Oxidation kinetics and nanostructure of model carbons based on TGA data

Transcription

1 Paper #070RK-0216 Topic: Reaction Kinetics U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, th Oxidation kinetics and nanostructure of model carbons based on TGA data I. C. Jaramillo 1, J. Levinthal 1, and J. S. Lighty C. K. Gaddam 2 and R. Vander Wal 2 1 Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112, USA 2 Department of Energy and Mineral Engineering, Penn State University, University Park, PA 16802, USA 1 Oxidation kinetics and fringe analysis studies of three model carbons, ranging from amorphous to onionlike nanostructures were performed in a thermogravimetric analyzer (TGA). The carbons were oxidized isothermally at temperatures ranging from 575 to 775 o C in air using a TGA Q600. Multiple tests were performed to obtain the most favorable operating conditions to minimize mass-transfer diffusion limitations. First-order reaction kinetics and an Arrhenius-type equation were used to extract the rate constants at each temperature. The activation energies for the oxidation of the carbon samples ranged from 124 to 204 kj/mol. The onion-like structure exhibited a slower kinetic rate compared to the amorphous M1300. For Regal 250 carbon, the kinetic rate changed during oxidation and Differential Scanning Calorimeter (DSC) analysis suggested the formation of a new compound (still to be identified) at higher conversions. The fringe analysis explained the differences in the kinetic parameters between carbon samples studied. The onion like nascent sample had a longer range of fringe lengths and smaller tortuosity, suggesting a flatter structure; the nascent M1300, had a much shorter fringe length and broader tortuosity, suggesting more curvature. The nascent R250 nanostructure was between the other two carbons. 1. Introduction Particulate air pollution is linked to adverse health effects and environmental impacts (Lighty et al., 2000; Chang, 2001). The combustion of fossil fuels in the transportation area accounts for a large portion of particulate matter (PM), which is comprised of highly carbonaceous material known as soot (Clague et al., 1999). Soot emissions from diesel engines are a big concern and strategies for reduction of its emissions have been performed over many years (Stratakis and Stamatelos, 2003; Neeft et al., 1997). A better understanding of soot oxidation, and its relation nanostructure will help to mitigate the emission from soot. Thermogravimetic analysis (TGA) is a widely-used technique to determine oxidation kinetics under well-defined conditions such as temperature and oxygen concentration (Stratakis and Stamatelos, 2003; Zouaoui et al., 2010; Kalogirou and.samaras, 2009; Kalogirou and.samaras, 2010). The

2 TGA technique offers the advantage that only a small amount of sample is needed, typically less than 20 mg, and the kinetic parameters (activation energy and pre-exponential factor) are determined by a limited number of experiments. The reactivity of soot and carbon blacks has been performed to examine the dependence of oxidation rate upon particle nanostructure by using high-resolution transmission electron microscopy (HRTEM) and imaging analysis (Boehman, et al., 2005;Vander Wal and Tomasek, 2003;Vander Wal et al., 2004; Vander Wal and Mueller, 2006; Vander Wal et al., 2007). HRTEM imaging is a powerful tool for the study of atomic scale structure of materials (Liati et al., 2013; Vander Wal et al., 2010). Curvature or tortuosity of lamella is a complimentary aspect of carbon nanostructure. Layer plane lengths are also critical to carbon chemical and physical properties (Vander Wal et al., 2010). It is possible to find a relationship between oxidation rates and nanostructure using model carbons, ranging from amorphous to onion-like nanostructures, which can help to a better understanding of the role of nanostructure on oxidation with surrogate mixtures. Previous work (Pahalagedara et al., 2012; Sharma et al., 2012) has studied the oxidation kinetics and structure activity relationship of diesel engine soot and commercial black carbon samples. Results indicate a unique correlation and trends between soot structure and reactivity. In this work, isothermal tests for three different model carbons were conducted to extract the kinetic parameters. In addition, HRTEM fringe analysis of the nascent and the partially oxidized samples were correlated with the oxidation kinetics. 2. Experimental section 2.1 Samples Three commercial carbons, Monarch 1300 (M1300), Regal 250 (R250) and onion-like carbon (OLC), were used in this study. Monarch 1300 and Regal 250 are materials obtained from Cabot. 2.2 Thermogravimetric analysis A SDT Q600 thermogravimetric analyzer (TGA) was used for the oxidation experiments. The samples were pretreated in an inert gas until the desired oxidation temperature was reached to drive off volatile matter with a ramp rate of 10 o C/min. The inert gas (N 2 ) was then switched to an oxidizing gas (air) at a volumetric flow rate of 500 ml/min. Depending on the sample evaluated, the conditions were constant to obtain the mass loss curve followed by a cooling down to room temperature. Experiments were conducted with a sample size of approximately 5 mg for M1300, 10 mg for onion OLC and 8 mg for R250.The sample was placed in a quartz crucible (3.92 mm diameter and 4.35 mm height). Isothermal tests were performed at temperatures from 550 to C for M1300, and R250 in increments of 25 0 C, while for OLC tests were performed from 575 to C. The mass loss data was recorded approximately every half second. 3. Data analysis and Results 3.1 Optimization of variables affecting TGA experiments Under certain operating conditions (oxidizer flow rate, initial sample mass and type of crucible), the oxidation inside the TGA furnace could be partially diffusion controlled (Stanmore at al., 1994; 2

