 1. Two-stage experimental apparatus with minimum wall interferences. Coal particles pass through the high purity Al2O3 tube (in yellow) in the upper furnace at a set temperature with a controlled residence time, and are trapped in the middle of the SiC tube (in black) by SiC particles and a SiC foam supported by a SiC rod at room temperature. TPD is conducted after a desirable amount of char sample is collected in the SiC tube. An Agilent model 6890N gas chromatograph, an Agilent Model 5973 mass spectrometer, and a HP PC with spectra libraries are online with the reactor. Carbon burnout is controlled by gas flowrate and O2 concentration.
of surface oxides on young chars produced with short oxidation times. Desorption is conducted in situ after oxidation, rendering minimal contact between the char and additional oxidants. As shown in Figure 1, the modified apparatus is a two-stage system involving two different types of reactor tubes, one for the production of young char and, the other, for the subsequent TPD. The two tubes are vertically placed in two furnaces in series. The reacting system also includes a particle feeder at the top of the furnaces. The upper reactor tube is made of Alsint 99.7, a sintered alumina of Bolt Technical Ceramics (BTC) containing 99.7% of Al2O3, with 1.91 cm I.D., 2.54 cm O.D., and 86.4 cm in length; it is in vanilla color in Figure 1. This straight tube is vertically placed in a Lindberg/Blue M model HTF 55122A furnace. The furnace temperature can be brought up to 1200 °C by a programmable temperature controller, Lindberg/Blue M model 58114. The upper furnace is usually used for oxidation and pyrolysis. The second reactor is a straight Hexoloy SA/SP SiC tube with 1.27 cm I.D., 2.54 cm O.D., and 864 mm in length, manufactured by Saint-Gobain Ceramics. This SiC tube is vertically placed in a Lindberg/Blue M Model 54494-V furnace equipped with 10 heating elements of 30.48 cm in length; it is in black color in Figure 1. The furnace temperature can be brought to 1700 °C by a programmable temperature controller, Lindberg/Blue M Model 59256-P-COM. During char preparation, coal particles pass through the Al2O3 tube at a specific temperature, with a controlled residence time, and are trapped in the middle of the SiC tube by about 10 g of SiC particles of two different sizes sitting on a SiC foam that, in turn, is supported by a SiC rod at room temperature. To facilitate the insertion of the foam into the SiC tube, it is cut from a SiC sheet with a 10-degree angle from the vertical position so that the lateral side of the foam is in a trapezoid shape. The diameter of circular foam at its larger base is about the same as the ID of the SiC tube. Small void space between the tube and the foam is filled (48) Senneca, O.; Salatino, P.; Masi, S. Proc. Combust. Inst. 2005, 30, 2223–2230. (49) Senneca, O.; Salatino, P.; Menghini, D. Proc. Combust. Inst. 2007, 31, 1889–1895. (50) Pan, Z.; Yang, R. T. Ind. Eng. Chem. Res. 1992, 31, 2675–2680.
with a small amount of the smaller beads, F60, of 212-300 µm from Industrial Supply, Inc. Larger beads, F12 of 1400-2000 µm from the same supplier, were placed on the top of the foam, and another layer of smaller beads, F60, is placed above the larger beads. ERG Materials Aerospace, Corp. and Saint-Gobain Ceramics Co. manufacture the SiC foam and rod, respectively. This arrangement prevents trapping of char particles in the pores of the foam so that we can reuse the expensive foam. The pressure drop across the bed and foam is negligible. TPD is conducted after a desirable amount of char sample is collected in the SiC tube. To chemically reduce the silicon oxides formed in previous TPD experiments (due to the reaction of SiC tube with the desorption product CO2) and therefore avoid undesirable CO emissions from the SiC tube and its supporting materials during TPD,19 the tube and its supporting materials have to be preheated in flowing ultrahigh pure He at 1650 °C for at least three times. During each 2-3 h isothermal treatment, CO concentration initially reaches a peak and steadily declines to a low level. Repeated treatment reduces the CO peak to an acceptable value, for example, about 130 ppm in our experiments. Gaseous products from both oxidation and desorption products are analyzed by an online Agilent Technologies 6890 gas chromatograph and 5973N mass spectrometer (GC/MS). Gas flows are controlled by rotameters calibrated at operating pressures. O2 concentration for coal oxidation varies from 1 to 21% in ultrahigh pure He from NexAir. The moisture in the products gas is removed by anhydrous calcium sulfate from W.A. Hammond Drierite Co. in a cartridge of 1.27 cm I.D. and 15.24 cm length immediately after the gas products exit from the reactor, whereas the main gaseous species CO and CO2 are not affected. Particle-injection Method. To produce accurate TPD results, it is necessary to remove and avoid oxidants from undesirable sources during both oxidation and TPD. One of these susceptible sources is the air leakage through the moving parts associated with a particle feeder. Thus, a novel particle feeder has been designed to deliver particles at a low and steady rate.51 It is located above the upper reactor and is only partly shown in Figure 1. Particles’ (51) Chen, W. Y.; Gowan, G. C.; Shi, G.; Wan, S. L. ReV. Sci. Instrum., 2008, 79, 083904.
