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Catalytic soot oxidation in microscale experiments
Applied Catalysis B: Environmental 55 (2005) 31–37 www.elsevier.com/locate/apcatb
Catalytic soot oxidation in microscale experiments Martin Seipenbuscha,*, Jan van Ervenb, Tobias Schalowc, Alfred P. Weberc, A. Dick van Langeveldd, Jan C.M. Marijnissenb, Sheldon K. Friedlandera a
Department of Chemical Engineering, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90024, USA Faculty of Applied Physics, Particle Technology Group, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands c Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe (TH), Kaiserstr. 12, 76128 Karlsruhe, Germany d Faculty of Applied Physics, Charged Particle Optics Group, Delft University of Technology, Lorentzweg, 12628CJ Delft, The Netherlands
b
Received 20 February 2004; received in revised form 15 July 2004; accepted 21 July 2004 Available online 8 September 2004
Abstract The oxidation of soot agglomerates over catalytically active surfaces is of interest for the development of catalytic reactors for the control of soot emissions. The process involves the transport and deposition of nanoparticle aggregates to a surface on which catalyst particles are deposited. To simulate this process, graphitized carbon nanoparticles and platinum nanoparticles were separately deposited on an oxidized silicon wafer by laser ablation and electro hydro dynamic atomization. Changes in particle morphology produced by the reaction were visualized ex situ by scanning electron microscopy. In this way chemical reaction data could be correlated with the local surface coverage and particle size of the catalytically active material and the morphology of the reacting particles, resulting in detailed local information on their interaction, which is not available in studies on bulk samples. The contact between catalyst and soot particles was loose, simulating the behavior of catalyst systems used in practice. The activation energy of the oxidation in air was found to be 40 kJ/mol whereas the activation energy in air/NO was found to be 160 kJ/mol, both in presence of Pt deposited on a SiO2 support. Notwithstanding the higher activation energy, the reaction rate of soot oxidation in air/NO is about two to three orders of magnitude higher than in air. A linear relationship between the relative Pt surface and reaction rate was found for the oxidation in an air/NO atmosphere. In air, the relationship has a minimum which indicates that there are different simultaneous mechanisms of reaction. Although activation energies are different from other studies, the oxidation temperatures are comparable. The EHDA and laser ablation produced platinum catalysts behave similarly and show potential to be used as model catalyst. # 2004 Elsevier B.V. All rights reserved. Keywords: Soot agglomerates; EHDA; Platinum catalysts
1. Introduction While the overall particulate mass emitted by diesel engines has been reduced in recent years, the fraction of the ultrafine particles has increased, which may pose a threat to human health [1,2]. These soot particles consist of a carbonaceous solid core with an adsorbed layer of hydrocarbons, sulfates and polycyclic aromatic hydrocarbons (PAH). The size of the particles makes them inhalable which causes serious concern with regard to adverse health * Corresponding author. Tel.: +1 721 608 8058; fax: +1 310 825 8805. E-mail address: [email protected] (M. Seipenbusch). 0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.07.007
effects, including carcinogenity [3]. To reduce particulate matter (PM) emissions from motor vehicles, Europe and the US have set standards of for PM and NOx that cannot be met within a few years by improving engine technology alone [4]. Therefore, particulate filters have to be employed to reduce the emissions of diesel engines. Moreover, properly designed filters could be retrofitted on existing diesel engines, thus, significantly improving environmental conditions. Due to the demanding operating conditions – high and rapidly alternating temperatures as well as a chemically aggressive environment – these filters are made of ceramic monoliths or woven fibers [5]. To prevent clogging and the accompanying rise in pressure drop over the filter element,
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Fig. 1. Schematic of reaction mechanisms for oxidation of carbon by NO/ NO2.
the accumulated soot has to be burned off periodically or continuously [4]. To increase the soot oxidation rate at the typical exhaust temperatures of diesel engines to a point where it equals or exceeds the deposition rate, oxidation catalysts are introduced into the soot traps. The most commonly used materials for this purpose are V2O5, CuO and MoO3 [6,7], Fe2O3 and Co3O4, [8,9] and Pt [10]. In experimental studies, measurements were mostly made with macroscopic mixtures of catalyst particles and various types of soot. Thermogravimetric oxidation ([11,6]), temperature programmed oxidation (TPO) ([10,11]) and packed-bed experiments ([12,13]), represent such studies on various catalysts. It has been generally accepted that the presence of NO strongly decreases the combustion temperature of soot when platinum is used as catalyst. It was proposed that platinum promotes the oxidation of carbon by oxidizing NO into NO2 [15], which is transported through the gas phase to the soot aggregates where it oxidizes carbon [13] while being reduced to NO (Fig. 1). Many authors emphasize the importance of the type of contact between soot and the active phase of the catalyst [11]. Some materials, e.g., metal oxides and alkali carbonates [8] only showed a catalytic effect when being in close contact to the carbon particles. In a particle trap the soot particles are deposited in a way that allows only indirect contact [19]. Therefore, to study the catalytic behavior under conditions similar to those of practical interest seems to be the most logic approach. However, the sample configuration in these experiments differs from those employed in practice, where the catalyst particles are dispersed over a ceramic support. Hence, at increased reaction rates, these macroscopic powder samples may experience heat and mass transfer limitations [14], thus, obscuring the intrinsic catalytic properties of the system under investigation. Two goals can be identified. The first goal is to measure the oxidation rate of soot aggregates deposited on a surface in the close proximity of Pt particles using a new experimental technique. Changes in the particle morphology upon the reaction were observed off-line by scanning electron microscopy. In this way, reaction data could be correlated to the local surface coverage and particle size of the active phase, as well as to the primary particle- and agglomerate size of the soot particles.