3 Gilot et al., 1995; Song et al., 2006; Song et al., 2010). In this case, the kinetic parameters cannot be directly obtained from TGA plots, and a combination of diffusive and kinetic models is necessary (Song et al., 2006; Song et al., 2010). To ensure that the TGA results reported in this study were not affected by diffusion limitations, multiple experiments were performed to find the most favorable conditions to estimate the kinetic parameters. Effect of the oxidizer flow rate Combustion of 5 mg of M1300 was carried out at 650 C at five oxidizer gas (air) flow rates ranging between 100 and 500 ml/min. Figure 1. shows the mass loss profiles of these for different gas flow rates. 1 Conversion ml/min 400 ml/min 200 ml/min 300 ml/min ml/min Time (min) Figur e 1. Isothermal conversion profiles of M1300 with different flow rates of air. Operating conditions: initial mass of 5mg, and temperature 650 o C. As seen in Figure 1, the oxidation rate at a gas flow rate of 100 ml/min was lower than the 200 ml/min and 300 ml/min, but continued constant for gas flow rates of 400 ml/min or higher. To avoid oxygen diffusion limitation, the gas flow rate used in these experiments was 500 ml/min. Effect of the initial mass To investigate initial mass, a set of runs were carried out at 600 C and 500 ml/min of oxidizer gas (air) with M1300 and initial mass loadings ranging from 2 to 10 mg. For OLC and R250 the loadings ranged from 5 to 16 mg. The mass-based oxidation profiles of M1300 carbon for the multiple experiments performed are presented in Figure 2. 3

4 mg 10 mg 5 mg Conversion mg Time (min) Figure 2. Isothermal conversion profiles of Monarch 1300 with different initial sample mass. Operating conditions: air flow rate of 500 ml/min, and temperature 600 o C. Figure 2 shows that the oxidation rate was slower when large initial sample mass was used indicating mass-transfer resistances through the sample layer. The smaller sample mass was discarded because the low reproducibility so 5 mg was used for M1300. An initial loading mass of 8-10 mg was selected as the optimum mass for additional experiments for R250 and OLC. 3.2 Estimation of Kinetic parameters The TGA mass loss curves were obtained under isothermal conditions and used to determine the soot-oxidation kinetic parameters. An Arrhenius-type equation was used to extract the rate constants at each temperature, as follows: dm n k c mpo2 (1) dt dm n Ea n kcmpo2 Aexp mpo2 (2) dt RT where the m is the mass of soot, t is the time, k c is the reaction rate constant, po 2 is the partial pressure of oxygen, A is the pre-exponential factor, E a is the activation energy of the reaction and n is the reaction order of oxygen. First-order reaction kinetics, n=1, was used for the analysis of the data generated in this study. Rearranging Equations 1 and 2 yields 4

5 d ln m m0 dt ln Apo2 Ea RT (3) d The mass loss curve slope gives d plot, ln m m0 dt against m m0 dt at different temperatures. The slope and intercept of the 1 yields E a and A. T A variety of tests were carried out at atmospheric pressure, in air, and temperature ranges from 550 to 775 C, depending on the sample evaluated. The kinetic parameters were extracted from Arrhenius -(m/m 0 function of time for OLC at different temperatures is presented in Figure 3. The figure suggests that, at lower temperatures, the oxidation rate is highest at the beginning and constantly decreases with degree of conversion as reported in the literature for black carbon and diesel soot (Vyazovkin et al., 2011). 775 C 700 C C 625 C Conversion Time (min) Figur e 3. Conversion profiles of OLC. The apparent kinetic parameters and rate constants 5 800