Early-stage Coal Oxidation by TPD
Energy & Fuels, Vol. 22, No. 6, 2008 3727 Table 1. Ultimate Analyses of Coals and Their Productsa
sample origin
history
% O2 in oxidation
pyrolysis or oxidation time, s
loss on drying %
C%
H%
N%
O % (by direct measurement)
S%
ash %
Illinois No. 6 bituminous coal
raw coal pyrolyzed char oxidized char
0 2
0.60 0.43 0.60 0.94
0.02 1.17 0.02 2.59
72.9 77.65 71.18 75.98 68.50
4.9 1.30 1.63 1.43 1.69
1.5 1.48 1.35 1.43 1.41
6.6 4.36 7.33 8.67 7.72
2.8 1.93 1.97 2.30 2.25
10.8 17.6 18.3 18.6 18.4
raw lignite pyrolyzed char oxidized char
0 1
0.60 0.43 0.60 0.73
4.47 1.66 1.50 1.86
64.14 67.44 66.14 67.16 67.40
4.33 1.57 1.28 1.31 1.28
0.97 1.10 0.96 1.03 1.07
18.44 12.61 12.35 11.65 11.84
1.24 0.77 1.41 1.16 1.24
9.8 21.2 29.3 24.5 25.1
North Dakota (Coteau) lignite
a
Dry basis for all measurements, except loss on drying.
Table 2. Oxygen Balancea column 1
char origin Illnois No. 6 bituminous coal
Coteau lignite
column 2
column 3
column 4
column 5
O in 1st O in 2nd diff. of O % O, column peak (730 °C) peak (1280 °C) pyrolysis O remained between 5 relative to % O, in TPD of in TPD of O remained column 1 the sum of or in column 5 oxidized oxidized in oxidized oxidation pyrolyzed % O2 in and column 2 relative to carbon char, mg char, mg char, mgb column 4 and column 3 column 3 burnout, % time, s char, mgb oxidation 0.12
0.064
0.20 0.31
0.061 0.056
0.12 0.20 0.24
0.233 0.192 0.196
2
1
0.033
0.099
0.103
0.039
29.5
39.4
12.6
0.04 0.037
0.088 0.098
0.114 0.112
0.053 0.056
41.4 41.5
60.2 57.1
8.5 16.4
0.127 0.114 0.075
0.193 0.155 0.126
0.187 0.173 0.176
-0.046 -0.018 -0.02
9.6 18.7 21
a This table is based on 1 mg of carbon in the char of interest. b From elemental analysis by Huffman Laboratories. Oxygen is measured directly by ASTM D5622, i.e., without taking into account the oxygen associated with most of the minerals in the sample.
feeding is mainly driven by gravity. It requires no aspirating gas and does not have moving parts at its periphery; therefore, it is much less susceptible to air leakage into the injection port. The feeder can be operated at both batch and near continuous injection modes. A patent application for this feeding device is pending. Oxidation and TPD. Before oxidation, the particle reservoir containing 2 g of dried, pulverized coal of 106 to 125 µm is purged with ultrahigh pure He followed by evacuation at least three times. During oxidation, particles are continuously fed into the Al2O3 reactor preheated at 1000 °C through a 0.635 cm I.D. stainless steel connection tube. The gas mixture enters the connection tube from its side. The oxidation time and, therefore, the carbon burnout are regulated by the gas flowrate; the carbon burnout is also controlled by O2 concentration in the feed. For the current work, we use the gas mixture containing 1, 2, or 21% O2 balanced with ultrahigh pure He. Oxidation of bituminous coal is conducted with particles feeding rates of 0.222 and 7.67 mg/s and with 2 and 21% O2, respectively. The reactor pressure is maintained at 1.68 atm for all the oxidation and TPD experiments. Gas flowrate is set at 300, 800, and 1300 mL/min for oxidizing bituminous coal by 2% O2, which correspond to 0.31, 0.20, and 0.12 s residence times in the heated zone, respectively. Bituminous coal is also oxidized by a mixture of 21% O2 at 4000 mL/min flowrate, which corresponds to a residence time about 0.04 s. On the basis of the elemental analysis reported by Huffman Laboratories, Inc., shown in Table 1, carbon burnout is about 13-21% for chars derived from bituminous coal when 2% O2 is used (seen in Table 2). Assuming the char after oxidation by 21% O2 has the same elemental compositions as those from oxidation by 2% O2, the complete weight loss during burning of this char by a Bunsen burner corresponds to about 75% carbon burnout. Lignite oxidation is conducted with 1% O2, particle feeding rate of 0.372 mg/s, and gas flowrate at 600, 800, and 1300 mL/min, which correspond to residence times of 0.24, 0.20, and 0.12 s, respectively. On the basis of the elemental analysis shown in Table 2, carbon burnout is about 15-20% for chars derived from the lignite. The transfer line between the reactor and the GC/MS is about 2.7 m long with a diameter of 0.36 cm.
TPD is carried out in the SiC tube as soon as a desirable amount char has been collected on the support materials. TPD is conducted between 100 and 1650 °C with 5 °C/min heating rate, with a 75 mL/min He flow that corresponds to a gas residence time of 20 s (or Peclet number 5040) in the transfer line kept at room temperature. Gas residence times of desorption products in the hot zone of the reactor vary from 87 to 17 s (or Peclet number from 41.2 to 10.8) when the temperature increases from 100 to 1650 °C, respectively. The reactor is maintained at 1650 °C for 90 min and then is cooled at 5 °C/min. Calibrations of MS sensitivities of CO and CO2 were conducted with a CO/CO2 gas mixture with certified concentrations on a regular basis. During the calibration, the gas mixture passed through the reactor system at the same pressure and flowrate as those used for the TPD. These MS sensitivities of CO and CO2 are used for quantifications of oxidation and TPD results. Ultrahigh-purity grade (NexAir) He with a minimum purity of 99.999% is used for oxidation and TPD. Oxidants impurities are guaranteed to be