The second goal was to produce platinum particles using two methods. Platinum catalysts in laboratories are normally produced by an impregnation method. First the support, e.g., SiO2 or g-Al2O3, is impregnated with a aqueous solution of the platinum precursor and dried. This is followed by calcination in air and a reduction step in hydrogen. The time and temperatures for each step differs from author to author. The currently employed production route for platinum particles was different because the support we used was a flat SiO2 surface where dispersed impregnation could not be achieved. Hence, the active phase was introduced by Electro HydroDynamic Atomization [16] and Laser Ablation [17].
2. Experimental 2.1. General The model catalyst consisted of platinum nanoparticles and carbon particles deposited on a SiO2 surface, such that there was loose contact between active phase and the carbon particles. These model catalysts were exposed to various gas phases at different temperatures in a tubular quartz reactor. Mass flow controllers were employed to adjust the flow rates of N2, O2 and NO. The NO inlet concentration was controlled by a NOx-monitor (Horiba). Reaction times ranged from 10 to 60 min. The interaction between active phase and carbon during the oxidation was determined analogously as by Schalow [20] and Jung et al. [21]. SEM micrographs of single carbon nanoparticle agglomerates were taken before and after oxidation, while Jung et al. used a TEM system. Structural changes induced by the reaction were monitored using the image analysis software ImageJ. This software was developed by the NIH and is available on the internet (SITE = http://rsb.info.nih.gov/ij/). The imaging was done using a field emission scanning electron microscope (Hitachi Model S-4700) at the UCLA Nanolab. The instrument has a resolution of 1.5 nm at 5 kV. By taking images at various magnifications of interest ‘‘road maps’’ of the surfaces were created thus allowing to find the same spot on the catalyst surfaces before and after the reaction. To distinguish between platinum and carbon particles, secondary electron (SE) and backscattering electron (BSE) micrographs were taken of the same spot. While platinum and carbon particles can both be seen on the SE micrographs, the BSE micrograph shows only platinum particles due to the much higher mass of the nuclei. 2.2. Soot preparation Depending on the conditions of formation, diesel soot includes graphitic components with varying amounts of associated aromatics, paraffins, oxygenated compounds and sulfates [18]. As a relatively simple surrogate for diesel soot, carbon nanoparticle agglomerates were produced by laser
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Fig. 2. Particles produced by EHDA: (a) chloroplatinic acid particles; (b) platinum particles produced by decomposition of particles similar to particles of (a).
ablation [17], an evaporation–condensation based technique. The energy of a pulsed laser (Lambda Physik model EMG 101 MSC) creates a short-lived plasma over a target of the desired material. During this short time, which is typical in the microsecond range, a very high aerosol concentration of carbon nanoparticles is generated. Hence, the collision frequency is high and coalescence occurs. Since, the cooling rate is extremely high, the coalescence is stopped early and colliding particles will agglomerate resulting in the formation of fractal-like structures of about 1 mm diameter. 2.3. Catalyst preparation The catalyst particles used in this work were made by two methods, which were electro hydro dynamic atomization (EHDA) and laser ablation. In all cases the catalyst support consisted of pieces of polished and superficially oxidized silicon wafers. The oxide layer was produced by oxidation of the silicon at 1000 8C in air for 1 h and had a thickness of 0.25 mm (http://www-mtl.mit.edu/6152j/appendix1.html). This is sufficient to neglect the influence of silicon. For the generation of Pt particles by EHDA a 0.2 wt.% solution of chloroplatinic acid (H2PtCl66H2O) in ethanol was used. The spraying nozzle had an o.d. of 0.16 mm and an i.d. of 0.06 mm. The flow rate was controlled by a high accuracy syringe pump (Harvard PHD 2000) and was kept constant at 8 ml/h while the potential between nozzle and the grounded counter electrode was maintained at 2.5 kV, leading to the cone-jet mode of operation. The jet broke up into very fine droplets, which yielded platinum salt particles after evaporation of the solvent. To form platinum particles the layers were put in an oven for 15 min which had a constant temperature of 700 8C. Upon thermal decomposition, the salt particles broke apart and platinum particles of a smaller size were formed. For lower coverages of Pt on the SiO2 surface, catalyst particles were also generated by laser ablation as described above. The target used was a Pt foil (Alfa Aesar 99.99% purity). Since non-agglomerated Pt
particles were desired, the aerosol was passed through a sintering furnace at 700 8C to reach total coalescence of the agglomerates.
2.4. Data acquisition and evaluation The analysis of the SEM images with ImageJ yielded the projected areas of agglomerates. Ullmann et al. [17] produced the agglomerates in the same way and found a fractal dimension of the agglomerates of 1.85. Therefore, we assume that the fractal dimension of the agglomerates is below two. In this case, the projected area of the agglomerate is meaningful for the calculation of particle mass, since there is very limited shading of primary particles mutually. Using the median diameter from the primary particle size distribution, the number of primary particles, N, with the same projected area as the agglomerate was calculated. From the primary particle diameter, the density and N, the mass of the particles could be calculated. The reaction rate was determined from the difference in carbon mass before and after the reaction and was normalized to the initial carbon mass to yield a mass specific reaction rate.