6 Table1. Apparent Kinetic Parameters sample name temperature range ( C ) E (kj/mol) A (pa -1 min -1 ) R 2 M E R E OLC E Table2. Kinetic rate constants estimated for a fractional conversion = 0.6. sample name k 575 C k 600 C k 625 C k 650 C k 700 C k 750 C k 775 C (Pa -1 min -1 ) (Pa -1 min -1 ) (Pa -1 min -1 ) (Pa -1 min -1 ) (Pa -1 min -1 ) (Pa -1 min -1 ) (Pa -1 min -1 ) M E E E E E E-05 R E E E E E E E-05 OLC 2.38E E E E E E E-06 The activation energies for the oxidation of the carbon samples ranged from 124 to 204 kj/mol. The rate constants were found to be in the same order reported by Sharman et al., 2012 for similar carbon black samples. For M1300 and R250 samples, the rate constants at temperatures higher than 700 o C appear to be affected by oxygen diffusion limitations (underlined-numbers in Table 2). The onionliked structure exhibited a slower kinetic rate compared to the amorphous M1300 as presented in Figure 4. 1 M R250 Conversion OLC Time (min) Figure 4. Conversion profiles of OLC, R 250 and M1300 carbons at 600 o C. The presence of a second peak in the Differential Scanning Calorimeter (DSC) suggested the formation of a new compound at higher conversions for the R 250 in the range of temperature between 575 and 650 o C. Figure 5 shows a plot of weight (%) versus time as well as the DSC profiles for four different temperatures. 6

7 575 C 600 C 625 C 650 C Figure 5 Weight (%) versus time and DSC profiles for four different temperatures. The presence of a second peak in the DSC at the end of the first slope at approximately 65 % mass loss was observed. Further study is needed for a better understanding of this observation. 3.3 High resolution transmission electron microscopy (HRTEM) Imaging analysis was performed on HRTEMs of partially-oxidized and nascent samples. The partially-oxidized samples were collected at 50 % weight loss in the TGA and placed in vials containing ethanol (200 proof). This mixture was filtered in a BHS Sonthofen pressurized filtration system. The remaining sample was then suspended in the ethanol and sonicated for 15 minutes. Upon completion, one drop from a glass micropipette was placed on a coated lacey carbon grid (Ted Pella 200-mesh Formvar). The grids were allowed to dry completely prior to being stored in the grid holder to be analyzed. Custom algorithms for fringe length and tortuosity were used for the analysis of soot nanostructure (Vander Wal and Tomasek, 2003;Vander Wal et al., 2004; Vander Wal and Mueller, 2006; Vander Wal et al., 2007). The preliminary fringe analysis for the three samples studied before and after oxidation is shown in Figure 6 7

8 Nascent Partially oxidized Nascent Partially oxidized Nascent Partially oxidized M1300 R 250 Onion Figure 7. Fringe length and tortuosity for M1300, carbon, R250, and OLC (from left). As in Figure 6 the OLC nascent sample had a longer range of fringe lengths and smaller tortuosity, suggesting a flatter structure; the nascent M1300, had much shorter fringe length and broader tortuosity, suggesting more curvature; the nascent R250 values were in the middle. The partially-oxidized OLC did not show significant changes in fringe length or fringe tortuosity. Partially-oxidized M1300 showed roughened surfaces that are indicative of oxidation and the oxidation increased the length of lamella. Finally the R250 oxidized sample showed a progressive increase in lamella length and decrease in tortuosity across the oxidation. 4. Conclusions The activation energy for the oxidation of the three-carbon black samples investigated ranged from 124 to 204 kj/mol, with the graphite onion-like carbon showing the largest activation energy and the smallest rate constant, and the amorphous M1300 showing the smallest activation energy and the largest rate constant. The activation energy of R250 was found to be in the middle. The M1300 carbon presents more amorphous structure with more edge sites, which have been found to be more reactive. In contrast, the OLC nascent sample had a longer range of fringe lengths and smaller tortuosity, suggesting a flatter structure. For R250 the kinetic rate changed during oxidation and DSC suggested the formation of a new compound at higher conversions. Acknowledgements This work is funded by the Department of Energy Basic Energy Sciences under Grant No DE-SC