3. Results and discussion 3.1. Platinum catalyst production Chloroplatinic acid salt particles produced by EHDA are shown in Fig. 2a. The salt particles decompose when heated for 15 min at 700 8C, thus, forming groups of smaller platinum particles. The original form of the salt particles can still be recognized, as shown in Fig. 2b [22]. Platinum particles produced by laser ablation are shown in Fig. 3. Clearly, the coverage of platinum particles is much lower than in case of EHDA. Also the sizes and structures differ from the EHDA produced particles. The
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Fig. 3. Platinum particles produced by laser ablation.
size distributions of the platinum particles produced by EHDA and laser ablation are shown in Fig. 4. The carbon particles produced using this method are shown in Fig. 5. The aggregated structure of the carbon particles resembles soot particles produced by diesel engines. The primary particle size distribution is shown in Fig. 6. From it, the specific surface area, Sv, of the carbon particles was calculated and found to be approximately 100 m2/g. This was done by dividing the total surface over the total mass of the particles, yielding: Sv ¼
S Npd2 6 ¼ ¼ m Nð1=6Þpd3 r dr
where d is the particle size and r is the density of the particle. The density was taken to be the density of the bulk material. The surface area was calculated because no BET measurements were possible with the small amounts produced. The particle size was the surface based mean diameter calculated by transforming the accumulated log–normal number based
Fig. 5. SEM micrograph of a carbon aggregate produced by laser ablation.
(Q0(x)) distribution into a surface based accumulated size distribution (Q2(x)). 3.2. Reaction data To determine the effect of local Pt coverage on the reaction rate, the projected area of the Pt-particles was determined from the SEM images. From the size distribution of Pt particles (Fig. 4), it was possible to calculate the local platinum surface area per unit surface area of the substrate. The rates of the carbon oxidation in air/NO were normalized to a temperature of 300 8C using the activation energy determined from the data. The results are plotted versus the relative surface area of the catalyst in Fig. 7. Since the data in
Fig. 4. Size distributions of the Pt particle size on samples produced by the electrospray (EHDA) method and by laser ablation with subsequent sintering. The mean particle size, x50, was about 12 nm for EHDA and 23 for laser ablation. The lines are best fits of log–normal functions.
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Fig. 6. The cumulative primary particle size distribution of carbon agglomerates produced by laser ablation. The curve is a log–normal function fitted to the data with an x0, 50 of 17 nm an a s of 0.5.
the diagram correlate linearly, the rate of carbon oxidation in air/NO is proportional to the Pt surface area. The linear relation points to the catalytic activity of the platinum. While we observed an influence of the surface area of Pt, the method of generation of the Pt particles (EHDA or laser ablation) did not seem to have an effect on their activity as catalysts. For the oxidation of carbon in air, the reaction rates for three different temperatures were plotted in a similar manner as a function of the relative surface area of the catalyst, see Fig. 8. In this case, the dependency of the oxidation rate on the relative Pt surface area is more complicated. In the absence of Pt, reaction rates of up to 2.5 104 s1 are reached. With increasing Pt surface area, the rates decrease steeply to approximately one-fifth of the initial value, and increased again for higher ratios of Pt surface area to
unit area of substrate surface. This behavior of the reaction rate may result from the effect of two different reaction mechanisms, one inhibited by the presence of small amounts of Pt and the other favored by larger amounts of Pt. The first mechanism would be the direct oxidation of carbon by O2 adsorbing from the gas phase while the second could be a spill-over of activated oxygen species from the Pt contacts to the carbon particles. While we lack an explanation for the inhibiting effect of Pt on the direct oxidation, the positive influence of a large Pt surface and a shorter average distance between carbon- and Pt-particles on a spill-over mechanism seems obvious. However, Dernaika and Uner [11] found that the presence of some transition metal oxides could raise the combustion temperature of soot, relative to the uncatalysed case, in a dry air atmosphere. This could be due to a similar phenomenon as in our observations.
Fig. 7. Calculated rates of oxidation of carbon in an Air/NO-atmosphere at a standardized temperature of 300 8C plotted vs. the surface area of catalyst per unit area of substrate. The open symbol represents data from catalysis with Pt produced by laser ablation while the full symbols show data from Pt produced by EHDA.
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Fig. 8. Rates of reaction of oxidation of carbon in air at 325, 350 and 375 8C plotted vs. the surface area of Pt relative to the unit surface area of the substrate. Open symbols represent reaction rates obtained for Pt-particles from laser ablation, full symbols are for EHDA. The line is only a guide to the eye.
Although the activation energies are different compared to other studies, the oxidation temperatures are comparable. To determine the activation energy for the catalytic combustion of soot in our system, two series of experiments were conducted for in a mixture of 20% O2 (Airgas, ultrahigh purity), in balance N2 (Airgas, ultra-high purity) and for an air/NO atmosphere consisting of 1000 ppm NO (Linde 0.5% NO in balance N2) in 20% O2 in balance N2 at temperatures ranging from 260 to 380 8C. Fig. 9 shows the reaction rates as a function of the reciprocal temperature for both sets of experiments. Since the data are plotted in the Arrhenius-form, the activation energies can be obtained from the slopes of the linear fits to the data points. For the oxidation of carbon in air, an activation energy of 40 20 kJ/mol was obtained by the least squares fit (see Fig. 5). This is low compared to data found by other authors. Dernaika and Uner [11] found an activation energy of
156 kJ/mol in the case of soot oxidation in dry air using a platinum catalyst on a SiO2 support. In many other studies, with a platinum catalyst but different support material, activation energies between 140 and 170 kJ/mol where found [23]. The discrepancy between the literature values and our study can be due to different preparation methods of the catalyst and carbon, or because of different experimental conditions. While all authors use TPO experiments, which are on the macroscopic scale, the oxidation in this study is observed on the microscale by analysis of SEM images. This difference can lead to discrepancies in observed activation energies. For the reaction in an air/NO-atmosphere a value of 160 8.5 kJ/mol was found from the least squares fit (see Fig. 9). The activation energy for the carbon oxidation in air/ NO is high compared to data in the literature. Jelles et al. [12] found an activation energy of 93 10 kJ/mol for a Pt
Fig. 9. Arrhenius-plot of the reaction rates of the catalytic combustion of carbon in air and Air/NO-atmospheres. Activation energies: for oxidation in air 40 (20) kJ/mol; for oxidation in Air/NO 160 (8.5) kJ/mol. Open symbols show data for Pt particles produced by laser ablation, full symbols for EHDA. Reaction rates calculated for a relative Pt surface area.
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based catalyst mixed with a cerium activated soot. From the data reported by Setiabudi et al. [13] on Pt/gAl2O3 we calculated an activation energy of 46 kJ/mol, which may indicate a mass transfer limitation in the bulk samples used in their studies. The catalyst generally lowers activation energy, typically by 30–80 kJ/mol (Stanmore2001). Thus, the activation energy found in this study is much higher compared to other studies. Notwithstanding the higher activation energy, Fig. 9 convincingly shows that the reaction rates in the presence of NO are about two to three orders of magnitude higher than those in absence of NO at comparable temperatures, in line with results of Setiabudi et al. [13]. This difference is supposedly caused by the pre-exponential factor (k) in the Arrhenius equation: Ea r ¼ k exp RT
bulk powder samples, which may be due to mass and heat transfer limitation in bulk systems. In summary, the experimental methods used in this study yield detailed local information on catalyst and carbon particles and their interaction during the reaction, which cannot be obtained in studies on bulk samples. Therefore, this method has potential for optimizing catalyst dispersion and also catalyst particle properties with respect to the catalytic activity, a process we continue to study.
where r is the reaction rate, k the pre-exponential factor, Ea the activation energy, R the gas constant, and T is the absolute temperature. For our data the pre-exponential factor of the oxidation in air is 0.23 s1 where in air/NO this factor is 3.7 1010 s1. The pre-exponential factor is a combination of two factors, i.e., a k0 and the number of active sites on the catalyst, N. Although the k0 may vary to some extend between various reactions, it is not likely to be expected to vary over 11 orders of magnitude, as could be inferred from the current results. Therefore, it can be concluded from our experiments that, apparently, the number of sites present on a platinum surface is higher for the NO reaction than for the oxidation of carbon in a nitrogen/oxygen atmosphere alone. Rates for the oxidation of carbon in air/NO found by Jelles et al. [12] using a Pt catalyst were lower by one to two orders of magnitude than our results; The reason for the higher reaction rates in our single particle experiment may be mass and heat transfer limitations in the bulk experiments.
References
4. Conclusions For Pt/SiO2-catalyzed oxidation of soot in air and air/NO, a strong dependency of the reactions rates on the Pt surface area is observed. For air/NO, the reaction rate is linearly proportional to the Pt-surface area. For carbon oxidation in air, two effects of the Pt seem to exist. However, the method of particle production, laser ablation with subsequent sintering or EHDA, did not seem to have an effect on the activity of the catalyst. The reaction rates and also the activation energy for the reaction in air/NO were higher than in comparable studies on
Acknowledgments Support for this work by the Alexander von HumboldtFoundation and the Parsons Foundation is gratefully acknowledged.