9 References Boehman, A.L., Song,J., Alam, A. (2005). Impact of Biodiesel Blending on Diesel Soot and the Regeneration of Particulate Filters. Energy and Fuels, 19, Chang, I.P. (2001). Climate change 2001: The scientific basis. Cambridge, UK: Cambridge University Press. Clague, A.D.H.; Donnet, J.B.; Wang, T.K.; Peng, J.C.M. (1999). A Comparison of diesel engine soot with carbon black. Carbon, 37 (10), Gilot, P., Brillard, A, Stanmore, B.R. (1995). Geometric effects on mass transfer during thermogravimetric analysis: Application to reactivity of diesel soot. Combustion and Flame, 102: Kalogirou, M. and Samaras, Z. (2009). A thermogravimetic kinetic study of uncatalyzed diesel soot oxidation. Journal of Thermal Analysis and Calorimetry, 98, Kalogirou, M. and Samaras, Z.(2010). Soot oxidation kinetics from TG experiments: Can they be used reliably in diesel particulate filter modeling tools?. Journal of Thermal Analysis and Calorimetry, 99, Liati A., Dimopoulos Eggenschwiler,P., Schreiber, D., Zelenay, V., and Ammann, M.(2013). Variations in diesel soot reactivity along the exhaust after-treatment system, based on the morphology and nanostructure of primary soot particles. Combustion and Flame, 160: Lighty, J.S., Veranth, J.M., and Sarofim, A.F.(2000). Combustion aerosols:factors governing their size and composition and implications to human health.journal of Air and Waste Management, 50, Neeft, J.P.A., Nijhuis, T.X., Makkee, M., and Moulijn, J.A. (1997). Kinetics of the oxidation of diesel soot. Fuel, 76, Pahalagedara, L., Sharma, H., Kuo, C.H., Dharmarathna, S., Joshi, A., Suib, S.L., and Mhadeshwar, A. B. (2012). Structure and Oxidation Activity Correlations for carbon Blacks and Diesel Soot. Energy and Fuels, 26, Sharma, H.N., Pahalagedara, L., Joshi, A., Suib, S. L.,and Mhadeshwar, A.B.(2012). Experimental study of Carbon Balck and diesel Engine Soot Oxidation Kinetics Using Thermogravimetric Analysis. Energy and Fuels, 26, J. Song, J. Chung-Hwan, and A. Boehman. Impacts of Oxygen Diffusion on the Combustion Rate of In-Bed Soot Particles. Energy Fuels, 24, , Song, Q., He, B., Yao, Q., Meng, Z., and Che, C.( 2006) Influence of Diffusion on Thermogravimetric Analysis of Carbon Black Oxidation. Energy Fuels, 20, Stanmore, B., Gilot, P., and Prado, G. (1994). The influence of mass transfer in DTG combustion tests. Thermochimica Acta, 240, Stratakis, G.A., and Stamatelos, A.M. (2003). Thermogravimetric analysis of soot emitted by a modern diesel engine run on catalyst-doped fuel. Combustion and Flame, 132, Vander Wal, R.L., and Tomasek, A.J. (2003) Soot oxidation: Dependence upon initial nanostructure. Combustion and Flame, 134, 1-9. Vander Wal, R.L., Tomasek, A.J., Street, K., Hull, D. R. and Thompson, W. K. (2004). Carbon Nanostructure Examined by Lattice Fringe Analysis of High-Resolution Transmission Electron Microscopy Images. Applied Spectroscopy, 58 (2), Vander Wal, R.L. and Mueller, C.J. (2006). Initial Investigation of Effects of Fuel Oxygenation on Nanostructure of Soot from Direct-Injection Diesel Engine. Energy & Fuels, 20, Vander Wal, R.L., Yezerets, A., Currier, N.W., Kim, D. H., and Wang, C. M. (2007). HRTEM Study of diesel soot collected from diesel particulate filters. Carbon, 45,

10 Vander Wal, R.L., Bryg, V.M., and Hays, M.D. (2010). Fingerprinting Soot (Towards Source Identification) Physical Structure and Chemical Composition. Aerosol science, 41, , Vyazovkin, S., Burnham, A.K., Criado, J.M., Perez-Maqueda, L.A., Popescu, C., and Sbirrazzuoli N. (2011). Thermochim. Acta, 520, Zouaoui, N., Brilhac, J.F., Mechati, F., Jequirim, M., Djellouli, B, and Gilot, P. (2010). Study of experimental and theoretical procedures when using thermogravimetric analysis to determine kinetic parameters of carbon black oxidation. Journal of Thermal Analysis and Calorimetry, 102,