[1] H.-E. Wichmann, A. Peters, Phil. Trans. R. Soc. Lond. A 358 (2000) 2751. [2] A. Sydbom, A. Blomberg, S. Parnia, N. Stenfors, T. Sandstro¨ m, S.-E. Dahle´ n, Eur. Respir. J. 17 (2001) 733. [3] M. Lipsett, S. Campleman, Am. J. Public Health 89 (1999) 1009. [4] G. Saracco, C. Badini, V. Specchia, Chem. Eng. Sci. 54 (1999) 3035. [5] Manufacturers of Emission Controls Association, Emission control of diesel fueled vehicles, Status report, 1997. [6] P. Ciambelli, M. D0 Amore, V. Palma, S. Vaccaro, Combust. Sci. Tech. 121 (1996) 67. [7] J. Trawczynski, React. Kinet. Catal. Lett. 63 (1998) 41. [8] J.P.A. Neeft, F. Hoornaert, M. Makkee, J.A. Moulijn, Thermochim. Acta 287 (1996) 261. [9] E.E. Miro´ , F. Ravelli, M.A. Ulla, L.M. Cornaglia, C.A. Querini, Catal. Today 53 (1999) 631. [10] G. Neri, L. Bonaccorsi, A. Donato, C. Milone, M.G. Musolino, A.M. Visco, Appl. Catal. B 11 (1997) 217. [11] B. Dernaika, D. Uner, Appl. Catal. B 40 (2003) 219. [12] S.J. Jelles, R.R. Krul, M. Makkee, J.A. Moulijn, Catal. Today 53 (1999) 623. [13] A. Setiabudi, B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Appl. Catal. B 35 (2002) 159. [14] R. Mahadevan, D. Lee, H. Sakurai, M.R. Zachariah, J. Phys. Chem. A 106 (2002) 11083. [15] J. Oi-Uchisawa, A. Obuchi, A. Ogata, R. Enomoto, S. Kushiyama, Appl. Catal. B 21 (1999) 9. [16] J.M. Grace, J.C.M. Marijnissen, J. Aerosol Sci. 25 (1994) 1005. [17] M. Ullmann, S.K. Friedlander, A. Schmidt-Ott, J. Nanoparticle Res. 4 (2002) 499. [18] U. Kirchner, R. Vogt, C. Natzeck, J. Goschnick, J. Aerosol Sci. 34 (2003) 1323. [19] J.P.A. Neeft, O.P. van Pruissen, M. Makkee, J.A. Moulijn, Appl. Catal. B 12 (1997) 21. [20] T. Schalow, Diploma Thesis, Universita¨ t Karlsruhe (TH), 2003. [21] H. Jung, D.B. Kittelson, M.R. Zachariah, Comb. Flame 136 (2004) 445. [22] J.O. Hernandez, E.A. Choren, Therm. Chim. Acta V71 (1983) 265. [23] B.R. Stanmore, J.F. Brilhac, P. Gilot, Carbon 39 (2001) 2247.
Catalytic soot oxidation in microscale experiments Martin Seipenbuscha,*, Jan van Ervenb, Tobias Schalowc, Alfred P. Weberc, A. Dick van Langeveldd, Jan C.M. Marijnissenb, Sheldon K. Friedlandera a
Department of Chemical Engineering, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90024, USA Faculty of Applied Physics, Particle Technology Group, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands c Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe (TH), Kaiserstr. 12, 76128 Karlsruhe, Germany d Faculty of Applied Physics, Charged Particle Optics Group, Delft University of Technology, Lorentzweg, 12628CJ Delft, The Netherlands
b
Received 20 February 2004; received in revised form 15 July 2004; accepted 21 July 2004 Available online 8 September 2004
Abstract The oxidation of soot agglomerates over catalytically active surfaces is of interest for the development of catalytic reactors for the control of soot emissions. The process involves the transport and deposition of nanoparticle aggregates to a surface on which catalyst particles are deposited. To simulate this process, graphitized carbon nanoparticles and platinum nanoparticles were separately deposited on an oxidized silicon wafer by laser ablation and electro hydro dynamic atomization. Changes in particle morphology produced by the reaction were visualized ex situ by scanning electron microscopy. In this way chemical reaction data could be correlated with the local surface coverage and particle size of the catalytically active material and the morphology of the reacting particles, resulting in detailed local information on their interaction, which is not available in studies on bulk samples. The contact between catalyst and soot particles was loose, simulating the behavior of catalyst systems used in practice. The activation energy of the oxidation in air was found to be 40 kJ/mol whereas the activation energy in air/NO was found to be 160 kJ/mol, both in presence of Pt deposited on a SiO2 support. Notwithstanding the higher activation energy, the reaction rate of soot oxidation in air/NO is about two to three orders of magnitude higher than in air. A linear relationship between the relative Pt surface and reaction rate was found for the oxidation in an air/NO atmosphere. In air, the relationship has a minimum which indicates that there are different simultaneous mechanisms of reaction. Although activation energies are different from other studies, the oxidation temperatures are comparable. The EHDA and laser ablation produced platinum catalysts behave similarly and show potential to be used as model catalyst. # 2004 Elsevier B.V. All rights reserved. Keywords: Soot agglomerates; EHDA; Platinum catalysts
1. Introduction While the overall particulate mass emitted by diesel engines has been reduced in recent years, the fraction of the ultrafine particles has increased, which may pose a threat to human health [1,2]. These soot particles consist of a carbonaceous solid core with an adsorbed layer of hydrocarbons, sulfates and polycyclic aromatic hydrocarbons (PAH). The size of the particles makes them inhalable which causes serious concern with regard to adverse health * Corresponding author. Tel.: +1 721 608 8058; fax: +1 310 825 8805. E-mail address: [email protected] (M. Seipenbusch). 0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.07.007
effects, including carcinogenity [3]. To reduce particulate matter (PM) emissions from motor vehicles, Europe and the US have set standards of for PM and NOx that cannot be met within a few years by improving engine technology alone [4]. Therefore, particulate filters have to be employed to reduce the emissions of diesel engines. Moreover, properly designed filters could be retrofitted on existing diesel engines, thus, significantly improving environmental conditions. Due to the demanding operating conditions – high and rapidly alternating temperatures as well as a chemically aggressive environment – these filters are made of ceramic monoliths or woven fibers [5]. To prevent clogging and the accompanying rise in pressure drop over the filter element,
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Fig. 1. Schematic of reaction mechanisms for oxidation of carbon by NO/ NO2.
the accumulated soot has to be burned off periodically or continuously [4]. To increase the soot oxidation rate at the typical exhaust temperatures of diesel engines to a point where it equals or exceeds the deposition rate, oxidation catalysts are introduced into the soot traps. The most commonly used materials for this purpose are V2O5, CuO and MoO3 [6,7], Fe2O3 and Co3O4, [8,9] and Pt [10]. In experimental studies, measurements were mostly made with macroscopic mixtures of catalyst particles and various types of soot. Thermogravimetric oxidation ([11,6]), temperature programmed oxidation (TPO) ([10,11]) and packed-bed experiments ([12,13]), represent such studies on various catalysts. It has been generally accepted that the presence of NO strongly decreases the combustion temperature of soot when platinum is used as catalyst. It was proposed that platinum promotes the oxidation of carbon by oxidizing NO into NO2 [15], which is transported through the gas phase to the soot aggregates where it oxidizes carbon [13] while being reduced to NO (Fig. 1). Many authors emphasize the importance of the type of contact between soot and the active phase of the catalyst [11]. Some materials, e.g., metal oxides and alkali carbonates [8] only showed a catalytic effect when being in close contact to the carbon particles. In a particle trap the soot particles are deposited in a way that allows only indirect contact [19]. Therefore, to study the catalytic behavior under conditions similar to those of practical interest seems to be the most logic approach. However, the sample configuration in these experiments differs from those employed in practice, where the catalyst particles are dispersed over a ceramic support. Hence, at increased reaction rates, these macroscopic powder samples may experience heat and mass transfer limitations [14], thus, obscuring the intrinsic catalytic properties of the system under investigation. Two goals can be identified. The first goal is to measure the oxidation rate of soot aggregates deposited on a surface in the close proximity of Pt particles using a new experimental technique. Changes in the particle morphology upon the reaction were observed off-line by scanning electron microscopy. In this way, reaction data could be correlated to the local surface coverage and particle size of the active phase, as well as to the primary particle- and agglomerate size of the soot particles.
The second goal was to produce platinum particles using two methods. Platinum catalysts in laboratories are normally produced by an impregnation method. First the support, e.g., SiO2 or g-Al2O3, is impregnated with a aqueous solution of the platinum precursor and dried. This is followed by calcination in air and a reduction step in hydrogen. The time and temperatures for each step differs from author to author. The currently employed production route for platinum particles was different because the support we used was a flat SiO2 surface where dispersed impregnation could not be achieved. Hence, the active phase was introduced by Electro HydroDynamic Atomization [16] and Laser Ablation [17].
2. Experimental 2.1. General The model catalyst consisted of platinum nanoparticles and carbon particles deposited on a SiO2 surface, such that there was loose contact between active phase and the carbon particles. These model catalysts were exposed to various gas phases at different temperatures in a tubular quartz reactor. Mass flow controllers were employed to adjust the flow rates of N2, O2 and NO. The NO inlet concentration was controlled by a NOx-monitor (Horiba). Reaction times ranged from 10 to 60 min. The interaction between active phase and carbon during the oxidation was determined analogously as by Schalow [20] and Jung et al. [21]. SEM micrographs of single carbon nanoparticle agglomerates were taken before and after oxidation, while Jung et al. used a TEM system. Structural changes induced by the reaction were monitored using the image analysis software ImageJ. This software was developed by the NIH and is available on the internet (SITE = http://rsb.info.nih.gov/ij/). The imaging was done using a field emission scanning electron microscope (Hitachi Model S-4700) at the UCLA Nanolab. The instrument has a resolution of 1.5 nm at 5 kV. By taking images at various magnifications of interest ‘‘road maps’’ of the surfaces were created thus allowing to find the same spot on the catalyst surfaces before and after the reaction. To distinguish between platinum and carbon particles, secondary electron (SE) and backscattering electron (BSE) micrographs were taken of the same spot. While platinum and carbon particles can both be seen on the SE micrographs, the BSE micrograph shows only platinum particles due to the much higher mass of the nuclei. 2.2. Soot preparation Depending on the conditions of formation, diesel soot includes graphitic components with varying amounts of associated aromatics, paraffins, oxygenated compounds and sulfates [18]. As a relatively simple surrogate for diesel soot, carbon nanoparticle agglomerates were produced by laser
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Fig. 2. Particles produced by EHDA: (a) chloroplatinic acid particles; (b) platinum particles produced by decomposition of particles similar to particles of (a).
ablation [17], an evaporation–condensation based technique. The energy of a pulsed laser (Lambda Physik model EMG 101 MSC) creates a short-lived plasma over a target of the desired material. During this short time, which is typical in the microsecond range, a very high aerosol concentration of carbon nanoparticles is generated. Hence, the collision frequency is high and coalescence occurs. Since, the cooling rate is extremely high, the coalescence is stopped early and colliding particles will agglomerate resulting in the formation of fractal-like structures of about 1 mm diameter. 2.3. Catalyst preparation The catalyst particles used in this work were made by two methods, which were electro hydro dynamic atomization (EHDA) and laser ablation. In all cases the catalyst support consisted of pieces of polished and superficially oxidized silicon wafers. The oxide layer was produced by oxidation of the silicon at 1000 8C in air for 1 h and had a thickness of 0.25 mm (http://www-mtl.mit.edu/6152j/appendix1.html). This is sufficient to neglect the influence of silicon. For the generation of Pt particles by EHDA a 0.2 wt.% solution of chloroplatinic acid (H2PtCl66H2O) in ethanol was used. The spraying nozzle had an o.d. of 0.16 mm and an i.d. of 0.06 mm. The flow rate was controlled by a high accuracy syringe pump (Harvard PHD 2000) and was kept constant at 8 ml/h while the potential between nozzle and the grounded counter electrode was maintained at 2.5 kV, leading to the cone-jet mode of operation. The jet broke up into very fine droplets, which yielded platinum salt particles after evaporation of the solvent. To form platinum particles the layers were put in an oven for 15 min which had a constant temperature of 700 8C. Upon thermal decomposition, the salt particles broke apart and platinum particles of a smaller size were formed. For lower coverages of Pt on the SiO2 surface, catalyst particles were also generated by laser ablation as described above. The target used was a Pt foil (Alfa Aesar 99.99% purity). Since non-agglomerated Pt
particles were desired, the aerosol was passed through a sintering furnace at 700 8C to reach total coalescence of the agglomerates.
2.4. Data acquisition and evaluation The analysis of the SEM images with ImageJ yielded the projected areas of agglomerates. Ullmann et al. [17] produced the agglomerates in the same way and found a fractal dimension of the agglomerates of 1.85. Therefore, we assume that the fractal dimension of the agglomerates is below two. In this case, the projected area of the agglomerate is meaningful for the calculation of particle mass, since there is very limited shading of primary particles mutually. Using the median diameter from the primary particle size distribution, the number of primary particles, N, with the same projected area as the agglomerate was calculated. From the primary particle diameter, the density and N, the mass of the particles could be calculated. The reaction rate was determined from the difference in carbon mass before and after the reaction and was normalized to the initial carbon mass to yield a mass specific reaction rate.
3. Results and discussion 3.1. Platinum catalyst production Chloroplatinic acid salt particles produced by EHDA are shown in Fig. 2a. The salt particles decompose when heated for 15 min at 700 8C, thus, forming groups of smaller platinum particles. The original form of the salt particles can still be recognized, as shown in Fig. 2b [22]. Platinum particles produced by laser ablation are shown in Fig. 3. Clearly, the coverage of platinum particles is much lower than in case of EHDA. Also the sizes and structures differ from the EHDA produced particles. The
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Fig. 3. Platinum particles produced by laser ablation.
size distributions of the platinum particles produced by EHDA and laser ablation are shown in Fig. 4. The carbon particles produced using this method are shown in Fig. 5. The aggregated structure of the carbon particles resembles soot particles produced by diesel engines. The primary particle size distribution is shown in Fig. 6. From it, the specific surface area, Sv, of the carbon particles was calculated and found to be approximately 100 m2/g. This was done by dividing the total surface over the total mass of the particles, yielding: Sv ¼
S Npd2 6 ¼ ¼ m Nð1=6Þpd3 r dr
where d is the particle size and r is the density of the particle. The density was taken to be the density of the bulk material. The surface area was calculated because no BET measurements were possible with the small amounts produced. The particle size was the surface based mean diameter calculated by transforming the accumulated log–normal number based
Fig. 5. SEM micrograph of a carbon aggregate produced by laser ablation.
(Q0(x)) distribution into a surface based accumulated size distribution (Q2(x)). 3.2. Reaction data To determine the effect of local Pt coverage on the reaction rate, the projected area of the Pt-particles was determined from the SEM images. From the size distribution of Pt particles (Fig. 4), it was possible to calculate the local platinum surface area per unit surface area of the substrate. The rates of the carbon oxidation in air/NO were normalized to a temperature of 300 8C using the activation energy determined from the data. The results are plotted versus the relative surface area of the catalyst in Fig. 7. Since the data in
Fig. 4. Size distributions of the Pt particle size on samples produced by the electrospray (EHDA) method and by laser ablation with subsequent sintering. The mean particle size, x50, was about 12 nm for EHDA and 23 for laser ablation. The lines are best fits of log–normal functions.
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Fig. 6. The cumulative primary particle size distribution of carbon agglomerates produced by laser ablation. The curve is a log–normal function fitted to the data with an x0, 50 of 17 nm an a s of 0.5.
the diagram correlate linearly, the rate of carbon oxidation in air/NO is proportional to the Pt surface area. The linear relation points to the catalytic activity of the platinum. While we observed an influence of the surface area of Pt, the method of generation of the Pt particles (EHDA or laser ablation) did not seem to have an effect on their activity as catalysts. For the oxidation of carbon in air, the reaction rates for three different temperatures were plotted in a similar manner as a function of the relative surface area of the catalyst, see Fig. 8. In this case, the dependency of the oxidation rate on the relative Pt surface area is more complicated. In the absence of Pt, reaction rates of up to 2.5 104 s1 are reached. With increasing Pt surface area, the rates decrease steeply to approximately one-fifth of the initial value, and increased again for higher ratios of Pt surface area to
unit area of substrate surface. This behavior of the reaction rate may result from the effect of two different reaction mechanisms, one inhibited by the presence of small amounts of Pt and the other favored by larger amounts of Pt. The first mechanism would be the direct oxidation of carbon by O2 adsorbing from the gas phase while the second could be a spill-over of activated oxygen species from the Pt contacts to the carbon particles. While we lack an explanation for the inhibiting effect of Pt on the direct oxidation, the positive influence of a large Pt surface and a shorter average distance between carbon- and Pt-particles on a spill-over mechanism seems obvious. However, Dernaika and Uner [11] found that the presence of some transition metal oxides could raise the combustion temperature of soot, relative to the uncatalysed case, in a dry air atmosphere. This could be due to a similar phenomenon as in our observations.
Fig. 7. Calculated rates of oxidation of carbon in an Air/NO-atmosphere at a standardized temperature of 300 8C plotted vs. the surface area of catalyst per unit area of substrate. The open symbol represents data from catalysis with Pt produced by laser ablation while the full symbols show data from Pt produced by EHDA.
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Fig. 8. Rates of reaction of oxidation of carbon in air at 325, 350 and 375 8C plotted vs. the surface area of Pt relative to the unit surface area of the substrate. Open symbols represent reaction rates obtained for Pt-particles from laser ablation, full symbols are for EHDA. The line is only a guide to the eye.
Although the activation energies are different compared to other studies, the oxidation temperatures are comparable. To determine the activation energy for the catalytic combustion of soot in our system, two series of experiments were conducted for in a mixture of 20% O2 (Airgas, ultrahigh purity), in balance N2 (Airgas, ultra-high purity) and for an air/NO atmosphere consisting of 1000 ppm NO (Linde 0.5% NO in balance N2) in 20% O2 in balance N2 at temperatures ranging from 260 to 380 8C. Fig. 9 shows the reaction rates as a function of the reciprocal temperature for both sets of experiments. Since the data are plotted in the Arrhenius-form, the activation energies can be obtained from the slopes of the linear fits to the data points. For the oxidation of carbon in air, an activation energy of 40 20 kJ/mol was obtained by the least squares fit (see Fig. 5). This is low compared to data found by other authors. Dernaika and Uner [11] found an activation energy of
156 kJ/mol in the case of soot oxidation in dry air using a platinum catalyst on a SiO2 support. In many other studies, with a platinum catalyst but different support material, activation energies between 140 and 170 kJ/mol where found [23]. The discrepancy between the literature values and our study can be due to different preparation methods of the catalyst and carbon, or because of different experimental conditions. While all authors use TPO experiments, which are on the macroscopic scale, the oxidation in this study is observed on the microscale by analysis of SEM images. This difference can lead to discrepancies in observed activation energies. For the reaction in an air/NO-atmosphere a value of 160 8.5 kJ/mol was found from the least squares fit (see Fig. 9). The activation energy for the carbon oxidation in air/ NO is high compared to data in the literature. Jelles et al. [12] found an activation energy of 93 10 kJ/mol for a Pt
Fig. 9. Arrhenius-plot of the reaction rates of the catalytic combustion of carbon in air and Air/NO-atmospheres. Activation energies: for oxidation in air 40 (20) kJ/mol; for oxidation in Air/NO 160 (8.5) kJ/mol. Open symbols show data for Pt particles produced by laser ablation, full symbols for EHDA. Reaction rates calculated for a relative Pt surface area.
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based catalyst mixed with a cerium activated soot. From the data reported by Setiabudi et al. [13] on Pt/gAl2O3 we calculated an activation energy of 46 kJ/mol, which may indicate a mass transfer limitation in the bulk samples used in their studies. The catalyst generally lowers activation energy, typically by 30–80 kJ/mol (Stanmore2001). Thus, the activation energy found in this study is much higher compared to other studies. Notwithstanding the higher activation energy, Fig. 9 convincingly shows that the reaction rates in the presence of NO are about two to three orders of magnitude higher than those in absence of NO at comparable temperatures, in line with results of Setiabudi et al. [13]. This difference is supposedly caused by the pre-exponential factor (k) in the Arrhenius equation: Ea r ¼ k exp RT
bulk powder samples, which may be due to mass and heat transfer limitation in bulk systems. In summary, the experimental methods used in this study yield detailed local information on catalyst and carbon particles and their interaction during the reaction, which cannot be obtained in studies on bulk samples. Therefore, this method has potential for optimizing catalyst dispersion and also catalyst particle properties with respect to the catalytic activity, a process we continue to study.
where r is the reaction rate, k the pre-exponential factor, Ea the activation energy, R the gas constant, and T is the absolute temperature. For our data the pre-exponential factor of the oxidation in air is 0.23 s1 where in air/NO this factor is 3.7 1010 s1. The pre-exponential factor is a combination of two factors, i.e., a k0 and the number of active sites on the catalyst, N. Although the k0 may vary to some extend between various reactions, it is not likely to be expected to vary over 11 orders of magnitude, as could be inferred from the current results. Therefore, it can be concluded from our experiments that, apparently, the number of sites present on a platinum surface is higher for the NO reaction than for the oxidation of carbon in a nitrogen/oxygen atmosphere alone. Rates for the oxidation of carbon in air/NO found by Jelles et al. [12] using a Pt catalyst were lower by one to two orders of magnitude than our results; The reason for the higher reaction rates in our single particle experiment may be mass and heat transfer limitations in the bulk experiments.
References
4. Conclusions For Pt/SiO2-catalyzed oxidation of soot in air and air/NO, a strong dependency of the reactions rates on the Pt surface area is observed. For air/NO, the reaction rate is linearly proportional to the Pt-surface area. For carbon oxidation in air, two effects of the Pt seem to exist. However, the method of particle production, laser ablation with subsequent sintering or EHDA, did not seem to have an effect on the activity of the catalyst. The reaction rates and also the activation energy for the reaction in air/NO were higher than in comparable studies on
Acknowledgments Support for this work by the Alexander von HumboldtFoundation and the Parsons Foundation is gratefully acknowledged.
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